Sunday, December 4, 2011

Dual Stack Support in LTE eNodeB - Technical bit

This technical bit summarizes the type of functionality expected out of Dual stack in user plane of eNodeB.


eNodeB connects to UE on Air interface side  and connect to multiple types of devices in the core network over backhaul network.  It communicates with MME,  S1 Gateways via Security Gateways.

GTP-U layer is the relay module which transfers the packets to/from UE to wireless core network.  GTP-U tunnel is normally terminated on S1 Gateway for normal traffic.  GTP-U tunnels are also terminated with other eNodeB in handover cases.

GTP-U packets can be optionally secured using IPsec.  Though transport mode is good enough, but tunnel model is commonly used.  IPSec tunnels are terminated at the Security Gateway in the core network.  Security Gateway are typically placed at the edge of core network and its placement is between eNodeB and S1 and other gateways in core network.

eNodeB Dual Stack Requirements:

eNodeB must ensure that it works with its peers.

  • IPv4 only UEs :  These UEs generate and consume Ipv4 packets only.
  • IPv6 only UEs:  These UEs generate and consume Ipv6 packets only.
  • Dual Stack UEs:  These UEs can generate and consume both IPv4 and IPv6 packets at the same time.
    • IPv4 Radio Bearers
    • IPv6 Radio Bearers
    • IPv4 and IPv6 Radio Bearers where both Ipv4 and IPv6 packets can be seen on the same RB.
S1 Gateways:
  • IPv4 Only S1 Gateways  :  GTP-U tunnels are IPv4 tunnels 
  • IPv6 only S1  Gateways -  GTP-U tunnels are IPv6 tunnels.
  • Dual Stack S1 Gateways  -  GTP-U tunnels to these gateways could be either IPv4 or IPv6.
  • In a given deployment, there is a possibility of having any of above types of S1 Gateways.
IPsec VPN Gateways:

eNB typically contains IPsec part of the eNB itself.  On the core network though,  Ipsec gateway is normally not combined with S1 Gateway.  It is a separate device/blade that sits in the core network.  Ipsec tunnels from eNB are terminated at this gateway whereas GTP tunnels are terminated at the S1 Gateway.

With this above background,  following functionality is expected typically in eNB:

GTP-U Layer:
  • Since there are different types of S1 gateways (IPv6 only,   IPv4 only and Dual stack),  MME can decide to put different UEs on various types of S1 gateways.  Hence GTP-U layer must be able to support IPv4 tunnels and IPv6 tunnels. 
  • GTP-U must be able to transport both Ipv4 and Ipv6 packets between UE and Core network on IPv4 based GTP tunnels and IPv6 based GTP tunnels.
  • GTP-U layer must be able to set DSCP values on outer IP header (IPv4 or IPv6) and should be able to copy DSCP values from inner IP packet (Could be IPv4 or IPv6).  Hence GTP layer should be able to copy DSCP value from  (For Uplink packets) 
    • IPv4 to IPv6
    • IPv6 to IPv6
    • IPv4 to IPv4
    • IPv6 to IPv4
IPSec Layer:

As noted above,  one security gateway might be front ending multiple S1 Gateways with respect to base stations.  Since there could be multiple GTP-U tunnels on one IPsec tunnel,  IPsec layer must support following:
  • Must be able to work with Security Gateway on core network whether Security Gateway in core network supports Ipv4 or IPv6 tunnel.
  • Must be able to transport both IPv4 and IPv6 packets (GTP-U tunnel packets) on one tunnel.
  • Must be able to do DSCP copy from GTP-U header to Outer IP header (could be IPv4 or IPv6). 
QoS Layer:

Since both Ipv4 and IPv6 packets traverse on the same Ethernet Port or VLAN port, it is necessary that the shaping and scheduling does not require two different types of configuration. It should be possible to create
  • QoS ACL  to have both Ipv4 and IPv6 rules.
  • A rule with both Ipv4 and Ipv6 address tuples.
  • Multiple rules pointing to the same Queues.

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