Internet Draft Seok Joo Koh Internet Engineering Task Force ETRI Expires August 2004 Qiaobing Xie February 2004 Motorola mSCTP with Mobile IP for Transport Layer Mobility 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 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 a "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 Mobile SCTP (mSCTP) is defined as SCTP with the ADDIP extension. The mSCTP can be used for providing seamless handover by exploiting its multi-homing feature. On the other hand, the Mobile IP basically provides the location management. In this document, we discuss the use of mSCTP along with Mobile IP for Internet mobility support in the transport layer. The use of SCTP with Mobile IP is focused on the mobile sessions that are initiated by CN to MN. Koh and Xie [Page 1] INTERNET DRAFT mSCTP with Mobile IP February 2004 Table of Contents 1. Introduction..................................................3 2. Terminology...................................................3 3. Mobility Support by Mobile IP.................................3 3.1 Location Management in Mobile IP..........................3 3.2 Handover Management in Mobile IP..........................4 4. Overview of mobile SCTP.......................................4 5. Location Management in mSCTP with Mobile IP...................5 6. Use of SCTP with Mobile IP....................................6 6.1 SCTP Initialization.......................................6 6.2 Association Initiation in mSCTP with Mobile IP............7 6.3 Data Transport and Handover during the Association........9 6.4 Usage Scenarios for SCTP with Mobile IPv6.................9 7. Discussion....................................................9 7.1 Requirements for SCTP over Mobile IP......................9 7.2 Route Optimization........................................9 7.3 Other Issues.............................................10 8. Security Considerations......................................10 9. Acknowledgement..............................................10 10. References..................................................10 Author's Addresses..............................................11 Koh and Xie [Page 2] INTERNET DRAFT mSCTP with Mobile IP February 2004 1. Introduction The Stream Control Transmission Protocol (SCTP) [3] that provides the multihoming feature. Without support of routers in the networks, the SCTP with the ADDIP extension [4] that is called mobile SCTP (mSCTP) can be used to provide seamless handover for the mobile sessions that are originated by Correspondent Nodes (CN) toward to Mobile Nodes (MN) [5, 6]. The mSCTP can also be used to support Internet mobility for the sessions that are initiated by CN toward MN, if it is used along with the Mobile IP [7, 8]. In this case, the Mobile IP is used for location management, and then the mSCTP will be used for seamless handover instead of the MIP-based handover schemes [9, 10]. Specifically, Mobile IP will be used only for the CN to find the current location of MN and to establish an SCTP association. Once the SCTP association has been established, the SCTP session will be supported by the mSCTP seamless handover procedures [5, 6]. This document is intended to continue a discussion to explore the use of SCTP for Internet mobility support. Please send comments to the mailing list . To subscribe to this mailing list, please send a mail to . 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]. 3. Mobility Support by Mobile IP Generically, Internet mobility issues are divided into Location Management and Handover Management. 3.1 Location Management in Mobile IP Location Management is to identify the current location of a mobile node and keep track of its changes as it moves on. Basically, the location management is done so as to prepare the call setup for the sessions that are requested to mobile nodes (MN) from correspondent nodes (CN). With help of location management, the CN will be able to locate the MN and to establish a session via an appropriate call setup process. Koh and Xie [Page 3] INTERNET DRAFT mSCTP with Mobile IP February 2004 In Mobile IPv4 [7], Home Agent (HA) and Foreign Agent (FA) are employed for location management. The Home Address (HoA) is used as a host identifier of an MN, whereas the Care-of Address (CoA) is used as a location identifier of the MN. The HA maintains the information on the current location of each mobile node by binding CoA of FA to HoA of MN. In case that a Collaborated CoA (CCoA) is used instead of CoA, MIPv4 need not use the FA. By this, an external CN will be able to establish a session with an MN. In Mobile IPv6 [8], the HA is employed for location management. Similar to MIPv4, HoA and CoA are used as a host identifier and a location identifier of the MN, respectively. The CoA of MIPv6 is the same as CCoA of MIPv4. By using MIPv6, an external CN will be able to establish a session with an MN. In Mobile IP, it is noted that the HoA is also used by the applications of an MN, since the MN binds its applications to the HoA. In this respect, HoA is also used for IP packet data transport. 