上下邻层实体之间的接口称为服务访问点(SAP)，网络层的服务访问点分为(18)两部分。

(131)

(130)

(129)

(128)

Flow control is a function that prevents network congestion by ensuring that(71)do not overwhelm(72)with data.

 There are three commonly used methods for handling network congestion.(73)is used by network devices to temporarily store bursts of excess data in memory until they can be processed(74)are used by receiving devices to help prevent their buffers from overflowing.(75)is a flow-control scheme in which the source device requires an acknowledgment from the destination aider a certain number of packets have been transmitted.

(126)

(125)

(124)

(123)

SNMPv2增加了一个非原子的Get命令，可以做到(63)，SNMPv2增加的Inform命令使得网络管理的结构可以是(64)。SNMPv1的报文中除版本号和SNMPPDU外，还包括(65)，在 SNMPv2中，在原PDU的基础上增加了(66)信息。RMON2监视(67)层的通信。