Chapter 2: Application Layer

Chapter 2: Application Layer
Our goals: conceptual, implementation aspects of network application protocols transport-layer service models client-server paradigm peer-to-peer paradigm learn about protocols by examining popular application-level protocols HTTP FTP SMTP / POP3 / IMAP DNS P2P programming network applications socket API 2: Application Layer

Chapter 2: Application layer
2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2.7 Socket programming with TCP 2.8 Socket programming with UDP 2: Application Layer

Some network apps e-mail web instant messaging remote login
P2P file sharing multi-user network games streaming stored video clips voice over IP real-time video conferencing social networking … 2: Application Layer

Creating a network app write programs that
application transport network data link physical write programs that run on (different) end systems communicate over network e.g., web server software communicates with browser software No need to write software for network-core devices Network-core devices do not run user applications applications on end systems allows for rapid app development, propagation application transport network data link physical application transport network data link physical 2: Application Layer

Application architectures
Client-server (C/S) Peer-to-peer (P2P) Hybrid of C/S and P2P 2: Application Layer

Client-server architecture
always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other client/server 2: Application Layer

Pure P2P architecture no always-on server
arbitrary end systems directly communicate peers are intermittently connected and change IP addresses Highly scalable but difficult to manage peer-peer 2: Application Layer

Hybrid of client-server and P2P
Skype voice-over-IP P2P application centralized server: finding address of remote party: client-client connection: direct (not through server) Instant messaging chatting between two users is P2P centralized service: client presence detection/location user registers its IP address with central server when it comes online user contacts central server to find IP addresses of buddies 2: Application Layer

Processes communicating
Client process: process that initiates communication Server process: process that waits to be contacted Process: program running within a host. within same host, two processes communicate using inter-process communication (defined by OS). processes in different hosts communicate by exchanging messages Note: applications with P2P architectures have client processes & server processes 2: Application Layer

Sockets process sends/receives messages to/from its socket
TCP with buffers, variables socket host or server process TCP with buffers, variables socket host or server process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process controlled by app developer Internet controlled by OS API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) 2: Application Layer

Addressing processes to receive messages, process must have identifier
host device has unique 32-bit IP address Q: does IP address of host suffice for identifying the process? 2: Application Layer

Addressing processes to receive messages, process must have identifier
host device has unique 32-bit IP address Q: does IP address of host on which process runs suffice for identifying the process? A: No, many processes can be running on same host identifier includes both IP address and port numbers associated with process on host. Example port numbers: HTTP server: 80 Mail server: 25 to send HTTP message to gaia.cs.umass.edu web server: IP address: Port number: 80 more shortly… 2: Application Layer

App-layer protocol defines
Types of messages exchanged, e.g., request, response Message syntax: what fields in messages & how fields are delineated Message semantics meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP Proprietary protocols: e.g., Skype 2: Application Layer

What transport service does an app need?
Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer, telnet) require 100% reliable data transfer Throughput some apps (e.g., multimedia) require minimum amount of throughput to be “effective” other apps (“elastic apps”) make use of whatever throughput they get Security Encryption, data integrity, … Timing some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” 2: Application Layer

Transport service requirements of common apps
Application file transfer Web documents real-time audio/video stored audio/video interactive games instant messaging Data loss no loss loss-tolerant Throughput elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up Time Sensitive no yes, 100’s msec yes, few secs yes and no 2: Application Layer

Internet transport protocols services
TCP service: connection-oriented: setup required between client and server processes 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 throughput guarantees, security UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security Q: why bother? Why is there a UDP? 2: Application Layer

Internet apps: application, transport protocols
layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (eg Youtube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) Underlying transport protocol TCP TCP or UDP typically UDP Application remote terminal access Web file transfer streaming multimedia Internet telephony 2: Application Layer

2.1 Principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2.7 Socket programming with TCP 2.8 Socket programming with UDP 2: Application Layer

Web/WWW and HTTP First some jargon Web page consists of objects
Object can be HTML file, JPEG image, Java applet, audio file,… Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: host name path name 2: Application Layer

WWW Architectural Overview
(a) A Web page (b) The page reached by clicking on Department of Animal Psychology. 2: Application Layer

WWW Architectural Overview
The parts of the Web model. 2: Application Layer

HTML – HyperText Markup Language
(a) The HTML for a sample Web page. (b) The formatted page. (b) 2: Application Layer

HTML A selection of common HTML tags. some can have additional parameters. 2: Application Layer

Steps in processing the information from an HTML form.
Dynamic Web Documents Steps in processing the information from an HTML form. 2: Application Layer

Dynamic Web Documents A sample HTML page with embedded PHP.
2: Application Layer

HTTP overview 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 HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator 2: Application Layer

HTTP overview (continued)
Uses TCP: 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 HTTP is “stateless” server maintains no information about past client requests 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 2: Application Layer

