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    Understanding Upstream and Downstream Processing of ToS Values in GTP-U Packets

    To provide the required QoS treatment for upstream and downstream subscriber traffic, GTP-U packets are processed at multiple points in the data path.

    This topic describes the upstream and downstream operations performed on GTP-U packets on the MobileNext Broadband Gateway.

    Processing of ToS Values for Upstream Subscriber Packets

    The broadband gateway processes upstream GTP-U packets from a Gn/S5 interface to a Gi/SGi interface.

    The following steps describe the processing of ToS values for upstream GTP-U packets:

    1. A GTP-U packet arrives on the mobile (Ethernet) interface, and a behavior aggregate (BA) classifier evaluates the ToS value of the subscriber packet to derive an appropriate Junos OS forwarding class and packet loss priority (PLP).
    2. The GTP-U packet is sent to the appropriate queue on the Packet Forwarding Engine. (The forwarding class determines the queue.)
    3. The packet is sent to the anchor Packet Forwarding Engine where the GTP packet header is decapsulated.

      Note: A classifier profile must be configured on the broadband gateway to provide a mapping from a traffic class/QCI to a forwarding class and PLP.

    4. Subscriber tunnel endpoint identifier (TEID) lookup identifies the traffic class or QCI for the packet. The traffic class or QCI is mapped to a forwarding class and PLP, based on the classifier profile configured on the broadband gateway.
    5. The packet is sent out on the anchor Packet Forwarding Engine where the egress rewrite rule applied on the mobile interface takes the forwarding class and PLP (Step 4) as input values to derive the appropriate DSCP marking before sending the packet to the SGi/Gi interface.

      Note: An egress rewrite rule must be configured and applied to each mobile interface to provide the required DSCP marking for subscriber traffic.

    6. The packet is sent out on the correct Gi or SGi interface.

    Processing of ToS Values for Downstream Subscriber Packets

    The broadband gateway processes downstream GTP-U packets from a Gi or SGi to a Gn or S5 interface.

    The following steps describe the processing of ToS values for downstream GTP-U packets:

    1. The GTP-U packet arrives from the Gi or SGi interface, and is sent to the anchor Packet Forwarding Engine associated with the virtual routing and forwarding (VRF) route.
    2. On the anchor Packet Forwarding Engine, an IP address lookup identifies the TEID for the GTP header and, before encapsulation, the traffic class/QCI maps to a forwarding class and PLP, based on the classifier profile configured on the broadband gateway.
    3. The packet is sent out from the anchor Packet Forwarding Engine where the ingress rewrite rule applied on the mobile interface takes the forwarding class and PLP (Step 2) as input values to derive the appropriate DSCP marking.
    4. The packet is encapsulated with TEID and outer IP address in the GTP header, which is used for route table lookup for the SGSN/S-GN and sent to the egress Packet Forwarding Engine interface.
    5. The packet is sent out on the correct Gn or S5 interface.

    Published: 2011-11-17