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diff --git a/doc/rfc/rfc7431.txt b/doc/rfc/rfc7431.txt new file mode 100644 index 0000000..a7530dd --- /dev/null +++ b/doc/rfc/rfc7431.txt @@ -0,0 +1,787 @@ + + + + + + +Internet Engineering Task Force (IETF) A. Karan +Request for Comments: 7431 C. Filsfils +Category: Informational IJ. Wijnands, Ed. +ISSN: 2070-1721 Cisco Systems, Inc. + B. Decraene + Orange + August 2015 + + + Multicast-Only Fast Reroute + +Abstract + + As IPTV deployments grow in number and size, service providers are + looking for solutions that minimize the service disruption due to + faults in the IP network carrying the packets for these services. + This document describes a mechanism for minimizing packet loss in a + network when node or link failures occur. Multicast-only Fast + Reroute (MoFRR) works by making simple enhancements to multicast + routing protocols such as Protocol Independent Multicast (PIM) and + Multipoint LDP (mLDP). + +Status of This Memo + + This document is not an Internet Standards Track specification; it is + published for informational purposes. + + 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). Not all documents + approved by the IESG are a candidate for any level of Internet + Standard; see Section 2 of RFC 5741. + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + http://www.rfc-editor.org/info/rfc7431. + + + + + + + + + + + + + + +Karan et al. Informational [Page 1] + +RFC 7431 MoFRR August 2015 + + +Copyright Notice + + Copyright (c) 2015 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) in effect on the date of + publication of this document. Please review these documents + carefully, as 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 are provided without warranty as + described in the Simplified BSD License. + +Table of Contents + + 1. Introduction ....................................................3 + 1.1. Conventions Used in This Document ..........................3 + 1.2. Terminology ................................................3 + 2. Basic Overview ..................................................4 + 3. Determination of the Secondary UMH ..............................5 + 3.1. ECMP-Mode MoFRR ............................................5 + 3.2. Non-ECMP-Mode MoFRR ........................................5 + 4. Upstream Multicast Hop Selection ................................6 + 4.1. PIM ........................................................6 + 4.2. mLDP .......................................................6 + 5. Detecting Failures ..............................................6 + 6. MoFRR Applicability to Dual-Plane Topology ......................7 + 7. Other Topologies ...............................................10 + 8. Capacity Planning for MoFRR ....................................11 + 9. PE Nodes .......................................................11 + 10. Other Applications ............................................11 + 11. Security Considerations .......................................12 + 12. References ....................................................12 + 12.1. Normative References .....................................12 + 12.2. Informative References ...................................12 + Acknowledgments ...................................................13 + Contributors ......................................................13 + Authors' Addresses ................................................14 + + + + + + + + + + + +Karan et al. Informational [Page 2] + +RFC 7431 MoFRR August 2015 + + +1. Introduction + + Different solutions have been developed and deployed to improve + service guarantees, both for multicast video traffic and Video on + Demand traffic. Most of these solutions are geared towards finding + an alternate path around one or more failed network elements (link, + node, or path failures). + + This document describes a mechanism for minimizing packet loss in a + network when node or link failures occur. Multicast-only Fast + Reroute (MoFRR) works by making simple changes to the way selected + routers use multicast protocols such as PIM and mLDP. No changes to + the protocols themselves are required. With MoFRR, in many cases, + multicast routing protocols don't necessarily have to depend on or + have to wait on unicast routing protocols to detect network failures; + see Section 5. + + On a Merge Point, MoFRR logic determines a primary Upstream Multicast + Hop (UMH) and a secondary UMH and joins the tree via both + simultaneously. Data packets are received over the primary and + secondary paths. Only the packets from the primary UMH are accepted + and forwarded down the tree; the packets from the secondary UMH are + discarded. The UMH determination is different for PIM and mLDP and + explained in Section 4. When a failure is detected on the path to + the primary UMH, the repair occurs by changing the secondary UMH into + the primary and the primary into the secondary. Since the repair is + local, it is fast -- greatly improving convergence times in the event + of node or link failures on the path to the primary UMH. + +1.1. 