RFC 3726 INFORMATIONAL SIGNALING · REQUIREMENTS · QOS
Signaling Requirements: Requirements for Signaling Protocols
What a signalling protocol must do if it is to work across networks, technologies and administrative borders
RFC EDITOR
Brunner, M., Ed., "Requirements for Signaling Protocols", RFC 3726, DOI 10.17487/RFC3726, April 2004, <https://www.rfc-editor.org/info/rfc3726>.
BIBTEX
@misc{rfc3726,
series = {Request for Comments},
number = 3726,
howpublished = {RFC 3726},
publisher = {RFC Editor},
doi = {10.17487/RFC3726},
url = {https://www.rfc-editor.org/info/rfc3726},
author = {M. Brunner},
title = {{Requirements for Signaling Protocols}},
pagetotal = 42,
year = 2004,
month = apr,
} RFC text
Contents
Network Working Group M. Brunner, Ed.
Request for Comments: 3726 NEC
Category: Informational April 2004
Requirements for Signaling Protocols
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines requirements for signaling across different
network environments, such as across administrative and/or technology
domains. Signaling is mainly considered for Quality of Service (Qos)
such as the Resource Reservation Protocol (RSVP). However, in recent
years, several other applications of signaling have been defined.
For example, signaling for label distribution in Multiprotocol Label
Switching (MPLS) or signaling to middleboxes. To achieve wide
applicability of the requirements, the starting point is a diverse
set of scenarios/use cases concerning various types of networks and
application interactions. This document presents the assumptions
before listing the requirements. The requirements are grouped
according to areas such as architecture and design goals, signaling
flows, layering, performance, flexibility, security, and mobility.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Problem Statement and Scope. . . . . . . . . . . . . . . . . . 6
4. Assumptions and Exclusions . . . . . . . . . . . . . . . . . . 8
4.1. Assumptions and Non-Assumptions. . . . . . . . . . . . . 8
4.2. Exclusions . . . . . . . . . . . . . . . . . . . . . . . 9
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Architecture and Design Goals. . . . . . . . . . . . . . 11
5.1.1. NSIS SHOULD Provide Availability Information
on Request . . . . . . . . . . . . . . . . . . . 11
5.1.2. NSIS MUST be Designed Modularly. . . . . . . . . 11
5.1.3. NSIS MUST Decouple Protocol and Information. . . 12
5.1.4. NSIS MUST Support Independence of Signaling and
Network Control Paradigm . . . . . . . . . . . . 12
5.1.5. NSIS SHOULD be Able to Carry Opaque Objects. . . 12
5.2. Signaling Flows. . . . . . . . . . . . . . . . . . . . . 12
5.2.1. The Placement of NSIS Initiator, Forwarder, and
Responder Anywhere in the Network MUST be
Allowed. . . . . . . . . . . . . . . . . . . . . 12
5.2.2. NSIS MUST Support Path-Coupled and MAY Support
Path-Decoupled Signaling . . . . . . . . . . . . 13
5.2.3. Concealment of Topology and Technology
Information SHOULD be Possible . . . . . . . . . 13
5.2.4. Transparent Signaling Through Networks SHOULD be
Possible . . . . . . . . . . . . . . . . . . . . 13
5.3. Messaging. . . . . . . . . . . . . . . . . . . . . . . . 13
5.3.1. Explicit Erasure of State MUST be Possible . . . 13
5.3.2. Automatic Release of State After Failure MUST be
Possible . . . . . . . . . . . . . . . . . . . . 14
5.3.3. NSIS SHOULD Allow for Sending Notifications
Upstream . . . . . . . . . . . . . . . . . . . . 14
5.3.4. Establishment and Refusal to set up State MUST
be Notified. . . . . . . . . . . . . . . . . . . 15
5.3.5. NSIS MUST Allow for Local Information Exchange . 15
5.4. Control Information. . . . . . . . . . . . . . . . . . . 16
5.4.1. Mutability Information on Parameters SHOULD be
Possible . . . . . . . . . . . . . . . . . . . . 16
5.4.2. It SHOULD be Possible to Add and Remove Local
Domain Information . . . . . . . . . . . . . . . 16
5.4.3. State MUST be Addressed Independent of Flow
Identification . . . . . . . . . . . . . . . . . 16
5.4.4. Modification of Already Established State SHOULD
be Seamless. . . . . . . . . . . . . . . . . . . 16
5.4.5. Grouping of Signaling for Several Micro-Flows
MAY be Provided. . . . . . . . . . . . . . . . . 17
5.5. Performance. . . . . . . . . . . . . . . . . . . . . . . 17
5.5.1. Scalability. . . . . . . . . . . . . . . . . . . 17
5.5.2. NSIS SHOULD Allow for Low Latency in Setup . . . 18
5.5.3. NSIS MUST Allow for Low Bandwidth Consumption
for the Signaling Protocol . . . . . . . . . . . 18
5.5.4. NSIS SHOULD Allow to Constrain Load on Devices . 18
5.5.5. NSIS SHOULD Target the Highest Possible Network
Utilization. . . . . . . . . . . . . . . . . . . 18
5.6. Flexibility. . . . . . . . . . . . . . . . . . . . . . . 19
5.6.1. Flow Aggregation . . . . . . . . . . . . . . . . 19
5.6.2. Flexibility in the Placement of the NSIS
Initiator/Responder. . . . . . . . . . . . . . . 19
5.6.3. Flexibility in the Initiation of State Change. . 19
5.6.4. SHOULD Support Network-Initiated State Change. . 19
5.6.5. Uni / Bi-directional State Setup . . . . . . . . 20
5.7. Security . . . . . . . . . . . . . . . . . . . . . . . . 20
5.7.1. Authentication of Signaling Requests . . . . . . 20
5.7.2. Request Authorization. . . . . . . . . . . . . . 20
5.7.3. Integrity Protection . . . . . . . . . . . . . . 20
5.7.4. Replay Protection. . . . . . . . . . . . . . . . 21
5.7.5. Hop-by-Hop Security. . . . . . . . . . . . . . . 21
5.7.6. Identity Confidentiality and Network Topology
Hiding . . . . . . . . . . . . . . . . . . . . . 21
5.7.7. Denial-of-Service Attacks. . . . . . . . . . . . 21
5.7.8. Confidentiality of Signaling Messages. . . . . . 22
5.7.9. Ownership of State . . . . . . . . . . . . . . . 22
5.8. Mobility . . . . . . . . . . . . . . . . . . . . . . . . 22
5.8.1. Allow Efficient Service Re-Establishment After
Handover . . . . . . . . . . . . . . . . . . . . 22
5.9. Interworking with Other Protocols and Techniques . . . . 22
5.9.1. MUST Interwork with IP Tunneling . . . . . . . . 22
5.9.2. MUST NOT Constrain Either to IPv4 or IPv6. . . . 23
5.9.3. MUST be Independent from Charging Model. . . . . 23
5.9.4. SHOULD Provide Hooks for AAA Protocols . . . . . 23
5.9.5. SHOULD work with Seamless Handoff Protocols. . . 23
5.9.6. MUST Work with Traditional Routing . . . . . . . 23
5.10. Operational. . . . . . . . . . . . . . . . . . . . . . . 23
5.10.1. Ability to Assign Transport Quality to Signaling
Messages . . . . . . . . . . . . . . . . . . . . 23
5.10.2. Graceful Fail Over . . . . . . . . . . . . . . . 24
5.10.3. Graceful Handling of NSIS Entity Problems. . . . 24
6. Security Considerations. . . . . . . . . . . . . . . . . . . . 24
7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 24
8. Appendix: Scenarios/Use Cases. . . . . . . . . . . . . . . . . 26
8.1. Terminal Mobility. . . . . . . . . . . . . . . . . . . . 26
8.2. Wireless Networks. . . . . . . . . . . . . . . . . . . . 28
8.3. An Example Scenario for 3G Wireless Networks . . . . . . 29
8.4. Wired Part of Wireless Network . . . . . . . . . . . . . 31
8.5. Session Mobility . . . . . . . . . . . . . . . . . . . . 33
8.6. QoS Reservation/Negotiation from Access to Core Network. 34
8.7. QoS Reservation/Negotiation Over Administrative
Boundaries . . . . . . . . . . . . . . . . . . . . . . . 34
