An Analysis of BGP Multiple Origin AS (MOAS) Conflicts Xiaoliang Zhao, NCSU S. Felix Wu, UC Davis Allison Mankin, Dan Massey, USC/ISI Dan Pei, Lan Wang, Lixia Zhang, UCLA IMW2001, November 1, 2001 Report another observed issue in BGP operation. Team work, Xiaoliang & Dan here addresses and AS numbers used in this presentation for illustration purpose

Outline Introduction of BGP Multiple Origin AS (MOAS) conflicts analysis Summary and recent work 2/22/2019 IMW2001 - San Francisco

Border Gateway Protocol 4 (BGP-4) To exchange inter-domain routing information Defined in RFC 1771, deployed since 1995 to support CIDR Path Vector Routing Protocol Includes the path information to the destination Loop detection Eliminates count-to-infinity problem, but still converge slowly [Labovitz97] More flexibility for local policy design 2/22/2019 IMW2001 - San Francisco

BGP operational environment Autonomous System (AS): a set of routers under a single technical administration e.g., AS4: ISI, AS3561: Cable & Wireless, etc. Each AS, the originator, advertises its own networks to its neighboring ASs, the neighboring ASs will propagate those advertisements to the rest of the Internet “I tell you, you tell your friends, and so on” A BGP route lists a prefix (destination) and the path of ASs to reach that prefix e.g., R=(p, <AS1, AS2, AS3>), and AS3 is the origin AS for the prefix p, AS2 provides the transit service for p. 2/22/2019 IMW2001 - San Francisco

BGP route updates and MOAS conflicts 128.9.0.0/16 nets AS 4 AS 226 128.9.0.0/16 Path: 226 128.9.0.0/16 Path: 4 MOAS conflict ! AS4 announcement goes away from time to time AS X AS Z AS Y 128.9.0.0/16 Path: Z, 226 128.9.0.0/16 Path: X, 4 2/22/2019 IMW2001 - San Francisco

Motivation It is recommended [RFC 1930] that each prefix should be originated by a single AS with a few possible exceptions However recommendation not followed in practice We want to answer the question that “what are the reasons for MOAS conflicts and what are the impacts?” Data talks... 2/22/2019 IMW2001 - San Francisco

Measurement Data Collection Data collected from the Oregon Route Views Peers with >50 routers from >40 different ASes. Our analysis uses data [11/08/9707/18/01] (1279 days total) At a randomly selected moment, The Route Views server observed 1364 MOAS conflicts The views from 3 individual ISPs showed 30, 12 and 228 MOAS conflicts More than 38000 MOAS conflicts observed during this time period. 2/22/2019 IMW2001 - San Francisco

Example MOAS Data Conflict# prefix start date end date days origin ASs 7 12.0.0.0/8 01/28/98 02/01/98 5 7018+1757 02/03/98 04/14/98 68 7018+1757 04/16/98 04/26/98 11 7018+1757 05/12/98 05/12/98 1 7018+1290 total lifetime for conflict #7 = 85 days ... 234 128.9.0.0/16 09/25/98 10/09/98 15 226+4 12/01/98 02/04/99 63 226+4 02/06/99 04/26/99 78 226+4 04/28/99 08/04/99 94 226+4 08/07/99 09/01/00 352 226+4 09/03/00 11/13/00 68 226+4 11/15/00 11/21/00 7 226+4 11/23/00 11/30/00 8 226+4 12/02/00 12/12/00 11 226+4 12/14/00 12/26/00 13 226+4 12/28/00 07/15/01 190 226+4 07/17/01 - 2 226+4 total lifetime for conflict #234 = 901 days (total 38225 MOAS conflicts) 2/22/2019 IMW2001 - San Francisco

MOAS Conflicts Do Exist Max: 11842 (11357 from a single AS) Max: 10226 (9177 from a single AS) For 04/07/1998, there are 11357 MOAS conflicts out of 12593 prefixes announced by AS 8584 (90.19%) For 04/07/2001, there are 9177 MOAS conflicts outof 9180 prefixes announced by AS 15412 (99.97%) 2/22/2019 IMW2001 - San Francisco

