2. Protocol Summary
From a 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. RFC 1542 [2] 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 a 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 finite 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."
Format of a DHCP message
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 (variable) |
+---------------------------------------------------------------+
Figure 1: Format of a DHCP message
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' is an opaque key, not to be interpreted by the server; for example, the 'client identifier' 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. The 'client identifier' chosen by a DHCP client MUST be unique to that client within the subnet to which the client is attached. If the client uses a 'client identifier' in one message, it MUST use that same identifier in all subsequent messages, to ensure that all servers correctly identify the client.
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.
Description of fields in a DHCP message
| Field | Octets | Description |
|---|---|---|
| op | 1 | Message op code / message type.<br/>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 began address acquisition or renewal process. |
| flags | 2 | Flags (see figure 2). |
| ciaddr | 4 | Client IP address; only filled in if client is in BOUND, RENEW or REBINDING state and can respond to ARP requests. |
| yiaddr | 4 | 'your' (client) IP address. |
| siaddr | 4 | IP address of next server to use in bootstrap; returned in DHCPOFFER, DHCPACK 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 | var | Optional parameters field. See the options documents for a list of defined options. |
Table 1: Description of fields in a DHCP message
The 'options' field is now variable length. A DHCP client must be prepared to receive DHCP messages with an 'options' field of at least length 312 octets. This requirement implies that a DHCP client must be prepared to receive a message of up to 576 octets, the minimum IP datagram size an IP 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.
In the case of a client using DHCP for initial configuration (before the client's TCP/IP software has been completely configured), DHCP requires creative use of the client's TCP/IP software and liberal interpretation of RFC 1122. The TCP/IP software SHOULD accept and forward to the IP layer any IP packets delivered to the client's hardware address before the IP address is configured; DHCP servers and BOOTP relay agents may not be able to deliver DHCP messages to clients that cannot accept hardware unicast datagrams before the TCP/IP software is configured.
To work around some clients that cannot accept IP unicast datagrams before the TCP/IP software is configured as discussed in the previous paragraph, 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.
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
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 is 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 DHCP client that has been moved to a different subnet or has changed its hardware address (perhaps because of a network interface failure and a replacement of hardware). The protocol defines that the key will be (IP-subnet-number, hardware-address) unless the client explicitly supplies an identifier using the 'client identifier' option. 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 clients. 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 (a set 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 a client 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 allocating server SHOULD probe the reused address before allocating the address, e.g., with an ICMP echo request, and the client SHOULD probe the newly received address, e.g., with ARP.