PDN Gateway Overview
▀ Features and Functionality - Base Software
▄ Cisco ASR 5x00 Packet Data Network Gateway Administration Guide
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Note
: The P-GW supports GTP-based S5/S8 and PMIPv6 S2a capabilities, with no commercial support for PMIPv6
S5/S8.
An EPS bearer uniquely identifies traffic flows that receive a common QoS treatment between a UE and P-GW in the
GTP-based S5/S8 design, and between a UE and HSGW in the PMIPv6 S2a approach. If different QoS scheduling
priorities are required between Service Data Flows, they should be assigned to separate EPS bearers. Packet filters are
signalled in the NAS procedures and associated with a unique packet filter identifier on a per-PDN connection basis.
One EPS bearer is established when the UE connects to a PDN, and that remains established throughout the lifetime of
the PDN connection to provide the UE with always-on IP connectivity to that PDN. That bearer is referred to as the
default bearer. A PDN connection represents a traffic flow aggregate between a mobile access terminal and an external
Packet Data Network (PDN) such as an IMS network, a walled garden application cloud or a back-end enterprise
network. Any additional EPS bearer that is established to the same PDN is referred to as a dedicated bearer. The EPS
bearer Traffic Flow Template (TFT) is the set of all 5-tuple packet filters associated with a given EPS bearer. The EPC
core elements assign a separate bearer ID for each established EPS bearer. At a given time a UE may have multiple
PDN connections on one or more P-GWs.
DHCP Support
The P-GW supports dynamic IP address assignment to subscriber IP PDN contexts using the Dynamic Host Control
Protocol (DHCP), as defined by the following standards:
RFC 2131, Dynamic Host Configuration Protocol
RFC 2132, DHCP Options and BOOTP Vendor Extensions
The method by which IP addresses are assigned to a PDN context is configured on an APN-by-APN basis. Each APN
template dictates whether it will support static or dynamic addresses. Dynamically assigned IP addresses for subscriber
PDN contexts can be assigned through the use of DHCP.
The P-GW acts as a DHCP server toward the UE and a DHCP client toward the external DHCP server. The DHCP
server function and DHCP client function on the P-GW are completely independent of each other; one can exist without
the other.
The P-GW does not support DHCP-relay.
Important:
Currently, the P-GW only supports DHCP with IPv4 addresses. IPv6 address support is planned at a
later date.
Deferred IPv4 Address Allocation
Apart from obtaining IP addresses during initial access signalling, a UE can indicate via PCO options that it prefers to
obtain IP address and related configuration via DHCP after default bearer has been established. This is also know as
Deferred Address Allocation.
IPv4 addresses are becoming an increasingly scarce resource. Since 4G networks like LTE are always on, scarce
resources such as IPv4 addresses cannot/should not be monopolized by UEs when they are in an ECM-IDLE state.
PDN-type IPv4v6 allows a dual stack implementing. The P-GW allocates an IPv6 address only by default for an IPv4v6
PDN type. The UE defers the allocation of IPv4 addresses based upon its needs, and relinquishes any IPv4 addresses to
the global pool once it is done. The P-GW may employ any IPv4 address scheme (local pool or external DHCP server)
when providing an IPv4 address on demand.
