a) What is the objective of FEC (Forwarding Equivalence Class)? R

Exercise 1:
a) What is the objective of FEC (Forwarding Equivalence Class)?
R- To switch the frames simply and to be scalable.
b) The field carrying the label that we add, for example, in Ethernet frame, is standardized.
It has 4 bytes, including a label of 20 bits, an experimental field of 3 bits, 1 bit indicating
if there is a label stack or not, and 8 bits of TTL (Time to Live). Doesn't this field seem
small? Why it contains a TTL field? (The three experimental bits can be used in various
ways to indicate, for example, a class of client. The bit of the label stack indicates if
several labels are stacked.)
R- Quite small, but thanks to the organization by FEC, which allows flow multiplexing, this
value is sufficient. Field TTL aims to destroy the frame after crossing a limited number of
hops in order to avoid loops in the networks.
c) What does it mean by an MPLS DiffServ network? Why this solution is widely used?
What is another equivalent solution that you can propose?
R- An MPLS DiffServ means an MPLS network in which the LSPs (Label Switched Path)
have the functionalities corresponding to the classes of service of DiffServ. This solution is
widely used because the customers can mark by themselves the IP packets with the class of
service negotiated with the operator. This marking can be also made or changed by the
operator. Alternative solution: specialized LSPs - for the telephone calls, for the
videoconference, for real time data, etc
Exercise 2:
Consider a network consisting of several interconnected ATM networks. To enter an ATM
network or to go from one ATM network to another, LSRs are used.
a) Show that this network is an MPLS network.
R- It is an MPLS network because to enter the network it is necessary to across an edge LSR.
b) Show that to transport IP signaling packets from one LSR to another, we need to use
ATM signaling.
R- As the ATM network interconnects two LSRs, it is necessary to open a virtual circuit in
the ATM network. This one can be established only by ATM signaling.
c) Show that we can use the existing ATM networks and transform them into MPLS
networks. What is the interest of this approach?
R- We can reuse the existing ATM network as sub-networks by putting LSRs, or more
exactly LERs, at all the entries of the network. The interest of this reuse is to be able to
migrate gently towards a pure MPLS network in which all the nodes become LSRs.
d) If the operator of this network wants to offer the MPLS DiffServ service, how he can do
it?
R- Offering the MPLS DiffServ service means that the qualities of service offered by the
operator are those corresponding to the DiffServ classes. In this case, the operator must
dedicate his LSPs (Label Switched Path) to classes of DiffServ service.
Exercise 3:
Consider an MPLS network having the LSRs inter-connected by the switched Ethernet links.
a) Show that it is necessary to add a label into the header fields of the Ethernet frame (the
“shim label”).
R- Because we have a network MPLS, it is necessary that the Ethernet frames carrying of
packets IP user are switched. As the MAC address cannot be used as label, MPLS standard
defines an additional field, the shim label, to carry the label.
b) Are the IP signaling packets also transported inside the Ethernet frames?
R- Yes, IP signaling packets are also carried by the Ethernet frames between LSRs. It is
always necessary to transport packets in frames, and here the level frame is Ethernet.
c) Can we support quality of service in this MPLS network?
R- Yes, it is possible to support QoS in MPLS. It is enough, for example, to choose the
DiffServ technology and to set appropriate priorities in the LSRs of the network.
d) Can we have an MPLS DiffServ network with all the generality of DiffServ?
R- Yes, it is possible to have an MPLS DiffServ network with all the generality of DiffServ
because the network is made up only of LSR DiffServ able to set up the priorities necessary
to carry out a DiffServ network.
Exercise 4:
Given an MPLS VPN, i.e. an MPLS network of an operator which allows providing their
customers with the VPN (Virtual Private Network) service. A VPN mainly aims at secure the
communications between two distant entities which cross a zone not belonging to the two
communicating entities.
a) Show that MPLS provides a basic solution allowing network operators to provide
connected companies with the VPN service.
R- The concept of LSP provides a basic solution to the VPN service. Only the packets of the
company having subscribed to the VPN service can use the dedicated LSP.
b) What is necessary to do in order to complete the security inside the MPLS network?
R- Mutual authentication between the two equipments located at the ends of the LSP and the
encryption of the packets sent over the LSP.
c) Can we introduce the concept of quality of service in these VPNs?
R- Yes, we can, for example, have an DiffServ MPLS VPN.
Exercise 5:
Consider the following MPLS network:
a) Node D sends an advertisement message “Label 40 for 192.168.20.0 /24” to nodes C, I,
E, J. Explain this advertisement message.
R- LSR D informs the neighbor LSR that its local label 40 is associated the network address
192.168.20.0 /24. LSRs C, I, E, J must use label 40 when they send traffic at destination of
network 192.168.20.0 /24 towards D.
b) Suppose that this MPLS network is a transit network. C, F, I, J are the edge LSRs. D and
E are interior LSRs.
The forwarding table of node C contains the following information:
IN
OUT
a.11 b.21
The forwarding table of node D contains the following information:
IN
OUT
a.21 b.Push 33
The forwarding table of node E contains the following information:
IN
OUT
a.33 b.Pop
The forwarding table of node F contains the following information:
IN
OUT
a.21 b.70
Complete the figure by indicating the label values associated with a packet which enters the
network domain by node C, follows this LSP by crossing nodes D and E, and leaves the network
domain by node F.
R-
I
IP 11
a
IP 21
C
b
IP 21 33
c
a
D
b
a
d
J
IP 21
E
b
IP 70
a
F
b

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