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Versions: 00                                                            
Internet Engineering Task Force                   G. Ruth, R. Yuan
INTERNET DRAFT                                    GTE Laboratories
                                                     6 August 1996

         Interworking Between CDPD and Mobile IP Networks


Two protocols, CDPD (Cellular Digital Packet Data) and Mobile-IP
have been developed by the CDPD Forum and IETF (Internet
Engineering Task Force) respectively to address the issue of
providing seamless network access to mobile data devices. In this
memo a scheme is proposed for the two networks to interwork
together and to support seamless migration of mobile data devices
between the networks.

Status of this Memo

This document is an Internet-Draft. 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

To learn the current status of any Internet-Draft, please check
the ``1id-abstracts.txt'' listing contained in the Internet-
Drafts Shadow Directories on ds.internic.net (US East Coast),
nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or
munnari.oz.au (Pacific Rim).

1. Introduction

Two protocols, CDPD and Mobile IP, have been developed in the past
few years to address the issue of network layer mobility support
for the general purpose data network. Both protocols enable a
mobile terminal to migrate seamlessly from one local area network
to another.

The CDPD (Cellular Digital Packet Data) standard was developed by
the CDPD Forum (an industrial association of cellular carriers,
equipment vendors and application developers) to provide packet
data services through the cellular telephony network. It specifies
a set of mobility enabling protocols for use in the CDPD networks.
CDPD networks is being deployed nationwide by the cellular
carriers. The latest specification CDPD Specification, version 1.1
was published in January 1995 [1].

The Mobile-IP protocol has been developed by the IETF to provide
mobility support in the current TCP/IP Internet. Mobile-IP is
designed to support transparent host migration among a variety of
IP subnetworks.

The concepts and principles for mobility management in both
protocols are the same and many of mobility support functions are
similar. However, while CDPD is designed to be a tariffed, carrier
operated service with uniform link layer infrastructure (it has
been widely deployed by many cellular carrier), Mobile IP is
designed to support a variety of heterogeneous subnetworks. Thus,
many differences exist between the two protocols. Currently, if a
Mobile IP host migrates into a CDPD coverage area (or vice versa),
its network connection will be terminated even with a CDPD
wireless modem. This is because the network layer protocols for
mobility support of the two networks cannot interoperable with
each other. Therefore, to enable universal network connectivity
for mobile hosts, it is necessary to provide methods for the two
networks to internetwork with each other.

This memo compares the mobility management functions for CDPD and
Mobile IP networks and suggests ways to support internetworking
between the two networks.

2. Mobility Support in CDPD Networks

CDPD is designed to exploit the unused capacity of the cellular
telephone network for packetized data delivery. It leverages the
existing cellular infrastructure by adding CDPD specific equipment
to the existing cell sites. The CDPD network architecture makes
use of three distinctive devices:

   o  MES: Mobile End System -- A mobile terminal with a wireless
      modem that accesses the CDPD network through an airlink.
      Each MES may have one or more Network Entity Identifiers
      (NEIs) which are IP or CLNP addresses. The CDPD modem also
      has a 48-bit CDPD equipment identifier assigned by the

   o  MDBS: Mobile Data Base Station -- provides the mobile
      data link relay function for the MES over the radio channel.
      It performs part of the radio resource management function
      to ensure that the data user doesn't interfere with the
      regular voice users.

   o  MDIS: Mobile Data Intermediate System -- controls mobility,
      performs registration, authentication and routing functions.
      It also controls the MDBS for radio resource management. The
      MDIS is a full-fledged network router.

Additionally, the CDPD architecture uses the term "Fixed End
System" (FES) to denote an ordinary hardwired network end system.

The logical CDPD network architecture is shown in Figure 1:.

                          |                  |
   MES....MDBS-----MDIS---| IP/CLNP Backbone |---Router--FES
                          |                  |

          Figure 1: Logical CDPD Network Architecture

In a CDPD network, each wireless local network (termed as AREA)
consists of one MDIS and up to 200 base stations (MDBSs). The
mobile end system (MES) uses a multiple access scheme (Digital
Sensing Multiple Access: DSMA) that gives packet data lower
priority than voice traffic to access the cellular network.
Because the MDBS is only involved in the data link relay function,
the MES can roam transparently within the AREA using the different
MDBSs for the data relay service, while maintaining the same data
link connection between the MDIS and MES. Thus the AREA can be
treated as a single network segment (e.g. Ethernet). Because there
is one and only one MDIS within one AREA, the MDIS serves as the
default gateway to/from the local network. It advertises the
reachability of this network segment to other routers in the
IP/CLNP backbone network.

The CDPD Forum has obtained several Class B IP addresses with
prefix 166 from IANA. Thus, all the CDPD network AREAs use 166 as
the network prefix.

If the MES roams from one AREA to another, the MES recognizes that
it is in a new AREA during the cell transfer by listening to the
channel identification message broadcasted from the base station
during the channel acquisition time. It then initiates a new
registration process using the MNRP (Mobile Network Registration
Protocol) with the new MDIS. The serving MDIS handles the
registration for the MES.  It also communicate with the home MDIS
of the MES so that appropriate authentication can be performed,
and an appropriate routing entry can be set up at the home MDIS to
forward packets destined to the mobile end system to the new
foreign area. Figure 2 depicts the information flow for such an
inter-AREA migration

                  old serving       new serving            home
 MES                  MDIS              MDIS               MDIS

 |--cell exit decision--|                 |                  |
 |                      |                 |                  |
 |--cell selection & entry decision ------|                  |
 |                      |                 |                  |
 |--data link establishment---------------|                  |
 |                      |                 |                  |
 |--end system hello ---+---------------->|                  |
 |                      |                 |                  |
 |                      |                 |---redirect req-->|
 |                      |                 |                  |
 |                      |                 |<--redirect con --|
 |                      |                 |                  |
 |<--intermediate system confirm (ISC)----|                  |
 |                      |                 |                  |
 |                      |<--------redirect flush (RDF)-------|

      Figure 2: Information Flow for Inter-AREA Migration

One key aspect of an MES migrating into a new area is the
associated authentication to verify the identity of the MES. In
the CDPD network, airlink security is accomplished by exchanging
secret keys between the serving MDIS and the visiting MES using a
Diffie-Hellman key exchange scheme. After the MES obtains the key
from the MDIS, it sends the authentication information tuple <NEI,
ARN, ASN> (where ARN = Authentication Sequence Number and ASN =
Authentication Sequence Number) to the serving MDIS in the End
System Hello message. This information is relayed to the home MDIS
for authentication in cleartext through the wired network.

After authenticating the MES, the home MDIS returns a success
message and assigns a new <authentication random number,
authentication sequence number> to the serving MDIS in the
Redirect Confirm message; the information is relayed to the MES
and can be used for authentication in the next registration.

The data packet forwarding from the home MDIS to the serving MDIS
is done by encapsulating each IP/CLNP packet into a new CLNP
packet. The destination address of the new CLNP packet is the
serving MDIS. When the serving MDIS receives the encapsulated CLNP
packet, it decapsulates the packet and delivers to the MES using
the established data link channel. This triangular routing scheme
(shown in Figure 3) is similar to Mobile IP triangular routing. As
with Mobile IP, the CDPD MES keeps its IP address at all times.

                         serving               home
                              \                 /\
                               \                |
                                \               |
                                 \              |

           === indicates an encapsulated flow

        Figure 3: Packet Forwarding in a CDPD Network

3. Mobility Support in Mobile IP networks

Mobile IP is designed to support host mobility in the current
Internet Protocol (IPv4). Therefore, any internet host with an
arbitrary IP address can be a mobile host migrating into a foreign
network. In addition, a local network segment may have multiple
routers attached, so that the routing path to/from the local
network is not unique. To address these issues, the basic
architecture of Mobile IP defines two entities: Home Agent (HA)
and Foreign Agent (FA). The FA is located in the serving (foreign)
network and provides direct network access to the mobile host (MH)
when needed. The HA is responsible for intercepting IP packets
destined to the mobile host and forwarding them to the serving FA
of the mobile host. Because the mobile host may not be able to
detect a subnet change through the link layer protocol, the FA/HA
explicitly advertise their presence using Agent Advertisement
messages (an extension of the ICMP router advertisement message, a
network layer service).

When a mobile host migrates into a new local area it recognizes the
new network from the Agent Advertisement message broadcasted
periodically from the FA. The network layer broadcast of the agent
advertisement message is necessary because there may not be a data
link layer mechanism to detect the network segment change. The Agent
Advertisement message includes one or more Care-of-Addresses (COAs)
from the FA, encapsulation type(s) supported by the FA, registration
lifetime and advertisement sequence number. The MH then initiates a
registration process with the home agent using UDP messages with
destination port 434. The registration message is relayed through
the serving FA in the foreign network. The registration process
enables the HA to update its mobility binding <MH, COA, last message
ID, registration lifetime> for the migrated mobile host so that
packets can be forwarded to the new location (COA).

To address the authentication and security concerns, Mobile IP
defines flexible authentication extensions that can be added to
the registration message using keyed-MD5. Both mobile-HA and
mobile-FA authenticators can be attached to the registration
message for proper authentication. While different authentication
schemes can be employed by the MH, FA and HA through service
agreement in advance, the Mobile IP standard specifies a default
authentication method using the MD5 algorithm (RFC 1321). The
algorithm computes a one-way hash function that produces a 128-bit
"message digest" for an arbitrary long registration message. The
shared secret key is pre-configured for the MH - HA
authentication. For MH-FA authentication, the key can either be
distributed manually, or using public key. The information flow of
the registration messages is depicted in Figure 4.

MH                  new FA            old FA                HA

 |--Registration Req-->|                 |                   |
 |                     |                 |                   |
 |                     |-----------Registration Req--------->|
 |                     |                 |                   |
 |                     |<---------Registration Reply---------|
 |                     |                 |                   |
 |<-Registration Reply-|                 |                   |

   Figure 4: Information Flow for MobileIP Registration Messages

The Mobile IP protocol also defines an option for the MH to act as
its own FA, if the foreign network has no FA and the MH can obtain
a local address from the DHCP server (e.g. using anycast
mechanism). In this case, the Care-of-Address is the newly
obtained the local IP address from DHCP server.

It is also noted that there is no registration cancellation
message sent to the old FA when registration at the new FA becomes
active. Because IP provides best effort datagram delivery, the
packets in transit will simply be dropped and the old registration
will expire after the validation period.

Similar to the CDPD approach, the packet forwarding in Mobile IP
is carried out using encapsulation/decapsulation. The HA
intercepts each packet destined to the MH and then encapsulate the
packet using the COA in the mobility binding of the MH. Upon
receiving the encapsulated packet, the FA decapsulates the packet
and sends it directly to the MH using its own link layer protocol.
Figure 5 shows the packet forwarding in Mobile IP.

               \                               /\
                \                              |
                 \                             |
                  \                            |

            === indicates an encapsulated flow

      Figure 5: Packet Forwarding in a Mobile IP Network

Two encapsulation methods are defined in the Mobile IP standard:
Minimum encapsulation and IP within IP (IPIP). IPIP encapsulation
is the recommended encapsulation method. The IPIP method handles
packet fragmentation easily but adds more overhead to the
encapsulated packet.

4. Comparison between CDPD and Mobile IP

CDPD and Mobile IP are designed to support general purpose network
layer mobility for packet data networks. In particular, both are
designed to support network layer mobility in the IP network, thus
enabling mobile host migration in the Internet. The basic mobility
management functions for CDPD and Mobile IP networks are based on
the same concepts and principles (e.g. packet encapsulation and

Although many of the mobility management concepts and functions in
CDPD and Mobile IP are similar, the detailed message formats
differs from each other. In addition, there are several notable
difference in the protocol:

   o  In CDPD, the MES can detect the network segment change from
      the link layer support, while in mobile IP, the explicit
      Agent Advertisement message is necessary for the mobile host
      to detect network change.

   o  In CDPD, the registration process is separated into two
      stages. First, the MES registers with the serving MDIS
      using the MNRP, where no authentication is required.
      Second, the serving MDIS uses a separate protocol, MNLP,
      to update the location information to the home MDIS and
      forward the authentication information from the MES to
      the home MDIS for authorization. In Mobile IP, the mobile
      host registers directly with the HA, while the FA provides
      the relay service to the registration services.

   o  In CDPD, the home MDIS informs the previous serving MDIS
      to flush the MES's registration record, while in Mobile IP,
      multiple simultaneous registration records with different
      FAs for a mobile host are permitted.

   o  Because of the uniqueness of the MDIS, it is guaranteed
      that the home MDIS can intercept the packet destined to
      the MES, while in mobile IP, the HA needs to use the proxy
      ARP protocol to advertise the mobile host reachability in
      order to intercept the packet.

   o  CDPD defines a single encapsulation method between the
      home MDIS and the serving MDIS. All the packets forwarded
      to the serving MDIS are encapsulated using a CLNP packet
      with minimum encapsulation header to increase efficiency.
      In Mobile IP, two encapsulation methods are defined with
      IP within IP as the recommended method.

