LISP Network Element Deployment Considerations
draft-jakab-lisp-deployment-00.txt

Abstract

This document discusses the different scenarios in which the LISP protocol may be deployed. Changes or extensions to other protocols needed by some of the scenarios are also highlighted.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on April 1, 2011.

Copyright Notice

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Table of Contents

1.  Introduction
2.  Tunnel Routers
    2.1.  Customer Edge
    2.2.  Provider Edge
    2.3.  Split ITR/ETR
    2.4.  Inter-Service Provider Traffic Engineering
    2.5.  Tunnel Routers behind NAT
        2.5.1.  ITR
        2.5.2.  ETR
    2.6.  Summary and Feature Matrix
3.  Proxy Tunnel Routers
    3.1.  P-ITR
        3.1.1.  EID registrar P-ITR services
        3.1.2.  LISP+BGP
        3.1.3.  Content Delivery Network P-ITR services
        3.1.4.  Content Delivery Network load balancing
    3.2.  P-ETR
4.  Map-Resolvers and Map-Servers
    4.1.  Map-Resolvers
    4.2.  Map-Servers
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgements
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Authors' Addresses

1.  Introduction

The Locator/Identifier Separation Protocol addresses the scaling issues of the global Internet routing system by separating the current addressing scheme into Endpoint IDentifiers (EIDs) and Routing LOCators (RLOCs). The main protocol specification [I‑D.ietf‑lisp] (Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, “Locator/ID Separation Protocol (LISP),” April 2010.) describes how the separation is achieved, which new network elements are introduced, and details the packet formats for the data and control planes.

While the boundary between the core and edge 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 of 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 draft we will use the term LISP site to refer to these networks/hosts behind a LISP Tunnel Router. We formally define it as:

LISP site:

A single host or a set of network elements in an edge network under the administrative control of a single organization, delimited from other network by LISP Tunnel Router(s).

Since LISP is a protocol which can be used for different purposes, it is important to identify possible deployment scenarios and the additional requierements they may impose on the protocol specification. The main specification [I‑D.ietf‑lisp] (Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, “Locator/ID Separation Protocol (LISP),” April 2010.) mentions positioning of tunnel routers, but without an in-depth discussion. This document fills that gap, by exploring the most common cases. While the theoretical combinations of device placements are quite numerous, the more practical scenarios are given preference in the following.

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).

Comments and discussions about this memo should be directed to the LISP working group: lisp@ietf.org.

2.  Tunnel Routers

LISP is a map-and-encap protocol, with the main goal of improving global routing scalability. To achieve its goal, it introduces several new network elements, each performing specific functions necessary to separate the edge from the core. The device that is the gateway between the edge and the core is called Tunnel Router (xTR), performing one or both of two separate functions:

1. Encapsulating packets originating from an end host to be transported over intermediary (transit) networks towards the other end-point of the communication
2. Decapsulating packets entering from intermediary (transit) networks, originated at a remote end host.

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 [I‑D.ietf‑lisp] (Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, “Locator/ID Separation Protocol (LISP),” April 2010.) 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.

2.1.  Customer Edge

As mentioned in the Introduction, LISP was designed with deployment at the core-edge boundary in mind, which can be approximated as the set of DFZ routers belonging to non-transit ASes. For the purposes of this document, we will consider this boundary to be consisting of the routers connecting LISP sites to their upstreams. 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|--+
|  +----+  +----+  |
|                  |
|     Customer     |
+------------------+

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 is 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 can be easily synchronized, 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 Loc-Status-Bits can be set correctly in the LISP data header of outgoing packets.

By choosing the encapsulate last, decapsulate first policy, 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 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 (Split ITR/ETR) for a discussion of this scenario).

Another thing to consider when placing tunnel routers are MTU issues. Since encapsulating packets increases overhead, the MTU of the end-to-end path may decrease, when encapsulated packets need to travel over segments having close to minimum MTU. 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 at the CE could possibly bring up MTU issues, depending on the link type to the provider.

2.2.  Provider Edge

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 (xTR at the PE).

+----------+        +------------------+
|   ISP1   |        |       ISP2       |
|          |        |                  |
|  +----+  |        |  +----+  +----+  |
+--|xTR1|--+        +--|xTR2|--|xTR3|--+
   +----+              +----+  +----+
      |                  |       |
      |                  |       |
      +--<[Customer]>----+-------+

While this approach can make transition easy for customers and may be cheaper for providers, the LISP site looses 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 (xTR at the PE): 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 simultanously, which may lead to unconsistent state for the mappings of the LISP site (e.g., weights for the same priority don't sum 100). 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 today's DNS.

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 correctly on encapsulated packets.

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.

2.3.  Split ITR/ETR

In a simple LISP deployment, xTRs are located at the border of the LISP site (see Section 2.1 (Customer Edge)). 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 considers that both entities can 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 packets exit the network through the best egress point. In turn, ETRs can be deployed at the border routers of the network, and packets are decapsulated as soon as possible. Again, once decapsulated packets are routed according to the EID, and can follow the best path.

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_B--| ISP_A |
|       +-------+         |         +-------+         +-------+
|  +-+        |           |             |
|  |S|        |    IGP    |             |
|  +-+        |           |             |
|       +-------+         |         +-------+         +-------+
|       | ITR_2 |---------+         | ETR_2 |-RLOC_B--| ISP_B |
|       +-------+                   +-------+         +-------+
|                                       |
+---------------------------------------+

This scenario has a set of implications:

• The site must carry at least partial BGP routes in order to choose the best egress point, increasing the complexity of the network. However, this is usually already the case for LISP sites that would benefit from this scenario.
• In LISP, ITRs set the reachability bits when encapsulating data packets. Hence, ITRs need a mechanism to be aware of the liveness of ETRs.
• ITRs encapsulate packets and in order to achieve efficient communications, the MTU of the site must be large enough to accommodate this extra header.
• In this scenario, each ITR is serving fewer hosts than in the case when it is deployed at the border of the network. It has been shown that cache hit ratio grows logarithmically with the amount of users [cache] (Jung, J., Sit, E., Balakrishnan, H., and R. Morris, “DNS performance and the effectiveness of caching,” 2002.). Taking this into account, when ITRs are deployed closer to the host the effectiveness of the mapping cache may be lower (i.e., the miss ratio is higher). Another consequence of this is that the site will transmit a higher amount of Map-Requests, increasing the load on the distributed mapping database. This could be solved by synchronizing ITR caches, via IGP or multicast.

2.4.  Inter-Service Provider Traffic Engineering

With LISP, two LISP sites can route packets among them and control their ingress TE policies. Typically, LISP is seen as applicable to stub networks, however the LISP protocol can also be applied to transit networks recursively.

Consider the scenario depicted in Figure 4 (Inter-Service provider TE scenario). Packets originating from the LISP site Stub1, client of ISP_A, with destination Stub4, client of ISP_B, are LISP encapsulated at their entry point into the ISP_A's network. The external IP header now has as the source RLOC an IP from ISP_A's address space and destination RLOC from ISP_B's address space. One or more ASes separate ISP_A from ISP_B. With a single level of LISP encapsulation, Stub4 has control over its ingress traffic. However, ISP_B only has the current tools (such as BGP prefix deaggregation) to control on which of his own upstream or peering links should packets 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.

                              _.--.
Stub1 ...   +-------+      ,-''     `--.      +-------+   ... Stub3
         \  |   R_A1|----,'             `. ---|R_B1   |  /
          --|   R_A2|---(     Transit     )   |       |--
Stub2 .../  |   R_A3|-----.             ,' ---|R_B2   |  \... Stub4
            +-------+      `--.     _.-'      +-------+
      ...     ISP_A            `--''            ISP_B     ...

A solution for this is to apply LISP recursively. ISP_A and ISP_B may reach a bilateral agreement to deploy their own private mapping system. ISP_A then encapsulates packets destined for the prefixes of ISP_B, which are listed in the shared mapping system. Note that in this case the packet is double-encapsulated. ISP_B's ETR removes the outer, second layer of LISP encapsulation from the incoming packet, and routes it towards the original RLOC, the ETR of Stub4, which does the final decapsulation.

If ISP_A and ISP_B agree to share a private distributed mapping database, both can control their ingress TE without the need of disaggregating prefixes. In this scenario the private database contains RLOC-to-RLOC bindings. The convergence time on the TE policies updates is fast, since ISPs only have to update/query a mapping to/from the database.

The main disadvantages of this deployment case are:

1. Extra LISP header is needed. This increases the packet size and, for efficient communications, it requires that the MTU between both ISPs can accomodate double-encapsulated packets.
2. The ISP ITR must encapsulate packets and therefore must know the RLOC-to-RLOC binding. These bindings are stored in a mapping database and may be cached in the ITR's mapping cache. Cache misses lead to an extra lookup latency, unless NERD [I‑D.lear‑lisp‑nerd] (Lear, E., “NERD: A Not-so-novel EID to RLOC Database,” March 2010.) is used for the lookups.
3. The operational overhead of maintaining the shared mapping database.

2.5.  Tunnel Routers behind NAT

2.5.1.  ITR

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:

1. Send all UDP packets with source port 4342, regardless of the destination port, to the RLOC of the ITR. The most simple way to achieve this is configuring 1:1 NAT mode from the external RLOC of the NAT device to the ITR's RLOC (Called "DMZ" mode in consumer broadband routers).
2. Rewrite the ITR-AFI and "Originating ITR RLOC Address" fields in the payload.

2.5.2.  ETR

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 the Map-Replies. Thus support for dynamic locators for the mapping database is needed in LISP equipment. Existing mechanisms used for updating dynamic DNS can be used to automate the process.

2.6.  Summary and Feature Matrix

Feature                         CE    PE    Split   Rec.
--------------------------------------------------------
Control of ingress TE            x     -      x      x
No modifications to existing
   int. network infrastructure   x     x      -      -
Loc-Status-Bits sync             x     -      x      x
No MTU/PMTUD issues              -     x      -      x

3.  Proxy Tunnel Routers

3.1.  P-ITR

Proxy Ingress Tunnel Routers (P-ITRs) are part of the LISP/non-LISP transition mechanism, allowing non-LISP sites to reach LISP sites. They announce certain aggregated EID prefixes into the global BGP routing tables 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 [I‑D.ietf‑lisp‑interworking] (Lewis, D., Meyer, D., Farinacci, D., and V. Fuller, “Interworking LISP with IPv4 and IPv6,” May 2009.) for a detailed description of P-ITR functionality.

A LISP site needs an interworking mechanism to communicate with non-LISP sites. A P-ITR can fulfill this role, enabling early adopters to see the benefits of LISP.

3.1.1.  EID registrar P-ITR services

EID registrars are in a good position to offer P-ITR services when allocating EID prefixes, unless the registrant wants to deploy it's own P-ITR infrastructure. Since the registrar is already providing Map-Server services, publishing registered prefixes into the mapping system, they can announce an aggregate of their allocated EID range into the DFZ. This way the P-ITR function can be colocated with the MS and the number of prefixes advertized to the DFZ reduced. On the other hand, at the early stages of the transition this may imply a significant portion of a LISP site's traffic always going through the P-ITR, which may not be acceptable for the registrar.

3.1.2.  LISP+BGP

At the other extreme exists the possibility to colocate the P-ITR function with the site's tunnel router(s). This would effectively keep today's funcionality for communications with legacy sites, and adding the benefits of LISP for communications with other early adopters. The downside is no decrease in global DFZ state.

However, in this case the P-ITR is not strictly necessary, since packets reaching the site border need no encapsulation to reach the network.

