Protocol Tower Encoding

Common Declarations describes an abstract model of RPC binding information referred to as a protocol_tower_t data type.

IDL Data Type Declarations defines the actual (concrete) IDL representation of a protocol tower data type as twr_t and twr_p_t data types as follows:

/*
 * Protocol Tower. The network representation of network addressing
 * information (e.g., RPC bindings).
 */
typedef struct {
    unsigned32          tower_length;
    [size_is(tower_length)]    
    byte                tower_octet_string[];
} twr_t, *twr_p_t;

This appendix defines the rules for encoding an protocol_tower_t (abstract) into the twr_t.tower_octet_string and twr_p_t->tower_octet_string fields (concrete). For historical reasons, this cannot be done using the standard NDR encoding rules for marshalling and unmarshalling. A special encoding is required.

Note that the twr_t and twr_p_t are mashalled as standard IDL data types, encoded in the standard transfer syntax (for example, NDR). As far as IDL and NDR are concerned, tower_octet_string is simply an opaque conformant byte array. This section only defines how to construct this opaque open array of octets, which contains the actual protocol tower information.

The tower_octet_string[] is a variable length array of octets that encodes a single, complete protocol tower. It is encoded as follows:

• Addresses increase, reading from left to right.

• Each tower_octet_string begins with a 2-byte floor count, encoded little-endian, followed by the tower floors as follows:

  +-------------+---------+---------+---------+---------+---------+
  | floor count | floor 1 | floor 2 | floor 3 |   ...   | floor n |
  +-------------+---------+---------+---------+---------+---------+
  
  

  The number of tower floors is specific to the particular protocol tower, also known as a protseq.

• Each tower floor contains the following:

  |<-   tower floor left hand side   ->|<-  tower floor right hand side  ->|
  +------------+-----------------------+------------+----------------------+
  |  LHS byte  |  protocol identifier  |  RHS byte  |  related or address  |
  |   count    |        data           |   count    |        data          |
  +------------+-----------------------+------------+----------------------+
  
  

  The LHS (Left Hand Side) of the floor contains protocol identifier information. Protocol identifier values and construction rules are defined in Protocol Identifiers .

  The RHS (Right Hand Side) of the floor contains related or addressing information. The type and encoding for the currently defined protocol identifiers are given in Protocol Identifiers .

  The floor count, LHS byte count and RHS byte count are all 2-bytes, in little endian format.

Protocol Tower Contents

All towers contain the 3 floors shown in Floors 1 to 3 Inclusive .

The content of floors 4 and 5 are protseq-specific. Floors 4 and 5 for TCP/IP Protocols shows the contents for the protocol sequences ncacn_ip_tcp and ncadg_ip_udp.

Implementations may optionally support the protocol sequence ncacn_dnet_nsp. Floors 4, 5 and 6 for DECnet Protocol shows the tower contents for this protocol.

Please note that the html version of this specification may contain formatting aberrations. The definitive version is available as an electronic publication on CD-ROM from The Open Group.