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

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### MPLS-TE Doesn’t ScaleAdrian FarrelOld Dog [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
- 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?

- 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?

- 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?

- 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

- 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

- 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

- 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

- 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!

- 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

- 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

- 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

- 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

- 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

- 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

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

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

- 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

- 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

- 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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