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MPLS-TE Doesn’t Scale Adrian Farrel Old Dog Consulting [email protected] www.mpls2007.com. Is the Sky Falling?. The only way to get your attention is to be alarmist MPLS-TE is perfectly functional in today’s networks But: MPLS-TE will not scale indefinitely

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Presentation Transcript
is the sky falling
Is the Sky Falling?
  • The only way to get your attention is to be alarmist
  • MPLS-TE is perfectly functional in today’s networks
  • But:
    • MPLS-TE will not scale indefinitely
    • The problem is the well-known “full mesh” or “n-squared” problem
      • The number of LSPs scales as the square of the number of PEs
what do we want to achieve
What Do We Want to Achieve?
  • MPLS-TE is an important feature for many SPs
    • Allow traffic to be groomed
    • Optimize use of network resources
    • Provide quality of service guaranties
  • Carriers look to provide edge-to-edge tunnels across their core networks
    • Differentiated Services
    • VPNs
    • VLANS and pseudowires
    • Multimedia content distribution
    • Normal IP traffic
what is the scope of the problem
What is the Scope of the Problem?
  • Consider a service provider network with 1000 PEs
    • This is not outrageously large
    • Such a network may be broken into areas or ASes
  • Consider a full mesh of PE-PE TE-LSPs
  • Consider parallel tunnels for different services, QoS levels, and for protection
  • May give rise to multiples of 999,000 LSPs in the core
    • What is the capacity of a core LSR?
    • What is the capacity of a management system?
what are the scaling limits
What Are the Scaling Limits?
  • Management
    • NMS
      • How many LSPs can the NMS process
    • Management protocols
      • Reporting on large numbers of LSPs may overload the management network
  • LSR issues
    • Memory capacity
      • Per LSP data requirements
    • CPU capacity – largely an RSVP-TE protocol issue
      • Degradation of LSP setup times
      • Soft state addressed by Refresh Reduction
    • MPLS forwarding plane
      • Number of labels (Only 1048559 per interface)
the snowflake topology
The Snowflake Topology
  • Example network for analysis
  • Meshed core of P nodes
    • Called P1 nodes
  • Each Pi+1 node connected to just one Pi node
  • PE nodes connected to just one Pn node
  • Well-defined connectivity and symmetry allows many important metrics to be computed
  • Number of levels & number of nodes per level may be varied
    • We can vary the number of P1 nodes
    • We can vary the ratio of Pi+1 to Pi
    • We can vary the value n
    • We can vary the number of PE nodes per Pn node

PE

P2

P1

analysing the snowflake topology
Analysing the Snowflake Topology
  • Define
    • Pn a node at the nth level (level 1 is core)
    • Sn the number of nodes at the nth level
    • Mn the multiplier at the nth level (how many Pn+1 nodes are connected to a Pn node)
    • Ln number of LSPs seen by a Pn node
  • Discover
    • LPE = 2*(SPE - 1)
    • L2 = M2*(2*SPE - M2 - 1)
    • L1 = M1*M2*(2*SPE – M2*(M1 + 1))
  • Practical numbers
    • S1 = 10, M1 = 10, and M2 = 20
    • SPE = 2000
    • LPE = 3998
    • L2 = 79580
    • L1 = 756000
the ladder topology
The Ladder Topology
  • Example network for analysis
  • Core of P1 nodes looks like a ladder
    • Similar to many nationalnetworks
  • Symmetrical trees subtendedto core
    • Each Pi+1 node connected to just one Pi node
    • Each PE node connected to just one P node
  • Again:
    • Well-defined connectivity and symmetry allows many important metrics to be computed
    • Number of levels & number of nodes per level may be varied
analysing the ladder topology
Analysing the Ladder Topology
  • Same definitions as for snowflake network
    • E the number of subtended edge nodes (PEs) to each spar-node (E = M1*M2)
  • Discover
    • LPE = 2*(SPE - 1)
    • L2 = 2*M2*(SPE - 1) - M2*(M2 - 1)
    • L1 ≈ E*E*S1*S1/2 + E*E*S1 + 3*E*E - E*M2
  • Practical numbers
    • S 1 = 10, M1 = 10, and M2 = 20
    • E = 200
    • SPE = 2000
    • LPE = 3998
    • L2 = 79580
    • L1 = 2516000
option 1 solve a different problem
Option 1 – Solve a Different Problem!
  • If a full mesh of PE-PE LSPs is too big, don’t build it!
    • This is the bottom line if we don’t fix the problem
  • The suggestion is to build a full mesh of Pn-to-Pn LSPs, and perform routing or routing-based MPLS between Pn and PE
  • Scaling improves from O(10002)to O(1002)
  • But we lose functionality
    • Why did we want a PE-PE mesh?
    • How do we handle private address spaces?
    • What if the traffic is not routable?
  • This may simply not be good enough to provide the function
option 2 lsp hierarchies
Option 2 – LSP Hierarchies
  • Well-known, core MPLS function
    • Label stacks
    • Forwarding Adjacencies (RFC 4206)
    • Configured or automatic grooming
  • Possible to build a full or partialmesh of hierarchical tunnels
  • For example connect all P2 nodes
    • Each P2 node must encapsulate each PE-PE LSP in the correct tunnel
    • Each P1 node only sees the P2-P2 tunnels
scaling properties of hierarchies snowflake
Scaling Properties of Hierarchies - Snowflake
  • Note that PE-PE tunnels don’t help
  • P1-P1 tunnels are also no benefit (core is fully meshed)
  • P2 nodes see all PE-PE LSPs and new tunnels
    • L2 = M2*(2*SPE - M2 - 1) + 2*(S2 - 1)
  • Situation at P1 nodes is much better
    • L1 = M1*(2*S2 - M1 - 1)
  • Numbers (S1 = 10, M1 = 10, and M2 = 20)

