Chap3.4 Transport Layer

Chap3.4 Transport Layer - 3/16/10 Principles of...

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Unformatted text preview: 3/16/10 Principles of Reliable data transfer •  important in app., transport, link layers •  top ­10 list of important networking topics! Transport Layer Part 2 Many slides in this document adapted from materials for Kurose and Ross, Computer Networking, 5th edition. copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved •  characterisDcs of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport Layer 3 ­2 Principles of Reliable data transfer •  important in app., transport, link layers •  top ­10 list of important networking topics! Principles of Reliable data transfer •  important in app., transport, link layers •  top ­10 list of important networking topics! •  characterisDcs of unreliable channel will determine complexity of reliable data transfer protocol (rdt) •  characterisDcs of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport Layer 3 ­3 Transport Layer 3 ­4 Reliable data transfer: geLng started rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer deliver_data(): called by rdt to deliver data to upper Reliable data transfer: geLng started We’ll: •  incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) •  consider only unidirecDonal data transfer –  but control info will flow on both direcDons! send side receive side •  use finite state machines (FSM) to specify sender, receiver event causing state transiDon acDons taken on state transiDon event acDons state: when in this “state” next state uniquely determined by next event udt_send(): called by rdt, to transfer packet over unreliable channel to receiver Transport Layer rdt_rcv(): called when packet arrives on rcv ­side of channel 3 ­5 state 1 state 2 Transport Layer 3 ­6 3/16/10 Rdt1.0: reliable transfer over a reliable channel •  underlying channel perfectly reliable –  no bit errors –  no loss of packets Rdt2.0: channel with bit errors •  underlying channel may flip bits in packet –  checksum to detect bit errors •  separate FSMs for sender, receiver: –  sender sends data into underlying channel –  receiver read data from underlying channel •  the quesDon: how to recover from errors: –  acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK –  nega6ve acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors –  sender retransmits pkt on receipt of NAK rdt_rcv(packet) extract (packet,data) deliver_data(data) Wait for call from above rdt_send(data) packet = make_pkt (data) udt_send(packet) Wait for call from below •  new mechanisms in rdt2.0 (beyond rdt1.0): –  error detecDon –  receiver feedback: control msgs (ACK,NAK) rcvr ­>sender sender receiver Transport Layer 3 ­7 Transport Layer 3 ­8 rdt2.0: FSM specificaDon rdt_send(data) sndpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for Wait for call ACK or from above udt_send(sndpkt) NAK rdt2.0: operaDon with no errors receiver rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for Wait for call from ACK or udt_send(sndpkt) above NAK rdt_rcv(rcvpkt) && isACK(rcvpkt) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) Wait for call from below rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) rdt_rcv(rcvpkt) && isACK(rcvpkt) Λ Wait for call from below Λ sender rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer 3 ­9 Transport Layer 3 ­10 rdt2.0: error scenario rdt_send(data) snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for Wait for call from ACK or udt_send(sndpkt) above NAK rdt_rcv(rcvpkt) && isACK(rcvpkt) Λ rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) Wait for call from below rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Transport Layer 3 ­11 rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted? •  sender doesn’t know what happened at receiver! •  can’t just retransmit: possible duplicate Handling duplicates: •  sender retransmits current pkt if ACK/NAK garbled •  sender adds sequence number to each pkt •  receiver discards (doesn’t deliver up) duplicate pkt stop and wait Sender sends one packet, then waits for receiver response Transport Layer 3 ­12 3/16/10 rdt2.1: sender, handles garbled ACK/NAKs rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && Wait for call 0 from above Wait for ACK or NAK 0 rdt2.1: receiver, handles garbled ACK/NAKs rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq0(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) Wait for 0 from below Wait for 1 from below rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) ( corrupt(rcvpkt) || isNAK(rcvpkt) ) udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) Λ rdt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) Λ Wait for ACK or NAK 1 rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) udt_send(sndpkt) Wait for call 1 from above rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_send(data) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) Transport Layer 3 ­13 Transport Layer 3 ­14 rdt2.1: discussion Sender: •  seq # added to pkt •  two seq. #’s (0,1) will suffice. Why? •  must check if received ACK/ NAK corrupted •  twice as many states –  state must “remember” whether “current” pkt has 0 or 1 seq. # rdt2.2: a NAK ­free protocol •  same funcDonality as rdt2.1, using ACKs only •  instead of NAK, receiver sends ACK for last pkt received OK –  receiver must explicitly include seq # of pkt being ACKed Receiver: •  must check if received packet is duplicate –  state indicates whether 0 or 1 is expected pkt seq # •  note: receiver can not know if its last ACK/NAK received OK at sender •  duplicate ACK at sender results in same acDon as NAK: retransmit current pkt Transport Layer 3 ­15 Transport Layer 3 ­16 rdt2.2: sender, receiver fragments rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || Wait for call Wait for ACK isACK(rcvpkt,1) ) 0 from 0 udt_send(sndpkt) above rdt3.0: channels with errors and loss New assumpDon: underlying channel can also lose packets (data or ACKs) –  checksum, seq. #, ACKs, retransmissions will be of help, but not enough Approach: sender waits “reasonable” amount of Dme for ACK •  retransmits if no ACK received in this Dme •  if pkt (or ACK) just delayed (not lost): –  retransmission will be duplicate, but use of seq. #’s already handles this –  receiver must specify seq # of pkt being ACKed •  requires countdown Dmer sender FSM fragment rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt)) udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) Λ Wait for 0 from below receiver FSM fragment rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK1, chksum) Transport udt_send(sndpkt) Layer 3 ­17 Transport Layer 3 ­18 3/16/10 rdt3.0 sender rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) start_timer Wait for call 0from above Wait for ACK0 rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) ) rdt_rcv(rcvpkt) rdt3.0 in acDon Λ timeout udt_send(sndpkt) start_timer rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) stop_timer Λ rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1) stop_timer timeout udt_send(sndpkt) start_timer Wait for ACK1 rdt_send(data) Wait for call 1 from above rdt_rcv(rcvpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,0) ) Λ Λ sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) start_timer Transport Layer 3 ­19 Transport Layer 3 ­20 rdt3.0 in acDon Performance of rdt3.0 •  rdt3.0 works, but performance sDnks •  ex: 1 Gbps link, 15 ms prop. delay, 8000 bit packet: ❍  U sender: uDlizaDon – fracDon of Dme sender busy sending ❍  ❍  1KB pkt every 30 msec  ­> 33kB/sec thruput over 1 Gbps link network protocol limits use of physical resources! Transport Layer 3 ­22 Transport Layer 3 ­21 rdt3.0: stop ­and ­wait operaDon sender first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R Pipelined protocols Pipelining: sender allows mulDple, “in ­flight”, yet ­to ­be ­ acknowledged pkts –  range of sequence numbers must be increased –  buffering at sender and/or receiver receive r RTT ACK arrives, send next packet, t = RTT + L / R first packet bit arrives last packet bit arrives, send ACK •  Two generic forms of pipelined protocols: go ­Back ­N, selec6ve repeat Transport Layer 3 ­23 Transport Layer 3 ­24 3/16/10 Pipelining: increased uDlizaDon sender first packet bit transmitted, t = 0 last bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK receiver Pipelining Protocols Go ­back ­N: overview •  sender: up to N unACKed pkts in pipeline •  receiver: only sends cumulaDve ACKs –  doesn’t ACK pkt if there’s a gap RTT ACK arrives, send next packet, t = RTT + L / R SelecDve Repeat: overview •  sender: up to N unACKed packets in pipeline •  receiver: ACKs individual pkts •  sender: maintains Dmer for each unACKed pkt –  if Dmer expires: retransmit only unACKed packet Increase uDlizaDon by a factor of 3! •  sender: has Dmer for oldest unACKed pkt –  if Dmer expires: retransmit all unACKed packets Transport Layer 3 ­25 Transport Layer 3 ­26 Go ­Back ­N Sender: •  k ­bit seq # in pkt header •  “window” of up to N, consecuDve unACKed pkts allowed Λ GBN: sender extended FSM rdt_send(data) if (nextseqnum < base+N) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ } else refuse_data(data) timeout start_timer udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) … udt_send(sndpkt [nextseqnum-1]) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) base=1 nextseqnum=1 Wait ❒  ACK(n): ACKs all pkts up to, including seq # n  ­ “cumulaDve ACK” ❍  rdt_rcv(rcvpkt) && corrupt(rcvpkt) may receive duplicate ACKs (see receiver) ❒  Dmer for each in ­flight pkt ❒  6meout(n): retransmit pkt n and all higher seq # pkts in window ❒  AnimaDon Transport Layer 3 ­27 base = getacknum(rcvpkt)+1 If (base == nextseqnum) stop_timer else start_timer Layer Transport 3 ­28 GBN: receiver extended FSM default udt_send(sndpkt) rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(expectedseqnum,ACK,chksum) udt_send(sndpkt) expectedseqnum++ GBN in acDon Wait expectedseqnum=1 sndpkt = make_pkt(expectedseqnum,ACK,chksum) Λ ACK ­only: always send ACK for correctly ­received pkt with highest in ­order seq # –  may generate duplicate ACKs –  need only remember expectedseqnum •  out ­of ­order pkt: –  discard (don’t buffer)  ­> no receiver buffering! –  Re ­ACK pkt with highest in ­order seq # Transport Layer 3 ­29 Transport Layer 3 ­30 3/16/10 SelecDve Repeat •  receiver individually acknowledges all correctly received pkts –  buffers pkts, as needed, for eventual in ­order delivery to upper layer SelecDve repeat: sender, receiver windows •  sender only resends pkts for which ACK not received –  sender Dmer for each unACKed pkt •  sender window –  N consecuDve seq #’s –  again limits seq #s of sent, unACKed pkts Transport Layer 3 ­31 Transport Layer 3 ­32 SelecDve repeat sender data from above : •  if next available seq # in window, send pkt SelecDve repeat in acDon receiver pkt n in [rcvbase, rcvbase+N ­1] ❒  send ACK(n) ❒  out ­of ­order: buffer ❒  in ­order: deliver (also deliver Dmeout(n): •  resend pkt n, restart Dmer ACK(n) in [sendbase,sendbase+N]: •  mark pkt n as received •  if n smallest unACKed pkt, advance window base to next unACKed seq # buffered, in ­order pkts), advance window to next not ­ yet ­received pkt pkt n in [rcvbase ­N,rcvbase ­1] ❒  ACK(n) otherwise: ❒  ignore Transport Layer 3 ­33 Transport Layer 3 ­34 Programming Assignment: AlternaDng Bit Protocol SelecDve repeat: dilemma Example: •  seq #’s: 0, 1, 2, 3 •  window size=3 •  receiver sees no difference in two scenarios! •  incorrectly passes duplicate data as new in (a) Q: what relaDonship between seq # size and window size? Transport Layer 3 ­36 ...
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