path: root/doc/rfc/rfc1541.txt

Network Working Group                                           R. Droms
Request for Comments: 1541                           Bucknell University
Obsoletes: 1531                                             October 1993
Category: Standards Track


                  Dynamic Host Configuration Protocol

Status of this memo

   This RFC specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" for the standardization state and status
   of this protocol.  Distribution of this memo is unlimited.

Abstract

   The Dynamic Host Configuration Protocol (DHCP) provides a framework
   for passing configuration information to hosts on a TCP/IP network.
   DHCP is based on the Bootstrap Protocol (BOOTP) [7], adding the
   capability of automatic allocation of reusable network addresses and
   additional configuration options [19].  DHCP captures the behavior of
   BOOTP relay agents [7, 23], and DHCP participants can interoperate
   with BOOTP participants [9].  Due to some errors introduced into RFC
   1531 in the editorial process, this memo is reissued as RFC 1541.


Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
   1.1 Related Work. . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2 Problem definition and issues . . . . . . . . . . . . . . . .  4
   1.3 Requirements. . . . . . . . . . . . . . . . . . . . . . . . .  5
   1.4 Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  6
   1.5 Design goals. . . . . . . . . . . . . . . . . . . . . . . . .  6
   2. Protocol Summary . . . . . . . . . . . . . . . . . . . . . . .  8
   2.1 Configuration parameters repository . . . . . . . . . . . . . 10
   2.2 Dynamic allocation of network addresses . . . . . . . . . . . 11
   3. The Client-Server Protocol . . . . . . . . . . . . . . . . . . 11
   3.1 Client-server interaction - allocating a network address. . . 12
   3.2 Client-server interaction - reusing a  previously allocated
       network address . . . . . . . . . . . . . . . . . . . . . . . 17
   3.3 Interpretation and representation of time values. . . . . . . 19
   3.4 Host parameters in DHCP . . . . . . . . . . . . . . . . . . . 19
   3.5 Use of DHCP in clients with multiple interfaces . . . . . . . 20
   3.6 When clients should use DHCP. . . . . . . . . . . . . . . . . 20
   4. Specification of the DHCP client-server protocol . . . . . . . 21



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   4.1 Constructing and sending DHCP messages. . . . . . . . . . . . 21
   4.2 DHCP server administrative controls . . . . . . . . . . . . . 23
   4.3 DHCP server behavior. . . . . . . . . . . . . . . . . . . . . 24
   4.3.1 DHCPDISCOVER message. . . . . . . . . . . . . . . . . . . . 24
   4.3.2 DHCPREQUEST message . . . . . . . . . . . . . . . . . . . . 27
   4.3.3 DHCPDECLINE message . . . . . . . . . . . . . . . . . . . . 29
   4.3.4 DHCPRELEASE message . . . . . . . . . . . . . . . . . . . . 29
   4.4 DHCP client behavior. . . . . . . . . . . . . . . . . . . . . 29
   4.4.1 Initialization and allocation of network address. . . . . . 29
   4.4.2 Initialization with known network address . . . . . . . . . 33
   4.4.3 Initialization with a known DHCP server address . . . . . . 34
   4.4.4 Reacquisition and expiration. . . . . . . . . . . . . . . . 34
   4.4.5 DHCPRELEASE . . . . . . . . . . . . . . . . . . . . . . . . 35
   5. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 35
   6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   7. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
   8. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
   A. Host Configuration Parameters  . . . . . . . . . . . . . . . . 39

List of Figures

   1. Format of a DHCP message . . . . . . . . . . . . . . . . . . .  9
   2. Format of the 'flags' field. . . . . . . . . . . . . . . . . . 10
   3. Timeline diagram of messages exchanged between DHCP client and
      servers when allocating a new network address. . . . . . . . . 15
   4. Timeline diagram of messages exchanged between DHCP client and
      servers when reusing a previously allocated network address. . 18
   5. State-transition diagram for DHCP clients. . . . . . . . . . . 31

List of Tables

   1. Description of fields in a DHCP message. . . . . . . . . . . . 14
   2. DHCP messages. . . . . . . . . . . . . . . . . . . . . . . . . 16
   3. Fields and options used by DHCP servers. . . . . . . . . . . . 25
   4. Fields and options used by DHCP clients. . . . . . . . . . . . 32

1. Introduction

   The Dynamic Host Configuration Protocol (DHCP) provides configuration
   parameters to Internet hosts.  DHCP consists of two components: a
   protocol for delivering host-specific configuration parameters from a
   DHCP server to a host and a mechanism for allocation of network
   addresses to hosts.

   DHCP is built on a client-server model, where designated DHCP server
   hosts allocate network addresses and deliver configuration parameters
   to dynamically configured hosts.  Throughout the remainder of this
   document, the term "server" refers to a host providing initialization



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   parameters through DHCP, and the term "client" refers to a host
   requesting initialization parameters from a DHCP server.

   A host should not act as a DHCP server unless explicitly configured
   to do so by a system administrator.  The diversity of hardware and
   protocol implementations in the Internet would preclude reliable
   operation if random hosts were allowed to respond to DHCP requests.
   For example, IP requires the setting of many parameters within the
   protocol implementation software.  Because IP can be used on many
   dissimilar kinds of network hardware, values for those parameters
   cannot be guessed or assumed to have correct defaults.  Also,
   distributed address allocation schemes depend on a polling/defense
   mechanism for discovery of addresses that are already in use.  IP
   hosts may not always be able to defend their network addresses, so
   that such a distributed address allocation scheme cannot be
   guaranteed to avoid allocation of duplicate network addresses.

   DHCP supports three mechanisms for IP address allocation.  In
   "automatic allocation", DHCP assigns a permanent IP address to a
   host.  In "dynamic allocation", DHCP assigns an IP address to a host
   for a limited period of time (or until the host explicitly
   relinquishes the address).  In "manual allocation", a host's IP
   address is assigned by the network administrator, and DHCP is used
   simply to convey the assigned address to the host.  A particular
   network will use one or more of these mechanisms, depending on the
   policies of the network administrator.

   Dynamic allocation is the only one of the three mechanisms that
   allows automatic reuse of an address that is no longer needed by the
   host to which it was assigned.  Thus, dynamic allocation is
   particularly useful for assigning an address to a host that will be
   connected to the network only temporarily or for sharing a limited
   pool of IP addresses among a group of hosts that do not need
   permanent IP addresses.  Dynamic allocation may also be a good choice
   for assigning an IP address to a new host being permanently connected
   to a network where IP addresses are sufficiently scarce that it is
   important to reclaim them when old hosts are retired.  Manual
   allocation allows DHCP to be used to eliminate the error-prone
   process of manually configuring hosts with IP addresses in
   environments where (for whatever reasons) it is desirable to manage
   IP address assignment outside of the DHCP mechanisms.

   The format of DHCP messages is based on the format of BOOTP messages,
   to capture the BOOTP relay agent behavior described as part of the
   BOOTP specification [7, 23] and to allow interoperability of existing
   BOOTP clients with DHCP servers.  Using BOOTP relaying agents
   eliminates the necessity of having a DHCP server on each physical
   network segment.



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1.1 Related Work

   There are several Internet protocols and related mechanisms that
   address some parts of the dynamic host configuration problem.  The
   Reverse Address Resolution Protocol (RARP) [10] (through the
   extensions defined in the Dynamic RARP (DRARP) [5]) explicitly
   addresses the problem of network address discovery, and includes an
   automatic IP address assignment mechanism.  The Trivial File Transfer
   Protocol (TFTP) [20] provides for transport of a boot image from a
   boot server.  The Internet Control Message Protocol (ICMP) [16]
   provides for informing hosts of additional routers via "ICMP
   redirect" messages.  ICMP also can provide subnet mask information
   through the "ICMP mask request" message and other information through
   the (obsolete) "ICMP information request" message.  Hosts can locate
   routers through the ICMP router discovery mechanism [8].

   BOOTP is a transport mechanism for a collection of configuration
   information.  BOOTP is also extensible, and official extensions [17]
   have been defined for several configuration parameters.  Morgan has
   proposed extensions to BOOTP for dynamic IP address assignment [15].
   The Network Information Protocol (NIP), used by the Athena project at
   MIT, is a distributed mechanism for dynamic IP address assignment
   [19].  The Resource Location Protocol RLP [1] provides for location
   of higher level services.  Sun Microsystems diskless workstations use
   a boot procedure that employs RARP, TFTP and an RPC mechanism called
   "bootparams" to deliver configuration information and operating
   system code to diskless hosts.  (Sun Microsystems, Sun Workstation
   and SunOS are trademarks of Sun Microsystems, Inc.)  Some Sun
   networks also use DRARP and an auto-installation mechanism to
   automate the configuration of new hosts in an existing network.

