1. MPLS Basics 2 2

2. IP plus ATM Integration
MPLS Applications
MPLS benefits and applications:
De-couples IP packet forwarding from the information carried in the IP header of the packet
Provides multiple routing paradigms (e.g., destination-based, explicit routing, etc…) over a common forwarding algorithm (label swapping)
Facilitates integration of ATM and IP - from control plane point of view an MPLS-capable ATM switch looks like a router
Provide value added services like IP VPNs and circuit transport over MPLS
IP plus ATM Integration
Traffic Engineering
FR/ATM E1
FR/ATM E2
IP VPN’s
Circuit Transport over MPLS

3. Agenda MPLS Concepts LSRs and labels Label Switch Paths
Label assignment and distribution
LDP overview

4. MPLS concepts Label Switch Routers
IGP domain with a label distribution protocol
An IP routing protocol is used within the routing domain (e.g.OSPF,ISIS)
A label distribution protocol is used to distribute address/label mappings between adjacent neighbors
The ingress LSR receives IP packets, performs packet classification (into a FEC), assign a label, and forward the labelled packet into the MPLS network
Core LSRs switch packets/cells based on the label value (no packet classification in the core)
The egress LSR removes the label before forwarding the IP packet outside the MPLS network

5. MPLS concepts Packet forwarding: FEC and Next-Hop
IP packets are classified in FECs
Forwarding Equivalence Class
A group of IP packets which are forwarded in the same manner
Over the same path
With the same forwarding treatment
Packet forwarding consists on
Assign a packet to a FEC
Determine the next-hop of each FEC

6. MPLS concepts Packet forwarding: FEC and Next-Hop
MPLS make use of FECs
MPLS nodes assign a label to each FEC
Packet classification (into a FEC) is done where the packet enters the core
No sub-sequent packet classification in the MPLS network

7. MPLS concepts Packet forwarding: FEC and Next-Hop
I/F In
Lab
Address
Prefix
Out
I/F
Out
Lab In
I/F In
Lab
Address
Prefix
Out
I/F
Out
Lab 4 x 3 5
Next-Hop
...
...
...
...
... 5 1 3
Next-Hop
...
...
...
...
...
In conventional IP forwarding, a particular router will typically
consider two packets to be in the same FEC if there is some address
prefix X in that router's routing tables such that X is the "longest
match" for each packet's destination address. As the packet traverses
the network, each hop in turn reexamines the packet and assigns it to
a FEC.
In MPLS, the assignment of a particular packet to a particular FEC is
done just once, as the packet enters the network. The FEC to which
the packet is assigned is encoded as a short fixed length value known
as a "label". When a packet is forwarded to its next hop, the label
is sent along with it; that is, the packets are "labeled" before they
are forwarded.
At subsequent hops, there is no further analysis of the packet's
network layer header. Rather, the label is used as an index into a
table(LFIB) which specifies the next hop, and a new label. The old label
is replaced with the new label, and the packet is forwarded to its
next hop. 3 1
/24 4
IP packet D=
Label = 5
IP packet D=
Label = 3
Rtr-B
Rtr-A
IP packet D=
Router-A forwards labelled packets by looking at the label value against the label table(LFIB). No packet classification into FEC is done.
Router-B classify the IP packet into a FEC and assign the corresponding label.

8. Label Switch Path (LSP)
Ingress-LSR
Egress-LSR
MPLS is responsible for directing a flow of IP packets along a predetermined path across a network. This path is called a label-switched path. Label-switched paths are similar to ATM PVCs in that they are simplex in nature; that is, the traffic flows in one direction from the ingress router to a egress router. Duplex traffic requires two label-switched paths; that is, one path to carry traffic in each direction. A label-switched path is created by the concatenation of one or more label-switched hops, allowing a packet to be forwarded from one label-switching router to another label-switching router across the MPLS domain. A label-switching router is a router that supports MPLS-based forwarding.
When an IP packet enters a label-switched path, the ingress router examines the packet and assigns it a label based on its destination, placing the label in the packet’s header. The label transforms the packet from one that is forwarded based on its IP routing information to one that is forwarded based on information associated with the label. The packet is then forwarded to the next router in the label-switched path. The key point in this scheme is that the physical path of the LSP is not limited to what the IGP would choose as the shortest path to reach the destination IP address.
LSP is the unidirectional sequence of LSRs through which the labelled packets have to go through in order to reach the egress LSR
FEC is determined in LSR-ingress
IGP domain with a label distribution protocol

9. Label Switch Path (LSP)
IGP domain with a label distribution protocol
LSP follows IGP shortest path
LSP diverges from IGP shortest path
LSPs derive from IGP routing information
LSPs may diverge from IGP shortest path
LSP tunnels (explicit routing) with Traffic Engineering

10. MPLS concepts Labels
Label | Exp|S| TTL
Label = 20 bits
Exp = Experimental, 3 bits
S = Bottom of stack, 1bit
TTL = Time to live, 8 bits
An MPLS header consists of:
20-bit label—Used to identify the packet to a particular LSP
Class of service value—Indicates queuing priority through the network. At each hop along the way, the class of service value determines which packets receive preferential treatment within the tunnel.
Stacking bit—Indicates that this MPLS packet has more than one label associated with it. If set to 1=>bottom of stack(last entry)
Time to live value—Contains a limit on the number of router “hops” this MPLS packet may travel through the network. It is decremented at each hop, and if the TTL value drops below one, the packet is discarded.
The label stack entries appear AFTER the data link layer headers, but BEFORE any network layer headers. The top of the label stack appears earliest in the packet, and the bottom appears latest. The network layer packet immediately follows the label stack entry which has the S bit set.
Generic: can be used over Ethernet, 802.3, PPP links, Frame Relay, ATM PVCs, etc.
Uses new Ethertypes/PPP PIDs/SNAP values/etc.
Can have label stacking of 4 octets each
draft-ietf-mpls-label-encaps-07.txt

