Chapter 3 a

Slides for Chapter 3:
Networking and Internetworking

From Coulouris, Dollimore, Kindberg and Blair
Distributed Systems:
Concepts and Design
Edition 5, © Addison-Wesley 2012

Network Performance

The main network performance parameters are those affecting the
speed with which individual messages can be transferred
between two interconnected computers.
Latency is the delay that occurs after a send operation is executed
and before data starts to arrive at the destination computer. It
can be measured as the time required to transfer an empty
message.
Data transfer rate is the speed at which data can be transferred
between two computers in the network once transmission has
begun, usually quoted in bits per second.
Message transmission time = latency + length ⁄ data transfer
rate
The total system bandwidth of a network is a measure of
throughput – the total volume of traffic that can be transferred
across the network in a given time. 2

Figure 3.1
Network performance

km

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5
© Pearson Education 2012

Figure 3.2
Conceptual layering of protocol software

Message sent Message received

Layer n

Layer 2

Layer 1

Sender Communication Recipient
medium


Figure 3.3
Encapsulation as it is applied in layered protocols

Application-layer message

Presentation header

Session header

Transport header

Network header


Figure 3.4
Protocol layers in the ISO Open Systems Interconnection (OSI) model

Message sent Message received
Layers
Application
Presentation
Session
Transport
Network
Data link
Physical
Sender Communication Recipient
medium


Figure 3.5
OSI protocol summary

Layer Description Examples
Application Protocols that are designed to meet the communication requirements of HTTP, FTP , SMTP,
specific applications, often defining the interface to a service. CORBA IIOP
Presentation Protocols at this level transmit data in a network representation that is Secure Sockets
independent of the representations used in individual computers, which may (SSL),CORBA Data
differ. Encryption is also performed in this layer, if required. Rep.
Session At this level reliability and adaptation are performed, such as detection of
failures and automatic recovery.
Transport This is the lowest level at which messages (rather than packets) are handled. TCP, UDP
Messages are addressed to communication ports attached to processes,
Protocols in this layer may be connection-oriented or connectionless.
Network Transfers data packets between computers in a specific network. In a WAN IP, ATM virtual
or an internetwork this involves the generation of a route passing through circuits
routers. In a single LAN no routing is required.
Data link Responsible for transmission of packets between nodes that are directly Ethernet MAC,
connected by a physical link. In a WAN transmission is between pairs of ATM cell transfer,
routers or between routers and hosts. In a LAN it is between any pair of hosts. PPP
Physical The circuits and hardware that drive the network. It transmits sequences of Ethernet base- band
binary data by analogue signalling, using amplitude or frequency modulation signalling, ISDN
of electrical signals (on cable circuits), light signals (on fibre optic circuits)
or other electromagnetic signals (on radio and microwave circuits).


Figure 3.6
Internetwork layers

Message
Layers

Application

Internetwork
Transport protocols

Internetwork
Internetwork packets

Network interface
Underlying
Network-specific packets network
protocols
Underlying network


Figure 3.7
Routing in a wide area network

A 1 B

2
Hosts
Links
or local 3 4
C
networks
5
D 6 E

Routers


Figure 3.8
Routing tables for the network in Figure 3.7

Routings from A Routings from B Routings from C
To Link Cost To Link Cost To Link Cost
A local 0 A 1 1 A 2 2
B 1 1 B local 0 B 2 1
C 1 2 C 2 1 C local 0
D 3 1 D 1 2 D 5 2
E 1 2 E 4 1 E 5 1

Routings from D Routings from E

To Link Cost To Link Cost
A 3 1 A 4 2
B 3 2 B 4 1
C 6 2 C 5 1
D local 0 D 6 1
E 6 1 E local 0


Figure 3.9
Pseudo-code for RIP routing algorithm

Send: Each t seconds or when Tl changes, send Tl on each non-faulty outgoing link.
Receive: Whenever a routing table Tr is received on link n:
for all rows Rr in Tr {
if (Rr.link | n) {
Rr.cost = Rr.cost + 1;
Rr.link = n;
if (Rr.destination is not in Tl) add Rr to Tl;
// add new destination to Tl
else for all rows Rl in Tl {
if (Rr.destination = Rl.destination and
(Rr.cost < Rl.cost or Rl.link = n)) Rl = Rr;
// Rr.cost < Rl.cost : remote node has better route
// Rl.link = n : remote node is more authoritative
}
}
}


Internetworking

To build an integrated network (an internetwork) we must
integrate many subnets. To make this possible, the following
are needed:
 a unified internetwork addressing scheme that enables
packets to be addressed to any host connected to any subnet;
 a protocol defining the format of internetwork packets and
giving rules according to which they are handled;
 interconnecting components that route packets to their
destinations in terms of internetwork addresses, transmitting
the packets using subnets with a variety of network
technologies.


Figure 3.10
Simplified view of part of a university campus network

router/
Campus 138.37.95.240/29 138.37.95.241 firewall
router subnet hammer
Staff subnet Student subnet
138.37.88 138.37.88.251 138.37.94.251 138.37.94
compute file server/
server Eswitch Eswitch gateway
bruno
138.37.88.249 custard
138.37.94.246 printers
dialup
server
☎
henry
138.37.88.230 other
file servers
server
hotpoint
138.37.88.162
web
server
copper
138.37.88.248
hub hub

desktop computers 138.37.88.xx desktop computers 138.37.94.xx

Campus sickle
138.37.95.248/29 100 Mbps Ethernet
router subnet router/
138.37.95.249 firewall
1000 Mbps Ethernet
Eswitch: Ethernet switch

Figure 3.11
Tunnelling for IPv6 migration

IPv6 encapsulated in IPv4 packets

IPv4 network
IPv6 IPv6
A B

Encapsulators


Figure 3.12
TCP/IP layers

TCP/IP, including the Web (HTTP), email (SMTP, POP),
file transfer (FTP) and Telnet (telnet)
Message
Layers

Application
Messages (UDP) or Streams (TCP)

Transport
UDP or TCP packets

Internet
IP datagrams

Network interface
Network-specific frames

Underlying network


Figure 3.13
Encapsulation in a message transmitted via TCP over an Ethernet

Application message

TCP header port

IP header TCP

Ethernet header IP

Ethernet frame


Figure 3.14
The programmer's conceptual view of a TCP/IP Internet

Application Application

TCP UDP

IP


IP Addressing

 Assigning host addresses to networks and the computers connected
to them had to satisfy the following requirements:
• It must be universal – any host must be able to send packets to any
other host in the Internet.
• It must be efficient in its use of the address space – it is impossible to
predict the ultimate size of the Internet and the number of network and
host addresses likely to be required.

 TCP/IP provision for 232 or approximately 4 billion addressable hosts.

 Short-sighted, for two reasons:
– The rate of growth of the Internet has far outstripped all predictions.
– The address space has been allocated and used much less efficiently than
expected.

18

Figure 3.15
Internet address structure, showing field sizes in bits

7 24
Class A: 0 Network ID Host ID

14 16
Class B: 1 0 Network ID Host ID

28 21 8
Class C: 1 1 0 Network ID Host ID

28
Class D (multicast): 1 1 1 0 Multicast address

27
Class E (reserved): 1 1 1 1 0 unused


Figure 3.16
Decimal representation of Internet addresses

octet 1 octet 2 octet 3 Range of addresses
Network ID Host ID
1.0.0.0 to
Class A: 1 to 127 0 to 255 0 to 255 0 to 255 127.255.255.255
Network ID Host ID
Class B: 128 to 191 0 to 255 0 to 255 0 to 255 128.0.0.0 to
191.255.255.255
Network ID Host ID
192.0.0.0 to
Class C: 192 to 223 0 to 255 0 to 255 1 to 254
223.255.255.255
Multicast address
Class D (multicast): 224 to 239 0 to 255 0 to 255 1 to 254 224.0.0.0 to
239.255.255.255

Class E (reserved): 240 to 255 0 to 255 0 to 255 1 to 254 240.0.0.0 to
255.255.255.255


Figure 3.17
IP packet layout

header
IP address of source IP address of destination data
up to 64 kilobytes


Unregistered addresses and Network Address Translation (NAT)

 Not all of the computers and devices that access
the Internet need to be assigned globally unique IP
addresses.
 Computers that are attached to a local network and
access to the Internet through a NAT-enabled
router can rely upon the router to redirect incoming
UDP and TCP packets for them.
 The network includes Internet-enabled computers
that are connected to the router by a wired
Ethernet connection as well as others that are
connected through a WiFi access point.
22

Figure 3.18
A typical NAT-based home network


Figure 3.19
IPv6 header layout

Version (4bits) Traffic class Flow label (20bits)
(8bits)
Payload length (16 bits) Next header (8bits) Hop limit (8 bits)

Source address
(128 bits)

Destination address
(128
bits)


Figure 3.20
The MobileIP routing mechanism

Sender Subsequent IP packets
tunnelled to FA Mobile host MH
Address of FA
returned to sender
First IP packet
addressed to MH
Internet
Foreign agent FA
Home
agent First IP packet
tunnelled to FA


Figure 3.21
Firewall configurations

a) Filtering router
Protected intranet
Router/
filter

Internet

web/ftp
server

b) Filtering router and bastion
R/filter Bastion

Internet

web/ftp
server

c) Screened subnet for bastion R/filter Bastion R/filter

Internet

web/ftp
server


Firewall

 IP packet filtering: This is a filter process examining
individual IP packets. It may make decisions based on the
destination and source addresses.
 It may also examine the service type field of IP packets and
interpret the contents of the packets based on the type.
 For example, it may filter TCP packets based on the port
number to which they are addressed, and since services are
generally located at well-known ports, this enables packets to
be filtered based on the service requested. For example,
many sites prohibit the use of NFS servers by external clients.

27

Firewall

TCP gateway: A TCP gateway process checks all TCP
connection requests and segment transmissions.
When a TCP gateway process is installed, the setting up of
TCP connections can be controlled and TCP segments can
be checked for correctness (some denial of service attacks
use malformed TCP segments to disrupt client operating
systems).
When desired, they can be routed through an application-
level gateway for content checking.

28

Firewall

 Application-level gateway: An application-level gateway process acts
as a proxy for an application process.
 For example, a policy may be desired that allows certain internal users to
make Telnet connections to certain external hosts.
 When a user runs a Telnet program on their local computer, it attempts to
establish a TCP connection with a remote host.
 The request is intercepted by the TCP gateway. The TCP gateway starts
a Telnet proxy process and the original TCP connection is routed to it. If
the proxy approves the Telnet operation (i.e., if the user is authorized to
use the requested host) it establishes another connection to the
requested host and relays all of the TCP packets in both directions.
 A similar proxy process would run on behalf of each Telnet client, and
similar proxies might be employed for FTP and other services.

29

Figure 3.22
IEEE 802 network standards

IEEE No. Name Title Reference

802.3 Ethernet CSMA/CD Networks (Ethernet) [IEEE 1985a]

802.4 Token Bus Networks [IEEE 1985b]

802.5 Token Ring Networks [IEEE 1985c]

802.6 Metropolitan Area Networks [IEEE 1994]

802.11 WiFi Wireless Local Area Networks [IEEE 1999]

802.15.1 Bluetooth Wireless Personal Area Networks [IEEE 2002]

802.15.4 ZigBee Wireless Sensor Networks [IEEE 2003]

802.16 WiMAX Wireless Metropolitan Area Networks [IEEE 2004a]


Figure 3.23
Ethernet ranges and speeds

10Base5 10BaseT 100BaseT 1000BaseT

Data rate 10 Mbps 10 Mbps 100 Mbps 1000 Mbps

Max. segment lengths:

Twisted wire (UTP) 100 m 100 m 100 m 25 m

Coaxial cable (STP) 500 m 500 m 500 m 25 m

Multi-mode fibre 2000 m 2000 m 500 m 500 m

Mono-mode fibre 25000 m 25000 m 20000 m 2000 m


Figure 3.24
Wireless LAN configuration

A B C
Laptops

radio obstruction
Wireless
D LAN
Palmtop E

Server Base station/
access point

LAN


Issues

 Hidden stations: Carrier sensing may fail to detect that
another station on the network is transmitting.
If tablet D is transmitting to the base station E, laptop A may not
be able to sense D’s signal because of the radio obstruction
shown. A might then start transmitting, causing a collision at E
unless steps are taken to prevent this.
 Fading: Due to the inverse square law of electromagnetic
wave propagation, the strength of radio signals diminishes
rapidly with the distance from the transmitter. Stations within a
wireless LAN may be out of range of other stations in the
same LAN.
Thus laptop A may not be able to detect a transmission by C,
although each of them can transmit successfully to B or E.
Fading defeats both carrier sensing and collision detection.
33

Issues

 Collision masking: The ‘listening’ technique used
in the Ethernet to detect collisions is not very
effective in radio networks.
Because of the inverse square law the locally
generated signal will always be much stronger than
any signal originating elsewhere, effectively
drowning out the remote transmission.
So, laptops A and C might both transmit
simultaneously to E and neither would detect that
collision, but E would receive only a garbled
transmission.

34

Carrier Sensing, Multiple Access with Collision Avoidance (CSMA/CA).

When a station is ready to transmit, it senses the medium. If it detects no
carrier signal it may assume that one of the following conditions is true:
1. The medium is available.
2. An out-of-range station is in the process of requesting a slot.
3. An out-of-range station is using a slot that it had previously reserved.

35

Figure 3.25
Bluetooth frame structure

bits: 72 18 18 18 0 - 2744

Access code Header Header Header Data for transmission
copy 1 copy 2 copy 3

Header

bits: 3 1 1 1 4 8

Destination Flow Ack Seq Type Header checksum

Address within = ACL, SCO,
Piconet poll, null

SCO packets (e.g. for voice data) have a 240-bit payload containing 80 bits
of data triplicated, filling exactly one timeslot.


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