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15-441 Computer Networking. Lecture 16 –TCP in detail Eric Anderson Fall 2013 www.cs.cmu.edu/~prs/15-441-F13. Good Ideas So Far…. Flow control Stop & wait Sliding window Loss recovery Timeouts Acknowledgement-driven recovery Selective repeat Cumulative acknowledgement

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15 441 computer networking

15-441 Computer Networking

Lecture 16 –TCP in detail

Eric Anderson

Fall 2013

www.cs.cmu.edu/~prs/15-441-F13

good ideas so far
Good Ideas So Far…
  • Flow control
    • Stop & wait
    • Sliding window
  • Loss recovery
    • Timeouts
    • Acknowledgement-driven recovery
      • Selective repeat
      • Cumulative acknowledgement
  • Congestion control
    • AIMD  fairness and efficiency
  • How does TCP actually implement these?
outline
Outline
  • TCP connection setup/data transfer
  • TCP Packet Loss and Retransmission
  • TCP congestion avoidance
  • TCP slow start
sequence number space
Sequence Number Space
  • Each byte in byte stream is numbered.
    • 32 bit value
    • Wraps around
    • Initial values selected at start up time
  • TCP breaks up the byte stream into packets.
    • Packet size is limited to the Maximum Segment Size
  • Each packet has a sequence number.
    • Indicates where it fits in the byte stream

13450

14950

16050

17550

packet 8

packet 9

packet 10

establishing connection three way handshake
Establishing Connection:Three-Way handshake
  • Each side notifies other of starting sequence number it will use for sending
    • Why not simply chose 0?
      • Must avoid overlap with earlier incarnation
      • Security issues
  • Each side acknowledges other’s sequence number
    • SYN-ACK: Acknowledge sequence number + 1
  • Can combine second SYN with first ACK

SYN: SeqC

ACK: SeqC+1

SYN: SeqS

ACK: SeqS+1

Client

Server

tcp connection setup example
TCP Connection Setup Example
  • Client SYN
    • SeqC: Seq. #4019802004, window 65535, max. seg. 1260
  • Server SYN-ACK+SYN
    • Receive: #4019802005 (= SeqC+1)
    • SeqS: Seq. #3428951569, window 5840, max. seg. 1460
  • Client SYN-ACK
    • Receive: #3428951570 (= SeqS+1)

09:23:33.042318 IP 128.2.222.198.3123 > 192.216.219.96.80: S 4019802004:4019802004(0) win 65535 <mss 1260,nop,nop,sackOK> (DF)

09:23:33.118329 IP 192.216.219.96.80 > 128.2.222.198.3123: S 3428951569:3428951569(0) ack 4019802005 win 5840 <mss 1460,nop,nop,sackOK> (DF)

09:23:33.118405 IP 128.2.222.198.3123 > 192.216.219.96.80: . ack 3428951570 win 65535 (DF)

tcp state diagram connection setup
TCP State Diagram: Connection Setup

CLOSED

Active open

/SYN

Passive open

Close

c

Close

LISTEN

s

SYN/SYN + ACK

Send/

SYN

SYN/SYN + ACK

SYN_RCVD

SYN_SENT

ACK

SYN + ACK/ACK

Close

/FIN

ESTABLISHED

Close

/FIN

FIN/ACK

FIN_WAIT_1

CLOSE_WAIT

FIN/ACK

ACK

Close

/FIN

ACK + FIN/ACK

FIN_WAIT_2

CLOSING

LAST_ACK

Timeout after two

ACK

ACK

segment lifetimes

FIN/ACK

TIME_WAIT

CLOSED

tearing down connection
Tearing Down Connection
  • Either side can initiate tear down
    • Send FIN signal
    • “I’m not going to send any more data”
  • Other side can continue sending data
    • Half open connection
    • Must continue to acknowledge
  • Acknowledging FIN
    • Acknowledge last sequence number + 1

A

B

FIN, SeqA

ACK, SeqA+1

Data

ACK

FIN, SeqB

ACK, SeqB+1

tcp connection teardown example
TCP Connection Teardown Example

09:54:17.585396 IP 128.2.222.198.4474 > 128.2.210.194.6616: F 1489294581:1489294581(0) ack 1909787689 win 65434 (DF)

09:54:17.585732 IP 128.2.210.194.6616 > 128.2.222.198.4474: F 1909787689:1909787689(0) ack 1489294582 win 5840 (DF)