   In other related work, the path minimum transmission unit (MTU)
   discovery algorithm can determine the MTU of an arbitrary internet
   path [14].  Comer and Droms have proposed the use of the Address
   Resolution Protocol (ARP) as a transport protocol for resource
   location and selection [6].  Finally, the Host Requirements RFCs [3,
   4] mention specific requirements for host reconfiguration and suggest
   a scenario for initial configuration of diskless hosts.

1.2 Problem definition and issues

   DHCP is designed to supply hosts with the configuration parameters
   defined in the Host Requirements RFCs.  After obtaining parameters
   via DHCP, a host should be able to exchange packets with any other
   host in the Internet.  The parameters supplied by DHCP are listed in
   Appendix A.





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   Not all of these parameters are required for a newly initialized
   host.  A client and server may negotiate for the transmission of only
   those parameters required by the client or specific to a particular
   subnet.

   DHCP allows but does not require the configuration of host parameters
   not directly related to the IP protocol.  DHCP also does not address
   registration of newly configured hosts with the Domain Name System
   (DNS) [12, 13].

   DHCP is not intended for use in configuring routers.

1.3 Requirements

   Throughout this document, the words that are used to define the
   significance of particular requirements are capitalized.  These words
   are:

      o "MUST"

        This word or the adjective "REQUIRED" means that the
        item is an absolute requirement of this specification.

      o "MUST NOT"

        This phrase means that the item is an absolute prohibition
        of this specification.

      o "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 should be understood and
        the case carefully weighed before choosing a different course.

      o "SHOULD NOT"

        This phrase means that there may exist valid reasons in
        particular circumstances when the listed behavior is acceptable
        or even useful, but the full implications should be understood
        and the case carefully weighed before implementing any behavior
        described with this label.









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      o "MAY"

        This word or the adjective "OPTIONAL" means that this item is
        truly optional.  One vendor may choose to include the item
        because a particular marketplace requires it or because it
        enhances the product, for example; another vendor may omit the
        same item.

1.4 Terminology

   This document uses the following terms:

      o "DHCP client"

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

      o "DHCP server"

        A DHCP server is an Internet host that returns configuration
        parameters to DHCP clients.

      o "BOOTP relay agent"

        A BOOTP relay agent is an Internet host or router that passes
        DHCP messages between DHCP clients and DHCP servers.  DHCP is
        designed to use the same relay agent behavior as specified in
        the BOOTP protocol specification.

      o "binding"

        A binding is a collection of configuration parameters, including
        at least an IP address, associated with or "bound to" a DHCP
        client.  Bindings are managed by DHCP servers.

1.5 Design goals

   The following list gives general design goals for DHCP.

      o 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.







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      o Hosts should require no manual configuration.  Each host should
        be able to discover appropriate local configuration parameters
        without user intervention and incorporate those parameters into
        its own configuration.

      o Networks should require no hand configuration for individual
        hosts.  Under normal circumstances, the network manager should
        not have to enter any per-host configuration parameters.

      o DHCP should not require a server on each subnet.  To allow for
        scale and economy, DHCP must work across routers or through the
        intervention of BOOTP/DHCP relay agents.

      o A DHCP host must be prepared to receive multiple responses to a
        request for configuration parameters.  Some installations may
        include multiple, overlapping DHCP servers to enhance
        reliability and increase performance.

      o DHCP must coexist with statically configured, non-participating
        hosts and with existing network protocol implementations.

      o DHCP must interoperate with the BOOTP relay agent behavior as
        described by RFC 951 and by Wimer [21].

      o DHCP must provide service to existing BOOTP clients.

   The following list gives design goals specific to the transmission of
   the network layer parameters.  DHCP must:

      o Guarantee that any specific network address will not be in
        use by more than one host at a time,

      o Retain host configuration across host reboot.  A host should,
        whenever possible, be assigned the same configuration parameters
        (e.g., network address) in response to each request,

      o Retain host configuration across server reboots, and, whenever
        possible, a host should be assigned the same configuration
        parameters despite restarts of the DHCP mechanism,

      o Allow automatic assignment of configuration parameters to new
        hosts to avoid hand configuration for new hosts,

      o Support fixed or permanent allocation of configuration
        parameters to specific hosts.






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2. Protocol Summary

   From the client's point of view, DHCP is an extension of the BOOTP
   mechanism.  This behavior allows existing BOOTP clients to
   interoperate with DHCP servers without requiring any change to the
   clients' initialization software.  A separate document details the
   interactions between BOOTP and DHCP clients and servers [9].  There
   are some new, optional transactions that optimize the interaction
   between DHCP clients and servers that are described in sections 3 and
   4.

   Figure 1 gives the format of a DHCP message and table 1 describes
   each of the fields in the DHCP message.  The numbers in parentheses
   indicate the size of each field in octets.  The names for the fields
   given in the figure will be used throughout this document to refer to
   the fields in DHCP messages.

   There are two primary differences between DHCP and BOOTP.  First,
   DHCP defines mechanisms through which clients can be assigned a
   network address for a fixed lease, allowing for serial reassignment
   of network addresses to different clients.  Second, DHCP provides the
   mechanism for a client to acquire all of the IP configuration
   parameters that it needs in order to operate.

   DHCP introduces a small change in terminology intended to clarify the
   meaning of one of the fields.  What was the "vendor extensions" field
   in BOOTP has been re-named the "options" field in DHCP. Similarly,
   the tagged data items that were used inside the BOOTP "vendor
   extensions" field, which were formerly referred to as "vendor
   extensions," are now termed simply "options."

   DHCP defines a new 'client identifier' option that is used to pass an
   explicit client identifier to a DHCP server.  This change eliminates
   the overloading of the 'chaddr' field in BOOTP messages, where
   'chaddr' is used both as a hardware address for transmission of BOOTP
   reply messages and as a client identifier.  The 'client identifier'
   option may contain a hardware address, identical to the contents of
   the 'chaddr' field, or it may contain another type of identifier,
   such as a DNS name.  Other client identifier types may be defined as
   needed for use with DHCP.  New client identifier types will be
   registered with the IANA [18] and will be included in new revisions
   of the Assigned Numbers document, as well as described in detail in
   future revisions of the DHCP Options [2].








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   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     op (1)    |   htype (1)   |   hlen (1)    |   hops (1)    |
   +---------------+---------------+---------------+---------------+
   |                            xid (4)                            |
   +-------------------------------+-------------------------------+
   |           secs (2)            |           flags (2)           |
   +-------------------------------+-------------------------------+
   |                          ciaddr  (4)                          |
   +---------------------------------------------------------------+
   |                          yiaddr  (4)                          |
   +---------------------------------------------------------------+
   |                          siaddr  (4)                          |
   +---------------------------------------------------------------+
   |                          giaddr  (4)                          |
   +---------------------------------------------------------------+
   |                                                               |
   |                          chaddr  (16)                         |
   |                                                               |
   |                                                               |
   +---------------------------------------------------------------+
   |                                                               |
   |                          sname   (64)                         |
   +---------------------------------------------------------------+
   |                                                               |
   |                          file    (128)                        |
   +---------------------------------------------------------------+
   |                                                               |
   |                          options (312)                        |
   +---------------------------------------------------------------+

                  Figure 1:  Format of a DHCP message

   DHCP clarifies the interpretation of the 'siaddr' field as the
   address of the server to use in the next step of the client's
   bootstrap process.  A DHCP server may return its own address in the
   'siaddr' field, if the server is prepared to supply the next
   bootstrap service (e.g., delivery of an operating system executable
   image).  A DHCP server always returns its own address in the 'server
   identifier' option.

   The options field is now variable length, with the minimum extended
   to 312 octets.  This brings the minimum size of a DHCP message up to
   576 octets, the minimum IP datagram size a host must be prepared to
   accept [3].  DHCP clients may negotiate the use of larger DHCP
   messages through the 'Maximum DHCP message size' option.  The options
   field may be further extended into the 'file' and 'sname' fields.



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   A new option, called 'vendor specific information', has been added to
   allow for expansion of the number of options that can be supported
   [2].  Options encapsulated as 'vendor specific information' must be
   carefully defined and documented so as to allow for interoperability
   between clients and servers from diferent vendors.  In particular,
   vendors defining 'vendor specific information' MUST document those
   options in the form of the DHCP Options document, MUST choose to
   represent those options either in data types already defined for DHCP
   options or in other well-defined data types, and MUST choose options
   that can be readily encoded in configuration files for exchange with
   servers provided by other vendors.  Options included as 'vendor
   specific options' MUST be readily supportable by all servers.

                                    1 1 1 1 1 1
                0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                B|             MBZ             |
                -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                B:  BROADCAST flag

                MBZ:  MUST BE ZERO (reserved for future use)

                Figure 2:  Format of the 'flags' field

   DHCP uses the 'flags' field [21].  The leftmost bit is defined as the
   BROADCAST (B) flag.  The semantics of this flag are discussed in
   section 4.1 of this document.  The remaining bits of the flags field
   are reserved for future use.  They MUST be set to zero by clients and
   ignored by servers and relay agents.  Figure 2 gives the format of
   the 'flags' field.