3.1.3.  Content Delivery Network P-ITR services

The main disadvantage of using P-ITRs is path stretch (unless colocated with the xTRs). Packets from non-LISP sites going through the P-ITR may take a suboptimal route. Because of this, large content providers may have the incentive to deploy geographically diverse P-ITRs, announcing their EID prefix with anycast. Existing content delivery networks (CDNs) could leverage their existing infrastructure to add this service to their portfolio, so that independent content providers could also offer improved latencies to their users. With this service in place a new LISP site need not deploy it's own P-ITR, but rather pay for the service and have low path stretch.

3.1.4.  Content Delivery Network load balancing

In addition to providing P-ITR services to third parties, CDNs can leverage the improvement brought by LISP for their own advantage. By deploying P-ITRs in strategic locations, traffic engineering could be improved beyond what is currently offered by DNS, by adjusting percentages of traffic flow to certain data centers, depending on their load. This can be achieved by setting the appropriate priorities, weights and loc-status-bits in mappings. And since the P-ITRs are controlled by the CDN operators, changes can take place instantaneously.

3.2.  P-ETR

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:

• LISP sites with unicast reverse path forwarding (uRPF) restrictions, and
• LISP sites without native IPv6 communicating with LISP nodes with IPv6-only locators.

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 originated from the LISP site is advertized to the global routing tables by a P-ITR, and the PE router has a route towards it. However, it 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 [I‑D.ietf‑lisp‑interworking] (Lewis, D., Meyer, D., Farinacci, D., and V. Fuller, “Interworking LISP with IPv4 and IPv6,” May 2009.) 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. 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. CDNs can also leverage their existing infrastructure, offering paid P-ETRs access.

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 existance 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. To enable autoconfiguration of the P-ETR locator, a DHCP option would be required.

As a security measure, access to P-ETRs should be limited to legitimate users by enforcing ACLs.

4.  Map-Resolvers and Map-Servers

4.1.  Map-Resolvers

A Map-Resolver a 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 cache or by consulting the distributed mapping database. The Map-Resolver is defined in [I‑D.ietf‑lisp‑ms] (Fuller, V. and D. Farinacci, “LISP Map Server,” April 2010.).

Map-Resolvers will be typically provided by ISPs, which also define the access policy the device. In medium or 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.

4.2.  Map-Servers

The Map-Server learns EID-to-RLOC mapping entries from an authoritative source and publishes them in the distributed mapping database. The Map-Server learns these entries 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 which is announcing and then re-encapsulates and forwards it to a matching ETR. The Map-Server is defined in [I‑D.ietf‑lisp‑ms] (Fuller, V. and D. Farinacci, “LISP Map Server,” April 2010.).

The Map-Server can be provided by the EID registrar or third parties. For instance, a LISP site (i.e., ISP) requests a Provider Independent (PI) set of addresses to an EID-registrar (i.e., RIPE). This registrar configures its own Map-Server to publish this prefix in the distributed mapping database and starts encapsulating and forwarding Map-Requests to the ETRs of the AS. These ETRs register this prefix at the Map-Server through periodic authenticated Map-Register messages. In this context there is a need for mechanisms to:

• Configure EID prefix(es) shared keys between the ETRs and the EID-registrar Map-Server.
• Configure the address of the Map-Server in the ETR of the AS.

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 ETRs authoritative for these prefixes. Upon the failure of a Map-Server, ITRs communicating with the set of EID-prefixes will be unable to reach any of the affected EID-prefixes. The only exception are the ITRs that contain in their map cache the EID-to-RLOC mappings. In this case ITRs can reach ETRs until the entry expires (typically 24 hours). For this reason redundant Map-Server server are desirable and the LISP specification should describe appropriate mechanisms.

5.  Security Considerations

Security implications of LISP deployments are to be discussed in a separate document.

6.  IANA Considerations

This memo includes no request to IANA.

7.  Acknowledgements

This draft includes material inspired by previous work from Darrel Lewis and Margaret Wasserman, presented at IETF76. Damien Saucez and Luigi Iannone provided helpful comments.

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses