1、Lecture 5: The Transport Layer (Basic Principles),Advanced Computer Network,Yang Qin Department of Computer Science Shenzhen Graduate School Harbin Institute of Technology,The Transport Service,Services Provided to the Upper Layers Transport Service Primitives Berkeley Sockets An Example of Socket P

2、rogramming: An Internet File Server,Services Provided to the Upper Layers,The network, transport, and application layers.,Transport Service Primitives,The primitives for a simple transport service.,Transport Service Primitives (2),The nesting of TPDUs, packets, and frames.,Transport Service Primitiv

3、es (3),A state diagram for a simple connection management scheme. Transitions labeled in italics are caused by packet arrivals. The solid lines show the clients state sequence. The dashed lines show the servers state sequence.,Berkeley Sockets,The socket primitives for TCP.,Socket Programming Exampl

4、e:Internet File Server,Client code using sockets.,6-6-1,Socket Programming Example:Internet File Server (2),Server code using sockets.,Elements of Transport Protocols,Addressing Connection Establishment Connection Release Flow Control and Buffering Multiplexing Crash Recovery,Transport Protocol,(a)

5、Environment of the data link layer. (b) Environment of the transport layer.,Addressing,TSAPs, NSAPs and transport connections.,Connection Establishment,How a user process in host 1 establishes a connection with a time-of-day server in host 2.,Connection Establishment (cont.),Three protocol scenarios

6、 for establishing a connection using a three-way handshake. CR denotes CONNECTION REQUEST. (a) Normal operation, (b) Old CONNECTION REQUEST appearing out of nowhere. (c) Duplicate CONNECTION REQUEST and duplicate ACK.,Connection Release,Abrupt disconnection with loss of data.,Connection Release (2),

7、The two-army problem.,Connection Release (3),Four protocol scenarios for releasing a connection. (a) Normal case of a three-way handshake. (b) final ACK lost.,6-14, a, b,Connection Release (4),(c) Response lost. (d) Response lost and subsequent DRs lost.,6-14, c,d,Flow Control and Buffering,(a) Chai

8、ned fixed-size buffers. (b) Chained variable-sized buffers. (c) One large circular buffer per connection.,Multiplexing,(a) Upward multiplexing. (b) Downward multiplexing.,Crash Recovery,Different combinations of client and server strategy.,A Simple Transport Protocol,The Example Service Primitives T

9、he Example Transport Entity The Example as a Finite State Machine,The Example Transport Entity,The network layer packets used in our example.,The Example Transport Entity (2),Each connection is in one of seven states: Idle Connection not established yet. Waiting CONNECT has been executed, CALL REQUE

10、ST sent. Queued A CALL REQUEST has arrived; no LISTEN yet. Established The connection has been established. Sending The user is waiting for permission to send a packet. Receiving A RECEIVE has been done. DISCONNECTING a DISCONNECT has been done locally.,The Example as a Finite State Machine (2),The

11、example protocol in graphical form. Transitions that leave the connection state unchanged have been omitted for simplicity.,The Real-Time Transport Protocol (2),The RTP header.,Performance Issues,Performance Problems in Computer Networks Network Performance Measurement System Design for Better Perfo

12、rmance Fast TPDU Processing Protocols for Gigabit Networks,Performance Problems in Computer Networks,The state of transmitting one megabit from San Diego to Boston (a) At t = 0, (b) After 500 sec, (c) After 20 msec, (d) after 40 msec.,Network Performance Measurement,The basic loop for improving network performance. Measure relevant network parameters, performance. Try to understand what is going on. Change one parameter.,System Design for Better Performance,Rules: CPU speed is more important than network speed. Reduce packe