8.8. QoS Signaling Between PSTN Gateways and Backbone Routers 35
8.9. PSTN Trunking Gateway. . . . . . . . . . . . . . . . . . 36
8.10. An Application Requests End-to-End QoS Path from the
Network. . . . . . . . . . . . . . . . . . . . . . . . . 38
8.11. QOS for Virtual Private Networks . . . . . . . . . . . . 39
8.11.1. Tunnel End Points at the Customer Premises . . . 39
8.11.2. Tunnel End Points at the Provider Premises . . . 39
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.1. Normative References . . . . . . . . . . . . . . . . . . 40
9.2. Informative References . . . . . . . . . . . . . . . . . 40
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 41
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 42
1. Introduction
This document is the product of the Next Steps in Signaling (NSIS)
Working Group. It defines requirements for signaling across
different network environments. It does not list any problems of
existing signaling protocols such as [RSVP].
In order to derive requirements for signaling it is necessary to
first have an idea of the scope within which they are applicable.
Therefore, we list use cases and scenarios where an NSIS protocol
could be applied. The scenarios are used to help derive requirements
and to test the requirements against use cases.
The requirements listed are independent of any application. However,
resource reservation and QoS related issues are used as examples
within the text. However, QoS is not the only field where signaling
is used in the Internet. Signaling might also be used as a
communication protocol to setup and maintain the state in middleboxes
[RFC3234].
This document does not cover requirements in relation to some
networking areas, in particular, interaction with host and site
multihoming. We leave these for future analysis.
1.1. Keywords
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 BCP 14, RFC 2119
[KEYWORDS].
2. Terminology
We list the most often used terms in the document. However, they
cannot be made precise without a more complete architectural model,
and they are not meant to prescribe any solution in the document.
Where applicable, they will be defined in protocol documents.
NSIS Entity (NE): The function within a node, which implements an
NSIS protocol. In the case of path-coupled signaling, the NE will
always be on the data path.
NSIS Forwarder (NF): NSIS Entity between a NI and NR, which may
interact with local state management functions in the network. It
also propagates NSIS signaling further through the network.
NSIS Initiator (NI): NSIS Entity that starts NSIS signaling to set up
or manipulate network state.
NSIS Responder (NR): NSIS Entity that terminates NSIS signaling and
can optionally interact with applications as well.
Flow: A traffic stream (sequence of IP packets between two end
systems) for which a specific packet level treatment is provided.
The flow can be unicast (uni- or bi-directional) or multicast. For
multicast, a flow can diverge into multiple flows as it propagates
toward the receiver. For multi-sender multicast, a flow can also
diverge when viewed in the reverse direction (toward the senders).
Data Path: The route across the networks taken by a flow or
aggregate, i.e., which domains/subdomains it passes through and the
egress/ingress points for each.
Signaling Path: The route across the networks taken by a signaling
flow or aggregate, i.e., which domains/subdomains it passes through
and the egress/ingress points for each.
Path-coupled signaling: A mode of signaling where the signaling
messages follow a path that is tied to the data packets. Signaling
messages are routed only through nodes (NEs) that are in the data
path.
Path-decoupled signaling: Signaling with independent data and
signaling paths. Signaling messages are routed to nodes (NEs) which
are not assumed to be on the data path, but which are (presumably)
aware of it. Signaling messages will always be directly addressed to
the neighbor NE, and the NI/NR may have no relation at all with the
ultimate data sender or receiver.
Service: A generic something provided by one entity and consumed by
another. It can be constructed by allocating resources. The network
can provide it to users or a network node can provide it to packets.
3. Problem Statement and Scope
We provide in the following a preliminary architectural picture as a
basis for discussion. We will refer to it in the following
requirement sections.
Note that this model is intended not to constrain the technical
approach taken subsequently, simply to allow concrete phrasing of
requirements (e.g., requirements about placement of the NSIS
Initiator.)
Roughly, the scope of NSIS is assumed to be the interaction between
the NSIS Initiator, NSIS Forwarder(s), and NSIS Responder including a
protocol to carry the information, and the syntax/semantics of the
information that is exchanged. Further statements on
assumptions/exclusions are given in the next Section.
The main elements are:
1. Something that starts the request for state to be set up in the
network, the NSIS Initiator.
This might be in the end system or within some other part of the
network. The distinguishing feature of the NSIS Initiator is that
it acts on triggers coming (directly or indirectly) from the
higher layers in the end systems. It needs to map the services
requested by them, and also provides feedback information to the
higher layers, which might be used by transport layer algorithms
or adaptive applications.
2. Something that assists in managing state further along the
signaling path, the NSIS Forwarder.
The NSIS Forwarder does not interact with higher layers, but
interacts with the NSIS Initiator, NSIS Responder, and possibly
one or more NSIS Forwarders on the signaling path, edge-to-edge or
end-to-end.
3. Something that terminates the signaling path, the NSIS Responder.
The NSIS responder might be in an end-system or within other
equipment. The distinguishing feature of the NSIS Responder is
that it responds to requests at the end of a signaling path.
4. The signaling path traverses an underlying network covering one or
more IP hops. The underlying network might use locally different
technology. For instance, QoS technology has to be provisioned
appropriately for the service requested. In the QoS example, an
NSIS Forwarder maps service-specific information to technology-
related QoS parameters and receives indications about success or
failure in response.
5. We can see the network at the level of domains/subdomains rather
than individual routers (except in the special case that the
domain contains one link). Domains are assumed to be
administrative entities. So security requirements might apply
differently for the signaling between the domains and within a
domain. Both cases we deal with in this document.
4. Assumptions and Exclusions
4.1. Assumptions and Non-Assumptions
1. The NSIS signaling could run end-to-end, end-to-edge, or edge-to-
edge, or network-to-network (between providers), depending on what
point in the network acts as NSIS initiator, and how far towards
the other end of the network the signaling propagates. In
general, we could expect NSIS Forwarders to become more 'dense'
towards the edges of the network, but this is not a requirement.
For example, in the case of QoS, an over-provisioned domain might
contain no NSIS Forwarders at all (and be NSIS transparent); at
the other extreme, NSIS Forwarders might be placed at every
router. In the latter case, QoS provisioning can be carried out
in a local implementation-dependent way without further signaling,
whereas in the case of remote NSIS Forwarders, a protocol might be
needed to control the routers along the path. This protocol is
then independent of the end-to-end NSIS signaling.
2. We do not consider 'pure' end-to-end signaling that is not
interpreted anywhere within the network. Such signaling is a
higher-layer issue and IETF protocols such as SIP etc. can be
used.
3. Where the signaling does cover several domains, we do not exclude
that different signaling protocols are used in each domain. We
only place requirements on the universality of the control
information that is being transported. (The goals here would be
to allow the use of signaling protocols, which are matched to the
characteristics of the portion of the network being traversed.)
Note that the outcome of NSIS work might result in various flavors
of the same protocol.
4. We assume that the service definitions a NSIS Initiator can ask
for are known in advance of the signaling protocol running. For
instance in the QoS example, the service definition includes QoS
parameters, lifetime of QoS guarantee etc., or any other service-
specific parameters.
There are many ways service requesters get to know about available
services. There might be standardized services, the definition
can be negotiated together with a contract, the service definition
is published in some on-line directory (e.g., at a Web page), and
so on.
5. We assume that there are means for the discovery of NSIS entities
in order to know the signaling peers (solutions include static
configuration, automatically discovered, or implicitly runs over
the right nodes along the data path, etc.). The discovery of the
NSIS entities has security implications that need to be addressed
properly. For some security mechanisms (i.e., Kerberos, pre-
shared secret) it is required to know the identity of the other
entity. Hence the discovery mechanism may provide means to learn
this identity, which is then later used to retrieve the required
keys and parameters.
6. NSIS assumes layer 3 routing and the determination of next data
node selection is not done by NSIS.
4.2. Exclusions
1. Development of specific mechanisms and algorithms for application
and transport layer adaptation are not considered, nor are the
protocols that would support it.
2. Specific mechanisms (APIs and so on) for interaction between
transport/applications and the network layer are not considered,
except to clarify the requirements on the negotiation
capabilities and information semantics that would be needed of
the signaling protocol.
3. Specific mechanisms and protocols for provisioning or other
network control functions within a domain/subdomain are not
considered. The goal is to reuse existing functions and
protocols unchanged. However, NSIS itself can be used for
signaling within a domain/subdomain.
For instance in the QoS example, it means that the setting of QoS
mechanisms in a domain is out of scope, but if we have a tunnel,
NSIS could also be used for tunnel setup with QoS guarantees. It
should be possible to exploit these mechanisms optimally within
the end-to-end context. Consideration of how to do this might
generate new requirements for NSIS however. For example, the
information needed by a NSIS Forwarder to manage a radio
subnetwork needs to be provided by the NSIS solution.
4. Specific mechanisms (APIs and so on) for interaction between the
network layer and underlying provisioning mechanisms are not
considered.
5. Interaction with resource management or other internal state
management capabilities is not considered. Standard protocols
might be used for this. This may imply requirements for the sort
of information that should be exchanged between the NSIS
entities.
6. Security implications related to multicasting are outside the
scope of the signaling protocol.
7. Service definitions and in particular QoS services and classes
are out of scope. Together with the service definition any
definition of service specific parameters are not considered in
this document. Only the base NSIS signaling protocol for
transporting the service information are addressed.
8. Similarly, specific methods, protocols, and ways to express
service information in the Application/Session level are not
considered (e.g., SDP, SIP, RTSP, etc.).
9. The specification of any extensions needed to signal information
via application level protocols (e.g., SDP), and the mapping to
NSIS information are considered outside of the scope of NSIS
working group, as this work is in the direct scope of other IETF
working groups (e.g., MMUSIC).
10. Handoff decision and trigger sources: An NSIS protocol is not
used to trigger handoffs in mobile IP, nor is it used to decide
whether to handoff or not. As soon as or in some situations even
before a handoff happened, an NSIS protocol might be used for
signaling for the particular service again. The basic underlying
assumption is that the route comes first (defining the path) and
the signaling comes after it (following the path). This doesn't
prevent a signaling application at some node interacting with
something that modifies the path, but the requirement is then
just for NSIS to live with that possibility. However, NSIS must
interwork with several protocols for mobility management.
11. Service monitoring is out of scope. It is heavily dependent on
the type of the application and or transport service, and in what
scenario it is used.
5. Requirements
This section defines more detailed requirements for a signaling
solution, respecting the problem statement, scoping assumptions, and
terminology considered earlier. The requirements are in subsections,
grouped roughly according to general technical aspects: architecture
and design goals, topology issues, parameters, performance, security,
information, and flexibility.
Two general (and potentially contradictory) goals for the solution
are that it should be applicable in a very wide range of scenarios,
and at the same time be lightweight in implementation complexity and
resource consumption requirements in NSIS Entities. We use the terms
'access' and 'core' informally in the discussion of some particular
requirements to refer to deployment conditions where particular
protocol attributes, especially performance characteristics, have
special importance. Specifically, 'access' refers to lower capacity
networks with fewer users and sessions. 'Core' refers to high
capacity networks with a large number of users and sessions.
One approach to this is that the solution could deal with certain
requirements via modular components or capabilities, which are
optional to implement or use in individual nodes.
5.1. Architecture and Design Goals
This section contains requirements related to desirable overall
characteristics of a solution, e.g., enabling flexibility, or
independence of parts of the framework.
5.1.1. NSIS SHOULD Provide Availability Information on Request
NSIS SHOULD provide a mechanism to check whether state to be setup is
available without setting it up. For the resource reservation
example this translates into checking resource availability without
performing resource reservation. In some scenarios, e.g., the mobile
terminal scenario, it is required to query, whether resources are
available, without performing a reservation on the resource.
5.1.2. NSIS MUST be Designed Modularly
A modular design allows for more lightweight implementations, if
fewer features are needed. Mutually exclusive solutions are
supported. Examples for modularity:
- Work over any kind of network (narrowband versus broadband,
error-prone versus reliable, ...). This implies low bandwidth
signaling, and elimination of redundant information MUST be
supported if necessary.
- State setup for uni- and bi-directional flows is possible.
- Extensible in the future with different add-ons for certain
environments or scenarios.
- Protocol layering, where appropriate. This means NSIS MUST
provide a base protocol, which can be adapted to different
environments.
5.1.3. NSIS MUST Decouple Protocol and Information
The signaling protocol MUST be clearly separated from the control
information being transported. This provides for the independent
development of these two aspects of the solution, and allows for this
control information to be carried within other protocols, including
application layer ones, existing ones or those being developed in the
future. The flexibility gained in the transport of information
allows for the applicability of the same protocol in various
scenarios.
