Network Working Group T. Melia, Ed.
Request for Comments: 5677 Alcatel-Lucent
Category: Standards Track G. Bajko
Nokia
S. Das
Telcordia Technologies Inc.
N. Golmie
NIST
JC. Zuniga
InterDigital Communications, LLC
December 2009
IEEE 802.21 Mobility Services Framework Design (MSFD)
Abstract
This document describes a mobility services framework design (MSFD)
for the IEEE 802.21 Media Independent Handover (MIH) protocol that
addresses identified issues associated with the transport of MIH
messages. The document also describes mechanisms for Mobility
Services (MoS) discovery and transport-layer mechanisms for the
reliable delivery of MIH messages. This document does not provide
mechanisms for securing the communication between a mobile node (MN)
and the Mobility Server. Instead, it is assumed that either lower-
layer (e.g., link-layer) security mechanisms or overall system-
specific proprietary security solutions are used.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
IESG Note
As described later in this specification, this protocol does not
provide security mechanisms. In some deployment situations lower-
layer security services may be sufficient. Other situations require
proprietary mechanisms or as yet incomplete standard mechanisms, such
as the ones currently considered by IEEE. For these reasons, the
specification recommends careful analysis before considering any
deployment.
Melia, et al. Standards Track [Page 1]RFC 5677 MSFD December 2009
The IESG emphasizes the importance of these recommendations. The
IESG also notes that this specification deviates from the traditional
IETF requirement that support for security in the open Internet
environment is a mandatory part of any Standards Track protocol
specification. An exception has been made for this specification,
but this should not be taken to mean that other future specifications
are free from this requirement.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Melia, et al. Standards Track [Page 2]
RFC 5677 MSFD December 2009
Table of Contents
1. Introduction ....................................................4
2. Terminology .....................................................4
2.1. Requirements Language ......................................7
3. Deployment Scenarios ............................................7
3.1. Scenario S1: Home Network MoS ..............................8
3.2. Scenario S2: Visited Network MoS ...........................8
3.3. Scenario S3: Third-Party MoS ...............................9
3.4. Scenario S4: Roaming MoS ...................................9
4. Solution Overview ..............................................10
4.1. Architecture ..............................................11
4.2. MIHF Identifiers (FQDN, NAI) ..............................12
5. MoS Discovery ..................................................12
5.1. MoS Discovery When MN and MoSh Are in the Home
Network (Scenario S1) .....................................13
5.2. MoS Discovery When MN and MoSv Both Are in Visited
Network (Scenario S2) .....................................14
5.3. MoS Discovery When MIH Services Are in a
Third-Party Remote Network (Scenario S3) ..................14
5.4. MoS Discovery When the MN Is in a Visited Network
and Services Are at the Home Network (Scenario S4) ........15
6. MIH Transport Options ..........................................15
6.1. MIH Message Size ..........................................16
6.2. MIH Message Rate ..........................................17
6.3. Retransmission ............................................17
6.4. NAT Traversal .............................................18
6.5. General Guidelines ........................................18
7. Operation Flows ................................................19
8. Security Considerations ........................................21
8.1. Security Considerations for MoS Discovery .................21
8.2. Security Considerations for MIH Transport .................21
9. IANA Considerations ............................................22
10. Acknowledgements ..............................................23
11. References ....................................................23
11.1. Normative References .....................................23
11.2. Informative References ...................................23
Melia, et al. Standards Track [Page 3]RFC 5677 MSFD December 2009
1. Introduction
This document proposes a solution to the issues identified in the
problem statement document [RFC5164] for the layer 3 transport of
IEEE 802.21 MIH protocols.
The MIH Layer 3 transport problem is divided into two main parts: the
discovery of a node that supports specific Mobility Services (MoS)
and the transport of the information between a mobile node (MN) and
the discovered node. The discovery process is required for the MN to
obtain the information needed for MIH protocol communication with a
peer node. The information includes the transport address (e.g., the
IP address) of the peer node and the types of MoS provided by the
peer node.
This document lists the major MoS deployment scenarios. It describes
the solution architecture, including the MSFD reference model and
MIHF identifiers. MoS discovery procedures explain how the MN
discovers Mobility Servers in its home network, in a visited network
or in a third-party network. The remainder of this document
describes the MIH transport architecture, example message flows for
several signaling scenarios, and security issues.
This document does not provide mechanisms for securing the
communication between a mobile node and the Mobility Server.
Instead, it is assumed that either lower layer (e.g., link layer)
security mechanisms, or overall system-specific proprietary security
solutions, are used. The details of such lower layer and/or
proprietary mechanisms are beyond the scope of this document. It is
RECOMMENDED against using this protocol without careful analysis that
these mechanisms meet the desired requirements, and encourages future
standardization work in this area. The IEEE 802.21a Task Group has
recently started work on MIH security issues that may provide some
solution in this area. For further information, please refer to
Section 8.
