Telecommunications - OSI Protocols - Session Layer

Services Provided to the presentation layer

Functions within the session layer

Data structure

SPDU (session protocol data unit)

PGI units (parameter group identifier)

PI units (parameter identifier)

Session Layer SPDU's

SPDU (session protocol data unit)

Parameters of the Connect SPDU

Parameters of the ACCEPT SPDU

Parameters of the REFUSE SPDU

Paremeters of the FINISH SPDU

Parameters of the DISCONNECT SPDU

Parameters of the NOT FINISHED SPDU

Parameters of the ABORT SPDU

Parameters of the DATA TRANSFER SPDU

Parameters of the EXPEDITED SPDU

Parameters of the TYPED DATA SPDU

Parameters of the CAPABILITY DATA SPDU

Parameters of the CAPABILITY DATA ACK SPDU

Parameters of the GIVE TOKENS SPDU

Parameters of the PLEASE TOKENS SPDU

Parameters of the MINOR SYNC POINT SPDU

Parameters of the MINOR SYNC ACK SPDU

Parameters of the MAJOR SYNC POINT SPDU

Parameters of the MAJOR SYNC ACK SPDU

Parameters of the RESYNCHRONIZE SPDU

Parameters of the RESYNCHRONIZE ACK SPDU

Parameters of the PREPARE SPDU

Parameters of the EXCEPTION DATA SPDU

Parameters of the ACTIVITY START SPDU

Parameters of the ACTIVITY RESUME SPDU

Parameters of the ACTIVITY INTERRUPT SPDU

Parameters of the ACTIVITY DISCARD SPDU

Parameters of the ACTIVITY END SPDU


1.0 - Introduction

slide OSI-S 0

slide S1 - session layer jargon

Purpose of the session layer

The purpose of the session layer is to provide the means necessary for co-operating presentation entities to organise and synchronise their dialogue and to manage their data exchange. To do this, the session layer provides services to establish a session-connection between two presentation entities and to support orderly data exchange interactions.


Physical v Logical sessions

PHYSICAL SESSIONS A physical connection may be a call or a connection through a network.
LOGICAL SESSIONS When two application-level processes wish to communicate, they do so by means of "sessions" which they perceive as one logical interaction (or, perhaps, a series of such interactions in parallel).

MAPPING OF LOGICAL SESSIONS ONTO PHYSICAL SESSIONS

a) One logical session may map onto a number of physical connections or a number of logical sessions can map onto a single physical connection.

EXAMPLE

An airline booking system, a terminal may be logged into a computer at the beginning of the day and logged out at the end. In between, there may be a large number of interactions. The user sees the whole day's work as one "session" but there is no reason why the phisical link need stay in place for the entire day (provided that the user is unaware that it isn't). If the terminal had sufficient intelligence, it could make and break the actual connections as frequently as neccessary (perhaps based on an algorithm to minimise costs if using a switched connection) such management would be handled by the session layer.


FUNCTIONS OF THE SESSION LAYER

Connection Establishment Phase Data Transfer Phase Connection Release Phase
Facilities negotiated which both ends support Expedited data exchange session connection synchrosation Orderly release or abort

SYNCHRONISATION

The reason for synchronisation is to allow recovery from errors. The sender places "marks" ("major" and "minor") in the data stream and these are acknowledged by the receiver. In the event of an error, the sender and receiver can resynchronise on one of these marks (corresponding to checkpointing in a computing system).

EXAMPLE

Distributed Database.


lh 6

SESSION PROTOCOL SUBSETS

SESSION SYNCHRONISATION TELEMATICS
KERNEL FACILITIES APPLICATIONS
BGS - BASIC COMBINED SUBSET NO
BSS - BASIC SYNCHRONISED SUBSET YES
BAS - BASIC ACTIVITY SUBSET YES

In many systems, the sophistication of the session layer will

not be required and a null session layer is permissible.


SESSION STANDARDS


CCITT ISO

X.215 8326 Basic connection - oriented session service definition.

X.225 8327 Basic connection - oriented session protocol definition.

X200 - Session Layer

Services Provided to the presentation layer

  1. Session-connection estabishment; presentation-entities identified by session addresses.
  2. Session-connection release;
  3. normal data exchange;
  4. quarantine service; an intergral number of session-service-data-units are not made availible to the receiving presentation-entity until explicitly released by the sending presentation entity.
  5. expedited data exchange;
  6. interaction management

(TWS two-way-simultaneous

TWA two-way-alternate

one-way-interaction)

7 - session-connection synchronization; allows presentation-entities to define and identify synchronization points and to reset a session-connection to a predefined state and to agree on a resynchronization point.

8 - exception reporting. such as unrecoverable session malfunctions.


Functions within the session layer

  1. session-connection to transport-connection mapping;
  2. session-connection flow control;
  3. expedited data transfer;
  4. session-connection recovery;
  5. session-connection release;
  6. session layer management.

slide S2


Data structure

SPDU (session protocol data unit)

SI LI parameter field user information field

PGI units (parameter group identifier)

PGI LI parameter field

PI units (parameter identifier)

PI LI parameter field

where LI= length indicator of parameter field in bytes (0=empty)


Session Layer SPDU's

SPDU (session protocol data unit)SPDU (session protocol data unit)

SI LI parameter field user information field

CN 13=0D Connect
AC 14=0E Accept
RF 12=0C Refuse
FN 9=09 Finish
DN 10=0A Disconnect
NF 8=08 Not finished
AB 25=19 Abort
AA 26=1A Abort accept
DT 1=01 Data transfer
EX 5=05 Expidited
TD 33=21 Typed data
CD 61= Caperbility data
CDA 62= Caperbility data ack
GT 1=01 Give tokens
PT 2=02 Please tokens
GTC 21=15 Give tokens confirm
GTA 22=16 Give tokens ack
MIP 49= Minor sync point
MIA 50= Minor sync ack
MAP 41= Major sync point
MAA 42= Major sync ack
RS 53= Resynchronize
RA 34= Resynchronize ack
PR 7= Prepare
ER 0=00 Exception report
ED 48=30 Exception data
AS 45= Activity start
AR 29= Activity resume
AI 25= Activity interrupt
AIA 26= Activity interrupt ac
AD 57= Activity Discard
ADA 58= Activity discard ack
AE 41= Activity end
AEA 42= Activity end ack

SI PGI PI
0D Connect 1 Connect Iden 10 Called SS-user ref
11 Common Reference
12 Additional Reference Information
5 Connect Accept Item 19 Protocol Options
21 TSDU Maximum size
22 Version Number
23 Initial Serial Number
26 Token Stting Item
20 Session User Requirements
51 Calling SSAP Identifier
52 Calling SSAP Identifier
193 User Data
0D Accept 1 Connect Iden 10 Called SS-user ref
11 Common Reference
12 Additional Reference Information
5 Connect Accept Item 19 Protocol Options
21 TSDU Maximum size
22 Version Number
23 Initial Serial Number
26 Token Setting Item
20 Session User Requirements
51 Calling SSAP Identifier
52 Calling SSAP Identifier
193 User Data
0D Refuse 1 Connect Iden 10 Called SS-user ref
11 Common Reference
12 Additional Reference Information
17 Transport Disconnect
20 Session User Requirments
22 Version Number
50 Reason Code
09 Finish 17 Transport Disconnect
193 User data
0A Disconnect 193 User data
08 Not finished 193 User data
AB Abort 17 Transport Disconnect
49 Reflect Parameter Values
193 User Data
01 Data transfer 25 Enclosure Item
05 Expidited
21 Typed data 25 Enclosure Item
3D Capability data 193 User Data
3E Capability data ack 193 User Data
01 Give tokens 16 Token Item
02 Please tokens 16 Token Item
193 User Data
15 Give tokens confirm
16 Give tokens ack
31 Minor sync point 15 Sync Type
42 Serial Number
193 User Data
32 Minor sync ack 42 Serial Number
46 User Data
29 Major sync point 15 Sync Type
42 Serial Number
193 User Data
2A Major sync ack 42 Serial Number
46 User Data
35 resynchronize 26 Token Setting
27 Resync Type
42 Serial Number
193 User Data
22 resynchronize ack 26 Token Setting
42 Serial Number
193 User Data
07 Prepare 24 Prepare Type
00 Exception report 49 Reflect Parameter Values
30 Exception data 50 Reason Code
193 User Data
2D Activity start 41 Activity Identifier
193 User Data
1D Activity resume 33 Linking Information 9 Called SS-user Reference
10 Calling SS-user Reference
11 CommonRReference
12 Additional Reference Information
41 Old Activity Identifier
42 Serial Number
41 New Activity
193 User Data
19 Activity interrupt 50 Reason Code
39 Activity discard 50 Reason Code
3A Activity ack
29 Activity end 42 Serial Numbe
193 User Data
2A Activity end ack

Parameters of the Connect SPDU

PGI m/ nm Code PI m/ nm Code Length
Connection Identifier m 1 Called SS-user Reference nm 10 64 octets maximum
Common Reference nm 11 14 octets maximum
Additional Reference Information nm 12 2 octets maximum
Connect/ Accept Item (see 8.3.2.6) nm 5 Protocol Options m 19 1 octet
TSDU Maximum size nm 21 4 octets
Version Number m 22 1 octet
Initial Serial Number nm 23 6 octets maximum
Token Setting Item nm 26 1 octet
Session User Requirements nm 20 2 octets
Calling SS Identifier nm 51 16 octets maximum
Calling SSAP Identifier nm 52 16 octets maximum
User Data nm 193 512 octets maximum

Key: m : mandatory

nm : not manatory (see 8.2.5)


Parameters of the ACCEPT SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Connection Identifier m 1 Called SS-user Reference nm 9 24 octets maximum 7.2.1.a1

8.3.2.3

Common Reference nm 11 14 octets maximum 7.2.1.a2

8.3.2.4

Additional Reference Information nm 12 2 octets maximum 7.2.1.a3

8.3.2.5

Connect/ Accept Item (see 8.3.2.6) nm 5 Protocol Options m 19 1 octet 7.2.1.b1

8.3.2.7

TSDU Maximum size nm 21 4 octets 7.2.1.b2

8.3.2.8

Version Number m 22 1 octet 7.2.1.b3

8.3.2.9

Initial Serial Number nm 23 6 octets maximum 7.2.1.b4

8.3.2.10

Token Setting Item nm 26 1 octet 7.2.1.b5
Session User Requirements nm 20 2 octets 7.2.1.d

8.3.2.13

Calling SSAP Identifier nm 51 16 octets maximum 7.2.1.d

8.3.2.14

Calling SSAP Identifier nm 52 16 octets maximum 7.2.1.e

8.3.2.15

User Data nm 193 512 octets maximum 7.1.2.f 8.3.2.16

Key: m : mandatory

nm : not manatory (see 8.2.5)


Parameters of the REFUSE SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Connection m 1 Called nm 9 24 octets 7.3.1.a1
Identifier SS-user maximum 8.3.3.3
Reference
Common Reference nm 11 14 octets maximum 7.3.1.a2

8.3.3.4

Additional Reference Information nm 12 2 octets maximum 7.3.1.a3

8.3.3.5

Transport Disconnect nm 17 1 octet 7.3.1.b

8.3.3.6

Session User Requirments nm 20 2 octets 7.3.1.c

8.3.3.7

Version Number nm 22 1 octet 7.3.1.d

8.3.3.8

Reason Code nm 50 513 octets 7.3.1.e
maximum 8.3.3.9

Key: nm : not mandatory (see 8.2.5)


Paremeters of the FINISH SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Transport disconnect nm 17 1 octet 7.4.1.a

8.3.4.3

User Data nm 193 512 octets maximum 7.4.1.b

8.3.4.4

Key: nm not mandatory (see 8.2.5)


Parameters of the DISCONNECT SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
User Data nm 193 512 octets maximum 7.5.1.b

8.3.5.3

Key: nm not mandatory (see 8.2.5)


Parameters of the NOT FINISHED SPDU

PGI m/ nm Code PI m/ nm Code Length Reference
and PV
User Data nm 193 512 octets maximum 7.6.1

8.3.6.3

Key: nm not mandatory (see 8.2.5)


Parameters of the ABORT SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Transport Disconnect m 17 1 octets 7.7.1.a

8.3.7.3

Reflect Parameter Values nm 49 9 octets maximum 7.7.1.b

8.3.7.4

User Data nm 193 9 octets maximum 7.7.1.c

8.3.7.5


Parameters of the DATA TRANSFER SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Enclosure Item nm 25 1 octet 7.9.1.a

8.3.9.3

User Information Field Unlimited 7.9.1.b

8.3.9.4

Key: nm : not mandatory (see 8.2.5)


Parameters of the EXPEDITED SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
User Information Field 14 octets maximum 7.10.1

8.3.10.3


Parameters of the TYPED DATA SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Enclosure Item nm 25 1 octet 7.11.1.a

3.11.3

User Information Field Unlimited 7.11.1.b

8.3.11.4


Parameters of the CAPABILITY DATA SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
User Data nm 193 512 octets maximum 7.12.1

8.3.12.3

Key: nm not mandatory (see 8.2.5)


Parameters of the CAPABILITY DATA ACK SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
User Data nm 193 512 octets maximum 7.13.1

8.3.13.3

Key: nm not mandatory (see 8.2.5)


Parameters of the GIVE TOKENS SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Token Item nm 16 1 octet 7.14.1

8.3.14.3

Key: nm not mandatory (see 8.2.5)


Parameters of the PLEASE TOKENS SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Token Item nm 16 1 octet 7.15.1

8.3.15.3

User Data nm 193 512 octets maximum 7.15.1.b

8.3.15.4

Key: nm not mandatory (see 8.2.5)


Parameters of the MINOR SYNC POINT SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Sync Type Item nm 15 1 octet 7.18.1a

8.3.18.3

Serial Number m 42 6 octets maximum 7.18.1b 8.3.18.5
User Data nm 193 512 octets maximum 7.18.1c

8.3.18.5

Key: m : mandatory

nm : not mandatory (see 8.2.5)


Parameters of the MINOR SYNC ACK SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Serial Number m 42 6 octets maximum 7.19.1.a

8.3.19.3

User Data nm 46 512 octets maximum 7.19.1.b

8.3.19.4


Parameters of the MAJOR SYNC POINT SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Sync Type Item nm 15 1 octet 7.20.1a

8.3.20.3

Serial Number m 42 6 octets maximum 7.20.1b

8.3.20.5

User Data nm 193 512 octets maximum 7.20.1.c

8.3.20.5

Key: m : mandatory

nm : not mandatory (see 8.2.5)


Parameters of the MAJOR SYNC ACK SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Serial Number m 42 6 octets maximum 7.21.1.a

8.3.21.3

User Data nm 512 octets maximum 7.21.1.b

8.3.21.4


Parameters of the RESYNCHRONIZE SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Token Setting Item nm 26 1 octet 7.22.1.a

8.3.22.3

Resync Type m 27 1 octet 7.22.1.b

8.3.22.4

Serial Number m 42 6 octets maximum 7.22.1.c

8.3.22.5

User Data nm 193 512 octets maximum 7.22.1.d

8.3.22.6


Parameters of the RESYNCHRONIZE ACK SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Token Setting Item nm 26 1 octet 7.23.1.a

8.3.23.3

Serial Number m 42 6 octets maximum 7.23.1.b

8.3.23.4

User Data nm 193 512 octets maximum 7.23.1.c

8.3.23.5


Parameters of the PREPARE SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Prepare Type m 24 1 octet 7.24.1

8.3.24.3


Parameters of the EXCEPTION REPORT SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Reflect Parameter Values m 49 65535 octets maximum 7.25.1

8.3.25.3


Parameters of the EXCEPTION DATA SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Reason Code m 50 1 octet 7.26.1.a

8.3.26.3

User Data nm 193 512 octets maximum 7.26.1.b

8.3.26.4


Parameters of the ACTIVITY START SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Activity Identifier m 41 6 octets maximum 7.27.1.a

8.3.27.3

User Data nm 193 512 octets maximum 7.27.1.b

8.3.27.4


Parameters of the ACTIVITY RESUME SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Linking Informaton m 33 Called SS-user Reference nm 9 24 octets maximum 7.28.1.1a

8.3.28.3

Calling SS-user Reference nm 10 24 octets maximum 7.28.1.a2

8.3.28.4

Common Reference nm 11 14 octets maximum 7.28.1.a3

8.3.28.5

Additional Reference Information nm 12 2 octets maximum 7.28.1.4

8.3.28.6

Old Activity Identifier m 41 6 octets maximum 7.28.1.a5

8.3.28.7

Serial Number m 42 6 octets maximum 7.28.1.a6

8.3.28.8

New Activity m 41 6 octets maximum 7.28.1.b

8.3.28.9

User Data nm 193 512 octets maximum 7.28.1.c

8.3.28.10

Key : m mandatory

nm not mandatory


Parameters of the ACTIVITY INTERRUPT SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Reason Code m 50 1 octet 7.29.1

8.3.29.3


Parameters of the ACTIVITY DISCARD SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Reason Code m 50 1 octet 7.31.1

8.3.31.3


Parameters of the ACTIVITY END SPDU

PGI m/ nm Code PI m/ nm Code Length Reference and PV
Serial Number m 42 6 octets maximum 7.34.1.a

8.3.33.4

User Data nm 193 512 octets maximum 7.34.1.a

8.3.33.4

Key : m : mandatory

nm : not mandatory


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