Flat 2-Level Hierarchy

SPE 2000 2000

LPE 3998 3998

L27958079778

L1 756000 1890

  • Maybe insert another layer (P3 ) to increase the scaling?
    • L3 remains high
scaling properties of hierarchies ladder
Scaling Properties of Hierarchies - Ladder
  • Note that PE-PE tunnels don’t help
  • But P1-P1 tunnels are good because core is not fully-meshed
    • L1≈ S1*S1/2 + 2*S1 + 2*E*E*(S1 - 1) - E*M2 - 2
  • Another level of hierarchy is also possible
    • Add a mesh of P2-P2 tunnels
      • L1 = S1*S1/2 + 2*S1 + 2*M1*M1*S1 - M1(M1 + 1) – 2
      • L2 = 2*M2*(S(PE) - 1) - M2*(M2 - 1) + 2*(S(1)*M(1) - 1)
  • Numbers (S 1 = 10, M1 = 10, and M2 = 20)

Flat 2-Level 3-Level

Hierarchy Hierarchy

SPE 2000 2000 2000

LPE 3998 3998 3998

L2 79580 79580 79778

L1 2516000716060 1958

issues and drawbacks for hierarchies
Issues and Drawbacks for Hierarchies
  • Scaling is not good enough!
    • Impact on layer adjacent to PEs is negligible
      • Actually impact is slightly negative
  • Management burden
    • Plan and operate a secondary mesh
      • Effectively the same burden as managing PEs or a layered network
      • Possible to consider auto-mesh techniques
  • Fast Reroute protection is a problem
    • FRR struggles to protect tunnel end-points
  • Not obvious how to arrange the hierarchy when the network is not symmetrical
    • E.g., some PEs closer to the core
option 3 multipoint to point lsps
Option 3 – Multipoint-to-Point LSPs
  • LSPs merge automatically as they converge on the destination
  • Reduces the number of LSPs toward the egress
  • Other LSP properties (e.g.,bandwidth) must be cumulative
  • TE is still possible, butde-merge is not considered
  • Should count “LSP state” not number of LSPs
    • New definition
      • Xn the amount of LSP state held at each Pn node
    • For flat and hierarchical networks:
      • Each LSP adds one state at ingress or egress
      • Each LSP adds two states at each transit node
scaling properties of mp2p lsps snowflake
Scaling Properties of MP2P LSPs - Snowflake

XPE = 2*(SPE - 1)

X2 = SPE*(M2 + 1)

X1=M1*M2*(S1 - 2) + SPE*(M1 + 1)

  • Numbers (S1 = 10, M1 = 10, and M2 = 20)

Flat 2-Level Hierarchy P2MP

SPE 2000 2000 2000

XPE 3998 3998 3998

X215916015935842000

X1 1512000 3780 23600

scaling properties of mp2p lsps ladder
Scaling Properties of MP2P LSPs - Ladder

XPE = 2*(SPE - 1)

X2 = (M2 + 1)*S1*E

X1≤ (4 + M1)*S1*E - M1*E

  • Numbers (S1 = 10, M1 = 10, and M2 = 20)

Flat 2-Level 3-Level P2MP

Hierarchy Hierarchy

SPE 2000 2000 2000 2000

XPE 3998 3998 3998 3998

X2159160 159160 159358 42000

X1 5032000 1433998 3898 26000

issues and drawbacks for mp2p lsps
Issues and Drawbacks for MP2P LSPs
  • Clear scaling benefits
    • Better than flat networks
    • Only thing that improves the situation adjacent to PEs
  • But…
    • Data plane support
      • This will only ever be a packet/frame/cell technology
    • Control plane support
      • RSVP does have MP2P support
      • RSVP-TE features not yet specified or implemented
    • De-aggregation and disambiguation
      • May be necessary to use label stack so that egress can detect sender of data
    • OAM may be more complex and require source labels
    • New management applications needed
    • FRR still to be designed
other topics for investigation
Other Topics for Investigation
  • Cost-effectiveness of the network
    • Revenue only generated by PEs
    • K = S(PE)/(S(1)+S(2) + ... + S(n))
    • Many ways to improve scaling reduce cost-effectiveness
  • Fast Reroute
    • What are the implications of FRR to scaling?
    • Can scaling contributions be designed that can be protected by FRR?
  • Point-to-multipoint
      • What are the scaling properties of P2MP MPLS-TE?
  • Domain boundaries (in particular AS boundaries)
    • Boundaries such as at area and AS borders cause constrictions
    • How can we reduce the number of LSPs seen by ABRs and ASBRs?
conclusions next steps and references
Conclusions, Next Steps, and References
  • MPLS-TE is not a scaling issue today
    • But it won’t scale arbitrarily
  • We need to plan now for tomorrow’s scalability
  • Hierarchical LSPs are not as good as expected
  • MP2P LSPs may offer a better solution
  • More research and implementation is needed
  • draft-ietf-mpls-te-scaling-analysis-01.txt
    • Seisho Yaukawa (NTT)
    • Adrian Farrel (Old Dog Consulting)
    • Olufemi Komolafe (Cisco Systems)
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