3.2 Handover Management in Mobile IP Handover Management is used to provide mobile hosts for seamless handover, whenever they move into different IP network regions during a session. The main objective of the seamless handover is to minimize the service disruption due to data loss and/or latency during the handover period. In Mobile IP, the Low Latency handover for MIPv4 [9] and Fast Handover for MIPv6 [10] have been designed for handover management. These MIP-based handover schemes rely on the tunneling between old and new Access Routers (ARs). 4. Overview of mobile SCTP In terms of mobile Internet services, a session involved by a mobile node can be classified into one of the following two types: a. Session originated from MN toward CN b. Session originated from CN toward MN The mobile sessions in (a) seem to be a natural extension of the classical 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. For this type of session, the location management is not a crucial requirement. Only the seamless handover will be required in terms of Internet mobility management. Koh and Xie [Page 4] INTERNET DRAFT mSCTP with Mobile IP February 2004 On the other hand, the case (b) requires the location management by which the session originator, CN, can locate the mobile host, MN, as supported in Mobile IP. Mobile SCTP (mSCTP) [5, 6], in its present form, is targeted for mobile sessions that are initiated by MNs toward CNs located in the fixed networks. The mSCTP is used to provide seamless handover for mobile nodes that change their IP addresses by continual moving across different IP subnets. These sessions do not require location management. The detailed schemes for seamless handover using mSCTP are described with some implementation issues in [5, 6]. The mSCTP can be used to provide an alternative scheme for seamless handover instead of the LMIPv4 and FMIPv6 schemes. The basic difference between the MIP-based handover schemes and mSCTP is that the mSCTP intrinsically realizes the handover in the transport layer without any support of network routers, whereas the MIP-based schemes rely on the support of routers support for tunneling between old and new ARs. To support the mobile sessions that are initiated by a CN toward an MN, the mSCTP may be used along with a location management scheme such as Mobile IP. In this scenario, the MIP will be used for a CN to locate an MN and to establish an SCTP association with the MN. After an SCTP association is successfully setup, the mobile SCTP will be used for providing seamless handover for the MN, as described in [5, 6]. 5. Location Management in mSCTP with Mobile IP For the present, the use of SCTP with MIP is focused on the mobile sessions that are initiated by CN to MN. The sessions initiated by MN can be supported only by mobile SCTP. Specifically, Mobile IP will be used only for location management, by which the CN locates the location of MN and establishes an SCTP association. Once the SCTP association has been established, the on-going SCTP session will be supported by the mSCTP seamless handover procedures [5, 6]. A part of the MIP functionality for data transport, will not be used in SCTP with MIP. If once the association is established, the data transport between MN and CN relies on SCTP over IP. The tunneling between HA and MN is not used. Furthermore, the Home Address (HoA) of MN is not used for the data transport. Note that the HoA is used for only the location management. Koh and Xie [Page 5] INTERNET DRAFT mSCTP with Mobile IP February 2004 6. Use of SCTP with Mobile IP 6.1 SCTP Initialization As described in RFC 2960 [1], the SCTP initialization is done from CN and MN as follow (also see Figure 1): a. CN sends INIT chunk to MN for triggering the association setup. b. MN responds with INIT-ACK chunk to CN. c. Then, CN and MN exchange COOKIE-ECHO and COOKIE-ACK each other. CN MN | | | INIT | |-------------------->| | | | INIT-ACK | |<--------------------| | | | COOKIE-ECHO | |-------------------->| | | | COOKIE-ACK | |<--------------------| Figure 1. SCTP Initialization As shown in the figure, the establishment of an SCTP association is ready by exchanging INIT and INIT-ACK and completed by exchanging COOKIE-ECHO and COOKIE-ACK between CN and MN. In Mobile IP, the HA will have information on the current location of an MN, which is updated by Registration or Binding Update procedures of the MN located at a foreign link. The location management of Mobile IP can be used to convey the INIT chunk message of CN to the MN via HA. After receiving the INIT chunk, the MN responds with INIT-ACK chunk directly to CN (not by way of HA) so as to complete the initiation of SCTP association. The responding INIT-ACK must contain the CCoA (in MIPv4) or CoA (in MIPv6), which can be addressable to the MN. Koh and Xie [Page 6] INTERNET DRAFT mSCTP with Mobile IP February 2004 6.2 Association Initiation in mSCTP with Mobile IP Mobile IPv4 considers two options to represent the location of the foreign link: CoA and CCoA. The CoA is the IP address of FA (possibly a router), whereas the CCoA is dynamically assigned