1. Advanced Computer Networks
CS716
Advanced Computer Networks
By Dr. Amir Qayyum 1

2. Lecture No. 28

3. Internetworking Basics of internetworking (heterogeneity) Routing
IP protocol, address resolution, control messages …
Routing
Global internets (scale)
Virtual geography and addresses
Hierarchical routing
Future internetworking: IPv6
Multicast traffic
MPLS

4. Multiprotocol Label Switching
Combines properties of virtual circuits with flexibility and robustness of datagrams
Relies on IP addresses and IP routing protocols
Forwards packets using short, fixed length labels with local scope
Marriage of two seemingly opposed technologies

5. MPLS - Capabilities
Enable IP capabilities on devices that do not have the capability to forward IP datagrams
Forward IP packets along explicit routes: routes precalculated separate to IP r protocol
Support certain types of VPN services
Where is performance improvement ?
Depends on factors other than header processing

6. Destination Based Forwarding
MPLS – attaching labels with IP datagrams
Router allocates a label for each prefix in its routing table

7. Destination Based Forwarding
Advertisement of label and prefixes to neighbors via Label Distribution Protocol (LDP)
Attach corresponding label to all packets sent to that router for that prefix

8. Destination Based Forwarding
Other routers store these labels as remote labels in their routing table along with prefixes

9. Destination Based Forwarding
Other routers store these labels as remote labels in their routing table along with prefixes

10. Destination Based Forwarding Example
R1 acts as Label Edge Router (LER)
Applies labels to arriving IP packets after complete IP lookup
Packet destined to arrives at R1
R1 matches the prefix , attaches label ‘15’ to packet, send to R2

11. Destination Based Forwarding Example
R2 checks the label of incoming packet, consults its table for outgoing interface (1), updates the label value to ‘24’ and forwards the packet to R3
R2 doesn’t consult IP address for forwarding packet to R3 !!!

12. MPLS – Achievements
Fixed-length label lookup instead of variable length IP prefix lookup
Simpler to implement exact match instead of longest match algorithm
Only forwarding algorithm is changed
Any standard routing algorithm may still be used
Packets will follow the path selected by IProute

13. How to Attach Labels to Packets ?
Depends upon type of link carrying packets
When IP packets carried as complete frames
Label is inserted as a “shim” between layer 2 header and layer 3 header
Like on most link types: Ethernet, Token Ring, PPP
When the switch function as an MPLS LSR
Label need to be in a place where switch can use it
In ATM, labels are inserted in ATM cell header (combination of VPI and VCI).

14. Methods of Attaching Labels

15. An MPLS Network
Mixture of conventional IP routers, label edge routers (LER) and ATM switches (as LSRs)
All are using the same routing protocols
Reduction in number of adjacencies that each router must maintain
Greatly reduce amount of work of each router
Edge routers have a full view of complete topology of the network
Edge routers pick a new path in case of node failure

16. MPLS – Example of MPLS Network
Conventional Network
Overlay of virtual circuits
Each router connect to other by a virtual circuit

17. MPLS – Example of MPLS Network
Routers peer directly with LSRs
No virtual circuits interconnecting routers
Each router has only one adjacency

18. MPLS – Explicit Routing
MPLS provides a convenient way to add capabilities similar to source routing to IP networks – Explicit routing
Not really source of packet that picks the route – different from source routing
One of the routers inside the service provider’s network does this
Application
Traffic Engineering

19. Example – Explicit Routing
“Fish” network
Route for traffic from R1 to R7 is R1-R3-R6-R7
Route for traffic from R2 to R7 is R2-R3-R4-R5-R7
Good use of capacity available along two distinct paths
Different from normal IP routing

20. MPLS – Explicit Routing
With MPLS enabled routers it is very easy to achieve desired routing
How can we make sure to choose different paths?
If labels of R1 and R2 are different, R3 can send the packets along different paths
How to agree on labels in a network?
Normal label switch forwarding doesn’t work
RSVP (Resource Reservation Protocol)

21. MPLS – Explicit Routing
Fast reroute – a capability to make networks more resilient in the face of failure
Possible to precalculate a path (backup path) between router ‘A’ to router ‘B’ that explicitly avoids a certain link ‘L’
Can significantly reduce the time taken to reroute packets around a point of failure
CSPF (Constrained Shortest Path First): Most common algorithm to calculate explicit routes

22. MPLS VPN - An ATM Circuit
Pseudowire Emulation
Routers interconnected by a tunnel
Tunnel header

23. Forwarding ATM Cells in MPLS VPN

24. Example of Layer 3 VPN