1

Routing with a clueRouting with a clue

Anat Bremler-Barr

Joint work with

Yehuda Afek & Sariel Har-Peled

Tel-Aviv University

2

IP LookupIP Lookup

• IP lookup - given an IP address, determine the next hop for reaching that destination

• Fast Address lookup key component for high performance routers

1011001101011011001111110101

Destination Address

Prefix NxtHop

* 400* 12011101110* 301111* 71* 210000001* 3 10110* 3101111* 510110011 * 210110011010* 4

Forwarding Table

3

Best Matching Prefix (BMP)

• Routers today use kind of trie data structure (Patricia)

• BMP lookup is expensive – Count in number of memory reference

1

1

0

Prefix NxtHop

* 100* 2000* 30001* 401* 2 1* 1110* 3Forwarding tableForwarding tabletrietrie

Example: The BMP of 0000 is 000*

0 1

0

0 1

4

• Faster data structureFaster data structure– [WVTP 97] log W (W=32)– [DBCP 97] [ DKVZ 99]

[CM99] [NK98] compression– [LSV 98] binary and 6-way search– [SV 98] version on tries – [ CP99] caching

• Hardware Hardware – CAMs [MF 93]– Pipeline [GLM 98], [HZPS99]

• Reducing the need of IP lookupReducing the need of IP lookup – Tag-Switching– CSR – ARIS– IP-Switching– threaded indices– MPLS

Three Approaches

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Our ApproachRouting with a Clue:Distributed IP-lookup

Clue: Telling a router where its upstream neighbor ended the IP lookup

R1 R2 R3

0 0

1

0

1

111

R4

0

11

1

Many times the BMP’s are identical

6

Distributed IP-lookup

Prefixlength

Work

in Trie

Workwitha clue

32backbone

backbone1

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Distributed IP lookup, why ?

Each router on the path to the destination performs a similar IP-lookup:

• Same destination address• Similar forwarding tables at

neighboring routers

010,00020,00030,00040,00050,00060,00070,000

AT&T1 AT&T2

AT&T1 AT&T2Equal 23,382 23,382Diff 32 36,093Total 23,414 60,475

Numberof

Prefixes

8

010,00020,00030,00040,00050,00060,00070,000

ISP-B1 ISP-B2

ISP-B1 ISP-B2Equal 55,540 55,540Diff 494 419Total 56,034 55,959

Forwarding tables are similar

Numberof

Prefixes

9

Even if the routers are not neighbors

010,00020,00030,00040,00050,00060,00070,000

MaeEast MaeWest

MaeEast MaeWestEqual 23,382 23,382Diff 18,868 741Total 42,250 24,123

Numberof

Prefixes

10

Outline:

• Simple method

• Clue table and fields

• Advanced method

• Experimental results

• MPLS improvements

• Conclusions

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In many cases the Clue directly determines the best matching

prefix:

When the clue vertex (in the trie):

(a) has no descendants,

(b) does not exist

Trie of R1 Trie of R2

(a)(a)

(b)(b)

R1 R2

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In other cases the clue is -

a good point to start IP lookup

•Search from the vertex that corresponds to the clue

•If search fails, then forward according to the BMP of the clue string

Trie of R1 Trie of R2

0 0

1

1

1

0 0

R1 R2

Dest. Add. :001101010...

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Clue Table

R1 R2 R3

Clue table

Cluetable

Cluestable

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The clue table

• An entry for each clue a router may receive from its neighbors

• Two fields, that are pre-computed: – Ptr Ptr - pointer to the vertex that

corresponds to the clue– FDFD - Final Decision (BMP, next hop):

• When clue fixes the BMP

• When the search from the clue fails

(two cases are distinguished by Ptr value)

Clue table size: Estimation 500KB

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Example

R1 R2

Prefix NxtHop

*00*0001*01*1*

Prefix NxtHop*00*0001*00010*0101*011*1*

Clue FD Ptr

*00* 00*0001*01* * 1* 1*

0

0

0

0

0

0

1

1

1

1

1

1

1

Forwarding table

Clue table

Forwarding table

0

1

0

16

Combining the clue table of several neighbors

• A clue table at each interface

• If not, combine them into one

table

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Clue encoding in the header

• 5 bits (IPv4) indicating the prefix of

the dest. address which is the clue

Example:

