The strangest cable modem issue

Mon Aug 23 16:25:50 UTC 2004

I think your problem was as a result of what happened to Comcast yesterday
http://www.miami.com/mld/miamiherald/news/local/states/florida/counties/brow
ard_county/9457328.htm?1c 

Was it only happening yesterday?

Coincidence??? 

--Samuel

  _____  

From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu] On Behalf Of
Drumm, Dan
Sent: Monday, August 23, 2004 11:10 AM
To: nanog at trapdoor.merit.edu
Subject: The strangest cable modem issue

I thought I'd ask NANOG this, since somebody may be from a large cable
operator and may know. I am a Comcast customer and don't want to call this
in through tech support, as I've tried that before without any success.

I use my cable modem normally - to surf the web, to play a game or two, and
to listen to stream casts. I have a buddy who often needs tech support, so I
use RAdmin (http://www.famatech.com/radmin/) to login to his computer and go
through stuff with him. I use large TCP packets, small UDP packets - ICMP,
ESP - large and small.

RAdmin can transfer files from desktop to desktop. Ethereal reveals this is
a standard TCP based protocol used on the RAdmin port tcp/4899. If I want
him to have a file, I push it that way. 

Last night - I decided to start IIS FTP server, and have him use his
SmartFTP (www.smartftp.com <http://www.smartftp.com/> ) client to pull
several larger files from me. I poked a hole in the Linksys router's port
forwarding for FTP and off we went. He grabs the first file, and it's going
along at roughly the correct uplink speed for Cable (20 KB/s or so) and all
is fine. Then, my cable modem reboots. Stops dead. All the lights go off and
start blinking, a full reload. It grabs the same IP address via DHCP and he
restarts. Again, about 15 seconds into the download, it crashes the cable
modem. We do this about 5 times to really make sure it's just FTP doing
this.

I have a DOCSIS 1.1 modem from RCA
(http://www.rca.com/product/viewdetail/0,2588,PI700502-CI700094,00.html?)

I have a Linksys BEFSR81
(http://www.linksys.com/products/product.asp?prid=155
<http://www.linksys.com/products/product.asp?prid=155&grid=5> &grid=5) with
the latest firmware

FTP is the only protocol that does this. Using RAdmin file transfer, I push
files to him over tcp/4899 and it doesn't happen.  

The cable modem is somewhat old, perhaps it needs to be flashed with a newer
image, but I am not sure what could cause it. 

Ideas?

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