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sef

join:2013-07-24

[Business] Comcast Business class -- uploading kills connection

My cable modem (SMC, of course) upgraded its firmware two weekends ago. Since then, I noticed some problems, and confirmed I could reproduce them, last night: if I do a large data transfer out of my network from one machine, all other network connections through the cable modem die horrible lossy deaths. As in, lost packets, extremely high latencies (I recorded some as high as 20 seconds), etc.

Comcast told me this is a known issue with the firmware, and I need to get a network load balancer. They do not have a fix for the firmware, I was told, nor can they downgrade it.

Anyone else got this? I have to admit, it took a while for my jaw to drop off the floor.



SMC_QoS

@comcast.net

Re: [Business] Comcast Business class -- uploading kills connect

The Comcast SMC gateway has never supported any kind of QoS, so this is not a new problem (although perhaps the new IPv6 enabled firmware is even worse). I had to ditch my SMC gateway because of problems with VoIP quality.

If you don't have a static IP account with Comcast, the easy solution is to get your own SB6121 modem and a good SMB/SOHO grade router with adequate QoS (and/or a good SMB/SOHO grade switch with adequate QoS).

If you have a static IP account with Comcast, then your only choice is to use the Comcast gateway, but minimize its IP footprint by using your own QoS enabled router as a true static IP device and don't use the SMC gateway's NAT or firewall functionality at all.

Comcast's published claims that their SMC gateway is equivalent to a PIX or SonicWall box in functionality has always been laughable (except to those who might have believed it):
»business.comcast.com/smb/service···faqs#04c

quote:
How does the Comcast firewall compare with a PIX or Sonicwall?

The Comcast IP Gateway's firewall performs many similar functions to a PIX or Sonicwall.


sef

join:2013-07-24

3 edits

No, this is a new problem. It did not happen before 2 weeks ago.

This is not a QoS issue; this is the fact that an outbound connection is *killing* the inbound connection, to the points that packets are lost. (Well, it's *similar* to QoS, except that the main point of QoS is to ensure that a particular protocol can get a minimum bandwidth; what's happening here is that the router has, for some reason, decided to stop letting packet contention happen in its buffers, meaning that after the first stream is established, everything else loses out.) (Alternately, it's so busy doing the uploading that it's not noticing downstream packets, except for when the TCP connection is waiting for ACKs. Either way, this is really buggy.)

Strangely, the bandwidth I get is higher than it should be. (On the machines where the problem is exhibited, I'm getting about 1.4MBytes/sec upload; on the systems where it doesn't, I'm getting about 900kbytes/sec upload; my connection is min. 22mbits/down and 5mbits/sec up.)

So, to reiterate: old firmware, this did not happen; new firmware, this does happen.

Also: I am not using the SMC as anything other than a router. (Although apparently the new firmware has caused known issues with that as well.)

Going back some number of years, I'd compare this to a synchronous instead of asynchronous connection. And maybe that is what's going on, in which case I'd say the new firmware is not time-slicing well-enough to notice that there are packets coming from outside.



EG
The wings of love
Premium
join:2006-11-18
Union, NJ
kudos:10
reply to SMC_QoS

said by SMC_QoS :

the easy solution is to get your own SB6121 modem

FWIW, why not get a newer and better SB6141 (eight downstream channel capable) device instead of an older only four downstream capable device ?

Apologies for the off topic post.


telcodad
Premium
join:2011-09-16
Lincroft, NJ
kudos:15

said by EG:

said by SMC_QoS :

the easy solution is to get your own SB6121 modem

FWIW, why not get a newer and better SB6141 (eight downstream channel capable) device instead of an older only four downstream capable device ?

Apologies for the off topic post.

I guess the SB6121 modem was suggested as it's not clear that the SB6141 is supported for Business Class service, as per the discussion in this previous thread: »Re: [Business] JUst had comcast installed - advice please

sef

join:2013-07-24

Correct, only that one is supported. Well... they said that Netgear CG3000DCR is also supported... but it has the same firmware because they (Comcast) insisted on it. I haven't done much research on that yet.



EG
The wings of love
Premium
join:2006-11-18
Union, NJ
kudos:10
reply to telcodad

O/k fair enough but I have read here and at other forums that some have had success with them provisioning it for the B.C. service.



SMC_QoS

@comcast.net
reply to sef

Actually "QoS" stands for Quality of Service, and it does not refer to any specific protocol, and it is not even necessarily bandwidth related; there are many different ways to implement QoS, depending on what symptoms you are trying to mitigate. In your case the problem sounds as if it is directly caused by the asymmetric (not asynchronous) nature of an HFC connection, and proper QoS (which again, has never been an SMC gateway feature) should relieve the symptom (if you can ditch the SMC gateway and use a standard cable modem and a good router/firewall).

If you must use the SMC gateway because you are a static IP customer, and you are already bypassing the NAT router and firewall in the SMC gateway, then you may be screwed. However, if you are not also currently using a good router with adequate QoS capability behind the SMC gateway...and before your LAN devices... you may still be able to mitigate the symptom you are seeing by doing so.



SMC_QoS

@comcast.net
reply to EG

said by EG:

said by SMC_QoS :

the easy solution is to get your own SB6121 modem

FWIW, why not get a newer and better SB6141 (eight downstream channel capable) device instead of an older only four downstream capable device ?

Apologies for the off topic post.

Because I don't see the SB6141 listed as an officially approved Comcast Business Class modem at »mydeviceinfo.comcast.net/ and I am not about to recommend that someone use a modem that Comcast may then use as an excuse for not providing support.

Here is the official list; do you see the SB6141 listed?



