I am writing a data display program where I receive the data through a serial port. The listener is written by others and it is quite complex. Now I need to transfer the received data to another program/pc. So I am thinking of the standard tcp communication from Qt.
Is there any class that come along with the tcp classes that does job like checksum?
If I am transmitting an array of 10 doubles each time but at high frequency. How could I write a client that received all the data correctly without writing those complex algorithms to check the validity of received data bytes?
TCP/IP includes these checks as part of the protocol itself. This includes guarantees for data integrity, as well as the correct re-assembly of data (i.e it will definitely be in the same order). You mentioned that TCP chops the datastream into packets; this is true, but it will re-assemble the packets in the correct order on the receiving end, or request a re-transmission if it needs to do so. All of this is taken care of by the Qt networking classes.
Related
I raised this question when reading the source code of muduo (C++ network library).
If a client sends a big size message which will be segmented by TCP, what happens in server side? (Does server know this message is already segmented?)
And is it necessary for network library to wait for the whole message and do not interrupt the upper layer?
When dealing with a stream protocol like TCP, you already have to reassemble received data into chunks of your own choosing. That's either a fixed number of bytes per chunk, or it's decided dynamically by parsing the data in terms of your application's protocol (e.g. HTTP).
You don't know when you receive a packet from the network layer that it has been segmented: you only know that you received some data. You may know (because you understand your own protocol) that you're expecting more data to finish the chunk, but you won't know whether there is any more data until you receive it. If you do receive it.
Conversely, a single TCP packet may well contain more than a single chunk of your application-layer data! Again, you need to be aware that there is no direct relationship between the two things.
You can, however, depend on the TCP packets being delivered in the same order in which they were sent, which is nice.
Simple analogy: a big ol' ship, carrying cargo. It may be carrying 40 cars, or it may be carrying just half the quantity of parts required to construct an airplane. Or it may be carrying both! You don't know until you read the shipping manifest and consult your own records on delivery. It's then your responsibility to unpack what you've received and do what you need to do with it.
And is it necessary for network library to wait for the whole message and do not interrupt the upper layer?
If the library wants to pass a full "message" to the upper layer, then usually yes. Some approaches will just block waiting for a full message, but that's not common nowadays. Asynchronous I/O is your friend.
(This was a generic answer, written with no knowledge of what muduo does specifically.)
I've been using boost asio sockets (UDP and TCP) to handle a custom protocol between my client server program. Its been working great until I discovered that on TCP async_send/async_recieve calls that data can arrived in combined chunks.
For example, if I make two send calls each with it's own packet, they can arrive combined at a single receive call. I wrongly assumed that every send corresponds to a receive, but I'm obviously wrong. It however has worked well for the longest time until I found the issue running the client for a different OS.
So my question is: are there any guarantees to the completeness of the data on arrival for every receive call? (e.g. async_send 128 bytes arrive in multiples of 128 bytes, or how it arrives must always be treated as random, like 1 bytes arrives then 127 bytes is possible)
More specifically, does this mean that:
Data can arrive concatenated or partial for every send call, and I
have to always handle the concatenated/partial data manually
Is this true for both UDP and TCP asio sockets?
I searched around and couldn't find any documentation on this so I was wondering if anyone have any idea.
First its important to understand that boost asio socket receive and sends methods just mean that they ordered the underlying network stack to receive or send data. By network stack this could be the windows socket API.
If you are sending data right to the same computer, via so called loopback addresses, the operating system (if there is any) can just "give" it to the listening i.e. receiving program. Thats the scenario where you would be most lucky to get things in order and always complete for all cases.
However if you want you are addressing another computer or because the operating system is in the mood, you will have different behaviour:
TCP was designed that you will get you data in the order you have send it. But the chunks or packet size if will be sent differs even on the same connection and is a key feature of TCP. Your OS or hardware network adapter might do some send or receive buffering too, before informing you. However things won't get lost.
So in short for TCP: You can make sure the data is complete by waiting for a certain point in your data async_read_until is just there for this case. Data from multiple send calls might be in one receive or many
UDP was designed to have a low latency in contrast to TCP, but without its ordering and completeness guarantees. So when you send a UDP datagram i.e. packet, usually the OS and network adapter will try to send it out ASAP. However on the way to the other computer, the internet might loose it, or hold one packet back until the one you send after the first, so that data you send later, could be received later, while you can also get the sent first, later, or might not. But when you receive a datagram it's complete in it self.
