Computer Communications
Lecture 17
The Real World - Standardization
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Standardization
• Standardisation is the process of establishing agreement
– Based on complete technical specifications
– Global scope
– Essential for equipment inter-operability (open systems)
– Essential for Inter-networking
• Standards require agreement on:
– Information, time packages and space (all the issues we’ve covered)
– Properties of electrical, optical and physical media
• Protocols underpin these agreements
• Protocols have static and dynamic features
• Static features are the content of agreements
• Protocols assist implementation of a service
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Protocols
• Case studies have illustrated:
– Use of particular protocols, and
– How protocols are stacked or layered
• MAP is example of use of sub-protocols
• Standardization of services and protocols is complex:
– Global agreement is required
– Agreements are ultimately political in nature, guided by technical issues
• Standards may evolve out of widespread use or explicit pre-agreement
– De Facto (widespread use) e.g. x86 CPU architecture, Internet protocols
– De Jure
(standardisation) e.g. ISO X.25 protocol, Food standardisation1
• De facto standards often given a post hoc official seal of approval
1 S.
White, “The Rise and Fall of Federal Food Standards in the United States: The Case of the Peanut Butter and Jelly
Sandwich”, Society for the Social History of Medicine, Spring Conference 1999 (Aberdeen, Scotland), also
http://www.fda.gov/oc/history/slideshow/default.htm
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Major Standards Organisations
• International Organization for Standardization:
– ISO supports national majorities of many interests
• International Telecommunication Union (ITU):
– United Nations agency
– Supports national interests of telecomms companies
• IEEE
– Focussed on computing and communications
• Internet Society:
– More open style than others and supports all interests
• Many others for more specialist communications
• We need standards though!
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ISO standards
• Centres on the Reference Model for Open Systems Interconnection
– Referred to as RM/OSI
• Model is the framework for protocol and service standards
• Model has seven layers
– Physical media at the bottom
– User applications at the top
– 5 layers of increasing abstraction in between
• Each standard protocol fits into one layer
– No protocol spans two or more layers (self contained)
• Each standard service falls within one layer
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ISO Standards
• Services of one layer are used to implement the protocols of the next
layer up.
• Thus RM/OSI is an architectural framework used for classifying
protocols and services.
• The framework is for guidance.
• The number of layers was contentious and was generally agreed on
past experience.
• Recall the design philosophy for layered protocols:
– Layers should be created when a new level of abstraction is required
– Each layer should perform a well-defined function
– There should be a natural mapping between each layer and an
internationally agreed group of standards
– Layer boundaries should minimise cross-boundary information flow
– Should be enough layers to separate functions that are distinct, but few
enough that the resulting protocols are not unwieldy.
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Example of useful layering
• Consider inter-network with 802.3 LAN and 802.11 WLAN
• Different physical media, but task is similar
• Needs differentiation at low levels but commonality higher up
Logical Link Control (LLC)
Knows nothing about
MAC layer, just relies
on its services
Medium Access Control
802.3
LAN
802.11
WLAN
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Knows nothing about
LLC layer, just
services requests
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The ISO / OSI 7-layer Reference Model
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Layers of the ISO model
• 7. Application layer: topmost
– Functions such as file transfer, terminal handling, RPCs and concurrent
transactions
– User application level in general
• 6. Presentation layer:
– Provides services for application layer
– Information representation
– Negotiates syntax, e.g. how bytes represent higher-level information
• 5. Session layer:
– Provides services for presentation layer
– Concerned with time period control and synchronization between two
parties.
