COMSCSEE 4140 Networking Laboratory Lecture 05

1
COMS/CSEE 4140 Networking LaboratoryLecture 05

• Salman Abdul Baset
• Spring 2008

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Announcements

• Lab 4 (1-4) due next week before your lab slot
• Assignment 2 due next Monday
• Class participation
• Help me update the router/linux commands
• Glossary
• Lab participation
• TAs / myself will ask random questions
• Midterm (March 10th, duration 1.5 hours)
• Projects

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Previous Lecture

• CIDR multi-homing and IP forwarding
• The Internet
• IETF, IRTF, IESG, IRB
• IANA, ICANN
• IETF (eight areas, 119 WGs)
• Routing protocols
• Distance vector vs. link state
• Intra-domain vs. inter-domain (IGP vs. EGP)
• Routing Information Protocol (RIP)

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Previous Lecture The Count-to-Infinity Problem
A
B
C
1
1
5
Agenda

• Routing Information Protocol (RIPv2)
• Link state protocols
• Open Shortest Path First (OSPF)
• Autonomous Systems (AS)

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The Gang of Four
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RIP - History

• Late 1960s Distance Vector protocols were used
  in the ARPANET
• Mid-1970s XNS (Xerox Network system) routing
  protocol is the precursor of RIP in IP (and
  Novells IPX RIP and Apples routing protocol)
• 1982 Release of routed for BSD Unix
• 1988 RIPv1 (RFC 1058) - classful routing
• 1993 RIPv2 (RFC 1388) - adds subnet masks
  with each route entry - allows classless
  routing
• 1997 RIPng (IPv6)
• 1998 Current version of RIPv2 (RFC 2453) and
  Internet standard (STD 56) (IPv4)

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Routing Information Protocol

• RIPv2
• Subnet masks, next hop addresses, authentication
  (plain text), multicast (instead of broad cast)
• Count-to-infinity solution
• Split-horizon
• Hold-down timer
• Triggered updates

A
B
C
1
1
A never advertises to B that its path to C goes
through B
A
B
C
1
1
B ignores any updates for the link B-C for a
hold-down time
A
B
C
1
1
B immediately advertises that its link is down.
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Routing Information Protocol

• Looping solution (for RIP messages)
• Maximum number of hops is 16.
• Link costs
• Always one or 16 (link-down)
• RIP timers
• per table update (30s /- 0 to 5) send complete
  routing table in unsolicited response to every
  neighbor router.
• per entry each entry has a timeout timer (180s)
• per entry route-flush timer (120s)
• Dedicated port
• UDP port 520 (msgs sent and rcvd on this port)
• Complete or partial routing table?
• Complete (may spread over multiple fragments)
• No reliable delivery
• Multicast
• 224.0.0.9

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RIPv1 Packet Format
1 RIPv1
1 request2 response
2 for IP 00 request full rou-ting table
Address of destination
Cost (measured in hops)
One RIP message can have up to 25 route
entries20x25500 bytes 8 (RIP hdr) 8 (UDP)
20 (IP)536 bytes
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RIPv2

• RIPv2 is an extends RIPv1
• Subnet masks are carried in the route information
• Authentication of routing messages
• Route information carries next-hop address
• Exploits IP multicasting
• Extensions of RIPv2 are carried in unused fields
  of RIPv1 messages

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RIPv2 Packet Format
2 RIPv2
1 request2 response
2 for IP 00 request full rou-ting table
Address of destination
Cost (measured in hops)
One RIP message can have up to 25 route entries
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RIPv2 Packet Format
2 RIPv2
Used to carry information from other routing
protocols (e.g., autonomous system number)
Subnet mask for IP address
Identifies a better next-hop address on the same
subnet than the advertising router, if one exists
(otherwise 0.0)
Any problems?
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RIP Messages

• Dedicated port for RIP is UDP port 520.
• Two types of messages
• Request messages
• used to ask neighboring nodes for an update
• Response messages
• contains an update

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Routing with RIP

• Initialization Send a request packet (command
  1, address family0..0) on all interfaces
• RIPv1 uses broadcast if possible,
• RIPv2 uses multicast address 224.0.0.9, if
  possible
• requesting routing tables from neighboring
  routers
• Request received Routers that receive above
  request send their entire routing table
• Response received Update the routing table
• Regular routing updates Every 30 /- 5 seconds,
  send all or part of the routing tables to every
  neighbor in an response message
• Triggered Updates Whenever the metric for a
  route change, send entire routing table.

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Agenda

• Routing Information Protocol (RIPv2)
• Link state protocols
• Open Shortest Path First (OSPF)
• Autonomous Systems

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Link State Routing

• Based on Dijkstra s Shortest-Path-First
  algorithm.
• Each router starts by knowing
• Prefixes of its attached networks.
• Links to its neighbors.
• Each router advertises to the entire network
  (flooding)
• Key idea synchronize state with directly
  connected routers
• Key idea ACK the flooded messages
• Prefixes of its directly connected networks
• Active links to its neighbors.
• Each router learns
• A complete topology of the network (routers,
  links).
• Each router computes shortest path to each
  destination.
• In a stable situation, all routers have the same
  graph, and compute the same paths.

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Dijkstras Shortest Path Algorithm for a Graph
Input Graph (N,E) with N the set of nodes
and E the set of edges cvw link cost (cvw 1
if (v,w) ? E, cvv 0) s source node. Output
Dn cost of the least-cost path from node s to
node n M s for each n ? M Dn
csn while (M ? all nodes) do Find w ? M
for which Dw minDj j ? M Add w to
M for each neighbor n of w and n ? M Dn
min Dn, Dw cwn Update route end for
end while end for
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Link state routing graphical illustration
Global view
b
3
1
2
a
c
d
6
Collecting all views yield a global complete
view of the network!
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Operation of a Link State Routing Protocol
IP Routing Table
Dijkstras Algorithm
Link StateDatabase
ReceivedLSAs
LSAs are flooded to other interfaces
LSA link-state advertisement
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Link State Routing Properties

• Each node requires complete topology information
• Link state information must be flooded to all
  nodes
• Guaranteed to converge

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Distance Vector vs. Link State Routing

• With distance vector routing, each node has
  information only about the next hop
• Node A to reach F go to B
• Node B to reach F go to D
• Node D to reach F go to E
• Node E go directly to F
• Distance vector routing makespoor routing
  decisions if directions are not
  completelycorrect (e.g., because a node is
  down).
• If parts of the directions incorrect, the routing
  may be incorrect until the routing algorithms has
  re-converged.

A
B
C
F
D
E
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Distance Vector vs. Link State Routing

• In link state routing, each node has a complete
  map of the topology
• If a node fails, each node can calculate the
  new route
• Difficulty All nodes need to have a consistent
  view of the network

A
B
C
F
D
E
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Distance Vector vs. Link State Routing
Link State
Vectoring

• Topology information is flooded within the
  routing domain
• Best end-to-end paths are computed locally at
  each router.
• Best end-to-end paths determine next-hops.
• Based on minimizing some notion of distance
• Works only if policy is shared and uniform
• Examples OSPF, IS-IS

• Each router knows little about network topology
• Only best next-hops are chosen by each router for
  each destination network.
• Best end-to-end paths result from composition of
  all next-hop choices
• Does not require any notion of distance
• Does not require uniform policies at all routers
• Examples RIP, BGP

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Agenda

• Routing Information Protocol (RIPv2)
• Link state protocols
• Open Shortest Path First (OSPF)
• Autonomous Systems

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OSPF

• OSPF Open Shortest Path First (Why Open?)
• The OSPF routing protocol is the most important
  link state routing protocol on the Internet
  (another link state routing protocol is IS-IS
  (intermediate system to intermediate system)
• The complexity of OSPF is significant
• RIP (RFC 2453 40 pages)
• OSPF (RFC 2328 250 pages)
• History
• 1989 RFC 1131 OSPF Version 1
• 1991 RFC 1247 OSPF Version 2
• 1994 RFC 1583 OSPF Version 2 (revised)
• 1997 RFC 2178 OSPF Version 2 (revised)
• 1998 RFC 2328 OSPF Version 2 (current version)

