TUT。RIAL MOBILE NETWORKING THROUGH MOBILE P CHARLES E. PERKINS Mobile IP is a proposed standard protocol that builds on the Internet Protocol by making mobility transparent to applications and higher level protocols like TCP to us as we move will giv net offers access us the tools to build new information computing environments sources worldwide, typi wherever we go. Those cally we do not expect 国变 who have little interest in nefit from that access mobility per se will still ntil we arrive at some familiar point-whether home, office, benefit from the ability to resume previous applications when or school. However, the increasing variety of wireless devices they reconnect. This is especially convenient in a wireless lan offering IP connectivity, such as PDAs, handhelds, and digi- office environment, where the boundaries between attachment tal cellular phones, is beginning to change our perceptions of points are not sharp and are often invisible the Internet The evolution of mobile networking will differ from that of To understand the contrast between the current realities telephony in some important respects. The endpoints of a tele- of IP connectivity and future possibilities, consider the tran- phone connection are typically human; computer applications sition toward mobility that has occurred in telephony over are likely to involve interactions between machines without the past 20 years. An analogous transition in the domain of human intervention. Obvious examples of this are mobile com networking, from dependence on fixed points of attachment puting devices on airplanes, ship aepen: on position-finding and automobiles. mobile to the flexibility afforded by mobility, has just begun. e Mobile computing and networking should not be con- devices, such as a satellite global positioning system, to work sed with the portable computing and networking we have in tandem with wireless access to the Internet. today. In mobile networking, computing activities are not dis- Another difference may well be rate of adoption. It took rupted when the user changes the computer's point of attach- many years for mobile phones to become cheap and light ment to the Internet. Instead, all the needed reconnection weight enough to be perceived as convenient. Because wireless o truly mobile computing offers many advantages. Confident nizers have already found user acceptance, mobile comput- cess to the Internet anytime, anywhere will help free us from ing may become popular much more quickly. the ties that bind us to our desktops. Consider how cellular However, there are still some technical obstacles that must phones have given people new freedom in carrying out their be overcome before mobile networking can become wide work. Taking along an entire computing environment has the spread. The most fundamental is the way the Internet Proto- potential not just to extend that flexibility but to fundamental- col, the protocol that connects the networks of todays Inter ly change the existing work ethic. Having the Internet available net, routes packets to their destinations according to IP 98/s100o1998IEE IEEE INTERNET COMPUTING
. TUTORIAL 58 1089-7801/98/$10.00 ©1998 IEEE IEEE INTERNET COMPUTING TUTORIAL MOBILE NETWORKING THROUGH MOBILE IP CHARLES E. PERKINS Sun Microsystems Although the Internet offers access to information sources worldwide, typically we do not expect to benefit from that access until we arrive at some familiar point—whether home, office, or school. However, the increasing variety of wireless devices offering IP connectivity, such as PDAs, handhelds, and digital cellular phones, is beginning to change our perceptions of the Internet. To understand the contrast between the current realities of IP connectivity and future possibilities, consider the transition toward mobility that has occurred in telephony over the past 20 years. An analogous transition in the domain of networking, from dependence on fixed points of attachment to the flexibility afforded by mobility, has just begun. Mobile computing and networking should not be confused with the portable computing and networking we have today. In mobile networking, computing activities are not disrupted when the user changes the computer’s point of attachment to the Internet. Instead, all the needed reconnection occurs automatically and noninteractively. Truly mobile computing offers many advantages. Confident access to the Internet anytime, anywhere will help free us from the ties that bind us to our desktops. Consider how cellular phones have given people new freedom in carrying out their work. Taking along an entire computing environment has the potential not just to extend that flexibility but to fundamentally change the existing work ethic. Having the Internet available to us as we move will give us the tools to build new computing environments wherever we go. Those who have little interest in mobility per se will still benefit from the ability to resume previous applications when they reconnect. This is especially convenient in a wireless LAN office environment, where the boundaries between attachment points are not sharp and are often invisible. The evolution of mobile networking will differ from that of telephony in some important respects. The endpoints of a telephone connection are typically human; computer applications are likely to involve interactions between machines without human intervention. Obvious examples of this are mobile computing devices on airplanes, ships, and automobiles. Mobile networking may well also come to depend on position-finding devices, such as a satellite global positioning system, to work in tandem with wireless access to the Internet. Another difference may well be rate of adoption. It took many years for mobile phones to become cheap and lightweight enough to be perceived as convenient. Because wireless mobile computing devices such as PDAs and pocket organizers have already found user acceptance, mobile computing may become popular much more quickly. However, there are still some technical obstacles that must be overcome before mobile networking can become widespread. The most fundamental is the way the Internet Protocol, the protocol that connects the networks of today’s Internet, routes packets to their destinations according to IP Mobile IP is a proposed standard protocol that builds on the Internet Protocol by making mobility transparent to applications and higher level protocols like TCP
