Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless : UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented : TCP segment structure reliable data transfer connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport services and protocols logical communication between processes on different hosts protocols run in end systems send side: breaks app messages into segments, passes to layer rcv side: reassembles segments into messages, passes to app layer more than one protocol available to apps Internet: TCP and UDP Transport ayer 3-1 Transport ayer 3-2 Internet -layer protocols Chapter 3 outline reliable, in-order delivery (TCP) congestion control connection setup unreliable, unordered delivery: UDP no-frills extension of best-effort IP services not available: delay guarantees bandwidth guarantees 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless : UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented : TCP segment structure reliable data transfer connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport ayer 3-3 Transport ayer 3-4 Multiplexing/demultiplexing Demultiplexing at rcv host: delivering received segments to correct socket link = socket P3 = process P1 P1 link host 1 host 2 Multiplexing at send host: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing) P2 P4 host 3 link Transport ayer 3-5 How demultiplexing works host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries 1 -layer segment each segment has source, destination port number host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields data (message) TCP/UDP segment format Transport ayer 3-6 1
Connectionless demux (cont) Connection-oriented demux DatagramSocket serversocket = new DatagramSocket(6428); P2 client IP: A SP: 9157 DP: 6428 SP: 6428 DP: 9157 SP provides return address P3 server IP: C SP: 6428 DP: 5775 SP: 5775 DP: 6428 P1 P1 Client IP:B TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number recv host uses all four values to direct segment to appropriate socket server host may support many simultaneous TCP sockets: each socket identified by its own 4-tuple web servers have different sockets for each connecting client non-persistent HTTP will have different socket for each request Transport ayer 3-7 Transport ayer 3-8 Connection-oriented demux (cont) Chapter 3 outline P1 client IP: A SP: 9157 DP: 80 S-IP: A D-IP:C P4 server IP: C P5 P6 P2 P1 P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 DP: 80 S-IP: B D-IP:C Client IP:B 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless : UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented : TCP segment structure reliable data transfer connection management 3.6 Principles of congestion control 3.7 TCP congestion control Transport ayer 3-9 Transport ayer 3-10 UDP: User Datagram Protocol [RFC 768] UDP: more Simple protocol UDP segments may be: lost delivered out of order connectionless: no handshaking between and each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at, small segment header no congestion control: UDP can blast away as fast as desired often used for streaming multimedia apps ength, in bytes of UDP loss tolerant segment, rate sensitive including other UDP uses header DNS SNMP reliable transfer over UDP: add reliability at layer source port # dest port # length 32 bits Application data (message) checksum UDP segment format Transport ayer 3-11 Transport ayer 3-12 2
UDP checksum Goal: detect errors in transmitted segment Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1 s complement sum) of segment contents puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected Internet Checksum Example Note: when adding numbers, a carryout from the most significant bit needs to be added to the result Example: add two 16-bit integers wraparound sum checksum 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 Transport ayer 3-13 Transport ayer 3-14 Chapter 3 outline Principles of Reliable data transfer 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless : UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented : TCP segment structure reliable data transfer connection management 3.6 Principles of congestion control 3.7 TCP congestion control important in app.,, link layers top-10 list of important ing topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport ayer 3-15 Transport ayer 3-16 Principles of Reliable data transfer Principles of Reliable data transfer important in app.,, link layers important in app.,, link layers top-10 list of important ing topics! top-10 list of important ing topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport ayer 3-17 Transport ayer 3-18 3
Reliable data transfer: getting started Reliable data transfer: getting started rdt_send(): called from, (e.g., by app.). Passed data to deliver to upper layer send side udt_send(): called by rdt, to transfer packet over unreliable channel to deliver_data(): called by rdt to deliver data to upper receive side rdt_rcv(): called when packet arrives on rcv-side of channel We ll: incrementally develop, sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer but control info will flow on both directions! use finite state machines (FSM) to specify, event causing state transition actions taken on state transition state: when in this state next state uniquely determined by next event state 1 event actions state 2 Transport ayer 3-19 Transport ayer 3-20 Rdt1.0: reliable transfer over a reliable channel underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for, : sends data into underlying channel read data from underlying channel call from packet = make_pkt(data) udt_send(packet) call from rdt_rcv(packet) extract (packet,data) Rdt2.0: channel with bit errors underlying channel may flip bits in packet checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): explicitly tells that pkt received OK How do humans recover from errors negative acknowledgements (NAKs): explicitly tells during that pkt conversation? had errors retransmits pkt on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): error detection feedback: control msgs (ACK,NAK) rcvr-> Transport ayer 3-21 Transport ayer 3-22 Rdt2.0: channel with bit errors underlying channel may flip bits in packet checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): explicitly tells that pkt received OK negative acknowledgements (NAKs): explicitly tells that pkt had errors retransmits pkt on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): error detection feedback: control msgs (ACK,NAK) rcvr-> rdt2.0: FSM specification sndpkt = make_pkt(data, checksum) call from isack(rcvpkt) ACK or NAK isnak(rcvpkt) corrupt(rcvpkt) udt_send(nak) call from notcorrupt(rcvpkt) udt_send(ack) Transport ayer 3-23 Transport ayer 3-24 4
rdt2.0 has a fatal flaw! rdt2.1:, handles garbled ACK/NAKs What happens if ACK/NAK corrupted? doesn t know what happened at! can t just retransmit: possible duplicate Handling duplicates: retransmits current pkt if ACK/NAK garbled adds sequence number to each pkt discards (doesn t deliver up) duplicate pkt stop and wait Sender sends one packet, then waits for response && isack(rcvpkt) isnak(rcvpkt) ) sndpkt = make_pkt(0, data, checksum) call 0 from ACK or NAK 1 ACK or NAK 0 call 1 from isnak(rcvpkt) ) && isack(rcvpkt) sndpkt = make_pkt(1, data, checksum) Transport ayer 3-25 Transport ayer 3-26 rdt2.1:, handles garbled ACK/NAKs rdt2.1: discussion (corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) not corrupt(rcvpkt) && has_seq1(rcvpkt) sndpkt = make_pkt(ack, chksum) notcorrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ack, chksum) 0 from 1 from notcorrupt(rcvpkt) && has_seq1(rcvpkt) sndpkt = make_pkt(ack, chksum) (corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ack, chksum) Transport ayer 3-27 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. # Receiver: must check if received packet is duplicate state indicates whether 0 or 1 is expected pkt seq # note: can not know if its last ACK/NAK received OK at Transport ayer 3-28 rdt2.2: a NAK-free protocol rdt2.2:, fragments same functionality as rdt2.1, using ACKs only instead of NAK, sends ACK for last pkt received OK must explicitly include seq # of pkt being ACKed duplicate ACK at results in same action as NAK: retransmit current pkt Transport ayer 3-29 (corrupt(rcvpkt) has_seq1(rcvpkt)) sndpkt = make_pkt(0, data, checksum) call 0 from 0 from ACK 0 FSM fragment FSM fragment notcorrupt(rcvpkt) && has_seq1(rcvpkt) sndpkt = make_pkt(ack1, chksum) isack(rcvpkt,1) ) && isack(rcvpkt,0) Transport ayer 3-30 5
rdt3.0: channels with errors and loss New assumption: underlying channel can also lose packets (data or ACKs) checksum, seq. #, ACKs, retransmissions will be of help, but not enough Approach: waits reasonable amount of time for ACK retransmits if no ACK received in this time if pkt (or ACK) just delayed (not lost): retransmission will be duplicate, but use of seq. # s already handles this must specify seq # of pkt being ACKed requires countdown timer rdt3.0 call 0from && isack(rcvpkt,1) stop_timer timeout isack(rcvpkt,0) ) sndpkt = make_pkt(0, data, checksum) Wait for ACK1 Wait for ACK0 call 1 from sndpkt = make_pkt(1, data, checksum) isack(rcvpkt,1) ) timeout && isack(rcvpkt,0) stop_timer Transport ayer 3-31 Transport ayer 3-32 rdt3.0 in action rdt3.0 in action Transport ayer 3-33 Transport ayer 3-34 Performance of rdt3.0 rdt3.0 works, but performance stinks ex: 1 Gbps link, 15 ms prop. delay, 8000 bit packet: R d trans U : utilization fraction of time busy sending U = 8000bits 10 bps 9 / R RTT + / R = 8microseconds.008 30.008 = 0.00027 microsec if RTT=30 msec, 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link protocol limits use of resources! rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 last packet bit transmitted, t = / R RTT ACK arrives, send next packet, t = RTT + / R U = / R RTT + / R =.008 30.008 first packet bit arrives last packet bit arrives, send ACK = 0.00027 microsec Transport ayer 3-35 Transport ayer 3-36 6
Pipelined protocols pipelining: allows multiple, in-flight, yet-tobe-acknowledged pkts range of sequence numbers must be increased buffering at and/or Pipelining: increased utilization first packet bit transmitted, t = 0 last bit transmitted, t = / R RTT ACK arrives, send next packet, t = RTT + / 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 two generic forms of pipelined protocols: go-back-n, selective repeat U = 3 * / R RTT + / R =.024 30.008 Increase utilization by a factor of 3! = 0.0008 microsecon Transport ayer 3-37 Transport ayer 3-38 Pipelined Protocols Go-back-N: big picture: can have up to N unacked packets in pipeline rcvr only sends cumulative acks doesn t ack packet if there s a gap has timer for oldest unacked packet if timer expires, retransmit all unack ed packets Selective Repeat: big pic can have up to N unack ed packets in pipeline rcvr sends individual ack for each packet maintains timer for each unacked packet when timer expires, retransmit only unack ed packet Go-Back-N Sender: k-bit seq # in pkt header window of up to N, consecutive unack ed pkts allowed ACK(n): ACKs all pkts up to, including seq # n - cumulative ACK may receive duplicate ACKs (see ) timer for each in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window Transport ayer 3-39 Transport ayer 3-40 GBN: extended FSM base=1 nextseqnum=1 && corrupt(rcvpkt) if (nextseqnum < base+n) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) nextseqnum++ } else refuse_data(data) Wait 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[nextseqnum-1]) Transport ayer 3-41 GBN: extended FSM default expectedseqnum=1 Wait sndpkt = make_pkt(expectedseqnum,ack,chksum) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) sndpkt = make_pkt(expectedseqnum,ack,chksum) expectedseqnum++ 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 buffering! Re-ACK pkt with highest in-order seq # Transport ayer 3-42 7
GBN in action Selective Repeat individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order delivery to upper layer only resends pkts for which ACK not received timer for each unacked pkt window N consecutive seq # s again limits seq #s of sent, unack ed pkts Transport ayer 3-43 Transport ayer 3-44 Selective repeat:, windows Selective repeat data from : if next available seq # in window, send pkt timeout(n): resend pkt n, restart timer ACK(n) in [sendbase,sendbase+n]: mark pkt n as received if n smallest unacked pkt, advance window base to next unacked seq # pkt n in [rcvbase, rcvbase+n-1] send ACK(n) out-of-order: buffer in-order: deliver (also deliver buffered, in-order pkts), advance window to next not-yet-received pkt pkt n in [rcvbase-n,rcvbase-1] ACK(n) otherwise: ignore Transport ayer 3-45 Transport ayer 3-46 Selective repeat in action Selective repeat: dilemma Example: seq # s: 0, 1, 2, 3 window size=3 sees no difference in two scenarios! incorrectly passes duplicate data as new in (a) Transport ayer 3-47 Q: what relationship between seq # size and window size? Transport ayer 3-48 8