English
English
In network programming, the construction of TCP/IP message headers is a key step in implementing the underlying communication protocol. As a flexible scripting language, although it is usually used for web page development, with its powerful binary data processing function pack() , we can also use it to construct TCP/IP message headers and even implement the design of custom protocols.
This article will introduce in detail how to use PHP's pack() function to construct TCP/IP packet headers, and use examples to show how to design a simple custom protocol.
The TCP message header consists of multiple fields, common ones include:
Source Port: 2 bytes
Destination Port: 2 bytes
Sequence Number: 4 bytes
Acknowledgment Number: 4 bytes
Data Offset and reserved bits: 1 byte
Flags: 1 byte
Window Size: 2 bytes
Checksum: 2 bytes
Urgent Pointer: 2 bytes
The IP header is composed of several fields.
To construct these message headers in PHP, the core is to package each field into a binary string in the specified format, and pack() is this key tool.
The pack() function packages the parameters into binary strings. It controls packaging rules through format characters:
n — Unsigned short integer (2 bytes, network endianness)
N — Unsigned long (4 bytes, network endianness)
C — Unsigned characters (1 byte)
a — NUL Fill string
Network byte order is the big-endian byte order, which complies with the provisions of the TCP/IP protocol.
