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Versions: 00                                                            
   IETF TSVWG Working Group
   Internet Draft                                          Killyeon Kim
                                                          Youngjun Park
                                                           Kyungjoo Suh
                                                          Yongseok Park
   Document: draft-kim-tsvwg-butrigger-00.txt                   Samsung
                                                            Electronics
   Expires: August 2004                                   February 2004


                    The BU-trigger method for improving
                      TCP performance over Mobile IPv6


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.


Abstract

   In Mobile IPv6 environment, TCP connections between MN (mobile node)
   and CN (correspondent node) suffer a significant degradation in
   performance in the form of poor throughput and very high interactive
   delays during handoff. This note describes an optional modification
   of TCP's congestion control mechanism for performance enhancement by
   using Binding Update messages to invoke TCP's congestion control
   processing. This modification is applied to TCP of CN side, and no
   additional processing requirement is added to MN to save MN's power.






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Conventions used in this document

   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.

Table of Contents

   1. Introduction...................................................2

   2. Overview of BU-trigger Mechanism...............................3

   3. Operation of BU-trigger Mechanism..............................4

      3.1. Mobile IPv6 Extension for BU-trigger Mechanism............4

      3.2. TCP Extension for BU-trigger Mechanism...........5

   4. Interoperability with Mobile IPv6/TCP..........................5

   References........................................................6

   Author's Addresses................................................6


1. Introduction

   Most of Internet services such as E-mail, ftp, and WWW employ TCP
   (Transmission Control Protocol) as their transport protocol. It is
   known that TCP is a reliable transport protocol tuned to perform well
   in traditional networks made up of wired links and stationary hosts.

   With the development of the wireless communication and Internet
   services, wireless networking is facing a rapidly growing need for
   efficient communication mechanisms that functionally integrate
   wireless and wired Internet components across their varying and
   distinctive transmission characteristics. Compared to wired networks,
   a wireless link has much less bandwidth. In addition, a wireless link
   has higher bit error rate due to its vulnerability to interference
   and disconnection. Therefore packet loss in wireless networks
   happens frequently and the response will take longer.

   In such environment, TCP suffers a significant degradation in
   performance in the form of poor throughput and very high interactive
   delays. This behavior arises due to the fact that errors and handoffs



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   on the wireless links are incorrectly interpreted as congestion by
   TCP at the sender.

   This note proposes a mechanism called BU-trigger to recover the end-
   to-end performance of TCP that is degraded during handoffs. The BU-
   trigger mechanism aims to extend the functions of TCP and Mobile IPv6
   protocols in CN side to improve the performance of TCP.

   The BU-trigger mechanism is motivated by the followings:

      (1) It increases the end-to-end performance of TCP without
          modification of TCP/Mobile IPv6 protocols in MN side.
      (2) It is interoperable with the original TCP/Mobile IPv6 protocol.
      (3) No additional processing power consumption is added to MN side.


2. Overview of BU-trigger Mechanism

   The BU-trigger mechanism is aimed to improve the end-to-end
   performance of TCP in Mobile IPv6 environment. This method uses the
   Binding Update messages which are sent from MN to CN to trigger TCP's
   operation in CN side in order to recover performance degradation
   during handoffs.

   The BU-trigger method is aimed to recover cwnd (congestion window
   size) of TCP connections which has been reduced to 1 because of
   timeout in CN side to improve the performance of TCP [APS99]. The BU-
   trigger extends Mobile IPv6 and TCP in CN side to achieve the
   performance enhancement goal. In Mobile IPv6 layer, MN will send
   Binding Update messages to CN to tell the new care-of-address when MN
   moves to another domain. The valid binding update message received by
   CN means handoff is over and CN can send packets to the MN. But
   because handoff latency is longer than TCP's RTO (Retransmission
   Timeout) in most of handoffs, the cwnd has been reduced.

   The key point of BU-trigger mechanism is summarized as follows:

      (1) Extension of Mobile IPv6 to generate BU-trigger signal to TCP
          layer.
      (2) Extension of TCP to remember the current congestion window size
          (cwnd) and slow start threshold size (ssthresh) when timeout
          occurs.
      (3) Extension of TCP to recover the cwnd and ssthresh to the backup
          values when TCP gets BU-trigger signal.








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3. Operation of BU-trigger Mechanism

   The BU-trigger mechanism is aimed to extend mobile IPv6 [Joh02] and
   TCP [Pos81] functions in CN side.

   Figure 1 shows the protocol stack of CN. The arrows in Figure 1 stand
   for BU-trigger signal sent from Mobile IPv6 to TCP layer when CN gets
   valid Binding Update messages.


