[AVT] Technical comments on draft-ietf-avt-rtp-hdrext-12

[Magnus Westerlund <magnus.westerlund at ericsson.com>]

Hi,

Sorry for not having read this in WGLC. However I think there is a 
serious error in the spec that needs to be fixed before this going further.

7.  BNF Syntax

   The syntax element 'URI-reference' is as defined in [RFC3986], except
   that only absolute URIs are permitted here.  The syntax element
   'extmap' is an attribute as defined in [RFC4566].
   Extensionattributes are not defined here, but by the specification
   that defines a specific extension name; there may be several.

     digit = "0"/"1"/"2"/"3"/"4"/"5"/"6"/"7"/"8"/"9"

     integer = 1*digit

     space = " "

     extensionname = URI-reference

     direction = "sendonly" / "recvonly" / "sendrecv" / "inactive"

     mapentry = "extmap:" integer ["/" direction]

     mapattrs = [space extensionattributes]

     extmap = mapentry space extensionname mapattrs


There are two serious issues with this. First RFC 4234 is not referenced 
to define that your using that type of BNF. Secondly and really 
seriously, the "extensionattributes" syntax is not defined. Unless one 
defines the form of it this exercise is useless as no parsers pre 
extension can handle and ignore the extension anyway.

I would propose to rewrite this section to the following:

7.  ABNF Syntax

The below syntax definition uses ABNF according to [RFC4234]. The syntax 
element 'URI' is as defined in [RFC3986]. The syntax element 'extmap' is 
an SDP attribute as defined in [RFC4566], i.e "a=" precede the extmap 
definition.

     extmap = mapentry SP extensionname mapattrs

     extensionname = URI

     URI = <Defined in RFC 3986>

     direction = "sendonly" / "recvonly" / "sendrecv" / "inactive"

     mapentry = "extmap:" 1*5DIGIT ["/" direction]

     mapattrs = *(SP extensionattribute)

     extensionattribute = non-ws-string

     non-ws-string = <Defined in RFC 4566>

     SP = <Defined in RFC 4234>

     DIGIT = <Defined in RFC 4234>

Some comments on this ABNF compared to the original one.
1. It limits the number of digits for the temporary identifier to 5, 
i.e. up to 16 bits supported if this ever is extended.

2. The extension can now be basically any string as long as it doesn't 
contain spaces. In addition there might be multiple additions with 
spaces in between them. The alternative would be to demand a space and 
then use byte-string (from rfc4566) to allow for any separators.

3. Uses RFC 4234 base syntax.

4. Uses URI rule instead of URI-Reference so that relative ones are not 
possible.


Cheers

Magnus Westerlund


IETF Transport Area Director & TSVWG Chair
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Multimedia Technologies, Ericsson Research EAB/TVM/M
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