Re: [dhcwg] WGLC on draft-ietf-dhc-mac-assign-01 and draft-ietf-dhc-slap-quadrant-01 - respond by Nov. 17

"Bernie Volz (volz)" <volz@cisco.com> Mon, 04 November 2019 15:17 UTC

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From: "Bernie Volz (volz)" <volz@cisco.com>
To: Alexandre Petrescu <alexandre.petrescu@gmail.com>, "dhcwg@ietf.org" <dhcwg@ietf.org>
Thread-Topic: [dhcwg] WGLC on draft-ietf-dhc-mac-assign-01 and draft-ietf-dhc-slap-quadrant-01 - respond by Nov. 17
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Date: Mon, 4 Nov 2019 15:17:20 +0000
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Subject: Re: [dhcwg] WGLC on draft-ietf-dhc-mac-assign-01 and draft-ietf-dhc-slap-quadrant-01 - respond by Nov. 17
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See below (BV>).

- Bernie

-----Original Message-----
From: dhcwg <dhcwg-bounces@ietf.org>; On Behalf Of Alexandre Petrescu
Sent: Thursday, October 31, 2019 4:14 PM
To: dhcwg@ietf.org
Subject: Re: [dhcwg] WGLC on draft-ietf-dhc-mac-assign-01 and draft-ietf-dhc-slap-quadrant-01 - respond by Nov. 17

I discuss because comments are requested, but I do not oppose advancemet of this draft.

The questions I make below are more like brainstorming, because I do not use the mac-assign mechanism myself.

Le 30/10/2019 à 02:51, Tomek Mrugalski a écrit :
> Hi, This mail initiates a Working Group Last Call on two related
> IDs:
> 
> Link-Layer Addresses Assignment Mechanism for DHCPv6
> draft-ietf-dhc-mac-assign-01
> https://tools.ietf.org/html/draft-ietf-dhc-mac-assign-01
> 
> and
> 
> SLAP quadrant selection options for DHCPv6
> draft-ietf-dhc-slap-quadrant-01
> https://tools.ietf.org/html/draft-ietf-dhc-slap-quadrant-01
> 
> Authors believe those drafts are ready for WGLC. Please post your 
> substantial comments and your opinion whether those are ready for 
> publication or not. If you have minor editorial comments, you may send 
> them to the authors directly.
> 
> Please post your comments by Nov. 17th.

draft says:
> DISCUSSION: A device may send its link-layer address in a LLADDR 
> option to ask the server to register that address to the client (if 
> available), making the assignment permanent for the lease duration.
> The client MUST be prepared to use a different address if the server  
> choses not to honor its hint.

I agree with the idea to request to register.

Which link-layer address may the device send in a LLADDR option?

Is it the link-layer address that was assigned previously by DHCP, or is it a temporary, or is it?

BV> This is whatever link-layer address that the device may have - such as a temporary address it may have generated. The server does not have to honor that request as the discussion points out.

The IoT devices and VMs  I have seen, if they have the capability to have a MAC address, then they already have something in there, like 0s, or like random bits, if not an IEEE-assigned address with OUI and so on.

Will this draft forbid these IoT devices and VMs to use their existing MAC addresses?

BV> No. I don't see how the draft could do that. Per above, if the device has an address in the non-global space, it can request to continue using it.

On another hand, can the MAC address that the device imposes (registers) to the DHCP server be used as a key to make the server always allocate the same IP address to the Client?

BV> That really is a policy issue for the DHCP server. There are many possibilities as to how this could be implemented such as:
1) Use an algorithm that given some input (such as client-id option), will always generate a stable mac address, OR
2) Once an address has been assigned, retain the binding between it and the client for a duration (either this is the valid lifetime or may be some additional lifetime beyond that that may not be communicated to the client)

> link-layer-len  Specifies the length of the link-layer-address field  
> (typically 6, for a link-layer-type of 1 (Ethernet)). A two octets 
> long field.

But the preceding figure says that the length is 128bits(?) precisely - there are no vertical dots, but vertical pipes '|'.  The rest of bits is padding 0s (leading? trailing?)?  MAC addresses longer than 128 bits arent accepted?

BV> 128 is never mentioned explicitly in this document. The figure using the vertical pipes in this manner is to say that this is a variable length field - there is no implication that of the size -- yes, it looks to be a multiple of 4 octets but that was just to make it easier to show the remaining fields (wrapping them between two rows makes them difficult to read). 

There are link-layers from IEEE that have lengths 16bit (short addresses in 802.15.4) and also 64bit (802.15.7 VLC Visible Light Comms).

BV> Correct. This is variable length field (link-layer-address is variable and depends on link-layer-type/link-layer-len).

> and may specify a specific address or address block as a hint for the 
> server.

I fully agree with the idea of requesting more than just one MAC address.  You call that a 'block'.  The question is how to represent (and implement) such a block.

BV> I'm not sure what the problem is here? 

> For example: link-layer-address: 02:04:06:08:0a and extra- addresses
> 3 designates a block of 4 addresses, starting from 02:04:06:08:0a
> (inclusive) and ending with 02:04:06:08:0d (inclusive).

Are the addresses ending in 0a, 0b, 0c and 0d each included in Figure3?
  In which field?

BV> Huh? A block is defined as the starting address plus the number of addresses. So, the 4 address ...0a, 0b, 0c, and 0d are represented by a starting address of ...0a and length of 3 (because it is "extra-addresses" - having 0 addresses assigned is useless).

Or only the starting address is included in full and the number of addresses in the field 'extra-addresses?

BV> See previous comment.

Shouldnt one rather encode the set of MAC addresses like the IPv6 address/plen notation? (rather than including fully each address).

BV> plen only allows allocating in powers of 2. 

What would be the relationship between a MAC block and an IP prefix?  In Prefix Delegation one would like to delegate always the same prefix to the same MAC address, to facilitate application continuity; with this address block concept here, would we talk about delegating an IP Prefix to a MAC block?

BV> Prefix delegation and mac address assignment are different and are used for different purposes. The mac-address is what the client will use to communicate at layer 2; prefix delegation is layer 3 issue. 

Finally, would one use a delegated MAC address to form an Interface ID and further prepend a prefix from RA to get a more privacy IP address?
(rather than using the built in MAC address that is centrally administered and has privacy risks).

BV> This is completely outside the scope of this document. Other documents cover how (or if) a link layer address is used for form an IPv6 address.

Alex


> 
> Cheers, Tomek
> 
> _______________________________________________ dhcwg mailing list 
> dhcwg@ietf.org https://www.ietf.org/mailman/listinfo/dhcwg
> 

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