pchMessageStart - what is for? (in BC 0.8)

[Many Coins]

Hello!

Tell me please, what is for:

Code:

pchMessageStart[0] = 0xfc;

and

Code:

pchMessageStart[4] = { 0xfb, 0xc0, 0xb6, 0xdb };

What does it do?

Thank you!

[achow101]

Those are the magic bytes. They prepend every single network message so that the message can be identified as being a Bitcoin message.

[Many Coins]

Those are the magic bytes. They prepend every single network message so that the message can be identified as being a Bitcoin message.

Is it different from, for example, litecoin? But why? Because they have different genesis blocks and seed-nodes, they will not be confused in any way.

And are these values ​​arbitrary? Or are there any rules for choosing them?

[Xynerise]

Is it different from, for example, litecoin? But why? Because they have different genesis blocks and seed-nodes, they will not be confused in any way.

Apparently Litecoin and Viacoin (the only 2 bitcoin code forks I checked) have the same magic bytes:

Code:

pchMessageStart[0] = 0xfb;
        pchMessageStart[1] = 0xc0;
        pchMessageStart[2] = 0xb6;
        pchMessageStart[3] = 0xdb;

https://github.com/litecoin-project/litecoin/blob/master/src/chainparams.cpp

And are these values ​​arbitrary? Or are there any rules for choosing them?

The same file linked above has this comment:

Code:

The message start string is designed to be unlikely to occur in normal data.
         * The characters are rarely used upper ASCII, not valid as UTF-8, and produce
         * a large 32-bit integer with any alignment.

[Many Coins]

Apparently Litecoin and Viacoin (the only 2 bitcoin code forks I checked) have the same magic bytes:

why not 

[achow101]

Is it different from, for example, litecoin?

Yes.

But why? Because they have different genesis blocks and seed-nodes, they will not be confused in any way.

To make sure that there's no crosstalk or confusion in the first place as figuring out that they have different genesis blocks still costs bandwidth and computing power.

And are these values ​​arbitrary? Or are there any rules for choosing them?

They're arbitrary. The common recommendation for new altcoins is to just randomly generate new magic bytes.

[Many Coins]

But why? Because they have different genesis blocks and seed-nodes, they will not be confused in any way.

To make sure that there's no crosstalk or confusion in the first place as figuring out that they have different genesis blocks still costs bandwidth and computing power.

And are these values ​​arbitrary? Or are there any rules for choosing them?

They're arbitrary. The common recommendation for new altcoins is to just randomly generate new magic bytes.

Thank you!

It is clear now.