09:54:17.585764 IP 128.2.222.198.4474 > 128.2.210.194.6616: . ack 1909787690 win 65434 (DF)

  • Session
    • Echo client on 128.2.222.198, server on 128.2.210.194
  • Client FIN
    • SeqC: 1489294581
  • Server ACK + FIN
    • Ack: 1489294582 (= SeqC+1)
    • SeqS: 1909787689
  • Client ACK
    • Ack: 1909787690 (= SeqS+1)
tcp state diagram connection teardown
TCP State Diagram: Connection Teardown

CLOSED

Active open

/SYN

Passive open

Close

Close

LISTEN

SYN/SYN + ACK

Send/

SYN

SYN/SYN + ACK

SYN_RCVD

SYN_SENT

ACK

SYN + ACK/ACK

“half-closed”

B→A still open

Close

/FIN

ESTABLISHED

Close

/FIN

FIN/ACK

FIN_WAIT_1

CLOSE_WAIT

FIN/ACK

A

B

ACK/

Close

/FIN

ACK + FIN/ACK

FIN_WAIT_2

CLOSING

LAST_ACK

Timeout after two

ACK

/ACK

segment lifetimes

FIN/ACK

TIME_WAIT

CLOSED

outline1
Outline
  • TCP connection setup/data transfer
  • Packet Loss and Retransmission
    • Recognizing packet loss
    • Identifying missing packets
    • Retransmission behavior
  • TCP congestion avoidance
  • TCP slow start
reliability challenges
Reliability Challenges
  • Congestion related losses
  • Variable packet delays
    • What should the timeout be?
  • Reordering of packets
    • How to tell the difference between a delayed packet and a lost one?
tcp go back n variant
TCP = Go-Back-N Variant
  • Sliding window with cumulative acks
    • Receiver can only return a single “ack” sequence number to the sender.
    • Acknowledges all bytes with a lower sequence number
    • Starting point for retransmission
    • Duplicate acks sent when out-of-order packet received
  • But: sender only retransmits a single packet.
    • Reason???
      • Only one that it knows is lost
      • Network is congested  shouldn’t overload it
  • Error control is based on byte sequences, not packets.
    • Retransmitted packet can be different from the original lost packet – Why?
outline2
Outline
  • TCP connection setup/data transfer
  • Packet Loss and Retransmission
    • Recognizing packet loss
    • Identifying missing packets
    • Retransmission behavior
  • TCP congestion avoidance
  • TCP slow start
retransmit timeout
Retransmit Timeout
  • How long is too long?
  • Well, how long does it usually take?

Early TCP: RTO = 2 x RTT

Last 20 years: RTO = RTT + 4x deviation

  • What’s the RTT? What’s the deviation?
round trip time estimation
Round-trip Time Estimation
  • Wait at least one RTT before retransmitting
  • Importance of accurate RTT estimators:
    • Low RTT estimate
      • unneeded retransmissions
    • High RTT estimate
      • poor throughput
  • RTT estimator must adapt to change in RTT
    • But not too fast, or too slow!
  • Spurious timeouts
    • “Conservation of packets” principle – never more than a window worth of packets in flight
original tcp round trip estimator
Original TCP Round-trip Estimator
  • Round trip times exponentially averaged:
    • New RTT = a (old RTT) + (1 - a) (new sample)
    • Recommended value for a: 0.8 - 0.9
      • 0.875 for most TCP’s
  • Retransmit timer set to (b * RTT), where b = 2
    • Every time timer expires, RTO exponentially backed-off
  • Not good at preventing spurious timeouts
    • Why?
jacobson s retransmission timeout
Jacobson’s Retransmission Timeout
  • Key observation:
    • At high loads, round trip variance is high
  • Solution:
    • Base RTO on RTT and standard deviation
      • RTO = RTT + 4 * rttvar
    • new_rttvar = b * dev + (1- b) old_rttvar
      • Dev = linear deviation
      • Inappropriately named – actually smoothed linear deviation
rtt sample ambiguity

A

B

Original transmission

RTO

ACK

Sample

RTT

retransmission

RTT Sample Ambiguity

A

B

  • Karn’s RTT Estimator
    • If a segment has been retransmitted:
      • Don’t count RTT sample on ACKs for this segment
      • Keep backed off time-out for next packet
      • Reuse RTT estimate only after one successful transmission

Original transmission

X

RTO

Sample

RTT

retransmission

ACK

timestamp extension
Timestamp Extension
  • Used to improve timeout mechanism by more accurate measurement of RTT
  • When sending a packet, insert current time into option
    • 4 bytes for time, 4 bytes for echo a received timestamp
  • Receiver echoes timestamp in ACK
    • Actually will echo whatever is in timestamp
  • Removes retransmission ambiguity
    • Can get RTT sample on any packet
timer granularity
Timer Granularity
  • Many TCP implementations set RTO in multiples of 200,500,1000ms
  • Why?
    • Avoid spurious timeouts – RTTs can vary quickly due to cross traffic
    • Make timers interrupts efficient
  • What happens for the first couple of packets?
    • Pick a very conservative value (seconds)
acks nacks
ACKs & NACKs
  • TCP has no NACK

Send 12

Send 13

Send 14

Send 15

Send 16

Send 17

Send 18

Send 19

ACK 12

ACK 13

ACK 14

ACK 14

ACK 14

ACK 14

duplicate acks fast retransmit
Duplicate ACKs (Fast Retransmit)
  • What are duplicate acks (dupacks)?
    • Repeated acks for the same sequence
  • When can duplicate acks occur?
    • Loss
    • Packet re-ordering
    • Window update – advertisement of new flow control window
  • Assume re-ordering is infrequent and not of large magnitude
    • Receipt of 3 or more duplicate acks is indication of loss
    • Don’t wait for timeout to retransmit packet
    • When does this fail?
duplicate acks fast retransmit1

Packets

Acks

Duplicate ACKs (Fast Retransmit)

Retransmission

X

Duplicate Acks

Sequence No

Time

outline3
Outline
  • TCP connection setup/data transfer
  • Packet Loss and Retransmission
    • Recognizing packet loss
    • Identifying missing packets
    • Retransmission behavior
  • TCP congestion avoidance
  • TCP slow start
tcp reno variant

Packets

Acks

TCP (Reno variant)

X

X

X

Now what? - timeout

X

Sequence No

Time

selective ack sack

Packets

Acks

Selective ACK (SACK )

X

X

X

X

Sequence No

Time

“Hole”

partial progress ack

Packets

Acks

“Partial Progress ACK”

X

X

X

X

Sequence No

Time

“Hole”

outline4
Outline
  • TCP connection setup/data transfer
  • Packet Loss and Retransmission
    • Recognizing packet loss
    • Identifying missing packets
    • Retransmission behavior
  • TCP congestion avoidance
  • TCP slow start
fast recovery
Fast Recovery
  • Each duplicate ack notifies sender that single packet has cleared network
  • When < newcwnd packets are outstanding
    • Allow new packets out with each new duplicate acknowledgement
  • Behavior
    • Sender is idle for some time – waiting for ½ cwnd worth of dupacks
    • Transmits at original rate after wait
      • Ack clocking rate is same as before loss
fast recovery1

Packets

Acks

Fast Recovery

Sent for each dupack after

W/2 dupacks arrive

Sequence No

X

Time

performance issues
Performance Issues
  • Timeout >> fast rexmit
  • Need 3 dupacks/sacks
  • Not great for small transfers
    • Don’t have 3 packets outstanding
  • What are real loss patterns like?
outline5
Outline
  • TCP connection setup/data transfer
  • Packet Loss and Retransmission
  • TCP congestion avoidance
  • TCP slow start
additive increase decrease
Additive Increase/Decrease
  • Both X1 and X2 increase/ decrease by the same amount over time
    • Additive increase improves fairness and additive decrease reduces fairness

Fairness Line

T1

User 2’s Allocation x2

T0

Efficiency Line

User 1’s Allocation x1

muliplicative increase decrease
Muliplicative Increase/Decrease
  • Both X1 and X2 increase by the same factor over time
    • Extension from origin – constant fairness

