tcpdump mailing list archives
Re: New link-type for Z-Wave serial interface
From: "Mikhail Gusarov" <dottedmag () dottedmag net>
Date: Sun, 21 Jul 2019 23:19:28 +0200
Hi Guy, On 21 Jul 2019, at 23:07, Guy Harris wrote:
So a "garbage frame" would be any frame that begins with a value other than 0x06, 0x15, 0x18, or 0x01?
Is there any reason for whatever capture mechanism produces these packets not to discard garbage frames rather than handing them to the libpcap caller (if this is implemented as a libpcap module) or writing them to a file (if this is implemented as a program that writes pcap or pcapng files)?
Not really. These bytes are useless without the framing, and there are at most 257 of them (frame length
is a single byte).
Now that I think about it, in a very rare case it is possible to not be able to synchronize at all: if a SOF byte is encountered inside the data frame, then the next byte will be interpreted as a length,If by "SOF byte" you mean "byte with the value 0x01", then an SOF byte will be encountered inside every frame of type Response, so it seems pretty clear that a data-frame-reading algorithm of "read andaccumulate bytes until you see an 0x01" won't work.Or do you mean that if the receiver has already determined that it's out of sync (for example, due to losing bytes), and it waits for an 0x01 (or possibly an ACK/NAK/CAN), it might see a 0x01 that's partof frame data, and think that's the beginning of a frame?
This case. I only expect it to happen in the beginning: traffic over Z-Wave is quite low, and the protocol is full of "send a frame and wait an acknowledgement from the chip that it was sent over the radio" so I don't expect to ever see UART overflows here (is "UART overflow" a thing with USB-serial
at all?). Best, Mikhail. _______________________________________________ tcpdump-workers mailing list tcpdump-workers () lists tcpdump org https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers
Current thread:
- New link-type for Z-Wave serial interface Mikhail Gusarov (Jul 21)
- Re: New link-type for Z-Wave serial interface Guy Harris (Jul 21)
- Re: New link-type for Z-Wave serial interface Mikhail Gusarov (Jul 21)
- Re: New link-type for Z-Wave serial interface Guy Harris (Jul 21)
- Re: New link-type for Z-Wave serial interface Guy Harris (Jul 21)
- Re: New link-type for Z-Wave serial interface Mikhail Gusarov (Jul 21)
- Re: New link-type for Z-Wave serial interface Guy Harris (Jul 22)
- Re: New link-type for Z-Wave serial interface Mikhail Gusarov (Jul 22)
- Re: New link-type for Z-Wave serial interface Guy Harris (Jul 23)
- Re: New link-type for Z-Wave serial interface Mikhail Gusarov (Jul 21)
- Re: New link-type for Z-Wave serial interface Guy Harris (Jul 21)