2.1 Configuration parameters repository

   The first service provided by DHCP is to provide persistent storage
   of network parameters for network clients.  The model of DHCP
   persistent storage is that the DHCP service stores a key-value entry
   for each client, where the key is some unique identifier (for
   example, an IP subnet number and a unique identifier within the
   subnet) and the value contains the configuration parameters for the
   client.

   For example, the key might be the pair (IP-subnet-number, hardware-
   address), allowing for serial or concurrent reuse of a hardware
   address on different subnets, and for hardware addresses that may not
   be globally unique.  Alternately, the key might be the pair (IP-
   subnet-number, hostname), allowing the server to assign parameters
   intelligently to a host that has been moved to a different subnet or



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   has changed hardware addresses (perhaps because the network interface
   failed and was replaced).

   A client can query the DHCP service to retrieve its configuration
   parameters.  The client interface to the configuration parameters
   repository consists of protocol messages to request configuration
   parameters and responses from the server carrying the configuration
   parameters.

2.2 Dynamic allocation of network addresses

   The second service provided by DHCP is the allocation of temporary or
   permanent network (IP) addresses to hosts.  The basic mechanism for
   the dynamic allocation of network addresses is simple: a client
   requests the use of an address for some period of time.  The
   allocation mechanism (the collection of DHCP servers) guarantees not
   to reallocate that address within the requested time and attempts to
   return the same network address each time the client requests an
   address.  In this document, the period over which a network address
   is allocated to a client is referred to as a "lease" [11].  The
   client may extend its lease with subsequent requests.  The client may
   issue a message to release the address back to the server when the
   client no longer needs the address.  The client may ask for a
   permanent assignment by asking for an infinite lease.  Even when
   assigning "permanent" addresses, a server may choose to give out
   lengthy but non-infinite leases to allow detection of the fact that
   the host has been retired.

   In some environments it will be necessary to reassign network
   addresses due to exhaustion of available addresses.  In such
   environments, the allocation mechanism will reuse addresses whose
   lease has expired.  The server should use whatever information is
   available in the configuration information repository to choose an
   address to reuse.  For example, the server may choose the least
   recently assigned address.  As a consistency check, the allocation
   mechanism may probe the reused address, e.g., with an ICMP echo
   request, before allocating the address, and the client will probe the
   newly received address, e.g., with ARP.

3. The Client-Server Protocol

   DHCP uses the BOOTP message format defined in RFC 951 and given in
   table 1 and figure 1.  The 'op' field of each DHCP message sent from
   a client to a server contains BOOTREQUEST. BOOTREPLY is used in the
   'op' field of each DHCP message sent from a server to a client.

   The first four octets of the 'options' field of the DHCP message
   contain the (decimal) values 99, 130, 83 and 99, respectively (this



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   is the same magic cookie as is defined in RFC 1497).  The remainder
   of the 'options' field consists a list of tagged parameters that are
   called "options".  All of the "vendor extensions" listed in RFC 1497
   are also DHCP options.  A separate document gives the complete set of
   options defined for use with DHCP [2].

   Several options have been defined so far.  One particular option -
   the "DHCP message type" option - must be included in every DHCP
   message.  This option defines the "type" of the DHCP message.
   Additional options may be allowed, required, or not allowed,
   depending on the DHCP message type.

   Throughout this document, DHCP messages that include a 'DHCP message
   type' option will be referred to by the type of the message; e.g., a
   DHCP message with 'DHCP message type' option type 1 will be referred
   to as a "DHCPDISCOVER" message.

3.1 Client-server interaction - allocating a network address

   The following summary of the protocol exchanges between clients and
   servers refers to the DHCP messages described in table 2.  The
   timeline diagram in figure 3 shows the timing relationships in a
   typical client-server interaction.  If the client already knows its
   address, some steps may be omitted; this abbreviated interaction is
   described in section 3.2.

   1. The client broadcasts a DHCPDISCOVER message on its local physical
      subnet.  The DHCPDISCOVER message may include options that suggest
      values for the network address and lease duration.  BOOTP relay
      agents may pass the message on to DHCP servers not on the same
      physical subnet.

   2. Each server may respond with a DHCPOFFER message that includes an
      available network address in the 'yiaddr' field (and other
      configuration parameters in DHCP options).  Servers need not
      reserve the offered network address, although the protocol will
      work more efficiently if the server avoids allocating the offered
      network address to another client.  The server unicasts the
      DHCPOFFER message to the client (using the DHCP/BOOTP relay agent
      if necessary) if possible, or may broadcast the message to a
      broadcast address (preferably 255.255.255.255) on the client's
      subnet.

   3. The client receives one or more DHCPOFFER messages from one or
      more servers.  The client may choose to wait for multiple
      responses.  The client chooses one server from which to request
      configuration parameters, based on the configuration parameters
      offered in the DHCPOFFER messages.  The client broadcasts a



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      DHCPREQUEST message that MUST include the 'server identifier'
      option to indicate which server it has selected, and may include
      other options specifying desired configuration values.  This
      DHCPREQUEST message is broadcast and relayed through DHCP/BOOTP
      relay agents.  To help ensure that any DHCP/BOOTP relay agents
      forward the DHCPREQUEST message to the same set of DHCP servers
      that received the original DHCPDISCOVER message, the DHCPREQUEST
      message must use the same value in the DHCP message header's
      'secs' field and be sent to the same IP broadcast address as the
      original DHCPDISCOVER message.  The client times out and
      retransmits the DHCPDISCOVER message if the client receives no
      DHCPOFFER messages.

   4. The servers receive the DHCPREQUEST broadcast from the client.
      Those servers not selected by the DHCPREQUEST message use the
      message as notification that the client has declined that server's
      offer.  The server selected in the DHCPREQUEST message commits the
      binding for the client to persistent storage and responds with a
      DHCPACK message containing the configuration parameters for the
      requesting client.  The combination of 'chaddr' and assigned
      network address constitute an unique identifier for the client's
      lease and are used by both the client and server to identify a
      lease referred to in any DHCP messages.  The 'yiaddr' field in the
      DHCPACK messages is filled in with the selected network address.

      If the selected server is unable to satisfy the DHCPREQUEST message
      (e.g., the requested network address has been allocated), the
      server SHOULD respond with a DHCPNAK message.

      A server may choose to mark addresses offered to clients in
      DHCPOFFER messages as unavailable.  The server should mark an
      address offered to a client in a DHCPOFFER message as available if
      the server receives no DHCPREQUEST message from that client.


















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   FIELD      OCTETS       DESCRIPTION
   -----      ------       -----------

   op            1  Message op code / message type.
                    1 = BOOTREQUEST, 2 = BOOTREPLY
   htype         1  Hardware address type, see ARP section in "Assigned
                    Numbers" RFC; e.g., '1' = 10mb ethernet.
   hlen          1  Hardware address length (e.g.  '6' for 10mb
                    ethernet).
   hops          1  Client sets to zero, optionally used by relay-agents
                    when booting via a relay-agent.
   xid           4  Transaction ID, a random number chosen by the
                    client, used by the client and server to associate
                    messages and responses between a client and a
                    server.
   secs          2  Filled in by client, seconds elapsed since client
                    started trying to boot.
   flags         2  Flags (see figure 2).
   ciaddr        4  Client IP address; filled in by client in
                    DHCPREQUEST if verifying previously allocated
                    configuration parameters.
   yiaddr        4  'your' (client) IP address.
   siaddr        4  IP address of next server to use in bootstrap;
                    returned in DHCPOFFER, DHCPACK and DHCPNAK by
                    server.
   giaddr        4  Relay agent IP address, used in booting via a
                    relay-agent.
   chaddr       16  Client hardware address.
   sname        64  Optional server host name, null terminated string.
   file        128  Boot file name, null terminated string; "generic"
                    name or null in DHCPDISCOVER, fully qualified
                    directory-path name in DHCPOFFER.
   options     312  Optional parameters field.  See the options
                    documents for a list of defined options.

             Table 1:  Description of fields in a DHCP message















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                Server          Client          Server
            (not selected)                    (selected)

                  v               v               v
                  |               |               |
                  |     Begins initialization     |
                  |               |               |
                  | _____________/|\_____________ |
                  |/ DHCPDISCOVER | DHCPDISCOVER \|
                  |               |               |
              Determines          |          Determines
             configuration        |         configuration
                  |               |               |
                  |\              |  ____________/|
                  | \_________    | /DHCPOFFER    |
                  |  DHCPOFFER\   |/              |
                  |            \  |               |
                  |       Collects replies        |
                  |              \|               |
                  |     Selects configuration     |
                  |               |               |
                  | _____________/|\_____________ |
                  |/ DHCPREQUEST  |  DHCPREQUEST \|
                  |               |               |
                  |               |     Commits configuration
                  |               |               |
                  |               | _____________/|
                  |               |/ DHCPACK      |
                  |               |               |
                  |    Initialization complete    |
                  |               |               |
                  .               .               .
                  .               .               .
                  |               |               |
                  |      Graceful shutdown        |
                  |               |               |
                  |               |\_____________ |
                  |               |  DHCPRELEASE \|
                  |               |               |
                  |               |        Discards lease
                  |               |               |
                  v               v               v

     Figure 3: Timeline diagram of messages exchanged between DHCP
               client and servers when allocating a new network address






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   Message         Use
   -------         ---

   DHCPDISCOVER -  Client broadcast to locate available servers.

