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  • Comcast woes....

    Since...oh, Friday maybe, I've been having almost no luck getting online. Well, I can get online otherwise I wouldn't be posting this, but pages take an insanely long time to load (sometimes not at all).

    ISP: Comcast *feh*
    Computer: HP laptop running Vista, up to date as far as I know
    Date problem started: 9/17. Up until now I have had no issues whatsoever with internet (and yes my bill is paid up ^_^).

    I do have Wireshark (packet sniffer) on the laptop, mainly installed in the attempt to catch any of the reset packets Comcast is not supposed to be using (so far this month I've been reset on eight occasions where there was no need to do so). I've noticed very little actual HTTP traffic, 98% of what I see is a storm of ARP broadcasts (machines trying to find each other) coming from the gateway I'm supposed to be using.

    I'd like to narrow the problem down a bit more before I try to call again; would it be possible or advisable for me to force use of a different gateway?

    My mom in the same neighborhood has also been reporting problems; later today I'll see if my computer works any better using her connection.

    Thoughts as to what the issue could be? It doesn't seem as if the problem is on my end...
    "I am quite confident that I do exist."
    "Excuse me, I'm making perfect sense. You're just not keeping up." The Doctor

  • #2
    First things first: Use of a different gateway will prevent your going anywhere except your local network. Depending on how Comcast sets things up, that could as limited as the piece of your town that is on Comcast's network.

    Don't change that.

    Second: The arp packets? Totally normal. Every machine will be seeing communications and talking constantly. The arp packets are having next to no impact on your connection.

    As to the cause? Several possibilities:
    • Security patches are issued to fix vulnerabilities after those vulnerabilities are found. During the time between discovery and patch installation a machine is vulnerable. Something could have gotten into your machine and is hijacking your connection. Possible, though not very plausible.
    • Your cable modem is going bad/has gone bad. Replacement might be in order. Plausible.
    • You don't mention if you have a home network. If you do, your router might have gone bad. Again, replacement might be in order. Plausible.
    • Local DNS servers in your area received a configuration change that results in DNS queries not resolving in a timely fashion. You will have to find a public DNS server and tell your computer to use that DNS server to determine if this is the cause. Possible, though not very plausible. Still worth trying.
    • A new splitter has been placed onto the line between the repeater on the street and your cable modem. With each split, the signal degrades. With each bit of degradation, your modem will have a harder time making the connection work reliably. Check your home wiring to make sure no splitters go between the street and your cable modem. Semi-plausible.
    • Something has gone bad on the line between the repeater on the street and your house. Basically, the signal degrades between the last repeater and your cable modem. If this has occurred, only Comcast can prove it an fix it. A technician will need to check the signal at your house to find out. Plausible. Likely if there was a severe weather event in your area around the time that your connection started falling apart.


    Anyway, that's what I got for ideas and suggestions. Hope that one of them helps.

    Comment


    • #3
      Check your modem signal levels. http://192.168.100.1 Make a note of your SNR, downstream and upstream power levels.

      Some modems do not have a user-accessible diagnostics page. Some modems lock out the diagnostics page once sync is achieved (notably, the Toshiba PCX 2600). Motorola Surfboard modems are always accessible however.

      Ballpark figures (per http://www.dslreports.com):
      Receive/Downstream: -15dbmV to +15dbmV
      Transmit/Upstream: 30dbmV to 55dbmV
      Signal to Noise Ratio (SNR): 30dB or Greater

      Mine, for comparison:
      SNR 37.2
      Down power -3.3
      Up power 41
      (with a Motorola SB5100)

      You may also want to find a server that is geographically close to you, and start pinging it and making notes on packet loss.

      And finally the simplest thing to check is DNS. Set your DNS servers to any two of 4.2.2.1-5, 208.67.220.220, or 208.67.222.222.
      On Vista -> From here Click Properties
      Select Internet Protocol version 4 and click Properties. Now you're here and can make the changes.
      Last edited by otakuneko; 09-22-2008, 03:08 AM.
      Supporting the idiots charged with protecting your personal information.

      Comment


      • #4
        i was just reading a slashdot article that may explain your woes. seems comcast is throtteling down bandwidth. its possible they have done so on a larger scale in your neighborhood.
        This is a drama-free zone; violators will be slapped. -Irving Patrick Freleigh
        my blog:http://steeledragon.wordpress.com/

        Comment


        • #5
          Aha. Go figure, I can connect perfectly fine at my mom's place and Wireshark does capture normal traffic there. When I'm at my place I am only seeing ARPs from a single IP (out of 6350 packets captured, 6340 were ARP requests with the other ten being pings from elsewhere on the network), no other packet types at all. That just seems weird.

          There are a few students in my building, it's possible that Comcast could be throttling the entire building based on their activity...if they're engaging in massive downloads. If not, then... A recent phone call went into a loop of "reboot router, reboot computer, the problem must be on the subscriber's end" [OK then what exactly is the failure point; wiring, modem on my desk, router elsewhere, what? it's not my computer and not the browser I'm using]
          "I am quite confident that I do exist."
          "Excuse me, I'm making perfect sense. You're just not keeping up." The Doctor

          Comment


          • #6
            That's not throttling, that's blocking.

            Some minor weirdness you wouldn't know unless you're a networking geek, or someone who looks at them for fun: ARP packets are restricted to a local network. They do not cross routers (well, unless you have a badly misconfigured router).

