Download BGP

Computer Networks
(Graduate level)
Lecture 7: Inter-domain Routing
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
Univ. of Tehran
Computer Network
1
Inter-Domain Routing


Border Gateway Protocol (BGP)
Assigned reading


[LAB00] Delayed Internet Routing Convergence
Sources





RFC1771: main BGP RFC
RFC1772-3-4: application, experiences, and analysis
of BGP
RFC1965: AS confederations for BGP
Christian Huitema: “Routing in the Internet”,
chapters 8 and 9.
John Stewart III: “BGP4 - Inter-domain routing in
the Internet”
Univ. of Tehran
Computer Network
2
Outline

External BGP (E-BGP)

Internal BGP (I-BGP)

Multi-Homing

Stability Issues
Univ. of Tehran
Computer Network
3
Internet’s Area Hierarchy

What is an Autonomous System (AS)?




A set of routers under a single technical
administration, using an interior gateway protocol
(IGP) and common metrics to route packets within
the AS and using an exterior gateway protocol (EGP)
to route packets to other AS’s
Sometimes AS’s use multiple IGPs and metrics, but
appear as single AS’s to other AS’s
Each AS assigned unique ID
AS’s peer at network exchange routing
information.
Univ. of Tehran
Computer Network
4
Example
1
2
IGP
2.1
IGP
EGP
1.1
2.2.1
1.2
EGP
EGP
EGP
3
IGP
4.1
EGP
5
3.1
5.1
Univ. of Tehran
2.2
IGP
IGP
4.2
4
3.2
5.2
Computer Network
5
History

Mid-80s: EGP




Reachability protocol (no shortest path)
Did not accommodate cycles (tree topology)
Evolved when all networks connected to NSF
backbone
Result: BGP introduced as routing protocol



Latest version = BGP 4
BGP-4 supports CIDR
Primary objective: connectivity not performance
Univ. of Tehran
Computer Network
6
Choices

Link state or distance vector?


Problems with distance-vector:


No universal metric – policy decisions
Bellman-Ford algorithm may not converge
Problems with link state:



Metric used by routers not the same – loops
LS database too large – entire Internet
May expose policies to other AS’s
Univ. of Tehran
Computer Network
7
Solution: Distance Vector with
Path


Each routing update carries the entire path
Loops are detected as follows:

When AS gets route check if AS already is in path



If yes, reject route
If no, add self and (possibly) advertise route further
Advantage:

Metrics are local - AS chooses path, protocol ensures
no loops
Univ. of Tehran
Computer Network
8
Interconnecting BGP Peers


BGP uses TCP to connect peers
Advantages:




Simplifies BGP
No need for periodic refresh - routes are
valid until withdrawn, or the connection is
lost
Incremental updates
Disadvantages


Congestion control on a routing protocol?
Poor interaction during high load
Univ. of Tehran
Computer Network
9
Hop-by-hop Model

BGP advertises to neighbors only those
routes that it uses


Consistent with the hop-by-hop Internet
paradigm
e.g., AS1 cannot tell AS2 to route to other
AS’s in a manner different than what AS2 has
chosen (need source routing for that)
Univ. of Tehran
Computer Network
10
AS Categories



Stub: an AS that has only a single
connection to one other AS - carries only
local traffic.
Multi-homed: an AS that has
connections to more than one AS, but
does not carry transit traffic
Transit: an AS that has connections to
more than one AS, and carries both
transit and local traffic (under certain
policy restrictions)
Univ. of Tehran
Computer Network
11
AS Categories
AS1
AS3
AS1
AS2
AS1
AS3
AS2
Transit
Stub
AS2
Multi-homed
Univ. of Tehran
Computer Network
12
Policy with BGP



BGP provides capability for enforcing
various policies
Policies are not part of BGP: they are
provided to BGP as configuration
information
BGP enforces policies by choosing paths
from multiple alternatives and controlling
advertisement to other AS’s
Univ. of Tehran
Computer Network
13
Examples of BGP Policies

A multi-homed AS refuses to act as
transit


A multi-homed AS can become transit for
some AS’s


Limit path advertisement
Only advertise paths to some AS’s
An AS can favor or disfavor certain AS’s
for traffic transit from itself
Univ. of Tehran
Computer Network
14
Routing Information Bases
(RIB)




Routes are stored in RIBs
Adj-RIBs-In: routing info that has been
learned from other routers (unprocessed
routing info)
Loc-RIB: local routing information
selected from Adj-RIBs-In (routes
selected locally)
Adj-RIBs-Out: info to be advertised to
peers (routes to be advertised)
Univ. of Tehran
Computer Network
15
BGP Common Header
1
0
2
3
Marker (security and message delineation)
16 bytes
Length (2 bytes)
Type (1 byte)
Types: OPEN, UPDATE, NOTIFICATION, KEEPALIVE
Univ. of Tehran
Computer Network
16
BGP OPEN message
1
0
2
3
Marker (security and message delineation)
Length
Type: open
version
My autonomous system
Hold time
BGP identifier
Parameter length
Optional parameters <type, length, value>
My AS: id assigned to that AS
Hold timer: max interval between KEEPALIVE or UPDATE messages
interval implies no keep_alive.
BGP ID: IP address of one interface (same for all messages)
Univ. of Tehran
Computer Network
17
BGP UPDATE message
1
0
2
3
Marker (security and message delineation)
Length
..routes len
Type: update
Withdrawn..
Withdrawn routes (variable)
...
Path attribute len
Path attributes (variable)
Network layer reachability information (NLRI) (variable)
•Many prefixes may be included in UPDATE, but must
share same attributes.
•UPDATE message may report multiple withdrawn routes.
Univ. of Tehran
Computer Network
18
BGP UPDATE Message


List of withdrawn routes
Network layer reachability information


Path attributes




List of reachable prefixes
Origin
Path
Metrics
All prefixes advertised in a message have
same path attributes
Univ. of Tehran
Computer Network
19
NLRI

Network Level Reachability Information

list of IP address prefixes encoded as follows:
Length (1 byte) Prefix (variable)
Univ. of Tehran
Computer Network
20
Path attributes
Type-Length-Value encoding
Attribute type (2 bytes) Attribute length (1-2 bytes)
Attribute Value (variable length)
Attribute type field
Attribute flags (1 byte) Attribute type code (1 byte)
Flags: optional, v.s. well-known
transitive, partial, extended length
Univ. of Tehran
Computer Network
21
BGP NOTIFICATION message
1
0
2
3
Marker (security and message delineation)
Length
Type: NOTIFICATION
Error code
Error sub-code
Data
•Used for error notification
TCP connection is closed immediately after notification
Univ. of Tehran
Computer Network
22
BGP KEEPALIVE message
0
1
2
3
Marker (security and message delineation)
Length
Type: KEEPALIVE
Sent periodically to peers to ensure connectivity.
If hold_time is zero, messages are not sent..
Sent in place of an UPDATE message
Univ. of Tehran
Computer Network
23
Path Selection Criteria



Information based on path attributes
Attributes + external (policy) information
Examples:


Hop count
Policy considerations




Preference for AS
Presence or absence of certain AS
Path origin
Link dynamics
Univ. of Tehran
Computer Network
24
Route Selection Summary
Highest Local Preference
Enforce relationships
Shortest ASPATH
Lowest MED
traffic engineering
i-BGP < e-BGP
Lowest IGP cost
to BGP egress
Throw up hands and
break ties
Lowest router ID
Univ. of Tehran
Computer Network
25
Back to Frank …
peer
provider
peer
Local preference only used in iBGP
customer
AS 4
local pref = 80
local pref = 90
AS 3
local pref = 100
AS 2
Higher Local
preference values
are more preferred
AS 1
13.13.0.0/16
26
Implementing Backup Links with
Local Preference (Outbound
Traffic)
AS 1
primary link
Set Local Pref = 100
for all routes from AS 1
backup link
AS 65000
Set Local Pref = 50
for all routes from AS 1
Forces outbound traffic to take primary link, unless link is down.
We’ll talk about inbound traffic soon …
27
Multihomed Backups
(Outbound Traffic)
AS 1
AS 3
provider
provider
primary link
backup link
Set Local Pref = 100
for all routes from AS 1
Set Local Pref = 50
for all routes from AS 3
AS 2
Forces outbound traffic to take primary link, unless link is down.
28
ASPATH Attribute
AS 1129
135.207.0.0/16
AS Path = 1755 1239 7018 6341
135.207.0.0/16
AS Path = 1239 7018 6341
AS 1239
Sprint
AS 1755
135.207.0.0/16
AS Path = 1129 1755 1239 7018 6341
Ebone
AS 12654
AS 6341
AT&T Research
RIPE NCC
RIS project
135.207.0.0/16
AS Path = 7018 6341
AS7018
135.207.0.0/16
AS Path = 6341
Global Access
135.207.0.0/16
AS Path = 3549 7018 6341
AT&T
135.207.0.0/16
AS Path = 7018 6341
AS 3549
Global Crossing
135.207.0.0/16
Prefix Originated
29
COMMUNITY Attribute to the Rescue!
AS 1
AS 3
provider
provider
AS 3: normal
customer local
pref is 100,
peer local pref is 90
192.0.2.0/24
ASPATH = 2
COMMUNITY = 3:70
192.0.2.0/24
ASPATH = 2
primary
backup
customer
AS 2
192.0.2.0/24
Customer import policy at AS 3:
If 3:90 in COMMUNITY then
set local preference to 90
If 3:80 in COMMUNITY then
set local preference to 80
If 3:70 in COMMUNITY then
set local preference to 70
30
Hot Potato Routing: Go for the
Closest Egress Point
192.44.78.0/24
egress 2
egress 1
15
56
IGP distances
This Router has two BGP routes to 192.44.78.0/24.
Hot potato: get traffic off of your network as
Soon as possible. Go for egress 1!
31
Getting Burned by the Hot Potato
2865
High bandwidth
Provider backbone
17
SFF
Low bandwidth
customer backbone
Heavy
Content
Web Farm
NYC
15
56
San Diego
Many customers want
their provider to
carry the bits!
tiny http request
huge http reply
32
Cold Potato Routing with MEDs
(Multi-Exit Discriminator Attribute)
Prefer lower
MED values
2865
17
Heavy
Content
Web Farm
192.44.78.0/24
MED = 56
192.44.78.0/24
MED = 15
15
56
192.44.78.0/24
This means that MEDs must be considered BEFORE
IGP distance!
Note1 : some providers will not listen to MEDs
Note2 : MEDs need not be tied to IGP distance
33
Route Selection Summary
Highest Local Preference
Enforce relationships
Shortest ASPATH
Lowest MED
traffic engineering
i-BGP < e-BGP
Lowest IGP cost
to BGP egress
Throw up hands and
break ties
Lowest router ID
This is somewhat simplified. Hey, what happened to ORIGIN??
Univ. of Tehran
Computer Network
34
Policies Can Interact Strangely
(“Route Pinning” Example)
backup
customer
1
3
2
Disaster strikes primary link
and the backup takes over
Univ. of Tehran
4
Install backup link using community
Primary link is restored but some
traffic remains pinned to backup
Computer Network
35
Path Attributes

Categories (recall flags):





well-known mandatory (passed on)
well-known discretionary (passed on)
optional transitive (passed on)
optional non-transitive (if unrecognized, not
passed on)
Optional attributes allow for BGP
extensions
Univ. of Tehran
Computer Network
36
Path attribute message format
(repeated)
Attribute flags
OTPE
Attribute type code
0
O: optional or well-known
T: transitive or local
P: partially evaluated
E: length in 1 or 2 bytes
Univ. of Tehran
Computer Network
Origin
AS_path
Next hop
etc.
37
ORIGIN path attribute


Well-known, mandatory attribute.
Describes how a prefix was generated at
the origin AS. Possible values:



IGP: prefix learned from IGP
EGP: prefix learned through EGP
INCOMPLETE: none of the above (often seen
for static routes)
Univ. of Tehran
Computer Network
38
AS_PATH attribute


Well-known, mandatory attribute.
Important components:


If forwarding to internal peer:


list of traversed AS’s
do not modify AS_PATH attribute
If forwarding to external peer:

prepend self into the path
Univ. of Tehran
Computer Network
39
Next hop path attribute




Well-known, mandatory attribute
NEXT_HOP: IP address of border router
to be used as next hop
Usually, next hop is the router sending
the UPDATE message
Useful when some routers do not speak
BGP
Univ. of Tehran
Computer Network
40
Example of NEXT_HOP
A
UPDATE MSG through BGP
(BGP)
B
(BGP)
Traffic to 138.39.0.0/16
C
(no BGP)
138.39.0.0/16
Univ. of Tehran
Computer Network
41
LOCAL PREF

Local (within an AS) mechanism to
provide relative priority among BGP
routers
R5
R1
AS 200
R2
AS 100
AS 300
R3 Local Pref = 500
Local Pref =800
R4
I-BGP
AS 256
Univ. of Tehran
Computer Network
42
AS_PATH

List of traversed AS’s
AS 200
AS 100
170.10.0.0/16
180.10.0.0/16
AS 300
AS 500
Univ. of Tehran
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200
Computer Network
43
CIDR and BGP
AS X
197.8.2.0/24
AS T (provider)
197.8.0.0/23
AS Z
AS Y
197.8.3.0/24
What should T announce to Z?
Univ. of Tehran
Computer Network
44
Options

Advertise all paths:




Path 1: through T can reach 197.8.0.0/23
Path 2: through T can reach 197.8.2.0/24
Path 3: through T can reach 197.8.3.0/24
But this does not reduce routing tables!
We would like to advertise:

Path 1: through T can reach 197.8.0.0/22
Univ. of Tehran
Computer Network
45
Sets and Sequences

Problem: what do we list in the route?



List T: omitting information not acceptable, may lead to
loops
List T, X, Y: misleading, appears as 3-hop path
Solution: restructure AS Path attribute as:


Path: (Sequence (T), Set (X, Y))
If Z wants to advertise path:


Path: (Sequence (Z, T), Set (X, Y))
In practice used only if paths in set have same attributes
Univ. of Tehran
Computer Network
46
Multi-Exit Discriminator (MED)

Hint to external neighbors about the
preferred path into an AS



Non-transitive attribute (we will see later
why)
Different AS choose different scales
Used when two AS’s connect to each
other in more than one place
Univ. of Tehran
Computer Network
47
MED


Hint to R1 to use R3 over R4 link
Cannot compare AS40’s values to AS30’s
180.10.0.0
MED = 50
R1
AS 10
R3
R2
AS 40
180.10.0.0
MED = 120
180.10.0.0
MED = 200
R4
AS 30
Univ. of Tehran
Computer Network
48
MED
• MED is typically used in provider/subscriber scenarios
• It can lead to unfairness if used between ISP because it
may force one ISP to carry more traffic:
ISP1
SF
ISP2
NY
• ISP1 ignores MED from ISP2
• ISP2 obeys MED from ISP1
• ISP2 ends up carrying traffic most of the way
Univ. of Tehran
Computer Network
49
Other Attributes

ORIGIN


NEXT_HOP



Source of route (IGP, EGP, other)
Address of next hop router to use
Used to direct traffic to non-BGP router
Check out http://www.cisco.com for full
explanation
Univ. of Tehran
Computer Network
50
Decision Process

Processing order of attributes:



Select route with highest LOCAL-PREF
Select route with shortest AS-PATH
Apply MED (if routes learned from same
neighbor)
Univ. of Tehran
Computer Network
51
Outline

External BGP (E-BGP)

Internal BGP (I-BGP)

Multi-Homing

Stability Issues
Univ. of Tehran
Computer Network
52
Internal vs. External BGP
•BGP can be used by R3 and R4 to learn routes
•How do R1 and R2 learn routes?
•Option 1: Inject routes in IGP
•Only works for small routing tables
•Option 2: Use I-BGP
R1
AS1
R3
E-BGP
R4
AS2
R2
Univ. of Tehran
Computer Network
53
I-BGP
Univ. of Tehran
Computer Network
54
Internal BGP (I-BGP)


Same messages as E-BGP
Different rules about re-advertising
prefixes:



Prefix learned from E-BGP can be advertised
to I-BGP neighbor and vice-versa, but
Prefix learned from one I-BGP neighbor
cannot be advertised to another I-BGP
neighbor
Reason: no AS PATH within the same AS and
thus danger of looping.
Univ. of Tehran
Computer Network
55
Internal BGP (I-BGP)
• R3 can tell R1 and R2 prefixes from R4
• R3 can tell R4 prefixes from R1 and R2
• R3 cannot tell R2 prefixes from R1
R2 can only find these prefixes through a direct connection to R1
Result: I-BGP routers must be fully connected (via TCP)!
• contrast with E-BGP sessions that map to physical links
R1
AS1
E-BGP
R3
R4
AS2
R2
I-BGP
Univ. of Tehran
Computer Network
56
Link Failures

Two types of link failures:




Failure on an E-BGP link
Failure on an I-BGP Link
These failures are treated completely
different in BGP
Why?
Univ. of Tehran
Computer Network
57
Failure on an E-BGP Link
• If the link R1-R2 goes down
• The TCP connection breaks
• BGP routes are removed
• This is the desired behavior
E-BGP session
AS1
R1
R2
AS2
Physical link
138.39.1.1/30
Univ. of Tehran
138.39.1.2/30
Computer Network
58
Failure on an I-BGP Link
•If link R1-R2 goes down, R1 and R2 should still be able to
exchange traffic
•The indirect path through R3 must be used
•Thus, E-BGP and I-BGP must use different conventions with
respect to TCP endpoints
138.39.1.2/30 R2
Physical link
138.39.1.1/30
R1
R3
I-BGP connection
Univ. of Tehran
Computer Network
59
Outline

External BGP (E-BGP)

Internal BGP (I-BGP)

Multi-Homing

Stability Issues
Univ. of Tehran
Computer Network
60
Multi-homing


With multi-homing, a single network has
more than one connection to the
Internet.
Improves reliability and performance:



Can accommodate link failure
Bandwidth is sum of links to Internet
Challenges


Getting policy right (MED, etc..)
Addressing
Univ. of Tehran
Computer Network
61
Multi-homing to a Single
Provider Case 1

Easy solution:


Use IMUX or Multi-link
PPP
ISP
Hard solution:


R1
Use BGP
Makes assumptions
about traffic (same
amount of prefixes
can be reached from
both links)
Univ. of Tehran
R2
Customer
Computer Network
62
Multi-homing to a single
provider: Case 2

If multiple prefixes,
may use MED


good if traffic load
from prefixes is equal
ISP
If single prefix, load
may be unequal

break-down prefix and
advertise different
prefixes over different
links
Univ. of Tehran
R1
138.39/16
Computer Network
R2
R3
Customer
204.70/16
63
Multi-homing to a single
provider: Case 3

For ISP-> customer
traffic, same as
before:



use MED
good if traffic load to
prefixes is equal
ISP
R1
For customer -> ISP
traffic:


R3 alternates links
multiple default routes
Univ. of Tehran
R2
R3
138.39/16
Computer Network
Customer
204.70/16
64
Multi-homing to a single
provider: Case 4

Most reliable
approach


Customer -> ISP:


no equipment sharing
ISP
same as case 2
R1
R2
R3
R4
ISP -> customer:

same as case 3
138.39/16
Univ. of Tehran
Computer Network
Customer
204.70/16
65
Multi-homing to Multiple
Providers

Major issues:



ISP3
Customer address space:





Addressing
Aggregation
Delegated by ISP1
Delegated by ISP2
Delegated by ISP1 and ISP2
Obtained independently
ISP1
Customer
Advantage and
disadvantage?
Univ. of Tehran
ISP2
Computer Network
66
Address Space from one ISP







Customer uses address
space from one, I.e ISP1
ISP1 advertises /16
aggregate
Customer advertises /24
route to ISP2
ISP2 relays route to ISP1
and ISP3
ISP2-3 use /24 route
ISP1 routes directly
Problems with traffic load?
Univ. of Tehran
ISP3
138.39/16
ISP1
Computer Network
ISP2
Customer
138.39.1/24
67
Pitfalls





ISP1 aggregates to a /19 at
border router to reduce
internal tables.
ISP1 still announces /16.
ISP1 hears /24 from ISP2.
ISP1 routes packets for
customer to ISP2!
Workaround: ISP1 must
inject /24 into I-BGP.
ISP3
138.39/16
ISP1
ISP2
138.39.0/19
Customer
138.39.1/24
Univ. of Tehran
Computer Network
68
Address Space from Both ISPs




ISP1 and ISP2 continue to
announce aggregates
Load sharing depends on
traffic to two prefixes
Lack of reliability: if ISP1
link goes down, part of
customer becomes
inaccessible.
Customer may announce
prefixes to both ISPs, but
still problems with longest
match as in case 1.
Univ. of Tehran
ISP3
ISP1
138.39.1/24
Computer Network
ISP2
204.70.1/24
Customer
69
Address Space Obtained
Independently


Offers the most control,
but at the cost of
aggregation.
Still need to control paths



suppose ISP1 large, ISP2-3
small
customer advertises long
path to ISP1, but local-pref
attribute used to override
ISP3 learns shorter path
from ISP2
Univ. of Tehran
ISP3
ISP1
Computer Network
ISP2
Customer
70
Outline

