《计算机网络——自顶向下方法与Internet特色》幻灯片Lecture31

• TCP socket identified by 4-tuple:
– – – – source IP address source port number dest IP address dest port number
• Server host may support many simultaneous TCP sockets:
Multiplexing at send host: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing)
application transport network link physical
Chapter 3: Transport Layer
Our goals: • understand principles behind transport layer services:
– multiplexing/demulti plexing – reliable data transfer – flow control – congestion control
3.2 Multiplexing/demultiplexing
Demultiplexing at rcv host: delivering received segments to correct socket
= socket = process P1 P1 P2 P4 application transport network link physical
3.1.2 Internet transport-layer protocols
• TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) • Segment and Datagram (also used in Network layer) • Internet’s Network Layer protocol ---- IP (Internet Protocol) . IP provides logical communication between hosts. The IP service model is a best-effort delivery service (尽力而为交互服务). • The most fundamental responsibility of UDP and TCP is to extend IP’s delivery service between two end system to a delivery service between two processes running on the end systems. • Extending host-to-host delivery to process-to-process delivery is called transport layer multiplexing (多路复用) and demultiplexing (多路分解).
DatagramSocket mySocket1 = new DatagramSocket(99111); DatagramSocket mySocket2 = new DatagramSocket(99222); • When host receives UDP segment: – checks destination port number in segment – directs UDP segment to socket with that port number • IP datagrams with different source IP addresses and/or source port numbers directed to same socket
• UDP socket identified by two-tuple:
(dest IP address, dest port number)
2、Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428);
network data link physical application transport network data link physical
3.1.1 Transport vs. network layer
• network layer: logical communication between hosts • transport layer: logical communication between processes
application transport network data link physical network data link physical
network data link physical
network data link physical
network data link physical
• 3.1 Transport-layer services • 3.2 Multiplexing and demultiplexing • 3.3 Connectionless transport: UDP • 3.4 Principles of reliable data transfer
• 3.5 Connection-oriented transport: TCP – segment structure – reliable data transfer – flow control – connection management • 3.6 Principles of congestion control • 3.7 TCP congestion control
network data link physical application transport network data link physical
Csport-layer services • 3.2 Multiplexing and demultiplexing • 3.3 Connectionless transport: UDP • 3.4 Principles of reliable data transfer
• 3.5 Connection-oriented transport: TCP – segment structure – reliable data transfer – flow control – connection management • 3.6 Principles of congestion control • 3.7 TCP congestion control
3.1 Transport services and protocols
• provide logical communication between app processes running on different hosts • transport protocols run in end systems – send side: breaks app messages into segments, passes to network layer – rcv side: reassembles segments into messages, passes to app layer • more than one transport protocol available to apps – Internet: TCP and UDP
P2 P3 P1 P1
SP: 6428 DP: 9157 SP: 9157
SP: 6428 DP: 5775 SP: 5775
client IP: A
DP: 6428
server IP: C
DP: 6428
Client
IP:B
SP provides “return address”
3、Connection-oriented demux
3.1.2 Internet transport-layer protocols
• reliable, in-order delivery (TCP) – congestion control – flow control – connection setup • unreliable, unordered delivery: UDP – no-frills extension of “best-effort” IP • services not available: – delay guarantees – bandwidth guarantees
– relies on, enhances, network layer services Household analogy: 12 kids sending letters to 12 kids • processes = kids • app messages = letters in envelopes • hosts = houses • transport protocol = Ann and Bill • network-layer protocol = postal service
32 bits source port # dest port #
other header fields
application data (message)
TCP/UDP segment format
2、Connectionless demultiplexing