The protocol architecture for registration in CDPD and Mobile IP
differ as follows. The CDPD registration procedure is separated
into two phases (MNRP and MNLP), different from the one phase
approach of Mobile IP. In addition, the CDPD's MNLP defines
several message to allow the MDISs to exchange location update
information without the involvement of MES. Furthermore, the
registration message contents of CDPD and Mobile IP is different.
The information fields contained in these messages are listed in
Table 1.

                       CDPD                   MobileIP

   Parameter      Field Name     M/O     Field Name       M/O

Permanent addr.   Source addr     M      Home addr         M
of mobile

Registration      Regist. seq     M      Registration      M
seq. control      Count                  identification

Authentication    Authentication  M      Mobile-home       M
(home-mobile)     parameter              auth. extension

Home agent id     NR                     Home Agent        M

Registration      NR                     Lifetime          M

Forwarding        Forwarding net  M      Care-of-Address   M
address           address

Multiple regist.  NR                     Code              M

Authentication    NR                     Mobile-foreign    O
(foreign-mobile)                         auth. extension

Encapsulation     NR                     Minimum encaps.   O
method                                   extension

Carrier identi-    Location info   O      NR

   M=Mandatory, O=Optional, NR=Not Relevant

      Table 1:  Information fields in registration messages

In a CDPD network, no authentication is required between the MES
and the serving MDIS. although an encryption key is exchanged
between the two entities using Diffie-Hellman algorithm. The MES
then authenticates itself within its home MDIS using the <NEI,
<ARN, ASN>> tuple; the serving MDIS will only issue an ISC message
to the MES if proper authorization from the home MDIS is obtained.
In a Mobile IP network, the registration message from the mobile
host can contain the FA-mobile host authentication extension to
allow the FA and the MH to authenticate each other.

When the mobile host/end system is roaming, the home network
should forward the packets to the serving/foreign network. In
CDPD, this task is being performed by the home MDIS. Since all
packets destined into the MES's home network go through the MDIS,
there is no need for the MDIS to make extra efforts to intercept
the packets. In Mobile IP, a subnet may have multiple paths for
packets to be routed to/from the subnet, and the mobile host's HA
may not be the gateway router. Thus the HA uses gratuitous ARP to
advertise the reachability of the mobile host once it receives the
mobile's registration from a foreign network (impersonating the
mobile host). When the MH returns to the home network and
deregisters from the HA, the normal packet delivery is resumed.

5. Internetworking between CDPD and Mobile IP

Due to the differences mentioned in Section 3, CDPD and Mobile IP
cannot interwork without any modifications. However, since many of
the mobility management concepts and functions are derived from
the same principles, CDPD and Mobile IP can support each other's
operation without major modification of the specification. This
section discusses how a CDPD network can support a Mobile IP user
through the use of middleware software that interfaces the CDPD
and Mobile IP networks. (A similar method can be used to enable
CDPD terminal support from a Mobile IP network.)

Suppose a Mobile IP host enters a CDPD domain and wants to
establish network access through the CDPD network. The MH can use
a CDPD docking station and/or a CDPD modem to access the CDPD
network. Following the CDPD network operation convention, the CDPD
modem must have a valid network address (IP address) registered
with the network operator. The CDPD network address uses prefix
166 and is different from the original IP address of the mobile

To obtain network service from the CDPD network and maintain a
Mobile IP connection, the MH must register with both the CDPD
network and its HA. Following the CDPD protocol, the CDPD modem
performs the CDPD registration with the serving MDIS using its
CDPD recognized IP address with prefix 166. From the CDPD
network's perspective, the MH is a valid CDPD MES with a valid
CDPD address, thus the Mobile IP aspect of the MH is completely
transparent from the CDPD network. The MH can then use the
standard Mobile IP protocol to register with its HA, using the
CDPD network address as the COA. In this case, the FA and the MH
are collocated and MH acts as its own agent. The CDPD network
address (NEI) is easily accessible from the modem memory/registers
using the standard AT command.

Upon completion of the registration process, the MH can continue
to send out IP packets to the network using the serving MDIS as
its default router. The CDPD network treats the MH as a
conventional MES with a valid CDPD address. The packets destined
to the MH will be encapsulated by the HA and forwarded to the MH
using the CDPD network address as the COA. The scenario is the
same as an IP-based FES communicating with an MES. The routing
scenario is depicted below in Figure 6. (The CDPD network
encapsulation is not shown.)