External BGP (e-BGP)

Internal BGP (i-BGP)

Multi-Homing

Stability Issues
Univ. of Tehran
Computer Network
71
Signs of Routing Instability


Record of BGP messages at major
exchanges
Discovered orders of magnitude larger
than expected updates

Bulk were duplicate withdrawals



Stateless implementation of BGP – did not keep
track of information passed to peers
Impact of few implementations
Strong frequency (30/60 sec) components

Interaction with other local routing/links etc.
Univ. of Tehran
Computer Network
72
Route Flap Storm





Overloaded routers fail to send
Keep_Alive message and marked as down
I-BGP peers find alternate paths
Overloaded router re-establishes peering
session
Must send large updates
Increased load causes more routers to
fail!
Univ. of Tehran
Computer Network
73
Route Flap Dampening


Routers now give higher priority to
BGP/Keep_Alive to avoid problem
Associate a penalty with each route



Increase when route flaps
Exponentially decay penalty with time
When penalty reaches threshold,
suppress route
Univ. of Tehran
Computer Network
74
BGP Limitations: Oscillations
AS 0
(*R,1R,2R)
R
AS 1
AS 2
(0R,1R,*R)
Univ. of Tehran
(0R,*R,2R)
Computer Network
75
BGP Limitations: Oscillations
AS 0
(-,*1R,2R)  (*R,1R,2R)
W
R
W
W
AS 1
AS 2
(0R,1R,*R)  (*0R,1R,-)
Univ. of Tehran
(*0R,-,2R)  (0R,*R,2R)
Computer Network
76
BGP Limitations: Oscillations
AS 0
(-,*1R,2R)  (-,*1R,2R)
01R
01R
R
AS 1
AS 2
(*0R,1R,-) (01R,*1R,-)
Univ. of Tehran
(-,-,*2R) (*0R,-,2R)
Computer Network
77
BGP Limitations: Oscillations
AS 0
(-,-,*2R) (-,*1R,2R)
10R
R
AS 1
AS 2
(01R,*1R,-)  (*01R,10R,-)
Univ. of Tehran
10R
Computer Network
(-,-,*2R) (-,-,*2R)
78
BGP Limitations: Oscillations
AS 0
(-,-,-)  (-,-,*2R)
20R
R
AS 1
AS 2
(*01R,10R,-)  (*01R,10R,-)
Univ. of Tehran
20R
Computer Network
(-,-,*20R)  (-,-,*2R)
79
BGP Limitations: Oscillations
AS 0
(-,*12R,-)  (-,-,-)
12R
R
AS 1
AS 2
(*01R,10R,-) (*01R,-,-)
Univ. of Tehran
12R
(-,-,*20R)  (-,-,*20R)
Computer Network
80
BGP Limitations: Oscillations
AS 0
(-,*12R,21R)  (-,*12R,-)
21R
R
AS 1
AS 2
(*01R,-,-) (*01R,-,-)
Univ. of Tehran
21R
Computer Network
(-,-,-)  (-,-,*20R)
81
BGP Oscillations


Can possible explore every possible path
through network  (n-1)! Combinations
Limit between update messages
(MinRouteAdver) reduces exploration


Forces router to process all outstanding messages
Typical Internet failover times

New/shorter link  60 seconds


Down link  180 seconds


Results in simple replacement at nodes
Results in search of possible options
Longer link  120 seconds

Results in replacement or search based on length
Univ. of Tehran
Computer Network
82
Problems

Routing table size


Need an entry for all paths to all networks
Required memory= O((N + M*A) * K)




N: number of networks
M: mean AS distance (in terms of hops)
A: number of AS’s
K: number of BGP peers
Univ. of Tehran
Computer Network
83
Routing Table Size
Networks

Mean AS Distance Number of AS’s
BGP Peers/Net
Memory
2,100
5
59
3
27,000
4,000
10
100
6
108,000
10,000
15
300
10
490,000
100,000
20
3,000
20
1,040,000
Problem reduced with CIDR
Univ. of Tehran
Computer Network
84
Next Lecture: TCP


Transport layer issues
Assigned reading


[S+99] The End-to-End Effects of Internet
Path Selection
[Tsu88] The Landmark Hierarchy: A New
Hierarchy for Routing in Very Large Networks
Univ. of Tehran
Computer Network
85