by using an address allocation mechanism such as DHCP. Unfortunately, the CoA in MIPv4 cannot be applicable to SCTP, since it is an address of FA and thus cannot be used within the MN host (for its SCTP association). Note that SCTP is an end-to-end transport layer protocol, not a network-layer one. On the other hand, the CCoA in MIPv4 can be used within the SCTP hosts. Specifically, the SCTP of MN could bind the CCoA to an SCTP association. For this reason, in this document, we focus on the use of SCTP over MIPv4 for the MN hosts with CCoA in a foreign link. The case of using CoA is for further study. Let us consider an example of MIPv4 networks, which consists of CN, MN and HA. The MN is now at Location A (a foreign link), and will then move into Location B, as shown in Figure 2. [1.1.1.2] +----+ | CN | +----+ || ******* *** *** ** ** ############## ** Internet **---# Home Agent # ** ** ############## ** ** [1.1.1.1] *** *** || ******** || || || ####### ####### # AR1 # # AR2 # ####### ####### | | Location A | | Location B | | +----+ +----+ | MN |=========>| MN | +----+ +----+ CCoA=[2.2.2.2] CCoA=[3.3.3.2] HoA=[1.1.1.2] HoA=[1.1.1.2] Figure 2. SCTP with Mobile IPv4 using CCoA Koh and Xie [Page 7] INTERNET DRAFT mSCTP with Mobile IP February 2004 We assume that the MN has already obtained a CCoA ([2.2.2.2]) from the DHCP server attached to AR1, and also registered the CCoA with HA by using the MIPv4 Registration Procedures. We also assume that the applications of MN can initially bind the CCoA as well as HoA via the socket interface. After initialization of SCTP association, the HoA may be released from the application, as described below. It is noted that the HoA will still be used for MN to update its new CCoAs to HA according to the MIPv4 mechanisms. Now the CN initiates an SCTP association with the MN by sending INIT chunk message over HoA ([1.1.1.2]). The INIT chunk will first be routed to HA, and the HA then forwards the INIT chunk to MN by referring to CCoA ([2.2.2.2]) and using a tunneling mechanism. CN HA MN | | | | INIT | INIT | |-------------------->|-------------------->| | | | | INIT-ACK (CCoA=primary address) | |<------------------------------------------| | | | | COOKIE-ECHO (over CCoA) | |------------------------------------------>| | | | | | COOKIE-ACK | |<------------------------------------------| | | | | SCTP Data Transport | |<----------------------------------------->| | | | Figure 3. SCTP Initiation in SCTP with MIPv4 In response to the INIT chunk, the MN sends INIT-ACK chunk to the CN. The INIT-ACK contains the CCoA address (as the Primary address) and HoA address. Here, the HoA address may only be used for the CN to check whether the responding MN is the authorized host or not (for somewhat security reason). In fact, the HoA will not be referred to by CN (see the Section 4 for more detailed discussion). The source address and destination address of IP packet containing the INIT chunk are CCoA [2.2.2.2] and CN [1.1.1.2], respectively. In turn, the COOKIE-ECHO and COOKIE-ACK chunks will be exchanged between CN and MN. Koh and Xie [Page 8] INTERNET DRAFT mSCTP with Mobile IP February 2004 6.3 Data Transport and Handover during the Association After the association is established, the CN transmits data chunks to MN over the CCoA address, and the MN sends data chunks to CN directly. When the MN moves from Location A to Location B, the MN gets a new CCoA address [3.3.3.2] from Location B. According to the MIPv4 mechanism, the MN will update its new location to HA, which is done in the Mobile IP layer regardless of the on-going SCTP association. On the other hand, the MN will perform the seamless handover, as it moves into a new IP subnet area, according to the mobile SCTP by adding the new IP address to the on-going association, as described in Section 4 of [6]. These procedures will be repeated until the association has been completed. 6.4 Usage Scenarios for SCTP with Mobile IPv6 The usage scenarios for SCTP with MIPv6 are similar to those for SCTP with MIPv4 that are described so far. In MIPv6, the CoA is used instead of CCoA in MIPv4. The CoA may be obtained from the foreign location via DHCPv6 or stateless address auto-configuration. 7. Discussion This section discusses comparison of SCTP/MIP with the MIP-only scheme and some issues. Again, this comparison is valid for the mobile sessions that are initiated by CN toward to MN. 7.1 Requirements for SCTP over Mobile IP The requirement for using SCTP over Mobile IP is that the CN and MN hosts must be aware of the mobile SCTP. In addition, the MN must be able to bind the CoA as well as HoA to its applications. In MIP, only HoA is bound to the applications of MN. 7.2 Route Optimization The SCTP intrinsically provides the route optimization for data transport between CN and MN. No additional route optimization procedures are required, differently from MIPv4. No binding update between MN and CN is needed, differently from MIPv6. As a result, the tunneling of data packets between HA and MN is not required too. Koh and Xie [Page 9] INTERNET DRAFT mSCTP with Mobile IP February 2004 7.3 Other Issues In the proposed scheme for use of SCTP over MIP, the home address (HoA) of MN is not involved in the data transport between CN and MN. The reason for this is to exploit the intrinsic route optimization feature of mobile SCTP. Note that the additional tunneling or binding update procedures are required in case that the HoA is used in the SCTP association. The HoA may be used as a backup IP address in the event of path failure of the primary address, CCoA or CoA. This is for further study. 