Download Networking and Internetworking

Chapter 3: Networking and Internetworking
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Introduction
Types of network
Network principles
Internet protocols
Network case studies:
Ethernet, wireless LAN and ATM
• Summary
Concepts
• Why should we study network technology?
– Network is the basis of distributed system
– The performance, reliability, scalability, … of the underlying networks
impact the behavior of DS and therefore affect their design
• Some concepts
– transmission media : wire, cable, fiber and wireless channels
– hardware devices : router, switch, bridge, hub, repeater and network
interface
– software components : protocol stacks, communication handler and
driver
– communication subsystem: the collection of hardware and software
components that provide the communication facilities for a distributed
system.
– host : computers and devices that use the network for communication
purpose
– node : computer or switching device attached to a network
– subnet : a unit of routing (delivering data from one part of Internet to
another); it’s a collection of nodes that can be reached on the same
physical network.
Networking issues for distributed systems
• Performance
– latency : the delay that occurs after a send operation is executed before data
starts to arrive at the destination computer. It can be measured as the time the
time to transfer an empty message. It forms a part of process-to-process
latency.
– data transfer rate : the speed at which data can be transferred between two
computers in the network once transmission has begun, bits/s
– Message transmission time = latency + length / data transfer rate
– data transfer rate is determined primarily by network physical characteristics,
whereas the latency is determined primarily by software overheads, routing
delays and delay of accessing to transmission channels
– In distributed systems, messages are always small in size, so the latency is
more significant than data transfer rate
– total system bandwidth : the total volume of traffic that can be transferred
across the network in a given time.
• Ethernet: system bandwidth is as same as data transfer rate
• WAN: multiple channels, deteriorates when there are too many messages
– Comparison of different communication channel
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•
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local network – a null message transmission time is under a millisecond
local memory - 1000 or more times faster than local network
local hard disk - 500 times slower than fast local network
Internet - round-trip latencies are in 300-600ms due to switching and contention
Networking issues for distributed systems … continued
• Scalability
– WWW: world wide wait ?
– future Internet: several billion nodes and hundreds of millions of active hosts,
new addressing and routing mechanisms
• Reliability
– Networks are highly reliable, whereas client and server computers and their
software often aren't, so error detection and recovery is best performed end-toend at the highest feasible level.
– Errors: e.g. failures in software of sender or receiver, or buffer overflow
• Security
– Firewall: to protect the resources in all of the computers inside the organization
from access by external users or processes and to control the use of resources
outside the firewall by users inside the organization, always runs on a gateway
– Secure network environment, e.g. VPN
• Mobility
– Although the current mechanisms have been adapted and extended to support
them, the expected future growth in the use of mobile devices will require
further extension
• QoS
– require guaranteed bandwidth and bounded latencies
• Multicasting: Need for one-to-many communication
Chapter 3: Networking and Internetworking
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Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless LAN and
ATM
• Summary
Types of network
• Local area networks (LANs)
– high speed, connected to a single communication medium, no routing of
messages, may have switches or hubs
– Ethernet, Token rings and … (1970s),
– 10M -> 100M -> 1000M (G)
• Wide area networks (WAN)
– lower speeds, links between different cities, countries or continents
– communication medium is a set of communication circuits linking a set of
dedicated computers called routers
• Metropolitan area networks (MANs)
– Ethernet, ATM, DSL(digital subscriber line)
• Wireless networks
– IEEE 802.11(WaveLAN): 2-11mbps over 150 meters
– WPANs (wireless personal area networks): infra-red links, BlueTooth (1-2
mbps over 10 meters)
– digital mobile phone network: European GSM/USA CDPD (up to 2 mbps)
– WAP (Wireless Application Protocol)
Types of networks … continued
• Internetworks
– several networks are linked together to provide common data
communication facilities that conceal the technology differences.
– They are interconnected by routers and gateways
– e.g. Internet
• Network comparisons
– different failure model : TCP vs. UDP
Chapter 3: Networking and Internetworking
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Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless LAN and
ATM
• Summary
Network Switching technology
• Packet switching
– Store and forward, share communication link, asynchronous
• Packet transmission
– Messages: arbitrary length
– Packet: restricted length
• Sufficient buffer storage at each node, avoid undue delays
• Data streaming
– Video stream: bandwidth requirement, continues flow e.g. frame N arrives
no more than N/24 seconds after the first frame arrives
– Bandwidth, latency and reliability must be guaranteed: predefined route, e.g.
