Interdomain Routing and The Border Gateway Protocol (BGP)
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Transcript of Interdomain Routing and The Border Gateway Protocol (BGP)
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Interdomain Routing and The Interdomain Routing and The Border Gateway Protocol Border Gateway Protocol
(BGP)(BGP)
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Today’s Big Picture
Large ISP Large ISP
GPRS AccessNetwork
Small ISP
Stub Stub
Stub
Large number of diverse networks
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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
• IANA– An AS owns unique IP prefixes– 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?
R border router internal router
BGPR2
R1
R3AS1
AS2
Two types of routers Border router (Edge)Internal router (Core)
Two border routers of different ASes will have a BGP session
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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.
RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System
… 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.
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IP Address Allocation and Assignment: Internet Registries
IANAwww.iana.org
RFC 2050 - Internet Registry IP Allocation Guidelines RFC 1918 - Address Allocation for Private Internets RFC 1518 - An Architecture for IP Address Allocation with CIDR
ARINwww.arin.org
APNICwww.apnic.org
RIPEwww.ripe.org
Allocate to National and local registries and ISPs Addresses assigned to customers by ISPs
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Whois servers (AS, IP)
• http://www.ripe.net/perl/whois– AS2588
• http://ws.arin.net/cgi-bin/whois.pl– AS701
• http://www.apnic.net/apnic-bin/whois.pl– AS4808
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AS Numbers (ASNs)
ASNs are 16 bit values.64512 through 65535 are “private”
• Genuity: 1 • MIT: 3• JANET: 786• 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
Currently over 20,000 in use.
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Partial View of www.cl.cam.ac.uk (128.232.0.20) Neighborhood
AS 786 ja.net
AS 1239 Sprint
AS 4373 Online Computer Library Center
Originates > 180 prefixes, Including 128.232.0.0/16
AS 3356Level 3
AS 6461AboveNet
AS 1213 HEAnet(Irish academic and research)
AS 7 UK Defense Research Agency
AS 5459 LINX
AS 702 UUNET
AS 20965 GEANT
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How Many ASNs are there today?
Thanks to Geoff Huston. http://bgp.potaroo.net on October 26, 2004
18,217
12,940origin only (notransit)
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IP network assignment process
IANA
RIR
Allo
catio
n
RIR
ISPA
lloca
tion
ISP
BGP
Ann
ounc
emen
t
IETF
IANA
Del
egat
ion
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RIR Allocations - Current
IETF Reserved, 20.1, 8%
Multicast, 16, 6%
IANA Pool, 89, 35%
RIR Pool, 14, 5%
Assigned, 116.9, 46%Allocated
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BGP Routing Table - Current
IETF Reserved, 20.1, 8%
Multicast, 16, 6%
IANA Pool, 89, 35%
RIR Pool, 14, 5%
Assigned, 42.4, 17%
Advertised, 74.5, 29%
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How many prefixes today?
Thanks to Geoff Huston. http://bgp.potaroo.net on October 26, 2004
179,903
Note: numbersactually dependspoint of view…
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The Gang of Four
Link State Vectoring
EGP
IGP
BGP
RIPOSPF
EIGRP
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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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The Border Gateway Protocol (BGP)
BGP = RFC 1771
+ “optional” extensionsRFC 1997 (communities) RFC 2439 (damping) RFC 2796 (reflection) RFC3065 (confederation) …
+ routing policy configurationlanguages (vendor-specific)
+ Current Best Practices in management of Interdomain Routing
BGP was not DESIGNED. It EVOLVED.
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BGP Operations (Simplified)
Establish session on TCP port 179
Exchange all active routes
Exchange incremental updates
AS1
AS2
While connection is ALIVE exchangeroute UPDATE messages
BGP session
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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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BGP Attributes
Value Code Reference----- --------------------------------- --------- 1 ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen] ... 255 reserved for development
From IANA: http://www.iana.org/assignments/bgp-parameters
Mostimportantattributes
Not all attributesneed to be present inevery announcement
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Attributes are Used to Select Best Routes
192.0.2.0/24pick me!
192.0.2.0/24pick me!
192.0.2.0/24pick me!
192.0.2.0/24pick me!
Given multipleroutes to the sameprefix, a BGP speakermust pick at mostone best route
(Note: it could reject them all!)
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BGP Route Processing
Best Route Selection
Apply Import Policies
Best Route Table
Apply Export Policies
Install forwardingEntries for bestRoutes.
ReceiveBGPUpdates
BestRoutes
TransmitBGP Updates
Apply Policy =filter routes & tweak attributes
Based onAttributeValues
IP Forwarding Table
Apply Policy =filter routes & tweak attributes
Open ended programming.Constrained only by vendor configuration language
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Route Selection Summary
Highest Local Preference
Shortest ASPATH
Lowest MED
i-BGP < e-BGP
Lowest IGP cost to BGP egress
Lowest router ID
traffic engineering
Enforce relationships
Throw up hands andbreak ties
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BGP Routing Tables
• Use “whois” queries to associate an ASN with “owner” (for example, http://www.arin.net/whois/arinwhois.html)
• 7018 = AT&T Worldnet, 701 =Uunet, 3561 = Cable & Wireless, …
show ip bgpBGP table version is 111849680, local router ID is 203.62.248.4Status codes: s suppressed, d damped, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
. . .*>i192.35.25.0 134.159.0.1 50 0 16779 1 701 703 i*>i192.35.29.0 166.49.251.25 50 0 5727 7018 14541 i*>i192.35.35.0 134.159.0.1 50 0 16779 1 701 1744 i*>i192.35.37.0 134.159.0.1 50 0 16779 1 3561 i*>i192.35.39.0 134.159.0.3 50 0 16779 1 701 80 i*>i192.35.44.0 166.49.251.25 50 0 5727 7018 1785 i*>i192.35.48.0 203.62.248.34 55 0 16779 209 7843 225 225 225 225 225 i*>i192.35.49.0 203.62.248.34 55 0 16779 209 7843 225 225 225 225 225 i*>i192.35.50.0 203.62.248.34 55 0 16779 3549 714 714 714 i*>i192.35.51.0/25 203.62.248.34 55 0 16779 3549 14744 14744 14744 14744 14744 14744 14744 14744 i. . .
Thanks to Geoff Huston. http://www.telstra.net/ops on July 6, 2001
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AS Graphs Can Be Fun
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Policy : Transit vs. Nontransit
AS 701
AS144
AS 701
A nontransit AS allows only traffic originating from AS or traffic with destination within AS
IP traffic
UUnet
Bell Labs
AT&T CBB
A transit AS allows traffic with neither source nor destination within AS to flow across the network
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Customers and Providers
Customer pays provider for access to the Internet
provider
customer
IP trafficprovider customer
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The “Peering” Relationship
peer peer
customerprovider
Peers provide transit between their respective customers
Peers do not provide transit between peers
Peers (often) do not exchange $$$trafficallowed
traffic NOTallowed
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Peering Provides Shortcuts
Peering also allows connectivity betweenthe customers of “Tier 1” providers.
peer peer
customerprovider
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Peering Wars
• 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
Peering struggles are by far the most contentious issues in the ISP world!
Peering agreements are often confidential.
Peer Don’t Peer
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Policy-Based vs. Distance-Based Routing?
ISP1
ISP2
ISP3
Cust1
Cust2Cust3
Host 1
Host 2
Minimizing “hop count” can violate commercial relationships thatconstrain inter-domain routing.
YES
NO
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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
• 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
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Routing Policy Example (cont)
• AS5 also imports “d” from AS4
• Which route does it prefer?– Does it matter?– Consider case where
• AS3 = Commercial Internet
• AS4 = Internet2
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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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ASPATH Attribute
AS7018135.207.0.0/16AS Path = 6341
AS 1239Sprint
AS 1755Ebone
AT&T
AS 3549Global Crossing
135.207.0.0/16AS Path = 7018 6341
135.207.0.0/16AS Path = 3549 7018 6341
AS 6341
135.207.0.0/16
AT&T Research
Prefix Originated
AS 12654RIPE NCCRIS project
AS 1129Global Access
135.207.0.0/16AS Path = 7018 6341
135.207.0.0/16AS Path = 1239 7018 6341
135.207.0.0/16AS Path = 1755 1239 7018 6341
135.207.0.0/16AS Path = 1129 1755 1239 7018 6341
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In fairness: could you do this “right” and still scale?