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 and discussion: Hee Young Jung, ETRI Mee Jeong Lee, Ewha Women University Moon Jung Chang, Ewha Women University Randall Stewart, Cisco Systems Maximilian Riegel, Siemens AG Michael Tuexen, University of Applied Science in Muenster 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 Koh and Xie [Page 10] NTERNET DRAFT mSCTP with Mobile IP February 2004 [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-03, August 2003 [6] Koh, S. J., et al., "Mobile SCTP for Transport Layer Mobility", draft-sjkoh-sctp-mobility-03, February 2004 [7] Perkins, C. (ed.), "IP mobility Support for IPv4", RFC 3344, August 2002 [8] Johnson, D., et al., "Mobility Support in IPv6", draft-ietf- mobileip-ipv6-24, June 2003 [9] Malki, K. L., et al., "Low Latency Handoffs in Mobile IPv4", draft-ietf-mobileip-lowlatency-handoffs-v4-07, October 2003 [10] Koodli, R., et al., "Fast Handovers for Mobile IPv6", draft- ietf-mipshop-fast-mipv6-00, October 2003 Author's Addresses Seok Joo Koh sjkoh@etri.re.kr ETRI, Korea Qiaobing Xie Qiaobing.Xie@motorola.com Motorola, Inc., USA Koh and Xie [Page 11] Internet Draft Seok J. Koh, ETRI Internet Engineering Task Force Mee Jeong Lee, EWU Expires August 2004 Maximilian Riegel, Siemens AG February 2004 Mary Li Ma, UBC Michael Tuexen, UASM Mobile SCTP for Transport Layer Mobility 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 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 a "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 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, Session Initiation Protocol or Reliable Server Pooling. Koh, Lee, Riegel, Ma, Tuexen [Page 1] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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..........................9 5.4 AAA Functionality........................................10 5.5 Link Layer Support for Multi-homing......................10 6. Location Management for mSCTP................................10 6.1 Use of mSCTP with Mobile IP..............................10 6.2 Use of SCTP with SIP.....................................11 6.3 Use of SCTP with RSerPool................................11 7. Comparison of mSCTP with SIP, Mobile IP and RSerPool.........12 8. Security Considerations......................................13 9. Acknowledgement..............................................13 10. References..................................................13 Author's Addresses..............................................14 Full Copyright Statement........................................15 Koh, Lee, Riegel, Ma, Tuexen [Page 2] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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), Reliable Server Pooling (RSerPool) [11] 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, MIP or RSerPool 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 . To subscribe to this mailing list, please send a mail to . 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". Koh, Lee, Riegel, Ma, Tuexen [Page 3] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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. Koh, Lee, Riegel, Ma, Tuexen [Page 4] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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, Mobile IP [9] or RSerPool. Koh, Lee, Riegel, Ma, Tuexen [Page 5] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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 Koh, Lee, Riegel, Ma, Tuexen [Page 6] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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. Koh, Lee, Riegel, Ma, Tuexen [Page 7] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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 "Address Configuration Parameter" set to "Set Primary Address". 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 "Set Primary Address". 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 Koh, Lee, Riegel, Ma, Tuexen [Page 8] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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. One particular further consideration in the handover procedures is that Steps 4.3 (adding), 4.4 (primary change) and 4.5 (deleting) can possibly be performed at the same time, which may depend on a specific implementation of mSCTP or network environment concerned. This issue needs to be investigated for further study. 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 a certain case, we may need to consider the multi-homed FS. It is noted that there are still many further issues on how to handle the mSCTP handover or which is better in the performance aspect for each of the single-homed and multi-homed FS cases. 