Submitted to DHC Working Group                        Peter De Schrijver
INTERNET DRAFT                                              Yves T'Joens
<draft-ietf-dhc-pv4-reconfigure-01.txt>                 Christian Hublet

                                                               June 2000
                                                   Expires December 2000

        Dynamic host configuration : DHCP reconfigure extension

Status of this memo

   This  document  is  an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   Distribution of this memo is unlimited.


   This draft  defines  extensions  to  DHCP  [DHCP]  to  allow  dynamic
   reconfiguration  of a single host triggered by the DHCP server (eg. a
   new IP address). This is  achieved  by  introducing  a  unicast  DHCP
   FORCERENEW message which forces the client to the RENEW state.

1. Introduction

   The  procedures  as  described  within  this  draft allow the dynamic
   reconfiguration of individual hosts.

1.1 Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",  "SHALL  NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2. DHCP force renew

   This section describes the DHCP force renew extension.

2.1 Terminology

   DHCP client : host to be reconfigured using DHCP.

   DHCP server : server which configured the DHCP client.

2.2 Force renew procedures

   The DHCP server sends a force renew message to the client. The client
   will change its state to the RENEW state. The client will then try to
   renew its lease according to normal DHCP procedures.  If  the  server
   wants  to assign a new IP address to the client, it will reply to the
   DHCP REQUEST with a DHCP NAK. The client will then  go  back  to  the
   init  state and broadcast a DHCP DISCOVER message. The server can now
   assign a new IP address to the client by replying with a DHCP  OFFER.
   If  the force renew message is lost, the DHCP server will not receive
   a DHCP REQUEST from the client and  it  should  retransmit  the  DHCP
   FORCERENEW  message using an exponential backoff algorithm. Depending
   on the bandwidth of the network between server and client, the server
   should   choose   a   delay.   This   delay  grows  exponentially  as
   retransmissions  fail.  The  amount  of  retransmissions  should   be

2.3 Rationale

   This  approach  has  a  number of advantages. It does not require new
   states to be added to the DHCP client implementation. This  minimizes
   the  amount of code to be changed. It also allows lease RENEWAL to be
   driven by the server, which can be used to optimize network usage  or
   DHCP server load.

3. Extended DHCP state diagram

+--------+                  +------+
| Init / |              +-->+ Init +<---------------+-------------------+
| Reboot |              |   +--+---+                |                   |
+---+----+          DHCPNAK/ -/Send DHCPDISCOVER    |                   |
    |               Restart    |     (broadcast)    |                   |
    |                   |      v   v-------------+  |                   |
 -/Send DHCPREQUEST     | +----+------+    DHCPOFFER/DHCPDECLINE        |
    |   (broadcast)     | | Selecting |----------+  |                   |
    v                   | +----+------+             |                   |
+---+----+              |   DHCPOFFER/DHCPREQUEST   |                   |
| Reboot +--------------+  (broadcast)              |                   |
+---+----+                     v                    |                   |
    |                     +----+-------+            DHCPNAK /halt network
    |                     + Requesting |            |       lease expired
   DHCPACK/               +----+-------+            |                   |
   Record lease                |                    |                   |
   set timers              DHCPACK/Record lease     |                   |
    |                          v   Set T1 & T2      |                   |
    |                       +--+----+DHCPFORCE  +---+---+           +---+----+
    +---------------------->+ Bound +---------->+ Renew +---------->+ Rebind |
                            +--+-+--+T1 expires +-+-+---+ T2 expires+--+--+--+
                               ^     /DHCPREQUEST | |     /broadcast      |
                          DHCPACK      to leasing | |     DHCPREQUEST     |
                               |        server    | |                     |

4. Message layout

   -----      ---------------
   'op'       BOOTREPLY
   'htype'    (From "Assigned Numbers" RFC)
   'hlen'     (Hardware address length in octets)
   'hops'     0
   'xid'      selected by server
   'secs'     0
   'ciaddr'   0
   'yiaddr'   0
   'siaddr'   0
   'flags'    0
   'giaddr'   0
   'chaddr'   client's hardware address
   'sname'    0
   'file'     0
   'options'  options

   DHCP  option  53  (DHCP  message type) is extended with a new value :

5. Failover Considerations

   A DHCP server should only send a DHCPFORCERENEW when it's fully aware
   of  the  current  state of the DHCP client. In practice this means it
   should  only  send  a  DHCPFORCERENEW   when   in   "PARTNER   DOWN",
   "COMMUNICATIONS  INTERRUPTED"  or  "NORMAL"  state, and only for DHCP
   clients of which the state is synchronised. (see [DHCP-FAILOVER])

6. IANA Considerations

   The new value for DHCP option 53 (DHCP message type)  to  indicate  a

7. Security Considerations

   As  in  some network environments DHCPFORCERENEW can be used to snoop
   and spoof traffic, the DHCPFORCERENEW message MUST  be  authenticated
   using a shared secret based mechanism as described in [DHCP-AUTH].

8. References

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

   [DHCP-AUTH] R. Droms, "Authentication for DHCP Messages", draft-ietf-
   dhc-authentication-12, October 1999, Work in progress.

   [DHCP-FAILOVER]  R.  Droms, K. Kinnear, M. Stapp, B. Volz, S. Gonczi,
   G. Rabil, M. Dooley, A. Kapur, "DHCP Failover Protocol",  draft-ietf-
   dhc-failover-06.txt, March 2000, Work in progress.

9. Contacts

   Peter De Schrijver
   Alcatel Network Strategy Group
   Francis Wellesplein 1, 2018 Antwerp, Belgium
   Phone : +32 3 240 8569
   E-mail :

   Yves T'joens
   Alcatel Network Strategy Group
   Francis Wellesplein 1, 2018 Antwerp, Belgium
   Phone : +32 3 240 7890
   E-mail :

   Christian Hublet
   Alcatel Carrier Internetworking Division
   De Villermontstraat 28, 2550 Kontich, Belgium
   Phone : +32 3 450 3322
   E-mail :