So in short for UDP: Data will arrive in datagram chunks, but some datagrams might be missing, or might arrive in another order than sent. The data from one send might be in one receive, might not, or later
So after some more testing here's what I concluded: the answer is no. Boost Asio sockets does not have magic that can enforce data completeness beyond what the TCP/UDP protocols enforces.
Edit:
So here's more of my research:
For TCP, it acts like a data stream. So packets may arrive partial or combined and is complete. So the user application need to handle deserialization of combined or partial data.
For UDP, because it is a datagram packet, if the packet arrives, it is guaranteed to be independent and complete. So there is no need to handle partial or combined packets.
I have a C++ application with GUI that runs (on PC 1) just like a network game, and receives data packets from another computer (2) via WiFi (ad-hoc, so it's quite reliable) at fairly regular intervals (like 40ms), once per loop on program (2). I use send/read.
Here is the problem:
- Packets are not always fully sent (but apparently you can simply keep send()ing the remaining data until all is sent, and thats works well)
- More importantly, packets are stacked in the socket during (1)'s loop until the read() occurs, and then there is no way to distinguish packets in the big stream of data, or know if you were already in the middle of a packet.
I tried to fix this with ID headers (you find an ID as first bytes and you know the length of the packet), but I often get lost (unknown ID : we are not at the beginning of the packet) and am forced to ignore all the remaining data.
So my question is:
Why do packets stack? (generally I have 400B of data whereas my packets are <100B long and fps (1) and (2) are not very different)
How can I have a more reliable way to receive actual packets, say, 80% of packets (discarding packet loss, it's not a question of UDP/TCP)?
Would a separate thread for receiving packets work? (on (1), the server)
How do real-time network games to that (including multiple client management)?
Thanks in advance.
(Sorry I do not have the code here, but I tried to be as clear as I could)
Well:
1) UDP transfers MESSAGES, but is unreliable.
2) TCP transfers BYTE STREAMS, and is reliable.
UDP cannot reliably transfer messages. Anything more reliable requires a protocol on top of UDP.
TCP cannot transfer messages unless they are one byte long. Anything more complex requires a protocol on top of TCP.
Why do packets stack? (generally I have 400B of data whereas my packets are <100B long and fps (1) and (2) are not very different)
Because the time to send packets across the net varies, it typically does not make sense to send packets at a high rate, so most networking libraries (e.g. RakNet) will queue up packets and do a send every 10 ms.
In the case of TCP, there is Nagle's algorithm which is a more principled way of doing the same thing. You can turn Nagle's off by setting the NO_DELAY TCP flag.
How can I have a more reliable way to receive actual packets, say, 80% of packets (discarding packet loss, it's not a question of UDP/TCP)?
If you use TCP, you will receive all of the packets and in the right order. The penalty for using TCP is if a packet is dropped, the packets after it wait until that packet can be resent before they are processed. This results in a noticeable delay, so any games that use TCP have sophisticated prediction techniques to hide this delay and other techniques to smoothly "catch up" once the missing packet arrives.
If you use UDP, you can implement a layer on top that gives you reliability but without the ordering if the order of the packets doesn't matter by sending a counter with each packet and having the receiver repeatedly notify the sender of gaps in the counts. You can also implement ordering by doing something similar. Of course, if you enforce both, then you are creating your own TCP layer. See http://www.jenkinssoftware.com/raknet/manual/reliabilitytypes.html for more details.
What you describe is what would happen if you are using TCP without a protocol on top of it to structure your transmitted data. Your idea of using an ID header and packet length is one such protocol. If you send a 4-byte ID followed by a 4-byte length followed by X number of bytes, then the receiver knows that it has to read 4 bytes followed by 4 bytes followed by X bytes to receive a complete packet. It doesn't get much simplier than that. The fact that you are still having problems reading packets with such a simple protocol suggests that your underlying socket reading code is flawed to begin with. Without seeing your actual code, it is difficult to tell you what you are doing wrong.
I don't mean how to connect to a socket. What should I know about UDP programming?
Do I need to worry about bad data in my socket?
I should assume if I send 200bytes I may get 120 and 60 bytes separately?
Should I worry about another connection sending me bad data on the same port?
If data doesnt arrive typically how long may I (typically) not see data for (250ms? 1 second? 1.75sec?)
What do I really need to know?
"i should assume if i send 200bytes i
may get 120 and 60bytes separately?"
When you're sending UDP datagrams your read size will equal your write size. This is because UDP is a datagram protocol, vs TCP's stream protocol. However, you can only write data up to the size of the MTU before the packet could be fragmented or dropped by a router. For general internet use, the safe MTU is 576 bytes including headers.
