《计算机网络与通讯》课程教学资源（PPT课件讲稿，英文版）Chapter 3 Transport Layer

Chapter 3 Transport Layer Chapter goals: Chapter Overview: o understand principles o transport layer services behind transport layer o multiplexing/demultiplexing services: o connectionless transport: UDP o multiplexing/demultiplex Ing o principles of reliable data transfer o reliable data transfer o flow control o connection-oriented transport o congestion control TCP g instantiation and o reliable transfer implementation in the f| ow contro Internet o connection management o principles of congestion contro o TCP congestion control 3: Transport Layer 3a-1

3: Transport Layer 3a-1 Chapter 3: Transport Layer Chapter goals:  understand principles behind transport layer services:  multiplexing/demultiplex ing  reliable data transfer  flow control  congestion control  instantiation and implementation in the Internet Chapter Overview:  transport layer services  multiplexing/demultiplexing  connectionless transport: UDP  principles of reliable data transfer  connection-oriented transport: TCP  reliable transfer  flow control  connection management  principles of congestion control  TCP congestion control

Transport services and protocols o provide logical communication lication between app processes running on different hosts data link networ data link o transport protocols run in end systems a transport vs network layer be, physical i network data link services. physical o network layer: data transfer 受 between end systems g transport layer: data application transfer between processes networ data link o relies on, enhances network ayer services 3: Transport Layer 3a-2

3: Transport Layer 3a-2 Transport services and protocols  provide logical communication between app’ processes running on different hosts  transport protocols run in end systems  transport vs network layer services:  network layer: data transfer between end systems  transport layer: data transfer between processes  relies on, enhances, network layer services application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link network physical data link physical

Transport-layer protocols Internet transport services: lication g reliable. in-order unicast data link networ delive ry (TCp) data link o congestion o flow contro o connection setup be, physical i network data link a unreliable (best-effort physical unordered unicast or 受 multicast delivery: UDP o services not available application o real-time networ data link o bandwidth guarantees o reliable multicast 3: Transport Layer 3a-3

3: Transport Layer 3a-3 Transport-layer protocols Internet transport services:  reliable, in-order unicast delivery (TCP)  congestion  flow control  connection setup  unreliable (“best-effort”), unordered unicast or multicast delivery: UDP  services not available:  real-time  bandwidth guarantees  reliable multicast application transport network data link physical application transport network data link physical network data link physical network data link physical network data link physical network data link network physical data link physical

Multiplexing/demultiplexing Recall: segment-unit of data Demultiplexing: deliverin exchanged between received segments to transport layer entities correct app layer processes o aka TPDU: transport protocol data unit receiver P3 application-layer P4 M(M data applicatio segment trarsport P2 eader M M net work applcation application segment→匠 M transport transport Hnl Metwork network 3: Transport Le 3a-4

3: Transport Layer 3a-4 application transport network M P2 application transport network Multiplexing/demultiplexing Recall: segment - unit of data exchanged between transport layer entities  aka TPDU: transport protocol data unit receiver Ht Hn Demultiplexing: delivering received segments to correct app layer processes segment segment M application transport network P1 M M M P3 P4 segment header application-layer data

Multiplexing/demultiplexing Multiplexing gathering data from multiple 32 bits app processes, enveloping data with header later used source port dest port for demultiplexing other header fields multiplexing/demultiplexing g based on sender receiver port numbers, IP addresses application o source, dest port #s in data each segment ( message) o recall: well-known port numbers for specific applications TCP/UDP segment format 3: Transport Layer 3a-5

3: Transport Layer 3a-5 Multiplexing/demultiplexing multiplexing/demultiplexing:  based on sender, receiver port numbers, IP addresses  source, dest port #s in each segment  recall: well-known port numbers for specific applications gathering data from multiple app processes, enveloping data with header (later used for demultiplexing) source port # dest port # 32 bits application data (message) other header fields TCP/UDP segment format Multiplexing:

Multiplexing/demultiplexing: examples source port:x Web client host a dest. port: 23 server b host c ource port: dest. port: x I Source IP: C Source IP: C Dest ip: B Dest ip: B source port:y source port:x port use: simple telnet app dest. port: 80 dest. port: 80 Source IP:A Dest ip: B Web Web client source port:x server b host a dest port: 80 port use: Web server 3: Transport Le 3a-6

3: Transport Layer 3a-6 Multiplexing/demultiplexing: examples host A server B source port: x dest. port: 23 source port:23 dest. port: x port use: simple telnet app Web client host A Web server B Web client host C Source IP: C Dest IP: B source port: x dest. port: 80 Source IP: C Dest IP: B source port: y dest. port: 80 port use: Web server Source IP: A Dest IP: B source port: x dest. port: 80

UDP: User datagram Protocol [RFC 768 口“ no frills"" bare bones" Internet transport Why is there a UDP? protocol o no connection 0" best effort"service, UDP establishment(which can segments may be add delay o lost g simple: no connection state o delivered out of order at sender receiver to app 口sma| segment header g connectionless g no congestion control: UDP o no handshaking between can blast away as fast as UDP sender receiver desired o each UDP segment handled independently of others 3: Transport Layer 3a-7

3: Transport Layer 3a-7 UDP: User Datagram Protocol [RFC 768]  “no frills,” “bare bones” Internet transport protocol  “best effort” service, UDP segments may be:  lost  delivered out of order to app  connectionless:  no handshaking between UDP sender, receiver  each UDP segment handled independently of others Why is there a UDP?  no connection establishment (which can add delay)  simple: no connection state at sender, receiver  small segment header  no congestion control: UDP can blast away as fast as desired

UDP: more o offen used for streaming multimedia apps 32 bits o loss tolerant Length source port dest port o rate sensitive bytes of UDP length checksum 口 other UDP uses segment IncludIng (why? ) eader O DNS O SNMP Application g reliable transfer over UDP data add reliability at message applicaTion layer o application-specific UDP segment format error recover 3: Transport Layer 3a-8

3: Transport Layer 3a-8 UDP: more  often used for streaming multimedia apps  loss tolerant  rate sensitive  other UDP uses (why?):  DNS  SNMP  reliable transfer over UDP: add reliability at application layer  application-specific error recover! source port # dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header

UDP checksum Goal: detect"errors"(e., flipped bits)in transmitted segment Sender. Receiver o treat segment contents 口 compute checksum of as sequence of 16-bit received segment Integers o check if computed checksum 口 checksum: addition(1s equals checksum field value complement sum)of o NO-error detected segment contents O yES- no error detected 口 sender puts checksum But maybe errors value into UDP checksum nonethless? More later field 3: Transport Layer 3a-9

3: Transport Layer 3a-9 UDP checksum Sender:  treat segment contents as sequence of 16-bit integers  checksum: addition (1’s complement sum) of segment contents  sender puts checksum value into UDP checksum field Receiver:  compute checksum of received segment  check if computed checksum equals checksum field value:  NO - error detected  YES - no error detected. But maybe errors nonethless? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment

Principles of reliable data transfer o important in app. transport, link layers o top-10 list of important networking topics COO sending recelver process process data data Reliable channel rat send()↓四 a ata deliver_data( reliable data reliable data transfer protocol transfer protocol (sending side) (receiving side uand0四[wxt- dt rcv( Unreliable channe (a) provided service (b) service implementation o characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt 3: Transport Layer 3a-10

3: Transport Layer 3a-10 Principles of Reliable data transfer  important in app., transport, link layers  top-10 list of important networking topics!  characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)