Internet%20Routing%20(COS%20598A)%20Today:%20Multi-Protocol%20Label%20Switching

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Internet Routing (COS 598A)Today Multi-Protocol
Label Switching

• Jennifer Rexford
• http//www.cs.princeton.edu/jrex/teaching/spring2
  005
• Tuesdays/Thursdays 1100am-1220pm

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Outline

• Circuit switching
• Packet switching vs. circuit switching
• Virtual circuits
• MPLS
• Labels and label-switching
• Forwarding Equivalence Classes
• Label distribution
• MPLS applications
• Feedback forms
• Fill out during last 20 minutes

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Packet Switching vs. Circuit Switching

• Packet switching
• Data traffic divided into packets
• Each packet contains its own header (with
  address)
• Packets sent separately through the network
• Destination reconstructs the message
• Example sending a letter through postal system
• Circuit switching
• Source first establishes a connection to the
  destination
• Each router on the path may reserve bandwidth
• Source ends data over the connection
• No destination address, since routers know the
  path
• Source tears down the connection when done
• Example voice conversation on telephone network

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Advantages of Circuit Switching

• Guaranteed bandwidth
• Predictable communication performance
• Not best-effort delivery with no real
  guarantees
• Simple abstraction
• Reliable communication channel between hosts
• No worries about lost or out-of-order packets
• Simple forwarding
• Forwarding based on time slot or frequency
• No longest prefix match on each packet
• Low per-packet overhead
• Forwarding based on time slot or frequency
• No IP (and TCP/UDP) header on each packet

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Disadvantages of Circuit Switching

• Wasted bandwidth
• Bursty traffic leads to idle connection during
  silent period
• Unable to achieve gains from statistical
  multiplexing
• Blocked connections
• Connection refused when resources are not
  sufficient
• Unable to offer okay service to everybody
• Connection set-up delay
• No communication until the connection is set up
• Unable to avoid extra latency for small data
  transfers
• Network state
• Routers must store per-connection information
• Unable to avoid per-connection storage and state
  failover

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Virtual Circuits

• Hybrid of packet and circuit switching
• Logical circuit between a source and destination
• Packets from different VCs multiplex on a link
• Virtual Circuit Identifier (VC ID)
• Source set-up establish path for the VC
• Switch mapping VC ID to an outgoing link
• Packet fixed length label in the header

1 7 2 7
1 14 2 8
link 7
1
link 14
2
link 8
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Swapping the Label at Each Hop

• Problem using VC ID along the whole path
• Each virtual circuit consumes a unique ID
• Starts to use up all of the ID space in the
  network
• Label swapping
• Map the VC ID to a new value at each hop
• Table has old ID, next link, and new ID
• Allows reuse of the IDs at different links

1 7 20 2 7 53
20 14 78 53 8 42
link 7
1
link 14
2
link 8
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Virtual Circuits Similar to IP Datagrams

• Data divided in to packets
• Sender divides the data into packets
• Packet has an address (e.g., IP address or VC ID)
• Store-and-forward transmission
• Multiple packets may arrive at once
• Need buffer space for temporary storage
• Multiplexing on a link
• No reservations statistical multiplexing
• Packets are interleaved without a fixed pattern
• Reservations resources for group of packets
• Guarantees to get a certain number of slots

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Virtual Circuits Differ from IP Datagrams

• Forwarding look-up
• Virtual circuits fixed-length connection id
• IP datagrams destination IP address
• Initiating data transmission
• Virtual circuits must signal along the path
• IP datagrams just start sending packets
• Router state
• Virtual circuits routers know about connections
• IP datagrams no state, easier failure recovery
• Quality of service
• Virtual circuits resources and scheduling per VC
• IP datagrams difficult to provide QoS

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Wide Range of Quality-of-Service Models

• Policies for allocating resources
• Admission control whether or not to accept the
  VC
• Link scheduling what order to send packets
• Buffer management which packets to drop
• One extreme best-effort service
• Accept all connections (unless table is full)
• Put all packets in a first-in-first-out queue
• Drop any packet arriving when queue is full
• Another extreme strict bandwidth guarantees
• Virtual circuit reserves bandwidth along the path
• Network edge must shape/police to enforce this
  rate
• Each link has a queue for packets from each VC
• Link schedules the packets using weighted fair
  queuing

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Multi-Protocol Label Switching
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Multi-Protocol Label Switching

• Multi-Protocol
• Encapsulate a data packet
• Could be IP, or some other protocol (e.g., IPX)
• Put an MPLS header in front of the packet
• Actually, can even build a stack of labels
• Label Switching
• MPLS header includes a label
• Label switching between MPLS-capable routers

MPLS header
IP packet
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Pushing, Swapping, and Popping

• Pushing add the initial in label
• Swapping map in label to out label
• Popping remove the out label

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Forwarding Equivalence Class (FEC)

• Rule for grouping packets
• Packets that should be treated the same way
• Identified just once, at the edge of the network
• Example FECs
• Destination prefix
• Longest-prefix match in forwarding table at entry
  point
• Useful for conventional destination-based
  forwarding
• Src/dest address, src/dest port, and protocol
• Five-tuple match at entry point
• Useful for fine-grain control over the traffic
• Sent by a particular customer site
• Incoming interface at entry point
• Useful for virtual private networks

A label is just a locally-significant identifier
for a FEC
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Label Distribution Protocol

• Distributing labels
• Learning the mapping from FEC to label
• Told by the downstream router
• Example destination-based forwarding

Im using label 43 for 12.1.1.0/24
Im using label 10 for 12.1.1.0/24
Pick in-label 10 for 12.1.1.0/24
In Link Out 43 to R4 10
R2
Map destinations in 12.1.1.0/24 to out-label 43
and link to R2
12.1.1.0/24
R1
R4
R3
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Supporting Explicitly-Routed Paths

• Explicitly routing from ingress to egress
• Set an explicit path (e.g., based on load)
• Perhaps reserve resources along the path
• Extend a protocol for resource reservation
• Start with ReSource Reservation Protocol (RSVP)
• Used for reserving resources along an IP path
• Extensions for label distribution explicit
  routing
• Extend a protocol for distributing labels
• Start with Label Distribution Protocol (LDP)
• Extensions for explicit routing reservation
• Two competing proposed standards

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Applications of MPLS
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TE With Constraint-Based Routing

• Path calculation
• Constrained shortest-path first
• Compute shortest path based on weights
• But, exclude paths that do not satisfy
  constraints
• E.g., do not consider links with insufficient
  bandwidth
• Information dissemination
• Extend OSPF/IS-IS to carry the extra information
• E.g., link-state attributes for available
  bandwidth
• Path signaling
• Establish label-switched path on explicit route
• Forwarding MPLS labels

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Surviving Failures Path Protection

• Path protection
• Reserve bandwidth on an alternate route
• Protect a label-switched path by having a
  stand-by
• Much better than conventional IP routing
• Precise control over where the traffic will go
• Stand-by path can be chosen to be disjoint

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Surviving Failures Fast Reroute

• Ensure fast recovery from a link failure
• Protect a link by having a protection sub-path
• Much faster recovery than switching paths
• Affected router can detect the link failure
• and start redirecting to the protection sub-path

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BGP-Free Core
iBGP
eBGP
C
12.1.1.0/24
A
R2
R1
R4
B
R3
D
FEC based on the destination prefix
Routers R2 and R3 dont need to speak BGP
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VPNs With Private Addresses
10.1.0.0/24
10.1.0.0/24
C
A
R2
Two FECs
R1
R4
B
R3
D
Direct traffic to orange
10.1.0.0/24
10.1.0.0/24
MPLS tags can differentiate green VPN from orange
VPN.
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Status of MPLS

• Deployed in practice
• BGP-free core
• Virtual Private Networks
• Traffic engineering
• Challenges
• Protocol complexity
• Configuration complexity
• Difficulty of collecting measurement data
• Continuing evolution
• Standards
• Operational practices and tools

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Conclusion

• MPLS is an overlay
• Tunneling on top of the network
• Built on top of an underlying routing algorithm
• Flexibility in mapping traffic to paths
• Associating packets with FECs, and then labels
• New protocols for creating label-switching tables
• Binding FECs to labels across a path
• Establishing explicit routes
• Many open questions
• Makes operations easier vs. harder?
• Trade-offs in exploiting the flexibility?
• Interdomain routing with MPLS?

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Rest of the Semester

• Rest of class
• Feedback forms
• Thanks (in advance) for your feedback
• Written reports for course projects
• Due Deans Date (May 10) by end of day
• Submitting via e-mail would be fine
• Oral presentations for course projects
• Monday May 16 at 130pm in room 302
• 15 minutes for single-person, 20 for groups