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| author | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
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| committer | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
| commit | 4bfd864f10b68b71482b35c818559068ef8d5797 (patch) | |
| tree | e3989f47a7994642eb325063d46e8f08ffa681dc /doc/rfc/rfc5439.txt | |
| parent | ea76e11061bda059ae9f9ad130a9895cc85607db (diff) | |
doc: Add RFC documents
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diff --git a/doc/rfc/rfc5439.txt b/doc/rfc/rfc5439.txt new file mode 100644 index 0000000..dbab3a1 --- /dev/null +++ b/doc/rfc/rfc5439.txt @@ -0,0 +1,2523 @@ + + + + + + +Network Working Group S. Yasukawa +Request for Comments: 5439 NTT +Category: Informational A. Farrel + Old Dog Consulting + O. Komolafe + Cisco Systems + February 2009 + + + An Analysis of Scaling Issues in MPLS-TE Core Networks + +Status of This Memo + + This memo provides information for the Internet community. It does + not specify an Internet standard of any kind. Distribution of this + memo is unlimited. + +Copyright Notice + + Copyright (c) 2009 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents (http://trustee.ietf.org/ + license-info) in effect on the date of publication of this document. + Please review these documents carefully, as they describe your rights + and restrictions with respect to this document. + +Abstract + + Traffic engineered Multiprotocol Label Switching (MPLS-TE) is + deployed in providers' core networks. As providers plan to grow + these networks, they need to understand whether existing protocols + and implementations can support the network sizes that they are + planning. + + This document presents an analysis of some of the scaling concerns + for the number of Label Switching Paths (LSPs) in MPLS-TE core + networks, and examines the value of two techniques (LSP hierarchies + and multipoint-to-point LSPs) for improving scaling. The intention + is to motivate the development of appropriate deployment techniques + and protocol extensions to enable the application of MPLS-TE in large + networks. + + This document only considers the question of achieving scalability + for the support of point-to-point MPLS-TE LSPs. Point-to-multipoint + MPLS-TE LSPs are for future study. + + + + +Yasukawa, et al. Informational [Page 1] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +Table of Contents + + 1. Introduction ....................................................3 + 1.1. Overview ...................................................3 + 1.2. Glossary of Notation .......................................5 + 2. Issues of Concern for Scaling ...................................5 + 2.1. LSP State ..................................................5 + 2.2. Processing Overhead ........................................6 + 2.3. RSVP-TE Implications .......................................6 + 2.4. Management .................................................7 + 3. Network Topologies ..............................................8 + 3.1. The Snowflake Network Topology .............................9 + 3.2. The Ladder Network Topology ...............................11 + 3.3. Commercial Drivers for Selected Configurations ............14 + 3.4. Other Network Topologies ..................................15 + 4. Required Network Sizes .........................................16 + 4.1. Practical Numbers .........................................16 + 5. Scaling in Flat Networks .......................................16 + 5.1. Snowflake Networks ........................................17 + 5.2. Ladder Networks ...........................................18 + 6. Scaling Snowflake Networks with Forwarding Adjacencies .........22 + 6.1. Two-Layer Hierarchy .......................................22 + 6.1.1. Tuning the Network Topology to Suit the + Two-Layer Hierarchy ................................23 + 6.2. Alternative Two-Layer Hierarchy ...........................24 + 6.3. Three-Layer Hierarchy .....................................25 + 6.4. Issues with Hierarchical LSPs .............................26 + 7. Scaling Ladder Networks with Forwarding Adjacencies ............27 + 7.1. Two-Layer Hierarchy .......................................27 + 7.2. Three-Layer Hierarchy .....................................28 + 7.3. Issues with Hierarchical LSPs .............................29 + 8. Scaling Improvements through Multipoint-to-Point LSPs ..........30 + 8.1. Overview of MP2P LSPs .....................................30 + 8.2. LSP State: A Better Measure of Scalability ................31 + 8.3. Scaling Improvements for Snowflake Networks ...............32 + 8.3.1. Comparison with Other Scenarios ....................33 + 8.4. Scaling Improvements for Ladder Networks ..................34 + 8.4.1. Comparison with Other Scenarios ....................36 + 8.4.2. LSP State Compared with LSP Numbers ................37 + 8.5. Issues with MP2P LSPs .....................................37 + 9. Combined Models ................................................39 + 10. An Alternate Solution .........................................39 + 10.1. Pros and Cons of the Alternate Solution ..................40 + 11. Management Considerations .....................................42 + 12. Security Considerations .......................................42 + 13. Recommendations ...............................................42 + + + + + +Yasukawa, et al. Informational [Page 2] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + 14. Acknowledgements ..............................................43 + 15. Normative References ..........................................43 + 16. Informative References ........................................43 + +1. Introduction + + Network operators and service providers are examining scaling issues + as they look to deploy ever-larger traffic engineered Multiprotocol + Label Switching (MPLS-TE) networks. Concerns have been raised about + the number of Label Switched Paths (LSPs) that need to be supported + at the edge and at the core of the network. The impact on control + plane and management plane resources threatens to outweigh the + benefits and popularity of MPLS-TE, while the physical limitations of + the routers may constrain the deployment options. + + Historically, it has been assumed that all MPLS-TE scaling issues can + be addressed using hierarchical LSP [RFC4206]. However, analysis + shows that the improvement gained by LSP hierarchies is not as + significant in all topologies and at all points in the network as + might have been presumed. Further, additional management issues are + introduced to determine the end-points of the hierarchical LSPs and + to operate them. Although this does not invalidate the benefits of + LSP hierarchies, it does indicate that additional techniques may be + desirable in order to fully scale MPLS-TE networks. + + This document examines the scaling properties of two generic MPLS-TE + network topologies and investigates the benefits of two scaling + techniques. + +1.1. Overview + + Physical topology scaling concerns are addressed by building networks + that are not fully meshed. Network topologies tend to be meshed in + the core but tree-shaped at the edges, giving rise to a snowflake + design. Alternatively, the core may be more of a ladder shape with + tree-shaped edges. + + MPLS-TE, however, establishes a logical full mesh between all edge + points in the network, and this is where the scaling problems arise + since the structure of the network tends to focus a large number of + LSPs within the core of the network. + + This document presents two generic network topologies (the snowflake + and the ladder) and attempts to parameterize the networks by making + some generalities. It introduces terminology for the different + scaling parameters and examines how many LSPs might be required to be + carried within the core of a network. + + + + +Yasukawa, et al. Informational [Page 3] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Two techniques (hierarchical LSPs and multipoint-to-point LSPs) are + introduced and an examination is made of the scaling benefits that + they offer as well as of some of the concerns with using these + techniques. + + Of necessity, this document makes many generalizations. Not least + among these is a set of assumptions about the symmetry and + connectivity of the physical network. It is hoped that these + generalizations will not impinge on the usefulness of the overview of + the scaling properties that this document attempts to give. Indeed, + the symmetry of the example topologies tends to highlight the scaling + issues of the different solution models, and this may be useful in + exposing the worst case scenarios. + + Although protection mechanisms like Fast Reroute (FRR) [RFC4090] are + briefly discussed, the main body of this document considers stable + network cases. It should be noted that make-before-break + re-optimisation after link failure may result in a significant number + of 'duplicate' LSPs. This issue is not addressed in this document. + + It should also be understood that certain deployment models where + separate traffic engineered LSPs are used to provide different + services (such as layer 3 Virtual Private Networks (VPNs) [RFC4110] + or pseudowires [RFC3985]) or different classes of service [RFC3270] + may result in 'duplicate' or 'parallel' LSPs running between any pair + of provider edge nodes (PEs). This scaling factor is also not + considered in this document, but may be easily applied as a linear + factor by the reader. + + The operation of security mechanisms in MPLS-TE networks [MPLS-SEC] + may have an impact on the ability of the network to scale. For + example, they may increase both the size and number of control plane + messages. Additionally, they may increase the processing overhead as + control plane messages are subject to processing algorithms (such as + encryption), and security keys need to be managed. Deployers will + need to consider the trade-offs between scaling objectives and + security objectives in their networks, and should resist the + temptation to respond to a degradation of scaling performance by + turning off security techniques that have previously been deemed as + necessary. Further analysis of the effects of security measures on + scalability are not considered further in this document. + + This document is designed to help service providers discover whether + existing protocols and implementations can support the network sizes + that they are planning. To do this, it presents an analysis of some + of the scaling concerns for MPLS-TE core networks and examines the + + + + + +Yasukawa, et al. Informational [Page 4] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + value of two techniques for