Spring 2006 EE 5304/EETS 7304 Internet Protocols

TO 2-14-06 p. 1

Spring 2006

EE 5304/EETS 7304 Internet Protocols

Tom OhDept of Electrical Engineering

[email protected]

Lecture 5

Routing protocols

TO 2-14-06 p. 2

Administrative Issues

Here are some useful books for learning OPNET.

Computer Networks – A Systems Approach--Third Edition by Larry L. Peterson & Bruce S. Davie • Network Simulation Experiments Manual (The Morgan

Kaufmann Series in Networking) by Emad Aboelela Modeling and Simulating Communications Networks: A

Hands-on Approach Using OPNET (Textbook Binding) by Irene Katzela

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Administrative Issues (cont)

Data and Computer Communications, Seventh Edition

Computer Networking with Internet Protocols, Fourth Edition

by William Stalling• Data and Computer Communications and Computer

Networking with internet Protocols and Technology: Opnet Lab Manual to Accompany the seventh edition and fourth edition (Paperback)

I have posted the second homework solution today.

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Outline

Distance-vector routing (Comer: Pg. 213-215)

Examples: RIP( Comer: Pg. 408-410), IGRP

Link-state routing( Comer: Pg. 216)

Example: OSPF (Comer: Pg. 410-412), IS-IS

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Distance-Vector Routing

Should routerA forward packet

to neighbor Bor C?

Packet todest. X

Dest. X

Neighborrouter B

Neighborrouter C

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Distance-Vector Routing (cont)

Packet todest. X

Dest. X

Neighborrouter B

Neighborrouter C

Bellman-Ford’s idea: if routers B and C know their least-cost routes to X, then router A’s least-cost choice is the neighbor offering the least-cost route to X

54

29

Choose router B because 5+4 < 2+9

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Packet todest. X

Dest. X

Neighborrouter C

Dest.

5

29

How does router A learn that router B

has route with cost 4?

Next hop Cost

X router F 4

Y router G 8

Z router H 5

Router B advertises part of its routing table

4

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Packet todest. X

Dest. X

Neighborrouter C

Dest.

5

29

Next hop Cost

X router B 9

Y router B 13

Z router C 15

How does router B set up its routing

table?

4

From routing advertisements

from its neighbors

Router A’s routing table

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Packet todest. X

Dest. X

Neighborrouter C

5

2

Originally router X advertised cost of

0 to itself...

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Packet todest. X

Dest. X

Neighborrouter C

5

2

Each neighbor updates its routing table, then

advertises its cost, and so on...

...Until all routers learn their least-cost routes to X

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Routers take turns to advertise their vectors of reachable destinations

and costs...

Basic operation

Dest. Next hop Cost

X : :

Y : :

Z : :

Dest. Next hop Cost

X : :

Y : :

Z : :

...Routers update their routing tables from

advertisements received from neighbors

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Example

Destination Distance RouteNet 1 0 directNet 2 0 directNet 4 8 router L

Net 17 5 router MNet 24 6 router JNet 30 2 router QNet 42 2 router J

Destination DistanceNet 1 2Net 4 3

Net 17 6Net 21 4Net 24 5Net 30 10Net 42 3

Destination Distance Route0 direct0 direct455624

Existing routing table at router K Advertisement from neighbor J

Updated routing table at router K

Changesroutingtablefor K

Net 1Net 2Net 4

Net 17

Net 24Net 30Net 42

Net 21

router Jrouter Mrouter J

router Qrouter J

router J

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Vector-Distance Routing Protocol: RIP

Early interior gateway protocol [RFC 1058]

Each router maintains a table where each destination address is represented by a pair (i,j)

i = next hop (node) along shortest route to that destination j = distance (number of hops) to that destination going

through node i

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RIP (cont)

Each router broadcasts its routing table of destinations and distances to its neighbors every 30 sec ("vector-distance" refers to these vectors of distances)

Each router updates its routing table after receiving updates from its neighbors

If a shorter route to a destination is found, that entry in routing table will be updated

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RIP (cont)

Advantage is simplicity: routers need to talk only to neighbors:

Disadvantages:

Eventually changes are propagated through network but convergence could be slow

Problem of inconsistency because each router is trusting the information advertised by its neighbor, which is relying on their neighbors, and so on

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”Count to infinity" problem

Network A Router1

Router2

distanced=1

distanced=2

Network A Router1

Router2

distanced=1

distanced=2

linkfailure

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Router1

Network A Router1

Router2

advertisesd=3

updates tod=4

Network A Router2

updates tod=3

advertisesd=2

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RIP (cont)

Also not scalable to larger networks:

More routers → longer to propagate changes through network

Each update message (vectors) becomes longer because more destinations in larger networks

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RIP Message Format

all zerofamily of network 1

command version all zero

address of network 1

distance to network 1

all zerofamily of network 2

address of network 2

distance to network 2

:

distancevectors

4 bytes

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RIP Message Format (cont) Command (1 byte): eg, request for information,

response to request

Version (1 byte): 1 (a new version 2, RIP-2 [RFC 1723] is the same protocol but fills in the zero-fields of the version 1 message with additional information)

Family of network (2 bytes): identifies protocol family related to address format, eg, 2 for IP addresses

Address of network (4 bytes): each destination address

Distance to network (4 bytes): integer distance in number of hops (max 15 to prevent routing loops)

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Vector-Distance Routing Protocol: IGRP

Interior Gateway Routing Protocol developed by Cisco in mid-1980s (after RIP)

RIP limited hop counts to 15 → limited network size RIP uses simple hop count

IGRP uses composite metric calculated by factoring weighted values for delay, bandwidth, reliability, load

Network administrators can adjust weights

Multipath routing is allowed

Single traffic stream can be split among multiple paths by round robin

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Enhanced IGRP

Enhanced IGRP (EIGRP) evolved from IGRP

Integrates capabilities of link-state routing with distance-vector routing

Partial updates (when route metrics change) instead of periodic updates

Supports multiple network protocols (IP, Appletalk, Novell NetWare,...)

Capabilities for routers to detect routing loops and find alternate routes without waiting for updates from other routers

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Link-State Routing

Link-state routing is also known as link-status routing or shortest path routing

Each router maintains a complete view of network topology (graph)

Graph is constructed from “link-state advertisements” broadcast by routers to all other routers

Updates consists of status of router’s links

Whenever router receives an update, it modifies its graph and recomputes least-cost paths by Dijkstra’s algorithm

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OSPF (cont)

Advantages:

Routing decisions should be consistent among all routers Each router performs its own computations on same

network map, therefore is not dependent on trustworthiness of neighbor’s data

Changes are propagated faster than distance-vector routing

Disadvantage: flooding of link-state advertisements increases with size of network, but ways to limit

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OSPF (cont)

Disadvantage: flooding of link-state advertisements increases with size of network, but ways to limit

Messages are constant length - depends on number of links per router, but does not depend on network size

Routing updates are sent only for significant changes OSPF allows hierarchical routing - network is divided into

areas, which reduces routing traffic

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Link-State Routing Protocol: OSPF

Open Shortest Path First proposed by IETF in late 1980s to overcome disadvantages of RIP [RFC 1583]

Based largely on research done at BBN Open means public standard SPF refers to Dijkstra’s algorithm

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OSPF Message Format

source router address

version type message length

area ID

authentication typechecksum

authentication

authentication

OSPFheader

number of link status advertisements

link status advertisement 1

link status advertisement 2

link statusupdates

4 bytes

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OSPF Message Format (cont)

Version (1 byte): 1

Type (1 byte): message type, eg, link status request, link status update

Message length (2 bytes): in bytes

Source router address (4 bytes)

Area ID (4 bytes): networks can divide itself into areas which hide their topology from other areas

Checksum (2 bytes): error detection

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OSPF (cont)

Authentication type (2 bytes): scheme for authentication, eg, 0 = none, 1 = password

Authentication (8 bytes): adds security against malicious, false routing information

Data in message depends on message type, eg, link status update (header type = 4)

Number of link status advertisements (4 bytes) Link status advertisements (4 bytes each)

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Link-State Routing Protocol: IS-IS

Intermediate System-to-Intermediate System developed by ISO

Intermediate system = router IS-IS routing protocol is for routers to determine routes

Similar to OSPF, IS-IS is a link-state routing protocol

Allows hierarchical routing

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Spring 2006

EE 5304/EETS 7304 Internet Protocols

Tom OhDept of Electrical Engineering

[email protected]

Network protocols and congestion control: X.25, ATM

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Outline

X.25

Sliding window congestion control

ATM (Comer: pg. 221-233)

Connection admission control

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X.25

ITU-T standard for public virtual circuit packet-switched networks (later basis for ISO standard 8208) popular in 1970s

Packetswitch

Packetswitch

X.25 X.25

DTE DCE

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X.25 (cont)

DCE = data circuit-terminating equipment (packet switch, node)

DTE = data terminal equipment (host, station, user, end system)

X.25 covers only DCE-DTE interface

X.25 layer 1 is also called X.21 X.25 layer 2 is LAP-B (link access procedure- balanced), a

subset of HDLC X.25 layer 3 describes packets and control across

interface to provide virtual circuit service

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X.25 (cont)

2 types of virtual circuits:

Permanent virtual circuits are set up and fixed by network operator

Virtual calls require call set-up (or establishment) before data transfer, and call disconnect (or clearing, termination) afterwards, using control packets

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[Stallings Fig 9.18]

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X.25 (cont)

Call setup is initiated by Call Request packet and confirmed by Call Accepted packet

Data packets can then be exchanged

Either party can request termination by Clear Request packet, acknowledged by Clear Confirmation packet

Clear Indication packet is forwarded to other party, acknowledged by Clear Confirmation packet

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X.25 (cont)

Virtual circuits are identified uniquely by number contained in packet header

Local significance only, translated at each node Global VC numbers have disadvantages: limit number of

connections, and troublesome to find unused numbers

2 types of packets: data and control packets

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X.25 Data Packet

3 byte header

Q (1 bit): qualified or unqualified data - use by higher layer protocols to identify different packet types

bits:

Q

channel

1 1

priority

1 1 1 1 1 1

D 0 1 group

M P(S) 0P(R)

data :

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X.25 Data Packet (cont)

D (1 bit): indicates significance of Piggyback field

0 means ACK requested from local DCE and not dest. DTE (does not guarantee delivery to dest. DTE)

1 means ACK from dest. DTE (guaranteed delivery)

Modulo (2 bits):

01 = both Sequence and Piggyback fields are modulo 8 10 = they are modulo 128 and header is extended with

extra byte (Sequence and Piggyback fields are extended to 7 bits each)

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Group (4 bits) + Channel (8 bits) = 12-bit virtual circuit number

DTE can have up to 4096 VCs to other DTEs using one physical link

Piggyback (3 bits): modulo 8 acknowledgement (next packet expected, P(R))

More (1 bit): indicates a group of packets belong together (eg, for higher layer protocol)

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Sequence (3 bits): modulo 8 sequence number P(S)

Control (1 bit): 0 = data packet, 1 = control packet

Data (variable length) = max. 128 bytes unless negotiated differently

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X.25 Control Packet

3 byte header

Same fields as data packet: Q, D, Group, Channel, Modulo

Control bit = 1

bits:

0

channel

1 1

priority

1 1 1 1 1 1

0 0 1 group

1packet type

additional fields :

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X.25 Control Packet (cont)

Packet Type (7 bits): indicates control function

eg, 0000101 = call request eg, PPP0010 = receive not ready (ACK but closes

sender's window until RR) eg, PPP0000 = receive ready (ACK when no reverse

packet is available for piggybacking, or ACK and opens sender's window after RNR)

eg, PPP0100 = reject (dest. DTE was forced to discard packet; use go-back-N to retransmit from packet PPP)

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X.25 Control Packet (cont)

Additional information (variable length)

eg, for call request: length of calling address length of called address calling address called address facilities (requests for special features, eg, collect calls) user data (eg, login, password)

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X.25 Congestion Control

Sliding window is used for flow and error control

Default window size = 2 unless otherwise negotiated up to max. 7 for 3-bit Sequence, and up to max. 127 for 7-bit Sequence

Error control is usually done by go-back-N ARQ

Negative ACK is REJ control packet Sender will retransmit specified packet and all following

packets

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Sliding Window Congestion Control

Same concept as sliding window control in data link layer

Idea is to limit number of packets in transit in network by window size W

Source can send up to W packets without waiting for ACK (or credit, permit)

Source will slow down if ACKs are delayed (or credits run out)

• Congestion starts to increase → delays along a route increase → ACKs are delayed → source will slow down

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Sliding Window (cont)

ACK may apply to single packet or multiple packets or specific bytes

ACKs are sent in special control packets or often piggybacked on reverse data packets

Window size may be static or dynamic

Performance of window control

Assume transmission times for ACKs are negligible (ie, ACKs are very short)

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T = packet transmission time = packet length/link rate

W = window size (in packets)

d = packet transmission time + roundtrip propagation delay

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Case 1: d > WT

d - WT = idle time between windows,

maximum source rate = W packets/d time

TWT

d

123

1

23

time

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Case 2: d < WT

sender can transmit continuously

max. source rate = 1 packet/T time

TWTd

123123

time

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Combining both cases:

source rate = min(W/d, 1/T)

1/T

Sourcerate

WT Roundtrip delay d

W/d

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Source will slow down when congestion causes long roundtrip delays

Source will automatically stop within W packet transmission times (if no ACKs returned)

Trade-off between response time (want W small to slow down a source quickly) and efficiency (want W > d/T so source can transmit continuously)

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OPNET

Login into linux or solaris machine

At prompt, type opnet

The first time a user runs OPNET, two directories are created:<opnet_user_home>\op_admin<opnet_user_home>\op_models

Under op_admin, OPNET creates the following directories and files:

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OPNET (cont)

Under op_admin, OPNET creates the following directories and files:

(1) bk directory => stores a copy of the OPNET files opened through the GUI.

Default backup interval time is set to 15 minutes.

(2) tmp directory => stores intermediate files needed for compiling or running

a simulation

(3) err_log file => records errors (such as during a compile or simulation run).

(4) session_log => records commands launched from the GUI; for example,

the command line used to start a simulation.

**Because these directories and files can grow to be very large (for example, up to 100 MB for an

err_log file), you may wish to occasionally clear these files. You can clear err_log and session_log files

using the Help -> xxx Log -> Clear menu item.

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OPNET (mod_dirs)

You can modify your preferences with the Edit / Preferences menu item in the OPNET GUI.

OPNET stores these user preferences in a file located in the <opnet_user_home>\op_admin directory.

In OPNET 11.5, the file is named "env_db11.5”

Your env_db11.5 is the "mod_dirs" preference, which stands for "model directories". When you add model directories, you must add the new directory in “mod_dirs” or env_db11.5.

Documents

Spring 2006 EE 5304/EETS 7304 Internet Protocols