Internet DRAFT - draft-estrela-timip
draft-estrela-timip
P. Estrela
Internet Draft A.Grilo
T. Vazƒo
M. Nunes
Document: draft-estrela-timip-00.txt INESC
Expires: September 2002 March 2002
Terminal Independent Mobile IP (TIMIP)
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
IP mobility protocols usually assume that the mobile nodes have a
mobility-aware IP stack, which is still a scenario that can seldom
be found nowadays. Most terminals, including laptops and PDAs, still
use legacy IP stacks, limiting their use to layer-2 mobility between
Access Points (APs) connected to the same Access Router (AR) within
a single IP subnet. This document presents Terminal Independent
Mobile IP (TIMIP), which supports IP mobility of mobility-unaware
mobile nodes with legacy IP stacks, while fully interoperating with
Mobile IP to provide macromobility across IP subnets.
Estrela 1
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
1. Introduction....................................................2
1.1 Applicability................................................2
2. Terminology.....................................................3
3. Architecture....................................................3
4. Registration....................................................4
5. Power-up........................................................5
6. Micromobility...................................................7
7. Macromobility...................................................8
7.1. Macromobility for LMNs......................................8
7.2. Macromobility for MIP terminals.............................9
8. References.....................................................10
9. Author's Addresses.............................................10
1. Introduction
TIMIP [1] is a micromobility architecture that allows Mobile Nodes
with legacy IP stacks to roam within an IP subnet. Macromobility,
i.e. mobility across subnet boundaries, still relies on Mobile IP
[2]. The main features of TIMIP are listed below, some of them being
already present in other micromobility architectures, namely CIP [3]
or HAWAII [4]:
- TIMIP does not require changes to the IP protocol stack of mobile
nodes, either for micromobility or macromobility.
- Refreshing of routing paths is performed by data packets sent by
the mobile terminals, with signaling being employed only when no
traffic is detected at the routers for a certain time interval
(CIP).
- Routing reconfiguration during handoff within a TIMIP domain only
needs to change the routing tables of the Access Routers located
in the shortest path between the New Access Point and the Old
Access Point (HAWAII).
- Routing of data packets within a TIMIP domain does not need to
reach the Access Network Gateway, involving only the Access
Routers located in the shortest path between the sender and the
receiver (HAWAII).
1.1 Applicability
TIMIP is applicable within LANs, and possibly larger corporate networks that
form one subnet only. To provide mobility between subnets
(macromobility), Mobile IP [2] should be supported in the Access
Network Gateway of the TIMIP domain.
It is assumed that the data link layer of the Access Points is able
to perform terminal power-up detection and to notify the TIMIP
layer.
Estrela [Page 2]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
It is also assumed that the Access Routers of the TIMIP domain form
a logical tree, with the Access Network Gateway located at the top.
2. Terminology
Access Network
An IP network that includes one or more Access Routers and a Access
Network Gateway.
Access Network Gateway (ANG)
An Access Router that separates the Access Network from a third
party network.
Access Point (AP)
An Access Router that offers layer-2 connectivity to Mobile Nodes.
Access Router (AR)
An IP router residing in an Access Network and connected to one or
more access points. An AR offers connectivity to Mobile Nodes. The
router may include intelligence beyond simple forwarding service
offered by ordinary IP routers. Some Access Routers can be Access
Points.
Legacy Mobile Node (LMN)
A Mobile Node whose IP layer is not aware of mobility.
Mobile Node (MN)
An IP end-node capable of changing its point of attachment to the
network.
Subnet
A range of IP address designed by a common prefix.
3. Architecture
A TIMIP domain is an IP subnet organized as a logical tree of Access
Routers (ARs) whose root router is the Access Network Gateway (ANG).
The Access Routers are also layer-2 Access Points (APs), which
interface directly with the MNs through the wireless medium (WM).
The architectural diagram of TIMIP is depicted in Figure 1.
Estrela [Page 3]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
| +-+ +-----+ +-----+
[] <--> | | -------| AR5 |-------------------| |
MN +-+ /+-----+ /| ANG |-
AP/AR1 / / | |
/ / +-----+
+-+ / /
| |---- /
+-+ /
AP/AR2 /
/
+-+ +-----+ /
| |--------| AR6 |----------
+-+ /+-----+
AP/AR3 /
/
+-+ /
| |----
+-+
AP/AR4
Figure 1: Architectural Diagram of a TIMIP domain.
As a Legacy Mobile Node (LMN) has a legacy IP stack and cannot issue
any signaling during handoff, handoff detection by the AR relies on
notification from layer-2. This notification triggers routing
reconfiguration of the TIMIP domain, leading to routing table
updating at the ARs.
