]> 5}="yes"?> China Mobile

China liuyisong@chinamobile.com

Futurewei

USA michael.mcbride@futurewei.com

Futurewei

USA tte@cs.fau.de

ZTE Corporation

China zhang.zheng@zte.com.cn

PIM LANs often have more than one upstream router. When PIM Sparse Mode (PIM-SM), including PIM-SSM, is used, this can lead to duplicate IP multicast packets being forwarded by these PIM routers. PIM Assert messages are used to elect a single forwarder for each IP multicast traffic flow between these routers. This document defines a mechanism to send and receive information for multiple IP multicast flows in a single PackedAssert message. This optimization reduces the total number of PIM packets on the LAN and can therefore speed up the election of the single forwarder, reducing the number of duplicate IP multicast packets incurred.

Introduction In PIM-SM shared LAN networks, there is typically more than one upstream router. When duplicate data packets appear on the LAN, from different upstream routers, assert packets are sent from these routers to elect a single forwarder according to . The PIM assert messages are sent periodically to keep the assert state. The PIM assert message carries information about a single multicast source and group, along with the corresponding metric-preference and metric of the route towards the source or RP. This document defines a mechanism to encode the information of multiple PIM Assert messages into a single PackedAssert message. This allows to send and receive information for multiple IP multicast flows in a single PackedAssert message without changing the PIM Assert state machinery. It reduces the total number of PIM packets on the LAN and can therefore speed up the election of the single forwarder, reducing the number of duplicate IP multicast packets. This can particularly be helpful when there is traffic for a large number of multicast groups or SSM channels and PIM packet processing performance of the routers is slow.

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

Terminology The reader is expected to be familiar with the terminology of . The following lists the abbreviations repeated in this document. AT: Assert Timer RP: Rendezvous Point RPF: Reverse Path Forwarding SPT: Shortest Path Tree RPT: RP Tree DR: Designated Router

Problem statement PIM Assert occur in many deployments. See for some explicit examples. PIM assert state depends mainly on the network topology. As long as there is a layer 2 network with more than 2 PIM routers, there may be multiple upstream routers, which can cause duplicate multicast traffic to be forwarded and assert process to occur. As the multicast services become widely deployed, the number of multicast entries increases, and a large number of assert messages may be sent in a very short period when multicast data packets trigger PIM assert processing in the shared LAN networks. The PIM routers need to process a large number of PIM assert small packets in a very short time. As a result, the device load is very large. The assert packet may not be processed in time or even discarded, thus extending the time of traffic duplication in the network. The PIM Assert mechanism can only be avoided by designing the network to be without transit subnets with multiple upstream routers. For example, an L2 ring between routers can sometimes be reconfigured to be a ring of point-to-point subnets connected by the routers. These L2/L3 topology changes are undesirable though, when they are only done to enable IP multicast with PIM because they increase the cost of introducing IP multicast with PIM. These designs are also not feasible when specific L2 technologies are needed. For example various L2 technologies for rings provide sub 50msec failover mechanisms, something not possible equally with an L3 subnet based ring. Likewise, IEEE Time Sensitive Networking mechanisms would require an L2 topology that can not simply be replaced by an L3 topology. L2 sub-topologies can also significantly reduce the cost of deployment.

Specification This document defines three elements in support of PIM assert packing: The PIM Hello Assert Packing Option. The encoding of PackedAssert messages. How to send and receive PackedAssert messages.

PIM Hello Assert Packing Option The PIM Assert Packing Hello Option () is used to announce support for the assert packing mechanisms specified in this document. PackedAssert messages () MUST only be sent when all PIM routers in the same subnet announce this option.

Assert Packing Message Formats The message body of an {RFC7761}}, Section 4.9.6, PIM Assert message, except for its first four bytes of header, describes the parameters of a (*,G) or (S,G) assert through the following information elements: R(PT), Source Address, Group Address, Metric and Metric Preference. This document calls this information an assert record. Assert packing introduces two new PIM Assert message encodings through the allocation and use of two specified flags in the PIM Assert message header, the P)acked and the A)ggregated flag. If the P)acked flag is 0, the message is a (non-packed) PIM Assert message as specified in . See . In this case, the A)ggregated flag MUST be set to 0. If the P)acked flag is 1), then the message is called a PackedAssert message and the type and hence encoding format of the payload is determined by the A)ggregated flag. If A=0, then the message body is a sequence of assert records preceeded by a count. This is called a "Simple PackedAssert" message. See . If A=1, then the message body is a sequence of aggregated assert records preceeded by a count. This is called an "Aggregated PackedAssert". See . Two aggregated assert record types are specified. The "Source Aggregated Assert Record", see , encodes one (common) Source Address, Metric and Metric Preference as well as a list of one or more Group Addresses with a Count. Source Aggregated Assert Records provide a more compact encoding than the Simple PackedAssert message format when multiple (S,G) flows share the same source S. A single Source Aggregated Assert Record with n Group Addresses represents the information of assert records for (S,G1)...(S,Gn). The "RP Aggregated Assert Record", see , encodes one common Metric and Metric Preference as well as a list of "Group Records", each of which encodes a Group Address and a list of zero or more Source Addresses with a count. This is called an "RP Aggregated Assert Record", because with standard RPF according to (), all the Group Addresses that use the same RP will have the same Metric and Metric Preference. RP Aggregation Records provide a more compact encoding than the Simple PackedAssert message format for (*,G) flows. The Source Address option is optionally used by assert procedures to indicate the source(s) that triggered the assert, otherwise it is 0. Both Source Aggregated Assert Records and RP Aggregated Assert Records also include the R)PT flag which maintains its semantic from but also distinguishes the encodings. Source Aggregated Assert Records have R=0, as (S,G) assert records do in . RP Aggregated Assert Records have R=1, as (*,G) assert records do in .

PackedAssert Mechanism PackedAsserts do not change the PIM assert state machine specification. Instead, sending and receiving of PackedAssert messages as specified in the following subsections is logically a layer in between sending/receiving of Assert messages and serialization/deserialization of their respective packets. There is no change in in the information elements of the transmitted information elements constituting the assert records not their semantics. Only the compactness of their encoding.

Sending PackedAssert messages When using assert packing, the regular Assert message encoding with A=0 and P=0 is still allowed to be sent. Router are free to choose which PackedAssert message type they send. If a router chooses to send PacketAssert messages, then it MUST comply to the requirements in the remainder of this section. Implementations SHOULD NOT send only Asserts (but no PackedAsserts) in all cases when all routers on the LAN do support Assert Packing. Routers SHOULD specify in documentation and/or management interfaces (such as a YANG model), which PackedAssert message types they can send and under which conditions they do - such as for data-triggered asserts or Assert Timer (AT) expiry based asserts. To send a PackedAssert message, when a PIM router has an assert record ready to sent according to the pseudocode "send Assert(...)" in , it will not immediately proceed in generating a PIM Assert message from it. Instead, it will remember it for assert packing and proceed with PIM assert processing for other (S,G) and (*,G) flows that will result in further (S,G) or (*,G) assert records until one or more of the following conditions is met and only then send the PackedAssert message(s). RECOMMENDED: Further processing would cause additional delay for sending the PackedAssert message. OPTIONAL: Further processing would cause "relevant"(*) delay for sending the PackedAssert message. OPTIONAL: The router has packed a "sufficient"(*) number of assert records for a PackedAssert message. There is no further space left in a possible PackedAssert message or the implementation does not want to pack further assert records. (*) "relevant" and "sufficient" are defined in this section below. Avoiding additional relevant delay is most critical for asserts that are triggered by reception of data or reception of asserts against which this router is the assert winner, because it needs to send out an assert to the (potential) assert looser(s) as soon as possible to minimize the time in which duplicate IP multicast packets can occur. To avoid additional delay in this case, the router SHOULD employ appropriate assert packing and scheduling mechanisms, such as for example the following. Asserts/PackedAsserts in this case are scheduled for serialization with highest priority, such that they bypass any potentially earlier scheduled other packets. When there is no such Assert/PacketAssert message scheduled for or being serialized, the router immediately serializes an Assert or PackedAssert message without further assert packing. If there are one or more Assert/PackedAssert messages serialized and/or scheduled to be serialized, then the router can pack assert records into new PackedAssert messages until shortly before the last of those Assert/PackedAssert packets has finished serializing. Asserts triggered by expiry of the AT on an assert winner are not time-critical because they can be scheduled in advance and because the Assert_Override_Interval parameter of already creates a 3 second window in which such assert records can be sent, received, and processed before an assert losers state would expire and duplicate IP multicast packets could occur. An example mechanism to allows packing of AT expiry triggered assert records on assert winners is to round the AT to an appropriate granularity such as 100msec. This will cause AT for multiple (S,G) and/or (*,G) states to expire at the same time, thus allowing them to be easily packed without changes to the assert state machinery. AssertCancel messages have assert records with an infinite metric and can use assert packing as any other Assert. They are sent on Override Timer (OT) expiry and can be packed for example with the same considerations as AT expiry triggered assert records. Additional delay is not "relevant" when it still causes the overall amount of (possible) duplicate IP multicast packets to decrease in a condition with large number of (S,G) and/or (*,G), compared to the situation in which no delay is added by the implementation. This can simply be the case because the implementation can not afford to implement the (more advanced) mechanisms described above, and some simpler mechanism that does introduce some additional delay still causes more overall reduction in duplicate IP multicast packets than not sending PackedAsserts at all, but only Asserts. "Relevant" is a highly implementation dependent metric and can typically only be measured against routers of the same type as receivers, and performance results with other routers will likely differ. Therefore it is optional. When Asserts are sent, a single packet loss will result only in continued or new duplicates from a single IP multicast flow. Loss of (non AssertCancel) PackedAssert impacts duplicates for all flows packed into the PackedAssert and may result in the need for re-sending more than one Assert/PacketAssert, because of the possible inability to pack them. Routers SHOULD therefore support mechanisms allowing for PackedAsserts and Asserts to be sent with an appropriate DSCP, such as Expedited Forwarding (EF), to minimize their loss, especially when duplicate IP multicast packets could cause congestion and loss. Routers MAY support a configurable option for sending PackedAssert messages twice in short order (such as 50msec apart), to overcome possible loss, but only a) if the total size of the two PackedAsserts is less than the total size of equivalent Assert messages, and b) if the conditions that caused the assert records in the PackedAssert message make the router believe that reception of either copies of the PackedAssert message will not trigger sending of Assert/PackedAssert. It is "sufficient" that assert records are not packed up to MTU size, but to a size that allows the router to achieve the required operating scale of (S,G) and (*,G) flows with minimum duplicates. This packing size may be larger when the network is operating with the maximum number of supported multicast flows, and it can be a smaller packing size when operating with fewer multicast flows. Larger than "sufficient" packets may then not provide additional benefits, because they only reduce the performance requirements for packet sending and reception, and other performance limiting factors may take over once a "sufficient" size is reached. And larger packets can incur more duplicates on loss. Considering a "sufficient" amount of packing minimizes the negative impacts of loss of PackedAssert packets without loss of (minimum packet duplication) performance. Like "relevant", "sufficient" is highly implementation dependent and hence only optional.

Receiving PackedAssert messages Upon reception of a PackedAssert message, the PIM router logically converts its payload into a sequence of assert records that are then processed as if an equivalent sequence of Assert messages where received according to .

Packet Formats This section describes the format of new PIM messages introduced by this document.

PIM Assert Packing Hello Option

OptionType TBD: PIM Packed Assert Capability Hello Option Including the PIM OptionType TBD indicates support for the ability to receive and process all the PackedAssert encodings defined in this document.

Assert Message Format

When assert packing is used on a subnet, routers MUST send Assert messages according to above format. This is exactly the same format as the one defined in but the A and P flags are not reserved, but distinguish this Assert Message Format from those newly defined in this document. P)acked: MUST be 0. A)ggregated: MUST be 0. All other field according to , Section 4.9.6.

Simple PackedAssert Message Format

PIM Version, Type, Checksum, Reserved: As specified in Section 4.9.6 of . Reserved(2): Set to zero on transmission. Ignored upon receipt. P)acked: MUST be 1. A)ggregated: MUST be 0. Count The number of packed Assert Records in the message. The format of each Assert Record is the same as the PIM assert message body as specified in Section 4.9.6 of .

Aggregated PackedAssert Message Format

PIM Version, Type, Reserved, Checksum: As specified in Section 4.9.6 of . P)acked: MUST be 1. A)ggregated: MUST be 1. Count: The number of Aggregated Assert Records following in the message. Each of these records can either be a Source Aggregated or RP aggregated Assert Record.

Source Aggregated Assert Record

Reserved: Set to zero on transmission. Ignored upon receipt. R: MUST be 0. R indicates both that the encoding format of the record is that of a Source Aggregated Assert Record, but also that all assert records represented by the Source Aggregated Assert Record have R=0 and are therefore (S,G) assert records according to the definition of R in , Section 4.9.6. Source Address, Metric Preference, Metric: As specified in Section 4.9.6 of . Source Address MUST NOT be zero. Number of Groups: The number of Group Address fields. Group Address: As specified in Section 4.9.6 of .

RP Aggregated Assert Record An RP Aggregation Assert record aggregates (*,G) assert records with the same Metric Preference and Metric. Typically this is the case for all (*,G) using the same RP, but the encoding is not limited to only (*,G) using the same RP because the RP address is not encoded as it is also not present in assert records.

R: MUST be 1. R indicates both that the encoding format of the record is that of an RP Aggregated Assert Record, and that all assert records represented by the RP Aggregated Assert Record have R=1 and are therefore (*,G) assert records according to the definition of R in , Section 4.9.6. Metric Preference, Metric: As specified in Section 4.9.6 of . Reserved: Set to zero on transmission. Ignored upon receipt. Number of Group Records: The number of packed Group Records. A record consists of a Group Address and a Source Address list with a number of sources. The format of each Group Record is:

Group Address and Reserved: As specified in Section 4.9.6 of . Reserved: Set to zero on transmission. Ignored upon receipt. Number of Sources: The Number of Sources is corresponding to the number of Source Address fields in the Group Record. If this number is 0, the Group Record indicates one assert record with a Source Address of 0. If this number is not 0 and one of the (*,G) assert records to be encoded should have the Source Address 0, then 0 needs to be encoded as one of the Source Address fields. Source Address: As specified in Section 4.9.6 of . But there can be multiple Source Address fields in the Group Record.

Updates from RFC3973 and RFC7761 This document introduces two new flag bits to the PIM Assert message type, as defined in , the P)acked and A)aggregated bits. When both bits are zero, the packet is an otherwise encoding and semantically unchanged PIM Assert message. Requirements for the use of P and A flag with other values than zero are specified in this document only for use with . These requirements do not apply to solely because of lack of interest in the use of the mechanisms specified in this document together with . [RFC-Editor: pls. remove this sentence: RFC3973 should be historic by now, and it is only mentioned at all because of the fact that the "PIM Message Type Registry" mentions it, and this document mentions that registry. Depending on IETF/IESG review, the authors would prefer if we could remove all mentioning of RFC3973 in this document, because it may just confuse readers.]

IANA Considerations

IANA is requested to allocate a new code points from the "PIM-Hello Options" registry for TBD.

IANA is asked to add the definition of the Aggregated and Packed Flags Bits for the PIM Assert Message Type to the "PIM Message Types" registry according to IANA considerations, and as shown in .

Security Considerations The security considerations of apply to the extensions defined in this document. This document packs multiple assert records in a single message. As described in Section 6.1 of , a forged Assert message could cause the legitimate designated forwarder to stop forwarding traffic to the LAN. The effect may be amplified when using a PackedAssert message.

Acknowledgments The authors would like to thank: Stig Venaas for the valuable contributions of this document, Alvaro Retana for his thorough and constructive RTG AD review.

Working Group considerations [RFC-Editor: please remove this section].

