an overview of the ip network layer.ppt
TRANSCRIPT
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An Overview of the IP Network Layer
Tim Griffin [email protected]
http://www.research.att.com/~griffin
December 1, 2000
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Goal
Today’s class should give you a basic understandingof how Internet Protocol (IP) data packets find their way from one end of the Internet to the other.
Understand connectivity in the IP world
This course will not say much about the applications that exploit this connectivity…
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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
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Networking Technologies
Circuit Based Packet Based
Virtual Circuits Connectionless
TDM TelephonySONET/SDH
Frame RelayATM
IP
X.25
CLNP (ISO)SNA (IBM)Appletalk IPX (Novell)
DWDM
Connection Oriented
(variable rate, store-and-forward)(constant rate)
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Connection Oriented vs. Connectionless
Connection Oriented
Connectionless
Send and Pray
• Connection set up. Signaling reserves resources along the end-to-end path
• Traffic flows • Connection torn down and resources freed
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Network Heterogeneity
SONET
ATM
IP
TCP
Circuits
Virtual Circuit
Connectionless
Virtual Connections
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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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Datagram
HEADER
DATA
1981, RFC 791
... up to 65,515 octets of data ...
::|+|+|
::|+|+|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
shaded fields little-used today
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Famous Members of the IP Protocol Family
IP (and ICMP)
TCP UDP
FTP HTTPTelnet RTPSNMPDNS
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IP is a Network Layer Protocol
Physical 1
Network
DataLink 1
Transport
Application
Session
Presentation
Network
Physical 1
DataLink 1
Physical 2
DataLink 2
Router
Physical 2
Network
DataLink 2
Transport
Application
Session
Presentation
Medium 1 Medium 2
Separate physical networks glued together into one logical network
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Encapsulation Example
Ethernet Header
IP Header
TCP Header
HTTP Header
….
HTTP Data ….
An Ethernet segment transmitting HTTP data.
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IP Hour Glass
IP
Networking Technologies
Networking Applications
Frame ATM
DWDMSONET
Webfile transfer
Ethernet
FDDI
Multimedia
X.25
HOST
Router
Link
Remote Access Voice
VPN
Minimalist network layer
TCP
e-stuff
IP is the mother of all disruptive technologies!
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QoS
Quality of Service vs.Quantity of Service
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Traditional Telco Network
The Brick. An internationally recognized symbol of dumbness
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IP Network
The user community now owns the compute power and software.
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How Dumb Can a Network Get?
• 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
The technical part of the “Bell-head vs. Net-head” divide.
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• “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
Internet Engineering Task Force (IETF)
You must visit this site!
The cultural part of the “Bell-head vs. Net-head” divide.
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Hosts, Networks, and Routers
Network A
Network B Network CRouter
Host 1
Host 2
Host 7
Host 1
Host 12 Host 2
Unique IP Address = Network Number + Host Number
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Actually, IP addresses Identify Interfaces
Network A
Network B Network C
Host 1
Host 2
Host 7
Host 1
Host 12 Host 2
Network C, Host 3
Network A, Host 3
Network B, Host 77
Machines can have more than one IP address.All routers do!
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IP Forwarding Table
Destination Next Hop Interface
Net ANet BNet C, Host 3
Router 1DirectRouter 2Router 1
INT 7
INT 7INT 3INT 4
A destination is usuallya network. May also be a host, or a “gateway of last resort” (default)
The next hop is either a directlyconnected network or a router on a directly connected network
A physical interface
Net C
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IP Forwarding Process
Forwarding Process
IP Forwarding Table Router
1. Remove a packet from an input queue
3. Match packet’s destination to a table entry
2. Check for sanity, decrement TTL field
4. Place packet on correct output queue
If queuesget full, just
drop packets!
If queuesget full, just
drop packets!
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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.
London
New York
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Routing = Implementing End-to-End Paths with Next Hop Forwarding Tables
R
R
RA
B
C
D
R1
R2
R3
R4 R5
EDest. Nxt Hop
R4R3R3R4DirectR4
Dest. Nxt Hop
A B C D Edefault
R2R2DirectR5R5R2
Dest. Nxt Hop
A B C D Edefault
R1DirectR3R1R3R1
Default toupstreamrouter
A B C D Edefault
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Something Fishy Here?
A
B
C
The next-hop forwarding paradigmdoes not allow router R to choosea route to A based on who originatedthe traffic, B or C.
R
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Bad Things Can Happen
R1 R2
Dest. Nxt Hop
A:
R2:
Dest. Nxt Hop
A:
R1:
Loops are routingproblems, not forwarding problems
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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
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How are Forwarding Tables Populated?
Statically DynamicallyRouters exchange information using ROUTING PROTOCOLS that compute “best” routes
Administrator manually configurestable entries
In practice : a mix of these….
+ More control+ Not restricted to destination-based forwarding - Doesn’t 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
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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
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Terminology Alert!
Static Routing
Dynamic Routing
Adaptive Routing
Possible Bell Head MeaningNet Head Meaning
Manual provisioningof routing table entries
Nailed up route
Automatically route around network failures.Alternate paths selecteddynamically.
Automatically route around network failures and congestion. Alternate paths often predetermined. May vary with time of day.
Automatically route around network failures, and congestion.
Use of these terms may vary ….
Net HeadTerm
????
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Architecture of Routing Protocols
IGP
IGP IGP
EGPEGP
EGP
AS 701
AS 6431 AS 7018
Interior Gateway Protocols (IGP) : inside autonomous systems
Exterior Gateway Protocols (EGP) : between autonomous systems
OSPF, IS-IS,RIP, EIGRP, ... BGPMetric Based Policy Based
UUNet
AT&T Common BackboneAT&T Research
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The Most Common Routing Protocols
Routing protocols exchange network reachability information between routers.
IP (and ICMP)
TCP UDP
BGP RIP
OSPF EIGRPIS-IS
Cisco proprietary
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What is a Routing Process?
Routing Process
Protocol-Specific Routing Table
import informationfrom other routers
exportinformationto other routers
IP Forwarding Table Router
Manual configuration
OS kernel
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Many routing processes can run on a single router
IP Forwarding TableOSPFDomain
RIPDomain
BGP
OS kernel
OSPF Process
OSPF Routing tables
RIP Process
RIP Routing tables
BGP Process
BGP Routing tables
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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
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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
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IPv4 Implementation of Addresses
Thirty Two Bits: 0 8 1
624
11111111 00010001 10000111 00000000
255 013417
255.17.134.0
Dotted Quad notation for “human readability”
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IP Addresses come in two parts
11111111 00010001 10000111 00000000
Network Number Host Number
Where is this dividing line?Well, that depends ....
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Classful Addresses
0nnnnnnn
10nnnnnn nnnnnnnn
nnnnnnnn nnnnnnnn110nnnnn
hhhhhhhh hhhhhhhh hhhhhhhh
hhhhhhhh hhhhhhhh
hhhhhhhh
n = network address bit h = host identifier bit
Class A
Class C
Class B
1981, RFC 791 (definition of IPv4)
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The Classful Address Space
Class Networks Hosts Share of IPaddress space
A 127 16,777,214 1/2
B 16,384 65,534 1/4
C 2,097,152 254 1/8
Leads to very inefficient allocation of addresses …
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Flat Network Addressing
This router needsat least12 table entries
Exports at least 12 network addresses
Z
Y
X
WT
UV
SQ
R
P
N
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Hierarchical Network Addressing
Network Z
This Router needsonly 3 table entries for networks Z-T, Z-S, and Z-U
Z
T
S
UNetwork Z-U-X
This is called Aggregation
Network Z-TExports onlyone AddressM P
Q W
WP
SX
MF
KJ
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Best Match Forwarding
Destination Address : Network X-W-Y-V, Host 12
Lookup in IP forwarding table isno longer based on exact match of network prefix.
Destination Next Hop
X
X-W-Y
X-W
R2
R7
R5
Match
Best Match
Match
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The IPv4 hack
IP Address : 12.4.0.0 IP Mask: 255.254.0.0
00001100 00000100 00000000 00000000
11111111 11111110 00000000 00000000
Address
Mask
for hosts Network Prefix
Use two 32 bit numbers to represent a network. Network number = IP address + Mask
Usually written as 12.4.0.0/15
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Which IP addresses are covered?
00001100 00000100 00000000 00000000
11111111 11111110 00000000 0000000012.4.0.0/15
00001100 00000101 00001001 00010000
00001100 00000111 00001001 00010000
12.5.9.16
12.7.9.16
IP address 12.5.9.16 is covered by network 12.4.0.0/15
IP address 12.7.9.16 is not
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Mask may vary with location
12.0.0.0/8
12.0.0.0/16
12.254.0.0/16
12.1.0.0/16
12.2.0.0/16
12.3.0.0/16
:::
12.253.0.0/16
12.3.0.0/2412.3.1.0/24
::
12.3.254.0/24
12.253.0.0/1912.253.32.0/1912.253.64.0/19
12.253.96.0/1912.253.128.0/1912.253.160.0/1912.253.192.0/19
:::
This allows all of these (sub)networksto be aggregated into one entry in an IP forwarding table
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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
An ongoing debate
NATs are not compatible with some VPN technologiessuch as IPSec andtunneling. Can breakother applications.
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What do routing protocols exchange?
Network numbers
Network number = IP address + Mask
Reachability information
How to reach a given network
+depends onprotocol
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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
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Basic Model: Finding Shortest Paths
A
D E
C
B
100
10020
20
80
80
20
20 10
10
10
10
10
10
Source
50
30
20 30
Find paths from a sourceto all other nodes that minimizes sum of arcmetrics
Arc metrics arestatically provisioned
IP routing protocols DO NOT route aroundnetwork congestion!
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Arcs, Nodes,Arc Weights
Directed Graph data structures
+Algorithm
Dijkstra’s or Bellman-Ford
=
Solution to shortest path problem
How can shortest path computation be decentralized and performed by many cooperating routers?
Distribute pathcomputation.
Keep only local link data.
Distribute all link data.
Perform pathcomputations locally.
RIP, EIGRP,BGP OSPF, IS-IS,
IDPR
Technology : Distance Vector vs. Link State Protocols
Distance Vector Link State
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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
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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
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RIP Routing Table Destination Next Hop Metric
Net ANet BNet C, Host 3
Router 1DirectRouter 2Router 1Default
3
50
A destination is either a network, a host, or a “gateway of last resort”
The next hop is either a directlyconnected network or a directly connected router
Measures how many“hops away” is the destination
0
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Basic RIP Protocol Periodically exchange list of destinations and metrics with all neighboring routers
Dest. Metric
ABC
305
Dest. Metric
AEF
120
RIP routers exchangetheir entire “distancevector” every 30 seconds
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Basic RIP Protocol (cont.)
c
Neighbor N
Is Dest. A in my RIP Table?
Receive
Dest. Metric
A m
From Neighbor N
Dest. Metric
A m + c
Nxt Hop
N
Add
to RIP table
Is m + c less than current metric for Dest. A?
NO
Yes
YesDest. Metric
A m + c
Nxt Hop
N
Replace current entry with
in RIP table
Is N my next hop for Dest. A andm + c is not the current metric?
NO
Yes
Trust your neighbor...
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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).
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Link State Database
A
D E
C
B100
10020
20
80
80
20
20 1010
10
10
1010
ME
Each Router has a database representing the entire networkthat is constructed from the local knowledge at each router
B
D
A
C
80100
20
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Building OSPF Routing Table
Compute locally using Link State Database!
A
D E
C
B100
10020
20
80
80
20
20 1010
10
10
1010
ME
50
30
20 30
Dijkstra
Dest. Nxt Hop Metric
B D 50
C D
D
D
D
E
3020
30
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That’s Easy!
Not so fast!
RIP RFC 1058 : 33 pages
OSPF RFC 2328 : 244 pages
Much of this complexity is related to thesynchronization ofthe distributed, replicated link statedatabase.Plus network modeling ….
Distance Vector vs. Link State….
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Area 1
Area 2
Area 3
Scalability: OSPF Areas
Area 0
Area K...
LS database unique within an area
Special OSPFprotocol to exchange routesbetween areas. This is a “distancevector” protocol!
• Decentralize administration
• Reduce memory usage per router
• Reduce bandwidth used by flooding
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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??
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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
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Interdomain routing = routing between autonomous systems
AS 701AS 1239
AS 7018
UUNet
AT&T Common Backbone
Sprint
Fidelity Investments
AT&T Research
207.104.168.0/24
AS 6431
AS 11040192.223.184.0/21
134.244.0.0/16
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Why not just use OSPF?
•Scale– The Internet is very large
•Policy– My good route might be your bad
route
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An ASN represents a unit of Routing Policy 834 Canada Long Distance Services1740 CerfNet1838 CerfNet2386 Data Communications Services2685 AGNS (IBM Network)2713 Jens Corporation2740 Canada Long Distance Services2751Paradyne 2767 Canada Long Distance Services2768 Canada Long Distance Services2855 Concert2915 Jens Corporation 3300 AT&T Unisource3448 AT&T Wireless3668 Network Systems4465 Easy Link Services4466 Easy Link Services4467 Easy Link Services 4468 Easy Link Services4661 Easy Link Asia-Pacific5074 Dial Platform (BMGS) 5075 FDDI Access Ring (BMGS) 5400 Concert5727 Asia-Pacific PVCs5728 WorldNet Dial Platform5729 WorldNet Services5730 FDDI Access Ring5731 WorldNet Services6269 AT&T-Internet26290 ALASCOM-DIAL6308 ALASCOM-MIS6431 AT&T Labs
6478 Worldnet Services 6537 CANADA LONG DISTANCE SERVICES 6742 AT&T 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 WorldNet10456 Government Markets11698 AT&T-PWSS13979 AT&T-IPFR14005 AT&T Y2K ICC14470 ATT-PWSS-215259 Solutions15290 Canada IES17224 Enhanced Network Services17225 Enhanced Network Services17226 Enhanced Network Services17227 Enhanced Network Services17228 Enhanced Network Services17229 Enhanced Network Services17230 Enhanced Network Services17231 Enhanced Network Services17232 Enhanced Network Services17233 Enhanced Network Services
64 ASNs known to beassigned to AT&T(list may not be complete!)
33 (in green)seen in BGProuting tables on11/25/2000(by RouteDB)
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How many ASNs are there?
From http://www.telstra.net/ops/bgptable.html on 11/26/2000
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Sample prefixes from announced to the CBB
These are called “networks” or “prefixes”or “CIDR blocks” or “network blocks” or“routes”. Yes, it is confusing.
• 207.104.168.0/24 : from AT&T 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
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How Many Prefixes are There?
From http://www.telstra.net/ops/bgptable.html on 11/26/2000
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How many hosts are there?
For more information, see http://www.isc.org/ds
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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
A transit AS allows traffic with neither source nor destination within AS to flow across the network
IP traffic
UUnet
Bell Labs
AT&T CBB
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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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Why not minimize “AS hop count”?
Regional ISP1
Regional ISP2
Regional ISP3
Cust2Cust3 Cust3
National ISP1
National ISP2
YES
NO
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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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BGP Operations Simplified
Establish Peering on TCP port 179
Peers Exchange All Routes
Exchange Incremental Updates
AS1
AS2
While connection is ALIVE exchangeroute UPDATE messages
BGP
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Two Types of BGP Neighbor Relationships
• External Neighbor (eBGP) in a different Autonomous Systems
• Internal Neighbor (iBGP) in the same Autonomous System
AS1
AS2
eBGP
iBGPPhysical Connection
Logical (TCP) Connection
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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 = Network prefix + attributes
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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 neighbor’s 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”
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AS Path Attribute
AS7018
135.104.0.0/16AS Path = 144
135.104.0.0/16AS Path = 144
AS701Alternet (Uunet)
AS702Alternet (Uunet)
135.104.0.0/16AS Path = 701 144
135.104.0.0/16AS Path = 702 701 144
AS1849Uunet UK
AT&T CBB
AS5413GXN
135.104.0.0/16AS Path = 7018 144
AS5459LINX
135.104.0.0/16AS Path = 5413 7018 144
135.104.0.0/16AS Path = 5459 5413 7018 144
AS144
135.104.0.0/16
Bell Labs
Route Originated
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AS Path Attribute (cont.)
BGP at AS YYY will never accept a route whose AS Path contains YYY. This avoids interdomain routing loops.
AS702UUnet
10.22.0.0/16AS Path = 1 333 702 877
Don’t Accept!
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Next Hop Attribute
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.
AS144Bell Labs
135.104.0.0/16Next Hop = 11.0.0.1
AS701UUnet
AS702UUnet
135.104.0.0/16Next Hop = 11.0.0.1
11.0.0.1
135.104.0.0/16Next Hop = 33.0.0.3
33.0.0.3
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Local Preference Attribute
AT&T CBB
AS 7018
Primary T3 lineBackup T1 line
Forces outboundtraffic to takeprimary link, unless link is down.
Set Local Pref = 100for all routes from 7018
Customer
Set Local Pref = 50for all routes from 7018
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Local Preference AttributeUsed only in iBGP to prefer a point of exit
Higher LocalPreference Valuesare more preferred
AS 1
AS 2
AS 4
AS 3
Frank’s Internet Barn
Frank’s Customer
Customer of Frank’s Customer
13.13.0.0/16AS Path = 4 1Loc pref = 80
13.13.0.0/16AS Path = 2 1Loc pref = 100
13.13.0.0/16AS Path = 3 1Loc pref = 90
Frank’s Upstream Provider
Frank’s Local Competition
13.13.0.0/16
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Multi Exit Discriminator
AT&T CBB
AS 7018
Set MED to metric of IGP at each border router
192.44.78.0/24
35 56
192.44.78.0/24MED = 56
192.44.78.0/24MED = 35
Used only in eBGP to suggestpreferred points of entry to a BGPneighbor
Lower MED valuesare more preferred
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BGP Route Processing
Best Route Selection
Apply Import Policies
BGP 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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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 < EGP <
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 ….
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BGP Routing Tables
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*>i 12.123.210.15 0 100 10409 10409 10409 i* i63.109.3.0/24 12.123.9.240 0 82 1239 16886 i*>i 12.123.9.240 0 82 1239 16886 i* 63.109.64.0/24 137.39.23.173 81 701 8143 i*>i 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 ?*>i 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/arinwhois.html)• 701 =Uunet, 10409 = allegro.net, 1239 = Sprint, 16886 = LVMH group, 8143 = Publicom, 3561
= Cable & Wireless
show ip bgp
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BGP Communities
Community Attribute is 32 bits
By convention, first 16 bits is ASN of owner of community definition
communitynumber
Very powerful BECAUSE it is meaningless
Two reserved communities
no_export (0xFFFFFF01) don’t export out of AS
no_advertise (0xFFFFFF02)don’t pass to BGP peers
A route can belong to more than one community
RFC 1997 (August 1996)
groups routes into logical collections
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Implementing “Backup” Policies with Communities
AS1221Telstra
AS4000Sprint Internat.
AS4805Global One
AS2764Connect Net
Connect Net wants to provide backup Internet access for Global One in case its connection to Sprint is lost.
1996, RFC 1998
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First, Without Communities ...
AS1221Telstra
AS4000Sprint Internat.
AS4805Global One
AS2764Connect Net
Local Pref = 80
Local Pref = 70
Connect Net and Global One would have to convince Telstra to do some custom configuration its Local Pref value for some routes…..
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With Communities ….
{1221,70} = I’ll set local pref to 70 (cust. provided backup){1221,80} = I’ll set local pref to 80 (other ISP routes){1221,90} = I’ll set local pref to 90 (my customer backup){1221,100} = I’ll set local pref to 100 (my customers)
AS4000Sprint Internat.
AS4805Global One
AS2764Connect Net
AS1221Telstra
Telstra’s communities:
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Global One Using Telstra’s Communities
AS1221Telstra
AS4000Sprint Internat.
AS4805Global One
AS2764Connect Net
Tag these routes with community {1221, 70}
Tag these routes with community {1221, 80}
All I have to do is pass along AS4805’sroutes to AS1221
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iBGP Peers Must be Fully MeshedeBGP update
iBGP updates
iBGP peers do not announce routes received via iBGP
• 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
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Route Reflectors
RR
RR
RR
RR
RR
RR
Route Reflectors must be fullymeshed
Route Reflectors pass along updatesto client routers
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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.
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Recommended Reading • IDR : http://www.ietf.org/html.charters/idr-charter.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.
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Links
www.research.att.com/~griffin/IPcourses.html
www.research.att.com/~griffin/interdomain.html
These slides :
Links on interdomain routing:
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What I Didn’t talk about…
IP Layer
TCP UDP
FTP HTTPTelnet RTPSNMPDNS
… Applications …
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Selected R&D 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