[dev] RTT

Sullivan (US), Scott C scott.c.sullivan at boeing.com
Thu Apr 2 20:38:22 EDT 2020

A link between 2 nodes (routers, hosts, etc.) in CORE is facilitated by a linux bridge.  So, if you have 2 routers directly connected to each other, CORE creates that by connecting each CORE node (i.e. via kernel virtualization) to the linux bridge.  If you have 3 nodes connected in a string (n1 - n2 - n3), nodes n1 and n2 are connected by one linux bridge and nodes n2 and n3 connected by another linux bridge.  You can see this by looking at the output of brctl when an emulation is running.

I would expect your RTT to increase as the number of hops increases.  If 1 hop is 80us, then it seems reasonable that 2 hops would be 160us, which is going to be system dependent.  I don't know how CORE implements bandwidth, but wouldn't be surprised if it's a tc configuration (shaper ?) on one of the node interfaces connected to a linux bridge.

CORE doesn't emulate software.  It runs objective software (e.g. ping, FRR, etc.) in each emulated node (i.e. via kernel virtualization) - the same software that you'd run outside of CORE.

From: dev [mailto:dev-bounces at lists.frrouting.org] On Behalf Of Lou Berger
Sent: Thursday, April 2, 2020 5:10 PM
To: Sofia Salgueiro <sof97 at live.com.pt>; dev at lists.frrouting.org
Subject: Re: [dev] RTT

This sounds like a core question to me....


On April 2, 2020 6:31:07 PM Sofia Salgueiro <sof97 at live.com.pt<mailto:sof97 at live.com.pt>> wrote:
Good afternoon,

I've been using CORE with FRRouting and I've noticed that the PING rtt between two directly connected router devices is approximately 80 us, even if I change the bandwidth of the links. I've also noticed that this value seems to increase with the number of hops, eg. between 2 routers with 4 links the value doubles to approximately 160 us.

So, I was wondering if maybe you could give me some insight on how the PING is emulated and, more specifically, why the rtt has this value?

Best regards,
Sofia Salgueiro
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