In MQTT Packets there is something called the remaining length field. This signifies the actual length of an MQTT Message.
The MQTT Documentation says that the remaining length is encoded by using something called "Continuation bit" Encoding Scheme.
My Question is this:
What is the need for "Encoding" the remaining length? Can't we just mention the length of the packet directly?
We can easily transmit the same length with fewer bytes. MQTT was designed to be lightweight right? Then why use this Encoding Scheme?
Please enlighten me. Or am I thinking about this in the wrong point of view?
Any help would be nice.
Thanks In Advance for your time.
Related
We are using fix earlier but client say implement OUCH for placing trade since that's faster . I checked on internet and message looks quite similar. How it get speed benefit. Please give me some example message
Basically, FIX message is transferred in text format, while OUCH transfers messages in binary format, which leads to OUCH could transfer message in slim size and occupy less bandwidth over the network; for achieving higher performance over transmission.
Everything has a trade-off. Below has listed some comparing items between both for a reference.
Data Format
FIX: Text
OUCH: Binary
Data Size
FIX: Big
OUCH: Slim
Data Length
FIX: Variable
OUCH: Fixed
Data Sequence
FIX: Explicit
OUCH: Implicit
Universality
FIX: Industrial standard; generally supported & accepted by most exchanges and clients
OUCH: Proprietary; limited to specific exchanges
The main difference is that OUCH is a protocol that is specific to the NASDAQ stock exchanges. The FIX protocol is a standard that is supported by most exchanges.
The OUCH protocol is a faster protocol as it is designed as a low level native (binary) protocol, but as it is specific to NASDAQ exchanges you will have to have another module based on FIX to connect and trade on other exchanges.
is there a way to easily add a Metadata to a verilog bit? My goal is to be able to identify certain bits that are well known prior to encryption, after an ethernet frame is being encrypted. I'd like to easily identify these bits location in the encrypted frame. I'd like this Metadata to be transparent to the actual design rtl (i.e. Allow it to flow naturally through external IPs that are not mine, and be recovered and analyzed on the other end).
Thanks
There is absolutely no way to do this using the original RTL path.
You were not clear about your reasoning for this, but sometimes people use a watermark which is encoding something into your data which is inconsequential to the design, but has meaning to your verification environment. For example, instead of sending completely random data in a packet, you send data with a specific checksum that has meaning to your verification environment.
I need to store email messages as soon as they are fetched from IMAP in the database for later processing. I extract the message using a FETCH request and data is returned using BODY.PEEK[].
From my understanding, all IMAP messages are returned as US-ASCII (the mail servers accept only that), but I could be wrong.
My options (in order of what I think it's right) are:
US-ASCII text column
Bytea
BLOB
I was thinking about using US-ASCII but I'm afraid of having problems with encoding, I don't know if there are "faulty" IMAP servers not returning us-ascii mails.
The alternative is Bytea, but I read you have to deal with encoding, so I'm not sure what's the advantage/disadvantage over US-ASCII.
BLOB is raw, and I'm not sure about the problems it deliver in this case. I assume I have to deal with the bytes-to-string conversion.
What's the recommended data type?
For small objects such as emails, I think you're going to be better off with Bytea. The storage and handling is different and since your objects are going to be small, it seems like it would be handled better as Bytea. See here for a comparison of the two by Microolap. That's not a full answer to your question but might take one option off the list.
You're making the very much unwarranted assumption that you can avoid dealing with encodings.
You can't.
Whether you use lob, bytea, or a text column that you assume contains 7-bit mail only... the mail is just arbitrary binary data. You do not know its text encoding. In practice mail clients have used 8-bit encoding forever; either standards-compliant via MIME quoted-printable, or often simply raw 8-bit text.
Some clients have even been known to include full 8-bit MIME segments that include null (zero) bytes. PostgreSQL won't tolerate that in a text column.
