Chisel: A Storage-efficient, Collision-free Hash-based Network Processing Architecture

1. Chisel: A Storage-efficient, Collision-free Hash-based Network Processing Architecture Author: Jahangir Hasan, SrihariCadambi, VenkattaJakkulaSrimatChakradhar Publisher: ISCA 2006 Presenter: Yuen-ShuoLi Date: 2012/10/03

2. Outline • Introduction • Bloomier Filter • Issue – False Positive • Issue - Wildcard • Issue - Update • Performance

3. Introduction(1/2) There are three major families of techniques for performing LPM • TCAM • Large cost and power dissipation • Trie-Based • Large memory requirements and long lookup latencies • Hash-Based • lower power, small memory requirement, and short lookup latencies • can’t handle wildcard and have collision problem

4. Introduction(2/2) • Chisel(Collision-free Hashing-Scheme for LPM) • use collision-free hashing scheme called Bloomier filter • solve two key problem • false positive • wildcard • support incremental updates

5. Bloomier Filter(1/5) • recall Bloom Filter • a bit vector • multiple hash function • false-positive problem

6. Bloomier Filter(2/5) • an extension of Bloom filter • collision-free hashing scheme • support storage and retrieval of arbitrary per-key information • support static sets of keys and not dynamic update • inherit Bloom filter’s false-positive problem

7. Bloomier Filter(3/5) HN: The set of k hash values of a key • Idea • find a T(t) among HN(t) such that there is a one-to-one mapping between all t and T(t) • setup the Index Table, so that a lookup for t return location T(t) • and store value in a separate Result Table at address T(t)

8. Bloomier Filter(4/5) • It is possible that this setup process can fail • use more hash function or expand the table • and use a little TCAM

9. Bloomier Filter(5/5) • In order to retrieve T(t), the solution is to store some value in T(t) • use XOR the value of the i’th hash function D[the data value in the ‘th location

10. Issue – False Positive(1/2) • can occur when lookup involves some key t which was not in the set of original keys used for setup • two way • add a checksum field to each hash bucket • still has false positives • use chisel architecture • eliminating false positives

11. Issue – False Positive(2/2) • encode a pointer p(t) for each t instead of • use two separate tables to store f(t) and t

12. Issue – Wildcard(1/3) • Because hash functions cannot operate on wildcard bits • the way to support wildcard bits • large number of tables • results in considerable hardware complexity and cost • CPE • resulting in huge amounts of memory • prefix collapsing

13. Issue – Wildcard(2/3)

14. Issue – Wildcard(3/3)

15. Issue - Update • update for existing prefix is easy • just change the entry in the Result Table • remove: temporarily mark the prefix dirty • a large fraction of updates are actually route-flaps • maintain shadow copy of the date structures in software • first incrementally update the shadow copy • then transfer the data structure to the hardware engine • route-flaps: a prefix is added back after being recently removed

16. Performance(1/3)

17. Performance(2/3)

18. Performance(3/3)