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EEC-484/584 Computer Networks

EEC-484/584 Computer Networks. Lecture 7 Wenbing Zhao wenbing@ieee.org (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer Networking book). Outline. Reliable data tranfer (part II) UDP TCP Segment header structure Connection management.

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EEC-484/584 Computer Networks

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  1. EEC-484/584Computer Networks Lecture 7 Wenbing Zhao wenbing@ieee.org (Part of the slides are based on Drs. Kurose & Ross’s slides for their Computer Networking book)

  2. Outline • Reliable data tranfer (part II) • UDP • TCP • Segment header structure • Connection management EEC-484/584: Computer Networks

  3. 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) EEC-484/584: Computer Networks

  4. 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) EEC-484/584: Computer Networks

  5. 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 EEC-484/584: Computer Networks

  6. 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 EEC-484/584: Computer Networks

  7. 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) EEC-484/584: Computer Networks

  8. 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 EEC-484/584: Computer Networks

  9. 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 EEC-484/584: Computer Networks

  10. rdt3.0 in action EEC-484/584: Computer Networks

  11. rdt3.0 in action EEC-484/584: Computer Networks

  12. rdt3.0 works, but performance stinks ex: 1 Gbps link, 15 ms prop. delay, 8000-bit packet: Performance of rdt3.0 • U sender: utilization – fraction of time sender busy sending • 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link • network protocol limits use of physical resources! EEC-484/584: Computer Networks

  13. 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 EEC-484/584: Computer Networks

  14. 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 EEC-484/584: Computer Networks

  15. 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! EEC-484/584: Computer Networks

  16. 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 EEC-484/584: Computer Networks

  17. Sender: k-bit seq # in pkt header “window” of up to N consecutive 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 EEC-484/584: Computer Networks

  18. 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 if first unacked pkt 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) // no more unacked pkts stop_timer else start_timer EEC-484/584: Computer Networks

  19. 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 # 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) EEC-484/584: Computer Networks

  20. GBN inaction EEC-484/584: Computer Networks

  21. Receiver individually acknowledges all correctly received pkts Buffers pkts, as needed, for eventual in-order delivery to upper layer Sender only resends pkts for which ACK not received Sender timer for each unacked pkt Sender window N consecutive seq #’s Again limits seq #s of sent, unacked pkts Selective Repeat EEC-484/584: Computer Networks

  22. Selective Repeat: Sender, Receiver Windows EEC-484/584: Computer Networks

  23. data from above : if next available seq # in window, send pkt timeout(n): resend pkt n, restart timer ACK(n) in [sendbase,sendbase+N-1]: mark pkt n as received if n smallest unACKed pkt, advance window base to next unACKed seq # Receiver Sender Selective Repeat 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 EEC-484/584: Computer Networks

  24. Selective Repeat In Action EEC-484/584: Computer Networks

  25. 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 relationship between seq # size and window size? Selective Repeat: Dilemma EEC-484/584: Computer Networks

  26. 0 0 3 3 6 6 2 2 5 5 1 1 4 4 7 7 0 3 6 0 3 6 2 5 2 5 1 4 7 1 4 7 Non-Sequential Receive Problem • The problem is caused by the overlap of sequence number between the new receiving window and the old receiving window 0 1 2 3 4 5 6 Overlap Overlap EEC-484/584: Computer Networks

  27. 0 0 3 3 6 6 2 2 5 5 1 1 4 4 7 7 0 3 6 0 3 6 2 5 2 5 1 4 7 1 4 7 No Overlap Non-Sequential Receive Problem • Solution: • make sure no overlap when receiver advances its window • Make window size w =1/2 range of seq numbers 0 1 2 3 EEC-484/584: Computer Networks

  28. “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 UDP: User Datagram Protocol EEC-484/584: Computer Networks

  29. Why is There a UDP? • No connection establishment (which can add delay) • Simple: no connection state at sender and receiver • Small segment header • No congestion control: UDP can blast away as fast as desired EEC-484/584: Computer Networks

  30. Often used for streaming multimedia apps Loss tolerant Rate sensitive Other UDP uses DNS SNMP Reliable transfer over UDP: add reliability at application layer UDP 32 bits source port # dest port # Length, in bytes of UDP segment, including header checksum length Application data (message) UDP segment format EEC-484/584: Computer Networks

  31. 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? UDP Checksum • Goal: detect “errors” (e.g., flipped bits) in transmitted segment EEC-484/584: Computer Networks

  32. Full duplex data: Bi-directional data flow in same connection MSS: maximum segment size Connection-oriented: Handshaking (exchange of control msgs) init’s sender, receiver state before data exchange Flow controlled: Sender will not overwhelm receiver Point-to-point: One sender, one receiver Reliable, in-order byte steam: No “message boundaries” Pipelined: TCP congestion and flow control set window size Send & receive buffers TCP: Overview EEC-484/584: Computer Networks

  33. TCP: Overview • TCP connection is byte stream, not message stream, no message boundaries • TCP may send immediately or buffer before sending • Receiver stores the received bytes in a buffer EEC-484/584: Computer Networks

  34. 32 bits source port # dest port # sequence number acknowledgement number head len not used Receive window U A P R S F checksum Urg data pnter Options (variable length) application data (variable length) TCP Segment Structure URG: urgent data (generally not used) counting by bytes of data (not segments!) ACK: ACK # valid PSH: push data now (generally not used) # bytes rcvr willing to accept RST, SYN, FIN: connection estab (setup, teardown commands) A TCP segment must fit into an IP datagram! Internet checksum (as in UDP) EEC-484/584: Computer Networks

  35. The TCP Segment Header • Source port and destination port: identify local end points of the connection • Source and destination end points together identify the connection • Sequence number: identify the byte in the stream of data that the first byte of data in this segment represents • Acknowledgement number: the next sequence number that the sender of the ack expects to receive • Ack # = Last received seq num + 1 • Ack is cumulative: an ack of 5 means 0-4 bytes have been received • TCP header length– number of 32-bit words in header EEC-484/584: Computer Networks

  36. The TCP Segment Header • URG– indicates urgent pointer field is set • Urgent pointer– points to the seq num of the last byte in a sequence of urgent data • ACK– acknowledgement number is valid • SYN– used to establish a connection • Connection request: ACK = 0, SYN = 1 • Connection confirm: ACK=1, SYN = 1 • FIN– release a connection, sender has no more data • RST– reset a connection that is confused • PSH– sender asked to send data immediately EEC-484/584: Computer Networks

  37. The TCP Segment Header • Receiver window size–number of bytes that may be sent beyond the byte acked • Checksum–add the header, the data, and the conceptual pseudoheader as 16-bit words, take 1’s complement of sum • For more info: http://www.netfor2.com/tcpsum.htmhttp://www.netfor2.com/checksum.html • Options– provides a way to add extra facilities not covered by the regular header • E.g., communicate buffer sizes during set up EEC-484/584: Computer Networks

  38. Sequence numbers: byte stream “number” of first byte in segment’s data ACKs: seq # of next byte expected from other side cumulative ACK time TCP Sequence Numbers and ACKs Host B Host A User types ‘C’ Seq=42, ACK=79, data = ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ Seq=79, ACK=43, data = ‘C’ host ACKs receipt of echoed ‘C’ Seq=43, ACK=80 simple telnet/ssh scenario EEC-484/584: Computer Networks

  39. TCP Connection Management TCP sender, receiver establish “connection” before exchanging data segments • Initialize TCP variables: • Sequence numbers • Buffers, flow control info (e.g. RcvWindow) • Client: connection initiator Socket clientSocket = new Socket("hostname","port number"); • Server: contacted by client Socket connectionSocket = welcomeSocket.accept(); EEC-484/584: Computer Networks

  40. TCP Connection Management Three way handshake: Step 1:client host sends TCP SYN segment to server • specifies initial sequence number • no data Step 2:server host receives SYN, replies with SYN/ACK segment • server allocates buffers • specifies server initial sequence number Step 3: client receives SYN/ACK, replies with ACK segment, which may contain data EEC-484/584: Computer Networks

  41. TCP Connection Management client server Three way handshake: • SYN segment is considered as 1 byte • SYN/ACK segment is also considered as 1 byte accept connect SYN (seq=x) SYN/ACK (seq=y, ACK=x+1) ACK (seq=x+1, ACK=y+1) EEC-484/584: Computer Networks

  42. Closing a connection: client closes socket:clientSocket.close(); Step 1:client end system sends TCP FIN control segment to server Step 2:server receives FIN, replies with ACK. Closes connection, sends FIN. TCP Connection Management client server close FIN ACK close FIN ACK timed wait closed EEC-484/584: Computer Networks

  43. Step 3:client receives FIN, replies with ACK. Enters “timed wait” - will respond with ACK to received FINs Step 4:server, receives ACK. Connection closed. Note:with small modification, can handle simultaneous FINs TCP Connection Management client server closing FIN ACK closing FIN ACK timed wait closed closed EEC-484/584: Computer Networks

  44. Exercise • A process at host A wants to establish a TCP connection with another process at host B. Assuming that host A chooses to use 1628 as the initial sequence number, and host B chooses to use 3217 as the initial sequence number for this connection, show the segments involved with the connection establishment process. You must include the following information for each such segment: (1) sequence number, (2) acknowledgement number (if applicable), (3) the SYN flag bit status, and (4) the ACK flag bit status. EEC-484/584: Computer Networks

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