Network Working Group D. Farinacci
Internet-Draft V. Fuller
Intended status: Experimental D. Meyer
Expires: November 27, 2009 D. Lewis
Cisco
May 26, 2009
LISP Alternative Topology (LISP+ALT)
draft-ietf-lisp-alt-00.txt
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Abstract
This document describes a method of building an alternative, logical
topology for managing Endpoint Identifier to Routing Locator mappings
using the Locator/ID Separation Protocol. The logical network is
built as an overlay on the public Internet using existing
technologies and tools, specifically the Border Gateway Protocol and
the Generic Routing Encapsulation. An important design goal for
LISP+ALT is to allow for the relatively easy deployment of an
efficient mapping system while minimizing changes to existing
hardware and software.
Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 6
4. The LISP 1.5 model . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Connectivity to non-LISP sites . . . . . . . . . . . . . . 8
4.2. Caveats on the use of Data Probes . . . . . . . . . . . . 9
5. LISP+ALT: Overview . . . . . . . . . . . . . . . . . . . . . . 10
5.1. ITR traffic handling . . . . . . . . . . . . . . . . . . . 10
5.2. EID Assignment - Hierarchy and Topology . . . . . . . . . 11
5.3. LISP+ALT Router . . . . . . . . . . . . . . . . . . . . . 12
5.4. ITR and ETR in a LISP+ALT Environment . . . . . . . . . . 12
5.5. Use of GRE and BGP between LISP+ALT Routers . . . . . . . 13
6. EID Prefix Propagation and Map-Request Forwarding . . . . . . 14
6.1. Changes to ITR behavior with LISP+ALT . . . . . . . . . . 14
6.2. Changes to ETR behavior with LISP+ALT . . . . . . . . . . 14
7. BGP configuration and protocol considerations . . . . . . . . 16
7.1. Autonomous System Numbers (ASNs) in LISP+ALT . . . . . . . 16
7.2. Sub-Address Family Identifier (SAFI) for LISP+ALT . . . . 16
8. EID-Prefix Aggregation . . . . . . . . . . . . . . . . . . . . 17
8.1. Traffic engineering with LISP and LISP+ALT . . . . . . . . 17
8.2. Edge aggregation and dampening . . . . . . . . . . . . . . 18
9. Connecting sites to the ALT network . . . . . . . . . . . . . 19
9.1. ETRs originating information into the ALT . . . . . . . . 19
9.2. ITRs Using the ALT . . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11.1. Apparent LISP+ALT Vulnerabilities . . . . . . . . . . . . 22
11.2. Survey of LISP+ALT Security Mechanisms . . . . . . . . . . 23
11.3. Using existing BGP Security mechanisms . . . . . . . . . . 23
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
13.1. Normative References . . . . . . . . . . . . . . . . . . . 25
13.2. Informative References . . . . . . . . . . . . . . . . . . 25
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Introduction
This document describes a method of building an alternative logical
topology for managing Endpoint identifier to Routing Locator mappings
using the Locator/ID Separation Protocol [LISP]. This logical
topology uses existing technology and tools, specifically the Border
Gateway Protocol [RFC4271] and its multi-protocol extension
[RFC2858], along with the Generic Routing Encapsulation [RFC2784]
protocol to construct an overlay network of devices that advertise
EID-prefixes only. These Endpoint Identifier Prefix Aggregators hold
hierarchically-assigned pieces of the Endpoint Identifier space
(i.e., prefixes) and their next hops toward the network element which
is authoritative for Endpoint Identifier-to-Routing Locator mapping
for that prefix. Tunnel routers can use this overlay to make queries
against and respond to mapping requests made against the distributed
Endpoint Identifier-to-Routing Locator mapping database. Note the
database is distributed (as described in [LISP]) and is stored in the
ETRs.
Note that an important design goal of LISP+ALT is to minimize the
number of changes to existing hardware and/or software that are
required to deploy the mapping system. It is envisioned that in most
cases existing technology can be used to implement and deploy LISP+
ALT. Since the deployment of LISP+ALT adds new devices to the
network, existing devices not need changes or upgrades. They can
function as they are to realize an underlying and robust physical
topology.
