Application Layer Socket API, HTTP, VoIP, DNS IS250 Spring 2010 John Chuang

2 Application Layer & Socket API Appl Trans port Net work Link Net work Link Net work Link Appl Trans port Net work Link Host AHost BRouter 1Router 2 end-to-end point-to-point end-to-end Software Operating System Hardware Socket API

John Chuang3 Socket  Socket: an API (application programming interface) that allows the abstraction of the underlying network (i.e., layers 4 and lower)  Sockets support send and receive methods  Sockets identified by  Two common types of sockets: -Stream socket (supported by TCP) -Datagram socket (supported by UDP)  Read: Comer

John Chuang4 Network Layers  Application (layer 7): specific to application need  Transport (layer 4): end-to-end delivery, congestion and flow control  Network (layer 3): addressing, routing  Data Link (layer 2): framing, error detection  Physical (layer 1): bits (0/1), voltages, frequencies, wires, pins, … IP TCP, UDP HTTP, FTP, NNTP, SMTP, telnet,... coax, twisted pair, fiber, wireless,... Ethernet FDDI, SONET Wi-Fi

John Chuang5 Representation v. Transfer  Application layer protocols specify two aspects of interaction: -Data representation: Syntax of data items that are exchanged, e.g., HTML -Data transfer: Interaction between client and server, e.g., HTTP

John Chuang6 L7 Alphabet Soup  Applications: -HTTP (Comer ) -FTP (Comer 4.11) - , SMTP (Comer ) -telnet, SSH -RTP, SIP (Comer 29) -…  Network support: -DNS (Comer ) -BGP, RIP, OSPF (Comer 27) -…

John Chuang7 Hypertext Transfer Protocol  HTTP specification (RFC 2616): -  HTTP uses TCP  Two types of HTTP messages -HTTP Request -HTTP Response Client Server HTTP Request HTTP Response

John Chuang8 HTTP Request Format Request line [ : ] … [Message body] Request header Entity body Note: = carriage-return-line-feed optional fields in [] General header Entity header Blank line

John Chuang9 HTTP Header Fields  HTTP Methods -GET -POST -HEAD -OPTIONS -PUT -DELETE -TRACE -CONNECT  General Header Fields -Cache-Control -Connection -Date -Pragma -Trailer -Transfer-Encoding -Upgrade -Via -Warning  Request Headers -Accept -Accept-Charset -Accept-Encoding -Accept-Language -Authorization -Cookie -Expect -From -Host * -If-Match If-Modified-Since -If-None-Match -If-Range -If-Unmodified-Since -Max-Forwards -Proxy-Authorization -Range -Referrer -TE -User-Agent * Mandatory in HTTP/1.1  Entity Headers -Allow -Content-Encoding -Content-Language -Content-Length -Content-Location -Content-MD5 -Content-Range -Content-Type -Expires -Last-Modified

John Chuang10 HTTP Request: Example Request line GET /index.html HTTP/1.1 Host: Blank line Request header HTTP message TCP header IP header Ethernet header Ethernet frame port TCP IP IP datagram/packet TCP segment HTTP message Adapted from Coulouris, Dollimore and Kindberg

John Chuang11 HTTP Response  Response Header Fields -Accept-Ranges -Age -Etag -Location -Proxy-Authenticate -Retry-After -Server -Set-Cookie -Vary -WWW-Authenticate  Status Codes -1xx: Informational -2xx: Success -3xx: Redirection -4xx: Client error -5xx: Server error  Some common codes: -200 OK -401 Unauthorized -404 Not Found Status line [ : ] … [Message body] Response header Entity body General header Entity header Blank line Note: = carriage-return-line-feed optional fields in []

