[chord] Chord and SIP

[Michael Walfish]

Hi Alan,

You seem to be interested in the possible interactions between SIP and
Chord or, more generally, between SIP and distributed hash tables (DHTs).
(Chord is a particular DHT design.) If I'm misunderstanding your question,
please let me know.

As you imply in your original note, there seem to be two ways DHTs and SIP
can interact, and these two are logically separate but probably exclusive:
(1) the DHT (assuming its nodes have public IP addresses) can help SIP with
the rendezvous function simply because the DHT implements a put-get
interface on (key,value) pairs or (2) SIP can help the DHT nodes (which
might be firewalled or NATed) establish point-to-point connections with
each other.

If the DHT is being run as a service (like OpenDHT, which is something you
should check out if you've not yet heard of it; http://www.openhash.org),
then the DHT could help with (1). If the DHT is running on end-users'
desktops to do file sharing or telephony (e.g., the Kademlia DHT, which is
inside Overnet), then SIP might help the DHT nodes communicate without the
DHT software having to implement firewall-punching logic, which logic has
been implemented in several peer-to-peer apps, skype being a notable
example, as you mention. A paper by Paul Francis at SIGCOMM's Network
Architecture workshop argues that SIP would be useful for exactly this
purpose. The paper is:

www.cs.cornell.edu/People/francis/nutss-fdna.pdf 

Also (and this is probably off-topic; sorry) some in the HIP community
have suggested that SIP could could be useful for HIP or vice-versa. If
you're curious about this, please let me know.

-Mike

| Date: Wed, 26 Jan 2005 01:47:18 +0000
| From: "Johnston, Alan" <alan.johnston at mci.com>
| To: Emil Sit <sit at mit.edu>
| Cc: chord at amsterdam.lcs.mit.edu
| Subject: RE: [chord] Chord and SIP
| 
| Hi Emil,
| 
| Thanks for your reply.  I'm reading up on Twine - it looks interesting.
| It seems CFS might also be useful to SIP as a lookup mechanism for
| rendezvous.
| 
| SIP is an application layer protocol used to establish, modify, and
| terminate media sessions over the Internet.  It has some built in
| rendezvous capabilities that allow devices with dynamic IP addresses or
| that change their point of attachment to the Internet to be reachable
| using a SIP URI.
| 
| I'm interested in finding out about the Chord protocol "on the wire"
| between peers over the Internet.  The documentation I've read so far
| says that it uses sends RPCs over UDP using ephemeral ports.  Are there
| any plans to use TLS as a transport?  Use of certificates for peer
| authentication?
| 
| Transporting Chord over SIP would allow the use of some of SIP's
| security and authentication mechanisms (such as TLS, S/MIME, etc.) but
| I'm still not sure that it is a good justification.
| 
| Thanks for all your help.
| 
| Thanks,
| Alan Johnston
| sip:alan at sipstation.com
| 
| > -----Original Message-----
| > From: Emil Sit [mailto:sit at MIT.EDU] 
| > Sent: Tuesday, January 25, 2005 11:02 AM
| > To: Johnston, Alan
| > Cc: chord at amsterdam.lcs.mit.edu
| > Subject: Re: [chord] Chord and SIP
| > 
| > 
| > Alan,
| > 
| > On Tue, 25 January 2005 at 13:34 (+0000), Johnston, Alan wrote:
| > > opinion with others looking into P2P.  I had assumed that SIP could 
| > > utilize the Chord protocol for discovery (use a SIP URI as a search 
| > > key and get back the IP address of the node that has the current 
| > > address of that URI) then run as normally.  However, others are 
| > > proposing that the Chord DHT algorithm be run *over* SIP.  That is, 
| > > messages between peers used to establish, modify, and query peers 
| > > would be carried over SIP messages.
| > 
| > I don't think any of us are very familiar with SIP and its 
| > capabilities.
| > 
| > Chord provides a mechanism for providing rendezvous services 
| > efficiently. If you needed, you could use Twine [1] for 
| > service discovery.  Or you could build your own service to 
| > layer on top of Chord so that you can provide the kinds of 
| > performance and data availability that your application requires.
| > 
| > Whether Chord is run separately (for example, using our 
| > implementation) or over SIP seems orthogonal to how it is 
| > used by an application. I imagine there would be some loss of 
| > efficiency in running the Chord protocols over SIP, depending 
| > on how things are encoded.
| > 
| > [1] http://nms.lcs.mit.edu/projects/twine/
| > 
| > -- 
| > Emil Sit / MIT CSAIL PDOS / http://pdos.lcs.mit.edu/chord/  
| > 
| 
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