Routing Algorithms and Traffic Engineering

1
Routing Algorithms and Traffic Engineering

• MPLS and OSPF
• traffic engineering
• minimum delay routing
• linear programming
• non-linear optimization

2
Network layer functions

• three important functions
• path determination route taken by packets from
  source to destination
• forwarding move packets from routers input to
  appropriate router output
• call setup some network architectures require
  router call setup along path before data flows

3
Network layer functions

• three important functions
• path determination route taken by packets from
  source to destination Routing algorithm
• forwarding move packets from routers input to
  appropriate router output
• call setup some network architectures require
  router call setup along path before data flows

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Routing
5
Goal determine good path (sequence of routers)
thru network from source to dest.
3
5
2
2
1
3

• Graph abstraction for routing algorithms
• graph nodes are routers
• graph edges are physical links
• link cost delay, cost, or congestion level

1
2
1

• good path
• typically means minimum cost path
• other defs possible

5
Routing Algorithm classification

• Global or decentralized information?
• Global
• all routers have complete topology, link cost
  info
• link state algorithms (Dijkstra)
• Decentralized
• router knows physically-connected neighbors, link
  costs to neighbors
• iterative process of computation, exchange of
  info with neighbors
• distance vector algorithms (Bellman Ford)

6
OSPF (Open Shortest Path First)

• link state protocol
• link costs between 0 and 65,535
• Cisco recommendation - link cost 1/(link
  capacity)
• rapid, loop-free convergence, scales well
• topology map at each node, route computation
  using Dijkstras algorithm
• OSPF advertisement carries one entry per neighbor
  router, advertisements flooded to entire
  Autonomous System
• multiple equal-cost paths allowed flow equally
  split on all outgoing links belonging to shortest
  paths
• IS IS (intermediate system-intermediate system)
  similar

7
Routing vs Switching

• routing based on address lookup, max prefix
  match
• search operation
• complexity O(logn) - 0(n)
• switching based on circuit numbers
• indexing operation
• complexity O(1)
• scalable to large networks
• ? MPLS

8
History Ipsilons IP Switching

• developed by Ipsilon
• routing software in every ATM switch
• initially, packets reassembled by routing
  software, forwarded to next hop
• long term flows transferred to separate VCs.

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Ipsilons IP Switching

• ATM VCs set up when new IP flows seen, i.e.,
• data-driven VC setup
• flow oriented traffic ftp, ssh, http, multimedia
• short-lived traffic DNS, SMTP, NTP
• Ipsilon claimed 90 of bytes flow-oriented
• runs as added software on ATM switch (12,000
  lines of code)

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Issues with Ipsilons IP switching

• VCI field used as ID
• VPI/VCI change at switch
• ? must run on every ATM switch
• ? non-IP switches not allowed between IP
  switches
• ? subnets limited to one switch
• cannot support VLANs
• scalability no. VCs gt no. flows
• ? VC explosion. 1000 setups/sec.
• QoS determined implicitly by flow class or RSVP
• ATM only

11
Tag Switching

• proposed by CISCO
• similar to VLAN tags
• tags explicit or implicit
• ingress router/host puts tag. exit router removes
  it

Key difference tags set up in background using
IP routing protocols (i.e. control-driven VC
setup)
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Alphabet Soup!

• CSR Cell Switched Router
• ISR Integrated Switch and Router
• LSR Label Switching Router
• TSR Tag Switching Router
• Multi layer switches
• Direct IP
• FastIP
• PowerIP
• MPLS - IETF standard

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MPLS concept route at edge, switch in core
IP
IP
IP Forwarding
IP Forwarding
LABEL SWITCHING
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MPLS Terminology

• LDP Label Distribution Protocol
• LSP Label Switched Path
• FEC Forwarding Equivalence Class
• LSR Label Switching Router
• LER Label Edge Router (Useful term not in
  standards)
• MPLS multi-protocol both in terms of protocols
  it supports ABOVE and BELOW in protocol stack!

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MPLS Header

• IP packet encapsulated in MPLS header and sent
  down LSP
• IP packet restored at end of LSP by egress router
• TTL adjusted by default

IP Packet
32-bit MPLS Header
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MPLS Header
TTL
Label
EXP
S

• label
• used to match packet to LSP
• experimental bits
• carries packet queuing priority (CoS)
• stacking bit can build stacks of labels
• qoal nested tunnels!
• time to live
• copied from IP TTL

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MPLS Forwarding Example

• IP packet destined to 134.112.1.5/32 arrives to
  SF
• San Francisco has route for 134.112/16
• next hop is LSP in New York

134.112/16
New York
134.112.1.5
0
San Francisco
1965
1026
Santa Fe
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MPLS Forwarding Example

• San Francisco pre-pends MPLS header onto IP
  packet, sends packet to first transit router on
  path

134.112/16
New York
San Francisco
Santa Fe
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MPLS Forwarding Example

• because packet arrived to Santa Fe with MPLS
  header, Santa Fe forwards it using MPLS
  forwarding table

134.112/16
New York
San Francisco
Santa Fe
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MPLS Forwarding Example

• packet arrives from penultimate router with label
  0
• egress router sees label 0, strips MPLS header
• egress router performs standard IP forwarding

134.112/16
New York
San Francisco
Santa Fe
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Regular IP Forwarding
47.1
1
IP 47.1.1.1
2
IP 47.1.1.1
1
3
2
IP 47.1.1.1
1
47.2
3
47.3
2
IP destination address unchanged in packet header!
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MPLS Label Distribution
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
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Label Switched Path (LSP)
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
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A General Vanilla LSP
- vanilla LSP actually part of tree from every
source to destination (unidirectional) - vanilla
LDP builds tree using existing IP forwarding
tables to route control messages
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Forwarding Equivalence Classes

• FEC - group of IP packets
• forwarded over same path, with same forwarding
  treatment
• FEC may correspond to
• destination IP subnet
• source, destination IP subnet
• QoS class

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Example
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Label Distribution Protocol (LDP)

• label distribution always done from downstream to
  upstream
• downstream-unsolicited new route gt send new
  label
• downstream-on-demand upstream LSR asks for label