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Can't see RTCP REMB packets

asked 2022-01-10 18:54:36 +0000

Beans gravatar image

updated 2022-01-10 20:39:35 +0000

Hello, I'm trying to view max bitrate fields (BR Exp and BR Mantissa) in the RTCP REMB packets which are filtered as packet type value 206 (payload-specific feedback) and feedback message type value 15 on Wireshark, but I can't see any information past media source SSRC. I captured these packets during a Google Meet call using Firefox.

Here is the packet capture (meet-firefox-Jan10.pcap) and screenshot I attached in the same folder: https://drive.google.com/drive/folder...

I expected to see the RTCP packet in the message format:

 0                   1                   2                   3               
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT=15  |   PT=206      |             length            |                               
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  SSRC of packet sender                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  SSRC of media source                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Unique identifier 'R' 'E' 'M' 'B'                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Num SSRC     | BR Exp    |  BR Mantissa                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   SSRC feedback                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  ...                                                          |

Note: I get an Expert Info (Warning/Protocol): Unknown Application Layer Feedback Type

Thank you so much for any help

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Comments

Can you share a capture, or at least display the RTCP packet?

grahamb gravatar imagegrahamb ( 2022-01-10 19:41:23 +0000 )edit

I tried attaching an image and attachment but ask.wireshark gives me an error: >60 points required to upload files..

Beans gravatar imageBeans ( 2022-01-10 19:52:51 +0000 )edit

Putting one on a publicly accessible file sharing site and posting the link to it here is the way to go.

Jaap gravatar imageJaap ( 2022-01-10 20:24:08 +0000 )edit

Ok I see, I attached a link in the post and here it is also: https://drive.google.com/drive/folder...

Beans gravatar imageBeans ( 2022-01-10 20:37:19 +0000 )edit

Hi, No one has written code to dissect https://datatracker.ietf.org/doc/html... so anything after the Aplication identifyer is unknown to Wireshark. You could write a bug request asking to have it implemented and see if someone picks it up.

Anders gravatar imageAnders ( 2022-01-11 07:14:22 +0000 )edit

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answered 2022-01-11 05:43:42 +0000

Chuckc gravatar image

Example pcap attached to 9757 - Add RTCP dissector for goog-remb (Receiver Estimated Maximum Bitrate)

Analyze -> Decode As... and set Current to RTCP.

(Wireshark User Guide example of Decode As...)

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Thanks for the reply, but UDP packets are already being decoded as RTP and RTCP.

Beans gravatar imageBeans ( 2022-01-11 05:53:09 +0000 )edit

Is the question how to capture REMB or how to decode and view when they are captured?

Chuckc gravatar imageChuckc ( 2022-01-11 06:13:32 +0000 )edit

I guess it's the prior.. My understanding is bandwidth estimation (specifically REMB technique) should be sent by default along with other RTP/RTCP packets when video calling over Firefox, but I don't see the REMB packets given that the SSRC of media source must be 0 (along with packet type=206 and fmt=15). Also, I attached a link with the mentioned packet capture along with a screenshot.

Beans gravatar imageBeans ( 2022-01-11 06:24:53 +0000 )edit
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answered 2022-01-11 04:54:55 +0000

Beans gravatar image

I had overlooked to filter SSRC of media source to be 0. It seems that I did not capture any REMB packets in the captured packets.

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Asked: 2022-01-10 18:53:56 +0000

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Last updated: Jan 11