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reaches the LSR, the incoming label is swapped by the outgoing label and the packet is
forwared to the next LSR. This label-switching process is similar to ATm VCI/VPI
processing. Inside the MPLS domain, packet classification, forwarding and QoS service
are determined using labels and COS. When the packet leaves the domain, the MPLS
header is removed.
Summarizing, MPLS provides following advatages:
1. Faster routing and processing.
2. Efficient tunneling mechanism.
Both these features make it effective for TE (Traffic Engineer-ing). Details can found in
[ea01] and [ea99].
MPLS can be used with DiffServ to provide effective QoS as discussed in next
section.
4) DiffServ over MPLS: [17] explains DiffServ over MPLS
as:
Although traffic engineering is realized by MPLS as discussed above, the DiffServ is
required for scalable QoS control. However, the MPLS encapsulates IP packets using
the shim header having the label, and the core router cannot refer to the DSCP.
Incompatibility with the DiffServ posed a problem. The IETF has proposed the DiffServ
over MPLS [ea00] to solve this problem.
The DiffServ over MPLS can map multiple BAs of DiffServ to a single LSP of
MPLS. By this, traffic on the LSP can be forwarded based on the PHB of BA. The E-
LSP to allow assigning multiple BAs to a single LSP using the EXP field and the L-LSP
to allow assigning a single LSP to a single BA (displays multiple packet discarding
priorities) can be used for LSP and BA mapping.
E-LSP: The E-LSP shows the PHB of a packet using the EXP field of MPLS
shim header. Up to eight BAs can be mapped in the EXP field.
L-LSP: The L-LSP determines the packet scheduling characteristics based on the
MPLS label and the packet discarding priority based on the shim header or layer-2 packet
discarding mechanism. The native ATM uses the L-LSP as it cannot use the EXP field. As
the NEs replace packet labels hop by hop, the label and DSCP mapping is difficult to
manage. While the E-LSP is easier to control than the L-LSP as it can previously determine
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