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Enclave.FastPacket

The FastPacket project provides efficient, zero-allocation mechanisms for reading and writing individual network packets.

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This project consists of two projects/packages:

Motivations

FastPacket aims to support real-time analysis/manipulation of network traffic, and underpins the Enclave (https://enclave.io) packet analysis behaviour needed for state-tracking of connections.

We aim for the highest-possible performance, trying to remove the cost of reading any value that you aren't actually interested in.

We also want to simplify defining packets, by allowing us to base how we lay out packets on the way RFCs and documentation represent them, rather than how C# code typically requires it.

History

For some time in Enclave, we were using the excellent PacketDotNet library to analyse ethernet packets "on the wire" as they were moving through a network adapter, but we saw that the number of memory allocations and overhead of reading packets with PacketDotNet was causing some slow downs when we needed to analyse "some" of the content of every packet.

So, we started a small, hand-rolled library internally to read individual fields of packets directly using ReadOnlySpan<byte>, rather than the ByteArraySegment approach used in PacketDotNet.

This then spun out into wanting a standard way to define packet structures, so the source generator was born, and decided it was useful enough to others to open-source.

Example

Here's an example of decoding an Ethernet packet containing IPv4 and TCP data, using FastPacket:

Span<byte> packetSpan = new byte[] { /** some packet data **/ };
        
var ethernetSpan = new EthernetPacketSpan(packetSpan);

if (ethernetSpan.Type == EthernetType.IPv4)
{
    // Most of our spans have a 'Payload' property, which contains all the
    // data the packet encapsulates.
    var ipSpan = new Ipv4PacketSpan(ethernetSpan.Payload);

    // Check the source address and protocol.
    if (ipSpan.Source == ValueIpAddress.Loopback &&
        ipSpan.Protocol == IpProtocol.Tcp)
    {
        // Use the IP payload.
        var tcpSpan = new TcpPacketSpan(ipSpan.Payload);

        // Change the destination port so everything goes to port 80. This directly writes to
        // the underlying buffer, at the right position.
        tcpSpan.DestinationPort = 80;
    }
}

Benchmarks

Here's an output of some of our benchmarks that compare the (aforementioned excellent) PacketDotNet library with FastPacket for parsing some common packet information such as UDP/TCP ports and payload size (you can find these in the bench folder).

All these benchmarks start from reading a raw ethernet frame.

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.22000
Intel Core i7-9750H CPU 2.60GHz, 1 CPU, 12 logical and 6 physical cores
.NET SDK=6.0.200
  [Host]     : .NET 6.0.2 (6.0.222.6406), X64 RyuJIT
  DefaultJob : .NET 6.0.2 (6.0.222.6406), X64 RyuJIT

|                                  Method |      Mean |    Error |   StdDev |  Gen 0 |  Gen 1 | Allocated |
|---------------------------------------- |----------:|---------:|---------:|-------:|-------:|----------:|
| GetEthernetHardwareAddress_PacketDotNet |  46.92 ns | 0.968 ns | 0.906 ns | 0.0523 |      - |     328 B |
|   GetEthernetHardwareAddress_FastPacket |  11.68 ns | 0.142 ns | 0.133 ns |      - |      - |         - |
|            GetIpv4UdpPorts_PacketDotNet | 201.34 ns | 3.841 ns | 3.592 ns | 0.1376 | 0.0002 |     864 B |
|              GetIpv4UdpPorts_FastPacket |  15.29 ns | 0.270 ns | 0.252 ns |      - |      - |         - |
|            GetIpv4TcpPorts_PacketDotNet | 202.72 ns | 3.963 ns | 3.892 ns | 0.1376 | 0.0002 |     864 B |
|              GetIpv4TcpPorts_FastPacket |  14.68 ns | 0.238 ns | 0.222 ns |      - |      - |         - |
|      GetIpv4TcpPayloadSize_PacketDotNet | 233.52 ns | 4.693 ns | 5.586 ns | 0.1488 |      - |     936 B |
|        GetIpv4TcpPayloadSize_FastPacket |  20.22 ns | 0.188 ns | 0.157 ns |      - |      - |         - |
|         CheckForIpv4TcpAck_PacketDotNet | 202.70 ns | 4.010 ns | 3.751 ns | 0.1376 | 0.0002 |     864 B |
|           CheckForIpv4TcpAck_FastPacket |  14.09 ns | 0.304 ns | 0.312 ns |      - |      - |         - |
|            GetIpv6UdpPorts_PacketDotNet | 238.48 ns | 4.573 ns | 5.444 ns | 0.1516 |      - |     952 B |
|              GetIpv6UdpPorts_FastPacket |  31.66 ns | 0.622 ns | 0.582 ns |      - |      - |         - |

Supported Protocols

We'll be adding more protocols as we need them, and extending support for things like TCP Option reading, checksum validation, and so on.

How It Works

FastPacket is backed by a C# Source Generator (Enclave.FastPacket.Generator) that generates efficient packet decoders from a simple type definition, that read directly from the underlying buffer.

The source generator approach lets us standardise on efficient patterns for reading packets, without needing a large amount of repetitive code everywhere. It also lets you create decoders based on the same principles, without writing out bitmasks, offset calculations and big-endian reads by hand.

Finally, the use of a source generator means we can remove any virtual method calls, which helps with our performance a bit more again.

With FastPacket, we define packet definitions, and let the generator create the actual decoder.

For example, this simple packet definition:

// Define the structure of an ethernet packet
internal ref struct EthernetPacketDefinition
{
    /// <summary>
    /// The destination hardware (MAC) address.
    /// </summary>
    public HardwareAddress Destination { get; set; }

    /// <summary>
    /// The source hardware (MAC) address.
    /// </summary>
    public HardwareAddress Source { get; set; }

    /// <summary>
    /// The EtherType field.
    /// </summary>
    public EthernetType Type { get; set; }

    /// <summary>
    /// The Ethernet Payload.
    /// </summary>
    public ReadOnlySpan<byte> Payload { get; set; }
}

// Define a type that should receive the implementation of the definition.
// Note that the implementation is a `readonly ref partial struct`, that's important!
[PacketImplementation(typeof(EthernetPacketDefinition))]
public readonly ref partial struct EthernetPacketSpan
{
}

will populate the rest of the EthernetPacketSpan type:

 readonly ref partial struct EthernetPacketSpan
{
    private readonly Span<byte> _span;

    public EthernetPacketSpan(Span<byte> packetData)
    {
        _span = packetData;
    }

    public Span<byte> GetRawData() => _span;

    /// <summary>
    /// The destination hardware (MAC) address.
    /// </summary>
    public Enclave.FastPacket.HardwareAddress Destination
    {
        get => new Enclave.FastPacket.HardwareAddress(_span.Slice(0, Enclave.FastPacket.HardwareAddress.Size));
        set => value.CopyTo(_span.Slice(0, Enclave.FastPacket.HardwareAddress.Size)); 
    }

    /// <summary>
    /// The source hardware (MAC) address.
    /// </summary>
    public Enclave.FastPacket.HardwareAddress Source
    {
        get => new Enclave.FastPacket.HardwareAddress(_span.Slice(0 + Enclave.FastPacket.HardwareAddress.Size, Enclave.FastPacket.HardwareAddress.Size));
        set => value.CopyTo(_span.Slice(0 + Enclave.FastPacket.HardwareAddress.Size, Enclave.FastPacket.HardwareAddress.Size)); 
    }
        
    /// <summary>
    /// The EtherType field.
    /// </summary>
    public Enclave.FastPacket.EthernetType Type
    {
        get => (Enclave.FastPacket.EthernetType)(BinaryPrimitives.ReadUInt16BigEndian(_span.Slice(0 + Enclave.FastPacket.HardwareAddress.Size + Enclave.FastPacket.HardwareAddress.Size)));
        set => BinaryPrimitives.WriteUInt16BigEndian(_span.Slice(0 + Enclave.FastPacket.HardwareAddress.Size + Enclave.FastPacket.HardwareAddress.Size), (ushort)(value)); 
    }

    /// <summary>
    /// The Ethernet Payload.
    /// </summary>
    public System.Span<byte> Payload
    {
        get => _span.Slice(0 + Enclave.FastPacket.HardwareAddress.Size + Enclave.FastPacket.HardwareAddress.Size + sizeof(ushort));
    }
}

At build time, the C# compiler will collapse all those constant values in the generated type into a single constant value as efficiently as it possibly can, so the eventual IL ASM after the JIT has run is very efficient (and largely inlined).

All of the generated code for the Enclave.FastPacket library is currently checked in to git here.

Source Generator Features

The FastPacket Source Generator gives us a number of features that make it easier to define packet structures.

Numeric Fields

FastPacket natively supports numeric fields that are:

Currently, FastPacket assumes that all numeric fields are big-endian, since that is the general network byte order. In future we may add support for handling little-endian values, but for now, assume that all byte order is taken care of as big-endian.

Automatic Size/Position Determination

By default, FastPacket generates automatic position/size constants for you as much as it can, based on the order of properties in the type.

For example, if you define the following packet and an implementation:

struct MyPacketDefinition
{
    public byte Field1 { get; set; }

    public byte Field2 { get; set; }

    public ushort Field2 { get; set; }
}

The generated properties are like so:

    public byte Field1
    {
        get => _span[0];
        set => _span[0] = value; 
    }        
        
    public byte Field2
    {
        get => _span[0 + sizeof(byte)];
        set => _span[0 + sizeof(byte)] = value; 
    }        
        
    public ushort Field3
    {
        get => BinaryPrimitives.ReadUInt16BigEndian(_span.Slice(0 + sizeof(byte) + sizeof(byte)));
        set => BinaryPrimitives.WriteUInt16BigEndian(_span.Slice(0 + sizeof(byte) + sizeof(byte)), value); 
    }

Wherever possible, we use constant computation to determine the position of fields in packets.

ReadOnly Implementations

You often may not have a writeable Span<byte> containing packet data, or you may want to prevent editing of packet fields.

If you add IsReadOnly = true to the PacketImplementation attribute, then the created implementation will not be modifiable, and will only depend on ReadOnlySpan<byte>.

struct MyPacketDefinition
{
    public ushort Value1 { get; set; }
}

[PacketImplementation(typeof(MyPacketDefinition), IsReadOnly = true)]
public readonly ref partial struct MyPacket
{
}

ToString

We will auto-generate a simple ToString implementation for you by default, with details of all the public properties of the packet included.

If you specify your own ToString override in your implementation struct, that replaces the default one we add.

ref struct MyPacketDefinition
{
    public ushort Value1 { get; set; }

    public ReadOnlySpan<byte> Payload { get; set; }
}

[PacketImplementation(typeof(MyPacketDefinition), IsReadOnly = true)]
public readonly ref partial struct MyPacket
{
}

var packetInstance = MyPacket(new byte[] { 0x00, 0x02, 0x00, 0x00, 0x00 });

// Prints:
// Value1: 2; Payload: 3 bytes
Console.WriteLine(packetInstance.ToString());

Field Visibility

The visibility of fields in the packet definition become the visibility of those fields in the implementation.

This is useful for fields that use up space in the packet, but are unused, so shouldn't be externally visible.

struct MyPacketDefinition
{
    // This field will use up space in the packet, but will be private in the implementation
    private ushort UnusedValue { get; set; }

    // This field will use up space in the packet, but will be internal in the implementation
    internal ushort InternalValue { get; set; }

    public ushort Value1 { get; set; }
}

Customising Fields

If you need to adjust individual fields, you can apply the PacketField attribute:

ref struct MyPacketDefinition
{
    // Customise this block to be 10 bytes in size.
    [PacketField(Size = 10)]
    public ReadOnlySpan<byte> DataBlock { get; set; }
}

This lets you do things like:

Blob Fields

You often want a field of data in a packet that represents a blob of data in some form.

You can add a Span<byte> or ReadOnlySpan<byte> field into your packet and FastPacket will give you the appropriate slice of data from the packet based on the size of fields preceding it.

If a blob field is the last property in the definition, we assume that it uses the remaining content of the packet's buffer, as with a lot of network payloads.

If a blob field is in the middle of the packet, you will need to tell us what size it is (either explicitly or with a SizeField or SizeFunction), otherwise we can't use it.

ref struct MyPacketDefinition
{
    public byte Field1 { get; set; }

    // Have to declare a size.
    [PacketField(Size = 12)]
    public ReadOnlySpan<byte> Options { get; set; }

    // Don't need to declare a size (it's just the rest of the buffer)
    public ReadOnlySpan<byte> Payload { get; set; }
}

To that end, if you need to calculate the size of a field dynamically, you can do that by specifiying a SizeField within the packet, or adding a static SizeFunction within the definition type.

Field-Based Size Determination

If the size of a field within your packet is determined by the size of another field (for example, a preceding 'Length' field ahead of a data segment), you can indicate that relationship using the SizeField property of the PacketField attribute:

ref struct MyPacketDefinition
{
    public ushort DataLength { get; set; }

    [PacketField(SizeField = nameof(DataLength))]
    public ReadOnlySpan<byte> Data1 { get; set; }
}

If the value of a single field alone isn't enough, you may need a SizeFunction.

Function-Based Size Determination

It's fairly common to determine the size of a particular field based on some other computed value, often a value in the same packet.

To that end, if you need to calculate the size of a field dynamically, you can do that by specifiying a static SizeFunction within the definition type.

ref struct MyPacketDefinition
{
    public ushort DataLength { get; set; }

    [PacketField(SizeFunction = nameof(GetDataSize))]
    public ReadOnlySpan<byte> Data { get; set; }

    public static int GetDataSize(ReadOnlySpan<byte> packet)
    {
        // I can safely access the `DataLength` property of my own definition!
        // That works just fine provided you are reading a field *before* the one you are computing the size of.
        // In future we'll add an analyzer to prevent accidental stack overflows.
        return new MyPacket(packet).DataLength;
    }
}

[PacketImplementation(typeof(MyPacketDefinition), IsReadOnly = true)]
readonly ref partial struct MyPacket
{
}

Size functions are defined either as:

public static int FuncName(ReadOnlySpan<byte> packet)

or

public static int FuncName(ReadOnlySpan<byte> packet, int position)`

In both versions, the packet parameter contains the entire packet buffer for inspection.

If you use the second version, the position parameter tells you the known location of the field in question in case the length is stored somewhere in that field.

Enum Support

Enums are just naturally supported in FastPacket; define your enum and then use that enum as a property type:

public enum FieldCodes : byte
{
    Code1 = 0x01,
    Code2 = 0x02
}

struct MyPacketDefinition
{
    public FieldCodes Code { get; set; }
}

The Code property here will read a single byte from the packet and cast it to your enum for you.

Note that I've specified a base numeric type for the enum; FastPacket can use this to figure out how much space your enum takes up in the packet.

If you aren't able to change the base type (perhaps it is an internal runtime enum), you can specify it on the property:

struct MyPacketDefinition
{
    [PacketField(EnumBackingType = typeof(byte))]
    public ProtocolType Type { get; set; }
}

Custom Types

Packet types are often more complex than just ushort, byte, int, etc. We may have custom structures we want to represent.

For an example, take a hardware, or MAC address. Hardware addresses are 6 bytes, but we will have other behaviour attached to them, so we don't want to just represent them as blobs of data.

You can use any custom type as a FastPacket field, as long as it implements the following:

For example, this is a valid custom type, that can be used in any FastPacket definition:

public struct MyCustomType
{
    private byte _b1;
    private byte _b2;

    public const int Size = 2;

    public MyCustomType(ReadOnlySpan<byte> data)
    {
        _b1 = data[0];
        _b2 = data[1];
    }

    public void CopyTo(Span<byte> data)
    {
        data[0] = _b1;
        data[1] = _b2;
    }
}

The Enclave.FastPacket library has already defined HardwareAddress and ValueIpAddress types designed for reducing allocations commonly seen with the PhysicalAddress and IpAddress types in the runtime.

Unions and Bit-fields

FastPacket allows us to represent 'unions', i.e. fields that share the same space, within a packet.

struct MyPacketDefinition
{
    // Any struct inside a definition is considered a union.
    // We need to define an explicit or computed size for a union right now, but that might change in future:
    [PacketField(Size = sizeof(uint))
    struct U1
    {
        public uint RawValue { get; set; }

        public CustomValue Value { get; set; }
    }
}

Once that definition gets converted to an implementation, those properties will be flattened into a single entry that looks a bit like:

[PacketImplementation(typeof(MyPacketDefinition))]
readonly ref partial struct MyPacketImplementation
{
    // In the generated code...

    public uint RawValue { get; set; }

    public CustomValue Value { get; set; }
}

However, both RawValue and Value occupy the same range of bytes in the packet as each other.

The real power of unions doesn't really reveal itself however until you start using bit-fields. Let's look at a snipped part of the TCP header definition, from the RFC:

   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |

The Data Offset, Reserved and Flags use up two bytes of space, but different parts of those two bytes mean different things. Let's look at how we can represent this easily with FastPacket:

internal ref struct TcpPacketDefinition
{
    // Snip...

    public uint AckNumber { get; set; }

    [PacketField(Size = sizeof(ushort))]
    public struct UFlags
    {
        [PacketFieldBits(0, 3)]
        public byte DataOffset { get; set; }

        [PacketFieldBits(10, 15)]
        public TcpFlags Flags { get; set; }
    }

    public ushort WindowSize { get; set; }

    public ushort Checksum { get; set; }

    public ushort UrgentPointer { get; set; }

    // Snip ...
}

The PacketFieldBits attribute lets us specify which bits a given field occupies within the union. Bits are specified as an inclusive range of bits within the union, using MSB-0 numbering to match a lot of RFC documentation.

The FastPacket generator automatically generates the necessary bitmasks and shifts to convert the MSB-0 numbering and means that each value you read is only the bits that you've specified.

Near-term tasks

Contributors

Got a protocol you want to support? Raise an issue for it so we can discuss, then we should be able to fold a packet definition (and tests) for that packet into the library fairly easily.

Build

.NET6 SDK required.

Run dotnet build to build.

Tests

Tests are in /tests. Run tests with dotnet test.