Histogram of MOAS Conflict Lifetime # of MOAS conflicts Total # of days a prefix experienced MOAS conflict 2/22/2019 IMW2001 - San Francisco

Distribution of MOAS Conflicts over Prefix Lengths ratio of # MOAS entries over total routing entries for the same prefix length 2/22/2019 IMW2001 - San Francisco

Classification of MOAS conflicts PSI.net event Classified into three categories: OrginTranAS: xn=yj (j<m) SplitView: xi=yj (i<n, j<m) DistinctPaths: xiyj (1 i  n, 1 j  m) Given a MOAS conflict for prefix p and two associated AS paths: asp1=(x1,x2,…xn) and asp2=(y1,y2,…ym) 2/22/2019 IMW2001 - San Francisco

Valid Causes of MOAS Conflicts (1) Exchange point addresses E.g.: 198.32.136.0/24 was originated by ASes 2914, 3561, 4006, 6079, 6453, 6461 and 7018. Few instances: 30 out of 38225 are identified as EP addresses Lifetime: 1226 days out of 1279 days for 198.32.138.0/24 AS sets typically only 12 prefixes out of 100K prefixes end with AS sets, and these AS sets were consistent with others Anycast addresses 2/22/2019 IMW2001 - San Francisco

Valid Causes of MOAS Conflicts (2) Multi-homing without BGP Private AS number Substitution 128.9/16 Path: 226 128.9/16 Path: 11422,4 131.179/16 Path: X 131.179/16 Path:Y AS 226 AS Y AS X AS 11422 131.179/16 Path: 64512 Static route or IGP route 128.9/16 Path: 4 AS 64512 AS 4 128.9/16 131.179/16 2/22/2019 IMW2001 - San Francisco

Invalid Causes of MOAS Conflicts Operational faults led to large spikes of MOAS conflicts 04/07/1998: one AS originated 12593 prefixes, out of which 11357 were MOAS conflicts 04/10/2001: another AS originated 9180 prefixes, out of which 9177 were MOAS conflicts There are many smaller scale examples of falsely originated routes Errors Intentional traffic hijacking 2/22/2019 IMW2001 - San Francisco

For more info about FNIISC project: Summary MOAS conflicts exist today Some due to operational need; some due to faults Blind acceptance of MOAS could be dangerous An open door for traffic hijacking A solution for determining MOAS validity is under development For more info about FNIISC project: http://fniisc.nge.isi.edu 2/22/2019 IMW2001 - San Francisco

Recent Work: MOAS Solutions Proposal 1: using BGP community attribute Proposal 2: DNS-based solution Solutions presented to NANOG 23 2/22/2019 IMW2001 - San Francisco

BGP-Based Solution Define a new community attribute Listing all the ASes allowed to originate a prefix Attach this MOAS community-attribute to BGP route announcement Enable BGP routers to detect faults and attacks At least in most cases, we hope! 2/22/2019 IMW2001 - San Francisco

Comm. Attribute Implementation Example 18/8, PATH<58>, MOAS{58,59} 18/8, PATH<59>, MOAS{58,59} AS58 18.0.0.0/8 AS52 18/8, PATH<4>, MOAS{4,58,59} 18/8, PATH<52>, MOAS{52, 58} AS59 Example configuration: router bgp 59 neighbor 1.2.3.4 remote-as 52 neighbor 1.2.3.4 send-community neighbor 1.2.3.4 route-map setcommunity out route-map setcommunity match ip address 18.0.0.0/8 set community 59:MOAS 58:MOAS additive 2/22/2019 IMW2001 - San Francisco

Another Proposal: DNS-based Solution Put the MOAS list in a new DNS Resource Record ftp://psg.com/pub/dnsind/draft-bates-bgp4-nlri-orig-verif-00.txt by Bates, Li, Rekhter, Bush, 1998 MOAS detected for 18/8, query DNS to verify Enhanced DNS service Query 18.bgp.in-addr.arpa: origin AS? Response 18.bgp.in-addr.arpa AS 58 8 AS 59 8 $ORIGIN 18.bpg.in-addr.arpa. ... AS 58 8 AS 59 8 Example configuration (zone file for 18.bgp.in-addr.arpa): 2/22/2019 IMW2001 - San Francisco