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 RFC 2119 [RFC2119]. + +1.2. Terminology + + MoFRR: Multicast-only Fast Reroute. + + ECMP: Equal-Cost Multipath. + + mLDP: Multipoint Label Distribution Protocol. + + PIM: Protocol Independent Multicast. + + UMH: Upstream Multicast Hop. A candidate next-hop that can be used + to reach the root of the tree. + + + + +Karan et al. Informational [Page 3] + +RFC 7431 MoFRR August 2015 + + + tree: Either a PIM (S,G)/(*,G) tree or an mLDP Point-to-Multipoint + (P2MP) or Multipoint-to-Multipoint (MP2MP) LSP. + + OIF: Outgoing interface. An interface used to forward multicast + packets down the tree towards the receivers. Either a PIM + (S,G)/(*,G) tree or an mLDP P2MP or MP2MP LSP. + + LFA: Loop-Free Alternate as defined in [RFC5286]. In unicast Fast + Reroute, this is an alternate next-hop that can be used to reach a + unicast destination without using the protected link or node. + + Merge Point: A router that joins a multicast stream via two divergent + upstream paths. + + RPF: Reverse Path Forwarding. + + RP: Rendezvous Point. + + LSP: Label Switched Path. + + LSR: Label Switching Router. + + BFD: Bidirectional Forwarding Detection. + + IGP: Interior Gateway Protocol. + + MVPN: Multicast Virtual Private Network. + + POP: Point Of Presence, an access point into the network. + +2. Basic Overview + + The basic idea of MoFRR is for a Merge Point router to join a + multicast tree via two divergent upstream paths in order to get + maximum redundancy. The determination of this alternate upstream is + defined in Section 3. + + In order to maximize robustness against any failure, the two paths + should be as diverse as possible. Ideally, they should not merge + upstream. Sometimes the topology guarantees maximal redundancy; + other times additional configuration or techniques are needed to + enforce it. See Section 6 for more discussion on the applicability + of MoFRR depending on the network topology. + + A Merge Point router should only accept and forward on one of the + upstream paths at a time in order to avoid duplicate packet + + + + + +Karan et al. Informational [Page 4] + +RFC 7431 MoFRR August 2015 + + + forwarding. The selection of the primary and secondary UMH is done + by the MoFRR logic and normally based on unicast routing to find + loop-free candidates. This is described in Section 4. + + Note, the impact of an additional amount of data on the network is + mitigated when tree membership is densely populated. When a part of + the network has redundant data flowing, join latency for new joining + members is reduced because it's likely a tree Merge Point is not far + away. + +3. Determination of the Secondary UMH + + The secondary UMH is a Loop-Free Alternate (LFA) as per [RFC5286]. + +3.1. ECMP-Mode MoFRR + + If the IGP installs two ECMP paths to the source, then as per + [RFC5286] the LFA is a primary next-hop. If the multicast tree is + enabled for ECMP-mode MoFRR, the router installs the paths as primary + and secondary UMHs. Before the failure, only packets received from + the primary UMH path are processed, while packets received from the + secondary UMH are dropped. + + The selected primary UMH SHOULD be the same as if the MoFRR extension + were not enabled. + + If more than two ECMP paths exist, one is selected as primary and + another as secondary UMH. The selection of the primary and secondary + is a local decision. Information from the IGP link-state topology + could be leveraged to optimize this selection such that the primary + and secondary paths are maximal divergent and don't lead to the same + upstream node. Note that MoFRR does not restrict the number of UMH + paths that are joined. Implementations may use as many paths as are + configured. + +3.2. Non-ECMP-Mode MoFRR + + A router X configured for non-ECMP-mode MoFRR for a multicast tree + joins a primary path to its primary UMH and a secondary path to its + LFA UMH. In order to prevent control-plane loops, a router MUST stop + joining the secondary UMH if this UMH is the only member in the OIF + list. + + To illustrate the reason for this rule, let's consider the example in + Figure 3. If two Provider Edge routers, PE1 and PE2, have received + an IGMP request for a multicast tree, they will both join the primary + path on their plane and a secondary path to the neighbor PE. If + their receivers leave at the same time, it's possible for the + + + +Karan et al. Informational [Page 5] + +RFC 7431 MoFRR August 2015 + + + multicast tree on PE1 and PE2 to never get deleted, as the PEs + refresh each other via the secondary path joins (remember that a + secondary path join is not distinguishable from a primary join). + +4. Upstream Multicast Hop Selection + + An Upstream Multicast Hop (UMH) is a candidate next-hop that can be + used to reach the root of the tree. This is normally based