Retransmission

• Retransmission Strategies
• Acknowledgements
• Stop-and-Wait retransmission
• Alternating Bit protocol

Retransmission strategies

• link level: when error rate on link is high, e.g. modem or wireless
• end-to-end: when error rate on link is low, e.g. optic fibers
• end-to-end + link level: when end-to-end transmission goes over some high-error-rate links and some reliable links, e.g. TCP over modem+fiber optic
• retransmissions use
  • special packets, acknowledgments, to signal that a packet has or has not been received,
  • timers to detect packet loss

Types of Acknowledgements

• cumulative: ack everything up to this sequence number, e.g. TCP
• non-cumulative: ack this packet
• negative: I am missing this specific packet

Stop and Wait protocol

• assumption: bounded delay T before receiver ack comes back to sender (i.e. either ack is received by sender within time T, or ack was never sent, or ack is lost)
• example: point-to-point connection
• one-bit sequence number for packets (0 or 1)
• if ack is received within time T, send next packet,
• otherwise, resend same packet
• sequence number helps receiver distinguish retransmission from new packet

Correctness of retransmission protocols

• correctness: protocol delivers exactly one copy of each packet, in the order originally sent
• assumption: if packet is sent often enough, it will make it through

Correctness of stop-and-wait

1. packet and ack received correctly
2. packet is lost: sender times out, retransmits.
3. ack is lost: sender times out, retransmits. Retransmission has same sequence number, so receiver discards it.

• receiver cannot miss packet, since sender keeps sending until positive ack is received
• receiver cannot duplicate packet, since successive packets with same sequence number are discarded.

Alternating Bit protocol

• assumption: FIFO (non-reordering) network
• example: ethernet
• single bit sequence/ack number
• only one possible outstanding (unacked) packet
• on receipt of packet s, send ack s
• if outstanding packet has sequence s:
  • ignore acks 1-s
  • ack s allows us to send next packet with sequence 1-s

Correctness of ABP

1. if network is FIFO,
2. any ack 1-s we receive is for the last packet
3. any ack s we receive is for the current packet, and
4. when we receive an ack s, there can be no ack 1-s left in the network.
5. therefore, any future ack 1-s must be for future packets, and
6. acks will be matched to packets