COMS/CSEE 4140 Networking Laboratory Lecture 05 Salman Abdul Baset Spring 2008.

COMS/CSEE 4140 Networking Laboratory

Lecture 05

Salman Abdul BasetSpring 2008

2

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

AA BB CC1 1

A's Routing Table B's Routing Table

C

to costvia(next hop)

2B C

to costvia(next hop)

1C

now link B-C goes down

C 2 C oo

C oo-C 2B

C oo C 3

C 3AC oo-

C 4 C oo

C oo-C 4B

5

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

6

The Gang of FourLink State Vectoring

EGP

IGP

BGP

RIPIS-IS

OSPF

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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 Novell’s IPX RIP and Apple’s 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

AA BB CC1 1A never advertises to B that its path to C goes through B

AA BB CC1 1

B ignores any updates for the link B-C for a hold-down time

AA BB CC1 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 FormatIP header UDP header RIP Message

Command Version Set to 00...0

32-bit address

Unused (Set to 00...0)

address family Set to 00.00


metric (1-16)

one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

One RIP message can have up to 25 route entries20x25=500 bytes + 8 (RIP hdr) + 8 (UDP) + 20 (IP)=536 bytes

1: request2: response

2: for IP0…0: request full rou-ting table

Address of destination

Cost (measured in hops)

1: RIPv1

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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 FormatIP header UDP header RIP Message

Command Version Set to 00...0

32-bit address


address family Set to 00.00


metric (1-16)

one

rout

e en

try(2

0 by

tes)


32 bits

One RIP message can have up to 25 route entries

1: request2: response

2: for IP0…0: request full rou-ting table

Address of destination

Cost (measured in hops)

2: RIPv2

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RIPv2 Packet FormatIP header UDP header RIPv2 Message

Command Version Set to 00.00

IP address

Subnet Mask

address family route tag

Next-Hop IP address

metric (1-16)

one

rout

e en

try(2

0 by

tes)


32 bits

Used to carry information from other routing protocols (e.g., autonomous system number)

Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0)

2: RIPv2

Subnet mask for IP address

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 family=0..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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Dijkstra’s 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 = min{Dj ; 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 whileend for

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Link state routing: graphical illustration

a

b

c d

3 1

62

a

36

b

c

a’s view:

a

b

c

3 1b’s view: c d2

d’s view:

Collecting all views yield a global & complete view of the network!

Global view:

a

b

c d

1

6

c’s view:

2

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Operation of a Link State Routing Protocol

ReceivedLSAs

IP Routing Table

Dijkstra’s

Algorithm

Link StateDatabase

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.

AA BB CC

DD EE FF

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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

AA BB CC

DD EE FF

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

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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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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)

OSPF

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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

Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address

3

4 2

5

1

1

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• Link costs are called Metric

• Metric is in the range [0 , 216]

• Metric can be asymmetric

10.1.1.0 / 24

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.4

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.6

10.1.1.2 10.1.4.4 10.1.7.6

10.1.2.3 10.1.5.5

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Link State Advertisement (LSA)

The LSA of router 10.1.1.1 is as follows:Link State ID: 10.1.1.1 = Router ID

Advertising Router: 10.1.1.1 = Router ID

Number of links: 3 = 2 links plus router itself

Description 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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10.1.1.2 10.1.4.4 10.1.7.6

10.1.2.3 10.1.5.5

4

3 2

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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

10.1.1.0 / 24

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.6

10.1.1.2 10.1.4.4 10.1.7.6

10.1.2.3 10.1.5.5

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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 MessageIP header

Body of OSPF MessageOSPF MessageHeader

Message TypeSpecific Data

LSA LSALSA ...

LSAHeader

LSAData

...

Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

TTL: set to 1 (in most cases)

OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89

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OSPF Packet Format

source router IP address

authentication

authentication

32 bits

version type message length

Area ID

checksum authentication type

Body of OSPF MessageOSPF MessageHeader

2: current version is OSPF V2

Message types:1: Hello (tests reachability)2: Database description3: Link state request4: Link state update5: Link state acknowledgement

ID of the Area from which the packet originated

Standard IP checksum taken over entire packet

0: no authentication1: Cleartext password2: MD5 checksum(added to end 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

Link State ID

link sequence number

advertising router

Link Age Link Type

checksum length

Link ID

Link Data

Link Type Metric#TOS metrics

LSA

LSAHeader

LSAData

Link ID

Link Data

Link Type Metric#TOS metrics

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

OSPF Hello

OSPF Hello: I heard 10.1.10.2

10.1.10.1 10.1.10.2

Scenario:Router 10.1.10.2 restarts

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Neighbor discovery and database synchronization

OSPF Hello

OSPF Hello: I heard 10.1.10.2

Database Description: Sequence = X

10.1.10.1 10.1.10.2

Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005

Database Description: Sequence = X+1

Sends empty database description

Scenario:Router 10.1.10.2 restarts

Discovery of adjacency

Sends database description. (description only contains LSA headers)

Database description of 10.1.10.2

Acknowledges receipt of description

After neighbors are discovered the nodes exchange their databases

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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

10.1.10.1 10.1.10.2

Link State Request packets, LSAs =

Router-LSA, 10.1.10.1,






Link State Update Packet, LSAs =

Router-LSA, 10.1.10.1, 0x80000006

Router-LSA, 10.1.10.2, 0x80000007

Router-LSA, 10.1.10.3, 0x80000003

Router-LSA, 10.1.10.4, 0x8000003a

Router-LSA, 10.1.10.5, 0x80000038

Router-LSA, 10.1.10.6, 0x80000005

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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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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)Ethernet

Router

Ethernet

Ethernet

RouterRouter

Ethernet

Ethernet

EthernetRouterRouter

Router

AutonomousSystem 2

AutonomousSystem 1

44

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

AS 6 AS 7

AS 4

AS 2 AS 5

AS 1

AS 3

45

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

EGP (e.g., BGP)

AS 2 AS 2

IGP (e.g., OSPF)IGP (e.g., RIP)

46

Interdomain Routing Interdomain routing is based on connectivity between autonomous

systems Interdomain routing can ignore many details of router

interconnection

AS 1 AS 2

AS 3

47

AS Graphs

From: T. Griffin, BGP Tutorial, ICNP 2002

AT&T North America

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Multiple Routing Protocols

Multiple routing protocols can run on the same router

Each routing protocol updates the routing table

routingtable

IP Forwarding

routing tablelookup

incoming IP datagrams

outgoing IP datagrams

routing protocol

routing protocol

RIP Process

OSPFProcess

BGP Process

routing table updates

49

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 >1

AS, but does not carry transit traffic Transit AS = has connection to >1

AS and carries transit traffic

50

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

AS 3

AS 1

AS 4

AS 2

AS 5

51

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.

AS 2AS 1

AS 3

AS 4

52

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

AS 5

AS 2

Customer/Provider

AS 6

Customer/Provider

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

53

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

AS 3

AS 5

AS 2Peers

Customer/Provider

AS 6

Customer/Provider

AS 1Peers

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

54

Shortcuts through peering

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

AS 3

AS 5

AS 2Peers

Customer/Provider

AS 6

Customer/Provider

AS 1Peers

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

Peers

55

This week’s 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.log Count-to-infinity

disable split-horizon, triggered updates and set hold-down timer to zero.

COMS/CSEE 4140 Networking Laboratory Lecture 05 Salman Abdul Baset Spring 2008.

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