However, note that the information carried needs to be standardized;
otherwise interoperability is difficult to achieve.
5.1.4. NSIS MUST Support Independence of Signaling and Network Control
Paradigm
The signaling MUST be independent of the paradigm and mechanism of
network control. E.g., in the case of signaling for QoS, the
independence of the signaling protocol from the QoS provisioning
allows for using the NSIS protocol together with various QoS
technologies in various scenarios.
5.1.5. NSIS SHOULD be Able to Carry Opaque Objects
NSIS SHOULD be able to pass around opaque objects, which are
interpreted only by some NSIS-capable nodes.
5.2. Signaling Flows
This section contains requirements related to the possible signaling
flows that should be supported, e.g., over what parts of the flow
path, between what entities (end-systems, routers, middleboxes,
management systems), in which direction.
5.2.1. The placement of NSIS Initiator, Forwarder, and Responder
Anywhere in the Network MUST be Allowed
The protocol MUST work in various scenarios such as host-to-network-
to-host, edge-to-edge, (e.g., just within one provider's domain),
user-to-network (from end system into the network, ending, e.g., at
the entry to the network and vice versa), and network-to-network
(e.g., between providers).
Placing the NSIS Forwarder and NSIS Initiator functions at different
locations allows for various scenarios to work with the same
protocol.
5.2.2. NSIS MUST Support Path-Coupled and MAY Support Path-Decoupled
Signaling.
The path-coupled signaling mode MUST be supported. NSIS signaling
messages are routed only through nodes (NEs) that are in the data
path.
However, there is a set of scenarios, where signaling is not on the
data path. Therefore, NSIS MAY support the path-decoupled signaling
mode, where signaling messages are routed to nodes (NEs), which are
not assumed to be on the data path, but which are aware of it.
5.2.3. Concealment of Topology and Technology Information SHOULD be
Possible
The NSIS protocol SHOULD allow for hiding the internal structure of a
NSIS domain from end-nodes and from other networks. Hence an
adversary should not be able to learn the internal structure of a
network with the help of the signaling protocol.
In various scenarios, topology information should be hidden for
various reasons. From a business point of view, some administrations
don't want to reveal the topology and technology used.
5.2.4. Transparent Signaling Through Networks SHOULD be Possible
It SHOULD be possible that the signaling for some flows traverses
path segments transparently, i.e., without interpretation at NSIS
Forwarders within the network. An example would be a subdomain
within a core network, which only interpreted signaling for
aggregates established at the domain edge, with the signaling for
individual flows passing transparently through it.
In other words, NSIS SHOULD work in hierarchical scenarios, where big
pipes/trunks are setup using NSIS signaling, but also flows which run
within that big pipe/trunk are setup using NSIS.
5.3. Messaging
5.3.1. Explicit Erasure of State MUST be Possible
When state along a path is no longer necessary, e.g., because the
application terminates, or because a mobile host experienced a hand-
off, it MUST be possible to erase the state explicitly.
5.3.2. Automatic Release of State After Failure MUST be Possible
When the NSIS Initiator goes down, the state it requested in the
network SHOULD be released, since it will most likely no longer be
necessary.
After detection of a failure in the network, any NSIS
Forwarder/Initiator MUST be able to release state it is involved in.
For example, this may require signaling of the "Release after
Failure" message upstream as well as downstream, or soft state timing
out.
The goal is to prevent stale state within the network and add
robustness to the operation of NSIS. So in other words, an NSIS
signaling protocol or mechanisms MUST provide means for an NSIS
entity to discover and remove local stale state.
Note that this might need to work together with a notification
mechanism. Note as well, that transient failures in NSIS processing
shouldn't necessarily have to cause all state to be released
immediately.
5.3.3. NSIS SHOULD Allow for Sending Notifications Upstream
NSIS Forwarders SHOULD notify the NSIS Initiator or any other NSIS
Forwarder upstream, if there is a state change inside the network.
There are various types of network changes for instance among them:
Recoverable errors: the network nodes can locally repair this type
error. The network nodes do not have to notify the users of the
error immediately. This is a condition when the danger of
degradation (or actual short term degradation) of the provided
service was overcome by the network (NSIS Forwarder) itself.
Unrecoverable errors: the network nodes cannot handle this type of
error, and have to notify the users as soon as possible.
Service degradation: In case the service cannot be provided
completely but only partially.
Repair indication: If an error occurred and it has been fixed, this
triggers the sending of a notification.
Service upgrade available: If a previously requested better service
becomes available.
The content of the notification is very service specific, but it is
must at least carry type information. Additionally, it may carry the
location of the state change.
The notifications may or may not be in response to a NSIS message.
This means an NSIS entity has to be able to handle notifications at
any time.
Note however, that there are a number of security consideration needs
to be solved with notification, even more important if the
notification is sent without prior request (asynchronously). The
problem basically is, that everybody could send notifications to any
NSIS entity and the NSIS entity most likely reacts on the
notification. For example, if it gets an error notification it might
erase state, even if everything is ok. So the notification might
depend on security associations between the sender of the
notification and its receiver. If a hop-by-hop security mechanism is
chosen, this implies also that notifications need to be sent on the
reverse path.
5.3.4. Establishment and Refusal to Set Up State MUST be Notified
A NR MUST acknowledge establishment of state on behalf of the NI
requesting establishment of that state. A refusal to set up state
MUST be replied with a negative acknowledgement by the NE refusing to
set up state. It MUST be sent to the NI. Depending on the signaling
application the (positive or negative) notifications may have to pass
through further NEs upstream. Information on the reason of the
refusal to set up state MAY be made available. For example, in the
resource reservation example, together with a negative answer, the
amount of resources available might also be returned.
5.3.5. NSIS MUST Allow for Local Information Exchange
The signaling protocol MUST be able to exchange local information
between NSIS Forwarders located within one single administrative
domain. The local information exchange is performed by a number of
separate messages not belonging to an end-to-end signaling process.
Local information might, for example, be IP addresses, notification
of successful or erroneous processing of signaling messages, or other
conditions.
In some cases, the NSIS signaling protocol MAY carry identification
of the NSIS Forwarders located at the boundaries of a domain.
However, the identification of edge should not be visible to the end
host (NSIS Initiator) and only applies within one administrative
domain.
5.4. Control Information
This section contains requirements related to the control information
that needs to be exchanged.
5.4.1. Mutability Information on Parameters SHOULD be Possible
It is possible that nodes modify parameters of a signaling message.
However, it SHOULD be possible for the NSIS Initiator to control the
mutability of the signaled information. For example, the NSIS
Initiator should be able to control what is requested end-to-end,
without the request being gradually mutated as it passes through a
sequence of nodes.
5.4.2. It SHOULD be Possible to Add and Remove Local Domain Information
It SHOULD be possible to add and remove local scope elements.
Compared to Requirement 5.3.5 this requirement does use the normal
signaling process and message exchange for transporting local
information. For example, at the entrance to a domain, domain-
specific information is added information is added, which is used in
this domain only, and the information is removed again when a
signaling message leaves the domain. The motivation is in the
economy of re-using the protocol for domain internal signaling of
various information pieces. Where additional information is needed
within a particular domain, it should be possible to carry this at
the same time as the end-to-end information.
5.4.3. State MUST be Addressed Independent of Flow Identification
Addressing or identifying state MUST be independent of the flow
identifier (flow end-points, topological addresses). Various
scenarios in the mobility area require this independence because
flows resulting from handoff might have changed end-points etc. but
still have the same service requirement. Also several proxy-based
signaling methods profit from such independence, though these are not
chartered work items for NSIS.
5.4.4. Modification of Already Established State SHOULD be Seamless
In many case, the established state needs to be updated (in QoS
example upgrade or downgrade of resource usage). This SHOULD happen
seamlessly without service interruption. At least the signaling
protocol should allow for it, even if some data path elements might
not be capable of doing so.
5.4.5. Grouping of Signaling for Several Micro-Flows MAY be Provided
NSIS MAY group signaling information for several micro-flows into one
signaling message. The goal of this is the optimization in terms of
setup delay, which can happen in parallel. This helps applications
requesting several flows at once. Also potential refreshes (in case
of a soft state solution) might profit from grouping.
However, the network need not know that a relationship between the
grouped flows exists. There MUST NOT be any transactional semantic
associated with the grouping. It is only meant for optimization
purposes.
5.5. Performance
This section discusses performance requirements and evaluation
criteria and the way in which these could and should be traded off
against each other in various parts of the solution.
Scalability is always an important requirement for signaling
protocols. However, the type of scalability and its importance
varies from one scenario to another.
Note that many of the performance issues are heavily dependent on the
scenario assumed and are normally a trade-off between speed,
reliability, complexity, and scalability. The trade-off varies in
different parts of the network. For example, in radio access
networks low bandwidth consumption will outweigh the low latency
requirement, while in core networks it may be reverse.
5.5.1. Scalability
NSIS MUST be scalable in the number of messages received by a
signaling communication partner (NSIS Initiator, NSIS Forwarder, and
NSIS Responder). The major concern lies in the core of the network,
where large numbers of messages arrive.
It MUST be scalable in number of hand-offs in mobile environments.
This mainly applies in access networks, because the core is
transparent to mobility in most cases.
It MUST be scalable in the number of interactions for setting up
state. This applies for end-systems setting up several states. Some
servers might be expected to setup a large number of states.
Scalability in the amount of state per entity MUST be achieved for
NSIS Forwarders in the core of the network.
Scalability in CPU usage MUST be achieved on end terminals and
intermediate nodes in case of many state setup processes at the same
time.
Specifically, NSIS MUST work in Internet scale deployments, where the
use of signaling by hosts becomes universal. Note that requirement
5.2.4 requires the functionality of transparently signaling through
networks without interpretation. Additionally, requirement 5.6.1
lists the capability to aggregate. Furthermore, requirement 5.5.4
states that NSIS should be able to constrain the load on devices.
Basically, the performance of the signaling MUST degrade gracefully
rather than catastrophically under overload conditions.
5.5.2. NSIS SHOULD Allow for Low Latency in Setup
NSIS SHOULD allow for low latency setup of states. This is only
needed in scenarios where state setups are required on a short time
scale (e.g., handover in mobile environments), or where human
interaction is immediately concerned (e.g., voice communication setup
delay).
5.5.3. NSIS MUST Allow for Low Bandwidth Consumption for the Signaling
Protocol
NSIS MUST allow for low bandwidth consumption in certain access
networks. Again only small sets of scenarios call for low bandwidth,
mainly those where wireless links are involved.
5.5.4. NSIS SHOULD Allow to Constrain Load on Devices
The NSIS architecture SHOULD give the ability to constrain the load
(CPU load, memory space, signaling bandwidth consumption and
signaling intensity) on devices where it is needed. One of the
reasons is that the protocol handling should have a minimal impact on
interior (core) nodes.
This can be achieved by many different methods. Examples include
message aggregation, header compression, minimizing functionality, or
ignoring signaling in core nodes. NSIS may choose any method as long
as the requirement is met.
5.5.5. NSIS SHOULD Target the Highest Possible Network Utilization
This requirement applies specifically to QoS signaling.
There are networking environments that require high network
utilization for various reasons, and the signaling protocol SHOULD to
its best ability support high resource utilization while maintaining
appropriate service quality.
In networks where resources are very expensive (as is the case for
many wireless networks), efficient network utilization for signaling
traffic is of critical financial importance. On the other hand there
are other parts of the network where high utilization is not
required.
5.6. Flexibility
This section lists the various ways the protocol can flexibly be
employed.
5.6.1. Flow Aggregation
NSIS MUST allow for flow aggregation, including the capability to
select and change the level of aggregation.
5.6.2. Flexibility in the Placement of the NSIS Initiator/Responder
NSIS MUST be flexible in placing an NSIS Initiator and NSIS
Responder. The NSIS Initiator might be located at the sending or the
receiving side of a data stream, and the NSIS Responder naturally on
the other side.
Also network-initiated signaling and termination MUST be allowed in
various scenarios such as PSTN gateways, some VPNs, and mobility.
This means the NSIS Initiator and NSIS Responder might not be at the
end points of the data stream.
5.6.3. Flexibility in the Initiation of State Change
The NSIS Initiator or the NSIS Responder SHOULD be able to initiate a
change of state. In the example of resource reservation this is
often referred to as resource re-negotiation. It can happen due to
various reasons, such as local resource shortage (CPU, memory on
end-system) or a user changed application preference/profiles.
5.6.4. SHOULD Support Network-Initiated State Change
NSIS SHOULD support network-initiated state change. In the QoS
example, this is used in cases, where the network is not able to
further guarantee resources and wants to e.g., downgrade a resource
reservation.
5.6.5. Uni / Bi-Directional State Setup
Both unidirectional as well as bi-direction state setup SHOULD be
possible. With bi-directional state setup we mean that the state for
bi-directional data flows is setup. The bi-directional data flows
have the same end-points, but the path in the two directions does not
need to be the same.
The goal of a bi-directional state setup is mainly an optimization in
terms of setup delay. There is no requirements on constrains such as
use of the same data path etc.
5.7. Security
This section discusses security-related requirements. The NSIS
protocol MUST provide means for security, but it MUST be allowed that
nodes implementing NSIS signaling do not have to use the security
means.
5.7.1. Authentication of Signaling Requests
A signaling protocol MUST make provision for enabling various
entities to be authenticated against each other using strong
authentication mechanisms. The term strong authentication points to
the fact that weak plain-text password mechanisms must not be used
for authentication.
5.7.2. Request Authorization
The signaling protocol MUST provide means to authorize state setup
requests. This requirement demands a hook to interact with a policy
entity to request authorization data. This allows an authenticated
entity to be associated with authorization data and to verify the
request. Authorization prevents state setup by unauthorized
entities, setups violating policies, and theft of service.
