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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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MPLS-TE Doesn’t Scale Adrian Farrel Old Dog Consulting [email protected]

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Mpls te doesn t scale adrian farrel old dog consulting adrian@olddog co uk

MPLS-TE Doesn’t ScaleAdrian FarrelOld Dog [email protected]

www.mpls2007.com


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)

    Flat2-Level Hierarchy

    SPE 20002000

    LPE39983998

    L27958079778

    L17560001890

  • 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)

    Flat2-Level 3-Level

    HierarchyHierarchy

    SPE200020002000

    LPE399839983998

    L2795807958079778

    L125160007160601958


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)

    Flat2-Level HierarchyP2MP

    SPE 200020002000

    XPE399839983998

    X215916015935842000

    X11512000378023600


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)

    Flat2-Level 3-LevelP2MP

    HierarchyHierarchy

    SPE200020002000 2000

    XPE3998399839983998

    X215916015916015935842000

    X150320001433998389826000


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)


    Mpls te doesn t scale adrian farrel old dog consulting adrian olddog co uk

    Questions?

    [email protected]


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