/*
* Copyright 2010 NCHOVY
*
* 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 org.krakenapps.pcap.decoder.ip;
import java.nio.ByteBuffer;
import org.krakenapps.pcap.util.Checksum;
/**
* Calculates IP checksum
*
* @author mindori
*/
public class IpChecksum {
private IpChecksum() {
}
public static int sum(Ipv4Packet p) {
ByteBuffer buf = build(p);
return sum(buf);
}
public static int sum(ByteBuffer buf) {
short[] words = new short[buf.limit() / 2];
for (int i = 0; i < words.length; i++)
words[i] = buf.getShort();
return Checksum.sum(words);
}
private static ByteBuffer build(Ipv4Packet p) {
ByteBuffer bb = ByteBuffer.allocate(100);
bb.put(getVersionAndIhl(p));
bb.put((byte) p.getTos());
bb.putShort((short) p.getTotalLength());
bb.putShort((short) p.getId());
bb.putShort(getFlagAndFragmentOffset(p));
bb.put((byte) p.getTtl());
bb.put((byte) p.getProtocol());
bb.putShort((short) 0); // zero header checksum
bb.putInt(p.getSource());
bb.putInt(p.getDestination());
if (p.getOptions() != null) {
bb.put(p.getOptions());
bb.put(p.getPadding());
}
bb.flip();
return bb;
}
private static byte getVersionAndIhl(Ipv4Packet p) {
return (byte) ((0x40) | ((p.getIhl() >> 2) & 0xf));
}
private static short getFlagAndFragmentOffset(Ipv4Packet p) {
byte flag = (byte) p.getFlags();
short offset = (short) p.getFragmentOffset();
return (short) (((flag << 13)) | (offset & 0x1fff));
}
}