Review of Previous Lecture

1. Review of Previous Lecture • Electronic Mail: SMTP, POP3, IMAP • DNS • Socket programming with TCP Some slides are in courtesy of J. Kurose and K. Ross

2. Announcement • Homework 1 and project 1 due Wed. midnight • Submission instruction posted in the newsgroup • Recitation materials online

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

4. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Principles of reliable data transfer Outline

5. provide logical communication between app processes running on different hosts transport protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer 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 Transport services and protocols

6. network layer: logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services On one host, there may be several processes communicating with processes on several other hosts, with different protocols Transport vs. network layer

7. reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP no-frills extension of “best-effort” IP services not available: delay guarantees 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 Internet transport-layer protocols

8. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Principles of reliable data transfer Outline

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

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

11. UDP socket identified by two-tuple: (dest IP address, dest port number) When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number IP datagrams with different source IP addresses and/or source port numbers directed to same socket Connectionless demultiplexing

12. P2 P1 P1 P3 SP: 9157 client IP: A DP: 6428 Client IP:B server IP: C SP: 6428 SP: 6428 SP: 5775 DP: 6428 DP: 9157 DP: 5775 Connectionless demux (cont) SP provides “return address”

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

14. SP: 5775 SP: 9157 P1 P1 P2 P4 P3 P6 P5 client IP: A DP: 80 DP: 80 Connection-oriented demux (cont) S-IP: B D-IP:C SP: 9157 DP: 80 Client IP:B server IP: C S-IP: A S-IP: B D-IP:C D-IP:C

15. SP: 5775 SP: 9157 P1 P1 P2 P3 client IP: A DP: 80 DP: 80 Connection-oriented demux: Threaded Web Server P4 S-IP: B D-IP:C SP: 9157 DP: 80 Client IP:B server IP: C S-IP: A S-IP: B D-IP:C D-IP:C

16. Transport-layer services Multiplexing and demultiplexing Connectionless transport: UDP Principles of reliable data transfer Outline

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

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

19. Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: addition of all segment contents + checksum check if all bits are 1: NO - error detected YES - no error detected. But maybe errors nonetheless? More later …. UDP checksum • Goal: detect “errors” (e.g., flipped bits) in transmitted segment 0110 0101 1011 0100 0110 0101 0100 1111 1’s complement sum: Addition: Addition: 1’s complement sum: