Diff Serv Tunneling Models

MPLS QoS Rule 4 causes an interesting behavior: Regardless of what the MPLS EXP value was changed to at the ingress LSR or any other LSR, that value is not copied to the exposed IP packet at the egress LSR of the MPLS network. In effect, this enables the operator of the MPLS cloud to carry the QoS value of the IP packet transparently through the MPLS network. No matter how many times the EXP bits are changed, by default, the IP precedence or DSCP bits of the IP packet are preserved; the value at the egress LSR is the same as when the IP packet entered the MPLS network. You can now tunnel the DiffServ value of the IP packet through the MPLS network (hence the name DiffServ Tunneling). The obvious advantage is that the MPLS network can have a different QoS scheme than the customers who connect to it, because the MPLS QoS scheme can be independent from the IP QoS scheme of the customers. The IETF has defined three models to tunnel the DiffServ information. All three models are distinct and have their own merits. Furthermore, the distinction between the three models is only at the edge LSRs. The P routers do not come into play with regard to the different DiffServ tunneling models.

Consider Figure 12-7.

Figure 12-7 General Operation of the MPLS DiffServ Tunneling Models

"LSP DiffServ Information' ->

"Tunneled DiffServ Information" ----------------------------------------------->

Ingress P P Egress

LSR LSR

Ingress P P Egress

LSR LSR

The Tunneled Diffserv information is the QoS of the labeled packets or the precedence/DSCP of the IP packets coming into the ingress LSR of the MPLS network. 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. The Tunneled DiffServ information is the QoS information that needs to get across the MPLS network transparently, whereas the LSP DiffServ information is the QoS information that all LSRs in this MPLS network use when forwarding the labeled packet.

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