[Cbor] changes to draft-ietf-cbor-network-addresses-01.txt

[Michael Richardson <[e-mail]>]

>        Title           : CBOR tags for IPv4 and IPv6 addresses and prefixes
>	Filename        : draft-ietf-cbor-network-addresses-01.txt
> A diff from the previous version is available at:
> https://www.ietf.org/rfcdiff?url2=draft-ietf-cbor-network-addresses-01


Brian E Carpenter <[e-mail]> wrote:
    bec> FYI, I am not tracking the CBOR list, but this caught my eye.

    bec> Since prefixes may be of any length and are not required to be
    bec> nibble-aligned, how is 2001:db8:1234::/47 encoded? Is it just this?

    bec>     TBD1([ 47, h'20010db81234'])

    bec> What is the specified behaviour if the byte string is too short or
    bec> too long?

The intention is that it is always zero-padded out to the needed prefix
length.

I have added sections in draft-ietf-cbor-network-addresses-01 which addresses
this as _Encoder Considerations_ and _Decoder Considerations_

I can't recall who asked about Covert Channels, but there are indeed an
opportunity.
In particular, with non-byte aligned prefixes (like 47 above), the 48th bit
(and bits 48 to 128) should be forced to zero.

Brian also laments:

> Reminder: in draft-ietf-anima-prefix-management-07 (RFC Editor queue), we
> have the CDDL below. Of course we could have used your tags if they had
> existed.

>     prefval /= pref6val
>     pref6val = [version6, length, ?prefix]
>     version6 = 6
>     length = 0..128             ; requested or offered prefix length
>     prefix = bytes .size 16     ; offered prefix in binary format

So, one byte for 4/6, vs two bytes for a 1+1 tag.
Unclear if "prefix" can be shortened though.

I just adjusted my code to omit trailing zeros, and to force bits to zero
when sending and receiving.
(Oh, I don't force trailing bits in a byte to zero when sending yet)

My C code is short, I could put it in an appendix if someone thought that was
useful, but I'm afraid it might cause more comments.

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]               Never tell me the odds!                 | ipv6 mesh networks [
]   Michael Richardson, Sandelman Software Works        |    IoT architect   [
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