Computer Networks (EENG 4810)

Computer Networks (EENG 4810)
Computer Networks- Course Objectives & Scope - 1

Course Objectives & Scope
Computer Networks- Course Objectives & Scope - 2

In this class, you are expected to learn -
A brief History of Computer Networks Categorization of Computer Networks Network Services and Internet Perspective Network Components- Nuts and Bolts View General Concepts of Network Design Protocols and Layered Communication Architecture Network Programming Computer Networks- Course Objectives & Scope - 3

This class, however, does not deal with -
Network Hardware Design Comparative analyses of different protocol standards Special purpose networks such as ad hoc sensor nets Applications of Queuing Theory to Network traffic control Computer Networks- Course Objectives & Scope - 4

Lesson 1: History of Computer Networks

History of Computer Networks - 1
Preview of the Lesson 1 In this lesson, we cover History of Computer Networks organized into approximately 5 decades. In passing, we get a hang of what all a computer network can do History of Computer Networks - 1

History of Computer Networks
Development of Packet Switching: Proprietary Networks and Internetworking: Proliferation of Networks: Internet Explosion: Developments of Last Decade: Bubble burst? Social Networks? History of Computer Networks- 2

Development of Packet Switching: 1961-72
Telephone network - World’s dominant communication network , uses circuit switching. (Early 1960s) Three research groups around the world independently invented packet switching (1964 – 1967) Leonard Kleinrock at MIT used queuing theory to demonstrate effectiveness of packet switching for bursty traffic Paul Baran of Rand Institute investigated packet switching for secure voice communication over military networks Donald Davies and Roger Scantlebury were developing ideas on packet switching at the National Physical Lab, England. Lesson 1: History of Computer Networks - 3

Development of Packet Switching: 1961-72 (continued)
J.C.R. Licklider and Laurence Roberts led the CS program at ARPA (Advanced Projects Research Agency) and published a plan for ARPAnet in 1967. Arpanet was the ancestor of today’s Internet. Early Packet switches were known as Interface Message Processors (IMPs). BBN got the contract. First IMP was installed at UCLA on Labor Day 1969 under Kleinrock’s supervision. Later 3 more at SRI, UCSB and University of Utah. Lesson 1: History of Computer Networks - 4

Leonard Kleinrock with IMP
Lesson 1: History of Computer Networks - 5

Development of Packet Switching: 1961-72 (continued)
First use of the net of 4 nodes was remote login from UCLA to SRI; it resulted in system crash. Robert Kahn demonstrated 15-node ARPAnet in 1972 ICCN. First host to host protocol was Network Control Protocol (NCP). Ray Tomlinson at BBN wrote the first program in 1972. Lesson 1: History of Computer Networks - 6

Proprietary Networks and Internet 1972-80
ALOHAnet- microwave satellite net linking universities on Hawaii islands (Norman Abramson 1970). Telenet- a BBN commercial packet network and Cyclades- a French Packet Net by Louis Pouzin. Time-sharing networks such as Tymnet and GE Information Services Net (late 60s and early 70s). Metcalfe’s PhD thesis proposing Ethernet. History of Computer Networks - 7

Proprietary Networks and Internet 1972-80 (Continued)
Proprietary Networks such as IBM’s ( ) System Network Architecture (SNA) paralleling the ARPAnet (Schwartz 1977). DEC’s DECnet and Xerox corporation’s XNA. Vincent Cerf and Robert Kahn (Cerf 1974)- Architecture for interconnecting Networks (They coined the word Internet for network of networks). DARPA’s packet satellite and packet-radio networks (Kahn 1978). History of Computer Networks - 8

Proprietary Networks and Internet 1972-80 (Early Internet Features)
Cerf and Kahn’s TCP (quite different from now) It combined reliable in-sequence delivery of data by end-system retransmission (as now) with forwarding (as IP now) Realization of usefulness of separation of unreliable, non-flow controlled end-to end transport service for applications such as packetized voice led to separation of IP. Three internet protocols TCP, IP and UDP - conceptually in place by the end of 1970’s. Main features of their InterNet- Minimalism, autonomy (no internal changes required for interconnection), Best effort delivery, stateless routers and decentalized control. History of Computer Networks - 9

Proprietary Networks and Internet 1972-80 (Early Ethernet Features)
Abramson’s ALOHA protocol- a multiple-access protocol for communication among geographically distributed users by a single shared broadcast medium. Metcalfe and Bogg’s EtherNet protocol for wire-based shared networks was originally motivated by the need to connect multiple PCprinters Lesson 1: History of Computer Networks - 10

Proliferation of Networks 1980-90
100 nodes by late 70’s New national networks (100,000 by the end of 80’s) BITNET for and FTP services among many North East Universities CSNET (computer Science Network) for researchers with no access to APRPAnet. NSF-net for access to NSF-sponsored super-computing centers Starting with a backbone of 56 kbps, NSF net was running at 1.5 Mbps by the end of the decade. History of Computer Networks - 11

Proliferation of Networks 1980-90 (Continued)
Simple Message Transfer Protocol (SMTP): 1982 Deployment of TCP /IP replacing NCP (Jan. 1, 1983) FTP- The File Transfer Protocol defined (1983). Host-based TCP Congestion Control (Jacobson 1988). Domain Name System (DNS)- mapping between human readable Internet computer name and 32-bit IP address. Lesson 1: History of Computer Networks - 12

Proliferation of Networks 1980-90 (The Minitel Project)
French Minitel project paralleling ARPAnet Ambitious projest sponsored by the French Government X.25 protocol suite using virtual circuits By mid-90’s, it offered more than 20, 000 services- from home banking to research database Used by more than 20% of the population Generated over $1 billion in revenue Was in most French homes 10 years before Americans had ever heard of the Internet. Lesson 1: History of Computer Networks - 14

Internet Explosion: The1990s
Early 90’s Arpanet decommissioned as Milnet and Defense Data Net grew enough to carry all defense-related traffic. NSF lifted restrictions on commercial use of NSFnet (1991). NSFnet began to serve as a backbone and was later decommissioned it in 1995. Web invented at CERN by Tim Berners-Lee (89-91) Developed intial versions of HTML, HTTP, a web server and a web browser - Based on the original work on Hypertext in 1940s by Bush (1945) and in 1960s by Ted Nelson Marc Andreesen developed Mosaic- Popular GUI browser. History of Computer Networks -15

Internet Explosion: First half of1990s
Marc Andreesen and Jim Clark formed Mosaic Communications in 1994(it later became Netscape). By 1995, University students were able surf web. Big and small companies started transacting on the web and transact commerce over the web. History of Computer Networks -16

Internet Explosion: Second half of1990s
Microsoft (MS) started making browsers (1996) and this started the war with NetScape which MS won later. evolved with address books, attachments, hot links, multimedia support. 4 Killer applications Web accessible Web browsing & internet commerce instant messaging with contact lists pioneered by ICQ peer-to-peer file sharing of MP3s , pioneered by Napster . By late 90’s, 50 million computers with 100+ million users on the web. 1 GBs Back bone link speeds achieved. History of Computer Networks -17

Developments of Last Decade
Financial turmoil, many start-ups collapsed. Still many companies like eBay, Yahoo, Amazon and Cisco emerged as winners despite setbacks in their stock prices. Advances in content distribution, internet telephony, high speed LANs and fast routers 3 Important developments High Speed Access Internet Access (Cable/DSL/Wireless LANs) Secure applications P2P (Point-to-point Networking) History of Computer Networks -18

Three Important Recent Developments I- High Speed Internet Access
Increased penetration of broadband residential Internet via Cable and DSL with applications such as high-quality Video on Demand and high quality Video Conferencing Increased ubiquity of public Wi-Fi nets (with 11 Mbps and higher speeds) Internet access via mobile phones of 3rd Generation & Beyond; proliferation of social networks History of Computer Networks -19

Three Important Recent Developments II- Security
Intrusion detection methods for early warning of denial of service attacks through worms (e.g. Blaster worm) that infect systems and clog networks. Use of Firewalls to filter unwanted traffic before it enters the network. Use of IP-traceback to pinpoint the origin of attacks. History of Computer Networks -20

Three Important Recent Developments III- P2P Networking
P2P application exploits resources (memory, disk-space, content and CPU cycles) in user’s computers. It gives significant autonomy from central servers. KaZaA is the most popular p2P-file sharing system. Currently, this network has 4 million connected systems and its traffic constitutes 20-50% of Internet traffic. History of Computer Networks -21

History of Computer Networks - 22
Summary and Follow-up In this lesson, we covered History of Computer Networks organized into approximately 5 decades. In passing, we found what all a computer networks can do. This will help you to write the first chapter of your project report i.e. to prepare a table of requirements for your own network! You got used to some terminology e.g. circuit switching, packet switching, firewalls, etc. If any of those concepts are not clear, you may search the web, discuss with me or wait on till we take them up in a greater detail later. Explore the concepts- Circuit/Virtual Circuit/Packet switching on the web. History of Computer Networks - 22

Lesson 2: Overview of Computer Networks

Overview of Computer Networks - 1
Preview of the Lesson 2 In this lesson, we try to answer the question- What is a Computer Network? We try to view computer networks from different perspectives. In other words, we try to answer the question: what are all the different types computer networks? We will have an overview of different components of a computer network (Internet). We also study a little bit of how the interconnected computers communicate with one another, that is, we will have cursory glance at protocol stacks. Overview of Computer Networks - 1

Computer Networks- Definition & Perspectives
Reference: What is a Computer Network? A system for communication among two or more computers. What are all the different types computer networks? Different ways of categorization of Computer networks are: Range or extent of the network Inter-nodal functional relationship Network Topology Specialized functions of the nodes Overview of Computer Networks - 2

Network Categorization based on the Range I- Personal Area Network (PAN) With a reach of a few meters, connects home/small office devices/computers or higher level net/Internet (in the latter case called an uplink) could be wired (using Universal Serial Bus, shortly USB, or Fire-wire) or wireless (using blue-tooth or IrDA, that is, Infrared Data Association) Blue Tooth PAN is also called Piconet IEEE adapts Physical and MAC layers from Bluetooth 1.1 Zigbeee is a proprietary technology for low power radios based on IEEE Overview of Computer Networks - 3

Network Categorization based on the Range II - Local Area Network (LAN) Range is less than 1000 m2 Could be used in home, small office or university. Earlier popular LAN was proprietary - DataPoint’s ArcNet IEEE later produced two LAN standards- Ether Net (IEEE 802.3) and Token Ring (IEEE 802.5) LAN speeds could be 10/100 Mbps (Ether Net) and 4/16/100 mbps/1 Gbps (Token Ring) Wireless LANs- IEEE (Wi-Fi)- speeds up to 56 Mbps Overview of Computer Networks - 4

Network Categorization based on the Range III - Metropolitan Area Network (MAN) Spans a city or a big campus with range up to 200 km (125 miles) Earlier technologies used for MANs were: Fiber Distributed Data Interface (FDDI) Switched Megabit Data Service (as defined by IEEE MAN standard) using either B-ISDN or Distributed Dual-Queue Dual Bus (DQDB) with speeds 1.5/45 Mbs. Asynchronous Transfer Mode (ATM) Above technologies are being displaced by 1GB Ether Net based Mans MAN links between LANs and WANs are usually microwave/ infra-red/radio. Overview of Computer Networks - 5

Network Categorization based on the Range IV - Wide Area Network (WAN)
Covers wide geographical areas spanning multiple cities. Works on leased lines and connects multiple LANs Uses protocols such as TCP/IP, x.25, Frame Relay and ATM Usually used to connect different sites of an organization or service provider. For this reason, it is being replaced by Virtual Private Networks (VPNs). VPNs are of two types- i) Secure (they use leased lines and use protocols like IPSEC ii) Trusted (They rely on security of single provider’s network and use protocols such as Multi-protocol label switching (MPLS) and Layer 2 Tunneling Protocol (L2TP) Overview of Computer Networks - 6

Network Categorization based on the Functional Relationship of the Nodes Client- Server Network Multi-tier architecture (GUI, business logic and DB could be in 3 separate tiers) Peer-to-Peer Network (each node acts as both a client and server, e.g. in case of ). Overview of Computer Networks - 7

Network Categorization based on the Network Topology
Bus Network Star Network Ring Network Grid Network Toroidal Networks and Hypercubes Tree and Hyper-tree Networks Overview of Computer Networks - 8

Network Categorization based on Specialized Function
Storage Area Network (SAN)- used for connecting multiple storage devices such as disk controllers and tape libraries to a server. Server Farms (Network of servers maintained by an enterprise) Process Control Network- transmits data between measurement and control units. Value Added Network (VAN)- a third party network put up to add value (e.g. maintenance & admin) to an enterprise network SOHO (small office home office) Network- use ethernet/Wi-Fi Wireless Community Networks- meant for hobbyists and use wireless LANs- outgrowths of amateur radio clubs. Overview of Computer Networks - 9

Nuts and Bolts view of Computer Network with Internet- Network of Networks Overview of Computer Networks - 10

Network Building Blocks
Switch - connects computing devices to host computers, allowing a large number of devices to share a limited number of ports Router - a Protocol-dependent device that connects sub-networks together Bridge - a device that interconnects local or remote networks Gateway - a device that can interconnect networks with different, incompatible communications Overview of Computer Networks - 11

Network Building Blocks (Continued)
Network hosts, workstations, etc. - they generally represent the source and sink (destination) of data traffic (packets) Multiplexer - telecommunications device that funnels multiple signals onto a single channel Transceiver - (short for transmitter-receiver), is a device that both transmits and receives analog or digital signals. Firewall - a system or group of systems that enforces an access control policy between an organization's network and the Internet for purposes of security. Overview of Computer Networks - 12

“Nuts and bolts” view of the Internet
It is a loosely hierarchical network of networks (some private intranets) with millions of connected computing devices: Hosts, end-systems (Network Edge) pc’s workstations, servers PDA (Personal Digital Assistant)’s phones, toasters running network apps : Communication links (Network Access) fiber, coaxial cable, copper, radio, satellite Switches, routers, bridges, gateways (Network Core) local ISP company network regional ISP router workstation server mobile Overview of Computer Networks - 13

What’s a protocol? Human protocols: A way of communication between humans Dictated by local culture Greeting, response, action taken Examples: “Hey, got time?,” “I have a dumb question,” This is so and so..” Network protocols: Machines rather than humans involved, but all Internet communication activity is governed by protocols Dictated by standards Protocols define format, order of messages sent and received among network entities, and actions taken on message transmission and receipt Example: TCP/IP, ISO Overview of Computer Networks - 14

