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Re: comments on draft-ietf-shim6-proto-05.txt {1}


El 08/10/2006, a las 17:09, Deguang Le escribió:



I don't mean that there are any situation where we could have a locator
change and the usage of the Update message wouldn't apply. ^-^
I mean: could the usage of the Update message apply in the mobility
situation where we could have a locator change? :-)



i guess so...
i mean, in the mobility scenario, the host learns a new address from the visited network. This address can be used as a locator for established contexts. The result is that there is a new locator available for the shim context. If the host wants to use the new locator for the estbalished communication, it must add it to the existent locator set and in order to do that, it should use the Update message
But in any case, imho this falls in the general case where the host learns a new locator and wants to use it in the established contexts (similar to the case where a new prefix is added for a multihomed site)
I think it would be better if this draft could provide some examples (cases) about the locator change in real applications. For examples, I would like to know if the follwoing cases belong to the "locator change" situation described in this draft: -A SHIM6 host is assigned a new usable locator at some interface of the SHIM6 host.
We can add in the draft that a locator change is either to add new locators to the locator set or to remove locators from it. 
-A SHIM6 host finds one of its locators is unusable or failure.
Please note that when a host finds that one of its locators is temporarily unusable e.g. it is unreachable, or there is a local failure, it can use the Update message to inform about the situation or it can simply convey Locator Preference infromation. This lasst option is a cheaper option in terms of processing because of the security checks 

You are right, I agree with you. However, there still may be the case
where the shim6 host determines one of its locators is permanently
unusable. I think in this case it is better to remove the unusable
locator from the locator set. right? :-)



yes, this seems the most neat approach


By the way, I would like to discuss the efficiency of using the Locator
List option for the addition/deletion of locator to/from the locator set 
further.
Because this draft specifies that "ALL" the locators must be carried in
the Locator List option (see page42), I think it is not efficient to use 
the Locator List option in the case where only one locator needs to be
add or removed while other locators keep unchanged. I think it is not
necessary to update those unchanged locators and repetitively perform
locator verification for those unchanged locators each time when only a
locator changes. As you said , it is expensive option. Right? :-)
this issue was discussed during the design and there were two possible approaches: the atomic approach, where each update message contains all the locators and the diferential approach where each message contains the diff from the existent locator set. As you mention the differential approach is more efficient in terms of overhead. The problem with the differential approach is that is more complex because both peers need to be synchronized i.e. both ends need to have the same idea of what the current locator set is. If a message is lost and the synchronization between the peers is lost, then they will end up with different perceptions of what the locator set is. This must be avoided. If we want to avoid this we need to build a reliable mechanism for exchanging the update message and its associated complexity, and also how to deal if the peers end up out of sync. We decided that it was a reasonable tradeoff to use the atomic approach and we have more overhead but less complexity. 


If my above issue is reasonable, do you think it is possible to define
two new separate options (e.g. Locator Addtion option and Locator
Deletion option) for adding a new locator to the locator set on a SHIM6
context and for deleting a failured locator from the locator set on a
SHIM6 context.



see above...
If the above two examples belong to the "locator change" situation, then I would like to know if a SHIM6 host must (or can or need to) send the Locator Request message with Locator List option to its peers when the SHIM6 host is assigned a new usable locator at some interface of the SHIM6 host or when the SHIM6 host finds one of its locators is unusable or failure.
but this would be up to the host to decide based in its local policy rather than a protocol issue imho 
You are right, and I know what you mean. But, I'd only like to know,
from the functionality point of view, if the Update Request message with 
Locator List option "CAN" be used for this purpose when the above two
situations occur. :-)
From the your following explanations, I think you agree the Update
Request message with Locator List option "CAN" do this.
absolutely, this is possible and a shim6 host can use it in the way you propose
I mean if a host has a new locator it can either add it to all of its established contexts, or to some of them or none of them depending of local considerations. Moreover, a host does not need to add all its locator to all its shim contexts. for instance a host could have a set of locators tha it uses for certain communications (e.g. communications within its own site) and some other set of locators that it uses for other communications (e.g. intersite communications) All these cases are perfectly ok and the spec supports any behaviour that the host decides to choose A similar consideration applies to the case where the host looses on locator. Depending on the situation it can either rmove from the locator set using an Update message or it can mark it as broken using the preference option or it can even do nothing and the peer will determine that the locator pairs contianing this locator are unreachable and will not use them. All these approaches are possibel and they are supported in the spec. So, i am not sure we need to add additional constraints on how ths needs to be used, what do you think? 

Thank you again, I think your answer confirms my understanding.

This draft specifies the messages, which are used for multihoming, but
it does not clearly present (or explain) if these specifications could
be appled for mobilty usage.
right, but this is a multihoming solution.... whether this can or not be used for mobility is another issue and i think it is explicitly out of the scope of the wg (see the charter)
However, the shim6 protocol can work with mobility protocolos like MIP and their interaction is detailed in the applibility statement draft 



So, the reasion why I ask these questions and make these comments is
that I am not sure if SHIM6 can be used in the mobile environments where a host may change its attachment/location to Internet, namely change the 
locator (including locator additon and locator deletion).

In fact, the above two examples I described are exactly the situations
that appear in the mobile environments. To my understanding, the
specified locator update messages also can be used for mobility
scenario. Now your answer confirms my understanding. Right?
i think you could, but as i mention this is explicitly out of the scope of the wg 

Regards, marcelo





Regards, marcelo


Cheers,
Deguang


Geoff Huston schrieb:


Hi,
This note starts the WG Last Call for comments on the three "base" Shim6 documents: draft-ietf-shim6-proto-05.txt "Level 3 multihoming shim protocol" 
 draft-ietf-shim6-hba-01               "Hash Based Addresses (HBA)"
draft-ietf-shim6-failure-detection-06 "Failure Detection and Locator Pair Exploration Protocol for IPv6 
                                       Multihoming"
They can be found at:
   http://www.ietf.org/internet-drafts/draft-ietf-shim6-proto-05.txt
   http://www.ietf.org/internet-drafts/draft-ietf-shim6-hba-01.txt
http://www.ietf.org/internet-drafts/draft-ietf-shim6-failure- detection-06.txt Please review the documents carefully, and send your feedback to the SHIM6 list. Please also indicate whether or not you believe that these documents are ready to go to the IESG for publication as a set of Proposed Standards. This Working Group Last Call will end in two weeks, on the 12th October 2006 at 0800, UTC+10 
  Thanks,
        Geoff & Kurtis