CSC 401 Data and Computer Communications Networks

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1 CSC 401 Data and Computer Communications Networks Transport Layer Pipelined Reliable Data Transfer Protocols: Go-Back-N and Selective Repeat Sec Prof. Lina Battestilli Fall 2017

2 Transport Layer Chapter 3 Outline 3.1 Transport-layer Services 3.2 Multiplexing and Demultiplexing 3.3 Connectionless Transport: UDP 3.4 Principles of Reliable Data Transfer building an rdt protocol pipelined rdt protocols: GBN, SR 3.5 Connection-oriented Transport: TCP 3.6 Principles of Congestion Control 3.7 TCP Congestion Control

3 Performance of rdt3.0 rdt3.0 is correct, but performance not so great e.g.: 1 Gbps link, 15ms one-way prop. delay, 30msec RTT, 8000 bit packet: D trans = L R U sender : utilization fraction of time sender busy sending U sender = 8000 bits = 10 9 = 8 microsecs bits/sec L / R RTT + L / R = Kbps throughput over 1 Gbps link = stop&wait network protocol limits use of physical resources!

4 rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R sender receiver RTT first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R U sender = L / R RTT + L / R = = Utilization: fraction of time the sender is busy sending bits into the channel 11

5 Pipelined protocols pipelining: sender allows multiple, in-flight, yet-to-be-acknowledged packets two generic forms of pipelined protocols: go-back-n, selective repeat

6 Pipelining: increased utilization first packet bit transmitted, t = 0 last bit transmitted, t = L / R sender receiver RTT ACK arrives, send next packet, t = RTT + L / R 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 3-packet pipelining increases utilization by a factor of 3! U sender = 3L / R RTT + L / R = = How fast can you send? Is there a limit? 13

7 Consequences of Pipelining Range of sequence numbers must be increased Sender and receiver may have to buffer packets At minimum the sender must have a buffer of packets that have been transmitted but not yet acknowledged These two approaches differ on how they respond to lost, corrupted &overly-delayed packets Go-back-N Selective Repeat 14

Pipelined protocols: Overview Go-back-N sender can have up to N unacked packets in pipeline receiver only sends cumulative ack doesn t ack packet if there s a gap sender has timer for oldest unacked

8 Pipelined protocols: Overview Go-back-N sender can have up to N unacked packets in pipeline receiver only sends cumulative ack doesn t ack packet if there s a gap sender has timer for oldest unacked packet when timer expires, retransmit all unacked packets Selective Repeat sender can have up to N unack ed packets in pipeline receiver sends individual ack for each packet sender maintains timer for each unacked packet when timer expires, retransmit only that unacked packet Any advantages of cumulative ACKs?

9 Go-Back-N: sender k-bit seq # in pkt header range [0, 2 k 1] window of up to N, consecutive unack ed pkts allowed window slides forward over the sequence number space sliding window protocol base -- sequence number of oldest unacknowledged packet nextseqnum smallest unused sequence number, i.e. next new packet that can be sent ACK(n): ACKs all pkts up to, including seq # n - cumulative ACK timer for oldest transmitted not acknowledged pkt timeout(n): retransmit packet n and ALL higher seq # pkts in window 16

$GBN: extended FSM sender L base=1 nextseqnum=1 rdt_rcv(rcvpkt) && corrupt(rcvpkt) L Event: Data from app received rdt_send(data) if (nextseqnum < base+n) { window is NOT full sndpkt[nextseqnum] =$

10 GBN: extended FSM sender L base=1 nextseqnum=1 rdt_rcv(rcvpkt) && corrupt(rcvpkt) L Event: Data from app received rdt_send(data) if (nextseqnum < base+n) { window is NOT full sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ } else refuse_data(data) window is full Wait rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) base = getacknum(rcvpkt)+1 If (base == nextseqnum) stop_timer else start_timer timeout start_timer udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) udt_send(sndpkt[nextseqnum-1]) Event: GOOD Case, Receipt of a Cumulative ACK Event: Timeout send all the already transmitted but un-acked packets. This is where the name of this protocol comes from. 17

11 GBN: extended FSM receiver default udt_send(sndpkt) L expectedseqnum=1 Wait sndpkt = make_pkt(expectedseqnum,ack,chksum) Event: Out-of-order packet received discards it and resends an ACK for the most recently received packet rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) extract(rcvpkt,data) deliver_data(data) Event: GOOD case, received the packets in order sndpkt = make_pkt(expectedseqnum,ack,chksum) udt_send(sndpkt) expectedseqnum++ Send ACK for correctly-received pkt with highest in-order seq # may generate duplicate ACKs need only remember expectedseqnum Out-of-order pkt received: discard (don t buffer): no receiver buffering! re-ack pkt with highest in-order seq # simple receiver but are there any disadvantages? 18

