One document matched: draft-yegin-dna-l2-hints-01.txt
Differences from draft-yegin-dna-l2-hints-00.txt
Internet Draft Alper Yegin (Editor)
Document: draft-yegin-dna-l2-hints-01.txt DoCoMo USA Labs
Expires: August 2004 Eric Njedjou
France Telecom R&D
Siva Veerepalli
Qualcomm, Inc
Nicolas Montavont
Thomas Noel
LSIIT -University Louis Pasteur
Link-layer Triggers and Hints for Detecting Network Attachments
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
Certain link-layer technologies are capable of providing various link
status information to the IP module. Link-layer event notifications
(triggers) along with optional auxiliary data (hints) can help the IP
module make expedited decisions regarding configuration changes. This
draft provides a non-exhaustive catalogue of triggers and hints from
well-known link-layer technologies.
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Table of Contents
1.0 Introduction.................................................2
2.0 Terminology..................................................3
3.0 Link-layer Triggers and Hints in Various Systems.............6
3.1. GPRS........................................................6
3.1.1. Network Reference Model...................................7
3.1.2. Link-layer Events and Information.........................7
3.2. 3GPP2......................................................11
3.2.1. Network Reference Model..................................11
3.2.2. Link-layer Events and Information........................13
3.3. WLAN.......................................................14
3.3.1. Network Reference Model..................................15
3.3.2. Link-layer Events and Information........................17
4.0 Triggers and Hints..........................................17
4.1. Link-up Trigger and Associated Hints.......................18
4.2. Link-down Trigger..........................................18
5.0 Security Considerations.....................................19
6.0 References..................................................19
7.0 Acknowledgements............................................20
Appendix A.......................................................20
A.1. Routing Area and Cell Change................................20
A.1.1. Link-layer Information....................................21
A.2. Sub-link-layer Information..................................21
Authors' Addresses...............................................22
Full Copyright Statement.........................................22
1.0 Introduction
It is not an uncommon occurrence for a host to change its point-of
attachment to the network. This can happen due to mobile usage
(e.g., a mobile phone moving among base stations) or nomadic usage
(e.g., road-warrior case).
Each time a host changes its point-of attachment, it is possible
that it will also have to change its IP-layer configurations, such
as its IP address and default gateway information. In order to make
these changes, the IP module has to detect the new network
attachment, realize that the old configuration is no longer valid
and obtain the new configuration parameters. The network detection
phase can usually use network-layer indications such as a change in
the advertised prefixes. But generally reliance on such indications
does not yield rapid detection, since these indications are not
readily available upon a link change.
Link-layer event notifications to the IP are considered link-layer
triggers. From detecting network attachment perspective, an
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auxiliary data delivered in association with a trigger is considered
a hint. It has been identified that receiving explicit triggers and
hints from the link-layer would expedite the detection process. The
link-layer indicating that the host has established a new connection
can be used as a trigger to further probe the network for a possible
configuration change. Additional hints when present can be used as
input to this process.
A link-layer trigger cannot be used to positively determine the need
for a configuration change as it might very well be the case that
the host is still connected to the same IP subnet despite the link
change. For example, there might be several IEEE 802.11b access
points connected to the same access router. Moving among these
access points does not warrant any IP-layer configuration change.
This is why the link-layer triggers should be used as "advisory-
only" unless stated otherwise.
In order to enable an enhanced network attachment detection scheme,
we need to identify types of link-layer triggers and hints that can
be realistically expected from various access technologies. The
objective of this draft is to provide a catalogue of existing link-
layer triggers and hints in various architectures.
The document limits itself to the minimum set of link-layer triggers
that are necessary for detecting network attachment. These triggers
are considered with hosts in mind, although they may also be
available on the network side (e.g., on the access router).
2.0 Terminology
Some of the terminology differs among the discussed architectures.
An architecture name is provided in parenthesis when a term has
limited applicability.
3GPP Third Generation Partnership Project
3GPP2 Third Generation Partnership Project 2
ANID Access Network Identifier. Identifies the packet switched
area served by a unique combination of RAN and MSC area.
(3GPP2)
AP Access Point. An access point is an entity that provides
bridging between the radio link and the wired network. (WLAN)
APN Access Point Name. A parameter of the PDP context, in the
form of a logical name that is used to select the GGSN or the
external IP network. (3GPP)
AT Access Terminal. Another term used for Mobile Terminal in
3GPP2 networks. (3GPP2)
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BSC Base Station Controller. A BSC controls a set of BTS. (3GPP,
3GPP2)
BSS Base Station System. The system that is made up of BTSs and
BSCs. (3GPP)
BSS Basic Service Set. A BSS is composed of one AP and its
attached MNs. (WLAN)
BSSID Basic Service Set Identification. A unique identifier of a
BSS. In infrastructure mode, it is the MAC address of the AP.
(WLAN)
BTS Base Transceiver Station. Mobile terminalÆs radio attachment
point to the network. A BTS is responsible for MTs within a
given radio cell.(3GPP, 3GPP2)
ESS Extended Service Set. The set composed of APs and associated
MNs(BSSs) that share a common distribution system. (WLAN)
FA Foreign Agent. A router on a mobile node's visited network
which provides routing services to the mobile node while
registered.
GGSN Gateway GPRS Support Node. A router between the GPRS core
network and an external IP network. (3GPP)
GMM GPRS Mobility Management. Sub-link-layer protocol between the
MT and the SGSN for handling MT movement. (3GPP)
GPRS General Packet Radio Service. Packet-switched data
transmission service on top of the GSM network. (3GPP)
GTP GPRS Tunneling Protocol. A protocol for encapsulating user
data traffic between the SGSN and the GGSN. (3GPP)
HA A router on a mobile node's home network which tunnels
datagrams for delivery to the mobile node when it is away
from home, and maintains current location information for the
mobile node.
IMSI International Mobile Subscriber Identity. A 12-digit number
that uniquely identifies a GPRS subscriber smart card. (3GPP)
LLC Logical Link Control. Data link protocol between the MT and
SGSN. (3GPP)
MN Mobile Node. A host or router that changes its point of
attachment from one network or subnet to another.
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MN Mobile Node. The conjunction of a Mobile Terminal, a SIM card
and Terminal Equipment. (3GPP)
MT Mobile Terminal. For example, a mobile phone handset or a
PCMCIA card. (3GPP)
MS Mobile Station. For example, a mobile phone or a combination
of mobile terminal (e.g., a phone) and terminal equipment
(e.g., a laptop). (3GPP2)
MUX Multiplex Layer. A link layer protocol used to multiplex
signaling and RLP protocols. (3GPP2)
NSAPI Network Layer Service Access Point Identifier. It is used to
identify a PDP context between MT and SGSN on top of the
Logical Link Control layer. It is set by the MT. (3GPP)
P-TMSI
Packet TMSI. A temporary IMSI allocated by the GPRS network
to the MT upon IMSI attach procedure.(3GPP)
PCF Packet Control Function (3GPP2)
PDP Address
Address of a MN for a given PDP context. (3GPP)
PDP Context
Soft state maintained between the Mobile Terminal, the SGSN
and the GGSN for guaranteeing a negotiated quality of service
for the IP flows exchanged between the GPRS Mobile Terminal
and an external Packet Data Network such as Internet. (3GPP)
PDSN Packet Data Serving Node. The default gateway router for MNs
in 3GPP2 networks. (3GPP2)
PLMN Public Land Mobile Network. A GPRS Network operated on a
national territory. (3GPP)
PPP Point-to-Point Protocol
RA Routing Area. Set of adjacent cells. A given number of RAs
are under the control of one SGSN. (3GPP)
RAN Radio Access Network. (3GPP, 3GPP2)
RLP Radio Link Protocol. A link-layer protocol used to improve
the physical-layer frame error rate over the air. (3GPP2)
R-P RAN-to-PDSN interface. Also known as the A10/A11 interface.
(3GPP2)
SGSN Serving GPRS Support Node. A router directly connected to the
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GPRS Radio Sub-System that handles the mobility of terminals
attached to the RAs under its authority. The SGSN is also the
Radio Sub-System interface to the GPRS IP core network. It
could be considered as an equivalent to the IEEE 802.11
access point. (3GPP)
SM Session Management. Sub-link-layer protocol between the MT
and the GGSN that handles the activation/deactivation of a
PDP Context. (3GPP)
SSID Service Set Identifier. Identifier of an ESS. (WLAN)
TE Terminal Equipment. A user's laptop for example. TE can
connect to the network via MT. (3GPP)
TI Transaction Identifier. This is the association between an
NSAPI and an identifier corresponding to an operation
performed on the associated PDP context. For example a
"Modify PDP Context Request" will be identified by a
Transaction Identifier. (3GPP)
TLLI Temporary Logical Link Identity. It is used by the SGSN to
identify a particular Mobile Terminal at the logical link
control layer. (3GPP)
3.0 Link-layer Triggers and Hints in Various Systems
This section provides an overview of various architectures and
discusses associated link-layer triggers and hints.
3.1. GPRS
Multi-interface terminals are changing the face of wireless IP
connectivity and GPRS [GPRS] is being one of the most pervasive
types of radio link for enabling multi-technology access to the
Internet.
GPRS is an enhancement to the GSM data transmission architecture and
capabilities, designed to allow for packet switching in user data
transmission within the GPRS network as well as for higher quality
of service for the IP traffic of Mobile Terminals with external
Packets Data Networks (PDN) such as the Internet or corporate LANs.
The GPRS architecture consists of a Radio Access Network and a
packet domain Core Network.
- The GPRS Radio Access Network is composed of Mobile Terminals, a
Base Station Subsystem (BSS) and Serving GPRS Support Nodes (SGSN).
The BSS is made up of radio cells called Base Transceiver Stations
(BTS) served under the control of Base Station Controllers (BSC).
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So-called Routing Areas are formed by the subdivision of BSCs. Each
SGSN in the GPRS architecture controls a set of RAs;
- An IP Core Network that acts as the transport backbone of user
datagrams between SGSNs and Gateway GPRS Support Nodes (GGSN). The
GGSN ensures the GPRS IP core network connectivity with external
networks, such as Internet or Local Area Networks.
