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A High Throughput String Matching Architecture for Intrusion Detection and Prevention. Lin Tan U of Illinois, Urbana Champaign Tim Sherwood UC, Santa Barbara. Outline. Why String Matching Matching against multiple strings The Aho-Corasick Algorithm The Devil in the Constants - PowerPoint PPT Presentation
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A High ThroughputString Matching Architecturefor Intrusion Detection and
Prevention
Lin TanU of Illinois, Urbana Champaign
Tim SherwoodUC, Santa Barbara
Outline• Why String Matching
– Matching against multiple strings
• The Aho-Corasick Algorithm
– The Devil in the Constants
• A Bit-Split Algorithm
• Hardware Design and Analysis
• Conclusions
To Protect and Serve
• Our machines are constantly under attack
• Cannot rely on end users, we need networks which actively defend themselves.
This requires the protection system to be able to operate at 10 to 40 Gb/s. (We aim at current and next generation networks.)
IDS/IPS are promising ways of providing protectionMarket for such systems: $918.9 million by the end of 2007.Snort: an widely accepted open source IDS
Our Contributions
• String Matching Architecture:– 0.4MB and 10Gbps for Snort rule set ( >10,000
characters)
• Bit-Split String Matching Algorithm– Reduces out edges from 256 to 2.
• Performance/area beats the best techniques we examined by a factor of 10 or more.
Scanning for Intrusions
Most IDS define a set of rules.A string defines a suspicious transmission.
We are not building a full IDS, rather building the primitives from which full systems can be built
CodeRed worm: web flow established uricontent with “/root.exe”
Traffic In Traffic Out
ScanSoftware
IDS
Multiple String Matching
• The multiple string matching algorithm:– Input: A set of strings/patterns S, and a buffer b– Output: Every occurrence of an element of S in b
– Extra constraint: b is really a stream
• How to implement: Option 1) search for each string independently
Option 2) combine strings together and search all at once
A B
A string can be anywhere in the payload of a packet.
A B D F C A B Input:
A BC AStrings:
Why hardware
• Snort: >1,000 rules, growing at 1 rule/day or more• Active research into automated rule building• Strings are not limited to be just [a-z]+
• We need a high speed string matching technique with stringent worst case performance.
• Many algorithms are targeted for average case performance. Aho-Corasick can scan once and output all matches. But it is too big to be on-chip.
Outline• Why String Matching
– Matching against multiple strings
• The Aho-Corasick Algorithm
– The Devil in the Constants
• A Bit-Split Algorithm
• Hardware Design and Analysis
• Conclusions
The Aho-Corasick Algorithm
• Given a finite set P of patterns, build a deterministic finite automaton G accepting the set of all patterns in P.
An AC Automaton Example
• Example: P = {he, she, his, hers}
0
1
h
2
9
8
6
3
4
57
e
s
i h
s er
s
Initial State
Accepting State
StateTransition Function
h Sh
hh
hh
S
SS
S
S
S
i
h
r
h
•The Construction: linear time.•The search of all patterns in P: linear time
(Edges pointing back to State 0 are not shown).
Linear Time: So what’s the problem
…
…
…
…
…
16,384
2
1
0
2553210
256 Next State Pointers
<14> <14> <14> <14> <14>
• How to implement it on chip?
• Problem: Size too big to be on-chip– ~ 10,000 nodes– 256 out edges per node– Requires 16,384*256*14 = ~10MB
• Solution: partition into small state machines– Less strings per machine– Less out edges per machine
Outline• Why String Matching
– Matching against multiple strings
• The Aho-Corasick Algorithm
– The Devil in the Constants
• A Bit-Split Algorithm
• Hardware Design and Analysis
• Conclusions
Our Main Idea: Bit-Split
• Partition rules (P) into smaller sets (P0 to Pn)
• Build AC state-machine for each subset
• For each DFA Pi, rip state-machine apart into 8 tiny state-machines (Bi0 through Bi7)
• Each of which searches for 1 bit in the 8 bit encoding of an input character– Only if all the different B machines agree can
there actually a match
An example of Bit-SplitP0 = { he, she, his, hers }
0
1
h
2
9
8
6
3
4
57
e
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i h
s er
s
hS
h
hh
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S
SS
S
S
S
i
h
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h
(Edges pointing back to State 0 are not shown).
