The Locator/ID Separation Protocol (LISP) for Multicast Environments
draft-ietf-lisp-rfc6831bis-00
| Document | Type | Active Internet-Draft (lisp WG) | |
|---|---|---|---|
| Authors | Dino Farinacci , David Meyer , John Zwiebel , Stig Venaas , Vengada Prasad Govindan | ||
| Last updated | 2024-08-27 | ||
| Replaces | draft-farinacci-lisp-rfc6831bis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-lisp-rfc6831bis-00
Network Working Group D. Farinacci
Internet-Draft D. Meyer
Obsoletes: 6831 (if approved) J. Zwiebel
Intended status: Standards Track S. Venaas
Expires: 27 February 2025 V. Govindan
Cisco Systems
26 August 2024
The Locator/ID Separation Protocol (LISP) for Multicast Environments
draft-ietf-lisp-rfc6831bis-00
Abstract
This document describes the design for inter-domain multicast
overlays using the Locator/ID Separation Protocol (LISP) architecture
and protocols. The document specifies how LISP multicast overlays
operate over multicast and unicast underlays. The mechanisms in this
specification indicate how a signal-based approach using the PIM
protocol can be used to program LISP encapsulators with a replication
list in a locator-set, where the replication list can be a mix of
multicast and unicast locators.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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."
This Internet-Draft will expire on 27 February 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 5
4. Basic Overview . . . . . . . . . . . . . . . . . . . . . . . 8
5. Source Addresses versus Group Addresses . . . . . . . . . . . 11
6. Locator Reachability Implications on LISP-Multicast . . . . . 12
7. Multicast Protocol Changes . . . . . . . . . . . . . . . . . 12
7.1. PIM Join Attributes for LISP . . . . . . . . . . . . . . 15
8. LISP-Multicast Data-Plane Architecture . . . . . . . . . . . 15
8.1. ITR Forwarding Procedure . . . . . . . . . . . . . . . . 15
8.1.1. Multiple RLOCs for an ITR . . . . . . . . . . . . . . 16
8.1.2. Multiple ITRs for a LISP Source Site . . . . . . . . 16
8.2. ETR Forwarding Procedure . . . . . . . . . . . . . . . . 16
8.3. Replication Locations . . . . . . . . . . . . . . . . . . 17
9. LISP-Multicast Interworking . . . . . . . . . . . . . . . . . 17
9.1. LISP and Non-LISP Mixed Sites . . . . . . . . . . . . . . 18
9.1.1. LISP Source Site to Non-LISP Receiver Sites . . . . . 19
9.1.2. Non-LISP Source Site to Non-LISP Receiver Sites . . . 21
9.1.3. Non-LISP Source Site to Any Receiver Site . . . . . . 21
9.1.4. Unicast LISP Source Site to Any Receiver Sites . . . 22
9.1.5. LISP Source Site to Any Receiver Sites . . . . . . . 22
9.2. LISP Sites with Mixed Address Families . . . . . . . . . 23
9.3. Making a Multicast Interworking Decision . . . . . . . . 25
10. Considerations When RP Addresses Are Embedded in Group
Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
11. Taking Advantage of Upgrades in the Core . . . . . . . . . . 26
12. Mtrace Considerations . . . . . . . . . . . . . . . . . . . . 26
13. Security Considerations . . . . . . . . . . . . . . . . . . . 26
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
15.1. Normative References . . . . . . . . . . . . . . . . . . 27
15.2. Informative References . . . . . . . . . . . . . . . . . 29
Appendix A. Document Change Log . . . . . . . . . . . . . . . . 30
A.1. Changes to draft-ietf-lisp-rfc6831bis-00 . . . . . . . . 30
A.2. Changes to draft-farinacci-lisp-rfc6831bis-02 . . . . . . 30
A.3. Changes to draft-farinacci-lisp-rfc6831bis-01 . . . . . . 30
A.4. Changes to draft-farinacci-lisp-rfc6831bis-00 . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction
The Locator/ID Separation Protocol [RFC9300] architecture provides a
mechanism to separate out Identification and Location semantics from
the current definition of an IP address. By creating two namespaces,
an Endpoint ID (EID) namespace used by sites and a Routing Locator
(RLOC) namespace used by core routing, the core routing
infrastructure can scale by doing topological aggregation of routing
information.
Since LISP creates a new namespace, a mapping function must exist to
map a site's EID-Prefixes to its associated Locators. For unicast
packets, both the source address and destination address must be
mapped. For multicast packets, only the source address needs to be
mapped. The destination group address doesn't need to be mapped
because the semantics of an IPv4 or IPv6 group address are logical in
nature and not topology dependent. Therefore, this specification
focuses on mapping a source EID address of a multicast flow during
distribution tree setup and packet delivery.
This specification will address the following scenarios:
1. How a multicast source host in a LISP site sends multicast
packets to receivers inside of its site as well as to receivers
in other sites that are LISP enabled.
2. How inter-domain (or between LISP sites) multicast distribution
trees are built and how forwarding of multicast packets leaving a
source site toward receivers sites is performed. While the
design principles of how such trees are built and maintained are
the same, there are some procedural differences based on whether
the underlay is unicast or multicast
3. What protocols are affected and what changes are required to such
multicast protocols.
4. How ASM-mode (Any Source Multicast), SSM-mode (Single Source
Multicast), and Bidir-mode (Bidirectional Shared Trees) service
models will operate.
5. How multicast packet flow will occur for multiple combinations of
LISP-enabled and non-LISP-enabled source and receiver sites. For
example:
a. How multicast packets from a source host in a LISP site are
sent to receivers in other sites when they are all non-LISP
sites.
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b. How multicast packets from a source host in a LISP site are
sent to receivers in both LISP-enabled sites and non-LISP
sites.
c. How multicast packets from a source host in a non-LISP site
are sent to receivers in other sites when they are all LISP-
enabled sites.
d. How multicast packets from a source host in a non-LISP site
are sent to receivers in both LISP-enabled sites and non-LISP
sites.
This specification focuses on what changes are needed to the
multicast routing protocols to support LISP-Multicast as well as
other protocols used for inter-domain multicast, such as
Multiprotocol BGP (MBGP) [RFC4760]. The approach proposed in this
specification requires no packet format changes to the protocols and
no operational procedural changes to the multicast infrastructure
inside of a site when all sources and receivers reside in that site,
even when the site is LISP enabled. That is, internal operation of
multicast is unchanged, regardless of whether or not the site is LISP
enabled or whether or not receivers exist in other sites that are
LISP enabled.
Therefore, we see only operational (and not protocol) changes for
PIM-ASM [RFC7761], Multicast Source Discovery Protocol (MSDP)
[RFC3618], Anycast-RP [RFC4610], and PIM-SSM [RFC4607]. BIDIR-PIM
[RFC5015], which typically does not run in an inter-domain
environment, is not addressed in depth in this RFC.
Also, the current version of this specification does not describe
multicast-based Traffic Engineering (TE) relative to the TE-ITR
(TE-based Ingress Tunnel Router) and TE-ETR (TE-based Egress Tunnel
Router) descriptions in [RFC9300]. Further work is also needed to
determine the detailed behavior for multicast Proxy-ITRs (mPITRs)
(Section 9.1.3), mtrace (Section 12), and locator reachability
(Section 6). Finally, further deployment and experimentation would
be useful to understand the real-life performance of the LISP-
Multicast solution. For instance, the design optimizes for minimal
state and control traffic in the core, but can in some cases cause
extra multicast traffic to be sent Section 8.1.2.