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. Koh, Lee, Riegel, Ma, Tuexen [Page 9] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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 this document. 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. It is noted that Cellular systems are expected to easily support the link-level multi-homing features, whereas the WLAN system case is for further study. 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. Koh, Lee, Riegel, Ma, Tuexen [Page 10] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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. 6.3 Use of SCTP with RSerPool RSerPool [11] can be used for location management. A mobile server registers a pool handle such that it becomes part of a pool. It is allowed that a pool consists of one pool element only. A client (mobile or not) must know the pool handle of the mobile server it wants to talk to. It sends a name resolution request to one of the ENRP servers and gets back the current IP-addresses. Since the ENRP servers within an operational scope share its state it is not important which ENRP server is contacted. If the MS changes its IP- address it re registers at the home ENRP server. So the pool handles can be used to address a server with changing IP- addresses. If the MC or the MS change their addresses due to handovers mSCTP can be used to handle this, except for the case were the MC and the MS change their addresses simultaneously. In this case mSCTP fails i.e. the SCTP association terminates. The RSerPool session concept can be used to reestablish a new SCTP association using the new addresses and continuing the RSerPool session. Depending on the application the impact of this session failover for the application can be very small. Koh, Lee, Riegel, Ma, Tuexen [Page 11] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 7. Comparison of mSCTP with SIP, Mobile IP and RSerPool Table 1 summarizes the comparison of mSCTP with Mobile IP, SIP and RSerPool. Table 1. mSCTP, MIP, SIP and RSerPool for Mobility Support +---------------+------------+-------------+------------+-----------+ | Category | mSCTP | Mobile IP | SIP | RSerPool | +---------------+------------+-------------+------------+-----------+ |Protocol Layer | Transport | Network |Application | Session | +---------------+------------+-------------+------------+-----------+ | Location | Not | Supported | Supported | Supported | | Management | Supported | | | | +---------------+------------+-------------+------------+-----------+ | Handover | Supported | FMIP needed | Supported | Supported | | Management | | | with mSCTP |with mSCTP | +---------------+------------+-------------+------------+-----------+ | Route | Provided | Binding | Not | Provided | | Optimization | Bacially |Update needed| Provided |with mSCTP | +---------------+------------+-------------+------------+-----------+ | Network | Not | Required | Not | Not | | Support | Required | | Required | Required | +---------------+------------+-------------+------------+-----------+ | Special | No | Home Agent, |SIP servers,| ENRP | | Agents | |Foreign Agent| Registrar | Server | +---------------+------------+-------------+------------+-----------+ 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 Koh, Lee, Riegel, Ma, Tuexen [Page 12] INTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 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. Reliable Server Pooling [11] can be used to provide the location management functionality. Used in combination with mSCTP it provides all functionalities required for a mobility solution. Simultaneous handovers of a MS and a MC will result in a session failover. 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: Moon Jeong Chang, Ewha Women University Hee Young Jung, ETRI Randall Stewart, Cisco Systems Qiabing Xie, Motorola 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-03, August 2003 Koh, Lee, Riegel, Ma, Tuexen [Page 13] NTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 [6] Coene, L. (ed.), "Multihoming issues in the SCTP", draft-coene- sctp-multihome-04, June 2003 [7] Perkins, C. (ed.), "IP Mobility Support for IPv4", RFC 3344, August 2002 [8] Johnson, D., et al., "Mobility Support in IPv6", draft-ietf- mobileip-ipv6-24, June 2003 [9] Koh, S. J., et al., "mSCTP with Mobile IP for Transport Layer Mobility", draft-sjkoh-mobile-sctp-mobileip-03, February 2004 [10] Stewart, R. Xie, Q., "Stream Control Transmission Protocol: A Reference Guide", Addison Wesley Longman, 2001. [11] Tuexen, M. et al., "Requirements for Reliable Server Pooling", RFC 3237, January 2002. Author's Addresses Seok J. 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) Michael Tuexen tuexen@fh-muenster.de University of Applied Science in Muenster (UASM) Koh, Lee, Riegel, Ma, Tuexen [Page 14] NTERNET DRAFT mSCTP for Transport Layer Mobility February 2004 Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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