HTTP connections Nonpersistent HTTP
At most one object is sent over a TCP connection. Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. 2: Application Layer

Nonpersistent HTTP (contains text, references to 10 jpeg images) Suppose user enters URL 1a. HTTP client initiates TCP connection to HTTP server (process) at on port 80 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. Message indicates that client wants object someDepartment/home.index 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time 2: Application Layer

Nonpersistent HTTP (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 time 6. Steps 1-5 repeated for each of 10 jpeg objects 2: Application Layer

Non-Persistent HTTP: Response time
Definition of RTT: time for a small packet to travel from client to server and back. Response time: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of HTTP response to return file transmission time total = 2RTT+transmit time time to transmit file initiate TCP connection RTT request received time 2: Application Layer

Persistent HTTP Nonpersistent HTTP issues: Persistent HTTP
requires 2 RTTs per object OS overhead for each TCP connection browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server sent over open connection client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects 2: Application Layer

HTTP request message two types of HTTP messages: request, response
ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) header lines Carriage return, line feed indicates end of message 2: Application Layer

HTTP request message: general format

Uploading form input Post method: Web page often includes form input
Input is uploaded to server in entity body URL method: Uses GET method Input is uploaded in URL field of request line: 2: Application Layer

Method types HTTP/1.0 GET POST HEAD HTTP/1.1 GET, POST, HEAD PUT
asks server to leave requested object out of response HTTP/1.1 GET, POST, HEAD PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field 2: Application Layer

HTTP response message status line (protocol status code status phrase)
HTTP/ OK Connection close 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 2: Application Layer

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 2: Application Layer

Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server: telnet cis.poly.edu 80 Opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. Anything typed in sent to port 80 at cis.poly.edu 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/ HTTP/1.1 Host: cis.poly.edu 3. Look at response message sent by HTTP server! 2: Application Layer

User-server state: cookies
Example: Susan always access Internet always from PC visits specific e-commerce site for first time when initial HTTP requests arrives at site, site creates: unique ID entry in backend database for ID Many major Web sites use cookies Four components: 1) cookie header line of HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user’s host, managed by user’s browser 4) back-end database at Web site 2: Application Layer

Cookies: keeping “state” (cont.)
client server ebay 8734 usual http request msg Amazon server creates ID 1678 for user create entry cookie file usual http response Set-cookie: 1678 ebay 8734 amazon 1678 usual http request msg cookie: 1678 cookie- specific action access usual http request msg cookie: 1678 cookie- spectific action access usual http response msg backend database one week later: ebay 8734 amazon 1678 usual http response msg 2: Application Layer

Cookies (continued) Cookies and privacy: What cookies can bring:
aside Cookies and privacy: cookies permit sites to learn a lot about you you may supply name and to sites What cookies can bring: authorization shopping carts recommendations user session state (Web ) How to keep “state”: protocol endpoints: maintain state at sender/receiver over multiple transactions cookies: http messages carry state 2: Application Layer

Web caches (proxy server)
Goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client origin server HTTP response Proxy server HTTP request client HTTP request HTTP response client origin server 2: Application Layer

More about Web caching cache acts as both client and server
typically cache is installed by ISP (university, company, residential ISP) Why Web caching? reduce response time for client request reduce traffic on an institution’s access link. Internet dense with caches: enables “poor” content providers to effectively deliver content (but so does P2P file sharing) 2: Application Layer

Caching example Assumptions Consequences origin servers
average object size = 100,000 bits avg. request rate from institution’s browsers to origin servers = 15/sec delay from institutional router to any origin server and back to router = 2 sec Consequences utilization on LAN = 15% utilization on access link = 100% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache 2: Application Layer

Caching example (cont)
origin servers possible solution increase bandwidth of access link to, say, 10 Mbps consequence utilization on LAN = 15% utilization on access link = 15% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade public Internet 10 Mbps access link institutional network 10 Mbps LAN institutional cache 2: Application Layer

Caching example (cont)
origin servers possible solution: install cache suppose hit rate is 0.4 consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin server utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache 2: Application Layer

If-modified-since: <date> If-modified-since: <date>
Conditional GET cache server Goal: don’t send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request If-modified-since: <date> server: response contains no object if cached copy is 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> 2: Application Layer

FTP: the file transfer protocol
user interface client file transfer FTP server user at host remote file system local 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 2: Application Layer

FTP: separate control, data connections
client server TCP control connection port 21 TCP data connection port 20 FTP client contacts FTP server at port 21, TCP is transport protocol client authorized over control connection client browses remote directory by sending commands over control connection. when server receives file transfer command, server opens 2nd TCP connection (for file) to client after transferring one file, server closes data connection. server opens another TCP data connection to transfer another file. control connection: “out of band” FTP server maintains “state”: current directory, earlier authentication 2: Application Layer

FTP commands, responses
Sample commands: sent as ASCII text over control channel USER username PASS password LIST return list of file in current directory RETR filename retrieves (gets) file STOR filename stores (puts) file onto 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 2: Application Layer

