CBOR tags for IPv4 and IPv6 addresses and prefixes
draft-ietf-cbor-network-addresses-05

CBOR Working Group                                         M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                              C. Bormann
Expires: 13 January 2022                          Universität Bremen TZI
                                                            12 July 2021

           CBOR tags for IPv4 and IPv6 addresses and prefixes
                  draft-ietf-cbor-network-addresses-05

Abstract

   This document describes two CBOR Tags to be used with IPv4 and IPv6
   addresses and prefixes.

   RFC-EDITOR-please remove: This work is tracked at https://github.com/
   cbor-wg/cbor-network-address

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  IPv6  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  IPv4  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Encoder Consideration for prefixes  . . . . . . . . . . . . .   4
   5.  Decoder Considerations for prefixes . . . . . . . . . . . . .   5
   6.  CDDL  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Tag 54 - IPv6 . . . . . . . . . . . . . . . . . . . . . .   7
     8.2.  Tag 52 - IPv4 . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Changelog  . . . . . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC8949] defines a number of CBOR Tags for common items.

   Tag 260 and tag 261 was later defined through IANA.  These tags cover
   addresses (260), and prefixes (261).  Tag 260 distinguishes between
   IPv4, IPv6 and Ethernet through the length of the byte string only.
   Tag 261 was not documented well enough to be used.

   The present specification achieves an explicit indication of IPv4 or
   IPv6, and the possibility to omit trailing zeroes.

   This document provides a format for IPv6 and IPv4 addresses,
   prefixes, and addresses with prefixes.  Prefixes MUST omit trailing
   zeroes in the address.  Due to the complexity of testing the value of
   omitting trailing zeros for addresses was considered non-essential
   and support for that was removed in this specification.

   This document does not deal with 6 or 8-byte Ethernet addressees.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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3.  Protocol

   These tags can applied to byte strings to represent a single address.

   When applied to an array that starts with a number, they represent a
   CIDR-style prefix of that length.  When a byte string (without
   prefix) appears in a context where a prefix is expected, then it is
   to be assumed that all bits are relevant.  That is, for IPv4, a /32
   is implied, and for IPv6, a /128 is implied.

   When applied to an array that starts with a byte string, that stands
   for an IP address, followed by the bit length of a prefix built out
   of the first "length" bits of the address.

3.1.  IPv6

   IANA has allocated tag 54 for IPv6 uses.  (Note that this is the
   ASCII code for '6'.)

   An IPv6 address is to be encoded as a sixteen-byte byte string
   (Section 3.1 of [RFC8949], major type 2), enclosed in Tag number 54.

   An IPv6 prefix, such as 2001:db8:1234::/48 is to be encoded as a two
   element array, with the length of the prefix first.  Trailing zero
   bytes MUST be omitted.

   For example:

   54([ 48, h'20010db81234'])

   An IPv6 address combined with a prefix length, such as being used for
   configuring an interface, is to be encoded as a two element array,
   with the (full-length) IPv6 address first and the length of the
   associated network the prefix next.

   For example:

   54([h'20010db81234DEEDBEEFCAFEFACEFEED', 56])

   Note that the address-with-prefix form can be reliably distinguished
   from the prefix form only in the sequence of the array elements.

3.2.  IPv4

   IANA has allocated tag 52 for IPv4 uses.  (Note that this is the
   ASCII code for '4'.)

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   An IPv4 address is to be encoded as a four-byte byte string
   (Section 3.1 of [RFC8949], major type 2), enclosed in Tag number 52.

   An IPv4 prefix, such as 192.0.2.0/24 is to be encoded as a two
   element array, with the length of the prefix first.  Trailing zero
   bytes MUST be omitted.

   For example:

   52([ 24, h'C00002'])

   An IPv4 address combined with a prefix length, such as being used for
   configuring an interface, is to be encoded as a two element array,
   with the (full-length) IPv4 address first and the length of the
   associated network the prefix next.

   For example, 192.0.2.1/24 is to be encoded as a two element array,
   with the length of the prefix (implied 192.0.2.0/24) last.

   52([ h'C0000201', 24])

   Note that the address-with-prefix form can be reliably distinguished
   from the prefix form only in the sequence of the array elements.

4.  Encoder Consideration for prefixes

   An encoder may omit as many right-hand (trailing) bytes which are all
   zero as it wishes.

