4 transport-sharing

Week 4Reliable transportSharing resources

Agenda

•Reliable transport

•Multiplexing

•Connection establishment

•Data transfer

•Connection release

•Sharing resources

Multiplexing applications

•How can we multiplex data from several applications running on the same host ?

Multiplexing

ServerClientSource port : 1234

Destination port: 5678

Request

Response

Source port : 5678Destination port: 1234

Agenda

•Reliable transport

•Multiplexing

•Connection establishment

•Data transfer

•Connection release

•Sharing resources

Connection establishment

•How to reliably open a connection ?

Connect.reqConnect.ind

CR

CAConnection established

Connect.respConnect.conf

Connection established

Segment loss

Connect.req()

Connect.ind()

Connect.conf() CA Connection established

Connection established

CR

CR Retransmission timer expires

Connect.resp()

Segments delayed

Connect.ind()

CR

Connect.conf() CA

CR

Old previous CR

First connection established

How to detect duplicates ?

Connect.req()

D

CA

Connect.respFirst connection established

First connection stopped First connection stopped

Delayed segments

•How to deal with delayed segments ?

•Network level guarantee

•No packet will survive more than MSL seconds inside the network

•Transport entities use on a local clock to detect duplicated connection establishment requests

Three way handshake

CR (seq=x)

CA (seq=y, ack=x)

CA (seq=x, ack=y)

Sequence number x readfrom local transport clock

Local state :Connection to B : - Wait for ack for CR (x)- Start retransmission timer

Sequence number y read fromlocal transport clockCA sent to ack CR

Local state :Connection to A :

- Wait for ack for CA(y)

Received CA acknowledges CRSend CA to ack received CA

Local state :Connection to B :

- established- current_seq = x

The sequence numbers used for the data segments will startfrom x

The sequence numbersused for the data segmentswill start from y

D(x)

D(y)

Local state :Connection to A :

- established- current_seq=yConnection established

Connection established

Host A Host B

Three way handshake (2)

CA (seq=y, ack=z)

CR (seq=z)

REJECT (ack=y)

Connection cancelled

No connection is established

Host A Host BSequence number y read from

local transport clockAcknowledges CR segment

Local state :Connection to A :

- Wait for ack for CA(y)

Local state :No connection to B

Send REJECT to cancelconnection establishment

Three way handshake (3)

CR (seq=z)

Current state does not contain a CR with seq=x

REJECT (ack=y)

Connection established

CR (seq=z)Retransmission timerexpires

CA (seq=w, ack=z)

CA (seq=z, ack=w)

CA (seq=y, ack=x)

Sequence number z readfrom local transport clock

Local state :Connection to B :

- Wait for ack for CR (z)- Start retransmission timer

Host A Host B

Current state does not contain a segment with seq=y

REJECT ignored

Sequence number w read fromlocal transport clockCA sent to ack CR

Local state :Connection to A :

- Wait for ack for CA(w)

Received CA acknowledges CRSend CA to ack received CA

Local state :Connection to B :

- established- current_seq = z

Three way handshake (4)

Invalid CA received from ASend REJECT

CA (seq=w, ack=z)

CR (seq=z)

CA (seq=z, ack=y)

REJECT (ack=w)

REJECT (ack=z)

Sequence number w read fromlocal transport clock

Acknowledges CR segmentLocal state :

Connection to A : - Wait for ack for CA(w)

Current state does not contain a CR with seq=z

Host A Host B

No connection is established

Agenda

•Reliable transport

•Multiplexing

•Connection establishment

•Data transfer

•Connection release

•Sharing resources

Reliable data transfer

•What are the differences with the reliable protocols of the datalink layer ?

•Segments can be reordered

•Buffers can change dynamically

•Bytestream service

Retransmission policies

•Which retransmission policy in reliable transport protocols ?

•Alternating Bit

•Go-back-n

•Selective repeat

Buffer management

•A transport entity serves a variable number of applications with a limited buffer

•The buffer/window allocated to a given connection may need to change dynamically as connections start and stop

Buffer management A B

Data.req(a)

Data.ind(a)

D(0,a)

C(OK,0, w=1)

C(OK,0,w=3)

Data.req(c)D(2,c)

2 new buffers becomeavailable

Data.req(b)

Data.ind(b)

D(1,b)

0 1 2 3

0 1 2 3

0 1 2 3

C(OK,1,w=3)

Rwin=1

0 1 2 3

Swin=3, rwin=1

0 1 2 3

Swin=3, rwin=1

0 1 2 3

Swin=3, rwin=1

0 1 2 3

Swin=3, rwin=1

Swin=3, rwin=3

0 1 2 3 Data.req(d)

D(3,d) 0 1 2 3

Buffer management A B

Data.req(b)

Data.ind(a)

Data.req(a)

D(0,a)

C(OK,0, w=0)

2 new buffers areavailable

0 1 2 3

Rwin=1

0 1 2 3

Swin=3, rwin=1

0 1 2 3

Swin=3, rwin=1

0 1 2 3

Receiver cannot handle segment immediately

C(OK,0,w=3)

Lost segment0 1 2 3

Swin=3, rwin=0

Window blockedNo transmission possible

Waits for control segment Waits for data segment

How to recover from deadlock ?Persitence timer on receiver, resend control segment after

timer expiration

Delayed segments

A BD(1,b)

Timer expirationRetransmission D(1,b)

D(3,d)

C(OK,0)

C(OK,0)

C(OK,3)

C(OK,0)

C(OK,1)

D(0,e) Data.ind(e)

Data.ind(b) !!!!!!!!!!!!

