Internet Draft: DHC-IPV4-AUTOCONFIG                             R. Troll
Document: draft-ietf-dhc-ipv4-autoconfig-00.txt           September 1998
Expires: March 1999

      Automaticly Choosing an IP Address in an Ad-Hoc IPv4 Network


Status of this memo

     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

     To view the entire list of current Internet-Drafts, please check
     the "1id-abstracts.txt" listing contained in the Internet-Drafts
     Shadow Directories on (Africa),
     (Northern Europe), (Southern Europe),
     (Pacific Rim), (US East Coast), or (US
     West Coast).


     With operating systems appearing in more and more devices, as well
     as computers appearing in more and more aspects of everyday life,
     communication between networked devices is increasingly important.
     The communication mechanism between these devices must be able to
     not only support the office LAN environment, but must also scale to
     larger WANS and the internet.

     This draft describes a method by which a host may automaticly give
     itself an IPv4 address, so that it will be able to use IP
     applications in all of the above environments.  This mechanism is
     in use today by a few operating systems, and additional information
     on those implementations is also provided.

1. Introduction

     Now that networked applications are becoming more prevalent,

Troll                                                           [Page 1]

Internet Draft          IPv4 Auto-Configuration           September 1998

     operating systems are migrating towards more scalable network
     protocols such as IP, allowing them to work in all sizes of
     environments.  However, there is a price to pay for this migration
     -- IP requires configuration that other protocols (IPX, Appletalk)
     do not require.

     Dynamic creation of usable ad-hoc networks is very useful when
     there are only a few machines on the entire network.  (For example,
     a dentist's office may only have a couple of machines.)  In order
     to allow a site such as this to use IP, the machines must each be
     configured with an IP address.  OS's wish to retain the minimal
     configuration that was necessary under their non-IP network stacks.

     Dynamic configuration protocols such as DHCP [DHCP] allow a site
     administrator to take care of the network configuration for a
     machine remotely.  By requesting network parameters via DHCP, the
     site administrator may provide all information necessary without
     the host's owner having to do anything.  However, not all sites
     have a central administrator to take care of this.

     To accommodate smaller networks, the OS may decide to intelligently
     choose an IP address for itself in the absence of a central
     configuration mechanism.

     This document describes a method by which an OS may determine
     whether or not to auto-configure itself an IP address, as well as
     how to inter-operate cleanly with an existing managed

1.1 Conventions Used in the Document

     The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
     in this document are to be interpreted as defined in "Key Words for
     Use in RFCs to Indicate Requirement Levels" [KEYWORDS]

1.2 Terminology

          Site Administrator
                         A Site Administrator is the person or
                         organization responsible for handing out IP
                         addresses to client machines.

          DHCP Client    A DHCP Client is an Internet host using DHCP to
                         obtain configuration parameters such as a
                         network address.

Troll                                                           [Page 2]

Internet Draft          IPv4 Auto-Configuration           September 1998

          DHCP Server    A DHCP Server is an Internet host that returns
                         configuration parameters to DHCP Clients.

2. To Choose or Not To Choose

     The first thing an Internet host should do is request an IP address
     via DHCP [DHCP].  This is done by sending out a DHCPDISCOVER
     message, with various tags set indicating what options the DHCP
     Client would like to receive information for [DHCPOPT].  The DHCP
     Client SHOULD also send the DHCP AutoConfigure option described in

     According to [DHCP], Section 4.4.1, the amount of time over which a
     DHCP Client should listen for DHCPOFFERS is implementation
     dependant.  During this time, if a DHCPOFFER is received, network
     configuration MUST occur as described in [DHCP] and [DHCPAC].

     If, during this time, no valid DHCPOFFERS are received, the DHCP
     Client is free to auto-configure an IP address according to section
     3 of this document.

2.1 Rebinding an Existing IP Address

     If the DHCP Client already has an existing IP address, it MUST
     follow the instructions outlined in [DHCP].  If the client winds up
     back in the INIT state, refer to section 2 of this document.

3. Choosing an IP Address

     Once a DHCP Client has determined it must auto-configure an IP
     address, it chooses an address.  The algorithm for choosing an
     address is implementation dependant.  The address range to use
     SHOULD be "169.254/16", which is registered with the IANA as the
     LINKLOCAL net.

     If choosing an address in this range, the DHCP Client MUST not use
     the first 256 or the last 256, as these are reserved for future

     When an address is chosen, the DHCP Client MUST test to see if the
     address is already in use.  For example, if the client is on a
     network that supports ARP, the client may issue an ARP request for
     the suggested request.  When broadcasting an ARP request for the
     suggested address, the client must fill in its own hardware address
     as the sender's hardware address, and 0 as the sender's IP address,
     to avoid confusing ARP caches in other hosts on the same subnet.
     If the network address appears to be in use, the client MUST choose
     another address, and try again.  The client MUST keep choosing

Troll                                                           [Page 3]

Internet Draft          IPv4 Auto-Configuration           September 1998

     addresses until it either finds one, or it has tried more then the
     autoconfig-retry count.  The autoconfig-retry count is
     implementation specific, and should be based on the algorithm used
     for choosing an IP address.  This retry count is present to make
     sure that DHCP Clients auto-configuring on busy auto-configured
     network segments do not loop infinitely looking for an IP address.

4. Ongoing Checks for a DHCP Server

     When the client originally sent out it's request, there may have
     been a network problem stopping the DHCP Server from responding.
     To make sure this is not the case, a DHCP Client with an auto-
     configured IP address MUST keep checking for an active DHCP Server.
     To do this, the DHCP Client MUST attempt to fetch an IP address as
     described in section 1 of this document.

     When rechecking, when the DHCP Client has determined no DHCP Server
     is responding, it MUST wait a period of time and try again.  For
     Ethernet implementations, the DHCP Client SHOULD check every 5

     If the DHCP Client receives a response from a DHCP Server, it MUST
     respond and attempt to obtain a lease from the server (per the DHCP
     specification).  If the client is successful in obtaining a new
     lease, and the internet host does not support multiple addresses on
     the interface being configured, it MUST drop any existing auto-
     configured IP address, and all active connections, while moving to
     the new address.  If the internet host does support multiple
     addresses on the interface, it MAY keep the auto-configured address

     If the DHCP response is an AutoConfigure [DHCPAC] response set to
     "DoNOTAutoConfigure", the host MUST drop all connections, give up
     any existing auto-configured IP address, and continue checking for
     a DHCP server.

5. Current Vendor Implementations

     As of this writing, Microsoft and Apple have operating systems that
     contain this functionality.  Descriptions of the implementation
     dependant parts are listed below.

5.1. Microsoft Windows 98

     With the initial release of Windows 98, Microsoft introduced auto-
     configuration functionality.  When developed, the AutoConfig

Troll                                                           [Page 4]

Internet Draft          IPv4 Auto-Configuration           September 1998

     [DHCPAC] specification did not exist, so the initial release does
     not contain this functionality.

     The Win98 DHCP Client sends out a total of 4 DHCPDISCOVERs, with an
     inter-packet interval of 6 seconds.  When no response is received
     after all 4 packets (24 seconds), it will autoconfigure an address.

     The auto-configure retry count for Windows 98 is 10.  After trying
     10 auto-configured IP addresses, and finding all are taken, the
     host will boot without an IP address.

5.2. Apple MacOS 8.5

     MacOS 8.5 sends three DHCPDISCOVER packets, with timeouts of 4, 8,
     and then 16 seconds.  When no response is received from all of
     these requests (28 seconds), it will autoconfigure.

6. Security Considerations

     All of the existing DHCP security considerations apply here.  This
     functionality does not introduce any new security concerns.

7. Acknowledgments

     I'd like to thank Microsoft and Apple for their help in writing
     this document.

8. Copyright

     Copyright (C) The Internet Society 1998. All Rights Reserved.

     This document and translations of it may be copied and furnished to
     others, and derivative works that comment on or otherwise explain
     it or assist in its implementation may be prepared, copied,
     published and distributed, in whole or in part, without restriction
     of any kind, provided that the above copyright notice and this
     paragraph are included on all such copies and derivative works.
     However, this document itself may not be modified in any way, such
     as by removing the copyright notice or references to the Internet
     Society or other Internet organizations, except as needed for the
     purpose of developing Internet standards in which case the
     procedures for copyrights defined in the Internet Standards process
     must be followed, or as required to translate it into languages
     other than English.

     The limited permissions granted above are perpetual and will not be

Troll                                                           [Page 5]

Internet Draft          IPv4 Auto-Configuration           September 1998

     revoked by the Internet Society or its successors or assigns.

     This document and the information contained herein is provided on

9. References

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


     [DHCPOPT] Alexander, S. and Droms, R., "DHCP Options and BOOTP
     Vendor Extension", RFC 2132, March 1997.


     [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
     Requirement Levels", RFC 2119, Harvard University, March 1997.


     [IPv6SAC] Thomson, S. and Narten, T. "IPv6 Stateless Address
     Autoconfiguration", RFC 1971, August 1996


     [DHCPAC] Troll, R. "DHCP Option to Disable Stateless Auto-
     Configuration in IPv4 Clients", RFC XXXXX, November 1998


10. Author's Address

     Ryan Troll
     Network Development
     Carnegie Mellon
     5000 Forbes Avenue
     Pittsburgh, PA 15213

     Phone: (412) 268-8691

Troll                                                           [Page 6]

Internet Draft          IPv4 Auto-Configuration           September 1998

     This document will expire March 1999

Troll                                                           [Page 7]