Submitted to MPLS Working Group D. Ooms INTERNET DRAFT W. Livens <draft-ooms-mpls-multicast-00.txt> B. Sales M. Ramalho Alcatel August, 1998 Expires February, 1999 Framework for IP Multicast in MPLS Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Distribution of this memo is unlimited. Abstract This document offers a framework for IP multicast deployment in an MPLS environment. Issues arising when MPLS techniques are applied to IP multicast are overviewed. The pros and cons of existing IP multicast routing protocols in the context of MPLS are described and the relation to the different trigger methods and LDP modes are discussed. Focus is on ATM as a L2 technology. Table of Contents 1. Introduction 2. MPLS and IP multicast: a winner combination 3. ATM as layer 2 4. Taxonomy of IP multicast routing protocols in the context of MPLS 4.1. Flood & Prune Ooms, et al. Expires February 1999 [Page 1]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 4.2. Source/Shared trees 4.3. Uni/Bi-directional Shared Trees 4.4. Loop-free-ness 4.5. RPF Check 4.6. Mapping of characteristics on existing protocols 5. Taxonomy of IP multicast LSP triggers 5.1. Request driven 5.1.1. General 5.1.2. Multicast routing messages 5.1.3. Resource reservation messages 5.2. Topology driven 5.3. Traffic driven 5.3.1. General 5.3.2. An implementation example 5.4. Combinations of triggers and LDP modes 6. Mixed L2/L3 forwarding in a single node 7. Piggy-backing 8. Explicit routing 9. QoS/CoS 9.1 DiffServ 9.2 IntServ and RSVP 10. More issues 10.1. TTL field 10.2. Shared media 10.3. Local control vs. egress control 10.4. Conservative vs. optimistic 10.5. Conservative vs. liberal 10.6. Scalability 11. Security Considerations 12. Acknowledgements Table of Abbreviations ATM Asynchronous Transfer Node CBT Core Based Tree CoS Class of Service DVMRP Distant Vector Multicast Routing Protocol IGMP Internet Group Management Protocol IP Internet Protocol L2 layer 2 (e.g. ATM) L3 layer 3 (e.g. IP) LSP Label Switched Path LSR Label Switching Router LSRd Downstream LSR LSRu Upstream LSR Ooms, et al. Expires February 1999 [Page 2]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 MIP Multicast Internet Protocol MOSPF Multicast OSPF mp2mp multipoint-to-multipoint p2mp point-to-multipoint PIM-DM Protocol Independent Multicast-Dense Mode PIM-SM Protocol Independent Multicast-Sparse Mode QoS Quality of Service RPF Reverse Path Forwarding RSVP Resource reSerVation Protocol TCP Transmission Control Protocol UDP User Datagram Protocol VC Virtual Circuit VCI Virtual Circuit Identifier VP Virtual Path VPI Virtual Path Identifier 1. Introduction In an MPLS cloud the routes are determined by a L3 routing protocol. These routes can then be mapped onto L2 paths to enhance network performance and to create a vehicle for enhanced network services (QoS/CoS, traffic engineering,...). Current unicast routing protocols generate a same (optimal) shortest path in steady state for a certain (source, destination)-pair. Remark that unicast protocols can behave slightly different with regard to equal cost paths. For multicast, the optimal solution would impose a Steiner tree computation. Unfortunately, no multicast routing protocol today is able to maintain such an optimal tree. Different multicast protocols will therefore, in general, generate different trees. The discussion is focused on intra-domain multicast routing protocols. Aspects of inter-domain routing are beyond the scope of this document. 2. MPLS and IP multicast: a winner combination Besides the better utilization of expensive L3 resources, multicast LSPs have even more benefits than unicast LSPs. First, multicast traffic flows are often those long-duration high-bandwidth flows that are prime candidate to be label switched (e.g. video streams). Next, the detection of these flows can be straightforward, as multicast flows are often setup using explicit routing messages (e.g. the receiver triggered Join messages in PIM-SM), which can be easily intercepted. Finally, IP multicast uses UDP, which does not have the Ooms, et al. Expires February 1999 [Page 3]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 congestion-avoiding behavior of TCP. A large scale deployment of multicast may therefore push aside regular TCP traffic, deteriorating the latter's performance. Label switching this multicast UDP traffic will therefore result in a better performance for non-label-switched TCP-based applications. 