Subject: RE: [dhcwg] dhcpv6-24: movement detection and Confirm message

From: "Bernie Volz (EUD)" <[email-masked]>
Date: Thu, 9 May 2002 08:24:25 -0500

Thomas:

My desire for the Confirm would be as follows:

- Client can send a Confirm to help in movement detection. It shouldn't be required for the client to do this since it may have other means of doing this.

- A server that does implement Confirm handles the message in three ways:
1. If it has the binding for the client and the addresses are valid, it can send a Reply with the success indication. This would be the case if the client hasn't really moved.

2. If it doesn't have a binding but in comparing the addresses provided in the Confirm, the server can determine that the prefixes for the addresses are NOT valid for the link, the server can Reply to the client that the addresses are bad.

3. The server discards the Confirm since it can not verify that the addresses are truely valid for the client.

(We could allow another Reply from the server that would indicate to the client that the prefixes appear to be valid, but it can't confirm the actual addresses. However, this isn't really necessary since no response to the Confirm essentially means that and it could give false information at times; that is why it would be best not to have servers do this.)

- While I think servers SHOULD implement Confirm, they don't have to. If they don't, they won't enhance movement detection but since there are other mechanisms that can help with this, it is not required.

- Confirm processing is much different than a Renew/Rebind. There is much less work involved on the server - it is only verifying the information. A Confirm can NOT be used to extend address lifetimes/leases. IMHO, it should only be used to help the client determine whether it moved.

- Bernie

-----Original Message-----
From: Thomas Narten [mailto:[email-masked]]
Sent: Thursday, May 09, 2002 9:00 AM
To: Ralph Droms
Cc: [email-masked]
Subject: Re: [dhcwg] dhcpv6-24: movement detection and Confirm message 

> Confirm - client has detected it may have moved, sends
>            Confirm to any available server to determine
>            if configuration is still valid
> Renew   - client unicasts to assigning server to extend
>            lease on IA
> Rebind  - client sends to any available server to extend
>            lease on IA

Including the above in the overview would be very helpful. It's not
clearly described there, and the above description really helps me to
understand why/how the messages differ.

> Thomas - rereading your original question, I'm not sure if you're 
> suggesting that a client not try to detect if it has moved to a new link, 
> or if it use a Renew instead of a Rebind when it detects it may have moved 
> to a new link.

I believe a client should definitely try to detect when it has
moved. What I was wondering about is whether DHC should be specifying
*how* it does movement detection (since this is a general topic), and
whether DHC should provide explicit mechanisms (beyond other existing
message types) that help the client determine if it has moved.

Seems to me the Confirm is (maybe?) just an optimization. Couldn't the
client just send a Renew to the assigning server?

I gather that one advantage of the Confirm is that it is sent to all
servers, so one will respond quickly. If the client sends a Renew to a
specific server, but the client has moved, the Renew will presumably
time out, which takes a lot longer. But in terms of the amount of work
a server has to do when processing Confirm vs. Renew, the work is
pretty much the same, I would think.  I assume that this is the
motivation for Confirm.

Question: when one sends a confirm, does only the server that has that
information respond, or are all servers expected to respond?

The text says:

>    In any situation when a client may have moved to a new link, the
>    client MUST initiate a Confirm/Reply message exchange.  The client
>    includes any IAs, along with the addresses associated with those IAs,
>    in its Confirm message.  Any responding servers will indicate the
>    acceptability of the addresses with the status in the Reply message
>    it returns to the client.

But how do other servers know that they are to respond positively? I
would expect that only server that originally allocated the addresses
to be able to respond positively that the config is valid. Maybe the
text isn't explicit enough about what a server does when it gets a
Confirm. Does it just check that the  addresses are on the link as
indicated by the link-address field in the relayed packet? This would
be sufficient to verify that client hasn't moved. If it needs to
verify that the actual addresses have been assigned  it's not clear
that any server can do that. This makes me wonder if this is really
very much better than just  using Renew (if only one server can
respond anyway).

