[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 RFC 3315

Internet Engineering Task Force                                 J. Bound
INTERNET DRAFT                                   Digital Equipment Corp.
DHC Working Group                                             C. Perkins
Obsoletes:  draft-ietf-dhc-dhcpv6-09.txt                Sun Microsystems
                                                             26 May 1997


         Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
                      draft-ietf-dhc-dhcpv6-10.txt


Status of This Memo

   This document is a submission to the DHC Working Group of the
   Internet Engineering Task Force (IETF). Comments should be submitted
   to the dhcp-v6@bucknell.edu mailing list.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   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.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

   Distribution of this document is unlimited.


Abstract

   The Dynamic Host Configuration Protocol (DHCPv6) provides a framework
   for passing configuration information, via extensions, to IPv6 nodes.
   It offers the capability of automatic allocation of reusable network
   addresses and additional configuration flexibility.  This protocol
   should be considered a stateful counterpart to the IPv6 Stateless
   Address Autoconfiguration protocol specification.










Bound, Perkins             Expires 26 November 1997             [Page i]


Internet Draft                DHCP Version 6                 26 May 1997




                                Contents



Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Terminology and Definitions                                        1
     2.1. IPv6 Terminology  . . . . . . . . . . . . . . . . . . . .    2
     2.2. DHCPv6 Terminology  . . . . . . . . . . . . . . . . . . .    3
     2.3. Specification Language  . . . . . . . . . . . . . . . . .    4

 3. Protocol Design Model                                              4
     3.1. Design Goals  . . . . . . . . . . . . . . . . . . . . . .    4
     3.2. DHCP Messages . . . . . . . . . . . . . . . . . . . . . .    6
     3.3. Request/Response Processing Model . . . . . . . . . . . .    7

 4. DHCP Message Formats and Field Definitions                         8
     4.1. DHCP Solicit Message Format . . . . . . . . . . . . . . .    8
     4.2. DHCP Advertise Message Format . . . . . . . . . . . . . .    9
     4.3. DHCP Request Message Format . . . . . . . . . . . . . . .   10
     4.4. DHCP Reply Message Format . . . . . . . . . . . . . . . .   12
     4.5. DHCP Release Message Format . . . . . . . . . . . . . . .   13
     4.6. DHCP Reconfigure Message Format . . . . . . . . . . . . .   14

 5. DHCP Client Considerations                                        15
     5.1. Verifying Resource Allocations After Restarts . . . . . .   15
     5.2. Sending DHCP Solicit Messages . . . . . . . . . . . . . .   16
     5.3. Receiving DHCP Advertise Messages . . . . . . . . . . . .   17
     5.4. Sending DHCP Request Messages . . . . . . . . . . . . . .   17
     5.5. Receiving DHCP Reply Messages . . . . . . . . . . . . . .   18
     5.6. Sending DHCP Release Messages . . . . . . . . . . . . . .   19
     5.7. Receiving DHCP Reconfigure Messages . . . . . . . . . . .   19

 6. DHCP Server Considerations                                        20
     6.1. Receiving DHCP Solicit Messages . . . . . . . . . . . . .   21
     6.2. Sending DHCP Advertise Messages . . . . . . . . . . . . .   21
     6.3. DHCP Request and Reply Message Processing . . . . . . . .   21
           6.3.1. Processing for Extensions to DHCP Request and Reply
                          Messages . . . . . . . . . . . . . . . . .  22
           6.3.2. Client Requests to Deallocate Unknown Resources .   23
     6.4. Receiving DHCP Release Messages . . . . . . . . . . . . .   23
     6.5. Sending DHCP Reconfigure Messages . . . . . . . . . . . .   24



Bound, Perkins            Expires 26 November 1997             [Page ii]


Internet Draft                DHCP Version 6                 26 May 1997


 7. DHCP Relay Considerations                                         24
     7.1. DHCP Solicit and DHCP Advertise Message Processing  . . .   24
     7.2. DHCP Request Message Processing . . . . . . . . . . . . .   25
     7.3. DHCP Reply Message Processing . . . . . . . . . . . . . .   26

 8. Retransmission and Configuration Variables                        26

 9. Security Considerations                                           29

10. Acknowledgements                                                  29

 A. Related Work in IPv6                                              29

 B. Comparison between DHCPv4 and DHCPv6                              31

Chair's Address                                                       36

Author's Address                                                      36

































Bound, Perkins            Expires 26 November 1997            [Page iii]


Internet Draft                DHCP Version 6                 26 May 1997


1. Introduction

   The Dynamic Host Configuration Protocol (DHCPv6, or in this
   document usually DHCP) provides configuration parameters to Internet
   nodes.  DHCP consists of a protocol for delivering node-specific
   configuration parameters from a DHCP server to a client, and a
   mechanism for allocation of network addresses and other related
   parameters to IPv6 [6] nodes.

   DHCP is built on a client-server model, where designated DHCP servers
   allocate network addresses and automatically deliver configuration
   parameters to dynamically configurable clients.  Throughout the
   remainder of this document, the term "server" refers to a node
   providing initialization parameters by way of the DHCP protocol,
   and the term "client" refers to a node requesting initialization
   parameters from a DHCP server.

   DHCPv6 uses Request and Reply messages to support a client/server
   processing model whereby both client and server are assured that
   requested configuration parameters have been received and accepted
   by the client.  DHCP supports optional configuration parameters and
   processing for nodes through extensions described in its companion
   document ``Extensions for the Dynamic Host Configuration Protocol for
   IPv6'' [11].

   The IPv6 Addressing Architecture [8] and IPv6 Stateless Address
   Autoconfiguration specifications provide new features not available
   in IP version 4 (IPv4) [14], which are used to simplify and
   generalize the operation of DHCP clients.

   Section 2 provides definitions for terminology used throughout
   this document.  Section 3 provides an overview of the protocol
   design model that guided the design choices in the specification;
   section 3.2 briefly describes the protocol messages and their
   semantics.  Section 4 provides the message formats and field
   definitions used for each message.  Sections 5,  6, and  7 specify
   how clients, servers, and relays interact.  Appendix A summarizes
   related work in IPv6 that will provide helpful context; it is not
   part of this specification, but included for informational purposes.
   Appendix B discusses the differences between DHCPv4 and DHCPv6.


2. Terminology and Definitions

   Relevant terminology from the IPv6 Protocol [6], IPv6 Addressing
   Architecture [8], and IPv6 Stateless Address Autoconfiguration [15]
   will be provided, and then the DHCPv6 terminology.




Bound, Perkins             Expires 26 November 1997             [Page 1]


Internet Draft                DHCP Version 6                 26 May 1997


2.1. IPv6 Terminology

      IP         Internet Protocol Version 6 (IPv6).  The terms IPv4 and
                 IPv6 are used only in contexts where necessary to avoid
                 ambiguity.

      node       A device that implements IP.

      router     A node that forwards IP datagrams not explicitly
                 addressed to itself.

      host       Any node that is not a router.

      link       A communication facility or medium over which nodes
                 can communicate at the link layer, i.e., the layer
                 immediately below IP. Examples are Ethernet (simple or
                 bridged); Token Ring; PPP links, X.25, Frame Relay, or
                 ATM networks; and internet (or higher) layer "tunnels",
                 such as tunnels over IPv4 or IPv6 itself.

      link-layer identifier
                 a link-layer identifier for an interface.  Examples
                 include IEEE 802 addresses for Ethernet or Token Ring
                 network interfaces, and E.164 addresses for ISDN links.

      link-local address
                 An IP address having link-only scope that can be used
                 to reach neighboring nodes attached to the same link.
                 All interfaces have a link-local address.

      neighbor   Nodes attached to the same link.

      interface
                 A node's attachment to the link.

      address    An IP layer identifier for an interface or a set of
                 interfaces.

      message    A unit of data carried in a datagram, exchanged between
                 DHCP agents and clients.

      datagram   An IP header plus payload.

      unicast address
                 An identifier for a single interface.  A datagram sent
                 to a unicast address is delivered to the interface
                 identified by that address.




Bound, Perkins             Expires 26 November 1997             [Page 2]


Internet Draft                DHCP Version 6                 26 May 1997


      multicast address
                 An identifier for a set of interfaces (typically
                 belonging to different nodes).  A datagram sent to
                 a multicast address is delivered to all interfaces
                 identified by that address.


2.2. DHCPv6 Terminology

      configuration parameter
                 Any parameter that can be used by a node to configure
                 its network environment and enable communication on a
                 link or internetwork.

      DHCP Client (or Client)
                 A node that initiates requests on a link to obtain
                 configuration parameters.

      DHCP Server (or Server)
                 A server is a node that responds to requests from
                 clients to provide:  addresses, prefix lengths, or
                 other configuration parameters.

      DHCP Relay (or Relay)
                 A node that acts as an intermediary to deliver DHCP
                 messages between clients and servers.

      DHCP Agent (or Agent)
                 Either a DHCP server or a DHCP relay.

      Agent Address
                 The address of a neighboring DHCP Agent on the same
                 link as the DHCP client.

      transaction-ID
                 The transaction-ID is a monotonically increasing
                 integer identifier specified by the client or server,
                 and used to match a response to a pending message.

      binding    A binding (or, client binding) in DHCP contains the
                 data which a DHCP server maintains for each of its
                 clients (see Section 6).









Bound, Perkins             Expires 26 November 1997             [Page 3]


Internet Draft                DHCP Version 6                 26 May 1997


2.3. Specification Language

   In this document, several words are used to signify the requirements
   of the specification, in accordance with RFC 2119 [3].  These words
   are often capitalized.

      MUST          This word, or the adjective "required", means that
                    the definition is an absolute requirement of the
                    specification.

      MUST NOT      This phrase means that the definition is an absolute
                    prohibition of the specification.

      SHOULD        This word, or the adjective "recommended", means
                    that there may exist valid reasons in particular
                    circumstances to ignore this item, but the full
                    implications must be understood and carefully
                    weighed before choosing a different course.
                    Unexpected results may result otherwise.

      MAY           This word, or the adjective "optional", means that
                    this item is one of an allowed set of alternatives.
                    An implementation which does not include this option
                    MUST be prepared to interoperate with another
                    implementation which does include the option.

      silently discard
                    The implementation discards the datagram without
                    further processing, and without indicating an error
                    to the sender.  The implementation SHOULD provide
                    the capability of logging the error, including the
                    contents of the discarded datagram, and SHOULD
                    record the event in a statistics counter.


3. Protocol Design Model

   This section is provided for implementors to understand the DHCPv6
   protocol design model from an architectural perspective.  The goals,
   conceptual models and implementation examples presented in this
   section do not specify requirements of the DHCPv6 protocol.


3.1. Design Goals

   The following list gives general design goals for this DHCP
   specification.




Bound, Perkins             Expires 26 November 1997             [Page 4]


Internet Draft                DHCP Version 6                 26 May 1997


    -  DHCP should be a mechanism rather than a policy.  DHCP MUST allow
       local system administrators control over configuration parameters
       where desired; e.g., local system administrators should be able
       to enforce local policies concerning allocation and access to
       local resources where desired.

    -  DHCP MUST NOT introduce any requirement for manual configuration
       of DHCP clients, except possibly for manually configured
       keys.  Each node should be able to discover appropriate
       local configuration parameters without user intervention, and
       incorporate those parameters into its own configuration.

    -  DHCP MUST NOT require a server on each link.  To allow for scale
       and economy, DHCP MUST work across DHCP relays.

    -  A DHCP client MUST be prepared to receive multiple (possibly
       different) responses to solicitations for DHCP servers.  Some
       installations may include multiple, overlapping DHCP servers to
       enhance reliability and/or to increase performance.

    -  DHCP MUST coexist with statically configured, non-participating
       nodes and with existing network protocol implementations.

    -  DHCPv6 MUST be compatible with IPv6 Stateless Address
       Autoconfiguration [15].

    -  DHCP MUST support the requirements of automated renumbering of IP
       addresses [4].

    -  DHCP servers should be able to support Dynamic Updates to
       DNS [16].

    -  DHCP servers MUST be able to handle multiple IPv6 addresses for
       each client.

    -  A DHCP server to server protocol is NOT part of this
       specification.

    -  It is NOT a design goal of DHCP to specify how a server
       configuration parameter database is maintained or determined.
       Methods for configuring DHCP servers are outside the scope of
       this document.









Bound, Perkins             Expires 26 November 1997             [Page 5]


Internet Draft                DHCP Version 6                 26 May 1997


3.2. DHCP Messages

   Each DHCP message contains a type, which defines their function
   within the protocol.  Processing details for these DHCP messages are
   specified in Sections 5, 6, and 7.  The message types are as follows:

      01 DHCP Solicit

         The DHCP Solicit message is a DHCP message sent to one or more
         DHCP Agents.

      02 DHCP Advertise

         The DHCP Advertise is an IP unicast message from a DHCP Agent
         in response to a client DHCP Solicit message.

      03 DHCP Request

         The DHCP Request is an IP unicast message from a client to a
         server to request configuration parameters on a network.

      04 DHCP Reply

         The DHCP Reply is an IP unicast message sent by a server to
         respond to a client's DHCP Request.  Extensions [11] to the
         DHCP Reply describe the resources that the DHCP Server has
         committed and allocated to the client, and may contain other
         information for use by the client.

      05 DHCP Release

         The DHCP Release message is used by a DHCP client to inform
         the server that the client is releasing a particular address,
         or set of addresses or resources, so that the server may
         subsequently mark the addresses as invalid, or release
         resources in the server's binding for the client.

      06 DHCP Reconfigure

         The DHCP Reconfigure message is used by a DHCP server to inform
         its client that the server has new configuration information of
         importance to the client.  The client is expected to initiate a
         new Request/Reply transaction.








Bound, Perkins             Expires 26 November 1997             [Page 6]


Internet Draft                DHCP Version 6                 26 May 1997


3.3. Request/Response Processing Model

   The request/response processing for DHCPv6 is transaction based
   and uses a best-effort set of messages to guarantee a completed
   transaction.

   Transactions are usually started by a client with a DHCP Request,
   which may be issued after the client knows the server's address.
   The response (DHCP Reply) is from the server (possibly via a DHCP
   Relay).  At this point in the flow all data has been transmitted
   and, hopefully, received.  To provide a method of recovery if either
   the client or server do not receive the messages to complete the
   transaction, the client is required to retransmit any DHCP Request
   message until it elicits the corresponding DHCP Reply or Replies,
   or until it can be reasonably certain that the desired DHCP Server
   is unavailable.  The timeout and retransmission guidelines and
   configuration variables are discussed in Section 8.  DHCP uses the
   UDP [13] protocol to communicate between clients and servers.  UDP is
   not reliable, but DHCP the retransmission scheme in the referenced
   section provides reliability between clients and servers.

   The following well-known multicast addresses are used by DHCP Agents:

      FF02:0:0:0:0:0:1:2
               All DHCP Agents (Servers and Relays) MUST join the
               link-local All-DHCP-Agents multicast group at the address
               FF02:0:0:0:0:0:1:2.

      FF05:0:0:0:0:0:1:3
               All DHCP Servers MUST, in addition, join the site-local
               All-DHCP-Servers multicast group at the address
               FF05:0:0:0:0:0:1:3.

      FF05:0:0:0:0:0:1:4
               All DHCP Relays MUST, on the other hand, join in addition
               the site-local All-DHCP-Relays multicast group at the
               address FF05:0:0:0:0:0:1:4.

   A client MUST transmit all messages over UDP using port 547 as the
   destination port.  A client MUST receive all messages from UDP port
   546.

   A DHCP Agent MUST transmit all messages to clients over UDP using
   port 546 as the destination port.  A DHCP Agent MUST receive all
   messages over UDP using port 547.  The source port for DHCP messages
   is arbitrary.





Bound, Perkins             Expires 26 November 1997             [Page 7]


Internet Draft                DHCP Version 6                 26 May 1997


4. DHCP Message Formats and Field Definitions

   All fields in DHCP messages MUST be initialized to binary zeroes by
   both the client and server unless otherwise noted.  DHCP message
   types not defined here (msg-types 0 and 7-255) are reserved.


4.1. DHCP Solicit Message Format

   A DHCP client (or DHCP relay on behalf of a client) transmits a DHCP
   Solicit message to obtain one or more DHCP server addresses.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |C|A|                 reserved                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         relay address                         |
     |                    (16 octets, if present)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      msg-type   1

      C          If set, the client requests that all servers receiving
                 the message deallocate the resources associated with
                 the client.

      A          If set, the relay's address is present

      reserved   0

      client's link-local address
                 The IP link-local address of the client interface from
                 which the client issued the DHCP Request message

      relay address
                 If present, the IP address of the interface on which
                 the relay received the client's DHCP Solicit message

   To discover a DHCP Agent address a DHCP client SHOULD send a DHCP
   Solicit message to the All-DHCP-Agents multicast address (see
   section 3.3).  Any DHCP Relay receiving the solicitation MUST forward
   it to the All-DHCP-Servers multicast address, to instruct DHCP
   Servers to send their advertisements to the prospective client.  In
   that case, the relay MUST insert the address of its interface from



Bound, Perkins             Expires 26 November 1997             [Page 8]


Internet Draft                DHCP Version 6                 26 May 1997


   which the client's solicitation was received into the agent's address
   field, and set the 'A' bit.

   DHCP clients MUST NOT issue any DHCP solicitation which is long
   enough so that inserting the Relay Address field would cause the
   message to exceed the MTU advertised on the link.


4.2. DHCP Advertise Message Format

   A DHCP agent sends a DHCP Advertise message to inform a prospective
   client about the IP address of a DHCP Agent to which a DHCP Request
   message may be sent.  When the client and server are on different
   links, the server sends the advertisement back through the DHCP Relay
   whence the solicitation came.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |S|                  reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         agent address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         server address                        |
     |                    (16 octets, if present)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              extensions (variable number and length) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      msg-type     2

      S            If set, the agent address is also a server address.

      reserved     0

      agent address
                   The IP address of a DHCP Agent interface on the same
                   link as the client.

      client's link-local address
                   The IP link-local address of the client interface
                   from which the client issued the DHCP Request message





Bound, Perkins             Expires 26 November 1997             [Page 9]


Internet Draft                DHCP Version 6                 26 May 1997


      server address
                   The IP address of the DHCP server

      extensions   See [11].

   Suppose that a DHCP server on the same link as a client issues the
   DHCP Advertise in response to a DHCP Solicit message sent to the
   All-DHCP-Agents multicast address.  Then the agent address will be
   the IP address of one of the server's interfaces, the 'S' bit will be
   set, the agent address will be an address of the server, and there
   will be no server address sent in the DHCP Advertise message.

   The DHCP Server MUST copy the client's link-local address into the
   advertisement which is sent in response to a DHCP Solicit.  Both
   Agent address and server address (if present) of the DHCP Advertise
   message MUST have sufficient scope to be reachable by the DHCP
   Client.  Moreover, the Agent address of any DHCP Advertise message
   sent by a DHCP relay MUST NOT be a link-local address.  In situations
   where there are no routers sending Router Advertisements, then a DHCP
   Server MUST be configured on the same link as prospective clients.
   The DHCPv6 protocol design does not apply to sitations where the
   client has no way to route messages to a server not on the same link


4.3. DHCP Request Message Format

   In order to request parameters from a DHCP server, a client sends a
   DHCP Request message, and MAY append extensions [11].  If the client
   does not know any DHCP server address, it MUST first obtain a server
   address by multicasting a DHCP Solicit message (see Section 4.1).  If
   the client does not have a valid IP address of sufficient scope for
   the DHCP server to communicate with the client, client MUST send the
   message to the local DHCP Relay and insert the DHCP Relay address as
   the agent address in the message header.  In this case, the client
   cannot send the message directly to the DHCP server because the
   server could not return any response to the client.  Otherwise, the
   client MAY omit the server address in the DHCP Request message; in
   this case, the client MUST send the DHCP Request message directly to













Bound, Perkins            Expires 26 November 1997             [Page 10]


Internet Draft                DHCP Version 6                 26 May 1997


   the server, using the server address as the IP destination address in
   the IP header.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |S|C| reserved  |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         server address                        |
     |                    (16 octets, if present)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         agent address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  extensions (variable number and length)   ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      msg-type   3

      S          If set, the server address is present

      C          If set, the client requests the server to clear all
                 other existing resources and bindings (not requested
                 in extensions) currently associated with the client,
                 deallocating as needed.

      reserved   0

      transaction-ID
                 A monotonically increasing number used to identify this
                 Request, and copied into the Reply.

      server address
                 If present, the IP address of the DHCP server which
                 should receive the client's DHCP Request message.

      agent address
                 The IP address of an agent interface, copied from a
                 DHCP Advertisement message.

      client's link-local address
                 The IP link-local address of the client interface from
                 which the client issued the DHCP Request message

      extensions See [11].



Bound, Perkins            Expires 26 November 1997             [Page 11]


Internet Draft                DHCP Version 6                 26 May 1997


4.4. DHCP Reply Message Format

   The server sends one DHCP Reply message in response to every DHCP
   Request or DHCP Release received.  If the request comes with the 'S'
   bit set, the client could not directly send the Request to the server
   and had to use a neighboring relay agent.  In that case, the server
   sends back the DHCP Reply with the 'L' bit set, and the DHCP Reply is
   addressed to the agent address found in the DHCP Request message.  If
   the 'L' bit is set, then the client's link-local address will also be
   present.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |L| error code  |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                     (16 octets, if present)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  extensions (variable number and length)   ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      msg-type   4

      L          If set, the link-local address is present

      error code
                 One of the following values:

                  0   Success
                 16   Failure, reason unspecified
                 17   Authentication failed or nonexistent
                 18   Poorly formed Request or Release
                 19   Resources unavailable
                 20   Client record unavailable
                 21   Invalid client IP address in Release
                 23   Relay cannot find Server Address
                 24   Cannot understand selected Character Set
                 64   Server unreachable (ICMP error)

      transaction-ID
                 A monotonically increasing number used to identify this
                 Reply, and copied from the client's Request.

      client's link-local address
                 If present, the IP address of the client interface
                 which issued the corresponding DHCP Request message.




Bound, Perkins            Expires 26 November 1997             [Page 12]


Internet Draft                DHCP Version 6                 26 May 1997


      extensions
                 See [11].

   If the 'L' bit is set, and thus the link-local address is present in
   the Reply message, the Reply is sent by the server to the relay's
   address which was specified as the agent address in the DHCP Request
   message, and the relay uses the link-local address to deliver the
   Reply message to the client.


4.5. DHCP Release Message Format

   The DHCP Release message is sent without the assistance of any DHCP
   relay.  When a client sends a Release message, it is assumed to
   have a valid IP address with sufficient scope to allow access to
   the target server.  Only the parameters which are specified in the
   extensions are released.  The DHCP server acknowledges the Release
   message by sending a DHCP Reply (Sections 4.4, 6.3).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |D|  reserved   |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         agent address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   client's link-local address                 |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        client address                         |
     |                    (16 octets, if present)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  extensions (variable number and length)   ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      msg-type   5

      D          If the 'D' ("Direct") flag is set, the client instructs
                 the server to send the DHCP Reply directly back to the
                 client, instead of using the given agent address and
                 link-local address to relay the Reply message.

      reserved   0

      transaction-ID
                 A monotonically increasing number used to identify this
                 Release, and copied into the Reply.



