Network Working Group Dino Farinacci
Internet Draft Cisco Systems
David Meyer
University of Oregon
Yakov Rekhter
Cisco Systems
Expiration Date: February 1997 August 1997
Intra-LIS IP multicast among routers over ATM using Sparse Mode PIM
<draft-ietf-ion-intralis-multicast-01.txt>
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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2. Abstract
This document describes how intra-LIS IP multicast can be efficiently
supported among routers over ATM without using the Multicast Address
Resolution Server (MARS). The method described here is specific to
Sparse Mode PIM [PIM-SM], and relies on the explicit join mechanism
inherent in PIM-SM to notify routers when they should create group
specific point-to-multipoint VCs.
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3. Overall model
This document focuses on forwarding of multicast traffic among PIM-SM
routers connected to an ATM network. Routers on an ATM network are
partitioned into Logical IP Subnets, or LISs. This document deals
with handling multicast within a single LIS. Handling inter-LIS
multicast traffic, including handling shortcuts, is outside the scope
of this document. In addition, this document does not address
forwarding of multicast traffic to or from hosts connected to an ATM
network.
4. Router behavior
This document requires that each router within a LIS knows IP and ATM
addresses of all other routers within the LIS. The mapping between IP
and ATM addresses may be provided by an ARP server [RFC1577], or by
any other means (e.g., static configuration).
Each PIM router within a LIS is required to maintain a single
(shared) point-to-multipoint distribution VC rooted at the router
with all other PIM routers in the LIS as the leaf nodes. The VC is
expected to be used for forwarding of multicast traffic (both data
and control) among routers within the LIS. For example, this VC would
be used for distributing PIM [PIM-SM] control messages (Join/Prune
messages).
In addition, if a PIM router receives a IGMP report from an non-PIM
neighbor, then the router may add the reporter to the existing shared
distribution VC or to the group specific distribution VC (if it
exists). The PIM router may also create a specific VC for this IGMP
proxy.
4.1. Establishing Dedicated, Per Group Point-to-Multipoint VCs
Routers may also maintain group specific, dedicated point-to-
multipoint VCs. In particular, an upstream router for a group may
choose to become the root of a group specific point-to-multipoint VC
whose leaves are the downstream routers that have directly connected
or downstream receivers for the group. While the criteria for
establishing a group specific point-to-multipoint VC are local to a
router, issues such as the volume of traffic associated with the
group and the fanout factor within the LIS should be considered.
Finally, note that a router must minimally support a single shared
point-to-multipoint VC for distribution of control messages and data
(to all group addresses).
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A router can choose to establish a dedicated point-to-multipoint VC
(or add another leaf to an already established dedicated point-to-
multipoint VC) when it receives a PIM Join or IGMP report messages
from another device in the same LIS. When a router that is the root
of a point-to-multipoint VC receives PIM Prune message or IGMP leave,
it removes the originator of the message from its dedicated point-
to-multipoint VC.
4.2. Switching to a Source-Rooted Tree
If at least one of the routers within a LIS decides to switch to a
source-rooted tree (by sending (S,G) PIM Joins), then all other
routers within the LIS that have downstream members for G should
switch to that source-rooted tree as well. Since a router that
switches to a source-rooted tree sends PIM Join messages for (S,G)
over its shared point-to-multipoint VC, the other routers within the
LIS are able to detect this. Once a router that has downstream
members for G detects this, the router should send (S,G) PIM Join
message as well (otherwise the router may receive duplicate traffic
from S).
Note that it is possible for a non-PIM router in the LIS to fail to
receive data if the injection point moves to router to which there is
not an existing VC.
4.2.1. Adding New Members to a Source-Rooted Tree
As mentioned above, this document requires that once one router in a
LIS decides to switch to the source tree for some (S,G), all routers
in the LIS that have downstream members must also switch to the (S,G)
source tree. Now, when a new router wants to receive traffic from G,
it starts sending (*,G)-Joins on it's shared point-to-multipoint VC
toward the RP for G. The root of the (S,G)-source-rooted tree will
know to add the new router to the point-to-multipoint VC servicing
the (S,G)-source-rooted tree by observing the (*,G)-joins on it's
shared point-to-multipoint VC. However, the new router must also
switch to the (S,G)-source-rooted tree. In order to accomplish this,
the newly added router must:
(i). Notice that it has been added to a new
point-to-multipoint VC
(ii). Notice (S,G) traffic coming down this new
point-to-multipoint VC
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Internet Draft draft-ietf-ion-intralis-multicast-01.txt August 1997
(iii). Send (S,G) joins toward S, causing it to switch to the
source-rooted tree. The router learns that the VC is
used to distribute (S,G) traffic in the previous
steps.
4.3. Handing the "Packet Reflection" Problem
When a router receives a multicast packet from another router in its
own LIS, the router should not send the packet on any of the routers
distribution point-to-multipoint VCs associate with the LIS. This
eliminates the problem of "packet reflection". Sending the packet on
the routers' distribution VCs associated with other LISs is
controlled by the multicast routing procedures.
5. Brief Comparison with MARS
The intra-LIS multicast scheme described in this document is intended
to be a less complex solution to an important subset of the
functionality provided by the Multicast Address Resolution Server, or
MARS [MARS]. In particular, it is designed to provide intra-LIS
multicast between routers using PIM-SM, and does not consider the
case of host-rooted point-to-multicast multicast distribution VCs.
Although MARS supports both of the current schemes for mapping the IP
multicast service model to ATM (multicast server and meshes of
point-to-multipoint VCs), it does so at at cost and complexity higher
than of the scheme described in this document. In addition, MARS
requires new encapsulations, whereas this proposal works with either
LLC/SNAP or with NLPID encapsulation. Another important difference is
that MARS allows point-to-multipoint VCs rooted either at a source or
at a multicast server (MCS). The approach taken here is to constrain
complexity by focusing on PIM-SM (taking advantage of information
available in explicit joins), and by allowing point-to-multipoint VCs
to be rooted only at the routers (which is roughly analogous to the
complexity and functionality of rooting point-to-multipoint VCs at
the sources).
In summary, the method described in this document is designed for the
router-to-router case, and takes advantage of the explicit-join
mechanism inherent in PIM-SM to provide a simple mechanism for
intra-LIS multicast between routers. MARS, on the other hand, accepts
different tradeoffs in complexity-functionality design space. In
particular, while the MARS paradigm provides a general neighbor
discovery mechanism, allows host to participate, and is protocol
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independent, it does so at considerable cost.
6. Security Considerations
Security issues are not discussed in this document.
7. References
[MARS] G. Armitage, "Support for Multicast over UNI
3.0/3.1 based ATM Networks.", RFC2022, November
1996.
[PIM-SM] Estrin, D, et. al., "Protocol Independent Multicast
Sparse Mode (PIM-SM): Protocol Specification",
draft-ietf-idmr-PIM-SM-spec-09.ps, October, 1996.
8. Acknowledgments
Petri Helenius provided several insightful comments on earlier
versions of this document.
9. Author Information
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Internet Draft draft-ietf-ion-intralis-multicast-01.txt August 1997
Dino Farinacci
Cisco Systems
170 Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-4696
e-mail: dino@cisco.com
David Meyer
University of Oregon
1225 Kincaid St.
Eugene, OR 97403
Phone: (541) 346-1747
e-mail: meyer@antc.uoregon.edu
Yakov Rekhter
cisco Systems, Inc.
170 Tasman Dr.
San Jose, CA 95134
Phone: (914) 528-0090
email: yakov@cisco.com
