[p2p-hackers] Why UDP and not TCP?

Greg Bildson gbildson at limepeer.com
Wed Dec 1 00:00:06 UTC 2004


Yeah, the code has been in beta for a while.

It is UDP based.  If all UDP is blocked then it won't work and wont be
attempted.  We do a lot to first ensure that you can receive solicited UDP
which is what this feature relies on.  i.e. If you send a packet to X via
UDP and you are behind a NAT/Firewall, you can receive a response back.  In
LimeWire terms, you are then said to be firewall capable and your searches
and responses indicate this.

There are many ways to negotiate the initiation of the connection on both
sides.  LimeWire has a concept of a push proxy for firewalled hosts so we
actually use that to deliver a special PUSH message that tells the host to
initiate a UDP connection to ip:port.  Both ends then start sending UDP
messages at each other and shortly thereafter, they should both be able to
receive those messages.  A type of TCP style connection negotiation begins
from there.  Just to be clear, this is not proxying.  The only thing that is
proxied, is the PUSH message to trigger the actions of the uploader.

The FAQ is out of date.

  -----Original Message-----
  From: p2p-hackers-bounces at zgp.org [mailto:p2p-hackers-bounces at zgp.org]On
Behalf Of David Barrett
  Sent: Tuesday, November 30, 2004 6:28 PM
  To: 'Peer-to-peer development.'
  Subject: RE: [p2p-hackers] Why UDP and not TCP?

  How does the Firewall-to-Firewall portion of Limewire work?  Does it use
un-firewalled clients as relay servers?  It doesn't sound like it, but I
thought that's the only solution that truly works in all situations.

  The "features history" page mentions this on the entry for 8.12.2004:

  "Firewall to Firewall transfers allows two people behind firewalls to
connect directly to each other and transfer data. This makes use of UDP, and
a third party to coordinate the initial messaging. . Normally, firewalled
users would only be able to download from other hosts who are not
firewalled, which is of course severely limited. With firewall to firewall
transfers, firewalled users can now access the full 100% of hosts."

  This implies something like the NAT-to-NAT trick works with firewalls
also.  I'm a little shaky on how UDP works with firewalls, do both clients
initiate a conversation with a third party, and then the third party hands
back information IP/port information of the pre-established out-bound
connection?  How does this work if the firewall simply blocks all UDP

  However, the website is either out of date or there's more to the story
because the FAQ says:


  "Q: What if I'm behind a firewall?

  A: LimeWire will work when a user is behind certain types of firewalls,
but will not work behind certain other types. If you are behind a firewall,
you will not be able to download anything from a user that's also behind a
firewall. In general, if you can connect (you will see your "connection
status" in the lower left hand corner of the application) using LimeWire,
you should be able to download and upload files, but LimeWire will not work
if you have either a web-only proxy or a SOCKS proxy."

  What's the full story?



  From: p2p-hackers-bounces at zgp.org [mailto:p2p-hackers-bounces at zgp.org] On
Behalf Of Greg Bildson
  Sent: Tuesday, November 30, 2004 2:12 PM
  To: Peer-to-peer development.
  Subject: RE: [p2p-hackers] Why UDP and not TCP?

  If you believe that there are problems with LimeWire, you should submit
them to bugs at limewire and they will be looked into promptly.   If you have
not already, you should also upgrade to version 4.2.3 to get rid of some
potential startup issues with old GWebcaches.

  LimeWire is a "good new" p2p application - check out that
firewall-to-firewall transfer in the new version.  ;)



    -----Original Message-----
    From: p2p-hackers-bounces at zgp.org [mailto:p2p-hackers-bounces at zgp.org]On
Behalf Of Digitalgruvmoves at aol.com
    Sent: Monday, November 29, 2004 9:10 PM
    To: p2p-hackers at zgp.org
    Subject: Re: [p2p-hackers] Why UDP and not TCP?

    Whats a good new p2p filesharing download to use? Limeware just started
acting nuts.
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