The Locator Identifier Separation Protocol (LISP)by David Meyer, Cisco Systems
The Internet Architecture Board's (IAB)'s October 2006 Routing and Addressing Workshop  renewed interest in the design of a scalable routing and addressing architecture for the Internet. Many concerns prompted this renewed interest, including the scalability of the routing system and the impending exhaustion of the IPv4 address space. Since the IAB workshop, several proposals have emerged that attempt to address the concerns expressed both at the workshop and in other forums [7,9,12,13,14]. All of these proposals are based on a common concept: the separation of locator and identifier in the numbering of Internet devices, often termed the "Loc/ID split." This article focuses on one proposal for implementing this concept: the Locator/Identifier Separation Protoco (LISP) .
The basic idea behind the Loc/ID split is that the current Internet routing and addressing architecture combines two functions: Routing Locators (RLOCs), which describe how a device is attached to the network, and Endpoint Identifiers (EIDs), which define "who" the device is, in a single numbering space, the IP address. Proponents of the Loc/ID split argue that this "overloading" of functions makes it virtually impossible to build an efficient routing system without forcing unacceptable constraints on end-system use of addresses. Splitting these functions apart by using different numbering spaces for EIDs and RLOCs yields several advantages, including improved scalability of the routing system through greater aggregation of RLOCs. To achieve this aggregation, we must allocate RLOCs in a way that is congruent with the topology of the network ("Rekhter's Law"). Today's "provider-allocated" IP address space is an example of such an allocation scheme. EIDs, on the other hand, are typically allocated along organizational boundaries. Because the network topology and organizational hierarchies are rarely congruent, it is difficult (if not impossible) to make a single numbering space efficiently serve both purposes without imposing unacceptable constraints (such as requiring renumbering upon provider changes) on the use of that space.
LISP, as a specific instance of the Loc/ID split, aims to decouple location and identity. This decoupling will facilitate improved aggregation of the RLOC space, implement persistent identity in the EID space, and, in some cases, increase the security and efficiency of network mobility.
Implementing the Locator/ID Separation
There are two basic approaches to implementing the Loc/ID split: map-and-encap and address rewriting. Each is briefly discussed in the following sections.
In the map-and-encap scheme (genenerally considered to have evolved from Bob Hinden's ENCAPS protocol ), when a source sends a packet to the EID of a destination outside of the source domain, the packet traverses the domain infrastructure to a border router (or other border element). The border router maps the destination EID to a RLOC that corresponds to an entry point in the destination domain (hence an EID-to-RLOC mapping system is needed; proposals are discussed later in the article). This phase is the "map" phase of map-and-encap. The border router then encapsulates the packet and sets the destinÃ‚Âation address to the RLOC returned by the mapping infrastructure (if any; it may be statically configured as well). This phase is the "encap" phase of the map-and-encap model.
Thus map-and-encap works by appending a new header to the existing packet; the "inner-header" source and destination addresses are EIDs, and the "outer-header" source and destination addresses are in most cases RLOCs. When an encapsulated packet arrives at the destination border router, the router decapsulates the packet and sends it on to its destination. Note that this process suggests that EIDs may need to be routable in some scope (likely scoped to the domain).
Map-and-encap schemes have the desirable property that they do not in general require host changes or changes to the core routing infrastructure. In addition, map-and-encap schemes work with both IPv4 and IPv6, and retain the original source address (a feature that is useful in various filtering scenarios). Controversy remains, however, as to whether or not the encapsulation overhead of map-and-encap schemes is problematic; opinions exist on both sides of this topic (see, for example, ).
The basic idea behind the address-rewriting schemes, originally proposed by Dave Clark and later by Mike O'Dell in his 8+8/GSE specification , is to take advantage of the 128-bit IPv6 address and use the top 64 bits as the routing locator ("Routing Goop," or RG), and the lower 64 bits as the endpoint identifier (hence rewriting works only for IPv6). In this scheme, when a host emits a packet destined for another domain, the source address contains its identifier (frequently a IEEE MAC address) in the lower 64 bits, and a special value (meaning unspecified) in the RG. The destination address contains the fully specified destination address (RG and EID).
When a packet destined for a remote domain arrives at the local domain egress router, the source RG is filled in (forming a full 128-bit address), and the packet is routed to the remote domain. On ingress to the remote domain, the destination RG is rewritten with the unspecified value, ensuring that the host does not know what its RG is.
This process, in theory, would enable the ease of renumbering that would be required to maintain congruence between prefix assignment and physical network topology that is required for the kind of "aggressive" renumbering envisioned in the 8+8/GSE specification.
The Locator/Identifier Separation Protocol (LISP)
LISP is designed to be a simple, incremental, network-based map-and-encap protocol that implements separation of Internet addresses into EIDs and RLOCs. Because LISP is a map-and-encap protocol, it requires no changes to host stacks and no major changes to existing database infrastructures. It is designed to be implemented in a relatively small number of routers. LISP is also an instance of what is architecturally called a "jack-up," because the existing network layer is "jacked up" and a new network layer is inserted below it (the term "jacked up" is attributed to Noel Chiappa). The LISP jack-up is depicted in Figure 1.
The LISP design aims to improve site multihoming (for example, by controlling site ingress without complex protocols), improve Internet Service Provider (ISP) multihoming, decouple site addressing from provider addressing, and reduce the size and dynamic properties of the core routing tables.
The LISP data plane (the map-and-encap operation) and the LISP control plane (the EID-to-RLOC mapping system) are very modular. In particular, although the base LISP specification defines the format of messages to query the mapping system and to receive responses from that system, it makes no assumptions on the architecture of potential mapping systems. As a result, several mapping systems have been proposed [0,1,4,5,6,10].
LISP Network Elements
The LISP specification defines two network elements: The Egress Tunnel Router (ETR) and the Ingress Tunnel Router (ITR).
A LISP Egress Tunnel Router (ETR) receives LISP-encapsulated IP packets from the Internet on one side and sends decapsulated IP packets to site end systems on the other side. In particular, an ETR accepts an IP packet where the destination address in the "outer" IP header is one of its own RLOCs. The router strips the "outer" header and forwards the packet based on the next IP header found.
A LISP Ingress Tunnel Router (ITR) accepts IP packets from site end systems on one side and sends LISP-encapsulated IP packets toward the Internet on the other side. In particular, an ITR accepts an IP packet with a single IP header (more precisely, an IP packet that does not contain a LISP header). The router treats this "inner" IP destination address as an EID and performs an EID-to-RLOC mapping lookup if necessary (that is, it does not already have an EID-to-RLOC mapping for the EID). The router then prepends an "outerÃ¢â‚¬?" IP header with one of its globally routable RLOCs in the Source Address field and the result of the mapping lookup in the Destination Address field. Note that this destination RLOC may be an intermediate, proxy device that has better knowledge of the EID-to-RLOC mapping closest to the destination EID.
LISP Data-Plane Operation
When a host in a LISP-capable domain emits a packet, it puts its EID in the packet source address, and EID of the correspondent host in its destination address (note that hosts will typically look up EIDs in the Domain Name System [DNS]). If the destination of the packet is in another domain, the packet traverses the source domain infrastructure to one of its ITRs. The ITR maps destination EID to a RLOC that corresponds to an ETR that is either in the destination domain or a proxy for the destination domain (how this mapping is accomplished in LISP is discussed later in the article). The ITR then encapsulates the packet, setting the destination address to the RLOC of the ETR returned by the mapping infrastructure or by static configuration. Note that LISP is address family-agnostic and as such can be used with both IPv4 and IPv6 (or any other address family). Figure 2 depicts the LISP IPv4 in IPv4 encapsulation.
When the packet arrives at the destination ETR, it decapsulates the packet and sends it on to its destination. Again, note that this scenario implies that EIDs need to be routable in some scope (likely scoped to the domain).
As mentioned previously, the LISP specification defines three packet types designed to support an EID-to-RLOC mapping system. The first type of packet, the Data Probe, is a data packet that an ITR may send into the mapping system to probe for the mapping; the authoritative ETR responds to the ITR with a Map-Reply message when it receives such a data packet. Note that in this case the ETR detects that the packet is a Data Probe by noticing that the inner Destination Address (DA) was copied to the outer DA by the ITR, that is, the inner DA equals the outer DA and is an EID. The second type of LISP packet used to support the mapping system is the Map Request. An ITR may query the mapping system by sending a Map-Request message into the mapping system to request a particular EID-to-RLOC mapping. As in the Data Probe case, the authoritative ETR responds with a Map-Reply message.
