Network Working Group O. Troan Internet-Draft B. Volz Intended status: Informational cisco Expires: September 29, 2012 March 28, 2012 Issues with multiple stateful DHCPv6 options draft-troan-dhc-dhcpv6-stateful-issues-00.txt Abstract [RFC3315] was not written with the expectation that other stateful DHCPv6 options would be developed. [RFC3633] shoe-horned the new options for Prefix Delegation options for DHCPv6 into DHCPv6. Implementation experience of the CPE model described in [RFC6204] has shown multiple issues with the DHCPv6 protocol in supporting multiple stateful options. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 29, 2012. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as Troan & Volz Expires September 29, 2012 [Page 1] Internet-Draft Multiple Stateful Option March 2012 described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Handling of multiple IA_* options . . . . . . . . . . . . . . . 3 3.1. Advertisement message . . . . . . . . . . . . . . . . . . . 3 3.2. Placement of Status codes . . . . . . . . . . . . . . . . . 4 3.3. T1/T2 timers . . . . . . . . . . . . . . . . . . . . . . . 5 3.4. Confirm message . . . . . . . . . . . . . . . . . . . . . . 5 3.5. Release / Decline messages . . . . . . . . . . . . . . . . 6 3.6. Unanswered options . . . . . . . . . . . . . . . . . . . . 6 3.7. Multiple provisioning domains . . . . . . . . . . . . . . . 6 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . . 7 7.2. Informative References . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 Troan & Volz Expires September 29, 2012 [Page 2] Internet-Draft Multiple Stateful Option March 2012 1. Introduction [RFC3315] was not written with the expectation that other stateful DHCPv6 options would be developed. [RFC3633] shoe-horned the new options for Prefix Delegation options for DHCPv6 into DHCPv6. Implementation experience of the CPE model described in [RFC6204] has shown multiple issues with the DHCPv6 protocol in supporting multiple stateful options. This document describes a number of problems encountered when introducing multiple IA_ options into DHCP. The document also suggest solutions with suggested edits to the protocol specification. The intention of this work is to modify the DHCP protocol specification to support multiple IA_ options within a single DHCP session. This problem can also be solved by implementing a separate DHCP session (separate client state machine) per IA_ option. This latter approach has a number of issues. Addition DHCP protocol traffic, 'collisions' between stateless options also included with the IA_ options, divergence in that each IA_ option specification basically specifies its own version of the DHCP protocol... 2. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 3. Handling of multiple IA_* options DHCPv6 was written with the assumption that the only stateful options where for assigning addresses. DHCPv6 PD describes how to extend the DHCPv6 protocol to handle prefix delegation, but RFC3633 did not consider how DHCP address assignment and prefix delegation could co- exist. 3.1. Advertisement message RFC3315 specifies that a client must ignore an Advertise message if a server will not assign any addresses to a client. A client requesting both IA_NA and IA_PD, with a server that only offers one of them, is not supported in the current protocol specification. Proposed solution: a client should accept Advertise messages, even when not all IA_ options are being offered. A client should ignore an Advertise message when no IA_ options at all are being offered. Troan & Volz Expires September 29, 2012 [Page 3] Internet-Draft Multiple Stateful Option March 2012 Replace Section 17.1.3: (existing errata) The client MUST ignore any Advertise message that includes a Status Code option containing the value NoAddrsAvail, with the exception that the client MAY display the associated status message to the user. With: The client MUST ignore any IAs in an Advertise message that includes a Status Code option containing the value NoAddrsAvail, with the exception that the client MAY display the associated status message to the user. An Advertise message without any IA options MUST be ignored. It is important to note that the receipt of a Advertisement without any IA options, does not imply that the client should restart the Solicit retransmissions timers. Doing so would lead to a Solicit/ Advertisement storm. 