Subject: [Capwap] Control channel packet classification

From: "Hasan Raza" <[email-masked]>
Date: Tue, 22 Aug 2006 02:06:12 -0700

Hi,

I have a basic question regarding the classification of packets exchanged over
control channel. It is stated in the draft that AC in Idle state can receive
Discovery as well as DTLS clientHello (+other dtls handshake) packets. The
clientHello packets are delivered to the SSL layer and Discovery packets are
consumed by the CAPWAP. Question is how? The draft assumes some magic
way of doing it. Since both of the packets are in plain, and are received on
a single udp port, which state/policy/field classifies such packets from one
another? Theoretically speaking, WTP can send a discovery packet which is
also a valid clientHello packet! Now AC would respond with a discovery
response or a clientHello reponse?

In thread http://lists.frascone.com/pipermail/capwap/msg03035.html, Pat talks about
the negotiated policy for the appropriate treatment of the received packets. That may
mean, if the AC is in join onwards state, it can be assumed that AC will only receive
encrypted packets. Though this policy can work, but there would be exceptions in
which WTP dies and comes back with a discovery packet. AC still being in join+
state, the received packets will be delivered to the SSL layer. Assumptions can be
made here that SSL would discard non-dtls packets and with timeouts in picture, WTP
and AC would eventually reach a consensus. But is this ok to assume?

In the same thread, Perkins suggested the mux header approach for control channel,
but it was not acknowledged? Is there some other hidden mechanism for the
classification? Or there is some basic point which I miss here?

Thanks
Hasan
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