Transport services and protocols Internet -layer protocols logical communication between processes 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 reliable, in-order delivery: TCP unreliable, unordered delivery: UDP services not available: delay guarantees bandwidth guarantees Transport ayer 3-1 Transport ayer 3-2 Sending and receiving = socket = process link P3 P1 P1 link P2 host 1 host 2 P4 link host 3 Receiving packets 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 right socket 32 bits source port # dest port # other header fields data (message) TCP/UDP segment format Transport ayer 3-3 Transport ayer 3-4 Connection-oriented (TCP) Connection-oriented TCP socket : source IP address source port number dest IP address dest port number All four values to direct segment to appropriate socket server host may support many simultaneous TCP sockets: web servers have different sockets for each connecting client P1 client IP: A SP: 9157 DP: 80 S-IP: A D-IP:C P4 server IP: C P5 P6 P2 P1P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 DP: 80 S-IP: B D-IP:C Client IP:B Transport ayer 3-5 Transport ayer 3-6 1
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 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 32 bits source port # dest port # length Application data (message) checksum UDP segment format Transport ayer 3-7 Transport ayer 3-8 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 3-9 Transport ayer 3-10 Principles of Reliable data transfer Principles of Reliable data transfer important in app.,, link layers top-10 list of important ing topics! 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) characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Transport ayer 3-11 Transport ayer 3-12 2
Principles of Reliable data transfer Reliable data transfer: getting started important in app.,, link layers top-10 list of important ing topics! rdt_send(): called from, (e.g., by app.). Passed data to deliver to upper layer deliver_data(): called by rdt to deliver data to upper send side receive side characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) udt_send(): called by rdt, to transfer packet over unreliable channel to rdt_rcv(): called when packet arrives on rcv-side of channel Transport ayer 3-13 Transport ayer 3-14 Reliable data transfer: getting started We ll: incrementally develop reliable data transfer protocol (rdt) only unidirectional data transfer but control info will flow on both directions! use finite state machines (FSM) 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 state: when in this state next state uniquely determined by next event state 1 event causing state transition actions taken on state transition event actions state 2 packet = make_pkt(data) udt_send(packet) rdt_rcv(packet) extract (packet,data) Transport ayer 3-15 Transport ayer 3-16 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 How do humans retransmits recover pkt on receipt from of errors NAK new mechanisms during conversation? in rdt2.0 (beyond rdt1.0): error detection feedback: control msgs (ACK,NAK) rcvr-> 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-> Transport ayer 3-17 Transport ayer 3-18 3
rdt2.0: FSM specification sndpkt = make_pkt(data, checksum) ACK or NAK isack(rcvpkt) isnak(rcvpkt) corrupt(rcvpkt) udt_send(nak) notcorrupt(rcvpkt) udt_send(ack) rdt2.0 has a fatal flaw! 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 Transport ayer 3-19 Transport ayer 3-20 rdt2.1:, handles garbled ACK/NAKs && 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 3-21 rdt2.1:, handles garbled ACK/NAKs (corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) not corrupt(rcvpkt) && has_seq1(rcvpkt) notcorrupt(rcvpkt) && has_seq0(rcvpkt) 0 from 1 from notcorrupt(rcvpkt) && has_seq1(rcvpkt) (corrupt(rcvpkt) sndpkt = make_pkt(nak, chksum) not corrupt(rcvpkt) && has_seq0(rcvpkt) Transport ayer 3-22 rdt2.1: discussion rdt2.2: a NAK-free protocol Sender: seq # added to pkt two seq. # s (0,1) will suffice. 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 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 3-23 Transport ayer 3-24 4
rdt2.2:, fragments rdt3.0: channels with errors and loss (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 3-25 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 Transport ayer 3-26 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 rdt3.0 in action Transport ayer 3-27 Transport ayer 3-28 rdt3.0 in action rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 last packet bit transmitted, t = d trans 2 d prop first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, 2 d prop + d trans d trans Utilization = 2 d prop + d trans Transport ayer 3-29 Transport ayer 3-30 5
Pipelined protocols pipelining: allows yet-to-be-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 =d trans 2 d prop ACK arrives, send next packet, t =2 d prop + d trans 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 3 d trans Utilization = 2 d prop + d trans Transport ayer 3-31 Transport ayer 3-32 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 3-33 Transport ayer 3-34 GBN in action Selective Repeat individually acknowledges correctly received pkts buffers pkts for 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 3-37 Transport ayer 3-38 6
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 3-39 Transport ayer 3-40 Selective repeat in action Transport ayer 3-41 7