Network Working Group T. Morin, Ed. Internet-Draft France Telecom R&D Intended status: Informational B. Niven-Jenkins, Ed. Expires: August 28, 2008 BT Y. Kamite NTT Communications R. Zhang BT N. Leymann Deutsche Telekom February 25, 2008 Considerations about Multicast for BGP/MPLS VPN Standardization draft-morin-l3vpn-mvpn-considerations-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 28, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract The current proposal for multicast in BGP/MPLS includes multiple Morin, et al. Expires August 28, 2008 [Page 1] Internet-Draft Multicast VPN Considerations February 2008 alternative mechanisms for some of the required building blocks of the solution. The aim of this document is to leverage previously documented requirements to identify the key elements and help move forward solution design, toward the definition of a standard having a well defined set of mandatory procedures. The different proposed alternative mechanisms are examined in the light of requirements identified for multicast in L3VPNs, and suggestions are made about which of these mechanisms standardization should favor. Issues related to existing deployments of early implementations are also addressed. Requirements Language 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]. Morin, et al. Expires August 28, 2008 [Page 2] Internet-Draft Multicast VPN Considerations February 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Examining alternatives mechanisms for MVPN functions . . . . . 4 3.1. MVPN auto-discovery . . . . . . . . . . . . . . . . . . . 4 3.2. S-PMSI Signaling . . . . . . . . . . . . . . . . . . . . . 6 3.3. PE-PE Transmission of C-Multicast Routing . . . . . . . . 8 3.3.1. PE-PE signalling scalability . . . . . . . . . . . . . 8 3.3.2. P-routers scalability . . . . . . . . . . . . . . . . 10 3.3.3. Impact of C-multicast routing on Inter-AS deployments . . . . . . . . . . . . . . . . . . . . . 10 3.3.4. Security and robustness . . . . . . . . . . . . . . . 11 3.3.5. C-multicast VPN join latency . . . . . . . . . . . . . 12 3.3.6. Architectural considerations . . . . . . . . . . . . . 13 3.3.7. Conclusion on C-multicast routing . . . . . . . . . . 14 3.4. Encapsulation techniques for P-multicast trees . . . . . . 15 3.5. Inter-AS deployments options . . . . . . . . . . . . . . . 16 4. Co-located RPs . . . . . . . . . . . . . . . . . . . . . . . . 18 5. Existing deployments . . . . . . . . . . . . . . . . . . . . . 19 6. Summary of recommendations . . . . . . . . . . . . . . . . . . 20 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 8. Security Considerations . . . . . . . . . . . . . . . . . . . 21 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21 10. Informative References . . . . . . . . . . . . . . . . . . . . 21 Appendix A. Handling the PIM routing processing load load . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Intellectual Property and Copyright Statements . . . . . . . . . . 25 Morin, et al. Expires August 28, 2008 [Page 3] Internet-Draft Multicast VPN Considerations February 2008 1. Introduction The current proposal for multicast in BGP/MPLS [I-D.ietf-l3vpn-2547bis-mcast] includes multiple alternative mechanisms for some of the required building blocks of the solution. However, it does not identify the core set of mechanisms which must be implemented in order to ensure interoperability. This may lead to a situation where implementations may support different subsets of the available optional mechanisms leading to implementations that do not interoperate. The aim of this document is to leverage the already expressed requirements [RFC4834] to identify the mechanisms that the authors believe are good candidates for being part of a core set of mandatory mechanisms which can be used to provide a base for interoperable solutions. This document will go through the different building blocks of the solution and provide the authors' recommendations as to which mechanisms the authors' favor for each building block, while considering the requirements already defined and the goal of a fully- interoperable standard. Considering the history of the multicast VPN proposals and implementations, the authors also consider it useful to discuss how existing deployments of early implementations [I-D.rosen-vpn-mcast][I-D.raggarwa-l3vpn-2547-mvpn] can fit in the picture, and provide suggestions in this respect. 2. Terminology Please refer to [I-D.ietf-l3vpn-2547bis-mcast] and [RFC4834]. 3. Examining alternatives mechanisms for MVPN functions 3.1. MVPN auto-discovery Section 5.2.10 of [RFC4834] states "The operation of a multicast VPN solution SHALL be as light as possible and providing automatic configuration and discovery SHOULD be a priority when designing a multicast VPN solution. Particularly the operational burden of setting up multicast on a PE or for a VR/VRF SHOULD be as low as possible". The current solution document [I-D.ietf-l3vpn-2547bis-mcast] addresses this requirement by proposing two different mechanisms for Morin, et al. Expires August 28, 2008 [Page 4] Internet-Draft Multicast VPN Considerations February 2008 MVPN auto-discovery: 1. BGP-based auto-discovery (described in section 4). 2. Discovery using PIM running on a MI-PMSI implemented with a shared tree using multicast ASM, or MP2MP LDP with the same common tree identifier configured in all VRFs of an MVPN. It is the recommendation of the authors that BGP-based auto-discovery is the preferred solution for auto-discovery and should be supported by all implementations while PIM/shared-tree based auto-discovery should be optionally considered for migration purpose only. Part of the rationale for this recommendation is also based on section 5.2.10 of [RFC4834] which states "as far as possible, the design of a solution SHOULD carefully consider the number of protocols within the core network: if any additional protocols are introduced compared with the unicast VPN service, the balance between their advantage and operational burden SHOULD be examined thoroughly". BGP is the auto-discovery protocol used in unicast (RFC4364) VPNs and therefore the use of BGP-based auto-discovery within multicast VPNs avoids the introduction of an additional auto-discovery protocol that would require additional OAM processes and