Human and Network Protocol Examples
Hi TCP connection req. Hi TCP connection reply. Got the time? Get 2:00 <file> time Overview of Computer Networks - 15

Protocols Building blocks of a network architecture Each protocol object has two different interfaces service interface: defines operations on this protocol peer-to-peer interface: defines messages exchanged with peer Term “protocol” is overloaded specification of peer-to-peer interface module that implements this interface Overview of Computer Networks - 16

Why Protocol “Layers?” Question: Simple Answer:
Networks are complex; they have many heterogeneous “pieces”: Hosts, routers, links of various media, Application entities, protocols, hardware, software … Question: How to achieve effective communication in this mess? Simple Answer: Divide & Conquer Overview of Computer Networks - 17

Why layering? Divide & Conquer Policy to handle Complex systems: Explicit structure allows identification of complex system’s pieces and their inter-relationships. Following slides present an example of a layered real-life protocol. Modularization eases maintenance and updating of system change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system Cost: Layering may affect efficiency, but is inevitable. Overview of Computer Networks - 18

Steps in Organization of air travel
ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing Overview of Computer Networks - 19

Layered services in air travel
Counter-to-counter delivery of person+bags baggage-claim-to-baggage-claim delivery people transfer: loading gate to arrival gate runway-to-runway delivery of plane airplane routing from source to destination Overview of Computer Networks - 20

Distributed implementation of layer functionality
ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing Departing airport arriving airport intermediate air traffic sites airplane routing airplane routing airplane routing Layers: each layer implements a service via its own intra-layer actions relying on services provided by layer below Overview of Computer Networks - 21

Internet protocol stack
Application: supporting network applications (e.g. ftp, smtp, http) Transport: host-host data transfer, defines quality and nature of data delivery (e.g. tcp, udp) application transport network link physical Network: addressing and routing of datagrams from source to destination (e,g. Ip & other routing protocols) Link: logical organization of data bits transmitted on a particular medium; framing, addressing, error correction/detection (check sum) e.g. ppp, ethernet Physical: bits “on the wire” Defines physical Properties of various media e.g. Ether-Net cable size 7-layer OSI protocol (of ISO) has session (reply and response packet pairing) and presentation layers (data syntax, encryption) above transport and below application layer. Overview of Computer Networks - 22

Layering: logical communication
Each layer: distributed “entities” implement layer functions at each node entities perform actions, exchange messages with peers application transport network link physical Overview of Computer Networks - 23

Layering: logical communication (continued)
data E.g.: transport Take data from app Add addressing, reliability check info to form “datagram” Send datagram to peer Wait for peer to ack receipt Analogy: post office application transport network link physical transport ack data data transport Overview of Computer Networks - 24

Layering: physical communication
data application transport network link physical network link physical application transport network link physical data application transport network link physical application transport network link physical Overview of Computer Networks - 25

Protocol layering and data
Each layer takes data from above, adds header information to create new data unit and passes new data unit to layer below source destination application transport network link physical application transport network link physical M H t n l message M H t n l segment datagram frame Overview of Computer Networks - 26

Protocol Data Units The combination of data from the next higher layer and control information is referred to as PDU. Control Information in the Transport Layer may include: Destination Service Access Point (DSAP) Sequence number Error-detection code Overview of Computer Networks - 27

Service Access Point A Service Access Point (SAP) is the location where a layer (N-1) entity provides service for a layer (N) entity. SDU: Service Data Unit ICI: Interface Control Information IDU: Interface Data Unit PDU: Protocol Data Unit Overview of Computer Networks - 28

Overview of Computer Networks -29
Summary of the Lesson 2 In this lesson, we addressed the question- What is a Computer Network? We studied the classification of computer networks from different perspectives i.e. had a taxonomic view. We had a components view of the computer network. We have also studied a little bit of how the interconnected computers communicate with one another, that is, we had cursory glance at protocol layers/stacks. Overview of Computer Networks -29

Lesson 3: Preview/Objectives
High level view of network application protocols client server paradigm service models learn about protocols by examining popular application-level protocols such as dns smtp pop ftp (Next Lesson) http (Next Lesson) Multimedia (Next Lesson) Lesson 3: Application Layer - 1

Application layer – Some Jargon
Applications (e.g., , file transfer, the Web): communicating, distributed processes running in network hosts in “user space” exchange messages to implement app Application-layer protocols one “piece” of an app define messages exchanged by apps and actions taken Depend on user services provided by lower layer protocols application transport network data link physical Lesson 3: Application Layer - 2

Network applications: some jargon
A process is a program that is running within a host. Within the same host, two processes communicate with inter-process communication defined by the OS. Processes running in different hosts communicate with an application-layer protocol A user agent is an interface between the user and the network application. Web-browser mail reader streaming audio/video: media player Lesson 3: Application Layer - 3

Client-server paradigm
Typical Application has two pieces: Client and Server application transport network data link physical reply request Client: initiates contact with server (“speaks first”) typically requests service from server, for Web, client is implemented in browser; for , in mail reader Server: provides requested service to client e.g., Web server sends requested Web page, mail server delivers Lesson 3: Application Layer - 4

Client-Server Communication
Client and Sever, as a matter of fact, any two applications on different hosts, communicate using what is called an API: application programming interface that defines interface between application and transport layer e.g. socket: the Internet API two processes communicate by writing data into socket and reading data out of socket How does a process “identify” the other process with which it wants to communicate? IP address of host running other process “Port number” - allows receiving host to determine to which local process the message should be delivered Lesson 3:Application Layer - 5

Services Provided by the Transport Layer to Applications
Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Bandwidth some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” other apps (“elastic apps”) make use of whatever bandwidth they get Timing some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Lesson 3:Application Layer - 6

Transport service requirements of common apps
Application file transfer Web documents real-time audio/video stored audio/video interactive games financial apps Data loss no loss loss-tolerant Bandwidth elastic audio: 5Kb-1Mb video:10Kb-5Mb same as above few Kbps up Time Sensitive no yes, 100’s msec yes, few secs yes and no Lesson 3:Application Layer - 7

Services provided by Internet transport protocols
TCP service: connection-oriented: setup required between client, server reliable transport between sending and receiving process flow control: sender won’t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum bandwidth guarantees UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? Lesson 3:Application Layer - 8

Internet application protocols and corresponding transport protocols
layer protocol smtp [RFC 821] telnet [RFC 854] http [RFC 2068] ftp [RFC 959] proprietary (e.g. RealNetworks) NFS (e.g., Vocaltec) Underlying transport protocol TCP TCP or UDP typically UDP Application remote terminal access Web file transfer streaming multimedia remote file server Internet telephony Lesson 3:Application Layer - 9

DNS: Domain Name System
Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “Name”, e.g., gaia.cs.umass.edu - used by humans People: many identifiers: SSN, Passport # Name Lesson 3: Application Layer - 10

DNS: Domain Name System
Application providing Mapping between IP addresses and domain name distributed database implemented in hierarchy of many name servers application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) note: core Internet function implemented as application-layer protocol complexity at network’s “edge” Lesson 3: Application Layer - 11

Lesson 3: Application Layer - 12
DNS name servers Why not centralize DNS? single point of failure traffic volume distant centralized database Maintenance doesn’t scale! Two types Name servers- Local name servers: each ISP, company has local (default) name server host DNS query first goes to local name server Authoritative name server: for a host: stores that host’s IP address, name can perform name/address translation for that host’s name Hence, the distributed organization where server has all name-to-IP address mappings. Lesson 3: Application Layer - 12

DNS: Root name servers contacted by local name server that can not resolve name root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server ~ dozen root name servers worldwide Lesson 3: Application Layer - 13

Simple DNS Scenario root name server Host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. Contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.umass.edu, if necessary 4, 5 & 6 are responses in reverse order. 2 4 3 5 local name server dns.eurecom.fr authorititive name server dns.umass.edu 1 6 requesting host surf.eurecom.fr gaia.cs.umass.edu Lesson 3: Application Layer - 14

A More Complex DNS Scenario
root name server Root name server: may not know authoratiative name server, but may know intermediate name server: who to contact to find authoritative name server 2 6 7 3 intermediate name server dns.umass.edu local name server dns.eurecom.fr 4 5 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu Lesson 3: Application Layer - 15

DNS: iterated queries recursive query: iterated query:
root name server recursive query: puts burden of name resolution on contacted name server heavy load? iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” iterated query 2 3 4 7 local name server dns.eurecom.fr intermediate name server dns.umass.edu 5 6 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu Lesson 3: Application Layer - 16

DNS: caching and updating records
once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time update/notify mechanisms under design by IETF RFC 2136 Lesson 3: Application Layer - 17

DNS records DNS: distributed db storing resource records (RR) Type=A
RR format: (name, value, type,ttl) Type=A name is hostname value is IP address Type=CNAME name is an alias name for some “cannonical” (the real) name value is cannonical name Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain Type=MX value is hostname of mail server associated with name Lesson 3: Application Layer - 18

DNS protocol & messages
DNS protocol : query and repy messages, both with same message format msg header identification: 16 bit # for query, repy to query uses same # flags: query or reply recursion desired recursion available reply is authoritative Lesson 3: Application Layer - 19

DNS protocol & messages (Continued)
Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used Lesson 3: Application Layer - 20

Electronic Mail user mailbox outgoing message queue Three major components: user agents mail servers simple mail transfer protocol: smtp User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on server mail server user agent SMTP Lesson 3: Application Layer - 21

Electronic Mail: mail servers
user agent SMTP Mail Servers mailbox contains incoming messages (yet to be read) for user message queue of outgoing (to be sent) mail messages smtp protocol between mail servers to send messages client: sending mail server “server”: receiving mail server Lesson 3: Application Layer - 22

Electronic Mail: smtp [RFC 821]
uses tcp to reliably transfer msg from client to server, port 25 direct transfer: sending server to receiving server three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII Lesson 3: Application Layer - 23

Try smtp interaction for yourself
telnet servername 25 see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send without using client (reader) Lesson 3: Application Layer - 24

Sample smtp interaction
S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection Lesson 3: Application Layer - 25

smtp: Some Observations
Comparison with http http: pull push both have ASCII command/response interaction, status codes http: each object is encapsulated in its own response message smtp: multiple objects message sent in a multipart message smtp uses persistent connections smtp requires that message (header & body) be in 7-bit ascii certain character strings are not permitted in message (e.g., CRLF.CRLF). Thus message has to be encoded (usually into either base-64 or quoted printable) smtp server uses CRLF.CRLF to determine end of message Lesson 3: Application Layer - 26

Mail message format smtp: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from smtp commands! body the “message”, ASCII characters only header blank line body Lesson 3: Application Layer - 27

Message format: multimedia extensions
MIME (Multipurpose Internet Mail extension): Contains multimedia mail extensions, RFC 2045, 2056 additional lines in msg header declare MIME content type From: To: Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......base64 encoded data MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data Lesson 3: Application Layer - 28

MIME types Content-Type: type/subtype; parameters
Text example subtypes: plain, html Image example subtypes: jpeg, gif Audio exampe subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding) Video example subtypes: mpeg, quicktime Application other data that must be processed by reader before “viewable” example subtypes: msword, octet-stream Lesson 3: Application Layer - 29

Multipart Type From: To: Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary= Content-Transfer-Encoding: quoted-printable Content-Type: text/plain Dear Bob, Please find a picture of a crepe. Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......base64 encoded data Lesson 3: Application Layer - 30

Mail access protocols POP3 or IMAP user agent user agent SMTP SMTP sender’s mail server receiver’s mail server SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP3: Post Office Protocol version 3 [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 2060] more features (more complex) manipulation of stored msgs on server Webmail/HTTP: Hotmail , Yahoo! Mail, etc. Lesson 3: Application Layer - 31

POP3 protocol S: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK user successfully logged on authorization phase client commands: user: declare username pass: password server responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> C: dele 1 C: retr 2 C: dele 2 C: quit S: +OK POP3 server signing off Lesson 3: Application Layer - 32

How POP3 Works? Note : DNS name or IP address of ISP server is typically configured when is set up. Lesson 3: Application Layer - 33

POP3 versus IMAP POP3 is widely used because of simplicity and robustness. Both allow downloads from different places, but POP3 assumes user will clear out all messages from server on every contact and works offline after that. This makes spread on different machines. IMAP (Internet Message Access Protocol) assumes messages remain indefinitely on the server. IMAP provides facilities to manipulate messages/ mailboxes on the server Lesson 3: Application Layer - 34

Lesson 3: Summary and Follow-up
We had a High level view of network application protocols using client server paradigm service models We learned about three of the most common application-level protocols dns smtp pop In the next class, we deal with three very popular application protocols ftp http Multimedia Lesson 3: Application Layer - 35

Lesson 4: More Application Layer Protocols

Learn about the following popular application-level protocols ftp http Multimedia Lesson 4: More Application Layer Protocols - 1

ftp: The file transfer protocol
user interface client file transfer FTP server user at host local file system remote file system transfer file to/from remote host client/server model client: side that initiates transfer (either to/from remote) server: remote host ftp: RFC 959 ftp server: port 21 Lesson 4: More Application Layer Protocols - 2

ftp: separate control, data connections
ftp client contacts ftp server at port 21, specifying TCP as transport protocol two parallel TCP connections opened: control: exchange commands, responses between client, server. “out of band control” data: file data to/from server ftp server maintains “state”: current directory, earlier authentication FTP client server TCP control connection port 21 TCP data connection port 20 Lesson 4: More Application Layer Protocols - 3

ftp commands, responses
Sample commands: sent as ASCII text over control channel USER username PASS password dir/ls return list of files in current directory Put filename retrieves (gets) file Get filename stores (puts) a local file on remote host Sample return codes status code and phrase (as in http) 331 Username OK, password required 125 data connection already open; transfer starting 425 Can’t open data connection 452 Error writing file Lesson 4: More Application Layer Protocols - 4

Lesson 4: More Application Layer Protocols - 5
The Web: some jargon Web page consists of “objects” addressed by a URL Most Web pages consist of: base HTML page, and several referenced objects. URL has three components: protocol, host name and path name: User agent for Web is called a browser: MS Internet Explorer Netscape Communicator Server for Web is called Web server: Apache (public domain) MS Internet Information Server Lesson 4: More Application Layer Protocols - 5

