| // Copyright 2016 The Netstack Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style |
| // license that can be found in the LICENSE file. |
| |
| // Package header provides the implementation of the encoding and decoding of |
| // network protocol headers. |
| package header |
| |
| import ( |
| "github.com/google/netstack/tcpip" |
| ) |
| |
| // Checksum calculates the checksum (as defined in RFC 1071) of the bytes in the |
| // given byte array. |
| func Checksum(buf []byte, initial uint16) uint16 { |
| v := uint32(initial) |
| |
| l := len(buf) |
| if l&1 != 0 { |
| l-- |
| v += uint32(buf[l]) << 8 |
| } |
| |
| for i := 0; i < l; i += 2 { |
| v += (uint32(buf[i]) << 8) + uint32(buf[i+1]) |
| } |
| |
| return ChecksumCombine(uint16(v), uint16(v>>16)) |
| } |
| |
| // ChecksumCombine combines the two uint16 to form their checksum. This is done |
| // by adding them and the carry. |
| func ChecksumCombine(a, b uint16) uint16 { |
| v := uint32(a) + uint32(b) |
| return uint16(v + v>>16) |
| } |
| |
| // PseudoHeaderChecksum calculates the pseudo-header checksum for the |
| // given destination protocol and network address, ignoring the length |
| // field. Pseudo-headers are needed by transport layers when calculating |
| // their own checksum. |
| func PseudoHeaderChecksum(protocol tcpip.TransportProtocolNumber, srcAddr tcpip.Address, dstAddr tcpip.Address) uint16 { |
| xsum := Checksum([]byte(srcAddr), 0) |
| xsum = Checksum([]byte(dstAddr), xsum) |
| return Checksum([]byte{0, uint8(protocol)}, xsum) |
| } |