1

Lecture, November 27, 2002

TCP

Other Internet Protocols; Internet Traffic

Scalability of Virtual Circuit Networks

QoS

2

IPv6flow label

payload length (bytes)

0 16 318

version priority

next header hop limit

128-bit source IP address

128-bit destination IP address

Payload

3

UDP

payload

0 16 318

source port

checksum length

destination port

4

TCP

Supports:Error control. For each segment:

Sequence numbers.Acknowledgment.TimeoutExample – stop and wait protocol. Very inefficient.Window based

Flow ControlCongestion Control

5

(a) no errors orlost PDUs

sender receiver

pdu0

ack0 pdu0

ack0 pdu1

pdu1 ack1

ack1 pdu0

pdu0 ack0

timeout

(detect duplicate)

(d) acknowledment lost

sender sender receiver

(e) duplicate pdu due to premature timeout

(b) lost PDUtime

time

sender receiver

pdu0 pdu0 ack0

timeout

ack0 pdu1

pdu1 pdu1 ack1

ack1 pdu0

sender receiver

pdu0 pdu0 ack0

timeout

ack0 pdu1

pdu1 pdu1 ack1

ack1 pdu0

pdu1(error detected)

(c) PDU in error

pdu0 pdu0 ack0

ack0 pdu1

pdu1 ack1

pdu1 pdu1

ack1 pdu0

pdu0

pdu0

receiver

ack0 ack0

pdu0

pdu1 pdu1 ack1

timeout

ack1 pdu1

pdu1(detect duplicate) pdu0

ack0 pdu0 ack1

6

TCP

options

acknowledgment number

destination port

payload

0 16 318

source port

sequence number

flags

checksum urgent pointer

advertized windowheaderlength

0

7

TCP 1 bit flagsACK – when set the ack value is valid

SYN, RST, FIN – used for connection establishment and tear-down

PUSH – data should be passed to the upper layer immediately.

URG – there is urgent information in the data

8

Data streaming. MSS- maximum segment size

0 536 1072 1608 536,000 536,536

segment 1 segment 2 segment 3 segment 1001

tcp hdr segment 3

ip hdr tcp hdr segment 3

576

556

536

9

TCP flow control window

Application

IP

TCP

Application

IP

TCP

lastByteAcknowledged

lastByteSent

lastByteFromApplication

Sender's Window

lastByteToApplication

nextByteExpected

lastByteReceived

Receiver's Window

Sender Receiver

10

SYN, SequenceNumber = c

Client

SYN+ACK, SequenceNumber = s, Acknowledgment=c+1

ACK, Acknowledgment=s+1

Server

data

11

TCP is a connection-oriented protocol for client-server communication

Three-wayhandshake

ClientProcess

Clientsocket

MainThread of

Server Process

Data

Serversocket

Newsocket

NewThread of

Server Process

Internet

12

CLOSED

LISTEN

SYN_SENTSYN_RECVD

ESTABLISHED

FIN_WAIT1

FIN_WAIT2

CLOSING

TIME_WAIT CLOSED

LAST_ACK

CLOSE_WAIT

Active OpenSYN

Close

ClosePassive Open

SendSYNSYN

SYN +ACK

CloseFIN

SYNSYN +ACK

ACK

CloseFIN

FINACK

ACK

FINACK

ACK

Timeout after twosegment lifetimes

ACK

CloseFIN

FINACK

SYN+ACKACK

Send

Receive

13

TCP congestion control

Host centric, feedback-based resource allocation policy.

The congestion control window is affected by the timing of the acknowledgments. A late or missing acknowledgment signals that the network is congested.

14

1

2

3

4

5

6

7

8

9

10

11

12

13

1 2 3 4 5 6 7 8 9 10 11 12

threshold

new threshold

time

timeout occurs

slowstart

window size

congestionavoidance

15

Other Internet protocols

ICMP – used by hosts and routers to exchange network layer information, e.g., error reporting

RIP – Routing Information Protocol

OSPF – Open Shortest Path First Protocol

16

Internet traffic

TCP 90 – 95 % of the Internet traffic 65-75 % of TCP traffic is Web related10 % of TCP traffic is due to News5 % of TCP traffic is due to Email5 % of TCP traffic is due to FTP1 % of TCP traffic is due to Napster

UDP 5 – 10 % of the Internet traffic DNSRealaudiogames

17

Scalability issues with VCs

Assume a router with n input and n output lines of b Gbpsaverage packet size l bytesdetermine the size of the routing table

18

Switch A Switch B

inPort = 6inVC = 9outPort = 2outVC = 13

2

Host a

6

Host b

5

1

inPort = 6inVC = 11outPort = 2outVC = 15

inPort = 5inVC = 4outPort = 1outVC = 9

inPort = 1inVC = 10outPort = 5outVC = 5

inPort = 2inVC = 12outPort = 6outVC = 8

inPort = 2inVC = 14outPort = 6outVC = 10

12 13 1415

8

9

10

11

Updated forwarding table of Switch A.

inPort inVC outPort outVC 2 12 6 8 6 9 2 13 2 14 6 10 6 11 2 15

3 456

Initial forwarding table of Switch B.

inPort inVC outPort outVC 1 8 5 3 5 4 1 9

Updated forwarding table of Switch B.

inPort inVC outPort outVC 1 8 5 3 5 4 1 9 1 10 5 5 5 6 1 11

Initial forwarding table of Switch A.

inPort inVC outPort outVC 2 12 6 8 6 9 2 13

inPort = 5inVC = 6outPort = 1outVC = 11

9

8

11

inPort = 1inVC = 8outPort = 5outVC = 3

22

19

Original Algorithm for Adaptive Retransmission

Measure SampleRTT for each segment/ACK pair Compute weighted average of RTTEstimatedRTT = x EstimatedRTT + (1- x SampleRTT

where 0.8 < 0.9

Set timeout based on EstimatedRTT TimeOut = 2 x EstimatedRTT

20

Karn/Partridge Algorithm

Do not sample RTT when re-transmitting

Double timeout after each retransmission

21

Karn/Partridge Algorithm

Sender SenderReceiver Receiver

Sample RTT

O rig ina l transm ission

R e-transm ission

A cknow ledgm ent

O rig ina l transm ission

A cknow ledgm ent

R e-transm ission

Sample RTT

22

Jacobson/Karels Algorithm

New calculation for average RTTDiff = SampleRTT - EstimatedRTT EstimatedRTT = EstimatedRTT + ( x Deviation = Deviation + (|Diff|- Deviation) where is a fraction between 0 and 1

Consider variance when setting timeout value TimeOut = x EstimatedRTT + x Deviation where = 1 and = 4

Notes algorithm only as good as granularity of clock (500

microseconds on Unix) accurate timeout mechanism important to congestion

control (later)

23

Congestion Control Mechanisms

The sender must perform retransmissions to compensate for lost packets due to buffer overflow.Unneeded retransmissions by the sender due to large delays causes a router to use link bandwidth to forward unneeded copies of a packet.

When a packet is dropped along a path the capacity used used at each upstream routers to forward packets to the point where it was dropped was wasted.

24

Delay/Throughput Tradeoffs Q uality o f S ervice

(D e lay)

Q uantity o f S ervice(Throughput)

25

1

2

3

4

5

6

7

8

9

10

11

12

13

1 2 3 4 5 6 7 8 9 10 11 12

threshold

new threshold

time

timeout occurs

slowstart

window size

congestionavoidance

26

Router with infinite buffer capacity

H ost A

H ost B

R outerinl

inl

outl

inl

outl

C

C/2

C/2

inlC/2

D elay

27

Fairness of TCP congestion mechanism

Equal bandwidthline

Fu ll bandw idthutiliza tion line

Throughput o fconnection 1

Throughput o fconnection 2

R

R

28

Flows and resource allocation

Flow: sequence of packets with a common characteristics

A layer-N flow the common attribute a layer-N attributeAll packets exchanged between two hosts

network layer flow All packets exchanged between two

processes transport layer flow

29

who makes decisions

host-centric

router-centric

basis fordecisions

the needs of the flow

the state of network

how are decisionsenforced

30

Min-max fair bandwidth allocation

Goal: fairness in a best-effort network.

Consider:Unidirectional flowsRouters with infinite buffer space

Link capacity is the only limiting factor.

31

AlgorithmStart with an allocation of zero Mbps for each flow.Increment equally the allocation for each flow until one of the links of the network becomes saturated. Now all the flows passing through the saturated link get an equal fraction of the link capacity.Increment equally the allocation for each flow that does not pass through the first saturated link until a second link becomes saturated. Now all the flows passing through the saturated link get an equal fraction of the link capacity.Continue by incrementing equally the allocations of all flows that do not use a saturated link until all flows use at least one saturated link.

32

QoS in a datagram network?

Packet Classification.

Buffer acceptance algorithms.

Explicit Congestion Notification.

Flow measurements

33

Packet classification

Identify the flow the packet belongs to.

The edge routers may be able to do that.

MPLS – multi protocol label switch. Add an extra header in front of the IP header. Now a router decides the output link based upon the input link and the MPLS header.

34

Buffer acceptance algorithms

Tail Drop.

RED – Random Early Detection

RIO – Random Early Detection with In and Out packet dropping strategies.

35

1.0

maxDropProb

minThr_out maxThr_out

minThr_in maxThr_in

avgQue

minThr_inmaxThr_in

sampleQueueLength

dropProb

low load

maxThr_out minThr_out

low load

medium load

medium load

high load

high load

out

in

(a)

(b)sampleQueueLength

inout

36

Explicit Congestion Notification (ECN)

Routers could prevent congestion by informing the source of the packets when they become lightly congested, but before they start dropping packets.

This strategy is called source quench.

37

Source quench

A router sets a congestion notification flag in the IP header to inform the destination that signs of congestion are visible.

The destination informs the source by setting a flag in the TCP header of segments carrying acknowledgments.

38

Problems with ECN

(1) TCP must be modified to support the new flag.

(2) Routers must be modified to distinguish between ECN-capable flows and those who do not support ECN.

(3) IP must be modified to support the congestion notification flag.

(4) TCP should allow the sender to confirm the congestion notification to the receiver, because acknowledgments could be lost.

39

Flow measurements

How to choose the measurement interval to accommodate bursty traffic?

Token bucket

40

The token bucket filter

Characterized by : (1) A token rate R, and (2) The depth of the bucket, B

Basic idea the sender is allocated tokens at a given rate and can accumulate tokens in the bucket until the bucket is filled. To send a byte the sender must have a token. The maximum burst can be of size B because at most B token can be accumulated.

41

Example

Flow A: generates data at a constant rate of 1 Mbps. Its filter will support a rate of 1 Mbps and a bucket depth of 1 byte,

Flow B: alternates between 0.5 and 2.0 Mbps. Its filter will support a rate of 1 Mbps and a bucket depth of 1 Mbps

Note: a single flow can be described by many token buckets.

42

Example

43

B: maximum bucket capacity

C: current number oftokens in bucket

Token Bucket

Router

Incoming packets Accepted packets

Droppedpackets

Initially: C = B1/r: filling rate of the bucket

During a period of T seconds thenumber of tokens accepted is:T x r.

r: the average rate of incoming flow

The number of packets acceptedduring a period of T seconds is atmost: B + T x r

A packet of length L: - is dropped if L > C - is accepted if L <= C andconsumes L tokens, C = C - L.

buffer

44

Token bucket

L = packet lengthC = # of tokens in the bucket---------------------------------------------------if ( L <= C ) { accept the packet; C = C - L; }else drop the packet;

45

A shaping buffer delays packets that do not confirm to the traffic shape

if ( L <= C ) {

accept the packet;

C = C - L;}

else { /* the packet arrived early, delay it */

while ( C < L ) {

wait; }

transmit the packet;

C = C - L;}

46

Packet Scheduling

PS and GPS – Processor Sharing & Generalized Processor Sharing

Round Robin, Weighted Round Robin

Priority Scheduling

Weighted Fair Queuing – practical version of GPS. Transmits packets in the order of their finishing time.

47

Weighted queuing

1q

2q

l

l

l

l

3q

4q

48

RSVP- Resource Reservation Protocol

Used to establish a path for a flow and reserve resources along the path.

Requirements:Accommodate faults – soft state.Support unicast as well as multicast.

PATH messages issued by sender includes TSpec

RESV messages issued by the receiver includes RSpec

49

RSVP

router R1

router R2

router R3

Sender

Receiver1

2

3 4

5

6

7

8

1,2,3,4 - PATH messages to establish a path from source to destination;5,6,7,8 - RESV messages sent by destination to reserve resources along the path

50

RSVP message

32-bit destination IP address

32-bit source IP address

header checksum

fragment identifier 13-bit fragment offset

protocol=46TTL -time to live

flags

0 16 318

packet length (bytes)ToShlenversion

checksumtype: PATH RESV

flagsversion

= 1

length of RSVP messagereservedSent_TTL

RSVP messageIt consists of one or more RSVP objects. Each objecthas a class number and type, length, and a value.

RSVPmessage

RSVPheader

IPheader

51

RSVP multicast

(1) Connection request

(3) RESV(TSpec,RSpec)

(2) PATH(TSpec)

(1) Connection request

(2) PATH(TSpec)

(3) RESV(TSpec,RSpec)

Routers

Internet

Receiver 1

Receiver 2

Receiver 3

Receiver 4

Sender 1

Sender 2

Router

Router

Router

Router

Router

Router

Router

Router

Router

52

Integrated Services

Support fine-grain QoS for individual flows.

Mechanisms:Specification of flow requirements - FlowspecsAdmission decisionsResource reservation and policingPolicy enforcement

53

Flowspecs

TSpec – specify the traffic characteristics

Rspec – describe services required from network.

54

Admission decisions

Two classes:Guaranteed Services – based upon token

bucketsControlled Load – approximates a best effort

model in a lightly loaded network.

55

Integrated Service Router

Admission Control

Resource Reservation

RoutingRSVP

RSVPMessages

Shaper

Policer

ClassifierDispatcherand Buffer

Acceptance IPpackets

RoutingMessages

IPpackets

56

Differentiated Services

Two classes of trafficRegularPremium

Edge routers mark the packets.

Premium packets enjoy EF – Expedited ForwardingAF – Assured Forwarding