Re: [Ice] Re-nomination and candidate pair switching in RFC 5245bis

[Bernard Aboba <[e-mail]>]

Christer said:

"The whole discussion began when I was given a comment that the text above
should be modified, to clarify that the pair used for media can change
after a pair has been selected.

But, if the outcome is that the pair can NOT change, maybe we need to
clarify THAT instead :)"

[BA] Currently, use of the "ice2" ICE option forestalls use of aggressive
nomination (e.g. setting the nominated flag on more than one pair).  Since
only the selected pair can be used to send media, that would seem to rule
out changing the pair used for media after a pair has been selected:

   Once a candidate pair has been selected
   only that candidate pair (referred to as selected pair) is used for
   sending media.


What about changing the pair used for media prior to selection?  On this
point, the text seems less clear than it could be.

Prior to nomination, the specification allows the sending of media on a
successful pair:

   o  Once there is at least one nominated pair in the VALID LIST for
      every component of at least one media stream and the state of the
      CHECK LIST is Running:


...

      *  The agent MUST continue to respond to any checks it may still
         receive for that media stream, and MUST perform triggered
         checks if required by the processing of Section 6.3
<https://tools.ietf.org/html/draft-ietf-ice-rfc5245bis-10#section-6.3>.

      *  The agent MAY begin transmitting media for this media stream as

         described in Section 11.1
<https://tools.ietf.org/html/draft-ietf-ice-rfc5245bis-10#section-11.1>.

However, the specification is not clear enough about the receiving side;
while it recommends that implementations be prepared to receive prior to
nomination, it does not require this. From Section 11.2:

   ICE implementations SHOULD by default be
   prepared to receive media on any of the candidates provided in the
   most recent candidate exchange with the peer.


What happens if an implementation is NOT prepared to receive media?

In WebRTC, an implementation cannot send without consent, which

suggests that perhaps an unwilling receiver could use consent to

influence the potential sender.


However, the specification does not even reference RFC 7675,

so it is left unclear about how this is to be done.

For example, a receiver might not reply to a consent

request if the inability to receive is temporary ("I'm not ready yet"),

but that might cause consent to time out prior to nomination and

might even influence pair selection inappropriately.


Another choice might be to revoke consent (which would

invalidate the pair).  But that's pretty drastic unless the

pair is truly unacceptable.
















>[BA] RFC 5245bis Section 7.1.1 continues to imply a single selected pair:
>
>   Eventually, there will be only a single nominated pair in the VALID
>   LIST for each component.  Once the state of the CHECK LIST is set to
>   Completed, that exact pair is selected by ICE for sending and
>   receiving media for that component.
>
>Based on that text, an implementation might still release resources (e.g.
unused TURN candidates) post-nomination. Given this, the "ice2" ICE option
doesn't address >potential interoperability issues resulting from different
resource release behaviors (although it does clear indicate lack of support
for aggressive nomination):

The whole discussion began when I was given a comment that the text above
should be modified, to clarify that the pair used for media can change
after a pair has been selected.

But, if the outcome is that the pair can NOT change, maybe we need to
clarify THAT instead :)

>   NOTE: A controlling agent that does not support this specification
>   (i.e. it is implemented according to RFC 5245) might nominate more
>   than one candidate pair.  This was referred to as aggressive
>   nomination in RFC 5245.  The usage of the 'ice2' ice option by
>   endpoints supporting this specifcation should prevent such
>   controlling agents from using aggressive nomination.
>
>Christer also said:
>
>"Also, my understanding was that endpoints supporting RFC 7675 might
maintain consent on pairs currently not
>used for media, in order to be able to re-nominate in case consent for the
currently nominated pair expires. However,
>RFC 7675 does not explicitly say anything about that."
>
>[BA] RFC 7675 Section 5 says:
>
>   Initial consent to send traffic is obtained using ICE [RFC5245].  An
>   endpoint gains consent to send on a candidate pair when the pair
>   enters the Succeeded ICE state.
>
>Given this, an RFC 5245bis implementation might request consent to send to
>multiple remote peer candidates, so as to keep them alive. However,
>there is nothing in RFC 7675 that requires the responder to grant
>consent for that.  For example, based on the text in RFC 5245bis
>Section 7.1.1, a conforming implementation might well revoke
>consent on local candidates other than the local candidate in the
>selected pair.

Sure - the responder is not mandated to grant consent to multiple
candidates after nomination. But, the option to do seems to be there
(unless I've understood the RFC wrong), and the only reason to do so would
be possible re-nomination.

Anyway, I don't have any strong feelings which way we go, but we do need to
make it clear in the spec whether re-nomination is allowed or not.

