SNA ( Systems Network Architecture )

• Physical
  Traditional SNA physical entities assume one of the following four forms: hosts, communications controllers, establishment controllers, and terminals. Hosts in SNA control all or part of a network and typically provide computation, program execution, database access, directory services, and network management.



• Data link control (DLC)
  Defines several protocols, including the Synchronous Data Link Control (SDLC) protocol for hierarchical communication, and the Token Ring Network communication protocol for LAN communication between peers



• Path control
  Performs many OSI network layer functions, including routing and datagram segmentation and reassembly (SAR)



• Transmission control
  Provides a reliable end-to-end connection service, as well as encrypting and decrypting services



• Data flow control
  Manages request and response processing, determines whose turn it is to communicate, groups messages, and interrupts data flow on request



• Presentation services
  Specifies data-transformation algorithms that translate data from one format to another, coordinate resource sharing, and synchronize transaction operations



• Transmission control
  Provides an interface for applications to use network services

• LU (Logical Unit): function as end-user access ports into an SNA network.


• PU (Physical Unit): provide users with access to network resources.


• SSCP (System Services Control Point): manage the transmission of information between end users.

• Subarea nodes.


• Peripheral nodes.

• Type 1 ¨C terminals.


• Type 2 - controllers and machines that manage terminals.


• Type 4 - front-end processors and machines that take some load off the main CPU. It usually contained within a communications controller. An example of a T4 node is an NCP, which routes data and controls flow between a front-end processor and other network resources.


• Type 5 - the main host. An example of a T5 node is the VTAM resident within an IBM mainframe. A VTAM controls the logical flow of data through a network, provides the interface between application subsystems and a network, and protects application subsystems from unauthorized access.

• SDLC
  SDLC has been widely implemented in SNA networks to interconnect communications and establishment controllers, and to move data via telecommunications links. It is used as the layer 2 of the SNA hierarchical network.



SDLC frame format

Link Header			Link Trailer
Flag	Address field	Control field	Information	FCS	Flag



• Flag
  The value of the flag is always (0x7E). In order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as Bit Stuffing is used by both the transmitter and the receiver.



• Address field
  The first byte of the frame after the header flag is known as the Address Field. SDLC is used on multipoint lines and it can support as many as 256 terminal control units or secondary stations per line.



• Control field
  The field following the Address Field is called the Control Field and serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type.



• Modes of operation
  In SDLC there is the notion of primary and secondary stations, defined simply as the initiator of a session and its respondent. The primary station sends commands and the secondary station sends responses.



• FCS
  The Frame Check Sequence (FCS) enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked. The sequence is first calculated by the transmitter using an algorithm based on the values of all the bits in the frame. The receiver then performs the same calculation on the received frame and compares its value to the CRC.



• Window size
  SDLC supports an extended window size (modulo 128) where the number of possible outstanding frames for acknowledgement is raised from 8 to 128. This extension is generally used for satellite transmissions where the acknowledgement delay is significantly greater than the frame transmission times.



• Frame types
  

  The following are the Supervisory Frame Types in SDLC:
  RR	Information frame acknowledgement and indication to receive more.
  REJ	Request for retransmission of all frames after a given sequence number.
  RNR	Indicates a state of temporary occupation of station (e.g., window full).
  The following are the Unnumbered Frame Types in SDLC:
  DISC	Request disconnection.
  UA	Acknowledgement frame.
  DM	Response to DISC indicating disconnected mode.
  FRMR	Frame reject
  CFGR	Configure.
  TEST	Sent from primary to secondary and back again.
  BCN	Beacon.
  SNRM	Initiator for normal response mode. Full master/slave relationship.
  SNRME	SNRM in extended mode.
  RD	Request disconnect.
  RIM	Secondary station request for initialization after disconnection.
  SIM	Set initialization mode.
  UP	Unnumbered poll.
  UI	Unnumbered information. Sends state information/data.
  XID	Identification exchange command.
  There is one Information Frame Type in SDLC:
  Info	Information frame.




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• QLLC

QLLC is a standard developed for interconnecting SNA LANs over packet switched WANs with X.25. The SDLC header and trailer is stripped off and replaced by similar fields of LAPB before transmission over the network. The standard also defines additional control bytes used to allow the receiving end of the network to reconstruct the original SDLC frame. The SNA information is passed over the network within the X.25 data packet.

SNA data and QLLC control frames are determined by the value of the Q-bit within the X.25 packet header.


QLLC Frame Types


QRR	Receive Ready.
QDISC	Disconnect.
QUA	Unnumbered Acknowledgement.
QDM	Disconnect Mode.
QFRMR	Frame Reject.
QTEST	Test.
QRD	Request Disconnect.
QXID	Exchange Identification.
QSM	Set Mode.




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• SNA

Frames format


Transmission Header (TH)

Request / Response Header (RH)

Request / Response Unit (RU)



• Transmission Header
  The TH field contains the Format Identifier value (FID). This value corresponds to the type of communication session and the environment in which it is used.
  
  FID2 is the format used between a T4 or T5 node and an adjacent T2.0 or T2.1 node, or between adjacent T2.1 nodes. FID3 is used on links to a PU T1 (such as AS/400 controllers). FID4 is used on links between PU T4s.
  
  The TH field also contains a mapping field (MPF) which indicates whether the frame is a complete SNA frame (containing TH, RH and RU) or just a segment. When the SNA frame is too large to be sent as one frame, it is divided into several segments (first, middle, last or whole). The first segment includes a TH (indicating that it is the first), an RH and the beginning of the RU. Other segments (middle and last) contain a TH (identical to the one of the first except for the MPF field) and the remainder of the RU.



• Request/Response Header
  The RH field denotes the SNA category of the frame, the format of the RU, whether requests are chained together, bracket indicators, pacing information and various other SNA frame properties.



• Request/Response Unit
  The RU contains the ¡®user data¡¯ that one LU sends to its session partner or a special SNA frame. A field within the RH distinguishes between cases and several classes of SNA frames. There are three categories of SNA frames: NS (function management data), DFC (data flow control) and SC (session control).



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• SNA TH0 & TH1

SNA TH0 and TH1 correspond to the FID header 0 and 1 respectively.
Packet format


4	6	7	8 bits
FID	MPF	EFI
DAF (2 bytes)
OAF (2 bytes)
SNF (2 bytes)
DCF (2 bytes)



• FID
  Format Identification: 0=FID 0, 1=FID 1.


• MPF
  Mapping field:
  

  0	Middle segment of a BIU
  1	Last segment of a BIU
  2	First segment of a BIU
  3	Whole BIU



• EFI
  Expedited flow indicator:
  

  0	Normal flow
  1	Expedited flow



• DAF
  Destination address field. Network address denoting the BIU¡¯s destination network addressable unit (NAU).



• OAF
  Origin address field. Network address denoting the originating NAU.



• SNF
  Sequence number field. Numerical identifier for the associated BIU.



• DCF
  Data count field. A binary count of the number of bytes in the BIU if the BIU segment is associated with the transmission header.



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• SNA TH5

SNATH 5 is the FID 5 header.
Packet format


4	6	7	8 bits
FID5 0101 (4 bits)	MPF (2 bits)	R	EFI
Reserved ( 1 byte)
SNF (2 bytes)
SA (8 bytes)



• FID 5
  The value of this field is 0101.



• MPF
  Mapping field.



• R
  Reserved bit.



• EFI
  Expedited flow indicator (1 bit).



• SNF
  Sequence number field.



• SA
  Session address.



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• APPN

Advanced Peer-to-Peer Networking (APPN) represents IBM's second-generation SNA. In creating APPN, IBM moved SNA from a hierarchical, mainframe-centric environment to a peer-based networking environment. At the heart of APPN is an IBM architecture that supports peer-based communications, directory services, and routing between two or more APPC systems that are not directly attached.


Components
In addition to the APPN environment, peer-based SNA networking specifies three additional key networking concepts: logical units (LUs), Advanced Program-to-Program Computing (APPC), and node type 2.1. Each plays an important role in the establishment of communication among SNA peers within the context of an SNA-based peer internetwork.

Node Types
Under APPN, peer-based communication occurs among several well-defined node types. These nodes can be broken down into three basic types:

• Low-entry network (LEN) node

LEN is a pre-APPN era peer-to-peer node. A LEN node participates in APPN networking by taking advantage of services provided by an adjacent network node (NN).


• End node (EN)

EN contains a subset of full APPN support. An end node accesses the network through an adjacent NN and uses the routing services of the same adjacent NN.


• Network node (NN)

NN contains full APPN functionality. The CP in an NN manages the resources of the NN, as well as the attached ENs and LEN nodes.



APPN Supports Several Well-Defined Node Types

• IBM APPN Services
  Basic APPN services fall into four general categories: configuration, directors, topology, and routing and session services. Each is summarized in the sections that follow.



• IBM APPN Configuration Services
  APPN configuration services are responsible for activating connections to the APPN network. Connection activation involves establishing a connection, establishing a session, and selecting an adjacency option.



• IBM APPN Directory Services
  APPN directory services are intended to help network devices locate service providers. These services are essential to establishing a session between end users. Directory services in APPN call for each NN to maintain a directory of local resources and a network directory that associate end users with NNs providing service. A distributed directory service then is formed from the collection of individual NN network directories. This section summarizes the nature of APPN databases, node-directory service handling, and the role of a centralized directory service.



• IBM APPN Topology and Routing Services
  In an APPN network topology, network nodes are connected by transmission groups (TGs). Each TG consists of a single link, and all NNs maintain a network topology database that contains a complete picture of all NNs and TGs in the network.
  
  A network's topology database is updated by information received in a topology database update (TDU) message.The network topology database contains information used when calculating routes with a particular class of service (CoS). APPN's routing services function uses information from directory and topology databases to determine a route based on CoS.



IBM APPN Session Services
Following route establishment, the APPN session-establishment process varies depending on the node type. If the originating end user is attached to an EN, a LOCATE reply containing the location of the destination and route is returned to the originating EN by the NN adjacent to the destination EN. The originating EN then sends a BIND on a session route. If originating, the end user is attached to a LEN node that sends a BIND to its adjacent NN. The adjacent NN converts the LEN BIND to APPN BIND, and sends a BIND on the session path.



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• HPR-APPN

HPR network is an extension of the SNA network. HPR (High Performance Routing) is an extension of the base-APPN that provides some key advancements. These new functions include:

Non-disruptive path switching.

• Better utilization of high-speed communication paths.


• An advanced congestion control methodology.

Additional functionality provided by two new components: Rapid Transport Protocol (RTP) and Automatic Network Routing (ANR). These components provide the added functionality exhibited by HPR nodes.


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• NHDR

The packet transported along an RTP connection has a specific format. It consists of 3 components. NHDR, THDR and data. The Network Layer Header (NHDR) begins the frame used by RTP (Rapid Transport Protocol) nodes. It provides addressing for the packet as it transverses the HPR network. The components of this header include the transmission priority and the ANR (Automatic Network Routing) labels. NHDR consists of some indicators that identify the packet as a network layer packet.


Format of the header


Switching mode(3 bits)

Transmission priority field(2 bits)

Function type (4 bits)

Time-sensitive packet indicator (1 bit)

Slowdown 1 (1 bit)

Slowdown 2 (1 bit)

ANR routing field or function routing field



• Switching mode

Switching mode may have the following values:


5	Function routing
6	Automatic network routing



• Transmission priority field (TPF)

TPF may have the following values


0	low (L)
1	medium (M)
2	high (H)
3	network (N)



• Function type (for switching mode 5)
  Function type of 1 indicates logical data link control.



• TSP
  Time-sensitive paket indicator



• Slowdown 1 and 2
  This field indicates when ever a minor (slowdown 1) or significant (slowdown 2) congestion condition exists. Possible values are:
  

  0	Does not exist
  1	Exists

  
  
• ANR routing field (for SM = 6)
  A string of ANR labels 1 or 2 bytes long. The string ends with 0xFF.



• Function routing field (for SM = 5)
  A 2 byte function routing address (FRA) followed by a 0xFF value.



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• NLP

In High Performance Routing (HPR),the message unit used to carry data over the route, Network Layer Packet is analogous to a datagram.


Sending SNA PIUs and NLPs.
Before any SNA packets may be communicated, PPP must reach the Network-Layer Protocol phase, and the appropriate SNA ControlProtocol must reach the Opened state.
The maximum length of a SNA packet transmitted over a PPP link is the same as the maximum length of the Information field of a PPP encapsulated packet.

Sending HPR Network Layer Packets (NLPs)
Exactly one HPR Network Layer Packet (NLP) is encapsulated in the PPP Information field, where the PPP Protocol field indicates type hex 004D (SNA).
It is architecturally possible to transport HPR NLPs over LLC over PPP using PPP Protocol field type hex 004B (SNA over LLC 802.2) ifthe optional HPR link-level error recover tower is included in the implementation.



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• THDR

THDR is the RTP Transport header. It is used by the RTP endpoints to provide correct processing of the packet. It is used for communication between the endpoints and to identify the RTP connection.


Format of header


TCID assignor(7 bytes)

Connection setup(1 bit)

Start-of-message indicator (1 bit)

End-of-message indicator (1 bit)

Status requested indicator (1 bit)

Respond ASAP indicator (1 bit)

Retry indicator (1 bit)

Last message indicator (1 bit)

Connection qualifier field indicator (2 bits)

Optional segments present indicator (1 bit)

Data offset (2 bytes)

Data length(2 bytes)

Byte sequence number(4 bytes)

Control vector 05

Optional segments



• TCID assignor

Transport connection identifier. There are 2 possible values:


0	TCID was assigned by the receiving RTP partner.
1	TCID was assigned by the sending RTP partner.



• Connection setup



0	presented
1	not presented



• Start of message indicator



0	not start of message
1	start of message



• End of message indicator



0	not end of message
1	end of message



• Status requested indicator



0	Receiver need not reply with a status segment.
1	Receiver must reply with a status segment.



• Respond ASAP indicator



1	Sender will retransmit reply ASAP.



• Retry indicator



0	Sender will retransmit this packet.



• Connection qualifier field indicator



0	none presented
1	originator



• Optional segments present indicator



0	not presented
1	presented



• Byte sequence number

Sequence number of the first byte of the data field.


• Optional segments

If present the optional segment can contain one or more of the following segments:
The structure of each segment is as follows:


0x0E	Status segment.
0x0D	Connection Setup segment.
0x10	Connection Identifier Exchange segment.
0x14	Switching Information segment.
0x22	Adaptive Rate-Based segment.
0x12	Connection Fault segment.
0x0F	Client Out-of-band Bits segment.



• The structure of each segment is as follows:



Byte	Content
0	Segment length/4
1	Segment type
2	Segment data



• Each segment may include control vectors.

Supported control vectors are:
Each segment may include control vectors. Supported control vectors are:


0x00	Node identifier Control Vector.
0x03	Network ID Control Vector.
0x05	Network Address Control Vector.
0x06	Cross-Domain Resource Manager Control Vector.
0x09	Activation Request/Response Sequence Identifier Control Vector.
0x0E	Network Name Control Vector.
0x10	Product Set ID Control Vector.
0x13	Gateway Support Capability Control Vector.
0x15	Network-Qualified Address Pair Control Vector.
0x18	SSCP Name Control Vector.
0x22	XID Negotiation Error Control Vector.
0x26	NCE Identifier Control Vector.
0x28	Topic Identifier Control Vector.
0x32	Short-Hold Mode Control Vector.
0x39	NCE Instant Identifier.
0x46	TG Descriptor Control Vector.
0x60	Fully qualified PCID Control Vector.
0x61	HPR Capabilities Control Vector.
0x67	ANR Path Control Vector.
0xFE	Control Vector Keys Not Recognized Control Vector.




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• DLSw

Data Link Switching (DLSw) is a forwarding mechanism for the IBM SNA (Systems Network Architecture) and IBM NetBIOS (Network Basic Input Output Services) protocols. Over IP networks, DLSw does not provide full routing, but instead provides switching at the SNA Data Link layer (i.e., layer 2 in the SNA architecture) and encapsulation in TCP/IP for transport over the Internet.


Message structure


8	16	Octets
Version number	Header length (=16)	0-1
Message length		2-3
Remote data link correlator		4-7
Remote DLC port ID		8-11
Reserved field		12-13
Message type	Flow control byte	14-15



• Version number

Set to 0x31 (ASCII 1) indicating a decimal value of 49. This is used to indicate DLSw version 1.


• Header length

Set to 0x48 for control messages and 0x10 for information and Independent Flow Control messages.


• Message length

Specifies the number of bytes within the data field following the header.


• Remote data link correlator / remote DLC port ID

The contents of the DLC and DLC Port ID have local significance only. The values received from a partner DLSw must not be interpreted by the DLSw that receives them and should be echoed "as is" to a partner DLSw in subsequent messages.


• Message type

The following message types are available:


CANUREACH_ex	Can U Reach Station-explorer
CANUREACH_cs	Can U Reach Station-circuit start
ICANREACH_ex	I Can Reach Station-explorer
ICANREACH_cs	I Can Reach Station-circuit start
REACH_ACK	Reach Acknowledgment
DGRMFRAME	Datagram Frame
XIDFRAME	XID Frame
CONTACT	Contact Remote Station
CONTACTED	Remote Station Contacted
RESTART_DL	Restart Data Link
DL_RESTARTED	Data Link Restarted
ENTER_BUSY	Enter Busy
EXIT_BUSY	Exit Busy
INFOFRAME	Information (I) Frame
HALT_DL	Halt Data Link
DL_HALTED	Data Link Halted
NETBIOS_NQ_ex	NETBIOS Name Query-explorer
NETBIOS_NQ_cs	NETBIOS Name Query-circuit setup
NETBIOS_NR_ex	NETBIOS Name Recognized-explorer
NETBIOS_NR_cs	NETBIOS Name Recog-circuit setup
DATAFRAME	Data Frame
HALT_DL_NOACK	Halt Data Link with no Ack
NETBIOS_ANQ	NETBIOSAdd Name Query
NETBIOS_ANR	NETBIOSAdd Name Response
KEEPALIVE	Transport Keepalive Message
CAP_EXCHANGE	Capabilities Exchange
IFCM	Independent Flow Control Message
TEST_CIRCUIT_REQ	Test Circuit Request
TEST_CIRCUIT_RSP	Test Circuit Response



• Flow control byte

Format of Flow Control is as follows:


FCI

FCA

reserved

FCO

■ Parent layer
■ Child layer

Ethernet 802.2

SNA