ATM and IPv6
• Switching schemes
–
–
–
–
Broadcast: no switching, Ethernet
Circuit switching: telephone system
Packet switching
Frame relay: combination of circuit switching and packet switching
• Delay: Internet -200 milliseconds, telephone - 50 milliseconds, ATM - a few
tens of microseconds
Network Protocols
• Protocol
– A specification of the sequence of messages that must be exchanged
– A specification of the format of the data in the messages
• Protocol layers
– Each layer presents an interface to the layers above it that extends the properties of
the underlying communication system
– Layer encapsulations (n1)
• Protocol suits ( protocol stack)
– Seven-layer Reference Model for OSI adopted by ISO
– Simplify and generalize the communication interface, but bring significant cost,
e.g., N copies
Mess age receiv ed
Mess age s ent
Lay ers
Applic ation
Pres entat ion
Sess ion
Transport
Netw ork
Data link
Phy sical
Sender
Communic at ion
medium
Recipient
Encapsulation of a packet
Applic ation-layer mess age
Pres entation header
Sess ion header
Transport header
Netw ork header
Other important concepts in network protocol
• Packet assembly
– Network–layer protocol packets: MTU (Maximum transfer unit),
1500Bytes in Ethernet, 64K in IP
• Ports
– Software definable destination points for communication within a host
computer
– Transport address = network address + port number,
– Port numbers above 1023 are available for general use
• Packet delivery
– Datagram packet delivery, e.g. IP, Ethernet, and most wired and wireless
LAN
– Virtual circuit packet delivery, e.g., ATM
– Different to the concepts of Connection oriented (TCP)/Connectionless
(UDP) in transport layer protocol
Routing
• Routing algorithm
– Determine the route taken by each packet.
• Predetermined for circuit switching (e.g. X.25) and frame relay
switching (e.g. ATM); determine on the fly for packet switching
– Dynamically update its knowledge of the network
• Distance vector
– Bellman-Ford protocol[1957]
– RIP (router information protocol)
• Periodically, and whenever the local routing table changes, send the
router table to all accessible neighbours
• When a table is received from a neighbouring router, make necessary
update
– Convergence problem
– Improvement: link cost include bandwidth information, speed
convergence, etc
Routing … continued
• Link state algorithm
– Each node maintains all knowledge of the network
– Each node can compute appropriate routes based on
the knowledge
– Avoid slow convergence and undesirable
intermediate states
– E.g. OSPF
Congestion control
• A rule of thumb
– when the load on a network exceeds 80% of its capacity, the
total throughput tends to drop as a result of packet losses
• Congestion control
– instead of allowing packets to travel through the network
until they reach over-congested nodes, hold them at earlier
nodes
• Control approaches
– Informing nodes along the congested route to reduce packet
transmission rate, i.e buffering for long time at intermediate
nodes or queue packets at source host
– In the Internet, congestion control rely on the end-to-end
traffic control, e.g. choke packets requesting a reduction in
transmission rate in TCP
Internetworking
• Internetwork
– integrate many subnets that use different network
technologies
• Requirements
– 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
• Example
Internetworking components
• Router
– Conduct routing, additionally link networks of different types
• Bridge
– link networks of different types, but not conduct routing
• Hub
– Connect hosts and extend segments of Ethernet and other
broadcast local network
• Switch
– Perform similar function to router, but for LANs only
• Tunnel
– A software layer that transmits packets through an alien
network environment
Chapter 3: Networking and Internetworking
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Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless
LAN and ATM
• Summary
Internet protocols
• Protocol layers (n1)
– TCP(UDP)/IP,(n2) web [HTTP], Email [SMTP,POP], news
[NNTP], FTP, SSL, etc
• Exceptions to the universal adoption of TCP/IP
– The use of WAP for wireless applications on portable
devices
– Special protocols to support multimedia streaming
applications
• Heterogeneous underlying networks support
– The success of TCP/IP: independence of the
underlying transmission technology (n3)
– E.g., IP over ATM, IP over Ethernet, IP over PPP, etc
Internet protocol layers
Message
Layers
Application
Messages (UDP) or Streams (TCP)
Transport
UDP or TCP packets
Internet
IP datagrams
Network interface
Network-specific frames
Underlying network
Encapsulation in a message transmitted via TCP
over an Ethernet
Application message
TCP header
IP header TCP
Ethernet header IP
Ethernet frame
port
The programmer’s conceptual view of a TCP/IP Internet
Applic ation
Applic ation
TCP
UDP
IP
IP addressing
• Schemes for naming and addressing hosts and for
routing IP packets to their destination is challenging.