improving scaling. This should motivate + the development of appropriate deployment techniques and protocol + extensions to enable the application of MPLS-TE in large networks. + + This document only considers the question of achieving scalability + for the support of point-to-point MPLS-TE LSPs. Point-to-multipoint + MPLS-TE LSPs are for future study. + +1.2. Glossary of Notation + + This document applies consistent notation to define various + parameters of the networks that are analyzed. These terms are + defined as they are introduced throughout the document, but are + grouped together here for quick reference. Refer to the full + definitions in the text for detailed explanations. + + n A network level. n = 1 is the core of the network. + See Section 3 for more details on the definition of a level. + P(n) A node at level n in the network. + S(n) The number of nodes at level n. That is, the number of P(n) + nodes. + L(n) The number of LSPs seen by a P(n) node. + X(n) The number of LSP segment states held by a P(n) node. + M(n) The number of P(n+1) nodes subtended to a P(n) node. + R The number of rungs in a ladder network. + E The number of edge nodes (PEs) subtended below (directly or + indirectly) a spar-node in a ladder network. + K The cost-effectiveness of the network expressed in terms of + the ratio of the number of PEs to the number of network nodes. + +2. Issues of Concern for Scaling + + This section presents some of the issues associated with the support + of LSPs at a Label Switching Router (LSR) or within the network. + These issues may mean that there is a limit to the number of LSPs + that can be supported. + +2.1. LSP State + + LSP state is the data (information) that must be stored at an LSR in + order to maintain an LSP. Here, we refer to the information that is + necessary to maintain forwarding plane state and the additional + information required when LSPs are established through control plane + protocols. While the size of the LSP state is implementation- + dependent, it is clear that any implementation will require some data + in order to maintain LSP state. + + + + + +Yasukawa, et al. Informational [Page 5] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Thus, LSP state becomes a scaling concern because as the number of + LSPs at an LSR increases, so the amount of memory required to + maintain the LSPs increases in direct proportion. Since the memory + capacity of an LSR is limited, there is a related limit placed on the + number LSPs that can be supported. + + Note that techniques to reduce the memory requirements (such as data + compression) may serve to increase the number of LSPs that can be + supported, but this will only achieve a moderate multiplier and may + significantly decrease the ability to process the state rapidly. + + In this document, we define X(n) as "the number of LSP segment states + held by a P(n) node." This definition observes that an LSR at the + end of an LSP only has to maintain state in one direction (i.e., into + the network), while a transit LSR must maintain state in both + directions (i.e., toward both ends of the LSP). Furthermore, in + multipoint-to-point (MP2P) LSPs (see Section 8), a transit LSR may + need to maintain LSP state for one downstream segment (toward the + destination) and multiple upstream segments (from multiple sources). + That is, we define LSP segment state as the state necessary to + maintain an LSP in one direction to one adjacent node. + +2.2. Processing Overhead + + Depending largely on implementation issues, the number of LSPs + passing through an LSR may impact the processing speed for each LSP. + For example, control block search times can increase with the number + of control blocks to be searched, and even excellent implementations + cannot completely mitigate this fact. Thus, since CPU power is + constrained in any LSR, there may be a practical limit to the number + of LSPs that can be supported. + + Further processing overhead considerations depend on issues specific + to the control plane protocols, and are discussed in the next + section. + +2.3. RSVP-TE Implications + + Like many connection-oriented signaling protocols, RSVP-TE (Resource + Reservation Protocol - Traffic Engineering) requires that state is + held within the network in order to maintain LSPs. The impact of + this is described in Section 2.1. Note that RSVP-TE requires that + separate information is maintained for upstream and downstream + relationships, but does not require any specific implementation of + that state. + + + + + + +Yasukawa, et al. Informational [Page 6] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + RSVP-TE is a soft-state protocol, which means that protocol messages + (refresh messages) must be regularly exchanged between signaling + neighbors in order to maintain the state for each LSP that runs + between the neighbors. A common period for the transmission (and + receipt) of refresh messages is 30 seconds, meaning that each LSR + must send and receive one message in each direction (upstream and + downstream) every 30 seconds for every LSP it supports. This has the + potential to be a significant constraint on the scaling of the + network, but various improvements [RFC2961] mean that this refresh + processing can be significantly reduced, allowing an implementation + to be optimized to remove nearly all concerns about soft-state + scaling in a stable network. + + Observations of existing implementations indicate that there may be a + threshold of around 50,000 LSPs above which an LSR struggles to + achieve sufficient processing to maintain LSP state. Although + refresh reduction [RFC2961] may substantially improve this situation, + it has also been observed that under these circumstances the size of + the Srefresh may become very large, and the processing required may + still cause significant disruption to an LSR. + + Another approach is to increase the refresh time. There is a + correlation between the percentage increase in refresh time and the + improvement in performance for the LSR. However, it should be noted + that RSVP-TE's soft-state nature depends on regular refresh messages; + thus, a degree of functionality is lost by increasing the refresh + time. This loss may be partially mitigated by the use of the RSVP-TE + Hello message, and can also be reduced by the use of various GMPLS + extensions [RFC3473], such as the use of [RFC2961] message + acknowledgements on all messages. + + RSVP-TE also requires that signaling adjacencies be maintained + through the use of Hello message exchanges. Although [RFC3209] + suggests that Hello messages should be retransmitted every 5 ms, in + practice, values of around 3 seconds are more common. Nevertheless, + the support of Hello messages can represent a scaling limitation on + an RSVP-TE implementation since one message must be sent and received + to/from each signaling adjacency every time period. This can impose + limits on the number of neighbors (physical or logical) that an LSR + supports, but does not impact the number of LSPs that the LSR can + handle. + +2.4. Management + + Another practical concern for the scalability of large MPLS-TE + networks is the ability to manage the network. This may be + constrained by the available tools, the practicality of managing + large numbers of LSPs, and the management protocols in use. + + + +Yasukawa, et al. Informational [Page 7] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Management tools are software implementations. Although such + implementations should not constrain the control plane protocols, it + is realistic to appreciate that network deployments will be limited + by the scalability of the available tools. In practice, most + existing tools have a limit to the number of LSPs that they can + support. While a Network Management System (NMS) may be able to + support a large number of LSPs, the number that can be supported by + an Element Management System (EMS) (or the number supported by an NMS + per-LSR) is more likely to be limited. + + Similarly, practical constraints may be imposed by the operation of + management protocols. For example, an LSR may be swamped by + management protocol requests to read information about the LSPs that + it supports, and this might impact its ability to sustain those LSPs + in the control plane. OAM (Operations, Administration, and + Management), alarms, and notifications can further add to the burden + placed on an LSR and limit the number of LSPs it can support. + + All of these considerations encourage a reduction in the number of + LSPs supported within the network and at any particular LSR. + +3. Network Topologies + + In order to provide some generic analysis of the potential scaling + issues for MPLS-TE networks, this document explores two network + topology models. These topologies are selected partly because of + their symmetry, which makes them more tractable to a formulaic + approach, and partly because they represent generalizations of real + deployment models. Section 3.3 provides a discussion of the + commercial drivers for deployed topologies and gives more analysis of + why it is reasonable to consider these two topologies. + + The first topology is the snowflake model. In this type of network, + only the very core of the network is meshed. The edges of the + network are formed as trees rooted in the core. + + The second network topology considered is the ladder model. In this + type of network, the core of the network is shaped and meshed in the + form of a ladder and trees are attached rooted to the edge of the + ladder. + + The sections that follow examine these topologies in detail in order + to parameterize them. + + + + + + + + +Yasukawa, et al. Informational [Page 8] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +3.1. The Snowflake Network Topology + + The snowflake topologies considered in this document are based on a + hierarchy of connectivity within the core network. PE nodes have + connectivity to P-nodes as shown in Figure 1. There is no direct + connectivity between the PEs. Dual homing of PEs to multiple P-nodes + is not considered in this document, although it may be a valuable + addition to a network configuration. + + P + /|\ + / | \ + / | \ + / | \ + PE PE PE + + Figure 1 : PE to P-Node Connectivity + + The relationship between P-nodes is also structured in a hierarchical + way. Thus, as shown in Figure 2, multiple P-nodes at one level are + connected to a P-node at a higher level. We number the levels such + that level 1 is the top level (top in our figure, and nearest to the + core of the network) and level (n) is immediately above level (n+1); + we denote a P-node at level n as a P(n). + + As with PEs, there is no direct connectivity between P(n+1) nodes. + Again, dual homing of P(n+1) nodes to multiple P(n) nodes is not + considered in this document, although it may be a valuable addition + to a network configuration. + + P(n) + /|\ + / | \ + / | \ + / | \ + P(n+1) P(n+1) P(n+1) + + Figure 2 : Relationship between P-Nodes + + At the top level, P(1) nodes are connected in a full mesh. In + reality, the level 1 part of the network may be slightly less well- + connected than this, but assuming a full mesh provides for + generality. Thus, the snowflake topology comprises a clique with + topologically equivalent trees subtended from each node in the + clique. + + + + + + +Yasukawa, et al. Informational [Page 9] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + The key multipliers for scalability are the number of P(1) nodes and + the multiplier relationship between P(n) and P(n+1) at each level, + down to and including PEs. + + We define the multiplier M(n) as the number of P(n+1) nodes at level + (n+1) attached to any one P(n). Assume that M(n) is constant for all + nodes at level n. Since nodes at the same level are not + interconnected (except at the top level), and since each P(n+1) node + is connected to precisely one P(n) node, M(n) is one less than the + degree of the node at level n (that is, the P(n) node is attached to + M(n) nodes at level (n+1) and to 1 node at level (n-1)). + + We define S(n) as the number of nodes at level (n). + + Thus: + + S(n) = S(1)*M(1)*M(2)*...