Transport Area WG Seok Joo Koh, ETRI
Internet Draft Mee Jeong Lee, EWU
Internet Engineering Task Force Maximilian Riegel, Siemens AG
Expires December 2003 Mary Li Ma, UBC
June 2003
Architecture of Mobile SCTP for IP Mobility Support
<draft-sjkoh-sctp-mobility-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026 [1].
Internet-Drafts are valid for a maximum of six months and may be
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Abstract
This document discusses the architecture of mobile SCTP (mSCTP) for
IP mobility support. The SCTP is the third transport layer protocol
next to TCP/UDP. It can also be used for IP mobility from the multi-
homing features. The SCTP with the ADDIP extension (or mSCTP) would
provide seamless or soft handover for the mobile host without support
of routers or agents in the networks. For location management, the
mSCTP could be used along with Mobile IP or Session Initiation
Protocol.
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Table of Contents
1. Introduction..................................................3
2. Terminology...................................................3
3. Motivations on Mobile SCTP....................................4
3.1 IP Mobility Issues........................................4
3.2 SCTP Multihoming Feature..................................5
3.3 Session Type considered in Mobile SCTP....................5
4. Procedures for mSCTP Handover.................................6
4.1 Session Initiation by Mobile Client.......................7
4.2 Obtaining an IP address for a new location................7
4.3 Adding the new IP address to the SCTP association.........7
4.4 Changing the Primary IP address...........................7
4.5 Deleting the old IP address from the SCTP association.....8
4.6 Repeating the handover procedures.........................9
5. Further Considerations for mSCTP Handover.....................9
5.1 Requirement for Mobile SCTP...............................9
5.2 Number of IP addresses used by Fixed Server...............9
5.3 Dynamic IP address configuration.........................10
5.4 AAA Functionality........................................10
5.5 Link Layer Support for Multi-homing......................11
6. Location Management for mSCTP................................11
6.1 Use of mSCTP with Mobile IP..............................11
6.2 Use of SCTP with SIP.....................................12
7. Comparison of mSCTP with SIP and Mobile IP...................13
8. Security Considerations......................................14
9. Acknowledgement..............................................14
10. References..................................................14
Author's Addresses..............................................15
Full Copyright Statement........................................16
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1. Introduction
SCTP (Stream Control Transmission Protocol), as defined in RFC 2960
[3], is the third transport layer protocol following TCP and UDP. The
SCTP is featured multi-streaming and multihoming, differently from
TCP. It is noted that the multihoming feature of SCTP enables the
SCTP to support the IP mobility.
More specifically, the SCTP with the ADDIP extension [4], which is
called mobile SCTP (mSCTP) in this document, can be used to provide
seamless handover for mobile hosts that are moving into different IP
network regions during the active session [5, 6].
The mSCTP may be used as an alternative scheme against the handover
schemes based on Mobile IP [7, 8]. Differently from the Mobile IP
based handover schemes, which rely on the support of network routers
for tunneling between access routers, the mobile SCTP provides the
handover management at the transport layer without help of routers.
The mSCTP can be used to provide seamless handover for mobile hosts
that are moving in to different IP networks. In other words, the
mSCTP is targeted for the client-server services, in which the mobile
client initiates an SCTP session with the fixed server. For
supporting the peer-to-peer services, in which a session is
terminated at the mobile host, the mSCTP must be used along with an
additional location management scheme such as Mobile IP [9],Session
Initiation Protocol (SIP) or Dynamic DNS (DDNS).
This document describes the architecture of SCTP for IP mobility
support. Specifically, we describe the use of SCTP for seamless
handover by using the SCTP ADDIP extension [5]. We also discuss how
to integrate SCTP with SIP or MIP for location management.
This document is intended to continue discussion to explore the use
of SCTP for IP mobility support. Please send comments to the mailing
list <mobile@sctp.de>. To subscribe to this mailing list, please send
a mail to <mobile-request@sctp.de>.
2. Terminology
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 [2].
In this document, "mSCTP" is short for "mobile SCTP".
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3. Motivations on Mobile SCTP
In this section, we discuss motivations on the use of SCTP for IP
mobility support, in the viewpoint of IP mobility management issues.
3.1 IP Mobility Issues
IP mobility issues have been focused and are regarded as a core
technology required for providing seamless mobility in the wireless
mobile networks such as WLAN, 3G Cellular. IP mobility issues can be
classified into Location Management and Handover Management.
