wdiff draft-ietf-lisp-interworking-00.txt draft-ietf-lisp-interworking-01.txt

Network Working Group D. Lewis
Internet-Draft D. Meyer
Intended status: Experimental D. Farinacci
Expires: November February 27, 2009 2011 V. Fuller
Cisco Systems, Inc.
May
August 26, 2009 2010

Interworking LISP with IPv4 and IPv6
draft-ietf-lisp-interworking-00
draft-ietf-lisp-interworking-01.txt

Abstract

This document describes techniques for allowing sites running the
Locator/ID Separation Protocol (LISP) to interoperate with Internet
sites (which may be using either IPv4, IPv6, or both) but which are
not running LISP. A fundamental property of LISP speaking sites is
that they use Endpoint Identifiers (EIDs), rather than traditional IP
addresses, in the source and destination fields of all traffic they
emit or receive. While EIDs are syntactically identical to IPv4 or
IPv6 addresses, normally routes to them are not carried in the global
routing system so an interoperability mechanism is needed for non-
LISP-speaking sites to exchange traffic with LISP-speaking sites.
This document introduces three such mechanisms. The first uses a new
network element, the LISP Proxy Ingress Tunnel Routers (PITR)
(Section 5) to act as a intermediate LISP Ingress Tunnel Router (ITR)
for non-LISP-speaking hosts. Second the document adds Network
Address Translation (NAT) functionality to LISP Ingress and LISP
Egress Tunnel Routers (xTRs) to substitute routable IP addresses for
non-routable EIDs. Finally, this document introduces a Proxy Egress
Tunnel Router (PETR) to handle cases where a LISP ITR cannot send
packets to non-LISP sites without encapsulation.

Status of this Memo

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Abstract

This Code Components extracted from this document describes techniques for allowing sites running must
include Simplified BSD License text as described in Section 4.e of
the
Locator/ID Separation Protocol (LISP [LISP]) to interoperate with
Internet sites not running LISP. A fundamental property of LISP-
speaking sites is that they use Endpoint Identifiers (EIDs), rather
than traditional IP addresses, in the source and destination fields
of all traffic they emit or receive. While EIDs are syntactically
identical to IP addresses, routes for them are not carried in the
global routing system so an interoperability mechanism is needed for
non-LISP-speaking sites to exchange traffic with LISP-speaking sites.
This document introduces two such mechanisms: the first uses a new
network element, the LISP Proxy Tunnel Router (PTR) (Section 5) to
act as a intermediate LISP Ingress Tunnel Router (ITR) for non-LISP-
speaking hosts while the second adds Network Address Translation
(NAT) functionality to LISP Ingress and LISP Egress Tunnel Routers
(xTRs) to substitute routable IP addresses for non-routable EIDs.

Table Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
2. LISP Interworking Models . . . . . . . . . . . . . . . . . . . 3 6
3. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 5 8
4. Routable EIDs . . . . . . . . . . . . . . . . . . . . . . . . 6 11
4.1. Impact on Routing Table . . . . . . . . . . . . . . . . . 6 11
4.2. Requirement for using BGP . . . . . . . . . . . . . . . . 6 11
4.3. Limiting the Impact of Routable EIDs . . . . . . . . . . . 6 11
4.4. Use of Routable EIDs for Testing sites transitioning to LISP . . . . . . . . . . 7 11
5. Proxy Ingress Tunnel Routers . . . . . . . . . . . . . . . . . . . . . 7 13
5.1. PTR PITR EID announcements . . . . . . . . . . . . . . . . . . 7 13
5.2. Packet Flow with PTRs PITRs . . . . . . . . . . . . . . . . . . 8 13
5.3. Scaling PTRs PITRs . . . . . . . . . . . . . . . . . . . . . . . 9 14
5.4. Impact of the PTRs PITRs placement in the network . . . . . . . 9 15
5.5. Benefit to Networks Deploying PTRs PITRs . . . . . . . . . . . . 9 15
6. LISP-NAT . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 16
6.1. Using LISP-NAT for with LISP-NR addressed hosts EIDs . . . . . . . . . . . 10 . . 16
6.2. LISP Sites with Hosts using RFC 1918 Addresses Sending
to non-LISP Sites . . . . . . . . . . . . . . . . . . . . 11 17
6.3. LISP Sites with Hosts using RFC 1918 Addresses
Communicating
Sending Packets to Other LISP Sites . . . . . . . . . . . . 11 17
6.4. LISP-NAT and multiple EIDs . . . . . . . . . . . . . . . . 12 18
6.5. When LISP-NAT and PTRs Together PITRs used by the same LISP Site . . . . 18
7. Proxy Egress Tunnel Routers . . . . . . . . . . . . 12
7. Security Considerations . . . . . 19
7.1. Packet Flow with Proxy Egress Tunnel Routers . . . . . . . 19
8. Discussion of Proxy ITRs (PITRs), LISP-NAT, and Proxy-ETRs
(PETRs) . . . . . . . . . . . . . 13
8. Acknowledgments . . . . . . . . . . . . . . 21
8.1. How Proxy-ITRs and Proxy-ETRs Interact . . . . . . . . . 13 . 21
9. IANA Security Considerations . . . . . . . . . . . . . . . . . . . 22
10. Acknowledgments . . . 14
10. References . . . . . . . . . . . . . . . . . . . . 23
11. IANA Considerations . . . . . . 14
10.1. Normative . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . 14
10.2. Informative . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . 25
12.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . 14 . . . . . . . . . . . . . . . . . . . 26

1. Introduction

This document describes two mechanisms for interoperation between LISP [LISP] sites, sites
which use non-globally-routed EIDs, and non-LISP
sites: sites. The first is
the use of PTRs, Proxy Ingress Tunnel router (PITRs), which create originate
highly-aggregated routes to EID prefixes for non-LISP sites to follow; and use.
It also describes the use of NAT by LISP
ETRs ITRs when communicating with sending packets to
non-LISP hosts. Finally, it describes Proxy Egress Tunnel routers
(PETRs) LISP for sites relying on PITRs, and which are faced with
certain restrictions.

A key behavior of the separation of Locators and End-Point-IDs is
that EID prefixes are normally not advertised to into the Internet's
Default Free Zone (DFZ). Specifically, only RLOCs are carried in the
Internet's DFZ. Existing Internet sites (and their hosts) who which do
not
participate run in the LISP system protocol must still be able to reach sites
numbered from this non routed LISP EID space. This draft describes a set
of three mechanisms
that can be used to provide reachability between sites that are LISP-capable LISP-
capable and those that are not. This document
introduces two such mechanisms: the

The first mechanism uses a new network element, the LISP Proxy
Ingress Tunnel Router (PTR) (Section 5) (PITR) to act as a intermediate LISP Ingress
Tunnel Router (ITR) for non-LISP-speaking
hosts while the hosts. The second
mechanism adds a form of Network Address Translation (NAT)
functionality to Tunnel Routers (xTRs) (xTRs), to substitute routable IP
addresses for non-routable EIDs. The final network element is the
LISP Proxy Egress Tunnel Routers (PETR), which act as an intermediate
Egress Tunnel Router (ETR) for LISP sites which need to encapsulate
packets LISP packets destined to non-LISP sites.

More detailed descriptions of these mechanisms and the network
elements involved may be found in the following sections:

- Section 2 describes the different cases where interworking
mechanisms are needed

- Section 3 defines terms used throughout the document

- Section 4 describes the relationship between the new EID prefix
space and the IP address space used by the current Internet

- Section 5 introduces and describes the operation of PTRs Proxy-ITRs

- Section 6 defines how NAT is used by ETRs to translate non-routable
EIDs into routable IP addresses.

- Section 7 introduces and describes the operations of Proxy-ETRs
- Section 8 describes the relationship between asymmetric and
Symmetric interworking mechanisms (Proxy-ITRs and Proxy-ETRs vs LISP-
NAT)

Note that any successful interworking model should be independent of
any particular EID-to-RLOC mapping algorithm. This document does not
comment on the value of any of the particular LISP mapping system. systems.

2. LISP Interworking Models

There are 4 unicast connectivity cases which describe how sites can
communicate with
send packets to each other:

1. Non-LISP site to Non-LISP site

2. LISP site to LISP site

3. LISP site to Non-LISP site

4. Non-LISP site to LISP site

Note that while Cases 3 and 4 seem similar, there are subtle
differences due to the way communications packets are originated.

The first case is the Internet as we know it today and as such will
not be discussed further here. The second case is documented in
[LISP] and, hence, and there are no new interworking requirements because there
are no new protocol requirements placed on intermediate non- LISP
routers.

In case 3, LISP site to Non-LISP site, a LISP site can (in most
cases) send packets to a non-LISP site because the non-LISP site
prefixes are routable. The non-LISP site need not do anything new to
receive packets. The only action the LISP site needs (with two
possible caveats introduced below) to take is to know when not to LISP-
encapsulate
LISP-encapsulate packets. This can be achieved via by using one of two
mechanisms:

1. At the ITR in the source site, if the destination of an IP packet
is found to match a prefix from the BGP routing table, then the
site is directly reachable by the BGP core that exists and
operates today.

2. Second, if (from the perspective of the ITR at the source site)
the destination address of an IP address is not found in the EID-
to-RLOC mapping database, the ITR could infer that it is not a
LISP-capable site, and decide to not LISP-encapsulate the packet.

3. In either of the two exceptions mentioned above there could be
some situations where (unencapsualted) packets originated by a
LISP site may not be forwarded to a non-LISP site. These cases
are reviewed in section 7, (Proxy-Egress Tunnel Routers).

Case 4, typically the most challenging, occurs when a host at a non-LISP non-
LISP site wishes to send traffic to a host at a LISP site. If the
source host uses a (non-globally-routable) EID as the destination IP
address, the packet is forwarded inside the source site until it
reaches a router which cannot forward it, tin (due to lack of a default
route), at which point the traffic is dropped. For traffic not to be
dropped, either some route must be exist for the some mechanism to make this destination EID outside of LISP-speaking part of the network or an
alternate mechanism
routable must be in place. Section 5 (PTRs) (PITRs) and Section 6
(LISP-NAT) (LISP-
NAT) describe two such mechanisms.

Note that case

Case 4 includes also applies to packets returning to the LISP Site site, in case Case 3.

3. Definition of Terms

Endpoint ID (EID): Endpoint ID (EID): A 32- 32-bit (for IPv4) or 128-bit
(for IPv6) value used in the source and destination address fields
of the first (most inner) LISP IP header of a packet. A packet that is emitted by The host obtains
a system contains EIDs in its
headers and LISP headers are prepended only when the packet
reaches destination EID the same way it obtains an Ingress Tunnel Router (ITR) on destination address
today, for example through a DNS lookup or SIP exchange. The
source EID is obtained via existing mechanisms used to set a
host's "local" IP address. An EID is allocated to a host from an
EID-prefix block associated with the data path site where the host is
located. An EID can be used by a host to refer to other hosts.
EIDs MUST NOT be used as LISP RLOCs. Note that EID blocks may be
assigned in a hierarchical manner, independent of the
destination EID. network
topology, to facilitate scaling of the mapping database. In
addition, an EID block assigned to a site may have site-local
structure (subnetting) for routing within the site; this structure
is not visible to the global routing system. When used in
discussions with other Locator/ID separation proposals, a LISP EID
will be called a "LEID". Throughout this document, any references
to "EID" refers to an LEID.

EID-Prefix: A power-of-2 block of EIDs which are allocated to a site
by an address allocation authority. EID-prefixes are associated
with a set of RLOC addresses which make up a "database mapping".
EID-prefix allocations can be broken up into smaller blocks when
an RLOC set is to be associated with the smaller EID- prefix. A
globally routed address block (whether PI or PA) is not an EID-
prefix. However, a globally routed address block may be removed
from global routing and reused as an EID-prefix. A site that
receives an explicitly allocated EID-prefix may not use that EID-
prefix as a globally routed prefix assigned to RLOCs

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. Provider
Independent (PI) Addresses: an address block assigned from a pool
where blocks are not associated with any particular location in
the network (e.g. from a particular service provider), and is
therefore not topologically aggregatable in the routing system.

Routing Locator (RLOC): An IP The IPv4 or IPv6 address of a LISP an egress tunnel router.
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 topologically-aggregatable blocks and 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) PA addresses. Multiple RLOCs can be
assigned to the same ETR device or to multiple ETR devices at a
site.

EID-to-RLOC Mapping: A binding between an EID and the RLOC-set that
can be used to reach the EID. We use the term "mapping" in this
document to refer to a 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) 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
[LISP].

LISP Routable (LISP-R) Site: A LISP site whose addresses are used as
both globally routable IP addresses and LISP EIDs.

LISP Non-Routable (LISP-NR) Site: A LISP site whose addresses are
EIDs only, these EIDs are not found in the legacy Internet routing
table.

LISP Proxy Ingress Tunnel Router (PTR): PTRs (PITR): PITRs are used to provide
interconnectivity between sites which use LISP EIDs and those
which do not. They act as a gateway gateways between those parts of the Legacy
Internet
and the which are not using LISP enabled Network. (the legacy Internet) A given PTR
PITR advertises one or more highly aggregated EID prefixes into
the public Internet and acts as the ITR for traffic received from
the public Internet. LISP Proxy Ingress Tunnel Routers are
described in Section 5.

LISP Network Address Translation (LISP-NAT): Network Address
Translation between EID space assigned to a site and RLOC space
also assigned to that site. LISP Network Address Translation is
described in Section 6.

LISP Proxy Egress Tunnel Router (PETR): PETRs provide a LISP
(Routable or Non-Routable EID) site's ITRs the ability to send
packets to non-LISP sites in cases where unencapsualted packets
(the default mechanism) would fail to be delivered. PETRs are
function by having an ITR encapsulate all non-LISP destined
traffic to a pre-configured PETR. LISP Proxy Egress Tunnel
Routers are described in Section 7.

