1. Fall 2005 By: H. Veisi Computer networks course Olum-fonoon Babol Chapter 6 The Transport Layer

2. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 2 Overview  It’s task is to provide efficient, reliable and cost-effective transport from the source to destination.  Hardware and/or Software within transport layer that does the work is called Transport entity.  Transport layer services:  Connectionless  Connection-oriented  Very similar to Network layer! So why we need this layer?!  Transport layer runs on user’s machine, Network layer mostly runs on routers and result inadequate services.  Provides Primitive functions to application layer for programmers.

3. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 3  Network layer: logical communication between hosts  Transport layer: logical communication between processes  relies on, enhances, network layer services University analogy: 7 students sending letters to 7 students  processes = students  app messages = letters in envelopes  hosts = universities  transport protocol = post office of universities  network-layer protocol = postal service Transport vs. Network Layer

4. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 4 Transport service primitives (1)  The primitives for a simple transport service:  Nesting of TPDU (Transport Protocol Data Unit), packets and frames:

5. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 5 Transport service primitives (2)  Connection/Transmission scenario: Host A Host B Listen Connect Connection Request TPDU Connection Accept TPDU Send Receive Send Receive Data Disconnect Disconnect TPDU Send Disconnect

6. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 6 Addressing Transport Service Access Pint = Port No. Network Service Access Pint = IP No.

7. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 7 The Internet Transport layer protocols  Two types:  UDP (User Datagram Protocol)  Connectionless transport, Like IP!  unreliable, unordered delivery  TCP: Transmission Control Protocol  Connection-oriented transport  reliable, in-order delivery (TCP)  Congestion control  Flow control  Connection setup

8. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 8 UDP (1)  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 segments may be:  lost  delivered out of order to app  Applications:  RPC: Remote Procedure Call  In DNS servers  RTP: Real-Time Transport Protocol  Audio, video, …

9. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 9  Reliable transfer over UDP: add reliability at application layer  application-specific error recovery!  UDP segment format source port # dest port # 32 bits Application data (message) length checksum Length, in bytes of UDP segment, including header UDP (2)

10. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 10 TCP (1) 

11. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 11  Full duplex data:  bi-directional data flow in same connection  Connection-oriented:  handshaking (exchange of control msgs) init’s sender, receiver state before data exchange  Flow controlled:  sender will not overwhelm receiver  Point-to-point:  one sender, one receiver  Reliable, in-order byte steam:  no “message boundaries”  Pipelined:  TCP congestion and flow control set window size  send & receive buffers Process writes data TCP send buffer Socket Process reads data TCP receive buffer Socket segment TCP (1)

12. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 12 TCP (2)  A TCP entity accepts user data stream and breaks them to segments.  For obtaining TCP services, both sender and receiver creating end points called sockets.  Socket No.= IP address of machine+ 16 bits local add. (Port No.)  Port No.s bellow 1024 are called Well-known ports  TCP does not support multicasting or broadcasting

13. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 13 TCP (3)  Some standard ports : PortProtocol Use 21 FTP File transfer 23 Telnet Remote login 25 SMTP E-mail 69 TFTP Trivial File Transfer Protocol 79 FingerLookup info about a user 80 HTTP World Wide Web 110 POP-3 Remote e-mail access 119 NNTP USENET news

14. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 14 source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number Receive window Urg data pnter checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept Internet checksum (as in UDP) window scaling factor, Time-stamping, maximum segment length,… RFCs: 854, 1323 [4Bytes] seq # is byte-stream number of first data byte in segment TCP (4)

15. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 15 TCP (5)  Connection Establishment  Tree-way handshaking  Use Sliding Window Protocol  Go-Back-N and Selective Repeat Host B Host A