Efficient packet classification using TCAMs

1. Efficient packet classification using TCAMs Authors:Derek Pao, Yiu Keung Li and Peng Zhou Publisher: Computer Networks 2006 Present:Chen-Yu Lin Date: Jan, 10, 2008

2. Outline • Introduction • Filter encoding scheme • Prefix Inclusion Coding scheme • Performance evaluation

3. Introduction • TCAM is currently the dominant solution method used by the industry. High cost and high power consumption are the two major drawbacks of TCAM-based lookup engines. Adoption of IPv6 with increased address length will further exacerbate. • In this paper, we present a filter encoding method, called prefix inclusion coding (PIC) to improve the efficiency of TCAM-based lookup engine. • Filters are stored in an encoded format to reduce storage requirement. • Codeword lookup is equivalent to finding the longest matching prefix in the codeword table.

4. Filter encoding scheme • Filter encoding is an effective approach to enhanced the efficiency of the TCAM lookup engine. • van Lunteren and engbersen proposed an encoding scheme called P2Cbased on the concept of primitive range hierarchy . • Ranges are mapped to different layers in the hierarchy such that ranges on the same layer must be disjoint and are represented by codeword.

5. Filter encoding scheme • Style 1 • Primitive ranges in a layer are assigned distinct non-zero codeword. • Style 2 • Allows 2 primitive ranges at the same layer to be assigned a common codeword if both ranges are subsets of two disjoint primitive ranges at other layers. • Style 3 • Primitive ranges on different layers are merged if the number of layers exceeds the physical limit.

6. Filter encoding scheme

7. Filter encoding scheme • The codeword lookup method is required to find all matching ranges and combine the code segments for each primitive layer to form the final codeword. • For example, the codeword corresponds to the input address y marked in coding style 2 of figure 2 is 010100. • The proposed method can have better compression ratio compared to P2C. • Our method can reduce the TCAM space requirement by 70% to over 90%

8. Prefix Inclusion Coding Scheme • A 5-tuple filter <F1,F2,F3,F4,F5> is stored in the TCAM in an encoded format <C1,C2,C3,C4,C5> where Ci is the codeword for field Fi. • Let p and q be two distinct address prefixes where the length if p is shorter than or equal to the length of q. Prefixes p and q are either disjoint, or q is enclosed by p (q is subrange of p). • The codeword assignment satisfies the following 3 requirement. • 1. A valid codeword must have non-zero value. • 2. The inclusion property is preserved. Cq is enclosed by Cp iff q is enclosed by p • 3. If q is enclosed by p, then Cqmust have a non-zero suffix extension from Cp

9. Prefix Inclusion Coding Scheme • The system maintains two separate codeword tables storing the <prefix, codeword> pairs for the source and destination prefixes. • An input address A is first translated to a codeword CA by finding the longest matching prefix in the corresponding codeword table. • Given a sorted list of n prefixes, a tree (i - tree) that represents the inclusion relationship can be constructed in O(hn) time where h is the depth of the tree. Depends on the maximum nesting level of prefixes.

10. Prefix Inclusion Coding Scheme • A leaf node in the i – tree can be assigned a full-length codeword that occupies a single point in the code space.

11. Prefix Inclusion Coding Scheme • The code space occupied by an internal node is greater than or equal to 1 plus the sum of the code space of its children. • The code space occupied by an internal node must be a power of 2. • The minimum codeword length is then equal to log2 of the code space occupied by root.

12. Prefix Inclusion Coding Scheme

13. Prefix Inclusion Coding Scheme • An address value is first translated to a value in the code space before the encoded filter table is searched. • In the codeword table, a don’t care bit in a codeword is substituted by a zero. • For example: • Input address is 1110 0111, the best matching prefix is J = 11100* and the return code value will be 10100 • The codeword lookup can be implemented using TCAM or other efficient algorithmic method.

14. Prefix Inclusion Coding Scheme • PIC can also applied to encode port range values • The port range [0-1023] and [1024-65535] are commonly found in filter sets, other arbitrary port ranges are, in general, subranges of these two ranges. • If two ranges are partially overlapping, one of the 2 ranges will be decomposed.

15. Prefix Inclusion Coding Scheme

16. Prefix Inclusion Coding Scheme • Using a pure PIC encoding scheme, there can be 2 options: • 1. range B is decomposed into 2 sub ranges. • 2. range C is decomposed into 2 sub ranges. • One may avoid port range decomposition by using a hybrid encoding method that combines PIC with P2C • Partially overlapped ranges are grouped into a composite range

17. Performance evaluation • Table 2 shows the required code length for the five fields in a filter.

18. Performance evaluation • Compare the raw space required by PIC and that of the conventional approach where filters are not encoded.

19. Performance evaluation • In PIC, TCAM space requirement consists of two components • The encoded filters • The codeword table for the two address fields. • n: total number of distinct prefixes.L: length of the overall codeword for a 5-tuple filter.r: number of replicated entries due to partially overlapping port range. • TCAM space for encoded filter table is (N + r) * L bits, and the space for the codeword table is n*32 bits.