Category: TCP/IP

Intro
This is probably the hardest case I’ve ever encountered. It’s so complicated many people needed to get involved to contribute to the solution.

Initial symptoms
It’s not easy to describe the problem while providing appropriate obfuscation. Over the course of a few days it came to light that in this particular large company for which I consult many people in office locations connected via an MPLS network were no longer able to log in to Cisco Jabber. That’s Cisco’s offering for Instant Messaging. When it works and used in combination with Cisco IP phones it’s pretty good – has some nice features. This major problem was first reported November 17th.

Knee-jerk reactions
Networking problem? No. Network guys say their networks are running fine. They may be a tad overloaded but they are planning to route Internet over the secondary links so all will be good in a few days.
Proxy problem? Nope. proxy guys say their Bluecoat appliances are running fine and besides everyone else is working.
Application problem? Application owner doesn’t see anything out of the ordinary.
Desktop problem? Maybe but it’s unclear.

Methodology
So of the 50+ users affected I recognized two power users that I knew personally and focussed on them. Over the course of days I learned:
– problem only occurs for WAN (MPLS) users
– problem only occurs when using one particular proxy
– if a user tries to connect often enough, they may eventually get in
– users can get in if they use their VPN client
– users at HQ were not affected

The application owner helpfully pointed out the URL for the web-based version of Cisco Jabber: https://loginp.webexconnect.com/… Anyone with the problem also could not log in to this site.

So working with these power users who patiently put up with many test suggestions we learned:

– setting the PC’s MTU to a small value, say 512 up to 696 made it work. Higher than that it generally failed.
– yet pings of up to 1500 bytes went through OK.
– the trace from one guy’s PC showed all his packets re-transmitted. We still don’t understand that.
– It’s a mess of communications to try to understand these modern, encrypted applications
– even the simplest trace contained over 1000 lines which is tough when you don’t know what you’re looking for!
– the helpful networking guy from the telecom company – let’s call him “Regal” – worked with us but all the while declaring how it’s impossible that it’s a networking issue
– proxy logs didn’t show any particular problem, but then again they cannot look into SSL communication since it is encrypted
– disabling Kaspersky helped some people but not others
– a PC with the problem had no problem when put onto the Internet directly
– if one proxy associated with the problem forwarded the requests to another, then it begins to work
– Is the problem reproducible? Yes, about 99% of the time.
– Do other web sites work from this PC? Yes.

From previous posts you will know that at some point I will treat every problem as a potential networking problem and insist on a trace.

Biases going in
So my philosophy of problem solving which had stood the test of time is either it’s a networking problem, or it’s a problem on the PC. Best is if there’s a competition of ideas in debugging so that the PC/application people seek to prove beyond a doubt it is a networking problem and the networking people likewise try to prove problem occurs on the PC. Only later did I realize the bias in this approach and that a third possibility existed.

So I enthused: what we need is a non-company PC – preferably on the same hardware – at the same IP address to see if there’s a problem. Well we couldn’t quite produce that but one power user suggested using a VM. He just happened to have a VM environment on his PC and could spin up a Windows 7 Professional generic image! So we do that – it shows the problem. But at least the trace form it is a lot cleaner without all the overhead of the company packages’ communication.

The hard work
So we do the heavy lifting and take a trace on both his VM with the problem and the proxy server and sit down to compare the two. My hope was to find a dropped packet, blame the network and let those guys figure it out. And I found it. After the client hello (this is a part of the initial SSL protocol) the server responds with its server hello. That packet – a largeish packet of 1414 bytes – was not coming through to the client! It gets re-transmitted multiple times and none of the re-transmits gets through to the PC. Instead the PC receives a packet the proxy never sent it which indicates a fatal SSL error has occurred.

So I tell Regal that look there’s a problem with these packets. Meanwhile Regal has just gotten a new PC and doesn’t even have Wireshark. Can you imagine such a world? It seems all he really has is his tongue and the ability to read a few emails. And he’s not convinced! He reasons after all that the network has no intelligent, application-level devices and certainly wouldn’t single out Jabber communication to be dropped while keeping everything else. I am no desktop expert so I admit that maybe some application on the PC could have done this to the packets, in effect admitting that packets could be intercepted and altered by the PC even before being recorded by Wireshark. After all I repeated this mantra many times throughout:

This explanation xyz is unlikely, and in fact any explanation we can conceive of is unlikely, yet one of them will prove to be correct in the end.

Meanwhile the problem wasn’t going away so I kludged their proxy PAC file to send everyone using jabber to the one proxy where it worked for all.