3. ATM as layer 2 Although MPLS is multiprotocol both at L3 and at L2, IP and ATM are the main L3 and L2 technologies. ATM offers big pipes, high switching capacities and QoS awareness, but in the context of MPLS it poses several limitations [DAVI]: - Limited ATM resources (VPI/VCI space): current ATM switches have a limited range of VPI/VCIs, limiting the number of LSPs that can be established. - No VC merging: a majority of current ATM switches does not support VC merging, it is an active area of research, not only in the context of MPLS, but also in the traditional ATM world. - No 'TTL-decrement' function in ATM. All three limitations will impact the implementation of multicast in MPLS as will be described in this document. 4. Taxonomy of IP multicast routing protocols in the context of MPLS At the moment, an abundance of IP multicast routing protocols is being proposed and developed. All these protocols have different characteristics (scalability, computational complexity, latency, control message overhead, tree type, etc...). It is not the purpose of this document to give a complete taxonomy of IP multicast routing protocols, only their characteristics relevant to the MPLS technology will be addressed. Following characteristics are considered: - Flood & Prune - Source/Shared trees - Uni/Bi-directional shared trees - Loop-free-ness - RPF check The discussion of these characteristics will not lead to the selection of one superior multicast routing protocol. It is even Ooms, et al. Expires February 1999 [Page 4]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 very probable that different IP multicast routing protocols will be deployed in the Internet. 4.1. Flood & Prune To establish the multicast tree some IP multicast routing protocols flood the network with multicast data. The branches can then be pruned by nodes which do not want to receive the data of the specific multicast group. This process is repeated periodically, thus generating a very volatile tree structure. Direct mapping of this dynamic layer 3 (L3) point-to-multipoint (p2mp) tree to a layer 2 (L2) p2mp LSP is problematic because of the limited ATM resources and the setup time of the LSPs. 4.2. Source/Shared trees IP multicast routing protocols create either source trees (S, G), i.e. a tree per source (S) and per multicast group (G), or shared trees (*, G), i.e. one tree per multicast group (Figure 1). Some protocols support a mixture of both tree types. R1 R1 R1 S1 / / / \ / / / \ / / / C---R2 S1---R2 S2---R2 / \ \ \ / \ \ \ S2 \ \ \ R3 R3 R3 Figure 1. Shared tree and Source trees The advantage of using shared trees, when label switching is applied, is that shared trees consume less labels (for ATM: less VPI/VCI space) than source trees (1 label per group versus 1 label per source and per group). However, mapping a shared tree end-to-end on ATM implies setting up multipoint-to-multipoint (mp2mp) LSPs. The problem of implementing mp2mp LSPs boils down to the VC merging problem. 4.3. Uni/Bi-directional Shared Trees Ooms, et al. Expires February 1999 [Page 5]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 Bidirectional shared trees have the disadvantage of creating a lot of merging points (M) in the nodes (N) of the shared tree. Figure 2 shows these merging points resulting from 2 senders S1 and S2 on a bidirectional tree. S1 S2 || || v| <- <- <- <- |v <- <- | -> -> -> -> | -> ----N----M----M----M----M----M----N || || || || || || |v |v |v |v |v |v | | | | | | Figure 2. Multicast traffic flows from 2 senders on a bidirectional tree In Figure 3 the same situation for unidirectional shared trees is depicted. In this case the data of the senders is tunneled towards the root node R, yielding only a single merging point, namely the root of the shared tree itself. S1 tunnel || S2 <----- v| tunnel || to R<------------------------- v| -> -> | -> -> -> -> | -> ----N----N----N----N----N----N----N || || || || || || |v |v |v |v |v |v | | | | | | Figure 3. Multicast traffic flows from 2 senders on a unidirectional tree In unidirectional shared trees the multicast traffic is sent encapsulated from the Designated Router (DR) of the source to the root node R. Hence, multicast traffic arriving at the root needs to be decapsulated first (L3 operation) before transmission over the (*, G) tree. Therefore, forwarding multicast packets in the root node can only be done at L3, so there is no issue of merging data from different sources at L2 in the root node. LSPs can only start from the root node, so the traffic can never be label switched end-to-end. 4.4. Loop-free-ness Multicast routing protocols which depend on a unicast routing Ooms, et al. Expires February 1999 [Page 6]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 protocol can suffer from the same transient loops as the unicast protocols do, however the effect of loops will be much worse in the case of multicast (multicast avalanche). Note that there exist multicast routing protocols which are guaranteed loop free [PARS]. This is however not an advantage if loop prevention is also performed by MPLS. 4.5. RPF Check Some protocols perform a Reverse Path Forwarding (RPF) check on the received multicast packets. This mechanism checks whether the packet is received on the interface which is on the shortest path to the source (or root). This mechanism can introduce problems when explicit routing is used (see section 8). Indeed, explicit routing can construct a tree yielding another incoming interface than the RPF-compatible one. 4.6. Mapping of characteristics on existing protocols The above characteristics are summarized in Table 1 for a non- exhaustive list of existing IP multicast routing protocols: DVMRP [PUSA], MOSPF [MOY], CBT [BALL], PIM-DM [DEER], PIM-SM [DEE2], MIP [PARS]. +------------------+------+------+------+------+------+-----+ | |DVMRP |MOSPF |CBT |PIM-DM|PIM-SM|MIP | +------------------+------+------+------+------+------+-----+ |Flood & Prune |yes |yes |no |yes |no |no | +------------------+------+------+------+------+------+-----+ |Tree Type |source|source|shared|source|both |both | +------------------+------+------+------+------+------+-----+ |Uni/Bi-directional|N/A |N/A |bi |N/A |uni |both | +------------------+------+------+------+------+------+-----+ |Loop Free |no |no |no |no |no |yes | +------------------+------+------+------+------+------+-----+ |RPF check |yes |yes |no |yes |yes |no | +------------------+------+------+------+------+------+-----+ Table 1. Taxonomy of IP Multicast Routing Protocols From Table 1 one can derive e.g. that DVMRP will consume a lot of L2 resources when the Flood & Prune L3 tree is mapped onto a L2 tree. Furthermore since DVMRP uses source trees it experiences no merging problem when label switching is applied. The table can be Ooms, et al. Expires February 1999 [Page 7]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 interpreted in the same way for the other protocols. 5. Taxonomy of IP multicast LSP triggers The creation of an LSP for multicast streams can be triggered by different events, which can be mapped on the well known categories of 'request driven', 'topology driven' and 'traffic driven'. a) Request driven: intercept the sending or receiving of control messages (e.g. multicast routing messages, resource reservation messages). b) Topology driven: map the L3 tree, which is available in the Multicast Routing Table, to a L2 tree. The mapping is done even if there is no traffic. c) Traffic driven: the L3 tree is mapped onto a L2 tree when data arrives on the tree. The granularity of the multicast streams will be (*, G) for the shared tree and (S, G) for a source tree, S being the source address and G the multicast group address. Whether the trigger by multicast routing messages is categorized as request or topology driven is debatable. The constructed L2 tree will be identical to the one constructed by topology driven methods, but the definition of request driven [CALL] includes all label assignments in response to control traffic. In [KATS] the multicast routing messages trigger is categorized as request driven, so we will continue using this convention. 5.1. Request driven 5.1.1. General The establishment of LSPs can be triggered by the interception of outgoing (requiring that the label is requested by the downstream LSR) or incoming (requiring that the label is requested by the upstream LSR) control messages. Figure 4 illustrates these two cases. Ooms, et al. Expires February 1999 [Page 8]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 LSRu LSRd LSRu LSRd -------+ +--- ---+ +------- | control | | control | <---*<-----message------- <-------message-------*---- | | | | | | trigger| | | | | |trigger | | bind | | bind | | +--------or---------> <---------or----------+ | bind-request | | bind-request | | | | | | | | | |----data----->| |-----data---->| incoming outgoing Figure 4. Request driven trigger (interception of resp. incoming and outgoing control messages) The downstream LSR (LSRd) sends a control message to the upstream LSR (LSRu). In the case that incoming control messages are intercepted and the MPLS module in LSRu decides to establish an LSP it will send an LDP bind (upstream mode) or an LDP bind request (downstream on demand mode) to LSRd. Currently, we can identify two important types of control messages: the multicast routing messages and the resource reservation messages. 