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Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

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Mauro Brenna

Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

Hello,

I am implementing a server which streams video using RTP. To read the
stream from a client I use gstreamer.
For the session protocol I would like to use RTSP and I am reading the RFC 2326.

I read in a passage that RTSP should be sent using TCP while you
should call RTSPU for UDP.
I'll cite from the RFC:

" The scheme rtsp requires that commands are issued via a reliable
protocol (within the Internet, TCP), while the scheme rtspu identifies
an unreliable protocol (within the Internet, UDP)."

I would like to know if the implementation of the RTSP provided by
gstreamer uses TCP or UDP. Is it possible to use RTSPU with
gstreamer? Anybody knows if RTPS over UDP or RTPSU is supported in
other media player like vlc?

Thanks,

Mauro

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Wim Taymans

Re: Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

On Thu, 2011-01-13 at 16:49 +0100, Mauro Brenna wrote:

> Hello,
>
> I am implementing a server which streams video using RTP. To read the
> stream from a client I use gstreamer.
> For the session protocol I would like to use RTSP and I am reading the RFC 2326.
>
> I read in a passage that RTSP should be sent using TCP while you
> should call RTSPU for UDP.
> I'll cite from the RFC:
>
> " The scheme rtsp requires that commands are issued via a reliable
> protocol (within the Internet, TCP), while the scheme rtspu identifies
> an unreliable protocol (within the Internet, UDP)."
>
> I would like to know if the implementation of the RTSP provided by
> gstreamer uses TCP or UDP. Is it possible to use RTSPU with
> gstreamer? Anybody knows if RTPS over UDP or RTPSU is supported in
> other media player like vlc?

rtspsrc supports data transport over TCP and UDP and RTSP over tcp. I am
not aware of any player doing the RTSP requests over UDP nor is it clear
from the spec how this is supposed to work, or how a server should
support it. I don't know why that part is in the spec or what it means.
rtspsrc supportes rtspu:// protocols but interpretes it that the RTP
transport is only done over UDP unicast or multicast. VLC doesn't seem
to understand the rtspu protocol.

Wim

>
> Thanks,
>
> Mauro
>
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Mauro Brenna

Re: Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

Thanks for the reply.
I think about your reply and I will implement a little rtspu (over
UDP) client for my application since the server does not implement the
TCP transport protocol.

Reading the RFC, I believe that rtsp means RTSP commands are sent over
TCP, RTSPU over UDP.
The way to specify if the RTP data packets would be sent (TCP/UDP) is
in the Transport Header as lower-transport. See RFC 2326, in section
"12.39 Transport".

I think the two things are independently so you could have:

RTSP (over TCP) + RTP data over TCP or UDP
RTSPu (over UDP) + RTP data over TCP or UDP.

Do you think I misunderstood something?

Bye,

Mauro

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Mauro Brenna

Re: Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

In reply to this post by Wim Taymans

Wim Taymans

Re: Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

On Tue, Jan 18, 2011 at 9:47 AM, Mauro Brenna <[hidden email]> wrote:

> Thanks for the reply.
> I think about your reply and I will implement a little rtspu (over
> UDP) client for my application since the server does not implement the
> TCP transport protocol.
>
> Reading the RFC, I believe that rtsp means RTSP commands are sent over
> TCP, RTSPU over UDP.
> The way to specify if the RTP data packets would be sent (TCP/UDP) is
> in the Transport Header as lower-transport. See RFC 2326, in section
> "12.39 Transport".

I think you misunderstood that part of the RFC, it talks about the
transport of RTP packets,
not RTSP.

>
> I think the two things are independently so you could have:
>
> RTSP (over TCP) + RTP data over TCP or UDP
> RTSPu (over UDP) + RTP data over TCP or UDP.

So how do you think it would work? you send the RTSP request over UDP?
How would you get the reply?
How does the server know where to send the reply to? How is this going
to work over NAT? What will happen
when the UDP packets get lost, duplicated or reordered? What would you
do if the RTSP request is too big to fit in
the MTU? What would you do if the reply is too big for the MTU? If you
manage to get RTSP over UDP
working, how and when are you going to open the TCP connection when
you decide to do RTP over TCP?

You need to answer those questions first before any of what you talk
about makes sense.

As I said: "I am not aware of any player doing the RTSP requests over
UDP nor is it clear
from the spec how this is supposed to work, or how a server should
support it. I don't know
why that part is in the spec or what it means."