Exporting internalstate would dramatically increase global instability and amount of routingstate
Shorter Doesn’t Always Mean Shorter
AS 4
AS 3
AS 2
AS 1
Mr. BGP says that path 4 1 is better than path 3 2 1
Duh!
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Routing Example 1
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Routing Example 2
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Tweak Tweak Tweak (TE)
• For inbound traffic– Filter outbound routes– Tweak attributes on
outbound routes in the hope of influencing your neighbor’s best route selection
• For outbound traffic– Filter inbound routes– Tweak attributes on
inbound routes to influence best route selection
outboundroutes
inboundroutes
inboundtraffic
outboundtraffic
In general, an AS has morecontrol over outbound traffic
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LOCAL PREFERENCE
AS 1AS 2
AS 4
AS 3
13.13.0.0/16
local pref = 80
local pref = 100
local pref = 90
Higher Localpreference valuesare more preferred
Local preference used ONLY in iBGP
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Implementing Backup Links with Local Preference (Outbound Traffic)
Forces outbound traffic to take primary link, unless link is down.
AS 1
primary link backup link
Set Local Pref = 100for all routes from AS 1 AS 65000
Set Local Pref = 50for all routes from AS 1
We’ll talk about inbound traffic soon …
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Multihomed Backups (Outbound Traffic)
Forces outbound traffic to take primary link, unless link is down.
AS 1
primary link backup link
Set Local Pref = 100for all routes from AS 1
AS 2
Set Local Pref = 50for all routes from AS 3
AS 3provider provider
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ASpath prepending
AS 1
192.0.2.0/24ASPATH = 2 2 2 2 2 2 2 2 2 2 2 2 2 2
customerAS 2
provider
192.0.2.0/24
192.0.2.0/24ASPATH = 2
AS 3provider
Padding in this way is oftenused as a form of loadbalancing
backupprimary
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COMMUNITY Attribute to the Rescue!
AS 1
customerAS 2
provider
192.0.2.0/24
192.0.2.0/24ASPATH = 2
AS 3provider
backupprimary
192.0.2.0/24ASPATH = 2 COMMUNITY = 3:70
Customer import policy at AS 3:If 3:90 in COMMUNITY then set local preference to 90If 3:80 in COMMUNITY then set local preference to 80If 3:70 in COMMUNITY then set local preference to 70
AS 3: normal customer local pref is 100,peer local pref is 90
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BGP Summary
• BGP4 is the protocol used on the Internet to exchange routing information between providers, and to propagate external routing information through networks.
• Each autonomous network is called an Autonomous System.
• ASs which inject routing information on their own behalf have ASNs.
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BGP Peering
• BGP-speaking routers peer with each other over TCP sessions, and exchange routes through the peering sessions.
• Providers typically try to peer at multiple places. Either by peering with the same AS multiple times, or because some ASs are multi-homed, a typical network will have many candidate paths to a given prefix.
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The BGP Route
• The BGP route is, conceptually, a “promise” to carry data to a section of IP space. The route is a “bag” of attributes.
• The section of IP space is called the “prefix” attribute of the route.
• As a BGP route travels from AS to AS, the ASN of each AS is stamped on it when it leaves that AS. Called the AS_PATH attribute, or “as-path” in Cisco-speak.
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BGP Route Attributes
• In addition to the prefix, the as-path, and the next-hop, the BGP route has other attributes, affectionately known as “knobs and twiddles” -– weight, rarely used - “sledgehammer”– local-pref, sometimes used - “hammer”– origin code, rarely used– MED (“metric”) - a gentle nudge