IP address allocation

A UE performs the address allocation procedures for at least one IP address (either IPv4 address or IPv6 prefix) after the default bearer activation if no IPv4 address is allocated during the default bearer activation.

One of the following ways is used to allocate IP addresses for the UE:

a) The HPLMN allocates the IP address to the UE when the default bearer is activated (dynamic or static HPLMN address);

b) The VPLMN allocates the IP address to the UE when the default bearer is activated (dynamic VPLMN address); or

c) The PDN operator or administrator allocates an (dynamic or static) IP address to the UE when the default bearer is activated (External PDN Address Allocation).

The IP address allocated for the default bearer is used for the dedicated bearers within the same PDN connection. IP address allocation for PDN connections, which are activated by the UE requested PDN connectivity procedure, is handled with the same set of mechanisms as those used within the Attach procedure.

PDN types IPv4, IPv6 and IPv4v6 are supported. An EPS Bearer of PDN type IPv4v6 may be associated with one IPv6 prefix only or with both one IPv4 address and one IPv6 prefix. PDN type IPv4 is associated with an IPv4 address. PDN type IPv6 is associated with an IPv6 prefix. PDN types IPv4 and IPv6 are utilised for the UE and/or the PDN GW support IPv4 addressing only or IPv6 prefix only; or operator preferences dictate the use of a single IP version only, or the subscription is limited to IPv4 only or IPv6 only for this APN. In addition, PDN type IPv4 and IPv6 are utilised for interworking with nodes of earlier releases.

The way that the UE sets the requested PDN type may be pre-configured in the device per APN. Unless otherwise configured (including when the UE does not send any APN), the UE sets the PDN type during the Attach or PDN Connectivity procedures based on its IP stack configuration as follows:

- A UE which is IPv6 and IPv4 capable shall request for PDN type IPv4v6.

- A UE which is only IPv4 capable shall request for PDN type IPv4.

- A UE which is only IPv6 capable shall request for PDN type IPv6.

- When the IP version capability of the UE is unknown in the UE (as in the case when the MT and TE are separated and the capability of the TE is not known in the MT), the UE requests for PDN type IPv4v6.

The HSS stores one or more PDN types per APN in the subscription data. During the Attach or UE requested PDN connectivity procedure MME compares the requested PDN type to the PDN type in the subscription records for the given APN and sets the PDN type as follows:

- If the requested PDN type is allowed by subscription, the MME sets the PDN type as requested.

- If the requested PDN type is IPv4v6 and subscription data only allows PDN type IPv4 or only allows PDN type IPv6, the MME sets the PDN type according to the subscribed value. A reason cause shall be returned to the UE indicating that only the assigned PDN type is allowed. In this case the UE shall not request another PDN connection to the same APN for the other IP version.

- If the requested PDN type is IPv4 or IPv6, and neither the requested PDN type nor PDN type IPv4v6 are subscribed, the PDN connection request is rejected.

- If the requested PDN type is IPv4v6, and both IPv4 and IPv6 PDN types are allowed by subscription but not IPv4v6, the MME shall set the PDN type to IPv4 or IPv6 where the selection between IPv4 and IPv6 is implementation specific. The UE should then initiate the UE requested PDN connectivity procedure to this APN in order to activate a second PDN connection with the other single address PDN type which was not allocated by the network.

NOTE 1: If the MT and TE are separated, the UE might not be able to use reason cause "single address bearers only" as a trigger for activating a second single-stack EPS bearer.

The PDN GW may restrict the usage of a PDN type IPv4v6 as follows.

- If the PDN GW receives a request for PDN type IPv4v6, but the PDN GW operator preferences dictate the use of IPv4 addressing only or IPv6 prefix only for this APN, the PDN type shall be changed to a single address PDN type (IPv4 or IPv6) and a reason cause shall be returned to the UE indicating that only the assigned PDN type is allowed. In this case the UE shall not request another PDN connection to the same APN for the other IP version.

- If the PDN GW receives a request for PDN type IPv4v6, but the MME does not set the Dual Address Bearer Flag due to the MME operator using single addressing per bearer to support interworking with nodes of earlier releases the PDN type shall be changed to a single IP version only and a reason cause shall be returned to the UE indicating that only single IP version per PDN connection is allowed. In this case the UE should request another PDN connection for the other IP version using the UE requested PDN connectivity procedure to the same APN with a single address PDN type (IPv4 or IPv6) other than the one already activated.

During inter-RAT mobility between E UTRAN and UTRAN/GERAN, an EPS bearer with PDN type IPv4v6 is mapped one-to-one to PDP type IPv4v6.

During inter-RAT mobility between E-UTRAN and UTRAN/GERAN, an EPS bearer with PDN type IPv4 is mapped one-to-one to a PDP context of PDP type IPv4. An EPS bearer with PDN type IPv6 is mapped one-to-one to a PDP context of PDP type IPv6.

It is the HPLMN operator that should define in the subscription whether a dynamic HPLMN or VPLMN address may be used.

The EPS UE may indicate to the network within the Protocol Configuration Options element that the UE wants to obtain the IPv4 address with DHCPv4:

- the UE may indicate that it prefers to obtain an IPv4 address as part of the default bearer activation procedure. In such a case, the UE relies on the EPS network to provide IPv4 address to the UE as part of the default bearer activation procedure.

- the UE may indicate that it prefers to obtain the IPv4 address after the default bearer setup by DHCPv4. That is, when the EPS network supports DHCPv4 and allows that, it does not provide the IPv4 address for the UE as part of the default bearer activation procedures. The network may respond to the UE by setting the PDN Address to 0.0.0.0. After the default bearer establishment procedure is completed, the UE uses the connectivity with the EPS and initiates the IPv4 address allocation on its own using DHCPv4. However, if the EPS network provides IPv4 address to the UE as part of the default bearer activation procedure, the UE should accept the IPv4 address indicated in the default bearer activation procedure.

- if the UE sends no Address Allocation Preference, the PDN GW determines whether to use DHCPv4 or not based on per APN configuration

Both EPS network elements and UE shall support the following mechanisms:

a. IPv4 address allocation via default bearer activation, if IPv4 is supported.

b. IPv6 prefix allocation via IPv6 Stateless Address autoconfiguration according to RFC 4862 [18], if IPv6 is supported;

Furthermore, the Protocol Configuration Options may be used during bearer activation to configure parameters which are needed for IP address allocation. Both EPS network elements and UE may support the following mechanisms:

a. IPv4 address allocation and IPv4 parameter configuration after the attach procedure via DHCPv4 according to RFC 2131 [19] and RFC 4039 [25];

b. IPv6 parameter configuration via Stateless DHCPv6 according to RFC 3736 [20].

c. Allocation of IPv6 prefixes using DHCPv6 according to RFC 3633 [21].

EPS network elements may support the following mechanism:

a. Allocation of a static IPv4 address and/or a static IPv6 prefix based on subscription data in the HSS.

If the static IP address/prefix is not stored in the HSS subscription record, it may be configured on a per-user per-APN basis in the DHCP/Radius/Diameter server and the PDN GW retrieves the IP address/prefix for the UE from the DHCP/Radius/Diameter server. In this case, static IP address/prefix is allocated by the same procedures as the dynamic IP address/prefix allocation.

The following clauses describe how the above listed IP address allocation mechanisms work when GTP based S5/S8 is used. The way of working of the IP address allocation mechanisms for PMIP based S5/S8 can be found in TS 23.402 [2].The procedures can be used both for PLMN (VPLMN/HPLMN) or external PDN based IP address allocation.

NOTE 2: It is transparent to the UE whether the PLMN or the external PDN allocates the IP address and whether the IP address is static or dynamic.

In order to support DHCP based IP address configuration, the PDN GW acts as the DHCP server for HPLMN assigned dynamic and static and VPLMN assigned dynamic IP addressing. When DHCP is used for external PDN assigned addressing and parameter configuration, the PDN GW shall act as the DHCP server towards the UE and it shall act as the DHCP client towards the external DHCP server. The Serving GW does not have any DHCP functionality. It forwards packets, including DHCP packets, between the UE and the PDN GW.

IPv6 Stateless Address autoconfiguration specified in RFC 4862 [18] is the basic mechanism to allocate /64 IPv6 prefix to the UE.

