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I'm doing R&D in SDN. In this case scenario, we use MPLS labeling, and Open vSwitch as software-switches. We are using 2 server nodes with ovs 2.6.0, with kernel modules loaded, and 2 hosts.

They are directly connected through 1 Gigabit Ethernet connections, and there is arround 1 ms of rtt, and in the case of the first packet less than 3 ms (using ping utility). Im using Iperf3 for doing the tests. The first test is the performance reached without using mpls labeling, and the second test is using mpls labeling. The MTU is adjusted to not doing fragmentation, so this is not the problem. I tried adjusting the congestion window and other parameters like TCP algorithm used.

mar jul  4 12:21:09 CEST 2017
Connecting to host 192.168.20.2, port 5201
[  4] local 192.168.20.1 port 43526 connected to 192.168.20.2 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec   112 MBytes   943 Mbits/sec    0    450 KBytes
[  4]   1.00-2.00   sec   112 MBytes   937 Mbits/sec    0    516 KBytes
[  4]   2.00-3.00   sec   112 MBytes   938 Mbits/sec    0    571 KBytes
[  4]   3.00-4.00   sec   112 MBytes   937 Mbits/sec    0    625 KBytes
[  4]   4.00-5.00   sec   112 MBytes   943 Mbits/sec    0    633 KBytes
[  4]   5.00-6.00   sec   111 MBytes   933 Mbits/sec    0    633 KBytes
[  4]   6.00-7.00   sec   111 MBytes   933 Mbits/sec    0    664 KBytes
[  4]   7.00-8.00   sec   112 MBytes   944 Mbits/sec    0    664 KBytes
[  4]   8.00-9.00   sec   111 MBytes   933 Mbits/sec    0    697 KBytes
[  4]   9.00-9.16   sec  18.8 MBytes   977 Mbits/sec    0    697 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-9.16   sec  1.00 GBytes   939 Mbits/sec    0             sender
[  4]   0.00-9.16   sec  1022 MBytes   935 Mbits/sec                  receiver

iperf Done.
<----------->
mar jul  4 12:40:10 CEST 2017
Connecting to host 192.168.20.2, port 5201
[  4] local 192.168.20.1 port 43530 connected to 192.168.20.2 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec   203 KBytes  1.66 Mbits/sec   57   2.82 KBytes
[  4]   1.00-2.00   sec   398 KBytes  3.26 Mbits/sec  124   2.82 KBytes
[  4]   2.00-3.00   sec   400 KBytes  3.28 Mbits/sec  124   2.82 KBytes
[  4]   3.00-4.00   sec   319 KBytes  2.61 Mbits/sec  124   2.82 KBytes
[  4]   4.00-5.00   sec   398 KBytes  3.26 Mbits/sec  126   2.82 KBytes
[  4]   5.00-6.00   sec   395 KBytes  3.24 Mbits/sec  124   2.82 KBytes
[  4]   6.00-7.00   sec   398 KBytes  3.26 Mbits/sec  126   2.82 KBytes
[  4]   7.00-8.00   sec   324 KBytes  2.66 Mbits/sec  124   2.82 KBytes
[  4]   8.00-9.00   sec   398 KBytes  3.26 Mbits/sec  124   2.82 KBytes
[  4]   9.00-10.00  sec   400 KBytes  3.28 Mbits/sec  126   2.82 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  3.55 MBytes  2.98 Mbits/sec  1179             sender
[  4]   0.00-10.00  sec  3.42 MBytes  2.87 Mbits/sec                  receiver

I know there are issues using MPLS and using ovs, but there are some facts that are weird in this case:

  • If i use UDP instead of TCP, there is one packet out of order, but the rest are good, so packets are using kernel datapath i guess.
  • There are 9 packets lost at the start of the TCP transmission, and there are more packets lost periodically. Looking the tcpdump traces, those packets are "missing" in the first node, because in the second hop they are not captured.
  • As you can see above, the performance using TCP w/o MPLS labeling is very good.

Is there someone who knows how to tackle this problem?

PD: Excuse me for my possible english typos.

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After doing some research, I managed to isolate the problem and to solve it. The NIC was with some checksum and offload options activated, forcing the packet to go to userspace, I guess that being encapsulated was the problem here.

Using ethtool utility we can disable this offloads. In this case, i used (as root):

ethtool -K <iface> gso off && ethtool -K <iface> tso off && ethtool -K <iface> gro off

To disable TCP Segmentation Offload (TSO), Generic Segmentation Offload (GSO) and Generic Receive Offload (GRO). Also rx and tx checksumming are disabled.

sources: Source 1 Source 2

I hope this helps someone when the time comes.

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