MTU Option should be included in ICMPv6 Router Advertisements

[Tore Anderson]

When using an overlay network on a physical network with standard Ethernet MTU (1500 octets), the instances' effective MTU is reduced.

The Neutron Router should inform the nodes about this fact, by including the MTU Option in the ICMPv6 Router Advertisements it sends. The current situation leads to blackholing of traffic, as the absence of the MTU Option causes the instance to believe it will be able to successfully transmit 1500 octets large frames to the network. However, these will be silently discarded. The symptom of is usually that the TCP three-way handshake succeeds, but that the connection appears to hang the moment payload starts being transmitted.

The MTU Option is documented here: https://tools.ietf.org/html/rfc4861#section-4.6.4. The corresponding radvd.conf option is called AdvLinkMTU. Note that the Neutron router is clearly aware of the reduced effective MTU, as it does use the corresponding DHCPv4 option to advertise it to instances/subnets using IPv4.

I observe this problem on OpenStack Kilo.

[Tore Anderson (toreanderson)]

I was mistaken regarding the Neutron router being aware of the reduced effective MTU. This had been hard-coded in /etc/neutron/dnsmasq-neutron.conf, in accordance with the documentation in http://docs.openstack.org/kilo/install-guide/install/apt/content/neutron-network-node.html.

Nevertheless, a similar method for configuring/hard-coding the MTU option in ICMPv6 RAs is required in order to prevent blackholing of IPv6 traffic in overlay networks with reduced MTU.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/244722

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/244722
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=47713f58701af9e27792230dd64daa3e20a4260b
Submitter: Jenkins
Branch: master

commit 47713f58701af9e27792230dd64daa3e20a4260b
Author: sridhargaddam <email address hidden>
Date: Thu Nov 12 15:49:15 2015 +0000

    Support MTU advertisement using IPv6 RAs

    RFC4861 allows us to specify the Link MTU using IPv6 RAs.
    When advertise_mtu is set in the config, this patch supports
    advertising the LinkMTU using Router Advertisements.

    Partially Implements: blueprint mtu-selection-and-advertisement
    Closes-Bug: #1495444
    Change-Id: I50d40cd3b8eabf1899461a80e729d5bd1e727f28

[Thierry Carrez (ttx)]

This issue was fixed in the openstack/neutron 8.0.0.0b3 development milestone.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/liberty
Review: https://review.openstack.org/308229

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/308229
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=0991b190df4eca710a4a613f9c9b14dca98e1edc
Submitter: Jenkins
Branch: stable/liberty

commit 0991b190df4eca710a4a613f9c9b14dca98e1edc
Author: sridhargaddam <email address hidden>
Date: Thu Nov 12 15:49:15 2015 +0000

    Support MTU advertisement using IPv6 RAs

    RFC4861 allows us to specify the Link MTU using IPv6 RAs.
    When advertise_mtu is set in the config, this patch supports
    advertising the LinkMTU using Router Advertisements.

    Conflicts:
     neutron/agent/linux/ra.py
     neutron/common/config.py
     neutron/db/l3_db.py
     neutron/tests/common/l3_test_common.py
     neutron/tests/unit/agent/l3/test_agent.py
     neutron/tests/unit/db/test_l3_db.py

    Liberty modifications:
    - In test_l3_db.py, added two test cases that were covering the
      change in Mitaka+.

    Partially Implements: blueprint mtu-selection-and-advertisement
    Closes-Bug: #1495444
    Change-Id: I50d40cd3b8eabf1899461a80e729d5bd1e727f28
    (cherry picked from commit 47713f58701af9e27792230dd64daa3e20a4260b)

[Thierry Carrez (ttx)]

This issue was fixed in the openstack/neutron 7.1.0 release.