《计算机网络 COMPUTER NETWORKS》课程教学资源（PPT课件讲稿）Chapter 18 互联网协议 Internet Protocols（IP）

COMPUTER NETWORKS Chapter 18 Internet protocols

1 COMPUTER NETWORKS Chapter 18 Internet Protocols

Protocol functions Small set of functions that form basis of all protocols Not all protocols have all functions Reduce duplication of effort May have same type of function in protocols at different levels Encapsulation(封装) Fragmentation and reassembly(分片和重装) Connection control(连接控制) Ordered delivery(顺序交付) Flow contro(流量控制 Error control(差错控制) Addressing(寻址) Multiplexing(复用) Transmission services(传输服务)

2/76 Protocol Functions • Small set of functions that form basis of all protocols • Not all protocols have all functions - Reduce duplication of effort - May have same type of function in protocols at different levels • Encapsulation(封装) • Fragmentation and reassembly (分片和重装) • Connection control (连接控制) • Ordered delivery (顺序交付) • Flow control (流量控制) • Error control (差错控制) • Addressing (寻址) • Multiplexing (复用) • Transmission services (传输服务)

Encapsulation Data usually transferred in blocks Protocol data units(PDUs) Each Pdu contains data and control information Some PdUs only control Three categories of control(三类控制字段) Address Of sender and/or receiver Error-detecting code E.g. frame check sequence Protocol control Additional information to implement protocol functions Addition of control information to data is encapsulation(在数据外添加控制信息称之为 封装) Data accepted or generated by entity and encapsulated into PDU Containing data plus control information e.g. TFTP, HDLC, frame relay, ATM, AAL5 Figure 11.15), LLC, IEEE 802.3, EEE802.11

3/76 Encapsulation • Data usually transferred in blocks - Protocol data units (PDUs) - Each PDU contains data and control information - Some PDUs only control • Three categories of control (三类控制字段) • Address - Of sender and/or receiver • Error-detecting code - E.g. frame check sequence • Protocol control - Additional information to implement protocol functions • Addition of control information to data is encapsulation(在数据外添加控制信息称之为 封装) • Data accepted or generated by entity and encapsulated into PDU - Containing data plus control information - e.g. TFTP, HDLC, frame relay, ATM, AAL5 (Figure 11.15), LLC, IEEE 802.3, IEEE 802.11

Fragmentation and Reassembly (Segmentation -OSI) EXchange data between two entities Characterized as sequence of pDUs of some bounded size Application level message in the form of datagram or continuous data stream R∈ eason Lower-level protocols may need to break data up into smaller blocks Communications network may only accept blocks of up to a certain size OATM 53 octets Ethernet 1526 octets olEEE802.11 2304 octets More efficient error control o Smaller retransmission -Faer(公平 oPrevent station monopolizing medium(防止长期霸占) Smaller buffers Provision of checkpoint and restart/recovery operations

4/76 Fragmentation and Reassembly (Segmentation – OSI) • Exchange data between two entities • Characterized as sequence of PDUs of some bounded size • Application level message in the form of datagram or continuous data stream • Reason: - Lower-level protocols may need to break data up into smaller blocks - Communications network may only accept blocks of up to a certain size oATM 53 octets oEthernet 1526 octets oIEEE802.11 2304 octets - More efficient error control oSmaller retransmission - Fairer(公平) oPrevent station monopolizing medium(防止长期霸占) - Smaller buffers - Provision of checkpoint and restart/recovery operations

Disadvantages of Fragmentation Make pdUs as large as possible because PDU contains some control information(分割都要加控制信息) Smaller block, larger overhead PDu arrival generates interrupt Smaller blocks, more interrupts More time processing smaller, more numerous pdus 5/76

5/76 Disadvantages of Fragmentation • Make PDUs as large as possible because - PDU contains some control information(分割都要加控制信息) - Smaller block, larger overhead • PDU arrival generates interrupt - Smaller blocks, more interrupts • More time processing smaller, more numerous PDUs

Reassembly Segmented data must be reassembled into messages More complex if pdUs out of order 6/76

6/76 Reassembly • Segmented data must be reassembled into messages • More complex if PDUs out of order

PDUS and Fragmentation (Copied from chapter 2 fig 2. 4) Application Data Transport I Header Transport rotocol data units Transport Header protocol data units (packets Network Network Header Header

7/76 PDUS and Fragmentation (Copied from chapter 2 fig 2.4)

Connection Control Connectionless data transfer(无连接的传输) Each PDU treated independently -Eg. datagram(数据报) Connection- oriented data transfer(面向连接的传输) Eg. virtual circuit(虚电路) Connection-oriented preferred (even required) for lengthy exchange of data Logical association, or connection, established between entities Three phases occur Connection establishment Data transfer Connection termination May be interrupt and recovery phases to handle errors

8/76 Connection Control • Connectionless data transfer(无连接的传输) - Each PDU treated independently - E.g. datagram(数据报) • Connection-oriented data transfer(面向连接的传输) - E.g. virtual circuit(虚电路) • Connection-oriented preferred (even required) for lengthy exchange of data • Logical association, or connection, established between entities • Three phases occur - Connection establishment - Data transfer - Connection termination • May be interrupt and recovery phases to handle errors

Phases of Connection Oriented Transfer Protocol Protocol Entity Entity C connection red quest Connection acc Data Multiple Acknowledgement exchanges Te erminate-connect ion request Termin te-connection accep

9/76 Phases of Connection Oriented Transfer

Connection establishment Entities agree to exchange data Typically, one station issues connection request Receiving entity accepts or rejects(simple) May include negotiation Syntax, semantics, and timing(语法、语义、定时) Both entities must use same protocol May allow optional features Must be agreed E.g. protocol may specify max PdU size 8000 octets one station may wish to restrict to 1000 octets

10/76 Connection Establishment • Entities agree to exchange data • Typically, one station issues connection request • Receiving entity accepts or rejects (simple) • May include negotiation • Syntax, semantics, and timing(语法、语义、定时) • Both entities must use same protocol • May allow optional features • Must be agreed • E.g. protocol may specify max PDU size 8000 octets; one station may wish to restrict to 1000 octets