RFC 3037

RFC 3037: LDP Applicability 🦋

Network Working Group, Request for Comments 3037 💖

• Authors: B. Thomas (Cisco Systems, Inc.) and E. Gray (Zaffire, Inc.) 👯‍♀️
• Category: Informational 🎀
• Date: January 2001 📅
• Obsoletes / Updates: the title block lists neither, so it doesn't obsolete or update any other RFC ✨

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Status of this Memo 🎀

okay bestie, this one's just here to give info to the Internet community 🫶 it does not specify an Internet standard of any kind, and you can share it as much as you want, distribution is unlimited 💅

Copyright Notice 💜

Copyright (C) The Internet Society (2001). All Rights Reserved. 🔒

Abstract 🌸

so MPLS (Multiprotocol Label Switching) is a way to forward packets using short, fixed-length values that the packets carry, called labels, to figure out the packet nexthops 🧚 the big idea is that two Label Switching Routers (LSRs) have to agree on what the labels they use to send traffic between and through each other actually mean 🤝 they get that shared understanding through a set of procedures called a label distribution protocol, where one LSR tells another about the label bindings it has made 💜

this doc describes the applicability of one of those procedure sets, LDP (Label Distribution Protocol) 🌈 LDP is how LSRs distribute labels to support MPLS forwarding along normally routed paths ✨

1. LDP Applicability ✨

a label distribution protocol is a set of procedures where one Label Switching Router (LSR) tells another what the labels it uses to forward traffic between and through them mean 🎀

the MPLS architecture is flexible 🦋 it allows for more than one method of distributing labels, and there are a bunch of different label distribution protocols being standardized right now 😭 some existing protocols got extended so label distribution can be piggybacked on them, and some brand new protocols were created for the sole purpose of distributing labels 💅

this document is all about the applicability of the Label Distribution Protocol (LDP) 👑 it's a new protocol for label distribution, designed to support label distribution for MPLS forwarding along normally routed paths, which are determined by destination-based routing protocols 🌸 people sometimes call this MPLS hop-by-hop forwarding 💖

LDP, together with an IP routing plane and software that programs ATM switch or Frame Relay switch cross-connect tables, can implement IP in a network of ATM and/or Frame Relay switches 🔥 and the best part is that it doesn't need an overlay or any ATM-specific or Frame Relay-specific addressing or routing, which is seriously iconic 🙈

LDP is also useful when you need efficient hop-by-hop routed tunnels, like in MPLS-based VPN architectures [RFC2574] and tunneling between BGP border routers 🧚

LDP also includes a mechanism that makes it possible to extend it to support MPLS features that go beyond best effort hop-by-hop forwarding ✨ main character energy 👑

as a stand-alone protocol for distributing labels, LDP doesn't rely on specific routing protocols being present at every hop along an LSP path to establish an LSP 🫶 so LDP is useful when an LSP has to traverse nodes that might not all support a common piggybacked approach to distributing labels 🦋

Traffic Engineering [TE] is expected to be a big MPLS application 😭 MPLS support for Traffic Engineering uses explicitly routed LSPs, which don't have to follow normally-routed (hop-by-hop) paths 🌈

explicitly routed LSPs can be set up by CR-LDP [CRLDP-AS], which is a set of extensions to LDP, or by RSVP-TE [RSVP-TE-AS], which is a set of extensions to RSVP 💅 ngl, there's currently no consensus on which of these protocols is technically superior 💀 so network administrators should make a choice between the two based on their needs and their particular situation 🎀

2. Requirement Level 💅

the "requirement level" [RFC2026] for LDP is:

Implementation of LDP is recommended for devices that perform MPLS forwarding along normally routed paths as determined by destination-based routing protocols. 🫶

3. Feature Overview 🦋

LDP associates a Forwarding Equivalence Class (FEC) [RFC3031] with each label it distributes 💖 two LSRs that use LDP to exchange FEC-label binding information are called "LDP Peers", and we say there's an "LDP Session" between them 👯‍♀️

LDP uses TCP for session communication 🧚 using TCP means session messages are reliably delivered, and the distributed labels and state information tied to LSPs don't need to be refreshed periodically 🔥 no constant re-sending, slay 💅

LDP includes a mechanism that lets an LSR discover potential LDP peers 🌸 because of that discovery mechanism, operators don't have to manually configure each LSR with its LDP peers 😭 bestie, that's a whole lot less work 🙈

when an LSR discovers another LSR, it follows the LDP session setup procedure to set up an LDP session 🦋 through this procedure, the LSRs establish a session TCP connection and use it to negotiate session parameters, like the label distribution method to be used (more on that below) ✨ once the LSRs agree on the parameters, the session is operational, and they use the TCP connection for label distribution 🌈

LDP supports two different methods for label distribution 💅 an LSR using Downstream Unsolicited distribution advertises FEC-label bindings to its peers when it's ready to forward packets in the FEC by means of MPLS 👑 an LSR using Downstream on Demand distribution gives FEC-label bindings to a peer in response to specific requests from that peer for a label for the FEC 🎀

LDP gives LSRs flexibility in their strategies for retaining learned labels 🧚 an LSR using liberal label retention stores all labels learned from peers, whether or not it currently needs them for forwarding, while one using conservative label retention only stores labels it has immediate use for and releases the unneeded labels back to the peer that advertised them 💜

LDP also gives flexibility in strategies for when to advertise FEC-label bindings 🦋 an LSR using independent control mode advertises FEC-label bindings to peers whenever it sees fit 🫶 one using ordered control only advertises bindings after it has received a label for the FEC from the FEC nexthop, or when it's an MPLS egress for the FEC 💅

Downstream on Demand distribution with conservative label retention and ordered control is a good fit when labels are a relatively scarce resource that has to be conserved 😭 Downstream Unsolicited distribution with liberal label retention and independent control is a good fit when labels are plentiful and don't need to be carefully conserved ✨ however, the protocol permits other combinations of distribution method, label retention mode and control mode, including hybrid variants of them 🌈

LDP defines a mechanism for loop detection, to protect against forwarding loops in LSPs that traverse non-TTL MPLS clouds 🔥 see [RFC3031] for a discussion of situations that may benefit from this mechanism 🙈 the loop detection mechanism is optional in the sense that it may be disabled by LSR configuration 🌸 but an LDP-compliant LSR must implement it 👑

LDP also includes an extension mechanism that supports the development of vendor-private and experimental features 💖 this mechanism defines procedures for introducing new types of messages and TLVs, methods an LSR can use to detect those messages and TLVs, and procedures an LSR must follow when it receives a message or TLV it doesn't implement 🧚 it's not possible to make every future enhancement backwards compatible, but these procedures make it easier to introduce new capabilities in MPLS networks that still include older implementations that don't recognize them 🦋

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more of RFC 3037 is coming in part 2 🎀 stay tuned, bestie 💅🫶

8. Full Copyright Statement ✨

okay bestie, this is the legal bit and it's giving very serious energy 💅 but it's still super important so let's go 🫶

Copyright (C) The Internet Society (2001). All Rights Reserved. 🌸

so you're totally allowed to copy this document and share it with your friends 💖 and you can make derivative works that comment on it or explain it or help people actually implement it 🧚 those can be prepared, copied, published and distributed, in whole or in part, with no restriction of any kind 🔥 as long as the copyright notice and this paragraph are included on every copy and derivative work 🎀

BUT (and this is a big but 🙈) the document itself may not be modified in any way. no removing the copyright notice, no removing the references to the Internet Society or other Internet organizations 😭 the only exceptions are when you need it for developing Internet standards, in which case you have to follow the copyright procedures in the Internet Standards process 📜, or when you need to translate it into languages other than English 🌈

the permissions above are perpetual, which means they will never be revoked by the Internet Society or its successors or assigns 💜 forever and ever, no cap ✨

and the whole document and everything in it is provided on an "AS IS" basis 🦋 THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED 😭 that includes (but isn't limited to) any warranty that using the information won't infringe any rights, and any implied warranties of merchantability or fitness for a particular purpose 💀 so basically they're not promising it'll be perfect for your project, periodt 🙈

Acknowledgement 🍓

funding for the RFC Editor function is currently provided by the Internet Society 💅 so that's who keeps the whole RFC machine running, iykyk 🫶