[Babel-users] DTLS and hmac co-existence

[Dave Taht]

David Schinazi <dschinazi.ietf at gmail.com> writes:

> Hi Dave,
>
> The Babel over DTLS spec states that
> - unicast traffic is sent over DTLS, where DTLS client uses an
> ephemeral port, and DTLS server uses a well-known port different from
> 6696
> - multicast traffic still uses 6696 for source and destination
>
> I don't see any issues with protecting Hellos with HMAC and everything
> else with DTLS.

Yes, the current spec will work. I was trying to solve a different
rate control problem by allowing ephemeral ports elsewhere.

a better method to negotiate a more relaxed approach to using ephemeral
ports, and not needed to be in the spec, either, would be to try an ack
request from an ephemeral port.

Has anyone put in for an official dtls port number from IANA?

>
> David
>
> On Wed, Nov 28, 2018 at 1:56 PM Dave Taht <dave at taht.net> wrote:
>
>     Have I been enough of a PITA this month?
>     
>     Section 4 of rfc6126bis-07 states:
>     
>     Both the source and destination UDP port are set to a well-known
>     port
>     number. A Babel packet MUST be silently ignored unless its source
>     address is either a link-local IPv6 address or an IPv4 address
>     belonging to the local network, and its source port is the
>     well-known
>     Babel port.
>     
>     Aside from the fact that all versions of babel prior to now
>     enforced
>     this requirement that the source and destination port numbers be
>     the
>     same, which is a major consideration, that idea leaked into the
>     hmac
>     implementation which does a:
>     
>     DO_HTONS(&port, (unsigned short)protocol_port);
>     if ((err = blake2s_init_key(&S, key->len, key->value,
>     BLAKE2S_DIGEST_LENGTH)) < 0) goto fail;
>     /* Hash the pseudo header. */
>     if ((err = blake2s_update(&S, src, 16)) < 0) goto fail;
>     if ((err = blake2s_update(&S, &port, 2)) < 0) goto fail;
>     ^^^^^
>     if ((err = blake2s_update(&S, dst, 16)) < 0) goto fail;
>     if ((err = blake2s_update(&S, &port, 2)) < 0) goto fail;
>     ^^^^
>     
>     the DTLS branch, on the other hand, mandates that ephemeral ports
>     be
>     used for DTLS communications. Section 2.1 of
>     https://tools.ietf.org/html/draft-ietf-babel-dtls-02
>     
>     The node acting as DTLS client initiates its DTLS connection from
>     an
>     ephemeral UDP port. Nodes SHOULD ensure that new client DTLS
>     connections use different ephemeral ports from recently used
>     connections to allow servers to differentiate between the new and
>     old
>     DTLS connections.
>     
>     The rub, here, ultimately, is that I could see HMAC being used to
>     protect multicast and DTLS used to protect unicast, in which case,
>     allowing for ephemeral src ports generally in the bis version
>     would be helpful. I do not see how requiring a fixed src port
>     helps
>     anything? (besides backward compatability)
>     
>     Selfishly, I have to admit, that by using an ephemeral udp port
>     for
>     route transfers and a different, either ephemeral or 6696 for
>     hello/ihu
>     - fq_codel automagically distinguishes between these two very
>     different
>     types of traffic and in congested network conditions will drop
>     routes
>     far more often than hellos.
>     
>     A backwards compatible mechanism would be to try hellos/ihus from
>     an
>     ephemeral port and note when you don't get one back, upon starting
>     an
>     association. A key indicator to other routers no longer respecting
>     the
>     fixed src port rule would be recieving such.
>     
>     ...
>     
>     As a side note, I had suggested using UDP-lite for development
>     purposes
>     or dtls communications. It's easier to use a different protocol
>     than it
>     is to get a new port number out of iana...
>     
>     I was queried as to why udp-lite, and my answer that I never got
>     around
>     to saying was that once you have an HMAC, you don't need a CRC,
>     and
>     also, to my knowledge, udp-lite is very widely deployed, just not
>     used.
>     It is the same in all other respects to udp. On a single hop over
>     ipv6,
>     udp-lite "just works" on the versions of linux and bsd I've tried
>     over
>     the past 5 years. It's impossible to compile out in linux
>     (although
>     netfilter support for it can be), for example.
>     
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