Building TCP IP packets A look at the computation steps which need to be performed for utilizing the TCP IP protocol NIC 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 to How 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 kernel TCP IP over Ethernet Here is a diagram from Intel s Software Developer s Manual of the widely deployed TCP IP Ethernet packet format UnixWare online tutorial recommended background reading The TCP IP Protocol Stack 2002 Caldera International Inc All rights reserved UnixWare 7 Release 7 1 3 30 October 2002 http uw713doc sco com en NET tcpip tcpN tcpip stack html TCP IP Protocol Stack by Caldera Incorporated Encapsulation and Decapsulation of Data within a network stack by Caldera Incorporated Network stack support for TCP IP by Caldera Incorporated Internet Protocol Header IP Header Little Endian layout Transport Control Protocol Header TCP Header Little Endian layout TCP Pseudo Header layout Our 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 Header Two checksum calculations The TCP Checksum gets inserted here The TCP Segment TCP Pseudo Header TCP Header packet data The TCP Checksum is computed over this array of words The IP Header Checksum gets inserted here Frame Header IP Header The IP Header Checksum is computed over this array of words TCP Header packet data Sample checksum algorithm to compute and insert the TCP Checksum unsigned 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 padding for int i 0 i nwords i cksum htons wp i cksum cksum 16 end around carries cksum 0xFFFF mask for 16 bits cksum 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 error In 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 Header
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