2. Terminology
The following acronyms and terminology are used in this document:
Media Independent Handover (MIH): the handover support architecture
defined by the IEEE 802.21 working group that consists of the MIH
Function (MIHF), MIH Network Entities, and MIH protocol messages.
Melia, et al. Standards Track [Page 4]RFC 5677 MSFD December 2009
Media Independent Handover Function (MIHF): a switching function that
provides handover services including the Event Service (ES),
Information Service (IS), and Command Service (CS), through
service access points (SAPs) defined by the IEEE 802.21 working
group [IEEE80221].
MIHF User: An entity that uses the MIH SAPs to access MIHF services,
and which is responsible for initiating and terminating MIH
signaling.
Media Independent Handover Function Identifier (MIHFID): an
identifier required to uniquely identify the MIHF endpoints for
delivering mobility services (MoS); it is implemented as either a
FQDN or NAI.
Mobility Services (MoS): composed of Information Service, Command
Service, and Event Service provided by the network to mobile nodes
to facilitate handover preparation and handover decision, as
described in [IEEE80221] and [RFC5164].
MoSh: Mobility Services provided by the mobile node's Home Network.
MoSv: Mobility Services provided by the Visited Network.
MoS3: Mobility Services provided by a third-party network, which is a
network that is neither the Home Network nor the current Visited
Network.
Mobile Node (MN): an Internet device whose location changes, along
with its point of connection to the network.
Mobility Services Transport Protocol (MSTP): a protocol that is used
to deliver MIH protocol messages from an MIHF to other MIH-aware
nodes in a network.
Information Service (IS): a MoS that originates at the lower or upper
layers of the protocol stack and sends information to the local or
remote upper or lower layers of the protocol stack. The purpose
of IS is to exchange information elements (IEs) relating to
various neighboring network information.
Event Service (ES): a MoS that originates at a remote MIHF or the
lower layers of the local protocol stack and sends information to
the local MIHF or local higher layers. The purpose of the ES is
to report changes in link status (e.g., Link Going Down messages)
and various lower layer events.
Melia, et al. Standards Track [Page 5]RFC 5677 MSFD December 2009
Command Service (CS): a MoS that sends commands from the remote MIHF
or local upper layers to the remote or local lower layers of the
protocol stack to switch links or to get link status.
Fully Qualified Domain Name (FQDN): a complete domain name for a host
on the Internet, showing (in reverse order) the full delegation
path from the DNS root and top-level domain down to the host name
(e.g., myexample.example.org).
Network Access Identifier (NAI): the user ID that a user submits
during network access authentication [RFC4282]. For mobile users,
the NAI identifies the user and helps to route the authentication
request message.
Network Address Translator (NAT): a device that implements the
Network Address Translation function described in [RFC3022], in
which local or private network layer addresses are mapped to
routable (outside the NAT domain) network addresses and port
numbers.
Dynamic Host Configuration Protocol (DHCP): protocols described in
[RFC2131] and [RFC3315] that allow Internet devices to obtain
respectively IPv4 and IPv6 addresses, subnet masks, default
gateway addresses, and other IP configuration information from
DHCP servers.
Domain Name System (DNS): a protocol described in [RFC1035] that
translates domain names to IP addresses.
Authentication, Authorization, and Accounting (AAA): a set of network
management services that respectively determine the validity of a
user's ID, determine whether a user is allowed to use network
resources, and track users' use of network resources.
Home AAA (AAAh): an AAA server located on the MN's home network.
Visited AAA (AAAv): an AAA server located in a visited network that
is not the MN's home network.
MIH Acknowledgement (MIH ACK): an MIH signaling message that an MIHF
sends in response to an MIH message from a sending MIHF.
Point of Service (PoS): a network-side MIHF instance that exchanges
MIH messages with an MN-based MIHF.
Melia, et al. Standards Track [Page 6]RFC 5677 MSFD December 2009
Network Access Server (NAS): a server to which an MN initially
connects when it is trying to gain a connection to a network and
that determines whether the MN is allowed to connect to the NAS's
network.
User Datagram Protocol (UDP): a connectionless transport-layer
protocol used to send datagrams between a source and a destination
at a given port, defined in RFC 768.
Transmission Control Protocol (TCP): a stream-oriented transport-
layer protocol that provides a reliable delivery service with
congestion control, defined in RFC 793.
Round-Trip Time (RTT): an estimation of the time required for a
segment to travel from a source to a destination and an
acknowledgement to return to the source that is used by TCP in
connection with timer expirations to determine when a segment is
considered lost and should be resent.
Maximum Transmission Unit (MTU): the largest size of an IP packet
that can be sent on a network segment without requiring
fragmentation [RFC1191].
Path MTU (PMTU): the largest size of an IP packet that can be sent on
an end-to-end network path without requiring IP fragmentation.