tools. Service providers with deployed unicast (RFC4364) VPNs already have extensive deployment and operations experience of using BGP as an auto- discovery protocol including OAM processes and tools. Such processes and tools will require modifications in order to support multicast auto-discovery but those modifications are anticipated to be less than those required to develop new processes and tools for a specific auto-discovery protocol. Additionally, BGP supports MD5 authentication of its peers for additional security. In contrast, there are no obvious authentication mechanisms to secure PIM communications in any known implementation. Furthermore, PIM based discovery is only applicable to deployments using a shared tree on an MI-PMSI, whereas BGP-based auto-discovery does not place any restrictions on the type of multicast trees that can be used. BGP-based auto-discovery is independent of the type of P-multicast tree used thus satisfying the requirement in section 5.2.4.1 of [RFC4834] that "a multicast VPN solution SHOULD be designed so that control and forwarding planes are not interdependent". Additionally, it is to be noted that a number of service providers have chosen to use SSM-based trees for the default MDTs within their current deployments, therefore relying already on BGP-based auto-discovery. Morin, et al. Expires August 28, 2008 [Page 5] Internet-Draft Multicast VPN Considerations February 2008 When shared trees are used, the use of BGP auto-discovery would allow inconsistencies in the addresses/identifiers used for the shared trees to be detected (e.g. the same shared tree identifier being used for different VPNs with distinct BGP route targets). This is particularly attractive in the context of inter-AS VPNs where the impact of any misconfiguration could be magnified and where a single service provider may not operate all the ASs. Note that this technique to detect some misconfiguration may not be usable during a transition period from a shared-tree autodiscovery to a BGP-based autodiscovery. Last, the use of the BGP-based autodiscovery is expected to be less prone to spoofing attacks (being based on a connection established with a three-way handshake), to which the PIM Hello over MI-PMSI procedures may be subject to (being datagram-based). ( the authors note that, in order to support the coexistence of both protocols (for example during migration scenarios), implementations could support both alternatives by providing a per-VRF configuration knob that would allow recognizing new PIM neighbors based on the reception of PIM hellos on a shared P-multicast tree, even for neighbors that did not advertise a BGP auto-discovery route ) 3.2. S-PMSI Signaling The current solution document [I-D.ietf-l3vpn-2547bis-mcast] proposes two mechanisms for S-PMSI Signaling: 1. A new UDP-based TLV protocol specifically for S-PMSI signaling (described in section 7.2.1). 2. A BGP-based mechanism for S-PMSI signaling (described in section 7.2.2). It is the recommendation of the authors that BGP is the preferred solution for S-PMSI signaling and should be supported by all implementations while the UDP-based S-PMSI signaling protocol should be considered optional. Part of the rationale for this recommendation is similar to that for BGP-based auto-discovery and is based on section 5.2.10 of [RFC4834] and the desire to avoid introducing and deploying additional protocols unless strictly necessary. Furthermore: o The BGP-based S-PMSI signaling mechanism can be used within MVPNs using either a UI-PMSI or a MI-PMSI while the UDP-based protocol Morin, et al. Expires August 28, 2008 [Page 6] Internet-Draft Multicast VPN Considerations February 2008 is restricted to use within MVPNs using an MI-PMSI. o The BGP-based S-PMSI signaling mechanism can be efficiently used in an inter-AS option B deployment context while the use of the UDP-based protocol does not preserve AS routing independence when used in an inter-AS option B context (i.e. the decision by a PE in an AS to use an S-PMSI for a given customer flow will impact routing state in other ASes). Co-existence with unicast inter-AS VPN options is strongly encouraged by section 5.2.6 of [RFC4834]. o The BGP-based S-PMSI signaling supports all the functionality and all the operational contexts that are supported by UDP-based protocol (and more). o BGP supports MD5 authentication of its peers for additional security. In contrast, there are no obvious authentication mechanisms to secure PIM communications in any known implementation. Therefore, it is the opinion of the authors that BGP is the preferred solution for performing S-PMSI signaling. However, the authors recognize that the UDP-based protocol has been in deployment for some time and would recommend that implementations supporting both protocols optionally provide a per-VRF configuration knob to allow an implementation to use the UDP-based TLV protocol for S-PMSI signaling for specific VRFs in order to support the coexistence of both protocols (for example during migration scenarios). Apart from such migration-facilitating mechanisms, the authors specifically do not recommend extending the already proposed UDP- based TLV protocol to new types of P-multicast trees. Section 7.2.2.3 of [I-D.ietf-l3vpn-2547bis-mcast] proposes two approaches for how a source PE can decide when to start transmitting customer multicast traffic on a S-PMSI: 1. The source PE sends multicast packets for the on both the I-PMSI P-multicast tree and the S-PMSI P-multicast tree simultaneously for a pre-configured period of time, letting the receiver PEs select the new tree for reception, before switching to only the S-PMSI. 2. The source PE waits for a pre-configured period of time after advertising the entry bound to the S-PMSI before fully switching the traffic onto the S-PMSI-bound P-multicast tree. Morin, et al. Expires August 28, 2008 [Page 7] Internet-Draft Multicast VPN Considerations February 2008 The first alternative has essentially two drawbacks: o traffic is sent twice for some period of time, which would appear to be at odds with the motivation for switching to an S-PMSI in order to optimize the bandwidth used by the multicast tree for that stream. o It is unlikely that the switchover can occur without packet loss or duplication if the transit delays of the I-PMSI P-multicast tree and the S-PMSI P-multicast tree differ. By contrast, the second alternative has none of these drawbacks, and satisfy the requirement in section 5.1.3 of [RFC4834], which states that "[...] a multicast VPN solution SHOULD as much as possible ensure that client multicast traffic packets are neither lost nor duplicated, even when changes occur in the way a client multicast data stream is carried over the provider network". The second alternative also happen to be the one used in existing deployments. For these reasons, it is the authors' recommendation to mandate the implementation of the second alternative for switching to S-PMSI. 3.3. PE-PE Transmission of C-Multicast Routing The current solution document [I-D.ietf-l3vpn-2547bis-mcast] proposes multiple mechanisms for PE-PE transmission of customer multicast routing information: 1. Full per-MVPN PIM peering across an MI-PMSI (described in section 5.2.1). 2. Lightweight PIM peering across an MI-PMSI (described in section 5.2.2) 3. The unicasting of PIM C-Join/Prune messages (described in section 5.2.3) 4. The use of BGP for carrying C-Multicast routing (described in section 5.3). 3.3.1. PE-PE signalling scalability Scalability being one of the core requirements for multicast VPN, it is useful to compare the proposed C-multicast routing mechanisms from this perspective : Section 4.2.4 of [RFC4834] recommends that "a multicast VPN solution SHOULD support several hundreds of PEs per multicast VPN, and MAY usefully scale up to thousands" and section 4.2.5 states that "a solution SHOULD scale up to thousands of PEs Morin, et al. Expires August 28, 2008 [Page 8] Internet-Draft Multicast VPN Considerations February 2008 having multicast service enabled". At such scales of multicast deployment, the first and third mechanisms require the PEs to maintain a large number of PIM adjacencies with other PEs of the same multicast VPN (which implies the regular exchange PIM Hellos with each other) and to refresh C-Join/Prune states, thus limiting the scalability of these approaches. The third mechanism would reduce the amount of C-Join/Prune processing for a given multicast flow for PEs that are not the upstream neighbor for this flow, but would require "explicit tracking" state to be maintained by the upstream PE, and would require refresh-reduction mechanisms to be used to mitigate the fact that PIM "Join suppression" cannot be used (what such a refresh- reduction mechanism would be has not been described yet). For these reasons, it seems that this approach is not suitable for higher scale scenarios. The second mechanism would operate in a similar manner to full per- MVPN PIM peering except that PIM hellos are not transmitted and PIM C-Join/Prune refresh-reduction would be used, thereby improving scalability, but this approach has been further developed and it is unclear if it is applicable. The first and second mechanisms can leverage the "Join suppression" behavior and thus improve the processing burden of an upstream PE, sparing the processing of one Join message for each remote PE joined to a multicast stream, but this improvement comes at the price of requiring all PEs of a multicast VPN to process all PIM Joins sent by any PE participating in the same multicast VPN whether they are the upstream PE or not. The fourth mechanism (the use of BGP for carrying C-Multicast routing) would have a comparable drawback of requiring all PEs to process a BGP C-multicast route only interesting a specific upstream PE. For this reason the C-multicast routing approach leverages the Route-Target constraint mechanisms, which specifically allows only the interested upstream PE to receive a BGP C-multicast route. When RT constraints are used the fourth mechanism reduces the processing load put on the provider infrastructure for customer multicast routing to the minimum (by avoiding any processing by "unrelated" PEs, that are nor the joining PE nor the upstream PE), and inherits BGP features that are expected to improve scalability (through, for instance, providing a means to offload some of the processing burden associated with client multicast routing onto BGP route-reflectors), and being based on TCP has no refresh-related scalability limit. (Please refer to Handling the PIM routing processing load load, for a Morin, et al. Expires August 28, 2008 [Page 9] Internet-Draft Multicast VPN Considerations February 2008 detailed explanation of the differences in ways of handling the C-multicast routing load, between the PIM-based approaches and the BGP-based approach) However, it is to be noted that offloading customer multicast routing processing onto BGP route-reflectors will increase the processing load placed on the route-reflector infrastructure, which, in the higher scale scenarios, is expected to call for adaptations such as: o a separation of resources for unicast and multicast VPN routing : using mvpn-dedicated BGP sessions and/or mvpn-dedicated BGP instances on route-reflectors, and/or mvpn-dedicated route- reflectors ; o the deployment of additional route-reflectors resources : increased processing resources on existing route reflectors or additional route-reflectors. 3.3.2. P-routers scalability Mechanisms (1) and (2) are restricted to use within multicast VPNs that use an MI-PMSI, thereby necessitating: the use of a P-multicast tree technique that allows shared trees (for example PIM-SM in ASM mode or MP2MP LDP) or the use of one P-multicast tree per PE per VPN, even for PEs that do not have sources in their directly attached sites for that VPN. By comparison, the fourth mechanism doesn't impose either of these restrictions, and when P2MP trees are used only necessitates the use of one tree per VPN per PE attached to a site with a multicast source or RP (or with a candidate BSR, if BSR is used), thereby improving the amount of state maintained by P-routers compared to the amount required to build an MI-PMSI with P2MP trees. 