Internet Engineering Task Force (IETF)                         V. Fuller
Request for Comments: 6836
Category: Experimental                                      D. Farinacci
ISSN: 2070-1721                                                 D. Meyer
                                                                D. Lewis
                                                           Cisco Systems
                                                            January 2013
Internet Engineering Task Force (IETF)                         V. Fuller
Request for Comments: 6836
Category: Experimental                                      D. Farinacci
ISSN: 2070-1721                                                 D. Meyer
                                                                D. Lewis
                                                           Cisco Systems
                                                            January 2013

Locator/ID Separation Protocol Alternative Logical Topology (LISP+ALT)




This document describes a simple distributed index system to be used by a Locator/ID Separation Protocol (LISP) Ingress Tunnel Router (ITR) or Map-Resolver (MR) to find the Egress Tunnel Router (ETR) that holds the mapping information for a particular Endpoint Identifier (EID). The MR can then query that ETR to obtain the actual mapping information, which consists of a list of Routing Locators (RLOCs) for the EID. Termed the Alternative Logical Topology (ALT), the index is built as an overlay network on the public Internet using the Border Gateway Protocol (BGP) and Generic Routing Encapsulation (GRE).


Status of This Memo


This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.


This document defines an Experimental Protocol for the Internet community. 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 a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

本文档为互联网社区定义了一个实验协议。本文件是互联网工程任务组(IETF)的产品。它代表了IETF社区的共识。它已经接受了公众审查,并已被互联网工程指导小组(IESG)批准出版。并非IESG批准的所有文件都适用于任何级别的互联网标准;见RFC 5741第2节。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at


Copyright Notice


Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

版权所有(c)2013 IETF信托基金和确定为文件作者的人员。版权所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(自本文件出版之日起生效。请仔细阅读这些文件,因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本,并提供简化BSD许可证中所述的无担保。

Table of Contents


   1. Introduction ....................................................3
   2. Definition of Terms .............................................5
   3. The LISP-ALT Model ..............................................8
      3.1. Routability of EIDs ........................................8
           3.1.1. Mechanisms for an ETR to Originate EID-Prefixes .....9
           3.1.2. Mechanisms for an ITR to Forward to EID-Prefixes ....9
           3.1.3. Map-Server Model Preferred ..........................9
      3.2. Connectivity to Non-LISP Sites ............................10
      3.3. Caveats on the Use of Data-Probes .........................10
   4. LISP+ALT: Overview .............................................10
      4.1. ITR Traffic Handling ......................................11
      4.2. EID Assignment - Hierarchy and Topology ...................12
      4.3. Use of GRE and BGP between LISP-ALT Routers ...............14
   5. EID-Prefix Propagation and Map-Request Forwarding ..............14
      5.1. Changes to ITR Behavior with LISP+ALT .....................15
      5.2. Changes to ETR Behavior with LISP+ALT .....................15
      5.3. ALT Datagram Forwarding Failure ...........................16
   6. BGP Configuration and Protocol Considerations ..................16
      6.1. Autonomous System Numbers (ASNs) in LISP+ALT ..............16
      6.2. Subsequent Address Family Identifier (SAFI) for LISP+ALT ..17
   7. EID-Prefix Aggregation .........................................17
      7.1. Stability of the ALT ......................................18
      7.2. Traffic Engineering Using LISP ............................18
      7.3. Edge Aggregation and Dampening ............................19
      7.4. EID Assignment Flexibility vs. ALT Scaling ................19
   8. Connecting Sites to the ALT Network ............................20
      8.1. ETRs Originating Information into the ALT .................20
      8.2. ITRs Using the ALT ........................................21
   9. Security Considerations ........................................22
      9.1. Apparent LISP+ALT Vulnerabilities .........................22
      9.2. Survey of LISP+ALT Security Mechanisms ....................23
      9.3. Use of Additional BGP Security Mechanisms .................24
   10. Acknowledgments ...............................................24
   11. References ....................................................24
      11.1. Normative References .....................................24
      11.2. Informative References ...................................25
   1. Introduction ....................................................3
   2. Definition of Terms .............................................5
   3. The LISP-ALT Model ..............................................8
      3.1. Routability of EIDs ........................................8
           3.1.1. Mechanisms for an ETR to Originate EID-Prefixes .....9
           3.1.2. Mechanisms for an ITR to Forward to EID-Prefixes ....9
           3.1.3. Map-Server Model Preferred ..........................9
      3.2. Connectivity to Non-LISP Sites ............................10
      3.3. Caveats on the Use of Data-Probes .........................10
   4. LISP+ALT: Overview .............................................10
      4.1. ITR Traffic Handling ......................................11
      4.2. EID Assignment - Hierarchy and Topology ...................12
      4.3. Use of GRE and BGP between LISP-ALT Routers ...............14
   5. EID-Prefix Propagation and Map-Request Forwarding ..............14
      5.1. Changes to ITR Behavior with LISP+ALT .....................15
      5.2. Changes to ETR Behavior with LISP+ALT .....................15
      5.3. ALT Datagram Forwarding Failure ...........................16
   6. BGP Configuration and Protocol Considerations ..................16
      6.1. Autonomous System Numbers (ASNs) in LISP+ALT ..............16
      6.2. Subsequent Address Family Identifier (SAFI) for LISP+ALT ..17
   7. EID-Prefix Aggregation .........................................17
      7.1. Stability of the ALT ......................................18
      7.2. Traffic Engineering Using LISP ............................18
      7.3. Edge Aggregation and Dampening ............................19
      7.4. EID Assignment Flexibility vs. ALT Scaling ................19
   8. Connecting Sites to the ALT Network ............................20
      8.1. ETRs Originating Information into the ALT .................20
      8.2. ITRs Using the ALT ........................................21
   9. Security Considerations ........................................22
      9.1. Apparent LISP+ALT Vulnerabilities .........................22
      9.2. Survey of LISP+ALT Security Mechanisms ....................23
      9.3. Use of Additional BGP Security Mechanisms .................24
   10. Acknowledgments ...............................................24
   11. References ....................................................24
      11.1. Normative References .....................................24
      11.2. Informative References ...................................25
1. Introduction
1. 介绍

This document describes the LISP+ALT system, used by an [RFC6830] Ingress Tunnel Router (ITR) or MR to find the Egress Tunnel Router (ETR) that holds the RLOC mapping information for a particular Endpoint Identifier (EID). The ALT network is built using the Border Gateway Protocol (BGP) [RFC4271], BGP multiprotocol extensions


[RFC4760], and Generic Routing Encapsulation (GRE) [RFC2784] to construct an overlay network of devices (ALT-Routers) that operate on EID-Prefixes and use EIDs as forwarding destinations.


ALT-Routers advertise hierarchically delegated segments of the EID namespace (i.e., prefixes) toward the rest of the ALT; they also forward traffic destined for an EID covered by one of those prefixes toward the network element that is authoritative for that EID and is the origin of the BGP advertisement for that EID-Prefix. An ITR uses this overlay to send a LISP Map-Request (defined in [RFC6830]) to the ETR that holds the EID-to-RLOC mapping for a matching EID-Prefix. In most cases, an ITR does not connect directly to the overlay network but instead sends Map-Requests via a Map-Resolver (described in [RFC6833]) that does. Likewise, in most cases, an ETR does not connect directly to the overlay network but instead registers its EID-Prefixes with a Map-Server that advertises those EID-Prefixes on to the ALT and forwards Map-Requests for them to the ETR.


