This document summarizes key aspects of two pipelined reliable data transfer protocols: Go-Back-N and Selective Repeat. Go-Back-N allows the sender to transmit multiple packets without waiting for acknowledgments, up to a maximum window size. If a timeout occurs, all unacknowledged packets are retransmitted. Selective Repeat allows the sender to retransmit only lost packets. It requires the receiver to buffer out-of-order packets and send selective acknowledgments. Both protocols aim to efficiently utilize available bandwidth while ensuring reliable data transfer.

1. Chapter 3
Transport Layer
Computer Networking:
A Top Down Approach,
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.

2. 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

3. 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

4. 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

5. 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

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

7. Selective Repeat
 Selective Repeat protocol avoids unnecessary
retransmissions
o Sender only retransmits packets that were lost or are
in error
o A window size N is used to limit the no of outstanding
unacknowledged packets in the pipeline

8. Selective Repeat
Sender:
 Data received from upper layers
o If window is full, returns data to upper layer
o Maintain synchronization mechanism
 Timeout
o Each packet has its own timer
o Single packet is retransmitted on timeout
 ACK received:
o Sender marked packet as received provided its in the
window
o Packets sequence no is equal to send_base,
• The window base is moved forward to the unacknowledged
packet

9. 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

10. Selective Repeat in Action
http://www.eecis.udel.edu/~amer/450/TransportApplets/SR/SRindex.html

11. 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