Brian Haley 7d139a013b Add RST linting to neutron

Start running the doc8 RST linter in the pep8 job. This
will catch obvious errors like line too long, etc. Tried
to fix most of the errors (there were a lot!), but added
some ignore directives so we can address some files later.

Did have to move a couple of files around as a double
include was causing some issues, but content is unchanged.

Change-Id: I336a9ee7729045da095be22ea0f58ee596967174

2024-10-01 11:18:46 -04:00

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Gaps from ML2/OVS

This is a list of some of the currently known gaps between ML2/OVS and OVN. It is not a complete list, but is enough to be used as a starting point for implementors working on closing these gaps. A TODO list for OVN is located at¹.

QoS minimum bandwidth allocation in Placement API

ML2/OVN integration with the Nova placement API to provide guaranteed minimum bandwidth for ports². Work in progress, see³
IPv6 Prefix Delegation

Currently ML2/OVN doesn't implement IPv6 prefix delegation. OVN logical routers have this capability implemented in⁴ and we have an open RFE to fill this gap⁵.
DHCP service for instances

ML2/OVS adds packet filtering rules to every instance that allow DHCP queries from instances to reach the DHCP agent. For OVN this traffic has to be explicitly allowed by security group rules attached to the instance. Note that the default security group does allow all outgoing traffic, so this only becomes relevant when using custom security groups⁶. Proposed patch is ⁷ but it needs to be revived and updated.
DNS resolution for instances

OVN cannot use the host's networking for DNS resolution, so Case 2b in⁸ can only be used when additional DHCP agents are deployed. For Case 2a a different configuration option has to be used in ml2_conf.ini:
```
[ovn]
dns_servers = 203.0.113.8, 198.51.100.53
```
OVN answers queries for hosts and IP addresses in tenant networks by spoofing responses from the configured DNS servers. This may lead to confusion in debugging.

OVN can only answer queries that are sent via UDP, queries that use TCP will be ignored by OVN and forwarded to the configured resolvers.

OVN can only answer queries with no additional options being set (EDNS). Such queries depending on the OVN version will either get broken responses or will also be forwarded to the configured resolvers.
IPv6 NDP proxy

The NDP proxy functionality for IPv6 addresses is not supported by OVN.
East/West Fragmentation

The core OVN implementation does not support fragmentation of East/West traffic using an OVN router between two private networks. This is being tracked in⁹ and¹⁰.
North/South Fragmentation and path MTU discovery

OVN does not correctly fragment IPv4 packets when the MTU of the target network is smaller than the MTU of the source network. Instead, affected packets could be silently dropped depending on the direction. OVN will also not generate ICMP "packet too big" responses for packets that have the DF bit set, even when the necessary configuration option is used in ml2_conf.ini:
```
[ovn]
ovn_emit_need_to_frag = true
```
This makes path MTU discovery fail, and is being tracked in¹¹ and¹².
Traffic metering

Currently neutron-metering-agent can only work with the Neutron L3 agent. It is not supported by the ovn-router service plugin nor by the neutron-ovn-agent. This is being reported and tracked in¹³.
Floating IP Port Forwarding in provider networks and with distributed routing

Currently, when provider network types like vlan or flat are plugged to a router as internal networks while the enable_distributed_floating_ip configuration option is enabled, Floating IP port forwardings which are using such router will not work properly. Due to an incompatible setting of the router to make traffic in the vlan/flat networks to be distributed but port forwardings are always centralized in ML2/OVN backend. This is being reported in¹⁴.

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Gaps from ML2/OVS

References

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