CBOR tags for IPv4 and IPv6 addresses and prefixes
draft-ietf-cbor-network-addresses-08

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

           CBOR tags for IPv4 and IPv6 addresses and prefixes
                  draft-ietf-cbor-network-addresses-08

Abstract

   This specification defines two CBOR Tags to be used with IPv6 and
   IPv4 addresses and prefixes.

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

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Three Forms . . . . . . . . . . . . . . . . . . . . . . .   3
       3.1.1.  Addresses . . . . . . . . . . . . . . . . . . . . . .   3
       3.1.2.  Prefixes  . . . . . . . . . . . . . . . . . . . . . .   3
       3.1.3.  Interface Definition  . . . . . . . . . . . . . . . .   3
     3.2.  IPv6  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  IPv4  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Encoder Considerations for Prefixes . . . . . . . . . . . . .   5
   5.  Decoder Considerations for Prefixes . . . . . . . . . . . . .   6
   6.  CDDL  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Tag 54 - IPv6 . . . . . . . . . . . . . . . . . . . . . .   8
     8.2.  Tag 52 - IPv4 . . . . . . . . . . . . . . . . . . . . . .   8
     8.3.  Tags 260 and 261  . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Changelog  . . . . . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC8949] defines a number of CBOR Tags for common items.  Tags 260
   and 261 were later defined through IANA [IANA.cbor-tags].  These tags
   cover addresses (260), and prefixes (261).  Tag 260 distinguishes
   between IPv6, IPv4 and Ethernet through the length of the byte string
   only.  Tag 261 was not documented well enough to be used.

   This specification defines tags 54 and 52.  These new tags are
   intended to be used in preference to tags 260 and 261.  They provide
   formats for IPv6 and IPv4 addresses, prefixes, and addresses with
   prefixes, achieving an explicit indication of IPv6 or IPv4.  The
   prefix format omits trailing zeroes in the address part.  (Due to the

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   complexity of testing, the value of omitting trailing zeros for the
   pure address format was considered non-essential and support for that
   is not provided in this specification.)  This specification does not
   deal with 6- or 8-byte Ethernet addresses.

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.

3.  Protocol

3.1.  Three Forms

3.1.1.  Addresses

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

   This form is called the Address Format.

3.1.2.  Prefixes

   When applied to an array that starts with an unsigned integer, they
   represent a CIDR-style prefix of that length.

   When the Address Format (i.e., 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.

   This form is called the Prefix Format.

3.1.3.  Interface Definition

   When applied to an array that starts with a byte string, which stands
   for an IP address, followed by an unsigned integer giving the bit
   length of a prefix built out of the first "length" bits of the
   address, they represent information that is commonly used to specify
   both the network prefix and the IP address of an interface.

   This form is called the Interface Format.

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3.2.  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.

   For example:

   54(h'20010db81234DEEDBEEFCAFEFACEFEED')

   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.

3.3.  IPv4

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

   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.

   For example:

   52(h'C0000201')

   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.

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   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 Considerations for Prefixes

   For the byte strings used in representing prefixes, an encoder MUST
   omit any right-aligned (trailing) sequence of bytes that are all
   zero.

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

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

   An encoder MUST take care to set all trailing bits in the final byte
   to zero, if any.  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 otherwise 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

   would be parsed in the exact same way.

   The same considerations apply to IPv4 prefixes.

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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 implementations is to first create an array of
   16 (or 4) zero bytes.

   Then taking whichever is smaller between (a) the length of the
   included byte-string, and (b) the number of bytes covered by the
   prefix-length rounded up to the next multiple of 8: fail if that
   number is greater than 16 (or 4), and then copy that many bytes from
   the byte-string into the array.

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

   unused_bits = (-prefix_length_in_bits) & 7;
   if (length_in_bytes > 0)
     address_bytes[length_in_bytes - 1] &= (0xFF << unused_bits);

   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.

   if (length_in_bytes > 0 &&
       (address_bytes[length_in_bytes - 1] & ~(0xFF << unused_bits))
       != 0)
     fail();

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.  That information, however, is likely to be found
   in the relevant RFCs anyway, so this is not a significant exposure.

   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.

8.  IANA Considerations

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

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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]

8.3.  Tags 260 and 261

   IANA is requested to add the note "DEPRECATED in favor of 52 and 54"
   to registrations 260 and 261"

9.  References

9.1.  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>.

9.2.  Informative References

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <http://www.iana.org/assignments/cbor-tags>.

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Appendix A.  Changelog

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

   *  03

   *  02

   *  01 added security considerations about covert channel

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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