Sample FTP interaction
[01] ftp nic.ddn.mil [02] connected to nic.ddn.mil [03] 220 nic FTP server (Sunos 4.1)ready. [04] Name: anonymous [05] 331 Guest login ok, send ident as password. [06] Password: [07] 230 Guest login ok, access restrictions apply. [08] ftp> cd rfc [09] 250 CWD command successful. [10] ftp> get rfc1261.txt nicinfo [11] 200 PORT command successful. [12] 150 ASCII data connection for rfc1261.txt ( ,1401) (4318 bytes). [13] 226 ASCII Transfer complete. local: nicinfo remote: rfc1261.txt 4488 bytes received in 15 seconds (0.3 Kbytes/s). [14] ftp> quit [15] 221 Goodbye. 2: Application Layer

Electronic Mail Some smileys. They will not be on the final exam :-).

Electronic Mail Three major components: SMTP SMTP SMTP user agents
user mailbox outgoing message queue user agent 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, Mozilla Thunderbird outgoing, incoming messages stored on server mail server user agent SMTP mail server user agent SMTP mail server SMTP user agent user agent user agent 2: Application Layer

Envelopes and messages. (a) Paper mail. (b) Electronic mail.
The User Agent Envelopes and messages. (a) Paper mail. (b) Electronic mail. 2: Application Layer

Electronic Mail: mail servers
user agent Mail Servers mailbox contains incoming messages 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 mail server user agent SMTP mail server user agent SMTP mail server SMTP user agent user agent user agent 2: Application Layer

Electronic Mail: SMTP [RFC 2821]
uses TCP to reliably transfer message 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 2: Application Layer

Scenario: Alice sends message to Bob
1) Alice uses UA to compose message and “to” 2) Alice’s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob’s mail server 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message mail server mail server 1 user agent user agent 2 3 6 4 5 2: Application Layer

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 2: Application Layer

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) 2: Application Layer

SMTP: final words Comparison with HTTP:
SMTP uses persistent connections SMTP requires message (header & body) to be in 7-bit ASCII SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull SMTP: push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in multipart msg 2: Application Layer

Mail message format header body
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 2: Application Layer

RFC 822 header fields related to message transport.
Message Formats – RFC 822 RFC 822 header fields related to message transport. 2: Application Layer

Message Formats – RFC 822 (2)
Some fields used in the RFC 822 message header. 2: Application Layer

MIME – Multipurpose Internet Mail Extensions
Problems with international languages: Languages with accents (French, German). Languages in non-Latin alphabets (Hebrew, Russian). Languages without alphabets (Chinese, Japanese). Messages not containing text at all (audio or images). 2: Application Layer

Message format: multimedia extensions
MIME: multimedia mail extension, 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 2: Application Layer

MIME RFC 822 headers added by MIME. 2: Application Layer

The MIME types and subtypes defined in RFC 2045.

A multipart message containing enriched and audio alternatives.
MIME A multipart message containing enriched and audio alternatives. 2: Application Layer

Mail access protocols SMTP SMTP access protocol
user agent user agent sender’s mail server receiver’s mail server SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: gmail, Hotmail, Yahoo! Mail, etc. 2: Application Layer

POP3 protocol authorization phase C: list transaction phase, client:
S: +OK POP3 server ready C: user bob 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 2: Application Layer

POP3 (more) and IMAP More about POP3 IMAP
Previous example uses “download and delete” mode. Bob cannot re-read if he changes client “Download-and-keep”: copies of messages on different clients POP3 is stateless across sessions IMAP Keep all messages in one place: the server Allows user to organize messages in folders IMAP keeps user state across sessions: names of folders and mappings between message IDs and folder name 2: Application Layer

Comparison of POP3 and IMAP

DNS: Domain Name System
People: many identifiers: SSN, name, passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., ww.yahoo.com - used by humans Q: map between IP addresses and name ? Domain Name System: 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” 2: Application Layer

The DNS Name Space A portion of the Internet domain name space.

DNS Why not centralize DNS? DNS services single point of failure
traffic volume distant centralized database maintenance doesn’t scale! DNS services hostname to IP address translation host aliasing Canonical, alias names mail server aliasing load distribution replicated Web servers: set of IP addresses for one canonical name 2: Application Layer

Distributed, Hierarchical Database
Root DNS Servers com DNS servers org DNS servers edu DNS servers poly.edu DNS servers umass.edu yahoo.com amazon.com pbs.org Client wants IP for 1st approx: client queries a root server to find com DNS server client queries com DNS server to get amazon.com DNS server client queries amazon.com DNS server to get IP address for 2: Application Layer

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 a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations) k RIPE London (also 16 other locations) i Autonomica, Stockholm (plus other locations) e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 36 other locations) m WIDE Tokyo (also Seoul, Paris, SF) 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA 2: Application Layer

