CS168 (and EE122) TAs past and present give you…..

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CS168 (and EE122) TAs past and present give you….

 Transport Algorithms  Stop-and-Wait  Go-Back-N  Selective Repeat  Router Architecture  Longest-Prefix-Match  Head of Line Blocking

 Reliable Data Transmission  ARQ: Automatic Repeat reQuest  FEC: Forward Error Correction  We use ARQ for reliable transport. (Why?)  Sliding Window  transmit window  receive window

 ARQ Examples  Stop-and-Wait ▪ tx window size: 1 ▪ rx window size: 1  Go-Back-N ▪ tx window size: N ▪ rx window size: 1  Selective Repeat ▪ tx window size: N ▪ rx window size: N

 Difference between Go-back-N and SR Alice Bob (1) (2) (3) ACK (1) Reject! Go-back-N Alice Bob (1) (2) (3) ACK (1) Selective Repeat ACK (3)

 TCP Reliable Transmission  A variation of Go-Back-N scheme  Many TCP implementations will buffer correctly received but out-of-order segments.  Selective Acknowledgement (SACK)  Selective Repeat  Cumulative ACKs  The sequence number is in bytes.  Therefore each packet may contain a continuous range of sequence numbers [seq, seq+len).  The receiver send an ACK seq+len for the packet, specifying it have received all data preceding seq+len bytes.  In TCP, an ACK specifies the sequence number of the next byte the receiver expects to receive.

Alice Bob (1) ACK(1) (3) ACK(2) (2) ACK(3) (3) ACK(3) (4) ACK(5) (5) Timeout!

 Tips and Tricks:  Always draw a timing diagram when trying to understand a new protocol!  If the homework/midterm asks for how long something takes, the timing diagram will help you calculate it.

Alice Bob (1) ACK(1) ACK(2) (2) ACK(3) (3) ACK(4) (4) ACK(5) (5) Transmission Delay for one packet: 2ms Propagation Delay for one packet: 30ms 10 packets sent. 32 * 10 = 320 ms

Alice Bob (1) (2) (3) Sliding Window

Alice Bob (1) (2) (3) ACK (1) (4) Sliding Window

Alice Bob (1) (2) (3) ACK (1) ACK (2) (4) (5) Sliding Window

Alice Bob (1) (2) (3) ACK (1) ACK (2) (4) (5) ACK (3) ACK (4) ACK (5) Sliding Window

Alice Bob (1) (2) (3) ACK (1) ACK (2) (4) (5) ACK (3) ACK (4) ACK (5) How long did it take for the LAST packet to be ACKed back to Alice?

Alice Bob (1) (2) (3) ACK (1) ACK (2) (4) (5) ACK (3) ACK (4) ACK (5) How long did it take for the LAST packet to be ACKed back to Alice? = 130ms 64ms 2ms

Alice Bob Alice Bob (1) (2) (3) ACK (1) ACK (2) ACK (3) (4) (5) ….timeout! (4) (5) ACK (4) ACK (5) How long did it take for the LAST packet to be ACKed back to Alice?

Alice Bob Alice Bob (1) (2) (3) ACK (1) ACK (2) ACK (3) (4) (5) ….timeout! (4) (5) ACK (4) ACK (5) 64ms 100ms 2ms 64ms How long did it take for the LAST packet to be ACKed back to Alice? = 230ms

Alice Bob (1) (2) (3) ACK (1) ACK (2) (4) ACK (3) How long did it take for the LAST packet to be ACKed back to Alice?

Alice Bob Alice Bob (1) (2) (3) ACK (1) ACK (2) ACK (3) (4) (5) (4) ACK (4) ACK (5) How long did it take for the LAST packet to be ACKed back to Alice? ….timeout!

Alice Bob Alice Bob (1) (2) (3) ACK (1) ACK (2) ACK (3) (4) (5) (4) ACK (4) ACK (5) How long did it take for the LAST packet to be ACKed back to Alice? = 228ms. ….timeout! 64ms 100ms

 Tips and Tricks:  Convert everything to binary first.  Always match from left-to-right.  Looking for the longest exact match starting from the first bit.

 Tips and Tricks:  Draw out your queues with the packets in them.  Cross out all of the packets that can go across the interconnect in a single round.  Mark “1”  Repeat with the remaining packets, marking 2 after you’ve “scheduled” one round.  Repeat again: 3, 4, etc until all packets are gone.

(1) (2) (3) (1) (2)(3)

(1) (2) (3) (1) (2) (3) (1)

(2) (3) (1) (3) (1,2)

(1) (2) (3) (1,2,3,4)

(1) (2) (3)

(1) (2) (3) (1)

(2) (3) (1)

(2) (3) (1,2)

(1) (2) (3) 3 (1,2,3)