M O addresses These addresses are associated with a fixed net- Ip address is connected. The network number is derived york location much as a nonmobile phone number is asso- from the IP address by masking off some of the low-order ciated with a physical jack in a wall. When the packets des- bits. Thus, the Ip address typically carries with it informa- tination is a mobile node, this means that each new point of tion that specifies the Ip nodes point of attachment attachment made by the node is associated with a new net- To maintain existing transport-layer connections(see the work number and, hence, a new IP address, making trans- sidebar"Nomadicity: How Mobility Will Affect the Protocol parent mobility impossible. Stack"on the next pages) as the mobile node moves from place Mobile IP(RFC 2002), a standard proposed by a work to place, it must keep its IP address the same InTCP(which ing group within the Internet Engineering Task Force, was accounts for the overwhelming majority of Internet connec- designed to solve this problem by allowing the mobile node tions), connections are indexed by a quadruplet that contains to use two IP addresses: a fixed home address and a care-of the IP addresses and port numbers of both connection end- address that changes at each new point of attachment. This points. Changing any of these four numbers will cause the con- article will present the Mobile IP standard in moderate nection to be disrupted and lost. On the other hand, correct technical detail and point the reader toward a wealth of fur- delivery of packets to the mobile node's current point of attach- ther information.2.In addition, readers can go to the side- ment depends on the network number contained within the bar Mobile IP Web Resources in this issue's IC Online at mobile node's IP address, which changes at new points of http://computer.org/internet/foraconvenientsetofhyperattachmentTochangetheroutingrequiresanewIpaddress linked resources associated with the new point I also describe how Mobile ip uses two iP addresses. of attachment Mobile ip has beel IP version 6, 4.5 the product a fixed home address and a care-of designed to solve this problem of a major effort within the address that changes at each new by allowing the mobile nodeto IETF to engineer an eventu- point of attachment use two IP addresses(see the al replacement for the cur- sidebar rent version of IP Although Terminology" for definitions Pv6 will support mobility to a greater degree than IPv4, it of italicized terms). In Mobile IP, the home address static and will still need Mobile IP to make mobility transparent to is used, for instance, to identify TCP connections. The care-of applications and higher level protocols such as TCP. address changes at each new point of attachment and can be There is a great deal of interest in mobile computing and thought of as the mobile node's topologically significant pparently in Mobile Ip as a way to provide for it. a quick address; it indicates the network number and thus identifies Web search for items related to Mobile IP returned over the mobile node's point of attachment with respect to the net 60,000 hits-impressive even given the notorious lack of work topology. The home address makes it appear that the selectivity for such procedures. Mobile IP forms the basis mobile node is continually able to receive data on its home net- either directly or indirectly of many current research efforts work, where Mobile IP requires the existence of a network and products. The Cellular Digital Packet Data(CDPD), node known as the home agent. Whenever the mobile node is for example, has created a widely deployed communications not attached to its home network(and is therefore attached infrastructure based on a previous draft specification of the to what is termed a foreign network), the home agent gets all protocol. In addition, most major router vendors have devel- the packets destined for the mobile node and arranges to deliv oped implementations for Mobile IP er them to the mobile node's current point of attachment The outlook for Mobile IP in the complex Internet mar- Whenever the mobile node moves, it registersits new care- ketplace is far from clear, and some technical problems remain, of address with its home agent. To get a packet to a mobile security being the most important. However, once the securi- node from its home network, the home agent delivers the ty solutions are solid, nomadic users may finally begin to enjoy packet from the home network to the care-of address. The the convenience of seamless untethered roaming and effective further delivery requires that the packet be modified so that application transparency that is the promise of Mobile IP. the care-of address appears as the destination IP address. This modification can be understood as a packet transformation HOW MOBILE IP WORKS or, more specifically, a redirection. When the packet arrives at IP routes packets from a source endpoint to a destination by the care-of address, the reverse transformation is applied so allowing routers to forward packets from incoming network that the packet once again appears to have the mobile node interfaces to outbound interfaces according to routing tables. home address as the destination IP address. When the pack The routing tables typically maintain the next-hop(out- et arrives at the mobile node, addressed to the home address bound interface)information for each destination IP it will be processed properly by TCP or whatever higher level address, according to the number of networks to which that protocol logically receives it from the mobile node's lp (that IEEE INTERNET COMPUTING http://computer.org/internet/JanUaRy.fEbruAry1998
addresses. These addresses are associated with a fixed network location much as a nonmobile phone number is associated with a physical jack in a wall. When the packet’s destination is a mobile node, this means that each new point of attachment made by the node is associated with a new network number and, hence, a new IP address, making transparent mobility impossible. Mobile IP (RFC 2002),1 a standard proposed by a working group within the Internet Engineering Task Force, was designed to solve this problem by allowing the mobile node to use two IP addresses: a fixed home address and a care-of address that changes at each new point of attachment. This article will present the Mobile IP standard in moderate technical detail and point the reader toward a wealth of further information.2,3 In addition, readers can go to the sidebar Mobile IP Web Resources in this issue’s IC Online at http://computer.org/internet/ for a convenient set of hyperlinked resources. I also describe how Mobile IP will change with IP version 6,4,5 the product of a major effort within the IETF to engineer an eventual replacement for the current version of IP.6 Although IPv6 will support mobility to a greater degree than IPv4, it will still need Mobile IP to make mobility transparent to applications and higher level protocols such as TCP. There is a great deal of interest in mobile computing and apparently in Mobile IP as a way to provide for it. A quick Web search for items related to Mobile IP returned over 60,000 hits—impressive even given the notorious lack of selectivity for such procedures. Mobile IP forms the basis either directly or indirectly of many current research efforts and products. The Cellular Digital Packet Data (CDPD),7 for example, has created a widely deployed communications infrastructure based on a previous draft specification of the protocol. In addition, most major router vendors have developed implementations for Mobile IP. The outlook for Mobile IP in the complex Internet marketplace is far from clear, and some technical problems remain, security being the most important. However, once the security solutions are solid, nomadic users may finally begin to enjoy the convenience of seamless untethered roaming and effective application transparency that is the promise of Mobile IP. HOW MOBILE IP WORKS IP routes packets from a source endpoint to a destination by allowing routers to forward packets from incoming network interfaces to outbound interfaces according to routing tables. The routing tables typically maintain the next-hop (outbound interface) information for each destination IP address, according to the number of networks to which that IP address is connected. The network number is derived from the IP address by masking off some of the low-order bits. Thus, the IP address typically carries with it information that specifies the IP node’s point of attachment. To maintain existing transport-layer connections (see the sidebar “Nomadicity: How Mobility Will Affect the Protocol Stack” on the next pages) as the mobile node moves from place to place, it must keep its IP address the same. In TCP (which accounts for the overwhelming majority of Internet connections), connections are indexed by a quadruplet that contains the IP addresses and port numbers of both connection endpoints. Changing any of these four numbers will cause the connection to be disrupted and lost. On the other hand, correct delivery of packets to the mobile node’s current point of attachment depends on the network number contained within the mobile node’s IP address, which changes at new points of attachment. To change the routing requires a new IP address associated with the new point of attachment. Mobile IP has been designed to solve this problem by allowing the mobile node to use two IP addresses (see the sidebar “Mobile Networking Terminology” for definitions of italicized terms). In Mobile IP, the home address is static and is used, for instance, to identify TCP connections. The care-of address changes at each new point of attachment and can be thought of as the mobile node’s topologically significant address; it indicates the network number and thus identifies the mobile node’s point of attachment with respect to the network topology. The home address makes it appear that the mobile node is continually able to receive data on its home network, where Mobile IP requires the existence of a network node known as the home agent. Whenever the mobile node is not attached to its home network (and is therefore attached to what is termed a foreign network), the home agent gets all the packets destined for the mobile node and arranges to deliver them to the mobile node’s current point of attachment. Whenever the mobile node moves, it registers its new careof address with its home agent. To get a packet to a mobile node from its home network, the home agent delivers the packet from the home network to the care-of address. The further delivery requires that the packet be modified so that the care-of address appears as the destination IP address. This modification can be understood as a packet transformation or, more specifically, a redirection. When the packet arrives at the care-of address, the reverse transformation is applied so that the packet once again appears to have the mobile node’s home address as the destination IP address. When the packet arrives at the mobile node, addressed to the home address, it will be processed properly by TCP or whatever higher level protocol logically receives it from the mobile node’s IP (that MOBILE IP 59 IEEE INTERNET COMPUTING http://computer.org/internet/ JANUARY • FEBRUARY 1998 . Mobile IP uses two IP addresses: a fixed home address and a care-of address that changes at each new point of attachment
MO B LEc。 M PUTIN C NOMADICITY: HOW MOBILITY WILL AFFECT THE PROTOCOL STACK Mobile IP is a large part, but by no means the only part, of the media typically introduce many other design requirements at story of mobile computing and networking. To see Mobile IP in layer two. In particular, the desire for confidentiality leads to its true place requires an understanding of the relationships the incorporation of encryption techniques, especially for wire- between the various layers of network protocols. Each layer less links. Often, lower bandwidth(compared with wired should present a clear model of operation to the architect. Once media) suggests the use of compression techniques. And, typ- the model is identified, the effects of mobility can be studied in ically, transmitting a signal causes the local receiver to lose relation to it. Nomadicity is the name used by the Cross-industry detection of any other signal because of the great difference Working Team (XIWT) at the Corporation for National in effective power levels between local and remote transmit Research Initiatives(CNRi) to denote an architecture for the ters. Thus, collision-detection techniques, such as those used with entire mobile computing environment Ethernet, must be replaced by less reliable collision-avoidance Figure A is a simplified view of the International Standards measures and careful etiquette Organizations protocol stack as it applies to Internet network- Other distinguishing characteristics of wireless communica- ing. The major goal of Mobile IP protocol design was to handle tions media include the difficulty of establishing a precise range mobility at the network layer and to leave transport and other (cell size)for connectivity to the medium, and the ability for sep- higher layers unaffected, so that the existing routing infra- arate stations to use the media without interference. This latter structure, nonmobile hosts, and current applications would not property of reuse depends upon avoiding interference between be required to change neighboring transmitters, and a great engineering discipline Protocol layer two, the data link layer, is responsible for link has been built up to understand optimal placement of such wire- establishment and maintenance. Thus, physical effects from less equipment as base stations. To reuse the physical wireless mobility are likely to require changes in the layer-two proto- medium to the fullest extent, the cell size should be as small as ols. Changes in position affect the signal-to-interference ratio possible. This means that as demand for wireless communica (SIR). Link layers that adapt forward error correction to SIR can tions increases, cell sizes will decrease, and the frequency with xhibit variable bandwidth but far fewer lost packets. Wireless which mobile computers will switch cells(change their point of attachment to the Internet) will grow correspondingly The Internet Protocol is at layer three, the network layer. Networking Layers Standard Protocols elects routes(determines paths) through a loosely confeder- Applications Http Nfs, Snmp Dns. Telnet ated association of independent network links. IP offers rout- ing from one network to another, in addition to some minor Window Mgr services such as fragmentation and reassembly, and check Sockets summing. Moving from one place to another can be modeled TCP UDP RTP as changing the network nodes point of attachment to the Inter IP ICMP IGMP IPSec. Mobile IP net Supporting mobility at this layer is therefore naturally mod- (IPX, Appletalk eled as changing the routing of datagrams destined for the Data Link IEEE 802* PPP mobile node so that they arrive at the new point of attachment. Network adapter This turns out to be a very convenient choice, and was the option chosen by the Mobile IP working group At the fransport layer, TCP(RFC 793)and other transport Figure A. The Internet networking stack showing protocols attempt to offer a more convenient abstraction for common protocols associated with each layer. data services than the characteristically chaotic stream of data rer 3)processing layer. More information on the abstract until it arrives at the care-of address. Such encapsulation is also ling as a way to perform layer 3 redirection on packets called tunneling which suggests that the packet burrows can be found in Bhagwat, Perkins, and Tripathi through the Internet, bypassing the usual effects of IP routing In Mobile IP the home agent redirects packets from the Mobile IP, then, is best understood as the cooperation of home network to the care-of address by constructing a new Ip three separable mechanisms header that contains the mobile node's care-of address as the destination IP address. This new header then shields or encap- Discovering the care-of address: sulates the original packet, causing the mobile node's home Registering the care-of address address to have no effect on the encapsulated packets routing Tunneling to the care-of address JanUarY.feBruAry1998http://computer.org/internet/ IEEE INTERNET COMPUTING
is, layer 3) processing layer. More information on the abstract modeling as a way to perform layer 3 redirection on packets can be found in Bhagwat, Perkins, and Tripathi.8 In Mobile IP the home agent redirects packets from the home network to the care-of address by constructing a new IP header that contains the mobile node’s care-of address as the destination IP address. This new header then shields or encapsulates the original packet, causing the mobile node’s home address to have no effect on the encapsulated packet’s routing until it arrives at the care-of address. Such encapsulation is also called tunneling, which suggests that the packet burrows through the Internet, bypassing the usual effects of IP routing. Mobile IP, then, is best understood as the cooperation of three separable mechanisms: ■ Discovering the care-of address; ■ Registering the care-of address; ■ Tunneling to the care-of address. MOBILE COMPUTING 60 JANUARY • FEBRUARY 1998 http://computer.org/internet/ IEEE INTERNET COMPUTING . Mobile IP is a large part, but by no means the only part, of the story of mobile computing and networking. To