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.
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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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