Transport Layer Security Protocol (TLS): an application layer
protocol that primarily assures privacy and data integrity between
two communicating network entities [RFC5246].
Sender Maximum Segment Size (SMSS): size of the largest segment that
the sender can transmit as per [RFC5681].
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in RFC 2119
[RFC2119].
3. Deployment Scenarios
This section describes the various possible deployment scenarios for
the MN and the Mobility Server. The relative positioning of the MN
and Mobility Server affects MoS discovery as well as the performance
of the MIH signaling service. This document addresses the scenarios
listed in [RFC5164] and specifies transport options to carry the MIH
protocol over IP.
Melia, et al. Standards Track [Page 7]RFC 5677 MSFD December 2009
3.1. Scenario S1: Home Network MoS
In this scenario, the MN and the services are located in the home
network. We refer to this set of services as MoSh as shown in Figure
1. The MoSh can be located at the access network the MN uses to
connect to the home network, or it can be located elsewhere.
+--------------+ +====+
| HOME NETWORK | |MoSh|
+--------------+ +====+
/\
||
\/
+--------+
| MN |
+--------+
Figure 1: MoS in the Home Network
3.2. Scenario S2: Visited Network MoS
In this scenario, the MN is in the visited network and mobility
services are provided by the visited network. We refer to this as
MoSv as shown in Figure 2.
+--------------+
| HOME NETWORK |
+--------------+
/\
||
\/
+====+ +-----------------+
|MoSv| | VISITED NETWORK |
+====+ +-----------------+
/\
||
\/
+--------+
| MN |
+--------+
Figure 2: MoSv in the Visited Network
Melia, et al. Standards Track [Page 8]RFC 5677 MSFD December 2009
3.3. Scenario S3: Third-Party MoS
In this scenario, the MN is in its home network or in a visited
network and services are provided by a third-party network. We refer
to this situation as MoS3 as shown in Figure 3. (Note that MoS can
exist both in home and in visited networks.)
+--------------+
| HOME NETWORK |
+====+ +--------------+ +--------------+
|MoS3| | THIRD PARTY | <===> /\
+====+ +--------------+ ||
\/
+-----------------+
| VISITED NETWORK |
+-----------------+
/\
||
\/
+--------+
| MN |
+--------+
Figure 3: MoS from a Third Party
3.4. Scenario S4: Roaming MoS
In this scenario, the MN is located in the visited network and all
MIH services are provided by the home network, as shown in Figure 4.
+====+ +--------------+
|MoSh| | HOME NETWORK |
+====+ +--------------+
/\
||
\/
+-----------------+
| VISITED NETWORK |
+-----------------+
/\
||
\/
+--------+
| MN |
+--------+
Figure 4: MoS Provided by the Home While in Visited
Melia, et al. Standards Track [Page 9]RFC 5677 MSFD December 2009
Different types of MoS can be provided independently of other types
and there is no strict relationship between ES, CS, and IS, nor is
there a requirement that the entities that provide these services
should be co-located. However, while IS tends to involve a large
amount of static information, ES and CS are dynamic services and some
relationships between them can be expected, e.g., a handover command
(CS) could be issued upon reception of a link event (ES). This
document does not make any assumption on the location of the MoS
(although there might be some preferred configurations), and aims at
flexible MSFD to discover different services in different locations
to optimize handover performance. MoS discovery is discussed in more
detail in Section 5.
4. Solution Overview
As mentioned in Section 1, the solution space is being divided into
two functional domains: discovery and transport. The following
assumptions have been made:
o The solution is primarily aimed at supporting IEEE 802.21 MIH
services -- namely, Information Service (IS), Event Service (ES),
and Command Service (CS).
o If the MIHFID is available, FQDN or NAI's realm is used for
mobility service discovery.
o The solutions are chosen to cover all possible deployment
scenarios as described in Section 3.
o MoS discovery can be performed during initial network attachment
or at any time thereafter.
The MN may know the realm of the Mobility Server to be discovered.
The MN may also be pre-configured with the address of the Mobility
Server to be used. In case the MN does not know what realm /
Mobility Server to query, dynamic assignment methods are described in
Section 5.
The discovery of the Mobility Server (and the related configuration
at MIHF level) is required to bind two MIHF peers (e.g., MN and
Mobility Server) with their respective IP addresses. Discovery MUST
be executed in the following conditions:
o Bootstrapping: upon successful Layer 2 network attachment, the MN
MAY be required to use DHCP for address configuration. These
procedures can carry the required information for MoS
configuration in specific DHCP options.
Melia, et al. Standards Track [Page 10]RFC 5677 MSFD December 2009
o If the MN does not receive MoS information during network
attachment and the MN does not have a pre-configured Mobility
Server, it MUST run a discovery procedure upon initial IP address
configuration.
o If the MN changes its IP address (e.g., upon handover), it MUST
refresh MIHF peer bindings (i.e., MIHF registration process). In
case the Mobility Server used is not suitable anymore (e.g., too
large RTT experienced), the MN MAY need to perform a new discovery
procedure.
o If the MN is a multi-homed device and it communicates with the
same Mobility Server via different IP addresses, it MAY run
discovery procedures if one of the IP addresses changes.
