Multicast On-path Telemetry using IOAM

Multicast is used by residential broadband customers across operator networks, private MPLS customers, and internal customers within corporate intranet. Multicast provides real time interactive online meetings or podcasts, IPTV, and financial markets real-time data, which all have a reliance on UDP's unreliable transport. End-to-end QOS, therefore, should be a critical component of multicast deployment in order to provide a good end user experience. In multicast video streaming, if a packet is dropped, and that packet happens to be a reference frame (i.e., I-Frame) in the video feed, the client receiver of the multicast feed goes into buffering mode resulting in a frozen window. Multicast packet drops and delay can severely affect the application performance and user experience. It is important to monitor the performance of the multicast traffic. New on-path telemetry techniques such as In-situ OAM (IOAM), IOAM Direct Export (DEX) IOAM Marking-based Postcard (PBT-M), and Hybrid Two-Step (HTS) are useful and complementary to the existing active OAM performance monitoring methods, provide promising means to directly monitor the network experience of multicast traffic. However, multicast traffic has some unique characteristics which pose some challenges on applying such techniques in an efficient way. The IP multicast packet data for a particular (S, G) state is identical from one branch to another on its way to multiple receivers. When adding IOAM trace data to multicast packets, each replicated packet would keep the telemetry data for its entire forwarding path. Since the replicated packets all share common path segments, redundant data will be collected for the same original multicast packet. Such redundancy consumes extra network bandwidth unnecessarily. For a large multicast tree, such redundancy is considerable. Alternatively, it could be more efficient to collect the telemetry data using solutions such as IOAM DEX to eliminate the data redundancy. However, IOAM DEX is lack of a branch identifier, making telemetry data correlation and multicast-tree reconstruction difficult. This draft provides two solutions to the IOAM data redundancy problem based on the IOAM standards. The requirements for multicast traffic telemetry are discussed along with the issues of the existing on-path telemetry techniques. We propose modifications to make these techniques adapt to multicast in order for the original multicast tree to be correctly reconstructed while eliminating redundant data.

Multicast traffic is forwarded through a multicast tree. With PIM and P2MP (e.g., MLDP, RSVP-TE) the forwarding tree is established and maintained by the multicast routing protocol. With BIER, no state is created in the network to establish a forwarding tree; instead, a bier header provides the necessary information for each packet to know the egress points. Multicast packets are only replicated at each tree branch fork node for efficiency. There are several requirements for multicast traffic telemetry, a few of which are: Reconstruct and visualize the multicast tree through data plane monitoring. Gather the multicast packet delay and jitter performance on each path. Find the multicast packet drop location and reason. Gather the VPN state and tunnel information in case of P2MP multicast. In order to meet these requirements, we need the ability to directly monitor the multicast traffic and derive data from the multicast packets. The conventional OAM mechanisms, such as multicast ping and trace, are not sufficient to meet these requirements.

On-path Telemetry techniques that directly retrieve data from multicast traffic's live network experience are ideal for addressing the aforementioned requirements. The representative techniques include In-situ OAM (IOAM) Trace option, IOAM Direct Export (DEX) option, and PBT-M. However, unlike unicast, multicast poses some unique challenges to applying these techniques. Multicast packets are replicated at each branch fork node in the corresponding multicast tree. Therefore, there are multiple copies of the original multicast packet in the network. If the IOAM trace option is used for on-path data collection, the partial trace data will also be replicated into the copy for each branch. The end result is that, at the multicast tree leaves, each copy of the multicast packet has a complete trace. Most of the data (except data from the last leaf branch), however, has redundant copies. Data redundancy introduces unnecessary header overhead, wastes network bandwidth, and complicates the data processing. The larger the multicast tree is or the longer the multicast path is, the more severe the redundancy problem becomes. The postcard-based solutions (e.g., IOAM DEX), can be used to eliminate such data redundancy, because each node on the tree only sends a postcard covering local data. However, they cannot track and correlate the tree branches properly due to the lack of branching information, so they can bring confusion about the multicast tree topology. For example, in a multicast tree, Node A has two branches, one to Node B and the other to node C; further, Node B leads to Node D and Node C leads to Node E. When applying postcard-based methods, one cannot tell whether or not Node D(E) is the next hop of Node B(C) from the received postcards. The fundamental reason for this problem is that there is not an identifier (either implicit or explicit) to correlate the data on each branch.

We provide two solutions to address the above issues. One is based on IOAM DEX and requires an extension to the instruction header of the IOAM DEX Option. The second solution combines the IOAM trace option and postcards for redundancy removal.

One way to mitigate the postcard-based telemetry's tree tracking weakness is to augment it with a branch identifier field. Note that this works for the IOAM DEX option but not for PBT-M because the IOAM DEX option has an instruction header which can be used to hold the branch identifier. To make the branch identifier globally unique, the branch fork node ID plus an index is used. For example, Node A has two branches: one to Node B and the other to Node C. Node A will use [A, 0] as the branch identifier for the branch to B, and [A, 1] for the branch to C. The identifier is carried with the multicast packet until the next branch fork node. Each node MUST export the branch identifier in the received IOAM DEX header in the postcards it sends. The branch identifier, along with the other fields such as flow ID and sequence number, is sufficient for the data collector to reconstruct the topology of the multicast tree. shows an example of this solution. "P" stands for the postcard packet. The square brackets contains the branch identifier. The curly brace contains the telemetry data about a specific node.

| C |------>| E |-... +-:-+ +-:-+ | : | | | | | | | |----+ : +---+ +---+ | A |------->| B | : | | | |--+ +-:-+ +---+ +---+ | | | +-->| D |--... | | +---+ ]]> Each branch fork node needs to generate a unique branch identifier (i.e., branch ID) for each branch in its multicast tree instance and include it in the IOAM DEX option header. The branch ID remains unchanged until the next branch fork node. The branch ID contains two parts: the branch fork node ID and an interface index. Conforming to the node ID specification in IOAM, the node ID is a 3-octet unsigned integer. The interface index is a two-octet unsigned integer. As shown in , the branch ID consumes 8 octets in total. The three unused octets MUST be set to 0.

shows that the branch ID is carried as an optional field after the flow ID and sequence number optional fields in the IOAM DEX option header. Two bits "N" and "I" (i.e., the third and fourth bits in the Extension-Flags field) are reserved to indicate the presence of the optional branch ID field. "N" stands for the Node ID and "I" stands for the interface index. If "N" and "I" are both set to 1, the optional multicast branch ID field is present; otherwise it is absent.

Once a node gets the branch ID information from the upstream, it MUST carry this information in its telemetry data export postcards, so the original multicast tree can be correctly reconstructed based on the postcards.

The second solution is a combination of the IOAM trace option and the postcard-based telemetry. To avoid data redundancy, at each branch fork node, the trace data accumulated up to this node is exported by a postcard before the packet is replicated. In this solution, each branch also needs to maintain some identifier to help correlate the postcards for each tree section. The natural way to accomplish this is to simply carry the branch fork node's data (including its ID) in the trace of each branch. This is also necessary because each replicated multicast packet can have different telemetry data pertaining to this particular copy (e.g., node delay, egress timestamp, and egress interface). As a consequence, the local data exported by each branch fork node can only contain partial data (e.g., ingress interface and ingress timestamp). shows an example in a segment of a multicast tree. Node B and D are two branch fork nodes and they will export a postcard covering the trace data for the previous section. The end node of each path will also need to export the data of the last section as a postcard.

| C |----->| D |-----... +---+ +---+ | | | | |--+ | | {A} | |--+ +---+ +---+ | | A |---->| B | +--... | | | |--+ +---+ {D3} +---+ +---+ | | |{B2,E} +-->| E |--... {B2} | | +---+ ]]> There is no need to modify the IOAM trace option header format as specified in . We just need to configure the branch fork nodes to export the postcards and refresh the IOAM header and data (e.g., clear the node data list and reset the Remaining Length field).

Mtrace version 2 (Mtrace2) is a protocol that allows the tracing of an IP multicast routing path. Mtrace2 provides additional information such as the packet rates and losses, as well as other diagnostic information. Unlike unicast traceroute, Mtrace2 traces the path that a packet would take from a source to a receiver. It is usually initiated from an Mtrace2 client by sending an Mtrace2 Query to a Last-Hop Router (LHR) or to a Rendezvous Point (RP). The LHR/RP turns the Query packet into an Mtrace2 Request, appends a Standard Response Block containing its interface addresses and packet statistics to the Request packet, and forwards the packet towards the source/RP. In a similar fashion, each router along the path to the source/RP appends a Standard Response Block to the end of the Request packet and forwards it to its upstream router. When the First-Hop Router (FHR) receives the Request packet, it appends its own Standard Response Block, turns the Request packet into a Reply, and unicasts the Reply back to the Mtrace2 client. New on-path telemetry techniques will enhance Mtrace2, and other existing OAM solutions, with more granular and realtime network status data through direct measurements. There are various multicast protocols that are used to forward the multicast data. Each will require their own unique on-path telemetry solution.

PIM-SM is the most widely used multicast routing protocol deployed today. Of the various PIM modes (PIM-SM, PIM-DM, BIDIR-PIM, PIM-SSM), PIM-SSM is the preferred method due to its simplicity and removal of network source discovery complexity. With all PIM modes, control plane state is established in the network in order to forward multicast UDP data packets. All PIM modes utilize network based source discovery except for PIM-SSM, which utilizes application based source discovery. IP Multicast packets fall within the range of 224.0.0.0 through 239.255.255.255. The telemetry solution will need to work within this address range and provide telemetry data for this UDP traffic. A proposed solution for encapsulating the telemetry instruction header and metadata in IPv6 packets is described in .

Multipoint Label Distribution Protocol (mLDP), P2MP RSVP-TE, Ingress Replication (IR), PIM MDT SAFI with GRE Transport, are commonly used within a Multicast VPN (MVPN) environment utilizing MVPN procedures Multicast in MPLS/BGP IP VPNs and BGP Encoding and Procedures for Multicast in MPLS/BGP IP VPNs. MLDP LDP Extension for P2MP and MP2MP LSPs provides extensions to LDP to establish point-to-multipoint (P2MP) and multipoint-to-multipoint (MP2MP) label switched paths (LSPs) in MPLS networks. P2MP RSVP-TE provides extensions to RSVP-TE RSVP-TE for P2MP LSPs for establish traffic-engineered P2MP LSPs in MPLS networks. Ingress Replication (IR) P2MP Trees Ingress Replication Tunnels in Multicast VPNs using unicast replication from parent node to child node over MPLS Unicast Tunnel. PIM MDT SAFI Multicast in BGP/MPLS IP VPNsutilizes PIM modes PIM-SSM, PIM-SM, PIM-BIDIR control plane with GRE transport data plane in the core for X-PMSI P-Tree using MVPN procedures. Replication SID SR Replication Segment for Multi-point Service Delivery replication segments for P2MP multicast service delivery in Segment Routing SR-MPLS networks. The telemetry solution will need to be able to follow these P2MP and MP2MP paths. The telemetry instruction header and data should be encapsulated into MPLS packets on P2MP and MP2MP paths. A corresponding proposal is described in .

BIER adds a new header to multicast packets and allows the multicast packets to be forwarded according to the header only. By eliminating the requirement of maintaining per multicast group state, BIER is more scalable than the traditional multicast solutions. OAM Requirements for BIER lists many of the requirements for OAM at the BIER layer which will help in the forming of on-path telemetry requirements as well. Depending on how the BIER header is encapsulated into packets with different transport protocols, the method to encapsulate the telemetry instruction header and metadata also varies. It is also possible to make the instruction header and metadata a part of the BIER header itself, such as in a TLV.

No new security issues are identified other than those discovered by the IOAM trace and DEX options.

The document requests two new extension flag registrations in the "IOAM DEX Extension-Flags" registry, as described in Section 4.1. Bit 2 "Multicast Branching Node ID [RFC XXXX] [RFC Editor: please replace with the RFC number of the current document]". Bit 3 "Multicast Branching Interface Index [RFC XXXX] [RFC Editor: please replace with the RFC number of the current document]".

The authors would like to thank Frank Brockners, Nils Warnke, Jake Holland, and Dino Farinacci for the comments and suggestions.