The Web: the http protocol
http: hypertext transfer protocol Web’s application layer protocol client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests http1.0: RFC 1945 http1.1: RFC 2068 Ip request Ip response DNS Server http request PC running Explorer http response http request Server running NCSA Web server http response The Internet Mac running Navigator Lesson 4: More Application Layer Protocols - 6

Navigation through The Web
Ip request Multiple servers may come into play The same client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests Browser determines URL and asks DNS for IP address Browser makes TCP connection on port 80 Ip response DNS Server http request ( following hyperlink to abc.com) PC running Explorer http response with a page having hyperlink to XYZ.com http response http request abc.com Web server The Internet XYZ.com Web server Lesson 4: More Application Layer Protocols - 7

More about the http protocol
http is “stateless” server maintains no information about past client requests http: TCP transport service: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client http messages (application-layer protocol messages) exchanged between browser (http client) and Web server (http server) TCP connection closed aside Protocols that maintain “state” are complex! past history (state) must be maintained if server/client crashes, their views of “state” may be inconsistent, must be reconciled Lesson 4: More Application Layer Protocols - 8

Further Details for the http example
Suppose user enters URL (contains text, references to 10 jpeg images) 1a. http client initiates TCP connection to http server (process) at Port 80 is default for http server. 1b. http server at host waiting for TCP connection at port 80. “accepts” connection, notifying client 2. http client sends http request message (containing URL) into TCP connection socket 3. http server receives request message, forms response message containing requested object (someDepartment/home.index), sends message into socket time Lesson 4: More Application Layer Protocols - 9

http example (cont.) 4. http server closes TCP connection. 5. http client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 6. Steps 1-5 repeated for each of 10 jpeg objects time Lesson 4: More Application Layer Protocols - 10

Non-persistent and persistent connections
HTTP/1.0 server parses request, responds, and closes TCP connection 2 Request-response messages to fetch each object Each object transfer suffers from slow start Persistent default for HTTP/1.1 on same TCP connection: server parses request, responds, parses new request,.. Client sends requests for all referenced objects as soon as it receives base HTML. Fewer Request-response messages and less slow start. But most 1.0 browsers use parallel TCP connections. Lesson 4: More Application Layer Protocols - 11

http message format: request
two types of http messages: request, response http request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr (extra carriage return, line feed) header lines Carriage return, line feed indicates end of message Lesson 4: More Application Layer Protocols - 12

http request message: general format
Lesson 4: More Application Layer Protocols - 13

Lesson 4: More Application Layer Protocols – 13.1
http Request Example Lesson 4: More Application Layer Protocols – 13.1

http message format: response
status line (protocol status code status phrase) HTTP/ OK Date: Thu, 06 Aug :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ... header lines data, e.g., requested html file Lesson 4: More Application Layer Protocols - 14

Lesson 4: More Application Layer Protocols – 14.1
http Response Example Lesson 4: More Application Layer Protocols – 14.1

http response status codes
In first line in server->client response message. A few sample codes: 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported Lesson 4: More Application Layer Protocols - 15

Trying out http (client side) for yourself
1. Telnet to your favorite Web server: telnet 80 Opens TCP connection to port 80 (default http server port) at Anything typed in sent to port 80 at 2. Type in a GET http request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to http server GET /~ross/index.html HTTP/1.0 3. Look at response message sent by http server! Lesson 4: More Application Layer Protocols - 16

User-server interaction: authentication
Authentication goal: control access to server documents stateless: client must present authorization in each request authorization: typically name, password authorization: header line in request if no authorization presented, server refuses access, sends WWW authenticate: header line in response client server usual http request msg 401: authorization req. WWW authenticate: usual http request msg + Authorization:line usual http response msg usual http request msg + Authorization:line time usual http response msg Browser caches name & password so that user does not have to repeatedly enter it. Lesson 4: More Application Layer Protocols - 17

User-server interaction: cookies
server sends “cookie” to client in response must Set-cookie: client presents cookie in later requests cookie: server matches presented-cookie with server-stored info authentication remembering user preferences, previous choices client server usual http request msg usual http response + Set-cookie: # usual http request msg cookie: # cookie- spectific action usual http response msg usual http request msg cookie: # cookie- spectific action usual http response msg Lesson 4: More Application Layer Protocols - 18

User-server interaction: conditional GET
client server Goal: don’t send object if client has up-to-date stored (cached) version client: specify date of cached copy in http request If-modified-since: <date> server: response contains no object if cached copy up-to-date: HTTP/ Not Modified http request msg If-modified-since: <date> object not modified http response HTTP/1.0 304 Not Modified http request msg If-modified-since: <date> object modified http response HTTP/ OK … <data> Lesson 4: More Application Layer Protocols - 19

Web Caches (proxy server)
Goal: satisfy client request without involving origin server user sets browser: Web accesses via web cache client sends all http requests to web cache if object at web cache, web cache immediately returns object in http response else requests object from origin server, then returns http response to client origin server Proxy server http request http request client http response http response http request http request http response http response client origin server Lesson 4: More Application Layer Protocols - 20

Why Web Caching? origin servers Assume: cache is “close” to client (e.g., in same network) smaller response time: cache “closer” to client decrease traffic to distant servers link out of institutional/local ISP network often bottleneck public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache Lesson 4: More Application Layer Protocols - 21

Streaming Audio (Music on Demand)
Some cases web-sever provides link to audio server. Media player gets the file using Real-time Streaming Protocol (RTSP). Lesson 4: More Application Layer Protocols - 22

Media Player Functions 1. User Interface Management 2. Transmission error handling 3. Decompression of music 4. Elimination of jitter. Lesson 4: More Application Layer Protocols - 23

Media Player Function: Elimination of Jitter
Concept of push and pull media servers Lesson 4: More Application Layer Protocols -24

Internet Radio Lesson 4: More Application Layer Protocols - 25

Internet Telephony The ITU Lesson 4: More Application Layer Protocols - 26

H.323 Protocol Stack RTP- Real-time Transport Protocol, RTCP- Real-time Transport Control Protocol, RAS- Registration/Admission/Status. H.245 channel is used to negotiate call parameters such as support for video or conference calls, Codecs supported, and so on. G.711, G.723.1, etc. Used for Congestion control Allows terminals join and leave zones , request and return bandwidths and provide status updates. Lesson 4: More Application Layer Protocols - 27

Call Flow in H.323 Lesson 4: More Application Layer Protocols - 28

Session Initiation Protocol (SIP)
A light-weight protocol designed to inter-work with existing internet applications. You can click and initiate telephone call A text based protocol modeled on HTTP. Interoperability could be a problem in the future. Lesson 4: More Application Layer Protocols -29

Video- Still and Moving Images
MPEG-1 output consists of 4 kinds of frames; I (Intra-coded) frames: Self-contained JPEG-encoded still pictures P (Predictive) frames: Block-by-block difference with last frame B (Bidirectional) frames: Differences between last and next frames D (DC-coded): Block averages used for last forward. Lesson 4: More Application Layer Protocols - 30

Video on Demand Here MPEG-2 is more applicable. It is similar to MPEG-1, but uses 10x10 blocks on place of 8x8. It also supports both progressive and interlaced images. Lesson 4: More Application Layer Protocols - 31

Video-servers RAM Zipf’s Law: Most popular movie is seven times as popular as the 7th popular movie. kth popular movie will have C/k of total requests where C= ? Magnetic Disk DVD Tape Lesson 4: More Application Layer Protocols - 32

Revisiting the client-server paradigm, we dealt with three very popular application protocols ftp http Multimedia Audio-servers H.323 SIP Video-on-Demand Next we will take up how to program applications using transport layer services (i.e. TCP/UDP sockets) Lesson 4: More Application Layer Protocols -33

Lesson 5: Writing Applications using Transport Layer Facilities

Learn about the usage of the following transport layer facilities for writing client-server applications UDP sockets TCP sockets Learn the difference between connection-oriented and connectionless transport layer services. Lesson 5: Writing Applications using Transport Layer Facilities-1

Socket programming Socket API introduced in BSD4.1 UNIX, 1981 socket
explicitly created, used, released by apps client/server paradigm two types of transport service via socket API: unreliable datagram reliable, byte stream-oriented a local-host created/owned application, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another (remote or local) application process socket Lesson 5: Writing Applications using Transport Layer Facilities-2

Socket-programming using TCP
Socket: a door between application process and end-end-transport protocol (UDP or TCP) TCP service: reliable transfer of bytes from one process to another controlled by application developer controlled by application developer process TCP with buffers, variables socket process TCP with buffers, variables socket controlled by operating system controlled by operating system internet host or server host or server Lesson 5: Writing Applications using Transport Layer Facilities-3

Socket programming with TCP
Client must contact server server process must first be running server must have created socket (door) that welcomes client’s contact Client contacts server by: creating client-local TCP socket specifying IP address, port number of server process When client creates socket: client TCP establishes connection to server TCP When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with multiple clients TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint Lesson 5: Writing Applications using Transport Layer Facilities-4

Socket programming with TCP
Example client-server app: client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream) server reads line from socket server converts line to uppercase, sends back to client client reads, prints modified line from socket (inFromServer stream) Input stream: sequence of bytes into process Output stream: sequence of bytes out of process inFromUser outToServer iinFromServer client socket Lesson 5: Writing Applications using Transport Layer Facilities-5

Client/server socket interaction: TCP
Server (running on hostid) Client create socket, port=x, for incoming request: welcomeSocket = ServerSocket() Unix 4.1c BSD: socket() bind() listen() accept() Unix 4.1c BSD: socket() connect() TCP connection setup wait for incoming connection request Socket connectionSocket = welcomeSocket.accept() create socket, connect to hostid, port=x clientSocket = Socket() send request using clientSocket read request from connectionSocket InputStream Socket.getInputStream() OutputStream Socket.getOutputStream() write reply to connectionSocket read reply from clientSocket connectionSocket.close() clientSocket.close() Lesson 5: Writing Applications using Transport Layer Facilities-6

Example: Java TCP client
import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream()); Create input stream Create client socket, connect to server Create output stream attached to socket Lesson 5: Writing Applications using Transport Layer Facilities-7

Example: Java TCP client (cont.)
Create input stream attached to socket BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } Send line to server Read line from server Lesson 5: Writing Applications using Transport Layer Facilities-8

Example: Java server (TCP)
import java.io.*; import java.net.*; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket Lesson 5: Writing Applications using Transport Layer Facilities-9

Example: Java TCP server (cont.)
DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } Create output stream, attached to socket Read in line from socket Write out line to socket End of while loop, loop back and wait for another client connection Lesson 5: Writing Applications on Transport Layer Facilities-10

Socket programming with UDP
UDP: no “connection” between client and server no handshaking sender explicitly attaches IP address and port of destination server must extract IP address, port of sender from received datagram UDP: transmitted data may be received out of order, or lost application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server Lesson 5: Writing Applications on Transport Layer Facilities-11

Client/Server socket interaction: UDP
Server (running on hostid) create socket, clientSocket = DatagramSocket() Client Create, address (hostid, port=x, send datagram request using clientSocket Unix 4.1c BSD: socket() bind() sendto() create socket, port=x, for incoming request: serverSocket = DatagramSocket() read request from serverSocket Unix 4.1c BSD: socket() bind() receivefrom() close clientSocket read reply from clientSocket write reply to serverSocket specifying client host address, port umber Lesson 5: Writing Applications on Transport Layer Facilities-12

Example: Java client (UDP)
import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine(); sendData = sentence.getBytes(); Create input stream Create client socket Translate hostname to IP address using DNS Lesson 5: Writing Applications on Transport Layer Facilities-13

Example: Java UDP client (cont.)
Create datagram with data-to-send, length, IP addr, port DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } Send datagram to server Read datagram from server Lesson 5: Writing Applications on Transport Layer Facilities-14

Example: Java server (UDP)
import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket); Create datagram socket at port 9876 Create space for received datagram Receive datagram Lesson 5: Writing Applications onTransport Layer Facilities-15

Example: Java UDP server (cont)
String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } Get IP addr port #, of sender Create datagram to send to client Write out datagram to socket End of while loop, loop back and wait for another datagram Lesson 5: Writing Applications on Transport Layer Facilities-16

In this class, Learned about the usage of the following transport layer facilities for writing application UDP sockets TCP sockets Learned the difference between connection-oriented and connectionless transport layer services. In the following classes, we study the transport layer itself. In other words, we find the ways of implementing transport layer functionalities. Lesson 5: Writing Applications on Transport Layer Facilities-17

Lesson 6: Transport Layer

Lesson 6: Preview and Objectives
Overview of transport layer services: Multiplexing/de-multiplexing Connectionless and unreliable data transport (UDP) Connection-oriented and reliable data transport (TCP) Study an Incremental Approach to the Design of Reliable Data Transfer Mechanisms in order to: Get an insight into how industrial products are usually evolved starting with simpler user-models/assumptions and proceeding on with more and more complex ones (big-bangs are rather rare!) Get a perspective on the TCP ‘s reliable data transfer mechanisms Transport Layer - 1

Transport services and protocols
Provide logical communication between app’ processes running on different hosts Transport protocols run in end systems Transport versus network layer services: network layer: data transfer between end systems transport layer: data transfer between processes relies on, enhances, network layer services application transport network data link physical network data link physical network data link physical network data link physical logical end-end transport network data link physical network data link physical application transport network data link physical Transport Layer - 2

Transport-layer Services
Internet transport services: Unreliable (“best-effort”), unordered unicast or multicast delivery (UDP) Reliable, in-order unicast delivery (TCP) congestion control flow control connection setup Services not available: real-time bandwidth guarantees reliable multicast application transport network data link physical network data link physical network data link physical network data link physical logical end-end transport network data link physical network data link physical application transport network data link physical Transport Layer - 3

Multiplexing/demultiplexing
Segment - unit of data exchanged between transport layer entities aka TPDU: transport protocol data unit Demultiplexing: delivering received segments to correct app layer processes receiver P3 P4 application-layer data M M application transport network segment header P1 P2 M M application transport network application transport network segment H t M H n segment Transport Layer -4

Multiplexing/Demultiplexing
Gathering data from multiple app processes, enveloping data with header (later used for demultiplexing) 32 bits source port # dest port # other header fields multiplexing/demultiplexing: Based on sender, receiver port numbers, IP addresses source, dest port #s in each segment recall: well-known port numbers for specific applications application data (message) TCP/UDP segment format Transport Layer -5