Issues and concerns about the deployment of LISP for Internet traffic
are discussed in [RFC9300]. Section 12 of that document provides
additional issues and concerns raised by this document.
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2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Definition of Terms
The terminology in this section is consistent with the definitions in
[RFC9300] but is extended specifically to deal with the application
of the terminology to multicast routing.
LISP-Multicast: a reference to the design in this specification.
That is, when any site that is participating in multicast
communication has been upgraded to be a LISP site, the operation
of control-plane and data-plane protocols is considered part of
the LISP-Multicast architecture.
Endpoint ID (EID): a 32-bit (for IPv4) or 128-bit (for IPv6) value
used in the source address field of the first (most inner) LISP
header of a multicast packet. The host obtains a destination
group address the same way it obtains one today, as it would when
it is a non-LISP site. The source EID is obtained via existing
mechanisms used to set a host's "local" IP address. An EID is
allocated to a host from an EID-Prefix block associated with the
site in which the host is located. An EID can be used by a host
to refer to another host, as when it joins an SSM (S-EID,G) route
using IGMP version 3 [RFC4604]. LISP uses Provider-Independent
(PI) blocks for EIDs; such EIDs MUST NOT be used as LISP RLOCs.
Note that EID blocks may be assigned in a hierarchical manner,
independent of the network topology, to facilitate scaling of the
mapping database. In addition, an EID block assigned to a site
may have site-local structure (subnetting) for routing within the
site; this structure is not visible to the global routing system.
Routing Locator (RLOC): the IPv4 or IPv6 address of an Ingress
Tunnel Router (ITR), the router in the multicast source host's
site that encapsulates multicast packets. It is the output of an
EID-to-RLOC mapping lookup. An EID maps to one or more RLOCs.
Typically, RLOCs are numbered from topologically aggregatable
blocks that are assigned to a site at each point to which it
attaches to the global Internet; where the topology is defined by
the connectivity of provider networks, RLOCs can be thought of as
Provider-Assigned (PA) addresses. Multiple RLOCs can be assigned
to the same ITR device or to multiple ITR devices at a site.
Ingress Tunnel Router (ITR): a router that accepts an IP multicast
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packet with a single IP header (more precisely, an IP packet that
does not contain a LISP header). The router treats this "inner"
IP destination multicast address opaquely so it doesn't need to
perform a map lookup on the group address because it is
topologically insignificant. The router then prepends an "outer"
IP header with one of its globally routable RLOCs as the source
address field. This RLOC is known to other multicast receiver
sites that have used the mapping database to join a multicast tree
for which the ITR is the root. In general, an ITR receives IP
packets from site end-systems on one side and sends LISP-
encapsulated multicast IP packets out all external interfaces that
have been joined.
An ITR would receive a multicast packet from a source inside of
its site when 1) it is on the path from the multicast source to
internally joined receivers, or 2) when it is on the path from the
multicast source to externally joined receivers.
Egress Tunnel Router (ETR): a router that is on the path from a
multicast source host in another site to a multicast receiver in
its own site. An ETR accepts a PIM Join/Prune message from a
site-internal PIM router destined for the source's EID in the
multicast source site. The ETR maps the source EID in the Join/
Prune message to an RLOC address based on the EID-to-RLOC mapping.
This sets up the ETR to accept multicast encapsulated packets from
the ITR in the source multicast site. A multicast ETR
decapsulates multicast encapsulated packets and replicates them on
interfaces leading to internal receivers.
xTR: is a reference to an ITR or ETR when direction of data flow is
not part of the context description. xTR refers to the router
that is the tunnel endpoint; it is used synonymously with the term
"tunnel router". For example, "an xTR can be located at the
Customer Edge (CE) router" means that both ITR and ETR
functionality can be at the CE router.
LISP Header: a term used in this document to refer to the outer IPv4
or IPv6 header, a UDP header, and a LISP header. An ITR prepends
headers, and an ETR strips headers. A LISP-encapsulated multicast
packet will have an "inner" header with the source EID in the
source field, an "outer" header with the source RLOC in the source
field, and the same globally unique group address in the
destination field of both the inner and outer header.
(S,G) State: the formal definition is in the PIM Sparse Mode
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[RFC7761] specification. For this specification, the term is used
generally to refer to multicast state. Based on its topological
location, the (S,G) state that resides in routers can be either
(S-EID,G) state (at a location where the (S,G) state resides) or
(S-RLOC,G) state (in the Internet core).
(S-EID,G) State: refers to multicast state in multicast source and
receiver sites where S-EID is the IP address of the multicast
source host (its EID). An S-EID can appear in an IGMPv3 report,
an MSDP SA message or a PIM Join/Prune message that travels inside
of a site.
(S-RLOC,G) State: refers to multicast state in the core where S is a
source locator (the IP address of a multicast ITR) of a site with
a multicast source. The (S-RLOC,G) is mapped from the (S-EID,G)
entry by doing a mapping database lookup for the EID-Prefix that
S-EID maps to. An S-RLOC can appear in a PIM Join/Prune message
when it travels from an ETR to an ITR over the Internet core.
uLISP Site: a unicast-only LISP site according to [RFC9300] that has
not deployed the procedures of this specification and, therefore,
for multicast purposes, follows the procedures from Section 9. A
uLISP site can be a traditional multicast site.
LISP Site: a unicast LISP site (uLISP Site) that is also multicast
capable according to the procedures in this specification.
mPETR: this is a multicast proxy-ETR that is responsible for
advertising a very coarse EID-Prefix to which non-LISP and uLISP
sites can target their (S-EID,G) PIM Join/Prune messages. mPETRs
are used so LISP source multicast sites can send multicast packets
using source addresses from the EID namespace. mPETRs act as
Proxy-ETRs for supporting multicast routing in a LISP
infrastructure. It is likely a uPITR [RFC6832] and an mPETR will
be co-located since the single device advertises a coarse EID-
Prefix in the underlying unicast routing system.
Mixed Locator-Sets: this is a Locator-Set for a LISP database
mapping entry where the RLOC addresses in the Locator-Set are in
both IPv4 and IPv6 format.
Unicast Encapsulated PIM Join/Prune Message: this is a standard PIM
Join/Prune message (LISP-encapsulated with destination UDP port
4341) that is sent by ETRs at multicast receiver sites to an ITR
at a multicast source site. This message is sent periodically as
long as there are interfaces in the OIF-list for the (S-EID,G)
entry for which the ETR is joining.
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OIF-list: this is notation to describe the outgoing interface list a
multicast router stores per multicast routing table entry so it
knows on which interfaces to replicate multicast packets.
RPF: Reverse Path Forwarding is a procedure used by multicast
routers. A router will accept a multicast packet for forwarding
if the packet was received on the path that the router would use
to forward unicast packets to the multicast packet's source.
4. Basic Overview
LISP, when used for unicast routing, increases the site's ability to
control ingress traffic flows. Egress traffic flows are controlled
by the IGP in the source site. For multicast, the IGP coupled with
PIM can decide which path multicast packets ingress. By using the
Traffic Engineering features of LISP [RFC9300], a multicast source
site can control the egress of its multicast traffic. By controlling
the priorities of Locators from a mapping database entry, a source
multicast site can control which way multicast receiver sites join to
the source site.
At this point in time, there is no requirement for different Locator-
Sets, priority, and weight policies for multicast than there is for
unicast. However, when Traffic Engineering policies are different
for unicast versus multicast flows, it will be desirable to use
multicast-based priority and weight values in Map-Reply messages.
The fundamental multicast forwarding model is to encapsulate a
multicast packet into another multicast packet or a unicast packet.
An ITR will encapsulate multicast packets received from sources that
it serves in a LISP-Multicast header. The destination group address
from the inner header is copied to the destination address of the
outer header. Alternately, the destination address of the outer
header could be the RLOC of the ETR. The inner source address is the
EID of the multicast source host and the outer source address is the
RLOC of the encapsulating ITR.
The LISP-Multicast architecture will follow this high-level protocol
and operational sequence:
1. Receiver hosts in multicast sites will join multicast content
the way they do today -- they use IGMP. When they use IGMPv3
where they specify source addresses, they use source EIDs; that
is, they join (S-EID,G). If the multicast source is external to
this receiver site, the PIM Join/Prune message flows toward the
ETRs, finding the shortest exit (that is, the closest exit for
the Join/Prune message and the closest entrance for the
multicast packet to the receiver).
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2. The ETR does a mapping database lookup for S-EID. If the
mapping is cached from a previous lookup (from either a previous
Join/Prune for the source multicast site or a unicast packet
that went to the site), it will use the RLOC information from
the mapping. The ETR will use the same priority and weighting
mechanism as for unicast. So, the source site can decide which
way multicast packets egress.
3. When using a multicast underlay, the ETR will build two PIM
Join/Prune messages, one that contains an (S-EID,G) entry that
is unicast to the ITR that matches the RLOC the ETR selects, and
the other that contains an (S-RLOC,G) entry so the core network
can create multicast state from this ETR to the ITR.
4. When using a unicast underlay, the ETR will build one PIM Join/
Prune message, one that contains an (S-EID,G) entry that is
unicast to the ITR that matches the RLOC the ETR selects. This
PIM Join/ Prune message contains enough information to help the
ITR build ingress-replicated unicast trees.
5. When the ITR gets the unicast Join/Prune message (see Section 3
for formal definition), it will process (S-EID,G) entries in the
message and propagate them inside of the site where it has
explicit routing information for EIDs via the IGP.
6. When using the multicast underlay the ITR receives the
(S-RLOC,G) PIM Join/Prune message, it will process it like any
other join it would get in today's Internet. The S-RLOC address
is the IP address of this ITR.
7. When using the unicast underlay the ITR create an explicitly
tracked list of unicast RLOC addresses that have joined the
tree. This information is used for ingress-replication.
8. At this point, there is (S-EID,G) state from the joining host in
the receiver multicast site to the ETR of the receiver multicast
site. There is (S-RLOC,G) state across the core network from
the ETR of the multicast receiver site to the ITR in the
multicast source site and (S-EID,G) state in the source
multicast site. Note, the (S-EID,G) state is the same S-EID in
each multicast site. As other ETRs join the same multicast
tree, they can join through the same ITR (in which case the
packet replication is done in the core) or a different ITR (in
which case the packet replication is done at the source site).
9. When a packet is originated by the multicast host in the source
site, the packet will flow to one or more ITRs that will prepend
a LISP header.
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10. For the unicast underlay, the ITR makes individual copies for
every ETR, the outer destination is set to the S-RLOC of the ETR
and the outer source address is set to the S-RLOC of the ITR.
11. For the multicast underlay, the ITR copies the group address to
the outer destination address field, the ITR inserts its own
locator address in the outer source address field. The ITR will
look at its (S-RLOC,G) state, where S-RLOC is its own locator
address, and replicate the packet on each interface on which an
(S-RLOC,G) join was received. The core has (S-RLOC,G) so where
fan-out occurs to multiple sites, a core router will do packet
replication.
12. When either the source site or the core replicates the packet,
the ETR will receive a LISP packet with a destination group
address. It will decapsulate packets because it has receivers
for the group. Otherwise, it would not have received the
packets because it would not have joined. The ETR decapsulates
and does an (S-EID,G) lookup in its multicast Forwarding
Information Base (FIB) to forward packets out one or more
interfaces to forward the packet to internal receivers.
This architecture is consistent and scalable with the architecture
presented in [RFC9300] where multicast state in the core operates on
Locators, and multicast state at the sites operates on EIDs.
Alternatively, [RFC9300] also has a mechanism where (S-EID,G) state
can reside in the core through the use of RPF Vectors [RFC5496] in
PIM Join/Prune messages. However, few PIM implementations support
RPF Vectors, and LISP should avoid S-EID state in the core. See
Section 5 for details.
However, some observations can be made on the algorithm above. The
control plane can scale but at the expense of sending data to sites
that may have not joined the distribution tree where the encapsulated
data is being delivered. For example, one site joins (S-EID1,G), and
another site joins (S-EID2,G). Both EIDs are in the same multicast
source site. Both multicast receiver sites join to the same ITR with
state (S-RLOC,G) where S-RLOC is the RLOC for the ITR. The ITR joins
both (S-EID1,G) and (S-EID2,G) inside of the site. The ITR receives
(S-RLOC,G) joins and populates the OIF-list state for for the
(S-RLOC,G) entry. Since both (S-EID1,G) and (S-EID2, G) map to the
one (S-RLOC,G), packets will be delivered by the core to both
multicast receiver sites even though each have joined a single
source-based distribution tree. This behavior is a consequence of
the many-to-one mapping between S-EIDs and a S-RLOC.
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There is a possible solution to this problem that reduces the number
of many-to-one occurrences of (S-EID,G) entries aggregating into a
single (S-RLOC,G) entry. If a physical ITR can be assigned multiple
RLOC addresses and these addresses are advertised in mapping database
entries, then ETRs at receiver sites have more RLOC address options
and therefore can join different (RLOC,G) entries for each (S-EID,G)
entry joined at the receiver site. It would not scale to have a one-
to-one relationship between the number of S-EID sources at a source
site and the number of RLOCs assigned to all ITRs at the site, but
"n" can reduce to a smaller number in the "n-to-1" relationship. And
in turn, this reduces the opportunity for data packets to be
delivered to sites for groups not joined.
5. Source Addresses versus Group Addresses
Multicast group addresses don't have to be associated with either the
EID or RLOC namespace. They actually are a namespace of their own
that can be treated as logical with relatively opaque allocation.
So, by their nature, they don't detract from an incremental
deployment of LISP-Multicast.
As for source addresses, as in the unicast LISP scenario, there is a
decoupling of identification from location. In a LISP site, packets
are originated from hosts using their allocated EIDs. EID addresses
are used to identify the host as well as where in the site's topology
the host resides but not how and where it is attached to the
Internet.
Therefore, when multicast distribution tree state is created anywhere
in the network on the path from any multicast receiver to a multicast
source, EID state is maintained at the source and receiver multicast
sites, and RLOC state is maintained in the core. That is, a
multicast distribution tree will be represented as a 3-tuple of
{(S-EID,G) (S-RLOC,G) (S-EID,G)}, where the first element of the
3-tuple is the state stored in routers from the source to one or more
ITRs in the source multicast site; the second element of the 3-tuple
is the state stored in routers downstream of the ITR, in the core, to
all LISP receiver multicast sites; and the third element in the
3-tuple is the state stored in the routers downstream of each ETR, in
each receiver multicast site, reaching each receiver. Note that
(S-EID,G) is the same in both the source and receiver multicast
sites.