EID Sub Namespace: A power-of-two block of aggregatable locators
set aside for LISP interworking.

4. Routable EIDs

An obvious way to achieve interworking between LISP and non-LISP
hosts is for a LISP site to simply announce EID prefixes into the
DFZ, much like the current routing system, effectively treating them
as "Provider Independent (PI)" prefixes. Doing Having a site do this is
undesirable as it defeats one of the primary goals of LISP - to
reduce global routing system state.

4.1. Impact on Routing Table

If EID prefixes are announced into the DFZ, the impact is similar to
the case in which LISP has not been deployed, because these EID
prefixes will be no more aggregatable than existing PI addressing.
This behavior is not desirable and such
Such a mechanism is not viewed as a viable long term solution. solution, but
may be a viable short term way for a site to transition a portion of
its address space to EID space without changing its existing routing
policy.

4.2. Requirement for using BGP

Non-LISP sites today use BGP to, among other things, enable ingress
traffic engineering. Relaxing this requirement is another primary
design goal of LISP.

4.3. Limiting the Impact of Routable EIDs

Two schemes are proposed to limit the impact of having EIDs announced
in the current global Internet routing table:

1. Section 5 discusses the LISP Proxy Tunnel Router, an approach
that provides ITR functionality to bridge LISP-capable and non-LISP-
capable non-
LISP-capable sites.

2. Section 6 discusses another approach, LISP-NAT, in which NAT
[RFC2993] is combined with ITR functionality to limit the the
impact of routable EIDs on the Internet routing infrastructure.

4.4. Use of Routable EIDs for Testing sites transitioning to LISP

A primary design goal for LISP (and other Locator/ID separation
proposals) is to facilitate topological aggregation of addresses namespace used
by the path computation, and, thus, decrease global routing system state.
overhead. Another goal is to achieve the benefits of improved
aggregation as soon as possible.
Advertising Individual sites advertising their
own routes for LISP EID prefixes into the global routing system is
therefore not recommended.

That being said, single homed sites (or multi-homed sites that are
not leaking more specific exceptions) and that are already using
provider-aggregated prefixes can use these prefixes as LISP EIDs
without adding state to the routing system; in system. In other words, such
sites do not cause additional prefixes to be advertised. For such
sites, connectivity to a non-LISP sites does not require interworking
machinery because the "PA" EIDs are already routable. routable (they are
effectively LISP-R type sites). Their EIDs are found in the LISP
mapping system, and their (aggregate) PA prefix(es) are found in the
DFZ Internet.

The continued announcements of an existing site's Provider
Independent (or "PI") prefix(es) is of course under control of that
site. Some period of transition, where a site is is found both in
the LISP mapping system, and as a discrete prefix in the Internet
routing system, may be a viable transition strategy. Care should be
taken not to advertise additional more specific LISP EID prefixes
into the DFZ.

5. Proxy Ingress Tunnel Routers

Proxy Ingress Tunnel Routers (PTRs) (PITRs) allow for non-LISP sites to communicate
with send
packets to LISP-NR sites. A PTR PITR is a new network element that
shares many characteristics with the LISP ITR. PTRs PITRs allow non-LISP
sites to send packets to LISP-NR sites without any changes to
protocols or equipment at the non-LISP site. PTRs PITRs have two primary
functions:

Originating EID Advertisements: PTRs PITRs advertise highly aggregated
EID-prefix space on behalf of LISP sites to so that non-LISP sites
can reach them.

Encapsulating Legacy Internet Traffic: PTRs PITRs also encapsulate non-
LISP Internet traffic into LISP packets and route them towards
their destination RLOCs.

5.1. PTR PITR EID announcements

A key part of PTR PITR functionality is to advertise routes for highly-
aggregated EID prefixes into part of the global routing system.
Aggressive aggregation is performed to minimize the number of new
announced routes. In addition, careful placement of PTRs PITRs can
greatly reduce the advertised scope of these new routes. To this
end, PTRs PITRs should be deployed close to non-LISP-speaking rather than
close to LISP sites. Such deployment not only limits the scope of
EID-prefix route advertisements, it also also allows traffic
forwarding load to be spread among many PTRs. PITRs.

5.2. Packet Flow with PTRs

Packets from a non-LISP site can reach a LISP-NR site with the aid of
a PTR. By advertising a route for a particular EID prefix into the
global routing system, traffic destined for that EID prefix is routed
to the PTR, which then performs LISP encapsulation. Once
encapsulated, traffic packets use the LISP (outer) header's
destination address to reach the destination ETR.

What follows is an example of the path PITRs

What follows is an example of the path a packet would take when using
a PTR. PITR. In this example, the LISP-NR site is given the EID prefix
240.0.0.0/24. For the purposes of this example, this prefix and no
covering aggregate is present in the global routing system. In other
words, if without the Proxy-ITR announcing 240.0.0.0/24, a packet with
this destination were to reach a router in the "Default Free Zone",
it would be dropped.

