Lecture 6: The Transport Layer

Transcription

1 Lecture 6: The Transport Layer

2 Reliable data delivery No lost data No duplicate data Computer Networks, Fall

3 applicabon transport network link physical Computer Networks, Fall

4 message app rdt_send() deliver_data() H t transport reliable data delivery network unreliable data delivery Computer Networks, Fall

5 rdt_send() deliver_data() app transport network network transport app Computer Networks, Fall

6 await rdt_send ( data ) data packet = make ( data ) udt_send ( packet ) Computer Networks, Fall

7 await rdt_rcv ( packet ) packet data = extract ( packet ) deliver_data ( data ) Computer Networks, Fall

8 Checksum Redundant informabon e.g., the binary sum of all data bytes sender adds checksum to each segment Used to detect (+correct) data corrupbon receiver recomputes checksum to detect corrup=on (poten=ally uses checksum to correct corrup=on) Computer Networks, Fall

9 rdt_send() NACK ACK deliver_data() Computer Networks, Fall

10 await data rdt_send ( data ) packet = make ( data ) udt_send ( packet ) rdt_rcv ( packet ) && is NACK ( packet ) udt_send ( packet ) await (N)ACK rdt_rcv ( packet ) && is ACK ( packet ) Computer Networks, Fall

11 rdt_rcv ( packet ) && corrupt( packet ) udt_send ( NACK ) rdt_rcv ( packet ) && not_corrupt ( packet ) udt_send ( ACK ) data = extract ( packet ) deliver_data ( data ) await packet Computer Networks, Fall

12 Acknowledgment Feedback from receiver to sender receiver determines whether segment corrupted sends posi=ve or nega=ve ACK Use to overcome data corrupbon if receiver sends a nega=ve ACK, sender retransmits Computer Networks, Fall

13 NACK Computer Networks, Fall

14 ?A?K Computer Networks, Fall

15 ACK seq# 1 Computer Networks, Fall

16 await data rdt_send ( data ) udt_send ( packet ) rdt_rcv ( packet ) && is NACK ( packet ) udt_send ( packet ) await (N)ACK rdt_rcv ( packet ) && is ACK ( packet ) Computer Networks, Fall

17 await data 0 rdt_send ( data ) udt_send ( packet ) rdt_rcv ( packet ) && is NACK ( packet ) udt_send ( packet ) await (N)ACK rdt_rcv ( packet ) && is ACK ( packet ) await (N)ACK await data 1 Computer Networks, Fall

18 rdt_rcv ( packet ) && corrupt ( packet ) udt_send ( NACK ) await packet rdt_rcv ( packet ) && not_corrupt ( packet ) udt_send ( ACK ) deliver_data ( data ) Computer Networks, Fall

19 rdt_rcv ( packet ) && corrupt ( packet ) udt_send ( NACK ) rdt_rcv ( packet ) && has_seq# ( 0 ) && not_corrupt ( packet ) udt_send ( ACK ) deliver_data ( data ) await packet 0 await packet 1 rdt_rcv ( packet ) && has_seq# ( 1 ) && not_corrupt ( packet ) udt_send ( ACK ) deliver_data ( data ) Computer Networks, Fall

20 NACK Computer Networks, Fall

21 ACK 1 Computer Networks, Fall

22 ACK 0 seq# 1 Computer Networks, Fall

23 ACK 1 Computer Networks, Fall

24 Sequence numbers An idenbfier for segments sender includes a sequence number in each segment receiver includes a sequence number in each ACK Used to disambiguate between segments sender indicates whether it sends new segment receiver indicates which was the last segment that was correctly received Computer Networks, Fall

25 X Bmeout! Computer Networks, Fall

26 X Bmeout! Computer Networks, Fall

27 X Bmeout! ACK 0 seq# 1 Computer Networks, Fall

28 Bmeout! ACK 0 seq# 1 ACK 0 Computer Networks, Fall

29 Timeout + ACK Timeout = an expected ACK is late the receiver communicates to the sender which was the last correct packet it received Used to overcome data loss if the sender s =meout expires, the sender retransmits Computer Networks, Fall

30 RDT elements Checksums detect data corrup=on at receiver ACKs + retransmissions overcome data corrup=on Timeouts + ACKs + retransmissions overcome data loss Computer Networks, Fall

31 transmission RTT ACK 0 transmission seq# 1 RTT ACK 1 Computer Networks, Fall

32 transmission rate R=1 Gbps segment size L = 1000 bytes transmission delay = L / R = 8 usec propagabon delay = 15 msec Computer Networks, Fall

33 0.008 msec 30 msec ACK 0 Busy for = sender/channel ublizabon Computer Networks, Fall

34 Sender ublizabon Stop and wait: poor sender ublizabon the sender does nothing while wai=ng for the receiver s ACK or the =meout Computer Networks, Fall

35 seq# 1 seq# 2 seq# 3 ACK 0 ACK 1 ACK 2 ACK 3 seq# 4 seq# 5 seq# 6 seq# 7 window size N = 4 Computer Networks, Fall

36 transmission seq# 1 seq# 2 RTT ACK 0 seq# 3 Busy for trans. N trans. + RTT Computer Networks, Fall

37 Sender ublizabon Stop and wait: poor sender ublizabon the sender does nothing while wai=ng for the receiver s ACK or the =meout Pipelining: becer ublizabon the sender sends up to N unacked segments N = sliding window size Computer Networks, Fall

38 seq# 1 seq# 2 seq# 3 ACK 0 ACK 1 ACK 2 ACK 3 seq# 4 seq# 5 seq# 6 seq# 7 Computer Networks, Fall

39 Bmeout for packet 2! seq# 1 seq# 2 seq# 3 ACK 0 X ACK 1 ACK 1 seq# 2 seq# 3 seq# 4 seq# 5 seq# 4 seq# 5 Computer Networks, Fall X X X

40 GobackN The receiver accepts 0 outoforder segments ACKs are cumulabve an ACK for segment 10 indicates that all segments un=l and including 10 have been received When the sender retransmits, it retransmits all the unacked segments Computer Networks, Fall

41 0 1 2 seq# 1 seq# 2 seq# 3 X ACK 0 ACK 1 ACK 3 seq# 4 seq# Bmeout for seq# 2 7 segment 2! Computer Networks, Fall

42 SelecBve Repeat The receiver accepts N1 outoforder packets ACKs are selecbve an ACK for segment 10 indicates that segment 10 has been received When the sender retransmits, it retransmits only one segment Computer Networks, Fall

43 Pipelining protocols GobackN the receiver accepts 0 outoforder segments the receiver sends cumula=ve ACKs the sender retransmits all unacked segments SelecBve repeat (SR) the receiver accepts N1 outoforder segments the receiver sends selec=ve ACKs the sender retransmits one segment Computer Networks, Fall