-% % ($) % % % * % + & ' ! $ % $ $. / 0$! /1 2! /3 = >? A = ! " #!! $ %! $ $! $ % " #, * % " # % $ " $ 4 5$6 /778 $6 4 5

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1 " # & ' ' () + " #, " # " 333 for call 0from above && notcorrupt(rcvpkt) && isack(rcvpkt,1) stop_timer timeout && ( corrupt(rcvpkt) isack(rcvpkt,0) ) rdt_send(data) sndpkt make_pkt(0, data, checksum) for ACK1 rdt_send(data) for ACK0 for call 1 from above sndpkt make_pkt(1, data, checksum) && ( corrupt(rcvpkt) isack(rcvpkt,1) ) timeout && notcorrupt(rcvpkt) && isack(rcvpkt,0) stop_timer / 0 /1 2 /3 A 6 / >? first packet bit transmitted, t 0 last packet bit transmitted, t L / R sender RTT ACK arrives, send next packet, t RTT + L / R receiver first packet bit arrives last packet bit arrives, send ACK : ; /3 2< 36 / A >?

2 (2) 22 2 " 6 : first packet bit transmitted, t 0 last bit transmitted, t L / R RTT ACK arrives, send next packet, t RTT + L / R sender receiver first packet bit arrives last packet bit arrives, send ACK last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK A >B C : " # (), " # 2 ( ) 2 " # EF base1 nextseqnum1 && corrupt(rcvpkt) rdt_send(data) if (nextseqnum < base+n) { sndpkt[nextseqnum] make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base nextseqnum) nextseqnum++ else refuse_data(data) && notcorrupt(rcvpkt) base getacknum(rcvpkt)+1 If (base nextseqnum) stop_timer else timeout udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) udt_send(sndpkt[nextseqnum1]) EF default expectedseqnum1 sndpkt make_pkt(0,ack,chksum) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) extract(rcvpkt,data) deliver_data(data) sndpkt make_pkt(expectedseqnum,ack,chksum) expectedseqnum " # expectedseqnum 22, 2 " #

3 @ 2 " #, " # " # GA2 /H " # G A2/H G22/H I J 22 2/ K 2/ A2/ L " #, / > :4 34 & " # : :' 30 3

4 //>> /> >/ >1/ / 2 > F M B 1 2 N? 31 socket door 22 2 ( ) : O application writes data TCP send buffer segment application reads data TCP receive buffer 2 F : 2, socket door # P EC C M > # J # 33 " #, " #, ( ), " # & 22, 2 Q, Q, K K 3 Seq42, ACK7, data C Seq7, ACK43, data C Seq43, ACK80 Q, SampleRTT ( ) + SampleRTT EstimatedRTT (1 α)estimatedrtt + αsamplertt L α />

5 L RTT (milliseconds) RTT: gaiacsumassedu to fantasiaeurecomfr EstimtedRTT ( ) EstimatedRTT evrtt (1β)evRTT + β SampleRTTEstimatedRTT time (seconnds) (typically, β 02) TimeoutInterval EstimatedRTT + 4evRTT SampleRTT Estimated RTT / 2 > F M B 1 2 N? C " # " # 2 TimeOutInterval C 361 NextSeqNum InitialSeqNum SendBase InitialSeqNum loop (forever) { switch(event) event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) pass segment to IP NextSeqNum NextSeqNum + length(data) event: timer timeout retransmit notyetacknowledged segment with smallest sequence number event: ACK received, with ACK field value of y if (y > SendBase) { SendBase y if (there are currently notyetacknowledged segments) / end of loop forever / R 2/, L R 2/ 4?/S 4??A S < 362

6 K K 3 K K 3 K K 3 Seq2, 8 bytes data 4 / Seq2, 8 bytes data X ACK100 Seq2, 8 bytes data ACK100 / 4 / 4 /> 4 /> "48> "48> Seq100, 20 bytes data ACK100 ACK120 Seq2, 8 bytes data ACK /> Seq2, 8 bytes data Seq100, 20 bytes data X ACK120 ACK G@E >1/H Event at Receiver Arrival of inorder segment with expected seq # All data up to expected seq # already ACKed Arrival of inorder segment with expected seq # One other segment has ACK pending Arrival of outoforder segment higherthanexpect seq # Gap detected Arrival of segment that partially or completely fills gap TCP Receiver action elayed ACK up to 00ms for next segment If no next segment, send ACK Immediately send single cumulative ACK, ACKing both inorder segments Immediately send duplicate ACK, indicating seq # of next expected byte Immediate send ACK, update next Expected byte 2 M 22 C C E event: ACK received, with ACK field value of y if (y > SendBase) { SendBase y if (there are currently notyetacknowledged segments) else { increment count of dup ACKs received for y if (count of dup ACKs received for y 3) { resend segment with sequence number y 367 M C F TimeoutInterval " M TimeoutInterval 368 6

7 Left Edge of 1st Block Right Edge of 1st Block / / Left Edge of nth Block Right Edge of nth Block / 2 > F M B 1 2 N? E,, 2, E 22 RcvWindow RcvBuffer[LastByteRcvd RcvWindow RcvWindow,

Chapter 3 Transport Layer

Chapter 3 Transport Layer Reti degli Elaboratori Canale AL Prof.ssa Chiara Petrioli a.a. 2013/2014 We thank for the support material Prof. Kurose-Ross All material copyright 1996-2012 J.F Kurose and K.W.