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Network Working Group                                          J. Arango
Internet-Draft                                                  J. Leong
Intended status: Experimental                              Cisco Systems
Expires: September 4, 2017                                 March 3, 2017

                        LISP Stateful Pull Model


   This document specifies a stateful pull model for LISP where ITRs can
   subscribe with the mapping system to be notified whenever a
   particular EID mapping changes.  The model uses a publish/subscribe
   mechanism that supports overlapping EID registrations without having
   to notify the ITR about every single prefix covered by a particular
   subscription.  The pull model is stateful in the sense that it
   requires that the mapping system maintain subscription state.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 4, 2017.

Copyright Notice

   Copyright (c) 2017 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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   2
   3.  Subscription Establishement . . . . . . . . . . . . . . . . .   2
   4.  Publication of Overlapping Prefixes . . . . . . . . . . . . .   3
   5.  Mobility and Barrier Prefixes . . . . . . . . . . . . . . . .   4
   6.  Negative Subscriptions  . . . . . . . . . . . . . . . . . . .   4
   7.  ITR Eviction of Subscription and Map-cache State  . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Subscription Establishement

   When operating in a stateful pull model, a lookup miss in the data
   plane's forwarding table results in the transmission of a
   subscription message to the mapping system.  The subscription message
   contains a query prefix field that is set to the destination host's
   address.  As in the stateless pull model, the ITR creates an
   incomplete map-cache entry to inhibit further signaling from the data
   plane until the corresponding mapping information is received from
   the mapping system.

   When processing a subscription message, the mapping system performs a
   longest-prefix match in the mapping database for the query prefix
   included in the subscription message.  The resulting mapping is sent
   to the ITR in a publication message.  The query prefix is also
   included as an explicit indication that the publication message is to
   be used by the ITR to create subscription state.

   The ITR creates an entry in a subscription table when it receives a
   publication message that includes a query prefix.  The subscription
   entry contains the longest-matching EID prefix returned by the
   mapping system in the publication message.  The query prefix is used
   by the ITR to perform an exact match lookup for an incomplete map-
   cache entry.  The map-cache entry is then linked as one of the
   sources (producers) of the subscription state.  The host address of

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   the map-cache entry is ovweritten with the EID prefix of the
   subscription entry.  The locators of the subscription entry are
   copied to the map-cache entry.  The map-cache entry is transitioned
   to complete state and reprogramed in the data plane.

4.  Publication of Overlapping Prefixes

   The mapping system must publish all mappings that are more specific
   than the subscription state, unless a mechanism is conceived to more
   efficiently support overlapping prefixes.  Publication of more
   specific mappings is necessary to ensure that the correct locators
   are used in the following two situations:

   a.  An ITR can quite possibly have a subscription entry that covers
       multiple destination hosts to which it is sending traffic.  The
       mapping system could later receive a registration that is more
       specific than the current subscription state and at the same time
       happens to be the longest matching prefix for one or multiple
       destination hosts currently being covered by the subscription.

   b.  An ITR starts forwarding traffic to a new destination host that
       is covered by an existing subscription entry, but for which the
       mapping system has a mapping that is even more specific.

   To efficiently support overlapping prefixes, the mapping system only
   publishes registered mappings that are one level below the
   subscription prefix.  To be formally precise, the mapping system only
   publishes registered mappings that are direct hirarchical children of
   the subscription prefix.  A more specific registered mapping is a
   hirarchichal child of a subscription prefix if there DOES NOT exist
   another registered mapping that is also more specific than the
   susbcription prefix and at the same time less specific than the
   prefix under consideration.

   The publication messages for child prefixes have a flag set
   indicating that a corresponding map-cache entry should be created
   with a "signal-and-forward" action in the data plane.  These Map-
   cache entries are also marked or linked as additional sources
   (producers) of the corresponding subscription.  When a longest
   matching forwarding lookup hits one of these map-cache entries, the
   packet will be encapulated using one of the locators in the entry and
   the control plane will be signaled to trigger a subscription for the
   destination host, as described in section Section 3.  The ITR is now
   subscribed to what used to be a child prefix.  The "signal-and-
   forward" map-cache entry is deleted.  The incomplete map-cache entry
   created during the data-plane signaling event inherits the EID prefix
   and the locators from the new susbcription state.  Any registered

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   mappings that happen to be hirarchical children of the new
   susbcription will be published by the mapping system.

