| // Copyright 2018 Google Inc. |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| // 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) |
| } |