Abstract:
A Ternary Content Addressable Memory (TCAM)-based Longest Prefix Match (LPM) lookup table including a TCAM holding a plurality of prefix entries for looking up results in an associated RAM, the associated RAM storing results corresponding to TCAM match indices; additional Random Access Memory (RAM) storing results from the associated RAM; and one entry in the TCAM representing at least two entries in the additional RAM from the associated RAM, whereby at least one entry in the TCAM is made available.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and device for improving the scalability of devices performing longest prefix match. 
     BACKGROUND OF THE INVENTION 
     Longest Prefix Match (LPM) is a problem of finding the longest prefix among a number of prefixes stored in a data base that matches a given lookup key. LPM can be used in many applications and is not limited to IP routing, but since IP routing is one of the major LPM applications, the present invention will be discussed in a context of IP routing, by way of non-limiting example, only. 
     A Forwarding Information Base (FIB), also known as a forwarding table, is most commonly used in network bridging, routing, and similar functions, to find the proper interface to which the input interface should send a packet to be transmitted by the router. Thus, a FIB contains a set of prefixes with corresponding output interfaces. A forwarding decision is made by finding the longest prefix matching the lookup key. The most commonly used lookup key at present is the IPv4 or IPv6 destination address of the packet. 
     A FIB, or other device for performing LPM, can be implemented with Random Access Memories (RAM) by using algorithms, such as M-Trie, Bitmap Tree, etc., or with fast hardware lookup mechanisms, such as Ternary Content Addressable Memories (TCAM). While advanced RAM-based FIB implementations/algorithms are scalable enough to hold millions of prefixes, they are not scalable from the lookup key width perspective, as the number of accesses to FIB memory depends on the lookup key width. In general, the wider the key, the more lookups are required. 
     With TCAM-based FIB or LPM implementation, the lookup time is constant. However, TCAM-based FIB is not scalable in terms of the number of prefixes. The most advanced TCAM device known today has a capacity of 40 Mbit, which can be used to hold up to 0.5M prefixes. 
     Accordingly, there is a long felt need for a method and apparatus for performing longest prefix match lookup that is scalable both in terms of the number of prefixes and in terms of the lookup key width. 
     SUMMARY OF THE INVENTION 
     The proposed solution is to improve LPM scalability by utilizing a TCAM device with its associated results RAM, adding an additional RAM, and inserting entries to the TCAM that stand for multiple entries in the additional RAM. The solution can also be used to hold a LPM lookup-table of a certain scale in fewer TCAM entries than in the conventional manner. In this way, it is possible either to use a smaller TCAM device or to free more TCAM entries for possible use by other applications. 
     There is provided according to the present invention a method for building a Longest Prefix Match lookup table including arranging a plurality of prefix entries in a TCAM and inserting results corresponding to TCAM match indices into an associated results RAM, the method including selecting a group of at least two prefix entries that can be reduced to a single entry in the TCAM, which single entry consists of a common prefix for all prefix entries in the group, arranging results from the associated RAM represented by the group of prefixes in additional RAM, replacing the group of prefix entries in the TCAM with the single TCAM entry pointing to multiple results in the additional RAM, thereby making available at least one entry in the TCAM, and replacing the entries in the associated RAM by an entry pointing to the entries in the additional RAM. 
     There is further provided, according to the invention, a method for performing Longest Prefix Match (LPM) lookup, the method including building and maintaining a lookup table including a TCAM and associated results Random Access Memory (RAM), and additional RAM, wherein at least two entries in the additional RAM are represented by one entry in the TCAM, performing a lookup in the TCAM, finding a results for an entry in the TCAM in the associated results RAM pointing to a group of results in the additional RAM, and, in response thereto, completing the lookup by reading an entry in the group of results in the additional RAM. 
