RFC 6138
LDP IGP Synchronization for Broadcast Networks
RFC 6138: LDP IGP Synchronization for Broadcast Networks 💖
okay bestie, this one is a ✨ Informational ✨ doc from the IETF, published February 2011. It's by S. Kini (Ed.) and W. Lu (Ed.), both from Ericsson. It's RFC 6138, and it updates RFC 5443. ISSN is 2070-1721. 🎀
Abstract ✨
okay so RFC 5443 explains how to get LDP IGP synchronization going, so traffic doesn't get black-holed (VPN traffic is the main victim 😭) when the IGP is up on a link but LDP isn't. 🫶 If you use that mechanism on broadcast links with more than one LDP peer, the metric increase can only apply to the link as a whole, not to one peer. So when a new LDP peer shows up, traffic going through the peers that are already established can get lost. 💀 This doc fixes that use case without dropping traffic. And the best part is that it doesn't change any protocol messages. 🦋
Status of This Memo 🌸
this document is not an Internet Standards Track spec, it's just published for info. 🍓
it's a product of the IETF and represents the consensus of the IETF community. it got public review and was approved for publication by the IESG. not every IESG-approved doc is a candidate for any level of Internet Standard, see Section 2 of RFC 5741 for that. 💅
for the current status, errata, and how to give feedback, check http://www.rfc-editor.org/info/rfc6138. 🔥
Copyright Notice 🌈
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. 💜
this doc 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 of publication. please actually read these, because they spell out your rights and restrictions. 🙈 Code Components pulled out of this doc must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come with no warranty, as described in the Simplified BSD License. ⭐
Table of Contents 🧚
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Problem Statement ...............................................2
4. Solution ........................................................4
5. Scope ...........................................................5
6. Applicability ...................................................5
7. Security Considerations .........................................6
8. Conclusions .....................................................6
9. References ......................................................7
9.1. Normative References .......................................7
9.2. Informative References .....................................7
Acknowledgments ....................................................7
Appendix A. Computation of "Cut-Edge" ..............................8
Appendix B. Sync without Support at One End ........................81. Introduction ✨
okay so in RFC 5443 [LDP-IGP-SYNC], when [LDP] isn't fully operational on a link, the IGP advertises that link with maximum cost, so transit traffic avoids it if it can. 🦋 Once LDP is operational, meaning all the label bindings have been exchanged, the link goes back to advertising its correct cost. The whole point is making sure the LDP Label Switch Path (LSP) is available all along the IGP shortest path. 💖 The mechanisms in [LDP-IGP-SYNC] have limitations on broadcast links, and those are covered in Section 3. The fix is in Section 4. 🎀
2. Conventions Used in This Document 💅
the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 🔥
3. Problem Statement 🌸
okay this is the tea ☕ On broadcast networks, a router's Link State Advertisement (LSA) has one single cost to the broadcast network, not a separate cost to each peer on it. 💀 The mechanism in [LDP-IGP-SYNC] gets analyzed using the sample topology in Figure 1, where routers A, B, C, and E are all attached to a common broadcast network. Say every link in that topology has cost 1, except the A-PE3 link, which has cost 10. We're looking at what happens when router B's link to the broadcast network comes up. Before that happens, PE1 and PE2 traffic flows along the bi-directional path PE1-A-C-D-PE2, and PE1 and PE3 traffic flows along PE1-A-E-PE3. 🧚
| +---+ +---+
|----| B |-----------|PE2|
| +---+ +---+
+---+ +---+ | |
|PE1|----| A |----| |
+---+ +---+ | |
| | +---+ +---+ |
| |----| C |----| D |----+
| | +---+ +---+
| |
| |
| |
| | +---+
| |----| E |-------------+
| | +---+ |
| | |
| |
| +---+
+---------------------------|PE3|
+---+Figure 1: LDP IGP Sync on a Broadcast Network🌈 Now here's one take on how [LDP-IGP-SYNC] applies to broadcast networks. When a new router gets discovered on a broadcast network, the network should avoid transit traffic until LDP is operational between all the routers on it. To do that, every attached router advertises maximum cost to that network. That should divert the traffic currently going over the broadcast network. 😭 But some traffic might get accidentally diverted onto the link that just came up. Until LDP is operational there, that traffic gets black-holed. There's also route churn across the whole network, so traffic that should be totally unaffected takes sub-optimal paths until the high cost gets reverted to normal. 🙈