Wim

>
> Do you think I misunderstood something?
>

> Bye,
>
> Mauro
>
> ------------------------------------------------------------------------------
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> malware threats, the impact they can have on your business, and how you
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>

Mauro Brenna

Re: Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

Hi,

As regards the possibility to transmit RTSP packets over UDP you can
also read in the RFC (2326):

" There is no notion of an RTSP connection; instead, a server maintains
a session labeled by an identifier. An RTSP session is in no way tied
to a transport-level connection such as a TCP connection. During an
RTSP session, an RTSP client may open and close many reliable
transport connections to the server to issue RTSP requests.
Alternatively, it may use a connectionless transport protocol such as
UDP."

So, I am practically convinced you could send also the RTSP commands,
not only the RTP data, via UDP.
Anywat, I don't want to start a post-reply war. I would like only to
understand what it is correct and what it is not. My application is
very specific and I will implement a very very simple RTSP over UDP
protocol.

Now, to answer your question:

- So how do you think it would work? you send the RTSP request over UDP?
Yes, I would do like that. Note the difference between sessionless
commands and commands that refers to a session. You need to mantain a
state for each session, if you would like to stream in unicast to more
than one client, otherwise you can use a single 'instance' of RTSP
server state and refuse to SETUP if a second client asks you so.

- How would you get the reply?
Via UDP, you send the SESSION id in the reply.

- How does the server know where to send the reply to?
I do not understand the question. The server should know specific
parameters via SETUP method. Or simply replies to the IP of the
sender. Which is the difference between UDP and TCP in this case in
your opinion?

- How is this going to work over NAT?
I don't use NAT. In any case when you use SETUP method you explicity
declare the client and server port to enable the firewall to learn
about them. That is what I understood by the RFC:

"A firewall may need to understand the SETUP method to open a "hole"
for the UDP media stream."

"For the benefit of any
intervening firewalls, a client must indicate the transport
parameters even if it has no influence over these parameters, for
example, where the server advertises a fixed multicast address.

Since SETUP includes all transport initialization information,
firewalls and other intermediate network devices (which need this
information) are spared the more arduous task of parsing the
DESCRIBE response, which has been reserved for media
initialization."

This could be a point in favour of TCP:
" Unlike HTTP, RTSP allows the media server to send requests to the
media client. However, this is only supported for persistent
connections, as the media server otherwise has no reliable way of
reaching the client. Also, this is the only way that requests from
media server to client are likely to traverse firewalls."

- What will happen when the UDP packets get lost, duplicated or reordered?
It is not clearly explained. In any case there is a section dedicated
to reliability:

"9.2 Reliability and Acknowledgements

Requests are acknowledged by the receiver unless they are sent to a
multicast group. If there is no acknowledgement, the sender may
resend the same message after a timeout of one round-trip time (RTT).
The round-trip time is estimated as in TCP (RFC 1123) [18], with an
initial round-trip value of 500 ms. An implementation MAY cache the
last RTT measurement as the initial value for future connections.

If a reliable transport protocol is used to carry RTSP, requests MUST
NOT be retransmitted; the RTSP application MUST instead rely on the
underlying transport to provide reliability."

- What would you do if the RTSP request is too big to fit in the MTU?
It does not happen in my case. If possible, you simply constraint the
client/server to use at maximum an MTU and send more than one request.
How is it implemented in TCP? Since the ordering is correct, you
simply send the RTSP packet fragmented and the server/client would
recompose it? How can the client/server understand the end of the RTSP
packet?

- What would you do if the reply is too big for the MTU?
Same as above.

- If you manage to get RTSP over UDP working, how and when are you
going to open the TCP connection when
you decide to do RTP over TCP?

I would open a TCP socket for RTP when the client issues a SETUP,
before sending the OK reply.
I would close the TCP connection associated to the UDP RTSP session
number X and ip Y, then the client with session X and ip Y issues the
command TEARDOWN.

I think I had formulated satisfactory answers but the one regarding
the fragmentation.

I'm looking forward to hearing from you your comments,

Mauro

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Wim Taymans

Re: Is RTSPU implemented in gstreamer? Can I use RTSP over UDP?

It sounds like you have it plan that could work.

