PEDS: Parallel Error Detection Scheme for TCAM Devices David Hay, Politecnico di Torino Joint work with Anat Bremler Barr (IDC), Danny Hendler (BGU) and Ron Roth (Technion) Supported by a Cisco grant

Packet Classification w/ TCAM Encoder Match lines Packet Header accept deny accept Dimensions: 100K-200K of rules, each entry is symbols TCAM Array Each entry represents a rule and is a word in {0,1,*} W

Types of Errors False Miss, False Hit, Indirect False Hit Very different than regular Error-Correction/Detection In fact, harder: we prove that on-the-fly error correcting is impossible, and that hardware change is needed for error detection * 1 * 1 * * 1 * 1 * No Match

Types of Errors False Miss, False Hit Very different than regular Error-Correction/Detection In fact, harder: we prove that on-the-fly error correcting is impossible, and that hardware change is needed for error detection * 1 * 1 * * 1 * 1 *

Types of Errors False Miss, False Hit, Indirect False Hit Very different from regular Error-Correction/Detection In fact, harder: we prove that on-the-fly error correcting is impossible, and that hardware change is needed for error detection * 1 * 1 * * 1 * 1 *

PEDS We add one or more check symbols to each entry as error detection code In most TCAM configuration, extra-bits are available anyway With the TCAM parallel lookup we provide a generic scheme with tradeoff points between: 1. Resilience 2. Number of extra-bits 3. Time till all errors are detected 4. Hardware changes Most tradeoff points can be set after deployment Always Depends on the entry width and not number of entries

Toy Example For each entry, duplicate each symbol 0*10  00**1100 → The entry is correct iff each pair has the equal symbols: 00, 11 or **. Applying two lookups for the first pair: 01****** 10****** If 00, 11 – none of the lookups match. If **, both lookups match If 01, 0*, *1 – only first lookup matches If 10, 1*, *0 – only second lookup matches Repeat for all other pairs. Even number of Matches Odd number of Matches

Hardware Change Encoder One XOR gate + One single-bit register per match lines (+control lines) Each register will save the result of the previous match and XOR it with the result of the current match. This will enable us to determine if we got even or odd number of matches.

One Generalization One check symbol for each k symbols Check symbol is the parity check Can be extended to each linear code Total Space: W/k extra bits. Resilience * : W/k A set of lookup keys such that the even-correct/odd- wrong property holds #keys for k symbols ~ 2 k+2 /3 #lookups ~ W[2 k+2 /(3k)] K# keys# lookups (W=100) # extra bits (W=100)

Wrap-up Detecting all errors using the built-in parallel lookup of the TCAM The number of lookups is a function of W, the width of the TCAM word, and not the number of entries in the database. Requires a hardware change, but only in the peripheral circuitry of the TCAM chip More generalizations and constructions in the paper…

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