| RFC 9164 | CBOR Tags for IP | December 2021 |
| Richardson & Bormann | Standards Track | [Page] |
This specification defines two Concise Binary Object Representation (CBOR) tags for use with IPv6 and IPv4 addresses and prefixes.¶
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 athttps://www.rfc-editor.org/info/rfc9164.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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[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 explicitlyindicate use of IPv6 or IPv4 by the tag number.These new tags are intended to be used in preference to tags 260 and261.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 trailingzeros for the pure address format was considered nonessential, andsupport for that is not provided in this specification.)This specification does not deal with MAC addresses (Section 2 of [RFC7042]).¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED","MAY", and "OPTIONAL" in this document are to be interpreted asdescribed in BCP 14[RFC2119][RFC8174] when, and only when, theyappear in all capitals, as shown here.¶
These tags can be applied to byte strings to represent a single address.¶
This form is called the "Address Format".¶
When applied to an array that starts with an unsigned integer, the tags represent aCIDR-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".¶
When applied to an array that starts with a byte string, which standsfor an IP address, followed by an unsigned integer giving the bitlength of a prefix built out of the firstlength bits of theaddress, the tags represent information that is commonly used to specifyboth 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 fromSection 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 representonly an address with a zone identifier and no interface prefix information, the prefix length may be replaced with the CBOR "null" (0xF6).¶
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.SeeSection 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 forconfiguring an interface, is to be encoded as a two-element array,with the (full-length) IPv6 address first and the length of theassociated network the prefix next; a third element can be added forthe zone identifier.¶
For example:¶
54([h'20010db81234deedbeefcafefacefeed', 56])¶
The address-with-prefix form can be reliably distinguishedfrom 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 significancefor Globally Unique Addresses (GUAs).¶
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.SeeSection 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 forconfiguring an interface, is to be encoded as a two-element array,with the (full-length) IPv4 address first and the length of theassociated network the prefix next; a third element can be added forthe 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 distinguishedfrom the prefix form only in the sequence of the array elements.¶
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 genericCBOR library or in the application.A generic CBOR library needs to document whether and how it handlesvalidity checking.¶
The ruleip-address-or-prefix inFigure 1 shows how to check theoverall structure of these tags and their content, the ranges ofinteger values, and the lengths of byte strings.An instance of tag 52 or 54 is valid if it matches that rule and, foripv6-prefix and ipv4-prefix, the considerations of Sections4.2 and4.3.¶
The tag validity rules, combined with the rules inSection 4.2.1 of [RFC8949], lead to deterministic encoding for tags 54 and 52 and requireno further additional deterministic encoding considerations as perSection 4.2.2 of [RFC8949].¶
For the byte strings used as the second element in the arrayrepresenting a prefix:¶
(1) An encoderMUST 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::/44MUST 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 encoderMUST then omit any right-aligned (trailing) sequence ofbytes 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'])¶
A decoderMUST check that all unused bits encoded in the byte stringipv6-prefix-bytes/ipv4-prefix-bytes, i.e., the bits to the right ofthe prefix length, are zero.¶
A decoderMUST also check that the byte string does not end in a zerobyte.¶
Since encoders are required to remove zero-valued trailing bytes, adecoderMUST handle cases where a prefix length specifies that morebits are relevant than are actually present in the byte string.¶
As an example, ::/128 is encoded as¶
54([128, h''])¶
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 theincluded byte string and (b) the number of bytes covered by theprefix length rounded up to the next multiple of 8, fail if thatnumber is greater than 16 (or 4) and then copy that many bytes fromthe byte string into the byte array.¶
Finally, when looking at the number of unused bits in the last byte (ifany) of the range covered by the prefix length, check that any unusedbits 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();¶
For use with Concise Data Definition Language (CDDL)[RFC8610], the type names defined inFigure 1are recommended:¶
ip-address-or-prefix = ipv6-address-or-prefix / ipv4-address-or-prefixipv6-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 16ipv4-address = bytes .size 4ipv6-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..128ipv4-prefix-length = 0..32ipv6-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
This document provides a CBOR encoding for IPv4 and IPv6 address information.Any applications using these encodings will need to consider the securityimplications of this data in their specific context. For example, identifyingwhich byte sequences in a protocol are addresses may allow an attacker oreavesdropper to better understand what parts of a packet to attack.¶
Applications need to check the validity (Section 4) of a tag beforeacting on any of its contents.If the validity checking is not done in the generic CBOR decoder, itneeds to be done in the application; in any case, it needs to be donebefore the tag is transformed into a platform-specific representationthat could conceal validity errors.¶
The right-hand bits of the prefix, after the prefix length, are set tozero by this protocol.(Otherwise, a malicious party could use them to transmit covert datain a way that would not affect the primary use of this encoding.Such abuse is detected by tag validity checking and can also bedetected by examination of the raw protocol bytes.)¶
IANA has allocated two tags from the Specification Required[RFC8126] area ofthe "Concise Binary Object Representation (CBOR) Tags" registry[IANA.cbor-tags]:¶
IANA has added the note "DEPRECATED in favor of 52 and 54 for IP addresses" to registrations 260 and 261.¶
Roman Danyliw,Donald Eastlake,Ben Kaduk,Barry Leiba, andÉric Vyncke reviewed the document and provided suggested text.Jürgen Schönwälder helped find the history of IPv4 zone identifiers.¶