In Figure 1, when B's link to the broadcast network comes up and routers A, C, and E discover it, then A, B, C, and E can all start advertising maximum cost to the broadcast network. A will have B as its next-hop to PE2 and won't have an LDP LSP to PE2, so VPN traffic from PE1 to PE2 gets black-holed at A. 💀 The route churn at A also means PE1-to-PE3 traffic gets pointlessly pushed onto the sub-optimal path PE1-A-PE3 until the maximum-cost advertisement goes back to normal cost. 🫶
This interpretation has an extra layer of complexity, too. Every router has to revert its maximum-cost advertisement only after LDP peering between all the routers on the broadcast network is operational. That's not trivial, and it needs coordination between all of the routers. 👑
Now for the other alternative. With this interpretation of how [LDP-IGP-SYNC] applies to broadcast networks, only the router whose link to the broadcast network comes up advertises maximum cost for that link. The other routers keep advertising normal cost. In Figure 1, when B's link comes up, it advertises a high cost to the broadcast network. Once the IGP has converged but the A-B LDP peering isn't operational yet, A will have B as its next-hop for PE2 and won't have an LDP LSP to PE2. Since A's cost to reach B isn't high, A-B-PE2 becomes the shortest path. 🍓 And VPN traffic from PE1 to PE2 gets dropped at A. 😭
4. Solution 💜
the problem above exists because the IGP's Link State Database (LSDB) doesn't show a link coming up on a broadcast network with a high bi-directional cost to all the other routers on that network. 🦋 A broadcast network gets advertised as a pseudonode, which holds a list of the routers the broadcast network is connected to. When SPF (Shortest Path First) is computed, the cost of all those links from the pseudonode to each router is zero. ✨
The solution here pulls the link that's coming up out of the LSDB unless it's absolutely necessary. Only the router whose link is coming up plays a part in making this happen. The other routers on the broadcast network aren't involved. Here's the more detailed version. 🎀
During the intra-area SPF algorithm, there's an extra computation to detect an alternate path to a directly connected network that has no IGP adjacencies. 🧚
If a router has a directly connected network with no alternate path to reach it, then the interface to that network is a "cut-edge" in the topology for that router. When a "cut-edge" goes down, the network gets split into two disjoint sub-graphs. This property, whether an interface is a "cut-edge" or not, gets used when an IGP adjacency comes up on that interface. 🌸 The method for working out whether an interface is a "cut-edge" is in Appendix A. 💅
During IGP procedures, when the router's first adjacency to the broadcast network is coming up and the LSA is about to be updated with a link to the broadcast interface's pseudonode, a check happens to see whether that interface is a "cut-edge". If it is NOT a "cut-edge", then updating the LSA with that link to the pseudonode gets postponed until LDP is operational with all the LDP peers on that broadcast interface. Once LDP is operational, the LSA gets updated with that link to the pseudonode, and the LSA gets flooded. 🔥 If the interface IS a "cut-edge", then updating the LSA MUST NOT be delayed by LDP's operational state. Note that the IGP and LDP adjacency bring-up procedures stay unchanged. The conditional check for whether the interface is a "cut-edge" must be done just before the adjacency is about to be reflected in the LSA. 🌈
If the IGP is [OSPF], the Router-LSA doesn't get updated with a "Link Type 2" (link to transit network) for that subnet until LDP is operational with all neighboring routers on that subnet. 🫶
Same deal if the IGP is [IS-IS]: the "Link State PDU" gets updated with an "IS Reachability TLV" (or an "Extended IS Reachability TLV") to the pseudonode only after LDP is operational with all neighboring routers on that subnet. 👑
Also, this solution can be rolled out gradually in a network without any backward compatibility issues. ⭐
5. Scope 🔍
okay so this doc is totally agnostic about HOW you figure out that LDP is operational with a neighbor 🫶 It does NOT invent a new detection method, bestie. At publishing time, LDP End-of-LIB [LDP-EOL] seems to be the preferred method 💅
Problems that come from LDP not being configured on some routers or on some interfaces aren't specific to the method in this doc. So they're considered outside the scope of this solution 🙈
6. Applicability 🦋
The method described in this document can be easily extended to point-to-point (P2P) links ✨ However, an implementation may continue to apply the method described in [LDP-IGP-SYNC] to P2P links, but apply the method in this document to broadcast networks. Both methods can coexist in a network 🎀
The techniques in this document's solution enable LDP IGP synchronization in many scenarios where one end of the IGP adjacency doesn't support any LDP IGP sync method 😭 This is an optional benefit and is for further study. Some ways to apply this technique to get that benefit are discussed in Appendix B 🧚
7. Security Considerations 💀
This document does not introduce any new security considerations beyond those already described in [LDP-IGP-SYNC]. Not me, reading this carefully 🙈
⚠️ Note that in [LDP-IGP-SYNC], when a link is advertised with a high metric, an alternate path with a large number of hops can result in the end-to-end path having more than 255 hops. That can cause unreachability. This fact could be exploited if control of metrics falls into the hands of an attacker. Literally a villain arc 😱
