RFC 5623
Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering
RFC 5623: Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering ✨
okay bestie, this one is a framework doc, so it's basically the map for how a Path Computation Element (PCE) helps with traffic engineering across multiple network layers. 🗺️🦋
Here's the title block, all the tea in one spot 🍵:
- RFC number: 5623
- Title: Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering
- Authors: E. Oki (University of Electro-Communications), T. Takeda (NTT), JL. Le Roux (France Telecom), and A. Farrel (Old Dog Consulting) 👑
- Category: Informational
- Date: September 2009
- Obsoletes/Updates: the title block doesn't list anything it obsoletes or updates, so it stands on its own 🌸
It's Informational, which means it shares information with the Internet community and isn't an Internet standard. Not a rule book, more like a really good guide 💅
Abstract 🎀
okay so a network can have multiple layers, and it's really important to optimize resource use across all of them together, not just doing each layer on its own. that's the whole idea. when you do it globally, the network gets way more efficient, and that process is what we call inter-layer traffic engineering 🔥 the Path Computation Element (PCE) can be a seriously powerful tool for pulling this off, no cap ✨
this document lays out a framework for using the PCE-based architecture for inter-layer Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) traffic engineering 💜 it gives suggestions for deploying PCE in multi-layer networks, describes network models where PCE does inter-layer traffic engineering, and explains how PCE relates to a functional component called the Virtual Network Topology Manager (VNTM) 🧚
Status of This Memo 🌈
This memo gives information to the Internet community. It is not an Internet standard of any kind. Distribution of this memo is unlimited 🫶
Copyright and License Notice 💖
Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved 🔒
okay, the fine print, but read it bestie, it's about your rights 😭 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info), as in effect on the date this document was published. Please review those documents carefully, because they spell out your rights and restrictions for this document. Any Code Components pulled out of this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come without warranty, as described in the BSD License 💅
Table of Contents 🍓
1. Introduction ....................................................3
1.1. Terminology ................................................3
2. Inter-Layer Path Computation ....................................4
3. Inter-Layer Path Computation Models .............................7
3.1. Single PCE Inter-Layer Path Computation ....................7
3.2. Multiple PCE Inter-Layer Path Computation ..................7
3.3. General Observations ......................................10
4. Inter-Layer Path Control .......................................10
4.1. VNT Management ............................................10
4.2. Inter-Layer Path Control Models ...........................11
4.2.1. PCE-VNTM Cooperation Model .........................11
4.2.2. Higher-Layer Signaling Trigger Model ...............13
4.2.3. NMS-VNTM Cooperation Model .........................16
4.2.4. Possible Combinations of Inter-Layer Path
Computation and Inter-Layer Path Control Models ....21
5. Choosing between Inter-Layer Path Control Models ...............22
5.1. VNTM Functions ............................................22
5.2. Border LSR Functions ......................................23
5.3. Complete Inter-Layer LSP Setup Time .......................24
5.4. Network Complexity ........................................24
5.5. Separation of Layer Management ............................25
6. Stability Considerations .......................................25
7. Manageability Considerations ...................................26
7.1. Control of Function and Policy ............................27
7.1.1. Control of Inter-Layer Computation Function ........27
7.1.2. Control of Per-Layer Policy ........................27
7.1.3. Control of Inter-Layer Policy ......................27
7.2. Information and Data Models ...............................28
7.3. Liveness Detection and Monitoring .........................28
7.4. Verifying Correct Operation ...............................29
7.5. Requirements on Other Protocols and Functional
Components ................................................29
7.6. Impact on Network Operation ...............................30
8. Security Considerations ........................................30
9. Acknowledgments ................................................31
10. References ....................................................32
10.1. Normative Reference ......................................32
10.2. Informative Reference ....................................321. Introduction 🦋
A network can have multiple layers 🌷 These layers might be separated by technology (for example, packet switch capable (PSC), time division multiplex (TDM), or lambda switch capable (LSC)) [RFC3945], by data plane switching granularity levels (for example, PSC-1, PSC-2, VC4, or VC12) [RFC5212], or by a difference between client and server networking roles. In a multi-layer network, Label Switched Paths (LSPs) in a lower layer carry higher-layer LSPs across the lower-layer network. The network topology made of those lower-layer LSPs, which get advertised as traffic engineering links (TE links) in the higher-layer network, is called the Virtual Network Topology (VNT) [RFC5212] 🧚
It can be really effective to optimize network resource use globally, meaning you take all layers into account instead of optimizing each layer on its own 🔥 That gets you better network efficiency, and it's what we call inter-layer traffic engineering. Inter-layer traffic engineering covers mechanisms that compute end-to-end paths across layers (known as inter-layer path computation), and mechanisms that control and manage the Virtual Network Topology (VNT) by setting up and releasing LSPs in the lower layers [RFC5212] 💅
Inter-layer traffic engineering is part of the scope of the Path Computation Element (PCE)-based architecture [RFC4655], and PCE can be a great mechanism for solving inter-layer path computation issues ✨
The PCE Communication Protocol requirements for inter-layer traffic engineering are set out in [PCC-PCE] 💜
This document describes a framework for applying the PCE-based architecture to inter-layer traffic engineering 🌸 It gives suggestions for deploying PCE in support of multi-layer networks. It also describes network models where PCE performs inter-layer traffic engineering, and explains the relationship between PCE and a functional component in charge of controlling and managing the VNT, called the Virtual Network Topology Manager (VNTM) 🎀
1.1. Terminology 🍓
This document uses terminology from the PCE-based path computation architecture [RFC4655], plus common terminology from Multi-Protocol Label Switching (MPLS) [RFC3031], Generalized MPLS (GMPLS) [RFC3945], and Multi-Layer Networks [RFC5212] 🫶
2. Inter-Layer Path Computation ✨
okay so this section is all about inter-layer path computation in MPLS and GMPLS networks 🌸 basically, what happens when a path has to cross from one layer to another 🫶
[RFC4206] defines a way to signal a higher-layer LSP that has an explicit route and includes the hops that LSPs in lower layers traverse. 💅 Figuring out the end-to-end paths across those layers is called inter-layer path computation, and it's giving very main character energy ✨
A Label Switching Router (LSR) in the higher layer might not have information about the lower layer's topology, especially in an overlay or augmented model deployment 🙈 So it might not be able to compute an end-to-end path across the layers. 💀
PCE-based inter-layer path computation means using one or more PCEs to compute an end-to-end path across layers 🦋 There are two ways this can go. The first is a single PCE path computation, where one PCE knows the topology of multiple layers and can compute the end-to-end path across all of them directly 👑 The second is a multiple PCE computation, where each PCE in a set knows the topology of one or more layers (but not all of them), and the PCEs work together to compute the end-to-end path 🧚
