n and reassembly of packets is resource intensive, and increases latency and network load. Thus, despite the specified 1500 byte MTU, many commercial UEs use smaller MTU sizes by default. The network can also communicate more optimal MTUs to UEs, as we will see in Section 4.2.2. 2.5.5 Multiple Packet Data Network Connections 3GPP networks support multiple PDN Connections - Primary PDP Contexts, or default EPS Bearers, hence, making the UE a multihomed host. However, 3GPP does not provide a mechanism to convey the routing information or policy. Thus, the routing policy has Deploying IPv6 in 3GPP Networks to be preconfigured in the host, or it is a bit random through which session the packets end up going. The routing policy can be application specific. For instance, some mobile operating systems allow application specific routing policy to be configured. Practically this means that the user interface asks which connection should be used, or the information is statically configured in the application settings. Some mobile phone systems may have inbuilt routing policy for specific applications such as MMS or IMS. The operators use multiple PDN Connections for various reasons. One reason is to differentiate billing and different services. For instance, for MMS the subscriber might pay per message, but not for data transfer, but for other services there is some kind of volume based charging. Using a different PDN Connection for the MMS traffic makes sure that data packets related to MMS are not charged, but the network is not connected to the Internet. The Internet connection is on another PDN Connection, and SO it is accounted for on data volume. In addition, operators may want to segregate important traffic from less important traffic. For this reason, the telephony traffic for IMS might have its own PDN Connection separated from the Internet traffic. We do not want to judge if this approach is the best possible way to achieve the target, but it is not necessarily very IP friendly either. As the modern smartphones a