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Transcript of CS168 (and EE122) TAs past and present give you…..
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Transport Algorithms &Some Router Architecture
CS168 (and EE122) TAs past and present give you….
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Today
Transport Algorithms Stop-and-Wait Go-Back-N Selective Repeat
Router Architecture Longest-Prefix-Match Head of Line Blocking
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Transport Algorithms
Reliable Data Transmission ARQ: Automatic Repeat reQuest FEC: Forward Error Correction We use ARQ for reliable transport. (Why?)
Sliding Window transmit window receive window
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Transport Algorithms
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
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Transport Algorithms
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)
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ARQ in the Real World: TCP 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.
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(i)Alice Bob
(1)
ACK(1)
(3)
ACK(2)
(2)
ACK(3)
(3)
ACK(3)
(4)
ACK(5)
(5)
Timeout!
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Transport Algorithms
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.
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(ii)Alice Bob
(1)
ACK(1)
ACK(2)
(2)
ACK(3)
(3)
ACK(4)
(4)
ACK(5)
(5)
Transmission Delay for one packet: 2msPropagation Delay for one packet: 30ms
10 packets sent.
32 * 10 = 320 ms
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(iii)Alice Bob
(1)
(2)
(3)
1 2 3 4 5
Sliding Window
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(iii)Alice Bob
(1)
(2)
(3)
1 2 3 4 5ACK (1)
(4)
Sliding Window
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(iii)Alice Bob
(1)
(2)
(3)
1 2 3 4 5ACK (1)
ACK (2) (4)
(5)
Sliding Window
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(iii)Alice Bob
(1)
(2)
(3)
1 2 3 4 5ACK (1)
ACK (2) (4)
(5)ACK (3)
ACK (4)
ACK (5)
Sliding Window
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(iii)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?
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(iii)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
64ms
2ms
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(iv)Alice BobAlice 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?
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(iv)Alice BobAlice 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
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(v)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?
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(v)Alice BobAlice 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!
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(v)Alice BobAlice 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
64ms
100ms
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Longest Prefix Match
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.
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2)
00001000 00000000 00000000 0000000000001001 00000000 00000000 0000000000001000 11000000 11000000 0000000000001000 11000000 10000000 0000000000001000 10000000 00000000 0000000000001000 11000000 00000000 00000000
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2i)
00001000 00000000 00000000 0000000000001001 00000000 00000000 0000000000001000 11000000 11000000 0000000000001000 11000000 10000000 0000000000001000 10000000 00000000 0000000000001000 11000000 00000000 00000000
9.8.4.56 00001001 000011000 00000100 00111000
2
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2ii)
00001000 00000000 00000000 0000000000001001 00000000 00000000 0000000000001000 11000000 11000000 0000000000001000 11000000 10000000 0000000000001000 10000000 00000000 0000000000001000 11000000 00000000 00000000
7.63.23.5 00000111 00111111 00010111 00000101
5
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2iii)
8.192.130.43 00001000 11000000 10000010 00101011
00001000 00000000 00000000 0000000000001001 00000000 00000000 0000000000001000 11000000 11000000 0000000000001000 11000000 10000000 0000000000001000 10000000 00000000 0000000000001000 11000000 00000000 00000000
1
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2iv)
00001000 00000000 00000000 0000000000001001 00000000 00000000 0000000000001000 11000000 11000000 0000000000001000 11000000 10000000 0000000000001000 10000000 00000000 0000000000001000 11000000 00000000 00000000
8.178.54.3 00001000 10110010 00110110 00000011
1
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2v)
00001000 00000000 00000000 0000000000001001 00000000 00000000 0000000000001000 11000000 11000000 0000000000001000 11000000 10000000 0000000000001000 10000000 00000000 0000000000001000 11000000 00000000 00000000
8.192.200.1400001000 11000000 11010000 00001110
2
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2vi)
00001000 00000000 00000000 0000000000001001 00000000 00000000 0000000000001000 11000000 11000000 0000000000001000 11000000 10000000 0000000000001000 10000000 00000000 0000000000001000 11000000 00000000 00000000
8.0.192.0 00001000 00000000 11000000 000000001
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Head of Line Blocking
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.
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3i)
(1)
(2)
(3)
(1)
(1)
(2)(3)
(3)
(3)
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3i)
(1)
(2)
(3)
(1)
(1) (2)
(3)
(3)
(3)
(1)
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3i)
(1)
(2)
(3)
(1)
(3)
(3)
(3)
(1,2)
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3i)
(1)
(2)
(3)
(3)
(3)
(1,2,3,4)
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3ii)
(1)
(2)
(3)
3
1
1
3
2
3
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3iii)
(1)
(2)
(3)
3
1
1
3
2
3
(1)
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3iii)
(1)
(2)
(3)3
1 1
32
3
(1)
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3iii)
(1)
(2)
(3)
1
3
3
(1,2)
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3iii)
(1)
(2)
(3)
3
(1,2,3)