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Home › Network Protocol Suite Directory Index › IBM SNA - Systems Network Architecture and Protocols › Path control

APPN: Advanced Peer-to-Peer Networking

Advanced Peer-to-Peer Networking (APPN) is an enhancement to the original IBM SNA architecture. APPN, which includes a group of protocols and processors, handles session establishment between peer nodes, dynamic transparent route calculation, and traffic prioritization. Using APPN, a group of computers can be automatically configured by one of the computers acting as a network controller so that peer programs in various computers will be able to communicate with other using specified network routing.

APPN features include:

  • Better distributed network control; because the organization is peer-to-peer rather than solely hierarchical, terminal failures can be isolated
  • Dynamic peer-to-peer exchange of information about network topology, which enables easier connections, reconfigurations, and routing
  • Dynamic definition of available network resources
  • Automation of resouce registration and directory lookup
  • Flexibility, which allows APPN to be used in any type of network topology

An APPN network is composed of three types of APPN node:

  • Low Entry Networking (LEN) Node - APPN LEN node provides peer to peer connectivity with all other APPN nodes.
  • End Node - An End Node is similar to a LEN node in that it participates at the periphery of an APPN network. An End Node includes a Control Point (CP) for network control information exchange with an adjacent network node.
  • Network Node - The backbone of an APPN network is composed of one or more Network Nodes which provide network services to attached LEN and End Nodes.

The APPN network have the following major functional processors:

Connectivity - The first phase of operation in an APPN network is to establish a physical link between two nodes. When it has been established, the capabilities of the two attached nodes are exchanged using XIDs. At this point, the newly attached node is integrated into the network.

Location of a Targeted LU - Information about the resources (currently only LUs) within the network is maintained in a database which is distributed across the End and Network Nodes in the network. End Nodes hold a directory of their local LUs. If the remote LU is found in the directory, a directed search message is sent across the network to the remote machine to ensure that the LU has not moved since it was last used or registered. If the local search is unsuccessful, a broadcast search is initiated across the network. When the node containing the remote LU receives a directed or broadcast search message, it sends back a positive response. A negative response is sent back if a directed or broadcast search fails to find the remote LU.

Route Selection - When a remote LU has been located, the originating Network Node server calculates the best route across the network for a session between the two LUs. Every Network Node in the APPN network backbone maintains a replicated topology database. This is used to calculate the best route for a particular session, based on the required class of service for that session. The class of service specifies acceptable values for session parameters such as propagation delay, throughput, cost and security. The route chosen by the originating Network Node server is encoded in a route selection control vector (RSCV).

Session Initiation - A BIND is used to establish the session. The RSCV describing the session route is appended to the BIND. The BIND traverses the network following this route. Each intermediate node puts a session connector for that session in place, which links the incoming and outgoing paths for data on the session. Data Transfer - Session data follows the path of the session connectors set up by the initial BIND. Adaptive pacing is used between each node on the route. The session connectors on each intermediate node are also responsible for segmentation and segment assembly when the incoming and outgoing links support different segment sizes.

Dependent LU Requestor - Dependent LUs require a host based System Services Control Point (SSCP) for LU-LU session initiation and management. This means that dependent LUs must be directly attached to a host via a single data link.

High-performance routing (HPR) - HPR is an extension to the APPN architecture. HPR can be implemented on an APPN network node or an APPN end node. HPR does not change the basic functions of the architecture. HPR has the following key functions:

  • Improves the performance of APPN routing by taking advantage of high-speed, reliable links
  • Improves data throughput by using a new rate-based congestion control mechanism
  • Supports nondisruptive re-routing of sessions around failed links or nodes
  • Reduces the storage and buffering required in intermediate nodes.

Protocol Structure

A simple APPN network is illustrated in the diagram below.

Advanced Peer-to-Peer Networking

APPN: Advanced Peer-to-Peer Networking

Related protocols: SNA, APPC

Sponsor Source: APPN is an IBM network architecture, extended from the IBM SNA

Reference:
http://publib-b.boulder.ibm.com/Redbooks.nsf/RedbookAbstracts/sg243669.html?Open: Inside APPN and HPR - The Essential Guide to the Next-Generation SNA
http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/D50L0000/CCONTENTS: SNA APPN Architecture Reference
http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2006.htm#17531: Designing APPN Internetworks
http://www.javvin.com/protocol/rfc2353.pdf: APPN/HPR in IP Networks (APPN Implementers" Workshop Closed Pages Document)

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