"i should worry about another
connection sending me bad data on the
same port?"
You don't have a connection, you have a port. You will receive any data sent to that port, regardless of where it's from. It's up to you to determine if it's from the right address.
If data doesnt arrive typically how
long may i (typically) not see data
for (250ms? 1 second? 1.75sec?)
Data can be lost forever, data can be delayed, and data can arrive out of order. If any of those things bother you, use TCP. Writing a reliable protocol on top of UDP is a very non trivial task and there is no reason to do so for almost all applications.
Should I worry about another
connection sending me bad data on the
same port?
Yes you should worry about it. Any application can send data to your open UDP port at any time. One of the big uses of UDP is many to one style communications where you multiplex communications with several peers on a single port using the addressed passed back during the recvfrom to differentiate between peers.
However, if you want to avoid this and only accept packets from a single peer you can actually call connect on your UDP socket. This cause the IP stack to reject packets coming from any host:port combo ( socket ) other than the one you want to talk to.
A second advantage of calling connect on your UDP socket is that in many OS's it gives a significant speed / latency improvement. When you call sendto on an unconnected UDP socket the OS actually temporarily connects the socket, sends your data and then disconnects the socket adding significant overhead.
A third advantage of using connected UDP sockets is it allows you to receive ICMP error messages back to your application, such as routing or host unknown due to a crash. If the UDP socket isn't connected the OS won't know where to deliver ICMP error messages from the network to and will silently discard them, potentially leading to your app hanging while waiting for a response from a crashed host ( or waiting for your select to time out ).
Your packet may not get there.
Your packet may get there twice or even more often.
Your packets may not be in order.
You have a size limitation on your packets imposed by the underlying network layers. The packet size may be quite small (possibly 576 bytes).
None of this says "don't use UDP". However you should be aware of all the above and think about what recovery options you may want to take.
Fragmentation and reassembly happens at the IP level, so you need not worry about that (Wikipedia). (This means that you won't receive split or truncated packets).
UDP packets have a checksum for the data and the header, so receiving bogus data is unlikely, but possible. Lost or duplicate packets are also possible. You should check your data in any case anyway.
There's no congestion control, so you may wish to consider that, if you plan on clogging the tubes with a lot of UDP packets.
UDP is a connectionless protocol. Sending data over UDP can get to the receiver, but can also get lost during transmission. UDP is ideal for things like broadcasting and streaming audio or video (i.e. a dropped packet is never a problem in those situations.) So if you need to ensure your data gets to the other side, stick with TCP.
UDP has less overhead than TCP and is therefore faster. (TCP needs to build a connection first and also checks data packets for data corruption which takes time.)
Fragmented UDP packets (i.e. packets bigger than about half a Kb) will probably be dropped by routers, so split your data into small chuncks before sending it over. (In some cases, the OS can take care of that.) Note that it is allways a packet that might make it, or not. Half packets aren't processed.
Latency over long distances can be quite big. If you want to do retransmission of data, I would go with something like 5 to 10 times the agerage latency time over the current connection. (You can measure the latency by sending and receiving a few packets.)
Hope this helps.
I won't follow suit with the other people who answered this, they all seem to push you toward TCP, and that's not for gaming at all, except maybe for login/chat info. Let's go in order:
Do I need to worry about bad data in my socket?
Yes. Even though UDP contains an extremely simple checksum for routers and such, it is not 100% efficient. You can add your own checksum device, but most of the time UDP is used when reliability is already not an issue, so data that doesn't conform should just be dropped.
I should assume if I send 200bytes I may get 120 and 60 bytes separately?
No, UDP is direct data write and read. However, if the data is too large, some routers will truncate and you lose part of the data permanently. Some have said roughly 576 bytes with header, I personally wouldn't use more than 256 bytes (nice round log2 number).
Should I worry about another connection sending me bad data on the same port?
UDP listens for any data from any computer on a port, so on this sense yes. Also note that UDP is a primitive and a raw format can be used to fake the sender, so you should use some sort of "key" in order for the listener to verify the sender against their IP.
If data doesnt arrive typically how long may I (typically) not see data for (250ms? 1 second? 1.75sec?)
Data sent on UDP is usually disposable, so if you don't receive data, then it can easily be ignored...however, sometimes you want "semi-reliable" but you don't want 'ordered reliable' like TCP uses, 1 second is a good estimate of a drop. You can number your packets on a rotation and write your own ACK communication. When a packet is received, it records the number and sends back a bitfield letting the sender know which packets it received. You can read this unfinished document for more information (although unfinished, it still yields valiable info):
http://gafferongames.com/networking-for-game-programmers/
The big thing to know when attempting to use UDP is:
Your packets might not all make it over the line, which means there is going to be possible data corruption.