The concatenation/mapping from the first element to the second
element of the 3-tuples is done by the ITR, and from the second
element to the third element is done at the ETRs.
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6. Locator Reachability Implications on LISP-Multicast
Multicast state as it is stored in the core is always (S,G) state as
it exists today or (S-RLOC,G) state as it will exist when LISP sites
are deployed. The core routers cannot distinguish one from the
other. They don't need to because it is state that uses RPF against
the core routing tables in the RLOC namespace. The difference is
where the root of the distribution tree for a particular source is.
In the traditional multicast core, the source S is the source host's
IP address. For LISP-Multicast, the source S is a single ITR of the
multicast source site.
An ITR is selected based on the LISP EID-to-RLOC mapping used when an
ETR propagates a PIM Join/Prune message out of a receiver multicast
site. The selection is based on the same algorithm an ITR would use
to select an ETR when sending a unicast packet to the site. In the
unicast case, the ITR can change on a per-packet basis depending on
the reachability of the ETR. So, an ITR can change relatively easily
using local reachability state. However, in the multicast case, when
an ITR becomes unreachable, new distribution tree state must be built
because the encapsulating root has changed. This is more significant
than an RPF-change event, where any router would typically locally
change its RPF-interface for its existing tree state. But when an
encapsulating LISP-Multicast ITR goes unreachable, new distribution
state must be built and reflect the new encapsulator. Therefore,
when an ITR goes unreachable, all ETRs that are currently joined to
that ITR will have to trigger a new Join/Prune message for (S-RLOC,G)
to the new ITR as well as send a unicast encapsulated Join/Prune
message telling the new ITR which (S-EID,G) is being joined.
This issue can be mitigated by using anycast addressing for the ITRs,
so the problem does reduce to an RPF change in the core, but still
requires a unicast encapsulated Join/Prune message to tell the new
ITR about (S-EID,G). The problem with this approach is that the ETR
really doesn't know when the ITR has changed, so the new anycast ITR
will get the (S-EID,G) state only when the ETR sends it the next time
during its periodic sending procedures.
7. Multicast Protocol Changes
A number of protocols are used today for inter-domain multicast
routing:
IGMPv1-v3, MLDv1-v2: These protocols [RFC4604] do not require any
changes for LISP-Multicast for two reasons. One is that they are
link-local and not used over site boundaries, and the second is
that they advertise group addresses that don't need translation.
Where source addresses are supplied in IGMPv3 and Multicast
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Listener Discovery version 2 (MLDv2) messages, they are
semantically regarded as EIDs and don't need to be converted to
RLOCs until the multicast tree-building protocol, such as PIM, is
received by the ETR at the site boundary. Addresses used for IGMP
and MLD come out of the source site's allocated addresses, which
are therefore from the EID namespace.
MBGP: Even though the Multiprotocol Extensions for BGP-4 (MBGP)
[RFC4760] are not part of a multicast routing protocol, they are
used to find multicast sources when the unicast BGP peering
topology and the multicast MBGP peering topology are not
congruent. When MBGP is used in a LISP-Multicast environment, the
prefixes that are advertised are from the RLOC namespace. This
allows receiver multicast sites to find a path to the source
multicast site's ITRs. MBGP peering addresses will be from the
RLOC namespace. There are no MBGP changes required to support
LISP-Multicast.
MSDP: MSDP [RFC3618] is used to announce active multicast sources to
other routing domains (or LISP sites). The announcements come
from the PIM Rendezvous Points (RPs) from sites where there are
active multicast sources sending to various groups. In the
context of LISP-Multicast, the source addresses advertised in MSDP
will semantically be from the EID namespace since they describe
the identity of a source multicast host. It will be true that the
state stored in MSDP caches from core routers will be from the EID
namespace. An RP address inside of the site will be from the EID
namespace so it can be advertised and reached by an internal
unicast routing mechanism. However, for MSDP peer-RPF checking to
work properly across sites, the RP addresses must be converted or
mapped into a routable address that is advertised and maintained
in the BGP routing tables in the core. MSDP peering addresses can
come out of either the EID or a routable address namespace. Also,
the choice can be made unilaterally because the ITR at the site
will determine which namespace the destination peer address is out
of by looking in the mapping database service. There are no MSDP
changes required to support LISP-Multicast.
PIM-SSM: In the simplest form of distribution tree building, when
PIM operates in SSM mode [RFC4607], a source distribution tree is
built and maintained across site boundaries. In this case, there
is a small modification to how PIM Join/Prune messages are sent by
the LISP-Multicast component. No modifications to any message
format, but to support taking a Join/Prune message originated
inside of a LISP site with embedded addresses from the EID
namespace and converting them to addresses from the RLOC namespace
when the Join/Prune message crosses a site boundary. This is
similar to the requirements documented in [RFC5135].
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BIDIR-PIM: Bidirectional PIM [RFC5015] is typically run inside of a
routing domain, but if deployed in an inter-domain environment,
one would have to decide if the RP address of the shared tree
would be from the EID namespace or the RLOC namespace. If the RP
resides in a site-based router, then the RP address is from the
EID namespace. If the RP resides in the core where RLOC addresses
are routed, then the RP address is from the RLOC namespace. This
could be easily distinguishable if the EID address were in a well-
known address allocation block from the RLOC namespace. Also,
when using Embedded-RP for RP determination [RFC3956], the format
of the group address could indicate the namespace the RP address
is from. However, refer to Section 10 for considerations core
routers need to make when using Embedded-RP IPv6 group addresses.
When using BIDIR-PIM for inter-domain multicast routing, it is
recommended to use statically configured RPs. This allows core
routers to associate a Bidir group's RP address with an ITR's RLOC
address, and site routers to associate the Bidir group's RP
address as an EID address. With respect to Designated Forwarder
(DF) election in BIDIR-PIM, no changes are required since all
messaging and addressing is link-local.
PIM-ASM: The ASM mode of PIM [RFC7761], the most popular form of
PIM, is deployed in the Internet today by having shared trees
within a site and using source trees across sites. By the use of
MSDP and PIM-SSM techniques described above, multicast
connectivity can occur across LISP sites. Having said that, that
means there are no special actions required for processing (*,G)
or (S,G,R) Join/Prune messages since they all operate against the
shared tree that is site resident. Just like with ASM, there is
no (*,G) in the core when LISP-Multicast is in use. This is also
true for the RP-mapping mechanisms Auto-RP and Bootstrap Router
(BSR) [RFC5059].
Based on the protocol description above, the conclusion is that there
are no protocol message format changes, just a translation function
performed at the control plane. This will make for an easier and
faster transition for LISP since fewer components in the network have
to change.
It should also be stated just like it is in [RFC9300] that no host
changes, whatsoever, are required to have a multicast source host
send multicast packets and for a multicast receiver host to receive
multicast packets.
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7.1. PIM Join Attributes for LISP
There are specific PIM Join/Prune attributes to build multicast
distribution trees with either a native Multicast or Unicast
underlay. Please refer to [RFC8059] for such attributes.
