Alert Standard Format (ASF) Specification

Alert Standard Format (ASF) Specification v2.0 DMTF Document DSP0136
DSP0136 23 April 2003 Page 26 of 94
If a session exists and the session is in the Message Transfer phase, RMCP passes the Session
ID, the RMCP message, and RMCP message Length, to the next lower-layer protocol (RSP) for
additional processing.
3.2.3.2.2 Message Encapsulation
When the sending device's RSP protocol engine receives a message from RMCP, RSP inserts
an RSP Header (see 3.2.3.1) at the beginning of the message and copies the Session ID value
into the header's Session ID field. RSP uses the Session ID to access the session state and
increments the session's Sequence Number and then inserts that value into the Sequence
Number field of the RSP Header.
Next, RSP creates an RSP Trailer (see 3.2.3.1) at the end of the message’s Data block. RSP
computes the amount of padding required (if any) to align the protected message's Integrity Data
field on a DWORD boundary. RSP then uses the RMCP message Length (passed from the
RMCP engine) to locate the end of the message’s Data block, and inserts the correct number of
pad bytes and the values for the RSP Trailer's Pad Length and Next Header fields.
Finally, RSP uses the Session ID to access the session state and determine which integrity
algorithm to use with the message, and computes the Integrity Data over the encapsulated
message (from Session ID to Next Header fields, inclusive). The calculated value is inserted into
the Integrity Data field of the RSP Trailer creating a protected RMCP message. Finally, RSP
updates the message Length to account for the addition of the RSP Header and Trailer, and
passes the UDP Source and Destination Port values (from the session state), the protected
RMCP message Length, and the protected RMCP message to the next lower-layer protocol
(UDP) for additional processing.
3.2.3.2.3 Lower-Layer Protocol Processing
When it receives a message from RSP, the sending device's UDP protocol engine inserts its
header at the beginning of the protected RMCP message. UDP copies the port values passed
from RSP into the UDP Header's Source Port and Destination Port fields, and then computes the
UDP packet length and checksum and inserts these values into the UDP Length and Checksum
fields. The resulting UDP packet is then passed to other lower-layer protocols (e.g. IP and
802.3/Ethernet) for additional processing and eventual transmission to its destination.
3.2.3.3 Inbound Message Processing
The follow sections and the figure below outline the processing steps used by an alert-sending
device or management console to remove security extensions from an inbound RMCP message.
Data IP Hd
r
Data UDP Hdr
Data RSP Tlr RSP Hdr
RMCP Message
Enet Hdr Data