text: [ 0037 ] fig1 is a block diagram illustrating relevant components of a networked system 110 employing one embodiment of the present invention . fig1 shows a pair of computer systems ( nodes ) 112 and 114 executing the tcp / ip protocol suite . in fig1 , nodes 112 and 114 are coupled to each other via a router or switch 116 and ethernet communication links 120 and 122 . in one embodiment , the present invention may take form in software executing on one or more processors within router or switch 116 . in another embodiment , the present invention may take form in an application specific integrated circuit ( asic ) in router or switch 116 . the present invention should not be limited to use within router or switch 116 . the present invention could find use within , for example , node 114 . for purposes of explanation , the present invention will be explained as being used within router 116 , it being understood that the present invention should not be limited thereto . nodes 112 and 114 are shown having four communication layers 130 - 136 and 140 - 146 , respectively . layers 130 - 136 and 140 - 146 take form in software instructions executing on one or more processors in nodes 112 and 114 , respectively . layers 132 - 136 and 142 - 146 are implemented in operating systems of nodes 12 and 14 , respectively . layers 130 and 140 and nodes 112 and 114 , respectively , take form in any one of many user applications including ftp , smtp , telnet , etc . for purposes of explanation , layers 132 and 142 will take form in either tcp or udp transport layers , layers 134 and 144 will take form in ip network layers , while layers 136 and 146 take form in ethernet link layers for interfacing with ethernet communication links 120 and 122 , respectively . in fig1 , node 112 is presented as a client to server node 114 . server application 40 provides some type of service ( e . g ., smtp ) to client application 130 in response to a request from client application 130 . client application 130 communicates with server application 140 by sending data down through layers 132 - 136 until the data , along with appended headers and / or trailers , is sent as a stream of bits to node 114 via router 116 and communication links 120 and 122 . the data received by node 114 is sent up through layers 146 - 142 until the data , less headers and / or trailers reaches server application 140 . the type of headers and / or trailers generally added to data generated by layers 130 - 136 are described in the background section above with reference to fig4 - 7 . router 116 shown in fig1 includes a filter 142 that checks frames it receives . if a frame received by router 116 passes the checks performed by filter 142 , the frame is passed to , for example , node 114 . if a frame received by router 116 does not pass one or more of the checks performed by filter 142 , the frame is dropped so that the frame does not reach its destination ( e . g ., node 114 ). filter 142 can perform many checks on frames received by router 116 . in one embodiment , filter 142 performs any one or more of the checks of the algorithm listed below : if ( protocol = tcp ) if ( fragment offset = 0 ) check length 1 ≧ ( p0 × 8 ) check ( 1 ) check length 2 ≧ ( p0 × 8 ) check ( 2 ) check tcp header length ≧ 5 check ( 3 ) else check fragment offset ≧ p0 check ( 4 ) if ( protocol = udp ) if ( fragment offset = 0 ) check length 1 ≧ ( p1 × 8 ) check ( 5 ) check length 2 ≧ ( p1 × 8 ) check ( 6 ) else check fragment offset ≧ p1 check ( 7 ) else if ( fragment offset = 0 ) check length 1 ≧ ( p2 × 8 ) check ( 8 ) check length 2 ≧ ( p2 × 8 ) check ( 9 ) else check fragment offset ≧ p2 , check ( 10 ) where length 1 and length 2 are calculated according to the following equations : length 1 ( in bytes ) = ( total length value in ip header of the received frame ) − (( ip header length value in ip header of the received frame ) × 4 ). length 2 ( in bytes ) = ( total number of counted bytes in the received frame ) − ( total number of data bytes in ethernet header and trailer of the received frame ) − (( ip header length value in ip header of the received frame ) × 4 ) p0 , p1 , and p2 in the above algorithm are programmable values stored within memory . p0 , p1 , and p2 may be equal to each other , or different from each other . it is noted that p0 , p1 , and p2 are multiplied by 8 in the above algorithms . in an alternative embodiment , p0 , p1 , and p2 may be multiplied by values other than 8 . length 2 is calculated as a function of the total number of counted bytes in the received frame to be checked . the total number of counted bytes of the received frame can be generated in one of many different ways . in one embodiment , a counting variable n in memory is initially to 0 . thereafter , n is incremented by one for each byte in the received frame until all bytes in the received frame are counted . the bytes of the frame can be counted as the bytes enter the router 116 , or the bytes can be counted after the received frame has been temporarily stored in memory of router 116 . in an alternative embodiment , length 2 can be calculated as follows : length 2 ( in bytes )=( total number of counted bytes in the datagram of the received frame )−(( ip header length value in ip header in the received frame )× 4 ) it is noted that in this alternative embodiment of calculating length 2 , only the bytes of the datagram of the received frame need be counted . the total number of counted bytes of the datagram can be generated in one of many different ways . in one embodiment , a counting variable m in memory is initially to 0 . thereafter , m is incremented by one for each byte of the datagram in the received frame until all bytes of the datagram are counted . the