Interdomain Routing
EECS 122: Lecture 11
Department of Electrical Engineering and Computer Sciences
University of California
Berkeley
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
2
Review
A
B
C
6
7
8
5
4
3
1
2
12
10
13
11
6
4
3
2
13
B
2
4
3
6
13
OSPF
RIP
IGRP
BGP
IntraDomain
IntraDomain
IntraDomain
Intradomain
Formulate the routing
problem as a Shortest
Path Problem
Link State v/s Distance
Vector
Both work reasonably well
in a well engineered
network
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
3
Today
Why Hierarchical Routing?
Interconnections
Addressing
Interdomain Routing
BGP
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
4
Hierarchical Routing
Is a natural way for routing
to scale
Size
Network Administration
Governance
Allows multiple metrics at
different levels of the
hierarchy
Exploits address
aggregation and allocation
6
4
3
2
13
2
4
3
6
13
7
8
5
1
12
10
11
Inter Domain
Routing
OSPF
RIP
IGRP
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
5
The Importance of Interconnections
The internet is an interconnection of unequal
networks
Interconnection arrangements drive
the competitive landscape
the robustness of the network
End
-
to
-
end performance
Interconnection is central to all large networks
Voice
Data
Wireless
Cable
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
6
Interconnections and Competition
www.thelist.com
How many ISP’s in the 415 area code?
That start with A
-
C: about 200…
Just DSL that start with A
-
C: about 80
In the telephone network
How many independent telephone companies in
1894
-
1902 in the US?
3039 commercial companies, 979 co
-
operatives
By controlling interconnection Bell got rid of them
Interconnection is now regulated (CLECs)
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
7
Big Picture
Large ISP
Large ISP
Dial
-
Up
ISP
Access
Network
Small ISP
Stub
Stub
Stub
The Internet contains a large number of diverse networks
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
8
Two ways to interconnect IP Networks…
Peering
The business relationship whereby ISPs
reciprocally provide to each other connectivity to
each others’ transit customers
Transit
The business relationship whereby one ISP
provides (usually sells) access to all destinations
in it’s routing table
William B. Norton, “Internet Service Providers and Peering”
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
9
Peering
Figure from
William B. Norton, “Internet Service Providers and Peering”
West and East Peer with USNet but they can’t reach each other
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
10
Transit
Figure from
William B. Norton, “Internet Service Providers and Peering”
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
11
Benefits of Transit v/s Peering
William B. Norton, “Internet Service Providers and Peering”
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
12
Moving from Transit to Peering
William B. Norton, “Internet Service Providers and Peering”
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
13
Name of the Game: Reachability
Interdomain routing is about implementing policies
of reachabilty
Routing efficiency and performance is important, but not
essential
ISPs could be competitors and do not want to share
internal network statistics such as load and topology
Use Border Gateway Protocol (BGP)
Border routers communicate over TCP port 179
A Path Vector Protocol
Communicate entire paths: Route Advertisements
A Router Can be involved multiple BGP sessions
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
14
R
border router
internal router
BGP
R2
R1
R3
A
AS1
AS2
you can reach
net A via me
traffic to A
table at R1:
dest
next hop
A
R2
Share connectivity information across ASes
BGP in concept
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
15
I
-
BGP and E
-
BGP
R
border router
internal router
R1
AS1
R4
R5
B
AS3
E
-
BGP
R2
R3
A
AS2
announce B
IGP: Interior Gateway Protocol.
Example: OSPF
I
-
BGP
IGP
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
16
BGP
Border Routers
from the same AS speak IBGP
from different AS’s speak EBGP
EBGP and IBGP are essentially the same
protocol
IBGP can only propagate routes it has learned
directly from its EBGP neighbors
All routers in the same AS form an IBGP mesh
Important to keep IBGP and EBGP in sync
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
17
Sharing routes
One router can participate in many BGP sessions.
Initially
… node advertises ALL routes it wants neighbor to
know (could be > 50K routes)
Ongoing
… only inform neighbor of changes
BGP Sessions
AS1
AS2
AS3
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
18
Four message types
Open: Session establishment id exchange
Notification: exception driven information
Keep Alive: soft state
Update: path vector information
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
19
BGP Update Message
Contains information about
New Routes
Withdrawn Routes: No longer valid
Path Attributes:
Attribute information allows policies to be
implemented
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
20
Issues
How are the routes advertised?
Addressing
How are routing policies implemented?
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
21
Addressing
Every router must be able to forward based
on *any* destination IP address
One strategy: Have a row for each address
There would be 10^8 rows!
Better strategy: Have a row for a range of
addresses
If addresses are assigned at random that wouldn’t work
too well
MAC addresses
Addresses allocation is a big deal.
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
22
Addressing reflects internet hierarchy
32 bits divided into 2 parts:
Class A
Class B
Class C
Class
-
base Addressing
network
host
0
0
network
host
1
16
0
network
host
1
24
0
~2 million nets
256 hosts
8
0
1
0
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
23
Class
-
based addresses did not scale
well
Example: an organization initially needs 100 addresses
Allocate it a class C address
Organization grows to need 300 addresses
Class B address is allocated. (~64K hosts)
That’s overkill
-
a huge waste
Only about 8200 class B addresses!
Artificial Address crises
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
24
Current Solution: Classless Internet
Domain Routing (CIDR)
CIDR allows networks to be assigned on arbitrary bit
boundaries
.
Address ranges can be assigned in chunks of 2
k
k=1…32
Idea
-
use aggregation
-
provide routing for a large
number of customers by advertising one common prefix.
This is possible because nature of addressing is hierarchical
Summarization reduces the size of routing tables, but
maintains connectivity.
Aggregation
Scalability and survivability of the Internet
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
25
Current Solution: Classless Internet
Domain Routing (CIDR)
Suppose fifty computers in a network are
assigned IP addresses
128.23.9.0
-
128.23.9.49
They share the
prefix
128.23.9
Is this the
longest
prefix?
Range is
01111111 00001111 00001001 00000000
to
01111111 00001111 00001001 00110001
How to write
01111111 00001111 00001001 00X
?
Convention:
128.23.9.0/26
There are
32
-
27=6
bits for the
50
computers
2
6
= 64
addresses
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
26
Specify a range of addresses by a prefix: X/Y
The prefix common to the entire range is the first Y bits of X.
X: The first address in the range has prefix X
Y: 2
32
-
Y
addresses in the range
Example 128.5.10/23
Common prefix is 23 bits: 01000000 00000101 0000101
Number of addresses: 2
9
= 512
Prefix aggregation
Combine two address ranges
128.5.10/24 and 128.5.11/24 gives 128.5.10/23
Routers match to longest prefix
Classless Inter
-
domain Routing Addresses
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
27
Assigning IP address (Ideally)
A host gets its IP address from the IP address block of its
organization
An organization gets an IP address block from its ISP’s
address block
An ISP gets its address block from its own provider OR
from one of the 3 routing registries:
ARIN: American Registry for Internet Numbers
RIPE: Reseaux IP Europeens
APNIC: Asia Pacific Network Information Center
Each Autonomous System (AS) is assigned a 16
-
bit
number (65536 total)
Currently 10,000 AS’s in use
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
28
Advertising a Route
One router telling another one
The prefix
IP address of the next hop
Path list of AS’s that the announcement has
passed through
Since announcement propagates from destination, this
yields the path
No refresh messages required
The announcing router will follow the path
itself
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
29
BGP Routing Table Scaling
Many small networks
Aggregation hides a lot…
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
30
BGP Update Message
Contains information about
New Routes
Withdrawn Routes: No longer valid
Path Attributes:
Path Weights
Multple Exit Discriminators
Local Preferences
Etc.
Attribute information allows policies to be
implemented
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
31
BGP: A Path
-
vector protocol
ner
-
routes>show ip bgp
BGP table version is 6128791, local router ID is 4.2.34.165
Status codes: s suppressed, d damped, h history, * valid, > best, i
-
internal
Origin codes: i
-
IGP, e
-
EGP, ?
-
incomplete
Network Next Hop Metric LocPrf Weight Path
* i3.0.0.0 4.0.6.142 1000 50 0
701 80
i
* i4.0.0.0 4.24.1.35 0 100 0 i
* i12.3.21.0/23 192.205.32.153 0 50 0
7018 4264 6468
?
