Clarification of sender behaviour in persist condition.

Clarification of sender behaviour in persist condition. This document attempts to clarify the notion of the Zero Window Probes (ZWP) described in RFC 1122 [RFC1122]. In particular, it clarifies the actions that can be taken on connections which are experiencing the ZWP condition. The motivation for this document stems from the belief that TCP implementations strictly adhering to the current RFC language have the potential to become vulnerable to Denial of Service (DoS) scenarios.