Bug 1908775

Summary:	MTU is not set correctly for IPsec case
Product:	OpenShift Container Platform	Reporter:	Mark Gray <mark.d.gray>
Component:	Networking	Assignee:	Mark Gray <mark.d.gray>
Networking sub component:	ovn-kubernetes	QA Contact:	Anurag saxena <anusaxen>
Status:	CLOSED CURRENTRELEASE	Docs Contact:
Severity:	medium
Priority:	high	CC:	anbhat, anusaxen, kewang, mark.d.gray
Version:	4.7
Target Milestone:	---
Target Release:	4.7.0
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	No Doc Update
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2021-02-08 17:18:09 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description Mark Gray 2020-12-17 15:15:40 UTC

Description of problem:

MTU should be adjusted when IPsec is enabled to account for packet overhead of adding IPsec headers.

Version-Release number of selected component (if applicable): OCP 4.7


How reproducible:


Steps to Reproduce:
1. Start cluster with IPsec disabled
2. Create Pod
3. Run `ip l | grep mtu` in pod to view MTU
4. Start cluster with IPsec enabled
5. Create Pod
6. Run `ip l | grep mtu` in pod to view MTU


Actual results:
* MTUs are the same value

Expected results:
* MTU with IPsec enabled should be adjusted to account for IPsec header

Additional info:

Comment 1 Anurag saxena 2020-12-17 15:31:38 UTC

@mark.d.gray Whats the ipsec mtu overhead supposed to be?

Comment 3 Mark Gray 2020-12-18 15:58:06 UTC

@anusaxen: The overhead is 46B at a maximum.

Comment 6 Ke Wang 2020-12-30 07:17:35 UTC

Per doc https://access.redhat.com/documentation/en-us/openshift_container_platform/3.11/html/cluster_administration/admin-guide-ipsec, the ipsec overhead is 62B.

Comment 8 Mark Gray 2021-01-04 18:00:14 UTC

(In reply to Ke Wang from comment #6)
> Per doc
> https://access.redhat.com/documentation/en-us/openshift_container_platform/3.
> 11/html/cluster_administration/admin-guide-ipsec, the ipsec overhead is 62B.

The overhead of IPsec depends on the IPsec mode (transport or tunnel) and the encryption algorithm used. I am unsure how it was configured in 3.11 but it may have been using different parameters. For 4.7, it should have a maximum overhead of 46B. The motivation for this is described in the commit message of the PR.