Subject: [rohc] Questions about RFC 3095

From: "Sarkar, Rajib" <[email-masked]>
Date: Mon, 14 Oct 2002 17:22:28 -0700

Hi,
	I have a few questions about RFC 3095.
1.	In section 5.2.2 at the end of page 44 RFC says 
"When the decompressor has determined the size of the feedback data
   field, it removes the feedback type octet and the Size field (if
   present) and hands the rest to the same-side associated compressor

   together with an indication of the size.  The feedback data received
   by the compressor has the following structure (feedback sent on a
dedicated feedback channel MAY also use this format)"
My question is : Is the presence of the decompressor is assumed only if
there is a piggybacking or interspersing?
2.	In section 5.7.7.3 through 5.7.7.6 Ipv6, Ipv4, RTP and UDP values
are initialized. Lets say for example in section 5.7.7.6, where RTP header
is initialized it is mentioned that 
Static part:

      SSRC.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      /             SSRC              /   4 octets
      +---+---+---+---+---+---+---+---+

   Dynamic part:

      P, X, CC, PT, M, sequence number, timestamp, timestamp stride,
      CSRC identifiers.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |  V=2  | P | RX|      CC       |  (RX is NOT the RTP X bit)
      +---+---+---+---+---+---+---+---+
      | M |            PT             |
      +---+---+---+---+---+---+---+---+
      /      RTP Sequence Number      /  2 octets
      +---+---+---+---+---+---+---+---+
      /   RTP Timestamp (absolute)    /  4 octets
      +---+---+---+---+---+---+---+---+
      /      Generic CSRC list        /  variable length
      +---+---+---+---+---+---+---+---+
      : Reserved  | X |  Mode |TIS|TSS:  if RX = 1
      +---+---+---+---+---+---+---+---+
      :         TS_Stride             :  1-4 octets, if TSS = 1
      +---+---+---+---+---+---+---+---+
      :         Time_Stride           :  1-4 octets, if TIS = 1
      +---+---+---+---+---+---+---+---+

   Eliminated:

      Nothing.

   Extras:

      RX: Controls presence of extension.

      Mode: Compression mode. 0 = Reserved,
                              1 = Unidirectional,
                              2 = Bidirectional Optimistic,
                              3 = Bidirectional Reliable.

   X: Copy of X bit from RTP header (presumed 0 if RX = 0)

   Reserved: Set to zero when sending, ignored when received.

   Generic CSRC list: CSRC list encoded according to section
          5.8.6.1, with all CSRC items present.

   CRC-DYNAMIC: Octets containing M-bit, sequence number field,
                and timestamp (octets 2-8).

CRC-STATIC: All other fields (octets 1, 9-12, original CSRC list).
That means that only SSRC is static part in the header. However, in appendix
A.1.4 it is said that
A.1.4. RTP header fields
      +-----------------+-------------+----------------+
      | Field           | Size (bits) |     Class      |
      +-----------------+-------------+----------------+
      | Version         |      2      |  STATIC-KNOWN  |
      | Padding         |      1      |     STATIC     |
      | Extension       |      1      |     STATIC     |
      | CSRC Counter    |      4      |    CHANGING    |
      | Marker          |      1      |    CHANGING    |
      | Payload Type    |      7      |    CHANGING    |
      | Sequence Number |     16      |    CHANGING    |
      | Timestamp       |     32      |    CHANGING    |
      | SSRC            |     32      |   STATIC-DEF   |
      | CSRC            |   0(-480)   |    CHANGING    |
      +-----------------+-------------+----------------+

   Version

      Only one working RTP version exists, namely version 2.  The field
      is therefore classified as STATIC-KNOWN.

   Padding

      The use of this field is application-dependent, but when payload
      padding is used it is likely to be present in all packets.  The
      field is therefore classified as STATIC.

   Extension

      If RTP extensions are used by the application, these extensions
      are likely to be present in all packets (but the use of extensions
      is very uncommon).  However, for safety's sake this field is
      classified as STATIC and not STATIC-KNOWN.

   SSRC

      This field is part of the definition of a stream and must thus be
      constant for all packets in the stream.  The field is therefore
      classified as STATIC-DEF.

   Total size of the fields in each class:

      +--------------+---------------+
      | Class        | Size (octets) |
      +--------------+---------------+
      | STATIC       |    2 bits     |
      | STATIC-DEF   |      4        |
      | STATIC-KNOWN |    2 bits     |
      | CHANGING     |  7.5(-67.5)   |
      +--------------+---------------+
So the appendix says that Padding, extension, version all are static. I am
confused here. Is the reason for this is 8 bit alignment? Any help will be
highly appreciated.
Thanks
Rajib
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