*M(n-1) + + So the number of PEs can be expressed as: + + S(PE) = S(1)*M(1)*M(2)*...*M(n) + + where the network has (n) layers of P-nodes. + + Thus, we may depict an example snowflake network as shown in Figure + 3. In this case: + + S(1) = 3 + M(1) = 3 + S(2) = S(1)*M(1) = 9 + M(2) = 2 + S(PE) = S(1)*M(1)*M(2) = 18 + + + + + + + + + + + + + + + + + + + +Yasukawa, et al. Informational [Page 10] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + PE PE PE PE PE PE + \ \/ \/ / + PE--P(2) P(2) P(2) P(2)--PE + \ | | / + \| |/ + PE--P(2)---P(1)------P(1)---P(2)--PE + / \ / \ + PE \ / PE + \/ + P(1) + /|\ + / | \ + / | \ + PE--P(2) P(2) P(2)--PE + / /\ \ + PE PE PE PE + + Figure 3 : An Example Snowflake Network + +3.2. The Ladder Network Topology + + The ladder networks considered in this section are based on an + arrangement of routers in the core network that resembles a ladder. + + Ladder networks typically have long and thin cores that are arranged + as conventional ladders. That is, they have one or more spars + connected by rungs. Each node on a spar may have: + + - connection to one or more other spars, + - connection to a tree of other core nodes, + - connection to customer nodes. + + Figure 4 shows a simplified example of a ladder network. A core of + twelve nodes makes up two spars connected by six rungs. + + + + + + + + + + + + + + + + + +Yasukawa, et al. Informational [Page 11] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + PE PE PE PE + PE PE PE | PE | PE PE PE | PE | PE + \| \|/ |/ | \| \|/ + PE-P-----P-----P-----P------P-----P--PE + | | | | | |\ + | | | | | | PE + | | | | | | + PE-P-----P-----P-----P------P-----P + /| /|\ |\ |\ |\ \ + PE PE PE | PE | PE | PE | PE PE + PE PE PE PE + + Figure 4 : A Simplified Ladder Network + + In practice, not all nodes on a spar (call them spar-nodes) need to + have subtended PEs. That is, they can exist simply to give + connectivity along the spar to other spar-nodes, or across a rung to + another spar. Similarly, the connectivity between spars can be more + complex with multiple connections from one spar-node to another spar. + Lastly, the network may be complicated by the inclusion of more than + two spars (or simplified by reduction to a single spar). + + These variables make the ladder network non-trivial to model. For + the sake of simplicity, we will make the following restrictions: + + - There are precisely two spars in the core network. + + - Every spar-node connects to precisely one spar-node on the other + spar. That is, each spar-node is attached to precisely one rung. + + - Each spar-node connects to either one (end-spar) or two (core-spar) + other spar-nodes on the same spar. + + - Every spar-node has the same number of PEs subtended. This does + not mean that there are no P-nodes subtended to the spar-nodes, but + does mean that the edge tree subtended to each spar-node is + identical. + + From these restrictions, we are able to quantify a ladder network as + follows: + + R - The number of rungs. That is, the number of spar-nodes on + each spar. + S(1) - The number of spar-nodes in the network. S(1)=2*R. + E - The number of subtended edge nodes (PEs) to each spar-node. + + + + + + +Yasukawa, et al. Informational [Page 12] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + The number of rungs may vary considerably. A number less than 3 is + unlikely (since that would not be a significantly connected network), + and a number greater than 100 seems improbable (because that would + represent a very long, thin network). + + E can be treated as for the snowflake network. That is, we can + consider a number of levels of attachment from P(1) nodes, which are + the spar-nodes, through P(i) down to P(n), which are the PEs. + Practically, we need to only consider n=2 (PEs attached direct to the + spar-nodes) and n=3 (one level of P-nodes between the PEs and the + spar-nodes). + + Let M(i) be the ratio of P(i) nodes to P(i-1) nodes, i.e., the + connectivity between levels of P-node as defined for the snowflake + topology. Hence, the number of nodes at any level (n) is: + + S(n) = S(1)*M(1)*M(2)*...*M(n-1) + + So the number of PEs subtended to a spar-node is: + + E = M(1)*M(2)*...*M(n) + + And the number of PEs can be expressed as: + + S(PE) = S(1)*M(1)*M(2)*...*M(n) + = S(1)*E + + Thus, we may depict an example ladder network as shown in Figure 5. + In this case: + + R = 5 + S(1) = 10 + M(1) = 2 + S(2) = S(1)*M(1) = 20 + M(2) = 2 + E = M(1)*M(2) = 4 + S(PE) = S(1)*E = 40 + + + + + + + + + + + + + + +Yasukawa, et al. Informational [Page 13] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + PE PE PE PE PE PE PE PE PE PE PE PE PE PE PE PE + \| \| \| \| |/ |/ |/ |/ + P(2) P(2) P(2) P(2) P(2) P(2) P(2) P(2) + \ \ | \ / | / / + PE \ \ | \ / | / / PE + \ \ \| \/ |/ / / + PE-P(2)---P(1)---P(1)---P(1)---P(1)---P(1)---P(2)-PE + | | | | | + | | | | | + | | | | | + PE-P(2)---P(1)---P(1)---P(1)---P(1)---P(1)---P(2)-PE + / / / | /\ |\ \ \ + PE / / | / \ | \ \ PE + / / | / \ | \ \ + P(2) P(2) P(2) P(2) P(2) P(2) P(2) P(2) + /| /| /| /| |\ |\ |\ |\ + PE PE PE PE PE PE PE PE PE PE PE PE PE PE PE PE + + Figure 5 : An Example Ladder Network + +3.3. Commercial Drivers for Selected Configurations + + It is reasonable to ask why these two particular network topologies + have been chosen. + + The most important consideration is physical scalability. Each node + (Label Switching Router - LSR) is only able to support a limited + number of physical interfaces. This necessarily reduces the ability + to fully mesh a network and leads to the tree-like structure of the + network toward the PEs. + + A realistic commercial consideration for an operator is the fact that + the only revenue-generating nodes in the network are the PEs. Other + nodes are needed only to support connectivity and scalability. + Therefore, there is a desire to maximize S(PE) while minimizing the + sum of S(n) for all values of (n). This could be achieved by + minimizing the number of levels and maximizing the connectivity at + each layer, M(n). Ultimately, however, this would produce a network + of just interconnected PEs, which is clearly in conflict with the + physical scaling situation. + + Therefore, the solution calls for a "few" levels with "relatively + large" connectivity at each level. We might say that the cost- + effectiveness of the network can be stated as: + + K = S(PE)/(S(1)+S(2) + ... + S(n)) where n is the level above the PEs + + + + + +Yasukawa, et al. Informational [Page 14] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + We should observe, however, that this equation may be naive in that + the cost of a network is not actually a function of the number of + routers (since a router chassis is often free or low cost), but is + really a function of the cost of the line cards, which is, itself, a + product of the capacity of the line cards. Thus, the relatively high + connectivity decreases the cost-effectiveness, while a topology that + tends to channel data through a network core tends to demand higher + capacity (and so, more expensive) line cards. + + A further consideration is the availability of connectivity (usually + fibers) between LSR sites. Although it is always possible to lay new + fiber, this may not be cost-effective or timely. The physical shape + and topography of the country in which the network is laid is likely + to be as much of a problem. If the country is 'long and thin', then + a ladder network is likely to be used. + + This document examines the implications for control plane and data + plane scalability of this type of network when MPLS-TE LSPs are used + to provide full connectivity between all PEs. + +3.4. Other Network Topologies + + As explained in Section 1, this document is using two symmetrical and + generalized network topologies for simplicity of modelling. In + practice, there are two other topological considerations. + + a. Multihoming + It is relatively common for a node at level (n) to be attached to + more than one node at level (n-1). This is particularly common at + PEs that may be connected to more than one P(n). + + b. Meshing within a level + A level in the network will often include links between P-nodes at + the same level, including the possibility of links between PEs. + This may result in a network that looks like a series of + concentric circles with spokes. + + Both of these features are likely to have some impact on the scaling + of the networks. However, for the purposes of establishing the + ground rules for scaling, this document restricts itself to the + consideration of the symmetrical networks described in Sections 2.1 + and 2.2. Discussion of other network formats is for future study. + + + + + + + + + +Yasukawa, et al. Informational [Page 15] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +4. Required Network Sizes + + An important question for this evaluation and analysis is the size of + the network that operators require. How many PEs are required? What + ratio of P to PE is acceptable? How many ports do devices have for + physical connectivity? What type of MPLS-TE connectivity between PEs + is required? + + Although presentation of figures for desired network sizes must be + treated with caution because history shows that networks grow beyond + all projections, it is useful to set some acceptable lower bounds. + That is, we can state that we are interested in networks of at least + a certain size. + + The most important features are: + + - The network should have at least 1000 PEs. + - Each pair of PEs should be connected by at least one LSP in each + direction. + +4.1. Practical Numbers + + In practice, reasonable target numbers are as follows. + + S(PE) >= 1000 + Number of levels is 3. That is: 1, 2, and PE. + M(2) <= 20 + M(1) <= 20 + S(1) <= 100 + +5. Scaling in Flat Networks + + Before proceeding to examine potential scaling improvements, we need + to examine how well the flat networks described in the previous + sections scale. + + Consider the requirement for a full mesh of LSPs linking all PEs. + That is, each PE has an LSP to and from every other LSP. Thus, if + there are S(PE) PEs in the network, there are S(PE)*(S(PE) - 1) LSPs. + + Define L(n) as the number of LSPs handled by a level (n) LSR. + + L(PE) = 2*(S(PE) - 1) + + + + + + + + +Yasukawa, et al. Informational [Page 16] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +5.1. Snowflake Networks + + There are a total of S(PE) PEs in the network and, since each PE + establishes an LSP with every other PE, it would be expected that + there are S(PE) - 1 LSPs incoming to each PE and the same number of + LSPs outgoing from the same PE, giving a total of 2(S(PE) - 1) on the + incident link. Hence, in a snowflake topology (see Figure 3), since + there are M(2) PEs attached to each P(2) node, it may tempting to + think that L(2) (the number of LSPs traversing each P(2) node) is + simply 2*(S(PE) - 1)*M(2). However, it should be noted that of the + S(PE) - 1 LSPs incoming to each PE, M(2) - 1 originated from nodes + attached to the same P(2) node, and so this value would count the + LSPs between the M(2) PEs attached to each P(2) node twice: once when + outgoing from the M(2) - 1 other nodes and once when incoming into a + particular PE. + + There are a total of M(2)*(M(2) - 1) LSPs between these M(2) PEs and, + since this value is erroneously included twice in 2*(S(PE) - 1)*M(2), + the correct value is: + + L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1) + = M(2)*(2*S(PE) - M(2) - 1) + + An alternative way of looking at this, that proves extensible for the + calculation of L(1), is to observe that each PE subtended to a P(2) + node has an LSP in each direction to all S(PE) - M(2) PEs in the rest + of the system, and there are M(2) such locally subtended PEs; thus, + 2*M(2)*(S(PE) - M(2)). Additionally, there are M(2)*(M(2) - 1) LSPs + between the locally subtended PEs. So: + + L(2) = 2*M(2)*(S(PE) - M(2)) + M(2)*(M(2) - 1) + = M(2)*(2*S(PE) - M(2) - 1) + + L(1) can be computed in the same way as this second evaluation of + L(2). Each PE subtended below a P(1) node has an LSP in each + direction to all PEs not below the P(1) node. There are M(1)*M(2) + PEs below the P(1) node, so this accounts for 2*M(1)*M(2)*(S(PE) - + M(1)*M(2)) LSPs. To this, we need to add the number of LSPs that + pass through the P(1) node and that run between the PEs subtended + below the P(1). Consider each P(2): it has M(2) PEs, each of which + has an LSP going to all of the PEs subtended to the other P(2) nodes + subtended to the P(1). There are M(1) - 1 such other P(2) nodes, and + so M(2)*(M(1) - 1) other such PEs. So the number of LSPs from the + PEs below a P(2) node is M(2)*M(2)*(M(1) - 1). And there are M(1) + P(2) nodes below the P(1), giving rise to a total of + M(2)*M(2)*M(1)*(M(1) - 1) LSPs. Thus: + + + + + +Yasukawa, et al. Informational [Page 17] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + L(1) = 2*M(1)*M(2)*(S(PE) - M(1)*M(2)) + M(2)*M(2)*M(1)*(M(1) - 1) + = M(1)*M(2)*(2*S(PE) - M(2)*(M(1) + 1)) + + So, for example, with S(1) = 5, M(1) = 10, and M(2) = 20, we see: + + S(PE) = 1000 + L(PE) = 1998 + L(2) = 39580 + L(1) = 356000 + + Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see: + + S(PE) = 2000 + L(PE) = 3998 + L(2) = 79580 + L(1) = 756000 + + In both examples, the number of LSPs at the core (P(1)) nodes is + probably unacceptably large, even though there are only a relatively + modest number of PEs. In fact, L(2) may even be too large in the + second example. + +5.2. Ladder Networks + + In ladder networks, L(PE) remains the same at 2*(S(PE) - 1). + + L(2) can be computed using the same mechanism as for the snowflake + topology because the subtended tree is the same format. Hence, + + L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1) + + But L(1) requires a different computation because each P(1) not only + sees LSPs for the subtended PEs, but is also a transit node for some + of the LSPs that cross the core (the core is not fully meshed). + + Each P(1) sees: + + o all of the LSPs between locally attached PEs, + o less those LSPs between locally attached PEs that can be served + exclusively by the attached P(2) nodes, + o all LSPs between locally attached PEs and remote PEs, and + o LSPs in transit that pass through the P(1). + + The first three numbers are easily determined and match what we have + seen from the snowflake network. They are: + + + + + + +Yasukawa, et al. Informational [Page 18] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + o E*(E-1) + o M(1)*M(2)*(M(2)-1) = E*(M(2) - 1) + o 2*E*E*(S(1) - 1) + + The number of LSPs in transit is more complicated to compute. It is + simplified by not considering the ends of the ladders but by + examining an arbitrary segment of the middle of the ladder, such as + shown in Figure 6. We look to compute and generalize the number of + LSPs traversing each core link (labeled a and b in Figure 6) and so + determine the number of transit LSPs seen by each P(1). + + : : : : : : + : : : : : : + P(2) P(2) P(2) P(2) P(2) P(2) + \ | \ / | / + \ | \ / | / + \| \/ |/ + ......P(1)---P(1)---P(1)...... + | a | | + | |b | + | | | + ......P(1)---P(1)---P(1)...... + /| /\ |\ + / | / \ | \ + / | / \ | \ + P(2) P(2) P(2) P(2) P(2) P(2) + : : : : : : + : : : : : : + + Figure 6 : An Arbitrary Section of a Ladder Network + + Of course, the number of LSPs carried on links a and b in Figure 6 + depends on how LSPs are routed through the core network. But if we + assume a symmetrical routing policy and an even distribution of LSPs + across all shortest paths, the result is the same. + + Now we can see that each P(1) sees half of 2a+b LSPs (since each LSP + would otherwise be counted twice as it passed through the P(1)), + except that some of the LSPs are locally terminated and so are only + included once in the sum 2a+b. + + So L(1) = a + b/2 - + (locally terminated transit LSPs)/2 + + (locally contained LSPs) + + + + + + + +Yasukawa, et al. Informational [Page 19] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Thus: + + L(1) = a + b/2 - + 2*E*E*(S(1) - 1)/2 + + E*(E-1) - E*(M(2) - 1) + = a + b/2 + + E*E*(2 - S(1)) - E*M(2) + + So all we have to do is work out a and b. + + Recall that the ladder length R = S(1)/2, and define X = E*E. + + Consider the contribution made by all of the LSPs that make n hops on + the ladder to the totals of each of a and b. If the ladder was + unbounded, then we could say that in the case of a, there are n*2X + LSPs along the spar only, and n(n-1)*2X/n = 2X(n-1) LSPs use a rung + and the spar. Thus, the LSPs that make n hops on the ladder + contribute (4n-2)X LSPs to a. Note that the edge cases are special + because LSPs that make only one hop on the ladder cannot transit a + P(1) but only start or end there. + + So with a ladder of length R = S(1)/2, we could say: + + R + a = SUM[(4i-2)*X] + 2RX + i=2 + + = 2*X*R*(R+1) + + And similarly, considering b in an unbounded ladder, the LSPs that + only travel one hop on the LSP are a special case, contributing 2X + LSPs, and every other LSP that traverses n hops on the ladder + contributes 2n*2X/n = 4X LSPs. So: + + R+1 + b = 2X + SUM[4X] + i=2 + + = 2*X + 4*X*R + + In fact, the ladders are bounded, and so the number of LSPs is + reduced because of the effect of the ends of the ladders. The links + that see the most LSPs are in the middle of the ladder. Consider a + ladder of length R; a node in the middle of the ladder is R/2 hops + away from the end of the ladder. So we see that the formula for the + contribution to the count of spar-only LSPs for a is only valid up to + n=R/2, and for spar-and-rung LSPs, up to n=1+R/2. Above these + limits, the contribution made by spar-only LSPs decays as (n-R/2)*2X. + + + +Yasukawa, et al. Informational [Page 20] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + However, for a first-order approximation, we will use the values of a + and b as computed above. This gives us an upper bound of the number + of LSPs without using a more complex formula for the reduction made + by the effect of the ends of the ladder. + + From this: + + L(1) = a + b/2 + + E*E*(2 - S(1)) - E*M(2) + = 2*X*R*(R+1) + + X + 2*X*R + + E*E*(2 - S(1)) - E*M(2) + = E*E*S(1)*(1 + S(1)/2) + + E*E + E*E*S(1) + + 2*E+E - E*E*S(1) - E*M(2) + = E*E*S(1)*(1 + S(1)/2) + 3*E+E - E*M(2) + = E*E*S(1)*S(1)/2 + E*E*S(1) + 3*E*E - E*M(2) + + So, for example, with S(1) = 6, M(1) = 10, and M(2) = 17, we see: + + E = 170 + S(PE) = 1020 + L(PE) = 2038 + L(2) = 34374 + L(1) = 777410 + + Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see: + + E = 200 + S(PE) = 2000 + L(PE) = 3998 + L(2) = 79580 + L(1) = 2516000 + + In both examples, the number of LSPs at the core (P(1)) nodes is + probably unacceptably large, even though there are only a relatively + modest number of PEs. In fact, L(2) may even be too large in the + second example. + + Compare the L(1) values with the total number of LSPs in the system + S(PE)*(S(PE) - 1), which is 1039380 and 3998000, respectively. + + + + + + + + + + +Yasukawa, et al. Informational [Page 21] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +6. Scaling Snowflake Networks with Forwarding Adjacencies + + One of the purposes of LSP hierarchies [RFC4206] is to improve the + scaling properties of MPLS-TE networks. LSP tunnels (sometimes known + as Forwarding Adjacencies (FAs)) may be established to provide + connectivity over the core of the network, and multiple edge-to-edge + LSPs may be tunneled down a single FA LSP. + + In our network we consider a mesh of FA LSPs between all core nodes + at the same level. We consider two possibilities here. In the + first, all P(2) nodes are connected to all other P(2) nodes by LSP + tunnels, and the PE-to-PE LSPs are tunneled across the core of the + network. In the second, an extra layer of LSP hierarchy is + introduced by connecting all P(1) nodes in an LSP mesh and tunneling + the P(2)-to-P(2) tunnels through these. + +6.1. Two-Layer Hierarchy + + In this hierarchy model, the P(2) nodes are connected by a mesh of + tunnels. This means that the P(1) nodes do not see the PE-to-PE + LSPs. + + It remains the case that: + + L(PE) = 2*(S(PE) - 1) + + L(2) is slightly increased. It can be computed as the sum of all + LSPs for all attached PEs, including the LSPs between the