0000 00000000 0001
0110 1000
b0 {0}
P0 B03
0
b1 { }0
1
b2 { },1 0,3
0001 0000
0111 0011
0 1
{ }0,3{ } 0,1,2,6b3
1
b3{0,1,2,6}
0
1
b4{0,1,4}
b6{0,1,2,5,6}
b5{0,3,7,8}
b7{0,3,9}
0
1
00
0
0
0
1
1
11
1
1
Compact State SetP0 = { he, she, his, hers }
0
1
h
2
9
8
6
3
4
57
e
s
i h
s er
s
hS
h
hh
hh
S
SS
S
S
S
i
h
r
h
(Edges pointing back to State 0 are not shown).
b0 { }
P0 B03
0
b1 { }
1
b2 { }1
b3{ 2 }
0b4 { }
b6{ 2,5 }
b5{7}
b7{9}
0
1
00
0
0
0
1
1
11
1
1
An example of Bit-SplitP0 = { he, she, his, hers }
(Edges pointing back to State 0 are not shown).
P0
0
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h
2
9
8
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3
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57
e
s
i h
s er
s
hS
h
hh
hh
S
SS
S
S
S
i
h
r
h
B03b0 {}
b1{}
b2{}
b3{2}
b4 {}
b6{2,5}
b5{7}
b7{9}
01
1
0
0
1
1
0
0
0
1
1
1
0
01
B04
1
b8{2,7}
b5 {}
b0 {}
b1{}
b2{}
b4{2}
b3 {}
b6{2,5}
b9{9}
0
1
b7 {}
0
0
0
0
00
0
00
1
1
1
11
1 1
1
Nice Properties
• The number of states in Bij is rigorously bounded by the number of states in Pi
• No exponential blow up in state
• Linear construction time
• Possible to traverse multiple edges at a time to multiply throughput
0
1
h
2
9
8
6
3
4
57
e
s
i h
s er
s
hS
h
hh
hh
S
SS
S
S
S
i
h
r
h
Matching on the example
h x h e rs Only scan the input stream once.
Input stream:
0
1
h
2
9
8
6
3
4
57
e
s
i h
s er
s
hS
h
hh
hh
S
SS
S
S
S
i
h
r
h
Matching on the example
P0 B03b0 {}
b1{}
b2{}
b3{2}
b4 {}
b6{2,5}
b5{7}
b7{9}
01
1
0
0
1
1
0
0
0
1
1
1
0
01
B04
1
b8{2,7}
b5 {}
b0 {}
b1{}
b2{}
b4{2}
b3 {}
b6{2,5}
b9{9}
0
1
b7 {}
0
0
0
0
00
0
00
1
1
1
11
1 1
1
h x h e 0 1 0 0 1 1 1 0
How do you “combine” the results from the different state machines?Only if all the state machines agree, is there actually a match.
2
How to Implement
• The AC state machine is equivalent to the 8 tiny state machines.
• The 8 tiny state machines can run independently, which means in parallel
• Intersection done with bit-wise AND.
• 8 is intuitive but not optimal
• How to build a system to implement this algorithm?– Our algorithm makes it feasible to be on-chip
A Hardware Implementation
• A rule module is equivalent to an AC state machine• Rule modules, tiles are structurally equivalent• All full match vectors are concatenated to indicate which
strings are matched• One tile stores one tiny bit-split state machine
8
4 Next State Pointers Partial Match Vector
<8> <8> <8> <8> <16> 0
1
2
255
…
3
deco
der
InputInput
Cur
rent
Sta
te <
8>C
urre
nt S
tate
<8>
2 bits fromeach byte
PartialMatchVector
Config Data
Output LatchOutput Latch
4:1 Mux 16
State Machine TileRule Module 0
Tile 0
Tile 1
Tile 3
Tile 2
Full Match Vector
2-bit Input [0:1]
Partial Match Vector
16
168
[6:7]
[2:3] [4:5]
ControlBlock
Rule Module 1
Byte
from
Pay
load
8
…
2
Rule Module N
8
8
Com
plet
