INTERNET DRAFT K. El Malki, N. A. Fikouras, S. R. Cvetkovic University of Sheffield 15th June 1999 Expires December 1999 Fast Handoff Method for Real-Time Traffic over Scaleable Mobile IP Networks 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 This document defines the operations to be performed by scaleable Mobile IP [1] networks during handoffs in order to support real-time traffic which has delay bounds. This method is based on the Regionalized Tunnel Management [2] approach. A method is described in this document which defines the operation of Mobile Nodes, Foreign Agents and Hierarchical Proxy Foreign Agents. This utilises multiple bindings in order to "multicast" traffic to potential Mobile Node movement locations in order to anticipate movement. This eliminates the service disruption period which is currently present during handoffs in Mobile IP networks due to registration delay. The information redundancy lasts only for very short periods and the waste of bandwidth is therefore minimal. El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 1] INTERNET-DRAFT Fast Handoff Method 15th June 1999 Table of Contents 1.0 Introduction 1.1 Assumptions 1.2 Acronyms 2.0 Description of the Fast Handoff Method 2.1 Overall Method and Architecture 2.2 Mobile Node (MN) functionality for Fast Handoffs 2.3 Foreign Agent (FA), Proxy Foreign Agent (PFA) and Top- level Proxy Foreign Agent (TPFA) functionality for Fast Handoffs 2.4 Support of "forward" and "backward" MN movement 2.5 Fast Handoffs applied to multiple BSs having common wireless overlap regions. 3.0 Security Considerations 4.0 References 5.0 Author's Address 1. Introduction Mobile Nodes (MNs) will vary their point of attachment to the Internet frequently. MNs which move in this way suffer a period of communication breakdown due to the time needed to update the MNs' location information (i.e. registration with the Home Agent). During these periods MNs experience a service disruption which undermines the quality of real-time services. In this document a Fast Handoff method is described. This method eliminates the service disruption period due to handoffs by anticipating the MN's movement and using multiple bindings to direct the MN's traffic to the multiple locations which it may move to. Although some waste of bandwidth will occur, this is minor since it will only last for the very short period in which a MN lies in the wireless overlap region of two network access points. We identify the movement between two access points as comprising both a movement between BSs (Base Stations: layer 2 mobility) and a movement between Mobility Agents (MAs). This method does not rely on the assumption of single-MA networks, and is applicable to multiple-MA networks. Finally, the Fast Handoff method may be applied to scenarios in which a Mobile Node enters the wireless overlap region of multiple BSs, and not only two as considered above. El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 2] INTERNET-DRAFT Fast Handoff Method 15th June 1999 1.1 Assumptions This method makes use of hierarchical agents and the Hierarchical Mobility Agent Extension [2]. It is also assumed that a FA will send advertisements containing its address and the addresses of all other PFAs in its hierarchical branch of the network using the PFA IP address extension [2]. These will be advertised in hierarchical order and this order cannot be modified. This method also assumes an underlying layer 2 protocol which allows a mobile node to receive and transmit contemporarily from/to multiple Base Stations (BSs) when in their wireless overlap region. A particular pattern of MN movement is not assumed, hence this method will provide seamless (loss-less) service to MNs moving "forward" and "backward" into any number of location and overlap regions. No limit is assumed regarding the number of MA levels in a hierarchy. 1.2 Acronyms MN Mobile Node FA Foreign Agent HA Home Agent PFA Proxy Foreign Agent TPFA Top-level Proxy Foreign Agent MA Mobility Agent. As such are described FAs, HAs, PFAs and TFAs. ISP Internet Service Provider COA Care-of address PM Prefix Matching ECS Eager Cell Switching LCS Lazy Cell Switching NAI Network Access Identifier 2.0 Description of Fast Handoffs Mechanism The Fast Handoff mechanism allows efficient handoffs for MNs. It supports Local, Metropolitan and Wide-Area mobility. This method is of greatest efficiency using Local and Metropolitan mobility, which are the most frequent scenarios for a MN. Security is considered in this method. El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 