CC: Sanjay Sane <ss...@nicira.com> CC: Justin Pettit <jpet...@nicira.com> CC: Hao Zheng <hzh...@nicira.com> --- PORTING | 43 +++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 43 insertions(+), 0 deletions(-)
diff --git a/PORTING b/PORTING index 1ac1c63..f50df6a 100644 --- a/PORTING +++ b/PORTING @@ -258,6 +258,49 @@ vswitchd/system-stats.c only knows how to obtain some statistics on Linux. Optionally you may implement them for your platform as well. +Why OVS Does Not Support Hybrid Providers +----------------------------------------- + +The "Porting Strategies" section above describes the "ofproto +provider" and "dpif provider" porting strategies. Only an ofproto +provider can take advantage of hardware TCAM support, and only a dpif +provider can take advantage of the OVS built-in implementations of +various features. It is therefore tempting to suggest a hybrid +approach that shares the advantages of both strategies. + +However, Open vSwitch does not support a hybrid approach. Doing so +may be possible, with a significant amount of extra development work, +but it does not yet seem worthwhile, for the reasons explained below. + +First, user surprise is likely when a switch supports a feature only +with a high performance penalty. For example, one user questioned why +adding a particular OpenFlow action to a flow caused a 1,058x slowdown +on a hardware OpenFlow implementation [1]. The action required the +flow to be implemented in software. + +Given that software-implemented flows cause a major slowdown, +software-implemented flows would only make sense for very low-volume +traffic. But many of the features built into the OVS software switch +implementation would need to apply to every flow to be useful. There +is no value, for example, in applying bonding or 802.1Q VLAN support +only to low-volume traffic. + +Besides supporting features of OpenFlow actions, a hybrid approach +could also support forms of matching not supported by particular +switching hardware, by sending all packets that might match a rule to +software. But again this can cause an unacceptable slowdown by +forcing bulk traffic through software. Consider, for example, a +hardware switch that can match on the IPv6 Ethernet type but not on +fields in IPv6 headers. An OpenFlow table that matched on the IPv6 +Ethernet type would perform well, but adding a rule that matched only +UDPv6 would force every IPv6 packet to software, slowing down not just +UDPv6 but all IPv6 processing. + +[1] Aaron Rosen, "Modify packet fields extremely slow", + openflow-discuss mailing list, June 26, 2011, archived at + https://mailman.stanford.edu/pipermail/openflow-discuss/2011-June/002386.html. + + Questions --------- -- 1.7.4.4 _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