Dest addressDest address:10110011111101011011001111110101

Clue is codedClue is coded:10001

The clueThe clue: 10110011111101011

• Need a hash-table

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To avoid hash-table

• Replace clue with <clue,index> pair: index is into the clue table (16 bits)

• Learning on the fly

Dest addressDest address:10110011111101011011001111110101

Clue is codedClue is coded:10001

The clueThe clue: 10110011111101011

Index:Index: 1110110101010110

1011…01 1110110101010110

Clue Ptr FD

Similar to the idea [CV96]

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Example

R1 R2

Prefix NxtHop

*00*0001*01*1*

Prefix NxtHop*00*0001*00010*0101*011*1*

Clue FD Ptr

*00* 00*0001*01* * 1* 1*

0

0

0

0

0

0

1

1

1

1

1

1

1

Forwarding table

Clue table

Forwarding table

0

1

0

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Advanced methodor

get more from a clue

Trie at R1 Trie at R2

clue clue

BMP of clue

R1 R2

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Advanced Method - Advanced Method - Claim:Claim:

No search is necessary, if on any path going down from the “clue” at the trie of R2, a prefix of R1 is encountered, before or at the same time that a prefix of R2 is encountered

If Claim holds on the received clue,

lookup costs one memory reference

Otherwise Problematic clues

need further search

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Problematic Clues

0

5000

10000

15000

20000

25000

AT&T1->AT&T2 575 22,839

Problematic Non-Prob

0

10000

20000

30000

40000

50000

60000

70000

AT&T2->AT&T1 52 60,423

Problematic Non-Prob

2.45%

0.0008%

AT&T1->AT&T2AT&T1->AT&T2

AT&T2->AT&T1AT&T2->AT&T1

Numberof

Prefixes

Numberof

Prefixes

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Problematic Clues

0

10000

20000

30000

40000

50000

60000

ISP-B1->ISP-B2 66 55,968

Problematic Non-Prob

0

10000

20000

30000

40000

50000

60000

70000

ISP-B2->ISP-B1 38 60,437

Problematic Non-Prob

0.0011%

0.0006%

ISP-B1->ISP-B2ISP-B1->ISP-B2

ISP-B2->ISP-B1ISP-B2->ISP-B1

Numberof

Prefixes

Numberof

Prefixes

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Combining the clue table of several neighbors

• A clue table at each interface• Advanced method - behavior depends on

which neighbor the clue came from:

Three solutions:– Intersection

• No search - if all neighbors agree that no further search is necessary

– Bit Map• Join the clues of neighbors in one table

• Add a Bit Map to each clue entry:

The j bit is set - further search is necessary if clue came from the j-th neighbor

– Sub-tables:• One table for the intersection: the clues that

behave the same,

• Specific table per neighbors for the other clue

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Method of continued search

Trie, Patricia[S93,LS88],

binary, 6-way[LSV98], Log W [WVTP97]

Clue from R1

Trie at R2

Prefix of R1

Prefix of R2

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Experimental results

AT&T1 AT&T2

0

5

10

15

20

25

Trie Patricia Binary 6-Way LogW

Common

Simple

advanced

Trie Patricia Binary 6-Way LogWCommon 23.5899 21 17 7 3.448Simple 2.0801 2.0565 2.1189 2.0456 3.0446advanced 1.0552 1.0442 1.049 1.019 1.0619

Mem

ref’s

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Experimental results

AT&T2 AT&T1

0

5

10

15

20

25

Trie Patricia Binary 6-Way LogW

Common

Simple

Advanced

Trie Patricia Binary 6-Way LogWCommon 23.241 19 16 6 3.6339Simple 2.0583 2.0396 2.0947 2.0367 3.0566Advanced 1.011 1.0011 1.0011 1.004 1.0029

Mem

ref’s

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Multi Protocol Label Switching

“Route once switch many”

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CLUE & MPLS

• MPLS still needs an IP-lookup– Data driven: at set up

– Control driven: at aggregation point

The label here is the clue

R1 R2 R3 R4

R5

R6

10.0.0/24 10.0.0/24 10.0.0/24 10.0.0/24

10.0.0.1/25

10.0.0.0/25

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• Simplicity

• 10 times faster

• Switching speed, without major changes (but MPLS w/o TE)

• Heterogeneous IP networks

• No coordination

• No setup time

• Helps MPLS too

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Future Work

• Clue with multidimensional classification

• Further experiments