These attributes are further enhanced in
[I-D.ietf-pim-jp-extensions-lisp]
8. LISP-Multicast Data-Plane Architecture
The LISP-Multicast data-plane operation conforms to the operation and
packet formats specified in [RFC9300]. However, encapsulating a
multicast packet from an ITR is a much simpler process. The process
is simply to copy the inner group address to the outer destination
address. And to have the ITR use its own IP address (its RLOC) as
the source address. The process is simpler for multicast because
there is no EID-to-RLOC mapping lookup performed during packet
forwarding.
In the decapsulation case, the ETR simply removes the outer header
and performs a multicast routing table lookup on the inner header
(S-EID,G) addresses. Then, the OIF-list for the (S-EID,G) entry is
used to replicate the packet on site-facing interfaces leading to
multicast receiver hosts.
There is no Data-Probe logic for ETRs as there can be in the unicast
forwarding case.
8.1. ITR Forwarding Procedure
The following procedure is used by an ITR, when it receives a
multicast packet from a source inside of its site:
1. A multicast data packet sent by a host in a LISP site will have
the source address equal to the host's EID and the destination
address equal to the address of the multicast group. It is
assumed the group information is obtained by current methods.
The same is true for a multicast receiver to obtain the source
and group address of a multicast flow.
2. When the ITR receives a multicast packet, it will have both S-EID
state and S-RLOC state stored. Since the packet was received on
a site-facing interface, the RPF lookup is based on the S-EID
state. If the RPF check succeeds, then the OIF-list contains
interfaces that are site facing and external facing. For the
site-facing interfaces, no LISP header is prepended. For the
external-facing interfaces a LISP header is prepended. When the
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ITR prepends a LISP header, it uses its own RLOC address as the
source address and copies the group address supplied by the IP
header that the host built as the outer destination address.
8.1.1. Multiple RLOCs for an ITR
Typically, an ITR will have a single RLOC address, but in some cases
there could be multiple RLOC addresses assigned from either the same
or different service providers. In this case, when (S-RLOC,G) Join/
Prune messages are received for each RLOC, there is a OIF-list
merging action that must take place. Therefore, when a packet is
received from a site-facing interface that matches on an (S-EID,G)
entry, the interfaces of the OIF-list from all (RLOC,G) entries
joined to the ITR as well as the site-facing OIF-list joined for
(S-EID,G) must be included in packet replication. In addition to
replicating for all types of OIF-lists, each OIF-list entry must be
tagged with the RLOC address, so encapsulation uses the outer source
address for the RLOC joined.
8.1.2. Multiple ITRs for a LISP Source Site
Note that when ETRs from different multicast receiver sites receive
(S-EID,G) joins, they may select a different S-RLOC for a multicast
source site due to policy (the multicast ITR can return different
multicast priority and weight values per ETR Map-Request). In this
case, the same (S-EID,G) is being realized by different (S-RLOC,G)
state in the core. This will not result in duplicate packets because
each ITR in the multicast source site will choose their own RLOC for
the source address for encapsulated multicast traffic. The RLOC
addresses are the ones joined by remote multicast ETRs.
When different (S-EID,G) traffic is combined into a single (RLOC,G)
core distribution tree, this may cause traffic to go to a receiver
multicast site when it does not need to. This happens when one
receiver multicast site joins (S1-EID,Gi) through a core distribution
tree of (RLOC1,Gi) and another multicast receiver site joins
(S2-EID,Gi) through the same core distribution tree of (RLOC1,Gi).
When ETRs decapsulate such traffic, they should know from their local
(S-EID,G) state if the packet should be forwarded. If there is no
(S-EID,G) state that matches the inner packet header, the packet is
discarded.
8.2. ETR Forwarding Procedure
The following procedure is used by an ETR, when it receives a
multicast packet from a source outside of its site:
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1. When a multicast data packet is received by an ETR on an
external-facing interface, it will do an RPF lookup on the S-RLOC
state it has stored. If the RPF check succeeds, the interfaces
from the OIF-list are used for replication to interfaces that are
site facing as well as interfaces that are external facing (this
ETR can also be a transit multicast router for receivers outside
of its site). When the packet is to be replicated for an
external-facing interface, the LISP encapsulation header is not
stripped. When the packet is replicated for a site-facing
interface, the encapsulation header is stripped.
2. The packet without a LISP header is now forwarded down the
(S-EID,G) distribution tree in the receiver multicast site.
8.3. Replication Locations
Multicast packet replication can happen in the following topological
locations:
* In an IGP multicast router inside a site that operates on S-EIDs.
* In a transit multicast router inside of the core that operates on
S-RLOCs.
* At one or more ETR routers depending on the path a Join/Prune
message exits a receiver multicast site.
* At one or more ITR routers in a source multicast site depending on
what priorities are returned in a Map-Reply to receiver multicast
sites.
In the last case, the source multicast site can do replication rather
than having a single exit from the site. But this can occur only
when the priorities in the Map-Reply are modified for different
receiver multicast sites so that the PIM Join/Prune messages arrive
at different ITRs.
This policy technique, also used in [RFC6836] for unicast, is useful
for multicast to mitigate the problems of changing distribution tree
state as discussed in Section 6.
9. LISP-Multicast Interworking
This section describes the multicast corollary to [RFC6832] regarding
the interworking of multicast routing among LISP and non-LISP sites.
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9.1. LISP and Non-LISP Mixed Sites
Since multicast communication can involve more than two entities to
communicate together, the combinations of interworking scenarios are
more involved. However, the state maintained for distribution trees
at the sites is the same, regardless of whether or not the site is
LISP enabled. So, most of the implications are in the core with
respect to storing routable EID-Prefixes from either PA or PI blocks.
Before enumerating the multicast interworking scenarios, let's define
three deployment states of a site:
* A non-LISP site that will run PIM-SSM or PIM-ASM with MSDP as it
does today. The addresses for the site are globally routable.
* A site that deploys LISP for unicast routing. The addresses for
the site are not globally routable. Let's define the name for
this type of site as a uLISP site.
* A site that deploys LISP for both unicast and multicast routing.
The addresses for the site are not globally routable. Let's
define the name for this type of site as a LISP-Multicast site.
A LISP site enabled for multicast purposes only will not be
considered in this document, but a uLISP site as documented in
[RFC6832] will be considered. In this section there is no discussion
of how a LISP site sends multicast packets when all receiver sites
are LISP-Multicast enabled; that has been discussed in previous
sections.
The following scenarios exist to make LISP-Multicast sites interwork
with non-LISP-Multicast sites:
1. A LISP site must be able to send multicast packets to receiver
sites that are a mix of non-LISP sites and uLISP sites.
2. A non-LISP site must be able to send multicast packets to
receiver sites that are a mix of non-LISP sites and uLISP sites.
3. A non-LISP site must be able to send multicast packets to
receiver sites that are a mix of LISP sites, uLISP sites, and
non-LISP sites.
4. A uLISP site must be able to send multicast packets to receiver
sites that are a mix of LISP sites, uLISP sites, and non-LISP
sites.
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5. A LISP site must be able to send multicast packets to receiver
sites which are a mix of LISP sites, uLISP sites, and non-LISP
sites.
9.1.1. LISP Source Site to Non-LISP Receiver Sites
In the first scenario, a site is LISP enabled for both unicast and
multicast traffic and as such operates on EIDs. Therefore, there is
a possibility that the EID-Prefix block is not routable in the core.