Bound, Perkins            Expires 26 November 1997             [Page 13]


Internet Draft                DHCP Version 6                 26 May 1997


      agent address
                 The IP address of the agent interface to which the
                 client issued the DHCP Request message

      client's link-local address
                 The IP link-local address of the client interface from
                 which the the client issued the DHCP Request message

      client address
                 The IP address of the client interface from which the
                 the client issued the DHCP Request message.  The client
                 address field is present whenever the 'D' bit is set,
                 even if it is equal to the link-local address.

      extensions See [11]

   Suppose that the client has an IP address that will still be valid
   after the server performs the operations requested in the extensions
   to the DHCP Release message.  In that case, and only then, the client
   SHOULD then specify the 'D' flag.  When the 'D' flag is set, the
   server MUST send the DHCP Reply back to the client's address as shown
   in the client address field of the Release message.  Otherwise, when
   the 'D' bit is not set, the server MUST send its DHCP Reply message
   to the agent address in the Release message, so that the relay agent
   can subsequently forward the Reply back to the releasing client at
   the client's link-local address indicated in the Reply message.
   Note that it is an error (Error Code 21) to include within the DHCP
   Release message both the 'D' bit and an IP address extension which
   has the IP address used as the client IP address field of the DHCP
   Release message header.


4.6. DHCP Reconfigure Message Format

   The DHCP Reconfigure message is sent without the assistance of any
   DHCP relay.  When a server sends a Reconfigure message, the client
   to which it is sent is assumed to have a valid IP address with
   sufficient scope to be accessible by the server.  Only the parameters













Bound, Perkins            Expires 26 November 1997             [Page 14]


Internet Draft                DHCP Version 6                 26 May 1997


   which are specified in the extensions to the Reconfigure message need
   be requested again by the client.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    msg-type   |   reserved    |        transaction-ID         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        server address                         |
     |                          (16 octets)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  extensions (variable number and length)   ....
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      msg-type     6

      reserved     0

      transaction-ID
                   A monotonically increasing number used to identify
                   this Reconfigure message, and copied into the
                   client's Request.

      server address
                   The IP address of the DHCP server issuing the DHCP
                   Reconfigure message.

      extensions   See [11]


5. DHCP Client Considerations

   A DHCP client MUST silently discard any DHCP Solicit, DHCP Request,
   or DHCP Release message it receives.


5.1. Verifying Resource Allocations After Restarts

   A DHCP client MAY retain its configured parameters and resources
   across client system reboots and DHCP client program restarts.
   However, in these circumstances a DHCP client MUST also formulate a
   DHCP Request message to verify that its configured parameters and
   resources are still valid.  This Request message MUST have the 'C'
   bit set, to clean up stale client binding information at the server
   which may no longer be in use by the client; stale information is
   that which the client does not include in extensions to such request
   messages.




Bound, Perkins            Expires 26 November 1997             [Page 15]


Internet Draft                DHCP Version 6                 26 May 1997


   If the server does not respond to the DHCP Request message, the
   client may still use any resources whose lifetimes have not yet
   expired.  In such cases, however, the client MUST begin to search
   for another server by multicasting a new DHCP Solicit message, again
   with the 'C' bit set, containing its IP address in the appropriate
   extension.

   This also handles the case wherein a client restarts on a new
   network, so that its IP address is no longer valid.  When the client
   multicasts a new DHCP Discover message, servers will respond with
   the information needed for the client to release its old address, if
   need be, and request an address reachable on the new network.  In
   this situation, when the client receives a new IP address and the old
   IP address is no longer reachable, the client MUST release its old
   IP address by issuing a DHCP Release message with the appropriate
   extension.


5.2. Sending DHCP Solicit Messages

   A DHCP client MUST have the address of a DHCP Server to send a
   Request message.  The client may locate a DHCP Server by multicasting
   a DHCP Solicit message to the All-DHCP-Agents link-local multicast
   address, setting the Hop Limit == 1 (see Section 3.3).  If there
   are no DHCP servers on the same link as the node, then a DHCP Relay
   MUST be present for further handling of the solicitation.  The
   prospective client SHOULD wait for ADV_WAIT seconds to get all the
   DHCP Advertisement messages which may be sent in response to the
   solicitation.

   If a DHCP client reboots and does not have a valid IP address,
   it MUST set the 'C' bit in the DHCP Solicit message it sends
   when restarting.  By setting the 'C' bit in the solicitation, a
   DHCP client requests that all the DHCP Servers that receive the
   solicitation should clean up their stale client records that match
   its link-local address.

   If a client sends a DHCP Solicit message after it reboots, the
   solicitation SHOULD be delayed after reception of the first Router
   Advertisement [10] message, by at least some random amount of time
   between MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY seconds.  This delay
   is intended to help stagger requests to DHCP Servers (and avoid
   link-layer collisions) after a power outage causes many nodes to
   reboot all at once.  Each subsequent DHCP Solicit message that is
   issued before receiving an advertisement MUST be delayed by twice the
   amount by which the previous DHCP Solicit message was delayed, plus
   a small random delay between MIN_SOLICIT_DELAY and MAX_SOLICIT_DELAY
   seconds.



Bound, Perkins            Expires 26 November 1997             [Page 16]


Internet Draft                DHCP Version 6                 26 May 1997


5.3. Receiving DHCP Advertise Messages

   After a DHCP Client has received a DHCP Advertise message, it has
   the address of a DHCP Server for subsequent DHCP Request messages.
   If the Advertise message has no server address field and does
   not have the 'S' bit set, the client MUST silently discard the
   message.  If the server's address is shown as a Multicast address,
   the advertisement MUST be silently discarded.

   If the 'S' bit is set, the DHCP Advertise message was transmitted
   by a DHCP server on the same link as the client.  In this case, any
   future DHCP message transactions sent to that server MUST be sent by
   the client to the agent address indicated by the 'S' bit.

   Advertisements may have extensions; this might allow the DHCP client
   to select the configuration that best meets its needs from among
   several prospective servers.


5.4. Sending DHCP Request Messages

   A DHCP client obtains configuration information from a DHCP server by
   sending a DHCP Request message.  The client MUST know the server's
   address before sending the Request message, and client MUST have
   acquired a (possibly identical) DHCP agent address.  If the client
   and server are on the same link, the agent address used by the client
   MUST be the same as the DHCP server's address.  A DHCP Request
   message MUST NOT be sent to any multicast address, since otherwise
   multiple DHCP agents would possibly allocate resources to the client
   in response to the same Request, and the client would have no way to
   know which servers had made the allocations, if any datagrams were
   lost due to collisions, etc.

   If the client has no valid IP address of sufficient scope, and the
   DHCP server is off-link, then the client MUST include the server
   address in the appropriate field of the DHCP Request message and set
   the 'S' bit.  In this case, the IP destination address of the Request
   message will be a DHCP relay address.

   Otherwise, if the client already has a valid IP address of sufficient
   scope and knows the IP address of a candidate DHCP server, it
   SHOULD send the Request message directly to the DHCP server without
   requiring the services of the local DHCP relay.

   If a client wishes to instruct a DHCP server to deallocate all
   unknown previous resources, configuration information, and bindings
   associated with its agent address and link-local address, it sets the
   'C' bit in the DHCP Request.  A client MAY send in such a Request



Bound, Perkins            Expires 26 November 1997             [Page 17]


Internet Draft                DHCP Version 6                 26 May 1997


   even when it is no longer attached to the link on which the relay
   address is attached.

   In any case, after choosing a transaction-ID which is numerically
   greater than its previous transaction-ID, and filling in the
   appropriate fields of the DHCP Request message, the client MAY append
   various DHCP Extensions to the message.  These extensions denote
   specific requests by the client; for example, a client may request
   a particular IP address, or request that the server send an update
   containing the client's new IP address to a Domain Name Server.  When
   all desired extensions have been applied, the DHCP client unicasts
   the DHCP Request to the appropriate DHCP Agent.

   For each pending DHCP Request message, a client MUST maintain the
   following information:

    - The transaction-ID of the request message,
    - The server address,
    - The agent address (which can be the same as the server address),
    - The time at which the next retransmission will be attempted, and
    - All extensions appended to the request message.

