[outages] Parital iTunes Podcast Outage?

[Mike Walter]

I can confirm I have also seen a few Podcasts experience the same thing, I have had to force download over and over until they finally complete.  I have not done any technical searching or captures yet.

-Mike

-----Original Message-----
From: Outages [mailto:outages-bounces at outages.org] On Behalf Of Jeremy Chadwick via Outages
Sent: Friday, July 11, 2014 4:26 PM
To: jrk1231-outml at nym.hush.com
Cc: outages at outages.org
Subject: Re: [outages] Parital iTunes Podcast Outage?

On Fri, Jul 11, 2014 at 03:18:10PM -0400, jrk1231-outml at nym.hush.com wrote:
> Sigh... Went back and tried pcap on a refresh several times, and got a
> ton of out of order and duplicate packets every time, followed by a
> fail like previous. I never saw the certificate but I did see several
> "Encrypted Alert" messages that Wireshark would not further detail. 
> 
> Jeremy, I will send you the small pcap off list if you don't mind, and
> maybe you can decipher what is going on.

I reviewed the pcap.  I'm going to ignore the out-of-order complaints
because a NAT'd router is involved (not saying it's to blame though, it
could be upstream network stuff, Internet stuff, or close-to-server-side
stuff).  The important part is that it doesn't impact the end goal in
this case (figuring out SSL cert validity).  I will say that the
out-of-order stuff is a bit annoying in this case (especially right
before socket closure).

The certificate is in packet #13.  The cert consists of a two-stage
VeriSign CA plus the itunes.apple.com cert itself.  If you expand each
of those (signedCertificate -> validity) you'll see the dates printed in
YY-MM-DD hh:mm:ss format per UTC time.  The dates are as follows:

itunes.apple.com - expires 2016-04-20
VeriSign Class 3 Extended - expires 2016-11-07
VeriSign Class 3 Public - expires 2021-11-07

I also verified this via openssl s_client -connect itunes.apple.com:443
(see madboa.com for details).  So it's not an expired cert that's
causing this behaviour, and there's more to verify that:

In the same TCP session I was looking at ("tcp.stream eq 0",
communication between lanip:55217 and 23.72.242.217:443), packets 21
through 29 seem to indicate (to me) that there is an actual conversion
going on between client and server.  The segments are smaller than
MSS/MTU and there's no reassembly.  If I had to take a guess, it looks
like after cipher negotiation, the client submits its HTTP headers +
payload (packets 21-23), the server responds with some HTTP headers +
possibly a small payload (packet 25), followed by the server issuing a
TLS Encrypted Alert message (packet 27) which the client sends back
(packet 29), followed by the client sending FIN+ACK (30), the server
sending FIN+ACK (31), then out-of-order stuff gets in the way (32, but
its in reply to sequence 1854 which is packet 30), followed by the
server sending 3 TCP RSTs 0.043 seconds later (which may be normal for
SSL).

The oddity of the last bits of communication of session 0 made me look
at session 1 ("tcp.stream eq 1") which looks like a significantly more
clear communication/"things are working" up until a certain point where
I can tell the capture snaplen was probably too small (packet 71)
followed by a bunch of duplicate ACKs and retransmissions.

The behaviour in session 1 is indicative of some kind of network-level
anomaly.  I see things like the client repeatedly sending duplicate ACK
packets back to the server for ack seq 22480 but the server never sends
that / acts in some way like it did (but the client never got it).  This
sort of behaviour continues for quite some time, combined with further
TCP retransmits.  Like I said, this kind of behaviour seems to imply
some kind of network-level anomaly and possibly packet loss but I
couldn't tell you where.

The TCP aspect of things is anomalous enough to make me think the true
issue is at a lower layer.  If everything looked "mostly" as clean as
session 0 then I'd be more concerned with the payload that's being
encrypted (and that's the biggest problem with use of SSL: if there is
something application-level / layer 7 going on, you have virtually no
way of debugging it.  I often laugh at SSL for this reason -- great, you
have your security, good for you and the assumption that nothing will
ever go wrong + troubleshooting will never be needed other than at the
physical client application + physical server application layer).

Sorry I can't be of more help, but I can at least rule out server
certificate expiry as the root cause.  :-)

-- 
| Jeremy Chadwick                                   jdc at koitsu.org |
| UNIX Systems Administrator                http://jdc.koitsu.org/ |
| Making life hard for others since 1977.             PGP 4BD6C0CB |

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