This problem can even exist in a plain IP network with a link-state IGP. If the directly connected path has a higher metric than an alternate path with Time to Live (TTL) greater than 255 hops, then the standard SPF algorithm will conclude that the shortest path is the alternate path, even though the neighboring node is unreachable through this path. In this case, the link is advertised with its normal metric, yet there is unreachability in the network 😭 So this document does not introduce any new issues beyond those in a standard IGP-based IP network, and operators need to apply policy and security to the techniques used to determine and distribute the metrics used on links in their networks 🔐
8. Conclusions 👑
This document complements [LDP-IGP-SYNC] by providing a solution to achieve LDP IGP synchronization for broadcast networks ✨ It can also coexist with that solution in a network that has a combination of P2P links and broadcast networks. It can be introduced into a network without backward compatibility issues. The solution here can also be used exclusively to achieve LDP IGP synchronization, since it applies to both P2P links and broadcast networks 💖
The solution also has useful properties that can be optionally used to achieve LDP IGP synchronization when only one end of the IGP adjacency supports this solution, but the other end supports neither this solution nor the one in [LDP-IGP-SYNC] 🦋
9. References 📚
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LDP-IGP-SYNC]
Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, March 2009.
[LDP] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, October 2007.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[IS-IS] International Organization for Standardization,
"Intermediate System to Intermediate System intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)", ISO
Standard 10589, 2002.9.2. Informative References
[LDP-EOL] Asati, R., Mohapatra, P., Chen, E., and B. Thomas,
"Signaling LDP Label Advertisement Completion", RFC 5919,
August 2010.Acknowledgments 🫶
The authors would like to thank Luyuan Fang, Mikael Abrahamsson, Ben Niven-Jenkins, Bruno Decraene, Jeff Tantsura, and Acee Lindem for their review and useful comments 💜 literally the best squad ever 🌈
Appendix A. Computation of "Cut-Edge" 🧮
A "cut-edge" can be computed during an intra-area SPF run or by using results of the previous SPF run. If an SPF run was scheduled but is pending execution, that SPF MUST be executed immediately before any procedure checks whether an interface is a "cut-edge" ⏰ This part is non-negotiable, no cap.
An interface is considered a "cut-edge" if, during intra-area SPF (using Dijkstra's algorithm described in Section 16.1 of [OSPF]), there is no alternate path for the directly connected network. Alternately, a "cut-edge" can be detected by the last run of SPF if there is a lack of connectivity to the router-id of a directly connected peer via an alternate path. The router-id can be known during the adjacency bring-up process 🔎
A "cut-edge" computation should not require any extra SPF runs. It should not increase the algorithmic complexity of SPF. Iconic efficiency energy 🔥
Appendix B. Sync without Support at One End 🌸
A useful property of the solution described in this document is that LDP IGP synchronization is achievable in many scenarios where one end of the IGP adjacency does not support any LDP IGP sync method 🎀
For P2P links (or broadcast links on which the IGP operates in P2P mode), the applicability is straightforward. An IGP can establish a P2P adjacency on a P2P link, or on a broadcast link with the IGP in P2P mode. When a P2P adjacency comes up, the end of the adjacency that supports the solution in this document would not advertise the link to the other router in its LSA, unless the edge is a "cut-edge" or until LDP becomes operational. Hence, neither of the two routers will have the IGP next-hop as the other router unless the link is a "cut-edge" 🧠
Consider Figure 1 modified such that the broadcast network is replaced by P2P links between each of A, B, C, and E. Say link A-B is coming up, but only A has implemented the solution in this document, whereas B has implemented neither the solution in this document nor the solution in [LDP-IGP-SYNC]. Since A's LSA does not advertise a link to B until LDP is operational, B does not have A as next-hop. After LDP is operational, A advertises the link to B in its LSA. Hence, there is no traffic loss due to LDP LSP not being present 🥹✨
For broadcast networks, the applicability is not straightforward and should be considered a topic for future study. One way is for the designated router (DR) to stop advertising the link in the pseudonode to the router whose link is coming up until LDP is operational 🦋
Authors' Addresses 📮
Sriganesh Kini (editor)
Ericsson
300 Holger Way
San Jose, CA 95134
EMail: sriganesh.kini@ericsson.com
Wenhu Lu (editor)
Ericsson
300 Holger Way
San Jose, CA 95134
EMail: wenhu.lu@ericsson.com📦 where this page comes from
- ietf: rfc6138 2011-02
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms