| Internet-Draft | CBOR-IP | July 2021 |
| Richardson & Bormann | Expires 26 January 2022 | [Page] |
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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[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.¶
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.¶
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 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.¶
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.¶
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:¶
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.¶
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.¶
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.¶
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.¶
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''])¶
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();
¶
For use with CDDL [RFC8610], the typenames 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]
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)
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.¶
IANA has allocated two tags from the Specification Required area of the Concise Binary Object Representation (CBOR) Tags:¶
Data Item: byte string or array Semantics: IPv6, [prefixlen,IPv6], [IPv6,prefixpart]¶
Data Item: byte string or array Semantics: IPv4, [prefixlen,IPv4], [IPv4,prefixpart]¶
This section is to be removed before publishing as an RFC.¶
none yet¶