1. Transport Layer3-1 Chapter 3 Transport Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in powerpoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.  Slides modified, and new ones created, by Merrie Bergmann 2/25/03 Thanks and enjoy! JFK/KWR All material copyright 1996-2002 J.F Kurose and K.W. Ross, All Rights Reserved

2. Transport Layer3-2 Chapter 3: Transport Layer Our goals: r understand principles behind transport layer services: m multiplexing/demulti- plexing m reliable data transfer m flow control m congestion control r learn about transport layer protocols in the Internet: m UDP: connectionless transport m TCP: connection-oriented transport m TCP congestion control

3. Transport Layer3-3 Chapter 3 outline: today’s lecture r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP

4. Transport Layer3-4 Internet protocol stack r application: supporting network applications m FTP, SMTP, HTTP r transport: host-host data transfer m TCP, UDP r network: routing of datagrams from source to destination m IP, routing protocols r link: data transfer between neighboring network elements m PPP, Ethernet r physical: bits “on the wire” application transport network link physical

5. Transport Layer3-5 Transport services and protocols r provide logical communication between app processes running on different hosts r transport protocols run in end systems m send side: breaks app messages into segments, passes to network layer m rcv side: reassembles segments into messages, passes to app layer r more than one transport protocol available to apps m Internet: TCP and UDP application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport

6. Transport Layer3-6 Transport vs. network layer r network layer: logical communication between hosts r transport layer: logical communication between processes m relies on, enhances, network layer services Household analogy: 12 kids sending letters to 12 kids r processes = kids r app messages = letters in envelopes r hosts = houses r transport protocol = Ann and Bill r network-layer protocol = postal service

7. Transport Layer3-7 Internet transport-layer protocols r reliable, in-order delivery (TCP) m congestion control m flow control m connection setup r unreliable, unordered delivery: UDP m no-frills extension of “best-effort” IP r services not available: m delay guarantees m bandwidth guarantees application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical logical end-end transport

8. Transport Layer3-8 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP

9. Transport Layer3-9 Multiplexing/demultiplexing application transport network link physical P1 application transport network link physical application transport network link physical P2 P3 P4 P1 host 1 host 2 host 3 = process= socket delivering received segments to correct socket Demultiplexing at rcv host: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) Multiplexing at send host:

10. Transport Layer3-10 How demultiplexing works r host receives IP datagrams m each datagram has source IP address, destination IP address m each datagram carries 1 transport-layer segment m each segment has source, destination port number (recall: well-known port numbers for specific applications) r host uses IP addresses & port numbers to direct segment to appropriate socket source port #dest port # 32 bits application data (message) other header fields TCP/UDP segment format

11. Transport Layer3-11 Protocol layering and data Each layer takes data from above r adds header information to create new data unit r passes new data unit to layer below application transport network link physical application transport network link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

12. Transport Layer3-12 Connectionless demultiplexing r Create sockets with port numbers: DatagramSocket mySocket1 = new DatagramSocket(99111); DatagramSocket mySocket2 = new DatagramSocket(99222); r UDP socket identified by two-tuple: ( dest IP address, dest port number) r When host receives UDP segment: m checks destination port number in segment m directs UDP segment to socket with that port number r IP datagrams with different source IP addresses and/or source port numbers directed to same socket

13. Transport Layer3-13 Connectionless demux (cont) DatagramSocket serverSocket = new DatagramSocket(6428); Client IP:B P3 client IP: A P1 P3 server IP: C SP: 6428 DP: 9157 SP: 9157 DP: 6428 SP: 6428 DP: 5775 SP: 5775 DP: 6428 SP: Source Port DP: Destination Port SP provides “return address”

14. Transport Layer3-14 Connection-oriented demux r TCP socket identified by 4-tuple: m source IP address m source port number m dest IP address m dest port number r recv host uses all four values to direct segment to appropriate socket r Server host may support many simultaneous TCP sockets: m each socket identified by its own 4-tuple r Web servers have different sockets for each connecting client m non-persistent HTTP will have different socket for each request

15. Transport Layer3-15 Connection-oriented demux (cont) Client IP:B P3 client IP: A P1 P3 server IP: C SP: 80 DP: 9157 SP: 9157 DP: 80 SP: 80 DP: 5775 SP: 5775 DP: 80 P4

16. Transport Layer3-16 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP

17. Transport Layer3-17 UDP: User Datagram Protocol [RFC 768] r “no frills,” “bare bones” Internet transport protocol r “best effort” service, UDP segments may be: m lost m delivered out of order to app r connectionless: m no handshaking between UDP sender, receiver m each UDP segment handled independently of others Why is there a UDP? r no connection establishment (which can add delay) r simple: no connection state at sender, receiver r small segment header r no congestion control: UDP can blast away as fast as desired

18. Transport Layer3-18 UDP: more r often used for streaming multimedia apps m loss tolerant m rate sensitive r other UDP uses m DNS m SNMP r reliable transfer over UDP: add reliability at application layer m application-specific error recovery! source port #dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header

19. Transport Layer3-19 UDP checksum Sender: r treat segment contents as sequence of 16-bit integers r checksum: addition (1’s complement sum) of segment contents r sender puts checksum value into UDP checksum field Receiver: r compute checksum of received segment r check if computed checksum equals checksum field value: m NO - error detected m YES - no error detected. But maybe errors nonetheless? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment