[VOIPSEC] RE: TLS as the SIP security mechanism

Brian Rosen br at brianrosen.net
Tue Aug 9 05:04:45 PDT 2005


There are one heck of a lot of web servers that do this; I can't imagine
what ills you are worried about that the web servers don't handle other than
the idea that you tend to keep the TLS connection longer lived on a SIP
connection than you do on most web connections.  There are "web services"
that DO keep connections alive.  

Of course, I'm not sure why you would want to have a single SIP proxy with
hundreds of thousands of connections on it; seems like a poor system design,
but your mileage may vary.

Then again, why would a failure that requires you to recreate the TLS
connection count as an outage?  Certainly it would not be visible to a
subscriber.  Remember you have three basic states:
	No call in progress, active registration
	Call in the middle of being established
	Call established, still in progress

Proxy outages typically have zero effect on the first and third cases.  It
may be that the session establishment gets abandoned on a proxy failure
(case 2), but a retry on the end user agent would succeed.  Not user
visible.

I know there is brain power being applied to recovery from restart
avalanche.  This is what happens if the service actually goes down hard, for
example, if a large area has a power outage that takes down the CPE, and
then the power comes back on.  

I'm watching DTLS.  May be interesting.

Brian

-----Original Message-----
From: Voipsec-bounces at voipsa.org [mailto:Voipsec-bounces at voipsa.org] On
Behalf Of Geoff Devine
Sent: Monday, August 08, 2005 9:03 AM
To: Voipsec at voipsa.org
Subject: [VOIPSEC] RE: TLS as the SIP security mechanism

I believe TLS is a poor choice as a security mechanism.  It's unfortunate
that it was placed in RFC 3261 as the recommended security method.  TLS must
be run over a reliable transport (TCP).  TCP has all kinds of scaling issues
when you're running hundreds of thousands of instances of TCP on a big
Carrier-class SIP proxy.  It's almost impossible to make TCP redundant so if
the SIP proxy bounces, all the SIP user agents have to establish new TCP
connections and then negotiate new security associations.  This creates an
avalanche of SYN messages and looks just like a classic SYN attack.  I don't
know how you achieve carrier-class five 9's availability in this kind of
environment since your 10 minute outage budget for the year is blown out of
the water any time you bounce a SIP proxy.
 
There's an IETF draft for a security mechanism called DTLS.  DTLS runs over
UDP so it doesn't suffer the TCP scaling and starup issues.  The only issue
with UDP-based SIP is that you need to keep the messages "small" to avoid
fragmentation.  SIP messages are typically fairly small but they can explode
in size when you start doing media security key exchanges embedded in your
SIP messaging using MIKEY or SDESCRIPTIONS.  We're seeing sizes get bigger
and bigger.  For example, in the session description protocol, you can't
offer buth SRTP and RTP due to a difficiency in the way SDP was specified.
The work-around is to offer two different instances of SIP as multipart
MIME.  Add in media security keys and Proxy-inserted headers to do things
like assert identity and you start blowing through 1500 bytes.  At that
point, you have two choices... either use a transport like TCP to deal with
message fragmentation (a poor choice, in my opinion), or start compressing
the signaling using methods like SIGCOMP (RFC 3320/3321).
 
Sigh.... so much for SIP being a simple, lightweight protocol.
 
Geoff
 
 

________________________________________

Date: Thu, 4 Aug 2005 14:41:45 -0700
From: John Todd <jtodd at loligo.com>
Subject: Re: [VOIPSEC] Are there any SIP client and proxy server that
        supports        IPSec or transport layer security ( hop-by-hop
protection)?
To: Voipsec at voipsa.org
Message-ID: <p06020411bf1839520b0f at loligo.com>
Content-Type: text/plain; charset="iso-8859-1" ; format="flowed"

At 9:27 AM -0600 8/4/05, Pankaj Shroff wrote:
>
>I think implementing TLS in the Sip User Agent is a prerogative of the User
>Agent developer. I think almost all commercial SIP products out there (such
>as software SIP User Agents, Proxies, etc., as well as Session Border
>Controllers) should support TLS. I am not so sure about IPSec support
>however. Also, S-RTP is still too new. I do not know of any product
>available today that supports S-RTP out of the box. Good luck on your
>project.
>  Pankaj
>
>  On 8/4/05, dennis <m8939605 at yahoo.com.tw> wrote:
>>
>>  I intend to start a project(client) that will
>>  implement TLS, IPsec(secure SIP) and secure RTP. I'm
>>  surveying the products(client and server) that support
>>  TLS or IPsec. But
>>  I don't find any SIP client product that support TLS
>>  or IPsec. Does anyone know such SIP security products ?
>>
>>  __________________________________________________
>>  ëzë¶éûùæìûêVêMí ímÅH
>>  înè"â?çZYahoo!äÔñÄë¶éûí êu
>  > http://messenger.yahoo.com.tw/
>  >
>--
>Pankaj Shroff
>shroffG at Gmail.com


I believe that the Sipura and Zultys platforms
support both TLS and SRTP.  However, both
companies have done what I belive to be a
terrible job in advertising and promoting this
capability, and there are very few SIP
proxies/gateways which support either protocol.
Zultys sells their own proxy/gateway, so is not
inclined to help develop anything.  I'm sure
there are others - Google should help you out
here.

More open-source efforts towards implementing TLS
and SRTP would be welcome, as far as I'm
concerned.  The number of locations using
Asterisk as both a SIP element and gateway
element would make it a natural choice for such a
stack to be implemented for open-source
environments.  The topic of SRTP and TLS comes up
every 6 months or so and gets a lot of approval,
but the important half of "rough consensus and
running code" never happens.

JT



------------------------------

Message: 3
Date: Sat, 6 Aug 2005 00:40:38 +0200
From: Johan Bilien <jobi at via.ecp.fr>
Subject: Re: [VOIPSEC] Are there any SIP client and proxy server that
        supports        IPSec or transport layer security ( hop-by-hop
protection)?
To: dennis <m8939605 at yahoo.com.tw>
Cc: Voipsec at voipsa.org
Message-ID: <20050805224037.GA28110 at via.ecp.fr>
Content-Type: text/plain; charset=us-ascii

On Thu, Aug 04, 2005, dennis wrote:
> I intend to start a project(client) that will
> implement TLS, IPsec(secure SIP) and secure RTP. I'm
> surveying the products(client and server) that support
> TLS or IPsec. But
> I don't find any SIP client product that support TLS
> or IPsec. Does anyone know such SIP security products ?

Have a look at minisip (http://www.minisip.org/). It supports TLS for
SIP messages, and both SRTP and IPsec to protect the media.

Johan.



------------------------------

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