Representation of bits and their serialisation in .Net
https://stackoverflow.com/questions/13148546
-
21-07-2021 - |
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I am practising implementing some basic layer 7 protocols but I am unsure of the best way of serialising and deserialising bits in the .Net framework.
According to the MSDN Data Type Summary, there is no bit data type. I have no idea how I would go about creating such a data type or even if it's possible so I am left with serialising/deserialising to a byte / byte array.
Given the following example from the top of an NTP packet:
0-1 LeapIndicator (LI) 2 bits
2-4 VersionNumber (VN) 3 bits
5-7 Mode 3 bits
8-15 Stratum 8 bits
I would like to encode into 2 bytes so I can send via the socket.
Also, I am currently using ints to represent the bits in enums, is it possible to use bits/hex or something a better than ints? For example the mode enum is defined as follows:
public enum Mode
{
/*
+-------+--------------------------+
| Value | Meaning |
+-------+--------------------------+
| 0 | reserved |
| 1 | symmetric active |
| 2 | symmetric passive |
| 3 | client |
| 4 | server |
| 5 | broadcast |
| 6 | NTP control message |
| 7 | reserved for private use |
+-------+--------------------------+
*/
Resevered = 0,
SymmetricActive = 1,
SymmetricPassive = 2,
Client = 3,
Server = 4,
Broadcast = 5,
ControlMessage = 6,
PrivateUse = 7
}
Side Note: The code for this project will eventually be open sourced, please bare in mind that if you answer. If you do not wish for the code to be shared, please say :) A link will be placed in the code back to this question.
Thanks in advance :)
Update: In case people are wondering what the NTP packet structure looks like, taken directly from RFC 5905, page 18
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LI | VN |Mode | Stratum | Poll | Precision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Dispersion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Reference Timestamp (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Origin Timestamp (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Receive Timestamp (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Transmit Timestamp (64) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Extension Field 1 (variable) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Extension Field 2 (variable) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| dgst (128) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Solution
I don't think I'd use an enum here at all. I'd probably create a struct to represent the packet header, storing the data in a ushort (16 bits):
public struct NtpHeader
{
private readonly ushort bits;
// Creates a header from a portion of a byte array, e.g
// given a complete packet and the index within it
public NtpHeader(byte[] data, int index)
{
bits = (ushort) (data[index] + (data[index] << 8));
}
public NtpHeader(int leapIndicator, int versionNumber,
int mode, int stratum)
{
// TODO: Validation
bits = (ushort) (leapIndicator |
(versionNumber << 2) |
(mode << 5) |
(stratum << 8));
}
public int LeapIndicator { get { return bits & 3; } }
public int VersionNumber { get { return (bits >> 2) & 7; } }
public int Mode { get { return (bits >> 5) & 7; } }
public int Stratum { get { return bits >> 8; } }
}
You'll want to check this though - it's not immediately clear what bit arrangement is really represented in the RFC. If you have sample packets with expected values, that would make things much clearer.
OTHER TIPS
FYI, there is a struct that represents a bit in .NET, it's System.Boolean. As mentioned by Marc, the protocol is in even bytes, so you could use an int (with each int holding 32 bits), or use enums in a bitmask style. Either way, you can use the System.BitConverter's static methods to do convert to and from byte arrays.
Have you considered using the Flags attribute? It allows you to treat enumerated type values as bits instead of ints: http://msdn.microsoft.com/en-us/library/system.flagsattribute.aspx
The smallest type of an enum in c# is byte (the other types available are explained here http://msdn.microsoft.com/en-us/library/sbbt4032.aspx). Define an enum of type byte:
enum Name:byte{}
in your example:
public enum Mode:byte
{
/*
+-------+--------------------------+
| Value | Meaning |
+-------+--------------------------+
| 0 | reserved |
| 1 | symmetric active |
| 2 | symmetric passive |
| 3 | client |
| 4 | server |
| 5 | broadcast |
| 6 | NTP control message |
| 7 | reserved for private use |
+-------+--------------------------+
*/
Resevered = 0,
SymmetricActive = 1,
SymmetricPassive = 2,
Client = 3,
Server = 4,
Broadcast = 5,
ControlMessage = 6,
PrivateUse = 7
}
If we wish to save space but have less readability, we can see that sizeof(LeapIndicator) + sizeof(VersionNumber) + sizeof(Mode) = 8 bits = 1 byte. and also sizeof(Sratum) = 8 bits = 1 byte.
Serialize: To put your packet fields into the result, simply multiply by 2 (left shift) by the appropriate number of bits and then OR with the cumulative result so far.
Deserialize: To extract your packet fields from the result, simple use an AND bitmask then divide by 2 (right shift) by the appropriate number of bits.
