draft-ietf-dhc-dhcpv6-opt-prefix-delegation-02.txt

DHCPv6协议在Linux操作系统下的一个客户端实现。

6. Overview of DHCP with Prefix Delegation

   Prefix delegation with DHCP is independent of address assignment with
   DHCP.  A requesting router can use DHCP for just prefix delegation or
   for prefix delegation along with address assignment and other
   configuration information.

   A requesting router first creates an IA_PD and assigns it an IAID.
   The requesting router then transmits a Solicit message containing an
   IA_PD option describing the IA_PD.  Delegating routers that can
   delegate prefixes to the IA_PD respond to the requesting router with
   an Advertise message.

   The requesting router may include prefixes in the IA_PDs as a hint to
   the delegating router about specific prefixes for which the
   requesting router has a preference.

   When the requesting router has identified a delegating router, the
   requesting router uses a Request message to populate the IA_PDs with
   prefixes.  The requesting router includes one or more IA_PD options
   in the Request message.  The delegating router returns prefixes and
   other information about the IA_PDs to the requesting router in IA_PD
   options in a Reply message.  The requesting router records the
   lifetimes for the delegated prefix(es) and uses the prefix(es) as
   described in the previous section.

   Before the valid lifetime on each delegated prefix expires, the
   requesting router includes the prefix in an IA_PD option sent in a
   Renew message to the delegating router.  The delegating router
   responds by returning the prefix with updated lifetimes to the
   requesting router.

7. Interface Selection

   Delegated prefixes are not associated with a particular interface in
   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 an



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   address from the upstream interface and which is suitable for use by
   the delegating router in responding to the requesting router.

8. 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 contacts
                     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 contacts
                     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.



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   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 recontacts 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 0 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, 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.

9. 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.







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   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 prefix

   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



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   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.

   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.

10. 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.

10.1 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 as
   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.

10.2 Delegating router behaviour

   The delegating router processes Solicit messages from requesting



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   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 Reply 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 status
   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.

11. 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.




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