– May manage 2 simplex channels to create duplex channel
– Allows marking of synchronization points within the dialogue
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Layers of the ISO model
• 4. Transport layer:
– Provides services for session layer
– Provides end-to-end reliable connection-oriented time package between two
computers
– Functions include flow control, error detection and correction, and
sequencing
– Need for these functions depends on the quality of the service provided by
the layer below
• 3. Network layer:
– Provides services for the transport layer
– Logically concatenates a set of links to create end-to-end connection
– Creates channels for communication space
– Provides routing and relaying within MSNs
– Several sub-layers to deal with network hierarchies from single protocols to
multi-protocol inter-networks
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Layers of the ISO model
• 2. Data link layer:
– provides services for network layer
– Provides time package between two computers linked by a physical channel
– Can include flow control, error detection and correction, and sequencing
– Depends on service needed by layer above
– Multiplexes channels onto multipeer for MBNs
– Highly dependent on nature of physical medium
– Each physical medium associated with its own Data Link Layer protocol
• 1. Physical layer: bottommost
– Provides services for data link layer
– Defines a physical transmission medium
– Provides electrical and mechanical procedures that allow bit transmission
over the medium
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Layered protocol analogy – postal mail
• Layer 1 – Physical
– Technology for transporting letters and parcels
– People, bicycles, main vans, trains, trucks, ships, air-freight
• Layer 2 – Data Link
– Mail collection, sorting, bagging (framing) at each post office
– Medium Access Control equivalent to road, rail & air traffic regulations
• Layer 3 – Network
– Postal network; a set of routing tables at each post office & sorting office + redirection hints
– Automated letter sorting is close analogy of packet routing in networks
• Layer 4 – Transport
– Postal service has no transport layer (I.e. reliable transport protocol)
– Alice & Bob could implement this informally by numbering their correspondence
• Layer 5 – Session
– Company may have incoming mail clerk and outgoing mail clerk
– Chief clerk responsible for two-way comms through them would be a Session Layer
• Layer 6 – Presentation
– Someone who translates mail from one language to another
• Layer 7 – Application
– Bulk mailing company, taking mailing lists and circulars and ensuring delivery
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RM/OSI controversies
• Session and presentation layers have found little use
– Argues to combine 3 upper layers
• Internet copes well without these layers
• Lower levels best suited to connection-oriented protocols over singleowner switching networks supported by reliable connection-oriented
services over physical links.
• COMPLETE CONTRAST to the Internet
• Problems with layering
– Can lead to poor performance
– Hides information that might otherwise be useful
– Consider a flow-control system guided by packet loss
» Packet loss may be due to congestion or transmission errors
» If error handling is at a lower layer than flow control, no way to tell the
cause of packet loss and flow control cannot know what to do.
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What OSI/RM does/does not contain
• OSI model is not a network architecture
– Does not specify exact services and protocols at each layer
– Specifies what each layer should do
• ISO has independent set of standards to implement OSI/RM
– Protocols for each layer
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Practical impact of RM/OSI
• Basis for ISO services and protocols
• Role of the model is guidance
• Most feel model should not be a standard
• Major impact has been in textbooks
• Undesirable influences
– Some equipment was designed strictly on layer by layer basis
• Thus inefficient at upper three layers
• It is an abstract model not implementation model
– Real implementations blur the boundaries slightly
– Majority of protocol stack operates within a single address space
» I.e. not as separate processes or threads communicating via messagepassing or O/S-based inter-process channels
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Impact on vocabulary
• ISO chose data unit instead of message.
– Hence protocol data unit (PDU) and
– Service data unit (SDU), also
– Transport layer TPDU
• Frame refers to data link layer message
• Packet refers to network layer message
• A bridge is a data link layer switch
• Router/Gateway is a network layer switch
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Earlier topics related to layers
• Application layer:
– MOTIS for electronic mail
– MAP associations between processes in MMS
• Presentation layer:
– ASN.1 not layer specific but relevant here
– Connection-oriented protocol in MAP set
• Session layer:
– Connection-oriented protocol in MAP set
• Transport layer:
– Five different classes, TP4 used in MAP.
• Network layer:
– Standard lifted from ITU-T X.25
– Highest layer that ISO did not define its own protocol.
– Connectionless protocol in MAP protocol set.
– Strongly influenced by Internet IP protocol.
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And the remaining layers …
• Data link layer:
– Message format used in HDLC procedures for compatibility with ITU-T
LAPB procedures.
– MAN and LAN standards taken from IEEE.
• Physical layer:
– Largely province of ITU-T.
– Number of standards here to deal with very specific physical arrangements.
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MAP revisited
• Recall MAP designed to comply with ISO.
• We discussed top 3 layers in FullMAP
– Build around OSI/RM using ISO protocol set
• In MAP they solve one specific problem
– How to get from a token bus physical link to an application-level connection
• One protocol to handle an MMS connection would have been a simpler
alternative
• Not really necessary and so messages are longer than necessary
• Lessons:
– Choose appropriate technology
– Don’t follow standards blindly - they must be genuinely helpful
– Standards should be the servant rather than the master
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POP revisited
• POP allows mail delivery to client machine from mail server
• Since introduction of POP, IMAP has become available
• IMAP – Internet Message Access Protocol
– More modern and flexible system than POP
– Allows access to email and stored mail folders
– Effective from anywhere on the Internet
• 25 Mb/user, better junk and virus monitors
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Internet standards
• An eminent Internet guru once said:
– “We reject kings, presidents and voting. We believe in rough consensus
and running code”
• Standards arrive tried and tested
• In ISO, it’s the other way around
• Like ISO, there is an Internet reference model – TCP/IP
– Useful for comparison with the ISO approach
– Originated from the ARPANET (US DoD, 1970s)
– Connected Universities and Govt establishments using leased lines
– Reference architecture needed to connect ARPANET to rest of world
– Result was the TCP/IP Reference Model, first defined 1974
– See Cerf and Kahn, “A Protocol for Packet Network Interconnection”, IEEE
Trans. on Commun., vol COM-22, pp.637-648, May 1974.
– Flexibility and resilience to network failure were main design goals
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ARPANET
• US phone network not resilient to nuclear attack
• DoD wanted new C&C network
– Paul Baran proposes distributed design
– AT&T dismiss idea as impractical (threat to phone?)
• 1957 Soviet Union beats US into space
• Eisenhower creates ARPA
– Larry Roberts, director
• 1967 ARPA turns to networking
– Revives Baran’s original proposal
• Wesley Clark suggests packet switching
• ARPA paper at ACM SIGOPS, 1967
• Similar NPL paper
– Also based on Baran’s proposal
• ARPANET project started
– BBN given contract
– 56 kbps lines
– IMPs at nodes
– First link (UCLA – SRI) Sept 1969
• 2004 Wesley Clark runs for presidential nomination…
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Standardisation continued
The Internet Model
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Internet model
• Only protocols are standardized
– Service implicit within protocol capabilities
• Protocols are a perfect fit to the model
• No surprise since model follows practice
• So not really a general purpose model
• Notion of a central transport/inter-network layer common to all comms
systems
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Internet model layering
Internet
applications
TCP, UDP
IP
Very simple model with four layers
Says little about Data Link and Physical layer functions – assumed
No separate Presentation and Session layers
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Original TCP/IP model protocols & apps
• The original applications, protocols and physical networks of the TCP/IP model
• Applications expanded over time
• Transport and Network layers remained largely intact
• Physical link is the least well specified aspect of the internet (most generic)
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Application layer
• Corresponds to top three layers of RM/OSI
• No separate Session or Presentation layer
– Not deemed necessary
– Experience has shown this to be correct
• Model contains protocols to suit particular applications:
– Telnet, FTP, SMTP, RFC 822,
– DNS - Domain Name System,
– NTP – Network Time Protocol,
– NNTP – USENET news articles
– HTTP – web pages
– And many more (look at /etc/services on your Linux machine)
• Management set similar to ISO but simpler
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Transport and Internet layers
• Originally one layer with TCP as transport protocol and TCP/IP as
combined layer protocol
• TCP is a reliable, connection-oriented protocol
• Provides byte-stream communication between computers
– Segments byte stream into discrete messages
– Passes them to the Internet later
– Receiver re-assembles messages into original byte stream
– Handles flow control using `Not Ready’ and `Ready’ control packets
• However unreliable connectionless required too:
– User Datagram Protocol (UDP) was added as a second transport protocol
– Used where prompt delivery more important than accuracy
• TCP and UDP are the only standardized protocols
– `Recommended’ status over IP, TCP the senior
• Thus TCP/IP often the descriptive of the Internet
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Host-to-Network layer
• Bottom and rather slim layer
– Supplies means of transmitting IP over networks
• Networks are the implementation primitive
• Must cope with all network standards
– Including those that emerge from other standards bodies
• Includes management and address resolution protocols
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Internet standards classification
• Protocol standards classified to layers, but...
• More important, standards classified as:
– Required: must be implemented,
– Recommended: should be implemented,
– Elective: may or may not be implemented.
• Examples of electives include electronic mail and message routing
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TCP protocols
• TCP/IP provides Ports and Sockets
– Port
» Identifies type, assigned by Internet Assigned Numbers Authority (IANA)
» Applications assigned unique ports (e.g. ssh is usually port 22, http is 80)
» 0-255
: public applications
» 255-1023 : assigned for commercial products
» 1024-49151 : not controlled by IANA but registered as a convenience
» 49152-65535 : dynamic ports
– Socket
» Identifies a specific connection
» Multiplexes application instances over a single TCP/IP link
» Socket is created at each end when connection is established
• For more protocol details, see Buchanan, “Distributed Systems and
Networks”, chapter 15, pp.352-376.
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Overview of TCP/IP protocols
ping
traceroute
Appl.
Appl.
Appl.
TCP
Appl.
ping
traceroute
UDP
ICMP
IGMP
IPv4
IPv6
32-bit
addresses
ICMP
v6
128-bit
addresses
ARP,
RARP
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Data
Link
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Internet versus OSI
• Internet protocol stack beats OSI in usage and industry support
• But, OSI was in the running until late 1980s
• So why did OSI fail?
– Slow standardisation process
– Tended to stifle innovation
– Lots of politically-motivated bickering
• Internet protocol development was free and easy
– Multicast service spread through efforts of handful of people
– OSI stack does not support multicast (probably never will)
• OSI was specified too early
– Before enough experience in large-scale network design
– Internet only standardises on tried and tested solutions
• Meta-stability of open choice
– User acceptance garners more user acceptance
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ITU-T standards
• Largely harmonised with ISO standards
– Includes OSI standards as part of its own set
» X.200 = OSI/RM
» X.700 = OSI management framework
» X.800 = OSI security framework
• ITU focus is on:
– Physical channels and networks, and
– Applications related to telecommunications
• ITU-T X.25 has 3 layers = OSI/RM lowest 3 layers
– X.21 or X21bis = Physical Layer
– LAPB and LAP = Data Link Layer
– X.25 packet protocol = Network Layer
• ITU-T X.75 covers inter-networking of X.25 networks
• Second area falls within top layer of ISO
– X.400 and X.500, basis for ISO MOTIS application (includes email)
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IEEE 802 standards
Covers LANs and MANs
Physical layer transmits bits
MAC layer multiplexes onto
multipeer channel, ie, 802-3, 4
and 5
LLC also mentioned earlier in
course
Logical Link Control Protocols – operate above MAC layer
LLC1 – unacknowledged connectionless
LLC2 – connection-oriented
LLC3 – acknowledged connectionless
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IEEE 802 standardisation activities
The 802 working groups. The important ones are marked with * The
ones marked with are hibernating. The one marked with † gave up.
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Proprietary standards
• Important since they led the way, before:
– ISO standards looked like dominating the field
– Internet standards actually managed to do so
• Almost all have a layered model of sorts
• Reflects the need to implement in stages and provide abstractions
• Can be fitted roughly to ISO and Internet models
• Enough to observe:
– Applications supported by Networking supported by Physical Media
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IBM System Network Architecture
SNA introduced (1974) in order to
rationalize hundreds of
communications products.
Interconnects IBM and OEMs.
SNA major influence in ISO model,
particularly layering.
SNA has seven similar layers.
Model is entirely connectionoriented – there are no
connectionless facilities.
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IBM System Network Architecture
• SNA also gifted its Synchronous Data Link Control (SDLC) protocol –
data link layer.
• Basis for HDLC, LAPB and LLC protocols.
• Original use with mainframe and front-end processors handling
communications below.
• No direct communication between terminals but all other
communications allowed.
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IBM System Network Architecture
• When PCs arrived, they were networked outside of SNA – token ring
LANs invented
• Then NETBIOS provided a connection-oriented service for IBM PCs
and clones.
• In 1985 PC networking brought into SNA.
• SNA still widely used in IBM environments
• Now supports both ISO and Internet standards and ATM, ethernet,…
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AppleTalk
• Networking was envisaged from the launch of the Macintosh computer.
• Specifications were made freely available.
• The core is the Datagram Delivery Protocol, DDP, which is an
unreliable connectionless service.
• Above sits the AppleTalk Transaction Protocol, ATP, which supplies a
request-response protocol.
• On top is AppleTalk Data Stream Protocol, ADSP, providing a reliable
connection-oriented service.
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Novell NetWare
• Reference model for NetWare:
• Default protocol is SPX/IPX.
• SPX – Sequenced Packet
Interchange, reliable connectionoriented service similar to TCP.
• IPX – Internet Packet Exchange,
unreliable connectionless similar to
IP.
Application
Transport - SPX
Network - IPX
Data Link
• Needs Network Interface Card
Ethernet or
Token ring
Physical
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Novell NetWare
• SPX/IPX similar role to TCP/IP but is incompatible with TCP/IP traffic,
however, NetWare/IP supports TCP/IP.
• Typical example of protocol stacking, that is, an IPX packet is included
in an IP packet for Internet and Unix communication.
• Supports DOS, Windows, Windows NT, Unix, OS/2 and Macintosh.
• Renamed IntranetWare in 1996.
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Suggested exercise
Review the content of the course, and examine how it relates to the
OSI model
Fill out the course questionnaire – think of this slides as a
Disconnection Request to our mutual communications over the past 9
weeks, which then requires a returned course questionnaire as a
Disconnection Acknowledgement.
Points to ponder
“‘What is the point of being alive,’ she said, ‘if you’re not going to
communicate?”’
– from Bluebeard by Kurt Vonnegut.
“This `telephone’ has too many shortcomings to be seriously considered
as a means of communication. The device is inherently of no value to us.”
– Western Union internal memo, 1876.
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