I am streaming raw UDP packets (rf data) from GNU Radio to Octave (or any other program). The data consists of 390625 4 byte floats per second. This is 1562500 bytes per second. When GNU Radio streams UDP, there is no header or sequence number in the UDP data, it's just raw floats. Because this is localhost to localhost, I am able to use a very large MTU.
Attached is a screenshot of Wireshark after right clicking and doing "Follow UDP Stream". There is a "blank" part of the Hex Dump at 0x6F38C8. I don't understand what this means? (I know that UDP does not provide reliable delivery and packets can be dropped and arrive out of order at any moment). Any help would be great!
The blank part is simply used as a barrier to differentiate between 2 UDP packets, solely for your convenience.
If you track down that exact data in the normal wireshark window you'll notice that the data before the blank part belongs to a certain UDP packet and that the data after the blank part belongs to the subsequent UDP packet.
The HEX numbers on the left specify the offset of the first byte of the line from the beginning of the stream in that direction (i.e. in the half blank line, the byte 0x08 has 0x6F38C8 bytes sent before it in that direction). Since the half blank line has just 8 bytes, the offset of the next line is 0x6F38C8 + 0x8 = 0x6F38D0. This is another indicator that the blank part isn't used as a filler for certain data that couldn't be represented due to some obscure reason.
Note that it would be impossible for wireshark to know whether data is lost and represent it to you in any manner, since UDP is unreliable (that is, unless the underlying protocol maintains certain counters by itself, and is parsed by wireshark).
Related
Digging around with/for HID reports, I ran into a strange problem within a USB HID device. I'm implementing an HID class device and have based my program on the HID USB program supplied by Keil. Some codes have been changed in this project and it seems working fine with 32 bytes input and 32 bytes output reports. Somehow, after thousands times data transferring, the Endpoint 1 out would hang and become a bad pipe. Then I searched the google for some tips, a topic remind me that we should write a data length zero packet after sending a length of packet match what you defined in the report description. But it's not working for me. Then I write a data length zero to the control pipe after I receive a out packet and magically, it works! It would never hang after million times transferring!
Here is my question: Why does it works after writing a data length zero to a control pipe. The data transferring in the out pipe should have no relationship with the data in the control pipe. It confuses me!
If you transfer any data that is less than the expected payload size, you must send a Zero Length Packet to indicate that data has transferred.
But it depends heavily on the implementation on the host controller, and not all devices follow the specification to the point and may stall.
Source:
When do USB Hosts require a zero-length IN packet at the end of a Control Read Transfer?
I need to send serialized data by satellite, which involves sending data as packets that have a maximum size of 2kB each.
Is there a built-in/easy way to serialize data with protobuf into packets limited to X size? And then rebuild them on the other end?
Protobuf always serializes a message into one byte array which is as long as is needed to represent the data.
Your best bet is to split the bytes into chunks an a lower layer, then re-assemble at the other end.
In all likelihood, these packets are not reliably delivered, and so you will also need a mechanism for acknowledging packets, re-transmitting dropped packets, congestion control, etc. These are all things that TCP normally does for you. If I were you, I would look for an existing TCP implementation -- or something like it -- that can sit on top of your satellite link.
I am using a SAM4E-EK board, and the processor is SAM4E. The board is equiped with a ADS7843 touch controller, contected from the processor through a SPI channel.
The chapter of SPI in datasheet of SAM4E said that
While the data in the Shift Register is shifted on the MOSI line, the MISO line is sampled and shifted in the Shift Register. Receiving data cannot occur without transmitting data.
But in an example for ADS7843 from ASF, it just sends data(8 bits) three times at first, and then it can receives data(8 bits) three times! I have test it, and it work fine.
So I think there is a hardware FIFO buffer in SPI receiver. But I can not find any related information in the datasheet and internet.
Am I right? or is there others mechanism making the example runs correctly?
The SAM4E manual says that SPI Receive Data Register has the size of 2 bytes
The ADS7843 manual says:
One complete conversion can be accomplished with three serial
communications, for a total of 24 clock cycles on the DCLK input
The figure 5 shows that byte0 is a request to ADS7843 and bytes1-2 are response.
You should send 1 byte of command and 2 dump bytes to provide SPI clocking while the ADS7843 is responsing (the manual says
Receiving data cannot occur without transmitting data
And when you reading 3 bytes, you get the 2 bytes of answer, stored in the SPI Receiver register
I am writing code for a USB device. Suppose the USB host starts a control read transfer to read some data from the device, and the amount of data requested (wLength in the Setup Packet) is a multiple of the Endpoint 0 max packet size. Then after the host has received all the data (in the form of several IN transactions with maximum-sized data packets), will it initiate another IN transaction to see if there is more data even though there can't be more?
Here's an example sequence of events that I am wondering about:
USB enumeration process: max packet size on endpoint 0 is reported to be 64.
SETUP-DATA-ACK transaction starts a control read transfer, wLength = 128.
IN-DATA-ACK transaction delivers first 64 bytes of data to host.
IN-DATA-ACK transaction delivers last 64 bytes of data to host.
IN-DATA-ACK with zero-length DATA packet? Does this transaction ever happen?
OUT-DATA-ACK transaction completes Status Phase of the transfer; transfer is over.
I tested this on my computer (Windows Vista, if it matters) and the answer was no: the host was smart enough to know that no more data can be received from the device, even though all the packets sent by the device were full (maximum size allowed on Endpoint 0). I'm wondering if there are any hosts that are not smart enough, and will try to perform another IN transaction and expect to receive a zero-length data packet.
