1. Interdomain routing V. Arun
College of Information and Computer Sciences
University of Massachusetts Amherst

2. Roadmap Hierarchical routing and BGP overview
Causes of delayed convergence in BGP
Stable Paths Problem and Simple Path Vector Protocol
Gao-Rexford conditions
Causes of inconsistency in BGP

3. Hierarchical routing
aggregate routers into regions, “autonomous systems” (AS)
routers in same AS run same routing protocol
“intra-AS” routing protocol
routers in different AS can run different intra-AS routing protocol
gateway router:
at “edge” of its own AS
has link to router in another AS
Network Layer

4. Interconnected ASes 3c 3a 2c 3b 2a AS3 2b 1c 1a 1b AS1 1d AS2
Intra-AS
Routing
algorithm
Inter-AS
Forwarding
table 3c
Network Layer

5. Inter-AS tasks
suppose router in AS1 receives datagram destined outside of AS1:
router should forward packet to gateway router, but which one?
AS1 must:
learn which dests are reachable through AS2, which through AS3
propagate this reachability info to all routers in AS1
job of inter-AS routing! 3c 3a 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2
Network Layer

6. Example: setting forwarding table in router 1d
suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2
inter-AS protocol propagates reachability info to all internal routers
router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c
installs forwarding table entry (x,I) … x 3c 3a 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2
Network Layer

7. Example: choosing among multiple ASes
now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x
this is also job of inter-AS routing protocol! … x 3c …… 3a 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2 ?
Network Layer

8. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto inter-domain routing protocol
“glue that holds the Internet together”
BGP provides each AS a means to:
eBGP: obtain subnet reachability information from neighboring ASs.
iBGP: propagate reachability information to all AS-internal routers.
determine “good” routes to other networks based on reachability information and policy.
allows subnet to advertise its existence to rest of Internet: “I am here”
Network Layer

9. BGP basics
BGP session: two BGP routers (“peers”) exchange BGP messages:
advertises paths to destination network prefixes (“path vector” protocol)
exchanged over semi-permanent TCP connections
when AS3 advertises a prefix to AS1:
AS3 promises it will forward datagrams towards that prefix
AS3 can aggregate prefixes in its advertisement 3c 3a
BGP
message 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2
Network Layer

10. BGP basics: distributing path information
using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.
1c can then use iBGP to distribute prefix info to all routers in AS1
1b can then re-advertise reachability info to AS2 over 1b-to-2a eBGP session
when router learns of new prefix, it creates entry for prefix in its forwarding table.
route propagation
eBGP session 3a 3b
iBGP session 2c
AS3
other
networks 1c 2a 2b
other
networks 1a 1b
AS2 1d
AS1
Network Layer

11. Path attributes and BGP routes
advertised prefix includes BGP attributes
prefix + attributes = “route”
two important attributes:
AS-PATH: contains ASs through which prefix advertisement has passed: e.g., [AS 67, AS 17, AS 24]
NEXT-HOP: indicates specific internal-AS router to next-hop AS. (multiple links may exist from self to next-hop-AS)
policy-based routing: gateway router receiving route advertisement uses import policy to select/reject route and export policy to re-advertise route
e.g., select cheaper route; or never route through AS x; or never advertise routes to AS y.
Network Layer

12. BGP route selection (import policy)
router may learn about more than 1 route to destination AS, selects route based on:
local preference value attribute: policy decision
shortest AS-PATH
Multi-exit discriminator (MED)
eBGP > iBGP
closest NEXT-HOP router: hot potato routing
router ID
Network Layer

13. BGP re-announce (export policy)
Routers commonly use “valley-free” routing export policy
Never advertise peer or provider routes to another peer or provider.
Network Layer

14. BGP routing policy A,B,C are provider networksX Y
legend:
customer
network:
provider
network
A,B,C are provider networks
X,W,Y are customer (of provider networks)
X is dual-homed: attached to two networks
X does not want to route from B via X to C
.. so X will not advertise to B a route to C
Network Layer

15. BGP routing policy (2) A advertises path AW to BX Y
legend:
customer
network:
provider
network
A advertises path AW to B
B advertises path BAW to X
Should B advertise path BAW to C?
No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers
B wants to force C to route to w via A
B wants to route only to/from its customers!
Network Layer

16. BGP re-announce (export policy)
Routers commonly use “valley-free” routing export policy
Never advertise peer or provider routes to another peer or provider.
Examples (arrows indicate $ flow or customer  provider relationship, else peering):
Q: Which of the above routes are permitted by “valley free” export policy?
Network Layer

