An Architectural Introduction to the LISP Location-Identity Separation System
draft-ietf-lisp-introduction-05
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| Document | Type | Active Internet-Draft (lisp WG) | |
|---|---|---|---|
| Authors | Albert Cabellos-Aparicio , Damien Saucez | ||
| Last updated | 2014-09-22 | ||
| Replaces | draft-chiappa-lisp-introduction | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
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draft-ietf-lisp-introduction-05
Network Working Group A. Cabellos
Internet-Draft UPC-BarcelonaTech
Intended status: Informational D. Saucez (Ed.)
Expires: March 26, 2015 INRIA
September 22, 2014
An Architectural Introduction to the LISP Location-Identity Separation
System
draft-ietf-lisp-introduction-05.txt
Abstract
This document describes the Locator/ID Separation Protocol (LISP)
architecture, its main operational mechanisms as well as its design
rationale.
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 March 26, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. LISP Architecture . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Design Principles . . . . . . . . . . . . . . . . . . . . 4
2.2. Overview of the Architecture . . . . . . . . . . . . . . 4
2.3. Data-Plane . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.1. LISP encapsulation . . . . . . . . . . . . . . . . . 7
2.3.2. LISP Forwarding State . . . . . . . . . . . . . . . . 8
2.4. Control-Plane . . . . . . . . . . . . . . . . . . . . . . 9
2.4.1. LISP Mappings . . . . . . . . . . . . . . . . . . . . 9
2.4.2. Mapping System Interface . . . . . . . . . . . . . . 9
2.4.3. Mapping System . . . . . . . . . . . . . . . . . . . 10
2.5. Internetworking Mechanisms . . . . . . . . . . . . . . . 13
3. LISP Operational Mechanisms . . . . . . . . . . . . . . . . . 13
3.1. Cache Management . . . . . . . . . . . . . . . . . . . . 14
3.2. RLOC Reachability . . . . . . . . . . . . . . . . . . . . 14
3.3. ETR Synchronization . . . . . . . . . . . . . . . . . . . 15
3.4. MTU Handling . . . . . . . . . . . . . . . . . . . . . . 16
4. Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Traffic Engineering . . . . . . . . . . . . . . . . . . . 18
7.2. LISP for IPv6 Transition . . . . . . . . . . . . . . . . 19
7.3. LISP for Network Virtualization . . . . . . . . . . . . . 19
7.4. LISP for Virtual Machine Mobility in Data Centers . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. A Brief History of Location/Identity Separation . . 23
A.1. Old LISP Models . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
There is a rough consensus that the Internet routing and addressing
system is facing severe scalability issues [RFC4984]. Specifically,
the growth in the size of the routing tables of the Default-Free Zone
(DFZ) is accelerating and showing a supra-linear slope [DFZ]. The
main driving force behind this growth is the de-aggregation of BGP
prefixes, which results from the existing BGP multihoming and traffic
engineering mechanisms that are used -at the time of this writing- on
the Internet, as well as non-aggregatable address allocations.
This issue has two profound implications, on the one hand Internet
core routers are exposed to the network dynamics of the edge. For
instance this typically leads to an increased amount of BGP UPDATE
messages (churn), which results in additional processing requirements
of Internet core routers in order to timely compute the DFZ RIB.
Secondly, the supra-linear growth imposes strong requirements on the
size of the memory storing the DFZ FIB. Both aspects lead to an
increase on the development and production cost of high-end routers,
and it is unclear if the semiconductor and router manufacturer
industries will be able to cope, in the long-term, with such
stringent requirements in a cost-effective way[RFC4984].
Although this important scalability issue is relatively new, the
architectural reasons behind it are well-known many years ago.
Indeed, and as pointed out by [Chiappa], IP addresses have overloaded
semantics. 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.
The Locator/ID Separation Protocol (LISP), specified in [RFC6830], is
built on top of this basic idea: decoupling the IP address overloaded
semantics. LISP creates two separate namespaces, EIDs (End-host
IDentifiers) and RLOCs (Routing LOCators), both are -typically, but
not limited to- 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- are architecturally separated and interconnected by
LISP-capable routers. LISP also introduces a publicly accessible
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
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location. By taking advantage of such separation between location
and identity, the Internet core is populated with RLOCs which can be
quasi-static and highly aggregatable, hence scalable [Quoitin].
