Another CDFA Based Multi-Pattern Matching Algorithm and Architecture for Packet Inspection

Presenter : Shi-qu YuDate : 2011/09/21

IntroductionPresent a method to optimize the potential

memory usage of DFA based algorithms for multi-pattern expression matching by the combining DFA’s paths (Named isomorphic path combination IMPC)

Propose a novel multi-pattern matching algorithm,called ACS

Cached DFA(CDFA)Cached DFA was firstly proposed by T.

Song [1], which is a simple extended model of DFA by adding one or more buffers (cache)

The extension is elegant and promising as a better basic theory for pattern matching algorithms.

The key contributions can be summarized as follows The lower boundary of traditional DFA based pattern matching algorithms is presented and analyzed. Isomorphic path combination (IMPC), an idea to optimize pattern matching algorithms, is addressed. Cached DFA (CDFA) based method is designed to achieve IMPC. Operational details are also addressed. A novel pattern matching algorithm, ACS, which are based on CDFA and IMPC is proposed. The related hardware design model is also presented. Experimental results show that 78.6% states can be saved by using ACS algorithm than DFA based solution.

PROBLEM ANALYSIS(DFA)a matching only occurs when the pattern

begins at a predefined location within the text to be matched-anchored matching

PROBLEM ANALYSIS(DFA)patterns may begin anywhere in the text

for the cases such as payload checking or spam filtering-anywhere matching

PROBLEM ANALYSIS(DFA)Four categories: basic transitions cross transitions failure transitions restartable transitions.For anywhere matching, basic and cross

transitions can cause memory’s explosion.

IMPC IDEAM ={K,Σ,δ , s0, F}

IMPC IDEA

CDFA Model

CDFA Model

Implicit State ColoringThere are two ways to represent the states’

colors. One is to explicitly use another several bits for each state, which may cause memory overhead. The other is to take advantage of current information and to implicitly color the states.

Implicit State Coloring

Implicit State Coloring

ACS ALGORITHM AND ARCHITECTUREThe algorithm is similar to AC algorithm

addition with the method of how to find isomorphic paths. We do not aim to find all isomorphic paths but the efficient ones.

Find Isomorphic PathFor easy implementation, some rules are given

for finding isomorphic paths. They are not strictly prerequisite for IMPC but the experienced ones for simplifying the issue.The basic idea is that all isomorphic paths are not overlapped and not confused for judging the next step on the diverging state.

Rules for Finding Isomorphic PathR1: The first character of all patterns is never counted as

part of isomorphic path.R2: For each converging state, there is only one

corresponding diverging state. That is, for an isomorphic path, the only exit corresponds to all entrances.

R3: For one pattern, there may be many potential isomorphic paths with other patterns, while only those chosen to be combined are called isomorphic paths.

Rules for Finding Isomorphic Path R4: For one pattern, there may be several isomorphic paths

to be combined with other patterns. However, any two ofthem are not overlapped.

R5: For one pattern, any of its isomorphic paths does notinclude another one.

R6: Along the isomorphic paths, there is no branch until thediverging state.

R7: Potential isomorphic paths can be over-lapped andincluded by others. The algorithm for choosing isomorphicpaths from potential ones is discussed in next section.

Greedy Algorithm for IMPC

Greedy Algorithm for IMPC