RFC 8395

Extensions to BGP-Signaled Pseudowires to Support Flow-Aware Transport Labels

📅 published June 2018, from ietf

Extensions to BGP-Signaled Pseudowires to Support Flow-Aware Transport Labels 💖

Okay bestie, this is RFC 8395 🎀, an Internet Standards Track document from the Internet Engineering Task Force (IETF), dated June 2018 ✨. It's written by K. Patel (Arrcus), S. Boutros (VMware), J. Liste (Cisco), B. Wen (Comcast), and J. Rabadan (Nokia) 🫶. It's identified as ISSN 2070-1721, and it updates RFC 4761 by defining new flags in the Control Flags field of the Layer2 Info Extended Community 🔥.

Abstract ✨

This doc defines the protocol extensions needed to sync up flow label states between Provider Edges (PEs) when you're using BGP-based signaling procedures 🦋. These extensions work just as well for point-to-point Layer 2 Virtual Private Networks (L2VPNs) 💜. It also updates RFC 4761, by defining new flags in the Control Flags field of the Layer2 Info Extended Community 🌈.

Status of This Memo 📜

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841 🌸.

Info about the current status of this document, any errata, and how to give feedback on it can be found at https://www.rfc-editor.org/info/rfc8395 🔗.

Copyright (c) 2018 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, because they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they are provided without warranty as described in the Simplified BSD License 💅.

Table of Contents 📑

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................3
   2. Modifications to the Layer2 Info Extended Community .............4
   3. Signaling the Presence of the Flow Label ........................5
   4. IANA Considerations .............................................6
   5. Security Considerations .........................................6
   6. References ......................................................7
      6.1. Normative References .......................................7
      6.2. Informative References .....................................7
   Acknowledgements ...................................................8
   Contributors .......................................................8
   Authors' Addresses .................................................9

1. Introduction 🌸

Okay so, [RFC6391] has the main trick 🧚. It adds an extra label (the Flow Label) into the MPLS label stack, which lets Label Switching Routers (LSRs) balance flows inside Pseudowires (PWs) at a finer granularity than the individual PWs. That balancing happens across the Equal Cost Multiple Paths (ECMPs) that exist within the Packet Switched Network (PSN) 🔥.

Furthermore, [RFC6391] defines the LDP protocol extensions required to synchronize the flow label states between the ingress and egress PEs when using the signaling procedures defined in the [RFC8077] 💜.

A PW [RFC3985] is transported over one single network path, even if ECMPs exist between the ingress and egress PW provider edge (PE) equipment 🛤️. This is required to preserve the characteristics of the emulated service ✨.

This document introduces an optional mode of operation 🎀 that lets a PW be transported over ECMPs. For example, this makes sense when using ECMPs is known to be beneficial to how the PW operates. This spec uses the principles defined in [RFC6391] and augments the BGP-signaling procedures of [RFC4761] and [RFC6624] 💅. Using a single path to preserve the packet delivery order is still the default mode of operation of a PW, and it's described in [RFC4385] and [RFC4928] 🦋.

High-bandwidth Ethernet-based services are a prime example of where the optional mode is worth it 🍓. They benefit from being able to load-balance flows in a PW over multiple PSN paths. In general, load-balancing is applicable when the PW attachment circuit bandwidth and the PSN core link bandwidth are of the same order of magnitude 📊.

To achieve the load-balancing goal, [RFC6391] introduces an additional Label Stack Entry (LSE), called the flow label, which sits at the bottom of the stack (right after the PW LSE) 🧱. LSRs commonly generate a hash of the label stack so they can tell flows apart and spread them over the available ECMPs 🌈. The presence of the flow label, which is closely tied to a flow determined by the ingress PE, will normally provide the greatest entropy 🤌.

Furthermore, following the procedures for inter-AS scenarios described in Section 3.4 of [RFC4761], the flow label should never be handled by the ASBRs 🙈. Only the terminating PEs on each AS are responsible for popping or pushing this label. This is equally applicable to Method B, as described in Section 3.4.2 of [RFC4761], where ASBRs swap the PW label as traffic goes from ASBR to PE and from ASBR to ASBR. Therefore, the flow label stays untouched across AS boundaries 👑.