5.1.2. Multicast routing messages In principle, this mechanism can only be used by IP multicast routing protocols which use explicit signaling: e.g. the Join messages in PIM-SM or CBT. Remark that DVMRP and PIM-DM can be adapted to support this type of trigger [FARI], however, at the cost of modifying the IP multicast routing protocol itself ! IP multicast routing messages can create both hard states (e.g. CBT Join + CBT Join-Ack) and soft states (e.g. PIM-SM Joins are sent periodically). The latter generates more control traffic for tree maintenance and thus requires more processing in the MPLS module. Triggers based on the multicast routing protocol messages have the disadvantage that the routing calculations performed by the multicast routing daemon to determine the Multicast Routing Table are repeated by the MPLS module. The former determines the tree that will be used at L3, the latter calculates an identical tree to be used by L2. Since the same task is performed twice, it is better to create the Ooms, et al. Expires February 1999 [Page 9]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 multicast LSP on the basis of information extracted from the Multicast Routing Table itself (see section 5.2 and 5.3). The routing calculations become more complex for protocols which support a switch-over from a (*, G) tree to a (S, G) tree because more messages have to be interpreted. When a host has a point-to-point connection to the first router one could create 'LSPs up to the end-user' by intercepting not only the multicast routing messages but the IGMP Join/Prune messages ([FENN]) as well. 5.1.3. Resource reservation messages As is the case for unicast the RSVP Resv message can be used as a trigger to establish LSPs. A source of a multicast group will send an RSVP Path message down the tree, the receivers can then reply with an RSVP Resv message. RSVP scales equally well for multicast as it does for unicast because: a) RSVP Resv messages can merge. b) RSVP Resv messages are only sent up to the first branch which made the required reservation. More on RSVP in the sections on Piggy-backing (section 7) and QoS (section 9). 5.2. Topology driven The Multicast Routing Table (MRT) is maintained by the IP multicast routing protocol daemon (e.g. PIM/pimd, DVMRP/mrouted). The MPLS module maps this L3 tree topology information to L2 p2mp LSPs. The MPLS module can poll the MRT to extract the tree topologies. Alternatively, the multicast daemon can be modified to notify the MPLS module directly of any change to the MRT. 5.3. Traffic driven 5.3.1. General A traffic driven trigger method will only construct LSPs for trees which carry traffic. It consumes less labels than the topology driven method, as labels are only allocated when there is traffic on the multicast tree. Ooms, et al. Expires February 1999 [Page 10]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 If the mixed L2/L3 forwarding capability (see section 6) is not supported, the traffic driven trigger requires an LDP mode in which the label is requested by the LSRu (downstream on demand or upstream mode). In Figure 5, suppose an LSP for a certain group exists to LSRd1 and another LSRd2 wants to join the tree. In order for LSRd2 to initiate a trigger, it must already receive the traffic from the tree. This can be either at L2 or at L3. The former case is a chicken and egg problem. The latter case requires a mixed L2/L3 forwarding capability in LSRu to add the L3 branch. +--------+ | LSRd1 | | | +--------+ | L3 | | LSRu | +--------+ | | | | | L3 | +--------------------------> +--------+ / | L2 | | | / +--------+ ->-------------+ | L2 | +--------+ +--------+ | LSRd2 | | | | L3 | +--------+ | | | | | L2 | +--------+ Figure 5. The LSRu has to request the label. 5.3.2. An implementation example Current implementations on Unix platforms of IP multicast routing protocols (DVMRP, PIM) have a Multicast Forwarding Cache (MFC). The MFC is a cached copy of the Multicast Routing Table. The MFC requests an entry for a certain multicast group when it experiences a 'cache miss' for an incoming multicast packet. The missing routing information is provided by the multicast daemon. If at a later point in time something changes to the route (outgoing interfaces added or removed), the multicast daemon will update the MFC. The MFC is implemented as a common component (part of the kernel), which makes this trigger very attractive because it can be transparently used for any IP multicast routing protocol. Ooms, et al. Expires February 1999 [Page 11]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 Entries in the MFC are removed when for a certain time no traffic is received anymore for this entry. When label switching is applied to a certain MFC-entry, the L3 will not see any packets arriving anymore. To obtain a normal MFC behavior the L3 counters of the MFC need to be updated by L2 measurements. 5.4. Combinations of triggers and LDP modes Table 2 shows the valid combinations of LDP modes and trigger types which were discussed in the previous sections. The (X) means that the combination is valid when the mixed L2/L3 forwarding feature is supported in the LSR (section 6). +----------------+-------------------------------------------+ | | label requested by | | | LSRu | LSRd | | +---------------------+---------------------+ | | upstream |downstream|downstream| upstream | | | |on demand | | on demand| +----------------+----------+----------+----------+----------+ |Request Driven | | | | | |(incoming msg) | X | X | | | +----------------+----------+----------+----------+----------+ |Request Driven | | | | | |(outgoing msg) | | | X | X | +----------------+----------+----------+----------+----------+ |Topology Driven | X | X | X | X | +----------------+----------+----------+----------+----------+ |Traffic Driven | X | X | (X) | (X) | +----------------+----------+----------+----------+----------+ Table 2. Valid combinations of triggers and LDP modes 6. Mixed L2/L3 forwarding in a single node Since unicast traffic has one incoming and one outgoing interface the traffic is either forwarded at L2 OR at L3 (Figure 6). Because multicast traffic can be forwarded to more than one outgoing interface one can consider the case that traffic to some branches is forwarded on L2 and to other branches on L3 (Figure 7). Ooms, et al. Expires February 1999 [Page 12]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 +--------+ +--------+ | L3 | | L3 | | +>>+ | | | | | | | | | +--|--|--+ +--------+ | | | | | | ->-----+ +-----> ->------>>-----> | L2 | | L2 | +--------+ +--------+ Figure 6. Unicast forwarding on resp. L3 or L2 +--------+ +--------+ +--------+ | L3 | | L3 | | L3 | | +>>++ | | +>>+ | | | | | || | | | | | | | +--|--||-+ +--|--|--+ +--------+ | | |+----> | | +-----> | +----> ->-----+ | | | |L2 | ->----->>-+ | | L2+-----> ->-----+>>------> | L2 +----> +--------+ +--------+ +--------+ Figure 7. Multicast forwarding on resp. L3, mixed L2/L3 or L2 Nodes which support this 'mixed L2/L3 forwarding' feature allow that a multicast tree splits in branches of which some are forwarded at L3 while others are switched at L2. The L3 forwarding has to take care that the traffic is not forwarded on those branches that already get their traffic on L2. This can be accomplished by e.g. providing an extra bit in the Multicast Routing Table. Although the mixed L2/L3 forwarding requires processing of the traffic at L3, the load on the L3 forwarding engine is generally less than in a pure L3 node. Supporting this 'mixed L2/L3 forwarding' feature has following advantages: Ooms, et al. Expires February 1999 [Page 13]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 a) Assume LSR A (Figure 8) is an MPLS edge node for the branch towards LSR B and an MPLS core node for the branch towards LSR C. The mixed L2/L3 forwarding allows that the branch towards C is not disturbed by a return to L3 in LSR A. +-------------+ | MPLS cloud | | N | | / \ | | / \ | | / \ | | A N | |/ \ \ | | \ \ | /| \ | B | C | | | +-------------+ Figure 8. Mixed L2/L3 forwarding in node A b) Allows a return to L3 for branches which requested lower QoS (section 9). c) Enables the use of the traffic driven trigger with the LDP downstream or upstream on demand mode, as explained in section 5.4. 7. Piggy-backing In Figure 4 (outgoing case) one can observe that instead of sending 2 separate messages the label advertisement can be piggy-backed on the existing control messages. However, some disadvantages can be identified: a) Since label advertisement is only one of the three functions of LDP (the two others are discovery and adjacency), LDP is still required for e.g. label range negotiation. b) Suppose piggy-backing is applied on the multicast routing protocol. In order to support unicast label switching, either piggy- backing has also to be implemented on the unicast routing protocol or LDP must be used. In the latter case, one may question the benefit of piggy-backing on the multicast routing protocol. As a result, piggy-backing introduces extra implementation effort. c) Piggy-backing assumes the LDP downstream mode, this excludes a Ooms, et al. Expires February 1999 [Page 14]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 number of trigger methods (see Table 2). d) Piggy-backing changes the LDP paradigm: LDP normally runs on top of TCP, assuring a reliable communication between peer nodes. Piggy-backed label advertisement often replaces the reliable communication with periodic soft-state label advertisements. Because of