                         +---------------------+
                         |     Application     |
                         +---------------------+
                         |         TCP         |
                         |       ^  ^  ^       |
                         +-------+--+--+-------+
                         |       |  |  |       |
                         |    Mobile IPv6      |
                         + - - - - - - - - - - +
                         |         IPv6        |
                         +---------------------+
                         |         MAC         |
                         +---------------------+
                         |         PHY         |
                         +---------------------+

                        Figure 1 CN Protocol Stack


   The operations of BU-trigger mechanism can be divided into Mobile
   IPv6 part and TCP part. The extension functions of these parts will
   be explained in the following sections.


   3.1. Mobile IPv6 Extension for BU-trigger Mechanism

   In Mobile IPv6 layer of CN side, the CN will perform some validation
   checks before accepting a Binding Update. When CN accepts a valid
   Binding Update, it will perform some extension functions needed by
   the BU-trigger mechanism.

   The operation of BU-trigger mechanism in Mobile IPv6 is as follows.

      (1) Validate the Binding Update
      (2) In the case of valid Binding Update, CN makes BU-trigger signal
          according to the information in Binding Update.
      (3) Invoke operation of TCP layer by sending BU-trigger signal to
          TCP.



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   The trigger signal sent from MIPv6 to TCP layer should include MN's
   home address because TCP layer needs to know which TCP connections
   are related to the MN that sends Binding Update. TCP just need to
   deal with the connections related to the current MN.

   The method for delivering BU-trigger signal to TCP layer depends on
   implementation. It can be delivered to TCP using IPC(Inter Process
   Call) method, or it can be included in local delivery TCP packet. The
   detailed description of BU-trigger signal contents and delivery
   methods is out of the scope of this document.


   3.2. TCP Extension for BU-trigger Mechanism

   In order to recover TCP performance, we need to recover the
   congestion window size(cwnd) and slow start threshold size(ssthresh)
   to the values when retransmission timer is timeout because of handoff.
   The BU-trigger mechanism adds previous cwnd (pcwnd) and previous
   ssthresh (pssthresh) fields to the TCP transmission control block to
   store previous cwnd and previous ssthresh. Moreover, every time the RTO
   expires, the TCP will copy the current cwnd and ssthresh to pcwnd
   and pssthresh.



   The overall operation flow of BU-trigger mechanism in TCP layer is as
   follows.

      (1) When BU-trigger signal arrived, get the home IP address of the
          MN.
      (2) Check TCP's transmission control block entry by entry, if the
          destination address of the entry is equal to MN's home IP
          address, apply the following procedures:
          (a) Recover the cwnd and ssthresh to pcwnd and pssthresh.
          (b) Reset the RTO.
          (c) Send the packets in sent_buffer.

   Then, TCP in CN side will send packets with the rate before the
   handoff occurs and the end-to-end performance of TCP will be recovered
   quickly.


4. Interoperability with Mobile IPv6/TCP

   The BU-trigger mechanism can be applied to Mobile IPv6 and TCP
   protocol in CN side. The extension functions of Mobile IPv6 and TCP
   due to BU-trigger mechanism have no effect to the TCP and Mobile
   IPv6 behavior in normal nodes. In other words, the CN applied BU-
   trigger mechanism will be interoperable well with hosts without BU-
   trigger functions.





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References

   [APS99] M. Allman, V. Paxson, and W. Stevens. TCP Congestion Control.
           RFC 2581, April 1999

   [Pos81] J. Postel. Transmission Control Protocol. RFC 793, September
           1981

   [Joh02] D. Johnson, C. Perkins, and J. Arkko. "Mobility Support in
           IPv6", draft-ietf-mobileip-ipv6-24.txt (work in progress)


Author's Addresses

   Questions about this memo can be directed to:

   Killyeon Kim
        Samsung Electronics Co., Ltd.
        Dong Suwon P.O.BOX 105
        416 Maetan-3Dong, Youngtong-Gu,
        Suwon-City, Gyeonggi-Do, Korea, 442-600
        Email: kimkl@samsung.com

   Youngjun Park
        Global Standards and Strategy team
        Telecommunication R & D Center
        Samsung Electronics Co., LTD.
        Dong Suwon P.O. BOX 105,
        416, Maetan-3dong, Youngtong-gu,
        Suwon-city, Gyeonggi-do, 442-600
        Korea
        Phone: +82-31-279-5979
        Email: youngjun74.park@samsung.com
        Fax: +82-31-279-5130

   Kyungjoo Suh (Joo Suh)
        Global Standards and Strategy team
        Telecommunication R & D Center
        Samsung Electronics Co., LTD.
        Dong Suwon P.O. BOX 105,
        416, Maetan-3dong, Youngtong-gu,
        Suwon-city, Gyeonggi-do, 442-600
        Korea
        Phone: +82-31-279-5123
        Email: joo.suh@samsung.com
        Fax: +82-31-279-5130

   Yongseok Park
        Samsung Electronics Co., Ltd.
        Dong Suwon P.O.BOX 105
        416 Maetan-3Dong, Youngtong-Gu,
        Suwon-City, Gyeonggi-Do, Korea, 442-600
        Email: yougseok.park@samsung.com


























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