15-441 Computer NetworkingGood Ideas So Far…OutlineSequence Number SpaceEstablishing Connection: Three-Way handshakeTCP Connection Setup ExampleTCP State Diagram: Connection SetupTearing Down ConnectionTCP Connection Teardown ExampleState Diagram: Connection Tear-downSlide 11Reliability ChallengesTCP = Go-Back-N VariantRound-trip Time EstimationOriginal TCP Round-trip EstimatorRTT Sample AmbiguityJacobson’s Retransmission TimeoutTimestamp ExtensionTimer GranularityFast RetransmitSlide 21TCP (Reno variant)SACKSACKPerformance IssuesSlide 26Additive Increase/DecreaseMuliplicative Increase/DecreaseWhat is the Right Choice?TCP Congestion ControlAIMDImplementation IssuePacket ConservationTCP Packet PacingCongestion AvoidanceCongestion Avoidance Sequence PlotCongestion Avoidance BehaviorHow to Change WindowFast RecoveryFast RecoveryImportant Lessons15-441 Computer NetworkingLecture 17 – More TCP & Congestion ControlCopyright ©, 2007-10 Carnegie Mellon University2Good Ideas So Far…•Flow control•Stop & wait•Parallel stop & wait•Sliding window (e.g., advertised windows)•Loss recovery•Timeouts•Acknowledgement-driven recovery (selective repeat or cumulative acknowledgement)•Congestion control•AIMD fairness and efficiency•How does TCP actually implement these?3Outline•TCP connection setup/data transfer•TCP reliability•TCP congestion avoidance4Sequence Number Space•Each byte in byte stream is numbered.•32 bit value•Wraps around•Initial values selected at start up time•TCP breaks up the byte stream into packets.•Packet size is limited to the Maximum Segment Size•Each packet has a sequence number.•Indicates where it fits in the byte streampacket 8 packet 9 packet 1013450 14950 16050 175505Establishing Connection:Three-Way handshake•Each side notifies other of starting sequence number it will use for sending•Why not simply chose 0?•Must avoid overlap with earlier incarnation•Security issues•Each side acknowledges other’s sequence number•SYN-ACK: Acknowledge sequence number + 1•Can combine second SYN with first ACKSYN: SeqCACK: SeqC+1SYN: SeqSACK: SeqS+1Client Server6TCP Connection Setup Example•Client SYN•SeqC: Seq. #4019802004, window 65535, max. seg. 1260•Server SYN-ACK+SYN•Receive: #4019802005 (= SeqC+1)•SeqS: Seq. #3428951569, window 5840, max. seg. 1460•Client SYN-ACK•Receive: #3428951570 (= SeqS+1)09:23:33.042318 IP 128.2.222.198.3123 > 192.216.219.96.80: S 4019802004:4019802004(0) win 65535 <mss 1260,nop,nop,sackOK> (DF)09:23:33.118329 IP 192.216.219.96.80 > 128.2.222.198.3123: S 3428951569:3428951569(0) ack 4019802005 win 5840 <mss 1460,nop,nop,sackOK> (DF)09:23:33.118405 IP 128.2.222.198.3123 > 192.216.219.96.80: . ack 3428951570 win 65535 (DF)7TCP State Diagram: Connection SetupCLOSEDSYNSENTSYNRCVDESTABLISTENactive OPENcreate TCBSnd SYN create TCBpassive OPENdelete TCBCLOSEdelete TCBCLOSEsnd SYNSENDsnd SYN ACKrcv SYNSend FINCLOSErcv ACK of SYNSnd ACKRcv SYN, ACKrcv SYNsnd ACKClientServer8Tearing Down Connection•Either side can initiate tear down•Send FIN signal•“I’m not going to send any more data”•Other side can continue sending data•Half open connection•Must continue to acknowledge•Acknowledging FIN•Acknowledge last sequence number + 1A BFIN, SeqAACK, SeqA+1ACKDataACK, SeqB+1FIN, SeqB9TCP Connection Teardown Example•Session•Echo client on 128.2.222.198, server on 128.2.210.194•Client FIN•SeqC: 1489294581•Server ACK + FIN•Ack: 1489294582 (= SeqC+1)•SeqS: 1909787689•Client ACK•Ack: 1909787690 (= SeqS+1)09:54:17.585396 IP 128.2.222.198.4474 > 128.2.210.194.6616: F 1489294581:1489294581(0) ack 1909787689 win 65434 (DF)09:54:17.585732 IP 128.2.210.194.6616 > 128.2.222.198.4474: F 1909787689:1909787689(0) ack 1489294582 win 5840 (DF)09:54:17.585764 IP 128.2.222.198.4474 > 128.2.210.194.6616: . ack 1909787690 win 65434 (DF)10State Diagram: Connection Tear-downCLOSINGCLOSEWAITFINWAIT-1ESTABTIME WAITsnd FINCLOSEsend FINCLOSErcv ACK of FINLAST-ACKCLOSEDFIN WAIT-2snd ACKrcv FINdelete TCBTimeout=2mslsend FINCLOSEsend ACKrcv FINsnd ACKrcv FINrcv ACK of FINsnd ACKrcv FIN+ACKACKActive ClosePassive Close11Outline•TCP connection setup/data transfer•TCP reliability•TCP congestion avoidance12Reliability Challenges•Congestion related losses•Variable packet delays•What should the timeout be?•Reordering of packets•How to tell the difference between a delayed packet and a lost one?13TCP = Go-Back-N Variant•Sliding window with cumulative acks•Receiver can only return a single “ack” sequence number to the sender.•Acknowledges all bytes with a lower sequence number•Starting point for retransmission•Duplicate acks sent when out-of-order packet received •But: sender only retransmits a single packet.•Reason???•Only one that it knows is lost•Network is congested shouldn’t overload it•Error control is based on byte sequences, not packets.•Retransmitted packet can be different from the original lost packet – Why?14Round-trip Time Estimation•Wait at least one RTT before retransmitting•Importance of accurate RTT estimators:•Low RTT estimate•unneeded retransmissions•High RTT estimate•poor throughput•RTT estimator must adapt to change in RTT•But not too fast, or too slow!•Spurious timeouts•“Conservation of packets” principle – never more than a window worth of packets in flight15Original TCP Round-trip Estimator•Round trip times exponentially averaged:•New RTT = (old RTT) + (1 - ) (new sample)•Recommended value for : 0.8 - 0.9•0.875 for most TCP’s•Retransmit timer set to (b * RTT), where b = 2•Every time timer expires, RTO exponentially backed-off•Not good at preventing spurious timeouts•Why?16RTT Sample Ambiguity•Karn’s RTT Estimator•If a segment has been retransmitted:•Don’t count RTT sample on ACKs for this segment•Keep backed off time-out for next packet•Reuse RTT estimate only after one successful transmissionA BACKSampleRTTOriginal transmissionretransmissionRTOA BOriginal transmissionretransmissionSampleRTTACKRTOX17Jacobson’s Retransmission Timeout•Key observation:•At high loads, round trip variance is high•Solution:•Base RTO on RTT and standard deviation•RTO = RTT + 4 * rttvar•new_rttvar = * dev + (1- ) old_rttvar•Dev = linear deviation •Inappropriately named – actually smoothed linear deviation18Timestamp Extension•Used to improve timeout
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