17. Why different Intra-, Inter-AS routing ?
policy:
inter-AS: admin wants control over how its traffic routed, who routes through its net.
intra-AS: single admin, so no policy decisions needed
scale:
hierarchical routing saves table size, reduced update traffic
performance:
intra-AS: can focus on performance
inter-AS: policy may dominate over performance
Network Layer

18. Min Route Advertisement Interval (MRAI)
Delayed BGP convergence
Min Route Advertisement Interval (MRAI)

19. Delayed BGP convergence (1)
Labovitz et al. fault injection measurement study: Tlong and Tdown take longer to converge, i.e., bad news propagates slower

20. Delayed BGP convergence (2)
Tdown events across 5 ISPs from Labovitz et al. study: minutes of convergence delay

21. BGP convergence delay: Why?
Q: What explains such high convergence delays on the order of several minutes in BGP?
Count-to-infinity problems? No because BGP is a path vector, not distance vector, protocol, so loops easy to detect
Policy routing? What is special about policy routing compared to shortest path routing?

22. BGP bouncing problem
Assumptions: shorter paths preferred over longer paths. No particular preference between equal length paths.

23. Key problem: path exploration
In an n-node clique with no export policy restrictions, there exist Omega((n-1)!) possible paths
(n-1)! paths of length n-1 + C(n-1,n-2).(n-2)! paths of length n-2 + … + C(n-1,2).2! paths of length 2 + (n-1) paths of length 1
Message ordering matters
Not every sequence of events results in exploring all or most of the Omega((n-1)!) paths
Need a way to limit spurious path propagation

24. Min Route Advertisement Interval timer
Key idea: wait long enough to see the effect of a change from all neighbors before reacting again
Clique example
Forces nodes to explore strictly increasing length paths
Convergence in O(n) rounds
Message complexity?

25. BGP bouncing with MRAI timers

26. Route Flap Damping (RFD)
Delayed BGP convergence
Route Flap Damping (RFD)

27. Route flap damping
MRAI timers can limit spurious path exploration in response to a common root cause, but are not designed to limit the number of root cause events
Route flap damping (RFD) intended to suppress persistent route flux due to say flaky routers
Key idea: Limit the frequency of route changes to a prefix, no matter the reason

28. RFD penalty function example

29. RFD: Things get murkier!
RFD can worsen convergence delay. Suppose node 1 in this 5-node clique flaps its route to d just twice, once withdrawing and then re-advertising.

30. RFD: Things get murkier!

31. Selective RFD scheme
Key idea: Do not apply RFD to secondary flaps, i.e., when announced routes are increasingly more preferred or increasingly less preferred but apply RFD when the preference direction changes, e.g., a node announces a more preferred route compared to the previous route followed by a less preferred route.
Secondary flaps are generally monotonically changing (increasing or decreasing) in preference

32. Stable Paths Problem & Simple Path Vector Protocol
Formal model of BGP
Stable Paths Problem & Simple Path Vector Protocol

33. Stable Paths Problem A simple formal model specifying
Single-node AS graph and single prefix
Strict path ranking functions at nodes
Adoption of highest-ranked peer route
Stability: when no node node wishes to change route
Can we determine if a stable configuration exists?
Do all ranking functions (or import policies) result in a stable configuration?

34. Examples: stable, shortest path

35. Examples: stable, non-shortest path

36. Examples: unsafe unstable non-shortest path
Unsafe: can persistently oscillate
Unstable: will persistently oscillate

37. Examples: stable, non-unique solution

38. Theoretical results
Solvability: The problem of determining whether an instance of SPP has a solution is NP-complete.
Sufficient condition: If an SPP instance has no dispute wheel, it is solvable and has a unique solution
Converse is not true, i.e., dispute wheel does not preclude stability, safety, or uniqueness of solution

39. Dispute wheel
Key idea: a cyclic in route preferences amongst a set of nodes, e.g., A prefers to route through B, B prefers to route through C, and C prefers to route through A
Each ui prefers the counter-clockwise neighbor route RiQi+1 over the direct route Qi
Subscripts are mod k, so uk-1 prefers Rk-1Q0

40. Simple Path Vector Protocol (SPVP)
A simple model of BGP that
receives into RIB most recent route announced by a neighbor
if FIB entry is not the best route available in RIB
Apply import policy to adopt best RIB route in FIB
Re-advertise adopted route to all neighbors
Fair activation sequence: any sent and queued message will be eventually processed by intended recipient
Convergence result: If an instance of SPP has no dispute wheel, then SPVP is guaranteed to converge.

41. Gao-Rexford Conditions

42. Causes of inconsistency in BGP