Fairness Line

T1

User 2’s Allocation x2

T0

Efficiency Line

User 1’s Allocation x1

what is the right choice

Fairness Line

x1

x0

User 2’s Allocation x2

x2

Efficiency Line

User 1’s Allocation x1

What is the Right Choice?
  • Constraints limit us to AIMD
    • Improves or keeps fairness constant at each step
    • AIMD moves towards optimal point
tcp congestion control
TCP Congestion Control
  • Changes to TCP motivated by ARPANET congestion collapse
  • Basic principles
    • AIMD
    • Packet conservation
    • Reaching steady state quickly
    • ACK clocking
slide38
AIMD
  • Distributed, fair and efficient
  • Packet loss is seen as sign of congestion and results in a multiplicative rate decrease
    • Factor of 2
  • TCP periodically probes for available bandwidth by increasing its rate

Rate

Time

implementation issue
Implementation Issue
  • Operating system timers are very coarse – how to pace packets out smoothly?
  • Implemented using a congestion window that limits how much data can be in the network.
    • TCP also keeps track of how much data is in transit
  • Data can only be sent when the amount of outstanding data is less than the congestion window.
    • The amount of outstanding data is increased on a “send” and decreased on “ack”
    • (last sent – last acked) < congestion window
  • Window limited by both congestion and buffering
    • Sender’s maximum window = Min (advertised window, cwnd)
packet conservation
Packet Conservation
  • At equilibrium, inject packet into network only when one is removed
    • Sliding window and not rate controlled
    • But still need to avoid sending burst of packets  would overflow links
      • Need to carefully pace out packets
      • Helps provide stability
  • Need to eliminate spurious retransmissions
    • Accurate RTO estimation
    • Better loss recovery techniques (e.g. fast retransmit)
tcp packet pacing
TCP Packet Pacing
  • Congestion window helps to “pace” the transmission of data packets
  • In steady state, a packet is sent when an ack is received
    • Data transmission remains smooth, once it is smooth
    • Self-clocking behavior

Pb

Pr

Sender

Receiver

As

Ar

Ab

congestion avoidance
Congestion Avoidance
  • If loss occurs when cwnd = W
    • Network can handle 0.5W ~ W segments
    • Set cwnd to 0.5W (multiplicative decrease)
  • Upon receiving ACK
    • Increase cwnd by (1 packet)/cwnd
      • What is 1 packet?  1 MSS worth of bytes
      • After cwnd packets have passed by  approximately increase of 1 MSS
  • Implements AIMD
congestion avoidance behavior
Congestion Avoidance Behavior

Congestion

Window

Time

Cut

Congestion

Window

and Rate

Grabbing

back

Bandwidth

Packet loss

+ retransmit

how to change window
How to Change Window
  • When a loss occurs have W packets outstanding
  • New cwnd = 0.5 * cwnd
    • How to get to new state without losing ack clocking?
outline6
Outline
  • TCP connection setup/data transfer
  • Packet Loss and Retransmission
  • TCP congestion avoidance
  • TCP slow start
reaching steady state
Reaching Steady State
  • Doing AIMD is fine in steady state but slow…
  • How does TCP know what is a good initial rate to start with?
    • Should work both for a CDPD (10s of Kbps or less) and for supercomputer links (10 Gbps and growing)
  • Quick initial phase to help get up to speed
  • Called “slow start” – Why?
slow start packet pacing
Slow Start Packet Pacing
  • How do we get this clocking behavior to start?
    • Initialize cwnd = 1
    • Upon receipt of every ack, cwnd = cwnd + 1
  • Implications
    • Window actually increases to W in RTT * log2(W)
    • Can overshoot window and cause packet loss
slow start example

One RTT

0R

1

One pkt time

1R

1

2

3

2R

2

3

4

6

5

7

4

5

6

7

3R

8

10

12

14

9

11

13

15

Slow Start Example
slow start sequence plot

Packets

Acks

Slow Start Sequence Plot

.

.

.

Sequence No

Time

return to slow start
Return to Slow Start
  • If packet is lost we lose our self clocking as well
    • Need to implement slow-start and congestion avoidance together
  • When retransmission occurs set ssthresh to 0.5w
    • If cwnd < ssthresh, use slow start
    • Else use congestion avoidance
tcp saw tooth behavior
TCP Saw Tooth Behavior

ssthresh

Timeouts

may still

occur

Congestion

Window

Time

Slowstart

to pace

packets

Fast

Retransmit

and Recovery

Initial

Slowstart

important lessons
Important Lessons
  • TCP state diagram  setup/teardown
  • TCP timeout calculation  how is RTT estimated
  • Modern TCP loss recovery
    • Why are timeouts bad?
    • How to avoid them?  e.g. fast retransmit