H.323 Fast Connect and Versioning

paulej at PACKETIZER.COM paulej at PACKETIZER.COM
Wed Sep 10 18:38:27 EDT 2003


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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