Internet DRAFT - draft-learmonth-rfc1226-bis
draft-learmonth-rfc1226-bis
Network Working Group I. Learmonth
Internet-Draft HamBSD
Obsoletes: 1226 (if approved) May 19, 2020
Intended status: Experimental
Expires: November 20, 2020
Internet Protocol Encapsulation of AX.25 Frames
draft-learmonth-rfc1226-bis-03
Abstract
This document describes a method for the encapsulation of AX.25 Link
Access Protocol for Amateur Packet Radio frames within IPv4 and IPv6
packets. Obsoletes RFC1226.
Note
Comments are solicited and should be addressed to the author(s).
The sources for this draft are at:
https://github.com/irl/draft-rfc1226-bis
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 20, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(https://trustee.ietf.org/license-info) in effect on the date of
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to this document.
1. Introduction
This document describes a method for the encapsulation of AX.25 Link
Access Protocol for Amateur Packet Radio [AX.25] frames within IPv4
and IPv6 packets. It obsoletes [RFC1226].
AX.25 is a data link layer protocol originally derived from layer 2
of the X.25 protocol suite and designed for use by amateur radio
operators. It is used extensively by amateur packet radio networks
worldwide.
In addition to specifying how packets should be encapsulated, 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.
2. Terminology
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 [RFC2119].
3. Internet Protocol Encapsulation
Each AX.25 frame is encapsulated in one IPv4 or IPv6 datagram using
protocol number 93 as assigned in the Assigned Internet Protocol
Numbers registry [protocol-numbers]. 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.
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3.1. Priority Frames
In normal operation, the DiffServ codepoint field [RFC2474] 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 [RFC2597]. 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. The receiver MUST NOT use the DiffServ
codepoint to infer anything about the nature of the encapsulated
packet. It has been shown that while the AF21 codepoint may be
remarked while crossing administrative boundaries, it is unlikely
that priority inversion will occur due to remarking where such
remarking occurs [Cust18].
3.2. Automatic Packet Reporting System
Automatic Packet Reporting System [APRS] is an amateur radio-based
system for real time digital communications for local situational
awareness. APRS uses AX.25 frames for addressing, and additionally
assigns special meaning to some of the reserved bits of an AX.25
frame header.
As a special case, when used with the Automatic Packet Reporting
System [APRS], priority frames will not occur. If a tunnel is
configured as carrying APRS data, the DiffServ codepoint SHOULD by
default be set to AF11 [RFC2597]. Where the "Precedence Bit"
[RR-bits] is set (i.e. it is zero) in an APRS packet, the DiffServ
codepoint should be set to BE. Where the "Operator Present Bit"
[RR-bits] is set (i.e. it is zero), the DiffServ codepoint MAY be set
to AF21 [RFC2597].
XXX: Open question, perhaps precendence bit should cause use of the
LE PHB.
Again, for the avoidance of doubt: on decapsulation the AX.25 frame
MUST NOT be modified based on the DiffServ codepoint on the received
encapsulating IP header. The receiver MUST NOT use the DiffServ
codepoint to infer anything about the nature of the encapsulated
packet. It has been shown that while AF codepoints may be remarked
while crossing administrative boundaries, it is unlikely that
priority inversion will occur, either with the BE traffic or between
AF PHBs due to remarking where such remarking occurs [Cust18].
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It is possible depending on the nature of the tunnel that
decapsulated packets would need to be treated as third-party traffic
according to the APRS specification [APRS]. In this case, the Third-
Party Network Identifier "IPENC" SHOULD be used. This is to
differentiate traffic using IP encapsulation from APRS-IS traffic
[APRS-IS].
4. IANA Considerations
Protocol number 93 is assigned in [protocol-numbers] and should be
updated to point to this document.
5. Security Considerations
With the exception of control signals exchanged between earth command
stations and space stations in the amateur-satellite service, amateur
radio transmissions cannot be encoded for the purpose of obscuring
their meaning. In essence, this means that cryptography that
requires the use of secrets to decipher a message cannot be used
where the possibility exists that a packet will be transmitted by an
amateur radio station.
The CRC-16-CCITT provides for an integrity check but does not
guarantee the authenticity of the packet. In many jurisdictions it
is a requirement for amateur radio stations that are Internet
connected that they verify that packets for transmission have
originated from licensed radio amateurs. In order to provide this
guarantee, IPSec [RFC4301] SHOULD be employed to provide
authentication of packets. A transport mode SA SHOULD be negotiated
between the IP endpoints to use ESP [RFC4303] with NULL encryption
[RFC2410] with the traffic selector matching packets with IP protocol
number 93. In cases where traffic is guaranteed to not pass via an
amateur radio link, non-NULL encryption MAY be used. Non-NULL
encryption MUST NOT be used where there is the possibility that the
encapsulating packet will be transmitted via an amateur radio link.
When transmitted by an amateur radio station, many propagation modes
will permit wide reception of a packet. As such, receivers MUST
implement anti-replay protection by verifying received sequence
numbers [RFC4303]. The size of the anti-replay window may need to be
scaled to account not only for the speed of the link, but also for
packet loss that may occur on amateur radio links. Following
extended packet loss a sender may have advanced the sequence number
beyond the window size allowed. Dead peer detection [RFC5996] can be
used to renegotiate SAs in this case and so SHOULD be enabled for any
SA expected to traverse an amateur radio link that is expected to
have varying propagation charachteristics.
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Given the need for anti-replay protection, it is not possible to
manually key the SAs. An automatic keying protocol such as IKEv1
[RFC2409] or IKEv2 [RFC5996] MUST be used to establish SAs. The
exact details of the automatic keying protocol to use and its
paramaters are not specified in this document.
6. Acknowledgements
The author would like to acknowledge the work of Brian Kantor who
authored the original specification [RFC1226] that this document
updates.
7. References
7.1. Normative References
[AX.25] Tucson Amateur Packet Radio Corporation, "AX.25 Link
Access Protocol for Amateur Packet Radio Version 2.2",
July 1998, <https://www.tapr.org/pdf/AX25.2.2.pdf>.
[protocol-numbers]
IANA, "Assigned Internet Protocol Numbers",
<http://www.iana.org/assignments/protocol-numbers/
protocol-numbers.xhtml>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, DOI 10.17487/RFC2410,
November 1998, <https://www.rfc-editor.org/info/rfc2410>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
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[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RR-bits] Bruninga, B., "APRS Future Use of AX.25 SSID RR Bits",
December 2012, <http://aprs.org/aprs12/RR-bits.txt>.
7.2. Informative References
[APRS] Wade, I., Ed., "APRS Protocol Reference", August 2000,
<http://www.aprs.org/doc/APRS101.PDF>.
[APRS-IS] Loveall, P., "APRS-IS", <http://www.aprs-is.net/>.
[Cust18] Custura, A., Secchi, R., and G. Fairhurst, "Exploring DSCP
modification pathologies in the Internet", Computer
Communications Vol. 127, pp. 86-94,
DOI 10.1016/j.comcom.2018.05.016, September 2018.
[RFC1226] Kantor, B., "Internet protocol encapsulation of AX.25
frames", RFC 1226, DOI 10.17487/RFC1226, May 1991,
<https://www.rfc-editor.org/info/rfc1226>.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
<https://www.rfc-editor.org/info/rfc2409>.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, DOI 10.17487/RFC5996, September 2010,
<https://www.rfc-editor.org/info/rfc5996>.
Author's Address
Iain R. Learmonth
HamBSD
Email: irl@hambsd.org
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