TCP and UDP: Connection-Oriented and Connectionless Protocols - Prof. Bradley Noble, Study notes of Electrical and Electronics Engineering

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Reliable Byte-Stream (TCP) Outline

Spring 2002

CS 461

Outline

Connection Establishment/TerminationSliding Window RevisitedFlow ControlAdaptive Timeout

End-to-End Protocols

-^

Underlying best-effort network^ –

drop messages

-^

re-orders messages

-^

delivers duplicate copies of a given message

-^

limits messages to some finite size

-^

delivers messages after an arbitrarily long delay

-^

Common end

• to
• end services

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-^

Common end

• to
• end services

–^

guarantee message delivery

-^

deliver messages in the same order they are sent

-^

deliver at most one copy of each message

-^

support arbitrarily large messages

-^

support synchronization

-^

allow the receiver to flow control the sender

-^

support multiple application processes on each host

TCP Overview

-^

Connection-oriented

-^

Byte-stream^ –

app writes bytes

-^

TCP sends

segments

–^

app reads bytes

-^

Full duplex

-^

Flow control: keep senderfrom overrunning receiver

-^

Congestion control: keepsender from overrunning network

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network

Application process

Writebytes

TCP Send buffer

Segment

Segment

Segment

Transmit segments

Application process

Readbytes

TCP Receive buffer

■ ■ ■

Data Link Versus Transport

-^

Potentially connects many different hosts^ – need explicit connection establishment and termination

-^

Potentially different RTT^ – need adaptive timeout mechanism

-^

Potentially long delay in network Spring 2002

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-^

Potentially long delay in network^ – need to be prepared for arrival of very old packets

-^

Potentially different capacity at destination^ – need to accommodate different node capacity

-^

Potentially different network capacity^ – need to be prepared for network congestion

Segment Format (cont)

-^

Each connection identified with 4-tuple:^ –

(SrcPort, SrcIPAddr, DsrPort, DstIPAddr)

-^

Sliding window + flow control^ –

acknowledgment, SequenceNum, AdvertisedWinow

Sender

Data

(SequenceNum)

Receiver

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-^

Flags^ –

SYN, FIN, RESET, PUSH, URG, ACK

-^

Checksum^ –

pseudo header + TCP header + data

Sender

Acknowledgment +AdvertisedWindow

Receiver

Connection Establishment and

Termination

Active participant

(client)

Passive participant

(server)

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Sliding Window Revisited

Sending application LastByteWritten

TCP LastByteSent

LastByteAcked

Receiving application

LastByteRead

TCP LastByteRcvd

NextByteExpected

(a)

(b)

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-^

Sending side^ –

LastByteAcked

LastByteSent

LastByteSent

LastByteWritten

-^

buffer bytes between LastByteAcked

and

LastByteWritten

-^

Receiving side^ –

LastByteRead

NextByteExpected

NextByteExpected

LastByteRcvd +

-^

buffer bytes between NextByteRead

and

LastByteRcvd

Flow Control

-^

Send buffer size:

MaxSendBuffer

-^

Receive buffer size:

MaxRcvBuffer

-^

Receiving side^ –

LastByteRcvd

-^

LastByteRead

MaxRcvBuffer

AdvertisedWindow

MaxRcvBuffer

NextByteExpected

NextByteRead

-^

Sending side Spring 2002

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11

-^

Sending side^ –

LastByteSent

-^

LastByteAcked

AdvertisedWindow

EffectiveWindow

AdvertisedWindow

LastByteSent

LastByteAcked

LastByteWritten

-^

LastByteAcked

MaxSendBuffer

–^

block sender if (

LastByteWritten

LastByteAcked

y

MaxSenderBuffer

-^

Always send ACK in response to arriving data segment

-^

Persist when

AdvertisedWindow =

Nagle’s Algorithm

-^

How long does sender delay sending data?^ – too long: hurts interactive applications– too short: poor network utilization^ –

strategies: timer

• based vs self
• clocking

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strategies: timer

• based vs self
• clocking -^

When application generates additional data^ – if fills a max segment (and window open): send it– else

•^

if there is unack’ed data in transit: buffer it until ACK arrives

• else: send it

Protection Against Wrap Around

-^

32-bit

SequenceNum

Bandwidth

Time Until Wrap Around

T1 (1.5 Mbps)

6.4 hours

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Ethernet (10 Mbps)

57 minutes

T3 (45 Mbps)

13 minutes

FDDI (100 Mbps)

6 minutes

STS-3 (155 Mbps)

4 minutes

STS-12 (622 Mbps)

55 seconds

STS-24 (1.2 Gbps)

28 seconds

TCP Extensions

-^

Implemented as header options

-^

Store timestamp in outgoing segments

-^

Extend sequence space with 32

• bit timestamp

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-^

Extend sequence space with 32

• bit timestamp

(PAWS)

-^

Shift (scale) advertised window

Adaptive Retransmission

(Original Algorithm)

-^

Measure

SampleRTT

for each segment / ACK pair

-^

Compute weighted average of RTT^ –

EstRTT

α

x

EstRTT

β

x

SampleRTT

where

α

β

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where

α

β

α

between 0.8 and 0.

β

between 0.1 and 0.

-^

Set timeout based on

EstRTT

TimeOut

x

EstRTT

Jacobson/ Karels Algorithm

-^

New Calculations for average RTT

Diff

SampleRTT

EstRTT

EstRTT = EstRTT + (

x

Diff)

Dev =

Dev +

|Diff| - Dev)

–^

where

δ

is a factor between 0 and 1

-^

Consider variance when setting timeout value

Spring 2002

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-^

Consider variance when setting timeout value

TimeOut

μ

x

EstRTT

φ

x

Dev

–^

where

= 1 and

φ^

-^

Notes^ –

algorithm only as good as granularity of clock (500ms on Unix)

-^

accurate timeout mechanism important to congestion control (later)