Capwap Mailing List: Re: Issue 168: DTLS and Retransmissions

[Pat Calhoun (pacalhou) (pcalhoun]

Scott/Charles, are we good with this? Can I close this issue with the
recommended text below?

PatC 

-----Original Message-----
From: Pasi.Eronen [at] nokia.com [mailto:Pasi.Eronen [at] nokia.com] 
Sent: Tuesday, August 19, 2008 1:10 PM
To: Pat Calhoun (pacalhou); skelly [at] arubanetworks.com; clancy [at] 
ltsnet.net
Cc: capwap [at] frascone.com
Subject: RE: [Capwap] Issue 168: DTLS and Retransmissions

Pat Calhoun wrote:

> So just to be clear, your request is to replace the first paragraph 
> only. The resulting section would read as follows:

No, the 2nd paragraph, the three bullets, and the paragraph following
the bullets also need work... but much of the text is really redundant
with the new 1st paragraph, so this could be shortened (while fixing the
incorrect parts at the same time) -- see below.

> <updated section>
> 2.4.3.  DTLS Error Handling
> 
>    If the AC or WTP does not respond to any DTLS handshake messages
>    sent by its peer, the DTLS specification calls for the message to
>    be retransmited.  Note that during the handshake, when both the
>    AC and the WTP are expecting additional handshake messages, they
>    both retransmit if an expected message has not been received
>    (note that retransmissions for CAPWAP Control messages work
>    differently: all CAPWAP Control messages are either requests or
>    responses, and the peer who sent the request is responsible for
>    retransmissions).
> 
>    If the WTP does not respond to any DTLS handshake messages sent
>    by the AC, the CAPWAP protocol allows for three possibilities,
>    listed below.  Note that DTLS MAY send a single TLS Alert message
>    to the AC to indicate session termination.
> 
>    o  The message was lost in transit; in this case, the DTLS
>       implementation on the WTP will retransmit its last outstanding
>       handshake message, since it did not receive a reply.
> 
>    o  The WTP sent a DTLS Alert, which was lost in transit; in this
>       case, the AC's WaitDTLS timer will expire, and the 
>       session will be
>       terminated.
> 
>    o  Communication with the WTP has completely failed; in this case,
>       the AC's WaitDTLS timer will expire, and the session will be
>       terminated.
> 
>    The DTLS specification provides for retransmission of 
>    unacknowledged
>    requests.  If retransmissions remain unacknowledged, the WaitDTLS
>    timer will eventually expire, at which time the CAPWAP 
>    component will
>    terminate the session.

Probably everything from the 2nd paragraph to here could be summarized
something like this:

   If the WTP or the AC does not receive an expected DTLS handshake
   message despite of retransmissions, the WaitDTLS timer will
   eventually expire, and the session will be terminated.  This can
   happen if communication between the peers has completely failed, or
   if one of the peers sent a DTLS Alert message which was lost in
   transit (DTLS does not retransmit Alert messages).

Best regards,
Pasi