Multiplexing/Demultiplexing: examples
source port: x dest. port: 23 Web client host C host A server B source port:23 dest. port: x Source IP: C Dest IP: B source port: y dest. port: 80 Source IP: C Dest IP: B source port: x dest. port: 80 port use: simple telnet app Source IP: A Dest IP: B source port: x dest. port: 80 Web server B Web client host A port use: Web server Transport Layer - 6

UDP: User Datagram Protocol [RFC 768]
“no frills,” “bare bones” Internet transport protocol “best effort” service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired Transport Layer - 7

More on UDP Other UDP uses (why?):
Often used for streaming multimedia apps loss tolerant rate sensitive Other UDP uses (why?): DNS SNMP Reliable transfer over UDP: add reliability at application layer application-specific error recovery! 32 bits source port # dest port # Length, in bytes of UDP segment, including header length checksum Application data (message) UDP segment format Transport Layer - 8

UDP checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment Sender: Treat segment contents as sequence of 16-bit integers Checksum: addition (1’s complement sum) of segment contents Sender puts checksum value into UDP checksum field Receiver: Compute checksum of received segment Check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. Transport Layer - 9

Principles of Reliable data transfer
Important in app., transport, link layers Top-10 list of important networking topics! Characteristics of unreliable channel will determine complexity of reliable data transfer protocol (RDT) Transport Layer - 10

Reliable data transfer: getting started
rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer deliver_data(): called by rdt to deliver data to upper send side receive side udt_send(): called by rdt, to transfer packet over unreliable channel to receiver rdt_rcv(): called when packet arrives on rcv-side of channel Transport Layer - 11

Reliable data transfer: getting started
We’ll: incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer but control info will flow on both directions! use finite state machines (FSM) to specify sender, receiver event causing state transition actions taken on state transition state: when in this “state” next state uniquely determined by next event state 1 state 2 event actions Transport Layer - 12

Rdt1.0: reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel Transport Layer - 13

Rdt2.0: channel with bit errors
underlying channel may flip bits in packet recall: UDP checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK human scenarios using ACKs, NAKs? new mechanisms in rdt2.0 (beyond rdt1.0): error detection receiver feedback: control msgs (ACK,NAK) rcvr->sender Transport Layer - 14

rdt2.0: FSM specification
sender FSM receiver FSM Transport Layer - 15

rdt2.0: in action (no errors)

rdt2.0: in action (error scenario)

rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted?
sender doesn’t know what happened at receiver! can’t just retransmit: possible duplicate What to do? sender ACKs/NAKs receiver’s ACK/NAK? What if sender ACK/NAK lost? retransmit, but this might cause retransmission of correctly received pkt! Handling duplicates: sender adds sequence number to each pkt sender retransmits current pkt if ACK/NAK garbled receiver discards (doesn’t deliver up) duplicate pkt stop and wait Sender sends one packet, then waits for receiver response Transport Layer - 18

rdt2.1: sender, handles garbled ACK/NAKs
Transport Layer - 19

rdt2.1: receiver, handles garbled ACK/NAKs
Transport Layer - 20

rdt2.1: discussion Sender: seq # added to pkt
two seq. #’s (0,1) will suffice. Why? must check if received ACK/NAK corrupted twice as many states state must “remember” whether “current” pkt has 0 or 1 seq. # Receiver: must check if received packet is duplicate state indicates whether 0 or 1 is expected pkt seq # note: receiver can not know if its last ACK/NAK received OK at sender Transport Layer - 21

rdt2.2: a NAK-free protocol
same functionality as rdt2.1, using ACKs only instead of NAK, receiver sends ACK for the last packet received OK receiver must explicitly include seq # of pkt being ACKed duplicate ACK at sender results in same action as NAK: retransmit current pkt sender FSM ! Transport Layer - 22

rdt3.0: channels with errors and loss
New assumption: underlying channel can also lose packets (data or ACKs) checksum, seq. #, ACKs, retransmissions will be of help, but not enough Q: how to deal with loss? sender waits until certain data or ACK lost, then retransmits yuck: drawbacks? Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this time if pkt (or ACK) just delayed (not lost): retransmission will be duplicate, but use of seq. #’s already handles this receiver must specify seq # of pkt being ACKed requires countdown timer Transport Layer -23

rdt3.0 sender Transport Layer - 24

rdt3.0 in action Transport Layer - 25

rdt3.0 in action Transport Layer - 26

Performance of rdt3.0 rdt3.0 works, but performance stinks
example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet: T transmit = 8kb/pkt 10**9 b/sec = 8 microsec Utilization = U = = 8 microsec msec fraction of time sender busy sending = 1KB pkt every 30 msec -> 33kB/sec throughput over 1 Gbps link network protocol limits use of physical resources! Transport Layer - 27

We had an overview of transport layer services: Multiplexing/de-multiplexing Connectionless and unreliable data transport (UDP) Connection-oriented and reliable data transport (TCP) We studied an Incremental Approach to the Design of Reliable Data Transfer Mechanisms (i.e. increasingly complex versions of RDT protocol) in order to: Get an insight into how industrial products are usually evolved starting with simpler user-models/assumptions and proceeding on with more and more complex ones (big-bangs are rather rare!) Get a perspective on the TCP ‘s reliable data transfer mechanisms Next class, we study TCP protocol with all the facilities it provides. Transport Layer - 28

Lesson 7: TCP

Lesson 7- TCP: Preview/Objectives
TCP Segment (Message) Format Study of Connection-oriented data transport (TCP) with facilities for: Connection Management Reliable data transfer with one of the two usual methods: Go back to N Selective Repeat Flow Control Congestion Control Lesson 7: TCP- 1

TCP Segment (Message) Structure
Lesson 7: TCP - 2

TCP Connection Management with 3-way Handshake
Lesson 7: TCP - 3

TCP Connection Closing Sequence
Lesson 7: TCP - 4

TCP Connection Management- Client Side State Transitions
Receive ACK/ Send Nothing CLOSING Receive FIN & ACK/ Send ACK Receive FIN/ Send ACK Sharp lines depict unusual states and transitions. Lesson 7: TCP - 5

TCP Connection Management- Server Side State Transitions
Sharp lines depict unusual states and transitions. Send SYN Receive RST/ Send Nothing SYN_SENT Receive SYN/ Send SYN &ACK (Simultaneous open) Lesson 7: TCP - 6

States of The TCP Connection Management FSM
Lesson 7: TCP - 7

Pipelined protocols Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender and/or receiver Two generic forms of pipelined protocols: go-Back-N, selective repeat Lesson 7: TCP - 8

Go-back-N ARQ It is the most commonly used sliding window protocol!
Here, the sender may send a series of frames. The number of unacknowledged frames is determined by the window size While no errors occur, the receiver will acknowledge the receipt of frames with RR# (receiver ready). A frame in error will be rejected with REJ# and discarded by the receiver. Upon receiving a REJ#, the sender must retransmit the frame in error and all frames that were sent thereafter. Lesson 7: TCP - 9

Go-Back-N Sender: k-bit seq # in pkt header
“window” of up to N, consecutive unack’ed pkts allowed ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window Lesson 7: TCP - 10

GBN: sender extended FSM
Lesson 7: TCP - 11

GBN: receiver extended FSM
receiver simple: ACK-only: always send ACK for correctly-received pkt with highest in-order seq # may generate duplicate ACKs need only remember expectedseqnum out-of-order pkt: discard (don’t buffer) -> no receiver buffering! ACK pkt with highest in-order seq # Lesson 7: TCP - 12

GBN in action Lesson 7: TCP - 13

Maximum Window Size The sequence number dilemma
Each frame has a k-bit field to represent its corresponding sequence number (0..2k-1) What is the maximum window size we can allow for Go-Back-N? Answer: 2k-1 Why not 2k ?? DISCUSS !! Lesson 7: TCP - 20

A Problem Similar To Circular-Q Problem
Example: Let’s say we use a 3-bit sequence number. Consider the following sequence of events Sender sends frame 0 Receiver sends Ack with expected seq.#1 Sender sends frames 1, 2, 3, 4, 5, 6, 7, 0 Sender receives Ack with seq.#1 and cannot decide whether all frames have been received correctly or all are lost in transit. Lesson 7: TCP - 21

Selective Repeat receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received sender timer for each unACKed pkt sender window N consecutive seq #’s again limits seq #s of sent, unACKed pkts Lesson 7: TCP - 14

Selective repeat: sender, receiver windows
Lesson 7: TCP - 15

Selective repeat pkt n in [rcvbase, rcvbase+N-1] data from above :
receiver sender pkt n in [rcvbase, rcvbase+N-1] send ACK(n) out-of-order: buffer in-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt pkt n in [rcvbase-N,rcvbase-1] ACK(n) otherwise: ignore data from above : if next available seq # in window, send pkt timeout(n): Send pkt n again, restart timer ACK(n) in [sendbase,sendbase+N]: mark pkt n as received if n smallest unACKed pkt, advance window base to next unACKed seq # Lesson 7: TCP - 16

Selective repeat in action
Lesson 7: TCP - 17

Selective repeat:dilemma
Example: seq #’s: 0, 1, 2, 3 window size=3 receiver sees no difference in two scenarios! incorrectly passes duplicate data as new in (a) Q: what relationship between seq # size and window size? Lesson 7: TCP - 18

Complementary Problem
Consider the following example: Assume a 3-bit sequence number Sender transmits segments 0-6 to the receiver Receiver gets all the segments in good shape and acknowledges with expected Seq.# 7. Now, lightning strikes and all Acks are lost Sender times out and retransmits segment 0 The receiver has advanced its window to accept segments 7, 0-5 and since frame 0 is one that is within that range, it is accepted. Lesson 7: TCP - 23

Actual Window Size The problem shown in the example is that there is an overlap between the sending and receiving windows. Hence, the solution to the window-size problem is to limit the maximum window size to half the range of the sequence number range That is, for a k-bit sequence number field: 2k-1, Show that: (MaxSeqNum + 1)/2 = 2k-1. Lesson 7: TCP - 24

Reliable Data Transfer Protocols- A Comparative Study
Stop-and-Wait Protocol Simple, but performance leaves much to be desired! Go-Back-N Better performance, but more complicated. Possibly wasteful if large blocks of packets need to be retransmitted Selective Repeat A pain to implement – needs multiple timers, but better performance through individual packet management Lesson 7: TCP - 19

Selective-Reject ARQ In this ARQ mechanism the sender only retransmits those frames for which a negative ACK (SREJ) has been received or for that timed out. The receiver does not discard frames which are delivered out of order. Question: What about the permissible window size? Lesson 7: TCP - 22

Flow Control in TCP RcvWindow = RcvBuffer – [LastByteRcvd – LastByteRead] LastByteSent – LastByteAcked <= RcvWindow Possible Sender -> TCP Solution? Lesson 7: TCP - 25

Silly Window Syndrome Sender is slow- Sends a byte at a time
Network bandwidth badly used Nagle’s algorithm- Wait, bunch and send Advisable to disable in interactive applications- cursor movement may look erratic and make user unhappy Receiver is slow- Takes a byte at a time for an interactive application Clarke’s solution- wait till a decent amount of space is available and advertise the receiver window size, Complementary to Nagle’s and both can work together Lesson 7: TCP - 26

General Congestion Control Mechanisms
End to End Congestion Control Network-assisted Congestion Control Direct feedback from router with a choke packet Router marks a field in packet. Upon receipt of the packet, receiver sends a notification to the sender. (Full RTT required!) Network-assisted Congestion not possible in TCP as there is no support from IP. Lesson 7: TCP - 27

Congestion Control in TCP
Three components of TCP congestion control algorithm Additive Increase Multiplicative Decrease Slow start Reaction to timeout events Lesson 7: TCP - 28

TCP Congestion Control States
Lesson 7: TCP - 29

Lesson 7- TCP: Summary & Follow up
We have studied TCP Segment (Message) Format and what each field of the message is meant for. Study of Connection-oriented data transport (TCP) with facilities for: Connection Management FSMs Reliable data transfer with one of the two usual methods: Go back to N Selective Repeat Flow Control with RcvWindow information 3 features of TCP Congestion Control Mechanism . Next class, we proceed on to the Network Layer. Lesson 7: TCP - 30

Lesson 8: Introduction to Network Layer

Lesson 8- Introduction to Network Layer: Preview/Objectives
Overview of network layer functions Forwarding Routing Call setup (sometimes) Network Models- Virtual Circuits versus Datagram Networks Routing Algorithms Desirable Characteristics Classification Different known types Overview of graph theory based algorithms Lesson 8: Introduction to Network Layer - 1

Network layer functions
Network layer protocols exist in every switch whether host (end system) or router (intermediate switch). application transport network data link physical Three important functions: Switching- Moving packets (frames) that come into a switch interface and forward them on the interface that leads to the destination. Switching implies forwarding- ability to determine the interface to which a frame should be directed. Switching has more of hardware connotation and forwarding refers to software aspect. Routing: Determination of path or route taken by packets from source to destination. There exist many routing algorithms for doing this. As against forwarding which refers to transfer of packets from an incoming link to an outgoing link, routing refers collective interaction via routing protocols for path determination. Call setup: some network architectures require router call setup along path before data flows network data link physical Lesson 8: Introduction to Network Layer - 2

Lesson 8: Introduction to Network Layer - 3
Network service model Q: What service model for “channel” transporting packets from sender to receiver? guaranteed bandwidth? preservation of inter-packet timing (no jitter)? loss-free delivery? in-order delivery? congestion feedback to sender? The most important abstraction provided by network layer: ? virtual circuit or datagram? ? ? service abstraction Lesson 8: Introduction to Network Layer - 3

Virtual circuits “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-destination path call setup for each call before data can flow and teardown each packet carries VC identifier (not destination host ID) every router on source-destination path s maintain “state” for each passing connection transport-layer connection only involved two end systems link, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like performance. Lesson 8: Introduction to Network Layer - 4

Virtual circuits: signaling protocols
used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet application transport network data link physical application transport network data link physical 5. Data flow begins 6. Receive data 4. Call connected 3. Accept call 1. Initiate call 2. incoming call Lesson 8: Introduction to Network Layer - 5

Datagram networks: the Internet model
no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets typically routed using destination host ID packets between same source-dest pair may take different paths application transport network data link physical application transport network data link physical 1. Send data 2. Receive data Lesson 8: Introduction to Network Layer - 6

Network layer service models:
Guarantees ? Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Congestion feedback no (inferred via loss) no congestion yes Bandwidth none constant rate guaranteed minimum Loss no yes Order no yes Timing no yes Internet model being extented: Intserv, Diffserv Lesson 8: Introduction to Network Layer - 7

Datagram or VC network: why?
Internet data exchange among computers “elastic” service, no strict timing req. “smart” end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at “edge” many link types different characteristics uniform service difficult ATM evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “dumb” end systems telephones complexity inside network Lesson 8: Introduction to Network Layer - 8

Routing The primary function of a packet network is to accept packets from a source and deliver them to a destination node. The process of forwarding the packets through the network is referred to a routing (routing has more of a global concept as against forwarding). Routing mechanisms have a set of requirements: correctness simplicity robustness stability fairness Lesson 8: Introduction to Network Layer - 9

Routing (Continued) Most important: optimality efficiency Routing directly impacts the performance of the network! WHY? In order to route packets on optimal routes through the network to their destinations, we must first decide what is to be optimized: delay cost throughput Lesson 8: Introduction to Network Layer - 10

Routing Information Routing decisions are generally based on some knowledge of the state of the network. Delay on certain links Cost through certain nodes Packet loss etc. This information may have to be dynamically collected. This leads to overhead which in turn reduces the utilization. Lesson 8: Introduction to Network Layer - 11

Routing Algorithms Graph abstraction for routing algorithms:
E D C B F 2 1 3 5 Goal: determine “good” path (sequence of routers) thru network from source to dest. Graph abstraction for routing algorithms: graph nodes are routers graph edges are physical links link cost: delay, $ cost, or congestion level “good” path: typically means minimum cost path other definitions possible Lesson 8: Introduction to Network Layer - 12

Routing Algorithm classification
Global or decentralized information? Global: all routers have complete topology, link cost info Example: “link state” algorithms Decentralized: router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors Example: “distance vector” algorithms Static or dynamic? Static: routes change slowly over time Dynamic: routes change more quickly Proactive (periodic update) Reactive (in response to link cost changes) Lesson 8: Introduction to Network Layer - 13

Different Types of Routing
Fixed Routing: Static Routing Tables, Pre-computed Routes Flooding: Simple but inefficient! WHY? Hot Potato Routing Simple, not very efficient, unpredictable Random Routing Simple, unpredictable, statistically fair (locally) Adaptive Routing sophisticated, expensive, efficient, complex... Lesson 8: Introduction to Network Layer - 14

Random Routing Sometimes called probabilistic routing! Here, the probability of a packet being forwarded on a particular link is a function of conditions on this link. Pi = Probability of link i being selected Ri = Data rate on link i Lesson 8: Introduction to Network Layer - 15

Random Routing (Continued)
Note: Random Routing is probabilistic, i.e., the link with the largest capacity may not be the one chosen for every transmission. We can formulate a static and dynamic (adaptive) version of the routing algorithm. Can you think of other measurements (metrics) to compute Pi ? Lesson 8: Introduction to Network Layer - 16

Adaptive Routing Adaptive Routing Techniques are used in almost all packet-switching networks. ARPANET Routing decisions change in response to changes in the network. Network Failure Congestion Adaptive routing strategies can improve performance. Adaptive routing strategies can aid congestion control. Lesson 8: Introduction to Network Layer - 17

Shortest Path Routing Algorithms
Shortest-path routing mechanisms are based on graph theoretic concepts. The challenge is to reformulate centralized forms of these algorithms to work in a distributed setting, such as a communication network. The information upon routing decisions are based may come from local measurements adjacent nodes all nodes in the network Lesson 8: Introduction to Network Layer - 18

Graph-Theoretic Formulation
Problem: Find a least cost path between any two nodes of a graph. Network viewed as a graph: Vertices (switches) Edges (links) Cost on each edge (congestion, actual cost, delay, etc.) A 3 9 B 2 F 1 E 6 4 C D 1 Lesson 8: Introduction to Network Layer - 19

Some of the established shortest-path algorithms in traditional graph theory are: Dijkstra’s shortest path algorithm Bellman-Ford Algorithm Floyd-Warshall Algorithm The main difference between the algorithms is the type of augmentation through each iteration. Dijkstra: nodes Bellman-Ford: number of arcs (links) in the path Floyd-Warshall: set of nodes in the path (all s-d pairs) These algorithms have been formulated in a centralized manner and must be mapped into a distributed environment. Lesson 8: Introduction to Network Layer - 20

Lesson 8- Introduction to Network Layer: Summary and Follow-up
We had an overview of network layer functions Forwarding Routing Call setup (sometimes) In passing studied the subtle differences between switching, forwarding and routing. We made a comparative study of Network Models- Virtual Circuits versus Datagram Networks We looked into the following aspects of Routing Algorithms Desirable Characteristics Classification Different known types Overview of graph theory based algorithms In the next class, we study in detail some of the shortest path routing algorithms. Lesson 8: Introduction to Network Layer - 21

Lesson 9: Routing Algorithms for Network Layer

Lesson 9: Routing Algorithms for Network Layer- Preview/Objectives
We study two routing algorithms Dikstra’s link State algorithm Distance vector (Bellman Ford) algorithm We work out examples We discuss the count-to-infinity problem Lesson 9: Routing Algorithms for Network Layer - 1

A Link-State Routing Algorithm
Dijkstra’s algorithm net topology, link costs known to all nodes accomplished via “link state broadcast” all nodes have same info computes least cost paths from one node (‘source”) to all other nodes gives routing table for that node iterative: after k iterations, know least cost path to k dest.’s Notation: c(i,j): link cost from node i to j. cost infinite if not direct neighbors D(v): current value of cost of path from source to destination V p(v): predecessor node along path from source to v, that is next v N: set of nodes whose least cost path definitively known Lesson 9: Routing Algorithms for Network Layer - 2

Lesson 9: Routing Algorithms for Network Layer - 3
Dijsktra’s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v if v adjacent to A then D(v) = c(A,v) else D(v) = infinity 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known shortest path cost to w plus cost from w to v */ 15 until all nodes in N Lesson 9: Routing Algorithms for Network Layer - 3

Dijkstra’s Algorithm: An Example
Step 1 2 3 4 5 start N A AD ADE ADEB ADEBC ADEBCF DIST (B),p(B) 2,A DIST(C),p(C) 5,A 4,D 3,E DIST(D),p(D) 1,A DIST(E),p(E) infinity 2,D DIST(F),p(F) infinity 4,E A E D C B F 2 1 3 5 Lesson 9: Routing Algorithms for Network Layer - 4

A Discussion on Dijkstra’s algorithm
Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N (the set) n*(n+1)/2 comparisons: O(n**2) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic A A D C B 2+e 1+e 1 A A D C B 2+e e 1+e 1 1 1+e 2+e D B D B e 1 C 1+e C 1 1 e … recompute … recompute … recompute routing initially Lesson 9: Routing Algorithms for Network Layer - 5

Bellman-Ford (Distance Vector)
The algorithm iterates on # of arcs in a path. The original algorithm is a single destination shortest path algorithm. Let D(h)i be the shortest ( h) path length from node i to node 1 (the destination). By definition, D(h)1= 0 h. Assumptions: There exists at least one path from every node to the destination All cycles not containing the destination have nonnegative length (cost). Lesson 9: Routing Algorithms for Network Layer - 6

Bellman Ford Algorithm- Preliminaries
NOTE: Let SD(i,j) be the shortest distance from node i to node j. In an undirected graph, we clearly have: SD(i,j) = SD(j,i). This may not be true for a Digraph. Why is the assumption of cycles with nonnegative cost important? Length (hops) is just one of many possible routing metrics. Can you think of others? Lesson 9: Routing Algorithms for Network Layer - 7

Bellman-Ford Algorithm
The Bellman-Ford Algorithm: Step 1: Set D(0)i =  i Step 2: For each h  0 compute D(h+1)i as D(h+1)i = minj[D(h)j + dj,i] i  1 where dj,i is the cost (length) of link lj,i We say that the algorithm has terminated when D(h)i = D(h-1)i i In a network with N nodes, the algorithm terminates after at most N iterations! Lesson 9: Routing Algorithms for Network Layer - 8

Distance Vector Routing Algorithm
Iterative: continues until no nodes exchange info. self-terminating: no “signal” to stop Asynchronous: nodes need not exchange info/iterate in lock step! Distributed: each node communicates only with directly-attached neighbors Distance Table data structure each node has its own row for each possible destination column for each directly-attached neighbor to node example: in node X, for destination Y via neighbor Z: D (Y,Z) X distance from X to Y, via Z as next hop c(X,Z) + min {D (Y,w)} Z w = Lesson 9: Routing Algorithms for Network Layer - 9

Distance Table: An Example
7 8 1 2 D () A B C D 1 7 6 4 14 8 9 11 5 2 E cost to destination via Destination D (C,D) E c(E,D) + min {D (C,w)} D w = 2+2 = 4 D (A,D) E c(E,D) + min {D (A,w)} D w = 2+3 = 5 loop! D (A,B) E c(E,B) + min {D (A,w)} B w = 8+6 = 14 loop! Lesson 9: Routing Algorithms for Network Layer - 10

Distance table gives routing table
1 7 6 4 14 8 9 11 5 2 E cost to destination via destination Outgoing link to use, cost A B C D A,1 D,5 D,4 D,2 destination Distance table Routing table Lesson 9: Routing Algorithms for Network Layer - 11

Distance Vector Routing: An Overview
Iterative, asynchronous: each local iteration caused by: local link cost change message from neighbor: its least cost path change from neighbor Distributed: each node notifies neighbors only when its least cost path to any destination changes neighbors then notify their neighbors if necessary Each node: wait for (change in local link cost of msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors Lesson 9: Routing Algorithms for Network Layer - 12

Distance Vector Algorithm
At all nodes, X: Initialization: for all adjacent nodes v: D (*,v) = infty /* the * operator means "for all rows" */ D (v,v) = c(X,v) for all destinations, y 6 send min D (y,w) to each neighbor /* w over all X's neighbors */ X X X w Lesson 9: Routing Algorithms for Network Layer - 13

Distance Vector Algorithm (cont.)
8 loop 9 wait (until I see a link cost change to neighbor V or until I receive update from neighbor V) 11 12 if (c(X,V) changes by d) /* change cost to all dest's via neighbor v by d */ /* note: d could be positive or negative */ for all destinations y: D (y,V) = D (y,V) + d 16 17 else if (update received from V wrt destination Y) /* shortest path from V to some Y has changed */ /* V has sent a new value for its min DV(Y,w) */ /* call this received new value is "newval" */ for the single destination y: D (Y,V) = c(X,V) + newval 22 23 if we have a new min D (Y,w) for any destination Y send new value of min D (Y,w) to all neighbors 25 26 forever X X w X X w X w Lesson 9: Routing Algorithms for Network Layer - 14

Distance Vector Algorithm: An Example
Z 1 2 7 Y Lesson 9: Routing Algorithms for Network Layer - 15

Distance Vector Algorithm: example (contd.)
Z 1 2 7 Y D (Y,Z) X c(X,Z) + min {D (Y,w)} w = 7+1 = 8 Z D (Z,Y) X c(X,Y) + min {D (Z,w)} w = 2+1 = 3 Y Lesson 9: Routing Algorithms for Network Layer - 16

Distance Vector: link cost changes
node detects local link cost change updates distance table (line 15) if cost change in least cost path, notify neighbors (lines 23,24) X Z 1 4 50 Y algorithm terminates “good news travels fast” Lesson 9: Routing Algorithms for Network Layer - 17

Distance Vector: link cost changes
good news travels fast bad news travels slow - “count to infinity” problem! X Z 1 4 50 Y 60 algorithm continues on! Lesson 9: Routing Algorithms for Network Layer - 18

Distance Vector: poisoned reverse
If Z routes through Y to get to X : Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) will this completely solve count to infinity problem? X Z 1 4 50 Y 60 algorithm terminates Lesson 9: Routing Algorithms for Network Layer - 19

Comparison of LS and DV algorithms
Message complexity LS: with n nodes, E links, O(nE) msgs sent each DV: exchange between neighbors only convergence time varies Speed of Convergence LS: O(n**2) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may be routing loops count-to-infinity problem Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes only its own table DV: DV node can advertise incorrect path cost each node’s table used by others error propagate thru network Lesson 9: Routing Algorithms for Network Layer - 20

Lesson 9: Routing Algorithms for Network Layer- Summary and Follow-up
We studied two routing algorithms Dikstra’s link State algorithm Distance vector (Bellman-Ford) algorithm We work ed out examples We discussed the count-to-infinity problem Next class, we continue with more on Internet & IP Lesson 9: Routing Algorithms for Network Layer - 21

Lesson 10: IP & The Internet

Lesson 10: IP & The Internet- Preview/Objectives
We see how the Internet- the network of networks works Study the IP message and address structures We study a number of Protocols & Algorithms ICMP ARP & RARP/BOOTP/DHCP RIP /OSPF & BGP We discuss how the count-to-infinity problem is addressed in the BGP. Lesson 10: IP & The Internet - 1

Lesson 10: IP & The Internet - 2

How Internet Handles Traffic Flow through Different Networks?
Lesson 10: IP & The Internet - 3

The IP Message Format Originally had Delay, Throughput and Reliability flags. Now it has 4 queuing priority classes, 3 discard probabilities and historical service classes. Don’t Fragment (e.g. Memory Image) Header Length in 32-bit words More Fragments (All but the last have it !) Tells to which datagram the newly arrived fragment belongs. Tells whether to give the datagram to TCP or UDP or some other process. Original Options: Security, strict source routing, loose source coding (gives list of routers not to be missed), Timestamp (enforces each router to append its address & Timestamp- useful for debugging) Variable Length field (in multiples of 32-bits) meant for inclusion by subsequent versions new Info. Lesson 10: IP & The Internet - 4

The IP Address Formats Lesson 10: IP & The Internet - 5

Reserved IP Addresses Lesson 10: IP & The Internet - 6

The Internet Network layer
Host, router network layer functions: Transport layer: TCP, UDP IP protocol addressing conventions datagram format packet handling conventions Routing protocols path selection ARP, RARP/BOOTP/ DHCP RIP/OSPF, BGP Network layer routing table ICMP protocol error reporting router “signaling” Link layer physical layer Lesson 10: IP & The Internet - 7

