mirror of
https://github.com/openvswitch/ovs
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19ef1b1f0f
Flagging packets with IP version is only needed at the netdev-dpdk level. In most cases, OVS is already inspecting the IP header in packet data, so maintaining such IP version metadata won't save much cycles (given the cost of additional branches necessary for handling outer/inner flags). Cleanup OVS shared code and only set these flags in netdev-dpdk.c. Signed-off-by: David Marchand <david.marchand@redhat.com> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
93 lines
3.8 KiB
ReStructuredText
93 lines
3.8 KiB
ReStructuredText
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Licensed under the Apache License, Version 2.0 (the "License"); you may
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not use this file except in compliance with the License. You may obtain
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a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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License for the specific language governing permissions and limitations
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under the License.
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Convention for heading levels in Open vSwitch documentation:
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======= Heading 0 (reserved for the title in a document)
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------- Heading 1
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~~~~~~~ Heading 2
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+++++++ Heading 3
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''''''' Heading 4
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Avoid deeper levels because they do not render well.
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========================================
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Userspace Datapath - Checksum Offloading
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========================================
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This document explains the internals of Open vSwitch support for checksum
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offloading in the userspace datapath.
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Design
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------
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Open vSwitch strives to forward packets as they arrive regardless of whether
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the checksum is correct or not. OVS is not responsible for fixing external
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checksum issues.
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The interface (internally referred to as a netdev) can set flags indicating
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whether each packet's checksum is good or bad upon receipt. If this flag is not
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set, OVS will consider the validity of the packet's checksum to be unknown.
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OVS will not re-calculate or update the packet's checksum if the checksum is
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already known to be correct, known to be explicitly incorrect, or destined for
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an egress interface that will recalculate the checksum anyways.
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If OVS does invalidate the checksum, and the packet ingresses the datapath with
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a checksum that is not known to be incorrect, OVS postpones checksum updates
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until the packet egresses the datapath. This recalculation can either be
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performed by OVS or, be offloaded onto the NIC if the egress NIC supports
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checksum offloading.
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When a packet egress the datapath, the packet flags and the egress interface
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flags are verified to make sure all required offload features to send out the
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packet are available on the egress interface. If not, the data path will fall
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back to equivalent software implementation.
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Interface (a.k.a. Netdev)
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-------------------------
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When the interface initiates, it should set the flags to tell the datapath
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which offload features are supported. For example, if the driver supports IP
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checksum offloading, then ``netdev->ol_flags`` should set the flag
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``NETDEV_TX_OFFLOAD_IPV4_CKSUM``.
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Rules
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-----
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1) OVS should strive to forward all packets regardless of checksum.
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2) OVS must not correct a known bad packet checksum.
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3) Packet with flag ``DP_PACKET_OL_RX_IP_CKSUM_GOOD`` means that the IP
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checksum is present in the packet and it is good.
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4) Packet with flag ``DP_PACKET_OL_RX_IP_CKSUM_BAD`` means that the IP
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checksum is present in the packet and it is bad. Extra care should be taken
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to not fix the packet during data path processing.
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5) The ingress packet parser can only set ``DP_PACKET_OL_TX_IP_CKSUM`` if the
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packet has ``DP_PACKET_OL_RX_IP_CKSUM_GOOD`` to not violate rule #2.
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6) Packet with flag ``DP_PACKET_OL_TX_IP_CKSUM`` tells the datapath to skip
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updating the IP checksum if the packet is modified. The IP checksum will be
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calculated by the egress interface if that supports IP checksum offload,
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otherwise the IP checksum will be performed in software before handing over
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the packet to the interface.
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7) When there are modifications to the packet that requires a checksum update,
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the datapath needs to remove the ``DP_PACKET_OL_RX_IP_CKSUM_GOOD`` flag,
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otherwise the checksum is assumed to be good in the packet.
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