| Internet-Draft | LISP Mcast Deployments | April 2025 |
| Govindan, et al. | Expires 4 October 2025 | [Page] |
We present our experiences in supporting deployments of LISP Multicast using unicast and multicast underlays. This document details design considerations that can be useful for anyone interested in deploying LISP multicast services over IP networks.¶
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This document describes deployment experiences of inter domain multicast routing function in a network where Locator/ID Separation is deployed using the Locator/ID Separation Protocol (LISP) architecture as described in [I-D.ietf-lisp-rfc6831bis]¶
All of the terminology used in this document comes from [RFC9301] [I-D.ietf-lisp-rfc6831bis], [I-D.ietf-lisp-vpn] and [I-D.moreno-lisp-uberlay].¶
This document covers the following aspects:¶
This document does not cover the following aspects:¶
The deployment experiences outlined in this document capture learnings from various industry segments listed below (not exhaustive:)¶
There are both advantages and costs in using a "PIM-over-PIM" design outlined in [I-D.ietf-lisp-rfc6831bis]:¶
A small but significant subset of deployments have been observed using the Head-End Replication (HER). This is primarily done for low-volume multicast or for deployments where there are restrictions in implementations for supporting an underlay with native multicast.¶
Another category of the deployments were early adopters of the technology when the software releases were limited to unicast underlay.¶
The primary characteristic of such networks is the presence of a limited number of LISP sites in which receivers are present. Please note that this does not necessarily mean that only a limited number of hosts receive the multicast.¶
Since the ASICs that form the data plane have very efficient methods to replicate multicast packets to local receivers, any deployment that has a good localization of receivers to a limited number of LISP sites can still use a unicast underlay with high efficiency.¶
On the positive side, there are widely deployed mechanisms for both traffic-engineering (e.g. Load balancing) and fast convergence due to link/ node failures in unicast that can be reused for overlay routed multicast when using a unicast underlay.¶
Another very important use-case for considering a unicast underlay is to have migration done from (say) IPv4 to IPv6.¶
It is also possible to perform the Head-End replication on an ITR or PITR.¶
Although not covered in this document, the design of HER allows for the co-existence of RLOCs using both unicast and multicast underlays.¶
Native multicast underlay presents notable advantages over Head-End Replication, particularly in network topologies where replication occurs at multiple nodes between the ETR and the ITR. Despite advancements in modern ASICs designed for high-performance multicast packet replication at the Head-End, bandwidth consumption remains a critical factor favoring the adoption of native multicast underlay.¶
In native multicast mode, there is a mapping between the overlay multicast group address and the underlay multicast group. This mapping must be consistent across network devices within a LISP domain to ensure uniformity. The simplest method involves a 1:1 mapping between the overlay LISP group address and the underlay multicast group address. To maintain uniqueness in this mapping process, implementations may incorporate additional parameters, such as the source IP address and LISP instance ID, providing sufficient entropy to ensure uniqueness across LISP instances.¶
There exists recent work like [I-D.ietf-lisp-group-mapping] that can be leveraged here.¶
Using native multicast in the underlay is used by the majority of the deployments known.¶
There are three deployment options that can be considered here for deployment:¶
One of the most critical design element is the choice of the RP Design. We have the following options:¶
LISP overlays extend ASM to networks lacking native multicast support traditionally. Native multicast in the core boosts ASM resilience and optimizes traffic distribution.¶
Head-end replication requires tuning to avoid ITR overload with many receivers or high traffic. LISP overlays enable ASM resilience by rerouting around underlay failures dynamically.¶
ASM deployments scale receiver counts flexibly without requiring underlay redesigns. Troubleshooting ASM demands monitoring both LISP overlay and underlay states concurrently.¶
Pre-validating underlay multicast capabilities ensures reliable ASM performance consistently. Using native multicast complicates failure diagnosis despite enhancing overall resilience.¶
In a Layer-3 overlay, the placement of RPs is critical for ensuring robust multicast service delivery. Unlike traditional PIM-ASM, LISP multicast relies on static Rendezvous Point (RP) configurations due to the lack of support for dynamic RP discovery mechanisms like Auto-RP or Bootstrap Router (BSR).¶