Additionally it limits denial of service attacks against parts of the
network or the entire network caused by unrestricted state setups.
Additionally it might be helpful to provide some means to inform
other protocols of participating nodes within the same administrative
domain about a previous successful authorization event.
5.7.3. Integrity Protection
The signaling protocol MUST provide means to protect the message
payloads against modifications. Integrity protection prevents an
adversary from modifying parts of the signaling message and from
mounting denial of service or theft of service type of attacks
against network elements participating in the protocol execution.
5.7.4. Replay Protection
To prevent replay of previous signaling messages the signaling
protocol MUST provide means to detect old i.e., already transmitted
signaling messages. A solution must cover issues of synchronization
problems in the case of a restart or a crash of a participating
network element.
5.7.5. Hop-by-Hop Security
Channel security between signaling entities MUST be implemented. It
is a well known and proven concept in Quality of Service and other
signaling protocols to have intermediate nodes that actively
participate in the protocol to modify the messages as it is required
by processing rules. Note that this requirement does not exclude
end-to-end or network-to-network security of a signaling message.
End-to-end security between the NSIS Initiator and the NSIS Responder
may be used to provide protection of non-mutable data fields.
Network-to-network security refers to the protection of messages over
various hops but not in an end-to-end manner i.e., protected over a
particular network.
5.7.6. Identity Confidentiality and Network Topology Hiding
Identity confidentiality SHOULD be supported. It enables privacy and
avoids profiling of entities by adversary eavesdropping the signaling
traffic along the path. The identity used in the process of
authentication may also be hidden to a limited extent from a network
to which the initiator is attached. However the identity MUST
provide enough information for the nodes in the access network to
collect accounting data.
Network topology hiding MAY be supported to prevent entities along
the path to learn the topology of a network. Supporting this
property might conflict with a diagnostic capability.
5.7.7. Denial-of-Service Attacks
A signaling protocol SHOULD provide prevention of Denial-of-service
attacks. To effectively prevent denial-of-service attacks it is
necessary that the used security and protocol mechanisms MUST have
low computational complexity to verify a state setup request prior to
authenticating the requesting entity. Additionally the signaling
protocol and the used security mechanisms SHOULD NOT require large
resource consumption on NSIS Entities (for example main memory or
other additional message exchanges) before a successful
authentication is done.
5.7.8. Confidentiality of Signaling Messages
Based on the signaling information exchanged between nodes
participating in the signaling protocol an adversary may learn both
the identities and the content of the signaling messages. Since the
ability to listen to signaling channels is a major guide to what data
channels are interesting ones.
To prevent this from happening, confidentiality of the signaling
message in a hop-by-hop manner SHOULD be provided. Note that most
messages must be protected on a hop-by-hop basis, since entities,
which actively participate in the signaling protocol, must be able to
read and eventually modify the signaling messages.
5.7.9. Ownership of State
When existing states have to be modified then there is a need to use
a session identifier to uniquely identify the established state. A
signaling protocol MUST provide means of security protection to
prevent adversaries from modifying state.
5.8. Mobility
5.8.1. Allow Efficient Service Re-Establishment After Handover
Handover is an essential function in wireless networks. After
handover, the states may need to be completely or partially re-
established due to route changes. The re-establishment may be
requested by the mobile node itself or triggered by the access point
that the mobile node is attached to. In the first case, the
signaling MUST allow efficient re-establishment after handover. Re-
establishment after handover MUST be as quick as possible so that the
mobile node does not experience service interruption or service
degradation. The re-establishment SHOULD be localized, and not
require end-to-end signaling.
5.9. Interworking with Other Protocols and Techniques
Hooks SHOULD be provided to enable efficient interworking between
various protocols and techniques including the following listed.
5.9.1. MUST Interwork with IP Tunneling
IP tunneling for various applications MUST be supported. More
specifically IPSec tunnels are of importance. This mainly impacts
the identification of flows. When using IPSec, parts of information
commonly used for flow identification (e.g., transport protocol
information and ports) may not be accessible due to encryption.
5.9.2. MUST NOT Constrain Either to IPv4 or IPv6
5.9.3. MUST be Independent from Charging Model
Signaling MUST NOT be constrained by charging models or the charging
infrastructure used.
5.9.4. SHOULD Provide Hooks for AAA Protocols
The NSIS protocol SHOULD be developed with respect to be able to
collect usage records from one or more network elements.
5.9.5. SHOULD Work with Seamless Handoff Protocols
An NSIS protocol SHOULD work with seamless handoff protocols such as
context transfer and candidate access router (CAR) discovery.
5.9.6. MUST Work with Traditional Routing
NSIS assumes traditional L3 routing, which is purely based on L3
destination addresses. NSIS MUST work with L3 routing, in particular
it MUST work in case of route changes. This means state on the old
route MUST be released and state on the new route MUST be established
by an NSIS protocol.
Networks, which do non-traditional routing, should not break NSIS
signaling. NSIS MAY work for some of these situations.
Particularly, combinations of NSIS unaware nodes and routing other
then traditional one causes some problems. Non-traditional routing
includes, for example, routing decisions based on port numbers, other
IP header fields than the destination address, or splitting traffic
based on header hash values. These routing environments result in
the signaling path being potentially different than the data path.
5.10. Operational
5.10.1. Ability to Assign Transport Quality to Signaling Messages
The NSIS architecture SHOULD allow the network operator to assign the
NSIS protocol messages a certain transport quality. As signaling
opens up the possibility of denial-of-service attacks, this
requirement gives the network operator a means, but also the
obligation, to trade-off between signaling latency and the impact
(from the signaling messages) on devices within the network. From
protocol design this requirement states that the protocol messages
SHOULD be detectable, at least where the control and assignment of
the messages priority is done.
Furthermore, the protocol design must take into account reliability
concerns. Communication reliability is seen as part of the quality
assigned to signaling messages. So procedures MUST be defined for
how an NSIS signaling system behaves if some kind of request it sent
stays unanswered. The basic transport protocol to be used between
adjacent NSIS Entities MAY ensure message integrity and reliable
transport.
5.10.2. Graceful Fail Over
Any unit participating in NSIS signaling MUST NOT cause further
damage to other systems involved in NSIS signaling when it has to go
out of service.
5.10.3. Graceful Handling of NSIS Entity Problems
NSIS entities SHOULD be able to detect a malfunctioning peer. It may
notify the NSIS Initiator or another NSIS entity involved in the
signaling process. The NSIS peer may handle the problem itself e.g.,
switching to a backup NSIS entity. In the latter case note that
synchronization of state between the primary and the backup entity is
needed.
6. Security Considerations
Section 5.7 of this document provides security related requirements
of a signaling protocol.
7. Acknowledgments
Quite a number of people have been involved in the discussion of the
document, adding some ideas, requirements, etc. We list them without
a guarantee on completeness: Changpeng Fan (Siemens), Krishna Paul
(NEC), Maurizio Molina (NEC), Mirko Schramm (Siemens), Andreas
Schrader (NEC), Hannes Hartenstein (NEC), Ralf Schmitz (NEC), Juergen
Quittek (NEC), Morihisa Momona (NEC), Holger Karl (Technical
University Berlin), Xiaoming Fu (Technical University Berlin), Hans-
Peter Schwefel (Siemens), Mathias Rautenberg (Siemens), Christoph
Niedermeier (Siemens), Andreas Kassler (University of Ulm), Ilya
Freytsis.
Some text and/or ideas for text, requirements, scenarios have been
taken from an Internet Draft written by the following authors: David
Partain (Ericsson), Anders Bergsten (Telia Research), Marc Greis
(Nokia), Georgios Karagiannis (Ericsson), Jukka Manner (University of
Helsinki), Ping Pan (Juniper), Vlora Rexhepi (Ericsson), Lars
Westberg (Ericsson), Haihong Zheng (Nokia). Some of those have
actively contributed new text to this document as well.
Another Internet Draft impacting this document has been written by
Sven Van den Bosch, Maarten Buchli, and Danny Goderis (all Alcatel).
These people contributed also new text.
Thanks also to Kwok Ho Chan (Nortel) for text changes. And finally
thanks Alison Mankin for the thorough AD review and thanks to Harald
Tveit Alvestrand and Steve Bellovin for the IESG review comments.
8. Appendix: Scenarios/Use Cases
In the following we describe scenarios, which are important to cover,
and which allow us to discuss various requirements. Some regard this
as use cases to be covered defining the use of a signaling protocol.
In general, these scenarios consider the specific case of signaling
for QoS (resource reservation), although many of the issues carry
over directly to other signaling types.
8.1. Terminal Mobility
The scenario we are looking at is the case where a mobile terminal
(MT) changes from one access point to another access point. The
access points are located in separate QoS domains. We assume Mobile
IP to handle mobility on the network layer in this scenario and
consider the various extensions (i.e., IETF proposals) to Mobile IP,
in order to provide 'fast handover' for roaming Mobile Terminals.
The goal to be achieved lies in providing, keeping, and adapting the
requested QoS for the ongoing IP sessions in case of handover.
Furthermore, the negotiation of QoS parameters with the new domain
via the old connection might be needed, in order to support the
different 'fast handover' proposals within the IETF.
The entities involved in this scenario include a mobile terminal,
access points, an access network manager, and communication partners
of the MT (the other end(s) of the communication association). From
a technical point of view, terminal mobility means changing the
access point of a mobile terminal (MT). However, technologies might
change in various directions (access technology, QoS technology,
administrative domain). If the access points are within one specific
QoS technology (independent of access technology) we call this
intra-QoS technology handoff. In the case of an inter-QoS technology
handoff, one changes from e.g., a DiffServ to an IntServ domain,
however still using the same access technology. Finally, if the
access points are using different access technologies we call it
inter-technology hand-off.
The following issues are of special importance in this scenario:
1) Handoff decision
- The QoS management requests handoff. The QoS management can
decide to change the access point, since the traffic conditions of
the new access point are better supporting the QoS requirements.
The metric may be different (optimized towards a single or a
group/class of users). Note that the MT or the network (see
below) might trigger the handoff.
- The mobility management forces handoff. This can have several
reasons. The operator optimizes his network, admission is no
longer granted (e.g., emptied prepaid condition). Or another
example is when the MT is reaching the focus of another base
station. However, this might be detected via measurements of QoS
on the physical layer and is therefore out of scope of QoS
signaling in IP. Note again that the MT or the network (see
below) might trigger the handoff.
- This scenario shows that local decisions might not be enough. The
rest of the path to the other end of the communication needs to be
considered as well. Hand-off decisions in a QoS domain do not
only depend on the local resource availability, e.g., the wireless
part, but involve the rest of the path as well. Additionally,
decomposition of an end-to-end signaling might be needed, in order
to change only parts of it.
2) Trigger sources
- Mobile terminal: If the end-system QoS management identifies
another (better-suited) access point, it will request the handoff
from the terminal itself. This will be especially likely in the
case that two different provider networks are involved. Another
important example is when the current access point bearer
disappears (e.g., removing the Ethernet cable). In this case, the
NSIS Initiator is basically located on the mobile terminal.
- Network (access network manager): Sometimes, the handoff trigger
will be issued from the network management to optimize the overall
load situation. Most likely this will result in changing the
base-station of a single providers network. Most likely the NSIS
Initiator is located on a system within the network.
3) Integration with other protocols
- Interworking with other protocol must be considered in one or the
other form. E.g., it might be worth combining QoS signaling
between different QoS domains with mobility signaling at hand-
over.
4) Handover rates
In mobile networks, the admission control process has to cope with
far more admission requests than call setups alone would generate.
For example, in the GSM (Global System for Mobile communications)
case, mobility usually generates an average of one to two handovers
per call. For third generation networks (such as UMTS), where it is
necessary to keep radio links to several cells simultaneously
(macro-diversity), the handover rate is significantly higher.
5) Fast state installation
Handover can also cause packet losses. This happens when the
processing of an admission request causes a delayed handover to the
new base station. In this situation, some packets might be
discarded, and the overall speech quality might be degraded
significantly. Moreover, a delay in handover may cause degradation
for other users. In the worst-case scenario, a delay in handover may
cause the connection to be dropped if the handover occurred due to
bad air link quality. Therefore, it is critical that QoS signaling
in connection with handover be carried out very quickly.
6) Call blocking in case of overload
Furthermore, when the network is overloaded, it is preferable to keep
states for previously established flows while blocking new requests.
Therefore, the resource reservation requests in connection with
handover should be given higher priority than new requests for
resource reservation.
8.2. Wireless Networks
In this scenario, the user is using the packet services of a wireless
system (such as the 3rd generation wireless system 3GPP/UMTS,
3GPP2/cdma2000). The region between the End Host and the Edge Node
(Edge Router) connecting the wireless network to another QoS domain
is considered to be a single QoS domain.
The issues in such an environment regarding QoS include:
1) The wireless networks provide their own QoS technology with
specialized parameters to coordinate the QoS provided by both the
radio access and wired access networks. Provisioning of QoS
technologies within a wireless network can be described mainly in
terms of calling bearer classes, service options, and service
instances. These QoS technologies need to be invoked with
suitable parameters when higher layers trigger a request for QoS.
Therefore these involve mapping of the requested higher layer QoS
parameters onto specific bearer classes or service instances. The
request for allocation of resources might be triggered by
signaling at the IP level that passes across the wireless system,
and possibly other QoS domains. Typically, wireless network
specific messages are invoked to setup the underlying bearer
classes or service instances in parallel with the IP layer QoS
negotiation, to allocate resources within the radio access
network.
2) The IP signaling messages are initiated by the NSIS initiator and
interpreted by the NSIS Forwarder. The most efficient placement
of the NSIS Initiator and NSIS Forwarder has not been determined
in wireless networks, but a few potential scenarios can be
envisioned. The NSIS Initiator could be located at the End Host
(e.g., 3G User equipment (UE)), the Access Gateway or at a node
that is not directly on the data path, such as a Policy Decision
Function. The Access Gateway could act as a proxy NSIS Initiator
on behalf of the End Host. The Policy Decision Function that
controls per-flow/aggregate resources with respect to the session
within its QoS domain (e.g., the 3G wireless network) may act as a
proxy NSIS Initiator for the end host or the Access Gateway.
