为什么使用 struct 打包生存时间（ttl）？

Question

I stumbled upon some code in the book PyMOTW3 ( https://pymotw.com/3/socket/multicast.html ) and I could not understand why the time to live(ttl) argument was packed as struct.pack('b',1) .

I tried searching the manuals to see if the arguments are supposed to be packed but it states that it can be integers. I tried inputting a normal integer and it seemed to work fine. It gave the same output as the code below. Then is there a specific reason why it has been packed like this ? I do know that packing it as 1 is unnecessary because the default value is 1 , but what if I need to use some other number. Do I need to pack it?

I have included the code from the book below.

import socket
import struct
import sys

message = b'very important data'
multicast_group = ('224.3.29.71', 10000)

# Create the datagram socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Set a timeout so the socket does not block
# indefinitely when trying to receive data.
sock.settimeout(0.2)

# Set the time-to-live for messages to 1 so they do not
# go past the local network segment.
ttl = struct.pack('b', 1)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)
print(s.getsockopt(socket.IPPROTO_IP,socket.IP_MULTICAST_TTL))

try:

    # Send data to the multicast group
    print('sending {!r}'.format(message))
    sent = sock.sendto(message, multicast_group)

    # Look for responses from all recipients
    while True:
        print('waiting to receive')
        try:
            data, server = sock.recvfrom(16)
        except socket.timeout:
            print('timed out, no more responses')
            break
        else:
            print('received {!r} from {}'.format(
                data, server))

finally:
    print('closing socket')
    sock.close()

This is the output I get whether I pack it or use a normal integer.

1

Sending very important message

Timed Out!

Closing Socket

Answer 1

All packets consist of bytes.

For the AF_INET or an IPv4 you can see the distribution of bits among the properties of the packet in this picture . You can see in the IPv4 header, TTL is the 9th octet of 20. TTL is an 8-bit (1-byte) field and can be from 0 to 255.

That is, each property should take a certain number of bits.

For this purpose, Python 3.7 has a module struct . This module performs conversions between Python values and C structs represented as Python bytes objects.

Since TTL is a positive integer of 1 byte in size, we have to select the unsigned char type, which is 'B'.

And so now the code looks like this:

ttl = struct.pack('B', 25)  # unsigned char as 1-byte object
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)