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11.12 LOOKUP-CHIP MODEL > 11.12 LOOKUP-CHIP MODEL - Pg. 264

264 CHAPTER 11 Prefix-Match Lookups describes a model of a lookup chip that does search and update. The chip has a Search and an Update process, both of which access a common SRAM memory that is either on or off chip (or both). The Update process allows incremental updates and (potentially) does a memory allocation/deallocation and a small amount of local compaction for every update. The actual updates can be done either completely on chip, partially in software, or completely in software. If a semiconductor company wishes to sell a lookup chip using a complex update algorithm (e.g., for compressed schemes), it may be wiser also to provide an update algorithm in hardware. If the lookup chip is part of a forwarding engine, how- ever, it may be simpler to relegate the update process completely to a separate CPU on the line card. Each access to SRAM can be fairly wide if needed, even up to 1000 bits. This is quite feasible today using a wide bus. The search and update logic can easily process 1000 bits in parallel in one memory cycle time. Recall that wide word accesses can help -- for example, in the tree bitmap and binary search on values schemes -- to reduce search times. Search and Update use time multiplexing to share access to the common SRAM that stores the lookup database. Thus the Search process is allowed S consecutive accesses to memory, and then the Update process is allowed K accesses to memory. If S is 20 and K is 2, this allows Update to steal a few cycles from Search while slowing down Search throughput by only a small fraction. Note that this increases the latency of Search by K memory accesses in the worst case; however, since the Search process is likely to be pipelined, this can be considered a small additional pipeline delay. The chip has pins to receive inputs for Search (e.g., keys) and Update (e.g., update type, key, result) and can return search outputs (e.g., result). The model can be instantiated for various types of lookups, including IP lookups (e.g., 32-bit IP addresses as keys and next hops as results), bridge lookups (48-bit MAC addresses as keys and output ports as results), and classification (e.g., packet headers as keys and matching rules as results). Each addition or deletion of a key can result in a call to deallocate a block and to allocate a different-size block. Each allocate request can be in any range from 1 to Max memory words. There is a total of M words that can be allocated. The actual memory can be either off chip, on chip, or both. Clearly, even off-chip solutions will cache the first levels of any lookup tree on chip. On-chip memory is attractive because of its speed and cost. Unfortunately, on-chip memory was limited by current processes to around 32 Mbits at the time of writing. This makes it difficult to support databases of 1 million prefixes. Internally, the chip will very likely be heavily pipelined. The lookup data structure is partitioned into several pieces, each of which is concurrently worked on by separate stages of logic. Thus the SRAM will likely be broken into several smaller SRAMs that can be accessed independently by each pipeline stage. There is a problem [SV00] with statically partitioning SRAM between pipeline stages, because memory needs for each stage can vary as prefixes are inserted and deleted. One possible solution is to break the single SRAM into a fairly large number of smaller SRAMs that can be dynamically allocated to the stages via a partial crossbar switch. However, designing such a crossbar at very high speeds is not easy. All the schemes described in this chapter can be pipelined because they are all fundamen- tally based on trees. All nodes at the same depth in a tree can be allocated to the same pipeline stage.