1

Outline

• Principles of congestion control

• TCP congestion control

2

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!

3

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

Two broad approaches towards congestion control:

4

TCP Congestion Control

• end-end control (no network assistance)

• sender limits transmission:

LastByteSent-LastByteAcked

CongWin

• Roughly,

• CongWin is dynamic, function of perceived network congestion

How does sender perceive congestion?

• loss event = timeout or 3 duplicate acks

• TCP sender reduces rate (CongWin) after loss event

three mechanisms:

– AIMD

– slow start

– conservative after timeout events

rate = CongWin

RTT Bytes/sec

5

TCP AIMD

8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow

multiplicative decrease: cut CongWin in half after loss event

additive increase: increase CongWin by 1 MSS every RTT in the absence of loss events: probing

Long-lived TCP connection

6

TCP Slow Start

• When connection begins, CongWin = 1 MSS

– Example: MSS = 500 bytes & RTT = 200 msec

– initial rate = 20 kbps

• available bandwidth may be >> MSS/RTT

– desirable to quickly ramp up to respectable rate

When connection begins, increase rate exponentially fast until first loss event

7

TCP Slow Start (more)

• When connection begins, increase rate exponentially until first loss event:

– double CongWin every RTT

– done by incrementing CongWin for every ACK received

• Summary: initial rate is slow but ramps up exponentially fast

Host A

one segment

RT

T

Host B

time

two segments

four segments

8

Refinement• After 3 dup ACKs:

– CongWin is cut in half

– window then grows linearly

• But after timeout event:

– CongWin instead set to 1 MSS;

– window then grows exponentially

– to a threshold, then grows linearly

• 3 dup ACKs indicates network capable of delivering some segments• timeout before 3 dup ACKs is “more alarming”

Philosophy:

9

Refinement (more)Q: When should the

exponential increase switch to linear?

A: When CongWin gets to 1/2 of its value before timeout.

Implementation:

• Variable Threshold

• At loss event, Threshold is set to 1/2 of CongWin just before loss event

10

Summary: TCP Congestion Control

• When CongWin is below Threshold, sender in slow-start phase, window grows exponentially.

• When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly.

• When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold.

• When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.

11

TCP sender congestion control

Event State TCP Sender Action Commentary

ACK receipt for previously unacked data

Slow Start (SS)

CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance”

Resulting in a doubling of CongWin every RTT

ACK receipt for previously unacked data

CongestionAvoidance (CA)

CongWin = CongWin+MSS * (MSS/CongWin)

Additive increase, resulting in increase of CongWin by 1 MSS every RTT

Loss event detected by triple duplicate ACK

SS or CA Threshold = CongWin/2, CongWin = Threshold,Set state to “Congestion Avoidance”

Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS.

Timeout SS or CA Threshold = CongWin/2, CongWin = 1 MSS,Set state to “Slow Start”

Enter slow start

Duplicate ACK

SS or CA Increment duplicate ACK count for segment being acked

CongWin and Threshold not changed

12

TCP throughput

• What’s the average throughout ot 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

13

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

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

14

Why is TCP fair?Two competing sessions:

• Additive increase gives slope of 1, as throughout increases

• multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughput

Con

nect

ion

2 th

roug

h pu t

congestion avoidance: additive increaseloss: decrease window by factor of 2

congestion avoidance: additive increaseloss: decrease window by factor of 2

15

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

• Research area: TCP friendly

Fairness and parallel TCP connections

• nothing prevents app from opening parallel cnctions between 2 hosts.

• Web browsers do this

• Example: link of rate R supporting 9 cnctions;

– new app asks for 1 TCP, gets rate R/10

– new app asks for 11 TCPs, gets R/2 !

Midterm Review

In class, 3:30-5:00 pm, Mon. 2/9

Closed Book

One 8.5” by 11” sheet of paper permitted (single side)

Lecture 1

• Internet Architecture

• Network Protocols

• Network Edge

• A taxonomy of communication networks

• The fundamental question: how is data transferred through net (including edge & core)?

• Communication networks can be classified based on how the nodes exchange information:

A Taxonomy of Communication Networks

Communication Networks

SwitchedCommunication

Network

BroadcastCommunication

Network

Circuit-Switched

Communication Network

Packet-Switched

Communication Network

Datagram Network

Virtual Circuit Network

TDM FDM

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern statistical multiplexing.

In TDM each host gets same slot in revolving TDM frame.

A

B

C10 MbsEthernet

1.5 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

Packet Switching versus Circuit Switching

• Circuit Switching

– Network resources (e.g., bandwidth) divided into “pieces” for allocation

– Resource piece idle if not used by owning call (no sharing)

– NOT efficient !

• Packet Switching:

– Great for bursty data

– Excessive congestion: packet delay and loss

– protocols needed for reliable data transfer, congestion control

Datagram Packet Switching

• Each packet is independently switched

– Each packet header contains destination address which determines next hop

– Routes may change during session

• No resources are pre-allocated (reserved) in advance

• Example: IP networks

Virtual-Circuit Packet Switching

• Hybrid of circuit switching and packet switching

– All packets from one packet stream are sent along a pre-established path (= virtual circuit)

– Each packet carries tag (virtual circuit ID), tag determines next hop

• Guarantees in-sequence delivery of packets

• However, packets from different virtual circuits may be interleaved

Lecture 2

• Network access and physical media

• Internet structure and ISPs

• Delay & loss in packet-switched networks

• Protocol layers, service models

Internet structure: network of networks

• “Tier-3” ISPs and local ISPs

– last hop (“access”) network (closest to end systems)

– Tier-3: Turkish Telecom, Minnesota Regional Network

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Four sources of packet delay

• 1. processing:

– check bit errors

– determine output link

A

B

propagation

transmission

processingqueueing

• 2. queueing

– time waiting at output link for transmission

– depends on congestion level of router

Delay in packet-switched networks

3. Transmission delay:

• R=link bandwidth (bps)

• L=packet length (bits)

• time to send bits into link = L/R

4. Propagation delay:

• d = length of physical link

• s = propagation speed in medium (~2x108 m/sec)

• propagation delay = d/s

A

B

propagation

transmission

processingqueueing

Note: s and R are very different quantities!

Internet protocol stack• application: supporting network

applications

– FTP, SMTP, STTP

• transport: host-host data transfer

– TCP, UDP

• network: routing of datagrams from source to destination

– IP, routing protocols

• link: data transfer between neighboring network elements

– PPP, Ethernet

• physical: bits “on the wire”

application

transport

network

link

physical

Application Layer• Principles of app layer protocols

• Web and HTTP

• FTP

• Electronic Mail: SMTP, POP3, IMAP

• DNS

• Socket Programming

• Web Caching

HTTP connections

Nonpersistent HTTP

• At most one object is sent over a TCP connection.

• HTTP/1.0 uses nonpersistent HTTP

Persistent HTTP

• Multiple objects can be sent over single TCP connection between client and server.

• HTTP/1.1 uses persistent connections in default mode

• HTTP Message, Format, Response, Methods• HTTP cookies

Response Time of HTTP

Nonpersistent HTTP issues:

• requires 2 RTTs per object

• OS must work and allocate host resources for each TCP connection

• but browsers often open parallel TCP connections to fetch referenced objects

Persistent HTTP

• server leaves connection open after sending response

• subsequent HTTP messages between same client/server are sent over connection

Persistent without pipelining:

• client issues new request only when previous response has been received

• one RTT for each referenced object

Persistent with pipelining:

• default in HTTP/1.1

• client sends requests as soon as it encounters a referenced object

• as little as one RTT for all the referenced objects

FTP: separate control, data connections

• FTP client contacts FTP server at port 21, specifying TCP as transport protocol

• Client obtains authorization over control connection

• Client browses remote directory by sending commands over control connection.

• When server receives a command for a file transfer, the server opens a TCP data connection to client

• After transferring one file, server closes connection.

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

• Server opens a second TCP data connection to transfer another file.

• Control connection: “out of band”

• FTP server maintains “state”: current directory, earlier authentication

Electronic Mail: SMTP [RFC 2821]• uses TCP to reliably transfer

email message from client to

server, port 25

• direct transfer: sending

server to receiving server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

DNS name servers

• no server has all name-to-IP address mappings

local name servers:

– each ISP, company has local (default) name server

– host DNS query first goes to local name server

authoritative name server:

– for a host: stores that host’s IP address, name

– can perform name/address translation for that host’s name

Why not centralize DNS?

• single point of failure

• traffic volume

• distant centralized database

• maintenance

doesn’t scale!

DNS example

Root name server:

• may not know authoritative name server

• may know intermediate name server: who to contact to find authoritative name server

requesting hostsurf.eurecom.fr

www.cs.nwu.edu

root name server

local name serverdns.eurecom.fr

1

23

4 5

6

authoritative name serverdns.cs.nwu.edu

intermediate name serverdns.nwu.edu

7

8

DNS: iterated queries

recursive query:

• puts burden of name resolution on contacted name server

• heavy load?

iterated query:

• contacted server replies with name of server to contact

• “I don’t know this name, but ask this server”

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4

5 6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated query

Web caches (proxy server)

• user sets browser: Web accesses via cache

• browser sends all HTTP requests to cache

– object in cache: cache returns object

– else cache requests object from origin server, then returns object to client

• Why web caching?

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

origin server

origin server

Caching example (3)Install cache

• suppose hit rate is .4

Consequence

• 40% requests will be satisfied almost immediately

• 60% requests satisfied by origin server

• utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)

• total delay = Internet delay + access delay + LAN delay

= .6*2 sec + .6*.01 secs + milliseconds < 1.3 secs

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Transport Layer

• Transport-layer services

• Multiplexing and demultiplexing

• Connectionless transport: UDP

• Principles of reliable data transfer

• TCP

– Segment structures

– Flow control

– Congestion control

Demultiplexing• UDP socket identified

by two-tuple:

(dest IP address, dest port number)

• When host receives UDP segment:

– checks destination port number in segment

– directs UDP segment to socket with that port number

• TCP socket identified by 4-tuple:

– source IP address

– source port number

– dest IP address

– dest port number

• recv host uses all four values to direct segment to appropriate socket

UDP: User Datagram Protocol [RFC 768]

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

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksum

UDP checksum

Sender:

• treat segment contents as sequence of 16-bit integers

• checksum: addition (1’s complement sum) of segment contents

• sender puts checksum value into UDP checksum field

Receiver:

• addition of all segment contents + checksum

• check if all bits are 1:

– NO - error detected

– YES - no error detected. But maybe errors nonetheless? More later ….

Goal: detect “errors” (e.g., flipped bits) in transmitted segment

0110010110110100

Addition:1’s complement sum:

0110010101001111

1’s complement sum:Addition:

Go-Back-NSender:

• k-bit seq # in pkt header

• “window” of up to N, consecutive unack’ed pkts allowed

• ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK”

– may deceive duplicate ACKs (see receiver)

• Single timer for all in-flight pkts

• timeout(n): retransmit pkt n and all higher seq # pkts in window

Selective Repeat

• receiver individually acknowledges all correctly received pkts

– buffers pkts, as needed, for eventual in-order delivery to upper layer

• sender only resends pkts for which ACK not received

– sender timer for each unACKed pkt

• sender window

– N consecutive seq #’s

– again limits seq #s of sent, unACKed pkts

Selective repeat: sender, receiver windows

TCP segment structure

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)