3] INTERNET-DRAFT Fast Handoff Method 15th June 1999 2.1 Overall Method and Architecture .................................................... . . . GLOBAL INTERNET . .................................................... / \ / \ +-------------------------+ +-------------------+ | ISP | | ISP | | ----- | | ----- | Top +---------|TPFA1|---------+ +-------|TPFA2|-----+ Level ----- ----- / \ | ----- \ | Local | PFA | \_______ | ----- \______ \ | / \ \ \ | Lowest ----- ----- ----- ----- ----- Level | FA1 | | FA2 | | FA3 | | FA4 | | FA5 | ----- ----- ----- ----- ----- | | | | | | ---------- | | | | | | BS BS BS BS | | | | MN -------> MN ------> MN ----------> MN 1 1ov2 2 2ov3 3 3ov4 4 Figure 1: Fast Handoff Network Topology Key: xovy = overlap area between the BSs in Pos. x and y The above diagram illustrates a general network architecture in which ISPs connect users through the Internet. Both single-agent and multiple-agent subnetworks are considered. In this scenario each ISP will host a Top-level MA or a Top-level Proxy Foreign Agent (TPFA). The function of a TPFA is to dynamically manage regional tunnels. Private networks (i.e. company networks) may have their own PFAs if they wish to manage tunnels locally. TPFAs and PFAs can provide fast handoffs by establishing multiple bindings (registrations) for moving MNs. The lowest levels of the MA hierarchy are occupied by FAs which provide network access to MNs. Although figure 1 presents only one level of FAs, any number of FA levels may exist below a given PFA. Furthermore, it is noted that FAs 1, 4 and 5 represent subnetworks with a single FA (i.e. single-agent subnetworks) El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 4] INTERNET-DRAFT Fast Handoff Method 15th June 1999 while FAs 2 and 3 exist within the same subnetwork (i.e. multiple-agent subnetworks). This configuration is typical of private organisations where multiple departments (each having a FA) are located within the same division or building floor and thus share communication resources. It is assumed in figure 1 that the MN has a HA connected to a PHA which is located anywhere on the Internet (as in [2]), but is not presented for simplicity. Link layer connectivity is provided by the BSs. Each BS can service MNs within its area of coverage. The overlap of multiple areas of coverage forms an overlap region. These regions are labelled as xovy, identifying the overlap between the two positions (i.e. x and y). It is assumed that a wireless technology is available at Layer 2 which allows the MN to communicate with N BSs when in the overlap region of their coverage areas. All FAs are required to include in their advertisements the PFA IP address extension [2] which will list all PFA's and TPFA's in the same hierarchical "branch" in descending order. Thus, in figure 1 the TPFA is listed first in FA advertisements. MNs are required to cache these lists in order to determine common routes as they move. Generally the TPFA is the single point of access to the Internet for a hierarchy of PFAs. In Figure 1 a scenario is depicted where a PFA denotes a local private or public administrative domain. The PFA can identify an organisational/residential network or it can provide network access to MNs on the street, although this can be done by the TPFA as shown in Figure 1. The following types of mobility are considered: 1)Local Mobilty (movements between Pos. 1 to Pos. 2) and Metropolitan-Area mobility (movement between Pos. 2 and Pos. 3) 2)Wide-Area Mobility (movement between Pos. 3 to Pos. 4) 2.2 Mobile Node (MN) functionality for Fast Handoffs The handoff mechanism is described in this section. Below is a flowchart specifying the MN operations when performing a Fast Handoff. As mentioned previously, TPFAs and PFAs are capable of supporting multiple simultaneous bindings for a MN. El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 5] INTERNET-DRAFT Fast Handoff Method 15th June 1999 It is assumed that the MN, upon movement from the Home network to a Foreign network, will cache the TPFA/PFA address hierarchy from the PFA IP address extension [2] present in FA agent advertisements. In the case of FA1 the extension would include the Local PFA and TPFA1. Some FAs may choose to advertise only Local PFAs. For example, an organisation comprising the Local PFA, FA1, FA2 and FA3 may not want to advertise the TPFA or any other higher PFAs which may exist in between. However this will prevent users from performing seamless handoffs between the organisation's network and the rest of the Internet. That is, fast and seamless handoffs would not be