3 transport.key

Computer Networking :
Principles, Protocols and Practice
Part 3 : Transport Layer

Olivier Bonaventure
http://inl.info.ucl.ac.be/

CNP3/2008.3. © O. Bonaventure 2008

Module 3 : Transport Layer
 Basics
 Building a reliable transport layer
 Reliable data transmission
 Connection establishment
 Connection release

 UDP : a simple connectionless transport protocol
 TCP : a reliable connection oriented transport
protocol

CNP3/2008.3. © O. Bonaventure, 2008

The transport layer

Transport Segments Transport
Network Network Network
Datalink Datalink Datalink
Physical Physical Physical

 Goals
 Improves the service provided by the network
layer to allow it to be useable by applications
 reliability
 multiplexing

 Transport layer services
 Unreliable connectionless service
 Reliable connection-oriented service

The network layer
Network Packets Network Packets Network
Datalink Datalink Datalink
Physical Physical Physical

 Network layer service in Internet
 Unreliable connectionless service
 Packets can be lost
 Packets can suffer from transmission errors
 Packet ordering is not preserved
 Packet can be duplicated
 Packet size is limited to about 64 KBytes

 How to build a service useable by applications ?


The transport layer

 Problems to be solved by transport layer
 Transport layer must allow two applications to
exchange information
 This requires a method to identify the applications

 The transport layer service must be useable by
applications
 detection of transmission errors
 correction of transmission errors
 recovery from packet losses and packet duplications
 different types of services
 connectionless
 connection-oriented
 request-response


The transport layer (2)
 Internal organisation
 The transport layer uses the service provide by the
network layer
 Two transport layer entities exchanges segments

Application ADU Application

Segment
Transport Transport Transport
layer entity entity

Packet Network
Network


Module 3 : Transport layer
 Basics
 Building a reliable transport layer
 Reliable data transmission
 Connection establishment
 Connection release

 UDP : a simple connectionless transport protocol
 TCP : a reliable connection oriented transport
protocol


Transport layer protocols
 How can we provide a reliable service in the
transport layer
 Hypotheses
1. The application sends small SDUs
2. The network layer provides a perfect service
1. There are no transmission errors inside the packets
2. No packet is lost
3. There is no packet reordering
4. There are no duplications of packets
3. Data transmission is unidirectional


Transport layer protocols (2)

 Reference environment

Application ADU Application

Segment
Transport Transport Transport
layer entity entity

Packet Network
Network

 Notations
 data.req and data.ind primitives for application/
transport interactions
 recv() and send() for interactions between transport
entity and network layer


Protocol 1 : Basics

A B

 Principle
 Upon reception of data.request(SDU), the transport
entity sends a segment containing this SDU through the
network layer (send(SDU))
 Upon reception of the contents of one packet from the
network layer (recv(SDU)), transport entity delivers the
SDU found in the packet to its user by using
data.ind(SDU)


Protocol 1 : Basics

A B
Data.request(a)

 Principle
data.ind(SDU)


Protocol 1 : Basics

A B
Data.request(a) Segment(a)

 Principle
data.ind(SDU)


Protocol 1 : Basics

A B

Data.ind(a)

 Principle
data.ind(SDU)


Protocol 1 as a FSM

 Sender

Data.req(SDU)
Wait
for
send(SDU)
SDU

 Receiver

recvd(SDU)
Wait
for
segment Data.ind(SDU)


Protocol 1 : Example

A B

 Issue
 What happens if the receiver is much slower than the
sender ?
 e.g. receiver can process one segment per second while sender is
producing 10 segments per second ?


A B

Data.ind(a)

 Issue
sender ?


A B
Data.request(b) Segment(b) Data.ind(a)

Data.ind(b)

 Issue
sender ?


A B
Data.request(b) Segment(b) Data.ind(a)

Data.request(c) Data.ind(b)
Segment(c)

Data.ind(c)

 Issue
sender ?

Protocol 2
 Principle
 Use a control segment (OK) that is sent by the
receiver after having processed the received
segment
 creates a feedback loop between sender and
receiver

 Consequences
 Two types of segments
 Data segment containing on SDU
 Notation : D(SDU)
 Control segment
 Notation : C(OK) Type
 Segment format
 At least one bit in the segment header is used to indicate
the type of segment

Protocol 2 (cont.)

