Abstract:
In general, in one aspect, the disclosure describes a method of searching for an address within a collection of addresses. The method includes identifying a section storing an ordered sub-set of a collection of addresses. The identifying is based on a comparison of the address with at least one section boundary value. The method also includes searching the identified section for a match for the address.

Description:
BACKGROUND 
     Computers, and other network devices, communicate by sending packets to one another over a network. By analogy, a packet is much like a mailed letter. That is, a letter is like a packet&#39;s payload while the mailing address is like a network destination address stored in a packet&#39;s header. 
     A packet traveling across a network may make many “hops” before reaching its final destination. For example, a packet may be handled by network devices known as routers that examine a packet&#39;s destination address to determine how to forward the packet further toward its destination. Typically routers perform this task by accessing routing tables that indicate where to forward packets destined for a particular address. 
     Because they are numeric, network addresses can be somewhat cryptic. For example, a computer may have an Internet Protocol (IP) address of “11000000101000000000110010000001”. For convenience, this string of binary bits can be expressed as “192.160.12.129” where each of the four numbers is the decimal expression of eight of the binary bits (e.g., 192 decimal  represents the first eight bits, 1100000 binary , of the address). 
     Routing tables often include entries for specific network addresses. For example, a table can specify where to forward a packet destined for address “192.160.12.129”. This is known as a “direct route” entry. 
     Routing tables may also include entries for collections of network addresses. For example, a routing table entry may specify where to forward packets starting with the address prefix of “192.160”. Thus, a packet destined for “192.160.11.11” and a packet destined for “192.160.99.99” would both be handled by the same table entry since they both start “192.160”. Entries for collections of addresses are known as “indirect routes”. An indirect route is often expressed as a network address followed by a prefix mask. For example, for the route “192.160.0.0/16” the “16” identifies the number of bits in the mask. Thus, in this example, the mask is 16-bits long and, therefore, the first two digits in the decimal string (i.e., “192.160”). 
     Potentially, multiple route entries in a routing table may overlap. For example, a direct route entry of “192.160.12.129/32” and an indirect route entry of “192.160.0.0/16” both encompass the network address “192.160.12.129”. Additionally, the routing table may include other indirect routes such as “192.160.12.0/24”. Potentially, these different entries may specify different handling information for a packet. To resolve this apparent dilemma over which route entry to use, routers can implement “best matching prefix” (a.k.a. “longest prefix matching”) algorithms. A best matching prefix algorithm selects the most specific route matching a network address (e.g., the matching route entry with the longest prefix). Thus, in this example, a packet destined for “192.160.12.129” would be handled by the direct route information stored for entry “192.160.12.129/32” which has a longer mask than “192.160.0.0/16” or “192.160.12.0/24”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are diagrams of a system for looking up an address. 
         FIG. 3  is a diagram of a section of addresses. 
         FIG. 4  is a flowchart of a process to lookup an address. 
         FIG. 5  is a flowchart of a process to insert an address. 
         FIG. 6  is a diagram of a network device that looks up addresses. 
     
    
    
     DETAILED DESCRIPTION 
     The continued growth of network traffic and continued increases in network connection speeds place greater demands on network devices to perform packet processing operations quickly. These operations include network address lookups, for example, used in packet classification, filtering, and forwarding.  FIG. 1  depicts an example of a system that illustrates techniques that can, potentially, not only speed address lookup operations, but can also, potentially, conserve memory used to store lookup information. 
     As shown, the system divides a collection of ordered addresses (e.g., the network addresses in a routing table) into a set of sections  100   a - 100   c . The addresses may be ordered in descending or ascending order. For the purposes of ordering, unmasked bits of indirect routes may be set to “0”. For example, an indirect IP route entry for “192.160.12.32/24” may be stored as “192.160.12.0” since the mask identifies the first three decimal digits of the address as the prefix. 
     As shown, the system stores shadow copies  102   a - 102   c  of boundary values of the sections  100 . For example, the system may include hardware registers to store the first or last network addresses of different sections  100 . For instance, in the implementation shown, shadow register  102   a  stores the value of the first network address in section  100   a , shadow register  102   b  stores the value of the first network address in section  100   b , and so forth. As section  100  contents change, the system updates the values of the shadow registers  102 . 
     To perform a lookup, an address  104  is compared to the shadow values  102 . For example, the shadow values  102  and address  104  may be applied to a collection of comparators  106  in parallel. In the implementation shown, the comparators  106  can output an indication of whether the network address  104  is greater than or equal to a boundary value  102 . That is, comparator  106   a  can output a “1” if the network address  104  is greater than or equal to the address stored in shadow register  102   a . Otherwise, the comparator  106   a  can output a “0”. 
     The outputs of the comparators  106  can build an array of bits  108  where individual bits identify whether the network address  104  is greater than or equal to the corresponding shadow value  102  of a section  100 . For example, assuming the network address  104  was stored in section  100   b , the bit array  108  would have a value of “110 . . . ”. The transition from “1” values to “0” values within the array  108  identifies the section  100   b  that will store an entry for the address if one exists. 
     As shown, search logic  110  can access the comparator  106  output to identify a section  100  to search. The search may be performed using a wide variety of searching algorithms (e.g., a linear or depth-first search). For example, the logic  110  may perform a binary search of entries within the identified section  100   b . Briefly, a binary search identifies a mid-point (or near mid-point) of a section and compares the mid-point with the value being searched for. Since section entries are ordered, this comparison conceptually divides the section in half. Thus, if the value exceeds the mid-point, the binary search repeats using the half of the section storing the higher values. If the value is less than the mid-point the binary search repeats using the half of the section storing the lesser values. This process repeats, narrowing the number of entries being considered, until a match is identified or determined to be absent. 
       FIG. 2  illustrates sample operation of the system. In the example shown, the network address  104  is greater than shadow values  102   a  and  102   b , but not subsequent shadow values  102   c . Thus, the bit-array of “110 . . . ” identifies the second section  100   b  as the section that would store an entry for the address  104  if one existed. Based on the bit-array (e.g., a “case” statement operating on the bit-array), the search logic  110  can search the identified section  100   b  for a match. 
     The techniques illustrated above can quickly search a very large set of addresses. As an example of potential speed savings, a pure binary search of a table for 2 n  entries may use n+1 cycles to determine if a match for an address exists, assuming one cycle per comparison. For example, for a 1024-entry table, a binary search may use up to 11 cycles. The techniques described above, however, can perform some of the comparisons in parallel. Thus, if the 1024-entry table is divided into eight 128-entry sections, the techniques described above can perform a search using 8 cycles to search the 128-entry section identified by the bit-array and 1 cycle to perform the parallel comparison of address to boundary values. This potential savings in cycles can help a device maintain wire speed processing even as connection rates reach 10 Gigabits/second, 40 Gigabits/second, and higher (e.g., Ethernet, OC-192, or OC-768). Additionally, these savings may be garnered without the expense of a content-addressable memory (CAM). 
       FIG. 3  depicts a sample section  120  in greater detail. In this example, the section  120  is implemented as a table. In addition to a set  122  of network addresses sorted from most-significant-bit to least-significant-bit in an ascending fashion, the table  120  stores the network mask associated with a network address. The table  120  may also store information associated with the address. For example, for routing, the table  120  may include an Ethernet address or outgoing interface used to reach the next hop for a packet. Instead of, or in addition to such forwarding information, the table  120  may identify filtering, quality of service, and/or processing policies (not shown) that should be applied to a packet having a particular source or destination address. 
     The techniques illustrated above may be used in a variety of lookup processes. For example,  FIG. 4  illustrates a process for looking up a best prefix match for a network address. As shown, after performing a parallel comparison  132  of a received  130  network address with the boundary values, the process performs a binary search  134  of the section identified by the comparison results for a direct route match. 
     If the search  134  for a direct match fails  136 , the process may search  138  the section for an indirect route. The search  138  for an indirect route can proceed like a binary search for an exact match. However, in searching for an indirect route, the mask of each route entry encountered during the search is applied to the network address. To increase speed of this operation, the mask may also be stored in a shadow register. If the masked network address and the route entry match, the process proceeds by linearly stepping through succeeding entries after the match. After encountering an entry that does not match, the preceding entry represents the best pattern match. 
     As an example, assume table  120  in  FIG. 3  is being searched for a best prefix match for the network address “192.160.12.5”. Assuming the middle entry is entry  122   f , “206.21.139.0/24”, the mask of entry  122   f , “/24” is applied to address “192.160.12.5” and yields “192.160.12.0”. Since “192.160.12.0” does not match “206.21.139.0”, the binary search continues with the middle entry  122   c  of the entries  122   a - 122   e  preceding entry  122   f . Thus, the mask of entry  122   c , “/16”, is applied to address “192.160.12.5” to yield “190.160.0.0”. This value equals the address value of entry  122   c . Once a match has been found, succeeding entries are tested for matches until one is found that does not match. Thus, the mask of entry  122   d , “/24”, is applied to address “192.160.12.5” to yield “192.160.12.0”. This value matches the address value of entry  122   d , thus, the examination of subsequent entries continues. Finally, the mask of entry  122   e , “/32”, is applied to the network address “192.160.12.5” to yield “192.160.12.5”. This value does not match the address, “192.160.12.129”, of entry  122   e . Thus, the preceding entry  122   d  represents the best prefix match for address “192.160.12.5”. 
       FIG. 5  depicts a process for inserting addresses into the sections. The process may be used to initially load the sections and/or to add new addresses, for example, to update a routing table. As shown, the process determines  142  a section to store a new entry  140 , for example, using the lookup techniques described above (e.g., comparing the new address with the shadow copies of section boundaries). After identifying a section, the process inserts  144  the new entry in section in its proper order. Potentially, the new entry constitutes a new boundary for the section. Thus, the process may update  146  the shadow value(s) for the section. 
     Insertion of the new entry may cause an overflow  148  in the section (e.g., a count of section entries may exceed a maximum number of section entries). If there already exists  152  a section beyond the current section, the process moves  156  the overflowing entry to the following section and updates the section&#39;s shadow value. Potentially, the overflow of one section may cause a chain reaction of overflows. That is, adding a new entry to a succeeding section may cause that section to overflow, and so forth. 
     If a section beyond the current section does not yet exist, the process may allocate  154  a new section and storage (e.g., an unused register) for the new section&#39;s shadow value. The ability to incrementally add sections as needed can potentially conserve memory. For example, in a system that allocates memory from a pool, the ability to allocate sections when needed allows for scalability and can limit the memory used to store lookup information to that needed for a current network configuration. Additionally, the number of sections can be independent of the search technique(s) used to search within a section. For example, though using a binary search to identify matching entry, the number of sections need not necessarily equal 2 n  number of sections. 
     The techniques described above may be used in a wide variety of network devices. For example,  FIG. 6  illustrates a network device  160  featuring one or more physical layer (PHY) devices  162  for handling physical transmission over a medium (e.g., copper wire, optical, or wireless connection). The device  160  also includes one or more link layer devices  164  such as an Ethernet medium access control (MAC) device or a Synchronous Optical Network (SONET) framer. The device  160  may also connect to a switch fabric (not shown) that enables the device to provide access to connections offered by other sets of PHY(s)  162  and link layer  164  devices. 
     The packet processing logic  166  operates on packets arriving via the PHY  162  and link layer  164  devices. The logic  166  may perform a variety of operations such as packet classification, filtering, and/or forwarding (e.g., routing and switching). As shown, the logic  166  includes address lookup logic  168  implementing techniques described above. 
     While described as applying to Internet Protocol addresses, the techniques described above may be used for a variety of addressing and sorted search schemes, for example, at the network level or other levels within a protocol stack. For example, the technique may be used to lookup Ethernet addresses. Additionally, while the above discussed 32-bit IP addresses of IPv4, the techniques may also be used with the longer addresses of IPv6 and other present and future standards. Further, in addition to searching for network addresses, the techniques can be used in other searching applications. 
     The techniques described above may be integrated into a wide variety of network devices and network components. For example, the techniques may be implemented for inclusion in a router, switch, or Transmission Control Protocol/Internet Protocol (TCP/IP) off-load engine (“TOE”) that handles TCP/IP operations (e.g., maintaining a TCP/IP state machine, generating ACK messages, and so forth) for a host or hosts. 
     While shown as featuring a combination of hardware (e.g., shadow registers and comparators) and software (search logic), the techniques may be implemented in entirely in hardware (e.g., as an Application-specific Integrated Circuit (ASIC)), firmware, software (e.g., computer instructions disposed on a computer readable medium), or some combination thereof. For example, the shadow values may exist in a partition of memory that can be read quickly while the actual sections may exist in multiple memory partitions (or on another storage device) that may take longer to access. The use of the shadow values, thus, can, potentially, avoid slower memory accesses. 
     Other implementations are within the scope of the following claims.