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Re: [AVT] Another question on RFC 4103
Marc,
My calculations end up a factor 10 lower than yours.
You are right that we did not think there was any reason to go deep in a
discussion on MTU sizes in RFC 4103.
Instead we specify the intended use, that is for human input, and human
reading in a real-time conversation.
The most rapid input I can imagine is from a speech-to-text application,
that maybe can reach 20 characters per second from a rapid speaker.
That is why the maximum required rate to support is specified to be 30
characters per second.
The recommended sample time is 300 ms, resulting in sending 3.3 packets per
second when there is text to send. With an even caracter rate, that results
in 9 new characters per packet.
With the recommended two generations of redundancy, it will mean that 27
characters are packed in each packet.
If they are from a language that make use of three UTF-8 bytes per
character, there will be a mean of 81 bytes text per packet.
Add the IPv4, UDP, RTP and RFC2198 overhead of -say 70 bytes, the packet
size that will be close to maximum is 151 bytes. That is so far from the
maximum MTU size
This is a maximum. In reality, text is usually entered by typing, resulting
in only 1-4 new characters per packet.
Even if the reasoning above tells that there is no risk at all for exceeding
the maximum MTU-size, the application should follow a reasonable strategy to
be sure: The application should check so that the maximum MTU is not
exceeded. If there is a risk for that, excess characters should be delayed
and transmitted in next regular packet to transmit.
Gunnar
-----Original Message-----
From: avt-bounces at ietf.org [mailto:avt-bounces at ietf.org] On Behalf Of Marc
Petit-Huguenin
Sent: Saturday, April 18, 2009 5:31 PM
To: avt at ietf.org
Subject: [AVT] Another question on RFC 4103
RFC 4103 does not give any advice on what to do when a t140block to send
will create an RTP packet that will exceed the PMTU. Should the PMTU be
considered as an implicit maximum character transmission rate, or should
multiple RTP packets with the same timestamp be sent? (for the latter there
is the additional issue that RFC 2198 does not provide any advice on the
same problem).
For example let say that the cps is 30 and the PMTU is unknown so
576 bytes is the maximum IPv4 packet size that can be sent. With 30 cps and
a language that makes use of three bytes per character, the maximum block
size is 900 bytes which is more than the maximum IPv4 packet size without
even counting the overhead. In this case the maximum implicit cps would be
17 cps.
--
Marc Petit-Huguenin
Home: marc at petit-huguenin.org
Work: petithug at acm.org
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