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Thus, the DiffServ network lets an application or flow packets to be marked at
the edge router which thus, can be differentiated from other packet flows and thus,
differentiated service can be provided. These marked packets are then provided
forwarding priority preferences in the core routers depending on the mark put on them by
the edge routers.
3) MPLS (Multi-Protocol Label Swithcing): It is a forward-ing scheme. It evolved from
Ciscos Tag Switching. In the OSI seven-layer model, it is between Layer 2 (L2, link
layer)and Layer 3 (L3, network layer).
Each MPLS packet has a header containing a 20-bit label, a 3-bit Class of
Service (COS) field, an 1-bit label stackindicator and an 8-bit TTL field. The MPLS
header is encapsulated between the link layer header and the network layer
header. A MPLS capable router, which is known as Label Switched Router (LSR),
examines only the label in forwarding the packet. The network protocol can be IP or
anything.
MPLS sets up labels across the LSRs by using some protocol known as Label
Distribution Protocol (LDP), which may not be the same in all LSRs. This LDP is used to
set upLabel Switched Paths (LSPs). A LSP is similar to ATM virtual circuit and is uni-
directional from sender to reciever. LSP set-up can be control driven or data driven. In
control-driven LSP setup, label distributions are triggered by control-driven traffic like
routing updates. In data-driven set up, the label distribution is triggered by request of a
flow or an aggregation of flows (known as traffic trunk). LSP set-up between 2 hosts can be
done explicitly. Then the path is known as explicit path (EP). This is one of the most useful
feature of MPLS and is heavily used in Traffic Engineering (TE). Thus, by using the
labels as a result of label distribution during set-up of a LSP, forwarding table is used to
forward or process packets based on their MPLS labels.
Normal packets are classified at the ingress router of the MPLS enabled network.
MPLS header is put on the packets depending on the kind of routing and service, we want
from the packet. These labeled packets are then used by the LSRs to forward the
packets. This is done by looking into the forwarding table which was constructed when
the labels were distributed during LSP set-up. This process of forwarding using labels
is faster than the forwading done by normal IP routing tables. When an incoming packet
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