Internet Engineering Task Force (IETF) A. Cabellos
Request for Comments: 9299
Universitat Politecnica de Catalunya
Category: Informational D. Saucez, Ed.
ISSN: 2070-1721 Inria
An Architectural Introduction to the Locator/ID Separation Protocol
This document describes the architecture of the Locator/ID Separation
Protocol (LISP), making it easier to read the rest of the LISP
specifications and providing a basis for discussion about the details
of the LISP protocols. This document is used for introductory
purposes; more details can be found in the protocol specifications,
RFCs 9300 and 9301.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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 candidates for any level of Internet
Standard; see Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9299
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Table of Contents 1.
Definitions of Terms 3.
LISP Architecture 3.1.
Design Principles 3.2.
Overview of the Architecture 3.3.
Data Plane 3.3.1.
LISP Encapsulation 3.3.2.
LISP Forwarding State 3.4.
Control Plane 3.4.1.
LISP Mappings 3.4.2.
Mapping System Interface 3.4.3.
Mapping System 3.5.
Internetworking Mechanisms 4.
LISP Operational Mechanisms 4.1.
Cache Management 4.2.
RLOC Reachability 4.3.
ETR Synchronization 4.4.
MTU Handling 5.
Use Cases 7.1.
Traffic Engineering 7.2.
LISP for IPv6 Co-existence 7.3.
LISP for Virtual Private Networks 7.4.
LISP for Virtual Machine Mobility in Data Centers 8.
Security Considerations 9.
IANA Considerations 10.
Normative References 10.2.
Informative References Appendix A
. A Brief History of Location/Identity Separation A.1.
Old LISP Models
This document introduces the Locator/ID Separation Protocol (LISP)
], its main operational mechanisms,
and its design rationale. Fundamentally, LISP is built following a
well-known architectural idea: decoupling the overloaded semantics of
IP addresses. As pointed out by Noel Chiappa [RFC4984
IP addresses identify both the topological location of a network
attachment point as well as the node's identity. However, nodes and
routing have fundamentally different requirements. On one hand,
routing systems require that addresses be aggregatable and have
topological meaning; on the other hand, nodes must be identified
independently of their current location [RFC4984
LISP creates two separate namespaces, Endpoint Identifiers (EIDs) and
Routing Locators (RLOCs). Both are syntactically identical to the
current IPv4 and IPv6 addresses. However, EIDs are used to uniquely
identify nodes irrespective of their topological location and are
typically routed intra-domain. RLOCs are assigned topologically to
network attachment points and are typically routed inter-domain.
With LISP, the edge of the Internet (where the nodes are connected)
and the core (where inter-domain routing occurs) can be logically
separated. LISP-capable routers interconnect the two logical spaces.
LISP also introduces a database, called the Mapping System, to store
and retrieve mappings between identity and location. LISP-capable
routers exchange packets over the Internet core by encapsulating them
to the appropriate location.
* RLOCs have meaning only in the underlay network, that is, the
underlying core routing system.
* EIDs have meaning only in the overlay network, which is the
encapsulation relationship between LISP-capable routers.
* The LISP edge maps EIDs to RLOCs.
* Within the underlay network, RLOCs have both Locator and
* An EID within a LISP site carries both identifier and Locator
semantics to other nodes within that site.
* An EID within a LISP site carries identifier and limited Locator
semantics to nodes at other LISP sites (i.e., enough Locator
information to tell that the EID is external to the site).
The relationship described above is not unique to LISP, and it is
common to other overlay technologies.
The initial motivation in the LISP effort is to be found in the
routing scalability problem [RFC4984
], where, if LISP were to be
completely deployed, the Internet core is populated with RLOCs while
Traffic Engineering (TE) mechanisms are pushed to the Mapping System.
In such a scenario, RLOCs are quasi-static (i.e., low churn), hence
making the routing system scalable [Quoitin], while EIDs can roam
anywhere with no churn to the underlying global routing system.
] discusses the impact of LISP on the global routing system
during the transition period. However, the separation between
location and identity that LISP offers makes it suitable for use in
additional scenarios, such as TE, multihoming, and mobility among
This document describes the LISP architecture and its main
operational mechanisms as well as its design rationale. It is
important to note that this document does not specify or complement
LISP. The interested reader should refer to the main LISP
specifications (see [RFC9300
] and [RFC9301
]), as well as the
complementary documents (i.e., [RFC6831
], and [RFC7052
]) for the protocol specifications
along with the LISP deployment guidelines [RFC7215
2. Definitions of Terms
Endpoint Identifier (EID): Addresses used to uniquely identify nodes
irrespective of their topological location. Typically routed
Routing Locator (RLOC): Addresses assigned topologically to network
attachment points. Typically routed inter-domain.
Ingress Tunnel Router (ITR): A LISP-capable router that encapsulates
packets from a LISP site towards the core network.
Egress Tunnel Router (ETR): A LISP-capable router that decapsulates
packets from the core of the network towards a LISP site.
xTR: A router that implements both ITR and ETR functionalities.
Map-Request: A LISP signaling message used to request an EID-to-RLOC
Map-Reply: A LISP signaling message sent in response to a Map-
Request that contains a resolved EID-to-RLOC mapping.
Map-Register: A LISP signaling message used to register an EID-to-
Map-Notify: A LISP signaling message sent in response of a Map-
Register to acknowledge the correct reception of an EID-to-RLOC
This document describes the LISP architecture and does not introduce
any new terms. The reader is referred to [RFC9300
] for the complete definition of terms.
3. LISP Architecture
This section presents the LISP architecture. It first details the
design principles of LISP, and then it proceeds to describe its main
aspects: data plane, control plane, and internetworking mechanisms.
3.1. Design Principles
The LISP architecture is built on top of four basic design
Locator/Identifier split: Decoupling the overloaded semantics of
current IP addresses allows devices to have identity-based
addresses that are separate from topologically meaningful
addresses. By allowing only the topologically meaningful
addresses to be exposed to the Internet core, those topologically
meaningful addresses can be aggregated to support substantial
scaling. Individual devices are assigned identity-based addresses
that are not used for forwarding in the Internet core.
