DHC Working Group                                               Ted Lemon
INTERNET DRAFT                                                    Nominum
Expires: August 2004                                      Bill Sommerfeld
Internet Draft                                           Sun Microsystems
Document: <draft-ietf-dhc-3315id-for-v4-02.txt>
Category: Standards Track                                  February, 2004

             Node-Specific Client Identifiers for DHCPv4

Status of this Memo

   This document is a submission by the Dynamic Host Configuration
   Working Group of the Internet Engineering Task Force (IETF). Comments
   should be submitted to the dhcwg@ietf.org mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   This document specifies the format that is to be used for encoding
   DHCPv4 [RFC2131 and RFC2132] client identifiers, so that those
   identifiers will be interchangeable with identifiers used in the
   DHCPv6 protocol [RFC3315].


   This document specifies the way in which DHCPv4 clients should
   identify themselves.  DHCPv4 client implementations that conform to
   this specification use a DHCPv6-style DHCP Unique Identifier (DUID)
   encapsulated in a DHCPv4 client identifier option.   This supersedes
   the behaviour specified in RFC2131 and RFC2132.

   The reason for making this change is that as we make the transition
   from IPv4 to IPv6, there will be network devices that must use both
   DHCPv4 and DHCPv6.  Users of these devices will have a smoother
   network experience if the devices identify themselves consistently,
   regardless of the version of DHCP they are using at any given
   moment.  Most obviously, DNS updates made by the DHCP server on

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   behalf of the client will not be handled correctly.  This change
   also addresses certain limitations in the functioning of
   RFC2131/2132-style DHCP client identifiers.

   This document first describes the problem to be solved.  It then
   states the new technique that is to be used to solve the problem.
   Finally, it describes the specific changes that one would have to
   make to RFC2131 and RFC2132 in order for those documents not to
   contradict what is described in this document.

1.0 Applicability

   This document updates RFC2131 and RFC2132.  DHCPv4 servers
   implementations SHOULD conform to this document.  DHCPv4 clients on
   network devices that are expected to support DHCPv6 in the future
   SHOULD conform to this document.  This document makes no changes to
   the behavior of DHCPv6 clients or servers.

   DHCPv4 clients and servers that are implemented according to this
   document should be implemented as if the changes specified in
   section 4.3 and 4.4 have been made to RFC2131 and RFC2132.

2.0 Problem Statement

2.1. Client identities are ephemeral

   RFC2132 recommends that client identifiers be generated by using
   the permanent link-layer address of the network interface that the
   client is trying to configure.  In cases where a network interface
   is removed from the client computer and replaced with a different
   network interface with a different permanent link-layer address,
   the identity of the client changes.  The client loses its IP
   address and any other resources associated with its old identifier
   - for example, its domain name as published through the DHCP

2.2. Clients can accidentally present multiple identities

   Consider a DHCP client that has two network interfaces, one of
   which is wired and one of which is wireless.  There are three
   interesting cases here:

   (a) Each network interface is attached to a different link.
   (b) Both network interface are connected to the same link.
   (c) Only one network interface is attached to a link.

   Case (a) is problematic, and is beyond the scope of this document.
   Briefly, in case (a), there is no obvious way to choose which of
   the two network interfaces represents the published identity of the
   client, and since the two network interfaces are connected to
   different network links, this could make a significant
   difference.   Also, if, as is likely, the two devices use two
   different identifiers, but wish to be identified as the same

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   client in the sense of the domain name on which their A record is
   published, they will compete for which interface identity gets the
   single available published identity, and there is no obvious way
   to write a DHCP client that produces the right result.

   Cases (b) and (c) are very common in practice, because many
   devices such as laptop computers that are popular at the time of
   this writing have both wireless and wired network interfaces that
   are installed simultaneously.   Both wired and wireless have
   advantages - wired has the advantage of speed, and wireless the
   advantage of mobility.

   So it seems likely that there will be devices that are in states
   (b) and (c) frequently, and indeed frequently make transitions
   between these states.   If the DHCP client that configures these
   devices uses the link-layer address of each device as an
   identifier, the two devices will appear to the DHCP server to be
   different nodes, and thus will be assigned different IP addresses.

   As in state (a), in state (b) only one of the two devices will be
   be able to acquire the public identity of the client, although this
   is less of a problem in case (b) because both interfaces are at
   least connected to the same network link.  Furthermore, if a device
   in state (b) makes the transition to state (c), it is quite
   possible that the lease for the device that has lost connectivity
   will remain valid for some time.  If the public identity of the
   client is associated with this now-defunct interface the client
   will not be reachable through its published domain name.

2.3. RFC2131/2132 and RFC3315 identifiers are incompatible

   The 'client identifier' option is used by DHCP clients and servers
   to identify clients.  In some cases, the value of the 'client
   identifier' option is used to mediate access to resources (for
   example, the client's domain name, as published through the DHCP
   server).  RFC2132 and RFC3315 specify different methods for
   deriving client identifiers.  These methods guarantee that the
   DHCPv4 and DHCPv6 identifier will be different.  This means that
   mediation of access to resources using these identifiers will not
   work correctly in cases where a node may be configured using DHCPv4
   in some cases and DHCPv6 in other cases.

2.4. RFC2131 does not require the use of a client identifier

   RFC2131 allows the DHCP server to identify clients either by
   using the client identifier option sent by the client, or, if the
   client did not send one, the client's link-layer address.   Like
   the client identifier format recommended by RFC2131, this suffers
   from the problems previously described in (2) and (3).

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3. Solutions

   The solution to problem 2.1 is to use a DHCP client identifier
   that is persistent - not tied to a particular piece of removable
   network hardware.  Then, when network hardware is swapped in and
   out, the client identifier does not change, and thus the client has
   a consistent IP address and consistent use of whatever resources
   have been associated with its identifier.

   This creates a new problem in case 2.2, however: if the two
   network interfaces are connected to the same link and use the same
   identifier, then the server's IP address assignment algorithm will
   assign the same IP address to both interfaces.   But if the DHCP
   client state machines configuring the two interfaces are
   sufficiently out of sync, the DHCPDISCOVER from the slower
   interface may be sent after the DHCPACK for the faster
   interface.   In this case, the DHCP server will detect a conflict
   and abandon the IP address, because the faster interface is
   responding to ICMP echo requests.   So we can't just use the same
   identifier on every interface.

   The solution to problem 2.3 is to use the DHCP Unique Identifier as
   defined in RFC3315 as a client identifier.  The DUID provides
   several different ways of producing persistent DHCP client
   identifiers, at least one of which is likely to be appropriate for
   any particular sort of network device.  This is also a valid way of
   addressing problem 2.1.

   To finish addressing problem 2.2, we modify the solution slightly.
   In addition to the DUID, RFC3315 defines an Identity Association ID
   (IAID).  The IAID, in combination with the DUID, identifies a
   particular identity with which to associate an IP address.  So a
   DHCP client has a single DUID, but has one IAID for each interface.
   The DHCP server associates IP addresses with the combination of
   (DUID, IAID), but uses the DUID to identify the client as a whole.

   The solution to problem (2.4) is to deprecate the use of the
   contents of the chaddr field in the DHCP packet as a means of
   identifying the client.

4. Implementation Requirements

   Here we specify changes to the behavior of DHCP clients and
   servers.   We also specify changes to the wording in RFC2131 and
   RFC2132.   DHCP clients, servers and relay agents that conform to
   this specification must implement RFC2131 and RFC2132 with the
   wording changes specified in sections 4.3 and 4.4.

4.1. DHCP Client behavior

   DHCP clients conforming to this specification MUST use stable DHCP
   node identifiers in the dhcp-client-identifier option.  DHCP
   clients MUST NOT use client identifiers based solely on layer two

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   addresses that are hard-wired to the layer two device (e.g., the
   ethernet MAC address) as suggested in RFC2131, except as allowed in
   section 9.2 of RFC3315.  DHCP clients MUST send a 'client
   identifier' option containing an Identity Association Unique
   Identifier, as defined in section 10 of RFC3315, and a DHCP Unique
   Identifier, as defined in section 9 of RFC3315.   These together
   constitute an RFC3315-style binding identifier.

   The general format of the DHCPv4 'client identifier' option is
   defined in section 9.14 of RFC2132.

   To send an RFC3315-style binding identifiier in a DHCPv4 'client
   identifier' option, the type of the 'client identifier' option is
   set to 255.  The type field is immediately followed by the IAID,
   which is an opaque 32-bit quantity.  The IAID is immediately
   followed by the DUID, which consumes the remaining contents of the
   'client identifier' option.   The format of the 'client
   identifier' option is as follows:

      Code  Len  Type  IAID                DUID
      | 61 | n  | 255 | i1 | i2 | i3 | i4 | d1 | d2 |...

   Any DHCPv4 or DHCPv6 client that conforms to this specification
   SHOULD provide a means by which an operator can learn what DUID the
   client has chosen.  Such clients SHOULD also provide a means by
   which the operator can configure the DUID.  A device that is
   normally configured with both a DHCPv4 and DHCPv6 client SHOULD
   automatically use the same DUID for DHCPv4 and DHCPv6 without any
   operator intervention.

   DHCP clients that support more than one network interface SHOULD
   use the same DUID on every interface.  DHCP clients that support
   more than one network interface SHOULD use a different IAID on
   each interface.

