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Lecture 7 TCP/IP Transport Layer (1)

Lecture 7 TCP/IP Transport Layer (1). Outline (Transport Layer). Principles behind transport layer services: multiplexing/demultiplexing principles of reliable data transfer learn about transport layer protocols in the Internet: UDP: connectionless transport

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Lecture 7 TCP/IP Transport Layer (1)

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  1. Lecture 7TCP/IP Transport Layer (1) Khaled Mahbub, IICT, BUET, 2008

  2. Outline (Transport Layer) • Principles behind transport layer services: • multiplexing/demultiplexing • principles of reliable data transfer • learn about transport layer protocols in the Internet: • UDP: connectionless transport • TCP: connection-oriented transport • segment structure • reliable data transfer • flow control • connection management Khaled Mahbub, IICT, BUET, 2008

  3. Transport Services and Protocols • provide logical communication between application processes running on different hosts • transport protocols run in end systems • sender side: breaks application messages into segments, passes to network layer • receiver side: reassembles segments into messages, passes to application layer • more than one transport protocol available to applications, e.g. • Internet: TCP and UDP Khaled Mahbub, IICT, BUET, 2008

  4. network layer: logical communication between hosts transport layer: logical communication between processes relies on, enhances, network layer services Transport vs. Network Layer Household analogy: • 5 kids from house A sending letters to 5 kids in house B • In house A, Ali collects letters from each kid and wrap them in single envelope. Ali also distributes received mails in house A. Babu does same task in house B. • Postal service is used to send mails • processes = kids • app messages = letters from each kid • hosts = houses • transport protocol = Ali and Babu • network-layer protocol = postal service Khaled Mahbub, IICT, BUET, 2008

  5. Internet Transport Layer protocols • User Datagram Protocol (UDP) • unreliable unordered data transfer between sending and receiving process • does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee • Transmission Control Protocol (TCP) • reliable, in-order delivery • reliable transport: acknowledgement, retransmissions • flow control: sender won’t overwhelm receiver • congestion control: for benefit of the Internet • Does not provide: timing, minimum bandwidth guarantees Khaled Mahbub, IICT, BUET, 2008

  6. Outline (Transport Layer) • Principles behind transport layer services: • multiplexing/demultiplexing • principles of reliable data transfer • learn about transport layer protocols in the Internet: • UDP: connectionless transport • TCP: connection-oriented transport • segment structure • reliable data transfer • flow control • connection management Khaled Mahbub, IICT, BUET, 2008

  7. application application application transport transport transport P4 P2 P3 P1 P1 network network network link link link physical physical physical Multiplexing at send host: Demultiplexing at rcv host: host 3 host 2 host 1 Multiplexing/demultiplexing delivering received segments to correct application layers processes. gathering data from multiple application layer process, enveloping data with header (later used for demultiplexing) = socket = process Khaled Mahbub, IICT, BUET, 2008

  8. host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries 1 transport-layer segment each segment has source, destination port number The port number is a 16-bit number, ranging from 0 to 65535. Port numbers ranging from 0 - 1023 are called well-known port numbers and are used by well-known application protocols such as HTTP, FTP, Telnet etc. host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields application data (message) TCP/UDP segment format How Demultiplexing Works Khaled Mahbub, IICT, BUET, 2008

  9. P2 P1 P1 P3 client IP: A Demultiplexing SP: 6428 SP: 6428 DP: 9157 DP: 5775 D-IP:A D-IP:B SP: 9157 SP: 5775 DP: 6428 DP: 6428 Client IP:B server IP: C D-IP:C D-IP:C SP provides “return address” Khaled Mahbub, IICT, BUET, 2008

  10. Outline (Transport Layer) • Principles behind transport layer services: • multiplexing/demultiplexing • principles of reliable data transfer • learn about transport layer protocols in the Internet: • UDP: connectionless transport • TCP: connection-oriented transport • segment structure • reliable data transfer • flow control • connection management Khaled Mahbub, IICT, BUET, 2008

  11. UDP: User Datagram 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 • often used for streaming multimedia applications • loss tolerant • rate sensitive • other UDP uses • DNS • SNMP • reliable transfer over UDP: add reliability at application layer • application-specific error recovery! Khaled Mahbub, IICT, BUET, 2008

  12. identifies receiving process (for multiplexing demultiplexing) 32 bits source port # dest port # Length, in bytes of UDP segment, including header checksum length For UDP header and UDP data (optional) Application data (message) UDP segment format UDP Segment Format Khaled Mahbub, IICT, BUET, 2008

  13. 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. Does not guarantee error free. UDP Checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment Khaled Mahbub, IICT, BUET, 2008

  14. Outline (Transport Layer) • Principles behind transport layer services: • multiplexing/demultiplexing • principles of reliable data transfer • learn about transport layer protocols in the Internet: • UDP: connectionless transport • TCP: connection-oriented transport • segment structure • reliable data transfer • flow control • connection management Khaled Mahbub, IICT, BUET, 2008

  15. important in application, transport, link layers top-10 list of important networking topics! characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) Principles of Reliable Data Transfer Khaled Mahbub, IICT, BUET, 2008

  16. 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 send side receive side Khaled Mahbub, IICT, BUET, 2008

  17. We will: 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 state 1 state 2 actions Reliable Data Transfer event causing state transition actions taken on state transition state: when in this “state” next state uniquely determined by next event Khaled Mahbub, IICT, BUET, 2008

  18. underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver reads data from underlying channel Rdt1.0: Reliable Transfer Over a Reliable Channel rdt_send(data) rdt_rcv(packet) Wait for call from below Wait for call from above packet = make_pkt(data) udt_send(packet) extract (packet,data) deliver_data(data) sender receiver Khaled Mahbub, IICT, BUET, 2008

  19. all transmitted packets are received in the order in which they were sent. underlying channel may flip bits in packet checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that packet received OK negative acknowledgements (NAKs): receiver explicitly tells sender that packet had errors sender retransmits packet on receipt of NAK new mechanisms in rdt2.0 (beyond rdt1.0): error detection receiver feedback: control msgs (ACK,NAK) receiver->sender Rdt2.0: Channel with Bit Errors Khaled Mahbub, IICT, BUET, 2008

  20. Wait for ACK or NAK rdt_rcv(rcvpkt) && corrupt(rcvpkt) stop and wait udt_send(NAK) Rdt2.0: FSM Specification rdt_send(data) 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) Wait for call from below L rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) Sender sends one packet, then waits for receiver response extract(rcvpkt,data) deliver_data(data) udt_send(ACK) Khaled Mahbub, IICT, BUET, 2008

  21. What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! add enough checksum bits to allow the sender to not only detect, but recover from, bit errors. resend the current data packet when it receives a garbled ACK or NAK packet. possible duplicate Handling duplicates: sender adds sequence number to each packet sender retransmits current packet if ACK/NAK garbled receiver discards (doesn’t deliver up) duplicate packet Rdt2.0 Flaws Khaled Mahbub, IICT, BUET, 2008

  22. Sender: sequence number added to packet two sequence numbers (0,1) will suffice. must check if received ACK/NAK corrupted twice as many states state must “remember” whether “current” packet has 0 or 1 sequence number. Receiver: must check if received packet is duplicate state indicates whether 0 or 1 is expected packet sequence number note: receiver can not know if its last ACK/NAK received OK at sender Rdt2.1: Mechanism Khaled Mahbub, IICT, BUET, 2008

  23. 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) Khaled Mahbub, IICT, BUET, 2008

  24. 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) Khaled Mahbub, IICT, BUET, 2008

  25. same functionality as rdt2.1, using ACKs only instead of NAK, receiver sends ACK for last packet received OK receiver must explicitly include sequence number of packet being ACKed duplicate ACK at sender results in same action as NAK: retransmit current packet Rdt2.2: a NAK-free Protocol Khaled Mahbub, IICT, BUET, 2008

  26. 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) Wait for ACK 0 Wait for call 0 from above rdt_rcv(rcvpkt) && (corrupt(rcvpkt) || has_seq1(rcvpkt)) L receiver FSM fragment udt_send(sndpkt) Wait for 0 from below rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && has_seq1(rcvpkt) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(ACK1, chksum) udt_send(sndpkt) Rdt2.2: Sender Receiver FSM Khaled Mahbub, IICT, BUET, 2008

