Import of the watch repository from Pebble

docs/pulse2/reliable-transport.md

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+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](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](https://www.itu.int/rec/T-REC-X.25-199610-I/en). 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.
+