4.10 Mobile IP for UMTS/GPRS End Users

23.1213GPPArchitectural requirements for Release 1999Release 1999TS

4.10.1 Mobile IP for UMTS/GPRS End Users

A single generic mobility handling mechanism that allows roaming between all types of access networks would allow the user to conveniently move between fixed and mobile networks, between public and private as well as between PLMN’s with different access technologies. The ongoing work in IETF Mobile IP working group [13] is targeted towards such a mechanism[3] and a set of standards are planned to be finalized during 1999. Thus, it is important to offer Mobile IP also to UMTS and GPRS users to allow them to roam to and from other access technologies while keeping ongoing data sessions, e.g. TCP or UDP. A typical UMTS network supporting Mobile IP is shown in figure 4.46.

Figure 4.46: Core network architecture with GPRS MM within the PLMN’s
and Mobile IP MM between different types of systems

As IP addresses in Ipv4 are scarce, it has to be assumed that Mobile Ipv4 preferably will be used with the Foreign Agent (FA) care-of addresses [10]. Compared to using co-located care-of addresses, FA care-of addresses does not only conserve IP addresses, it is also more efficient over the radio interface. We assume here that the MS keeps the same care-of address as long as the PDP context is activated, i.e. does not change GGSN/FA during a UMTS/GPRS session. It is further assumed that PDP type "IP" is used. It is, however, likely that PDP type" PPP" also could be used. Roaming between PLMN’s can be realized with GPRS roaming or Mobile IP.

To offer Mobile IP with FA care-of addresses over the UMTS/GPRS network, some requirements need to be fulfilled. Some of these will cause changes to the current GPRS standards.

A signalling scheme, shown in figure 4.47, is described below. The PPP setup and the UMTS/GPRS attach procedures and "GGSN-Initiated PDP Context modification procedure" have been omitted for clarity.

Figure 4.47: PDP Context activation with Mobile IP registration
(the PPP setup and UMTS/GPRS attach procedures not included)

1. The AT command carries parameters that the MT needs to request the PDP Context Activation. The important parameter here, is the APN (Access Point Name), see clause A below. The AT command is followed by a setup of the PPP connection between the MT and the TE, which are not included in the figure.

2. The MT sends the "Activate PDP Context Request" to the SGSN. The message includes various parameters of which the "APN" (Access Point Name) and the "Requested PDP Address" are of interest here. The MS may use APN to select a reference point to a certain external network and/or to select a service. APN is a logical name referring to the external packet data network and/or to a service that the subscriber wishes to connect to. The "Requested PDP Address" should be omitted for all MS’s using Mobile IP. This is done irrespective of if the MT has a permanently assigned Mobile IP address from its Mobile IP home network, a previously assigned dynamic home address from its Mobile IP home network or if it wishes the Mobile IP home network to allocate a "new" dynamic home address.

A. The SGSN will base the choice of GGSN on the APN that is given by the MS.

The APN consists of two parts: the Network ID and the Operator ID. If no APN is given and PDP type is "IP", the SGSN chooses a suitable GGSN according to operator’s configuration of the SGSN. Similarly, a Network ID of the format vvv (one label, no dots) can be used to specify any GGSN with a specific service (vvv), e.g. Internet access, gateway for voice over IP, Mobile IP FA. If the SGSN is not configured to identify the requested service it may try with a DNS interrogation for vvv.current-operator.current-country.gprs or, if that is not successful, with vvv.home-operator.home-country.gprs, where the home parameters are taken from the subscription data.

3. The SGSN requests the selected GGSN to set up a PDP Context for the MS. The PDP address and APN fields are the same as in the "Activate PDP Context Request" message.

4. A Create PDP Context Response is sent from the GGSN/FA to the SGSN. If the creation of PDP Context was successful, some parameters will be returned to the SGSN, if not, error code will be returned. If the GGSN has been configured by the operator to use a Foreign Agent for the requested APN, the PDP address returned by the GGSN shall be set to 0.0.0.0. indicating that the PDP address shall be negotiated by the MS with a Home Agent after the PDP context activation procedure.

5. The Activate PDP Context Accept message is sent by the SGSN to the MS and contains similar information as the Create PDP Context Response message.

6. The Agent Advertisement [10] is an ICMP (Internet Control Message Protocol) Router Advertisement message with a mobility agent advertisement extension. The latter part contains parameters of the FA that the mobile node needs, among those are one or more care-of addresses that the FA offers. This message should be sent, in the UMTS/GPRS user plane, as an IP limited broadcast message, i.e. destination address 255.255.255.255, however only on the TEID for the newly arrived MS to avoid broadcast over the radio interface.

7. The Mobile IP Registration Request is sent from the mobile node to the GGSN/FA across the GPRS/UMTS backbone as user traffic. The mobile node includes its (permanent) home address as a parameter [10]. Alternatively, it can request a temporary address assigned by the home network by including the Network Access Identifier (NAI) in a Mobile-Node-NAI Extension [12], [11].

8. The FA forwards the Mobile IP Registration Request to the home network of the mobile node, where it get processed by a Home Agent (HA). Meanwhile, the GGSN/FA needs to store the home address of the mobile node or the NAI and the local link address of the MS, i.e. the TEID.

9. The Registration Reply is sent from the home network to the FA, which extracts the information it needs (e.g. the home address of the mobile node if allocated by the home network).

10. The FA forwards the message to the mobile node in the UMTS/GPRS user plane. As the FA/GGSN knows the TEID and the NAI or home address, it can pass it on to the correct MS. A home address of the MS allocated by the home network is sent to the SGSN by means of the "GGSN-Initiated PDP Context modification procedure" described in [23.060].

4.10.1.1 Alterations of and Additions to Current GPRS Standards

To support Mobile IP as described above, the following alterations and additions to the GPRS specifications are necessary:

1) the functionality of the GGSN needs to be enhanced with FA functionality, according to IETF specifications. For interoperability, a set of RFC’s should be recommended. There is no need to standardize an interface between the GGSN and the FA, as it is considered being one integrated node;

2) the GGSN/FA node should send a FA Advertisement message after sending the Create PDP Context Response;

3) the GGSN should not give the MS a temporary UMTS/GPRS IP (PDP) address if the Mobile IP FA service has been requested;

4) the GGSN/FA and the MS shall exchange Mobile IP 57ignalling messages in the UMTS/GPRS user plane;

5) allow the APN to mean a GGSN with a specific service, not only a physical node. To support this, the operator must have the possibility to configure the SGSN or DNS with the choice of GGSN depending on service. A default mechanism is also needed to use a GGSN in the MS’s home network if the visited SGSN does not support the requested service. Finally, an agreement between operators is needed on the possible APN’s.