A full protocol exchange example follows:

1. Source The source host makes a DNS lookup EID for destination, and gets
240.1.1.1 in return.

2. Source The source host has a default route to customer Edge (CE) router
and forwards the packet to the CE.

3. The CE has a default route to its Provider Edge (PE) router, and
forwards the packet to the PE.

4. The PE has route to 240.0.0.0/24 and the next hop is the PTR. PITR.

5. The PTR PITR has or acquires a mapping for 240.1.1.1 and LISP
encapsulates,
encapsulates the packet packet. The outer IP header now has a
destination address of one of the
RLOC. destination EID's RLOCs. The
outer source address of this encapsulated packet is the
PTR's PITR's
RLOC.

6. The PTR PITR looks up the RLOC, and forwards LISP packet to the next
hop.
hop, after which, it is forwarded by other routers to the ETR's
RLOC.

7. The ETR decapsulates the packet and delivers the packet to the
240.1.1.1 host in the destination LISP site.

8. Packets from host 240.1.1.1 will flow back through the LISP
site's ITR. Such packets are not encapsulated because the ITR
knows that the destination (the original source) is a non-LISP
site. The ITR knows this because it can check the LISP mapping
database for the destination EID, and on a failure determine that
the destination site is not LISP enabled.

9. Packets are then routed natively and directly to the destination
(original source) site.

Note that in this example the return path is asymmetric, so return
traffic will not go back through the PTR. PITR. This is because the
LISP-NR site's ITR will discover that the originating site is not a
LISP site, and not encapsulate the returning packet (see [LISP] for
details of ITR behavior).

The asymmetric nature of traffic flows allows the PTR PITR to be
relatively simple - it will only have to encapsulate LISP packets.

5.3. Scaling PTRs

PTRs PITRs

PITRs attract traffic by announcing the LISP EID namespace into parts
of the non-LISP-speaking global routing system. There are several
ways that a network could control how traffic reaches a particular
PTR
PITR to prevent it from receiving more traffic than it can handle:

First, the PTR's

1. The PITR's aggregate routes might be selectively announced,
giving a coarse way to control the quantity of traffic attracted
by that PTR.

Second, PITR. For example, some of the routes being announced
might be tagged with a BGP community and their scope of
announcement limited by the routing policy of the provider.

2. The same address might be announced by multiple PTRs PITRs in order to
share the traffic using IP Anycast. The asymmetric nature of
traffic flows allows through the PTR Proxy ITR means that operationally,
deploying a set PITRs would be very similar to existing Anycasted
services like DNS caches. Multiple Proxy ITRs could advertise
the same BGP Next Hop IP address as their RLOC, and traffic would
be relatively simple -
it will only have attracted to encapsulate LISP packets. the nearest Next Hop according to the the
network's IGP.

5.4. Impact of the PTRs PITRs placement in the network

There are several approaches that a network could take in placing
PTRs.
PITRs. Placing the PTR PITR near the ingress source of traffic allows for the
communication between the non-LISP site and the LISP site to have the
least "stretch" (i.e. the least number of forwarding hops when
compared to an optimal path between the sites).

Some proposals, for example CRIO [CRIO], have suggested grouping PTRs
PITRs near an arbitrary subset of ETRs and announcing a 'local'
subset of EID space. This model cannot guarantee minimum stretch if
the EID prefix route advertisement points are changed (such a change
might occur if a site adds, removes, or replaces one or more ISPs of its
ISP connections).

5.5. Benefit to Networks Deploying PTRs PITRs

When traffic packets destined for LISP-NR site arrives sites arrive and is are encapsulated
at a PTR, Proxy-ITR, a new LISP packet header is pre-pended. This causes
the packet's destination to be set to the destination site ETRs RLOC.
Because
traffic is packets are thus routed towards RLOCs, it can potentially
better follow the Proxy-ITR network's traffic engineering policies
(such as closest exit routing). This also means that providers who which
are not default-
free default-free and do not deploy PTRs Proxy-ITRs end up sending more
traffic to expensive transit links (assuming their upstreams have
deployed Proxy-ITRs) rather than to the ETR's RLOC addresses, to
which they may well have cheaper and closer connectivity to (via, for
example, settlement-free peering. For peering). A corollary to this would be that
large transit providers, deploying PTRs PITRs may attract more traffic,
and therefore more revenue, from their customers.

6. LISP-NAT

LISP Network Address Translation (LISP-NAT) is a limited form of NAT
[RFC2993]. LISP-NAT is designed to enable the interworking of non-
LISP sites and LISP-NR sites by ensuring that the LISP-NR's site
addresses are always routable. LISP-NAT accomplishes this by
translating a host's source address from an 'inner' (LISP-NR EID)
value to an 'outer' (LISP-R) value and keeping this translation in a
table that it can reference for subsequent packets.

In addition, existing RFC 1918 [RFC1918] sites can use LISP-NAT to
talk to both LISP or non-LISP sites.

The basic concept of LISP-NAT is that when transmitting a packet, the
ITR replaces a non-routable EID source address with a routable source
address, which enables packets to return to the site.

There are two main cases that involve LISP-NAT:

1. Hosts at LISP sites that use non-routable global EIDs speaking to
non-LISP sites using global addresses.

2. Hosts at LISP sites that use RFC 1918 private EIDs speaking to
other sites, who may be either LISP or non-LISP.

Note that LISP-NAT is not needed in the case of LISP-R (routable
global EIDs) sources. This case occurs when a site is announcing its
prefix into both the LISP mapping system as well as the Internet DFZ.
This is because the LISP-R source's address is routable, and return
packets will be able to natively reach the site.

6.1. Using LISP-NAT for with LISP-NR addressed hosts EIDs

LISP-NAT allows a host with a LISP-NR EID to communicate with non-
LISP send packets to non-LISP
hosts by translating the LISP-NR EID to a globally unique
address. address (a
LISP-R EID). This globally unique address may be a either a PI or PA
address.

An example of this translation follows. For this example, a site has
been assigned a LISP-NR EID of 220.1.1.0/24. In order to utilize
LISP-NAT, the site has also been provided the PA EID of
128.200.1.0/24, and uses the first address (128.200.1.1) as the
site's RLOC. The rest of this PA space (128.200.1.2 to
128.200.1.254) is used as a translation pool for this site's hosts
who need to communicate with send packets to non-LISP hosts.

The translation table might look like the following:

Site NR-EID Site R-EID Site's RLOC Translation Pool
=========================================================================
==============================================================
220.1.1.0/24 128.200.1.0/24 128.200.1.1 128.200.1.2 - 128.200.1.254 128.200.1.2-254

Figure 1: Example Translation Table

The Host 220.1.1.2 sends a packet destined for a non-LISP site to its
default route (the ITR). The ITR receives the packet, and determines
that the destination is not a LISP site. How the ITR makes this
determination is up to the ITRs implementation of the EID-to-RLOC
mapping system used (see, for example [LISP-ALT]).

The ITR then rewrites the source address of the packet from 220.1.1.2
to 128.200.1.2, which is the first available address in the LISP-R
EID space available to it. The ITR keeps this translation in a table
in order to reverse this process when receiving packets destined to
128.200.1.2.

Finally, when the ITR forwards this packet without encapsulating it,
it uses the entry in its LISP-NAT table to translate the returning
packets' destination IPs to the proper host.

6.2. LISP Sites with Hosts using RFC 1918 Addresses Sending to non-LISP
Sites

In the case where hosts using RFC 1918 addressed hosts addresses desire to communicate with send
packets to non-LISP hosts hosts, the LISP-NAT implementation acts much like
an existing IPv4 NAT device. The ITR providing the NAT service must
use LISP-R EIDs for its global address pool as well as providing all
the standard NAT functions required today.

The source of the packet must be translated to a LISP-R EID in a
manner similar to Section 6, and this packet must be forwarded to the
ITR's next hop for the destination, without LISP encapsulation.

6.3. LISP Sites with Hosts using RFC 1918 Addresses Communicating Sending Packets
to Other LISP Sites

LISP-NAT allows a host with a an RFC 1918 address to communicate with send packets to
LISP hosts by translating the RFC 1918 address to a LISP EID. After
translation, the communication between source and destination ITR and
ETRs continues as described in [LISP].

An example of this translation and encapsulation follows. For this
example, a host has been assigned a RFC 1918 address of 192.168.1.2.
In order to utilize LISP-NAT, the site also has been provided the
LISP-R EID prefix of 192.0.2.0/24, and uses the first address
(192.0.2.1) as the site's RLOC. The rest of this PA space (192.0.2.2
to 192.0.2.254) is used as a translation pool for this site's hosts
who need to communicate with send packets to both non-LISP and LISP hosts.

The Host 192.168.1.2 sends a packet destined for a non-LISP site to
its default route (the ITR). The ITR receives the packet and
determines that the destination is a LISP site. How the ITR makes
this determination is up to the ITRs implementation of the EID/RLOC
mapping system.

The ITR then rewrites the source address of the packet from
192.168.1.2 to 192.0.2.2, which is the first available address in the
LISP EID space available to it. The ITR keeps this translation in a
table in order to reverse this process when receiving packets
destined to 192.0.2.2.

The ITR then LISP encapsulates this packet (see [LISP] for details).
The ITR uses the site's RLOC as the LISP outer header's source and
the translation address as the LISP inner header's source. Once it
decapsulates returning traffic, it uses the entry in its LISP-NAT
table to translate the returning packet's destination IP address and
then forward to the proper host.

6.4. LISP-NAT and multiple EIDs

When a site has two addresses that a host might use for global
reachability, care must be chosen on which EID is found in DNS. For
example, whether applications such as DNS use the LISP-R EID or the
LISP-NR EID. This problem exists for NAT in general, but the
specific issue described above is unique to LISP. Using PTRs PITRs can
mitigate this problem, since the LISP-NR EID can be reached in all
cases.

6.5. When LISP-NAT and PTRs Together PITRs used by the same LISP Site

With LISP-NAT, there are two EIDs possible for a given host, the
LISP-R EID and the LISP-NR EID. When a site has two addresses that a
host might use for global reachability, name-to-address directories
may need to be modified.

This problem, global addressability, exists for NAT in general, but
the specific issue described above is unique to LOC/ID split location/identity
separation schemes. Some schemes [ref: 6-1 proxy] of these have suggested running a separate
DNS instance for legacy new types of EIDs. This solves the problem but
introduces complexity for the site. Alternatively, using PTRs PITRs can
mitigate this problem, because the LISP-NR EID can hbe be reached in all
cases.

In summary, there are two options

7. Proxy Egress Tunnel Routers

Proxy Egress Tunnel Routers (PETRs) allow for interworking LISP with IPv4 and
V6. In sites to send
packets to non-LISP sites in the NAT case where the access network does
not allow for the LISP site can use NAT and manage send packets with the
transition on its own. In source address of
the PTR case, we add site's EID(s). A PETR is a new network element
called that,
conceptually, acts as an ETR for traffic destined to non-LISP sites.
This also has the effect of allowing an ITR avoid having to decide
whether to encapsulate packets or not - it can always encapsulate
packets. An ITR would encapsulate packets destined for LISP sites
(no change here) and these would be routed directly to the
corespondent site's ETR. All other packets (those destined to non-
LISP sites) will be sent to the originating site's PETR.

There are two primary reasons why sites would want to utilize a PTR that PETR:

Avoiding strict uRPF failures: Some provider's access networks
require the source of the packets emitted to be within the
addressing scope of the access networks. (see section 9)

Traversing a different IP Protocol: A LISP site may want to transmit
packets to a non-LISP site where the some of the intermediate
network does not support the particular IP protocol desired (v4 or
v6). PETRs can relieve that burden allow this LISP site's data to 'hop over' this by
utilizing LISP's support for mixed protocol encapsulation.

7.1. Packet Flow with Proxy Egress Tunnel Routers

Packets from a LISP site can reach a non-LISP site with the aid of a
Proxy-ETR (or PETR). An ITR is simply configured to send all non-
LISP traffic, which it normally would have forwarded natively (non-
encapsulated), to a PETR. In the case where the ITR uses the Map-
Resolver interface the ITR will encapsulate packets that match its
Negative Map-Cache to the configured Proxy-ETR(s). In the case where
the ITR is connected to the mapping system directly it would
encapsulate all packets to the configured Proxy-ETR that are cache
misses. Note that this outer encapsulation to the Proxy-ETR may be
in an IP protocol other than the (inner) encapsulated data. Routers
then use the LISP (outer) header's destination address to route the
packets toward the configured Proxy-ETR.

A PETR should verify the (inner) source EID of the packet at time of
decapsulation in order to verify that this is from a configured LISP
site. This is to prevent spoofed inner sources from being
encapsulated through the Proxy-ETR.

What follows is an example of the path a packet would take when using
a PETR. In this example, the LISP-NR (or LISP-R) site is given the
EID prefix 240.2.0.0/24, and it is trying to reach host at a non-LISP
site with the IP prefix of 192.0.2.0/24. For the purposes of this
example, the destination is a non-LISP site and 192.0.2.0/24 is found
in the Internet's routing system.

A full protocol exchange example follows:

1. The source host makes a DNS lookup for the destination, and gets
192.0.2.100 (a host in a non-LISP site) in return.

2. The source host has a default route to customer Edge (CE) router
and forwards the packet towards the CE.

3. The CE is a LISP ITR, and is configured to encapsulate traffic
destined for non-LISP sites to a Proxy-ETR.

4. The Proxy ETR decapsulates the LISP packet and forwards the
original packet to its next hop.

5. The packet is then routed natively and directly to the
destination (non-LISP) site 192.0.2.0/24.

Note that in this example the return path is asymmetric, so return
traffic will not go back through the Proxy-ETR. This means that in
order to reach LISP-NR sites, non-LISP sites must still use Proxy
ITRs.

