RE: [dhcwg] Assigning DHCPv6 option codes

Richard Barr Hibbs <rbhibbs@pacbell.net> Thu, 24 January 2002 05:33 UTC

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Date: Wed, 23 Jan 2002 21:23:18 -0800
From: Richard Barr Hibbs <rbhibbs@pacbell.net>
Subject: RE: [dhcwg] Assigning DHCPv6 option codes
In-reply-to: <200201232355.g0NNtWFh026414@calcite.rhyolite.com>
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> -----Original Message-----
> From: Vernon Schryver
> Sent: Wednesday, January 23, 2002 15:56
>
> I thought RFC 2132 clearly separated the site-specific range
> [128,254] from the vendor-specific range defined in section 8.4.
> Where is the overlap?
>
...hmmmm...  the imprecision of words.....

yes, vendor-specific <<information>> is encoded as specified in section 8.4
into the value field of a single DHCP <<option>> (number 43), but that's not
quite the same thing as I was pointing out in my reply:  Wyse, Citrix, and
other vendors of "thin" clients, several vendors of VoIP telephones, and a
few other vendors such as cisco and Intel use <<option>> numbers 128-254 to
request and/or supply data for their products.

While option 43 is defined for passing data whose code point values are not
required to be specified by an RFC, it does permit the use of
"vendor-specific" code points 1-254, which are permitted to redefine or
duplicate any "standard" DHCP option, or to define completely new ones.
Options 128-254 of the site-specific option set may also do this, so what is
the difference?

The total length of the encapsulated data in option 43 is limited to 255
octets:  the total length of data in the site-specific options is 127 * 255
= 32,385 octets.  On the other hand, option 43 allows 254 different,
presumably unique, code points while the site-specific options permit only
127.  The downside to using the site-specific options for conveying data is
that server support is far from uniform because of the lack of specification
for dealing with overlapping or conflicting vendor implementations.  Of
course, a similar problem exists with option 43:  it is not possible to
examine an arbitrary set of vendor-encapsulated options and know the
identity of the vendor, since there is no standard way to specify this.
That's why in several server implementations the vendor class identifier
(option 60) is presumed (though not required by RFC 2132) to be used in
conjunction with option 43.

To stave off an argument, RFC 2132 does not require (by using MUST phrases)
that option 43 only be used with option 60 -- the RFC uses weaker SHOULD
phrases -- and does not specify server behavior if either option appears
alone.

--Barr


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