Subject: [dhcwg] RFC 2132: Minimum DHCP message size question

From: "Santosh Chandwani" <[email-masked]>
Date: Mon, 2 May 2005 21:26:54 +0800

RFC 2132, Section 9.10 describes the Maximum DHCP Message Size option
and states that the minimum size that a DHCP service must accept is 576
bytes.

Microsoft is trying to address an issue with the DHCP client, where the
DHCP service running on some smaller routers do not accept DHCP messages
of size larger than 576 bytes. This becomes an issue in cases where the
client tries to send a vendor-specific or another option of large size,
which causes the message size to grow beyond 576 bytes. To properly
address the issue, we want to verify that the constraint applies only to
DHCP servers. Although it's highly unlikely, we want to ensure that
there are no DHCP Relay Agents which impose a restriction on the size of
the DHCP message they will forward. Insights you can share on the
subject will be appreciated.

Thanks,

Santosh

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---

From: Ted Lemon <[email-masked]>
Date: Mon, 2 May 2005 10:10:20 -0500

On May 2, 2005, at 8:26 AM, Santosh Chandwani wrote:
>
> Microsoft is trying to address an issue with the DHCP client, where  
> the DHCP service running on some smaller routers do not accept DHCP  
> messages of size larger than 576 bytes. This becomes an issue in  
> cases where the client tries to send a vendor-specific or another  
> option of large size, which causes the message size to grow beyond  
> 576 bytes. To properly address the issue, we want to verify that  
> the constraint applies only to DHCP servers. Although it’s highly  
> unlikely, we want to ensure that there are no DHCP Relay Agents  
> which impose a restriction on the size of the DHCP message they  
> will forward. Insights you can share on the subject will be  
> appreciated.
>
>

BOOTP relay agents follow the BOOTP RFC, not the DHCP RFC, and the  
bootp RFC doesn't require them to pass messages larger than 576  
bytes. So  it's quite possible, unfortunately, that there is some  
relay agent out there that doesn't support large DHCP packets.   And  
unfortunately, that relay agent would not be out of spec.   I don't  
know of any place in RFC2131 or RFC2132 where any requirement is  
placed on BOOTP relay agents.

Now, is a lame relay agent like this broken?   I would say it is.    
It should certainly never be advertised as supporting DHCP.   But is  
it out of spec?   Unfortunately no.