AS Path Prepending
draft-ietf-grow-as-path-prepending-04
Network Working Group M. McBride
Internet-Draft Futurewei
Updates: 7454, 8195 (if approved) D. Madory
Intended status: Best Current Practice Kentik
Expires: January 9, 2022 J. Tantsura
Microsoft
R. Raszuk
NTT Network Innovations
H. Li
HPE
J. Heitz
Cisco
G. Mishra
Verizon Inc.
July 8, 2021
AS Path Prepending
draft-ietf-grow-as-path-prepending-04
Abstract
AS Path Prepending provides a tool to manipulate the BGP AS_Path
attribute through prepending multiple entries of an AS. AS Path
Prepending is used to deprioritize a route or alternate path. By
prepending the local ASN multiple times, ASs can make advertised AS
paths appear artificially longer. Excessive AS Path Prepending has
caused routing issues in the internet. This document provides
guidance with the use of AS Path Prepending, including alternative
solutions, in order to avoid negatively affecting the internet.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 9, 2022.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Cascading and ripple affect of prepending across the
internet . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Excessive Prepending . . . . . . . . . . . . . . . . . . 5
3.3. Prepending during a routing leak . . . . . . . . . . . . 6
3.4. Prepending to All . . . . . . . . . . . . . . . . . . . . 7
3.5. Memory . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Errant announcement . . . . . . . . . . . . . . . . . . . 7
4. Alternatives to AS Path Prepend . . . . . . . . . . . . . . . 8
5. Best Practices . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Border Gateway Protocol (BGP) [RFC4271] specifies the AS_PATH
attribute which enumerates ASs a route update has traversed. If the
UPDATE message is propagated over an external link, then the local AS
number is prepended to the AS_PATH attribute, and the NEXT_HOP
attribute is updated with an IP address of the router that should be
used as a next hop to the network. If the UPDATE message is
propagated over an internal link, then the AS_PATH attribute and the
NEXT_HOP attribute are passed unmodified.
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A common practice among operators is to prepend multiple entries of
an AS (known as AS Path Prepending) in order to deprioritize a route
or a path. This has worked well in practice but the practice is
increasing, with both IPv4 and IPv6, and there are inherit risks to
the global internet especially with excessive AS Path Prepending.
Prepending is frequently employed in an excessive manner such that it
renders routes vulnerable to disruption or misdirection. AS Path
Prepending is discussed in Use of BGP Large Communities [RFC8195] and
this document provides additional, and specific, guidance to
operators on how to be a good internet citizen with the proper use of
AS Path Prepending.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Use Cases
There are various reasons that AS Path Prepending is in use today
including:
o Preferring one ISP over another ISP on the same ASBR or across
different ASBRs.
o Preferring one ASBR over another ASBR in the same site or between
sister sites.
o Utilize one path exclusively and another path solely as a backup.
o Signal to indicate that one path may have a different amount of
capacity than another where the lower capacity link still takes
traffic.
o Conditionally prefer one ASBR over another at the same site or
between sites for lowest latency path based on geographic
location.
o An ISP doesn't accept traffic engineering using BGP communities.
Prepending is the only option.
The following illustration, from Geoff Hustons Path Prepending in BGP
[1], shows how AS Prepending is typically used:
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+---+ +---+
+---| D |----| F |
| +---+ +---+
+---+ +---+ |
| A |---| B | |
+---+ +---+ 2x<- |
| +---+ +---+
+---| C |----| E |
+---+ +---+
In the diagram above, A, B, C, D, E, and F all have a different AS.
B will normally prefer the path via C to send traffic to E, as this
represents the shorter AS path for B. If E were to prepend a further
two instances of its own AS number when advertising its routes to C,
then B will now see a different situation, where the AS Path via D
represents the shorter path. Through the use of selective prepending
E is able to alter the routing decision of B, even though B is not an
adjacent neighbour of E. The result is that traffic from A and B
will be passed via D and F to reach E, rather than via C. In this
way prepending implements action at a distance where the routing
decisions made by non-adjacent ASs can be influenced by selective AS
Path prepending.
3. Problems
Since it is so commonly used, what is the problem with the excessive
use of AS Path Prepending? Here are a few examples:
3.1. Cascading and ripple affect of prepending across the internet
Care must be taken in prepending, as prepending can result in a
ripple affect with multiple AS's performing the same set of prepend
in the same direction can result in route leaks where the valid
preferred path becomes now de-preferred.
