EEC-484/584 Computer Networks

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

2. Outline Reminder No class next Monday (Columbus Day) Reliable data transfer Pipelining protocols UDP EEC-484/584: Computer Networks

3. 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: 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

4. rdt3.0 sender Wait for ACK0 Wait for ACK1 Wait for call 1 from above Wait for call 0from above 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

5. rdt3.0 in action 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

6. rdt3.0 in action 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

7. Performance of rdt3.0 rdt3.0 works, but performance stinks ex: 1 Gbps link, 15 ms prop. delay, 8000 bit packet: • 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! 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

8. Pipelined Protocols 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 EEC-484/584: Computer Networks

9. 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

10. Pipelining Protocols 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 EEC-484/584: Computer Networks

11. 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 receiver) • timer for oldest in-flight pkt • timeout(n): retransmit pkt n and all higher seq # pkts in window EEC-484/584: Computer Networks

12. GBN: Sender Extended FSM Wait 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

13. GBN: Receiver Extended FSM 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 # 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

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

15. Selective Repeat 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 EEC-484/584: Computer Networks

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

17. Selective Repeat 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 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

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

19. Selective Repeat: Dilemma 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? EEC-484/584: Computer Networks

20. 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 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 0 1 2 3 4 5 6 Overlap Overlap EEC-484/584: Computer Networks

21. 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 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 0 1 2 3 EEC-484/584: Computer Networks

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

23. 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

24. UDP Often used for streaming multimedia apps Loss tolerant Rate sensitive Other UDP uses DNS SNMP Reliable transfer over UDP: add reliability at application layer 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

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

26. TCP: Overview 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

27. 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

28. TCP Segment Structure 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) 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) 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

29. 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

30. 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

31. 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao

32. TCP Sequence Numbers and ACKs Sequence numbers: byte stream “number” of first byte in segment’s data ACKs: seq # of next byte expected from other side cumulative ACK time 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 9/19/2014 EEC-484/584: Computer Networks Wenbing Zhao