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
draft-saucez-lisp-itr-graceful-00.txt
Abstract
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
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
Saucez, et al. Expires January 2, 2013 [Page 1]
Internet-Draft LISP Graceful Restart July 2012
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.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Saucez, et al. Expires January 2, 2013 [Page 2]
Internet-Draft LISP Graceful Restart July 2012
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
Saucez, et al. Expires January 2, 2013 [Page 3]
Internet-Draft LISP Graceful Restart July 2012
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.
Saucez, et al. Expires January 2, 2013 [Page 4]
Internet-Draft LISP Graceful Restart July 2012
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
description.
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
Saucez, et al. Expires January 2, 2013 [Page 5]
Internet-Draft LISP Graceful Restart July 2012
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
site.
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
Saucez, et al. Expires January 2, 2013 [Page 6]
Internet-Draft LISP Graceful Restart July 2012
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
system.
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.
Saucez, et al. Expires January 2, 2013 [Page 7]
Internet-Draft LISP Graceful Restart July 2012
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
Saucez, et al. Expires January 2, 2013 [Page 8]
Internet-Draft LISP Graceful Restart July 2012
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 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
[I-D.ietf-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
Saucez, et al. Expires January 2, 2013 [Page 9]
Internet-Draft LISP Graceful Restart July 2012
draft-ietf-lisp-22 (work in progress), February 2012.
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.
[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
Saucez, et al. Expires January 2, 2013 [Page 10]
Internet-Draft LISP Graceful Restart July 2012
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
Saucez, et al. Expires January 2, 2013 [Page 11]