But even for clients using compliant MIME, quoted-printable escaping text bodies, etc... the mail may contain non-ASCII chars, they're just escaped. Indexing these and ignoring the escapes will yield weird and wrong results. Also, attachments will usually be arbitrary base64 data. Indexing this as text is utterly meaningless. Then there's all the HTML bodies, multi-part/alternative segments, CSS, etc...
When dealing with email, assume that anything a client or server can do wrong, it will do wrong. For storage, treat the email as raw bytes of unknown encoding. That's exactly what bytea is for.
If you want to do anything with the mail you'll need a defensive MIME parser that can extract the MIME parts, cope with broken parts, etc. It'll need to check the declared encoding (if any) against the actual mime-part body, and guess encodings if none are declared or the declared encoding is obviously wrong. It'll have to deal with all sorts of bogus MIME structure and contents; quoted-printable bodies that aren't really quoted-printable, and all that.
So if you plan to index this email, it's definitely not as simple as "create a fulltext index and merrily carry on". The question with that is not if it will fail but when.
Personally, if I had to do this (and given the choice I wouldn't) I'd store the raw email as bytea. Then for search I'd decompose it into MIME parts, detect text-like parts, do encoding detection and dequoting, etc, and inject the decoded and cleaned up text bodies into a separate table for text indexing.
There are some useful Perl modules for this that you can possibly use via plperlu, but I'd likely do it in an outside script/tool instead. Then you have your choice of MIME processors, languages, etc.
I'm working on a project where I need to send a value between two pieces of hardware using CoDeSys. The comms system in use is CAN and is only capable of transmitting in Bytes, making the maximum value 255.
I need to send a value higher than 255, I'm capable of splitting this over more than one byte and reconstructing it on the receiving machine to get the original value.
I'm thinking I can divide the REAL value by 255 and if the result is over 1 then deconstruct the value in to one byte holding the remainders and one byte holding the amount of 255's in the whole number.
For example 355 would amount to one byte of 100 and another of 1.
Whilst I can describe this, I'm having a really hard time figuring out how to actually write this in logic.
Can anyone help here?
This is all handled for you in CoDeSys if I understand you correctly.
1. CAN - Yes it's in byte but you must not be using CANopen you are using the low level FB that ask you to send a CAN frame of an 8 byte array?
If it is your own two custom controllers ( you are programming both of them in CoDeSys) just use netvariables. Netvariables allows you to transfer any type of variable and you can take the variable list from one controller and import it to another controller and all the data will show up. You don't have to do any variable manipulation it's handle under the hood for you. But I don't know the specifics of your system and what you are trying to do.
If you are trying to deconstruct construct variables from one size to another that is easy and I can share that code with you.
I am using go and protocol buffers. go program sends encoded protocol buffer messages to clients connected to the socket. Now since protocol-buffers is not delimited, clients don't know how much data to read from the socket.
I am planning to prefix the message with the message length, a 32bit integer. So clients can read 4 bytes, get the length and read the full message.
I can put an integer value into the bytes array using binary package. Something like,
binary.Write(buf, binary.LittleEndian, value)
Now the question is, write needs a byte order and how will the receiving end know what is the byte order? Is there a way to deal with this without specifying explicit byte order?
The convention is that network byte order is big endian, but your client and server must agree in any case. Specify the byte order explicitly.
Edit: Reading the Protobuf docs, it might be better to use proto.EncodeVarint and proto.DecodeVarint to write the length before the message, for the sake of consistency.
You should always explicitly define and document the byte order (and layout) on such things. In general communications, big-endian direct layout seems to be the norm (so 47 would be 4 bytes with hex values 00 00 00 2F). However, you might want to take specific context into consideration. For example, if you are already talking protobuf, you could also use the "varint" encoding format, in which case 47 is a single byte: 2F - many protobuf implementations will provide pre-rolled methods to consume or produce a varint (otherwise, it is documented here - but can be summarised as 7-bit payload with continuation bit, with the least-significant 7-bit group first)