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TCP Maintenance and Minor                                        F. Gont
Extensions (tcpm)                                                UTN/FRH
Internet-Draft                                          January 29, 2009
Intended status: Informational
Expires: August 2, 2009


On the problem of long delays between connection-establishment attempts
                                 in TCP
                draft-gont-tcpm-connection-delays-00.txt

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Abstract

   This document discusses a number of solutions to the problem of long
   delays between connection establishment attempts in TCP.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  A more conservative approach  . . . . . . . . . . . . . . . . . 3
   3.  Asynchronous Application Notification . . . . . . . . . . . . . 4
   4.  Issuing several connection requests in parallel . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5
































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1.  Introduction

   This document discusses a number of alternative solutions to the one
   proposed in [I-D.ietf-tcpm-tcp-soft-errors] for the problem of long
   delays between connection establishment attempts in TCP.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


2.  A more conservative approach

   A more conservative approach would be to abort a connection in the
   SYN-SENT or SYN-RECEIVED states only after an ICMP Destination
   Unreachable has been received a specified number of times, and the
   SYN segment has been retransmitted more than some specified number of
   times.

   Two new parameters would have to be introduced to TCP, to be used
   only during the connection-establishment phase: MAXSYNREXMIT and
   MAXSOFTERROR.  MAXSYNREXMIT would specify the number of times the SYN
   segment would have to be retransmitted before a connection is
   aborted.  MAXSOFTERROR would specify the number of ICMP messages
   indicating soft errors that would have to be received before a
   connection is aborted.

   Two additional variables would need to be introduced to store
   additional state information during the connection-establishment
   phase: "nsynrexmit" and "nsofterror".  Both would be initialized to
   zero. "nsynrexmit" would be incremented by one every time the SYN
   segment is retransmitted. "nsofterror" would be incremented by one
   every time an ICMP message that indicates a soft error is received.

   A connection in the SYN-SENT or SYN-RECEIVED states would be aborted
   if nsynrexmit was greater than MAXSYNREXMIT and "nsofterror" was
   simultaneously greater than MAXSOFTERROR.

   This approach would give the network more time to solve the
   connectivity problem.  However, it should be noted that depending on
   the values chosen for the MAXSYNREXMIT and MAXSOFTERROR parameters,
   this approach could still lead to long delays between connection
   establishment attempts, thus not solving the problem.  For example,
   BSD systems abort connections in the SYN-SENT or the SYN-RECEIVED
   state when a second ICMP error is received, and the SYN segment has
   been retransmitted more than three times.  They also set up a
   "connection-establishment timer" that imposes an upper limit on the
   time the connection establishment attempt has to succeed, which



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   expires after 75 seconds [Stevens2].  Even when this policy may be
   better than the three-minutes timeout policy specified in [RFC1122],
   it may still be inappropriate for handling the potential problems
   described in this document.  This more conservative approach has been
   implemented in BSD systems since, at least, 1994 [Stevens2].


3.  Asynchronous Application Notification

   In section 4.2.4.1, [RFC1122] states that there MUST be a mechanism
   for reporting soft TCP error conditions to the application.  Such a
   mechanism (assuming one is implemented) could be used by applications
   to cycle through the destination IP addresses.  However, this
   approach would increase application complexity, and would take a long
   time to kick in, as it would require all existing applications to be
   modified.


4.  Issuing several connection requests in parallel

   For those scenarios in which a domain name maps to several IP
   addresses, several connection requests could be issued in parallel,
   each one to a different destination IP address.  The host would then
   use the first connection attempt to succeed, eliminating the
   potential delay in establishing a connection with the destination
   host.  However, this would mean that every attempt to connect to a
   multihomed host would imply sending several SYN segments, making it
   hard for network operators to distinguish valid connection attempts
   from those performing Denial of Service (DoS) attacks.

   An alternative approach would be as follows.  A host would issue a
   connection request to the first IP address in the list returned by
   the name-to-address mapping function.  If this connection request
   didn't succeed in some time, a connection request to the second IP
   address in the list would be issued in parallel.  If none of these
   connection requests succeeded in some time, and there were still more
   addresses left in the list, they would be tried in the same way.
   While this approach would, in principle, avoid the problems of the
   previous approach, it might be hard to define the time interval to
   wait before issuing each parallel connection request.  A short time
   interval would lead to the problems caused by the previous approach,
   while a long time interval would likely still lead to long delays in
   establishing a connection with the destination host.  In any case, it
   must be noted that both approaches have the same drawbacks as the
   solution described in Section 2: they would increase application
   complexity, and would take too long to begin to be used by
   applications.




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5.  Security Considerations

   To be included in future revisions of this document.


6.  IANA Considerations

   This document has no actions for IANA.


7.  Acknowledgements


8.  References

8.1.  Normative References

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

   [I-D.ietf-tcpm-tcp-soft-errors]
              Gont, F., "TCP's Reaction to Soft Errors",
              draft-ietf-tcpm-tcp-soft-errors-09 (work in progress),
              November 2008.

   [Stevens2]
              Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
              The Implementation", Addison-Wesley , 1994.


Author's Address

   Fernando Gont
   Universidad Tecnologica Nacional / Facultad Regional Haedo
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fernando@gont.com.ar
   URI:   http://www.gont.com.ar





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