   If a client does not receive a DHCP Reply message (Section 5.5) with
   the same transaction-ID as a pending DHCP Request message within
   REPLY_MSG_INITIAL_TIMEOUT seconds, or if the received DHCP Reply
   message contains a DHCP Authentication extension which fails to
   provide the correct authentication information, the client MUST
   retransmit the Request with the same transaction-ID and continue to
   retransmit according to the rules in Section 8.

   If the client transmits a DHCP Request in response to a DHCP
   Reconfigure message (see Section5.7), the client can continue to
   operate with its existing configuration information and resources
   until it receives the corresponding DHCP Reply from the server.  The
   same retransmission rules apply as for any other DHCP Request message
   from the client.


5.5. Receiving DHCP Reply Messages

   When a client receives a DHCP Reply message, it MUST check whether
   the transaction-ID in the Reply message matches the transaction-ID
   of a pending DHCP Request message.  If no match is found, the Reply
   message MUST be silently discarded.

   If the Reply message is acceptable, the client processes each
   Extension [11], extracting the relevant configuration information
   and parameters for its network operation.  The client can determine



Bound, Perkins            Expires 26 November 1997             [Page 18]


Internet Draft                DHCP Version 6                 26 May 1997


   when all extensions in the Reply have been processed by using the
   Length field of the Reply.  Some extensions in the Reply may have
   error codes, when the server was unable to honor the request, which
   will indicate to the client the reason for failure.  If the server is
   unable to honor the request in an extension included by the client,
   that extension may simply be omitted from the Reply.

   Some configuration information extracted from the extensions to the
   DHCP Reply message MUST remain associated with the DHCP server that
   sent the message.  The particular extensions that require this extra
   measure of association with the server are indicated in the DHCP
   Extensions document [11].  These "resource-server" associations are
   used when sending DHCP Release messages.


5.6. Sending DHCP Release Messages

   If a DHCP client determines that some of its network configuration
   parameters are no longer needed, it SHOULD enable the DHCP server to
   release allocated resources which are no longer in use by sending a
   DHCP Release message to the server.  The client consults its list
   of resource-server associations in order to determine which server
   should receive the desired Release message.  If a client wishes to
   ask the server to release all information and resources relevant to
   the client, the client specifies no extensions; this is preferable
   to sending a DHCP Request message with the 'C' bit set and no
   extensions.

   Suppose a client wishes to release resources which were granted to
   it on another link.  In that case, the client MUST instruct the
   server to send the DHCP Reply directly back to the client, instead
   of performing the default processing of sending the DHCP Reply back
   through the agent-address included in the DHCP Release.  This is done
   by setting the 'D' bit in the DHCP Release message (see section 4.5).


5.7. Receiving DHCP Reconfigure Messages

   Each DHCP client MUST listen at UDP port 546 to receive possible
   DHCP Reconfigure messages, except in cases where the client knows
   that no Reconfigure message will ever be issued.  In some cases,
   the IP address at which the client listens will be a multicast
   address sent to the client by the DHCP server in an extension to
   an earlier DHCP Reply message.  If the client does not listen for
   DHCP Reconfigure messages, it is possible that the client will
   not receive notification that its DHCP server has deallocated the
   client's IP address and/or other resources allocated to the client.
   See discussion in 6.5.  The client MAY receive an update to the



Bound, Perkins            Expires 26 November 1997             [Page 19]


Internet Draft                DHCP Version 6                 26 May 1997


   prefix for their addresses and then MUST use that prefix for their
   addresses.

   If a DHCP client receives a DHCP Reconfigure message, it is a request
   for the client to initiate a new DHCP Request/Reply transaction with
   the server which sent the Reconfigure message.  The server sending
   the Reconfigure message MAY be different than the server which sent a
   DHCP Reply message containing the original configuration information.

   For each Extension which is present in the Reconfigure message, the
   client appends a matching Extension to its Request message, which
   it formulates to send to the server specified in the server address
   field of the message.  The client also copies a transaction-ID from
   the Reconfigure message into the Request message.  From then on,
   processing is the same as specified above in Section 5.4.

   Resources held by the client which are not identified by Extensions
   in the server's Reconfigure message are not affected.

   Note that a server may ask its client to join a multicast group
   for the purpose of receiving DHCP Reconfigure messages.  When a
   Reconfigure message is delivered to the client by way of the selected
   multicast address, the client MUST delay its further response for
   a random amount of time uniformly distributed within the interval
   between RECONF_MSG_MIN_RESP and RECONF_MSG_MAX_RESP seconds.  This
   will minimize the likelihood that the server will be bombarded with
   DHCP Request messages all at the same time.


6. DHCP Server Considerations

   A server MUST ignore any DHCP Advertise, DHCP Reply, or DHCP
   Reconfigure message it receives.

   A server maintains a collection of client records, called
   ``bindings''.  Each binding is uniquely identifiable by the ordered
   pair <link-local address, agent address>, since the link-local
   address is guaranteed to be unique [15] on the link identified
   by the agent address.  An implementation MUST support bindings
   consisting of at least a client's link-local address, agent address,
   preferred lifetime and valid lifetime [15] for each client address,
   and the transaction-ID. A client binding may be used to store any
   other information, resources, and configuration data which will be
   associated with the client.  A DHCP server MUST retain its clients'
   bindings across server reboots, and, whenever possible, a DHCP client
   should be assigned the same configuration parameters despite server
   system reboots and DHCP server program restarts.  A DHCP server MUST




Bound, Perkins            Expires 26 November 1997             [Page 20]


Internet Draft                DHCP Version 6                 26 May 1997


   support fixed or permanent allocation of configuration parameters to
   specific clients.


6.1. Receiving DHCP Solicit Messages

   If the DHCP Solicit message was received at the All-DHCP-Servers
   multicast address, the DHCP Server MUST check to make sure that the
   agent address is present, and not a link-local address.  Otherwise,
   if the agent address is not present, or if it is a link-local
   address, the server MUST silently discard the packet.  If the UDP
   length disagrees with the length determined by the format of the
   DHCP Solicit message, the server MUST drop the packet and SHOULD log
   the error.  Note that if the client sends a DHCP Solicit message
   from a link-local address, the multicast destination will be the
   All-DHCP-Agents address, not the All-DHCP-Servers address.


6.2. Sending DHCP Advertise Messages

   Upon receiving and verifying the correctness of a DHCP Solicit
   message, a server constructs a DHCP Advertise message and transmits
   it on the same link as the solicitation was received from.  If the
   'A' bit is set, the advertisement MUST be sent to the relay address;
   otherwise, the server MUST send the advertisement to the client's
   link-local address.  An IP address of the interface on which the
   server receives the Solicit message MUST appear in the server address
   field of the corresponding advertisement.

   The DHCP server MAY append extensions to the Advertisement, in order
   to offer the soliciting node the best possible information about
   the services and resources which the server may be able to make
   available.


6.3. DHCP Request and Reply Message Processing

   The DHCP server MUST check to ensure that the client's link-local
   address field of the Request message contains a link-local address.
   If not, the message MUST be silently discarded.  Otherwise, it checks
   for the presence of the 'S' bit.  If the 'S' bit is set, the server
   MUST check that the server address matches the destination IP address
   at which the Request message was received by the server.  If the
   server address does not match, the Request message MUST be silently
   discarded.

   If the received agent address and link-local address do not
   correspond to any binding known to the server, then the server



Bound, Perkins            Expires 26 November 1997             [Page 21]


Internet Draft                DHCP Version 6                 26 May 1997


   MAY create a new binding for the previously unknown client, and
   send a DHCP Reply with any resources allocated to the new binding.
   Otherwise, it SHOULD return a DHCP Reply with an error code of 19.

   While processing the Request, the server MUST first determine whether
   or not the Request is a retransmission of an earlier DHCP Request
   from the same client.  This is done by comparing the transaction-ID
   to all those transaction-IDs received from the same client during the
   previous XID_TIMEOUT seconds.  If the transaction-ID is the same as
   one received during that time, the server MUST take the same action
   (e.g., retransmit the same DHCP Reply to the client) as it did after
   processing the previous DHCP Request with the same transaction-ID.

