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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Client/Server Implementation, Features and Issues

DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA) 1 2 DHCP and BOOTP Interoperability

Conflict Detection Operation

The idea behind conflict detection is very simple. Suppose a DHCP server receives a DHCPDISCOVER message from a client and decides to offer it a lease. Before sending the DHCPOFFER message, the server conducts a probe by sending ICMP Echo (ping) messages out to the address it plans to offer. It then waits a short period of time to hear if it receives any ICMP Echo Reply messages back. If it does, it knows the IP address is in use and chooses a different one.

If all DHCP servers are configured to do this before offering an address, then it is possible to give all of them the same, overlapping addresses for assignment. They won't have any way of coordinating with each other, but as long as they “ask first” by doing an ICMP check, there won't be any problems. This provides an administrator with the advantages of overlapping address ranges—simplicity and access to all addresses by all servers—without risk of address conflicts. The only small drawback is a little bit of extra network traffic to perform the check, and possibly a few milliseconds of server CPU time if a new address needs to be chosen.

If you were paying attention when you read about the DHCP allocation process, you may have noticed that what I am describing here sounds familiar. In fact, it's true: this feature really isn't anything new. The use of ICMP to check an address before offering it is actually mentioned in RFC 2131 as part of the standard DHCP allocation process, and you can find it mentioned as step #5 in the allocation process in this Guide.

So why was “conflict detection” required to be an extra feature? Simple: the use of ICMP wasn't mandatory because the standard says servers SHOULD do it and not that they MUST do it. That's why they capitalize those words in the standards. J This choice was made to provide flexibility in implementing DHCP, but that flexibility comes at a cost. So, if you want to use this feature, you need to look for support for it in your server software.

DHCP Autoconfiguration / Automatic Private IP Addressing (APIPA) 1 2 DHCP and BOOTP Interoperability