Overlay architecture: This architecture overlays route packets over
the current Internet, allowing deployment of new protocols without
changing the current infrastructure; hence, this results in a low
Decoupled data plane and control plane: Separating the data plane
from the control plane allows them to scale independently and use
different architectural approaches. This is important given that
they typically have different requirements and allows for other
data planes to be added. Even though the data plane and the
control plane are decoupled, they are not completely isolated,
because the LISP data plane may trigger control plane activity.
Incremental deployability: This principle ensures that the protocol
interoperates with the legacy Internet while providing some of the
targeted benefits to early adopters.
3.2. Overview of the Architecture
LISP architecturally splits the core from the edge of the Internet by
creating two separate namespaces: Endpoint Identifiers (EIDs) and
Routing Locators (RLOCs). The edge consists of LISP sites (e.g., an
Autonomous System) that use EID addresses. EIDs are IPv4 or IPv6
addresses that uniquely identify communication end hosts and are
assigned and configured by the same mechanisms that exist at the time
of this writing. EIDs do not contain inter-domain topological
information, and because of this, EIDs are usually routable at the
edge (within LISP sites) but not in the core; see Section 3.5
discussion of LISP site internetworking with non-LISP sites and
domains in the Internet.
LISP sites (at the edge) are connected to the interconnecting core of
the Internet by means of LISP-capable routers (e.g., border routers).
LISP sites are connected across the interconnecting core of the
Internet using tunnels between the LISP-capable routers. When
packets originated from a LISP site are flowing towards the core
network, they ingress into an encapsulated tunnel via an Ingress
Tunnel Router (ITR). When packets flow from the core network to a
LISP site, they egress from an encapsulated tunnel to an Egress
Tunnel Router (ETR). An xTR is a router that can perform both ITR
and ETR operations. In this context, ITRs encapsulate packets, while
ETRs decapsulate them; hence, LISP operates as an overlay on top of
the current Internet core.
| Mapping | |
. System | | Control
-| |`, | Plane
,' \-----------------/ . |
/ | ---
,.., - _,....,, | ,.., |
/ ` ,' ,-` `', | / ` |
/ \ +-----+ ,' `, +-----+ / \ |
| EID |-| xTR |--/ RLOC ,--| xTR |-| EID | | Data
| Space |-| |--| Space |--| |-| Space | | Plane
\ / +-----+ . / +-----+ \ / |
`. .' `. ,' `. .' |
`'-` `., ,.' `'-` ---
LISP Site (Edge) Core LISP Site (Edge)
Figure 1: A Schema of the LISP Architecture
With LISP, the core uses RLOCs. An RLOC is an IPv4 or IPv6 address
assigned to a core-facing network interface of an ITR or ETR.
A database that is typically distributed, called the Mapping System,
stores mappings between EIDs and RLOCs. Such mappings relate the
identity of the devices attached to LISP sites (EIDs) to the set of
RLOCs configured at the LISP-capable routers servicing the site.
Furthermore, the mappings also include TE policies and can be
configured to achieve multihoming and load balancing. The LISP
Mapping System is conceptually similar to the DNS, where it is
organized as a distributed multi-organization network database. With
LISP, ETRs register mappings, while ITRs retrieve them.
Finally, the LISP architecture emphasizes incremental deployment.
Given that LISP represents an overlay to the current Internet
architecture, end hosts, as well as intra-domain and inter-domain
routers, remain unchanged. The only required changes to the existing
infrastructure are to routers connecting the EID space with the RLOC
space. Additionally, LISP requires the deployment of an independent
Mapping System; such a distributed database is a new network entity.
The following describes a simplified packet flow sequence between two
nodes that are attached to LISP sites. Please note that typical
LISP-capable routers are xTRs (both ITR and ETR). Client HostA wants
to send a packet to server HostB.
| Mapping |
| System |
` \----------------/ `.
,' _,..-..,, ',
/ -` `-, \
.' ,' \ `,
` ' \ '
+-----+ | | RLOC_B1+-----+
HostA | | | RLOC |-------| | HostB
EID_A--|ITR_A|----| Space | |ETR_B|--EID_B
| | RLOC_A1 |-------| |
+-----+ | | RLOC_B2+-----+
Figure 2: Packet Flow Sequence in LISP 1.
HostA retrieves the EID_B of HostB, typically querying the DNS
and obtaining an A or AAAA record. Then, it generates an IP
packet as in the Internet. The packet has source address EID_A
and destination address EID_B. 2.
The packet is forwarded towards ITR_A in the LISP site using
standard intra-domain mechanisms. 3.
ITR_A, upon receiving the packet, queries the Mapping System to
retrieve the Locator of ETR_B that is servicing HostB's EID_B.
In order to do so, it uses a LISP control message called Map-
Request. The message contains EID_B as the lookup key. In turn,
it receives another LISP control message called Map-Reply. The
message contains two Locators: RLOC_B1 and RLOC_B2. It also
contains TE policies: priority and weight per Locator. Note that
a Map-Reply can contain more Locators if needed. ITR_A can cache
the mapping in local storage to speed up forwarding of subsequent
ITR_A encapsulates the packet towards RLOC_B1 (chosen according
to the priorities/weights specified in the mapping). The packet
contains two IP headers. The outer header has RLOC_A1 as source
and RLOC_B1 as destination. The inner original header has EID_A
as source and EID_B as destination. Furthermore, ITR_A adds a
LISP header. More details about LISP encapsulation can be found
in Section 3.3.1
5. The encapsulated packet is forwarded over the interconnecting
core as a normal IP packet, making the EID invisible from the
6. Upon reception of the encapsulated packet by ETR_B, it
decapsulates the packet and forwards it to HostB.
3.3. Data Plane
This section provides a high-level description of the LISP data
plane, which is specified in detail in [RFC9300
]. The LISP data
plane is responsible for encapsulating and decapsulating data packets
and caching the appropriate forwarding state. It includes two main
entities, the ITR and the ETR. Both are LISP-capable routers that
connect the EID with the RLOC space (ITR) and vice versa (ETR).
