.net qr code NETWORK DATA AND NETWORK ERRORS in Font

Creation QR Code in Font NETWORK DATA AND NETWORK ERRORS

CHAPTER 5 NETWORK DATA AND NETWORK ERRORS
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Fixed-length messages are a bit rare since so little data these days seems to fit within static boundaries, but when transmitting binary data in particular, you might find it a good fit for certain situations. A fourth pattern is to somehow delimit your messages with special characters. The receiver would wait in a recv() loop like the one just cited, but wait until the reply string it was accumulating finally contained the delimiter indicating the end-of-message. If the bytes or characters in the message are guaranteed to fall within some limited range, then the obvious choice is to end each message with a symbol chosen from outside that range. If you were sending ASCII strings, for example, you might choose the null character '\0' as the delimiter. If instead the message can include arbitrary data, then using a delimiter is a problem: what if the character you are trying to use as the delimiter turns up as part of the data The answer, of course, is quoting, just like having to represent a single-quote character as \' in the middle of a Python string that is itself delimited by single-quote characters: 'All\'s well that ends well.' I recommend using a delimiter scheme only where your message alphabet is constrained; it is too much trouble if you have to handle arbitrary data. For one thing, your test for whether the delimiter has arrived now has to make sure that you are not confusing a quoted delimiter for a real one that actually ends the message. A second complexity is that you then have to make a pass over the message to remove the quote characters that were protecting literal occurrences of the delimiter. Finally, it means that message length cannot be measured until you have performed decoding a message of length 400 could be 400 symbols long, or it could be 200 instances of the delimiter accompanied by the quoting character, or anything in between. A fifth pattern is to prefix each message with its length. This is a very popular choice for highperformance protocols since blocks of binary data can be sent verbatim without having to be analyzed, quoted, or interpolated. Of course, the length itself has to be framed using one of the techniques given previously often it is simply a fixed-width binary integer, or else a variable-length decimal string followed by a delimiter. But either way, once the length has been read and decoded, the receiver can enter a loop and call recv() repeatedly until the whole message has arrived. The loop can look exactly like the one in Listing 3-1, but with a length variable in place of the number 16. Finally, what if you want the simplicity and efficiency of this fifth pattern but you do not know ahead of time how long each message will be perhaps because the sender is himself reading data from a source whose length he cannot predict In such cases, do you have to abandon elegance and slog through the data looking for delimiters Unknown lengths are no problem if you use a final, and sixth, pattern. Instead of sending just one, try sending several blocks of data that are each prefixed with their length. This means that as each chunk of new information becomes available to the sender, it can be labeled with its length and placed on the outgoing stream. When the end finally arrives, the sender can emit an agreed-upon signal perhaps a length field giving the number zero that tells the receiver that the series of blocks is complete. A very simple example of this idea is shown in Listing 5 2. Like the previous listing, this sends data in only one direction from the client to the server but the data structure is much more interesting. Each message is prefixed with a 4-byte length; in a struct, 'I' means a 32-bit unsigned integer, meaning that these messages can be up to 4GB in length. A series of three such messages is sent to the server, followed by a zero-length message which is essentially just a length field with zeros inside and then no message data after it to signal that the series of blocks is over. Listing 5 2. Sending Blocks of Data #!/usr/bin/env python # Foundations of Python Network Programming - 5 - blocks.py # Sending data one block at a time. import socket, struct, sys
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