[pathanalysis] BDDReachability chooses IP protocol 0 when unconstrained, confusing users

[dhalperi]

@anothermattbrown I know why this is happening, but this is probably counter intuitive. E.g., updating refs:

@@ -861,13 +861,13 @@
                 "dstPort" : 0,
                 "ecn" : 0,
                 "fragmentOffset" : 0,
-                "icmpCode" : 255,
-                "icmpVar" : 255,
+                "icmpCode" : 0,
+                "icmpVar" : 0,
                 "ingressNode" : "as3core1",
                 "ingressVrf" : "default",
-                "ipProtocol" : "IP",
+                "ipProtocol" : "HOPOPT",
                 "packetLength" : 0,
-                "srcIp" : "2.255.255.255",
+                "srcIp" : "2.0.0.0",
                 "srcPort" : 0,
                 "state" : "NEW",
                 "tag" : "BASE",

it's nice that values are now taking on minimums, but maybe there's a "smarter" thing we can do for picking a representative IP protocol?

[anothermattbrown]

When we prefer certain options, we can simply add an additional constraint before asking for the representative. If the BDD is unsatisfiable with that constraint, then we rollback. We can easily generalize this to a prioritized list of values, etc.

If this seems reasonable, what's a good list of (in descending order of preference) for the values to try? Do we want to do this for other fields as well? Combinations of fields?

[anothermattbrown]

Another approach: we can test whether the field is constrained in the BDD and if so avoid picking a representative for that field. One complication is that there are in general many ways of satisfying a BDD, and only some of them include a constraint on the field. So we could first choose the shortest implicant (a single path to the 1 node), and then test which fields are constrained by that path. This may be too complicated in case we need to explain it to users.

[dhalperi]

Aside priority-wise -- certainly don't think this is a blocker -- we may have been "getting lucky" with NoD since it tends to pick values it sees used... but worth thinking about.

I think your idea of "favorite parameters (TCP -> UDP -> ?)" may be a good one. not sure tho.

[saparikh]

Perhaps start with ICMP with type and codes for echo request and then go for UDP with the port commonly used for traceroute and then well known TCP ports (SSH, HTTP, HTTPS, DNS, etc...)?

[yifeiyuan]

Done