draft-ietf-grow-va-04.txt   draft-ietf-grow-va-05.txt 
Network Working Group P. Francis Network Working Group P. Francis
Internet-Draft MPI-SWS Internet-Draft MPI-SWS
Intended status: Informational X. Xu Intended status: Informational X. Xu
Expires: August 26, 2011 Huawei Expires: January 2, 2012 Huawei
H. Ballani H. Ballani
Cornell U. Cornell U.
D. Jen D. Jen
UCLA UCLA
R. Raszuk R. Raszuk
Cisco Cisco
L. Zhang L. Zhang
UCLA UCLA
February 22, 2011 July 1, 2011
FIB Suppression with Virtual Aggregation FIB Suppression with Virtual Aggregation
draft-ietf-grow-va-04.txt draft-ietf-grow-va-05.txt
Abstract Abstract
The continued growth in the Default Free Routing Table (DFRT) The continued growth in the Default Free Routing Table (DFRT)
stresses the global routing system in a number of ways. One of the stresses the global routing system in a number of ways. One of the
most costly stresses is FIB size: ISPs often must upgrade router most costly stresses is FIB size: ISPs often must upgrade router
hardware simply because the FIB has run out of space, and router hardware simply because the FIB has run out of space, and router
vendors must design routers that have adequate FIB. FIB suppression vendors must design routers that have adequate FIB. FIB suppression
is an approach to relieving stress on the FIB by NOT loading selected is an approach to relieving stress on the FIB by not loading selected
RIB entries into the FIB. Virtual Aggregation (VA) allows ISPs to RIB entries into the FIB. Virtual Aggregation (VA) allows ISPs to
shrink the FIBs of any and all routers, easily by an order of shrink the FIBs of any and all routers, easily by an order of
magnitude with negligible increase in path length and load. FIB magnitude with negligible increase in path length and load. FIB
suppression deployed autonomously by an ISP (cooperation between ISPs suppression can be deployed autonomously by an ISP without requiring
is not required), and can co-exist with legacy routers in the ISP. cooperation between adjacent ISPs, and can co-exist with legacy
There are no changes from the 03 version. routers in the ISP.
Status of this Memo Status of this Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Scope of this Document . . . . . . . . . . . . . . . . . . 5 1.1. Scope of this Document . . . . . . . . . . . . . . . . . . 5
1.2. Requirements notation . . . . . . . . . . . . . . . . . . 6 1.2. Requirements notation . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview of Virtual Aggregation (VA) . . . . . . . . . . . . . 7 2. Overview of Virtual Aggregation (VA) . . . . . . . . . . . . . 6
2.1. Mix of legacy and VA routers . . . . . . . . . . . . . . . 9 2.1. Mix of Legacy and VA Routers . . . . . . . . . . . . . . . 9
2.2. Summary of Tunnels and Paths . . . . . . . . . . . . . . . 10 2.2. Summary of Tunnels and Paths . . . . . . . . . . . . . . . 9
3. Specification of VA . . . . . . . . . . . . . . . . . . . . . 11 3. Specification of VA . . . . . . . . . . . . . . . . . . . . . 11
3.1. VA Operation . . . . . . . . . . . . . . . . . . . . . . . 12 3.1. Legacy Routers . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1. Legacy Routers . . . . . . . . . . . . . . . . . . . . 12 3.2. Advertising and Handling Virtual Prefixes (VP) . . . . . . 12
3.1.2. Advertising and Handling Virtual Prefixes (VP) . . . . 12 3.2.1. Distinguishing VPs from Sub-prefixes . . . . . . . . . 12
3.1.3. Border VA Routers . . . . . . . . . . . . . . . . . . 16 3.2.2. Limitations on Virtual Prefixes . . . . . . . . . . . 12
3.1.4. Advertising and Handling Sub-Prefixes . . . . . . . . 16 3.2.3. Aggregation Point Routers (APR) . . . . . . . . . . . 13
3.1.5. Suppressing FIB Sub-prefix Routes . . . . . . . . . . 17 3.2.4. Non-APR Routers . . . . . . . . . . . . . . . . . . . 14
3.2. New Configuration . . . . . . . . . . . . . . . . . . . . 18 3.2.5. Adding and deleting VPs . . . . . . . . . . . . . . . 14
4. Usage of Tunnels . . . . . . . . . . . . . . . . . . . . . . . 19 3.3. Border VA Routers . . . . . . . . . . . . . . . . . . . . 15
4.1. MPLS tunnels . . . . . . . . . . . . . . . . . . . . . . . 19 3.4. Advertising and Handling Sub-Prefixes . . . . . . . . . . 15
4.2. Usage of Inner Label . . . . . . . . . . . . . . . . . . . 20 3.5. Suppressing FIB Sub-prefix Routes . . . . . . . . . . . . 16
3.5.1. Selecting Popular Prefixes . . . . . . . . . . . . . . 16
3.6. New Configuration . . . . . . . . . . . . . . . . . . . . 17
3.7. Interaction with Traffic Engineering . . . . . . . . . . . 18
4. Usage of MPLS Tunnels . . . . . . . . . . . . . . . . . . . . 19
4.1. Usage of Inner Label . . . . . . . . . . . . . . . . . . . 19
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
6.1. Properly Configured VA . . . . . . . . . . . . . . . . . . 20 6.1. Properly Configured VA . . . . . . . . . . . . . . . . . . 20
6.2. Mis-configured VA . . . . . . . . . . . . . . . . . . . . 21 6.2. Mis-configured VA . . . . . . . . . . . . . . . . . . . . 21
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . . 22 8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . . 22 8.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
ISPs today manage constant DFRT growth in a number of ways. One way, ISPs today manage constant DFRT growth in a number of ways. One way,
of course, is for ISPs to upgrade their router hardware before DFRT of course, is for ISPs to upgrade their router hardware before DFRT
growth outstrips the size of the FIB. This is too expensive for many growth outstrips the size of the FIB. This may be too expensive for
ISPs. They would prefer to extend the lifetime of routers whose FIBs many ISPs. They would prefer to extend the lifetime of routers whose
can no longer hold the full DFRT. FIBs can no longer hold the full DFRT.
A common approach taken by lower-tier ISPs is to default route to A common approach taken by lower-tier ISPs is to default route to
their providers. Routes to customers and peer ISPs are maintained, their transit providers. Routes to customers and peer ISPs are
but everything else defaults to the provider. This approach has maintained, but everything else defaults to the provider. This
several disadvantages. First, packets to Internet destinations may approach has several disadvantages. First, packets to Internet
take longer-than-necessary AS paths. This problem can be mitigated destinations may take longer-than-necessary Autonomous System (AS)
through careful configuration of partial defaults, but this can paths. This problem can be mitigated through careful configuration
require substantial configuration overhead. A second problem with of partial defaults, but this can require substantial configuration
defaulting to providers is that the ISP is no longer able to provide overhead. A second problem with defaulting to providers is that the
the full DFRT to its customers. Finally, provider defaults prevents ISP is no longer able to provide the full DFRT to its customers.
the ISP from being able to detect martian packets. As a result, the Finally, provider defaults prevents the ISP from being able to detect
ISP transmits packets that could otherwise have been dropped over its martian packets. As a result, the ISP transmits packets that could
expensive provider links. otherwise have been dropped over its expensive provider links.
An alternative is for the ISP to maintain full routes in its core An alternative is for the ISP to maintain full routes in its core
routers, but to filter routes from edge routers that do not require a routers, but to filter routes from edge routers that do not require a
full DFRT. These edge routers can then default route to the core full DFRT. These edge routers can then default route to the core
routers. This is often possible with edge routers that interface to routers. This is often possible with edge routers that interface to
customer networks. The problem with this approach is that it cannot customer networks. The problem with this approach is that it cannot
be used for all edge routers. For instance, it cannot be used for be used for all edge routers. For instance, it cannot be used for
routers that connect to transits. It of course also does not help in routers that connect to transits. It of course also does not help in
cases where core routers themselves have inadequate FIB capacity. cases where core routers themselves have inadequate FIB capacity.
FIB Suppression is an approach to shrinking FIB size that requires no FIB Suppression is an approach to shrinking FIB size that requires no
changes to BGP, no changes to packet forwarding mechanisms in changes to BGP, no changes to packet forwarding mechanisms in
routers, and relatively minor changes to control mechanisms in routers, and relatively minor changes to control mechanisms in
routers and configuration of those mechanisms. The core idea behind routers and configuration of those mechanisms. The core idea behind
FIB suppression is to run BGP as normal, and in particular to not FIB suppression is to run BGP as normal, and in particular to not
shrink the RIB, but rather to not load certain RIB entries into the shrink the RIB, but rather to not load certain RIB entries into the
FIB. This approach minimizes changes to routers, and in particular FIB. This approach minimizes changes to routers, and in particular
is simpler than more general routing architectures that try to shrink is simpler than more general routing architectures that try to shrink
both RIB and FIB. With FIB suppression, there are no changes to BGP both RIB and FIB. With FIB suppression, there are no changes to BGP
per se. The BGP decision process does not change. The selected AS- per se. The BGP decision process does not change, the selected AS-
path does not change, and except on rare occasion the exit router PATH does not change, and except on rare occasion the exit router
does not change. ISPs can deploy FIB suppression autonomously and does not change. ISPs can deploy FIB suppression autonomously and
with no coordination with neighboring ASes. with no coordination with neighboring ASes.
This document describes an approach to FIB suppression called This document describes an approach to FIB suppression called
"Virtual Aggregation" (VA). VA operates by organizing the IP (v4 or "Virtual Aggregation" (VA). VA operates by organizing the IP (v4 or
v6) address space into Virtual Prefixes (VP), and using tunnels to v6) address space into Virtual Prefixes (VP), and using tunnels to
aggregate the (regular) sub-prefixes within each VP. The decrease in aggregate the (regular) sub-prefixes within each VP. The decrease in
FIB size can be dramatic, easily 5x or 10x with only a slight path FIB size can be dramatic, easily 5x or 10x with only a slight path
length and router load increase [nsdi09]. The VPs can be organized length and router load increase [nsdi09].
such that all routers in an ISP see FIB size decrease, or in such a
way that "core" routers keep the full FIB, and "edge" routers have
almost no FIB (i.e. by defining a VP of 0/0). This "core-edge" style
of VA deployment is much simpler than a "full" VA deployment, whereby
multiple VPs are defined, and any router, core or otherwise, can have
reduced FIB size. This simpler "core-edge" style of deployment is
specified in a separate draft in order to make it more easily
understandable [I-D.ietf-grow-simple-va].
VA has the following characteristics:
o it is robust to router failure,
o it allows for traffic engineering,
o it allows for existing inter-domain routing policies,
o it operates in a predictable manner and is therefore possible to
test, debug, and reason about performance (i.e. establish SLAs),
o it can be safely installed, tested, and started up,
o it can be configured and reconfigured without service
interruption,
o it can be incrementally deployed, and in particular can be
operated in an AS with a mix of VA-capable and legacy routers,
o it accommodates existing security mechanisms such as unicast
Reverse Path Forwarding (uRPF) ingress filtering and DoS defense,
o does not introduce significant new security vulnerabilities.
1.1. Scope of this Document 1.1. Scope of this Document
The scope of this document is limited to Intra-domain VA operation. The scope of this document is limited to intra-domain VA operation.
In other words, the case where a single ISP autonomously operates VA Individual ASs autonomously operate VA internally without any
internally without any coordination with neighboring ISPs. coordination with neighboring ASs. For the remainder of this
document, the terms ISP, AS, and domain are used interchangeably.
Note that this document assumes that the VA "domain" (i.e. the unit
of autonomy) is the AS (that is, different ASes run VA independently
and without coordination). For the remainder of this document, the
terms ISP, AS, and domain are used interchangeably.
This document applies equally to IPv4 and IPv6. This document applies equally to IPv4 and IPv6.
This document is limited to the following tunnel types: MPLS Label
Switched Paths (LSP), and of MPLS inner labels tunneled over either
LSPs or IP headers.
VA may operate with a mix of upgraded routers and legacy routers. VA may operate with a mix of upgraded routers and legacy routers.
There are no topological restrictions placed on the mix of routers. There are no topological restrictions placed on the mix of routers.
In order to avoid loops between upgraded and legacy routers, packets In order to avoid loops between upgraded and legacy routers, packets
are always tunneled by the VA routers to the BGP NEXT_HOPs of the are always tunneled by the VA routers to the BGP NEXT_HOPs of the
matched BGP routes. If a given local ASBR is a legacy router, it matched BGP routes. If a given local ASBR (Autonomous System Border
must be able to terminate tunnels. Router) is a legacy router, it must be able to terminate tunnels.
1.2. Requirements notation 1.2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" when
document are to be interpreted as described in [RFC2119]. capitalized in this document are to be interpreted as described in
[RFC2119].
1.3. Terminology 1.3. Terminology
Aggregation Point Router (APR): An Aggregation Point Router (APR) is Aggregation Point Router (APR): An Aggregation Point Router (APR) is
a router that aggregates a Virtual Prefix (VP) by installing a router that aggregates a Virtual Prefix (VP) by installing
routes (into the FIB) for all of the sub-prefixes within the VP. routes (into the FIB) for all of the sub-prefixes within the VP.
APRs advertise the VP to other routers with BGP. For each sub- APRs advertise the VP to other routers with BGP. For each sub-
prefix within the VP, APRs have a tunnel from themselves to the prefix within the VP, APRs have a tunnel from themselves to the
remote ASBR (Autonomous System Border Router) where packets for remote ASBR (Autonomous System Border Router) where packets for
that prefix should be delivered. that prefix should be delivered.
Install and Suppress: The terms "install" and "suppress" are used to Install and Suppress: The terms "install" and "suppress" are used to
describe whether a RIB entry has been loaded or not loaded into describe whether a RIB entry has been loaded or not loaded into
the FIB. In other words, the phrase "install a route" means the FIB. In particular, "install a route" means "install a route
"install a route into the FIB", and the phrase "suppress a route" into the FIB", and "suppress a route" means "do not install a
means "do not install a route into the FIB". route into the FIB".
Legacy Router: A router that does not run VA, and has no knowledge Legacy Router: A router that does not run VA, and has no knowledge
of VA. Legacy routers, however, must be able to terminate tunnels of VA. Legacy routers, however, must be able to terminate tunnels
when they are local ASBRs. when they are local ASBRs.
non-APR Router: In discussing VPs, it is often necessary to Non-APR Router: In discussing VPs, it is often necessary to
distinguish between routers that are APRs for that VP, and routers distinguish between routers that are APRs for that VP, and routers
that are not APRs for that VP (but of course may be APRs for other that are not APRs for that VP (but of course may be APRs for other
VPs not under discussion). In these cases, the term "APR" is VPs not under discussion). In these cases, the term "APR" is
taken to mean "a VA router that is an APR for the given VP", and taken to mean "a VA router that is an APR for the given VP", and
the term "non-APR" is taken to mean "a VA router that is not an the term "non-APR" is taken to mean "a VA router that is not an
APR for the given VP". The term non-APR router is not used to APR for the given VP". The term non-APR router is not used to
refer to legacy routers. refer to legacy routers.
Popular Prefix: A Popular Prefix is a sub-prefix that is installed Popular Prefix: A Popular Prefix is a sub-prefix that is installed
in a router in addition to the sub-prefixes it holds by virtue of in a router in addition to the sub-prefixes it holds by virtue of
being a Aggregation Point Router. The Popular Prefix allows being a Aggregation Point Router. The Popular Prefix allows
skipping to change at page 7, line 4 skipping to change at page 6, line 32
in this document to refer either to the loc-RIB (as used in in this document to refer either to the loc-RIB (as used in
[RFC4271]), or to the combined Adj-RIBs-In, the Loc-RIB, and the [RFC4271]), or to the combined Adj-RIBs-In, the Loc-RIB, and the
Adj-RIBs-Out. Adj-RIBs-Out.
Sub-Prefix: A regular (physically aggregatable) prefix. These are Sub-Prefix: A regular (physically aggregatable) prefix. These are
equivalent to the prefixes that would normally comprise the DFRT equivalent to the prefixes that would normally comprise the DFRT
in the absence of VA. A VA router will contain a sub-prefix entry in the absence of VA. A VA router will contain a sub-prefix entry
either because the sub-prefix falls within a Virtual Prefix for either because the sub-prefix falls within a Virtual Prefix for
which the router is an APR, or because the sub-prefix is installed which the router is an APR, or because the sub-prefix is installed
as a Popular Prefix. Legacy routers hold the same sub-prefixes as a Popular Prefix. Legacy routers hold the same sub-prefixes
that they hold today. that they hold today.
Tunnel: This document specifies the use of MPLS Label Switched Paths
Tunnel: This draft specifies the use of MPLS Label Switched Paths
(LSP), and of MPLS inner labels tunneled over either LSPs or IP (LSP), and of MPLS inner labels tunneled over either LSPs or IP
headers. Other types of tunnels may be used, but are not headers. While in principle other types of tunnels may be used,
specified here. This document generically uses the term tunnel to they are not specified here. This document uses the term tunnel
refer to any of these tunnel types. to refer to the above MPLS encapsulations.
VA router: A router that operates Virtual Aggregation according to VA router: A router that operates Virtual Aggregation according to
this document. this document.
Virtual Prefix (VP): A Virtual Prefix (VP) is a prefix used to Virtual Prefix (VP): A Virtual Prefix (VP) is a prefix used to
aggregate its contained regular prefixes (sub-prefixes). The set aggregate its contained regular prefixes (sub-prefixes). The set
of sub-prefixes in a VP are not physically aggregatable, and so of sub-prefixes in a VP are not physically aggregatable, and so
they are aggregated at APRs through the use of tunnels. they are aggregated at APRs through the use of tunnels.
VP-List: A list of defined VPs. All routers must agree on the VP-List: A list of defined VPs. All routers must agree on the
contents of this list (which is statically configured into every contents of this list.
VA router).
2. Overview of Virtual Aggregation (VA) 2. Overview of Virtual Aggregation (VA)
For descriptive simplicity, this section starts by describing VA For descriptive simplicity, this section starts by describing VA
assuming that there are no legacy routers in the domain. Section 2.1 assuming that there are no legacy routers in the domain. Section 2.1
overviews the additional functions required by VA routers to overviews the additional functions required by VA routers to
accommodate legacy routers. accommodate legacy routers.
A key concept behind VA is to operate BGP as normal, and in A key concept behind VA is to operate BGP as normal, and in
particular to populate the RIB with the full DFRT, but to suppress particular to populate the RIB with the full DFRT, but to suppress
many or most prefixes from being loaded into the FIB. By populating many or most prefixes from being loaded into the FIB. By populating
the RIB as normal, we avoid any changes to BGP, and changes to router the RIB as normal, we avoid any changes to BGP, and changes to router
operation are relatively minor. The basic idea behind VA is quite operation are relatively minor. The basic idea behind VA is as
simple. The address space is partitioned into large prefixes --- follows: The address space is partitioned into large prefixes ---
larger than any aggregatable prefix in use today. These prefixes are larger than any aggregatable prefix in use today. These prefixes are
called Virtual Prefixes (VP). Different VPs do not need to be the called Virtual Prefixes (VP). Different VPs do not need to be the
same size. They may be a mix of /6, /7, /8 (for IPv4), and so on. same size. They may be a mix of /6, /7, /8 (for IPv4), and so on.
Indeed, an ISP can define a single /0 VP, and use it for a core/edge Indeed, an ISP can define a single /0 VP, and use it for a core/edge
type of configuration [I-D.ietf-grow-simple-va]. That is, the core type of configuration. That is, the core routers would maintain full
routers would maintain full FIBs, and edge routers could maintain FIBs, and edge routers could maintain default routes to the core
default routes to the core routers, and suppress as much of the FIB routers, and suppress as much of the FIB as they wish. Each ISP can
as they wish. Each ISP can independently select the size of its VPs. independently select the size of its VPs.
VPs are not themselves topologically aggregatable. VA makes the VPs VPs are not themselves topologically aggregatable. VA makes the VPs
aggregatable through the use of tunnels, as follows. Associated with aggregatable through the use of tunnels, as follows. Associated with
each VP are one or more "Aggregation Point Routers" (APR). An APR each VP are one or more "Aggregation Point Routers" (APR). An APR
(for a given VP) is a router that installs routes for all sub- (for a given VP) is a router that installs routes for all sub-
prefixes (i.e. real physically aggregatable prefixes) within the VP. prefixes (i.e. real physically aggregatable prefixes) within the VP.
Note that an APR is not a special router per se---it is an otherwise By "install routes" here, we mean:
normal router that is configured to operate as an APR. By "install
routes" here, we mean:
1. The route for each of the sub-prefixes is loaded into the FIB, 1. The route for each of the sub-prefixes is loaded into the FIB,
and and
2. there is a tunnel from the APR to the BGP NEXT_HOP for the route. 2. there is a tunnel from the APR to the BGP NEXT_HOP for the route.
The APR originates a BGP route to the VP. This route is distributed The APR originates a BGP route to the VP. This route is distributed
within the domain, but not outside the domain. With this structure within the domain, but not outside the domain. With this structure
in place, a packet transiting the ISP goes from the ingress router to in place, a packet transiting the ISP goes from the ingress router to
the APR (usually via a tunnel), and then from the APR to the BGP the APR (usually via a tunnel), and then from the APR to the BGP
NEXT_HOP router via a tunnel. VA can operate with MPLS LSPs, or with NEXT_HOP router via a tunnel. VA can operate with MPLS LSPs, or with
MPLS inner labels over LSPs or IP headers. Section 4 specifies the MPLS inner labels over LSPs or IP headers. Section 4 specifies the
usage of tunnels. usage of MPLS tunnels. Other tunnel types (i.e., GRE) may be used,
but are not specified in this document.
The BGP NEXT_HOP can be either the local ASBR or the remote ASBR. In The BGP NEXT_HOP can be either the local ASBR or the remote ASBR. In
the former case, an inner label is used to tunnel packets the former case, an inner label is used to tunnel packets
(Section 4.2). In either case, all tunner headers are stripped by (Section 4.1). In either case, all tunnel headers are stripped by
the local ASBR before the packet is delivered to the remote ASBR. In the local ASBR before the packet is delivered to the remote ASBR. In
other words, the remote ASBR sees a normal IP packet, and is other words, the remote ASBR sees a normal IP packet, and is
completely unaware of the existence of VA in the neighboring ISP. completely unaware of the existence of VA in the neighboring ISP.
Note that legacy ASBRs MUST set themselves as the BGP NEXT_HOP.
Note that the AS-path is not effected at all by VA. This means among Note that the AS-PATH is not effected at all by VA. This means among
other things that AS-level policies are not effected by VA. The other things that AS-level policies are not effected by VA. The
packet may not, however, follow the shortest path within the ISP packet may not, however, follow the shortest path within the ISP
(where shortest path is defined here as the path that would have been (where shortest path is defined here as the path that would have been
taken if VA were not operating), because the APR may not be on the taken if VA were not operating), because the APR may not be on the
shortest path between the ingress and egress routers. When this shortest path between the ingress and egress routers. When this
happens, the packet experiences additional latency and creates extra happens, the packet experiences additional latency and creates extra
load (by virtue of taking more hops than it otherwise would have). load (by virtue of taking more hops than it otherwise would have).
Note also that, with VA, a packet may occasionally take a different Note also that, with VA, a packet may occasionally take a different
exit point than it otherwise would have. exit point than it otherwise would have. This can occur for instance
when the exit point nearest to the selected APR is different than the
exit point nearest to the router initiating the tunnel to the APR.
VA can avoid traversing the APR for selected routes by installing VA can avoid traversing the APR for selected routes by installing
these routes in non-APR routers. In other words, even if an ingress these routes in non-APR routers. In other words, even if an ingress
router is not an APR for a given sub-prefix, it MAY install that sub- router is not an APR for a given sub-prefix, it MAY install that sub-
prefix into its FIB. Packets in this case are tunneled directly from prefix into its FIB. Packets in this case are tunneled directly from
the ingress to the BGP NEXT_HOP. These extra routes are called the ingress to the BGP NEXT_HOP. These extra routes are called
"Popular Prefixes", and are typically installed for policy reasons "Popular Prefixes", and are typically installed for policy reasons
(e.g. customer routes are always installed), or for sub-prefixes that (e.g. customer routes are always installed), or for sub-prefixes that
carry a high volume of traffic (Section 3.1.5.1). Different routers carry a high volume of traffic (Section 3.5.1). Different routers
MAY have different Popular Prefixes. As such, an ISP MAY assign may have different Popular Prefixes. As such, an ISP may assign
Popular Prefixes per router, per POP, or uniformly across the ISP. A Popular Prefixes per router, per POP, or uniformly across the ISP. A
given router MAY have zero Popular Prefixes, or the majority of its given router may have zero Popular Prefixes, or the majority of its
FIB MAY consist of Popular Prefixes. The effectiveness of Popular FIB may consist of Popular Prefixes. The effectiveness of Popular
Prefixes to reduce traffic load relies on the fact that traffic Prefixes to reduce traffic load relies on the fact that traffic
volumes follow something like a power-law distribution: i.e. that 90% volumes follow something like a power-law distribution: i.e. that 90%
of traffic is destined to 10% of the destinations. Internet traffic of traffic is destined to 10% of the destinations. Internet traffic
measurement studies over the years have consistently shown that measurement studies over the years have consistently shown that
traffic patterns follow this distribution, though there is no traffic patterns follow this distribution [nsdi09], though there is
guarantee that they always will. no guarantee that they always will.
Note that for routing to work properly, every packet must sooner or Note that for routing to work properly, every packet must sooner or
later reach a router that has installed a sub-prefix route that later reach a router that has installed a sub-prefix route that
matches the packet. This would obviously be the case for a given matches the packet. This would obviously be the case for a given
sub-prefix if every router has installed a route for that sub-prefix sub-prefix if every router has installed a route for that sub-prefix.
(which of course is the situation in the absence of VA). If this is If this is not the case, then there MUST be at least one Aggregation
not the case, then there MUST be at least one Aggregation Point Point Router (APR) for the sub-prefix's Virtual Prefix (VP).
Router (APR) for the sub-prefix's Virtual Prefix (VP). Ideally, Ideally, every POP contains at least two APRs for every Virtual
every POP contains at least two APRs for every Virtual Prefix. By Prefix. By having APRs in every POP, the latency imposed by routing
having APRs in every POP, the latency imposed by routing to the APR to the APR is minimal (the extra hop is within the POP). By having
is minimal (the extra hop is within the POP). By having more than more than one APR, there is a redundant APR should one fail. In
one APR, there is a redundant APR should one fail. In practice it is practice it is often not possible to have an APR for every VP in
often not possible to have an APR for every VP in every POP. This is every POP. This is because some POPs may have only one or a few
because some POPs may have only one or a few routers, and therefore routers, and therefore there may not have enough cumulative FIB space
there may not have enough cumulative FIB space in the POP to hold in the POP to hold every sub-prefix. Note that any router ("edge",
every sub-prefix. Note that any router ("edge", "core", etc.) MAY "core", etc.) MAY be an APR.
be an APR.
It is important that both the contents of BGP RIBs, as well as the It is important that both the contents of BGP RIBs, as well as the
contents of the Routing Table (as defined in Section 3.2 of contents of the Routing Table (as defined in Section 3.2 of
[RFC4271]) not be modified by VA (other than the introduction of [RFC4271]) not be modified by VA (other than the introduction of
routes to VPs). This is because PIM-SM [RFC4601] relies on the routes to VPs). This is because PIM-SM [RFC4601] relies on the
contents of the Routing Table to build its own trees and forwarding contents of the Routing Table to build its own trees and forwarding
table. Therefore, FIB suppression MUST take place between the table. Therefore, FIB suppression MUST take place between the
Routing Table and the actual FIB(s). Routing Table and the actual FIB(s).
2.1. Mix of legacy and VA routers 2.1. Mix of Legacy and VA Routers
It is important that an ISP be able to operate with a mix of "VA It is important that an ISP be able to operate with a mix of "VA
routers" (routers upgraded to operate VA as described in the routers" and "legacy routers". This allows ISPs to deploy VA in an
document) and "legacy routers". This allows ISPs to deploy VA in an
incremental fashion and to continue to use routers that for whatever incremental fashion and to continue to use routers that for whatever
reason cannot be upgraded. This document allows such a mix, and reason cannot be upgraded. This document allows such a mix, and
indeed places no topological restrictions on that mix. It does, indeed places no topological restrictions on that mix. It does,
however, require that legacy routers (and VA routers for that matter) however, require that legacy routers are able to forward tunneled
are able to forward already-tunneled packets, are able to serve as packets, are able to serve as tunnel endpoints, and are able to
tunnel endpoints, and are able to participate in distribution of participate in distribution of tunnel information required to
tunnel information required to establish themselves as tunnel establish themselves as tunnel endpoints. Depending on the tunnel
endpoints. (This is listed as Requirement R5 in the companion type, legacy routers MAY also be able to initiate tunneled packets,
tunneling documents.) Depending on the tunnel type, legacy routers though this is an OPTIONAL requirement. Legacy routers MUST use
MAY also be able to initiate tunneled packets, though this is an their own address as the BGP NEXT_HOP.
OPTIONAL requirement. (This is listed as Requirement R4 in the
companion tunneling documents.) Legacy routers MUST use their own
address as the BGP NEXT_HOP, and MUST FIB-install routes for which
they are the BGP NEXT_HOP.
2.2. Summary of Tunnels and Paths 2.2. Summary of Tunnels and Paths
To summarize, the following tunnels are created: To summarize, the following tunnels are created:
1. From all VA routers to all BGP NEXT_HOP addresses (where the BGP 1. From all VA routers to all BGP NEXT_HOP addresses (where the BGP
NEXT_HOP address is either an APR, a local ASBR, or the remote NEXT_HOP address is either an APR, a local ASBR, or the remote
ASBR neighbor of a VA router). Note that this is listed as ASBR neighbor of a VA router).
Requirement R3 in the companion tunneling documents.
2. Optionally, from all legacy routers to all BGP NEXT_HOP 2. Optionally, from all legacy routers to all BGP NEXT_HOP
addresses. addresses.
There are a number of possible paths that packets may take through an There are a number of possible paths that packets may take through an
ISP, summarized in the following diagram. Here, "VA" is a VA router, ISP, summarized in the following diagram. Here, "VA" is a VA router,
"LR" is a legacy router, the symbol "==>" represents a tunneled "LR" is a legacy router, the symbol "==>" represents a tunneled
packet (through zero or more routers), "-->" represents an untunneled packet (through zero or more routers), "-->" represents an untunneled
packet, and "(pop)" represents stripping the tunnel header. The packet, and "(pop)" represents stripping the tunnel header. The
symbol "::>" represents the portion of the path where although the symbol "::>" represents the portion of the path where although the
tunnel is targeted to the receiving node, the outer header has been tunnel is targeted to the receiving node, the outer header has been
stripped. (Note that the remote ASBR may actually be a legacy router stripped.
or a VA router---it doesn't matter (and isn't known) to the ISP.)
Egress Egress
Router Router
Ingress Some APR (Local Remote Ingress Some APR (Local Remote
Router Router Router ASBR) ASBR Router Router Router ASBR) ASBR
------- ------ ------ ------ -------- ------- ------ ------ ------ --------
1. VA===================>VA=========>VA(pop)::::>LR 1. VA===================>VA=========>VA(pop)::::>Peer ASBR
2. VA===================>VA=========>LR--------->LR 2. VA===================>VA=========>LR--------->Peer ASBR
3. VA===============================>VA(pop)::::>LR 3. VA===============================>VA(pop)::::>Peer ASBR
4. VA===============================>LR--------->LR 4. VA===============================>LR--------->Peer ASBR
(The following two exist in the case where legacy routers (The following two exist in the case where legacy routers
can initiate tunneled packets.) can initiate tunneled packets.)
5. LR===============================>VA(pop)::::>LR 5. LR===============================>VA(pop)::::>Peer ASBR
6. LR===============================>LR--------->LR 6. LR===============================>LR--------->Peer ASBR
(The following two exist in the case where legacy routers (The following two exist in the case where legacy routers
cannot initiate tunneled packets.) cannot initiate tunneled packets.)
7. LR------->VA (remaining paths as in 1 to 4 above) 7. LR------->VA (remaining paths as in 1 to 4 above)
8. LR------->LR--------------------->LR--------->LR 8. LR------->LR--------------------->LR--------->Peer ASBR
The first and second paths represent the case where the ingress The first and second paths represent the case where the ingress
router does not have a Popular Prefix for the destination, and MUST router does not have a Popular Prefix for the destination, and MUST
tunnel the packet to an APR. The third and fourth paths represent tunnel the packet to an APR. The third and fourth paths represent
the case where the ingress router does have a Popular Prefix for the the case where the ingress router does have a Popular Prefix for the
destination, and so tunnels the packet directly to the egress. The destination, and so tunnels the packet directly to the egress. The
fifth and sixth paths are similar to the third and fourth paths fifth and sixth paths are similar to the third and fourth paths
respectively, but where the ingress is a legacy router that can respectively, but where the ingress is a legacy router that can
initiate tunneled packets, and effectively has the Popular Prefix by initiate tunneled packets, and effectively has the Popular Prefix by
virtue of holding the entire DFRT. (Note that some ISPs have only virtue of holding the entire DFRT. (Note that some ISPs have only
partial RIBs in their customer-facing edge routers, and default route partial RIBs in their customer-facing edge routers, and default route
to a router that holds the full DFRT. This case is not shown here, to a router that holds the full DFRT. This case is not shown here,
but works perfectly well.) Finally, paths 7 and 8 represent the case but works perfectly well.) Finally, paths 7 and 8 represent the case
where legacy routers cannot initiate a tunneled packet. where legacy routers cannot initiate a tunneled packet.
VA prevents the routing loops that might otherwise occur when VA VA prevents the routing loops that might otherwise occur when VA
routers and legacy routers are mixed. The trick is avoiding the case routers and legacy routers are mixed. In particular, VA avoids the
where a legacy router is forwarding packets towards the BGP NEXT_HOP, case where a legacy router is forwarding packets towards the BGP
while a VA router is forwarding packets towards the APR, with each NEXT_HOP, while a VA router is forwarding packets towards the APR,
router thinking that the other is on the shortest path to their with each router thinking that the other is on the shortest path to
respective targets. their respective targets.
In the first four types of path, the loop is avoided because tunnels In the first four types of path, the loop is avoided because tunnels
are used all the way to the egress. As a result, there is never an are used all the way to the egress. As a result, there is never an
opportunity for a legacy router to try to route based on the opportunity for a legacy router to try to route based on the
destination address unless the legacy router is the egress, in which destination address unless the legacy router is the egress, in which
case it forwards the packet to the remote ASBR. case it forwards the packet to the remote ASBR.
In the 5th and 6th cases, the ingress is a legacy router, but this In the 5th and 6th cases, the ingress is a legacy router, but this
router can initiate tunnels and has the full FIB, and so simply router can initiate tunnels and has the full FIB, and so simply
tunnels the packet to the egress router. tunnels the packet to the egress router.
In the 7th and 8th cases, the legacy ingress cannot initiate tunnels, In the 7th and 8th cases, the legacy ingress cannot initiate tunnels,
and so forwards the packet hop-by-hop towards the BGP NEXT_HOP. The and so forwards the packet hop-by-hop towards the BGP NEXT_HOP. The
packet will work its way towards the egress router, and will either packet will work its way towards the egress router, and will either
progress through a series of legacy routers (in which case the IGP progress through a series of legacy routers (in which case the IGP
prevents loops), or it will eventually reach a VA router, after which prevents loops), or it will eventually reach a VA router, after which
it will take tunnels as in the 1st and 2nd cases. it will take tunnels as in the 1st and 2nd cases.
3. Specification of VA 3. Specification of VA
This section describes in detail how to operate VA. It starts with a This section describes in detail how to operate VA.
brief discussion of requirements, followed by a specification of
router support for VA.
3.1. VA Operation
In this section, the detailed operation of VA is specified.
3.1.1. Legacy Routers 3.1. Legacy Routers
VA can operate with a mix of VA and legacy routers. To prevent the VA can operate with a mix of VA and legacy routers. To prevent the
types of loops described in Section 2.2, however, legacy routers MUST types of loops described in Section 2.2, however, legacy routers MUST
satisfy the following requirements: satisfy the following requirements:
1. When forwarding externally-received routes over iBGP, the BGP 1. When forwarding externally-received routes over iBGP, the BGP
NEXT_HOP attribute MUST be set to the legacy router itself. NEXT_HOP attribute MUST be set to the legacy router itself.
2. Legacy routers MUST be able to detunnel packets addressed to 2. Legacy routers MUST be able to detunnel packets addressed to
themselves at the BGP NEXT_HOP address. They MUST also be able themselves at the BGP NEXT_HOP address. They MUST also be able
to convey the tunnel information needed by other routers to to convey the tunnel information needed by other routers to
initiate tunneled packets to them. This is listed as initiate tunneled packets to them. If a legacy router cannot
"Requirement R1" in the companion tunneling documents. If a detunnel and convey tunnel parameters, then the AS cannot use VA.
legacy router cannot detunnel and convey tunnel parameters, then
the AS cannot use VA.
3. Legacy routers MUST be able to forward all tunneled packets. 3. Legacy routers MUST be able to forward all tunneled packets.
4. Every legacy router MUST hold its complete FIB. (Note, of 4. Every legacy router MUST hold its complete FIB. Note, however,
course, that this FIB does not necessarily need to contain the that this FIB does not necessarily need to contain the full DFRT.
full DFRT. This might be the case, for instance, if the router This might be the case, for instance, if the router is an edge
is an edge router that defaults to a core router.) router that defaults to a core router.
As long as legacy routers participating in tunneling as described As long as legacy routers participating in tunneling as described
above there are no topological restrictions on the legacy routers. above there are no topological restrictions on the legacy routers.
They may be freely mixed with VA routers without the possibility of They may be freely mixed with VA routers without the possibility of
forming sustained loops (Section 2.2). forming sustained loops (Section 2.2).
3.1.2. Advertising and Handling Virtual Prefixes (VP) 3.2. Advertising and Handling Virtual Prefixes (VP)
3.1.2.1. Distinguishing VPs from Sub-prefixes 3.2.1. Distinguishing VPs from Sub-prefixes
VA routers MUST be able to distinguish VPs from sub-prefixes. This VA routers MUST be able to distinguish VPs from sub-prefixes. This
is primarily in order to know which routes to install. In is primarily in order to know which routes to install. In
particular, non-APR routers MUST know which prefixes are VPs before particular, non-APR routers SHOULD know which prefixes are VPs before
they receive routes for those VPs, for instance when they first boot they receive routes for those VPs, for instance when they first boot
up. This is in order to avoid the situation where they unnecessarily up. This is in order to avoid the situation where they unnecessarily
start filling their FIBs with routes that they ultimately don't need start filling their FIBs with routes that they ultimately don't need
to install (Section 3.1.5). This leads to the following requirement: to install (Section 3.5). This leads to the following requirement:
It MUST be possible to statically configure the complete list of VPs It MUST be possible to configure the complete list of VPs into all VA
into all VA routers. This list is known as the VP-List. routers. This list is known as the VP-List.
3.1.2.2. Limitations on Virtual Prefixes 3.2.2. Limitations on Virtual Prefixes
From the point of view of best-match routing semantics, VPs are From the point of view of best-match routing semantics, VPs are
treated identically to any other prefix. In other words, if the treated identically to any other prefix. In other words, if the
longest matching prefix is a VP, then the packet is routed towards longest matching prefix is a VP, then the packet is routed towards
the VP. If a packet matching a VP reaches an Aggregation Point the VP. If a packet matching a VP reaches an APR for that VP, and
Router (APR) for that VP, and the APR does not have a better matching the APR does not have a better matching route, then the packet is
route, then the packet is discarded by the APR (just as a router that discarded by the APR (just as a router that originates any prefix
originates any prefix will discard a packet that does not have a will discard a packet that does not have a better match).
better match).
The overall semantics of VPs, however, are slightly different from The overall semantics of VPs, however, are slightly different from
those of real prefixes. Without VA, when a router originates a route those of real prefixes. Without VA, when a router originates a route
for a (real) prefix, the expectation is that the addresses within the for a (real) prefix, the expectation is that the addresses within the
prefix are within the originating AS (or a customer of the AS). For prefix are within the originating AS (or a customer of the AS). For
VPs, this is not the case. APRs originate VPs whose sub-prefixes VPs, this is not the case. APRs originate VPs whose sub-prefixes
exist in different ASes. Because of this, it is important that VPs exist in different ASes. Because of this, VPs MUST not be advertised
not be advertised across AS boundaries. across AS boundaries. This is done with NO_EXPORT Communities
Attribute (Section 3.2.3).
It is up to individual domains to define their own VPs. VPs MUST be It is up to individual domains to define their own VPs. VPs MUST be
"larger" (span a larger address space) than any real sub-prefix. If "larger" (span a larger address space) than any real sub-prefix. If
a VP is smaller than a real prefix, then packets that match the real a VP is smaller than a real prefix, then packets that match the real
prefix will nevertheless be routed to an APR owning the VP, at which prefix will nevertheless be routed to an APR owning the VP, at which
point the packet will be dropped if it does not match a sub-prefix point the packet will be dropped if it does not match a sub-prefix
within the VP (Section 6). within the VP (Section 6).
(Note that, in principle there are cases where a VP could be smaller (Note that, in principle there are cases where a VP could be smaller
than a real prefix. This is where the egress router to the real than a real prefix. This is where the egress router to the real
skipping to change at page 13, line 45 skipping to change at page 13, line 11
forward the packet correctly. On the other hand, if the egress forward the packet correctly. On the other hand, if the egress
router is a legacy router, then the APR could not tunnel matching router is a legacy router, then the APR could not tunnel matching
packets to the egress. This is because the egress would view the VP packets to the egress. This is because the egress would view the VP
as a better match, and would loop the packet back to the APR. For as a better match, and would loop the packet back to the APR. For
this reason we require that VPs be larger than any real prefixes, and this reason we require that VPs be larger than any real prefixes, and
that APRs never install prefixes larger than a VP in their FIBs.) that APRs never install prefixes larger than a VP in their FIBs.)
It is valid for a VP to be a subset of another VP. For example, 20/7 It is valid for a VP to be a subset of another VP. For example, 20/7
and 20/8 can both be VPs. In fact, this capability is necessary for and 20/8 can both be VPs. In fact, this capability is necessary for
"splitting" a VP without temporarily increasing the FIB size in any "splitting" a VP without temporarily increasing the FIB size in any
router. (Section 3.1.2.5). router. (Section 3.2.5).
3.1.2.3. Aggregation Point Routers (APR) 3.2.3. Aggregation Point Routers (APR)
Any router MAY be configured as an Aggregation Point Router (APR) for For each VP for which a router is an APR, the router does the
one or more Virtual Prefixes (VP). For each VP for which a router is following:
an APR, the router does the following:
1. The APR MUST originate a BGP route to the VP [RFC4271]. In this 1. The APR MUST originate a BGP route to the VP. In this route, the
route, the NLRI are all of the VPs for which the router is an NLRI are all of the VPs for which the router is an APR. This is
APR. This is true even for VPs that are a subset of another VP. true even for VPs that are a subset of another VP. The ORIGIN is
The ORIGIN is set to INCOMPLETE (value 2), the AS number of the set to INCOMPLETE (value 2), the AS number of the APR's AS is
APR's AS is used in the AS_PATH, and the BGP NEXT_HOP is set to used in the AS_PATH, and the BGP NEXT_HOP is set to the address
the address of the APR. The ATOMIC_AGGREGATE and AGGREGATOR of the APR. The ATOMIC_AGGREGATE and AGGREGATOR attributes are
attributes are not included. not included.
2. The APR MUST attach a NO_EXPORT Communities Attribute [RFC1997] 2. The APR MUST attach a NO_EXPORT Communities Attribute [RFC1997]
to the route. to the route.
3. The APR MUST be able to detunnel packets addressed to itself at 3. The APR MUST be able to detunnel packets addressed to itself at
its BGP NEXT_HOP address. It MUST also be able to convey the its BGP NEXT_HOP address. It MUST also be able to convey the
tunnel information needed by other routers to initiate tunneled tunnel information needed by other routers to initiate tunneled
packets to them (Requirement R1). packets to them.
4. If a packet is received at the APR whose best match route is the 4. If a packet is received at the APR whose best match route is the
VP (i.e. it matches the VP but not any sub-prefixes within the VP (i.e. it matches the VP but not any sub-prefixes within the
VP), then the packet MUST be discarded (see Section 3.1.2.2). VP), then the packet MUST be discarded (see Section 3.2.2). This
This can be accomplished by never installing a prefix larger than can be accomplished by never installing a prefix larger than the
the VP into the FIB, or by installing the VP as a route to VP into the FIB, or by installing the VP as a route to \dev\null.
\dev\null.
3.1.2.3.1. Selecting APRs 3.2.3.1. Selecting APRs
An ISP is free to select APRs however it chooses. The details of An ISP is free to select APRs however it chooses. The details of
this are outside the scope of this document. Nevertheless, a few this are outside the scope of this document. Nevertheless, a few
comments are made here. In general, APRs should be selected such comments are made here. In general, APRs should be selected such
that the distance to the nearest APR for any VP is small---ideally that the distance to the nearest APR for any VP is small---ideally
within the same POP. Depending on the number of routers in a POP, within the same POP. Depending on the number of routers in a POP,
and the sizes of the FIBs in the routers relative to the DFRT size, and the sizes of the FIBs in the routers relative to the DFRT size,
it may not be possible for all VPs to be represented in a given POP. it may not be possible for all VPs to be represented in a given POP.
In addition, there should be multiple APRs for each VP, again ideally In addition, there should be multiple APRs for each VP, again ideally
in each POP, so that the failure of one does not unduly disrupt in each POP, so that the failure of one does not unduly disrupt
traffic. traffic.
Note that, although VPs MUST be larger than real prefixes, there is 3.2.4. Non-APR Routers
intentionally no mechanism designed to automatically insure that this
is the case. Such a mechanisms would be dangerous. For instance, if
an ISP somewhere advertised a very large prefix (a /4, say), then
this would cause APRs to throw out all VPs that are smaller than
this. For this reason, VPs MUST be set through static configuration
only.
3.1.2.4. Non-APR Routers
A non-APR router MUST install at least the following routes: A non-APR router MUST install at least the following routes:
1. Routes to VPs (identifiable using the VP-List). 1. Routes to VPs (identifiable using the VP-List).
2. Routes to all sub-prefixes that are not covered by any VP in the 2. Routes to all sub-prefixes that are not covered by any VP in the
VP-List. VP-List.
If the non-APR has a tunnel to the BGP NEXT_HOP of any such route, it If the non-APR has a tunnel to the BGP NEXT_HOP of any such route, it
MUST use the tunnel to forward packets to the BGP NEXT_HOP. MUST use the tunnel to forward packets to the BGP NEXT_HOP.
When an APR fails, routers must select another APR to send packets to When an APR fails, routers must select another APR to send packets to
(if there is one). This happens, however, through normal internal (if there is one). This happens, however, through normal internal
BGP convergence mechanisms. BGP convergence mechanisms.
skipping to change at page 15, line 15 skipping to change at page 14, line 20
2. Routes to all sub-prefixes that are not covered by any VP in the 2. Routes to all sub-prefixes that are not covered by any VP in the
VP-List. VP-List.
If the non-APR has a tunnel to the BGP NEXT_HOP of any such route, it If the non-APR has a tunnel to the BGP NEXT_HOP of any such route, it
MUST use the tunnel to forward packets to the BGP NEXT_HOP. MUST use the tunnel to forward packets to the BGP NEXT_HOP.
When an APR fails, routers must select another APR to send packets to When an APR fails, routers must select another APR to send packets to
(if there is one). This happens, however, through normal internal (if there is one). This happens, however, through normal internal
BGP convergence mechanisms. BGP convergence mechanisms.
3.1.2.5. Adding and deleting VPs 3.2.5. Adding and deleting VPs
An ISP may from time to time wish to reconfigure its VP-List. There An ISP may from time to time wish to reconfigure its VP-List. There
are a number of reasons for this. For instance, early in its are a number of reasons for this. For instance, early in its
deployment an ISP may configure one or a small number of VPs in order deployment an ISP may configure one or a small number of VPs in order
to test VA. As the ISP gets more confident with VA, it may increase to test VA. As the ISP gets more confident with VA, it may increase
the number of VPs. Or, an ISP may start with a small number of large the number of VPs. Or, an ISP may start with a small number of large
VPs (i.e. /4's or even one /0), and over time move to more smaller VPs (i.e. /4's or even one /0), and over time move to more smaller
VPs in order to save even more FIB. In this case, the ISP will need VPs in order to save even more FIB. In this case, the ISP will need
to "split" a VP. Finally, since the address space is not uniformly to "split" a VP. Finally, since the address space is not uniformly
populated with prefixes, the ISP may want to change the size of VPs populated with prefixes, the ISP may want to change the size of VPs
in order to balance FIB size across routers. This can involve both in order to balance FIB size across routers. This can involve both
splitting and merging VPs. Of course, an ISP must be able to modify splitting and merging VPs. Of course, an ISP must be able to modify
its VP-List without 1) interrupting service to any destinations, or its VP-List without 1) interrupting service to any destinations, or
2) temporarily increasing the size of any FIB (i.e. where the FIB 2) temporarily increasing the size of any FIB (i.e. where the FIB
size during the change is no bigger than its size either before or size during the change is no bigger than its largest size either
after the change). before or after the change).
Adding a VP is straightforward. The first step is to configure the The first step for adding a VP is to configure the APRs for the VP.
APRs for the VP. This causes the APRs to originate routes for the This causes the APRs to originate routes for the VP. Non-APR routers
VP. Non-APR routers will install this route according to the rules will install this route according to the rules in Section 3.2.4 even
in Section 3.1.2.4 even though they do not yet recognize that the though they do not yet recognize that the prefix is a VP.
prefix is a VP. Subsequently the VP is added to the VP-List of non- Subsequently the VP is added to the VP-List of non-APR routers. The
APR routers. The Non-APR routers can then start suppressing the sub- Non-APR routers can then start suppressing the sub-prefixes with no
prefixes with no loss of service. loss of service.
To delete a VP, the process is reversed. First, the VP is removed To delete a VP, the process is reversed. First, the VP is removed
from the VP-Lists of non-APRs. This causes the non-APRs to install from the VP-Lists of non-APRs. This causes the non-APRs to install
the sub-prefixes. After all sub-prefixes have been installed, the VP the sub-prefixes. After all sub-prefixes have been installed, the VP
may be removed from the APRs. may be removed from the APRs.
In many cases, it is desirable to split a VP. For instance, consider In many cases, it is desirable to split a VP. For instance, consider
the case where two routers, Ra and Rb, are APRs for the same prefix. the case where two routers, Ra and Rb, are APRs for the same prefix.
It would be possible to shrink the FIB in both routers by splitting It would be possible to shrink the FIB in both routers by splitting
the VP into two VPs (i.e. split one /6 into two /7's), and assigning the VP into two VPs (i.e. split one /6 into two /7's), and assigning
each router to one of the VPs. While this could in theory be done by each router to one of the VPs. While this could in theory be done by
first deleting the larger VP, and then adding the smaller VPs, doing first deleting the larger VP, and then adding the smaller VPs, doing
so would temporarily increase the FIB size in non-APRs, which may not so would temporarily increase the FIB size in non-APRs, which may not
have adequate space for such an increase. For this reason, we allow have adequate space for such an increase. For this reason, we allow
overlapping VPs. overlapping VPs.
To split a VP, first the two smaller VPs are added to the VP-Lists of To split a VP, first the two smaller VPs are added to the VP-Lists of
all non-APR routers (in addition to the larger superset VP). Next, all non-APR routers (in addition to the larger superset VP). Next,
skipping to change at page 16, line 22 skipping to change at page 15, line 28
forward packets to the APRs for the smaller VPs. Next, the larger VP forward packets to the APRs for the smaller VPs. Next, the larger VP
can be removed from the VP-Lists of all non-APR routers. Finally, can be removed from the VP-Lists of all non-APR routers. Finally,
the larger VP can be removed from its APRs. the larger VP can be removed from its APRs.
To merge two VPs, the new larger VP is configured in all non-APRs. To merge two VPs, the new larger VP is configured in all non-APRs.
This has no effect on FIB size or APR selection, since the smaller This has no effect on FIB size or APR selection, since the smaller
VPs are better matches. Next the larger VP is configured in its VPs are better matches. Next the larger VP is configured in its
selected APRs. Next the smaller VPs are deleted from all non-APRs. selected APRs. Next the smaller VPs are deleted from all non-APRs.
Finally, the smaller VPs are deleted from their corresponding APRs. Finally, the smaller VPs are deleted from their corresponding APRs.
3.1.3. Border VA Routers 3.3. Border VA Routers
A VA router that is an ASBR MUST do the following: A VA router that is an ASBR MUST do the following:
1. When forwarding externally-received routes over iBGP, if a tunnel 1. When forwarding externally-received routes over iBGP, if a tunnel
with an inner label is used, the ASBR MUST set the BGP NEXT_HOP with an inner label is used, the ASBR MUST set the BGP NEXT_HOP
attribute to itself. Otherwise, the BGP NEXT_HOP attribute is attribute to itself. Otherwise, the BGP NEXT_HOP attribute is
left unchanged. left unchanged.
2. They MUST establish tunnels as described in Section 4. 2. They MUST establish tunnels as described in Section 4.
3. The ASBR MUST detunnel the packet before forwarding the packet to 3. The ASBR MUST detunnel the packet before forwarding the packet to
the remote ASBR. In other words, the remote ASBR receives a the remote ASBR.
normal untunneled packet identical to the packet it would receive
without VA.
4. The ASBR MUST be able to forward the packet without a FIB lookup. 4. The ASBR MUST be able to forward the packet without a FIB lookup.
In other words, the tunnel information itself contains all the In other words, the tunnel information itself contains all the
information needed by the border router to know which remote ASBR information needed by the border router to know which remote ASBR
should receive the packet. should receive the packet.
3.1.4. Advertising and Handling Sub-Prefixes 3.4. Advertising and Handling Sub-Prefixes
Sub-prefixes are advertised and handled by BGP as normal. VA does Sub-prefixes are advertised and handled by BGP as normal. VA does
not effect this behavior. The only difference in the handling of not effect this behavior. The only difference in the handling of
sub-prefixes is that they might not be installed in the FIB, as sub-prefixes is that they might not be installed in the FIB, as
described in Section 3.1.5. described in Section 3.5.
In those cases where the route is installed, packets forwarded to In those cases where the route is installed, packets forwarded to
prefixes external to the AS MUST be transmitted via the tunnel prefixes external to the AS MUST be transmitted via the tunnel
established as described in Section 3.1.3. established as described in Section 3.3.
3.1.5. Suppressing FIB Sub-prefix Routes 3.5. Suppressing FIB Sub-prefix Routes
Any route not for a known VP (i.e. not in the VP-List) is taken to be Any route not for a known VP (i.e. not in the VP-List) is taken to be
a sub-prefix. The following rules are used to determine if a sub- a sub-prefix. The following rules are used to determine if a sub-
prefix route can be suppressed. prefix route can be suppressed.
1. A VA router MUST NOT FIB-install a sub-prefix route for which 1. A VA router MUST NOT FIB-install a sub-prefix route for which
there is no tunnel to the BGP NEXT_HOP address. This is to there is no tunnel to the BGP NEXT_HOP address. This is to
prevent a loop whereby the APR forwards the packet hop-by-hop prevent a loop whereby the APR forwards the packet hop-by-hop
towards the next hop, but a router on the path that has FIB- towards the next hop, but a router on the path that has FIB-
suppressed the sub-prefix forwards it back to the APR. If there suppressed the sub-prefix forwards it back to the APR.
is an alternate route to the sub-prefix for which there is a
tunnel, then that route SHOULD be selected, even if it is less
attractive according to the normal BGP best path selection
algorithm.
2. If the router is an APR, a route for every sub-prefix within the 2. If the router is an APR, a route for every sub-prefix within the
VP MUST be FIB-installed (subject to the above limitation that VP MUST be FIB-installed (subject to the above limitation that
there be a tunnel). there be a tunnel).
3. If a non-APR router has a sub-prefix route that does not fall 3. If a non-APR router has a sub-prefix route that does not fall
within any VP (as determined by the VP-List), then the route MUST within any VP (as determined by the VP-List), then the route MUST
be installed. This may occur because the ISP hasn't defined a VP be installed. This may occur because the ISP hasn't defined a VP
covering that prefix, for instance during an incremental covering that prefix, for instance during an incremental
deployment buildup. deployment build-up.
4. If an ASBR is using strict uRPF to do ingress filtering, then it 4. If an ASBR is using strict uRPF to do ingress filtering, then it
MUST install routes for which the remote ASBR is the BGP NEXT_HOP MUST install routes for which the remote ASBR is the BGP NEXT_HOP
[RFC2827]. Note that only a APR may do loose uRPF filtering, and [RFC2827]. Note that only an APR may do loose uRPF filtering,
then only for routes to sub-prefixes within its VPs. and then only for routes to sub-prefixes within its VPs.
5. All other sub-prefix routes MAY be suppressed. Such "optional" 5. All other sub-prefix routes MAY be suppressed. Such "optional"
sub-prefixes that are nevertheless installed are referred to as sub-prefixes that are nevertheless installed are referred to as
Popular Prefixes. Note, however, that whether or not to install Popular Prefixes. Note, however, that whether or not to install
a given sub-prefix SHOULD NOT be based on whether or not there is a given sub-prefix SHOULD NOT be based on whether or not there is
an active route to a VP in the VP-List. This avoids the an active route to a VP in the VP-List. This avoids the
situation whereby, during BGP initialization, the router receives situation whereby, during BGP initialization, the router receives
some sub-prefix routes before receiving the corresponding VP some sub-prefix routes before receiving the corresponding VP
route, with the result that it installs routes in its FIB that it route, with the result that it installs routes in its FIB that it
will only remove a short time later, possibly even overflowing will only remove a short time later, possibly even overflowing
its FIB. its FIB.
3.1.5.1. Selecting Popular Prefixes 3.5.1. Selecting Popular Prefixes
Individual routers MAY independently choose which sub-prefixes are Individual routers MAY independently choose which sub-prefixes are
Popular Prefixes. There is no need for different routers to install Popular Prefixes. There is no need for different routers to install
the same sub-prefixes. There is therefore significant leeway as to the same sub-prefixes. There is therefore significant leeway as to
how routers select Popular Prefixes. As a general rule, routers how routers select Popular Prefixes. As a general rule, routers
should fill the FIB as much as possible, because the cost of doing so should fill the FIB as much as possible, because the cost of doing so
is relatively small, and more FIB entries leads to fewer packets is relatively small, and more FIB entries leads to fewer packets
taking a longer path. Broadly speaking, an ISP may choose to fill taking a longer path. Broadly speaking, an ISP may choose to fill
the FIB by making routers APRs for as many VPs as possible, or by the FIB by making routers APRs for as many VPs as possible, or by
assigning relatively few APRs and rather filling the FIB with Popular assigning relatively few APRs and rather filling the FIB with Popular
skipping to change at page 18, line 18 skipping to change at page 17, line 16
1. Policy-based: The simplest approach for network administrators is 1. Policy-based: The simplest approach for network administrators is
to have broad policies that routers use to determine which sub- to have broad policies that routers use to determine which sub-
prefixes are designated as popular. An obvious policy would be a prefixes are designated as popular. An obvious policy would be a
"customer routes" policy, whereby all customer routes are "customer routes" policy, whereby all customer routes are
installed (as identified for instance by appropriate community installed (as identified for instance by appropriate community
attribute tags). Another policy would be for a router to install attribute tags). Another policy would be for a router to install
prefixes originated by specific ASes. For instance, two ISPs prefixes originated by specific ASes. For instance, two ISPs
could mutually agree to install each other's originated prefixes. could mutually agree to install each other's originated prefixes.
A third policy might be to install prefixes with the shortest AS- A third policy might be to install prefixes with the shortest AS-
path. PATH.
2. Static list: Another approach would be to configure static lists 2. Static list: Another approach would be to configure static lists
of specific prefixes to install. For instance, prefixes of specific prefixes to install. For instance, prefixes
associated with an SLA might be configured. Or, a list of associated with an SLA might be configured. Or, a list of
prefixes for the most popular websites might be installed. prefixes for the most popular websites might be installed.
3. High-volume prefixes: By installing high-volume prefixes as 3. High-volume prefixes: By installing high-volume prefixes as
Popular Prefixes, the latency and load associated with the longer Popular Prefixes, the latency and load associated with the longer
path required by VA is minimized. One approach would be for an path required by VA is minimized. One approach would be for an
ISP to measure its traffic volume over time (days or a few ISP to measure its traffic volume over time (days or a few
weeks), and statically configure high-volume prefixes as Popular weeks), and statically configure high-volume prefixes as Popular
Prefixes. There is strong evidence that prefixes that are high- Prefixes. There is strong evidence that prefixes that are high-
volume tend to remain high-volume over multi-day or multi-week volume tend to remain high-volume over multi-day or multi-week
timeframes (though not necessarily at short timeframes like timeframes (though not necessarily at short timeframes like
minutes or seconds). High-volume prefixes MAY also be installed minutes or seconds). High-volume prefixes MAY also be installed
dynamically. In other words, a router measures its own traffic automatically. For this, a router measures its own traffic
volumes, and installs and removes Popular Prefixes in response to volumes, and installs and removes Popular Prefixes in response to
short term traffic load. The downside of this approach is that changes in traffic load. The downside of this approach is that
it complicates debugging network problems. If packets are being it complicates debugging network problems. If packets are being
dropped somewhere in the network, it is more difficult to find dropped somewhere in the network, it is more difficult to find
out where if the selected path can change dynamically. out where if the selected path can change dynamically.
3.2. New Configuration 3.6. New Configuration
VA places new configuration requirements on ISP administrators. VA places new configuration requirements on ISP administrators.
Namely, the administrator must: Namely, the administrator does the following.
1. Select VPs, and configure the VP-List into all VA routers. As a 1. Select VPs, and configure the VP-List into all VA routers. As a
general rule, having a larger number of relatively small prefixes general rule, having a larger number of relatively small prefixes
gives administrators the most flexibility in terms of filling gives administrators the most flexibility in terms of filling
available FIB with sub-prefixes, and in terms of balancing load available FIB with sub-prefixes, and in terms of balancing load
across routers. Once an administrator has selected a VP-List, it across routers. Once an administrator has selected a VP-List, it
is just as easy to configure routers with a large list as a small is just as easy to configure routers with a large list as a small
list. We can expect network operator groups like NANOG to list. A good list might be one where the number of VPs is
compile good VP-Lists that ISPs can then adopt. A good list relatively large, say 100 or so (noting again that each VP must
would be one where the number of VPs is relatively large, say 100 be smaller than a real prefix), and the number of sub-prefixes
or so (noting again that each VP must be smaller than a real within each VP is roughly the same.
prefix), and the number of sub-prefixes within each VP is roughly
the same.
2. Select and configure APRs. There are three primary 2. Select and configure APRs. There are three primary
considerations here. First, there must be enough APRs to handle considerations here. First, there needs to be enough APRs to
reasonable APR failure scenarios. Second, APR assignment should failover to should one or more APRs crash. Second, APR
not result in router overload. Third, particularly long paths assignment should not result in router overload. Third,
should be avoided. Ideally there should be two APRs for each VP excessively long paths should be avoided. Ideally there should
within each PoP, but this may not be possible for small PoPs. be two APRs for each VP within each PoP, but this may not be
Failing this, there should be at least two APRs in each possible for small PoPs. Failing this, there should be at least
geographical region, so as to minimize path length increase. two APRs in each geographical region, so as to minimize path
Routers should have the appropriate counters to allow length increase. Routers should have the appropriate counters to
administrators to know the volume of APR traffic each router is allow administrators to know the volume of APR traffic each
handling so as to adjust load by adding or removing APR router is handling so as to adjust load by adding or removing APR
assignments. assignments.
3. Select and configure Popular Prefixes or Popular Prefix policies. 3. Select and configure Popular Prefixes or Popular Prefix policies.
There are two general goals here. The first is to minimize load There are two general goals here. The first is to minimize load
overall by minimizing the number of packets that take longer overall by minimizing the number of packets that take longer
paths. The second is to insure that specific selected prefixes paths. The second is to insure that specific selected prefixes
don't have overly long paths. These goals must be weighed don't have overly long paths. These goals have to be weighed
against the administrative overhead of configuring potentially against the administrative overhead of configuring potentially
thousands of Popular Prefixes. As one example a small ISP may thousands of Popular Prefixes. As one example a small ISP may
wish to keep it simple by doing nothing more than indicating that wish to keep it simple by doing nothing more than indicating that
customer routes should be installed. In this case, the customer routes should be installed. In this case, the
administrator could otherwise assign as many APRs as possible administrator could otherwise assign as many APRs as possible
while leaving enough FIB space for customer routes. As another while leaving enough FIB space for customer routes. As another
example, a large ISP could build a management system that takes example, a large ISP could build a management system that takes
into consideration the traffic matrix, customer SLAs, robustness into consideration the traffic matrix, customer SLAs, robustness
requirements, FIB sizes, topology, and router capacity, and requirements, FIB sizes, topology, and router capacity, and
periodically automatically computes APR and Popular Prefix periodically automatically computes APR and Popular Prefix
assignments. assignments.
4. Usage of Tunnels 3.7. Interaction with Traffic Engineering
4.1. MPLS tunnels In VA, some traffic traverses an APR as an intermediate "hop", and
some does not. For that traffic that does not, there is no
difference between how that traffic is handled and how it is handled
in a non-VA network with edge-to-edge tunnels. As a result, there
should be no difference in how traffic engineering operates on that
traffic.
For traffic that does traverse APR "hop", the following holds: Any
traffic engineering decisions that affect the BGP NEXT_HOP must be
made at the APR. Traffic engineering decisions that effects the
router path through the AS may be handled in one of two ways. First,
the path decision may simply be made twice independently, once for
the ingress-to-APR tunnel, and once for the APR-to-egress tunnel.
This approach requires no changes to the traffic engineering
mechanisms per se, but it may not make optimal path selection
decisions. Second, the traffic engineering decision may take into
account both tunnels, even to the point of choosing among multiple
transit APRs. This approach may be more optimal, but is more complex
and requires changes to existing mechanisms.
Overall, if the majority of traffic does not involve an APR "hop",
for instance through the use of popular prefixes, then VA in any
event has a minimal impact on traffic engineering, and so the impact
of VA may potentially be ignored.
4. Usage of MPLS Tunnels
VA utilizes a straight-forward application of MPLS. The tunnels are VA utilizes a straight-forward application of MPLS. The tunnels are
MPLS Label Switched Paths (LSP), and are signaled using either the MPLS Label Switched Paths (LSP), and are signaled using either the
Label Distribution Protocol (LDP) [RFC5036] or RSVP-TE [RFC3209]. Label Distribution Protocol (LDP) [RFC5036] or RSVP-TE [RFC3209].
Both VA and legacy routers MUST participate in this signaling. Both VA and legacy routers MUST participate in this signaling.
APRs and ASBRs initiate tunnels. In both cases, Downstream APRs and ASBRs initiate tunnels. In both cases, Downstream
Unsolicited tunnels are initiated to all IGP neighbors with the full Unsolicited tunnels are initiated to all IGP neighbors with the full
BGP NEXT_HOP address as the Forwarding Equivalence Class (FEC). In BGP NEXT_HOP address as the Forwarding Equivalence Class (FEC). In
the case of APRs, the BGP NEXT_HOP is the APR's own address. In the the case of APRs, the BGP NEXT_HOP is the APR's own address. In the
case of legacy ASBRs, the BGP NEXT_HOP is the ASBR's own address. In case of legacy ASBRs, the BGP NEXT_HOP is the ASBR's own address. In
the case of VA ASBRs, the BGP NEXT_HOP is that of the remote ASBR. the case of VA ASBRs, the BGP NEXT_HOP is that of the remote ASBR.
Existing Penultimate Hop Popping (PHP) mechanisms in the data plane Existing Penultimate Hop Popping (PHP) mechanisms in the data plane
can be used for forwarding packets to remote ASBRs. can be used for forwarding packets to remote ASBRs.
4.2. Usage of Inner Label 4.1. Usage of Inner Label
Besides using a separate LSP to identify the remote ASBR as described Besides using a separate LSP to identify the remote ASBR as described
above, it is also possible to use an inner label to identify the above, it is also possible to use an inner label to identify the
remote ASBR. Either an outer label or an IP tunnel identifies the remote ASBR. Either an outer label or an IP tunnel identifies the
local ASBR. local ASBR.
When a local ASBR advertises a route into iBGP, it sets the NEXT_HOP When a local ASBR advertises a route into iBGP, it sets the NEXT_HOP
to itself, and assigns a label to the route. This label is used as to itself, and assigns a label to the route. This label is used as
the inner label, and identifies the remote ASBR from which the route the inner label, and identifies the remote ASBR from which the route
was received [RFC3107]. was received [RFC3107].
skipping to change at page 20, line 35 skipping to change at page 20, line 12
packet to the NEXT_HOP address. Otherwise, an IP header address to packet to the NEXT_HOP address. Otherwise, an IP header address to
the NEXT_HOP address is used. the NEXT_HOP address is used.
5. IANA Considerations 5. IANA Considerations
There are no IANA considerations. There are no IANA considerations.
6. Security Considerations 6. Security Considerations
We consider the security implications of VA under two scenarios, one We consider the security implications of VA under two scenarios, one
where VA is configured and operated correctly, and one where it is where VA is assumed to be configured and operated correctly, and one
mis-configured. A cornerstone of VA operation is that the basic where it is mis-configured. A cornerstone of VA operation is that
behavior of BGP doesn't change, especially inter-domain. Among other the basic behavior of BGP doesn't change, especially inter-domain.
things, this makes it easier to reason about security. Among other things, this makes it easier to reason about security.
6.1. Properly Configured VA 6.1. Properly Configured VA
If VA is configured and operated properly, then the external behavior If VA is configured and operated properly, then the external behavior
of an AS does not change. The same upstream ASes are selected, and of an AS does not change. The same upstream ASes are selected, and
the same prefixes and AS-paths are advertised. Therefore, a properly the same prefixes and AS-PATHs are advertised. Therefore, a properly
configured VA domain has no security impact on other domains. configured VA domain has no security impact on other domains.
If another ISP starts advertising a prefix that is larger than a If another ISP starts advertising a prefix that is larger than a
given VP, this prefix will be ignored by APRs that have a VP that given VP, this prefix will be ignored by APRs that have a VP that
falls within the larger prefix (Section 3.1.2.3). As a result, falls within the larger prefix (Section 3.2.3). As a result, packets
packets that might otherwise have been routed to the new larger that might otherwise have been routed to the new larger prefix will
prefix will be dropped at the APRs. Note that the trend in the be dropped at the APRs. Note that the trend in the Internet is
Internet is towards large prefixes being broken up into smaller ones, towards large prefixes being broken up into smaller ones, not the
not the reverse. Therefore, such a larger prefix is likely to be reverse. Therefore, such a larger prefix is likely to be invalid.
invalid. If it is determined without a doubt that the larger prefix If it is determined without a doubt that the larger prefix is valid,
is valid, then the ISP will have to reconfigure its VPs. then the ISP will have to reconfigure its VPs.
VA does not change an ISP's ability to do ingress filtering using VA does not change an ISP's ability to do ingress filtering using
strict uRPF (Section 3.1.5). strict uRPF (Section 3.5).
Regarding DoS attacks, there are two issues that need to be Regarding DoS attacks, there are two issues that need to be
considered. First, does VA result in new types of DoS attacks? considered. First, does VA result in new types of DoS attacks?
Second, does VA make it more difficult to deploy DoS defense systems. Second, does VA make it more difficult to deploy DoS defense systems.
Regarding the first issue, one possibility is that an attacker Regarding the first issue, one possibility is that an attacker
targets a given router by flooding the network with traffic to targets a given router by flooding the network with traffic to
prefixes that are not popular, and for which that router is an APR. prefixes that are not popular, and for which that router is an APR.
This would cause a disproportionate amount of traffic to be forwarded This would cause a disproportionate amount of traffic to be forwarded
to the APR(s). While it is up to individual ISPs to decide if this to the APR(s). While it is up to individual ISPs to decide if this
attack is a concern, it does not strike the authors that this attack attack is a concern, it does not strike the authors that this attack
skipping to change at page 21, line 34 skipping to change at page 21, line 9
Many DoS defense systems use dynamically established Routing Table Many DoS defense systems use dynamically established Routing Table
entries to divert victims' traffic into LSPs that carry the traffic entries to divert victims' traffic into LSPs that carry the traffic
to scrubbers. This mechanism works with VA---it simply over-rides to scrubbers. This mechanism works with VA---it simply over-rides
whatever route is in place. This mechanism works equally well with whatever route is in place. This mechanism works equally well with
APRs and non-APRs. APRs and non-APRs.
6.2. Mis-configured VA 6.2. Mis-configured VA
VA introduces the possibility that a VP is advertised outside of an VA introduces the possibility that a VP is advertised outside of an
AS. This in fact should be a low probability event, but it is AS. This in fact should be a low probability event since routers
considered here none-the-less. filter these, but it is considered here none-the-less.
If an AS leaks a large VP (i.e. larger than any real prefixes), then If an AS leaks a large VP (i.e. larger than any real prefixes), then
the impact is minimal. Smaller prefixes will be preferred because of the impact is minimal. Smaller prefixes will be preferred because of
best-match semantics, and so the only impact is that packets that best-match semantics, and so the only impact is that packets that
otherwise have no matching routes will be sent to the misbehaving AS otherwise have no matching routes will be sent to the misbehaving AS
and dropped there. If an AS leaks a small VP (i.e. smaller than a and dropped there. If an AS leaks a small VP (i.e. smaller than a
real prefix), then packets to that AS will be hijacked by the real prefix), then packets to that AS will be hijacked by the
misbehaving AS and dropped. This can happen with or without VA, and misbehaving AS and dropped. (This can happen with or without VA, and
so doesn't represent a new security problem per se. so doesn't represent a new security problem per se.)
Although VPs MUST be larger than real prefixes, there is
intentionally no mechanism designed to automatically insure that this
is the case. Such a mechanisms would be dangerous. For instance, if
an ISP somewhere advertised a very large prefix (a /4, say), then
this would cause APRs to throw out all VPs that are smaller than
this. For this reason, VPs must be set through configuration only.
7. Acknowledgements 7. Acknowledgements
The authors would like to acknowledge the efforts of Xinyang Zhang The authors would like to acknowledge the efforts of Xinyang Zhang
and Jia Wang, who worked on CRIO (Core Router Integrated Overlay), an and Jia Wang, who worked on CRIO (Core Router Integrated Overlay), an
early inter-domain variant of FIB suppression, and the efforts of early inter-domain variant of FIB suppression, and the efforts of
Hitesh Ballani and Tuan Cao, who worked on the configuration-only Hitesh Ballani and Tuan Cao, who worked on the configuration-only
variant of VA that works with legacy routers. We would also like to variant of VA that works with legacy routers. We would also like to
thank Scott Brim, Daniel Ginsburg, and Rajiv Asati for their helpful thank Scott Brim, Daniel Ginsburg, and Rajiv Asati for their helpful
comments. In particular, Daniel's comments significantly simplified comments. In particular, Daniel's comments significantly simplified
the spec (eliminating the need for a new External Communities the spec (eliminating the need for a new Extended Communities
Attribute). Attribute). Finally, we would like to thank Wes George, Med
Boucadair, and Bruno Decraene for their reviews and suggestions.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP [RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
Communities Attribute", RFC 1997, August 1996. Communities Attribute", RFC 1997, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
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