Network Working Group Camilo Cardona
Internet-Draft IMDEA Networks/UC3M
Intended status: Informational Pierre Francois
Expires: May 11, 2016 Paolo Lucente
Cisco Systems
November 08, 2015
Impact of BGP filtering on Inter-Domain Routing Policies
draft-ietf-grow-filtering-threats-08
Abstract
This document describes how unexpected traffic flows can emerge
across an autonomous system, as the result of other autonomous
systems filtering, or restricting the propagation of more specific
prefixes. We provide a review of the techniques to detect the
occurrence of this issue and defend against it.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Unexpected Traffic Flows . . . . . . . . . . . . . . . . . . 4
2.1. Local filtering . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Unexpected traffic flows caused by local filtering of
more specific prefixes . . . . . . . . . . . . . . . 5
2.2. Remote filtering . . . . . . . . . . . . . . . . . . . . 6
2.2.1. Unexpected traffic flows caused by remotely triggered
filtering of more specific prefixes . . . . . . . . . 7
3. Techniques to detect unexpected traffic flows caused by
filtering of more specific prefixes . . . . . . . . . . . . . 8
3.1. Existence of unexpected traffic flows within an AS . . . 8
3.2. Contribution to the existence of unexpected traffic flows
in another AS . . . . . . . . . . . . . . . . . . . . . . 9
4. Techniques to Traffic Engineer unexpected flows . . . . . . . 10
4.1. Reactive Traffic Engineering . . . . . . . . . . . . . . 11
4.2. Proactive measures . . . . . . . . . . . . . . . . . . . 12
4.2.1. Access lists . . . . . . . . . . . . . . . . . . . . 12
4.2.2. Neighbor-specific forwarding . . . . . . . . . . . . 13
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
It is common practice for network operators to propagate a more
specific prefix in the BGP routing system, along with the less
specific prefix that they originate. It is also possible for some
Autonomous Systems (ASes) to apply different policies to the more
specific and the less specific prefix.
Although BGP makes independent, policy driven decisions for the
selection of the best path to be used for a given IP prefix, routers
must forward packets using the longest-prefix-match rule, which
"precedes" any BGP policy (RFC1812 [RFC1812]). The existence of a
prefix p that is more specific than a prefix p' in the Forwarding
Information Base (FIB) will let packets whose destination matches p
be forwarded according to the next hop selected as best for p (the
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more specific prefix). This process takes place by disregarding the
policies applied in the control plane for the selection of the best
next-hop for p'. When an Autonomous System filters more specific
prefixes and forwards packets according to the less specific prefix,
the discrepancy among the routing policies applied to the less and
the more specific prefixes can create unexpected traffic flows.
These may infringe the policies of other ASes, still holding a path
towards the more specific prefix.
The objective of this draft is to shed light on possible side effects
associated with more specific prefix filtering. Such actions can be
explained by traffic engineering action, misconfiguration, or
malicious intent. This document presents examples of such side
effects and discusses approaches towards solutions to the problem.
The rest of the document is organized as follows: In Section 2 we
provide some scenarios in which the filtering of more specific
prefixes leads to the creation of unexpected traffic flows.
Section 3 and Section 4 discuss some techniques that ASes can use
for, respectively, detecting and reacting to unexpected traffic
flows. The document concludes in Section 5.
1.1. Terminology
More specific prefix: A prefix in the routing table with an address
range that is covered by a shorter prefix also present in the routing
table.
Less specific prefix: A prefix in the routing table with an address
range partially covered by other prefixes.
This document reuses the definitions of customer-transit peering and
settlement-free peering of RFC4384 [RFC4384].
Selective advertisement: The behavior of only advertising a self
originated BGP path for a prefix over a strict subset of the eBGP
sessions of the AS.
Selective propagation: The behavior of only propagating a BGP path
for a prefix over a strict subset of the eBGP sessions of an AS.
Local filtering: The behavior of explicitly ignoring a BGP path
received over an eBGP session.
Remote filtering: The behavior of triggering selective propagation of
a BGP path at a distant AS. Note that this is typically achieved by
tagging a self-originated path with BGP communities defined by the
distant AS.
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Unexpected traffic flow: Traffic flowing between two neighboring ASes
of an AS, although the transit policy of that AS is to not provide
connectivity between these two neighbors. A traffic flow across an
AS, between two of its transit providers, or between a transit
provider and one of its settlement-free peers, are classical examples
of unexpected traffic flows.
2. Unexpected Traffic Flows
In this section, we describe how more specific prefix filtering can
lead to unexpected traffic flows in other, remote, ASes. We
differentiate cases in which the filtering is performed locally from
those where the filtering is triggered remotely.
2.1. Local filtering
Different reasons motivate local filtering, for example: (1) Traffic
engineering, where an AS wants to control its local outbound traffic
distribution using only the policy applied to the less specific
prefix. Such a practice was notably documented in [INIT7-RIPE63] (2)
Enforcing contract compliance, where, for instance, an AS avoids a
settlement-free peer to attract traffic to one link by using
selective advertisement, when this is not allowed by their peering
agreement. (3) The need for Forwarding Information Base memory
preservation sometimes pushes ISP operators to filter more specific
prefixes.
Figure 1 illustrates a scenario where one AS performs local filtering
due to outbound traffic engineering. The figure depicts AS64504, and
two of its neighboring ASes, AS64502, and AS64505. AS64504 has a
settlement-free peering with AS64502 and is a customer of AS64505.
AS64504 receives from AS64505 prefixes 2001:DB8::/32 and
2001:DB8::/34. AS64504 only receives the less specific prefix
2001:DB8::/32 from AS64502.
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,-----.
/ \
( AS64505 )
\ /
`--+--'
2001:DB8::/32 | |
2001:DB8::/34 v |
|
,--+--. 2001:DB8::/32 ,-----.
/ \ <-- / \
( AS64504 )-------------( AS64502 )
\ / \ /
`-----' `-----'
Figure 1: Local Filtering
Due to economic reasons, AS64504 might prefer to send traffic to
AS64502 instead of AS64505. However, even if paths received from
AS64502 are given a large local preference, routers in AS64504 will
still send traffic to prefix 2001:DB8::/34 via neighbor AS64505.
This situation may push AS64504 to apply an inbound filter for the
more specific prefix, 2001:DB8::/34, on the session with AS64505.
After applying the filter, AS64504 will send traffic for the more
specific prefix to AS64502.
2.1.1. Unexpected traffic flows caused by local filtering of more
specific prefixes
In this section, we show how the decision of AS64504 to perform local
filtering creates unexpected traffic flows in AS64502. Figure 2
shows the whole picture of the scenario; where AS64501 is a customer
of AS64503 and AS64502. AS64503 is a settlement-free peer with
AS64502. AS64503 and AS64502 are customers of AS64505. The AS
originating the two prefixes, AS64501, performs selective
advertisement with the more specific prefix and only announces it to
AS64503.
After AS64504 locally filters the more specific prefix, traffic from
AS64504 to prefix 2001:DB8::/34 is forwarded towards AS64502.
Because AS64502 receives the more specific prefix from AS64503,
traffic from AS64504 to 2001:DB8::/34 follows the path
AS64504-AS64502-AS64503-AS64501. AS64502's BGP policies are
implemented to avoid transporting traffic between AS64504 and
AS64503. However, due to the discrepancies of routes between the
more and the less specific prefixes, unexpected traffic flows between
AS64504 and AS64503 exist in AS64502's network.
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____,,................______
_,.---'''' `''---..._
,-'' AS64505 `-.
[ /
-.._ __.-'
. `'---....______ ______...---''
+ |/32 `''''''''''''''' |
| |/34 + |/32 |
v | v |/34 |
| | ^ |
| ^ |/32 | |/32
| + | + |/34
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS64504 \ <-+ / AS64502 \ / AS64503 \
| |_________| |________| |
| | /32 | |/32 /32| |
'. ,' . ,' /34 . ,'
`. ,' `. ,' +-> <-+ `. ,'
``---'' ``---'' ``---''
| ^ |
^ |2001:DB8::/32 | |2001:DB8::/32
| | + |2001:DB8::/34
+ | _....---------...._|
,-'AS64501 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''
Figure 2: Unexpected traffic flows due to local filtering
2.2. Remote filtering
ISPs can tag the BGP paths that they propagate to neighboring ASes
with communities, in order to tweak the propagation behavior of the
ASes that handle these paths [on_BGP_communities]. Some ISPs allow
their customers to use such communities to let the receiving AS not
export the path to some selected neighboring ASes. By combining
communities, the prefix could be advertised only to a given peer of
the AS providing this feature. A network operator can leverage
remote filtering to, for instance, limit the scope of prefixes and
hence perform a more granular inbound traffic engineering.
Figure 3 illustrates a scenario in which an AS uses BGP communities
to command its provider to selectively propagate a more specific
prefix. Let AS64501 be a customer of AS64502 and AS64503. AS64501
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originates prefix 2001:DB8::/32, which it advertises to AS64502 and
AS64503. AS64502 and AS64503 are settlement-free peers. Let AS64501
do selective advertisement and only propagate 2001:DB8::/34 over
AS64503. AS64503 would normally propagate this prefix to its
customers, providers, and peers, including AS64502.
Let us consider that AS64501 decides to limit the scope of the more
specific prefix. AS64501 can make this decision based on its traffic
engineering strategy. To achieve this, AS64501 can tag the more
specific prefix with a set of communities that leads AS64503 to only
propagate the path to AS64502.
^ \ / ^ ^ \ / ^
| /32 \ / /32 | | /32 \ / /32 |
,-----. ,-----.
,' `. ,' `.
/ AS64502 \ / AS64503 \
( )-------------( )
\ / /32 /32 \ /
`. ,' -> /34 `. ,'
'-----; <- / '-----'
\ /
^ \ / ^
| \ / |
| \ / |
| \ ,-----.' | 2001:DB8::/32
| ,' `. | 2001:DB8::/34
2001:DB8::/32 +-- / AS64501 \ --+
( )
\ /
`. ,'
'-----'
Figure 3: Remote triggered filtering
2.2.1. Unexpected traffic flows caused by remotely triggered filtering
of more specific prefixes
Figure 4 expands the scenario from Figure 3 and includes other ASes
peering with AS64502 and AS64503. Due to the limitation on the scope
performed on the more specific prefix, ASes that are not customers of
AS64502 will not receive a path for 2001:DB8::/34. These ASes will
forward packets destined to 2001:DB8::/34 according to their routing
state for 2001:DB8::/32. Let us assume that AS64505 is such an AS,
and that its best path towards 2001:DB8::/32 is through AS64502.
Packets sent towards 2001:DB8::1 by AS64505 will reach AS64502.
However, in the data-plane of the nodes of AS64502, the longest
prefix match for 2001:DB8::1 is 2001:DB8::/34, which is reached
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through AS64503, a settlement-free peer of AS64502. Since AS64505 is
not in the customer branch of AS64502, traffic flows between two non-
customer ASes in AS64502.
,-----.
,' `.
/ AS64505 \
( )
\ /
,`. ,' \
/ '-----' \
/ ^ ^ \
/32 | | /32 '
,-----.' + + ,-----.
,' `. ,' `.
/ AS64502 \ / AS64503 \
( )-------------( )
\ / /32 /32 \ /
`. ,' +-> /34 `. ,'
'-----; <-+ / '-----'
\ /
^ \ / ^
| \ / |
| \ / |
| \ ,-----.' | 2001:DB8::/32
| ,' `. | 2001:DB8::/34
2001:DB8::/32 +--+ / AS64501 \ +--+
( )
\ /
`. ,'
'-----'
Figure 4: Unexpected traffic flows due to remote triggered filtering
3. Techniques to detect unexpected traffic flows caused by filtering of
more specific prefixes
3.1. Existence of unexpected traffic flows within an AS
To detect if unexpected traffic flows are taking place in its
network, an ISP can monitor its traffic data to check if it is
providing transit between two of its peers, although this policy is
configured to not provide such transit. IPFIX (RFC7011 [RFC7011]) is
an example of a technology that can be used to export information
regarding traffic flows across the network. Traffic information must
be analyzed under the perspective of the business relationships with
neighboring ASes to detect the flows not fitting the policy.
Operators can use collection systems that combine traffic statistics
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with policy information for this end. Check [PMACCT] for an open
source application meeting these requirements.
Note that the AS detecting the unexpected traffic flow may simply
realize that this policy configuration is broken. The first
recommended action upon detection of an unexpected traffic flow is to
verify the correctness of the BGP configuration.
Once the local configuration is confirmed correct, the operator
should check if the unexpected flow arose due to filtering of BGP
paths for more-specific prefixes by neighboring ASes. This can be
performed in two steps. First, the operator should check whether the
neighboring AS originating the unexpected flow is forwarding traffic
using a less-specific prefix that is announced to it by the affected
network. The second step is to try to infer the reason why the
neighboring AS does not use the more specific path for forwarding,
i.e., finding why the more specific prefix was filtered. Due to the
distributed nature and restricted visibility of the steering of BGP
policies, this second step does not identify the origin of the
problem with guaranteed accuracy.
For the first step, the operator should check if the destination
address of the unexpected traffic flow is locally routed as per a
more specific prefix only received from non-customer peers. The
operator should also check if there are paths to a less specific
prefix received from a customer, and hence propagated to peers. If
these two situations happen at the same time, the neighboring AS at
the entry point of the unexpected flow is routing the traffic based
on the less specific prefix, although the ISP is actually forwarding
the traffic via non-customer links.
For the second step, one can rely on human interaction or looking
glasses to find out whether local filtering, remote filtering, or
selective propagation was performed on the more specific prefix. No
openly available tools that can automatically perform this operation
have been identified.
3.2. Contribution to the existence of unexpected traffic flows in
another AS
It can be considered problematic to trigger unexpected traffic flows
in another AS. It is thus advisable for an AS to assess the risks of
filtering more specific prefixes before implementing them, by
obtaining as much information as possible about its surrounding
routing environment.
There may be justifiable reasons for one ISP to perform filtering;
either to enforce established policies or to provide prefix
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advertisement scoping features to its customers. These can vary from
trouble-shooting purposes to business relationship implementations.
Restricting the use of these features for the sake of avoiding the
creation of unexpected traffic flows is not a practical option.
In order to assess the risk of filtering more specific prefixes, the
AS would need information on the routing policies and the
relationships among external ASes, to detect if its actions could
trigger the appearance of unexpected traffic flows. With this
information, the operator could detect other ASes receiving the more
specific prefix from non-customer ASes, while announcing the less
specific prefix to other non-customer ASes. If the filtering of the
more specific prefix leads other ASes to send traffic for the more
specific prefix to these ASes, an unexpected traffic flow can arise.
However, the information required for this operation is difficult to
obtain since it is frequently considered confidential.
4. Techniques to Traffic Engineer unexpected flows
Network Operators can adopt different approaches with respect to
unexpected traffic flows. Note that due the complexity of inter-
domain routing policies, there is not a single solution that can be
applied to all situations. This section provides potential solutions
that ISPs must evaluate against each particular case. We classify
these actions according to whether they are proactive or reactive.
Reactive approaches are those in which the operator tries to detect
the situations via monitoring and solve unexpected traffic flows,
manually, on a case-by-case basis.
Anticipant or preventive approaches are those in which the routing
system will not let the unexpected traffic flows actually take place
when the scenario arises.
We use the scenario depicted in Figure 5 to describe these two kinds
of approaches. Since proactive approaches can be complex to
implement and can lead to undesired effects, the reactive approach is
the more reasonable recommendation to deal with unexpected flows.
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____,,................______
_,.---'''' `''---..._
,-'' AS64505 `-.
[ /
-.._ __.-'
. `'---....______ ______...---''
+ |/32 `''''''''''''''' |
| |/34 + |/32 |
v | v |/34 |
| | ^ |
| ^ |/32 | |/32
| + | + |/34
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS64504 \ <-+ / AS64502 \ / AS64503 \
| |_________| | | |
| | /32 | | | |
'. ,' . ,' . ,'
`. ,' `. ,' `. ,'
``---'' ``---'' ``---''
| ^ |
^ |2001:DB8::/32 | |2001:DB8::/32
| | + |2001:DB8::/34
+ | _....---------...._|
,-'AS64501 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''
Figure 5: Traffic Engineering of unexpected traffic flows - Base
example
4.1. Reactive Traffic Engineering
An operator who detects unexpected traffic flows originated by any of
the cases described in Section 2 can contact the ASes that are likely
to have performed the propagation tweaks, inform them of the
situation, and persuade them to change their behavior.
If the situation remains, the operator can implement prefix filtering
in order to stop the unexpected flows. The operator can decide to
perform this action over the session with the operator announcing the
more specific prefix or over the session with the neighboring AS from
which it is receiving the traffic. Each of these options carry a
different repercussion for the affected AS. We briefly describe the
two alternatives.
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o An operator can decide to stop announcing the less specific prefix
at the peering session with the neighboring AS from which it is
receiving traffic to the more specific prefix. In the example of
Figure 5, AS64502 would filter out the prefix 2001:DB8::/32 from
the eBGP session with AS64504. In this case, traffic heading to
the prefix 2001:DB8::/32 from AS64501 would no longer traverse
AS64502. AS64502 should evaluate if solving the issues originated
by the unexpected traffic flows are worth the loss of this traffic
share.
o An operator can decide to filter out the more specific prefix at
the peering session over which it was received. In the example of
Figure 5, AS64502 would filter out the incoming prefix
2001:DB8::/34 from the eBGP session with AS64505. As a result,
the traffic destined to that /32 would be forwarded by AS64502
along its link with AS64501, despite the actions performed by
AS64501 to have this traffic coming in through its link with
AS64503. However, as AS64502 will no longer know a route to the
more specific prefix, it risks losing the traffic share from
customers different from AS64501 to that prefix. Furthermore,
this action can generate conflicts between AS64502 and AS64501,
since AS64502 does not follow the routing information expressed by
AS64501 in its BGP announcements.
Note that it is possible that the behavior of the neighboring AS
causing the unexpected traffic flows violates a contractual agreement
between the two networks.
4.2. Proactive measures
4.2.1. Access lists
An operator could install access-lists to prevent unexpected traffic
flows from happening in the first place. In the example of Figure 5,
AS64502 would install an access-list denying packets matching
2001:DB8::/34 associated with the interface connecting to AS64504.
As a result, traffic destined to that prefix would be dropped,
despite the existence of a valid route towards 2001:DB8::/32.
The operational overhead of such a solution is considered high, as
the operator would have to constantly adapt these access-lists to
accommodate inter-domain routing changes. Moreover, this technique
lets packets destined to a valid prefix be dropped while they are
sent from a neighboring AS that may not know about the policy
conflict, and hence had no means to avoid the creation of unexpected
traffic flows. For this reason, this technique can be considered
harmful.
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4.2.2. Neighbor-specific forwarding
An operator can technically ensure that traffic destined to a given
prefix will be forwarded from an entry point of the network based
only on the set of paths that have been advertised over that entry
point.
As an example, let us analyze the scenario of Figure 5 from the point
of view of AS64502. The edge router connecting to the AS64504
forwards packets destined to prefix 2001:DB8::/34 towards AS64505.
Likewise, it forwards packets destined to prefix 2001:DB8::/32
towards AS64501. The router, however, only propagates the path of
the less specific prefix (2001:DB8::/32) to AS64504. An operator
could implement the necessary techniques to force the edge router to
forward packets coming from AS64504 based only on the paths
propagated to AS64504. Thus, the edge router would forward packets
destined to 2001:DB8::/34 towards AS64501 in which case no unexpected
traffic flow would occur.
Different techniques could provide this functionality; however, their
technical implementation can be complex to design and operate. An
operator could, for instance, employ Virtual Routing Forwarding (VRF)
tables (RFC4364 [RFC4364]) to store the routes announced to a
neighbor and forward traffic exclusively based on those routes.
[on_BGP_RS_VPNs] describes the use of such an architecture for
Internet routing, and provides a description of its limitations.
In such architecture, packets received from a peer would be forwarded
solely based on the paths that fit the path propagation policy for
that peer, and not based on the global routing table of the router.
As a result, a more specific path that would not be propagated to a
peer will not be used to forward a packet from that peer, and the
unexpected flow will not take place. Packets will be forwarded based
on the policy compliant less specific prefix. However, note that an
operator must make sure that all their routers could support the
potential performance impact of this approach.
Note that similarly to the solution described in Section 4.1, this
approach could create conflicts between AS64502 and AS64501, since
the traffic forwarding performed by AS64502 goes against the policy
of AS64501.
5. Conclusions
This document describes how filtering and selective propagation of
more-specific prefixes can potentially create unexpected traffic
flows across some ASes. We provided examples of scenarios where
these practices lead to unexpected traffic flows, and introduce some
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techniques for their detection and prevention. Although there are
reasonable situations in which ASes could filter more-specific
prefixes, network operators are encouraged to implement this type of
filters considering the cases described in this document. Operators
can implement monitoring systems to detect unexpected traffic flows
and react to them according to their own policy.
6. Security Considerations
It is possible for an AS to use any of the methods described in this
document to deliberately reroute traffic flowing through another AS.
This document informed on the potential routing security issue, and
analyzed ways for operators to defend against it.
It must be noted that, at the time of this document, there are no
existing or proposed tools to automatically protect against such
behavior. Operators can use network monitoring and collection tools
to detect unexpected flows and deal with them on a case-by-case
basis.
7. IANA Considerations
This document has no IANA actions.
8. Acknowledgments
The authors would like to thank Wes George, Jon Mitchell, Bruno
Decraene, and Job Snijders for their useful suggestions and comments.
9. References
9.1. Normative References
[RFC4384] Meyer, D., "BGP Communities for Data Collection", RFC
4384, February 2006.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[RFC7011] Claise, B., Trammell, B., and P. Aitken, "Specification of
the IP Flow Information Export (IPFIX) Protocol for the
Exchange of Flow Information", RFC 7011, September 2013.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
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9.2. Informative References
[on_BGP_communities]
Donnet, B. and O. Bonaventure, "On BGP Communities", ACM
SIGCOMM Computer Communication Review vol. 38, no. 2, pp.
55-59, April 2008.
[on_BGP_RS_VPNs]
Vanbever, L., Francois, P., Bonaventure, O., and J.
Rexford, "Customized BGP Route Selection Using BGP/MPLS
VPNs", Cisco Systems, Routing Symposium
http://www.cs.princeton.edu/~jrex/talks/cisconag09.pdf,
October 2009.
[INIT7-RIPE63]
"INIT7-RIPE63", <http://ripe63.ripe.net/presentations/48-
How-more-specifics-increase-your-transit-bill-v0.2.pdf>.
[PMACCT] "pmacct project: IP accounting iconoclasm",
<http://www.pmacct.net>.
Authors' Addresses
Camilo Cardona
IMDEA Networks/UC3M
Avenida del Mar Mediterraneo, 22
Leganes 28919
Spain
Email: juancamilo.cardona@imdea.org
Pierre Francois
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: pifranco@cisco.com
Paolo Lucente
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: plucente@cisco.com
Camilo Cardona, et al. Expires May 11, 2016 [Page 15]
