Questions about packet format

[Elefant]

I'm trying to write a python interface for the freeciv server, and I'm having trouble understanding the packet formats.
This packet is able to trick the server into thinking a client is trying to connect:

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\x00T\x04username\x00+Freeciv-3.1-network city-original rsdesc32 obsinv ids32\x00\x00\x00\x00\x00\x03\x00\x00\x00\x01\x00\x00\x00\x03

and here is the packets.def definition for it:

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PACKET_SERVER_JOIN_REQ = 4; cs, dsend, no-delta, no-handle
  STRING username[48];
  STRING capability[512];
  STRING version_label[48];
  UINT32 major_version, minor_version, patch_version;
end

A few questions:
What does the \x00T at the beginning mean?
I am assuming that 04username is the packet number and the username string.
Why, then, is the username string shorter than 48 characters?
Also, why aren't they separate?
The next chunk is obviously the capability string.
Why is the version_label string empty?
Also, there are 5 x00 blocks between capability and major_version. I am assuming that each block is a byte, in hexadecimal. So then, three are the most significant three bytes of major_version, and one would be the empty version_label. What is the last one for?

[cazfi]

See README.delta.

What does the \x00T at the beginning mean?

I think that's the two byte length of the packet (to know where's the boundary between packets) ASCII "T" = 84.

I am assuming that 04username is the packet number and the username string.
Why, then, is the username string shorter than 48 characters?

It's transferred NULL-terminated, i.e., the \x00 terminates it.

Also, why aren't they separate?

What do you mean? Hmm... Note that it was \x04, and not "04" like you wrote.

One complication is that historically packet type was 8bit value, and for that reason it's just one byte in these initial packets. But as soon as the server and the client have established that the other party is new enough to understand 16bit type fields, the following packets will use 2 bytes for the type.

The next chunk is obviously the capability string.
Why is the version_label string empty?

Because you are using a version that has no version label?

Also, there are 5 x00 blocks between capability and major_version. I am assuming that each block is a byte, in hexadecimal. So then, three are the most significant three bytes of major_version, and one would be the empty version_label. What is the last one for?

First one terminates capability string.