Polytechnic University  M. Veeraraghavan 1 Review of networking concepts Prof. Malathi Veeraraghavan University of Virginia

Polytechnic University  M. Veeraraghavan 2 Outline Review of basic concepts in networking –Prerequisite: A first course on networking –Communication links and switches –Types of networks –Shared links: media access control (MAC)

Polytechnic University  M. Veeraraghavan 3 What is a communication network? Simplest “network” –Single link between two pieces of end-user equipment (e.g., PC, telephone) End-user equipment End-user equipment –Types of communication links Twisted pair Coaxial cable Optical fiber Wireless links –Radio frequencies –Infra-red frequencies

Polytechnic University  M. Veeraraghavan 4 What is needed to send data on communication links? Error control –Error detection: Parity checks, Checksum, Cyclic Redundancy Code (CRC) –Error correction: ARQ (Automatic Repeat reQuest) FEC (Forward Error Correction) Flow control: handles rate mismatch between sender and receiver –x-ON/x-OFF –Window based flow control –Rate based flow control

Polytechnic University  M. Veeraraghavan 5 Switches Connect multiple links and route traffic from one link to another End-user equipment End-user equipment End-user equipment End-user equipment Switch

Polytechnic University  M. Veeraraghavan 6 Why use a switch? If there are N endpoints (end-user equipment), then how many links are needed for full mesh connectivity? How many physical links are needed if these endpoints are connected through a switch?

Polytechnic University  M. Veeraraghavan 7 Answers Number of direct links needed to connect N nodes is N links – since we only need one link from an endpoint to a switch

Polytechnic University  M. Veeraraghavan 8 Cost of using a switch? Switch cost Can all endpoints have full connectivity at all times to all other endpoints? –Yes, with multiplexing on the links

Polytechnic University  M. Veeraraghavan 9 Concept of multiplexing Time division multiplexing –Allows data from different sessions to be combined at different times on to the same line –How many DS0s in a T1? Wavelength division multiplexing –Difference between FDM (Frequency Division Multiplexing) and WDM? –Relation between frequency and wavelength

Polytechnic University  M. Veeraraghavan 10 Answers 24 DS0s in a T1 Term WDM is the same as FDM at optical frequencies – see EM spectrum chart Speed of light c = f : wavelength; f: frequency

Polytechnic University  M. Veeraraghavan 11 Transceiver rate Rate of transmission and reception at endpoints and the switch –Needs to be sufficient for “full mesh” connectivity “all the time” –e.g., if DS0s used between endpoints in full mesh network, then T1s can be used in 25 endpoint network with a switch for full mesh connectivity

Polytechnic University  M. Veeraraghavan 12 Types of switches Circuit switches: Position-based switching –Switch consults a table to determine output port on which to send data bits based on their arriving position “Position”: Interface (space), time slot and/or wavelength –Space division switch: switch based on input interface –Time division switching: interface + time slot –Wavelength division switching: interface + wavelength –No buffers Packet switches: Label-based switching –Switch consults a table to determine output port on which to send the packet based on value of label (in packet header) –Label could be changed on outgoing port or could stay the same –Have buffers to hold packets

Polytechnic University  M. Veeraraghavan 13 Switch designs See lectures on circuit switching and packet switching in Course on Data Networks Compare unfolded view of a CS with that of a PS See relevance of queueing theory to delays of calls or packets through switches

Polytechnic University  M. Veeraraghavan 14 Network of switches Expand 1-switch network to a multi-switch network Why not build one gigantic switch? –Scalability limitations End-user equipment End-user equipment Switch End-user equipment Switch

Polytechnic University  M. Veeraraghavan 15 Different types of networks A network is defined by its “switching mode” and its “networking mode” Circuit switching vs. packet switching –Circuit-switching: switching based on position (space, time, ) of arriving bits –Packet-switching: switching based on information in packet headers Connectionless vs. connection-oriented networking: –CL: Packets routed based on address information in headers –CO: Connection set up (resources reserved) prior to data transfer Packet-switching Circuit-switching Switching modes Connectionless Connection-oriented Networking modes ATM, X.25 IP, SS7 MPLS IP + RSVP Telephone network, SONET/SDH, WDM

Polytechnic University  M. Veeraraghavan 16 Types of data transfers Sending end Consuming end Live Stored Live Stored Interactive/ Live streaming Recording Stored streamingFile transfers An application could consist of different types of data transfers —An http session has an interactive component, but could also have a non-real-time transfer

Polytechnic University  M. Veeraraghavan 17 Types of data transfers Sending end Consuming end Live Stored Live Stored Interactive/ Live streaming Recording Stored streamingFile transfers An application could consist of different types of data transfers —An http session has an interactive component, but could also have a non-real-time transfer