1.1. Requirements Language 🔑

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here 🫶.

2. Modifications to the Layer2 Info Extended Community 🎀

The Layer2 Info Extended Community is how control information about the PWs to be set up gets signaled 📨. The Extended Community format is described in [RFC4761]. The format of this Extended Community looks like this:

         +------------------------------------+
         | Extended Community type (2 octets) |
         +------------------------------------+
         |  Encaps Type (1 octet)             |
         +------------------------------------+
         |  Control Flags (1 octet)           |
         +------------------------------------+
         |  Layer-2 MTU (2 octets)            |
         +------------------------------------+
         |  Reserved (2 octets)               |
         +------------------------------------+

Figure 1: Layer2 Info Extended Community

Control Flags:

This field holds bit flags about the control information for PWs 🧠. It's getting a definition for two new flags. ✨

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |Z|Z|Z|Z|T|R|C|S|      (Z = MUST Be Zero)
         +-+-+-+-+-+-+-+-+

Figure 2: Control Flags Bit Vector

Here's how the bits in the Control Flags Bit Vector are defined. The remaining bits, marked "Z", MUST be set to zero when sending, and MUST be ignored when receiving this Extended Community 🙅‍♀️.

  • T: When the bit value is 1, the PE announces that it can send a PW packet that includes a flow label 🦋. When the bit value is 0, the PE is saying that it will not send a PW packet containing a flow label.
  • R: When the bit value is 1, the PE is able to receive a PW packet with a flow label present 🫧. When the bit value is 0, the PE is unable to receive a PW packet with the flow label present.
  • C: Defined in [RFC4761].
  • S: Defined in [RFC4761].

3. Signaling the Presence of the Flow Label 🦋

As part of the PW signaling procedures described in [RFC4761], a Layer2 Info Extended Community gets advertised in the Virtual Private LAN Service (VPLS) BGP Network Layer Reachability Information (NLRI) 📣.

A PE that wants to send a flow label in a PW packet MUST include, in its VPLS BGP NLRI, a Layer2 Info Extended Community using the Control Flags field with T = 1 💖.

A PE that is willing to receive a flow label in a PW packet MUST include, in its VPLS BGP NLRI, a Layer2 Info Extended Community using the Control Flags field with R = 1 🌸.

A PE that receives a VPLS BGP NLRI containing a Layer2 Info Extended Community with R = 0 MUST NOT include a flow label in the PW packet 🙅‍♀️.

So, a PE that sends a Control Flags field with T = 1 and receives a Control Flags field with R = 1 MUST include a flow label in the PW packet ✅. With any other combination, a PE MUST NOT include a flow label in the PW packet 🚫.

A PE MAY support configuring the flow label (the T and R bits) on a per-service basis, for example on a VPLS VPN Forwarding Instance (VFI) 🎛️. It's also possible that, on a given service, PEs don't all share the same flow label settings 🤷‍♀️. So the presence of a flow label is decided on a per-peer basis, based on the local and remote T and R bit values 🧩. For example, a PE that's part of a VPLS, and has a local T = 1, must only transmit traffic with a flow label to those peers that signaled R = 1 📤. If that same PE has local R = 1, it must only expect to receive traffic with a flow label from peers with T = 1 📥. Any other traffic must not have a flow label. 😭 A PE expecting to receive traffic from a remote peer with a flow label MAY drop traffic that has no flow label 🗑️. A PE expecting to receive traffic from a remote peer with no flow label MAY drop traffic that has a flow label 🗑️.

Changing flow label settings may impact traffic over a PW 🌊. These changes could trigger changes in the PEs' data-plane programming, meaning the imposition and disposition of the flow label 🛠️. This is an implementation-specific behavior and is outside the scope of this document 🤐.

