Abstract:
A series of hardware pipeline units each processing a stride during prefix search operations on a multi-bit trie includes, within at least one pipeline unit other than the last pipeline unit, a mechanism for retiring search results from the respective pipeline unit rather than passing the search results through the remaining pipeline units. Early retirement may be triggered by either the absence of subsequent strides to be processed or completion (a miss or end node match) of the search, together with an absence of active search operations in subsequent pipeline units. The early retirement mechanism may be included in those pipeline units corresponding to a last stride for a maximum prefix length shorter than the pipeline (e.g., 20 or 32 bits rather than 64 bits), in pipeline units selected on some other basis, or in every pipeline unit. Worst-case and/or average latency for prefix search operations is reduced.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to multi-bit trie based network search engines and, more specifically, to improving the performance of hardware pipelined multi-bit trie based network search engines. 
     BACKGROUND OF THE INVENTION 
     Network routers for packet-based communications protocols such as Internet Protocol (IP) direct incoming information to the next neighbor along a route to the intended destination for the packet. To do this, typically each router along the route must perform route address prefix (normally referred to as just “prefix”) lookup operations on a prefix (or routing) table to determine the appropriate next hop address for the destination IP prefix. Such operations are performed by either an embedded network processor or, more commonly, by a separate network search engine. 
     Originally the hardware for network search engines employed content addressable memory (CAM), a type of memory consisting of a bit comparator and two memory elements, one for storing data and the other storing a compare mask. The CAM compares incoming data with the value stored in the data memory under the control of the mask value, which may be programmed to override the comparison result to “always match” (i.e., “don&#39;t care”). In operation, a CAM-based network search engine functions by storing all prefixes of a routing table in a CAM array in a specific, prioritized order, with each prefix&#39;s associated next hop information stored in a corresponding (linked) location in another memory. During prefix lookup, a key is placed on the comparand (compare operand) bus of the CAM array and compared against all prefixes in the memory. The array of match results from all comparisons is sent through a priority logic unit to determine the highest priority match, with the winning match used to address the next hop memory from which the corresponding next hop information is read and returned. 
     More recently, software based network search engines employing a general-purpose processor and a normal memory have been developed. Within such devices, the processor performs prefix searches with a series of memory read and comparison operations. The routing table prefixes and next hop information are typically stored in the memory in data structures built according to one of various software algorithms developed to reduce memory usage in storing the routing table and the number of memory accesses during lookup. For these purposes, a multi-bit trie and the corresponding algorithm are among the data structures and algorithms that achieve the best data compression with a bounded number of memory accesses for search operations. 
     A trie (from the middle four letters of “retrieve”) is a tree-based data structure built to represent binary strings, where each bit or group of bits in the string determines the direction taken among branches within the tree. A binary (unibit or single bit) trie proceeds bit-by-bit and has at most two branches from each node, while a multi-bit consumes multiple bits at a time and has several branches at each node, each branch leading to the next level. The number of bits consumed or examined during branch selection at each node is referred to as a stride. A uniform width stride trie is a trie with all strides having the same width, except possibly the last stride, which may be the remainder of the prefix length after being divided by the stride width. 
     Generally, the multi-bit trie algorithm works by storing and retrieving prefixes in a uniform stride width trie, grouping all branches in the same level with the same parent (next higher stride level) stride value into a table, referred to as a trie table. At each level, the corresponding stride value provides an index into a trie table entry containing the information needed to get to the next level. A multi-bit trie has the advantage that prefixes with common high order bits (strides) will share the same parent trie tables, reducing the memory required to store the prefixes. 
     If a prefix of length l is divided into m strides each of n bits, the maximum possible number of entries within the next level trie table is 2 n . The algorithm encodes all next level stride values from the same parent into a 2 n  bit data field stored in the entry within the parent trie table, along with a pointer containing the base address of the next level (child) trie table, in a data structure referred to as a trie node. Table compression is achieved by allocating memory for the actual number of table entries that exist, instead of the maximum size 2 n . For the last stride of each prefix, a similar type of data structure, referred to as an end node, is used, except in this case the pointer points to a table containing next hop information instead of a next level trie table. 
     Routing table lookup is also performed in same width strides, with the value of the next level stride from the input search key (typically an IP address of 32 or 64 bits) decoded and processed together with the associated data field in the stride value&#39;s parent table entry. If a stored route with the same prefix stride value is determined to exist within the trie, an index is calculated using the information in the parent table, then the search continues using the table pointer and the calculated index to form an address leading to the next level trie table entry. If a match is not found, the search terminates without success. If a search reaches an end node and a match is found, the search is successful and the associated next hop information is read from the next hop table. 