The third type of LISP packet used to support the mapping system is the Map Reply. An ETR emits a Map Reply under two conditions. First, if the ETR receives a LISP-encapsulated packet in which the outer-header destination address is the same as that of the inner header, it knows that the packet is a Data Probe and can respond with a Map Reply to the source ITR. The ETR may also receive a Map Request, in which case it replies to the requesting ITR with the mapping.
LISP Control Plane
Both map-and-encap and address-rewriting models rely on an additional of level of indirection in the addressing architecture to make the routing system scale reasonably. Because packets are sourced with an EID in the Destination Address field and EIDs are not in general routable on the global Internet, the destination EID must be mapped to an RLOC in order to deliver the packet to another domain (that is, across the Internet). In the case of the map-and-encap schemes, it is a direct translation: an EID is mapped to a RLOC. The situation is subtly different for the rewriting schemes; in general such schemes must look up the entire destination address (usually proposed to reside in the DNS) [11,13], but must somehow determine the source RG when rewriting the source address at the domain border.
In either Loc/ID split model, an EID-to-RLOC mapping service is needed to make the system scale reasonably and to make it operationally viable. There are three important scale parameters to consider when architecting a mapping service: the rate of updates to the mapping database, the state of the mapping service required, and the latency incurred during database lookup. The scaling properties of the database are frequently characterized as a (Rate × State) problem (ignoring for the moment the subject of lookup latency); because most estimates put the size of the mapping database at O(1010), the database update rate must be small (note that this situation is a primary reason that current mapping proposals do not incorporate reachability information into the mapping database). In addition, the choice of push vs. pull also affects latency: if you push the entire database close to the edge, you improve lookup latency at the cost of increased state; if you architect a service that requires a mapping request and you find an authoritative server for that mapping (that is, pull), you reduce state at the cost of increased lookup latency.
LISP-Alternative-Topology: A LISP Control Plane
The basic idea behind LISP-Alternative-Topology (LISP-ALT)  is to build an alternative logical topology for managing EID-to-RLOC mappings for LISP. This logical topology uses existing technology and tools, specifically the Border Gateway Protocol (BGP)  and its multiprotocol extension, along with the Generic Routing Encapsulation (GRE)  protocol to construct an overlay network of devices that advertise EID prefixes only.
As was the case for the LISP data plane, an important design goal of LISP-ALT is to minimize the number of changes to existing hardware and software that are required to deploy the mapping system. Therefore, LISP-ALT requires modifications to neither BGP nor GRE.
Note that LISP-ALT is a hybrid push/pull architecture. Aggregated EID prefixes are "pushed" among the LISP-ALT routers and, optionally, to ITRs (which may elect to receive the aggregated information, as opposed to simply using a default mapping). Specific EID-to-RLOC mappings are "pulled" by ITRs either by Map Requests or Data Probes, both of which are routed over the alternate topology and result in Map Replies being generated by ETRs.
The basic idea behind in LISP-ALT, then, is to use BGP running over a GRE overlay to build the reachability required to route Data Probes, Map Requests, and Map Replies over the alternate topology. The ALT Routing Information Base (RIB) comprises EID prefixes and associated next hops. The LISP-ALT routers talk External BGP (eBGP) to each other in order to propagate EID prefix update information, which is learned either over eBGP connections from the authoritative ETR or by configuration. ITRs may also eBGP peer with one or more LISP-ALT routers in order to route Data Probe packets or Map Requests
In summary, the LISP-ALT uses BGP to propagate EID-prefix reachability information used by ITRs and ETRs to forward Map Requests, Map Replies, and Data Probes. This reachability is carried as IPv4 or IPv6 Network Layer Reachability Information (NLRI) without modification (because the EID space has the same syntax as IPv4 or IPv6). LISP-ALT routers eBGP peer with one another, forming the overlay network. A LISP-ALT router near the edge learns EID prefixes that originate with authoritative ETRs. In general then, LISP-ALT routers aggregate EID prefixes, and forward Data Probes, Map-Requests, and Map-Replies.
Threat Models and Mitigation
As in any Loc/ID split approach, a critical operation is the creation of locator-to-ID binding state that devices will use over time. In the case of LISP, the critical operation is the creation of EID-to-RLOC mappings in the ITR and the ETR. We can obtain these mappings in three ways:
LISP mitigates attacks on the first two techniques by including a nonce in the LISP header; the nonce is a 32-bit randomly generated number (generated by the source ITR) that is used to test route returnability.
More specifically, an ETR echoes the nonce back to the ITR in a Map-Reply message. That is, the nonce, combined with the ITR accepting only solicited Map Replies, provides a base level of authentication for Map Replies. Note however, that these techniques do not protect against man-in-the-middle attacks.
The LISP design assumes that many (if not most) security mechanisms are part of the mapping database service when using control-plane procedures for obtaining EID-to-RLOC mappings. Denial-of-Service (DoS) attack prevention, on the other hand, depends on the ability of an impleÃ‚Âmentation to rate-limit Map Requests and Map Replies (in the control plane), as well as its ability to rate limit the number of data-triggered Map Replies (for example, in response to Data Probe packets).
Refer to  for a more detailed preliminary threat analysis for LISP.
LISP and Fast Endpoint Mobility
Fast endpoint mobility occurs when an endpoint moves relatively rapidly, changing its IP layer network attachment point, and maintenance of session continuity is a goal. Mobile IPv4  and Mobile IPv6 [21,22,27] mechanisms can be used in this case; note however, that the interaction of Mobile IP with LISP needs further exploration. Refer to the LISP specification  for additional details.
In summary, the major problem introduced by a Loc/ID split scheme is that as an endpoint moves, changes to the mapping between its EID and a set of RLOCs for its new network location may be required. When this change is added to the overhead of mobile IP binding updates, some packets might be delayed or dropped. In general, the problem is controlling the update rate (that is, the [Rate x State] product described previously), and is an area of ongoing research.
A multicast group address, as defined in the original Internet architecture, is an identifier of a grouping of topologically independent receiver host locations. The address encoding itself does not determine the location of the receiver(s). The multicast routing protocol and the network-based state the protocol creates determine the location of the receivers.
In the LISP context, a multicast group address is both an EID and a RLOC. As such, no specific action is necessary for destination addresses; a group address that appears in an inner IP header (built by a source host) is used as the destination EID by an ITR as a destination address when it LISP-encapsulates the packet (that is, the ITR uses the same group address as the destination RLOC).
The source RLOC, as is usually the case, is the ITR IP address (that is, one of its RLOCs).
At the receiving side, Protocol Independent Multicast (PIM)  has to translate the source-address Join/Prune messages from RLOCs to EIDs when multicast packets are forwarded by the ETR. However, in contrast to the unicast case (where a Map Request is sent by the ITR at forwarding time), a Map Request can be sent when the multicast tree is being built.
Putting It All Together: A Day in the Life of a LISP Packet
When a host in a LISP-capable domain wants to send a packet, it first looks up the correspondent host's EID in the DNS. It then puts its EID in the packet source address, and EID of the correspondent host in its destination address; if the destination of the packet is in another domain, the packet traverses the source domain infrastructure to one of the domain ITRs.
If the ITR has cached the EID-to-RLOC mapping for the destination EID, it sets the destination RLOC in the outer (encapsulated) header to the cached RLOC, and the source RLOC to its RLOC (note that the inner header has the source host's EID as the source and the destination's EID in the Destination field). The packet is then sent over the Internet to the ETR indicated in the destination RLOC, which decapsulates the packet and sends it on to the destination EID.
If, on the other hand, the ITR does not have a EID-to-RLOC mapping for the destination EID, it encapsulates the packet in a LISP header in which the destination address is the same as the inner header destination address, namely, the EID of the destination host. This packet is a Data Probe packet, and is routed over the LISP-ALT topology to the LISP-ALT router (typically an ETR, but this type of router is not required) that is authoritative for the EID-to-RLOC mapping. When the ETR receives the Data Probe packet, it decapsulates the packet and sends it on to the destination EID and sends a Map Reply to the source ITR so subsequent packets are sent natively over the Internet (as opposed to over the LISP-ALT overlay network). This query/response transaction is required only for the first packet sent between sites; all subsequent packets are sent LISP-encapsulated directly between the ITR and the ETR (and in particular, not over the LISP-ALT topology). Finally, note that the ITR could also preload its cache with mappings for popular destinations using the Map-Request message, avoiding the Data Probe packet (and associated latency, if any) altogether.
For example, consider the scenario depicted in Figure 3. In this case, a source S with EID 220.127.116.11 wants to send a packet to destination D whose EID is 18.104.22.168. The packet arrives at ITR S2, which does not have an EID-to-RLOC mapping for 22.214.171.124. S2 LISP-encapsulates the packet with the outer header having its RLOC (126.96.36.199) as the source address, copies the destination EID (188.8.131.52) from the inner header to the outer-header destination, and sends the data packet (a Data Probe) into the LISP-ALT topology. The packet follows the paths computed by BGP in the LISP-ALT topology to ETR D2. When D2 receives the packet, it decapsulates it and forwards the packet to the destination 184.108.40.206; D2 also responds with a Map-Reply message that tells S2 (220.127.116.11) that the EID-to-RLOC mapping for 18.104.22.168/8 has two elements, ETR D1 (whose RLOC is 22.214.171.124) and ETR D2 (whose RLOC is 126.96.36.199). After receiving the Map Reply, ITR S2 can send LISP-encapsulated packets natively over the Internet (that is, not over the ALT topology).
Note that the mapping has priority (p) and weight (w) attributes. Priorities tell the ITR which ETRs to use in which order, and weights tell the ITR how to split load across ETRs of a given priority (w is a percentage of traffic that should go to each ETR). In this case, both ETRs have the same priority (1), and have weight 50 (that is, each ETR should receive 50 percent of the traffic).
New Functions Enabled by the Mapping System
Weights and priorities provide new capabilities for multihomed sites, which can use these features to control how traffic ingressing to the site is spread across its links without the complexity and overhead of running BGP. In particular, a multihomed site can configure its mapping database so that its links are used in an "active-active' configuration (that is, both links are in use). This situation is depicted in Figure 3, where the mapping databases entry 188.8.131.52/8 has two ETRs at the same priority that are equally weighted, meaning that the ITR will spread flows equally among the two ETRs.
This function is particularly attractive for Small Office or Home Office (SOHO) sites that desire both redundancy in their Internet connections and the ability to easily load share across those links in an active-active configuration, without the complexity and operational expense of running BGP.
Another interesting functionality enabled by the LISP control plane is the ability to mitagate some types of DoS attacks. In particular, if an ETR notices that it the subject of a DoS attack from behind an ITR (that is, DoS packets are destined to an EID-prefix for which it is authorative), it can use the LISP locator reachability bits (see Figure 2) to tell the the source ITR that the RLOC for that EID-prefix is not available. The ETR accomplishes this by sending a locator-reachability bit of zero for the RLOC to the offending ITR. Note that this functionality is similar to Ioannidis and Bellovin's "ICMP Pushback" proposal .
LISP and its associated mapping protocol(s) have two primary performance concerns:
In the case of encapsulation overhead, the concern is that the addition of the LISP header will cause the encapsulate packet to exceed the path Maximum Transmission Unit (MTU). As mentioned previously, this area of research is still active (see, for example, ).
In the case of lookup latency and packet loss, because LISP-ALT uses BGP to find a particular EID-to-RLOC mapping, there could be latency associated with the first few packets in the first flow between sites (note that it is only the first flow; subsequent flows can use the mapping installed in the ITR). However, this latency is mitigated, and the initial packets are not lost because LISP can send the first few data packets over the control plane; these packets are the Data Probe packets. There is additional latency associated with the time required for the destination ETR to return the Map Reply. However, after this initial transaction is completed, no additional latency is injected by the mapping system.
As mentioned previously, there is a trade-off in the mapping system among the state required to be held by network elements, the rate of updates to the mapping system, and the latency incurred when looking up an EID-to-RLOC mapping. LISP-ALT is a hybrid (push/pull) architecture that attempts to minimize the state requirements on ITRs, while at the same time minimizing lookup latency.
LISP is a new protocol that implements the Loc/ID split using a map-and-encap protocol. It obtains the advantages of the level of indirection afforded by the Loc/ID split while minimizing changes to hosts and to the core routing system. In addition, LISP enables new functions such as BGP-free multihoming in an active-active configuration.
The LISP specification and supporting documents are the work of many people, including Scott Brim, Noel Chiappa, Dino Farinacci, Vince Fuller, Eliot Lear, Darrel Lewis, and Dave Oran.