----- ----- ----- -----
| LSR |--| LSR |................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 1: A Simple Example of a Multi-Layer NetworkLet's look at the two-layer network in Figure 1 🍓 The higher layer (LSRs H1, H2, H3, and H4) is a packet-based IP/MPLS or GMPLS network, and the lower layer (LSRs, H2, L1, L2, and H3) is a GMPLS optical network. The ingress LSR in the higher layer (H1) wants to set up an LSP to an egress LSR (H4), also in the higher layer, and it has to go across the lower layer. For that, it needs a path in the higher layer. 🎀 Now, suppose there's no TE link in the higher layer between the border LSRs on the boundary between the two layers (H2 and H3). Suppose also that the ingress LSR has no topology visibility into the lower layer. 😭
If you run a single-layer path computation in the higher layer, it fails because that TE link is missing 💔 But inter-layer path computation can still give you a route in the higher layer (H1-H2-H3-H4) and suggest that a lower-layer LSP be set up between the border LSRs (H2-L1-L2-H3). It's literally the glow-up we needed 🌈
Lower-layer LSPs that get advertised as TE links into the higher-layer network form a Virtual Network Topology (VNT) ✨ That VNT can be used for routing higher-layer LSPs. Inter-layer path computation for end-to-end LSPs in the higher-layer network that span the lower-layer network may use the VNT, and the PCE is a candidate for computing the paths of those higher-layer LSPs within the higher layer 🫶 Alternatively, the PCE-based path computation model can do any of the following:
- Perform a single computation on behalf of the ingress LSR, using information gathered from more than one layer. This mode is referred to as single PCE computation in [RFC4655]. 👑
- Compute a path on behalf of the ingress LSR by cooperating with the PCEs responsible for each layer. This mode is referred to as multiple PCE computation with inter-PCE communication in [RFC4655]. 🤝
- Perform separate path computations on behalf of the TE-LSP head-end and each transit border LSR that is the entry point to a new layer. This mode is referred to as multiple PCE computation (without inter-PCE communication) in [RFC4655]. This option uses per-layer path computation, which successive PCEs perform independently. 🦋
Now, when a network has more than two layers (for example, MPLS over SONET over Optical Transport Network (OTN)) and you need to compute a path that crosses more than two of them, you can combine multiple PCE-based path computation models 🧠 For example, the single PCE computation model could compute a path across the SONET and OTN layers, while the multiple PCE computation with inter-PCE communication model computes a path across the MPLS layer (computed by the higher-layer PCE) and the SONET layer (computed by the lower-layer PCE) 💖
The PCE invoked by the head-end LSR computes a path that the LSR can use to signal an MPLS-TE or GMPLS LSP. Once the path information has been converted to an Explicit Route Object (ERO), it's used for RSVP-TE signaling. There are two options 💅
- Option 1: Mono-Layer Path
The PCE computes a "mono-layer" path, meaning a path that includes only TE links from the same layer. There are two cases for this option 🌸 In the first case, the PCE computes a path that includes lower-layer LSPs that are already established, or lower-layer LSPs that will be established on demand. The resulting ERO includes subobject(s) for the lower-layer hierarchical LSPs, expressed as the TE link identifiers of those hierarchical LSPs when they're advertised as TE links in the higher-layer network. The TE link might be a regular TE link that's actually established, or a virtual TE link that isn't established yet (see [RFC5212]). If it's a virtual TE link, reaching the head-end of the lower-layer LSP during signaling triggers a setup attempt for a new lower-layer LSP. 🚀 Keep in mind that the path of a virtual TE link isn't necessarily known in advance, so this may need a further (lower-layer) path computation. 🫠
The second case is that the PCE computes a path that includes a loose hop spanning the lower-layer network. The higher-layer path computation picks which lower-layer network to use and its entry and exit points, but it does not pick the path across that lower-layer network. A transit LSR that is the entry point to the lower-layer network is expected to expand the loose hop, either by itself or by relying on the services of a PCE. The expansion on the border LSR may result in selecting an existing lower-layer LSP, or in computing and setting up a new lower-layer LSP. 🎀
Note that even if a PCE computes a path with a loose hop that's expected to be expanded across the lower-layer network, the LSR at the entry point to that lower-layer network may simply expand the loose hop within the same layer. 🙈 If you need stricter control over how the LSR establishes the path, mechanisms such as Path Key [RFC5520] could be applied. 🔐
- Option 2: Multi-Layer Path
The PCE computes a "multi-layer" path, meaning a path that includes TE links from distinct layers [RFC4206]. This kind of path can include the complete path of one or more lower-layer LSPs that already exist, or ones that aren't established yet. In the latter case, signaling the higher-layer LSP triggers the establishment of the lower-layer LSPs ⭐
3. Inter-Layer Path Computation Models ✨
In Section 2, three models are defined for PCE-based inter-layer path computation: single PCE computation, multiple PCE computation with inter-PCE communication, and multiple PCE computation without inter-PCE communication 🦋 Single PCE computation is covered in Section 3.1 below, and multiple PCE computation (with and without inter-PCE communication) is covered in Section 3.2 below.
3.1. Single PCE Inter-Layer Path Computation 👑
In this model, a single PCE that has topology visibility into all layers performs the inter-layer path computation. That kind of PCE is called a multi-layer PCE 💅
In Figure 2, the network has two layers. LSRs H1, H2, H3, and H4 belong to the higher layer, and LSRs H2, H3, L1, and L2 belong to the lower layer. The PCE is a multi-layer PCE with visibility into both layers, so it can perform end-to-end path computation across layers (a single PCE path computation) 🧚 For instance, it can compute an optimal path H1-H2-L1-L2-H3-H4 for a higher-layer LSP from H1 to H4. This path includes the path of a lower-layer LSP from H2 to H3, which either already exists or hasn't been established yet 🌈
-----
| PCE |
-----
----- ----- ----- -----
| LSR |--| LSR |................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 2: Single PCE Inter-Layer Path Computation3.2. Multiple PCE Inter-Layer Path Computation ✨
okay so in this model there's at least one PCE per layer, and each PCE can only see the topology of its own layer 🧚 Some providers wanna keep those layer boundaries because of organizational and/or service management stuff. Choosing multiple PCEs instead of a single PCE can also be about scalability, since each PCE only has to keep topology info for one layer, which means a smaller Traffic Engineering Database (TED) 💅
These PCEs are called mono-layer PCEs 🦋 They team up to compute an end-to-end optimal path across layers, and it's giving teamwork 🫶
Figure 3 shows multiple PCE inter-layer computation with inter-PCE communication. There's one PCE in each layer, and they collaborate to compute an end-to-end path across layers. PCE Hi handles the higher layer and may "consult" with PCE Lo to compute paths across the lower layer, while PCE Lo handles path computation in the lower layer. Here's a simple example of how they could cooperate:
- LSR H1 sends a request to PCE Hi for a path H1-H4.
- PCE Hi selects H2 as the entry point to the lower layer and H3 as the exit point.
- PCE Hi requests a path H2-H3 from PCE Lo.
- PCE Lo returns H2-L1-L2-H3 to PCE Hi.
- PCE Hi is now able to compute the full path (H1-H2-L1-L2-H3-H4) and return it to H1.
