14. Reliability of Client Initiated Message Exchanges

14. Reliability of Client Initiated Message Exchanges

A DHCP client is responsible for reliable delivery of messages in client-initiated message exchanges described in sections 17 and 18. If a DHCP client fails to receive an expected response from a server, the client MUST retransmit its message. This section describes the retransmission strategy used by clients in client-initiated message exchanges.

Note that the procedure described in this section is modified slightly when used with Solicit messages. The modified procedure is described in section 17.1.2.

A client begins a message exchange by sending a message to a server. The message exchange terminates when the client successfully receives an appropriate response or reply from a server or servers, or when the message exchange is considered to have failed according to the retransmission mechanism described below.

Client retransmission behavior is controlled and described by the following variables:

RT
Retransmission timeout

IRT
Initial retransmission time

MRC
Maximum retransmission count

MRT
Maximum retransmission time

MRD
Maximum retransmission duration

RAND
Randomization factor

For each message transmission or retransmission, the client sets RT according to the rules given below. If RT expires before the message exchange terminates, the client recomputes RT and retransmits the message.

Each calculation of a new RT includes a randomization factor (RAND), which is a random number chosen with a uniform distribution between -0.1 and +0.1. The randomization factor is included to minimize synchronization of messages sent by DHCP clients.

The algorithm for choosing the random number does not need to be cryptographically secure. The algorithm SHOULD produce a different sequence of random numbers from each invocation of the DHCP client.

The RT for the first message transmission is based on IRT:

RT = IRT + RAND*IRT

The RT for each subsequent message transmission is based on the previous value of RT:

RT = 2*RTprev + RAND*RTprev

MRT specifies an upper bound on the value of RT (disregarding the randomization added by using RAND). If the value of MRT is 0, there is no upper bound on the value of RT. Otherwise:

if (RT > MRT)
   RT = MRT + RAND*MRT

MRC specifies an upper bound on the number of times a client can retransmit a message. Unless MRC is zero, the message exchange fails once the client has transmitted the message MRC times.

MRD specifies an upper bound on the length of time over which a client can retransmit a message. Unless MRD is zero, the message exchange fails once MRD seconds have elapsed since the client first transmitted the message.

If both MRC and MRD are non-zero, the message exchange fails as soon as either of the conditions specified in the previous two paragraphs is met.

If both MRC and MRD are zero, the client will continue to transmit the message until it receives a response.