TBGP: A more scalable and functional BGP. Paul Francis Jan. 2004

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1 TBGP: A more scalable and functional BGP Paul Francis Jan. 2004

2 BGP: Border Gateway Protocol BGP is the top-level routing protocol in the Internet It holds the Internet together BGP allows routers to tell each other how to reach every IP prefix in the Internet TBGP = Tunneled BGP This is our proposal to alleviate the scaling issues faced by BGP

3 What s wrong with BGP? BGP itself is ok Well lacks traffic engineering features But it is overloaded---operating well beyond its imagined capacity When BGP was designed, there were perhaps a few thousand IP prefixes Now there are 150,000 or so!

4 150,000 prefixes... That s not so many! 150,000 prefixes (and associated forwarding information) can be stored in a few Mbytes of memory Doesn t seem like much what s the big deal?

5 The cost of a prefix Each router keeps a copy of what its neighbor routers told it Routers may have dozens of neighbors Whenever a change occurs, the router must Recalculate its routes (can be a substantial calculation), Inform its neighbors of any changes, Rebuild its forwarding structures, Update all copies of its forwarding table (a highend router may have many such copies)

6 BGP converges slowly Because of all this work, BGP may take many seconds or even minutes to converge This has prompted overlay networks that detect and route around BGP failures! BGP will hold down routes that flap a few times In order to minimize BGP updates

7 Other ramifications of BGP s bloat Multi-homing is limited to organizations with large address spaces Multi-homing is where a network connects to multiple ISPs, usually for redundancy Generally, it is just plain hard to understand what BGP is doing The prefixes for major hosting centers aren t treated differently from prefixes for small companies its all mixed together

8 Route Aggregation Basics Address hierarchy topological hierarchy Internet (other ISPs) Hosts * Site Y Y 20.1.* 20.2.* ISP A ISP B * X Site X X1 X2 Y1 Y

9 Multihoming violates the clean hierarchical structure 20.1.* * Internet (other ISPs) 20.2.* * * Y ISP A ISP B * X1 X2 Y1 Y X *

10 IPv6 solves this by giving hosts multiple addresses * Y Internet (other ISPs) 20.1.* 20.2.* ISP A ISP B * X1 X2 Y1 Y X *

11 IPv6 Multiple addresses approach Actually, first proposed by Francis in 1992 As a way of solving scaling problem and giving hosts control over routes Lots of criticism of this approach It isn t the end sites problem, so they are not motivated to be the solution Rather, end sites will ask ISPs to advertise their prefixes, and ISPs will do it

12 Goals of TBGP First and foremost, shrink the size of BGP routing tables Solution must allow deployment by ISPs, not sites Solution should reuse existing BGP to the extent possible Second, improve traffic engineering For both ISPs and sites

13 Structure of the Internet Transit ISPs (200x) POP Access ISPs (3000x) Transit Access Peer Peer Transit Access Routers Customer Sites (100s of thousands)

14 Internet paths tend to have an up-across-down structure ACROSS DOWN UP

15 Routing UP or DOWN requires very little routing information 200x DOWN UP 3000x 10?? x

16 Most routing information is required for routing ACROSS 200x 3000x 10?? x

17 TBGP idea Routing ACROSS is hard because BGP must compute routes to all ISPs and all multihomed sites across the entire Internet TBGP idea: Break the ACROSS path into two separate steps: BGP only computes how to get to all POPs A Mapping Table tells which POP to use for which prefixes

18 BGP TBGP BGP Table Prefix NH Pr1 R1 Pr2 R2 Pr3 R1 Pr4 R3 Pr5 R2 Pr6 R1.. Pr R4 Pr R3 Pr R1 Pr R2 Pr R2 Pr R3 BGP Table POP NH P1 R1 P2 R2.. P1999 R2 P2000 R3 Mapping Table Prefix Pop Pr1 P1 Pr2 P1771 Pr3 P20 Pr4 P158 Pr5 P912 Pr6 P Pr P334 Pr P1388 Pr P141 Pr P672 Pr P838 Pr P1771

19 Why is this an improvement??? BGP computation is very expensive Every router has a slightly different view A global distributed computation is required to generate these views Mapping table distribution is cheap by comparison Every router has the same mapping table With TBGP, BGP computation is much smaller, and the targets are more stable Transit ISP POPs

20 By adding tunnels, we can also shrink the Mapping Table way down Carve IP address space into arbitrary superprefixes (i.e. /10) There is a partial mapping table corresponding to each super-prefix Only contains map entries for prefixes within the super-prefix Assign different partial mapping tables to different routers

21 Each super-prefix has an associated Partial Mapping Table Partial Mapping Table 1 Partial Mapping Table 3 Partial Mapping Table N Partial Mapping Table 2 Partial Mapping Table 3 IP Address Space The true prefixes within a give super prefixes represent sites spread all over the Internet

22 Different routers contain different partial mapping tables Each router advertises the associated super-prefix into BGP Partial Mapping Table 3 Partial Mapping Table 1 Partial Mapping Table 2

23 Path of packet to address in Superprefix 1 ACROSS tunneled to the access POP of the destination PMT1 PMT3 PMT2 Detunnel, and DOWN to the destination UP to router advertising super-prefix 1

24 Packets routed via super-prefixes are not necessarily shortest path Super-prefix path PMT1 PMT3 PMT2 Shortest path

25 Tunnels are very flexible Any router can choose to have any mapping table entries it wants Not just those of its super-prefix Because tunnel allows packets to pass through routers that don t have the mapping Routers can hold mapping table entries for destinations the receive a lot of traffic Thus getting shortest path for the most popular destinations

26 Path length versus total routing table size

27 Many other issues Authentication of mapping table entries Appears straight-forward Cost of tunneling and detunneling Detunneling today is expense (twice the cost of not detunneling), but it could be cheap DoS threat created by tunneling Attacker uses tunnels to spread an attack out Improved traffic engineering Can exploit mapping table

28 Status Design is largely worked out Completed simulation study to determine table size versus path length Preparing Sigcomm submission Next steps: Implement and run on NLR next generation Internet architecture testbed???

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