[ovs-discuss] How to support traceroute using OVS
blp at ovn.org
Sun Nov 29 19:38:35 UTC 2015
On Fri, Nov 27, 2015 at 11:23:09PM -1000, Nicholas Bastin wrote:
> On Thu, Nov 26, 2015 at 9:44 PM, Chao Hu <chao.hu at ericsson.com> wrote:
> > In the ovs-ofctl manual, it’s said in dec_ttl action that “If controller
> > ids are not specified, the “packet_in” message will be sent only to the
> > controllers having controller id zero which have registered for the invalid
> > ttl packets.” In my test, I didn’t specify the controller’s controller-id,
> > which should have 0 by default. And it’s said in the code of ofproto.c
> > that “OFPC_INVALID_TTL_TO_CONTROLLER is deprecated in OF 1.3”, does it
> > mean in OF1.3, “packet_in” on invalid ttl will be sent by default and no
> > register is needed? Or such packet-in is not supported in OF1.3?
> The spec is unclear. In fact it doesn't even specify what an "invalid TTL"
> value would actually be. (The OF 1.1 spec specified "invalid" as 0 or 1 in
> a code comment, but future specs seem to have eliminated even that hint of
> what values qualifies as "invalid" - I did not then, nor do I now, know why
> 1 would be regarded as an invalid TTL value for a packet transiting a
> layer-2 switch).
Well, that's a mess.
I've filed a bug report EXT-559 with the ONF working group, so perhaps
we can at least get "invalid TTL" defined for OF1.6.
> > BTW, match on “nw_ttl” seems also not supported in OF1.3.
> This was never supported in the spec, but it's possible that OVS supported
> that match field at some point (I'm not sure).
Yes, OVS has supported matching on the IPv4/v6 TTL/hop limit since Open
vSwitch version 1.4.
> > How to solve this problem?
> You probably can't, at least not in a wholly satisfactory way, without
> modifying the source to OVS, or inlining a function (linux tap interface
> driver, etc.) in the path on each port. What you really want to do is get
> OVS to issue a packet-out of an ICMP Time Exceeded when an incoming packet
> reaches a TTL of 0 within the pipeline (modulo the times when you actually
> should send a Destination Unreachable reply), but this has all kinds of
> nasty edge cases.
We're working to make this possible. What kinds of nasty edge cases do
you have in mind? I'd like to make everything as correct as possible,
More information about the discuss