TLD and Authoritative Servers
Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. Network Solutions maintains servers for com TLD Educause for edu TLD Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail). can be maintained by organization or service provider 2: Application Layer

Local Name Server does not strictly belong to hierarchy
each ISP (residential ISP, company, university) has one. also called “default name server” when host makes DNS query, query is sent to its local DNS server acts as proxy, forwards query into hierarchy 2: Application Layer

DNS name resolution example
root DNS server 2 Host at cis.poly.edu wants IP address for gaia.cs.umass.edu 3 TLD DNS server 4 5 iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” local DNS server dns.poly.edu 7 6 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu 2: Application Layer

DNS name resolution example
requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 4 5 6 authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 recursive query: puts burden of name resolution on contacted name server heavy load? 2: Application Layer

DNS: caching and updating records
once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time TLD servers typically cached in local name servers Thus root name servers not often visited update/notify mechanisms under design by IETF RFC 2136 2: Application Layer

RR format: (name, value, type, ttl)
DNS records DNS: distributed db storing resource records (RR) RR format: (name, value, type, ttl) Type=A name is hostname value is IP address Type=CNAME name is alias name for some “canonical” (the real) name is really servereast.backup2.ibm.com value is canonical name Type=NS name is domain (e.g. foo.com) value is hostname of authoritative name server for this domain Type=MX value is name of mailserver associated with name 2: Application Layer

DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header identification: 16 bit # for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative 2: Application Layer

DNS protocol, messages Name, type fields for a query RRs in response
to query records for authoritative servers additional “helpful” info that may be used 2: Application Layer

Inserting records into DNS
example: new startup “Network Utopia” register name networkuptopia.com at DNS registrar (e.g., Network Solutions) provide names, IP addresses of authoritative name server (primary and secondary) registrar inserts two RRs into com TLD server: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, , A) create authoritative server Type A record for Type MX record for networkutopia.com How do people get IP address of your Web site? 2: Application Layer

2.1 Principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2.7 Socket programming with TCP 2.8 Socket programming with UDP 2: Application Layer

What is P2P? “the sharing of computer resources and services by direct exchange of information” 2: Application Layer

What is P2P? “P2P is a class of applications that take advantage of resources – storage, cycles, content, human presence – available at the edges of the Internet. Because accessing these decentralized resources means operating in an environment of unstable and unpredictable IP addresses P2P nodes must operate outside the DNS system and have significant, or total autonomy from central servers” 2: Application Layer

What is P2P? “A distributed network architecture may be called a P2P network if the participants share a part of their own resources. These shared resources are necessary to provide the service offered by the network. The participants of such a network are both resource providers and resource consumers” 2: Application Layer

What is P2P? Various definitions seem to agree on sharing of resources
direct communication between equals (peers) no centralized control 2: Application Layer

What is a peer? “…an entity with capabilities similar to other entities in the system.” “…a network entity with which one performs peering operations.” “…a participant that acts as both client and server.” What is wrong with this definition? 2: Application Layer

Client/Server Architecture
Well known, powerful, reliable server is a data source Clients request data from server Very successful model WWW (HTTP), FTP, Web services, etc. Server Client Internet * Figure from 2: Application Layer

Client/Server Limitations
Scalability is hard to achieve Presents a single point of failure Requires administration Unused resources at the network edge P2P systems try to address these limitations 2: Application Layer

P2P Architecture All nodes are both clients and servers Node
Provide and consume data Any node can initiate a connection No centralized data source “The ultimate form of democracy on the Internet” “The ultimate threat to copy-right protection on the Internet” Node Internet * Content from 2: Application Layer

P2P Network Characteristics
Clients are also servers and routers Nodes contribute content, storage, memory, CPU Nodes are autonomous (no administrative authority) Network is dynamic: nodes enter and leave the network “frequently” Nodes collaborate directly with each other (not through well-known servers) Nodes have widely varying capabilities 2: Application Layer

P2P Goals and Benefits Efficient use of resources
Unused bandwidth, storage, processing power at the “edge of the network” Scalability No central information, communication and computation bottleneck Aggregate resources grow naturally with utilization Reliability Replicas Geographic distribution No single point of failure Ease of administration Nodes self-organize Built-in fault tolerance, replication, and load balancing Increased autonomy Anonymity – Privacy not easy in a centralized system Dynamism highly dynamic environment ad-hoc communication and collaboration 2: Application Layer

P2P Applications File sharing (Napster, Gnutella, Kazaa, …)
Multiplayer games (Unreal Tournament, DOOM, …) Collaborative applications (ICQ, shared whiteboard, …) Distributed computation …) Ad-hoc networks (…) … 2: Application Layer

P2P Goals/Benefits Cost sharing Resource aggregation
Improved scalability/reliability Increased autonomy Anonymity/privacy Dynamism Ad-hoc communication 2: Application Layer

