draft-ietf-grow-as-path-prepending-01.txt		draft-ietf-grow-as-path-prepending-02.txt
	Network Working Group M. McBride		Network Working Group M. McBride
	Internet-Draft Futurewei		Internet-Draft Futurewei
	Intended status: Best Current Practice D. Madory		Intended status: Best Current Practice D. Madory
	Expires: May 3, 2021 Oracle		Expires: May 5, 2021 Oracle
	J. Tantsura		J. Tantsura
	Apstra		Apstra
	R. Raszuk		R. Raszuk
	Bloomberg LP		Bloomberg LP
	H. Li		H. Li
	HPE		HPE
	October 30, 2020		J. Heitz
			Cisco
			November 1, 2020
	AS Path Prepending		AS Path Prepending
	draft-ietf-grow-as-path-prepending-01		draft-ietf-grow-as-path-prepending-02
	Abstract		Abstract
	AS Path Prepending provides a tool to manipulate the BGP AS_Path		AS Path Prepending provides a tool to manipulate the BGP AS_Path
	attribute through prepending multiple entries of an AS. AS Path		attribute through prepending multiple entries of an AS. AS Path
	Prepending is used to deprioritize a route or alternate path. By		Prepending is used to deprioritize a route or alternate path. By
	prepending the local ASN multiple times, ASs can make advertised AS		prepending the local ASN multiple times, ASs can make advertised AS
	paths appear artificially longer. Excessive AS Path Prepending has		paths appear artificially longer. Excessive AS Path Prepending has
	caused routing issues in the internet. This document provides		caused routing issues in the internet. This document provides
	guidance,to the internet community, with how best to utilize AS Path		guidance,to the internet community, with how best to utilize AS Path
	skipping to change at page 1, line 44 ¶		skipping to change at page 1, line 46 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on May 3, 2021.		This Internet-Draft will expire on May 5, 2021.
	Copyright Notice		Copyright Notice
	Copyright (c) 2020 IETF Trust and the persons identified as the		Copyright (c) 2020 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 28 ¶		skipping to change at page 2, line 28 ¶
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3		1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
	2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3		2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
	3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . 4		3. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . 4
	3.1. Excessive Prepending . . . . . . . . . . . . . . . . . . 4		3.1. Excessive Prepending . . . . . . . . . . . . . . . . . . 4
	3.2. Prepending during a routing leak . . . . . . . . . . . . 5		3.2. Prepending during a routing leak . . . . . . . . . . . . 5
	3.3. Prepending to All . . . . . . . . . . . . . . . . . . . . 6		3.3. Prepending to All . . . . . . . . . . . . . . . . . . . . 5
	3.4. Memory . . . . . . . . . . . . . . . . . . . . . . . . . 6		3.4. Memory . . . . . . . . . . . . . . . . . . . . . . . . . 6
	3.5. Errant announcement . . . . . . . . . . . . . . . . . . . 7		3.5. Errant announcement . . . . . . . . . . . . . . . . . . . 6
	4. Alternatives to AS Path Prepend . . . . . . . . . . . . . . . 7		4. Alternatives to AS Path Prepend . . . . . . . . . . . . . . . 7
	5. Best Practices . . . . . . . . . . . . . . . . . . . . . . . 7		5. Best Practices . . . . . . . . . . . . . . . . . . . . . . . 7
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 9		7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9		8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 9
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
	9.1. Normative References . . . . . . . . . . . . . . . . . . 9		9.1. Normative References . . . . . . . . . . . . . . . . . . 9
	9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9		9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
	1. Introduction		1. Introduction
	The Border Gateway Protocol (BGP) [RFC4271] specifies the AS_PATH		The Border Gateway Protocol (BGP) [RFC4271] specifies the AS_PATH
	attribute which enumerates ASs a route update has traversed. If the		attribute which enumerates ASs a route update has traversed. If the
	UPDATE message is propagated over an external link, then the local AS		UPDATE message is propagated over an external link, then the local AS
	number is prepended to the AS_PATH attribute, and the NEXT_HOP		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		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		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		propagated over an internal link, then the AS_PATH attribute and the
	skipping to change at page 4, line 27 ¶		skipping to change at page 4, line 27 ¶
	two instances of its own AS number when advertising its routes to C,		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		then B will now see a different situation, where the AS Path via D
	represents the shorter path. Through the use of selective prepending		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		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		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		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		way prepending implements action at a distance where the routing
	decisions made by non-adjacent ASs can be influenced by selective AS		decisions made by non-adjacent ASs can be influenced by selective AS
	Path prepending.		Path prepending.
	To illustrate, in August 2020 a large ISP had a network outage that
	affected their customers and other ISPs. One major problem was that
	the ISP wasn't withdrawing BGP routes, the stale routes were
	continuing to be announced as legitimate by the down ISP. This
	caused blackholing of traffic even when customers had backup ISPs.
	What could customers do in this situation? They could change local
	preference to help send traffic to the backup ISP. They could send
	more specifics to the backup ISP. They could also pre-provision the
	use of AS Path Prepend to prepend the same AS amount to both primary
	and backup ISPs before failure. Customers could then, during a
	failure, remove one prepend to the backup ISP to make it more
	preferred over the down ISP. This is one, of several, scenarios
	where using AS Path Prepend can be beneficial.
	3. Problems		3. Problems
	Since it is so commonly used, what is the problem with the excessive		Since it is so commonly used, what is the problem with the excessive
	use of AS Path Prepending? Here are a few examples:		use of AS Path Prepending? Here are a few examples:
	3.1. Excessive Prepending		3.1. Excessive Prepending
	The risk of excessive use of AS Path Prepending can be illustrated		The risk of excessive use of AS Path Prepending can be illustrated
	with real-world examples that have been anonymized using documention		with real-world examples that have been anonymized using documention
	prefixes [RFC5737] and ASs [RFC5398] . Consider the prefix		prefixes [RFC5737] and ASs [RFC5398] . Consider the prefix
	skipping to change at page 6, line 47 ¶		skipping to change at page 6, line 33 ¶
	errors in BGP routing. Consider the prepended AS path below:		errors in BGP routing. Consider the prepended AS path below:
	64496 64501 64501 64510 64510 64501 64510 64501 64501 64510 64510		64496 64501 64501 64510 64510 64501 64510 64501 64501 64510 64510
	64501 64501 64510		64501 64501 64510
	The prepending here involves a mix of two distinct ASNs (64501 and		The prepending here involves a mix of two distinct ASNs (64501 and
	64510) with the last two digits transposed.		64510) with the last two digits transposed.
	3.4. Memory		3.4. Memory
	BGP attribute sets are shared among stored routes, ie, if two stored		Long AS Paths cause an increase in memory usage by BGP speakers. The
	routes have the same attribute sets, the attribute set is stored only		memory usage is not so much a concern in the control plane BGP
	once. AS Paths are shared among attribute sets so that if two stored		implementations, but more so when AS Paths are included in Netflow
	attribute sets have the same AS Path, then the AS Path is stored only		messages. Netflow is processed in the forwarding plane, where memory
	once. Storing them in the control plane is not a big problem.		is more expensive than in the control plane.
	However, AS Paths can be sent in Netflow which is generated in the		A concern about an AS Path longer than 255 is the extra complexity
	forwarding plane. AS Paths are not stored in expensive fast memory		required to process it, because it needs to be encoded in more than
	on the forwarding plane, but still, using memory on the forwarding		one AS_SEQUENCE in the AS_PATH BGP path attribute.
	plane has greater impact than on the control plane. An AS Path
	consists of AS_SEQUENCE (and other elements). An AS_SEQUENCE can
	contain a maximum of 255 ASNs. If the AS Path is longer, then
	multiple AS_SEQUENCE's are required. The code to parse them and
	create them is not often exercised and is a potential for bugs in
	fresh code. The older implementations have these bugs well and truly
	shaken out of them. Some BGP implementations have had memory
	corruption/fragmentation problems with long AS Paths.
	3.5. Errant announcement		3.5. Errant announcement
	There was an Internet-wide outage caused by a single errant routing		There was an Internet-wide outage caused by a single errant routing
	announcement. In this incident, AS64496 announced its one prefix		announcement. In this incident, AS64496 announced its one prefix
	with an extremely long AS path. Someone entered their ASN instead of		with an extremely long AS path. Someone entered their ASN instead of
	the prepend count 64496 modulo 256 = 252 prepends and when a path		the prepend count 64496 modulo 256 = 252 prepends and when a path
	lengths exceeded 255, routers crashed		lengths exceeded 255, routers crashed
	4. Alternatives to AS Path Prepend		4. Alternatives to AS Path Prepend
	skipping to change at page 9, line 14 ¶		skipping to change at page 9, line 8 ¶
	7. Security Considerations		7. Security Considerations
	Long prepending may make a network more vulernable to route hijacking		Long prepending may make a network more vulernable to route hijacking
	which will exist whenever there is a well connected peer that is		which will exist whenever there is a well connected peer that is
	willing to forge their AS_PATH or allows announcements with a		willing to forge their AS_PATH or allows announcements with a
	fabricated AS path.		fabricated AS path.
	8. Acknowledgement		8. Acknowledgement
	The authors would like to thank Greg Skinner, Randy Bush, Dave		The authors would like to thank Greg Skinner, Randy Bush, Dave
	Farmer, Nick Hilliard, Martijn Schmidt, Jakob Heitz, Michael Still,		Farmer, Nick Hilliard, Martijn Schmidt, Michael Still, Geoff Huston
	Geoff Huston and Jeffrey Haas for contributing to this document.		and Jeffrey Haas for contributing to this document.
	9. References		9. References
	9.1. Normative References		9.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	skipping to change at line 440 ¶		skipping to change at page 10, line 28 ¶
	Robert Raszuk		Robert Raszuk
	Bloomberg LP		Bloomberg LP
	Email: robert@raszuk.net		Email: robert@raszuk.net
	Hongwei Li		Hongwei Li
	HPE		HPE
	Email: flycoolman@gmail.com		Email: flycoolman@gmail.com
			Jakob Heitz
			Cisco
			170 West Tasman Drive
			San Jose, CA 95134
			USA
			Email: jheitz@cisco.com
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