Internet Routing Protocols Lecture 01 · 1 Internet Routing Protocols Lecture 01 Timothy G. Griffin Computer Lab Cambridge UK Advanced Systems Topics Lent Term, 2008 Common View of
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Internet Routing ProtocolsLecture 01
Timothy G. Griffin Computer Lab Cambridge UK
Advanced Systems Topics
Lent Term, 2008
Common View of the TelcoNetwork
Brick
2
Common View of the IP Network(Layer 3)
IP routing is the little bit-o-smarts left in the IPnetwork layer
• Dynamic Routing protocols are used toimplement and maintain connectivity in theInternet.
• Which protocols are used?• How do they work?• How do they behave?• What are some of the fundamental
tradeoffs in the design space of routingprotocols?
3
Outline
• Lecture 1 : Routing vs. Forwarding.Internet routing architecture
• Lecture 2: Intra-domain routing with“shortest paths”. Link-state vs. distance-vector.
• Lecture 3 : Inter-domain routing. TheBorder Gateway Protocol (BGP)
• Lecture 4 : BGP continued• Lecture 5 : BGP dynamics• Lecture 6 : BGP routing anomalies
4
5
GARR-B
6
(Winter '02)
(Win
ter '0
2)
(Summer '03)
UW-Superior
UW-StoutUW-River Falls
Fox Valley TC
UW-Oshkosh
UW-Milwaukee
UW-ParksideUW-Whitewater
UW-Madison
UW-Platteville
UW-La Crosse
UW-Eau Claire
UW-Stevens Point
UW-Green Bay
Marshfield
Rhinelander
Rice Lake
Clintonville
Stiles
Jct.
Portage
Dodgeville
La Crosse
Genuity
OC-3 (155Mbps)
DS-3 (45Mbps)
T1 (1.5Mbps)
OC-12 (622Mbps)
(Summer '02)
Qwest
and Other
Provider(s)
(Sum
mer '02)
Internet 2
& Qwest
! Peering - Public and Private
! Commodity Internet Transit
! Internet2
! Merit and Other State Networks
! National Education Network
! Regional Research Peers
Wausau
Gigabit Ethernet
(Summer '02)
(Su
mm
er '0
3)
Chicago - 1
Chicago - 2
(Winter '02)
Chicago
wiscnet.net
GO BUCKY!
7
WorldCom (UUNet)
Internet Initiative Japan (IIJ)
8
Telstra international
Charlotte
Portland
Providence
Newark
Cedar Knolls
Syracuse
Buffalo
WhitePlains
Rochester
Columbia
New Orleans
Nashville
AustinHouston
TulsaOklahom
aCity
Albuquerque
Phoenix
AnaheimAnaheim
LasVegas
Salt LakeCity
ColoradoSprings
Milwaukee
Detroit
Columbus
Cincinnati
Seattle Spokane
Portland
Louisville
Little Rock
Jacksonville
Ft. Lauderdale
Miami
RaleighRichmond
DenverIndianapolis
Pittsburgh
Baltimore
Plymouth
Atlanta
Minneapolis
Gardena
Tampa
SanBernardino
Arlington
Ft. Worth
Rochelle Pk
Honolulu
Orlando
Sherman Oaks
Ojus
HamiltonSquareSilver
Springs
Wayne
Chicago
RollingMeadows
Omaha
St Louis
San Diego
Anchorage, AK
N X OC48
Backbone Node
Gateway Node
N X DS3N X OC3
Remote Access Router
R Remote GSR Access Router
N X OC12
NX OC192
Cambridge
Framingham
StamfordBridgeport
Grand Rapids Providence
Glenview
Albany
Sacramento
Oakland
Redwood City
San Jose
San Francisco
ChicagoSan Francisco
Florissant
Davenport
Worcester
Madison
Camden, NJ
Norcross
New Brunswick
Birmingham
San Antonio
Oak Brook
South Bend
Dayton
Bohemia
Hartford
San Juan PR
W. Palm Beach
HarrisburgDes Moines
Memphis
Greensboro
Norfolk
R
KansasCity
AkronRR
R
Los Angeles
Dallas
Wash.DC
St. Paul
Freehold
R
Manchester
R
RR
Ft.Lauderdale
Dunwoody
Note: Connectivity andnodes shown aretargeted for deployment;actual deploymentmay vary. Maps shouldnot be used to predictservice availability.
R
R
R
Phil
NYC
Cleveland
R
RNYC-Bdwy
Birmingham
LA-AirportBlvd
AT&T IP BackboneYear end 2001
Rev. 6-4-01
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Architecture of DynamicRouting
AS 1
AS 2
BGP
EGP = Exterior Gateway Protocol. Policy Based.
IGP = Interior Gateway Protocol. Metric based.
OSPF, IS-IS, RIP, EIGRP (cisco)
Only one: BGP
The Routing Domain of BGP is the entire Internet
IGP
IGP
How many ASN are used today?
Jan 28. 2008
http://bgp.potaroo.net
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Routers Talking to Routers(The “control plane”)
Routing info
Routing info
• Routing computation is distributed among routers within arouting domain
• Computation of best next hop based on routing informationis the most CPU/memory intensive task on a router
• Routing messages are usually not routed, but exchangedvia layer 2 between physically adjacent routers (internalBGP and multi-hop external BGP are exceptions)
• Topology information isflooded within the routingdomain
• Best end-to-end paths arecomputed locally at each router.
