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Network Working Group                                          D. Saucez
Internet-Draft                                                     INRIA
Intended status: Experimental                             O. Bonaventure
Expires: January 2, 2013                                       UCLouvain
                                                              L. Iannone
                                                       Telecom ParisTech
                                                             C. Filsfils
                                                           Cisco Systems
                                                            July 1, 2012

                       LISP ITR Graceful Restart


   The Locator/ID Separation Protocol (LISP) is a map-and-encap
   mechanism to enable the communication 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 the EID-to-RLOC cache of an ITR, which is initially empty, is
   populated progressively according to the traffic going through 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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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 January 2, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

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

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1.  Introduction

   The Locator/ID Separation Protocol (LISP) [I-D.ietf-lisp] 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).  The EIDs are
   not directly routable on the global Internet, only the 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 the ITR that serves its site.  This 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 encapsulation defined in [I-D.ietf-lisp] 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), this can cause a
   high miss rate (i.e., no EID-to-RLOC Cache entry matching the
   destination RLOC) and hence packet loss.  Such a miss storm includes
   a burst of Map-Requests that may overload the mapping system.

   This memo aims at starting a discussion about ITR graceful (re)start
   in LISP networks.  In this memo, we discuss the problem of ITR
   (re)start with the associated risk of miss storm and discuss EID-to-
   RLOC cache synchronization to provide ITR graceful restart without
   overwhelming the mapping system or packet loss.

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.
   o  Cache Miss Storm: a period during which the observed cache miss
      rate at an ITR is significantly higher than the rate observed at
      the steady state on the ITR is called a Cache Miss Storm.

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   o  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 Cache Miss Storms.
   o  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,
      actually at its first boot as part of the specific network) or an
      ITR actually re-booting due to reasons like, for instance,
      maintenance, temporary outage, etc.

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
   o  Ingress Tunnel Router (ITR) [I-D.ietf-lisp]: 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).
   o  Egress Tunnel Router (ETR) [I-D.ietf-lisp]: 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.
   o  Routing Locator (RLOC) [I-D.ietf-lisp]: 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

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      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 PA addresses.  Multiple RLOCs can be
      assigned to the same ETR device or to multiple ETR devices at a
   o  Endpoint ID (EID) [I-D.ietf-lisp]: 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 a host 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 references to "EID" refers
      to an LEID.
   o  EID-to-RLOC Cache [I-D.ietf-lisp]: 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.
   o  EID-to-RLOC Database [I-D.ietf-lisp]: 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
      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

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      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 make 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
   that caused the cache miss.  The ITR then caches the mapping for any
   subsequent packet toward the same destination.  LISP [I-D.ietf-lisp]
   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 impact of a
   miss is thus 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

   When an ITR restarts, its EID-to-RLOC cache is initially empty, and
   grows 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 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
   when the ITR is stopped (or fails) and, the second when the ITR is
   again available for encapsulation.  The first 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 miss storm can be
   observed when the ITR is restarted, because it might have to
   encapsulate all the traffic redirected to it.  Indeed, when the ITR
   is freshly restarted, its cache is empty meaning that every packet
   will cause misses at that particular time.

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   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 miss
   storm upon ITRs restarts.

   In this memo, we open the discussion on techniques that can be used
   to avoid 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 ITRs and hence risks to cause miss storm.  Indeed,
   the cache of an ITR is empty when it starts so every packet received
   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.
   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.
   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 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

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   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

   ITR 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 the 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.

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 storm which results in packet drops and
   mapping system overload.  To tackle this issue, we propose 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

              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",

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              draft-ietf-lisp-22 (work in progress), February 2012.

8.2.  Informative References

              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,

   [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.

   [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
   2004 route des Lucioles BP 93
   Sophia Antipolis Cedex,   06902

   Email: damien.saucez@inria.fr

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

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

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

   Email: luigi.iannone@telecom-paristech.fr

   Clarence Filsfils
   Cisco Systems
   Brussels,   1000

   Email: cf@cisco.com

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