Network Working Group A. Cabellos
Internet-Draft UPC-BarcelonaTech
Intended status: Informational D. Saucez (Ed.)
Expires: April 27, 2015 INRIA
October 24, 2014
An Architectural Introduction to the Locator/ID Separation Protocol
(LISP)
draft-ietf-lisp-introduction-07.txt
Abstract
This document describes the architecture of the Locator/ID Separation
Protocol (LISP), making it easier to read the rest of the LISP
specifications and providing a basis for discussion about the details
of the LISP protocols. This document is used for introductory
purposes, more details can be found in RFC6830, the protocol
specification.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 27, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition of Terms . . . . . . . . . . . . . . . . . . . . . 4
3. LISP Architecture . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Design Principles . . . . . . . . . . . . . . . . . . . . 4
3.2. Overview of the Architecture . . . . . . . . . . . . . . 4
3.3. Data-Plane . . . . . . . . . . . . . . . . . . . . . . . 7
3.3.1. LISP Encapsulation . . . . . . . . . . . . . . . . . 7
3.3.2. LISP Forwarding State . . . . . . . . . . . . . . . . 8
3.4. Control-Plane . . . . . . . . . . . . . . . . . . . . . . 9
3.4.1. LISP Mappings . . . . . . . . . . . . . . . . . . . . 9
3.4.2. Mapping System Interface . . . . . . . . . . . . . . 9
3.4.3. Mapping System . . . . . . . . . . . . . . . . . . . 10
3.5. Interworking Mechanisms . . . . . . . . . . . . . . . . . 13
4. LISP Operational Mechanisms . . . . . . . . . . . . . . . . . 13
4.1. Cache Management . . . . . . . . . . . . . . . . . . . . 14
4.2. RLOC Reachability . . . . . . . . . . . . . . . . . . . . 14
4.3. ETR Synchronization . . . . . . . . . . . . . . . . . . . 16
4.4. MTU Handling . . . . . . . . . . . . . . . . . . . . . . 16
5. Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Traffic Engineering . . . . . . . . . . . . . . . . . . . 19
8.2. LISP for IPv6 Co-existence . . . . . . . . . . . . . . . 19
8.3. LISP for Virtual Private Networks . . . . . . . . . . . . 20
8.4. LISP for Virtual Machine Mobility in Data Centers . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. A Brief History of Location/Identity Separation . . 24
A.1. Old LISP Models . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This document introduces the Locator/ID Separation Protocol (LISP)
[RFC6830] architecture, its main operational mechanisms and its
design rationale. Fundamentally, LISP is built following a well-
known architectural idea: decoupling the IP address overloaded
semantics. Indeed and as pointed out by [Chiappa], currently IP
addresses both identify the topological location of a network
attachment point as well as the node's identity. However, nodes and
routing have fundamentally different requirements, routing systems
require that addresses are aggregatable and have topological meaning,
while nodes require to be identified independently of their current
location [RFC4984].
LISP creates two separate namespaces, EIDs (End-host IDentifiers) and
RLOCs (Routing LOCators), both are typically syntactically identical
to the current IPv4 and IPv6 addresses. EIDs are used to uniquely
identify nodes irrespective of their topological location and are
typically routed intra-domain. RLOCs are assigned topologically to
network attachment points and are typically routed inter-domain.
With LISP, the edge of the Internet (where the nodes are connected)
and the core (where inter-domain routing occurs) can be logically
separated and interconnected by LISP-capable routers. LISP also
introduces a database, called the Mapping System, to store and
retrieve mappings between identity and location. LISP-capable
routers exchange packets over the Internet core by encapsulating them
to the appropriate location.
By taking advantage of such separation between location and identity
LISP offers Traffic Engineering, multihoming, and mobility among
others benefits. Additionally, LISP's approach to solve the routing
scalability problem [RFC4984] is that with LISP the Internet core is
populated with RLOCs while Traffic Engineering mechanisms are pushed
to the Mapping System. With this RLOCs are quasi-static (i.e., low
churn) and hence, the routing system scalable [Quoitin] while EIDs
can roam anywhere with no churn to the underlying routing system.
This document describes the LISP architecture, its main operational
mechanisms as its design rationale. It is important to note that
this document does not specify or complement the LISP protocol. The
interested reader should refer to the main LISP specifications
[RFC6830] and the complementary documents [RFC6831], [RFC6832],
[RFC6833], [RFC6834], [RFC6835], [RFC6836], [RFC7052] for the
protocol specifications along with the LISP deployment guidelines
[RFC7215].
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2. Definition of Terms
This document describes the LISP architecture and does not define or
introduce any new term. The reader is referred to [RFC6830],
[RFC6831], [RFC6832], [RFC6833], [RFC6834], [RFC6835], [RFC6836],
[RFC7052], [RFC7215] for the LISP definition of terms.
3. LISP Architecture
This section presents the LISP architecture, it first details the
design principles of LISP and then it proceeds to describe its main
aspects: data-plane, control-plane, and inetrworking mechanisms.
3.1. Design Principles
The LISP architecture is built on top of four basic design
principles:
o Locator/Identifier split: By decoupling the overloaded semantics
of the current IP addresses the Internet core can be assigned
identity meaningful addresses and hence, can use aggregation to
scale. Devices are assigned with relatively opaque identity
meaningful addresses that are independent of their topological
location.
o Overlay architecture: Overlays route packets over the current
Internet, allowing deployment of new protocols without changing
the current infrastructure hence, resulting into a low deployment
cost.
o Decoupled data and control-plane: Separating the data-plane from
the control-plane allows them to scale independently and use
different architectural approaches. This is important given that
they typically have different requirements and allows for other
data-planes to be added.
o Incremental deployability: This principle ensures that the
protocol interoperates with the legacy Internet while providing
some of the targeted benefits to early adopters.
3.2. Overview of the Architecture
LISP splits architecturally the core from the edge of the Internet by
creating two separate namespaces: Endpoint Identifiers (EIDs) and
Routing LOCators (RLOCs). The edge consists of LISP sites (e.g., an
Autonomous System) that use EID addresses. EIDs are typically -but
not limited to- IPv4 or IPv6 addresses that uniquely identify
communication end-hosts and are assigned and configured by the same
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mechanisms that exist at the time of this writing. EIDs do not
contain inter-domain topological information and can be thought as an
analogy to Provider Independent (PI [RFC4116]) addresses. Because of
this, EIDs are usually only routable at the edge with a LISP site.
With LISP, LISP sites (edge) and the core of the Internet are
interconnected by means of LISP-capable routers (e.g., border
routers) using tunnels. When packets originated from a LISP site are
flowing towards the core network, they ingress into an encapsulated
tunnel via an Ingress Tunnel Router (ITR). When packets flow from
the core network to a LISP site, they egress from an encapsulated
tunnel to an Egress Tunnel Router (ETR). An xTR is a router which
can perform both ITR and ETR operations. In this context ITRs
encapsulate packets while ETRs decapsulate them, hence LISP operates
as an overlay on top of the current Internet core.
/-----------------\ ---
| Mapping | |
. System | | Control
-| |`, | Plane
,' \-----------------/ . |
/ \ ---
,.., - _,..--..,, `, ,.., |
/ ` ,' ,-` `', . / ` |
/ \ +-----+ ,' `, +--'--+ / \ |
| EID |-| xTR |---/ RLOC ,---| xTR |-| EID | | Data
| Space |-| |---| Space |---| |-| Space | | Plane
\ / +-----+ . / +-----+ \ / |
`. .' `. ,' `. .' |
`'-` `., ,.' `'-` ---
``''--''``
LISP Site (Edge) Core LISP Site (Edge)
Figure 1.- A schema of the LISP Architecture
With LISP, the core uses RLOCs, an RLOC is typically -but not limited
to- an IPv4 or IPv6 address assigned to an Internet-facing network
interface of an ITR or ETR. Typically RLOCs are numbered from
topologically aggregatable blocks assigned to a site at each point to
which it attaches to the global Internet. The topology is defined by
the connectivity of networks, in this context RLOCs can be thought of
Provider Aggregatable addresses [RFC4116].
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A typically distributed database, called the Mapping System, stores
mappings between EIDs and RLOCs. Such mappings relate the identity
of the devices attached to LISP sites (EIDs) to the set of RLOCs
configured at the LISP-capable routers servicing the site.
Furthermore, the mappings also include traffic engineering policies
and can be configured to achieve multihoming and load balancing. The
LISP Mapping System is conceptually similar to the DNS where it is
organized as a distributed multi-organization network database. With
LISP, ETRs register mappings while ITRs retrieve them.
Finally, the LISP architecture emphasizes a cost effective
incremental deployment. Given that LISP represents an overlay to the
current Internet architecture, endhosts as well as intra and inter-
domain routers remain unchanged, and the only required changes to the
existing infrastructure are to routers connecting the EID with the
RLOC space. Such LISP capable routers, in most cases, only require a
software upgrade. Additionally, LISP requires the deployment of an
independent Mapping System, such distributed database is a new
network entity.
The following describes a simplified packet flow sequence between two
nodes that are attached to LISP sites. Client HostA wants to send a
packet to server HostB.
/----------------\
| Mapping |
| System |
.| |-
` \----------------/ `.
,` \
/ `.
,' _,..-..,, ',
/ -` `-, \
.' ,' \ `,
` ' \ '
+-----+ | | RLOC_B1+-----+
HostA | | | RLOC |-------| | HostB
EID_A--|ITR_A|----| Space | |ETR_B|--EID_B
| | RLOC_A1 |-------| |
+-----+ | | RLOC_B2+-----+
, /
\ /
`', ,-`
``''-''``
Figure 2.- Packet flow sequence in LISP
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1. HostA retrieves the EID_B of HostB (typically querying the DNS)
and generates an IP packet as in the Internet, the packet has
source address EID_A and destination address EID_B.
2. The packet is routed towards ITR_A in the LISP site using
standard intra-domain mechanisms.
3. ITR_A upon receiving the packet queries the Mapping System to
retrieve the locator of ETR_B that is servicing HostB's EID_B.
In order to do so it uses a LISP control message called Map-
Request, the message contains EID_B as the lookup key. In turn
it receives another LISP control message called Map-Reply, the
message contains two locators: RLOC_B1 and RLOC_B2 along with
traffic engineering policies: priority and weight per locator.
ITR_A also stores the mapping in a local cache to speed-up
forwarding of subsequent packets.
4. ITR_A encapsulates the packet towards RLOC_B1 (chosen according
to the priorities/weights specified in the mapping). The packet
contains two IP headers, the outer header has RLOC_A1 as source
and RLOC_B2 as destination, the inner original header has EID_A
as source and EID_B as destination. Furthermore ITR_A adds a
LISP header, more details about LISP encapsulation can be found
in Section 3.3.1.
5. The encapsulated packet is forwarded by the Internet core as a
normal IP packet, making the EID invisible from the Internet
core.
6. Upon reception of the encapsulated packet by ETR_B, it
decapsulates the packet and forwards it to HostB.
3.3. Data-Plane
This section provides a high-level description of the LISP data-
plane, which is specified in detail in [RFC6830]. The LISP data-
plane is responsible for encapsulating and decapsulating data packets
and caching the appropriate forwarding state. It includes two main
entities, the ITR and the ETR, both are LISP capable routers that
connect the EID with the RLOC space (ITR) and vice versa (ETR).
3.3.1. LISP Encapsulation
ITRs encapsulate data packets towards ETRs. LISP data packets are
encapsulated using UDP (port 4341). A particularity of LISP is that
UDP packets should include a zero checksum [RFC6935] [RFC6936] that
it is not verified in reception, LISP also supports non-zero
checksums that may be verified. This decision was made because the
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typical transport protocols used by the applications already include
a checksum, by neglecting the additional UDP encapsulation checksum
xTRs can forward packets more efficiently.
LISP-encapsulated packets also include a LISP header (after the UDP
header and before the original IP header). The LISP header is
prepended by ITRs and striped by ETRs. It carries reachability
information (see more details in Section 4.2) and the Instance ID
field. The Instance ID field is used to distinguish traffic to/from
different tenant address spaces at the LISP site and that may use
overlapped but logically separated EID addressing.
Overall, LISP works on 4 headers, the inner header the source
constructed, and the 3 headers a LISP encapsulator prepends ("outer"
to "inner"):
1. Outer IP header containing RLOCs as source and destination
addresses. This header is originated by ITRs and stripped by
ETRs.
2. UDP header (port 4341) with zero checksum. This header is
originated by ITRs and stripped by ETRs.
3. LISP header that contains various forwarding-plane features (such
as reachability) and an Instance ID field. This header is
originated by ITRs and stripped by ETRs.
4. Inner IP header containing EIDs as source and destination
addresses. This header is created by the source end-host and is
left unchanged by LISP data plane processing on the ITR and ETR.
Finally, in some scenarios Recursive and/or Re-encapsulating tunnels
can be used for Traffic Engineering and re-routing. Re-encapsulating
tunnels are consecutive LISP tunnels and occur when a decapsulator
(an ETR action) removes a LISP header and then acts as an encapsultor
(an ITR action) to prepend another one. On the other hand, Recursive
tunnels are nested tunnels and are implemented by using multiple LISP
encapsulations on a packet. Typically such functions are implemented
by Reencapsulating Tunnel Routers (RTRs).
3.3.2. LISP Forwarding State
ITRs retrieve from the LISP Mapping System mappings between EID
prefixes and RLOCs that are used to encapsulate packets. Such
mappings are stored in a local cache called the Map-Cache for
subsequent packets addressed to the same EID prefix. Mappings
include a (Time-to-Live) TTL (set by the ETR). More details about
the Map-Cache management can be found in Section 4.1.
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3.4. Control-Plane
The LISP control-plane, specified in [RFC6833], provides a standard
interface to register and request mappings. The LISP Mapping System
is a database that stores such mappings. The following first
describes the mappings, then the standard interface to the Mapping
System, and finally its architecture.
3.4.1. LISP Mappings
Each mapping includes the bindings between EID prefix(es) and set of
RLOCs as well as traffic engineering policies, in the form of
priorities and weights for the RLOCs. Priorities allow the ETR to
configure active/backup policies while weights are used to load-
balance traffic among the RLOCs (on a per-flow basis).
Typical mappings in LISP bind EIDs in the form of IP prefixes with a
set of RLOCs, also in the form of IPs. IPv4 and IPv6 addresses are
encoded using the appropriate Address Family Identifier (AFI)
[RFC3232]. However LISP can also support more general address
encoding by means of the ongoing effort around the LISP Canonical
Address Format (LCAF) [I-D.ietf-lisp-lcaf].
With such a general syntax for address encoding in place, LISP aims
to provide flexibility to current and future applications. For
instance LCAFs could support MAC addresses, geo-coordinates, ASCII
names and application specific data.
3.4.2. Mapping System Interface
LISP defines a standard interface between data and control planes.
The interface is specified in [RFC6833] and defines two entities:
Map-Server: A network infrastructure component that learns mappings
from ETRs and publishes them into the LISP Mapping System.
Typically Map-Servers are not authoritative to reply to queries
and hence, they forward them to the ETR. However they can also
operate in proxy-mode, where the ETRs delegate replying to queries
to Map-Servers. This setup is useful when the ETR has limited
resources (i.e., CPU or power).
Map-Resolver: A network infrastructure component that interfaces
ITRs with the Mapping System by proxying queries and in some cases
responses.
The interface defines four LISP control messages which are sent as
UDP datagrams (port 4342):
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Map-Register: This message is used by ETRs to register mappings in
the Mapping System and it is authenticated using a shared key
between the ETR and the Map-Server.
Map-Notify: When requested by the ETR, this message is sent by the
Map-Server in response to a Map-Register to acknowledge the
correct reception of the mapping and convey the latest Map-Server
state on the EID to RLOC mapping. In some cases a Map-Notify can
be sent to the previous RLOCs when an EID is registered by a new
set of RLOCs.
Map-Request: This message is used by ITRs or Map-Resolvers to
resolve the mapping of a given EID.
Map-Reply: This message is sent by Map-Servers or ETRs in response
to a Map-Request and contains the resolved mapping. Please note
that a Map-Reply may contain a negative reply if, for example, the
queried EID is not part of the LISP EID space. In such cases the
ITR typically forwards the traffic natively (non encapsulated) to
the public Internet, this behavior is defined to support
incremental deployment of LISP.
3.4.3. Mapping System
LISP architecturally decouples control and data-plane by means of a
standard interface. This interface glues the data-plane, routers
responsible for forwarding data-packets, with the LISP Mapping
System, a database responsible for storing mappings.
With this separation in place the data and control-plane can use
different architectures if needed and scale independently. Typically
the data-plane is optimized to route packets according to
hierarchical IP addresses. However the control-plane may have
different requirements, for instance and by taking advantage of the
LCAFs, the Mapping System may be used to store non-hierarchical keys
(such as MAC addresses), requiring different architectural approaches
for scalability. Another important difference between the LISP
control and data-planes is that, and as a result of the local mapping
cache available at ITR, the Mapping System does not need to operate
at line-rate.
The LISP WG has explored application of the following distributed
system techniques to the Mapping System architecture: graph-based
databases in the form of LISP+ALT [RFC6836], hierarchical databases
in the form of LISP-DDT [I-D.ietf-lisp-ddt], monolithic databases in
the form of LISP-NERD [RFC6837], flat databases in the form of LISP-
DHT [I-D.cheng-lisp-shdht],[I-D.mathy-lisp-dht] and, a multicast-
based database [I-D.curran-lisp-emacs]. Furthermore it is worth
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noting that, in some scenarios such as private deployments, the
Mapping System can operate as logically centralized. In such cases
it is typically composed of a single Map-Server/Map-Resolver.
The following focuses on the two mapping systems that have been
implemented and deployed (LISP-ALT and LISP+DDT).
3.4.3.1. LISP+ALT
The LISP Alternative Topology (LISP+ALT) [RFC6836] was the first
Mapping System proposed, developed and deployed on the LISP pilot
network. It is based on a distributed BGP overlay participated by
Map-Servers and Map-Resolvers. The nodes connect to their peers
through static tunnels. Each Map-Server involved in the ALT topology
advertises the EID-prefixes registered by the serviced ETRs, making
the EID routable on the ALT topology.
When an ITR needs a mapping it sends a Map-Request to a Map-Resolver
that, using the ALT topology, forwards the Map-Request towards the
Map-Server responsible for the mapping. Upon reception the Map-
Server forwards the request to the ETR that in turn, replies directly
to the ITR using the native Internet core.
3.4.3.2. LISP-DDT
LISP-DDT [I-D.ietf-lisp-ddt] is conceptually similar to the DNS, a
hierarchical directory whose internal structure mirrors the
hierarchical nature of the EID address space. The DDT hierarchy is
composed of DDT nodes forming a tree structure, the leafs of the tree
are Map-Servers. On top of the structure there is the DDT root node
[DDT-ROOT], which is a particular instance of a DDT node and that
matches the entire address space. As in the case of DNS, DDT
supports multiple redundant DDT nodes and/or DDT roots. Finally,
Map-Resolvers are the clients of the DDT hierarchy and can query
either the DDT root and/or other DDT nodes.
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/---------\
| |
| DDT Root|
| /0 |
,.\---------/-,
,-'` | `'.,
-'` | `-
/-------\ /-------\ /-------\
| DDT | | DDT | | DDT |
| Node | | Node | | Note | ...
| 0/8 | | 1/8 | | 2/8 |
\-------/ \-------/ \-------/
_. _. . -..,,,_
-` -` \ ````''--
+------------+ +------------+ +------------+ +------------+
| Map-Server | | Map-Server | | Map-Server | | Map-Server |
| EID-prefix1| | EID-prefix2| | EID-prefix3| | EID-prefix4|
+------------+ +------------+ +------------+ +------------+
Figure 3.- A schematic representation of the DDT tree structure,
please note that the prefixes and the structure depicted
should be only considered as an example.
The DDT structure does not actually index EID-prefixes but eXtended
EID-prefixes (XEID). An XEID-prefix is just the concatenation of the
following fields (from most significant bit to less significant bit):
Database-ID, Instance ID, Address Family Identifier and the actual
EID-prefix. The Database-ID is provided for possible future
requirements of higher levels in the hierarchy and to enable the
creation of multiple and separate database trees.
In order to resolve a query LISP-DDT operates in a similar way to the
DNS but only supports iterative lookups. DDT clients (usually Map-
Resolvers) generate Map-Requests to the DDT root node. In response
they receive a newly introduced LISP-control message: a Map-Referral.
A Map-Referral provides the list of RLOCs of the set of DDT nodes
matching a configured XEID delegation. That is, the information
contained in the Map-Referral points to the child of the queried DDT
node that has more specific information about the queried XEID-
prefix. This process is repeated until the DDT client walks the tree
structure (downwards) and discovers the Map-Server servicing the
queried XEID. At this point the client sends a Map-Request and
receives a Map-Reply containing the mappings. It is important to
note that DDT clients can also cache the information contained in
Map-Referrals, that is, they cache the DDT structure. This is used
to reduce the mapping retrieving latency[Jakab].
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The DDT Mapping System relies on manual configuration. That is Map-
Resolvers are manually configured with the set of available DDT root
nodes while DDT nodes are manually configured with the appropriate
XEID delegations. Configuration changes in the DDT nodes are only
required when the tree structure changes itself, but it doesn't
depend on EID dynamics (RLOC allocation or traffic engineering policy
changes).
3.5. Interworking Mechanisms
EIDs are typically identical to either IPv4 or IPv6 addresses and
they are stored in the LISP Mapping System, however they are usually
not announced in the Internet global routing system. As a result
LISP requires an inetrworking mechanism to allow LISP sites to speak
with non-LISP sites and vice versa. LISP inetrworking mechanisms are
specified in [RFC6832].