   DHCPOFFER    -  Server to client in response to DHCPDISCOVER with
                   offer of configuration parameters.

   DHCPREQUEST  -  Client broadcast to servers requesting offered
                   parameters from one server and implicitly declining
                   offers from all others.

   DHCPACK      -  Server to client with configuration parameters,
                   including committed network address.

   DHCPNAK      -  Server to client refusing request for configuration
                   parameters (e.g., requested network address already
                   allocated).

   DHCPDECLINE  -  Client to server indicating configuration parameters
                   (e.g., network address) invalid.

   DHCPRELEASE  -  Client to server relinquishing network address and
                   cancelling remaining lease.

                          Table 2:  DHCP messages

   5. The client receives the DHCPACK message with configuration
      parameters.  The client performs a final check on the parameters
      (e.g., ARP for allocated network address), and notes the duration
      of the lease and the lease identification cookie specified in the
      DHCPACK message.  At this point, the client is configured.  If the
      client detects a problem with the parameters in the DHCPACK
      message, the client sends a DHCPDECLINE message to the server and
      restarts the configuration process.  The client should wait a
      minimum of ten seconds before restarting the configuration process
      to avoid excessive network traffic in case of looping.

      If the client receives a DHCPNAK message, the client restarts the
      configuration process.

      The client times out and retransmits the DHCPREQUEST message if the
      client receives neither a DHCPACK or a DHCPNAK message.  The client
      retransmits the DHCPREQUEST according to the retransmission
      algorithm in section 4.1.  If the client receives neither a DHCPACK
      or a DHCPNAK message after ten retransmissions of the DHCPREQUEST
      message, the client reverts to INIT state and restarts the
      initialization process.  The client SHOULD notify the user that the



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      initialization process has failed and is restarting.

   6. The client may choose to relinquish its lease on a network address
      by sending a DHCPRELEASE message to the server.  The client
      identifies the lease to be released by including its network
      address in the 'ciaddr' field and its hardware address in the
      'chaddr' field.

3.2 Client-server interaction - reusing a previously allocated network
    address

   If a client remembers and wishes to reuse a previously allocated
   network address (allocated either by DHCP or some means outside the
   protocol), a client may choose to omit some of the steps described in
   the previous section.  The timeline diagram in figure 4 shows the
   timing relationships in a typical client-server interaction for a
   client reusing a previously allocated network address.

      1. The client broadcasts a DHCPREQUEST message on its local subnet.
         The DHCPREQUEST message includes the client's network address in
         the 'ciaddr' field.  DHCP/BOOTP relay agents pass the message on
         to DHCP servers not on the same subnet.

      2. Servers with knowledge of the client's configuration parameters
         respond with a DHCPACK message to the client.

         If the client's request is invalid (e.g., the client has moved
         to a new subnet), servers may respond with a DHCPNAK message to
         the client.

      3. The client receives the DHCPACK message with configuration
         prameters.  The client performs a final check on the parameters
         (as in section 3.1), and notes the duration of the lease and
         the lease identification cookie specified in the DHCPACK
         message.  At this point, the client is configured.

         If the client detects a problem with the parameters in the
         DHCPACK message, the client sends a DHCPDECLINE message to the
         server and restarts the configuration process by requesting a
         new network address.  This action corresponds to the client
         moving to the INIT state in the DHCP state diagram, which is
         described in section 4.4.









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                Server          Client          Server

                  v               v               v
                  |               |               |
                  |             Begins            |
                  |         initialization        |
                  |               |               |
                  |              /|\              |
                  |  ___________/ | \___________  |
                  | /DHCPREQUEST  |  DHCPREQUEST\ |
                  |/              |              \|
                  |               |               |
               Locates            |            Locates
            configuration         |         configuration
                  |               |               |
                  |\              |              /|
                  | \             |  ___________/ |
                  |  \            | /  DHCPACK    |
                  |   \_______    |/              |
                  |    DHCPACK\   |               |
                  |         Initialization        |
                  |            complete           |
                  |              \|               |
                  |               |               |
                  |          (Subsequent          |
                  |            DHCPACKS           |
                  |            ignored)           |
                  |               |               |
                  |               |               |
                  v               v               v

     Figure 4: Timeline diagram of messages exchanged between DHCP
               client and servers when reusing a previously allocated
               network address

         If the client receives a DHCPNAK message, it cannot reuse its
         remembered network address.  It must instead request a new
         address by restarting the configuration process, this time
         using the (non-abbreviated) procedure described in section
         3.1.  This action also corresponds to the client moving to
         the INIT state in the DHCP state diagram.

         The client times out and retransmits the DHCPREQUEST message if
         the client receives neither a DHCPACK nor a DHCPNAK message.
         The   time between retransmission MUST be chosen according to
         the algorithm given in section 4.1.  If the client receives no
         answer after transmitting 4 DHCPREQUEST messages, the client
         MAY choose to use the previously allocated network address and



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         configuration parameters for the remainder of the unexpired
         lease.  This corresponds to moving to BOUND state in the client
         state transition diagram shown in figure 5.

      4. The client may choose to relinquish its lease on a network
         address by sending a DHCPRELEASE message to the server.  The
         client identifies the lease to be released with the lease
         identification cookie.

         Note that in this case, where the client retains its network
         address locally, the client will not normally relinquish its
         lease during a graceful shutdown.  Only in the case where the
         client explicitly needs to relinquish its lease, e.g., the client
         is about to be moved to a different subnet, will the client send
         a DHCPRELEASE message.

3.3 Interpretation and representation of time values

   A client acquires a lease for a network address for a fixed period of
   time (which may be infinite).  Throughout the protocol, times are to
   be represented in units of seconds.  The time value of 0xffffffff is
   reserved to represent "infinity".  The minimum lease duration is one
   hour.

   As clients and servers may not have synchronized clocks, times are
   represented in DHCP messages as relative times, to be interpreted
   with respect to the client's local clock.  Representing relative
   times in units of seconds in an unsigned 32 bit word gives a range of
   relative times from 0 to approximately 100 years, which is sufficient
   for the relative times to be measured using DHCP.

   The algorithm for lease duration interpretation given in the previous
   paragraph assumes that client and server clocks are stable relative
   to each other.  If there is drift between the two clocks, the server
   may consider the lease expired before the client does.  To
   compensate, the server may return a shorter lease duration to the
   client than the server commits to its local database of client
   information.

3.4 Host parameters in DHCP

   Not all clients require initialization of all parameters listed in
   Appendix A.  Two techniques are used to reduce the number of
   parameters transmitted from the server to the client.  First, most of
   the parameters have defaults defined in the Host Requirements RFCs;
   if the client receives no parameters from the server that override
   the defaults, a client uses those default values.  Second, in its
   initial DHCPDISCOVER or DHCPREQUEST message, a client may provide the



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   server with a list of specific parameters the client is interested
   in.

   The client SHOULD include the 'maximum DHCP message size' option to
   let the server know how large the server may make its DHCP messages.
   The parameters returned to a client may still exceed the space
   allocated to options in a DHCP message.  In this case, two additional
   options flags (which must appear in the 'options' field of the
   message) indicate that the 'file' and 'sname' fields are to be used
   for options.

   The client can inform the server which configuration parameters the
   client is interested in by including the 'parameter request list'
   option.  The data portion of this option explicitly lists the options
   requested by tag number.

   In addition, the client may suggest values for the network address
   and lease time in the DHCPDISCOVER message.  The client may include
   the 'requested IP address' option to suggest that a particular IP
   address be assigned, and may include the 'IP address lease time'
   option to suggest the lease time it would like.  No other options
   representing "hints" at configuration parameters are allowed in a
   DHCPDISCOVER or DHCPREQUEST message.  The 'ciaddr' field is to be
   filled in only in a DHCPREQUEST message when the client is requesting
   use of a previously allocated IP address.

   If a server receives a DHCPREQUEST message with an invalid 'ciaddr',
   the server SHOULD respond to the client with a DHCPNAK message and
   may choose to report the problem to the system administrator.  The
   server may include an error message in the 'message' option.

3.5 Use of DHCP in clients with multiple interfaces

   A host with multiple network interfaces must use DHCP through each
   interface independently to obtain configuration information
   parameters for those separate interfaces.

3.6 When clients should use DHCP

   A host should use DHCP to reacquire or verify its IP address and
   network parameters whenever the local network parameters may have
   changed; e.g., at system boot time or after a disconnection from the
   local network, as the local network configuration may change without
   the host's or user's knowledge.

