Instability of BGP ASPP

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Instability of BGP ASPP

• Supervised by Prof. Chiu and Prof. John
• Presented by Hui Wang

2
Outline

• Introduction to Routing and BGP
• AS Relationship and Routing Policy
• Inbound Traffic Engineering and ASPP
• ASPP at Different Level (my work)
• Instability of BGP ASPP (my work)
• Future Work
• Conclusion

3
Routing In Internet

• Intradomain vs. Interdomain Routing
• Intradomain
• Router only knows the topology of its domain.
• Three types of IGP (Interior Gateway Protocol)
• Static routing Only useful in very small domains
• Distance vector routing RIP (used in small
  domain)
• Link-state routing
• OSPF (enterprise networks)
• Intermediate System- Intermediate-System (IS-IS)
  (widely used by ISPs)

4
Routing In Internet

• Intradomain vs. Interdomain Routing
• Interdomain Routing (EGP)
• Router knows the domain topology of Internet
• But every domain is a blackbox for EGP
• EGP should be scalable (13,000 AS, 120,000
  routes)
• Border Gateway protocol
• Designed in early 1990s
• Current version 4 1995

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Introduction BGP Basics

• The two variants of BGP
• eBGP between border routers of distinct AS
• Used to distribute the interdomain routes to all
  border routers over eBGP sessions.
• iBGP between BGP routers inside AS
• Used to distribute the interdomain routes to all
  routers in same AS over iBGP sessions. In small
  networks, these iBGP sessions are established in
  a fullmesh. In larger networks, this full-mesh is
  replaced by the utilization of route reflectors
  or confederations.

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Introduction BGP Basics

• An Example eBGP vs. iBGP

eBGP
R1
R2
R5
R6
R3
R4
iBGP
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Introduction BGP Basics

• Autonomous Systems (AS)
• An AS is defined as
• a set of routers under a single technical
  administration. It means that an AS presents a
  consistent picture of what destinations are
  reachable through it.
• Each AS is identified by its AS number (16
  bits)
• The private AS numbers (range 64512 through
  65535) are reserved for private use and should
  not be advertised on the global Internet.
  (currently, more than 13000 ASes)
• A domain is often equivalent to an AS
• A domain may be composed of several ASes
• Many domains do not have an AS number

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Introduction BGP Principle

• Based on TCP
• Distance vector protocol
• - BGP router advertises its best route to each
  neighbor
• - Advertisements are only sent when their routes
  change (No periodic advertisement of routes as
  with RIP)
• Five Types of Message
• - Update
• - Withdrawal
• - Control Message Open, Notification and
  Keepalive

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Introduction BGP Message

• Some Fields in BGP Update Message
• IP prefixes List of reachable IP prefixes
• AS-PATH the list of AS through which the
  announcement passed
• NEXT-HOP the IP address of the router that
  advertised the route
• LOCAL-PREF can be used for traffic control
  purposes
• MED (Multi-Exit-Discriminator) used for traffic
  control between neighbors
• ORIGIN how the route was learned (IGP, EGP,
  Incomplete)
• ATOMIC AGGREGATE and AGGREGATOR

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Introduction BGP Message

• An Example

AS4
AS1
AS2
If AS1 send Update Message to AS2 12.0.0.0/16,
AS1, (AS3,AS1)
It means AS1 says I can reach this prefix
through AS3, if you have traffic to this prefix,
you can send to AS1.
AS3
Prefix 12.0.0.0/16
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Introduction BGP Feature

• An important feature BGP allows each AS to
  define its own routing policy...

• But how does BGP support different policies?

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Introduction BGP Router Organization

• Three tables and two filters

best route to each destination
BGP RIB
Only send to some (not all) destination
Export Filter
Import Filter
All acceptable routes
BGP decision process selects the best route
towards each destination 1.Prefer routes with
highest local-pref 2. shortest ASPath 3. with the
lowest ORIGIN attribute 3. smallest MED 4.Prefer
routes learned via eBGP over via iBGP 5. closest
next-hop 6. learned from router with lowest
router id
Receive routes from neighbors
Import Policy
Export Policy
Determines which BGP Msgs are acceptable from
neighbors. (maybe assign local-pref to each route)
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Introduction BGP Feature

• Why BGP want to support different routing
  policies?