For LISP receiver multicast sites, this isn't a problem, but for non-
LISP or uLISP receiver multicast sites, when a PIM Join/Prune message
is received by the edge router, it has no route to propagate the
Join/Prune message out of the site. This is no different than the
unicast case that LISP Network Address Translation (LISP-NAT) in
[RFC6832] solves.
LISP-NAT allows a unicast packet that exits a LISP site to get its
source address mapped to a globally routable address before the ITR
realizes that it should not encapsulate the packet destined to a non-
LISP site. For a multicast packet to leave a LISP site, distribution
tree state needs to be built so the ITR can know where to send the
packet. So, the receiver multicast sites need to know about the
multicast source host by its routable address and not its EID
address. When this is the case, the routable address is the
(S-RLOC,G) state that is stored and maintained in the core routers.
It is important to note that the routable address for the host cannot
be the same as an RLOC for the site because it is desirable for ITRs
to process a PIM Join/Prune message that is received from an
external-facing interface. If the message will be propagated inside
of the site, the site-part of the distribution tree is built.
Using a globally routable source address allows non-LISP and uLISP
multicast receivers to join, create, and maintain a multicast
distribution tree. However, the LISP-Multicast receiver site will
want to perform an EID-to-RLOC mapping table lookup when a PIM Join/
Prune message is received on a site-facing interface. It does this
because it wants to find an (S-RLOC,G) entry to Join in the core.
So, there is a conflict of behavior between the two types of sites.
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The solution to this problem is the same as when an ITR wants to send
a unicast packet to a destination site but needs to determine if the
site is LISP enabled or not. When it is not LISP enabled, the ITR
does not encapsulate the packet. So, for the multicast case, when
the ETR receives a PIM Join/Prune message for (S-EID,G) state, it
will do a mapping table lookup on S-EID. In this case, S-EID is not
in the mapping database because the source multicast site is using a
routable address and not an EID-Prefix address. So, the ETR knows to
simply propagate the PIM Join/Prune message to an external-facing
interface without converting the (S-EID,G) because it is an (S,G),
where S is routable and reachable via core routing tables.
Now that the multicast distribution tree is built and maintained from
any non-LISP or uLISP receiver multicast site, the way the packet
forwarding model is used can be explained.
Since the ITR in the source multicast site has never received a
unicast encapsulated PIM Join/Prune message from any ETR in a
receiver multicast site, it knows there are no LISP-Multicast
receiver sites. Therefore, there is no need for the ITR to
encapsulate data. Since it will know a priori (via configuration)
that its site's EIDs are not routable (and not registered to the
mapping database system), it assumes that the multicast packets from
the source host are sent by a routable address. That is, it is the
responsibility of the multicast source host's system administrator to
ensure that the source host sends multicast traffic using a routable
source address. When this happens, the ITR acts simply as a router
and forwards the multicast packet like an ordinary multicast router.
There is an alternative to using a LISP-NAT scheme just as there is
an alternative to using unicast [RFC6832] forwarding by employing
Proxy Tunnel Routers (PxTRs). This can work the same way for
multicast routing as well, but the difference is that non-LISP and
uLISP sites will send PIM Join/Prune messages for (S-EID,G) that make
their way in the core to multicast PxTRs. Let's call this use of a
PxTR as a "Multicast Proxy-ETR" (or mPETR). Since the mPETRs
advertise very coarse EID-Prefixes, they draw the PIM Join/Prune
control traffic making them the target of the distribution tree. To
get multicast packets from the LISP source multicast sites, the tree
needs to be built on the path from the mPETR to the LISP source
multicast site. To make this happen, the mPETR acts as a "Proxy-ETR"
(where in unicast it acts as a "Proxy-ITR", or an uPITR [RFC6832]).
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The existence of mPETRs in the core allows source multicast site ITRs
to encapsulate multicast packets according to (S-RLOC,G) state. The
(S-RLOC,G) state is built from the mPETRs to the multicast ITRs. The
encapsulated multicast packets are decapsulated by mPETRs and then
forwarded according to (S-EID,G) state. The (S-EID,G) state is built
from the non-LISP and uLISP receiver multicast sites to the mPETRs.
9.1.2. Non-LISP Source Site to Non-LISP Receiver Sites
Clearly non-LISP-Multicast sites can send multicast packets to non-
LISP receiver multicast sites. That is what they do today. However,
discussion is required to show how non-LISP-Multicast sites send
multicast packets to uLISP receiver multicast sites.
Since uLISP receiver multicast sites are not targets of any (S,G)
state, they simply send (S,G) PIM Join/Prune messages toward the non-
LISP source multicast site. Since the source multicast site in this
case has not been upgraded to LISP, all multicast source host
addresses are routable. So, this case is simplified to where a uLISP
receiver multicast site appears to the source multicast site to be a
non-LISP receiver multicast site.
9.1.3. Non-LISP Source Site to Any Receiver Site
When a non-LISP source multicast site has receivers in either a non-
LISP/uLISP site or a LISP site, one needs to decide how the LISP
receiver multicast site will attach to the distribution tree. It is
known from Section 9.1.2 that non-LISP and uLISP receiver multicast
sites can join the distribution tree, but a LISP receiver multicast
site ETR will need to know if the source address of the multicast
source host is routable or not. It has been shown in Section 9.1.1
that an ETR, before it sends a PIM Join/Prune message on an external-
facing interface, does an EID-to-RLOC mapping lookup to determine if
it should convert the (S,G) state from a PIM Join/Prune message
received on a site-facing interface to an (S-RLOC,G). If the lookup
fails, the ETR can conclude the source multicast site is a non-LISP
site, so it simply forwards the Join/Prune message. (It also doesn't
need to send a unicast encapsulated Join/Prune message because there
is no ITR in a non-LISP site and there is namespace continuity
between the ETR and source.)
For a non-LISP source multicast site, (S-EID,G) state could be
limited to the edges of the network with the use of multicast proxy-
ITRs (mPITRs). The mPITRs can take native, unencapsulated multicast
packets from non-LISP source multicast and uLISP sites and
encapsulate them to ETRs in receiver multicast sites or to mPETRs
that can decapsulate for non-LISP receiver multicast or uLISP sites.
The mPITRs are responsible for sending (S-EID,G) joins to the non-
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LISP source multicast site. To connect the distribution trees
together, multicast ETRs will need to be configured with the mPITR's
RLOC addresses so they can send both (S-RLOC,G) joins to build a
distribution tree to the mPITR as well as configured for sending
unicast joins to mPITRs so they can propagate (S-EID,G) joins into
source multicast sites. The use of mPITRs is undergoing more study
and is a work in progress.
9.1.4. Unicast LISP Source Site to Any Receiver Sites
In the last section, it was explained how an ETR in a multicast
receiver site can determine if a source multicast site is LISP
enabled by looking into the mapping database. When the source
multicast site is a uLISP site, it is LISP enabled, but the ITR, by
definition, is not capable of doing multicast encapsulation. So, for
the purposes of multicast routing, the uLISP source multicast site is
treated as a non-LISP source multicast site.
Non-LISP receiver multicast sites can join distribution trees to a
uLISP source multicast site since the source site behaves, from a
forwarding perspective, as a non-LISP source site. This is also the
case for a uLISP receiver multicast site since the ETR does not have
multicast functionality built-in or enabled.
Special considerations are required for LISP receiver multicast
sites; since they think the source multicast site is LISP enabled,
the ETR cannot know if the ITR is LISP-Multicast enabled. To solve
this problem, each mapping database entry will have a multicast
2-tuple (Mpriority, Mweight) per RLOC [RFC9300]. When the Mpriority
is set to 255, the site is considered not multicast capable. So, an
ETR in a LISP receiver multicast site can distinguish whether a LISP
source multicast site is a LISP-Multicast site or a uLISP site.
9.1.5. LISP Source Site to Any Receiver Sites
When a LISP source multicast site has receivers in LISP, non-LISP,
and uLISP receiver multicast sites, it has a conflict about how it
sends multicast packets. The ITR can either encapsulate or natively
forward multicast packets. Since the receiver multicast sites are
heterogeneous in their behavior, one packet-forwarding mechanism
cannot satisfy both. However, if a LISP receiver multicast site acts
like a uLISP site, then it could receive packets like a non-LISP
receiver multicast site, thereby making all receiver multicast sites
have homogeneous behavior. However, this poses the following issues:
* LISP-NAT techniques with routable addresses would be required in
all cases.
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* Or, alternatively, mPETR deployment would be required, thus
forcing coarse EID-Prefix advertisement in the core.
* But, what is most disturbing is that when all sites that
participate are LISP-Multicast sites but a non-LISP or uLISP site
joins the distribution tree, then the existing joined LISP
receiver multicast sites would have to change their behavior.
This would create too much dynamic tree-building churn to be a
viable alternative.
So, the solution space options are:
1. Make the LISP ITR in the source multicast site send two packets,
one that is encapsulated with (S-RLOC,G) to reach LISP receiver
multicast sites and another that is not encapsulated with
(S-EID,G) to reach non-LISP and uLISP receiver multicast sites.
2. Make the LISP ITR always encapsulate packets with (S-RLOC,G) to
reach LISP-Multicast sites and to reach mPETRs that can
decapsulate and forward (S-EID,G) packets to non-LISP and uLISP
receiver multicast sites.
9.2. LISP Sites with Mixed Address Families
A LISP database mapping entry that describes the Locator-Set,
Mpriority, and Mweight per locator address (RLOC), for an EID-Prefix
associated with a site could have RLOC addresses in either IPv4 or
IPv6 format. When a mapping entry has a mix of RLOC-formatted
addresses, it is an implicit advertisement by the site that it is a
dual-stack site. That is, the site can receive IPv4 or IPv6 unicast
packets.
To distinguish if the site can receive dual-stack unicast packets as
well as dual-stack multicast packets, the Mpriority value setting
will be relative to an IPv4 or IPv6 RLOC See [RFC9300] for packet
format details.
If one considers the combinations of LISP, non-LISP, and uLISP sites
sharing the same distribution tree and considering the capabilities
of supporting IPv4, IPv6, or dual-stack, the number of total
combinations grows beyond comprehension.
Using some combinatorial math, the following profiles of a site and
the combinations that can occur:
1. LISP-Multicast IPv4 Site
2. LISP-Multicast IPv6 Site
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3. LISP-Multicast Dual-Stack Site
4. uLISP IPv4 Site
5. uLISP IPv6 Site
6. uLISP Dual-Stack Site
7. non-LISP IPv4 Site
8. non-LISP IPv6 Site
9. non-LISP Dual-Stack Site
Let's define (m n) = m!/(n!*(m-n)!), pronounced "m choose n" to
illustrate some combinatorial math below.
When 1 site talks to another site, the combinatorial is (9 2), when 1
site talks to another 2 sites, the combinatorial is (9 3). If we sum
this up to (9 9), then:
(9 2) + (9 3) + (9 4) + (9 5) + (9 6) + (9 7) + (9 8) + (9 9) =
36 + 84 + 126 + 126 + 84 + 36 + 9 + 1
which results in 502 as the total number of cases to be considered.
This combinatorial gets even worse when one considers a site using
one address family inside of the site and the xTRs using the other
address family (as in using IPv4 EIDs with IPv6 RLOCs or IPv6 EIDs
with IPv4 RLOCs).
To rationalize this combinatorial nightmare, there are some
guidelines that need to be put in place:
* Each distribution tree shared between sites will either be an IPv4
distribution tree or an IPv6 distribution tree. Therefore, head-
end replication can be avoided by building and sending packets on
each address-family-based distribution tree. Even though there
might be an urge to do multicast packet translation from one
address family format to the other, it is a non-viable over-
complicated urge. Multicast ITRs will only encapsulate packets
where the inner and outer headers are from the same address
family.
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* All LISP sites on a multicast distribution tree must share a
common address family that is determined by the source site's
Locator-Set in its LISP database mapping entry. All receiver
multicast sites will use the best RLOC priority controlled by the
source multicast site. This is true when the source site is
either LISP-Multicast or uLISP enabled. This means that priority-
based policy modification is prohibited. When a receiver
multicast site ETR receives an (S-EID,G) join, it must select a
S-RLOC for the same address family as S-EID.
* When a multicast Locator-Set has more than one locator, only
locators from the same address family MUST be set to the same best
priority value. A mixed Locator-Set can exist (for unicast use),
but the multicast priorities MUST be the set for the same address
family locators.
* When the source site is not LISP enabled, determining the address
family for the flow is up to how receivers find the source and
group information for a multicast flow.
9.3. Making a Multicast Interworking Decision
Thus far, Section 9 has shown all combinations of multicast
connectivity that could occur. As already concluded, this can be
quite complicated, and, if the design is too ambitious, the dynamics
of the protocol could cause a lot of instability.
The trade-off decisions are hard to make, and so the same single
solution is desirable to work for both IPv4 and IPv6 multicast. It
is imperative to have an incrementally deployable solution for all of
IPv4 unicast and multicast and IPv6 unicast and multicast while
minimizing (or eliminating) both unicast and multicast EID namespace
state.
Therefore, the design decision to go with uPITRs [RFC6832] for
unicast routing and mPETRs for multicast routing seems to be the
sweet spot in the solution space in order to optimize state
requirements and avoid head-end data replication at ITRs.
10. Considerations When RP Addresses Are Embedded in Group Addresses
When ASM and PIM-BIDIR are used in an IPv6 inter-domain environment,
a technique exists to embed the unicast address of an RP in an IPv6
group address [RFC3956]. When routers in end sites process a PIM
Join/Prune message that contains an Embedded-RP group address, they
extract the RP address from the group address and treat it from the
EID namespace. However, core routers do not have state for the EID
namespace and need to extract an RP address from the RLOC namespace.
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Therefore, it is the responsibility of ETRs in multicast receiver
sites to map the group address into a group address where the
Embedded-RP address is from the RLOC namespace. The mapped RP
address is obtained from an EID-to-RLOC mapping database lookup. The
ETR will also send a unicast (*,G) Join/Prune message to the ITR so
the branch of the distribution tree from the source site resident RP
to the ITR is created.
This technique is no different than the techniques described in this
specification for translating (S,G) state and propagating Join/Prune
messages into the core. The only difference is that the (*,G) state
in Join/Prune messages are mapped because they contain unicast
addresses encoded in an Embedded-RP group address.
11. Taking Advantage of Upgrades in the Core
If the core routers are upgraded to support [RFC5496], then the EID-
specific data can be passed through the core without, possibly,
having to store the state in the core.
By doing this, one can eliminate the ETR from unicast encapsulated
PIM Join/Prune messages to the source site's ITR.
However, this solution is restricted to a small set of workable cases
that would not be good for general use of LISP-Multicast. In
addition, due to slow convergence properties, it is not recommended
for LISP-Multicast.
12. Mtrace Considerations
Mtrace functionality MUST be consistent with unicast traceroute
functionality where all hops from multicast receiver to multicast
source are visible.
The design for mtrace for use in LISP-Multicast environments is to be
determined but should build upon mtrace version 2 specified in
[MTRACE].
13. Security Considerations
The security concerns for LISP-Multicast are mainly the same as for
the base LISP specification [RFC9300] and for multicast in general,
including PIM-ASM [RFC7761].
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There may be a security concern with respect to unicast PIM messages.
When multiple receiver sites are joining an (S-EID1,G) distribution
tree that maps to a (RLOC1,G) core distribution tree, and a malicious
receiver site joins an (S-EID2,G) distribution tree that also maps to
the (RLOC1,G) core distribution tree, the legitimate sites will
receive data from S-EID2 when they did not ask for it.
Other than as noted above, there are currently no known security
differences between multicast with LISP and multicast without LISP.
However, this has not been a topic that has been investigated deeply
so far; therefore, additional issues might arise in future.
14. Acknowledgments
The authors would like to gratefully acknowledge the people who have
contributed discussion, ideas, and commentary to the making of this
proposal and specification. People who provided expert review were
Scott Brim, Greg Shepherd, and Dave Oran. Other commentary from
discussions at the Summer 2008 IETF in Dublin were Toerless Eckert
and IJsbrand Wijnands.
The authors would also like to thank the MBONED working group for
constructive and civil verbal feedback when this document was
presented at the Fall 2008 IETF in Minneapolis. In particular, good
commentary came from Tom Pusateri, Steve Casner, Marshall Eubanks,
Dimitri Papadimitriou, Ron Bonica, Lenny Guardino, Alia Atlas, Jesus
Arango, and Jari Arkko.
An expert review of this specification was done by Yiqun Cai and
Liming Wei. The authors thank them for their detailed comments.
This work originated in the Routing Research Group (RRG) of the IRTF.
An individual submission was converted into a LISP working group
document.
15. References
15.1. Normative References
[I-D.ietf-pim-jp-extensions-lisp]
Govindan, V. P. and S. Venaas, "PIM Join/ Prune Attributes
for LISP Environments using Underlay Multicast", Work in
Progress, Internet-Draft, draft-ietf-pim-jp-extensions-
lisp-05, 26 February 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-pim-jp-
extensions-lisp-05>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3618] Fenner, B., Ed. and D. Meyer, Ed., "Multicast Source
Discovery Protocol (MSDP)", RFC 3618,
DOI 10.17487/RFC3618, October 2003,
<https://www.rfc-editor.org/info/rfc3618>.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, DOI 10.17487/RFC3956, November 2004,
<https://www.rfc-editor.org/info/rfc3956>.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, DOI 10.17487/RFC4604,
August 2006, <https://www.rfc-editor.org/info/rfc4604>.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<https://www.rfc-editor.org/info/rfc4607>.
[RFC4610] Farinacci, D. and Y. Cai, "Anycast-RP Using Protocol
Independent Multicast (PIM)", RFC 4610,
DOI 10.17487/RFC4610, August 2006,
<https://www.rfc-editor.org/info/rfc4610>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
<https://www.rfc-editor.org/info/rfc5015>.
[RFC5135] Wing, D. and T. Eckert, "IP Multicast Requirements for a
Network Address Translator (NAT) and a Network Address
Port Translator (NAPT)", BCP 135, RFC 5135,
DOI 10.17487/RFC5135, February 2008,
<https://www.rfc-editor.org/info/rfc5135>.
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[RFC5496] Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
Forwarding (RPF) Vector TLV", RFC 5496,
DOI 10.17487/RFC5496, March 2009,
<https://www.rfc-editor.org/info/rfc5496>.
[RFC6831] Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
Locator/ID Separation Protocol (LISP) for Multicast
Environments", RFC 6831, DOI 10.17487/RFC6831, January
2013, <https://www.rfc-editor.org/info/rfc6831>.
[RFC6832] Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
"Interworking between Locator/ID Separation Protocol
(LISP) and Non-LISP Sites", RFC 6832,
DOI 10.17487/RFC6832, January 2013,
<https://www.rfc-editor.org/info/rfc6832>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC8059] Arango, J., Venaas, S., Kouvelas, I., and D. Farinacci,
"PIM Join Attributes for Locator/ID Separation Protocol
(LISP) Environments", RFC 8059, DOI 10.17487/RFC8059,
January 2017, <https://www.rfc-editor.org/info/rfc8059>.
[RFC9300] Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
Cabellos, Ed., "The Locator/ID Separation Protocol
(LISP)", RFC 9300, DOI 10.17487/RFC9300, October 2022,
<https://www.rfc-editor.org/info/rfc9300>.
15.2. Informative References
[MTRACE] Asaeda, H. and W. Lee, Ed., "Mtrace Version 2: Traceroute
Facility for IP Multicast", Work in Progress, October
2012.
[RFC5059] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
"Bootstrap Router (BSR) Mechanism for Protocol Independent
Multicast (PIM)", RFC 5059, DOI 10.17487/RFC5059, January
2008, <https://www.rfc-editor.org/info/rfc5059>.
[RFC6836] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol Alternative Topology
(LISP+ALT)", RFC 6836, January 2013,
<https://www.rfc-editor.org/rfc/rfc6836>.
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Appendix A. Document Change Log
[RFC Editor: Please delete this section on publication as RFC.]
A.1. Changes to draft-ietf-lisp-rfc6831bis-00
* Posted August 2024.
* Make draft-farinacci-lisp-rfc6831bis-02 working group document
draft-ietf-lisp-rfc6831bis-00.
A.2. Changes to draft-farinacci-lisp-rfc6831bis-02
* Posted May 2024 (Vengada Prasad Govindan).
* Added reference to RFC8059
A.3. Changes to draft-farinacci-lisp-rfc6831bis-01
* Posted May 2024 (Vengada Prasad Govindan).
* Add support for unicast underlay in signal-based approach
(RFC6831)
A.4. Changes to draft-farinacci-lisp-rfc6831bis-00
* Posted May 2024.
* Starting with [RFC6831] to move it to a bis document for standards
track.
* Changed references to standards track RFCs.
* Address comments from Prasad Govindan.
Authors' Addresses
Dino Farinacci
Cisco Systems
Tasman Drive
San Jose, CA
United States of America
Email: farinacci@gmail.com
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Dave Meyer
Cisco Systems
Tasman Drive
San Jose, CA
United States of America
Email: dmm@1-4-5.net
John Zwiebel
Cisco Systems
Tasman Drive
San Jose, CA
United States of America
Email: jzwiebel@cruzio.com
Stig Venaas
Cisco Systems
Tasman Drive
San Jose, CA
United States of America
Email: stig@cisco.com
Vengada Prasad Govindan
Cisco Systems
Email: venggovi@cisco.com
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