PIM协议翻译及深入理解分析（rfc2362第二章）

PIM-SM Protocol Overview

A router receives explicit Join/Prune messages from those neighboring  routers that have downstream group members. The router then forwards  data packets addressed to a multicast group, G, only onto those  interfaces on which explicit joins have been received. Note that all  routers mentioned in this document are assumed to be PIM-SM capable,  unless otherwise specified.文件中提及的路由器被假定具有PIM-SM能力（除非另有指定） 一个路由器收到明确的加入剪出消息（来自上行组成员的邻居），路由器转发数据包的地址给多播组，G，在这些（准确收到明确的加入消息的）接口上。    A Designated Router (DR) sends periodic Join/Prune messages toward a    group-specific Rendezvous Point (RP) for each group for which it has    active members. Each router along the path toward the RP builds a    wildcard (any-source) state for the group and sends Join/Prune    messages on toward the RP. We use the term route entry to refer to    the state maintained in a router to represent the distribution tree.    A route entry may include such fields as the source address, the    group address, the incoming interface from which packets are    accepted, the list of outgoing interfaces to which packets are sent,    timers, flag bits, etc. The wildcard route entry's incoming interface    points toward the RP; the outgoing interfaces point to the    neighboring downstream routers that have sent Join/Prune messages    toward the RP. This state creates a shared, RP-centered, distribution    tree that reaches all group members.  对于每一个组，DR周期发送加入/剪出消息朝向一个指定组RP，每个路由器沿着朝向RP的路径，为这个组建立一个通配符（*，G）状态并且发送加入剪出消息。 我们使用路由条目指定路由器维持状态来建立分布树。路由条目包括：源地址，组地址，包被接受的输入接口，包被发送的输出接口列表， 计时器，flag位等等。这个通配路由条目输入接口指向RP，输出接口指向相邻的下行路由器（朝着RP方向发送加入/剪出消息）。 这个状态创建了一个共享的，以RP为中心，可以到达所有组成员分布树。 When a data source first sends  to a group, its DR unicasts Register messages to the RP with the  source's data packets encapsulated within. If the data rate is high,  the RP can send source-specific Join/Prune messages back towards the  source and the source's data packets will follow the resulting  forwarding state and travel unencapsulated to the RP.  Whether they  arrive encapsulated or natively, the RP forwards the source's  decapsulated data packets down the RP-centered distribution tree  toward group members.  If the data rate warrants it, routers with  local receivers can join a source-specific, shortest path,  distribution tree, and prune this source's packets off of the shared  RP-centered tree. For low data rate sources, neither the RP, nor  last-hop routers need join a source-specific shortest path tree and  data packets can be delivered via the shared, RP-tree. 当一个数据源第一次发送到一个组上时，DR以单播方式将注册消息送达RP（伴随着源数据包的封包）。 如果数据速率高，RP可以发送指定源（S,G）指定的加入剪出消息返回给组播源，源的数据包按照所得到的转发状态并解包给RP。无论他们是否封装到达，RP都将解包到RP分布树上。

如果数据速率许可，本地接收者路由器可以加入一个指定源，最短路径，分布树，从RP树上剪出这个源的包

对低速率的源，RP和最后一跳路由都不需要加入最短路径树，包被传递给共享的RP树。    2.1 Local hosts joining a group本地主机加入一个组    In order to join a multicast group, G, a host conveys its membership    information through the Internet Group Management Protocol (IGMP), as    specified in [4][5], (see figure 1). From this point on we refer to    such a host as a receiver, R, (or member) of the group G.    Note that all figures used in this section are for illustration and    are not intended to be complete. For complete and detailed protocol    action see Section 3.