4. Registration
In order to allow attachment of an MN to a TIMIP domain, the MN must
be previously registered at the ANG. This is accomplished offline
through management procedures. For each MN recognized in a TIMIP
domain, registration information consists on the following:
- MAC address of the MN;
- IP Address of the MN;
- MIP capability;
- IP address of the MIP Home Agent;
- Authentication Key;
- Authentication option.
The MIP capability parameter specifies if MIP is required, either
implemented at the ANG on behalf of legacy terminal (surrogate MIP)
or implemented at the terminal itself. If the terminal has a legacy
IP protocol stack, the next two parameters specify respectively the
IP address of its Home Agent and the authentication key to be used
between the terminal and the ANG when the authentication option is
turned on. It should be noted that TIMIP authentication is mandatory
for macromobility scenarios for both MIP and legacy terminals. The
IP address of the Home Agent is not used when the terminal
implements MIP, as the terminal itself is responsible for
registering with the Home Agent, bypassing the ANG.
Estrela [Page 4]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
Once this data is configured at the ANG, it is forwarded to the APs
(except the authentication key) so that they are able to know the IP
address of newly associated terminals based on their MAC address
provided by layer-2 as explained below.
5. Power-up
When a terminal firstly appears in a TIMIP domain, a routing path is
created along the hierarchy of ARs. Consider the scenario in Figure
1. When the MN attaches itself to AR1, the creation of the routing
path takes the following steps:
1. The MT performs a layer-2 association with an AP that belongs to
the local TIMIP domain.
2. At the AP, layer-2 notifies the IP layer about the presence of
the MT in its wireless interface, triggering the routing
reconfiguration procedure. Layer-2 sends the MAC address of the
terminal to the IP layer. The MAC address is matched against the
terminal registration information broadcasted by the ANG and the
respective IP address is found. As the New AP has currently no
routing table entry for the MT, the routing table is updated with
the addition of this new entry.
3. The New AP sends a RoutingUpdate message up to the AR at
hierarchical level 2. This AR acknowledges with a
RoutingUpdateAck message, and updates its routing table
accordingly with the addition of a new entry relative to the MT.
This entry points to the source of the RoutingUpdate message (in
this case the AP) in order to specify the path through which the
terminal can be reached.
4. Exchange of RoutingUpdate/RoutingUpdateAck messages climbs up the
hierarchy levels. At each level, the routing table is updated
with the creation of a new entry relative to the MT. This entry
always points to the source of the RoutingUpdate message in order
to specify the path through which the MT can be reached.
5. Exchange of RoutingUpdate/RoutingUpdateAck messages reaches the
ANG, completing the creation of the new routing path.
The MT is now reachable through the routing path established by the
above procedures. The ARs that do not belong to this path have no
routing entry for the MT. At these ARs, all packets destined to the
MT are forwarded up the hierarchy of routers by default. All packets
that arrive at an AR whose routing table has an entry to the
destination, are forwarded down the hierarchy of routers until they
reach the radio interface in which the MT is located. Packets
destined to a terminal located in the same TIMIP domain as the
source, only in the worst case reach the ANG.
Estrela [Page 5]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
The RoutingUpdate and RoutingUpdateAck messages include a timestamp
generated at the New AP. As in TIMIP all APs are synchronized by
means of the Network Time Protocol (NTP) [5], this guarantees
consistency even when the MT moves faster than the route
reconfiguration.
It should also be noted that the routing path is soft-state and
after its establishment, it is refreshed by the data packets sent by
the MT. Nevertheless, as the packets are routed within the TIMIP
domain, some of the ARs may not be refreshed. When this occurs, the
routing entry for the MT becomes invalid after a predefined timeout
(e.g. 10s). The AR where the timer expired starts to send ICMP
EchoRequest messages to the terminal, filling the source address
field of the IP header with the IP address of the ANG. This forces
the MT to reply with EchoReply messages destined to the ANG, which
will refresh all the routing path within the TIMIP domain. If the MT
does not reply within a predefined timeout (e.g. 60s), the routing
entry for the MT is removed.
This basic TIMIP configuration is adequate to have micromobility in
wireless access networks where security is not an issue.
Nevertheless, just like in other unprotected IP networks, it allows
MTs to power-on with false MAC and IP addresses. In order to avoid
this, a minimal security functionality must be implemented at the MT
itself. However, this can be done in the application layer with no
need to change the IP protocol stack. When the authentication option
is turned on, it is assumed that the MT runs a special security
application, which uses a database of authentication keys for the
different TIMIP domains in which the MT is allowed to power-up. This
database is indexed by the IP addresses of the ANGs that are the
root of the respective networks.
