On 18/8/17 11:33 am, Mike Karels wrote:
Another $.02 (inline):
On 17 Aug 2017, at 18:39, Gopakumar Pillai wrote:
Thank You Bjoern. Could you please point me to the RFC?
I don’t know if there is anything more recent than RFC1122 on this.
IIRC, it requires queuing at least one packet. Queing one packet is
what BSD has done essentially since ARP was implemented.
This asks the question: One physical packet or one logical packet?
Gopakumar's change effectively changes the queuing from one physical
packet to the logical one.
The next question becomes "how much extra work do we do to achieve
this and does it affect anything else"?
If this is not a MUST behavior in RFC, would my fix be good? I
agree that this would affect only ICMP/UDP traffic.
People have been asking for queuing of multiple packets for years.
That is a more general change. Consider another dumb application
that starts out by sending multiple UDP packets back-to-back.
However, well-designed application protocols don’t experience
problems like this. I’ll quickly note that ping isn’t an
application, but a network measuring tool. If you ask the question
“what happens if I start off a session with a single large packet
and I don’t support retransmission”, ping answers that question
correctly.
If badly-designed protocols get bad performance, that doesn’t seem
like a bug to me, but a feature.
On 8/17/17, 2:40 PM, "Bjoern A. Zeeb"
<bzeeb-li...@lists.zabbadoz.net> wrote:
On 17 Aug 2017, at 21:16, Gopakumar Pillai wrote:
> Hi FreeBSD Networking Gurus,
> I came across an issue with an old version of FreeBSD and
looking at
> the latest FreeBSD code, seems it exists even now. I am
assuming that
> this issue is not reported.
>
> Observation:
> When a ping was performed with larger payload than MTU, the
first ping
> failed when the ARP entry was absent for that IP.
That is because ping/ICMP has no retransmit.
> Noticed on the wire that the last IP fragment was sent for
the first
> request and then the subsequent requests were fine.
>
> Root Cause:
> * ip_output fragments the packets and loops through the
fragments to
> send them to ether_output.
> * ether_output does an arpresolve and if there is no
existing ARP
> entry it'll return EWOULDBLOCK after sending ARP Request.
> * ether_output ignores the error and propagates success to
ip_output
> and it continues to send the remaining fragments.
> * llentry keeps only one mbuf and the last fragment is
retained when
> the ARP Reply comes and the fragment is sent.
Yes, according to the spec (RFC) we are supposed to throw the
packet
away entirely and simply report that to the next upper layer.
However
over the years people realised that this sucks for a TCP SYN
packet with
a retransmit timer and hence we store one of them.
A large UDP packet would btw see the same behaviour to your ping.
There’s no guarantee any of these packets will not be dropped
anywhere
on the network, so we can as well.
Just my 2ct
/bz
Mike
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