Transcript What is Routing Policy

Today’s Big Picture
Large ISP
Large ISP
Stub
Small ISP
Dial-Up
ISP
Stub
Stub
Large number of diverse networks
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Access
Network
Internet AS Map: caida.org
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Autonomous System(AS)
Internet is not a single network
 Collection of networks controlled by different
administrations
 An autonomous system is a network under a
single administrative control
 An AS owns an IP prefix
 Every AS has a unique AS number
 ASes need to inter-network themselves to form
a single virtual global network
 Need a common protocol for communication

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Who speaks Inter-AS routing?
AS2
BGP
AS1
R2
R3
R1
R
border router
internal router
 Two types of routers
 Border router(Edge), Internal router(Core)
 Two border routers of different ASes will have a BGP
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Intra-AS vs Inter-AS



An AS is a routing domain
Within an AS:
 Can run a link-state routing protocol
 Trust other routers
 Scale of network is relatively small
Between ASes:
 Lack of information about other AS’s network (Linkstate not possible)
 Crossing trust boundaries
 Link-state protocol will not scale
 Routing protocol based on route propagation
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Autonomous Systems (ASes)
 An
autonomous system is an autonomous routing
domain that has been assigned an Autonomous System
Number (ASN).
All parts within an AS remain connected.
… the administration of an AS appears to other ASes to
have a single coherent interior routing plan and presents a
consistent picture of what networks are reachable through it.
RFC 1930: Guidelines for creation, selection,
and registration of an Autonomous System
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IP Address Allocation and Assignment:
Internet Registries
IANA
www.iana.org
ARIN
www.arin.org
RIPE
www.ripe.org
APNIC
www.apnic.org
Allocate to National and local registries and ISPs
Addresses assigned to customers by ISPs
RFC 2050 - Internet Registry IP Allocation Guidelines
RFC 1918 - Address Allocation for Private Internets
RFC 1518 - An Architecture for IP Address Allocation with CIDR
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AS Numbers (ASNs)
ASNs are 16 bit values.
64512 through 65535 are “private”
•
•
•
•
•
•
•
•
Currently over 11,000 in use.
Genuity: 1
MIT: 3
Harvard: 11
UC San Diego: 7377
AT&T: 7018, 6341, 5074, …
UUNET: 701, 702, 284, 12199, …
Sprint: 1239, 1240, 6211, 6242, …
…
ASNs represent units of routing policy
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Nontransit vs. Transit ASes
ISP 2
ISP 1
Traffic NEVER
flows from ISP 1
through NET A to ISP 2
NET A
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Internet Service
providers (ISPs)
have transit
networks
Nontransit AS
might be a corporate
or campus network.
Could be a “content
provider”
Selective Transit
NET B
NET A DOES NOT
provide transit
Between NET D
and NET B
NET C
NET A
NET A provides transit
between NET B and NET C
and between NET D
and NET C
NET D
Most transit ASes allow only selective transit
key impact of commercialization
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Customers and Providers
provider
provider
IP traffic
customer
customer
Customer pays provider for access to the Internet
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Customer-Provider Hierarchy
provider
IP traffic
customer
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The Peering Relationship
peer
provider
peer
customer
Peers provide transit between
their respective customers
Peers do not provide transit
between peers
traffic
allowed
traffic NOT
allowed
Peers (often) do not exchange $$$
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BGP-4

BGP = Border Gateway Protocol

Is a Policy-Based routing protocol

Is the de facto EGP of today’s global Internet

Relatively simple protocol, but configuration is complex
and the entire world can see, and be impacted by, your
mistakes.
•
1989 : BGP-1 [RFC 1105]
–
•
Replacement for EGP (1984, RFC 904)
1990 : BGP-2 [RFC 1163]
• 1991 : BGP-3 [RFC 1267]
•
1995 : BGP-4 [RFC 1771]
–
Support for Classless Interdomain Routing (CIDR)
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BGP Operations (Simplified)
Establish session on
TCP port 179
AS1
BGP session
Exchange all
active routes
AS2
While connection
is ALIVE exchange
route UPDATE messages
Exchange incremental
updates
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Four Types of BGP Messages

Open : Establish a peering session.

Keep Alive : Handshake at regular intervals.

Notification : Shuts down a peering session.

Update : Announcing new routes or withdrawing
previously announced routes.
announcement
=
prefix + attributes values
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What is Routing Policy

Policy refers to arbitrary preference among a menu of
available routes (based upon routes’ attributes)
 Public description of the relationship between external
BGP peers
 Can also describe internal BGP peer relationship

Eg: Who are my BGP peers
What routes are
 Originated by a peer
 Imported from each peer
 Exported to each peer
 Preferred when multiple routes exist
What to do if no route exists?


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Routing Policy Example




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AS1 originates prefix “d”
AS1 exports “d” to AS2,
AS2 imports
AS2 exports “d” to AS3,
AS3 imports
AS3 exports “d” to AS5,
AS5 imports
Routing Policy Example (cont)


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AS5 also imports “d” from
AS4
Which route does it
prefer?
 Does it matter?
 Consider case where
 AS3 = Commercial
Internet
 AS4 = Internet2
Import and Export Policies



Inbound filtering controls outbound traffic
 filters route updates received from other peers
 filtering based on IP prefixes, AS_PATH, community
Outbound Filtering controls inbound traffic
 forwarding a route means others may choose to reach
the prefix through you
 not forwarding a route means others must use another
router to reach the prefix
Attribute Manipulation
 Import: LOCAL_PREF (manipulate trust)
 Export: AS_PATH and MEDs
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Attributes are Used to Select Best
Routes
192.0.2.0/24
pick me!
192.0.2.0/24
pick me!
192.0.2.0/24
pick me!
Given multiple
routes to the same
prefix, a BGP speaker
must pick at most
one best route
192.0.2.0/24
pick me!
(Note: it could reject
them all!)
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BGP Policy Knob: Attributes
Value
----1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
...
255
Code
--------------------------------ORIGIN
AS_PATH
NEXT_HOP
MULTI_EXIT_DISC
LOCAL_PREF
ATOMIC_AGGREGATE
AGGREGATOR
COMMUNITY
ORIGINATOR_ID
CLUSTER_LIST
DPA
ADVERTISER
RCID_PATH / CLUSTER_ID
MP_REACH_NLRI
MP_UNREACH_NLRI
EXTENDED COMMUNITIES
Reference
--------[RFC1771]
[RFC1771]
[RFC1771]
[RFC1771]
[RFC1771]
[RFC1771]
[RFC1771]
[RFC1997]
[RFC2796]
[RFC2796]
[Chen]
[RFC1863]
[RFC1863]
[RFC2283]
[RFC2283]
[Rosen]
reserved for development
From IANA: http://www.iana.org/assignments/bgp-parameters
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We will cover a
subset of these
attributes
Not all attributes
need to be present in
every announcement
BGP Route Processing
Apply Policy =
Receive
filter routes &
BGP
tweak
Updates
attributes
Apply Import
Policies
Based on
Attribute
Values
Best
Routes
Best Route
Selection
Best Route
Table
Apply Policy =
filter routes &
tweak
attributes
Apply Export
Policies
Install forwarding
Entries for best
Routes.
IP Forwarding Table
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Transmit
BGP
Updates
Import and Export Policies


For inbound traffic
 Filter outbound routes
 Tweak attributes on
outbound
outbound routes in the
inbound
routes
hope of influencing your traffic
neighbor’s best route
selection
For outbound traffic
 Filter inbound routes
inbound
outbound
 Tweak attributes on
routes
traffic
inbound routes to
influence best route
selection
In general, an AS has more
control over outbound traffic
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Policy Implementation Flow
Incoming
Adj
RIB
In
Main
BGP
RIB
IGPs
Main
RIB/
FIB
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Adj
RIB
Out
Static
&
HW
Info
Outgoing
Conceptual Model of BGP Operation
RIB : Routing Information Base
 Adj-RIB-In: Prefixes learned from neighbors. As
many Adj-RIB-In as there are peers
 Loc-RIB: Prefixes selected for local use after
analyzing Adj-RIB-Ins. This RIB is advertised
internally.
 Adj-RIB-Out : Stores prefixes advertised to a
particular neighbor. As many Adj-RIB-Out as
there are neighbors

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Path Attributes: ORIGIN

ORIGIN:
 Describes how a prefix came to BGP at the
origin AS
 Prefixes are learned from a source and
“injected” into BGP:
 Directly connected interfaces, manually
configured static routes, dynamic IGP or EGP
 Values:
IGP (EGP): Prefix learnt from IGP (EGP)
INCOMPLETE: Static routes
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Path Attributes: AS-PATH
List of ASs thru which the prefix announcement
has passed. AS on path adds ASN to AS-PATH
 Eg: 138.39.0.0/16 originates at AS1 and is
advertised to AS3 via AS2.
 Eg: AS-SEQUENCE: “100 200”
 Used for loop detection and path selection

AS1
(100)
138.39.0.0/16
AS3
(15)
AS2
(200)
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Traffic Often Follows ASPATH
135.207.0.0/16
ASPATH = 3 2 1
AS 1
AS 3
AS 2
AS 4
135.207.0.0/16
IP Packet
Dest =
135.207.44.66
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… But It Might Not
135.207.0.0/16
ASPATH = 1
AS 2 filters all
subnets with masks
longer than /24
135.207.0.0/16
ASPATH = 3 2 1
135.207.44.0/25
ASPATH = 5
AS 1
AS 3
AS 2
AS 4
135.207.0.0/16
IP Packet
Dest =
135.207.44.66
AS 5
135.207.44.0/25
From AS 4, it may look like this
packet will take path 3 2 1, but it
actually takes path 3 2 5
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Shorter AS-PATH Doesn’t Mean Shorter #
Hops
BGP says that
path 4 1 is better
than path 3 2 1
Duh!
AS 4
AS 3
AS 2
AS 1
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ASPATH Padding: Shed inbound traffic
AS 1
provider
192.0.2.0/24
ASPATH = 2 2 2
192.0.2.0/24
ASPATH = 2
primary
backup
customer
AS 2
192.0.2.0/24
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Padding will (usually)
force inbound
traffic from AS 1
to take primary link
Padding May Not Shut Off All Traffic
AS 1
AS 3
provider
provider
192.0.2.0/24
ASPATH = 2
192.0.2.0/24
ASPATH = 2 2 2 2 2 2 2 2 2 2 2 2 2 2
primary
backup
customer
AS 2
192.0.2.0/24
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AS 3 will send traffic on
“backup” link because it
prefers customer routes and
local preference is
considered before ASPATH
length!
Padding in this way is often
used as a form of load
balancing
Deaggregation + Multihoming
If AS 1 does
not announce the
more specific prefix,
then most traffic
to AS 2 will go
through AS 3
because it is a
longer match
12.2.0.0/16
12.2.0.0/16
12.0.0.0/8
AS 3
AS 1
provider
provider
AS 2
customer
12.2.0.0/16
AS 2 is
“punching a hole” in the CIDR block
of AS 1=> subverts CIDR
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BGP Table Growth
Thanks:
Geoff Huston.
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Polytechnic
Institute
http://www.telstra.net/ops/bgptable.html
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Large BGP Tables Considered Harmful
• Routing tables must store best
routes and alternate routes
• Burden can be large for routers with
many alternate routes (route
reflectors for example)
• Routers have been known to die
• Increases CPU load, especially
during session reset
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ASNs Growth
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From: Institute
Geoff
Huston. http://www.telstra.net/ops
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Dealing with ASN growth…

Make ASNs larger than 16 bits
 How about 32 bits?
 See Internet Draft: “BGP support for four-octet AS
number space” (draft-ietf-idr-as4bytes-03.txt)
 Requires protocol change and wide deployment

Change the way ASNs are used
 Allow multihomed, non-transit networks to use
private ASNs
 Uses ASE (AS number Substitution on Egress )
 See Internet Draft: “Autonomous System Number
Substitution on Egress” (draft-jhaas-ase-00.txt)
 Works at edge, requires protocol change (for loop
prevention)
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Daily Update Count
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A Few Bad Apples …
Most prefixes are
stable most of the time.
On this day, about 83% of the prefixes
were not updated.
Typically, 80% of
the updates are
for less than 5%
Of the prefixes.
Percent of BGP table prefixes
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Thanks to Madanlal Musuvathi for this
40plot.
Data source: RIPE NCC
Route Flap Dampening (RFC 2439)
Routes are given a penalty for changing.
If penalty exceeds suppress limit, the
route is dampened. When the route is not changing,
its penalty decays exponentially. If the penalty goes
below reuse limit, then it is announced again.
• Can dramatically reduce the number of
BGP updates
• Requires additional router resources
• Applied on eBGP inbound only
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Route Flap Dampening Example
route dampened
for nearly 1 hour
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penalty for each flap = 1000
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How Long Does BGP Take to Adapt to
Changes?
100
Cumulative Percentage of Events
90
80
70
60
Tup
Tshort
50
Tlong
40
Tdow n
30
20
10
0
0
20
40
60
80
100
120
Seconds Until Convergence
From: Abha Ahuja and Craig Labovitz
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140
160
Two Main Factors in Delayed
Convergence
Rate limiting timer slows everything down
 BGP can explore many alternate paths before
giving up or arriving at a new path
 No global knowledge in vectoring protocols

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Implementation Does Matter!
stateless withdraws
widely deployed
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stateful withdraws
widely deployed