Network Working Group O. Troan
Internet-Draft B. Volz
Updates: 3315,3633 (if approved) Cisco Systems, Inc.
Intended status: Standards Track January 1, 2014
Expires: July 5, 2014
Issues with multiple stateful DHCPv6 options
draft-ietf-dhc-dhcpv6-stateful-issues-05.txt
Abstract
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) was not written
with the expectation that additional stateful DHCPv6 options would be
developed. IPv6 Prefix Options for Dynamic Host Configuration
Protocol (DHCP) version 6 shoe-horned the new options for Prefix
Delegation 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
updates RFC3315 and indirectly RFC3633.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on July 5, 2014.
Copyright Notice
Copyright (c) 2014 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
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Handling of multiple IA options types . . . . . . . . . . . . 3
4.1. Advertise Message . . . . . . . . . . . . . . . . . . . . 4
4.2. Placement of Status Codes . . . . . . . . . . . . . . . . 5
4.3. T1/T2 Timers . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Renew Message . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Rebind Message . . . . . . . . . . . . . . . . . . . . . 7
4.6. Confirm Message . . . . . . . . . . . . . . . . . . . . . 8
4.7. Release Message . . . . . . . . . . . . . . . . . . . . . 9
4.8. Multiple Provisioning Domains . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
DHCPv6 [RFC3315] was not written with the expectation that additional
stateful DHCPv6 options would be developed. DHCPv6 Prefix Delegation
[RFC3633] shoe-horned the new options for Prefix Delegation 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 with
multiple IA option types into DHCP and recommended changes to the
DHCPv6 protocol specifications.
The intention of this work is to modify the DHCP protocol
specification to support multiple IA option types within a single
DHCP session. This problem can also be solved by implementing a
separate DHCP session (separate client state machine) per IA option
type. This latter approach has a number of issues: additional DHCP
protocol traffic, 'collisions' between stateless options also
included with the IA options, divergence in that each IA option type
specification specifies its 'own' version of the DHCP protocol.
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The changes described in this document will be incorporated in a new
revision of the DHCPv6 protocol specification [RFC3315].
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. Terminology
Stateful options Options that require dynamic binding
state per client on the server.
Identity association (IA): A collection of stateful options assigned
to a client. Each IA has an associated
IAID. A client may have more than one IA
assigned to it; for example, one for each
of its interfaces. Each IA holds one
type of IA option; for example, an
identity association for temporary
addresses (IA_TA) holds temporary
addresses (see "identity association for
temporary addresses"). Throughout this
document, "IA" is used to refer to an
identity association without identifying
the type of stateful option in the IA.
4. Handling of multiple IA options types
DHCPv6 was written with the assumption that the only stateful options
were 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.
If a client requests multiple IA option types, but the server is
configured 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 option type and
continue to Solicit for the unfulfilled IA options, or it could
continue with the single session, and include the unfulfilled IA
options on subsequent messages to the server.
Proposed solution: the client should keep a single session with the
server and include the missing options on subsequent messages
(Request, Renew, and Rebind) to the server.
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4.1. Advertise 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 option types are being offered. A client SHOULD
ignore an Advertise message when no bindings at all are being
offered. The client SHOULD include the not offered IA option types
in its Request.
Replace Section 17.1.3 of [RFC3315]: (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(s) to the
user.
With:
The client MUST ignore any Advertise message that contains no
bindings (if only IA_NA and/or IA_TA options were requested,
this is a message that includes a Status Code option containing the
value NoAddrsAvail), with the exception that the client MAY display
the associated status message(s) to the user.
And, replace:
- The client MAY choose a less-preferred server if that server
has a better set of advertised parameters, such as the
available addresses advertised in IAs.
With:
- The client MAY choose a less-preferred server if that server
has a better set of advertised parameters, such as the
available options advertised in IAs.
It is important to note that the receipt of a Advertisement without
any bindings does not imply that the client should restart the
Solicit retransmissions timers. Doing so would lead to a Solicit/
Advertisement storm.
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4.2. Placement of Status Codes
In Reply messages IA specific status codes (i.e., NoAddrsAvail,
NotOnlink, NoBinding, NoPrefixAvail) 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
option types, there might be a case where a server is not willing to
offer addresses, but might be willing to offer other stateful option
types.
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. Ideally the Status Code option
should be encapsulated in the IA option for all DHCP messages. This
makes Advertisement messages equal to Reply messages.
Proposed solution: No change. For backwards compatibility, the
NoAddrsAvail Status Code option when no addresses are available will
be kept in the global scope for Advertise messages. Other IA option
types MUST encapsulate the Status Code option within the IA option.
To clarify further: when a client requests both IA_NA and IA_PD, and
the server can offer IA_PD but not IA_NA, the server sends an
Advertise response containing no IA_NA option, a status code option
of NoAddrsAvail, and one or more IA_PD options containing IAPREFIX
options.
4.3. 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 option types 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
options in Reply and Advertise messages to the same value. To deal
with the case where servers have not yet been updated to do that,
clients MUST use the shortest (explicit or implicit) T1/T2 timer
(larger than 0) in any IA options in the Reply. Longer T1/T2 timers
are ignored.
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4.4. Renew Message
The Renew message, as described in [RFC3315], allows a client to only
renew bindings assigned via a Request message.
In a multiple IA option type model, the Renew does not support the
ability for the client to renew one IA option type while requesting
bindings for other IA option types that were not available when the
client sent the Request.
Proposed solution: The client should continue with the IA options
received, while continuing to include the other IA options in
subsequent messages to the server. The client and server processing
need to be modified. Note that this change makes the server's IA
processing of Renew and Rebind similar to the Request processing.
Replace Section 18.1.3 of [RFC3315]:
At time T1 for an IA, the client initiates a Renew/Reply message
exchange to extend the lifetimes on any addresses in the IA. The
client includes an IA option with all addresses currently assigned
to the IA in its Renew message.
With:
At time T1 for an IA, the client initiates a Renew/Reply message
exchange to extend the lifetimes on any addresses in the IA. The
client includes an IA option with all addresses currently assigned
to the IA in its Renew message. The client also includes an IA
option for each binding it desires but has been unable to obtain.
Replace Section 18.2.3 of [RFC3315]:
If the server cannot find a client entry for the IA the server
returns the IA containing no addresses with a Status Code option
set to NoBinding in the Reply message.
With:
If the server cannot find a client entry for the IA the server
creates the bindings for that client according to the server's
policy and configuration information and records the IAs and
other information requested by the client.
Note that clients that communicate with servers that do not support
this updated Renew processing will receive the NoBinding status for
the IA which had no bindings. The client MUST continue to process
the other IAs in the Reply. The client MAY attempt a Solicit/
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Advertise/Request/Reply sequence periodically to obtain bindings for
these IAs. However, it MUST limit the frequency at which is does
this to no more often than the renewal frequency.
4.5. Rebind Message
The Rebind message, as described in [RFC3315] does not explicitly
specify what a server should do when an IA option which contains no
addresses is present.
Replace Section 18.1.4 of [RFC3315]:
At time T2 for an IA (which will only be reached if the server to
which the Renew message was sent at time T1 has not responded), the
client initiates a Rebind/Reply message exchange with any available
server. The client includes an IA option with all addresses
currently assigned to the IA in its Rebind message.
With:
At time T2 for an IA (which will only be reached if the server to
which the Renew message was sent at time T1 has not responded), the
client initiates a Rebind/Reply message exchange with any available
server. The client includes an IA option with all addresses
currently assigned to the IA in its Rebind message. The client
also includes an IA option for each binding it desires but has been
unable to obtain.
Replace Section 18.2.4 of [RFC3315] with the following text to
clarify how the server should handle all of the possible conditions:
When the server receives a Rebind message that contains an IA
option from a client, it locates the client's binding and verifies
that the information in the IA from the client matches the
information stored for that client.
