/// <summary>
/// Resolves the specified IPv4 destination address to a physical
/// address and hands the specified IPv4 packet down to the link
/// layer.
/// </summary>
/// <param name="ifc">The interface through which to output the
/// data.</param>
/// <param name="destination">The logical address of the destination
/// host, which can different from the final destination address
/// contained in the IP packet.</param>
/// <param name="packet">The IPv4 packet to transmit.</param>
/// <exception cref="ArgumentNullException">Thrown if any of the arguments
/// is null.</exception>
void Output(Interface ifc, IpAddress destination, IpPacket packet)
{
ifc.ThrowIfNull("ifc");
destination.ThrowIfNull("destination");
packet.ThrowIfNull("packet");
// Translate IP address into MAC-Address.
var macDestination = arp.Lookup(ifc, destination);
// IP address is not in our ARP table.
if (macDestination == null)
{
// Put packet on hold until the MAC-48 destination address has
// been figured out.
WaitingPacketsOf(ifc).Add(
new Tuple <IpAddress, IpPacket>(destination, packet));
WriteLine(ifc.FullName + " is putting IP packet on-hold due to pending ARP request.");
// Schedule a new ARP request.
arp.Resolve(ifc, destination);
}
else
{
WriteLine(ifc.FullName + " is queueing IP packet for " + destination);
Output(ifc, macDestination, packet);
}
}
/// <summary>
/// Computes the 16-bit checksum of the specified IPv4 packet.
/// </summary>
/// <param name="packet">The packet to compute the checksum for.</param>
/// <param name="withChecksumField">true to include the packet's
/// checksum field in the calculation; otherwise false.</param>
/// <returns>The checksum of the specified IPv4 packet.</returns>
/// <exception cref="ArgumentNullException">Thrown if the packet
/// argument is null.</exception>
public static ushort ComputeChecksum(IpPacket packet,
bool withChecksumField = false)
{
packet.ThrowIfNull("packet");
// The version and IHL fields are 4 bit wide each.
var vi = (byte)(((packet.Ihl & 0x0F) << 4) |
(((int)packet.Version) & 0x0F));
// The flags field is 3 bits and the fragment offset 13 bits wide.
var ffo = (ushort)(((packet.FragmentOffset & 0x1FFF) << 3) |
((int)packet.Flags & 0x07));
var bytes = new ByteBuilder()
.Append(vi)
.Append(packet.Dscp)
.Append(packet.TotalLength)
.Append(packet.Identification)
.Append(ffo)
.Append(packet.TimeToLive)
.Append((byte)packet.Protocol)
.Append(withChecksumField ? packet.Checksum : (ushort)0)
.Append(packet.Source.Bytes)
.Append(packet.Destination.Bytes)
.ToArray();
var sum = 0;
// Treat the header bytes as a sequence of unsigned 16-bit values and
// sum them up.
for (var n = 0; n < bytes.Length; n += 2)
{
sum += BitConverter.ToUInt16(bytes, n);
}
// Use carries to compute the 1's complement sum.
sum = (sum >> 16) + (sum & 0xFFFF);
// Return the inverted 16-bit result.
return((ushort)(~sum));
}
/// <summary>
/// Computes the 16-bit checksum of the specified IPv4 packet.
/// </summary>
/// <param name="packet">The packet to compute the checksum for.</param>
/// <param name="withChecksumField">true to include the packet's
/// checksum field in the calculation; otherwise false.</param>
/// <returns>The checksum of the specified IPv4 packet.</returns>
/// <exception cref="ArgumentNullException">Thrown if the packet
/// argument is null.</exception>
public static ushort ComputeChecksum(IpPacket packet,
bool withChecksumField = false) {
packet.ThrowIfNull("packet");
// The version and IHL fields are 4 bit wide each.
var vi = (byte) (((packet.Ihl & 0x0F) << 4) |
(((int) packet.Version) & 0x0F));
// The flags field is 3 bits and the fragment offset 13 bits wide.
var ffo = (ushort) (((packet.FragmentOffset & 0x1FFF) << 3) |
((int) packet.Flags & 0x07));
var bytes = new ByteBuilder()
.Append(vi)
.Append(packet.Dscp)
.Append(packet.TotalLength)
.Append(packet.Identification)
.Append(ffo)
.Append(packet.TimeToLive)
.Append((byte) packet.Protocol)
.Append(withChecksumField ? packet.Checksum : (ushort)0)
.Append(packet.Source.Bytes)
.Append(packet.Destination.Bytes)
.ToArray();
var sum = 0;
// Treat the header bytes as a sequence of unsigned 16-bit values and
// sum them up.
for (var n = 0; n < bytes.Length; n += 2)
sum += BitConverter.ToUInt16(bytes, n);
// Use carries to compute the 1's complement sum.
sum = (sum >> 16) + (sum & 0xFFFF);
// Return the inverted 16-bit result.
return (ushort)(~ sum);
}