Transport Layer 3-1

Outline

TCP Congestion control Flow control

Transport Layer 3-2

Principles of Congestion Control

Congestion: informally: “too many sources sending too

much data too fast for network to handle”

manifestations: lost packets (buffer overflow at routers) long delays (queuing in router buffers)

Transport Layer 3-3

Causes/costs of congestion: scenario 1

two senders, two receivers

one router, infinite buffers

large delays when congested

unlimited shared output link buffers

Host Ain : original data

Host B

out

Transport Layer 3-4

Causes/costs of congestion: scenario 2

one router, finite buffers sender retransmission of lost packet

finite shared output link buffers

Host A in : original data

Host B

out

'in : original data, plus retransmitted data

Transport Layer 3-5

Causes/costs of congestion: scenario 3 four senders multihop paths timeout/retransmit

Q: what happens as number of senders increase?

finite shared output link buffers

Host Ain : original data

Host B

out

'in : original data, plus retransmitted data

Transport Layer 3-6

TCP congestion control: cwnd goal: TCP sender should transmit as fast as

possible, but without congesting network Q: how to find rate just below congestion level

each TCP sender sets its own rate, called congestion window (cwnd) based on implicit feedback: ACK: segment received (a good thing!),

network not congested so increase sending rate

lost segment: assume loss due to congested network, so decrease sending rate

Transport Layer 3-7

TCP Slow Start

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer 3-8

TCP: congestion avoidance

Increasing sending rate:

How far would the doubling of cwnd go?

Till, it reaches a threshold

After that it increases linearly

Decrease sending rate

Set the threshold value to half of current cwnd

loss: decrease cwnd to 1 and start the slow-start again

What if a loss happens?

Transport Layer 3-9

Popular “flavors” of TCP

ssthresh

ssthresh

TCP Tahoe

TCP Reno

Transmission round

cwnd w

indow

siz

e (

in

segm

ents

)

Transport Layer 3-10

Summary: TCP Congestion Control

when cwnd < ssthresh, sender in slow-start phase, window grows exponentially.

when cwnd >= ssthresh, sender is in congestion-avoidance phase, window grows linearly.

when loss/timeout occurs, ssthresh set to cwnd/2, cwnd set to 1

Transport Layer 3-11

TCP Congestion Control

Numerical example: Assume TCP Tahoe

Initial ssthreshold = 14

First loss occurrence after 9 transmissions. What would be the current congestion window and ssthreshold?

Transport Layer 3-12

TCP Flow Control

receive side of TCP connection has a receive buffer:

speed-matching service: matching send rate to receiving application’s drain rate

app process may be slow at reading from buffer

sender won’t overflow

receiver’s buffer bytransmitting too

much, too fast

flow control

IPdatagrams

TCP data(in buffer)

(currently)unused buffer

space

applicationprocess

Transport Layer 3-13

TCP Flow control: how it works

(suppose TCP receiver discards out-of-order segments)

unused buffer space:= rwnd

= RcvBuffer-[LastByteRcvd - LastByteRead]

receiver: advertises unused buffer space by including rwnd value in segment header

sender: limits # of unACKed bytes to rwnd guarantees receiver’s

buffer doesn’t overflow

IPdatagrams

TCP data(in buffer)

(currently)unused buffer

space

applicationprocess

rwndRcvBuffer

Transport Layer 3-14

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

Transport Layer 3-15

UDP: User Datagram Protocol [RFC 768]

“no frills,” “bare bones” Internet transport protocol

“best effort” service, UDP segments may be: lost delivered out of order

to app connectionless:

no handshaking between UDP sender, receiver

each UDP segment handled independently of others

Why is there a UDP? no connection

establishment (which can add delay)

simple: no connection state at sender, receiver

small segment header no congestion control:

UDP can blast away as fast as desired

Transport Layer 3-16

UDP: more

often used for streaming multimedia apps loss tolerant rate sensitive

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header