# How does Packet Header Generator module handle tags **URL:** \ **Category:** GNU Radio\ **Created:** [July 20, 2014, 2:58pm UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081 "2014-07-20T14:58:20Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [July 20, 2014, 2:58pm UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/1 "2014-07-20T14:58:20Z")
Hi Experts, I was reading tx\_ofdm.py. I found this module use ‘set\_tag\_propagation\_policy(TPP\_DONT)’ to handle tags by itself, but I cann’t find the code that used to handle tag in this module. I knew this module will call ‘packet\_header\_ofdm.header\_formatter()’ and ‘packet\_header\_default.header\_formatter()’, but I also cann’t find the handle tag code in these functions. Anyone can help me figure out where does this module process tags? Thanks Tiankun ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 20, 2014, 3:17pm UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/2 "2014-07-20T15:17:31Z")
Hi Tiankun, what’s your GNU Radio version [1]? Where did you find that tx\_ofdm.py? There is no tx\_ofdm.py in my GNU Radio source tree (in none of v3.7.4, v3.6.5 or v3.4.2). There is a tx\_ofdm.grc, which can be used to generate a tx\_ofdm.py, but that does not (and shouldn’t) contain a “set\_tag\_propagation\_policy” line. Greetings, Marcus [1] you can find out by running “gnuradio-config-info -v” ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 20, 2014, 3:52pm UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/3 "2014-07-20T15:52:01Z")
Hi Marcus, My tx\_ofdm.py is gen by tx\_ofdm.grc, there are a ‘packet\_headergenerator\_bb’ module in it. This module use ‘set\_tag\_propagation\_policy(TPP\_DONT)’ to handle tags by itself. I didn’t find any code has touch tags in this module, but when I connect it to Tag Debug module, I can find the tags output. In my opinion, if one module use ‘set\_tag\_propagation\_policy(TPP\_DONT)’, and it does not handle it, tags will not pass down. Thanks Tiankun 于 2014年07月20日 21:15, Marcus Müller 写道: ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 20, 2014, 4:56pm UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/4 "2014-07-20T16:56:22Z")
Hi Tiankun, On 20.07.2014 15:50, hutiantian wrote: > Hi Marcus, > My tx\_ofdm.py is gen by tx\_ofdm.grc, there are a > ‘packet\_headergenerator\_bb’ module in it. This module use > ‘set\_tag\_propagation\_policy(TPP\_DONT)’ to handle tags by itself. > Ah ok! > I didn’t find any code has touch tags in this module, but when I > connect it to Tag Debug module, I can find the tags output. > This is indeed strange. However, packet\_headergenerator\_bb is a subclass > of tagged\_stream\_block [1]. > In my opinion, if one module use > ‘set\_tag\_propagation\_policy(TPP\_DONT)’, and it does not handle it, > tags will not pass down. > That’s generally correct, but there’s the tagged stream block exception: > [1], “A note on tag propagation” tells us that one most of the time > would use TPP\_DONT to handle all non-length tags, whilst length tags are > always passed forward. Hope that helped! Greetings, Marcus [1] [http://gnuradio.org/doc/doxygen/page\_tagged\_stream\_blocks.html](http://gnuradio.org/doc/doxygen/page_tagged_stream_blocks.html) ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 21, 2014, 11:55am UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/5 "2014-07-21T11:55:57Z")
Before myths about tagged stream blocks start perpetuating the webs: TPP\_DONT on a tagged stream blocks does the same as on any other type of block; however, TSB tags still get through. A closer inspection of packet\_headergenerator\_bb (either by reading the source code or the manual page ([GNU Radio Manual and C++ API Reference: gr::digital::packet\_headergenerator\_bb Class Reference](http://gnuradio.org/doc/doxygen/classgr_1_1digital_1_1packet __headergenerator__ bb.html#details)) reveals that the actual processing is done in a separate kernel, the header formatter object, which is user-defined. See packet\_header\_default for an example. M ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 21, 2014, 2:05pm UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/6 "2014-07-21T14:05:52Z")
Hi Martin, What’s the TSB tags? Does the tags have the different types? I think the tags just a pair of key-value. Thanks Tiankun 于 2014年07月21日 17:54, Martin B. 写道: ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 21, 2014, 1:54pm UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/7 "2014-07-21T13:54:58Z")
Hi Marcus, What’s the “non-length tags” mean? the “length tags” you mentioned is that input parameter, right? I only find this one tags. When I remove ‘set\_tag\_propagation\_policy(TPP\_DONT)’, this module seems still works fine. Why should we need set this flag in this module? Thanks Tiankun 于 2014年07月20日 22:54, Marcus Müller 写道: ---
**Author:** ![Tiankun\_Hu](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/46a35a/32.png) [@Tiankun\_Hu](https://www.ruby-forum.com/u/Tiankun_Hu)\ **Post date:** [July 22, 2014, 10:52am UTC](https://www.ruby-forum.com/t/how-does-packet-header-generator-module-handle-tags/231081/8 "2014-07-22T10:52:55Z")
On 07/21/2014 02:04 PM, hutiantian wrote: > Hi Martin, > What’s the TSB tags? Does the tags have the different types? I think the > tags just a pair of key-value. [http://gnuradio.org/doc/doxygen/page\_tagged\_stream\_blocks.html](http://gnuradio.org/doc/doxygen/page_tagged_stream_blocks.html) M