Wireshark · Ethereal-dev: [Ethereal-dev] RTCP - SDES - PRIV item length interpreted incorrectly

[dankom@xxxxxxxxxxxx]

It seems to me, that Ethereal processes incorrectly PRIV item in the
RTCP protocol, packet type SDES. Ethereal requires that the length
field of this item contain the length of the value string, instead of
the length of the whole item.
Although this is not explicitly specified in the PRIV item description
of RFC 1889, the length field definition in the general SDES item
description renders probable the latter interpretation:

"Each item consists of an 8-bit type field, an 8-bit octet count
describing the length of the text (thus, not including this two-octet
header), and the text itself."


I also found some evidence in the PRIV description:
(marked with ^)

6.4.8 PRIV: Private extensions SDES item

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     PRIV=8    |     length    | prefix length | prefix string...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ...              |                  value string                ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This item is used to define experimental or application-specific SDES
   extensions. The item contains a prefix consisting of a length-string
   pair, followed by the value string filling the remainder of the item
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   and carrying the desired information. The prefix length field is 8
   bits long. The prefix string is a name chosen by the person defining
   the PRIV item to be unique with respect to other PRIV items this
   application might receive. The application creator might choose to
   use the application name plus an additional subtype identification if
   needed.  Alternatively, it is recommended that others choose a name
   based on the entity they represent, then coordinate the use of the
   name within that entity.

   Note that the prefix consumes some space within the item's total
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
   length of 255 octets, so the prefix should be kept as short as
   ^^^^^^^^^^^^^^^^^^^^^
   possible. This facility and the constrained RTCP bandwidth should not
   be overloaded; it is not intended to satisfy all the control
   communication requirements of all applications.

                                         Miklos Danko, M.D.
mailto:dankom@xxxxxxxxxxxx                   (Hungary)