Page 315 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 301

From an overall perspective it is interesting to see how mobile device‐specific 3GPP
signaling is mixed with standard IPv6 stateless auto‐configuration functionality. This
makes the whole process somewhat more complicated but in theory also allows the
network to change the IPv6 prefix at any time, e.g., again for privacy purposes.
As noted above IPv6 connectivity no longer requires Network Address Translation
(NAT) in the network and hence mobile devices can host services that are reachable
from the Internet. In practice it can be observed, however, that mobile network opera-
tors supporting IPv6 for standard Internet connectivity are blocking incoming connection
requests, most likely for security reasons. While in most cases this is beneficial it unduly
limits customers if they are not offered a way of disabling such a filter.

4.17.2 IPv6 and International Roaming

In practice requesting an IPv4v6 default bearer during LTE attach might not work in all
networks when roaming abroad. If a 3GPP network has not yet been updated to recog-
nize IPv6 signaling parameters, the initial attach procedure can fail, which means that
not even IPv4 connectivity will be available. This is why network operators only forward
IPv6‐related subscription information to other networks abroad when they are certain
that the network can handle or at least ignore the IPv6‐related subscription parameters
without failing. On the mobile device side, some operating systems disable IPv6 by
default when they detect that the home network is not available without giving the user
the choice of manually enabling it. Other operating systems, such as Android, are by
default configured to establish an IPv4‐only default bearer when roaming. This param-
eter can be changed by the user, however, as it is part of the APN profile in the network
settings.

4.17.3 IPv6 and Tethering
A popular function today is to use a smartphone or other cellular device as a Wi‐Fi
access point and router to connect other devices such as notebooks to the Internet. This
is referred to as Wi‐Fi tethering. To support IPv6 in addition to IPv4 for tethered
devices, the tethering software in mobile devices needs to be extended. While mobile
operating systems support this functionality today, it can be observed in practice that
not all vendors have activated the IPv6 tethering extension in practice.
When connecting a notebook or other device to the Internet via the tethering func-
tion of a smartphone the first action the device performs after connecting to the
Wi‐Fi access point created by the cellular device is to get an IPv4 address and an IPv4
DNS server address via DHCP (Dynamic Host Configuration Protocol). In addition,
IPv6‐capable devices also try to get a global IPv6 address, which requires quite a
number of steps:
First the notebook checks if its IPv6 link local address with the interface identifier set
to the MAC address of its Wi‐Fi interface is already used in the network with a Neighbor
Solicitation message. If no answer is received the device then goes ahead and uses this
IPv6 link local address for a number of purposes.
In the next step the notebook finds out if an IPv6 router is available in the Wi‐Fi
network. This is done by sending a Router Solicitation Message from the link local
address to the IPv6 ‘all routers’ multicast address. If the cellular device has implemented