1. Inter-Domain Routing: BGP, Routing Policies, etc.
BGP Path Selection and Policy Routing
Stable Path Problem and Policy Conflicts
Stable BGP routing: Gao and Jennifer’s path selection guidelines
Readings: Do the required readings
CSci5221: BGP Policies

2. BGP Is Not Guaranteed to Converge!
BGP is not guaranteed to converge to a stable routing. Policy inconsistencies can lead to “livelock” protocol oscillations.
Goal:
Design a simple, tractable and complete model of BGP modeling
Example application: sufficient condition to guarantee convergence.
CSci5221: BGP Policies

3. BGP is Solving What Problem?
Underlying problem
Shortest Paths
Distributed means of
computing a solution. X?
RIP, OSPF, IS-IS
BGP
X can
aid in the design of policy analysis algorithms and heuristics,
aid in the analysis and design of BGP and extensions,
help explain some BGP routing anomalies,
provide a fun way of thinking about the protocol
CSci5221: BGP Policies

4. Abstract Model of BGP A graph of nodes and edges,
Each node represents an AS
Each edge “connectivity” between two ASes
Focus on only one prefix originated from node 0
Node 0 is thus referred as the origin
For each non-zero node v, a set of permitted paths to the origin, Pv
it always contains “null path”.
Ranking function of permitted paths at node v
Null path is always least preferred
CSci5221: BGP Policies

5. Separate Dynamic v Static Semantics
BGP policies  Stable Paths Problem
Dynamic semantics:
BGP  SPVP
SPVP: Simple Path Vector Protocol
A distributed algorithm for solving Stable Paths Problem
CSci5221: BGP Policies

6. Ranking BGP Paths Highest local Preference Shortest AS path Length
Origin: IGP < EGP < INCOMPLETE
-- i.e., prefer the lower ranked origin
Lowest MED value
EBGP is preferred over IBGP
Lowest IGP cost
Tie breaking (e.g. using router id)
CSci5221: BGP Policies

7. Stable Paths Problem 5
2 1 0
2 0
1 3 0
1 0
3 0
4 2 0
4 3 0 3 4 2 1
Ideal “Static” Version of BGP Path Selection
Given set of permissible paths and ranking of paths at each node,
Is there a “global” assignment of paths (Pv at each node v) such that
Pv is permissible
Pv is highest ranked path among paths that are consistent with those advertised/selected by neighbors
If such a path assignment exists, we say SPP has a solution 2
most preferred …
least preferred (not null)
CSci5221: BGP Policies

8. A Solution to SPP Restated
A solution is an assignment of permitted paths to each node such that
node u’s assigned path is either the null path or is a path uwP, where wP is assigned to node w and {u,w} is an edge in the graph,
each node is assigned the highest ranked path among those consistent with the paths assigned to its neighbors
CSci5221: BGP Policies

9. A Solution to SPP 5
2 1 0
2 0
1 3 0
1 0
3 0
4 2 0
4 3 0 3 4 2 1
A solution need not represent a shortest path tree or a spanning tree
CSci5221: BGP Policies

10. Multiple Solutions May Exist1 2
1 2 0
1 0
2 1 0
2 0
First solution
DISAGREE
Second solution
CSci5221: BGP Policies

11. Multiple Solutions Can Occur Due to Recovery:
1 0
1 0 1 1 1 2 3
primary
link
2 3 0
2 1 0
2 3 0
3 1 0 2 2
backup
link
3 0 3 3
3 0
Remove primary link
Restore primary link
CSci5221: BGP Policies

12. Bad Gadget: No Solution
Stage 1:
1: [10]
2: [210]
3: [30]
Stage 2:
1:[130]
2:[20]
3:[320]
Back to stage 1 2 3 1
2 1 0
2 0
1 3 0
1 0
3 2 0
3 0 4
CSci5221: BGP Policies

13. Bad Gadget: No Solution2 3 1
2 1 0
2 0
1 3 0
1 0
3 2 0
3 0 4
Stage 1:
1: [10]
2: [20]
3: [320]
Stage 2:
1:[130]
2:[210]
3:[30]
Back to stage 1
CSci5221: BGP Policies

14. Has A Solution, But Can Get Trapped:1 2
1 2 0
1 0
2 1 0
2 0 3 4 5 6
3 1 0
As with DISAGREE, this part
has two distinct solutions
This part has a solution only
when node 1 is assigned the
direct path (1 0).
CSci5221: BGP Policies

15. Has A Solution, But Can Get Trapped:4
1 2 0
1 0 1
3 1 0 3 6 5 2
2 1 0
2 0
This part has a solution only
when node 1 is assigned the
direct path (1 0).
As with DISAGREE, this part
has two distinct solutions
CSci5221: BGP Policies

16. How To Solve An SPP? Exponential complexity
Just enumerate all path assignments, And check stability of each….
NP-complete
3-SAT can be reduced to SPP
CSci5221: BGP Policies

17. SPVP Protocol SPVP: Simple Path Vector Protocol
Distributed Algorithms to Solve SPP
“dynamic” version of path selection
Pick the best path available at any time
process spvp[u] {
receive P from w 
{ rib-in(uw) := u P
if rib(u) != best(u) {
rib(u) := best(u)
foreach v in peers(u) {
send rib(u) to v }
CSci5221: BGP Policies

18. SPVP and SPP SPVP wanders around assignment space SPP Solvable
SPVP Can Diverge
must diverge
must converge
CSci5221: BGP Policies

19. A sufficient condition for sanity
If an instance of SPP has an acyclic dispute digraph, then
Static (SPP)
solvable
Dynamic (SPVP)
unique solution
safe (can’t diverge)
predictable restoration
all sub-problems
uniquely solvable
robust with respect to link/node failures
See the optional paper [G+99] “Policy Disputed in Path Vector Protocols” by Timothy G. Griffin et al, Section 3.1 (pp.5-6) for a formal and detailed definition of dispute digraph
CSci5221: BGP Policies

20. Dispute Digraph
Given a set of permitted paths and their rankings at each AS, Dispute Digraph (DD) is constructed as follows
each permitted path P is a node in dispute digraph
two types of directed edges (arcs) in dispute digraph
transmission arcs (denoted by a dotted arrow in DD)
for two neighboring ASes u and v, if vP is permitted at v and (u,v)P is permitted at u, then we have a transmission arc vP ……> (u,v)P
dispute arcs (denoted by a solid arrow in DD)
for two neighboring ASes u and v, we have a dispute arc from Q P:=(u,v)R if the following conditions hold: 1) P is permitted at u, 2) Q is permitted at v; 3) (u,v)Q is not permitted at u, or ranking of (u,v)Q is lower than P at u ; 4) R is permitted at v, but is lower ranked than Q at v;
CSci5221: BGP Policies

21. Dispute Digraph Example
2 0
1 0 2 1 4 3
1 3 0
1 0
2 1 0
2 0
4 2 0
4 3 0
3 0
4 2 0
2 1 0
CYCLE
4 3 0
1 3 0
BAD GADGET II
3 0
CSci5221: BGP Policies

22. Dispute Wheels
u_0
At u_i, rank of Q_i is less than or equal to rank of R_iQ_(i+1)
There exists a dispute wheel iff there exists
cycle in the dispute digraph
R_k
R_0
u_k
u_1
R_1
Q_0
u_2
Q_1
Q_k
Q_2
Q_(I+1)
Q_i
u_(i+1)
R_i
u_i
CSci5221: BGP Policies

23. Dispute Wheel Example
1 2 0
1 0
2 3 0
2 0 3 1 1 2 2 1 3 3 2
3 1 0
3 0
CSci5221: BGP Policies

24. Avoiding Divergence: A Dynamic Solution?
Extend SPVP with a history attribute,
A route’s history contains a path in the dispute digraph that “explains” how the route was obtained,
A route history will contain a dispute cycle if and only if a policy dispute is dynamically realized.
If a route’s history contains a cycle, then suppress it ….
CSci5221: BGP Policies

25. How to Ensure No Policy Conflicts
Strawman Proposal: Perform Global Policy Check
Require each AS to publish its policies
Detect and resolve conflicts
Problems:
ASes typically unwilling to reveal policies
Checking for convergence is NP-complete
Failures may still cause oscillations
CSci5221: BGP Policies

26. Think Globally, Act Locally
Key features of a good solution
Safety: guaranteed convergence
Expressiveness: allow diverse policies for each AS
Autonomy: do not require revelation/coordination
Backwards-compatibility: no changes to BGP
Local restrictions on configuration semantics
Ranking
Filtering
CSci5221: BGP Policies

27. Gao and Rexford Scheme Permit only two business arrangements
Gao & Rexford, “Stable Internet Routing without Global Coordination”, IEEE/ACM ToN, 2001
Permit only two business arrangements
Customer-provider
Peering
Constrain both filtering and ranking based on these arrangements to guarantee safety
Surprising result: these arrangements correspond to today’s (common) behavior
CSci5221: BGP Policies

28. Relationship #1: Customer-Provider
Filtering
Routes from customer: to everyone
Routes from provider: only to customers
From the customer
To other destinations
From other destinations
To the customer
providers
customer
advertisements
traffic
CSci5221: BGP Policies

29. Relationship #2: Peering
Filtering
Routes from peer: only to customers
No routes from other peers or providers
advertisements
traffic
customer
peer
CSci5221: BGP Policies

30. Rankings Prefer routes from customers over routes from peers
Prefer routes from peers over routes from providers
provider
peer
customer
CSci5221: BGP Policies

31. Additional Assumption: Consistent AS Hierarchy
Disallowed!
CSci5221: BGP Policies

32. Safety: Proof Sketch System state: the current route at each AS
Activation sequence: revisit some router’s selection based on those of neighboring ASes
CSci5221: BGP Policies

33. Activation Sequence: Intuition
Activation: emulates a message ordering
Activated router has received and processed all messages corresponding to the system state
“Fair” activation: all routers receive and process outstanding messages
CSci5221: BGP Policies

34. Safety: Proof Sketch State: the current route at each AS
Activation sequence: revisit some router’s selection based on those of neighboring ASes
Goal: find an activation sequence that leads to a stable state
Safety: satisfied if that activation sequence is contained within any “fair” activation sequence
CSci5221: BGP Policies

35. Proof, Step 1: Customer Routes
Activate ASes from customer to provider
AS picks a customer route if one exists
Decision of one AS cannot cause an earlier AS to change its mind
An AS picks a customer
route when one exists
CSci5221: BGP Policies

36. Proof, Step 2: Peer & Provider Routes
Activate remaining ASes from provider to customer
Decision of one Step-2 AS cannot cause an earlier Step-2 AS to change its mind
Decision of Step-2 AS cannot affect a Step-1 AS
AS picks a peer or provider
route when no customer
route is available
CSci5221: BGP Policies

37. Ranking and Filtering Interactions
Allowing more flexibility in ranking
Allow same preference for peer and customer routes
Never choose a peer route over a shorter customer route
… at the expense of stricter AS graph assumptions
Hierarchical provider-customer relationship (as before)
No private peering with (direct or indirect) providers
Peering
CSci5221: BGP Policies

38. Some problems Requires acyclic hierarchy (global condition)
Cannot express many business relationships
Abovenet
Verio
PSINet
Sprint
Customer
Hang customers off of above net
Label peering edges
Question: Can we relax the constraints on filtering? What happens to rankings?
CSci5221: BGP Policies