* e128.32.0.0/16 192.205.32.153 0 50 0
7018 4264 6468 25
e
Every route advertisement contains the entire AS path
Generalization of distance vector
Can implement policies for choosing best route
Can detect loops at an AS level
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
32
Multihoming
Two or more
interdomain
connections between
the same AS’s
Two or more
interdomain
connections between a
customer and different
ISPs
6
4
3
2
A
B
2
4
3
6
7
8
5
1
RIP
IGRP
IBGP
IBGP
a
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
33
Multiexit Discriminators (MEDs)
6
4
3
2
A
B
2
4
3
6
7
8
5
1
RIP
IGRP
IBGP
IBGP
x
One AS influences the decisions
of a neighboring AS
•
AS_A wants to tell AS_B that network
x
is closer to router 2
than to router 3
•
Router 2 advertises a smaller MED value for
x
than Router 3
•
AS_B prefers the path to
x
that does not go through 6 and 3
•
AS_B does not propagate MEDS from AS_A any further
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
34
Routing Process Overview
Import
Policy
Engine
Decision
process
Routes
used by
router
Routes
received
from
neighbors
Routes
sent to
neighbors
BGP table
IP routing
table
Choose
best route
accept,
deny, set
preferences
forward,
not forward
set MEDs
Export
Policy
Engine
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
35
Attribute: Local Preference
Used to indicate preference
among multiple paths for the
same prefix
anywhere
in the
Internet.
The higher the value the
more preferred
Exchanged between IBGP
peers only. Local to the AS.
Often used to select a
specific exit point for a
particular destination
AS4
AS2
AS3
AS1
140.20.1.0/24
BGP table at AS4:
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
36
Choosing best route
Choose route with highest
LOCAL_PREF
Preference
-
based routing
If multiple choices, select route with shortest
hop
-
count
If multiple choices for same neighboring AS,
choose path with max MED value
Choose route based on lowest origin type
IGP < EGP < INCOMPLETE
Among IGP paths, choose one with lowest cost
Finally use router ID to break the tie.
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
37
BGP Policies
Multiple ways to implement a “policy”
Decide not propagate advertisements
I’m not carrying your traffic
Decide not to consider MEDs but use shortest hop
Hot potato routing
Prepend own AS# multiple times to fool BGP into
not thinking AS further away
Many others…
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
38
Transit vs. Nontransit AS
AS1
ISP1
ISP2
r1
r2
r2
r3
r2
r1
r3
AS1
ISP1
ISP2
r1
r2,r3
r2,r1
r3
r2
r1
r3
Transit traffic = traffic whose source and destination are outside the AS
Nontransit AS
: does not carry transit traffic
Transit AS
: does carry transit traffic
•
Advertise own routes only
•
Do not propagate routes learned from other AS’s
•
Advertises its own routes PLUS routes
learned from other AS’s
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
39
Customer
-
Transit Problem
Small ISP
Large ISP
Large ISP
r1
r2,r3
r2,r1
r3
r2
r1
r3
Assume that the small ISP is a customer of two large ISPs
If customer ISP does not obey export rules
forwards advertisements from one large ISP to another
Carries huge volume of transit traffic between two large
ISPs
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
40
BGP and Performance
BGP isn’t designed for policy routing not performance
Hot Potato routing is most common but suboptimal
Performance isn’t the greatest
20% of internet paths inflated by at least 5 router hops
Very susceptible to router misconfiguration
Blackholes: announce a route you cannot reach
October 1997 one router brought down the internet for 2 hours
Flood update messages (don’t store routes, but keep asking your
neighbors to clue you in). 3
-
5 million useless withdrawals!
In principle, BGP could diverge
Various solutions proposed to limit the set of allowable policies
Focuses on avoiding “policy cycles”
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
41
4/1
-
4/16 2002
•
1,224,733 IP addresses,
•
2,093,194 IP links,
•
932,000 destinations,
•
70% of globally routable network prefixes;
•
10,999 ASes (84% of ASes),
•
34,209 peering sessions
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
42
Skitter Legend
Plot the AS based on polar co
-
ordinates (r,
θ
):
r = 1
-
log (As degree +1 / Max Degree+1)
Higher the degree lower the radius
Θ
= longitude of AS headquarters
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
43
February 25, 2003
Abhay Parekh, EE122 S2003:Updated from
Stoica EE122 F2002
44
Summary
The Internet is composed of various ASes which
use BGP to inter
-
network themselves
The Internet uses classless addressing
BGP as a routing protocol
Path
-
vector based
Supports route
-
aggregation
Supports preferential routing
Uses Import and Export policies
BGP is the protocol that “holds” the Internet
together
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