Why do messages going through websockets always start with \x00 and end with \xff, as in \x00Your message\xff?
This documentation might help...
Excerpt from section 1.2:-
Data is sent in the form of UTF-8 text. Each frame of data starts
with a 0x00 byte and ends with a 0xFF byte, with the UTF-8 text in
between.
The WebSocket protocol uses this framing so that specifications that
use the WebSocket protocol can expose such connections using an
event-based mechanism instead of requiring users of those
specifications to implement buffering and piecing together of
messages manually.
To close the connection cleanly, a frame consisting of just a 0xFF
byte followed by a 0x00 byte is sent from one peer to ask that the
other peer close the connection.
The protocol is designed to support other frame types in future.
Instead of the 0x00 and 0xFF bytes, other bytes might in future be
defined. Frames denoted by bytes that do not have the high bit set
(0x00 to 0x7F) are treated as a stream of bytes terminated by 0xFF.
Frames denoted by bytes that have the high bit set (0x80 to 0xFF)
have a leading length indicator, which is encoded as a series of
7-bit bytes stored in octets with the 8th bit being set for all but
the last byte. The remainder of the frame is then as much data as
was specified. (The closing handshake contains no data and therefore
has a length byte of 0x00.)
The working spec has changed and no longer uses 0x00 and 0xFF as start and end bytes
http://tools.ietf.org/id/draft-ietf-hybi-thewebsocketprotocol-04.html
I am not 100% sure about this but my guess would be to signify the start and end of the message. Since x00 is a single byte representation of 0 and xFF is a single byte representation of 255
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I m trying to write the code for on switch board touch sensor which communication with mcu(esp32) using the uart protocol. There we have frame of packet which we have to write to uart to get the reading. Let me share some documentation,
1.
In API frame structure following is the fixed definition of any command frame
Every first byte of frame is fixed 0x7B (“{” in ASCII, 123 in decimal).
Every second byte of frame is ‘command type’ byte, it informs what you need to do with rest of the data. This will act as a frame Identifier in the data received from touch panel (response frame and event frame)
Third byte is length of frame. It is 1-byte value (L - 4) where L is total numbers of bytes of whole frame.
Second Last byte is checksum. Checksum is a lower byte of addition of whole individual bytes of frame except First byte (Start byte), Second byte (Command type byte),
Second Last byte (Checksum byte itself) and Last byte is 0x7D.
Last byte is 0x7D, it is End code to indicate the end of frame. (“}” in ASCII, 125 in decimal).
For Example, consider following frame.
Table 1.1 Frame example 1.
1st Byte 2nd Byte 3rd Byte 4th Byte 5th Byte 6th Byte
0x7B 0x03 0x02 0x05 0x07 0x7D
Start Code Command Length Data Checksum End Code
So the checksum will be lower byte of addition of third byte and fourth byte.
0x02 + 0x05 = 0x07 so we will consider 0x07 as checksum here.
Example 2, consider following frame.
Table 1.2 Frame example 2.
1st Byte 2nd Byte 3rd Byte 4th Byte 5th Byte 6th Byte 7th Byte 8th Byte
0x7B 0x52 0x04 0x02 0xFF 0x14 0x19 0x7D
Start Code Frame Length Data Data Data Checksum End Code
Identifier
In this example 2 the checksum will be lower byte of addition of third to sixth byte.
0x04 + 0x02 + 0xFF + 0x14 = 0x0119 so we will consider 0x19 as checksum here.
2.
Blink LED (All slider LED) control.
1.Command
This package is used to control blinking of LED. Hardware Version 2.0 has dedicated status LED. Which will be used to indicate status of product as on need.
Table 1.6 Blink LED command package detail.
Status 1st Byte 2nd Byte 3rd Byte 4th Byte 5th Byte 6th Byte 8th Byte
Start 0x7B 0x05 0x03 0x01 (Start) (0x01 to 0xNN*) Checksum 0x7D
Stop 0x7B 0x05 0x03 0x00 (Stop) 0x00 Checksum 0x7D
Start Code Command Length Pulse with (x100ms) Checksum End Code
To start status LED blinking, the start command frame is sent with required value of 4th byte as 0x01. For Example, to make status LED blinking as time duration 200ms, the value of 5th byte is 0x02.
And to stop status LED blinking the stop frame is sent
2.Response
Table 1.7 Blink LED response detail.
1st Byte 2nd Byte 3rd Byte 4th Byte 5th Byte
0x7B 0x55 0x01 0x01 0x7D
n 1 point, we can able to see how uart frame should be. In 2 point, I want to read and write the frame command to stop and start blinkin the led.
My question is
how should I send multiple bytes over uart?
Does I need to send make a frame of packets? If Yes then How should I do
that?
Also, how should I read the response of it?
I did research on how frame the packet and send frame over uart but not found any useful blogs and answer.
More Info:
Language: C
Compiler: GCC
MCU: ESP32
Hope I m able to explain it.
Thanks in advance for the help!!
Sending multiple bytes
Sending multiple bytes is straight-forward with the ESP-IDF framework. Let's assume your command frame is in an array called frame and the length (in bytes) of the frame is stored in frame_length:
uint8_t frame[] = { 0x7B, 0x03, 0x02, 0x05, 0x07, 0x7D };
int frame_length = 6;
uart_write_bytes(uart_port, frame, frame_length);
The bigger challenge is probably how to construct the frame in the first place, in particular how to calculate the checksum.
Sending multiple frames
Sending multiple frames is straight-forward as well. Just call the above function multiple times. The protocol has been carefully designed such that the receiver is able to split the stream of bytes into frames.
You should prevent however that multiple tasks sends frames concurrently. That way the communication could get mixed up.
Receiving frames
Receiving isn't a problem either. Just read frame by frame. It's a two step process:
Read 3 bytes. The third byte provides the length of the frame.
Read the remaining bytes.
It could look like so:
#define MAX_FRAME_LENGTH 80
uint8_t frame[MAX_FRAME_LENGTH];
int frame_length = 0;
int read_frame(uint8_t* frame) {
uart_read_bytes(uart_port, frame, 3, portMAX_DELAY);
uart_read_bytes(uart_port, frame + 3, frame[2] + 4, portMAX_DELAY);
return frame[2] + 4;
}
I am using the TRX library to process the ISO8583 Message. I am receiving a Raw data EOF character. But the last byte is not removed from the buffer as it's not defined in the packager and it's causing an issue in parsing the next transaction. How to manage this?
And while sending a response back how to add EOF character?
Normally, this kind of stuff (protocol characters) is removed before decoding ISO8583 data.
For example, you read 100 bytes from the socket. And it's ISO data + EOF character. You would remove EOF character and process 99 bytes of ISO data through a decoder.
And reverse is true when you send data. You encode your data first then add EOF character. The resulting byte array goes into the socket.
Sorry, I don't know anything about TRX library, but, hopefully, general advice helps you somewhat.
i want to know if we have a simple udp packet that has a string in it how much the size of udp packet will be
for exmaple we have a udp packet that has a string in it
the string is:stackoverflow.com
ok now how much the size of the udp packet will be?
i was thinking that may the size of udp packets that have a text in it be like a size of text file with the same text in packet?
so if the file have this text and the size is 1 kilobites
test
if the packet has same text will the size be same as text file?
The size of a UDP datagram is the size of the data inside it (payload) plus the size of the UDP and IP headers. The payload can be up to 65507 bytes for IPv4 transported over IP with no additional options in the header.
It's up to you how you create the payload. "test" could be as little as 4 bytes using a single-byte character set with no terminator or length indicator. Really, it's up to you to create a payload structure that you are able to decode at your receiver.
Add 8 octets for UDP header, another 12 for IPv4.
It gets a bit more complicated if you need to calculate the lower layer, e.g. ethernet frame
This is the question about bzip2 archive format. Any Bzip2 archive consists of file header, one or more blocks and tail structure. All blocks should start with "1AY&SY", 6 bytes of BCD-encoded digits of the Pi number, 0x314159265359. According to the source of bzip2:
/*--
A 6-byte block header, the value chosen arbitrarily
as 0x314159265359 :-). A 32 bit value does not really
give a strong enough guarantee that the value will not
appear by chance in the compressed datastream. Worst-case
probability of this event, for a 900k block, is about
2.0e-3 for 32 bits, 1.0e-5 for 40 bits and 4.0e-8 for 48 bits.
For a compressed file of size 100Gb -- about 100000 blocks --
only a 48-bit marker will do. NB: normal compression/
decompression do *not* rely on these statistical properties.
They are only important when trying to recover blocks from
damaged files.
--*/
The question is: Is it true, that all bzip2 archives will have blocks with start aligned to byte boundary? I mean all archives created by reference implementation of bzip2, the bzip2-1.0.5+ utility.
I think that bzip2 may parse the stream not as byte stream but as bit stream (the block itself is encoded by huffman, which is not byte-aligned by design).
So, in other words: If grep -c 1AY&SY greater (huffman may generate 1AY&SY inside block) or equal to count of bzip2 blocks in the file?
BZIP2 looks at a bit stream.
From http://blastedbio.blogspot.com/2011/11/random-access-to-bzip2.html:
Anyway, the important bits are that a BZIP2 file contains one or more
"streams", which are byte aligned, each containing one (zero?) or more
"blocks", which are not byte aligned, followed by an end of stream
marker (the six bytes 0x177245385090 which is the square root of pi as
a binary coded decimal (BCD), a four byte checksum, and empty bits for
byte alignment).
The bzip2 wikipedia article also alludes to bit-block alignment (see the File Format section), which seems to be inline from what I remember from school (had to implement the algorithm...).
With reference to this question and to ASIO libary, I would like to know what is the maximum size of ICMP v6 reply packet. I'm using ASIO library to listen for and receive ICMPv6 packets, but I don't know what size of buffer to use in order to prevent buffer over flow. I'm confused by IPv6 supporting extension headers.
Ex code:
asio::streambuf replyBuffer;
replyBuffer.consume(replyBuffer.size());
size_t length = icmpV6Socket->receive(replyBuffer.prepare(65536) );
One of the great features of Boost.Asio's buffers is that they provide protection against buffer overruns. Boost.Asio's buffers pair together a handle to the actual memory and the size. Thus, as long as the size is properly set or deduced, then Boost.Asio operations will not produce buffer overruns.
Nevertheless, the details for ICMPv6 sizes are as follows. IPv6 Header is 40 bytes, and reserves 2 bytes to represent the Payload Length. Thus, the max payload for IPv6, including extension headers, is 65,535. This differs from IPv4, where the Total Length included the header itself. The ICMPv6 Echo Reply header is 8 bytes.
Therefore:
The maximum IPv6 packet is 65,575 bytes (max payload of 65,535 + header of 40).
The maximum IPv6 payload is 65,535 bytes.
The maximum ICMPv6 Echo Reply body is 65,527 bytes. (max payload of 65,535 - ICMPv6 Echo Reply Header of 8).