8. Discussion of Proxy ITRs (PITRs), LISP-NAT, and Proxy-ETRs (PETRs)

In summary, there are three mechanisms for interworking LISP with
non-LISP Sites (for both IPv4 and IPv6). In the LISP-NAT option the
LISP site can manage and control the interworking on its own. In the site,
PITR case, we the site is not required to manage the advertisement of
it's EID prefix into the DFZ, with the
downside cost of potentially adding
stretch to the connections of non-LISP sites trying sending packets to reach the
LISP site.

7. The third option is Proxy-ETRs, which are optionally used
by sites relying on PITRs case to mitigate two caveats for LISP sites
sending packets to non-LISP sites. This means Proxy-ETRs are not
usually expected to be deployed by themselves, rather they will be
used to assist LISP-NR sites which are already using PITRs.

8.1. How Proxy-ITRs and Proxy-ETRs Interact

There is a subtle difference between Symmetrical (LISP-NAT) vs
Asymmetrical (Proxy-ITR and Proxy-ETR) Interworking techniques.
Operationally, Proxy-ITRs (PITRs) and Proxy-ETRs (PETRs) can (and
likely should) be decoupled since Proxy-ITRs are best deployed
closest to non-LISP sites, and Proxy-ETRs are best located close to
the LISP sites they are decapsulating for. This asymmetric placement
of the two network elements minimizes the stretch imposed on each
direction of the packet flow, while still allowing for coarsely
aggregated announcements of EIDs into the Internet's routing table.

9. Security Considerations

Like any router or LISP ITR, PTRs PITRs will have the ability opportunity to
inspect traffic at the time that they encapsulate. More work needs to be done to see if
this ability can be exploited by the control plane along The location of
these devices in the lines network can have implications for discarding
malicious traffic on behalf of
Remote Triggered BGP Black Holes. XXX:Reference? ETRs which request this behavior (via
the drop action bit in Map-Reply packets for an EID or EID prefix).

As with traditional NAT, LISP-NAT will hide obscure the actual host ID
LISP-NR EID behind the RLOCs LISP-R addresses used as the NAT pool.

When LISP Sites reply sites send packets to non-LISP sites and (these non-LISP sites
rely on PTRs PITRs to enable
Interworking, Interworking), packets will be sourced from addresses have the Site's
EID as its source IP address. These EIDs may not be recognized by
their Internet Service Provider's Unicast Reverse Path Forwarding
(uRPF) rules enabled on the Provider Edge Router. Several options
are available to the service provider. For example they could enable
a less strict version of uRPF, where they only look for the existence
of the the EID prefix in the routing table. Another, more secure,
option is to add a static route for the customer on the PE router,
but not redistribute this route into the provider's routing table.

8.
Finally, Proxy-ETRs can enable LISP sites to bypass this uRPF check
by encapsulating all of their egressing traffic destined to non-LISP
sites to the Proxy-ETR (thus ensuring the outer IP source address is
the site's RLOC).

10. Acknowledgments

Thanks goes to Christian Vogt, Lixia Zhang and Zhang, Robin Whittle Whittle, Michael
Menth, and Xuewei Wang, and Noel Chiappa who have made insightful
comments with respect to interworking LISP Interworking and transition mechanisms.

A special thanks goes to Scott Brim for his initial brainstorming of
these ideas and also for his careful review.

11. IANA Considerations

This document creates no new requirements on IANA namespaces
[RFC2434].

10.

12. References

10.1.

12.1. Normative References

[LISP] Farinacci, D., Fuller, V., Oran, Meyer, D., and D. Meyer, Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-00
draft-ietf-lisp-06 (work in progress), May 2009. January 2010.

[LISP-ALT]
Farinacci, D., Fuller, V., Meyer, D., and D. Meyer, Lewis, "LISP
Alternative Topology (LISP-ALT)", draft-ietf-lisp-alt-00 (LISP+ALT)",
draft-ietf-lisp-alt-03.txt (work in progress),
Febuary 2010.

[LISP-MS] Farinacci, D. and V. Fuller, "LISP Map Server",
draft-ietf-lisp-ms-03.txt (work in progress), May 2009. Feb 2010.

[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.

[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.

10.2.

12.2. Informative References

[CRIO] Zhang, X., Francis, P., Wang, J., and K. Yoshida, "CRIO:
Scaling IP Routing with the Core Router-Integrated
Overlay".

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.

[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.

Authors' Addresses

Darrel Lewis
Cisco Systems, Inc.

Email: darlewis@cisco.com

David Meyer
Cisco Systems, Inc.

Email: dmm@cisco.com

Dino Farinacci
Cisco Systems, Inc.

Email: dino@cisco.com

Vince Fuller
Cisco Systems, Inc.

Email: vaf@cisco.com