Data Transfer and termination Both data and control information exchanged e.g. flow control, error control Data flow and acknowledgements may be in one or both directions One side may send termination request Or central authority might terminate 1176
11/76 Data Transfer and Termination • Both data and control information exchanged - e.g. flow control, error control • Data flow and acknowledgements may be in one or both directions • One side may send termination request • Or central authority might terminate
Sequencing Many connection-oriented protocols use sequencing e.g. HDLC, IEEE 802.11 PDUs numbered sequentially(给数据报编序列号) Each side keeps track of outgoing and incoming numbers Supports three main functions Ordered delivery Flow control Error control Not found in all connection-oriented protocols E.g. frame relay and ATm All connection-oriented protocols include some way of identifying connection Unique connection identifier Combination of source and destination addresses 1276
12/76 Sequencing • Many connection-oriented protocols use sequencing - e.g. HDLC, IEEE 802.11 • PDUs numbered sequentially (给数据报编序列号) • Each side keeps track of outgoing and incoming numbers • Supports three main functions - Ordered delivery - Flow control - Error control • Not found in all connection-oriented protocols - E.g.frame relay and ATM • All connection-oriented protocols include some way of identifying connection - Unique connection identifier - Combination of source and destination addresses
Ordered Delivery PDUs may arrive out of order Different paths through network Pdu order must be maintained Number PDUs sequentially Easy to reorder received PDUs Finite sequence number field(有限的序列号字段) - Numbers repeat modulo maximum number(以最大序列号为模) Maximum sequence number greater than maximum number of PDUs that could be outstanding(序列号的最大值要比未经确认的 PDU的最大数量要大) In fact, maximum number may need to be twice maximum number of PdUs that could be outstanding oe.g, selective-repeat ARQ(在选择重发ARQ中,序列号必须是窗口 大小的两倍) 1376
13/76 Ordered Delivery • PDUs may arrive out of order - Different paths through network • PDU order must be maintained • Number PDUs sequentially • Easy to reorder received PDUs • Finite sequence number field(有限的序列号字段) - Numbers repeat modulo maximum number(以最大序列号为模) - Maximum sequence number greater than maximum number of PDUs that could be outstanding(序列号的最大值要比未经确认的 PDU的最大数量要大) - In fact, maximum number may need to be twice maximum number of PDUs that could be outstanding oe.g. selective-repeat ARQ(在选择重发ARQ中，序列号必须是窗口 大小的两倍)
Flow Control Performed by receiving entity to limit amount or rate of data sent Stop-and-wait Each Pdu must be acknowledged before next sent Cred信贷) Amount of data that can be sent without acknowledgment E.g. HDLC sliding-window Must be implemented in several protocols Network traffic control Buffer space Application overflow oE.g. waiting for disk access 14/76
14/76 Flow Control • Performed by receiving entity to limit amount or rate of data sent • Stop-and-wait - Each PDU must be acknowledged before next sent • Credit(信贷) - Amount of data that can be sent without acknowledgment - E.g. HDLC sliding-window • Must be implemented in several protocols - Network traffic control - Buffer space - Application overflow oE.g. waiting for disk access
Error control Guard against loss or damage Error detection and retransmission Sender inserts error-detecting code in PDU o Function of other bits in pdu Receiver checks code on incoming PDU If error, discard If transmitter doesn't get acknowledgment in reasonable time retransmit Error-correction code Enables receiver to detect and possibly correct errors Error control is performed at various layers of protocol Between station and network Inside network 15/76
15/76 Error Control • Guard against loss or damage • Error detection and retransmission - Sender inserts error-detecting code in PDU oFunction of other bits in PDU - Receiver checks code on incoming PDU - If error, discard - If transmitter doesn’t get acknowledgment in reasonable time, retransmit • Error-correction code • Enables receiver to detect and possibly correct errors • Error control is performed at various layers of protocol - Between station and network - Inside network