passing values to client: how big?

Simon Hobson dhcp1 at
Tue Jan 13 19:52:45 UTC 2009

David W. Hankins wrote:

>The packet size issue is also another thorny one.  576 is the magic
>number named in RFC 2131, but this was never intended to be a maximum
>packet size, I don't think.  My interpretation of the language is that
>it is an absolute minimum you must support.
>So there is some confusion here; 576 octets, measuring the UDP payload,
>IP payload, or link layer payload (full IP packet)?
>I think all three interpretations exist in some form.  The most
>nefarious is DHCP relay agents that presume a 576 octet full IP packet
>size, and DHCP clients that presume a 576 DHCP packet size (UDP
>payload).  This combined with client brain damage that they must
>'pad' the packet out to the 576 octet size (init to zero), and the two
>don't interoperate, even when they don't even need the extra space.
>The client hint that it can handle a larger payload is all well and
>good, but it does us no good if we're connected by a relay that drops
>packets larger than 576 octets.  There is no relay agent signal for
>its supported packet size ranges.  You're actually better off ignoring
>the client hint unless you're directly attached or unicasting (giaddr
>is zero), although we don't do this (no RFC describes this behaviour
>It does mean that even clients that can receive larger packet contents
>really shouldn't advertise their ability to do so, or they might dupe
>servers into believing it despite relay incompatibility.  This is why
>I've never updated dhclient to send a default MMS option on interface
>MTU (we support single, non-fragmented packets up to 64KB).

It's bound to be the case - if you make something optional, then 
there is bound to be an implementation that doesn't support it. So 
large packets are bound to be a problem somewhere. But I guess, when 
the spec was written, the packet size was considered to be quite 
Simon Hobson

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