P2P Challenges Decentralization Scalability and Performance Anonymity
Fairness Dynamism Security Transparency Fault Resilience and Robustness 2: Application Layer

Popular file sharing P2P Systems
Napster, Gnutella, Kazaa, Freenet Large scale sharing of files. User A makes files (music, video, etc.) on their computer available to others User B connects to the network, searches for files and downloads files directly from user A Issues of copyright infringement 2: Application Layer

Research Areas Peer discovery and group management
Data placement and searching Reliable and efficient file exchange Security/privacy/anonymity/trust 2: Application Layer

Design Concerns Group Management Search Per-node state Load balancing
Fault tolerance/resiliency Search Bandwidth usage Time to locate item Success rate 2: Application Layer

Approaches Centralized Unstructured
Structured (Distributed Hash Tables) 2: Application Layer

Centralized index original “Napster” design
directory server peers Alice Bob 1 2 3 original “Napster” design 1) when peer connects, it informs central server: IP address content 2) Alice queries for “Hey Jude” 3) Alice requests file from Bob 2: Application Layer

Centralized model Bob Alice Jane Judy file transfer is decentralized, but locating content is highly centralized Upload index to central server when you come online To search, consult central server Request doc directly 2: Application Layer

Centralized Benefits: Drawbacks: Low per-node state
Limited bandwidth usage Short location time High success rate Fault tolerant Drawbacks: Single point of failure Limited scale Possibly unbalanced load copyright infringement Bob Alice Upload index to central server when you come online To search, consult central server Request doc directly Judy Jane 2: Application Layer

Napster program for sharing files over the Internet
a “disruptive” application/technology? history: 5/99: Shawn Fanning (freshman, Northeasten U.) founds Napster Online music service 12/99: first lawsuit 3/00: 25% UWisc traffic Napster 2000: est. 60M users 2/01: US Circuit Court of Appeals: Napster knew users violating copyright laws 7/01: # simultaneous online users: Napster 160K, Gnutella: 40K, Morpheus: 300K 2: Application Layer

Napster: how does it work
Application-level, client-server protocol over point-to-point TCP Four steps: Connect to Napster server Upload your list of files (push) to server. Give server keywords to search the full list with. Select “best” of correct answers. (pings) 2: Application Layer

Napster File list is uploaded 1. napster.com users

Napster 2. User requests search at server. napster.com user Request
and results user 2: Application Layer

Napster 3. User pings hosts that apparently have data.
Looks for best transfer rate. napster.com pings pings user 2: Application Layer

Napster 4. User retrieves file napster.com Retrieves file user

Napster Central Napster server
Can ensure correct results Fast search Bottleneck for scalability Single point of failure Susceptible to denial of service Malicious users Lawsuits, legislation Hybrid P2P system – “all peers are equal but some are more equal than others” Search is centralized File transfer is direct (peer-to-peer) 2: Application Layer

Unstructured overlay network: graph fully distributed
edge between peer X and Y if there’s a TCP connection all active peers and edges form overlay net edge: virtual (not physical) link given peer typically connected with < 10 overlay neighbors fully distributed no central server used by Gnutella Each peer indexes the files it makes available for sharing (and no other files) 2: Application Layer

Gnutella: Query flooding
File transfer: HTTP Query message sent over existing TCP connections peers forward Query message QueryHit sent over reverse path Query QueryHit 2: Application Layer

Gnutella: Peer joining
joining peer Alice must find another peer in Gnutella network: use list of candidate peers Alice sequentially attempts TCP connections with candidate peers until connection setup with Bob Flooding: Alice sends Ping message to Bob; Bob forwards Ping message to his overlay neighbors (who then forward to their neighbors….) peers receiving Ping message respond to Alice with Pong message Alice receives many Pong messages, and can then setup additional TCP connections 2: Application Layer

Unstructured Scalability: limited scope flooding 2: Application Layer
Bob Alice Jane Judy Carl Scalability: limited scope flooding Everyone knows about some small number of nodes To find a file, ask everyone you know When you find out who has the doc, ask directly 2: Application Layer

Unstructured Gnutella model Benefits: Drawbacks:
Carl Jane Gnutella model Benefits: Limited per-node state Fault tolerant Drawbacks: High bandwidth usage Long time to locate item No guarantee on success rate Possibly unbalanced load Bob Everyone knows about some small number of nodes To find a file, ask everyone you know When you find out who has the doc, ask directly Alice Judy 2: Application Layer

Gnutella Searching by flooding:
If you don’t have the file you want, query 7 of your neighbors. If they don’t have it, they contact 7 of their neighbors, for a maximum hop count of 10. Requests are flooded, but there is no tree structure. No looping but packets may be received twice. Reverse path forwarding * Figure from 2: Application Layer

Gnutella fool.* ? TTL = 2 2: Application Layer

Gnutella X IPX:fool.her TTL = 1 TTL = 1 TTL = 1 fool.her

Gnutella fool.me Y fool.you IPY:fool.me fool.you 2: Application Layer

Gnutella IPY:fool.me fool.you 2: Application Layer

Gnutella: strengths and weaknesses
pros: flexibility in query processing complete decentralization simplicity fault tolerance/self-organization cons: severe scalability problems susceptible to attacks Pure P2P system 2: Application Layer

Gnutella: initial problems and fixes
2000: avg size of reachable network only hosts. Why so small? modem users: not enough bandwidth to provide search routing capabilities: routing black holes Fix: create peer hierarchy based on capabilities previously: all peers identical, most modem black holes preferential connection: favors routing to well-connected peers favors reply to clients that themselves serve large number of files: prevent freeloading 2: Application Layer

Structured FreeNet, Chord, CAN, Tapestry, Pastry model
001 012 FreeNet, Chord, CAN, Tapestry, Pastry model 212 ? 212 ? 332 212 305 More systematic approach Ids for docs and nodes Store doc at node with closest id Keep track of small number of nodes with ids close to yours Route requests toward the document 2: Application Layer

Structured FreeNet, Chord, CAN, Tapestry, Pastry model Benefits:
001 012 FreeNet, Chord, CAN, Tapestry, Pastry model Benefits: Manageable per-node state Manageable bandwidth usage and time to locate item Guaranteed success Drawbacks: Possibly unbalanced load Harder to support fault tolerance 212 ? 212 ? 332 212 305 More systematic approach Ids for docs and nodes Store doc at node with closest id Keep track of small number of nodes with ids close to yours Route requests toward the document 2: Application Layer

Unstructured vs Structured P2P
The systems we described do not offer any guarantees about their performance (or even correctness) Structured P2P Scalable guarantees on numbers of hops to answer a query Maintain all other P2P properties (load balance, self-organization, dynamic nature) Approach: Distributed Hash Tables (DHT) 2: Application Layer

Improvements: SuperPeers
KaZaA model Hybrid centralized and unstructured Advantages and disadvantages? 2: Application Layer

Hierarchical Overlay between centralized index, query flooding approaches each peer is either a super node or assigned to a super node TCP connection between peer and its super node. TCP connections between some pairs of super nodes. Super node tracks content in its children 2: Application Layer

Kazaa (Fasttrack network)
Hybrid of centralized Napster and decentralized Gnutella hybrid P2P system Super-peers act as local search hubs Each super-peer is similar to a Napster server for a small portion of the network Super-peers are automatically chosen by the system based on their capacities (storage, bandwidth, etc.) and availability (connection time) Users upload their list of files to a super-peer Super-peers periodically exchange file lists You send queries to a super-peer for files of interest 2: Application Layer

File Distribution: Server-Client vs P2P
Question : How much time to distribute file from one server to N peers? us: server upload bandwidth Server ui: peer i upload bandwidth u1 d1 u2 d2 us di: peer i download bandwidth File, size F dN Network (with abundant bandwidth) uN 2: Application Layer

File distribution time: server-client
server sequentially sends N copies: NF/us time client i takes F/di time to download F u2 u1 d1 d2 us Network (with abundant bandwidth) dN uN = dcs = max { NF/us, F/min(di) } i Time to distribute F to N clients using client/server approach increases linearly in N (for large N) 2: Application Layer

File distribution time: P2P
Server server must send one copy: F/us time client i takes F/di time to download NF bits must be downloaded (aggregate) F u2 u1 d1 d2 us Network (with abundant bandwidth) dN uN fastest possible upload rate: us + Sui dP2P = max { F/us, F/min(di) , NF/(us + Sui) } i 2: Application Layer

Server-client vs. P2P: example
Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us 2: Application Layer

File distribution: BitTorrent
Efficient content distribution system using file swarming. Usually does not perform all the functions of a typical p2p system, like searching. CacheLogic estimated (around 2003 or so) that BitTorrent Traffic accounts for roughly 35% of all traffic on the Internet. Author: Bram Cohen 2: Application Layer

File distribution: BitTorrent
P2P file distribution tracker: tracks peers participating in torrent torrent: group of peers exchanging chunks of a file obtain list of peers trading chunks peer 2: Application Layer

BitTorrent file divided into 256KB chunks. peer joining torrent:
has no chunks, but will accumulate them over time registers with tracker to get list of peers, connects to subset of peers (“neighbors”) while downloading, peer uploads chunks to other peers. peers may come and go once peer has entire file, it may (selfishly) leave or (altruistically) remain 2: Application Layer

BT: File sharing To share a file or group of files, a peer first creates a .torrent file, a small file that contains: metadata about the files to be shared, and Information about the tracker, the computer that coordinates the file distribution. Peers first obtain a .torrent file, and then connect to the specified tracker, which tells them from which other peers to download the pieces of the file. 2: Application Layer

