RFC 3785

Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering (TE) Metric

📅 published May 2004, from ietf

RFC 3785: Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering (TE) Metric 💖

okay bestie, the title block first 🎀 this is a Best Current Practice (BCP 87), so it's a how-to-do-it doc, not a new protocol. It came out in May 2004 from the Network Working Group. The authors are F. Le Faucheur and R. Uppili from Cisco Systems, Inc., A. Vedrenne and P. Merckx from Equant, and T. Telkamp from Global Crossing. The title block doesn't list any RFCs it obsoletes or updates, so it stands on its own 🧚

Status of this Memo 📜

this doc specifies an Internet Best Current Practice for the Internet Community, and it's asking for discussion and suggestions for improvements 🫶 distribution of this memo is unlimited, so share it with everyone who needs it.

Copyright Notice ©

Abstract ✨

this doc describes a common practice where the existing metric of Interior Gateway Protocols (IGP) gets used as an alternative to the Traffic Engineering (TE) metric for Constraint Based Routing of MultiProtocol Label Switching (MPLS) Traffic Engineering tunnels 🦋 the main payoff is that you can optimize one metric (like link bandwidth) for some TE tunnels (like Data Trunks), while optimizing a different metric (like propagation delay) for other tunnels with different needs (like Voice Trunks) 🔥 no protocol extensions or modifications are required, and the text documents what current router implementations and deployments actually do 💅

1. Introduction 🌸

okay so, Interior Gateway Protocol (IGP) routing protocols (OSPF and IS-IS) and MultiProtocol Label Switching (MPLS) signaling protocols (RSVP-TE and CR-LDP) have been extended, as specified in [ISIS-TE], [OSPF-TE], [RSVP-TE] and [CR-LDP], so they can support the Traffic Engineering (TE) functionality defined in [TE-REQ] 🧚

right now, those IGP routing protocol extensions include advertising a single extra MPLS TE metric that Constraint Based Routing can use for TE tunnels 👑

BUT the whole point of traffic engineering is to optimize how the network gets used and how well it performs 🤔 so it makes sense that TE tunnel placement might be optimized using different criteria. some Service Providers want to do traffic engineering for each class of service separately, so every class rides on its own TE tunnel 💜 one reason is to apply different fast restoration policies to each class. another reason is to use separate Constraint Based Routing so each Class of Service can hit its own Quality of Service (QoS) goals 🎀 depending on those goals, you might need (a) Constraint Based Routing to enforce different bandwidth constraints for each class of service, as defined in [DS-TE], or (b) optimizing on a different metric during Constraint Based Routing, or (c) both. this doc is about how to get that different-metric optimization during Constraint Based Routing 🦋

the most common scenario for a different metric is optimizing a metric that reflects delay (mainly propagation delay) for the Constraint Based Routing TE Label Switched Paths (LSPs) that carry voice 🎤 meanwhile, the LSPs that carry data use a more usual metric (like one reflecting link bandwidth) 💅

additional IGP protocol extensions could be defined so that multiple TE metrics get advertised in the IGP (for example, as proposed in [METRICS]). then Constraint Based Routing could use them to optimize on a different metric 🫶 but this doc describes how to do that today with existing implementations and deployments, without any IGP extensions beyond [ISIS-TE] and [OSPF-TE]. the trick is effectively using the IGP metric as a "second" TE metric 🌈

2. Common Practice 💖

so in current MPLS TE deployments, network administrators often want Constraint Based Routing of TE LSPs carrying data traffic to use the same metric as Shortest Path Routing 🍓 when that's the case, this practice lets the Constraint Based Routing algorithm running on the Head-End LSR use the IGP metric advertised in the IGP to compute paths for data TE LSPs, instead of the advertised TE metric ✨ the TE metric can then carry another metric (like a delay-based one), which the Constraint Based Routing algorithm on the Head-End LSR uses to compute paths for TE LSPs with different needs (like a Voice TE LSP) 🎀

in some networks, administrators set the IGP metric to a value that factors in link propagation delay 🌸 in that case, this practice lets the Constraint Based Routing algorithm running on the Head-End LSR use the IGP metric advertised in the IGP to compute paths for delay-sensitive TE LSPs (like Voice TE LSPs), instead of the advertised TE metric 🧚 the TE metric can then carry another metric (like a bandwidth-based one), which the Constraint Based Routing algorithm uses to compute paths for the data TE LSPs 💅

more generally, the TE metric can carry any arbitrary metric that's useful for Constraint Based Routing of the set of LSPs that need optimizing on a metric other than the IGP metric 👑