   If a host has knowledge of a previous network address and is unable
   to contact a local DHCP server, the host may continue to use the
   previous network address until the lease for that address expires.



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   If the lease expires before the host can contact a DHCP server, the
   host must immediately discontinue use of the previous network address
   and may inform local users of the problem.

4. Specification of the DHCP client-server protocol

   In this section, we assume that a DHCP server has a block of network
   addresses from which it can satisfy requests for new addresses.  Each
   server also maintains a database of allocated addresses and leases in
   local permanent storage.

4.1 Constructing and sending DHCP messages

   DHCP clients and servers both construct DHCP messages by filling in
   fields in the fixed format section of the message and appending
   tagged data items in the variable length option area.  The options
   area includes first a four-octet 'magic cookie' (which was described
   in section 3), followed by the options.  The last option must always
   be the 'end' option.

   DHCP uses UDP as its transport protocol.  DHCP messages from a client
   to a server are sent to the 'DHCP server' port (67), and DHCP
   messages from a server to a client are sent to the 'DHCP client' port
   (68).

   DHCP messages broadcast by a client prior to that client obtaining
   its IP address must have the source address field in the IP header
   set to 0.

   If the 'giaddr' field in a DHCP message from a client is non-zero,
   the server sends any return messages to the 'DHCP server' port on the
   DHCP relaying agent whose address appears in 'giaddr'.  If the
   'giaddr' field is zero, the client is on the same subnet, and the
   server sends any return messages to either the client's network
   address, if that address was supplied in the 'ciaddr' field, or to
   the client's hardware address or to the local subnet broadcast
   address.

   If the options in a DHCP message extend into the 'sname' and 'file'
   fields, the 'option overload' option MUST appear in the 'options'
   field, with value 1, 2 or 3, as specified in the DHCP options
   document [2].  If the 'option overload' option is present in the
   'options' field, the options in the 'options' field MUST be
   terminated by an 'end' option, and MAY contain one or more 'pad'
   options to fill the options field.  The options in the 'sname' and
   'file' fields (if in use as indicated by the 'options overload'
   option) MUST begin with the first octet of the field, MUST be
   terminated by an 'end' option, and MUST be followed by 'pad' options



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   to fill the remainder of the field.  Any individual option in the
   'options', 'sname' and 'file' fields MUST be entirely contained in
   that field.  The options in the 'options' field MUST be interpreted
   first, so that any 'option overload' options may be interpreted.  The
   'file' field MUST be interpreted next (if the 'option overload'
   option indicates that the 'file' field contains DHCP options),
   followed by the 'sname' field.

   DHCP clients are responsible for all message retransmission.  The
   client MUST adopt a retransmission strategy that incorporates a
   randomized exponential backoff algorithm to determine the delay
   between retransmissions.  The delay before the first retransmission
   MUST be 4 seconds randomized by the value of a uniform random number
   chosen from the range -1 to +1.  Clients with clocks that provide
   resolution granularity of less than one second may choose a non-
   integer randomization value.  The delay before the next
   retransmission MUST be 8 seconds randomized by the value of a uniform
   number chosen from the range -1 to +1.  The retransmission delay MUST
   be doubled with subsequent retransmissions up to a maximum of 64
   seconds.  The client MAY provide an indication of retransmission
   attempts to the user as an indication of the progress of the
   configuration process.  The protocol specification in the remainder
   of this section will describe, for each DHCP message, when it is
   appropriate for the client to retransmit that message forever, and
   when it is appropriate for a client to abandon that message and
   attempt to use a different DHCP message.

   Normally, DHCP servers and BOOTP relay agents attempt to deliver
   DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using
   unicast delivery.  The IP destination address (in the IP header) is
   set to the DHCP 'yiaddr' address and the link-layer destination
   address is set to the DHCP 'chaddr' address.  Unfortunately, some
   client implementations are unable to receive such unicast IP
   datagrams until the implementation has been configured with a valid
   IP address (leading to a deadlock in which the client's IP address
   cannot be delivered until the client has been configured with an IP
   address).

   A client that cannot receive unicast IP datagrams until its protocol
   software has been configured with an IP address SHOULD set the
   BROADCAST bit in the 'flags' field to 1 in any DHCPDISCOVER or
   DHCPREQUEST messages that client sends.  The BROADCAST bit will
   provide a hint to the DHCP server and BOOTP relay agent to broadcast
   any messages to the client on the client's subnet.  A client that can
   receive unicast IP datagrams before its protocol software has been
   configured SHOULD clear the BROADCAST bit to 0.  The BOOTP
   clarifications document discusses the ramifications of the use of the
   BROADCAST bit [21].



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   A server or relay agent sending or relaying a DHCP message directly
   to a DHCP client (i.e., not to a relay agent specified in the
   'giaddr' field) SHOULD examine the BROADCAST bit in the 'flags'
   field.  If this bit is set to 1, the DHCP message SHOULD be sent as
   an IP broadcast using an IP broadcast address (preferably
   255.255.255.255) as the IP destination address and the link-layer
   broadcast address as the link-layer destination address.  If the
   BROADCAST bit is cleared to 0, the message SHOULD be sent as an IP
   unicast to the IP address specified in the 'yiaddr' field and the
   link-layer address specified in the 'chaddr' field.  If unicasting is
   not possible, the message MAY be sent as an IP broadcast using an IP
   broadcast address (preferably 255.255.255.255) as the IP destination
   address and the link-layer broadcast address as the link-layer
   destination address.

4.2 DHCP server administrative controls

   DHCP servers are not required to respond to every DHCPDISCOVER and
   DHCPREQUEST message they receive.  For example, a network
   administrator, to retain stringent control over the hosts attached to
   the network, may choose to configure DHCP servers to respond only to
   hosts that have been previously registered through some external
   mechanism.  The DHCP specification describes only the interactions
   between clients and servers when the clients and servers choose to
   interact; it is beyond the scope of the DHCP specification to
   describe all of the administrative controls that system
   administrators might want to use.  Specific DHCP server
   implementations may incorporate any controls or policies desired by a
   network administrator.

   In some environments, a DHCP server will have to consider the values
   of the 'chaddr' field and/or the 'class-identifier' option included
   in the DHCPDISCOVER or DHCPREQUEST messages when determining the
   correct parameters for a particular client.  For example, an
   organization might have a separate bootstrap server for each type of
   client it uses, requiring the DHCP server to examine the 'class-
   identifier' to determine which bootstrap server address to return in
   the 'siaddr' field of a DHCPOFFER or DHCPACK message.

   A DHCP server must use some unique identifier to associate a client
   with its lease.  The client may choose to explicitly provide the
   identifier through the 'client identifier' option.  If the client
   does not provide a 'client identifier' option, the server MUST use
   the contents of the 'chaddr' field to identify the client.

   DHCP clients are free to use any strategy in selecting a DHCP server
   among those from which the client receives a DHCPOFFER message.  The
   client implementation of DHCP should provide a mechanism for the user



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   to select directly the 'class-identifier' value.

4.3 DHCP server behavior

   A DHCP server processes incoming DHCP messages from a client based on
   the current state of the binding for that client.  A DHCP server can
   receive the following messages from a client:

      o DHCPDISCOVER

      o DHCPREQUEST

      o DHCPDECLINE

      o DHCPRELEASE

   Table 3 gives the use of the fields and options in a DHCP message by
   a server.  The remainder of this section describes the action of the
   DHCP server for each possible incoming message.

4.3.1 DHCPDISCOVER message

   When a server receives a DHCPDISCOVER message from a client, the
   server chooses a network address for the requesting client.  If no
   address is available, the server may choose to report the problem to
   the system administrator and may choose to reply to the client with a
   DHCPNAK message.  If the server chooses to respond to the client, it
   may include an error message in the 'message' option.  If an address
   is available, the new address should be chosen as follows:

   o The client's previous address as recorded in the client's binding,
     if that address is in the server's pool of available addresses and
     not already allocated, else

   o The address requested in the 'Requested IP Address' option, if that
     address is valid and not already allocated, else

   o A new address allocated from the server's pool of available
     addresses.












