Is the turn server relaying DSCP?

[Yu Bao]

Hi,

I'm wondering what is the behavior of the turn server when it comes to relay the DSCP field.

RFC5766 states:

Differentiated Services Code Point (DSCP) field [RFC2474]

      Preferred Behavior: Set the outgoing value to the incoming value,
      unless the server includes a differentiated services classifier
      and marker [RFC2474].

      Alternate Behavior: Set the outgoing value to a fixed value, which
      by default is Best Effort unless configured otherwise.

      In both cases, if the server is immediately adjacent to a
      differentiated services classifier and marker, then DSCP MAY be
      set to any arbitrary value in the direction towards the
      classifier.

And I do see this in the STATUS file:

9) TOS (DiffServ and ECN) field handling (preferred behavior of RFC 5766) implemented,
for Linux. Other platforms support the alternative behavior of RFC 5766.

I'm only concerned about Linux - does the turn server only implements the preferred behavior when the TURN client is connected via TURN/UDP AND the relaying protocol is also UDP? 
What if one leg uses UDP and the other leg is on TCP? 

Also I don't find any configuration option to tune this, is the behavior hardcoded? Could the user set an arbitrary value for it?

Thanks!
Yu Bao

[Oleg Moskalenko]

On Linux, it uses the preferred behavior, always, unconditionally.

The "other leg" is always UDP in RFC 5766 - it uses only UDP relay. Even when client-to-server connection is TCP, the relay is always UDP. 

On other platforms (like FreeBSD), the TURN Server uses the alternative behavior, the field value is whatever the OS default value for that field is. You set that value in your OS settings (the network stack settings), not in the TURN Server.

[Yu Bao]

Thanks for the clarification Oleg.

[Eva]

Hello,

You answered before that on linux preferred behaviour for DSCP is always used.

However when I set a DSCP tag (by using googDscp constraint : true when setting up a peer connection), packets from client to TURN server are marked (AF) but

packets after going through the TURN server have their DSCP set back to zero (default).

Is there any additional configuration to implement so the DSCP would not be changed?

Thank you in advance,

Eva

[Oleg Moskalenko]

DSCP and AF are two different fields. Which one are you talking about ?

Oleg

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[Ewa Janczukowicz]

Hello,

Sorry for the confusion. I am talking about DSCP field. 

When I set DSCP, the packets from client to server are (from Wireshark):

Differentiated Services Field: 0x88 (DSCP 0x22: Assured Forwarding 41; (...)

1000 10 .. = Differentiated Services Codepoint: Assured Forwarding 41 (0x22)

And packets after going through the server are:

Differentiated Services Field: 0x00 (DSCP 0x00: Default; (...)

0000 00 .. = Differentiated Services Codepoint: Default (0x00)

Thank you in advance,

Ewa

[Oleg Moskalenko]

If you are sure that this is not working properly, then file a bug in the project bugs list, we'll fix it.

Oleg

[Oleg Moskalenko]

By the way, you are talking about UDP-to-UDP forwarding, right ? We do not support IP headers forwarding for TCP traffic.

Oleg

[Ewa Janczukowicz]

Yes, udp-to-udp.

[Oleg Moskalenko]

give me some time for investigation.

[Ewa Janczukowicz]

Ok, no problem.

Thank you so much.

[Oleg Moskalenko]

This functionality is supposed to work in Linux and in FreeBSD. I tested the latest versions, and I found that both TOS and TTL handling is broken. I am working on fixes. The fix will be available in the next builds of coturn and rfc5766-turn-server.

Oleg

Yes, udp-to-udp.

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[Oleg Moskalenko]

you can check version 4.4.2.1, it has that bug fixed.

[Ewa Janczukowicz]

Thank you. I will look into that.

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