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Internet Engineering Task Force (IETF) V. Fuller
Request for Comments: 6836
Category: Experimental D. Farinacci
ISSN: 2070-1721 D. Meyer
D. Lewis
Cisco Systems
January 2013
Locator/ID Separation Protocol Alternative Logical Topology (LISP+ALT)
Abstract
This document describes a simple distributed index system to be used
by a Locator/ID Separation Protocol (LISP) Ingress Tunnel Router
(ITR) or Map-Resolver (MR) to find the Egress Tunnel Router (ETR)
that holds the mapping information for a particular Endpoint
Identifier (EID). The MR can then query that ETR to obtain the
actual mapping information, which consists of a list of Routing
Locators (RLOCs) for the EID. Termed the Alternative Logical
Topology (ALT), the index is built as an overlay network on the
public Internet using the Border Gateway Protocol (BGP) and Generic
Routing Encapsulation (GRE).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6836.
Fuller, et al. Experimental [Page 1]
RFC 6836 LISP+ALT January 2013
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
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.
Fuller, et al. Experimental [Page 2]
RFC 6836 LISP+ALT January 2013
Table of Contents
1. Introduction ....................................................3
2. Definition of Terms .............................................5
3. The LISP-ALT Model ..............................................8
3.1. Routability of EIDs ........................................8
3.1.1. Mechanisms for an ETR to Originate EID-Prefixes .....9
3.1.2. Mechanisms for an ITR to Forward to EID-Prefixes ....9
3.1.3. Map-Server Model Preferred ..........................9
3.2. Connectivity to Non-LISP Sites ............................10
3.3. Caveats on the Use of Data-Probes .........................10
4. LISP+ALT: Overview .............................................10
4.1. ITR Traffic Handling ......................................11
4.2. EID Assignment - Hierarchy and Topology ...................12
4.3. Use of GRE and BGP between LISP-ALT Routers ...............14
5. EID-Prefix Propagation and Map-Request Forwarding ..............14
5.1. Changes to ITR Behavior with LISP+ALT .....................15
5.2. Changes to ETR Behavior with LISP+ALT .....................15
5.3. ALT Datagram Forwarding Failure ...........................16
6. BGP Configuration and Protocol Considerations ..................16
6.1. Autonomous System Numbers (ASNs) in LISP+ALT ..............16
6.2. Subsequent Address Family Identifier (SAFI) for LISP+ALT ..17
7. EID-Prefix Aggregation .........................................17
7.1. Stability of the ALT ......................................18
7.2. Traffic Engineering Using LISP ............................18
7.3. Edge Aggregation and Dampening ............................19
7.4. EID Assignment Flexibility vs. ALT Scaling ................19
8. Connecting Sites to the ALT Network ............................20
8.1. ETRs Originating Information into the ALT .................20
8.2. ITRs Using the ALT ........................................21
9. Security Considerations ........................................22
9.1. Apparent LISP+ALT Vulnerabilities .........................22
9.2. Survey of LISP+ALT Security Mechanisms ....................23
9.3. Use of Additional BGP Security Mechanisms .................24
10. Acknowledgments ...............................................24
11. References ....................................................24
11.1. Normative References .....................................24
11.2. Informative References ...................................25
1. Introduction
This document describes the LISP+ALT system, used by an [RFC6830]
Ingress Tunnel Router (ITR) or MR to find the Egress Tunnel Router
(ETR) that holds the RLOC mapping information for a particular
Endpoint Identifier (EID). The ALT network is built using the Border
Gateway Protocol (BGP) [RFC4271], BGP multiprotocol extensions
Fuller, et al. Experimental [Page 3]
RFC 6836 LISP+ALT January 2013
[RFC4760], and Generic Routing Encapsulation (GRE) [RFC2784] to
construct an overlay network of devices (ALT-Routers) that operate on
EID-Prefixes and use EIDs as forwarding destinations.
ALT-Routers advertise hierarchically delegated segments of the EID
namespace (i.e., prefixes) toward the rest of the ALT; they also
forward traffic destined for an EID covered by one of those prefixes
toward the network element that is authoritative for that EID and is
the origin of the BGP advertisement for that EID-Prefix. An ITR uses
this overlay to send a LISP Map-Request (defined in [RFC6830]) to the
ETR that holds the EID-to-RLOC mapping for a matching EID-Prefix. In
most cases, an ITR does not connect directly to the overlay network
but instead sends Map-Requests via a Map-Resolver (described in
[RFC6833]) that does. Likewise, in most cases, an ETR does not
connect directly to the overlay network but instead registers its
EID-Prefixes with a Map-Server that advertises those EID-Prefixes on
to the ALT and forwards Map-Requests for them to the ETR.
It is important to note that the ALT does not distribute actual
EID-to-RLOC mappings. What it does provide is a forwarding path from
an ITR (or MR) that requires an EID-to-RLOC mapping to an ETR that
holds that mapping. The ITR/MR uses this path to send an ALT
Datagram (see Section 3) to an ETR, which then responds with a
Map-Reply containing the needed mapping information.
One design goal for LISP+ALT is to use existing technology wherever
possible. To this end, the ALT is intended to be built using
off-the-shelf routers that already implement the required protocols
(BGP and GRE); little, if any, LISP-specific modifications should be
needed for such devices to be deployed on the ALT (see Section 7 for
aggregation requirements). Note, though, that organizational and
operational considerations suggest that ALT-Routers be both logically
and physically separate from the "native" Internet packet transport
system; deploying this overlay on those routers that are already
participating in the global routing system and actively forwarding
Internet traffic is not recommended.
This specification is experimental, and there are areas where further
experience is needed to understand the best implementation strategy,
operational model, and effects on Internet operations. These areas
include:
o application effects of on-demand route map discovery
o tradeoff in connection setup time vs. ALT design and performance
when using a Map Request instead of carrying initial user data in
a Data-Probe
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RFC 6836 LISP+ALT January 2013
o best practical ways to build ALT hierarchies
o effects of route leakage from ALT to the current Internet,
particularly for LISP-to-non-LISP interworking
o effects of exceptional situations, such as denial-of-service (DoS)
attacks
Experimentation, measurements, and deployment experience on these
aspects is appreciated. While these issues are conceptually well-
understood (e.g., an ALT lookup causes potential delay for the first
packet destined to a given network), the real-world operational
effects are much less clear.
The remainder of this document is organized as follows: Section 2
provides the definitions of terms used in this document. Section 3
outlines the LISP-ALT model, where EID-Prefixes are advertised using
BGP on an overlay network (the "ALT") and Map-Requests are forwarded
across it. Section 4 provides a basic overview of the LISP
Alternative Logical Topology architecture, and Section 5 describes
how the ALT uses BGP to propagate EID reachability over the overlay
network. Section 6 describes other considerations for using BGP on
the ALT. Section 7 describes the construction of the ALT aggregation
hierarchy, and Section 8 discusses how LISP-ALT elements are
connected to form the overlay network. Section 9 discusses security
considerations relevant to LISP+ALT.
2. Definition of Terms
This section provides high-level definitions of LISP concepts and
components involved with and affected by LISP+ALT.
Alternative Logical Topology (ALT): The virtual overlay network
made up of tunnels between LISP-ALT Routers. The Border Gateway
Protocol (BGP) runs between ALT-Routers and is used to carry
reachability information for EID-Prefixes. The ALT provides a way
to forward Map-Requests (and, if supported, Data-Probes) toward
the ETR that "owns" an EID-Prefix. As a tunneled overlay, its
performance is expected to be quite limited, so using it to
forward high-bandwidth flows of Data-Probes is strongly
discouraged (see Section 3.3 for additional discussion).
ALT-Router: The device that runs on the ALT. The ALT is a static
network built using tunnels between ALT-Routers. These routers
are deployed in a roughly hierarchical mesh in which routers at
each level in the topology are responsible for aggregating
EID-Prefixes learned from those logically "below" them and
advertising summary prefixes to those logically "above" them.
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RFC 6836 LISP+ALT January 2013
Prefix learning and propagation between ALT-Routers is done using
BGP. An ALT-Router at the lowest level, or "edge" of the ALT,
learns EID-Prefixes from its "client" ETRs. See Section 3.1 for a
description of how EID-Prefixes are learned at the "edge" of the
ALT. See also Section 6 for details on how BGP is configured
between the different network elements. When an ALT-Router
receives an ALT Datagram, it looks up the destination EID in its
forwarding table (composed of EID-Prefix routes it learned from
neighboring ALT-Routers) and forwards it to the logical next hop
on the overlay network.
Endpoint ID (EID): A 32-bit (for IPv4) or 128-bit (for IPv6) value
used to identify the ultimate source or destination for a LISP-
encapsulated packet. See [RFC6830] for details.
EID-Prefix: A set of EIDs delegated in a power-of-two block.
Information about EID-Prefixes is exchanged among ALT-Routers (not
on the global Internet) using BGP, and EID-Prefixes are expected
to be assigned in a hierarchical manner such that they can be
aggregated by ALT-Routers. Such a block is characterized by a
prefix and a length. Note that while the ALT routing system
considers an EID-Prefix to be an opaque block of EIDs, an end site
may put site-local, topologically relevant structure (subnetting)
into an EID-Prefix for intra-site routing.
Aggregated EID-Prefixes: A set of individual EID-Prefixes that have
been aggregated in the [RFC4632] sense.
Map-Server (MS): An edge ALT-Router that provides a registration
function for non-ALT-connected ETRs, originates EID-Prefixes into
the ALT on behalf of those ETRs, and forwards Map-Requests to
them. See [RFC6833] for details.
Map-Resolver (MR): An edge ALT-Router that accepts an Encapsulated
Map-Request from a non-ALT-connected ITR, decapsulates it, and
forwards it on to the ALT toward the ETR that owns the requested
EID-Prefix. See [RFC6833] for details.
Ingress Tunnel Router (ITR): A router that sends LISP Map-Requests
or encapsulates IP datagrams with LISP headers, as defined in
[RFC6830]. In this document, "ITR" refers to any device
implementing ITR functionality, including a Proxy-ITR (see
[RFC6832]). Under some circumstances, a LISP Map-Resolver may
also originate Map-Requests (see [RFC6833]).
Fuller, et al. Experimental [Page 6]
RFC 6836 LISP+ALT January 2013
Egress Tunnel Router (ETR): A router that sends LISP Map-Replies
in response to LISP Map-Requests and decapsulates LISP-
encapsulated IP datagrams for delivery to end-systems, as defined
in [RFC6830]. In this document, "ETR" refers to any device
implementing ETR functionality, including a Proxy-ETR (see
[RFC6832]). Under some circumstances, a LISP Map-Server may also
respond to Map-Requests (see [RFC6833]).
Routing Locator (RLOC): A routable IP address for a LISP Tunnel
Router (ITR or ETR). Interchangeably referred to as a "locator"
in this document. An RLOC is also the output of an EID-to-RLOC
mapping lookup; an EID-Prefix maps to one or more RLOCs.
Typically, RLOCs are numbered from topologically aggregatable
blocks that are assigned to a site at each point where it attaches
to the global Internet; where the topology is defined by the
connectivity of provider networks, RLOCs can be thought of as
Provider-Assigned (PA) addresses. Routing for RLOCs is not
carried on the ALT.
EID-to-RLOC Mapping: A binding between an EID-Prefix and the set of
RLOCs that can be used to reach it; sometimes simply referred to
as a "mapping".
EID-Prefix Reachability: An EID-Prefix is said to be "reachable" if
at least one of its Locators is reachable. That is, an EID-Prefix
is reachable if the ETR that is authoritative for a given
EID-to-RLOC mapping is reachable.
Default Mapping: A mapping entry for EID-Prefix 0.0.0.0/0 (::/0 for
IPv6). It maps to a Locator-Set used for all EIDs in the
Internet. If there is a more-specific EID-Prefix in the
map-cache, it overrides the Default Mapping entry. The Default
Mapping entry can be learned by configuration or from a Map-Reply
message.
ALT Default Route: An EID-Prefix value of 0.0.0.0/0 (or ::/0 for
IPv6) that may be learned from the ALT or statically configured on
an edge ALT-Router. The ALT Default Route defines a forwarding
path for a packet to be sent into the ALT on a router that does
not have a full ALT forwarding database.
Fuller, et al. Experimental [Page 7]
RFC 6836 LISP+ALT January 2013
3. The LISP-ALT Model
The LISP-ALT model uses the same basic query/response protocol that
is documented in [RFC6830]. In particular, LISP+ALT provides two
types of packets that an ITR can originate to obtain EID-to-RLOC
mappings:
Map-Request: A Map-Request message is sent into the ALT to request
an EID-to-RLOC mapping. The ETR that owns the mapping will
respond to the ITR with a Map-Reply message. Since the ALT only
forwards on EID destinations, the destination address of the
Map-Request sent on the ALT must be an EID.
Data-Probe: Alternatively, an ITR may encapsulate and send the first
data packet destined for an EID with no known RLOCs into the ALT
as a Data-Probe. This might be done to minimize packet loss and
to probe for the mapping. As above, the authoritative ETR for the
EID-Prefix will respond to the ITR with a Map-Reply message when
it receives the data packet over the ALT. As a side-effect, the
encapsulated data packet is delivered to the end-system at the ETR
site. Note that the Data-Probe's inner IP destination address,
which is an EID, is copied to the outer IP destination address so
that the resulting packet can be routed over the ALT. See
Section 3.3 for caveats on the usability of Data-Probes.
The term "ALT Datagram" is shorthand for a Map-Request or Data-Probe
to be sent into or forwarded on the ALT. Note that such packets use
an RLOC as the outer-header source IP address and an EID as the
outer-header destination IP address.
Detailed descriptions of the LISP packet types referenced by this
document may be found in [RFC6830].
3.1. Routability of EIDs
A LISP EID has the same syntax as an IP address and can be used,
unaltered, as the source or destination of an IP datagram. In
general, though, EIDs are not routable on the public Internet; LISP+
ALT provides a separate, virtual network, known as the LISP
Alternative Logical Topology (ALT) on which a datagram using an EID
as an IP destination address may be transmitted. This network is
built as an overlay on the public Internet using tunnels to
interconnect ALT-Routers. BGP runs over these tunnels to propagate
path information needed to forward ALT Datagrams. Importantly, while
the ETRs are the source(s) of the unaggregated EID-Prefixes, LISP+ALT
uses existing BGP mechanisms to aggregate this information.
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RFC 6836 LISP+ALT January 2013
3.1.1. Mechanisms for an ETR to Originate EID-Prefixes
There are three ways that an ETR may originate its mappings into
the ALT:
1. By registration with a Map-Server, as documented in [RFC6833].
This is the common case and is expected to be used by the
majority of ETRs.
2. Using a "static route" on the ALT. Where no Map-Server is
available, an edge ALT-Router may be configured with a "static
EID-Prefix route" pointing to an ETR.
3. Edge connection to the ALT. If a site requires fine-grained
control over how its EID-Prefixes are advertised into the ALT, it
may configure its ETR(s) with tunnel and BGP connections to edge
ALT-Routers.
3.1.2. Mechanisms for an ITR to Forward to EID-Prefixes
There are three ways that an ITR may send ALT Datagrams:
1. Through a Map-Resolver, as documented in [RFC6833]. This is the
common case and is expected to be used by the majority of ITRs.
2. Using a "default route". Where a Map-Resolver is not available,
an ITR may be configured with a static ALT Default Route pointing
to an edge ALT-Router.
3. Edge connection to the ALT. If a site requires fine-grained
knowledge of what prefixes exist on the ALT, it may configure its
ITR(s) with tunnel and BGP connections to edge ALT-Routers.
3.1.3. Map-Server Model Preferred
The ALT-connected ITR and ETR cases are expected to be rare, as the
Map-Server/Map-Resolver model is simpler for an ITR/ETR operator to
use and also provides a more general service interface to not only
the ALT but to other mapping databases that may be developed in the
future.
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RFC 6836 LISP+ALT January 2013
3.2. Connectivity to Non-LISP Sites
As stated above, EIDs used as IP addresses by LISP sites are not
routable on the public Internet. This implies that, absent a
mechanism for communication between LISP and non-LISP sites,
connectivity between them is not possible. To resolve this problem,
an "interworking" technology has been defined; see [RFC6832] for
details.
3.3. Caveats on the Use of Data-Probes
It is worth noting that there has been a great deal of discussion and
controversy about whether Data-Probes are a good idea. On the one
hand, using them offers a method of avoiding the "first packet drop"
problem when an ITR does not have a mapping for a particular
EID-Prefix. On the other hand, forwarding data packets on the ALT
would require that it either be engineered to support relatively high
traffic rates, which is not generally feasible for a tunneled
network, or that it be carefully designed to aggressively rate-limit
traffic to avoid congestion or DoS attacks. There may also be issues
caused by different latency or other performance characteristics
between the ALT path taken by an initial Data-Probe and the
"Internet" path taken by subsequent packets on the same flow once a
mapping is in place on an ITR. For these reasons, the use of
Data-Probes is not recommended at this time; they should only be
originated from an ITR when explicitly configured to do so, and such
configuration should only be enabled when performing experiments
intended to test the viability of using Data-Probes.
4. LISP+ALT: Overview
LISP+ALT is a hybrid push/pull architecture. Aggregated EID-Prefixes
are advertised among the ALT-Routers and to those (rare) ITRs that
are directly connected via a tunnel and BGP to the ALT. Specific
EID-to-RLOC mappings are requested by an ITR (and returned by an ETR)
using LISP when it sends a request either via a Map-Resolver or to an
edge ALT-Router.
The basic idea embodied in LISP+ALT is to use BGP, running on a
tunneled overlay network (the ALT), to establish reachability between
ALT-Routers. The ALT BGP Routing Information Base (RIB) is comprised
of EID-Prefixes and associated next hops. ALT-Routers interconnect
using BGP and propagate EID-Prefix updates among themselves.
EID-Prefix information is learned from ETRs at the "edge" of the ALT
either through the use of the Map-Server interface (the common case),
by static configuration, or by BGP-speaking ETRs.
Fuller, et al. Experimental [Page 10]
RFC 6836 LISP+ALT January 2013
Map-Resolvers learns paths through the ALT to Map-Servers for
EID-Prefixes. An ITR will normally use a Map-Resolver to send its
ALT Datagrams on to the ALT but may, in unusual cases (see
Section 3.1.2), use a static ALT Default Route or connect to the ALT
using BGP. Likewise, an ETR will normally register its prefixes in
the mapping database using a Map-Server or can sometimes (see
Section 3.1.1) connect directly to the ALT using BGP. See [RFC6833]
for details on Map-Servers and Map-Resolvers.
Note that while this document specifies the use of Generic Routing
Encapsulation (GRE) as a tunneling mechanism, there is no reason that
parts of the ALT cannot be built using other tunneling technologies,
particularly in cases where GRE does not meet security, management,
or other operational requirements. References to "GRE tunnel" in
later sections of this document should therefore not be taken as
prohibiting or precluding the use of other tunneling mechanisms.
Note also that two ALT-Routers that are directly adjacent (with no
layer-3 router hops between them) need not use a tunnel between them;
in this case, BGP may be configured across the interfaces that
connect to their common subnet, and that subnet is then considered to
be part of the ALT topology. The use of techniques such as "eBGP
multihop" to connect ALT-Routers that do not share a tunnel or common
subnet is not recommended, as the non-ALT routers in between the
ALT-Routers in such a configuration may not have information
necessary to forward ALT Datagrams destined to EID-Prefixes exchanged
across that BGP session.
In summary, LISP+ALT uses BGP to build paths through ALT-Routers so
that an ALT Datagram sent into the ALT can be forwarded to the ETR
that holds the EID-to-RLOC mapping for that EID-Prefix. This
reachability is carried as IPv4 or IPv6 Network Layer Reachability
Information (NLRI) without modification (since an EID-Prefix has the
same syntax as an IPv4 or IPv6 address prefix). ALT-Routers
establish BGP sessions with one another, forming the ALT. An
ALT-Router at the "edge" of the topology learns EID-Prefixes
originated by authoritative ETRs. Learning may be through the
Map-Server interface, by static configuration, or via BGP with the
ETRs. An ALT-Router may also be configured to aggregate EID-Prefixes
received from ETRs or from other LISP-ALT Routers that are
topologically "downstream" from it.
4.1. ITR Traffic Handling
When an ITR receives a packet originated by an end-system within its
site (i.e., a host for which the ITR is the exit path out of the
site) and the destination EID for that packet is not known in the
ITR's map-cache, the ITR creates either a Map-Request for the
destination EID or the original packet encapsulated as a Data-Probe
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RFC 6836 LISP+ALT January 2013
(see Section 3.3 for caveats on the usability of Data-Probes). The
result, known as an ALT Datagram, is then sent to an ALT-Router (see
also [RFC6833] for non-ALT-connected ITRs, noting that Data-Probes
cannot be sent to a Map-Resolver). This "first-hop" ALT-Router uses
EID-Prefix routing information learned from other ALT-Routers via BGP
to guide the packet to the ETR that "owns" the prefix. Upon receipt
by the ETR, normal LISP processing occurs: the ETR responds to the
ITR with a LISP Map-Reply that lists the RLOCs (and, thus, the ETRs
to use) for the EID-Prefix. For Data-Probes, the ETR also
decapsulates the packet and transmits it toward its destination.
Upon receipt of the Map-Reply, the ITR installs the RLOC information
for a given prefix into a local mapping database. With these mapping
entries stored, additional packets destined to the given EID-Prefix
are routed directly to an RLOC without use of the ALT, until either
the entry's Time to Live (TTL) has expired or the ITR can otherwise
find no reachable ETR. Note that a current mapping may exist that
contains no reachable RLOCs; this is known as a Negative Cache Entry,
and it indicates that packets destined to the EID-Prefix are to be
dropped.
Full details on Map-Request/Map-Reply processing may be found in
[RFC6830].
Traffic routed on to the ALT consists solely of ALT Datagrams, i.e.,
Map-Requests and Data-Probes (if supported). Given the relatively
low performance expected of a tunneled topology, ALT-Routers (and
Map-Resolvers) should aggressively rate-limit the ingress of ALT
Datagrams from ITRs and, if possible, should be configured to not
accept packets that are not ALT Datagrams.
4.2. EID Assignment - Hierarchy and Topology
The ALT database is organized in a hierarchical manner with
EID-Prefixes aggregated on power-of-2 block boundaries. Where a LISP
site has multiple EID-Prefixes that are aligned on a power-of-2 block
boundary, they should be aggregated into a single EID-Prefix for
advertisement. The ALT network is built in a roughly hierarchical,
partial mesh that is intended to allow aggregation where clearly
defined hierarchical boundaries exist. Building such a structure
should minimize the number of EID-Prefixes carried by LISP+ALT nodes
near the top of the hierarchy.
Routes on the ALT do not need to respond to changes in policy,
subscription, or underlying physical connectivity, so the topology
can remain relatively static and aggregation can be sustained.
Because routing on the ALT uses BGP, the same rules apply for
generating aggregates; in particular, an ALT-Router should only be
Fuller, et al. Experimental [Page 12]
RFC 6836 LISP+ALT January 2013
configured to generate an aggregate if it is configured with BGP
sessions to all of the originators of components (more-specific
prefixes) of that aggregate. Not all of the components need to be
present for the aggregate to be originated (some may be holes in the
covering prefix, and some may be down), but the aggregating router
must be configured to learn the state of all of the components.
Under what circumstances the ALT-Router actually generates the
aggregate is a matter of local policy: in some cases, it will be
statically configured to do so at all times with a "static discard"
route. In other cases, it may be configured to only generate the
aggregate prefix if at least one of the components of the aggregate
is learned via BGP.
An ALT-Router must not generate an aggregate that includes a
non-LISP-speaking hole unless it can be configured to return a
Negative Map-Reply with action="Natively-Forward" (see [RFC6830]) if
it receives an ALT Datagram that matches that hole. If it receives
an ALT Datagram that matches a LISP-speaking hole that is currently
not reachable, it should return a Negative Map-Reply with
action="drop". Negative Map-Replies should be returned with a short
TTL, as specified in [RFC6833]. Note that an off-the-shelf,
non-LISP-speaking router configured as an aggregating ALT-Router
cannot send Negative Map-Replies, so such a router must never
originate an aggregate that includes a non-LISP-speaking hole.
This implies that two ALT-Routers that share an overlapping set of
prefixes must exchange those prefixes if either is to generate and
export a covering aggregate for those prefixes. It also implies that
an ETR that connects to the ALT using BGP must maintain BGP sessions
with all of the ALT-Routers that are configured to originate an
aggregate that covers that prefix and that each of those ALT-Routers
must be explicitly configured to know the set of EID-Prefixes that
make up any aggregate that it originates. See also [RFC6833] for an
example of other ways that prefix origin consistency and aggregation
can be maintained.
As an example, consider ETRs that are originating EID-Prefixes for
10.1.0.0/24, 10.1.64.0/24, 10.1.128.0/24, and 10.1.192.0/24. An
ALT-Router should only be configured to generate an aggregate for
10.1.0.0/16 if it has BGP sessions configured with all of these ETRs,
in other words, only if it has sufficient knowledge about the state
of those prefixes to summarize them. If the Router originating
10.1.0.0/16 receives an ALT Datagram destined for 10.1.77.88, a
non-LISP destination covered by the aggregate, it returns a Negative
Map-Reply with action "Natively-Forward". If it receives an ALT
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RFC 6836 LISP+ALT January 2013
Datagram destined for 10.1.128.199 but the configured LISP prefix
10.1.128.0/24 is unreachable, it returns a Negative Map-Reply with
action "drop".
Note: Much is currently uncertain about the best way to build the ALT
network; as testing and prototype deployment proceed, a guide to how
to best build the ALT network will be developed.
4.3. Use of GRE and BGP between LISP-ALT Routers
The ALT network is built using GRE tunnels between ALT-Routers. BGP
sessions are configured over those tunnels, with each ALT-Router
acting as a separate Autonomous System (AS) "hop" in a Path Vector
for BGP. For the purposes of LISP+ALT, the AS-path is used solely as
a shortest-path determination and loop-avoidance mechanism. Because
all next hops are on tunnel interfaces, no IGP is required to resolve
those next hops to exit interfaces.
LISP+ALT's use of GRE and BGP facilitates deployment and operation of
LISP because no new protocols need to be defined, implemented, or
used on the overlay topology; existing BGP/GRE tools and operational
expertise are also re-used. Tunnel address assignment is also easy:
since the addresses on an ALT tunnel are only used by the pair of
routers connected to the tunnel, the only requirement of the IP
addresses used to establish that tunnel is that the attached routers
be reachable by each other; any addressing plan, including private
addressing, can therefore be used for ALT tunnels.
5. EID-Prefix Propagation and Map-Request Forwarding
As described in Section 8.2, an ITR sends an ALT Datagram to a given
EID-to-RLOC mapping. The ALT provides the infrastructure that allows
these requests to reach the authoritative ETR.
Note that under normal circumstances Map-Replies are not sent over
the ALT; an ETR sends a Map-Reply to one of the ITR RLOCs learned
from the original Map-Request. See Sections 6.1.2 and 6.2 of
[RFC6830] for more information on the use of the Map-Request 'ITR
RLOC Address' field. Keep in mind that the 'ITR RLOC Address' field
supports multiple RLOCs in multiple address families, so a Map-Reply
sent in response to a Map-Request is not necessarily sent back to the
Map-Request RLOC source.
There may be scenarios, perhaps to encourage caching of EID-to-RLOC
mappings by ALT-Routers, where Map-Replies could be sent over the ALT
or where a "first-hop" ALT-Router might modify the originating RLOC
on a Map-Request received from an ITR to force the Map-Reply to be
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returned to the "first-hop" ALT-Router. These cases will not be
supported by initial LISP+ALT implementations but may be subject to
future experimentation.
ALT-Routers propagate path information via BGP ([RFC4271]) that is
used by ITRs to send ALT Datagrams toward the appropriate ETR for
each EID-Prefix. BGP is run on the inter-ALT-Router links, and
possibly between an edge ("last-hop") ALT-Router and an ETR or
between an edge ("first-hop") ALT-Router and an ITR. The ALT BGP RIB
consists of aggregated EID-Prefixes and their next hops toward the
authoritative ETR for that EID-Prefix.
5.1. Changes to ITR Behavior with LISP+ALT
As previously described, an ITR will usually use the Map-Resolver
interface and will send its Map Requests to a Map-Resolver. When an
ITR instead connects via tunnels and BGP to the ALT, it sends ALT
Datagrams to one of its "upstream" ALT-Routers; these are sent only
to obtain new EID-to-RLOC mappings -- RLOC probe and cache TTL
refresh Map-Requests are not sent on the ALT. As in basic LISP, it
should use one of its RLOCs as the source address of these queries;
it should not use a tunnel interface as the source address, as doing
so will cause replies to be forwarded over the tunneled topology and
may be problematic if the tunnel interface address is not routed
throughout the ALT. If the ITR is running BGP with the LISP-ALT
Router(s), it selects the appropriate ALT-Router based on the BGP
information received. If it is not running BGP, it uses a statically
configured ALT Default Route to select an ALT-Router.
5.2. Changes to ETR Behavior with LISP+ALT
As previously described, an ETR will usually use the Map-Server
interface (see [RFC6833]) and will register its EID-Prefixes with its
configured Map-Servers. When an ETR instead connects using BGP to
one or more ALT-Routers, it announces its EID-Prefix(es) to those
ALT-Routers.
As documented in [RFC6830], when an ETR generates a Map-Reply message
to return to a querying ITR, it sets the outer-header IP destination
address to one of the requesting ITR's RLOCs so that the Map-Reply
will be sent on the underlying Internet topology, not on the ALT;
this avoids any latency penalty (or "stretch") that might be incurred
by sending the Map-Reply via the ALT, reduces load on the ALT, and
ensures that the Map-Reply can be routed even if the original ITR
does not have an ALT-routed EID. For details on how an ETR selects
which ITR RLOC to use, see Section 6.1.5 of [RFC6830].
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5.3. ALT Datagram Forwarding Failure
Intermediate ALT-Routers forward ALT Datagrams using normal,
hop-by-hop routing on the ALT overlay network. Should an ALT-Router
not be able to forward an ALT Datagram, whether due to an unreachable
next hop, TTL exceeded, or other problem, it has several choices:
o If the ALT-Router understands LISP, as is the case for a
Map-Resolver or Map-Server, it may respond to a forwarding failure
by returning a Negative Map-Reply, as described in Section 4.2 and
[RFC6833].
o If the ALT-Router does not understand LISP, it may attempt to
return an ICMP message to the source IP address of the packet that
cannot be forwarded. Since the source address is an RLOC, an
ALT-Router would send this ICMP message using "native" Internet
connectivity, not via the ALT overlay.
o A non-LISP-capable ALT-Router may also choose to silently drop the
non-forwardable ALT Datagram.
[RFC6830] and [RFC6833] define how the source of an ALT Datagram
should handle each of these cases. The last case, where an ALT
Datagram is silently discarded, will generally result in several
retransmissions by the source, followed by treating the destination
as unreachable via LISP when no Map-Reply is received. If a problem
on the ALT is severe enough to prevent ALT Datagrams from being
delivered to a specific EID, this is probably the only sensible way
to handle this case.
Note that the use of GRE tunnels should prevent MTU problems from
ever occurring on the ALT; an ALT Datagram that exceeds an
intermediate MTU will be fragmented at that point and will be
reassembled by the target of the GRE tunnel.
6. BGP Configuration and Protocol Considerations
6.1. Autonomous System Numbers (ASNs) in LISP+ALT
The primary use of BGP today is to define the global Internet routing
topology in terms of its participants, known as Autonomous Systems.
LISP+ALT specifies the use of BGP to create a global overlay network
(the ALT) for finding EID-to-RLOC mappings. While related to the
global routing database, the ALT serves a very different purpose and
is organized into a very different hierarchy. Because LISP+ALT does
use BGP, however, it uses ASNs in the paths that are propagated among
ALT-Routers. To avoid confusion, LISP+ALT should use newly assigned
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AS numbers that are unrelated to the ASNs used by the global routing
system. Exactly how this new space will be assigned and managed will
be determined during the deployment of LISP+ALT.
Note that the ALT-Routers that make up the "core" of the ALT will not
be associated with any existing core-Internet ASN because the ALT
topology is completely separate from, and independent of, the global
Internet routing system.
6.2. Subsequent Address Family Identifier (SAFI) for LISP+ALT
As defined by this document, LISP+ALT may be implemented using BGP
without modification. Given the fundamental operational difference
between propagating global Internet routing information (the current
dominant use of BGP) and creating an overlay network for finding
EID-to-RLOC mappings (the use of BGP as proposed by this document),
it may be desirable to assign a new SAFI [RFC4760] to prevent
operational confusion and difficulties, including the inadvertent
leaking of information from one domain to the other. The use of a
separate SAFI would make it easier to debug many operational problems
but would come at a significant cost: unmodified, off-the-shelf
routers that do not understand the new SAFI could not be used to
build any part of the ALT network. At present, this document does
not request the assignment of a new SAFI; additional experimentation
may suggest the need for one in the future.
7. EID-Prefix Aggregation
To facilitate EID-Prefix aggregation, the ALT BGP topology is
provisioned in a hierarchical manner; the fact that all inter-node
links are tunnels means that topology can be constrained to follow
the EID-Prefix assignment hierarchy. Redundant links are provisioned
to compensate for node and link failures. A basic assumption is that
as long as the routers are up and running, the underlying Internet
will provide alternative routes to maintain tunnel and BGP
connectivity among ALT-Routers.
Note that, as mentioned in Section 4.2, the use of BGP by LISP+ALT
requires that information only be aggregated where all active more-
specific prefixes of a generated aggregate prefix are known. This is
no different than the way that BGP route aggregation works in the
existing global routing system: a service provider only generates an
aggregate route if it is configured to learn all prefixes that make
up that aggregate.
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7.1. Stability of the ALT
It is worth noting that LISP+ALT does not directly propagate
EID-to-RLOC mappings. What it does is provide a mechanism for an ITR
to communicate with the ETR that holds the mapping for a particular
EID-Prefix. This distinction is important when considering the
stability of BGP on the ALT network as compared to the global routing
system. It also has implications for how site-specific EID-Prefix
information may be used by LISP but not propagated by LISP+ALT (see
Section 7.2 below).
RLOC prefixes are not propagated through the ALT, so their
reachability is not determined through the use of LISP+ALT. Instead,
reachability of RLOCs is learned through the LISP ITR-ETR exchange.
This means that link failures or other service disruptions that may
cause the reachability of an RLOC to change are not known to the ALT.
Changes to the presence of an EID-Prefix on the ALT occur much less
frequently: only at subscription time or in the event of a failure of
the ALT infrastructure itself. This means that "flapping" (frequent
BGP updates and withdrawals due to prefix state changes) is not
likely and mapping information cannot become "stale" due to slow
propagation through the ALT BGP mesh.
7.2. Traffic Engineering Using LISP
Since an ITR learns an EID-to-RLOC mapping directly from the ETR that
owns it, it is possible to perform site-to-site Traffic Engineering
by setting the preference and/or weight fields, and by including
more-specific EID-to-RLOC information in Map-Reply messages.
This is a powerful mechanism that can conceivably replace the
traditional practice of routing prefix deaggregation for Traffic
Engineering purposes. Rather than propagating more-specific
information into the global routing system for local or regional
optimization of traffic flows, such more-specific information can be
exchanged, through LISP (not LISP+ALT), on an as-needed basis between
only those ITRs/ETRs (and, thus, site pairs) that need it. Such an
exchange of "more-specifics" between sites facilitates Traffic
Engineering by allowing richer and more fine-grained policies to be
applied without advertising additional prefixes into either the ALT
or the global routing system.
Note that these new Traffic Engineering capabilities are an attribute
of LISP and are not specific to LISP+ALT; discussion is included here
because the BGP-based global routing system has traditionally used
propagation of more-specific routes as a crude form of Traffic
Engineering.
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7.3. Edge Aggregation and Dampening
Normal BGP best common practices apply to the ALT network. In
particular, first-hop ALT-Routers will aggregate EID-Prefixes and
dampen changes to them in the face of excessive updates. Since
EID-Prefix assignments are not expected to change as frequently as
global routing BGP prefix reachability, such dampening should be very
rare and might be worthy of logging as an exceptional event. It is
again worth noting that the ALT carries only EID-Prefixes, used to
construct a BGP path to each ETR (or Map-Server) that originates each
prefix; the ALT does not carry reachability information about RLOCs.
In addition, EID-Prefix information may be aggregated as the topology
and address assignment hierarchy allow. Since the topology is all
tunneled and can be modified as needed, reasonably good aggregation
should be possible. In addition, since most ETRs are expected to
connect to the ALT using the Map-Server interface, Map-Servers will
implement a natural "edge" for the ALT where dampening and
aggregation can be applied. For these reasons, the set of prefix
information on the ALT can be expected to be both better aggregated
and considerably less volatile than the actual EID-to-RLOC mappings.
7.4. EID Assignment Flexibility vs. ALT Scaling
There are major open questions regarding how the ALT will be deployed
and what organization(s) will operate it. In a simple,
non-distributed world, centralized administration of EID-Prefix
assignment and ALT network design would facilitate a well-aggregated
ALT routing system. Business and other realities will likely result
in a more complex, distributed system involving multiple levels of
prefix delegation, multiple operators of parts of the ALT
infrastructure, and a combination of competition and cooperation
among the participants. In addition, the re-use of existing IP
address assignments, both Provider-Independent ("PI") and Provider-
Assigned ("PA"), to avoid renumbering when sites transition to LISP
will further complicate the processes of building and operating
the ALT.
A number of conflicting considerations need to be kept in mind when
designing and building the ALT. Among them are:
1. Target ALT routing state size and level of aggregation. As
described in Section 7.1, the ALT should not suffer from the same
performance constraints or stability issues as does the Internet
global routing system, so some reasonable level of deaggregation
and an increased number of EID-Prefixes beyond what might be
considered ideal should be acceptable. That said, measures, such
as tunnel rehoming to preserve aggregation when sites move from
one mapping provider to another and implementing aggregation at
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RFC 6836 LISP+ALT January 2013
multiple levels in the hierarchy to collapse deaggregation at
lower levels, should be taken to reduce unnecessary explosion of
ALT routing state.
2. Number of operators of parts of the ALT and how they will be
organized (hierarchical delegation vs. shared administration).
This will determine not only how EID-Prefixes are assigned but
also how tunnels are configured and how EID-Prefixes can be
aggregated between different parts of the ALT.
3. Number of connections between different parts of the ALT.
Tradeoffs will need to be made among resilience, performance, and
placement of aggregation boundaries.
4. EID-Prefix portability between competing operators of the ALT
infrastructure. A significant benefit for an end site to adopt
LISP is the availability of EID space that is not tied to a
specific connectivity provider; it is important to ensure that an
end site doesn't trade lock-in to a connectivity provider for
lock-in to a provider of its EID assignment, ALT connectivity, or
Map-Server facilities.
This is, by no means, an exhaustive list.
While resolving these issues is beyond the scope of this document,
the authors recommend that existing distributed resource structures,
such as the IANA/Regional Internet Registries and the ICANN/Domain
Registrar, be carefully considered when designing and deploying the
ALT infrastructure.
8. Connecting Sites to the ALT Network
8.1. ETRs Originating Information into the ALT
EID-Prefix information is originated into the ALT by three different
mechanisms:
Map-Server: In most cases, a site will configure its ETR(s) to
register with one or more Map-Servers (see [RFC6833]) and does not
participate directly in the ALT.
BGP: For sites requiring complex control over their EID-Prefix
origination into the ALT, an ETR may connect to the LISP+ALT
overlay network by running BGP to one or more ALT-Routers over
tunnel(s). The ETR advertises reachability for its EID-Prefixes
over these BGP connection(s). The edge ALT-Router(s) that
receive(s) these prefixes then propagate(s) them into the ALT.
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Here, the ETR is simply a BGP peer of ALT-Router(s) at the edge of
the ALT. Where possible, an ALT-Router that receives EID-Prefixes
from an ETR via BGP should aggregate that information.
Configuration: One or more ALT-Routers may be configured to
originate an EID-Prefix on behalf of the non-BGP-speaking ETR that
is authoritative for a prefix. As in the case above, the ETR is
connected to ALT-Router(s) using GRE tunnel(s), but rather than
BGP being used, the ALT-Router(s) are configured with what are in
effect "static routes" for the EID-Prefixes "owned" by the ETR.
The GRE tunnel is used to route Map-Requests to the ETR.
Note: In all cases, an ETR may register to multiple Map-Servers or
connect to multiple ALT-Routers for the following reasons:
* redundancy, so that a particular ETR is still reachable even if
one path or tunnel is unavailable.
* to connect to different parts of the ALT hierarchy if the ETR
"owns" multiple EID-to-RLOC mappings for EID-Prefixes that
cannot be aggregated by the same ALT-Router (i.e., are not
topologically "close" to each other in the ALT).
8.2. ITRs Using the ALT
In the common configuration, an ITR does not need to know anything
about the ALT, since it sends Map-Requests to one of its configured
Map-Resolvers (see [RFC6833]). There are two exceptional cases:
Static default: If a Map-Resolver is not available but an ITR is
adjacent to an ALT-Router (either over a common subnet or through
the use of a tunnel), it can use an ALT Default Route to cause all
ALT Datagrams to be sent to that ALT-Router. This case is
expected to be rare.
Connection to ALT: A site with complex Internet connectivity may
need more fine-grained distinction between traffic to LISP-capable
and non-LISP-capable sites. Such a site may configure each of its
ITRs to connect directly to the ALT, using a tunnel and BGP
connection. In this case, the ITR will receive EID-Prefix routes
from its BGP connection to the ALT-Router and will LISP-
encapsulate and send ALT Datagrams through the tunnel to the
ALT-Router. Traffic to other destinations may be forwarded
(without LISP encapsulation) to non-LISP next-hop routers that the
ITR knows.
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In general, an ITR that connects to the ALT does so only to
ALT-Routers at the "edge" of the ALT (typically two for
redundancy). There may, though, be situations where an ITR would
connect to other ALT-Routers to receive additional, shorter-path
information about a portion of the ALT of interest to it. This
can be accomplished by establishing GRE tunnels between the ITR
and the set of ALT-Routers with the additional information. This
is a purely local policy issue between the ITR and the ALT-Routers
in question.
As described in [RFC6833], Map-Resolvers do not accept or forward
Data-Probes; in the rare scenario that an ITR does support and
originate Data-Probes, it must do so using one of the exceptional
configurations described above. Note that the use of Data-Probes is
discouraged at this time (see Section 3.3).
9. Security Considerations
LISP+ALT shares many of the security characteristics of BGP. Its
security mechanisms are comprised of existing technologies in wide
operational use today, so securing the ALT should be mostly a matter
of applying the same technology that is used to secure the BGP-based
global routing system (see Section 9.3 below).
9.1. Apparent LISP+ALT Vulnerabilities
This section briefly lists the known potential vulnerabilities of
LISP+ALT.
Mapping integrity: Potential for an attacker to insert bogus
mappings to black-hole (create a DoS attack) or intercept LISP
data-plane packets.
ALT-Router availability: Can an attacker DoS the ALT-Routers
connected to a given ETR? If a site's ETR cannot advertise its
EID-to-RLOC mappings, the site is essentially unavailable.
ITR mapping/resources: Can an attacker force an ITR or ALT-Router to
drop legitimate mapping requests by flooding it with random
destinations for which it will generate large numbers of
Map-Requests and fill its map-cache? Further study is required to
see the impact of admission control on the overlay network.
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RFC 6836 LISP+ALT January 2013
EID Map-Request exploits for reconnaissance: Can an attacker learn
about a LISP site's TE policy by sending legitimate mapping
requests and then observing the RLOC mapping replies? Is this
information useful in attacking or subverting peer relationships?
Note that any public LISP mapping database will have similar
data-plane reconnaissance issues.
Scaling of ALT-Router resources: Paths through the ALT may be of
lesser bandwidth than more "direct" paths; this may make them more
prone to high-volume DoS attacks. For this reason, all components
of the ALT (ETRs and ALT-Routers) should be prepared to rate-limit
traffic (ALT Datagrams) that could be received across the ALT.
UDP Map-Reply from ETR: Since Map-Replies are sent directly from the
ETR to the ITR's RLOC, the ITR's RLOC may be vulnerable to various
types of DoS attacks (this is a general property of LISP, not a
LISP+ALT vulnerability).
More-specific prefix leakage: Because EID-Prefixes on the ALT are
expected to be fairly well-aggregated and EID-Prefixes propagated
out to the global Internet (see [RFC6832]) much more so,
accidental leaking or malicious advertisement of an EID-Prefix
into the global routing system could cause traffic redirection
away from a LISP site. This is not really a new problem, though,
and its solution can only be achieved by much more strict prefix
filtering and authentication on the global routing system.
Section 9.3 describes an existing approach to solving this
problem.
9.2. Survey of LISP+ALT Security Mechanisms
Explicit peering: The devices themselves can prioritize incoming
packets as well as potentially do key checks in hardware to
protect the control plane.
Use of TCP to connect elements: This makes it difficult for third
parties to inject packets.
Use of HMAC to protect BGP/TCP connections: Hashed Message
Authentication Code (HMAC) [RFC5925] is used to verify the
integrity and authenticity of TCP connections used to exchange BGP
messages, making it nearly impossible for third-party devices to
either insert or modify messages.
Message sequence numbers and nonce values in messages: This allows
an ITR to verify that the Map-Reply from an ETR is in response to
a Map-Request originated by that ITR (this is a general property
of LISP; LISP+ALT does not change this behavior).
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9.3. Use of Additional BGP Security Mechanisms
LISP+ALT's use of BGP allows it to take advantage of BGP security
features designed for existing Internet BGP use. This means that
LISP+ALT can and should use technology developed for adding security
to BGP (in the IETF SIDR working group or elsewhere) to provide
authentication of EID-Prefix origination and EID-to-RLOC mappings.
10. Acknowledgments
The authors would like to specially thank J. Noel Chiappa, who was a
key contributor to the design of the Content distribution Overlay
Network Service for LISP (LISP-CONS) mapping database (many ideas
from which made their way into LISP+ALT) and who has continued to
provide invaluable insight as the LISP effort has evolved. Others
who have provided valuable contributions include John Zwiebel, Hannu
Flinck, Amit Jain, John Scudder, Scott Brim, and Jari Arkko.
11. References
11.1. Normative References
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
January 2007.
[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.
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RFC 6836 LISP+ALT January 2013
11.2. Informative References
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC6832] Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
"Interworking between Locator/ID Separation Protocol
(LISP) and Non-LISP Sites", RFC 6832, January 2013.
Authors' Addresses
Vince Fuller
EMail: vaf@vaf.net
Dino Farinacci
Cisco Systems
Tasman Drive
San Jose, CA 95134
USA
EMail: farinacci@gmail.com
Dave Meyer
Cisco Systems
Tasman Drive
San Jose, CA 95134
USA
EMail: dmm@1-4-5.net
Darrel Lewis
Cisco Systems
Tasman Drive
San Jose, CA 95134
USA
EMail: darlewis@cisco.com
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