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] for the protocol
specifications along with the LISP deployment guidelines [RFC7215].
2. LISP Architecture
This section presents the LISP architecture, we first detail the
design principles of LISP and then we proceed to describe its main
aspects: data-plane, control-plane, and internetworking mechanisms.
2.1. 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 with
topological meaningful address and hence, can use aggregation to
scale. Devices are assigned with identity meaningful address that
are independent of its topological location.
o Overlay architecture: Overlays route packets over the current
Internet, allowing to deploy new protocols without changing the
current infrastructure hence, resulting from 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.
o Incremental deployability: This principle ensures that the
protocol is compatible with the legacy Internet while providing
some of the targeted benefits to early adopters.
2.2. 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 (RLOC). The edge are LISP sites (e.g., an
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Autonomous System) that use EID addresses. EIDs are typically -but
not limited to- IPv4 or IPv6 addresses that uniquely identify
endhosts and are assigned and configured by the same mechanisms that
we have at the time of this writing. EIDs can be are typically
Provider Independent (PI [RFC4116]) addresses and can be thought as
they don't contain intra-domain topological information. Because of
this, EIDs are usually only routable in the edge.
With LISP, LISP sites (edge) and the core of the Internet are inter-
connected by means of LISP-capable routers (e.g., border routers).
When they provide egress (from the core perspective) to a LISP site
they are called Egress Tunnel Routers (ETR), Ingress Tunnel Routers
(ITR) when they provide ingress, and xTR when they provide both.
ITRs and ETRs exchange packets by encapsulating them, hence LISP
operates as an overlay to 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 though as
Provider Aggregatable addresses [RFC4116].
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A publicly accessible and usually 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 can be thought as the
equivalent of a DNS that would be accessed by ETRs to register
mappings and by ITRs to retrieve them.
Finally, the LISP architecture has a strong emphasis in 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
typically require only a software upgrade. Additionally, LISP
requires the deployment of an independent Mapping System, this
distributed database is a new network entity.
In what follows we describe a simplified packet flow sequence between
two nodes that are attached to LISP sites. Client hostA wants to
send a packt 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. In order
to do so it uses a LISP control message called Map-Request, the
message contains EID_A 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 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 2.3.1.
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.
2.3. Data-Plane
This section describes the LISP data-plane, which is specified in
[RFC6830]. The LISP data-plane is responsible of 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 viceversa (ETR). We first describe how packets are LISP-
encapsulated and then we proceed to explain how ITRs cache forwarding
state.
2.3.1. 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
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it is not verified in reception, LISP also supports non-zero
checksums that may be verified. This decision was made because the
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). The LISP header is prepended by ITRs and striped by ETRs.
It carries reachability information (see more details in Section 3.2)
and the Instance ID field. The Instance ID field is used to
distinguish traffic that belongs to multiple tenants inside a LISP
site, and that may use overlapped but logically separated addressing
space.
Overall, LISP encapsulated data packets carry 4 headers [RFC6830]
("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 may contain reachability information 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
remains unchanged.
Finally and 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 an
ETR removes a LISP header and then acts as an ITR to prepend another
one. On the other hand, Recursive tunnels are nested tunnels and are
implemented by using multiple LISP encapsulations on a packet.
2.3.2. 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- to
increase the forwarding speed of subsequent packets addressed to the
same EID prefix. Mappings include a (Time-to-Live) TTL (set by the
ETR) and are expired according to this value, more details about the
Map-Cache management can be found in Section 3.1.
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2.4. Control-Plane
The LISP control-plane, specified in [RFC6833], provides a standard
interface to register, query, and retrieve mappings. The LISP
Mapping System, is a publicly accessible database that stores such
mappings. In what follows we first describe the mappings, then the
standard interface, and finally the Mapping System architecture.
2.4.1. 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. Such addresses are encoded
using a general syntax called LISP Canonical Address Format (LCAF),
specified in [I-D.ietf-lisp-lcaf]. The syntax is general enough to
support encoding of IPv4 and IPv6 addresses and any other type of
value.
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.
2.4.2. 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 low
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.
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 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.
2.4.3. Mapping System
LISP architecturally decouples control and data-plane by means of a
standard interface. This interface glues the data-plane, routers
responsible of forwarding data-packets, with the LISP Mapping System,
a publicly accessible database responsible of 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 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 discussed for the Mapping System architecture the
four main techniques available in distributed systems, namely: 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 [I-D.lear-lisp-nerd] and flat
databases in the form of LISP-DHT
[I-D.cheng-lisp-shdht],[I-D.mathy-lisp-dht]. Furthermore it is worth
noting that, in some scenarios such as private deployments, the
Mapping System can operate logically centralized. In such cases it
is typically composed of a single Map-Server/Map-Resolver.
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In what follows we focus on the two mapping systems that have been
implemented and deployed (LISP-ALT and LISP+DDT).
2.4.3.1. 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. All the
participating nodes connect to their peers through static tunnels.
Every ETR involved in the ALT topology advertises its EID prefixes
making the EID routable on the overlay.
When an ITR needs a mapping, it sends a Map-Request to a nearby ALT
router. The ALT routers then forward the Map-Request on the overlay
by inspecting their ALT routing tables. When the Map-Request reaches
the ETR responsible for the mapping, a Map-Reply is generated and
directly sent to the ITR's RLOC, without using the ALT overlay.
2.4.3.2. 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. The
following figure presents a schematic representation of the DDT
hierarchy.
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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|
+------------+ +------------+ +------------+ +------------+
Figre 3.- An schematic representation of the DDT tree structure,
please note that the prefixes and the structure depitected
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 iteratively and in a
similar way to the DNS. 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).
2.5. Internetworking Mechanisms
EIDs are typically identical to either IPv4 or IPv6 addresses and
they are announced at the LISP Mapping System, however they are
usually not announced in the Internet global routing system. As a
result LISP requires an internetworking mechanism to allow LISP sites
to speak with non-LISP sites and viceversa. LISP internetworking
mechanisms are specified in [RFC6832].
LISP defines two entities to provide internetworking:
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 data-packet, 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 Engress 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 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 allows to
overcome such limitations by encapsulating packets over the
network. Finally, the RLOC of PETRs must be statically configured
in ITRs.
3. LISP Operational Mechanisms
In this section we detail the main operational mechanisms defined in
LISP.
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3.1. 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 could refresh the mapping by sending
a new Map-Request. Typical values for TTL defined by LISP are
24h.
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 it a SMR to notify it that a new
mapping is available.
3.2. 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
architectures require of explicit mechanisms to propagate
reachability information. LISP defines a set of mechanisms to inform
ITRs and PITRS about the reachability of the cached RLOCs:
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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 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 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:
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.
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.
3.3. 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.
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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
3.4. 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 defienes two mechanisms:
Stateless: With this mechanism ITRs fragment packets that are too
big, typically 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.
In both cases if the packet cannot be framgneted (IPv4 with DF=1 or
IPv6) then the ITR drops it and replies with a ICMP Too Big message
to the source.
4. Mobility
LISP can also be used to enable mobility of devices not located in
LISP networks. The problem with mobility of such devices is that
their IP address changes whenever they change location, interrupting
so flows.
To enable mobility on such devices, the device can implement the xTR
functionality where the IP address presented to applications is an
EID that never changes while the IP address obtained from the network
is used by the xTR as RLOC. Packets are then transported on the
network using the IP address assigned to the device by the visited
network while at the application level IP addresses remain
independent of the location of the device.
Whenever the device changes of RLOC, the ITR 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 signalling is strictly limited the Map-Server and to
hosts with which communications are ongoing.
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5. Multicast
LISP also supports multicast environments, the operational changes
required to the multicast protocols are documented in [RFC6831].
In such scenarios, LISP creates multicast state both at the core and
at the sites (both source and receiver). In order 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 that belongs to a LISP site transmits a PIM Join/
Prune message (S-EID,G) to join a multicast group.
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 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
receiving ETRs that decapsulates the packets and sends them using
the receiver's site multicast state.
6. 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 must be
envisioned.
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
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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
though carefully so to not overload the slow path and rate limiting
should be used as specified in [RFC6830].
Care must also been 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 control informations,
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-Server that strictly respect [RFC6833]
and the lightweight authentication mechanism proposed by LISP-Sec
[I-D.ietf-lisp-sec] is a possibility to reduce the risk. In more
critical environments, stronger authentication may have to be used.
Packets are transported encapsulated with LISP meaning that devices
on the path between an ITR (or PITR) and an ETR (or PETR) cannot
correctly inspect the content of packets unless they implement
methods to strip the headers added by LISP. Similarly, mappings
enable triangular routing (i.e., packets of a flow cross different
border routers depending on their direction) which means that
intermediate boxes may have incomplete view on the traffic they
inspect or manipulate.
More details about security implications of LISP can be found in
[I-D.ietf-lisp-threats].
7. Use Cases
7.1. 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.
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As opposed to BGP, a LISP site can strictly impose via which ETRs the
traffic must enter the 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 the owner of 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 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 owner of the EID and 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.
7.2. LISP for IPv6 Transition
LISP encapsulations permits to transport packets using EIDs from a
given address family (e.g., IPv6) with packets with addresses
belonging to another address family (e.g., IPv4). The absence of
correlation between the address family of RLOCs and EIDs makes LISP a
candidate to ease the transition to IPv4.
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.
7.3. LISP for Network Virtualization
It is nowadays common to operate several virtual networks over the
same physical infrastructure. The current approach usually rely on
BGP/MPLS VPNs, where BGP is used to exchange routing information and
MPLS to segregate packets of the different logical networks. This
functionality could be achieved with LISP where the mappings and the
mapping system are used instead of BGP and the LISP encapsulation is
used to replace MPLS.
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In virtual networks, it is essential to distinguish to 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 threat the packet.
Appart from the simplicity of managing mappings, the advantage of
using LISP for virtual network is that it does not impose any
requirement on the underlying network, except running IP.
7.4. 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
subnetworks where servers are hosted as forming the EID space. A
LISP router is placed at the border between the backbone and each
sub-network. When a virtual machine is moved to another subnetwork,
it can (temporarily) keep the address of the sub-network it was
hosted before the move so to allow ongoing communications to subsist.
When a subnetwork detects the presence of a host with an address that
does not belong to the subnetwork (e.g., via a message sent by the
hypervisor), the LISP router of the new subnetwork registers the IP
address of the virtual machine as an EID to the Map-Server of the
subnetwork and associates its own address as RLOC.
To inform the other LISP routers that the machine moved and where,
and then to avoid detours via the initial subnetwork, every Map-
Server can listen on a predefined multicast address that is used as
source address for Map-Register. As a result, the Map-Notify sent
back by the Map-Server will be received by all the LISP routers that
hence automatically learn the new location of the virtual machine.
8. Security Considerations
This document does not specify any protocol or operational practices
and hence, does not have any security considerations.
9. IANA Considerations
This memo includes no request to IANA.
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10. Acknowledgements
To Do.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935, April 2013.
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[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, April 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.
11.2. 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.
[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.ermagan-lisp-nat-traversal]
Ermagan, V., Farinacci, D., Lewis, D., Skriver, J., Maino,
F., and C. White, "NAT traversal for LISP", draft-ermagan-
lisp-nat-traversal-03 (work in progress), March 2013.
[I-D.ietf-lisp-ddt]
Fuller, V., Lewis, D., Ermagan, V., and A. Jain, "LISP
Delegated Database Tree", draft-ietf-lisp-ddt-01 (work in
progress), March 2013.
[I-D.ietf-lisp-lcaf]
Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
Address Format (LCAF)", draft-ietf-lisp-lcaf-05 (work in
progress), May 2014.
[I-D.ietf-lisp-sec]
Maino, F., Ermagan, V., Cabellos-Aparicio, A., and D.
Saucez, "LISP-Security (LISP-SEC)", draft-ietf-lisp-sec-06
(work in progress), April 2014.
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[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.lear-lisp-nerd]
Lear, E., "NERD: A Not-so-novel EID to RLOC Database",
draft-lear-lisp-nerd-08 (work in progress), March 2010.
[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.
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. The first Internet-Draft on LISP appeared in January,
2007, along with a LISP mailing list at the IETF.
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
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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 initilly conceived, had a number of potential operating
modes, named 'models'. Although they are now obsolete, 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'
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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