The remainder of this document is organized as follows: Section 3
provides the definitions of terms used in this document. Section 4
outlines the basic LISP 1.5 model. Section 5 provides a basic
overview of the LISP Alternate Topology architecture, and Section 6
describes how the ALT uses BGP to propagate Endpoint Identifier
reachability over the overlay network. Section 8 describes the
construction of the ALT aggregation hierarchy, and Section 9
discusses how LISP+ALT elements are connected to form the overlay
network.
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3. Definition of Terms
LISP+ALT operates on two name spaces and introduces a new network
element, the LISP+ALT Router (see below). This section provides
high-level definitions of the LISP+ALT name spaces, network elements,
and message types.
The Alternative Logical Topology (ALT): The virtual overlay network
made up of tunnels between EID Prefix Aggregators. The Border
Gateway Protocol (BGP) runs between LISP+ALT routers and is used
to carry reachability information for EID prefixes.
Legacy Internet: The portion of the Internet which does not run LISP
and does not participate in LISP+ALT.
LISP+ALT Router: The devices which run on the ALT. The ALT is a
static network built using tunnels between LISP+ALT routers.
These routers are deployed in a hierarchy in which routers at each
level in the this hierarchy are responsible for aggregating all
EID prefixes learned from those logically "below" them and
advertising summary prefixes to the routers logically "above"
them. All prefix learning and propagation between levels is done
using BGP. LISP+ALT routers at the lowest level, or "edge", of
the ALT learn EID prefixes either over a BGP session to ETRs or
through static routes (in the case of the "low-opex ETR"). See
Section 7 for details on how BGP is configured between the
different network elements.
The primary function of LISP+ALT routers is to provide a
lightweight forwarding infrastructure for LISP control-plane
messages (Map-Request and Map-Reply), and to transport data
packets when the packet has the same destination address in both
the inner (encapsulating) destination and outer destination
addresses ((i.e., a Data Probe packet).
Endpoint ID (EID): 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. A packet that is emitted by
a system contains EIDs in its headers and LISP headers are
prepended only when the packet reaches an Ingress Tunnel Router
(ITR) on the data path to the destination EID.
In LISP+ALT, EID-prefixes MUST BE assigned in a hierarchical
manner (in power-of-two) such that they can be aggregated by LISP+
ALT routers. In addition, a site may have site-local structure in
how EIDs are topologically organized (subnetting) for routing
within the site; this structure is not visible to the global
routing system.
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EID-Prefix Aggregate: A set of EID-prefixes said to be aggregatable
in the [RFC4632] sense. That is, an EID-Prefix aggregate is
defined to be a single contiguous power-of-two EID-prefix block.
Such a block is characterized by a prefix and a length.
Routing Locator (RLOC): An IP address of an egress tunnel router
(ETR). It is the output of a EID-to-RLOC mapping lookup. An EID
maps to one or more RLOCs. Typically, RLOCs are numbered from
topologically-aggregatable blocks that are assigned to a site 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 Provider Aggregatable (PA) addresses.
Note that in LISP+ALT, RLOCs are not carried by LISP+ALT routers.
EID-to-RLOC Mapping: A binding between an EID and the RLOC-set that
can be used to reach the EID. The term "mapping" refers to an
EID-to-RLOC mapping.
EID Prefix Reachability: An EID prefix is said to be "reachable" if
one or more of its locators are reachable. That is, an EID prefix
is reachable if the ETR (or its proxy) that is authoritative for a
given EID-to-RLOC mapping is reachable.
Default Mapping: A Default Mapping is a mapping entry for EID-
prefix 0.0.0.0/0. It maps to a locator-set used for all EIDs in
the Internet. If there is a more specific EID-prefix in the
mapping cache it overrides the Default Mapping entry. The Default
Mapping route can be learned by configuration or from a Map-Reply
message.
Default Route: A Default Route in the context of LISP+ALT is a EID-
prefix value of 0.0.0.0/0 which is advertised by BGP on top of the
ALT. The Default Route is used to realize a path for Data Probe
or Map-Request packets.
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4. The LISP 1.5 model
As documented in [LISP], the LISP 1.5 model uses the same basic
query/response protocol machinery as LISP 1.0. In particular, LISP+
ALT provides two mechanisms for an ITR to obtain EID-to-RLOC mappings
(both of these techniques are described in more detail in
Section 9.2):
Data Probe: An ITR may send the first few data packets into the ALT
to minimize packet loss and to probe for the mapping; the
authoritative ETR will respond to the ITR with a Map-Reply message
when it receives the data packet over the ALT. Note that in this
case, the inner Destination Address (DA), which is an EID, is
copied to the outer DA and is routed over the ALT.
Map-Request: An ITR may also send a Map-Request message into the ALT
to request the mapping. As in the Data Probe case, the
authoritative ETR will respond to the ITR with a Map-Reply
message. In this case, the DA of the Map-Request MUST be an EID.
See [LISP] for the format of Map-Request and Map-Reply packets.
As with LISP 1.0, EIDs are routable and can be used, unaltered, as
the source and destination addresses in IP datagrams. Unlike in LISP
1.0, LISP 1.5 EIDs are not routable on the public Internet; instead,
they are only routed over a separate, virtual topology referred to as
the LISP Alternative Virtual Network. This network is built as an
overlay on the public Internet using tunnels to interconnect LISP+ALT
routers. BGP is run over these tunnels to propagate the information
needed to route Data Probes and Map-Request/Replies. Importantly,
while the ETRs are the source(s) of the unaggregated EID prefix data,
LISP+ALT uses existing BGP mechanisms to aggressively aggregate this
information. Note that ETRs are not required to participate (or
prevented from participating) in LISP+ALT; they may choose
communicate their mappings to their serving LISP+ALT router(s) at
subscription time via configuration. ITRs are also not required to
participate in (nor prevented from participating in) LISP+ALT.
4.1. 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 [Interworking] for
details.
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4.2. 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 are also other
issues involving latency or other differences between the ALT path
that initial a Data Probe would take and the path that subsequent
packets on the same flow would take once a mapping were in place on
an ITR. For these and other reasons use of Data Probes should be
considered experimental and should be disabled by default in all ITR
implementations.
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5. LISP+ALT: Overview
LISP+ALT is a hybrid push/pull architecture. Aggregated EID prefixes
are "pushed" among the LISP+ALT routers and, optionally, out to ITRs
(which may elect to receive the aggregated information, as opposed to
simply using a default mapping). Specific EID-to-RLOC mappings are
"pulled" by ITRs when they either send explicit LISP requests or data
packets on the alternate topology that result in triggered replies
being generated by ETRs.
The basic idea embodied in LISP+ALT is to use BGP, running over
tunneled overlay network, to establish reachability required to route
Data Probes and Map-Requests over an alternate logical topology
(ALT). The ALT BGPRoute Information Base (RIB) is comprised of EID
prefixes and associated next hops. LISP+ALT routers interconnect
using eBGP and propagate EID prefix updates, which are learned over
eBGP connections to authoritative ETRs, or by static configuration.
ITRs may also eBGP peer with one or more LISP+ALT to learn the best
ALT router to use to forward a Data Proble or Map-Request for a
particular prefix; in most cases, an ITR will have a default EID
mapping pointing to one or more LISP+ALT routers.
Note that while this document specifies the use of Generic Routing
Encapsulation (GRE) as a tunneling mechanism, there is no reason that
an ALT cannot be built using other tunneling technologies. In cases
where GRE does not meet security, management, or other operational
requirements, it is reasonable to use another tunneling technology
that does. References to "GRE tunnel" in later sections of this
document should therefore not be taken as prohibiting or precluding
the use of other, available tunneling mechanisms.
In summary, LISP+ALT uses BGP to propagate EID-prefix update
information to facilitate forwarding a Map-Reqeusts or Data Probe to
the ETR that holds the EID-to-RLOC mapping for that EID-prefix. This
reachability is carried as IPv4 or IPv6 NLRI without modification
(since an EID prefix has the same syntax as IPv4 or IPv6 address
prefix). LISP+ALT routers eBGP peer with one another, forming the
ALT. A LISP+ALT router near the edge learns EID prefixes originated
by authoritative ETRs, either by eBGP peering with them or by
configuration. LISP+ALT routers aggregate EID prefixes, and forward
Data Probes and Map-Requests.
5.1. 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 for that packet is not known in the ITR's mapping
cache, the ITR encapsulates the packet in a LISP header, copying the
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inner destination address (EID) to the outer destination address
(RLOC), and transmits it through a GRE tunnel to a LISP+ALT router in
the ALT. This "first hop" LISP+ALT router uses EID-prefix routing
information learned from other LISP+ALT routers via BGP to guide the
packet to the ETR which "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. The ETR also de-encapsulates 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 a viable ETR without use of the ALT, until
either the entry's TTL has expired, or the ITR can otherwise find no
reachable ETR. Note that a valid mapping (not timed-out) may exist
that contains no reachable RLOCs (i.e. all paths to that ETR are
down); in this case, packets destined to the EID prefix are dropped,
not routed through the ALT.
Traffic routed over the ALT therefore consists of:
o EID prefix Map-Requests, and
o data packets destined for those EID prefixes while the ITR awaits
map replies
5.2. EID Assignment - Hierarchy and Topology
EID-prefixes will be allocated to a LISP site by Internet Registries.
Multiple allocations may not be in power-of-2 blocks. But when they
are, they will be aggregated into a single, advertised EID-prefix.
The ALT network is built in a tree-structured hierarchy to allow
aggregation at merge points in the tree. Building such a structure
should minimize the number of EID-prefixes carried by LISP+ALT nodes
near the top of the hierarchy.
Since the ALT will not need to change due to subscription or policy
reasons, 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, a LISP+ALT
router should only be configured to generate an aggregate if it is
configured with BGP sessions to all of the originators of components
(more-specifics prefixes) of that aggregae; not all of the components
of 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.
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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.
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.
This implies that two ALTs 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 originates a prefix must maintain BGP sessions with all ALT
routers that are configured to originate an aggregate which covers
that prefix.
Note: much is currently uncertain about the best way to build the ALT
network; as testing and prototype deployment proceeds, a guide to how
to best build the ALT network will be developed.
5.3. LISP+ALT Router
A LISP+ALT Router has the following functionality:
1. It runs, at a minimum, the eBGP part of the BGP protocol.
2. It supports a separate RIB which uses next-hop GRE tunnel
interfaces for forwarding Data Probes and Map-Requests.
3. It can act as a "proxy-ITR" to support non-LISP sites.
4. It can act as an ETR, or as a recursive or re-encapsulating ITR
to reduce mapping tables in site-based LISP routers.
5.4. ITR and ETR in a LISP+ALT Environment
An ITR using LISP+ALT may have additional functionality as follows:
1. If it is also acting as a LISP+ALT Router, it sends Data Probes
or Map-Requests on the BGP best path computed GRE tunnel for each
EID prefix.
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2. When acting solely as a ITR, it sends Data Probes or Map-Requests
directly to a configured LISP+ALT router.
An ETR using LISP+ALT may also behave slightly differently:
1. If it is also acting as a LISP+ALT router, it advertises its
configured EID-prefixes into BGP for distribution through the
ALT.
2. It receives Data Probes and Map-Requests only over GRE tunnel(s)
to its "upstream" LISP+ALT router(s) and responds with Map-
Replies for the EID prefixes that it "owns".
5.5. Use of GRE and BGP between LISP+ALT Routers
The ALT network is built using GRE tunnels between LISP+ALT routers.
eBGP sessions are configured over those tunnels, with each LISP+ALT
router acting as a separate 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 reduces provider Operational Expense
(OPEX) because no new protocols need to be either defined or used on
the overlay topology. Also, since tunnel IP addresses are local in
scope, no coordination is needed for their assignment; any addressing
scheme (including private addressing) can be used for tunnel
addressing.
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6. EID Prefix Propagation and Map-Request Forwarding
As described in Section 9.2, an ITR may send either a Map-Request or
a data probe to find 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 the source RLOC learned from
the original Map-Request. 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 sent to it; these cases will not be
supported by initial LISP+ALT implementations but may be subject to
future experimentation.
LISP+ALT routers propagate mapping information for use by ITRs (when
making Map-Requests or sending Data Probes) using eBGP [RFC4271].
eBGP is run on the inter-LISP+ALT router links, and and possibly
between an edge ("last hop") LISP+ALT router and an ETR or between an
edge ("first hop") LISP+ALT router and an ITR. The ALT eBGP RIB
consists of aggregated EID prefixes and their next hops toward the
authoritative ETR for that EID prefix.
6.1. Changes to ITR behavior with LISP+ALT
When using LISP+ALT, an ITR always sends either Data Probes or Map-
Requests to one of its "upstream" LISP+ALT routers. As in basic
LISP, it should use one of its RLOCs as the source address of these
queries; it should explicitly 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 explicitly routed throughout the ALT. If the ITR is
running BGP with the LISP+ALT router(s), it selects the appropriate
LISP+ALT router based on the BGP information received. If it is not
running BGP, it uses static configuration to select a LISP+ALT
router; in the general case, this will effectively be an "EID-prefix
default route".
6.2. Changes to ETR behavior with LISP+ALT
If an ETR connects using BGP to one or more LISP+ALT router(s), it
simply announces its EID-prefix to those LISP+ALT routers. In the
"low-opex" case, where the ETR does not use BGP, it will still have a
GRE tunnel to one or more LISP+ALT routers; these LISP+ALT router(s)
the ETR must route Map-Requests and Data Probes to the ETR and
contain configuration (in effect, static routes) for the ETR's EID-
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prefixes. Note that in either case, when an ETR generates a Map-
Reply message to return to a querying ITR, it sends it to the ITR's
source-RLOC (i.e., on the underlying Internet topology, not on the
ALT; this avoids any latency penalty that might be incurred by
routing over the ALT).
See also Section 9 for more details about the "low-opex" ETR and ITR
configurations.
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7. BGP configuration and protocol considerations
7.1. 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 EID-to-RLOC
mapping database which, while related to the global routing database,
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 LISP+ALT routers. To
avoid confusion, it needs to be stressed that that these LISP+ALT
ASNs use a new numbering space that is unrelated to the ASNs used by
the global routing system. Exactly how this new space will be
assigned and managed will be determined during experimental
deployment of LISP+ALT.
Note that the LISP+ALT routers that make up the "core" of the ALT
will not be associated with any existing core-Internet ASN because
topology, hierarchy, and aggregation boundaries are completely
separate from and independent of the global Internet routing system.
7.2. Sub-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 managing the global EID-to-RLOC database
(the use of BGP proposed by this document), it may be desirable to
assign a new SAFI [RFC2858] to prevent operational confusion and
difficulties, including the inadvertent leaking of information from
one domain to the other. At present, this document does not require
the assignment of a new SAFI but the authors anticipate that
experimentation may suggest the need for one in the future.
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8. EID-Prefix Aggregation
The ALT BGP peering topology should be arranged in a tree-like
fashion (with some meshiness), with redundancy to deal with node and
link failures. A basic assumption is that as long as the routers are
up and running, the underlying topology will provide alternative
routes to maintain BGP connectivity among LISP+ALT routers.
Note that, as mentioned in Section 5.2, the use of BGP by LISP+ALT
requires that information can only be aggregated where all active
more-specific prefixes of a generated aggregate prefix are known.
This implies, for example, that if a given set of prefixes is used by
multiple, ALT networks, those networks must interconnect and share
information about all of the prefixes if either were to generate an
aggregate prefix that covered all of them. 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 to all prefixes that make up that
aggregate.
8.1. Traffic engineering with LISP and LISP+ALT
It is worth noting that LISP+ALT does not directly propagate EID-to-
RLOC mappings. What it does is provide a mechanism for a LISP ITR to
find the ETR that holds the mapping for a particular EID prefix.
This distinction is important for several reasons. First, it means
that the reachability of RLOCs is learned through the LISP ITR-ETR
exchange so "flapping" of state information through BGP is not likely
nor can mapping information become "stale" by slow propagation
through the ALT BGP mesh. Second, by deferring EID-to-RLOC mapping
to an ITR-ETR exchange, it is possible to perform site-to-site
traffic engineering through a combination of setting the preference
and weight fields and by returning more-specific EID-to-RLOC
information in LISP 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; should a receiving ITR decide that it does not
wish to store such more-specific information, it has the option of
discarding it as long as a shorter, covering EID prefix exists. Not
only does this greatly improve the scalability of the global routing
system but it also allows improved traffic engineering techniques by
allowing richer and more fine-grained policies to be applied.
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8.2. Edge aggregation and dampening
Note also that 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 with anywhere
as frequently BGP prefix reachability on the Internet, 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, along with BGP-generated paths to the ETRs that source
those prefixes as advertisements travel over the logical topology;
this set of information is considerablly less volitile than the
actual EID-to-RLOC mappings.
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9. Connecting sites to the ALT network
9.1. ETRs originating information into the ALT
EID prefix information is originated into the ALT by two different
mechanisms:
eBGP: An ETR may participate in the LISP+ALT overlay network by
running eBGP to one or more LISP+ALT router(s) over GRE tunnel(s).
In this case, the ETR advertises reachability for its EID prefixes
over these eBGP connection(s). The LISP+ALT router(s) that
receive(s) these prefixes then propagate(s) them into the ALT.
Here the ETR is simply an eBGP peer of LISP+ALT router(s) at the
edge of the ALT. Where possible, a LISP+ALT router that receives
EID prefixes from an ETR via eBGP should aggregate that
information.
Configuration: One or more LISP+ALT router(s) 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 LISP+ALT router(s) using GRE tunnel(s) but rather
than BGP being used, the LISP+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 that the LISP+ALT router could also serve as a proxy for its
TCP-connected ETRs.
Note: in both cases, an ETR may have connections to to multiple
LISP+ALT routers for the following reasons:
* redundancy, so that a particular ETR is still reachable through
the ALT 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 LISP+ALT router (i.e. are not
topologically "close" to each other in the ALT).
9.2. ITRs Using the ALT
In order to source Map-Requests to the ALT or to route a Data Probe
packet over the ALT, each ITR participating in the ALT establishes a
connection to one or more LISP+ALT routers. These connections can be
either eBGP or TCP (as described above).
In the case in which the ITR is running eBGP, the peer LISP+ALT
routers use these connections to advertise highly aggregated EID-
prefixes to the peer ITRs. The ITR then installs the received
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prefixes into a forwarding table that is used to to send LISP Map-
Requests to the appropriate LISP+ALT router. In most cases, a LISP+
ALT router will send a default mapping to its client ITRs so that
they can send request for any EID prefix into the ALT.
In the case in which the ITR is connected to some set of LISP+ALT
routers without eBGP, the ITR sends Map-Requests to any of its
connected LISP+ALT routers.
An ITR may also choose to send the first few data packets over the
ALT to minimize packet loss and reduce mapping latency. In this
case, the data packet serves as a mapping probe (Data Probe) and the
ETR which receives the data packet (over the ALT) responds with a
Map-Reply is sent to the ITR's source-RLOC using the underlying
topology. Note that the use of Data Probes is discouraged at this
time (see Section 4.2).
In general, an ITR will establish connections only to LISP+ALT
routers at the "edge" of the ALT (typically two for redundancy) but
there may also be situations where an ITR would connect to other
LISP+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 LISP+ALT routers with the additional information. This is a
purely local policy issue between the ITR and the LISP+ALT routers in
question.
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10. IANA Considerations
This document makes no request of the IANA.
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11. 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. Securing LISP+ALT is much simpler than
securing BGP.
Compared to BGP, LISP+ALT routers are not topologically bound,
allowing them to be put in locations away from the vulnerable AS
border (unlike eBGP speakers).
11.1. Apparent LISP+ALT Vulnerabilities
This section briefly lists of the apparent vulnerabilities of LISP+
ALT.
Mapping Integrity: Can an attacker insert bogus mappings to black-
hole (create a DoS) or intercept LISP data-plane packets?
LISP+ALT router Availability: Can an attacker DoS the LISP+ALT
routers connected to a given ETR? without access to its mappings,
a site is essentially unavailable.
ITR Mapping/Resources: Can an attacker force an ITR or LISP+ALT
router to drop legitimate mapping requests by flooding it with
random destinations that it will have to query for. Further study
is required to see the impact of admission control on the overlay
network.
EID Map-Request Exploits for Reconnaissance: Can an attacker learn
about a LISP destination sites' TE policy by sending legitimate
mapping requests messages and then observing the RLOC mapping
replies? Is this information useful in attacking or subverting
peer relationships? Note that LISP 1.0 has a similar data-plane
reconnaissance issue.
Scaling of LISP+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 denial-of-service attacks. For this
reason, all components of the ALT (ETRs and ALT routers) should be
prepared to rate-limit traffic that could be received across the
ALT (Map-Requests and Data Probes).
UDP Map-Reply from ETR: Since Map-Replies packets are sent directly
from the ETR to the ITR's RLOC, the ITR's RLOC may be vulnerable
to various types of DoS attacks.
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11.2. Survey of LISP+ALT Security Mechanisms
Explicit peering: The devices themselves can both 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 Protected TCP Connections: HMAC is used to verify
message integrity and authenticity, making it nearly impossible
for third party devices to either insert or modify messages.
Message Sequence Numbers and Nonce Values in Messages: This allows
for devices to verify that the mapping-reply packet was in
response to the mapping-request that they sent.
11.3. Using existing BGP Security mechanisms
LISP+ALT's use of BGP allows for the ALT to take advantage of BGP
security features designed for existing Internet BGP use.
For example, should either sBGP [I-D.murphy-bgp-secr] or soBGP
[I-D.white-sobgparchitecture] become widely deployed it expected that
LISP+ALT could use these mechanisms to provide authentication of EID-
to-RLOC mappings, and EID origination.
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12. Acknowledgments
Many of the ideas described in this document were developed during
detailed discussions with Scott Brim and Darrel Lewis, who made many
insightful comments on earlier versions of this document.
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13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
"Multiprotocol Extensions for BGP-4", RFC 2858, June 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.
13.2. Informative References
[I-D.murphy-bgp-secr]
Murphy, S., "BGP Security Analysis",
draft-murphy-bgp-secr-04 (work in progress),
November 2001.
[I-D.white-sobgparchitecture]
White, R., "Architecture and Deployment Considerations for
Secure Origin BGP (soBGP)",
draft-white-sobgparchitecture-00 (work in progress),
May 2004.
[Interworking]
Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
"Interworking LISP with IPv4 and ipv6",
draft-lewis-lisp-interworking-02.txt (work in progress),
January 2009.
[LISP] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-farinacci-lisp-12.txt (work in progress),
March 2009.
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Authors' Addresses
Dino Farinacci
Cisco
Tasman Drive
San Jose, CA 95134
USA
Email: dino@cisco.com
Vince Fuller
Cisco
Tasman Drive
San Jose, CA 95134
USA
Email: vaf@cisco.com
Dave Meyer
Cisco
Tasman Drive
San Jose, CA 95134
USA
Email: dmm@cisco.com
Darrel Lewis
Cisco
Tasman Drive
San Jose, CA 95134
USA
Email: darlewis@cisco.com
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