this periodic label advertisement the control traffic will increase. e) If a VCID notification mechanism [NAGA] is required, the (in-band) notification can be done by sending the LDP bind through the newly established VC. This way only one message is required. This method cannot be combined with piggy-backing because the routing message is sent before the VC can be established. An extra handshake message is thus required, diminishing the benefit of piggy-backing. For multicast two piggy-back candidates exist: a) Multicast routing messages: protocols as PIM-SM and CBT have explicit Join messages which could carry the label mappings. This approach is described in [FARI]. When different multicast routing protocols are deployed, an extension to each of these protocols has to be defined. b) RSVP Resv messages: a label mapping extension object for RSVP, also applicable to multicast, is proposed in [DAVI]. Piggy-backing is not incompatible with multicast, but one has to consider the disadvantages carefully. 8. Explicit routing Explicit routing is a powerful concept to control the multicast network topology. It provides network engineers with a tool which allows explicitly defined LSPs to be selected for multicast traffic. Such tools can be very useful for ISPs [UUNE] so that new IP multicast services could be directed to particular links which are under traffic engineering control, unlike conventional dynamic IP routing. In case of explicit routing, (part of) the tree is calculated or configured by a single node and subsequently mapped on an LSP. If one central node would know all receivers, it can construct a more optimal tree (more optimal than a shortest-path-tree). However, maintaining the knowledge of all receivers introduces a lot of signaling towards this central node. This can be a bottleneck if Ooms, et al. Expires February 1999 [Page 15]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 group membership is very dynamic. If routing is based on multiple constraints (instead of just hop- count), it can occur that two branches of the same tree cross a same node. An example of this is shown in Figure 9. The two additive cost metrics associated with a certain link are represented by (c1,c2). Suppose two users A and B both require that sum(c1)<12 and sum(c2)<7. The only solution implies that the traffic from S is split in M1 and carried two times over the M2-M3 link. (1,5) A3---User A A1-----A2 / / \ /(10,1) / +-----+ S-----M1 M2 M3 \ +-----+ \ / \(1,5) B1-----B2 \ (10,1) B3---User B Figure 9. Two branches of a tree crossing a same node Such forwarding state cannot be represented by the usual (S,G) or (*,G) table lookup and RPF check. Only explicit routing can do this. Note that if an 'explicit routed' LSP for a certain part of the tree is established, one must make sure that the L3 forwarding engine of the LSRs at the end of the LSP is notified of the explicit path in order not to do an (incorrect) RPF check. 9. QoS/CoS 9.1. DiffServ The Differentiated Services approach can be applied to multicast as well. It introduces finer stream granularities (Class of Service (CoS) as an extra differentiator). A sender can construct one or more trees with different CoS bits. These (S, G, CoS) or (*, G, CoS) trees can be mapped very easily onto LSPs when the traffic driven trigger is used. In this case one can create different LSPs for the different classes. Note however that these LSPs still use the same route. The situation becomes more Ooms, et al. Expires February 1999 [Page 16]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 complicated when one wants to combine DiffServ with QoS Routing [NEVE]. 9.2. IntServ and RSVP RSVP can be used to setup multicast trees with QoS. An important multicast issue is the problem of how to map the 'heterogeneous receivers' paradigm onto ATM (remark that it is not solved in IP either). This subject is tackled in [CRAW]. Pragmatic approaches are the 'Limited Heterogeneity Model' which allows a best effort service and a single alternate QoS (e.g. a QoS proposed by the sender in a RSVP Path message) and the 'Homogeneous Model' which allows only a single QoS. The first approach will construct full trees for each service class. The sender has to send its traffic twice across the network (1 best- effort and 1 QoS tree). Both trees can be label switched. The second approach constructs one tree and the best-effort users are connected to the QoS tree. If the branches created for best-effort users are not to be label switched, (thus carried by a hop-by-hop default VC) the QoS multicast traffic has to be merged onto these default VCs. This function can be provided by the 'mixed L2/L3 forwarding' feature described in section 6. If this is not available VC merging is necessary to avoid a return to L3 in the QoS LSP. The mapping of the IntServ service categories onto ATM service categories is studied in [GARR]. 