Wednesday, May 4, 2011

BRAS/Server initiated renewal for DHCPv6-PD leases - When?

One major issue when dealing with IPv6 CPEs is the currently missing capability to renew automatically the IPv6 addresses on the CPE's LAN after a disconnect/reconnect of the subscriber's dynamic session.

Although there are some tricks (#1, #2) for client (subscriber) initiated renewal, not all CPE vendors support those tricks. Also many times it is preferable to have the BRAS/BNG, or generally the ISPs, control this renewal, since all the AAA (and BSS/OSS) systems are usually managed by them.

The DHCPv6 "Reconfigure" message was made to help in the above case. According to RFC 3315:

RECONFIGURE (10) A server sends a Reconfigure message to a client to inform the client that the server has new or updated configuration parameters, and that the client is to initiate a Renew/Reply or Information-request/Reply transaction with the server in order to receive the updated information.
The client includes a Reconfigure Accept option if the client is willing to accept Reconfigure messages from the server.

It's obvious that without this support, a client must wait until it renews its lease to get configuration updates, which might be from some hours to many days. Btw, shouldn't the change of the WAN interface state on the CPE automatically cause the renewal of the delegated prefix on its LAN???

Also, according to the recently approved informational RFC 6204, the support of the DHCPv6 Reconfigure option is a MUST for IPv6 CPEs.

WAA-4: The IPv6 CE router MUST be able to support the following DHCPv6 options: IA_NA, Reconfigure Accept and DNS_SERVERS.

Now, someone malicious might translate the above "MUST be able to support" phrase into "ok, it's not actually required to support it now, but you must be able to support it in the future". It definitely would be better to have it as "MUST support".

A recent "IPv6 CE Router Interoperability Whitepaper" from UNH-IOL shows that none of the CPEs that were tested, supported this option.

The last issue discovered during the testing was IPv6 CE router lack of support for DHCP Reconfigure. According to draft-ietf-v6ops-ipv6-cpe-router-09, “WAA-4: The IPv6 CE router MUST be able to support the following DHCPv6 options: IA_NA, Reconfigure Accept [RFC3315], DNS_SERVERS [RFC3646].” Therefore the IPv6 CE routers should have included the Reconfigure Accept in DHCPv6 Request or Solicit messages.

It gets a little bit more complicated, if you check what RFC 3633 says about the "Reconfigure" message when it is used for Prefix Delegation:

13.1. Delegating Router behavior

The delegating router initiates a configuration message exchange with a requesting router, as described in section 19, "DHCP Server-Initiated Configuration Exchange" of RFC 3315, by sending a Reconfigure message (acting as a DHCP server) to the requesting router, as described in section 19.1, "Server Behavior" of RFC 3315. The delegating router specifies the IA_PD option in the Option Request option to cause the requesting router to include an IA_PD option to obtain new information about delegated prefix(es).

13.2. Requesting Router behavior

The requesting router responds to a Reconfigure message, acting as a DHCP client, received from a delegating router as described in section 19.4, "Client Behavior" of RFC 3315. The requesting router MUST include the IA_PD Prefix option(s) (in an IA_PD option) for prefix(es) that have been delegated to the requesting router by the delegating router from which the Reconfigure message was received.

So, if someone claims support of the "Reconfigure" option, where does it refer to? DHCPv6 or DHCPv6-PD? What about Relay?

On the server side, Juniper MX series already support it (it's called "dynamic reconfiguration for DHCPv6"), but Cisco ASR1k doesn't. Cisco CNR 7.x also supports it, so does (or will) Dibbler 0.8.0. ISC DHCPv6 server and Windows Server 2008 probably don't.


  • Our experience with IPv6 CPEs until now is disappointing on this matter. Although we have feedback from various CPE vendors that they will support it, none of them actually supports it now.
  • Wouldn't it be interesting to have the "Reconfigure" message be sent by the BRAS/BNG DHCPv6 server to the client, when the router receives a Radius CoA (RFC 3576) packet for this specific subscriber?

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