3.3.1. LISP Encapsulation
ITRs encapsulate data packets towards ETRs. LISP data packets are
encapsulated using UDP (port 4341). The source port is usually
selected by the ITR using a 5-tuple hash of the inner header (so as
to be consistent in case of multipath solutions, such as ECMP
]) and ignored on reception. LISP data packets are often
encapsulated in UDP packets that include a zero checksum [RFC6935
] that may not be verified when it is received, because LISP
data packets typically include an inner transport protocol header
with a non-zero checksum. The use of UDP zero checksums over IPv6
for all tunneling protocols like LISP is subject to the applicability
statement in [RFC6936
]. If LISP data packets are encapsulated in UDP
packets with non-zero checksums, the outer UDP checksums are verified
when the UDP packets are received, as part of normal UDP processing.
LISP-encapsulated packets also include a LISP header (after the UDP
header and before the original IP header). The LISP header is
prepended by ITRs and stripped by ETRs. It carries reachability
information (see more details in Section 4.2
) and the 'Instance ID'
field. The 'Instance ID' field is used to distinguish traffic to/
from different tenant address spaces at the LISP site, and this use
of the Instance ID may use overlapped but logically separated EID
Overall, LISP works on 4 headers: the inner header the source
constructed and the 3 headers a LISP encapsulator prepends ("outer"
to "inner"): 1.
Outer IP header containing RLOCs as source and destination
addresses. This header is originated by ITRs and stripped by
UDP header (port 4341), usually with zero checksum. This header
is originated by ITRs and stripped by ETRs. 3.
LISP header that contains various forwarding-plane features (such
as reachability) and an 'Instance ID' field. This header is
originated by ITRs and stripped by ETRs. 4.
Inner IP header containing EIDs as source and destination
addresses. This header is created by the source end host and is
left unchanged by the LISP data plane processing on the ITR and
Finally, in some scenarios, re-encapsulating and/or recursive tunnels
are useful to choose a specified path in the underlay network, for
instance, to avoid congestion or failure. Re-encapsulating tunnels
are consecutive LISP tunnels and occur when a decapsulator (an ETR
action) removes a LISP header and then acts as an encapsulator (an
ITR action) to prepend another one. On the other hand, recursive
tunnels are nested tunnels and are implemented by using multiple LISP
encapsulations on a packet. Such functions are implemented by Re-
encapsulating Tunnel Routers (RTRs). An RTR can be thought of as a
router that first acts as an ETR by decapsulating packets and then as
an ITR by encapsulating them towards another Locator; more
information can be found in [RFC9300
] and [RFC9301
3.3.2. LISP Forwarding State
In the LISP architecture, ITRs keep just enough information to route
traffic flowing through them. In other words, ITRs only need to
retrieve from the LISP Mapping System mappings between EID-Prefixes
(blocks of EIDs) and RLOCs that are used to encapsulate packets.
Such mappings are stored in a local cache called the LISP Map-Cache
for subsequent packets addressed to the same EID-Prefix. Note that
in the case of overlapping EID-Prefixes, after a request, the ITR may
receive a set of mappings covering the requested EID-Prefix and all
more-specific EID-Prefixes (cf., Section 5.5 of [RFC9301
include a Time to Live (TTL) (set by the ETR). More details about
the Map-Cache management can be found in Section 4.1
3.4. Control Plane
The LISP control plane, specified in [RFC9301
], provides a standard
interface to register and request mappings. The LISP Mapping System
is a database that stores such mappings. The following sub-sections
first describe the mappings, then the standard interface to the
Mapping System, and finally its architecture.
3.4.1. LISP Mappings
Each mapping includes the bindings between EID-Prefix(es) and a set
of RLOCs as well as TE policies, in the form of priorities and
weights for the RLOCs. Priorities allow the ETR to configure active/
backup policies, while weights are used to load-balance traffic among
the RLOCs (on a per-flow basis).
Typical mappings in LISP bind EIDs in the form of IP prefixes with a
set of RLOCs, also in the form of IP addresses. IPv4 and IPv6
addresses are encoded using the appropriate Address Family Identifier
]. However, LISP can also support more general address
encoding by means of the ongoing effort around the LISP Canonical
Address Format (LCAF) [RFC8060
With such a general syntax for address encoding in place, LISP aims
to provide flexibility to current and future applications. For
instance, LCAFs could support Media Access Control (MAC) addresses,
geocoordinates, ASCII names, and application-specific data.
3.4.2. Mapping System Interface
LISP defines a standard interface between data and control planes.
The interface is specified in [RFC9301
] and defines two entities:
Map-Server: A network infrastructure component that learns mappings
from ETRs and publishes them into the LISP Mapping System.
Typically, Map-Servers are not authoritative to reply to queries;
hence, they forward them to the ETR. However, they can also
operate in proxy-mode, where the ETRs delegate replying to queries
to Map-Servers. This setup is useful when the ETR has limited
resources (e.g., CPU or power).
Map-Resolver: A network infrastructure component that interfaces
ITRs with the Mapping System by proxying queries and, in some
The interface defines four LISP control messages that are sent as UDP
datagrams (port 4342):
Map-Register: This message is used by ETRs to register mappings in
the Mapping System, and it is authenticated using a shared key
between the ETR and the Map-Server.
Map-Notify: When requested by the ETR, this message is sent by the
Map-Server in response to a Map-Register to acknowledge the
correct reception of the mapping and convey the latest Map-Server
state on the EID-to-RLOC mapping. In some cases, a Map-Notify can
be sent to the previous RLOCs when an EID is registered by a new
set of RLOCs.
Map-Request: This message is used by ITRs or Map-Resolvers to
resolve the mapping of a given EID.
Map-Reply: This message is sent by Map-Servers or ETRs in response
to a Map-Request and contains the resolved mapping. Please note
that a Map-Reply may contain a negative reply if, for example, the
queried EID is not part of the LISP EID space. In such cases, the
ITR typically forwards the traffic as is (non-encapsulated) to the
public Internet. This behavior is defined to support incremental
deployment of LISP.
3.4.3. Mapping System
LISP architecturally decouples control and data planes by means of a
standard interface. This interface glues the data plane -- routers
responsible for forwarding data packets -- with the LISP Mapping
System -- a database responsible for storing mappings.
With this separation in place, the data and control planes can use
different architectures if needed and scale independently.
