1. EEC-484/584 Computer 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. 2 6/25/2015EEC-484/584: Computer Networks Outline Reliable data tranfer (part II) UDP TCP –Segment header structure –Connection management Wenbing Zhao

3. 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 6/25/2015EEC-484/584: Computer NetworksWenbing Zhao

4. 4 rdt3.0 sender sndpkt = make_pkt(0, data, checksum) udt_send(sndpkt) start_timer rdt_send(data) Wait for ACK0 rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,1) ) Wait for call 1 from above sndpkt = make_pkt(1, data, checksum) udt_send(sndpkt) start_timer rdt_send(data) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,0) rdt_rcv(rcvpkt) && ( corrupt(rcvpkt) || isACK(rcvpkt,0) ) rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) && isACK(rcvpkt,1) stop_timer udt_send(sndpkt) start_timer timeout udt_send(sndpkt) start_timer timeout rdt_rcv(rcvpkt) Wait for call 0from above Wait for ACK1  rdt_rcv(rcvpkt)    6/25/2015EEC-484/584: Computer NetworksWenbing Zhao

5. 5 rdt3.0 in action 6/25/2015EEC-484/584: Computer NetworksWenbing Zhao

6. 6 rdt3.0 in action 6/25/2015EEC-484/584: Computer NetworksWenbing Zhao

7. 7 Performance of rdt3.0 rdt3.0 works, but performance stinks ex: 1 Gbps link, 15 ms prop. delay, 8000-bit packet: m U sender : utilization – fraction of time sender busy sending m 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link m network protocol limits use of physical resources! 6/25/2015EEC-484/584: Computer NetworksWenbing Zhao

8. 8 rdt3.0: stop-and-wait operation first packet bit transmitted, t = 0 senderreceiver RTT last packet bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R 6/25/2015EEC-484/584: Computer NetworksWenbing Zhao

9. 9 EEC-484/584: Computer Networks 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 6/25/2015Wenbing Zhao

10. 10 EEC-484/584: Computer Networks Pipelining: Increased Utilization first packet bit transmitted, t = 0 senderreceiver RTT last bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK Increase utilization by a factor of 3! 6/25/2015Wenbing Zhao

11. 11 EEC-484/584: Computer Networks 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 6/25/2015Wenbing Zhao

12. 12 EEC-484/584: Computer Networks Go-Back-N Sender: k-bit seq # in pkt header “window” of up to N consecutive unack’ed pkts allowed r ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” m may receive duplicate ACKs (see receiver) r timer for oldest in-flight pkt r timeout(n): retransmit pkt n and all higher seq # pkts in window 6/25/2015Wenbing Zhao

13. 13 EEC-484/584: Computer Networks GBN: Sender Extended FSM Wait start_timer udt_send(sndpkt[base]) udt_send(sndpkt[base+1]) … udt_send(sndpkt[nextseqnum-1]) timeout 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) base = getacknum(rcvpkt)+1 If (base == nextseqnum) // no more unacked pkts stop_timer else start_timer rdt_rcv(rcvpkt) && notcorrupt(rcvpkt) base=1 nextseqnum=1 rdt_rcv(rcvpkt) && corrupt(rcvpkt)  6/25/2015Wenbing Zhao

14. 14 EEC-484/584: Computer Networks 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 # Wait udt_send(sndpkt) default rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum) extract(rcvpkt,data) deliver_data(data) sndpkt = make_pkt(expectedseqnum,ACK,chksum) udt_send(sndpkt) expectedseqnum++ expectedseqnum=1 sndpkt = make_pkt(expectedseqnum,ACK,chksum)  6/25/2015Wenbing Zhao

15. 15 EEC-484/584: Computer Networks GBN in action 6/25/2015Wenbing Zhao

16. 16 EEC-484/584: Computer Networks 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 6/25/2015Wenbing Zhao

17. 17 EEC-484/584: Computer Networks Selective Repeat: Sender, Receiver Windows 6/25/2015Wenbing Zhao