The signaling procedures in [RFC4761] say that the unspecified bits in the Control Flags field (bits 0-5) MUST be set to zero when sending, and MUST be ignored when receiving 🔒. So the signaling procedure described here is backwards compatible with existing implementations 🫶. A PE that doesn't support the extensions in this document will always advertise a value of zero in the R bit. That means a flow label will never be included in a packet sent to it by one of its peers 🙌. Similarly, it will always advertise a value of zero in the T bit. That means a peer will know a flow label will never be included in a packet sent by it 💅.

Note that what gets signaled is the desire to include the flow LSE in the label stack 💭. The value of the flow label is a local matter for the ingress PE, and the label value itself is not signaled 🤫✨.

4. IANA Considerations ✨

okay so bestie, [RFC4761] came along and defined a Control Flags Bit Vector inside the Layer2 Info Extended Community 🎀 but it never asked for a registry to be made for it 🥺. So this document fixes that! IANA has now created the "Layer2 Info Extended Community Control Flags Bit Vector" registry 🦋 over at <https://www.iana.org/assignments/bgp-extended-communities>.

Based on [RFC4761] and this document, the initial contents of this registry look like this 💅:

Value   Name                               Reference
-----   --------------------------------   --------------
T       Request to send a flow label       This document
R       Ability to receive a flow label    This document
C       Presence of a Control Word         RFC 4761
S       Sequenced delivery of frames       RFC 4761

As per [RFC4761] and this document, the remaining bits are unassigned 🌸, and they MUST be set to zero when sending and MUST be ignored when receiving the Layer2 Info Extended Community. Main character rules, no exceptions 👑.

5. Security Considerations 🔐

Okay, this part is serious bestie 😭. This extension to BGP does not change the underlying security issues inherent in [RFC4271] and [RFC4761]. So if your network already has security worries from those two, this doesn't make them better or worse 🫶.

6. References 📚

6.1. Normative References 💜

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, <https://www.rfc-editor.org/info/rfc4271>.

[RFC4761] Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007, <https://www.rfc-editor.org/info/rfc4761>.

[RFC6391] Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V., Regan, J., and S. Amante, "Flow-Aware Transport of Pseudowires over an MPLS Packet Switched Network", RFC 6391, DOI 10.17487/RFC6391, November 2011, <https://www.rfc-editor.org/info/rfc6391>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

6.2. Informative References 🌈

[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985, DOI 10.17487/RFC3985, March 2005, <https://www.rfc-editor.org/info/rfc3985>.

[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385, February 2006, <https://www.rfc-editor.org/info/rfc4385>.

[RFC8077] Martini, L., Ed., and G. Heron, Ed., "Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)", STD 84, RFC 8077, DOI 10.17487/RFC8077, February 2017, <https://www.rfc-editor.org/info/rfc8077>.

[RFC4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal Cost Multipath Treatment in MPLS Networks", BCP 128, RFC 4928, DOI 10.17487/RFC4928, June 2007, <https://www.rfc-editor.org/info/rfc4928>.

[RFC6624] Kompella, K., Kothari, B., and R. Cherukuri, "Layer 2 Virtual Private Networks Using BGP for Auto-Discovery and Signaling", RFC 6624, DOI 10.17487/RFC6624, May 2012, <https://www.rfc-editor.org/info/rfc6624>.

Acknowledgements 🙏

The authors would like to thank Bertrand Duvivier and John Drake for their review and comments 💖.

Contributors 🧚

In addition to the authors listed above, the following individuals also contributed to this document:

  • Eric Lent
  • John Brzozowski
  • Steven Cotter

Authors' Addresses 💌

Keyur Patel Arrcus

Email: keyur@arrcus.com

Sami Boutros VMware

Email: boutros.sami@gmail.com

Jose Liste Cisco

Email: jliste@cisco.com

Bin Wen Comcast

Email: bin_wen@cable.comcast.com

Jorge Rabadan Nokia

Email: jorge.rabadan@nokia.com

📦 where this page comes from