1. Chapter 3: Networking and Internetworking
Introduction
Types of network
Network principles
Internet protocols
Network case studies:
Ethernet, wireless LAN and ATM
Summary

2. Concepts Why should we study network technology? Some concepts
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.

3. 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
local network – a null message transmission time is under a millisecond
local memory or more times faster than local network
local hard disk times slower than fast local network
Internet - round-trip latencies are in ms due to switching and contention

4. 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-to-end 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

5. Chapter 3: Networking and Internetworking
Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless LAN and ATM
Summary

6. Local area networks (LANs)
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 (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)

7. 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

8. Chapter 3: Networking and Internetworking
Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless LAN and ATM
Summary

9. 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
Frame relay: combination of circuit switching and packet switching
Delay: Internet -200 milliseconds, telephone - 50 milliseconds, ATM - a few tens of microseconds

10. Network Protocols Protocol Protocol layers
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

11. Encapsulation of a packet

12. 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

13. Routing Routing algorithm Distance vector
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

14. 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

15. Congestion control A rule of thumb Congestion control
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

16. Internetworking Internetwork Requirements Example
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

17. 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

18. Chapter 3: Networking and Internetworking
Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless LAN and ATM
Summary

19. Internet protocols Protocol layers (n1)
TCP(UDP)/IP,(n2) web [HTTP], [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

20. Internet protocol layers
Messages (UDP) or Streams (TCP)
Application
Transport
Internet
UDP or TCP packets
IP datagrams
Network-specific frames
Message
Layers
Underlying network
Network interface

21. Encapsulation in a message transmitted via TCP over an Ethernet
Application message
TCP header
IP header
Ethernet header
Ethernet frame
port
TCP IP

22. The programmer’s conceptual view of a TCP/IP Internet

23. 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

24. Internet address structure28

25. Two steps were taken: IPv6, Classless interdomain routing (CIDR)
Decimal representation of Internet addresses
octet 1
octet 2
octet 3
Class A:
1 to 127
0 to 255
1 to 254
Class B:
128 to 191
Class C:
192 to 223
224 to 239
Class D (multicast):
Network ID
Host ID
Multicast address
240 to 255
Class E (reserved): to to to to to
Range of addresses
Two steps were taken: IPv6, Classless interdomain routing (CIDR)

26. 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

27. 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 /24

28. Internet topology

29. Future of IP IPv6(n1) Migration from IPv4:
2128 (3*1038) addresses, 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

30. IPv6 header layout

31. The MobileIP routing mechanism
Sender
Home
Mobile host MH
Foreign agent FA
Internet
agent
First IP packet
addressed to MH
Address of FA
returned to sender
tunnelled to FA
Subsequent IP packets

32. TCP and UDP Use of ports UDP features TCP features
Provide process-to-process communication
UDP features
TCP features

33. UDP features Connectionless Datagram delivery Con Pro
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

34. TCP Connection oriented Message delivery Flow control
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

35. Symbolic names for hosts and networks
Domain names
Symbolic names for hosts and networks
pku.edu.cn
The DNS would not workable without the extensive use of caching.

36. 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

37. Firewall configurations

38. Chapter 3: Networking and Internetworking
Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless LAN and ATM
Summary

39. Ethernet IEEE 802.3[Xerox 1973] Ethernet packet layout (n1)
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

40. Ethernet frame layout Destination address Source Length of data
Data for transmission
Frame check
sequence
7bytes 1byte bytes 6 bytes
2 bytes bytes ≤
length
1500bytes bytes S
preamble

41. Ethernet … continued Ethernet efficiency Affected by
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

42. Collision detection failures in 802.11
Wireless LAN
Wireless LAN types
Infrastructure network, e.g. IEEE (n1)
Ad hoc network: network built on the fly
Collision detection failures in
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

43. Wireless LAN configuration

44. introduction
Slot reservation added to the MAC protocol in
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
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
Begin to transmit

45. 802.11 introduction … continued
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
Shared-key authentication mechanism
XOR operation on the base of shared key to prevent from eavesdropping

46. 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

47. ATM protocol layers

48. 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

49. Switching virtual paths in an ATM network

50. Chapter 3: Networking and Internetworking
Introduction
Types of network
Network principles
Internet protocols
Network case studies: Ethernet, wireless LAN and ATM
Summary

51. Summary Layered protocols Delivery approach Routing mechanism
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

52. OSI protocol summary FTP 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.
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
connected by a physical link. In a WAN transmission is between pairs of
routers 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 of
binary data by analogue signalling, using amplitude or frequency modulation
of electrical signals (on cable circuits), light signals (on fibre optic circuits)
or other electromagnetic signals (on radio and microwave circuits).
Ethernet base- band
signalling,
ISDN
EJB

53. Distance-Vector Routing table for the network
Routings from A
Routings from B
Routings from C To
Link
Cost A B C D E
local 1 3 2 4 5
Routings from D To
Link
Cost A B C D E 3 6
local 1 2
Hosts
Links
or local
networks A D E B C 1 2 5 4 3 6
Routers
Routings from E To
Link
Cost A B C D E 4 5 6
local 2 1

54. 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 }

55. Simplified view of the QMW Computer Science network
file
compute
dialup
hammer
henry
hotpoint
bruno
router/
sickle
/29
copper
firewall
web
/29
server
desktop computers xx
subnet
Eswitch
custard
hub
Student subnet
Staff subnet
other
servers %
1000 Mbps Ethernet
Eswitch: Ethernet switch
100 Mbps Ethernet
file server/
gateway
printers
Campus
router xx

56. Tunnelling A B IPv6 IPv6 encapsulated in IPv4 packets Encapsulators
IPv4 network A B IP
IP encapsulated in PPP packets
Encapsulators
PPP network

57. ATM cell layout