IPv6 oddness in Comcast land...

valdis.kletnieks at vt.edu valdis.kletnieks at vt.edu
Sun Mar 19 23:16:11 UTC 2017

Trying to figure out what the heck is going on here.  Any good
explanations cheerfully accepted.

Background:  Home internet router is a Linksys WRT1200AC that had been
running OpenWRT 15.05.01. IPv6 worked just fine - Comcast handed me a /60
via DHCP-PD and no issues.  I reflashed it to Lede 17.01, and after doing
all the reconfig, I'm hitting a really strange IPv6 issue.

Symptoms - IPv6 still configures correctly, but IPv6 packets appear to go out
and disappear into the ether when they leave the Linksys.  Doing a traceroute
to any IPv6 destination makes things work again - for a while (from 15 minutes
to an hour or two).

As seen from my laptop (I have the matching tcpdump from the outbound
interface on the Linksys):

[~] ping -6 -c 3 listserv.vt.edu
PING listserv.vt.edu(listserv.ipv6.vt.edu (2001:468:c80:2105:211:43ff:feda:d769)) 56 data bytes

--- listserv.vt.edu ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2070ms

[~] traceroute -6 listserv.vt.edu
traceroute to listserv.vt.edu (2001:468:c80:2105:211:43ff:feda:d769), 30 hops max, 80 byte packets
 1  2601:5c0:c001:69e2::1 (2601:5c0:c001:69e2::1)  2.417 ms  3.077 ms  5.358 ms
 2  * * *
 3  * * *
 4  * * *
 5  * * *
 6  * hu-0-10-0-7-pe04.ashburn.va.ibone.comcast.net (2001:558:0:f5c1::2)  31.478 ms  31.975 ms
 7  2001:559::d16 (2001:559::d16)  32.406 ms  17.102 ms  24.751 ms
 8  2001:550:2:2f::a (2001:550:2:2f::a)  23.245 ms  23.519 ms  22.185 ms
 9  2607:b400:f0:2003::f0 (2607:b400:f0:2003::f0)  29.782 ms  28.604 ms  29.891 ms
10  2607:b400:90:ff05::f1 (2607:b400:90:ff05::f1)  30.423 ms *  30.680 ms
11  * * *
12  listserv.ipv6.vt.edu (2001:468:c80:2105:211:43ff:feda:d769)  34.562 ms  39.072 ms  24.633 ms
[~] ping -6 -c 3 listserv.vt.edu
PING listserv.vt.edu(listserv.ipv6.vt.edu (2001:468:c80:2105:211:43ff:feda:d769)) 56 data bytes
64 bytes from listserv.ipv6.vt.edu (2001:468:c80:2105:211:43ff:feda:d769): icmp_seq=1 ttl=53 time=33.3 ms
64 bytes from listserv.ipv6.vt.edu (2001:468:c80:2105:211:43ff:feda:d769): icmp_seq=2 ttl=53 time=24.3 ms
64 bytes from listserv.ipv6.vt.edu (2001:468:c80:2105:211:43ff:feda:d769): icmp_seq=3 ttl=53 time=46.0 ms

--- listserv.vt.edu ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 24.334/34.595/46.093/8.927 ms

So it looks like something times out somewhere and fails to pass packets

TCP connections don't keep IPv6 alive. I have a browser window that
auto-updates every 5 minutes, and a SmokePing process on a Raspberri Pi uploads
to a server at work every few minutes, and those eventually drop back to IPv4
when the IPv6 TCP fails to connect. And normal UDP doesn't seem to keep it
alive - NTP pointing at IPv6 peers loses connectivity as well.

But a traceroute wakes it up. It's almost like some router is losing the
route to me out of the FIB, and fixes it when it has to handle a packet
on the CPU slow path (like send back a 'time exceeded').  But I'm mystified
why this started when I reflashed my router.

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