Open Issues

Changelog This document is hosted starting with -06 on https://github.com/toerless/pim-assert-packing.

draft-ietf-pim-assert-packing-07 Included changes from review of Alvaro Retana (https://mailarchive.ietf.org/arch/msg/pim/Cp4o5glUFge2b84X9CQMwCWZjAk/) Please see the following emails discussing the changes: https://raw.githubusercontent.com/toerless/pim-assert-packing/main/emails/05-alvaro-review-reply.txt https://raw.githubusercontent.com/toerless/pim-assert-packing/main/emails/07-pim-wg-announce.txt

draft-ietf-pim-assert-packing-06 This version was converted from txt format into markdown for better editing later, but is otherwise text identical to -05. It was posted to DataTracker to make diffs easier. Functional changes:

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies Protocol Independent Multicast - Dense Mode (PIM-DM). PIM-DM is a multicast routing protocol that uses the underlying unicast routing information base to flood multicast datagrams to all multicast routers. Prune messages are used to prevent future messages from propagating to routers without group membership information. This memo defines an Experimental Protocol for the Internet community. This document specifies Protocol Independent Multicast - Sparse Mode (PIM-SM). PIM-SM is a multicast routing protocol that can use the underlying unicast routing information base or a separate multicast-capable routing information base. It builds unidirectional shared trees rooted at a Rendezvous Point (RP) per group, and it optionally creates shortest-path trees per source.This document obsoletes RFC 4601 by replacing it, addresses the errata filed against it, removes the optional (*,*,RP), PIM Multicast Border Router features and authentication using IPsec that lack sufficient deployment experience (see Appendix A), and moves the PIM specification to Internet Standard. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. The PIM version 2 messages share a common message header format. The common header definition contains eight reserved bits. This document specifies how these bits may be used by individual message types and creates a registry containing the per-message-type usage. This document also extends the PIM type space by defining three new message types. For each of the new types, four of the previously reserved bits are used to form an extended type range.This document updates RFCs 7761 and 3973 by defining the use of the currently Reserved field in the PIM common header. This document further updates RFCs 7761 and 3973, along with RFCs 5015, 5059, 6754, and 8364, by specifying the use of the currently reserved bits for each PIM message.This document obsoletes RFC 6166. This document describes the MVPN (Multicast in BGP/MPLS IP VPNs) solution designed and deployed by Cisco Systems. The procedures specified in this document are largely a subset of the generalized MVPN framework recently standardized by the IETF. However, as the deployment of the procedures specified herein predates the publication of IETF standards (in some cases by over five years), an implementation based on these procedures differs in some respects from a fully standards-compliant implementation. These differences are pointed out in the document. This document defines a Historic Document for the Internet community.

Use case examples

Enterprise network When an Enterprise network is connected through a layer-2 network, the intra-enterprise runs layer-3 PIM multicast. The different sites of the enterprise are equivalent to the PIM connection through the shared LAN network. Depending upon the locations and amount of groups there could be many asserts on the first-hop routers.

Video surveillance Video surveillance deployments have migrated from analog based systems to IP-based systems oftentimes using multicast. In the shared LAN network deployments, when there are many cameras streaming to many groups there may be issues with many asserts on first-hop routers.

Financial Services Financial services extensively rely on IP Multicast to deliver stock market data and its derivatives, and current multicast solution PIM is usually deployed. As the number of multicast flows grow, there are many stock data with many groups may result in many PIM asserts on a shared LAN network from publisher to the subscribers.

IPTV broadcast Video PIM DR deployments are often used in host-side network for IPTV broadcast video services. Host-side access network failure scenario may be benefitted by assert packing when many groups are being used. According to the DR will be elected to forward multicast traffic in the shared access network. When the DR recovers from a failure, the original DR starts to send traffic, and the current DR is still forwarding traffic. In the situation multicast traffic duplication maybe happen in the shared access network and can trigger the assert progress.

MVPN MDT As described in , MDT (Multicast Distribution Tree) is used as tunnels for MVPN. The configuration of multicast-enabled VRF (VPN routing and forwarding) or interface that is in a VRF changing may cause many assert packets to be sent in a same time.

Special L2 services Additionally, future backhaul, or fronthaul, networks may want to connect L3 across an L2 underlay supporting Time Sensitive Networks (TSN). The infrastructure may run DetNet over TSN. These transit L2 LANs would have multiple upstreams and downstreams. This document is taking a proactive approach to prevention of possible future assert issues in these types of environments.