• Best end-to-end pathsdetermine next-hops.
• Based on minimizing somenotion of distance
• Works only if policy is sharedand uniform
• Examples: OSPF, IS-IS
• Each router knows little aboutnetwork topology
• Only best next-hops are chosenby each router for eachdestination network.
• Best end-to-end paths resultfrom composition of all next-hop choices
• Does not require any notion ofdistance
• Does not require uniformpolicies at all routers
• Examples: RIP, BGP
Link State Vectoring
Technology of Distributed Routing
11
The Gang of Four
Link State Vectoring
EGP
IGP
BGP
RIPIS-IS
OSPF
22
The standard model
Physical
Network
DataLink
Transport
Application
Session
Presentation
Physical
Network
DataLink
Transport
Application
Session
Presentation
data sentdata received
12
23
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All Hail the IP Datagram!
HEADER
DATA
1981, RFC 791
... up to 65,515 octets of data ...
::|+|+|
::|+|+|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
shaded fields little-used today
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IP Hour Glass
IP
Networking Technologies
Networking Applications
Frame ATM
DWDMSONET
emailWeb
file transfer
Ethernet
FDDI
Multimedia
X.25
Remote Access Voice
VPN
Minimalist network layer
TCP
e-stuff
13
Best Effort, Connectionless,Connectivity
The is the fundamental service provided by Internet Service Providers (ISPs)
All other IP services depend on connectivity: DNS, email, VPNs, Web Hosting, …
IP traffic
135.207.49.8 192.0.2.153
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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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27
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 IdentifyInterfaces
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!
15
29
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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31
IPv4 Addresses are 32 BitValues
11111111 00010001 10000111 00000000
255 013417
255.17.134.0Dotted quad notation
IPv6 addresses have 128 bits
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IP Addresses come in twoparts
11111111 00010001 10000111 00000000
Network Number Host Number
Where is this dividing line?Well, that depends ....
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33
Classful Addresses
0nnnnnnn
10nnnnnn nnnnnnnn
nnnnnnnn nnnnnnnn110nnnnn
hhhhhhhh hhhhhhhh hhhhhhhh
hhhhhhhh hhhhhhhh
hhhhhhhhn = network address bit h = host identifier bit
Class A
Class C
Class B
Leads to a rigid, flat, inefficient use of address space …
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RFC 1519: Classless Inter-DomainRouting (CIDR)
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
18
Which IP Addresses are Covered by aPrefix?
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
12.5.9.16 is covered by prefix 12.4.0.0/15
12.7.9.16 is not covered by prefix 12.4.0.0/15
36
CIDR allows Hierarchy in Addressing
12.0.0.0/8
12.0.0.0/16
12.254.0.0/16
12.1.0.0/1612.2.0.0/1612.3.0.0/16
:::
12.253.0.0/16
12.3.0.0/2412.3.1.0/24
::
12.3.254.0/24
12.253.0.0/1912.253.32.0/1912.253.64.0/1912.253.96.0/1912.253.128.0/1912.253.160.0/1912.253.192.0/19
:::
19
Classless Forwarding
Destination =12.5.9.16------------------------------- payload
Prefix Interface Next Hop
12.0.0.0/8 10.14.22.19 ATM 5/0/8
12.4.0.0/15
12.5.8.0/23 attached
Ethernet 0/1/3
Serial 1/0/7
10.1.3.77
IP Forwarding Table
0.0.0.0/0 10.14.11.33 ATM 5/0/9
even better
OK
better
best!
38
How Are Forwarding TablesPopulated to implement Routing?
Statically DynamicallyRouters exchange network reachabilityinformation using ROUTING PROTOCOLS.Routers use this to compute best routes
Administratormanually configuresforwarding table entries
In practice : a mix of these.Static routing mostly at the “edge”
+ 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 difficult- Current protocols are destination-based
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39
Routing vs. Forwarding
R
R
RA
B
C
D
R1R2
R3
R4 R5
ENet Nxt Hop
R4R3R3R4DirectR4
Net Nxt Hop
A B C D Edefault
R2R2DirectR5R5R2
Net Nxt Hop
A B C D Edefault
R1DirectR3R1R3R1
Default toupstreamrouter
A B C D Edefault
Forwarding: determine next hop
Routing: establish end-to-end paths
Forwarding always works
Routing can be badly broken
40
Happy Packets: The Internet Does Not Exist Only toPopulated Routing Tables
Forwarding Table
OSPFDomain
RIPDomain
BGPOSPF Process
OSPF Routing tables
RIP Process
RIP Routing tables
BGP Process
BGP Routing tables
Forwarding Table Manager
21
Before We Go Any Further…
IP ROUTING PROTOCOLS DO NOT DYNAMICALLY ROUTE AROUND NETWORK CONGESTION
• IP traffic can be very bursty
• Dynamic adjustments in routing typicallyoperate more slowly than fluctuations intraffic load
• Dynamically adapting routing to accountfor traffic load can lead to wild, unstableoscillations of routing system
42
Next Lecture: Shortest PathRouting
This is what IS-IS, OSPF, and RIP do, more or less.
A
D E
C
B100
100 20
20
35
35
2020 20
20
10
10
1010
55
35
20 30
Dest. Nxt HopB CC C
DD
DE
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