wdiff draft-ietf-grow-filtering-threats-02.txt draft-ietf-grow-filtering-threats-03.txt

Network Working Group Camilo Cardona
Internet-Draft IMDEA Networks/UC3M
Intended status: Informational Pierre Francois
Expires: August 17, 2014 February 5, 2015 IMDEA Networks
Paolo Lucente
Cisco Systems
February 13,
August 4, 2014

Making BGP filtering a habit: Impact on policies
draft-ietf-grow-filtering-threats-02
draft-ietf-grow-filtering-threats-03

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 document describes how unexpected traffic flows can emerge in
across an autonomous system, as the result of other autonomous
systems due to filtering, or restricting the filtering propagation of overlapping
BGP prefixes by neighboring domains.
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
2. Filtering overlapping prefixes . . . . .
1.1. Terminology . . . . . . . . . . 3
2.1. Local filtering . . . . . . . . . . . . . . . . . 3
2. Unexpected Traffic Flows . . . . 3
2.2. Remotely triggered filtering . . . . . . . . . . . . . . 6
3. Uses of overlapping prefix 4
2.1. Local filtering that create unexpected
traffic flows . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. 4
2.1.1. Unexpected traffic Flows flows caused by local filtering of
overlapping prefixes . . . . . . . . . . . . . . . . 7
3.1.1. Unexpected traffic flows caused by local 5
2.2. Remote filtering of
overlapping prefixes . . . . . . . . . . . . . . . . 8
3.1.2. . . . . 6
2.2.1. Unexpected traffic flows caused by remotely triggered
filtering of overlapping prefixes . . . . . . . . . . 12
4. 7
3. Techniques to detect unexpected traffic flows caused by
filtering of overlapping prefixes . . . . . . . . . . . . . . 15
4.1. Being the 'victim' 8
3.1. Existence of unexpected traffic flows within an AS . . . . . 15
4.2. Being a contributor 8
3.2. Contribution to the existence of unexpected traffic flows
in other networks another AS . . . . . . . . . . . . . 15
5. . . . . . . . . . 9
4. Techniques to counter unexpected traffic flows due to the
filtering of overlapping prefixes . . . . . . . 10
4.1. Reactive counter-measures . . . . . . . 16
5.1. Reactive . . . . . . . . . 11
4.2. Anticipant counter-measures . . . . . . . . . . . . . . . 12
4.2.1. Access lists . 17
5.2. Anticipant counter-measures . . . . . . . . . . . . . . . 18
5.2.1. Access lists . . . . 12
4.2.2. Neighbor-specific forwarding . . . . . . . . . . . . 13
5. Conclusions . . . . . 18
5.2.2. Automatic overlapping prefix filtering . . . . . . . 19
5.2.3. Neighbor-specific forwarding . . . . . . . . . . . . 19 . 13
6. Conclusions Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . 19
7. References . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . 20
7.1. . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . 0
7.2. URIs . . 14
9.1. References . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. URIs . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20

1. Introduction

It is common practice for . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

It is common practice for network operators to propagate overlapping
prefixes a more
specific (overlapping) prefix in the BGP routing system, along with
the prefixes covering prefix 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.

While BGP makes independent, policy driven decisions for the
selection of the best path to be used for a given IP prefix. However, prefix, routers
must forward packets using the longest-prefix-match rule, which
"precedes" any BGP policy (RFC1812 [1]). Indeed, the 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). p'. When an Autonomous System filters overlapping prefixes and
forwards packets according to the covering prefix, the discrepancy in
the routing policies applied to covering and overlapping prefixes can
create unexpected traffic flows that infringe the policies of other ASes
ASes, 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 possible side effects
associated with overlapping prefix filtering by making the routing community aware filtering. This document presents
examples of the
cases where the such side effects of filtering might turn and discusses approaches towards
solutions to be negative for the business of Internet Service Providers (ISPs). problem.

The rest of the document is organized as follows: Section 2
illustrates the motivation to filter overlapping prefixes. In Section 3, 2 we
provide some scenarios in which the filtering of overlapping prefixes lead
leads to the creation of unexpected traffic flows
on other ASes. flows. Section 4 3 and
Section 5 4 discuss some techniques that ASes can use for,
respectively, detect and react to unexpected 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
conclude in Section 5.

1.1. Terminology

Overlapping prefix: A prefix in which the
filtering routing table with an address
range that is performed locally from those where covered by another prefix present in the filtering is
triggered remotely. These scenarios will be used as a base routing table.

Covering prefix: A prefix in
Section 3 for describing side effects bound the routing table with such practices.

2.1. Local filtering

Let us first analyze an address range
partially covered by other prefixes.

We re-use the scenario depicted in Figure 1. AS1 and AS2
are two autonomous systems spanning a large geographical area definitions of customer-transit peering and settlement-
free peering in 3 different physical locations. Let AS1 announce of RFC4384 [2].

Selective advertisement: The behavior of only advertising a self
originated BGP path for a 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 strict subset of the link
closer to this subnet, AS1 could announce eBGP
sessions of the overlapping prefix AS.

Selective propagation: The behavior of only propagating a BGP path
for a prefix over this specific session. At the time a strict subset of the establishment eBGP sessions of the
peering, it can be defined by both ASes that hot potato routing would
happen in both directions an AS.

Local filtering: The behavior of traffic. In other words, it was agreed 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 each AS will deliver this is typically achieved by
tagging a self-originated path with BGP communities defined by the
distant AS.

Unexpected traffic to flow: Traffic flowing between two neighboring ASes
of an AS, although the other transit policy of that AS on the nearest
peering link. In this scenario, it becomes relevant to AS2 is to
enforce such practice by detecting the described situations and
automatically issuing the appropriate filtering. In this case, by
implementing not provide
connectivity between 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 two neighbors. A traffic flow across an
AS, between two of local filtering

Local filtering could be required in other cases. For example, its transit providers, or between a
dual homed AS receiving an overlapping prefix from only transit
provider and one of its
providers. Figure 2 depicts a simple example settlement-free peers, are classical examples
of this case.

_..._
,' `.
/ 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 [2].

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
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 unexpected traffic flows at other, distant, ASes.

3. Uses of overlapping prefix filtering that create unexpected traffic
flows

In this section, we define the concept of unexpected 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.

3.1. Unexpected 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 [3]). 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 AS1. In the figure, we
also show a customer of AS2, AS4, which is announcing prefix P4/p4.
AS2 propagates this prefix to AS1.

,-----. ,-----. ,-----.
,' `. ,' `. ,' `.
/ AS1 \ / AS2 \ / AS3 \
( )-----( )-----( )
\ / P4/p4 \ / \ P3/p3 /
`. ,' `. ,' `. ,'
'-----' '-----' '-----'
|
|
|
,-----.
,' `.
/ AS4 \
( )
\ P4/p4 /
`. ,'
'-----'

Figure 4: Prefix exchange among four autonomous systems

Although ISPs usually implement the aforementioned policies,
unexpected traffic flows may still appear. In Figure 4, unexpected
traffic flows are created, when, despite AS2's policy, traffic
arriving from peer AS1 is received and transported to AS3 by AS2.
These types 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 unexpected traffic flows on ASes.
We provide examples of these cases in the next sections.

3.1.1. Unexpected 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 unexpected 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
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 as best path to AS1 the one
received from AS3.

3.1.1.2. Unexpected 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 unexpected
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:

2. Unexpected Traffic Flows

In this section, we describe how overlapping prefix filtering can
lead to unexpected traffic flows by local in other, remote, ASes. We
differentiate cases in which the filtering - Case 1

Let us assume is performed locally from
those where the scenario illustrated in Figure 6. For this case,
AS1 filtering is triggered remotely.

2.1. Local filtering

Local filtering can be motivated by different reasons, such as: (1)
Traffic engineering, where an AS wants to control their local
outbound traffic distribution using only propagates the overlapping prefix policy applied to AS3. AS4 receives the
overlapping prefix only from its transit provider, AS5.

AS4 now
covering prefix. (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 in not allowed
by their peering agreement.

Figure 1 illustrates a situation scenario in which it would be favorable for it one AS is motivated to
filter the announcement of prefix 10.0.0.0/24 from AS5.
Subsequently, traffic from AS4
perform local filtering due to prefix 10.0.0.0/24 outbound traffic engineering. The
figure depicts AS64504, and two of its neighboring ASes, AS64502 and
AS64505. AS 64504 has a settlement-free peering with AS64502 and is forwarded
towards AS2. Because AS2
a customer of AS64505. AS64504 receives from AS64505 prefixes
2001:DB8::/32 and 2001:DB8::/34, covering and overlapping prefixes,
respectively. AS64504 receives only the more specific covering prefix
2001:DB8::/32 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 AS64502.

,-----.
/ \
( AS64505 )
\ /
`--+--'
2001:DB8::/32 | |
2001:DB8::/34 v |
|
,--+--. 2001:DB8::/32 ,-----.
/ \ <-- / \
( AS64504 )-------------( AS64502 )
\ / \ /
`-----' `-----'

Figure 1: Local Filtering

Due to avoid using
itself economical reasons, AS64504 might prefer to exchange send traffic between AS4 and AS3. However, due to the
discrepancies
AS64502 instead of routes AS64505. However, even if paths received from the overlapping and covering prefixes,
unexpected traffic flows between AS4 and AS3
AS64502 are given a large local preference, routers in AS64504 will
still exist on AS2's
network. send traffic to prefix 2001:DB8::/34 via neighbor AS64505.
This situation may push AS64504 to apply an inbound filter for the
overlapping prefix, 2001:DB8::/34, on the session with AS64505.
After the filter is economically detrimental for AS2, since
it forwards applied, traffic from a peer to a non-customer neighbor.

3.1.1.3. the overlapping prefix will
be sent to AS64502

2.1.1. Unexpected traffic flows caused 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 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''

Figure 7: Unexpected traffic flows after of
overlapping prefixes

In this section, we show how the decision of AS64504 to perform local
filtering - Case creates unexpected traffic flows in AS64502. Figure 2

Let us assume a second case
shows the whole picture of the scenario; where AS2 AS64501 is a customer
of AS64503 and AS3 are not peering AS64502. AS64503 is a settlement-free peer with
AS64502. AS64503 and AS1
only propagates AS64502 are customers of AS64505. The AS
originating the overlapping prefix to AS3. AS4 receives two prefixes, AS64501, performs selective
advertisement with the overlapping prefix and 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 announces it would be favorable to filter
AS64503.

After AS64504 locally filters the announcement
of prefix 10.0.0.0/24 from AS5. Subsequently, overlapping prefix, traffic from AS4
AS64504 to prefix 10.0.0.0/24 would be 2001:DB8::/34 is forwarded towards AS2. Due to the
existence of a route to prefix 10.0.0.0/24, AS2 AS64502.
Because AS64502 receives the traffic
heading to this more specific prefix from AS4 and sends it to AS5. This situation
creates unexpected traffic flows that contradict AS2's BGP policy,
since the AS ends up forwarding AS64503,
traffic from a peer AS64504 to a transit
network.

3.1.2. Unexpected traffic flows caused by remotely triggered filtering
of overlapping prefixes

We present a configuration scenario in which an AS, using 2001:DB8::/34 follows the
mechanism described in Section 2.2, informs its provider path
AS64504-AS64502-AS64503-AS64501. AS64502's BGP policies are
implemented to
selectively propagate an overlapping prefix, leading avoid transporting traffic between AS64504 and
AS64503. However, due to the creation discrepancies of routes from the
overlapping and covering prefixes, unexpected 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, between
AS64504 and peers. Some remote ASes will route traffic
destined to 10.0.0.1 through AS2 while others will route traffic
through AS3.

\ / \ /
/22 \ / /22 /22 \ AS64503 exist in AS64502's network.

____,,................______
_,.---'''' `''---..._
,-'' AS64505 `-.
[ / /22
,-----. ,-----.
-.._ __.-'
. `'---....______ ______...---''
+ |/32 `''''''''''''''' |
| |/34 + |/32 |
v | v |/34 |
| | ^ |
| ^ |/32 | |/32
| + | + |/34
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS2 AS64504 \ /22 <-+ / AS3 \
( )-------------( ) AS64502 \ / /22 AS64503 \ /
| |_________| |________| |
| | /32 | |/32 /32| |
'. ,' . ,' /34 . ,'
`. ,' `. ,'
'-----; / '-----'
\ /
\ /
10.0.0.0/22\ /10.0.0.0/22
\ /
\ ,-----.' +-> <-+ `. ,'
``---'' ``---'' ``---''
| ^ |
^ |2001:DB8::/32 | |2001:DB8::/32
| | + |2001:DB8::/34
+ | _....---------...._|
,-'AS64501 ``-.
/' `.
/ AS1 \
( )
\ /
`. ,'
'-----' _,
`-.._ _,,,'
`''---------'''

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 [1]. 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. Remote filtering can be leveraged by an AS to, for
instance, limit the scope of prefixes and hence perform a more
granular inbound traffic engineering.

Figure 8: Example 3 illustrates a scenario

3.1.2.2. Injection of in which an AS uses BGP communities
to command its provider to selectively propagate an overlapping
prefix. Let AS64501 be a customer of AS64502 and AS64503. AS64501
originates prefix 2001:DB8::/32, which it advertises through AS64502
and AS64503. AS64502 and AS64503 are settlement-free peers. Let AS1 advertise 10.0.0.0/24
AS64501 do selective advertisement and only propagate 2001:DB8::/34
over AS3 only. AS3 AS64503. AS64503 would normally 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 AS64502.

Let us consider that AS64501 decides to limit the /24 that contains
AS3 and AS2. Some multi-homed customers scope 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.

\ / /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 prefix. AS64501 can make this case. AS3 could receive traffic
destined to 10.0.0.0/24 from its customers, providers, and peers,
which it directly forwards to decision based on its customer AS1.

3.1.2.3. Creation of unexpected
traffic flows by limiting the scope of engineering strategy. To achieve this, AS64501 can tag 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.

,-------.
,' `.
/ AS5 \
( /22:AS2 ) with a set of communities that leads AS64503 to
only propagate the path to AS64502.

^ \ /
`. ,'
'-------' ^ ^ \ / ^
| /32 \ /
/22 \ //22 /22 /32 | | /32 \ //22 / /32 |
,-----. ,-----.
,' `. /22 ,' `.
/ AS2 AS64502 \ /24 / AS3 AS64503 \
( /22:AS1 )-------------( /22:AS1 )
\ /24:AS3 / /22 /32 /32 \ /24:AS1 /
/22 /`.
`. ,' -> /34 `. ,'
/24/
'-----; <- / '-----'
/
\ /
,---./
^ \ /
/ \ 10.0.0.0/22\ /10.0.0.0/22
( AS4 ) ^
| \ / 10.0.0.0/24 |
| \ / |
| \ ,-----.'
`---' | 2001:DB8::/32
| ,' `. | 2001:DB8::/34
2001:DB8::/32 +-- / AS1 AS64501 \ --+
( )
\ /
`. ,'
'-----'

Figure 10: More Specific Injection

From AS2's point 3: Remote triggered filtering

2.2.1. Unexpected traffic flows caused by remotely triggered filtering
of view, such a path is a "peer path" overlapping prefixes

Figure 4 expands the scenario from Figure 3 and will only
be advertised by AS2 includes other AS
peering with ASes 64502 and 64503. Due to its customers. the limitation on the
scope performed on the overlapping prefix, ASes that are not
customers of AS2 AS64502 will not receive a path for
10.0.0.0/24. 2001:DB8::/34.
These ASes will forward packets destined to 10.0.0.0/24 2001:DB8::/34 according
to their routing state for 10.0.0.0/22. 2001:DB8::/32. Let us assume that
AS5 AS64505
is such an AS, and that its best path towards 10.0.0.0/22 2001:DB8::/32 is
through AS2. Then, packets AS64502. Packets sent towards 10.0.0.1 2001:DB8::1 by AS5 AS64505 will
eventually
reach AS2. AS64502. However, in the data-plane of the nodes of
AS2, AS64502,
the longest prefix match for 10.0.0.1 2001:DB8::1 is 10.0.0.0/24, 2001:DB8::/34, which is
reached through AS3, AS64503, a settlement-free peer of AS2. AS64502. Since AS5
AS64505 is not in the customer branch of AS2, AS64502, we are in a
situation in which traffic flows between non-customer ASes take place
in AS2.

4. AS64502.

,-----.
,' `.
/ AS64505 \
( )
\ /
`. ,'
'-----'
^ \ / ^ ^ \ / ^
| /32 \ / /32 | | /32 \ / /32 |
+ ,-----. + + ,-----. +
,' `. ,' `.
/ AS64502 \ / AS64503 \
( )-------------( )
,-----. \ / /32 /32 \ /
,' `.---------`. ,' +-> /34 `. ,'
/ AS64504 \ /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
overlapping prefixes

We differentiate the techniques available for detecting

3.1. Existence of unexpected traffic flows caused by the described scenarios from the cases in
which the interested 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 his policy is
configured to not provide such transit. IPFIX (RFC7011 [3]) 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 victim or contributor perspective of the business relationships with
neighboring ASes. Open source tools such
operations.

4.1. Being as [4] can be used to this
end.

Note that the AS detecting the 'victim' of unexpected traffic flows

To detect if unexpected 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 may simply
realize that his policy configuration is forwarded to
a non-customer next-hop.

As mentioned in Section 3.1, broken. The first
recommended action upon detection of an unexpected traffic flows might appear
due flow is to different situations. To discover
verify the correctness of the BGP configuration.

Once it has been assessed that the local configuration is correct,
the operator should check if the problem arose after detected in the data-plane
arose due to filtering of prefixes BGP paths by neighboring ASes, an ASes. The
operator can
analyze available BGP data. For instance, should check if the destination address of the unexpected
traffic flow is locally routed as per an ISP can seek for overlapping prefixes for which prefix only
received from non-customer peers. The operator should also checks if
there are paths to a covering prefix received from a customer, and
hence propagated to peers. If these two situations happen at the next-hop
same time, the neighboring AS at the entry point of the unexpected
flow is through a provider (or
peer), while routing the traffic based on the next-hop for their covering prefix(es) prefix, although
the ISP is through a
client. Direct communication actually forwarding the traffic via non-customer links.

To detect the origin of the problem, human interaction or looking
glasses can be used in order to check find out whether non-customer neighboring ASes are propagating a path towards
the covering prefix and not the path towards local filtering,
remote filtering, or selective propagation was performed on the
overlapping prefix.
This situation should trigger a warning, as this would mean that ASes
in Due to the distributed nature and restricted
visibility of the steering of BGP policies, such analysis is deemed
to not identify the surrounding area origin of the current AS problem with guaranteed accuracy.
We are forwarding packets
based on the routing entry for not aware, at the less specific prefix only.

4.2. Being a contributor time of this writing, of any openly
available tool that can automatically perform this operation.

3.2. Contribution to the existence of unexpected traffic flows in other networks
another AS

It can be considered problematic to be causing unexpected traffic
flows on in 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, 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 relationship implementations.
Restricting such the use of these features for the sake of avoiding the
creation of unexpected 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 unexpected traffic flow.

It is
thus advisable for an AS to obtain assess the risks of filtering
overlapping prefixes before implementing them by obtaining as much
data information as possible about the
Internet environment its surrounding routing
environment. The AS would need information of the AS routing policies
and assess the risks relationships among external ASes to detect if its actions
could trigger the appearance of filtering unexpected traffic flows. With this
information, the operator could detect other ASes receiving the
overlapping prefixes before implementing them.

Monitoring prefix from non-customer ASes, while announcing the
covering prefix to other non-customer ASes. If the manipulation filtering of the communities that implement
overlapping prefix leads other ASes to send traffic for the
scoping of prefixes
overlapping prefix to these ASes, an unexpected traffic flow can
arise. However, the information required for this operation is recommended
difficult to obtain, due to the ISPs that provide these
features. The monitored behavior should then be compared with their
terms distributed nature of use.

5. BGP policies.
We are not aware, at the time of this writing, of any openly
available tool that can automatically perform this procedure.

4. Techniques to counter unexpected traffic flows due to the filtering
of overlapping prefixes

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. We provide potential solutions that ISPs
must evaluate against each particular case. 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 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 configuration scenario is set up. arises.

We use the scenario depicted in Figure 11 5 to describe these two kinds
of approaches. Based on our analysis, we observe that anticipant
approaches can be complex to implement and can lead to undesired
repercussions.
effects. Therefore, we conclude that the reactive approach is the
more reasonable recommendation to deal with unexpected flows.

____,,................______
_,.---'''' `''---..._
,-'' AS5 AS64505 `-.
[ /
-.._ __.-'
. `'---....______ ______...---''
|/22
+ |/32 `''''''''''''''' |
|/24 |/22
| |/34 + |/32 |
v | v |/34 | |/24
| | ^ |
| ^ |/32 | |/22 |/22 |/32
| + | |/24 + |/34
_,,---.:_ _,,---.._ _,,---.._
,' `. ,' `. ,' `.
/ AS4 AS64504 \ <-+ / AS2 AS64502 \ / AS3 AS64503 \
| |_________| | | |
| | /22 /32 | | | |
'. ,' . ,' . ,'
`. ,' `. ,' `. ,'
``---'' ``---'' ``---''
| ^ |
^ |2001:DB8::/32 | |10.0.0.0/24
|10.0.0.0/22 |10.0.0.0/22
_;,..---------...._|
,-'AS1 |2001:DB8::/32
| | + |2001:DB8::/34
+ | _....---------...._|
,-'AS64501 ``-.
/' `.
`. _,
`-.._ _,,,'
`''---------'''

Figure 11: Anticipant counter-measures 5: Counter-measures for unexpected traffic flows - Base
example

5.1.

4.1. Reactive counter-measures

An operator who detects unexpected traffic flows originated by any of
the cases described in Section 3 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
overlapping 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 describe briefly the
two alternatives.

o An operator can decide to stop announcing the covering prefix at
the peering session with the neighboring AS from which it is
receiving traffic to the overlapping prefix. In the example of
Figure 11, AS2 5, AS64502 would filter out the prefix 10.0.0.0/22 2001:DB8::/32 from
the eBGP session with AS4. AS64504. In this case, not all the traffic
heading to the prefix 10.0.0.0/22 2001:DB8::/32 from AS1 AS64501 would not no longer
traverse AS2.
AS2 AS64502. AS64502 should evaluate if solving the inconvenient issues
originated by the unexpected traffic flows are worth the loss of
this traffic share.

o An operator can decide to filter-out filter out the concerned overlapping prefix at the
peering session over which it was received. In the example of
Figure 11, AS2 5, AS64502 would filter out the incoming prefix
10.0.0.0/24
2001:DB8::/34 from the eBGP session with AS5. AS64505. As a result,
the traffic destined to that /24 /32 would be forwarded by AS2 AS64502
along its link with AS1, AS64501, despite the actions performed by AS1
AS64501 to have this traffic coming in through its link with AS3.
AS64503. However, as AS2 AS64502 will no longer possess know a route to the
overlapping prefix, it risks losing the traffic share from
customers different from AS1 AS64501 to that prefix. Furthermore,
this action can generate conflicts between
AS2 AS64502 and AS1, AS64501,
since AS2 AS64502 does not follow the policy routing information expressed by AS1
AS64501 in its BGP announcements.

It is possible that the behavior from of the neighboring AS that is causing the
unexpected traffic flows opposes the peering agreement. In this
case, an operator can could account the amount of traffic that has been
subject to the unexpected flows flows, using traffic measurement protocols
such as IPFIX, 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.

5.2.

4.2. Anticipant counter-measures

5.2.1.

4.2.1. 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 unexpected traffic flows.
In the example of Figure 11, AS2 5, AS64502 would install an access-list
denying packets matching 10.0.0.0/24 2001:DB8::/34 associated with the interface
connecting to AS4. AS64504. As a result, traffic destined to that prefix
would be dropped, despite the existence of a valid route towards 10.0.0.0/22.

Note that this
2001:DB8::/32.

This technique actually lets packets destined to a valid prefix be
dropped while they are sent from a neighboring AS that
cannot may not know
about the policy conflicts conflict and hence had no means to avoid the
creation of unexpected traffic flows.

5.2.2. Automatic overlapping prefix filtering

As described in Section 3, filtering of overlapping prefixes can in
some scenarios lead to unexpected traffic flows. Nevertheless,
depending on the autonomous system implementing such practice, For this
operation can prevent these cases. This reason, this
technique 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 unexpected traffic flow in AS2. Nevertheless,
as described in Section 5.1, the filtering of overlapping prefixes
can generate conflicts between AS1 considered harmful and AS2, since AS2 would is thus not
forward traffic according to AS1's policy. Additionally, AS2 can
lose traffic share recommended for the overlapping prefix from customers
different from AS1.

5.2.3.
implementation.

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 11 5 from the point
of view of AS2. AS64502. The edge router connecting to the AS4 forward AS64504
forwards packets destined to prefix 10.0.0.0/24 2001:DB8::/34 towards AS5. AS64505.
Likewise, it
will forward forwards packets destined to prefix 10.0.0.0/22 2001:DB8::/32
towards AS1. AS64501. The router, however, only propagates the path of
the covering prefix
(10.0.0.0/22) (2001:DB8::/32) to AS4. AS64504. An operator could
implement the necessary techniques to force the edge router to
forward packets coming from
AS4 AS64504 based only on the paths
propagated to AS4. AS64504. Thus, the edge router would forward packets
destined to 10.0.0.0/24 2001:DB8::/34 towards AS1 AS64501 in which case no unexpected
traffic flow would occur.

Different techniques could provide the functionality just described; 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 [4] to store the routes announced to a neighbor and
forward traffic exclusively based on those routes. [2] describes an approach to implement this behavior.
Similar
solution and provides a description of its limitations. In the
future, new network protocols and architectures could provide this
functionality with less overhead for management and device resources.

Note that similarly to the solution described in Section 5.2.2, 4.1, this
approach could create conflicts between AS2 AS64502 and AS1, AS64501, since
the traffic forwarding performed by A2 AS64502 goes against the policy
of AS1.

6. AS64501.

5. Conclusions

In this document, we described threats to policies of autonomous
systems caused by how the filtering of overlapping
prefixes performed by
external networks. can potentially create unexpected traffic flows in remote
ASes. We provide provided examples of scenarios in which unexpected traffic
flows are caused by these practices and introduce some techniques for
their detection and prevention. Analyzing the different options for
dealing with this kind of problems, we recommend potential victims ASes affected by
unexpected traffic flows to implement monitoring systems that can
detect them and react to them according to the specific situation.
Although we observe that there are reasonable situations in which
ASes could filter overlapping prefixes, we encourage that network
operators to implement this type of filters only after considering the cases
described in this document.

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.
The objective of this document is to inform on this potential routing
security issue.

7. IANA Considerations

This document has no IANA actions.

8. Acknowledgments

The authors would like to thank Wes George, Jon Mitchell, and Bruno
Decraene for their useful suggestions and comments.

9. References

9.1. 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>.

7.2. <http://ripe63.ripe.net/presentations/48-
How-more-specifics-increase-your-transit-bill-v0.2.pdf>.

[4] "pmacct project: IP accounting iconoclasm",
<http://www.pmacct.net>.

9.2. URIs

[1] http://www.ietf.org/rfc/rfc1812.txt

[2] http://tools.ietf.org/html/draft-white-grow-overlapping-routes-02

[3] http://www.ietf.org/rfc/rfc4384.txt

[3] http://www.ietf.org/rfc/rfc7011.txt

[4] http://www.ietf.org/rfc/rfc4364.txt

Authors' Addresses

Camilo Cardona
IMDEA Networks/UC3M
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

Paolo Lucente
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA

Email: plucente@cisco.com