RE: [Syslog-sec] I-D ACTION:draft-ietf-syslog-transport-udp-02.txt

["Rainer Gerhards" <rgerhards@hq.adiscon.com>]

Anton, 

> > - In 3.4, the draft says:
> > 
> > ######
> >    All implementations MUST support sending and receiving syslog
> >    messages up to and including the size which does not require
> >    fragmentation (507 bytes for IPv4 and 1191 bytes for IPv6).  This
> >    size excludes the overhead of the syslog transport and UDP/IP
> >    headers.  Support for larger messages is encouraged.
> Implementors
> >    SHOULD clearly state the maximum supported message size in
> >    documentation.
> > ######
> > 
> > Actually, all implementations MUST support a message size of 
> > 65,000 bytes, because this is the required minimum in 
> > -protocol 4.1. We agreed on this minimum after long 
> > discussions on the message size. I'd actually prefer not to 
> > re-iterate this. But if we must, let me say that a guaranteed 
> > message size of 507 bytes looks far too low to me.
> > 
> > Please note that the -protcol requirement meant that all 
> > implementations MUST support fragmentation.
> > 
> > Sorry I didn't spot this in the -01 version...
> > 
> > I've also noticed that in -protocol, I have limited that max 
> > message size to 16,000,000 bytes in -protocol. It might make 
> > sense to sync this. I can go up with my number.
> 
> Indeed we discussed this at length, but my recollection of conclusions
> was different. :)  Let's revisit.  We decided (I thought) that we
> can't force transport implementors into supporting fragmentation as it
> was too complicated. Plus, we can't them into supporting the maximum
> message size as the common denominator may be too small. 

I've tried to find something definitive in the mailing list archive.
Actually, I couldn't come up with such a thing... So it looks like there
were some private conversations or we both had a wrong understanding of
concensus. Anyhow, let's try to finalize it now.

I thought that the concensus was that we can not force implementors to
support very large messages. Thus, the minimum-to-be-supported size
should be just 64K. Please remember that the IHE healthcare initiative
tries to use syslog for auditing and has a size requirement of at least
32K. So there is some industry need for larger size messages.
> 
> Hence, only the size that can be transported without fragmentation
> must be supported by transport implementors.  

This means we are going down with the actual message limit from 1K
(which already was very small) to just 480 octets. Given the additional
overhead we have introduced, there is not much left for the acutal
message.

Of course, you can argue that it is just the minimum size that MUST be
supported. But I think if we face reality, that minimum will soon become
the maximum supported by most implementations. I am very concerend that
such a short guaranteed message size really makes sense... If we really
take that route, we should eventually be bold enough to go back to 1K
without fragmentation, even though we have some reliability concerns. At
least that seems to have worked for several years now. I don't think we
will get much acceptance for a standard that leaves around 400 bytes for
the actual message (not counting meta data in structured ids....).

> However, a full
> transport implementation will support fragmentation and size of up to
> 16777216 bytes.   
> 
> I think what we need is a statement in -protocol that it supports a
> message size up to 16777216, but transports MAY impose a different
> limit. 

That's already there ;)

> 
> It was mentioned that some devices will physically not be able to
> handle large messages, where "large" can be quite a small number.
> Restricting all messages to this common denominator is not good, so we
> have a multi-tier support system. People are not likely to support
> streaming of message parts in their implementations, so available
> memory will be an issue.  
> 
> Administrators will choose the appropriate implementations. Should we
> add a statement that transport implementations MUST document maximum
> supported message size? 
> 
> I know it is less than optimal, but requiring all transports to
> support some huge size is not a good idea either IMO. 
> 
> You also had an idea for doing fragmentation within the -protocol by
> upper layer. I originally liked it.  But later people commented that
> this was improper layering and shifted responsibilities of transport
> layer up the stack.  I agree with this.  It would have introduced
> potential fragmentation on several layers and we would have still
> forced the higher layer to deal with UDP sizing constraints. 
> 
> I agree this issue deserves discussion. Let's have it and put it bed
> for good. 
> 
> > I do not fully understand what you mean in 4.2 by:
> > 
> > #####
> >  No concurrent port reuse on the same host is
> >    allowed.
> > #####
> > 
> > It's probably just my English, but I do not get it. I'd 
> > appreciate if you could elaborate a little.
> 
> What this refers to is the use of SO_REUSEPORT socket option which
> allows multiple sockets to be bound to the same port.  I guess it
> would not be a problem if the two different consumers of the socket
> (maybe threads) use different MessageId numbers.  And this is stated
> as requirement elsewhere: all message parts must be send using the
> same MessageId and source port among other things.   
> 
> Following your suggestion, we already allow multiple different ports
> to be used concurrent within a single process, so I have no problem
> with removing this restriction. It is too implementation-specific
> anyway.  What do you think?

I agree. I simply did not fully understand what you were saying ;)
Eventually the sentence could be a bit more verbose - but this might
just have been my personal problem ;)

Rainer
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