During default bearer establishment, the PDN GW sends the IPv6 prefix and Interface Identifier to the S GW, and then the S GW forwards the IPv6 prefix and Interface Identifier to the MME or to the SGSN. The MME or the SGSN forwards the IPv6 Interface Identifier to the UE. The MME does not forward the IPv6 prefix to the UE. If the UE receives the IPv6 prefix from the SGSN during PDP Context Activation procedure, it ignores it.

NAS Attach Request

The UE initiates the Attach procedure by the transmission, to the eNodeB, of an Attach Request. Following parameters are included in this message.

IMSI or old GUTI, last visited TAI (if available), UE Core Network Capability, UE Specific DRX parameters, Attach Type, ESM message container (Request Type, PDN Type, Protocol Configuration Options, Ciphered Options Transfer Flag), KSIASME, NAS sequence number, NAS-MAC, additional GUTI, P-TMSI signature, Voice domain preference and UE's usage setting) message together with RRC parameters indicating the Selected Network and the old GUMMEI.

The old GUTI may be derived from a P TMSI and RAI.

IMSI is included

• if the UE does not have a valid GUTI or
• if the UE does not have a valid P TMSI available, or
• if the UE is configured to perform Attach with IMSI at PLMN change and is accessing a new PLMN. 

The UE stores the TIN in detached state.

• If the UE's TIN indicates "GUTI" or "RAT-related TMSI" and the UE holds a valid GUTI then the old GUTI indicates this valid GUTI. 
• If the UE's TIN indicates "P TMSI" and the UE holds a valid P TMSI and related RAI then these two elements are indicated as the old GUTI. Mapping a P TMSI and RAI to a GUTI 
• If the UE holds a valid GUTI and the old GUTI indicates a GUTI mapped from a P-TMSI and RAI, then the UE indicates the GUTI as additional GUTI. If the old GUTI indicates a GUTI mapped from a P-TMSI and RAI and the UE has a valid P-TMSI signature associated to it, the P-TMSI signature shall be included.

The UE sets the voice domain preference and UE's usage setting according to its configuration. If available, the last visited TAI shall be included in order to help the MME produce a good list of TAIs for any subsequent Attach Accept message. Selected Network indicates the PLMN that is selected for network sharing purposes. The RRC parameter "old GUMMEI" takes its value from the "old GUTI" contained in the Attach Request.

If the UE has valid security parameters, the Attach Request message is integrity protected by the NAS-MAC in order to allow validation of the UE by the MME. KSIASME, NAS sequence number and NAS-MAC are included if the UE has valid EPS security parameters. NAS sequence number indicates the sequential number of the NAS message. If the UE does not have a valid EPS security association, then the Attach Request message is not integrity protected.

The UE network capabilities indicate also the supported NAS and AS security algorithms. PDN type indicates the requested IP version (IPv4, IPv4/IPv6, IPv6). Protocol Configuration Options (PCO) are used to transfer parameters between the UE and the PDN GW, and are sent transparently through the MME and the Serving GW. The Protocol Configuration Options may include the Address Allocation Preference indicating that the UE prefers to obtain an IPv4 address only after the default bearer activation by means of DHCPv4.

If the UE intends to send PCO which require ciphering (e.g., PAP/CHAP usernames and passwords) or send an APN, or both, the UE shall set the Ciphered Options Transfer Flag and send PCO or APN or both only after authentication and NAS security setup have been completed (see below). If the UE has UTRAN or GERAN capabilities, it sends the NRSU in the PCO to indicate the support of the network requested bearer control in UTRAN/GERAN.

Request Type is included in the ESM message container and indicates "Handover" when the UE has already an activated PDN GW/HA due to mobility with non-3GPP accesses. Attach Type indicates whether it is an EPS attach or a combined EPS/IMSI attach or an Emergency Attach.

For an Emergency Attach the UE sets both the Attach Type and the Request Type to "Emergency" and the IMSI shall be included if the UE does not have a valid GUTI or a valid P-TMSI available. The IMEI shall be included when the UE has no IMSI, no valid GUTI and no valid P-TMSI.