If the server finds the addresses in the IA for the client and the
server determines that the addresses in the IA are appropriate for
the link to which the client's interface is attached according to
the server's explicit configuration information, the server SHOULD
send back the IA to the client with new lifetimes and T1/T2 times.
If the server cannot find a client entry for the IA and the server
determines that the addresses in the IA are not appropriate for the
link to which the client's interface is attached according to the
server's explicit configuration information, the server MAY send a
Reply message to the client containing the client's IA, with the
lifetimes for the addresses in the IA set to zero. This Reply
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constitutes an explicit notification to the client that the
addresses in the IA are no longer valid. In this situation, if the
server does not send a Reply message it silently discards the
Rebind message.
If the server cannot find a client entry for the IA and the server
determines that the addresses in the IA are appropriate for the
link to which the client's interface is attached according to the
server's explicit configuration information, and the addresses are
not in use, the server MAY assign the addresses to the client and
send a Reply message to the client with new lifetimes and T1/T2
times for the bindings. If the server cannot assign the addresses
to the client, the server returns the IA containing no addresses
with a Status Code option set to NoBinding in the Reply message.
Note: A server SHOULD NOT provide additional bindings to the client
as the client could accept a Reply from a different server (this is
the same issue as in the Discussion under the Rapid Commit option,
see section 22.14).
The server constructs a Reply message by setting the "msg-type"
field to REPLY, and copying the transaction ID from the Rebind
message into the transaction-id field.
The server MUST include a Server Identifier option containing the
server's DUID and the Client Identifier option from the Rebind
message in the Reply message.
The server includes other options containing configuration
information to be returned to the client as described in section
18.2.
4.6. Confirm Message
The Confirm message, as described in [RFC3315], is specific to
address assignment. It allows a server without a binding reply to
the message, under the assumption that the server only needs
knowledge about the prefix(es) on the link, to inform the client that
the address is likely valid or not. This message is sent when e.g.
the client has moved and needs to validate its addresses. Not all
bindings can be validated by servers and the Confirm message provides
for this by specifying that a server that is unable to determine the
on-link status MUST NOT send a Reply.
Note: Confirm has a specific meaning and does not overload Renew/
Rebind. It also is lower processing cost as the server does NOT need
to extend lease times or otherwise send back other configuration
options.
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Proposed solution: Allow and specify the Confirm message for other IA
option types. The server performs the same test as for addresses on
the delegated prefixes (see [RFC3315], section 18.2.2).
Replace Section 12.1 of [RFC3633]:
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.
...
The Confirm and Decline message types are not used with Prefix
Delegation.
With:
If such verification is needed the requesting router MUST initiate
a Confirm message exchange as 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 Confirm message.
...
The Decline message type is not used with Prefix Delegation.
4.7. Release Message
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.
4.8. Multiple Provisioning Domains
This document has assumed that all DHCP servers on a network are in a
single provisioning domain and thus should be "equal" in the service
that they offer. This was also assumed by [RFC3315] and [RFC3633].
One could envision a network where the DHCP servers are in multiple
provisioning domains, and it may be desirable to have the DHCP client
obtain different IA types from different provisioning domains. How a
client detects the multiple provisioning domains and how it would
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interact with the multiple servers in these different domains is
outside the scope of this document.
5. IANA Considerations
This specification does not require any IANA actions.
6. Security Considerations
There are no new security considerations pertaining to this document.
7. Acknowledgements
8. References
8.1. 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.
8.2. Informative References
[RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O.
Troan, "Basic Requirements for IPv6 Customer Edge
Routers", RFC 6204, April 2011.
Authors' Addresses
Ole Troan
Cisco Systems, Inc.
Philip Pedersens vei 20
N-1324 Lysaker
Norway
Email: ot@cisco.com
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Bernie Volz
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
USA
Email: volz@cisco.com
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