Of course, if you want a truly optimal end-to-end path, the cooperation could get more complex 🧠
-----
| PCE |
| Hi |
--+--
|
----- ----- | ----- -----
| LSR |--| LSR |............|...........| LSR |--| LSR |
| H1 | | H2 | | | H3 | | H4 |
----- -----\ --+-- /----- -----
\ | PCE | /
\ | Lo | /
\ ----- /
\ /
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 3: Multiple PCE Inter-Layer Path Computation
with Inter-PCE CommunicationFigure 4 shows multiple PCE inter-layer path computation without inter-PCE communication. As covered in Section 2, separate path computations are done on behalf of the TE-LSP head-end and each transit border LSR that is the entry point to a new layer 🌸
-----
| PCE |
| Hi |
-----
----- ----- ----- -----
| LSR |--| LSR |........................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ ----- /----- -----
\ | PCE | /
\ | Lo | /
\ ----- /
\ /
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 4: Multiple PCE Inter-Layer Path Computation
without Inter-PCE Communication3.3. General Observations
okay the vibes here are all about tradeoffs 💜
- Depending on implementation details, the time to do inter-layer path computation in the single PCE model may be less than in the multiple PCE model with cooperating mono-layer PCEs, because there's no requirement to exchange messages between cooperating PCEs ⚡
- When TE topology for all layer networks is visible within one routing domain, the single PCE model may be adopted, because a PCE can collect every layer's TE topology by participating in just one routing domain 🎀
- As the single PCE model uses more TE topology information in one computation than the PCEs in the multiple PCE model do, it needs more computation power and memory 🥵
When there are multiple candidate layer border nodes (we may say the higher layer is multi-homed), optimal path computation requires that all the possible paths transiting different layer border nodes or links get examined 👀 This is pretty simple in the single PCE model because the PCE has full visibility, so the computation looks a lot like the one inside a single domain of a single layer 💅 In the multiple PCE model, the backward-recursive techniques described in [RFC5441] could be used by treating the layers as separate domains 🦋
4. Inter-Layer Path Control
4.1. VNT Management
Here's the tea 🍵 After mono-layer path computation, a PCE may figure out that there isn't enough bandwidth in the higher-layer network to support this or future higher-layer LSPs. The fix might be to provision new LSPs across the lower-layer network. And modifying, reorganizing, or newly provisioning lower-layer LSPs may lead to better use of lower-layer network resources, given what the higher-layer network needs. So basically, the VNT needs to be controlled or managed together with inter-layer path computation 🧩
A VNT Manager (VNTM) is defined as a functional element that manages and controls the VNT. The PCE and the VNT Manager are distinct functional elements, and they may or may not be collocated 🌈
4.2. Inter-Layer Path Control Models
4.2.1. PCE-VNTM Cooperation Model
----- ------
| PCE |--->| VNTM |
----- ------
^ :
: :
: :
v V
----- ----- ----- -----
| LSR |----| LSR |................| LSR |----| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 5: PCE-VNTM Cooperation ModelOkay so picture a multi-layer network with higher-layer and lower-layer networks 🦋 LSRs H1, H2, H3, and H4 belong to the higher-layer network, and LSRs H2, L1, L2, and H3 belong to the lower-layer network, as shown in Figure 5. We're looking at single PCE inter-layer path computation to explain the PCE-VNTM cooperation model, but multiple PCE path computation, with or without inter-PCE communication, can work with this model too 💖
Say H1 asks the PCE to compute an inter-layer path between H1 and H4. There's no TE link in the higher layer between H2 and H3 before the request, so the request fails 😭 BUT the PCE can still give useful info to the VNT Manager responsible for the lower-layer network, which might help sort things out for future higher-layer LSP setup ✨
Here's how the roles split up. The PCE does the inter-layer path computation and can't supply a path, because there's no TE link between H2 and H3. The computation fails, but the PCE suggests to the VNTM that a lower-layer LSP (H2-H3) could be set up to support future LSP requests 🎀 The messages from the PCE to the VNTM carry info about the higher-layer demand (from H2 to H3), and may include a suggested path in the lower layer (if the PCE can see into the lower-layer network). The VNTM uses local policy and possibly management/configuration input to decide how to handle the PCE's suggestion, and it may ask an ingress LSR (e.g., H2) to establish a lower-layer LSP. The VNTM or the ingress LSR (H2) may also use a PCE with lower-layer visibility to compute the path for this new LSP 🧚
When the higher-layer PCE fails to compute a path and notifies the VNTM, it may wait for the lower-layer LSP to be set up and advertised as a TE link ⏳ The PCE may have a timer. Once the TED is updated within a specified duration, the PCE will know about a new TE link. It can then compute the complete end-to-end path for the higher-layer LSP and return the result to the PCC. In this case the PCC may be kept waiting for a while, and it's important that the PCC understands this 🥺 It's also important that the PCE and VNTM have an agreement that the lower-layer LSP will be set up in a timely manner, or that the VNTM will tell the PCE no new LSP will become available. Either way, if the PCE decides to wait, it must operate a timeout 🚨 Here's an example of this cooperative procedure between PCE and VNTM, using the example network in Figure 4:
Step 1: H1 (PCC) requests PCE to compute a path between H1 and H4.
Step 2: The path computation fails because there is no TE link
across the lower-layer network.
Step 3: PCE suggests to VNTM that a new TE link connecting H2 and
H3 would be useful. The PCE notifies VNTM that it will be
waiting for the TE link to be created. VNTM considers
whether lower-layer LSPs should be established, if
necessary and acceptable within VNTM's policy constraints.
Step 4: VNTM requests an ingress LSR in the lower-layer network
(e.g., H2) to establish a lower-layer LSP. The request
message may include a lower-layer LSP route obtained from
the PCE responsible for the lower-layer network.
Step 5: The ingress LSR signals to establish the lower-layer LSP.
Step 6: If the lower-layer LSP setup is successful, the ingress
LSR notifies VNTM that the LSP is complete and supplies
the tunnel information.
Step 7: The ingress LSR (H2) advertises the new LSP as a TE link
in the higher-layer network routing instance.
Step 8: PCE notices the new TE link advertisement and recomputes
the requested path.
Step 9: PCE replies to H1 (PCC) with a computed higher-layer LSP
route. The computed path is categorized as a mono-layer
path that includes the already-established lower-layer LSP
as a single hop in the higher layer. The higher-layer
route is specified as H1-H2-H3-H4, where all hops are
strict.
