openstack/neutron
Code Issues Proposed changes

Merge "ML2: Configure path_mtu to default to 1500 bytes"

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Jenkins 2016-02-11 06:38:54 +00:00 committed by Gerrit Code Review
commit 65eda0eacd
2 changed files with 30 additions and 1 deletions
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2
neutron/plugins/ml2/config.py
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@ -39,7 +39,7 @@ ml2_opts = [
"entrypoints to be loaded from the " "entrypoints to be loaded from the "
"neutron.ml2.extension_drivers namespace. " "neutron.ml2.extension_drivers namespace. "
"For example: extension_drivers = port_security,qos")), "For example: extension_drivers = port_security,qos")),
cfg.IntOpt('path_mtu', default=0, cfg.IntOpt('path_mtu', default=1500,
help=_('Maximum size of an IP packet (MTU) that can traverse ' help=_('Maximum size of an IP packet (MTU) that can traverse '
'the underlying physical network infrastructure without ' 'the underlying physical network infrastructure without '
'fragmentation. For instances using a ' 'fragmentation. For instances using a '

29
releasenotes/notes/1500-default-mtu-b0d6e4ab193b62a4.yaml Normal file
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@ -0,0 +1,29 @@
---
prelude: >
- The ML2 plug-in supports calculating the MTU for instances
using overlay networks by subtracting the overlay protocol
overhead from the value of 'path_mtu', ideally the physical
(underlying) network MTU, and providing the smaller value to
instances via DHCP. Prior to Mitaka, 'path_mtu' defaults to
0 which disables this feature. In Mitaka, 'path_mtu' defaults
to 1500, a typical MTU for physical networks, to improve the
"out of box" experience for typical deployments.
features:
- In Mitaka, the combination of 'path_mtu' defaulting to 1500 and
'advertise_mtu' defaulting to True provides a value of MTU accounting
for any overlay protocol overhead on the network to instances using
DHCP. For example, an instance attaching to a VXLAN network receives
a 1450 MTU from DHCP accounting for 50 bytes of overhead from the
VXLAN overlay protocol if using IPv4 endpoints.
issues:
- The combination of 'path_mtu' and 'advertise_mtu' only adjusts the
MTU for instances rather than all virtual network components between
instances and provider/public networks. In particular, setting
'path_mtu' to a value greater than 1500 can cause packet loss even
if the physical network supports it. Also, the calculation does not
consider additional overhead from IPv6 endpoints.
upgrade:
- Operators using the ML2 plug-in with 'path_mtu' defaulting to 0 may need
to perform a database migration to update the MTU for existing networks
and possibly disable existing workarounds for MTU problems such as
increasing the physical network MTU to 1550.