MPLS QoS tunneling: Diffserv over MPLS

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MPLS has exp bits reserved for QoS usage. You can tunnel the DiffServ value of the IP packet through the MPLS network. That provides an advantage to service provider so that they can have different QoS policies than their customers. The IETF has defined three models to tunnel the DiffServ information.

  • Pipe Model
  • Short Pipe Model
  • Uniform Model

For describing these models I have to define 2 terms, LSP Diffserv and Tunneled Diffserv.

  • LSP Diffserv: The LSP DiffServ information is the QoS (the value of the EXP bits) of the MPLS packets transported on the LSP from the ingress LSR to the egress LSR.
  • Tunneled Diffserv: The Tunneled DiffServ information is the QoS information that needs to get across the MPLS network transparently,

Please find below a brief description of all three models:

Pipe Model

In the Pipe model, the following rules apply:

  • The LSP DiffServ information is not necessarily (but might be) derived from the Tunneled DiffServ information on the ingress LSR.
  • On an intermediate LSR (a P router), the LSP DiffServ information of the outgoing label is derived from the LSP DiffServ information of the incoming label.
  • On the egress LSR, the forwarding treatment of the packet is based on the LSP DiffServ information, and the LSP DiffServ information is not propagated to the Tunneled DiffServ information.

Short Pipe Model

  • The Short Pipe model is similar to the Pipe model, with one difference. The forwarding treatment on the egress LSR is different for the Short Pipe model. Therefore, the third bullet becomes this:
  • On the egress LSR, the forwarding treatment of the packet is based on the Tunneled DiffServ information, and the LSP DiffServ information is not propagated to the Tunneled DiffServ information.

Uniform Model

In the Uniform model, the following rules apply:

  • The LSP DiffServ information must be derived from the Tunneled DiffServ information on the ingress LSR.
  • On an intermediate LSR (a P router), the LSP DiffServ information of the outgoing label is derived from the LSP DiffServ information of the incoming label.
  • On the egress LSR, the LSP DiffServ information must be propagated to the Tunneled DiffServ information.

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