12 GBN in action sender window (N=4) sender send pkt0 send pkt1 send pkt2 send pkt3 (wait) rcv ack0, send pkt4 rcv ack1, send pkt5 ignore duplicate ACK pkt 2 timeout send pkt2 send pkt3 send pkt4 send pkt5 Xloss receiver receive pkt0, send ack0 receive pkt1, send ack1 receive pkt3, discard, (re)send ack1 receive pkt4, discard, (re)send ack1 receive pkt5, discard, (re)send ack1 rcv pkt2, deliver, send ack2 rcv pkt3, deliver, send ack3 rcv pkt4, deliver, send ack4 rcv pkt5, deliver, send ack5 19

13 Performance of GBN GBN has sequence numbers, cumulative ACKs, checksums, and timeout/retransmit we will see these in TCP as well It can potentially fill the pipeline with packets It could suffer performance problems: If Window size or Bandwidth-Delay product are very large (i.e.many pkts in pipeline) a single pkt error results in lots of retransmissions 20

14 -21 Selective Repeat receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer sender only resends pkts for which ACK not received sender timer for each unacked pkt sender window N consecutive seq # s limits seq #s of sent, unacked pkts

15 Selective Repeat: sender, receiver windows 22

Selective Repeat - psudocode sender data from above: if next available seq # in window, send pkt timeout(n): resend pkt n, restart its timer ACK(n) in [send_base,send_base+n-1]: mark pkt n as

16 Selective Repeat - psudocode sender data from above: if next available seq # in window, send pkt timeout(n): resend pkt n, restart its timer ACK(n) in [send_base,send_base+n-1]: mark pkt n as received if n was the smallest unacked pkt, advance window start at next unacked seq # receiver pkt n in [rcv_base, rcv_base+n-1] send ACK(n) out-of-order: buffer in-order: deliver (also deliver other buffered, in-order pkts), advance window to next not-yetreceived pkt pkt n in [rcv-base-n,rcv-base-1] Send ACK(n):receiver must have not gotten the previous ACK otherwise: ignore [rcv-base-n to rcv-base-1] 23

pkt3, buffer, send ack3 receive pkt4, buffer, send ack4 receive pkt5, buffer, send ack5 rcv pkt2; deliver pkt2, pkt3, pkt4, pkt5; send ack2

17 Selective Repeat in action sender window (N=4) sender send pkt0 send pkt1 send pkt2 send pkt3 (wait) rcv ack0, send pkt4 rcv ack1, send pkt5 record ack3 arrived pkt 2 timeout send pkt2 record ack4 arrived record ack5 arrived Xloss receiver receive pkt0, send ack0 receive pkt1, send ack1 receive pkt3, buffer, send ack3 receive pkt4, buffer, send ack4 receive pkt5, buffer, send ack5 rcv pkt2; deliver pkt2, pkt3, pkt4, pkt5; send ack2 t/interactiveanimations/selective-repeat-protocol/index.html Q: what happens when ack2 arrives? 24

18 Selective Repeat: dilemma example: seq # s space: 0, 1, 2, 3 Window Size=3 sender window (after receipt) pkt0 pkt1 pkt2 pkt3 (a) no problem pkt0 X receiver window (after receipt) will accept packet with seq number 0 receiver sees no difference in two scenarios! duplicate data accepted as new in (b) Q: what relationship between seq # size and window size to avoid problem in (b)? receiver can t see sender side. receiver behavior identical in both cases! something s (very) wrong! pkt0 pkt1 pkt2 X X timeout retransmit pkt0 X pkt0 (b) oops! will accept packet with seq number 0 25

19 Reliable Transfer Mechanisms Checksums Timers Sequence Numbers ACKs, Negative ACKs Window/Pipelining Note: re-ordered packets addressed by packets can not live in the network for longer than some fixed max amount of time 26

20 References Some of the slides are identical or derived from 1. Slides for the 7 th edition of the book Kurose & Ross, Computer Networking: A Top-Down Approach, 2. Computer Networking, Nick McKeown and Philip Levis, 2014 Stanford University

Chapter 3 Transport Layer

Chapter 3 Transport Layer A note on the use of these Powerpoint slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations;