AX.25 over IP: Internet protocol encapsulation of AX.25 frames
Carries amateur radio packet protocol inside Internet packets
Kantor, B., "Internet protocol encapsulation of AX.25 frames", RFC 1226, DOI 10.17487/RFC1226, May 1991, <https://www.rfc-editor.org/info/rfc1226>.
@misc{rfc1226,
series = {Request for Comments},
number = 1226,
howpublished = {RFC 1226},
publisher = {RFC Editor},
doi = {10.17487/RFC1226},
url = {https://www.rfc-editor.org/info/rfc1226},
author = {B. Kantor},
title = {{Internet protocol encapsulation of AX.25 frames}},
pagetotal = 2,
year = 1991,
month = may,
} RFC text
Contents
Network Working Group B. Kantor
Request for Comments: 1226 Univ. of Calif San Diego
May 1991
Internet Protocol Encapsulation of AX.25 Frames
Status of this Memo
This memo describes a method for the encapsulation of AX.25 (the
Amateur Packet-Radio Link-Layer Protocol) frames within IP packets.
This technique is an Experimental Protocol for the Internet
community. Discussion and suggestions for improvement are requested.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
The AX.25 Amateur Packet-Radio Link-Layer Protocol
The AX.25 Amateur Packet-Radio Link-Layer Protocol is described in
the publication by that name [1], incorporated here by reference.
Each AX.25 packet ("frame") is encapsulated in one IP datagram.
Normally no AX.25 frame will exceed 330 octets, so fragmentation at
the IP layer should not be necessary. However, experiments with
larger AX.25 frame sizes may require the use of standard IP
fragmentation and reassembly procedures.
When an AX.25 frame is encapsulated within an IP packet, HDLC framing
elements (flags and zero-stuffing) are omitted, as the IP datagram
adequately delimits the beginning and end of each AX.25 frame. The
16-bit CRC-CCITT frame check sequence (normally generated by the HDLC
transmission hardware) is included. In all other respects, AX.25
frames are encapsulated unaltered.
Each such IP datagram shall have a protocol number of 93.
Reference
[1] AX.25 Amateur Packet-Radio Link-Layer Protocol Version 2.0
October 1984. Available from the American Radio Relay League,
Newington CT USA 06111, and other sources.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Brian Kantor
University of California at San Diego
Network Operations C-024
La Jolla, CA 92093-0214
Phone: (619) 534-6865
EMail: brian@UCSD.EDU
AI ELI5
On this page
§1Amateur Radio over IP
The AX.25 Amateur Packet-Radio Link-Layer Protocol defines how amateur (ham) radio operators exchange messages via packet radio. To allow AX.25 frames to travel across the Internet, they must be encapsulated in IP datagrams. Each AX.25 frame becomes the payload of one IP packet, assigned IP protocol number 93.
Ham radio messages need to be packed into Internet packets, so they can travel across the Internet and still be recognized when they arrive.
§2Frame Encapsulation
When an AX.25 frame is encapsulated in IP, the HDLC framing elements (flags and zero-stuffing) are removed because the IP datagram already marks the frame boundaries. The 16-bit CRC-CCITT frame check sequence is preserved intact. AX.25 frames normally do not exceed 330 octets, so IP fragmentation is typically not necessary, though larger frames can use standard IP fragmentation if needed.
The ham radio message part is copied directly into the Internet packet, but some of the radio packaging is removed because the Internet packet provides its own wrapping.
§3Protocol Numbers and Standards
Each IP datagram carrying an AX.25 frame uses protocol number 93. The document references the AX.25 Amateur Packet-Radio Link-Layer Protocol Version 2.0 from October 1984, available from the American Radio Relay League. The RFC specifies only the basic encapsulation; security considerations for amateur radio networks over IP are not addressed in this document.
The Internet uses protocol number 93 to mean "this packet carries an AX.25 radio message". The details of AX.25 itself come from the ham radio standard.