Subject: [rtcweb] RTP Two-Byte Header usage in WebRTC

From: Iñaki Baz Castillo <[email-masked]>
Date: Sun, 15 Jan 2017 16:12:58 +0100

Hi, is there any RTP header extension defined for WebRTC that requires
usage of the Two-Byte Header format defined in RFC 5285? AFAIS current
WebRTC implementations always use the One-Byte Header format.

Thanks a lot.

-- 
Iñaki Baz Castillo
<[email-masked]>

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From: Magnus Westerlund <[email-masked]>
Date: Mon, 16 Jan 2017 10:16:04 +0100

> WebRTC implementations always use the One-Byte Header format.

Hi,

You will have to support two-byte header extension formats, even if 
one-byte will be almost always used. The reason for this is that the 
SDES RTP header extensions can't guarantee that the value they carry is 
no more than 16 bytes. SDES items can be up to 255 bytes.

So the MID is recommended to be short (few bytes), but mandated to be 
support (WebRTC endpoints) in RTCWEB through the support of BUNDLE. But, 
so far no requirement to not use longer than 16 bytes

I would also expect CNAME RTP header extension to show up, especially in 
combination with the rapid synchronization RTP header extension. The 
current CNAME generation recommendation RTP Usage -> RFC7022 does not 
restrict the CNAME length to no longer than 16 bytes, in fact it only 
specifies a minimal length of 16 bytes (Due to 96 bits + base64 
encoding). Thus, also CNAMEs could result in two-byte headers.

I also don't see how we can mandate in RTCWeb context to not use fields 
that results in only one-byte header extensions.

Cheers

Magnus Westerlund

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---

From: Iñaki Baz Castillo <[email-masked]>
Date: Mon, 16 Jan 2017 11:46:08 +0100

2017-01-16 10:16 GMT+01:00 Magnus Westerlund <[email-masked]>:
> Hi,
>
> You will have to support two-byte header extension formats, even if one-byte
> will be almost always used. The reason for this is that the SDES RTP header
> extensions can't guarantee that the value they carry is no more than 16
> bytes. SDES items can be up to 255 bytes.
>
> support (WebRTC endpoints) in RTCWEB through the support of BUNDLE. But, so
> far no requirement to not use longer than 16 bytes
>
> combination with the rapid synchronization RTP header extension. The current
> CNAME generation recommendation RTP Usage -> RFC7022 does not restrict the
> CNAME length to no longer than 16 bytes, in fact it only specifies a minimal
> length of 16 bytes (Due to 96 bits + base64 encoding). Thus, also CNAMEs
> could result in two-byte headers.
>
> I also don't see how we can mandate in RTCWeb context to not use fields that
> results in only one-byte header extensions.

Clear, so now I should realize of whether Chrome and Firefox
understand Two-Bytes header extensions or not. AFAIS (by taking
traces) they just generate One-Byte extensions.

Thanks a lot.

-- 
Iñaki Baz Castillo
<[email-masked]>