Inter-domain Routing: Today and Tomorrow

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Inter-domain Routing Today and Tomorrow
Dr. Jia Wang jiawang_at_research.att.com ATT Labs
Research Florham Park, NJ 07932,
USA http//www.research.att.com/jiawang/
Prof. Zhuoqing Morley Mao zmao_at_umich.edu Departmen
t of EECS University of Michigan Ann Arbor, MI
48109, USA http//www.eecs.umich.edu/zmao/
IEEE INFOCOM 2004 Tutorial March 8, 2004
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Outline

• Overview of Inter-domain routing
• Routing policies
• Measuring inter-domain paths
• Routing instability
• BGP Beacon - measurement infrastructure
• Implication on network engineering
• Security issues

Our opinions should not be taken to represent
ATT policies
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Part I Overview of Inter-domain Routing
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Internet

• Loose cooperative effort of Internet Service
  Providers (ISPs)
• E.g., ATT, Sprint, UUNet, AOL
• Best effort service
• Connectedness
• Anyone connected to the Internet can exchange
  traffic with anyone else connected to the
  Internet

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Internet routing
rusty.cs.berkeley.edu IP169.229.62.116 Prefix169
.229.0.0/16
www.cnn.com IP64.236.16.52 Prefix64.236.16.0/20
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Internet routing dictates application performance
www.cnn.com IP64.236.16.52 Prefix64.236.16.0/20
rusty.cs.berkeley.edu IP169.229.62.116 Prefix169
.229.0.0/16
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Internet routing domain

• Network devices under same technical and
  administrative control
• Common routing policy
• E.g., ISPs, enterprise networks

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Autonomous System (AS)

• Autonomous routing domain with an AS number (ASN)
• AS numbers
• 16 bits integer
• Public AS number 1 64511
• Private AS number 64512 65535
• Examples
• ATT 7018, 6431,
• Sprint 1239, 1240,
• MIT 3

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More than 14,000 ASes today
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Internet Initiative Japan (IIJ)
11
IIJ, Tokyo
12
Telstra international
13
WorldCom (UUNet)
14
UUNet, Europe
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Sprint, USA
16
ATT IP Backbone, USA
17
GARR-B
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Gigabit research network
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wiscnet.net
GO BUCKY!
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MIT.edu
http//bgp.lcs.mit.edu/
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Internet routing architecture
Inter-domain routing
Intra-domain routing
IP traffic
Internet
Berkeley
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Intra-domain routing

• Run within a certain network infrastructure
• Optimize routes taken between points within a
  network
• Internal Gateway Protocols (IGPs)
• Metrics based
• OSPF (Open Shortest Path First)
• RIP (Routing Information Protocol)
• IS-IS (Intermediate System to Intermediate System)

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Inter-domain routing

• Run between networks
• Provide full connectivity of entire Internet
• External Gateway Protocol (EBGP)
• Policy based
• BGP (Border Gateway Protocol)

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Inter-domain routing and BGP

• Static routing
• Mainly for stub networks
• Default routing
• Small stub networks
• Dynamic routing
• Via BGP

No need to run BGP in static routing and default
routing.
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Link state

• Examples OSPF, IS-IS
• Based on Dijkstras shortest path computation
• Each router periodically floods immediate
  reachability information to other routers
• Fast convergence
• High communication and computation overhead
• Not scalable for large networks
• Requires periodic refreshes

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Vectoring

• Distance vs. Path Vector
• Distance hop count (RIP)
• Path entire path (BGP)
• Helps identify loops
• Supports policy-based routing based on path
• Minimal communication overhead
• Takes longer to converge, i.e., in proportion to
  the maximum path length

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Link state vs. vectoring
Link state
Vectoring
IGP
EGP
BGP is a path vector protocol
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Classful addressing

• IPv4 32 bits
• Five classes of networks

Improve scaling factor of routing in the Internet
gt classless
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RFC1519 Classless Inter-domain Routing (CIDR)

• No implicit mask based on the class of the
  network
• Explicit masks passed in the routing protocol
• Allow aggregation and hierarchical routing

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CIDR addressing
IP address 12.70.0.0
Mask 255.255.252.0
00001100 00100110 00000000 00000000
Address
00001100 00100110 00000000 00000000
Mask
11111111 11111111 11000000 00000000
11111111 11111111 11000000 00000000
Host identifier
Network prefix
CIDR representation 12.70.0.0/22
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Address aggregation
Internet
12.70.3.0/24
12.70.0.0/24
ISP A
12.70.1.0/24
ISP B
12.71.0.0/16
12.70.2.0/24
12.70.0.0/22 12.71.0.0/16
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Routing and forwarding

• Routing
• The decision process of choosing optimal path
  that is consistent with the administrative or
  technical policy
• Forwarding
• The act of receiving a packet, doing a lookup,
  and copying a packet to the next hop

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Classless forwarding
Internet
12.70.0.20
10.20.128.10
10.20.128.1
10.20.0.1
IP traffic
10.20.1.1
Prefix Next hop 12.70.0.0/24 10.20.0.1 12.70.0.0/
16 10.20.1.1 12.0.0.0/8 10.20.128.1 0.0.0.0
10.20.128.10
135.120.0.1
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Inter-domain routing with CIDR support

• BGP-4 RFC1771
• De facto EGP
• Path vector protocol
• Run on top of TCP for reliability
• Carry routing information between ASes
• Policy based routing

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BGP basic operations

• Set up BGP session
• Exchange all candidate routes
• Send incremental updates

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Establish BGP session
Establish neighboring session between 12.10.0.1
and 12.10.0.2
TCP 179
12.10.0.1
12.10.0.2
Prefix Next hop 12.70.0.0/24 10.20.0.1 12.9.0.0/1
6 10.20.1.1
Prefix Next hop 135.120.0.0/24 10.128.0.1 68.35.0
.0/16 10.192.1.1
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Exchange all candidate routes
12.70.0.0/24 10.20.0.1 12.9.0.0/16 10.20.1.1
12.10.0.1
12.10.0.2
135.120.0.0/24 10.128.0.1 68.35.0.0/16 10.192.1.1
Prefix Next hop 12.70.0.0/24 10.20.0.1 12.9.0.0/1
6 10.20.1.1 135.120.0.0/24 10.128.0.1 68.35.0.0/16
10.192.1.1
Prefix Next hop 135.120.0.0/24 10.128.0.1 68.35.0
.0/16 10.192.1.1 12.70.0.0/24 10.20.0.1 12.9.0.0/1
6 10.20.1.1
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Send incremental updates
Withdraw 12.9.0.0/16
12.10.0.1
12.10.0.2
Prefix Next hop 12.70.0.0/24 10.20.0.1 12.9.0.0/1
6 10.20.1.1 135.120.0.0/24 10.128.0.1 68.35.0.0/16
10.192.1.1
Prefix Next hop 135.120.0.0/24 10.128.0.1 68.35.0
.0/16 10.192.1.1 12.70.0.0/24 10.20.0.1 12.9.0.0/1
6 10.20.1.1
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BGP messages

• OPEN set up a peering session
• UPDATE announce new routes or withdraw
  previously announced routes
• NOTIFICATION shut down a peering session
• KEEPALIVE confirm active connection at regular
  interval

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Internal vs. external BGP
Internet
I-BGP
AS B
E-BGP
AS C
AS A
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I-BGP mesh
I-BGP update
E-BGP update
I-BGP update
I-BGP update
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Make I-BGP scale for large AS

• Route reflectors
• Confederations

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Route reflector
E-BGP update
RR
RR
Only best paths being sent by RR
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Confederation
AS 1000
EBGP
IBGP
EBGP
IBGP
AS 65020
AS 65010
EBGP
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BGP updates

• Three blocks
• Prefix
• Path attributes
• Unreachable routes

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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 RFC1998
• 10 CLUSTER_LIST RFC1998

• 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
• 17 NEW_AS_PATH E.Chen
• 18 NEW_AGGREGATOR E.Chen
• 19 SAFI Specific Attribute (SSA) Nalawade
• 20-254 Unassigned
• 255 reserved for development

http//www.iana.org/assignments/bgp-parameters
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Establish connectivity
Prefix Next hop AS path 135.120.0.0/16 12.10.0.5
2 1
AS 3
Prefix Next hop AS path 135.120.0.0/16 12.10.0.1
1
12.10.0.6
IBGP
EBGP
12.10.0.5
AS 1
AS 2
135.120.0.0/16
EBGP
12.10.0.2
IBGP
12.10.0.1
IBGP
Prefix Next hop AS path 135.120.0.0/16 12.10.0.1
1
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IGP and BGP working together
Prefix Next hop AS path 135.120.0.0/16 12.10.0.1
1
AS 3
Prefix Next hop 12.10.0.0/30 10.10.0.1 135.120.0.
0/16 10.10.0.1
12.10.0.6
IBGP
EBGP
12.10.0.5
AS 1
AS 2
12.10.0.1
135.120.0.0/16
EBGP
12.10.0.2
10.10.0.1
IBGP
12.10.0.0/30
IBGP
Prefix Next hop AS path 135.120.0.0/16 12.10.0.1
1