Performance Analysis of Packet Classification Algorithms on

Network Processors

Deepa Srinivasan, IBM Corporation Wu-chang Feng, Portland State University

November 18, 2004IEEE Local Computer Networks

Network Processors

• Emerging platform for high-speed packet processing– Splice in a statistic here?– Provide device programmability while keeping

performance

• Architectures differ, but common features include…– Multiple processing units executing in parallel– Instruction set customized for network applications– Binary image pre-determined at compile time

Example: Intel’s IXP

IXP Architecture

• Multi-processor– StrongARM core for slow-path processing– 6 microengines for fast-path processing

• Hardware support for multi threading• Each microengine has 4 thread contexts• Zero or minimal overhead context switch

Motivation for study

• NPs offer a programmable, parallel alternative, but current packet processing algorithms are– Written for sequential execution or– Designed using custom, invariant ASICs

• To use them on NPs– Algorithms must be mapped onto NPs in different ways

with each mapping having varying performance

Our study

• Examine several mappings of a packet classification algorithm onto NP hardware

• Identify general problems in performing such mappings

Why packet classification?

• Fundamental function performed by all network devices– Routers, switches, bridges, firewalls, IDS

• Increasing complexity makes packet classification the bottleneck– Increase in size of rulesets– Increase in dimension of rulesets– Algorithms must perform at high-speed on the fast-path

Picking an algorithm

• Many algorithms sequential– Do not leverage inherent parallelism in NPs

• Several parallel algorithms– BitVector [Lakshman98]

• Parallel lookup implemented via FPGA• Maps well onto NP platform

Bit Vector algorithm

• T.V. Lakshman, D. Stiliadis, “High-speed policy-based packet forwarding using efficient multi-dimensional range matching”, SIGCOMM 1998.– Parallel search algorithm– Preprocessing phase– Two-stage classification phase

• Perform lookup for each dimension in parallel• Combine results to determine matching rule

Example ruleset

Rule Field 1 Field 2 Field 3 Action

r1 (10, 11) (2, 4) (8, 11) Allow

r2 (4, 6) (8, 11) (1, 4) Allow

r3 (9, 11) (5, 7) (12, 14) Deny

r4 (6, 8) (1, 3) (5, 9) Allow

Number of rules (N) = 4

Number of dimensions (d) = 3

Width of dimension (W) = 4 (bits)

BitVector example

Packet = {6, 10, 2}

Matching rule = r2

Two design mappings

• Consider multiple mappings of BitVector onto Intel’s IXP1200 microengines– Option 1: All processing for a single packet handled by

one microengine (μEngine) - Parallel– Option 2: Processing for a single packet is split across

μEngines - Pipelined

Recall: IXP has 6 μEngines

Parallel Mapping

Pipelined Mapping

Memory allocation

Purpose Type of memory

Queue for inter-microengine communication SRAM

List of rules actions SRAM

Tries representing ranges SDRAM

Bit Vectors SDRAM

Evaluation platform

• Intel IXP1200 Developer Workbench– Graphical IDE– Cycle-accurate simulator– Performance statistics

• All experiments run within simulator– Configurable– Logging facility

Simulator configuration

• IXP1200 chip– 1K microstore– Core frequency (~ 165 MHz)– 4 ports receive data

• Simulations run until 75000 packets received by IXP– Simulator sends packets as fast as possible

• Rulesets used– Experiments use a small, fixed set of rules– Availability of real-world firewall rulesets limited

Performance metrics

Performance Metric Description

Transmit rate (Mbps) The overall packet transmit rate of the IXP, for all the ports that are configured to send packets.

Microengine execution time (%) The percentage of the total number of microengine cycles that a microengine spent in performing useful tasks.

Microengine aborted time (%) The percentage of the total time of a microengine that was wasted due to instructions in its pipeline being aborted, typically due to branch instructions.

Microengine idle time (%) The percentage of the total time of a microengine that was wasted due to none of the 4 hardware threads being available to run, typically due to memory access wait time.

SDRAM access (%) The total percentage of SDRAM bandwidth utilized by all microengines.

SRAM access (%) The total percentage of SRAM bandwidth utilized by all microengines.

Results and Analysis

Throughput

Packets sent/receive ratio

Analysis

• Overall, Parallel performs better than Pipelined

• Pipelined : A single packet header in SDRAM is read multiple (3) times

Microengine utilization

Microengine aborted time

Analysis

• Aborted time is typically caused by branch instructions

• Algorithms must reduce branch instructions to maximize throughput

Microengine idle time

Distribution of microengine time

0

20

40

60

80

100

120

1 2 3 4 5 6

Microengine

Tim

e (%

) Idle

Aborted

Executing

0

20

40

60

80

100

120

1 2 3 4 5 6

Microengine

Tim

e (%

) Idle

Aborted

Executing

Parallel Pipelined

Analysis

• High microengine idle time in Pipelined due to memory latency

• Lower microengine aborted time in Pipelined due to what?

Discussion

• Pipelined mappings can bottleneck through memory– Repeated memory reads to send work from μEngine to

μEngine– Direct hardware support for pipelining required

• IXP2xxx = next-neighbor registers• Currently re-examining our results on IXP2400

• Algorithms with fewer branch instructions result in better microengine utilization (lower aborted time)

Conclusion

• Packet classification is a fundamental function

• Parallel nature of NPs well-suited for parallel search algorithms

Conclusion

• Network processors offer high packet processing speed and programmability– Performance of an algorithm depends on the design

mapping chosen

• Contributions– Demonstrated that mapping has considerable impact on

performance• Pipelined mappings benefit from hardware support• Algorithms with fewer branch instructions result in better

processor utilization

Future work

• Analyze other mappings– Split work across different hardware threads in a single microengine– Placement of data structures in different memory banks

• IXP2400– Examine how hardware features change trade-offs in algorithm

mapping

• Algorithms designed specifically for network processors

Backup Slides

Definitions

• Process of categorizing packets according to pre-defined rules

• Classifier or ruleset: collection of rules

• Dimension or field: packet header used

• Rule: range of field values and action

Packet classification algorithms

Algorithm Time complexity

Storage complexity

Linear Search Nd Nd

Set-pruning Tries dW NddW

Grid of tries Wd-1 NdW

Cross producting dW Nd

Fat Inverted Segment tree (l + 1)W l x N1+1/l

Recursive Flow Classification d Nd

Hierarchical Intelligent Cuttings d Nd

Tuple Space Search m N

Bit Vector dW + N/memory-width

dN2

N: number of rules d: number of dimensionsW: maximum number of bitsl : number of levels occupied by a FIS-tree