Once the MIHF peer has been discovered, MIH information can be
exchanged between MIH peers over a transport protocol such as UDP or
TCP. The usage of transport protocols is described in Section 6 and
packing of the MIH messages does not require extra framing since the
MIH protocol defined in [IEEE80221] already contains a length field.
4.1. Architecture
Figure 5 depicts the MSFD reference model and its components within a
node. The topmost layer is the MIHF user. This set of applications
consists of one or more MIH clients that are responsible for
operations such as generating query and response, processing Layer 2
triggers as part of the ES, and initiating and carrying out handover
operations as part of the CS. Beneath the MIHF user is the MIHF
itself. This function is responsible for MoS discovery, as well as
creating, maintaining, modifying, and destroying MIH signaling
associations with other MIHFs located in MIH peer nodes. Below the
MIHF are various transport-layer protocols as well as address
discovery functions.
Melia, et al. Standards Track [Page 11]RFC 5677 MSFD December 2009
+--------------------------+
| MIHF User |
+--------------------------+
||
+--------------------------+
| MIHF |
+--------------------------+
|| || ||
|| +------+ +-----+
|| | DHCP | | DNS |
|| +------+ +-----+
|| || ||
+--------------------------+
| TCP/UDP |
+--------------------------+
Figure 5: MN Stack
The MIHF relies on the services provided by TCP and UDP for
transporting MIH messages, and relies on DHCP and DNS for peer
discovery. In cases where the peer MIHF IP address is not pre-
configured, the source MIHF needs to discover it either via DHCP or
DNS as described in Section 5. Once the peer MIHF is discovered, the
MIHF must exchange messages with its peer over either UDP or TCP.
Specific recommendations regarding the choice of transport protocols
are provided in Section 6.
There are no security features currently defined as part of the MIH
protocol level. However, security can be provided either at the
transport or IP layer where it is necessary. Section 8 provides
guidelines and recommendations for security.
4.2. MIHF Identifiers (FQDN, NAI)
MIHFID is required to uniquely identify the MIHF end points for
delivering the mobility services (MoS). Thus an MIHF identifier
needs to be unique within a domain where mobility services are
provided and independent of the configured IP address(es). An MIHFID
MUST be represented either in the form of an FQDN [RFC2181] or NAI
[RFC4282]. An MIHFID can be pre-configured or discovered through the
discovery methods described in Section 5.
5. MoS Discovery
The MoS discovery method depends on whether the MN attempts to
discover a Mobility Server in the home network, in the visited
network, or in a third-party remote network that is neither the home
network nor the visited network. In the case where the MN already
Melia, et al. Standards Track [Page 12]RFC 5677 MSFD December 2009
has a Mobility Server address pre-configured, it is not necessary to
run the discovery procedure. If the MN does not have pre-configured
Mobility Server, the following procedure applies.
In the case where a Mobility Server is provided locally (scenarios S1
and S2), the discovery techniques described in [RFC5678] and
[RFC5679] are both applicable as described in Sections 5.1 and 5.2.
In the case where a Mobility Server is located in the home network
while the MN is in the visited network (scenario S4), the DNS-based
discovery described in [RFC5679] is applicable.
In the case where a Mobility Server is located in a third-party
network that is different from the current visited network (scenario
S3), only the DNS-based discovery method described in [RFC5679] is
applicable.
It should be noted that authorization of an MN to use a specific
Mobility Server is neither in scope of this document nor is currently
specified in [IEEE80221]. We further assume all devices can access
discovered MoS. In case future deployments will implement
authorization policies, the mobile nodes should fall back to other
learned MoS if authorization is denied.
5.1. MoS Discovery When MN and MoSh Are in the Home Network (Scenario
S1)
To discover a Mobility Server in the home network, the MN SHOULD use
the DNS-based MoS discovery method described in [RFC5679]. In order
to use that mechanism, the MN MUST have its home domain pre-
configured (i.e., subscription is tied to a network). The DNS query
option is shown in Figure 6a. Alternatively, the MN MAY use the DHCP
options for MoS discovery [RFC5678] as shown in Figure 6b (in some
deployments, a DHCP relay may not be present).
Melia, et al. Standards Track [Page 13]RFC 5677 MSFD December 2009
(a) +-------+
+----+ |Domain |
| MN |-------->|Name |
+----+ |Server |
MN@example.org +-------+
(b)
+-----+ +------+
+----+ | | |DHCP |
| MN |<----->| DHCP|<---->|Server|
+----+ |Relay| | |
+-----+ +------+
Figure 6: MOS Discovery (a) Using DNS Query, (b) Using DHCP Option
5.2. MoS Discovery When MN and MoSv Both Are in Visited Network
(Scenario S2)
To discover a Mobility Server in the visited network, the MN SHOULD
attempt to use the DHCP options for MoS discovery [RFC5678] as shown
in Figure 7.
