Internet Research Task Force F. Templin, Ed.
(IRTF) Boeing Research & Technology
Internet-Draft August 12, 2010
Intended status: Experimental
Expires: February 13, 2011
The Internet Routing Overlay Network (IRON)
draft-templin-iron-10.txt
Abstract
Since the Internet must continue to support escalating growth due to
increasing demand, it is clear that current routing architectures and
operational practices must be updated. This document proposes an
Internet Routing Overlay Network for supporting sustainable growth
through Provider Independent addressing while requiring no changes to
end systems and no changes to the existing routing system. While
business considerations are an important determining factor for
widespread adoption, they are out of scope for this document. This
document is a product of the IRTF Routing Research Group.
Status of this Memo
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This Internet-Draft will expire on February 13, 2011.
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The Internet Routing Overlay Network . . . . . . . . . . . . . 6
3.1. IR[CE] - IRON Customer Edge Router . . . . . . . . . . . . 8
3.2. IR[VE] - IRON Virtual Prefix Company Edge Router . . . . . 8
3.3. IR[VC] - IRON Virtual Prefix Company Core Router . . . . . 9
3.4. IR[VP] - IRON Virtual Prefix Company Combined Router . . . 10
4. IRON Organizational Principles . . . . . . . . . . . . . . . . 11
5. IRON Initialization . . . . . . . . . . . . . . . . . . . . . 12
5.1. IR[VC] Initialization . . . . . . . . . . . . . . . . . . 13
5.2. IR[VE] Initialization . . . . . . . . . . . . . . . . . . 13
5.3. IR[CE] Initialization . . . . . . . . . . . . . . . . . . 14
6. IRON Operation . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. IR[CE] Operation . . . . . . . . . . . . . . . . . . . . . 15
6.2. IR[VE] Operation . . . . . . . . . . . . . . . . . . . . . 17
6.3. IR(VC) Operation . . . . . . . . . . . . . . . . . . . . . 18
6.4. IRON Reference Operating Scenarios . . . . . . . . . . . . 19
6.4.1. Both Hosts Within IRON EUNs . . . . . . . . . . . . . 19
6.4.2. Mixed IRON and Non-IRON Hosts . . . . . . . . . . . . 24
6.5. Mobility, Multihoming and Traffic Engineering
Considerations . . . . . . . . . . . . . . . . . . . . . . 27
6.5.1. Mobility Management . . . . . . . . . . . . . . . . . 27
6.5.2. Multihoming . . . . . . . . . . . . . . . . . . . . . 28
6.5.3. Inbound Traffic Engineering . . . . . . . . . . . . . 28
6.5.4. Outbound Traffic Engineering . . . . . . . . . . . . . 28
6.6. Renumbering Considerations . . . . . . . . . . . . . . . . 28
6.7. NAT Traversal Considerations . . . . . . . . . . . . . . . 29
6.8. Nested EUN Considerations . . . . . . . . . . . . . . . . 29
6.8.1. Host A Sends Packets to Host Z . . . . . . . . . . . . 30
6.8.2. Host Z Sends Packets to Host A . . . . . . . . . . . . 32
7. Additional Considerations . . . . . . . . . . . . . . . . . . 33
8. Related Initiatives . . . . . . . . . . . . . . . . . . . . . 33
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 34
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
12.1. Normative References . . . . . . . . . . . . . . . . . . . 34
12.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. IRON VPs Over Internetworks with Different
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Address Families . . . . . . . . . . . . . . . . . . 37
Appendix B. Scaling Considerations . . . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38
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1. Introduction
Growth in the number of entries instantiated in the Internet routing
system has led to concerns for unsustainable routing scaling
[I-D.narten-radir-problem-statement]. Operational practices such as
increased use of multihoming with IPv4 Provider-Independent (PI)
addressing are resulting in more and more fine-grained prefixes
injected into the routing system from more and more end-user
networks. Furthermore, the forthcoming depletion of the public IPv4
address space has raised concerns for both increased deaggregation
(leading to yet further routing table entries) and an impending
address space run-out scenario. At the same time, the IPv6 routing
system is beginning to see growth in IPv6 Provider-Aggregated (PA)
prefixes [BGPMON] which must be managed in order to avoid the same
routing scaling issues the IPv4 Internet now faces. Since the
Internet must continue to scale to accommodate increasing demand, it
is clear that new routing methodologies and operational practices are
needed.
Several related works have investigated routing scaling issues and
proposed solutions. Virtual Aggregation (VA) [I-D.ietf-grow-va] and
Aggregation in Increasing Scopes (AIS) [I-D.zhang-evolution] are
global routing proposals that introduce routing overlays with Virtual
Prefixes (VPs) to reduce the number of entries required in each
router's Forwarding Information Base (FIB) and Routing Information
Base (RIB). Routing and Addressing in Networks with Global
Enterprise Recursion (RANGER) [RFC5720] examines recursive
arrangements of enterprise networks that can apply to a very broad
set of use case scenarios [I-D.russert-rangers]. In particular,
RANGER supports encapsulation and secure redirection by treating each
layer in the recursive hierarchy as a virtual non-broadcast, multiple
access (NBMA) "link". RANGER is an architectural framework that
includes Virtual Enterprise Traversal (VET) [I-D.templin-intarea-vet]
and the Subnetwork Adaptation and Encapsulation Layer (SEAL)
(including the SEAL Control Message Protocol (SCMP))
[I-D.templin-intarea-seal] as its functional building blocks.
This document proposes an Internet Routing Overlay Network (IRON)
with goals of supporting sustainable growth while requiring no
changes to the existing routing system. IRON borrows concepts from
VA, AIS and RANGER, and further borrows concepts from the Internet
Vastly Improved Plumbing (Ivip) [I-D.whittle-ivip-arch] architecture
proposal along with its associated Translating Tunnel Router (TTR)
mobility extensions [TTRMOB]. Indeed, the TTR model to a great
degree inspired the IRON mobility architecture design discussed in
this document. The Network Address Translator (NAT) traversal
techniques adapted for IRON were inspired by the Simple Address
Mapping for Premises Legacy Equipment (SAMPLE) proposal
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[I-D.carpenter-softwire-sample].
IRON specifically seeks to provide scalable PI addressing without
changing the current BGP [RFC4271] routing system. IRON observes the
Internet Protocol standards [RFC0791][RFC2460]. Other network layer
protocols that can be encapsulated within IP packets (e.g., OSI/CLNP
[RFC1070], etc.) are also within scope.
The IRON is a global routing system comprising virtual overlay
networks managed by Virtual Prefix Companies (VPCs) that own and
manage Virtual Prefixes (VPs) from which End User Network (EUN) PI
prefixes (EPs) are delegated to customer sites. The IRON is
motivated by a growing customer demand for multihoming, mobility
management and traffic engineering while using stable PI addressing
to avoid network renumbering [RFC4192][RFC5887]. The IRON uses the
existing IPv4 and IPv6 global Internet routing systems as virtual
links for tunneling inner network protocol packets within outer IPv4
or IPv6 headers (see: Section 3). The IRON requires deployment of a
small number of new BGP core routers and supporting servers, as well
as IRON-aware routers/servers in customer EUNs. No modifications to
hosts, and no modifications to most routers are required.
Note: This document is offered in compliance with Internet Research
Task Force (IRTF) document stream procedures [RFC5743]; it is not an
IETF product and is not a standard. The views in this document were
considered controversial by the IRTF Routing Research Group (RRG) but
the RG reached a consensus that the document should still be
published. The document will undergo a period of review within the
RRG and through selected expert reviewers prior to publication. The
following sections discuss details of the IRON architecture.
2. Terminology
This document makes use of the following terms:
End User Network (EUN)
an edge network that connects an organization's devices (e.g.,
computers, routers, printers, etc.) to the Internet and possibly
also the IRON.
Internet Service Provider (ISP)
a service provider which physically connects customer EUNs to the
Internet. In other words, an ISP is responsible for providing IP
connectivity to a customer owning an EUN.
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Provider Aggregated (PA) address or prefix
a network layer address or prefix delegated to an EUN by an ISP.
Provider Independent (PI) address or prefix
a network layer address or prefix delegated to an EUN by a third
party independently of the EUN's ISP arrangements.
Virtual Prefix (VP)
a PI prefix block (e.g., an IPv4 /16, an IPv6 /20, an OSI NSAP
prefix, etc.) that is owned and managed by a Virtual Prefix
Company (VPC).
End User Network PI prefix (EP)
a more-specific PI prefix derived from a VP (e.g., an IPv4 /28, an
IPv6 /56, etc.) and delegated to an EUN by a VPC.
EP Address (EPA)
a network layer address belonging to an EP and assigned to the
interface of an end system in an EUN.
Locator
an IP address assigned to the interface of a router or end system
within a public or private network. Locators taken from public IP
prefixes are routable on a global basis, while locators taken from
private IP prefixes are made public via Network Address
Translation (NAT).
Virtual Prefix Company (VPC)
a company that owns and manages a set of VPs from which it
delegates End User Network PI Prefixes (EPs) to EUNs
Internet Routing Overlay Network (IRON)
an overlay network configured over the global Internet. The IRON
supports routing through encapsulation of inner packets with EPA
addresses within outer headers that use locator addresses.
3. The Internet Routing Overlay Network
The Internet Routing Overlay Network (IRON) consists of IRON Routers
(IRs) that automatically tunnel the packets of end-to-end
communication sessions within encapsulating headers used for
Internetwork routing. IRs use Virtual Enterprise Traversal (VET)
[I-D.templin-intarea-vet] in conjunction with the Subnetwork
Encapsulation and Adaptation Layer (SEAL) [I-D.templin-intarea-seal]
to encapsulate inner network layer packets within outer headers as
shown in Figure 1:
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+-------------------------+
| Outer headers with |
~ locator addresses ~
| (IPv4 or IPv6) |
+-------------------------+
| SEAL Header |
+-------------------------+ +-------------------------+
| Inner Packet Header | --> | Inner Packet Header |
~ with EP addresses ~ --> ~ with EP addresses ~
| (IPv4, IPv6, OSI, etc.) | --> | (IPv4, IPv6, OSI, etc.) |
+-------------------------+ +-------------------------+
| | --> | |
~ Inner Packet Body ~ --> ~ Inner Packet Body ~
| | --> | |
+-------------------------+ +-------------------------+
Inner packet before Outer packet after
before encapsulation after encapsulation
Figure 1: Encapsulation of Inner Packets Within Outer IP Headers
VET specifies the automatic tunneling mechanisms used for
encapsulation, while SEAL specifies the format and usage of the SEAL
header as well as a set of control messages. Most notably, IRs use
SEAL to deterministically exchange and authenticate control messages
such as indications of Path Maximum Transmission Unit (PMTU)
limitations.
The IRON is manifested through a business model in which Virtual
Prefix Companies (VPCs) own and manage virtual overlay networks
comprising a set of IRs that are distributed throughout the Internet
and serve highly-aggregated Virtual Prefixes (VPs). VPCs delegate
sub-prefixes from their VPs which they lease to customers as End User
Network PI prefixes (EPs). The customers in turn assign the EPs to
their customer edge IRs which connect their End User Networks (EUNs)
to the IRON.
VPCs may have no affiliation with the ISP networks from which
customers obtain their basic Internet connectivity. Therefore,
unless the ISP also acts as a VPC the customer must have two business
relationships - one with the ISP and a second with the VPC. In that
case, the VPC can open for business and begin serving their customers
immediately without the need to coordinate their activities with ISPs
or with other VPCs. Further details on business considerations are
out of scope for this document.
The IRON requires no changes to end systems and no changes to most
routers in the Internet. Instead, the IRON comprises IRs that are
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deployed either as new platforms or as modifications to existing
platforms. IRs may be deployed incrementally without disturbing the
existing Internet routing system, and act as waypoints (or "cairns")
for navigating the IRON. The functional roles for IRs are described
in the following sections.
3.1. IR[CE] - IRON Customer Edge Router
An IR[CE] is a Customer Edge router (or host with embedded gateway
function) that logically connects the customer's EUNs and their
associated EPs to the IRON via tunnels as shown in Figure 2. IR[CE]s
obtain EPs from VPCs and use them to number subnets and interfaces
within their EUNs. An IR[CE] can be deployed on the same physical
platform that also connects the customer's EUNs to its ISPs, but it
may also be a separate router or even a standalone server system
located within the EUN. (This model applies even if the EUN connects
to the ISP via a Network Address Translator (NAT) - see Section 6.7).
.-.
,-( _)-.
+--------+ .-(_ (_ )-.
| IR[CE] |--(_ ISP )
+---+----+ `-(______)-'
| <= T \ .-.
.-. u \ ,-( _)-.
,-( _)-. n .-(_ (- )-.
.-(_ (_ )-. n (_ Internet )
(_ EUN ) e `-(______)-
`-(______)-' l ___
| s => (:::)-.
+----+---+ .-(::::::::)
| Host | .-(::::::::::::)-.
+--------+ (:::: The IRON ::::)
`-(::::::::::::)-'
`-(::::::)-'
Figure 2: IR[CE] Connecting EUN to the IRON
3.2. IR[VE] - IRON Virtual Prefix Company Edge Router
An IR[VE] is a VPC's overlay network edge router that provides
forwarding and mapping services for the EPs owned by customer
IR[CE]s. In typical deployments, a VPC will deploy many IR[VE]s
around the IRON in a globally-distributed fashion (e.g., as depicted
in Figure 3) so that IR[CE] clients can discover those that are
nearby.
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+--------+ +--------+
| IR[VE] | | IR[VE] |
| Boston | | Tokyo |
+--+-----+ ++-------+
+--------+ \ /
| IR[VE] | \ ___ /
| Seattle| \ (:::)-. +--------+
+------+-+ .-(::::::::)------+ IR[VE] |
\.-(::::::::::::)-. | Paris |
(:::: The IRON ::::) +--------+
`-(::::::::::::)-'
+--------+ / `-(::::::)-' \ +--------+
| IR[VE] + | \--- + IR[VE] |
| Moscow | +----+---+ | Sydney |
+--------+ | IR[VE] | +--------+
| Cairo |
+--------+
Figure 3: IR[VE] Global Distribution Example
Each IR[VE] serves as a customer-facing tunnel endpoint router that
IR[CE]s form bidirectional tunnels with over the IRON. Each IR[VE]
also associates with an Internet-facing IR[VC] that can forward
packets from the IRON out to the native public Internet and vice-
versa as discussed in the next section.
3.3. IR[VC] - IRON Virtual Prefix Company Core Router
An IR[VC] is a VPC's overlay network core router that acts as a
gateway between the IRON and the native public Internet. It
therefore also serves as an Autonomous System Border Router (ASBR)
that is owned and managed by the VPC.
Each VPC configures one or more IR[VC]s which advertise the company's
VPs into the IPv4 and IPv6 global Internet BGP routing systems. Each
IR[VC] associates with all of the VPC's overlay network edge routers,
e.g., via tunnels over the IRON, via a direct interconnect such as an
Ethernet cable, etc. The IR[VC] role is depicted in Figure 4:
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,-( _)-.
.-(_ (_ )-.
(_ Internet )
`-(______)-'
|
+----+---+
| IR[VC] |
+----+---+
_|_
(:::)-.
.-(::::::::)
+--------+ .-(::::::::::::)-. +--------+
| IR[VE] | (:::: The IRON ::::) | IR[VE] |
+--------+ `-(::::::::::::)-' +--------+
`-(::::::)-'
+--------+
| IR[VE] |
+--------+
Figure 4: IR[VC] Connecting IRON to Native Internet
3.4. IR[VP] - IRON Virtual Prefix Company Combined Router
An IR[VP] is a VPC's overlay network router that combines the
functions of both the IR[VE] and IR[VC]. While not in itself a
fundamental building block of the architecture, it is mentioned here
to clarify an implementation option available to VPCs.
In the IR[VP] model, the IR[VE] and IR[VC] functions can be thought
of as "half-gateway" functions that together comprise a unified
IR[VP]. The IR[VE] and IR[VC] functions can therefore be discussed
separately even when both functions reside within the same physical
IR[VP] platform as shown in Figure 5:
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,-( _)-.
.-(_ (_ )-.
(_ Internet )
`-(______)-'
|
+----------+----------+
| IR[VC] half-gateway |
+---------------------+
| IR[VE] half-gateway |
+----------+----------+
<- IR[VP] Unified Gateway ->
_|_
(:::)-.
.-(::::::::)
.-(::::::::::::)-.
(:::: The IRON ::::)
`-(::::::::::::)-'
`-(::::::)-'
Figure 5: IR[VP] Combining IR[VE] and IR[VC] Functions
4. IRON Organizational Principles
The IRON consists of the union of all VPC overlay networks worldwide
(where each VPC configures one or more overlay networks). Each such
overlay network represents a distinct "patch" on the Internet
"quilt", where the patches are stitched together by tunnels over the
links, routers, bridges, etc. that connect the public Internet. When
a new VPC overlay network is deployed, it becomes yet another patch
on the quilt. The IRON is therefore a composite overlay network
consisting of multiple individual patches, where each patch can
coordinate its activities independently of all others (with the
exception that each patch must be aware of all VP's in the IRON).
Each VPC overlay network in the IRON maintains a set of IR[VC]s that
connect the overlay network directly to the public IPv4 and IPv6
Internets. Each IR[VC] advertises the VPC overlay network's IPv4 VPs
into the IPv4 BGP routing system and advertises the overlay network's
IPv6 VPs into the IPv6 BGP routing system. IR[VC]s will therefore
receive packets with EPA destination addresses sent by end systems in
the Internet then re-encapsulate and forward them toward the correct
EPA-addressed end systems connected to the VPC overlay network.
Each VPC overlay network also manages a set of IR[VE]s that connect
customer EUNs to the IRON and to the IPv6 and IPv4 Internets via
their associations with IR[VC]s. IR[VE]s therefore need not be BGP
routers themselves and can be simple commodity hardware platforms.
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Moreover, the IR[VE] and IR[VC] functions can be deployed together on
the same physical platform as an IR[VP] or they may be deployed on
separate platforms (e.g., for load balancing purposes).
Each IR[VE] maintains a working set of IR[CE]s for which it caches
EP-to-IR[CE] mappings in its Forwarding Information Base (FIB). Each
IR[VE] also in turn propagates the list of EPs in its working set to
each of the IR[VC]s in the VPC overlay network via a dynamic routing
protocol (e.g., an overlay network internal BGP instance that carries
only the EP-to-IR[VE] mappings and does not interact with the
external BGP routing system). Each IR[VE] therefore only needs to
track the EPs for its current working set of IR[CE]s, while each
IR[VC] will maintain a full EP-to-IR[VE] mapping table that
represents reachability information for all EPs in the VPC overlay
network.
Customers establish IR[CE]s to connect their EUNs to both the VPC
overlay network and to the rest of the IRON. Each EUN can connect to
the IRON via one or multiple IR[CE]s as long as the multiple IR[CE]s
coordinate with one another, e.g., to mitigate EUN partitions.
Unlike IR[VC]s and IR[VE]s, IR[CE]s may use private addresses behind
one or several layers of NATs. The IR[CE] initially discovers a list
of nearby IR[VE]s through an anycast discovery process. It then
selects one of these nearby IR[VE]s as its server and forms a two-way
tunnel with the IR[VE] through an initial exchange followed by
periodic keepalives.
After the IR[CE] selects a serving IR[VE], it forwards outbound
packets from its EUNs by tunneling them to an IR[VC]/IR[VE] within
the IRON that serves the final destination. When the IR[CE] cannot
tunnel packets directly to an IR[VC]/IR[VE] that serves the final
destination (e.g., when the destination address is a non-EPA address)
it instead tunnels them to its own serving IR[VE].
The IRON can also be used to support VPs of network layer address
families that cannot be routed natively in the underlying
Internetwork (e.g., OSI/CLNP within the public Internet, IPv6 within
IPv4-only Internetworks, IPv4 within IPv6-only Internetworks, etc.).
Further details for support of IRON VPs over non-native Internetworks
are discussed in Appendix A.
5. IRON Initialization
IRON initialization entails the startup actions of IRs within the VPC
overlay network and customer EUNs. The following sections discuss
these startups procedures.
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5.1. IR[VC] Initialization
Before its first operational use, each IR[VC] in a VPC overlay
network is provisioned with the list of VPs that it will serve as
well as the locators for all IR[VE]s that belong to the same overlay
network. The IR[VC] is also provisioned with external BGP
interconnections the same as for any BGP router.
Upon startup, the IR[VC] engages in BGP routing exchanges with its
peers in the IPv4 and IPv6 Internets the same as for any BGP router.
It then connects to all of the IR[VE]s in the overlay network (e.g.,
via a TCP connection over a two-way tunnel, via an iBGP route
reflector, etc.) for the purpose of discovering EP->IR[VE] mappings.
After the IR[VC] has fully populated its EP->IR[VE] mapping
information database, it is said to be "synchronized" wrt its VPs.
After this initial synchronization procedure, the IR[VC] then
advertises the overlay network's VPs externally. In particular, the
IR[VC] advertises the IPv6 VPs into the IPv6 BGP routing system and
advertises the IPv4 VPs into the IPv4 BGP routing system. If the
IR[VC] only services IPv6 VPs (e.g., 2001:DB8::/32), it advertises
the IPv6 VPs into the IPv6 routing system and also advertises a
companion IPv4 prefix (e.g., 192.0.2.0/24) into the IPv4 routing
system that can be used by IR[CE]s/IR[VE]s from other VPC overlay
networks for anycast discovery purposes. Similarly, if the IR[VC]
only services IPv4 VPs, it also advertises a companion IPv6 prefix
(e.g., 2001:DB8::/56) into the IPv6 routing system. (See Appendix A
for more information on the discovery and use of companion prefixes.)
The IR[VC] then engages in ordinary packet forwarding operations.
5.2. IR[VE] Initialization
Before its first operational use, each IR[VE] in a VPC overlay
network is provisioned with the locators for all IR[VC]s that serve
the overlay network's VPs. In order to support route optimization,
the IR[VE] must also be provisioned with the list of all VPs in the
IRON (i.e., and not just the VPs of it own overlay network) so that
it can discern EPA and non-EPA addresses. The IR[VE] should also
discover the VP companion prefix relationships discussed in Section
5.1, e.g., via a global database such as discussed in Appendix A.
Upon startup, each IR[VE] must connect to all of the IR[VC]s within
its overlay network (e.g., via a TCP connection over a two-way
tunnel, via an iBGP route reflector, etc.) for the purpose of
reporting its EP->IR[VE] mappings. The IR[VE] then actively listens
for IR[CE] customers which register their EP prefixes as part of
establishing a two-way tunnel. When a new IR[CE] registers its EP
prefixes, the IR[VE] announces the new EP additions to all IR[VC]s;
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when an existing IR[CE] unregisters its EP prefixes, the IR[VE]
withdraws its announcements.
5.3. IR[CE] Initialization
Before its first operational use, each IR[CE] must obtain one or more
EPs from its VPC as well as any companion prefixes of other address
families (see Section 5.1) associated with the EPs. The IR[CE] must
also be provisioned with the list of all VPs in the IRON (i.e., and
not just the VPs of its own overlay network) so that it can discern
EPA and non-EPA addresses. The IR[CE] could therefore be greatly
simplified if the list of VPs could be covered within a small number
of very short prefixes, e.g., one or a few IPv6 ::/20's.
The IR[CE] must also obtain a certificate and a public/private key
pair from the VPC that it can later use to prove ownership of its
EPs. This implies that each VPC must run its own key infrastructure
to be used only for the purpose of verifying a customer's claimed
right to use an EP. Hence, the VPC need not coordinate its key
infrastructure with any other organization.
Upon startup, the IR[CE] sends a SEAL Control Message Protocol (SCMP)
Router Solicitation (SRS) message using an implicit anycast procedure
to discover the nearest IR[VC] in its VPC overlay network. The
IR[VC] will in turn return a list of locators of the company's nearby
IR[VE]s. (This list is analogous to the ISATAP Potential Router List
(PRL) [RFC5214].)I
To perform the implicit anycast procedure, the IR[CE] sets the source
address of the SRS message to one of its locator addresses and sets
the destination address of the message to any EPA taken from one of
its own EPs. (If the EP is of a different address family than the
IR[CE]'s locators, however, the IR[CE] instead sets the destination
address to any address taken from the companion prefix associated
with the EP.) This SRS message will be delivered to the nearest
IR[VC] that attaches the VPC overlay network to the Internet. When
the IR[VC] receives the SRS message, it sends back an SCMP Router
Advertisement (SRA) message that lists the locator addresses of one
or more nearby IR[VE] routers.
After the IR[CE] receives an SRA message from the nearby IR[VC]
listing the locator addresses of nearby IR[VE]s, it sends SRS test
messages to one or more of the locator addresses to elicit SRA
messages. The IR[VE] that configures the locator will include the
header of the soliciting SRS message in its SRA message so that the
IR[CE] can determine the number of hops along the forward path. The
IR[VE] also includes a metric in its SRA messages indicating its
service availability so that the IR[CE] can avoid selecting IR[VE]s
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that are overloaded. The IR[VE] also includes a challenge/response
puzzle that the IR[CE] must answer if it wishes to enlist this
IR[VE]'s services.
When the IR[CE] receives these SRA messages, it can measure the round
trip time between sending the SRS and receiving the SRA as an
indication of round-trip delay. If the IR[CE] wishes the enlist the
services of a specific IR[VE] (e.g., based on the measured
performance), it then calculates the answer to the puzzle using its
keying information and sends the answer back to the IR[VE] in a new
SRS message that also contains all of the IR[CE]'s EP prefixes for
which it claims ownership. If the IR[CE] answered the puzzle
correctly, the IR[VE] will send back a new SRA message that includes
a non-zero default router lifetime and that signifies the
establishment of a two-way tunnel. (A zero default router lifetime
on the other hand signifies that the IR[VE] is currently unable to
establish a two-way tunnel, e.g., due to heavy load, due to
challenge/response failure, etc.)
Note that in the above procedure it is essential that the IR[CE]
select one and only one IR[VE]. This is to allow the VPC overlay
network mapping system to have one and only one active EP-to-IR[VE]
mapping at any point in time which shares fate with the IR[VE]
itself. If this IR[VE] fails, the IR[CE] will quickly select a new
one which will automatically update the VPC overlay network mapping
system with a new EP-to-IR[VE] mapping.
6. IRON Operation
Following the IRON initialization detailed in Section 5, IRs engage
in the steady-state process of receiving and forwarding packets. All
IRs forward encapsulated packets over the IRON using the mechanisms
of VET [I-D.templin-intarea-vet] and SEAL [I-D.templin-intarea-seal],
while IR[VC]s and IR[VE]s additionally forward packets to and from
the native IPv6 and IPv4 Internets. IRs also use the SEAL Control
Message Protocol (SCMP) to coordinate with other IRs, including the
process of sending and receiving redirect messages for route
optimization. Each IR operates as specified in the following sub-
sections.
6.1. IR[CE] Operation
After selecting its serving IR[VE] as specified in Section 5.3, the
IR[CE] should register each of its ISP connections with the IR[VE] in
order to establish multiple two-way tunnels for multihoming purposes.
To do so, it sends periodic SRS messages to its serving IR[VE] via
each of its ISPs to establish additional two-way tunnels and to keep
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each two-way tunnel alive. These messages need not include
challenge/response mechanisms since prefix proof of ownership was
already established in the initial exchange and a nonce in the SEAL
header can be used to confirm that the SRS message was sent by the
correct IR[CE]. This implies that a single nonce is used to
represent the set of all two-way tunnels between the IR[CE] and the
IR[VE]. Therefore, there are multiple two-way tunnels, and the nonce
names this "bundle" of tunnels.
If the IR[CE] ceases to receive SRA messages from its serving IR[VE]
via a specific ISP connection, it marks the IR[VE] as unreachable
from that address and therefore over that ISP connection. (The
IR[CE] must also inform its serving IR[VE] of this outage via one of
its working ISP connections.) If the IR[CE] ceases to receive SRA
messages from its serving IR[VE] via multiple ISP connections, it
marks the IR[VE] as unusable and quickly attempts to establish a
connection with a new IR[VE]. The act of establishing the connection
with a new serving IR[VE] will automatically purge the stale mapping
state associated with the old serving IR[VE].
When an end system in an EUN has a packet to send, the packet is
forwarded through the EUN via normal routing until it reaches the
IR[CE], which then tunnels the packet either to its serving IR[VE]s
or to an IR[VC]/IR[VE] that serves the packet's final destination.
When the IR[CE] does not know an outer destination locator address
that can be used to reach an IR[VC]/IR[VE] that serves the packet's
final destination (or, if the final destination is a non-EPA address)
the IR[CE] encapsulates the packet in an outer header with its
locator as the source address and the locator of its serving IR[VE]
as the destination address.
Otherwise, when the inner destination address matches the address
family of the IR[CE]'s locator, the IR[CE] encapsulates the packet in
an outer header with its locator as the source address and the
destination address of the inner packet copied into the destination
address of the outer packet. When the inner destination address does
not match the address family of the IR[CE]'s locator, but the IR[CE]
knows of an outer locator address that can reach an IR/[VC]/IR[VE]
that serves the final destination, the IR[CE] encapsulates the packet
with the outer destination address set to this outer locator address.
The IR[CE] then forwards the encapsulated packet via one of its ISP
connections, where normal Internet routing will convey it to an
IR[VC]/IR[VE] that services the destination.
The IR[CE] uses the mechanisms specified in VET and SEAL to
encapsulate each forwarded packet. The IR[CE] further uses the SCMP
protocol to coordinate with other IRs, including accepting redirect
messages that indicate a better next hop. When the IR[CE] receives
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an SCMP redirect, it checks the identification field of the
encapsulated message to verify that the redirect corresponds to a
packet that it had previously sent and accepts the redirect if there
is a match. Thereafter, subsequent packets forwarded by the source
IR[CE] will follow a route-optimized path.
6.2. IR[VE] Operation
After an IR[VE] is initialized, it responds to SRSs from IR[CE]s by
sending SRAs as described in Section 6.1. When the IR[VE] receives
an SRS message from a new IR[CE], it sends back an SRA message with a
challenge/response puzzle. The IR[CE] in turn sends an SRS message
with an answer to the puzzle. If this authentication fails, the
IR[VE] discards the message. Otherwise, it creates tunnel state for
this new IR[CE], records the EPs in its FIB, and records the locator
address from the SCMP message as the link-layer address of the next
hop. The IR[VE] next sends an SRA message back to the IR[CE] to
complete the tunnel establishment.
When the IR[VE] receives a SEAL-encapsulated packet from one of its
IR[CE] tunnel endpoints, it examines the inner destination address.
If the inner destination address is not an EPA, the IR[VE]
decapsulates the packet and forwards it unencapsulated into the
Internet if it is able to do so without loss due to ingress
filtering. Otherwise, the IR[VE] re-encapsulates the packet (i.e.,
it removes the outer header and replaces it with a new outer header
of the same address family) and sets the outer destination address to
the locator address of an IR[VC] within its VPC overlay network. It
then forwards the re-encapsulated packet to the IR[VC], which will in
turn decapsulate it and forward it into the Internet.
If the inner destination address is an EPA, however, the IR[VE] re-
encapsulates the packet, sets the outer source address to one of its
own locator address, and sets the outer destination address to the
inner destination address. (If the outer header is of a different
address family than the inner header, however, the IR[VE] instead
sets the destination address to any address taken from the companion
prefix associated with the inner destination address.) The IR[VE]
then forwards the re-encapsulated packet into the Internet via a
default or more-specific route. The IR[VE] may then receives SCMP
redirect messages from an IR[VC]/IR[VE] that serves the destination
EUN. In that case, the IR[VE] forwards the redirect message to the
IR[CE] that sent the original inner packet. The source and
destination addresses of the forwarded SCMP redirect message use the
outer destination and source addresses of the original packet,
respectively. This arrangement is necessary to allow the redirect
messages to flow through any NATs on the path.
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When the IR[VE] receives a SEAL-encapsulated packet from an IR[VC] or
from the Internet, if the inner destination address matches an EP in
its FIB the IR[VE] 'A' re-encapsulates the packet and forwards it to
its client IR[CE] 'B' which in turn decapsulates the packet and
forwards it to the correct end system in the EUN. If 'B' has left
notice with 'A' that it has moved to a new IR[VE] 'C', however, 'A'
will instead forward the re-encapsulated packet to 'C' and also send
an SCMP redirect message back to the source of the packet. In this
way, IR[CE]s can change between IR[VE]s (e.g., due to mobility
events) without exposing packets to loss.
6.3. IR(VC) Operation
After an IR[VC] has synchronized its VPs (see: Section 5.1) it
advertises the full set of the company's VP's into the IPv4 and IPv6
Internet BGP routing systems. The VPs will be represented as
ordinary routing information in the BGP, and any packets originating
from the IPv4 or IPv6 Internet destined to an EPA covered by one of
the VPs will be forwarded into the VPC's overlay network by an
IR[VC].
When an IR[VC] receives a packet from the Internet destined to an EPA
covered by one of its VPs, it examines the packet format to determine
the proper handling procedures as follows:
o If the packet is an SCMP SRS message, the IR[VC] sends an SRA
message back to the source listing the locator addresses of nearby
IR[VE] routers then discards the message. The IR[VC] silently
discards all other SCMP messages.
o If the packet is not SEAL-encapsulated the IR[VC] looks in its FIB
to discover a locator of the IR[VE] that serves the destination
address. The IR[VC] then simply encapsulates the packet with its
own locator as the outer source address and the locator of the
IR[VE] as the outer destination address and forwards the packet to
the IR[VE].
o If the packet is SEAL-encapsulated the IR[VC] sends an SCMP
redirect message of the same address family back to the source
with the locator of the serving IR[VE] as the redirected target.
The source and destination addresses of the SCMP redirect message
use the outer destination and source addresses of the original
packet, respectively. This arrangement is necessary to allow the
redirect messages to flow through any NATs on the path. After
sending the redirect message, the IR[VC] then rewrites the outer
source address to one of its own locators, rewrites the outer
destination address to the locator of the IR[VE] and forwards the
packet to the IR[VE] (*).
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(*) Note that in this arrangement any errors that occur on the path
between the IR[VC] to the IR[VE] will not be delivered to the
original source. This implies that the path between the IR[VC] and
IR[VE] should be made as free from errors as possible (e.g., such as
when the IR[VC] and IR[VE] are connected to the same physical link).
6.4. IRON Reference Operating Scenarios
The IRON is used to support communications when one or both hosts are
located within EP-addressed EUNs regardless of whether the EPs are
provisioned by the same VPC or by different VPCs . When both hosts
are within IRON EUNs, route redirections that eliminate unnecessary
IR[VC]s (and sometimes also IR[VE]s) from the path are possible.
When only one host is within an IRON EUN, however, route optimization
cannot be used.
The following sections discuss the two scenarios. Note that it is
sufficient to discuss the scenarios in a unidirectional fashion,
i.e., by tracing packet flows only in the forward direction from the
source host to destination host. The reverse direction can be
considered separately, and incurs the same considerations as for the
forward direction.
6.4.1. Both Hosts Within IRON EUNs
When both hosts are within EP-addressed EUNs, the initial packets of
the flow may need to involve an IR[VC] of the destination host but
route optimization can eliminate the IR[VC] from the path for
subsequent packets. Two sub-scenarios exist based on whether or not
the IR[CE] of the source host configures a locator of the same
address family as the inner packet. The sub-cases are discussed in
the following sections.
6.4.1.1. IR[CE] of Source Host Configures a Locator of the Same
Protocol Version as the EPA
Figure 6 shows the flow of initial packets from host A to host B
within two EP-addressed EUNs when the IR[CE] of the source host A
configures a locator of the same protocol version as the inner
packet:
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________________________________________
.-( .-. )-.
.-( ,-( _)-. )-.
.-( +=================+ _ +========+ )-.
.( // (_|| Internet|| _) || ).
.( // ||-(______)|| vv ).
.( // || || +------------+ ).
( // vv || | IR[VE](B) |====+ )
( // +------------+ +------------+ \\ )
( // .-. | IR[VC](B) | .-. \\ )
( //,-( _)-. +------------+ ,-( _)-\\ )
( .||_ (_ )-. / .-(_ (_ ||. )
( _|| ISP A .) / (redirect) (__ ISP B ||_))
( ||-(______)-' / `-(______)|| )
( || | / | vv )
( +-----+-----+ <=/ +-----+-----+ )
| IR[CE](A) | | IR[CE](B) |
+-----+-----+ The IRON +-----+-----+
| ( (Overlaid on the native Internet) ) |
.-. .-( .-) .-.
,-( _)-. .-(________________________)-. ,-( _)-.
.-(_ (_ )-. .-(_ (_ )-.
(_ IRON EUN A ) (_ IRON EUN B )
`-(______)-' `-(______)-'
| |
+---+----+ +---+----+
| Host A | | Host B |
+--------+ +--------+
Figure 6: EPA/Locator Matching Scenario Before Redirects
In this scenario, host A sends packets destined to host B (i.e.,
packets with source address A and destination address B) via its
network interface connected to EUN A. (This interface could be a
physical interface such as an Ethernet NIC, an ISATAP or VET tunnel
virtual interface with IR[CE](A) as a PRL router, etc.) Routing with
EUN A will direct the packets to IR[CE](A) as a default router for
the EUN which then uses VET and SEAL to encapsulate them in outer
headers with its locator address as the outer source address and B as
the outer destination address (i.e., the inner and outer destination
address will be the same). IR[CE](A) then releases the encapsulated
packets into its ISP network connection that provided its locator.
The ISP will release the packet into the Internet without filtering
since the (outer) source address is topologically correct. Once the
packets have been released into the Internet, routing will direct
them to the nearest IR[VC] that advertises reachability to a VP that
covers destination address B (namely, IR[VC](B)).
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IR[VC](B) will receive the encapsulated packets from IR[CE](A) then
check its FIB to discover an entry that covers address B with
IR[VE](B) as the next hop. IR[VC](B) will then issue SCMP redirect
messages to inform IR[CE](A) that IR[VE](B) is a better next hop (*).
IR[VC](B) then rewrites the outer source address of the encapsulated
packets to its own locator address and rewrites the destination
address of the encapsulated packets to the locator address of
IR[VE](B). IR[VC](B) then forwards these re-encapsulated packets to
IR[VE](B).
IR[VE](B) will receive the encapsulated packets from IR[VC](B) then
check its FIB to discover an entry that covers destination address B
with IR[CE](B) as the next hop. IR[VE](B) then rewrites the outer
source address of the packets to its own locator address and rewrites
the outer destination address to the locator address of IR[CE](B).
IR[VE](B) then tunnels these re-encapsulated packets to IR[CE](B),
which will in turn decapsulate the packets and forward the inner
packets to host B via EUN B.
(*) Note that after the initial flow of packets, IR[CE](A) will have
received one or more SCMP redirect messages from IR[VC](B) informing
it of IR[VE](B) as a better next hop. Thereafter, IR[CE](A) will
forward its encapsulated packets directly to the locator address of
IR[VE](B) without involving IR[VC](B) as shown in Figure 7:
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________________________________________
.-( .-. )-.
.-( ,-( _)-. )-.
.-( +=============> .-(_ (_ )-.======+ )-.
.( // (__ Internet _) || ).
.( // `-(______)-' vv ).
.( // +------------+ ).
( // | IR[VE](B) |====+ )
( // +------------+ \\ )
( // .-. .-. \\ )
( //,-( _)-. ,-( _)-\\ )
( .||_ (_ )-. .-(_ (_ ||. )
( _|| ISP A .) (__ ISP B ||_))
( ||-(______)-' `-(______)|| )
( || | | vv )
( +-----+-----+ The IRON +-----+-----+ )
| IR[CE](A) | (Overlaid on the native Internet) | IR[CE](B) |
+-----+-----+ +-----+-----+
| ( ) |
.-. .-( .-) .-.
,-( _)-. .-(________________________)-. ,-( _)-.
.-(_ (_ )-. .-(_ (_ )-.
(_ IRON EUN A ) (_ IRON EUN B )
`-(______)-' `-(______)-'
| |
+---+----+ +---+----+
| Host A | | Host B |
+--------+ +--------+
Figure 7: EPA/Locator Matching Scenario After Redirects
6.4.1.2. IR[CE] of Source Host Configures a Locator of a Different
Protocol Version than the EPA
Figure 8 shows the flow of initial packets from host A to host B
within two EP-addressed EUNs when the IR[CE] of source host A cannot
configure a locator of the same address family as the inner network
layer protocol. For example, if the IR[CE] configures only an IPv4
locator, but EUN A uses IPv6 natively, IR[CE] is obliged to forward
its initial packets through its serving IR[VE].
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________________________________________
.-( .-. )-.
.-( ,-( _)-. )-.
.-( +========+(_ (_ +=====+ )-.
.( || (_|| Internet ||_) || ).
.( || ||-(______)-|| vv ).
.( +--------++--+ || || +------------+ ).
( +==>| IR[VE](A) | vv || | IR[VE](B) |====+ )
( // +------------+ +--++----++--+ +------------+ \\ )
( // .-. | IR[VC](B) | .-. \\ )
( //,-( _)-. +------------+ ,-( _)-\\ )
( .||_ (_ )-. / .-(_ (_ ||. )
( _|| ISP A .) / (redirect) (__ ISP B ||_))
( ||-(______)-' / `-(______)|| )
( || | / | vv )
( +-----+-----+ <=/ +-----+-----+ )
| IR[CE](A) | | IR[CE](B) |
+-----+-----+ The IRON +-----+-----+
| ( (Overlaid on the native Internet) ) |
.-. .-( .-) .-.
,-( _)-. .-(________________________)-. ,-( _)-.
.-(_ (_ )-. .-(_ (_ )-.
(_ IRON EUN A ) (_ IRON EUN B )
`-(______)-' `-(______)-'
| |
+---+----+ +---+----+
| Host A | | Host B |
+--------+ +--------+
Figure 8: EPA/Locator Mismatching Scenario Before Redirects
In this scenario, host A sends packets destined to host B via its
network interface connected to EUN A. Routing with EUN A will direct
the packets to IR[CE](A) as a default router for the EUN which then
uses VET and SEAL to encapsulate them in outer headers with its
locator address as the outer source address and the locator address
of its serving IR[VE](A) as the outer destination address. IR[CE](A)
then simply releases the encapsulated packets into its ISP network
connection that provided its locator. The ISP will release the
packets into the Internet without filtering since the (outer) source
address is topologically correct. Once the packets have been
released into the Internet, routing will direct them to IR[VE](A).
IR[VE](A) receives the encapsulated packets from IR[CE](A) then
rewrites the outer source address to its own locator address and
rewrites the outer destination address to an address taken from the
companion prefix associated with the VP that matches B. IR[VE](A)
then releases the re-encapsulated packets into the Internet where
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routing will direct them to IR[VC](B) which advertises the companion
prefix..
IR[VC](B) will receive the encapsulated packets from IR[VE](A) then
check its FIB to discover an entry that covers inner destination
address B with IR[VE](B) as the next hop. IR[VC](B) will then issue
SCMP redirect messages to inform IR[VE](A) that IR[VE](B) is a better
next hop (*). IR[VC](B) then rewrites the outer source address of
the encapsulated packets to its own locator address and rewrites the
outer destination address to the locator address of IR[VE](B).
IR[VC](B) then forwards these re-encapsulated packets to IR[VE](B).
IR[VE](B) will receive the encapsulated packets from IR[VC](B) then
check its FIB to discover an entry that covers destination address B
with IR[CE](B) as the next hop. IR[VE](B) then re-encapsulates the
packet in an outer header with its own locator address as the outer
source address and the locator address of IR[CE](B) as the outer
destination address. IR[VE](B) then releases these re-encapsulated
packets into the Internet, where routing will direct them to
IR[CE](B). IR[CE](B) will in turn decapsulate the packets and
forward the inner packets to host B via EUN B.
(*) Note that after the initial flow of packets, IR[VE](A) will have
received one or more SCMP redirect messages from IR[VC](B) informing
it of IR[VE](B) as a better next hop. IR[VE](A) will in turn forward
the redirects to IR[CE](A), which will thereafter forward its
encapsulated packets directly to the locator address of IR[VE](B)
without involving either IR[VE](A) or IR[VC](B) as shown earlier in
Figure 7.
6.4.2. Mixed IRON and Non-IRON Hosts
When one host is within an IRON EUN and the other is in a non-IRON
EUN (i.e., one that connects to the native Internet instead of the
IRON), the IR elements involved depend on the packet flow directions.
The cases are described in the following sections.
6.4.2.1. From IRON Host A to Non-IRON Host B
Figure 9 depicts the IRON reference operating scenario for packets
flowing from Host A in an IRON EUN to Host B in a non-IRON EUN:
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_________________________________________
.-( )-. )-.
.-( +-------)----+ )-.
.-( | IR[VC](A) |--------------+ )-.
.( +------------+ \ ).
.( +=======>| IR[VE](A) | \ ).
.( // +--------)---+ \ ).
( // ) \ )
( // The IRON ) \ )
( // .-. ) \ .-. )
( //,-( _)-. ) \ ,-( _)-. )
( .||_ (_ )-. ) The Native Internet .-|_ (_ )-. )
( _|| ISP A ) ) (_ | ISP B ))
( ||-(______)-' ) |-(______)-' )
( || | )-. v | )
( +-----+ ----+ )-. +-----+-----+ )
| IR[CE](A) |)-. | Router B |
+-----+-----+ +-----+-----+
| ( ) |
.-. .-(____________________________________)-. .-.
,-( _)-. ,-( _)-.
.-(_ (_ )-. .-(_ (_ )-.
(_ IRON EUN A ) (_ non-IRON EUN )
`-(______)-' `-(___B___)-'
| |
+---+----+ +---+----+
| Host A | | Host B |
+--------+ +--------+
Figure 9: From IRON Host A to Non-IRON Host B
In this scenario, host A sends packets destined to host B via its
network interface connected to IRON EUN A. Routing with EUN A will
direct the packets to IR[CE](A) as a default router for the EUN which
then uses VET and SEAL to encapsulate them in outer headers with its
locator address as the outer source address and the locator address
of a serving IR[VE] (i.e., IR[VE](A) as the outer destination
address. The ISP will pass the packets without filtering since the
(outer) source address is topologically correct. Once the packets
have been released into the native Internet, routing will direct them
to IR[VE](A).
IR[VE](A) receives the encapsulated packets from IR[CE](A) then
forwards them to IR[VC](A) which simply decapsulates them and
releases the unencapsulated packets into the Internet. Once the
packets are released into the Internet, routing will direct them to
the final destination B. (Note that in this diagram IR[VE](A) and
IR[VC](A) are depicted as two halves of a unified IR[VP](A). In that
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case, the "forwarding" between IR[VE](A) and IR[VC](A) is a zero-
instruction imaginary operation.)
Note that this scenario always involves an IR[VE](A) and IR[VC](A)
owned by the VPC that provides service to IRON EUN A. This scenario
therefore imparts a cost that would need to be borne by either the
VPC or its customers.
6.4.2.2. From Non-IRON Host B to IRON Host A
Figure 10 depicts the IRON reference operating scenario for packets
flowing from Host B in an Non-IRON EUN to Host A in an IRON EUN:
_______________________________________
.-( )-. )-.
.-( +-------)----+ )-.
.-( | IR[VC](A) |<-------------+ )-.
.( +------------+ \ ).
.( +========| IR[VE](A) | \ ).
.( // +--------)---+ \ ).
( // ) \ )
( // The IRON ) \ )
( // .-. ) \ .-. )
( //,-( _)-. ) \ ,-( _)-. )
( .||_ (_ )-. ) The Native Internet .-|_ (_ )-. )
( _|| ISP A ) ) (_ | ISP B ))
( ||-(______)-' ) |-(______)-' )
( vv | )-. | | )
( +-----+ ----+ )-. +-----+-----+ )
| IR[CE](A) |)-. | Router B |
+-----+-----+ +-----+-----+
| ( ) |
.-. .-(____________________________________)-. .-.
,-( _)-. ,-( _)-.
.-(_ (_ )-. .-(_ (_ )-.
(_ IRON EUN A ) (_ non-IRON EUN )
`-(______)-' `-(___B___)-'
| |
+---+----+ +---+----+
| Host A | | Host B |
+--------+ +--------+
Figure 10: From Non-IRON Host B to IRON Host A
In this scenario, host B sends packets destined to host A via its
network interface connected to non-IRON EUN B. Routing will direct
the packets to IR[VC](A) which then forwards them to IR[VE](A) using
encapsulation if necessary. (Note that in this diagram IR[VE](A) and
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IR[VC](A) are depicted as two halves of a unified IR[VP](A). In that
case, the "forwarding" between IR[VE](A) and IR[VC](A) is a zero-
instruction imaginary operation.)
IR[VE](A) will then check its FIB to discover an entry that covers
destination address A with IR[CE](A) as the next hop. IR[VE](A) then
encapsulates the packets using its own locator address as the outer
source address and the locator address of IR[CE](A) as the outer
destination address. IR[VE](A) then releases these encapsulated
packets into the Internet, where routing will direct them to
IR[CE](A). IR[CE](A) will in turn decapsulate the packets and
forward the inner packets to host A via its network interface
connected to IRON EUN A.
Note that this scenario always involves an IR[VE](A) and IR[VC](A)
owned by the VPC that provides service to IRON EUN A. This scenario
therefore imparts a cost that would need to be borne by either the
VPC or its customers.
6.5. Mobility, Multihoming and Traffic Engineering Considerations
While IR[VE]s and IR[VC]s can be considered as fixed infrastructure,
IR[CE]s may need to move between different network points of
attachment, connect to multiple ISPs, or explicitly manage their
traffic flows. The following sections discuss mobility, multi-homing
and traffic engineering considerations for IR[CE]s.
6.5.1. Mobility Management
When an IR[CE] changes its network point of attachment (e.g., due to
a mobility event), it configures one or more new locators. If the
IR[CE] has not moved far away from its previous network point of
attachment, it simply informs its serving IR[VE] of any locator
additions or deletions. This operation is performance-sensitive, and
should be conducted immediately to avoid packet loss.
If the IR[CE] has moved far away from its previous network point of
attachment, however, it re-issues the implicit anycast discovery
procedure described in Section 6.1 to discover whether its candidate
set of serving IR[VE]s has changed. If the IR[CE]'s current serving
IR[VE] is also included in the new list received from the VPC, this
serves as indication that the IR[CE] has not moved far enough to
warrant changing to a new serving IR[VE]. Otherwise, the IR[CE] may
wish to move to a new serving IR[VE] in order to maintain optimal
routing. This operation is not performance-critical, and therefore
can be conducted over a matter of seconds/minutes instead of
milliseconds/microseconds.
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To move to a new IR[VE], the IR[CE] first engages in the EP
registration process with the new IR[VE] and maintains the
registrations through periodic SRS/SRA exchanges the same as
described in Section 6.1. The IR[CE] then informs its former IR[VE]
that it has moved by providing it with the locator address of the new
IR[VE]. The IR[CE] then discontinues the SRS/SRA keepalive process
with the former IR[VE], which will garbage-collect the stale FIB
entries when their lifetime expires. This will allow the former
IR[VE] to redirect existing correspondents to the new IR[VE] so that
no packets are lost.
6.5.2. Multihoming
An IR[CE] may register multiple locators with its serving IR[VE]. It
can assign metrics with its registrations to inform its IR[VE] of
preferred locators, and can select outgoing locators according to its
local preferences. Multihoming is therefore naturally supported.
6.5.3. Inbound Traffic Engineering
An IR[CE] can dynamically adjust the priorities of its prefix
registrations with its serving IR[VE] in order to influence inbound
traffic flows. It can also change between serving IR[VE]s when
multiple IR[VE]s are available, but should strive for stability in
its IR[VE] selection in order to limit routing churn.
6.5.4. Outbound Traffic Engineering
An IR[CE] can select outgoing locators, e.g., based on current QoS
considerations such as minimizing one-way delay or one-way delay
variation.
6.6. Renumbering Considerations
As better link layer technologies and service plans emerge, customers
will be motivated to select their service providers through healthy
competition between ISPs. If a customer's EUN addresses are tied to
a specific ISP, however, the customer may be forced to undergo a
painstaking EUN renumbering process if it wishes to changes to a
different ISP [RFC4192][RFC5887].
When a customer obtains EP prefixes from a VPC, it can change between
ISPs seamlessly and without need to renumber. If the VPC itself
applies unreasonable costing structures for use of the EPs, however,
the customer may be compelled to seek a different VPC and would again
be required to confront a renumbering scenario. The IRON approach to
renumbering avoidance therefore depends on VPCs conducting ethical
business practices and offering reasonable rates.
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6.7. NAT Traversal Considerations
The Internet today consists of a global public IPv4 routing and
addressing system with non-IRON EUNs that use either public or
private IPv4 addressing. The latter class of EUNs connect to the
public Internet via Network Address Translators (NATs). When an
IR[CE] is located behind a NAT, its selects IR[VE]s using the same
procedures as for IR[CE]s with public addresses, i.e., it will send
SRS messages to IR[VE]s in order to get SRA messages in return. The
only requirement is that the IR[CE] must configure its SEAL
encapsulation to use a transport protocol that supports NAT
traversal, namely UDP.
Since the IR[VE] maintains state about its IR[CE] customers, it can
discover locator information for each IR[CE] by examining the UDP
port number and IP address in the outer headers of SRS messages.
When there is a NAT in the path, the UDP port number and IP address
in the SRS message will correspond to state in the NAT box and might
not correspond to the actual values assigned to the IR[CE]. The
IR[VE] can then encapsulate packets destined to hosts serviced by the
IR[CE] within outer headers that use this IP address and UDP port
number. The NAT box will receive the packets, translate the values
in the outer headers to match those assigned to the IR[CE], then
forward the packets to the IR[CE]. In this sense, the IR[VE]'s
"locator" for the IR[CE] consists of the concatenation of the IP
address and UDP port number.
IRON does not introduce any new issues to complications raised for
NAT traversal or for applications embedding address referrals in
their payload.
6.8. Nested EUN Considerations
Each IR[CE] configures a locator that may be taken from an ordinary
non-EPA address assigned by an ISP or from an EPA address taken from
an EP assigned to another IR[CE]. In that case, the IR[CE] is said
to be "nested" within the EUN of another IR[CE].
For example, assume a configuration in which IR[CE](A) configures a
locator EPA(B) taken from the EP assigned to EUN(B). IR[CE](B) in
turn configures a locator EPA(C) taken from the EP assigned to
EUN(C). Finally, IR[CE](C) assigns a locator ISP(D) taken from a
non-EPA address delegated by an ordinary ISP(D). Using this example,
the "nested-IRON" case must be examined in which a host A which
configures the address EPA(A) within EUN(A) exchanges packets with
host Z located elsewhere in the Internet. The example configuration
is depicted in Figure 11:
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.-.
EPA(D) ,-( _)-.
+-----------+ .-(_ (_ )-.
| IR[CE](C) |--(_ ISP(D) )
+-----+-----+ `-(______)-'
| <= T \ .-.
.-. u \ ,-( _)-.
,-( _)-. n .-(_ (- )-.
.-(_ (_ )-. n (_ Internet )
(_ EUN(C) ) e `-(______)- +--------+
`-(______)-' l ___ | Host Z |
| EPA(C) s => (:::)-. +--------+
+-----+-----+ .-(::::::::)
| IR[CE](B) | .-(::::::::::::)-.
+-----+-----+ (:::: The IRON ::::)
| `-(::::::::::::)-'
.-. `-(::::::)-'
,-( _)-.
.-(_ (_ )-. +-----------------+
(_ EUN(B) ) | IR[VP/VC/VE]'s] |
`-(______)-' +-----------------+
| EPA(B)
+-----+-----+
| IR[CE](A) |
+-----------+
|
.-.
,-( _)-. EPA(A)
.-(_ (_ )-. +--------+
(_ EUN(A) )---| Host A |
`-(______)-' +--------+
Figure 11: Nested EUN Example
The two cases of host A sending packets to host Z, and host Z sending
packets to host A, must be considered separately as described below.
6.8.1. Host A Sends Packets to Host Z
6.8.1.1. Nested IRON Example When Z Configures an EPA Address
Host A first forwards a packet with source address EPA(A) and
destination address EPA(Z) into EUN(A). Routing within EUN(A) will
direct the packet to IR[CE](A), which encapsulates it in an outer
header with EPA(B) as the outer source address and EPA(Z) as the
outer destination address then forwards the once-encapsulated packet
into EUN(B). Routing within EUN[B] will direct the packet to
IR[CE](B), which encapsulates it in an outer header with EPA(C) as
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the outer source address and EPA(Z) as the outer destination address
then forwards the twice-encapsulated packet into EUN(C). Routing
within EUN(C) will direct the packet to IR[CE](C), which encapsulates
it in an outer header with ISP(D) as the outer source address and
EPA(Z) as the outer destination address. IR[CE](C) then sends this
triple-encapsulated packet into the ISP(D) network, where it will be
routed into the Internet to an IR[VC](Z) that advertises a VP that
covers destination address EPA(Z).
When IR[VC](Z) receives the triple-encapsulated packet, it consults
its FIB to determine that IR[VE](Z) is the serving router for EP(Z).
It then re-encapsulates the packet by changing the outer source
address to its own locator address and the outer destination address
to the locator address for IR[VE](Z). It also sends a redirect
message back to IR[CE](C) as normal. When IR[VE](Z) receives the
triple-encapsulated packet, it strips off all outer layers of
encapsulation and re-encapsulates the inner packet in a single outer
header using its own locator address as the source address and the
locator address of IR[CE](Z) as the destination address. IR[VE](Z)
then tunnels the packet to IR[CE](Z), which decapsulates the packet
and forwards it to host Z.
The key architectural requirement derived from this case is that each
IR[VE] must iteratively decapsulate each layer of a multi-
encapsulated packet when the outer destination address matches an EPA
assigned to one of its IR[CE] customers. When the final such layer
of encapsulation is reached, the IR[VE] must re-encapsulate the
packet and forward it to the correct customer IR[CE].
6.8.1.2. Nested IRON Example when Z Configures a non-EPA Address
Host A first forwards a packet with source address EPA(A) and
destination address Z into EUN(A). Routing within EUN(A) will direct
the packet to IR[CE](A), which encapsulates it in an outer header
with EPA(B) as the outer source address and IR[VE](A) as the outer
destination address then forwards this once-encapsulated packet into
EUN(B). (Note that IR[CE](A) must forward this packet via its
serving IR[VE](A) for reasons explained in Section 6.4.2). Routing
within EUN[B] will direct the packet to IR[CE](B), which encapsulates
it in an outer header with EPA(C) as the outer source address and
IR[VE](B) as the outer destination address then forwards this twice-
encapsulated packet into EUN(C). Routing within EUN(C) will direct
the packet to IR[CE](C), which encapsulates it in an outer header
with ISP(D) as the outer source address and IR[VE](C) as the outer
destination address. IR[CE](C) then sends this triple-encapsulated
packet into its ISP network, where it will be routed to IR[VE](C).
To ease in discussion of this case, now consider that each IR[VE]
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named above is half of a unified IR[VP] that combines both the IR[VC]
and IR[VE] functions. With this simplification in mind, when
IR[VP](C) receives the triple-encapsulated packet, it removes the
outermost layer of encapsulation and forwards the twice-encapsulated
packet into the Internet where Internet routing will direct it to
IR[VP](B). IR[VP](B) in turn removes the next layer of encapsulation
and forwards the once-encapsulated packet into the Internet where
Internet routing will direct it to IR[VP](A). IR[VP](A) will finally
remove the final layer of encapsulation and forward the packet into
the Internet where Internet routing will direct it to host Z.
The key architectural requirement derived from this case is that each
IR[VE] must iteratively decapsulate each layer of a multi-
encapsulated packet when the outer destination address is one of its
own locator addresses. When the final such layer of encapsulation is
reached, the IR[VE] forwards the packet into the Internet.
6.8.2. Host Z Sends Packets to Host A
Whether or not host Z configures an EPA address, its packets destined
to Host A will eventually reach IR[VE](A). IR[VE](A) will have a
mapping that lists IR[CE](A) as the next hop toward EPA(A).
IR[VE](A) will then encapsulate the packet with EPA(B) as the outer
destination address and forward the packet into the Internet.
Internet routing will convey this once-encapsulated packet to
IR[VE](B) which will have a mapping that lists IR[CE](B) as the next
hop toward EPA(B). IR[VE](B) will then encapsulate the packet with
EPA(C) as the outer destination address and forward the packet into
the Internet. Internet routing will then convey this twice-
encapsulated packet to IR[VE](C) which will have a mapping that lists
IR[CE](C) as the next hop toward EPA(C). IR[VE](C) will then
encapsulate the packet with ISP(D) as the outer destination address
and forward the packet into the Internet. Internet routing will then
convey this triple-encapsulated packet to IR[CE](C).
When the triple-encapsulated packet arrives at IR[CE](C), it strips
the outer layer of encapsulation and forwards the twice-encapsulated
packet to EPA(C) which is the locator address of IR[CE](B). When
IR[CE](B) receives the twice-encapsulated packet, it strips the outer
layer of encapsulation and forwards the once-encapsulated packet to
EPA(B) which is the locator address of IR[CE](A). When IR[CE](A)
receives the once-encapsulated packet, it strips the outer layer of
encapsulation and forwards the unencapsulated packet to EPA(A) which
is the host address of host A.
The key architectural requirement derived from this case is that each
IR[CE] must decapsulate only the outermost layer of a multi-
encapsulated packet when the outer destination address matches an EPA
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assigned to a device in its EUN. This class of packets can be
considered as "inbound" wrt the IR[CE]'s EUNs. The outbound cases
are discussed in Section 6.8.1
7. Additional Considerations
Considerations for the scalability of Internet Routing due to
multihoming, traffic engineering and provider-independent addressing
are discussed in [I-D.narten-radir-problem-statement]. Route
optimization considerations for mobile networks are found in
[RFC5522].
8. Related Initiatives
IRON builds upon the concepts RANGER architecture [RFC5720], and
therefore inherits the same set of related initiatives.
Virtual Aggregation (VA) [I-D.ietf-grow-va] and Aggregation in
Increasing Scopes (AIS) [I-D.zhang-evolution] provide the basis for
the Virtual Prefix concepts.
Internet vastly improved plumbing (Ivip) [I-D.whittle-ivip-arch] has
contributed valuable insights, including the use of real-time
mapping. The use of IR[VE]s as mobility anchor points is directly
influenced by Ivip's associated TTR mobility extensions [TTRMOB].
[I-D.bernardos-mext-nemo-ro-cr] discussed a route optimization
approach using a Correspondent Router (CR) model. The IRON IR[VE]
construct is similar to the CR concept described in this work,
however the manner in which customer EUNs coordinates with IR[VE]s is
different and based on the redirection model associated with NBMA
links.
Numerous publications have proposed NAT traversal techniques. The
NAT traversal techniques adapted for IRON were inspired by the Simple
Address Mapping for Premises Legacy Equipment (SAMPLE) proposal
[I-D.carpenter-softwire-sample].
9. IANA Considerations
There are no IANA considerations for this document.
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10. Security Considerations
Security considerations that apply to tunneling in general are
discussed in [I-D.ietf-v6ops-tunnel-security-concerns]. Additional
considerations that apply also to IRON are discussed in RANGER
[RFC5720], VET [I-D.templin-intarea-vet] and SEAL
[I-D.templin-intarea-seal].
IR[CE]s require a means for securely registering their EP-to-locator
bindings with their VPC. Each VPC provides its customer IR[CE]s with
a secure means for registering and re-registering their mappings.
11. Acknowledgements
This ideas behind this work have benefited greatly from discussions
with colleagues; some of which appear on the RRG and other IRTF/IETF
mailing lists. Robin Whittle and Steve Russert co-authored the TTR
mobility architecture which strongly influenced IRON. Eric
Fleischman pointed out the opportunity to leverage anycast for
discovering topologically-close servers. Thomas Henderson
recommended a quantitative analysis of scaling properties.
The following individuals provided essential review input: Mohamed
Boucadair, Wesley Eddy, Dae Young Kim and Robin Whittle.
12. References
12.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
12.2. Informative References
[BGPMON] net, B., "BGPmon.net - Monitoring Your Prefixes,
http://bgpmon.net/stat.php", June 2010.
[I-D.bernardos-mext-nemo-ro-cr]
Bernardos, C., Calderon, M., and I. Soto, "Correspondent
Router based Route Optimisation for NEMO (CRON)",
draft-bernardos-mext-nemo-ro-cr-00 (work in progress),
July 2008.
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[I-D.carpenter-softwire-sample]
Carpenter, B. and S. Jiang, "Legacy NAT Traversal for
IPv6: Simple Address Mapping for Premises Legacy Equipment
(SAMPLE)", draft-carpenter-softwire-sample-00 (work in
progress), June 2010.
[I-D.ietf-grow-va]
Francis, P., Xu, X., Ballani, H., Jen, D., Raszuk, R., and
L. Zhang, "FIB Suppression with Virtual Aggregation",
draft-ietf-grow-va-02 (work in progress), March 2010.
[I-D.ietf-v6ops-tunnel-security-concerns]
Hoagland, J., Krishnan, S., and D. Thaler, "Security
Concerns With IP Tunneling",
draft-ietf-v6ops-tunnel-security-concerns-02 (work in
progress), March 2010.
[I-D.narten-radir-problem-statement]
Narten, T., "On the Scalability of Internet Routing",
draft-narten-radir-problem-statement-05 (work in
progress), February 2010.
[I-D.russert-rangers]
Russert, S., Fleischman, E., and F. Templin, "RANGER
Scenarios", draft-russert-rangers-05 (work in progress),
July 2010.
[I-D.templin-intarea-seal]
Templin, F., "The Subnetwork Encapsulation and Adaptation
Layer (SEAL)", draft-templin-intarea-seal-16 (work in
progress), July 2010.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-16 (work in progress),
July 2010.
[I-D.whittle-ivip-arch]
Whittle, R., "Ivip (Internet Vastly Improved Plumbing)
Architecture", draft-whittle-ivip-arch-04 (work in
progress), March 2010.
[I-D.zhang-evolution]
Zhang, B. and L. Zhang, "Evolution Towards Global Routing
Scalability", draft-zhang-evolution-02 (work in progress),
October 2009.
[RFC1070] Hagens, R., Hall, N., and M. Rose, "Use of the Internet as
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a subnetwork for experimentation with the OSI network
layer", RFC 1070, February 1989.
[RFC3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
Reserved for Documentation", RFC 3849, July 2004.
[RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day", RFC 4192,
September 2005.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4548] Gray, E., Rutemiller, J., and G. Swallow, "Internet Code
Point (ICP) Assignments for NSAP Addresses", RFC 4548,
May 2006.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
Route Optimization Requirements for Operational Use in
Aeronautics and Space Exploration Mobile Networks",
RFC 5522, October 2009.
[RFC5720] Templin, F., "Routing and Addressing in Networks with
Global Enterprise Recursion (RANGER)", RFC 5720,
February 2010.
[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737, January 2010.
[RFC5743] Falk, A., "Definition of an Internet Research Task Force
(IRTF) Document Stream", RFC 5743, December 2009.
[RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
Still Needs Work", RFC 5887, May 2010.
[TTRMOB] Whittle, R. and S. Russert, "TTR Mobility Extensions for
Core-Edge Separation Solutions to the Internet's Routing
Scaling Problem,
http://www.firstpr.com.au/ip/ivip/TTR-Mobility.pdf",
August 2008.
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Appendix A. IRON VPs Over Internetworks with Different Address Families
The IRON architecture leverages the routing system by providing
generally shortest-path routing for packets with EPA addresses from
VPs that match the address family of the underlying Internetwork.
When the VPs are of an address family that is not routable within the
underlying Internetwork, however, (e.g., when OSI/NSAP [RFC4548] VPs
are used within an IPv4 Internetwork) a global mapping database is
required to allow IR[VE]s to map VPs to companion prefixes taken from
address families that are routable within the Internetwork. For
example, an IPv6 VP (e.g., 2001:DB8::/32) could be paired with a
companion IPv4 prefix (e.g., 192.0.2.0/24) so that encapsulated IPv6
packets can be forwarded over IPv4-only Internetworks.
Every VP in the IRON must therefore be represented in a globally
distributed Master VP database (MVPd) that maintains VP-to-companion
prefix mappings for all VPs in the IRON. The MVPd is maintained by a
globally-managed assigned numbers authority in the same manner as the
Internet Assigned Numbers Authority (IANA) currently maintains the
master list of all top-level IPv4 and IPv6 delegations. The database
can be replicated across multiple servers for load balancing much in
the same way that FTP mirror sites are used to manage software
distributions.
Upon startup, each IR[VE] discovers the full set of VPs for the IRON
by reading the MVPd. The IR[VE] reads the MVPd from a nearby server
and periodically checks the server for deltas since the database was
last read. After reading the MVPd, the IR[VE] has a full list of VP
to companion prefix mappings and is said to be "synchronized with the
IRON".
The IR[VE] can then forward packets toward EPAs covered by a VP by
encapsulating them in an outer header of the VP's companion prefix
address family and using any address taken from the companion prefix
as the outer destination address. The companion prefix therefore
serves as an implicit anycast prefix.
Possible encapsulations in this model include IPv6-in-IPv4, IPv4-in-
IPv6, OSI/CLNP-in-IPv6, OSI/CLNP-in-IPv4, etc.
Appendix B. Scaling Considerations
Scaling aspects of the IRON architecture have strong implications for
its applicability in practical deployments. Scaling must be
considered along multiple vectors including Interdomain core routing
scaling, scaling to accommodate large numbers of customer EUNs,
traffic scaling, state requirements, etc.
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In terms of routing scaling, each VPC will advertise one or more VPs
from which EPs are delegated to customer EUNs. Routing scaling will
therefore be minimized when each VP covers many EPs. For example,
the IPv6 prefix 2001:DB8::/32 contains 2^16 /56 prefixes for
assignment to EUNs. Therefore, 2^16 EUNs can be represented as a
single VP in the interdomain routing core. The IRON could therefore
accommodate 10^10 IPv6 ::/56 EPs with only 625 IPv6 ::/32 VPs
advertised in the interdomain routing core.
In terms of traffic scaling for IR[VC]s, each IR[VC] represents an
ASBR of a "shell" enterprise network that simply turns arriving
traffic packets with EPA destination addresses back out into the
Internet towards IR[VE]s that service customer EUNs. Moreover, the
IR[VC] sheds traffic destined to EPAs through redirection which
removes it from the path for the vast majority of traffic packets.
On the other hand, IR[VC]s must handle all traffic packets forwarded
between EUNs and the non-IRON Internet. The scaling concerns for
this latter class of traffic are no different than for ASBR routers
that connect large enterprise networks to the Internet. In terms of
traffic scaling for IR[VE]s, each IR[VE] services a set of the VPC
overlay network's customer EUNs. The IR[VE] services all traffic
packets destined to its EUNs but only services the initial packets of
flows initiated from the EUNs. Therefore, traffic scaling is an
asymmetric consideration and is proportional to the number of EUNs
each IR[VE] serves.
In terms of state requirements for IR[VC]s, each IR[VC] maintains a
list of all IR[VE]s in the VPC overlay network as well as all
customer EUNs that each IR[VE] serves. This state is therefore
dominated by the number of EUNs in the VPC overlay network. Sizing
the IR[VC] to accommodate state information for all EUNs is therefore
required during VPC overlay network planning. In terms of state
requirements for IR[VE]s, each IR[VE] maintains two-way tunnel state
for each of the customer EUNs it serves but need not keep state for
all EUNs in the VPC overlay network. Finally, neither IR[VC]s nor
IR[VE] need keep state for final destinations of outbound traffic.
IR[CE]s source and sink all traffic packets originating from or
destined to the customer EUN. Therefore traffic scaling
considerations for IR[CE]s are the same as for any site border
router. IR[CE]s also retain state for the final destinations of
outbound traffic flows. This can be managed as soft state, since
stale entries purged from the cache will be refreshed when new
traffic packets are sent.
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Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
entire. Box 3707 MC 7L-49
Seattle, WA 98124
USA
Email: fltemplin@acm.org
Templin Expires February 13, 2011 [Page 39]
