Keep your Communities Clean: Exploring the Routing Message Impact of BGP Communities Thomas Krenc Robert Beverly Georgios Smaragdakis Naval Postgraduate School Naval Postgraduate School TU Berlin
[email protected] [email protected] [email protected] ABSTRACT BGP communities [10, 26] are one such example of an optional BGP communities are widely used to tag prefix aggregates for policy, attribute that adds new meta-information to routing messages. traffic engineering, and inter-AS signaling. Because individual ASes Communities are used to conveniently tag (an aggregate of) pre- define their own community semantics, many ASes blindly propa- fixes to enable particular actions or policies. However, community gate communities they do not recognize. Prior research has shown values and semantics are not standardized – the meaning of a spe- the potential security vulnerabilities when communities are not cific community value is defined by individual autonomous systems filtered. This work sheds light on a second unintended side-effect of (ASes). Benoit et al. first defined a taxonomy where, broadly speak- communities and permissive propagation: an increase in unneces- ing, communities are used to tag received prefixes to aid an AS’s sary BGP routing messages. Due to its transitive property, a change internal routing decisions, or added to outbound announcements in the community attribute induces update messages throughout as a convenient signaling mechanism [10]. Today, communities established routes, just updating communities. We ground our work are known to encode location identifiers [15, 17] express policy by characterizing the handling of updates with communities, in- preferences upstream [18], enable selective advertisement in In- cluding when filtered, on multiple real-world BGP implementations ternet Exchange Points (IXPs) [19, 34], and as a DDoS mitigation in controlled laboratory experiments.