It is because BGP is a routing protocol between
ASes. Different ASes have different commercial
interests. The routing policy is constrained by
commercial agreements.
AS1 should announce AS2 the route to other
ASes AS2 can choose to not announce AS1 the route
to AS3
AS3
AS3
Provider
AS1
AS1
Customer
Customer
AS2
AS2
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Summary Introduction to BGP

• Routing in Internet
• Intradomain vs. Interdomain
• BGP Basics
• eBGP vs. iBGP
• AS vs. Domain
• BGP Principle
• TCP
• Distance Vector
• BGP Message
• Five types of Msgs
• Some Fields (attribute)
• BGP Feature
• Support Different Routing Policy
• BGP Router Organization
• Three Tables

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• I mentioned that BGP want to support different
  routing policies because there are different
  agreements between two ASes. Now

How the agreements (AS relationship) affect the
routing policy?
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Outline

• Introduction to Routing and BGP
• AS Relationship and Routing Policy
• Inbound Traffic Engineering and ASPP
• ASPP at Different Level (my work)
• Instability of BGP ASPP (my work)
• Future Work
• Conclusion

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AS Relationship

• Agreement Relationship
• In practice Classify AS relationship into
• Provider to Customer
• Peer to Peer
• Provider to Customer Customer c buys Internet
  connectivity from provider P.
• Peer to Peer AS x and y agree to exchange
  traffic between their customers free of charge.

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Routing Policy

• Two kinds of Routing Policy
• Export Routing Policy vs. Import Routing Policy
• Import filter
• -Specifies which routes can be accepted by the
  router among all the received routes from a given
  neighbor
• Export filter
• -Specifies which routes can be advertised by the
  router to a given neighbor

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Export Routing Policy

• It is a conclusion from real world
• An AS does not provide transit service between
  its providers and peers.

Consider AS u AS v is provider(u) or peer(u) For
each best route r of u If first(r.as_path) is
provider(u) or peer(u) Then export(v,u)r

AS3
AS1
AS2
Its Selective Export Rule!
AS4
customer
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Export Routing Policy

• It has import influence on Routing Table Entry
  Pattern

Look at some AS Paths in routes of a Routing Table
AS2
AS3
AS1
AS2
AS2
AS4
AS3
AS1
AS1
AS3
AS1,AS2,AS3
AS1,AS2,AS3
AS1,AS2,AS3,AS4
It will not appear in BGP routing table!
This one is OK!
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Export Routing Policy

• All AS Path should be Valley Free
• After traversing a provider to customer edge or a
  peer to peer edge, the AS Path can not traverse a
  customer to provider or peer to peer edge.

AS Path in Routing Table
Reference will be given in the last part
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Export Routing Policy

• A example in a simulation Topology
• See Simulation Tool

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Import Routing Policy

• It is implemented by this attribute
  local-preference

Then AS2-AS4 will be selected as the best route
by AS4!
Prefix 12.0.0.0/16
(12.0.0.0/16, AS2, (AS1,AS2))
AS2
If AS4 prefer link AS2-AS4,assign high local
preference to this route! (for example 100)
AS4
AS1
AS3
(12.0.0.0/16, AS3, (AS1,AS3))
If AS4 prefer link AS2-AS4,assign low local
preference to this route! (for example 80)
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Import Routing Policy

• In real world, how do the router assign local
  preference to affect routing?

In most case, the settings corresponds to the
economical relationships between the ASes.
Typical Local Preference!

• Customer Routes have higher local preference than
  other routes. (Since provider is paid to carry
  packets to customer. Also AS thinks customer is
  closer to the destination )
• Peer Routes have higher local preference than
  Provider Routes. (Since he has to pay for the
  traffic to his provider)
• Due to the utilization of the local preference
  attribute, some paths on the Internet are longer
  than their shortest length.

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Import Routing Policy

• A example in a simulation Topology
• See Simulation Tool

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Summary Relationship and Policy

• Relationship
• Provider to Customer
• Peer to Peer
• Routing Policy
• Export Routing Policy
• Selective Export Rule
• Valley Free Routing Table Entry Pattern
• Import Routing Policy
• Typical Local Preference

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Outline

• Introduction to Routing and BGP
• AS Relationship and Routing Policy
• Inbound Traffic Engineering and ASPP
• ASPP at Different Level (my work)
• Instability of BGP ASPP (my work)
• Future Work
• Conclusion

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Inbound Traffic Engineering Mutihomed

• With the development of Internet, many AS became
  multihomed, which means it has more than one
  physical links to its provider or has more than
  one providers.
• So it will try to control the distribution of
  inbound traffic and outbound traffic on each link
• Only talk Inbound Traffic Engineering in this
  presentation.

