Lecture 4: BGP

1. Lecture 4: BGP • Presentations • Lab information • H/W update

2. Inter-Domain routing • At the beginning: the Internet was a single network • Funded by the US Government, ARPAnet • Started in 1969 and lasted until 1985 • As its size grew things became unworkable (see RFC-827) • Time to introduce hierarchy: • All the routers of the same organization belong to the same Administrative System (AS) • Routing is between ASes now

3. Administrative Systems (AS) • Single organization • Same routing policy • Has a unique id – AS Number (ASN) • 16 bits right now • Valid ids between 1-64511 • 64 bits are coming • Right now 22,500 are visible in the Internet

4. Exterior Routing or Inter-domain routing • Certain routers in an AS talk to routers in other ASes and exchange routing information • Then they introduce this information inside their AS • EXAMPLE

5. So every Inter-domain protocol has to do the following • Establish the adjacency with the neighbor • Monitor the status of this connection • Exchange routing information • Some similarities with intra-domain routing protocols but: • Can not do flooding now, network is too big • Routing information is different now • Reachability information • I can reach network a.b/16 through next-hop nh • EXAMPLE

6. What is the cost now? • It is the cost of transiting an AS network • What does it mean? • Can not compare costs from two different AS • Cost may have to do more with business than network engineering • Sending traffic to a given route may cost me more

7. Policy Routing • By controlling what I export I control what traffic I carry • I can have backdoor links for example • By controlling what I import I control where my traffic goes • All these are based on business reasons and do not have to do much with routing itself • I have a contract to provide or buy service with a customer or by a provider • Configured manually on each border router as a list of import and export policies • Can be long, tedious and error-prone

8. A Naïve Approach: EGP • For NSFnet: • Assume 2 level hierarchy with a backbone/stub networks • No problem with loops • Periodic exchange of reachability information • Works fine as long as the routes are not too many • Advertise a single integer cost with each reachable destination • But what do I do with it?

9. Internet Evolution • Single network until 1982-84 • ARPAnet • NSFnet • 2 level hierarchy • Internet gradually becomes private around 1985 • Many independent operators • Complex hierarchy • Address exhaustion and CIDR in 1992 • Route table size explosion

10. Classless Inter-Domain Routing (CIDR) • We saw that the / prefix len can be arbitrary • Well it was not like that all the time • Class A, B, C addresses and a large waste of addresses • I need 1000 IP addresses I get a a.b/16 and I waste the other 15K addresses! • In 1991 class B was in danger of being exhausted (expected around March 1994) • A variable prefix length allows more accurate allocation of addresses and reduces the address waste • Instead of a a.b/16 get a.b.c/24, a.b.d/24, a.b.e/24, a.b.f/24 and I do not waste any address • BUT routers need to know three networks now, LARGER routing tables

11. Hierarchical Address allocation • This is why the second component of CIDR is the hierarchical address allocation, routers still know only a.b/16 • IP addresses are allocated by Internet Assigned Numbers Authority (IANA) • and given to Regional Internet Registries (RIRs) • 5 for each major region of the world • They assign from the address allocation to other entities in the region

12. Protocols adapt to Internet evolution • EGP (1984) • When things were simple • BGP-1, BGP-2, BGP-3 between 1989-1994 • And BGP-4 at 1995 • To include CIDR and arbitrary hierarchies • BGP-4 still there with extensions • Multi-protocol • To handle new protocols, IPv6 mostly, multicast, VPNs • RR • Better scalability • Communities • Better management

13. Internet evolved to… • Something very large and complex • Structure is definitely not an acyclic graph • Dual homing, peering etc… • I can have routing loops • Reachable destinations are MANY • Around 90K unique prefixes these days • Many more if we count multiple routes to a prefix • Hard to exchange them periodically • Each AS has its own internal policies and notion of cost • It is not possible to compare between different ASes

14. How to deal with arbitrary AS topologies? • BGP-4 • Path vector, CIDR, policies • Path vector • I list all the ASes in the path • Loop avoidance is trivial: • make sure that I am not listed in the path • Add myself in the path when I advertise a prefix • Of course there is no free lunch: • Route advertisements are getting large… • It really depends on the topology of the internet • Some attempt to measure is at RFC 1774

15. Paths • BGP manages paths • Path consists of • Network Layer Reachability Information (NLRI) e.g 12.50.45/24 • A sequence of PATH attributes that give info related to this destination • PATH attributes • Each have a Flags field • Optional or well known (well known must be supported by all routers) • Transitive or local (Transitive gets propagated, local not) • Partial or not (partial applies only to part of the path)

16. Important path attributes • ORIGIN (well known) • Is this path learned from IGP, BGP or other • AS_PATH • The list of ASes (well known) • NEXT_HOP • Next hop to reach the prefix (well known) • MULTI_EXIT_DISC (MED) • Helps selection of paths (local, optional) • LOCAL_PREF • Helps selection of paths (well known)

17. BGP next hop • EXAMPLE • Can be third party • IGP knows how to reach the next hop • Recursive route lookup • Can use the best route to reach the next hop • Next hop usually is the loopback address • Never goes down

18. Internal BGP • An AS will have multiple border routers talking to different peers • May learn multiple routes for the same prefix • How do I choose which one to use? • Border routers must make a consistent decision • Else I may have routing loops • All border routers in my AS talk to each other • Internal BGP or iBGP • Over multiple IGP hops, not directly connected • Must be a full-mesh

19. MED and Local Pref • EXAMPLE • Local pref has effect on outgoing traffic • MED on incoming traffic