Dynamic Host Configuration Protocol (DHCP) is a mechanism that allows a client to obtain an IP address and other configuration information about the IPv4 or IPv6 network that it has joined. By using this information to configure its own parameters, a client can join a network and move between networks without advance knowledge of the configuration required by the network.
DHCP operates on a client-server model, with one or more DHCP servers providing services for a network. DHCP servers listen for requests from clients and provide the information requested. If a client requests an IP address, the DHCP server creates and maintains a binding with a specific lifetime between the client and the IP address the server allocated to it. A client can renew the address binding before the lifetime, or lease, expires or release the address when it leaves the network. When a client releases an address or allows the lease to expire, the server deletes the binding and returns the address to the pool of addresses available for new clients.
When DHCP services are not available on the client's network, another host can act as a relay agent, intercepting the client's requests and forwarding them to a DHCP server. The relay agent also receives the responses from the server and forwards them to the client. Since the relay agent must be able to communicate with the client before it has been assigned an IP address, they must reside in the same subnet and the relay agent must be able to send messages to the client's hardware (MAC) address.
With the DHCP Server Emulation feature, you can add a DHCP Server Node test case to your test session to provide simple IP address allocation services to clients either directly or via a relay agent. The server node maintains IP address pools that you define with the IP Pool Manager and, in response to requests, allocates addresses from the pools and manages client bindings. You can include the Renewal Time Value (T1) and Rebinding Time Value (T2) options in the acknowledgement messages to the client and define the maximum lease time supported by the node.
The server node only responds to messages sent to its IP address. All DHCP broadcast messages are ignored to prevent the node from inadvertently processing requests broadcasted by devices that are in the test network but outside of the scope of the test. User Class is the only DHCP option recognized by the node, and the node will always allocate an address from one of its pools for the maximum lease time regardless of whether a specific address or lease time is requested by the client.
DHCP client support is available in the GGSN Nodal and IP Application Node test cases. You can configure the client to request a specific lease time, include the User Class option to request an address from specific address pools, and include a Parameter Request List. When multiple DHCP servers respond to a request, the client will always accept the first response and ignore subsequent responses.
|
NOTES: Neither the simulated client nor the server node attempt to test an allocated IP address' use in the network during the allocation process. It is assumed that the address is not being used by any other host. |
A GGSN can be configured to act as either a DHCP client or as a relay agent. When acting as a client, the GGSN obtains IP addresses for primary PDP contexts as it creates the contexts, renews the address leases as necessary, and releases the addresses when the contexts are deleted. When the GGSN acts as a relay agent, the MN is responsible for initiating an address request to the DHCP server after a primary PDP context is established, renewing the address lease prior to expiration, and releasing the address after the context is deleted. The GGSN must recognize that the client is using DHCP and update the context when the address allocation transaction is complete. In this case, you can configure the GGSN Nodal test case to simulate a DHCP client. In both cases, you can add a DHCP Server Node to the test to simulate the DHCP server.
When you configure a DHCP client in the IP Application Node test case, the Client Nodes request addresses from a DHCP server as they are activated. Client Nodes can communicate directly with a DHCP server in their subnet or accept the assistance of a relay agent located in the test network.