Polytechnic University  M. Veeraraghavan 18 Matching applications & networks Data transfers Non-real-time (stored at sender and receiver ends) Real-time (consumed or sent live) Interactive (two-way) (consumed and sent live) e.g. telephony, telnet, ftp, http Streaming (one-way) (consumed live; sent from live or stored source) e.g. radio/TV broadcasts Recording (one-way) (stored at receiver end; sent from live source); e.g. Replay Short transfers (e.g. short ) Long transfers (e.g. large image, audio, video or data) Ideal networks Connectionless networks Circuit-switched networks Packet-switched CO networks

Polytechnic University  M. Veeraraghavan 19 Congestion control What is it? –The purpose of a network is to allow sharing of resources –This means if demand is high, there could be competition for resources from multiple users –What are network resources: Link capacity (bandwidth) Switch buffer space (only in packet switches)

Polytechnic University  M. Veeraraghavan 20 Congestion control In CO networks –Congestion control: mostly preventive –Connection Admission Control (CAC) Check availability of bandwidth and buffer resources before admitting a connection CS CO networks: congestion will not occur once circuits are admitted PS CO networks: congestion can occur after connection is admitted if connection admission is based on statistical multiplexing –Have some supplemental reactive congestion control scheme

Polytechnic University  M. Veeraraghavan 21 Congestion control In CL networks –Have packet switches detect congestion and send reactive messages asking sender to slow down –e.g., datagram routers in SS7 networks send such messages; SRP (Spatial Reuse Protocol) switches in MANs send such messages –IP routers implement Explicit Congestion Notification (ECN) procedures

Polytechnic University  M. Veeraraghavan 22 End-to-end path Transport protocols –Ensure reliable transfer across a communication path consisting of many links (“zero” loss) –OR ensure delay-controlled path across a communication path consisting of many links –Error control and flow control –Delay control (e.g., RTP) –Congestion control and connection control – special in TCP

Polytechnic University  M. Veeraraghavan 23 Applications Most Internet applications are client-server based Web server End-user equipment Network Web clients (Usually runs on fixed hosts) Network End-user equipment -sending clients (outlook, messenger) Outgoing servers (pop, imap) Network End-user equipment -receiving clients (outlook, messenger) Incoming servers (smtp)

Polytechnic University  M. Veeraraghavan 24 Protocol Stacks OSI model: two more layers between AL and TL –Session layer and presentation layer PHY: Physical; DLL: Data Link Layer; NL: Network Layer; TL: Transport Layer; AL: Application Layer DLL NL TL AL PHY Endpoint Switch NL DLL PHY DLL IP TCP/UDP AL PHY Endpoint Switch NL DLL PHY DLL

Polytechnic University  M. Veeraraghavan 25 Example protocols AL protocols: http, smtp, ftp, PCM voice TL protocols: TCP, UDP, RTP, AAL NL protocols: IP, ATM DLL protocols: PPP, HDLC PHY protocols: DS0, DS1 Ethernet: PHY+DLL+NL

Polytechnic University  M. Veeraraghavan 26 Functions of protocol layers PHY: sends bits across a link DLL: error control and flow control on a link NL: switching (routing), multiplexing, congestion control TL: error control and flow control on an end-to-end basis AL: Functions specific to the application

Polytechnic University  M. Veeraraghavan 27 Congestion control and connection control in TCP IP routers did not implement ECN until recently –TCP performs congestion control –Senses whether network switches (routers) are congested or not –Adjusts rate accordingly –Slow start and congestion avoidance Concept of a “connection” at the TL –End hosts maintain state information regarding a TCP connection to track sequence numbers and ACKs –Connection open (SYN) and close (FIN) procedures –Contrast with a “connection” at the NL, where each switch maintains state about the connection

Polytechnic University  M. Veeraraghavan 28 User plane, control plane, and management plane Management plane: consists of all the protocols needed to “configure” data tables for the operation of the network –For example, protocols for routing data dissemination (distributed or centralized) –Other functions: performance, fault mgmt., accounting, security Control plane: –Connection control protocols in CO networks, this includes connection setup at each switch (connections at the network layer) in CL networks, this includes connection setup only at the endpoints (connections at the transport layer, if the TL protocol is reliable) –Call control protocols User plane: protocols for the actual flow of data

Polytechnic University  M. Veeraraghavan 29 Routing protocol in all three types of networks - Phase 1 Host A Host B I IV V III II Routing protocol Dest. Next hop III-* IV Dest. Next hop III-* III Dest. Next hop B B Routing tables Routing protocols exchange topology/loading/reachability information Routes to destinations are precomputed and stored in routing tables

