What is a packet checksum Here we investigate the NIC s capabilities for computing and detecting errors using checksums Frame is extended to occupy a slot time of 512 bytes Carrier Extension Frame Check Sequence Data Type Lenth Source Address Destination Address Start Of Frame Delimiter Preamble Gigabit Ethernet frame format Some lowest level details The frame s Preamble consists of 7 bytes of an alternating bit pattern of 1 s and 0 s The Start of Frame Delimiter is a one byte bit pattern which continues the alternation of 1 s and 0 s until the final bit is reached which breaks the pattern of alternation 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011 The 64 bit Preamble and SFD In hexadecimal notation 0xAA 0xAA 0xAA 0xAA 0xAA 0xAA 0xAA 0xAB The Start of Frame Delimiter The FCS field The Frame Check Sequence is a four byte integer usually computed by the hardware according to a sophisticated mathematical error detection scheme known as CyclicRedundancy Check CRC The CRC is calculated by performing a modulo 2 division of the data by a generator polynomial and recording the remainder after division CRC 32 x32 x26 x23 x22 x16 x12 x11 x10 x8 x7 x5 x4 x2 x 1 The CRCERRS register Our Intel Pro1000 ethernet controller has a statistical counter register the first one at offset 0x4000 in the memory mapped I Ospace which counts any received frames which arrived with a CRC error indicated Our nic2 c was programmed to strip the 4 byte FCS field from all received frames by setting the SECRC bit in register RCTL Another simpler checksum The Pro1000 s legacy format Receive Descriptor 16 bytes Base address 64 bits The device driver initializes this base address field with the physical address of a packet buffer Packetlength Packetstatus errors checksum VLAN tag The network controller will write back the values for these fields when it has transferred a received packet s data into the packet buffer What is this packet checksum According to the Intel documentation the packet checksum is the unadjusted 16 bit ones complement of the packet page 24 Now what exactly does that mean And why did Intel s hardware designers believe that device driver software would need this value to be available for every ethernet packet that the NIC receives The idea of complements Whenever a whole object gets divided into two parts those pieces often referred to as complements Example complementary angles in a right triangle B ABC C A BAC 90 Set Theory Venn Diagram A B Within the universe represented here by the box the orange set B is the complement of the green set A Arithmetic Among the digits in our base ten system the values 1 and 9 are complements the values 2 and 8 are complements the values 3 and 7 are complements the values 4 and 6 are complements Complements are useful when performing subtractions in modular arithmetic 4 6 mod 10 4 4 mod 10 Two s complement Digital computers use modular arithmetic in the base two number system Two 8 bit numbers are complements if their sum equals 28 256 00000001 is the twos complement of 11111111 00000010 is the twos complement of 11111110 00000011 is the twos complement of 11111101 As a consequence subtractions can be done with the same circuits as additions One s complement But for some purposes there is a different kind of complement that results in better arithmetical properties it s known as the diminished radix complement For the case of radix ten it s called nine s complement and for the case of radix two it s called one s complement Nine s complements A pair of 3 digit numbers x and y in the radix ten number system would be called nine s complements if x y 103 1 999 Thus 321 and 678 are nines complements A pair of 3 digit numbers x and y in the radix two number system would be called one s complements if x y 23 1 111 Thus 110 and 001 are ones complements end around carry When you want to add two binary numbers using the one s complement system you can do it by first performing ordinary binary addition and then adding in the carry bit 10101010 8 bit number 11110000 1 10011010 1 10011011 8 bit number 9 bits in the normal total apply end around carry 8 bits in the ones complement total NIC uses one s complement For network programming nowadays it is common practice for one s complement to be used when computing checksums It is also common practice for multi byte integers to be sent out over the wire in so called big endian byte order i e the most significant byte goes out ahead of the bytes having lesser significance Intel s cpu uses little endian Whenever our x86 processor stores a multi byte value into a series of cells in memory it puts the least significant byte first i e at the earliest memory address followed by the remaining bytes having greater significance AX 0x3456 mov ax buf buf 0x56 0x34 Checksum using C unsigned char unsigned int unsigned int unsigned short unsigned int cp phys to virt rxring rxhead base address nbytes rxring rxhead packet length nwords nbytes 2 wp unsigned short cp i checksum 0 if nbytes 1 cp nbytes 0 nwords for i 0 i nwords i checksum wp i checksum checksum 16 checksum htons checksum checksum checksum checksum 0xFFFF pad odd length packet two s complement sum do end around carries adjustment 1 swap the bytes adjustment 2 and flip the bits mask to lowest 16 bits Let s compare our checksum calculation with the one done by the PRO1000 printk cpu computed checksum 04X checksum printk nic s rx packet checksum 04X rxring rxhead packet chksum printk n In class demonstration 1 TIMEOUT We will insert into our nic2 c device driver s read function our C code that computes and displays the unadjusted 16 bit ones complement sum for each received packet and compare our calculation with the NIC s packet checksum Checksum offloading Our Intel 82573L network controller has the capability of performing several useful checksum calculations on normal network packets if this desired by a device driver Receive CheckSum control register 31 RXCSUM 0x5000 9 8 reserved 0 T U O F L D I P O F L D 7 0 PCSS Legend PCSS Packet Checksum Start default 0x00 IPOFLD IP checksum Off load Enable 1 yes 0 no TUOFLD TCP UDP checksum Off load Enable 1 yes 0 no Using nicecho c To compensate for the modifications made to the DA and SA fields by our echo c we can omit the first six words 12 bytes from the checksum calculations done both by our read code and by the nic hardware we start our addition loop at i 6 instead of i 0 for i 6 i nwords i checksum wp i