[standards-jig] Pub/Sub for JNG?

Dave dave at dave.tj
Wed May 1 17:12:41 CDT 2002


If we allow entities to decide on a connection-by-connection basis
whether to use UDP or TCP and how (if at all) to compress the streams,
we're not losing much (since entities can always just continue to use
the standard method), but we're gaining a major feature, being able to
add XML transfer methods as we wish.  We can even implement the HTTP
proxying gunk as nothing but another XML transfer method.

Our goal for JNG should be simplification.  There are plenty of JEPs
in the current Jabber system that are only necessary because of a
lack of generality in the Jabber protocol itself.  When we generalize
stuff, there's no mandate for every entity to support every conceivable
possibility; however, we obviate the need for JEPs just so some entities
can support what should theoretically be quite obvious if the basic
protocol is flexible enough.

 - Dave


David Waite wrote:
> 
> Iain Shigeoka wrote:
> 
> >Wow, this thread sure has taken off.  :)  
> >
> >1) compressed streams
> >
> >I'm mixed on this.  As mentioned earlier, it was shown
> >that you can compress Jabber streams to lower bandwidth.
> >IM servers are typically network bound so the computation
> >overhead of compression should not slow the overall
> >performance of the server.
> >
> >I believe the last time this came up, it was eventually dropped
> >because the benefits just didn't justify the added complexity.
> >
> >Ideally, I suppose we should try to make this a negotiable
> >transport feature.  It seems to come up enough that no
> >matter what the group decides, there will be enough people
> >in the other camp that will want to do the opposite.
> >
> >I believe we can design a transport system that can accomodate
> >this.
> >
> I'll add a few more comments.
> 
> The effects of using something like gzip over a 100-200 byte text block 
> (a typical message) will not give any significant performance gains. 
> Using gzip over multiple messages might, but that will increase latency 
> if you cache messages for transmission in a block. Transmission as a 
> gzip stream will not work, because you cannot guarantee that a message 
> will end on a byte boundary; a message might not be transmitted until 
> additional data is sent down the pipe. Finally, I believe the gzip/bzip 
> dictionaries have a minimum in-memory size of about a megabyte; AFAIK 
> this is one reason why a lot of linux installation systems used to 
> require more memory to install than to actually run linux.
> 
> For something like server to server, this might be justified. You would 
> still want to make sure to have blocks of traffic, but the conception is 
> that you have less server connections than client connections, so the 
> memory usage will be less noticable, and the higher CPU usage may 
> balence itself off.
> 
> I still think the best solution to the bandwidth problem (assuming for 
> the moment that it is a problem) is a jabber-specific compression 
> system, with a default (negotiated) dictionary which is appended by both 
> sides as traffic continues. Ideally this compression system would also 
> be a binary representation of the XML, but this becomes significantly 
> difficult with things like prefixed namespaces :-)
> 
> In the end, its a really cool project, I would be really interested in 
> seeing it done as a project and probably would contribute a bit of 
> brainpower to the project. It might never be an actual standard 
> recognised by the JSF though.
> 
> >2) UDP
> >
> >Technically, I think UDP is good idea.  Especially in situations
> >where we could exploit multicast.
> >
> >I imagine that the main reason people avoid it though is difficulty
> >in making a good implementation based on UDP, and firewall
> >issues.  The latter would seem to me to be the largest issue.
> >For all its technical advantages, I think UDP is simply a non-starter
> >for us if we want to get inside enterprises.
> >
> UDP issues include:
> - Difficulty (impossibility) getting through firewalls in either direction
> - datagram-based sending requires more logic
> - datagrams are recognised as being disposable during periods of congestion.
> - things like traffic retrying, sequencing, congestion notification and 
> other flow control are all lost, and also all needed by Jabber. 
> Basically for something which _is_ stream-based like the protocol of 
> Jabber, you wind up trying to reimplement a tweaked, non-conformant TCP 
>  implementation within a client. I might be crazy, but I think that the 
> Linux kernel group, the *BSD respective kernel groups, and even 
> Microsoft can design a better TCP implementation than I can, especially 
> when I work in langauges other than C or C++.
> 
> Again, it would be cool to see, but you aren't going to get a lot of 
> people to jump at it until there is an actual implementation. Even, then 
> it is not suitable for a lot of applications - the fact that every 
> commercial instant messaging system now uses TCP illustrates this quite 
> well.
> 
> -David Waite
> 
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