Re: [dpdk-dev] tcpdump support in DPDK 2.3

[Matthew Hall <mhall@mhcomputing.net>]

On Mon, Dec 14, 2015 at 04:29:41PM -0500, Kyle Larose wrote:
> I've seen lots of ideas and options tossed around which would solve
> some or all of the above items, but nobody actually committing to
> anything. What can we do to actually agree on a solution to go and
> implement? I'm relatively new to the community, so I don't really know
> how this stuff works. Do people typically form a working group where
> they go off and discuss the problem, and then come back to the main
> community with a proposal? Or do people just submit RFCs independently
> with their own ideas?
> 
> Thanks,
> Kyle

I am getting the impression of a misplaced sense of urgency / panic. I don't 
think anybody came up with a reason why we have to answer all these questions 
tremendously quickly. It will take some more time, particularly with the 
holidays, for the developers to finish the last bug fixes on the current 
release before they have time to discuss 2.3 features.

When that happens, someone working on DPDK full time will be identified as the 
leader for the feature, that will lead the effort on PCAP, and help us 
formulate the plan. Until then, what we really could use at this point is not 
necessarily more writings and speculation, but an answer on some key tech 
questions, particularly from some kernel guys:

1) How do we get the pcap filter string and/or BPF opcode vector from libpcap 
/ tcpdump / tshark / wireshark, into the DPDK application? There we can 
compile it using the user-space bpfjit, so we can filter the packets at very 
high speeds and not end up breaking everything doing a ton of stupid copies 
when somebody does a capture of one flow on his i40e device or such. libpcap 
is crappy about this, as it sends it all over syscalls which are always 
assuming the kernel is on the other end, which is a bad assumption on their 
part but many decades old and not so easy to fix.

2) How do we get the matched packets back out to the extcap or libpcap? From 
what I saw extcap is tshark / wireshark only, which are 1) GPL licensed in 
various ways, 2) not as widely used as libpcap. So using only extcap might be 
kind of crappy.

3) For libpcap to work, maybe it will help if some of our kernel guys can help 
us find out how to "detect" the kernel put a BPF capture filter onto a TUN / 
TAP interface, and copy that filter to the DPDK app. Then, take any matched 
packets and write them back onto the TUN / TAP. This would also be super 
efficient and work with more off-the-shelf tools besides just tshark / 
wireshark.

If we don't find the answers for these items I don't think we have a path to a 
working solution, forgetting about all the nice-to-have points such as UX 
issues, troubleshooting, debugging, etc.

Matthew.