Subject: Routing for UDP/real-time vs TCP

From: "Bogdan Ghita" <[email-masked]>
Date: Thu, 5 Oct 2000 09:04:25 +0100

Dear all

I have a question related to general routing issues:
I am looking into measuring RTT for a specific path using 2 different methods: TCP acknowledgements and RTCP NTP/LSR/DLSR fields.
Is there any specification regarding different handling by the servers of packets that carry TCP segments vs packets carrying UDP datagrams? I had several discussions lately about how some/all routers tend to drop first ICMP traffic, then TCP traffic and, at the end, UDP traffic, as the first category is not critical and the second can recover. If so, it would mean that there are several different transport performance rates for the same path, depending on the type of traffic being sent. Are the assumption/conclusion correct? Could you point me to some materials that detail this matter?

Thank you very much

Best regards
Bogdan Ghita

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From: [email-masked]
Date: Thu, 05 Oct 2000 18:05:54 +0200

> I am looking into measuring RTT for a specific path using 2
> different methods: TCP acknowledgements and RTCP NTP/LSR/DLSR
> fields.
>
> Is there any specification regarding different handling by the
> servers of packets that carry TCP segments vs packets carrying UDP
> datagrams?

Not that I know of.

However, there are some operational practices which are fairly
widespread.  These operational practices are typically motivated by
the desire to reduce the impact of certan classes of denial-of-
service attacks.  This typically translates into at least a rate
limiting for ICMP Echo Reply traffic on a per-link basis.

I've never heard of anyone consciously dropping TCP traffic, thus
preferring UDP traffic -- in fact I can think of no good reason to
do so.  However, there are a number of protocols which run over UDP
which do not behave as a good citizen and respond to congestion, and
the net effect if such protocols are in use and try to consume a
significant amount of the available capacity on a FIFO-handled queue
will be that TCP experiences packet loss and backs off to adapt to
the available capacity.  Fair Queueing may counter some of that
effect.

Regards,

- Håvard

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From: Kavé Salamatian <[email-masked]>
Date: Wed, 25 Oct 2000 23:38:36 +0200

Dear All,

Anybody is aware of a paper of research work that has been done about relating the loss statistics seen by two flows sharing the same paths, but with different inter-packet delay distribution. 

A simple example of application, how to relate to loss process seen by a UDP connection to the loss process seen by a competing TCP connection ?

Thank you very much.

Best Regards

Kavé Salamatian

Assistant Professor, LIP6, UPMC, Paris France