3.3.3. Impact of C-multicast routing on Inter-AS deployments Furthermore, co-existence with unicast inter-AS VPN options, and an equal level of security for multicast and unicast including in an inter-AS context, are specifically mentioned in sections 5.2.6, 5.2.8 and 5.2.12 of [RFC4834]. The first three mechanisms impose direct PE to PE communications : this does not apply well to an inter-AS option B context, because of security and robustness issues that are involved by such a level of reachability and interaction between PEs in different ASes. Morin, et al. Expires August 28, 2008 [Page 10] Internet-Draft Multicast VPN Considerations February 2008 Their use in an inter-AS context is possible, but not without limitations or additional engineering design trade-offs depending upon the interconnect types. By comparison, the fourth option (the use of BGP for carrying C-Multicast routing) does not have any of the above limitations related to inter-AS deployments, and also provides an additional alternative to facilitate such deployments through the possibility of using segmented inter-AS trees. 3.3.4. Security and robustness BGP supports MD5 authentication of its peers for additional security, thereby possibly benefit directly to multicast VPN customer multicast routing, whether for intra-AS or inter-AS communications. By contrast, with a PIM-based approach, no mechanism providing a comparable level of security to authenticate communications between remote PEs has been yet fully described yet [I-D.ietf-pim-sm-linklocal][], and in any case would require significant additional operations for the provider to be usable in a multicast VPN context. The robustness of the infrastructure, especially the existing infrastructure providing unicast VPN connectivity, is key. The C-multicast routing function, especially under load, will compete with the unicast routing infrastructure. With the PIM-based approaches, unicast and multicast VPN routing are expected to only compete in the PE, for routing plane processing resources. In the case of the BGP-based approach, they will compete on the PE for processing resources, and in the route-reflector if they are used. It is identified that in both cases, mechanisms will be required to arbitrate resources (e.g. processing priorities). In the case of PIM-based procedures, between the different control plane routing instances in the PE. And in the case of the BGP-based approach, this is likely to require using distinct BGP sessions for multicast and unicast, possibly toward distinct route-reflectors. Multicast routing is dynamic by nature, and multicast VPN routing has to follow the VPN customers multicast routing events. The different approaches can be compared on how they are expected to behave in scenarios where multicast routing in the VPNs is subject to an intense activity. Such a load would be comparable to the higher scale scenarios described in xx (Section 3.3.1) and the fourth (BGP- based) approach - when deployed to handle a significant multicast VPN routing load - is expected to be the most efficient approach in a such case. On the other hand, while the BGP-based approach is likely to suffer a slowdown under a load raising beyond processing resources (because of possibly congested TCP sockets), the PIM-based approaches Morin, et al. Expires August 28, 2008 [Page 11] Internet-Draft Multicast VPN Considerations February 2008 would react to such a load by dropping messages, with later failure recovery through message refreshes, this being at the expense of some predictability. In fact both situations are problematic, and what seems important is the ability for the VPN backbone operator to (a) limit the amount of multicast routing activity that can be triggered by a multicast VPN customer, and to (b) provide the best possible independence between distinct VPNs. It seems that both of these can be addressed through local implementation improvements, and that both the BGP-based and PIM-based approach could be engineered to provide (a) and (b). It can be noted though that the BGP approach proposes ways to dampen C-multicast route withdrawals and/or advertisements, and thus already describes a way to provide (a), while nothing hasn't been yet described for the PIM-based approach, though these type of approaches rely on a per VPN dataplane to carry the mvpn control plane, and thus might naturally benefit from this first level of separation to solve (b). 3.3.5. C-multicast VPN join latency Section 5.1.3 of [RFC4834] states that "the group join delay [...] is also considered one important QoS parameter. It is thus RECOMMENDED that a multicast VPN solution be designed appropriately in this regard.". In a multicast VPN context, the "group join delay"of interest is the time between a CE sending a PIM Join to its PE and the first packet of the corresponding multicast stream being received by the CE. The different approaches proposed seem to have different characteristics in how they are expected to impact join latency: o the PIM-based approaches minimize the number of control plane processing hops between the PE of a new receiver and the PE of the multicast source, and being datagram based introduce minimal delay, thereby possibly having a best-case join latency as good as possible depending on implementation efficiency o the BGP-based approach uses TCP exchanges, that may introduce an additional delay depending on BGP implementation performances, but are expected to control the worst-case join latency under load o the BGP-based approaches is designed to allow the introduction of route-reflectors which will introduce an additional processing delay between the receiver-PE and the source-PE o in higher scale scenarios, the BGP-based approach is expected to provide some control of the worst-case join latency whereas the Morin, et al. Expires August 28, 2008 [Page 12] Internet-Draft Multicast VPN Considerations February 2008 PIM-based approaches may behave less efficiently if PIM messages are lost o in higher scale scenarios, the introduction of route-reflectors in the BGP architecture are expected to provide processing efficiency which is expected to improve latency compared to the PIM-based approaches This qualitative comparison of approaches tend to highlight that the BGP based approach is designed for controlling the "worst-case" join latency whereas for the PIM-based approaches seem to structurally be able to reach the shorter "best-case" group join latency (especially compared to deployment of the BGP-based approach where route- reflectors are used). Doing a quantitative comparison is not possible without referring to specific implementations and benchmarking procedures, and would possibly expose different conclusions, especially for best-case group join latency for which performance is expected vary with implementations. We can also note that improving a BGP implementation for reduced latency of route processing would not only benefit multicast VPN group join latency, but the whole BGP-based routing. Last, it is to be noted that the C-multicast routing procedures will only impact the group join latency of a said multicast stream for the first receiver that is located across the provider backbone from the multicast source. 