            Let me reiterate that, because it's so important: ARP packets do not cross routers.

            Therefore, every single bit of traffic you were seeing captured by Wireshark was local traffic. This means that no traffic is coming from outside the network into your machine. None at all.

            Now, here's the next set of tests to perform (the first one is done, and is documented for completeness sake):
            1. Take the computer to another location and try to access the internet there. This eliminates a firewall on the machine (mostly). You've already done this.
            2. Take the cable modem you use elsewhere and test it elsewhere. In this case, take your computer and modem over to your mom's place. Disconnect her modem, plug yours in, and attach her computer. Try to get online. with her computer. If this fails, plug her modem back in, and make sure she can get online with hers. If so, your modem is fried.
            3. If that works, plug your computer into your modem. Now try to get online. If you get online here, using your modem, your computer, and your wiring to connect to the modem, you now have absolute proof it's not your equipment.


            Now, if those steps fail, you're still not getting online, we go to a desperation move: Disconnect all cable modems, and wait one hour. Plug yours in, and try again. If that still fails, and hers being plugged in still works, your modem is toast.

            If, at any point, using all your equipment and just changing location, you're getting online, then you have definite proof positive that the problem is Comcast. Get them back on the phone and demand a tech come out. Explain the proof if need be.

            Hope this winds up helping a little bit, at least.

            Comment


            • #7
              Note for #2, depending on how tight their configuration is, you may have to call and give them the MAC address of the cable modem. It is unlikely, but possible.
              Supporting the idiots charged with protecting your personal information.

              Comment


              • #8
                ARP requests are very normal on a cable network. The cable modem isn't a router, it really is just a modem, so you do see the broadcasts. If you're at all worried, add up the bandwidth they consume - it should be very small indeed.

                So you can filter ARP out of your sniffing. That will make the rest of it so much easier to read.

                There are legitimate reasons to receive RST packets:

                - Your computer initiates a TCP connection (SYN) but the other computer isn't listening on that port. This will look like SYN, RST, SYN, RST, SYN, RST for a couple of retries.

                - A connection is closed, and the remote computer is in a hurry to free the resources and get on with serving the next client. This looks like: FIN, FIN/ACK, RST.

                - A remote computer is rebooted or otherwise completely forgets about the connection. This is an abnormal condition, but the RST packets tell your computer that it happened and to deal with it. This looks like DATA/ACK ... <retry> ... <retry>, RST.

                What Comcast was doing is somewhat different. They were selecting specific connections and resetting them in mid-flow. This is easy enough to identify, due to round-trip latency and the fact that the Sandvine, being out-of-line, canot actually filter the remains of the connection as it collapses. This looks like: DATA/ACK, DATA/ACK, RST (recv), DATA/ACK (recv), RST (sent).

                HTH.

                Comment


                • #9
                  I love all the troubleshooting steps, and I'm sure this issue has already been resolved.

                  But, I can't help but wonder if anyone ever thought to call Comcast. The reason why I say that is when I was having issues with my internet back in February, I believe. I did the normal things. I rebooted my modem and router and when that didn't work, I rebooted again and connected directly the modem.

                  When that didn't work, I booted into safe mode with networking to see if it could possibly be like a firewall or a background programming not letting me on. I checked for possible DNS Server issues, there were non.

                  So, I gave my ISP, Charter, a call. They apparently looked at my modem from their end and noticed that I had really weak signals.

                  So, they send a technician out to my house and he noted that there were squirrel chews on the line. He also said that when it started getting cold, the line becomes a bit more brittle and that's probably why we're noticing the problem now.

                  Then, he went ahead and fixed the line. My internet connection has been golden since.
                  When will the fantasy end? When will the heaven begin?

                  Comment


                  • #10
                    Quoth Fashion Lad! View Post
                    But, I can't help but wonder if anyone ever thought to call Comcast.
                    I did, and said so:

                    Quoth Pedersen View Post
                    If, at any point, using all your equipment and just changing location, you're getting online, then you have definite proof positive that the problem is Comcast. Get them back on the phone and demand a tech come out. Explain the proof if need be.
                    All of the rest of my post was about proving it one way or the other. Comcast has a reputation for being downright customer hostile (just google for "comcast sucks" for a few months' worth of reading). My steps eliminate the possibility of it being your equipment, and avoid the necessity of even trying to get assistance from them in the event that it does turn out to be your equipment.

                    And, if it's not your equipment, you have all the proof you need to get them to send a technician out. They'll still make you do some crap troubleshooting steps, but you'll already know the answers.

                    Comment


                    • #11
                      So, I gave my ISP, Charter, a call. They apparently looked at my modem from their end and noticed that I had really weak signals.
                      Hence why I suggested checking the signal levels.

                      I wonder what became of the OP, anyway?
                      Supporting the idiots charged with protecting your personal information.

                      Comment


                      • #12
                        After being out of town for a week, I now seem to be having no issues (or less than last time, now it seems that any website using google-analytics.com will hang)....I was actually hoping I'd get a chance to test my modem at my mom's place and vice versa (would that even work?).

                        Her connection is also fine on all computers in the apartment. Keeping my ear to the wire for the rest of this week though...
                        Last edited by Dreamstalker; 10-01-2008, 11:33 PM.
                        "I am quite confident that I do exist."
                        "Excuse me, I'm making perfect sense. You're just not keeping up." The Doctor

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