+-----+ +------+
+----+ | | |DHCP |
| MN |<----->| DHCP|<---->|Server|
+----+ |Relay| | |
+-----+ +------+
Figure 7: MoS Discovery Using DHCP Options
5.3. MoS Discovery When MIH Services Are in a Third-Party Remote
Network (Scenario S3)
To discover a Mobility Server in a remote network other than home
network, the MN MUST use the DNS-based MoS discovery method described
in [RFC5679]. The MN MUST first learn the domain name of the network
containing the MoS it is searching for. The MN can query its current
Mobility Server to find out the domain name of a specific network or
the domain name of a network at a specific location (as in Figure
8a). IEEE 802.21 defines information elements such as OPERATOR ID
and SERVICE PROVIDER ID that can be a domain name. An IS query can
provide this information, see [IEEE80221].
Alternatively, the MN MAY query a Mobility Server previously known to
learn the domain name of the desired network. Finally, the MN MUST
use DNS-based discovery mechanisms to find a Mobility Server in the
Melia, et al. Standards Track [Page 14]RFC 5677 MSFD December 2009
remote network as in Figure 8b. It should be noted that step b can
only be performed upon obtaining the domain name of the remote
network.
(a)
+------------+
+----+ | |
| | |Information |
| MN |-------->| Server |
| | |(previously |
+----+ |discovered) |
+------------+
(b)
+-------+
+----+ |Domain |
| MN |-------->|Name |
+----+ |Server |
MN@example.org +-------+
Figure 8: MOS Discovery Using (a) IS Query to a Known IS Server,
(b) DNS Query
5.4. MoS Discovery When the MN Is in a Visited Network and Services Are
at the Home Network (Scenario S4)
To discover a Mobility Server in the visited network when MIH
services are provided by the home network, the DNS-based discovery
method described in [RFC5679] is applicable. To discover the
Mobility Server at home while in a visited network using DNS, the MN
SHOULD use the procedures described in Section 5.1.
6. MIH Transport Options
Once the MoS have been discovered, MIH peers run a capability
discovery and subscription procedure as specified in [IEEE80221].
MIH peers MAY exchange information over TCP, UDP, or any other
transport supported by both the server and the client. The client
MAY use the DNS discovery mechanism to discover which transport
protocols are supported by the server in addition to TCP and UDP that
are recommended in this document. While either protocol can provide
the basic transport functionality required, there are performance
trade-offs and unique characteristics associated with each that need
to be considered in the context of the MIH services for different
network loss and congestion conditions. The objectives of this
section are to discuss these trade-offs for different MIH settings
such as the MIH message size and rate, and the retransmission
parameters. In addition, factors such as NAT traversal are also
Melia, et al. Standards Track [Page 15]RFC 5677 MSFD December 2009
discussed. Given the reliability requirements for the MIH transport,
it is assumed in this discussion that the MIH ACK mechanism is to be
used in conjunction with UDP, while it MUST NOT be used with TCP
since TCP includes acknowledgement and retransmission functionality.
6.1. MIH Message Size
Although the MIH message size varies widely from about 30 bytes (for
a capability discovery request) to around 65000 bytes (for an IS
MIH_Get_Information response primitive), a typical MIH message size
for the ES or CS ranges between 50 to 100 bytes [IEEE80221]. Thus,
considering the effects of the MIH message size on the performance of
the transport protocol brings us to discussing two main issues,
related to fragmentation of long messages in the context of UDP and
the concatenation of short messages in the context of TCP.
Since transporting long MIH messages may require fragmentation that
is not available in UDP, if MIH is using UDP a limit MUST be set on
the size of the MIH message based on the path MTU to destination (or
the Minimum MTU where PMTU is not implemented). The Minimum MTU
depends on the IP version used for transmission, and is the lesser of
the first hop MTU, and 576 or 1280 bytes for IPv4 [RFC1122] or for
IPv6 [RFC2460], respectively, although applications may reduce these
values to guard against the presence of tunnels.
According to [IEEE80221], when an MIH message is sent using an L3 or
higher-layer transport, L3 takes care of any fragmentation issue and
the MIH protocol does not handle fragmentation in such cases. Thus,
MIH layer fragmentation MUST NOT be used together with IP layer
fragmentation and MUST not be used when MIH packets are carried over
TCP.
