Survey of Packet Classification Algorithms. Outline Background and problem definition Classification...
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Transcript of Survey of Packet Classification Algorithms. Outline Background and problem definition Classification...
Survey of
Packet Classification
Algorithms
Outline
Background and problem definition
Classification schemes
– One dimensional classification
– Two dimensional classification
Background
Flow-aware vs. Flow-unaware Routers Flow-aware router
– Keep track of flows and perform similar processing on packets in a flow
Flow-unaware router
– Packet-by-packet router
– Treat each incoming packet individually
Why Flow-aware Router? Additional mechanisms required
– Admission control, resource reservation, per-flow queueing, fair scheduling etc.
Provision of DiffService in ISPs
– Capability to distinguish and isolate traffic belonging to different flows based on negotiated service agreements
Classification
Rules or Policies
Need for DiffService Service
Traffic shaping Traffic filtering Policy routing
ISP1
NAP
E1
E2
ISP2
ISP3Z
X
Y
More Value added Services DiffService
– Regard traffic from Autonomous System #33 as `platinum grade’
Accounting and billing– Treat all video traffic as highest priority and perf
orm accounting for this type of traffic Committed access rate (rate limiting)
– Rate limit WWW traffic from sub interface#739 to 10Mbps
Flow-aware Router-Basic Architectural Components
Special processing
Control
Datapath:per-packet processing
Routing lookup
Routing, resource reservation, admission control
Packet classification
Switching
Scheduling
Flow Classification
Forwarding Engine
Flow Classification
HEADER
Flow Index
Classifier (Policy Database)
Predicate Action
IncomingPacket
Classful Addresses
0
10
110
Network Host
Network
Network
Host
Host
Class A
Class B
Class C
7 24
21 8
14 16
Every address was class A or B or C, easily determined by the first three bits of the address
Classless InterDomain Routing (CIDR)
Prefix can be of arbitrary length
208.12.16/24 208.12.21/24 208.12.31/24
0 232-1Total IPv4 address space
Prefix ranges
208.12.21/24
0 232-1Total IPv4 address space
208.12.16/20
These addresses match both prefixes
An exception prefix
Table Growth of a Backbone Router
From http://www.telstra.net/ops/bgptable.html
Prefix Length Distribution
Problem
Definition-
Packet
Classification
Given a classifier C with N rules, Rj, 1 j N, where Rj consists of three entities
– A regular expression Rj[i], 1 i d, on each of the d header fields,
– A number, pri(Rj), indicating the priority of the rule in the classifier, and
– An action, referred to as action(Rj)
Classification is a Generalization of Lookup
Classifier = routing table
One-dimension (destination address)
Rule = routing table entry
Regular expression = prefix
Action = (next-hop-address, port)
Priority = prefix-length
Metrics for Classification Algorithms Speed
Storage requirements
Low update time
Ability to handle large classifiers
Flexibility in implementation
Low preprocessing time
Scalability in the number of header fields
Flexibility in rule specification
One Dimensional
Packet Classification
–
IP Address Lookup
Algorithms
Binary Tries
Prefixesa 0*b 01000*c 011*d 1*e 100*f 1100*g 1101*h 1110*i 1111*
a d
c
b
e
h if g
0
0
0
0
0
0
0
0 0
1
1
1 1
1
11
Path-Compressed Trie
Prefixesa 0*b 01000*c 011*d 1*e 100*f 1100*g 1101*h 1110*i 1111*
a d
ec
h if g
0
0
0
0 0
1
1 1
1
11
b
0
1
3 2
3
4 4
Legend: x indicates to inspect which bit
Disjoint-prefix Binary Trie
Prefixesa 0*b 01000*c 011*d 1*e 100*f 1100*g 1101*h 1110*i 1111*
c
b
e
h if g
0
0
0
0
0
0
0
0 0
1
1
1 1
1
11
a1
0
a3
1
a2
1
d1
1
Leaf pushing Disjoint prefixes do not overlap No prefix is itself a prefix of another
Variable-stride Multibit Trie
a
c
01 10
a d d
00 11
c
b
ihgfe
00
0 1
0 101 1011 00 11
01 10
stride=2stride=1
Prefixesa 0*b 01000*c 011*d 1*e 100*f 1100*g 1101*h 1110*i 1111*
Reduced number of memory accesses Greater wasted space
Caching Addresses
CPU
MAC
LocalBuffer
Memory
LineCard
DMA
MAC
LocalBuffer
Memory
Fast Path
Slow Path
Advantages Increased average lookup performance
Disadvantages Decreased locality in backbone traffic Cache size Cache management overhead Hardware implementation difficult
LineCard
LocalBuffer
Memory
LineCard
DMA DMA
MAC
BufferMemory
Hash-based Scheme Store a hash table for each prefix length Hash key is the prefix value and prefix
length Search scheme
– Linear search on prefix lengths
– Binary search on prefix lengths Need to provide intermediate markers
– Guide to more specific prefix
Need precomputation per marker– Avoid backtracking
Linear Search on Prefix Lengths
Prefixesa 0*b 01000*c 011*d 1*e 100*f 1100*g 1101*h 1110*i 1111*j 01*k 1100001*p 101*
a d
j
c
b
e
h if g
0
0
0
0
0
0
0
0 0
1
1
1 1
1
11
p1
0
0
k1
1
3
2
5
7
6
4
Linear searchon length
Binary Search on Prefix Lengths
Prefixesa 0*b 01000*c 011*d 1*e 100*f 1100*g 1101*h 1110*i 1111*j 01*k 1100001*p 101*
a d
j
c
b
e
h if g
0
0
0
0
0
0
0
0 0
1
1
1 1
1
11
p1
0
0
k1
1
3
2
5
7
6
4
Binary search on length
Lookups with Ternary-CAM
Memory array Priority
encoder
Next-hopmemory
Next-hop
TCAM RAM
01
23
M
0
1
00
1
DestinationAddress
Lookups with Ternary-CAM
Advantages– Suitable for multiple fields– Fast: 16-20 ns (50-66 Mpps)– Simple to understand
Disadvantages– Inflexible: range-to-prefix blow
up– Density: largest available in 200
0 is 32K x 128 (but can be cascaded)
– Management software, and on-chip logic: non-trivial complexity
– Power: 5-8 W– Incremental updates: slow– DRAM-based CAMs: higher dens
ity but soft-error is a problem– Cost: $30-$160 for 1Mb
Two Dimensional
Packet Classification
Set-pruning Tries
Rule DA SA
R1 0* 10*
R2 0* 01*
R3 0* 1*
R4 00* 1*
R5 00* 11*
R6 10* 1*
R7 * 00*
Dimension SA
Dimension DA
R7 R2 R1 R5 R7 R2 R1
R3
R7
R6
R7
R4
Hierarchical Tries
Dimension DA
Dimension SA
R5 R2 R1
R3R6
R7
R4
Rule DA SA
R1 0* 10*
R2 0* 01*
R3 0* 1*
R4 00* 1*
R5 00* 11*
R6 10* 1*
R7 * 00*
Grid-of-Tries
Dimension DA
Dimension SA
R5 R2R1
R3R6
R7
R4
Rule DA SA
R1 0* 10*
R2 0* 01*
R3 0* 1*
R4 00* 1*
R5 00* 11*
R6 10* 1*
R7 * 00*
Grid-of-Tries – cont.
Advantages
Good solution for two dimensions
Disadvantages
Static solution Not easily extensible to more than two dimensions
20K entries: 2MB, 9 memory accesses (with expansion)
Bitmap-intersection
R4 R3 R2R1
1
1
0
0
1
0
1
1
R3
R4
R1
R2
Bitmap-intersection – cont.
Advantages
Good solution for multiple dimensions, for small classifiers
Disadvantages
Static solution Large memory bandwidth (scales linearly in N) Large amount of memory (scales quadratically in N) Hardware-optimized
512 rules: 1Mpps with single FPGA (33MHz) and five 1Mb SRAM chips
Cross-producting
R4 R3R2
R1
5
4
3
2
1
6
21 7 8 94 5 63
P1P2
(1,3)
(8,4)
Cross-producting – cont.
Advantages
Fast accesses Suitable for multiple fields
Disadvantages
Large amount of memory Need caching for bigger classifiers (> 50 rules)
50 rules: 1.5MB, need caching (on-demand cross-producting) for bigger classifiers
Need: d 1-D lookups + 1 memory access, O(Nd) space