Sandeep,

|Good day to you, I hope that you are well?

I think we are moving forward, but my engineer is now experiencing
problems whereby it seems that the message is becoming corrupted before the
outgoing message can be transmitted.  The following is my engineer’s
description of the problem:

Using “transport_adapter_sample.c” as an example, I am trying to add an
attribute to the outgoing SDP message.

I have modified the example code as follows:

    /* You may do anything to the local_sdp, e.g. adding new

attributes, or

    * even modifying the SDP if you want.

    */

    if (1)

    {

           /* Say we add a proprietary attribute here.. */

           pjmedia_sdp_attr *mikey_attr;



           mikey_attr = PJ_POOL_ALLOC_T(sdp_pool, pjmedia_sdp_attr);



           pj_strdup2(sdp_pool, &mikey_attr->name, "key-mgmt");

           pj_strdup2(sdp_pool, &mikey_attr->value, "mikey ");

Create_DH_HMAC_SDP_Attribute(sdp_pool, local_sdp, mikey_attr);

           pjmedia_sdp_attr_add(&local_sdp->attr_count,

local_sdp->attr, my_attr);

    }

Create_DH_HMAC_SDP_Attribute creates a base64 encoded attribute and
appends it to “mikey_attr->value”.

using

pj_strcat2(&mikey_atrr->value, base64_buffer);

This appears to work okay, and the attribute is added to the local_sdp,
and results in an attribute pj_string something like

         “mikey AQcF/kkSBAA………….”            About 157 bytes in length.

When I step through the routine pjsua_media_channel_create_sdp in
pjsua_media.c a call is made to

sdp->conn = pjmedia_sdp_conn_clone(pool, m->conn);

which then overwrites my attribute, resulting in a corrupted message being
sent.

Any thoughts/suggestions would be extremely welcome.

Many thanks…

Tim

From: McLeod, Tim
Sent: 11 January 2013 11:02
To: 'pjsip list'
Subject: RE: [pjsip] Custom Session Negotiation

Sandeep,

Thank you for your very quick response.  I have passed your suggestion to
my engineer who has confirmed that this is exactly the information he
needed.  Excellent work.

Thanks again…

Tim

From: pjsip [mailto:pjsip-bounces@lists.pjsip.org] *On Behalf Of *Sandeep
Karanth
Sent: 11 January 2013 05:17
To: pjsip list
Subject: Re: [pjsip] Custom Session Negotiation

Hi Tim,

      I guess probably you are using below part of *transport_encode_sdp()

*function * *to add your custom attribute to sdp.

if (1)

{

     /* Say we add a proprietary attribute here.. */

     pjmedia_sdp_attr *my_attr;



     my_attr = PJ_POOL_ALLOC_T(sdp_pool, pjmedia_sdp_attr);

     pj_strdup2(sdp_pool, &my_attr->name, "X-adapter");

     pj_strdup2(sdp_pool, &my_attr->value, "some value");



     pjmedia_sdp_attr_add(&local_sdp->media[media_index]->attr_count,

local_sdp->media[media_index]->attr, my_attr);

}

I think you are probably substituting in the above block 'X-adapter' with
your custom attribute. The problem is that pjmedia_sdp_attr_add function
highlighted above is adding that parameter as a media parameter and hence
it will come as a media parameter.

You can instead substitute the above line with
pjmedia_sdp_attr_add(&local_sdp->attr_count,local_sdp->attr,my_attr); and
try again. Hopefully this should solve your problem!

Regards,

Sandeep

On Fri, Jan 11, 2013 at 10:20 AM, Sandeep Karanth sandeepk.kdp@gmail.com
wrote:

Correct me if am wrong! So what you are looking to do is to add a session
level attribute i.e a=somevalue before any of the media parameter (m) lines

but instead it is getting added after some (m) parameter and hence it is
effect your custom "a" attribute in effect is becoming a media level
attribute??

On Thu, Jan 10, 2013 at 7:40 PM, McLeod, Tim Tim.McLeod@cassidian.com
wrote:

Furthering our project for experimenting with various methods of
providing secure VoIP communications we have a need to provide custom
negotiation mechanisms.  My engineer is currently looking into how this
might be incorporated into the existing pjsua functionality.  He is
encountering difficulties.

He is attempting to insert MIKEY session attributes into SDP message, i.e.
a=key-mgmt: mikey {base64 encoded mikey message} by using the function
“transport_encode_sdp” in the transport adapter to add the desired mikey
message.  When capturing the SIP invite message in Wireshark, the coding is
translated as a media attribute, the suspicion being that this is the case
because it is added to the message after the media description.  Is there a
clean way to add a session description before the media attribute?

Your help would (once again) be very much appreciated.

Tim McLeod MBCS CITP

Principal Engineer

Tel: +44 1633 715097

Mob: +44 7765 088364

Email: tim.mcleod@cassidian.com

RLI: tim.mcleod@eads.r.mil.uk

Website: www.cassidian.com

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I think we are moving forward, but my engineer is now experiencing problems whereby it seems that the message is becoming corrupted before the outgoing message can be transmitted.  The following is my engineer's description of the problem:

Using "transport_adapter_sample.c" as an example, I am trying to add an attribute to the outgoing SDP message.

I have modified the example code as follows:

    /* You may do anything to the local_sdp, e.g. adding new attributes, or
    * even modifying the SDP if you want.
    */
    if (1)
    {
           /* Say we add a proprietary attribute here.. */
           pjmedia_sdp_attr *mikey_attr;

           mikey_attr = PJ_POOL_ALLOC_T(sdp_pool, pjmedia_sdp_attr);

           pj_strdup2(sdp_pool, &mikey_attr->name, "key-mgmt");
           pj_strdup2(sdp_pool, &mikey_attr->value, "mikey ");

Create_DH_HMAC_SDP_Attribute(sdp_pool, local_sdp, mikey_attr);

           pjmedia_sdp_attr_add(&local_sdp->attr_count, local_sdp->attr, my_attr);
    }

Create_DH_HMAC_SDP_Attribute creates a base64 encoded attribute and appends it to "mikey_attr->value".

using
pj_strcat2(&mikey_atrr->value, base64_buffer);

This appears to work okay, and the attribute is added to the local_sdp, and results in an attribute pj_string something like

         "mikey AQcF/kkSBAA............."            About 157 bytes in length.

When I step through the routine pjsua_media_channel_create_sdp in pjsua_media.c a call is made to

sdp->conn = pjmedia_sdp_conn_clone(pool, m->conn);

which then overwrites my attribute, resulting in a corrupted message being sent.

Any thoughts/suggestions would be extremely welcome.

I didn't look into the source of strcat2() ATM but all pjlib string
functions do not care about the allocation (and so they cannot possibly
enlarge it - if it is actually possible at all).

So what happens is IMHO:

• you create the attribute correct and successful (at least at that
  moment)
• you append the base64 string (and it looks then correct) but the pool
  doesn't know that it should touch the space after
  mikey_attr->value.ptr+mikey_attr->value.slen (because you only
  pj_strdup2() "mikey " into and that reserves probably just 7 bytes
  or so).
• on the next allocation from that pool and the subsequent write into
  it, the base64 string is plain simply overwritten.

So better allocate enough space for "mikey " and the base64 string (and
not only around 7 bytes ....).

How much space is enough space ;o)

BTW pj_strdup2 allocates exactly six bytes, while
pj_strdup2_with_null allocates seven in this case.

IMHO the proposed "void pj_strcat3(pj_pool_t*,pj_str_t*,const
char*)" solution leans more to the PJ library programming style.