   Otherwise, if the server has no record of a message from the client
   with the same transaction-ID, the server identifies and allocates
   all the relevant information, resources, and configuration data that
   is associated with the client.  Then it sends that information to
   its DHCP client by constructing a DHCP Reply message and including
   the client's information in DHCP Extensions to the Reply message.
   The DHCP Reply message uses the same transaction-ID as found in the
   received DHCP Request message.  Note that the reply message MAY
   contain information not specifically requested by the client.

   If the DHCP Request message has the 'S' bit set in the message
   header, then the Request was sent to the server by a DHCP Relay.  In
   this case, the DHCP server MUST send the corresponding DHCP Reply
   message to the agent address found in the Request (see section 7.2).


6.3.1. Processing for Extensions to DHCP Request and Reply Messages

   The DHCP Request may contain extensions, which are interpreted
   (by default) as advisory information from the client about its
   configuration preferences.  For instance, if the IP Address Extension
   is present, the DHCP server SHOULD attempt to allocate or extend the
   lifetime of the address indicated by the extension.  Some extensions
   may be marked by the client as required.

   The DHCP server may accept some extensions for successful processing
   and allocation, while still rejecting others, or the server may
   reject various extensions for different reasons.  The server sets
   the Error Code appropriately for those extensions which return error
   status to the client.  The DHCP server sends a single Reply message
   in response to each DHCP Request, with the same transaction-ID as the
   Request.

   Whenever it is able to, the server includes an extension in the
   Reply message for every extension sent by the client in the Request



Bound, Perkins            Expires 26 November 1997             [Page 22]


Internet Draft                DHCP Version 6                 26 May 1997


   message.  If the client requests some extensions that cannot be
   supplied by the server, the server can simply fail to provide them,
   not including them in the Reply.  Other extensions can be rejected by
   including them in the Reply with an appropriate error code indicating
   failure.


6.3.2. Client Requests to Deallocate Unknown Resources

   When a client DHCP Request is received that has the 'C' bit set, the
   server should check to find out whether the extensions listed in the
   Request message match those which it has associated with the client's
   binding.  Any resources which are not indicated by the client are
   presumed to be unknown by the client, and thus possible candidates
   for reallocation to satisfy requests from other clients.  The DHCP
   Server MUST deallocate all resources associated with the client
   upon reception of a DHCP Request with the 'C' bit set, except for
   those which the server is willing to reallocate in response to the
   client's request.  It may be more efficient to avoid deallocating any
   resources until after the list of extensions to the request have been
   inspected.


6.4. Receiving DHCP Release Messages

   If the server receives a DHCP Release Message, it MUST verify that
   the link-local address field of the message contains an address
   which could be a valid link-local address (i.e., one with the prefix
   FE80::0000/64).  If not, the message MUST be silently discarded.

   In response to a DHCP Release Message with a valid client's
   link-local address and agent address, the DHCP server formulates a
   DHCP Reply message that will be sent back to the releasing client by
   way of the client's link-local address.  A DHCP Reply message sent
   in response to a DHCP Release message MUST be sent to the client's
   link-local address via the agent address in the Release message
   and set the 'L' bit in the Reply, unless the 'D' bit is set in the
   Release message.

   If the received agent address and link-local address do not
   correspond to any binding known to the server, then the server SHOULD
   return a DHCP Reply with an error code of 20.

   Otherwise, if the agent address and link-local address indicate a
   binding known to the server, then the server continues processing the
   Release message.  If there are any extensions, the server releases
   the particular configuration items specified in the extensions.




Bound, Perkins            Expires 26 November 1997             [Page 23]


Internet Draft                DHCP Version 6                 26 May 1997


   Otherwise, if there are no extensions, the server releases all
   configuration information in the client's binding.

   After performing the operations indicated in the DHCP Release message
   and its extensions, the DHCP server formulates a DHCP Reply message,
   copying the transaction-ID, from the DHCP Release message.  For
   each Extension in the DHCP Release message successfully processed
   by the server, a matching Extension is appended to the DHCP Reply
   message.  For extensions in the DHCP Release message which cannot be
   successfully processed by the server, a DHCP Reply message containing
   extensions with the appropriate error codes MUST be returned by the
   server.


6.5. Sending DHCP Reconfigure Messages

   If a DHCP server needs to change the configuration associated to any
   of its clients, it constructs a DHCP Reconfigure message and sends it
   to each such client [11].  The Reconfigure MAY be sent to a multicast
   address chosen by the server and sent to each of its clients in an
   extension to a previous DHCP Reply message.


7. DHCP Relay Considerations

   The DHCP protocol is constructed so that a relay does not have
   to maintain any state in order to facilitate DHCP client/server
   interactions.

   All relays MUST send DHCP Request messages out from an IP address of
   the interface from which the DHCP request was received.

   The main purpose of the DHCP Relay is to enable clients and servers
   to carry out DHCP protocol transactions.  DHCP Solicit messages are
   issued by the relay when initiated by prospective DHCP clients.
   By default, the relay discovers local DHCP Servers by use of
   multicasting DHCP solicitations to the All-DHCP-Servers multicast
   address, but relays SHOULD allow this behavior to be configurable.
   The relay SHOULD NOT send such a multicast solicitation on the
   interface from which it received the solicitation from the client.


7.1. DHCP Solicit and DHCP Advertise Message Processing

   Upon receiving a DHCP Solicit message from a prospective client, a
   relay, by default, forwards the message to all DHCP Servers at a site
   according to the following procedure:




Bound, Perkins            Expires 26 November 1997             [Page 24]


Internet Draft                DHCP Version 6                 26 May 1997


    -  copying the prospective client's solicitation message fields into
       the appropriate fields of the outgoing solicitation,

    -  setting the 'A' bit,

    -  copying the address of its interface from which the solicitation
       was received from the client into the DHCP Relay address field,
       and

    -  finally, sending the resulting message to the All-DHCP-Servers
       multicast address, FF05:0:0:0:0:0:1:3, over all interfaces except
       that from which the client's solicitation was received.

   When the relay receives a DHCP advertisement, it relays the
   advertisement to the client at the client's link-local address by way
   of the interface indicated in the agent's address field.


7.2. DHCP Request Message Processing

   When a relay receives a DHCP Request message, it SHOULD check
   that the message is received from a link-local address, that the
   link-local address matches the link-local address field in the
   Request message header, and that the agent address field of the
   message matches an IP address associated to the interface from which
   the DHCP Request message was received.  If any of these checks fail,
   the Relay MUST silently discard the Request message.

   The relay MUST also check whether the 'S' bit is set in the message
   header.  If not, the datagram is discarded, and the relay SHOULD
   return a DHCP Reply message to the address contained in the client's
   link-local address field of the Request message, with error code 18.

   If the received request message is acceptable, the relay then
   transmits the DHCP Request message to the address of the DHCP
   server found in the Server IP Address field of the received DHCP
   Request message.  All of the fields of DHCP Request message header
   transmitted by the relay are copied over unchanged from the DHCP
   Request received from the client.  Only the fields in the IP header
   will differ from the datagram received from the client, not the
   payload.  If the Relay receives an ICMP error, the Relay SHOULD
   return a DHCP Reply message to the client address (which can be found
   in the payload of the ICMP message [5]), with error code 64.








Bound, Perkins            Expires 26 November 1997             [Page 25]


Internet Draft                DHCP Version 6                 26 May 1997


7.3. DHCP Reply Message Processing

   When the relay receives a DHCP Reply, it MUST check whether the
   message has the 'L' bit set.  It MUST check whether the link-local
   address field contains a link-local address.  If all the checks are
   satisfied, the relay MUST send a DHCP Reply message to the link-local
   address listed in the received Reply message.  Only the fields in the
   IP header will differ from the datagram received from the server, not
   the payload.


8. Retransmission and Configuration Variables

   When a DHCP client does not receive a DHCP Reply in response to a
   pending DHCP Request, the client MUST retransmit the identical DHCP
   Request, with the same transaction-ID, to the same server again
   until it can be reasonably sure that the DHCP server is unavailable
   and an alternative can be chosen.  The DHCP Server assumes that the
   client has received the configuration information included with the
   extensions to the DHCP Reply message, and it is up to the client
   to continue to try for a reasonable amount of time to complete the
   transaction in order to make that assumption hold true.  All the
   actions specified for DHCP Request in this section hold also for DHCP
   Release messages sent by the DHCP Client.