     There is also provided according to the invention, a Ternary Content Addressable Memory (TCAM)-based Longest Prefix Match (LPM) lookup table including a TCAM holding a plurality of prefix entries for looking up results in an associated RAM, the associated RAM storing results corresponding to TCAM match indices, additional Random Access Memory (RAM) storing results from the associated RAM, and one entry in the TCAM representing at least two entries in the additional RAM from the associated RAM, whereby at least one entry in the TCAM is made available 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be further understood and appreciated from the following detailed description taken in conjunction with the drawings in which: 
         FIG. 1   a  is a schematic illustration of representation of LPM device entries by TCAM entries, in accordance with the prior art; 
         FIG. 1   b  is a schematic illustration of representation of the LPM entries of  FIG. 1   a  by TCAM and additional RAM entries, in accordance with one exemplary embodiment of the present invention; 
         FIG. 2   a  is a schematic illustration of representation of LPM device entries by TCAM entries, in accordance with another example of the prior art; 
         FIG. 2   b  is a schematic illustration of representation of the LPM entries of  FIG. 2   a  by TCAM and additional RAM entries, in accordance with another exemplary embodiment of the present invention; 
         FIG. 3   a  is a schematic illustration of representation of LPM device entries by TCAM entries, in accordance with another example of the prior art; and 
         FIG. 3   b  is a schematic illustration of representation of the LPM entries of  FIG. 3   a  by TCAM and additional RAM entries, in accordance with a further exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a method and apparatus for performing longest-prefix-match lookup, using a TCAM (with its associated RAM serving as a results memory) and additional RAM, in order to achieve better scale (for the number of entries) than in standard TCAM-based solutions. The method improves LPM scalability by storing a portion of the results data in the RAM associated with the TCAM and a portion of the data in the additional RAM, and inserting entries to the TCAM that stand for multiple entries in the additional RAM, thereby making available in the TCAM entries that would be occupied in conventional TCAM-based devices. 
     In conventional TCAM based LPM solutions, each LPM entry is represented by one TCAM entry, which points to a result in an associated results memory (the associated RAM). In the present invention, a group of LPM entries can be represented by a single TCAM entry, whose prefix is a common prefix of all the entries in that group. In this case, the respective result in the associated RAM points to an array of results in the additional RAM, which contains the results of each entry in that group of LPM entries. 
     The entries in the additional RAM can then be searched using a narrow portion of the lookup key, whereby results can be obtained in a single RAM access. In this way, some of the results in the results memory will be found during the lookup in the TCAM and some of the results will be found during the subsequent lookup in the additional RAM. It will be appreciated that, if desired, one (or more) further additional RAM access can be provided, i.e., several layers of RAM arrays can be enabled, one pointing to the other An algorithm that uses multiple levels of RAM might achieve better scale for LPM entries for certain sizes of TCAM and RAM. The disadvantage of this arrangement is the lookup time, since multiple RAM accesses will be performed. 
     A TCAM entry, whose prefix is of some length M, may refer to an array of results in the additional RAM that contains all the possible prefixes of length M+N that match this TCAM entry. Such an array will contain 2 N  entries (which is the number of prefixes of length M+N that match a certain prefix of length M), and will represent all the LPM entries with prefixes of lengths M to M+N, inclusive, that match the prefix (of length M) of the TCAM entry that points to the array. When a group of LPM entries can be represented in the TCAM by a single entry, the group of results is moved from the associated results RAM to the additional RAM. The LPM entries whose results are represented in the array in the additional RAM can be removed from the TCAM and replaced by a single entry pointing to a new result in the associated results RAM which, in turn, points to the corresponding array in the additional RAM, thus allowing representation of a larger LPM lookup table with a given TCAM size. A subsequent lookup in the corresponding additional RAM array will only be over bits M to (M+N−1), so that results can be obtained during a single access of the additional RAM. 
     The result in the associated RAM entry indicated by the TCAM lookup no longer contains the original results (which are now in the additional RAM) but contains three parameters permitting reading of the actual results—start bit, stride size and an index calculated based on the lookup key, start bit and stride size. These parameters indicate the location of the start of the array in the additional RAM and the relevant bits of the look up key (for example, the relevant bits of the IP address, in a FIB) over which the lookup in the additional RAM is performed, that should be used to retrieve the index inside the array in the additional RAM. Typically, the relevant bits are bits M through (M+N−1) of the lookup key. The LPM lookup is completed by reading an entry in the additional RAM corresponding to the calculated index within the array pointed to by the TCAM match index. The result address is calculated according to these values and fetched from the additional RAM memory. 
     The results for TCAM entries that are not grouped together in arrays can be placed in the results RAM associated with the TCAM. One of the bits in the result in the associated results RAM may be used to indicate whether the rest of the result (in associated RAM) is the actual result, or contains a pointer to an array in the additional RAM where the actual result stands, as explained above. While this method is not mandatory, it is useful in order to avoid the additional memory access when the result prefix is not grouped in an array. 
     For purposes of the present application, for an array with 2 N  entries, N is referred to as the array&#39;s stride size. Thus, a TCAM entry with prefix-length M that points to an array with stride size N stands for up to (2 N+1 −1) LPM entries of length ranging from M to M+N, inclusive. It will be appreciated that several stride-sizes may be supported and arrays of different sizes can coexist in the additional RAM. 