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Features of OSPF

• Provides authentication of routing messages
• Enables load balancing by allowing traffic to be
  split evenly across routes with equal cost
  (problem reordering)
• Type-of-Service routing allows to setup different
  routes dependent on the TOS field
• Supports subnetting
• Supports multicasting
• Allows hierarchical routing

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Hierarchical OSPF
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Hierarchical OSPF

• Two-level hierarchy local area, backbone.
• Link-state advertisements only in area
• each nodes has detailed area topology only know
  direction (shortest path) to nets in other
  areas.
• Area border routers summarize distances to
  nets in own area, advertise to other Area Border
  routers.
• Backbone routers run OSPF routing limited to
  backbone.

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Example Network
10.1.7.6
10.1.1.1
10.1.1.2
10.1.4.4
.1
.2
.2
.4
.4
.6
10.1.7.0 / 24
10.1.4.0 / 24
10.1.1.0 / 24
.1
.2
.4
.6
Router IDs can be selected independent of
interface addresses, but usually chosen to be the
smallest interface address
10.1.6.0 / 24
10.1.3.0 / 24
10.1.8.0 / 24
10.1.2.0 / 24
.3
.5
.3
.5
.5
.3
10.1.5.0/24
10.1.2.3
10.1.5.5

• Link costs are called Metric
• Metric is in the range 0 , 216
• Metric can be asymmetric

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Link State Advertisement (LSA)
4
3
2

• The LSA of router 10.1.1.1 is as followsLink
  State ID 10.1.1.1 Router IDAdvertising
  Router 10.1.1.1 Router IDNumber of links
  3 2 links plus router itselfDescription of
  Link 1 Link ID 10.1.1.2, Metric
  4Description of Link 2 Link ID 10.1.2.2,
  Metric 3Description of Link 3 Link ID
  10.1.1.1, Metric 0

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Network and Link State Database
Each router has a database which contains the
LSAs from all other routers
LS Type
Link
StateID
Adv. Router
Checksum
LS
SeqNo
LS Age
Router-LSA
10.1.1.1
10.1.1.1
0x9b47
0x80000006
0
Router-LSA
10.1.1.2
10.1.1.2
0x219e
0x80000007
1618
Router-LSA
10.1.2.3
10.1.2.3
0x6b53
0x80000003
1712
Router-LSA
10.1.4.4
10.1.4.4
0xe39a
0x8000003a
20
Router-LSA
10.1.5.5
10.1.5.5
0xd2a6
0x80000038
18
Router-LSA
10.1.7.6
10.1.7.6
0x05c3
0x80000005
1680
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Link State Database

• The collection of all LSAs is called the
  link-state database
• Each router has an identical link-state database
• Useful for debugging Each router has a complete
  description of the network
• If neighboring routers discover each other for
  the first time, they will exchange their
  link-state databases
• The link-state databases are synchronized using
  reliable flooding (flooded packets are
  acknowledged using Link State Acknowledgement
  packet)

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OSPF Packet Format
OSPF packets are not carried as UDP payload! OSPF
has its own IP protocol number 89
TTL set to 1 (in most cases)
Destination IP neighbors IP address or
224.0.0.5 (ALLSPFRouters) or 224.0.0.6
(AllDRouters)
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OSPF Packet Format
2 current version is OSPF V2
ID of the Area from which the packet originated
Message types 1 Hello (tests reachability) 2
Database description 3 Link state request 4
Link state update 5 Link state acknowledgement
0 no authentication 1 Cleartext password 2 MD5
checksum (added to end packet)
Standard IP checksum taken over entire packet
Authentication passwd 1 64 cleartext
password Authentication passwd 2 0x0000
(16 bits) KeyID (8 bits)
Length of MD5 checksum (8 bits)
Nondecreasing sequence number (32 bits)
Prevents replay attacks
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OSPF LSA Format
LSA Header
Link 1
Link 2
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Discovery of Neighbors

• Routers multicasts OSPF Hello packets on all
  OSPF-enabled interfaces.
• If two routers share a link, they can become
  neighbors, and establish an adjacency
• After becoming a neighbor, routers exchange their
  link state databases

