Subject: [midcom] Does midcom work with RTSP?

From: Johan Gerhardsson <[email-masked]>
Date: Thu, 23 Aug 2001 16:31:06 +0200

Hello all,

my name is Johan Gerhardsson, I'm thinking of a use case where a RTSP proxy
"implements" midcom agent procuderes (i.e. a RTSP proxy uses midcom to do
nat bindings and opens a pin-hole in a middlebox in order to let media
through to an end-user application that is located in a private network).
Question: Does RTSP really work with midcom?

RFC2326 (RTSP), page.59, general parameters: destination: "A server SHOULD
not allow a client to direct media streams to an address that differs from
the address commands are coming from." 
I.e. the RTSP protocol recomend RTSP servers to NOT allow clients to specify
an alternate IP address to receive media (e.g. RTP) other than what was used
in the RTSP signalling (the TCP stream where RTSP is in use). 

How is it then possible to use midcom in order to open a pinhole through a
middlebox that routes the RTP packets from the server directly to the client
(not through the proxy) when the proxy opens a second TCP stream connection
towards the server which implicitely tells the server to send the RTP
packets to the proxy?

I think fig.2 in <draft-ietf-midcom-framework-03.txt> isn't in harmony with
the statement above from the RTSP RFC.

Have I missed something?

Thanks

Johan Gerhardsson
Reddo Networks AB
email: [email-masked]
http://reddo.net

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From: Scott Brim <[email-masked]>
Date: Thu, 23 Aug 2001 10:59:27 -0400

> the statement above from the RTSP RFC.

The issue raised in rfc2326 would be between a client and and an RTSP
proxy.  The midcom machinery is separate and wouldn't even know there's
an issue.

The midcom protocol has not been specified yet.  However, the general
feeling is that it will be very simple.  An authorized agent will be
able to request a middlebox to install packet matching and modifying
rules, for example "let packets between X and Y to pass through".  The
requirements on the midcom protocol, to the extent that they have been
worked out, do not say anything about restricting which addresses or
group of addresses can be in such a rule.

Similarly, there is nothing in the midcom protocol that would require a
middlebox to know about any relationship between agent and endpoint
client.  Unless new requirements come up, the middlebox has no reason or
means to know which particular endpoint activity caused an agent to
generate a particular ruleset request.  In fact the whole point of the
Midcom WG is to remove such intelligence from the middlebox.

So, if there is an issue, it is with the RTSP proxy -- it needs to
follow rfc2326 in deciding how it grants client requests.  Once those
decisions are made, the requests it makes to the middlebox via the
midcom protocol will fall out automatically.

..Scott