e Se
t of M
atch
es fo
r All R
ules
String Match Engine
16
An efficient Implementation
00 01 10 11 PMV
0 0 1 0 0 0000
1 0 2 0 0 0000
2 0 3 0 0 1000
3 0 4 0 0 1110
4 0 4 0 0 1111
5
6
7
8
9
00 01 10 11 PMV
0 1 0 2 0 0000
1 1 0 3 0 0000
2 1 0 5 0 0000
3 1 6 5 0 0000
4 7 0 2 0 1000
5 0 4 5 0 0000
6 7 0 2 0 1100
7 9 0 3 0 0000
8 1 0 3 0 0010
9 1 0 3 0 0001
00 01 10 11 PMV
0 1 0 0 2 0000
1 1 3 0 2 0000
2 4 0 0 2 0000
3 1 0 5 6 1000
4 1 7 0 2 0000
5 1 0 0 8 0000
6 4 0 0 2 0010
7 1 0 5 6 1100
8 4 0 0 2 0001
9
00 01 10 11 PMV
0 0 0 1 2 0000
1 0 0 3 2 0000
2 0 0 4 2 0000
3 0 0 3 5 1000
4 0 0 6 2 0000
5 0 0 4 7 0010
6 0 0 3 5 1100
7 0 0 4 2 0001
8
9
Tile 0 Tile 2Tile 1 Tile 3
Cycle 3 e 01 10 01 01Cycle 2 h 01 10 10 00Cycle 1 x 01 11 10 00Cycle 0 h 01 10 10 00
h
h
x
e
h
h
x
e
h
h
e
xh
x
he
e 1100h 0000x 0000h 0000
e 1111h 1110x 1000h 0000
e 1000h 0000x 0000h 0000
e 1000h 0000x 0000h 0000
Cycle 3 + P 1000Cycle 2 + P 0000Cycle 1 + P 0000Cycle 0 + P 0000
22 2
2
Related Work• Software based
– Good for ~100Mb/s, common case
• FPGA-based– Many schemes map rules down to a specialized circuit
• Near optimal utilization of hardware resources– Implementing state machines on block-RAMs [Cho and Mangione-
Smith]– Concurrent to our work: mapping state machines to on-chip SRAM
[Aldwairi et. al.]– Bloom filters [Dharmapurikar et al.]
• Excellent filter in the common case
• TCAM-based– Require all patterns to be shorter or equal to TCAM width – Cutting long patterns: 2Gbps with 295KB TCAM [Yu et. al.]
Conclusions
• New Tile-based Architecture– 0.4MB and 10Gbps for Snort rule set ( >10,000
characters)– Possible to be used for other applications, e.g. IP
lookups, packet classification.
• New Bit-split Algorithm: – General purpose enough for many other applications, e.g.
spam detection, peephole optimization, IP lookups, packet classification, etc.
– Feasible to be implemented on other tile-based architecture.
An efficient Implementation
00 01 10 11 PMV
0 0 1 0 0 0000
1 0 2 0 0 0000
2 0 3 0 0 1000
3 0 4 0 0 1110
4 0 4 0 0 1111
5
6
7
8
9
00 01 10 11 PMV
0 1 0 2 0 0000
1 1 0 3 0 0000
2 1 0 5 0 0000
3 1 6 5 0 0000
4 7 0 2 0 1000
5 0 4 5 0 0000
6 7 0 2 0 1100
7 9 0 3 0 0000
8 1 0 3 0 0010
9 1 0 3 0 0001
00 01 10 11 PMV
0 1 0 0 2 0000
1 1 3 0 2 0000
2 4 0 0 2 0000
3 1 0 5 6 1000
4 1 7 0 2 0000
5 1 0 0 8 0000
6 4 0 0 2 0010
7 1 0 5 6 1100
8 4 0 0 2 0001
9
00 01 10 11 PMV
0 0 0 1 2 0000
1 0 0 3 2 0000
2 0 0 4 2 0000
3 0 0 3 5 1000
4 0 0 6 2 0000
5 0 0 4 7 0010
6 0 0 3 5 1100
7 0 0 4 2 0001
8
9
Tile 0 Tile 2Tile 1 Tile 3
Cycle 3 e 01 10 01 01Cycle 2 h 01 10 10 00Cycle 1 x 01 11 10 00Cycle 0 h 01 10 10 00
h
h
x
e
h
h
x
e
h
h
e
xh
x
he
e 1100h 0000x 0000h 0000
e 1111h 1110x 1000h 0000
e 1000h 0000x 0000h 0000
e 1000h 0000x 0000h 0000
Cycle 3 + P 1000Cycle 2 + P 0000Cycle 1 + P 0000Cycle 0 + P 0000
22 2
2