 Sender
Data.req(SDU)
send(D(SDU))

Wait Wait
for for
SDU OK
Recvd(C(OK))
 Receiver -

Recvd(D(SDU))
Data.ind(SDU)
Wait Process
for SDU
segment
-
Send(C(OK))


A B

The sender only sends segments when authorised
by the receiver


A B
Data.req(a) D(a)

Data.ind(a)

by the receiver


A B
Data.req(a) D(a)

Data.req(b) Data.ind(a)

by the receiver


A B
Data.req(a) D(a)

Data.req(b) C(OK) Data.ind(a)

by the receiver


A B
Data.req(a) D(a)


D(b)

Data.ind(b)

by the receiver


A B
Data.req(a) D(a)


D(b)
C(OK)
Data.ind(b)

by the receiver

Protocol 3
transport layer
 Hypotheses
1. Transmission errors are possible
2. No packet is lost


Transmission errors

 Which types of transmission errors do we
need to consider in the transport layer ?

Network

 Physical-layer transmission errors caused by
nature
 Random isolated error
 one bit is ﬂipped in the segment
 Random burst error
 a group of n bits inside the segment is errored
 most of the bits in the group are ﬂipped


Security issues versus
transmission errors
 Information sent over a network may
become corrupted for other reasons than
transmission errors

Network


Security issues versus
transmission errors
 Information sent over a network may
become corrupted for other reasons than
transmission errors

Network

 These attacks are dealt by using special
security protocols and mechanisms outside
CNP3/2008.3.
the transport layer © O. Bonaventure, 2008

How to detect transmission errors ?

 Principle

 Sender adds some control information inside the
segment
 control information is computed over the entire segment
and placed in the segment header or trailer

Type+ Control Type Control

 Receiver checks that the received control
information is correct by recomputing it


Parity bits

 Simple solution to detect transmission errors
 Used on slow-speed serial lines
 e.g. modems connected to the telephone network

 Odd Parity
 For each group of n bits, sender computes the
n+1th bit so that the n+1 group contains an odd
number of bits set to 1
 Examples
0011010 1101100

 Even Parity

Parity bits


 Odd Parity
 Examples
0011010 0 1101100

 Even Parity

Parity bits


 Odd Parity
 Examples
0011010 0 1101100 1

 Even Parity

Internet checksum

 Motivation
 Internet protocols are implemented in software
and we would like to have efﬁcient algorithms to
detect transmission errors that are easy to
implement

 Solution
 Internet checksum
 Sender computes for each segment and over the entire
segment the 1s complement of the sum of all the 16
bits words in the segment
 Receiver recomputes the checksum over each received
segment and veriﬁes that it is correct. Otherwise, the


Internet checksum : example
 Assume a segment composed of 48 bits

0110011001101100 0101010101010101 0000111100001111



0110011001101100 0000111100001111
0101010101010101



0110011001101100 0000111100001111
0101010101010101

1011101110111011



0110011001101100
0101010101010101

1011101110111011
0000111100001111



0110011001101100
0101010101010101

1011101110111011
0000111100001111

1100101011001010



0110011001101100
0101010101010101

1011101110111011
0000111100001111

1100101011001010 0011010100110101


Cyclical Redundancy Check
(CRC)
 Principle
 Improve the performance of the Internet
checksum by using polynomial codes
 Sender and receiver agree on r+1 bits pattern called
Generator (G)
 Sender adds r bits of CRC to a d bits data segment
such that the d+r bits pattern is exactly divisible by G
using modulo 2 arithmetic
 D * 2^r XOR R = n*G

d bits r bits

 All computations are done in modulo 2 arithmetic by
using XOR
 1011 + 0101 = 1110 1001 + 1101 = 0100
 1011 - 0101 = 1110 1001 - 1101 = 0100

Detection of transmission errors (2)

 Behaviour of the receiver
 If the checksum is correct
 Send an OK control segment to the sender to
 conﬁrm the reception of the data segment
 allow the sender to send the next segment

 If the checksum is incorrect
 The content of the segment is corrupted and must be
discarded
 Send a special control segment (NAK) to the sender to
ask it to retransmit the corrupted data segment


Protocol 3a : Sender

 Sender

Data.req(SDU) Recvd(C(NAK))
send(D(SDU,CRC)) send(D(SDU,CRC))

Wait Wait
for for
SDU OK/NAK
Recvd(C(OK))
-


Protocol 3a : Receiver

 Receiver Recvd(D(SDU,CRC))
AND IsOK(CRC,SDU)
Data.ind(SDU) Process
segment
OK
Wait
for
segment Send(C(OK))
-

Send(C(NAK))
-

Recvd(D(SDU,CRC)) Process
segment
AND Not(IsOK(CRC,SDU)) KO
-


Protocol 3a : Example

A B



A B
Data.req(a) D(a)

Data.ind(a)



A B
Data.req(a) D(a)

Data.req(b) Data.ind(a)



A B
Data.req(a) D(a)




A B
Data.req(a) D(a)

Transmission error
D(b')
Invalid checksum



A B
Data.req(a) D(a)

Transmission error
D(b')
C(NAK) Invalid checksum



A B
Data.req(a) D(a)

Transmission error
D(b')

Retransmission



A B
Data.req(a) D(a)

Transmission error
D(b')

Retransmission D(b)

Data.ind(b)



A B
Data.req(a) D(a)

Transmission error
D(b')

Retransmission D(b)

C(OK)
Data.ind(b)


Protocol 3b

transport layer ?
 Hypotheses
1. Transmission errors are possible
2. Packets can be lost

2. How to deal with these problems ?


Protocol 3a and segment losses

 How do segment losses affect protocol 3a ?

A B




A B
Data.req(a) D(a)

Data.ind(a)




A B
Data.req(a) D(a)

C(OK) Data.ind(a)




A B
Data.req(a) D(a)

C(OK) Data.ind(a)
Data.req(b)




A B
Data.req(a) D(a)

C(OK) Data.ind(a)
Data.req(b) Lost segment
D(b)




A B
Data.req(a) D(a)

C(OK) Data.ind(a)
Data.req(b) Lost segment
D(b)

DEADLOCK
A is waiting for a B is waiting for a
control segment data segment

Protocol 3b

 Modiﬁcation to the sender
 Add a retransmission timer to retransmit the lost
segment after some time
Data.req(SDU) Recvd(C(NAK))
send(D(SDU,CRC)) send(D(SDU,CRC))
start_timer() restart_timer()
Wait Wait
for for
SDU OK/NAK
Recvd(C(OK))
cancel_timer()
Timer expires
send(D(SDU,CRC))
restart_timer()
 No modiﬁcation to the receiver

Protocol 3b : Example

A B



A B
Data.req(a) D(a)
start timer

Data.ind(a)



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)
start timer



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)
start timer D(b) Lost segment



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)

timer expires



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)

D(b)
timer expires

Data.ind(b)



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)

D(b)
timer expires

C(OK)
Data.ind(b)



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)

D(b)
timer expires

C(OK)
Data.ind(b)

 Does this protocol always work ?


A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)
start timer D(b)

Data.ind(b)



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)
start timer D(b)
Lost segment C(OK)
Data.ind(b)



A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)
start timer D(b)
Lost segment C(OK)
Data.ind(b)
D(b)
timer expires

C(OK)
Data.ind(b)


A B
Data.req(a) D(a)
start timer

C(OK) Data.ind(a)
cancel timer
Data.req(b)
start timer D(b)
Lost segment C(OK)
Data.ind(b)
D(b)
timer expires

C(OK)
Data.ind(b)
 How to solve this problem ?

Alternating bit protocol

 Principles of the solution
 Add sequence numbers to each data segment sent by
sender
 By looking at the sequence number, the receiver can
check whether it has already received this segment

 Contents of each segment
 Data segments
 Control segments
Type, Seq. number CRC

Type, Seq. number CRC
 How many bits do we need for the sequence number?
 a single bit is enough

Sender
Recvd(C(NAK?)) OR
Recvd(C(OK1))
Data.req(SDU) or timer expires
send(D(0,SDU,CRC)) send(D(0,SDU,CRC))
start_timer() restart_timer()
Wait Wait
for for
D(0,...) OK0/NAK

Recvd(C(OK1)) Recvd(C(OK0))
cancel_timer() cancel_timer()
Data.req(SDU)
send(D(1,SDU,CRC)) All corrupted
start_timer() segments are
Wait Wait discarded in all states
for for
OK1/NAK D(1,...)

Recvd(C(NAK?)) or recvd(C(OK0)) or timer expires
send(D(1,SDU,CRC))
CNP3/2008.3. restart-timer() © O. Bonaventure, 2008

Receiver
Recvd(D(1,SDU,CRC)) Recvd(D(0,SDU,CRC))
AND IsOK(CRC,SDU) AND IsOK(CRC,SDU)
Send(C(OK1)) Recvd(D(0,SDU,CRC)) Send(C(OK0))
AND IsOK(CRC,SDU)
-
Data.ind(SDU) Process Send(C(OK0))Wait
Wait
for SDU0 for
D(0,...) OK D(1,...)

Bad(D(?,SDU,CRC))
Bad(D(?,SDU,CRC))
-
-
Recvd(D(1,SDU,CRC))
AND IsOK(CRC,SDU)
-
- Data.ind(SDU) Send(C(NAK1))
Send(C(NAK0))
-
Process Send(C(OK1))
Process Process
SDU0 SDU1 SDU1
KO OK KO


Example
A B


Example
A B
Data.req(a) D(0,a)

C(OK0) Data.ind(a)


Example
A B
Data.req(a) D(0,a)

Data.req(b) C(OK0) Data.ind(a)

D(1,b)

Data.ind(b)


Example
A B
Data.req(a) D(0,a)


D(1,b)
Retransmission timer
D(1,b) Data.ind(b)
D(1,b) recvd C(OK1) Duplicate detected
C(OK1)
D(1,b) recvd


Example
A B
Data.req(a) D(0,a)


D(1,b)
D(1,b) Data.ind(b)
Data.req(c) C(OK1)
D(1,b) recvd Lost segment
D(0,c)


Example
A B
Data.req(a) D(0,a)


D(1,b)
D(1,b) Data.ind(b)
Data.req(c) C(OK1)
D(0,c)


Example
A B
Data.req(a) D(0,a)


D(1,b)
D(1,b) Data.ind(b)
Data.req(c) C(OK1)
D(0,c)
D(0,c) Data.ind(c)

Performance of the alternating bit
protocol
 What is the performance of the ABP in this case
 One-way delay : 250 msec
 Physical layer throughput : 50 kbps
 segment size : 1000 bits


protocol

Data
1000/50000 = 20 msec 250 msec
520 msec
Ack


protocol

Data
1000/50000 = 20 msec 250 msec
520 msec
Ack

 -> Performance is function of
CNP3/2008.3. bandwidth * round-trip-time © O. Bonaventure, 2008

How to improve
the alternating bit protocol ?
 Use a pipeline
 Principle
 The sender should be allowed to send more than
one segment while waiting for an
acknowledgement from the receiver


How to improve
the alternating bit protocol ?
 Use a pipeline
 Principle
 The sender should be allowed to send more than
one segment while waiting for an
acknowledgement from the receiver
A B
Data.req(a) D(0,a)
Data.req(b)
...
Data.ind(a)
Data.req(e) ...

D(4,e)
Data.ind(e)


How to improve
the alternating bit protocol ? (2)
 Modiﬁcations to alternating bit protocol
 Sequence numbers inside each segment
 Each data segment contains its own sequence number
 Each control segment indicates the sequence number of
the data segment being acknowledged (OK/NAK)

 Sender
 Needs enough buffers to store the data segments that
have not yet been acknowledged to be able to retransmit
them if required

 Receiver
 Needs enough buffers to store the out-of-sequence
segments


How to improve
the alternating bit protocol ? (2)
 Modiﬁcations to alternating bit protocol
 Sequence numbers inside each segment
 Each data segment contains its own sequence number
 Each control segment indicates the sequence number of
the data segment being acknowledged (OK/NAK)

 Sender
 Needs enough buffers to store the data segments that
have not yet been acknowledged to be able to retransmit
them if required

 Receiver
 Needs enough buffers to store the out-of-sequence
segments
How to avoid an overﬂow of the receiverʼs buffers ?

Sliding window
 Principle
 Sender keeps a list of all the segments that it is allowed
to send
 sending_window

... 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ....
Acked segments Available seq. nums Forbidden seq. num.
Unacknowledged segments

 Receiver also maintains a receiving window with the list
of acceptable sequence number
 receiving_window

 Sender and receiver must use compatible windows
 sending_window ≤ receiving window
 For example, window size is a constant for a given protocol or negotiated
during connection establishment phase

Sliding windows : example

 Sending and receiving window : 3 segments
A B
Sending window
012345678



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
D(0,a)

Data.ind(a)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)

D(1,b)
Data.ind(a)
Data.ind(b)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
Data.ind(c)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
012345678 C(OK0) Data.ind(c)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
012345678 C(OK0) Data.ind(c)
C(OK1)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
012345678 C(OK0) Data.ind(c)
C(OK1)
012345678



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
012345678 C(OK0) Data.ind(c)
C(OK1)
0 1 2 3 4 5 6 7 8 Data.req(d)
012345678
D(3,d)
Data.ind(d)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
012345678 C(OK0) Data.ind(c)
C(OK1)
0 1 2 3 4 5 6 7 8 Data.req(d)
C(OK2)
012345678
D(3,d)
Data.ind(d)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
012345678 C(OK0) Data.ind(c)
C(OK1)
0 1 2 3 4 5 6 7 8 Data.req(d)
C(OK2)
012345678
012345678 D(3,d)
Data.ind(d)



A B
Sending window
0 1 2 3 4 5 6 7 8 Data.req(a)
012345678
0 1 2 3 4 5 6 7 8 Data.req(b) D(0,a)
012345678 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
012345678 C(OK0) Data.ind(c)
C(OK1)
0 1 2 3 4 5 6 7 8 Data.req(d)
C(OK2)
012345678
0 1 2 3 4 5 6 7 8 Data.req(e) D(3,d)
012345678 Data.ind(d)
D(4,e)


Encoding sequence numbers

 Problem
 How many bits do we have in the segment
header to encode the sequence number
 N bits means 2N different sequence numbers

 Solution
 place inside each transmitted segment its
sequence number modulo 2N
 The same sequence number will be used for
several different segments
 be careful, this could cause problems...

 Sliding window
 List of consecutive sequence numbers (modulo 2N) that
the sender is allowed to transmit


Sliding window : second
example
3 segments sending and receiving window
 Sequence number encoded as 2 bits ﬁeld
A B
Sending window
0123

OK(0)


example
A B
Sending window
0123 Data.req(a)
0123
D(0,a)

Data.ind(a)

OK(0)


example
A B
Sending window
0123 Data.req(a)
0123
0123 Data.req(b) D(0,a)

D(1,b)
Data.ind(a)
Data.ind(b)
OK(0)


example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
OK(0)
Data.ind(c)


example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
C(OK0)
OK(0)
Data.ind(c)


example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)


example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)


example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)
0123


example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)
0123 Data.req(d)

D(3,d)
Data.ind(d)

example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)
0123 Data.req(d)
0123
D(3,d)
Data.ind(d)

example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)
0123 Data.req(d) C(OK2)
0123
D(3,d)
Data.ind(d)

example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)
0123
0123 D(3,d)
Data.ind(d)

example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)
0123
0123 Data.req(e) D(3,d)
Data.ind(d)
D(0,e)

example
A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Data.ind(b)
0123 C(OK0)
OK(0)
Data.ind(c)
C(OK1)
0123
0123 Data.req(e) D(3,d)
0123 Data.ind(d)
D(0,e)

Reliable transfer with a sliding window

 How to provide a reliable data transfer with a
sliding window
 How to react upon reception of a control segment ?
 Senderʼs and receiverʼs behaviours

 Basic solutions
 Go-Back-N
 simple implementation, in particular on receiving side
 throughput will be limited when losses occur

 Selective Repeat
 more difﬁcult from an implementation viewpoint
 throughput can remain high when limited losses occur


GO-BACK-N

 Principle
 Receiver must be as simple as possible
 Receiver
 Only accepts consecutive in-sequence data segments
 Meaning of control segments
 Upon reception of data segment
 OKX means that all data segments, up to and including X have
been received correctly
 NAKX means that the data segment whose sequence number is
X contained an error or was lost

 Sender
 Relies on a retransmission timer to detect segment losses
 Upon expiration of retransmission time or arrival of a NAK
segment : retransmit all the unacknowledged data
segments
 the sender may thus retransmit a segment that was already
received correctly but out-of-sequence at destination

Go-Back-N : Receiver
 State variable
 next : sequence number of expected data segment

Recvd(D(next,SDU,CRC)) Recvd(D(next,SDU,CRC))
AND NOT(IsOK(CRC,SDU)) AND IsOK(CRC,SDU)
discard(SDU); Data.ind(SDU)
send(C(NAK,next,CRC));
Process
Wait SDU
OK

-
send(C(OK,next,CRC));
Recvd(D(t<>next,SDU,CRC)) next=(next+1);
AND IsOK(CRC,SDU)
discard(SDU);
send(C(OK,(next-1) ,CRC));


Go-Back-N : Sender

 State variables
 base : sequence number of oldest data segment
 seq : ﬁrst available sequence number
 W : size of sending window
Recvd(C(?,?,CRC)) Data.req(SDU)
and NOT( CRCOK(C(?,?,CRC))) AND ( seq < (base+w) )
- if (seq==base) { start_timer ; }
insert_in_buffer(SDU);
send(D(seq,SDU,CRC));
Wait seq=seq+1 ;

[ Recvd(C(NAK,?,CRC))
and CRCOK(C(NAK,?,CRC))]
Recvd(C(OK,t,CRC)) or timer expires
and CRCOK(C(OK,t,CRC)) for (i=base;i<seq; i=i+1)
base=(t+1); { send(D(i,SDU,CRC)); }
if (base==seq) restart_timer();
{ cancel_timer();}
else
{ restart_timer(); }

Go-Back-N : Example

A B
Sending window
0123


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
D(0,a)

Data.ind(a)


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
D(1,b)
Data.ind(a)

Invalid CRC, discarded


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Invalid CRC, discarded
Not expected seq num,
discarded


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
0123 C(OK,0) Invalid CRC, discarded
discarded


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
C(NAK,1) Not expected seq num,
0123
discarded


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
C(NAK,1) Not expected seq num,
0123 C(OK,0) discarded


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
Retransmission C(NAK,1) Not expected seq num,
0123 C(OK,0) discarded
0123
D(1,b)
0123
D(2,c)
Data.ind(b)
Data.ind(c)


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
0123 C(OK,0)
Data.req(d) discarded
0123
D(1,b)
0123
D(2,c)
Data.ind(b)
D(3,d) Data.ind(c)
Data.ind(d)


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
0123 C(OK,0)
0123
D(1,b)
0123 Data.req(e)
D(2,c)
Data.ind(b)
D(3,d) Data.ind(c)
Data.ind(d)


Go-Back-N : Example

A B
Sending window
0123 Data.req(a)
0123
Transmission error
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
0123 C(OK,0)
0123
D(1,b)
0123 Data.req(e)
D(2,c)
Data.ind(b)
Sending window is full
D(3,d) Data.ind(c)
Application blocked
Data.ind(d)
e will be accepted
and sent later © O. Bonaventure, 2008
CNP3/2008.3.

Go-Back-N : Example (2)

A B
Sending window
0123



A B
Sending window
0123 Data.req(a)
0123
0123 D(0,a)

Data.ind(a)



A B
Sending window
0123 Data.req(a)
0123
0123 Data.req(b) D(0,a) Segment lost
D(1,b)
Data.ind(a)



A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)

discarded



A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
0123 C(OK,0)
discarded



A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
0123 C(OK,0)
C(OK,0) discarded



A B
Sending window
0123 Data.req(a)
0123
0123 D(1,b)
Data.req(c)
Data.ind(a)
D(2,c)
0123 Retransmission C(OK,0)
timer expires Not expected seq num,
C(OK,0) discarded
0123
D(1,b)
0123
D(2,c)
Data.ind(b)
Data.ind(c)


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