An Overview of the IP Network Layer

1
An Overview of the IP Network Layer

• Tim Griffin
• griffin_at_research.att.com
• http//www.research.att.com/griffin
• December 1, 2000

2
Goal
Understand connectivity in the IP world
Todays class should give you a basic
understanding of how Internet Protocol (IP) data
packets find their way from one end of the
Internet to the other.
This course will not say much about the
applications that exploit this connectivity
3
Outline

• What is special about the IP network layer?
• What is an IP Routing Protocol?
• How is Addressing Implemented in IPv4?
• Routing inside an ISP.
• RIP
• OSPF
• Routing In the Global Internet
• How do ISPs exchange routing information?
• BGP

4
Networking Technologies
Packet Based
Circuit Based
(variable rate, store-and-forward)
(constant rate)
TDM Telephony
SONET/SDH
DWDM
Virtual Circuits
Connectionless
CLNP (ISO)
Frame Relay
SNA (IBM)
ATM
Appletalk
X.25
IPX (Novell)
IP
Connection Oriented
5
Connection Oriented vs. Connectionless
Connection Oriented

• Connection set up. Signaling reserves resources
  along the end-to-end path
• Traffic flows
• Connection torn down and resources freed

Connectionless
Send and Pray
6
Network Heterogeneity
Virtual Connections
Connectionless
Virtual Circuit
Circuits
7
IP Datagram
0 1 2
3 0 1 2 3 4 5 6 7 8 9 0 1 2
3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
-------------------------
------- Version IHL Service Type
Total Length
-------------------------
------- Identification
Flags Fragment Offset
-------------------------
------- Time to Live Protocol
Header Checksum
-------------------------
------- Source
Address
-------------------------
-------
Destination Address
-------------------------
------- Options
Padding
-------------------------
-------
H E A D E R
D A T A


... up to 65,515 octets of data ...
-------------------------
-------
shaded fields little-used today
1981, RFC 791
8
Famous Members of the IP Protocol Family
9
IP is a Network Layer Protocol
Separate physical networks glued together into
one logical network
Application
Application
Router
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
DataLink 1
DataLink 2
Physical 1
Physical 2
10
Encapsulation Example
An Ethernet segment transmitting HTTP data.
11
IP Hour Glass
Networking Applications
Remote Access
Voice
e-stuff
HOST
email
file transfer
Multimedia
Web
VPN
TCP
IP
Router
Minimalist network layer
Frame
ATM
Ethernet
DWDM
SONET
FDDI
Link
X.25
Networking Technologies
IP is the mother of all disruptive technologies!
12
QoS
Quality of Service vs. Quantity of
Service
13
Traditional Telco Network
The Brick. An internationally recognized symbol
of dumbness
14
IP Network
The user community now owns the compute power and
software.
15
How Dumb Can a Network Get?
The technical part of the Bell-head vs.
Net-head divide.

• In the Internet, intelligence is in Hosts
• IP is connectionless, best effort.
• Routing protocols today provide only connectivity
  and supports only one type of service best
  effort datagram
• Only real smarts left in the network layer
  Dynamic Routing Protocols, which provide
  end-to-end connectivity

16
Internet Engineering Task Force (IETF)
The cultural part of the Bell-head vs. Net-head
divide.

• We reject kings, presidents, and voting. We
  believe in rough consensus and running code. ---
  Dave Clark
• Publishes Requests for Comments or RFCs
• Some are designated as Internet Standards
• Working Groups
• Internet Drafts
• http//www.ietf.org

You must visit this site!
17
Hosts, Networks, and Routers
Host 7
Host 1
Network A
Host 2
Host 1
Router
Network C
Network B
Unique IP Address Network Number
Host Number
Host 12
Host 2
18
Actually, IP addresses Identify Interfaces
Host 7
Host 1
Network A
Host 2
Host 1
Network C
Network B
Machines can have more than one IP address. All
routers do!
Host 12
Host 2
19
IP Forwarding Table
Destination
Next Hop
Interface
Net A
Router 1
INT 7
Net B
Direct
INT 4
Net C, Host 3
Router 2
INT 3
Net C
Router 1
INT 7
A destination is usually a network. May also be
a host, or a gateway of last resort (default)
The next hop is either a directly connected
network or a router on a directly connected
network
A physical interface
20
IP Forwarding Process
1. Remove a packet from an input
queue
2. Check for sanity, decrement TTL
field
4. Place packet on correct output
queue
Forwarding Process
3. Match packets destination to a
table entry
If queues get full, just drop packets!
If queues get full, just drop packets!
IP Forwarding Table
Router
21
Route vs. Route

• Route sometimes means the end-to-end path
  traversed by data traffic
• Route often means the destination next hop
  entry in an IP forwarding table.

22
Routing Implementing End-to-End Paths with
Next Hop Forwarding Tables
23
Something Fishy Here?
B
R
A
C
The next-hop forwarding paradigm does not allow
router R to choose a route to A based on who
originated the traffic, B or C.
24
Bad Things Can Happen
Nxt Hop
Dest.
A
R1
Loops are routing problems, not forwarding
problems
Nxt Hop
Dest.
A
R2
25
Outline

• What is special about the IP network layer?
• What is an IP Routing Protocol?
• How is Addressing Implemented in IPv4?
• Routing inside an ISP.
• RIP
• OSPF
• Routing In the Global Internet
• How do ISPs exchange routing information?
• BGP

26
How are Forwarding Tables Populated?
Statically
Dynamically
Routers exchange information using ROUTING
PROTOCOLS that compute best routes
Administrator manually configures table entries
More control Not restricted to
destination-based forwarding - Doesnt
scale - Slow to adapt to network failures
Can rapidly adapt to changes in network
topology Can be made to scale well - Complex
distributed algorithms - Consume CPU,
Bandwidth, Memory - Debugging can be hell -
Current protocols are destination-based
In practice a mix of these.
27
Dynamically route around network congestion? NO!

• IP traffic is very bursty
• Dynamic adjustments in routing typically operate
  more slowly than fluctuations in traffic load
• Attempt to adapt routing to account for load can
  lead to wild, unstable oscillations of routing
  system

28
Terminology Alert!
Net Head Term
Possible Bell Head Meaning
Net Head Meaning
Static Routing
Manual provisioning of routing table entries
Nailed up route
Dynamic Routing
Automatically route around network
failures. Alternate paths selected dynamically.
Automatically route around network failures and
congestion. Alternate paths often predetermined.
May vary with time of day.
Adaptive Routing
Automatically route around network failures,
and congestion.
????
Use of these terms may vary .
29
Architecture of Routing Protocols
Interior Gateway Protocols (IGP) inside
autonomous systems
Exterior Gateway Protocols (EGP) between
autonomous systems
AS 701
UUNet
OSPF, IS-IS, RIP, EIGRP, ...
BGP
Metric Based
Policy Based
ATT Common Backbone
ATT Research
AS 6431
AS 7018
30
The Most Common Routing Protocols
BGP
RIP
Cisco proprietary
UDP
OSPF
IS-IS
TCP
EIGRP
IP (and ICMP)
Routing protocols exchange network reachability
information between routers.
31
What is a Routing Process?
Manual configuration
import information from other routers
export information to other routers
Routing Process
Protocol-Specific Routing Table
OS kernel
IP Forwarding Table
Router
32
Many routing processes can run on a single router
BGP
OS kernel
RIP Domain
OSPF Domain
IP Forwarding Table
33
Routing Configuration for IP Routers

• Configure List of Interfaces, with IP addresses
  and subnet masks
• List of Neighbors
• Link weights
• Define static routes
• Import routes from other routing domains
• Configure protocol-specific, vendor-specific
  parameters for each routing protocol

Expressed in low-level configuration files
34
Outline

• What is special about the IP network layer?
• What is an IP Routing Protocol?
• How is Addressing Implemented in IPv4?
• Routing inside an ISP.
• RIP
• OSPF
• Routing In the Global Internet
• How do ISPs exchange routing information?
• BGP

35
IPv4 Implementation of Addresses
Thirty Two Bits
0
8
16
24
11111111
00010001
10000111
00000000
0
255
134
17
255.17.134.0
Dotted Quad notation for human readability
36
IP Addresses come in two parts
Where is this dividing line? Well, that depends
....
37
Classful Addresses
0nnnnnnn
hhhhhhhh
hhhhhhhh
hhhhhhhh
Class A
10nnnnnn
nnnnnnnn
hhhhhhhh
hhhhhhhh
Class B
nnnnnnnn
nnnnnnnn
hhhhhhhh
110nnnnn
Class C
h host identifier bit
n network address bit
1981, RFC 791 (definition of IPv4)
38
The Classful Address Space
Leads to very inefficient allocation of addresses

39
Flat Network Addressing
Y
P
Exports at least 12 network addresses
W
T
This router needs at least 12 table entries
X
R
Q
N
U
S
Z
V
40
Hierarchical Network Addressing
Network Z-T
T
Exports only one Address
P
M
This is called Aggregation
Q
W
Z
This Router needs only 3 table entries for
networks Z-T, Z-S, and Z-U
S
P
M
W
F
S
K
X
J
U
Network Z-U-X
Network Z
41
Best Match Forwarding
Lookup in IP forwarding table is no longer based
on exact match of network prefix.
Destination Address Network X-W-Y-V, Host 12
Destination
Next Hop
X
R2
Match
X-W-Y
R7
Best Match
X-W
R5
Match
42
The IPv4 hack
Use two 32 bit numbers to represent a network.
Network number IP address Mask
IP Address 12.4.0.0 IP Mask 255.254.0.0
Usually written as 12.4.0.0/15
43
Which IP addresses are covered?
IP address 12.5.9.16 is covered by network
12.4.0.0/15
12.5.9.16
12.4.0.0/15
12.7.9.16
IP address 12.7.9.16 is not
44
Mask may vary with location
12.0.0.0/16

12.1.0.0/16
12.3.0.0/24
12.2.0.0/16
12.3.1.0/24

12.3.0.0/16

12.0.0.0/8
12.3.254.0/24
This allows all of these (sub)networks to be
aggregated into one entry in an IP forwarding
table
12.253.0.0/16
12.254.0.0/16
45
Private Address Space
10.0.0.0/8 (10.0.0.0 ---
10.255.255.255) 172.16.0.0/12 (172.16.0.0 ---
172.31.255.255) 192.168.0.0/16 (192.168.0.0 ---
192.168.255.255)
RFC 1918
Private Addresses Network Address
Translation (NAT) Firewalls
No need for IPv6
NATs are not compatible with some VPN
technologies such as IPSec and tunneling. Can
break other applications.
An ongoing debate
46
What do routing protocols exchange?
Network numbers Network number IP
address Mask

Reachability information How to reach a
given network
depends on protocol
47
Outline

• What is special about the IP network layer?
• What is an IP Routing Protocol?
• How is Addressing Implemented in IPv4?
• Routing inside an ISP.
• RIP
• OSPF
• Routing In the Global Internet
• How do ISPs exchange routing information?
• BGP

48
Basic Model Finding Shortest Paths
Find paths from a source to all other nodes that
minimizes sum of arc metrics
B
50
100
20
100
20
Arc metrics are statically provisioned IP
routing protocols DO NOT route around network
congestion!
80
C
A
30
Source
80
20
10
10
20
10
10
10
20
30
10
D
E
49
Technology Distance Vector vs. Link State
Protocols
How can shortest path computation be
decentralized and performed by many cooperating
routers?
Solution to shortest path problem

Directed Graph data structures
Distance Vector
Link State
Arcs, Nodes, Arc Weights
Distribute path computation. Keep only local
link data.
Distribute all link data. Perform
path computations locally.

Algorithm
Dijkstras or Bellman-Ford
RIP, EIGRP, BGP
OSPF, IS-IS, IDPR
50
RIP

• RIP Routing Information Protocol
• Does not scale well, designed for small LANs
• Is a distance vector protocol
• Very simple, easy to configure, easy to implement
• Is most widely used routing protocol

51
RIP History

• Developed at Xerox PARC in early 1980s
• Reimplemented in Berkeley UNIX
• 1988 Standardized in RFC 1058
• 1994 RIP-2, RFC 1723
• Support CIDR addressing
• Authentication
• 1997 RIPng for IPv6, RFC 2080

52
RIP Routing Table
Destination
Next Hop
Metric
Net A
Router 1
3
Net B
Direct
0
Net C, Host 3
Router 2
5
Router 1
0
Default
The next hop is either a directly connected
network or a directly connected router
A destination is either a network, a host, or a
gateway of last resort
Measures how many hops away is the destination
53
Basic RIP Protocol
Periodically exchange list of destinations and
metrics with all neighboring routers
RIP routers exchange their entire
distance vector every 30 seconds
54
Basic RIP Protocol (cont.)
Trust your neighbor...
Is Dest. A in my RIP Table?
NO
Yes
Is N my next hop for Dest. A and m c is not the
current metric?
Yes
NO
Is m c less than current metric for Dest. A?
Yes
55
OSPF

• OSPF Open Shortest Path First
• Developed to address shortcomings of RIP
• has rapid, loop-free convergence
• does not count to infinity
• Link metrics between 0 and 65,535, no limit on
  path metric
• Is a link state protocol
• Has reputation for being complex
• Scales well
• Defined in RFCs 1247 (1991), 1583 (1994), 2178
  (1997), 2328 (1998).

56
Link State Database
Each Router has a database representing the
entire network that is constructed from the local
knowledge at each router
57
Building OSPF Routing Table
Dijkstra
B
Dest.
Nxt Hop
Metric
50
100
20
B
D
50
100
20
80
C
A
C
D
30
ME
30
80
D
D
20
20
10
10
20
10
10
D
E
30
10
20
30
10
D
E
Compute locally using Link State Database!
58
Thats Easy!
Not so fast!
Much of this complexity is related to
the synchronization of the distributed,
replicated link state database. Plus network
modeling .
RIP RFC 1058 33 pages
OSPF RFC 2328 244 pages
59
Scalability OSPF Areas
LS database unique within an area

• Decentralize administration
• Reduce memory usage per router
• Reduce bandwidth used by flooding

Area 0
...
Special OSPF protocol to exchange routes between
areas. This is a distance vector protocol!
60
Traffic engineering is hard with current protocols

• Link metrics attract or repel all traffic
• Results in congestion and unused capacity
• Destination based, next-hop forwarding paradigm
  has very coarse granularity
• Routing and forwarding are distinct, but still
  very tightly coupled

MPLS to the rescue??
61
Outline

• What is special about the IP network layer?
• What is an IP Routing Protocol?
• How is Addressing Implemented in IPv4?
• Routing inside an ISP.
• RIP
• OSPF
• Routing In the Global Internet
• How do ISPs exchange routing information?
• BGP

62
Interdomain routing routing between autonomous
systems
UUNet
Sprint
134.244.0.0/16
AS 701
AS 1239
ATT Common Backbone
AS 7018
ATT Research
Fidelity Investments
AS 6431
AS 11040
207.104.168.0/24
192.223.184.0/21
63
Why not just use OSPF?

• Scale
• The Internet is very large
• Policy
• My good route might be your bad route

64
An ASN represents a unit of Routing Policy
834 Canada Long Distance Services 1740
CerfNet 1838 CerfNet 2386 Data Communications
Services 2685 AGNS (IBM Network) 2713 Jens
Corporation 2740 Canada Long Distance
Services 2751Paradyne 2767 Canada Long Distance
Services 2768 Canada Long Distance Services 2855
Concert 2915 Jens Corporation 3300 ATT
Unisource 3448 ATT Wireless 3668 Network
Systems 4465 Easy Link Services 4466 Easy Link
Services 4467 Easy Link Services 4468 Easy Link
Services 4661 Easy Link Asia-Pacific 5074 Dial
Platform (BMGS) 5075 FDDI Access Ring (BMGS)
5400 Concert 5727 Asia-Pacific PVCs 5728
WorldNet Dial Platform 5729 WorldNet
Services 5730 FDDI Access Ring 5731 WorldNet
Services 6269 ATT-Internet2 6290
ALASCOM-DIAL 6308 ALASCOM-MIS 6431 ATT Labs
6478 Worldnet Services 6537 CANADA LONG
DISTANCE SERVICES 6742 ATT London UK 6905
European Internal 6934 Columbus 7018
WorldNet Backbone 7170 Government Markets
7329 CMS 7948 GEOPLEX 8030 WorldNet 8031
WorldNet 8032 WorldNet 8033 WorldNet 8034
WorldNet 8035 WorldNet 10456 Government
Markets 11698 ATT-PWSS 13979 ATT-IPFR 14005
ATT Y2K ICC 14470 ATT-PWSS-2 15259
Solutions 15290 Canada IES 17224 Enhanced Network
Services 17225 Enhanced Network Services 17226
Enhanced Network Services 17227 Enhanced Network
Services 17228 Enhanced Network Services 17229
Enhanced Network Services 17230 Enhanced Network
Services 17231 Enhanced Network Services 17232
Enhanced Network Services 17233 Enhanced Network
Services
64 ASNs known to be assigned to ATT (list may
not be complete!)
33 (in green) seen in BGP routing tables
on 11/25/2000 (by RouteDB)
65
How many ASNs are there?
From http//www.telstra.net/ops/bgptable.html on
11/26/2000
66
Sample prefixes from announced to the CBB

• 207.104.168.0/24 from ATT Labs (covers 256 IP
  addresses in range 207.104.168.0 to
  207.104.168.255)
• 192.223.184.0/21 from Fidelity Investments
• 134.244.0.0/16 from UUnet

These are called networks or prefixes or
CIDR blocks or network blocks or routes.
Yes, it is confusing.
67
How Many Prefixes are There?
From http//www.telstra.net/ops/bgptable.html on
11/26/2000
68
How many hosts are there?
For more information, see http//www.isc.org/ds
69
Policy Transit vs. Nontransit
A transit AS allows traffic with neither source
nor destination within AS to flow across the
network
AS 701
ATT CBB
AS 701
UUnet
AS144
A nontransit AS allows only traffic originating
from AS or traffic with destination within AS
Bell Labs
IP traffic
70
Policy-Based vs. Distance-Based Routing?
Host 1
Cust1
Minimizing hop count can violate commercial
relationships that constrain inter- domain
routing.
ISP1
ISP3
Host 2
ISP2
Cust3
Cust2
71
Why not minimize AS hop count?
National ISP1
National ISP2
Regional ISP3
Regional ISP1
Regional ISP2
Cust2
Cust3
Cust3
72
BGP-4

• BGP Border Gateway Protocol
• Is a Policy-Based routing protocol
• Is the de facto EGP of todays 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)

73
BGP Operations Simplified
Establish Peering on TCP port 179
BGP
Peers Exchange All Routes
While connection is ALIVE exchange route UPDATE
messages
Exchange Incremental Updates
74
Two Types of BGP Neighbor Relationships

• External Neighbor (eBGP) in a different
  Autonomous Systems
• Internal Neighbor (iBGP) in the same Autonomous
  System

AS1
eBGP
iBGP
Physical Connection
AS2
Logical (TCP) Connection
75
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
Network prefix attributes
76
BGP Attributes

• Local Preference Used by IBGP to rank routes
  within an autonomous system
• AS path list of ASNs the announcement traversed
• Next Hop IP address of external neighbors
  interface
• Origin How was the route originated?
• Multi Exit Discriminator preference of egress
  points
• Community arbitrary colors for routes
• Atomic Aggregate indicates if info has been lost
• Aggregator which ASN lost the information
• Originator ID for ibgp route reflectors
• .

RIP distance vector, BGP path vector
77
AS Path Attribute
AS1849
135.104.0.0/16 AS Path 702 701 144
Uunet UK
AS702
135.104.0.0/16 AS Path 701 144
135.104.0.0/16 AS Path 5459 5413 7018 144
Alternet (Uunet)
AS5459
LINX
135.104.0.0/16 AS Path 5413 7018 144
135.104.0.0/16 AS Path 144
AS7018
AS5413
GXN
ATT CBB
135.104.0.0/16 AS Path 144
135.104.0.0/16 AS Path 7018 144
78
AS Path Attribute (cont.)
BGP at AS YYY will never accept a route whose AS
Path contains YYY. This avoids interdomain
routing loops.
AS702
UUnet
10.22.0.0/16 AS Path 1 333 702 877
Dont Accept!
79
Next Hop Attribute
33.0.0.3
AS701
11.0.0.1
UUnet
AS144
AS702
UUnet
Bell Labs
135.104.0.0/16 Next Hop 33.0.0.3
135.104.0.0/16 Next Hop 11.0.0.1
135.104.0.0/16 Next Hop 11.0.0.1
Every time a route announcement crosses an AS
boundary, the Next Hop attribute is changed to
the IP address of the border router that
announced the route.
80
Local Preference Attribute
Forces outbound traffic to take primary link,
unless link is down.
AS 7018
ATT CBB
Backup T1 line
Primary T3 line
Set Local Pref 50 for all routes from 7018
Set Local Pref 100 for all routes from 7018
Customer
81
Local Preference Attribute
Used only in iBGP to prefer a point of exit
Franks Upstream Provider
AS 4
13.13.0.0/16 AS Path 4 1 Loc pref 80
13.13.0.0/16 AS Path 3 1 Loc pref 90
Franks Internet Barn
Franks Local Competition
AS 3
13.13.0.0/16 AS Path 2 1 Loc pref 100
Franks Customer
AS 2
Customer of Franks Customer
Higher Local Preference Values are more preferred
AS 1
13.13.0.0/16
82
Multi Exit Discriminator
Used only in eBGP to suggest preferred points
of entry to a BGP neighbor
AS 7018
ATT CBB
192.44.78.0/24 MED 56
192.44.78.0/24 MED 35
56
35
192.44.78.0/24
Lower MED values are more preferred
Set MED to metric of IGP at each border router
83
BGP Route Processing
Open ended programming. Constrain
ed only by vendor configuration language
Apply Policy filter routes tweak attributes
Apply Policy filter routes tweak attributes
Receive BGP Updates
Best Routes
Transmit BGP Updates
Based on Attribute Values
Best Route Selection
Apply Import Policies
BGP Route Table
Apply Export Policies
Install forwarding Entries for best Routes.
IP Forwarding Table
84
BGP Best Route Selection Process
Can select at most one route to any given prefix

• Prefer routes with highest local preference
• Then prefer routes with shortest AS-paths
• Then prefer routes with lowest origin (IGP lt EGP
  lt INCOMPLETE)
• Then prefer lowest MEDs (if routes are from same
  neighbor ASN)
• Then prefer EBGP learned routes over IBGP learned
  routes
• Then prefer routes with lowest IGP cost to BGP
  next-hop
• Then break ties by selecting route with lowest
  BGP next-hop

This is somewhat simplified .
85
BGP Routing Tables
show ip bgp
Network Next Hop Metric
LocPrf Path 63.108.216.0/21 137.39.23.173
81 701 701 10409 i i
12.123.210.15 0 100
10409 10409 10409 i gti
12.123.210.15 0 100 10409
10409 10409 i i63.109.3.0/24 12.123.9.240
0 82 1239 16886 i gti
12.123.9.240 0 82 1239
16886 i 63.109.64.0/24 137.39.23.173
81 701 8143 i gti
12.123.21.242 0 82 3561 8143
i i 12.123.21.242 0
82 3561 8143 i i63.109.65.0/24
12.123.21.242 0 82 3561 8143
? gti 12.123.21.242 0
82 3561 8143 ?
137.39.23.173 81 701 8143 ?

• CBB tables often have 100K 200K routes.
• Use whois queries to associate an ASN with
  owner (for example, http//www.arin.net/whois/ar
  inwhois.html)
• 701 Uunet, 10409 allegro.net, 1239 Sprint,
  16886 LVMH group, 8143 Publicom, 3561 Cable
  Wireless

86
BGP Communities
Very powerful BECAUSE it is meaningless
Community Attribute is 32 bits
By convention, first 16 bits is ASN of owner of
community definition
community number
groups routes into logical collections
Two reserved communities
no_export (0xFFFFFF01) dont export out
of AS
A route can belong to more than one community
no_advertise (0xFFFFFF02) dont pass to BGP
peers
RFC 1997 (August 1996)
87
Implementing Backup Policies with Communities
Connect Net wants to provide backup Internet
access for Global One in case its connection to
Sprint is lost.
1996, RFC 1998
88
First, Without Communities ...
Connect Net and Global One would have to convince
Telstra to do some custom configuration its Local
Pref value for some routes..
89
With Communities .
Telstras communities
1221,70 Ill set local pref to 70 (cust.
provided backup) 1221,80 Ill set local pref
to 80 (other ISP routes) 1221,90 Ill set
local pref to 90 (my customer
backup) 1221,100 Ill set local pref to 100
(my customers)
90
Global One Using Telstras Communities
AS1221
Telstra
Tag these routes with community 1221, 80
Tag these routes with community 1221, 70
All I have to do is pass along AS4805s routes
to AS1221
91
iBGP Peers Must be Fully Meshed

• N border routers means N(N-1)/2 peering sessions
  -- this does not scale
• Currently three solutions
• Break an AS up into smaller Autonomous Systems
• Route Reflectors
• Confederations

iBGP peers do not announce routes received via
iBGP
92
Route Reflectors
Route Reflectors must be fully meshed
Route Reflectors pass along updates to client
routers
93
Route Flapping
Routes that go up and down are said to be
flapping
Flapping routes are often penalized, and if
penalty goes over a threshold, they are
Suppressed for some period of time.
94
Recommended Reading

• IDR http//www.ietf.org/html.charters/idr-charte
  r.html
• Internet Routing Architectures. Bassam Halabi. .
  The BGP Bible! Get second edition (2000).
• BGP4 Inter-domain Routing in the Internet. John
  W. Stewart, III. 1999. Good high-level
  introduction.
• Routing in the Internet. Christian Huitema. 2000.
• OSPF -- Anatomy of an Internet Routing Protocol.
  John T. Moy. 1998.
• Managing IP Networks with Cisco Routers. Scott M.
  Ballew. 1997.

95
Links
These slides
www.research.att.com/griffin/IPcourses.html
Links on interdomain routing
www.research.att.com/griffin/interdomain.html
96
What I Didnt talk about
Applications
FTP
HTTP
Telnet
RTP
SNMP
DNS
UDP
TCP
IP Layer
97
Selected RD work on IP Network Management

• Bravo backbone routing analysis, visualization,
  and optimization
• Tap collection and analysis of traffic flow
  data
• Metrica SNMP polling
• Netdb router configuration management
• Peermon monitoring CBB from the outside
• Routdb monitoring CBB routes from the inside
• Topology server building live network maps
• Falcon fault correlation, visualization
• Wipm active delay measurement