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Network Working Group                                         M. McBride
Internet-Draft                                                 Futurewei
Intended status: Best Current Practice                         D. Madory
Expires: January 14, 2021                                         Oracle
                                                             J. Tantsura
                                                           July 13, 2020

                            AS-Path Prepend


   AS_Path prepending provides a tool to manipulate the BGP AS_Path
   attribute through prepending multiple entries of an AS.  AS_Path
   prepend is used to deprioritize a route or alternate path.  By
   prepending the local ASN multiple times, ASes can make advertised AS
   paths appear artificially longer.  Excessive AS_Path prepending has
   caused routing issues in the internet.  This document provides
   guidance,to the internet community, with how best to utilize AS_Path
   prepend 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.

   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 https://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 January 14, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Problems  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Excessive Prepending  . . . . . . . . . . . . . . . . . .   3
     2.2.  Prepending during a routing leak  . . . . . . . . . . . .   3
     2.3.  Route Competition . . . . . . . . . . . . . . . . . . . .   4
     2.4.  Prepending to All . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Memory  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.6.  Errant announcement . . . . . . . . . . . . . . . . . . .   6
   3.  Best Practices  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The Border Gateway Protocol (BGP) [RFC4271] specifies the AS_Path
   attribute which enumerates the ASs that must be traversed to reach
   the networks listed in the BGP UPDATE message.  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.

   A common practice among operators is to prepend multiple entries of
   an AS (known as AS_Path prepend) 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 prepend.

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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Problems

   Since it it so commonly used, what is the problem with the excessive
   use of AS_Path prepend?  Here are a few examples:

2.1.  Excessive Prepending

   The risk of excessive use of AS_Path prepend can be illustrated with
   real-world examples.  Consider the Ukranian prefix (
   which is normally announced with an inordinate amount of prepending.
   A recent analysis revealed that is announced to the
   world along the following AS path:

   3255 197158 197158 197158 197158 197158 197158 197158 197158 197158
   197158 197158 197158 197158 197158 197158 197158 197158 197158 197158
   197158 197158 197158 197158

   In this example, the origin AS197158 appears 23 consecutive times
   before being passed on to a single upstream (AS3255), which passes it
   on to the global internet, prepended-to-all.  An attacker wanting to
   intercept or manipulate traffic to this prefix might enlist a
   datacenter of questionable morals who would allow announcements of
   the same prefix with a fabricated AS path such as 999999 3255 197158.
   Here the fictional AS999999 represents the shady datacenter.  This
   malicious route would be pretty popular 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.

2.2.  Prepending during a routing leak

   In April 2010, China Telecom 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 ASes accepted the leaked routes, most of the impact was
   constrained to Chinese routes.  However, two of the top five most-
   propagated leaked routes (listed in the table below) were US routes.
   Was there some grand conspiracy to intercept traffic destined for

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   these routes?  Actually, it was due to something much more troubling:
   gratuitous AS path prepending.

   During the routing leak, nearly all of the ASes of the internet
   preferred the Chinese leaked routes for and because, at the time, these two US prefixes were being
   announced to the entire internet along the following excessively
   prepended AS path: 3257 7795 12163 12163 12163 12163 12163 12163.
   With this odd configuration, virtually any illegitimate route,
   whether a deliberate hijack or an inadvertent leak, 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 while incurring risk
   of hijacked traffic during a routing leak or hijack.  You'd 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 ASes to choose another origin if one were to suddenly
   appear whether by mistake or otherwise.

2.3.  Route Competition

   So what happens when a non-prepended route competes against an
   excessively prepended route?  Let's consider a real-world example.
   The Polish route is normally announced with the
   origin prepended three times (41952 41952 41952) to three providers
   and prepended twice to a fourth.  Beginning at 15:28:14 UTC on June
   6, a new origin that was not prepended appeared in the routing table
   for this route.  As is illustrated in the graphic below, AS60781
   quickly became the most popular version of this route for the next
   week until it disappeared.

   When both AS41952 and AS60781 were in contention for being considered
   the origin of this prefix, the non-prepended route was dominating as
   we would expect.  In some cases, the impact of prepending isn't as
   straightforward.  Let's take as an example.  This
   prefix is prepended but isn't one of the 60,000 prepended-to-all
   routes mentioned earlier because its prepending is only visible to a
   little more than half of our BGP sources.  In any event, this prefix
   is announced to the internet in two ways: it's prepended to AS6939
   and not prepended to AS174:

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   ...6939 17356 17356 17356 17356 17356 17356 17356 17356 17356 17356

   ...174 17356

   From these two route options, one might reasonably infer that it is
   17356's intention to deprioritize routes to AS6939 by prepending
   itself 10 times on routes to that upstream.  It may seem to follow
   that the non-prepended path to AS174 would be the most popular.
   However, the opposite is true.  Despite extensive prepending, AS6939
   is the more popular choice.  In this case, prepending is going up
   against the local preferences of a legion of ASes: AS6939 has an
   extensive peering base of thousands of ASes.  These ASes opt to send
   traffic for free through their AS6939 peering links instead of having
   to pay to send traffic through a transit provider (and via AS174)
   regardless of the AS path length.  AS17356 could prepend their routes
   to AS6939 100 times (please don't!) and AS6939 would still be the
   more popular provider.  Keep in mind that the average AS diameter of
   the internet is only around 4 hops, so prepending more than a couple
   of times buys you nothing.

2.4.  Prepending to All

   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

   Much of the worst propagation of leaked routes during big leak events
   have been due to routes being prepended-to-all.  AS4671 leak of April
   2014 (>320,000 prefixes) was prepended-to-all.  And the AS4788 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 ASes of the internet.  In this configuration,
   prepending is no longer shaping route propagation but is simply
   incentivizing ASes 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 of below:

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   52981 267429 267429 267492 267492 267429 267429 267492 267492 267429
   267429 267492 267492 267429

   The prepending here involves a mix of two distinct ASNs (267429 and
   267492) with the last two digits transposed.

2.5.  Memory

   Some BGP implementations have had memory corruption/fragmentation
   problems with long AS_PATHS.

2.6.  Errant announcement

   There was an Internet-wide outage caused by a single errant routing
   announcement.  In this incident, AS47868 announced its one prefix
   with an extremely long AS path.  Someone entered their ASN instead of
   the prepend count 47868 modulo 256 = 252 prepends and when a path
   lengths exceeded 255, routers crashed

3.  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 of using AS-Path prepend:

   o  Network operators should ensure prepending is absolutely
      necessary.  Many of your networks have excessive prepending

   o  Prepending more than a couple of times buys you nothing.  So don't
      do it.

   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  It is not typical to see more than 20 ASes in a AS_PATH in the
      Internet today even with the use of AS_Path prepend.  The Internet
      is typically around 5 ASes 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

4.  IANA Considerations

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5.  Security Considerations

   There are no security issues introduced by this draft.

6.  Acknowledgement

7.  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,

   [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,

   [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>.

Authors' Addresses

   Mike McBride

   Email: michael.mcbride@futurewei.com

   Doug Madory

   Email: douglas.madory@oracle.com

   Jeff Tantsura

   Email: jefftant.ietf@gmail.com

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