The Internet Control Message Protocol
Each ICMP message is encapsulated in an IP packet Lesson 10: IP & The Internet - 8

ARP (Address Resolution Protocol)
Used in IPV4 (over Ethernet) to get the hardware/link/MAC address of the machine with IP address ARP message of the form “I am X1.X2.X3.X4, tell me who is Y1.Y2,Y3,Y4 is sent using LAN (say, ETHERNET) broadcast address (all 1’s) in an ethernet packet. Only the concerned system sends ARP response; others discard. Lesson 1: History of Computer Networks - 2

RARP, BOOTP and DHCP RARP- Reverse Address Resolution Protocol Useful for diskless workstations getting binary image of O/S from remote file server. BOOTP (Bootstrap Protocol) Invented because destination address of all 1’s in RARP is not portable to RARP server across network Uses UDP. DHCP (Dynamic Host Configuration Protocol) has largely replaced RARP & BOOTP DHCP relay agents, in the network of the source, intercept all DHCP discover packets and unicast them to the DHCP server across the network. DHCP. Lesson 10: IP & The Internet - 10

DHCP Lesson 10: IP & The Internet - 11

OSPF (Open Shortest Path First)
Interior Gateway Protocol for routing within Autonomous Systems (ASes). It Supports point-to-point routing between two routers multi-access networks with Broadcasting (e.g. LANs) and multi-access networks without broad casting (e.g. WANs). Lesson 10: IP & The Internet - 12

OSPF (Open Shortest Path First)
Interior Gateway Protocol (routing within Autonomous Systems (ASes). Supports- point-to-point routing between two routers, multi-access networks with Broadcasting (e.g. LANs) and multi-access networks without broad casting (e.g. WANs). Lesson 10: IP & The Internet - 13

OSPF (Continued) Original Interior gateway protocol was RIP (Routing Information Protocol) based on the Bellman-Ford algorithm in ARPANET. Now replaced by an extension of the LS algorithm. It is open, dynamic (adaptable to changes), supports other metrics e.g. delay, routing based on types of service, hierarchical systems, security, tunneling, and does load balancing Lesson 10: IP & The Internet - 14

BGP (Boarder Gateway Protocol)
Exterior Gateway Protocol used between ASes Uses Distance Vector (DV) routing, but solves the count to infinity problem by keeping track paths , not just the costs to destination. Policies based on political, security or economic considerations configured into BGP routers by Scripts. Lesson 10: IP & The Internet - 15

Lesson 10: IP & The Internet- Summary and Follow-up
We have seen how the Internet- the network of networks works (particularly, the tunneling concept) We Studied the IP message and address structures We studied a number of Protocols & Algorithms ICMP ARP & RARP/BOOTP/DHCP RIP /OSPF & BGP We discussed how the count-to-infinity problem is addressed in the BGP. Next class, we proceed on to Data-link layer. Lesson 10: IP & The Internet - 16

Lesson 11: Introduction to Data Link Layer

Lesson 11: Introduction to Data Link Layer -Preview/Objectives
We study the principles behind various link layer services such as Error Detection and correction Multiple access (sharing the broadcast channel) Point-to-point (Single wire e.g. SLIP/PPP) Broadcast (Shared wire e.g. Ethernet, WaveLan etc. Switched (e.g. Switched Ethernet, ATM, etc.) Link layer Addressing (ARP- already done!) Reliable Data Transfer & Flow control (already done in the context of TCP) We study Pure and Slotted Protocols- precursors of CSMA/CD Lesson 11: Introduction to Data Link Layer - 1

Link Layer: Setting the Context
Lesson 11: Introduction to Data Link Layer - 2

Link Layer & Data Link Protocol
two physically connected devices: host-router, router-router, host-host unit of data: frame application transport network link physical M H t n l network link physical data link protocol H l H t n M frame phys. link adapter card Lesson 11: Introduction to Data Link Layer - 3

Lesson 11: Introduction to Data Link Layer - 4
Link Layer Services Framing and link access: encapsulate datagram into frame, adding header, trailer implement channel access if shared medium, ‘physical addresses’ used in frame headers to identify source and destination different from IP address! Reliable delivery between two physically connected devices: we learned how to do this already (in the context of TCP)! seldom used on low bit error link (fiber, some twisted pair) wireless links: high error rates Q: why both link-level and end-end reliability? Lesson 11: Introduction to Data Link Layer - 4

More Link Layer Services
Flow Control: pacing between sender and receivers Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors and signals sender for retransmission or drops frame Error Correction: receiver identifies and corrects bit error(s) without resorting to retransmission Lesson 11: Introduction to Data Link Layer - 5

Link Layer: Implementation
implemented in “adapter” e.g., PCMCIA card, Ethernet card typically includes: RAM, DSP chips, host bus interface, and link interface application transport network link physical M H t n l network link physical data link protocol H l H t n M frame phys. link adapter card Lesson 11: Introduction to Data Link Layer - 6

Error Detection in Link Layer
Parity bit (single bit indication, but even number of flips can’t be detected) Check Sum is simple, but not enough (even number of flips in the opposite direction give the same value) Cyclic Redundancy Check is more rigorous and hence used in link layer Transport layer relies on this and manages with simpler Check Sum. Lesson 11: Introduction to Data Link Layer - 7

Cyclic Redundancy Check Code
For r-bit CRC code, (r+1)-bit Generator (G) is required. Most Significant Bit of G = 1 8-, 12-, 16-, 32-bit G’s defined by International standards 8-bit G used for protecting 5-byte ATM headers GCRC-32 = Lesson 11: Introduction to Data Link Layer - 8

More About CRC CRC is also known as polynomial code CRC Formula Derivation: D.2r XOR R = n G D.2r = n G XOR R R = remainder (D.2r/G ) when subtraction in the division is done by XOR. CRC can detect Burst errors (consecutive bit errors) of size < r+1 Under some assumptions, bust errors of size > r+1 can be detected with probability 1 – 0.5 r Each CRC standard can detect any odd number of bit errors. Lesson 11: Introduction to Data Link Layer - 9

Multiple Access Links and Protocols
Three types of “links”: point-to-point (single wire, e.g. PPP, SLIP) broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.) switched (e.g., switched Ethernet, ATM etc) Lesson 11: Introduction to Data Link Layer - 10

Multiple Access protocols
single shared communication channel two or more simultaneous transmissions by nodes: interference only one node can send successfully at a time multiple access protocol: distributed algorithm that determines how stations share channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols: synchronous or asynchronous information needed about other stations robustness (e.g., to channel errors) performance Lesson 11: Introduction to Data Link Layer - 11

MAC Protocols: A Taxonomy
Three broad classes: Channel Partitioning divide channel into smaller “pieces” (time slots, frequency) allocate piece to node for exclusive use Random Access allow collisions “recover” from collisions “Taking turns” tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized Lesson 11: Introduction to Data Link Layer - 12

Random Access protocols
When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes -> “collision”, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA and CSMA/CD Lesson 11: Introduction to Data Link Layer - 13

Pure (Unslotted) ALOHA
Users are not synchronized. Each user transmits a data packet when ready. In the event of two or more packets collide (overlap in time), each user involved realized this and retransmit the packet after a randomized delay. Lesson 11: Introduction to Data Link Layer - 14

Pure ALOHA (Continued)
unslotted Aloha: simpler, no synchronization packet needs transmission: send without awaiting for beginning of slot collision probability includes two overlapping intervals: packet sent at t0 collide with other packets sent in [t0-1, t0+1] Lesson 11: Introduction to Data Link Layer - 15

Slotted ALOHA Like Pure-ALOHA with additional requirements: The channel is slotted in time Each user is required to synchronize the start of packet transmission to coincide with the slot boundary (only complete collision would occur, avoid partial collision) Lesson 11: Introduction to Data Link Layer - 16

Slotted Aloha - Further Details
time is divided into equal size slots (= packet trans. time) node with new arriving packets: transmit at beginning of next slot if collision: retransmit packet in future slots with probability p, until successful. Success (S), Collision (C), Empty (E) slots Lesson 11: Introduction to Data Link Layer - 17

Limit on the Slotted Aloha efficiency
Q: what is max fraction slots successful? A: Suppose N stations have packets to send each transmits in slot with probability p prob. successful transmission S is: by single node: S= p (1-p)(N-1) by any of N nodes S = Probability (only one transmits) = N p (1-p)(N-1) … choosing optimum p as N -> infinity ... = 1/e = .37 as N -> infinity (we will see in the next slide) At best: channel use for useful transmissions 37% of time! Lesson 11: Introduction to Data Link Layer - 18

Derivation of Slotted Aloha efficiency Limit
S = Probability of success of any of the N nodes (i.e. only one transmits) = N p (1-p)(N-1) Find the maximum value of S using the established formula: Solution: Setting ds/dp = 0, we get, N. (1-p)(N-1) _ N p (N-1) (1-p)(N-2) = 0 p = 1/N Putting this value “p” in S and taking limits we get, S = 1/e Lesson 11: Introduction to Data Link Layer - 19

Pure & Slotted Aloha Efficiency Limits
P(success by given node) = P(node transmits) . P(no other node transmits in [p0-1,p0] . P(no other node transmits in [p0,p0+1] = p . (1-p)(N-1) .(1-p)(N-1) P(success by any of N nodes) = N p . (1-p)(N-1). (1-p)(N-1) … choosing optimum p as N -> infty ... = 1/(2e) using similar derivation = .18 G = offered load = Np 0.5 1.0 1.5 2.0 0.1 0.2 0.3 0.4 Pure Aloha Slotted Aloha S = throughput = “goodput” (success rate) protocol constrains effective channel throughput! Lesson 11: Introduction to Data Link Layer - 20

Lesson 11: Introduction to Data Link Layer –Summary & Follow-up
We studied the principles behind various link layer services e.g. Error Detection and correction Multiple access (sharing the broadcast channel) Point-to-point (Single wire e.g. SLIP/PP) Broadcast (Shared wire e.g. Ethernet, WaveLan etc. Switched (e.g. Switched Ethernet, ATH, etc.) Link layer Addressing Reliable Data Transfer & Flow control (already done in the TCP class) We studied and analyzed Pure and Slotted ALOHA Protocols- precursors of CSMA/CD. Next class, we proceed on to Link layer technologies and study CSMA/CD, Ethernet and other protocols & Technologies. Lesson 11: Introduction to Data Link Layer - 21

Lesson 12: Link Layer Technologies

Lesson 12: Link Layer Technologies-Preview/Objectives
We study specific link layer technologies and their implementation Current Multiple Access MAC (Medium Access Control) Protocols- CSMA/CD Channel Partitioning “Taking Turns” type – Token Ring Ethernet Hubs, Bridges and Switches PPP ATM IEEE LANs Lesson 12: Link Layer Technologies - 1

Carrier Sense Multiple Access (CSMA)
Used in radio network. Propagation delay is small compared to packet transmission time. Avoid collision by listening to the carrier before transmission. Lesson 12: Link Layer Technologies - 2

CSMA: Carrier Sense Multiple Access)
CSMA: listen before transmit: If channel sensed idle: transmit entire packet If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others! Good Manners protocol. Lesson 12: Link Layer Technologies - 3

Lesson 12: Link Layer Technologies - 4
CSMA collisions spatial layout of nodes along ethernet Collisions can occur: Propagation delay means two nodes may not yet hear each other’s transmission Collision: Entire packet transmission time wasted Note: Role of distance and propagation delay in determining collision probability. Lesson 12: Link Layer Technologies - 4

CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage persistent or non-persistent retransmission Collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting Same human analogy of the polite conversationalist Lesson 12: Link Layer Technologies - 5

IEEE CSMA/CD Uses 1-persistent CSMA algorithm. Rules: if the channel is idle then transmit if the channel is busy, then continue to listen until idle then transmit immediately if a collision is detected during the transmission, immediately cease transmitting the frame and transmit a jamming signal to ensure everyone knows the collision, hence the name collision detection (CD) After transmitting the jamming signal, then wait a random time and attempt to transmit again Lesson 12: Link Layer Technologies - 6

CSMA/CD Collision Detection
Lesson 12: Link Layer Technologies - 7

“Taking Turns” MAC protocols
Channel partitioning MAC protocols: share channel efficiently at high load inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols efficient at low load: single node can fully utilize channel high load: collision overhead “Taking turns” protocols look for best of both worlds! Lesson 12: Link Layer Technologies - 8

“Taking Turns” MAC protocols
Polling: master node “invites” slave nodes to transmit in turn Request to Send, Clear to Send messages concerns: polling overhead latency single point of failure (master) Token passing: control token passed from one node to next sequentially. token message concerns: token overhead latency single point of failure (token) Lesson 12: Link Layer Technologies - 9

Reservation-based protocols
Distributed Polling: time divided into slots begins with N short reservation slots reservation slot time equal to channel end-end propagation delay station with message to send posts reservation reservation seen by all stations after reservation slots, message transmissions ordered by known priority Lesson 12: Link Layer Technologies - 10

Summary of MAC protocols
What can we do with a shared media? Channel Partitioning, by time, frequency or code Time Division,Code Division, Frequency Division Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technoligies (wire), hard in others (wireless) CSMA/CD used in Ethernet Taking Turns polling from a central cite, token passing Lesson 12: Link Layer Technologies - 11

Ethernet “Dominant” LAN technology: Cheap $20 for 100Mbs! First wildey used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Etheret sketch Lesson 12: Link Layer Technologies - 12

Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern followed by one byte with pattern Used to synchronize receiver, sender clock rates Last two 11’s of the 8th for alerting about something important to come. Lesson 12: Link Layer Technologies - 13

Ethernet Frame Structure (Continued)
Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped 8 bytes 6 bytes 6 bytes 2 bytes bytes 4 bytes Lesson 12: Link Layer Technologies - 14

Ethernet: CSMA/CD Algorithm
A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} else {wait until ongoing transmission is over and goto A} Lesson 12: Link Layer Technologies - 15

Ethernet’s CSMA/CD- Finer Details
Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K x 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten or more collisions, choose K from {0,1,2,3,4,…,1023} Lesson 12: Link Layer Technologies - 16

Ethernet Technologies: 10Base2
10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! Lesson 12: Link Layer Technologies - 17

10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus “star topology” CSMA/CD implemented at hub Lesson 12: Link Layer Technologies - 18

More on10BaseT and 100BaseT Max distance from node to Hub is 100 meters Hub can disconnect “jabbering adapter Hub can gather monitoring information, statistics for display to LAN administrators Lesson 12: Link Layer Technologies - 19

Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared broadcast channels in shared mode, CSMA/CD is used; short distances between nodes to be efficient uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links Lesson 12: Link Layer Technologies - 20

PPP- Format Flag field mark the beginning and end of the PPP frame What is the use of the same address and control fields? Protocol- values depend on the upper layer (network) protocol receiving the data: AppleTalk (29), IPCP (8021) Lesson 12: Link Layer Technologies - 21

PPP- Format- How differentiate Data and Control Info in the Header?
Answer: A technique called byte stuffing. An escape byte precedes the flags byte appearing as data. What about escape byte itself? Lesson 12: Link Layer Technologies - 22

PPP- State Model PPP’s Link Control Protocol (LCP) manages the states. Physical layer presence indicated by carrier detection or admin action Terminate request and ACK exchange Configure-request frame (a PPP Frame with protocol set to LCP value- Co21) and configure-ack/configure-nak/ configure-reject responses received. Lesson 12: Link Layer Technologies - 23

ATM AAL1- Constant bit rate services AAL2- Variable bit rate (e.g. video) services AAL5- IP Services AAL (ATM Adaptation Layer)- Performs error detection; Equivalent to Transport layer as it is responsible for segmentation & Reassembly. Lesson 12: Link Layer Technologies - 24

AAL5 PDU Lesson 12: Link Layer Technologies - 25

ATM Cell Header VCI- Virtual circuit identifier PT- payload type CLP- Cell Priority Bit HEC- Header Error Control Lesson 12: Link Layer Technologies - 26

ATM Physical Layer At the bottom of the ATM protocol stack Uses T1/T3, SONET/SDH (synchronous Optical Network/Synchronous Digital Hierarchy) over a single-mode fiber. T1/T3 frames over fiber, microwave and copper Like T1/T3, SONET/SDH have frame structures to establish sync between transmitters and receivers. Cell based with no frames (clock at receiver is derived from a transmitted signal) Standardized rates for SONET OC-1: Mbps OC-3: Mbps OC-12: Mbps OC-48: 2.5 Gbps Lesson 12: Link Layer Technologies - 27

Wi-Fi: Wireless LANs Building Block of Wi-Fi LAN architecture is Basic Service Set (BSS) containing a base station, known as access point (AP) One or more wireless stations WI-FI Uses CSMA/CA LANs that deploy APs are called Infrastructure Wireless LANs Lesson 12: Link Layer Technologies - 28

IEEE Standards Standard Frequency Range Data Range 802.11b GHZ up to 11 Mbps 802.11a GHZ up to 54 Mbps 802.11g GHZ up to 54 Mbps 802.11b mostly sufficient for home networks with DSL or broadband Cable. 802a have higher bit rates, but have lesser transmission distance for the same power. 802g’s have both high speed and low power advantages. Lesson 12: Link Layer Technologies - 29

Lesson 12: Link Layer Technologies -30
How Wi-Fi works Once AP is installed, it is given 1 or 2 word Service Set Identifier (SSID). It is also given channel numbers- 85 MHz in b, for example, divided into 11 channels. As per wifi standard, AP periodically transmits beacon frames with its SSID and MAC Address Wireless station tries to access an AP using association protocol. When channel is sensed idle, a station (AP or other station) transmits frame after a time called Distributed Inter-Frame Space (DIFS) Lesson 12: Link Layer Technologies -30

How Wi-Fi works (continued)
When channel is busy, it takes a random back off value and freezes it. Only when it is idle, it starts counting down and transmits when count is zero. This is for collision avoidance. Once the frame is transmitted, waits for ACK. If ACK is received and another frame is required and starts again with a random back off value. If ACK is not received, same process is repeated with a larger back-off value. Collision is avoided for large frames by Request to Send (RTS) and Clear To Send (CTS) protocol message exchanges before data transmission and ACK. Lesson 12: Link Layer Technologies -31

Lesson 12: Link Layer Technologies-Summary/Follow-up
We studied the following link layer technologies and their implementation Current Multiple Access MAC (Medium Access Control) Protocols- CSMA/CD Channel Partitioning “Taking Turns” type – Token Ring Ethernet Hubs, Bridges and Switches PPP ATM IEEE LANs Next class, we take up Physical Layer Lesson 12: Link Layer Technologies - 32

Lesson 13: Introduction to Physical Layer

Lesson 13: Introduction to Physical Layer -Preview/Objectives
We study physical layer functionality and 3 types of transmission Simplex Half Duplex Full Duplex Signals and their properties Relation between bandwidth and data rate Lesson 13: Introduction to Physical Layer -1

Lesson 13: Introduction to Physical Layer - 2
Physical layer is concerned with data transmission Data transmission occurs between a transmitter and a receiver. The media may be guided or unguided: guided: twisted pair, coaxial cable, and fiber. unguided: through air, water, or vacuum. Either type of transmission is based on electromagnetic waves. A direct link is the signal transmission path between two devices with no intermediate device other than repeaters and amplifiers. Lesson 13: Introduction to Physical Layer - 2

Data Transmission- Some Terminology
A guided medium is point-to-point if it provides a direct link between two devices; the medium is shared by only those two devices; In a multi-point configuration, more than two devices share the transmission medium. We distinguish 3 forms of transmission: Simplex Half Duplex Full Duplex Lesson 13: Introduction to Physical Layer - 3

Simplex Transmission Transmission in only one direction; one station is the transmitter, the other the receiver. Examples: One-Way Street Keyboard-Computer connection Computer-Monitor connection TV Broadcast Can you think of other simplex examples? Lesson 13: Introduction to Physical Layer - 4

Half-Duplex Transmission
Half Duplex: Transmission in both directions possible, but NOT at the same time. Here, the attached stations are both, sender and receiver. Examples: One-Lane Road with access control lights. While cars go in one directions, cars going the opposite way must wait. Walkie-Talkies CB-Radios Traditional Ethernet (Coax or 10baseT) Lesson 13: Introduction to Physical Layer - 5

Full Duplex Transmission
Transmission in both directions simultaneously. Both stations can send and receive at the same time. Examples: Regular 2-way street Full-Duplex repeated Ethernet (Gbit Ethernet) Full Duplex transmission can be accomplished in two ways: Separated physical transmission media Divided channel capacity and separation of signals in different directions. Lesson 13: Introduction to Physical Layer - 6

What is transmitted? Signals are transmitted; could be electrical, optical , etc. Signals can be expressed in two ways: in the Time-Domain, the signal intensity varies over time; i.e., as a function of time, f(t) in the Frequency-Domain, the signal is expressed as a function of the constituent frequencies, the set of sinusoid signals which make up the signal. We need to distinguish between 2 types of signals: Continuous; Discrete Lesson 13: Introduction to Physical Layer - 7

Continuous and Discrete Signals
A continuous signal is one in which the signal intensity varies in a smooth fashion over time. There are no breaks (poles) or discontinuities. A discrete signal is one in which the signal intensity maintains a constant level for some period of time and then changes to another constant level. Note: A discrete signal may consist of more than just 2 constant levels; i.e., discrete does not mean binary! Lesson 13: Introduction to Physical Layer - 8

Periodic Signal The simplest sort of signal is a periodic signal. Here, T is said to be the period. T is the smallest value that satisfies the equation. Definition: a signal s(t) is periodic if and only if Lesson 13: Introduction to Physical Layer - 9

Sinusoid- The Fundamental Continuous Signal
The sine wave is the fundamental continuous signal. We can represent the sine wave by 3 parameters: Amplitude (A) Frequency (f) Phase () Lesson 13: Introduction to Physical Layer - 10

Amplitude, Frequency and Phase
Amplitude (A): is the peak value or strength of the signal over time. (in Volts, Watts, etc.) Frequency (f): is the rate (in cycles per second, or Hertz (Hz)) at which the signal repeats. The period T can be computed as T=1/f. T is the amount of time taken for one repetition. Phase (): is the measure of the relative position in time within a single period of the signal. Lesson 13: Introduction to Physical Layer - 11

Wavelength of a Signal The Wavelength () of a signal is the distance occupied by a single cycle (or period). In other words, it is the distance between to points of corresponding phase of two consecutive cycles. Here, v represents the velocity of the signal. Lesson 13: Introduction to Physical Layer - 12

Frequency Domain Representation of Signals
The Frequency-Domain Concept allows us to represent a signal as the sum of constituent frequencies. For example: The components of s(t) are sine waves of frequencies f1 and 3f1. Fourier analysis is the method of decomposing signals into the constituent sinusoids. s(t) = sin(2f1t) + 1/3 sin(2(3f1)t) Lesson 13: Introduction to Physical Layer - 13

Frequency Domain Analysis
When all of the frequency components are integer multiples of one frequency f1, f1 is called the fundamental frequency. The period of the total signal is equal to the period of the fundamental frequency. The spectrum of a signal is the range of frequencies that it contains. In our example, the spectrum extends from f1 to 3 f1. Lesson 13: Introduction to Physical Layer - 14

Bandwidth Physical property of the transmission medium Depends on length, thickness, construction, etc. Range of frequencies transmitted without being strongly attenuated In our example, the bandwidth required to send the signal without distortion is 3f1- f1 = 2f1. Note that most of the energy in the signal is contained in a relative narrow band of frequencies. This is referred to as the effective bandwidth required. In this case, a medium with lower bandwidth can transmit this signal with tolerable distortion. Lesson 13: Introduction to Physical Layer - 15

Fourier Analysis- An Overview
Any reasonably behaved periodic signal can be expressed as a sum (possibly infinite) of sines and cosines as follows: Sine and cosine term pair for a value of n is called nth harmonic. Root Mean Square (RMS) amplitude √an2+bn2 indicates the significance of the nth harmonic. s(t)=c/2 + Σn=1 to ∞ansin(2nft) + Σn=1 to ∞bncos(2nft) Lesson 13: Introduction to Physical Layer - 16

Relation between Data Rate and Bandwidth
At b bits/sec, time required to send 8-bits = 8/b sec. Freq. of 1st harmonic will be b/8 Hz. How many harmonica pass through a voice grade line with 3000 Hz cut-off? Lesson 13: Introduction to Physical Layer - 17

Lesson 13: Introduction to Physical Layer –Summary and Follow-up
We studied physical layer functionality and 3 types of transmission Simplex Half Duplex Full Duplex We studied Signals and their properties (particularly Fourier Analysis) Relation between bandwidth and data rate Next class, we study about wireless access technologies. Lesson 13: Introduction to Physical Layer -18

Lesson 14: Physical Layer (Wireless Access)

Lesson 14: Physical Layer (Wireless Access)-Preview/Objectives
We study in this lesson Two kinds of wireless access Fixed (e.g. fixed wireless systems using traditional mobile access technologies, wi-fi) Mobile Mobile Access: Generations 1-3, 2.5, Evolutionary Technologies- FDMA (e.g. AMPS), TDMA (e.g. GSM), CDMA (e.g IS-95/CDMA-2000), WCDMA Mobility Management Lesson 14: Physical Layer (Wireless Access) - 1

How Wireless Systems Work?
RNC in UMTS jargon Depending upon in which cell mobile is, it will be able to access a particular base station. Call will be se up via a Base Station controller (BSC) and a Mobile Switching Center (MSC) after a lot of call processing (control or signaling messages) back and forth. Phone could be stationary (fixed) or mobile- but in case of mobile phones a technique called hand-over/hand-off is used. MSC or PDSN/GGSN BSC-X BSC-Y A B C D Lesson 14: Physical Layer (Wireless Access) - 2

Lesson 14: Physical Layer (Wireless Access) - 3
Multiple Access Each pair of users enjoy a dedicated, private circuit through the transmission medium (air in case of wireless systems), unaware of the existence of other users. Lesson 14: Physical Layer (Wireless Access) - 3

Generations of Wireless Technologies
1st Generation Mobile Phones (Analog Voice) Push to Talk Systems (e.g. CB radios, police radios) in late 1950s IMTS (Improved Mobile Telephone Systems) 1960s AMPS (Advanced Mobile Phone Systems) 1982 by Bell Labs 2nd Generation (Digital Voice) D-AMPS, GSM and CDMA (IS-95) 3rd Generation 1XRTT, CDMA-200 and UMTS (Universal Mobile Telecommunications System) based on W-CDMA. Beyond 3g (B3g)- Evolutionary (1xEVDV, 1xEVDO, etc.) 2.5 G Enhanced Data Rates for GSM (Edge) and GPRS (General Packet Radio Services) Lesson 14: Physical Layer (Wireless Access) - 4

CDMA-Spread Spectrum Slow varying (low frequency) data signal is spread over a large spectrum using a fast (high frequency signal CDMA spreading principle- Anything we can do , we can undo. Lesson 14: Physical Layer (Wireless Access) - 5

How do you do & Undo? Lesson 14: Physical Layer (Wireless Access) - 6

Spreading Example Lesson 14: Physical Layer (Wireless Access) - 7

De-spreading (Recovery of Previously Spread Data) for the same Example
Lesson 14: Physical Layer (Wireless Access) - 8

How do you handle mixture of signals from multiple users?
Use orthogonal signals (e.g. Walsh codes) for spreading. Two signals are orthogonal if their XOR sum has equal number of 1’s and 0’s (e.g and ) Lesson 14: Physical Layer (Wireless Access) - 9

Mobility Management Hand-off/Hand-over Two types Soft-handoff (Continuous connection with two base stations and seamless transfer) Hard-handoff (mobile stops transmitting, adjusts its parameters and restarts) Intersystem (control is passed to a new MSC) Intra-system Lesson 14: Physical Layer (Wireless Access) - 10

Lesson 14: Physical Layer (Wireless Access)-Summary/Follow-up
We studied in this lesson Two kinds of wireless access Fixed (e.g. fixed wireless systems using traditional mobile access technologies, wi-fi) Mobile Mobile Access: Generations 1-3 Technologies- FDMA (e.g. AMPS), TDMA (e.g. GSM), CDMA (e.g IS-95/CDMA-2000), WCDMA Mobility Management Lesson 14: Physical Layer (Wireless Access) - 11

Lesson 15: Introduction to Network Security

Lesson 15: Introduction Network Security-Preview/Objectives
We study in this lesson What is security? What all it entails? Cryptography Authentication Message Integrity Types of Keys for encryption, their distribution and certification Famous Public Key Algorithm (RSA) Lesson 15: Introduction to Network Security - 1

