Wrong packet decision in FO state

[Klaus Warnke]

Test ipv4/ipv4/udp/rtp/mp3-variable-pt packet 12 uor-2-id can not be decompressed.
outer ip-id is constant 0, inner is nbo +1.
c_generic.c: decide_FO_packet() line 2522

                        if( (!is_ip_v4 || is_rnd) && (!is_ip2_v4 || is_rnd2))
                                packet = PACKET_UOR_2_RTP;
                        else if(nr_ts_bits <= MAX_BITS_IN_4_BYTE_SDVL &&
                                (is_ip_v4 && nr_ip_id_bits > 0 &&
                                 (!is_ip_v4 || is_rnd2 || nr_ip_id_bits2 == 0)))
                                packet = PACKET_UOR_2_ID;
                        else
                                packet = PACKET_UOR_2_TS;

(gdb) p is_rnd
$38 = 1
(gdb) p is_ip_v4
$36 = 1
$37 = 16
(gdb) p is_rnd2
$41 = 0
(gdb) p nr_ip_id_bits2
$42 = 0

outer ip is rnd, therefore nr_ip_id_bits==16.
In this case, the ip-id is transmitted in packet remainder, not here.
but

                                (is_ip_v4 && nr_ip_id_bits > 0 &&

does not take this into account.
fix:

                                (is_ip_v4 && !is_rnd && nr_ip_id_bits > 0 &&

This also needs improvement:

                                 (!is_ip_v4 || is_rnd2 || nr_ip_id_bits2 == 0)))

Because it is possible to use uor-2-id to send <=5bits of ip-id-2 bits,
if outer ip-id is rnd/not present (->not compressible).

regards, Klaus

[Klaus Warnke (k-warnke)]

--- src/comp/c_generic.c 2010-08-22 14:00:49 +0000
+++ src/comp/c_generic.c 2011-05-11 15:43:40 +0000
@@ -2713,7 +2719,7 @@
                                packet = PACKET_UOR_2_RTP;
                        }
                        else if(nr_ts_bits <= MAX_BITS_IN_4_BYTE_SDVL &&
- (is_ip_v4 && nr_ip_id_bits > 0 &&
+ (is_ip_v4 && !is_rnd && nr_ip_id_bits > 0 &&
                                 (!is_ip_v4 || is_rnd2 || nr_ip_id_bits2 == 0)))
                        {
                                packet = PACKET_UOR_2_ID;

[Didier Barvaux (didier-barvaux)]

Confirmed in 1.2.x releases.

[Didier Barvaux (didier-barvaux)]

Confirmed in 1.3.x releases.

[Didier Barvaux (didier-barvaux)]

Confirmed in trunk.

[Didier Barvaux (didier-barvaux)]

Fix committed in trunk, see http://bazaar.launchpad.net/~didier-barvaux/rohc/main/revision/311. Scheduled for release 1.4.0. Thanks Klaus for the bug report.

Fix the way IP-ID is handled in case there are 2 IP headers: the base headers
contains the IP-ID bits of the innermost IPv4 header with a non-random IP-ID,
not the IP-ID bits of the outer IPv4 header.

Do achieve that:
 - add some code to determine the innermost IPv4 header with a non-random IP-ID,
 - add debug traces to print the conditions that make the library select a
   packet type,
 - force values in the packet_type_t enum to be sure they do not change from one
   platform to another,
 - introduce a new non-regression test that causes the IP-ID problems,
 - introduce a new rohc_utils.h headers that defines some utils (min and max
   macros currently),
 - fix a small problem that transmitted TS bits even if they were deductible
   from the SN value (small compression gain expected).

Note 1: the format of the ROHC is altered by this changeset, so compatibility
               with previous revision is not ensured.

Note 2: the algorithm that decides the packet type is not optimal, it should
               be reworked in the future.

[Didier Barvaux (didier-barvaux)]

The bug will not be fixed in the 1.3.x branch since it would break compatibility between 1.3.x releases. A note about the bug was added in the README file.
See http://bazaar.launchpad.net/~didier-barvaux/rohc/1.3.x/revision/180

[Didier Barvaux (didier-barvaux)]

The bug will not be fixed in the 1.2.x branch since it would break compatibility between 1.2.x releases. A note about the bug was added in the README file.
See http://bazaar.launchpad.net/~didier-barvaux/rohc/1.2.x/revision/151