From the point of view prevailing in detecting network attachment, the
GPRS access network will be only seen as providing layer 1-2
reachability even if it is able to provide IP connectivity alone.
3.1.1. Network Reference Model
Most of the triggers described in this document come from messages
exchanged on top of the Logical Link Control protocol (LLC) running
between the Mobile Terminal and the SGSN. The messages are part of
the GPRS Mobility Management (GMM) and Session Management (SM)
protocols and ensure functionalities such as GPRS attach, detach,
PDP Context activation and deactivation, Routing Area update.
|
+----| <-------------GMM/SM--------------> +-----+
| | <--------------LLC----------------> | |
| | | |
| | \ / | |
| MT | +-----+ |SGSN |
| | Radio interface | |<---------------->| |
| |<----Protocols--->| BSS | | |
+----+ (RLC, MAC, L1) +-----+ +-----+
Figure 1. Signaling protocol stack between MT and SGSN
3.1.2. Link-layer Events and Information
In GPRS networks, only network attachment/detachment and subsequent
PDP context changing events will directly impact the IP
configurations, hence should be used as link-layer triggers by IP.
Other events such as routing area and cell change do not directly
imply potential configuration change. More details on those
secondary types of events can be found in Appendix A.
When connecting, first a GPRS attach needs to be made to the SGSN.
The procedure is attempted whenever a GPRS-enabled Mobile Terminal
is being switched on. The attachment can also take place at any time
while the MT is switched on, for example following a detach forced
by the network. The MT provides its identity during the attach
request to the network in the form of a so-called Packet Temporary
Mobile Subscriber Identity (P-TMSI). If the MT has no valid P-TMSI,
it provides its IMSI.
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Before the MT becomes GPRS attached, it scans for available GPRS
networks, as well as acquires the identities of their cells in the
covered area. It is also possible for the MT to obtain the radio
capabilities of these cells.
When a MT has performed the GPRS attach, it becomes in READY state.
In this state, the MT is reachable (using the logical link layer -
LLC) by the GPRS Radio Access Point called the SGSN. Otherwise, its
state is said to be IDLE. During the IDLE state, no IP level
communication is possible with an external network, such as
Internet. The SGSN identifies the logical link with the MT by the
Temporary Logical Link Identifier (TLLI) it derives from the P-TMSI
that was assigned to the MT. It has to be noted that the MT or SGSN
may initiate a detach procedure (Mobile or Network Initiated
Detach). The MT returns from READY to IDLE STATE upon detachment.
The MT is actually considered GPRS attached when it has received an
"Attach Accept" message from the SGSN. The MT is considered detached
from the GPRS Network when it has sent or received a "Detach Accept
message" from/to the SGSN. This is an indication that the link-layer
connectivity is being lost.
The "Detach Accept" message is also preceded by a "Detach Request"
message from the side initiating the detachment procedure. This
message is an indication that a detachment from the GPRS network is
about to take place. The network-layer could then anticipate the
loss of connectivity.
The "Attach Accept" message comes along with an update of the Mobile
Terminal Mobility Management context held at the GMM/MM level. This
message contains:
- The Packet Temporary Mobile Station Identifier (P-TMSI). The P-
TMSI is a temporary IMSI allocated by the GPRS network upon attach
(if no P-TMSI was already present). It is used for subscriber
location hiding purpose in substitution to the IMSI.
- The current Cell Identity (CI)
- The current Routing Area Identity (RAI) which identifies the
serving SGSN
- The ciphering algorithm, key (Kc) and sequence number (CKSN)
Once the GPRS MT is attached, the attached network information can
be sent to it via the "MM information" message that contains:
- The network name known as Public Land Mobile Network ID in 3GPP
terminology
- Network registration type (Home or Roaming)
A MN that wants to establish IP-level connections through the GPRS
MT should first request the GPRS network to settle the necessary
soft state mechanism (GPRS tunneling protocol) between its serving
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SGSN and the GGSN corresponding to the APN specified in the PDP
Context parameters. Only after this tunneling mechanism takes place
can the MN's IP packets be forwarded to and from its remote IP
peers. The process by which this is made possible is designated as a
PDP Context Request.
The aim of this function is also to provide IP-level configuration
on top of the GPRS link-layer attachment, in order for the MN to get
IP reachability with external networks, such as Internet. The
establishment of a PDP context is partially based on link-layer
characteristics negotiated between the MT and the GPRS network (SGSN
and GGSN). These characteristics include the QoS profile that will
be guaranteed by the SGSN and GGSN (e.g., maximum delay, link
reliability, peak and mean throughputs). When the MT requests a PDP
context, it selects a Network Service Access Point Identifier
(NSAPI) that it sends to the SGSN with the request. The NSAPI is
sent (as part of the PDP Context request message) on top the Logical
Link Control layer identified for that MT by the TLLI. In this way,
the SGSN is able to uniquely identify the PDP context.
A PDP context Activation procedure can also be initiated by the GGSN
(Network-requested PDP Context Activation) but this alternative is
not likely to happen so often.
The network may also decide to modify an existing PDP Context with a
given MN at any time. Such a modification might be prompted by the
MN's serving SGSN when it estimates that the negotiated QoS profile
can no longer be respected. For instance, the GPRS Network might
temporarily not be able to guarantee the contracted delay, in which
case it would force the related PDP context parameter to be
renegotiated. Note that, a MT can decide not to accept such an
update of its PDP context, in which case it should start a PDP
context deactivation procedure. Furthermore, a PDP context may be
deleted at any time at the request of the MT or the network. After a
PDP context is deleted, the MT returns to simply attached state
(READY STATE). Finally, a Mobile Terminal can hold several PDP
contexts, each corresponding to a different NSAPI.
+--------------+
| PDP Context1 | +-------+
| NSAPI 1 | | |
| ------------ | +------+ | |
| GPRS MT +-------+ TLLI +---------| SGSN |
| ------------ | +------+ | |
| PDP Context2 | | |
| NSAPI 2 | +-------+
+--------------+
Figure 2. NSAPI and TLLI (link identifier).
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A PDP context is considered activated on the MT side as soon as an
"Activate PDP Context Accept" message has been received from the
GGSN. The reception of this message can be considered as a trigger
that the GPRS network will be providing a certain link-layer
quality-of service for which parameters (e.g., delay, reliability,
throughput) are included with the messages described below.
When the network is about to modify a PDP Context, it informs the MT
by sending a "Modify PDP Context Request" message. This can also be
an indication at the MN's network-layer that the link-layer
characteristics on the GPRS attachment are about to change. The MN
could then be able to anticipate such a change, which would likely
be a drop or an increase of service quality. The "Modify PDP Context
Accept" message confirms the modification and is an indication that
the initially negotiated PDP context characteristics are no longer
valid.
A "Deactivate PDP Context Request" message is sent by the MN or
received from the SGSN depending on which side has initiated the
deactivation procedure. The transmission or reception of this
message can serve as a trigger that the IP configuration of the MN's
GPRS interface or one of its IP configuration (in case multiple PDP
Contexts are present on the MT), is about to be deleted. This could
help the MN anticipate the coming loss of IP attachment. A
"Deactivate PDP Context Accept" sent or received by the MT is a
confirmation that the PDP context is being deleted.
The "Activate PDP Context Accept" message comes along with a
modification of the GMM context that contains the following
information:
- The TI (transaction identifier) associated to the procedure of
activating a PDP context. It consists of the NSAPI generated by the
MT for that PDP context and an operation identifier,
- The IP address for that PDP context,
- The QoS Profile negotiated with the network,
- The Radio Priority level for data transmission.
The "Modify PDP Context Accept" comes along with the following
information:
-The TI associated to the procedure,
-The new QoS profile negotiated with the network,
-The radio priority level for data transmission.
The "Deactivate PDP Context Accept" message comes along with the TI
associated to the procedure.
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3.2. 3GPP2
3GPP2 (cdma2000) packet data services provide mobile users wide area
high-speed access to packet switched networks. 3GPP2 consists of
multiple radio access technologies, namely 1x EV, 1x EV-DO and 1x
EV-DV, where the order shows the evolution of technology in the
industry. 1x Evolution Data Only (1x EV-DO) and 1x Evolution Data-
Voice (1x EV-DV) are enhanced air interface technologies that are
optimized for higher data rates.
The aforementioned 3GPP2 technologies share a common core network
infrastructure which enables easy transition to enhanced air
interface technologies. 3GPP2 networks use the Point-to-Point
Protocol (PPP) as the link-layer protocol between the mobile node
and the network access server. Hence, link-layer mechanisms are
pretty consistent across all air interface technologies. Unless
specifically called out, all link-layer mechanisms specified in this
document apply to all 3GPP2 air interface technologies.
3.2.1. Network Reference Model
Some of the major components of the 3GPP2 packet network
architecture (see Figure 3) consist of:
- Mobile Node (also known as Mobile Station or Access Terminal in
3GPP2), which allows mobile access to packet-switched networks over
a wireless connection,
- Radio Access Network, which consists of the Base Station
Transceivers (BTS), Base Station Controllers (BSC), and the Packet
Control Function (PCF),
- Network Access Server known as the Packet Data Switching Node
(PDSN). The PDSN also serves as the Foreign Agent (FA), in the case
of Mobile IP service.
+-------------------------+
| RAN |
+====+ | +=====+ +=====+ | +======+
| | | | BSC/| | | | | |
| MN |-----------| | BTS |-------| PCF |--|-------| PDSN |
| | | | | A8/A9 | | |A10/A11| |
+====+ | +=====+ +=====+ | +======+
| |
+-------------------------+
Figure 3. Packet Network Reference Model
Figure 4 shows the hierarchical relationship between the RAN,
PDSN/FA and HA. The control and bearer interfaces between the BSC
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and PCF are known as the A9 and A8 interface respectively, while the
control and bearer interfaces between PCF and PDSN are known as the
A11 and A10 interfaces respectively. Note that, the A11/A10
interface is also known as the R-P interface (for RAN-PDSN
interface). The A9 and A11 interfaces are used to establish A8 and
A10 connections. The A8 and A10 connections are used to tunnel link
layer data (PPP frames) between the BSC and PDSN.
+======+
| |
| HA |
| |
+======+
|
|
+--------------+---------------+
| | |
+======+ +======+ +======+
| | | | | |
| PDSN | | PDSN | | PDSN |
| | | | | |
+======+ +======+ +======+
/ \ / \ / \
A10/A11---------/---\------------/---\----------/---\---------
/ \ / \ / \
/ \ / \ / \
+======+ +======+ +======+ \ / \
| | | | | | +======+ +======+
| PCF | | PCF | | PCF | | | | |
| | | | | | | PCF | | PCF |
+======+ +======+ +======+ | | | |
| / \ | +======+ +======+
A8/A9 ----|--------/---\------|----------|-------------|-----
| / \ | | |
+====+ +====+ +====+ +====+ +====+ +====+
| | | | | | | | | | | |
|BSC | |BSC | |BSC | |BSC | |BSC | |BSC |
| | | | | | | | | | | |
+====+ +====+ +====+ +====+ +====+ +====+
+====+
| |
| MS | ------->
| |
+====+
Figure 4. Hierarchical relationship between RAN, PDSN and HA
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A PCF controls one or more BSCs. The area served by each PCF is
identified by the Access Network Identifier (ANID). This is referred
to as the SUBNET ID in the 1x EV-DO system. Any given BSC is
associated with one and only one PCF. The combination of BSC and PCF
is also known as the RAN. Each PCF can communicate with one or more
PDSNs. However, for a given mobile user, the PCF typically
establishes a connection with a specific PDSN.
Link-layer-related (e.g., handover) information is provided by the
RAN to the MS via 3GPP2 overhead signaling messages broadcast over
the air interface.
A number of other important components of the architecture that
enable call setup (such as the MSC, HLR, AC and/or AAA servers) are
left out for the sake of simplicity. None of these components have a
direct impact on the discussion of link-layer hints.
3.2.2. Link-layer Events and Information
While a PPP connection is in ESTABLISHED state at the MN and PDSN,
the packet data service state at the MN can be in ACTIVE or DORMANT
state. In the ACTIVE state, all the bearers between the MN and the
PDSN are in the established state. In the DORMANT state, the radio
link bearer and the A8 connection are torn down to conserve radio
resources. However, the A10 bearer still remains connected, and the
PPP state is maintained both at the MN and PDSN.
MN transitions from DORMANT to ACTIVE state when the MN has some
data to send or the network (PDSN) has data to send to the MN. When
a MN in ACTIVE packet data service state hands off from one RAN to
another, it results in an ANID change. An ANID change may or may not
result in a change in the MN point of attachment to the network
(i.e., PDSN).
If the ANID changes, but no change in the network attachment point,
a new A10 connection between the new PCF and serving PDSN is
established. If the ANID change results in a change in network
attachment point (i.e., PDSN), the new PDSN initiates a new PPP
connection setup with the MN, resulting in an update of the network
configuration information such as IP address and DNS server address
on the mobile node. In the case of Mobile IP, PPP resynchronization
is followed by Mobile IP registration to update the FA (PDSN)
address in the Mobile IP binding at the HA.
Hence, a PPP resynchronization from the PDSN could be viewed as a
link-layer event that updates network configuration information in
the MN and further provides an indication to the MN that Mobile IP
registration is required to update the binding in the HA with the
new FA address.
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On the other hand, when a DORMANT mobile moves, the RAN is not aware
of the presence of the mobile in its area (as the radio link is not
in established state). The RAN relies on the MN to inform it of the
MN's presence. The ANID for the RAN, which is broadcast on the
overhead channel, is used for determining RAN changes by the MN.
When a dormant MN moves and the ANID changes, the MN registers with
the RAN to initiate a new A10 connection between the new RAN and
PDSN. If the ANID change also results in a change in the network
attachment point, not only is a new A10 connection established, but
also a new PPP connection is established between the new PDSN and
MN. The RAN transitions the MN from DORMANT to ACTIVE state in order
to resynchronize the PPP connection. This results in an update in
the network layer configuration information such as IP address and
DNS server address in the MN. In the case of Mobile IP, PPP
resynchronization is followed by Mobile IP registration to update
the FA (PDSN) address in the Mobile IP binding at the HA.
As described above, a lower-layer indication (ANID change) allows a
MN to discover a potential change in the network point of
attachment. From IP's perspective, changes in the PPP link status
provide a trigger and hints about the network attachment change.
3.3. WLAN
WLANs are the wireless extension of the Local Area Networks. A WLAN
offers MNs short range network access at high rate. The maximum
coverage area of a node is usually from few meters indoors to more
than one hundred meter outdoor. The raw bandwidth varies between
1Mbps to 54Mbps depending on the norm used and the configuration of
the equipment.
The IEEE 802.11 series are specified by IEEE since 1997 and the
currently available standards are IEEE 802.11b [802.11b] and IEEE
802.11g [802.11g] operating in the 2.4GHz band, and IEEE 802.11a
[802.11a] operating in the 5GHz band. The specification defines both
the MAC-layer and the physical-layer (e.g., modulation techniques
and propagation). The MAC level is the same for all these
technologies.
Two operating modes are available in the IEEE 802.11 series. In ad-
hoc mode, each equipment in range may directly communicate with
other, i.e. without any infrastructure or intermediate hop. In
infrastructure mode, all link-layer frames are transmitted to an
access point (AP) which then forwards them to the final receiver.