3.2. Placement of Status codes In Reply messages IA specific status codes (NoAddrsAvail, NotOnlink, NoBinding) are encapsulated in the IA option. In Advertisement messages the Status code option with the NoAddrsAvail code is in the "global" scope. That makes sense when the failure case is fatal. With the introduction of multiple IA options, there might be a case where a server is not willing to offer addresses, but might be willing to offer other options. While a Status Code option is implicitly bound to a specific type of IA. E.g. NoPrefixAvail is only applicable to IA_PD, and NoAddrsAvail is only applicable to IA_NA/IA_TA, it may be problematic to make this assumption for all status codes. If so the Status Code must be embedded in the IA_ option for all DHCP messages. This makes Advertisement messages equal to Reply messages. Proposed solution: Embedding the Status code option in the specific IA. For backwards compatibility, the IA_NA status code can be kept in the global scope. Replace Section 17.2.2: (existing errata) Troan & Volz Expires September 29, 2012 [Page 4] Internet-Draft Multiple Stateful Option March 2012 If the server will not assign any addresses to any IAs in a subsequent Request from the client, the server MUST send an Advertise message to the client that includes only a Status Code option with code NoAddrsAvail and a status message for the user, a Server Identifier option with the server's DUID, and a Client Identifier option with the client's DUID. With: If the server will not assign any addresses to an IA in a subsequent Request from the client, the server MUST send an Advertise message to the client that includes the IA containing a Status Code option with status code NoAddrsAvail and a status message for the user, a Server Identifier option with the server's DUID, and a Client Identifier option with the client's DUID. The server SHOULD include other stateful IA options (like IA_PD) and other configuration options in the Advertise message. 3.3. T1/T2 timers The T1 and T2 timers determine when the client will contact the server to extend lifetimes of information received in an IA. How should a client handle the case where multiple IA_ options have different T1 and T2 timers? In a multiple IA_ options model, the T1/T2 timers are protocol timers, that should be independent of the IA_ options themselves. If we were to redo the DHCP protocol from scratch the T1/T2 timers should be carried in a separate DHCP option. Proposed solution: The server should set the T1/T2 timers in all IA_* to the same value. To deal with the case where servers have not yet been updated to do that, clients must use the shortest T1/T2 timer (larger than 0) in any IA_ option. Longer T1/T2 timers are ignored. 3.4. Confirm message The Confirm message is specific to address assignment. It lets a server without a binding to reply to the message, under the assumption that the server has knowledge of the prefix on the link, while not having the binding, can inform the client that the address is likely valid. This message is sent when e.g. the client has moved and needs to validate its addresses. Not all IA_ options can be validated by servers without the binding. Note: Confirm has a specific meaning and doesn't overload Renew/ Rebind. It also is lower processing cost as the server does NOT need Troan & Volz Expires September 29, 2012 [Page 5] Internet-Draft Multiple Stateful Option March 2012 to extend lease times or otherwise send back options. Proposed solution: Allow and specify the Confirm message for other IA options. It is encouraged to use Renew instead of Confirm. 3.5. Release / Decline messages A client can release any individual lease at any time. A client can get "back" a lease by using a Renew message. It MAY do this at any time, though must avoid creating a Renew storm. E.g. wait until T1. 3.6. Unanswered options If a client requests multiple IA_ options, but the server is willing to only offer a subset of them, the client could react in several ways. Reset the state machine and continue to send Solicit messages, create separate DHCP sessions for each IA and continue to Solicit for the missing options, or it could continue with the single session, and include the missing options on subsequent messages to the server. Proposed solution: the client should keep a single session with the server. The client should continue with the IA_ options received, while continuing to request the other IA options in subsequent messages to the server. That means continue to include the empty unanswered IAs in subsequent Renew and Rebind messages. For the IAs that the server has no binding for, it must reply using the same behaviour as for a Request message. That is with a "NoAddrsAvail" status. (As opposed to the currently specified NoBinding). This behaviour will not require the server to remember the IAs that it is not willing to server. I.e. the change is to allow the client to include IAs (bindings) in Renew/Rebind messages which it has not received (yet). 3.7. Multiple provisioning domains Out of scope for this set of proposed changes to RFC3315/RFC3633. 4. IANA Considerations This specification does not require any IANA actions. 5. Security Considerations There are no new security considerations pertaining to this document. Troan & Volz Expires September 29, 2012 [Page 6] Internet-Draft Multiple Stateful Option March 2012 6. Acknowledgements 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6", RFC 3633, December 2003. [RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O. Troan, "Basic Requirements for IPv6 Customer Edge Routers", RFC 6204, April 2011. 7.2. Informative References Authors' Addresses Ole Troan cisco Oslo, Norway Email: ot@cisco.com Bernie Volz cisco Out in the Woods somewhere, US of A Email: volz@cisco.com Troan & Volz Expires September 29, 2012 [Page 7]