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Transport Layer

Transport Layer. Transport Layer – Topics. Review: multiplexing, connection and connectionless transport, services provided by a transport layer UDP Tools for transport layer Error detection, ACK/NACK, ARQ Approaches to transport Go-Back-N Selective repeat TCP Services

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Transport Layer

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  1. Transport Layer

  2. Transport Layer – Topics • Review: multiplexing, connection and connectionless transport, services provided by a transport layer • UDP • Tools for transport layer • Error detection, ACK/NACK, ARQ • Approaches to transport • Go-Back-N • Selective repeat • TCP • Services • TCP: Connection setup, acks and seq num, timeout and triple-dup ack, slow-start, congestion avoidance.

  3. Transport layer • Transfers messages between application in hosts • For ftp you exchange files and directory information. • For http you exchange requests and replies/files • For smtp messages are exchanged • Services possibly provided • Reliability • Error detection/correction • Flow/congestion control • Multiplexing (support several messages being transported simultaneously)

  4. Connection oriented / connectionless • TCP supports the idea of a connection • Once listen and connect complete, there is a logical connection between the hosts. • The state of the connection can be determined (the connection is cut or not) • But TCP does not have a heartbeat message • UDP is connectionless • Packets are just sent. There is no concept (supported by the transport layer) of a connection • The application can make a connection over UDP. So the application is each host will support the hand-shaking and monitoring the state of the “connection.” • There are several other transport layer protocols besides TCP and UDP, but TCP and UDP are the most popular

  5. Connection oriented Connections must be set up The state of the connection can be determined Flow/congestion control Limits congestion in the network and end hosts Control how fast data can be sent Larger Packet header Retransmits lost packets and reports if packets were not successfully transmitted Check sum for error detection Connectionless Connections does not need to be set-up The state of the connection is unknown No flow/congestion control Could cause excessive congestion and unfair usage Data can be sent exactly when it needs to be. Low overhead No feedback provided as to whether packets were successfully transmitted. Check sum for error detection TCP vs UCP

  6. Applications and Transport Protocols • Smtp/mail TCP • telnet TCP • http TCP • ftp TCP • NFS UDP or TCP (why udp, I do not know) • Multimedia streaming UDP or TCP • Voice over ip – UDP • Routing –UDP, its own, or TCP • DNS -UDP

  7. SP: 5775 SP: 9157 P1 P1 P2 P4 P3 P6 P5 client IP: A DP: 80 DP: 80 Multiplexing with ports Transport layer packet headers always contain source and destination port IP headers have source and destination IPs S-IP: B D-IP:C SP: 9157 DP: 80 Client IP:B server IP: C S-IP: A S-IP: B D-IP:C D-IP:C

  8. 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Chapter 3 outline

  9. “no frills,” “bare bones” Internet transport protocol “best effort” service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired UDP: User Datagram Protocol [RFC 768]

  10. often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recovery! 32 bits source port # dest port # Length, in bytes of UDP segment, including header checksum length Application data (message) UDP segment format UDP: more

  11. Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender 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. But maybe errors nonetheless? More later …. UDP checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment

  12. 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 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 wraparound sum checksum

  13. 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Chapter 3 outline

  14. Principles of Reliable data transfer

  15. Principles of Reliable data transfer

  16. Principles of Reliable data transfer

  17. 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 udt_send():called by rdt, to transfer packet over unreliable channel to receiver rdt_rcv():called when packet arrives on rcv-side of channel Reliable data transfer: getting started send side receive side

  18. We’ll: incrementally develop sender, receiver 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 sender, receiver event causing state transition actions taken on state transition state: when in this “state” next state uniquely determined by next event event state 1 state 2 actions Reliable data transfer: getting started

  19. underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel Rdt1.0: reliable transfer over a reliable channel

  20. underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver read data from underlying channel rdt_send(data) rdt_rcv(packet) Wait for call from below Wait for call from above extract (packet,data) deliver_data(data) sender receiver Rdt1.0: reliable transfer over a reliable channel packet = make_pkt(data) udt_send(packet)

  21. underlying channel may flip bits in packets checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): error detection receiver feedback: control msgs (ACK,NAK) rcvr->sender Rdt2.0: channel with bit errors

  22. rdt2.0: FSM specification

  23. Wait for ACK or NAK rdt_rcv(rcvpkt) && corrupt(rcvpkt) udt_send(NAK) Wait for call from below rdt2.0: FSM specification rdt_send(data) receiver snkpkt = make_pkt(data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && isNAK(rcvpkt) Wait for call from above udt_send(sndpkt) rdt_rcv(rcvpkt) && isACK(rcvpkt) L sender rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) extract(rcvpkt,data) deliver_data(data) udt_send(ACK)

  24. 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 rdt2.0 has a fatal flaw!

  25. rdt2.1: sender, handles garbled ACK/NAKs

  26. rdt2.1: receiver, handles garbled ACK/NAKs

  27. Wait for ACK or NAK 0 Wait for call 1 from above Wait for ACK or NAK 1 rdt2.1: sender, handles garbled ACK/NAKs rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) Wait for call 0 from above udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt) L L rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isNAK(rcvpkt) ) rdt_send(data) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) udt_send(sndpkt)

  28. Wait for 0 from below Wait for 1 from below 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) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) sndpkt = make_pkt(NAK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq1(rcvpkt) rdt_rcv(rcvpkt) && not corrupt(rcvpkt) && has_seq0(rcvpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK, chksum) udt_send(sndpkt)

  29. 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: receiver can not know if its last ACK/NAK received OK at sender rdt2.1: discussion

  30. same functionality 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 duplicate ACK at sender results in same action as NAK: retransmit current pkt rdt2.2: a NAK-free protocol

  31. rdt2.2: sender, receiver fragments

  32. Wait for call 0 from above Wait for ACK 0 Wait for 0 from below rdt2.2: sender, receiver fragments rdt_send(data) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) ) udt_send(sndpkt) sender FSM fragment rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt)) L receiver FSM fragment udt_send(sndpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK1, chksum) udt_send(sndpkt) What happens if a pkt is duplicated?

  33. New assumption: 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 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 receiver must specify seq # of pkt being ACKed requires countdown timer rdt3.0: channels with errors and loss

  34. rdt3.0 sender

  35. Wait for ACK0 Wait for ACK1 Wait for call 1 from above Wait for call 0from above rdt3.0 sender rdt_send(data) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) ) sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) start_timer L rdt_rcv(rcvpkt) L timeout udt_send(sndpkt) start_timer rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) stop_timer stop_timer timeout udt_send(sndpkt) start_timer rdt_rcv(rcvpkt) L rdt_send(data) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,0) ) sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) start_timer L

  36. rdt3.0 in action receiver sender receiver sender send pkt0 send pkt0 rec pkt0 send ack0 rec ack0 rec pkt0 send pkt1 send ack0 rec ack0 TO send pkt1 rec pkt1 resend pkt1 send ack1 rec ack1 rec pkt1 send pkt1 send ack1 rec ack1 rec pkt1 send pkt2 time rec pkt2 time

  37. rdt3.0 in action receiver sender send pkt0 receiver sender rec pkt0 send pkt0 send ack0 rec ack0 rec pkt0 send pkt1 send ack0 rec pkt1 TO rec ack0 send ack1 send pkt1 send pkt1 rec pkt1 TO send ack1 rec ack1 rec pkt1 send pkt2 send ack1 send pkt1 rec ack1 rec pkt2 rec pkt1 send no pkt (dupACK) send pkt? send ack2 send ack1 rec ack1 rec ack2 send pkt2 send pkt2 time time

  38. rdt3.0 works, but performance stinks ex: 1 Gbps link, 15 ms prop. delay, 8000 bit packet and 100bit ACK: What is the total delay Data transmission delay 8000/109 = 810-6 ACK Transmission delay 100/109 = 10-7 sec Total Delay 215ms + .008 + .0001=30.0081ms Utilization Time transmitting / total time .008 / 30.0081 = 0.00027 This is one pkt every 30msec or 33 kB/sec over a 1 Gbps link! Performance of rdt3.0

  39. rdt3.0: stop-and-wait operation sender receiver first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R

  40. Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender and/or receiver Two generic forms of pipelined protocols: go-Back-N, selective repeat Pipelined protocols

  41. Pipelining: increased utilization sender receiver first packet bit transmitted, t = 0 last bit transmitted, t = L / R first packet bit arrives RTT last packet bit arrives, send ACK last bit of 2nd packet arrives, send ACK last bit of 3rd packet arrives, send ACK ACK arrives, send next packet, t = RTT + L / R Increase utilization by a factor of 3!

  42. Go-back-N: big picture: Sender can have up to N unacked packets in pipeline Rcvr only sends cumulative acks Doesn’t ack packet if there’s a gap Sender has timer for oldest unacked packet If timer expires, retransmit all unacked packets Selective Repeat: big pic Sender can have up to N unacked packets in pipeline Rcvr acks individual packets Sender maintains timer for each unacked packet When timer expires, retransmit only unack packet Pipelining Protocols

  43. Selective repeat: big picture • Sender can have up to N unacked packets in pipeline • Receiver acks individual packets • Sender maintains timer for each unacked packet • When timer expires, retransmit only unack packet

  44. Sender: k-bit seq # in pkt header “window” of up to N, unack’ed pkts allowed Go-Back-N • ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” • may receive duplicate ACKs (see receiver) • timer for each in-flight pkt • timeout(n): retransmit pkt n and all higher seq # pkts in window

  45. State of pkts pkts start 0 unACKed pkts window N=12 1 unACKed pkts window Next pkt to be sent N unACKed pkts window N-1 unACKed pkts window Sliding window N unACKed pkts Go-Back-N unACKed pkt Pkt that could be sent ACKed pkt Unused pkt send pkt send pkts ACK arrives Send pkt window N=12

  46. N-1 unACKed pkts window N unACKed pkts window N unACKed pkts window 0 unACKed pkts window unACKed pkt Pkt that could be sent Go-Back-N ACKed pkt Unused pkt N unACKed pkts window ACK arrives Send pkt No ACK arrives …. timeout

  47. GBN: sender extended Activity Diagram

  48. GBN: Receiver Activity Diagram

  49. GBN: sender extended Activity Diagram Waiting for file Set N Set NextPktToSend=0 Set LastACKed=-1 Clear Timers(LastACKed+1 to NextPktToSend-1) NextPktToSend = LastACKed+1 otherwise Timer expires Wait NextPktToSend – LastACKed<N Send pkt[NextPktToSend] with SeqNum= NextPktToSend NextPktToSend++ Set Timer(NextPktToSend) = Now + TO ACK arrived with ACKNum = AN Clear Timers(LastACKed+1 to AN) LastACKed = AN

  50. GBN: Receiver Activity Diagram start Set NextPktToRec = 0 Clear ReceiverBuffer Clear ReceivedPkts ReceiverBase = 0 wait Place Pkt in ReceiverBuffer[SeqNum] ReceivedPkts[SeqNum]=1 otherwise Send ACK with ACKNum = NextPktToRec - 1 ReceivedPkts[NextPktToRec] == 1 NextPktToRec++ Send pkt to app Actually, there is not need for a receiver buffer

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