SNAC(01,21)  SRV_EXT_STATUS 

This is an own extended status notification. You'll get this when you have server-stored icon or if you setup iChat "available" message. It's also used to tell the client whether or not it needs to upload an SSI buddy icon. Check for buddy icons info here.

About icon flags byte. Its purpose is unknown, but i found that bit8=1 is a command to upload icon to server and it is appeared after changing corresponding ssi item.

 00 01   word   SNAC family
 00 21   word   SNAC subtype
 00 00   word   SNAC flags
 xx xx xx xx   dword   SNAC request-id
 00 01   word   notice type (0x0000 & 0x0001 - SSBI info)
 xx   byte   icon flags (bitmask)
 xx   byte   icon md5 hash length (allways 16)
 xx ..   array   icon md5 hash array

 00 02   word   notice type (0x0002 - iChat "available" message)
 xx xx   word   length of message
 xx ..   string   iChat available message

Example SNAC dump with flap header:

  2A 02 26 0F 00 1E 00 01  00 21 00 00 C8 4C DF CE  *.&..&...!...L..
  00 01 41 10 FC 23 B3 F7  C8 00 D3 92 EF 5E 06 8A  ..A..#.......^..
  D3 66 7F C7                                       .f..


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