So what we really needed was to create a span port on the switch where the PC was plugged in and connect a 2nd PC to a port enabled in promiscuous mode with that mirrored traffic. That’s quite a lot of setup and we were almost there when our power user began to work so we couldn’t reproduce the problem. That was about Dec 1st. Then our 2nd power user also fell through and could no longer reproduce the problem either a day later.

10,000 foot view
What we had so far is a whole bunch of contradictory evidence. Network? Desktop? We still could not say due to the contradictions, the likes of which I’ve never witnessed.

Affiliates affected and find the problem
Meanwhile an affiliate began to see the problem and independently examined it. They made much faster progress than we did. Within a day they found the reason (suggested by their networking person from the telecom, who apparently is much better than ours): the server hello packet has the expedited forwarding (EF) flag set in the differentiated code services point (DSCP) section of the IP header.

Say what?
So I really got schooled on this one. I was saying It has to be an application-aware “something” on the network or PC that is purposefully messing around with the SSL communication. That’s what the evidence screamed to me. So a PC-based firewall seemed a strong contender and that is how Regal was thinking.

So the affiliate explained it this way: the company uses QOS on their routers. Phone (VOIP) gets priority and is the only application where the EF bit is expected to be set. VOIP packets are small, by the way. Regular applications like web sites should just use the default QOS. And according to Wikipedia, many organizations who do use QOS will impose thresholds on the EF pakcets such that if the traffic exceeds say 30% of link capacity drop all packets with EF set that are over a certain size. OK, maybe it doesn’t say that, but that is what I’ve come to understand happens. Which makes the dropping of these particular packets the correct behaviour as per the company’s own WAN contract and design. Imagine that!

Smoking gun no more
So now my smoking gun – blame it on the network for dropped packets – is turned on its head. Cisco has set this EF bit on its server hello response on the loginp.webexconnect.com web site. This is undesirable behaviour. It’s not a phone call after all which requires a minimum jitter in packet timing.

So next time I did a trace I found that instead of EF flag being set, the AF (Assured Forwarding) flag was set. I suppose that will make handling more forgiving inside the company’s network, but I was told that even that was too much. Only default value of 0 should be set for the DSCP value. This is an open issue in Cisco’s hands now.

But at least this explains most observations. Small MTU worked? Yup, those packets are looked upon more favorably by the routers. One proxy worked, the other did not? Yup, they are in different data centers which have different bandwidth utilization. The one where it was not working has higher utilization. Only affected users are at WAN sites? Yup, probably only the WAN routers are enforcing QOS. Worked over VPN, even on a PC showing the problem? Yup – all VPN users use a LAN connection for their proxy settings. Fabricated SSL fatal error packet? I’m still not sure about that one – guess the router sent it as a courtesy after it decided to drop the server hello – just a guess. Problem fixed by shutting down Kaspersky? Nope, guess that was a red herring. Every problem has dead ends and red herrings, just a fact of life. And anyway that behaviour was not very consistent. Problem started November 17th? Yup, the affiliate just happened to have a baseline packet trace from November 2nd which showed that DSCP was not in use at that time. So Cisco definitely changed the behaviour of Cisco Jabber sometime in the intervening weeks. Other web sites worked, except this one? Yup, other web sites do not use the DSCP section of the IP header so it has the default value of 0.

Conclusion
Cisco has decided to remove the DSCP flag from these packets, which will fix everything. Perhaps EF was introduced in support of Cisco Jabber’s extended use as a soft phone??? Then this company may have some re-design of their QOS to take care of because I don’t see an easy solution. Dropping the MTU on the proxy to 512 seems pretty drastic and inefficient, though it would be possible. My reading of TCP is that nothing prevents QOS from being set on any sort of TCP packet even though there may be a gentleman’s agreement to not ordinarily do so in all except VOIP packets or a few other special classes. I don’t know. I’ve really never looked at QOS before this problem came along.

The company is wisely looking for a way to set all packets with DSCP = 0 on the Intranet, except of course those like VOIP where it is explicitly supposed to be used. This will be done on the Internet router. In Cisco IOS it is possible with a policy map and police setting where you can set set-dscp-transmit default. Apparently VPN and other things that may check the integrity of packets won’t mind the DSCP value being altered – it can happen anywhere along the route of the packet.

Boy applications these days are complicated! And those rare times when they go wrong really require a bunch of cooperating experts to figure things out. No one person holds all the expertise any longer.

My simplistic paradigm of its either the PC or the network had to make room for a new reality: it’s the web site in the cloud that did them in.

Could other web sites be similarly affected? Yes it certainly seems a possibility. So I now know to check for use of DSCP if a particular web site is not working, but all others are.

References and related
This Wikipedia article is a good description of DSCP: https://en.wikipedia.org/wiki/Differentiated_services