TCP

1
TCP

• Jean Walrand
• U.C. Berkeley
• www.eecs.berkeley.edu/wlr

2
Outline

• TCP Service
• State Diagram
• Flow Control
• Congestion Control
• RTT Estimation
• Fast Recovery
• Fast Retransmit

3
TCP Service

• Error-Free Byte Stream

Reliable byte stream
TCP
IP
Unreliable packet delivery
Actual data path
4
TCP Service (cd)

• Multiplexing Ports

A B p1 p2
5
State Diagram

• Key Steps

6
State Diagram (cd)

• States

7
State Diagram (cd)

8
Flow Control

• Objective Avoid saturating receiver
• Mechanism Receiver advertises window Wrec

W ? Wrec Outstanding where Oustanding Last
sent last Acked
9
Congestion Control

• Objective Avoid saturating routers
• Mechanism Loss gt Reduce W No loss gt Increase
  W
• Details See book.

10
Congestion Control (cd)

• Slow Start ACK gt W W 1

11
Congestion Control (cd)

• Slow Start ACK gt W W 1

12
Congestion Control (cd)

• Congestion Avoidance ACK gt W W 1/W

13
Congestion Control (cd)

• Congestion Avoidance ACK gt W W 1/W

14
RTT Estimation

• Timeout if delay gt average 4 deviations
• Average delay and deviation first order filters

Average A(n) calculated when n-th ACK arrives
A(n) a?A(n 1) (1 a)?Delay(n)
Deviation D(n) calculated when n-th ACK arrives
D(n) a?D(n 1) (1 a)?Delay(n) A(n)
Note - Obvious corrections when retransmission
Do not include delays of retransmitted packets
Double timeout after retransmission
15
Fast Retransmission

• When 3 duplicated ACKs, retransmit and W W/2
• Motivation 3 dup to avoid retransmission after
  misordering

16
Fast Recovery

• Objective Keep buffer full after setting W
  W/2
• Illustration Window goes from 2K to K

L, L 2K 1, , L 4
L 2K, L, L 2K 1, , L 4 gt W 2K 1
L 2K 1, L 2K, L, L 2K 1, , L 5 gt W
2K 2
. . . . . . . . .
L 3K - 2,, L 2K 1, L 2K, L gt W 3K - 1
L 3K - 1,, L 2K 1, L 2K gt W K