# Patent

## Title
Switching circuit implementing variable string matching

## Abstract

A content matching engine (CME) uses a content addressable memory (CAM) array that stores a plurality of strings in separate entries. The strings define one or more rules to be matched. The strings of each rule are linked, thereby providing a required order. The strings of each rule can be linked by per-entry counters associated with each string, or by a state machine. The strings in the CAM array are compared with a packet, which is shifted one symbol at a time (because the strings can start on any boundary). When the CAM detects a match, the CAM skips over the string that resulted in the match, thereby preventing erroneous matches. The CAM allows parallel matching to be performed for multiple rules. If the contents of a packet match all of the strings of a rule, in order, then the CME asserts a match/index signal that identifies the matched rule.

## Background
FIELD OF THE INVENTION
The present invention relates to the switching of Internet protocol (IP) packets. More specifically, the present invention relates to a content-aware switching application that examines the content of IP packets at non-deterministic locations, and switches the IP packets in response to the detected contents.
DISCUSSION OF RELATED ART
Wide scale adoption of network-based applications is projected to occur over time, with tremendous increases in the number of Internet data centers deployed worldwide, and tens of billions of dollars in hosting revenue. Most data centers are not engineered to be reliable or scalable enough to support such growth. Transferring both dynamic and static content instantaneously within data centers will demand a whole new level of intelligence, power and control. Achieving quality of service (QoS) for application delivery from the data center will require more then just bandwidth and speed. Deeper visibility into IP traffic and application protocols at the transaction level will also be necessary. To address these issues, Internet data centers will have to replace limited, enterprise-class legacy platforms with new purpose-built hardware and software.
FIG. 1 is a schematic diagram of a conventional TCP/IP packet 100. Packet 100 includes information designated layer 3 (L3), layer 4 (L4) and layer 7 (L7). Layer 3 includes a source address (SRC) 101 and a destination address (DST) 102. Layer 4 includes a source port identifier 103 and a destination port identifier 104. Layers 3 and 4 are located at deterministic locations of IP packet 100. Thus, it is relatively easy to locate and extract the information in Layers 3 and 4. As a result, most routing switches operate on the information present in Layers 3 and 4. However, as described above, it will become difficult, if not impossible, to switch packets at high speeds (OC-48, OC-192) based on Layers 3 and 4 as the number of Internet data centers increase.
IP packet 100 also includes Layer 7 information, which is located at non-deterministic locations of the packet. Layer 7 information provides deeper visibility into IP traffic and application protocols at the transaction level. Examples of Layer 7 information include the strings “.com” 105 and “.gif” 106. Because Layer 7 information is located at non-deterministic locations of packet 100, it is difficult to switch IP packet 100 based on the Layer 7 information.
Current known solutions for improving IP packet switching speed include the following. Multiple standard network processing units (NPUs) can be cascaded to obtain 1 gigabit/second (Gbps) switching speed. However, it is relatively expensive and power intensive to use multiple NPUs. Application specific integrated circuit (ASIC) solutions have also been used to obtain faster switching speeds. However, ASIC solutions require costly custom design and typically require a relatively large amount of memory.
PMC-Sierra Corp. has recently announced the ClassiPITM Network Classification Processor, which is available as part number PM2329. This processor claims to enable analysis and classification for all layers of a packet, including payload, at wire speeds from 100 Mb/s to Gigabit LANs, as well as OC-48 and higher speed WANS. However, this processor is limited to 900 Mbps for Layer 7 processing.
Problems associated with matching a sequence of characters located at non-deterministic location of a packet (e.g., Layer 7) include the following. There will typically be a long list of expressions to match, making it difficult to match all expressions at an acceptable speed. There may also be multiple rule matches per packet. Moreover, the number of active expressions may vary with the particular input, which can have a large impact on performance. In addition, it may be difficult to update rules on the fly, such that dynamic applications (e.g., virtual private networks (VPNs)) may be implemented. Moreover, certain substrings, such as “.com” may match other strings, such as “.complaint”, making comparisons difficult.
It would therefore be desirable to have a system and method that overcomes the above-described deficiencies and difficulties of the prior art.
SUMMARY
Accordingly, the present application provides a content aware switching application that examines the Layer 7 content of TCP/IP packets. The present invention uses a content matching engine (CME) to compare the sequence of characters in a packet against a list of “regular expressions” or “rules” to determine if there is a match. Based on a detected match, the packet will be switched to a server that is well suited to process the packet. For example, if the Layer 7 content of a packet identifies an image, then the packet will be switched to a server that specializes in processing images. Other rules can be used to implement functions such as load balancing, service priority, SPAM filtering, virus detection and mail forwarding.
Advantages of the CME include: the ability to offload NPU or ASIC task load; the ability to match a sequence of characters at real OC-48; the ability to store thousands of regular expressions or rules in a single device; and the ability to be updated on the fly.