• Requirement:
– It must be universal
– It must be efficient
– The addressing scheme must lend itself to the development of
routing scheme
• The scheme
– A 32-bit numeric identifier containing a network identifier and
a host identifier
– There are four allocated classed of Internet address-A,B,C,D
Internet address structure
Clas s A:
Clas s B:
0
7
24
Netw ork ID
Host ID
1 0
14
16
Netw ork ID
Host ID
21
Clas s C:
1 1 0
8
Netw ork ID
Host ID
28
Clas s D (multicast ):
1 1 1 0
Multicast address
28
27
Clas s E (reserved):
1 1 1 1 0
unused
Decimal representation of Internet addresses
octet 1
octet 2
Network ID
Class A:
octet 3
Host ID
1.0.0.0 to
1 to 127
0 to 255
0 to 255
Network ID
Class B:
Range of addresses
128 to 191
0 to 255
127.255.255.255
Host ID
0 to 255
0 to 255
0 to 255
128.0.0.0 to
191.255.255.255
Network ID
Class C:
192 to 223
0 to 255
Host ID
0 to 255
1 to 254
192.0.0.0 to
223.255.255.255
Multicast address
Class D (multicast):
Class E (reserved):
224 to 239
0 to 255
0 to 255
1 to 254
240 to 255
0 to 255
0 to 255
1 to 254
224.0.0.0 to
239.255.255.255
240.0.0.0 to
255.255.255.255
Two steps were taken: IPv6, Classless interdomain routing (CIDR)
IP protocol
• Transmits datagrams from one host to another, if
necessary via intermediate routers
– Unreliable (best-effort) delivery semantics
• packets can be lost, duplicated, delayed or delivered out of
order
– Address resolution: Address Resolution Module(ARP)
• IP address -> Ethernet address mapping, (IP address,
Ethernet address) pairs cache on each host
header
IP addres s of s ource
IP addres s of des tination
up to 64 kiloby tes
data
IP routing
• Routs packets from source to destination
– Internet topology: Autonomous System, Areas(n1)
– Routing algorithms:
• RIP -1
• RIP-2
• Open Short Path First (OSPF)
– Default routes: trade routing efficiency for table size
– Classless interdomain routing (CIDR): create subnet
by means of subdividing address or aggregating
addresses by mask field, e.g. 162.105.203.0/24
Internet topology
Future of IP
• IPv6(n1)
– 2128 (3*1038) addresses, 1000 IP addresses per square meter of
the Earth’s surface
– Routing speed : no checksum, no fragmentation
– Real time : priority and flow label which is used to reserve
resources
– Extension header ( information of router, authentication, etc),
– multicast and anycast
– Security through extension header type
• Migration from IPv4:
– IPv6 router island,
– depend on economics
IPv6 header layout
Version (4 bits) Priority (4 bits )
Pay load length (16 bits )
Flow label (24 bits)
Next header (8 bits)
Sourc e address
(128 bits )
Destination addres s
(128 bits )
Hop limit (8 bits)
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
TCP and UDP
• Use of ports
– Provide process-to-process communication
• UDP features
• TCP features
UDP features
• Connectionless
• Datagram delivery
– A UDP datagram is encapsulated inside an IP
packet, up to 64kb in size
• Con
– unreliable delivery due to unreliable IP
• Pro
– minimal additional cost and transmission delays
TCP
• Connection oriented
– two side must shake hands to establish a bi-directional communication
channel
• Message delivery
– Deliver arbitrary long sequences of bytes via stream-based programming
abstraction
– Sequencing: divide stream into data segments, sequence number on each
segment
– Checksum: cover the header and the data in the segment
• Flow control
– Receiver send the highest number of received segment and window size to
sender by acknowledge message
– Buffering: receiver buffer and sender buffer used for flow control
– In interactive application, receiver inform sender when timeout or the buffer
reaches the MTU limit
– Retransmission: retransmit the segment when no acknowledgement within a
specified timeout
Domain names
• Symbolic names for hosts and networks
– pku.edu.cn
• The DNS would not workable without the
extensive use of caching.
Firewall
• The purpose of a firewall is to monitor and control
all communication into and out of an intranet
– including service control,
– behavior control
– and user control
• Filter approaches (n1)
– IP packet filtering, e.g. router/filter
– TCP gateway, e.g. bastion
– Application level gateway, e.g. telnet proxy process
• Virtual private networks (VPN)
– Secure connections located at different sites using public Internet links
– By the use of cryptographically protected secure channels at the IP level
Firewall configurations
a) Filt ering router
Rout er/
f ilter
Protect ed intranet
Internet
w eb/f tp
s erv er
b) Filt ering router and bastion
R/ f ilter
Bast ion
Internet
w eb/f tp
s erv er
c ) Sc reened s ubnet f or bas tion
R/ f ilter
Internet
w eb/f tp
s erv er
Bast ion
R/ f ilter
Chapter 3: Networking and Internetworking
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Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless
LAN and ATM
• Summary
Ethernet
• IEEE 802.3[Xerox 1973]
– Carrier sensing, multiple access with collision detection
– Frame broadcasting
– Bandwidth: 3m -> 10m -> 100m -> 1000m
• Ethernet packet layout (n1)
– 248 different addresses
• Packet collisions
– Carrier sensing
• wait until no signal is present then transmit
– Collision detection
• When transmit through output port, also listen on the input port, and
compare the two signals, If differ, send jamming signal
– Back-off
• wait a time n before retransmitting, n: a random integer
Ethernet frame layout
7bytes 1byte
preamble
S
6 bytes 6 bytes
Destination
address
2 bytes
Source Length
address
of data
46 bytes ≤ length ≤
1500bytes
Data for transmission
4 bytes
Frame check
sequence
Ethernet … continued
• Ethernet efficiency
– Efficiency = number of packets transmitted successfully /
theoretical maximum number without collision
– Affected by
• A finite time for a signal inserted at a point in the media to
reach all other points
• number of stations on the network
• stations’ level of activity
Wireless LAN
• Wireless LAN types
– Infrastructure network, e.g. IEEE 802.11 (n1)
– Ad hoc network: network built on the fly
• Collision detection failures in 802.11
– Hidden stations: carrier sensing fail to detect that
another station on the network is transmitting, lead to
collision at base station
– Fading: the strength of radio signals diminishes
rapidly with the distance from the transmitter, so that
defeating both carrier sensing and collision detection
– Collision masking
Wireless LAN configuration
A
B
C
Laptops
radio obs truction
Palmtop
Server
D
E
Wireless
LAN
Base s tation/
acc es s point
LAN
802.11 introduction
•
Slot reservation added to the MAC protocol in 802.11
1.
Firstly, sense the medium, if no carrier signal, then
•
•
•
2.
Sender send a RTS (Request To Send) frame to receiver;
Receiver reply a CTS (Clear To Send) frame to sender. The
effect of the exchange is
•
•
3.
medium is available
an out-of-range station is in the process of requesting a slot
an out-of-range station is using a slot
the station within range of sender will pick up the RTS frame and
take note of the duration
the station within range of receiver will pick up the CTS frame and
take note of the duration
Begin to transmit
802.11 introduction … continued
• 802.11 avoid collisions in ways
– CTS frames avoid the hidden station and fading problem
– If RTS/CTS is corrupted, then a back-off period is used
– When RTS/CTS exchange correctly, there is no collisions in the
following communication except intermittent fading prevents a
third party from receiving either of them
• Security in 802.11
– Shared-key authentication mechanism
– XOR operation on the base of shared key to prevent from
eavesdropping
Asynchronous Transfer Mode networks (ATM)
• Deploy ATM on top of other networks
– Can be implemented over existing digital telephony networks,
Bandwidth from 32 kbps (voice) to 622mbps
– Native mode: Over optical fiber, copper and other transmission
media, bandwidth up to several gigabits per seconds
• ATM layers (n1)
– Adaptation layer
• end-to-end layer implemented at the sending and receiving host
– ATM layer
• connection-oriented service that transmits fixed length packets called
cells, avoid flow control and error checking at the switching, provide
bandwidth and latency guarantees
• VC (virtual channel): a logical unidirectional association between two
endpoints of a link in the physical path from source to destination
• VP (virtual path): a bundle of virtual channel that are associated with a
physical path between two switching nodes
ATM protocol layers
Mess age
Lay ers
Applic ation
Higher-lay er protoc ols
ATM adaption layer
ATM cells
ATM layer
ATM virtual channels
Phy sical
ATM… continued
• The nodes in a ATM network can play three distinct roles
(n1)
– Hosts: send and receive messages
– VP switches: hold tables showing the correspondence
information between incoming and outgoing VPs
– VP/VC switches: correspondence information for both VPs and
VCs
• ATM cell: 5-bytes header and a 48-byte data field
Header: 5 by tes
Virtual path id
Virtual channel id
Flags
53 bytes
Data
Switching virtual paths in an ATM network
Host
VPI = 2
VPI = 3
VPI = 4
VPI in VPI out
2
3
VP/VC
s w itch
VP sw itch
4
5
VPI = 5
VP sw itch
Host
VPI : virtual path identifier
Virtual path
Virtual channels
Chapter 3: Networking and Internetworking
•
•
•
•
•
Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless
LAN and ATM
• Summary
Summary
• Layered protocols
– 7 layers in OSI model / 5 layers in the Internet
• Delivery approach
– Packet switch, frame relay
• Routing mechanism
– distance vector / link state
• Congestion control
• The Internet
– TCP/IP
• Network cases
– Ethernet, WLAN, ATM
OSI protocol summary
Layer
Description
Examples
Application
Protocols that are designed to meet the communication requirements of
specific applications, often defining the interface to a service.
HTTP, FTP , SMTP,
CORBA IIOP
Presentation
Protocols at this level transmit data in a network representation that is
independent of the representations used in individual computers, which may
differ. Encryption is also performed in this layer, if required.
Secure Sockets
(SSL),CORBA Data
Rep.
Session
At this level reliability and adaptation are performed, such as detection of
failures and automatic recovery.
EJB
Transport
This is the lowest level at which messages (rather than packets) are handled.
Messages are addressed to communication ports attached to processes,
Protocols in this layer may be connection-oriented or connectionless.
TCP, UDP
Network
Transfers data packets between computers in a specific network. In a WAN
or an internetwork this involves the generation of a route passing through
routers. In a single LAN no routing is required.
IP, ATM virtual
circuits
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).
Distance-Vector Routing table for the network
Routings from A
To
Link
Cost
A
local
0
B
1
1
C
1
2
D
3
1
E
1
2
A
Hosts
or local
networks
Routings from B
To
Link
Cost
A
1
1
B
local
0
C
2
1
D
1
2
E
4
1
1
3
Routings from D
To
Link
Cost
A
3
1
B
3
2
C
6
2
D
local
0
E
6
1
B
2
Links
4
C
5
D
6
Routings from C
To
Link
Cost
A
2
2
B
2
1
C
local
0
D
5
2
E
5
1
E
Routers
Routings from E
To
Link
Cost
A
4
2
B
4
1
C
5
1
D
6
1
E
local
0
Psudo-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
}
}
}
Simplified view of the QMW Computer Science network
Campus
router
138.37.95.240/29
subnet
138.37.95.241
router/
firewall
hammer
Staff subnet
Student subnet
138.37.88.251
138.37.88
compute
server
138.37.94.251
Eswitch
file server/
gateway
Eswitch
bruno
138.37.88.249
%
138.37.94
custard
138.37.94.246
dialup
server
henry
138.37.88.230
printers
other
servers
file
server
hotpoint
138.37.88.162
web
server
copper
138.37.88.248
hub
desktop computers
Campus
router
138.37.95.248/29
subnet
hub
138.37.88.xx
desktop computers
138.37.94.xx
sickle
138.37.95.249
router/
firewall
100 Mbps Ethernet
1000 Mbps Ethernet
Eswitch: Ethernet switch
Tunnelling
IPv6 encapsulated in IPv4 packets
IPv4 network
A
IPv6
IPv6
B
Encapsulators
IP encapsulated in PPP packets
PPP network
A
IP
IP
Encapsulators
B
ATM cell layout
Header: 5 by tes
Virtual path id
Virtual channel id
Flags
53 bytes
Data