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
- Session-connection estabishment; presentation-entities identified by session addresses.
- Session-connection release;
- normal data exchange;
- 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.
- expedited data exchange;
- 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
- session-connection to transport-connection mapping;
- session-connection flow control;
- expedited data transfer;
- session-connection recovery;
- session-connection release;
- 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