Friends and enemies: Alice, Bob, Trudy
Figure 7.1 goes here Well-known in network security world Bob, Alice (close friends) want to communicate “securely” Trudy, the “intruder” may intercept, delete, add messages Lesson 15: Introduction to Network Security - 2

What is network security?
Secrecy: only sender, intended receiver should “understand” message contents sender encrypts message receiver decrypts message Authentication: sender, receiver want to confirm identity of each other Message Integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection Lesson 15: Introduction to Network Security - 3

Internet security threats I- Packet Sniffing
Packet sniffing is possible because the media is broadcast type promiscuous NIC reads all packets passing by any one can read all unencrypted data (e.g. passwords) e.g.: C sniffs B’s packets A C src:B dest:A payload B Lesson 15: Introduction to Network Security - 4

Internet security threats II- IP Spoofing
IP Spoofing (e.g. C pretending to be B) is done by: Generation of “raw” IP packets directly from application, putting any value into IP source address field such that receiver can’t tell if source is spoofed More generic name for this kind of attack- Sybil attack where even bogus messages can be introduced in the network. A C src:B dest:A payload B Lesson 15: Introduction to Network Security - 5

Internet security threats III: Denial of Service Attack
This attack is done by A flood of maliciously generated packets that “swamp” receiver Distributed DOS (DDOS): multiple coordinated sources that swamp receiver e.g. C and remote host SYN-attack A A C SYN SYN SYN SYN SYN B SYN SYN Lesson 15: Introduction to Network Security - 6

Jargon of cryptography
plaintext K A K B plaintext ciphertext Figure 7.3 goes here symmetric key crypto: sender, receiver keys identical public-key crypto: encrypt key public, decrypt key secret Lesson 15: Introduction to Network Security - 7

Symmetric key cryptography
Substitution cipher: substituting one thing for another monoalphabetic cipher: substitute one letter for another plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq E.g.: Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc Q: How hard to break this simple cipher?: brute force (how hard?) other? Lesson 15: Introduction to Network Security - 8

Symmetric key crypto: DES
DES: Data Encryption Standard US encryption standard [NIST 1993] 56-bit symmetric key, 64 bit plaintext input How secure is DES? DES Challenge: 56-bit-key-encrypted phrase (“Strong cryptography makes the world a safer place”) decrypted (brute force) in 4 months no known “backdoor” decryption approach making DES more secure use three keys sequentially (3-DES) on each datum use cipher-block chaining Lesson 15: Introduction to Network Security - 9

Symmetric key crypto: DES
DES operation initial permutation 16 identical “rounds” of function application, each using different 48 bits of key final permutation Lesson 15: Introduction to Network Security - 10

Public Key Cryptography
symmetric key crypto requires sender, receiver know shared secret key Q: how to agree on key in first place (particularly if never “met”)? public key cryptography radically different approach [Diffie-Hellman76, RSA78] sender, receiver do not share secret key encryption key public (known to all) decryption key private (known only to receiver) Lesson 15: Introduction to Network Security - 11

Public key cryptography
Figure 7.7 goes here Lesson 15: Introduction to Network Security - 12

Public key encryption algorithms
Two inter-related requirements: need d ( ) and e ( ) such that 1 B B d (e (m)) = m B B 2 need private and public keys for d ( ) and e ( ), respectively B B RSA: Rivest, Shamir, Adelson algorithm Lesson 15: Introduction to Network Security - 13

RSA: Encryption, decryption
0. Given (n,e) and (n,d) as computed above 1. To encrypt bit pattern, m, compute c = m mod n e e (i.e., remainder when m is divided by n) 2. To decrypt received bit pattern, c, compute m = c mod n d d (i.e., remainder when c is divided by n) m = (m mod n) e mod n d Magic happens! Lesson 15: Introduction to Network Security - 14

Lesson 15: Introduction to Network Security - 15
RSA: Choosing keys 1. Choose two large prime numbers p, q. (e.g., 1024 bits each) 2. Compute n = pq, z = (p-1)(q-1) 3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”). 4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ). 5. Public key is (n,e). Private key is (n,d). Lesson 15: Introduction to Network Security - 15

RSA example: Bob chooses p=5, q=7. Then n=35, z=24.
e=5 (so e, z relatively prime). d=29 (so ed-1 exactly divisible by z. e c = m mod n e m m encrypt: 12 248832 17 c d m = c mod n d c decrypt: 17 12 Lesson 15: Introduction to Network Security - 16

Lesson 15: Introduction to Network Security - 17
RSA: How does it work? e d To prove: m = (m mod n) mod n , we use two theorems: Fermat’s little theorem :(x p-1 mod p = 1), when p is prime and x is prime to p. Chinese Reminder Theorem : If a = b mod p and a=b mod q where p and q are relatively prime, a=b mod pq. (me)d =med= med-1.m= mh(p-1)(q-1).m = 1h(q-1).m (mod p) = m (mod p) (me)d =med= med-1.m= mh(p-1)(q-1).m = 1h(p-1).m (mod q) = m (mod q) Hence, (me)d = m (mod pq) by Chinese Reminder Theorem In the above, h is an integer . Since ed-1 is divisible by z=(p-1)(q-1), ed-1 = hz =h(p-1)(q-1). Lesson 15: Introduction to Network Security - 17

RSA: Is it the end of Public Key Cryptography?
No. Recently, another algorithm called Elliptic Curve Cryptography is getting popular as it is even more difficult to break. Lesson 15: Introduction to Network Security - 18

Lesson 15: Introduction to Network Security-Summary/Follow-up
We studied in this lesson What is security? What all it entails? Cryptography Authentication Message Integrity Types of Keys for encryption, their distribution and certification Famous Public Key Algorithm (RSA) In the next class, we take up other security issues (e.g. authentication) and some applications. Lesson 15: Introduction to Network Security - 19

Lesson 16: Network Security (Continued)

Lesson 16: Network Security (Continued)-Preview/Objectives
We study in this lesson A more detailed view of the following security features: Authentication Message Integrity Key distribution and certification Security in practice: Application layer: secure Transport layer: Internet commerce, SSL, SET Network layer: IP security Lesson 16: Network Security (Continued) - 1

Authentication Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice” Failure scenario?? Lesson 16: Network Security (Continued) - 2

Authentication: another try
Protocol ap2.0: Alice says “I am Alice” and sends her IP address along to “prove” it. Failure scenario?? Lesson 16: Network Security (Continued) - 3

Authentication: another try
Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Failure scenario? Lesson 16: Network Security (Continued) - 4

Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. I am Alice encrypt(password) Failure scenario? Lesson 16: Network Security (Continued) - 5

Authentication: yet another try
Goal: avoid playback attack Nonce: number (R) used only once in a lifetime ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key Figure 7.11 goes here Failures, drawbacks? Lesson 16: Network Security (Continued) - 6

Lesson 16: Network Security (Continued) - 7
Authentication: ap5.0 ap4.0 requires shared symmetric key problem: how do Bob, Alice agree on key can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography Figure 7.12 goes here Lesson 16: Network Security (Continued) - 7

ap5.0: security hole Man (woman) in the middle attack: Trudy poses as Alice (to Bob) and as Bob (to Alice) Figure 7.14 goes here Need “certified” public keys (more later …) Lesson 16: Network Security (Continued) - 8

Digital Signatures Cryptographic technique analogous to hand-written signatures. Sender (Bob) digitally signs document, establishing he is document owner/creator. Verifiable, nonforgeable: recipient (Alice) can verify that Bob, and no one else, signed document. Simple digital signature for message m: Bob encrypts m with his private key dB, creating signed message, dB(m). Bob sends m and dB(m) to Alice. Lesson 16: Network Security (Continued) - 9

More on Digital Signatures
Suppose Alice receives msg m, and digital signature dB(m) Alice verifies m signed by Bob by applying Bob’s public key eB to dB(m) then checks eB(dB(m) ) = m. If eB(dB(m) ) = m, whoever signed m must have used Bob’s private key. Alice thus verifies that: Bob signed m. No one else signed m. Bob signed m and not m’. Non-repudiation: Alice can take m, and signature dB(m) to court and prove that Bob signed m. Lesson 16: Network Security (Continued) - 10

Message Digests Computationally expensive to public-key-encrypt long messages Goal: fixed-length,easy to compute digital signature, “fingerprint” apply hash function H to m, get fixed size message digest, H(m). Hash function properties: Many-to-1 Produces fixed-size msg digest (fingerprint) Given message digest x, computationally infeasible to find m such that x = H(m) computationally infeasible to find any two messages m and m’ such that H(m) = H(m’). Lesson 16: Network Security (Continued) - 11

Digital signature = Signed message digest
Alice verifies signature and integrity of digitally signed message: Bob sends digitally signed message: Lesson 16: Network Security (Continued) - 12

Hash Function Algorithms
Internet checksum would make a poor message digest. Too easy to find two messages with same checksum. MD5 hash function widely used. Computes 128-bit message digest in 4-step process. arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x. SHA-1 is also used. US standard 160-bit message digest Lesson 16: Network Security (Continued) - 13

Trusted Intermediaries
Problem: How do two entities establish shared secret key over network? Solution: trusted key distribution center (KDC) acting as intermediary between entities Problem: When Alice obtains Bob’s public key (from web site, , diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: trusted certification authority (CA) Lesson 16: Network Security (Continued) - 14

Key Distribution Center (KDC)
Alice,Bob need shared symmetric key. KDC: server shares different secret key with each registered user. Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC. Alice communicates with KDC, gets session key R1, and KB-KDC(A,R1) Alice sends Bob KB-KDC(A,R1), Bob extracts R1 Alice, Bob now share the symmetric key R1. Lesson 16: Network Security (Continued) - 15

Certification Authorities
Certification authority (CA) binds public key to particular entity. Entity (person, router, etc.) can register its public key with CA. Entity provides “proof of identity” to CA. CA creates certificate binding entity to public key. Certificate digitally signed by CA. When Alice wants Bob’s public key: gets Bob’s certificate (Bob or elsewhere). Apply CA’s public key to Bob’s certificate, get Bob’s public key Lesson 16: Network Security (Continued) - 16

Secure Alice wants to send secret message, m, to Bob. generates random symmetric private key, KS. encrypts message with KS also encrypts KS with Bob’s public key. sends both KS(m) and eB(KS) to Bob. Lesson 16: Network Security (Continued) - 17

Secure e-mail (continued)
Alice wants to provide sender authentication message integrity. Alice digitally signs message. sends both message (in the clear) and digital signature. Lesson 16: Network Security (Continued) - 18

Secure e-mail (continued)
Alice wants to provide secrecy, sender authentication, message integrity. Note: Alice uses both her private key, Bob’s public key. Lesson 16: Network Security (Continued) - 19

Pretty good privacy (PGP)
Internet encryption scheme, a de-facto standard. Uses symmetric key cryptography, public key cryptography, hash function, and digital signature as described. Provides secrecy, sender authentication, integrity. Inventor, Phil Zimmerman, was target of 3-year federal investigation. A PGP signed message: ---BEGIN PGP SIGNED MESSAGE--- Hash: SHA1 Bob:My husband is out of town tonight.Passionately yours, Alice ---BEGIN PGP SIGNATURE--- Version: PGP 5.0 Charset: noconv yhHJRHhGJGhgg/12EpJ+lo8gE4vB3mqJhFEvZP9t6n7G6m5Gw2 ---END PGP SIGNATURE--- Lesson 16: Network Security (Continued) - 20

Secure sockets layer (SSL)
Server authentication: SSL-enabled browser includes public keys for trusted CAs. Browser requests server certificate, issued by trusted CA. Browser uses CA’s public key to extract server’s public key from certificate. Visit your browser’s security menu to see its trusted CAs. PGP provides security for a specific network app. SSL works at transport layer. Provides security to any TCP-based app using SSL services. SSL: used between WWW browsers, servers for I-commerce (shttp). SSL security services: server authentication data encryption client authentication (optional) Lesson 16: Network Security (Continued) - 21

SSL (continued) Encrypted SSL session: Browser generates symmetric session key, encrypts it with server’s public key, sends encrypted key to server. Using its private key, server decrypts session key. Browser, server agree that future messages will be encrypted. All data sent into TCP socket (by client or server) is encrypted with session key. SSL: basis of IETF Transport Layer Security (TLS). SSL can be used for non-Web applications, e.g., IMAP. Client authentication can be done with client certificates. Lesson 16: Network Security (Continued) - 22

Secure electronic transactions (SET)
designed for payment-card transactions over Internet. provides security services among 3 players: customer merchant merchant’s bank All must have certificates. SET specifies legal meanings of certificates. apportionment of liabilities for transactions Customer’s card number passed to merchant’s bank without merchant ever seeing number in plain text. Prevents merchants from stealing, leaking payment card numbers. Three software components: Browser wallet Merchant server Acquirer gateway Lesson 16: Network Security (Continued) - 23

IPSEC: Network Layer Security
Network-layer secrecy: sending host encrypts the data in IP datagram TCP and UDP segments; ICMP and SNMP messages. Network-layer authentication destination host can authenticate source IP address Two principle protocols: authentication header (AH) protocol encapsulation security payload (ESP) protocol For both AH and ESP, source, destination handshake: create network-layer logical channel called a service agreement (SA) Each SA unidirectional. Uniquely determined by: security protocol (AH or ESP) source IP address 32-bit connection ID Lesson 16: Network Security (Continued) - 24

ESP Protocol Provides secrecy, host authentication, data integrity. Data, ESP trailer encrypted. Next header field is in ESP trailer. ESP authentication field is similar to AH authentication field. Protocol = 50. Lesson 16: Network Security (Continued) - 25

Authentication Header (AH) Protocol
Provides source host authentication, data integrity, but not secrecy. AH header inserted between IP header and IP data field. Protocol field = 51. Intermediate routers process datagrams as usual. AH header includes: connection identifier authentication data: signed message digest, calculated over original IP datagram, providing source authentication, data integrity. Next header field: specifies type of data (TCP, UDP, ICMP, etc.) Lesson 16: Network Security (Continued) - 26

Lesson 16: Network Security (Continued)-Summary/Follow-up
We studied in this lesson A more detailed view of the following security features: Authentication Message Integrity Key distribution and certification Application of those security features in practice: Application layer: secure Transport layer: Internet commerce, SSL, SET Network layer: IP security (IPSec) Lesson 16: Network Security (Continued) - 27

Computer Networks (EENG 4810)

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