on + unicast routing to find loop-free candidate(s). With MoFRR + procedures, we select a primary and a backup UMH. The procedures for + determining the UMH are different for PIM and mLDP. + +4.1. PIM + + The UMH selection in PIM is also known as the Reverse Path Forwarding + (RPF) procedure. Based on a unicast route lookup on either the + source address or Rendezvous Point (RP) [RFC4601], an upstream + interface is selected for sending the PIM Joins/Prunes AND accepting + the multicast packets. The interface the packets are received on is + used to pass or fail the RPF check. If packets are received on an + interface that was not selected as the primary by the RPF procedure, + the packets are discarded. + +4.2. mLDP + + The UMH selection in mLDP also depends on unicast routing, but the + difference from PIM is that the acceptance of multicast packets is + based on MPLS labels and is independent of the interface on which the + packet is received. Using the procedures as defined in [RFC6388], an + upstream Label Switching Router (LSR) is elected. The upstream LSR + that was elected for a Label Switched Path (LSP) gets a unique local + MPLS label allocated. Multicast packets are only forwarded if the + MPLS label matches the MPLS label that was allocated for that LSP's + (primary) upstream LSR. + +5. Detecting Failures + + Once the two paths are established, the next step is detecting a + failure on the primary path to know when to switch to the backup + path. This is a local issue, but this section explores some + possibilities. + + The first (and simplest) option is to detect the failure of the local + interface as it's done for unicast Fast Reroute. Detection can be + performed using the loss of signal or the loss of probing packets + (e.g., BFD). This option can be used in combination with the other + options as documented below. Just like for unicast fast reroute, + 50 msec switchover is possible. + + + +Karan et al. Informational [Page 6] + +RFC 7431 MoFRR August 2015 + + + A second option consists of comparing the packets received on the + primary and secondary streams but only forwarding one of them -- the + first one received, no matter which interface it is received on. + Zero packet loss is possible for RTP-based streams. + + A third option assumes a minimum known packet rate for a given data + stream. If a packet is not received on the primary RPF within this + time frame, the router assumes primary path failure and switches to + the secondary RPF interface. 50 msec switchover may be possible for + high-rate streams (e.g., IPTV where SD video has a continuous inter- + packet gap of about 3 msec), but in general the delay is dependent on + the rate of the multicast stream. + + A fourth option leverages the significant improvements of the IGP + convergence speed. When the primary path to the source is withdrawn + by the IGP, the MoFRR-enabled router switches over to the backup + path, and the UMH is changed to the secondary UMH. Since the + secondary path is already in place, and assuming it is disjoint from + the primary path, convergence times would not include the time + required to build a new tree and hence are smaller. Sub-second to + sub-200 msec switchover should be possible. + +6. MoFRR Applicability to Dual-Plane Topology + + MoFRR applicability is topology dependent. The applicability is the + same as LFA FRR, which is discussed in [RFC6571]. + + The following section will discuss MoFRR applicability to dual-plane + network topologies. + + MoFRR works best in dual-planes topologies as illustrated in the + figures below. MoFRR may be enabled on any router in the network. + In the figures below, MoFRR is shown enabled on the Provider Edge + (PE) routers to illustrate one way in which the technology may be + deployed. + + + + + + + + + + + + + + + + +Karan et al. Informational [Page 7] + +RFC 7431 MoFRR August 2015 + + + S + P / \ P + / \ + ^ G1 R1 ^ + P / \ P + / \ + G2----------R2 ^ + | \ | \ P + ^ | \ | \ + P | G3----------R3 + | | | | + | | | | ^ + G4---|------R4 | P + ^ \ | \ | + P \ | \ | + G5----------R5 + ^ | | ^ + P | | P + | | + Gi Ri + \ \__ ^ /| + \ \ S1/ | ^ + ^ \ ^\ / |P2 + P1 \ S2\_/__ | + \ / \| + PE1 PE2 + + P = Primary path + S = Secondary path + + Figure 1: Two-Plane Network Design + + The topology has two planes, a primary plane and a secondary plane + that are fully disjoint from each other all the way into the POPs. + This two-plane design is common in service provider networks as it + eliminates single point of failures in their core network. The links + marked P indicate the normal (primary) path of how the PIM Joins flow + from the POPs towards the source of the network. Multicast streams, + especially for the densely watched channels, typically flow along + both the planes in the network anyway. + + The only change MoFRR adds to this is on the links marked S where the + PE routers join a secondary path to their secondary ECMP UMH. As a + result of this, each PE router receives two