| Network Working Group | L. Jakab |
| Internet-Draft | Cisco Systems |
| Intended status: Informational | A. Cabellos-Aparicio |
| Expires: January 10, 2014 | F. Coras |
| J. Domingo-Pascual | |
| Technical University of Catalonia | |
| D. Lewis | |
| Cisco Systems | |
| July 09, 2013 |
LISP Network Element Deployment Considerations
draft-ietf-lisp-deployment-09.txt
This document discusses the different scenarios for the deployment of the new network elements introduced by the Locator/Identifier Separation Protocol (LISP).
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The Locator/Identifier Separation Protocol (LISP) is designed to address the scaling issues of the global Internet routing system identified in [RFC4984] by separating the current addressing scheme into Endpoint IDentifiers (EIDs) and Routing LOCators (RLOCs). The main protocol specification [RFC6830] describes how the separation is achieved, which new network elements are introduced, and details the packet formats for the data and control planes.
LISP assumes that such separation is between the edge and core and uses mapping and encapsulation for forwarding. While the boundary between both is not strictly defined, one widely accepted definition places it at the border routers of stub autonomous systems, which may carry a partial or complete default-free zone (DFZ) routing table. The initial design of LISP took this location as a baseline for protocol development. However, the applications of LISP go beyond just decreasing the size of the DFZ routing table, and include improved multihoming and ingress traffic engineering (TE) support for edge networks, and even individual hosts. Throughout the document we will use the term LISP site to refer to these networks/hosts behind a LISP Tunnel Router. We formally define the following two terms:
Since LISP is a protocol which can be used for different purposes, it is important to identify possible deployment scenarios and the additional requirements they may impose on the protocol specification and other protocols. Additionally, this document is intended as a guide for the operational community for LISP deployments in their networks. It is expected to evolve as LISP deployment progresses, and the described scenarios are better understood or new scenarios are discovered.
Each subsection considers an element type, discussing the impact of deployment scenarios on the protocol specification. For definition of terms, please refer to the appropriate documents (as cited in the respective sections).
The device that is the gateway between the edge and the core is called a Tunnel Router (xTR), performing one or both of two separate functions:
The first function is performed by an Ingress Tunnel Router (ITR), the second by an Egress Tunnel Router (ETR).
Section 8 of the main LISP specification [RFC6830] has a short discussion of where Tunnel Routers can be deployed and some of the associated advantages and disadvantages. This section adds more detail to the scenarios presented there, and provides additional scenarios as well.
The first scenario we discuss is customer edge, when xTR functionality is placed on the router(s) that connect the LISP site to its upstream(s), but are under its control. As such, this is the most common expected scenario for xTRs, and this document considers it the reference location, comparing the other scenarios to this one.
ISP1 ISP2
| |
| |
+----+ +----+
+--|xTR1|--|xTR2|--+
| +----+ +----+ |
| |
| LISP site |
+------------------+
Figure 1: xTRs at the customer edge
From the LISP site perspective the main advantage of this type of deployment (compared to the one described in the next section) is having direct control over its ingress traffic engineering. This makes it easy to set up and maintain active/active, active/backup, or more complex TE policies, without involving third parties.
Being under the same administrative control, reachability information of all ETRs is easier to synchronize, because the necessary control traffic can be allowed between the locators of the ETRs. A correct synchronous global view of the reachability status is thus available, and the Locator Status Bits (Loc-Status-Bits, defined in [RFC6830]) can be set correctly in the LISP data header of outgoing packets.
By placing the tunnel router at the edge of the site, existing internal network configuration does not need to be modified. Firewall rules, router configurations and address assignments inside the LISP site remain unchanged. This helps with incremental deployment and allows a quick upgrade path to LISP. For larger sites with many external connections, distributed in geographically diverse points of presence (PoPs), and complex internal topology, it may however make more sense to both encapsulate and decapsulate as soon as possible, to benefit from the information in the IGP to choose the best path (see Section 2.3 for a discussion of this scenario).
Another thing to consider when placing tunnel routers is MTU issues. Encapsulation increases the amount of overhead associated with each packet. This added overhead decreases the effective end-to-end path MTU (unless fragmentation and reassembly is used). Some transit networks are known to provide larger MTU than the typical value of 1500 bytes of popular access technologies used at end hosts (e.g., IEEE 802.3 and 802.11). However, placing the LISP router connecting to such a network at the customer edge could possibly bring up MTU issues, depending on the link type to the provider as opposed to the following scenario. See [RFC4459] for MTU considerations of tunneling protocols on how to mitigate potential issues. Still, even with these mitigations, path MTU issues are still possible.
