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..
Licensed under the Apache License, Version 2.0 (the "License"); you may
not use this file except in compliance with the License. You may obtain
a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations
under the License.
Convention for heading levels in Open vSwitch documentation:
======= Heading 0 (reserved for the title in a document)
------- Heading 1
~~~~~~~ Heading 2
+++++++ Heading 3
''''''' Heading 4
Avoid deeper levels because they do not render well.
===
OVN
===
Q: Why does OVN use STT and Geneve instead of VLANs or VXLAN (or GRE)?
A: OVN implements a fairly sophisticated packet processing pipeline in
"logical datapaths" that can implement switching or routing functionality.
A logical datapath has an ingress pipeline and an egress pipeline, and each
of these pipelines can include logic based on packet fields as well as
packet metadata such as the logical ingress and egress ports (the latter
only in the egress pipeline).
The processing for a logical datapath can be split across hypervisors. In
particular, when a logical ingress pipeline executes an "output" action,
OVN passes the packet to the egress pipeline on the hypervisor (or, in the
case of output to a logical multicast group, hypervisors) on which the
logical egress port is located. If this hypervisor is not the same as the
ingress hypervisor, then the packet has to be transmitted across a physical
network.
This situation is where tunneling comes in. To send the packet to another
hypervisor, OVN encapsulates it with a tunnel protocol and sends the
encapsulated packet across the physical network. When the remote
hypervisor receives the tunnel packet, it decapsulates it and passes it
through the logical egress pipeline. To do so, it also needs the metadata,
that is, the logical ingress and egress ports.
Thus, to implement OVN logical packet processing, at least the following
metadata must pass across the physical network:
* Logical datapath ID, a 24-bit identifier. In Geneve, OVN uses the VNI to
hold the logical datapath ID; in STT, OVN uses 24 bits of STT's 64-bit
context ID.
* Logical ingress port, a 15-bit identifier. In Geneve, OVN uses an option
to hold the logical ingress port; in STT, 15 bits of the context ID.
* Logical egress port, a 16-bit identifier. In Geneve, OVN uses an option
to hold the logical egress port; in STT, 16 bits of the context ID.
See ``ovn-architecture(7)``, under "Tunnel Encapsulations", for details.
Together, these metadata require 24 + 15 + 16 = 55 bits. GRE provides 32
bits, VXLAN provides 24, and VLAN only provides 12. Most notably, if
logical egress pipelines do not match on the logical ingress port, thereby
restricting the class of ACLs available to users, then this eliminates 15
bits, bringing the requirement down to 40 bits. At this point, one can
choose to limit the size of the OVN logical network in various ways, e.g.:
* 16 bits of logical datapaths + 16 bits of logical egress ports. This
combination fits within a 32-bit GRE tunnel key.
* 12 bits of logical datapaths + 12 bits of logical egress ports. This
combination fits within a 24-bit VXLAN VNI.
* It's difficult to identify an acceptable compromise for a VLAN-based
deployment.
These compromises wouldn't suit every site, since some deployments
may need to allocate more bits to the datapath or egress port
identifiers.
As a side note, OVN does support VXLAN for use with ASIC-based top of rack
switches, using ``ovn-controller-vtep(8)`` and the OVSDB VTEP schema
described in ``vtep(5)``, but this limits the features available from OVN
to the subset available from the VTEP schema.
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