svn commit: r289276 - in head/sys: conf kern netinet sys

[Bryan Drewery]

On 10/13/2015 5:35 PM, Hiren Panchasara wrote:
> Author: hiren
> Date: Wed Oct 14 00:35:37 2015
> New Revision: 289276
> URL: https://svnweb.freebsd.org/changeset/base/289276
> 
> Log:
>   There are times when it would be really nice to have a record of the last few
>   packets and/or state transitions from each TCP socket. That would help with
>   narrowing down certain problems we see in the field that are hard to reproduce
>   without understanding the history of how we got into a certain state. This
>   change provides just that.
>   
>   It saves copies of the last N packets in a list in the tcpcb. When the tcpcb is
>   destroyed, the list is freed. I thought this was likely to be more
>   performance-friendly than saving copies of the tcpcb. Plus, with the packets,
>   you should be able to reverse-engineer what happened to the tcpcb.
>   
>   To enable the feature, you will need to compile a kernel with the TCPPCAP
>   option. Even then, the feature defaults to being deactivated. You can activate
>   it by setting a positive value for the number of captured packets. You can do
>   that on either a global basis or on a per-socket basis (via a setsockopt call).
>   
>   There is no way to get the packets out of the kernel other than using kmem or
>   getting a coredump. I thought that would help some of the legal/privacy concerns
>   regarding such a feature. However, it should be possible to add a future effort
>   to export them in PCAP format.
>   
>   I tested this at low scale, and found that there were no mbuf leaks and the peak
>   mbuf usage appeared to be unchanged with and without the feature.
>   
>   The main performance concern I can envision is the number of mbufs that would be
>   used on systems with a large number of sockets. If you save five packets per
>   direction per socket and have 3,000 sockets, that will consume at least 30,000
>   mbufs just to keep these packets. I tried to reduce the concerns associated with
>   this by limiting the number of clusters (not mbufs) that could be used for this
>   feature. Again, in my testing, that appears to work correctly.
>   
>   Differential Revision:	D3100

You're supposed to use the full URL here which will auto close the review.

I also replied to the review with style findings just now.

>   Submitted by:		Jonathan Looney <jlooney at juniper dot net>
>   Reviewed by:		gnn, hiren


-- 
Regards,
Bryan Drewery

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