     The software algorithm that is responsible for the creation of the LPM table is also responsible for spotting candidate prefixes for creation of arrays, and deciding to create new arrays or to decompose existing arrays. One possible way to choose which arrays should be created is to prioritize candidate arrays by their “utilization”, which is defined as the ratio between the number of TCAM entries that can be reduced when creating the array, and the array size (2 N  for an array with stride size of N). 
     When prioritizing according to ‘utilization’, for example, a possible implementation involves using global utilization thresholds, so that candidate arrays with utilization higher than a certain threshold are created, and existing arrays whose utilization falls below some other threshold are decomposed and their LPM entries are returned to the TCAM individually. Since the lookup table can change over time, especially in the case of FIB tables, these arrays can be created and decomposed during operation. In this exemplary implementation, the values of the utilization thresholds can be optimized by performing tests on the specific application using the LPM lookup, and will also be highly influenced by the ratio between the available TCAM size and the available RAM size dedicated for the use of the algorithm. The larger the RAM size compared to the TCAM size, the lower the utilization threshold that can be allowed. 
     For example, looking at IP routing, tests show that, utilizing the distribution of Internet IPv4 entries in the forwarding table of the Internet in April 2009, the method of the present invention could achieve the following scale results: 
                                             Ratio between additional    Ratio between number of FIB entries that           RAM entries to   can be achieved using the proposed           TCAM entries   algorithm to number of TCAM entries                           8:1   ~3.3:1            4:1   ~2.65:1           2:1   ~1.67:1                        
Looking at the middle line in the table, for example, if the additional RAM memory (used to store the prefix arrays) has 4 times the number of entries that the TCAM has, the algorithm can improve the scale of the number of prefixes in the FIB table by ˜2.65 times compared to a conventional TCAM solution. Putting numbers to the example, using a TCAM device that can hold 1M entries and an additional RAM memory with 4M entries, the algorithm can achieve a scale of ˜2.65M FIB entries
 
     Referring to  FIG. 1   a , there is shown a schematic illustration of an LPM table, here illustrated as a FIB table  20 , illustrating the representation of a plurality of FIB entries by TCAM entries, in accordance with one example of the prior art. As can be seen, each FIB entry  12  (illustrated as black dots in the FIB table trie  10 ) is represented by one entry  14  in the TCAM  22 , which points to a result (A to E)  16  in the associated results RAM  24 . Nodes  18  (illustrated as white dots in the FIB table trie  10 ) are not real FIB entries, they merely show junctions in the FIB table trie, so do not appear in the TCAM. 
       FIG. 1   b  is a schematic illustration of a representation of a FIB table  30  representing a FIB table trie  31 , corresponding to the FIB table trie  10  of  FIG. 1   a , in accordance with one exemplary embodiment of the present invention. FIB table  30  includes a TCAM  22  with its associated results RAM  24  and additional RAM  26 . The small dotted triangles  19  on  FIGS. 1   a  and  1   b  indicate the candidate portion of the FIB trie that can be reduced to a single TCAM entry (i.e., removed to the additional RAM for searching therein). The corresponding results from the associated results memory are now stored in an array  36  (A,B,C,D) in the additional RAM  26 . Thus, in  FIG. 1   b , a single TCAM entry  32 , prefix 0100* in TCAM  22 , replaces the 4 former TCAM entries 0100*, 01001*, 010000* and 010001*, thereby leaving three entries available in the TCAM. Preferably, the prefix in the TCAM that replaces the group of prefixes is the common prefix of all the entries in the group. 
     The result  34  now retrieved by the TCAM lookup contains the location of the start of the array  36  in the additional RAM and the relevant bits of the entry (e.g., IP address) over which the lookup is performed, so as to retrieve the index inside the array (here illustrated as bits  4  and  5 ). The utilization of the reduction shown in this example is the number of TCAM prefixes that become available, here 3, divided by the size of the array, which is 2 2 =4. Thus, this reduction has a utilization of ¾ or 75%. 
       FIG. 2   a  is a schematic illustration of representation of LPM device entries by TCAM entries, in accordance with another example of the prior art. This FIB table trie  40  is similar to trie  10  in  FIG. 1   b , except that node 0100* is not a real FIB entry. In a conventional TCAM based LPM device, the entries of trie  40  are stored in a TCAM  42  and an associated results memory  44 . Since prefix 0100* is not a real FIB entry, both the TCAM  42  and the results memory  44  contains one fewer entry than in  FIG. 1   a.    