3.1.1 Location Management
Location Management is used to identify the current location of
mobile nodes and also to keep track of the their location changes as
they move on.
In Mobile IP [7, 8], the mobility agents such as Home Agent (HA) and
Foreign Agent (only for IPv4) are employed for location management as
well as data transport. In the schemes, Home Address (HoA) and Care-
of Address (CoA) are used for a terminal identifier and a location
identifier of the IP host, respectively. For location management, the
Mobile IP uses the binding update messages, in which a mobile node
has to inform its current location (CoA) to its HA.
SIP can also be used for location management. In SIP, a UA registers
its new location with the location database via SIP Registrar server
by sending an SIP Register message containing a Contact Header.
So far, the Dynamic DNS (DDNS) has also been discussed as one of the
candidate approaches for location management, which would not require
special servers or procedures for mSCTP client.
3.1.2 Handover Management
The handover management is targeted to provide the mobile hosts for
seamless handover whenever they change their point of attachment to
IP networks (as represented by cell regions or IP subnets). The main
objective of the handover management is to minimize the service
disruption due to data loss and/or handover latency during handover.
In Mobile IP, the Low Latency or Fast handover schemes have been
designed for handover management. These schemes rely on the tunneling
between old and new access routers for seamless handover.
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The mobile SCTP can be used as an alternative scheme against such
Mobile IP based handover schemes. Differently from the Mobile IP
handover schemes that rely on the help of network routers for
tunneling between access routers, the mobile SCTP provides the
handover management at the transport layer without help of routers.
3.2 SCTP Multihoming Feature
The SCTP intrinsically provides the multihoming feature [3], in which
a mobile node is allowed to simultaneously bind multiple IP addresses
to its network interface.
The recent works on the SCTP include the ADDIP extension [4]. The
ADDIP extension enables the SCTP to add, delete and change the IP
addresses during active SCTP association.
In this document, the SCTP implementation with the ADDIP extension is
called the mobile SCTP (mSCTP) [5]. The mSCTP can be used for
seamless handover while the mobile node is moving into different IP
network regions over the session period. This document aims at
discussing the use of mSCTP for seamless handover, which includes the
specific handover procedures and associated implementation issues.
3.3 Session Type considered in Mobile SCTP
Sessions considered in mobile communications can be classified into
the following two types:
a. Session originated from mobile host toward fixed host
b. Session originated from fixed host toward mobile host
The mobile sessions in (a) seem to be a natural extension of the
client-server model, in which the mobile host originating the session
can be viewed as a client, while the counter endpoint will function
as a server.
On the other hand, the case (b) requires the additional location
management functionality for the session originator to find the
current location of the mobile host and to keep track of the location
changes, which has so far been addressed by Mobile IP [7, 8].
The mobile SCTP, in the present form, is targeted for seamless
handover of mobile session associated with the case (a). To support
the session type of the case (b), the mSCTP must be used along with
an additional location management scheme such as SIP or Mobile IP,
which is discussed in [9].
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4. Procedures for mSCTP Handover
In this section, we describe the generic algorithm of mobile SCTP for
handover in the procedural manner, which is designed based on the
scheme in [5].
The mSCTP handover needs to be triggered by MC because only the MC
knows the movement of itself and the signal strength from the old and
new ARs.
More specifically, we consider a mobile client (MC) that initiates an
SCTP association with a fixed server (FS), and then moves from
Location A (2.2.2.x domain) to Location B (3.3.3.x domain), as shown
in Figure 1.
[1.1.1.2]
+----+
| FS |
+----+
||
##########
# Router # [1.1.1.1]
##########
||
*******
*** ***
** **
** Internet **
** **
** **
*** ***
|| ******** ||
|| ||
####### #######
[2.2.2.1] # AR1 # # AR2 # [3.3.3.1]
####### #######
| |
Location A | | Location B
| |
+----+ +----+
| MC |=========>| MC |
+----+ +----+
[2.2.2.2] [3.3.3.2]
Figure 1. SCTP for Seamless Handover
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Figure 1 illustrates an example of the use of mobile SCTP for
seamless handover in IPv4 networks. Based on this figure, the
handover procedures are described in the succeeding sections.
4.1 Session Initiation by Mobile Client
We assume that the MC initiates an SCTP association with the FS. The
resulting SCTP association has the set of IP addresses with [2.2.2.2]
for MC and [1.1.1.2] for FS. It is also assumed that the MC can get
an IP address ([2.2.2.2]) with help of DHCP or IPv6 stateless address
auto-configuration.