In accordance with the present invention, the CME is implemented using a content addressable memory (CAM) array. Each rule is divided into a plurality of strings, which are stored in the CAM. The strings of each rule are linked, thereby providing a required order to the strings of the rule. In one embodiment, the strings of each rule are linked by per-entry counters associated with each string. In another embodiment, the strings of each rule are linked by a state machine. If the contents of a packet match all of the strings of a rule, in order, then the CME asserts a match signal along with an index value that identifies the matched rule. The basic approach requires that the CAM implement a search through a packet one character at a time, because the strings can start on any boundary. When the CAM detects a match, the CAM-skips over the string that resulted in the match, thereby preventing subsequent erroneous matches. The CAM allows parallel matching to be performed for multiple rules. As a result, the processing time does not increase as the number of rules increases.
In one embodiment, the CME includes a shift register configured to receive and shift a packet in a byte-wise manner, thereby providing a pattern that changes by one byte every clock cycle. The pattern is provided to a CAM array having a plurality of entries configured to store a corresponding plurality of strings that identify a first rule. Each of the entries is coupled to receive the pattern in parallel from the shift register, and assert a match output signal if the string stored in the CAM entry matches the pattern.
Each of the CAM entries is coupled to provide a match output signal to a corresponding entry counter. Each of the entry counters stores a count value associated with the length of the string stored in the corresponding CAM entry. When a CAM entry asserts a match signal, the corresponding entry counter starts to count down to zero. Upon reaching zero, the entry counter asserts an enable signal, which is applied to a subsequent CAM entry (or to a priority encoder, if the entry counter is the last entry counter of the rule). This manner, the various strings stored in the CAM entries are linked.
In another embodiment, the CME includes a shift register configured to receive and shift a packet in a byte-wise manner, thereby providing a pattern that changes by one byte every clock cycle. The pattern is provided to a CAM array having a plurality of entries configured to store a corresponding plurality of strings that identify a first rule. Each of the entries is coupled to receive the pattern in parallel from the shift register.
Each of the CAM entries has a corresponding state identifier block that stores a corresponding state identifier value. A state register stores a current state identifier value. A CAM entry is only enabled if the state identifier value corresponding to the CAM entry matches the current state identifier value stored in the state register. If a CAM entry is enabled, and the contents of the CAM entry matches the pattern, the CAM entry will assert a match signal.
The CME further includes a priority encoder that receives the match signal, and in response, asserts a match signal and an index signal. The match and index signals are used to access a corresponding entry in an SRAM array. This entry includes a new state identifier value and a count value. The new state identifier value is loaded into the state register, thereby becoming the current state identifier value. The count value, which corresponds to the length of the CAM entry that caused the match, is loaded into a counter. The counter is decremented to a zero count over one or more clock cycles. The CAM array is only enabled when the counter has a zero count, thereby preventing erroneous matches. In this manner, the various strings of the rule are linked in the desired order.
In one variation, each CAM entry can also include a byte location indicator, which indicates the byte location within the packet where the corresponding string must exist in order to result in a match. A byte counter is reset upon receiving each new packet, and is incremented each time the packet is shifted in the shift register, such that the count of the byte counter identifies the original byte location of the first byte of the shifted pattern. The count of the byte counter is applied to the byte location indicator in each of the CAM entries, such that actual byte location must match the byte location of the byte location indicator in order for a match to exist.
The present invention will be more fully understood in view of the following description and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of a conventional Internet protocol packet.
FIG. 2 is a block diagram of a content matching engine (CME) in accordance with one embodiment of the present invention.
FIG. 3 is a block diagram of a system that uses the CME of FIG. 2 in accordance with one embodiment of the present invention.
FIG. 4 is a block diagram of a system that uses the CME of FIG. 2 in accordance with another embodiment of the present invention.
FIG. 5 is a flow diagram illustrating a rule that includes the strings: “http://www.”, “*”, “.com/”, “*”, and “.gif” or “.jpg”, wherein “*” represents “any symbol”.
FIG. 6 is a block diagram illustrating the CME of FIG. 2 in more detail in accordance with one embodiment of the present invention.
FIG. 7 is a block diagram illustrating the CME of FIG. 6 configured to implement the rule of FIG. 5 in accordance with one embodiment of the present invention.
FIG. 8 is a block diagram illustrating the CME of FIG. 7, when the packet is shifted such that the string “http://www.” is applied to a CAM array of the CME.
FIG. 9 is a block diagram that illustrates the manner in which the string “http://www.” is shifted over eleven clock cycles, along with the count of a corresponding entry counter.
FIG. 10 is a block diagram illustrating a CME that implements a plurality of CAM arrays connected in parallel in accordance with one embodiment of the present invention.
FIG. 11 is a block diagram of a CME which implements a state machine in accordance with another embodiment of the present invention.
FIG. 12 is a flow diagram which illustrates a rule that requires the strings “www.”, “*”, “com/” and either “com/” or “sram/” to be detected, in order.
FIG. 13 is a block diagram illustrating the CME of FIG. 11 configured to implement the rule of FIG. 12 in accordance with one embodiment of the present invention.
FIG. 14 is a flow diagram of a rule that requires a destination address (Layer 3) of “164.145.*.*” beginning at byte location 16 of the packet, a destination port address (Layer 4) of “33” beginning at byte location 24, followed in order by “www.”, “*” (any symbol), “com/” and “com/” or “sram/”.
FIG. 15 is a block diagram of a CME configured to implement the rule of FIG. 14 in accordance with one embodiment of the present invention.