The loss of an IP fragment leads to the retransmission of an entire
MIH message, which in turn leads to poor end-to-end delay performance
in addition to wasted bandwidth. Additional recommendations in
[RFC5405] apply for limiting the size of the MIH message when using
UDP and assuming IP layer fragmentation. In terms of dealing with
short messages, TCP has the capability to concatenate very short
messages in order to reduce the overall bandwidth overhead. However,
this reduced overhead comes at the cost of additional delay to
complete an MIH transaction, which may not be acceptable for CS and
ES. Note also that TCP is a stream-oriented protocol and measures
data flow in terms of bytes, not messages. Thus, it is possible to
split messages across multiple TCP segments if they are long enough.
Even short messages can be split across two segments. This can also
cause unacceptable delays, especially if the link quality is severely
degraded as is likely to happen when the MN is exiting a wireless
access coverage area. The use of the TCP_NODELAY option can
Melia, et al. Standards Track [Page 16]
RFC 5677 MSFD December 2009
alleviate this problem by triggering transmission of a segment less
than the SMSS. (It should be noted that [RFC4960] addresses both of
these problems, but discussion of SCTP is omitted here, as it is
generally not used for the mobility services discussed in this
document.)
6.2. MIH Message Rate
The frequency of MIH messages varies according to the MIH service
type. It is expected that CS/ES messages arrive at a rate of one in
hundreds of milliseconds in order to capture quick changes in the
environment and/or process handover commands. On the other hand, IS
messages are exchanged mainly every time a new network is visited,
which may be in order of hours or days. Therefore, a burst of either
short CS/ES messages or long IS message exchanges (in the case where
multiple MIH nodes request information) may lead to network
congestion. While the built-in rate-limiting controls available in
TCP may be well suited for dealing with these congestion conditions,
this may result in large transmission delays that may be unacceptable
for the timely delivery of ES or CS messages. On the other hand, if
UDP is used, a rate-limiting effect similar to the one obtained with
TCP SHOULD be obtained by adequately adjusting the parameters of a
token bucket regulator as defined in the MIH specifications
[IEEE80221]. Recommendations for token bucket parameter settings are
as follows:
o If the MIHF knows the RTT (e.g., based on the request/response MIH
protocol exchange between two MIH peers), the rate can be based
upon this as specified in [IEEE80221].
o If not, then on average it SHOULD NOT send more than one UDP
message every 3 seconds.
6.3. Retransmission
For TCP, the retransmission timeout is adjusted according to the
measured RTT. However due to the exponential backoff mechanism, the
delay associated with retransmission timeouts may increase
significantly with increased packet loss.
If UDP is being used to carry MIH messages, MIH MUST use MIH ACKs.
An MIH message is retransmitted if its corresponding MIH ACK is not
received by the generating node within a timeout interval set by the
MIHF. The maximum number of retransmissions is configurable and the
value of the retransmission timer is computed according to the
algorithm defined in [RFC2988]. The default maximum number of
Melia, et al. Standards Track [Page 17]RFC 5677 MSFD December 2009
retransmissions is set to 2 and the initial retransmission timer
(TMO) is set to 3s when RTT is not known. The maximum TMO is set to
30s.
6.4. NAT Traversal
There are no known issues for NAT traversal when using TCP. The
default connection timeout of 2 hours 4 minutes [RFC5382] (assuming a
2-hour TCP keep-alive) is considered adequate for MIH transport
purposes. However, issues with NAT traversal using UDP are
documented in [RFC5405]. Communication failures are experienced when
middleboxes destroy the per-flow state associated with an application
session during periods when the application does not exchange any UDP
traffic. Hence, communication between the MN and the Mobility Server
SHOULD be able to gracefully handle such failures and implement
mechanisms to re-establish their UDP sessions. In addition and in
order to avoid such failures, MIH messages MAY be sent periodically,
similarly to keep-alive messages, in an attempt to refresh middlebox
state. As [RFC4787] requires a minimum state timeout of 2 minutes or
more, MIH messages using UDP as transport SHOULD be sent once every 2
minutes. Re-registration or event indication messages as defined in
[IEEE80221] MAY be used for this purpose.
6.5. General Guidelines
The ES and CS messages are small in nature and have tight latency
requirements. On the other hand, IS messages are more resilient in
terms of latency constraints, and some long IS messages could exceed
the MTU of the path to the destination. TCP SHOULD be used as the
default transport for all messages. However, UDP in combination with
MIH acknowledgement SHOULD be used for transporting ES and CS
messages that are shorter than or equal to the path MTU as described
in Section 6.1.
For both UDP and TCP cases, if a port number is not explicitly
assigned (e.g., by the DNS SRV), MIH messages sent over UDP, TCP, or
other supported transport MUST use the default port number defined in
Section 9 for that particular transport.
A Mobility Server MUST support both UDP and TCP for MIH transport and
the MN MUST support TCP. Additionally, the server and MN MAY support
additional transport mechanisms. The MN MAY use the procedures
defined in [RFC5679] to discover additional transport protocols
supported by the server (e.g., SCTP).
