p4c-bm2-ss compiler bug: could not convert Checksum16 to Json

[tomrodeheffer]

I wrote the following program to try out the Checksum16 extern: checksum.p4.txt
But p4c-bm2-ss says:

In file: ../backends/bmv2/expression.cpp:82
Compiler Bug: checksum.p4(38): ck_1/ck;: could not convert to Json
    hdr.h.cksm = ck.get(hdr.h);
                 ^^

Is Checksum16 supported in p4c-bm2-ss? If not, I would expect to see some remark about that in the documentation and a better error message from the compiler.

[mihaibudiu]

The documentation is really lacking, indeed.
Checksums can be only instantiated in the verifyChecksum and updateChecksum controls.
The error message is bad, so we'll keep this as a bug.

[jafingerhut]

I was running a test case yesterday with someone using v1model.p4 using the 2017-Jun-19 latest versions of p4c-bm2-ss and p4lang/behavioral-model's simple_switch, and it seems that nothing in the verifyCheckum and updateChecksum controls is executed in simple_switch. Not 100% sure, but we had some evidence pointing that way.

[mihaibudiu]

BMv2 ignores checksum verify statements, even in P4-14.
If the update does not work then you can't really modify IPv4 headers.
We have issue #655, which still seems to be open. @hanw assigned himself for this one.

[antoninbas]

Actually bmv2 now logs an error message if a packet has an invalid checksum, but does not drop the packet. This was implemented a couple months ago.

[mihaibudiu]

I hope that the new checksum APIs in v1model make this a non-issue,