Note that the FS is in the single homing with [1.1.1.2], and the MC
is also in single homing state, in which the IP address [2.2.2.2] is
set to its primary IP address in the SCTP initiation process.
4.2 Obtaining an IP address for a new location
Let us assume that the MC moves from Location A to B. In this phase,
we also need to assume that the MC can obtain a new IP address
belonging to the Location B, which may be possible with help of the
DHCP or IPv6 address auto-configuration capability in the Location B.
Obtaining a new IP address may also rely on the support of the
wireless signaling control at the physical layer, in order for the MC
to get the IP address information via IP control packets from the
Location B.
By SCTP, the newly obtained IP address ([3.3.3.2] in the figure) MUST
be signaled or informed to the SCTP protocol stack, and then the SCTP
will bind the new IP address to the existing SCTP association.
4.3 Adding the new IP address to the SCTP association
After obtaining a new IP address, the SCTP of MC MUST inform the
Fixed Server about the new IP address by sending Address
Configuration Change (ASCONF) Chunk to the FS. The MC may receive the
corresponding ASCONF-ACK Chunk from the FS.
4.4 Changing the Primary IP address
While the MC continues to move toward the Location B, it needs to
change its primary IP address to the new IP address according at an
appropriate rule.
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Actually, the configuration of a specific rule for changing the
primary IP address is a challenging issue of the mobile SCTP.
Some of the possible rules for triggering the primary IP address
change are listed below:
a. As soon as a new IP address is detected
When the MC receives the ASCONF-ACK from FS, it sends another
ASCONF with ôAddress Configuration Parameterö set to ôSet Primary
Addressö.
FS sends back ASCONF-ACK once receives the second ASCONF to confirm
the handover successfully.
This choice may be preferred in terms of the handover latency, in
particular, for the fast-moving MC. However, it is less suitable
when the MC shows a so-called oscillation (or ping-pong) behavior
across those two locations.
b. By using an explicit indication from the underlying layer
In this case, the underlying PHY layer of MC detects and compares
the signal strength, and determines the time on when the SCTP sends
an ASCONF with ôSet Primary Addressö.
If the underlying physical layer can detect and compare the signal
strength of the physical media, and also inform the SCTP about a
certain indication (possibly by using a up-call), then the MC may
trigger the primary address change according to the indication.
This rule is a more preferred choice, but seems to depend on the
wireless system concerned and its implementations.
If once the primary address is changed, the FS will send the upcoming
data over the new primary IP address.
There is still a further issue on how the MC should handle the data
packets queued in the outgoing buffer with the source IP address of
the old primary IP address.
4.5 Deleting the old IP address from the SCTP association
As the MC progresses to move toward the Location B, if the old IP
address gets inactive, the MC MUST delete the IP address from the
address list. The rule for determining that the IP address is
inactive may also be implemented by using additional information from
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the underlying network or physical layer, as done in the previous
step (for changing the primary address.)
4.6 Repeating the handover procedures
The procedural steps for seamless handover described above will be
repeated whenever the MC moves to a new location, until the SCTP
association will be released.
5. Further Considerations for mSCTP Handover
5.1 Requirement for Mobile SCTP
The only requirement for providing the seamless handover based on
SCTP is that the MC and FS hosts are equipped with the Mobile SCTP
implementations, (i.e., SCTP with ADDIP extension.)
5.2 Number of IP addresses used by Fixed Server
In this document, we assume that the FS is in the single homing, i.e.
the FS and MC are in a 1-to-2 asymmetric multi-homing configuration
[10]. In such a case, the performance of providing fault resilience
to the communication may possibly be degraded, because the following
routing tables are in use:
o Before Handover:
----------------------- -------------------------
MC FS
======================= =========================
Destination AR1 Destination AR
======================= =========================
1.1.1.0 2.2.2.1 2.2.2.0(Primary) 1.1.1.1
3.3.3.0 1.1.1.1
----------------------- --------------------------
o After Handover:
------------------------ --------------------------
MC FS
======================== ==========================
Destination AR2 Destination AR
======================== ==========================
1.1.1.0 3.3.3.1 2.2.2.0 1.1.1.1
3.3.3.0(Primary) 1.1.1.1
------------------------ ---------------------------
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This particular routing arrangement of MC tends to prevent the
association from taking advantage of the existence of the second
interface. This is because the number of possible different paths,
which can be used to route data between two endpoints, can never be
larger than the minimum of IP addresses used by the endpoint.