   Similarly, when a client sends a DHCP Request message in response to
   a Reconfigure message from the server, the client assumes that the
   DHCP server has received the Request.  The server MUST retransmit the
   identical DHCP Reconfigure to the client for a reasonable amount of
   time, to try to elicit the Request message from the client, in order
   to make the best effort for that assumption to hold true.  If no
   corresponding DHCP Request is ever received by the server, the server
   MAY erase or deallocate information as needed from the client's
   binding.

   These retransmissions occur using the following configuration
   variables for a DHCP implementation that MUST be configurable by a
   client or server:

      ADV_WAIT

         The amount of time a client waits to hear DHCP Advertisements
         after issuing a DHCP Solicit to the All-DHCP Agents multicast
         address.

         Default:  5 seconds





Bound, Perkins            Expires 26 November 1997             [Page 26]


Internet Draft                DHCP Version 6                 26 May 1997


      REPLY_MSG_INITIAL_TIMEOUT

         The time in seconds that a DHCP client waits to receive a
         server's DHCP Reply before retransmitting a DHCP Request.

         Default:  2 seconds.

      REPLY_MSG_MIN_RETRANS

         The minimum number of DHCP Request transmissions that a DHCP
         client should retransmit, before aborting the request, possibly
         retrying the Request with another Server, and logging a DHCP
         System Error.

         Default:  10 retransmissions.

      REPLY_MSG_RETRANS_INTERVAL

         The time between successive retransmissions of DHCP Request
         messages.

         Default:  2 seconds.

      RECONF_MSG_INITIAL_TIMEOUT

         The time in seconds that a DHCP server waits to receive
         a client's DHCP Request before retransmitting its DHCP
         Reconfigure.

         Default:  2 seconds.

      RECONF_MSG_MIN_RETRANS

         The minimum number of DHCP Reconfigure messages that a DHCP
         server should retransmit, before assuming the the client is
         unavailable and that the server can proceed with the needed
         reconfiguration of that client's resources, and logging a DHCP
         System Error.

         Default:  10 retransmissions.

      RECONF_MSG_RETRANS_INTERVAL

         The least time between successive retransmissions of DHCP
         Reconfigure messages.

         Default:  2 seconds.




Bound, Perkins            Expires 26 November 1997             [Page 27]


Internet Draft                DHCP Version 6                 26 May 1997


      RECONF_MSG_MIN_RESP

         The minimum amount of time before a client can respond to a
         DHCP Reconfigure message sent to a multicast address.

         Default:  2 second.

      RECONF_MSG_MAX_RESP

         The maximum amount of time before a client MUST respond to a
         DHCP Reconfigure message sent to a multicast address.

         Default:  10 seconds.

      MIN_SOLICIT_DELAY

         The maximum amount of time a prospective client is required
         to wait, after determining from a Router Discovery message
         that the client should perform stateful address configuration,
         before sending a DHCP Solicit to a DHCP Server.

         Default:  1 second

      MAX_SOLICIT_DELAY

         The maximum amount of time a prospective client is required
         to wait, after determining from a Router Discovery message
         that the client should perform stateful address configuration,
         before sending a DHCP Solicit to a DHCP Server.

         Default:  5 seconds

      XID_TIMEOUT

         The amount of time a DHCP server has to keep track of
         client transaction-IDs in order to make sure that client
         retransmissions using the same transaction-ID are idempotent.

         Default:  600 seconds

   Note that, if a client receives a DHCP message which fails
   authentication, it should continue to wait for another message which
   might be correctly authenticated just as if the failed message had
   never arrived; however, receiving such failed messages SHOULD be
   logged.






Bound, Perkins            Expires 26 November 1997             [Page 28]


Internet Draft                DHCP Version 6                 26 May 1997


9. Security Considerations

   DHCP clients and servers often have to authenticate the messages they
   exchange.  For instance, a DHCP server may wish to be certain that a
   DHCP Request originated from the client identified by the <link-local
   address, agent address> fields included within the Request message
   header.  Conversely, it is often essential for a DHCP client to
   be certain that the configuration parameters and addresses it has
   received were sent to it by an authoritative DHCP server.  Similarly,
   a DHCP server should only accept a DHCP Release message which seems
   to be from one of its clients, if it has some assurance that the
   client actually did transmit the Release message.  At the time of
   this writing, there is no generally accepted mechanism useful with
   DHCPv4 that is appropriate for use with DHCPv6.

   The IPv6 Authentication Header can provide security for DHCPv6
   messages when both endpoints have a suitable IP address.  However,
   a client often has only a link-local address, and such an address
   is not sufficient for a DHCP server which is off-link.  In those
   circumstances the DHCP relay is involved, so that the DHCP message
   MUST have the relay's address in the IP destination address field,
   even though the client aims to deliver the message to the DHCP
   server.  The DHCP Client-Server Authentication Extension [11] is
   intended to be used in these circumstances.


10. Acknowledgements

   Thanks to the DHC Working Group for their time and input into the
   specification.  Ralph Droms and Thomas Narten have had a major role
   in shaping the continued improvement of the protocol by their careful
   reviews.  Thanks also for the consistent input, ideas, and review by
   (in alphabetical order) Brian Carpenter, Jack McCann, Yakov Rekhter,
   Matt Thomas, Sue Thomson, and Phil Wells.

   Thanks to Steve Deering and Bob Hinden, who have consistently
   taken the time to discuss the more complex parts of the IPv6
   specifications.  Thanks to Stuart Cheshire for his excellent minutes.


A. Related Work in IPv6

   The related work in IPv6 that would best serve an implementor
   to study is the IPv6 Specification [6], the IPv6 Addressing
   Architecture [8], IPv6 Stateless Address Autoconfiguration [15], IPv6
   Neighbor Discovery Processing [10], and Dynamic Updates to DNS [16].
   These specifications enable DHCP to build upon the IPv6 work to




Bound, Perkins            Expires 26 November 1997             [Page 29]


Internet Draft                DHCP Version 6                 26 May 1997


   provide both robust stateful autoconfiguration and autoregistration
   of DNS Host Names.

   The IPv6 Specification provides the base architecture and design of
   IPv6.  A key point for DHCP implementors to understand is that IPv6
   requires that every link in the internet have an MTU of 576 octets
   or greater (in IPv4 the requirement is 68 octets).  This means that
   a UDP datagram of 536 octets will always pass through an internet
   (less 40 octets for the IPv6 header), as long as there are no IP
   options prior to the UDP header in the datagram.  But, IPv6 does
   not support fragmentation at routers, so that fragmentation takes
   place end-to-end between hosts.  If a DHCP implementation needs
   to send a datagram greater than 536 octets it can either fragment
   the UDP datagram in UDP or use Path MTU Discovery [9] to determine
   the size of the datagram that will traverse a network path.  It is
   implementation dependent how this is accomplished in DHCP.

   The IPv6 Addressing Architecture specification [8] defines the
   address scope that can be used in an IPv6 implementation, and the
   various configuration architecture guidelines for network designers
   of the IPv6 address space.  Two advantages of IPv6 are that multicast
   addressing is required, and nodes can create link-local addresses
   during initialization of the nodes environment.  This means that a
   client immediately can configure an IP address at initialization
   for an interface, before communicating in any manner on the link.
   The client can then use a well-known multicast address to begin
   communications to discover neighbors on the link, or to send a DHCP
   Solicit and locate a DHCP server or relay.

   IPv6 Stateless Address Autoconfiguration [15] (addrconf) specifies
   procedures by which a node may autoconfigure addresses based on
   router advertisements [10], and the use of a validation lifetime to
   support renumbering of addresses on the Internet.  In addition the
   protocol interaction by which a node begins stateless or stateful
   autoconfiguration is specified.  DHCP is one vehicle to perform
   stateful autoconfiguration.  Compatibility with addrconf is a design
   requirement of DHCP (see Section 3.1).

   IPv6 Neighbor Discovery [10] is the node discovery protocol in IPv6
   (replaces and enhances functions of ARP [12]).  To truly understand
   IPv6 and addrconf it is strongly recommended that implementors
   understand IPv6 Neighbor Discovery.

   Dynamic Updates to DNS [16] is a specification that supports the
   dynamic update of DNS records for both IPv4 and IPv6.  DHCP can use
   the dynamic updates to DNS to now integrate addresses and name space
   to not only support autoconfiguration, but also autoregistration in




Bound, Perkins            Expires 26 November 1997             [Page 30]


Internet Draft                DHCP Version 6                 26 May 1997


   IPv6.  The security model to be used with DHCPv6 should conform as
   closely as possible to that outlined in RFC 1825 [2].


B. Comparison between DHCPv4 and DHCPv6

   This appendix is provided for readers who will find it useful to see
   a model and architecture comparison between DHCPv4 [7, 1] and DHCPv6.
   There are three key reasons for the differences:

     o IPv6 inherently supports a new model and architecture for
       communications and autoconfiguration of addresses.

     o DHCPv6 in its design was able to take advantage of the inherent
       benefits of IPv6.

     o New features were added to support the evolution and the
       existence of mature Internet users in the industry.

   IPv6 Architecture/Model Changes:

     o The link-local address permits a node to have an address
       immediately when the node boots, which means all clients have a
       source IP address at all times to locate a server or relay agent
       on the local link.

     o The need for bootp compatibility and broadcast flags are removed,
       which permitted a great deal of freedom in designing the new
       packet formats for the client and server interaction.

     o Multicast and the scoping methods in IPv6 permitted the design of
       discovery packets that would inherently define their range by the
       multicast address for the function required.

     o Stateful autoconfiguration has to coexist and integrate with
       stateless autoconfiguration supporting Duplicate Address
       Detection and the two IPv6 lifetimes, to facilitate the dynamic
       renumbering of addresses and the management of those addresses.

     o Multiple addresses per interface are inherently supported in
       IPv6.

     o Most DHCPv4 options are unnecessary now because the configuration
       parameters are either obtained through IPv6 Neighbor Discovery or
       the Service Location protocol.

   DHCPv6 Architecture/Model Changes:




Bound, Perkins            Expires 26 November 1997             [Page 31]


Internet Draft                DHCP Version 6                 26 May 1997


     o The message type is the first byte in the packet.

     o IPv6 Address allocations are now handled in a message extension
       as opposed to the main header.

     o Client/Server bindings are now mandatory and take advantage of
       the client's link-local address to always permit communications
       either directly from an on-link server, or from a remote server
       through an on-link relay-agent.

     o Servers are now discovered by a client solicit and server or
       relay-agent advertisement model.

     o The client will know if the server is on-link or off-link.

     o The client after a solicit will be returned the addresses of
       available servers either from an on-link server or from an
       on-link relay-agent as agents providing the advertisements.

     o The on-link relay-agent will obtain the location of remote server
       addresses from system configuration or by the use of a site wide
       DHCPv6 Multicast packet.

     o The protocol is optimized and removes the use of ACKs and NAKs
       once the client and server set-up is complete.

     o The server assumes the client receives its responses unless it
       receives a retransmission of the same client request.  This
       permits recovery in the case where the network has faulted.

     o DHCPINFORM is inherent in the new packet design; a client can
       request configuration parameters other than IPv6 addresses in the
       optional extension headers.

     o Clients MUST listen to their UDP port for the new Reconfigure
       message type from servers, unless they join the appropriate
       multicast group as specified by the DHCP server.

     o Dynamic Updates to DNS are supported in the IPv6 Address
       extension.

     o New extensions have been defined.

   New Internet User Features:

     o Configuration of Dynamic Updates to DNS to support multiple
       implementation policy requirements.




Bound, Perkins            Expires 26 November 1997             [Page 32]


Internet Draft                DHCP Version 6                 26 May 1997


     o Configuration of what policy is enforced when addresses are
       deprecated for dynamic renumbering can be implemented.

     o Configuration of how relay-agents locate remote servers for a
       link can be implemented.

     o An Authentication extension has been added.

     o Configuration of additional addresses for server applications can
       be requested by a client in an implementation.

     o Reclaiming addresses allocated with very long lifetimes can be
       implemented using the Reconfigure message type.

     o Configuration of tightly coupled integration between stateless
       and stateful address autoconfiguration can be implemented.



































Bound, Perkins            Expires 26 November 1997             [Page 33]


Internet Draft                DHCP Version 6                 26 May 1997


References

    [1] S. Alexander and R. Droms.  DHCP Options and BOOTP Vendor
        Extensions.  RFC 2132, March 1997.

    [2] R. Atkinson.  Security Architecture for the Internet Protocol.
        RFC 1825, August 1995.

    [3] S. Bradner.  Key words for use in RFCs to Indicate Requirement
        Levels.  RFC 2119, March 1997.

    [4] S. Bradner and A. Mankin.  The Recommendation for the IP Next
        Generation Protocol.  RFC 1752, January 1995.

    [5] A. Conta and S. Deering.  Internet Control Message Protocol
        (ICMPv6) for the Internet Protocol Version 6 (IPv6).  RFC 1885,
        December 1995.

    [6] S. Deering and R. Hinden.  Internet Protocol, Version 6 (IPv6)
        Specification.  RFC 1883, December 1995.

    [7] R. Droms.  Dynamic Host Configuration Protocol.  RFC 2131, March
        1997.

    [8] R. Hinden and S. Deering.  IP Version 6 Addressing Architecture.
        RFC 1884, December 1995.

    [9] J. McCann, S. Deering, and J. Mogul.  Path MTU Discovery for IP
        version 6.  RFC 1981, August 1996.

   [10] T. Narten, E. Nordmark, and W. Simpson.  Neighbor Discovery for
        IP version 6 (IPv6).  RFC 1970, August 1996.

   [11] C. Perkins.  Extensions to DHCPv6.
        draft-ietf-dhc-dhcpv6ext-06.txt (work in progress), May 1997.

   [12] David C. Plummer.  An Ethernet Address Resolution Protocol:
        Or Converting Network Protocol Addresses to 48.bit Ethernet
        Addresses for Transmission on Ethernet Hardware.  RFC 826,
        November 1982.

   [13] J. B. Postel.  User Datagram Protocol.  RFC 768, August 1980.

   [14] J. B. Postel, Editor.  Internet Protocol.  RFC 791, September
        1981.

   [15] S. Thomson and T. Narten.  IPv6 stateless address
        autoconfiguration.  RFC 1971, August 1996.



Bound, Perkins            Expires 26 November 1997             [Page 34]


Internet Draft                DHCP Version 6                 26 May 1997


   [16] P. Vixie, S. Thomson, Y. Rekhter, and J. Bound.  Dynamic Updates
        in the Domain Name System (DNS).  RFC 2136, April 1997.

















































Bound, Perkins            Expires 26 November 1997             [Page 35]


Internet Draft                DHCP Version 6                 26 May 1997


Chair's Address

   The working group can be contacted via the current chair:

      Ralph Droms
      Computer Science Department
      323 Dana Engineering
      Bucknell University
      Lewisburg, PA 17837

      Phone:  (717) 524-1145
      E-mail:  droms@bucknell.edu



Author's Address

   Questions about this memo can be directed to:

   Jim Bound                            Charles Perkins
   Digital Equipment Corporation        Netcentricity Group
   110 Spitbrook Road, ZKO3-3/U14       Sun Microsystems, Inc.
   Nashua, NH 03062                     2550 Garcia Avenue.
                                        Mountain View, CA  94043
   Phone: +1-603-881-0400               +1-415-336-7153
   Fax:                                 +1-415-336-0673
   E-mail: bound@zk3.dec.com            charles.perkins@corp.sun.com
























Bound, Perkins            Expires 26 November 1997             [Page 36]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/