[dhcwg] draft-troan-dhc-dhcpv6-stateful-issues-00
"Bernie Volz (volz)" <volz@cisco.com> Mon, 02 April 2012 15:56 UTC
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Date: Mon, 02 Apr 2012 10:56:08 -0500
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Thread-Topic: draft-troan-dhc-dhcpv6-stateful-issues-00
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From: "Bernie Volz (volz)" <volz@cisco.com>
To: dhcwg@ietf.org
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Subject: [dhcwg] draft-troan-dhc-dhcpv6-stateful-issues-00
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Ole submitted the draft-troan-dhc-dhcpv6-stateful-issues-00 draft just before the Paris IETF DHC WG meeting and I presented the http://www.ietf.org/proceedings/83/slides/slides-83-dhc-15.pdf slides at that meeting. Please look over the draft and slides as we are hoping to fast-track these RFC 3315/3633 changes for the RFC 6204bis work. In thinking about this some more since, I believe that encouraging use of Renew (section 3.4 and "Confirm / Renew Handling" slide) is not the best. This "Renew" was to follow the RFC 3633 (section 12.1) requirements: If such verification is needed the requesting router MUST initiate a Rebind/Reply message exchange as described in section 18.1.4, "Creation and Transmission of Rebind Messages" of RFC 3315, with the exception that the retransmission parameters should be set as for the Confirm message, described in section 18.1.2, "Creation and Transmission of Confirm Messages" of RFC 3315. The requesting router includes any IA_PDs, along with prefixes associated with those IA_PDs in its Rebind message. >From RFC 3315: The first Confirm message from the client on the interface MUST be delayed by a random amount of time between 0 and CNF_MAX_DELAY. The client transmits the message according to section 14, using the following parameters: IRT CNF_TIMEOUT MRT CNF_MAX_RT MRC 0 MRD CNF_MAX_RD CNF_MAX_DELAY 1 sec Max delay of first Confirm CNF_TIMEOUT 1 sec Initial Confirm timeout CNF_MAX_RT 4 secs Max Confirm timeout CNF_MAX_RD 10 secs Max Confirm duration Renew is effectively "unicast" to the server that granted the lease(s). Thus: 1. If the client has indeed moved to a new location, or 2. If the client has not moved, but that server is down, The client must always wait 10 seconds (CNF_MAX_RD) before transitioning to sending Rebinds before it can hope to get any feedback (if there is another server present). In many cases, perhaps other information (such as receipt of RAs) might already indicate to the client whether it has likely moved or not? In these cases either using the Confirm or immediately starting with a Rebind has a 10 second advantage. Note also that Confirm has the other advantages outlined in the presentation: 1. It is clear what the client is attempting to do as it doesn't overload the Renew/Rebind requests. 2. It has significant less load on the server since the server does not need to (potentially) extend lifetimes or otherwise record anything about the client/server interaction with respect to these leases (which eliminates any possibility of a write by the server which is generally expensive - especially in a situation where lots of clients might be doing this). The one disadvantage to Confirm is that it provides no useful information to any Relay Agent (CMTS in particular) that might be wanting to gleam information from the transaction. There may also be situations where the Confirm, if used, can't determine if the client has or has not moved in which case the client will get no response to its Confirm - RFC 3315, Section 18.1.2 then states the client SHOULD continue to use the previously obtained information. So what do others think? Do we not consider the 10 second delay to be significant and worth addressing? Do we consider the additional server cost of doing Renew/Rebind processing an issue? - Bernie
- [dhcwg] draft-troan-dhc-dhcpv6-stateful-issues-00 Bernie Volz (volz)