DofusFrameDecoder.cs

    public class DofusFrameDecoder : HeaderFrameDecoder<DofusMetadata>
    {
        protected override bool TryParseHeader(ref SequenceReader<byte> input, out DofusMetadata frame, out int length)
        {
            frame = default;
            length = 0;
            if (!input.TryReadBigEndian(out short header)) return false;
            if (!input.TryReadBigEndian(out int instanceId)) return false;

            var messageId = header >> 2;
            var lengthByteCount = header & 0x3;

            if (messageId <= 0) return false;
            if (input.Remaining < lengthByteCount) return false;

            Span<byte> lengthBuffer = stackalloc byte[lengthByteCount];
            input.Sequence.Slice(2 + 4, lengthByteCount).CopyTo(lengthBuffer);

            for (int i = lengthByteCount - 1, j = 0; i >= 0; i--, j++)
                length |= lengthBuffer[j] << (i * 8);

            frame = new DofusMetadata(messageId, (uint)instanceId, lengthByteCount);
            return true;
        }
    }

DofusFrameEncoder.cs

    public class DofusFrameEncoder : HeaderFrameEncoder<DofusMetadata>
    {
        protected override int GetHeaderLength(DofusMetadata frame) => sizeof(short) + frame.LengthByteCount;

        protected override void WriteHeader(Span<byte> buffer, OutputFrame<DofusMetadata> frame)
        {
            var metadata = frame.Frame;

            var header = (short)(metadata.MessageId << 2 | (byte)metadata.LengthByteCount);
            BinaryPrimitives.WriteInt16BigEndian(buffer, header);

            if (metadata.LengthByteCount == 0) return;
            var span = buffer.Slice(2);
            switch (metadata.LengthByteCount)
            {
                case 1:
                    span[0] = (byte)frame.Payload.Length;
                    break;
                case 2:
                    BinaryPrimitives.WriteInt16BigEndian(span, (short)frame.Payload.Length);
                    break;
                case 3:
                    span[0] = (byte)(frame.Payload.Length >> 16 & byte.MaxValue);
                    BinaryPrimitives.WriteInt16BigEndian(span.Slice(1), (short)(frame.Payload.Length & ushort.MaxValue));
                    break;
                default:
                    throw new ArgumentOutOfRangeException(nameof(metadata.LengthByteCount));
            }
        }
    }

DofusMetadata.cs

    public class DofusMetadata
    {
        public int LengthByteCount { get; set; }
        public uint InstanceId { get; set; }
        public int MessageId { get; set; }
        
        public DofusMetadata(int messageId, uint instanceId, int lengthByteCount) // from client
        {
            LengthByteCount = lengthByteCount;
            MessageId = messageId;
            InstanceId = instanceId;
        }

        public DofusMetadata(int messageId, int lengthByteCount) // from server
        {
            LengthByteCount = lengthByteCount;
            MessageId = messageId;
        }
    }