3Com Switch 8800 Advanced Software V5 Configuration Guide

Protocols and Standards 719
Figure 214 DHCP message format
■ op: Message type defined in option field. 1 = REQUEST, 2 = REPLY
■ htype,hlen: Hardware address type and length of a DHCP client.
■ hops: Number of relay agents a request message traveled.
■ xid: Transaction ID, a random number chosen by the client to identify an IP
address allocation.
■ secs: Filled in by the client, the number of seconds elapsed since the client
began address acquisition or renewal process. Currently this field is reserved
and set to 0.
■ flags: The leftmost bit is defined as the BROADCAST (B) flag. If this flag is set to
0, the DHCP server sent a reply back by unicast; if this flag is set to 1, the DHCP
server sent a reply back by broadcast. The remaining bits of the flags field are
reserved for future use.
■ ciaddr: Client IP address.
■ yiaddr: ’your’ (client) IP address, assigned by the server.
■ siaddr: Server IP address, from which the clients obtained configuration
parameters.
■ giaddr: The first relay agent IP address a request message traveled.
■ chaddr: Client hardware address.
■ sname: The server host name, from which the client obtained configuration
parameters.
■ file: Bootfile name and routing information, defined by the server to the client.
■ options: Optional parameters field that is variable in length, which includes the
message type, lease, DNS IP address, WINS IP address and so forth.
Protocols and
Standards
■ RFC2131:Dynamic Host Configuration Protocol
■ RFC2132:DHCP Options and BOOTP Vendor Extensions
■ RFC1542:Clarifications and Extensions for the Bootstrap Protocol
op (1)
0715
htype (1) hlen (1) hops (1)
xid (4)
23 31
secs (2) flags (2)
ciaddr (4)
yiaddr (4)
siaddr (4)
giaddr (4)
chaddr (16)
sname (64)
file (128)
options (variable)