LISP defines two entities to provide inetrworking:
Proxy Ingress Tunnel Router (PITR): PITRs provide connectivity from
the legacy Internet to LISP sites. PITRs announce in the global
routing system blocks of EID prefixes (aggregating when possible)
to attract traffic. For each incoming packet from a source not in
a LISP site (a non-EID), the PITR LISP-encapsulates it towards the
RLOC(s) of the appropriate LISP site. The impact of PITRs in the
routing table size of the DFZ is, in the worst-case, similar to
the case in which LISP is not deployed. EID-prefixes will be
aggregated as much as possible both by the PITR and by the global
routing system.
Proxy Egress Tunnel Router (PETR): PETRs provide connectivity from
LISP sites to the legacy Internet. In some scenarios, LISP sites
may be unable to send encapsulated packets with a local EID
address as a source to the legacy Internet. For instance when
Unicast Reverse Path Forwarding (uRPF) is used by Provider Edge
routers, or when an intermediate network between a LISP site and a
non-LISP site does not support the desired version of IP (IPv4 or
IPv6). In both cases the PETR overcomes such limitations by
encapsulating packets over the network. There is no specified
provision for the distribution of PETR RLOC addresses to the ITRs.
4. LISP Operational Mechanisms
This section details the main operational mechanisms defined in LISP.
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4.1. Cache Management
LISP's decoupled control and data-plane, where mappings are stored in
the control-plane and used for forwarding in the data plane, requires
of a local cache in ITRs to reduce signaling overhead (Map-Request/
Map-Reply) and increase forwarding speed. The local cache available
at the ITRs, called Map-Cache, is used by the router to LISP-
encapsulate packets. The Map-Cache is indexed by (Instance ID, EID-
prefix) and contains basically the set of RLOCs with the associated
traffic engineering policies (priorities and weights).
The Map-Cache, as any other cache, requires cache coherence
mechanisms to maintain up-to-date information. LISP defines three
main mechanisms for cache coherence:
Time-To-Live (TTL): Each mapping contains a TTL set by the ETR, upon
expiration of the TTL the ITR has to refresh the mapping by
sending a new Map-Request. Typical values for TTL defined by LISP
are 24 hours.
Solicit-Map-Request (SMR): SMR is an explicit mechanism to update
mapping information. In particular a special type of Map-Request
can be sent on demand by ETRs to request refreshing a mapping.
Upon reception of a SMR message, the ITR must refresh the bindings
by sending a Map-Request to the Mapping System.
Map-Versioning: This optional mechanism piggybacks in the LISP
header of data-packets the version number of the mappings used by
an xTR. This way, when an xTR receives a LISP-encapsulated packet
from a remote xTR, it can check whether its own Map-Cache or the
one of the remote xTR is outdated. If its Map-Cache is outdated,
it sends a Map-Request for the remote EID so to obtain the newest
mappings. On the contrary, if it detects that the remote xTR Map-
Cache is outdated, it sends a SMR to notify it that a new mapping
is available.
Finally it is worth noting that in some cases an entry in the map-
cache can be proactively refreshed using the mechanisms described in
the section below.
4.2. RLOC Reachability
The LISP architecture is an edge to edge pull architecture, where the
network state is stored in the control-plane while the data-plane
pulls it on demand. On the contrary BGP is a push architecture,
where the required network state is pushed by means of BGP UPDATE
messages to BGP speakers. In push architectures, reachability
information is also pushed to the interested routers. However pull
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architectures require explicit mechanisms to propagate reachability
information. LISP defines a set of mechanisms to inform ITRs and
PITRS about the reachability of the cached RLOCs:
Locator Status Bits (LSB): LSB is a passive technique, the LSB field
is carried by data-packets in the LISP header and can be set by a
ETRs to specify which RLOCs of the ETR site are up/down. This
information can be used by the ITRs as a hint about the reachability
to perform additional checks. Also note that LSB does not provide
path reachability status, only hints on the status of RLOCs.
Echo-nonce: This is also a passive technique, that can only operate
effectively when data flows bi-directionally between two
communicating xTRs. Basically, an ITR piggybacks a random number
(called nonce) in LISP data packets, if the path and the probed
locator are up, the ETR will piggyback the same random number on the
next data-packet, if this is not the case the ITR can set the locator
as unreachable. When traffic flow is unidirectional or when the ETR
receiving the traffic is not the same as the ITR that transmits it
back, additional mechanisms are required.
RLOC-probing: This is an active probing algorithm where ITRs send
probes to specific locators, this effectively probes both the locator
and the path. In particular this is done by sending a Map-Request
(with certain flags activated) on the data-plane (RLOC space) and
waiting in return a Map-Reply, also sent on the data-plane. The
active nature of RLOC-probing provides an effective mechanism to
determine reachability and, in case of failure, switching to a
different locator. Furthermore the mechanism also provides useful
RTT estimates of the delay of the path that can be used by other
network algorithms.
Additionally, LISP also recommends inferring reachability of locators
by using information provided by the underlay, in particular:
It is worth noting that RLOC probing and Echo-nonce can work
together. Specifically if a nonce is not echoed, an ITR could RLOC-
probe to determine if the path is up when it cannot tell the
difference between a failed bidirectional path or the return path is
not used (a unidirectional path).
ICMP signaling: The LISP underlay -the current Internet- uses the
ICMP protocol to signal unreachability (among other things). LISP
can take advantage of this and the reception of a ICMP Network
Unreachable or ICMP Host Unreachable message can be seen as a hint
that a locator might be unreachable, this should lead to perform
additional checks.
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Underlay routing: Both BGP and IBGP carry reachability information,
LISP-capable routers that have access to underlay routing information
can use it to determine if a given locator or path are reachable.
4.3. ETR Synchronization
All the ETRs that are authoritative to a particular EID-prefix must
announce the same mapping to the requesters, this means that ETRs
must be aware of the status of the RLOCs of the remaining ETRs. This
is known as ETR synchronization.
At the time of this writing LISP does not specify a mechanism to
achieve ETR synchronization. Although many well-known techniques
could be applied to solve this issue it is still under research, as a
result operators must rely on coherent manual configuration
4.4. MTU Handling
Since LISP encapsulates packets it requires dealing with packets that
exceed the MTU of the path between the ITR and the ETR. Specifically
LISP defines two mechanisms:
Stateless: With this mechanism the effective MTU is assumed from the
ITR's perspective. If a payload packet is too big for the
effective MTU, and can be fragmented, the payload packet is
fragmented on the ITR, such that reassembly is performed at the
destination host.
Stateful: With this mechanism ITRs keep track of the MTU of the
paths towards the destination locators by parsing the ICMP Too Big
packets sent by intermediate routers. Additionally ITRs will send
ICMP Too Big messages to inform the sources about the effective
MTU.
In both cases if the packet cannot be fragmented (IPv4 with DF=1 or
IPv6) then the ITR drops it and replies with a ICMP Too Big message
to the source.
5. Mobility
The separation between locators and identifiers in LISP was initially
proposed for traffic engineering purpose where LISP sites can change
their attachment points to the Internet (i.e., RLOCs) without
impacting endpoints or the Internet core. In this context, the
border routers operate the xTR functionality and endpoints are not
aware of the existence of LISP. However, this mode of operation does
not allow seamless mobility of endpoints between different LISP sites
as the EID address might not be routable in a visited site.
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Nevertheless, LISP can be used to enable seamless IP mobility when
LISP is directly implemented in the endpoint or when the endpoint
roams to an attached xTR. Each endpoint is then an xTR and the EID
address is the one presented to the network stack used by
applications while the RLOC is the address gathered from the network
when it is visited.
Whenever the device changes of RLOC, the xTR updates the RLOC of its
local mapping and registers it to its Map-Server. To avoid the need
of a home gateway, the ITR also indicates the RLOC change to all
remote devices that have ongoing communications with the device that
moved. The combination of both methods ensures the scalability of
the system as signaling is strictly limited the Map-Server and to
hosts with which communications are ongoing.
6. Multicast
LISP also supports transporting IP multicast packets sent from the
EID space, the operational changes required to the multicast
protocols are documented in [RFC6831].
In such scenarios, LISP may create multicast state both at the core
and at the sites (both source and receiver). When signaling is used
to create multicast state at the sites, LISP routers unicast
encapsulate PIM Join/Prune messages from receiver to source sites.
At the core, ETRs build a new PIM Join/Prune message addressed to the
RLOC of the ITR servicing the source. An simplified sequence is
shown below
1. An end-host willing to join a multicast channel sends an IGMP
report. Multicast PIM routers at the LISP site propagate PIM
Join/Prune messages (S-EID, G) towards the ETR.
2. The join message flows to the ETR, upon reception the ETR builds
two join messages, the first one unicast LISP-encapsulates the
original join message towards the RLOC of the ITR servicing the
source. This message creates (S-EID, G) multicast state at the
source site. The second join message contains as destination
address the RLOC of the ITR servicing the source (S-RLOC, G) and
creates multicast state at the core.
3. Multicast data packets originated by the source (S-EID, G) flow
from the source to the ITR. The ITR LISP-encapsulates the
multicast packets, the outter header includes its own RLOC as the
source (S-RLOC) and the original multicast group address (G) as
the destination. Please note that multicast group address are
logical and are not resolved by the mapping system. Then the
multicast packet is transmitted through the core towards the
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receiving ETRs that decapsulates the packets and sends them using
the receiver's site multicast state.
LISP can also support non-PIM mechanisms to maintain multicast state.
7. Security
LISP uses a pull architecture to learn mappings. While in a push
system, the state necessary to forward packets is learned
independently of the traffic itself, with a pull architecture, the
system becomes reactive and data-plane events (e.g., the arrival of a
packet for an unknown destination) may trigger control-plane events.
This on-demand learning of mappings provides many advantages as
discussed above but may also affect the way security is enforced.
Usually, the data-plane is implemented in the fast path of routers to
provide high performance forwarding capabilities while the control-
plane features are implemented in the slow path to offer high
flexibility and a performance gap of several order of magnitude can
be observed between the slow and the fast paths. As a consequence,
the way data-plane events are notified to the control-plane must be
thought carefully so to not overload the slow path and rate limiting
should be used as specified in [RFC6830].
Care must also be taken so to not overload the mapping system (i.e.,
the control plane infrastructure) as the operations to be performed
by the mapping system may be more complex than those on the data-
plane, for that reason [RFC6830] recommends to rate limit the sending
of messages to the mapping system.
To improve resiliency and reduce the overall number of messages
exchanged, LISP offers the possibility to leak information, such as
reachabilty of locators, directly into data plane packets. In
environments that are not fully trusted, control informations gleaned
from data-plane packets should be verified before using them.
Mappings are the centrepiece of LISP and all precautions must be
taken to avoid them to be manipulated or misused by malicious
entities. Using trustable Map-Servers that strictly respect
[RFC6833] and the lightweight authentication mechanism proposed by
LISP-Sec [I-D.ietf-lisp-sec] reduces the risk of attacks to the
mapping integrity. In more critical environments, secure measures
may be needed.
As with any other tunneling mechanism, middleboxes on the path
between an ITR (or PITR) and an ETR (or PETR) must implement
mechanisms to strip the LISP encapsulation to correctly inspect the
content of LISP encapsulated packets.
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Like other map-and-encap mechanisms, LISP enables triangular routing
(i.e., packets of a flow cross different border routers depending on
their direction). This means that intermediate boxes may have
incomplete view on the traffic they inspect or manipulate.
More details about security implications of LISP are discussed in
[I-D.ietf-lisp-threats].
8. Use Cases
8.1. Traffic Engineering
BGP is the standard protocol to implement inter-domain routing. With
BGP, routing informations are propagated along the network and each
autonomous system can implement its own routing policy that will
influence the way routing information are propagated. The direct
consequence is that an autonomous system cannot precisely control the
way the traffic will enter the network.
As opposed to BGP, a LISP site can strictly impose via which ETRs the
traffic must enter the the LISP site network even though the path
followed to reach the ETR is not under the control of the LISP site.
This fine control is implemented with the mappings. When a remote
site is willing to send traffic to a LISP site, it retrieves the
mapping associated to the destination EID via the mapping system.
The mapping is sent directly by an authoritative ETR of the EID and
is not altered by any intermediate network.
A mapping associates a list of RLOCs to an EID prefix. Each RLOC
corresponds to an interface of an ETR (or set of ETRs) that is able
to correctly forward packets to EIDs in the prefix. Each RLOC is
tagged with a priority and a weight in the mapping. The priority is
used to indicates which RLOCs should be preferred to send packets
(the least preferred ones being provided for backup purpose). The
weight permits to balance the load between the RLOCs with the same
priority, proportionally to the weight value.
As mappings are directly issued by the authoritative ETR of the EID
and are not altered while transmitted to the remote site, it offers
highly flexible incoming inter-domain traffic engineering with even
the possibility for a site to issue a different mapping for each
remote site, implementing so precise routing policies.
8.2. LISP for IPv6 Co-existence
LISP encapsulations permits to transport packets using EIDs from a
given address family (e.g., IPv6) with packets from other address
families (e.g., IPv4). The absence of correlation between the
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address family of RLOCs and EIDs makes LISP a candidate to allow,
e.g., IPv6 to be deployed when all of the core network may not have
IPv6 enabled.
For example, two IPv6-only data centers could be interconnected via
the legacy IPv4 Internet. If their border routers are LISP capable,
sending packets between the data center is done without any form of
translation as the native IPv6 packets (in the EID space) will be
LISP encapsulated and transmitted over the IPv4 legacy Internet by
the mean of IPv4 RLOCs.
8.3. LISP for Virtual Private Networks
It is common to operate several virtual networks over the same
physical infrastructure. In such virtual private networks, it is
essential to distinguish which virtual network a packet belongs and
tags or labels are used for that purpose. With LISP, the distinction
can be made with the Instance ID field. When an ITR encapsulates a
packet from a particular virtual network (e.g., known via the VRF or
VLAN), it tags the encapsulated packet with the Instance ID
corresponding to the virtual network of the packet. When an ETR
receives a packet tagged with an Instance ID it uses the Instance ID
to determine how to treat the packet.
The main advantage of using LISP for virtual networks, on top of the
simplicity of managing the mappings, is that it does not impose any
requirement on the underlying network, as long as it is running IP.
8.4. LISP for Virtual Machine Mobility in Data Centers
A way to enable seamless virtual machine mobility in data center is
to conceive the datacenter backbone as the RLOC space and the subnet
where servers are hosted as forming the EID space. A LISP router is
placed at the border between the backbone and each subnet. When a
virtual machine is moved to another subnet, it can keep (temporarily)
the address it had before the move so to continue without a transport
layer connection reset. When an xTR detects a source address
received on a subnet to be an address not assigned to the subnet, it
registers the address to the Mapping System.
To inform the other LISP routers that the machine moved and where,
and then to avoid detours via the initial subnetwork, mechanisms such
as the Solicit-Map-Request messages are used.
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9. Security Considerations
This document does not specify any protocol or operational practices
and hence, does not have any security considerations.
10. IANA Considerations
This memo includes no request to IANA.
11. Acknowledgements
This document was initiated by Noel Chiappa and much of the core
philosophy came from him. The authors acknowledge the important
contributions he has made to this work and thank him for his past
efforts.
The authors would also like to thank Dino Farinacci, Fabio Maino,
Luigi Iannone, Sharon Barakai, Isidoros Kouvelas, Christian Cassar,
Florin Coras, Marc Binderberger, Alberto Rodriguez-Natal, Ronald
Bonica, Chad Hintz, Robert Raszuk, Joel M. Halpern, Darrel Lewis, as
well as every people acknowledged in [RFC6830].
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
Gill, "IPv4 Multihoming Practices and Limitations", RFC
4116, July 2005.
[RFC4984] Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
Workshop on Routing and Addressing", RFC 4984, September
2007.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830, January
2013.
[RFC6831] Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
Locator/ID Separation Protocol (LISP) for Multicast
Environments", RFC 6831, January 2013.
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[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.
[RFC6834] Iannone, L., Saucez, D., and O. Bonaventure, "Locator/ID
Separation Protocol (LISP) Map-Versioning", RFC 6834,
January 2013.
[RFC6835] Farinacci, D. and D. Meyer, "The Locator/ID Separation
Protocol Internet Groper (LIG)", RFC 6835, January 2013.
[RFC6836] Fuller, V., Farinacci, D., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol Alternative Logical
Topology (LISP+ALT)", RFC 6836, January 2013.
[RFC6837] Lear, E., "NERD: A Not-so-novel Endpoint ID (EID) to
Routing Locator (RLOC) Database", RFC 6837, January 2013.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935, April 2013.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, April 2013.
[RFC7052] Schudel, G., Jain, A., and V. Moreno, "Locator/ID
Separation Protocol (LISP) MIB", RFC 7052, October 2013.
[RFC7215] Jakab, L., Cabellos-Aparicio, A., Coras, F., Domingo-
Pascual, J., and D. Lewis, "Locator/Identifier Separation
Protocol (LISP) Network Element Deployment
Considerations", RFC 7215, April 2014.
12.2. Informative References
[Chiappa] Chiappa, J., "Endpoints and Endpoint names: A Propose
Enhancement to the Internet Architecture,
http://mercury.lcs.mit.edu/~jnc/tech/endpoints.txt", 1999.
[DDT-ROOT]
LISP DDT ROOT, , "http://ddt-root.org/", August 2013.
[DFZ] Huston, Geoff., "Growth of the BGP Table - 1994 to Present
http://bgp.potaroo.net/", August 2013.
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[I-D.cheng-lisp-shdht]
Cheng, L. and J. Wang, "LISP Single-Hop DHT Mapping
Overlay", draft-cheng-lisp-shdht-04 (work in progress),
July 2013.
[I-D.curran-lisp-emacs]
Brim, S., Farinacci, D., Meyer, D., and J. Curran, "EID
Mappings Multicast Across Cooperating Systems for LISP",
draft-curran-lisp-emacs-00 (work in progress), November
2007.
[I-D.ietf-lisp-ddt]
Fuller, V., Lewis, D., Ermagan, V., and A. Jain, "LISP
Delegated Database Tree", draft-ietf-lisp-ddt-02 (work in
progress), October 2014.
[I-D.ietf-lisp-lcaf]
Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
Address Format (LCAF)", draft-ietf-lisp-lcaf-06 (work in
progress), October 2014.
[I-D.ietf-lisp-sec]
Maino, F., Ermagan, V., Cabellos-Aparicio, A., and D.
Saucez, "LISP-Security (LISP-SEC)", draft-ietf-lisp-sec-07
(work in progress), October 2014.
[I-D.ietf-lisp-threats]
Saucez, D., Iannone, L., and O. Bonaventure, "LISP Threats
Analysis", draft-ietf-lisp-threats-10 (work in progress),
July 2014.
[I-D.mathy-lisp-dht]
Mathy, L., Iannone, L., and O. Bonaventure, ""LISP-DHT:
Towards a DHT to map identifiers onto locators" draft-
mathy-lisp-dht-00 (work in progress)", April 2008.
[Jakab] Jakab, L., Cabellos, A., Saucez, D., and O. Bonaventure,
"LISP-TREE: A DNS Hierarchy to Support the LISP Mapping
System, IEEE Journal on Selected Areas in Communications,
vol. 28, no. 8, pp. 1332-1343", October 2010.
[Quoitin] Quoitin, B., Iannone, L., Launois, C., and O. Bonaventure,
""Evaluating the Benefits of the Locator/Identifier
Separation" in Proceedings of 2Nd ACM/IEEE International
Workshop on Mobility in the Evolving Internet
Architecture", 2007.
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Appendix A. A Brief History of Location/Identity Separation
The LISP system for separation of location and identity resulted from
the discussions of this topic at the Amsterdam IAB Routing and
Addressing Workshop, which took place in October 2006 [RFC4984].
A small group of like-minded personnel from various scattered
locations within Cisco, spontaneously formed immediately after that
workshop, to work on an idea that came out of informal discussions at
the workshop and on various mailing lists. The first Internet-Draft
on LISP appeared in January, 2007.
Trial implementations started at that time, with initial trial
deployments underway since June 2007; the results of early experience
have been fed back into the design in a continuous, ongoing process
over several years. LISP at this point represents a moderately
mature system, having undergone a long organic series of changes and
updates.
LISP transitioned from an IRTF activity to an IETF WG in March 2009,
and after numerous revisions, the basic specifications moved to
becoming RFCs at the start of 2013 (although work to expand and
improve it, and find new uses for it, continues, and undoubtly will
for a long time to come).
A.1. Old LISP Models
LISP, as initially conceived, had a number of potential operating
modes, named 'models'. Although they are note used anymore, one
occasionally sees mention of them, so they are briefly described
here.
LISP 1: EIDs all appear in the normal routing and forwarding tables
of the network (i.e. they are 'routable');this property is used to
'bootstrap' operation, by using this to load EID->RLOC mappings.
Packets were sent with the EID as the destination in the outer
wrapper; when an ETR saw such a packet, it would send a Map-Reply
to the source ITR, giving the full mapping.
LISP 1.5: Similar to LISP 1, but the routability of EIDs happens on
a separate network.
LISP 2: EIDs are not routable; EID->RLOC mappings are available from
the DNS.
LISP 3: EIDs are not routable; and have to be looked up in in a new
EID->RLOC mapping database (in the initial concept, a system using
Distributed Hash Tables). Two variants were possible: a 'push'
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system, in which all mappings were distributed to all ITRs, and a
'pull' system in which ITRs load the mappings they need, as
needed.
Authors' Addresses
Albert Cabellos
UPC-BarcelonaTech
c/ Jordi Girona 1-3
Barcelona, Catalonia 08034
Spain
Email: acabello@ac.upc.edu
Damien Saucez (Ed.)
INRIA
2004 route des Lucioles BP 93
Sophia Antipolis Cedex 06902
France
Email: damien.saucez@inria.fr
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