see Mobile IP in its true place requires an understanding of the relationships between the various layers of network protocols. Each layer should present a clear model of operation to the architect. Once the model is identified, the effects of mobility can be studied in relation to it. Nomadicity is the name used by the Cross-Industry Working Team (XIWT) at the Corporation for National Research Initiatives (CNRI) to denote an architecture for the entire mobile computing environment.1 Figure A is a simplified view of the International Standards Organization’s protocol stack as it applies to Internet networking. The major goal of Mobile IP protocol design was to handle mobility at the network layer and to leave transport and other higher layers unaffected, so that the existing routing infrastructure, nonmobile hosts, and current applications would not be required to change. Protocol layer two, the data link layer, is responsible for link establishment and maintenance. Thus, physical effects from mobility are likely to require changes in the layer-two protocols. Changes in position affect the signal-to-interference ratio (SIR). Link layers that adapt forward error correction to SIR can exhibit variable bandwidth but far fewer lost packets. Wireless media typically introduce many other design requirements at layer two. In particular, the desire for confidentiality leads to the incorporation of encryption techniques, especially for wireless links. Often, lower bandwidth (compared with wired media) suggests the use of compression techniques. And, typically, transmitting a signal causes the local receiver to lose detection of any other signal because of the great difference in effective power levels between local and remote transmitters. Thus, collision-detection techniques, such as those used with Ethernet, must be replaced by less reliable collision-avoidance measures and careful etiquette. Other distinguishing characteristics of wireless communications media include the difficulty of establishing a precise range (cell size) for connectivity to the medium, and the ability for separate stations to use the media without interference. This latter property of reuse depends upon avoiding interference between neighboring transmitters, and a great engineering discipline has been built up to understand optimal placement of such wireless equipment as base stations. To reuse the physical wireless medium to the fullest extent, the cell size should be as small as possible. This means that as demand for wireless communications increases, cell sizes will decrease, and the frequency with which mobile computers will switch cells (change their point of attachment to the Internet) will grow correspondingly. The Internet Protocol is at layer three, the network layer. IP selects routes (determines paths) through a loosely confederated association of independent network links. IP offers routing from one network to another, in addition to some minor services such as fragmentation and reassembly, and checksumming. Moving from one place to another can be modeled as changing the network node’s point of attachment to the Internet. Supporting mobility at this layer is therefore naturally modeled as changing the routing of datagrams destined for the mobile node so that they arrive at the new point of attachment. This turns out to be a very convenient choice, and was the option chosen by the Mobile IP working group. At the transport layer, TCP (RFC 793)2 and other transport protocols attempt to offer a more convenient abstraction for data services than the characteristically chaotic stream of data NOMADICITY: HOW MOBILITY WILL AFFECT THE PROTOCOL STACK Networking Layers Standard Protocols Applications HTTP, NFS, SNMP, DNS, Telnet, FTP, ... Window Mgr Sockets Transport TCP, UDP, RTP Network IP, ICMP, IGMP, IPSec, Mobile IP ... (IPX, Appletalk) Data Link IEEE 802.*, PPP Physical Network adapter Figure A. The Internet networking stack showing common protocols associated with each layer
M O emanating from IP. The vagaries and time dependencies of can easily affect the applications desired operation. The simplest routers and Internet congestion often cause datagrams to be example is the need for Web applications to adjust their presen- delivered out of order, duplicated, or even dropped entirely tation of graphical data depending on the available end-to-end before reaching their destination. TCP attempts to solve those bandwidth. Today that bandwidth is largely constrained by the problems, but offers little help in supplying a steady (constant link conditions at the endpoints and the congestion status of infra- bandwidth)stream of data, or in delivering data within spec- structure connectivity. Mobile computers introduce more variabl- fied time bounds. Over time, TCP has been modified to treat ity into this mix and thus exacerbate the growing need for multi- dropped packets as an indication of network congestion, and media applications to detect and act on dynamic connection therefore to throttle transmissions as soon as a lost packet is parameters, such as link bandwidth, error rate, and round-trip detected (by managing sequence numbers). This is the wrong times. Other logical parameters, such as cost and security, may strategy when packets are corrupted by transmission over a eventually exhibit similar dynamic behavior and further compli- noisy wireless channel, because for such packets immediate cate application response to connection status information retransmission is much better than delayed retransmission. Lastly, a word should be said about the granularity of pro Ways to change this behavior are still under debate tocol response to node movement. Today's typical user must be At the top layer are the application protocols Depending content with portable computing, which requires reinitializing on the transport model employed, application protocols are and reestablishing connections at each new point of attach- largely freed from much of the drudgery of error correction, ment