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.fEbruAry1998Discovering 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 adver￾tisements are extended to also contain the needed care-of address, they are known as agent advertisements. Home agents and foreign agents typically broadcast agent adver￾tisements 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 spec￾ified 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 middle￾ware services. For example, a mobile computer might need to be reconfigured differently at each different point of attach￾ment. 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 Con￾figuration 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 presen￾tation 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 infra￾structure connectivity. Mobile computers introduce more variabil￾ity into this mix and thus exacerbate the growing need for multi￾media 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 compli￾cate application response to connection status information. Lastly, a word should be said about the granularity of pro￾tocol 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 attach￾ment to the Internet. However, acceptance of this mode of oper￾ation 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 automat￾ic, 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 Exten￾sions,” RFC 2132, Mar. 1997. 6. J. Veizades, et al., “Service Location Protocol,” RFC 2165, July 1997