18. 18 EEC-484/584: Computer Networks 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 # Sender pkt n in [rcvbase, rcvbase+N-1] r send ACK(n) r out-of-order: buffer r in-order: deliver (also deliver buffered, in-order pkts), advance window to next not- yet-received pkt pkt n in [rcvbase-N,rcvbase-1] r ACK(n) otherwise: r ignore Receiver 6/25/2015Wenbing Zhao

19. 19 EEC-484/584: Computer Networks Selective Repeat In Action 6/25/2015Wenbing Zhao

20. 20 EEC-484/584: Computer Networks 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? 6/25/2015Wenbing Zhao

21. 21 6/25/2015EEC-484/584: Computer Networks 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 7 0 1 2 3 4 5 6 7 0123456 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Overlap Wenbing Zhao

22. 22 6/25/2015EEC-484/584: Computer Networks 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 4 5 6 7 0 1 2 3 4 5 6 7 0123 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 No Overlap Wenbing Zhao

23. 23 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

24. 24 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

25. 25 6/25/2015EEC-484/584: Computer Networks 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 source port #dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header Wenbing Zhao

26. 26 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

27. 27 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

28. 28 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

29. 29 6/25/2015EEC-484/584: Computer Networks TCP Segment Structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number Receive window Urg data pnter checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP) A TCP segment must fit into an IP datagram! Wenbing Zhao

30. 30 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

31. 31 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

32. 32 6/25/2015EEC-484/584: Computer Networks 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.htm http://www.netfor2.com/checksum.htmlhttp://www.netfor2.com/tcpsum.htm http://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 Wenbing Zhao

33. 33 6/25/2015EEC-484/584: Computer Networks 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 Host A Host B Seq=42, ACK=79, data = ‘C’ Seq=79, ACK=43, data = ‘C’ Seq=43, ACK=80 User types ‘C’ host ACKs receipt of echoed ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ time simple telnet/ssh scenario Wenbing Zhao

34. 34 6/25/2015EEC-484/584: Computer Networks 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(); Wenbing Zhao

35. 35 6/25/2015EEC-484/584: Computer Networks 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 Wenbing Zhao

36. 36 6/25/2015EEC-484/584: Computer Networks TCP Connection Management Three way handshake: SYN segment is considered as 1 byte SYN/ACK segment is also considered as 1 byte client SYN (seq=x) server SYN/ACK (seq=y, ACK=x+1) ACK (seq=x+1, ACK=y+1) connect accept Wenbing Zhao

37. 37 6/25/2015EEC-484/584: Computer Networks TCP Connection Management 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. client FIN server ACK FIN close closed timed wait Wenbing Zhao

38. 38 6/25/2015EEC-484/584: Computer Networks TCP Connection Management 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 client FIN server ACK FIN closing closed timed wait closed Wenbing Zhao

39. 39Exercise 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. 6/25/2015EEC-484/584: Computer NetworksWenbing Zhao

40. 40 6/25/2015EEC-484/584: Computer Networks Sending Window Start from empty and grow to a maximum size Within sending window, packets sent but not acked –Sender must keep those packets for possible retransmission –If max window size = w, need w buffers 0 1 2 3 4 5 6 7 1 st outstanding packet Last packet sent 0 1 2 3 4 5 6 7 A new packet sent (if send window allows) Sent window enlarges when more packet is sent When new packet arrives from application layer, it is given next highest sequence number, and upper edge of window is incremented Wenbing Zhao

41. 41 6/25/2015EEC-484/584: Computer Networks Sending Window 0 1 2 3 4 5 6 7 3 ack 0 1 2 3 4 5 6 7 Sent window shrinks when the ack corresponding to the 1 st outstanding packet Is received When ack arrives from receiver, lower edge of window is incremented In general, ack is cumulative Wenbing Zhao

42. 42 6/25/2015EEC-484/584: Computer Networks Receiving Window List of consecutive sequence numbers of packets that receiver is permitted to accept When packet with (seq num = lower edge of window) arrives –Packet is passed to higher layer –Ack is generated –Window slid down by 1 (remains same size as was initially) 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 3 Wenbing Zhao