The other location at the core-edge boundary for deploying LISP routers is at the Internet service provider edge. The main incentive for this case is that the customer does not have to upgrade the CE router(s), or change the configuration of any equipment. Encapsulation/decapsulation happens in the provider's network, which may be able to serve several customers with a single device. For large ISPs with many residential/business customers asking for LISP this can lead to important savings, since there is no need to upgrade the software (or hardware, if it's the case) at each client's location. Instead, they can upgrade the software (or hardware) on a few PE routers serving the customers. This scenario is depicted in Figure 2.
+----------+ +------------------+
| ISP1 | | ISP2 |
| | | |
| +----+ | | +----+ +----+ |
+--|xTR1|--+ +--|xTR2|--|xTR3|--+
+----+ +----+ +----+
| | |
| | |
+--<[LISP site]>---+-------+
Figure 2: xTR at the PE
While this approach can make transition easy for customers and may be cheaper for providers, the LISP site loses one of the main benefits of LISP: ingress traffic engineering. Since the provider controls the ETRs, additional complexity would be needed to allow customers to modify their mapping entries.
The problem is aggravated when the LISP site is multihomed. Consider the scenario in Figure 2: whenever a change to TE policies is required, the customer contacts both ISP1 and ISP2 to make the necessary changes on the routers (if they provide this possibility). It is however unlikely, that both ISPs will apply changes simultaneously, which may lead to inconsistent state for the mappings of the LISP site. Since the different upstream ISPs are usually competing business entities, the ETRs may even be configured to compete, either to attract all the traffic or to get no traffic. The former will happen if the customer pays per volume, the latter if the connectivity has a fixed price. A solution could be to have the mappings in the Map Server(s), and have their operator give control over the entries to customer, much like in the Domain Name System at the time of this writing.
Additionally, since xTR1, xTR2, and xTR3 are in different administrative domains, locator reachability information is unlikely to be exchanged among them, making it difficult to set Loc-Status-Bits (LSB) correctly on encapsulated packets. Because of this, and due to the security concerns about LSB described in [I-D.ietf-lisp-threats] their use is discouraged without verifying ETR reachability through the mapping system or other means. Mapping versioning is another alternative [RFC6834].
Compared to the customer edge scenario, deploying LISP at the provider edge might have the advantage of diminishing potential MTU issues, because the tunnel router is closer to the core, where links typically have higher MTUs than edge network links.
In a simple LISP deployment, xTRs are located at the border of the LISP site (see Section 2.1). In this scenario packets are routed inside the domain according to the EID. However, more complex networks may want to route packets according to the destination RLOC. This would enable them to choose the best egress point.
The LISP specification separates the ITR and ETR functionality and allows both entities to be deployed in separated network equipment. ITRs can be deployed closer to the host (i.e., access routers). This way packets are encapsulated as soon as possible, and egress point selection is driven by operational policy. In turn, ETRs can be deployed at the border routers of the network, and packets are decapsulated as soon as possible. Once decapsulated, packets are routed based on destination EID, according to internal routing policy.
In the following figure we can see an example. The Source (S) transmits packets using its EID and in this particular case packets are encapsulated at ITR_1. The encapsulated packets are routed inside the domain according to the destination RLOC, and can egress the network through the best point (i.e., closer to the RLOC's AS). On the other hand, inbound packets are received by ETR_1 which decapsulates them. Then packets are routed towards S according to the EID, again following the best path.
+---------------------------------------+
| |
| +-------+ +-------+ +-------+
| | ITR_1 |---------+ | ETR_1 |-RLOC_A--| ISP_A |
| +-------+ | +-------+ +-------+
| +-+ | | |
| |S| | IGP | |
| +-+ | | |
| +-------+ | +-------+ +-------+
| | ITR_2 |---------+ | ETR_2 |-RLOC_B--| ISP_B |
| +-------+ +-------+ +-------+
| |
+---------------------------------------+
Figure 3: Split ITR/ETR Scenario
This scenario has a set of implications:
At the time of this writing, if two ISPs want to control their ingress TE policies for transit traffic between them, they need to rely on existing BGP mechanisms. This typically means deaggregating prefixes to choose on which upstream link packets should enter. This is either not feasible (if fine-grained per-customer control is required, the very specific prefixes may not be propagated) or increases DFZ table size.
Typically, LISP is seen applicable only to stub networks, however the LISP protocol can be also applied in a recursive manner, providing service provider ingress/egress TE capabilities without impacting the DFZ table size.
In order to implement this functionality with LISP consider the scenario depicted in Figure 4. The two ISPs willing to achieve ingress/egress TE are labeled as ISP_A and ISP_B, they are servicing Stub1 and Stub2 respectively, both are required to be LISP sites. In this scenario we assume that Stub1 and Stub2 are communicating and thus, ISP_A and ISP_B offer transit for such communications. ISP_A has RLOC_A1 and RLOC_A2 as upstream IP addresses while ISP_B has RLOC_B1 and RLOC_B2. The shared goal among ISP_A and ISP_B is to control the transit traffic flow between RLOC_A1/A2 and RLOC_B1/B2.
_.--.
Stub1 ... +-------+ ,-'' `--. +-------+ ... Stub2
\ | R_A1|----,' `. ---|R_B1 | /
--| | ( Transit ) | |--
... .../ | R_A2|-----. ,' ---|R_B2 | \... ...
+-------+ `--. _.-' +-------+
... ... ISP_A `--'' ISP_B ... ...
Figure 4: Inter-Service provider TE scenario
Both ISPs deploy xTRs on on RLOC_A1/A2 and RLOC_B1/B2 respectively and reach a bilateral agreement to deploy their own private mapping system. This mapping system contains bindings between the RLOCs of Stub1 and Stub2 (owned by ISP_A and ISP_B respectively) and RLOC_A1/A2 and RLOC_B1/B2. Such bindings are in fact the TE policies between both ISPs and the convergence time is expected to be fast, since ISPs only have to update/query a mapping to/from the database.
The packet flow is as follows. First, a packet originated at Stub1 towards Stub2 is LISP encapsulated by Stub1's xTR. The xTR of ISP_A reencapsulates it and, according to the TE policies stored in the private mapping system, the ISP_A xTR chooses RLOC_B1 or RLOC_B2 as the reencapsulation destination. Note that the packet transits between ISP_A and ISP_B double-encapsulated. Upon reception at the xTR of ISP_B the packet is decapsulated and sent towards Stub2 which performs the last decapsulation.
This deployment scenario, which uses recursive LISP, includes two important caveats. First, it is intended to be deployed between only two ISPs. If more than two ISPs use this approach, then the xTRs deployed at the participating ISPs must either query multiple mapping systems, or the ISPs must agree on a common shared mapping system. Furthemore, keeping this deployment scenario restricted to only two ISPs maintains the solution scalable, given that only two entities need to agree on using recursive LISP, and only one private mapping system is involved.
Second, the scenario is only recommended for ISPs providing connectivity to LISP sites, such that source RLOCs of packets to be reencapsulated belong to said ISP. Otherwise the participating ISPs must register prefixes they do not own in the above mentioned private mapping system. Failure to follow these recommendations may lead to operational and security issues when deploying this scenario.
Besides these recommendations, the main disadvantages of this deployment case are:
NAT in this section refers to IPv4 network address and port translation.
Packets encapsulated by an ITR are just UDP packets from a NAT device's point of view, and they are handled like any UDP packet, there are no additional requirements for LISP data packets.
Map-Requests sent by an ITR, which create the state in the NAT table, have a different 5-tuple in the IP header than the Map-Reply generated by the authoritative ETR. Since the source address of this packet is different from the destination address of the request packet, no state will be matched in the NAT table and the packet will be dropped. To avoid this, the NAT device has to do the following:
This setup supports only a single ITR behind the NAT device.
An ETR placed behind NAT is reachable from the outside by the Internet-facing locator of the NAT device. It needs to know this locator (and configure a loopback interface with it), so that it can use it in Map-Reply and Map-Register messages. Thus support for dynamic locators for the mapping database is needed in LISP equipment.
Again, only one ETR behind the NAT device is supported.
An implication of the issues described above is that LISP sites with xTRs can not be behind carrier based NATs, since two different sites would collide on the port forwarding. An alternative to static hole-punching to explore is the use of the Port Control Protocol (PCP) [RFC6887].
The following table gives a quick overview of the features supported by each of the deployment scenarios discussed above (marked with an "x") in the appropriate column: "CE" for customer edge, "PE" for provider edge, "Split" for split ITR/ETR, and "Recursive" for inter-service provider traffic engineering. The discussed features include:
Feature CE PE Split Recursive
-------------------------------------------------------------
Control of ingress TE x - x x
No modifications to existing
int. network infrastructure x x - -
Loc-Status-Bits sync x - x x
MTU/PMTUD issues minimized - x - -
Map Resolvers and Map Servers make up the LISP mapping system and provide a means to find authoritative EID-to-RLOC mapping information, conforming to [RFC6833]. They are meant to be deployed in RLOC space, and their operation behind NAT is not supported.
The Map Server learns EID-to-RLOC mapping entries from an authoritative source and publishes them in the distributed mapping database. These entries are learned through authenticated Map-Register messages sent by authoritative ETRs. Also, upon reception of a Map-Request, the Map Server verifies that the destination EID matches an EID-prefix for which it is authoritative for, and then re-encapsulates and forwards it to a matching ETR. Map Server functionality is described in detail in [RFC6833].
The Map Server is provided by a Mapping Service Provider (MSP). The MSP participates in the global distributed mapping database infrastructure, by setting up connections to other participants, according to the specific mapping system that is employed (e.g., ALT [RFC6836], DDT [I-D.ietf-lisp-ddt]). Participation in the mapping database, and the storing of EID-to-RLOC mapping data is subject to the policies of the "root" operators, who should check ownership rights for the EID prefixes stored in the database by participants. These policies are out of the scope of this document.
In all cases, the MSP configures its Map Server(s) to publish the prefixes of its clients in the distributed mapping database and start encapsulating and forwarding Map-Requests to the ETRs of the AS. These ETRs register their prefix(es) with the Map Server(s) through periodic authenticated Map-Register messages. In this context, for some LISP sites, there is a need for mechanisms to:
The Map Server plays a key role in the reachability of the EID-prefixes it is serving. On the one hand it is publishing these prefixes into the distributed mapping database and on the other hand it is encapsulating and forwarding Map-Requests to the authoritative ETRs of these prefixes. ITRs encapsulating towards EIDs under the responsibility of a failed Map Server will be unable to look up any of their covering prefixes. The only exception are the ITRs that already contain the mappings in their local cache. In this case ITRs can reach ETRs until the entry expires (typically 24 hours). For this reason, redundant Map Server deployments are desirable. A set of Map Servers providing high-availability service to the same set of prefixes is called a redundancy group. ETRs are configured to send Map-Register messages to all Map Servers in the redundancy group. The configuration for fail-over (or load-balancing, if desired) among the members of the group depends on the technology behind the mapping system being deployed. Since ALT is based on BGP and DDT was inspired from DNS, deployments can leverage current industry best practices for redundancy in BGP and DNS. These best practices are out of the scope of this document.
Additionally, if a Map Server has no reachability for any ETR serving a given EID block, it should not originate that block into the mapping system.
A Map Resolver is a network infrastructure component which accepts LISP encapsulated Map-Requests, typically from an ITR, and finds the appropriate EID-to-RLOC mapping by either consulting its local cache or by consulting the distributed mapping database. Map Resolver functionality is described in detail in [RFC6833].
Anyone with access to the distributed mapping database can set up a Map Resolver and provide EID-to-RLOC mapping lookup service. Database access setup is mapping system specific.
For performance reasons, it is recommended that LISP sites use Map Resolvers that are topologically close to their ITRs. ISPs supporting LISP will provide this service to their customers, possibly restricting access to their user base. LISP sites not in this position can use open access Map Resolvers, if available. However, regardless of the availability of open access resolvers, the MSP providing the Map Server(s) for a LISP site should also make available Map Resolver(s) for the use of that site.
In medium to large-size ASes, ITRs must be configured with the RLOC of a Map Resolver, operation which can be done manually. However, in Small Office Home Office (SOHO) scenarios a mechanism for autoconfiguration should be provided.
One solution to avoid manual configuration in LISP sites of any size is the use of anycast RLOCs [RFC4786] for Map Resolvers similar to the DNS root server infrastructure. Since LISP uses UDP encapsulation, the use of anycast would not affect reliability. LISP routers are then shipped with a preconfigured list of well know Map Resolver RLOCs, which can be edited by the network administrator, if needed.
The use of anycast also helps improve mapping lookup performance. Large MSPs can increase the number and geographical diversity of their Map Resolver infrastructure, using a single anycasted RLOC. Once LISP deployment is advanced enough, very large content providers may also be interested running this kind of setup, to ensure minimal connection setup latency for those connecting to their network from LISP sites.
While Map Servers and Map Resolvers implement different functionalities within the LISP mapping system, they can coexist on the same device. For example, MSPs offering both services, can deploy a single Map Resolver/Map Server in each PoP where they have a presence.
Proxy Ingress Tunnel Routers (P-ITRs) are part of the non-LISP/LISP transition mechanism, allowing non-LISP sites to reach LISP sites. They announce via BGP certain EID prefixes (aggregated, whenever possible) to attract traffic from non-LISP sites towards EIDs in the covered range. They do the mapping system lookup, and encapsulate received packets towards the appropriate ETR. Note that for the reverse path LISP sites can reach non-LISP sites simply by not encapsulating traffic. See [RFC6832] for a detailed description of P-ITR functionality.
The success of new protocols depends greatly on their ability to maintain backwards compatibility and inter-operate with the protocol(s) they intend to enhance or replace, and on the incentives to deploy the necessary new software or equipment. A LISP site needs an interworking mechanism to be reachable from non-LISP sites. A P-ITR can fulfill this role, enabling early adopters to see the benefits of LISP, similar to tunnel brokers helping the transition from IPv4 to IPv6. A site benefits from new LISP functionality (proportionally with existing global LISP deployment) when going LISP, so it has the incentives to deploy the necessary tunnel routers. In order to be reachable from non-LISP sites it has two options: keep announcing its prefix(es) with BGP, or have a P-ITR announce prefix(es) covering them.
If the goal of reducing the DFZ routing table size is to be reached, the second option is preferred. Moreover, the second option allows LISP-based ingress traffic engineering from all sites. However, the placement of P-ITRs significantly influences performance and deployment incentives. Section 5 is dedicated to the migration to a LISP-enabled Internet, and includes deployment scenarios for P-ITRs.
In contrast to P-ITRs, P-ETRs are not required for the correct functioning of all LISP sites. There are two cases, where they can be of great help:
In the first case, uRPF filtering is applied at their upstream PE router. When forwarding traffic to non-LISP sites, an ITR does not encapsulate packets, leaving the original IP headers intact. As a result, packets will have EIDs in their source address. Since we are discussing the transition period, we can assume that a prefix covering the EIDs belonging to the LISP site is advertised to the global routing tables by a P-ITR, and the PE router has a route towards it. However, the next hop will not be on the interface towards the CE router, so non-encapsulated packets will fail uRPF checks.
To avoid this filtering, the affected ITR encapsulates packets towards the locator of the P-ETR for non-LISP destinations. Now the source address of the packets, as seen by the PE router is the ITR's locator, which will not fail the uRPF check. The P-ETR then decapsulates and forwards the packets.
The second use case is IPv4-to-IPv6 transition. Service providers using older access network hardware, which only supports IPv4 can still offer IPv6 to their clients, by providing a CPE device running LISP, and P-ETR(s) for accessing IPv6-only non-LISP sites and LISP sites, with IPv6-only locators. Packets originating from the client LISP site for these destinations would be encapsulated towards the P-ETR's IPv4 locator. The P-ETR is in a native IPv6 network, decapsulating and forwarding packets. For non-LISP destination, the packet travels natively from the P-ETR. For LISP destinations with IPv6-only locators, the packet will go through a P-ITR, in order to reach its destination.
For more details on P-ETRs see the [RFC6832] draft.
P-ETRs can be deployed by ISPs wishing to offer value-added services to their customers. As is the case with P-ITRs, P-ETRs too may introduce path stretch (the ratio between the cost of the selected path and that of the optimal path). Because of this the ISP needs to consider the tradeoff of using several devices, close to the customers, to minimize it, or few devices, farther away from the customers, minimizing cost instead.
Since the deployment incentives for P-ITRs and P-ETRs are different, it is likely they will be deployed in separate devices, except for the CDN case, which may deploy both in a single device.
In all cases, the existence of a P-ETR involves another step in the configuration of a LISP router. CPE routers, which are typically configured by DHCP, stand to benefit most from P-ETRs. Autoconfiguration of the P-ETR locator could be achieved by a DHCP option, or adding a P-ETR field to either Map-Notifys or Map-Replies.