Overall Architecture Tracker Web Server .torrent C A [Seed] Peer
Web page with link to .torrent A B C Peer [Leech] Downloader “US” [Seed] Tracker Web Server .torrent 2: Application Layer

Overall Architecture Tracker Web Server Get-announce C A [Seed] Peer
Web page with link to .torrent A B C Peer [Leech] Downloader “US” [Seed] Tracker Get-announce Web Server 2: Application Layer

Overall Architecture Tracker Web Server Response-peer list C A [Seed]
Web page with link to .torrent A B C Peer [Leech] Downloader “US” [Seed] Tracker Response-peer list Web Server 2: Application Layer

Overall Architecture Tracker Web Server Shake-hand C A [Seed] Peer
Web page with link to .torrent A B C Peer [Leech] Downloader “US” [Seed] Tracker Shake-hand Web Server 2: Application Layer

Overall Architecture Tracker Web Server C A pieces [Seed] Peer [Leech]
Web page with link to .torrent A B C Peer [Leech] Downloader “US” [Seed] Tracker pieces Web Server 2: Application Layer

BT: The .torrent file The URL of the tracker
Pieces <hash1, hash 2,…, hash n> Piece length Name of the file Length of the file 2: Application Layer

BT: The Tracker IP address, port, peer id
State information (Completed or Downloading) Returns a random list of peers 2: Application Layer

BT: Pulling Chunks at any given time, different peers have different subsets of file chunks periodically, a peer (Alice) asks each neighbor for list of chunks that they have. Alice sends requests for her missing chunks 2: Application Layer

BT: Pieces and Sub-Pieces
A piece is broken into sub-pieces ... typically 16KB in size Until a piece is assembled, only download the sub-pieces of that piece only This policy lets pieces assemble quickly 2: Application Layer

BT: Pipelining When transferring data over TCP, always have several requests pending at once, to avoid a delay between pieces being sent. At any point in time, some number, typically 5, are requested simultaneously. Every time a piece or a sub-piece arrives, a new request is sent out. 2: Application Layer

BT: Piece Selection The order in which pieces are selected by different peers is critical for good performance If an inefficient policy is used, then peers may end up in a situation where each has all identical set of easily available pieces, and none of the missing ones. If the original seed is prematurely taken down, then the file cannot be completely downloaded! What are “good policies?” 2: Application Layer

BT: Chunk Selection Strict Priority Rarest First Random First Piece
First Priority Rarest First General rule Random First Piece Special case, at the beginning Endgame Mode Special case 2: Application Layer 158

Random First Piece Initially, a peer has nothing to trade
Important to get a complete piece ASAP Select a random piece of the file and download it 2: Application Layer

Rarest Piece First Determine the pieces that are most rare among your peers, and download those first. This ensures that the most commonly available pieces are left till the end to download. 2: Application Layer

BT: Sending Chunks tit-for-tat
Alice sends chunks to four neighbors currently sending her chunks at the highest rate re-evaluate top 4 every 10 secs every 30 secs: randomly select another peer, starts sending chunks newly chosen peer may join top 4 “optimistically unchoke” 2: Application Layer

BT: Choking Choking is a temporary refusal to upload. It is one of BitTorrent’s most powerful idea to deal with free riders (those who only download but never upload). Tit-for-tat strategy is based on game-theoretic concepts. 2: Application Layer

BitTorrent: Tit-for-tat
(1) Alice “optimistically unchokes” Bob (2) Alice becomes one of Bob’s top-four providers; Bob reciprocates (3) Bob becomes one of Alice’s top-four providers With higher upload rate, can find better trading partners & get file faster! 2: Application Layer

Optimistic unchoking A BitTorrent peer has a single “optimistic unchoke” to which it uploads regardless of the current download rate from it. This peer rotates every 30s Reasons: To discover currently unused connections are better than the ones being used To provide minimal service to new peers 2: Application Layer 164

Upload-Only mode Once download is complete, a peer has no download rates to use for comparison nor has any need to use them. The question is, which nodes to upload to? Policy: Upload to those with the best upload rate. This ensures that pieces get replicated faster, and new seeders are created fast 2: Application Layer

Questions about BT Which features contribute to the efficiency of BitTorrent? What is the effect of bandwidth constraints? Is the Rarest First policy really necessary? Must nodes perform seeding after downloading is complete? How serious is the Last Piece Problem? Does the incentive mechanism affect the performance much? 2: Application Layer 166

Trackerless torrents 2: Application Layer
BitTorrent also supports "trackerless" torrents, featuring a DHT implementation that allows the client to download torrents that have been created without using a BitTorrent tracker. 2: Application Layer

P2P: searching for information
Index in P2P system: maps information to peer location (location = IP address & port number) . Instant messaging Index maps user names to locations. When user starts IM application, it needs to inform index of its location Peers search index to determine IP address of user. File sharing (eg e-mule) Index dynamically tracks the locations of files that peers share. Peers need to tell index what they have. Peers search index to determine where files can be found 2: Application Layer