to the Internet. However, acceptance of this mode of oper transmission, flow control, and the like. However, mobility ation may well evaporate if the reinitialization process has to be creates new needs at the application layer, which require performed a lot more frequently Left unchecked, the expected additional protocol support: automatic configuration, service decreases in cell sizes will require exactly that in the future. The discovery, link awareness, and environment awareness existing methods typify portable network computing, which These protocol support mechanisms form a set of middle- means establishing the availability of network computing when are services. For example, a mobile computer might need to one arrives at a new point of attachment but being unable to be reconfigured differently at each different point of attach- continue previous computing activities. The point of Mobile IP. ment. Among other things, a new DNS server, IP address, link DHCP and similar protocols is to provide completely automat MTU, and default router may be required. These configuration ic, noninteractive reconnection to those activities items are usually thought of as being worked out at setup time for desktop systems, but for mobile computers no single answer REFERENCES can be sufficient. Recent deployment of the Dynamic Host Con- 1. Corporation for National Research Initiatives. XIWT: Cross-Industry figuration Protocol(RFCs 2131, 2132)", goes some way WorkingTeam,1994,http://www.cnri.restonva.us3000/xiwt/ toward resolving configuration difficulties, but is not the whole public. html. answer. Discovering services can be modeled as a requirement 2. "Transmission Control Protocol, "J.B. Postel, ed, RFC 793, Sept. 1981 for automatic configuration, but is more naturally useful when 3. W. Stevens, "TCP Sow Start, Congestion Avoidance, Fast Retransmit, services are located upon demand and according to the needs and Fast Recovery Algorithms, "RFC 2001, Jan 1997 of application protocols. This need is just now being met by the 4. R. Droms, ' Dynamic Host Configuration Protocol, "RFC 2131, Mar Service Location Protocol(RFC 2165).6 1997, ftp: //ds internic. net/rfc/rfc2131. txt. One of the more challenging aspects of architecting such 5. S. Alexander and R Droms, "DHCP Options and BOOTP Vendor Exten- iddleware lies in offering applications the opportunity to detect sions, " RFC 2132. Mar 1997 he state of the physical link, which changes dynamically and 6. J. Veizades, et al., "Service Location Protocol, "RFC 2165, July 1997 Discovering the Care-of Address tisements are extended to also contain the needed care-of The Mobile IP discoveryprocess has been built on top of an address, they are known as agent advertisements Home existing standard protocol, Router Advertisement, specified agents and foreign agents typically broadcast agent adver- in RFC 1256. Mobile ip discovery does not modify the tisements at regular intervals(for example, once a second or original fields of existing router advertisements but simply once every few seconds ). If a mobile node needs to get extends them to associate mobility functions. Thus, a router care-of address and does not wish to wait for the periodic advertisement can carry information about default routers, advertisement, the mobile node can broadcast or multicast just as before, and in addition carry further information solicitation that will be answered by any foreign agent about one or more care-of addresses. When the router adver- home agent that receives it IEEE INTERNET COMPUTING http://computer.org/internet/JanUaRy.fEbruAry1998
Discovering the Care-of Address The Mobile IP discovery process has been built on top of an existing standard protocol, Router Advertisement, specified in RFC 1256.9 Mobile IP discovery does not modify the original fields of existing router advertisements but simply extends them to associate mobility functions. Thus, a router advertisement can carry information about default routers, just as before, and in addition carry further information about one or more care-of addresses. When the router advertisements are extended to also contain the needed care-of address, they are known as agent advertisements. Home agents and foreign agents typically broadcast agent advertisements at regular intervals (for example, once a second or once every few seconds). If a mobile node needs to get a care-of address and does not wish to wait for the periodic advertisement, the mobile node can broadcast or multicast a solicitation that will be answered by any foreign agent or home agent that receives it. MOBILE IP 61 IEEE INTERNET COMPUTING http://computer.org/internet/ JANUARY • FEBRUARY 1998 . emanating from IP. The vagaries and time dependencies of routers and Internet congestion often cause datagrams to be delivered out of order, duplicated, or even dropped entirely before reaching their destination. TCP attempts to solve those problems, but offers little help in supplying a steady (constant bandwidth) stream of data, or in delivering data within specified time bounds. Over time, TCP has been modified to treat dropped packets as an indication of network congestion, and therefore to throttle transmissions as soon as a lost packet is detected (by managing sequence numbers).3 This is the wrong strategy when packets are corrupted by transmission over a noisy wireless channel, because for such packets immediate retransmission is much better than delayed retransmission. Ways to change this behavior are still under debate. At the top layer are the application protocols. Depending on the transport model employed, application protocols are largely freed from much of the drudgery of error correction, retransmission, flow control, and the like. However, mobility creates new needs at the application layer, which require additional protocol support: automatic configuration, service discovery, link awareness, and environment awareness. These protocol support mechanisms form a set of middleware services. For example, a