John Chuang12 HTTP Response: Example Status line Response header Entity body General header Entity header Blank line HTTP/ OK Date: Mon, 12 Nov :14:04 GMT Server: Apache/ (Red Hat) X-Powered-By: PHP/4.3.9 Set-Cookie: PHPSESSID=f49e0b5ec; expires=Wed, 05-Dec :47:24 GMT; path=/ Last-Modified: Mon, 12 Nov :14:04 GMT Cache-Control: no-store, no-cache, must-revalidate Cache-Control: post-check=0, pre-check=0 Pragma: no-cache Transfer-Encoding: chunked Content-Type: text/html; charset=utf-8 …

John Chuang13 HTTP/1.1 and Persistent TCP Connections  An HTML object often has other embedded objects (e.g., images)  HTTP/1.0 requires a new TCP session for each object -Overhead of session establishment  HTTP/1.1 permits reuse of one TCP session for multiple requests -Use “Connection: Keep-Alive” header

John Chuang14 L7 Alphabet Soup  Applications: -HTTP (Comer ) -FTP (Comer 4.11) - , SMTP (Comer ) -telnet, SSH -RTP, SIP (Comer 29) -…  Network support: -DNS (Comer ) -BGP, RIP, OSPF (Comer 27) -…

John Chuang15 Real-Time Multimedia  How to support real-time multimedia applications (e.g., video, audio) over a best- effort IP network? -Packets may be delivered out of order or lost -Retransmission is too slow to be helpful  Technique: delayed playback using timestamps and jitter buffer Source: Douglas Comer

John Chuang16 Real-Time Transport Protocol (RTP)  Protocol for transport of real-time multi- media data -Sequence number and timestamp in RTP header Source: Douglas Comer

John Chuang17 Real-Time Transport Protocol (RTP)  RTP messages encapsulated in UDP datagrams -Why not TCP? Source: Douglas Comer

John Chuang18 RTP Companion Protocols  RTCP: Real-time Transport Control Protocol -Control packets sent periodically to report quality such as loss rate and jitter  RTSP: Real-time Transport Streaming Protocol -Used by streaming media players to provide remote control like capabilities (e.g., play, stop, fast forward)

John Chuang19 IP Telephony (VoIP)  Companies around the world are replacing traditional telephone switches with IP routers -Routers cost much less than traditional telephone switches -Sending both data and voice as IP packets lowers cost because underlying infrastructure is shared  The basic idea behind IP telephony: -continuously sample audio, convert to digital (PCM) -send digitized stream across IP network (RTP) -convert stream back to analog for playback  Complications arise in signaling and backward compatibility with PSTN

John Chuang20 IP Telephony (VoIP)  IP telephone  Media gateway controller  Media gateway  Signaling gateway Source: Douglas Comer

John Chuang21 VoIP Protocols  RTP for user multimedia data  Multiple protocols for signaling: -SIP: Session Initiation Protocol [RFC 2543] (IETF) -H.323 (ITU) -ENUM: converts telephone number to URI; facilitates DNS lookup Source: Douglas Comer

John Chuang22 Example SIP Session Source: Douglas Comer

John Chuang23 L7 Alphabet Soup  Applications: -HTTP (Comer ) -FTP (Comer 4.11) - , SMTP (Comer ) -telnet, SSH -RTP, SIP (Comer 29) -…  Network support: -DNS (Comer ) -BGP, RIP, OSPF (Comer 27) -…

John Chuang24 Domain Name Service (DNS)  Routers and end-hosts use IP addresses -e.g.,  Human beings remember hostnames -e.g., ischool.berkeley.edu  Need translation service!  Client sends query message (hostname) to domain name server using UDP  Name server sends response message (with resolved IP address) back to client

John Chuang25 Local Name Server (ns1.berkeley.edu) Server ( Client (client.ischool.berkeley.edu) DNS Query (1) (2) (3)

John Chuang26 Local Name Server (ns1.berkeley.edu) Server ( DNS Query (1) (2) Remote Name Server (ns.publisher.com) Root Name Server (f.root-servers.net) (3) (4) (5) (6) (7) Client (client.ischool.berkeley.edu)

John Chuang27 Hostnames: [a-m].root-servers.net Source:

John Chuang28 Next Time: Overlays  Build your own networks at the application layer