10. More issues 10.1. TTL The TTL field in the IP header is typically used for loop detection. In IP multicast it is also used to limit the scope of the multicast packets by setting an appropriate TTL value. Since the TTL value is not decremented in an LSP, the scope restriction function is affected. Suppose one could calculate in advance the number of hops an LSP traverses. In a unicast LSP the TTL value could then be decremented at the ingress node. This is impossible for multicast since all the branches of the tree can have different lengths. 10.2. Shared media Ooms, et al. Expires February 1999 [Page 17]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 Multicast on shared media requires label space partitioning, otherwise the danger exists that two downstream LSRs will use the same label to subscribe to different multicast groups. A label space partitioning mechanism is described in [FAR2]. 10.3. Local control vs. egress control In local control (also called independent mode [ANDE]) each LSR can take the initiative to set up a LSP. In egress control (also called ordered mode [ANDE]) the LSP is set up on the initiative of the egress node. All the previously described trigger methods (section 5) combine with LDP local control. In case of the request driven approach the label distribution in fact behaves as egress controlled: the control messages flow from the egress node upstream, imposing the same sequence to the label advertisement. In case of piggy-backing the label advertisements themselves are flowing from the egress node upstream. 10.4. Conservative vs. optimistic The conservative mode ([DAVI]) only accepts an upstream label for a certain stream if it already has a downstream label for this stream. The optimistic mode always accepts the label. The conservative mode cannot be used in combination with a traffic driven trigger in case the node does not support mixed L2/L3 forwarding. According to Table 2, this case implies that labels are requested by the upstream LSR. Suppose in Figure 10 that an LSP exists from S to R1 and a new branch must be added to R2. B will only accept a label on the A-B link if a label is already assigned on the B-C link. However, to establish a label on the B-C link, B must already receive traffic on the A-B link. This is not possible at L2 nor at L3 (since A does not support mixed L2/L3). N---N---R1 / / S -----A \ \ B---C---R2 Figure 10. Ooms, et al. Expires February 1999 [Page 18]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 10.5. Conservative vs. liberal In the conservative mode ([ANDE]) only the labels that are required for forwarding data are allocated and maintained. In the liberal mode labels are advertised and maintained to all neighbors. This mode does not scale in ATM due to the the limited VPI/VCI space. In some cases (see below) it is not known by an LSR to which neighbor it has to request a label. Therefore, it has to send the request to all its neighbors. In such case supporting the liberal mode requires no extra messages. However, it still requires extra state information and label space. Table 3 shows the cases where it is known by an LSR where to send its label requests. +---------+----------------------------------+ | | label requested by | | | LSRu | LSRd | +---------+----------------+-----------------| |unicast | Yes | No | +---------+----------------+-----------------| |multicast| Yes | Yes | +---------+----------------+-----------------+ Table 3. Does an LSR know where to send its label requests ? For a unicast flow, an LSR can determine the next hop LSR, which is the one to send the request to in case of upstream or downstream-on- demand mode. The LSR is however not able to find the previous hop. The previous hop is not necessarily the next hop towards the source, because the path from A to B is not necessarily the same as the path from B to A. Such a situation can occur as a result of asymmetric link measures or in the event that multiple equal cost paths exist [PAXS]. In the case of multicast, an LSR knows both the next hop(s) and the previous hop. Because multicast trees are constructed using the reverse shortest path method, the previous hop is always the next hop towards the source or towards the root of the tree. As a result, multicast maps very naturally on the conservative mode. 10.6. Scalability Sparse mode multicast routing protocols (CBT, PIM-SM) are more scalable than dense mode protocols. But even the sparse mode Ooms, et al. Expires February 1999 [Page 19]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 protocols introduce state in each node of the tree. An enhancement to this is proposed in [TIAN]. In this proposal tunnels are created which span the non-branching nodes. An appropriate trigger could map these tunnels to LSPs. 11. Security Considerations Security considerations are not discussed in this version of the document. 12. Acknowledgements The authors would like to thank Piet Van Mieghem, Philip Dumortier, Hans De Neve, Jan Vanhoutte, Alex Mondrus and Gerard Gastaud for the fruitful discussions and/or their thorough revision of this document. References [ANDE] L. Andersson, P. Doolan, N. Feldman, A. Fredette and R. Thomas, "Label Distribution Protocol", IETF Draft, draft-mpls-ldp- 00.txt, March 1998. [BALL] A. Ballardie, "Core Based Trees (CBT, v2) Multicast Routing - Protocol Specification", IETF Draft, draft-ietf-idmr-cbt-spec- 09.txt, 1997. [CALL] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow and A. Viswanathan, "A Framework for Multiprotocol Label Switching", IETF Draft, draft-ietf-mpls-framework-02.txt, November 1997. [CRAW] E. Crawley, Editor, L. Berger, S. Berson, F. Baker, M. Borden and J. Krawczyk, "A Framework for Integrated Services and RSVP over ATM", IETF Draft, draft-ietf-issll-atm-framework-04.txt, May 1998. [DAVI] B. Davie, J. Lawrence, K. McCloghrie, Y. Rekhter, E. Rosen, G. Swallow and P. Doolan, "Use of Label Switching With ATM", IETF Draft, draft-davie-mpls-atm-00.txt, November 1997. [DAV2] B. Davie, Y. Rekhter, E. Rosen, A. Viswanathan, V. Srinivasan and S. Blake, "Use of Label Switching With RSVP", IETF Draft, draft-ietf-mpls-rsvp-00.txt, March 1998 [DEER] S. Deering, D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Ooms, et al. Expires February 1999 [Page 20]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 Deering, M. Handley, V. Jacobson, C. Liu, P. Sharma and L Wei, "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC 2117, June 1997. [DEE2] S. Deering, D. Estrin, D. Farinacci, V. Jacobson, Protocol Independent Multicast (PIM), Dense Mode Protocol: Specifica- tion", IETF Draft, 1994. [FARI] D. Farinacci and Y. Rekhter, "Multicast Tag Binding and Distri- bution using PIM", IETF Draft, draft-farinacci-multicast-tagsw- 00.txt, December 1996. [FAR2] D. Farinacci and Y. Rekhter, "Partitioning Tag Space among Mul- ticast Routers on a Common Subnet", IETF Draft, draft- farinacci-multicast-tag-part-00.txt, December 1996. [FENN] W. Fenner, "Internet Group Management Protocol, IGMP, version 2", RFC 2236, November 1997. [GARR] M. Garrett and M. Borden, "Interoperation of Controlled-Load Service and Guaranteed Service with ATM", IETF Draft, draft- ietf-issll-atm-mapping-06.txt, March 1998. [KATS] Y. Katsube, Y. Ohba and K. Nagami, "Two Modes of MPLS Explicit Label Distribution Protocol", IETF Draft, draft-katsube-mpls- two-ldp-00.txt, September 1997. [MOY] J. Moy, "Multicast extensions to OSPF", RFC 1584, March 1994. [NAGA] K. Nagami, N. Demizu, H. Esaki and P. Doolan, "VCID Notification over ATM link", IETF Draft, draft-ietf-mpls-vcid-atm-00.txt; March 1998. [NEVE] H. De Neve and P. Van Mieghem, "Hop-by-Hop Quality of Service Routing", submitted to INFOCOM '99. [PUSA] T. Pusateri, "Distance Vector Multicast Routing Protocol", IETF Draft, draft-ietf-idmr-dvmrp-v3-05, October 1997. [PARS] M. Parsa and J. Garcia-Luna-Aceves, "A protocol for scaleable loop-free multicast routing", IEEE JSAC, vol.15, no.3, p.316- 331, April 1997 [PAXS] V. Paxson, "End-to-End Routing Behavior in the Internet", IEEE/ACM Transactions on Networking 5(5), pp. 601-615. [TIAN] J. Tian, G. Neufeld, "Forwarding State Reduction for Sparse Mode Multicast Communication" Ooms, et al. Expires February 1999 [Page 21]
Internet Draft draft-ooms-mpls-multicast-00.txt August 1998 [UUNE] D. Awduche, J. Malcolm, J. Agogbua, M. O'Dell, J. McManus, "Requirements for Traffic Engineering Over MPLS", <draft- awduche-mpls-traffic-eng-00.txt>, April 1998. Authors Addresses Dirk Ooms Alcatel Corporate Research Center Fr. Wellesplein 1, 2018 Antwerpen, Belgium. Phone : 32-3-240-4732 Fax : 32-3-240-9932 E-mail: oomsd@rc.bel.alcatel.be Wim Livens Alcatel Corporate Research Center Fr. Wellesplein 1, 2018 Antwerpen, Belgium. Phone : 32-3-240-7570 E-mail: livensw@rc.bel.alcatel.be Bernard Sales Alcatel Corporate Research Center Fr. Wellesplein 1, 2018 Antwerpen, Belgium. Phone : 32-3-240-9574 E-mail: salesb@btmaa.bel.alcatel.be Maria Fernanda Ramalho Alcatel Corporate Research Center Fr. Wellesplein 1, 2018 Antwerpen, Belgium. Phone : 32-3-240-9725 E-mail: ramalhom@rc.bel.alcatel.be Ooms, et al. Expires February 1999 [Page 22]