Typically, the data plane is optimized to route packets according to
hierarchical IP addresses. However, the control plane may have
different requirements, for instance, and by taking advantage of the
LCAFs, the Mapping System may be used to store nonhierarchical keys
(such as MAC addresses), requiring different architectural approaches
for scalability. Another important difference between the LISP
control and data planes is that, and as a result of the local mapping
cache available at the ITR, the Mapping System does not need to
operate at line-rate.
Many of the existing mechanisms to create distributed systems have
been explored and considered for the Mapping System architecture:
graph-based databases in the form of LISP Alternative Logical
Topology (LISP-ALT) [RFC6836
], hierarchical databases in the form of
the LISP Delegated Database Tree (LISP-DDT) [RFC8111
databases in the form of the LISP Not-so-novel EID-to-RLOC Database
], flat databases in the form of the LISP
Distributed Hash Table (LISP-DHT) [LISP-SHDHT] [Mathy], and a
multicast-based database [LISP-EMACS]. Furthermore, it is worth
noting that, in some scenarios, such as private deployments, the
Mapping System can operate as logically centralized. In such cases,
it is typically composed of a single Map-Server/Map-Resolver.
The following sub-sections focus on the two Mapping Systems that have
been implemented and deployed (LISP-ALT and LISP-DDT).
] was the first Mapping System proposed, developed,
and deployed on the LISP pilot network. It is based on a distributed
BGP overlay in which Map-Servers and Map-Resolvers participate. The
nodes connect to their peers through static tunnels. Each Map-Server
involved in the ALT topology advertises the EID-Prefixes registered
by the serviced ETRs, making the EID routable on the ALT topology.
When an ITR needs a mapping, it sends a Map-Request to a Map-Resolver
that, using the ALT topology, forwards the Map-Request towards the
Map-Server responsible for the mapping. Upon reception, the Map-
Server forwards the request to the ETR, which in turn replies
directly to the ITR.
] is conceptually similar to the DNS, a hierarchical
directory whose internal structure mirrors the hierarchical nature of
the EID address space. The DDT hierarchy is composed of DDT nodes
forming a tree structure; the leafs of the tree are Map-Servers. On
top of the structure, there is the DDT root node, which is a
particular instance of a DDT node, that matches the entire address
space. As in the case of DNS, DDT supports multiple redundant DDT
nodes and/or DDT roots. Finally, Map-Resolvers are the clients of
the DDT hierarchy and can query the DDT root and/or other DDT nodes.
| DDT Root|
| /0 |
,-'` | `'.,
-'` | `-
/-------\ /-------\ /-------\
| DDT | | DDT | | DDT |
| Node | | Node | | Node | ...
| 0/8 | | 1/8 | | 2/8 |
\-------/ \-------/ \-------/
_. _. . -..,,,_
-` -` \ ````''--
+------------+ +------------+ +------------+ +------------+
| Map-Server | | Map-Server | | Map-Server | | Map-Server |
| EID-Prefix1| | EID-Prefix2| | EID-Prefix3| | EID-Prefix4|
+------------+ +------------+ +------------+ +------------+
Figure 3: A Schematic Representation of the DDT Tree Structure
Please note that the prefixes and the structure depicted in the
figure above should only be considered as an example.
The DDT structure does not actually index EID-Prefixes; rather, it
indexes Extended EID-Prefixes (XEID-Prefixes). An XEID-Prefix is
just the concatenation of the following fields (from most significant
bit to less significant bits): Database-ID, Instance ID, Address
Family Identifier, and the actual EID-Prefix. The Database-ID is
provided for possible future requirements of higher levels in the
hierarchy and to enable the creation of multiple and separate
In order to resolve a query, LISP-DDT operates in a similar way to
the DNS but only supports iterative lookups. DDT clients (usually
Map-Resolvers) generate Map-Requests to the DDT root node. In
response, they receive a newly introduced LISP control message: a
Map-Referral. A Map-Referral provides the list of RLOCs of the set
of DDT nodes matching a configured XEID delegation. That is, the
information contained in the Map-Referral points to the child of the
queried DDT node that has more specific information about the queried
XEID-Prefix. This process is repeated until the DDT client walks the
tree structure (downwards) and discovers the Map-Server servicing the
queried XEID. At this point, the client sends a Map-Request and
receives a Map-Reply containing the mappings. It is important to
note that DDT clients can also cache the information contained in
Map-Referrals; that is, they cache the DDT structure. This is used
to reduce the time required to retrieve mappings [Jakab].
The DDT Mapping System relies on manual configuration. That is, Map-
Resolvers are configured with the set of available DDT root nodes,
while DDT nodes are configured with the appropriate XEID delegations.
Configuration changes in the DDT nodes are only required when the
tree structure changes itself, but it doesn't depend on EID dynamics
(RLOC allocation or TE policy changes).
3.5. Internetworking Mechanisms
EIDs are typically identical to either IPv4 or IPv6 addresses, and
they are stored in the LISP Mapping System. However, they are
usually not announced in the routing system beyond the local LISP
domain. As a result, LISP requires an internetworking mechanism to
allow LISP sites to speak with non-LISP sites and vice versa. LISP
internetworking mechanisms are specified in [RFC6832
LISP defines two entities to provide internetworking:
Proxy Ingress Tunnel Router (PITR): PITRs provide connectivity from
the legacy Internet to LISP sites. PITRs announce in the global
routing system blocks of EID-Prefixes (aggregating when possible)
to attract traffic. For each incoming packet from a source not in
a LISP site (a non-EID), the PITR LISP-encapsulates it towards the
RLOC(s) of the appropriate LISP site. The impact of PITRs on the
routing table size of the Default-Free Zone (DFZ) is, in the worst
case, similar to the case in which LISP is not deployed. EID-
Prefixes will be aggregated as much as possible, both by the PITR
and by the global routing system.
Proxy Egress Tunnel Router (PETR): PETRs provide connectivity from
LISP sites to the legacy Internet. In some scenarios, LISP sites
may be unable to send encapsulated packets with a local EID
address as a source to the legacy Internet, for instance, when
Unicast Reverse Path Forwarding (uRPF) is used by Provider Edge
routers or when an intermediate network between a LISP site and a
non-LISP site does not support the desired version of IP (IPv4 or
IPv6). In both cases, the PETR overcomes such limitations by
encapsulating packets over the network. There is no specified
provision for the distribution of PETR RLOC addresses to the ITRs.
Additionally, LISP also defines mechanisms to operate with private
] by means of LISP-NAT [RFC6832
]. In this case, the xTR
replaces a private EID source address with a routable one. At the
time of this writing, work is ongoing to define NAT-traversal
capabilities, that is, xTRs behind a NAT using non-routable RLOCs.
PITRs, PETRs, and LISP-NAT enable incremental deployment of LISP by
providing significant flexibility in the placement of the boundaries
between the LISP and non-LISP portions of the network and making it
easy to change those boundaries over time.
4. LISP Operational Mechanisms
This section details the main operational mechanisms defined in LISP.
4.1. Cache Management
LISP's decoupled control and data planes, where mappings are stored
in the control plane and used for forwarding in the data plane,
require a local cache in ITRs to reduce signaling overhead (Map-
Request/Map-Reply) and increase forwarding speed. The local cache
available at the ITRs, called Map-Cache, is used by the router to
LISP-encapsulate packets. The Map-Cache is indexed by (Instance ID,
EID-Prefix) and contains basically the set of RLOCs with the
associated TE policies (priorities and weights).
The Map-Cache, as with any other cache, requires cache coherence
mechanisms to maintain up-to-date information. LISP defines three
main mechanisms for cache coherence:
Record Time To Live (TTL): Each mapping record contains a TTL set by
the ETR. Upon expiration of the TTL, the ITR can't use the
mapping until it is refreshed by sending a new Map-Request.
Solicit-Map-Request (SMR): SMR is an explicit mechanism to update
mapping information. In particular, a special type of Map-Request
can be sent on demand by ETRs to request refreshing a mapping.
Upon reception of an SMR message, the ITR must refresh the
bindings by sending a Map-Request to the Mapping System. Further
uses of SMRs are documented in [RFC9301
Map-Versioning: This optional mechanism piggybacks, in the LISP
header of data packets, the version number of the mappings used by
an xTR. This way, when an xTR receives a LISP-encapsulated packet
from a remote xTR, it can check whether its own Map-Cache or the
one of the remote xTR is outdated. If its Map-Cache is outdated,
it sends a Map-Request for the remote EID so as to obtain the
newest mappings. On the contrary, if it detects that the remote
xTR Map-Cache is outdated, it sends an SMR to notify it that a new
mapping is available. Further details are available in [RFC9302
Finally, it is worth noting that, in some cases, an entry in the Map-
Cache can be proactively refreshed using the mechanisms described in
the section below.
4.2. RLOC Reachability
In most cases, LISP operates with a pull-based Mapping System (e.g.,
DDT). This results in an edge-to-edge pull architecture. In such a
scenario, the network state is stored in the control plane while the
data plane pulls it on demand. This has consequences concerning the
propagation of xTRs' reachability/liveness information, since pull
architectures require explicit mechanisms to propagate this
information. As a result, LISP defines a set of mechanisms to inform
ITRs and PITRs about the reachability of the cached RLOCs:
Locator-Status-Bits (LSBs): Using LSBs is a passive technique. The
'LSB' field is carried by data packets in the LISP header and can
be set by ETRs to specify which RLOCs of the ETR site are up/down.
This information can be used by the ITRs as a hint about the
reachability to perform additional checks. Also note that LSBs do
not provide path reachability status; they only provide hints
about the status of RLOCs. As such, they must not be used over
the public Internet and should be coupled with Map-Versioning to
prevent race conditions where LSBs are interpreted as referring to
different RLOCs than intended.
Echo-Nonce: This is also a passive technique that can only operate
effectively when data flows bidirectionally between two
communicating xTRs. Basically, an ITR piggybacks a random number
(called a nonce) in LISP data packets. If the path and the probed
Locator are up, the ETR will piggyback the same random number on
the next data packet; if this is not the case, the ITR can set the
Locator as unreachable. When traffic flow is unidirectional or
when the ETR receiving the traffic is not the same as the ITR that
transmits it back, additional mechanisms are required. The Echo-
Nonce mechanism must be used in trusted environments only, not
over the public Internet.
RLOC-Probing: This is an active probing algorithm where ITRs send
probes to specific Locators. This effectively probes both the
Locator and the path. In particular, this is done by sending a
Map-Request (with certain flags activated) on the data plane (RLOC
space) and then waiting for a Map-Reply (also sent on the data
plane). The active nature of RLOC-Probing provides an effective
mechanism for determining reachability and, in case of failure,
switching to a different Locator. Furthermore, the mechanism also
provides useful RTT estimates of the delay of the path that can be
used by other network algorithms.
It is worth noting that RLOC-Probing and the Echo-Nonce can work
together. Specifically, if a nonce is not echoed, an ITR cannot
determine which path direction has failed. In this scenario, an ITR
can use RLOC-Probing.
Additionally, LISP also recommends inferring the reachability of
Locators by using information provided by the underlay, particularly:
ICMP signaling: The LISP underlay -- the current Internet -- uses
ICMP to signal unreachability (among other things). LISP can take
advantage of this, and the reception of an ICMP Network
Unreachable or ICMP Host Unreachable message can be seen as a hint
that a Locator might be unreachable. This should lead to
performing additional checks.
Underlay routing: Both BGP and IGP carry reachability information.
LISP-capable routers that have access to underlay routing
information can use it to determine if a given Locator or path is
4.3. ETR Synchronization
All the ETRs that are authoritative to a particular EID-Prefix must
announce the same mapping to the requesters. This means that ETRs
must be aware of the status of the RLOCs of the remaining ETRs. This
is known as ETR synchronization.
At the time of this writing, LISP does not specify a mechanism to
achieve ETR synchronization. Although many well-known techniques
could be applied to solve this issue, it is still under research. As
a result, operators must rely on coherent manual configuration.
4.4. MTU Handling
Since LISP encapsulates packets, it requires dealing with packets
that exceed the MTU of the path between the ITR and the ETR.
Specifically, LISP defines two mechanisms:
Stateless: With this mechanism, the effective MTU is assumed from
the ITR's perspective. If a payload packet is too big for the
effective MTU and can be fragmented, the payload packet is
fragmented on the ITR, such that reassembly is performed at the
Stateful: With this mechanism, ITRs keep track of the MTU of the
paths towards the destination Locators by parsing the ICMP Too Big
packets sent by intermediate routers. ITRs will send ICMP Too Big
messages to inform the sources about the effective MTU.
Additionally, ITRs can use mechanisms such as Path MTU Discovery
] or Packetization Layer Path MTU Discovery
] to keep track of the MTU towards the Locators.
In both cases, if the packet cannot be fragmented (IPv4 with DF=1 or
IPv6), then the ITR drops it and replies with an ICMP Too Big message
to the source.5.
The separation between Locators and identifiers in LISP is suitable
for TE purposes where LISP sites can change their attachment points
to the Internet (i.e., RLOCs) without impacting endpoints or the
Internet core. In this context, the border routers operate the xTR
functionality, and endpoints are not aware of the existence of LISP.
This functionality is similar to Network Mobility [RFC3963
However, this mode of operation does not allow seamless mobility of
endpoints between different LISP sites, as the EID address might not
be routable in a visited site. Nevertheless, LISP can be used to
enable seamless IP mobility when LISP is directly implemented in the
endpoint or when the endpoint roams to an attached xTR. Each
endpoint is then an xTR, and the EID address is the one presented to
the network stack used by applications while the RLOC is the address
gathered from the network when it is visited. This functionality is
similar to Mobile IP ([RFC5944
] and [RFC6275
Whenever a device changes its RLOC, the xTR updates the RLOC of its
local mapping and registers it to its Map-Server, typically with a
low TTL value (1 min). To avoid the need for a home gateway, the ITR
also indicates the RLOC change to all remote devices that have
ongoing communications with the device that moved. The combination
of both methods ensures the scalability of the system, as signaling
is strictly limited to the Map-Server and to hosts with which
communications are ongoing. In the mobility case, the EID-Prefix can
be as small as a full /32 or /128 (IPv4 or IPv6, respectively),
depending on the specific use case (e.g., subnet mobility vs. single
VM/Mobile node mobility).
The decoupled identity and location provided by LISP allow it to
operate with other Layer 2 and Layer 3 mobility solutions.6.
LISP also supports transporting IP multicast packets sent from the
EID space. The required operational changes to the multicast
protocols are documented in [RFC6831
In such scenarios, LISP may create multicast state both at the core
and at the sites (both source and receiver). When signaling is used
to create multicast state at the sites, LISP routers encapsulate PIM
Join/Prune messages from receiver to source sites as unicast packets.
At the core, ETRs build a new PIM Join/Prune message addressed to the
RLOC of the ITR servicing the source. A simplified sequence is shown
An end host willing to join a multicast channel sends an IGMP
report. Multicast PIM routers at the LISP site propagate PIM
Join/Prune messages (S-EID, G) towards the ETR. 2.
The Join message flows to the ETR. Upon reception, the ETR
builds two Join messages. The first one unicast LISP-
encapsulates the original Join message towards the RLOC of the
ITR servicing the source. This message creates (S-EID, G)
multicast state at the source site. The second Join message
contains, as a destination address, the RLOC of the ITR servicing
the source (S-RLOC, G) and creates multicast state at the core. 3.
Multicast data packets originated by the source (S-EID, G) flow
from the source to the ITR. The ITR LISP-encapsulates the
multicast packets. The outer header includes its own RLOC as the
source (S-RLOC) and the original multicast group address (G) as
the destination. Please note that multicast group addresses are
logical and are not resolved by the Mapping System. Then, the
multicast packets are transmitted through the core towards the
receiving ETRs, which decapsulate the packets and forward them
using the receiver site's multicast state.
Please note that the inner and outer multicast addresses are
generally different, except in specific cases where the underlay
provider implements tight control on the overlay. LISP
specifications already support all PIM modes [RFC6831
Additionally, LISP can also support non-PIM mechanisms in order to
maintain multicast state.
When multicast sources and receivers are active at LISP sites and the
core network between the sites does not provide multicast support, a
signal-free mechanism can be used to create an overlay that will
allow multicast traffic to flow between sites and connect the
multicast trees at the different sites [RFC8378
]. Registrations from
the different receiver sites will be merged in the Mapping System to
assemble a multicast replication list inclusive of all RLOCs that
lead to receivers for a particular multicast group or multicast
channel. The replication list for each specific multicast entry is
maintained as a database mapping entry in the LISP Mapping System.
7. Use Cases
7.1. Traffic Engineering
A LISP site can strictly impose via which ETRs the traffic must enter
the LISP site network even though the path followed to reach the ETR
is not under the control of the LISP site. This fine control is
implemented with the mappings. When a remote site is willing to send
traffic to a LISP site, it retrieves the mapping associated with the
destination EID via the Mapping System. The mapping is sent directly
by an authoritative ETR of the EID and is not altered by any
A mapping associates a list of RLOCs with an EID-Prefix. Each RLOC
corresponds to an interface of an ETR (or set of ETRs) that is able
to correctly forward packets to EIDs in the prefix. Each RLOC is
tagged with a priority and a weight in the mapping. The priority is
used to indicate which RLOCs should be preferred for sending packets
(the least preferred ones being provided for backup purposes). The
weight permits balancing the load between the RLOCs with the same
priority, in proportion to the weight value.
As mappings are directly issued by the authoritative ETR of the EID
and are not altered when transmitted to the remote site, it offers
highly flexible incoming inter-domain TE and even makes it possible
for a site to support a different mapping policy for each remote
7.2. LISP for IPv6 Co-existence
LISP encapsulations allow transporting packets using EIDs from a
given address family (e.g., IPv6) with packets from other address
families (e.g., IPv4). The absence of correlation between the
address families of RLOCs and EIDs makes LISP a candidate to allow,
e.g., IPv6 to be deployed when all of the core network may not have
For example, two IPv6-only data centers could be interconnected via
the legacy IPv4 Internet. If their border routers are LISP capable,
sending packets between the data centers is done without any form of
translation, as the original IPv6 packets (in the EID space) will be
LISP encapsulated and transmitted over the IPv4 legacy Internet via
7.3. LISP for Virtual Private Networks
It is common to operate several virtual networks over the same
physical infrastructure. In such virtual private networks,
determining to which virtual network a packet belongs is essential;
tags or labels are used for that purpose. When using LISP, the
distinction can be made with the 'Instance ID' field. When an ITR
encapsulates a packet from a particular virtual network (e.g., known
via Virtual Routing and Forwarding (VRF) or the VLAN), it tags the
encapsulated packet with the Instance ID corresponding to the virtual
network of the packet. When an ETR receives a packet tagged with an
Instance ID, it uses the Instance ID to determine how to treat the
The main usage of LISP for virtual private networks does not
introduce additional requirements on the underlying network, as long
as it runs IP.
7.4. LISP for Virtual Machine Mobility in Data Centers
A way to enable seamless virtual machine (VM) mobility in the data
center is to conceive the data center backbone as the RLOC space and
the subnet where servers are hosted as forming the EID space. A LISP
router is placed at the border between the backbone and each subnet.
When a VM is moved to another subnet, it can keep (temporarily) the
address it had before the move so as to continue without a transport-
layer connection reset. When an xTR detects a source address
received on a subnet to be an address not assigned to the subnet, it
registers the address to the Mapping System.
To inform the other LISP routers that the machine moved and where,
and then to avoid detours via the initial subnetwork, mechanisms such
as the Solicit-Map-Request messages are used.
8. Security Considerations
This section describes the security considerations associated with
In a push Mapping System, the state necessary to forward packets is
learned independently of the traffic itself. However, with a pull
architecture, the system becomes reactive, and data plane events
(e.g., the arrival of a packet with an unknown destination address)
may trigger control plane events. This on-demand learning of
mappings provides many advantages, as discussed above, but may also
affect the way security is enforced.
Usually, the data plane is implemented in the fast path of routers to
provide high-performance forwarding capabilities, while the control
plane features are implemented in the slow path to offer high
flexibility, and a performance gap of several orders of magnitude can
be observed between the slow and fast paths. As a consequence, the
way to notify the control plane of data plane events must be
considered carefully so as not to overload the slow path, and rate
limiting should be used as specified in [RFC9300
] and [RFC9301
Care must also be taken not to overload the Mapping System (i.e., the
control plane infrastructure), as the operations to be performed by
the Mapping System may be more complex than those on the data plane.
For that reason, [RFC9300
] and [RFC9301
] recommend rate limiting the
sending of messages to the Mapping System.
To improve resiliency and reduce the overall number of messages
exchanged, LISP makes it possible to leak certain information, such
as the reachability of Locators, directly into data plane packets.
In environments that are not fully trusted, like the open Internet,
control information gleaned from data plane packets must not be used
or must be verified before using it.
Mappings are the centerpiece of LISP, and all precautions must be
taken to prevent malicious entities from manipulating or misusing
them. Using trustable Map-Servers that strictly respect [RFC9301
and the authentication mechanism proposed by LISP-SEC [RFC9303
reduces the risk of attacks on mapping integrity. In more critical
environments, secure measures may be needed. The way security is
implemented for a given Mapping System strongly depends on the
architecture of the Mapping System itself and the threat model
assumed for the deployment. Thus, Mapping System security has to be
discussed in the relevant documents proposing the Mapping System
As with any other tunneling mechanism, middleboxes on the path
between an ITR (or PITR) and an ETR (or PETR) must implement
mechanisms to strip the LISP encapsulation to correctly inspect the
content of LISP-encapsulated packets.
Like other map-and-encap mechanisms, LISP enables triangular routing
(i.e., packets of a flow cross different border routers, depending on
their direction). This means that intermediate boxes may have an
incomplete view of the traffic they inspect or manipulate. Moreover,
LISP-encapsulated packets are routed based on the outer IP address
(i.e., the RLOC) and can be delivered to an ETR that is not
responsible for the destination EID of the packet or even delivered
to a network element that is not an ETR. Mitigation consists of
applying appropriate filtering techniques on the network elements
that can potentially receive unexpected LISP-encapsulated packets.
More details about security implications of LISP are discussed in
9. IANA Considerations
This document has no IANA actions.
10.1. Normative References
] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191
, November 1990,
] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918
, DOI 10.17487/RFC1918
February 1996, <https://www.rfc-editor.org/info/rfc1918
] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992
, DOI 10.17487/RFC2992
, November 2000,
] Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
Thubert, "Network Mobility (NEMO) Basic Support Protocol", RFC 3963
, DOI 10.17487/RFC3963
, January 2005,
] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821
, DOI 10.17487/RFC4821
, March 2007,
] Meyer, D., Ed., Zhang, L., Ed., and K. Fall, Ed., "Report
from the IAB Workshop on Routing and Addressing", RFC 4984
, DOI 10.17487/RFC4984
, September 2007,
] Perkins, C., Ed., "IP Mobility Support for IPv4, Revised", RFC 5944
, DOI 10.17487/RFC5944
, November 2010,
] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275
, DOI 10.17487/RFC6275
] Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
Locator/ID Separation Protocol (LISP) for Multicast
Environments", RFC 6831
, DOI 10.17487/RFC6831
] Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
"Interworking between Locator/ID Separation Protocol
(LISP) and Non-LISP Sites", RFC 6832
, January 2013,
] Farinacci, D. and D. Meyer, "The Locator/ID Separation
Protocol Internet Groper (LIG)", RFC 6835
, January 2013,
] Fuller, V., Farinacci, D., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol Alternative Logical
Topology (LISP+ALT)", RFC 6836
, DOI 10.17487/RFC6836
January 2013, <https://www.rfc-editor.org/info/rfc6836
] Lear, E., "NERD: A Not-so-novel Endpoint ID (EID) to
Routing Locator (RLOC) Database", RFC 6837
, January 2013,
] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935
, April 2013,
] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums", RFC 6936
, DOI 10.17487/RFC6936
, April 2013,
] Schudel, G., Jain, A., and V. Moreno, "Locator/ID
Separation Protocol (LISP) MIB", RFC 7052
, October 2013,
] Jakab, L., Cabellos-Aparicio, A., Coras, F., Domingo-
Pascual, J., and D. Lewis, "Locator/Identifier Separation
Protocol (LISP) Network Element Deployment
Considerations", RFC 7215
, DOI 10.17487/RFC7215
] Saucez, D., Iannone, L., and O. Bonaventure, "Locator/ID
Separation Protocol (LISP) Threat Analysis", RFC 7835
, April 2016,
] Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
Address Format (LCAF)", RFC 8060
, DOI 10.17487/RFC8060
February 2017, <https://www.rfc-editor.org/info/rfc8060
] Fuller, V., Lewis, D., Ermagan, V., Jain, A., and A.
