Lecture 5 Transport [compatibility Mode]

Advanced Computer Networks Lecture 5 The Transport Layer, Transmission Control Protocol (TCP), and User Datagram Protocol (UDP)

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Outline The Transport p Layer y ™ The TCP Protocol ™

¾ ¾ ¾ ¾ ¾ ¾ ¾

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TCP Characteristics TCP Connection setup TCP Segments TCP Sequence Numbers TCP Sliding Window Timeouts and Retransmission (Congestion Control and Avoidance)

The UDP Protocol 2

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The Transport Layer What is the transport layer for? ™ What characteristics might it have? ™

¾ Reliable delivery ¾ Flow control ¾…

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Review of the transport layer Leland.Stanford.edu

Athena.MIT.edu

Application Layer

Nick

Dave Transport Layer

O.S. D

Data

Header

Data

O.S.

Header

Network Layer y

H

D

H

D

D

H

H

D

D

H

H

Link Layer 4

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Layering: The OSI Model layer-to-layer communication Application

Application

Presentation

Presentation

7 6 5 4 3 2 1

7 6

Session

Session

Peer-layer communication

Transport

Transport

Router

Router

Network

Network

Network

Network

Link

Link

Link

Link

Physical

Physical

Physical

Physical

5 4 3 2 1

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Layering: Our FTP Example Application Presentation

FTP ASCII/Binary

Session TCP

Transport

IP

Network

Ethernet

Link

Transport Network Li k Link Physical

Application

The 7-layer OSI Model

The 4-layer Internet Model 6

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TCP Characteristics ™

TCP is connection-oriented. ¾

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TCP provides a stream-of-bytes service. TCP is reliable: ¾ ¾ ¾ ¾ ¾ ¾

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3-way handshake used for connection setup.

Acknowledgements indicate delivery of data. Checksums are used to detect corrupted data. Sequence numbers detect missing, or mis-sequenced data. Corrupted data is retransmitted after a timeout. Mis-sequenced q data is re-sequenced. q (Window-based) Flow control prevents over-run of receiver.

TCP uses congestion control to share network capacity among users. We’ll study this in the next lecture. 7

TCP is connection-oriented (Active) Client

Syn

(Passive) Server

Syn + Ack Ack

(Active) Client

Fin

(Passive) Server

(Data +) Ack Fin Ack

Connection Setup 3-way handshake

Connection Close/Teardown 2 x 2-way handshake 8

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TCP supports a “stream of bytes” service Host A

Host B

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…which is emulated using TCP “segments” Host A

Segment sent when:

TCP Data

Host B

1. Segment full (MSS bytes), 2. Not full, but times out, or 3. “Pushed” by application. TCP Data

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The TCP Segment Format IP Data TCP Data

0

TCP Hdr

15 Src port

31 Dst port

Sequence # Ack Sequence # HLEN 4

RSVD 6

Flags

URG ACK PSH RST SYN FIN

TCP Header and Data + IP Addresses

IP Hdr

Checksum

Window Size

Src/dst port numbers and IP addresses uniquely identify socket

Urg Pointer

(TCP Options)

TCP Data 11

Sequence Numbers Host A

ISN (initial sequence number)

Sequence number = 1st byte

Host B

TCP Data

TCP HDR

TCP Data

Ack sequence number = next expected byte TCP P HDR

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Initial Sequence Numbers (Active) Client

(Passive) Server

Syn +ISNA

Syn + Ack +ISNB Ack

Connection Setup 3-way handshake 13

TCP Sliding Window How much data can a TCP sender have outstanding d in the h network? k ™ How much data should TCP retransmit when an error occurs? Just selectively repeat the missing data? ™ How does the TCP sender avoid overrunning the receiver’s buffers? ™

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TCP Sliding Window Window Size

Data ACK’d

Outstanding Un-ack’d data

Data OK to send

Data not OK to send yet

Window is meaningful to the sender. Current window size is “advertised” by receiver (usually 4k – 8k Bytes when connection set-up). ™ TCP’s Retransmission policy is “Go Back N”. ™ ™

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TCP Sliding Window Round-trip time

Round-trip time Window Size

Host A

Host B

???

Window Size

ACK (1) RTT > Window size

Window Size

ACK ACK (2) RTT = Window size

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TCP: Retransmission and Timeouts Round-trip time (RTT)

Retransmission TimeOut (RTO) Guard Band

Host A

Estimated RTT

Data1

Data2

ACK

ACK

Host B TCP uses an adaptive retransmission timeout value: Congestion RTT changes Changes in Routing frequently 17

TCP: Retransmission and Timeouts Picking the RTO is important: ™ ™

Pick a values that’s too big and it will wait too long to retransmit a packet, Pick a value too small, and it will unnecessarily retransmit packets.

The original algorithm for picking RTO: 1. EstimatedRTTk= α EstimatedRTTk-1 + (1 - α) SampleRTT 2. RTO = 2 * EstimatedRTT Determined empirically

Characteristics of the original algorithm: ™ ™

Variance is assumed to be fixed. But in practice, variance increases as congestion increases.

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TCP: Retransmission and Timeouts ™ ™

Router queues grow when there is more traffic, until they become unstable. As load grows, variance of delay grows rapidly. idl

Average Queueing Delay

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There will be some (unknown) distribution of RTTs. We are trying to estimate an RTO to minimize i i i th the probability b bilit of f a false f l timeout. ti t Probability

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Variance grows rapidly with load

variance

mean

RTT

Load (Amount of traffic arriving to router) 19

TCP: Retransmission and Timeouts Newer Algorithm includes estimate of variance in RTT:

Same as

Difference = SampleRTT - EstimatedRTT before ™ EstimatedRTTk = EstimatedRTTk-1 + (δ*Difference) ™ Deviation = Deviation + δ*( |Difference| - Deviation ) ™

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RTO = μ * EstimatedRTT + φ * Deviation μ≈1 φ≈4

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TCP: Retransmission and Timeouts Karn’s Algorithm

Host A

Host B

Host A

Retransmission Wrong RTT Sample

Host B

Retransmission Wrong RTT Sample

Problem:

How can we estimate RTT when packets are retransmitted?

Solution:

On retransmission, don’t update estimated RTT (and double RTO). 21

User Datagram Protocol (UDP) Characteristics

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UDP is a connectionless datagram service.

¾ There is no connection establishment: packets may show up at any time.

™ ™

UDP packets are self-contained. UDP is unreliable:

¾ No acknowledgements to indicate delivery of data. ¾ Checksums cover the header, and only optionally cover the data. ¾ Contains no mechanism to detect missing or mis-sequenced packets. ¾ No mechanism for automatic retransmission. ¾ No mechanism for flow control, and so can over-run the receiver.

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User-Datagram Protocol (UDP) A1

A2

B1

B2

App

App

App

App

OS UDP

Like TCP, UDP uses port number to demultiplex packets

IP

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User-Datagram Protocol (UDP) Packet format

By default, default only covers the header.

SRC port

DST port

checksum

length DATA

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Why do we have ha e UDP? D ?

¾ It is used by applications that don’t need reliable delivery, or ¾ Applications that have their own special needs, such as streaming of real-time audio/video.

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