Fidonet Packets

FidoNet Packets are all somewhat based on the “Type-2” packet format defined in Section F (Network Layer) of FTS-1, a.k.a. FTS-0001 which contains:

1. A 58-byte “Packet Header”
2. Zero or more “Packed Messages” (each with its own header and body text, null terminated)
3. Packet terminator (16-bits of 0)

“Type-1”, “Type-3” and other Fidonet packet formats are not discussed here since they are not widely used.

The original “Type-2” packet format is also sometimes called the “Stone-Age” packet format.

All Type-2 Fidonet Packet Headers (and their derivations) are 58 bytes in length.

• Each Packet Header contains multiple single-byte and multi-byte fields.
• All multi-byte integer fields are stored in little endian byte-order.
• Text fields (e.g. password and domains) contain US-ASCII characters or nulls ('\0').

The “Type-2” is the most basic of Packet headers, but it lacks support for points (so-called “4D addresses”) or domains (“5D addresses”). Early revisions of FTS-1 lacked support for zones (“3D addresses”). The support for points in Fidonet Packet Headers was added in the Type-2 Packet Extension Proposal (FSC-39), as well as the Type-2+ and Type-2.2 packet header format/extension proposals.

None of the proposed “Type-2” extensions were ever formally standardized, though they became ubiquitous in the early 1990s and have remained in wide-spread use ever since.

Type-2+ packets appear to be the most widely supported (accepted and generated) by Fidonet software in use today. However, there appears to be some debate about whether FSC-39[.4] defines the most-commonly-accepted packets or FSC-48 defines the most-commonly-accepted packets. To add to this confusion, only FSC-48 actually mentions the term “Type-2+”.

The only differences between the packet headers defined in FSC-39.1 and FSC-48 are the capValid (CWvalidationCopy) fields and the auxNet fields: both fields are present in FSC-48 but absent in FSC-39.1. In FSC-39.4, the capValid field was added, but the auxNet field remains absent. Fidonet software that accepts an extended Type-2 packet but does not recognize the auxNet field may be confused about the originator of packets from point nodes (where the origNet field is set to 0xffff and the auxNet field is then used).

FSC-48 offers this explanation:

Although it (FSC-39 packet header) implements fields for  zone 
and point number,  an FTS-0001 compliant application is  not 
required to look at these fields.  When a point mails  using 
these  fields  to implement its 4D address,  a  system  only 
looking  at the net/node info,  as is required by  FTS-0001, 
still sees it as a boss node, causing the obvious problems.

This document provides a way for transparent point handling, 
using   a  technique  already  exploited  by  many   mailers 
internally.  It  will allow this document to be  implemented 
and used by mailers not supporting it.  At the same time the 
danger that a point is seen as the boss node is eliminated.

• FTS-1: A Basic FidoNet(r) Technical Standard, Randy Bush, version 16
• FSC-39: A Type-2 Packet Extension Proposal, Mark A. Howard, version 1
• FSC-39: A Type-2 Packet Extension Proposal, Mark A. Howard, version 4
• FSC-48: A Proposed Type-2 Packet Extension, Jan Vroonhof, version 1
• FSC-48: A Proposed Type-2 Packet Extension, Jan Vroonhof, version 2
• FSC-45: A Proposal for A New Packet Header Format, Thom Henderson, version 1
• References