DETAILED DESCRIPTION
FIG. 2 is a block diagram of a content matching engine (CME) 200 in accordance with one embodiment of the present invention. CME 200 has an input port for receiving packet data at a 2.4 Gbps (OC-48) or a 10 Gbps rate (OC-192). CME 200 also has an input port for receiving a set of rules, which are stored in CME 200. CME 200 determines whether each incoming packet matches any of the rules. If CME 200 determines that a match exists, then CME 200 asserts a MATCH signal and provides an INDEX signal associated with the matched rule.
FIG. 3 is a block diagram of a system 300 that uses CME 200. In addition to CME 200, system 300 includes control processor 301, network processing unit (NPU)/ASIC 302 and framer/PHY 303. Control processor 301 loads the rules into CME 200. Control processor 301 also loads instructions into NPU/ASIC 302. These instructions define the manner in which NPU/ASIC 302 processes a received packet when the received packet matches one of the rules. Framer/PHY 303 receives and aligns data packets from a fiber-optic network, and provides the aligned data packets to CME 200 and NPU/ASIC 302. If CME 200 determines that a received packet matches one of the rules, CME 200 asserts the MATCH signal and provides an INDEX value associated with the matched rule to NPU/ASIC 302. In response to the MATCH/INDEX signals, NPU/ASIC 302 routes the packet through a switch fabric to an output port, which is designed to efficiently process the packet in view of the matched rule.
FIG. 4 is a block diagram of an alternate system 400 that uses CME 200. In addition to CME 200, system 400 includes control processor 401, network processing unit (NPU)/ASIC 402 and framer/PHY 403. System 400 is controlled in a manner similar to system 300. However, system 400 uses quad data rate (QDR) packets, whereas system 300 uses a SPI3 or SPI4.2 style packet interface.
The list of rules stored in CME 200 might be defined as set forth below in Table 1.









TABLE 1









Is the packet an image request?



Is the packet a CGI script transaction?



Does the packet include a cookie?



Is the packet an FTP transaction?



Is the packet from a paying customer?



Is the packet SPAM?










The first rule in Table 1 may be expressed as follows in accordance with one embodiment of the present invention.
http://www.*.com*(.gif/.jpg)
In this rule, the string “*” represents “any symbol”. FIG. 5 is a flow diagram illustrating this rule, which is hereinafter referred to as rule 500. Thus, in accordance with rule 500, an incoming packet is examined to determine whether the packet contains the string “http://www.” (Step 501). If the string “http://www.” is detected, then the incoming packet is further examined to determine whether the packet contains any subsequent symbol (Step 502). If a subsequent symbol is detected, the packet is further examined to determine whether subsequent symbols of the packet contain the string “.com/” (Step 503). If the string “.com/” is detected, then the incoming packet is further examined to determine whether the packet contains any subsequent symbol (Step 504). If a subsequent symbol is detected, the packet is further examined to determine whether subsequent symbols of the packet contain either the string “.gif” (Step 505) or the string “.jpg” (Step 506). If either one of the strings “.gif” or “.jpg” are detected in Steps 505 or 506, then CME 200 will assert the MATCH signal (Step 507). CME 200 will also provide a corresponding INDEX signal in response to the asserted MATCH signal. These MATCH and INDEX signals are then used by an NPU/ASIC to route the packet to a server that is suited to processing images. Note that the symbols required to create a match with rule 500 are typically found at non-deterministic locations within Layer 7 of a packet that includes an image request.
Other rules can be used to implement functions such as load balancing, service priority, SPAM filtering, virus detection and mail forwarding. Appendix A lists additional exemplary rules that may be used in accordance with the present invention. It is understood that the rules are not limited by these examples.
FIG. 6 is a block diagram of CME 200 in accordance with one embodiment of the present invention. CME 200 includes shift register 601 and content addressable memory (CAM) array 602. In the described embodiment, CAM array 602 includes a plurality of CAM entries 610, a corresponding plurality of entry counters 611, and an encoder 612. Each of CAM entries 610 is capable of storing a string of symbols and each of entry counters 611 is capable of storing a count value for the corresponding entry. The rules are loaded into one or more entries of CAM array 602. Shift register 601 is coupled to receive the incoming data packet. The contents of shift register 601 are provided to CAM array 602 as a symbol pattern (PATTERN). The PATTERN provided by shift register 601 is compared (in parallel) with the strings stored in CAM entries 610.
FIG. 7 is a block diagram illustrating the CAM array 602 loaded with strings associated with rule 500. Each of the strings is assigned a count corresponding to the number of symbols in the string. Although not illustrated in FIG. 7, many other rules can be stored in CAM array 602. All of these rules are processed in parallel in the manner described below.
In the described example, a packet 701 that includes the symbols
“ . . . http://www.idt.com/ipc/77510/blk.gif . . . ” at non-deterministic locations is loaded into shift register 601. When packet 701 is initially loaded into shift register 601, the PATTERN provided by shift register 601 does not match the contents of any of the CAM entries 610. Each clock cycle, shift register 601 is controlled to shift the contents of packet 701 to the left by one symbol.
As illustrated in FIG. 8, packet 701 is eventually shifted such that the PATTERN begins with the string “http://www.”. At this time, the first entry of CAM array 602 detects a match, and enables the counter associated with the first entry of CAM array 602 (hereinafter, the first entry counter). At this time, the first entry counter stores a count value of “11”.
FIG. 9 illustrates the PATTERN and the associated count of the first entry counter over the next eleven clock cycles. Thus, during the next clock cycle, packet 701 is shifted left by one symbol, and the first entry counter is decremented by one to a count of “10”. This shifting/decrementing continues until the count of the first entry counter reaches “0”. When the count of the first entry counter reaches “0”, the first entry counter asserts an enable signal, which is applied to the second entry of CAM array 602. The first entry counter is then reset to a count of “11”. Note that processing does not continue to the next string of the rule until the matched string of the rule has been shifted out of register 601. This prevents erroneous matches from occurring.
When the first entry counter asserts the enable signal, the second entry of CAM array 602 (i.e., “*”, or “any symbol”) will match the PATTERN (shown as the last PATTERN of FIG. 9). As a result, the counter associated with the second entry of CAM array 602 (hereinafter, the second entry counter) is enabled. During the next clock cycle, the second entry counter will be decremented to a count of “0”. Upon reaching a count of “0”, the second entry counter will assert an enable signal, which is applied to the third entry of CAM array 602.
Packet 701 continues to be shifted through register 601. After two more clock cycles, the PATTERN begins with the string “.com/”. At this time, the third entry of CAM array 602 detects a match, and enables the counter associated with the third entry of CAM array 602 (hereinafter, the third entry counter). At this time, the third entry counter has a value of “5”. Processing continues, with the string “.com/” being shifted out of register 601 and the third entry counter being decremented to a count of “0” over the next five clock cycles. Upon reaching a count of “0”, the third entry counter asserts an enable signal, which is applied to the fourth entry of CAM array 602.
When the third entry counter asserts the enable signal, the fourth entry of CAM array 602 (i.e., “any symbol”) will match the PATTERN (i.e., “ipc/7750/blk.gif”). As a result, the counter associated with the fourth entry of CAM array 602 (hereinafter, the fourth entry counter) is enabled. During the next clock cycle, the fourth entry counter will be decremented to a count of “0”. Upon reaching a count of “0”, the fourth entry counter will assert an enable signal, which is applied to both the fifth and sixth entries of CAM array 602.
Packet 701 continues to be shifted through register 601. After twelve more clock cycles, the PATTERN begins with the string “.gif”. At this time, the fifth entry of CAM array 602 detects a match, and enables the counter associated with the fifth entry of CAM array 602 (hereinafter, the fifth entry counter). At this time, the fifth entry counter has a value of “4”. Processing continues, with the string “.gif” being shifted out of register 601 and the fifth entry counter being decremented to a count of “0” over the next four clock cycles. Upon reaching a count of “0”, the fifth entry counter asserts a Match signal, which is applied to encoder 612. In response, encoder 612 asserts a MATCH signal and an INDEX signal, wherein the INDEX signal identifies the rule that resulted in the match (i.e., rule 500).
Note that if the PACKET included the string “.jpg” instead of the string “.gif”, then the sixth entry of CAM array 602 would have detected a match, and the sixth entry counter would have asserted a Match signal.
Also note that CAM array 602 is capable of storing many rules. For example, CAM array 602 can include 16K entries, each having a width of 64-bits (or eight 8-bit bytes), in one embodiment of the present invention. The PATTERN is compared with all 16K entries in parallel, thereby reducing the required comparison time. In the described embodiment, CAM array 602 operates at a frequency in the range of 200 to 300 MHz. With a symbol (byte) size of 8-bits, this enables comparisons to be performed at OC-48. Advantageously, CME 200 exhibits relatively low power consumption, because a limited number of entries are enabled at any given time. In addition, CME 200 is able to offload task load from the associated NPU/ASIC. Moreover, CME 200 can be easily updated on the fly by modifying the contents of CAM array 602.
In one variation, the PATTERN can be provided to parallel CAM arrays. FIG. 10 is a block diagram illustrating a CME 1000, which implements parallel CAM arrays 602-605. In the described embodiment, CAM arrays 603-605 are identical to CAM array 602, although this is not necessary. Shift register 601 applies the PATTERN to CAM arrays 602-605 in parallel. In response, CAM arrays 602-605 generate match signals MATCH0-MATCH3 and index signals INDEX0-INDEX3, respectively, in the manner described above. Priority encoder 606 selects the MATCH and INDEX signals generated by one of the CAM arrays 602-605, and transmits these MATCH and INDEX signals as the output signals, MATCHout and INDEXout. CME 1000 advantageously increases the number of entries/rules, without increasing the required comparison time. CME 1000 enables comparisons to be performed at OC-192.
FIG. 11 is a block diagram of a CME 1100 in accordance with another embodiment of the present invention. CME 1100 includes shift register 1101, relatively small CAM block 1102, SRAM block 1103, state register 1104 and counter 1105. In the described embodiment, CAM block 1102 includes 64 entries, with each entry having 32 symbols. The relatively small size of CAM block 1102 ensures that the CAM block exhibits a relatively fast cycle time (e.g., 3.3 nanoseconds). Each of the entries of CAM array 1102 includes a string and a corresponding state identifier. Each rule is written to CAM array 1102 as a series of strings and state identifiers.
Encoder 1112 receives the Match signals generated by the entries, and in response, provides an asserted MATCH signal and a corresponding INDEX value to SRAM 1103. SRAM 1103 includes a plurality of entries, wherein each entry includes a state identifier and a corresponding count value. Each of the entries of SRAM 1103 corresponds with an entry in CAM array 1102. In the described example, the Nth entry of CAM array 1102 corresponds with the Nth entry of SRAM 1103, although other mapping schemes can be used in other embodiments. One of the state identifiers from SRAM 1103 is loaded into state register 1104, and the corresponding count value is loaded into counter 1105. The state identifier stored by state register 1104 is provided to CAM array 1102. CAM array 1102 enables only the entries having the same state identifier as the state identifier stored in state register 1104.
The operation of CME 1100 will now be described using the rule 1200 illustrated in FIG. 12, which requires the strings “www.” (1201), “*” (1202), “com/” (1203) and either “com/” (1204) or “sram/” (1205) to be detected, in order.
FIG. 13 is a block diagram illustrating the manner in which CME 1100 is loaded to implement rule 1200. In the described example, a packet that includes the string . . . www.idt.com/sram/ . . . is applied to shift register 1101. At the beginning of the comparison, state register 1104 is loaded with the initial state identifier of “100”. This state identifier of “100” is applied to CAM array 1102, thereby enabling the first entry of CAM array 1102, which stores a state identifier equal to “100”. Also at the beginning of the comparison, the count value stored by counter 1105 is set to “0”. Counter 1105 is configured to enable SRAM 1103 only when the count value stored in counter 1105 has a value of “0”. Counter 1105 is configured to disable SPAM 1103 when the count value has a value other than “0”.
The packet is shifted into shift register 1101 until the PATTERN begins with the string “www.”. At this time, the first entry of CAM array 1102 detects a match, and asserts a corresponding Match signal. In response, encoder 1112 asserts a MATCH signal and generates an INDEX signal corresponding with the first entry of SPAM 1103. In response, the first entry of SRAM 103 is accessed, thereby loading state register 1104 with a value of “102”, and counter 1105 with a count value of “3”.
The state identifier value of “102” is routed to CAM array 1102, thereby enabling the second and third entries of CAM array 1102, which store state identifiers equal to “102”. Over the next three clock cycles, the string “www.” is shifted out of shift register 1101, and counter 1105 decrements the count value to “0”. At this time, the PATTERN begins with string “idt.com/sram/”, and the third entry of CAM array 1102 (i.e., “*”) detects a match. As a result, the third entry of CAM array 1102 asserts a corresponding Match signal, thereby causing encoder 1112 to assert a MATCH signal and generate an INDEX signal corresponding with the third entry of SRAM 1103. In response, the third entry of SRAM 103 is accessed, thereby loading state register 1104 with a value of “102”, and counter 1105 with a count value of “0”. The state identifier value of “102” continues to enable the second and third entries of CAM array 1102, while the count value of “0” enables SRAM 1103.
The packet is shifted for three more cycles, until the PATTERN begins with the string “com/sram/”. At this time, the second entry of CAM array 1102 detects a match, and asserts a corresponding Match signal. In response, encoder 1112 asserts a MATCH signal, and generates an INDEX signal corresponding with the second entry of SRAM 1103. In response, the second entry of SRAM 103 is accessed, thereby loading state register 1104 with a value of “103”, and counter 1105 with a count value of “3”. The state identifier value of “103” is provided to CAM array 1102, thereby enabling the fourth and fifth entries of CAM array 1102, which store state identifier values of “103”.
Over the next three clock cycles, the string “com/” Is shifted out of shift register 1101, and counter 1105 decrements to a count value of “0”. At this time, the PATTERN begins with string “sram/”, and the fifth entry of CAM array 1102 detects a match. As a result, the fifth entry of CAM array 1102 asserts a corresponding Match signal, thereby causing encoder 1112 to assert a MATCH signal and generate an INDEX signal corresponding with the fifth entry of SRAM 1103. In response, the fifth entry of SRAM 103 is accessed, thereby loading state register 1104 with a MATCH signal and an associated INDEX value. This MATCH signal and INDEX value are provided as an output of CME 1100.
In another embodiment of the present invention, certain rules may require that certain strings be located at predetermined locations of in the packet. FIG. 14 is an example of a rule 1400 that requires a destination address (Layer 3) of “164.145.*.*” beginning at byte location 16 of the packet, a destination port address (Layer 4) of “33” (i.e., “http://”) beginning at byte location 24, followed in order by “www.”, “*” (any symbol), “com/” and “com/” or “sram/”.
In order to implement these additional rules, CME 1100 must be modified to keep track of the current byte location of the shifted PACKET. In addition, the entries of CAM array 1102 must be modified to store the relevant predetermined byte locations.
FIG. 15 is a block diagram of a CME 1500, which is configured to process rule 1400. CME 1500 includes shift register 1501, byte location counter 1501A, CAM block 1502, SRAM block 1503, state register 1504 and counter 1505. Shift register 1501, CAM array 1502, SRAM block 1503, state register 1504 and counter 1505 operate in substantially the same manner as shift register 1101, CAM array 1102, SRAM block 1103, state register 1104 and counter 1105 of CME 1100, with the differences noted below.
Byte location counter 1501A is reset to a “0” value upon loading a PACKET into shift register 1501. Each time that the PACKET is shifted within shift register 1501, the byte location counter 1501A is incremented. Thus, the count of byte location counter 1501A identifies the byte location of the first byte of the PATTERN. CAM array 1502 is subdivided to include a third column that stores the predetermined byte locations associated with the rule. Thus, the first entry of CAM array 1502 stores a predetermined byte location of “16” and the second entry of CAM array 1502 stores a predetermined byte location of “24”. The other entries of CAM array 1502 associated with rule 1400 store “don't care” values (*).
In the described example, a packet that includes the string “ . . . 164.145.6.7 . . . 33 . . . www.idt.com/sram/ . . . ” is applied to shift register 1501, wherein the byte “164” is aligned with the 16th byte location of the packet, and the byte “33” is aligned with the 24th byte location of the packet. At the beginning of the comparison, byte location counter 1501A is loaded with “0”, state register 1504 is loaded with the initial state value of “100”, and counter 1505 is loaded with “0”.
When the PACKET has been shifted such that the PATTERN begins with “164.145.6.7 . . . ”, the COUNT of counter 1501A will be “16”. At this time, the both the COUNT of counter 1501A and the PATTERN match the first entry of CAM array 1502. As a result, the first entry of SRAM 1503 is accessed, with state indicator “101” being loaded into state register 1504, and “4” being loaded into counter 1505. The state indicator “101” enables the second entry of CAM array 1502.
When the PACKET has been shifted such that the PATTERN begins with “33 . . . ”, the COUNT of counter 1501A will be “24”. At this time, the both the COUNT of counter 1501A and the PATTERN match the second entry of CAM array 1502. As a result, the second entry of SRAM 1503 is accessed, with state indicator “102” being loaded into state register 1504, and “3” being loaded into counter 1505. The state indicator “102” enables the third entry of CAM array 1502. Processing then continues in a manner consistent with that described above in connection with FIG. 13, with the third, fourth, fifth and seventh entries of CAM array 1502 sequentially matching the contents of the PACKET, such that CME 1500 provides the MATCH and INDEX signals stored in the seventh entry of SRAM 1503 as output signals.
Although byte location counter 1501A has been described in combination with the circuitry of CME 1100, it is understood that this byte location counter can also be implemented with the circuitry of CME 200.
The present invention advantageously addresses the market trend towards intelligent network applications. The CAM core of the CME advantageously provides for position independent string matches for Layer 7 information. The CAM core is also capable of providing position dependent string matches for Layer 3 and Layer 4 information. The CME of the present invention provides for higher performance than existing NPUs, provides a more robust solution for regular expressions, and provides a lower part count than existing solutions using NPUs. Moreover, the CME of the present invention can be expanded to OC-192.
The CME of the present invention can be used to implement many different content aware applications. For example, the CME of the present invention can be used to implement web switching applications which perform load balancing of HTML servers, controlling routing to image servers and CGI transaction servers, and providing service priority by user or activity. The CME of the present invention can also be used to filter web content, or implement a security firewall. Moreover, the CME of the present invention can be used to filter and steer e-mail (e.g., SPAM filtering, virus detection and e-mail forwarding).
Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to a person skilled in the art. Thus, the invention is limited only by the following claims.

## Claims

1. A content matching engine comprising:
a shift register configured to receive and shift a packet in a byte-wise manner, thereby providing a pattern;
a content addressable memory (CAM) array having a plurality of entries configured to store a corresponding plurality of strings that identify a first rule, wherein each of the entries is coupled to receive the pattern in parallel from the shift register, and assert a match output signal if the string stored in the entry matches the pattern;
a plurality of entry counters, each being coupled to receive a match output signal from a corresponding entry of the CAM array, and each storing a count value associated with the length of the string stored in the corresponding entry of the CAM array;
connections between output terminals of a first set of the entry counters and enable terminals of a set of the entries; and
a priority encoder coupled to output terminals of a second set of the entry counters.

2. The content matching engine of claim 1, wherein each of the entry counters is configured to count down toward a zero count upon detecting an asserted match output signal.

3. The content matching engine of claim 2, wherein each of the entry counters is configured to assert an output signal in response to reaching a zero count.

4. The content matching engine of claim 1, wherein the shift register is configured to shift the packet at a frequency of one byte per clock cycle.

5. The content matching engine of claim 1, wherein the CAM array is a ternary CAM array capable of storing “don't care” values.

6. The content matching engine of claim 1, wherein the CAM array further comprises a plurality of entries configured to store a corresponding plurality of strings that identify a second rule.

7. The content matching engine of claim 1, wherein at least one of the connections connects the output terminal of one of the entry counters to the enable terminals of a plurality of entries.

8. The content matching engine of claim 1, wherein the CAM array comprises a plurality of CAM blocks connected in parallel.