the same way as addresses are for address assignment, and the rules   described in the section "Client Source Address and Interface   Selection" of the DHCP specification [6] do not apply.   When a requesting router sends a DHCP message, it SHOULD be sent on   the interface associated with the upstream router (ISP network).  The   upstream interface is typically determined by configuration.  This   rule applies even in the case where a separate IA_PD is used for each   downstream interface.   When a requesting router sends a DHCP message directly to a   delegating router using unicast (after receiving the Server Unicast   option from that delegating router), the source address SHOULD be anTroan & Droms          Expires September 1, 2003                [Page 7]Internet-Draft       IPv6 Prefix Options for DHCPv6           March 2003   address from the upstream interface and which is suitable for use by   the delegating router in responding to the requesting router.9. Identity Association for Prefix Delegation Option   The IA_PD option is used to carry a prefix delegation identity   association, the parameters associated with the IA_PD and the   prefixes associated with it.   The format of the IA_PD option is:     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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |         OPTION_IA_PD          |         option-length         |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                         IAID (4 octets)                       |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                              T1                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                              T2                               |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    .                                                               .    .                          IA_PD-options                        .    .                                                               .    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   option-code:      OPTION_IA_PD (TBD)   option-length:    12 + length of IA_PD-options field.   IAID              The unique identifier for this IA_PD; the IAID must                     be unique among the identifiers for all of this                     requesting router's IA_PDs.   T1                The time at which the requesting router should                     contact the delegating router from which the                     prefixes in the IA_PD were obtained to extend the                     lifetimes of the prefixes delegated to the IA_PD;                     T1 is a time duration relative to the current time                     expressed in units of seconds.   T2                The time at which the requesting router should                     contact any available delegating router to extend                     the lifetimes of the prefixes assigned to the                     IA_PD; T2 is a time duration relative to the                     current time expressed in units of seconds.Troan & Droms          Expires September 1, 2003                [Page 8]Internet-Draft       IPv6 Prefix Options for DHCPv6           March 2003   IA_PD-options     Options associated with this IA_PD.   The IA_PD-options field encapsulates those options that are specific   to this IA_PD.  For example, all of the IA_PD Prefix Options carrying   the prefixes associated with this IA_PD are in the IA_PD-options   field.   An IA_PD option may only appear in the options area of a DHCP   message.  A DHCP message may contain multiple IA_PD options.   The status of any operations involving this IA_PD is indicated in a   Status Code option in the IA_PD-options field.   Note that an IA_PD has no explicit "lifetime" or "lease length" of   its own.  When the valid lifetimes of all of the prefixes in a IA_PD   have expired, the IA_PD can be considered as having expired.  T1 and   T2 are included to give delegating routers explicit control over when   a requesting router should contact the delegating router about a   specific IA_PD.   In a message sent by a requesting router to a delegating router,   values in the T1 and T2 fields indicate the requesting router's   preference for those parameters.  The requesting router sets T1 and   T2 to zero if it has no preference for those values.  In a message   sent by a delegating router to a requesting router, the requesting   router MUST use the values in the T1 and T2 fields for the T1 and T2   parameters.  The values in the T1 and T2 fields are the number of   seconds until T1 and T2.   The delegating router selects the T1 and T2 times to allow the   requesting router to extend the lifetimes of any prefixes in the   IA_PD before the lifetimes expire, even if the delegating router is   unavailable for some short period of time.  Recommended values for T1   and T2 are .5 and .8 times the shortest preferred lifetime of the   prefixes in the IA_PD that the delegating router is willing to   extend, respectively.  If the time at which the prefixes in an IA_PD   are to be renewed is to be left to the discretion of the requesting   router, the delegating router sets T1 and T2 to 0.   If a delegating router receives an IA_PD with T1 greater than T2, and   both T1 and T2 are greater than 0, the delegating router ignores the   invalid values of T1 and T2 and processes the IA_PD as though the   delegating router had set T1 and T2 to 0.   If a requesting router receives an IA_PD with T1 greater than T2, and   both T1 and T2 are greater than 0, the client discards the IA_PD   option and processes the remainder of the message as though the   delegating router had not included the IA_PD option.Troan & Droms          Expires September 1, 2003                [Page 9]Internet-Draft       IPv6 Prefix Options for DHCPv6           March 200310. IA_PD Prefix option   The IA_PD Prefix option is used to specify IPv6 address prefixes   associated with an IA_PD.  The IA_PD Prefix option must be   encapsulated in the IA_PD-options field of an IA_PD option.   The format of the IA_PD Prefix option is:     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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |        OPTION_IAPREFIX        |         option-length         |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                      preferred-lifetime                       |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |                        valid-lifetime                         |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    | prefix-length |                                               |    +-+-+-+-+-+-+-+-+          IPv6 prefix                          |    |                           (16 octets)                         |    |                                                               |    |                                                               |    |                                                               |    |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |               |                                               .    +-+-+-+-+-+-+-+-+                                               .    .                       IAprefix-options                        .    .                                                               .    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   option-code:      OPTION_IAPREFIX (TBD)   option-length:    25 + length of IAprefix-options field   preferred-lifetime: The recommended preferred lifetime for the IPv6                     prefix in the option, expressed in units of                     seconds.  A value of 0xFFFFFFFF represents                     infinity.   valid-lifetime:   The valid lifetime for the IPv6 prefix in the                     option, expressed in units of seconds.  A value of                     0xFFFFFFFF represents infinity.   prefix-length:    Length for this prefix in bits   IPv6-prefix:      An IPv6 prefixTroan & Droms          Expires September 1, 2003               [Page 10]Internet-Draft       IPv6 Prefix Options for DHCPv6           March 2003   IAprefix-options: Options associated with this prefix   In a message sent by a requesting router to a delegating router, the   values in the fields can be used to indicate the requesting router's   preference for those values.  The requesting router may send a value   of zero to indicate no preference.  A requesting router may set the   IPv6 prefix field to zero and a given value in the prefix-length   field to indicate a preference for the size of the prefix to be   delegated.   In a message sent by a delegating router the preferred and valid   lifetimes should be set to the values of AdvPreferredLifetime and   AdvValidLifetime as specified in section "Router Configuration   Variables" of RFC2461 [3], unless administratively configured.   A requesting router discards any prefixes for which the preferred   lifetime is greater than the valid lifetime.  A delegating router   ignores the lifetimes set by the requesting router if the preferred   lifetime is greater than the valid lifetime and ignores the values   for T1 and T2 set by the requesting router if those values are   greater than the preferred lifetime.   The values in the preferred and valid lifetimes are the number of   seconds remaining for each lifetime.   An IA_PD Prefix option may appear only in an IA_PD option.  More than   one IA_PD Prefix Option can appear in a single IA_PD option.   The status of any operations involving this IA_PD Prefix option is   indicated in a Status Code option in the IAprefix-options field.11. Delegating Router Solicitation   The requesting router locates and selects a delegating router in the   same way as described in section "DHCP Server Solicitation" of the   DHCP specification [6].  The details of the solicitation process are   described in this section.11.1 Requesting router behaviour   The requesting router creates and transmits a Solicit message as   described in sections "Creation of Solicit Messages" and   "Transmission of Solicit Messages" of the DHCP specification [6].   The requesting router creates an IA_PD and assigns it an IAID.  The   requesting router MUST include the IA_PD option in the Solicit   message.   The requesting router processes any received Advertise messages asTroan & Droms          Expires September 1, 2003               [Page 11]Internet-Draft       IPv6 Prefix Options for DHCPv6           March 2003   described in section "Receipt of Advertise Messages" in the DHCP   specification [6].  The requesting router MAY choose to consider the   presence of advertised prefixes in its decision about which   delegating router to respond to.   