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AS-Path Prepending: there is no rose without a thorn
P. Marcos, L. Prehn, L. Leal, A. Dainotti, A. Feldmann, and M. Barcellos, "AS-Path Prepending: there is no rose without a thorn", in ACM Internet Measurement Conference (IMC), Oct 2020.
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AS-Path Prepending: there is no rose without a thorn

Pedro  Marcos2
Lars Prehn
Lucas Leal3
Alberto Dainotti1
Anja Feldmann
Marinho Barcellos4
1

CAIDA, San Diego Supercomputer Center, University of California San Diego

2

Universidade Federal do Rio Grande (FURG)

3

Universidade Federal do Rio Grande do Sul (UFRGS)

4

University of Waikato

Inbound traffic engineering (ITE)—the process of announcing routes to, e.g., maximize revenue or minimize congestion—is an essential task for Autonomous Systems (ASes). AS Path Prepending (ASPP) is an easy to use and well-known ITE technique that routing manuals show as one of the first alternatives to influence other ASes’ routing decisions. We observe that origin ASes currently prepend more than 25% of all IPv4 prefixes. ASPP consists of inflating the BGP AS path. Since the length of the AS path is the second tie-breaker in the BGP best path selection, ASPP can steer traffic to other routes. Despite being simple and easy to use, the appreciation of ASPP among operators and researchers is diverse. Some have questioned its need, effectiveness, and predictability, as well as voiced security concerns. Motivated by these mixed views, we revisit ASPP. Our longitudinal study shows that ASes widely deploy ASPP, and its utilization has slightly increased despite public statements against it. We surprisingly spot roughly 6k ASes originating at least one prefix with prepends that achieve no ITE goal. With active measurements, we show that ASPP effectiveness as an ITE tool depends on the AS location and the number of available upstreams; that ASPP security implications are practical; identify that more than 18% of the prepended prefixes contain unnecessary prepends that achieve no apparent goal other than amplifying existing routing security risks. We validate our findings in interviews with 20 network operators.

Keywords: AS Path, BGP, routing, security, topology
  Last Modified: Tue Dec-15-2020 18:12:48 UTC
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