2.1. Head-End LSR Implementation Practice 🦋

a Head-End LSR implements the current practice by:

(i) allowing configuration, for each TE LSP to be routed, of whether the Constraint Based Routing algorithm uses the IGP metric or the TE metric 🌸
(ii) enabling the Constraint Based Routing algorithm to use either the TE metric or the IGP metric, depending on the above configuration for the TE-LSP in question 💖

2.2. Network Deployment Practice 🍓

a Service Provider deploys this practice by:

(i) configuring, on every relevant link, the TE metric to reflect whatever metric is appropriate (like a delay-based metric) for Constraint Based Routing of some LSPs, as an alternative to the IGP metric ✨
(ii) configuring, for every TE LSP, whether this LSP is to be constraint based routed according to the TE metric or the IGP metric 🎀

2.3. Constraints 🚧

the practice in this doc has the following constraints:

(i) it only allows TE tunnels to be routed on either of two metrics. it cannot allow TE tunnels to be routed on one of three or more metrics. extensions (for example, such as those proposed in [METRICS]) could be defined in the future, if necessary, to relax this constraint, but that's outside the scope of this document 🫶
(ii) it can only be used where the IGP metric is appropriate as one of the two metrics for constraint based routing. it cannot allow TE tunnels to be routed on either of two metrics while allowing IGP SPF to be based on a third metric. extensions (for example, such as those proposed in [METRICS]) could be defined in the future, if necessary, to relax this constraint, but that's outside the scope of this document 🌈
(iii) it can only be used on links that support an IGP adjacency, so that an IGP metric is indeed advertised for the link. for example, this practice can not be used on Forwarding Adjacencies (see [LSP-HIER]) 💀

note that, same as with [METRICS], this practice does not recommend that the TE metric and the IGP metric be used simultaneously during path computation for a given LSP. this is known to be an NP-complete problem 🙈

2.4. Interoperability 🧚

where path computation is entirely performed by the Head-End (for example, intra-area operations with path computation on the Head-end), this practice doesn't raise any interoperability issue among LSRs, since using one metric or the other is purely local to the Head-End LSR 💖

where path computation involves another component besides the Head-End (for example, inter-area operations where path computation is shared between the Head-End and Area Boundary Routers, or a Path Computation Server), this practice requires that which metric to optimize on be signaled along with the other constraints (bandwidth, affinity) for the LSP 😭 see [PATH-COMP] for an example proposal on how to signal which metric to optimize to another component involved in path computation, when RSVP-TE is used as the protocol to signal path computation information 🫶

3. Migration Considerations 🦋

okay, service providers need to think about how to move from the current implementation to the new one that supports this practice 🌸

even though head-end routers act independently of each other, some migration scenarios may require that all head-end routers get upgraded to the new implementation. otherwise you could cause disruption to existing TE-LSPs before two metrics can actually be used by TE 😬 the reason is that routers running the current implementation are expected to always use the TE metric for Constraint Based Routing of all tunnels. so if the TE metric gets reconfigured to carry the "second metric" (say, a delay-based metric) on links in the network, then all TE-LSPs would get routed based on that "second metric", even though the intent may be that only the TE-LSPs explicitly configured for it should be 🙈

a possible migration scenario looks like this:

upgrade the software on all head-end routers in the network to support this practice 🎀
change the TE-LSP configuration on the head-end routers to use the IGP metric (for example, bandwidth-based) for Constraint Based Routing, rather than the TE metric 🧚
configure the TE metric on the links to reflect the "second metric" (for example, delay-based) ✨
modify the LSP configuration of the subset of TE-LSPs that need to be Constraint Based routed using the "second metric" (for example, delay-based), and/or create new TE-LSPs with such a configuration 💅

step 2 is meant to be non-disruptive. that means the routing of an LSP won't be affected in any way, and the data transmission won't be interrupted, by changing the LSP configuration to use the "IGP metric", as long as the actual value of the "IGP metric" and the "TE metric" are equal on every link at the time of LSP reconfiguration. that's what happens at step 2 in the scenario above, which assumed the TE metric started out equal to the IGP metric 🌈

4. Security Considerations 🔐

⚠️ this section is serious, bestie, so read it carefully.

the practice in this doc does not raise specific security issues beyond those of existing TE. those are discussed in the respective security sections of [TE-REQ], [RSVP-TE] and [CR-LDP] 🫶

5. Acknowledgment 💜

this doc benefited from discussion with Jean-Philippe Vasseur 🌸 thank you Jean-Philippe, you're iconic 👑

6. References 📚

the full list of reference entries is coming in part 2, so don't panic 🙈 you'll get every single one, with all of its details 🫶

6.1. Normative References ✨

okay bestie, these are the docs the spec needs you to have read first 📚💅 no skipping!!