copies of the same + stream, one from the primary plane and the other from the secondary + plane. As a result of normal UMH behavior, the multicast stream + + + + + +Karan et al. Informational [Page 8] + +RFC 7431 MoFRR August 2015 + + + received over the primary path is accepted and forwarded to the + downstream receivers. The copy of the stream received from the + secondary UMH is discarded. + + When a router detects a routing failure on the path to its primary + UMH, it will switch to the secondary UMH and accept packets for that + stream. If the failure is repaired, the router may switch back. The + primary and secondary UMHs have only local context and not end-to-end + context. + + As one can see, MoFRR achieves the faster convergence by pre-building + the secondary multicast tree and receiving the traffic on that + secondary path. The example discussed above is a simple case where + there are two ECMP paths from each PE device towards the source, one + along the primary plane and one along the secondary. In cases where + the topology is asymmetric or is a ring, this ECMP nature does not + hold, and additional rules have to be taken into account to choose + when and where to join the secondary path. + + MoFRR is appealing in such topologies for the following reasons: + + 1. Ease of deployment and simplicity: the functionality is only + required on the PE devices, although it may be configured on all + routers in the topology. Furthermore, each PE device can be + enabled separately; there is no need for network-wide + coordination in order to deploy MoFRR. Interoperability testing + is not required as there are no PIM or mLDP protocol changes. + + 2. End-to-end failure detection and recovery: any failure along the + path from the source to the PE can be detected and repaired with + the secondary disjoint stream. (See the second, third, and + fourth options in Section 5.) + + 3. Capacity efficiency: as illustrated in the previous example, the + multicast trees corresponding to IPTV channels cover the backbone + and distribution topology in a very dense manner. As a + consequence, the secondary path grafts onto the normal multicast + trees (i.e., trees signaled by PIM or mLDP without the MoFRR + extension) at the aggregation level and hence does not demand any + extra capacity either on the distribution links or in the + backbone. The secondary path simply uses the capacity that is + normally used, without any duplication. This is different from + conventional FRR mechanisms that often duplicate the capacity + requirements when the backup path crosses links/nodes that + already carry the primary/normal tree, and thus twice as much + capacity is required. + + + + + +Karan et al. Informational [Page 9] + +RFC 7431 MoFRR August 2015 + + + 4. Loop-free: the secondary path join is sent on an ECMP disjoint + path. By definition, the neighbor receiving this request is + closer to the source and hence will not cause a loop. + + The topology we just analyzed is very frequent and can be modeled as + per Figure 2. The PE has two ECMP disjoint paths to the source. + Each ECMP path uses a disjoint plane of the network. + + Source + / \ + Plane1 Plane2 + | | + A1 A2 + \ / + PE + + Figure 2: PE is Dual-Homed to Dual-Plane Backbone + + Another frequent topology is described in Figure 3. PEs are grouped + by pairs. In each pair, each PE is connected to a different plane. + Each PE has one single shortest-path to a source (via its connected + plane). There is no ECMP like in Figure 2. However, there is + clearly a way to provide MoFRR benefits as each PE can offer a + disjoint secondary path to the PE in the other plane (via the + disjoint path). + + The MoFRR secondary neighbor selection process needs to be extended + in this case as one cannot simply rely on using an ECMP path as + secondary neighbor. This extension is referred to as non-ECMP-mode + MoFRR and is described in Section 3.2. + + Source + / \ + Plane1 Plane2 + | | + A1 A2 + | | + PE1----PE2 + + Figure 3: PEs Are Connected in Pairs to Dual-Plane Backbone + +7. Other Topologies + + As mentioned in Section 6, MoFRR works best in dual-plane topologies. + If MoFRR is applied to non-dual-plane networks, it's possible that + the secondary path is affected by the same failure that affected the + + + + + +Karan et al. Informational [Page 10] + +RFC 7431 MoFRR August 2015 + + + primary path. In that case, there is no guarantee that the backup + path will provide an uninterrupted traffic flow of packets without + loss or duplication. + +8. Capacity Planning for MoFRR + + The previous section has described two very frequent designs (Figures + 2 and 3) which provide maximum MoFRR benefits. + + Designers with topologies different than Figures 2 and 3 can still + benefit from MoFRR, thanks to the use of capacity planning tools. + + Such tools are able to simulate the ability of each PE to build two + disjoint branches of the same tree. This simulation could be for + hundreds of PEs and