Introduction: Part-11 CSCI 6781 Advanced Computer Networks Ram Dantu

Introduction: Part-1 1

CSCI 6781Advanced Computer Networks

Ram Dantu


Course Overview


Syllabus

This class IS about... Recent advances in Computer Networks New developments in Physical Layer, Layer

2 (ATM/MPLS), Layer 3 (mobile IP, ForCES), Layer 4 (SCTP) and application layers like SIP, MIDCOM, MobileIP, etc.

Vulnerability of these protocols, threat models

Wireless networks and handoff issues Working together and learning together ….


Background

My background Your background, thesis topic and why

are you talking this course Need some course to finish the degree Want to be network security engineer Want to be a hacker

Check your email often (atleast twice a day)

Office Hours and appointment (available most of the week days, get an appointment)


Grading

See website


Issues in Internet

Scalability QoS Security Availability


MPLS, Why

Full mesh of routers leads to O(NxN) flooding when a link goes down. This is way of expressing scalability of flooding mechanism.

Decoupling routing and forwarding Better integration of the IP and ATM worlds Basis for building next generation network applications and

services such as provider-provided VPNs Traffic engineering

Optimizing network utilization Handling unexpected congestion Handling link and node failures

Quality of Service Any Transport over MPLS. MPLS supports applications that

facilitates carrying Layer 2 traffic, such as Frame Relay, Ethernet and ATM over MPLS cloud.


Why SCTP

• SCTP preserves message boundaries TCP is byte-oriented. Applications must add their own record marking to delineate messages.

• concept of chunks

• security against SYN flooding attack

• multi-homing

• multi-streaming

• message fragmentation and bundling

• congestion control


Why SCTP• SCTP PDU = 1 common header + 1 or more chunks ( control or data)

• Association setup = 4 way handshake (INIT, INIT-ACK, COOKIE-ECHO,

COOKIE_ACK)

• COOKIE mechanism to prevent SYN flooding attack

• Graceful shutdown(SHUTDOWN, SHUTDOWN-ACK, SHUTDOWN-COMPLETE) no half-close as in TCP

• Separates reliability from ordered delivery

• Preserves message boundaries

• SACK chunks to ack cumulative TSN + gap ack blocks + duplicate TSNs

• Achieves link / path redundancy by supporting multi-homed hosts along with

reachability check


Forces

Forwarding and Control separation Scalability Interoperability between network

processors Managing Middleboxes


Voice Over IP

lower costs per call, especially for long-distance calls lower infrastructure costs: once IP infrastructure is

installed, no or little additional telephony infrastructure is needed. Note that voice over IP traffic does not necessarily have to travel over the public Internet: it may also be deployed on private IP networks.

End user can manage services Setup and rerouting Voice and data convergence Maintainability


What’s the Internet: “nuts and bolts” view

millions of connected computing devices: hosts, end-systems pc’s workstations, servers PDA’s phones, toasters

running network apps communication links

fiber, copper, radio, satellite

routers: forward packets (chunks) of data thru network

local ISP

companynetwork

regional ISP

router workstation

servermobile


What’s the Internet: “nuts and bolts” view protocols: control sending,

receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP

Internet: “network of networks” loosely hierarchical public Internet versus private

intranet

Internet standards RFC: Request for comments IETF: Internet Engineering

Task Force

local ISP

companynetwork

regional ISP

router workstation

servermobile


What’s the Internet: a service view

communication infrastructure enables distributed applications: WWW, email, games, e-

commerce, database., voting,

more? communication services

provided: connectionless connection-oriented

cyberspace [Gibson]:“a consensual hallucination experienced

daily by billions of operators, in every nation, ...."


Perspective

Network users: Does the network support the users’ applications Reliability Error free service Speed of data transfer

Network designers: Cost efficient network design Good utilization of network resources Cost of building the network Types of services to be supported Security


Perspective

Network providers: Network administration and customer service Maximize Revenue Minimize Operations Expenses Survivability and Resiliency (Why)

• Discuss the difference!• Give examples


A closer look at network structure: network edge:

applications and hosts network core:

routers network of networks

access networks, physical media: communication links


Building Blocks

Switches, Routers, Gateways Special network components responsible for

“moving” packets across the network from source to destination.

Network hosts, workstations, etc. they generally represent the source and sink

(destination) of data traffic (packets)

We can recursively build large networks by interconnecting networks via gateways and routers.


