H.323 Fast Connect and Versioning

paulej at PACKETIZER.COM paulej at PACKETIZER.COM
Fri Sep 12 16:56:42 EDT 2003


Peter,

The danger with adding the codepoint for "syntax2002" is that it does not necessarily encompass all of the rules for version 2, 3, etc.  Since those future versions do not exist, it presents us with certain problems.

Perhaps the right solution is two-fold:
  1.. Add a new "syntax2002" field
  2.. Allow the called endpoint to modify the version number field in the Fast Connect proposal.  It could *not* change it if the calling device is version 0 or not using the new "syntax2002" field, but we could add a rule that says that if the calling device included "syntax2002", it also means that the called device may change the version number in the reply to indicate the actual supported version.
If we do (2), then we need to change the language in H.323 to say that parameters shall not be changed, unless explicitly allowed by the particular "controlling media profile document".  For T.38, that would be Annex D/H.323.  For V.150.1, that would be Annex P/H.323.

The video codec issue is an interesting one... several options can be proposed with various capabilities, but they can't be changed.

There is an implementation approach that could be used to solve these kinds of issues, but some folks don't like it.  That is: don't use Fast Connect at all-- just do termcap exchange and only open media channels and ring the remote phone once caps are exchanged and media is opened.  Regardless of whether H.245 is tunneled or on a separate connection, the exchange of all required messages can be done in about 3 TCP packets per side.

As for the requirement that H.245 tunneling be used with Fast Connect--- yes, the requirement is there, but folks ignore that like they do other requirements in the standard ;-)  The wording might be "must support tunneling", which does not mean it has to be used.

Fast Connect certainly has certain advantages over H.245, but if we had never introduced Fast Connect in the first place, I suspect nobody would think something is missing.  Most likely, folks would have engineered their products to send TCS right away, would not have alerted the user until media was established, etc.  They would have optimized their code to send TCS,MSDet,OLC in the first outgoing message, replied with TCS,TCSAck,MSDet,OLCAck,OLC in the reply TCP packet, and then TCSAck,OLCAck in the sender's second TCP packet.  In the rare case where the proposed OLC is not acceptable, it would require an extra exchange of messages, but certainly no worse than Fast Connect today.

I'm actually working on a new extension to H.323 to allowing the calling endpoint to explicitly request that the call establishment be delayed until a certain point (e.g., bi-directional media channels are opened).  The calling side can control when it lets the call proceed.  Likewise, the called side can control it by not acknowledging that the requested "delay point" has been reached.  This might be the better way to handle T.38.... except that, as you point out, there are Fast Connect-only T.38 devices.

My head hurts...

I really hate to break the rules about changing the attributes of a Fast Connect proposal.

Here's another thought:  What if we add text to Annex D/H.323 that says that if the proposed version is not supported, then it shall not accept the proposal.  If it wants to offer a "counter proposal", it has two means: H.245 signaling or H.460.6 (Extended Fast Connect).

Paul

  ----- Original Message ----- 
  From: Peter Price 
  To: 'Paul E. Jones' ; itu-sg16 at external.cisco.com ; tsg16q14 at itu.int 
  Sent: Thursday, September 11, 2003 3:53 AM
  Subject: RE: H.323 Fast Connect and Versioning


  Paul,

  as I said before any solution is likely to be a bit dirty because of the nature of the problem.  I think your "syntax2002" suggestion is perfectly valid - it is a smaller more localised change and, especially given the reluctance to add new code points, is probably a better alternative than t39faxV2.  It only requires a single change to T38FaxProfile rather than changes to both DataApplicationCapability and DataApplicationMode.  There is no danger that the change could affect anything other than T.38 aware endpoints.

  On the subject of H245 tunneling, the problem scenario we're discussing is Fast Connect and I thought that H.323 V4 says that endpoints using Fast Connect shall use H.245 tunneling!  It doesn't do much for pure T.38 endpoints which don't do any H.245 though.

  The thread has suggested two approaches 
  1. resolve the issue  within the Fast Connect proposal (or any subsequent requestMode/OLC etc)
  2. resolve the issue by modifying some other part of the standard by introducing "special cases"

  As an implementor, I would prefer to see a solution within the Fast Connect proposal rather than force other changes in the standard - the danger of going that route is you don't know what the downstream consequences are going to be.  Containing the solution in T38FaxProfile keeps implementation simpler - you receive a message and you know exactly what you are doing without having to go looking for other information.  Logically, the tunneled H245 messages arrive after the Setup message and its easier to process the Setup completely before starting to look at the H.245.

  Furthermore, if H.323 endpoints are to remain interoperable with pure T.38 endpoints (are there any?) then the solution *must* be contained within the Fast Connect proposal.

  You suggest that "syntax002" is a bit of a kludge.  It probably is but it does have the advantage of being isolated.  I think that "special cases" in the standard that may have unforeseen consequences for endpoints that are not interested in T.38 are very much worse.

  ------------

  A slight aside here (but related).

  Your remark about the way that all endpoints appear to decode and re-encode the Fast Connect proposals implies that the rule for not changing the proposals is effectively impossible to maintain.

  I only work with audio endpoints that use the basic audio codecs and T.38 so until this discussion started hadn't really thought about this issue.  It's easy to say you musn't change, say, the frame count of G729 but for codecs that are defined as extensible like T.38 (and all the video codecs) there will always be a problem when new endpoints offer new features to old endpoints.

  Perhaps the Fast Connect rule needs some review to address the specific issue of extensible capabilities.
  Have video endpoints already encountered this problem? 
  Do any video endpoint implementors have any relevent comments here?

  Peter
    -----Original Message-----
    From: Paul E. Jones [mailto:paulej at packetizer.com]
    Sent: 10 September 2003 23:35
    To: Peter Price; itu-sg16 at external.cisco.com; tsg16q14 at itu.int
    Subject: Re: H.323 Fast Connect and Versioning


    Peter,

    I've only seen this problem with fax.  To answer your last question, I've only seen T.38 use this kind of version tag.  However, V.150.1 also has versioning information as part of the object identifier that identifies the capability.  This will be interesting to see if we introduce the same kind of problem there.  In general, it's just not good to advertise the version through an OLC... it's better to perform a full caps exchange.  The trouble is that modem and (to some extent) fax timings are such that we must open channels ASAP... before a caps exchange.  (Actually, we could transmit the termcap set in the Setup message, but few devices support that.)

    We have had non-compatible payload specifications before and we resolved that by adding new code points.  However, we've been trying to avoid that.  Even so, we could do it again... it's just less desirable.

    I had another idea.  What we could do is, within the t38faxProtocol SEQUENCE, we could indicate which syntax is to be used.  Older devices would not see this field and would not decode it.  So, when the reply is re-encoded, it would not be present.  So, even if the version was set to "2", the "Syntax2002" field, say, would not be present.  This would mean that the 1998 syntax has to be used.  A newer endpoint would see the field and would properly re-encode it in the reply.  This is a bit of a kludge and works only because of the way the ASN.1 encoding/decoding works with every device I've seen.

    Another solution to the problem might be to require that endpoint use H.245 tunneling and to advertise their capabilities in the Setup message.  That could allow us to avoid this problem entirely.  I'm just not sure how excited people would be to be forced to use H.245 tunneling every time they use fax, modem, or text relay.

    Paul

      ----- Original Message ----- 
      From: Peter Price 
      To: 'Paul E. Jones' ; itu-sg16 at external.cisco.com ; tsg16q14 at itu.int 
      Sent: Wednesday, September 10, 2003 12:34 PM
      Subject: RE: H.323 Fast Connect and Versioning


      Paul,

      I wrote the first part of this email and then reread yours - I was bogged down in the fax issue but I think you are actually raising a wider issue aren't you?  See part 2 below.

      -------------------------- Part 1 
      I don't believe that 5 versions of T38 would result in 5 offered channels.

      The need for the different capability is due to the fact that what you are offering is a payload that is encoded in a different and incompatible way.  ie its a bit like offering G.729 and the sending packets encoded according to G.723.1,  they both represent speech but they are not going to be played out properly.  The single extra bit introduced into the T.38 payload packet by the 2002 ASN.1 is backwards incompatible.

      The problem only exists for endpoints that only know about the 1998 ASN.1 and are unaware of the incompatibility - it is important that they do not think they can accept the offered channel.

      Once an endpoint is aware of the problem (ie it knows about the 2002 ASN.1) then it can handle versions >= V2 (as well as V0 and V1).  Of course, this does assume that a similar incompatibilty does not creep into the payload ASN.1 in future versions - but that's down to careful work in the standard development and editing stage.

      I still think adding t38faxV2 (say) to DataApplicationCapability and DataApplicationMode is the simplest solution 
      [ t38faxV2 would use the same definitions for t38FaxProtocol and t38FaxProfile - its only the payload that has changed ].  This protects the existing T.38 implementations and avoids the need to break the rule about modifying Fast Connect proposals.

      The change in the T.38 payload ASN.1 breaks the fundamental backwards compatibility that ASN.1 is supposed to guarantee and thus whatever the final solution there has to be an element of a hack involved - I think that this change would isolate the change and protect the rest of the standard.

      -------------------------- Part 2

      The versioning issue applies to any form of payload,  voice/video/fax/whatever.

      The problem is still going to exist in early versions of endpoints that don't understand the consequence of accepting versions that they do not understand fully.  If a new version of a codec's payload is not backwards compatible then I would assert that it is a new codec and must be signalled as a different capability.

      The issue of multiple variations already exists anyway although not (to my knowledge) with version numbers.
      Endpoints already offer multiple packet sizes for exactly the reason that you are not supposed to alter the Fast Connect proposal.  What happens when somebody starts to offer g729Extensions and has to offer all the combinations of Annexes because they don't know what the other end can use ( I make that 64 proposals in each direction without adding further annexes! )?

      I don't see that relaxing the rule about modifying the version in a Fast Connect channel will help resolve the problem of having to offer multiple proposals.  You either have to allow *anything* to be modified or stick to the current rule. Exceptions allowing certain fields to be modified just makes life much more difficult and confusing.

      T.38 is the only codec I am aware of that actually uses a version number in this manner.  Are there any others? Why was it introduced in T.38?  Perhaps this is a lesson for the future about the value of introducing of such a field in other codecs.

      Peter
        -----Original Message-----
        From: Paul E. Jones [mailto:paulej at packetizer.com]
        Sent: 10 September 2003 16:01
        To: Peter Price; itu-sg16 at external.cisco.com; tsg16q14 at itu.int
        Subject: Re: H.323 Fast Connect and Versioning


        Peter,

        I think you're on the right track.  We could avoid ASN.1 changes by introducing the new capability as a generic data capability, but a new capability is required here, I think.

        The problem, as I see it, is that when we use Fast Connect, we can't alert the calling side as to what version the called side actually supports.  This suggests that if we have 5 versions of T.38, the calling side would have to propose a channel for each version independently.  That's horrible.  It's only complicated further by the fact that T.38 may not be signaled by itself-- it might be part of audio proposals that also include modem, text over IP, VBD, or other media.  It might even be that there are several versions of the modem (V.150.1) protocol advertised.

        I think the only real solution to this problem is to allow the Fast Connect proposals to be altered by the called endpoint such that they can change the version number.. and nothing else.  H.323 has an explicit statement that says that the proposals can't be modified before returning them, but perhaps this simple exception might resolve these issues.  I think without such, it's going to be terrible complicated.

        Paul

          ----- Original Message ----- 
          From: Peter Price 
          To: 'paulej at PACKETIZER.COM' ; itu-sg16 at external.cisco.com 
          Sent: Wednesday, September 10, 2003 3:23 AM
          Subject: RE: H.323 Fast Connect and Versioning


          Paul wrote

          "Perhaps we can require the calling device to not transmit any data until it receives at least one IFP packet from the called side and determines the ASN.1 version used to encode the message."

          Unfortunately this won't work - although typically the called endpoint will provide the first IFP (Probably a CED) this doesn't work when you poll for a fax - in that case the calling endpoint will probably want to send the first IFP.

          The only way I can see out of this is to add a new data application (say, t38faxV2) to DataApplicationCapability etc in the H.245 ASN.1.    t38fax would use the 1998 ASN.1 and t38faxV2 would use the 2002 ASN.1 - and future carefully checked modifications ;-).  Now there's no problem, a 2002 aware endpoint can offer both versions and a 1998 aware endpoint can only accept the ASN.1 it understands.


          Pete Price
          Vegastream Limited

            -----Original Message-----
            From: paulej at PACKETIZER.COM [mailto:paulej at PACKETIZER.COM]
            Sent: 09 September 2003 20:32
            To: itu-sg16 at external.cisco.com
            Subject: H.323 Fast Connect and Versioning


            Folks,

            Today, I was exchanging e-mail with somebody over the fax version number issue and the different syntax that is used (1998 vs 2002).

            If we open H.245 and exchange a full set of capabilities, and H.323 endpoint could determine the version supported by the other side and open a channel supporting that particular version.  However, I don't think any text is explicitly clear on that.

            Another scenario-- and one I have more trouble with-- is Fast Connect.  If a calling endpoint populates the fastStart element with "version 2" proposals, for example, the called side (say, a version 0 device) might accept the proposal and return the response.  However, it is not allowed to modify the version field.  The reason is that Fast Connect proposals are not ordered in a way such that replies must be ordered the same way.  Rather, the calling device determines which proposals are accepted based on characteristics of the proposals returned (e.g., codec type, samples per packet, or other information).  In some cases, a calling endpoint will actually not try to "match" the proposal returned, but just accept it as a proposal and run with it.

            The problem is that if a calling device proposes version 2 and the called device returns version 2 (but is actually a v0 device), then the wrong syntax will be transmitted on the wire.  Thus, the text needs to state somewhere one of these options (or something similar):
              1.. The calling device must offer a proposal for each version it wants to potentially use and the called device must accept the first proposal it can accept (in order of the proposals) and the called device must not accept any proposal for a version it does not support 
              2.. The calling device must wait for capability exchange to complete to determine the actual supported version of the other device
            Alternatively, we could make an allowance for the endpoint to change the version number in the Fast Connect proposal, but I don't think that's a good idea, as it would quite possibly break interoperability with some devices.

            What would a version 0 device do today if it received a Fast Connect proposal advertising version 2?  Would it accept it?  I suspect so and I'm afraid that we might have some interop problems regardless of the direction we go.

            Perhaps we can require the calling device to not transmit any data until it receives at least one IFP packet from the called side and determines the ASN.1 version used to encode the message.  As much as we can push onto the shoulders of a v2 device, the better, as I don't think we have any real deployments in the field (yet)... might be wrong, but I think it would be a far less significant impact on that side.

            I'm open to suggestions.  Perhaps this issue is even addressed and I've simply overlooked it.

            Thanks,
            Paul

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