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TCP Transport Layer Overview

Learn about the TCP transport layer, including its features, protocols, and scenarios. This overview covers topics such as flow control, congestion control, retransmission scenarios, and TCP connection management.

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TCP Transport Layer Overview

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  1. 09-Transport Layer: TCP Transport Layer

  2. ANNOUNCEMENTS Please read chapter 3! Exam 1 next Thursday Please read chapter 3! Please read chapter 3! Lab 2 due at 6:00 PM today Candidate seminar 10th hour, Olin 267 Transport Layer

  3. GBN and SR • To the applets… Transport Layer

  4. full duplex data: bi-directional data flow in same connection connection-oriented: handshaking (exchange of control msgs) flow control: sender will not overwhelm receiver congestion control point-to-point: one sender, one receiver reliable, in-order byte stream: no “message boundaries” pipelined: TCP congestion and flow control set window size send & receive buffers TCP: OverviewRFCs: 793, 1122, 1323, 2018, 2581 Transport Layer

  5. 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) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data up now (generally not used) # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) Transport Layer

  6. Seq. #’s: byte stream “number” of first byte in segment’s data ACKs: seq # of next byte expected from other side cumulative ACK Q: how receiver handles out-of-order segments A: TCP spec doesn’t say, - up to implementor time TCP seq. #’s and ACKs 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 simple telnet scenario Transport Layer

  7. Host A Host B Seq=92, 8 bytes data ACK=100 Seq=92 timeout timeout X loss Seq=92, 8 bytes data ACK=100 time time lost ACK scenario TCP: retransmission scenarios Host A Host B Seq=92, 8 bytes data Seq=100, 20 bytes data ACK=100 ACK=120 Seq=92, 8 bytes data Sendbase = 100 SendBase = 120 ACK=120 Seq=92 timeout SendBase = 100 SendBase = 120 premature timeout Transport Layer

  8. Host A Host B Seq=92, 8 bytes data ACK=100 Seq=100, 20 bytes data timeout X loss ACK=120 time Cumulative ACK scenario TCP retransmission scenarios (more) SendBase = 120 Transport Layer

  9. TCP ACK generation[RFC 1122, RFC 2581] TCP Receiver action Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Immediately send single cumulative ACK, ACKing both in-order segments Immediately send duplicate ACK, indicating seq. # of next expected byte Immediate send ACK, provided that segment starts at lower end of gap Event at Receiver Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. # . Gap detected Arrival of segment that partially or completely fills gap Transport Layer

  10. 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 Transport Layer

  11. (suppose TCP receiver discards out-of-order segments) spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] rcvr advertises spare room by including value of RcvWindow in segments sender limits unACKed data to RcvWindow guarantees receive buffer doesn’t overflow TCP Flow control: how it works Transport Layer

  12. Recall:TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) client: connection initiator Socket clientSocket = new Socket("hostname","port number"); server: contacted by client Socket connectionSocket = welcomeSocket.accept(); 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 Transport Layer

  13. Closing a connection: client closes socket:clientSocket.close(); Step 1:client end system sends TCP FIN control segment to server Step 2:server receives FIN, replies with ACK. Closes connection, sends FIN. client server close FIN ACK close FIN ACK timed wait closed TCP Connection Management (cont.) Transport Layer

  14. Step 3:client receives FIN, replies with ACK. Enters “timed wait” - will respond with ACK to received FINs Step 4:server, receives ACK. Connection closed. Note:with small modification, can handle simultaneous FINs. TCP Connection Management (cont.) client server closing FIN ACK closing FIN ACK timed wait closed closed Transport Layer

  15. RST and FIN • FIN is the bit set when a client is ready to close the connection. • RST is used by the server to indicate that the port and IP address do not match any sockets on the server. Transport Layer

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