bytes of the datagram can be counted as the bytes of the datagram enter the router 116 , or the bytes of the datagram can be counted after the datagram have been temporarily stored in memory of router 116 . in operation , filter 142 will drop any received frame if any one or more of the various checks ( 1 )-( 10 ) are not passed . it is noted that filter 142 need not perform all checks ( 1 )-( 10 ) listed above for each frame received by router 116 . for example , filter 142 at one point in time , may perform only check ( 2 ) or only check ( 3 ), or filter 142 may perform only checks ( 2 ), ( 3 ) and ( 4 ) on frames received by router 116 . at another point in time , filter 142 may perform all checks ( 1 )-( 10 ) on frames received by router 116 . for purposes of explanation , it will be presumed that filter 142 performs all checks ( 1 )-( 10 ) on all frames received by router 116 . thus , if router 116 receives a frame , regardless of whether its datagram contains a udp or tcp segment as identified in the protocol field of the ip header , if the fragment offset of the ip header is set to 0 , and if lengths 1 or 2 are calculated to be less than p2 × 8 , filter 142 will drop the frame such that it never reaches its destination ( e . g ., node 14 ) in accordance with checks ( 8 ) and ( 9 ), respectively . if router 116 receives a frame , regardless of whether its datagram contains a udp or tcp segment as identified in the protocol field of the ip header , filter 142 will drop the frame if the fragment offset defined in the ip header is not equal to 0 but is less than p2 in accordance with check ( 10 ). if router 116 receives a frame having a udp segment in its datagram ( as identified in the protocol field of the ip header of the received frame ) and if the fragment offset set forth in the ip header of the received frame is set to 0 , then filter 142 will drop the received frame if length 1 or length 2 is less than p1 × 8 in accordance with checks ( 5 ) and ( 6 ), respectively . if router 116 receives a frame having a udp segment , filter 142 will drop the frame if the fragment offset set forth in the ip header is not set to 0 but is set to a value less than p1 in accordance with check ( 7 ). if router 116 receives a frame having a tcp segment in its datagram ( as identified in the protocol field of the ip header of the received frame ), filter 142 will drop the frame if the fragment offset value set forth in the ip header is set to 0 , and if length 1 or length 2 is less than p0 × 8 in accordance with checks ( 1 ) and ( 2 ), respectively . if router 116 receives a frame having a tcp segment in its datagram ( as identified in the protocol field of the ip header of the received frame ), filter 142 will drop the frame if the value of the tcp header length field is than 5 in accordance with check ( 3 ). if router 116 receives a frame having a tcp segment , filter 142 will drop the frame if the fragment offset set forth in the ip header is not set to 0 but is set to a value less than p0 in accordance with check ( 4 ). as described in the background section above frame 108 in fig1 is capable of passing the rfc 3128 algorithm . if router 116 receives frame 108 , filter 142 executing the algorithm set forth above , will drop frame 108 if , for example , p0 is set to 2 such that p0 × 8 is 16 bytes . when router 116 receives frame 108 , lengths 1 and 2 are calculated . for purposes of explanation the total number of bytes of frame 108 including the bytes in the ethernet header and trailer , is counted . in the illustrated example , because each line of frame 108 is 32 - bits long , a total number of 46 bytes will be counted . the total number of bytes in the ethernet header and trailer is 18 . the ip header length value in the ip header of frame 108 is 5 . accordingly , length 2 ( in bytes )= 46 − 18 −( 5 × 4 )= 8 . because 8 bytes is less than p0 × 8 = 16 bytes , frame 108 shown in fig1 will be dropped by filter 142 in accordance with check ( 2 ). as noted above , the checks ( 1 )-( 10 ) above can be performed by one or more processors within router 116 executing software instructions . alternatively , the checks ( 1 )-( 10 ) above can be performed by one or more asics within router 116 . fig1 illustrates one non - software implemented filter embodiment for checking frames in accordance with checks ( 1 )-( 10 ). more particularly , fig1 shows in block diagram form , a media access control ( mac ) circuit 144 coupled to a parser circuit 146 and asic 148 . in operation , mac circuit 144 receives an ethernet frame directly or indirectly from node 112 via ethernet communication link 120 . media access controller may store the received frame within a memory ( not shown ). mac circuit 144 counts the total number of bytes within the received frame , including the number of bytes of the ethernet frame header and trailer . this value is provided as the total number of counted bytes to asic 148 . additionally , mac circuit 144 provides the frame data to asic 148 and parser 146 . it is noted that mac circuit 144 processes the received frame into a format that can be understood by asic 148 and parser 146 . asic 148 in response to receiving the frame data and the total number of counter bytes from mac circuit 144 , performs one or more of the checks ( 1 )-( 10 ) above . if the frame received by asic 148 does not meet one or more of the checks set forth above , then the frame is dropped . although the present invention has been described in connection with several embodiments , the invention is not intended to be limited to the embodiments described herein . on the contrary , it is intended to cover such alternatives , modifications , and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims .