Advanced Computer Networks: Lecture 28 - Dr. Amir Qayyum
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CS716
Advanced Computer Networks
By Dr. Amir Qayyum
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Lecture No. 28
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Internetworking
• Basics of internetworking (heterogeneity)
– IP protocol, address resolution, control messages …
• Routing
• Global internets (scale)
– Virtual geography and addresses
– Hierarchical routing
• Future internetworking: IPv6
• Multicast traffic
• MPLS
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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
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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
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Destination Based Forwarding
• MPLS – attaching labels with IP datagrams
• Router allocates a label for each prefix in
its routing table
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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
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Destination Based Forwarding
• Other routers store these labels as remote
labels in their routing table along with
prefixes
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Destination Based Forwarding
• Other routers store these labels as remote
labels in their routing table along with
prefixes
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Destination Based Forwarding
Example
• R1 acts as Label Edge Router (LER)
– Applies labels to arriving IP packets after complete IP lookup
• Packet destined to 10.1.1.5 arrives at R1
– R1 matches the prefix 10.1.1, attaches label ‘15’ to packet,
send to R2
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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 !!!
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MPLS – Achievements
• Fixedlength 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
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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).
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Methods of Attaching Labels
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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
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MPLS – Example of MPLS Network
• Conventional Network
– Overlay of virtual circuits
– Each router connect to other by a virtual circuit
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MPLS – Example of MPLS Network
• MPLS Network
– Routers peer directly with LSRs
– No virtual circuits interconnecting routers
– Each router has only one adjacency
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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
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Example – Explicit Routing
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“Fish” network
Route for traffic from R1 to R7 is R1R3R6R7
Route for traffic from R2 to R7 is R2R3R4R5R7
Good use of capacity available along two distinct paths
Different from normal IP routing
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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)
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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
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explicit routes
MPLS VPN An ATM Circuit
• Pseudowire Emulation
•Routers interconnected by a tunnel
• Tunnel header
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Forwarding ATM Cells in MPLS VPN
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Example of Layer 3 VPN
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