5.  Mobility and Barrier Prefixes

   Dynamic EID prefixes are mobility prefixes configured at the ETRs to
   enable detection and registration of individual hosts covered by the
   prefix, as opposed to just registering the entire block as a single
   prefix.  Depending on the prefix length, dynamic EID prefixes can
   result in a very large fan-out of individual host mappings that are
   hirarchical children of a less specific mapping.

   An ITR that sends a subscription where the query prefix is a mobile
   host that is not yet registered in the mapping system can potentially
   end up being subscribed to a less specific prefix with a huge fan-out
   of mobile host mappings.  These mobile host mappings are hirarchical
   children of the less specific mapping and will therefore be published
   to the ITR.

   To avoid publishing all the mobile hosts to the ITR, the dynamic EID
   prefix is configured in the mapping system as a barrier prefix.  A
   barrier prefix has the following properties:

   a.  If the barrier prefix is the longest matching prefix for a
       subscription request, the ITR gets subscribed to the query
       prefix, not the barrier prefix.  If the mobile node corresponding
       do the query prefix is not yet registered, the publication
       message can have an empty locator set.

   b.  A barrier prefix that is a hierarchical child of a less specific
       subscription gets published to the ITR with an explicit
       indication that it MUST be programed in the data plane with a
       "signal" action.  This ensures that future traffic covered by the
       dynamic EID prefix triggers a subscription, even if the ITR has a
       map-cache entry that is less specific than the dynamic EID

6.  Negative Subscriptions

   In the LISP stateless pull model, a Map-Request whose longest
   matching lookup in the mapping system results in a lookup miss will
   trigger a negative Map-Reply.  The prefix included in the reply is
   the least specific gap or hole in the mapping system that also covers
   the prefix in the Map-Request.  This minimizes the amount of map-
   cache entries necessary to forward traffic not covered by mapping

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   In the stateful pull model, a subscription message could similarly
   result in a lookup miss in the mapping system.  The least specific
   gap or hole in the mapping database that covers the subscription
   request gets self-registered by the mapping system as a negative
   mapping.  A negative mapping can contain an empty locator set.
   Alternatively, if one or more PETRs are registering the 0/0 prefix,
   the locator set of a negative mappings could inherit the merged set
   of locators from the 0/0 mapping.  The publication message sent to
   the ITR in response to the subscription contains the negative
   mapping.  According to the subscription procedure described in
   section Section 3, the ITR creates a subscription entry and a map-
   cache entry for the negative mapping.

   If and when a prefix gets registered that is more specific than a
   negative mapping, said prefix will effectively be a hierarchical
   child of the negative mapping and will therefore be published to any
   ITR currently subscribed to the negative prefix.

   Negative mappings get evicted from the mapping system when the set of
   ITRs subscribed to the negative mapping becomes empty.  Subscriptions
   to prefixes that are more specific than the negative mapping do not
   prevent the eviction of the negative mapping.  What is important is
   that there are no subscriptions on the negative mapping itself.

7.  ITR Eviction of Subscription and Map-cache State

   An ITR subscription entry may be sourced by one map-cache entry whose
   prefix is equal to that of the subscription entry, and multiple map-
   cache entries whose prefixes are more specific than that of the
   subscription entry.  Any of these map-cache entries may be evicted
   when an unpublish message is received from the mapping system.  An
   unpublish message is an indication that the corresponding mapping is
   no longer registered in the mapping system.

   A subscription entry can be evicted from the ITR when the set of map-
   cache entries sourcing the subscription becomes empty.  The ITR sends
   an unsubscribe message when a suscription entry is evicted.

   The ITR can collectively evict a subscription entry and all
   associated map-cache entries in a single atomic operation if none of
   the map-cache entries have been hit by forwarding traffic after an
   unspecified amount of time.  Note that while evicting a single map-
   cache entry for lack of use is possible, it will not prevent the
   mapping system for publishing it again if there is a change in the
   corresponding mapping.

   Note that the mapping system MUST keep track of a subscription even
   if there are no registered mappings that are covered by the

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   subscription.  The Mapping system can only evict a subscription entry
   if it receives an unsubscribe message from the ITR or if it looses
   communication with the ITR

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,

Authors' Addresses

   Jesus Arango
   Cisco Systems
   170 Tasman Drive
   San Jose, CA  95134

   Email: jearango@cisco.com

   Johnson Leong
   Cisco Systems
   170 Tasman Drive
   San Jose, CA  95134

   Email: joleong@cisco.com

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