There are 2 possible solutions to this problem:
1) Once MC issues the second ASCONF with ôAddress Configuration
Parameterö setting to ôSet Primary Addressö to request FS to
perform the change-over, the MC transfers the sending buffer of
the old link to the new link. Once MC receives the ASCONF_ACK
from FS which confirms the handover success, the MC starts to
sending buffered data on the new network.
2) As an alternative of this asymmetric multi-homing configuration,
the FS may assign two IP addresses to the network interface to
let both MC and FS in the dual homing state to enable easy
distinction of the two links at the MC. This allows SCTP to
represent different links by different entries in the host
routing table of the MC. The handover is beneficial in terms of
fault resilience for both MC and FS to use all the IP addresses
available to them when handover is performed. For more
information on this issue, please refer to [5, 6].
5.3 Dynamic IP address configuration
The basic assumption for seamless handover to a new IP subnet region
is that the MC is able to obtain a new IP address from the new
location. Typically, this will be implemented by using DHCP in IPv4
networks and DHCPv6 or Stateless address auto-configuration in IPv6
networks.
The handover latency incurred for obtaining the new IP address via
DHCP or IPv6 needs to be examined by experiments. The concerned
handover latency also includes the delay required for the handover
delay between the wireless links.
5.4 AAA Functionality
It is envisioned that the deployment of mSCTP will be done along with
the appropriate AAA server in the respective access network domains.
The AAA server is used to authenticate and the MC in the locations,
and also to authorize the new IP address configuration via DHCP and
IPv6 stateless configuration. However, this issue is outside the
scope of mSCTP.
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5.5 Link Layer Support for Multi-homing
To support the multi-homing capability for Mobile Client, we need to
consider the characteristics of the wireless links such as WLAN and
Cellular systems.
In the legacy WLAN systems, it may not be allowed for an MC to
simultaneously bind two different Access Points (APs), in which the
multi-homing is not easy to support. Accordingly, to enable mSCTP in
the WLAN networks, a special driver to WLAN NIC will be required to
realize the concurrent binding to two different APs belonging to
different IP access networks.
On the other hand, the Cellular systems are expected to easily
support the link-level multi-homing features.
The multi-homing feature enables the mSCTP to support seamless
handover by simultaneous binding of two different addresses while
staying the overlapping region. Time interval for an MC to stay in
the overlapping region will give impact on the performance of the
handover procedures.
It is also noted that the handover based on mSCTP depends on the
support of the underlying physical and link layers to measure the
wireless signal strength. The measured signal strength information
can helpfully used for the SCTP to trigger the addition and deletion
of IP addresses, and the change of the primary address. The handover
performance will depend on such capability in terms of data loss and
delay during handover.
6. Location Management for mSCTP
The mSCTP can provide only the handover for mobile hosts or the
sessions initiated by mobile hosts. To support the mobile sessions
that are terminated at mobile hosts, the mSCTP needs to be used along
with a location management scheme such as Mobile IP or SIP.
6.1 Use of mSCTP with Mobile IP
In this scenario, Mobile IP will be used to locate a mobile host and
then for Home Agent to forward the data packet (SCTP INIT chunk) to
the mobile host. The succeeding process for SCTP association
initiation, including SCTP INIT-ACK, COOKIE-ECHO, and COOKIE-ACK,
will be done directly between the mobile host and the peering host,
not via Home Agent.
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After an SCTP association is successfully setup, the mSCTP will be
used for providing seamless handover for the mobile host. Details of
SCTP with MIP are described in [9].
6.2 Use of SCTP with SIP
In this scenario, each host uses SCTP instead of TCP/UDP as the
transport protocol. After the call setup by SIP signaling, the SCTP
will be used for data transport and seamless handover.
The SIP provides location management functionality by using SIP
REGISTER messages. When a mobile host moves into a visiting network,
it will update its current location (e.g., IP address or SIP URL) by
sending a SIP Register (with a Contact Header) to the (home) SIP
Registrar server. The Registrar server will then update the location
database as indicated by the REGISTER message.
When a call setup is requested with a mobile host, the (home) SIP
proxy server will interrogate the location database to locate the
mobile host and then relay the SIP INVITE message to the (visiting)
SIP Proxy server up to the mobile host.
If once the SCTP association is established via the SIP signaling,
the data transport between two concerned hosts will be done according
to the mSCTP handover mechanisms.
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7. Comparison of mSCTP with SIP and Mobile IP
Table 1 summarizes the comparison of mSCTP with Mobile IP and SIP.
Table 1. mSCTP, MIP and SIP for Mobility Support
+------------------+--------------+---------------+---------------+
| Category | mSCTP | Mobile IP | SIP |
+------------------+--------------+---------------+---------------+
| Protocol Layer | Transport | Network | Application |
+------------------+--------------+---------------+---------------+
| Location Mgt. |Not Supported | Supported | Supported |
+------------------+--------------+---------------+---------------+
| Handover Mgt. | Supported |FMIP is needed | Not Supported |
+------------------+--------------+---------------+---------------+
| Route |Intrinsically | Provided by | Not Provided |
| Optimization | Provided |Binding Update | |
+------------------+--------------+---------------+---------------+
| Network Support | Not Required | Required | Not Required |
+------------------+--------------+---------------+---------------+
| Special Agents | Not Required | Home Agent, | SIP Proxy, |
| Required | | Foreign Agent |Registrar, etc |
+------------------+--------------+---------------+---------------+
As described in Table 1, the mSCTP can be used for seamless handover
in the transport layer. To use mSCTP, it is required that the CN and
MN hosts should be aware of the mobile SCTP. Instead, mSCTP does not
need the support of network routers for seamless handover.
Furthermore, the mSCTP intrinsically provides the Route Optimization
without using any additional Binding Update procedures.
For location management, the mSCTP may be used along with MIP or SIP.
In case of using MIP for location management, only the MN needs to be
aware of MIP, whereas the CN need not use MIP. Using mSCTP with MIP,
the MN must also be able to bind the CoA as well as HoA to its
applications. The HoA will be used only for location management.
After establishment of an SCTP session, the HoA will not be used for
data transport. Instead, the CoA is employed for the SCTP data
transport. On the other hand, in MIP, only HoA is bound to the
applications of MN regardless of the different CoAs.
The MIP provides the location and management in the network layer,
and it can support seamless handover with help of neighboring routers
such as tunneling between old and new ARs. On the other hand, SIP is
an application signaling protocol that supports the location
management for user or personal mobility. SIP itself does not provide
seamless handover. It may be used together with mSCTP for seamless
handover.
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8. Security Considerations
This document discusses architecture of SCTP mobility support. The
associated security issues will be identified as further works go on.
9. Acknowledgement
The Authors would like to give special thanks to the following people
for their valuable contributions:
Jun Seob Lee, ETRI
Moon Jeong Chang, Ewha Womans University
Michael Tuexen, University of Applied Science in Muenster
Randall Stewart, Cisco Systems
Hee Young Jung, ETRI
Sung Han Kim, ETRI
10. References
[1] S. Bradner, "The Internet Standards Process -- Revision 3", BCP,
RFC 2026, October 1996.
[2] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP, RFC 2119, March 1997.
[3] Stewart, R., et al., "Stream Control Transmission Protocol", RFC
2960, October 2000
[4] Stewart, R., "Stream Control Transmission Protocol (SCTP) Dynamic
Address Reconfiguration", draft-ietf-tsvwg-addip-sctp-07, February
2003
[5] Riegel, M. and Tuexen M., "Mobile SCTP", draft-riegel-tuexen-
mobile-sctp-02, February 2003
[6] Coene, L. (ed.), "Multihoming issues in the SCTP", draft-coene-
sctp-multihome-03, February 2002
[7] Perkins, C. (ed.), "IP Mobility Support for IPv4", RFC 3344,
August 2002
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[8] Johnson, D., et al., "Mobility Support in IPv6", draft-ietf-
mobileip-ipv6-21, May 2003
[9] Koh, S. J., et al., "SCTP with Mobile IP for IP Mobility Support",
draft-sjkoh-mobile-sctp-mobileip-01, June 2003
[10]Stewart, R. Xie, Q., ôStream Control Transmission Protocol û A
Reference Guideö, Addison Wesley Longman, 2001.
Author's Addresses
Seok Joo Koh
sjkoh@etri.re.kr
ETRI, Korea
Mee Jeong Lee
lmj@ewha.ac.kr
Ewha Womans University (EWU), Korea
Maximilian Riegel
Maximilian.Riegel@icn.siemens.de
Siemens AG, Germany
Mary Li Ma
maryma@interchange.ubc.ca
University of British Columbia (UBC)
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