Melia, et al. Standards Track [Page 18]
RFC 5677 MSFD December 2009
7. Operation Flows
Figure 9 gives an example operation flow between MIHF peers when an
MIH user requests an IS and both the MN and the Mobility Server are
in the MN's home network. DHCP is used for Mobility Services (MoS)
discovery, and TCP is used for establishing a transport connection to
carry the IS messages. When the Mobility Server is not pre-
configured, the MIH user needs to discover the IP address of the
Mobility Server to communicate with the remote MIHF. Therefore, the
MIH user sends a discovery request message to the local MIHF as
defined in [IEEE80221].
In this example (one could draw similar mechanisms with DHCPv6), we
assume that MoS discovery is performed before a transport connection
is established with the remote MIHF, and the DHCP client process is
invoked via some internal APIs. The DHCP client sends a DHCP INFORM
message according to standard DHCP and with the MoS option as defined
in [RFC5678]. The DHCP server replies via a DHCP ACK message with
the IP address of the Mobility Server. The Mobility Server address
is then passed to the MIHF locally via some internal APIs. The MIHF
generates the discovery response message and passes it on to the
corresponding MIH user. The MIH user generates an IS query addressed
to the remote Mobility Server. The MIHF invokes the underlying TCP
client, which establishes a transport connection with the remote
peer. Once the transport connection is established, the MIHF sends
the IS query via an MIH protocol REQUEST message. The message and
query arrive at the destination MIHF and MIH user, respectively. The
Mobility Server MIH user responds to the corresponding IS query and
the Mobility Server MIHF sends the IS response via an MIH protocol
RESPONSE message. The message arrives at the source MIHF, which
passes the IS response on to the corresponding MIH user.
Melia, et al. Standards Track [Page 19]
RFC 5677 MSFD December 2009
MN MoS
|===================================| |======| |===================|
+ ---------+ +---------+
| MIH USER | +------+ +------+ +------+ +------+ | MIH USER|
| +------+ | | TCP | |DHCP | |DHCP | | TCP | | +------+|
| | MIHF | | |Client| |Client| |Server| |Server| | | MIHF ||
+----------+ +------+ +------+ +------+ +------++----------+
| | | | | |
MIH Discovery | | | | |
Request | | | | |
| | | | | |
|Invoke DHCP Client | | | |
|(Internal process with MoS)|DHCP INFORM| | |
|==========================>|==========>| | |
| | | | | |
| Inform Mobility Server | DHCP ACK | | |
| Address |<==========| | |
|<==========================| | | |
| (internal process) | | | |
| | | | | |
MIH Discovery | | | | |
Response | | | | |
| | | | | |
IS Query | | | | |
MIH User-> MIHF | | | | |
| | | | | |
|Invoke TCP Client| | | | |
|================>| TCP connection established | |
Internal process |<=============================>| |
| | | | | |
| IS QUERY REQUEST (via MIH protocol) |
|===========================================================>|
| | | | | IS QUERY|
| | | | | REQUEST|
| | | | MIHF-> MIH User |
| | | | | QUERY|
| | | | | RESPONSE|
| | | | MIHF <-MIH User |
| | | | | |
| | IS QUERY RESPONSE (via MIH protocol) |
|<===========================================================|
| | | | | |
IS RESPONSE | | | | |
MIH User <-MIHF | | | | |
| | | | | |
Figure 9: Example Flow of Operation Involving MIH User
Melia, et al. Standards Track [Page 20]RFC 5677 MSFD December 2009
8. Security Considerations
There are two components to the security considerations: MoS
discovery and MIH transport. For MoS discovery, DHCP and DNS
recommendations are hereby provided per IETF guidelines. For MIH
transport, we describe the security threats and expect that the
system deployment will have means to mitigate such threats when
sensitive information is being exchanged between the mobile node and
Mobility Server. Since IEEE 802.21 base specification does not
provide MIH protocol level security, it is assumed that either lower
layer security (e.g., link layer) or overall system-specific (e.g.,
proprietary) security solutions are available. The present document
does not provide any guidelines in this regard. It is stressed that
the IEEE 802.21a Task Group has recently started work on MIH security
issues that may provide some solution in this area. Finally,
authorization of an MN to use a specific Mobility Server, as stated
in Section 5, is neither in scope of this document nor is currently
specified in [IEEE80221].
8.1. Security Considerations for MoS Discovery
There are a number of security issues that need to be taken into
account during node discovery. In the case where DHCP is used for
node discovery and authentication of the source and content of DHCP
messages is required, network administrators SHOULD use the DHCP
authentication option described in [RFC3118], where available, or
rely upon link layer security. [RFC3118] provides mechanisms for
both entity authentication and message authentication. In the case
where the DHCP authentication mechanism is not available,
administrators may need to rely upon the underlying link layer
security. In such cases, the link between the DHCP client and Layer
2 termination point may be protected, but the DHCP message source and
its messages cannot be authenticated or the integrity of the latter
checked unless there exits a security binding between link layer and
DHCP layer.