The text says (under server processing):

>    If the server finds that the information for the client does not
>    match what is in the binding for that client or the configuration
>    information is not valid, the server sends a Reply message containing
>    a Status Code option with the value ConfNoMatch.

I think more text would be good to make it clear that a server can also
respond positively even if it can't verify the actual
binding. Likewise, a positive response in that case DOSE NOT actually
confirm the binding, just that the node hasn't moved.

Make sense?

> I think from:

> >if a client detects that it has
> >moved, then just have it redo the normal DHC exchanges.

> you mean keep movement detection but use different DHCP messages.  Exactly 
> what did you have in mind for "normal DHCP exchange"?

Renew, Rebind and or just restarting with solicit.

> The idea behind 
> Confirm is (as is the case in DHCPv4) if the client has not moved (e.g., 
> the client has been power cycled), the client uses a two message exchange 
> with any available server rather than a four message exchange.

This assumes that *any* server is in a position to respond
definitively. This is not immediately clear to me per above.

> Perhaps Confirm overlaps with Rapid Commit?

Hmm.

Thomas

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---

From: Thomas Narten <[email-masked]>
Date: Thu, 09 May 2002 09:42:14 -0400

> - Client can send a Confirm to help in movement detection. It
>    shouldn't be required for the client to do this since it may have
>   other means of doing this.

OK

> 1. If it has the binding for the client and the addresses are valid,
>it can send a Reply with the success indication. This would be the
>case if the client hasn't really moved.

OK

> 2. If it doesn't have a binding but in comparing the addresses
>provided in the Confirm, the server can determine that the prefixes
>for the addresses are NOT valid for the link, the server can Reply to
>the client that the addresses are bad.

I.e., with a NotOnLink.

> 3. The server discards the Confirm since it can not verify that the
>addresses are truely valid for the client.

Hmm. If a client has moved to a completely new site, will servers in
general respond "NotOnLink" or will they think "I don't know"? And if
the latter, and they don't respond, won't the client retransmit and
won't it take a while to time out? I would assume its important for
Confirm to be quick.

> (We could allow another Reply from the server that would indicate to
> the client that the prefixes appear to be valid, but it can't
> confirm the actual addresses. However, this isn't really necessary
> since no response to the Confirm essentially means that and it could
> give false information at times; that is why it would be best not to
> have servers do this.)

I agree its not necessary, so long as the semantics of the response
make it clear the client can only assume it hasn't moved and needs to
not change the state of its bindings in anyway (i.e., it should not
restart the T1/T2 timers, for example)

> - While I think servers SHOULD implement Confirm, they don't have
>to. If they don't, they won't enhance movement detection but since
>there are other mechanisms that can help with this, it is not
>required.

Hmm. If this message is OPTIONAL to implement, this needs to be made
clear in the spec. I assumed it was mandatory. I think it should be
mandatory. If it is not, there is little point in clients implementing
it. I.e., it doesn't make sense for clients to implement basic
features that servers aren't required to implement. This leads to bad
interoperability.

> - Confirm processing is much different than a Renew/Rebind. There is
>much less work involved on the server - it is only verifying the
>information. A Confirm can NOT be used to extend address
>lifetimes/leases. IMHO, it should only be used to help the client
>determine whether it moved.

OK.

Thanks,
Thomas

---

From: Ted Lemon <[email-masked]>
Date: Thu, 9 May 2002 11:47:19 -0500

> features that servers aren't required to implement. This leads to bad

This is a case where the market will quickly punish servers that don't 
implement it that should.   We aren't even insisting that address 
allocation is mandatory, AFAIK, so saying that confirm is mandatory seems 
inconsistent.   That is, it is fine to implement a DHCPv6 server that only 
responds to information requests and that does not allocate IP addresses.   
A DHCP server configured to just provide information might not even have 
enough information to determine whether or not a client is on the link on 
which its addresses are valid.