Internet Protocol Encapsulation of AX.25 Frames

This document describes a method for the encapsulation of AX.25 (the AX.25 Link Access Protocol for Amateur Packet Radio ) frames within IP version 4 and version 6 packets. It obsoletes . This document explicitly states that it applies both to Internet Protocol version 4 and version 6 and now mentions negotiation of larger frame sizes possible in AX.25 version 2.2. Additionally, it gives recommendations for DiffServ codepoint marking of the encapsulating headers based on the AX.25 frame content and provides security considerations for the use of this encapsulation method.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

Each AX.25 frame is encapsulated in one IP version 4 or version 6 datagram. For AX.25 version 2.0, the maximum frame size expected is 330 bytes and implementations MUST be prepared to handle frames of this size. Higher frame sizes can be negotiated by AX.25 version 2.2 and so this is a minimum requirement and not a limit. 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 CRC-16-CCITT frame check sequence (normally generated by the HDLC transmission hardware) is included trailing the information field. In all other respects, AX.25 frames are encapsulated unaltered.

In normal operation, the DiffServ codepoint field in the encapsulating IP header SHOULD be set to best effort (BE). The exception to this is "priority frames" as specified for AX.25 version 2.2, including acknowledgement and digipeat frames, which SHOULD have the DiffServ codepoint set to AF21 . A slot is reserved on the radio channel for the transmission of these frames and the use of this codepoint will permit the frames to arrive promptly at the station for transmission. For the avoidance of doubt: on decapsulation the AX.25 frame MUST NOT be modified regardless of the DiffServ codepoint on the received encapsulating IP header.

As a special case, when used with the Automatic Packet Reporting System , priority frames will not occur. If a tunnel is configured as carrying APRS data, the DiffServ codepoint SHOULD by default be set to AF11 . Where the "Precedence Bit" is set (i.e. it is zero) in an APRS packet, the DiffServ codepoint should be set to BE. Where the "Operator Present Bit" is set (i.e. it is zero), the DiffServ codepoint MAY be set to AF21 . Again, for the avoidance of doubt: on decapsulation the AX.25 frame MUST NOT be modified regardless of the DiffServ codepoint on the received encapsulating IP header.

Protocol number 93 is assigned in and should be updated to point to this document.

XXX Left the hard part for last, but the basics of it: You should use something to guarantee integrity My advice is to use IPsec Use ESP on the Internet, use AH on amateur radio links Use AH if possibility that packet will go via amateur radio Tunnels will be configured statically (can't think of other use cases) so certificates are good Routing via IPsec is not required, transport mode suffices, tunnel mode for cases where there is NAT

The author would like to acknowledge the work of Brian Kantor who authored the original specification that this document updates.