11. MPLS concepts Labels ATM Cell Header PPP Header
GFC
VPI
VCI
PTI
CLP
HEC
DATA
Label
PPP Header
(Packet over SONET/SDH)
PPP Header
Label
Layer 3 Header
Shim header
LAN MAC Label Header
MAC Header
Label
Layer 3 Header

12. Label assignment and distribution
Labels have local significance
Each LSR binds his own label mappings
Each LSR assign labels to his FECs
Labels are assigned and exchanged between adjacent LSRs
Downstream to Upstream
Applications may require non-adjacent neighbors
e.g VPN

13. Label Distribution Protocols
Several protocols for label exchange
LDP
Maps unicast IP destinations into labels
RSVP, CR-LDP
Used for traffic engineering and resource reservation
BGP
External labels (VPN)

14. Upstream and Downstream LSRs
/24
/24
Rtr-A
Rtr-B
Rtr-C
Rtr-C is the downstream neighbor of Rtr-B for destination /24
Rtr-B is the downstream neighbor of Rtr-A for destination /24
LSRs know their downstream neighbors through the IP routing protocol
Next-hop address is the downstream neighbor

15. Upstream and Downstream LSRs
LSRs assign a label to each FEC
Label distribution may be upstream or downstream driven
Most implementations use downstream with two variants
Unsolicited Downstream
Downstream on demand

16. Unsolicited Downstream distribution
Use label 5 for destination /24
Use label 7 for destination /24
/24
/24
Rtr-A
Rtr-B
Rtr-C
Next-Hop In
Lab -
...
Address
Prefix
Out
I/F 1 5
Next-Hop In
Lab 5
...
Address
Prefix
Out
I/F 1 7
Next-Hop In
Lab 7
...
Address
Prefix
Out
I/F 1 -
IGP derived routes
LSRs assign a label to each FEC
LSRs distribute labels to the upstream neighbors

17. Downstream on demand distribution
Use label 5 for destination /24
Use label 7 for destination /24
/24
/24
Rtr-A
Rtr-B
Rtr-C
Request label for destination /24
Request label for destination /24
LSRs assign a label to each FEC
Upstream LSRs request labels to downstream neighbors
Downstream LSRs distribute labels upon request

18. Penultimate Hop Popping
The label at the top of the stack is removed (popped) by the upstream neighbor of the egress LSR
The egress LSR requests the “popping” through the label distribution protocol
Egress LSR advertises implicit-null label
The egress LSR will not have to do a lookup and remove the label itself
One lookup is saved in the egress LSR

19. Penultimate Hop Popping
Address
Prefix and mask
/24
Next-Hop
Interface
Serial1
/24
Serial2
171.68/16
...
Null
Next-Hop In
Lab
Address
Prefix
Out
I/F -
171.68/16 1 4
...
Next-Hop In
Lab
Address
Prefix
Out
I/F 4
171.68/16 2
pop
...
Summary route for /16
Summary route for /16 1 1
Use label 4 for FEC /16
Use label “implicit-null” for FEC /16
/24
Summary route is propagate through the IGP and label is assigned by each LSR
Egress LSR summarises more specific routes and advertises a label for the new FEC
/24
Egress LSR needs to do an IP lookup for finding more specific route
Egress LSR need NOT receive a labelled packet
label will have to be popped anyway

20. Penultimate Hop Popping
Address
Prefix and mask
/24
Next-Hop
Interface
Serial1
/24
Serial2
171.68/16
...
Null
Next-Hop In
Lab
Address
Prefix
Out
I/F -
171.68/16 1 4
...
Next-Hop In
Lab
Address
Prefix
Out
I/F 4
171.68/16 2
pop
... 1 1
IP packet D=
Label = 4
IP packet D=
IP packet D=
/24
IP packet D=
/24
Packet arrives without the label at the egress LSR. Egress LSR needs to do an IP lookup to match more specific routes
Packet is MPLS forwarded Top label is removed
IP packet enters the MPLS network Ingress LSR assign a label and forward the packet

21. LDP Concepts One of several standardised label distribution protocol
draft-ietf-mpls-ldp-09.txt
A set of procedures and messages to distribute mappings between labels and FECs
Two LSRs which use LDP to exchange label/FEC mapping information are known as "LDP Peers"
Peers exchange LDP messages
Uses TLV encoded message structure
All LDP messages have a common structure that uses a Type-Length-
Value (TLV) encoding scheme

22. Concepts LDP Messages Discovery messages Session messages
Used to discover and maintain the presence of new peers
Hello packets (UDP) sent to all-routers-in-subnet multicast address
Once neighbor is discovered, the LDP session is established over TCP
Session messages
Establish, maintain and terminate LDP sessions
Advertisement messages
Create, modify, delete label mappings
Notification messages
Error signalling
Discovery messages provide a mechanism whereby LSRs indicate their
presence in a network by sending a Hello message periodically. This
is transmitted as a UDP packet to the LDP port at the `all routers on
this subnet' group multicast address. When an LSR chooses to
establish a session with another LSR learned via the Hello message,
it uses the LDP initialization procedure over TCP transport. Upon
successful completion of the initialization procedure, the two LSRs
are LDP peers, and may exchange advertisement messages.
Session Messages e.g Initialization, Keepalives
Advertisement Messages e.g. Label Mapping, Label Withdrawal, Label Request,..