It is important to note that the ALT does not distribute actual EID-to-RLOC mappings. What it does provide is a forwarding path from an ITR (or MR) that requires an EID-to-RLOC mapping to an ETR that holds that mapping. The ITR/MR uses this path to send an ALT Datagram (see Section 3) to an ETR, which then responds with a Map-Reply containing the needed mapping information.


One design goal for LISP+ALT is to use existing technology wherever possible. To this end, the ALT is intended to be built using off-the-shelf routers that already implement the required protocols (BGP and GRE); little, if any, LISP-specific modifications should be needed for such devices to be deployed on the ALT (see Section 7 for aggregation requirements). Note, though, that organizational and operational considerations suggest that ALT-Routers be both logically and physically separate from the "native" Internet packet transport system; deploying this overlay on those routers that are already participating in the global routing system and actively forwarding Internet traffic is not recommended.


This specification is experimental, and there are areas where further experience is needed to understand the best implementation strategy, operational model, and effects on Internet operations. These areas include:


o application effects of on-demand route map discovery

o 按需路线图发现的应用效果

o tradeoff in connection setup time vs. ALT design and performance when using a Map Request instead of carrying initial user data in a Data-Probe

o 使用映射请求而不是在数据探测器中携带初始用户数据时,连接设置时间与ALT设计和性能之间的权衡

o best practical ways to build ALT hierarchies

o 构建ALT层次结构的最佳实用方法

o effects of route leakage from ALT to the current Internet, particularly for LISP-to-non-LISP interworking

o 从ALT到当前Internet的路由泄漏的影响,尤其是LISP到非LISP互通

o effects of exceptional situations, such as denial-of-service (DoS) attacks

o 异常情况的影响,例如拒绝服务(DoS)攻击

Experimentation, measurements, and deployment experience on these aspects is appreciated. While these issues are conceptually well-understood (e.g., an ALT lookup causes potential delay for the first packet destined to a given network), the real-world operational effects are much less clear.


The remainder of this document is organized as follows: Section 2 provides the definitions of terms used in this document. Section 3 outlines the LISP-ALT model, where EID-Prefixes are advertised using BGP on an overlay network (the "ALT") and Map-Requests are forwarded across it. Section 4 provides a basic overview of the LISP Alternative Logical Topology architecture, and Section 5 describes how the ALT uses BGP to propagate EID reachability over the overlay network. Section 6 describes other considerations for using BGP on the ALT. Section 7 describes the construction of the ALT aggregation hierarchy, and Section 8 discusses how LISP-ALT elements are connected to form the overlay network. Section 9 discusses security considerations relevant to LISP+ALT.


2. Definition of Terms
2. 术语的定义

This section provides high-level definitions of LISP concepts and components involved with and affected by LISP+ALT.


Alternative Logical Topology (ALT): The virtual overlay network made up of tunnels between LISP-ALT Routers. The Border Gateway Protocol (BGP) runs between ALT-Routers and is used to carry reachability information for EID-Prefixes. The ALT provides a way to forward Map-Requests (and, if supported, Data-Probes) toward the ETR that "owns" an EID-Prefix. As a tunneled overlay, its performance is expected to be quite limited, so using it to forward high-bandwidth flows of Data-Probes is strongly discouraged (see Section 3.3 for additional discussion).


ALT-Router: The device that runs on the ALT. The ALT is a static network built using tunnels between ALT-Routers. These routers are deployed in a roughly hierarchical mesh in which routers at each level in the topology are responsible for aggregating EID-Prefixes learned from those logically "below" them and advertising summary prefixes to those logically "above" them.


Prefix learning and propagation between ALT-Routers is done using BGP. An ALT-Router at the lowest level, or "edge" of the ALT, learns EID-Prefixes from its "client" ETRs. See Section 3.1 for a description of how EID-Prefixes are learned at the "edge" of the ALT. See also Section 6 for details on how BGP is configured between the different network elements. When an ALT-Router receives an ALT Datagram, it looks up the destination EID in its forwarding table (composed of EID-Prefix routes it learned from neighboring ALT-Routers) and forwards it to the logical next hop on the overlay network.


Endpoint ID (EID): A 32-bit (for IPv4) or 128-bit (for IPv6) value used to identify the ultimate source or destination for a LISP-encapsulated packet. See [RFC6830] for details.


EID-Prefix: A set of EIDs delegated in a power-of-two block. Information about EID-Prefixes is exchanged among ALT-Routers (not on the global Internet) using BGP, and EID-Prefixes are expected to be assigned in a hierarchical manner such that they can be aggregated by ALT-Routers. Such a block is characterized by a prefix and a length. Note that while the ALT routing system considers an EID-Prefix to be an opaque block of EIDs, an end site may put site-local, topologically relevant structure (subnetting) into an EID-Prefix for intra-site routing.


Aggregated EID-Prefixes: A set of individual EID-Prefixes that have been aggregated in the [RFC4632] sense.


Map-Server (MS): An edge ALT-Router that provides a registration function for non-ALT-connected ETRs, originates EID-Prefixes into the ALT on behalf of those ETRs, and forwards Map-Requests to them. See [RFC6833] for details.


Map-Resolver (MR): An edge ALT-Router that accepts an Encapsulated Map-Request from a non-ALT-connected ITR, decapsulates it, and forwards it on to the ALT toward the ETR that owns the requested EID-Prefix. See [RFC6833] for details.


Ingress Tunnel Router (ITR): A router that sends LISP Map-Requests or encapsulates IP datagrams with LISP headers, as defined in [RFC6830]. In this document, "ITR" refers to any device implementing ITR functionality, including a Proxy-ITR (see [RFC6832]). Under some circumstances, a LISP Map-Resolver may also originate Map-Requests (see [RFC6833]).


Egress Tunnel Router (ETR): A router that sends LISP Map-Replies in response to LISP Map-Requests and decapsulates LISP-encapsulated IP datagrams for delivery to end-systems, as defined in [RFC6830]. In this document, "ETR" refers to any device implementing ETR functionality, including a Proxy-ETR (see [RFC6832]). Under some circumstances, a LISP Map-Server may also respond to Map-Requests (see [RFC6833]).

出口隧道路由器(ETR):根据[RFC6830]中的定义,发送LISP Map回复以响应LISP Map请求,并对LISP封装的IP数据报进行去封装以交付给终端系统的路由器。在本文件中,“ETR”指实现ETR功能的任何设备,包括代理ETR(见[RFC6832])。在某些情况下,LISP映射服务器也可能响应映射请求(请参见[RFC6833])。

Routing Locator (RLOC): A routable IP address for a LISP Tunnel Router (ITR or ETR). Interchangeably referred to as a "locator" in this document. An RLOC is also the output of an EID-to-RLOC mapping lookup; an EID-Prefix maps to one or more RLOCs. Typically, RLOCs are numbered from topologically aggregatable blocks that are assigned to a site at each point where it attaches to the global Internet; where the topology is defined by the connectivity of provider networks, RLOCs can be thought of as Provider-Assigned (PA) addresses. Routing for RLOCs is not carried on the ALT.


EID-to-RLOC Mapping: A binding between an EID-Prefix and the set of RLOCs that can be used to reach it; sometimes simply referred to as a "mapping".


EID-Prefix Reachability: An EID-Prefix is said to be "reachable" if at least one of its Locators is reachable. That is, an EID-Prefix is reachable if the ETR that is authoritative for a given EID-to-RLOC mapping is reachable.


Default Mapping: A mapping entry for EID-Prefix (::/0 for IPv6). It maps to a Locator-Set used for all EIDs in the Internet. If there is a more-specific EID-Prefix in the map-cache, it overrides the Default Mapping entry. The Default Mapping entry can be learned by configuration or from a Map-Reply message.


ALT Default Route: An EID-Prefix value of (or ::/0 for IPv6) that may be learned from the ALT or statically configured on an edge ALT-Router. The ALT Default Route defines a forwarding path for a packet to be sent into the ALT on a router that does not have a full ALT forwarding database.


3. The LISP-ALT Model

The LISP-ALT model uses the same basic query/response protocol that is documented in [RFC6830]. In particular, LISP+ALT provides two types of packets that an ITR can originate to obtain EID-to-RLOC mappings:


Map-Request: A Map-Request message is sent into the ALT to request an EID-to-RLOC mapping. The ETR that owns the mapping will respond to the ITR with a Map-Reply message. Since the ALT only forwards on EID destinations, the destination address of the Map-Request sent on the ALT must be an EID.


Data-Probe: Alternatively, an ITR may encapsulate and send the first data packet destined for an EID with no known RLOCs into the ALT as a Data-Probe. This might be done to minimize packet loss and to probe for the mapping. As above, the authoritative ETR for the EID-Prefix will respond to the ITR with a Map-Reply message when it receives the data packet over the ALT. As a side-effect, the encapsulated data packet is delivered to the end-system at the ETR site. Note that the Data-Probe's inner IP destination address, which is an EID, is copied to the outer IP destination address so that the resulting packet can be routed over the ALT. See Section 3.3 for caveats on the usability of Data-Probes.


The term "ALT Datagram" is shorthand for a Map-Request or Data-Probe to be sent into or forwarded on the ALT. Note that such packets use an RLOC as the outer-header source IP address and an EID as the outer-header destination IP address.


Detailed descriptions of the LISP packet types referenced by this document may be found in [RFC6830].


3.1. Routability of EIDs
3.1. EIDs的路由性

A LISP EID has the same syntax as an IP address and can be used, unaltered, as the source or destination of an IP datagram. In general, though, EIDs are not routable on the public Internet; LISP+ ALT provides a separate, virtual network, known as the LISP Alternative Logical Topology (ALT) on which a datagram using an EID as an IP destination address may be transmitted. This network is built as an overlay on the public Internet using tunnels to interconnect ALT-Routers. BGP runs over these tunnels to propagate path information needed to forward ALT Datagrams. Importantly, while the ETRs are the source(s) of the unaggregated EID-Prefixes, LISP+ALT uses existing BGP mechanisms to aggregate this information.

LISP EID与IP地址具有相同的语法,可以作为IP数据报的源或目标使用,不作更改。不过,一般来说,EID不能在公共互联网上路由;LISP+ALT提供了一个独立的虚拟网络,称为LISP替代逻辑拓扑(ALT),在该网络上可以传输使用EID作为IP目标地址的数据报。该网络构建为覆盖在公共互联网上,使用隧道互连ALT路由器。BGP通过这些隧道传播转发ALT数据报所需的路径信息。重要的是,虽然ETR是未聚合EID前缀的来源,但LISP+ALT使用现有的BGP机制来聚合此信息。

3.1.1. Mechanisms for an ETR to Originate EID-Prefixes
3.1.1. ETR生成EID前缀的机制

There are three ways that an ETR may originate its mappings into the ALT:


1. By registration with a Map-Server, as documented in [RFC6833]. This is the common case and is expected to be used by the majority of ETRs.

1. 通过向地图服务器注册,如[RFC6833]中所述。这是常见的情况,预计大多数ETR都会使用。

2. Using a "static route" on the ALT. Where no Map-Server is available, an edge ALT-Router may be configured with a "static EID-Prefix route" pointing to an ETR.

2. 在ALT上使用“静态路由”。在没有地图服务器的情况下,边缘ALT路由器可以配置指向ETR的“静态EID前缀路由”。

3. Edge connection to the ALT. If a site requires fine-grained control over how its EID-Prefixes are advertised into the ALT, it may configure its ETR(s) with tunnel and BGP connections to edge ALT-Routers.

3. 到ALT的边缘连接。如果站点需要对其EID前缀在ALT中的播发方式进行细粒度控制,则可以使用到边缘ALT路由器的隧道和BGP连接配置其ETR。

3.1.2. Mechanisms for an ITR to Forward to EID-Prefixes
3.1.2. ITR转发到EID前缀的机制

There are three ways that an ITR may send ALT Datagrams:


1. Through a Map-Resolver, as documented in [RFC6833]. This is the common case and is expected to be used by the majority of ITRs.

1. 通过映射解析器,如[RFC6833]中所述。这是常见的情况,预计大多数ITR都会使用。

2. Using a "default route". Where a Map-Resolver is not available, an ITR may be configured with a static ALT Default Route pointing to an edge ALT-Router.

2. 使用“默认路由”。在地图解析器不可用的情况下,可以使用指向边缘ALT路由器的静态ALT默认路由配置ITR。

3. Edge connection to the ALT. If a site requires fine-grained knowledge of what prefixes exist on the ALT, it may configure its ITR(s) with tunnel and BGP connections to edge ALT-Routers.

3. 到ALT的边缘连接。如果站点需要关于ALT上存在哪些前缀的细粒度知识,则可以使用到边缘ALT路由器的隧道和BGP连接配置其ITR。

3.1.3. Map-Server Model Preferred
3.1.3. 首选地图服务器型号

The ALT-connected ITR and ETR cases are expected to be rare, as the Map-Server/Map-Resolver model is simpler for an ITR/ETR operator to use and also provides a more general service interface to not only the ALT but to other mapping databases that may be developed in the future.


3.2. Connectivity to Non-LISP Sites
3.2. 与非LISP站点的连接

As stated above, EIDs used as IP addresses by LISP sites are not routable on the public Internet. This implies that, absent a mechanism for communication between LISP and non-LISP sites, connectivity between them is not possible. To resolve this problem, an "interworking" technology has been defined; see [RFC6832] for details.


3.3. Caveats on the Use of Data-Probes
3.3. 关于使用数据探测器的注意事项

It is worth noting that there has been a great deal of discussion and controversy about whether Data-Probes are a good idea. On the one hand, using them offers a method of avoiding the "first packet drop" problem when an ITR does not have a mapping for a particular EID-Prefix. On the other hand, forwarding data packets on the ALT would require that it either be engineered to support relatively high traffic rates, which is not generally feasible for a tunneled network, or that it be carefully designed to aggressively rate-limit traffic to avoid congestion or DoS attacks. There may also be issues caused by different latency or other performance characteristics between the ALT path taken by an initial Data-Probe and the "Internet" path taken by subsequent packets on the same flow once a mapping is in place on an ITR. For these reasons, the use of Data-Probes is not recommended at this time; they should only be originated from an ITR when explicitly configured to do so, and such configuration should only be enabled when performing experiments intended to test the viability of using Data-Probes.


4. LISP+ALT: Overview
4. LISP+ALT:概述

LISP+ALT is a hybrid push/pull architecture. Aggregated EID-Prefixes are advertised among the ALT-Routers and to those (rare) ITRs that are directly connected via a tunnel and BGP to the ALT. Specific EID-to-RLOC mappings are requested by an ITR (and returned by an ETR) using LISP when it sends a request either via a Map-Resolver or to an edge ALT-Router.


The basic idea embodied in LISP+ALT is to use BGP, running on a tunneled overlay network (the ALT), to establish reachability between ALT-Routers. The ALT BGP Routing Information Base (RIB) is comprised of EID-Prefixes and associated next hops. ALT-Routers interconnect using BGP and propagate EID-Prefix updates among themselves. EID-Prefix information is learned from ETRs at the "edge" of the ALT either through the use of the Map-Server interface (the common case), by static configuration, or by BGP-speaking ETRs.

LISP+ALT中包含的基本思想是使用运行在隧道覆盖网络(ALT)上的BGP在ALT路由器之间建立可达性。ALT BGP路由信息库(RIB)由EID前缀和相关的下一跳组成。ALT路由器使用BGP互连,并在它们之间传播EID前缀更新。EID前缀信息可通过使用地图服务器接口(常见情况)、静态配置或讲BGP的ETR从ALT“边缘”的ETR中获取。

Map-Resolvers learns paths through the ALT to Map-Servers for EID-Prefixes. An ITR will normally use a Map-Resolver to send its ALT Datagrams on to the ALT but may, in unusual cases (see Section 3.1.2), use a static ALT Default Route or connect to the ALT using BGP. Likewise, an ETR will normally register its prefixes in the mapping database using a Map-Server or can sometimes (see Section 3.1.1) connect directly to the ALT using BGP. See [RFC6833] for details on Map-Servers and Map-Resolvers.


Note that while this document specifies the use of Generic Routing Encapsulation (GRE) as a tunneling mechanism, there is no reason that parts of the ALT cannot be built using other tunneling technologies, particularly in cases where GRE does not meet security, management, or other operational requirements. References to "GRE tunnel" in later sections of this document should therefore not be taken as prohibiting or precluding the use of other tunneling mechanisms. Note also that two ALT-Routers that are directly adjacent (with no layer-3 router hops between them) need not use a tunnel between them; in this case, BGP may be configured across the interfaces that connect to their common subnet, and that subnet is then considered to be part of the ALT topology. The use of techniques such as "eBGP multihop" to connect ALT-Routers that do not share a tunnel or common subnet is not recommended, as the non-ALT routers in between the ALT-Routers in such a configuration may not have information necessary to forward ALT Datagrams destined to EID-Prefixes exchanged across that BGP session.


In summary, LISP+ALT uses BGP to build paths through ALT-Routers so that an ALT Datagram sent into the ALT can be forwarded to the ETR that holds the EID-to-RLOC mapping for that EID-Prefix. This reachability is carried as IPv4 or IPv6 Network Layer Reachability Information (NLRI) without modification (since an EID-Prefix has the same syntax as an IPv4 or IPv6 address prefix). ALT-Routers establish BGP sessions with one another, forming the ALT. An ALT-Router at the "edge" of the topology learns EID-Prefixes originated by authoritative ETRs. Learning may be through the Map-Server interface, by static configuration, or via BGP with the ETRs. An ALT-Router may also be configured to aggregate EID-Prefixes received from ETRs or from other LISP-ALT Routers that are topologically "downstream" from it.


4.1. ITR Traffic Handling
4.1. ITR流量处理

When an ITR receives a packet originated by an end-system within its site (i.e., a host for which the ITR is the exit path out of the site) and the destination EID for that packet is not known in the ITR's map-cache, the ITR creates either a Map-Request for the destination EID or the original packet encapsulated as a Data-Probe


(see Section 3.3 for caveats on the usability of Data-Probes). The result, known as an ALT Datagram, is then sent to an ALT-Router (see also [RFC6833] for non-ALT-connected ITRs, noting that Data-Probes cannot be sent to a Map-Resolver). This "first-hop" ALT-Router uses EID-Prefix routing information learned from other ALT-Routers via BGP to guide the packet to the ETR that "owns" the prefix. Upon receipt by the ETR, normal LISP processing occurs: the ETR responds to the ITR with a LISP Map-Reply that lists the RLOCs (and, thus, the ETRs to use) for the EID-Prefix. For Data-Probes, the ETR also decapsulates the packet and transmits it toward its destination.


Upon receipt of the Map-Reply, the ITR installs the RLOC information for a given prefix into a local mapping database. With these mapping entries stored, additional packets destined to the given EID-Prefix are routed directly to an RLOC without use of the ALT, until either the entry's Time to Live (TTL) has expired or the ITR can otherwise find no reachable ETR. Note that a current mapping may exist that contains no reachable RLOCs; this is known as a Negative Cache Entry, and it indicates that packets destined to the EID-Prefix are to be dropped.


Full details on Map-Request/Map-Reply processing may be found in [RFC6830].


Traffic routed on to the ALT consists solely of ALT Datagrams, i.e., Map-Requests and Data-Probes (if supported). Given the relatively low performance expected of a tunneled topology, ALT-Routers (and Map-Resolvers) should aggressively rate-limit the ingress of ALT Datagrams from ITRs and, if possible, should be configured to not accept packets that are not ALT Datagrams.


4.2. EID Assignment - Hierarchy and Topology
4.2. EID分配-层次结构和拓扑

The ALT database is organized in a hierarchical manner with EID-Prefixes aggregated on power-of-2 block boundaries. Where a LISP site has multiple EID-Prefixes that are aligned on a power-of-2 block boundary, they should be aggregated into a single EID-Prefix for advertisement. The ALT network is built in a roughly hierarchical, partial mesh that is intended to allow aggregation where clearly defined hierarchical boundaries exist. Building such a structure should minimize the number of EID-Prefixes carried by LISP+ALT nodes near the top of the hierarchy.


Routes on the ALT do not need to respond to changes in policy, subscription, or underlying physical connectivity, so the topology can remain relatively static and aggregation can be sustained. Because routing on the ALT uses BGP, the same rules apply for generating aggregates; in particular, an ALT-Router should only be


configured to generate an aggregate if it is configured with BGP sessions to all of the originators of components (more-specific prefixes) of that aggregate. Not all of the components need to be present for the aggregate to be originated (some may be holes in the covering prefix, and some may be down), but the aggregating router must be configured to learn the state of all of the components.


Under what circumstances the ALT-Router actually generates the aggregate is a matter of local policy: in some cases, it will be statically configured to do so at all times with a "static discard" route. In other cases, it may be configured to only generate the aggregate prefix if at least one of the components of the aggregate is learned via BGP.


An ALT-Router must not generate an aggregate that includes a non-LISP-speaking hole unless it can be configured to return a Negative Map-Reply with action="Natively-Forward" (see [RFC6830]) if it receives an ALT Datagram that matches that hole. If it receives an ALT Datagram that matches a LISP-speaking hole that is currently not reachable, it should return a Negative Map-Reply with action="drop". Negative Map-Replies should be returned with a short TTL, as specified in [RFC6833]. Note that an off-the-shelf, non-LISP-speaking router configured as an aggregating ALT-Router cannot send Negative Map-Replies, so such a router must never originate an aggregate that includes a non-LISP-speaking hole.

ALT路由器不得生成包含非LISP语音孔的聚合,除非可以配置为在收到与该孔匹配的ALT数据报时返回带有action=“native Forward”(请参见[RFC6830])的否定映射回复。如果它接收到一个ALT数据报,该数据报与当前无法访问的LISP语音孔相匹配,它应该返回一个带有action=“drop”的否定映射回复。按照[RFC6833]中的规定,负面映射回复应返回一个简短的TTL。请注意,配置为聚合ALT路由器的现成非LISP语音路由器不能发送否定的Map回复,因此此类路由器不得发起包含非LISP语音孔的聚合。

This implies that two ALT-Routers that share an overlapping set of prefixes must exchange those prefixes if either is to generate and export a covering aggregate for those prefixes. It also implies that an ETR that connects to the ALT using BGP must maintain BGP sessions with all of the ALT-Routers that are configured to originate an aggregate that covers that prefix and that each of those ALT-Routers must be explicitly configured to know the set of EID-Prefixes that make up any aggregate that it originates. See also [RFC6833] for an example of other ways that prefix origin consistency and aggregation can be maintained.


As an example, consider ETRs that are originating EID-Prefixes for,,, and An ALT-Router should only be configured to generate an aggregate for if it has BGP sessions configured with all of these ETRs, in other words, only if it has sufficient knowledge about the state of those prefixes to summarize them. If the Router originating receives an ALT Datagram destined for, a non-LISP destination covered by the aggregate, it returns a Negative Map-Reply with action "Natively-Forward". If it receives an ALT

作为一个例子,考虑ESTR是EID前缀为101.0.0/24,101.64.0/24,101.128 0/24,和101.192.0/24。如果ALT路由器的BGP会话配置了所有这些ETR,则只应将其配置为生成10.1.0.0/16的聚合,换句话说,只有当它对这些前缀的状态有足够的了解以对其进行汇总时,才应将其配置为生成聚合。如果源于10.1.0.0/16的路由器接收到一个目的地为10.1.77.88的ALT数据报(聚合所覆盖的非LISP目的地),它将返回一个带有“本机转发”操作的否定映射回复。如果它收到ALT

Datagram destined for but the configured LISP prefix is unreachable, it returns a Negative Map-Reply with action "drop".


Note: Much is currently uncertain about the best way to build the ALT network; as testing and prototype deployment proceed, a guide to how to best build the ALT network will be developed.


4.3. Use of GRE and BGP between LISP-ALT Routers
4.3. 在LISP-ALT路由器之间使用GRE和BGP

The ALT network is built using GRE tunnels between ALT-Routers. BGP sessions are configured over those tunnels, with each ALT-Router acting as a separate Autonomous System (AS) "hop" in a Path Vector for BGP. For the purposes of LISP+ALT, the AS-path is used solely as a shortest-path determination and loop-avoidance mechanism. Because all next hops are on tunnel interfaces, no IGP is required to resolve those next hops to exit interfaces.


LISP+ALT's use of GRE and BGP facilitates deployment and operation of LISP because no new protocols need to be defined, implemented, or used on the overlay topology; existing BGP/GRE tools and operational expertise are also re-used. Tunnel address assignment is also easy: since the addresses on an ALT tunnel are only used by the pair of routers connected to the tunnel, the only requirement of the IP addresses used to establish that tunnel is that the attached routers be reachable by each other; any addressing plan, including private addressing, can therefore be used for ALT tunnels.


5. EID-Prefix Propagation and Map-Request Forwarding
5. EID前缀传播和映射请求转发

As described in Section 8.2, an ITR sends an ALT Datagram to a given EID-to-RLOC mapping. The ALT provides the infrastructure that allows these requests to reach the authoritative ETR.


Note that under normal circumstances Map-Replies are not sent over the ALT; an ETR sends a Map-Reply to one of the ITR RLOCs learned from the original Map-Request. See Sections 6.1.2 and 6.2 of [RFC6830] for more information on the use of the Map-Request 'ITR RLOC Address' field. Keep in mind that the 'ITR RLOC Address' field supports multiple RLOCs in multiple address families, so a Map-Reply sent in response to a Map-Request is not necessarily sent back to the Map-Request RLOC source.

请注意,在正常情况下,Map回复不会通过ALT发送;ETR向从原始Map请求中学习的其中一个ITR RLOC发送Map回复。有关Map请求“ITR RLOC地址”字段使用的更多信息,请参见[RFC6830]第6.1.2节和第6.2节。请记住,“ITR RLOC地址”字段支持多个地址族中的多个RLOC,因此响应Map请求发送的Map应答不一定发送回Map请求RLOC源。

There may be scenarios, perhaps to encourage caching of EID-to-RLOC mappings by ALT-Routers, where Map-Replies could be sent over the ALT or where a "first-hop" ALT-Router might modify the originating RLOC on a Map-Request received from an ITR to force the Map-Reply to be


returned to the "first-hop" ALT-Router. These cases will not be supported by initial LISP+ALT implementations but may be subject to future experimentation.


ALT-Routers propagate path information via BGP ([RFC4271]) that is used by ITRs to send ALT Datagrams toward the appropriate ETR for each EID-Prefix. BGP is run on the inter-ALT-Router links, and possibly between an edge ("last-hop") ALT-Router and an ETR or between an edge ("first-hop") ALT-Router and an ITR. The ALT BGP RIB consists of aggregated EID-Prefixes and their next hops toward the authoritative ETR for that EID-Prefix.

ALT路由器通过BGP([RFC4271])传播路径信息,ITRs使用BGP向每个EID前缀的相应ETR发送ALT数据报。BGP在ALT路由器间链路上运行,可能在边缘(“最后一跳”)ALT路由器和ETR之间或边缘(“第一跳”)ALT路由器和ITR之间运行。ALT BGP RIB由聚合的EID前缀及其向该EID前缀的权威ETR的下一跳组成。

5.1. Changes to ITR Behavior with LISP+ALT
5.1. 使用LISP+ALT更改ITR行为

As previously described, an ITR will usually use the Map-Resolver interface and will send its Map Requests to a Map-Resolver. When an ITR instead connects via tunnels and BGP to the ALT, it sends ALT Datagrams to one of its "upstream" ALT-Routers; these are sent only to obtain new EID-to-RLOC mappings -- RLOC probe and cache TTL refresh Map-Requests are not sent on the ALT. As in basic LISP, it should use one of its RLOCs as the source address of these queries; it should not use a tunnel interface as the source address, as doing so will cause replies to be forwarded over the tunneled topology and may be problematic if the tunnel interface address is not routed throughout the ALT. If the ITR is running BGP with the LISP-ALT Router(s), it selects the appropriate ALT-Router based on the BGP information received. If it is not running BGP, it uses a statically configured ALT Default Route to select an ALT-Router.


5.2. Changes to ETR Behavior with LISP+ALT
5.2. 使用LISP+ALT更改ETR行为

As previously described, an ETR will usually use the Map-Server interface (see [RFC6833]) and will register its EID-Prefixes with its configured Map-Servers. When an ETR instead connects using BGP to one or more ALT-Routers, it announces its EID-Prefix(es) to those ALT-Routers.


As documented in [RFC6830], when an ETR generates a Map-Reply message to return to a querying ITR, it sets the outer-header IP destination address to one of the requesting ITR's RLOCs so that the Map-Reply will be sent on the underlying Internet topology, not on the ALT; this avoids any latency penalty (or "stretch") that might be incurred by sending the Map-Reply via the ALT, reduces load on the ALT, and ensures that the Map-Reply can be routed even if the original ITR does not have an ALT-routed EID. For details on how an ETR selects which ITR RLOC to use, see Section 6.1.5 of [RFC6830].

如[RFC6830]中所述,当ETR生成映射回复消息以返回查询ITR时,它将外部报头IP目的地地址设置为请求ITR的RLOC之一,以便映射回复将在基础互联网拓扑上发送,而不是在ALT上发送;这避免了通过ALT发送Map应答可能引起的任何延迟损失(或“拉伸”),减少了ALT上的负载,并确保即使原始ITR没有ALT路由EID,也可以路由Map应答。有关ETR如何选择使用哪个ITR RLOC的详细信息,请参见[RFC6830]第6.1.5节。

5.3. ALT Datagram Forwarding Failure
5.3. ALT数据报转发失败

Intermediate ALT-Routers forward ALT Datagrams using normal, hop-by-hop routing on the ALT overlay network. Should an ALT-Router not be able to forward an ALT Datagram, whether due to an unreachable next hop, TTL exceeded, or other problem, it has several choices:


o If the ALT-Router understands LISP, as is the case for a Map-Resolver or Map-Server, it may respond to a forwarding failure by returning a Negative Map-Reply, as described in Section 4.2 and [RFC6833].

o 如果ALT路由器理解LISP,就像Map解析器或Map服务器一样,它可以通过返回否定的Map应答来响应转发失败,如第4.2节和[RFC6833]所述。

o If the ALT-Router does not understand LISP, it may attempt to return an ICMP message to the source IP address of the packet that cannot be forwarded. Since the source address is an RLOC, an ALT-Router would send this ICMP message using "native" Internet connectivity, not via the ALT overlay.

o 如果ALT路由器不理解LISP,它可能会尝试将ICMP消息返回到无法转发的数据包的源IP地址。由于源地址是RLOC,ALT路由器将使用“本机”互联网连接发送此ICMP消息,而不是通过ALT覆盖。

o A non-LISP-capable ALT-Router may also choose to silently drop the non-forwardable ALT Datagram.

o 不支持LISP的ALT路由器也可以选择静默地丢弃不可转发的ALT数据报。

[RFC6830] and [RFC6833] define how the source of an ALT Datagram should handle each of these cases. The last case, where an ALT Datagram is silently discarded, will generally result in several retransmissions by the source, followed by treating the destination as unreachable via LISP when no Map-Reply is received. If a problem on the ALT is severe enough to prevent ALT Datagrams from being delivered to a specific EID, this is probably the only sensible way to handle this case.


Note that the use of GRE tunnels should prevent MTU problems from ever occurring on the ALT; an ALT Datagram that exceeds an intermediate MTU will be fragmented at that point and will be reassembled by the target of the GRE tunnel.


6. BGP Configuration and Protocol Considerations
6. BGP配置和协议注意事项
6.1. Autonomous System Numbers (ASNs) in LISP+ALT
6.1. LISP+ALT中的自治系统编号(ASN)

The primary use of BGP today is to define the global Internet routing topology in terms of its participants, known as Autonomous Systems. LISP+ALT specifies the use of BGP to create a global overlay network (the ALT) for finding EID-to-RLOC mappings. While related to the global routing database, the ALT serves a very different purpose and is organized into a very different hierarchy. Because LISP+ALT does use BGP, however, it uses ASNs in the paths that are propagated among ALT-Routers. To avoid confusion, LISP+ALT should use newly assigned


AS numbers that are unrelated to the ASNs used by the global routing system. Exactly how this new space will be assigned and managed will be determined during the deployment of LISP+ALT.


Note that the ALT-Routers that make up the "core" of the ALT will not be associated with any existing core-Internet ASN because the ALT topology is completely separate from, and independent of, the global Internet routing system.

请注意,构成ALT“核心”的ALT路由器将不会与任何现有的核心Internet ASN关联,因为ALT拓扑完全独立于全局Internet路由系统。

6.2. Subsequent Address Family Identifier (SAFI) for LISP+ALT
6.2. LISP+ALT的后续地址族标识符(SAFI)

As defined by this document, LISP+ALT may be implemented using BGP without modification. Given the fundamental operational difference between propagating global Internet routing information (the current dominant use of BGP) and creating an overlay network for finding EID-to-RLOC mappings (the use of BGP as proposed by this document), it may be desirable to assign a new SAFI [RFC4760] to prevent operational confusion and difficulties, including the inadvertent leaking of information from one domain to the other. The use of a separate SAFI would make it easier to debug many operational problems but would come at a significant cost: unmodified, off-the-shelf routers that do not understand the new SAFI could not be used to build any part of the ALT network. At present, this document does not request the assignment of a new SAFI; additional experimentation may suggest the need for one in the future.


7. EID-Prefix Aggregation
7. EID前缀聚合

To facilitate EID-Prefix aggregation, the ALT BGP topology is provisioned in a hierarchical manner; the fact that all inter-node links are tunnels means that topology can be constrained to follow the EID-Prefix assignment hierarchy. Redundant links are provisioned to compensate for node and link failures. A basic assumption is that as long as the routers are up and running, the underlying Internet will provide alternative routes to maintain tunnel and BGP connectivity among ALT-Routers.

为了便于EID前缀聚合,以分层方式提供ALT BGP拓扑;所有节点间链路都是隧道这一事实意味着拓扑可以约束为遵循EID前缀分配层次结构。提供冗余链路以补偿节点和链路故障。一个基本假设是,只要路由器启动并运行,底层互联网将提供替代路由,以维持ALT路由器之间的隧道和BGP连接。

Note that, as mentioned in Section 4.2, the use of BGP by LISP+ALT requires that information only be aggregated where all active more-specific prefixes of a generated aggregate prefix are known. This is no different than the way that BGP route aggregation works in the existing global routing system: a service provider only generates an aggregate route if it is configured to learn all prefixes that make up that aggregate.


7.1. Stability of the ALT
7.1. 丙氨酸氨基转移酶的稳定性

It is worth noting that LISP+ALT does not directly propagate EID-to-RLOC mappings. What it does is provide a mechanism for an ITR to communicate with the ETR that holds the mapping for a particular EID-Prefix. This distinction is important when considering the stability of BGP on the ALT network as compared to the global routing system. It also has implications for how site-specific EID-Prefix information may be used by LISP but not propagated by LISP+ALT (see Section 7.2 below).


RLOC prefixes are not propagated through the ALT, so their reachability is not determined through the use of LISP+ALT. Instead, reachability of RLOCs is learned through the LISP ITR-ETR exchange. This means that link failures or other service disruptions that may cause the reachability of an RLOC to change are not known to the ALT. Changes to the presence of an EID-Prefix on the ALT occur much less frequently: only at subscription time or in the event of a failure of the ALT infrastructure itself. This means that "flapping" (frequent BGP updates and withdrawals due to prefix state changes) is not likely and mapping information cannot become "stale" due to slow propagation through the ALT BGP mesh.

RLOC前缀不是通过ALT传播的,因此它们的可达性不是通过使用LISP+ALT来确定的。相反,RLOC的可达性是通过LISP ITR-ETR交换来学习的。这意味着ALT不知道可能导致RLOC可达性改变的链路故障或其他服务中断。ALT上EID前缀存在的改变发生的频率要低得多:仅在订阅时或ALT基础设施本身发生故障时发生。这意味着“拍打”(由于前缀状态更改而导致频繁的BGP更新和撤销)不太可能发生,并且映射信息不会因为通过ALT BGP网格的传播速度慢而变得“过时”。

7.2. Traffic Engineering Using LISP
7.2. 基于LISP的流量工程

Since an ITR learns an EID-to-RLOC mapping directly from the ETR that owns it, it is possible to perform site-to-site Traffic Engineering by setting the preference and/or weight fields, and by including more-specific EID-to-RLOC information in Map-Reply messages.


This is a powerful mechanism that can conceivably replace the traditional practice of routing prefix deaggregation for Traffic Engineering purposes. Rather than propagating more-specific information into the global routing system for local or regional optimization of traffic flows, such more-specific information can be exchanged, through LISP (not LISP+ALT), on an as-needed basis between only those ITRs/ETRs (and, thus, site pairs) that need it. Such an exchange of "more-specifics" between sites facilitates Traffic Engineering by allowing richer and more fine-grained policies to be applied without advertising additional prefixes into either the ALT or the global routing system.


Note that these new Traffic Engineering capabilities are an attribute of LISP and are not specific to LISP+ALT; discussion is included here because the BGP-based global routing system has traditionally used propagation of more-specific routes as a crude form of Traffic Engineering.


7.3. Edge Aggregation and Dampening
7.3. 边缘聚集和衰减

Normal BGP best common practices apply to the ALT network. In particular, first-hop ALT-Routers will aggregate EID-Prefixes and dampen changes to them in the face of excessive updates. Since EID-Prefix assignments are not expected to change as frequently as global routing BGP prefix reachability, such dampening should be very rare and might be worthy of logging as an exceptional event. It is again worth noting that the ALT carries only EID-Prefixes, used to construct a BGP path to each ETR (or Map-Server) that originates each prefix; the ALT does not carry reachability information about RLOCs. In addition, EID-Prefix information may be aggregated as the topology and address assignment hierarchy allow. Since the topology is all tunneled and can be modified as needed, reasonably good aggregation should be possible. In addition, since most ETRs are expected to connect to the ALT using the Map-Server interface, Map-Servers will implement a natural "edge" for the ALT where dampening and aggregation can be applied. For these reasons, the set of prefix information on the ALT can be expected to be both better aggregated and considerably less volatile than the actual EID-to-RLOC mappings.


7.4. EID Assignment Flexibility vs. ALT Scaling
7.4. EID分配灵活性与ALT缩放

There are major open questions regarding how the ALT will be deployed and what organization(s) will operate it. In a simple, non-distributed world, centralized administration of EID-Prefix assignment and ALT network design would facilitate a well-aggregated ALT routing system. Business and other realities will likely result in a more complex, distributed system involving multiple levels of prefix delegation, multiple operators of parts of the ALT infrastructure, and a combination of competition and cooperation among the participants. In addition, the re-use of existing IP address assignments, both Provider-Independent ("PI") and Provider-Assigned ("PA"), to avoid renumbering when sites transition to LISP will further complicate the processes of building and operating the ALT.


A number of conflicting considerations need to be kept in mind when designing and building the ALT. Among them are:


1. Target ALT routing state size and level of aggregation. As described in Section 7.1, the ALT should not suffer from the same performance constraints or stability issues as does the Internet global routing system, so some reasonable level of deaggregation and an increased number of EID-Prefixes beyond what might be considered ideal should be acceptable. That said, measures, such as tunnel rehoming to preserve aggregation when sites move from one mapping provider to another and implementing aggregation at

1. 目标ALT路由状态大小和聚合级别。如第7.1节所述,ALT不应受到与Internet全局路由系统相同的性能约束或稳定性问题,因此一些合理的解聚集级别和超出理想值的EID前缀数量的增加是可以接受的。也就是说,当站点从一个映射提供者移动到另一个映射提供者时,需要采取一些措施,例如隧道重新命名以保持聚合,并在

multiple levels in the hierarchy to collapse deaggregation at lower levels, should be taken to reduce unnecessary explosion of ALT routing state.


2. Number of operators of parts of the ALT and how they will be organized (hierarchical delegation vs. shared administration). This will determine not only how EID-Prefixes are assigned but also how tunnels are configured and how EID-Prefixes can be aggregated between different parts of the ALT.

2. ALT部分的操作员数量及其组织方式(分层委托与共享管理)。这不仅将决定EID前缀的分配方式,还将决定隧道的配置方式以及如何在ALT的不同部分之间聚合EID前缀。

3. Number of connections between different parts of the ALT. Tradeoffs will need to be made among resilience, performance, and placement of aggregation boundaries.

3. ALT不同部分之间的连接数。需要在恢复能力、性能和聚合边界的放置之间进行权衡。

4. EID-Prefix portability between competing operators of the ALT infrastructure. A significant benefit for an end site to adopt LISP is the availability of EID space that is not tied to a specific connectivity provider; it is important to ensure that an end site doesn't trade lock-in to a connectivity provider for lock-in to a provider of its EID assignment, ALT connectivity, or Map-Server facilities.

4. EID前缀在ALT基础设施的竞争运营商之间的可移植性。终端站点采用LISP的一个重要好处是EID空间的可用性,而不受特定连接提供商的限制;确保终端站点不会将对连接提供程序的锁定转换为对其EID分配、ALT连接或地图服务器设施的提供程序的锁定,这一点很重要。

This is, by no means, an exhaustive list.


While resolving these issues is beyond the scope of this document, the authors recommend that existing distributed resource structures, such as the IANA/Regional Internet Registries and the ICANN/Domain Registrar, be carefully considered when designing and deploying the ALT infrastructure.


8. Connecting Sites to the ALT Network
8. 将站点连接到ALT网络
8.1. ETRs Originating Information into the ALT
8.1. ETRs将原始信息发送到ALT

EID-Prefix information is originated into the ALT by three different mechanisms:


Map-Server: In most cases, a site will configure its ETR(s) to register with one or more Map-Servers (see [RFC6833]) and does not participate directly in the ALT.


BGP: For sites requiring complex control over their EID-Prefix origination into the ALT, an ETR may connect to the LISP+ALT overlay network by running BGP to one or more ALT-Routers over tunnel(s). The ETR advertises reachability for its EID-Prefixes over these BGP connection(s). The edge ALT-Router(s) that receive(s) these prefixes then propagate(s) them into the ALT.


Here, the ETR is simply a BGP peer of ALT-Router(s) at the edge of the ALT. Where possible, an ALT-Router that receives EID-Prefixes from an ETR via BGP should aggregate that information.


Configuration: One or more ALT-Routers may be configured to originate an EID-Prefix on behalf of the non-BGP-speaking ETR that is authoritative for a prefix. As in the case above, the ETR is connected to ALT-Router(s) using GRE tunnel(s), but rather than BGP being used, the ALT-Router(s) are configured with what are in effect "static routes" for the EID-Prefixes "owned" by the ETR. The GRE tunnel is used to route Map-Requests to the ETR.


Note: In all cases, an ETR may register to multiple Map-Servers or connect to multiple ALT-Routers for the following reasons:


* redundancy, so that a particular ETR is still reachable even if one path or tunnel is unavailable.

* 冗余,因此即使一条路径或隧道不可用,特定ETR仍然可以访问。

* to connect to different parts of the ALT hierarchy if the ETR "owns" multiple EID-to-RLOC mappings for EID-Prefixes that cannot be aggregated by the same ALT-Router (i.e., are not topologically "close" to each other in the ALT).

* 如果ETR“拥有”EID前缀的多个EID到RLOC映射,且不能由同一个ALT路由器聚合(即,在ALT中拓扑上彼此不“接近”),则连接到ALT层次结构的不同部分。

8.2. ITRs Using the ALT
8.2. ITRs使用ALT

In the common configuration, an ITR does not need to know anything about the ALT, since it sends Map-Requests to one of its configured Map-Resolvers (see [RFC6833]). There are two exceptional cases:


Static default: If a Map-Resolver is not available but an ITR is adjacent to an ALT-Router (either over a common subnet or through the use of a tunnel), it can use an ALT Default Route to cause all ALT Datagrams to be sent to that ALT-Router. This case is expected to be rare.


Connection to ALT: A site with complex Internet connectivity may need more fine-grained distinction between traffic to LISP-capable and non-LISP-capable sites. Such a site may configure each of its ITRs to connect directly to the ALT, using a tunnel and BGP connection. In this case, the ITR will receive EID-Prefix routes from its BGP connection to the ALT-Router and will LISP-encapsulate and send ALT Datagrams through the tunnel to the ALT-Router. Traffic to other destinations may be forwarded (without LISP encapsulation) to non-LISP next-hop routers that the ITR knows.


