Path information unit (PIU)

In TCP/IP, the unit that is transferred through the network is called a packet. A packet includes the data (payload) and the IP and TCP headers. The headers are used to route the packet and manage the TCP session. In SNA the unit that flows in the network is called a path information unit (PIU).

As with every networking protocol, messages are routed to an address in the network. In an IP network, the address is the IP address of the host and the address is assigned either dynamically using a DHCP, or it is a static IP address.

In SNA, the messages are sent to a network accessible unit (NAU). The addresses are assigned to the network accessible unit by the control point when the physical units or logical units are activated.

The path information unit carries three fields that are used by the network accessible unit to route the information in the network; see Figure 1.

Figure 1. Path information unit

Transmission header (TH)

The transmission header (TH) is used to route message units through the network. The transmission header contains routing information for the transport network. SNA defines different transmission header formats and identifies the different formats by a form indicator (FID) type.

Transmission headers vary in length according to their FID type. Path control uses the different FID types to route data between different types of nodes. Two important FID types are:

FID2 - This format is used to route data between a subarea boundary node and an adjacent peripheral node, or between adjacent APPN or LEN nodes.
FID4 - This format is used to route data between subarea nodes.

Figure 2 shows the layout of FID2 and FID4 transmission headers and the structure of the addressing field for these two FIDs.

In FID4, the network accessible unit is formed from the subarea and element address. In FID2, used by T2.0 nodes, the element address is 8 bits long, limiting the number of elements to 255. T2.1 also uses FID2, and handles the destination address field (DAF) and origin address field (OAF) as one 16-bit field.

Figure 2. SNA addressing with FID2 and FID4 transmission headers

Request header (RH)

Each request that an NAU sends begins with a request header (RH). A request header is a 3-byte field that identifies the type of data in the associated request unit. The request header also provides information about the format of the data and specifies protocols for the session. Only NAUs use request header information.

Request unit (RU)

Each request that an NAU sends also contains a request unit (RU). A request unit is a field of variable length that contains either end-user data (data RUs) or an SNA command (command RUs). Data RUs contain information that is exchanged between end users. Command RUs control the operation of the network.

Response header (RH)

Each response that an NAU sends includes a response header (RH). Like a request header, a response header is a 3-byte field that identifies the type of data in the associated response unit. A bit called the request/response indicator (RRI) distinguishes a response header from a request header.

The receiving NAU indicates whether the response being returned to the request sender is positive or negative by setting a single bit.

Response unit (RU)

A response unit (RU) contains information about the request. Positive responses to command requests generally contain a 1-3 byte response unit that identifies the command request. Positive responses to data requests contain response headers, but no response unit. Negative response units are 4-7 bytes long and are always returned with a negative response.