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1、15-441 Computer NetworkingLecture 18 More TCP & Congestion Control1Good Ideas So Far Flow controlStop & waitParallel stop & waitSliding window (e.g., advertised windows) Loss recoveryTimeoutsAcknowledgement-driven recovery (selective repeat or cumulative acknowledgement) Congestion controlAIMD fairn
2、ess and efficiency How does TCP actually implement these?2Lecture 18: TCP DetailsOutline THE SPOOKY PARTS of TCPIf it doesnt scare you now it will on the Final! TCP connection setup/data transferThe Candy-exchange Protocol (TCP) TCP reliability How to recover your DEAD packets TCP congestion avoidan
3、ceAvoiding the death-traps of overloaded routers3Lecture 18: TCP DetailsSequence Number Space Each byte in byte stream is numbered.32 bit valueWraps aroundInitial values selected at start up time TCP breaks up the byte stream into packets.Packet size is limited to the Maximum Segment Size Each packe
4、t has a sequence number.Indicates where it fits in the byte streampacket 8packet 9packet 10134501495016050175504Lecture 18: TCP DetailsEstablishing Connection: Three-Way handshake Each side notifies other of starting sequence number it will use for sendingWhy not simply chose 0?Must avoid overlap wi
5、th earlier incarnationSecurity issues Each side acknowledges others sequence numberSYN-ACK: Acknowledge sequence number + 1 Can combine second SYN with first ACKSYN: SeqCACK: SeqC+1 SYN: SeqSACK: SeqS+1ClientServer5Lecture 18: TCP DetailsTCP Connection Setup ExampleClient SYNSeqC: Seq. #4019802004,
6、window 65535, max. seg. 1260 Server SYN-ACK+SYNReceive: #4019802005 (= SeqC+1)SeqS: Seq. #3428951569, window 5840, max. seg. 1460 Client SYN-ACKReceive: #3428951570 (= SeqS+1)09:23:33.042318 IP 128.2.222.198.3123 192.216.219.96.80: S 4019802004:4019802004(0) win 65535 (DF)09:23:33.118329 IP 192.216.
7、219.96.80 128.2.222.198.3123: S 3428951569:3428951569(0) ack 4019802005 win 5840 (DF)09:23:33.118405 IP 128.2.222.198.3123 192.216.219.96.80: . ack 3428951570 win 65535 (DF)6Lecture 18: TCP DetailsTCP State Diagram: Connection SetupCLOSEDSYN SENTSYN RCVDESTABLISTENactive OPEN create TCB Snd SYN crea
8、te TCBpassive OPENdelete TCBCLOSEdelete TCBCLOSEsnd SYNSEND snd SYN ACKrcv SYNSend FINCLOSErcv ACK of SYN Snd ACKRcv SYN, ACKrcv SYN snd ACKClientServer7Lecture 18: TCP DetailsTearing Down Connection Either side can initiate tear downSend FIN signal“Im not going to send any more data” Other side can
9、 continue sending dataHalf open connectionMust continue to acknowledge Acknowledging FINAcknowledge last sequence number + 1ABFIN, SeqAACK, SeqA+1ACKDataACK, SeqB+1FIN, SeqB8Lecture 18: TCP DetailsTCP Connection Teardown ExampleSessionEcho client on 128.2.222.198, server on 128.2.210.194 Client FINS
10、eqC: 1489294581 Server ACK + FINAck: 1489294582 (= SeqC+1)SeqS: 1909787689 Client ACKAck: 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 1909787689:1909
11、787689(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)9Lecture 18: TCP DetailsState Diagram: Connection Tear-downCLOSINGCLOSE WAITFIN WAIT-1ESTABTIME WAITsnd FINCLOSEsend FINCLOSErcv ACK of FINLAST-ACKCLOSEDFIN WAIT-2snd ACKrcv
12、 FINdelete TCBTimeout=2mslsend FINCLOSE send ACKrcv FINsnd ACKrcv FINrcv ACK of FINsnd ACKrcv FIN+ACKACKActive ClosePassive Close10Lecture 18: TCP DetailsOutline TCP connection setup/data transfer TCP reliability11Lecture 18: TCP DetailsReliability Challenges Congestion related losses Variable packe
13、t delaysWhat should the timeout be? Reordering of packetsHow to tell the difference between a delayed packet and a lost one?12Lecture 18: TCP DetailsTCP = Go-Back-N Variant Sliding window with cumulative acksReceiver can only return a single “ack” sequence number to the sender.Acknowledges all bytes
14、 with a lower sequence numberStarting point for retransmissionDuplicate acks sent when out-of-order packet received But: sender only retransmits a single packet.Reason?Only one that it knows is lostNetwork is congested shouldnt overload it Error control is based on byte sequences, not packets.Retran
15、smitted packet can be different from the original lost packet Why?13Lecture 18: TCP DetailsRound-trip Time Estimation Wait at least one RTT before retransmitting Importance of accurate RTT estimators:Low RTT estimateunneeded retransmissionsHigh RTT estimatepoor throughput RTT estimator must adapt to
16、 change in RTTBut not too fast, or too slow! Spurious timeouts“Conservation of packets” principle never more than a window worth of packets in flight14Lecture 18: TCP DetailsOriginal TCP Round-trip Estimator Round trip times exponentially averaged:New RTT = a (old RTT) + (1 - a) (new sample)Recommended value for a: 0.8 - 0.90.875 for mos