available for movement from position 2 to 3 or 2 to 4. In figure 1 it is assumed that the MN has powered up in position 1. Having no prior record of a hierarchy it operates as per [2] and issues a Registration Request using as COA (Care-of-address) the IP address of the TPFA. It is assumed that this Request updates the HA's MN location information, that the MN is granted network access and that the MN caches the list of PFAs (CURRENT PFA list) received in the FA agent advertisement. The MA activity in response to the Request is presented in section 2.3. It is assumed that MNs keep record of all the FAs from which they have received an agent advertisement ("known" Agents) even if they belong to different subnetworks. Similarly, the MN keeps record of "known" subnetworks. These are subnetworks in which "known" Agents lie. Furthermore, MNs are required to keep record of all established bindings. Optionally the MN may keep record of PFA IP address lists. Even though all "known" FAs hold the same status, only one FA is the "primary" FA. The binding associated with that FA is always updated by requesting a long lifetime and the PFA list associated with the "primary" FA is considered as CURRENT. Bindings associated with all the FAs, except for the "primary" FA, are updated by requesting a short lifetime (3 * advertisement rate). In this way the method is able to distinguish between the primary traffic flow and auxiliary traffic flows. The purpose of auxiliary flows is to anticipate MN movement into a new subnetwork. In Figure 1, when the MN moves from Position 1 to 1ov2, FA1 will be the "primary" FA while FA2 will receive and provide the MN with the auxiliary traffic flow. "Known" FAs whose lifetime has expired are removed, and so is all cached information associated with them (i.e. bindings, entries in "known" subnetwork list and PFA IP address lists). Using the list of "known" agents and subnetworks, in conjunction El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 6] INTERNET-DRAFT Fast Handoff Method 15th June 1999 with PM (Prefix Matching) and the NAI (Network Access Identifier), can help detect MN movement into new subnetworks. Since a MN may maintain simultaneous bindings with MAs, this method assumes a hybrid LCS-ECS movement detection method with PM or NAI support. That is, MNs are Eager to request simultaneous bindings when movement is detected by PM or NAI domain comparison, but are Lazy to abandon established bindings. This behaviour has been integrated in the flowchart presented below. The following flowchart illustrates the Fast Handoffs procedure for MNs when moving between FAs: .-----------> .-> Has the lifetime of a "known" FA expired ? .NO | | | | | | |YES | | | | | | | Remove FA from the "known" FAs list. | | | De-cache associated PFA list. | | | Delete all associated bindings. | | | (MN is Lazy to abandon bindings) | | | | | | | | | | | | | | | Has the "primary" FA expired ? ------------>| | | | NO | | | |YES | | | | | | | Use FA with longest outstanding lifetime | | | as "primary" FA. | /|\ /|\ Get a new CURRENT PFA IP address | | | extension list. | | | | | | | | | | | | | | | Received any successful Registration <-----. | | Replies ? ---------------------->. | | | NO | | | |YES | | | | | | | Cache associated binding and | | | optionally PFA IP address list. | | | Add FA to "known" FAs list. | | | Add FA's subnet to "known" subnetworks list. | | | | | | | | | | | | | El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 7] INTERNET-DRAFT Fast Handoff Method 15th June 1999 | | Are there any bindings about <---------. | | to timeout ? ----------->. | | | NO | /|\ /|\ |YES | | | | | | | Update bindings by sending a | | | Registration Request with the | | | IP address of the corresponding PFA | | | or TPFA as the COA. | | | Do not set the S bit ON. | | | | | | | | | | |<-- Received new FA Advertisements ? <-------. | | NO (not a "known" FA) | | | | | |YES | | | | | Does PM or NAI comparison indicate | .<---- discovery of a new subnet ? | NO | | |YES | | | Movement has been detected. | Cache PFA IP address extension | list (NEW PFA list). /|\ | | | | Find Common PFA by comparing | CURRENT and NEW PFA lists. | | | | | Is there a Common PFA ? | / \ | YES / \ NO | / \ | Send a Registration Request Send a Registration Request | using the Common PFA as COA using the TPFA as COA. Set the | Set the S bit ON to request a S bit ON to request a simultaneous | simultaneous binding. Use a binding. Use a short lifetime | short lifetime (i.e. (i.e. 3 * advertisement rate). | 3 * advertisement rate). The MN is Eager in establishing | The MN is Eager in establishing new bindings. | new bindings. | | | | | | | | | | \|/ \|/ .