1. A system for managing network communications to a Session Initiation Protocol (SIP) server, comprising: a processor; a packet limiter programmatically-located below an Internet Protocol (IP) processing component in an IP processing stack and arranged to, in cooperation with the processor: receive packets relating to SIP messages from a network; process the packets; and selectively forward a subset of the packets to the IP processing component based on the processing, wherein a packet identifies a source and the packet limiter is arranged to selectively forward packets from a first set of one or more sources in preference to packets from a second set of one of more sources, wherein the packet limiter is arranged to reassign sources to the first set and the second set following a time interval, wherein the IP processing component is arranged to perform IP processing and pass processed data from the subset of the packets to a SIP processing stack for initial SIP processing, and wherein the initial SIP processing comprises parsing SIP messages within the processed data; and a work scheduler programmatically-located in the SIP processing stack and arranged to, in cooperation with the processor, access the result of the initial SIP processing and determine whether to proceed with further SIP processing of the SIP messages based on a level of system activity.