3.3.6. Architectural considerations The fourth mechanism (the use of BGP for carrying C-Multicast routing) would appear to fit well with the current unicast architecture as BGP is the customer routing distribution protocol used in unicast VPNs and therefore using BGP for customer routing distribution within multicast VPNs avoids the introduction of an additional protocol that would require additional OAM processes and tools. Service provider's with deployed unicast (RFC4364) VPNs already have extensive deployment and operations experience of using BGP as a customer routing distribution protocol including OAM processes and tools. Such processes and tools will require modification in order to support customer multicast routing but those modifications are anticipated to be less than those required to develop new processes and tools for a distinct customer routing protocol. It should be noted that because PIM will be used as the CE-PE customer routing distribution protocol, service providers will still need OAM processes and tools in order to manage the PIM protocol, so this rationale only applies to a subset of the tools and processes Morin, et al. Expires August 28, 2008 [Page 13] Internet-Draft Multicast VPN Considerations February 2008 already in place. An illustrative example of the benefit brought by consistency with unicast design is how the "extranet" feature can be implemented : when BGP-based mechanisms are used, the already defined and well understood BGP route target import/export semantics are just reused and applied to BGP mVPN routes. By contrast, it is not specified how implementing the same feature would be done in the context of other alternative mechanisms, and unclear if this is possible without significant engineering trade-offs given that their control plane is tied to a specific MI-PMSI tunnel. Note that the support for the Extranet feature is stated as a MUST in sections 5.1.6 of [RFC4834]. Section 5.2.10 of [RFC4834] states that "as far as possible, the design of a solution SHOULD carefully consider the number of protocols within the core network: if any additional protocols are introduced compared with the unicast VPN service, the balance between their advantage and operational burden SHOULD be examined thoroughly". Considering that the recommendation of the authors would be BGP for auto-discovery and S-PMSI signaling, the choice of BGP for customer multicast routing would be consistent with the protocol choice for unicast VPNs and would adequately address this requirement. 3.3.7. Conclusion on C-multicast routing The fourth approach (BGP-based) for customer multicast routing clearly presents some advantages over the PIM-based alternatives. However it has yet to be deployed within an operational MVPN, and only limited experience exists with its implementations. By contrast, PIM-based mechanisms lack many of these benefits and have identified limitations in how they can handle customer multicast routing load in higher-scale scenarios. Despite these, experience showed that the "Full PIM peering" approach is operationally viable. Consequently, at the present time and until there is experience with all of the proposed mechanisms it is not clear which of the above mechanisms should be recommended as the preferred solution to implementers. However, it would appear prudent for implementations to consider supporting both the fourth (BGP-based) and first (full per-MPVN PIM peering) mechanisms. Further experience on both implementations is likely to be required before some best practice can be defined. The first mechanism (full per-MVPN PIM peering across an MI-PMSI) is the mechanism used by [I-D.rosen-vpn-mcast] and therefore it is deployed and operating in MVPNs today. The authors recognize that because full per-MVPN PIM peering has been in deployment for some Morin, et al. Expires August 28, 2008 [Page 14] Internet-Draft Multicast VPN Considerations February 2008 time, the support for this mechanism may be helpful for backwards compatibility and in order to facilitate migration towards the BGP- based approach. Moreover to improve the clarity of the proposed specifications, considering that neither hello suppression nor refresh-reduction procedures are currently specified or documented and that it is not clear what the impact to the PIM state machine of these additional procedures may be, the authors recommend that the proposals for lightweight PIM peering across an MI-PMSI (the second mechanism) and for the unicasting of PIM C-Join/Prune messages (the third mechanism) be removed from the current solution document [I-D.ietf-l3vpn-2547bis-mcast] (at least until they have been further specified and both their impact and benefit on a multicast VPN deployment is spelled out). 3.4. Encapsulation techniques for P-multicast trees In this section the authors will not make any restricting recommendations since the appropriateness of a specific provider core data plane technology will depend on a large number of factors, for example the service provider's currently deployed unicast data plane, many of which are service provider specific. However, implementations should not unreasonably restrict the data plane technology that can be used, and should not force the use of the same technology for different VPNs attached to a single PE. Initial implementations may only support a reduced set of encapsulation techniques and data plane technologies but this should not be a limiting factor that hinders future support for other encapsulation techniques, data plane technologies or interoperability. Section 5.2.4.1 of [RFC4834] states "In a multicast VPN solution extending a unicast L3 PPVPN solution, consistency in the tunneling technology has to be favored: such a solution SHOULD allow the use of the same tunneling technology for multicast as for unicast. Deployment consistency, ease of operation and potential migrations are the main motivations behind this requirement." Current unicast VPN deployments use a variety of LDP, RSVP-TE and GRE/IP-Multicast for encapsulating customer packets for transport across the provider core of VPN services. It is recommended that implementations support the three corresponding multicast tree encapsulations techniques, namely: mLDP, P2MP RSVP-TE and GRE/ IP-multicast in order to allow the same encapsulations to be used for unicast and multicast traffic as well as facilitating migration from [I-D.rosen-vpn-mcast] to an MPLS label based encapsulation. Morin, et al. Expires August 28, 2008 [Page 15] Internet-Draft Multicast VPN Considerations February 2008 All three of the above encapsulation techniques support the building of P2MP multicast trees. In addition mLDP and GRE/IP-ASM-Multicast implementations may also support the building of MP2MP multicast trees. The use of MP2MP trees may provide some scaling benefits to the service provider as only a single MP2MP tree need be deployed per VPN, thus reducing the amount of multicast state that needs to be maintained by P routers. This gain in state is at the expect of bandwidth optimization, since sites that do not have multicast receivers for multicast streams sourced behind a said PE group will still receive packets of such streams, leading to non-optimal bandwidth utilization across the VPN core. One thing to consider is that the use of MP2MP multicast tree will require configuring the same tree identifier or multicast ASM group address in all PEs, and will not provide the kind of autoconfiguration possible with P2MP trees. MVPN services can also be supported over a unicast VPN core through the use of ingress PE replication whereby the ingress PE replicates any multicast traffic over the P2P tunnels used to support unicast traffic. While this option does not require the service provider to modify their existing P routers (in terms of protocol support) and does not require maintaining multicast-specific state on the P routers in order for the service provider to be able deploy a multicast VPN service, the use of ingress PE replication obviously leads to non-optimal bandwidth utilization and it is therefore unlikely to be the long term solution chosen by service providers. However ingress PE replication may be useful during some migration scenarios or where a service provider considers the level of multicast traffic on their network to be too low to justify deploying multicast specific support within their VPN core. All proposed approaches for control plane and dataplane can be used to provide aggregation amongst multicast groups within a VPN and amongst different multicast VPNs, and potentially reduce the amount of state to be maintained by P routers. However the latter -- the aggregation amongst different multicast VPNs will require support for upstream-assigned labels on the PEs. Support for upstream-assigned labels may require changes to the data plane processing of the PEs and this should be taken into consideration by service providers considering the use of aggregate S-PMSI tunnels for the specific platforms that the service provider has deployed. 3.5. Inter-AS deployments options There are a number of scenarios that lead to the requirement for inter-AS multicast VPNs, including: Morin, et al. Expires August 28, 2008 [Page 16] Internet-Draft Multicast VPN Considerations February 2008 1. A service provider may have a large network that they have segmented into a number of ASs. 2. A service provider's multicast VPN may consist of a number of ASs due to acquisitions and mergers with other service providers. 3. A service provider may wish to interconnect their multicast VPN platform with that of another service provider. The first scenario can be considered the "simplest" because the network is wholly managed by a single service provider under a single strategy and is therefore likely to use a consistent set of technologies across each AS. The second scenario may be more complex than the first because the strategy and technology choices made for each AS may have been different due to their differing history and the service provider may not have (or may be unwilling to) unified the strategy and technology choices for each AS. The third scenario is the most complex because in addition to the complexity of the second scenario, the ASs are managed by different service providers and therefore may be subject to a different trust model than the other scenarios. Section 5.2.6 of [RFC4834] states "A solution MUST support inter-AS multicast VPNs, and SHOULD support inter-provider multicast VPNs. Considerations about coexistence with unicast inter-AS VPN Options A, B and C (as described in section 10 of [RFC4364]) are strongly encouraged." and "A multicast VPN solution SHOULD provide inter-AS mechanisms requiring the least possible coordination between providers, and keep the need for detailed knowledge of providers' networks to a minimum - all this being in comparison with corresponding unicast VPN options." Section 8 of [I-D.ietf-l3vpn-2547bis-mcast] addresses these requirements by proposing two approaches for mVPN inter-AS deployments: 1. Segmented inter-AS tunnels where each AS constructs its own separate multicast tunnels which are then 'stitched' together by the ASBRs (described in section 8.2). 