4.3 KiB
PULSEv2 Reliable Transport
The purpose of this document is to describe the procedures for the PULSEv2 reliable transport (TRAIN) to be used in the initial implementations, with support for only Stop-and-Wait ARQ (Automatic Repeat reQuest). Hopefully, limiting the scope in this way will make it simpler to implement compared to a more performant Go-Back-N ARQ. This document is a supplement to the description of TRAIN in pulse2.md.
The PULSEv2 reliable transport (TRAIN) is based on X.25 LAPB, which implements reliable datagram delivery using a Go-Back-N ARQ (Automatic Repeat reQuest) procedure. Since a Stop-and-Wait ARQ is equivalent to Go-Back-N with a window size of 1, LAPB can be readily adapted for Stop-and-Wait ARQ. The description in this document should hopefully be compatible with an implementation supporting the full Go-Back-N LAPB procedures when that implementation is configured with a window size of 1, so that there is a smooth upgrade path which doesn't require special cases or compatibility breakages.
Documentation conventions
This document relies heavily on the terminology used in ITU-T Recommendation X.25. Readers are also assumed to have some familiarity with section 2 of that document.
The term "station" is used in this document to mean "DCE or DTE".
Procedures for information transfer
There is no support for communicating a busy condition. It is assumed that a station in a busy condition will silently drop packets, and that the timer recovery procedure will be sufficient to ensure reliable delivery of the dropped packets once the busy condition is cleared. An implementation need not support sending or receiving RNR packets.
Sending I packets
All Information transfer packets must be sent with the Poll bit set to 1. The procedures from X.25 §2.4.5.1 apply otherwise.
Receiving an I packet
When the DCE receives a valid I packet whose send sequence number N(S) is equal to the DCE receive state variable V(R), the DCE will accept the information fields of this packet, increment by one its receive state variable V(R), and transmit an RR response packet with N(R) equal to the value of the DCE receive state variable V(R). If the received I packet has the Poll bit set to 1, the transmitted RR packet must be a response packet with Final bit set to 1. Otherwise the transmitted RR packet should have the Final bit set to 0.
Reception of out-of-sequence I packets
Since the DTE should not have more than one packet in-flight at once, an out-of-sequence I packet would be due to a retransmit: RR response for the most recently received I packet got lost, so the DTE re-sent the I packet. Discard the information fields of the packet and send an RR packet with N(R)=V(R).
Receiving acknowledgement
When correctly receiving a RR packet, the DCE will consider this packet as an acknowledgement of the most recently-sent I packet if N(S) of the most recently-sent I packet is equal to the received N(R)-1. The DCE will stop timer T1 when it correctly receives an acknowledgement of the most recently-sent I packet.
Since all I packets are sent with P=1, the receiving station is obligated to respond with a supervisory packet. Therefore it is unnecessary to support acknowledgements embedded in I packets.
Receiving an REJ packet
Since only one I packet may be in-flight at once, the REJ packet is due to the RR acknowledgement from the DTE getting lost and the DCE retransmitting the I packet. Treat it like an RR.
Waiting acknowledgement
The DCE maintains an internal transmission attempt variable which is set to 0 when the transport NCP signals a This-Layer-Up event, and when the DCE correctly receives an acknowledgement of a sent I packet.
If Timer T1 runs out waiting for the acknowledgement from the DTE for an I packet transmitted, the DCE will add one to its transmission attempt variable, restart Timer T1 and retransmit the unacknowledged I packet.
If the transmission attempt variable is equal to N2 (a system parameter), the DCE will initiate a restart of the transport link.