EG
The wings of love
Premium
join:2006-11-18
Union, NJ
kudos:10

I've already conceded to this... Sorry for the intrusion...


sef

join:2013-07-24
reply to SMC_QoS

said by SMC_QoS :

In your case the problem sounds as if it is directly caused by the asymmetric (not asynchronous) nature of an HFC connection

Note that I said it's behaving AS IF it is synchronous instead of asynchronous.

The asymmetric nature is irrelevant; you can have this particular class of problem with a fully symmetric connection. If it's a synchronous communication, however, then data can only be transmitted in one direction at a time. And that's how it's behaving.

I know IP and above rather well.


SMC_QoS

@comcast.net

said by sef:

said by SMC_QoS :

In your case the problem sounds as if it is directly caused by the asymmetric (not asynchronous) nature of an HFC connection

Note that I said it's behaving AS IF it is synchronous instead of asynchronous.

The asymmetric nature is irrelevant; you can have this particular class of problem with a fully symmetric connection. If it's a synchronous communication, however, then data can only be transmitted in one direction at a time. And that's how it's behaving.

I know IP and above rather well.

In that case, possibly what you really meant to say was half vs full duplex, not asynchronous vs synchronous (and neither of those terms are directly related to IP networking).

And FWIW, the symptom you describe is a classic asymmetric upstream capacity problem, and is a more likely explanation than either a half/full duplex or asynchronous/synchronous misconfiguration in the SMC gateway firmware (although it might not hurt to check the SMC gateway's switch port interface status/settings).

And good luck resolving your problem.

sef

join:2013-07-24
reply to sef

It turns out that the managed switch I got has a bandwidth option. Limiting outbound bandwidth to about 7mbits/sec works around the issue.

The SMC firmware is still badly broken.



SMC_QoS

@comcast.net

said by sef:

It turns out that the managed switch I got has a bandwidth option. Limiting outbound bandwidth to about 7mbits/sec works around the issue...

Some would call that a QoS adjustment (at least that is what my D-Link routers and Netgear managed switches call it).

And you are welcome.


same_ole

@comcast.net

1 edit
reply to SMC_QoS

said by SMC_QoS :

And FWIW, the symptom you describe is a classic asymmetric upstream capacity problem, and is a more likely explanation than either a half/full duplex or asynchronous/synchronous misconfiguration in the SMC gateway firmware (although it might not hurt to check the SMC gateway's switch port interface status/settings).

I think he explained things as clearly as possible with this:

No, this is a new problem. It did not happen before 2 weeks ago.

So, to reiterate: old firmware, this did not happen; new firmware, this does happen.

So how is this not a firmware issue? You completely missed one of the main points.
Expand your moderator at work


mb

join:2000-07-23
Washington, NJ
reply to same_ole

Re: [Business] Comcast Business class -- uploading kills connect

The timing relationship to the firmware update could be anecdotal and the issue could be related to something else. There was a lengthy thread here a while back about how saturating your uplink negatively affects your downlink.


FirebirdTN

join:2012-12-13
Brighton, TN
kudos:1
Reviews:
·Comcast
reply to sef

Sounds to me like the new firmware must have adjusted the modem's buffer size some how.

Your symptoms sound EXACTLY like what happened to me....for three months straight. Glad you mitigated the issue quickly; I lost a lot of hair pulling it out.

And the thread mb is refering too sounds like it was the one I created on this issue:

»Bufferbloat-long read..

-Alan



anon111

@pnap.net

FWIW, I have the same issue with the business that I support- but we were saturating he upload speeds. I found that once we hit out throughput limit comcast starts dropping/delaying inbound traffic with the SMC modem that we have. Long story short we threw more bandwidth (money) at the problem to fix it and have not had an issue since.



SMC_QoS

@comcast.net
reply to same_ole

said by same_ole :

said by SMC_QoS :

And FWIW, the symptom you describe is a classic asymmetric upstream capacity problem, and is a more likely explanation than either a half/full duplex or asynchronous/synchronous misconfiguration in the SMC gateway firmware (although it might not hurt to check the SMC gateway's switch port interface status/settings).

I think he explained things as clearly as possible with this:

No, this is a new problem. It did not happen before 2 weeks ago.

So, to reiterate: old firmware, this did not happen; new firmware, this does happen.

So how is this not a firmware issue? You completely missed one of the main points.

You will not find anywhere in any post I made in this thread any suggestion that the problem was not a firmware issue. The point it that the firmware issue is a lack of QoS support, and that problem has always been present in the SMC gateway (that the OP has only noticed it since the most recent firmware update is simply coincidental/irrelevant).

But speaking of missing the point, apparently you missed the simple point that by externally applying rate limiting QoS, the OP was able to eliminate the upstream saturation problem:

said by sef:

It turns out that the managed switch I got has a bandwidth option. Limiting outbound bandwidth to about 7mbits/sec works around the issue.



SpottedCat

join:2004-06-27
Miami, FL
reply to sef

Here's the problem:

1) Your upload begins. PowerBoost is in effect, so you upload several times faster than the speed you're paying for. (In my case, around 20 megabits, on my 2 megabit upload). TCP windows open up to give you the maximum speed. All is wonderful for a few seconds.
2) PowerBoost period ends. Your upload bandwidth is clamped down to what you are paying for. Since the TCP windows are large for the high transfer speed, packets pile up in the buffers of whatever router is doing the throttling. This introduces high latency.

This latency is what you are feeling. Since there is a ton of latency outbound, transfers in both directions suffer; downloads because the ACKs take forever to go out, and uploads because the data packets are competing with all the others backed up in the pipe.

The solution is a router that does proper outbound QoS and drops packets rather than delay them when they get sent too fast. Unfortunately, you have to forego PowerBoost for this to be fully effective.