Discovering RPs via the mapping system is possible but not covered in this document.¶
RPs can be positioned both inside and outside the LISP domain. The typical configuration involves static RP setup and redundancy through the Anycast RP concept, which allows multiple RPs to share the same IP address, providing load balancing and fault tolerance. This setup requires synchronization between RPs using the MSDP to exchange information about active sources.¶
In some deployments, RP placements are a combination of RPs placed inside together with RPs placed outside the LISP domains. This configuration leverages advanced MSDP peering or group mesh peering to enhance multicast service resilience and efficiency.¶
The RP placement significantly affects the convergence between the shared and source trees. It is essential that all xTRs within a given LISP instance use a consistent address scheme with identical mapping to ensure efficient multicast routing. The RP facilitates the initial setup of the sharedi tree, allowing sources and receivers to establish direct multicast data flows.¶
When working with short lived streams (e.g. PA systems for airports) it was observed that using the shared tree was optimal. The cost of switching to the shortest-path tree can be avoided in such scenarios. However such choices are better done on a case-by-case basis e.g. based on the range of group addresses.¶
SSM services over a Layer-3 LISP domain connect multicast sources and receivers via the overlay. Receivers join source trees (S,G) by signalling IGMPv3, which then is transported as PIM packets over the LISP overlay. The typical SSM services would be represented by Financial Data, IPTV and Live streaming applications.¶
The traffic within a LISP domain, similar to the ASM would be subject to encapsulation and depending on the multicast mode it would be either head-end replication or native (overlay to underlay multicast mapping).¶
The sources and receivers can be connected to the LISP site or be located outside of the LISP domain. LISP overlay provides a resiliency by rerouting traffic dynamically.¶
SSM services eliminate RPs and shared trees, simplifying tree management. Direct (S,G) trees enhance scalability and reduce latency for one-to-many uses.¶
Receivers must support IGMPv3 (or MLDv2) to specify sources, avoiding ASM fallback. Replication strategies need tuning to balance ITR load and underlay bandwidth.¶
[I-D.moreno-lisp-uberlay] provides methods to operate and interconnect multiple LISP sites using Border xTR nodes.¶
[I-D.ietf-lisp-vpn] describes the use of the LISP to create Virtual Private Networks (VPNs). LISP is used to provide segmentation in both the LISP data plane and control plane.¶
Using the procedures and construct of the above two references, we have been to able to build and deploy solutions that cater to multiple tenants connected to multiple LISP sites spread across multiple site overlays.¶
One of the primary deployment use cases involves delivering multicast services across multiple site overlays or RLOC spaces [I-D.moreno-lisp-uberlay]. There are several methods for routing multicast packets when sources and recievers are connected to LISP site overlays that are connected through VPNs. Two most common methods are given below:¶
Choosing between these options has significant design implications for both unicast and multicast flows:¶
In the native forwarding approach, traffic leaving a site overlay is decapsulated at the border xTRs and placed into the appropriate VRF corresponding to the Forwarding Context. This scenario creates considerable overhead in deploying multicast configurations across multiple site overlays, as each LISP Forwarding Context must be mapped to an individual VRF in uberlay.¶
Implementing an overlay over the LISP uberlay network offers advantages by extending the LISP Forwarding Context between different LISP domains. In this case, Border xTRs in each LISP domain are responsible for decapsulating and re-encapsulating traffic between different site overlays. This can be achieved by using disjoint underlay multicast groups in the different site overlays/ uberlay. PIM can be leveraged for signaling the different underlay group mappings for the site-overlays and uberlay. [RFC8059]¶
This feature is beyond the scope of this document.¶
This memo includes no request to IANA.¶
This informational document does not introduce any new security considerations.¶
A very special and sincere thanks to Dino Farinacci for his extensive and insightful comments for improving the content and the readability of this document.¶
The authors would like to acknowledge Stig Venaas for his review. Many individuals also contributed to the discussions for the material of this draft including Arunkumar Nandakumar, Aswin Kuppusami, Rajavel Ganesamoorthy, Sankar S and Sundara Moorthy. All contributions are gratefully acknowledged.¶
TBD¶