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TCP

TCP. Jean Walrand U.C. Berkeley www.eecs.berkeley.edu/~wlr. Outline. TCP Service State Diagram Flow Control Congestion Control RTT Estimation Fast Recovery Fast Retransmit. TCP Service. Error-Free Byte Stream. Reliable byte stream. TCP. IP. Unreliable packet delivery.

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TCP

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  1. TCP Jean Walrand U.C. Berkeley www.eecs.berkeley.edu/~wlr

  2. Outline • TCP Service • State Diagram • Flow Control • Congestion Control • RTT Estimation • Fast Recovery • Fast Retransmit

  3. TCP Service • Error-Free Byte Stream Reliable byte stream TCP IP Unreliable packet delivery Actual data path

  4. ports p1 p2 p1 p2 p3 p1 p2 TCP IP A B C TCP Service (c’d) • Multiplexing: Ports [A | B | p1 | p2 | …]

  5. A (3) Data: k + 1 ACK: n + 1 (4) FIN (1) SYN: k (5) FIN.ack (2) SYN: n ACK: k + 1 (7) FIN.ack ACK B (6) FIN • SYN = Synchronize: A tells B that its bytes start with sequence number k + 1 • SYN = Synchronize: B tells A that its bytes start with sequence number n + 1 • ACK = Acknowledgement: B tells A that it agrees that A’s bytes start with k+1 (3) Data: A starts sending bytes with sequence number k + 1 ACK = Acknowledgement: A tells B that it agrees that B’s bytes start with n+1 (4) FIN = Finish: A tells B that it closes the “half-connection” from A to B; (5) B acks (6)-(7) Similar to (4)-(5) State Diagram • Key Steps: …

  6. Timed Wait SYN sent FIN Wait-1 Closed Closed Established FIN Wait-2 (1) A SYN Data + ACK FIN Established Listen FIN.ack Last Ack … FIN FIN.ack SYN + ACK ACK B SYN received Closed Close Wait (1): A waits in case B retransmits FIN and A must ack again State Diagram (c’d) • States:

  7. State Diagram (c’d)

  8. Wrec [ACK | Wrec | …] Flow Control • Objective: Avoid saturating receiver • Mechanism: Receiver advertises window Wrec W  Wrec – Outstanding where Oustanding = Last sent – last Acked

  9. Congestion Control • Objective: Avoid saturating routers • Mechanism: Loss => Reduce W; No loss => Increase W • Details: See book.

  10. W = 1 W = 2 W = 4 W = 8 Congestion Control (c’d) • Slow Start:ACK => W = W + 1

  11. W RTT Congestion Control (c’d) • Slow Start:ACK => W = W + 1

  12. W = 8.125 + 1/8.125  8 + 2/8 W = 8 + 1/8 W  8 + 8/8 = 9 W = 8 W  9 + 9/9 = 10 Congestion Control (c’d) • Congestion Avoidance: ACK => W = W + 1/W

  13. W RTT Congestion Control (c’d) • Congestion Avoidance: ACK => W = W + 1/W

  14. RTT Estimation • Timeout if delay > average + 4 deviations • Average delay and deviation: first order filters: Average A(n) calculated when n-th ACK arrives: A(n) = aA(n – 1) + (1 – a)Delay(n) Deviation D(n) calculated when n-th ACK arrives: D(n) = aD(n – 1) + (1 – a)|Delay(n) – A(n)| Note - Obvious corrections when retransmission: Do not include delays of retransmitted packets Double timeout after retransmission

  15. ACK n ACK n ACK n Fast Retransmission • When 3 duplicated ACKs, retransmit and W = W/2 • Motivation: 3 dup to avoid retransmission after misordering

  16. L + 2K – 1, …, L + 4, L+3, L+2, L +1, L, L - 1 Last Received 3 Dup Ack Fast Recovery • Objective: Keep buffer full after setting W = W/2 • Illustration: Window goes from 2K to K L, L + 2K – 1, …, L + 4 L + 2K, L, L + 2K – 1, …, L + 4 => W = 2K + 1 L + 2K + 1, L + 2K, L, L + 2K – 1, …, L + 5 => W = 2K + 2 . . . . . . . . . L + 3K - 2,…, L + 2K + 1, L + 2K, L => W = 3K - 1 L + 3K - 1,…, L + 2K + 1, L + 2K => W = K

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