     Referring now to  FIG. 2   b , there is shown a representation of the LPM entries of  FIG. 2   a  by TCAM and additional RAM entries, in accordance with another exemplary embodiment of the present invention, illustrated in FIB table trie  50 . Despite the fact that prefix 0100* is not a real FIB entry, it can still be used in reduction of a candidate portion of the trie. Thus, instead of 5 entries, in TCAM  51  there are three entries. Entry  52  is the 0100* prefix which points to result  56  in the associated results RAM  54 . The results in the results RAM corresponding to the TCAM entries that were reduced are stored in array  57  in the additional RAM  58 . As can be seen, the array in additional RAM  58  includes a prefix 010001* that is not represented by an LPM entry in the group of entries reduced to the array. In this case, the array will contain a “backtracking” result, i.e., the result of the first node above it in the LPM table trie which is a real node, here 010*, result E. In other words, according to the invention, if the array includes a prefix of length M+N that is not represented by any LPM entry in the reduced group, the array will contain a backtracking result, i.e., a duplication of the result of the longest prefix in the table that is a sub-prefix of M+N and has length less than M. It will be appreciated that, in this case, while the array size is also 2 2 =4, the number of TCAM prefixes is reduced only by 2, thus, this reduction has a utilization of 2/4 or 50%. 
       FIG. 3   a  is a schematic illustration of representation of LPM device entries by TCAM entries, in accordance with a further example of the prior art. This FIB table trie  60  is similar to trie  40  in  FIG. 2   b , except that node 010010* is not a real FIB entry. In a conventional TCAM based LPM device, the entries of trie  60  are stored in a TCAM  62  and an associated results memory  64 . Since prefix 010010* is not a real FIB entry, both the TCAM  62  and the results memory  64  contains one fewer entry than in  FIG. 2   a.    
       FIG. 3   b  is a schematic illustration of representation of the LPM entries of  FIG. 3   a  by TCAM and additional RAM entries, in accordance with a further exemplary embodiment of the present invention, illustrated in FIB table trie  70 . In this example, there are also three entries in TCAM  71 . Here, too, entry  72  is the 0100* prefix which points to result  76  in the associated results RAM  74 . The results in the results RAM corresponding to the TCAM entries that were reduced are stored in array  77  in the additional RAM  78 . As can be seen, the array in additional RAM  78  includes two additional prefixes 010010* and 010011* that are not represented by any LPM entry. In this case, the array will contain a “duplication” result, i.e., both of these entries will have the same result, result C. In other words, according to the invention, an LPM entry with prefix-length less than M+N that is represented in the array may be duplicated to several results in the array that stand for all the possible prefixes of length M+N represented by this prefix, unless there is a more specific prefix (i.e., of longer length) in the array that represents this M+N prefix. As can be seen, in this case, while the array size is also 2 2 =4, the number of TCAM prefixes is reduced only by 1, thus, this reduction has a utilization of ¼ or 25%. 
     To sum up, an LPM lookup is performed on the LPM device of the present invention as follows. First, a TCAM lookup is performed, for a given key, in the TCAM of the LPM device. An associated results RAM entry corresponding to the TCAM match index is read, and an index is calculated, based on the lookup key and two parameters (start bit and stride size) in the associated results RAM entry. The LPM lookup is completed by reading an entry in the additional RAM corresponding to the calculated index within the array pointed to by a third parameter from the associated results RAM entry. This additional RAM entry contains the application specific result for the longest prefix matching the given key. 
     The LPM table is maintained by adding new prefixes to the table and removing old prefixes from the table, as required. A new prefix can be added to the table in one of three ways. First, as in conventional LPM tables, a new entry can be added to the TCAM for a given prefix, and the result is written into the corresponding result RAM entry. Alternatively, if a suitable array has already been created in additional RAM, the prefix result can be added to the corresponding entry in the existing array in the additional RAM. If appropriate, and if no corresponding array exists, a new array can be created in the additional RAM, containing the new prefix results with some of the existing prefix results. The new common prefix result, pointing to the newly created array, is written into the results RAM and the new common prefix is added to the TCAM. Any existing array corresponding to the prefixes included in the new array are now decomposed and all the respective resources (e.g., TCAM resources) are returned to TCAM and associated results RAM. 
     A prefix result can be updated by overwriting the old result in the results RAM entry or in additional RAM entries. 
     An old prefix can be removed from the table by either overwriting the prefix result in the corresponding additional RAM entry by the result of the longest prefix matching the deleted prefix, or by invalidating the corresponding TCAM entry for the given prefix so that it no longer participates in the lookup. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. It will further be appreciated that the invention is not limited to what has been described hereinabove merely by way of example. Rather, the invention is limited solely by the claims which follow.