主机通过IGMP加入一个组播组。主机就可以作为一个组G的成员（接收者）    When a DR (e.g., router A in figure 1) gets a membership indication    from IGMP for a new group, G, the DR looks up the associated RP. The    DR creates a wildcard multicast route entry for the group, referred    to here as a (*,G) entry; if there is no more specific match for a    particular source, the packet will be forwarded according to this    entry. DR通过IGMP得到一个成员，DR查找相关RP，DR为这个组创建一个

组播路由条目（*，G），如果对于一个特殊源不再有指定的匹配，包就通过这个条目俩转发。    The RP address is included in a special field in the route entry and    is included in periodic upstream Join/Prune messages. The outgoing    interface is set to that included in the IGMP membership indication    for the new member. The incoming interface is set to the interface    used to send unicast packets to the RP.    When there are no longer directly connected members for the group,    IGMP notifies the DR.  If the DR has neither local members nor    downstream receivers, the (*,G) state is deleted. RP地址即包括在路由条目中，也包括在周期的上行加入剪出消息中。

输出接口：IGMP新的成员；输出接口：发送单播包给RP 当没有与组直接相连的成员时，IGMP告知DR。如果DR即没有本地成员也没有下行接收者，（*，G）状态将被删除。 2.2 Establishing the RP-rooted shared tree建立以RP为根的共享树    Triggered by the (*,G) state, the DR creates a Join/Prune message    with the RP address in its join list and the the wildcard bit (WC-    bit) and RP-tree bit (RPT-bit) set to 1. The WC-bit indicates that    any source may match and be forwarded according to this entry if    there is no longer match; the RPT-bit indicates that this join is    being sent up the shared, RP-tree. The prune list is left empty. When    the RPT-bit is set to 1 it indicates that the join is associated with    the shared RP-tree and therefore the Join/Prune message is propagated传播    along the RP-tree. When the WC-bit is set to 1 it indicates that the    address is an RP and the downstream receivers expect to receive    packets from all sources via this (shared tree) path. The term RPT-    bit is used to refer to both the RPT-bit flags associated with route    entries, and the RPT-bit included in each encoded address in a    Join/Prune message. 由（*，G）状态触发，DR创建一个加入剪出消息，RP地址本身加入列表，WC-bit和RPT-bit置1。 WC-bit表示任何源都可以匹配,根据条目被转发（如果无匹配了），RPT-bit表示这个加入被发送到共享树上。 剪出列表剩余为空。 the RPT-bit is set to 1消息沿着RP树被传播。 WC-bit is set to 1这个地址就是一个RP，下行接收者    通过这个路径（RP树路径）   期望收到     来自所有源的包 。     RPT-bit被用来 RPT-bit flags 关联路由条目，RPT-bit也包括在加入剪出消息的编码地址中。    Each upstream router creates or updates its multicast route entry for    (*,G) when it receives a Join/Prune with the RPT-bit and WC-bit set.    The interface on which the Join/Prune message arrived is added to the    list of outgoing interfaces (oifs) for (*,G). Based on this entry    each upstream router between the receiver and the RP sends a    Join/Prune message in which the join list includes the RP. The packet    payload contains Multicast-Address=G, Join=RP,WC-bit,RPT-bit,    Prune=NULL. 收到带有带有RPT-bit and WC-bit加入剪出消息之后，每一个上行创建或者更新

多播路由条目。

这个接口（消息到达）被加入到输出列表中(即（*,G）)， 基于这个条目，每个上行路由器在接收者与RP之间都会发送一个消息，加入列表就包括RP。 该包  包括 G，RP，WC-bit，RPT-bit，Prune=NULL。 2.3 Hosts sending to a group主机发送到一个组

   When a host starts sending multicast data packets to a group,    initially its DR must deliver each packet to the RP for distribution    down the RP-tree (see figure 2).  The sender's DR initially    encapsulates each data packet in a Register message and unicasts it    to the RP for that group. The RP decapsulates each Register message    and forwards the enclosed data packet natively to downstream members    on the shared RP-tree.

当主机开始发送一个多播数据包到一个组，首先DR必须传递每个包到RP。

发送者DR初始在注册消息中封装每一个数据包，单播给那个组的RP。这个RP解包并且

在RP树上转发这个封闭的包到下行成员。    If the data rate of the source warrants the use of a source-specific    shortest path tree (SPT), the RP may construct a new multicast route    entry that is specific to the source, hereafter referred to as (S,G)    state, and send periodic Join/Prune messages toward the source. Note    that over time, the rules for when to switch can be modified without    global coordination.  When and if the RP does switch to the SPT, the    routers between the source and the RP build and maintain (S,G) state    in response to these messages and send (S,G) messages upstream toward    the source. 源数据的速率允许使用SPT，RP针对源构建一个新的多播路由条目，（S,G）状态

周期性朝向源发送加入剪出消息。如果超时，没有全局协调，规则在要转变的时候就被修改。当如果RP改变到SPT，这路由器就会在源和（RP建立和维持（S,G）状态）之间响应这些消息和发送朝向上行的源的这些消息（S,G）。      The source's DR must stop encapsulating data packets in Registers    when (and so long as) it receives Register-Stop messages from the RP.    The RP triggers Register-Stop messages in response to Registers, if    the RP has no downstream receivers for the group (or for that    particular source), or if the RP has already joined the (S,G) tree    and is receiving the data packets natively.  Each source's DR    maintains, per (S,G), a Register-Suppression-timer.  The Register-    Suppression-timer is started by the Register-Stop message; upon    expiration, the source's DR resumes sending data packets to the RP,    encapsulated in Register messages. 当收到来自DR的注册停止消息源的DR必须停止在注册消息中封包，RP触发注册停止消息响应注册者，如果RP没有这个组上行接收者，或者RP已经加入（S,G）树，自然就接收到数据包。每个源DR的维持，每个（S,G），一个注册抑制定时器。注册抑制定时器由注册停止消息开启，一旦到期，源DR恢复向RP发送数据包（封装在注册消息中）。 2.4 Switching from shared tree (RP-tree)  to  shortest  path  tree    (SP-tree)}来自共享树转变为最短路径树（SPT：shortest path tree）    A router with directly-connected members first joins the shared RP-    tree.  The router can switch to a source's shortest path tree (SP-    tree) after receiving packets from that source over the shared RP-    tree. The recommended policy is to initiate the switch to the SP-tree    after receiving a significant number of data packets during a    specified time interval from a particular source. To realize this    policy the router can monitor data packets from sources for which it    has no source-specific multicast route entry and initiate such an    entry when the data rate exceeds the configured threshold.  As shown    in figure 3, router `A' initiates a (S,G) state. 一个直接连接成员的路由器第一次加入RP树，当通过共享树接收到来自源的包之后，

这个路由器可以转为一个源的SPT。在一个指定的时间间隔中，收到来自源的一些重要的数据包之后，就可以建议开始转变为SPT。实现这个策略，路由器可以监控来自源的数据包（没有特殊源的组播路由器条目），启用一个条目当数据速率超过配置的阀值。    When a (S,G) entry is activated (and periodically so long as the    state exists), a Join/Prune message is sent upstream towards the    source, S, with S in the join list. The payload contains Multicast-    Address=G, Join=S, Prune=NULL. When the (S,G) entry is created, the    outgoing interface list is copied from (*,G), i.e., all local shared    tree branches are replicated in the new shortest path tree. In this    way when a data packet from S arrives and matches on this entry, all    receivers will continue to receive the source's packets along this    path. (In more complicated scenarios, other entries in the router    have to be considered, as described in Section 3). Note that (S,G)    state must be maintained in each last-hop router that is responsible    for initiating and maintaining an SP-tree. Even when (*,G) and (S,G)    overlap, both states are needed to trigger the source-specific    Join/Prune messages.  (S,G) state is kept alive by data packets    arriving from that source. A timer, Entry-timer, is set for the (S,G) 当一个条目被激活（维持一个状态），一个加入剪出消息被发送到上行朝着源的方向，

