Chapter 1 

communication links

fiber, copper, radio, satellite

transmission rate:

 bandwidth

Packet switches:

o  
forward
packets (chunks of data)

o  
Routers and switches

Protocols

·       
control
sending, and receiving of msgs

·       
all
communication activity on the Internet is governed by protocols

·       
protocols
define the format, order of msgs sent and received among network entities, and
actions taken on msg transmission, receipt

Internet standards

·       
RFC:
Request for comments

·       
IETF:
Internet Engineering Task Force

network structure:

·       
network edge:

o  
Host
may be a client or server.

o  
Servers
are knowns as data centers.

·       
access networks, physical media:

o  
wired or wireless

·       
network core:

o  
routers

o  
networks

digital subscriber line (DSL)

·       
data
over DSL phone line goes to Internet

·       
voice
over DSL phone line goes to telephone net

cable network

·       
frequency division multiplexing: different channels transmitted in different frequency bands

·       
HFC
(hybrid fiber coax): asymmetric: up to 30Mbps downstream transmission rate, 2
Mbps upstream transmission rate

·       
network
of cable: fiber attaches homes to ISP router homes share access network to
cable headend unlike DSL, which has dedicated access to central office

Ethernet

·       
typically
used in companies, universities, etc

·       
10
Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates

·       
today,
end systems typically connect into Ethernet switch

Wireless access networks

v shared wireless access network
connects end system to router

§  via base station aka “access point”

Host

host sending function:

v  takes
application message

v  breaks
into smaller chunks, known as packets, of length L bits

v  transmits
packet into access network at transmission rate R

§  link
transmission rate, aka link capacity, aka link bandwidth

Physical media

v bit: propagates between transmitter/receiver
pairs

v physical link: what lies between
transmitter & receiver

v guided media:

§  signals propagate in solid media:
copper, fiber, coax

v unguided media:

§  signals propagate freely, e.g., radio

v twisted pair (TP)

two insulated copper wires

v Category 5: 100 Mbps, 1 Gpbs Ethernet

v Category 6: 10Gbps

v coaxial cable:

§  two concentric copper conductors

§  bidirectional

§  broadband:

v  multiple channels on cable

v  HFC

v fiber optic cable:

§  glass fiber carrying light pulses,
each pulse a bit

§  high-speed operation:

v high-speed point-to-point
transmission (e.g., 10’s-100’s Gpbs transmission rate)

§  low error rate:

v repeaters spaced far apart

v immune to electromagnetic noise

v radio:

§  signal carried in the electromagnetic
spectrum

§  No physical “wire”

§  bidirectional

§  propagation environment effects:

v Reflection

v  obstruction by objects

v interference

v radio link types:

§  Terrestrial  microwave: up to 45 Mbps channels

§  LAN (e.g., WiFi): 11Mbps, 54 Mbps

§  wide-area (e.g., cellular): 3G
cellular: ~ few Mbps

§  satellite:

v Kbps to 45Mbps channel (or multiple
smaller channels)

v 270 msec end-end delay

v  geosynchronous versus low altitude

Network core:

§  mesh of interconnected routers

§  packet-switching

Packet-switching:

hosts break application-layer messages into packets

§  forward packets from one
router to the next, across links on a path from source to destination

§  each packet transmitted at full link
capacity

·       
store-and-forward:

entire packet must arrive at router
before it can be transmitted on next link

·       
queuing
and loss:

If
arrival rate (in bits) to link exceeds transmission rate of link for a period
of time:

§  packets will queue, wait to be
transmitted on link

§  packets can be dropped (lost) if
memory (buffer) fills up

Two key network-core functions:

·       
routing:

determines source-destination route
taken by packets

§  routing algorithms

·       
forwarding:

 move packets from router’s input to
appropriate router output

circuit switching:

end-end resources allocated to, reserved for “call” between
source & dest:

v dedicated resources: no sharing

§  circuit-like (guaranteed) performance

v circuit segment idle if not used by
call (no sharing)

v Commonly used in traditional
telephone networks

Packet switching versus circuit switching:

In circuit switching, a path is established between the source
and destination before the transmission occurs. On the other hand, packet
switching is connectionless which means a dynamic route is decided for each
packet while transmission.

Packet switching is more efficient than circuit switching
because it ensures that more of the bandwidth of all cables is fully utilized.
As it makes better use of resources, packet switching is more likely to reduce
congestion than circuit switching

loss and delay

packets queue
in router buffers

v packet arrival rate to link
(temporarily) exceeds output link capacity

v packets queue, wait for turn

Four sources of packet delay

·       
d_proc: nodal processing

o  
check
bit errors

o  
determine
output link

o  
typically
< msec

·       
d_queue: queueing delay

o  
time
waiting at output link for transmission

o  
depends
on congestion level of router

·       
d_trans: transmission delay:

o  
L: packet
length (bits)

o  
R: link bandwidth
(bps)

o  
d_trans = L/R

·       
d_prop: propagation delay:

o  
d: length of
physical link

o  
s: propagation
speed in medium (~2×10⁸ m/sec)

o  
d_prop = d/s

Internet delays and routes

v traceroute program: provides delay
measurement from source to router along end-end Internet path towards
destination.  For all i:

§  sends three packets that will reach
router i on path towards destination

§  router i will return packets
to sender

§  sender times interval between transmission
and reply.

Packet loss

v queue (aka buffer) preceding link in
buffer has finite capacity

v packet arriving to full queue dropped
(aka lost)

v lost packet may be retransmitted by
previous node, by source end system, or not at all

Throughput

v throughput: rate (bits/time unit) at which bits
transferred between sender/receiver

§  instantaneous: rate at given point in time

§  average: rate over longer period of time

v bottleneck link

link on end-end path that
constrains  end-end throughput

layers:

each layer
implements a service

§  via its own internal-layer actions

§  relying on services provided by layer
below

Internet protocol stack

v application: supporting network applications

§  FTP, SMTP, HTTP

v transport: process-process data transfer

§  TCP, UDP

v network: routing of datagrams from source to
destination

§  IP, routing protocols

v link: data transfer between
neighboring  network elements

§  Ethernet, 802.111 (WiFi), PPP

v physical: bits “on the wire”

ISO/OSI reference model

v presentation: allow applications to interpret
meaning of data, e.g., encryption, compression, machine-specific conventions

v session: synchronization, checkpointing,
recovery of data exchange

v Internet stack “missing” these
layers!

§  these services, if needed,
must be implemented in application

Malware:

§  virus: self-replicating infection by
receiving/executing  object (e.g., e-mail
attachment)

§  worm: self-replicating infection by
passively receiving object that gets itself executed

v spyware malware can record
keystrokes, web sites visited, upload info to collection site

v infected host can be enrolled in  botnet, used for spam. DDoS attacks

Denial of Service (DoS):

attackers make resources (server, bandwidth) unavailable to
legitimate traffic by overwhelming resource with bogus traffic

packet “sniffing”:

§  broadcast media (shared ethernet,
wireless)

§  promiscuous network interface
reads/records all packets (e.g., including passwords!) passing by

§  wireshark software used for
end-of-chapter labs is a (free) packet-sniffer

IP spoofing:

                send packet with false source address