Step 10: H1 initiates signaling with the computed path H2-H3-H4 to
establish the higher-layer LSP.4.2.2. Higher-Layer Signaling Trigger Model ✨
Okay bestie, let's look at Figure 6 🦋
-----
| PCE |
-----
^
:
:
v
----- ----- ----- -----
| LSR |----| LSR |................| LSR |--| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----
Figure 6: Higher-Layer Signaling Trigger ModelFigure 6 is the higher-layer signaling trigger model 💖 We're explaining it with single PCE path computation, but multiple PCE path computation, with or without inter-PCE communication, can also be applied to this model. 🌸
Same setup as Section 4.2.1: H1 asks the PCE for a path between H1 and H4 🎀 Before that request, there's no TE link in the higher layer between H2 and H3. Sad, but real 😭
So here's the tea. The PCE can't find a mono-layer path, but it might decide that setting up a lower-layer LSP between H2 and H3 would give good connectivity 🧚 If the PCE has inter-layer visibility, it can return a multi-layer path that includes lower-layer hops (H1-H2-L1-L2-H3-H4). If it has no visibility into the lower layer, it can return a mono-layer path with a loose hop from H2 to H3 (H1-H2-H3(loose)-H4). The first one is the multi-layer path, and the second one is the mono-layer path with loose hops. Two different vibes, both valid 💅
Now, with a multi-layer path, the LSP route from the PCE already includes a lower-layer LSP route that doesn't exist yet. The border LSR sitting at the edge between the higher and lower layers (H2 in this example) gets a higher-layer signaling message, notices the next hop is in the lower layer, and starts setting up that lower-layer LSP as described in [RFC4206]. Heads up: all of this depends on a policy applied at the border LSR 👑 Here's how it goes:
Step 1: H1 (PCC) asks the PCE to compute a path between H1 and H4. The request says inter-layer path computation is allowed. ✨
Step 2: After the inter-layer computation, the PCE decides a new lower-layer LSP needs to be set up. 🔥
Step 3: The PCE replies to H1 (PCC) with a multi-layer route that has both higher-layer and lower-layer LSP routes. It can be written as H1-H2-L1-L2-H3-H4, where every hop is strict. 💜
Step 4: H1 starts higher-layer signaling using the explicit route H2-L1-L2-H3-H4 that was computed. 🦋
Step 5: The border LSR (H2) gets the higher-layer signaling message and starts lower-layer signaling to set up a lower-layer LSP along the specified lower-layer route H2-L1-L2-H3. It spots the hops in the explicit route that belong to the lower layer, checks with local policy that a new LSP is okay, and then sets up the lower-layer LSP it needs. Also, it's possible that a suitable lower-layer LSP was already set up (or became available) between when the path was computed and when the signaling message reached the border LSR. In that case, the border LSR can just pick that existing lower-layer LSP without signaling a new one, as long as it satisfies the explicit route in the higher-layer signaling request. Efficient and iconic 🌈
Step 6: Once the lower-layer LSP is up, higher-layer signaling keeps going along the specified higher-layer route H2-H3-H4 using hierarchical signaling [RFC4206]. Ta-da! 🎀
Now, the mono-layer path version is a different story 🧚 Here, the higher-layer LSP route has a loose hop that goes through the lower-layer network between the two border LSRs. When a border LSR gets the higher-layer signaling message, it has to work out a path for a new lower-layer LSP itself. It applies local policy to check that a new LSP is acceptable, and then it either asks a PCE responsible for the lower-layer network or computes the path on its own. After that, it starts signaling to set up the lower-layer LSP. Same as before, a suitable lower-layer LSP might already exist or have become available. In that case, the border LSR can use it without signaling a new LSP, as long as the existing LSP satisfies the explicit route in the higher-layer signaling request. Because the higher-layer request used a loose hop and didn't spell out any details of the path in the lower layer, the border LSR has more freedom to pick a lower-layer LSP than it did in the previous example. Main character energy 👑
The only real difference between the multi-layer and mono-layer procedures is Step 5. Here's the mono-layer version:
Step 5': The border LSR (H2) that gets the higher-layer signaling message applies local policy to check that a new LSP is acceptable, and then starts setting up a lower-layer LSP. It either asks a PCE responsible for the lower-layer network or computes the route itself, in order to expand the loose hop route in the higher-layer path. 🫶
One more thing! A virtual TE link might already have been advertised into the higher-layer network 💀 That makes the PCE return H1-H2-H3-H4, where all the hops are strict. But when the higher-layer signaling message reaches the layer border node H2 (the one that advertised the virtual TE link), H2 realizes the TE link doesn't actually exist yet. So it signals the needed LSP across the lower-layer network using its own path determination (just like it would for a loose hop in the higher layer) before it keeps going with higher-layer signaling. Plot twist, but handled 🙈
Now let's look at the multi-layer network in Figure 7 🌸
PCE
^
:
:
V
H1--H2 H3--H4
\ /
L1==L2==L3--L4--L5
|
|
L6--L7
\
H5--H6
Figure 7: Example of a Multi-Layer NetworkFigure 7 shows how an ERO that can include subobjects in both the higher and lower layers helps set up lower-layer LSPs in the higher-layer signaling trigger model. Examples of multi-layer EROs are explained with this figure. The higher-layer signaling trigger model gives you several options for the ERO when it gets to the last LSR in the higher layer (H2) 💖
- The next subobject is a loose hop to H3 (mono-layer ERO).
- The next subobject is a strict hop to L1, followed by a loose hop to H3.
- The next subobjects are a series of hops (strict or loose) in the lower layer, followed by H3. For example,
{L1(strict), L3(loose), L5(loose), H3(strict)}.
In the first example, the lower layer can use any LSP tunnel that delivers the end-to-end LSP to H3 ✨ In the third case, the lower layer must pick an LSP tunnel that goes through L3 and L5. That doesn't mean the lower layer can or should use one LSP from L1 to L3 and another from L3 to L5, though. Not how we do it, bestie 🙅♀️💅
4.2.3. NMS-VNTM Cooperation Model 🦋
okay so in this model the NMS and VNTM team up to set up a lower-layer LSP 🫶 There are two flavors. In the first one, the layers interact during path computation at the PCE level, and it's called the "integrated flavor" 💅 In the second one, the layers interact through NMS and VNTM cooperation instead, which could be a point of application of administrative, billing, and security policy. That's the "separated flavor" 🌸
NMS-VNTM Cooperation Model (integrated flavor)
------ -----
| NMS |<-->| PCE |
| | -----
| ---- |
||VNTM||
| ---- |
------
: :
: ---------
: :
V V
----- ----- ----- -----
| LSR |----| LSR |................| LSR |----| LSR |
| H1 | | H2 | | H3 | | H4 |
----- -----\ /----- -----
\----- -----/
| LSR |--| LSR |
| L1 | | L2 |
----- -----Figure 8: NMS-VNTM Cooperation Model (integrated flavor) ✨
Figure 8 shows the NMS-VNTM cooperation model (integrated flavor) 🎀 The case of single PCE path computation is used to explain it here, but multiple PCE path computation with inter-PCE communication can also be applied to this model. Note that multiple PCE path computation without inter-PCE communication does not fit in with this model. For this model to have meaning, the VNTM and NMS are closely coupled 💜
The NMS sends the path computation request to the PCE. The PCE returns the inter-layer path computation result. When the NMS receives that result, the NMS works with the VNTM and sends the request to LSR H2 to set up the lower-layer LSP. The VNTM uses local policy and possibly management/configuration input to decide how to process the computation result from the PCE 🧚
An example procedure for the NMS-VNTM cooperation model (integrated flavor) goes like this 🍓
Step 1: The NMS requests the PCE to compute a path between H1 and H4. The request indicates that inter-layer path computation is allowed.
Step 2: The PCE computes a path. The result (H1-H2-L1-L2-H3-H4) is sent back to the NMS.
