Internet DRAFT - draft-cardona-filtering-threats
draft-cardona-filtering-threats
Network Working Group Camilo Cardona
Internet-Draft Pierre Francois
Intended status: Informational IMDEA Networks
Expires: January 10, 2014 July 9, 2013
Making BGP filtering a habit: Impact on policies
draft-cardona-filtering-threats-02
Abstract
Network operators define their BGP policies based on the business
relationships that they maintain with their peers. By limiting the
propagation of BGP prefixes, an autonomous system avoids the
existence of flows between BGP peers that do not provide any
economical gain. This draft describes how undesired flows can emerge
in autonomous systems due to the filtering of overlapping BGP
prefixes by neighboring domains.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 10, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Filtering overlapping prefixes . . . . . . . . . . . . . . . . 3
2.1. Local filtering . . . . . . . . . . . . . . . . . . . . . 4
2.2. Remotely triggered filtering . . . . . . . . . . . . . . . 5
3. Uses of overlapping prefix filtering that create undesired
traffic flows . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Undesired Traffic Flows . . . . . . . . . . . . . . . . . 7
3.1.1. Undesired traffic flows caused by local filtering
of overlapping prefixes . . . . . . . . . . . . . . . 8
3.1.2. Undesired traffic flows caused by remotely
triggered filtering of overlapping prefixes . . . . . 11
4. Techniques to detect undesired traffic flows caused by
filtering of overlapping prefixes . . . . . . . . . . . . . . 14
4.1. Being the 'victim' . . . . . . . . . . . . . . . . . . . . 15
4.2. Being a contributor to the existence of undesired
traffic flows in other networks . . . . . . . . . . . . . 15
5. Techniques to counter undesired traffic flows due to the
filtering of overlapping prefixes . . . . . . . . . . . . . . 16
5.1. Reactive counter-measures . . . . . . . . . . . . . . . . 17
5.2. Anticipant counter-measures . . . . . . . . . . . . . . . 18
5.2.1. Access lists . . . . . . . . . . . . . . . . . . . . . 18
5.2.2. Automatic filtering . . . . . . . . . . . . . . . . . 18
5.2.3. Neighbor-specific forwarding . . . . . . . . . . . . . 18
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
It is common practice for network operators to propagate overlapping
prefixes along with the prefixes that they originate. It is also
possible for some Autonomous Systems (ASes) to apply different
policies to the overlapping (more specific) and the covering (less
specific) prefix. Some ASes can even benefit from filtering the
overlapping prefixes.
BGP makes independent, policy driven decisions for the selection of
the best path to be used for a given IP prefix. However, routers
must forward packets using the longest-prefix-match rule, which
"precedes" any BGP policy (RFC1812 [4]). Indeed, 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
overlapping prefix). This process takes place by disregarding the
policies applied in the control plane for the selection of the best
next-hop for p' (the covering prefix). When overlapping prefixes are
filtered and packets are forwarded according to the covering prefix,
the discrepancy in the routing policies applied to covering and
overlapping prefixes can create undesired traffic flows that infringe
the policies of Internet Service Providing (ISPs) still holding a
path towards the overlapping prefix.
This document presents examples of such cases and discusses solutions
to the problem. The objective of this draft is to shed light on the
use of prefix filtering by making the routing community aware of the
cases where the effects of filtering might turn to be negative for
the business of ISPs.
The rest of the document is organized as follows: Section 2
illustrates the motivation to filter overlapping prefixes. In
Section 3, we provide some scenarios in which the filtering of
overlapping prefixes lead to the creation of undesired traffic flows
on other ASes. Section 4 and Section 5 discuss some techniques that
ASes can use for, respectively, detect and react to undesired traffic
flows.
2. Filtering overlapping prefixes
There are several scenarios where filtering an overlapping prefix is
relevant to the operations of an AS. In this section, we provide
examples of these scenarios. We differentiate cases in which the
filtering is performed locally from those where the filtering is
triggered remotely. These scenarios will be used as a base in
Section 3 for describing side effects bound with such practices.
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2.1. Local filtering
Let us first analyze the scenario depicted in Figure 1. AS1 and AS2
are two autonomous systems spanning a large geographical area and
peering in 3 different physical locations. Let AS1 announce prefix
10.0.0.0/22 over all peering links with AS1. Additionally, let us
define that there is part of AS1's network which exclusively uses
prefix 10.0.0.0/24 and which is closer to a peering point than to
others.
To receive the traffic destined to prefix 10.0.0.0/24 on the link
closer to this subnet, AS1 could announce the overlapping prefix only
over this specific session. At the time of the establishment of the
peering, it can be defined by both ASes that hot potato routing would
happen in both directions of traffic. In other words, it was agreed
that each AS will deliver the traffic to the other AS on the nearest
peering link. In this scenario, it becomes relevant to AS2 to
enforce such practice by detecting the described situations and
automatically issuing the appropriate filtering. In this case, by
implementing these automatic procedures, AS2 would legitimately
detect and filter prefix 10.0.0.0/24.
___....-----------....___
,.--' AS2 `--..
,' `.
| |
`._ _.'
`--..__ _,,.--'
. `'''-----------'''' |
| | |
| | |
10.0.0.0/22| 10.0.0.0/22| |10.0.0.0/22
| ___....-----------....___ |10.0.0.0/24
,.--'AS1 `--..
,' ...........`.
| |10.0.0.0/24 |
`._ |........._.'
`--..__ _,,.--'
`'''-----------''''
Figure 1: Basic scenario of local filtering
Local filtering could be required in other cases. For example, a
dual homed AS receiving an overlapping prefix from only one of its
providers. Figure 2 depicts a simple example of this case.
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_..._
,' `.
/ AS4 \
| |
\ /
,`-...-'.
/ '.
10.0.0.0/22 ,' \
10.0.0.0/24 / \ 10.0.0.0/22
..:_ >..._
,' `. ,' `.
/ AS2 \ / AS3 \
| | | |
\ / \ /
`-...-', `-...-'
\ /
\ /
10.0.0.0/22 \_..._ '10.0.0.0/22
10.0.0.0/24,' `.
/ AS1 \
| |
\ /
`-...-'
Figure 2: Basic scenario of local filtering
In this scenario, prefix 10.0.0.0/22 is advertised by AS1 to AS2 and
AS3. Both ASes propagate the prefix to AS4. Additionally, AS1
advertises prefix 10.0.0.0/24 to AS2, which subsequently propagates
the prefix to AS4.
It is possible that AS4 resolves to filter the more specific prefix
10.0.0.0/24. One potential motivation could be the economical
preference of the path via AS2 over AS3. Another feasible reason is
the existence of a technical policy by AS4 of aggregating incoming
prefixes longer than /23.
The above examples illustrate two of the many motivations to
configure routing within an AS with the aim of ignoring more specific
prefixes. Operators have reported applying these filters in a manual
fashion [3]. The relevance of such practice led to investigate
automated filtering procedures in I-D.WHITE [5].
2.2. Remotely triggered filtering
ISPs can tag the BGP paths that they propagate to neighboring ASes
with communities, in order to tweak the propagation behavior of the
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ASes that handle these paths [1].
Some ISPs allow their direct and indirect customers to use such
communities to let the receiving AS not export the path to some
selected neighboring AS. By combining communities, the prefix could
be advertised only to a given peer of the AS providing this feature.
Figure 3 illustrates an example of this case.
10.0.0.0/22 ,' \
10.0.0.0/24 / \ 10.0.0.0/22
..:_ >..._
,' `. ,' `.
/ AS2 \________ / AS3 \
| |/22 /22| |
\ / \ /
`-...-', `-...-'
\ /
\ /
10.0.0.0/22 \_..._ '10.0.0.0/22
10.0.0.0/24,' `.
/ AS1 \
| |
\ /
`-...-'
Figure 3: Remote triggered filtering
AS2 and AS3 are peers. Both ASes are providers of AS1. For traffic
engineering purposes, AS1 could use communities to prevent AS2 from
announcing prefix 10.0.0.0/24 to AS3.
Such technique is useful for operators to tweak routing decisions in
order to align with complex transit policies. We will see in later
sections that by producing the same effect as filtering, they can
also lead to undesired traffic flows at other, distant, ASes.
3. Uses of overlapping prefix filtering that create undesired traffic
flows
In this section we define the concept of undesired traffic flows and
describe three configuration scenarios that lead to their creation.
Note that these examples do not capture all the cases where such
issues can take place. More examples will be provided in future
revisions of this document.
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3.1. Undesired Traffic Flows
The BGP policy of an Internet Service provider includes all actions
performed over its originated routes and the routes received
externally. One important part of the BGP policy is the selection of
the routes that are propagated to each neighboring AS. One of the
goals of these policies is to allow ISPs to avoid transporting
traffic between two ASes without economical gain. For instance, ISPs
typically propagate to their peers only routes coming from its
customers (RFC4384 [6]). We briefly illustrate this operation in
Figure 4. In the figure, AS2 is establishing a settlement free
peering with AS1 and AS3. AS2 receives prefix P3/p3, from AS3. AS2,
however, is not interested in transporting traffic from AS1 to AS3,
therefore it does not propagate the prefix to AS3. In the figure, we
also show a customer of AS2, AS4, which is announcing prefix P4/p4.
AS2 propagates this prefix to AS1.
,-'''-.. ,-'''-.. ,-'''-..
,' \ ,' \ ,' \
.' \ .' \ .' \
| |------| | ----| P3/p3 |
\ AS1 / P4/p4\ AS2 / \ AS3/
\ ,' \ ,' \ ,'
`-...-' `-...-' `-...-'
|
|
|
,-'''-..
,' P4/p4 \
.' \
| |
\ AS4 /
\ ,'
`-...-'
Figure 4: Prefix exchange among four autonomous systems
Although ISPs usually implement the aforementioned policies,
undesired traffic flows may still appear. In Figure 4, undesired
traffic flows are created, when, despite AS2's policy, traffic
arriving from peer AS1 is received and transported to AS3 by AS2.
These type of traffic flows can arise due to a number of reasons.
Specifically, in this document we explain how the filtering of
overlapping prefixes might cause undesired traffic flows on ASes. We
provide examples of these cases in the next sections.
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3.1.1. Undesired traffic flows caused by local filtering of overlapping
prefixes
In this section we describe cases in which an AS locally filters an
overlapping prefix. We show that, depending on the BGP policies
applied by surrounding ASes, this decision can lead to undesired
traffic flows.
3.1.1.1. Initial setup
We start by describing the basic scenario of this case in Figure 5.
____,,................______
_,.---'''' `''---..._
,-'' AS5 `-.
[ /
-.._ __.-'
. `'---....______ ______...---''
|/22 `''''''''''''''' |
|/24 |/22 |
| |/24 |
| | |
| |/22 |/22
| |/24 |/24
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS4 \ / AS2 \ / AS3 \
| |_________| |________| |
| | /22 | |/22 /22| |
'. ,' /24 . ,'/24 /24 . ,'
`. ,' `. ,' `. ,'
``---'' ``---'' ``---''
| |
|10.0.0.0/24 |10.0.0.0/24
|10.0.0.0/22 |10.0.0.0/22
| _....---------...._|
,-'AS1 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''
Figure 5: Initial Setup Local
AS1 is a customer of AS2 and AS3. AS2, AS3, and AS4 are customers of
AS5. AS2 is establishing a peering with AS3 and AS4. AS1 is
announcing a covering prefix, 10.0.0.0/22, and an overlapping prefix
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10.0.0.0/24 to its providers. In the initial setup, AS2 and AS3
announce the two prefixes to their peers and transit providers. AS4
receives both prefixes from its peer (AS2) and transit provider
(AS5). We will consider that AS5 chooses the path through AS3 to
reach AS1.
3.1.1.2. Undesired traffic flows by local filtering - Case 1
In the next scenarios, we show that if AS4 filters the incoming
overlapping prefix from AS5, there is a situation in which undesired
traffic flows are created on other ASes.
____,,................______
_,.---'''' `''---..._
,-'' AS5 `-.
[ /
-.._ __.-'
. `'---....______ ______...---''
|/22 `''''''''''''''' |
|/24 |/22 |
| |/24 |
| | |
| |/22 |/22
| | |/24
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS4 \ / AS2 \ / AS3 \
| |_________| |________| |
| | /22 | |/22 /22| |
'. ,' . ,' /24 . ,'
`. ,' `. ,' `. ,'
``---'' ``---'' ``---''
| |
| |10.0.0.0/24
|10.0.0.0/22 |10.0.0.0/22
| _,,..---------...._|
,-'AS1 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''
Figure 6: Undesired traffic flows by local filtering - Case 1
Let us assume the scenario illustrated in Figure 6. For this case,
AS1 only propagates the overlapping prefix to AS3. AS4 receives the
overlapping prefix only from its transit provider, AS5.
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AS4 now is in a situation in which it would be favorable for it to
filter the announcement of prefix 10.0.0.0/24 from AS5.
Subsequently, traffic from AS4 to prefix 10.0.0.0/24 is forwarded
towards AS2. Because AS2 receives the more specific prefix from AS3,
traffic from AS4 to prefix 10.0.0.0/24 follows the path AS4-AS2-AS3-
AS1. AS2's BGP policies are implemented to avoid AS2 to exchange
traffic between AS4 and AS3. However, due to the discrepancies of
routes from the overlapping and covering prefixes, undesired traffic
flows between AS4 and AS3 still exist on AS2's network. This
situation is economically detrimental for AS2, since it forwards
traffic from a peer to a non-customer neighbor.
3.1.1.3. Undesired traffic flows by local filtering - Case 2
____,,................______
_,.---'''' `''---..._
,-'' AS5 `-.
[ /
-.._ __.-'
. `'---....______ ______...---''
|/22 `''''''''''''''' |
|/24 |/22 |
| |/24 |
| | |
| |/22 |/22
| | |/24
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS4 \ / AS2 \ / AS3 \
| |_________| | | |
| | /22 | | | |
'. ,' . ,' . ,'
`. ,' `. ,' `. ,'
``---'' ``---'' ``---''
| |
| |10.0.0.0/24
|10.0.0.0/22 |10.0.0.0/22
_;,..---------...._|
,-'AS1 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''
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Figure 7: Undesired traffic flows after local filtering - Case 2
Let us assume a second case where AS2 and AS3 are not peering and AS1
only propagates the overlapping prefix to AS3. AS4 receives the
overlapping prefix only from its transit provider, AS5. This case is
illustrated in Figure 7.
Similar to the scenario described in Section 3.1.1.2, AS4 is in a
situation in which it would be favorable to filter the announcement
of prefix 10.0.0.0/24 from AS5. Subsequently, traffic from AS4 to
prefix 10.0.0.0/24 is forwarded towards AS2. Due to the existence of
a route to prefix 10.0.0.0/24, AS2 receives the traffic heading to
this prefix from AS4, and sends it to AS5. This situation creates
undesired traffic flows that contradict AS2's BGP policy, since the
AS ends up forwarding traffic from a peer to a transit network.
3.1.2. Undesired traffic flows caused by remotely triggered filtering
of overlapping prefixes
We present a configuration scenario in which an AS, using the
mechanism described in Section 2.2, informs its provider to
selectively propagate an overlapping prefix, leading to the creation
of undesired traffic flows in another AS.
3.1.2.1. Initial setup
Let AS1 be a customer of AS2 and AS3. AS1 owns 10.0.0.0/22, which it
advertises through AS2 and AS3. Additionally, AS2 and AS3 are peers.
Both AS2 and AS3 select A1's path as best, and propagate it to their
customers, providers, and peers. Some remote ASes will route traffic
destined to 10.0.0.1 through AS2 while others will route traffic
through AS3.
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\ / \ /
/22 \ / /22 /22 \ / /22
,-----. ,-----.
,' `. ,' `.
/ AS2 \ /22 / AS3 \
( )-------------( )
\ / /22 \ /
`. ,' `. ,'
'-----; / '-----'
\ /
\ /
10.0.0.0/22\ /10.0.0.0/22
\ /
\ ,-----.'
,' `.
/ AS1 \
( )
\ /
`. ,'
'-----'
Figure 8: Example scenario
3.1.2.2. Injection of an overlapping prefix
Let AS1 advertise 10.0.0.0/24 over AS3 only. AS3 would propagate
this prefix to its customers, providers, and peers, including AS2.
From AS2's point of view, the path towards 10.0.0.0/24 is a "peer
path" and AS2 will only advertise it to its customers. ASes in the
customer branch of AS2 will receive a path to the /24 that contains
AS3 and AS2. Some multi-homed customers of AS2 may also receive a
path through AS3, but not through AS2, from other peering or provider
links. Any remote AS that is not lying in the customer branch of
AS2, will receive a path for 10.0.0.0/24 through AS3 and not through
AS2.
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\ / /22\ / /22
/22 \ / /22 /24 \ / /24
,-----. ,-----.
,' `. /22 ,' `.
/ AS2 \ /24 / AS3 \
( /22:AS1 )-------------( /22:AS1 )
\ /24:AS3 / /22 \ /24:AS1 /
/22 /`. ,' `. ,'
/24/ '-----; / '-----'
/ \ /
,---./ \ /
/ \ 10.0.0.0/22\ /10.0.0.0/22
| AS4 ) \ / 10.0.0.0/24
\ / \ ,-----.'
`---' ,' `.
/ AS1 \
( )
\ /
`. ,'
'-----'
Figure 9: Injection of overlapping prefix
AS2 only receives traffic destined to 10.0.0.0/24 from its customers,
which it forwards to its peer AS3. Routing is consistent with usual
Internet Routing Policies in this case. AS3 could receive traffic
destined to 10.0.0.0/24 from its customers, providers, and peers,
which it directly forwards to its customer AS1.
3.1.2.3. Creation of undesired traffic flows by limiting the scope of
the overlapping prefix
Now, let us assume that 10.0.0.0/24, which is propagated by AS1 to
AS3, is tagged to have AS3 only propagate that path to AS2, using the
techniques described in Section 2.2.
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,-------.
,' `.
/ AS5 \
( /22:AS2 )
\ /
`. ,'
'-------' \ / \ /
/22 \ //22 /22 \ //22
,-----. ,-----.
,' `. /22 ,' `.
/ AS2 \ /24 / AS3 \
( /22:AS1 )-------------( /22:AS1 )
\ /24:AS3 / /22 \ /24:AS1 /
/22 /`. ,' `. ,'
/24/ '-----; / '-----'
/ \ /
,---./ \ /
/ \ 10.0.0.0/22\ /10.0.0.0/22
( AS4 ) \ / 10.0.0.0/24
\ / \ ,-----.'
`---' ,' `.
/ AS1 \
( )
\ /
`. ,'
'-----'
Figure 10: More Specific Injection
From AS2's point of view, such a path is a "peer path" and will only
be advertised by AS2 to its customers.
ASes that are not customers of AS2 will not receive a path for
10.0.0.0/24. These ASes will forward packets destined to 10.0.0.0/24
according to their routing state for 10.0.0.0/22. Let us assume that
AS5 is such an AS, and that its best path towards 10.0.0.0/22 is
through AS2. Then, packets sent towards 10.0.0.1 by AS5 will
eventually reach AS2. However, in the data-plane of the nodes of
AS2, the longest prefix match for 10.0.0.1 is 10.0.0.0/24, which is
reached through AS3, a peer of AS2. Since AS5 is not in the customer
branch of AS2, we are in a situation in which traffic flows between
non-customer AS take place in AS2.
4. Techniques to detect undesired traffic flows caused by filtering of
overlapping prefixes
We differentiate the techniques available for detecting undesired
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traffic flows caused by the described scenarios from the cases in
which the interested AS is the victim or contributor of such
operations.
4.1. Being the 'victim'
To detect if undesired traffic flows are taking place in its network,
an ISP can monitor its traffic data and validate if any flow entering
the ISP network through a non-customer link is forwarded to a non-
customer next-hop.
As mentioned in Section 3.1, undesired traffic flows might appear due
to different situations. To discover if the problem arose after the
filtering of prefixes by neighboring ASes, an operator can analyze
available BGP data. For instance, an ISP can seek for overlapping
prefixes for which the next-hop is through a provider (or peer),
while the next-hop for their covering prefix(es) is through a client.
Direct communication or looking glasses can be used to check whether
non-customer neighboring ASes are propagating a path towards the
covering prefix to their own customers, peers, or providers. This
should trigger a warning, as this would mean that ASes in the
surrounding area of the current AS are forwarding packets based on
the routing entry for the less specific prefix only.
4.2. Being a contributor to the existence of undesired traffic flows in
other networks
It can be considered problematic to be causing undesired traffic
flows on other ASes. This situation may appear as an abuse to the
network resources of other ISPs.
There may be justifiable reasons for one ISP to perform filtering,
either to enforce established policies or to provide prefix
advertisement scoping features to its customers. These can vary from
trouble-shooting purposes to business relationships implementations.
Restricting such features for the sake of avoiding the creation of
undesired traffic flows is not a practical option.
Traffic data does not help an ISP detect that it is acting as a
contributor of the creation of the undesired traffic flow. It is
thus advisable to obtain as much information as possible about the
Internet environment of the AS and assess the risks of filtering
overlapping prefixes before implementing them.
Monitoring the manipulation of the communities that implement the
scoping of prefixes is recommended to the ISPs that provide these
features. The monitored behavior should then be faced against their
terms of use.
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5. Techniques to counter undesired traffic flows due to the filtering
of overlapping prefixes
Network Operators can adopt different approaches with respect to
undesired traffic flows. We classify these actions according to
whether they are anticipant or reactive.
Reactive approaches are those in which the operator tries to detect
the situations and solve undesired traffic flows, manually, on a
case-by-case basis.
Anticipant or preventive approaches are those in which the routing
system will not let the undesired traffic flows actually take place
when the configuration scenario is set up.
We will describe these two kinds of approaches in the following part
of this Section. We will use the scenario depicted in Figure 11 to
provide examples for the different techniques.
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____,,................______
_,.---'''' `''---..._
,-'' AS5 `-.
[ /
-.._ __.-'
. `'---....______ ______...---''
|/22 `''''''''''''''' |
|/24 |/22 |
| |/24 |
| | |
| |/22 |/22
| | |/24
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS4 \ / AS2 \ / AS3 \
| |_________| | | |
| | /22 | | | |
'. ,' . ,' . ,'
`. ,' `. ,' `. ,'
``---'' ``---'' ``---''
| |
| |10.0.0.0/24
|10.0.0.0/22 |10.0.0.0/22
_;,..---------...._|
,-'AS1 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''
Figure 11: Anticipant counter-measures - Base example
5.1. Reactive counter-measures
An operator who detects that its policies are threatened by undesired
traffic flows 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.
For some cases, if the external ASes maintain their behavior, an
operator can account the amount of traffic that has been subject to
the undesired flows and charge the peer for that traffic. That is,
the operator can claim that it has been a provider of that peer for
the traffic that transited between the two ASes.
An operator can decide to filter-out the concerned overlapping prefix
at the peering session over which it was received. In the example of
Figure 11, AS2 would filter out the incoming prefix 10.0.0.0/24 from
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the eBGP session with AS5. As a result, the traffic destined to that
/24 would be forwarded by AS2 along its link with AS1, despite the
actions performed by AS1 to have this traffic coming in through its
link with AS3.
5.2. Anticipant counter-measures
5.2.1. Access lists
An operator can configure its routers to install dynamically an
access-list made of the prefixes towards which the forwarding of
traffic from that interface would lead to undesired traffic flows.
Note that this technique actually lets packets destined to a valid
prefix be dropped while they are sent from a neighboring AS that
cannot know about policy conflicts and hence had no means to avoid
the creation of undesired traffic flows.
In the example of Figure 11, AS2 would install an access-list denying
packets matching 10.0.0.0/24 associated with the interface connecting
to AS4. As a result, traffic destined to that prefix would be
dropped, despite the existence of a valid route towards 10.0.0.0/22.
5.2.2. Automatic filtering
As described in Section 3, filtering of overlapping prefixes can in
some scenarios lead to undesired traffic flows. Nevertheless,
depending on the autonomous system implementing such practice, this
operation can prevent these cases. This can be illustrated using the
example described in Figure 11: if AS2 or AS3 filter prefix
10.0.0.0/24, there would be no undesired traffic flow in AS2.
5.2.3. 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 11 from the
point of view of AS2. The edge router connecting to the AS4 forward
packets destined to prefix 10.0.0.0/24 towards AS5. Likewise, it
will forward packets destined to prefix 10.0.0.0/22 towards AS1. The
router, however, only propagates the path of the covering prefix
(10.0.0.0/22) to AS4. An operator could implement the necessary
techniques to force the edge router to forward packets coming from
AS4 based only on the paths propagated to AS4. Thus, the edge router
would forward packets destined to 10.0.0.0/24 towards AS1 in which
case no undesired traffic flow would occur. This functionality could
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be implemented in different ways. [2] describes an approach to
implement this Behavior.
6. Conclusions
In this document, we described threats to policies of autonomous
systems caused by the filtering of overlapping prefixes by external
networks. We provide examples of scenarios in which undesired
traffic flows are caused by these practices and introduce some
techniques for their detection and prevention. We observe that there
are reasonable situations in which ASes could filter overlapping
prefixes, however, we encourage that network operators implement this
type of filters only after considering the cases described in this
document.
7. References
[1] Donnet, B. and O. Bonaventure, "On BGP Communities", ACM SIGCOMM
Computer Communication Review vol. 38, no. 2, pp. 55-59,
April 2008.
[2] 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.
[3] "INIT7-RIPE63", <http://ripe63.ripe.net/presentations/
48-How-more-specifics-increase-your-transit-bill-v0.2.pdf>.
[4] <http://www.ietf.org/rfc/rfc1812.txt>
[5] <http://tools.ietf.org/html/
draft-white-grow-overlapping-routes-00>
[6] <http://www.ietf.org/rfc/rfc4384.txt>
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Authors' Addresses
Camilo Cardona
IMDEA Networks
Avenida del Mar Mediterraneo, 22
Leganes 28919
Spain
Email: juancamilo.cardona@imdea.org
Pierre Francois
IMDEA Networks
Avenida del Mar Mediterraneo, 22
Leganes 28919
Spain
Email: pierre.francois@imdea.org
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