In this section MN refers to a IEEE 802.11 station without the AP
functionality.
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3.3.1. Network Reference Model
In the infrastructure mode, the network connectivity is offered to
MN (IEEE 802.11 station) through an AP. An AP is a bridge between
the wireless domain and the wired domain. The coverage area of an AP
defines a cell. A BSS (Basic Service Set) is composed of one AP and
at least one attached MN. A common IEEE 802.11 infrastructure is
shown in Figure 5.
Access Router-----Internet-----Access Router
| |
| LAN LAN |
-+----+---- -+--------+------+-
| | |
| ~~~~~~~~~~~AP2~~~~~~~~~ |
~~~~~~~AP1~~~~~~~~ ~~~~~~~AP3~~~~~~~~
~ ~ ~ ~ ~ ~
~ ~ ~ ~ ~ ~
~ MN ~ ~ ~ ~ ~
~ ~ ~ ~ ~ MN ~
~ ~ MN ~ ~ ~ ~
~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
BSS of AP1 ~ ~ BSS of AP3
~ MN ~
~ ~
~~~~~~~~~~~~~~~~~~~~~~~
BSS of AP2
Figure 5: Architecture of IEEE 802.11 access.
When several APs are connected together through a DS (Distribution
System), the set of all cells is called ESS for Extended Service
Set. The structure of the DS is not defined in the IEEE 802.11
standard and any technology can be used as DS medium. The document
IEEE 802.11f [IAPP] proposes the Inter-AP protocol (IAPP) to be used
between APs of the same ESS. Some information on attached MNs may be
exchanged in an ESS in order to enable context transfers and enhance
MN's roaming.
When a MN moves out of the coverage area of its current AP, it may
attach to a new AP. The new AP can be connected to the same access
network as well as it can be connected to a different access
network, this makes no difference at the link layer. However, if the
new AP is connected to the same subnet as the old AP, the MN can
continue its IP communication through the new AP without any
configuration change at the network-layer. But if the new AP is
connected to a different subnet, the MN needs to configure a new IP
address valid for the new subnet and use some additional mechanism
to maintain its ongoing communication sessions, such as Mobile IP
[MIPv4, MIPv6].
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A MN must be associated and authenticated with an AP in order to
send and receive data frames. At any given time, a MN can be
associated with only one AP on each IEEE 802.11 radio interface.
When a MN moves between two APs, it has to switch into promiscuous
mode to discover and initiate a connection with a new AP. A MN
cannot send IP packets during the establishment of a connection with
an AP. In an RSN (Robust Secure Network [802.11i]) the data packets
are still blocked until the IEEE 802.1X authentication and key
management is successfully completed.
Being associated implies that the MN has established a relationship
with the AP.
In a WLAN that does not support RSNA (RSN Association), three
different steps are required for the MN to be associated with an AP.
First the MN evaluates the potential APs in its range. In active
mode, the MN scans its default channel to identify the available APs
(exchange of Probe Request and Probe Response). If the MN does not
receive any response from AP (e.g., no APs are operating in this
channel), the MN switches to the next channel and continues the
scanning. In passive mode, the MN only listens to beacons sent by AP
to discover the potential APs.
Once the MN discovers its target AP and its parameters, an
authentication phase begins (exchange of Authentication
Request/Response).
When a MN succeeds the authentication process, it can associate with
the AP (exchange of Association Request/Response). The MN sends its
different link-layer parameters and the AP may accept to include the
MN in the BSS. A MN may also issue a Re-association Request when the
new AP belongs to the same ESS as the old AP of the MN. The
Re-association message contains the MAC address of the old AP of the
MN, allowing the new AP to inform old AP that the MN will now be
associated with it. Note that even if the two APs belong to the same
ESS, they can be on different IP subnets. No assumption is made on
the location of APs in IEEE 802.11 series.
In a IEEE 802.11 RSN, IEEE 802.1X might be used as the
authentication and key management mechanism. In this framework, the
authentication is performed after the MN is connected to the AP by
utilizing protocols above the MAC layer. The process to be
associated with an AP is the same as in the model described above,
except that authentication at the MAC layer must not take place. The
(re-)association of a MN is mapped to an IEEE 802.1X port on the AP,
and the controlled IEEE 802.1X port blocks every data packets. Only
the EAPOL packets are authorized to be sent through the uncontrolled
IEEE 802.1X port. Once the authentication successfully completes,
the 4-way handshake protocol takes place. The 4-way handshake
consists of four EAPOL messages sent between the MN and the AP which
guarantee the liveness of both the AP and the MN, the freshness and
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the synchronization of the session key. This triggers the change of
the state of the IEEE 802.1X port from blocked to unblocked.
Subsequently, data packets can be exchanged on the link.
3.3.2. Link-layer Events and Information
The roaming of MNs between APs is managed by the link-layer protocol
and is known as link-layer handover. As long as the MN moves between
APs in the same access network, the IP layer is not involved in the
movement management. However, when the MN handovers to a new AP in
another IP subnet, the MN needs to perform operations to maintain
its existing communications [MIPv4, MIPv6]. Therefore, even if a
link-layer handover occurs at the link layer, it doesn't necessarily
imply a network-layer handover.