S在加入列表中，    entry and this timer is restarted whenever data packets for (S,G) are    forwarded out at least one oif, or Registers are sent.  When the    Entry-timer expires, the state is deleted. The last-hop router is the    router that delivers the packets to their ultimate end-system    destination.  This is the router that monitors if there is group    membership and joins or prunes the appropriate distribution trees in    response.  In general the last-hop router is the Designated Router    (DR) for the LAN. However, under various conditions described later,    a parallel router connected to the same LAN may take over as the    last-hop router in place of the DR.    Only the RP and routers with local members can initiate switching to    the SP-tree; intermediate routers do not. Consequently, last-hop    routers create (S,G) state in response to data packets from the    source, S; whereas intermediate routers only create (S,G) state in    response to Join/Prune messages from downstream that have S in the    Join list.    The (S,G) entry is initialized with the SPT-bit cleared, indicating    that the shortest path tree branch from S has not yet been setup    completely, and the router can still accept packets from S that    arrive on the (*,G) entry's indicated incoming interface (iif). Each    PIM multicast entry has an associated incoming interface on which    packets are expected to arrive.    When a router with a (S,G) entry and a cleared SPT-bit starts to    receive packets from the new source S on the iif for the (S,G) entry,    and that iif differs from the (*,G) entry's iif, the router sets the    SPT-bit, and sends a Join/Prune message towards the RP, indicating    that the router no longer wants to receive packets from S via the    shared RP-tree. The Join/Prune message sent towards the RP includes S    in the prune list, with the RPT-bit set indicating that S's packets    must not be forwarded down this branch of the shared tree. If the    router receiving the Join/Prune message has (S,G) state (with or    without the route entry's RPT-bit flag set), it deletes the arriving    interface from the (S,G) oif list.  If the router has only (*,G)    state, it creates an entry with the RPT-bit flag set to 1. For    brevity we refer to an (S,G) entry that has the RPT-bit flag set to 1    as an (S,G)RPT-bit entry. This notational distinction is useful to    point out the different actions taken for (S,G) entries depending on    the setting of the RPT-bit flag. Note that a router can have no more    than one active (S,G) entry for any particular S and G, at any    particular time; whether the RPT-bit flag is set or not. In other    words, a router never has both an (S,G) and an (S,G)RPT-bit entry for    the same S and G at the same time. The Join/Prune message payload    contains Multicast-Address=G, Join=NULL, Prune=S,RPT-bit.    A new receiver may join an existing RP-tree on which source-specific    prune state has been established (e.g., because downstream receivers    have switched to SP-trees). In this case the prune state must be    eradicated upstream of the new receiver to bring all sources' data    packets down to the new receiver. Therefore, when a (*,G) Join    arrives at a router that has any (Si,G)RPT-bit entries (i.e., entries    that cause the router to send source-specific prunes toward the RP),    these entries must be updated upstream of the router so as to bring    all sources' packets down to the new member. To accomplish this, each    router that receives a (*,G) Join/Prune message updates all existing    (S,G)RPT-bit entries. The router may also trigger a (*,G) Join/Prune    message upstream to cause the same updating of RPT-bit settings    upstream and pull down all active sources' packets. If the arriving    (*,G) join has some sources included in its prune list, then the    corresponding (S,G)RPT-bit entries are left unchanged (i.e., the    RPT-bit remains set and no oif is added). 2.5 Steady state maintenance of distribution tree (i.e., router state)}    In the steady state each router sends periodic Join/Prune messages    for each active PIM route entry; the Join/Prune messages are sent to    the neighbor indicated in the corresponding entry. These messages are    sent periodically to capture state, topology, and membership changes.    A Join/Prune message is also sent on an event-triggered basis each    time a new route entry is established for some new source (note that    some damping function may be applied, e.g., a short delay to allow    for merging of new Join information). Join/Prune messages do not    elicit any form of explicit acknowledgment; routers recover from lost    packets using the periodic refresh mechanism. 2.6 Obtaining RP information获得    To obtain the RP information, all routers within a PIM domain collect    Bootstrap messages. Bootstrap messages are sent hop-by-hop within the    domain; the domain's bootstrap router (BSR) is responsible for    originating the Bootstrap messages. Bootstrap messages are used to    carry out a dynamic BSR election when needed and to distribute RP    information in steady state.    A domain in this context is a contiguous set of routers that all    implement PIM and are configured to operate within a common boundary    defined by PIM Multicast Border Routers (PMBRs). PMBRs connect each    PIM domain to the rest of the internet.    Routers use a set of available RPs (called the RP-Set) distributed in    Bootstrap messages to get the proper Group to RP mapping. The    following paragraphs summarize the mechanism; details of the    mechanism may be found in Sections 3.6 and Appendix 6.2. A (small)    set of routers, within a domain, are configured as candidate BSRs    and, through a simple election mechanism, a single BSR is selected    for that domain. A set of routers within a domain are also configured    as candidate RPs (C-RPs); typically these will be the same routers    that are configured as C-BSRs.  Candidate RPs periodically unicast    Candidate-RP-Advertisement messages (C-RP-Advs) to the BSR of that    domain. C-RP-Advs include the address of the advertising C-RP, as    well as an optional group address and a mask length field, indicating    the group prefix(es) for which the candidacy is advertised. The BSR    then includes a set of these Candidate-RPs (the RP-Set), along with    the corresponding group prefixes, in Bootstrap messages it    periodically originates.  Bootstrap messages are distributed hop-by-    hop throughout the domain.    Routers receive and store Bootstrap messages originated by the BSR.    When a DR gets a membership indication from IGMP for (or a data    packet from) a directly connected host, for a group for which it has    no entry, the DR uses a hash function to map the group address to one    of the C-RPs whose Group-prefix includes the group (see Section 3.7).    The DR then sends a Join/Prune message towards (or unicasts Registers    to) that RP.    The Bootstrap message indicates liveness of the RPs included therein.    If an RP is included in the message, then it is tagged as `up' at the    routers; while RPs not included in the message are removed from the    list of RPs over which the hash algorithm acts. Each router continues    to use the contents of the most recently received Bootstrap message    until it receives a new Bootstrap message.    If a PIM domain partitions, each area separated from the old BSR will    elect its own BSR, which will distribute an RP-Set containing RPs    that are reachable within that partition. When the partition heals,    another election will occur automatically and only one of the BSRs    will continue to send out Bootstrap messages. As is expected at the    time of a partition or healing, some disruption in packet delivery    may occur. This time will be on the order of the region's round-trip    time and the bootstrap router timeout value. 2.7 Interoperation with dense mode  protocols such as DVMRP    In order to interoperate with networks that run dense-mode, broadcast    and prune, protocols, such as DVMRP, all packets generated within a    PIM-SM region must be pulled out to that region's PIM Multicast    Border Routers (PMBRs) and injected (i.e., broadcast) into the DVMRP    network. A PMBR is a router that sits at the boundary of a PIM-SM    domain and interoperates with other types of multicast routers such    as those that run DVMRP.  Generally a PMBR would speak both protocols    and implement interoperability functions not required by regular PIM    routers. To support interoperability, a special entry type, referred    to as (*,*,RP), must be supported by all PIM routers.  For this    reason we include details about (*,*,RP) entry handling in this    general PIM specification.    A data packet will match on a (*,*,RP) entry if there is no more    specific entry (such as (S,G) or (*,G)) and the destination group    address in the packet maps to the RP listed in the (*,*,RP) entry. In    this sense, a (*,*,RP) entry represents an aggregation of all the    groups that hash to that RP. PMBRs initialize (*,*,RP) state for each    RP in the domain's RPset. The (*,*,RP) state causes the PMBRs to send    (*,*,RP) Join/Prune messages toward each of the active RPs in the    domain.  As a result distribution trees are built that carry all data    packets originated within the PIM domain (and sent to the RPs) down    to the PMBRs.    PMBRs are also responsible for delivering externally-generated    packets to routers within the PIM domain. To do so, PMBRs initially    encapsulate externally-originated packets (i.e., received on DVMRP    interfaces) in Register messages and unicast them to the    corresponding RP within the PIM domain. The Register message has a    bit indicating that it was originated by a border router and the RP    caches the originating PMBR's address in the route entry so that    duplicate Registers from other PMBRs can be declined with a    Register-Stop message.    All PIM routers must be capable of supporting (*,*,RP) state and    interpreting associated Join/Prune messages. We describe the handling    of (*,*,RP) entries and messages throughout this document; however,    detailed PIM Multicast Border Router (PMBR) functions will be    specified in a separate interoperability document (see directory,    http://catarina.usc.edu/pim/interop/). 2.8 Multicast data packet processing    Data packets are processed in a manner similar to other multicast    schemes.  A router first performs a longest match on the source and    group address in the data packet. A (S,G) entry is matched first if    one exists; a (*,G) entry is matched otherwise. If neither state    exists, then a (*,*,RP) entry match is attempted as follows: the    router hashes on G to identify the RP for group G, and looks for a    (*,*,RP) entry that has this RP address associated with it. If none    of the above exists, then the packet is dropped. If a state is    matched, the router compares the interface on which the packet    arrived to the incoming interface field in the matched route entry.    If the iif check fails the packet is dropped, otherwise the packet is    forwarded to all interfaces listed in the outgoing interface list.    Some special actions are needed to deliver packets continuously while    switching from the shared to shortest-path tree. In particular, when    a (S,G) entry is matched, incoming packets are forwarded as follows:       1 If the SPT-bit is set, then:            1 if the incoming interface is the same as a matching              (S,G) iif, the packet is forwarded to the oif-list of              (S,G).            2 if the incoming interface is different than a matching              (S,G) iif , the packet is discarded.       2 If the SPT-bit is cleared, then:           1 if the incoming interface is the same as a matching              (S,G) iif, the packet is forwarded to the oif-list of              (S,G). In addition, the SPT bit is set for that entry if              the incoming interface differs from the incoming interface              of the (*,G) or (*,*,RP) entry.            2 if the incoming interface is different than a matching              (S,G) iif, the incoming interface is tested against a              matching (*,G) or (*,*,RP) entry. If the iif is the same as              one of those, the packet is forwarded to the oif-list of              the matching entry.            3 Otherwise the iif does not match any entry for G and              the packet is discarded.    Data packets never trigger prunes.  However, data packets may trigger    actions that in turn trigger prunes. For example, when router B in    figure 3 decides to switch to SP-tree at step 3, it creates a (S,G)    entry with SPT-bit set to 0. When data packets from S arrive at    interface 2 of B, B sets the SPT-bit to 1 since the iif for (*,G) is    different than that for (S,G). This triggers the sending of prunes    towards the RP. 2.9 Operation over Multi-access Networks    This section describes a few additional protocol mechanisms needed to    operate PIM over multi-access networks: Designated Router election,    Assert messages to resolve parallel paths, and the Join/Prune-    Suppression-Timer to suppress redundant Joins on multi-access    networks.    Designated router election:    When there are multiple routers connected to a multi-access network,    one of them must be chosen to operate as the designated router (DR)    at any point in time.  The DR is responsible for sending triggered    Join/Prune and Register messages toward the RP.    A simple designated router (DR) election mechanism is used for both    SM and traditional IP multicast routing.  Neighboring routers send    Hello messages to each other. The sender with the largest network    layer address assumes the role of DR. Each router connected to the    multi-access LAN sends the Hellos periodically in order to adapt to    changes in router status.    Parallel paths to a source or the RP--Assert process:    If a router receives a multicast datagram on a multi-access LAN from    a source whose corresponding (S,G) outgoing interface list includes    the interface to that LAN, the packet must be a duplicate.  In this    case a single forwarder must be elected.  Using Assert messages    addressed to `224.0.0.13' (ALL-PIM-ROUTERS group) on the LAN,    upstream routers can resolve which one will act as the forwarder.    Downstream routers listen to the Asserts so they know which one was    elected, and therefore where to send subsequent Joins. Typically this    is the same as the downstream router's RPF (Reverse Path Forwarding)    neighbor; but there are circumstances where this might not be the    case, e.g., when using multiple unicast routing protocols on that    LAN. The RPF neighbor for a particular source (or RP) is the next-hop    router to which packets are forwarded en route to that source (or    RP); and therefore is considered a good path via which to accept    packets from that source.    