Hi everybody
I have been having some problems trying to decode some SNA, eventually
reverted to using the clunky old sniffer and found that the sniffer decode
did not match the ethereal decode.
The problem seems to revolve around BIU's that are split into multiple
frames.
from http://www.protocols.com/pbook/sna.htm
When the SNA frame is too large to be sent as one frame, it is divided into
several segments (first, middle, last or whole). The first segment includes
a TH (indicating that it is the first), an RH and the beginning of the RU.
Other segments (middle and last) contain a TH (identical to the one of the
first except for the MPF field) and the remainder of the RU.
This snippet of a trace shows that this frame is the last segment of a BIU
but a phantom RH has been decoded from the following SNA data.
Systems Network Architecture
Transmission Header
Transmission Header Byte 0: 0x24
0010 .... = Format Identifer: Subarea Node <--> PU2 (0x02)
.... 01.. = Mapping Field: Last segment of a BIU (1)
.... ..0. = ODAI Assignment Indicator: 0
.... ...0 = Expedited Flow Indicator: Normal Flow (0)
Reserved
Destination Address Field: 0x39
Origin Address Field: 0x01
Sequence Number Field: 258
Request/Response Header
Request/Response Header Byte 0: 0xc7
1... .... = Request/Response Indicator: Response (1)
.10. .... = Request/Response Unit Category: Data Flow Control
.... 0... = Format Indicator: No FM Header
.... .1.. = Sense Data Included: Included
.... ..1. = Begin Chain Indicator: First in Chain
.... ...1 = End Chain Indicator: Last in Chain
Request/Response Header Byte 1: 0xf1
1... .... = Definite Response 1 Indicator: True
..1. .... = Definite Response 2 Indicator: True
...1 .... = Response Type Indicator: Negative
.... ..0. = Queued Response Indicator: Response bypasses TC
queues
.... ...1 = Pacing Indicator: True
Request/Response Header Byte 2: 0x4b
Thanks in advance
Nick
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