Rare project developers

[Alexander Gall <>]

Hi

I have RARE/freertr running on my Tofino2 switch. The switch is
connected to a Cisco 8201 running IOS-XR 7.5.2. I just tested LLDP and
ping and that works fine AFAICS.

However, freertr reports 2 packets per minute with failed checksums,
e.g,

2022-06-09 21:33:16 info ipCor4.parseIPheader:ipCor4.java:91 got bad checksum 
from 192.168.10.2
2022-06-09 21:33:26 info ipCor4.parseIPheader:ipCor4.java:91 got bad checksum 
from 192.168.10.2

192.168.10.2 is the address of the 8201. I'm not sure whats's the best
way to analyze this. Here are the dumps of the packets taken on the
bf_pci0 interface (the destination MAC addresses correspond to
224.0.0.1 and 224.0.0.22)

21:33:16.726974 00:01:68:87:c6:06 > 00:a8:01:00:5e:00, ethertype Unknown 
(0x2240), length 62:
        0x0000:  00a8 0100 5e00 0001 6887 c606 2240 0800
        0x0010:  46c0 0024 326e 0000 0102 dafe c0a8 0a02
        0x0020:  e000 0001 9404 0000 1164 ec5f 0000 0000
        0x0030:  023c 0000 0000 0000 0000 0000 0000

21:33:26.127290 00:16:68:87:c6:06 > 00:a8:01:00:5e:00, ethertype Unknown 
(0x2240), length 72:
        0x0000:  00a8 0100 5e00 0016 6887 c606 2240 0800
        0x0010:  46c0 0038 3276 0000 0102 dacd c0a8 0a02
        0x0020:  e000 0016 9404 0000 2200 37d5 0000 0003
        0x0030:  0200 0000 e000 0002 0200 0000 e000 000d
        0x0040:  0200 0000 e000 0016

The first byte is the CPU header with the source port 168 (front-panel
port 5/0 on this device). The first packet is a IGMP membership query,
the second a IGMPv3 membership report. I don't have a
"hexdump-to-ipchecksum" tool handy, so didn't check the sums in these
dumps.

Generic checksum-offloading is disabled on the interface (and I would
expect this to not be a problem since the kernel would not be able to
determine that this is a IPv4 packet due to the CPU header). Also, the
ICMP echo packets appear to have correct checksum (at least freertr
doesn't complain about those).

The IGMP packets have the router alert IP option, but I'm sure freertr
is handling that correctly :)

I'm also sure you'll see immediately what's wrong here :)

-- 
Alex