The upstream router elected is the one that has the shortest distance    to the source. Therefore, when a packet is received on an outgoing    interface a router sends an Assert message on the multi-access LAN    indicating what metric it uses to reach the source of the data    packet. The router with the smallest numerical metric (with ties    broken by highest address) will become the forwarder. All other    upstream routers will delete the interface from their outgoing    interface list. The downstream routers also do the comparison in case    the forwarder is different than the RPF neighbor.    Associated with the metric is a metric preference value. This is    provided to deal with the case where the upstream routers may run    different unicast routing protocols. The numerically smaller metric    preference is always preferred. The metric preference is treated as    the high-order part of an assert metric comparison.  Therefore, a    metric value can be compared with another metric value provided both    metric preferences are the same.  A metric preference can be assigned    per unicast routing protocol and needs to be consistent for all    routers on the multi-access network.    Asserts are also needed for (*,G) entries since an RP-Tree and an    SP-Tree for the same group may both cross the same multi-access    network. When an assert is sent for a (*,G) entry, the first bit in    the metric preference (RPT-bit) is always set to 1 to indicate that    this path corresponds to the RP tree, and that the match must be done    on (*,G) if it exists. Furthermore, the RPT-bit is always cleared for    metric preferences that refer to SP-tree entries; this causes an SP-    tree path to always look better than an RP-tree path. When the SP-    tree and RPtree cross the same LAN, this mechanism eliminates the    duplicates that would otherwise be carried over the LAN.    In case the packet, or the Assert message, matches on oif for    (*,*,RP) entry, a (*,G) entry is created, and asserts take place as    if the matching state were (*,G).    The DR may lose the (*,G) Assert process to another router on the LAN    if there are multiple paths to the RP through the LAN.  From then on,    the DR is no longer the last-hop router for local receivers and    removes the LAN from its (*,G) oif list. The winning router becomes    the last-hop router and is responsible for sending (*,G) join    messages to the RP.    Join/Prune suppression:    Join/Prune suppression may be used on multi-access LANs to reduce    duplicate control message overhead; it is not required for correct    performance of the protocol. If a Join/Prune message arrives and    matches on the incoming interface for an existing (S,G), (*,G), or    (*,*,RP) route entry, and the Holdtime included in the Join/Prune    message is greater than the recipient's own [Join/Prune-Holdtime]    (with ties resolved in favor of the higher network layer address), a    timer (the Join/Prune-Suppression-timer) in the recipient's route    entry may be started to suppress further Join/Prune messages. After    this timer expires, the recipient triggers a Join/Prune message, and    resumes sending periodic Join/Prunes, for this entry. The    Join/Prune-Suppression-timer should be restarted each time a    Join/Prune message is received with a higher Holdtime. 2.10 Unicast Routing Changes    When unicast routing changes, an RPF check is done on all active    (S,G), (*,G) and (*,*,RP) entries, and all affected expected incoming    interfaces are updated.  In particular, if the new incoming interface    appears in the outgoing interface list, it is deleted from the    outgoing interface list. The previous incoming interface may be added    to the outgoing interface list by a subsequent Join/Prune from    downstream.  Join/Prune messages received on the current incoming    interface are ignored.  Join/Prune messages received on new    interfaces or existing outgoing interfaces are not ignored. Other    outgoing interfaces are left as is until they are explicitly pruned    by downstream routers or are timed out due to lack of appropriate    Join/Prune messages. If the router has a (S,G) entry with the SPT-bit    set, and the updated iif(S,G) does not differ from iif(*,G) or    iif(*,*,RP), then the router resets the SPT-bit.    The router must send a Join/Prune message with S in the Join list out    any new incoming interfaces to inform upstream routers that it    expects multicast datagrams over the interface.  It may also send a    Join/Prune message with S in the Prune list out the old incoming    interface, if the link is operational, to inform upstream routers    that this part of the distribution tree is going away. 2.11 PIM-SM for Inter-Domain Multicast    Future documents will address the use of PIM-SM as a backbone inter-    domain multicast routing protocol. Design choices center primarily    around the distribution and usage of RP information for wide area,    inter-domain groups. 2.12 Security    All PIM control messages may use IPsec [6] to address security    concerns.  Security mechanisms are likely to be enhanced in the near    future.