<------------------------------------------. El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 8] INTERNET-DRAFT Fast Handoff Method 15th June 1999 When the MN moves into the overlapping region of multiple subnetworks, it will receive agent advertisements containing the PFA IP address extension lists. The MN will therefore be in the position to determine if there is a Common PFA which could potentially send traffic for the MN to both its old and new location. If PM or NAI indicate movement into a new subnetwork then the MN will send a Registration Request. If there is a Common PFA the MN sends a Registration Request with the Common PFA as COA (N.B. the TPFA could be the Common PFA). Otherwise, if there is no Common PFA, the MN will use the TPFA as the COA. The S bit is set ON only when a Common PFA is found, such that the Common PFA maintains simultaneous bindings with the MN. In this case the Common PFA will "multicast" traffic to both MN's registered locations. When the "primary" FA changes, a new Registration Request is issued without setting the S bit. This causes the Common PFA/TPFA to replace the simultaneous bindings held previously with a single binding for the MN's current location. 2.3 Foreign Agent (FA), Proxy Foreign Agent (PFA) and Top-level Proxy Foreign Agent (TPFA) functionality for Fast Handoffs Upon receipt of a Registration Request, the FA will add the Hierarchical Mobility Extension. The FA will add its own address to this extension. The FA is aware of the next higher PFA which it already includes in its advertisements. It will therefore send the packet to the next higher PFA. Intermediate FA levels act as simple routers and relay the Request. Each PFA will check whether its address corresponds to the COA. If they do not correspond then the PFA adds its address to the hierarchical agent extension list (hence forming the PFA IP address extension list) and sends the Request onto the next higher PFA which is known and included in all MA advertisements. However, if the PFA's address corresponds to the Request's COA then the PFA will authenticate the MN and terminate the Request. If the Request has the S-bit set then the PFA will add a simultaneous binding for the MN using the next lower level PFA (or FA) as destination. If the S bit is not set then it will replace the old binding it had for the MN with a new one using the next lower level PFA (or FA) as destination. El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 9] INTERNET-DRAFT Fast Handoff Method 15th June 1999 If the Request reaches the TPFA, and the TPFA is not the COA, then the TPFA relays the registration to the PHA. However, if the TPFA is the COA then it acts as a regular PFA. Registration Requests that are accepted produce a Registration Reply. All Registration Replies contain the PFA IP address extension list (Hierarchical Mobility Agent extension). This is the same extension received in the Request, but inverted. It is always relayed to the next lower level PFA. Its address is known since it is present at the top of the PFA IP address list of the Registration Reply. Upon receiving a Registration Reply the PFAs will check if their address is contained in the PFA IP address extension list. If so the PFA will add a binding for the MN using the next PFA (or FA) address in the list as destination. It will then send the Reply to the next lower PFA (or FA) by using the next PFA IP address in the list. In this way every TPFA/PFA in the hierarchy branch will create a binding for the MN with a PFA below in the hierarchy. The advantage of this approach is that when the MN traffic flow has to change part of its route as a result of MN movement then this will affect only certain TPFA/PFAs and not every MA in the branch. The disadvantage of this approach is that every TPFA/PFA is the tunnel endpoint for a MA above itself in the hierarchy but also a tunnel startpoint for a MA below in the hierarchy. As the process of detunneling and tunneling can prove very demanding it is proposed that MAs will in this case avoid detunneling. Instead it is preferred that the MA only change the appropriate fields in the outer (encapsulation) header of incoming encapsulated traffic. "Multicasting" can occur by cloning that same encapsulated traffic. The TPFA or PFA will authenticate the MN upon receiving its Registration Request. If the Request cannot be authenticated then the TPFA or PFA will issue a Registration Reply with a code of 67 (MN failed authentication). (see 3.0) 2.4 