Chapter 3Transport Layer

Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

Pipelined Reliable Data Transfer Protocols

Pipelining has the following consequences for reliable data transfer Range of sequence numbers must be

increased Sender and receiver sides may have to

buffer more than one packet.

Two basic approaches towards pipeline error recovery:

Go-Back-N, Selective Repeat

Go-Back-N (GBN)

Sender: Sender is allowed to transmit multiple packets without

waiting for an acknowledgement Constrained to a certain maximum number N.

Base or send_base Sequence number of oldest unacknowledged packet

Nextseqnum Sequence number of next packet to be sent

The range of sequence numbers for transmitted but not acknowledged packets can be viewed as a window of size N.

This window slides forward as the protocol operates

Go-Back-N

GBN sender must respond to three types of events

Invocation from above (rdt_send() is called): If window is full, returns data to upper layer Maintain synchronization mechanism

Receipt of an ACK: ACK for packet with seq # n is taken as“Cumulative ACK” More shortly in receiver

Time out event: Sender has timer for oldest unacknowledged packet

• If timer expires, retransmit all unacknowledged packets

Go-Back-N

Receiver: If a packet with seq # n is received correctly and

is in order ACK is sent and data is delivered to upper layers

For all other cases Receiver discards the packet and resends ACK for most

recently received in order packet Packets are delivered one at a time to upper

layers If a packet k has been received and delivered, then all

packets with seq # lower than k have also been delivered.

Receiver discards out of order packets No receiver buffering Need only remember expectedseqnum

GBN inaction

http://www.eecis.udel.edu/~amer/450/TransportApplets/GBN/GBNindex.html

Selective Repeat Selective Repeat protocol avoids unnecessary

retransmissionso Sender only retransmits packets that were lost or

are in erroro A window size N is used to limit the no of

outstanding unacknowledged packets in the pipeline

Selective Repeat

Sender: Data received from upper layers

o If window is full, returns data to upper layero Maintain synchronization mechanism

Timeouto Each packet has its own timero Single packet is retransmitted on timeout

ACK received:o Sender marked packet as received provided its in

the windowo Packets sequence no is equal to send_base,

• The window base is moved forward to the unacknowledged packet

Selective Repeat

Receiver:

Packets with sequence no in the window Selective ACK is sent to the sender whether or not

it is in order. Out-of-order: buffer but send ACK for that packet Deliver base plus buffered packets

Packets with sequence number below the window base An ACK must be generated even though the packet

has already been acknowledged by the receiver

Selective Repeat in Action

http://www.eecis.udel.edu/~amer/450/TransportApplets/SR/SRindex.html

Selective Repeat: Dilemma

Finite range of sequence numbers

Example:

Seq #’s: 0, 1, 2, 3 Window size=3 Receiver sees no difference

in two scenarios! Incorrectly passes duplicate

data as new in (a) Window size of one less than

the sequence number space does not work

Window size must be less than or equal to half the size of sequence no. space