   There is no relationship between the number of bytes omitted and the
   prefix length.  For instance, the prefix 2001:db8::/64 is optimally
   encoded as:

   54([64, h'20010db8'])

   An encoder MUST take care to set all trailing bits to zero.  While
   decoders are expected to ignore them, such garbage entities could be
   used as a covert channel, or may reveal the state of what would
   otherewise be private memory contents.  So for example,
   2001:db8:1230::/44 MUST be encoded as:

   52([44, h'20010db81230'])

   even though variations like:

   54([44, h'20010db81233'])  WRONG
   54([45, h'20010db8123f'])  WRONG

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   would be parsed in the exact same way.

   The same considerations apply to IPv4 prefixes.

5.  Decoder Considerations for prefixes

   A decoder MUST consider all bits to the right of the prefix length to
   be zero.

   A decoder MUST handle the case where a prefix length specifies that
   more bits are relevant than are actually present in the byte-string.
   As a pathological case, ::/128 can be encoded as

   54([128, h''])

   A recommendation for implementation is to first create an array of 16
   (or 4) bytes in size, set it all to zero.

   Then looking at the length of the included byte-string, and of the
   prefix-length, rounded up to the next multiple of 8, and taking
   whichever is smaller, copy that many bytes from the byte-string into
   the array.

   Finally, looking at the last three bits of the prefix-length (that
   is, the prefix-length modulo 8), use a static array of 8 values to
   force the lower bits, non-relevant bits to zero.

   A particularly paranoid decoder could examine the lower non-relevant
   bits to determine if they are non-zero, and reject the prefix.  This
   would detect non-compliant encoders, or a possible covert channel.

6.  CDDL

   For use with CDDL [RFC8610], the typenames defined in Figure 1 are
   recommended:

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   ip-address-or-prefix = ipv6-address-or-prefix /
                          ipv4-address-or-prefix

   ipv6-address-or-prefix = #6.54(ipv6-address /
                                  ipv6-address-with-prefix /
                                  ipv6-prefix)
   ipv4-address-or-prefix = #6.52(ipv4-address /
                                  ipv4-address-with-prefix /
                                  ipv4-prefix)

   ipv6-address = bytes .size 16
   ipv4-address = bytes .size 4

   ipv6-address-with-prefix = [ipv6-address, ipv6-prefix-length]
   ipv4-address-with-prefix = [ipv4-address, ipv4-prefix-length]

   ipv6-prefix-length = 0..128
   ipv4-prefix-length = 0..32

   ipv6-prefix = [ipv6-prefix-length, ipv6-prefix-bytes]
   ipv4-prefix = [ipv4-prefix-length, ipv4-prefix-bytes]

   ipv6-prefix-bytes = bytes .size (uint .le 16)
   ipv4-prefix-bytes = bytes .size (uint .le 4)

                                  Figure 1

7.  Security Considerations

   Identifying which byte sequences in a protocol are addresses may
   allow an attacker or eavesdropper to better understand what parts of
   a packet to attack.

   Reading the relevant RFC may provide more information, so it would
   seem that any additional security that was provided by not being able
   to identify what are IP addresses falls into the security by
   obscurity category.

   The right-hand bits of the prefix, after the prefix-length, are
   ignored by this protocol.  A malicious party could use them to
   transmit covert data in a way that would not affect the primary use
   of this encoding.  Such abuse would be detected by examination of the
   raw protocol bytes.  Users of this encoding should be aware of this
   possibility.

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8.  IANA Considerations

   IANA has allocated two tags from the Specification Required area of
   the Concise Binary Object Representation (CBOR) Tags:

8.1.  Tag 54 - IPv6

   Data Item: byte string or array
   Semantics: IPv6, [prefixlen,IPv6], [IPv6,prefixpart]

8.2.  Tag 52 - IPv4

   Data Item: byte string or array
   Semantics: IPv4, [prefixlen,IPv4], [IPv4,prefixpart]

9.  Normative References

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

Appendix A.  Changelog

   This section is to be removed before publishing as an RFC.

   *  03

   *  02

   *  01 added security considerations about covert channel

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Acknowledgements

   none yet

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

   Carsten Bormann
   Universität Bremen TZI
   Germany

   Email: cabo@tzi.org

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