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  Field      DHCPOFFER            DHCPACK             DHCPNAK
  -----      ---------            -------             -------

  'op'       BOOTREPLY            BOOTREPLY           BOOTREPLY
  'htype'    (From "Assigned Numbers" RFC)
  'hlen'     (Hardware address length in octets)
  'hops'     0                    0                   0
  'xid'      'xid' from client    'xid' from client   'xid' from client
             DHCPDISCOVER         DHCPREQUEST         DHCPREQUEST
             message              message             message
  'secs'     0                    0                   0
  'ciaddr'   0                    'ciaddr' from       'ciaddr' from
                                  DHCPREQUEST or 0    DHCPREQUEST or 0
  'yiaddr'   IP address offered   IP address          0
             to client            assigned to client
  'siaddr'   IP address of next   IP address of next  0
             bootstrap server     bootstrap server
  'flags'    if 'giaddr' is not 0 then 'flags' from client message else 0
  'giaddr'   0                    0                   0
  'chaddr'   'chaddr' from        'chaddr' from       'chaddr' from
             client               client DHCPREQUEST  client DHCPREQUEST
             DHCPDISCOVER         message             message
             message
  'sname'    Server host name     Server host name    (unused)
             or options           or options
  'file'     Client boot file     Client boot file    (unused)
             name or options      name or options
  'options'  options              options

  Option                   DHCPOFFER        DHCPACK          DHCPNAK
  ------                   ---------        -------          -------

  Requested IP address     MUST NOT         MUST NOT         MUST NOT
  IP address lease time    MUST             MUST             MUST NOT
  Use 'file'/'sname'       MAY              MAY              MUST NOT
  fields
  DHCP message type        DHCPOFFER        DHCPACK          DHCPNAK
  Parameter request list   MUST NOT         MUST NOT         MUST NOT
  Message                  SHOULD           SHOULD           SHOULD
  Client identifier        MUST NOT         MUST NOT         MUST NOT
  Class identifier         MUST NOT         MUST NOT         MUST NOT
  Server identifier        MUST             MAY              MAY
  Maximum message size     MUST NOT         MUST NOT         MUST NOT
  All others               MAY              MAY              MUST NOT

           Table 3:  Fields and options used by DHCP servers





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   As described in section 4.2, a server MAY, for administrative
   reasons, assign an address other than the one requested, or may
   refuse to allocate an address to a particular client even though free
   addresses are available.

   While not required for correct operation of DHCP, the server should
   not reuse the selected network address before the client responds to
   the server's DHCPOFFER message.  The server may choose to record the
   address as offered to the client.

   The server must also choose an expiration time for the lease, as
   follows:

   o IF the client has not requested a specific lease in the
     DHCPDISCOVER message and the client already has an assigned network
     address, the server returns the lease expiration time previously
     assigned to that address (note that the client must explicitly
     request a specific lease to extend the expiration time on a
     previously assigned address), ELSE

   o IF the client has not requested a specific lease in the
     DHCPDISCOVER message and the client does not have an assigned
     network address, the server assigns a locally configured default
     lease time, ELSE

   o IF the client has requested a specific lease in the DHCPDISCOVER
     message (regardless of whether the client has an assigned network
     address), the server may choose either to return the requested
     lease (if the lease is acceptable to local policy) or select
     another lease.

   Once the network address and lease have been determined, the server
   constructs a DHCPOFFER message with the offered configuration
   parameters.  It is important for all DHCP servers to return the same
   parameters (with the possible exception of a newly allocated network
   address) to ensure predictable host behavior regardless of the which
   server the client selects.  The configuration parameters MUST be
   selected by applying the following rules in the order given below.
   The network administrator is responsible for configuring multiple
   DHCP servers to ensure uniform responses from those servers.  The
   server MUST return to the client:










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   o The client's network address, as determined by the rules given
     earlier in this section, and the subnet mask for the network to
     which the client is connected,

   o The expiration time for the client's lease, as determined by the
     rules given earlier in this section,

   o Parameters requested by the client, according to the following
     rules:

        -- IF the server has been explicitly configured with a default
           value for the parameter, the server MUST include that value
           in an appropriate option in the 'option' field, ELSE

        -- IF the server recognizes the parameter as a parameter
           defined in the Host Requirements Document, the server MUST
           include the default value for that parameter as given in the
           Host Requirements Document in an appropriate option in the
           'option' field, ELSE

        -- The server MUST NOT return a value for that parameter,

   o Any parameters from the existing binding that differ from the Host
     Requirements documents defaults,

   o Any parameters specific to this client (as identified by
     the contents of 'chaddr' in the DHCPDISCOVER or DHCPREQUEST
     message), e.g., as configured by the network administrator,

   o Any parameters specific to this client's class (as identified
     by the contents of the 'class identifier' option in the
     DHCPDISCOVER or DHCPREQUEST message), e.g., as configured by
     the network administrator; the parameters MUST be identified
     by an exact match between the client's 'client class' and the
     client class identified in the server,

   o Parameters with non-default values on the client's subnet.

   The server inserts the 'xid' field from the DHCPDISCOVER message into
   the 'xid' field of the DHCPOFFER message and sends the DHCPOFFER
   message to the requesting client.

4.3.2 DHCPREQUEST message

   A DHCPREQUEST message may come from a client responding to a
   DHCPOFFER message from a server, or from a client verifying a
   previously allocated IP address.  If the DHCPREQUEST message contains
   a 'server identifier' option, the message is in response to a



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RFC 1541          Dynamic Host Configuration Protocol       October 1993


   DHCPOFFER message.  Otherwise, the message is a request to renew or
   extend an existing lease.

   Consider first the case of a DHCPREQUEST message in response to a
   DHCPOFFER message.  If the server is identified in the 'server
   identifier' option in the DHCPREQUEST message, the server checks to
   confirm that the requested parameters are acceptable.  Usually, the
   requested parameters will match those returned to the client in the
   DHCPOFFER message; however, the client may choose to request a
   different lease duration.  Also, there is no requirement that the
   server cache the parameters from the DHCPOFFER message.  The server
   must simply check that the parameters requested in the DHCPREQUEST
   are acceptable.  If the parameters are acceptable, the server records
   the new client binding and returns a DHCPACK message to the client.

   If the requested parameters are unacceptable, e.g., the requested
   lease time is unacceptable to local policy, the server sends a
   DHCPNAK message to the client.  The server may choose to return an
   error message in the 'message' option.

   If a different server is identified in the 'server identifier' field,
   the client has selected a different server from which to obtain
   configuration parameters.  The server may discard any information it
   may have cached about the client's request, and may free the network
   address that it had offered to the client.

   Note that the client may choose to collect several DHCPOFFER messages
   and select the "best" offer.  The client indicates its selection by
   identifying the offering server in the DHCPREQUEST message.  If the
   client receives no acceptable offers, the client may choose to try
   another DHCPDISCOVER message.  Therefore, the servers may not receive
   a specific DHCPREQUEST from which they can decide whether or not the
   client has accepted the offer.  Because the servers have not
   committed any network address assignments on the basis of a
   DHCPOFFER, servers are free to reuse offered network addresses in
   response to subsequent requests.  As an implementation detail,
   servers should not reuse offered addresses and may use an
   implementation-specific timeout mechanism to decide when to reuse an
   offered address.

   In the second case, when there is no 'server identifier' option, the
   client is renewing or extending a previously allocated IP address.
   The server checks to confirm that the requested parameters are
   acceptable.  If the parameters specified in the DHCPREQUEST message
   match the previous parameters, or if the request for an extension of
   the lease (indicated by an extended 'IP address lease time' option)
   is acceptable, the server returns a DHCPACK message to the requesting
   client.  Otherwise, the server returns a DHCPNAK message to the



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   client.  In particular, if the previously allocated network address
   in the 'ciaddr' field from the client does not match the network
   address recorded by the server for that client, the server sends a
   DHCPNAK to the client.

   A DHCP server chooses the parameters to return in a DHCPACK message
   according to the same rules as used in constructing a DHCPOFFER
   message, as given in section 4.3.1.

4.3.3 DHCPDECLINE message

   If the server receives a DHCPDECLINE message, the client has
   discovered through some other means that the suggested network
   address is already in use.  The server MUST mark the network address
   as not allocated and SHOULD notify the local system administrator of
   a possible configuration problem.

4.3.4 DHCPRELEASE message

   Upon receipt of a DHCPRELEASE message, the server marks the network
   address as not allocated.  The server should retain a record of the
   client's initialization parameters for possible reuse in response to
   subsequent requests from the client.

4.4 DHCP client behavior

   Figure 5 gives a state-transition diagram for a DHCP client.  A
   client can receive the following messages from a server:

      o DHCPOFFER

      o DHCPACK

      o DHCPNAK

   Table 4 gives the use of the fields and options in a DHCP message by
   a client.  The remainder of this section describes the action of the
   DHCP client for each possible incoming message.  The description in
   the following section corresponds to the full configuration procedure
   previously described in section 3.1, and the text in the subsequent
   section corresponds to the abbreviated configuration procedure
   described in section 3.2.

4.4.1 Initialization and allocation of network address

   The client begins in INIT state and forms a DHCPDISCOVER message.
   The client should wait a random time between one and ten seconds to
   desynchronize the use of DHCP at startup.  The client sets 'ciaddr'



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   to 0x00000000.  The client MAY request specific parameters by
   including the 'parameter request list' option.  The client MAY
   suggest a network address and/or lease time by including the
   'requested IP address' and 'IP address lease time' options.  The
   client MUST include its hardware address in the 'chaddr' field for
   use in delivery of DHCP reply messages.  The client MAY include a
   different unique identifier in the 'client identifier' option.  If
   the client does not include the 'client identifier' option, the
   server will use the contents of the 'chaddr' field to identify the
   client's lease.