Wim

On Wed, Jan 19, 2011 at 9:59 AM, Mauro Brenna <[hidden email]> wrote:

> Hi,
>
> As regards the possibility to transmit RTSP packets over UDP you can
> also read in the RFC (2326):
>
> " There is no notion of an RTSP connection; instead, a server maintains
> a session labeled by an identifier. An RTSP session is in no way tied
> to a transport-level connection such as a TCP connection. During an
> RTSP session, an RTSP client may open and close many reliable
> transport connections to the server to issue RTSP requests.
> Alternatively, it may use a connectionless transport protocol such as
> UDP."
>
> So, I am practically convinced you could send also the RTSP commands,
> not only the RTP data, via UDP.
> Anywat, I don't want to start a post-reply war. I would like only to
> understand what it is correct and what it is not. My application is
> very specific and I will implement a very very simple RTSP over UDP
> protocol.
>
> Now, to answer your question:
>
> - So how do you think it would work? you send the RTSP request over UDP?
> Yes, I would do like that. Note the difference between sessionless
> commands and commands that refers to a session. You need to mantain a
> state for each session, if you would like to stream in unicast to more
> than one client, otherwise you can use a single 'instance' of RTSP
> server state and refuse to SETUP if a second client asks you so.
>
> - How would you get the reply?
> Via UDP, you send the SESSION id in the reply.
>
> - How does the server know where to send the reply to?
> I do not understand the question. The server should know specific
> parameters via SETUP method. Or simply replies to the IP of the
> sender. Which is the difference between UDP and TCP in this case in
> your opinion?
>
> - How is this going to work over NAT?
> I don't use NAT. In any case when you use SETUP method you explicity
> declare the client and server port to enable the firewall to learn
> about them. That is what I understood by the RFC:
>
> "A firewall may need to understand the SETUP method to open a "hole"
> for the UDP media stream."
>
> "For the benefit of any
> intervening firewalls, a client must indicate the transport
> parameters even if it has no influence over these parameters, for
> example, where the server advertises a fixed multicast address.
>
> Since SETUP includes all transport initialization information,
> firewalls and other intermediate network devices (which need this
> information) are spared the more arduous task of parsing the
> DESCRIBE response, which has been reserved for media
> initialization."
>
> This could be a point in favour of TCP:
> " Unlike HTTP, RTSP allows the media server to send requests to the
> media client. However, this is only supported for persistent
> connections, as the media server otherwise has no reliable way of
> reaching the client. Also, this is the only way that requests from
> media server to client are likely to traverse firewalls."
>
>
> - What will happen when the UDP packets get lost, duplicated or reordered?
> It is not clearly explained. In any case there is a section dedicated
> to reliability:
>
> "9.2 Reliability and Acknowledgements
>
> Requests are acknowledged by the receiver unless they are sent to a
> multicast group. If there is no acknowledgement, the sender may
> resend the same message after a timeout of one round-trip time (RTT).
> The round-trip time is estimated as in TCP (RFC 1123) [18], with an
> initial round-trip value of 500 ms. An implementation MAY cache the
> last RTT measurement as the initial value for future connections.
>
> If a reliable transport protocol is used to carry RTSP, requests MUST
> NOT be retransmitted; the RTSP application MUST instead rely on the
> underlying transport to provide reliability."
>
>
> - What would you do if the RTSP request is too big to fit in the MTU?
> It does not happen in my case. If possible, you simply constraint the
> client/server to use at maximum an MTU and send more than one request.
> How is it implemented in TCP? Since the ordering is correct, you
> simply send the RTSP packet fragmented and the server/client would
> recompose it? How can the client/server understand the end of the RTSP
> packet?
>
>
> - What would you do if the reply is too big for the MTU?
> Same as above.
>
> - If you manage to get RTSP over UDP working, how and when are you
> going to open the TCP connection when
> you decide to do RTP over TCP?
>
> I would open a TCP socket for RTP when the client issues a SETUP,
> before sending the OK reply.
> I would close the TCP connection associated to the UDP RTSP session
> number X and ip Y, then the client with session X and ip Y issues the
> command TEARDOWN.
>
>
>
> I think I had formulated satisfactory answers but the one regarding
> the fragmentation.
>
>
> I'm looking forward to hearing from you your comments,
>
> Mauro
>
> ------------------------------------------------------------------------------
> Protect Your Site and Customers from Malware Attacks
> Learn about various malware tactics and how to avoid them. Understand
> malware threats, the impact they can have on your business, and how you
> can protect your company and customers by using code signing.
> http://p.sf.net/sfu/oracle-sfdevnl
> _______________________________________________
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> [hidden email]
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>