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• Introduction• Types of network• Network principles• Internet protocols• Network case studies:
Ethernet, wireless LAN and ATM• Summary
Chapter 3: Networking and Internetworking
• 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.
Concepts
• 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 • 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
• Scalability – WWW: world wide wait ?– future Internet: several billion nodes and hundreds of millions of active host
s, 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-to-end at the highest fea
sible 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 organizat
ion 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
Networking issues for distributed systems … continued
• Introduction• Types of network• Network principles• Internet protocols• Network case studies: Ethernet, wireless LAN and
ATM• Summary
Chapter 3: Networking and Internetworking
• 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 se
t 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 mbp
s)– WAP (Wireless Application Protocol)
Types of network
• Internetworks– several networks are linked together to provide common data c
ommunication facilities that conceal the technology differences.– They are interconnected by routers and gateways– e.g. Internet
• Network comparisons– different failure model : TCP vs. UDP
Types of networks … continued
• Introduction• Types of network• Network principles• Internet protocols• Network case studies: Ethernet, wireless LAN and
ATM• Summary
Chapter 3: Networking and Internetworking
• 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 Switching technology
• 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
Network Protocols
Application
Presentation
Session
Transport
Network
Data link
Physical
Message sent Message received
Sender Recipient
Layers
Communicationmedium
Encapsulation of a packet
Presentation header
Application-layer message
Session header
Transport header
Network header
• 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
Other important concepts in network protocol
• 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
• 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
Routing … continued
• 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
Congestion control
• Internetwork – integrate many subnets that use different network technologi
es
• 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 g
iving rules according to which they are handled– Interconnecting components that route packets to their destin
ations in terms of internetwork addresses, transmitting the packets using subnets with a variety of network technologies
• Example
Internetworking
• 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
Internetworking components
• Introduction
• Types of network
• Network principles
• Internet protocols
• Network case studies: Ethernet, wireless LAN and ATM
• Summary
Chapter 3: Networking and Internetworking
• 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 protocols
Internet protocol layers
Messages (UDP) or Streams (TCP)
Application
Transport
Internet
UDP or TCP packets
IP datagrams
Network-specific frames
MessageLayers
Underlying network
Network interface
Encapsulation in a message transmitted via TCP over an Ethernet
Application message
TCP header
IP header
Ethernet header
Ethernet frame
port
TCP
IP
The programmer’s conceptual view of a TCP/IP Internet
IP
Application Application
TCP UDP
• 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
IP addressing
Internet address structure
7 24
Class A: 0 Network ID Host ID
14 16
Class B: 1 0 Network ID Host ID
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 unused0
28
Decimal representation of Internet addresses
octet 1 octet 2 octet 3
Class A: 1 to 127
0 to 255 0 to 255 1 to 254
Class B: 128 to 191
Class C: 192 to 223
224 to 239 Class D (multicast):
Network ID
Network ID
Network ID
Host ID
Host ID
Host ID
Multicast address
0 to 255 0 to 255 1 to 254
0 to 255 0 to 255 0 to 255
0 to 255 0 to 255 0 to 255
Multicast address
0 to 255 0 to 255 1 to 254240 to 255 Class E (reserved):
1.0.0.0 to
127.255.255.255
128.0.0.0 to
191.255.255.255
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
240.0.0.0 to
255.255.255.255
Range of addresses
Two steps were taken: IPv6, Classless interdomain routing (CIDR)
• 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, Etherne
t address) pairs cache on each host
IP protocol
dataIP address of destinationIP address of source
header
up to 64 kilobytes
• 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
IP routing
Internet topology
• IPv6(n1)– 2128 (3*1038) addresses, 1000 IP addresses per square meter of t
he Earth’s surface– Routing speed : no checksum, no fragmentation– Real time : priority and flow label which is used to reserve reso
urces– Extension header ( information of router, authentication, etc),– multicast and anycast– Security through extension header type
• Migration from IPv4: – IPv6 router island, – depend on economics
Future of IP
IPv6 header layout
Source address(128 bits)
Destination address(128 bits)
Version (4 bits) Priority (4 bits) Flow label (24 bits)
Payload length (16 bits) Hop limit (8 bits)Next header (8 bits)
The MobileIP routing mechanism
Sender
Home
Mobile host MH
Foreign agent FAInternet
agent
First IP packet addressed to MH
Address of FAreturned to sender
First IP packettunnelled to FA
Subsequent IP packetstunnelled to FA
• Use of ports– Provide process-to-process communication
• UDP features• TCP features
TCP and UDP
• 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
UDP features
• 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
TCP
• Symbolic names for hosts and networks– pku.edu.cn
• The DNS would not workable without the extensive use of caching.
Domain names
• 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
Firewall configurations
Internet
Router/Protected intraneta) Filtering router
Internet
b) Filtering router and bastion
filter
Internet
R/filterc) Screened subnet for bastion R/filter Bastion
R/filter Bastion
web/ftpserver
web/ftpserver
web/ftpserver
• Introduction
• Types of network
• Network principles
• Internet protocols
• Network case studies: Ethernet, wireless LAN and ATM
• Summary
Chapter 3: Networking and Internetworking
• 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
Ethernet frame layout
Destinationaddress
Sourceaddress
Length
of data
Data for transmission Frame checksequence
7bytes 1byte 6 bytes 6 bytes 2 bytes 46 bytes ≤ length ≤ 1500bytes 4 bytes
Spreamble
• 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
Ethernet … continued
• 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
Wireless LAN configuration
LAN
Server
WirelessLAN
Laptops
Base station/access point
Palmtop
radio obstruction
A B C
DE
• Slot reservation added to the MAC protocol in 802.11 1. Firstly, sense the medium, if no carrier signal, then
• 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
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
• 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
3. Begin to transmit
802.11 introduction
• 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
802.11 introduction … continued
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
Physical
Application
ATM layer
Higher-layer protocols
ATM cells
ATM virtual channels
MessageLayers
ATM adaption layer
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
Flags DataVirtual channel idVirtual path id
53 bytes
Header: 5 bytes
Switching virtual paths in an ATM network
VPI in VPI out
23
45
VPI = 3
VPI = 5
VPI = 4
Virtual path Virtual channels
VPI = 2
VPI : virtual path identifier
VP switch VP/VCswitch
VP switch
Host
Host
• Introduction
• Types of network
• Network principles
• Internet protocols
• Network case studies: Ethernet, wireless LAN and ATM
• Summary
Chapter 3: Networking and Internetworking
• 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
Summary
OSI protocol summary
Layer Description Examples
Application Protocols that are designed to meet the communication requirements ofspecific 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 isindependent of the representations used in individual computers, which maydiffer. Encryption is also performed in this layer, if required.
Secure Sockets(SSL),CORBA DataRep.
Session At this level reliability and adaptation are performed, such as detection offailures and automatic recovery.
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 WANor an internetwork this involves the generation of a route passing throughrouters. In a single LAN no routing is required.
IP, ATM virtualcircuits
Data link Responsible for transmission of packets between nodes that are directlyconnected by a physical link. In a WAN transmission is between pairs ofrouters or between routers and hosts. In a LAN it is between any pair of hosts.
Ethernet MAC,ATM cell transfer,PPP
Physical The circuits and hardware that drive the network. It transmits sequences ofbinary data by analogue signalling, using amplitude or frequency modulationof electrical signals (on cable circuits), light signals (on fibre optic circuits)or other electromagnetic signals (on radio and microwave circuits).
Ethernet base- bandsignalling, ISDN
EJB
Distance-Vector Routing table for the network
Routings from D
To Link CostABCDE
336
local6
12201
Routings from E
To Link CostABCDE
4456
local
21110
Routings from A Routings from B Routings from C
To Link Cost To Link Cost To Link CostABCDE
local1131
01212
ABCDE
1local
214
10121
ABCDE
22
local55
21021
Hosts Linksor local networks
A
D E
B
C
1
2
5
43
6
Routers
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 Tlelse 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
file
compute
dialup
hammer
henry
hotpoint
138.37.88.230
138.37.88.162
bruno138.37.88.249
router/sickle
138.37.95.241138.37.95.240/29
138.37.95.249
copper138.37.88.248
firewall
web
138.37.95.248/29
server
desktop computers 138.37.88.xx
subnet
subnet
Eswitch
138.37.88
server
server
server
138.37.88.251
custard138.37.94.246
desktop computers
Eswitch
138.37.94
hubhub
Student subnetStaff subnet
otherservers
router/firewall
138.37.94.251
1000 Mbps EthernetEswitch: Ethernet switch
100 Mbps Ethernet
file server/gateway
printers
Campusrouter
Campusrouter
138.37.94.xx
Tunnelling
A BIPv6 IPv6
IPv6 encapsulated in IPv4 packets
Encapsulators
IPv4 network
A BIP IP
IP encapsulated in PPP packets
Encapsulators
PPP network
ATM cell layout
Flags DataVirtual channel idVirtual path id
53 bytes
Header: 5 bytes