     Successfully completing a lookup in a multi-bit trie requires multiple levels of the trie to be traversed. At each level, the tree node is accessed from memory and used, along with the stride value, to determine the address of the trie node at the next level. 
     However, latencies within hardware multi-bit trie network search engines can contribute to the overall latencies within the network router, and therefore within the network itself. 
     There is, therefore, a need in the art for an improved multi-bit trie network search engine. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a multi-bit trie network search engine for a network router, a series of hardware pipeline units each processing a stride during prefix search operations on a multi-bit trie and including, within at least one pipeline unit other than the last pipeline unit, a mechanism for retiring search results from the respective pipeline unit rather than passing the search results through the remaining pipeline units. Early retirement may be triggered by either the absence of subsequent strides to be processed or completion (a miss or end node match) of the search at the current stride, together with an absence of active search operations in subsequent pipeline units for systems not supporting out-of-order completion of pipelined search operations. The early retirement mechanism may be included in those pipeline units corresponding to a last stride for a maximum prefix length shorter than the pipeline (e.g., 20 or 32 bit prefixes rather than 64 bit prefixes), in pipeline units selected on some other basis, or in every pipeline unit. Worst-case and/or average latency for prefix search operations is reduced. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
         FIG. 1  depicts a processing system utilizing a configurable multi-stage pipeline multi-bit trie network search engine according to one embodiment of the present invention; 
         FIG. 2  is a diagram of a microarchitecture for a configurable multi-stage pipelined multi-bit trie network search engine according to one embodiment of the present invention; 
         FIG. 3  illustrates hardware pipelining of multi-bit trie searches within a configurable multi-stage pipelined multi-bit trie network search engine according to one embodiment of the present invention; and 
         FIGS. 4A-4B  depict in greater detail the structure of a pipeline unit for a process level within a configurable multi-stage pipelined multi-bit trie network search engine according to various embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS.  1  through  4 A- 4 B, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device. 
       FIG. 1  depicts a processing system utilizing a configurable multi-stage pipeline multi-bit trie network search engine according to one embodiment of the present invention. Processing system  100  implements a portion of an Internet Protocol (IP) network router and includes a system controller or network processing unit (NPU)  101  coupled to a network packet search engine (NPSE)  102 , which in turn is coupled to external memory  103 . NPSE  102  receives addresses and/or commands from system controller  101  as well as prefixes for the next hop address, and returns the next hop address to system controller  101 . NPSE  102  transmits data table memory addresses to external memory, together with a data pointer to the next hop address within a data table, determined from prefix tables  104 , to external memory  103 . NPSE  102  receives from external memory  103  the next hop address from the table and entry within data tables  105  identified by the address and data pointer. System controller  101  is coupled to a backplane  106  and through a network interface  107  to a network. 
     NPSE  102  is preferably coupled to system controller  101  by a high-speed 32-bit network processor unit (NPU) interface and to external memory  103  by a high-speed 32-bit static random access memory (SRAM) interface. Each interface is implemented by a quad data rate (QDR) architecture, with the system controller  101  as the master for the NPU interface and the external memory  103  as the slave for the SRAM interface, providing 32 bit simultaneous data input/output (I/O) at two words per clock with a maximum clock frequency of 250 mega-Hertz (MHz) for peak aggregate bandwidth of 4.5 gigabytes/second (GB/s). Those skilled in the art will recognize that other configurations of the NPU and SRAM interfaces are possible. 
       FIG. 2  is a diagram of a microarchitecture for a configurable multi-stage pipelined multi-bit trie network search engine according to one embodiment of the present invention. NPSE  102  includes: an NPU interface  200  coupling the NPSE  102  to the system controller  101 ; an arbiter  201 ; a central processor unit (CPU)  202  with associated memory (SRAM)  203  containing the programs executed by CPU  202 ; an SRAM controller  204  coupling the NPSE  102  to the external memory  103 ; and an array (sixteen in the exemplary embodiment) of pipeline logic units  205   a - 205   n  and a corresponding set (also sixteen in the exemplary embodiment) of configurable memory blocks forming a series of virtual memory banks  206   a - 206   n , with pipeline logic units  205   a - 205   n  and memory banks  206   a - 206   n  coupled by a crossbar  207  enabling the virtual bank configurations. The major tasks performed by NPSE  102  are route insert, delete and search, which expect NPSE  102  to maintain the routing table by inserting routes into and deleting routes from the internal and external memory, and to perform a route search on the prefixes presented and return the associated next hop information. 