This section discusses a deployment architecture to support the migration to a LISP-enabled Internet. The loosely defined terms of "early transition phase", "late transition phase", and "LISP Internet phase" refer to time periods when LISP sites are a minority, a majority, or represent all edge networks respectively.
For sites wishing to go LISP with their PI prefix the least disruptive way is to upgrade their border routers to support LISP, register the prefix into the LISP mapping system, but keep announcing it with BGP as well. This way LISP sites will reach them over LISP, while legacy sites will be unaffected by the change. The main disadvantage of this approach is that no decrease in the DFZ routing table size is achieved. Still, just increasing the number of LISP sites is an important gain, as an increasing LISP/non-LISP site ratio may decrease the need for BGP-based traffic engineering that leads to prefix deaggregation. That, in turn, may lead to a decrease in the DFZ size and churn in the late transition phase.
This scenario is not limited to sites that already have their prefixes announced with BGP. Newly allocated EID blocks could follow this strategy as well during the early LISP deployment phase, depending on the cost/benefit analysis of the individual networks. Since this leads to an increase in the DFZ size, the following architecture should be preferred for new allocations.
In addition to publishing their clients' registered prefixes in the mapping system, MSPs with enough transit capacity can offer them P-ITR service as a separate service. This service is especially useful for new PI allocations, to sites without existing BGP infrastructure, that wish to avoid BGP altogether. The MSP announces the prefix into the DFZ, and the client benefits from ingress traffic engineering without prefix deaggregation. The downside of this scenario is adding path stretch.
Routing all non-LISP ingress traffic through a third party which is not one of its ISPs is only feasible for sites with modest amounts of traffic (like those using the IPv6 tunnel broker services today), especially in the first stage of the transition to LISP, with a significant number of legacy sites. This is because the handling of said traffic is likely to result in additional costs, which would be passed down to the client. When the LISP/non-LISP site ratio becomes high enough, this approach can prove increasingly attractive.
Compared to LISP+BGP, this approach avoids DFZ bloat caused by prefix deaggregation for traffic engineering purposes, resulting in slower routing table increase in the case of new allocations and potential decrease for existing ones. Moreover, MSPs serving different clients with adjacent aggregatable prefixes may lead to additional decrease, but quantifying this decrease is subject to future research study.
Instead of a LISP site, or the MSP, announcing their EIDs with BGP to the DFZ, this function can be outsourced to a third party, a P-ITR Service Provider (PSP). This will result in a decrease of the operational complexity both at the site and at the MSP.
The PSP manages a set of distributed P-ITR(s) that will advertise the corresponding EID prefixes through BGP to the DFZ. These P-ITR(s) will then encapsulate the traffic they receive for those EIDs towards the RLOCs of the LISP site, ensuring their reachability from non-LISP sites.
While it is possible for a PSP to manually configure each client's EID routes to be announced, this approach offers little flexibility and is not scalable. This section presents a scalable architecture that offers automatic distribution of EID routes to LISP sites and service providers.
The architecture requires no modification to existing LISP network elements, but it introduces a new (conceptual) network element, the EID Route Server, defined as a router that either propagates routes learned from other EID Route Servers, or it originates EID Routes. The EID-Routes that it originates are those that it is authoritative for. It propagates these routes to Proxy-ITRs within the AS of the EID Route Server. It is worth to note that a BGP capable router can be also considered as an EID Route Server.
Further, an EID-Route is defined as a prefix originated via the Route Server of the mapping service provider, which should be aggregated if the MSP has multiple customers inside a single large continuous prefix. This prefix is propagated to other P-ITRs both within the MSP and to other P-ITR operators it peers with. EID Route Servers are operated either by the LISP site, MSPs or PSPs, and they may be collocated with a Map Server or P-ITR, but are a functionally discrete entity. They distribute EID-Routes, using BGP, to other domains, according to policies set by participants.
MSP (AS64500)
RS ---> P-ITR
| /
| _.--./
,-'' /`--.
LISP site ---,' | v `.
( | DFZ )----- Mapping system
non-LISP site ----. | ^ ,'
`--. / _.-'
| `--''
v /
P-ITR
PSP (AS64501)
Figure 5: The P-ITR Route Distribution architecture
The architecture described above decouples EID origination from route propagation, with the following benefits:
In the example in the figure we have a MSP providing services to the LISP site. The LISP site does not run BGP, and gets an EID allocation directly from a RIR, or from the MSP, who may be a LIR. Existing PI allocations can be migrated as well. The MSP ensures the presence of the prefix in the mapping system, and runs an EID Route Server to distribute it to P-ITR service providers. Since the LISP site does not run BGP, the prefix will be originated with the AS number of the MSP.
In the simple case depicted in Figure 5 the EID-Route of LISP site will be originated by the Route Server, and announced to the DFZ by the PSP's P-ITRs with AS path 64501 64500. From that point on, the usual BGP dynamics apply. This way, routes announced by P-ITR are still originated by the authoritative Route Server. Note that the peering relationships between MSP/PSPs and those in the underlying forwarding plane may not be congruent, making the AS path to a P-ITR shorter than it is in reality.
The non-LISP site will select the best path towards the EID-prefix, according to its local BGP policies. Since AS-path length is usually an important metric for selecting paths, a careful placement of P-ITR could significantly reduce path-stretch between LISP and non-LISP sites.
The architecture allows for flexible policies between MSP/PSPs. Consider the EID Route Server networks as control plane overlays, facilitating the implementation of policies necessary to reflect the business relationships between participants. The results are then injected to the common underlying forwarding plane. For example, some MSP/PSPs may agree to exchange EID-Prefixes and only announce them to each of their forwarding plane customers. Global reachability of an EID-prefix depends on the MSP the LISP site buys service from, and is also subject to agreement between the mentioned parties.
In terms of impact on the DFZ, this architecture results in a slower routing table increase for new allocations, since traffic engineering will be done at the LISP level. For existing allocations migrating to LISP, the DFZ may decrease since MSPs may be able to aggregate the prefixes announced.
Compared to LISP+BGP, this approach avoids DFZ bloat caused by prefix deaggregation for traffic engineering purposes, resulting in slower routing table increase in the case of new allocations and potential decrease for existing ones. Moreover, MSPs serving different clients with adjacent aggregatable prefixes may lead to additional decrease, but quantifying this decrease is subject to future research study.
The flexibility and scalability of this architecture does not come without a cost however: A PSP operator has to establish either transit or peering relationships to improve their connectivity.
Phase | LISP+BGP | MSP P-ITR | PITR-RD
-----------------+--------------+-----------------+----------------
Early transition | no change | slower increase | slower increase
Late transition | may decrease | slower increase | slower increase
LISP Internet | considerable decrease
The following table presents the expected effects of the different transition scenarios during a certain phase on the DFZ routing table size:
It is expected that PITR-RD will co-exist with LISP+BGP during the migration, with the latter being more popular in the early transition phase. As the transition progresses and the MSP P-ITR and PITR-RD ecosystem gets more ubiquitous, LISP+BGP should become less attractive, slowing down the increase of the number of routes in the DFZ.
Note that throughout Section 5 we focused on the effects of LISP deployment on the DFZ route table size. Other metrics may be impacted as well, but to the best of our knowlegde have not been measured as of yet.
Security implications of LISP deployments are to be discussed in separate documents. [I-D.ietf-lisp-threats] gives an overview of LISP threat models, while securing mapping lookups is discussed in [I-D.ietf-lisp-sec].
This memo includes no request to IANA.
Many thanks to Margaret Wasserman for her contribution to the IETF76 presentation that kickstarted this work. The authors would also like to thank Damien Saucez, Luigi Iannone, Joel Halpern, Vince Fuller, Dino Farinacci, Terry Manderson, Noel Chiappa, Hannu Flinck, Paul Vinciguerra, Fred Templin, Brian Haberman, and everyone else who provided input.
| [RFC6830] | Farinacci, D., Fuller, V., Meyer, D. and D. Lewis, "The Locator/ID Separation Protocol (LISP)", RFC 6830, January 2013. |
| [RFC6832] | Lewis, D., Meyer, D., Farinacci, D. and V. Fuller, "Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites", RFC 6832, January 2013. |
| [RFC6833] | Fuller, V. and D. Farinacci, "Locator/ID Separation Protocol (LISP) Map-Server Interface", RFC 6833, January 2013. |
To help the operational community deploy LISP, this informative section offers a step-by-step guide for migrating a BGP based Internet presence to a LISP site. It includes a pre-install/pre-turn-up checklist, and customer and provider activation procedures.
LISP, like any tunneling protocol, will increase the size of packets when the LISP header is appended. If increasing the MTU of the access links is not possible, care must be taken that ICMP is not being filtered in order to allow for Path MTU Discovery to take place.