Getting it there in one piece

Transcription

1 Getting it there in ne piece Service mdel and implementatin Principles f reliable data transfer CS242 Cmputer Netwrks Department f Cmputer Science Wellesley Cllege Reliable transfer 9-2 Terminlgy Finite state machines rdt_send(): called frm abve t pass data t deliver deliver_data(): called by rdt t deliver data t upper Event causing the transitin Unique start state udt_send(): called by rdt t transfer packet ver unreliable channel t receiver rdt_rcv(): called when packet arrives n receiving side f channel N actin taken when event ccurs Actin taken when event ccurs Reliable transfer 9-3 Reliable transfer 9-4 1

2 Reliable transfer ver a perfectly reliable channel*: rdt1.0 *N bit errrs, n lss f packets, and everything arrives in the rder sent. Reliable transfer 9-5 Underlying channel may crrupt bits We cntinue t assume that all transmitted packets arrive in the rder sent. We use an apprach similar t prtcl used n the telephne t give directins. Prtcl requires Errr detectin Receiver feedback in the frm f ACKs and NAKs Retransmissin *In netwrking wrld: reliable data transfer prtcls based n retransmissins are knw as ARQ (Autmatic Repeat request) prtcls. Reliable transfer 9-6 Reliable transfer ver channel with bit errrs: rdt2.0 Stp-and-wait prtcls When the sender is in the wait-fr-ack-r-nak state, it cannt get mre data frm the upper layer*. Reliable transfer 9-7 *Actually, this prtcl suffers frm an even mre serius flaw. What? Reliable transfer 9-8 2

3 ACK r NAK is crrupted Expressing urselves clearly If speaker didn t understand the OK r Please repeat, she might ask What did yu say? (new packet type). We culd add enugh checksum bits t allw the speaker nt nly t detect, but recver frm bit errrs.* If the speaker s What did yu say? is crrupted, the receiver wuld be very cnfused and prbably ask What did yu say? Or, we culd have the sender simply resend the current packet when it receives a garble ACK r NAK packet. *Wrks prvided channel desn't lse entire packets. Reliable transfer 9-9 Reliable transfer 9-10 Resending the ACK (r NAK) Sender Receiver Create a new field t include the sequence number f the packet Sender Receiver ACK ACK Quack r pkt1? Quack Reliable transfer 9-11 *Fr stp and wait a ne-bit sequence number suffices. Why? Reliable transfer

4 rdt2.1 sender rdt2.1 receiver Reliable transfer 9-13 Reliable transfer 9-14 Getting the NAK ut Prtcl rdt2.1 uses ACKs t acknwledge packets prperly received, and uses NAKs when a crrupted packet is received. We can d away with NAKs by sending an ACK fr the last crrectly received packet. A sender that receives tw ACKs fr the same packet knws the receiver did nt crrectly receive the packet fllwing the ne ACKed twice. rdt2.2 sender The sender nw gets bth an ack and the sequence # f the acked packet. Reliable transfer 9-15 Reliable transfer

5 rdt2.2 receiver send Crrupted send Sender ack0 Receiver receive send ack0 rdt2.2 in actin ack0 receive send ack0 receive ack0 send pkt1 pkt1 ack0 crrupted send ack0 Reliable transfer 9-17 receive ack0 send pkt1 pkt1 receive pkt1 send ack1 Reliable transfer 9-18 Reliable data transfer ver a lssy channel with bit errrs Suppse nw that in additin t crrupting bits, the underlying channel can lse packets as well. Tw additinal cncerns must be addressed hw t detect packet lss; what t d when a packet disappears. We put the burden n the sender Suppse the sender transmits a packet and either that packet r the receiver s ACK is lst. In either case, n reply. If the sender is willing t wait lng enugh t be certain f n reply... Reliable transfer 9-19 Reliable transfer

6 But hw lng is lng enugh? Well, the sender must wait at least as lng as a rund-trip delay. But in many netwrks, this wrst-case max delay is difficult t estimate. Mrever, the netwrk shuld recver frm packet lss as sn as pssible. rdt3.0 sender We are still assuming that packets cannt be received ut f rder. Reliable transfer 9-21 Reliable transfer 9-22 Operatin f rdt3.0 alternating-bit prtcl Operatin f rdt3.0 alternating-bit prtcl Reliable transfer 9-23 Reliable transfer