计算机网络与互联网(英语)chap16.ppt

1、Chapter 16,Protocols and Protocol Layering,Protocol,Agreement about communication A network protocol or computer communication protocol is a set of rules that specify the format and meaning of messages exchanged between computers across a network Specifies Format of messages (syntax) Meaning of mess

2、ages (semantics) Rules for exchange Procedure for handling problems,Need for Protocols,Hardware is low level Many problems can occur Bits corrupted or destroyed Entire packet lost Packet duplicated Packets delivered out of order Need mechanisms to distinguish among Multiple computers on a network Mu

3、ltiple applications on a computer Multiple copies of a single application on a computer,Set of Protocols,Complexity requires multiple protocols, each of which manages a part of the problem A set of related protocols that are designed for compatibility is called a protocol suite Protocol suite design

4、ers Analyze communication problem Divide problems into subproblems Design a protocol for each subproblem Work together,Set of Protocols,Each protocol solves part of communication problem Protocols are implemented by protocol software Known as Protocol suite Protocol family Designed in layers,Plan fo

5、r Protocol Design,Intended for protocol designers Divides protocols into layers Each layer devoted to one subproblem Example: ISO 7-layer reference model,Illustration of the 7-Layer Model,ISO - International Organization for Standards Defined early Now somewhat dated Does not include internet layer!

6、,ISO Layers,Layer 1: Physical Underlying hardware- such as RS-232 or Ethernet (chap5) Layer 2: Data Link (media access) Hardware frame definitions: Format of data in frames and delivery of frames through network interface (chap7:frame, stuffing, checksum) Layer 3: Network Packet forwarding: Address

7、assignment and data delivery across a physical network (chap9:addressing chap13:forwarding) Layer 4: Transport Reliability: Reliable delivery of data between computers (chap16-17),ISO Layers(continued),Layer 5: Session Login and passwords: Management of sessions such as login to a remote computer La

8、yer 6: Presentation Data representation:Common formats for representation of data Layer 7: Application Services for common applications: Application-specific protocols such as FTP and SMTP (electronic mail),Layers and Protocol Software,Protocol software follows layering model One software module per

9、 layer Modules cooperate Incoming or outgoing data passes from one module to another Entire set of modules known as stack,Layered Software and Stacks,Two constraints: Software for each layer depends only on the services of the software provided by lower layers Software at layer n at the destination

10、receives exactly the same protocol message sent by layer n at the sender These constraints mean that protocols can be tested independently and can be replaced within a protocol stack,Layers and Packet Headers,On the sender, each layer: Accepts an outgoing message from the layer above Adds a header a

11、nd other processing Passes resulting message to next lower layer,Layers and Packet Headers,On the receiver, each layer: Receives an incoming message from the layer below Removes the header for that layer and performs other processing Passes the resulting message to the next higher layer,Scientific L

12、ayering Principle,Application data need to be transformed into packets (the basic transmission unit) Peer entities in layer N+1 communicate with each other by communication services provided by layer N (below them) Each layer has specific tasks and functionality. It also provides services to the lay

13、ers above and below it Peer entities communicate by exchanging messages,Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source,Illustration of Layering Principle,Peer-to-peer communications,In order to travel data from the so

14、urce to the destination, each layer of the OSI model at the source must communicate with its peer layer at the destination. The protocols of each layer exchange information, called protocol data units (PDUs). Each layer of communication on the source computer communicates with a layer-specific PDU,

15、and with its peer layer on the destination computer.,Protocol Data Units,Different layers in the OSI model have different groupings for data(PDU): Application layer deals with APDU presentation layer deals with PPDU session layer deals with SPDU Transport layer deals with segments Network layer enca

16、psulates segments into packets Data Link layer encapsulates packets into frames Physical layer converts frames to bit streams,PDU,Data Flow,Protocol Techniques,Need to ensure reliable delivery of a block of data from one computer to another For bit corruption - Data link layer (chap4:detect error) P

17、arity Checksum CRC For out-of-order delivery(chap13:best path, chap 15:connectionless network) Transport layer:Sequence numbers Duplication Sequence numbers,Protocol Techniques (continued),For lost packets Positive acknowledgement and retransmission(带重发的确认) For replay(重播) (excessive delay) Unique me

18、ssage ID For data overrun (数据过载) Flow control (流控制),Out-of-order Delivery,Packets may be delivered out of order - especially in systems that include multiple networks Out of order delivery can be detected and corrected through sequencing Sender attaches sequence number to each outgoing packet Receiv

19、ed uses sequence numbers to put packets in order and detect missing packets List,Duplicate Delivery,Packets may be duplicated during transmission Can caused by malfunctioning hardware Sequencing can be used to. Detect duplicate packets with duplicated sequence numbers Discard those duplicate packets

20、,Retransmission,Protocols use positive acknowledgment with retransmission to detect and correct lost packets Receiver sends short message acknowledging receipt of packets Sender infers lost packets from missing acknowledgments Sender retransmits lost packets Sender sets timer for each outgoing packe

21、t Saves copy of packet If timer expires before acknowledgment is received, sender can retransmit saved copy Protocols define upper bound on retransmission to detect unrecoverable network failure,Replay,One source of delay in a packet switching system arises from the store-and-forward approach-queue

22、Extraordinary delays can lead to replay errors Replay means that an old, delayed packet affect later communication For example:,Two computers agree to communicate at 1 PM One computer sends a sequence of ten packets to the other A hardware problem causes packet 3 to be delayed Router change to avoid

23、 the hardware problem Protocol software on the sending computer retransmitting packet 3, and the remaining packets are transmitted without error At 1:05 PM the two computers agree to communicate again After the second packet arrives, the delayed copy of packet 3 arrives from the earlier session Pack

24、et 3 arrives from the second session,Replay,Sufficiently delayed packets may be inserted into later sessions Protocols attach session number to each packet in a protocol session to differentiate packets from different sessions,Flow Control,Needed because Sending computer system faster than receiving

25、 computer Sending application faster than receiving application Protocols use flow control mechanisms through which receiver can control rate of data transmission Related to buffering Two forms Stop-and-go (简单停等) Sliding window (滑动窗口),Stop-And-Go Flow Control,Sending Side Transmits one packet Waits

26、for signal from receiver Receiving side Receives and consumes packets Transmits signal to sender Can be very inefficient of network bandwidth if delivery time is large,发送方,接收方,Example,Network:a packet size of 1000 octets, a throughput capacity of 2 Mbps, a delay of 50 millisecond The hardware can tr

27、ansport 2 Mbps from one computer to another After transmitting a packet, the sender must wait 100 msec before sending another packet(50 msec for the packet to reach the receiver and 50 msec for an acknowledgement to travel back) The maximum rate at which data can be sent using stop-and-go is one pac

28、ket every 100 millisecond When expressed as a bit rate, the maximum rate stop-and-go can achieved is 80,000bps,which is only 4% of the hardware capacity,Sliding Window Flow Control,Allows sender to transmit multiple packets before receiving an acknowledgment Number of packets that can be sent is def

29、ined by the protocol and called the window As acknowledgments arrive from the receiver, the window is moved along the data packets; hence sliding window,Sliding Window Flow Control,Receiving side Establishes multiple buffers and informs sender Sending side Transmits packets for all available buffers

30、 Only waits if no signal arrives before transmission Receiving side Sends signals as packets arrive,Illustration of SlidingWindow on Sending Side,Window tells how many packets can be sent Window moves as acknowledgements arrive,Comparison of Flow Control,Stop-and-go Slow Useful only in special cases

31、 Sliding window Fast Needed in high-speed network,time : 8N 2N+,N,N,N,N,N,N,N,N,N,Why Sliding Window?,Simultaneously Increase throughput Control flow Speedup Tw = min(B, TG * W) where B is underlying hardware bandwidth TW is sliding window throughput TG is stop-and-go throughput W is the window size

32、,Congestion,Fundamental problem in networks Analogous to congestion on a highway Principal cause of delay Network congestion arises in network systems that include multiple links If input to some link exceeds maximum bandwidth, packets will queue up at connection to that link Eventually, packets wil

33、l be discarded and packets will be retransmitted,Illustration of ArchitectureThat Can Experience Congestion,Multiple sources Bottleneck,Congestion and Loss,Modern network hardware works well; most packet loss results from congestion, not from hardware failure,Rate control (e.g. temporarily reducing Windows