2: Mike Ash s Deep Dive Into Peer-to-Peer Networking in Objective-C

Creating DataMatrix in Objective-C 2: Mike Ash s Deep Dive Into Peer-to-Peer Networking

CHAPTER 2: Mike Ash s Deep Dive Into Peer-to-Peer Networking
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It will also pad out the end of structs so that the next struct will start on a nice address if it s being used in an array. All of this padding is compiler dependent and has no place being part of a network protocol. The #pragmas tell the compiler to stop padding and cram everything into as little space as possible, which is exactly what we want if we re going to be sending stuff out over the network. Figure 2-4 shows the original packet structure without the #pragma, and Figure 2-5 shows the corrected version.
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Figure 2-4. The packet structure on the network with a typical compiler
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Figure 2-5. The layout of the packet structure with #pragma pack(1) enabled
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We don t plan for spheres to change color, so sending the color in every single packet is redundant. Surely, it would be better to simply send the color once and then send only position changes after that It s a good idea, but the trouble is the unreliable nature of UDP. If that initial color transmission happened to be lost, the color information would be gone forever. On a typical Wi-Fi network you can count on a roughly 1 percent chance of losing any given packet, so this is a very real possibility! If we did send the color data only with the first transmission, we d also have to have some way for the other side to acknowledge receipt of the color packet and for
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CHAPTER 2: Mike Ash s Deep Dive Into Peer-to-Peer Networking
the transmitter to resend it if it got lost. Suddenly, the code becomes a lot more complex. It s much simpler, if slightly more wasteful, to just tack 3 bytes onto every position update to ensure that the color is always known to the other side. Now, you re probably wondering what happens if a position update is lost. After all, doesn t that have the same problem The beauty of the position updates is that SphereNet doesn t need to see all of them. If it misses one in the middle, the next one will correct the problem. As a result, the sphere may follow a slightly different path than it did on the sending side, but that s not a big problem. By sending a position update every time the sphere moves, and periodically sending them even when the sphere is not moving at all, we can easily ensure that every other copy of SphereNet stays reasonably up to date with the position of all spheres even in the face of the occasional lost packet. There s one more piece to the puzzle: figuring out where to send these packets. Apple s Bonjour technology comes to the rescue here. SphereNet can broadcast its presence to the network, and all other copies of SphereNet can find it using Bonjour.
Understanding Endianness
There is one detail that I glossed over in the previous description of the packet format: endianness. If you aren t familiar with it, you may be surprised to learn that computers have two different ways to represent integers in memory: big-endian and little-endian. And these two ways are incompatible. The difference comes about due to the order in which the bytes of a multibyte integer are written. Take the integer 305,419,896 for example. In hexadecimal, this integer would be written out as 0x12345678. The question is, how does it look in memory, for example, as an array of unsigned chars One obvious way would be to simply write it down in order:
unsigned char myInt[4] = { 0x12, 0x34, 0x56, 0x78 };
But it s just as reasonable, albeit somewhat less natural, to write it down in the opposite order, with the least-significant byte first:
unsigned char myInt[4] = { 0x78, 0x56, 0x34, 0x12 };
The former system is called big-endian, and the latter is called little-endian. The Intel x86 CPU used in Macs these days is little-endian, as is the ARM CPU used in the iPhone. The PowerPC processors used in older Macs are big-endian, and in general, it s common to find either version being used on various platforms. Reading data using the wrong endian will give you garbled, useless numbers, so it s important to get this right.
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