6. The Transport Layer and protocols

Transcription

1 6. The Transport Layer and protocols 1 Dr.Z.Sun

2 Outline Transport layer services Transmission Control Protocol Connection set-up and tear-down Ports and Well-know-ports Flow control and Congestion control User Datagram Protocol (UDP) The Socket Interface The Socket API Socket procedures 2 Dr.Z.Sun

3 The transport layer services To provide reliable, cost effective data transport from source to destination machine. Provide addresses to different applications in the same machine Two types of network service: connection oriented (establishment, data transfer and release) and connectionless (similar to the network layer connectionless service) Flow control Quality of service 3 Dr.Z.Sun

4 Transport segment, packet and frame 4 Dr.Z.Sun

5 Addressing 5 Dr.Z.Sun

6 Transmission Control Protocol (TCP) Service: reliable, ordered bytestream delivery connection-oriented Connection set-up: active open: threeway handshake passive open local connection name Connection tear-down: clean termination abortive termination (option) LCN: Logical Connection Name client user A active open open LCN timewait closed close request TCP network SYN SYN ACK SYN ACK FIN ACK 6 Dr.Z.Sun FIN FIN FIN ACK TCP (listen) server passive open open LCN user B close indication (remaining data + ACKs) close request closed 6

7 TCP Segment Format (31) source port destination port sequence number acknowledgement number data offset reserved u a p r s f checksum window urgent pointer urg ack psh rst syn fin check urgent ptr field check ack num. field push reset synchronise seq num. no more data options options padding data Well-known ports (RFC 1700) 20: FTP data 21: FTP control 23: telnet 25: SMTP 53: domain server 80: www 42: host name server 110: POP3 79: finger 7 Dr.Z.Sun 8

8 The TCP segment Every byte on a TCP connection has its own 32-bit Sequence number A segment consists of a fixed 20-bytes header (plus an option): (536+20=556) to (65, =65,495) bytes Fit into 65,535 byte IP payload Fit into Max Transfer Unit (MTU) of other networks Segmentation and reassembly Variable-size sliding window is used Acknowledgement up to (acknowledge number 1) received Flow and congestion control 8 Dr.Z.Sun

9 TCP byte stream A TCP connection is a bytes stream, not a message stream. Sending process of writing 4 x 512 bytes can be delivered as 4x512, byte-chunks 2x1024 byte-chunks, or 1x2048 byte-chunk 9 Dr.Z.Sun

10 TCP congestion control No congestion control in IP Layers above IP must have congestion control: if not, network will collapse! TCP has built-in congestion control: user has no control over the congestion control behaviour all TCP implementations have (roughly) the same congestion control algorithm TCP results in (roughly) fair share of the network Real-time applications (e.g. using RTP): congestion control must be part of application 10 Dr.Z.Sun 9

11 Why congestion control and avoidance? There are two potential problems: Network capacity Receiver capacity To deal with the problem, need two windows: Receiver s window Congestion window Need slow start Need a third parameter: slow start threshold (ssthresh) 11 Dr.Z.Sun

12 Slow start & congestion avoidance TCP should maximise throughput Bandwidth delay product: what is the RTT? what is the bandwidth? Network load changes: time of day, day of week new applications Dynamically adaptive: slow start congestion avoidance slow start: cwnd = 1, ssthresh = 64K on congestion: ssthresh = wnd/2 cwnd = 1 on ACK: if cwnd <= ssthresh double cwnd else if (cwnd < rwnd) cwnd = cwnd + 1 (congestion avoidance) Conservative back-off 12 Dr.Z.Sun 10

13 Illustrate slow start & congestion control 13 Dr.Z.Sun 11

14 TCP time management RTT = a RTT + (1 - a) M typically a = 7/8 D = a D + (1 - a) RTT - M Timeout = RTT + 4 * D 14 Dr.Z.Sun

15 Wireless TCP (indirect TCP) 15 Dr.Z.Sun

16 User Datagram Protocol (UDP) Connectionless, unreliable, unordered, datagram service No error control No flow control No congestion control Port numbers Must be used for real-time data: TCP automatic congestion control and flow control behaviour is unsuitable (31) source port destination port length checksum data 16 Dr.Z.Sun 12

17 The Real-Time Transport Protocol (a) The position of RTP in the protocol stack. (b) Packet nesting. 17 Dr.Z.Sun

18 The Real-Time Transport Protocol (2) Ver: version of the protocol; P: Padding; X: extension header; CC: number of contribution sources; Payload type: encoding algorithm used; Timestamp: when the first sample in the packet was made; Synchronisation source identifier: which stream the packet belong to; Contribution source identifier: used when mixers are present. 18 Dr.Z.Sun

19 The socket API It is an application interface with transport protocol software is known as an Application Program Interface (API). The socket API (abbreviated as socket) is available in many operating systems (such Microsoft s Windows, Sun Microsystems Unix systems and Linux systems ). The socket API become the de facto standard in the industry. The sockets are also integrated with Unix I/O that an application communicates through a socket similar to the way the application transfers data to or from a file. 19 Dr.Z.Sun 13

20 Socket procedures sock_descriptor = socket (protocol _family, type, protocol); bind(socket, local address, addresslen); listen (socket, queue_size); connect(socket, server_address, server_address_len); newsock = accept(socket, sock_address, sock_address_len); send (socket, data, length, flags); recv (socket, buffer length, flags); close(sock_descriptor ); There are also many other procedures available to get information about the networks. 20 Dr.Z.Sun 14

21 An example of a client and server 21 Dr.Z.Sun 15

22 Socket Programming Example: Internet File Server Client code using sockets. 22 Dr.Z.Sun

23 Socket Programming Example: Internet File Server (2) Client code using sockets. 23 Dr.Z.Sun

24 Summary Transport layer services The Internet TCP Connection set-up and tear-down Ports and Well-know-ports Flow control and Congestion control The Internet UDP The Socket API 24 Dr.Z.Sun 16