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From: Rich <rich@example.invalid>
Newsgroups: comp.os.linux.misc
Subject: Re: I never thought of this scenario
Date: Sat, 20 Apr 2024 18:07:08 -0000 (UTC)
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Richard Kettlewell <invalid@invalid.invalid> wrote:
> Rich <rich@example.invalid> writes:
>> And, the protocol "must" be routable:
>>
>> RFC2131: https://www.rfc-editor.org/rfc/rfc2131 - page 6:
>>
>>         DHCP should not require a server on each subnet.  To allow for
>>         scale and economy, DHCP must work across routers or through the
>>         intervention of BOOTP relay agents.
>>
>> Note they use "must" above in the statement "DHCP must work across
>> routers".  Page 4 defines "must" as:
>>
>>         o "MUST"
>>         
>>         This word or the adjective "REQUIRED" means that the item is an
>>         absolute requirement of this specification.
> 
> You missed a bit:
> 
>    Throughout this document, the words that are used to define the
>    significance of particular requirements are capitalized.  These words
>                                                ^^^^^^^^^^^
>    are:
> 
> 
> The ‘must’ in the design goals is not capitalized.

Indeed, I did miss that.

>> Therefore the RFC explicitly allows for DHCP to be routed.
> 
> A protocol is not its design goals. You can’t conclude that a protocol
> actually achieves a goal just by looking at the what the goals were. A
> good recent example would be SIKE, which totally failed to meet its
> design goals.

Fair enough, however, given:

1) no explicit statement requiring non-routability in the RFC (if the 
designers had wanted it to be "non-routable" as Lawrence continues to 
asssert, they would have said so);

2) an explicit statement in the design goals of "working across 
routers"

it therefore becomes reasonable to presume that "routability" was 
at a minimum, not excluded, and was likely intended.

> I don’t personally care how DHCP gets across routers but from a quick
> skim it looks like it relies some kind of relay agent. Table 1 or
> section 3.1 might be reasonable references.

It relies on a BOOTP Relay agent only for the initial, unconfigured, no 
IP address state, of the client.  Once the client has an IP, other DHCP 
protocol interactions happen using the client IP, and no BOOTP Relay 
agents are involved.

DHCP is also not a "transport layer" protocol.  Instead, it uses UDP 
for its transport layer (see RFC url above, page 22):

    "DHCP uses UDP as its transport protocol."

Since UDP is itself routable, DHCP is also routable, because DHCP is 
simply a protocol definition for sending particular "messages" inside 
of UDP packets.