2. Non-segmented inter-AS tunnels where the multicast tunnels are end-to-end across ASes, so even though the PEs belonging to a given MVPN may be in different ASs the ASBRs play no special role and function merely as P routers (described in section 8.1). Morin, et al. Expires August 28, 2008 [Page 17] Internet-Draft Multicast VPN Considerations February 2008 Section 5.2.6 of [RFC4834] also states "Within each service provider the service provider SHOULD be able on its own to pick the most appropriate tunneling mechanism to carry (multicast) traffic among PEs (just like what is done today for unicast)". Segmented inter-AS tunnels is the only solution that is capable of meeting this requirement. The segmented inter-AS solution would appear to offer the largest degree of deployment flexibility to operators, however the non- segmented inter-AS solution can simplify deployment in a restricted number of scenarios and [I-D.rosen-vpn-mcast] only supports the non- segmented inter-AS solution and therefore the non-segmented inter-AS solution is likely to be required by some operators for backward compatibility and during migration from [I-D.rosen-vpn-mcast] to [I-D.ietf-l3vpn-2547bis-mcast]. The applicability of segmented or non-segmented inter-AS tunnels to a given deployment or inter-provider interconnect will depend on a number of factors specific to each service provider. However, due to the additional deployment flexibility offered by segmented inter-AS tunnels, it is the recommendation of the authors that all implementations should support the segmented inter-AS model. Additionally, the authors recommend that implementations should consider supporting the non-segmented inter-AS model in order to facilitate co-existence with existing deployments, and as a feature to provide a lighter engineering in a restricted set of scenarios, although it is recognized that initial implementations may only support one or the other. Additionally, the authors note that the proposed BGP-based approaches for S-PMSI signaling and C-multicast routing information distribution provide a good fit with both segmented and non-segmented inter-AS tunnels. In contrast the UDP-TLV based approach for S-PMSI signaling appears to be incompatible with segmented inter-AS tunnels, and it is unclear if the proposed PIM-based approaches for C-multicast routing information distribution would be fully applicable to segmented inter-AS tunnels. 4. Co-located RPs Section 5.1.10.1 of [RFC4834] states "In the case of PIM-SM in ASM mode, engineering of the RP function requires the deployment of specific protocols and associated configurations. A service provider may offer to manage customers' multicast protocol operation on their behalf. This implies that it is necessary to consider cases where a customer's RPs are outsourced (e.g., on PEs). Consequently, a VPN solution MAY support the hosting of the RP function in a VR or VRF." Morin, et al. Expires August 28, 2008 [Page 18] Internet-Draft Multicast VPN Considerations February 2008 Co-locating a customer's RP on a PE can provide some benefits to the customer as outlined in section 5.1.10.3 of [RFC4834] which states "In the case of PIM-SM, when a source starts to emit traffic toward a group (in ASM mode), if sources and receivers are located in VPN sites that are different than that of the RP, then traffic may transiently flow twice through the SP network and the CE-PE link of the RP (from source to RP, and then from RP to receivers). This traffic peak, even short, may not be convenient depending on the traffic and link bandwidth. However, customers who have already deployed multicast within their networks and have therefore already deployed their own internal RPs are often reluctant to hand over the control of their RPs to their service provider and make use of a co-located RP model. Also, providing collocating the RP on PE will require the activation of MSDP or the processing of PIM Registers on the PE. Securing the PE routers for such activity requires special care, additional work, and will likely rely on specific features to be provided by the routers themselves. The applicability of the co-located RP model to a given MVPN will thus depend on a number of factors specific to each customer and service provider. It is therefore the recommendation of the authors that implementations should support a co-located RP model, but that support for a co-located RP model within an implementation should not restrict deployments to using a co-located RP model : implementations MUST support deployments when activation of a PIM RP function (PIM Register processing and RP-specific PIM procedures) or VRF MSDP instance is not required on any PE router and where all the RPs are deployed within the customers' networks or CEs. 5. Existing deployments Some suggestions provided in this document can be used to incrementally modify currently deployed implementations without hindering these deployments, and without hindering the consistency of the standardized solution by providing optional per-VRF configuration knobs to support modes of operation compatible with currently deployed implementations, while at the same time using the recommended approach on implementations supporting the standard. In cases where this may not be easily achieved, a recommended approach would be to provide a per-VRF configuration knob that allows incremental per-VPN migration of the mechanisms used by a PE device, which would allow migration with some per-VPN interruption of service (e.g. during a maintenance window). Morin, et al. Expires August 28, 2008 [Page 19] Internet-Draft Multicast VPN Considerations February 2008 Mechanisms allowing "live" migration by providing concurrent use of multiple alternatives for a given PE and a given VPN, is not seen as a priority considering the expected implementation complexity associated with such mechanisms. However, if there happen to be cases where they could be viably implemented relatively simply, such mechanisms may help improve migration management. 6. Summary of recommendations The following list summarizes the authors' recommendations. These recommendations are not intended to prevent the implementation of alternative solutions, rather they are the authors' recommendations for the mechanisms that should be made mandatory in [I-D.ietf-l3vpn-2547bis-mcast] and therefore be supported by all implementations. It is the authors' recommendation: o that BGP-based auto-discovery be the mandated solution for auto- discovery ; o that BGP be the mandated solution for S-PMSI signaling ; o that the mandated solution for S-PMSI switch-over be the mechanism based on the source-connected PE switching traffic from the I-PMSI tunnel to the S-PMSI tunnel, without transmitting traffic on both at the time ; o that implementations support both the BGP-based and the full per- MPVN PIM peering solutions for PE-PE transmission of customer multicast routing until further operational experience is gained with both solutions ; o that implementations support the following multicast tree encapsulations: mLDP, P2MP RSVP-TE and GRE/IP-Multicast ; o that implementations support segmented inter-AS tunnels and consider supporting non-segmented inter-AS tunnels (in order to maintain backwards compatibility and for migration) ; o implementations MUST support deployments when activation of a PIM RP function (PIM Register processing and RP-specific PIM procedures) or VRF MSDP instance is not required on any PE router. Morin, et al. Expires August 28, 2008 [Page 20] Internet-Draft Multicast VPN Considerations February 2008 7. IANA Considerations This document makes no request to IANA. [ Note to RFC Editor: this section may be removed on publication as an RFC. ] 8. Security Considerations This document does not by itself raise any particular security considerations. 