RFC 9164

Internet Engineering Task Force (IETF)                     M. Richardson
Request for Comments: 9164                      Sandelman Software Works
Category: Standards Track                                     C. Bormann
ISSN: 2070-1721                                   Universität Bremen TZI
                                                           December 2021

   Concise Binary Object Representation (CBOR) Tags for IPv4 and IPv6
                         Addresses and Prefixes


   This specification defines two Concise Binary Object Representation
   (CBOR) tags for use with IPv6 and IPv4 addresses and prefixes.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

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

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Protocol
     3.1.  Three Forms
       3.1.1.  Addresses
       3.1.2.  Prefixes
       3.1.3.  Interface Definition
     3.2.  IPv6
     3.3.  IPv4
   4.  Tag Validity
     4.1.  Deterministic Encoding
     4.2.  Encoder Considerations for Prefixes
     4.3.  Decoder Considerations for Prefixes
       4.3.1.  Example Implementation
   5.  CDDL
   6.  Security Considerations
   7.  IANA Considerations
     7.1.  Tag 54 - IPv6
     7.2.  Tag 52 - IPv4
     7.3.  Tags 260 and 261
   8.  References
     8.1.  Normative References
     8.2.  Informative References

   Authors' Addresses

1.  Introduction

   [RFC8949] defines a number of CBOR tags for common items.  Tags 260
   and 261 were later defined in documents 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

   This specification defines tags 54 and 52 to explicitly indicate use
   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, while
   explicitly indicating use 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 nonessential, and support for that is not
   provided in this specification.)  This specification does not deal
   with MAC addresses (Section 2 of [RFC7042]).

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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

   This form is called the "Address Format".

3.1.2.  Prefixes

   When applied to an array that starts with an unsigned integer, the
   tags 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,
   the tags 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 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, the prefix length may be replaced with the CBOR "null"

3.2.  IPv6

   IANA has allocated tag 54 for IPv6 uses.  (This is the ASCII code for

   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:


   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.

   For example:

   54([48, h'20010db81234'])

   An IPv6 address combined with a prefix length, such as one 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.

   An 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 (GUAs).

3.3.  IPv4

   IANA has allocated tag 52 for IPv4 uses.  (This is the ASCII code for

   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:


   An IPv4 prefix, such as, 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, is to be encoded as a two-element array,
   with the length of the prefix (implied 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 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

   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:

   (1) An encoder MUST set any unused bytes and any unused bits in the
   final byte, if any, to zero.  Unused bytes (or bits) are bytes (or
   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 in which the bytes are all zeros.

   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

   Since encoders are required to remove zero-valued trailing bytes, a
   decoder MUST handle cases 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.

   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, when 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)

5.  CDDL

   For use with Concise Data Definition Language (CDDL) [RFC8610], the
   type names defined in Figure 1 are recommended:

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

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

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

   ipv6-address-with-prefix = [ipv6-address,
                               ipv6-prefix-length / null,
   ipv4-address-with-prefix = [ipv4-address,
                               ipv4-prefix-length / null,

   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 a CBOR encoding for IPv4 and IPv6 address
   information.  Any applications using these encodings will need to
   consider the security implications of this 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.

   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

7.  IANA Considerations

   IANA has allocated two tags from the Specification Required [RFC8126]
   area of the "Concise Binary Object Representation (CBOR) Tags"
   registry [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 has added 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,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [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,

8.2.  Informative References

              IANA, "Concise Binary Object Representation (CBOR) Tags",

   [RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
              Schoenwaelder, "Textual Conventions for Internet Network
              Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              DOI 10.17487/RFC4007, March 2005,

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,

   [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>.


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

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

   Carsten Bormann
   Universität Bremen TZI

   Email: cabo@tzi.org