2.5. Message Sizes

The maximum size of a SNMPv2 message is limited to the minimum of:

1. the maximum message size which the destination SNMPv2 entity can accept; and,

2. the maximum message size which the source SNMPv2 entity can generate.

The former may be known on a per-recipient basis; and in the absence of such knowledge, is indicated by transport domain used when sending the message. The latter is imposed by implementation-specific local constraints.

Each transport mapping for the SNMPv2 indicates the minimum message size which a SNMPv2 implementation must be able to produce or consume. Although implementations are encouraged to support larger values whenever possible, a conformant implementation must never generate messages larger than allowed by the receiving SNMPv2 entity.

One of the aims of the GetBulkRequest-PDU, specified in this protocol, is to minimize the number of protocol exchanges required to retrieve a large amount of management information. As such, this PDU type allows a SNMPv2 entity acting in a manager role to request that the response be as large as possible given the constraints on message sizes. These constraints include the limits on the size of messages which the SNMPv2 entity acting in an agent role can generate, and the SNMPv2 entity acting in a manager role can receive.

However, it is possible that such maximum sized messages may be larger than the Path MTU of the path across the network traversed by the messages. In this situation, such messages are subject to fragmentation. Fragmentation is generally considered to be harmful [4], since among other problems, it leads to a decrease in the reliability of the transfer of the messages. Thus, a SNMPv2 entity which sends a GetBulkRequest-PDU must take care to set its parameters accordingly, so as to reduce the risk of fragmentation. In particular, under conditions of network stress, only small values should be used for max-repetitions.