The authentication takes place in step 2 of the power-on procedure,
immediately after layer-2 notifies the IP layer of the AP about the
association of the MT. The AP sends an SignatureRequest message to a
well known UDP port in the MT. This message carries <IP address of
the MT, IP address of the ANG, rand, timestamp>, where rand is a
random value and the timestamp is an NTP formatted 64-bit value. The
same message is sent to the ANG. Both the MT and the ANG answer the
AP with a SignatureReply message containing the same fields present
in the SignatureRequest message, plus its 128-bit MD5 message digest
[6] calculated with the authentication key of the MT for this
network. The latter is only known by the MT (based on the
authentication key database and the IP address of the ANG) and the
ANG (based on the registration information). The AP compares the
signatures of the two SignatureReply messages, and proceeds with the
routing reconfiguration procedures in case there is a match.
Estrela [Page 6]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
6. Micromobility
An example of handoff between two APs that belong to the same access
subnetwork is depicted in 2.
| +-+ +-----+ +-----+
[] <--> | | -------| AR5 |-------------------| |
MN +-+ /+-----+ /| ANG |-
AP/AR1 / / | |
| / / +-----+
| +-+ / /
| | |---- /
(handoff)V +-+ /
AP/AR2 /
/
+-+ +-----+ /
| |--------| AR6 |----------
+-+ /+-----+
AP/AR3 /
/
+-+ /
| |----
+-+
AP/AR4
Figure 2: Handoff between APs.
The Handoff procedure takes the following steps:
The first four steps of the Handoff procedure are the same as those
of the Power-up case. The remaining steps are the following:
5. Exchange of RoutingUpdate/RoutingUpdateAck messages climbs up the
hierarchy levels, until the crossover AR (the AR that belongs
simultaneously to the old path and to the new path) is reached
(in the example above, node AR5). Now that the new routing path
is completely created, the old path must be deleted. This
procedure starts when the crossover AR sends a RoutingUpdate
message addressed to the MT through the old routing path. The AR
that receives the message realizes that the MT is no longer
accessible through it, updates its routing path by deleting the
entry that corresponds to the MT and replies with a
RoutingUpdateAck message.
6. Exchange of RoutingUpdate/RoutingUpdateAck messages goes down the
AR tree following the old path, until the Old AP is reached. At
each level, the routing table is updated by deleting the entry
relative to the MT.
In a normal shared media LAN, when a terminal has a packet destined
to an address within the same IP subnet (which is known through the
analysis of the IP address prefix), it tries to obtain the MAC
Estrela [Page 7]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
address of the destination through an ARP request and send the
packet directly to it. In TIMIP, as the APs are also ARs, if the
destination is associated to a different AP, the ARP request will
never be answered. In order to prevent this situation, the terminal
is forced to send all packets to the IP Gateway of the local
subnetwork (which corresponds to the ANG) through the AP, which is
accomplished by setting the subnetwork mask configuration to
255.255.255.255. When the terminal sends an ARP request to obtain
the MAC address of the Gateway, the local AP answers with its own
MAC address.
7. Macromobility
TIMIP relies on MIP to support macromobility. The ANG is the HA for
MNs whose home network is its TIMIP domain. As LMNs do not implement
MIP, the ANG implements all the needed functionality on their
behalf. On the other hand, when a foreign MN supports MIP, the ANG
acts as a Foreign Agent for the local TIMIP domain.
7.1. Macromobility for LMNs
When a MT enters a TIMIP domain different from its current location,
the terminal is locally authenticated and a routing path is created
between the terminal and the ANG. Packets are then sent/received
to/from the outside through the ANG. If the home network of the MT
is a different MIP domain, its HA must be notified so that packets
can be correctly routed through an IP tunnel established from the HA
to the FA located at the ANG.
After consulting the registration information of the MT (namely the
IP address of the HA and the MIP capability), the ANG realizes that
it is a foreign MT and that it does not implement MIP. Consequently,
the ANG must act as a MIP proxy on behalf of the MT, generating all
MIP signaling as the MT would. Firstly it has to notify the HA about
the MTÆs new location and CoAddr by means of a MIP Registration
Request message, which requires authentication using the
authentication key between the MT and the HA. As the ANG does not
know this key, it is the MT that must sign the message. The ANG
sends the MT an AuthenticationRequest message containing <IP address
of the ANG, IP address of the HA, MIP Registration Request,
timestamp> authenticated by the ANG with MD5(K1,
AuthenticationRequest), where K1 is the authentication key between
the MT and ANG for this TIMIP domain. The MT finds K1 in the key
database based on the IP address of the ANG and obtains the
authentication key of his home network (K2) in the key database,
based on the IP address of the HA. It then answers with an
AuthenticationReply message containing <IP address of the ANG, IP
address of the HA, MD5(K2, MIP Registration Request), timestamp>.
This message is also authenticated by the terminal with MD5(K1,
AuthenticationReply). The ANG can now send a correctly authenticated
MIP Registration Request message to the HA adding the message digest
provided by the MT as a Mobile-Home Authentication Extension field.
Estrela [Page 8]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
The HA answers with an authenticated MIP Registration Reply message,
which has a message digest MD5(K2, MIP Registration Reply) appended
as a Mobile-Home Authentication Extension field. In order to verify
the identity of the HA, the ANG must again rely on the MT. It sends
an AuthenticationRequest message to the MT, containing <IP address
of the ANG, IP address of the HA, MIP Registration Reply (except the
Mobile-Home Authentication Extension), timestamp>, authenticated
with MD5(K1, Authentication Request). The terminal answers with an
AuthenticationReply message containing <IP address of the ANG, IP
address of the HA, MD5(K2, MIP Registration Reply), timestamp>. If
the MD5 digest of the MIP Registration Reply provided by the MT
matches that present in the Mobile-Home Authentication Extension of
the Registration Reply message sent by the HA, the ANG can resume
communication with the HA.
After the communication with the HA is established, the ANG de-
encapsulates the tunneled IP packets that come from the HA addressed
to the MT and forwards them normally to the MT according to the
routing path established in the AR tree. Packets generated by the MT
are routed normally according to the same procedures described above
for micromobility.
As already seen, all traffic that crosses the boundary of the TIMIP
domain must pass through the ANG, which is the IP Gateway to the
core network. Nevertheless, whenever an MT moves to a different
domain, the IP address of the ANG changes. In order to keep
consistency, the MT must change its IP Gateway configuration at each
handoff between TIMIP domains, otherwise the ARP requests to obtain
the MAC address of the IP Gateway would not be answered by the APs.
This inconvenience is avoided by configuring the MTs with a well-
known ANG IP address recognized by all APs of all TIMIP domains. The
latter broadcast gratuitous ARP messages associating their own MAC
addresses with the well-known ANG IP address each time a terminal
performs an association.
7.2. Macromobility for MIP terminals
When the MT supports MIP but belongs to a different domain, the ANG
plays the role of FA. The MT powers-on in the same way as legacy MTs
in the TIMIP domain. Once this is completed, the MIP signaling
starts. The ANG broadcasts RFC 1256 [7] Router Advertisement
messages periodically, specifying its IP address as the MIP CoAddr.
In order to haste the process, the MT can request the advertisement
by broadcasting a Router Solicitation message, which is then
forwarded by the AP to the ANG.
After the MT receives a Router Advertisement message, it notifies
its HA about the CoAddr through the ANG. The HA is then able to
forward the incoming packets to the CoAddr through an IP tunnel. The
ANG de-encapsulates the IP packets and forwards them normally to the
MT according to the routing path established in the AR tree. Packets
generated by the MT are also routed normally within the TIMIP
Estrela [Page 9]
Internet-draft Terminal Independent March 2002
Mobility for IP (TIMIP)
domain. Handoff between APs within the foreign TIMIP domain are
dealt with TIMIP micromobility procedures only (see above) as the FA
is always the same (i.e. the ANG).
It should be noted that in this case it is the MT itself that
authenticates the MIP messages when communicating with the HA.
8. References
1 Grilo A., Estrela P., Nunes M., Terminal Independent Mobility for
IP (TIMIP)ö, IEEE Communications, Vol. 39 N¦12, December 2001.
2 Perkins C. (Editor), "IP Mobility Support", RFC 2002, IETF,
October 1996.
3 A. Campbell et al, ôDesign, Implementation and Evaluation of
Cellular IPö, IEEE Personal Communications, Vol. 7 N¦4, August
2000.
4 R. Ramjee et al, ôIP-Based Access Network Infrastructure for
Next-Generation Wireless Data Networksö, IEEE Personal
Communications, Vol. 7 N¦4, August 2000.
5 Mills D. ôNetwork Time Protocol (Version 3), Specification,
Implementation and Analysisö, RFC 1305, IETF, March 1992.
6 Rivest R., ôThe MD5 Message-Digest Algorithmö, RFC 1321, IETF,
April 1992.
7 Deering S. (Editor), ôICMP Router Discovery Messagesö, RFC 1256,
IETF, September 1991.
9. Author's Addresses
Pedro Estrela
INESC
Rua Alves Redol, N.9 Phone: +351-213100324
1000 Lisboa, Portugal Email: pedro.estrela@inesc.pt
Antnio Grilo
INESC
Rua Alves Redol, N.9 Phone: +351-213100226
1000 Lisboa, Portugal Email: amg@cris.inesc.pt
Teresa Vazƒo
INESC
Rua Alves Redol, N.9 Phone: +351-213100286
1000 Lisboa, Portugal Email: teresa.vazao@inesc.pt
Mßrio Nunes
INESC
Rua Alves Redol, N.9 Phone: +351-213100256
1000 Lisboa, Portugal Email: msn@inesc.pt
Estrela [Page 10]
