1. EEC-484/584 Computer Networks Lecture 13 Wenbing Zhao wenbing@ieee.org (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer Networking book)

2. 2 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Outline Sliding window protocols UDP TCP –Segment header structure –Connection management

3. 3 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Go Back n Sender –Stores all packets in output buffer –Must get acks in order in which packets are sent Receiver –Discards all packets following lost or damaged one Works well –If transmission errors rare and few retransmissions –If lot of traffic in both directions

4. 4 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Selective Repeat Receiver accepts and buffers packets following lost or damaged packets Both sender and receiver maintain windows –Sender ’ s window starts at 0, grows to MAX SEQ –Receiver ’ s window fixed at MAX SEQ Receiver has buffer reserved for each seq num in its window 0 1 2 3 4 5 6 7

5. 5 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Selective Repeat When packet arrives, receiver checks if seq num in window If so and if not already received, this packet is accepted and stored If all lower numbered packets delivered, this packet is delivered as well 0 1 2 3 4 5 6 7 5 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 3

6. 6 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Selective Repeat If no reverse traffic before timer goes off, separate ack is sent When receiver suspects error, sends NAK back to sender (request for retransmission) Two circumstances that trigger NAK –Damaged packet arrives –Packet other than expected one arrives, suspect expected one is lost Receiver sends only one NAK for packet expected

7. 7 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Non-Sequential Receive Problem New range of valid sequence numbers for receiver can overlap old range Overlap can contain duplicates Example: n = 3-bit seq num (8 possible numbers, 0 through 7, back to 0) 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

8. 8 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Non-Sequential Receive Problem Sender sends 0,1,2,3,4,5,6 Receiver –Receives 0,1,2,3,4,5,6 –Sends ack but ack gets lost –Expects to receive 7,0,1,2,3,4,5 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

9. 9 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Non-Sequential Receive Problem Sender times out, retransmits 0,1,2,3,4,5,6 Receiver checks 0 is in new window, thinks new 0 because has sent ack for old 6, waits for 7 0123456 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

10. 10 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Non-Sequential Receive Problem Sender receives ack for 0-6, sends 7 Receiver receives 7, delivers 7 and old 0 (as new 0 !) 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

11. 11 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

12. 12 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

13. 13 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

14. 14 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

15. 15 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

16. 16 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao Internet Checksum Example 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 0 0 1 1 0 0 1 1 0 0 1 1 0 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 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 wraparound sum checksum To know more: http://www.netfor2.com/udpsum.htm http://www.netfor2.com/checksum.html

17. 17 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

18. 18 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

19. 19 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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!

20. 20 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

21. 21 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

22. 22 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

23. 23 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

24. 24 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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();

25. 25 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

26. 26 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

27. 27 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

28. 28 Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao 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

29. 29Exercise 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. Fall Semester 2008EEC-484/584: Computer NetworksWenbing Zhao