Sliding Window Protocol - Extensions

Extensions

There are many ways that the protocol can be extended:

  • The above examples assumed that packets are never reordered in transmission; they may be lost in transit (error detection makes corruption equivalent to loss), but will never appear out of order.

The protocol can be extended to support packet reordering, as long as the distance can be bounded; the sequence number modulus N must be expanded by the maximum misordering distance.

  • It is possible to not acknowledge every packet, as long as an acknowledgment is sent after not receiving any packets for a while. For example, TCP normally acknowledges every second packet.
  • It is common to inform the transmitter immediately if a gap in the packet sequence is detected. HDLC has a special REJ (reject) packet for this.
  • The transmit and receive window sizes may be changed during communication, as long as their sum remains within the limit of N. In particular, it is common to reduce the transmit window size to slow down transmission to match the link's speed, avoiding saturation or congestion.
  • One common simplification of selective-repeat is so called SREJ-REJ ARQ. This operates with wr=2 and accepts following packets, but only allows a single lost packet; if a second packet is lost, no more packets are buffered. (I.e. wr=1 while waiting.) This gives most of the performance benefits of the full selective-repeat protocol, with a simpler implementation.

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