Step 3: The NMS discovers that a lower-layer LSP is needed. It works with the VNTM to determine whether the new TE LSP H2-L1-L2-H3 is permitted according to policy, etc. 🔥
Step 4: The VNTM requests the ingress LSR in the lower-layer network (H2) to establish a lower-layer LSP. The request message includes the lower-layer LSP route obtained from the PCE.
Step 5: H2 signals to establish the lower-layer LSP.
Step 6: If the lower-layer LSP setup is successful, H2 notifies the VNTM that the LSP is complete and supplies the tunnel information.
Step 7: H2 advertises the new LSP as a TE link in the higher-layer network routing instance.
Step 8: The VNTM notifies the NMS that the underlying lower-layer LSP has been set up, and the NMS notices the new TE link advertisement.
Step 9: The NMS requests H1 to set up a higher-layer LSP between H1 and H4 with the path computed in Step 2. The lower-layer links are replaced by the corresponding higher-layer TE link. Hence, the NMS sends the path H1-H2-H3-H4 to H1 👑
Step 10: H1 initiates signaling with the path H2-H3-H4 to establish the higher-layer LSP. iconic 💀
NMS-VNTM Cooperation Model (separate flavor)
-----
| NMS |
| | -----
----- | PCE |
^ ^ | Hi |
: : -----
: : ^
: : :
: : :
: v v
: ------ ----- ----- ------
: | LSR |--| LSR |........................| LSR |--| LSR |
: | H1 | | H2 | | H3 | | H4 |
: ------ -----\ /----- ------
: ^ \ /
: : \ /
: -------- \ /
v : \ /
------ ----- \----- -----/
| VNTM |<-->| PCE | | LSR |--| LSR |
| | | Lo | | L1 | | L2 |
------ ----- ----- -----Figure 9: NMS-VNTM Cooperation Model (separate flavor) 🌈
Figure 9 shows the NMS-VNTM cooperation model (separate flavor) 💖 The NMS manages the higher layer here. The case of multiple PCE computation without inter-PCE communication is used to explain this model, but single PCE path computation could also be applied to it. Note that multiple PCE path computation with inter-PCE communication does not fit in with this model 🙈
The NMS requests a head-end LSR (H1 in this example) to set up a higher-layer LSP between the head-end and tail-end LSRs without specifying any route. The head-end LSR, which is a PCC, requests the higher-layer PCE to compute a path between the head-end and tail-end LSRs. There is no TE link in the higher layer between the border LSRs (H2 and H3 in this example). When the PCE fails to compute a path, it informs the PCC (i.e., the head-end LSR), which notifies the NMS. The notification may include information about the reason for failure, such as that there is no TE link between the border LSRs or that computation constraints cannot be met 😭
Note that it is equally valid for the higher-layer PCE to be consulted by the NMS rather than by the head-end LSR. In this case, the result is the same: the NMS discovers that an end-to-end LSP cannot be provisioned owing to the lack of a TE link between H2 and H3 🥺
The NMS may now suggest (or request) to the VNTM that a lower-layer LSP between the border LSRs be established and be advertised as a TE link in the higher layer, to support future higher-layer LSP requests. The communication between the NMS and the VNTM may be performed in an automatic or a manual manner. It is a key interaction between layers, and the layers may also be separate administrative domains. Thus, this communication is potentially a point of application of administrative, billing, and security policy 🔐 The NMS may wait for the lower-layer LSP to be set up and advertised as a TE link, or it may reject the operator's request for the service that requires the higher-layer LSP, with a suggestion that the operator try again later ⏳
The VNTM requests the lower-layer PCE to compute a path, and then requests H2 to establish a lower-layer LSP. Alternatively, the VNTM may make a direct request to H2 for the LSP, and H2 may consult the lower-layer PCE. After the NMS is informed or notices that the lower-layer LSP has been established, it can request the head-end LSR (H1) to set up the higher-layer end-to-end LSP between H1 and H4 ✨
So, cooperation between the higher layer and the lower layer is performed through communication between the NMS and the VNTM. An example of this procedure for the NMS-VNTM cooperation model, using the example network in Figure 6, is as follows 🦋
Step 1: The NMS requests a head-end LSR (H1) to set up a higher-layer LSP between H1 and H4 without specifying any route.
Step 2: H1 (PCC) requests the PCE to compute a path between H2 and H3.
Step 3: The path computation fails because there is no TE link across the lower-layer network.
Step 4: H1 (PCC) notifies the NMS. The notification may include an indication that there is no TE link between H2 and H4.
Step 5: The NMS suggests (or requests) to the VNTM that a new TE link connecting H2 and H3 would be useful. The NMS notifies the VNTM that it will be waiting for the TE link to be created. The VNTM considers whether lower-layer LSPs should be established, if necessary and acceptable within the VNTM's policy constraints 🤔
Step 6: The VNTM requests the lower-layer PCE for path computation.
Step 7: The VNTM requests the ingress LSR in the lower-layer network (H2) to establish a lower-layer LSP. The request message includes a lower-layer LSP route obtained from the lower-layer PCE responsible for the lower-layer network.
Step 8: H2 signals the lower-layer LSP.
Step 9: If the lower-layer LSP setup is successful, H2 notifies the VNTM that the LSP is complete and supplies the tunnel information.
Step 10: H2 advertises the new LSP as a TE link in the higher-layer network routing instance.
Step 11: The VNTM notifies the NMS that the underlying lower-layer LSP has been set up, and the NMS notices the new TE link advertisement.
Step 12: The NMS again requests H1 to set up a higher-layer LSP between H1 and H4.
