Userspace Datapath - Checksum Offloading

This document explains the internals of Open vSwitch support for checksum offloading in the userspace datapath.

Design

Open vSwitch strives to forward packets as they arrive regardless of whether the checksum is correct or not. OVS is not responsible for fixing external checksum issues.

The interface (internally referred to as a netdev) can set flags indicating whether each packet’s checksum is good or bad upon receipt. If this flag is not set, OVS will consider the validity of the packet’s checksum to be unknown.

OVS will not re-calculate or update the packet’s checksum if the checksum is already known to be correct, known to be explicitly incorrect, or destined for an egress interface that will recalculate the checksum anyways.

If OVS does invalidate the checksum, and the packet ingresses the datapath with a checksum that is not known to be incorrect, OVS postpones checksum updates until the packet egresses the datapath. This recalculation can either be performed by OVS or, be offloaded onto the NIC if the egress NIC supports checksum offloading.

When a packet egress the datapath, the packet flags and the egress interface flags are verified to make sure all required offload features to send out the packet are available on the egress interface. If not, the data path will fall back to equivalent software implementation.

Interface (a.k.a. Netdev)

When the interface initiates, it should set the flags to tell the datapath which offload features are supported. For example, if the driver supports IP checksum offloading, then netdev->ol_flags should set the flag NETDEV_TX_OFFLOAD_IPV4_CKSUM.

Rules

  1. OVS should strive to forward all packets regardless of checksum.

  2. OVS must not correct a known bad packet checksum.

  3. Packet with flag DP_PACKET_OL_RX_IP_CKSUM_GOOD means that the IP checksum is present in the packet and it is good.

  4. Packet with flag DP_PACKET_OL_RX_IP_CKSUM_BAD means that the IP checksum is present in the packet and it is bad. Extra care should be taken to not fix the packet during data path processing.

  5. The ingress packet parser can only set DP_PACKET_OL_TX_IP_CKSUM if the packet has DP_PACKET_OL_RX_IP_CKSUM_GOOD to not violate rule #2.

  6. Packet with flag DP_PACKET_OL_TX_IPV4 is an IPv4 packet.

  7. Packet with flag DP_PACKET_OL_TX_IPV6 is an IPv6 packet.

  8. Packet with flag DP_PACKET_OL_TX_IP_CKSUM tells the datapath to skip updating the IP checksum if the packet is modified. The IP checksum will be calculated by the egress interface if that supports IP checksum offload, otherwise the IP checksum will be performed in software before handing over the packet to the interface.

  9. When there are modifications to the packet that requires a checksum update, the datapath needs to remove the DP_PACKET_OL_RX_IP_CKSUM_GOOD flag, otherwise the checksum is assumed to be good in the packet.

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