[ovs-discuss] TCP Load Balancers and PMTU discovery

Miguel Angel Ajo majopela at redhat.com
Fri Dec 11 16:53:11 UTC 2020


Do we know if OVN Loadbalancers will forward ICMPs related to a load
balancer connection, or who could I ask?

Please note that
 - 172.30.23.156 is the VIP of my TCP load balancer, attached to a router.
 - 10.129.2.2 is the gateway which will find the bigger than can transit
packet and respond with the unreachable / need to frag packet
 - 10.134.0.46 is the client, behind the gateway, that connects
to 172.30.23.156 on port 1234

At the start the client will be seen sending a 3200 packet to the VIP that
works, but that's probably a reassembled packet after delivery, since
the path between the gateway and that client is a tunnel with <1500 MTU.

16:40:57.953197 IP 10.134.0.46.35443 > 172.30.23.156.1234: Flags [S], seq
3131207079, win 26583, options [mss 8861,sackOK,TS val 2995034450 ecr
0,nop,wscale 7], length 0
16:40:57.954352 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [S.], seq
1743781087, ack 3131207080, win 26547, options [mss 8861,sackOK,TS val
301081336 ecr 2995034450,nop,wscale 7], length 0
16:40:57.955760 IP 10.134.0.46.35443 > 172.30.23.156.1234: Flags [.], ack
1, win 208, options [nop,nop,TS val 2995034453 ecr 301081336], length 0
16:40:57.955827 IP 10.134.0.46.35443 > 172.30.23.156.1234: Flags [F.], seq
3201, ack 1, win 208, options [nop,nop,TS val 2995034453 ecr 301081336],
length 0
16:40:57.956467 IP 10.134.0.46.35443 > 172.30.23.156.1234: Flags [P.], seq
1:3201, ack 1, win 208, options [nop,nop,TS val 2995034453 ecr 301081336],
length 3200
16:40:57.956478 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [.], ack
1, win 208, options [nop,nop,TS val 301081339 ecr 2995034453,nop,nop,sack 1
{3201:3202}], length 0
16:40:57.956507 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [.], ack
3202, win 258, options [nop,nop,TS val 301081339 ecr 2995034453], length 0
16:40:57.957069 IP 10.134.0.46.35443 > 172.30.23.156.1234: Flags [.], seq
1:1387, ack 1, win 208, options [nop,nop,TS val 2995034455 ecr 301081339],
length 1386
16:40:57.957097 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [.], ack
3202, win 258, options [nop,nop,TS val 301081340 ecr
2995034453,nop,nop,sack 1 {1:1387}], length 0
16:40:57.960404 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [P.], seq
1:3151, ack 3202, win 258, options [nop,nop,TS val 301081343 ecr
2995034453], length 3150
16:40:57.960469 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [F.], seq
3151, ack 3202, win 258, options [nop,nop,TS val 301081343 ecr 2995034453],
length 0
*16:40:57.960939 IP 10.129.2.2 > 172.30.23.156 <http://172.30.23.156>: ICMP
10.134.0.46 unreachable - need to frag (mtu 1438), length 556*
16:40:57.961017 IP 10.134.0.46.35443 > 172.30.23.156.1234: Flags [.], ack
1, win 208, options [nop,nop,TS val 2995034459 ecr 301081340,nop,nop,sack 1
{3151:3152}], length 0
16:40:57.964937 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [P.], seq
1:3151, ack 3202, win 258, options [nop,nop,TS val 301081348 ecr
2995034459], length 3150
*16:40:57.965003 IP 10.129.2.2 > 172.30.23.156 <http://172.30.23.156>: ICMP
10.134.0.46 unreachable - need to frag (mtu 1438), length 556*
16:40:58.169939 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [P.], seq
1:3151, ack 3202, win 258, options [nop,nop,TS val 301081553 ecr
2995034459], length 3150
*16:40:58.170014 IP 10.129.2.2 > 172.30.23.156 <http://172.30.23.156>: ICMP
10.134.0.46 unreachable - need to frag (mtu 1438), length 556*
16:40:58.577939 IP 172.30.23.156.1234 > 10.134.0.46.35443: Flags [P.], seq
1:3151, ack 3202, win 258, options [nop,nop,TS val 301081961 ecr
2995034459], length 3150
*16:40:58.578015 IP 10.129.2.2 > 172.30.23.156 <http://172.30.23.156>: ICMP
10.134.0.46 unreachable - need to frag (mtu 1438), length 556*


Thank you very much, and best regards,
Miguel Ángel.

-- 
Miguel Ángel Ajo  @mangel_ajo <https://twitter.com/mangel_ajo>
OpenShift / Kubernetes / Multi-cluster Networking team.
ex OSP / Networking DFG, OVN Squad Engineering
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