linux-kernel - 什么是 base_reachable_time(ARP 相关)proc 条目

Question

根据 arp(7) - Linux 手册页:

base_reachable_time (since Linux 2.2) Once a neighbor has been found, the entry is considered to be valid for at least a random value between base_reachable_time/2 and 3*base_reachable_time/2. An entry's validity will be extended if it receives positive feedback from higher level protocols. Defaults to 30 seconds. This file is now obsolete in favor of base_reachable_time_ms. base_reachable_time_ms (since Linux 2.6.12) As for base_reachable_time, but measures time in milliseconds. Defaults to 30000 milliseconds.

我不明白这个解释 - 特别是关于来自更高级别协议(protocol)的积极反馈的声明。有人请澄清一下吗？

Answer 1

来自arp(7) man page ,

When there is no positive feedback for an existing mapping after some time (see the /proc interfaces below), a neighbor cache entry is considered stale. Positive feedback can be gotten from a higher layer; for example from a successful TCP ACK. Other protocols can signal forward progress using the MSG_CONFIRM flag to sendmsg(2). When there is no forward progress, ARP tries to reprobe.

基本上这意味着，如果有很多成功的 ACK 发生类似持续 TCP 连接的事情，那么它假定它拥有的 IP/MAC 对是有效的，并且当表通常会过期。