Re: Last Call: 'A Lightweight UDP Transfer Protocol for the the Internet Registry Information Service' to Proposed Standard (draft-ietf-crisp-iris-lwz)

[Andrew Newton <andy at hxr.us>]

Lars Eggert wrote:
> > I just checked the simple user-drive, cli client I wrote and it 
> > doesn't retransmit at all (perhaps not the best UI experience).
>
> the issue isn't with retransmissions. If - to use Harald's example - no 
> reply arrives for 70 out of 100 issued requests, this is a pretty strong 
> indication that the server or something on the way to or from the server 
> is congested. In response to such a signal, the request rate should be 
> reduced.

I'm not disagreeing.  I was merely pointing to one reason why this never 
came up as feed back from implementations.

> I'd also like to bring up a second, somewhat related issue. Section 3 says:
>
>    Each IRIS-LWZ query or response is contained in a single UDP packet.
>    Servers MUST be prepared to accepted packets as large as 4000 octets,
>    and clients MUST NOT send packets larger than 4000 octets.
>
> and Section 4 says
>
>    2.  If a request is less than or equal to 4000 octets, send it
>        uncompressed.
>
>    3.  If a request can be compressed to a size less than or equal to
>        4000 octets, send the request using compression.  Otherwise use
>        another transfer protocol.
>
> Sending packets larger than the minimum MTU of ~512 bytes is likely to 
> cause fragmentation, which can significantly reduce transmission 
> efficiency and reliability [1] and cause data corruption [2].

This section came about as feedback from a server implementation where the 
implementor wanted to know how big of a buffer to reserve.  To answer your 
question below, 4000 was picked using DNSSEC as guidance.  However, looking 
at my code I see that MTU is the actual maximum value (I'll ask what others 
have done).  There likely should be a step 0: determine maximum size of 
packet (the lesser of mtu or 4000, which is almost always likely to be mtu).

> Fragmentation becomes more likely as packets get larger. With packets of 
> 4000 bytes (where does this number come from, anyway?), fragmentation is 
> pretty much guaranteed across most links. If IRIS-LZW messages are 
> expected to be larger than ~512 bytes in general, the use of a transport 
> protocol other than UDP is probably a good idea.

That section does say: "Not all IRIS requests and responses will be able to 
utilize UDP and may require the use of other transfer protocols (i.e. 
IRIS-XPC and/or BEEP)."

-andy


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