P2P Case study: Skype inherently P2P: pairs of users communicate.
Skype clients (SC) inherently P2P: pairs of users communicate. proprietary application-layer protocol (inferred via reverse engineering) hierarchical overlay with SNs Index maps usernames to IP addresses; distributed over SNs Skype login server Supernode (SN) 2: Application Layer

Peers as relays Problem when both Alice and Bob are behind “NATs”.
NAT prevents an outside peer from initiating a call to insider peer Solution: Using Alice’s and Bob’s SNs, Relay is chosen Each peer initiates session with relay. Peers can now communicate through NATs via relay 2: Application Layer

P2P Review Two key functions of P2P systems Sharing content
Finding content Direct transfer between peers All systems do this Structured vs. unstructured placement of data Automatic replication of data Centralized (Napster) Decentralized (Gnutella) Probabilistic guarantees (DHTs) 2: Application Layer

Issues with P2P Free Riding (Free Loading)
Two types of free riding Downloading but not sharing any data Not sharing any interesting data On Gnutella 15% of users contribute 94% of content 63% of users never responded to a query Didn’t have “interesting” data No ranking: what is a trusted source? 2: Application Layer

Socket programming Goal: learn how to build client/server application that communicate using sockets Socket API introduced in BSD4.1 UNIX, 1981 explicitly created, used, released by apps client/server paradigm two types of transport service via socket API: unreliable datagram reliable, byte stream-oriented a host-local, application-created, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another application process socket 2: Application Layer

Socket-programming using TCP
Socket: a door between application process and end-end-transport protocol (UCP 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 2: Application Layer

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 source port numbers used to distinguish clients (more in Chap 3) TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint 2: Application Layer

Client/server socket interaction: TCP
Server (running on hostid) Client create socket, port=x, for incoming request: welcomeSocket = ServerSocket() TCP connection setup close connectionSocket read reply from clientSocket create socket, connect to hostid, port=x clientSocket = Socket() wait for incoming connection request connectionSocket = welcomeSocket.accept() send request using clientSocket read request from connectionSocket write reply to 2: Application Layer

Stream jargon A stream is a sequence of characters that flow into or out of a process. An input stream is attached to some input source for the process, e.g., keyboard or socket. An output stream is attached to an output source, e.g., monitor or socket. Client process client TCP socket 2: Application Layer

Socket programming with TCP
Example client-server app: 1) client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream) 2) server reads line from socket 3) server converts line to uppercase, sends back to client 4) client reads, prints modified line from socket (inFromServer stream) 2: Application Layer

Example: Java client (TCP)
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 2: Application Layer

Example: Java client (TCP), 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 2: Application Layer

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 2: Application Layer

Example: Java server (TCP), 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 2: Application Layer

Socket programming with UDP
UDP: no “connection” between client and server no handshaking sender explicitly attaches IP address and port of destination to each packet server must extract IP address, port of sender from received packet 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 2: Application Layer

Client/server socket interaction: UDP
Server (running on hostid) create socket, clientSocket = DatagramSocket() Client Create datagram with server IP and port=x; send datagram via clientSocket create socket, port= x. serverSocket = DatagramSocket() read datagram from serverSocket close clientSocket read datagram from write reply to serverSocket specifying client address, port number 2: Application Layer

Example: Java client (UDP)
process Input: receives packet (recall thatTCP received “byte stream”) Output: sends packet (recall that TCP sent “byte stream”) client UDP socket 2: Application Layer

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 2: Application Layer

Example: Java client (UDP), 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 2: Application Layer

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 2: Application Layer

Example: Java server (UDP), 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 2: Application Layer

Chapter 2: Summary our study of network apps now complete!
specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P: BitTorrent socket programming application architectures client-server P2P hybrid application service requirements: reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP 2: Application Layer

Chapter 2: Summary Most importantly: learned about protocols
typical request/reply message exchange: client requests info or service server responds with data, status code message formats: headers: fields giving info about data data: info being communicated Important themes: control vs. data msgs in-band, out-of-band centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer “complexity at network edge” 2: Application Layer

Chapter 2: 1rt Homework 1. P2 2. How to solve the problem of free-riding in P2P applications? Describe as many solutions as possible. Hand in at once by Monitors ONE week later. No print! Hand-writing with you ID and name. 2: Application Layer

Review Questions See the textbook R1, R2, R10, R11, R14, R24, R27, R28
P7, P22 Web, FTP, , DNS等在设计上有哪些缺陷? 如何解决（或避免）P2P应用中的版权问题? 2: Application Layer

After-class exercise To learn: Socket programming in C

Chapter 2: Application Layer

Similar presentations

Presentation on theme: "Chapter 2: Application Layer"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Chapter 2: Application Layer

Similar presentations

Presentation on theme: "Chapter 2: Application Layer"— Presentation transcript:

Similar presentations

About project

Feedback