       FIG. 3  illustrates hardware pipelining of multi-bit trie searches within a configurable multi-stage pipelined multi-bit trie network search engine according to one embodiment of the present invention. In a hardware implementation of a multi-bit trie search algorithm, the search through multiple trie levels may be pipelined as shown, with each process level operating on a different stride for a given search key and processing of a new search key initiated each cycle. At any given time, each level of the pipeline is working on a different search key, while processing the stride bits (for that search key) at that level. In one implementation, a miss or an end node match within any of pipeline units other than the last (pipeline unit  205   n ) produces a result that is passed without further processing through the remainder of the pipeline units. 
     Most multi-bit trie algorithms are implemented as software programs running on a general-purpose programmable processor. To the extent hardware implementations of multi-bit trie algorithms have been proposed, latencies associated with the search through such a hardware trie are not discussed or addressed. 
     An importation component of hardware implementation of a multi-bit trie algorithm is the amount of on-chip storage required to store the trie tables. On the one hand, the memory must be large enough to store large routing tables with several hundred thousand prefixes in core routers. At the same time, due to the high throughput lookup requirement of several hundred million searches per second, the access to the memory must be pipelined, which means the memory must be configurable and flexible for simultaneous accessibility in each stage of the pipeline or level of the trie. 
     One consequence of having a configurable memory structure on-chip is that processing a stride at each trie level takes longer. In the example NPSE  102 , there are sixteen pipeline units  205   a - 205   n  each processing a 4 bit stride. A 64 bit address prefix for Internet Protocol version 6 (IPv6) packets (typically only 64 bits of the 128 bit prefixes for IPv6 packets are relevant in a search) can be examined at wire speed, but the work done at each level is greater: the configurable on-chip SRAM memory bank  206   a - 206   n  needs to be accessed, the trie node entry extracted, and the next trie level address computed. Up to four cycles may be required for these processes in each pipeline unit  205   a - 205   n , resulting in a 64 cycle latency. 
     Since the same NPSE  102  can accommodate multiple routing tables each with different maximum prefix lengths, e.g., 32 bit prefixes for Internet Protocol version 4 (IPv4) packets or 20 bit prefixes for virtual private network (VPN) packets, the latency of pipeline units  205   a - 205   n  imposes an unnecessary restriction on the search completion time for prefixes that are less than 64 bits in length. In addition, the pipelined implementation depicted suffers from the drawback that the entire 64 cycle latency is wasted even when a prefix lookup find the longest prefix match in the first one or two levels, so that average-case lookup latency also suffers. 
       FIGS. 4A and 4B  depict in greater detail the structure of a pipeline unit for a process level within a configurable multi-stage pipelined multi-bit trie network search engine according to various embodiments of the present invention.  FIG. 4A  illustrates the details of a pipeline unit for one embodiment addressing worst-case latency for a given table. Each table that is built and traversed in NPSE  102  is for a specific networking application—that is, IPv4/IPv6, VPN, etc., which means that generally the maximum prefix length l in the table is known. 
     The maximum prefix length determines the maximum number of trie levels L max  for a search with NPSE  102  is L max =ceiling(l%4), which in turn determines the worst case latency for searches in that table. Thus, an IPv4 table will have a maximum prefix length of 32 bits and a maximum number of search levels (for 4 bit strides) of eight. In NPSE  102 , where each stride level search within a pipeline unit  205   a - 205   n  requires four cycles, the worst-case latency for a 32 bit IPv4 search key should be no more than 32 cycles. However, if results of the eighth pipeline unit must be passed through the remaining eight pipeline units as described above, the full 64 cycles will be required. 
       FIG. 4A  illustrates a structure that, assuming all lookups are handled in-order, reduces the worst-case latency from 64 cycles to 32 cycles. Each pipeline unit  205   i  within the series of pipeline units  205   a - 205   n  includes processing logic  401  for the corresponding process level i receiving an input, if any, from the previous process level i−1 through a buffer  402 . As a lookup operation proceeds down the pipeline, the number of input bits within the input search key remaining to be processed as strides are consumed at each level is tracked. Within at least one pipeline unit (the eighth pipeline unit in the example of IPv4/IPv6 lookups), a signal  403  is generated indicating when there are no strides remaining. This signal  403  is employed by logic  404  to generate an input to a control unit  405  (e.g., a demultiplexer) determining whether the search result continues through the pipeline (i.e., is passed to the buffer for process level i+1) or is retired. 
