Module rel 

Reliable provisional responses (RFC 3262): 100rel, RSeq, RAck and PRACK.

An ordinary 180 Ringing is fire-and-forget. Over UDP it is simply lost sometimes, and the caller hears nothing while the callee’s phone rings — or, worse, an early-media answer never arrives and the call connects to silence. RFC 3262 makes a provisional response an acknowledged message: the UAS numbers it, retransmits it until a PRACK comes back, and gives up on the whole invitation if none ever does.

Some carriers will not accept a call without it, which is the practical reason this exists.

Everything here is pure: sequence numbers, the ordering rule, and the decision about whether a request may or must be answered reliably. The retransmission clock lives a layer up.

Structs

Numbering

The UAS’s numbering of reliable provisionals within one transaction (RFC 3262 §3).

Offered

What the peer said about 100rel in its request.

RAck

The RAck header (RFC 3262 §7.2): response-num CSeq-num Method.

RSeq

The RSeq header (RFC 3262 §7.1).

Sequence

The UAC’s view of the reliable provisionals arriving for one request (RFC 3262 §4).

Enums

Received

What the UAC should do with a reliable provisional it has just received.

Reliability

What a UAS must do about reliability for an incoming request.

Constants

MAX_FIRST_RSEQ

The largest first sequence number the RFC allows.

OPTION_TAG

The option tag, in Supported, Require and Unsupported (RFC 3262 §8.1).

Functions

reliability

Decide the UAS side (RFC 3262 §3).