Re: [AVT] RFC 4103 - RTP Payload for Text Conversation

[Marc Petit-Huguenin <petithug at acm.org>]

Gunnar Hellstrom wrote:
> Marc,
> See answers inline.
> 
> 
> -----Original Message-----
> From: avt-bounces at ietf.org [mailto:avt-bounces at ietf.org] On Behalf Of Marc
> Petit-Huguenin
> Sent: Tuesday, April 14, 2009 5:44 PM
> To: avt at ietf.org
> Subject: [AVT] RFC 4103 - RTP Payload for Text Conversation
> 
> I hope that this is the right place for questions on RFC 4103.
> 
> - Section 5.2, 3rd paragraph:  "The last packet before an idle period will
> contain only one non-empty T140block as redundant data..."
> 
> I am not sure how to reconcile this with the first paragraph in the same
> section that states that the beginning of an idle period is when the empty
> T140block is sent.  Does it means that the idle period starts with the empty
> T140block when no redundancy is used, but with the last retransmitted packet
> when redundancy is used?
> 
> <GH: Read te first paragraph of 5.2 as valid for the non-redundancy case. In
> that case you transmit one packet with an empty T140 block at the sample
> time after the last valid data. Then there will be an equal opportunity to
> detect if valid data is missing regardless of if the data is within a text
> burst or at the end of it. The idle period starts after transmission of that
> packet.
> Next multi-line paragraph is for the redundancy case.  Then you send in this
> pattern: 
> data1, data2, data3.   
> 
> data2, data3, empty.  
> 
> data3, empty, empty 
> 
> -----  idle period     ----           
> 
> empty, empty, data4    
> 
> This matches your assumption, and the text in the RFC.>

So if I understand correctly the idle period starts 300 ms after
sending the last text when there is no redundancy, and 600 ms after
sending the last text in the primary block when using redundancy.

> 
> 
> This also does not fit well with the next sentence; "Any empty T140block
> sent as primary data MUST be included as redundant T140block in subsequent
> packets, just as normal text."  If it was the case, wouldn't the last
> redundant packet sent entirely be composed of empty T140block?
> <GH: Yes, it fits. If the idle period is short, the latest transmitted empty
> T140 block that has not been transmitted the full number of redundant
> generations, will be transmitted again, stepped up in redundancy
> generations, and followed by the new primary data. But if the idle period
> was longer, the old empty blocks will be dropped. >

OK.

> 
> - T-140 says that the byte order mark should be inserted at the beginning of
> the session, which I understand as the first packet sent (as RFC 4103 does
> not say anything about it).  But the byte order mark is useless after been
> converted to UTF-8 so why a sender should send it?
> <GH: One good reason for this is that some network equipment requires
> outgoing packets in order to be able to receive incoming packets. So, one
> reason for this transmission is to "prime the channel". 

Right.  NATs.


Thanks.

-- 
Marc Petit-Huguenin
Home: marc at petit-huguenin.org
Work: petithug at acm.org