mobile computer might need to be reconfigured differently at each different point of attachment. Among other things, a new DNS server, IP address, link MTU, and default router may be required. These configuration items are usually thought of as being worked out at setup time for desktop systems, but for mobile computers no single answer can be sufficient. Recent deployment of the Dynamic Host Configuration Protocol (RFCs 2131, 2132)4,5 goes some way toward resolving configuration difficulties, but is not the whole answer. Discovering services can be modeled as a requirement for automatic configuration, but is more naturally useful when services are located upon demand and according to the needs of application protocols. This need is just now being met by the Service Location Protocol (RFC 2165).6 One of the more challenging aspects of architecting such middleware lies in offering applications the opportunity to detect the state of the physical link, which changes dynamically and can easily affect the application’s desired operation. The simplest example is the need for Web applications to adjust their presentation of graphical data depending on the available end-to-end bandwidth. Today that bandwidth is largely constrained by the link conditions at the endpoints and the congestion status of infrastructure connectivity. Mobile computers introduce more variability into this mix and thus exacerbate the growing need for multimedia applications to detect and act on dynamic connection parameters, such as link bandwidth, error rate, and round-trip times. Other logical parameters, such as cost and security, may eventually exhibit similar dynamic behavior and further complicate application response to connection status information. Lastly, a word should be said about the granularity of protocol response to node movement. Today’s typical user must be content with portable computing, which requires reinitializing and reestablishing connections at each new point of attachment to the Internet. However, acceptance of this mode of operation may well evaporate if the reinitialization process has to be performed a lot more frequently. Left unchecked, the expected decreases in cell sizes will require exactly that in the future. The existing methods typify portable network computing, which means establishing the availability of network computing when one arrives at a new point of attachment but being unable to continue previous computing activities. The point of Mobile IP, DHCP and similar protocols is to provide completely automatic, noninteractive reconnection to those activities. REFERENCES 1. Corporation for National Research Initiatives. XIWT: Cross-Industry Working Team, 1994, http://www.cnri.reston.va.us:3000/XIWT/ public.html. 2. “Transmission Control Protocol,” J. B. Postel, ed., RFC 793, Sept. 1981. 3. W. Stevens, “TCP Sow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms,” RFC 2001, Jan. 1997. 4. R. Droms, “Dynamic Host Configuration Protocol,” RFC 2131, Mar. 1997, ftp://ds.internic.net/rfc/rfc2131.txt. 5. S. Alexander and R. Droms, “DHCP Options and BOOTP Vendor Extensions,” RFC 2132, Mar. 1997. 6. J. Veizades, et al., “Service Location Protocol,” RFC 2165, July 1997
MO B LEc。 M PUTIN C mobile nodes network interface. In this sit uation, the mobile node should begin to FA advertises service hunt for a new care-of address, or possibly se a care-of address known from advertise ments it is still receiving. The mobile node MH requests service may choose to wait for another advertise- FA relays request to HA ment if it has not received any recently adver tised care-of addresses, or it may send an nt Registering the Care-of Address Once a mobile node has a care-of address FA relays status to MH HA accepts or denies its home agent must find out about it. Fig- ure 1 shows the registration process defined by Mobile IP for this purpose. The process begins when the mobile node, possibly with the assistance of a foreign agent, sends a reg Figure 1. Registration operations in Mobile IP. FA is foreign agent, HA is istration request with the care-of address home address and MH is mobile host information. When the home agent receives this request, it(typically) adds the necessary information to its routing table, approves Home agents use agent advertisements to make them- the request, and sends a registration reply back to the mobile selves known, even if they do not offer any care-of address- node. Although the home agent is not required by the es. However, it is not possible to associate preferences to the Mobile IP protocol to handle registration requests by updat various care-of addresses in the router advertisement, as is ing entries in its routing table, doing so offers a natural the case with default routers. The IETF working group was implementation strategy, and all implementations I know concerned that dynamic preference values might destabilize of take this approach the operation of Mobile IP. Because no one could defend static preference assignments except for backup mobility Authentication. Registration requests contain parameters agents, which do not help distribute the routing load, the and flags that characterize the tunnel through which the group eventually decided not to use the preference assign- home agent will deliver packets to the care-of address. Tun ments with the care-of address list nels can be constructed in various ways, described briefly in Thus, an agent advertisement performs the following the next section. 0. I When a home agent accepts the functions. request, it begins to associate the home address of the mobile node with the care-of address and maintains this associa allows for the detection of mobility agents tion until the registration lifetime expires. The triplet that a lists one or more available care-of addresses contains the home address, care-of address, and registration forms the mobile node about special features provided lifetime is called a binding for the mobile node. a registra by foreign agents, for example, alternative encapsulation tion request can be considered a binding update sent by the a lets mobile nodes determine the network number and mobile node techniques a binding update is an example of a remote redirect, status of their link to the internet and because it is sent remotely to the home agent to affect the lets the mobile node know whether the agent is a home home agents routing table. This view of registration makes agent, a foreign agent, or both, and therefore whether it the need for authentication very clear. 