Re: [tcpm] Is this a problem?

MURALI BASHYAM <> Tue, 30 October 2007 06:11 UTC

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Date: Mon, 29 Oct 2007 22:44:52 -0700
Subject: Re: [tcpm] Is this a problem?
To: Joe Touch <touch@ISI.EDU>, Mahesh Jethanandani <>
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----- Original Message ----
From: Joe Touch <touch@ISI.EDU>
To: Mahesh Jethanandani <>
Sent: Monday, October 29, 2007 3:42:41 PM
Subject: Re: [tcpm] Is this a problem?

Mahesh Jethanandani wrote:
> Folks,
> We have documented a case of HTTP servers that are prone to resource
> starvation with the use of a small user level program. The program
> not require any special privileges or changes in the kernel. The user
> level program on the client opens a connection to a HTTP server,
 sends a
> GET request for a large file (larger than the advertised window of
> client) but never reads the response.
> Three well-known, public sites were tested for this vulnerability. 
> The two most common HTTP servers, Apache and IIS were the target.
> While one site had put mitigation technique in place, the others had
> none. With the latter two we were able to hold connections in
> ESTABLISHED state for days. The former site had a mitigation in place
> with a fixed timeout of 11 min., which was easy to guess and work
> around.
> We (the authors) believe that this is a huge problem. What do you
> folks feel?

I agree with Caitlin and your previous responses; this is an

So we are placing the responsibility for protecting system-wide TCP resources such as buffers
and connections on all the TCP based applications out there? Which "application"
are you referring to here? BGP? NFS ? CIFS? HTTP? SMTP? FTP? The entire application
community? Potentially several or all of these apps are vulnerable. How do we bring all these 
protocols into a single agreeable framework and  accomplish the right thing? 

> Previous responses to this documentation has been that it is a
> application problem. It is clear from our experimentation that most
> servers (and FTP too) have not implemented any mitigation techniques.
> believe that this problem exists across the whole range of TCP based
> applications prevalent on the internet, although our experiments were
> limited to the web application. Where applications have tried to put
> mitigation techniques in place, workaround has been easy. This is
> because applications do not have the same amount of visibility as TCP
> does on the state of the connection.

Applications know when a connection hasn't closed. They also know when
write would block due to insufficient socket resources. Either or both
of these provide sufficient visibility to avoid the problem entirely.

Applications cannot discriminate between a connection that's stalled due to network
congestion and a connection that's being stalled by a malicious receiver who has closed
his receive window forever. And for how long the connection has been in in this stalled state etc.

And what about multi-application servers? Surely each application here is running as an
independent process and indvidually they don't have the system wide awareness that TCP has...

As you note, applications can solve this easily. Since it's their
obligation to do so, and it's easy for them to do so, there's no reason
to complicate the transport layer with this responsibility.

Applications can implement a fixed or even an adaptive timeout, but unless they can make
the above distinction i am pointing out, they will be knocking out legitimate (albeit slow) 
connections, i.e there will be a fairness issue. Applications do not have visibility into TCP connection
sender's state to solve the fairness issue. A solution which penalizes legitimate connections is clearly a broken one.

Furthermore, does the propose solution solve the DOS problem? Aren't
there other ways to keep a source stalled? (i.e., what about continued
SACKs? or repeated ACKs indicating a lost segment? at the very least,
could ACK a byte at a time, sending 3-4 duplicate ACKs for each byte,
which would keep the sender from opening its window and stall things a

How do you propose to carry out these attacks without changing the client side TCP stack in the 
windows or linux kernel? The vulnerability which we have demonstrated and addressed requires a simple user 
level program which anyone with a little, basic programming background can write today. Clearly the bar to creating these attacks you are pointing out is higher.


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