1. COS 561: Advanced Computer Networks
BGP Policies
Jennifer Rexford
Fall 2016 (TTh 3:00-4:20 in CS 105)
COS 561: Advanced Computer Networks

2. Outline BGP route selection Ensuring BGP stability
(Multi-homed traffic engineering)
Propane paper (Ryan Beckett)

3. BGP Route Selection

4. Border Gateway Protocol
ASes exchange info about who they can reach
IP prefix: block of destination IP addresses
AS path: sequence of ASes along the path
Policies configured by the AS’s operator
Path selection: which of the paths to use?
Path export: which neighbors to tell? 3
“ /24:
path (2,1)”
“ /24:
path (1)” 2 1
data traffic
data traffic

5. Applying Policy to Routes
Import policy
Filter unwanted routes from neighbor
E.g. prefix that your customer doesn’t own
Manipulate attributes to influence path selection
E.g., assign local preference to favored routes
Export policy
Filter routes you don’t want to tell your neighbor
E.g., don’t tell a peer a route learned from other peer
Manipulate attributes to control what they see
E.g., make a path look artificially longer than it is

6. BGP Policy: Influencing Decisions
Open ended programming.
Constrained only by vendor configuration language
Apply Policy =
filter routes &
tweak attributes
Apply Policy =
filter routes &
tweak attributes
Receive
BGP
Updates
Based on
Attribute
Values
Transmit
BGP
Updates
Best
Routes
Apply Import
Policies
Best Route
Selection
Best Route
Table
Apply Export
Policies
Install forwarding
Entries for best
Routes.
IP Forwarding Table

7. BGP Decision Process on a Router
Routing Information Base
Store all BGP routes for each destination prefix
Withdrawal message: remove the route entry
Advertisement message: update the route entry
Selecting the best route
Consider all BGP routes for the prefix
Apply rules for comparing the routes
Select the one best route
Use this route in the forwarding table
Send this route to neighbors

8. BGP Decision Process Highest local preference Shortest AS path
Set by import policies upon receiving advertisement
Shortest AS path
Included in the route advertisement
Lowest origin type
Included in advertisement or reset by import policy
Smallest multiple exit discriminator
Included in the advertisement or reset by import policy
Smallest internal path cost to the next hop
Based on intradomain routing protocol (e.g., OSPF)
Smallest next-hop router id
Final tie-break

9. Hot-Potato Routing dest multiple egress points New York San Francisco9 10
ISP network
this and the next slide explain the problem. explain egress point
link weights determine both intradomain path and selection of egress points
Dallas
Hot-potato routing = route to closest egress point when there is more than one
best BGP route to destination

10. Routing Policies Economics Traffic engineering
Enforce business relationships
Pick routes based on revenue and cost
Get traffic out of the network as early as possible
Traffic engineering
Balance traffic over edge links
Select routes with good end-to-end performance
Security and scalability
Filter routes that seem erroneous
Prevent the delivery of unwanted traffic
Limit the dissemination of small address blocks

11. BGP Stability Without Global Coordination

12. Customer-Provider Relationship
Customer pays provider for Internet access
Provider exports customer’s routes to everybody
Customer exports only to downstream customers
Traffic to the customer
Traffic from the customer
advertisements d
provider
traffic
provider
customer d
customer

13. Peer-Peer Relationship
Peers exchange traffic between customers
AS exports only customer routes to a peer
AS exports a peer’s routes only to its customers
Traffic to/from the peer and its customers
advertisements
peer
peer
traffic d

14. Three Restrictions on Policies
Route export
Do not export a route learned from one peers or provider, to another
Route selection
Prefer a route learned from a customer over a route learned from a peer or provider
AS graph
No cycle of provider-customer relationships
Together: guarantee convergence to unique, stable route assignment

15. Valid and Invalid Paths
Valid paths: “6 4 3 d” and “8 5 d”
Invalid paths: “6 5 d” and “1 4 3 d”
Valid paths: “1 2 d” and “7 d”
Invalid path: “5 8 d” 1 2 3 4 d 5 6
Provider-Customer
Peer-Peer 7 8

16. Solving the Convergence Problem
Result
Safety: guaranteed convergence to unique stable solution
Inherent safety: holds under failures and policy changes
Definitions
System state: current best route at each AS
Activating AS: re-do decision based on neighbor choices
Sketch of (constructive) proof
Find an activation sequence that leads to a stable state
Any “fair” sequence (eventually) includes this sequence

17. Rough Sketch of the Proof
Two phases
Walking up the customer-provider hierarchy
Walking down the provider-customer hierarchy 1 2 3 4 d 5 6
Provider-Customer
Peer-Peer 7 8

18. System is stable because ASes act like this
Two Interpretations
System is stable because ASes act like this
High-level argument
Export and topology assumptions are reasonable
Path selection rule matches with financial incentives
Empirical results
BGP routes for popular prefixes stable for ~10 days
Most instability from a few flapping destinations
ASes should follow rules for system stability
Encourage operators to obey these guidelines
… and provide ways to verify the configuration
Need to consider more complex relationships

19. Multi-Homing

20. Why Connect to Multiple Providers?
Reliability
Reduced fate sharing
Survive ISP failure
Performance
Multiple paths
Select the best
Financial
Leverage through competition
Game 95th-percentile billing model
Provider 1
Provider 2

21. Outbound Traffic: Pick a BGP Route
Easier to control than inbound traffic
IP routing is destination based
Sender determines where the packets go
Control only by selecting the next hop
Border router can pick the next-hop AS
Cannot control selection of the entire path
Provider 1
Provider 2
“(1, 3, 4)”
“(2, 7, 8, 4)”

22. Outbound Traffic: Shortest AS Path
No import policy on border router
Pick route with shortest AS path
Arbitrary tie break (e.g., router-id)
Performance?
Shortest path is not necessarily best
Propagation delay or congestion
Load balancing?
Could lead to uneven split in traffic
E.g., one provider with shorter paths
E.g., too many ties with a skewed tie-break d s

23. Outbound Traffic: Primary and Backup
Single policy for all prefixes
High local-pref for session to primary provider
Low load-pref for session to backup provider
Outcome of BGP decision process
Choose the primary provider whenever possible
Use the backup provider when necessary
But…
What if you want to balance traffic load?
What if you want to select better paths?

24. Outbound Traffic: Load Balancing
Selectively use each provider
Assign local-pref across destination prefixes
Change the local-pref assignments over time
Useful inputs to load balancing
End-to-end path performance data
E.g., active measurements along each path
Outbound traffic statistics per destination prefix
E.g., packet monitors or router-level support
Link capacity to each provider
Billing model of each provider

25. Outbound Traffic: What Kind of Probing?
Lots of options
HTTP transfer
UDP traffic
TCP traffic
Traceroute
Ping
Pros and cons for each
Accuracy
Overhead
Dropped by routers
Sets off intrusion detection systems
How to monitor the “paths not taken”?

26. Outbound Traffic: How Often to Change?
Stub ASes have no BGP customers
So, routing changes do not trigger BGP updates
TCP flows that switch paths
Out-of-order packets during transition
Change in round-trip-time (RTT)
Impact on the providers
Uncertainty in the offered load
Interaction with their own traffic engineering?
Impact on other end users
Good: move traffic off of congested paths
Bad: potential oscillation as other stub ASes adapt?

27. Propane Paper
Ryan Beckett