2 The home agent is on its home network or a foreign network must be certain registration was originated by the mobile node and not by some other malicious node pretending to Mobile nodes use router solicitations as defined in rfc 1256 be the mobile node. a malicious node could cause the home to detect any change in the set of mobility agents available at agent to alter its routing table with erroneous care-of address the current point of attachment. (In Mobile IP this is then information, and the mobile node would be unreachable to termed agent solicitation. If advertisements are no longer all incoming communications from the Internet. detectable from a foreign agent that previously had offered a The need to authenticate registration information has care-of address to the mobile node, the mobile node should played a major role in determining the acceptable design presume that foreign agent is no longer within range of the parameters for Mobile IP. Each mobile node and home agent JanUarY.feBruAry1998http://computer.org/internet/ IEEE INTERNET COMPUTING
Home agents use agent advertisements to make themselves known, even if they do not offer any care-of addresses. However, it is not possible to associate preferences to the various care-of addresses in the router advertisement, as is the case with default routers. The IETF working group was concerned that dynamic preference values might destabilize the operation of Mobile IP. Because no one could defend static preference assignments except for backup mobility agents, which do not help distribute the routing load, the group eventually decided not to use the preference assignments with the care-of address list. Thus, an agent advertisement performs the following functions: ■ allows for the detection of mobility agents; ■ lists one or more available care-of addresses; ■ informs the mobile node about special features provided by foreign agents, for example, alternative encapsulation techniques; ■ lets mobile nodes determine the network number and status of their link to the Internet; and ■ lets the mobile node know whether the agent is a home agent, a foreign agent, or both, and therefore whether it is on its home network or a foreign network. Mobile nodes use router solicitations as defined in RFC 1256 to detect any change in the set of mobility agents available at the current point of attachment. (In Mobile IP this is then termed agent solicitation.) If advertisements are no longer detectable from a foreign agent that previously had offered a care-of address to the mobile node, the mobile node should presume that foreign agent is no longer within range of the mobile node’s network interface. In this situation, the mobile node should begin to hunt for a new care-of address, or possibly use a care-of address known from advertisements it is still receiving. The mobile node may choose to wait for another advertisement if it has not received any recently advertised care-of addresses, or it may send an agent solicitation. Registering the Care-of Address Once a mobile node has a care-of address, its home agent must find out about it. Figure 1 shows the registration process defined by Mobile IP for this purpose. The process begins when the mobile node, possibly with the assistance of a foreign agent, sends a registration request with the care-of address information. When the home agent receives this request, it (typically) adds the necessary information to its routing table, approves the request, and sends a registration reply back to the mobile node. Although the home agent is not required by the Mobile IP protocol to handle registration requests by updating entries in its routing table, doing so offers a natural implementation strategy, and all implementations I know of take this approach. Authentication. Registration requests contain parameters and flags that characterize the tunnel through which the home agent will deliver packets to the care-of address. Tunnels can be constructed in various ways, described briefly in the next section.10,11 When a home agent accepts the request, it begins to associate the home address of the mobile node with the care-of address, and maintains this association until the registration lifetime expires. The triplet that contains the home address, care-of address, and registration lifetime is called a binding for the mobile node. A registration request can be considered a binding update sent by the mobile node. A binding update is an example of a remote redirect, because it is sent remotely to the home agent to affect the home agent’s routing table. This view of registration makes the need for authentication very clear.12 The home agent must be certain registration was originated by the mobile node and not by some other malicious node pretending to be the mobile node. A malicious node could cause the home agent to alter its routing table with erroneous care-of address information, and the mobile node would be unreachable to all incoming communications from the Internet. The need to authenticate registration information has played a major role in determining the acceptable design parameters for Mobile IP. Each mobile node and home agent MOBILE COMPUTING 62 JANUARY • FEBRUARY 1998 http://computer.org/internet/ IEEE INTERNET COMPUTING . FA FA HA FA FA advertises service MH requests service FA relays request to HA FA relays status to MH HA accepts or denies Figure 1. Registration operations in Mobile IP. FA is foreign agent, HA is home address, and MH is mobile host