It has many ways to do inbound traffic
engineering!
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Inbound Traffic Engineering Ways

• Obviously, in order to do inbound traffic
  engineering, AS needs to send different BGP
  messages on different links to influence the BGP
  decision process of routers in distant AS.
• Selective announcements
• Prefix splitting announce a large prefix on all
  links for redundancy, but prefer some links for
  parts of this prefix
• Remember When forwarding an IP packet, a router
  will always select the longest match in its
  routing table
• AS Path Prepending artificially increase the
  length of AS-Path

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Inbound Traffic Engineering Ways

• Advantages and drawbacks
• Selective announcements
• always work, but if one prefix is advertised on
  a single link, it may become unreachable in case
  of failure
• Prefix Splitting
• better than selective announcements in case of
  failure. but increases significantly the size of
  all BGP tables. some ISPs filter announcements
  for long prefixes
• AS-Path prepending
• useful for backup link, but besides that, it is
  difficult to find the amount of prepending...

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AS Path Prepending

• AS Path Prepending artificially increase the
  length of AS-Path

AS5
Here, the traffic from AS5 to AS4 will follow
this Path AS5-AS2-AS4 If AS4 want to shift the
traffic to link AS3-AS4, it will send route to
AS2 in this way (Prefix, AS4, (AS4,AS4,AS4)) So
AS5 will select other way because he thinks
AS2-AS4-AS4-AS4 is longer than AS1-AS3-AS4 It is
called AS4 prepend link AS2-AS4 twice (or by
two). (Note prepending is directional, AS2 can
prepend link AS4-AS2)
AS1
AS2
AS3
AS4
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Community-based ASPP

• AS can ask its provider, to do prepending for it

BGP support the attribute community. AS can
attach special community value to request down
stream router to perform a special action, for
example, to ask the router prepend for him. So
it is called Community-based AS-Path prepending.
AS5
AS1
AS2
AS3
In this case, AS4 can ask AS2 to prepend AS-Path
when announcing to AS5. It is called AS2
prepends AS5-AS2 for AS4
AS4
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Summary Inbound Traffic Engineering and ASPP

• Multihomed
• Three Way to Do Inbound Traffic Engineering
• AS Path Prepending
• Community-based ASPP

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• Because BGP support Community-based ASPP, so we
  can do ASPP at different levels, for example,
  prepending at its parents or prepending at its
  grandfather.

How does ASPP affect the whole network? How does
ASPP at different levels affect the network?
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Outline

• Introduction to Routing and BGP
• AS Relationship and Routing Policy
• Inbound Traffic Engineering and ASPP
• ASPP at Different Level (my work)
• Instability of BGP ASPP (my work)
• Future Work
• Conclusion

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ASPP at Different Levels

• How to evaluate the influence of ASPP?
• The change of total traffic in the network.
• Add up all the traffic on all links.
• The number of affected AS pairs
• Affected AS pairs the routing path between them
  changes because of ASPP.
• Network traffic model
• Uniform Distribution means for any ASi and ASj,
  the traffic from ASi to ASj is 10 units.
• See Simulation Tools

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ASPP at Different Levels

• Findings
• ASPP will increase the total traffic of the
  network.
• ASPP at high level has less influence on local
  traffic and the whole Internet.

• Attention These are simulation results of some
  random topology, not general conclusion. We need
  a good network topology model for real-world
  Internet!

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ASPP at Different Levels

• If it is true in real Internet
• When we want to shift a little traffic to the
  other link, we can ask the parents to do ASPP for
  us. It has less influence on the whole Internet.

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Outline

• Introduction to Routing and BGP
• AS Relationship and Routing Policy
• Inbound Traffic Engineering and ASPP
• ASPP at Different Level (my work)
• Instability of BGP ASPP (my work)
• Future Work
• Conclusion

40
Instability of BGP ASPP

• My Network Model
• Every AS obeys Exporting Routing Policy
• AS does not provide transit service between any
  two of its providers or peers. So all AS paths in
  routing table should be valley-free.
• Every AS obeys Importing Routing Policy
• Customer route is preferred over peer route, and
  peer route over provider route.
• Uniform Traffic Distribution

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Instability of BGP ASPP

• Network Model
• Every AS wants to do inbound traffic load balance
  through ASPP.
• So he will try to prepend the most heavy-loaded
  link and predict what will happen. If the
  prepending helps him to balance his traffic, he
  will do it. Otherwise, he will give up and thinks
  he has got optimal result.

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Instability of BGP ASPP

• It is an abstract simple example

At time1, Internet is stable. In time slot 1, AS1
find link1 has heavy traffic and prepending this
link is helpful, he do it. This prepending
affects the whole Internet. After some time, at
time2, Internet become stable. But AS2 find his
traffic is skewed, so he do prepending. AS2 also
affects AS1, then in time slot 3, AS1 do
prepending againIf every AS find he has had
nothing to do to get better results, I call this
process converged. Will this process converge
after some time?
Internet
Link1
Link4
Link3
Link2
AS1
AS2

Time Slot 1
Time Slot 2
Time 1
Time 2
Time 3
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Instability of BGP ASPP

• Distributed Greedy ASPP Algorithm
• Does ASi have more than one providers?
• If it doesnt have, stop
• Otherwise, continue.
• Find the heavy-loaded link
• Predict if it will have better balance after
  prepend this link by one.
• If it will have, then do it.
• Otherwise, predict the result of prepending it by
  two. If it helps him, do it.
• Otherwise, predict the result of prepending it by
  three. If it helps him, do it.
• If all these prepending dont help, it has got
  best results, it stops.
• If all ASes stop, the process is converged.

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Instability of BGP ASPP

• How to evaluate the traffic balance?
• Formula (AS has n links to providers)

Balance coefficient
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Instability of BGP ASPP

• See the simulation
• Two cases which will never converge.
• Analyze these cases
• Do prepending in sequence will reduce the
  probability of non-convergence.

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Outline

• Introduction to Routing and BGP
• AS Relationship and Routing Policy
• Inbound Traffic Engineering and ASPP
• ASPP at Different Level (my work)
• Instability of BGP ASPP (my work)
• Future Work
• Conclusion

47
Future Work on ASPP

• Try to find a way to model the AS topology of
  real-world Internet, so I can draw some general
  conclusions.
• Based on the model, try to conclude the reason
  and the pattern of ASPP cycle (or loop).
• Based on it, try to find if there is a practical
  way to prevent ASPP cycle.
• If there is no practical way, try to find all the
  AS that form the cycle. (John said maybe they
  could negotiate and find some solution-)

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My Current Question

• Some papers said ASPP is not a effective way to
  do inbound traffic engineering, so will my future
  work be useful in real world?

However, BGP is really an important research
area. Any successful research results in this
area will be significant for the development of
Internet. In fact, it has been a hot topic in
these three years.
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Why BGP is hot Growth in Table Size

• The reasons for the recent growth
• Fraction of IPv4 address space advertised
• 24 of total IPv4 space in 2000
• 28 of total IPv4 space in April 2003
• Increase in number of ASes
• About 3000 ASes in early 1998
• More than 13000 ASes in April 2003
• Increase in multi-homing
• Less than 1000 multi-homed stub ASes in early
  1998
• More than 6000 multi-homed stub ASes April 2003
• Increase in advertisement of small prefixes
• Number of IPv4 addresses advertised per prefix
• In late 1999, 16k IPv4 addr. per prefix in BGP
  tables
• In April 2003, 8k IPv4 addr. per prefix in BGP
  tables

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Why BGP is hot Issues and Challenges

• How to sustain the growth of the Internet?
• In theory anyone can announce its routes with
  BGP, In practice, BGP routing tables cannot be
  infinite...
• BGP should react quickly to link failures.
• An ISP should be able to control the flow of its
  interdomain traffic.
• Security of interdomain routing.
• Path length was a good indicator only for 50 of
  the considered paths.

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Conclusion

• '' BGP is running on more than 100K routers (my
  estimate), making it one of the world's largest
  and most visible distributed system. Global
  dynamics and scaling principles are still not
  well understood...''
• Tim Griffin, ATT Research

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Reference

• 1. Lixin Gao, On inferring autonomous system
  relationships in the internet, IEEE/ACM
  Transactions on Networking (TON), v.9 n.6,
  p.733-745, December 2001
• 2. Feng Wang, Lixin Gao, On inferring and
  characterizing internet routing policies,
  Proceedings of the conference on Internet
  measurement conference, October 27-29, 2003,
  Miami Beach, FL, USA

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• Thank You.