Subject: some questions about MTU, template

From: Yasuji Takeuchi <[email-masked]>
Date: Fri, 24 Dec 1999 11:41:15 +0900

Dear SVRLOC WG members,

I read the specifications for SLP,both v1 and v2. And I have several items
for implementation.

Concern 1. Path MTU
What is the definition of MTU? I think this definition is not clear.
I think MTU concerns only between Datalink layer and Network layer. 
The fragmentation is processed in Network layer, and when we make a udp
appilication, we don't have to care about MTU. Why does MTU concern UDP?

Concern 2. Template implementation
When the device(i.e printer) that behaves as SA has a template, can I add
any private attributes to the template? And when UA issued any type of 
requst with the template(i.e. service:printer:lpr), can SA issue AttrRqst
with added one?

regards,
Yasuji Takeuchi
  [email-masked]

---

From: [email-masked]
Date: Wed, 29 Dec 1999 05:09:42 -0500 (EST)

Yasuiji,

>> appilication, we don't have to care about MTU. Why does MTU concern UDP?

The reason that SLP messages which use unicast are limited to 1400 bytes is that
on 802 type media this size will not fragment when it is transmitted.  Messages in
SLP may be unicast or multicast (the application which sends the request does
not know which will be used).  It is very bad to fragment a message which will
be sent by multicast (or broadcast) - I am not even sure that it is possible in 
some cases.  An additional concern is that UDP implementations do not have
to support delivery of datagrams which are larger than 576 bytes.  In practice,
they support 4k these days and on many platforms 64k.

Overflow is only visible on replies.  The reply will still be only 1400 bytes in size,
with as much of the reply as will fit included.  The requester, noting the overflow
flag, may retransmit the request using TCP or elect to make do with the partial
reply.

The reason a configuration option is specified for MTU is that in some cases
a smaller MTU may have to be used.  Host requirements for IP support specify
only that 576 bytes must be supported at the IP layer.  For some challenged
media you may want to specify 520 bytes maximum size or the SLP message
(leaving enough room for IPv4 or IPv6 headers and the UDP header), for instance.
Many UDP based protocols require applications to use this size so they will work
on any link technology.  Since SLP is mainly meant to function on a set of multicast
enabled interconnected LANs, we chose a MTU value which will work on 802
nets.

Jim replied:
> An SA can add any parameters it wants to the template, and the UA               
> can issue a request for added parameters. RFC 2608 specifies a                  
> convention that additional attributes should begin with "x-" and                
> I believe Erik recently issued an Internet Draft                                
> (draft-guttman-svrloc-vendor-ext-00.txt) which describes additional             
> conventions for safe vendor extension.     

SLP allows the UA to include any attribute in the SrvRqst or AttrRqst.
The SA or DA which responds may not support that attribute - in which
case the UA will not get it.  

In the vendor extension document I describe how this works and conventions
to use for vendor specific extensions.  Please look at the internet draft (it is now 
draft-guttman-svrloc-vendor-ext-01.txt) and let me know what you think.

Erik