                  +---------+    +--------------+
      MH/MES/FA<==|  CDPD   |<===|              |<===HA
           \      |         |    |              |    /\
            \     | Network |    |   INTERNET   |    |
             ---->|         |--->|              |-->host
                  +---------+    +--------------+

            === indicates an encapsulated flow

     Figure 6: Supporting Mobile IP host in a CDPD Network:
               One-directional Encapsulation Approach


The one directional encapsulation approach described above may
create an accounting problem for the CDPD network. As dictated by
some CDPD network operators, any packet originated from the CDPD
network must have a valid CDPD network address (with prefix 166)
as its source address. Such networks use the source to create
accounting data for billing purposes. The one way encapsulation
approach allows a packet originating from the MH to use its home
address as the source address, which cannot be properly accounted
by the account meter in the MDIS. Therefore, for accounting
purposes, every packet originating from the MH should be
encapsulated using the CDPD network address as the source address.
However, this creates another problem for the corresponding host
for the MH, since the corresponding host may not have the
capability to decapsulate the packet.

A bidirectional encapsulation approach is proposed to solve the
accounting problem and keep the corresponding host transparent at
the same time. The MH encapsulates outgoing packets using the HA's
address as the destination address and the CDPD NEI as the source
address. Upon receiving the encapsulated packets, the HA
decapsulates and forwards them to the correct destination.
Therefore, to support the migration of a mobile host into a CDPD
network, the HA must also provide a decapsulation function. This
is relatively simple because the HA already has the encapsulation
capability. The bidirectional encapsulation tunnel established
between the MH and HA serves as a virtual private network (VPN)
connection for the MH and its home network.

The bidirectional encapsulation method is depicted in Figure 7.

                +---------+    +--------------+
                | CDPD    |    |              |
   MH/MES/FA<==>| Network |<==>|   INTERNET   |<===>HA<--->host
                |         |    |              |
                +---------+    +--------------+

            === indicates an encapsulated flow

     Figure 7: Supporting Mobile IP host in a CDPD Network:
               Bi-directional Encapsulation Approach

Note that in the initial approach, the only interaction between
the Mobile IP software and the CDPD network is for the Mobile IP
software to retrieve the CDPD network address associated with the
CDPD modem. No modification on the part of CDPD network
infrastructure is needed. For the one directional encapsulation
approach, no change on the Mobile IP HA is required. On the other
hand, for the VPN approach, the Mobile IP software on the MH and
its HA should be enhanced so that a bidirectional encapsulation
tunnel can be established between the two entities.

If the MH/MES roams into a new serving MDIS, both the registration
and packet forwarding will be performed by the CDPD network
without impact on the Mobile IP protocol.

6. Summary

This memo has investigated the mobility management functions for
the two prominent technologies being developed and deployed in the
communication industry: CDPD and Mobile IP. While these two are
based on the same principles and concepts, they can not interwork
with each other due to the differences in their approaches in the
registration protocol, encapsulation method, and security
extensions. Historically, the two protocols have different design
goals in terms of uniformity/heterogeneity, tariff and security

An approach was identified for interworking between CDPD and
Mobile IP networks while keeping the existing protocols unchanged.
The scheme calls for adding relative simple middleware to the
mobile host software to enable its usage of CDPD network as a
Mobile IP subnet.

With a large deployed base of CDPD networks and the ubiquity of
the IP based Internet it is important to explore schemes that
allow the two networks to interconnect with each other and provide
mobility services to the ever increasing population of mobile
computing devices.

7. Security Considerations

Security considerations are not discussed in this memo.

8. References

[1] CDPD Forum, "Cellular Digital Packet Data System
Specification", Release 1.1, January 19, 1995.

[2] IETF Mobile IP working group, "IP Mobility Support - Draft-
IETF-Mobileip-Protocol-17", May 31, 1996.

9. Authors' Addresses

Greg Ruth
GTE Laboratories, Inc.
40 Sylvan Street
Waltham, MA  02254
617 466 2448

Ruixi Yuan
GTE Laboratories, Inc.
40 Sylvan Street
Waltham, MA  02254
617 466 2050

Internet Draft    CDPD-MobileIP Interoperability    6 August 1996

Ruth & Yuan              Expires Nov 1996              [Page 12]

Internet Draft       CDPD-MobileIP Interoperability        31 July