rohc team mailing list archive

[宋鑫 <songxinwest@xxxxxxxxx>]

Thanks for reply!

> First, the packet is a IP/UDP/RTP packet, so where is the origion
> packet from?

>I'm not sure to understand you question. The IP/UDP/RTP packet comes
>from any application that generates such a packet: software SIP phone,
>PIBX...

I means that if these Ip/UDP/RTP packet's headers are constructed by the
application itself?
can you explain the principle of the iprohc's tunnel to me.I don't
understand it well...

Thanks


2013/6/1 Didier Barvaux <didier@xxxxxxxxxxx>

> Hello 宋鑫,
>
> > In iprohc reference, it creates a tunnel and a udp socket, if  the
> > virtual interface get a packet, first it will compress the packet to
> > a rohc packet,then tunnel the rohc packet to the udp socket,at last
> > use "sendto" to send the rohc packet. but I have some confusions
> > about it.
>
> Not an UDP socket, but a RAW socket. So encapsulation is IP only
> instead of IP/UDP.
>
>
> > First, the packet is a IP/UDP/RTP packet, so where is the origion
> > packet from?
>
> I'm not sure to understand you question. The IP/UDP/RTP packet comes
> from any application that generates such a packet: software SIP phone,
> PIBX...
>
>
> > Second, after compress, the  IP/UDP/RTP packet is transformed to a
> > rohc packet,but when transport is UDP,using the "sendto" function, it
> > adds the IP and UDP header again.If this is not efficient?
>
> Yes, kind of. It would more efficient to avoid the IP overhead, but you
> need it to transport ROHC packets over Internet without help from the
> link layer. If you control the link layer on the path between the 2
> ROHC endpoints, you don't need it.
>
> Back on efficiency, let's compute the IP/ROHC efficiency:
>  - The original IP/UDP/RTP is composed of 40 bytes of headers.
>  - The resulting ROHC packet is composed of 1 byte of headers (best
>    case).
>  - The IP/ROHC packet is composed of 21 bytes of headers.
>  - So, we saved 19 bytes of headers per packet, ie. 47% of headers.
>  - If the RTP payload is short (20 bytes for example), we saved 19
>    bytes of a 60-byte packet, ie. 31% of the packet.
>
> A third of bandwidth is great, but we can do better. Let's pack several
> ROHC packets together in one IP packet (as IP/ROHC does). It will save
> use a part of the overhead added by the IP encapsulation.
>
> Let's do the computation with packing set to 5 ROHC packets:
>  - 5 IP/UDP/RTP/data packets weight 5 * (40 + 20) = 300 bytes.
>  - 5 ROHC packets weight 5 * (1 + 20) = 105 bytes.
>  - one IP/(5*ROHC) packet weights 20 + 105 = 125 bytes.
>  - so, we saved 175 bytes, ie. 58% of the whole packets.
>
> Saving 58% of bandwidth seems "efficient" to me :)
>
> Regards,
> Didier
>
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