9. Acknowledgements We would like to thank Adrian Farrel, Eric Rosen, Yakov Rekhter, and Maria Napierala for their feedback that helped shape this document. 10. Informative References [RFC4834] Morin, T., "Requirements for Multicast in L3 Provider- Provisioned Virtual Private Networks (PPVPNs)", RFC 4834, April 2007. [I-D.ietf-l3vpn-2547bis-mcast] Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP VPNs", draft-ietf-l3vpn-2547bis-mcast-06 (work in progress), October 2006. [I-D.rosen-vpn-mcast] Rosen, E., "Multicast in MPLS/BGP VPNs", draft-rosen-vpn-mcast-08 (work in progress), December 2004. [I-D.raggarwa-l3vpn-2547-mvpn] Aggarwal, R., "Base Specification for Multicast in BGP/ MPLS VPNs", draft-raggarwa-l3vpn-2547-mvpn-00 (work in progress), June 2004. [I-D.ietf-pim-sm-linklocal] Atwood, J., "Authentication and Confidentiality in PIM-SM Link-local Messages", draft-ietf-pim-sm-linklocal-02 (work in progress), November 2007. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Morin, et al. Expires August 28, 2008 [Page 21] Internet-Draft Multicast VPN Considerations February 2008 Appendix A. Handling the PIM routing processing load load The main role of multicast routing is to let routers determine that they should start or stop forwarding a said multicast stream on a said link. In the multicast VPN context, this has to be made for each VPN, and the associated function is thus named "customer- multicast routing" or "C-multicast routing" and its role is to let PE routers determine that they should start of stop forwarding the traffic of a said multicast stream toward the remote PEs, on some S-PMSI tunnel. When some "join" message is received by a PE, this PE knows that it should be sending traffic for the corresponding multicast group of the corresponding VPN. But the reception of a "prune" message from a remote PE is not enough by itself for a PE to know that it should stop forwarding the corresponding multicast traffic : it has to make sure that they aren't any other PEs that still have receivers for this traffic. There are many ways that the "C-multicast routing" building block can be designed so that a PE can determine when it can stop forwarding a said multicast stream toward other PEs: PIM LAN Procedures, by default By default when PIM LAN procedures are used, when a PE Prunes itself from a multicast tree, all other PEs check their own state to known if they are on the tree, in which case they send a PIM Join message to override the Prune. The "did the last receiver leave?" question is thus implicitly replied to by all PE routers, for each PIM Prune message. PIM LAN Procedures, with explicit tracking : PIM LAN procedures can use an "explicit tracking" approach, where a PE which is the upstream router for a multicast stream maintains an updated list of all neighbors who are joined to the tree. Thus, when it receives a Leave message from a PIM neighbor, it instantly knows the answer to the "did the last receiver leave?" question. In this case, the question is replied to by the upstream router alone. The side effect of this "explicit tracking" is that "Join suppression" is not used : the downstream PEs will always send Joins toward the upstream PE, which will have to process them all. BGP-based C-multicast routing When BGP-based procedures are used for C-multicast routing, if no BGP route reflector is used, the "did the last receiver leave?" question is answered like in the PIM "explicit tracking" approach. But, when a BGP route-reflector is used (which is expected to be Morin, et al. Expires August 28, 2008 [Page 22] Internet-Draft Multicast VPN Considerations February 2008 the recommended approach), the role of maintaining an updated list of the PE part of a said multicast tree is taken care of by the route-reflector(s). Using plain BGP route selection procedures, the route-reflector will withdraw a C-multicast Source Tree Join for a said (C-S,C-G) when there is no PE advertising one anymore. In this context, the "did the last receiver leave?" question can be said to be answered by the route-reflector alone. Furthermore, the BGP route distribution can leverage more than one route-reflector : if a hierarchy of Route Reflectors is used, the "did the last receiver leave?" question is partly answered by each route-reflector in the hierarchy. We can see that answering the "last receiver leaves" questions is a significant proportion of the work that the C-multicast routing building block has to make, and that there are many ways that the related load can be spread among the different functions. Authors' Addresses Thomas Morin (editor) France Telecom R&D 2 rue Pierre Marzin Lannion 22307 France Email: thomas.morin@orange-ftgroup.com Ben Niven-Jenkins (editor) BT 208 Callisto House, Adastral Park Ipswich, Suffolk IP5 3RE UK Email: benjamin.niven-jenkins@bt.com Yuji Kamite NTT Communications Corporation Tokyo Opera City Tower 3-20-2 Nishi Shinjuku, Shinjuku-ku Tokyo 163-1421 Japan Email: y.kamite@ntt.com Morin, et al. Expires August 28, 2008 [Page 23] Internet-Draft Multicast VPN Considerations February 2008 Raymond Zhang BT 2160 E. Grand Ave. El Segundo CA 90025 USA Email: raymond.zhang@bt.com Nicolai Leymann Deutsche Telekom Goslarer Ufer 35 10589 Berlin Germany Email: nicolai.leymann@t-systems.com Morin, et al. Expires August 28, 2008 [Page 24] Internet-Draft Multicast VPN Considerations February 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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