11 CS716 Advanced Computer Networks By Dr. Amir Qayyum

2 Lecture No. 30

3 TCP Outline TCP vs a sliding window on a direct link Model of use Segment header format and options States and state diagram (think-pair-share) Sliding window implementation details Flow control issues Bit allocation limitations Adaptive retransmission algorithms

4 TCP vs Sliding Window on Direct Link RTT varies –Among peers (hosts at other end of connections) –Over time –Requires adaptive approach to retransmission (and window sizes)

5 TCP vs Sliding Window on Direct Link Packets can be –Delayed for long periods (assume 60 seconds) –Reordered in the network –Must be prepared for arrival of very old packets

6 TCP vs Sliding Window on Direct Link Peer capabilities vary –Minimum link speed on route –Buffering capacity at destination –Requires adaptive approach to window sizes

7 TCP vs Sliding Window on Direct Link Network capacity varies –Other traffic competes for most links –Requires (global) congestion control strategy Why not implement more functionality in IP, e.g. ordering guarantees or congestion control ?

8 End-to-End Argument A function should not be provided at a given layer unless it can be completely and correctly implemented at that layer In-order delivery: hop-by-hop ordering guarantee is not robust to path changes or multiple paths A B C D E F

9 End-to-End Argument Congestion control –Should be stopped at source –But network can provide feedback A B C D E F 100Mbps 10Mbps 1Mbps 100Mbps 5Mbps 1Mbps 100Mbps 5Mbps 10Mbps blue should get 9Mbps, but gets only 5Mbps with hop-by-hop drops

10 TCP Model of Use Connection setup via 3-way handshake Data transport –Sender writes some data –TCP Breaks data into segments Sends each segment over IP Retransmits, reorders, and removes duplicates as necessary –Receiver reads some data Teardown through 4-step exchange

11 TCP Connection Setup TCP Connection Setup via 3-way Handshake –J and K are (different) sequence numbers for messages –Sequence numbers need not start at zero Active participant (client) Passive participant (server) SYN J SYN K ACK J+1 ACK K+1

12 TCP Data Transport Model Data broken into segments –Limited by maximum segment size (MSS) –Defaults to 536 bytes –Negotiable during connection setup –Typically set to MTU of directly connected network minus size of IP and TCP headers (40 bytes) Three events send segment –>= MSS bytes of data ready to be sent –Explicit PUSH operation by application –Periodic timeout

13 TCP Connection Teardown TCP connection teardown in 4 steps –Either client or server can initiate connection teardown –FIN is associated with sequence number space ClientServer FIN J FIN K ACK K+1 ACK J+1 active close passive close closes connection

14 TCP Segment Header & Pseudo-Header source portdestination port sequence number ACK sequence number hdrlenadvertised window 0 flags TCP checksumurgent pointer options (variable) 410 source IP address destination IP address TCP length6 (TCP) 8

15 TCP Segment Header 16-bit source and destination ports 32-bit send and ACK sequence numbers 4-bit header length in 4-byte words (minimum 5)

16 TCP Segment Header Six 1-bit flags –URG: segment contains urgent data –ACK: ACK sequence number is valid –PSH: do not delay delivery of data –RST: reset connection (rejection or abnormal termination); no sequence number associated –SYN: synchronize segment for setup –FIN: final segment for teardown