  27. New assumption: underlying channel can also lose packets (data or ACKs) how to detect packet loss and what to do when this occurs. use of checksum, sequence numbers, ACK packets, and retransmissions Approach: sender waits “reasonable” amount of time for ACK retransmits if no ACK received in this time if packet (or ACK) just delayed (not lost): retransmission will be duplicate, but use of sequence numbers already handles this receiver must specify sequence number of packet being ACKed requires countdown timer Rdt3.0: Channels with Errors and Loss Khaled Mahbub, IICT, BUET, 2008

  28. 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 Khaled Mahbub, IICT, BUET, 2008

  29. Rdt3.0 in Action Khaled Mahbub, IICT, BUET, 2008

  30. Rdt3.0 in Action Khaled Mahbub, IICT, BUET, 2008

  31. Performance of Rdt3.0 • rdt3.0 works, but performance stinks • example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet: L (packet length in bits) 8kb/pkt T = = = 8 microsec transmit R (transmission rate, bps) 10**9 b/sec • U sender: utilization – fraction of time sender busy sending • 1KB packet every 30 msec -> 33kB/sec throughput over 1 Gbps link • network protocol limits use of physical resources! Khaled Mahbub, IICT, BUET, 2008

  32. 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 Khaled Mahbub, IICT, BUET, 2008

  33. Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged packets 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 Khaled Mahbub, IICT, BUET, 2008

  34. 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 Khaled Mahbub, IICT, BUET, 2008

  35. Sender: k-bit sequence number in packet header “window” of up to N, consecutive unack’ed packets allowed Go-Back-N • ACK(n): ACKs all packets up to, including sequence number n - “cumulative ACK” • may deceive duplicate ACKs (see receiver) • timer for each in-flight packet • timeout(n): retransmit packet n and all higher sequence number packets in window Khaled Mahbub, IICT, BUET, 2008

  36. Wait 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) L base=1 nextseqnum=1 timeout start_timer udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) … udt_send(sndpkt[nextseqnum-1]) rdt_rcv(rcvpkt) && corrupt(rcvpkt) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) base = getacknum(rcvpkt)+1 If (base == nextseqnum) stop_timer else start_timer Khaled Mahbub, IICT, BUET, 2008

  37. ACK-only: always send ACK for correctly-received packet with highest in-order sequence number may generate duplicate ACKs need only remember expectedseqnum out-of-order packet: discard (don’t buffer) -> no receiver buffering! Re-ACK packet with highest in-order sequence GBN: Receiver Extended FSM default udt_send(sndpkt) rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) L Wait extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(expectedseqnum,ACK,chksum) udt_send(sndpkt) expectedseqnum++ expectedseqnum=1 sndpkt = make_pkt(expectedseqnum,ACK,chksum) Khaled Mahbub, IICT, BUET, 2008

  38. GBN in Action Khaled Mahbub, IICT, BUET, 2008

  39. receiver individually acknowledges all correctly received packets buffers packets, as needed, for eventual in-order delivery to upper layer sender only resends packets for which ACK not received sender timer for each unACKed packet sender window N consecutive sequence numbers again limits sequence numbers of sent, unACKed packets Selective Repeat Khaled Mahbub, IICT, BUET, 2008

  40. Selective Repeat: Sender, Receiver Windows Khaled Mahbub, IICT, BUET, 2008

  41. data from above : if next available sequence number in window, send packet timeout(n): resend packet n, restart timer ACK(n) in [sendbase,sendbase+N]: mark packet n as received if n smallest unACKed packet, advance window base to next unACKed sequence number receiver sender Selective Repeat packet n in [rcvbase, rcvbase+N-1] • send ACK(n) • out-of-order: buffer • in-order: deliver (also deliver buffered, in-order packets), advance window to next not-yet-received packet packet n in [rcvbase-N,rcvbase-1] • ACK(n) otherwise: • ignore Khaled Mahbub, IICT, BUET, 2008

  42. Selective Repeat in Action Khaled Mahbub, IICT, BUET, 2008

  43. 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) a window size that is one smaller than the size of the sequence number space won't work. the window size must be less than or equal to half the size of the sequence number space. Selective Repeat: Dilemma Khaled Mahbub, IICT, BUET, 2008

  44. Reading Material • Chapter 3 – text3 (Kurose) • Chapter 6 – text2 (Tanenbaum) Khaled Mahbub, IICT, BUET, 2008

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