Network Working Group D. Farinacci
Internet-Draft D. Meyer
Intended status: Experimental J. Zwiebel
Expires: November 27, 2009 S. Venaas
cisco Systems
May 26, 2009
LISP for Multicast Environments
draft-ietf-lisp-multicast-00.txt
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Abstract
This draft describes how inter-domain multicast routing will function
in an environment where Locator/ID Separation is deployed using the
LISP architecture.
Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 6
4. Basic Overview . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Source Addresses versus Group Addresses . . . . . . . . . . . 12
6. Locator Reachability Implications on LISP-Multicast . . . . . 13
7. Multicast Protocol Changes . . . . . . . . . . . . . . . . . . 14
8. LISP-Multicast Data-Plane Architecture . . . . . . . . . . . . 16
8.1. ITR Forwarding Procedure . . . . . . . . . . . . . . . . . 16
8.2. ETR Forwarding Procedure . . . . . . . . . . . . . . . . . 16
8.3. Replication Locations . . . . . . . . . . . . . . . . . . 17
9. LISP-Multicast Interworking . . . . . . . . . . . . . . . . . 18
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 . . . 20
9.1.3. Non-LISP Source Site to Any Receiver Site . . . . . . 21
9.1.4. Unicast LISP Source Site to Any Receiver Sites . . . 21
9.1.5. LISP Source Site to Any Receiver Sites . . . . . . . . 22
9.2. LISP Sites with Mixed Address Families . . . . . . . . . . 22
9.3. Making a Multicast Interworking Decision . . . . . . . . . 24
10. Considerations when RP Addresses are Embedded in Group
Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11. Taking Advantage of Upgrades in the Core . . . . . . . . . . . 26
12. Security Considerations . . . . . . . . . . . . . . . . . . . 27
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
14.1. Normative References . . . . . . . . . . . . . . . . . . . 29
14.2. Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
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1. 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].
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2. Introduction
The Locator/ID Separation Architecture [LISP] provides a mechanism to
separate out Identification and Location semantics from the current
definition of an IP address. By creating two namespaces, an EID
namespace used by sites and a 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 specifications
focuses on to map 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.
3. What protocols are affected and what changes are required to such
multicast protocols.
4. How ASM-mode, SSM-mode, and Bidir-mode service models will
operate.
5. How multicast packet flow will occur for multiple combinations of
LISP and non-LISP capable 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.
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-
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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 Multi-
protocol BGP (MBGP) [RFC4760]. The approach proposed in this
specification requires no 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 which are
LISP-enabled.
Therefore, we see changes only to PIM-ASM [RFC4601], MSDP [RFC3618],
and PIM-SSM [RFC4607]. Bidir-PIM [RFC5015], which typically does not
run in an inter-domain environment is not addressed in depth in this
version of the specification.
Also, the current version of this specification does not describe
multicast-based Traffic Engineering relative to the TE-ITR and TE-ETR
descriptions in [LISP].
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3. Definition of Terms
The terminology in this section is consistent with the definitions in
[LISP] 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 the host is located in. 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 a
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 which accepts an IP multicast
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
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sites which 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
which 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. Used synonymously with the term "Tunnel
Router". For example, "An xTR can be located at the Customer Edge
(CE) router", meaning 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
[RFC4601] specification. For this specification, the term is used
generally to refer to multicast state. Based on its topological
location, the (S,G) state 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
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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 (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 [LISP] which has
not deployed the procedures of this specification and therefore,
for multicast purposes, follows the procedures from Section 9.
mPTR: this is a multicast PTR that is responsible for advertising a
very coarse EID prefix which non-LISP and uLISP sites can target
their (S-EID,G) PIM Join/Prune message to. mPTRs are used so LISP
source multicast sites can send multicast packets using source
addresses from the EID namespace. mPTRs act as Proxy ETRs for
supporting multicast routing in a LISP infrastructure.
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 PIM Join/Prune Message: this is a standard PIM Join/Prune
message (encapsulated in an IP header with protocol number 103)
which 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
the ETR is joining for.
