Building TCP/IP packetsNIC’s ‘offloading’ capabilitiesHow much work?TCP/IP over EthernetUnixWare: online tutorialTCP/IP Protocol StackEncapsulation and Decapsulation of Data within a network stackNetwork stack support for TCP/IPInternet Protocol HeaderIP Header (Little Endian layout)Transport Control Protocol HeaderTCP Header (Little-Endian layout)TCP Pseudo Header layoutOur ‘nictcp.c’ demoTwo checksum calculationsSample checksum algorithmIn-class demonstrationIn-class exerciseBuilding TCP/IP packetsA look at the computation-steps which need to be performed for utilizing the TCP/IP protocolNIC’s ‘offloading’ capabilities•An advanced feature of Intel’s most recent PRO1000 gigabit ethernet controllers is an ability to perform many of the calculations associated with the processing of network packets which otherwise would have to be programmed by device-driver authors and then executed at runtime by a CPU that has much other vital work to attend toHow much work?•To gain a concrete idea of the work that is required to set up a conventional network packet for transmission, and then to take it apart when it’s received, we have created a character-mode Linux driver that shows us all of the essential calculation-steps, as it does not rely on the usual networking software included within Linux’s kernelTCP/IP over Ethernet Here is a diagram, from Intel’s Software Developer’s Manual, of the widely deployed TCP/IP Ethernet packet formatUnixWare: online tutorial <http://uw713doc.sco.com/en/NET_tcpip/tcpN.tcpip_stack.html>© 2002 Caldera International, Inc. All rights reserved. UnixWare 7 Release 7.1.3 - 30 October 2002 “The TCP/IP Protocol Stack” recommended background readingTCP/IP Protocol Stackby Caldera, IncorporatedEncapsulation and Decapsulation of Data within a network stackby Caldera, IncorporatedNetwork stack support for TCP/IPby Caldera, IncorporatedInternet Protocol HeaderIP Header (Little Endian layout)Transport Control Protocol HeaderTCP Header (Little-Endian layout)TCP Pseudo Header layoutOur ‘nictcp.c’ demo•This device-driver’s ‘write()’ function will show the sequence of steps needed for building a packet with the TCP/IP format:•Setup the packet’s data ‘payload’•Prepend a TCP packet-header •Then prepend a TCP ‘pseudo’ packet-header•Compute and insert the TCP checksum•Replace the pseudo-header with an IP header•Compute and insert the IP Header Checksum•Finally prepend the Ethernet Headerpacket-dataTwo checksum calculationsTCP HeaderTCP Pseudo-HeaderThe TCP SegmentThe TCP Checksum is computed over this array of wordspacket-dataTCP Header The IP Header Checksum is computed over this array of wordsIP HeaderFrame HeaderThe TCP Checksum gets inserted hereThe IP Header Checksum gets inserted hereSample checksum algorithm// to compute and insert the TCP Checksumunsigned char *cp = phys_to_virt( txring[ txtail ].base_addr );unsigned short *wp = (unsigned short *)( cp + 22 );unsigned int nbytes = 12 + 20 + len;unsigned int nwords = (nbytes / 2) + (nbytes % 2);unsigned int cksum = 0;if ( len & 1 ) cp[ 14 + 20 + 20 + len ] = 0; // paddingfor (int i = 0; i < nwords; i++) cksum += htons( wp[ i ] );cksum += (cksum >> 16); // end-around-carriescksum & 0xFFFF; // mask for 16-bitscksum ^= 0xFFFF; // flip the low 16-bits*(unsigned short*)(cp + 50) = htons( cksum );In-class demonstration•We can use our ‘pktsplit.c’ demo-module to confirm the correctness of our TCP/IP packet-format as far as the NIC hardware is concerned – it reports (in its descriptor ‘status’ and ‘errors’ fields) that both of our checksums were computed by the NIC on reception -- and that neither was in errorIn-class exercise•Can you adapt these ideas in our ‘nictcp.c’ driver to produce the analogous ‘nicudp.c’ demonstration module?•A similar ‘pseudo’ UDP Header is used to calculate the UDP Checksum value (see the Intel Software Developer’s Manual for the layout and size of the UDP
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