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Chapter 5 End-to-End Protocols

Outline5.1 UDP5.2 TCP5.3 Remote Procedure Call

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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– 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

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5.1 Simple Demultiplexor (UDP)• Unreliable and unordered datagram service• Adds multiplexing• No flow control• Endpoints identified by ports

– servers have well-known ports– see /etc/services on Unix

• Header format

• Optional checksum– psuedo header + UDP header + data

SrcPort DstPort

ChecksumLength

Data

0 16 31

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5.2 Reliable Byte-Stream (TCP)Outline

5.2.1 End-to-end Issues5.2.2 Segment Format5.2.3 Connection Establishment/Termination5.2.4 Sliding Window Revisited 5.2.5 Triggering Transmission

Silly Window Syndrome Nagle’s Algorithm5.2.6 Adaptive Retransmission Original Algorithm Karn/Partridge Algorithm Jacobson/Karels Algorithm5.2.7 Record Boundaries5.2.8 TCP Extensions

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TCP Overview

• Connection-oriented• Byte-stream

– app writes bytes– TCP sends segments– app reads bytes

• Full duplex• Flow control: keep sender from

overrunning receiver• Congestion control: keep sender

from overrunning network

Application process

Writebytes

TCP

Send buffer

Segment Segment Segment

Transmit segments

Application process

Readbytes

TCP

Receive buffer

■ ■ ■

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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– 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

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5.2.2 Segment Format

Options (variable)

Data

Checksum

SrcPort DstPort

HdrLen 0 Flags

UrgPtr

AdvertisedWindow

SequenceNum

Acknowledgment

0 4 10 16 31

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Segment Format (cont)• Each connection identified with 4-tuple:

– (SrcPort, SrcIPAddr, DsrPort, DstIPAddr)

• Sliding window + flow control– acknowledgment, SequenceNum, AdvertisedWinow

• Flags– SYN, FIN, RESET, PUSH, URG, ACK

• Checksum– pseudo header + TCP header + data

Sender

Data (SequenceNum)

Acknowledgment +AdvertisedWindow

Receiver

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5.2.3 Connection Establishment and Termination

Active participant(client)

Passive participant(server)

SYN, SequenceNum =x

ACK, Acknowledgment =y+1

Acknowledgment =x+1

SYN+ACK,

SequenceNum=y,

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State Transition DiagramCLOSED

LISTEN

SYN_RCVD SYN_SENT

ESTABLISHED

CLOSE_WAIT

LAST_ACKCLOSING

TIME_WAIT

FIN_WAIT_2

FIN_WAIT_1

Passive open Close

Send/SYNSYN/SYN + ACK

SYN + ACK/ACK

SYN/SYN + ACK

ACK

Close/FIN

FIN/ACKClose/FIN

FIN/ACKACK + FIN/ACK

Timeout after twosegment lifetimes

FIN/ACK

ACK

ACK

ACK

Close/FIN

Close

CLOSED

Active open/SYN

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5.2.4 Sliding Window Revisited

• Sending side– LastByteAcked < = LastByteSent

– LastByteSent < = LastByteWritten

– buffer bytes between LastByteAcked and LastByteWritten

• Receiving side– LastByteRead < NextByteExpected

– NextByteExpected < = LastByteRcvd +1

– buffer bytes between NextByteRead and LastByteRcvd

Sending application

LastByteWrittenTCP

LastByteSentLastByteAcked

Receiving application

LastByteReadTCP

LastByteRcvdNextByteExpected

(a) (b)

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Flow Control• Send buffer size: MaxSendBuffer• Receive buffer size: MaxRcvBuffer• Receiving side

– LastByteRcvd - LastByteRead < = MaxRcvBuffer– AdvertisedWindow = MaxRcvBuffer - (NextByteExpected - NextByteRead)

• 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 = 0

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Protection Against Wrap Around

• 32-bit SequenceNum

Bandwidth Time Until Wrap AroundT1 (1.5 Mbps) 6.4 hoursEthernet (10 Mbps) 57 minutesT3 (45 Mbps) 13 minutesFDDI (100 Mbps) 6 minutesSTS-3 (155 Mbps) 4 minutesSTS-12 (622 Mbps) 55 secondsSTS-24 (1.2 Gbps) 28 seconds

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Keeping the Pipe Full

• 16-bit AdvertisedWindow

Bandwidth Delay x Bandwidth ProductT1 (1.5 Mbps) 18KBEthernet (10 Mbps) 122KBT3 (45 Mbps) 549KBFDDI (100 Mbps) 1.2MBSTS-3 (155 Mbps) 1.8MBSTS-12 (622 Mbps) 7.4MBSTS-24 (1.2 Gbps) 14.8MB

assuming 100ms RTT

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5.2.5 Triggering Transmission

• How aggressively does sender exploit open window?

• If the sender aggressively fills an empty container as soon as it arrives, then any small container introduced into the system remains in the system indefinitely.

• Receiver-side solutions– after advertising zero window, wait for space equal to a maximum segment

size (MSS)– delayed acknowledgements

Sender Receiver

Silly Window Syndrome

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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 (ack)

• 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

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5.2.6 Adaptive Retransmission(Original Algorithm)

• Measure SampleRTT for each segment / ACK pair• Compute weighted average of RTT

– EstRTT = x EstRTT + x SampleRTT– where + = 1 between 0.8 and 0.9 between 0.1 and 0.2

• Set timeout based on EstRTT– TimeOut = 2 x EstRTT

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Karn/Partridge Algorithm

• Do not sample RTT when retransmitting • Double timeout after each retransmission

Sender Receiver

Original transmission

ACK

Retransmission

Sender Receiver

Original transmission

ACK

Retransmission

(a) (b)

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Jacobson/ Karels Algorithm• New Calculations for average RTT• Diff = SampleRTT - EstRTT• EstRTT = EstRTT + ( x Diff)• Dev = Dev + ( |Diff| - Dev)

– where is a factor between 0 and 1

• Consider variance when setting timeout value• TimeOut = x EstRTT + x Dev

– where = 1 and = 4

• Notes– algorithm only as good as granularity of clock (500ms on Unix)– accurate timeout mechanism important to congestion control (later)

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5.2.7 Record Boundaries

• TCP is a byte-stream protocol, the number of bytes written by the sender are not necessarily the same as the number of bytes read by the receiver.

• How to insert “record boundaries” into this byte stream ?

• URG flag and UrgPtr field (out-of-band data)

• PUSH operation

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5.2.8 TCP Extensions

• Mitigate some problem that TCP is facing as the underlying network gets faster.

• Implemented as header options– Store timestamp in outgoing segments to improve

TCP’s timeout mechanism.– Extend sequence space with 32-bit timestamp– Shift (scale) advertised window (larger window)