I think I read the relevant parts of the USB 2.0 and USB 3.0 specifications from usb.org but I did not find this issue addressed. I would appreciate it if someone can point me to the right section in either of those documents.
I know that a zero-length packet can be necessary if the device chooses to send less data than the host requested in wLength.
I know that I could make my code flexible enough to handle either case, but I'm hoping I don't have to.
Thanks to anyone who can answer this question!
Read carefully USB specification:
The Data stage of a control transfer from an endpoint to the host is complete when the endpoint does one of
the following:
Has transferred exactly the amount of data specified during the Setup stage
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
So, in your case, when wLength == transfer size, answer is NO, you don't need ZLP.
In case wLength > transfer size, and (transfer size % ep0 size) == 0 answer is YES, you need ZLP.
In general, USB uses a less-than-max-length packet to demarcate an end-of-transfer. So in the case of a transfer which is an integer multiple of max-packet-length, a ZLP is used for demarcation.
You see this in bulk pipes a lot. For example, if you have a 4096 byte transfer, that will be broken down into an integer number of max-length packets plus one zero-length-packet. If the SW driver has a big enough receive buffer set up, higher-level SW receives the entire transfer at once, when the ZLP occurs.
Control transfers are a special case because they have the wLength field, so ZLP isn't strictly necessary.
But I'd strongly suggest SW be flexible to both, as you may see variations with different USB host silicon or low-level HCD drivers.
I would like to expand on MBR's answer. The USB specification 2.0, in section 5.5.3, says:
The Data stage of a control transfer from an endpoint to the host is
complete when the endpoint does one of the following:
Has transferred exactly the amount of data specified during the Setup stage
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
When a Data stage is complete, the Host Controller advances to the
Status stage instead of continuing on with another data transaction.
If the Host Controller does not advance to the Status stage when the
Data stage is complete, the endpoint halts the pipe as was outlined in
Section 5.3.2. If a larger-than-expected data payload is received from
the endpoint, the IRP for the control transfer will be
aborted/retired.
I added emphasis to one of the sentences in that quote because it seems to specifically say what the device should do: it should "halt" the pipe if the host tries to continue the data phase after it was done, and it is done if all the requested data has been transmitted (i.e. the number of bytes transferred is greater than or equal to wLength). I think halting refers to sending a STALL packet.
In other words, the device does not need a zero-length packet in this situation and in fact the USB specification says it should not provide one.
You don't have to. (*)
The whole point of wLength is to tell the host the maximum number of bytes it should attempt to read (but it might read less !)
(*) I have seen devices that crash when IN/OUT requests were made at incorrect time during control transfers (when debugging our host solution). So any host doing what you are worried about, would of killed those devices and is hopefully not in the market.
I am trying to write an app that exchanges data with other iPhones running the app through the Game Kit framework. The iPhones discover each other and connect fine, but the problems happens when I send the data. I know the iPhones are connected properly because when I serialize an NSString and send it through the connection it comes out on the other end fine. But when I try to archive a larger object (using NSKeyedArchiver) I get the error message "AGPSessionBroadcast failed (801c0001)".
I am assuming this is because the data I am sending is too large (my files are about 500k in size, Apple seems to recommend a max of 95k). I have tried splitting up the data into several transfers, but I can never get it to unarchive properly at the other end. I'm wondering if anyone else has come up against this problem, and how you solved it.
I had the same problem w/ files around 300K. The trouble is the sender needs to know when the receiver has emptied the pipe before sending the next chunk.
I ended up with a simple state engine that ran on both sides. The sender transmits a header with how many total bytes will be sent and the packet size, then waits for acknowledgement from the other side. Once it gets the handshake it proceeds to send fixed size packets each stamped with a sequence number.
The receiver gets each one, reads it and appends it to a buffer, then writes back to the pipe that it got packet with the sequence #. Sender reads the packet #, slices out another buffer's worth, and so on and so forth. Each side keeps track of the state they're in (idle, sending header, receiving header, sending data, receiving data, error, done etc.) The two sides have to keep track of when to read/write the last fragment since it's likely to be smaller than the full buffer size.
This works fine (albeit a bit slow) and it can scale to any size. I started with 5K packet sizes but it ran pretty slow. Pushed it to 10K but it started causing problems so I backed off and held it at 8096. It works fine for both binary and text data.
Bear in mind that the GameKit isn't a general file-transfer API; it's more meant for updates of where the player is, what the current location or other objects are etc. So sending 300k for a game doesn't seem that sensible, though I can understand hijacking the API for general sharing mechanisms.
The problem is that it isn't a TCP connection; it's more a UDP (datagram) connection. In these cases, the data isn't a stream (which gets packeted by TCP) but rather a giant chunk of data. (Technically, UDP can be fragmented into multiple IP packets - but lose one of those, and the entire UDP is lost, as opposed to TCP, which will re-try).
The MTU for most wired networks is ~1.5k; for bluetooth, it's around ~0.5k. So any UDP packet that you sent (a) may get lost, (b) may be split into multiple MTU-sized IP packets, and (c) if one of those packets is lost, then you will automatically lose the entire set.
Your best strategy is to emulate TCP - it sends out packets with a sequence number. The receiving end can then request dupe transmissions of packets which went missing afterwards. If you're using the equivalent of an NSKeyedArchiver, then one suggestion is to iterate through the keys and write those out as individual keys (assuming each keyed value isn't that big on its own). You'll need to have some kind of ACK for each packet that gets sent back, and a total ACK when you're done, so the sender knows it's OK to drop the data from memory.