CBOR tags for IPv4 and IPv6 addresses and prefixes
draft-ietf-cbor-network-addresses-12

CBOR Working Group                                         M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                              C. Bormann
Expires: 23 April 2022                            Universität Bremen TZI
                                                         20 October 2021

           CBOR tags for IPv4 and IPv6 addresses and prefixes
                  draft-ietf-cbor-network-addresses-12

Abstract

   This specification defines two CBOR Tags for use with IPv6 and IPv4
   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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Copyright Notice

   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  . . . . . . . . . . . . . . . .   4
     3.2.  IPv6  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  IPv4  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Tag validity  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Deterministic Encoding  . . . . . . . . . . . . . . . . .   6
     4.2.  Encoder Considerations for Prefixes . . . . . . . . . . .   6
     4.3.  Decoder Considerations for Prefixes . . . . . . . . . . .   7
       4.3.1.  Example implementation  . . . . . . . . . . . . . . .   7
   5.  CDDL  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Tag 54 - IPv6 . . . . . . . . . . . . . . . . . . . . . .  10
     7.2.  Tag 52 - IPv4 . . . . . . . . . . . . . . . . . . . . . .  10
     7.3.  Tags 260 and 261  . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Changelog  . . . . . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [RFC8949] defines a number of CBOR Tags for common items.  Tags 260
   and 261 were later defined in drafts listed with IANA
   [IANA.cbor-tags].  These tags were intended to cover addresses (260)
   and prefixes (261).  Tag 260 distinguishes between IPv6, IPv4, and
   MAC [RFC7042] addresses only through the length of the byte string,
   making it impossible, for example, to drop trailing zeros in the
   encoding of IP addresses.  Tag 261 was not documented well enough for
   use.

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   This specification defines tags 54 and 52 achieving an explicit
   indication of IPv6 or IPv4 by the tag number.  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
   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 MAC addresses (Section 2 of [RFC7042]) such as they are
   used for Ethernet.

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.

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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.

   The length of the byte string is always 16 bytes (for IPv6) and 4
   bytes (for IPv4).

   This form is called the Interface Format.

   Interface Format definitions support an optional third element to the
   array, which is to be used as the IPv6 Link-Local zone identifier
   from Section 4 of [RFC3542] and Section 6 of [RFC4007]; for symmetry
   this is also provided for IPv4 as in [RFC4001] and [RFC6991].  The
   zone identifier may be an integer, in which case it is to be
   interpreted as the interface index.  It may be a text string, in
   which case it is to be interpreted as an interface name.

   As explained in [RFC4007] the zone identifiers are strictly local to
   the node.  They are useful for communications within a node about
   connected addresses (for instance, where a link-local peer is
   discovered by one daemon, and another daemon needs to be informed).
   They may also have utility in some management protocols.

   In the cases where the Interface Format is being used to represent
   only an address with a zone identifier, and no interface prefix
   information, then the prefix length may be replaced with the CBOR
   "null" (0xF6).

3.2.  IPv6

   IANA has allocated tag 54 for IPv6 uses.  (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.  See Section 4
   for the detailed construction of the second element.

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   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; a third element can be added for
   the zone identifier.

   For example:

   54([h'20010db81234deedbeefcafefacefeed', 56])

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

   Some example of a link-local IPv6 address with a 64-bit prefix:

   54([h'fe8000000000020202fffffffe030303', 64, 'eth0'])

   with a numeric zone identifier:

   54([h'fe8000000000020202fffffffe030303', 64, 42])

   An IPv6 link-local address without a prefix length:

   54([h'fe8000000000020202fffffffe030303', null, 42])

   Zone identifiers may be used with any kind of IP address, not just
   Link-Local addresses.  In particular, they are valid for multicast
   addresses, and there may still be some significance for Globally
   Unique Addresses (GUA).

3.3.  IPv4

   IANA has allocated tag 52 for IPv4 uses.  (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')

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   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.  See Section 4
   for the detailed construction of the second element.

   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; a third element can be added for
   the zone identifier.

   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])

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

4.  Tag validity

   This section discusses when a tag 54 or tag 52 is valid
   (Section 5.3.2 of [RFC8949]).  As with all CBOR tags, validity
   checking can be handled in a generic CBOR library or in the
   application.  A generic CBOR library needs to document whether and
   how it handles validity checking.