   The client generates and records a random transaction identifier and
   inserts that identifier into the 'xid' field.  The client records its
   own local time for later use in computing the lease expiration.  The
   client then broadcasts the DHCPDISCOVER on the local hardware
   broadcast address to 0xffffffff IP broadcast address and 'DHCP
   server' UDP port.

   If the 'xid' of an arriving DHCPOFFER message does not match the
   'xid' of the most recent DHCPDISCOVER message, the DHCPOFFER message
   must be silently discarded.  Any arriving DHCPACK messages must be
   silently discarded.

   The client collects DHCPOFFER messages over a period of time, selects
   one DHCPOFFER message from the (possibly many) incoming DHCPOFFER
   messages (e.g., the first DHCPOFFER message or the DHCPOFFER message
   from the previously used server) and extracts the server address from
   the 'server identifier' option in the DHCPOFFER message.  The time
   over which the client collects messages and the mechanism used to
   select one DHCPOFFER are implementation dependent.  The client may
   perform a check on the suggested address to ensure that the address
   is not 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 sends a
   DHCPDECLINE message to the server and waits for another DHCPOFFER. As
   the client does not have a valid network address, the client must
   broadcast the DHCPDECLINE message.










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RFC 1541          Dynamic Host Configuration Protocol       October 1993


 --------                               -------
|        | +-------------------------->|       |<-------------------+
| INIT/  | |     +-------------------->| INIT  |                    |
| REBOOT |DHCPNAK/         +---------->|       |<---+               |
|        |Restart|         |            -------     |               |
 --------  |  DHCPNAK/     |               |                        |
    |      Discard offer   |      -/Send DHCPDISCOVER               |
-/Send DHCPREQUEST         |               |                        |
    |      |     |      DHCPACK            v        |               |
 -----------     |   (not accept.)/   -----------   |               |
|           |    |  Send DHCPDECLINE |           |  |               |
| REBOOTING |    |         |         | SELECTING |  |               |
|           |    |        /          |           |  |               |
 -----------     |       /            -----------   |               |
    |            |      /                  |        |               |
DHCPACK/         |     /  +----------------+        |               |
Record lease,    |    |   v                         |               |
set timers      ------------                        |               |
    |   +----->|            |             DHCPNAK, Lease expired/   |
    |   |      | REQUESTING |                  Halt network         |
    DHCPOFFER/ |            |                       |               |
    Discard     ------------                        |               |
    |   |        |        |                   -----------           |
    |   +--------+     DHCPACK/              |           |          |
    |              Record lease, set    -----| REBINDING |          |
    |                timers T1, T2     /     |           |          |
    |                     |        DHCPACK/   -----------           |
    |                     v     Record lease, set   ^               |
    +----------------> -------      /Timers T1,T2   |               |
               +----->|       |<---+                |               |
               |      | BOUND |<---+                |               |
  DHCPOFFER, DHCPACK, |       |    |            T2 expires/   DHCPNAK/
   DHCPNAK/Discard     -------     |             Broadcast  Halt network
               |       | |         |            DHCPREQUEST         |
               +-------+ |        DHCPACK/          |               |
                    T1 expires/   Record lease, set |               |
                 Send DHCPREQUEST timers T1, T2     |               |
                 to leasing server |                |               |
                         |   ----------             |               |
                         |  |          |------------+               |
                         +->| RENEWING |                            |
                            |          |----------------------------+
                             ----------

          Figure 5:  State-transition diagram for DHCP clients






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RFC 1541          Dynamic Host Configuration Protocol       October 1993


  Field      DHCPDISCOVER          DHCPREQUEST           DHCPDECLINE,
                                                         DHCPRELEASE
  -----      ------------          -----------           -----------

  'op'       BOOTREQUEST           BOOTREQUEST           BOOTREQUEST
  'htype'    (From "Assigned Numbers" RFC)
  'hlen'     (Hardware address length in octets)
  'hops'     0                     0                     0
  'xid'      selected by client    selected by client    selected by
                                                         client
  'secs'     (opt.)                (opt.)                0
  'flags'    Set 'BROADCAST'       Set 'BROADCAST'
             flag if client        flag if client
             requires broadcast    requires broadcast
             reply                 reply
             0
  'ciaddr'   0                     previously            ciaddr
                                   allocated newtork
                                   address
  'yiaddr'   0                     0                     0
  'siaddr'   0                     0                     0
  'giaddr'   0                     0                     0
  'chaddr'   client's hardware     client's hardware     client's
                                                         hardware
             address               address               address
  'sname'    options, if           options, if           (unused)
             indicated in          indicated in
             'sname/file'          'sname/file'
             option; otherwise     option; otherwise
             unused                unused
  'file'     options, if           options, if           (unused)
             indicated in          indicated in
             'sname/file'          'sname/file'
             option; otherwise     option; otherwise
             'generic' name or     'generic' name or
             null                  null
  'options'  options               options               (unused)














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  Option                     DHCPDISCOVER  DHCPREQUEST      DHCPDECLINE,
                                                            DHCPRELEASE
  ------                     ------------  -----------      -----------

  Requested IP address       MAY           MUST NOT         MUST NOT
  IP address lease time      MAY           MAY              MUST NOT
  Use 'file'/'sname' fields  MAY           MAY              MAY
  DHCP message type          DHCPDISCOVER  DHCPREQUEST      DHCPDECLINE/
                                                            DHCPRELEASE
  Client identifier          MAY           MAY              MAY
  Class identifier           SHOULD        SHOULD           MUST NOT
  Server identifier          MUST NOT      MUST (after      MUST
                                           DHCPDISCOVER),
                                           MUST NOT (when
                                           renewing)
  Parameter request list     MAY           MAY              MUST NOT
  Maximum message size       MAY           MAY              MUST NOT
  Message                    SHOULD NOT    SHOULD NOT       SHOULD
  Site-specific              MAY           MAY              MUST NOT
  All others                 MUST NOT      MUST NOT         MUST NOT

           Table 4:  Fields and options used by DHCP clients

   If the parameters are acceptable, the client records the address of
   the server that supplied the parameters from the 'server identifier'
   field and sends that address in the 'server identifier' field of a
   DHCPREQUEST broadcast message.  Once the DHCPACK message from the
   server arrives, the client is initialized and moves to BOUND state.
   The DHCPREQUEST message contains the same 'xid' as the DHCPOFFER
   message.  The client records the lease expiration time as the sum of
   the time at which the original request was sent and the duration of
   the lease from the DHCPOFFER message.  The client SHOULD broadcast an
   ARP reply to announce the client's new IP address and clear any
   outdated ARP cache entries in hosts on the client's subnet.

4.4.2 Initialization with known network address

   The client begins in INIT-REBOOT state and sends a DHCPREQUEST message
   with the 'ciaddr' field set to the client's network address.  The
   client may request specific configuration parameters by including
   the 'parameter request list' option.  The client generates and records a
   random transaction identifier and inserts that identifier into the 'xid'
   field.  The client records its own local time for later use in
   computing the lease expiration.  The client MUST NOT incldue a 'server
   identifier' in the DHCPREQUEST message.  The client then broadcasts
   the DHCPREQUEST on the local hardware broadcast address to the 'DHCP
   server' UDP port.




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   Once a DHCPACK message with an 'xid' field matching that in the
   client's DHCPREQUEST message arrives from any server, the client is
   initialized and moves to BOUND state.  The client records the lease
   expiration time as the sum of the time at which the DHCPREQUEST
   message was sent and the duration of the lease from the DHCPACK
   message.

4.4.3 Initialization with a known DHCP server address

   When the DHCP client knows the address of a DHCP server, in either
   INIT or REBOOTING state, the client may use that address in the
   DHCPDISCOVER or DHCPREQUEST rather than the IP broadcast address.  If
   the client receives no response to DHCP messages sent to the IP
   address of a known DHCP server, the DHCP client reverts to using the
   IP broadcast address.

4.4.4 Reacquisition and expiration

   The client maintains two times, T1 and T2, that specify the times at
   which the client tries to extend its lease on its network address.  T1
   is the time at which the client enters the RENEWING state and attempts
   to contact the server that originally issued the client's network
   address.  T2 is the time at which the client enters the REBINDING
   state and attempts to contact any server.

   At time T1 after the client accepts the lease on its network address,
   the client moves to RENEWING state and sends (via unicast) a
   DHCPREQUEST message to the server to extend its lease.  The client
   generates a random transaction identifier and inserts that identifier
   into the 'xid' field in the DHCPREQUEST. The client records the local
   time at which the DHCPREQUEST message is sent for computation of the
   lease expiration time.  The client MUST NOT include a 'server
   identifier' in the DHCPREQUEST message.

   Any DHCPACK messages that arrive with an 'xid' that does not match
   the 'xid' of the client's DHCPREQUEST message are silently discarded.
   When the client receives a DHCPACK from the server, the client
   computes the lease expiration time as the sum of the time at which the
   client sent the DHCPREQUEST message and the duration of the lease in
   the DHCPACK message.  The client has successfully reacquired its
   network address, returns to BOUND state and may continue network
   processing.

   If no DHCPACK arrives before time T2 (T2 > T1) before the expiration
   of the client's lease on its network address, the client moves to
   REBINDING state and sends (via broadcast) a DHCPREQUEST message to
   extend its lease.  The client sets the 'ciaddr' field in the
   DHCPREQUEST to its current network address.  The client MUST NOT



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RFC 1541          Dynamic Host Configuration Protocol       October 1993


   include a 'server identifier' in the DHCPREQUEST message.

   Times T1 and T2 are configurable by the server through options.  T1
   defaults to (0.5 * duration_of_lease).  T2 defaults to (0.875 *
   duration_of_lease).  Times T1 and T2 should be chosen with some random
   "fuzz" around a fixed value, to avoid synchronization of client
   reacquisition.

   In both RENEWING and REBINDING state, if the client receives no
   response to its DHCPREQUEST message, the client should wait one-half
   the remaining time until the expiration of T1 (in RENEWING state) and
   T2 (in REBINDING state) down to a minimum of 60 seconds, before
   retransmitting the DHCPREQUEST message.

   If the lease expires before the client receives a DHCPACK, the client
   moves to INIT state, MUST immediately stop any other network
   processing and requests network initialization parameters as if the
   client were uninitialized.  If the client then receives a DHCPACK
   allocating that client its previous network address, the client SHOULD
   continue network processing.  If the client is given a new network
   address, it MUST NOT continue using the previous network address and
   SHOULD notify the local users of the problem.

4.4.5 DHCPRELEASE

   If the client no longer requires use of its assigned network address
   (e.g., the client is gracefully shut down), the client sends a
   DHCPRELEASE message to the server.  Note that the correct operation of
   DHCP does not depend on the transmission of DHCPRELEASE messages.

5. Acknowledgments

   Greg Minshall, Leo McLaughlin and John Veizades have patiently
   contributed to the the design of DHCP through innumerable discussions,
   meetings and mail conversations.  Jeff Mogul first proposed the
   client-server based model for DHCP.  Steve Deering searched the
   various IP RFCs to put together the list of network parameters
   supplied by DHCP.  Walt Wimer contributed a wealth of practical
   experience with BOOTP and wrote a document clarifying the behavior of
   BOOTP/DHCP relay agents.  Jesse Walker analyzed DHCP in detail,
   pointing out several inconsistencies in earlier specifications of the
   protocol.  Steve Alexander reviewed Walker's analysis and the fixes to
   the protocol based on Walker's work.  And, of course, all the members
   of the Dynamic Host Configuration Working Group of the IETF have
   contributed to the design of the protocol through discussion and
   review of the protocol design.





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6. References

   [1] Acetta, M., "Resource Location Protocol", RFC 887, CMU, December
       1983.

   [2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor
       Extensions", RFC 1533, Lachman Technology, Inc., Bucknell
       University, October 1993.

   [3] Braden, R., Editor, "Requirements for Internet Hosts --
       Communication Layers", STD 3, RFC 1122, USC/Information Sciences
       Institute, October 1989.

   [4] Braden, R., Editor, "Requirements for Internet Hosts --
       Application and Support, STD 3, RFC 1123, USC/Information
       Sciences Institute, October 1989.

   [5] Brownell, D, "Dynamic Reverse Address Resolution Protocol
       (DRARP)", Work in Progress.

   [6] Comer, D., and R. Droms, "Uniform Access to Internet Directory
       Services", Proc. of ACM SIGCOMM '90 (Special issue of Computer
       Communications Review), 20(4):50--59, 1990.

   [7] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951,
       Stanford and SUN Microsystems, September 1985.

   [8] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox
       PARC, September 1991.

   [9] Droms, D., "Interoperation between DHCP an BOOTP" RFC 1534,
       Bucknell University, October 1993.

  [10] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse
       Address Resolution Protocol", RFC 903, Stanford, June 1984.

  [11] Gray C., and D. Cheriton, "Leases: An Efficient Fault-Tolerant
       Mechanism for Distributed File Cache Consistency", In Proc. of
       the Twelfth ACM Symposium on Operating Systems Design, 1989.

  [12] Mockapetris, P., "Domain Names -- Concepts and Facilities", STD
       13, RFC 1034, USC/Information Sciences Institute, November 1987.

  [13] Mockapetris, P., "Domain Names -- Implementation and
       Specification", STD 13, RFC 1035, USC/Information Sciences
       Institute, November 1987.





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RFC 1541          Dynamic Host Configuration Protocol       October 1993


  [14] Mogul J., and S. Deering, "Path MTU Discovery", RFC 1191,
       November 1990.

  [15] Morgan, R., "Dynamic IP Address Assignment for Ethernet Attached
       Hosts", Work in Progress.

  [16] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
       USC/Information Sciences Institute, September 1981.

  [17] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497,
       USC/Information Sciences Institute, August 1993.

  [18] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, July 1992.

  [19] Jeffrey Schiller and Mark Rosenstein. A Protocol for the Dynamic
       Assignment of IP Addresses for use on an Ethernet. (Available
       from the Athena Project, MIT), 1989.

  [20] Sollins, K., "The TFTP Protocol (Revision 2)",  RFC 783, NIC,
       June 1981.

  [21] Wimer, W., "Clarifications and Extensions for the Bootstrap
       Protocol", RFC 1542, Carnegie Mellon University, October 1993.

7. Security Considerations

   DHCP is built directly on UDP and IP which are as yet inherently
   insecure.  Furthermore, DHCP is generally intended to make
   maintenance of remote and/or diskless hosts easier.  While perhaps
   not impossible, configuring such hosts with passwords or keys may be
   difficult and inconvenient.  Therefore, DHCP in its current form is
   quite insecure.

   Unauthorized DHCP servers may be easily set up.  Such servers can
   then send false and potentially disruptive information to clients
   such as incorrect or duplicate IP addresses, incorrect routing
   information (including spoof routers, etc.), incorrect domain
   nameserver addresses (such as spoof nameservers), and so on.
   Clearly, once this seed information is in place, an attacker can
   further compromise affected systems.

   Malicious DHCP clients could masquerade as legitimate clients and
   retrieve information intended for those legitimate clients.  Where
   dynamic allocation of resources is used, a malicious client could
   claim all resources for itself, thereby denying resources to
   legitimate clients.




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RFC 1541          Dynamic Host Configuration Protocol       October 1993


8. Author's Address

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

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









































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RFC 1541          Dynamic Host Configuration Protocol       October 1993


A. Host Configuration Parameters

   IP-layer_parameters,_per_host:_

   Be a router                     on/off                 HRC 3.1
   Non-local source routing        on/off                 HRC 3.3.5
   Policy filters for
   non-local source routing        (list)                 HRC 3.3.5
   Maximum reassembly size         integer                HRC 3.3.2
   Default TTL                     integer                HRC 3.2.1.7
   PMTU aging timeout              integer                MTU 6.6
   MTU plateau table               (list)                 MTU 7
   IP-layer_parameters,_per_interface:_
   IP address                      (address)              HRC 3.3.1.6
   Subnet mask                     (address mask)         HRC 3.3.1.6
   MTU                             integer                HRC 3.3.3
   All-subnets-MTU                 on/off                 HRC 3.3.3
   Broadcast address flavor        0x00000000/0xffffffff  HRC 3.3.6
   Perform mask discovery          on/off                 HRC 3.2.2.9
   Be a mask supplier              on/off                 HRC 3.2.2.9
   Perform router discovery        on/off                 RD 5.1
   Router solicitation address     (address)              RD 5.1
   Default routers, list of:
          router address          (address)              HRC 3.3.1.6
          preference level        integer                HRC 3.3.1.6
   Static routes, list of:
          destination             (host/subnet/net)      HRC 3.3.1.2
          destination mask        (address mask)         HRC 3.3.1.2
          type-of-service         integer                HRC 3.3.1.2
          first-hop router        (address)              HRC 3.3.1.2
          ignore redirects        on/off                 HRC 3.3.1.2
          PMTU                    integer                MTU 6.6
          perform PMTU discovery  on/off                 MTU 6.6

   Link-layer_parameters,_per_interface:_
   Trailers                       on/off                 HRC 2.3.1
   ARP cache timeout              integer                HRC 2.3.2.1
   Ethernet encapsulation         (RFC 894/RFC 1042)     HRC 2.3.3

   TCP_parameters,_per_host:_
   TTL                            integer                HRC 4.2.2.19
   Keep-alive interval            integer                HRC 4.2.3.6
   Keep-alive data size           0/1                    HRC 4.2.3.6

Key:

   MTU = Path MTU Discovery (RFC 1191, Proposed Standard)
   RD = Router Discovery (RFC 1256, Proposed Standard)



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