Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 3815-441 Computer NetworkingLecture 18 – More TCP & Congestion Control2Lecture 18: TCP DetailsGood 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?3Lecture 18: TCP DetailsOutline•The devilish details of TCP•TCP connection setup and data transfer•TCP reliability•Be nice to your data•TCP congestion avoidance•Be nice to your routers34Lecture 18: TCP DetailsSequence 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 175505Lecture 18: TCP DetailsEstablishing 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 Server6Lecture 18: TCP DetailsTCP 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)7Lecture 18: TCP DetailsTCP State Diagram: Connection SetupCLOSEDSYNSENTSYNRCVDESTABLISTENactive OPENcreate TCBSnd SYN create TCBpassive OPENdelete TCBCLOSEdelete TCBCLOSEsnd SYNSENDsnd SYN ACKrcv SYNSend FINCLOSErcv ACK of SYNSnd ACKRcv SYN, ACKrcv SYNsnd ACKClientServer8Lecture 18: TCP DetailsTearing 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, SeqB9Lecture 18: TCP DetailsTCP 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)10Lecture 18: TCP DetailsState 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 Close11Lecture 18: TCP DetailsOutline•TCP connection setup/data transfer•TCP reliability12Lecture 18: TCP DetailsReliability 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?13Lecture 18: TCP DetailsTCP = 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?14Lecture 18: TCP Details•How to set timeout?•Wait until sender knows it should have seen an ACK•How long should this be?1415Lecture 18: TCP DetailsRound-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 flight16Lecture 18: TCP DetailsOriginal TCP Round-trip Estimator•Round trip times exponentially averaged:•New RTT = α (old RTT) + (1 - α) (new sample)•Recommended value for α: 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?17Lecture 18: TCP DetailsRTT 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 transmissionretransmissionSampleRTTACKRTOX18Lecture 18: TCP DetailsJacobson’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 = β * dev + (1- β) old_rttvar•Dev = linear deviation •Inappropriately named – actually smoothed linear deviation19Lecture 18: TCP DetailsTimestamp Extension•Used to improve timeout mechanism by more accurate
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