SNA Networking

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SNA Networking

• ...connecting the dots
• ...many of them

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SNA history

• Before OSI model...handle all functions
• Mainframe to terminal application use
• Move characters only to dumb terminals
• Central control of network
• Specialists to pre-define and maintain
• Complex responsibilities
• Understanding programs/databases
• Understanding peripheral devices

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SNA Node Roles (subarea)

• Heirarchy is defined in Nodes
• Subarea nodes
• Peripheral nodes
• Subarea nodes control action of Peripheral nodes

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SNA Node Roles (subarea)
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SNA hardware entities (subarea)

• Mainframe
• Front-end processor
• Cluster controller
• 3270 terminal

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SNA software entities (subarea)

• VTAM
• NCP
• SSCP

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SNA Network Accessible Units (subarea)

• PU
• LU
• CP (earlier edition showed session here, which
  is incorrect)

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Network Activation(subarea)

• Done by mainframe operator
• Heirarchical
• Careful sequence of nodes and links
• Must correspond to network gen
• When the T5 Network node comes up, most (if not
  all) of the remainder of the network is brought
  up as well

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Establishing Routes(subarea)

• Links along a physical path are statically
  predefined in subarea network tables
• Characteristics determines Class of Service
• Reliability
• Performance
• Class of Service is important
• When LU wants to initiate a session, it petitions
  SSCP
• SSCP will establish permanent routing between
  requested LUs based on class of service requested
  and class of service available
• Once session is established (routing is
  established) the route cannot be changed
• If the route goes down, the session goes down

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Transmission Groups in a Heirarchical Network
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SNA after the PC and AS/400

• Smart device, simple message
• Binary data
• Offload complex responsibilities
• programs and databases
• peripheral devices
• Exchange by peer over token-ring
• central control not needed

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OSI model corresponds to SNA

• Layer 7 Transaction CPI-C
• Layer 6 Presentation APPC
• Layer 5 Data flow LU6.2
• Layer 4 Transmission LU6.2
• Layer 3 Path APPN
• Layer 2 Data link Token Ring
• Layer 1 Physical Token Ring

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APPN Goal

• Handle diversity
• Platforms, topologies, applications
• Heirarchical and APPN simultaneously
• Older dependent LUs can use APPN
• Intelligent technology
• Present throughout the network
• Dynamic (automatic) response to changes
• Local or wide-area
• Fast or slow links

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SNA software entities with APPN
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SNA hardware entities

• Mainframe
• Front-end processor
• Cluster controller
• 3270 terminal
• AS/400 minicomputer
• PC microcomputer
• Token-ring LAN

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APPN functions

• Keep track of resources in network
• Dynamic exchange of information between nodes
• Paths between nodes
• Resources within node
• Select best path for data route
• Handle mix of traffic
• Class of Service routing
• Enhance reliability
• Non-disruptive path switch

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SNA Node Roles(APPN)

• Low-entry networking node
• Passive interaction with Network node
• Relies on Network node for all network services
• APPN End node
• No pass-thru traffic
• Active interaction with Network node
• Network node (intermediate routing network)
• Handles pass-thru traffic (routing)
• Finds location of partner node and exchanges
  information
• Places resource information in Central Directory
• Reduce broadcast searches
• Computes potential routes to partner
• Selects best route from among those computed

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SNA Node Roles(APPN Interchange)

• Interchange node (specialized for this function)
• Both heirarchical and peer-oriented functionality
• Handle search requests between subarea node and
  APPN node
• Speaks SSCP-SSCP on one side (CDINIT)
• Speaks CP-CP on other side (LOCATE)
• Allow gradual migration from heirarchical to
  peer-oriented APPN
• Permits routing that passes through intervening
  heirarchical node

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SNA Node Roles(APPN Interchange)
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SNA Node Roles(APPN Composite)

• Mainframe node
• Handling heirarchical and peer-oriented functions)

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SNA Node Roles(APPN Composite)
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Links andTransmission Groups

• Link connects adjacent nodes
• Link station at each end handles communication
  across data line
• Lower layers of communications protocol
• Typically SDLC or Token-Ring
• Links are formally known as Transmission Groups
• There may be several transmission groups between
  nodes
• Increase reliability
• Increase bandwith

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APPN Control Points

• CP in a Network node handles routing and resource
  management
• Dynamic configuration
• Reveal its resources
• Discover other resources
• Initial route determination
• Pass thru routing

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APPN Control Points

• CP in an End node handles resource management
• Dynamic configuration
• Reveals its resource to it adjacent Network node
• Requests help from Network node for initial route
  determination
• Controls only its local resources

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APPN Control Points
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APPN Network Accessible Units

• Components that establish a session
• Establish routing for data between end users
• Physical Unit (PU)
• Logical Unit (LU)
• Control Point (CP)

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PU duties in APPN

• PUs activate and de-activate links between nodes
• Between APPN nodes this function is handled by
  the CP

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LU duties in APPN

• Work on behalf of end users
• Application programs
• Terminal users
• Pass user data into network
• Send session-activation requests directly to
  their LU partners
• This was handled by SSCP in heirarchical
• APPN LUs are Independent LU
• Not handled by CP or SSCP

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CP duties in APPN

• Entirely dynamic
• Manage resources in its domain
• Does not maintain sessions with LUs
• APPN LUs are Independent LUs
• Talk directly with their partner LUs
• Session setup and control
• Share resource data with CP in adjacent node
• Routing data
• Directory services

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Network Activation(APPN)

• Local operator decides when APPN node becomes
  active network component
• Any node can be first or last or in-between
• No node is subordinate to any other node
• Local operator gives command to CP to bring up
  links
• Automatic Topology Database Update when a network
  node comes up
• New Network node reveals its resources to
  adjacent Network node
• Propagated among all Network nodes in network
• Cached in the Central Directory Services database

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Phases of Link Activation

• Connect
• Dialup
• Modem training
• Pre-negotiation (is adjacent node active?)
• Link level poll
• APPN uses XID-XID pairs pre-negotiation flag
• Contact (link stations negotiate by exchanging
  XID pairs)
• Link station roles
• Which link station is primary and which is
  secondary
• The primary link station controls the links and
  the secondary link station
• Link characteristics
• Maximum BTU (basic transmission unit) sizes
  allowed on the link
• Node capabilities
• End node or Low-entry end node or Network node
  capability
• Segmentation and re-assembly capability of the
  node

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Node connection example
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Node connection example

• CP activates local LS (link station)
• CP commands LS to connect to adjacent LS
• LS sends successful connect reply
• CPs exchange identifications and protocol
  commands
• CPs establish sessions between them
• CPs exchange information on their capabilities
• List of resources (LUs)
• Topology database is updated

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Node connection example
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Activating CP-CP sessions

• CPs establish sessions after link has been
  established to adjacent node
• Both CPs initiate a session between them
• The initiator of a LU6.2 session is the
  contention winner
• Send via the contention-winner session
• Receive via the contention-loser session
• Request capabilities of partner CP
• Management services capabilities
• Entry-point or focal-point
• Kinds of directory objects which can be searched
• Exchange directory or network topology
  information
• Reveal LUs to be cached in Network Directory

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Session setup services

• Central Directory Server
• Locate partner LUs
• One Network nodes used by others
• After local searches have failed
• Network Topology Database
• Calculate route to partner LU
• Every Network node can do this
• Has its own copy of the database

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Session setup sequence

• Locate the destination LU
• Local node table search
• Network node table search
• Central Directory server database search
• Directed search of neighbor Network nodes
• Broadcast search
• Calculate the best route
• Network nodes topology database

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Route Selection Control Vector

• Components of a route
• Chain of network nodes
• From origin to destination
• Conforms to Class of Service requested
• Is appended to the BIND request

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Session Protocols between LUs (APPN)

• Partner LUs at both ends must use same protocol
• Response requirements
• Message size requirements
• Number of multiple simultaneous requests
• Called the BIND request
• There is a particular protocol used for
  peer-oriented LU-to-LU communication
• LU6.2
• Advanced-Program-to-Program Communcation (APPC)

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LU6.2 or APPC Overview

• Designed for peer-oriented LU-to-LU communication
• Program-to-program
• Program-to-device
• Device-to-device
• Application Programming Interface to encapsulate
  product platform details of LU6.2
• CPI-C
• Common Programming Interface-Communications

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Establishing Routes for LUs(APPN)

• Uses Class of Service
• Locate the session partner
• Not predefined in a static table like subarea
• Find it in real-time since it is dynamic
• Determine the route between the session partners
• When the session partner is found CP in Network
  node server can determine route

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Routing in APPN Networks

• Session identifier in Transmission Header
• Matching session identifier stored at
  intermediate node
• Only for duration of session

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Automatic Network Routing

• Reduces overhead at intermediate nodes
• Packets header contains path that packet is to
  take through the network
• Sequence of transmission groups
• No session table lookups

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APPN Application Support

• Can now push from both ends of network
• API (application program interfaces)
• technical details required to talk across
  network
• Called CPI-C
• Dynamic definition of LUs
• Independent LUs

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APPN node connection

• CP-a activates LS-a1
• CP-a issues CONNECT_OUT
• Command that requests LS-a1 to connect with LS-b1
• LS-a1 informs CP-a that is has successfully
  connected with LS-b1
• CP-a and CP-b exchange identifications and data
  link protocol commands necessary to activate the
  link
• CP-a and CP-b activate two CP-CP sessions between
  them
• CP-a and CP-b exchange information on their
  respective capabilities

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APPN node connection

• CP-a sends CP-b a list of its resources
• For example, a list of its LUs
• CP-b updates its local directory

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Initiating LU-LU session example
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Initiating LU-LU session

• SESSION-INITIATION request from LU-a to CP-a
• Identify the two LUs that are to participate in
  the session
• Specify the MODE name for the session
• MODE name identifies CLASS of SERVICE
• CP-a checks directory to see if LU-c is a local
  resource
• LOCATE request from CP-a to CP-b
• CP-b is CP-as Network Node Server
• Finds partner LU-c
• Calculate route based on MODE (CLASS of SERVICE)
• Build Route-Selection-Control-Vector
• LOCATE-REPLY from CP-b to CP-a
• Contains Route-Selection-Control-Vector

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Initiating LU-LU session

• CONTROL-INITIATE (CINT) from CP-a to LU-a
• CINT enables LU-a to activate session with LU-c
• CP-a includes Route-Selection-Control-Vector
• SESSION-ACTIVATION (BIND) from LU-a to LU-c
• Include Route-Selection-Control-Vector in BIND
• CP-a CP-b CP-c use transmission group vectors in
  RSCV to route the BIND along consecutive
  transmissions groups to LU-c
• As the BIND flows from node to node,
  half-sessions are initialized in partner LU-a and
  LU-c as well as session connectors in
  intermediate routing node B
• Half-sessions and session-connectors built in
  reverse direction when LU-c sends BIND response
  to LU-a

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Example Startup Script

• open tw1
• allow errors on
• log twh1aa
• add line tw1,tndm twh1,protocol SDLC,type
  SECONDARY
• add pu putwh1,address H01, line tw1,xid YES
• add mode LU62,maxsess 32,minconwin 31,minconlos
  1,rcvw 7, sendw 7
• add appl twqa,open twqa,llu UMAG002,prot CPIC
• add appl twqb,open twqb,llu UMAG002,prot CPIC
• add appl twqc,open twqc,llu UMAG002,prot CPIC
• add appl twqd,open twqd,llu UMAG002,prot CPIC
• add appl twqe,open twqe,llu UMAG002,prot CPIC
• add appl twqf,open twqf,llu UMAG002,prot CPIC
• add appl twqg,open twqg,llu UMAG002,prot CPIC

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Example Startup Script

• add RLU TRW1.CICSCVP1,pu putwh1,parsess yes
• add RLU TRW1.trw1bbp1,pu putwh1,parsess yes
• add RLUALIAS twqa,RLU TRW1.CICSCVP1
• add RLUALIAS twqb,RLU TRW1.CICSCVP1
• add RLUALIAS twqc,RLU TRW1.CICSCVP1
• add RLUALIAS twqd,RLU TRW1.CICSCVP1
• add RLUALIAS twqe,RLU TRW1.CICSCVP1
• add RLUALIAS twqf,RLU TRW1.CICSCVP1
• add RLUALIAS twqg,RLU TRW1.CICSCVP1
• start line tw1,sub all
• start cnos LU62,llu UMAG002,rlu TRW1.CICSCVP1
• start cnos LU62,llu UMAG002,rlu TRW1.TRW1BBP1

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Congestion Causes

• Network sized for typical peak data flow
• Excessive data flow will choke network to a
  standstill
• Response times lengthen without ceasing
• Buffers are depleted
• This happens very rapidly
• Remains choked while network is at capacity
• Remains choked until data flow falls below
  network capacity

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Congestion Solutions

• Message repackaging
• Larger or smaller units
• Blocking
• Segmentation
• Accommodate efficiency of link
• Message pacing
• Number of end-to-end messages that source can
  send to destination without acknowledgement
• Session level pacing
• Intermediate session connectors pace their
  adjacent links as well
• Pacing back-pressure lowers message rate by
  propagating backwards toward sender LU

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References

• www.cisco.com
• http//publibfp.boulder.ibm.com
• www.oreilly.com
• www.interskill.com
• www.navdesign.com
• www.support.3com.com
• Insession Labs Pty. Ltd ICE Intersystem
  Communications Environment Product Summary