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<-5x <-5x <-5x
+---+ +---+ +---+ +---+
+---| D |----| F |----| H |----| J |
| +---+ +---+ +---+ +---+
+---+ +---+ | |
| A |---| B | | |
+---+ +---+ 13x<-| |
| +---+ +---+ +---+ +---+
+---| C |----| E |----| G |----| I |
+---+ +---+ +---+ +---+
In the diagram above A, B, C, D, E, F G, H, I, J are all part of a
different AS. B will normally prefer the path via D to send traffic
to J, as this represents the preferred path to B, due to E prepending
13 instances of its own AS number when advertising routes to C. ISP
J decides to prepend 5 instances of its own AS when advertising H,
and ISP H decides to do the same and prepends 5 instances of its own
AS when advertising to F. ISP F decides to as well prepend 5
instances of its own AS when advertising to D. B now sees 19 AS hops
for prefixes coming from D to get to J which should be the preferred
path compare to 18 AS hops coming from C which is now preferred. We
now have a route leak to I as B now sends all of its traffic through
I to reach J. This is the typical scenario where route leaks occur
where providers decide to de-prefer a path, however as the same de-
prefer of a path gets cascaded in the same direction, as a result,
the path that should never be preferred as its as-path is very high
in this case 18 AS hops ends up being the preferred path resulting in
a route leak. BGP large communties along with conditional
prepending, along with care being taken when prepending is performed
between providers can help mitigate the adverse impacts of
prepending.
3.2. Excessive Prepending
The risk of excessive use of AS Path Prepending can be illustrated
with real-world examples that have been anonymized using
documentation prefixes [RFC5737] and ASs [RFC5398] . Consider the
prefix 198.51.100.0/24 which is normally announced with an inordinate
amount of prepending. A recent analysis revealed that
198.51.100.0/24 is announced to the world along the following AS
path:
64496 64511 64511 64511 64511 64511 64511 64511 64511 64511 64511
64511 64511 64511 64511 64511 64511 64511 64511 64511 64511 64511
64511 64511
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In this example, the origin AS64511 appears 23 consecutive times
before being passed on to a single upstream (AS64496), which passes
it on to the global internet, prepended-to-all. An attacker, wanting
to intercept or manipulate traffic to this prefix, could enlist a
datacenter to allow announcements of the same prefix with a
fabricated AS path such as 999999 64496 64511. Here the fictional
AS999999 represents the shady datacenter. This malicious route would
be preferred due to the shortened AS path length and might go
unnoticed by the true origin, even if route-monitoring had been
implemented. Standard BGP route monitoring checks a route's origin
and upstream and both would be intact in this scenario. The length
of the prepending gives the attacker room to craft an AS path that
would appear plausible to the casual observer, comply with origin
validation mechanisms, and not be detected by off-the-shelf route
monitoring.
3.3. Prepending during a routing leak
In April 2010, a service provider experienced a routing leak. While
analyzing the leak something peculiar was noticed. When we ranked
the approximately 50,000 prefixes involved in the leak based on how
many ASs accepted the leaked routes, most of the impact was
constrained to Country A routes. However, two of the top five most-
propagated leaked routes (listed in the table below) were Country B
routes.
During the routing leak, nearly all of the ASs of the internet
preferred the Country A leaked routes for 192.0.2.0/21 and
198.51.100.0/22 because, at the time, these two Country B prefixes
were being announced to the entire internet along the following
excessively prepended AS path: 64496 64500 64511 64511 64511 64511
64511 64511. Virtually any illegitimate route would be preferred
over the legitimate route. In this case, the victim is all but
ensuring their victimhood.
There was only a single upstream seen in the prepending example from
above, so the prepending was achieving nothing except incurring risk.
You would think such mistakes would be relatively rare, especially
now, 10 years later. As it turns out, there is quite a lot of
prepending-to-all going on right now and during leaks, it doesn't go
well for those who make this mistake. While one can debate the
merits of prepending to a subset of multiple transit providers, it is
difficult to see the utility in prepending to every provider. In
this configuration, the prepending is no longer shaping route
propagation. It is simply incentivizing ASs to choose another origin
if one were to suddenly appear whether by mistake or otherwise.