Smirnov, "Locator/ID Separation Protocol Delegated
Database Tree (LISP-DDT)", RFC 8111
, DOI 10.17487/RFC8111
May 2017, <https://www.rfc-editor.org/info/rfc8111
] Moreno, V. and D. Farinacci, "Signal-Free Locator/ID
Separation Protocol (LISP) Multicast", RFC 8378
, May 2018,
] Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
Cabellos, Ed., "The Locator/ID Separation Protocol
(LISP)", RFC 9300
, DOI 10.17487/RFC9300
, October 2022,
] Farinacci, D., Maino, F., Fuller, V., and A. Cabellos,
Ed., "Locator/ID Separation Protocol (LISP) Control
Plane", RFC 9301
, DOI 10.17487/RFC9301
, October 2022,
] Iannone, L., Saucez, D., and O. Bonaventure, "Locator/ID
Separation Protocol (LISP) Map-Versioning", RFC 9302
, October 2022,
] Maino, F., Ermagan, V., Cabellos, A., and D. Saucez,
"Locator/ID Separation Protocol Security (LISP-SEC)", RFC 9303
, DOI 10.17487/RFC9303
, October 2022,
10.2. Informative References
[Jakab] Jakab, L., Cabellos-Aparicio, A., Coras, F., Saucez, D.,
and O. Bonaventure, "LISP-TREE: A DNS Hierarchy to Support
the LISP Mapping System", IEEE Journal on Selected Areas
in Communications, vol. 28, no. 8, pp. 1332-1343,
DOI 10.1109/JSAC.2010.101011, October 2010,
Brim, S., Farinacci, D., Meyer, D., and J. Curran, "EID
Mappings Multicast Across Cooperating Systems for LISP",
Work in Progress, Internet-Draft, draft-curran-lisp-emacs-
00, 9 November 2007, <https://www.ietf.org/archive/id/
Cheng, L. and M. Sun, "LISP Single-Hop DHT Mapping
Overlay", Work in Progress, Internet-Draft, draft-cheng-
lisp-shdht-04, 15 July 2013,
[Mathy] Mathy, L. and L. Iannone, "LISP-DHT: Towards a DHT to map
identifiers onto locators", CoNEXT '08: Proceedings of the
2008 ACM CoNEXT Conference, ReArch '08 - Re-Architecting
the Internet, DOI 10.1145/1544012.1544073, December 2008,
[Quoitin] Quoitin, B., Iannone, L., de Launois, C., and O.
Bonaventure, "Evaluating the Benefits of the Locator/
Identifier Separation", Proceedings of 2nd ACM/IEEE
International Workshop on Mobility in the Evolving
Internet Architecture, DOI 10.1145/1366919.1366926, August
Appendix A. A Brief History of Location/Identity Separation
The LISP architecture for separation of location and identity
resulted from the discussions of this topic at the Amsterdam IAB
Routing and Addressing Workshop, which took place in October 2006
A small group of like-minded personnel spontaneously formed
immediately after that workshop to work on an idea that came out of
informal discussions at the workshop and on various mailing lists.
The first Internet-Draft on LISP appeared in January 2007.
Trial implementations started at that time, with initial trial
deployments underway since June 2007; the results of early experience
have been fed back into the design in a continuous, ongoing process
over several years. At this point, LISP represents a moderately
mature system, having undergone a long, organic series of changes and
LISP transitioned from an IRTF activity to an IETF WG in March 2009.
After numerous revisions, the basic specifications moved to becoming
RFCs at the start of 2013; work to expand, improve, and find new uses
for it continues (and undoubtedly will for a long time to come). The
LISP WG was rechartered in 2018 to continue work on the LISP base
protocol and produce Standards Track documents.
A.1. Old LISP Models
LISP, as initially conceived, had a number of potential operating
modes, named 'models'. Although they are not used anymore, one
occasionally sees mention of them, so they are briefly described
LISP 1: EIDs all appear in the normal routing and forwarding tables
of the network (i.e., they are 'routable'). This property is used
to load EID-to-RLOC mappings via bootstrapping operations.
Packets are sent with the EID as the destination in the outer
wrapper; when an ETR sees such a packet, it sends a Map-Reply to
the source ITR, giving the full mapping.
LISP 1.5: LISP 1.5 is similar to LISP 1, but the routability of EIDs
happens on a separate network.
LISP 2: EIDs are not routable; EID-to-RLOC mappings are available
from the DNS.
LISP 3: EIDs are not routable and have to be looked up in a new EID-
to-RLOC mapping database (in the initial concept, a system using
Distributed Hash Tables). Two variants were possible: a 'push'
system in which all mappings were distributed to all ITRs and a
'pull' system in which ITRs load the mappings when they need them.
This document was initiated by Noel Chiappa, and much of the core
philosophy came from him. The authors acknowledge the important
contributions he has made to this work and thank him for his past
The authors would also like to thank Dino Farinacci, Fabio Maino,
Luigi Iannone, Sharon Barkai, Isidoros Kouvelas, Christian Cassar,
Florin Coras, Marc Binderberger, Alberto Rodriguez-Natal, Ronald
Bonica, Chad Hintz, Robert Raszuk, Joel M. Halpern, Darrel Lewis, and
Universitat Politecnica de Catalunya
c/ Jordi Girona s/n
Damien Saucez (editor)
2004 route des Lucioles - BP 93