4.2. DHCPv4 Server behavior

   This document does not require any change to DHCPv4 or DHCPv6
   servers that follow RFC2131 and RFC2132.  However, some DHCPv4
   servers can be configured not to conform to RFC2131 and RFC2131, in
   the sense that they ignore the 'client identifier' option and use
   the client's hardware address instead.

   DHCP servers that conform to this specification MUST use the
   'client identifier' option to identify the client if the client
   sends it.

   DHCP servers MAY use administrator-supplied values for chaddr and
   htype to identify the client in the case where the administrator is
   assigning a fixed IP address to the client, even if the client
   sends an client identifier option.  This is ONLY permitted in the

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   case where the DHCP server administrator has provided the values
   for chaddr and htype, because in this case if it causes a problem,
   the administrator can correct the problem by removing the offending
   configuration information.

4.3. Changes from RFC2131

   In section 2 of RFC2131, on page 9, the text that reads "; for
   example, the 'client identifier' may contain a hardware address,
   identical to the contents of the 'chaddr' field, or it may contain
   another type of identifier, such as a DNS name" is deleted.

   In section 4.2 of RFC2131, the text "The client MAY choose to
   explicitly provide the identifier through the 'client identifier'
   option.  If the client supplies a 'client identifier', the client
   MUST use the same 'client identifier' in all subsequent messages,
   and the server MUST use that identifier to identify the client.  If
   the client does not provide a 'client identifier' option, the
   server MUST use the contents of the 'chaddr' field to identify the
   client." is replaced by the text "The client MUST explicitly
   provide a client identifier through the 'client identifier'

   In the same section, the text "Use of 'chaddr' as the client's
   unique identifier may cause unexpected results, as that identifier
   may be associated with a hardware interface that could be moved to
   a new client.  Some sites may choose to use a manufacturer's serial
   number as the 'client identifier', to avoid unexpected changes in a
   clients network address due to transfer of hardware interfaces
   among computers.  Sites may also choose to use a DNS name as the
   'client identifier', causing address leases to be associated with
   the DNS name rather than a specific hardware box." is replaced by
   the text "The DHCP client MUST NOT rely on the 'chaddr' field to
   identify it."

   In section 4.4.1 of RFC2131, the text "The client MAY include a
   different unique identifier" is replaced with "The client MUST
   include a unique identifier".

   In sections 3.1, item 4 and 6, 3.2 item 3 and 4, and 4.3.1, where
   RFC2131 says that 'chaddr' may be used instead of the 'client
   identifier' option, the text that says "or 'chaddr'" and "'chaddr'
   or" is deleted.

   Note that these changes do not relieve the DHCP server of the
   obligation to use 'chaddr' as an identifier if the client does not
   send a 'client identifier' option.  Rather, they oblige clients
   that conform with this document to send a 'client identifier'
   option, and not rely on 'chaddr' for identification.  DHCP servers
   MUST use 'chaddr' as an identifier in cases where 'client
   identifier' is not sent, in order to support old clients that do
   not conform with this document.

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4.4. Changes from RFC2132

   The text in section 9.14, beginning with "The client identifier MAY
   consist of" through "that meet this requirement for uniqueness." is
   replaced with "the client identifier consists of a type field whose
   value is normally 255, followed by a two-byte type field, followed
   by the contents of the identifier.  The two-byte type field and the
   format of the contents of the identifier are defined in RF3315,
   section 9."  The text "its minimum length is 2" in the following
   paragraph is deleted.

5. Security Considerations

   This document raises no new security issues.  Potential exposure to
   attack in the DHCPv4 protocol are discussed in section 7 of the
   DHCP protocol specification [RFC2131] and in Authentication for
   DHCP messages [RFC3118].  Potential exposure to attack in the
   DHCPv6 protocol is discussed in section 23 of RFC3315.

6. IANA Considerations

   This document defines no new name spaces that need to be
   administered by the IANA.  This document deprecates all 'client
   identifier' type codes other than 255, and thus there is no need
   for the IANA to track possible values for the type field of the
   'client identifier' option.

7. Normative References

   [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
             March 1997.
   [RFC2132] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor
             Extensions", RFC2132, March, 1997
   [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
             Carney, M., "Dynamic Host Configuration Protocol for
             IPv6 (DHCPV6)", July, 2003

8. Informative References

   [RFC3118] Droms, R., Arbaugh, W., "Authentication for DHCP
             Messages", RFC3118, June, 2001

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Author's Addresses

   Ted Lemon
   2385 Bay Road
   Redwood City, CA 94063 USA
   +1 650 381 6000

   Bill Sommerfeld
   Sun Microsystems
   1 Network Drive
   Burlington, MA 01824
   +1 781 442 3458

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