In general, an ITR that connects to the ALT does so only to ALT-Routers at the "edge" of the ALT (typically two for redundancy). There may, though, be situations where an ITR would connect to other ALT-Routers to receive additional, shorter-path information about a portion of the ALT of interest to it. This can be accomplished by establishing GRE tunnels between the ITR and the set of ALT-Routers with the additional information. This is a purely local policy issue between the ITR and the ALT-Routers in question.


As described in [RFC6833], Map-Resolvers do not accept or forward Data-Probes; in the rare scenario that an ITR does support and originate Data-Probes, it must do so using one of the exceptional configurations described above. Note that the use of Data-Probes is discouraged at this time (see Section 3.3).


9. Security Considerations
9. 安全考虑

LISP+ALT shares many of the security characteristics of BGP. Its security mechanisms are comprised of existing technologies in wide operational use today, so securing the ALT should be mostly a matter of applying the same technology that is used to secure the BGP-based global routing system (see Section 9.3 below).


9.1. Apparent LISP+ALT Vulnerabilities
9.1. 明显的LISP+ALT漏洞

This section briefly lists the known potential vulnerabilities of LISP+ALT.


Mapping integrity: Potential for an attacker to insert bogus mappings to black-hole (create a DoS attack) or intercept LISP data-plane packets.


ALT-Router availability: Can an attacker DoS the ALT-Routers connected to a given ETR? If a site's ETR cannot advertise its EID-to-RLOC mappings, the site is essentially unavailable.


ITR mapping/resources: Can an attacker force an ITR or ALT-Router to drop legitimate mapping requests by flooding it with random destinations for which it will generate large numbers of Map-Requests and fill its map-cache? Further study is required to see the impact of admission control on the overlay network.


EID Map-Request exploits for reconnaissance: Can an attacker learn about a LISP site's TE policy by sending legitimate mapping requests and then observing the RLOC mapping replies? Is this information useful in attacking or subverting peer relationships? Note that any public LISP mapping database will have similar data-plane reconnaissance issues.


Scaling of ALT-Router resources: Paths through the ALT may be of lesser bandwidth than more "direct" paths; this may make them more prone to high-volume DoS attacks. For this reason, all components of the ALT (ETRs and ALT-Routers) should be prepared to rate-limit traffic (ALT Datagrams) that could be received across the ALT.


UDP Map-Reply from ETR: Since Map-Replies are sent directly from the ETR to the ITR's RLOC, the ITR's RLOC may be vulnerable to various types of DoS attacks (this is a general property of LISP, not a LISP+ALT vulnerability).


More-specific prefix leakage: Because EID-Prefixes on the ALT are expected to be fairly well-aggregated and EID-Prefixes propagated out to the global Internet (see [RFC6832]) much more so, accidental leaking or malicious advertisement of an EID-Prefix into the global routing system could cause traffic redirection away from a LISP site. This is not really a new problem, though, and its solution can only be achieved by much more strict prefix filtering and authentication on the global routing system. Section 9.3 describes an existing approach to solving this problem.


9.2. Survey of LISP+ALT Security Mechanisms
9.2. LISP+ALT安全机制综述

Explicit peering: The devices themselves can prioritize incoming packets as well as potentially do key checks in hardware to protect the control plane.


Use of TCP to connect elements: This makes it difficult for third parties to inject packets.


Use of HMAC to protect BGP/TCP connections: Hashed Message Authentication Code (HMAC) [RFC5925] is used to verify the integrity and authenticity of TCP connections used to exchange BGP messages, making it nearly impossible for third-party devices to either insert or modify messages.


Message sequence numbers and nonce values in messages: This allows an ITR to verify that the Map-Reply from an ETR is in response to a Map-Request originated by that ITR (this is a general property of LISP; LISP+ALT does not change this behavior).


9.3. Use of Additional BGP Security Mechanisms
9.3. 使用额外的BGP安全机制

LISP+ALT's use of BGP allows it to take advantage of BGP security features designed for existing Internet BGP use. This means that LISP+ALT can and should use technology developed for adding security to BGP (in the IETF SIDR working group or elsewhere) to provide authentication of EID-Prefix origination and EID-to-RLOC mappings.

LISP+ALT对BGP的使用允许它利用为现有互联网BGP使用而设计的BGP安全功能。这意味着LISP+ALT可以而且应该使用为BGP(在IETF SIDR工作组或其他地方)增加安全性而开发的技术,以提供EID前缀发起和EID到RLOC映射的身份验证。

10. Acknowledgments
10. 致谢

The authors would like to specially thank J. Noel Chiappa, who was a key contributor to the design of the Content distribution Overlay Network Service for LISP (LISP-CONS) mapping database (many ideas from which made their way into LISP+ALT) and who has continued to provide invaluable insight as the LISP effort has evolved. Others who have provided valuable contributions include John Zwiebel, Hannu Flinck, Amit Jain, John Scudder, Scott Brim, and Jari Arkko.

作者要特别感谢J.Noel Chiappa,他是LISP(LISP-CONS)映射数据库内容分发覆盖网络服务设计的关键贡献者(许多想法由此进入LISP+ALT),并且随着LISP工作的发展,他继续提供了宝贵的见解。其他做出宝贵贡献的人包括约翰·兹维贝尔、汉努·弗林克、阿米特·贾因、约翰·斯卡德尔、斯科特·布里姆和贾里·阿尔科。

11. References
11. 工具书类
11.1. Normative References
11.1. 规范性引用文件

[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

[RFC2784]Farinaci,D.,Li,T.,Hanks,S.,Meyer,D.,和P.Traina,“通用路由封装(GRE)”,RFC 27842000年3月。

[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

[RFC4271]Rekhter,Y.,Li,T.,和S.Hares,“边境网关协议4(BGP-4)”,RFC 42712006年1月。

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

[RFC4632]Fuller,V.和T.Li,“无类域间路由(CIDR):互联网地址分配和聚合计划”,BCP 122,RFC 4632,2006年8月。

[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007.

[RFC4760]Bates,T.,Chandra,R.,Katz,D.,和Y.Rekhter,“BGP-4的多协议扩展”,RFC 4760,2007年1月。

[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The Locator/ID Separation Protocol (LISP)", RFC 6830, January 2013.

[RFC6830]Farinaci,D.,Fuller,V.,Meyer,D.,和D.Lewis,“定位器/身份分离协议(LISP)”,RFC 6830,2013年1月。

[RFC6833] Fuller, V. and D. Farinacci, "Locator/ID Separation Protocol (LISP) Map-Server Interface", RFC 6833, January 2013.

[RFC6833]Fuller,V.和D.Farinaci,“定位器/ID分离协议(LISP)地图服务器接口”,RFC 6833,2013年1月。

11.2. Informative References
11.2. 资料性引用

[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", RFC 5925, June 2010.

[RFC5925]Touch,J.,Mankin,A.,和R.Bonica,“TCP认证选项”,RFC 59252010年6月。

[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.

[RFC6832]Lewis,D.,Meyer,D.,Farinaci,D.,和V.Fuller,“定位器/ID分离协议(LISP)和非LISP站点之间的互通”,RFC 6832,2013年1月。

Authors' Addresses


Vince Fuller



Dino Farinacci Cisco Systems Tasman Drive San Jose, CA 95134 USA



Dave Meyer Cisco Systems Tasman Drive San Jose, CA 95134 USA



Darrel Lewis Cisco Systems Tasman Drive San Jose, CA 95134 USA