D(0,a)

Data.req(a)

Data.ind(a)

Data.ind(b)

Data.ind(e)

Delayed segments

•How to deal with them ?

•Packets cannot live more than MSL seconds inside the network

•Only one segment carrying sequence number x can be transmitted during MSL seconds

•upper bound on maximum throughput

Bidirectional transfer

•How to efficiently carry data in both directions ?

PiggybackingA B

Data.req(a)

Data.ind(a)

D(0,0,a)Data.req(b)

D(1,0,b)

Error

Discarded

Data.req(c)

D(2,0,c)

Segment -> bufferRetransmission

Data.ind(b)

D(1,5,b)

Data.ind(c)

Data.req(d)

D(3,6,d)

Data.ind(d)C(OK,2)

C(OK,3)

Data.req(x)D(5,0,w)

Data.req(w)

Data.ind(w)

D(6,0,x)

Data.ind(x)

D(5,0,w) acks D(0,0,a)

Bytestream

•How to provide a bytestream service ?

Byte stream service (2)

A B

Data.req(ijkl)

Data.req(mnop)

Data.req(abcdef)

Data.ind(ab)

D(0,ab)

C(OK,1)

C(OK,1)

D(2,cd)

Lost segment

D(4,ef)

Placed in buffer

Data.ind(cdef)

D(2,cd)

Expiration timerRetransmission

D(6,ijklmnop)

Data.ind(ijklmnop)C(OK,5)

C(OK,13)

Agenda

•Reliable transport

•Multiplexing

•Connection establishment

•Data transfer

•Connection release

•Sharing resources

Connection release

•Graceful release

•Data transfer is finished and connection must be terminated

•Abrupt release

•Something went wrong and the connection must be closed immediately

•Data can be lost !

Graceful releaseD(‘a’,1233)

DISCONNECT.req (A-B)

DISCONNECT.ind(A-B)

ACK,1234DISCONNECT.conf(A-B)

ACK,4567DISCONNECT.conf(A-B)

DISCONNECT.req(B-A)

DISCONNECT.ind(B-A)

DR(B-A,4567)Outgoing connection (A->B)

closed

Incoming connection (A->B)closed

Incoming connection (B->A)closed

Outgoing connection (B->A)closed

DR(A-B,1234)

DATA.ind(‘a’)

Abrupt release

CR (seq=z)

CA (seq=w, ack=z)

CA (seq=z, ack=w)

D Data.req()

Data.ind()Disc.req()

D Data.req()

DR Disc.req()Connection closed

Connection closed

This segment will not be delivered !

Agenda

•Reliable transport

•Sharing resources

•Which resources need to be shared

•Medium Access Control

•Congestion Control

Network resources

•What are the resources that are shared by multiple users inside a network ?

Sharing bandwidth

•Several nodes on a single link

Agenda

•Reliable transport

•Sharing resources

•Which resources need to be shared

•Medium Access Control

•Congestion Control

How to share access to a link ?

•Deterministic solutions

•Probabilistic solutions

Time Division Multiplexing

ALOHA

The collision problem

AB

collision

Medium Access Control : ALOHAN=1;while ( N<= max) do

send frame;wait for ack on return channel or timeout:if ack on return channel

exit while;else

/* timeout *//* retransmission is needed */N=N+1;

end do/* too many attempts */

CSMA

•Key idea

•Listen to the link before transmitting and only transmit when nobody else transmits

CSMA/CD•Key idea

•Listen to link before transmitting

•Detect collisions

•If a collision occurs, stop transmitting

•Caveat

•Is it possible to detect all collisions ?

CSMA/CA

•Key idea

•In wireless networks, we need to avoid collisions by deferring transmissions

•Possibility of “reserving” transmission slots

Agenda

•Reliable transport

•Sharing resources

•Which resources need to be shared

•Medium Access Control

•Congestion Control

Adapting to different

bandwidth

Self-clocking

The congestion problem

Fairness

•What is the final objective of congestion control ?

•On a single link

•Fair share

•In a large network

•Max-min fairness

Max-min fairness•a max-min allocation of bandwidth is an allocation of bandwidth which maximises the allocation of bandwidth to the sources receiving the smallest allocation

•a max-min fair allocation is such that in order to increase the bandwidth allocated to one source, it is necessary to decrease the bandwidth allocated to another source which already receives a lower allocation

Congestion control

Congestion control•Additive Increase / Multiplicative

Decrease

# Additive Increase Multiplicative Decreaseif congestion : rate=rate*betaC # MD, betaC<1else rate=rate+alphaN # AI

How to detect congestion ?

•Host-based solutions

•Router-based solutions