   The rule ip-address-or-prefix in Figure 1 shows how to check the
   overall structure of these tags and their content, the ranges of
   integer values, and the lengths of byte strings.  An instance of tag
   52 or 54 is valid if it matches that rule and, for ipv6-prefix and
   ipv4-prefix, the considerations of Sections 4.2 and 4.3.

4.1.  Deterministic Encoding

   The tag validity rules, combined with the rules in Section 4.2.1 of
   [RFC8949], lead to deterministic encoding for tags 54 and 52 and
   require no further Additional Deterministic Encoding Considerations
   as per Section 4.2.2 of [RFC8949].

4.2.  Encoder Considerations for Prefixes

   For the byte strings used as the second element in the array
   representing a prefix:

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   (1) An encoder MUST set any unused bytes, and any unused bits in the
   final byte, if any, to zero.  Unused bytes/bits are bytes/bits that
   are not covered by the prefix length given.  So for example,
   2001:db8:1230::/44 MUST be encoded as:

   54([44, h'20010db81230'])

   even though variations like:

   54([44, h'20010db81233'])
   54([44, h'20010db8123f'])
   54([44, h'20010db8123012'])

   start with the same 44 bits, but are not valid.

   (Analogous examples can be constructed for IPv4 prefixes.)

   (2) An encoder MUST then 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'])

4.3.  Decoder Considerations for Prefixes

   A decoder MUST check that all unused bits encoded in the byte string
   ipv6-prefix-bytes/ipv4-prefix-bytes, i.e., the bits to the right of
   the prefix length, are zero.

   A decoder MUST also check that the byte string does not end in a zero
   byte.

   Since encoders are required to remove zero-valued trailing bytes, 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 an example, ::/128 is encoded as

   54([128, h''])

4.3.1.  Example implementation

   A recommendation for prefix decoder implementations is to first
   create an array of 16 (or 4) zero bytes.

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   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 byte array.

   Finally, looking at the number of unused bits in the last byte (if
   any) of the range covered by the prefix length, check that any unused
   bits in the byte string are zero:

   unused_bits = (8 - (prefix_length_in_bits % 8)) % 8;
   if (length_in_bytes > 0 &&
       (address_bytes[length_in_bytes - 1] & ~(0xFF << unused_bits))
          != 0)
     fail();

5.  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 / null,
                               ?ip-zone-identifier]
   ipv4-address-with-prefix = [ipv4-address,
                               ipv4-prefix-length / null,
                               ?ip-zone-identifier]

   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)

   ip-zone-identifier = uint / text

                  Figure 1: CDDL types for tags 54 and 52

6.  Security Considerations

   This document provides an CBOR encoding for IPv4 and IPv6 address
   information.  Any applications using these encodings will need to
   consider the security implications of these data in their specific
   context.  For example, 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.

   Applications need to check the validity (Section 4) of a tag before
   acting on any of its contents.  If the validity checking is not done
   in the generic CBOR decoder, it needs to be done in the application;
   in any case it needs to be done before the tag is transformed into a
   platform-specific representation that could conceal validity errors.

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   The right-hand bits of the prefix, after the prefix-length, are set
   to zero by this protocol.  (Otherwise, 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 is detected by tag validity
   checking, and can also be detected by examination of the raw protocol
   bytes.)

7.  IANA Considerations

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

7.1.  Tag 54 - IPv6

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

7.2.  Tag 52 - IPv4

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

7.3.  Tags 260 and 261

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

8.  References

8.1.  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>.

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   [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>.

8.2.  Informative References

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

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003,
              <https://www.rfc-editor.org/info/rfc3542>.

   [RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
              Schoenwaelder, "Textual Conventions for Internet Network
              Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,
              <https://www.rfc-editor.org/info/rfc4001>.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              DOI 10.17487/RFC4007, March 2005,
              <https://www.rfc-editor.org/info/rfc4007>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <https://www.rfc-editor.org/info/rfc6991>.

   [RFC7042]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
              IETF Protocol and Documentation Usage for IEEE 802
              Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042,
              October 2013, <https://www.rfc-editor.org/info/rfc7042>.

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

   Roman Danyliw, Donald Eastlake, Ben Kaduk, Barry Leiba, and Éric
   Vyncke reviewed the document and provided suggested text.  Jürgen
   Schönwälder helped finding the history of IPv4 zone identifiers.

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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