attached PE + (this figure is unchanged from Section 5.1, i.e., M(2)*(2*S(PE) - + M(2) - 1)), plus the number of FA LSPs providing a mesh to the other + P(2) nodes. Since the number of P(2) nodes is S(2), each P(2) node + sees 2*(S(2) - 1) FA LSPs. Thus: + + L(2) = M(2)*(2*S(PE) - M(2) - 1) + 2*(S(2) - 1) + + L(1), however, is significantly reduced and can be computed as the + sum of the number of FA LSPs to and from each attached P(2) to each + other P(2) in the network, including (but counting only once) the FA + LSPs between attached P(2) nodes. In fact, the problem is identical + to the L(2) computation in Section 5.1. So: + + L(1) = M(1)*(2*S(2) - M(1) - 1) + + + + + + + + + +Yasukawa, et al. Informational [Page 22] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + So, for example, with S(1) = 5, M(1) = 10, and M(2) = 20, we see: + + S(PE) = 1000 + S(2) = 50 + L(PE) = 1998 + L(2) = 39678 + L(1) = 890 + + Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see: + + S(PE) = 2000 + S(2) = 100 + L(PE) = 3998 + L(2) = 79778 + L(1) = 1890 + + So, in both examples, potential problems at the core (P(1)) nodes + caused by an excessive number of LSPs can be avoided, but any problem + with L(2) is made slightly worse, as can be seen from the table + below. + + Example| Count | Unmodified | 2-Layer + | | (Section 5.1) | Hierarchy + -------+-------+---------------+---------- + A | L(2) | 39580 | 39678 + | L(1) | 356000 | 890 + -------+-------+---------------+---------- + B | L(2) | 79580 | 79778 + | L(1) | 756000 | 1890 + +6.1.1. Tuning the Network Topology to Suit the Two-Layer Hierarchy + + Clearly, we can reduce L(2) by selecting appropriate values of S(1), + M(1), and M(2). We can do this without negative consequences, since + no change will affect L(PE) and since a large percentage increase in + L(1) is sustainable now that L(1) is so small. + + Observe that: + + L(2) = M(2)*(2*S(PE) - M(2) - 1) + 2*(S(2) - 1) + + where S(PE) = S(1)*M(1)*M(2) and S(2) = S(1)*M(1). So L(2) scales + with M(2)^2 and we can have the most impact by reducing M(2) while + keeping S(PE) constant. + + + + + + + +Yasukawa, et al. Informational [Page 23] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + For example, with S(1) = 10, M(1) = 10, and M(2) = 10, we see: + + S(PE) = 1000 + S(2) = 100 + L(PE) = 1998 + L(2) = 20088 + L(1) = 1890 + + And similarly, with S(1) = 20, M(1) = 20, and M(2) = 5, we see: + + S(PE) = 2000 + S(2) = 400 + L(PE) = 3998 + L(2) = 20768 + L(1) = 15580 + + These considerable scaling benefits must be offset against the cost- + effectiveness of the network. Recall from Section 3.3 that: + + K = S(PE)/(S(1)+S(2) ... + S(n)) + + where n is the level above the PEs, so that for our network: + + K = S(PE) / (S(1) + S(2)) + + Thus, in the first example the cost-effectiveness has been halved + from 1000/55 to 1000/110. In the second example, it has been reduced + to roughly one quarter, changing from 2000/110 to 2000/420. + + So, although the tuning changes may be necessary to reach the desired + network size, they come at a considerable cost to the operator. + +6.2. Alternative Two-Layer Hierarchy + + An alternative to the two-layer hierarchy presented in Section 6.1 is + to provide a full mesh of FA LSPs between P(1) nodes. This technique + is only of benefit to any nodes in the core of the level 1 network. + It makes no difference to the PE and P(2) nodes since they continue + to see only the PE-to-PE LSPs. Furthermore, this approach increases + the burden at the P(1) nodes since they have to support all of the + PE-to-PE LSPs as in the flat model plus the additional 2*(S(1) - 1) + P(1)-to-P(1) FA LSPs. Thus, this approach should only be considered + where there is a mesh of P-nodes within the ring of P(1) nodes, and + is not considered further in this document. + + + + + + + +Yasukawa, et al. Informational [Page 24] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +6.3. Three-Layer Hierarchy + + As demonstrated by Section 6.2, introducing a mesh of FA LSPs at the + top level (P(1)) has no benefit, but if we introduce an additional + level in the network (P(3) between P(2) and PE) to make a four-level + snowflake, we can introduce a new layer of FA LSPs so that we have a + full mesh of FA LSPs between all P(3) nodes to carry the PE-to-PE + LSPs, and a full mesh of FA LSPs between all P(2) nodes to carry the + P(3)-to-P(3) LSPs. + + The number of PEs is S(PE) = S(1)*M(1)*M(2)*M(3), and the number of + PE-to-PE LSPs at a PE remains L(PE) = 2*(S(PE) - 1). + + The number of LSPs at a P(3) can be deduced from Section 6.1. It is + the sum of all LSPs for all attached PEs, including the LSPs between + the attached PE, plus the number of FA LSPs providing a mesh to the + other P(3) nodes. + + L(3) = M(3)*(2*S(PE) - M(3) - 1) + 2*(S(3) - 1) + + The number of LSPs at P(2) can also be deduced from Section 6.1 since + it is the sum of all LSPs for all attached P(3) nodes, including the + LSPs between the attached PE plus the number of FA LSPs providing a + mesh to the other P(2) nodes. + + L(2) = M(2)*(2*S(3) - M(2) - 1) + 2*(S(2) - 1) + + Finally, L(1) can be copied straight from 6.1. + + L(1) = M(1)*(2*S(2) - M(1) - 1) + + For example, with S(1) = 5, M(1) = 5, M(2) = 5, and M(3) = 8, we see: + + S(PE) = 1000 + S(3) = 125 + S(2) = 25 + L(PE) = 1998 + L(3) = 16176 + L(2) = 1268 + L(1) = 220 + + + + + + + + + + + +Yasukawa, et al. Informational [Page 25] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Similarly, with S(1) = 5, M(1) = 5, M(2) = 8, and M(3) = 10, we see: + + S(PE) = 2000 + S(3) = 200 + S(2) = 25 + L(PE) = 3998 + L(3) = 40038 + L(2) = 3184 + L(1) = 220 + + Clearly, there are considerable scaling improvements with this three- + layer hierarchy, and all of the numbers (even L(3) in the second + example) are manageable. + + Of course, the extra level in the network tends to reduce the cost- + effectiveness of the networks with values of K = 1000/155 and K = + 2000/230 (from 1000/55 and 2000/110) for the examples above. That is + a reduction by a factor of 3 in the first case and 2 in the second + case. Such a change in cost-effectiveness has to be weighed against + the desire to deploy such a large network. If LSP hierarchies are + the only scaling tool available, and networks this size are required, + the cost-effectiveness may need to be sacrificed. + +6.4. Issues with Hierarchical LSPs + + A basic observation for hierarchical scaling techniques is that it is + hard to have any impact on the number of LSPs that must be supported + by the level of P(n) nodes adjacent to the PEs (for example, it is + hard to reduce L(3) in Section 6.3). In fact, the only way we can + change the number of LSPs supported by these nodes is to change the + scaling ratio M(n) in the network -- in other words, to change the + number of PEs subtended to any P(n). But such a change has a direct + effect on the number of PEs in the network and so the cost- + effectiveness is impacted. + + Another concern with the hierarchical approach is that it must be + configured and managed. This may not seem like a large burden, but + it must be recalled that the P(n) nodes are not at the edge of the + network -- they are a set of nodes that must be identified so that + the FA LSPs can be configured and provisioned. Effectively, the + operator must plan and construct a layered network with a ring of + P(n) nodes giving access to the level (n) network. This design + activity is open to considerable risk as failing to close the ring + (i.e., allowing a node to be at both level (n+1) and at level (n)) + may cause operational confusion. + + + + + + +Yasukawa, et al. Informational [Page 26] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Protocol techniques (such as IGP automesh [RFC4972]) have been + developed to reduce the configuration necessary to build this type of + multi-level network. In the case of automesh, the routing protocol + is used to advertise the membership of a 'mesh group', and all + members of the mesh group can discover each other and connect with + LSP tunnels. Thus, the P(n) nodes giving access to level (n) can + advertise their existence to each other, and it is not necessary to + configure each with information about all of the others. Although + this process can help to reduce the configuration overhead, it does + not eliminate it, as each member of the mesh group must still be + planned and configured for membership. + + An important consideration for the use of hierarchical LSPs is how + they can be protected using MPLS Fast Reroute (FRR) [RFC4090]. FRR + may provide link protection either by protecting the tunnels as they + traverse a broken link or by treating each level (n) tunnel LSP as a + link in level (n+1) and providing protection for the level (n+1) LSPs + (although in this model, fault detection and propagation time may be + an issue). Node protection may be performed in a similar way, but + protection of the first and last nodes of a hierarchical LSP is + particularly difficult. Additionally, the whole notion of scaling + with regard to FRR gives rise to separate concerns that are outside + the scope of this document as currently formulated. + + Finally, observe that we have been explaining these techniques using + conveniently symmetrical networks. Consider how we would arrange the + hierarchical LSPs in a network where some PEs are connected closer to + the center of the network than others. + +7. Scaling Ladder Networks with Forwarding Adjacencies + +7.1. Two-Layer Hierarchy + + In Section 6.2, we observed that there is no value to placing FA LSPs + between the P(1) nodes of our example snowflake topologies. This is + because those LSPs would be just one hop long and would, in fact, + only serve to increase the burden at the P(1) nodes. However, in the + ladder model, there is value to this approach. The P(1) nodes are + the spar-nodes of the ladder, and they are not all mutually adjacent. + That is, the P(1)-to-P(1) hierarchical LSPs can create a full mesh of + P(1) nodes where one does not exist in the physical topology. + + The number of LSPs seen by a P(1) node is then: + + o all of the tunnels terminating at the P(1) node, + o any transit tunnels, and + o all of the LSPs due to subtended PEs. + + + + +Yasukawa, et al. Informational [Page 27] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + This is a substantial reduction; all of the transit LSPs are reduced + to just one per remote P(1) that causes any transit LSP. So: + + L(1) = 2*(S(1) - 1) + + O(S(1)*S(1)/2) + + 2*E*E*(S(1) - 1) + E*(E-1) - E*(M(2) - 1) + + where O(S(1)*S(1)/2) gives an upper bound order of magnitude. So: + + L(1) = S(1)*S(1)/2 + 2*S(1) + 2*E*E*(S(1) - 1) - E*M(2) - 2 + + So, in our two examples: + + With S(1) = 6, M(1) = 10, and M(2) = 17, we see: + + E = 170 + S(PE) = 1020 + L(PE) = 2038 + L(2) = 34374 + L(1) = 286138 + + Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see: + + E = 200 + S(PE) = 2000 + L(PE) = 3998 + L(2) = 79580 + L(1) = 716060 + + Both of these show significant improvements over the previous L(1) + figures of 777410 and 2516000. But the numbers are still too large + to manage, and there is no improvement in the L(2) figures. + +7.2. Three-Layer Hierarchy + + We can also apply the three-layer hierarchy to the ladder model. In + this case, the number of LSPs between P(1) nodes is not reduced, but + tunnels are also set up between all P(2) nodes. Thus, the number of + LSPs seen by a P(1) node is: + + o all of the tunnels terminating at the P(1) node, + o any transit tunnels between P(1) nodes, and + o all of the LSPs due to subtended P(2) nodes. + + No PE-to-PE LSPs are seen at the P(1) nodes. + + + + + + +Yasukawa, et al. Informational [Page 28] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + L(1) = 2*(S(1) - 1) + + O(S(1)*S(1)/2) + + 2*(S(1) - 1)*M(1)*M(1) + M(1)*(M(1) - 1) + + where O(S(1)*S(1)/2) gives an upper bound order of magnitude. So: + + L(1) = S(1)*S(1)/2 + 2*S(1) + 2*M(1)*M(1)*S(1) - M(1)(M(1) + 1) - 2 + + Unfortunately, there is a small increase in the number of LSPs seen + by the P(2) nodes. Each P(2) now sees all of the PE-to-PE LSPs that + it saw before and is also an end-point for a set of P(2)-to-P(2) + tunnels. Thus, L(2) increases to: + + L(2) = 2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1) + 2*(S(1)*M(1) - 1) + + So, in our two examples: + + With S(1) = 6, M(1) = 10, and M(2) = 17, we see: + + E = 170 + S(PE) = 1020 + L(PE) = 2038 + L(2) = 34492 + L(1) = 1118 + + Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see: + + E = 200 + S(PE) = 2000 + L(PE) = 3998 + L(2) = 79778 + L(1) = 1958 + + This represents a very dramatic decrease in LSPs across the core. + +7.3. Issues with Hierarchical LSPs + + The same issues exist for hierarchical LSPs as described in Section + 6.4. Although dramatic improvements can be made to the scaling + numbers for the number of LSPs at core nodes, this can only be done + at the cost of configuring P(2) to P(2) tunnels. The mesh of P(1) + tunnels is not enough. + + But the sheer number of P(2) to P(2) tunnels that must be configured + is a significant management burden that can only be eased by using a + technique like automesh [RFC4972]. + + + + + +Yasukawa, et al. Informational [Page 29] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + It is significant, however, that the scaling problem at the P(2) + nodes cannot be improved by using tunnels and that the only solution + to ease this in the hierarchical approach would be to institute + another layer of hierarchy (that is, P(3) nodes) between the P(2) + nodes and the PEs. This is, of course, a significant expense. + +8. Scaling Improvements through Multipoint-to-Point LSPs + + An alternative (or complementary) scaling technique has been proposed + using multipoint-to-point (MP2P) LSPs. The fundamental improvement + in this case is achieved by reducing the number of LSPs toward the + destination as LSPs toward the same destination are merged. + + This section presents an overview of MP2P LSPs and describes their + applicability and scaling benefits. + +8.1. Overview of MP2P LSPs + + Note that the MP2P LSPs discussed here are for MPLS-TE and are not + the same concept familiar in the Label Distribution Protocol (LDP) + described in [RFC5036]. + + Traffic flows generally converge toward their destination and this + can be utilized by MPLS in constructing an MP2P LSP. With such an + LSP, the Label Forwarding Information Base (LFIB) mappings at each + LSR are many-to-one so that multiple pairs {incoming interface, + incoming label} are mapped to a single pair {outgoing interface, + outgoing label}. Obviously, if per-platform labels are used, this + mapping may be optimized within an implementation. + + It is important to note that with MP2P MPLS-TE LSPs, the traffic + flows are merged. That is, some additional form of identifier is + required if de-merging is required. For example, if the payload is + IP traffic belonging to the same client network, no additional de- + merging information is required since the IP packet contains + sufficient data. On the other hand, if the data comes, for example, + from a variety of VPN client networks, then the flows will need to be + labeled in their own right as point-to-point (P2P) flows, so that + traffic can be disambiguated at the egress of the MP2P LSPs. + + Techniques for establishing MP2P MPLS-TE LSPs and for assigning the + correct bandwidth downstream of LSP merge points are out of the scope + of this document. + + + + + + + + +Yasukawa, et al. Informational [Page 30] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +8.2. LSP State: A Better Measure of Scalability + + Consider the network topology shown in Figure 3. Suppose that we + establish MP2P LSP tunnels such that there is one tunnel terminating + at each PE, and that that tunnel has every other PE as an ingress. + Thus, a PE-to-PE MP2P LSP tunnel would have S(PE)-1 ingresses and one + egress, and there would be S(PE) such tunnels. + + Note that there still remain 2*(S(PE) - 1) PE-to-PE P2P LSPs that are + carried through these tunnels. + + Let's consider the number of LSPs handled at each node in the + network. + + The PEs continue to handle the same number of PE-to-PE P2P LSPs, and + must also handle the MP2P LSPs. So: + + L(PE) = 2*(S(PE) - 1) + S(PE) + + But all P(n) nodes in the network only handle the MP2P LSP tunnels. + Nominally, this means that L(n) = S(PE) for all values of n. This + would appear to be a great success with the number of LSPs cut to + completely manageable levels. + + However, the number of LSPs is not the only issue (although it may + have some impact for some of the scaling concerns listed in Section + 4). We are more interested in the amount of LSP state that is + maintained by an LSR. This reflects the amount of storage required + at the LSR, the amount of protocol processing, and the amount of + information that needs to be managed. + + In fact, we were also interested in this measure of scalability in + the earlier sections of this document, but in those cases we could + see a direct correlation between the number of LSPs and the amount of + LSP state since transit LSPs had two pieces of state information (one + on the incoming and one on the outgoing interface), and ingress or + egress LSPs had just one piece of state. + + We can quantify the amount of LSP state according to the number of + LSP segments managed by an LSR. So (as above), in the case of a P2P + LSP, an ingress or egress has one segment to maintain, while a + transit has two segments. Similarly, for an MP2P LSP, an LSR must + maintain one set of state information for each upstream segment + (which, we can assume, is in a one-to-one relationship with the + number of upstream neighbors) and exactly one downstream segment -- + ingresses obviously have no upstream neighbors, and egresses have no + downstream segments. + + + + +Yasukawa, et al. Informational [Page 31] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + So we can start again on our examination of the scaling properties of + MP2P LSPs using X(n) to represent the amount of LSP state held at + each P(n) node. + +8.3. Scaling Improvements for Snowflake Networks + + At the PEs, there is only connectivity to one other network node: the + P(2) node. But note that if P2P LSPs need to be used to allow + disambiguation of data at the MP2P LSP egresses, then these P2P LSPs + are tunneled within the MP2P LSPs. So X(PE) is: + + X(PE) = 2*(S(PE) - 1) if no disambiguation is required, + + and + + X(PE) = 4*(S(PE) - 1) if disambiguation is required. + + Each P(2) node has M(2) downstream PEs. The P(2) sees a single MP2P + LSP targeted at each downstream PE with one downstream segment (to + that PE) and M(2) - 1 upstream segments from the other subtended PEs. + Additionally, each of these LSPs has an upstream segment from the one + upstream P(1). This gives a total of M(2)*(1 + M(2)) LSP segments. + + There are also LSPs running from the subtended PEs to every other PE + in the network. There are S(PE) - M(2) such PEs, and the P(2) sees + one upstream segment for each of these from each subtended PE. It + also has one downstream segment for each of these LSPs. This gives + (M(2) + 1)*(S(PE) - M(2)) LSP segments. + + Thus: + + X(2) = M(2)*(1 + M(2)) + (M(2) + 1)*(S(PE) - M(2)) + = S(PE)*(M(2) + 1) + + Similarly, at each P(1) node there are M(1) downstream P(2) nodes and + so a total of M(1)*M(2) downstream PEs. Each P(1) is connected in a + full mesh with the other P(1) nodes and so has (S(1) - 1) neighbors. + + The P(1) sees a single MP2P LSP targeted at each downstream PE. This + has one downstream segment (to the P(2) to which the PE is connected) + and M(1) - 1 upstream segments from the other subtended P(2) nodes. + Additionally, each of these LSPs has an upstream segment from each of + the P(1) neighbors. This gives a total number of LSP segments of + M(1)*M(2)*(M(1) + S(1) - 1). + + There are also LSPs running from each of the subtended PEs to every + other PE in the network. There are S(PE) - M(1)M(2) such PEs, and + the P(1) sees one upstream segment for each of these from each + + + +Yasukawa, et al. Informational [Page 32] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + subtended P(2) (since the aggregation from the subtended PEs has + already happened at the P(2) nodes). It also has one downstream + segment to the appropriate next hop P(1) neighbor for each of these + LSPs. This gives (M(1) + 1)*(S(PE) - M(1)*M(2)) LSP segments. + + Thus: + + X(1) = M(1)*M(2)*(M(1) + S(1) - 1) + + (M(1) + 1)*(S(PE) - M(1)*M(2)) + = M(1)*M(2)*(S(1) - 2) + S(PE)*(M(1) + 1) + + So, for example, with S(1) = 10, M(1) = 10, and M(2) = 10, we see: + + S(PE) = 1000 + S(2) = 100 + X(PE) = 3996 (or 1998) + X(2) = 11000 + X(1) = 11800 + + And similarly, with S(1) = 20, M(1) = 20, and M(2) = 5, we see: + + S(PE) = 2000 + S(2) = 400 + X(PE) = 5996 (or 2998) + X(2) = 12000 + X(1) = 39800 + +8.3.1. Comparison with Other Scenarios + + For comparison with the examples in Sections 5 and 6, we need to + convert those LSP-based figures to our new measure of LSP state. + + Observe that each LSP in Sections 5 and 6 generates two state units + at a transit LSR and one at an ingress or egress. So we can provide + conversions as follows: + + Section 5 (flat snowflake network) + + L(PE) = 2*(S(PE) - 1) + L(2) = M(2)*(2*S(PE) - M(2) - 1) + L(1) = M(1)*M(2)*(2*S(PE) - M(2)*(M(1) + 1)) + X(PE) = 2*(S(PE) - 1) + X(2) = 2*M(2)*(2*S(PE) - M(2) - 1) + X(1) = 2*M(1)*M(2)*(2*S(PE) - M(2)*(M(1) + 1)) + + For the example with S(1) = 10, M(1) = 10, and M(2) = 10, this + gives a comparison table as follows: + + + + +Yasukawa, et al. Informational [Page 33] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Count | Unmodified | MP2P + ------+-------------+---------- + X(PE) | 1998 | 3996 + X(2) | 39780 | 11000 + X(1) | 378000 | 11800 + + Clearly, this technique is a significant improvement over the flat + network within the core of the network, although the PEs are more + heavily stressed if disambiguation is required. + + Section 6.1 (two-layer hierarchy snowflake network) + + L(PE) = 2*(S(PE) - 1) + L(2) = M(2)*(2*S(PE) - M(2) - 1) + 2*(S(2) - 1) + L(1) = M(1)*(2*S(2) - M(1) - 1) + X(PE) = 2*(S(PE) - 1) + X(2) = 2*M(2)*(2*S(PE) - M(2) - 1) + 2*(S(2) - 1) + X(1) = 2*M(1)*(2*S(2) - M(1) - 1) + + Note that in the computation of X(2) the hierarchical LSPs only add + one state at each P(2) node. + + For the same example with S(1) = 10, M(1) = 10, and M(2) = 10, this + gives a comparison table as follows: + + Count | 2-Layer | MP2P + | Hierarchy | + ------+-------------+---------- + X(PE) | 1998 | 3996 + X(2) | 39978 | 11000 + X(1) | 3780 | 11800 + + We can observe that the MP2P model is better at P(2), but the + hierarchical model is better at P(1). + + In fact, this comparison can be generalized to observe that the MP2P + model produces its best effects toward the edge of the network, while + the hierarchical model makes most impression at the core. However, + the requirement for disambiguation of P2P LSPs tunneled within the + MP2P LSPs does cause a double burden at the PEs. + +8.4. Scaling Improvements for Ladder Networks + + MP2P LSPs applied just within the ladder will not make a significant + difference, but applying MP2P for all LSPs and at all nodes makes a + very big difference without requiring any further configuration. + + + + + +Yasukawa, et al. Informational [Page 34] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + LSP state at a spar-node may be divided into those LSPs' segments + that enter or leave the spar-node due to subtended PEs (local LSP + segments), and those that enter or leave the spar-node due to remote + PEs (remote segments). + + The local segments may be counted as: + + o E LSPs targeting local PEs + o (S(1)-1)*E*M(1) LSPs targeting remote PEs + + The remote segments may be counted as: + + o (S(1)-1)*E outgoing LSPs targeting remote PEs + o <= 3*S(1)*E incoming LSPs targeting any PE (there are precisely + P(1) nodes attached to any other P(1) node) + + Hence, using X(1) as a measure of LSP state rather than a count of + LSPs, we get: + + X(1) <= E + (S(1)-1)*E*M(1) + (S(1)-1)*E + 3*S(1)*E + <= (4 + M(1))*S(1)*E - M(1)*E + + The number of LSPs at the P(2) nodes is also improved. We may also + count the LSP state in the same way so that there are: + + o M(2) LSPs targeting local PEs, + o M(2)*(S(1)*E) LSPs from local PEs to all other PEs, and + o S(1)*E - M(2) LSPs to remote PEs. + + So using X(2) as a measure of LSP state and not a count of LSPs, we + have: + + X(2) = M(2) + M(2)*(S(1)*E) + S(1)*E - M(2) + = (M(2) + 1)*S(1)*E + + Our examples from Section 5.2 give us the following numbers: + + With S(1) = 6, M(1) = 10, and M(2) = 17, we see: + + E = 170 + S(PE) = 1020 + X(PE) = 2038 + X(2) = 18360 + X(1) <= 12580 + + + + + + + +Yasukawa, et al. Informational [Page 35] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Alternatively, with S(1) = 10, M(1) = 10, and M(2) = 20, we see: + + E = 200 + S(PE) = 2000 + X(PE) = 3998 + X(2) = 42000 + X(1) <= 26000 + +8.4.1. Comparison with Other Scenarios + + The use of MP2P compares very favorably with all scaling scenarios. + It is the only technique able to reduce the value of X(2), and it + does this by a factor of almost two. The impact on X(1) is better + than everything except the three-level hierarchy. + + The following table provides a quick cross-reference for the figures + for the example ladder networks. Note that the previous figures are + modified to provide counts of LSP state rather than LSP numbers. + Again, each LSP contributes one state at its end points and two + states at transit nodes. + + Thus, for the all cases we have: + + X(PE) = 2*(S(PE) - 1) or + X(PE) = 4*(S(PE) - 1) if disambiguation is required. + + In the unmodified (flat) case, we have: + + X(2) = 2*(M(2)*(2*S(PE) - M(2) - 1)) + X(1) = 2*(M(1)*M(2)*(2*S(PE) - M(2)*(M(1) + 1))) + + In the two-level hierarchy, we have: + + X(2) = 2*(2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1)) + X(1) = S(1)*S(1) + 2*S(1) + 4*E*E*(S(1) - 1) - 2*E*M(2) - 2 + + In the three-level hierarchy, we have: + + X(2) = 2*(2*M(2)*(S(PE) - 1) - M(2)*(M(2) - 1)) + 2*(S(1)*M(1) - 1) + X(1) = S(1)*S(1) + 2*S(1) + 4*M(1)*M(1)*S(1) - 2*M(1)(M(1) + 1) - 2 + + Example A: S(1) = 6, M(1) = 10, and M(2) = 17 + Example B: S(1) = 10, M(1) = 10, and M(2) = 20 + + + + + + + + +Yasukawa, et al. Informational [Page 36] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + Example| Count | Unmodified | 2-Level | 3-Level | MP2P + | | | Hierarchy | Hierarchy | + -------+-------+------------+------------+-------------+------- + A | X(2) | 68748 | 68748 | 68866 | 18360 + | X(1) | 1554820 | 572266 | 2226 | 12580 + -------+-------+------------+------------+-------------+------- + B | X(2) | 159160 | 159160 | 159358 | 42000 + | X(1) | 5032000 | 1433998 | 3898 | 26000 + +8.4.2. LSP State Compared with LSP Numbers + + Recall that in Section 8.3, the true benefit of MP2P was analyzed + with respect to the LSP segment state required, rather than the + actual number of LSPs. This proved to be a more accurate comparison + of the techniques because the MP2P LSPs require state on each branch + of the LSP, so the saving is not linear with the reduced number of + LSPs. + + A similar analysis could be performed here for the ladder network. + The net effect is that it increases the state by an order of two for + all transit LSPs in the P2P models, and by a multiplier equal to the + degree of a node in the MP2P model. + + A rough estimate shows that, as with snowflake networks, MP2P + provides better scaling than the one-level hierarchical model and is + considerably better at the core. But MP2P compares less will with + the two-level hierarchy especially in the core. + +8.5. Issues with MP2P LSPs + + The biggest challenges for MP2P LSPs are the provision of support in + the control and data planes. To some extent, support must also be + provided in the management plane. + + Control plane support is just a matter of defining the protocols and + procedures [MP2P-RSVP], although it must be clearly understood that + this will introduce some complexity to the control plane. + + Hardware issues may be a little more tricky. For example, the + capacity of the upstream segments must never (allowing for + statistical over-subscription) exceed the capacity of the downstream + segment. Similarly, data planes must be equipped with sufficient + buffers to handle incoming packet collisions. + + The management plane will be impacted in several ways. Firstly, the + management applications will need to handle LSPs with multiple + senders. This means that, although the applications need to process + fewer LSPs, they will be more complicated and will, in fact, need to + + + +Yasukawa, et al. Informational [Page 37] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + process the same number of ingresses and egresses. Other issues like + diagnostics and OAM would also need to be enhanced to support MP2P, + but might be borrowed heavily from LDP networks. + + Lastly, note that when the MP2P solution is used, the receiver (the + single egress PE of an MP2P tunnel) cannot use the incoming label as + an indicator of the source of the data. Contrast this with P2P LSPs. + Depending on deployment, this might not be an issue since the PE-PE + connectivity may in any case be a tunnel with inner labels to + discriminate the data flows. + + In other deployments, it may be considered necessary to include + additional PE-PE P2P LSPs and tunnel these through the MP2P LSPs. + This would require the PEs to support twice as many LSPs. Since PEs + are not usually as fully specified as P-routers, this may cause some + concern; however, the use of penultimate hop popping on the MP2P LSPs + might help to reduce this issue. + + In all cases, care must be taken not to confuse the reduction in the + number of LSPs with a reduction in the LSP state that is required. + In fact, the discussion in Section 8.3 is slightly optimistic since + LSP state toward the destination will probably need to include sender + information and so will increase depending on the number of senders + for the MP2P LSP. Section 8.4, on the other hand, counts LSP state + rather than LSPs. This issue is clearly dependent on the protocol + solution for MP2P RSVP-TE, which is out of scope for this document. + + MPLS Fast Reroute (FRR) [RFC4090] is an attractive scheme for + providing rapid local protection from node or link failures. Such a + scheme has, however, not been designed for MP2P at the time of + writing, so it remains to be seen how practical it could be, + especially in the case of the failure of a merge node. Initial + examination of this case suggests that FRR would not be a problem for + MP2P, given that each flow can be handled separately. + + As a final note, observe that the MP2P scenario presented in this + document may be optimistic. MP2P LSP merging may be hard to achieve + between LSPs with significantly different traffic and Quality of + Service (QoS) parameters. Therefore, it may be necessary to increase + the number of MP2P LSPs arriving at an egress. + + + + + + + + + + + +Yasukawa, et al. Informational [Page 38] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +9. Combined Models + + There is nothing to prevent the combination of hierarchical and MP2P + solutions within a network. + + Note that if MP2P LSPs are tunneled through P2P FA LSPs across the + core, none of the benefit of LSP merging is seen for the hops during + which the MP2P LSPs are tunneled. + + On the other hand, it is possible to construct solutions where MP2P + FA LSPs are constructed within the network, resulting in savings from + both modes of operation. + +10. An Alternate Solution + + A simple solution to reducing the number of LSP tunnels handled by + any node in the network has been proposed. In this solution it is + observed that part of the problem is caused purely by the total + number of LSP in the network, and that this is a function of the + number of PEs since a full mesh of PE-PE LSPs is required. The + conclusion of this observation is to move the tunnel end-points + further into the network so that, instead of having a full mesh of + PE-PE tunnels, we have only a full mesh of P(n)-P(n) tunnels. + + Obviously, there is no change in the physical network topology, so + the PEs remain subtended to the P(n) nodes, and the consequence is + that there is no TE on the links between PEs and P(n) nodes. + + In