:: SID ::
After a forward connect the two mailbox system are exchanging some characters
which are called SID (System IDentifier). This SID is used for defining the
forward protocol.

Overview of basic SID:
- the SID has following structure: "[f1-f2-f3]"
- (-) defines the end of first field and the beginning of the next field.Felds.
- f1, f2 and f3 does not contain the characters "[" or "]".
- f1 is the identifier of the software.
- f2 contains software specific data, e.g. the version number of the software.
- f3 is a string of alphanumeric characters and defines the set of features
  the mailbox know. Every character defines one feature. numbers can be added
  after each character to define a revision number of that feature. if a number
  is missing, revisionnumber 0 is used be default.

OpenBCM mailbox is sending the following SID:
                  ^known features
             ^version number
     ^name of the software

Following features are known at the moment:
- A: ACK-Mails are generated and send
- B: compressed forward like F6FBB
- B1: Resume mode when doing forward like F6FBB
- D: DieBox extensions
- D1: block CRCs when doing forward like F6FBB
- F: batch forward like F6FBB
- H: hierarchical address can be handled
- M: MIDs can be handled (BIDs for personal mails)
- R: reject messages can be handled
- W: WPROT like OE3DZW
- Z: "BCMNET Loginkonzept" is handled (german citizen band)
- $: BIDs (Bulletin-IDs) can be handled

The Feature D is defined as following:
- boardnames up to 8 characters are supported.
- lifetime are send to partner mailbox system.
- AutoBIN mails are supported and forwarded.
- forward of Erase-/MyBBS-mails (E/M-Mails)

The character D was defined by DL8HBS and means "D"L8HBS or "D"ieBox.
The D-extensions are used at the moment by following systems:
- BayCom/OpenBCM-Mailbox (DL8MBT, OE3DZW, DG9MHZ, DH3MB, DK2UI, DH8YMB)
- Digi-Point-Mailbox (DL8HBS)
- DieBox (DF3AV, DL1BDY) - jedoch derzeit teilweise ohne "D" im SID
- WinGT-Mailbox (DG8NDL)
- MCuT-Mailbox (DG6VJ, DG4IAD)

The existens of a SID, implicated that the system can change the forward
way and can handle OK/NO-commands.

OpenBCM mailbox passes each received SID and searches for $ character. If
the $ character is not included into SID, forward will be disconnected by
OpenBCM mailbox, because systems without BID are not supported.

The SID of other mailbox system can also contain:
- A (F6FBB):   ACK messages are known (forward command SA)
- B (F6FBB):   only in addition with F: Huffman-compressed S&F
- B1 (F6FBB):  as B, but with CRC at beginning of each mail; resume-mode added
- C (CBBS):    automatic clock setting (obsolet)
- D (DL8HBS):  boardnames up to 8 characters, CRC-16, in addition with B also
               compressed, in addition with B1 also better CRC
- D1 (DL8HBS): as D plus block CRC-mode in compressed forward
- F (F6FBB):   Batch forward like F6FBB, use F6FBB syntax
- H (W0RLI):   system can handle hierarchical address
- I (W0RLI):   system can handle "zero command", a line beginning with (;)
- L (G1NNA):   compression like G1NNA (obsolet)
- M (W0RLI):   can handle MIDs (BIDs for personal mails)
- R (AA4RE):   reject messages can be handled
- X (W0RLI):   X-Forward
- Y (WA7MBL):  forward with YAPP protocol
- $ (WA7MBL):  Bulletin IDentifier can be handled; the dollar character must be
               the last sign in field f3!

Some examples of known SIDs:
 w0rli V17.5, using BID, H-Adr., Null-commands and X-Forward.

 ag3f Version der rli/gyq cbbs.

 wa8bxn V1.16, using BID and H-Adr.

 wa7mbl V5.12, using BID and H-Adr.

 aa4re V2.3, using MID, BID, and H-Adr.

 DigiPoint V4.10, F6FBB & DieBox options are used.