If you're working on an application where 100% of the data needs to arrive reliably to provide functionality, use TCP. If you're working on an application where some loss is allowable (streaming media, etc.) then go for UDP but don't expect everything to get from one of the pipe to the other intact.
One way to look at the difference between applications appropriate for UDP vs. TCP is that TCP is good when data delivery is "better late than never", UDP is good when data delivery is "better never than late".
Another aspect is that the stateless, best-effort nature of most UDP-based applications can make scalability a bit easier to achieve. Also note that UDP can be multicast while TCP can't.
In addition to don.neufeld's recommendation to use TCP.
For most applications TCP is easier to implement. If you need to maintain packet boundaries in a TCP stream, a good way is to transmit a two byte header before the data to delimit the messages. The header should contain the message length. At the receiving end just read two bytes and evaluate the value. Then just wait until you have received that many bytes. You then have a complete message and are ready to receive the next 2-byte header.
This gives you some of the benefit of UDP without the hassle of lost data, out-of-order packet arrival etc.
And don't assume that if you send a packet it got there.
If there is a packet size limitation imposed by some router along the way, your UDP packets could be silently truncated to that size.
Two things:
1) You may or may not received what was sent
2) Whatever you receive may not be in the same order it was sent.
So I'm almost done an assignment involving Win32 programming and sockets, but I have to generate and analyze some statistics about the transfers. The only part I'm having trouble with is how to figure out the number of packets that were sent to the server from the client.
The data sent can be variable-length, so I can't just divide the total bytes received by a #define'd value.
We have to use asynchronous calls to do everything, so I've been trying to increment a counter with every FD_READ message I get for the server's socket. However, because I have to be able to accept a potentially large file size, I have to call recv/recvfrom with a buffer size around 64k. If I send a small packet (a-z), there are no problems. But if I send a string of 1024 characters 10x, the server reports 2 or 3 packets received, but 0% data loss in terms of bytes sent/received.
Any idea how to get the number of packets?
Thanks in advance :)
This really boils down to what you mean by 'packet.'
As you are probably aware, when a TCP/UDP message is sent on the wire, the data being sent is 'wrapped,' or prepended, with a corresponding TCP/UDP header. This is then 'wrapped' in an IP header, which is in turn 'wrapped' in an Ethernet frame. You can see this breakout if you use a sniffing package like Wireshark.
The point is this. When I hear the term 'packet,' I think of data at the IP level. IP data is truly packetized on the wire, so packet counts make sense when talking about IP. However, if you're using regular sockets to send and receive your data, the IP headers, as well as the TCP/UDP headers, are stripped off, i.e., you don't get this information from the socket. And without that information, it is impossible to determine the number of 'packets' (again, I'm thinking IP) that were transmitted.
You could do what others are suggesting by adding your own header with a length and a counter. This information will help you accurately size your receive buffers, but it won't help you determine the number of packets (again, IP...), especially if you're doing TCP.
If you want to accurately determine the number of packets using Winsock sockets, I would suggest creating a 'raw' socket as suggested here. This socket will collect all IP traffic seen by your local NIC. Use the IP and TCP/UDP headers to filter the data based on your client and server sockets, i.e., IP addresses and port numbers. This will give an accurate picture of how many IP packets were actually used to transmit your data.
Not a direct answer to your question but rather a suggestion for a different solution.
What if you send a length-descriptor in front of the data you want to transfer? That way you can already allocate the correct buffer size (not too much, not too little) on the client and also check if there were any losses when the transfer is over.
With TCP you should have no problem at all because the protocol itself handles the error-free transmission or otherwise you should get a meaningful error.
Maybe with UDP you could also split up your transfer into fixed-size chunks with a propper sequence-id. You'd have to accumulate all incoming packages before you sort them (UDP makes no guarantee on the receive-order) and paste the data together.
On the other hand you should think about it if it is really necessary to support UDP as there is quite some manual overhead if you want to get that protocol error-safe... (see the Wikipedia Article on TCP for a list of the problems to get around)
Do your packets have a fixed header, or are you allowed to define your own. If you can define your own, include a packet counter in the header, along with the length. You'll have to keep a running total that accounts for rollover in your counter, but this will ensure you're counting packets sent, rather than packets received. For an simple assignment, you probably won't be encountering loss (with UDP, obviously) but if you were, a packet counter would make sure your statistics reflected the sent message accurately.