Polytechnic University  M. Veeraraghavan 30 Signaling protocol for NL connection setup in a PS CO network - Phase 2 Connection setup consists of each switch on the path –Route lookup for next hop node to reach destination –CAC (Connection Admission Control) for buffer and BW –Writing the input/output label mapping tables and programming the scheduler Host A Host B I IV V III II Connection setup a b c a b c d dc a b IN Port /Label OUT Port/Label a/L1 c/L2 IN Port /Label OUT Port/Label a/L2 c/L1 IN Port /Label OUT Port/Label d/L1 b/L3 Connection setup (B) Connection setup Virtual circuit

Polytechnic University  M. Veeraraghavan 31 Signaling protocol for NL connection setup in a CS CO network - Phase 2 Connection setup consists of each switch on the path –Route lookup for next hop node to reach destination –CAC (Connection Admission Control) for BW (note: no buffers) –Writing the port/timeslot/  mapping table Host A Host B I IV V III II Connection setup a b c a b c d dc a b IN Port /Timeslot OUT Port/Timeslot a/1 c/2 IN Port /Timeslot OUT Port/Timeslot a/2 c/2 IN Port /Timeslot OUT Port/Timeslot d/2 b/1 Connection setup (B) Connection setup Circuit

Polytechnic University  M. Veeraraghavan 32 TL connection setup in a CL PS network - Phase 2 Notion of transport layer connections –Exchange initial sequence numbers end-to-end to allow for ARQ (Automatic Repeat reQuest) based error correction, i.e., retransmissions in case of errors Host A Host B I IV V III II Dest. Next hop B II Dest. Next hop B III Dest. Next hop B B Routing tables SYN ACK

Polytechnic University  M. Veeraraghavan 33 User-plane packet forwarding in a PS CO network - Phase 3 Labels are VPI/VCIs in ATM Labels are translated from link-to-link Host A Host B I IV V III II a b c a b c d dc a b L2 L1 L3 IN Port /Label OUT Port/Label a/L1 c/L2

Polytechnic University  M. Veeraraghavan 34 User-plane actions in a circuit-switched network - Phase 3 Bits arriving at switch I on time slot 1 on port a are switched to time slot 2 of port c Host A Host B I IV V III II a b c a b c d dc a b OUT Port/Timeslot IN Port /Timeslot a/1 c/2

Polytechnic University  M. Veeraraghavan 35 User-plane packet forwarding in a CL PS network - Phase 3 Packet headers carry destination host address (unchanged as it passes hop by hop) Each CL packet switch does a route lookup to determine the outgoing port/next hop node Host A Host B I IV V III II a b c a b c d dc a b B B BB

Polytechnic University  M. Veeraraghavan 36 Addressing Where are endpoint addresses used: –In CL PS networks, endpoint addresses are carried in packet headers –In CO networks, be it PS or CS, endpoint addresses are carried in connection setup messages

Polytechnic University  M. Veeraraghavan 37 Summarized addresses What are summarized addresses? Why summarize addresses?

Polytechnic University  M. Veeraraghavan 38 Summarized addresses What are summarized addresses? –An address that represents a group of endpoint addresses –e.g., all 212 numbers, IP addresses Why summarize addresses? –Reduces routing table sizes – hold one entry for a summarized address instead of a large number of individual addresses –Reduces routing message lengths that convey reachability information

Polytechnic University  M. Veeraraghavan 39 Examples of signaling protocols SS7 (Signaling System No. 7) network (with its SS7 protocol stack) carries signaling messages to set up and release circuits in a telephone network

Polytechnic University  M. Veeraraghavan 40 Examples of routing protocols In an Ethernet network –Spanning tree algorithm and address learning In the Internet: –Link-state routing protocols, such as Open Path Shortest First (OSPF) –Distance-vector based routing protocols, such as Routing Information Protocol (RIP) In telephone networks: –Real-Time Network Routing (RTNR)

Polytechnic University  M. Veeraraghavan 41 Examples of addressing schemes Internet –4-byte IP addresses Telephone networks –8-byte E.164 address (telephone number) ATM networks –20-byte ATM End System Address (AESA)

Polytechnic University  M. Veeraraghavan 42 Broadcast links Wireless Copper: ethernet hubs Optical fiber: Passive star couplers Ethernet hub or WDM Passive Star Coupler Blind broadcast Ethernet switch (packet switch) Dest: A A

Polytechnic University  M. Veeraraghavan 43 MAC protocols Medium Access Control (MAC) protocols are used in broadcast links to allow a node to access medium and send information As if “switch” is in endpoints Wasteful of resources because all endpoints receive all packets End-user equipment A End-user equipment B End-user equipment C To B B’s MAC layer checks destination address to determine whether the packet should be “switched” to the application or dropped C’s MAC layer checks destination address to determine whether the packet should be “switched” to the application or dropped

