PIM Join/ Prune Attributes for LISP Environments using Underlay Multicast
Ciscovenggovi@cisco.com
Routing
Internet Engineering Task ForcetemplateThis document specifies an extension to PIM Join/ Prune messages.
This document defines one PIM Join/ Prune attribute that support the
construction of multicast distribution trees where the root and
receivers are located in different Locator/ID Separation Protocol
(LISP) sites using underlay IP Multicast. This attribute allows the receiver site to signal
the underlay multicast group to the control plane of the root ITR (Ingress Tunnel Router).
Introduction
The construction of multicast distribution trees where the root and
receivers are located in different LISP sites is defined in
. specifies that (root-EID,G) data packets are to be LISP-
encapsulated into (root-RLOC,G) multicast packets. This document defines a TLV that facilitates the
construction of trees for (root-RLOC, G).
Specifically, the assignment of the underlay multicast group needs to be done in consonance with
the downstream xTR nodes and avoid unnecessary replication or traffic hairpinning.
Since the Receiver RLOC Attribute TLV defined in only
addresses the Ingress Replication case, an additional TLV is defined by this draft to include
scenarios where the underlay uses Multicast transport. The TLV definition proposed here complies
with the base specification .
This document uses terminology defined in , such as EID,
RLOC, ITR, and ETR. Requirements LanguageThe 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 RFC 2119.When LISP based Multicast trees are built using IP Multicast in the underlay, the mapping between
the overlay group address and the underlay group address becomes a very crucial. It is possible
that under certain circumstances, differnt subsets of xTRs subscribing to the same overlay
multicast stream would be constrained to use different underlay multicast mapping ranges.
This definitely involves a trade-off between replication and the flexibility in
assigning address ranges and could be required in certain situations as below:
Inter-site PxTR:
When multiple LISP sites are connected through a LISP based transit, the site border node
interconnects the site-facing interfaces and the external LISP based core. Under such circumstances,
there could be different ranges of multicast group addresses used for building the (S-RLOC, G) trees
inside the LISP site and the external LISP based core. This is desired for various reasons:
Other Use-cases:
TBD
Editorial Note: Comments from Stig: There should be some text indicating that the group address used should ideally only be used for LISP encapsulation (if ASM), and perhaps that it is preferrable to use an SSM group. Also, that the group obviously must be a group that the underlay supports/allows. I think it is also worth noting that ideally, different ETRs should request the same group.
F-bit:
The Transitive bit. Specifies whether this attribute is transitive or non-transitive. MUST be set to zero. This attribute is ALWAYS non-transitive.
E-bit:
End-of-Attributes bit. Specifies whether this attribute is the last. Set to zero if there are more attributes. Set to 1 if this is the last attribute.
Type:
The Receiver Group Attribute type is TBD.
Length:
The length in octets of the attribute value. MUST be set to the length in octets of the receiver group address plus one octet to account for the Address Family field.
Addr Family:
The PIM Address Family of the receiver group as defined in .
Receiver Group:
The Multicast Group address on which the receiver ETR wishes to receive the IP multicast encapsulated flow.
Acknowledgements
The authors would like to thank Stig Venaas for his valuable comments.
ContributorsCiscosankt@cisco.comCiscokumaram3@cisco.comIANA ConsiderationsThis memo includes the following request to IANA:
One new PIM Join/ Prune attribute types have been requested: value TBD
for the Receiver Group Attribute. Security Considerations
There is perhaps a new attack vector where an attacker can send a bunch of joins with different group addresses. It may interfere with other multicast traffic if those group addresses overlap. Also, it may take up a lot of resources if replication for thousands of groups are requested. However PIM authentication (?) should come to the rescue here. TBD
Since explicit tracking would be done, perhaps it is worth enforcing that for each ETR RLOC (the RLOC used as the source of the overlay join), there should be only one group, whatever is in the last join would override what was there earlier? Or is it to strict to only allow a single group? Might there be reasons to maybe split different LISP payload into different groups in some cases. TBD.
Ed Note: To be addressed - Comments from Stig: Regarding security considerations and PIM authentication. The only solution we have here is to use IP-Sec to sign the J/P messages. I don’t know if anyone has tried to us IPSec between LISP RLOCs. Are there any LISP security mechanisms that would help here for authenticating LISP encapsulated messages between xTRs?