RFC 7881

Seamless Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6, and MPLS

📅 published July 2016, from ietf

Seamless Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6, and MPLS 🎀

Okay bestie, this is RFC 7881 and it's a Standards Track doc 💅 It came out in July 2016 with ISSN 2070-1721. The authors are C. Pignataro and D. Ward from Cisco, plus N. Akiya from Big Switch Networks ✨ The header doesn't list any RFCs it obsoletes or updates, so it's standing completely on its own 🦋

Abstract

Super short and sweet: this doc defines the procedures for using Seamless Bidirectional Forwarding Detection (S-BFD) in IPv4, IPv6, and MPLS environments 💖

Status of This Memo

This is an Internet Standards Track document 👑

It's a product of the Internet Engineering Task Force (IETF), so it represents the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more info on Internet Standards, check Section 2 of RFC 7841 🧚

For the current status, any errata, and how to send feedback, go to http://www.rfc-editor.org/info/rfc7881 🔗

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved 🌈

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as in effect on the date of publication. Please read these carefully, because they spell out your rights and restrictions for this document 🙈 Code Components pulled out of this document must include the Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions, and they come without warranty, as described in the Simplified BSD License.

Table of Contents ✨

1. Introduction ....................................................2
2. S-BFD UDP Port ..................................................2
3. S-BFD Echo UDP Port .............................................3
4. S-BFD Control Packet Demultiplexing .............................3
5. Initiator Procedures ............................................3
   5.1. Details of S-BFD Control Packets Sent by SBFDInitiator .....4
        5.1.1. Target versus Remote Entity (S-BFD Discriminator) ...4
6. Responder Procedures ............................................5
   6.1. Details of S-BFD Control Packets Sent by SBFDReflector .....5
7. Security Considerations .........................................6
8. IANA Considerations .............................................6
9. References ......................................................7
   9.1. Normative References .......................................7
   9.2. Informative References .....................................7
Acknowledgements ...................................................8
Contributors .......................................................8
Authors' Addresses .................................................8

1. Introduction 🌸

Okay so S-BFD [RFC7880] is a generalized mechanism that lets network nodes do seamless continuity checks to remote entities 💫 This document is where the necessary procedures for using S-BFD in IPv4, IPv6, and MPLS environments get spelled out.

You're expected to already know your IP [RFC791] [RFC2460], BFD [RFC5880], MPLS BFD [RFC5884], and S-BFD [RFC7880] terms and protocol constructs, no cap 🧚

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 RFC 2119 [RFC2119] 🎀

2. S-BFD UDP Port 🦋

A brand new UDP port is defined for S-BFD in IPv4, IPv6, and MPLS environments: 7784 ✨

Here's the vibe for S-BFD Control packets going from the SBFDInitiator to the SBFDReflector. The SBFDReflector session MUST listen for incoming S-BFD Control packets on port 7784. SBFDInitiator sessions MUST transmit S-BFD Control packets with destination port 7784. The source port on those packets can be any port, with one exception: it MUST NOT be 7784. The same UDP source port number MUST be used for all S-BFD Control packets associated with a particular SBFDInitiator session. The source port number is unique among all SBFDInitiator sessions on the system 🔥

Now for S-BFD Control packets going the other way, from the SBFDReflector to the SBFDInitiator: the SBFDInitiator session MUST listen for reflected S-BFD Control packets at its source port 💜

3. S-BFD Echo UDP Port 🌈

The BFD Echo port defined by [RFC5881], port 3785, is used for the S-BFD Echo function in IPv4, IPv6, and MPLS environments. SBFDInitiator sessions MUST transmit S-BFD Echo packets with destination port 3785. The setting of the UDP source port [RFC5881] and the procedures [RFC7880] for the S-BFD Echo function are outside the scope of this document, so that's a different story 💀

4. S-BFD Control Packet Demultiplexing 💖

S-BFD Control packet demultiplexing follows the procedure specified in Section 7.1 of [RFC7880]. A received S-BFD Control packet MUST be demultiplexed with the destination UDP port field 🧠

This procedure for an S-BFD packet runs on both the initiator and the reflector, so it's a whole vibe. If the port is 7784 (i.e., an S-BFD packet for the SBFDReflector), then the packet MUST be looked up to locate a corresponding SBFDReflector session, based on the value from the Your Discriminator field in the table describing S-BFD Discriminators. If the port is not 7784 but the packet is demultiplexed to be for an SBFDInitiator, then the packet MUST be looked up to locate a corresponding SBFDInitiator session, based on the value from the Your Discriminator field in the table describing BFD Discriminators. In that case, the destination IP address of the packet SHOULD be validated to be for itself. If the packet demultiplexes to a classical BFD session, then the procedures from [RFC5880] apply 🫶

5. Initiator Procedures 👑

S-BFD Control packets are transmitted with an IP header, a UDP header, and a BFD Control packet ([RFC5880]). When S-BFD Control packets are explicitly label switched (i.e., not IP routed and forwarded over a Label Switched Path (LSP), but explicitly sent on a specific LSP), the former is prepended with a label stack. Note that this document does not make a distinction between a single-hop S-BFD scenario and a multi-hop S-BFD scenario; both scenarios are supported ✨

The necessary values in the BFD control headers are described in [RFC7880]. Section 5.1 describes the necessary values in the MPLS header, IP header, and UDP header when an SBFDInitiator on the initiator is sending S-BFD Control packets 🌸

5.1. Details of S-BFD Control Packets Sent by SBFDInitiator 💅

  • Specifications common to both IP-routed S-BFD Control packets and explicitly label-switched S-BFD Control packets:
    • The Source IP Address field of the IP header MUST be set to a local IP address that is expected to be routable by the target (i.e., not an IPv6 link-local address when the target is multiple hops away).
    • The UDP destination port MUST be set to a well-known UDP destination port assigned for S-BFD, i.e., 7784.
    • The UDP source port MUST NOT be set to 7784.
  • Specifications for IP-routed S-BFD Control packets:
    • The Destination IP Address field of the IP header MUST be set to an IP address of the target.
    • The TTL / Hop Limit field of the IP header SHOULD be set to 255.
  • Specifications for explicitly label-switched S-BFD Control packets:
    • S-BFD Control packets MUST have the label stack that is expected to reach the target.
    • The TTL field of the topmost label SHOULD be 255.
    • The destination IP address MUST be chosen from the 127/8 range for IPv4 and from the 0:0:0:0:0:ffff:7f00:0/104 range for IPv6, as per [RFC5884].
    • The TTL / Hop Limit field of the IP header MUST be set to 1.

5.1.1. Target versus Remote Entity (S-BFD Discriminator) 🦋

Typically, an S-BFD Control packet will have the Your Discriminator field corresponding to an S-BFD Discriminator of the remote entity located on the target network node defined by the destination IP address or the label stack. It is, however, possible for an SBFDInitiator to carefully set the Your Discriminator and TTL fields to perform a continuity test in the direction towards a target, but destined to a transit network node and not to the target itself 👀

Section 5.1 intentionally uses the word "target" instead of "remote entity" to accommodate this possible S-BFD usage through TTL expiry. This also requires that S-BFD Control packets not be dropped by the responder node due to TTL expiry. Thus, implementations on the responder MUST allow received S-BFD Control packets taking a TTL expiry exception path to reach the corresponding SBFDReflector session. This is an existing packet-processing exception practice for Operations, Administration, and Maintenance (OAM) packets, where the control plane further identifies the type of OAM by the protocol and port numbers 🔥

8. IANA Considerations ✨

okay bestie, so IANA got to give out a brand new port number 💖 The number is 7784, and it got added to the "Service Name and Transport Protocol Port Number Registry" 🎀 Here's the entry, and every single field is marked as a must-have except the port number itself, which is optional 🫶

Service Name (REQUIRED)
  s-bfd

Transport Protocol(s) (REQUIRED)
  udp

Assignee (REQUIRED)
  IESG <iesg@ietf.org>

Contact (REQUIRED)
  IETF Chair <chair@ietf.org>

Description (REQUIRED)
  Seamless Bidirectional Forwarding Detection (S-BFD)

Reference (REQUIRED)
  RFC 7881

Port Number (OPTIONAL)
  7784

9. References 📚

9.1. Normative References 💅

These are the ones you absolutely NEED to read, no skipping 😭

9.2. Informative References 🧚

These are the extra-credit reads, lowkey but still iconic 🌈

Acknowledgements 🫶

The authors wanted to thank the BFD WG members for helping shape this doc, and a bunch of people made really big contributions: Marc Binderberger, Jeffrey Haas, Santosh Pallagatti, Greg Mirsky, Sam Aldrin, Vengada Prasad Govindan, Mallik Mudigonda, and Srihari Raghavan. Iconic squad 👑🌸

Contributors 💖

The following are the key contributors to this document:

  • Tarek Saad, Cisco Systems, Inc. ✨
  • Siva Sivabalan, Cisco Systems, Inc. 🦋
  • Nagendra Kumar, Cisco Systems, Inc. 🌈

Authors' Addresses 💅

Carlos Pignataro Cisco Systems, Inc.

Email: cpignata@cisco.com

Dave Ward Cisco Systems, Inc.

Email: wardd@cisco.com

Nobo Akiya Big Switch Networks

Email: nobo.akiya.dev@gmail.com

📦 where this page comes from