this case, we have already done the hard work for computing the + number of LSPs in the previous sections. The power of the analysis + in the earlier sections is demonstrated by its applicability to this + new model -- all we need to do is make minor changes to the formulae. + This is most simply done by removing a layer from the network. We + introduce the term "tunnel end-point" (TEP) and replace the P(n) + nodes with TEPs. Thus, the example of a flat snowflake network in + Figure 3 becomes as shown in Figure 7. Corresponding changes can be + made to all of the sample topologies. + + + + + + + + + + + + + + +Yasukawa, et al. Informational [Page 39] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + PE PE PE PE PE PE + \ \/ \/ / + PE--TEP TEP TEP TEP--PE + \ | | / + \| |/ + PE--TEP---P(1)------P(1)---TEP--PE + / \ / \ + PE \ / PE + \/ + P(1) + /|\ + / | \ + / | \ + PE--TEP TEP TEP--PE + / /\ \ + PE PE PE PE + + Figure 7 : An Example Snowflake Network with Tunnel End-Points + + To perform the scaling calculations we need only replace the PE + counts in the formulae with TEP counts, and observe that there is one + fewer layer in the network. For example, in the flat snowflake + network shown in Figure 7, we can see that the number of LSPs seen at + a TEP is: + + L(TEP) = 2*(S(TPE) - 1) + + In our sample networks, S(TPE) is typically of the order of 50 or 100 + (the original values of S(2)), so L(TEP) is less than 200, which is + quite manageable. + + Similarly, the number of LSPs handled by a P(1) node can be derived + from the original formula for the number of LSPs seen at a P(2) node, + since all we have done is reduce n in P(n) from 2 to 1. So our new + formula is: + + L(1) = M(1)*(2*S(TEP) - M(1) - 1) + + With figures for M(1) = 10 and S(TEP) = 100, this gives us L(1) = + 1890. This is also very manageable. + +10.1. Pros and Cons of the Alternate Solution + + On the face of it, this alternate solution seems very attractive. + Simply by contracting the edges of the tunnels into the network, we + have shown a dramatic reduction in the number of tunnels needed, and + there is no requirement to apply any additional scaling techniques. + + + + +Yasukawa, et al. Informational [Page 40] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + But what of the PE-P(n) links? In the earlier sections of this + document, we have assumed that there was some requirement for PE-PE + LSPs with TE properties that extended to the PE-P(n) links at both + ends of each LSP. That means that there was a requirement to provide + reservation-based QoS on those links, to be able to discriminate + traffic flows for priority-based treatment, and to be able to + distinguish applications and sources that send data based on the LSPs + that carry the data. + + It might be argued that, since the PE-P(n) links do not offer any + routing options (each such link provides the only access to the + network for a PE), most of the benefits of tunnels are lost on these + peripheral links. However, TE is not just about routing. Just as + important are the abilities to make resource reservations, to + prioritize traffic, and to discriminate between traffic from + different applications, customers, or VPNs. + + Furthermore, in multihoming scenarios where each PE is connected to + more than one P(n) or where a PE has multiple links to a single P(n), + there may be a desire to pre-select the link to be used and to direct + the traffic to that link using a PE-PE LSP. Note that multihoming + has not been considered in this document. + + Operationally, P(n)-P(n) LSPs offer the additional management + overhead that is seen for hierarchical LSPs described in Section 6. + That is, the LSPs have to be configured and established through + additional configuration or management operations that are not + carried out at the PEs. As described in Section 6, automesh + [RFC4972] could be used to ease this task. But it must be noted + that, as mentioned above, some of the key uses of tunnels require + that traffic is classified and placed in an appropriate tunnel + according to its traffic class, end-points, originating application, + and customer (such as client VPN). This information may not be + readily available for each packet at the P(n) nodes since it is PE- + based information. Of course, it is possible to conceive of + techniques to make this information available, such as assigning a + different label for each class of traffic, but this gives rise to the + original problem of larger numbers of LSPs. + + Our conclusion is, therefore, that this alternate technique may be + suitable for the general distribution of traffic based solely on the + destination, or on a combination of the destination and key fields + carried in the IP header. In this case, it can provide a very + satisfactory answer to the scaling issues in an MPLS-TE network. But + if more sophisticated packet classification and discrimination is + required, this technique will make the desired function hard to + + + + + +Yasukawa, et al. Informational [Page 41] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + achieve, and the trade-off between scaling and feature-level will + swing too far towards solving the scaling issue at the expense of + delivery of function to the customer. + +11. Management Considerations + + The management issues of the models presented in this document have + been discussed in-line. No one solution is without its management + overhead. + + Note, however, that scalability of management tools is one of the + motivators for this work and that network scaling solutions that + reduce the active management of LSPs at the cost of additional effort + to manage the more static elements of the network represent a + benefit. That is, it is worth the additional effort to set up MP2P + or FA LSPs if it means that the network can be scaled to a larger + size without being constrained by the management tools. + + The MP2P technique may prove harder to debug through OAM methods than + the FA LSP approach. + +12. Security Considerations + + The techniques described in this document use existing or yet-to-be- + defined signaling protocol extensions and are subject to the security + provided by those extensions. Note that we are talking about + tunneling techniques used within the network and that both approaches + are vulnerable to the creation of bogus tunnels that deliver data to + an egress or consume network resources. + + The fact that the MP2P technique may prove harder to debug through + OAM methods than the FA LSP approach is a security concern since it + is important to be able to detect misconnections. + + General issues of the relationship between scaling and security are + covered in Section 1.1, but the details are beyond the scope of this + document. Readers are referred to [MPLS-SEC] for details of MPLS + security techniques. + +13. Recommendations + + The analysis in this document suggests that the ability to signal + MP2P MPLS-TE LSPs is a desirable addition to the operator's MPLS-TE + toolkit. + + At this stage, no further recommendations are made, but it would be + valuable to consult more widely to discover: + + + + +Yasukawa, et al. Informational [Page 42] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + - The concerns of other service providers with respect to network + scalability. + + - More opinions on the realistic constraints to the network + parameters listed in Section 4. + + - Desirable values for the cost-effectiveness of the network + (parameter K). + + - The applicability, manageability, and support for the two + techniques described. + + - The feasibility of combining the two techniques, as discussed in + Section 9. + + - The level of concern over the loss of functionality that would + occur if the alternate solution described in Section 10 was + adopted. + +14. Acknowledgements + + The authors are grateful to Jean-Louis Le Roux for discussions and + review input. Thanks to Ben Niven-Jenkins, JP Vasseur, Loa + Andersson, Anders Gavler, Ben Campbell, and Tim Polk for their + comments. Thanks to Dave Allen for useful discussion of the math. + +15. Normative References + + [RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP) + Hierarchy with Generalized Multi-Protocol Label Switching + (GMPLS) Traffic Engineering (TE)", RFC 4206, October + 2005. + +16. Informative References + + [RFC2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F., + and S. Molendini, "RSVP Refresh Overhead Reduction + Extensions", RFC 2961, April 2001. + + [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., + and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP + Tunnels", RFC 3209, December 2001. + + [RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, + P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi- + Protocol Label Switching (MPLS) Support of Differentiated + Services", RFC 3270, May 2002. + + + + +Yasukawa, et al. Informational [Page 43] + +RFC 5439 Scaling in MPLS-TE February 2009 + + + [RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label + Switching (GMPLS) Signaling Resource ReserVation + Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC + 3473, January 2003. + + [RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation + Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005. + + [RFC4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast + Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090, + May 2005. + + [RFC4110] Callon, R. and M. Suzuki, "A Framework for Layer 3 + Provider-Provisioned Virtual Private Networks (PPVPNs)", + RFC 4110, July 2005. + + [RFC4972] Vasseur, JP., Ed., Leroux, JL., Ed., Yasukawa, S., + Previdi, S., Psenak, P., and P. Mabbey, "Routing + Extensions for Discovery of Multiprotocol (MPLS) Label + Switch Router (LSR) Traffic Engineering (TE) Mesh + Membership", RFC 4972, July 2007. + + [RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed., + "LDP Specification", RFC 5036, October 2007. + + [MP2P-RSVP] Yasukawa, Y., "Supporting Multipoint-to-Point Label + Switched Paths in Multiprotocol Label Switching Traffic + Engineering", Work in Progress, October 2008. + + [MPLS-SEC] Fang, L., Ed., "Security Framework for MPLS and GMPLS + Networks", Work in Progress, November 2008. + + + + + + + + + + + + + + + + + + + + +Yasukawa, et al. Informational [Page 44] + +RFC 5439 Scaling in MPLS-TE February 2009 + + +Authors' Addresses + + Seisho Yasukawa + NTT Corporation + 9-11, Midori-Cho 3-Chome + Musashino-Shi, Tokyo 180-8585 Japan + Phone: +81 422 59 4769 + EMail: s.yasukawa@hco.ntt.co.jp + + Adrian Farrel + Old Dog Consulting + EMail: adrian@olddog.co.uk + + Olufemi Komolafe + Cisco Systems + 96 Commercial Street + Edinburgh + EH6 6LX + United Kingdom + EMail: femi@cisco.com + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Yasukawa, et al. Informational [Page 45] + |