Keepalive messages in SIP are not originally part of the RFC. However, many SIP implementations actually use them, sending UDP packets (containing only whitespaces) to maintain the connection. These packets are accepted if they are not longer than a preset value (see the max_keepalive_size attribute of the AbstractSipProxy proxy class) and interprets them as keepalive messages. Such packets are uniformly replaced with UDP packets containing only a single line-feed.