Support of "forward" and "backward" MN movement This method does not assume that the MN will only move "forward" to a new network. The MN may move into the overlap region of networks and then move back to its original position. Consider the following MN movement: Pos.1 --> Pos. 1ov2 --> Pos.1 El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 10] INTERNET-DRAFT Fast Handoff Method 15th June 1999 When the MN moves to Pos. 1ov2, the Fast Handoff method will cause the Local PFA to "multicast" traffic for the MN to both FA1 and FA2. This is done by establishing a new auxiliary route (simultaneous binding) between the Local PFA and FA2. This has a short lifetime and will be renewed as long as the MN stays in Pos. 1ov2. However, when the MN moves back to Pos. 1 the lifetime of the binding between the Local PFA and FA2 will elapse and the "multicasting" will stop. The Fast Handoff method is therefore independent of particular MN movement patterns. 2.5 Fast Handoffs applied to multiple subnetworks having a wireless overlap region. Let us consider a situation in which the MN lies in the overlap region of multiple subnetworks. Moreover, let us assume that at least one of the subnetworks consists of multiple advertising MAs. Depending on the protocol that communicates PFA IP addresses to all the MAs in the hierarchy and determines its structure, it is possible that even MAs in the same subnet might advertise different PFA IP address extension lists. Normal ECS [3] operation would force the MN to request a binding for every newly discovered MA. In the case presented this might cause traffic "multicasting" at multiple PFAs in the hierarchy. For this reason the presented method requires a "known" subnetworks list and the use of PM [3] or NAI to distinguish between advertisements coming in from known and unknown subnetworks. Unknown subnetworks are only the ones for which a MN should request a new binding. In the described method this functionality is provided by keeping a list of "known" FAs and subnetworks and using either PM or NAI (tenth step in the flowchart). Thus, as the MN enters a new multiple-agent subnetwork it will only request an additional binding for the first encountered MA. 3.0 Security Considerations It is assumed that there is a secure association between the PFAs (including the TPFA) and all the FAs in its hierarchy, in order to prevent intruders from impersonating as PFAs. Moreover, a secure association between the TPFA and the HA or PHA as in [2] is assumed. El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 11] INTERNET-DRAFT Fast Handoff Method 15th June 1999 Potentially a MN can be authenticated by multiple PFAs/TPFA in a hierarchy. A Registration Request that is positively authenticated will result in a positive Registration Reply. As the Reply is being relayed through all the PFAs in the hierarchy new bindings are created and the MN authentication is recorded for future reference. This means that in the future, when a PFA receives a Registration Request with its IP address as COA it may authenticate it and issue a Reply. Otherwise, if the Request cannot be authenticated, a Reply with code 67 (MN failed authentication) is issued. On receipt of a positive Registration Reply, the FA which hosts the MN may grant it with network access. If this Reply is not received or is negative the FA will block network access to the MN (this case is considered by RFC 2002). It is assumed that encryption will exist on the wireless medium at the link-layer. This will stop intruders from intercepting data on the wireless segments. 4.0 References [1] C. Perkins, Editor, "IP Mobility Support", RFC 2002, October 1996. [2] P. R. Calhoun, G. Montenegro, C.E. Perkins, "Mobile IP Regionalized Tunnel Management", Internet draft (work in progress), draft-ietf-mobileip-reg-tunnel-00.txt, November 1998. [3] C.E. Perkins, "Mobile IP", Addison-Wesley, ISBN 0-201-63469-4, 1998. 5.0 Author's Address Any query may be directed to: Karim El Malki Nicholas A. Fikouras Srba R. Cvetkovic Dept. of Computer Science Dept. of Computer Science Dept. of Computer Science The University of Sheffield The University of Sheffield The University of Sheffield 211 Portobello Street 211 Portobello Street 211 Portobello Street Sheffield S1 4DP, U.K. Sheffield S1 4DP, U.K. Sheffield S1 4DP, U.K. Phone: +44-114-2221935 Phone: +44-114-2221873 Phone:+44 (0)114 222 1825 Fax: +44-114-2221810 Fax: +44-114-2221810 Fax: +44-114-2228299 E-Mail: karim@dcs.shef.ac.uk E-Mail: nick@dcs.shef.ac.uk E-mail: srba@dcs.shef.ac.uk El Malki, Fikouras, Cvetkovic Expires December 1999 [Page 12]