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Transport Protocols

Transport Protocols. Lesson 12 NETS2150/2850 http://www.ug.cs.usyd.edu.au/~nets2150/. School of Information Technologies. Lesson Outcome. understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control

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Transport Protocols

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  1. Transport Protocols Lesson 12 NETS2150/2850 http://www.ug.cs.usyd.edu.au/~nets2150/ School of Information Technologies

  2. Lesson Outcome • 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

  3. Position of transport layer

  4. Transport layer duties

  5. provide logical communication between app processes running on different hosts transport protocols run in end systems(not in ISs) 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. Transport vs. Network Layer • network layer: logical communication between hosts • transport layer: logical communication between processes • relies on, enhances, network layer services

  7. Types of data deliveries

  8. SCTP TCP & UDP • Transmission Control Protocol • Connection oriented • RFC 793 • User Datagram Protocol (UDP) • Connectionless • RFC 768 • Stream Control Transmission Protocol (SCTP) – for VoIP Some protocols in the TCP/IP protocol suite

  9. IP addresses & port numbers Port Numbers IP addresses versus port numbers

  10. Well-known ports

  11. reliable, in-order delivery (TCP) congestion control flow control connection setup unreliable, unordered delivery: UDP extension of “best-effort” IP 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

  12. Issues in a Simple Transport Protocol • Multiplexing and Addressing • Flow Control • Connection establishment and termination

  13. Socket Address Process-to-process delivery needs two IDs at each end: - IP address - Port number

  14. delivering received segments to correct socket/process gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) Multiplexing at send host: Demultiplexing at rcv host: Multiplexing/Demultiplexing

  15. host receives IP packets each packet has source IP address, destination IP address each packet carries 1 transport-layer segment each segment has source, destination port number host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields application data (message) TCP/UDP segment format How Demultiplexing Works

  16. Connectionless demultiplexing • 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 packets with different source IP addresses and/or source port numbers directed to same socket

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

  18. 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 E.g.: Web servers have different sockets for each connecting client Connection-oriented demux

  19. P2 P1 P1 P3 P4 SP: 80 SP: 5775 SP: 80 DP: 9157 DP: 80 DP: 5775 Connection-oriented demux (cont) Eg.: TCP SP: 9157 client IP: A DP: 80 Client IP:B server IP: C

  20. receive side of TCP connection has a receive buffer: speed-matching service: matching the send rate to the receiving app’s drain rate flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast TCP Flow Control • app process may be slow at reading from buffer J. Kurose and Ross, Computer Networking

  21. Connection Establishment and Termination • Allow each end to know the other exists • Negotiation of optional parameters by mutual agreement (3-way handshake) • Triggers allocation of transport entity resources • Only in TCP

  22. “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? fast: 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]

  23. often used for streaming multimedia apps loss tolerant other UDP uses DNS (domain name resolution) SNMP (network management) Need to perform application-specific error recovery! UDP (2) 32 bits source port # dest port # Length, in octets, including header checksum length Application data (message) UDP segment format

  24. full duplex data: bi-directional data flow in same connection Specifies maximum segment size (MSS) 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 send & receive buffers TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581

  25. 32 bits source port # dest port # sequence number acknowledgement number head len not used Receive window U A P R S F checksum Urg data pnter Options (variable length) application data (variable length) TCP segment structure URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) J. Kurose and Ross, Computer Networking

  26. Seq. #’s: byte stream “number” of first octet in segment’s data ACKs: seq # of next octet expected from other side cumulative ACK Host B Host A User types ‘C’ Seq=42, ACK=79, data = ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ Seq=79, ACK=43, data = ‘C’ host ACKs receipt of echoed ‘C’ Seq=43, ACK=80 time simple Telnet scenario TCP seq. #’s and ACKs

  27. Recall:TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) Three way handshake: Step 1:client host sends TCP SYN segment to server specifies initial seq # no data Step 2:server host receives SYN, replies with SYNACK segment server allocates buffers specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data TCP Connection Management

  28. Closing a connection: Step 1:client end system sends TCP FIN control segment to server Step 2:server receives FIN, replies with ACK. Closes connection, sends FIN. Step 3:client receives FIN, replies with ACK. Step 4:server, receives ACK. Connection closed. client server close FIN ACK close FIN ACK timed wait closed TCP Connection Management (cont.)

  29. TCP Connection Management (cont.) Could be combined (3-way) Setup Termination

  30. Congestion: informally: “too many sources sending too much data too fast for network to handle” different from flow control!! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) a top-10 problem! Principles of Congestion Control

  31. TCP congestion control • There is no explicit feedback from network • Congestion inferred from end-system observed loss, delay • When this happens, TCP reduces its window size • Using CWnd and RcvWnd • More details in NETS3303/3603!

  32. Summary • Transport layer establishes the process-to-process communication • TCP uses reliable, connection oriented approach • Used by FTP, HTTP, Telnet apps • UDP uses unreliable, connectionless approach • Used by DNS, SNMP and streaming apps like Realplayer and MS Windows Media Player • Stallings 20.1, 20.2 and 204.

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