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Network Working Group                                          D. Saucez
Internet-Draft                                                     INRIA
Intended status: Experimental                             O. Bonaventure
Expires: June 23, 2014                                         UCLouvain
                                                              L. Iannone
                                                       Telecom ParisTech
                                                             C. Filsfils
                                                           Cisco Systems
                                                       December 20, 2013


                       LISP ITR Graceful Restart
                 draft-saucez-lisp-itr-graceful-03.txt

Abstract

   The Locator/ID Separation Protocol (LISP) is a map-and-encap
   mechanism to enable communications between hosts identified with
   their Endpoint IDentifier (EID) over the Internet where EIDs are not
   routable.  To do so, packets toward EIDs are encapsulated in packets
   with routing locators (RLOCs) to form dynamic tunnels.  An Ingress
   Tunnel Router (ITR) that encapsulates EID packets determines tunnel
   endpoints via mappings that associate EIDs to RLOCs.  Before
   encapsulating a packet, the ITR queries the mapping system to obtain
   the mapping associated to the EID of the packet it must encapsulate.
   Such mapping is cached by the ITR in its local EID-to-RLOC cache for
   any subsequent encapsulation for the same EID.  LISP is scalable
   because EID-to-RLOC mappings are cached on ITRs.  Initially, the
   cache is empty and is populated progressively according to the
   traffic traversing the ITR.  However, after an ITR is restarted,
   e.g., for maintenance reason, its cache is empty which means that all
   packets that are re-routed to the freshly restarted ITR will cause
   cache misses and a potentially high loss rate.  In this draft, we
   present mechanisms to reduce the negative impact on traffic caused by
   the restart of an ITR in a LISP network.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   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 June 23, 2014.

Copyright Notice

   Copyright (c) 2013 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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definition of terms . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  LISP Definition of Terms  . . . . . . . . . . . . . . . .   4
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   6
   4.  ITR Graceful Restart  . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Locator/ID Separation Protocol (LISP) [RFC6830] relies on two
   principles.  First, Endpoint Identifiers (EIDs) are allocated to
   hosts while Routing Locators (RLOCs) are allocated to LISP Ingress
   Tunnel Routers (ITR) and Egress Tunnel Routers (ETR).  EIDs are not
   directly routable on the global Internet, only RLOCs are.  Second,
   LISP relies on mapping and encapsulation.  Hosts are located on sites
   and are served by ITRs and ETRs.  When host A.1 in site A needs to
   send a packet to host B.2 in site B, its packet is intercepted by an
   ITR that serves its site.  The ITR queries a mapping system to find
   the RLOC of the ETR that serves EID B.2.  Once the RLOC of the ETR
   serving B's site is known, the ITR encapsulates the packet using the



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   encapsulation defined in [RFC6830] so that it can reach B's ETR.  B's
   ETR decapsulates the packet and forwards it to host B.

   Packets from a LISP site are routed to their closest ITR by the mean
   of the routing system (e.g., IGP).  In case of an ITR that just
   booted (either because it has just been added to the network or
   because it has been restarted due to maintenance) a large portion of
   the traffic can potentially be routed to the freshly started ITR.
   However, in this case, its EID-to-RLOC cache is empty.  While with
   traditional routing, such a massive redirection has minor impact on
   the traffic (except for path stretch and latency), in the context of
   LISP, this can cause a high volume of cache misses (i.e., no EID-to-
   RLOC cache entry matching the destination RLOC) resulting in a high
   volume of dropped packets, hence, potentially leading to severe
   traffic disruption.  Furthermore, such a high number of cache misses
   triggers a burst of Map-Requests that may overload the mapping system
   (or Map Resolvers if [RFC6833] is used).

   This memo opens the question about how to perform graceful (re)start
   of ITRs in LISP networks.  It aims at documenting the problem of ITR
   (re)start with the associated risk of "miss storm" and discusses EID-
   to-RLOC cache synchronization solutions to provide ITR graceful
   restart without overwhelming the mapping system and without high
   packet losses.

2.  Definition of terms

   This section introduces the definition of the main elements and terms
   used throughout the whole document.  More specifically, hereafter the
   terms introduced by this document are defined, while in Section 2.1
   the definitions related to the LISP's architecture are provided in
   order to ease the read of the present document.

   EID-to-RLOC cache miss storm:  A sudden raise of the cache miss rate
         at an ITR to a level significantly higher than the rate
         observed at steady state on the ITR.

   Map-Request storm:  The side effect of a EID-to-RLOC cache miss
         storm, is the generation of a high number of Map-Requests,
         which is called a Map-Request storm.

   Synchronization Set:  The set of ITRs that are potentially on the
         path of the same traffic should have their EID-to-RLOC cache
         synchronized in order to avoid EID-to-RLOC cache miss storms.

   ITR Restart:  Generic term indicating an ITR that has just completed
         the bootstrap phase and resuming normal operation.  It can be
         either an ITR that has been added to the network (hence,



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         actually at its first boot as part of the specific network) or
         an ITR actually re-booting due to various reasons such as
         maintenance or outage.

2.1.  LISP Definition of Terms

   LISP operates on two name spaces and introduces several new network
   elements.  This section provides high-level definitions of the LISP
   name spaces and network elements and as such, it MUST NOT be
   considered as an authoritative source.  The reference to the
   authoritative document for each term is included in every term
   description.

   Ingress Tunnel Router (ITR)  [RFC6830]: An ITR is a router that
         resides in a LISP site.  Packets sent by sources inside of the
         LISP site to destinations outside of the site are candidates
         for encapsulation by the ITR.  The ITR treats the IP
         destination address as an EID and performs an EID-to-RLOC
         mapping lookup.  The router then prepends an "outer" IP header
         with one of its globally routable RLOCs in the source address
         field and the result of the mapping lookup in the destination
         address field.  Note that this destination RLOC MAY be an
         intermediate, proxy device that has better knowledge of the
         EID-to-RLOC mapping closer to the destination EID.  In general,
         an ITR receives IP packets from site end-systems on one side
         and sends LISP-encapsulated IP packets toward the Internet on
         the other side.  Specifically, when a service provider prepends
         a LISP header for Traffic Engineering purposes, the router that
         does this is also regarded as an ITR.  The outer RLOC the ISP
         ITR uses can be based on the outer destination address (the
         originating ITR's supplied RLOC) or the inner destination
         address (the originating hosts supplied EID).

   Egress Tunnel Router (ETR)  [RFC6830]: An ETR is a router that
         accepts an IP packet where the destination address in the
         "outer" IP header is one of its own RLOCs.  The router strips
         the "outer" header and forwards the packet based on the next IP
         header found.  In general, an ETR receives LISP-encapsulated IP
         packets from the Internet on one side and sends decapsulated IP
         packets to site end-systems on the other side.  ETR
         functionality does not have to be limited to a router device.
         A server host can be the endpoint of a LISP tunnel as well.

   Routing LOCator (RLOC)  [RFC6830]: A RLOC is an IPv4 [RFC0791] or
         IPv6 [RFC2460] address of an egress tunnel router (ETR).  A
         RLOC 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



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         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 PA addresses.  Multiple
         RLOCs can be assigned to the same ETR device or to multiple ETR
         devices at a site.

   Endpoint ID (EID)  [RFC6830]: An EID is a 32-bit (for IPv4) or
         128-bit (for IPv6) value used in the source and destination
         address fields of the first (most inner) LISP header of a
         packet.  The host obtains a destination EID the same way it
         obtains an destination address today, for example through a
         Domain Name System (DNS) [RFC1034] lookup or Session Invitation
         Protocol (SIP) [RFC3261] exchange.  The source EID is obtained
         via existing mechanisms used to set a host's "local" IP
         address.  An EID used on the public Internet must have the same
         properties as any other IP address used in that manner; this
         means, among other things, that it must be globally unique.  An
         EID is allocated to a host from an EID-prefix block associated
         with the site where the host is located.  An EID can be used by
         hosts to refer to other hosts.  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.  In theory, the bit string that
         represents an EID for one device can represent an RLOC for a
         different device.  As the architecture is realized, if a given
         bit string is both an RLOC and an EID, it must refer to the
         same entity in both cases.  When used in discussions with other
         Locator/ID separation proposals, a LISP EID will be called a
         "LEID".  Throughout this document, any reference to "EID"
         refers to an LEID.

   EID-to-RLOC cache  [RFC6830]: The EID-to-RLOC cache is a short-lived,
         on- demand table in an ITR that stores, tracks, and is
         responsible for timing-out and otherwise validating EID-to-RLOC
         mappings.  This cache is distinct from the full "database" of
         EID-to-RLOC mappings, it is dynamic, local to the ITR(s), and
         relatively small while the database is distributed, relatively
         static, and much more global in scope.

   EID-to-RLOC Database  [RFC6830]: The EID-to-RLOC database is a global
         distributed database that contains all known EID-prefix to RLOC
         mappings.  Each potential ETR typically contains a small piece
         of the database: the EID-to-RLOC mappings for the EID prefixes
         "behind" the router.  These map to one of the router's own,
         globally visible, IP addresses.  The same database mapping



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         entries MUST be configured on all ETRs for a given site.  In a
         steady state the EID-prefixes for the site and the locator-set
         for each EID-prefix MUST be the same on all ETRs.  Procedures
         to enforce and/or verify this are outside the scope of this
         document.  Note that there MAY be transient conditions when the
         EID-prefix for the site and locator-set for each EID-prefix may
         not be the same on all ETRs.  This has no negative implications
         since a partial set of locators can be used.

3.  Problem Statement

   LISP is a map-and-encap mechanism where an ITR dynamically learns the
   mappings when it receives a packet for a destination EID for which it
   did not do encapsulation before.  When such a packet is received, a
   cache miss occurs and the ITR sends a Map-Request to the mapping
   system to retrieve the mapping that corresponds to the destination of
   the packet that caused the cache miss.  The ITR then caches the
   mapping for any subsequent packet toward the same destination.  LISP
   [RFC6830] does not specify how a packet that causes a cache miss must
   be handled.  However, to the best of our knowledge, the current
   implementations drop packets causing a cache miss.  The consequences
   of such a current practice in case of cache miss is two-fold.  On the
   one hand, misses imply packet losses and hence performance issues.
   On the other hand, due to the consequent Map-Request, cache misses
   cause load on the mapping system.

   When an ITR restarts, its EID-to-RLOC cache is initially empty, and
   is populated, growing in size, progressively with the traffic.
   However, because mappings have a limited lifetime, the EID-to-RLOC
   cache size converges to a stable value and it is expected to always
   observe misses.  As shown in [Networking12], at the steady state,
   networks experience a rather stable, and limited, miss rate.
   However, when an ITR is restarted, e.g., for a maintenance operation,
   a cache miss storm can be observed.  A EID-to-RLOC cache miss storm
   is a phenomenon during which the miss rate is significantly higher
   than the miss rate normally observed in the network.  A miss storm
   has two sever side effects, first, it abruptly increases the load on
   the mapping system, and second, many packets are dropped, which
   causes performance issues.  When an ITR is restarted, actually two
   cache miss storms can be observed.  The first one happens when the
   ITR is stopped (or fails); while the second one happens when the ITR
   is again available for encapsulation.  The first EID-to-RLOC cache
   miss storm is due to the fact that all the traffic is suddenly
   redirected to the other ITRs in the network, which might not have the
   mappings for all the EIDs of ongoing communications.  The second EID-
   to-RLOC cache miss storm can be observed when the ITR is restarted,
   because it might have to encapsulate all the traffic redirected to
   it.  As a matter of fact, when the ITR is freshly restarted, its



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   cache is empty meaning that every packet will cause misses at that
   particular time.

   Cache misses are normal in a LISP network.  However, these misses
   normally happen only when the first packet of the first flow toward
   an EID is received by an ITR which have no significant impact on the
   traffic at steady state in the network.  On the contrary, when an ITR
   restarts, cache misses happen on elapsing, potentially high
   throughput, flows for which high loss rate is not acceptable.  For
   this particular reason, techniques must be applied to avoid EID-to-
   RLOC cache miss storm upon ITRs restarts.

   It can be argued that if a router fails and is out of order for a
   long time, avoiding the EID-to-RLOC cache miss storm, which lasts in
   the order of minutes, is not worth.  This is not actually accurate.
   When a router fails, there are usually already deployed backup
   solutions in order to re-direct the traffic instantaneously, with
   almost no losses.  Such redirection remains in place until the
   failure is fixed, without any consequence on the traffic except for
   using a different path.  Similarly, when the router is back online,
   booting, traffic will flow again trough it only when the state of the
   router is consistent with the rest of the network, making re-
   directing the traffic through it disruptionless.  All of this is not
   true for ITRs.  Even if with existing techniques we are able to re-
   direct the traffic with no losses, the LISP encapsulation engine will
   drop packets because of the lack of mappings in the cache, creating
   traffic disruption and a raise in signaling traffic on the mapping
   system.

   In this memo, we open the discussion on techniques that can be used
   to avoid EID-to-RLOC cache miss storms in the case of a planned ITR
   restart.  In other words, we discuss how to achieve ITR graceful
   restart.

4.  ITR Graceful Restart

   The addition of an ITR causes the traffic to be redirected to the
   freshly started ITR and hence risks to cause miss storm.  As the
   cache of an ITR is empty when it starts, every received packet
   potentially causes a miss.  We can isolate three techniques to
   protect the network from miss storm when an ITR is added (or
   restarted) in the network.  All the ITRs that are potentially used by
   the same node in the network are grouped in synchronization sets.

   o  Non-volatile mapping storage: when an ITR has to be stopped, its
      EID-to-RLOC cache is stored on a non-volatile medium (e.g., a hard
      drive) such that when it is restarted, it can load the EID-to-RLOC
      cache to be equivalent of the cache it had before it restarted.



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   o  ITR deflection: when a miss occurs at an ITR while it is starting
      up, the ITR deflects the packet that caused a miss to an ITR in
      its synchronization set and, in parallel, sends a Map-Request for
      the EID that caused the miss.  Note that the Map-Request can even
      be sent to another ITR of the site or a Map Resolver working in
      proxy mode.  In this manner mapping retrieval latency can be
      shortened.

   o  ITR cache synchronization: upon startup, the ITR synchronizes its
      cache with the other ITRs in its synchronization set.  The ITR is
      marked as available only after the cache is synchronized.

   The non-volatile storage offers the advantage to be transparent for
   the network and is adapted to short unavailability periods (e.g., the
   ITR reboots after an upgrade).  However, this technique is not
   adapted for long unavailability periods where most of the entries
   might be outdated and new prefixes unknown, or when an ITR is added
   for the first time in the network.  This technique is thus
   recommended only for network with a low mapping caching dynamics.

   Traffic deflection to other ITRs (or a PxTR) upon misses causes
   several issues.  On the one hand, the ITR that is restarting must
   determine the ITR to which the packet must be deflected.  On the
   other hand, packets must be marked as deflected in order to avoid
   loops.  In addition, the ITR must determine its graceful restart
   period such that it stops deflecting traffic once at steady state.
   The deflection from one ITR to another can be done directly in LISP
   where the ITR that started LISP encapsulates and forwards the packet
   to another ITR.  This last ITR must then also run the ETR
   functionality to decapsulate the packet.

   ITR EID-to_RLOC cache synchronization is the most adapted to graceful
   restart.  When the ITR starts, it sends requests to an ITR in its
   synchronization set (or its MR) to obtain the full cache.  When the
   synchronization is finished, the ITR advertises itself as an ITR in
   the network such that the ITR does receive traffic to encapsulate
   only once its cache is synchronized.

5.  Security Considerations

   Security considerations have to be written accordingly to the
   technique finally chosen for ITR graceful restart.  However, as a
   general security recommendation, we can say that mappings must be
   authenticated in order to avoid relay attacks or denial of service.
   However, ITR graceful restart should not introduce any new threat in
   the core LISP mechanism.





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6.  Conclusion

   In this memo, we highlighted the implication of the addition or the
   restart of an ITR in a LISP network.  When an ITR is added into a
   LISP network, its EID-to-RLOC cache is initially empty.  Therefore,
   when on-going flows are routed to the freshly started ITR, their
   packets cause potential miss storms which result in packet drops and
   mapping system overload.  To tackle this issue, we propose and
   discuss three different techniques to reduce the impact of a planed
   ITR restart.

7.  Acknowledgments

   The authors would like to acknowledge Dino Farinacci, Vince Fuller,
   Darrel Lewis, Fabio Maino, and Simon van der Linden.

8.  References

8.1.  Normative References

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

   [RFC6833]  Fuller, V. and D. Farinacci, "Locator/ID Separation
              Protocol (LISP) Map-Server Interface", RFC 6833, January
              2013.

8.2.  Informative References

   [Networking12]
              Saucez, D., Kim, J., Iannone, L., Bonaventure, O., and C.
              Filsfils, "A local Approach to Fast Failure Recovery of
              LISP Ingress Tunnel Routers", The 11th International
              Conference on Networking (Networking'12) , May 2012,
              <[Networking12]>.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.






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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

Authors' Addresses

   Damien Saucez
   INRIA
   2004 route des Lucioles BP 93
   Sophia Antipolis Cedex  06902
   France

   Email: damien.saucez@inria.fr


   Olivier Bonaventure
   UCLouvain
   Universite catholique de Louvain, Place Sainte Barbe 2
   Louvain-la-Neuve  1348
   Belgium

   Email: olivier.bonaventure@uclouvain.be
   URI:   http://inl.info.ucl.ac.be


   Luigi Iannone
   Telecom ParisTech
   23, Avenue d'Italie
   75013 Paris
   France

   Email: luigi.iannone@telecom-paristech.fr


   Clarence Filsfils
   Cisco Systems
   Brussels  1000
   Belgium

   Email: cf@cisco.com










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