Hello.
I have encountered a weird performance problem in Linux IP fragmentation
when using video streaming services behind the NAT. Also I have studied
a possible bug in the DF bit (don't fragment) handling when forwarding
the IP packets.
First the system setup description:
[host1]-int lan-(eth1)[linux router](eth0)-extlan-[fibre router]-internet
where:
host1: is a Netgem N7800 "cable box" for online video streaming services
provided by local telco (Can access Netflix, HBO nordic, "live TV", etc.)
linux router: Linux computer with Dualcore Intel Celeron G1840, running
currently Linux kernel 4.20.0-rc2, and openSUSE Leap 15.0
eth1: Linux Routers internal (NAT) interface, 192.168.0.1/24 network,
mtu set to 1500, RTL8169sb/8110sb
eth0: Linux Routers internet facing interface, public ip address, mtu
set to 1500, RTL8168evl/8111evl
fibre router: Alcatel Lucent fibre router (I-241G-Q), directly connected
to the eth0 of the Linux router.
And now when using the Netgem N7800 with online video services (Netflix,
HBO nordic, etc) the Linux router will receive very BIG IP packets in
the eth0 upto ~20kB, this seems to lead to the following problems in the
Linux IP stack.
IP fragmentation performance:
When the Linux router receives these large IP packets in the eth0
everything works, but it seems that them cause very large performance
degradation from internal network to the internet regarding the latency
when the IP fragmentation is performed. The ping latency from internal
network to the internel network increases from stable 15ms-20ms up to
700-800ms AND also the ping from the internal network to the linux
router eth1 (192.168.0.). However up link works perfectly, the ping is
still stable when streaming the online services (From linux router to
the internet). It seems that the IP fragmentation is somehow blocking
the eth1 reception or transmission for very long time (which it
shouldn't). I'm able to test and debug the issue further, but advice
regarding where to look would be appreciated.
DF Bit, mtu bug when forwarding:
I have started to study the above mentioned problem and have found a
possible bug in the DF bit and mtu handling in IP forwarding. The BIG
packets received from streaming services all have the "DF bit" set and
the question is that should we be forwarding them at all as that would
result them being fragmented? Apparently we currently are... I have
traced this down to the ip_forward.c function ip_exceeds_mtu(), and the
following patch seems to fix that.
--- net/ipv4/ip_forward.c.orig 2018-12-02 11:09:32.764320780 +0200
+++ net/ipv4/ip_forward.c 2018-12-02 12:53:25.031232347 +0200
@@ -49,7 +49,7 @@ static bool ip_exceeds_mtu(const struct
return false;
/* original fragment exceeds mtu and DF is set */
- if (unlikely(IPCB(skb)->frag_max_size > mtu))
+ if (unlikely(skb->len > mtu))
return true;
if (skb->ignore_df)
This seems to work (in some ways) - after the change IP packets that are
too large to the internal network get dropped and we are sending "ICMP
Destination unreachable, The datagram is too big" messages to the
originator (as we should?). However it seems that not all services
really like this... Netflix behaves as expected and ping is stable from
internal network to the internet, but for example HBO nordic will not
work anymore (too little buffering? Retransimissions not working?). So
it seems the original issue should be also fixed (And the fragmention
should be allowed?).
Any advice would be appreciated. Thanks!
PS. Watching TV was not this intensive 20 years ago :)
--
Risto Pajula
or.paj...@gmail.com
