1 Chapter 3 Transport Layer Computer Networking: A Top Down Approach 4 th edition. Jim Kurose, Keith Ross Addison-Wesley, July A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

2 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP r 3.4 Principles of reliable data transfer r 3.5 Connection-oriented transport: TCP m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

3 TCP Connection Establishment Three-way handshake Step 1: client host sends TCP SYN segment to server m specifies initial seq # m no data Step 2: server host receives SYN, replies with SYN+ACK segment m server allocates buffers m specifies server initial seq. # Step 3: client receives SYN+ACK, replies with ACK segment, which may contain data ACK

4 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 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)

5 TCP Connection Termination Step 1: client end system sends TCP FIN control segment to server Step 2: server receives FIN, replies with ACK. When server is done sending all data, it closes connection by sending FIN. Step 3: client receives FIN, replies with ACK. m Enters “timed wait” - will respond with ACK to received FINs Step 4: server, receives ACK. Connection closed. client FIN server ACK FIN close closed timed wait

6 TCP Connection Management (cont) TCP client lifecycle TCP server lifecycle SYN SYN + ACK ACK FIN client FIN serv er ACK FIN close closed timed wait

7 TCP state-transition diagram CLOSED LISTEN SYN_RCVDSYN_SENT ESTABLISHED CLOSE_WAIT LAST_ACKCLOSING TIME_WAIT FIN_WAIT_2 FIN_WAIT_1 Passive openClose Send/SYN SYN/SYN + ACK SYN + ACK/ACK SYN/SYN + ACK ACK Close/FIN FIN/ACKClose/FIN FIN/ACK ACK + FIN/ACK Timeout after two segment lifetimes FIN/ACK ACK Close/FIN Close CLOSED Active open/SYN

8 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP r 3.4 Principles of reliable data transfer r 3.5 Connection-oriented transport: TCP m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

9 Principles of Congestion Control Congestion: r informally: “too many sources sending too much data too fast for network to handle” m the load on the network is more than its capacity r different from flow control! r manifestations: m lost packets (buffer overflow at routers) m long delays (queueing in router buffers) r a top-10 problem!

9/13/05 10 Network Performance

11 Approaches towards congestion control End-end congestion control: r no explicit feedback from network r congestion inferred from end-system observed loss, delay r approach taken by TCP Network-assisted congestion control: r routers provide feedback to end systems m single bit indicating congestion m explicit rate sender should send at Two broad approaches towards congestion control:

12 Network-Assisted Congestion Control

13 TCP Congestion Control r Approach m Limit the rate at which sender sends data as a function of perceived congestion in the network r Issues m How does it perceive that there is congestion in the network? m What algorithm does it use to change the sending rate as a function of the perceived level of congestion?