1. Washington University in St. Louis
CSE 465 – Spring Digital Systems Laboratory Lecture 6 : String Matching in Reconfigurable Hardware
John W Lockwood
Washington University in St. Louis
Copyright 2005

2. String Scanning : 32-bit Data FormatH0
Packet Header
.. Header ..
Packet Header (N Words)
[31 downto 0] of Words H0 .. HN
Packet Payload (M bytes)
Bytes P0 .. PM HN P0 P1 P2 P3
Packet
Payload P4 P5 P6 P7 P8 P9
P10
P11
.. Payload .. PM
32 bits

3. MP6 Problem Statement Problem Statement Details
Implement a plug-in module that monitors a traffic flow. For cells with payloads that begin with “HELLO”
Details
Scan Flows destined for particular port
Match the content of the cell for the “HELLO”
ASCII: “HELLO”
Hex: 48 – 45 – 4C – 4C – 4F
Binary: 0100,1000 ; 0100,0101; 0100,1100 ; 0100,1100 ; 0100,1111

4. String Scanning : 32-bit Data FormatH0
Packet Header
.. Header ..
“HELLO” at even word offset in packet HN P0 P1 P2 ..
Packet
Payload
‘H’
‘E’
‘L’
‘L’
‘O’ .. .. ..
.. Payload .. PM
32 bits

5. String Scanning : 32-bit Data FormatH0
Packet Header
.. Header ..
“HELLO” offset by one byte in packet HN P0 P1 P2 P3
Packet
Payload ..
‘H’
‘E’
‘L’
‘L’
‘O’ .. ..
.. Payload .. PM
32 bits

6. String Scanning : 32-bit Data FormatH0
Packet Header
.. Header ..
“HELLO” offset by two bytes in packet HN P0 P1 P2 P3
Packet
Payload P4 ..
‘H’
‘E’
‘L’
‘L’
‘O’ ..
.. Payload .. PM
32 bits

7. String Scanning : 32-bit Data FormatH0
Packet Header
.. Header ..
“HELLO” offset by three bytes in packet HN P0 P1 P2 P3
Packet
Payload P4 P5 ..
‘H’
‘E’
‘L’
‘L’
‘O’
.. Payload .. PM
32 bits

8. String Scanning : Simplified State Machine
Packet Header
Init
.. Header .. HN
Header
Match P0 P1 P2 ..
Packet
Payload
‘H’
‘E’
‘L’
‘L’
‘HELL’
Match
‘O’ .. .. ..
.. Payload ..
‘HELLO’
Match PM
32 bits
Basic State Transition Diagram (simplified)

9. String Scanning : Simplified State Machine
Packet Header
Init
.. Header .. HN
Header
Match P0 P1 P2 P3
Packet
Payload ..
‘H’
‘E’
‘L’
‘HEL’
Match
‘L’
‘O’ .. ..
.. Payload ..
‘HELLO’
Match PM
32 bits
Basic State Transition Diagram (simplified)

10. String Scanning : Simplified State Machine
Packet Header
Init
.. Header .. HN
Header
Match P0 P1 P2 P3
Packet
Payload P4 ..
‘H’
‘E’
‘HE’
Match
‘L’
‘L’
‘O’ ..
.. Payload ..
‘HELLO’
Match PM
32 bits
Basic State Transition Diagram (simplified)

11. String Scanning : Detailed State Machine
Packet Header
Init
.. Header ..
End of Packet HN
Matching Port
Header
Match P0 P1 P2 P3
End of
Packet
‘HXXX’
Packet
Payload P4 P5 ..
‘H’
‘H’
Match
‘E’
‘L’
‘L’
‘O’
Not ‘HXXX’
‘ELLO’
.. Payload ..
‘HELLO’
Match
‘HXXX’ PM
32 bits
End of
Packet
Additional State Transitions (more detail provided)

12. Search Modules
Specification: Search for: “HELLO” n
“HELLO”
match

13. Parallel Matching Engines
Final Value
( end of document )
Control
Signals 8 32
Start 8
End 8
Data_Enable … 32 = 8
Streaming Data Input 8
Count Register
Match
Adder 32 8 8
Initial Value
( start of
Document ) 32
Streaming
Input
Data 8
Parallel Finite State Machines
(one for each offset)

14. Matching different length strings
Length=2 String : “my”
my**
*my*
**my
***m + y***
Length=3 String : “cry”
cry*
*cry
**cr + y***
***c + ry**
Length=5 String : “laugh”
laug + h***
*lau + gh**
**la + ugh*
***l + augh

15. Special Cases : Multiple Matches in a clock
For streams > 1,000 bytes, multiple packets are sent from the traffic generator
State machines should only transit when Data Enabled, otherwise – stay in the same state.
“MY’ count += 2
“CRY” count += 2
M Y M Y
x x C R
Y C R Y

16. Process data only when enabled
Data streams can be larger than 1,000 bytes
For streams > 1,000 bytes, multiple packets are sent fom the traffic generator
State machines should only transit when Data Enabled, otherwise – stay in the same state.
Hello Count = 1
Hello Count = 2
M Y M Y
L L O H
x x x x
E L L O
Data
Enabled
Data
Enabled
Data
Enabled No
Data

17. Data Enable Data streams can be larger than 1,000 bytes
For streams > 1,000 bytes, multiple packets are sent from the traffic generator
State machines should only transit when Data Enabled, otherwise – stay in the same state.
Hello Count = 1
Hello Count = 2
x H E L
L L O H
x x x x
E L L O
Data
Enabled
Data
Enabled
Data
Enabled No
Data