The requesting router MUST ignore any Advertise message that includes   a Status Code option containing the value NoPrefixAvail, with the   exception that the requesting router MAY display the associated   status message to the user.11.2 Delegating router behaviour   The delegating router processes Solicit messages from requesting   routers in the same way as described in section "Receipt of Solicit   messages" of the DHCP specification [6].  If the message contains an   IA_PD option and the delegating router is configured to delegate   prefix(es) to the requesting router, the delegating router selects   the prefix(es) to be delegated to the requesting router.  The   mechanism through which the delegating router selects prefix(es) for   delegation is not specified in this document.  Examples of ways in   which the delegating router might select prefix(es) for a requesting   router include: static assignment based on subscription to an ISP;   dynamic assignment from a pool of available prefixes; selection based   on an external authority such as a RADIUS server using the Framed-   IPv6-Prefix option as described in RFC 3162 [7].   If the delegating router cannot delegate any prefixes to an IA_PD in   the message from the requesting router, the delegating router MUST   include the IA_PD in the Advertise message with no prefixes in the   IA_PD and a Status Code option in the IA_PD containing status code   NoPrefixAvail.   If the requesting router includes an IA_PD Prefix option in the IA_PD   option in its Solicit message, the delegating router MAY choose to   use the information in that option to select the prefix(es) or prefix   size to be delegated to the requesting router.   The delegating router sends an Advertise message to the requesting   router in the same way as described in section "Creation and   transmission of Advertise messages" in the DHCP specification [6].   The delegating router MUST include an IA_PD option, identifying any   prefix(es) that the delegating router will delegate to the requesting   router.   If the delegating router will not assign any prefixes to any IA_PDs   in a subsequent Request from the requesting router, the delegating   router MUST send an Advertise message to the requesting router that   includes a Status Code option with code NoPrefixAvail and a statusTroan & Droms          Expires September 1, 2003               [Page 12]Internet-Draft       IPv6 Prefix Options for DHCPv6           March 2003   message for the user, a Server Identifier option with the delegating   router's DUID and a Client Identifier option with the requesting   router's DUID.12. Requesting router initiated prefix delegation   A requesting router uses the same message exchanges as described in   section "DHCP Client-Initiated Configuration Exchange" of the DHCP   specification [6] to obtain or update prefix(es) from a delegating   router.  The requesting router and the delegating router use the   IA_PD Prefix option to exchange information about prefix(es) in much   the same way IA Address options are used for assigned addresses.12.1 Requesting router behaviour   The requesting router uses a Request message to populate IA_PDs with   prefixes.  The requesting router includes one or more IA_PD options   in the Request message.  The delegating router then returns the   prefixes for the IA_PDs to the requesting router in IA_PD options in   a Reply message.   The requesting router includes IA_PD options in any Renew, or Rebind   messages sent by the requesting router.  The IA_PD option include all   of the prefixes the requesting router currently has associated with   that IA_PD.   In some circumstances the requesting router may need verification   that the delegating router still has a valid binding for the   requesting router.  Examples of times when a requesting router may   ask for such verification include:   o  The requesting router reboots.   o  The requesting router's upstream link flaps.   o  The requesting router is physically disconnected from a wired      connection.   If such verification is needed the requesting router MUST initiate a   Rebind/Reply message exchange as described in the section "Creation   and Transmission of Rebind Messages" of the DHCP specification [6],   with the exception that the retransmission parameters should be set   as for the Confirm message, described in the section "Creation and   Transmission of Confirm Messages" of the DHCP specification [6].  The   requesting router includes any IA_PDs, along with prefixes associated   with those IA_PDs in its Rebind message.   Each prefix has valid and preferred lifetimes whose durations areTroan & Droms          Expires September 1, 2003               [Page 13]Internet-Draft       IPv6 Prefix Options for DHCPv6           March 2003   specified in the IA_PD Prefix option for that prefix.  The requesting   router uses Renew and Rebind messages to request the extension of the   lifetimes of a delegated prefix.   The requesting router uses a Release message to return a delegated   prefix to a delegating router.  The prefixes to be released MUST be   included in the IA_PDs.   The Confirm and Decline message types are not used with Prefix   Delegation.   Upon the receipt of a valid Reply message, for each IA_PD the   requesting router assigns a subnet from each of the delegated   prefixes to each of the links to which the associated interfaces are   attached, with the following exception: the requesting router MUST   NOT assign any delegated prefixes or subnets from the delegated   prefix(es) to the link through which it received the DHCP message