The PGW's router advertisement on the ims netdev offers a default
router, and on IPv4-only wifi that is the phone's only IPv6 default
route. Every AAAA-first client (apk, browsers) then sends its SYN into
the IMS walled garden, which drops it: apk sat in SYN-SENT for nine
minutes on the dev phone, and a field user's remedy was to stop imsd
before package work.
Set accept_ra_defrtr=0 on the muxed netdev before link-up (the router
solicitation goes out once the link-local address passes DAD), flush a
default route an earlier RA installed (an imsd restart), and give the
P-CSCF a host route over the netdev instead. imsd talks to the P-CSCF
only and already installs a host route per media peer; the RA's prefix
and MTU still apply. The host route makes the P-CSCF reachable before
the RA has been processed, so the PDN MTU ModemManager reports is set
on the link as well, the way NetworkManager does for its bearers. A
failed host route, or a P-CSCF that is not an IPv6 address (the PDN is
brought up IPv6-only), fails the unit with a clear message.
Dev phone (KPN, 0.3.8-r0): restart path drops the RA default and
registers over the host route, apk update in 6 s where it hung; MO and
MT calls with audio both ways; cold boot never installs the default
route, the link carries the 1280 MTU before the host route exists,
fresh registration in 1 s.
Jorijn van der Graaf
8b376bbf95