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https://github.com/soypat/lneto.git
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fba97c990a
* Simplified checksum calculation and verification * Fixed tests * UDP: using NewBoundedFrame to create a Frame instance limited to the actual frame size. * Minor: renamed some functions * Commented and made more readable consecutive SetCRC calls. * Fixed icmp checksum calculation after rebase * Replaced NewBoundedFrame with explicit validation
72 lines
2.0 KiB
Go
72 lines
2.0 KiB
Go
package lneto
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import (
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"encoding/binary"
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)
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// CRC791 function as defined by RFC 791. The Checksum field for TCP+IP
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// is the 16-bit ones' complement of the ones' complement sum of
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// all 16-bit words in the header. In case of uneven number of octet the
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// last word is LSB padded with zeros.
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//
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// The zero value of CRC791 is ready to use.
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type CRC791 struct {
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sum uint32
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}
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func sum16(sum uint32) uint16 {
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sum = (sum & 0xffff) + sum>>16
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// the max value of sum at this point is 0x1fffe, so an additional round is enough
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return ^uint16(sum + sum>>16)
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}
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func sumWriteEven(sum uint32, buff []byte) uint32 {
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for i := 0; i < len(buff); i += 2 {
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sum += uint32(binary.BigEndian.Uint16(buff[i:]))
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}
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return sum
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}
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// Write adds the bytes in p to the running checksum. The buffer size must be even or the function will panic.
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func (c *CRC791) WriteEven(buff []byte) {
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c.sum = sumWriteEven(c.sum, buff)
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}
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// AddUint32 adds a 32 bit value to the running checksum interpreted as BigEndian (network order).
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func (c *CRC791) AddUint32(value uint32) {
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c.AddUint16(uint16(value >> 16))
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c.AddUint16(uint16(value))
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}
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// Add16 adds a 16 bit value to the running checksum interpreted as BigEndian (network order).
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func (c *CRC791) AddUint16(value uint16) {
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c.sum += uint32(value)
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}
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// Sum16 calculates the checksum with the data written to c thus far.
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func (c *CRC791) Sum16() uint16 {
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return sum16(c.sum)
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}
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// PayloadSum16 returns the checksum resulting by adding the bytes in p to the running checksum.
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func (c *CRC791) PayloadSum16(buff []byte) uint16 {
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odd := len(buff) & 1
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sum := sumWriteEven(c.sum, buff[:len(buff)-odd])
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if odd > 0 {
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sum += uint32(buff[len(buff)-1]) << 8
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}
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return sum16(sum)
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}
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// Reset zeros out the CRC791, resetting it to the initial state.
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func (c *CRC791) Reset() { *c = CRC791{} }
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// NonZeroChecksum ensures that the given checksum is not zero, by returning 0xffff instead.
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func NeverZeroSum(sum16 uint16) uint16 {
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// 0x0000 and 0xffff are the same number in ones' complement math
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if sum16 == 0 {
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return 0xffff
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}
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return sum16
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}
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