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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, 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, we don't see a requirement for different
locator-sets, priority, and weight policies for multicast than we
have for unicast.
The fundamental multicast forwarding model is to encapsulate a
multicast packet into another multicast packet. An ITR will
encapsulate multicast packets received from sources that it serves in
another LISP multicast header. The destination group address from
the inner header is copied to the destination address of the outer
header. 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 S-EID is a local multicast source host.
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 but it is the closest entrance for the multicast packet
to the receiver).
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.
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3. The ETR will build two PIM Join/Prune messages, one that contains
a (S-EID,G) entry that is unicast to the ITR that matches the
RLOC the ETR selects, and the other which contains a (S-RLOC,G)
entry so the core network can create multicast state from this
ETR to the ITR.
4. 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. When 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.
5. 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).
6. When a packet is originated by the multicast host in the source
site, it will flow to one or more ITRs which will prepend a LISP
header by copying the group address to the outer destination
address field and insert 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 a (S-RLOC,G) joined was received on. The core
has (S-RLOC,G) so where fanout occurs to multiple sites, a core
router will do packet replication.
7. 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 also decapsulate packets because it has
receivers for the group. Otherwise, it would have not received
the packets because it would not have joined. The ETR
decapsulates and does a (S-EID,G) lookup in its multicast 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 [LISP] where multicast state in the core operates on
locators and multicast state at the sites operates on EIDs.
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Alternatively, [LISP] does present a mechanism where (S-EID,G) state
can reside in the core through the use of RPF-vectors [RPFV] in PIM
Join/Prune messages. However, this will require EID state in core as
well as the use of RPF-vector formatted Join/Prune messages which are
not the default implementation choice. So we choose a design that
can allow the separation of namespaces as unicast LISP provides. It
will be at the expense of creating new (S-RLOC,G) state when ITRs go
unreachable. See Section 5 for details.
However, we have some observations on the algorithm above. We can
scale the control plane but at the expense of sending data to sites
which 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 it. 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.
There is a possible solution to this problem which 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 we
can reduce the "n" to a smaller number in the "n-to-1" relationship.
And in turn, reduce the opportunity for data packets to be delivered
to sites for groups not joined.
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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, those 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 the 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 RPFs 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 goes 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 rebuilt 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 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 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.
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7. Multicast Protocol Changes
A number of protocols are used today for inter-domain multicast
routing:
IGMPv1-v3, MLDv1-v2: These protocols do not require any changes for
LISP-Multicast for two reasons. One being that they are link-
local and not used over site boundaries and second they advertise
group addresses that don't need translation. Where source
addresses are supplied in IGMPv3 and 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 MBGP is not a multicast routing protocol, it is
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 which 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.
MSDP: MSDP 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 site will be from the EID
namespace so it can be advertised and reached by 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. And
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.
PIM-SSM: In the simplest form of distribution tree building, when
PIM operates in SSM mode, a source distribution tree is built and
maintained across site boundaries. In this case, there is a small
modification to the operation of the PIM protocol (but not to any
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message format) 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 [MNAT].
PIM-Bidir: Bidirectional PIM 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 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. With respect
to DF-election in Bidir PIM, no changes are required since all
messaging and addressing is link-local.
PIM-ASM: The way ASM mode PIM, the most popular form of PIM, is
deployed in the Internet today is by having shared-trees within a
site and using source-trees across sites. By the use of MSDP and
PIM-SSM techniques described above, we can get multicast
connectivity 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 which is site resident. This is also true for the RP-
mapping mechanisms Auto-RP and BSR.
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 [LISP] 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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8. LISP-Multicast Data-Plane Architecture
The LISP-Multicast data-plane operation conforms to the operation and
packet formats specified in [LISP]. 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), and
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 group 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
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 the host built as the outer destination address.
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 are 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:
o In an IGP multicast router inside a site which operates on S-EIDs.
o In a transit multicast router inside of the core which operates on
S-RLOCs.
o At one or more ETR routers depending on the path a Join/Prune
message exits a receiver multicast site.
o 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 only can occur
when the priorities in the Map-Reply are modified for different
receiver multicast site so that the PIM Join/Prune messages arrive at
different ITRs.
This policy technique, also used in [ALT] for unicast, is useful for
multicast to mitigate the problems of changing distribution tree
state as discussed in Section 6.
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9. LISP-Multicast Interworking
This section will describe the multicast corollary to [INTWORK] which
describes the interworking of multicast routing among LISP and non-
LISP sites.
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 or not. So most of the implications are in the core
with respect to storing routable EID prefixes from either PA or PI
blocks.
Before we enumerate the multicast interworking scenarios, we must
define 3 deployment states of a site:
o A non-LISP site which will run PIM-SSM or PIM-ASM with MSDP as it
does today. The addresses for the site are globally routable.
o 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.
o 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.
We will not consider a LISP site enabled for multicast purposes only
but do consider a uLISP site as documented in [INTWORK]. In this
section we don't discuss 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 which 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 which 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 which are a mix of LISP sites, uLISP sites, and
non-LISP sites.
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4. A uLISP site must be able to send multicast packets to receiver
sites which are a mix of LISP sites, uLISP sites, and non-LISP
sites.
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 capable 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-NAT in [INTWORK] 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 we want the ITRs to
process a received PIM Join/Prune message from an external-facing
interface to be propagated inside of the site so the site-part of the
distribution tree is built.
Using a globally routable source address allows non-LISP and uLISP
multicast receiver 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 a (S-RLOC,G) entry to Join in the core. So
we have a conflict of behavior between the two types of sites.
The solution to this problem is the same as when an ITR wants to send
a unicast packet to a destination site but needs determine if the
site is LISP capable or not. When it is not LISP capable, the ITR
does not encapsulate the packet. So for the multicast case, when 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
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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 a 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 packet
forwarding model is performed can be explained.
Since the ITR in the source multicast site has never received a
unicast 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, 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 like there is
for unicast [INTWORK] forwarding by using Proxy Tunnel Routers
(PTRs). 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) which make their way in the core to
PTRs. Let's call this use of a PTR as a "Multicast PTR" (or mPTR).
Since the PTRs 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 mPTR to the LISP
source multicast site. To make this happen the mPTR acts as a "Proxy
ETR" (where in unicast it acts as a "Proxy ITR").
The existence of mPTRs in the core allows LISP source multicast site
ITRs to encapsulate multicast packets so the state built between the
ITRs and the mPTRs is (S-RLOC,G) state. Then the mPTRs can
decapsulate packets and forward natively to the non-LISP and uLISP
receiver multicast sites.
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.
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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 looks to the source multicast site as 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. We
know 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. We showed in Section 9.1.1 that an
ETR, before it sends a PIM Join/Prune message on an external-facing
interface, does a 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 a (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 Join/Prune message because there is no ITR in a non-LISP
site and there is namespace continuity between the ETR and source).
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 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 capable, the ETR
cannot know if ITR is LISP-Multicast capable. To solve this problem,
each mapping database entry will have a multicast 2-tuple (Mpriority,
Mweight) per RLOC. When the Mpriority is set to 255, the site is
considered not multicast capable. So an ETR in a LISP receiver
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multicast site can distinguish whether a LISP source multicast site
is LISP-Multicast site from 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 making all receiver multicast sites have
homogeneous behavior. However, this poses the following issues:
o LISP-NAT techniques with routable addresses would be required in
all cases.
o Or alternatively, mPTR deployment would be required forcing coarse
EID prefix advertisement in the core.
o But what is most disturbing is that when all sites that
participate are LISP-Multicast sites but then 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 mPTRs 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
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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 [LISP] for packet format
details.
If you consider 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, we have the following profiles of a
site and the combinations that can occur:
1. LISP-Multicast IPv4 Site
2. LISP-Multicast IPv6 Site
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
Lets 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 sum
this up to (9 9), we have:
(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 the total number of cases to be considered at 502.