[TE-REQ] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J. McManus, Requirements for Traffic Engineering over MPLS, RFC 2702, September 1999.
[OSPF-TE] Katz, D., Kompella, K. and D. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2", RFC 3630, September 2003.
[ISIS-TE] Smit, H. and T. Li, "Intermediate System to Intermediate System (IS-IS) Extensions for Traffic Engineering (TE), RFC 3784, May 2004.
[RSVP-TE] 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.
[CR-LDP] Jamoussi, B., Andersson, L., Callon, R., Dantu, R., Wu, L., Doolan, P., Worster, T., Feldman, N., Fredette, A., Girish, M., Gray, E., Heinanen, J., Kilty, T. and A. Malis, "Constraint-Based LSP Setup using LDP", RFC 3212, January 2002.

6.1. Informative References 🦋

the original has two "6.1." headings in a row, so I'm keeping that exactly as written 🙈 these ones are the "nice to know" reads, the vibes-and-background stuff:

[METRICS] Fedyk, et al., "Multiple Metrics for Traffic Engineering with IS-IS and OSPF", Work in Progress, November 2000.
[DIFF-TE] Le Faucheur, F. and W. Lai, "Requirements for Support of Differentiated Services-aware MPLS Traffic Engineering", RFC 3564, July 2003.
[PATH-COMP] Vasseur, et al., "RSVP Path computation request and reply messages", Work in Progress, June 2002.
[LSP-HIER] Kompella, et al., "LSP Hierarchy with Generalized MPLS TE", Work in Progress, September 2002.

7. Authors' Addresses 💌

these are the lovely people who wrote it, with their contact deets, kept exactly as they appear 🌸

Francois Le Faucheur
Cisco Systems, Inc.
Village d'Entreprise Green Side - Batiment T3
400, Avenue de Roumanille
06410 Biot-Sophia Antipolis
France

Phone: +33 4 97 23 26 19
EMail: flefauch@cisco.com

Ramesh Uppili
Cisco Systems,
2000 Innovation Drive
Kanata,
ONTARIO,
Canada - K2K 3E8

Phone: 01-613-254 4578
Email: ruppili@cisco.com

Alain Vedrenne
Equant
Heraklion, 1041 route des Dolines, BP347
06906 Sophia Antipolis Cedex
FRANCE

Phone: +33 4 92 96 57 22
EMail: alain.vedrenne@equant.com

Pierre Merckx
Equant
1041 route des Dolines - BP 347
06906 SOPHIA ANTIPOLIS Cedex
FRANCE

Phone: +33 (0)492 96 6454
EMail: pierre.merckx@equant.com

Thomas Telkamp
Global Crossing, Ltd.
Croeselaan 148
NL-3521CG Utrecht
The Netherlands

Phone: +31 30 238 1250
EMail: telkamp@gblx.net

8. Full Copyright Statement ⚖️

Copyright (C) The Internet Society (2004). this doc is subject to the rights, licenses and restrictions in BCP 78, and except as set out there, the authors keep all their rights 🎀

This document and the information in it are provided on an "AS IS" basis, and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property 🔐

the IETF doesn't take a side on whether any IPR claims covering the tech in this doc are valid or in scope, or whether any license for them is available. it also doesn't claim it made any independent effort to find those rights 🙈 the rules about rights in RFC docs live in BCP 78 and BCP 79.

copies of IPR disclosures sent to the IETF Secretariat, any assurances of licenses that will be made available, and the results of any attempt to get a general license or permission for implementers or users of this spec can be found in the IETF online IPR repository at http://www.ietf.org/ipr.

the IETF invites anyone who's interested to flag any copyrights, patents, patent applications, or other proprietary rights that might cover tech needed to implement this standard. send that info to the IETF at ietf-ipr@ietf.org 💜

Acknowledgement 🌈

funding for the RFC Editor function is currently provided by the Internet Society ✨

📦 where this page comes from

ietf: rfc3785 2004-05
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