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3.4. Prepending to All
Based on analysis done in 2019, Excessive AS Path Prepending [2], out
of approximately 750,000 routes in the IPv4 global routing table,
nearly 60,000 BGP routes are prepended to 95% or more of hundreds of
BGP sources. About 8% of the global routing table, or 1 out of every
12 BGP routes, is configured with prepends to virtually the entire
internet. The 60,000 routes include entities of every stripe:
governments, financial institutions, even important parts of internet
infrastructure.
Much of the worst propagation of leaked routes during big leak events
have been due to routes being prepended-to-all. AS64505 leak of
April 2014 (>320,000 prefixes) was prepended-to-all. And the AS64506
leak of June 2015 (>260,000 prefixes) was also prepended-to-all.
Prepended-to-all prefixes are those seen as prepended by all (or
nearly all) of the ASs of the internet. In this configuration,
prepending is no longer shaping route propagation but is simply
incentivizing ASs to choose another origin if one were to suddenly
appear whether by mistake or otherwise. The percentage of the IPv4
table that is prepended-to-all is growing at 0.5% per year. The IPv6
table is growing slower at 0.2% per year. The reasons for using
prepend-to-all appears to be due to 1) the AS forgetting to remove
the prepending for one of its transit providers when it is no longer
needed and 2) the AS attempting to de-prioritize traffic from transit
providers over settlement-free peers and 3) there are simply a lot of
errors in BGP routing. Consider the prepended AS path below:
64496 64501 64501 64510 64510 64501 64510 64501 64501 64510 64510
64501 64501 64510
The prepending here involves a mix of two distinct ASNs (64501 and
64510) with the last two digits transposed.
3.5. Memory
Long AS Paths cause an increase in memory usage by BGP speakers. A
concern about an AS Path longer than 255 is the extra complexity
required to process it, because it needs to be encoded in more than
one AS_SEQUENCE in the AS_PATH BGP path attribute.
3.6. Errant announcement
There was an Internet-wide outage caused by a single errant routing
announcement. In this incident, AS64496 announced its one prefix
with an extremely long AS path. Someone entered their ASN instead of
the prepend count 64496 modulo 256 = 252 prepends and when a path
lengths exceeded 255, routers crashed
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4. Alternatives to AS Path Prepend
Various options, to provide path preference without needing to use AS
Path Prepend, include:
o Use predefined communities that are mapped to a particular
behavior when propagated.
o Announce more specific routes on the preferred path.
o The BGP Origin Code is an attribute that is used for path
selection and can be used as a high order tie-breaker. The three
origin codes are IGP, EGP and INCOMPLETE. When AS Paths are of
equivalent length, users could advertise paths, with IGP or EGP
origin, over the preferred path while the other ASBRs (which would
otherwise need to prepend N times) advertises with an INCOMPLETE
origin code.
o The Multi Exit Discriminator (MED) is an optional non-transitive
attribute that can be used to influence path preference instead of
using as-path. MED is non transitive so it cannot influence an AS
more then 1 AS hop away.
o Local-preference optional non-transitive attribute, above as-path
in bgp path selection, can be used to influence route preference
within the local operators AS administrative domain. Local-
preference can shield the operator domain from traffic shifts off
the preferred path to a de-preferred path caused by excess
prepending done by service providers across the internet. If all
service providers across the internet set local-preference inbound
conditionally with Large Community set on preferred paths,
essentially the impacts of route leaks as well as other routing
issues resulting from excess prepending can be mitigated.
<-5x <-5x <-5x
LP 110 +---+ +---+ +---+ +---+
+---| D |----| F |----| H |----| J |
| +---+ +---+ +---+ +---+
+---+ +---+ | |
| A |---| B | | |
+---+ +---+ 13x<-| |
| +---+ +---+ +---+ +---+
+---| C |----| E |----| G |----| I |
+---+ +---+ +---+ +---+
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In the diagram above A, B, C, D, E, F G, H, I, J are all part of a
different AS. B will normally prefer the path via D to send traffic
to J, as this represents the preferred path to B, due to E prepending
13 instances of its own AS number when advertising routes to C. ISP
J decides to prepend 5 instances of its own AS when advertising to H,
and ISP H decides to do the same and prepends 5 instances of its own
AS when advertising to F. ISP F decides to also prepend 5 instances
of its own AS when advertising to D. B now sees 19 AS hops for
prefixes coming from D to get to J which should be the preferred path
compare to 18 AS hops coming from C which is now preferred. We now
have a route leak to I as B now sends all of its traffic through I to
reach J. Route leak on B can be prevented locally within the
operator domain by setting local-preference inbound, which is above
as-path length in the best path selection, to higher then default 100
to 110 inbound from D, thus shielding the operator B from being
influenced by the excessive prepend cascading ripple affect by F, H,
J. Note that A still sees the cascading ripple affect of excess
prepending, however A, or any service provider AS downstream of B,
ingressing B, will be shunted to D via local-preference and the route
leak is now mitigated for all downstream AS to the left of B that
prefer the path through B.
5. Best Practices
Many of the best practices, or lack thereof, can be illustrated from
the preceeding examples. Here's a summary of the best current
practices when using AS Path Prepending:
o Network operators should ensure prepending is absolutely necessary
as many networks have excessive prepending. It is best to
innumerate what the routing policies are intended to achieve
before concluding that prepending is a solution
o The neighbor you are prepending may have an unconditional
preference for customer routes and prepending doesn't work. It's
helpful to check with neighbors to see if they will honor the
prepend to avoid wasting effort and potentially causing further
vulnerabilities.
o Use of local-preference inbound on preferred paths between service
providers to help mitigate the adverse affects of prepending
o There is no need to prepend more than 5 ASs. The following
diagram, from the previously referenced AS Path Prepending
analysis from 2019, shows that 90% of AS path lengths are 5 ASNs
or fewer in length.
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+------------------------------------+
90| |
| X |
80| X X |
| X X |
70| X X |
| X X |
60| X X |
| X X |
50| X X |
| X X |
40| X X |
| X X |
30| X X |
| X X |
20| X XX |
| XX XX |
10| XX XXXX |
|XX XXXXXXXXXXXXXXXXX|
+------------------------------------+
5 10 15
AS Path Length in IPv4
X Axis = unique AS Paths in millions
o Don't prepend ASNs that you don't own.
o Prepending-to-all is a self-inflicted and needless risk that
serves little purpose. Those excessively prepending their routes
should consider this risk and adjust their routing configuration.
o The Internet is typically around 5 ASs deep with the largest
AS_PATH being 16-20 ASNs. Some have added 100 or more AS Path
Prepends and operators should therefore consider limiting the
maximum AS-path length being accepted through aggressive filter
policies.
6. IANA Considerations
7. Security Considerations
Long prepending may make a network more vulernable to route hijacking
which will exist whenever there is a well connected peer that is
willing to forge their AS_PATH or allows announcements with a
fabricated AS path.
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8. Acknowledgement
The authors would like to thank Greg Skinner, Randy Bush, Dave
Farmer, Nick Hilliard, Martijn Schmidt, Michael Still, Geoff Huston
and Jeffrey Haas for contributing to this document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC5398] Huston, G., "Autonomous System (AS) Number Reservation for
Documentation Use", RFC 5398, DOI 10.17487/RFC5398,
December 2008, <https://www.rfc-editor.org/info/rfc5398>.
[RFC5737] Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
Reserved for Documentation", RFC 5737,
DOI 10.17487/RFC5737, January 2010,
<https://www.rfc-editor.org/info/rfc5737>.
[RFC8195] Snijders, J., Heasley, J., and M. Schmidt, "Use of BGP
Large Communities", RFC 8195, DOI 10.17487/RFC8195, June
2017, <https://www.rfc-editor.org/info/rfc8195>.
9.2. URIs
[1] https://labs.apnic.net/?p=1264
[2] https://blogs.oracle.com/internetintelligence/excessive-as-path-
prepending-is-a-self-inflicted-vulnerability
Authors' Addresses
Mike McBride
Futurewei
Email: michael.mcbride@futurewei.com
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Doug Madory
Kentik
Email: dmadory@kentik.com
Jeff Tantsura
Microsoft
Email: jefftant.ietf@gmail.com
Robert Raszuk
NTT Network Innovations
940 Stewart Dr
Sunnyvale, CA 94085
USA
Email: robert@raszuk.net
Hongwei Li
HPE
Email: flycoolman@gmail.com
Jakob Heitz
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: jheitz@cisco.com
Gyan Mishra
Verizon Inc.
Email: gyan.s.mishra@verizon.com
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