An Interconnection Network


The network edge: end systems (hosts):

run application programs e.g., WWW, email at “edge of network”

client/server model client host requests,

receives service from server

e.g., WWW client (browser)/ server; email client/server

peer-peer model: host interaction symmetric e.g.: teleconferencing


Network edge: connection-oriented service

Goal: data transfer between end sys.

handshaking: setup (prepare for) data transfer ahead of time Hello, hello back

human protocol set up “state” in two

communicating hosts TCP - Transmission

Control Protocol Internet’s connection-

oriented service

TCP service [RFC 793] reliable, in-order byte-

stream data transfer loss: acknowledgements

and retransmissions

flow control: sender won’t overwhelm

receiver

congestion control: senders “slow down

sending rate” when network congested


Network edge: connectionless service

Goal: data transfer between end systems same as before!

UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service unreliable data

transfer no flow control no congestion

control

App’s using TCP: HTTP (WWW), FTP

(file transfer), Telnet (remote login), SMTP (email)

App’s using UDP: streaming media,

teleconferencing, Internet telephony


The Network Core

mesh of interconnected routers

the fundamental question: how is data transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”


Different Types of Switching

Different Types of Switching: Circuit Switching (telephone network)

• dedicated circuit, sending and receiving bit streams

Message Switching Packet Switching

• store and forward, sending and receiving packets Virtual Circuit Switching Cell Switching (ATM)

What are Packets? Data to be transmitted is divided into

discrete blocks


Network Core: Circuit Switching

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required


Cost-Effective Resource Sharing Must share (multiplex) network

resources among multiple users.

Common Multiplexing Strategies Time-Division Multiplexing (TDM) Synchronous TDM (STDM) Frequency-Division Multiplexing (FDM)

Multiplexing multiple logical flows over a single physical link.


Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls resource piece idle if not

used by owning call (no sharing)

dividing link bandwidth into “pieces” frequency division time division


Network Core: Packet Switching

each end-end data stream divided into packets

user A, B packets share network resources

each packet uses full link bandwidth

resources used as needed,

resource contention: aggregate resource

demand can exceed amount available

congestion: packets queue, wait for link use

store and forward: packets move one hop at a time transmit over link wait turn at next

link

Bandwidth division into “pieces”Dedicated allocationResource reservation


Network Core: Packet Switching

Packet-switching versus circuit switching: human restaurant analogy

other human analogies?

A

B

C10 MbsEthernet

1.5 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link


Network Core: Packet SwitchingPacket-switching:

store and forward behavior


Packet switching versus circuit switching

1 Mbit link each user:

100Kbps when “active”

active 10% of time

circuit-switching: 10 users

packet switching: with 35 users,

probability > 10 active less that .004

Packet switching allows more users to use network!

N users

1 Mbps link


Packet switching versus circuit switching

Great for bursty data resource sharing no call setup

Excessive congestion: packet delay and loss protocols needed for reliable data transfer,

congestion control Q: How to provide circuit-like behavior?

bandwidth guarantees needed for audio/video apps

still an unsolved problem (chapter 6)

Is packet switching a “slam dunk winner?”


Packet-switched networks: routing

Goal: move packets among routers from source to destination we’ll study several path selection algorithms (chapter

4) datagram network:

destination address determines next hop routes may change during session analogy: driving, asking directions

virtual circuit network: each packet carries tag (virtual circuit ID), tag

determines next hop fixed path determined at call setup time, remains fixed

thru call routers maintain per-call state


Access networks and physical media

Q: How to connect end systems to edge router?

residential access nets institutional access

networks (school, company)

mobile access networks

Keep in mind: bandwidth (bits per

second) of access network?

shared or dedicated?


Residential access: point to point access

Dialup via modem up to 56Kbps direct access to

router (conceptually) ISDN: intergrated services

digital network: 128Kbps all-digital connect to router

ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router up to 8 Mbps router-to-home ADSL deployment: UPDATE

THIS


Residential access: cable modems

HFC: hybrid fiber coax asymmetric: up to 10Mbps

upstream, 1 Mbps downstream

network of cable and fiber attaches homes to ISP router shared access to router

among home issues: congestion,

dimensioning deployment: available

via cable companies, e.g., MediaOne


Institutional access: local area networks

company/univ local area network (LAN) connects end system to edge router

Ethernet: shared or dedicated

cable connects end system and router

10 Mbs, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs soon

LANs: chapter 5


Wireless access networks

shared wireless access network connects end system to router

wireless LANs: radio spectrum replaces

wire e.g., Lucent Wavelan 10

Mbps

wider-area wireless access CDPD: wireless access

to ISP router via cellular network

basestation

mobilehosts

router


Physical Media

physical link: transmitted data bit propagates across link

guided media: signals propagate in

solid media: copper, fiber

unguided media: signals propagate

freelye.g., radio

Twisted Pair (TP) two insulated copper

wires Category 3: traditional

phone wires, 10 Mbps ethernet

Category 5 TP: 100Mbps ethernet


Physical Media: coax, fiber

Coaxial cable: wire (signal carrier)

within a wire (shield) baseband: single

channel on cable broadband: multiple

channel on cable

bidirectional common use in

10Mbs Ethernet

Fiber optic cable: glass fiber carrying

light pulses high-speed operation:

100Mbps Ethernet high-speed point-to-

point transmission (e.g., 5 Gps)

low error rate


Physical media: radio

signal carried in electromagnetic spectrum

no physical “wire” bidirectional propagation

environment effects: reflection obstruction by objects interference

Radio link types: microwave

e.g. up to 45 Mbps channels

LAN (e.g., waveLAN) 2Mbps, 11Mbps

wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps

satellite up to 50Mbps channel (or multiple

smaller channels) 270 Msec end-end delay geosynchronous versus LEOS


Different Types of Links

Sometimes you install your own!Category 5 twisted pair 10-100Mbps, 100m50-ohm coax (ThinNet) 10-100Mbps, 200m75-ohm coax (ThickNet) 10-100Mbps, 500mMultimode fiber 100Mbps, 2kmSingle-mode fiber 100-2400Mbps, 40km


Bigger Pipes!

Sometimes leased from the phone company

STS: Synchronous Transport Signal

Service to ask for Bandwidth you getISDN 64 KbpsT1 1.544 MbpsT3 44.736 MbpsSTS-1 51.840 MbpsSTS-3 155.250 MbpsSTS-12 622.080 MbpsSTS-24 1.244160 GbpsSTS-48 2.488320 Gbps


Delay in packet-switched networkspackets experience delay

on end-to-end path four sources of delay at

each hop

nodal processing: check bit errors determine output link

queueing time waiting at output

link for transmission depends on

congestion level of router

A

B

propagation

transmission

nodalprocessing queueing


Delay in packet-switched networksTransmission delay: R=link bandwidth

(bps) L=packet length (bits) time to send bits into

link = L/R

Propagation delay: d = length of physical

link s = propagation speed in

medium (~2x108 m/sec) propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are very different quantitites!


Bandwidth (throughput)

Amount of data that can be transmitted per time unit

Example: 10Mbps

link versus end-to-end

Notation KB = 210 bytes Mbps = 106 bits per second

Performance


Bandwidth related to “bit width”

Performance

a)

b)

1 second

1 second


Latency (delay) Time it takes to send message from

point A to point B Example: 24 milliseconds (ms) Sometimes interested in in round-trip

time (RTT) Components of latency

Latency = Propagation + Transmit + Queue + Proc.

Propagation = Distance / SpeedOfLight

Transmit = Size / Bandwidth


The “hard” limit

Speed of light 3.0 x 108 meters/second in a vacuum 2.3 x 108 meters/second in a cable 2.0 x 108 meters/second in a fiber

Notes no queuing delays in direct link bandwidth not relevant if Size = 1 bit process-to-process latency includes

software overhead software overhead can dominate when

Distance is small


Relative importance of bandwidth and latency small message (e.g., 1 byte): 1ms vs. 100ms

dominates 1Mbps vs. 100Mbps large message (e.g., 25 MB): 1Mbps vs.

100Mbps dominates 1ms vs. 100ms

Consider two channels of 1Mbps and 100 Mbps respectively. For a 1 byte message, the available bandwidth is relatively insignificant given a RTT of 1 ms. The transmit delay for each channel is 8 s and 0.08 s, respectively.


Delay x Bandwidth Product

e.g., 100ms RTT and 45Mbps Bandwidth = 560KB of data

We have to view the network as a buffer. This may have interesting consequences: How much data did the sender transmit

before a response can be received?

Bandwidth

Delay


Application Needs bandwidth requirements: burst versus peak

rate jitter: variance in latency (inter-packet gap)

Average Bandwidth Requirement is Not enough: consider a source with an avg. BW-

requirement of 2Mbps. If the application generates 1 Mbit in one second interval and 3Mbit in a second, a channel that can support 2 Mbps max. will have a tough time.

Other Quality of Service (QOS) Parameters: max. and min. delay max. and min. bandwidth demand rates for dynamic increase of demands Cell-Loss Rate


Queueing delay (revisited)

R=link bandwidth (bps) L=packet length (bits) a=average packet

arrival rate

traffic intensity = La/R

La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can

be serviced, average delay infinite!


What Goes Wrong in the Network? Different types of Error:

Bit-level errors (electrical interference)• 1 in 106-107 in copper - 1 in 1012 - 1014 in fiber

Packet-level errors (congestion) Link and node failures

How should we deal with these types of error?

What are the consequences of errors?


Other types of problems in the Network?

Messages are delayed Messages are deliver out-of-order Third parties eavesdrop

The key problem is to fill in the gap between what applications expect and what the underlying technology provides.

Documents

Introduction: Part-11 CSCI 6781 Advanced Computer Networks Ram Dantu