In the case where DNS is used for discovering MoS, fake DNS requests
and responses may cause denial of service (DoS) and the inability of
the MN to perform a proper handover, respectively. Where networks
are exposed to such DoS, it is RECOMMENDED that DNS service providers
use the Domain Name System Security Extensions (DNSSEC) as described
in [RFC4033]. Readers may also refer to [RFC4641] to consider the
aspects of DNSSEC operational practices.
8.2. Security Considerations for MIH Transport
The communication between an MN and a Mobility Server is exposed to a
number of security threats:
Melia, et al. Standards Track [Page 21]
RFC 5677 MSFD December 2009
o Mobility Server identity spoofing. A fake Mobility Server could
provide the MNs with bogus data and force them to select the wrong
network or to make a wrong handover decision.
o Tampering. Tampering with the information provided by a Mobility
Server may result in the MN making wrong network selection or
handover decisions.
o Replay attack. Since Mobility Services as defined in [IEEE80221]
support a 'PUSH model', they can send large amounts of data to the
MNs whenever the Mobility Server thinks that the data is relevant
for the MN. An attacker may intercept the data sent by the
Mobility Server to the MNs and replay it at a later time, causing
the MNs to make network selection or handover decisions that are
not valid at that point in time.
o Eavesdropping. By snooping the communication between an MN and a
Mobility Server, an attacker may be able to trace a user's
movement between networks or cells, or predict future movements,
by inspecting handover service messages.
There are many deployment-specific system security solutions
available, which can be used to countermeasure the above mentioned
threats. For example, for the MoSh and MoSv scenarios (including
roaming scenarios), link layer security may be sufficient to protect
the communication between the MN and Mobility Server. This is a
typical mobile operator environment where link layer security
provides authentication, data confidentiality, and integrity. In
other scenarios, such as the third-party MoS, link layer security
solutions may not be sufficient to protect the communication path
between the MN and the Mobility Server. The communication channel
between MN and Mobility Server needs to be secured by other means.
The present document does not provide any specific guidelines about
the way these security solutions should be deployed. However, if in
the future the IEEE 802.21 Working Group amends the specification
with MIH protocol level security or recommends the deployment
scenarios, IETF may revisit the security considerations and recommend
specific transport-layer security as appropriate.
9. IANA Considerations
This document registers the following TCP and UDP ports with IANA:
Keyword Decimal Description
-------- --------------- ------------
ieee-mih 4551/tcp MIH Services
ieee-mih 4551/udp MIH Services
Melia, et al. Standards Track [Page 22]RFC 5677 MSFD December 2009
10. Acknowledgements
The authors would like to thank Yoshihiro Ohba, David Griffith, Kevin
Noll, Vijay Devarapalli, Patrick Stupar, and Sam Xia for their
valuable comments, reviews, and fruitful discussions.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC3118] Droms, R., Ed., and W. Arbaugh, Ed., "Authentication for
DHCP Messages", RFC 3118, June 2001.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC5678] Bajko, G. and S. Das, "Dynamic Host Configuration
Protocol (DHCPv4 and DHCPv6) Options for IEEE 802.21
Mobility Services (MoS) Discovery", RFC 5678, December
2009.
[RFC5679] Bajko, G., "Locating IEEE 802.21 Mobility Services Using
DNS", RFC 5679, December 2009.
11.2. Informative References
[IEEE80221] "IEEE Standard for Local and Metropolitan Area Networks -
Part 21: Media Independent Handover Services", IEEE
LAN/MAN Std 802.21-2008, January 2009,
http://www.ieee802.org/21/private/Published%20Spec/
802.21-2008.pdf (access to the document requires
membership).
Melia, et al. Standards Track [Page 23]RFC 5677 MSFD December 2009
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
[RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational
Practices", RFC 4641, September 2006.
[RFC4787] Audet, F., Ed., and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, January 2007.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5164] Melia, T., Ed., "Mobility Services Transport: Problem
Statement", RFC 5164, March 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and
P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP
142, RFC 5382, October 2008.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage
Guidelines for Application Designers", BCP 145, RFC 5405,
November 2008.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
Melia, et al. Standards Track [Page 24]RFC 5677 MSFD December 2009
Authors' Addresses
Telemaco Melia (editor)
Alcatel-Lucent
Route de Villejust
Nozay 91620
France
EMail: telemaco.melia@alcatel-lucent.com
Gabor Bajko
Nokia
EMail: Gabor.Bajko@nokia.com
Subir Das
Telcordia Technologies Inc.
EMail: subir@research.telcordia.com
Nada Golmie
NIST
EMail: nada.golmie@nist.gov
Juan Carlos Zuniga
InterDigital Communications, LLC
EMail: j.c.zuniga@ieee.org
Melia, et al. Standards Track [Page 25]