In a WLAN that does not support RSN, upon reception of the
Association (or Re-association) response message from the AP
indicating that the association is accepted, the MN is associated to
the AP. It can then transmit and receive data packets through this
AP. This association is valid as long as the MN does not receive a
De-authentication message or a De-association message from its AP,
or the MN moves to a new AP.
So the reception of a (Re-)Association Response that completes a
successful association conveys that the MN's point of attachment to
the network may have changed. There is no mechanism at the link-
layer that allows the MN to know if it has also changed the IP
subnet.
In a RSN, data packets between the MN and the AP are allowed upon
successful completion of a 4-way handshake. In this case, the
reception of a (Re-)Association Response does not imply the link is
established yet.
When the MN receives a De-authentication message (in the case of the
MAC layer authentication) or a Disassociation message, it means that
the MN is no longer authenticated or associated with the AP that
sent the message. So this message indicates that IP packets can not
be sent through this link until the reception of a subsequent
(Re)Association Response or 4-way handshake.
4.0 Triggers and Hints
A small set of useful link-layer triggers and hints can be
identified based on the technologies described in Section 3.0. This
document limits the scope to those that are relevant to network-
layer configuration changes.
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4.1. Link-up Trigger and Associated Hints
This trigger is asynchronously provided to IP when a new link
instance is created. This trigger may be generated even when the
host does not change its physical point-of attachment but creates a
new link instance with the current link-layer access device.
Network-layer may interpret this trigger as a sign of possible
configuration change. It may react to link-up trigger by
reconfirming its current configuration (e.g.: sending a router
solicitation in the case of stateless IPv6 address auto-
configuration). The detailed use of link-up trigger for detecting
network attachment is outside the scope of this draft.
Creation of a new PDP context can be used to generate a link-up
trigger in GPRS networks. Similarly, a new PPP link establishment
can lead to a trigger in 3GPP2 networks. Both of these mechanisms
also provide network-layer configuration on the host. The IP
addresses configured via these mechanisms can be considered as link-
layer hints. In fact, this type of strong hint simplifies the task
of detecting network attachment at the network-layer. This hint
indicates the already configured parameters, hence further network
attachment detection is generally not necessary.
Association and re-association events in non-RSN, and completion of
4-way handshake in RSN can be used to generate a link-up trigger in
IEEE 802.11 networks. Unlike the technologies used in 3GPP and
3GPP2, networkùlayer configuration is not provided as part of link-
layer establishment in IEEE 802.11 networks. Aside from not
providing the IP address configuration, this link-layer does not
present a useful hint to be used with the network attachment
detection process either. This is due to lack of a one-to-one
mapping between IP subnets and link-layer parameters. See Appendix A
of [DNA4] for a detailed discussion.
4.2. Link-down Trigger
This trigger is asynchronously provided to IP when an existing link
instance is removed.
Network-layer may interpret this trigger as a sign of possible
configuration change. This trigger might be followed by a link-up
trigger in the case of a handover. The detailed use of link-down
trigger for detecting network attachment is outside the scope of
this draft.
The deactivation of PDP context in GPRS networks can be used to
generate the link-down trigger. Bringing down a PPP connection in
3GPP2 would have the same effect. De-authentication and
disassociation events in IEEE 802.11 non-RSN, and disassociation
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event in IEEE 802.11 RSN can be used to generate a link-down trigger
being sent to IP.
5.0 Security Considerations
The link-layer triggers and hints are advisory only. They SHOULD be
used as indications of possible network-layer configuration change,
not an absolute change. When used in this context, potential
security threats from their use is limited but not necessarily
completely eliminated. A faked link-layer trigger can still be used
to launch a denial-of service attack on the host and the associated
network. Secure generation and delivery of these triggers and hints
MUST be ensured. This is a subject for lower-layer designs and
therefore it is outside the scope of this document.
6.0 References
[3GPP2/TIA] "IS-835 - cdma2000 Wireless IP Network Standard"
[3GPP2/TIA] "IS-2001 û Interoperability Specification (IOS) for
cdma2000 Access Network Interfaces"
[GPRS-AT] "Digital cellular telecommunications system (Phase 2+); AT
command set for GPRS Mobile Equipment (ME), (GSM 07.07 version 7.8.0
Release 98).
[GPRS] "Digital cellular telecommunications system (Phase 2+);
General Packet Radio Service (GPRS) Service description; Stage 2",
(3GPP TS 03.60 version 7.9.0 Release 98).
[GPRS-LINK]"Digital cellular telecommunications system (Phase 2+);
Radio subsystem link control", (GSM 03.05 version 7.0.0 Release 98).
[IAPP] IEEE Std. 802.11f/D3, Draft supplement to IEEE Std 802.11,
1999 Edition, "Recommanded Practice for Multi-Vendor Access Point
Interoperability via an Inter-Access Point Protocol Across
Distribution Systems Supporting IEEE 802.11 Operation", January
2002.
[802.11b] IEEE Std 802 Part 11, "Information technology -
Telecomunications anbd information exchange between systems - Local
and metropolitan area networks - Specific requirements - Part 11:
Wireless Lan Medium Access Control (MAC) And Physical Layer (PHY)
Specifications", August 1999.
[802.11a] IEEE Std 802.11a-1999, supplement to IEEE Std 802.11-1999,
"Part 11: Wireless MAN Medium Access Control (MAC) and Physical
Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ
band, September 1999.
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[Page 20] L2 Triggers and Hints February 2004
[802.11g] IEEE Std 802.11g-2003, Amendment to IEEE Std 802.11, 1999
edition, "Part 11: Wireless "Part 11: Wireless MAN Medium Access
Control (MAC) and Physical Layer (PHY) specifications. Amendemnt 4:
Further Higher Data Rate Extension in the 2.4 GHz Band, June 2003.
[80211i] IEEE Draft 802.11I/D5.0, "Draft Supplement to STANDARD FOR
Telecommunications and Information Exchange between Systems û
LAN/MAN Specific Requirements - Part 11: Wireless Medium Access
Control (MAC) and physical layer specifications: Specification for
Enhanced Security", August 2003.
[MIPv4] Perkins, C., "IP Mobility Support for IPv4", RFC3344, August
2002.
[MIPv6] Perkins, C. and J. Arko, "Mobility Support in IPv6", I-D
draft-ietf-mobileip-ipv6-24.txt, June 2003.
[DNA4] Aboba, B., "Detection of Network Attachment (DNA) in IPv4",
I-D draft-ietf-dhc-dna-ipv4-05.txt, January 2004.
7.0 Acknowledgements
The authors would like to acknowledge Sanjeev Athalye (Qualcomm) and
Muhammad Mukarram bin Tariq (DoCoMo USA Labs) for their useful
comments and suggestions.
Appendix A
This section describes the additional events and the associated
information with the GPRS networks. Unlike the PDP context changes,
the following events do not directly imply potential IP
configuration change.
A.1. Routing Area and Cell Change
The GPRS Radio Sub-System is organized in sets of Routing Areas
(RAs), each set managed by a unique SGSN. The RAs are in turn
divided into cells.
A GPRS MT detects that it has entered a new cell by comparing the
cell's identity just received with the cell identity stored in the
MT's Mobility Management context. A cell update procedure with the
network then takes place between the MT and the SGSN.
If the new cell is inside a new RA, the MT detects it by
periodically comparing the RA identifier stored in its MM context
with that just received from the new cell and initiates a RA update
procedure with the SGSN. If the SGSN receiving the RA update request
realizes that the old RA is not handled by itself, then it knows
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that an inter-SGSN RA update is required. Necessary updates are
performed in the GPRS Network Sub-System:
- The new SGSN starts a handover procedure whereby it requests and
receives the MM and PDP contexts from the old SGSN of the MT, before
packet tunneling can start to the GGSN.
- The MT location is updated in the network.
A.1.1. Link-layer Information
The MT initiates the RA update procedure by sending a "Routing Area
Update Request" to the new SGSN. This is potentially an indication
of
an imminent SGSN change (GPRS Access Point).
The network confirms that it has updated the RA (SGSN) by sending a
"Routing Area Update Accept" to the MT. The MN can utilize this
message as an indication that the MT's SGSN has changed following
the handover procedure.
The accept message comes along with an update of the MM context with
new information as described below:
- New P-TMSI
- New Cell Identity
- New RA Identity
- New ciphering algorithm, key and sequence number
A.2. Sub-link-layer Information
Some of the information, such as the signal quality (e.g., channel's
Bit Error Rate) and signal level, are not link-layer information but
rather GPRS Radio Link Control (RLC) parameters. Nonetheless, their
knowledge at the network-layer might be useful to assess the
pertinence of deciding to attach in the case where their values are
below the limits which are deemed necessary or required for the
attachment.
The RLC parameters corresponding to the two identified information
are:
- RXLEV, the received signal strength
- RXQUAL, the received signal quality
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Authors' Addresses
Alper E. Yegin
DoCoMo Communications Laboratories USA, Inc.
181 Metro Drive, Suite 300 Phone: +1 408 451 4743
San Jose, CA 95110 Fax: +1 408 451 1090
USA email: alper@docomolabs-usa.com
Eric Njedjou
France Telecom R & D
4, Rue du Clos Courtel BP 91226 Phone: +33 299124202
35512 Cesson-S‰vign‰ email: eric.njedjou@france-telecom.com
France
Siva Veerepalli
Qualcomm, Incorporated.
5775 Morehouse Drive Phone : +1 858 658 4628
San Diego, CA 92131 Fax : +1 734 661 1812
USA email : sivav@qualcomm.com
Nicolas Montavont
LSIIT - University Louis Pasteur Phone: (33) 390244587
Pole API, bureau C430 email: montavont@dpt-info.u-strasbg.fr
Boulevard Sebastien Brant URI: www-r2.u-strasbg.fr/~montavont
Illkirch 67400
France
Thomas Noel
LSIIT - University Louis Pasteur Phone: (33) 390244592
Pole API, bureau C428 email: noel@dpt-info.u-strasbg.fr
Boulevard Sebastien Brant URI: www-r2.u-strasbg.fr/~noel/
Illkirch 67400
France
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