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
Abstract
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
https://www.rfc-editor.org/info/rfc9164.
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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
Acknowledgements
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
use.
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",
"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, 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"
(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.
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
'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. 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 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:
(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
byte.
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)
fail();
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 /
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 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
bytes.)
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,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
8.2. Informative References
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://www.iana.org/assignments/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,
<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>.
Acknowledgements
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
Germany