Chapter 3 Transport Layer

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Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.

Thanks and enjoy! JFK/KWR All material copyright 1996-2010 J.F Kurose and K.W. Ross, All Rights Reserved Transport Layer

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Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer

3.5 Connection-oriented transport: TCP        

segment structure reliable data transfer flow control connection management

3.6 Principles of congestion control 3.7 TCP congestion control

Transport Layer

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Principles of Congestion Control Congestion:      

 

informally: “too many sources sending too much data too fast for network to handle” different from flow control! manifestations:   lost packets (buffer overflow at routers)   long delays (queueing in router buffers) a top-10 problem!

Transport Layer

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Causes/costs of congestion: scenario 1      

two senders, two receivers one router, infinite buffers no retransmission

Host A

Host B

λout

λin : original data

unlimited shared output link buffers

   

large delays when congested maximum achievable throughput Transport Layer

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Causes/costs of congestion: scenario 2    

one router, finite buffers sender retransmission of timed-out packet   application-layer input = application-layer output: λin = λout   transport-layer input includes retransmissions : λ‘in λin λin : original data λ'in: original data, plus

λout

retransmitted data

Host B Host A

finite shared output link buffers Transport Layer

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Congestion scenario 2a: ideal case sender sends only when router buffers available

λout

 

R/2

λin

λin : original data λ'in: original data, plus

copy

R/2

λout

retransmitted data

Host B Host A

free buffer space!

finite shared output link buffers Transport Layer

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Congestion scenario 2b: known loss  

packets may get dropped at router due to full buffers   sometimes lost

 

sender only resends if packet known to be lost (admittedly idealized) λin : original data λ'in: original data, plus

copy

λout

retransmitted data

Host B Host A

no buffer space!

Transport Layer

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Congestion scenario 2b: known loss packets may get dropped at router due to full buffers   sometimes not lost  

sender only resends if packet known to be lost (admittedly idealized)

R/2

when sending at R/ 2, some packets are retransmissions but asymptotic goodput is still R/2 (why?)

λout

 

λin

λin : original data λ'in: original data, plus

R/2

λout

retransmitted data

Host B Host A

free buffer space!

Transport Layer

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Congestion scenario 2c: duplicates

 

packets may get dropped at router due to full buffers sender times out prematurely, sending two copies, both of which are delivered

R/2

λin

λin λ'in

timeout copy

when sending at R/ 2, some packets are retransmissions including duplicated that are delivered!

λout

 

R/2

λout

Host B Host A

free buffer space!

Transport Layer

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Congestion scenario 2c: duplicates

 

packets may get dropped at router due to full buffers sender times out prematurely, sending two copies, both of which are delivered

R/2

when sending at R/ 2, some packets are retransmissions including duplicated that are delivered!

λout

 

λin

R/2

“costs” of congestion:    

more work (retrans) for given “goodput” unneeded retransmissions: link carries multiple copies of pkt   decreasing goodput

Transport Layer

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Causes/costs of congestion: scenario 3      

four senders multihop paths timeout/retransmit

Q: what happens as λ

in and λ

increase ? in

Host A

λin : original data

λout

λ'in : original data, plus retransmitted data

finite shared output link buffers

Host B

Transport Layer

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Causes/costs of congestion: scenario 3 H o st A

λ o u t

H o st B

another “cost” of congestion:   when packet dropped, any “upstream transmission capacity used for that packet was wasted! Transport Layer

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Approaches towards congestion control Two broad approaches towards congestion control: end-end congestion control:    

 

no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP

network-assisted congestion control:  

routers provide feedback to end systems   single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)   explicit rate sender should send at

Transport Layer

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Case study: ATM ABR congestion control ABR: available bit rate:    

 

“elastic service” if sender’s path “underloaded”:   sender should use available bandwidth if sender’s path congested:   sender throttled to minimum guaranteed rate

RM (resource management) cells:    

 

sent by sender, interspersed with data cells bits in RM cell set by switches (“network-assisted”)   NI bit: no increase in rate (mild congestion)   CI bit: congestion indication RM cells returned to sender by receiver, with bits intact

Transport Layer

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Case study: ATM ABR congestion control

 

two-byte ER (explicit rate) field in RM cell

  congested switch may lower ER value in cell   sender’ send rate thus maximum supportable rate on path

 

EFCI bit in data cells: set to 1 in congested switch

  if data cell preceding RM cell has EFCI set, sender sets CI bit in returned RM cell Transport Layer

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Chapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer

3.5 Connection-oriented transport: TCP        

segment structure reliable data transfer flow control connection management

3.6 Principles of congestion control 3.7 TCP congestion control

Transport Layer

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TCP congestion control: additive increase, multiplicative decrease

approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs   additive increase: increase cwnd by 1 MSS every RTT until loss detected   multiplicative decrease: cut cwnd in half after loss

saw tooth behavior: probing for bandwidth

cwnd: congestion window size

 

congestion window

24 Kbytes

16 Kbytes

8 Kbytes

time time Transport Layer

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TCP Congestion Control: details  

sender limits transmission: LastByteSent-LastByteAcked ≤ cwnd

 

roughly, rate =

 

cwnd RTT

Bytes/sec

cwnd is dynamic, function of perceived network congestion

How does sender perceive congestion?   loss event = timeout or 3 duplicate acks   TCP sender reduces rate (cwnd) after loss event three mechanisms:   AIMD   slow start   conservative after timeout events

Transport Layer

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TCP Slow Start when connection begins, increase rate exponentially until first loss event:   initially cwnd = 1 MSS   double cwnd every RTT   done by incrementing cwnd for every ACK received  

summary: initial rate is slow but ramps up exponentially fast

Host A RTT

 

Host B one segm ent

two segm ents

four segm

ents

time

Transport Layer

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Refinement: inferring loss  

 

after 3 dup ACKs:   cwnd is cut in half   window then grows linearly but after timeout event:   cwnd instead set to 1 MSS;   window then grows exponentially   to a threshold, then grows linearly

Philosophy:   3

dup ACKs indicates network capable of delivering some segments   timeout indicates a “more alarming” congestion scenario

Transport Layer

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Refinement Q: when should the exponential increase switch to linear? A: when cwnd gets to 1/2 of its value before timeout.

Implementation:    

variable ssthresh on loss event, ssthresh is set to 1/2 of cwnd just before loss event

Transport Layer

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Summary: TCP Congestion Control duplicate ACK dupACKcount++ Λ

cwnd = 1 MSS ssthresh = 64 KB dupACKcount = 0

slow start

timeout ssthresh = cwnd/2 cwnd = 1 MSS dupACKcount = 0 retransmit missing segment

dupACKcount == 3 ssthresh= cwnd/2 cwnd = ssthresh + 3 retransmit missing segment

New ACK! new ACK cwnd = cwnd+MSS dupACKcount = 0 transmit new segment(s), as allowed cwnd > ssthresh Λ

timeout ssthresh = cwnd/2 cwnd = 1 MSS dupACKcount = 0 retransmit missing segment

timeout ssthresh = cwnd/2 cwnd = 1 dupACKcount = 0 retransmit missing segment

New ACK!

.

new ACK cwnd = cwnd + MSS (MSS/cwnd) dupACKcount = 0 transmit new segment(s), as allowed

congestion avoidance duplicate ACK dupACKcount++

New ACK! New ACK cwnd = ssthresh dupACKcount = 0

fast recovery

dupACKcount == 3 ssthresh= cwnd/2 cwnd = ssthresh + 3 retransmit missing segment

duplicate ACK cwnd = cwnd + MSS transmit new segment(s), as allowed

Transport Layer

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TCP throughput   what’s

the average throughout of TCP as a function of window size and RTT?   ignore slow start

  let

W be the window size when loss occurs.

  when window is W, throughput is W/RTT   just after loss, window drops to W/2, throughput to W/2RTT.   average throughout: .75 W/RTT

Transport Layer

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TCP Futures: TCP over “long, fat pipes”      

example: 1500 byte segments, 100ms RTT, want 10 Gbps throughput requires window size W = 83,333 in-flight segments throughput in terms of loss rate:

1.22⋅ MSS RTT L    

➜ L = 2·10-10 Wow – a very small loss rate! new versions of TCP for high-speed

Transport Layer

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TCP Fairness fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K TCP connection 1

TCP connection 2

bottleneck router capacity R

Transport Layer

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Why is TCP fair? two competing sessions:  

additive increase gives slope of 1, as throughout increases multiplicative decrease decreases throughput proportionally equal bandwidth share

R Connection 2 throughput

 

loss: decrease window by factor of 2 congestion avoidance: additive increase loss: decrease window by factor of 2 congestion avoidance: additive increase

Connection 1 throughput R Transport Layer

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Fairness (more) Fairness and UDP   multimedia apps often do not use TCP

  do not want rate throttled by congestion control

 

instead use UDP:

  pump audio/video at constant rate, tolerate packet loss

Fairness and parallel TCP connections   nothing prevents app from opening parallel connections between 2 hosts.   web browsers do this   example: link of rate R supporting 9 connections;

  new app asks for 1 TCP, gets rate R/10   new app asks for 11 TCPs, gets R/2 !

Transport Layer

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Chapter 3: Summary  

 

principles behind transport layer services:   multiplexing, demultiplexing   reliable data transfer   flow control   congestion control instantiation and implementation in the Internet   UDP   TCP

Next:   leaving the network “edge” (application, transport layers)   into the network “core” Transport Layer

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