Regards,

Christer

On Mon, Jul 24, 2017 at 4:33 PM, Christer Holmberg <
[e-mail]> wrote:

> Hi
> ,
> >[BA] RFC 5245bis Section 7.1.1 continues to imply a single selected pair:
> >
> >   Eventually, there will be only a single nominated pair in the VALID
> >   LIST for each component.  Once the state of the CHECK LIST is set to
> >   Completed, that exact pair is selected by ICE for sending and
> >   receiving media for that component.
> >
> >Based on that text, an implementation might still release resources (e.g.
> unused TURN candidates) post-nomination. Given this, the "ice2" ICE option
> doesn't address >potential interoperability issues resulting from different
> resource release behaviors (although it does clear indicate lack of support
> for aggressive nomination):
>
> The whole discussion began when I was given a comment that the text above
> should be modified, to clarify that the pair used for media can change
> after a pair has been selected.
>
> But, if the outcome is that the pair can NOT change, maybe we need to
> clarify THAT instead :)
>
> >   NOTE: A controlling agent that does not support this specification
> >   (i.e. it is implemented according to RFC 5245) might nominate more
> >   than one candidate pair.  This was referred to as aggressive
> >   nomination in RFC 5245.  The usage of the 'ice2' ice option by
> >   endpoints supporting this specifcation should prevent such
> >   controlling agents from using aggressive nomination.
> >
> >Christer also said:
> >
> >"Also, my understanding was that endpoints supporting RFC 7675 might
> maintain consent on pairs currently not
> >used for media, in order to be able to re-nominate in case consent for
> the currently nominated pair expires. However,
> >RFC 7675 does not explicitly say anything about that."
> >
> >[BA] RFC 7675 Section 5 says:
> >
> >   Initial consent to send traffic is obtained using ICE [RFC5245].  An
> >   endpoint gains consent to send on a candidate pair when the pair
> >   enters the Succeeded ICE state.
> >
> >Given this, an RFC 5245bis implementation might request consent to send to
> >multiple remote peer candidates, so as to keep them alive. However,
> >there is nothing in RFC 7675 that requires the responder to grant
> >consent for that.  For example, based on the text in RFC 5245bis
> >Section 7.1.1, a conforming implementation might well revoke
> >consent on local candidates other than the local candidate in the
> >selected pair.
>
> Sure - the responder is not mandated to grant consent to multiple
> candidates after nomination. But, the option to do seems to be there
> (unless I've understood the RFC wrong), and the only reason to do so would
> be possible re-nomination.
>
> Anyway, I don't have any strong feelings which way we go, but we do need
> to make it clear in the spec whether re-nomination is allowed or not.
>
> Regards,
>
> Christer
>
>
>
>
> On Fri, Jul 21, 2017 at 12:33 PM, Christer Holmberg <
> [e-mail]> wrote:
> Hi Bernard,
>
> Support of 5245bis is also negotiated, using the “ice2” ICE option.
>
> Also, my understanding was that endpoints supporting RFC 7675 might
> maintain consent on pairs currently not used for media, in order to be able
> to re-nominate in case consent for the currently nominated pair expires.
> However, RFC 7675 does not explicitly say anything about that.
>
> Regards,
>
> Christer
>
> From: Ice [mailto:[e-mail]] On Behalf Of Bernard Aboba
> Sent: 20 July 2017 14:22
> To: [e-mail]
> Subject: [Ice] Re-nomination and candidate pair switching in RFC 5245bis
>
> During the ICE WG meeting today, there was discussion of whether
> RFC5245bis should indicate that it is possible to re-nominate pairs
> (proposed by Peter), or whether it is possible to switch from one interface
> to another (Cullen).  While these capabilities are desirable, attempting to
> add them to RFC 5245bis without negotiation has the potential to break
> interoperability with existing RFC 5245 implementations.
>
> In my experience, this is an area where RFC 5245 implementations have very
> different interpretations. For example, some implementations (e.g. ones
> that did not support aggressive) discard non-selected candidate pairs after
> nomination. These implementations (e.g. particularly ones included in
> previous product releases) cannot be assumed to change their behavior after
> RFC 5245bis is published.  This raises the possibility that that
> interoperability could be impacted.
>
> Since in practice the desired candidate pair switching capabilities are
> most likely to be supported in WebRTC implementations supporting Trickle
> ICE, my recommendation is to think of candidate pair switching as a Trickle
> ICE capability.   Since Trickle-ICE support is negotiated, clarifications
> relating to candidate-pair switching can be linked to that negotiation.
>
> This provides a potential way forward that bypasses potential
> interoperability issues.  For example, if text on candidate-pair switching
> is to be added to (either to RFC 5245bis or Trickle-ICE) then the text
> could say that support for these behaviors can only be assumed if they are
> explicitly negotiated. The Trickle-ICE document could then create normative
> requirements for support of the new behaviors by stating that support for
> them is mandatory when supporting full-Trickle.
>
>
>
>