THE ISO/OSI APPLICATIONS MODEL

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Goals:
Topics:
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D
CC
Y. Yemini, 1996
c
THE ISO/OSI APPLICATIONS MODEL
Review the OSI approach and failure
1.
The OSI model
2.
Example: X.400
3.
Critical review
Do not duplicate without written authorization.
Y. Yemini, 1996
c
The OSI Layer Model
Service access points (SAP)
Protocol data units (PDUs)
Peer procedures
1. OSI
Protocol
Entity
Protocol
Entity
Peer Interactions
X.Request X.IndicationX.ResponseX.Confirm
1
4
3
2
10-1-2
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Y. Yemini, 1996
c
Design principles
Connection-oriented network layer
Standardize all protocol aspects: SAP, PDU, procedures
Confirmed interactions (mostly)
A single comprehensive standard per layer
Conformance testing, formal specifications
Absence: handling heterogeneity
The OSI Reference Model
1. OSI
Ph
y
sical
(
X.21
)
Link
(
LAPD
)
Network
(
X.25
)
Transport
(
TP
)
Session
Presentation
Application
Ph
y
sical
(
X.21
)
Link
(
LAPD
)
Network
(
X.25
)
Transport
(
TP
)
Session
Presentation
Application
Physical (X.21)
Link (LAPD)
Network( X.25)
Ph
y
sical
(
X.21
)
Link
(
LAPD
)
Network
(
X.25
)
Ph
y
sical
(
X.21
)
Link
(
LAPD
)
Network
(
X.25
)
Do not duplicate without written authorization.
Y. Yemini, 1996
c
1. OSI
The OSI Application La
y
er Model
Service elements (SE) provide common services
Association Control SE (ACSE): manage associations
Remote Operations SE (ROSE): manage interactions
Reliable Transfer SE (RTSE): manage bulk transfer
Commitment, Concurrency, Recovery SE (CCRSE)
Common Management Information SE (CMISE)
Application elements (AE) use common SEs
FTAM: File transfer access method (AE and SE)
X.400: Email
X.500: Directory
Transport Layer
Session
Presentation
Application
RTSE
ROSE
ACSE
X.400
Session
Presentation
Application
RTSE
ROSE
ACSE
X.400
10-3-4
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Y. Yemini, 1996
c
1. OSI
Example: Association Control Element (ASCE)
Establish/terminate a parametrized association among peers.
Primitives (verbs)
A-Associate.Request/Response: Establish a parametrized association
Parameters (>20): Mode, calling& calledAE context parameters, user information,
presentation context parameters, quality of service, session parameters
A-Release. request: Confirmed termination of association
Parameters: Reason, user information
A-Abort. Request: Unconfirmed termination of an association by an AE
Parameters: User information
A-P-Abort. Indication: Provider-initiated abort
Parameters: Reason
Transport Layer
Session
Presentation
Application
RTSE
ROSE
ACSE
X.400
Session
Presentation
Application
RTSE
ROSE
ACSE
X.400
Do not duplicate without written authorization.
Y. Yemini, 1996
c
SE Define API To Common Services
SE define API to common services
Services are defined in terms of OSI stack
Session
Presentation
Application
RTSE
ROSE
ACSE
Application
10-5-6
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Y. Yemini, 1996
c
Scope of standard:
Models: operations, message structure, message store..
Services
Protocols for various peers
Conformance testing
2. X.400
X.400: Overall Architecture
UA
User
P3: UA<-->MTA
MS
P2:
user<-->UA
MTA
MTA
MTA
MTA
P1:
MTA<-->MTA
MTS
MHS
UA
UA
User
User
P7: UA<-->MS
UA
Do not duplicate without written authorization.
Y. Yemini, 1996
c
The Messa
g
e
IPMS Header
Body
IP-MessageID
From:
To:
Subject:
UMPDUId
Originator
Contents type
Priority
.....
Recipients
MTS Envelope
10-7-8
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Y. Yemini, 1996
c
X.400 Services
2. X400
Basic services:
Access management, Message identification, Contents type selection,Conversions,
Submission/delivery time stamp,Non-delivery notification......
Optional services:
Alternate recipients Deferred delivery, Delivery notification Multidestination delivery
...............................
Basic services:
IP message identification, Typed message body
Optional services:
Originator indication,Primary & copy recipients, Replying IP message, Subject
indication, Blind copy recipients Authorizing users, Importance
Sensitivity...............................
MTA services
Interpersonal ms
g
services:
Do not duplicate without written authorization.
Y. Yemini, 1996
c
Service Elements
Generic Service elements: ACSE, ROSE...
Message Delivery Service Element (MDSE)
Supports MTA to UA delivery of messages (half of P3)
Message Submission Service Element (MSSE)
Supports UA to MTA delivery of messages ( other half of P3)
Message Transfer Service Element (MTSE)
Supports MTA to MTA transfer of messages (P1)
Message Administration Service Element (MASE)
Supports MTA/MS interactions with UA concerning message administration (access
control, message status information..)
Message Retrieval Service Element (MRSE)
Supports retrieval of messages by UA from MS (P7)
2. X400
10-9-10
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Y. Yemini, 1996
c
Compare With The Internet Approach
Internet design principles
Minimalistic design: keep protocols as simple as possible
Focus on interoperability and scalability
Network layer provides packet routing services
Connections, reliability etc is end-node function
Standardize PDUs and peer interactions only
Allow arbitrary physical layers under the network layer
Keep agile, fast, open, participatory standardization process
Use reference implementations to assure conformance
3 Critical Review
Do not duplicate without written authorization.
Y. Yemini, 1996
c
What Can We Learn From The OSI Experience
A dynamic technology market cannot sustain complexity
Standard proposals must compete in a rapidly evolving market
Complex standards take too long to develop and implement
Complex standards are difficult to implement and maintain
Market tends to select the simplest and most available timely solution
Can complex closed organizations create viable standards
Contrast the simplicity of RFC's and the ease of getting them with OSI standards
10-11-12