This combinatorial gets even worse when you consider a site using one
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address family inside of the site and the xTRs use 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 which need to be put in place:
o Each distribution tree shared among sites will either be an IPv4
distribution tree or an IPv6 distribution tree. Therefore, we can
avoid head-end replication 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.
o All LISP sites on a multicast distribution tree must share a
common address family which 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 capable. This means that priority-
based policy modification is prohibited.
o When the source site is not LISP capable, it is up to how
receivers find the source and group information for a multicast
flow. That mechanism decides the address family for the flow.
9.3. Making a Multicast Interworking Decision
This Multicast Interworking section has shown all combinations of
multicast connectivity that could occur. As you might have 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 we want the same single
solution 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 PTRs for unicast routing and
mPTRs for multicast routing seems to be the sweet spot in the
solution space so we can optimize state requirements and avoid head-
end data replication at ITRs.
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10. Considerations when RP Addresses are Embedded in Group Addresses
When ASM and PIM-Bidir is used in an IPv6 inter-domain environment, a
technique exists to embed the unicast address of an RP in a IPv6
group address [RFC3956]. When routers in end sites process a PIM
Join/Prune message which contain 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, need to extract an RP address from the RLOC namespace.
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 a 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.
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11. Taking Advantage of Upgrades in the Core
If the core routers are upgraded to support [RPFV] and [JOIN-ATTR],
then we can pass EID specific data through the core without,
possibly, having to store the state in the core.
By doing this we can eliminate the ETR from unicasting PIM Join/Prune
messages to the source site's ITR.
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12. Security Considerations
Refer to the [LISP] specification.
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13. 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 Summer 2008 Dublin IETF were Toerless Eckert and
Ijsbrand Wijnands.
We would also like to thank the MBONED working group for constructive
and civil verbal feedback when this draft 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, and Lenny Guardino.
This work originated in the Routing Research Group (RRG) of the IRTF.
The individual submission [MLISP] was converted into this IETF LISP
working group draft.
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14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3618] Fenner, B. and D. Meyer, "Multicast Source Discovery
Protocol (MSDP)", RFC 3618, October 2003.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[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, August 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, October 2007.
14.2. Informative References
[ALT] Farinacci, D., Fuller, V., and D. Meyer, "LISP Alternative
Topology (LISP-ALT)", draft-fuller-lisp-alt-02.txt (work
in progress), April 2008.
[INTWORK] Lewis, D., Meyer, D., and D. Farinacci, "Interworking LISP
with IPv4 and IPv6", draft-lewis-lisp-interworking-00.txt
(work in progress), December 2007.
[JOIN-ATTR]
Wijnands, IJ. and A. Boers, "Format for using TLVs in PIM
messages", draft-ietf-pim-join-attributes-03.txt (work in
progress), May 2007.
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[LISP] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-00.txt (work in progress), May 2009.
[MLISP] Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas,
"LISP for Multicast Environments",
draft-farinacci-lisp-multicast-01.txt (work in progress),
November 2008.
[MNAT] Wing, D. and T. Eckert, "Multicast Requirements for a
Network Address (and port) Translator (NAT)",
draft-ietf-behave-multicast-07.txt (work in progress),
June 2007.
[RPFV] Wijnands, IJ., Boers, A., and E. Rosen, "The RPF Vector
TLV", draft-ietf-pim-rpf-vector-06.txt (work in progress),
February 2008.
Farinacci, et al. Expires November 27, 2009 [Page 30]
Internet-Draft LISP for Multicast Environments May 2009
Authors' Addresses
Dino Farinacci
cisco Systems
Tasman Drive
San Jose, CA
USA
Email: dino@cisco.com
Dave Meyer
cisco Systems
Tasman Drive
San Jose, CA
USA
Email: dmm@cisco.com
John Zwiebel
cisco Systems
Tasman Drive
San Jose, CA
USA
Email: jzwiebel@cisco.com
Stig Venaas
cisco Systems
Tasman Drive
San Jose, CA
USA
Email: stig@cisco.com
Farinacci, et al. Expires November 27, 2009 [Page 31]
