CS641 Advanced Computer Networks Lecture 04 Bhaskaran Raman - PowerPoint PPT Presentation

CS641 Advanced Computer Networks Lecture 04 Bhaskaran Raman Department of CSE, IIT Bombay http://www.cse.iitb.ac.in/~br/ http://www.cse.iitb.ac.in/synerg/doku.php?id=public:courses:cs641-autumn10:start

Outline for Today ● Recall: routing mechanisms – DV, LS, Path-Vector, Hierarchical routing ● Border Gateway Protocol (BGP) basics

Routing (recall) ● Routing protocols – Link-State – Distance-Vector – Path-Vector ● Hierarchical routing ● Address and network mask

Border Gateway Protocol (BGP) ● Autonomous System (AS): unit of routing on the Internet ● BGP is the routing protocol in the AS graph – Path-vector based protocol – Data path is reverse of control path – Allows for policy-based routing ● Each AS has border routers which speak BGP ● BGP is implemented on top of TCP!

BGP Messages ● OPEN – For initialization, exchange of AS numbers ● UPDATE – Main routing messages – Route announcements and withdrawals ● KEEP-ALIVE – Periodic heart-beat to keep BGP session alive ● NOTIFICATION – Before closing a connection

BGP Path Attributes ● AS-PATH – To implement the path vector protocol ● NEXT-HOP – IP address of the external next-hop for this route ● ORIGIN – How the route to the prefix was learnt – Three possibilities: IGP, EGP, Redistributed

Path Attributes (continued) ● MULTI-EXIT-DISCRIMINATOR (MED) ● AS2 announces about AS3 to AS1 – With MED=10 on Link A, MED=50 on Link B Link AS3 A AS1 AS2 AS4 Link B

Path Attributes (continued) ● LOCAL-PREF ● AS4 may prefer route via AS3 138.39.0.0/16 AS1 AS2 AS3 AS4

Some Remarks ● I-BGP versus E-BGP – I-BGP used between routers of the same AS – This is NOT the same as IGP ● Route selection: order of rules – LOCAL-PREF – AS-PATH – MED – IGP origin preferred over EGP origin – IGP cost

More Remarks ● BGP not guaranteed to converge! – Has poor convergence properties in practice – Can take up to a few min. to recover from failure – Overlay networks (later in course) ● About 100K BGP routers – Largest distributed system to date – A router may have as many as 250,000 entries! ● http://bgp.potaroo.net/ – Its a surprise that it works!

Upcoming Topics ● ISPs, peering ● Border Gateway Protocol (BGP) – Policy control, convergence, and other issues ● Note: upcoming designated reading – [LMJ97] C. Labovitz, G. R. Malan, and F. Jahanian, "Internet Routing Instability", Proceedings of SIGCOMM'97, September 1997. ● IGP convergence