[Int-area] SOCKS6 and UDP

[Ben Schwartz <[e-mail]>]

I have a few notes on the SOCKS 6 draft, related to UDP.

1. I would like to require servers to share resumption between TLS and
DTLS.  This would allow clients to keep a single resumption cache,
instead of two.

2. I think the protocol should support sending UDP datagrams over the
TCP socket being used for UDP ASSOCIATE.  If the client sends a
datagram on this channel, it would become the mapped channel for the
association.  (Alternatively, it could serve as a "fallback channel"
when there is no UDP-based channel available.)  This would be faster
for short-lived UDP transactions, because the client could include the
first upstream packet in the first flight, without needing to wait for
a reply containing the Association ID.  It would also help in cases
where the client-proxy path is UDP-intolerant.

3. The 64-bit Association ID seems like a potential security
vulnerability when using DTLS.  Suppose an attacker observes a DTLS
connection attempt from a client.  If the attacker can block the true
client's DTLS, they can start enumerating Association IDs until they
find the one that works.  Enumerating a 64-bit space is not easy, but
it is too small to be considered cryptographically strong.

Putting a very large Association ID at the beginning of every packet
is not appealing, but with DTLS there may be a more secure and more
efficient solution.  For example, the client could include a longer
Association ID only until it gets an acknowledgement from the server,
at which point both sides know that the UDP association is mapped to
this DTLS connection.  Alternatively, the client could send some
information about its DTLS ClientHello in the TLS channel that sent
the UDP ASSOCIATE, creating a secure binding.

4. SOCKS6-UDP is not compatible with PLPMTUD
(https://tools.ietf.org/html/draft-ietf-tsvwg-datagram-plpmtud),
because the client cannot control fragmentation at the proxy.  I think
we should add a "Don't Fragment" bit to the SOCKS 6 datagram header to
support PLPMTUD.

(I would love it if SOCKS 6 could pass ICMP messages for classical
PMTUD (and ping! and traceroute!) but I don't expect that to happen.)

--Ben