     With the structure of  FIG. 4A , when a lookup operation reaches a level where there are no more bits to be processed in the input search key, the operation&#39;s results can retire provided there are no active searches further down in the pipeline. For this reason, logic  404  receives active search signals from each subsequent pipeline unit within the series  205   a - 205   n —that is, pipeline units that are physically “ahead” or further along in the pipeline chain, but processing, at a moment of interest, searches earlier in time (issued before the search being processed in the current pipeline unit was issued). However, if NPSE  102  only processes prefixes of a single length that is less than a maximum supported length (i.e., while adapted to handle either IPv4 or IPv6 prefixes, in operation NPSE  102  is employed to handle only IPv4 prefixes), such active search signals are not required. 
     Signal  403 , logic  404  and control unit  405  ensure that the worst-case latency for tables with a 32 bit search key will be 32 cycles instead of 64 cycles, provided there are no active, outstanding lookups in process levels greater than the eighth. On early retirement, a successful lookup is forwarded to the external SRAM  103  with the next hop pointer information, while a failed lookup returns to the NPU  101 . 
     In addition, multiple instances of the structure in  FIG. 4A  may be implemented within pipeline units  205   a - 205   n . For instance, in addition to being implemented at the eighth pipeline unit to handle IPv4/IPv6 lookups, the structure of  FIG. 4A  may also be implemented at the fifth pipeline unit to allow early retirement of lookups on 20 bit VPN prefixes in 20 cycles rather than 32 or 64 cycles, again provided that there are no active, outstanding lookups in process levels greater than the fifth if NPSE  102  handles lookups for different length prefixes. 
       FIG. 4B  addresses average-case latency. Even with the structure of  FIG. 4A , a 32 bit prefix lookup that completed, either successfully or unsuccessfully, at a process level earlier than the eighth would still retire only after the eighth process level (since strides remain to be processed). However, assuming that all lookups are processed in order, the structure of  FIG. 4B  enables searches to retire from the pipeline earlier. Rather than a signal dependent on whether there are any further strides remaining to be processed, the processing logic  401  generates a signal  406  indicating whether the search is complete (i.e., either a miss or a match to an end node has been determined). 
     With the system of  FIG. 4B , when a search operation completes successfully or unsuccessfully, a search complete signal  406  is asserted. When signal  406  is asserted and there are no active searches further down in the pipeline, then this particular search can retire from the pipeline regardless of whether there are strides remaining in the prefix to be processed. The search results no longer need be staged through the rest of the pipeline. 
     The search completion detection system of  FIG. 4B  may be implemented within every pipeline unit  205   a - 205   n  (i.e., at every process level) to allow retirement of each search at the earliest possible point. However, interconnect routing and timing constraints, as well as other practical reasons, may justify using the structure of  FIG. 4B  only at certain levels or prefix lengths that can benefit the average-case latency. The mechanisms of  FIGS. 4A and 4B  are not required in the last pipeline unit  205   n , although an active search signal from that pipeline unit  205   n  may be required for preceding pipeline units for process levels i through n−1. 
     In addition, if an extra level of complexity is added to the pipeline implementation of  FIG. 2  and searches are allowed to complete out-of-order in the pipeline with a transaction identifier for every search request, searches may be retired as soon as complete, even when later process levels within the pipeline are still processing lookup requests. Depending on whether the NPU  101  can accept out-of-order search results and the nature of the interface between the NPU  101  and the NPSE  102 , a reorder buffer may be used to return search responses in-order, reducing average latency at the cost of increased design complexity. 
     The present invention reduces the latency of lookup requests in a pipelined multi-bit trie network search engine. NPUs that cannot generate a sufficient number of threads to take full advantage of the long latency in the search engine pipeline often find their input queues on the router line card backed up while waiting for the current threads to complete the associated lookup request. With a reduced worst-case or average-case latency, lookups will complete faster and reduce or avoid such congestion. 
     Although the present invention has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, enhancements, nuances, gradations, lesser forms, alterations, revisions, improvements and knock-offs of the invention disclosed herein may be made without departing from the spirit and scope of the invention in its broadest form.