(Webrtc) VP8 RTX padding packets have wrong padding length

Site Outreach Issue #18992016

Details

Author
Francesco D.
Created
Sep 14, 2018
Privacy
This issue is public.
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Reported by 1 person

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Steps to reproduce

We noticed in these days, that EDGE send padding (I think for probing) in the RTX channel.
The problem is that the last byte of the padding contains a wrong info about the length of the padding.
As per RFC, an RTX packet must contain the OSN field, which is 2 bytes.
https://tools.ietf.org/html/rfc4588#section-4

Inspecting the incoming packets, for example this one:

LENGHT: 272
***************
RTP PACKET
Header: 
    version: 2
    has padding: true
    has extention: true
    cc: 0
    marker: true
    type: 96
    sequence number: 29031
    timestamp: 201402033
    synchronization source: 3004
Extention:
    code: 0XBEDE
    length: 1
Extentions:
    id: 1    data: 00101100 11001010 00110110 
    RTX_OSN: 43845
Padding:
   bytes: 252

The packet is long 272 bytes, the last byte of the padding indicates that the padding is long 252.
This means we have 20 bytes left >12 for the header, 8 (4 extension header + 4 of extension (abs-send-time in this case)).
But we also have OSN that is other 2 bytes.
The padding length in the last byte should be 250.

In fact, if for whatever reason this packet contained an RTX with a vp8 fragment and padding, when parsing the frame would be corrupted since stripping the padding we would have deleted the last 2 bytes of the frame.

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    Comments and activity

    • Microsoft Edge Team

      Changed Assigned To to “Gurpreet V.”

      Changed Status to “Site Outreach”

    • Seems that other browsers are behaving the same.
      So it might be my misinterpretation of the RFC or the RFC not be so clear.
      Can close

    • Hi Francesco!

      Thank you for contacting us about this issue! We have resolved this issue as “External” which means the problem is not due to an issue with Microsoft Edge. If you are still experiencing this problem then please feel free to reopen this issue and I will resume the investigation.

      Thanks,
      Zach

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