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