Re: [dhcwg] MTU option for DHCPv6?

"Templin, Fred L" <Fred.L.Templin@boeing.com> Thu, 28 July 2016 15:18 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: "Bernie Volz (volz)" <volz@cisco.com>, Lorenzo Colitti <lorenzo@google.com>
Thread-Topic: [dhcwg] MTU option for DHCPv6?
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Date: Thu, 28 Jul 2016 15:18:16 +0000
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Subject: Re: [dhcwg] MTU option for DHCPv6?
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Hi Bernie,

The MTU option I am talking about allows link administrators to set the MTU value that
all nodes on the link must use. draft-van-beijunm-multi-mtu proposed having different
MTU values among the nodes no the link, but there is at least one problem with that.
In particular, IP multicast expects that all nodes on the link will be able to receive multicast
packets as large as the link MTU. So, for example, if node A on the link configured an MTU
of 8KB and node B configured an MTU of 4KB, the 8KB multicasts sent by node A would
not reach node B – and this violates the reasonable expectation of nodes attached to
the same link. Therefore, the DHCPv6 MTU option should encode the minimum MTU
among the (potential) MTUs of all nodes on the link, which in this case would be 4KB.

Interestingly, draft-van-beijnum-multi-mtu does not have a section on multicast.

Thanks – Fred
fred.l.templin@boeing.com

From: Bernie Volz (volz) [mailto:volz@cisco.com]
Sent: Thursday, July 28, 2016 8:05 AM
To: Lorenzo Colitti <lorenzo@google.com>
Cc: otroan@employees.org; Ian Farrer <ianfarrer@gmx.com>om>; Templin, Fred L <Fred.L.Templin@boeing.com>om>; <dhcwg@ietf.org> <dhcwg@ietf.org>
Subject: RE: [dhcwg] MTU option for DHCPv6?

> it looks like reconfigure messages MUST be discarded if they do not include authentication.

It has always been that way (see https://tools.ietf.org/html/rfc3315#section-15.11). Reconfigure uses the Reconfigure Key Authentication Protocol (the server provided the client the key at some point earlier in the client/server interaction). Of course, this does require the client to indicate it is willing to do Reconfigure.

BTW: If I understood draft-van-beijnum-multi-mtu, it is about using ND, not RAs. And this allows [link local] destination specific MTUs to be used.


-          Bernie

From: Lorenzo Colitti [mailto:lorenzo@google.com]
Sent: Thursday, July 28, 2016 10:57 AM
To: Bernie Volz (volz) <volz@cisco.com<mailto:volz@cisco.com>>
Cc: otroan@employees.org<mailto:otroan@employees.org>; Ian Farrer <ianfarrer@gmx.com<mailto:ianfarrer@gmx.com>>; Templin, Fred L <Fred.L.Templin@boeing.com<mailto:Fred.L.Templin@boeing.com>>; <dhcwg@ietf.org<mailto:dhcwg@ietf.org>> <dhcwg@ietf.org<mailto:dhcwg@ietf.org>>
Subject: Re: [dhcwg] MTU option for DHCPv6?

On Thu, Jul 28, 2016 at 10:52 PM, Bernie Volz (volz) <volz@cisco.com<mailto:volz@cisco.com>> wrote:
And, note that Fred had indicated "I'm operating on a link where I don't need to get any configuration information from RS/RA - everything comes from DHCPv6." So, looks like at least he wants DHCPv6 option(s).

Yes, but it doesn't have to be that way. Sending an RA would work just as well. Like all RA parameters, it also has the advantage that it is easier to update dynamically if needed. Doing that in DHCPv6 is more difficult, because at least as of RFC3315bis, it looks like reconfigure messages MUST be discarded if they do not include authentication.