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Abstract
This document discusses tunneling RTSP in HTTP and more specifically
RTSP with interleaved RTP data.
1. Introduction
There is a need for tunneling RTP streams inside RTSP in HTTP
because in some cases users are located behind a firewall that is
configured to only let HTTP through.
After discussing the context of the problem we will give the
requirements for a solution and outline what a solution could be.
2. Motivation
RTSP [1, 10.12] clearly defines how RTP stream data can be embedded
with RTSP methods. It also recommends that RTSP should be
transported using TCP.
RTSP使用TCP
Note that tunneling RTSP (only) in HTTP would not be useful since
the RTP data transported using UDP would be blocked.
HTTP 只能传送TCP,所以以UDP封包的RTP不能通过http 传送
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With this TCP based transport of RTP-inside-RTSP, firewalls
configured to exclude UDP traffic can be traversed, which is already
very useful.
However for many end users the situation is even worse; indeed some
ISPs and many corporate Internet access are protected by strict
firewalls. Typically these firewalls are configured to exclude all
traffic except HTTP. Thus there is need to transport media through
HTTP.
Although it is recognized that doing so is not optimal (see [1] and
[2] for a discussion on why RTP/UDP is a better idea) it should be
obvious that the core reason why tunneling streaming in HTTP in not
a good idea is due to the use of TCP for transport, which is not an
issue we need to discuss here since transporting RTP inside RTSP on
TCP (as described in [1]) suffers from the same problem.
RTP通过RTSP传不好，但是没有办法
In fact the need for such a solution is so strong that most media
streaming products implement (pseudo) streaming through HTTP in one
way or another.
It is also obvious that although in many cases the reason why a
corporate or ISP firewall is configured for HTTP only is pure
paranoia, there are cases when suppressing media streaming is a
genuine concern for an IT administrator. In this last case providing
a standard technology that makes it possible to filter RTSP in HTTP
would be a very good idea.
It is however of fundamental importance to stress that one of the
contexts where such tunneling is needed is in environments where IT
management is not leading edge. Indeed paranoia very often goes
together with ignorance and/or lack of capability to trustfully
communicate between decision makers and knowledgeable technical
people. In such environments firewalls are set for maximum security
i.e. HTTP-only and solutions that are known to work will be kept as
long as possible. Therefore the solution we seek MUST work with all
deployed firewalls otherwise it will be a very little value since we
cannot expect this key target population to upgrade their firewalls
if this is what is required to enable RTSP tunneling in HTTP.
On the other hand the situation is very different for more forward-
looking organizations. We have two cases then. In places where IT
administrators opened the required UDP ports in their firewalls so
as to enable streaming, new solutions i.e. upgrading the firewall-
will be easily adopted. There are also places where IT administrator
did not open UDP ports for streaming upon explicit instructions from
their management to stop media streaming. In such places surely the
emergence of a standard technology enabling to deploy firewalls that
can also block tunneling of media in HTTP will be well received.
Therefore the solution we seek should also provide the ability to
configure filtering mechanisms so as to give full control on what
can and what cannot traverse the firewall.
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Tunneling RTSP in HTTP July 2001
3. Requirements
The requirements for tunneling RTSP in HTTP are therefore the
following:
Requirement 1: to traverse existing (deployed) HTTP-only firewalls
Requirement 2: to allow the development of new HTTP-level firewalls
where RTSP tunneling can be detected and eventually filtered.
4. Solutions
One solution is described in [3]. We will not discuss this solution
in full detail but instead we will focus on what seems to be the
most controversial issue.
5. Discussion
As described in [3] there is a need to prevent deployed HTTP proxy
agents from trying to parse the RTSP syntax that lies after the HTTP
header. Indeed HTTP-level firewalls are there to do exactly that:
check that TCP connections carry HTTP data and nothing else.
Unfortunately it seems that some deployed implementations will try
to check the correctness of the HTTP syntax and in doing so stumble
upon the RTSP syntax, causing the service to be denied. The solution
proposed in [3] is therefore to hide RTSP syntax by trans-coding it
in base64.
One obvious problem with this solution is that it may be seen as
some kind of a cheat. We pretend as discussed in section 2 that this
is not a true concern. Actually, as mentioned above, tunneling
streaming media in HTTP is already performed on very large scales by
a number of proprietary solutions and firewall administrators are
actually lacking standard-based solutions to recover control upon
such bandwidth-intensive traffic.
On the other hand, if a solution such as the one described in [3]
was to become an IETF standard, proxy agents could detect this
scenario by looking for an Accept or Content-Type header containing
"application/x-rtsp-tunnelled". Classical filtering techniques could
then be applied.
Alternatively other marking schemes could be designed to allow
detection of RTSP tunneling into HTTP.
6. Security Considerations
Tunneling RTSP in HTTP does not have different security
considerations than RTSP on TCP (covered by [1]) nor HTTP.
7. Acknowledgements
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Tunneling RTSP in HTTP July 2001
This work has been started after a discussion in the Internet
Streaming Media Alliance forum; authors wish to thank the people of
this forum for raising interesting points.
8. References
[1] Schulzrinne, Rao, Lanphier, RTSP: Real Time Streaming Protocol
RFC 2326, Internet Engineering Task Force, April 1998.
[2] Schulzrinne, Casner, Frederick, Jacobson RTP: A Transport
Protocol for Real Time Applications RFC 1889, Internet Engineering
Task Force, January 1996.
[3] http://index.apple.com/~singer/qt/rtspthroughhttp.html
9. Authors' Addresses
Philippe Gentric
Philips
51 rue Carnot
92156 Suresnes
France
e-mail: philippe.gentric@philips.com
anne jones
Apple
1 Infinite Loop
Cupertino, CA 95014
e-mail: astoria@apple.com
Philippe Gentric
Software architect
Philips Digital Networks - MP4Net
51 rue Carnot B.P. 301
92156 Suresnes FRANCE
tel: +33(0)147283740
fax: +33(0)147283725
philippe.gentric@philips.com
http://www.mpeg-4player.com