CBOR Working Group M. Richardson
Internet-Draft Sandelman Software Works
Intended status: Standards Track C. Bormann
Expires: 26 January 2022 Universität Bremen TZI
25 July 2021
CBOR tags for IPv4 and IPv6 addresses and prefixes
draft-ietf-cbor-network-addresses-06
Abstract
This specification describes two CBOR Tags to be used with IPv4 and
IPv6 addresses and prefixes.
// RFC-EDITOR-please-remove: This work is tracked at
// https://github.com/cbor-wg/cbor-network-address
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.3. IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Encoder Considerations for Prefixes . . . . . . . . . . . . . 5
5. Decoder Considerations for Prefixes . . . . . . . . . . . . . 5
6. CDDL . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8.1. Tag 54 - IPv6 . . . . . . . . . . . . . . . . . . . . . . 7
8.2. Tag 52 - IPv4 . . . . . . . . . . . . . . . . . . . . . . 8
9. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[RFC8949] defines a number of CBOR Tags for common items. Tags 260
and 261 were later defined through IANA. These tags cover addresses
(260), and prefixes (261). Tag 260 distinguishes between IPv4, IPv6
and Ethernet through the length of the byte string only. Tag 261 was
not documented well enough to be used.
This specification provides a format for IPv6 and IPv4 addresses,
prefixes, and addresses with prefixes, achieving an explicit
indication of IPv4 or IPv6. Prefixes omit trailing zeroes in the
address. (Due to the complexity of testing, the value of omitting
trailing zeros for addresses was considered non-essential and support
for that was removed in this specification.)
This specification does not deal with 6 or 8-byte Ethernet addresses.
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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 a number, 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, that stands
for an IP address, followed by the bit length of a prefix built out
of the first "length" bits of the address.
This form is called the Interface Format.
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:
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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.
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.
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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.
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''])
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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:
8.1. Tag 54 - IPv6
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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]
9. 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>.
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
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Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
Carsten Bormann
Universität Bremen TZI
Germany
Email: cabo@tzi.org
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