hundreds of sources. + + This allows an assessment of the MoFRR protection coverage of a given + network, for a set of sources. + + If the protection coverage is deemed insufficient, the designer can + use such a tool to optimize the topology (add links, change IGP + metrics). + +9. PE Nodes + + Many Service Providers devise their topology such that PEs have + disjoint paths to the multicast sources. MoFRR leverages the + existence of these disjoint paths without any PIM or mLDP protocol + modification. Interoperability testing is thus not required. In + such topologies, MoFRR only needs to be deployed on the PE devices. + Each PE device can be enabled one by one. + +10. Other Applications + + While all the examples in this document show the MoFRR applicability + on PE devices, it is clear that MoFRR could be enabled on aggregation + or core routers. + + MoFRR can be popular in data center network configurations. With the + advent of lower-cost Ethernet and increasing port density in routers, + there is more meshed connectivity than ever before. When using a + three-level access, distribution, and core layers in a data center, + there is a lot of inexpensive bandwidth connecting the layers. This + will lend itself to more opportunities for ECMP paths at multiple + layers. This allows for multiple layers of redundancy protecting + link and node failure at each layer with minimal redundancy cost. + + + + + +Karan et al. Informational [Page 11] + +RFC 7431 MoFRR August 2015 + + + Redundancy costs are reduced because only one packet is forwarded at + every link along the primary and secondary data paths so there is no + duplication of data on any link thereby providing make-before-break + protection at a very small cost. + + A MoFRR router only accepts packets from the primary path and + discards packets from the secondary path. For that reason, + management applications (like ping and mtrace) will not work when + verifying the secondary path. + + The MoFRR principle may be applied to MVPNs. + +11. Security Considerations + + There are no security considerations for this design other than what + is already in the main PIM specification [RFC4601] and mLDP + specification [RFC6388]. + +12. References + +12.1. 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, + <http://www.rfc-editor.org/info/rfc2119>. + + [RFC5286] Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification + for IP Fast Reroute: Loop-Free Alternates", RFC 5286, + DOI 10.17487/RFC5286, September 2008, + <http://www.rfc-editor.org/info/rfc5286>. + +12.2. Informative References + + [RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, + "Protocol Independent Multicast - Sparse Mode (PIM-SM): + Protocol Specification (Revised)", RFC 4601, + DOI 10.17487/RFC4601, August 2006, + <http://www.rfc-editor.org/info/rfc4601>. + + [RFC6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B. + Thomas, "Label Distribution Protocol Extensions for Point- + to-Multipoint and Multipoint-to-Multipoint Label Switched + Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011, + <http://www.rfc-editor.org/info/rfc6388>. + + + + + + +Karan et al. Informational [Page 12] + +RFC 7431 MoFRR August 2015 + + + [RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene, + B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free + Alternate (LFA) Applicability in Service Provider (SP) + Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012, + <http://www.rfc-editor.org/info/rfc6571>. + +Acknowledgments + + Thanks to Dave Oran and Alvaro Retana for their review and comments + on this document. + + The authors would like to especially acknowledge Dino Farinacci, John + Zwiebel, and Greg Shepherd for the genesis of the MoFRR concept. + +Contributors + + Below is a list of the contributors in alphabetical order: + + Dino Farinacci + Email: farinacci@gmail.com + + Wim Henderickx + Alcatel-Lucent + Copernicuslaan 50 + Antwerp 2018 + Belgium + Email: wim.henderickx@alcatel-lucent.com + + Uwe Joorde + Deutsche Telekom + Dahlweg 100 + D-48153 Muenster + Germany + Email: Uwe.Joorde@telekom.de + + Nicolai Leymann + Deutsche Telekom + Winterfeldtstrasse 21 + Berlin 10781 + Germany + Email: N.Leymann@telekom.de + + Jeff Tantsura + Ericsson + 300 Holger Way + San Jose, CA 95134 + United States + Email: jeff.tantsura@ericsson.com + + + +Karan et al. Informational [Page 13] + +RFC 7431 MoFRR August 2015 + + +Authors' Addresses + + Apoorva Karan + Cisco Systems, Inc. + 3750 Cisco Way + San Jose, CA 95134 + United States + + Email: apoorva@cisco.com + + + Clarence Filsfils + Cisco Systems, Inc. + De kleetlaan 6a + Diegem BRABANT 1831 + Belgium + + Email: cfilsfil@cisco.com + + + IJsbrand Wijnands (editor) + Cisco Systems, Inc. + De Kleetlaan 6a + Diegem 1831 + Belgium + + Email: ice@cisco.com + + + Bruno Decraene + Orange + 38-40 rue du General Leclerc + Issy Moulineaux Cedex 9, 92794 + France + + Email: bruno.decraene@orange.com + + + + + + + + + + + + + + + +Karan et al. Informational [Page 14] + |