Polytechnic University  M. Veeraraghavan 44 Consider wireless links Naturally broadcast medium –One transmitter sends data; multiple receivers can receive the signal and obtain the data –Need a MAC (Medium Access Control) protocol to share the “naturally broadcast” wireless medium Endpoint

Polytechnic University  M. Veeraraghavan 45 Shared links in wired domain Distance limitation between farthest hosts – Shannon’s capacity; SNR; attenuation outbound inbound Multipoint drops: potential interference on inbound line – polling; e.g. multidrop telephone lines Hub or optical passive star coupler Host Hubs/Optical passive star couplers: any data received on one line is broadcast to all other lines

Polytechnic University  M. Veeraraghavan 46 Classification of MAC protocols MAC protocols Fixed-assignment schemes Random-access schemes Demand assignment schemes Circuit-switched (e.g., FDMA, TDMA) Connectionless packet-switched (e.g., Ethernet, ) Connection-oriented packet-switched (e.g., CDMA, polling) Channelization

Polytechnic University  M. Veeraraghavan 47 Shared link as a LAN: relation between MAC protocols and LANs A shared link allows multiple end stations to hear a transmission from any station No node is serving as a “forwarding engine” for packets in a controlled fashion –hubs, passive star couplers, ring adapters, taps blindly send data UNLIKE switches, routers, bridges This shared link concept works well as a local area network –if too large a network – with many hosts – each host will get a small percentage of bandwidth

Polytechnic University  M. Veeraraghavan 48 Shared links as “access” links Two reasons for using shared links on the access segment –individual endpoints (hosts/phones) generate small quantities of data traffic –Costs should be kept low for end users Consequence: access links are often shared MAC protocols in the upstream direction

Polytechnic University  M. Veeraraghavan 49 Shared link in the presence of basestations/APs? Is it still one shared link if basestations/APs forward data between two endpoints that cannot “hear” each other –No, basestations/APs become forwarding engines, i.e., switches –If a cell phone under one basestation calls another cell phone under the same basestation and the basestation allocates frequencies for both ends and forwards data bits Not different from a circuit switch forwarding bits received on one DS0 to another DS0 –Same thing when an AP uses destination addresses to rebroadcast data – it acts as a packet switch

Polytechnic University  M. Veeraraghavan 50 Compare TDMA on an access link with TDM on an inter-switch link Similar in concept: sharing resources on one link among many users Difference: –Multiple senders on access link –One sender in each direction on inter-switch link Basestation Endpoint Timeslot 1Timeslot 2Timeslot 3 Circuit switch Circuit switch T1 line carrying 24 different DS0s (phone calls)

Polytechnic University  M. Veeraraghavan 51 Internetworking An internet –A path that traverses multiple networks possibly ones using different networking techniques

Polytechnic University  M. Veeraraghavan 52 Simplest network – one link Endpoint Switch One network – same type of switches – link rates can be different Single networks Endpoint A shared link: often used to create a LAN

Polytechnic University  M. Veeraraghavan 53 Endpoint Switch Network 1 Endpoint Switch Network 2 IP router An internetwork The Internet approach to internetworking Have all endpoints speak the IP (Internet Protocol) in addition to their own network protocols For loss-sensitive applications: run TCP, an end-to-end transport protocol, irrespective of whether –both ends are within the same network –the two ends are on different networks IP routers are connectionless packet switches –they forward IP packets from one network to another based on the destination IP address carried in the IP header and information stored in their routing tables Network 3

Polytechnic University  M. Veeraraghavan 54 Protocol stacks in the Internet

Polytechnic University  M. Veeraraghavan 55 Today’s most common networks in the Internet Ethernet within enterprises using a combination of –shared-medium Ethernet LANs with hubs, or –with Ethernet switches – which are connectionless packet switches PDH/SONET networks in the MAN and WAN domains –Routers are interconnected by T1, T3, OC3 connections that are set up through a PDH/SONET circuit-switched network –PPP, Point-to-Point Protocol, is executed on these circuits

Polytechnic University  M. Veeraraghavan 56 Need Internet address and Network address Host A Host B Switch 1 Switch 2 Ethernet 1 Host C Host D Switch 3 Switch 4 Ethernet 2 IP router Internetwork Host E Host F Switch Ethernet 3 Host A sends a packet to Host C: -Places Host C’s IP address in IP header -To get through Ethernet 1, it needs Ethernet address of IP router’s interface 1 -Switch 1 and Switch 2 forward packets based on destination Ethernet address of IP router’s interface 1 -IP router forwards packet to port 2 to reach Host C (based on IP level routing data using destination IP address of host C) -IP router needs Ethernet address of Host C to send the packet through Ethernet 2 -Switch 3 and 4 forward packets based on destination Ethernet address of Host C 1 2 3

Polytechnic University  M. Veeraraghavan 57 Summary Reviewed networking concepts