Step 13: H1 requests the higher-layer PCE to compute a path and obtains a successful result that includes the higher-layer route specified as H1-H2-H3-H4, where all hops are strict 🫶
Step 14: H1 initiates signaling with the computed path H2-H3-H4 to establish the higher-layer LSP. periodt 👑
4.2.4. Possible Combinations of Inter-Layer Path Computation and Inter-Layer Path Control Models ✨
Okay bestie, Table 1 is basically the cheat sheet 📋 for which computation and control models can vibe together. 💅 There are three inter-layer path computation models: the single PCE path computation model, the multiple PCE path computation with inter-PCE communication model, and the multiple PCE path computation without inter-PCE communication model. There are also four inter-layer path control models: the PCE-VNTM cooperation model, the higher-layer signaling trigger model, the NMS-VNTM cooperation model (integrated flavor), and the NMS-VNTM cooperation model (separate flavor). Every combo of computation and control model is possible EXCEPT for the multiple PCE path computation with inter-layer PCE communication model paired with the NMS-VNTM cooperation model. 🙈
Table 1: Possible Combinations of Inter-Layer Path Computation
and Inter-Layer Path Control Models
------------------------------------------------------
| Path computation | Single | Multiple | Multiple |
| \ | PCE | PCE with | PCE w/o |
| Path control | | inter-PCE | inter-PCE |
|---------------------+--------------------------------|
| PCE-VNTM | Yes | Yes | Yes |
| cooperation | | | |
|---------------------+--------+-----------+-----------|
| Higher-layer | Yes | Yes | Yes |
| signaling trigger | | | |
|---------------------+--------+-----------+-----------|
| NMS-VNTM | Yes | Yes | No |
| cooperation | | | |
| (integrated flavor) | | | |
|---------------------+--------+-----------+-----------|
| NMS-VNTM | No* | No | Yes |
| cooperation | | | |
| (separate flavor) | | | |
---------------------+--------+-----------+------------ Note that, in case of NSM-VNTM cooperation (separate flavor) and single PCE inter-layer path computation, the PCE function used by NMS and VNTM may be collocated, but it will operate on separate TEDs. 🧚
5. Choosing between Inter-Layer Path Control Models 🎀
Let's talk about how to pick your model, bestie! 👑 This section compares the PCE-VNTM cooperation model, the higher-layer signaling trigger model, and the NMS-VNTM cooperation model in terms of VNTM functions, border LSR functions, higher-layer signaling time, and complexity (in terms of number of states and messages). A network operator can choose an appropriate model for their deployment scenario, taking all of these considerations into account. ✨
5.1. VNTM Functions 🦋
VNTM functions are required in both the PCE-VNTM cooperation model and the NMS-VNTM model. In the PCE-VNTM cooperation model, communications are required between PCE and VNTM and between VNTM and a border LSR. Communications between a higher-layer PCE and the VNTM are event notifications and may use Simple Network Management Protocol (SNMP) notifications from the PCE MIB modules [PCE-MIB]. Note that communications from the PCE to the VNTM do not have any acknowledgements, so no receipts, bestie 🙃. VNTM-LSR communication can use existing GMPLS-TE MIB modules [RFC4802]. 💖
In the NMS-VNTM cooperation model, communications are required between NMS and VNTM, between VNTM and a lower-layer PCE, and between VNTM and a border LSR. NMS-VNTM communications, which are out of scope of this document, may use proprietary or standard interfaces, some of which, for example, are standardized in TM Forum. Communications between VNTM and a lower-layer PCE use the Path Computation Element Communication Protocol (PCEP) [RFC5440]. VNTM-LSR communications are the same as in the PCE-VNTM cooperation model. 🌸
In the higher-layer signaling trigger model, no VNTM functions are required, and no such communications are required. Main character energy, no extra drama 💅.
If VNTM functions are not supported in a multi-layer network, the higher-layer signaling trigger model has to be chosen. 🍓
The inclusion of VNTM functionality allows better coordination of cross-network LSP tunnels and application of network-wide policy that is far harder to apply in the trigger model, since it requires the coordination of policy between multiple border LSRs. Literally so much easier with VNTM 😭🔥.
Also, VNTM functions could be applied to establish LSPs (or connections) in non-MPLS/GMPLS networks, which do not have signaling capabilities, by configuring each node along the path from the VNTM. It's giving versatility 🌈.
5.2. Border LSR Functions 🧚
In the higher-layer signaling trigger model, a border LSR must have some additional functions. It needs to trigger lower-layer signaling when a higher-layer Path message suggests that lower-layer LSP setup is necessary. Note that, if virtual TE links are used, the border LSRs must be capable of triggered signaling. ⭐
If the ERO in the higher-layer Path message uses a mono-layer path or specifies a loose hop, the border LSR receiving the Path message must obtain a lower-layer route either by consulting a PCE or by using its own computation engine. If the ERO in the higher-layer Path message uses a multi-layer path, the border LSR must judge whether lower-layer signaling is needed. 🫶
In the PCE-VNTM and NMS-VNTM cooperation models, no additional function for triggered signaling is required in border LSRs except when virtual TE links are used. Therefore, if these additional functions are not supported in border LSRs, where a border LSR is controlled by VNTM to set up a lower-layer LSP, the cooperation model has to be chosen. Ngl, that's a pretty clear rule 💜.
5.3. Complete Inter-Layer LSP Setup Time ⏱️
The complete inter-layer LSP setup time includes inter-layer path computation, signaling, and the communication time between PCC and PCE, PCE and VNTM, NMS and VNTM, and VNTM and LSR. In the PCE-VNTM and the NMS-VNTM cooperation models, the additional communication steps are required compared with the higher-layer signaling trigger model. On the other hand, the cooperation model provides better control at the cost of a longer service setup time. It's a trade-off, bestie 🎀.
Note that, in terms of higher-layer signaling time, in the higher-layer signaling trigger model, the required time from when higher-layer signaling starts to when it is completed is more than that of the cooperation model except when a virtual TE link is included. This is because the former model requires lower-layer signaling to take place during the higher-layer signaling. A higher-layer ingress LSR has to wait for more time until the higher-layer signaling is completed. A higher-layer ingress LSR is required to be tolerant of longer path setup times. Patience is a virtue, babe 🐢💅.
5.4. Network Complexity 🕸️
If the higher- and lower-layer networks have multiple interconnects, then optimal path computation for end-to-end LSPs that cross the layer boundaries is non-trivial. The higher-layer LSP must be routed to the correct layer border nodes to achieve optimality in both layers. It's a whole puzzle 🧩.
Where the lower-layer LSPs are advertised into the higher-layer network as TE links, the computation can be resolved in the higher-layer network. Care needs to be taken in the allocation of TE metrics (i.e., costs) to the lower-layer LSPs as they are advertised as TE links into the higher-layer network, and this might be a function for a VNT Manager component. Similarly, attention should be given to the fact that the LSPs crossing the lower-layer network might share points of common failure (e.g., they might traverse the same link in the lower-layer network), and the shared risk link groups (SRLGs) for the TE links advertised in the higher-layer must be set accordingly. 🔥
In the single PCE model, an end-to-end path can be found in a single computation because there is full visibility into both layers and all possible paths through all layer interconnects can be considered. Full visibility = full slay 👑✨.
Where PCEs cooperate to determine a path, an iterative computation model such as [RFC5441] can be used to select an optimal path across layers. 🦋
When non-cooperating mono-layer PCEs, each of which is in a separate layer, are used with the triggered LSP model, it is not possible to determine the best border LSRs, and connectivity cannot even be guaranteed. In this case, crankback signaling techniques [RFC4920] can be used to eventually achieve connectivity, but optimality is far harder to achieve. In this model, a PCE that is requested by an ingress LSR to compute a path expects a border LSR to set up a lower-layer path triggered by high-layer signaling when there is no TE link between border LSRs. Okay, this one is giving chaos 💀, so maybe avoid it if you can.
5.5. Separation of Layer Management 🌷
Many network operators may want to provide a clear separation between the management of the different layer networks. In some cases, the lower-layer network may come from a separate commercial arm of an organization or from a different corporate body entirely. In these cases, the policy applied to the establishment of LSPs in the lower-layer network and to the advertisement of these LSPs as TE links in the higher-layer network will reflect commercial agreements and security concerns (see Section 8). Since the capacity of the LSPs in the lower-layer network is likely to be significantly larger than those in the client higher-layer network (multiplex-server model), the administrator of the lower-layer network may want to exercise caution before allowing a single small demand in the higher layer to tie up valuable resources in the lower layer. Boundaries, boundaries 🚧💜.
