BGP Policy

1
BGP Policy

• Jennifer Rexford

2
Challenges of BGP

• Large distributed system
• More than 20,000 nodes
• Autonomous nodes
• Diverse policy goals
• Trade-off of goals
• Flexible policy
• Convergence speed
• Large scale
• Policies in practice
• Business relationships, traffic engineering,
  scalability, security,

3
Outline

• BGP policy mechanics
• Import and export policies
• Route attributes
• Decision process
• BGP policies in practice
• Business relationships
• Distributing routes inside the AS
• Traffic engineering
• BGP security

4
Components of BGP

• BGP protocol
• Definition of how two BGP neighbors communicate
• Message formats, state machine, route attributes,
  etc.
• Standardized by the IETF
• Policy specification
• Flexible language for filtering and manipulating
  routes
• Indirectly affects the selection of the best
  route
• Varies across vendors, though constructs are
  similar
• BGP decision process
• Complex sequence of rules for selecting the best
  route
• De facto standard applied by router vendors
• Being codified in a new RFC for BGP coming soon

5
Border Gateway Protocol

• ASes exchange reachability information
• IP prefix block of destination addresses
• AS path sequence of ASes along the path
• Policies configured by the network operator
• Path selection which of the paths to use?
• Path export which neighbors to tell?

I can reach 12.34.158.0/24 via AS 1
I can reach 12.34.158.0/24
1
2
3
data traffic
data traffic
12.34.158.5
6
BGP Protocol Update Messages

• Update messages
• Advertisement
• New route for the prefix (e.g., 12.34.158.0/24)
• Attributes such as the AS path (e.g., 2 1)
• Withdrawal
• Announcing that the route is no longer available
• Numerous BGP attributes
• AS path
• Next-hop IP address
• Local preference
• Multiple-Exit Discriminator

7
BGP Policy Influencing Decisions
Open ended programming. Constrain
ed only by vendor configuration language
Apply Policy filter routes tweak attributes
Apply Policy filter routes tweak attributes
Receive BGP Updates
Best Routes
Transmit BGP Updates
Based on Attribute Values
Best Route Selection
Apply Import Policies
Best Route Table
Apply Export Policies
Install forwarding Entries for best Routes.
IP Forwarding Table
8
BGP Decision Process Path Selection on a Router

• Routing Information Base
• Store all BGP routes for each destination prefix
• Withdrawal message remove the route entry
• Announcement 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

9
BGP Decision Process Multiple Steps

• Highest local preference
• 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

10
Import Policy Local Preference

• Favor one path over another
• Override the influence of AS path length
• Apply local policies to prefer a path
• Example prefer customer over peer

Local-pref 90
Sprint
ATT
Local-pref 100
Tier-2
Yale
Tier-3
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Import Policy Filtering

• Discard some route announcements
• Detect configuration mistakes and attacks
• Examples on session to a customer
• Discard route if prefix not owned by the customer
• Discard route with other large ISP in the AS path

ATT
USLEC
Princeton
128.112.0.0/16
12
Export Policy Filtering

• Discard some route announcements
• Limit propagation of routing information
• Examples
• Dont announce routes from one peer to another
• Dont announce routes for management hosts

Sprint
UUNET
ATT
network operator
Princeton
128.112.0.0/16
13
Export Policy Attribute Manipulation

• Modify attributes of the active route
• To influence the way other ASes behave
• Example AS prepending
• Artificially inflate AS path length seen by
  others
• Convince some ASes to send traffic another way

ATT
USLEC
Sprint
88
Princeton
88 88
128.112.0.0/16
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BGP Policy Configuration

• Routing policy languages are vendor-specific
• Not part of the BGP protocol specification
• Different languages for Cisco, Juniper, etc.
• Still, all languages have some key features
• Policy as a list of clauses
• Each clause matches on route attributes
• and discards or modifies the matching routes
• Configuration done by human operators
• Implementing the policies of their AS
• Business relationships, traffic engineering,
  security

15
BGP Policies in Practice
16
Business Relationships

• Common relationships
• Customer-provider
• Peer-peer
• Backup, sibling,
• Implementing in BGP
• Import policy
• Ranking customer routes over peer routes
• Export policy
• Export only customer routes to peers and providers

17
Customer-Provider Relationship

• Customer pays provider for access to Internet
• Provider exports customers routes to everybody
• Customer exports providers routes to customers

Traffic to the customer
Traffic from the customer
d
ATT
ATT
Princeton
d
Princeton
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Peer-Peer Relationship

• Peers exchange traffic between customers
• AS exports only customer routes to a peer
• AS exports a peers routes only to its customers

Traffic to/from the peer and its customers
Sprint
ATT
d
Princeton
UBC
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How Peering Decisions are Made?
Peer
Dont Peer

• Reduces upstream transit costs
• Can increase end-to-end performance
• May be the only way to connect your customers to
  some part of the Internet (Tier 1)

• You would rather have customers
• Peers are usually your competition
• Peering relationships may require periodic
  renegotiation

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Backup Relationship

• Backup provider
• Only used if the primary link fails
• Routes through other paths

USLEC
ATT
Princeton
128.112.0.0/16
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Sibling Relationship

• Two ASes owned by the same institution
• E.g., two ASes that have merged
• E.g., two ASes simply for scaling reasons
• Essentially act as a single AS

CerfNet
ATT
22
Internal BGP
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An AS is Not a Single Node

• Multiple routers in an AS
• Need to distribute BGP information within the AS
• Internal BGP (iBGP) sessions between routers

eBGP
AS1
iBGP
AS2
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Internal BGP and Local Preference

• Example
• Both routers prefer the path through AS 100 on
  the left
• even though the right router learns an external
  path

AS 200
AS 100
AS 300
Local Pref 100
Local Pref 90
I-BGP
AS 256
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Example Customer to Provider
import policies
route selection
export policies
router
A
local pref 100
select UPMC route
send to other iBGP neighbors
B
Wanadoo
A
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Example Peers
import policies
route selection
export policies
router
C
A
B
Wanadoo
Suppose DT, FT, and BT are peers
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Example Customers vs. Peers
import policies
route selection
export policies
router
A
select DT route
send to other iBGP and eBGP neighbors
B
A

• Suppose
• DT is a customer
• of FT and BT
• FT and BT are peers

Wanadoo
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Example Multiple Egress Points
import policies
route selection
export policies
router
A
What will router D choose?
Wanadoo
B
D
C
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Hot-Potato (Early-Exit) Routing
route to UPMC
A
7
B
2
1
2
2
D
1
5
1
C
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Traffic Engineering
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Traffic Engineering Goals

• Load balancing
• Making good use of network resources
• Alleviating network congestion
• End-to-end performance
• Avoiding paths with downstream congestion
• By moving traffic to alternate paths
• Mechanisms
• Preferring some paths over other paths
• E.g., by setting local-preference attribute
• Among routes within the same business class

32
BGP Decision Process in Action
But, what if the path (3,4,1) would be better?
33
Manipulating Policy to Move the Traffic

• Assign local preference to
• Prefer one neighbor over another for a prefix
• Prefer certain AS paths over others
• Router configuration languages
• Specifying rules for setting local-pref attribute
• if path(3, , 1), then local-pref110
• else, local-pref100
• Allow policy to over-ride shortest AS path
• Indirect way of making one path look better or
  worse than another
• Main way to do BGP traffic engineering today

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BGP Security
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Security Goals for BGP

• Secure message exchange between neighbors
• Confidential BGP message exchange
• Can ASes exchange messages w/o someone watching?
• No denial of service
• Prevent overload, session reset, tampered
  messages?
• Validity of the routing information
• Origin authentication
• Is the prefix owned by the AS announcing it?
• AS path authentication
• Is AS path the sequence of ASes the update
  traversed?
• AS path policy
• Does AS path adhere to the routing policies of
  each AS?

36
IP Address Ownership

• IP address block assignment
• Regional Internet Registries (ARIN, RIPE, APNIC)
• Internet Service Providers
• Proper origination of a prefix into BGP
• By the AS who owns the prefix
• or, by its upstream provider(s) in its behalf
• However, whats to stop someone else?
• Prefix hijacking another AS originates the
  prefix
• BGP does not verify that the AS is authorized
• Registries of prefix ownership are inaccurate

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Address Ownership Prefix Hijacking
12.34.0.0/16
12.34.0.0/16

• Consequences for the affected ASes
• Blackhole data traffic is discarded
• Snooping data traffic is inspected, and then
  redirected
• Impersonation data traffic is sent to bogus
  destinations

38
Address Ownership Subprefix Hijacking
12.34.0.0/16
12.34.158.0/24

• Originating a more-specific prefix
• Every AS picks the bogus route for that prefix
• Traffic follows the longest matching prefix

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Preventing (Sub)Prefix Hijacking

• Best common practice for route filtering
• Each AS filters routes announced by customers
• E.g., based on the prefixes the customer owns
• However, not everyone applies these practices
• Hard to filter routes initiated from far away
• So, BGP remains very vulnerable to hijacks
• Other techniques
• Secure extensions to BGP (e.g., S-BGP, soBGP)
• Anomaly detection of suspected hijacks

40
BGP Attributes Bogus Paths

• AS tampers with AS path
• Deletes ASes from the AS path
• Prepends with a bogus AS number
• Goal influence the path-selection process
• Attract data traffic to the route
• E.g., by making AS path look shorter
• E.g., delete AS that might trigger route
  filtering
• Create blackholes for parts of the Internet
• E.g., prepend bogus AS to trigger loop detection
• Very hard to defend against these attacks
• How can you tell that the route is bogus?

41
BGP Attributes Invalid Paths

• AS exports a route it shouldnt
• AS path is a valid sequence, but violated policy
• Example customer misconfiguration
• Exports routes from one provider to another
• interacts with provider policy
• Provider prefers routes learned from customers
• so provider picks these as the best route
• leading the dire consequences
• E.g., directing all Internet traffic through
  customer
• Main defense
• Filtering routes based on prefixes and AS path

42
BGP Attributes Missing/Inconsistent Routes

• Peering agreements require consistent export
• Prefix advertised at all peering points
• Prefix advertised with same AS path length
• Reasons for violating the policy
• Trick neighbor into cold potato
• Configuration mistake
• Main defense
• Analyzing BGP updates
• or data traffic
• for signs of inconsistency

dest
Bad AS
BGP
data
src
43
BGP Security Today

• Applying best common practices (BCPs)
• Securing the session (authentication, encryption)
• Filtering routes by prefix and AS path
• Resetting attributes to default values
• Packet filters to block unexpected control
  traffic
• This is not good enough
• Depends on vigilant application of BCPs
• and not making configuration mistakes!
• Doesnt address fundamental problems
• Cant tell who owns the IP address block
• Cant tell if the AS path is bogus or invalid
• Cant be sure the data packets follow the chosen
  route

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Conclusion

• BGP protocol vs. policy
• Protocol is simple
• Policy is complicated
• BGP policy is a black art
• Indirect way of specifying policy
• Manipulating attributes to influence decisions
• Filtering routes to scope the routing information
• Common examples of policy today
• Business relationships
• Traffic engineering
• Security

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Discussion

• Is BGP trying to do too many things?
• Policy
• Scalability
• Convergence
• Is BGP too indirect for its own good?
• AS only learns some routes from its neighbors
• And applies policies to indirectly pick the
  routes
• Too many protocols involved?
• External BGP
• Internal BGP
• Intradomain protocol

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Gao Paper

• Inferring AS relationships
• Customer-provider
• Peer-peer
• Every path tells a story
• E.g., a path 701 7018 46
• Implies edges (701, 7018) and (7018, 46)
• Implies that 7018 (ATT) allows AS 701 (UUNet) to
  transit to AS 46 (Rutgers)
• Can limit certain possibilities
• E.g., 701-7018 and 7018-46 cant both be peers
• E.g., 7018 cannot be the customer of both ASes

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Valid and Invalid Paths

• AS relationships limit the kinds of valid paths
• Uphill portion customer-provider relationships
• Plateau zero or one peer-peer edge
• Downhill portion provider-customer relationships

Valid
Invalid
Invalid
48
Characterizations of AS Topology

• Tier-1 small number of tier-1 ASes
• A near-clique of 15 ASes with no providers
• ATT, Sprint, UUNET,
• Transit core peer with tier-1s and each other
• Around 100-200 large ASes
• UUNET Europe, KDDI, and Singapore Telecom
• Regional ISPs non-stubs near the edge
• Around 2000 medium-sized ASes
• Minnesota Regional Network, US West
• Stub ASes no peer or customer neighbors
• Princeton, Rutgers, MIT, ATT Research,