ScenarioRouter 10.1.10.2 restarts
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Neighbor discovery and database synchronization
ScenarioRouter 10.1.10.2 restarts
After neighbors are discovered the nodes exchange
their databases
Sends database description. (description only
contains LSA headers)
Sends empty database description
Acknowledges receipt of description
Database description of 10.1.10.2
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Regular LSA exchanges
10.1.10.2 explicitly requests each LSA from
10.1.10.1
10.1.10.1 sends requested LSAs
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Dissemination of LSA-Update

• A router sends and refloods LSA-Updates, whenever
  the topology or link cost changes. (If a received
  LSA does not contain new information, the router
  will not flood the packet)
• Exception Infrequently (every 30 minutes), a
  router will flood LSAs even if there are not new
  changes.
• Acknowledgements of LSA-updates
• explicit ACK, or
• implicit via reception of an LSA-Update
• Question If a new node comes up, it could build
  the database from regular LSA-Updates (rather
  than exchange of database description). What role
  do the database description packets play?

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Agenda

• Routing Information Protocol (RIPv2)
• Link state protocols
• Open Shortest Path First (OSPF)
• Autonomous Systems

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Autonomous Systems

• An autonomous system (AS) is a region of the
  Internet that is administered by a single entity
  and that has a unified routing policy
• Each autonomous system is assigned an Autonomous
  System Number (ASN).
• Columbia campus network (AS14)
• Rogers Cable Inc. (AS812)
• Sprint (AS1239, AS1240, AS 6211, )
• Interdomain routing is concerned with determining
  paths between autonomous systems (interdomain
  routing)
• Routing protocols for interdomain routing are
  called exterior gateway protocols (EGP)

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Autonomous Systems (AS)
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Interdomain and Intradomain Routing

• Routing protocols for intradomain routing are
  called interior gateway protocols (IGP)
• Objective shortest path
• Routing protocols for interdomain routing are
  called exterior gateway protocols (EGP)
• Objective satisfy policy of the AS

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Interdomain vs. Intradomain

• Intradomain routing
• Routing is done based on metrics
• Routing domain is one autonomous system
• Interdomain routing
• Routing is done based on policies
• Routing domain is the entire Internet

46
Interdomain Routing

• Interdomain routing is based on connectivity
  between autonomous systems
• Interdomain routing can ignore many details of
  router interconnection

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AS Graphs
ATT North America
From T. Griffin, BGP Tutorial, ICNP 2002
48
Multiple Routing Protocols

• Multiple routing protocols can run on the same
  router
• Each routing protocol updates the routing table

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Autonomous Systems Terminology

• local traffic traffic with source or
  destination in AS
• transit traffic traffic that passes through
  the AS
• Stub AS has connection to only one AS, only
  carry local traffic
• Multihomed AS has connection to gt1 AS, but
  does not carry transit traffic
• Transit AS has connection to gt1 AS and
  carries transit traffic

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Stub and Transit Networks

• AS 1, AS 2, and AS 5 are stub networks
• AS 2 is a multi-homed stub network
• AS 3 and AS 4 are transit networks

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Selective Transit

• Example
• Transit AS 3 carries traffic between AS 1 and AS
  4 and between AS 2 and AS 4
• But AS 3 does not carry traffic between AS 1 and
  AS 2
• The example shows a routing policy.

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Customer/Provider

• A stub network typically obtains access to the
  Internet through a transit network.
• Transit network that is a provider may be a
  customer for another network
• Customer pays provider for service

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Customer/Provider and Peers

• Transit networks can have a peer relationship
• Peers provide transit between their respective
  customers
• Peers do not provide transit between peers
• Peers normally do not pay each other for service

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Shortcuts through peering

• Note that peering reduces upstream traffic
• Delays can be reduced through peering
• But Peering may not generate revenue

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This weeks lab

• /etc/quagga/ripd.conf
• eth1 does not work on some machines (PC1 and PC2
  of rack 3)
• Set eth1 to a completely different IP address
  e.g., 202.11.12.15 and use eth2
• Enable debugging and observe /etc/quagga/ripd.lo
  g
• Count-to-infinity
• disable split-horizon, triggered updates and set
  hold-down timer to zero.