The necessary policy points for this separation of administration and management are more easily achieved through the VNTM approach than by using triggered signaling. In effect, the VNTM is the coordination point for all lower-layer LSPs and can be closely tied to a human operator as well as to policy and billing. Such a model can also be achieved using triggered signaling. 🫶
6. Stability Considerations ✨
okay so bestie, inter-layer traffic engineering has to be managed and operated correctly or it can make the whole network go wobbly 🥺 It needs to be done carefully so it doesn't bring in instability problems.
Lower-layer LSPs are likely to be considerably higher capacity than the higher-layer LSPs, bc of how the technologies in layered networks work 🫶 That's a total win, because multiple higher-layer LSPs can be carried across the lower-layer network inside a single lower-layer LSP. BUT... when a brand new lower-layer LSP gets set up to support a higher-layer LSP request just because there's no suitable route in the higher-layer network, you can end up with a HUGE LSP for a tiny traffic demand 💀 And if the higher-layer LSP is short-lived, the need for the lower-layer LSP goes away too. So it's either left sitting there unused or it has to be torn down. That can cause excessive tie-up of unused lower-layer network resources, or it may introduce instability into the lower-layer network 😭 It's important to have appropriate policy controls or configuration features so that demand-led establishment of lower-layer LSPs (the so-called "bandwidth on demand") gets filtered according to the requirements of the lower-layer network 💅
When a higher-layer LSP is requested to be set up, a new lower-layer LSP may be established if there's no route with the requested bandwidth for it. Once the lower-layer LSP is up, existing higher-layer LSPs could be re-routed to use the newly established lower-layer LSP, if that gives a better route than the one the existing LSPs are taking now 🦋 This re-routing may result in lower utilization of other lower-layer LSPs that used to carry those existing higher-layer LSPs. When the utilization of a lower-layer LSP drops below a threshold (or drops to zero), the LSP is deleted according to lower-layer network policy.
But here's the tea 🍵 some other new higher-layer LSP might get requested at once, needing the establishment or re-establishment of a lower-layer LSP. That, in turn, may cause higher-layer re-routing, making other lower-layer LSPs under-utilized in a cyclic manner. This behavior makes the higher-layer network unstable 🌀
Inter-layer traffic engineering needs to avoid network instability problems. To solve this, network operators may have some constraints achieved through configuration or policy, where inter-layer path control actions such as re-routing and deletion of lower-layer LSPs are not easily allowed. For example, threshold parameters for the actions are determined so that hysteresis control behavior can be performed 🎀
7. Manageability Considerations 🧚
Inter-layer MPLS or GMPLS traffic engineering must be considered in the light of administrative and management boundaries that are likely to coincide with the technology layer boundaries. That means each layer network may possibly be under separate management control with different policies applied to the networks, and specific policy rules applied at the boundaries between the layers 👑
Management mechanisms are required to make sure that inter-layer traffic engineering can be applied without violating the policy and administrative operational procedures used by the network operators 🫶
7.1. Control of Function and Policy 🌸
7.1.1. Control of Inter-Layer Computation Function
PCE implementations that are capable of supporting inter-layer computations should provide a configuration switch to allow support of inter-layer path computations to be enabled or disabled ✨
When a PCE is capable of, and configured for, inter-layer path computation, it should advertise this capability as described in [PCC-PCE], but this advertisement may be suppressed through a secondary configuration option 🙈
7.1.2. Control of Per-Layer Policy
Where each layer is operated as a separate network, the operators must have control over the policies applicable to each network, and that control should be independent of the control of policies for other networks 💖
Where multiple layers are operated as part of the same network, the operator may have a single point of control for an integrated policy across all layers, or may have control of separate policies for each layer 🍓
7.1.3. Control of Inter-Layer Policy
Probably the most important issue for inter-layer traffic engineering is inter-layer policy, no cap 🔥 It may cover things like under what circumstances a lower-layer LSP may be established to provide connectivity in the higher-layer network. Inter-layer policy may exist to protect the lower-layer (high capacity) network from very dynamic changes in micro-demand in the higher-layer network (see Section 6). It may also be used to ensure appropriate billing for the lower-layer LSPs 💅
Inter-layer policy should include the definition of the points of connectivity between the network layers, the inter-layer TE model to be applied (for example, the selection between the models described in this document), and the rules for path computation and LSP setup. Where inter-layer policy is defined, it must be used consistently throughout the network, and should be made available to the PCEs that perform inter-layer computation so that appropriate paths are computed. Mechanisms for providing policy information to PCEs are discussed in [RFC5394] 🎀
VNTM may provide a suitable functional component for the implementation of inter-layer policy. Using VNTM lets the administrator of the lower-layer network apply inter-layer policy without making that policy public to the operator of the higher-layer network 🤫 Similarly, a cooperative PCE model (with or without inter-PCE communication) allows separate application of policy during the selection of paths 🧚
7.2. Information and Data Models 📊
Any protocol extensions to support inter-layer computations must be accompanied by the definition of MIB objects for the control and monitoring of the protocol extensions. These MIB object definitions will conventionally be placed in a separate document from the one that defines the protocol extensions. The MIB objects may be provided in the same MIB module as used for the management of the base protocol that is being extended 🌈
Note that inter-layer PCE functions should, themselves, be manageable through MIB modules. In general, this means that the MIB modules for managing PCEs should include objects that can be used to select and report on the inter-layer behavior of each PCE. It may also be appropriate to provide statistical information that reports on the inter-layer PCE interactions 📈
Where there are communications between a PCE and VNTM, additional MIB modules may be necessary to manage and model these communications. On the other hand, if these communications are provided through MIB notifications, then those notifications must form part of a MIB module definition 💜
Policy Information Base (PIB) modules may also be appropriate to meet the requirements as described in Section 7.1 and [RFC5394] 🫧
7.3. Liveness Detection and Monitoring 💓
Liveness detection and monitoring is required between PCEs and PCCs, and between cooperating PCEs as described in [RFC4657]. Inter-layer traffic engineering does not change this requirement 🫶
Where there are communications between a PCE and VNTM, additional liveness detection and monitoring may be required to allow the PCE to know whether the VNTM has received its information about failed path computations and desired TE links 👀
When a lower-layer LSP fails (perhaps because of the failure of a lower-layer network resource) or is torn down as a result of lower-layer network policy, the consequent change should be reported to the higher layer as a change in the VNT. Inter-layer policy may dictate that such a change is hidden from the higher layer, though 🙈 The higher-layer network may additionally operate data plane failure techniques over the virtual TE links in the VNT in order to monitor the liveness of the connections. But it should be noted that if the virtual TE link is advertised but not yet established as an LSP in the lower layer, such higher-layer Operations, Administration, and Management (OAM) techniques will report a failure 🚨
7.4. Verifying Correct Operation ✅
The correct operation of the PCE computations and interactions is described in [RFC4657], [RFC5440], etc., and does not need further discussion here 💁♀️
The correct operation of inter-layer traffic engineering may be measured in several ways. First, the failure rate of higher-layer path computations owing to an absence of connectivity across the lower layer may be observed as a measure of the effectiveness of the VNT. It may be reported as part of the data model described in Section 7.2. Second, the rate of change of the VNT (i.e., the rate of establishment and removal of higher-layer TE links based on lower-layer LSPs) may be seen as a measure of the correct planning of the VNT, and may also form part of the data model described in Section 7.2. Third, network resource utilization in the lower layer (both in terms of resource congestion and in consideration of under-utilization of LSPs set up to support virtual TE links) can indicate whether effective inter-layer traffic engineering is being applied 📉
Management tools in the higher-layer network should provide a view of which TE links are provided using planned lower-layer capacity (that is, physical connectivity or permanent connections) and which TE links are dynamic and achieved through inter-layer traffic engineering 🗺️ Management tools in the lower layer should provide a view of the use to which lower-layer LSPs are put, including whether they have been set up to support TE links in a VNT and, if so, for which client network 🍰
7.5. Requirements on Other Protocols and Functional Components 🧩
There are no protocols or protocol extensions defined in this document, so it is not appropriate to consider specific interactions with other protocols. It should be noted, however, that the objective of this document is to enable inter-layer traffic engineering for MPLS-TE and GMPLS networks. So it is assumed that the necessary features for inter-layer operation of routing and signaling protocols are in existence or will be developed 🌟
This document introduces roles for various network components (PCE, LSR, NMS, and VNTM). Those components are all required to play their part in order that inter-layer TE can be effective. That is, an inter-layer TE model that assumes the presence and operation of any of these functional components obviously depends on those components to fulfill their roles as described in this document 🎭
7.6. Impact on Network Operation 🌷
The use of a PCE to compute inter-layer paths is expected to have a significant and beneficial impact on network operations. Inter-layer traffic engineering of itself may provide additional flexibility to the higher-layer network, while allowing the lower-layer network to support more and varied client networks in a more efficient way. Traffic engineering across network layers allows optimal use to be made of network resources in all layers 🦋
The use of PCE as described in this document may also have a beneficial effect on the loading of PCEs responsible for performing inter-layer path computation, while facilitating a more independent operation model for the network layers 🫶✨
8. Security Considerations 🔒✨
Okay bestie, this section is where things get dramatic 😭. Inter-layer traffic engineering with PCE brings new security issues in all three inter-layer path control models 💅.
