Maintaining statistics counters in router line cards

摘要

Packet switches (that is, IP routers and ATM and Ethernet switches) maintain statistics for performance monitoring, network management, security, network tracing, and traffic engineering. Counters usually collect such statistics as the number of arrivals of a specific packet type or they count a particular event, such as when the network drops a packet. A packet's arrival can lead to the updating of several different statistics counters. The number of statistics counters in a network device and their rate of update are often limited by memory technology. On-chip registers or SRAM (on-or off-chip) can hold a few counters. Often, a network device has to maintain many counters and therefore must store them in off-chip DRAM. But the large random access times of DRAMs make their use difficult when supporting high-bandwidth links. The time it takes to read, update, and write a single counter would be too long, and worse still, each arriving packet can trigger the update of multiple counters. To alleviate these problems, we use a well-known architecture for storing and updating statistics counters. This approach maintains smaller-size counters in fast (potentially on-chip) SRAM, while maintaining full-size counters in a large, slower DRAM. Our goal is to ensure that the system always correctly maintains counter values at line rate. An optimal counter management algorithm (CMA) minimizes the required SRAM size while ensuring correct line-rate operation for a large number of counters.