short notes from 29.061

Posted: August 24, 2010 in technical
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TS 29.061, more precisely the Interworking between PGW and PDN, sections 11 to 13.

TS 23.401, sections 5.3.1 – IP Address Allocation, 5.3.2 – Attach Procedure

It seems that:

1. an UE can simultaneously connect to multiple APNs;

2. an UE can have multiple default bearers per APN connection: for example, one for IPv4 and one for IPv6;

2.a) 2 default bearers per APN connection are possible when the MME does NOT set the Dual Address Bearer Flag; this way, the MME forces the sending of separate IPv4 and IPv6 requests for PDN connectivity;

2.b) if the MME sets the Dual Address Bearer Flag, then it can send a request with dual-stack IPv4v6 and the APN can provide both of these IP addresses at once – this means that there are 2 IP addresses (one IPv4 and one IPv6 ONLY one of each type !) for a SINGLE default bearer;

3. Allocation of these IP addresses to the UE can happen from a local PGW pool or from the PDN. In the later case, the Create Session Response message sent from the PGW to the SGW (and further on to the MME) has PAA =, following the later completion of this address;

3.a) If the PGW has nothing to do with the further negotiation of the IP addresses, we are talking about a direct transparent access IP allocation; the PGW is just a proxy;

3.b) If the PGW is actively (protocol dependent) implicated in the IP address allocation, then we are talking about a non transparent access; the PGW is an active part in the IP negotiation: for instance, it may act as a DHCP server for the UE (via SGW, of course) and in the same time as DHCP client – when talking to the APN’s actual DHCP server;

*Note: the role of the PGW in the DHCP allocation case is different from the role of its 3G homologous, the GGSN – this entity playing the role of a DHCP relay agent in this scenario;

*Note: We are talking about the so-called IP-CAN (Connectivity to Access Network) session establishment – which, as far as I understand from TS 29.061, refers to the process of allocating an IP address (IPv4, IPv6 or IPv4v6) by a process other than gathering it from the PGW pool, for instance: via DHCP/DHCP-PD, PPP, IMS CN IM process…etc…

4. IP-CAN can be established at:

a) Initial Attach (default bearer activation) to the APN (in EPC) – Primary PDP Context Activation to the APN (in 3G)

b) after the initial attach, via a dedicated bearer/secondary PDP context

5. The IP assignment can take place:

a) either at the subscription phase – in which case we are talking about a static address

b) or at the IP-CAN session establishment – in which case we are talking about a dynamic address

*Note: Usually, as part of the IP-CAN negotiation ( no matter if this takes place at initial attach of afterwards), the PGW may request the UE to authenticate to the external APN’s AAA server


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