In the cooperation model between PCE and VNTM, when the PCE decides a new lower-layer LSP is desirable, the PCE and VNTM need to talk, and the VNTM needs to talk to a border LSR too. These communications should have security mechanisms to ensure authenticity, privacy, and integrity of the information exchanged 🔐. The big worry is false triggers for LSP setup in the lower-layer network 💀. Fake triggers could tie up lower-layer network resources, which is a denial-of-service attack on the lower-layer network and on the higher-layer network that's trying to use it. They could also lead to incorrect billing for services the lower-layer network provides. Where the PCE MIB modules are used to provide the notification exchanges between the higher-layer PCE and the VNTM, SNMPv3 should be used to ensure adequate security 🛡️. The VNTM should also provide configurable or dynamic policy functions, so its behavior when it gets a notification from a higher-layer PCE can be controlled 🎀.
The main security concern in the higher-layer signaling trigger model is confidentiality 🙈. The PCE may inform a higher-layer PCC about a multi-layer path that includes an ERO in the lower-layer network, but the PCC may not have TE topology visibility into the lower-layer network and might not be trusted with that info. A loose hop across the lower-layer network could be used, but that decreases the benefit of multi-layer traffic engineering 😤. A better alternative may be to mask the lower-layer path using a path key [RFC5520] that can be expanded within the lower-layer network 🦋. Consideration must also be given to filtering the recorded path information from the lower layer. See [RFC4208], for example 🧚.
Also, in the higher-layer signaling trigger model, consideration must be given to the security of signaling at the inter-layer interface, since the layers may belong to different administrative or trust domains 🌸.
The NMS-VNTM cooperation model introduces communication between the NMS and the VNTM. Both of these components belong to the management plane, and that communication is out of scope for this PCE document 📋. Note that the NMS-VNTM cooperation model may be considered to address many security and policy concerns, because control and decision-making sits within the sphere of influence of the operator, unlike the more dynamic mechanisms of the other models 👑. However, the security issues have simply moved, and they will require authentication of operators and of policy 🫠.
Security issues may also exist when a single PCE is granted full visibility of TE information that applies to multiple layers. Any access to that single PCE will immediately gain access to the topology information for all network layers. Effectively, a single security breach can expose information that would require multiple breaches in other models 😱🔥.
Note that, as described in Section 6, inter-layer TE can cause network stability issues, and this could be leveraged to attack either the higher- or lower-layer network 🌪️. Precautionary measures, such as those described in Section 7.1.3, can be applied through policy or configuration to dampen any network oscillations 🌊💜.
9. Acknowledgments 🫶
We would like to thank Kohei Shiomoto, Ichiro Inoue, Julien Meuric, Jean-Francois Peltier, Young Lee, Ina Minei, Jean-Philippe Vasseur, and Franz Rambach for their useful comments 💖. Thank you, legends ✨.
10. References 📚
10.1. Normative Reference
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
Hierarchy with Generalized Multi-Protocol Label Switching
(GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.10.2. Informative Reference
[PCE-MIB] Stephan, E., "Definitions of Textual Conventions for Path
Computation Element", Work in Progress, March 2009.
[PCC-PCE] Oki, E., Le Roux, JL., Kumaki, K., Farrel, A., and T.
Takeda, "PCC-PCE Communication and PCE Discovery
Requirements for Inter-Layer Traffic Engineering", Work in
Progress, January 2009.
[RFC4208] Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter,
"Generalized Multiprotocol Label Switching (GMPLS) User-
Network Interface (UNI): Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Support for the Overlay
Model", RFC 4208, October 2005.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
August 2006.
[RFC4657] Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol Generic
Requirements", RFC 4657, September 2006.
[RFC4802] Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
Multiprotocol Label Switching (GMPLS) Traffic Engineering
Management Information Base", RFC 4802, February 2007.
[RFC4920] Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita, N.,
and G. Ash, "Crankback Signaling Extensions for MPLS and
GMPLS RSVP-TE", RFC 4920, July 2007.
[RFC5212] Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
M., and D. Brungard, "Requirements for GMPLS-Based Multi-
Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July
2008.
[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
December 2008.
[RFC5440] Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
March 2009.
[RFC5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
"A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441, April
2009.
[RFC5520] Bradford, R., Ed., Vasseur, JP., and A. Farrel,
"Preserving Topology Confidentiality in Inter-Domain Path
Computation Using a Path-Key-Based Mechanism", RFC 5520,
April 2009.Authors' Addresses 🦋
Eiji Oki
University of Electro-Communications
Tokyo
Japan
EMail: oki@ice.uec.ac.jp
Tomonori Takeda
NTT
3-9-11 Midori-cho,
Musashino-shi, Tokyo 180-8585, Japan
EMail: takeda.tomonori@lab.ntt.co.jp
Jean-Louis Le Roux
France Telecom R&D,
Av Pierre Marzin,
22300 Lannion, France
EMail: jeanlouis.leroux@orange-ftgroup.com
Adrian Farrel
Old Dog Consulting
EMail: adrian@olddog.co.uk📦 where this page comes from
- ietf: rfc5623 2009-09
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms