Network Working Group Camilo Cardona
Internet-Draft IMDEA Networks/UC3M
Intended status: Informational Pierre Francois
Expires: August 17, 2014 IMDEA Networks
Paolo Lucente
Cisco Systems
February 13, 2014
Making BGP filtering a habit: Impact on policies
draft-ietf-grow-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 unexpected traffic 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 17, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Camilo Cardona, et al. Expires August 17, 2014 [Page 1]
Internet-Draft Making BGP filtering a habit February 2014
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Filtering overlapping prefixes . . . . . . . . . . . . . . . 3
2.1. Local filtering . . . . . . . . . . . . . . . . . . . . . 3
2.2. Remotely triggered filtering . . . . . . . . . . . . . . 6
3. Uses of overlapping prefix filtering that create unexpected
traffic flows . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Unexpected traffic Flows . . . . . . . . . . . . . . . . 7
3.1.1. Unexpected traffic flows caused by local filtering of
overlapping prefixes . . . . . . . . . . . . . . . . 8
3.1.2. Unexpected traffic flows caused by remotely triggered
filtering of overlapping prefixes . . . . . . . . . . 12
4. Techniques to detect unexpected traffic flows caused by
filtering of overlapping prefixes . . . . . . . . . . . . . . 15
4.1. Being the 'victim' of unexpected traffic flows . . . . . 15
4.2. Being a contributor to the existence of unexpected
traffic flows in other networks . . . . . . . . . . . . . 15
5. Techniques to counter unexpected 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 overlapping prefix filtering . . . . . . . 19
5.2.3. Neighbor-specific forwarding . . . . . . . . . . . . 19
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. References . . . . . . . . . . . . . . . . . . . . . . . 0
7.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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
Camilo Cardona, et al. Expires August 17, 2014 [Page 2]
Internet-Draft Making BGP filtering a habit February 2014
must forward packets using the longest-prefix-match rule, which
"precedes" any BGP policy (RFC1812 [1]). 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 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 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 Internet Service Providers (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 unexpected traffic flows
on other ASes. Section 4 and Section 5 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 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.
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.
Camilo Cardona, et al. Expires August 17, 2014 [Page 3]
Internet-Draft Making BGP filtering a habit February 2014
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.
Camilo Cardona, et al. Expires August 17, 2014 [Page 4]
Internet-Draft Making BGP filtering a habit February 2014
_..._
,' `.
/ 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].
Camilo Cardona, et al. Expires August 17, 2014 [Page 5]
Internet-Draft Making BGP filtering a habit February 2014
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.
Camilo Cardona, et al. Expires August 17, 2014 [Page 6]
Internet-Draft Making BGP filtering a habit February 2014
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.
Camilo Cardona, et al. Expires August 17, 2014 [Page 7]
Internet-Draft Making BGP filtering a habit February 2014
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
Camilo Cardona, et al. Expires August 17, 2014 [Page 8]
Internet-Draft Making BGP filtering a habit February 2014
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: Unexpected 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.
Camilo Cardona, et al. Expires August 17, 2014 [Page 9]
Internet-Draft Making BGP filtering a habit February 2014
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 using
itself to exchange traffic between AS4 and AS3. However, due to the
discrepancies of routes from the overlapping and covering prefixes,
unexpected 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. Unexpected traffic flows by local filtering - Case 2
Camilo Cardona, et al. Expires August 17, 2014 [Page 10]
Internet-Draft Making BGP filtering a habit February 2014
____,,................______
_,.---'''' `''---..._
,-'' 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 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 would be 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 unexpected traffic flows that contradict AS2's BGP policy,
Camilo Cardona, et al. Expires August 17, 2014 [Page 11]
Internet-Draft Making BGP filtering a habit February 2014
since the AS ends up forwarding traffic from a peer 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 the
mechanism described in Section 2.2, informs its provider to
selectively propagate an overlapping prefix, leading to the creation
of 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, 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 \ / /22
,-----. ,-----.
,' `. ,' `.
/ AS2 \ /22 / AS3 \
( )-------------( )
\ / /22 \ /
`. ,' `. ,'
'-----; / '-----'
\ /
\ /
10.0.0.0/22\ /10.0.0.0/22
\ /
\ ,-----.'
,' `.
/ AS1 \
( )
\ /
`. ,'
'-----'
Figure 8: Example scenario
Camilo Cardona, et al. Expires August 17, 2014 [Page 12]
Internet-Draft Making BGP filtering a habit February 2014
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.
\ / /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.
Camilo Cardona, et al. Expires August 17, 2014 [Page 13]
Internet-Draft Making BGP filtering a habit February 2014
3.1.2.3. Creation of unexpected 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.
,-------.
,' `.
/ 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
Camilo Cardona, et al. Expires August 17, 2014 [Page 14]
Internet-Draft Making BGP filtering a habit February 2014
branch of AS2, we are in a situation in which traffic flows between
non-customer ASes take place in AS2.
4. Techniques to detect unexpected traffic flows caused by filtering of
overlapping prefixes
We differentiate the techniques available for detecting unexpected
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' 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 is forwarded to
a non-customer next-hop.
As mentioned in Section 3.1, unexpected 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 and not the path towards the overlapping prefix.
This situation 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 unexpected traffic flows
in other networks
It can be considered problematic to be causing unexpected 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
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 to obtain as much information as possible about the
Camilo Cardona, et al. Expires August 17, 2014 [Page 15]
Internet-Draft Making BGP filtering a habit February 2014
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 compared with their
terms of use.
5. 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. 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.
We use the scenario depicted in Figure 11 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. Therefore, we conclude that the reactive approach is
the more reasonable recommendation to deal with unexpected flows.
Camilo Cardona, et al. Expires August 17, 2014 [Page 16]
Internet-Draft Making BGP filtering a habit February 2014
____,,................______
_,.---'''' `''---..._
,-'' 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 unexpected traffic flows originated by any of
the cases described in Section 3 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.
Camilo Cardona, et al. Expires August 17, 2014 [Page 17]
Internet-Draft Making BGP filtering a habit February 2014
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 would filter out the prefix 10.0.0.0/22 from the
eBGP session with AS4. In this case, all the traffic heading to
the prefix 10.0.0.0/22 from AS1 would not longer traverse AS2.
AS2 should evaluate if solving the inconvenient originated by the
unexpected traffic flows are worth the loss of this traffic share.
o 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 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. However, as AS2 will
no longer possess a route to the overlapping prefix, it risks
losing the traffic share from customers different from AS1 to that
prefix. Furthermore, this action can generate conflicts between
AS2 and AS1, since AS2 does not follow the policy expressed by AS1
in its BGP announcements.
It is possible that the behavior from the neighboring AS that is
causing the unexpected traffic flows opposes the peering agreement.
In this case, an operator can account the amount of traffic that has
been subject to the unexpected 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.
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 unexpected 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.
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 unexpected traffic flows.
Camilo Cardona, et al. Expires August 17, 2014 [Page 18]
Internet-Draft Making BGP filtering a habit February 2014
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, 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 unexpected traffic flow in AS2. Nevertheless,
as described in Section 5.1, the filtering of overlapping prefixes
can generate conflicts between AS1 and AS2, since AS2 would not
forward traffic according to AS1's policy. Additionally, AS2 can
lose traffic share for the overlapping prefix from customers
different from AS1.
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 unexpected traffic flow would occur.
Different techniques could provide the functionality just described;
however, their technical implementation can be complex to design and
operate. [2] describes an approach to implement this behavior.
Similar to the solution described in Section 5.2.2, this approach
could create conflicts between AS2 and AS1, since the traffic
forwarding performed by A2 goes against the policy of AS1.
6. Conclusions
In this document, we described threats to policies of autonomous
systems caused by the filtering of overlapping prefixes performed by
external networks. We provide 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 to implement monitoring systems that can
Camilo Cardona, et al. Expires August 17, 2014 [Page 19]
Internet-Draft Making BGP filtering a habit February 2014
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 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>.
7.2. 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
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
Camilo Cardona, et al. Expires August 17, 2014 [Page 20]
Internet-Draft Making BGP filtering a habit February 2014
Paolo Lucente
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: plucente@cisco.com
Camilo Cardona, et al. Expires August 17, 2014 [Page 21]