Depending on the placement of the NSIS Initiator, the NSIS
Forwarder may be located at an appropriate point in the wireless
network.
3) The need for re-negotiation of resources in a new wireless domain
due to host mobility. In this case the NSIS Initiator and the
NSIS Forwarder should detect mobility events and autonomously
trigger re-negotiation of resources.
8.3. An Example Scenario for 3G Wireless Networks
The following example is a pure hypothetical scenario, where an NSIS
signaling protocol might be used in a 3G environment. We do not
impose in any way, how a potential integration might be done. Terms
from the 3GPP architecture are used (P-CSCF, IMS, expanded below) in
order to give specificity, but in a hypothetical design, one that
reflects neither development nor review by 3GPP. The example should
help in the design of a NSIS signaling protocol such that it could be
used in various environments.
The 3G wireless access scenario is shown in Figure 1. The Proxy-Call
State Control Function (P-CSCF) is the outbound SIP proxy (only used
in IP Multimedia Subsystems (IMS)). The Access Gateway is the egress
router of the 3G wireless domain and it connects the radio access
network to the Edge Router (ER) of the backbone IP network. The
Policy Decision Function (PDF) is an entity responsible for
controlling bearer level resource allocations/de-allocations in
relation to session level services e.g., SIP. The Policy Decision
Function may also control the Access Gateway to open and close the
gates and to configure per-flow policies, i.e., to authorize or
forbid user traffic. The P-CSCF (only used in IMS) and the Access
Gateway communicate with the Policy Decision Function, for network
resource allocation/de-allocation decisions. The User Equipment (UE)
or the Mobile Station (MS) consists of a Mobile Terminal (MT) and
Terminal Equipment (TE), e.g., a laptop.
+--------+
+--------->| P-CSCF |---------> SIP signaling
/ +--------+
/ SIP |
| |
| +-----+ +----------------+
| | PDF |<---------->| NSIS Forwarder |<--->
| +-----+ +----------------+
| | ^
| | |
| | |
| |COPS |
| | |
+------+ +---------+ |
| UE/MS|----------| Access |<-----------+ +----+
+------+ | Gateway |------------------| ER |
+---------+ +----+
Figure 1: 3G wireless access scenario
The PDF has all the required QoS information for per-flow or
aggregate admission control in 3G wireless networks. It receives
resource allocation/de-allocation requests from the P-CSCF and/or
Access Gateway etc. and responds with policy decisions. Hence the
PDF may be a candidate entity to host the functionality of the NSIS
Initiator, initiating the NSIS QoS signaling towards the backbone IP
network. On the other hand, the UE/MS may act as the NSIS Initiator
or the Access Gateway may act as a Proxy NSIS Initiator on behalf of
the UE/MS. In the former case, the P-CSCF/PDF has to do the mapping
from codec types and media descriptors (derived from SIP/SDP
signaling) to IP traffic descriptor. In the latter case, the UE/MS
may use any appropriate QoS signaling mechanism as the NSIS
Initiator. If the Access Gateway is acting as the Proxy NSIS
initiator on behalf of the UE/MS, then it may have to do the mapping
of parameters from radio access specific QoS to IP QoS traffic
parameters before forwarding the request to the NSIS Forwarder.
The NSIS Forwarder is currently not part of the standard 3G wireless
architecture. However, to achieve end-to-end QoS a NSIS Forwarder is
needed such that the NSIS Initiators can request a QoS connection to
the IP network. As in the previous example, the NSIS Forwarder could
manage a set of pre-provisioned resources in the IP network, i.e.,
bandwidth pipes, and the NSIS Forwarder perform per-flow admission
control into these pipes. In this way, a connection can be made
between two 3G wireless access networks, and hence, end-to-end QoS
can be achieved. In this case the NSIS Initiator and NSIS Forwarder
are clearly two separate logical entities. The Access Gateway or/and
the Edge Router in Fig.1 may contain the NSIS Forwarder
functionality, depending upon the placement of the NSIS Initiator as
discussed in scenario 2 in section 8.2. This use case clearly
illustrates the need for an NSIS QoS signaling protocol between NSIS
Initiator and NSIS Forwarder. An important application of such a
protocol may be its use in the end-to-end establishment of a
connection with specific QoS characteristics between a mobile host
and another party (e.g., end host or content server).
8.4. Wired Part of Wireless Network
A wireless network, seen from a QoS domain perspective, usually
consists of three parts: a wireless interface part (the "radio
interface"), a wired part of the wireless network (i.e., Radio Access
Network) and the backbone of the wireless network, as shown in Figure
2. Note that this figure should not be seen as an architectural
overview of wireless networks but rather as showing the conceptual
QoS domains in a wireless network.
In this scenario, a mobile host can roam and perform a handover
procedure between base stations/access routers. In this scenario the
NSIS QoS protocol can be applied between a base station and the
gateway (GW). In this case a GW can also be considered as a local
handover anchor point. Furthermore, in this scenario the NSIS QoS
protocol can also be applied either between two GWs, or between two
edge routers (ER).
|--|
|GW|
|--| |--|
|MH|--- .
|--| / |-------| .
/--|base | |--| .
|station|-|ER|...
|-------| |--| . |--| back- |--| |---| |----|
..|ER|.......|ER|..|BGW|.."Internet"..|host|
-- |-------| |--| . |--| bone |--| |---| |----|
|--| \ |base |-|ER|... .
|MH| \ |station| |--| .
|--|--- |-------| . MH = mobile host
|--| ER = edge router
<----> |GW| GW = gateway
Wireless link |--| BGW = border gateway
... = interior nodes
<------------------->
Wired part of wireless network
<---------------------------------------->
Wireless Network
Figure 2. QoS architecture of wired part of wireless network
Each of these parts of the wireless network impose different issues
to be solved on the QoS signaling solution being used:
1) Wireless interface: The solution for the air interface link has to
ensure flexibility and spectrum efficient transmission of IP
packets. However, this link layer QoS can be solved in the same
way as any other last hop problem by allowing a host to request
the proper QoS profile.
2) Wired part of the wireless network: This is the part of the
network that is closest to the base stations/access routers. It
is an IP network although some parts logically perform tunneling
of the end user data. In cellular networks, the wired part of the
wireless network is denoted as a radio access network.
This part of the wireless network has different requirements for
signaling protocol characteristics when compared to traditional IP
networks:
- The network must support mobility. Many wireless networks are
able to provide a combination of soft and hard handover
procedures. When handover occurs, reservations need to be
established on new paths. The establishment time has to be as
short as possible since long establishment times for s degrade
the performance of the wireless network. Moreover, for maximal
utilization of the radio spectrum, frequent handover operations
are required.
- These links are typically rather bandwidth-limited.
- The wired transmission in such a network contains a relatively
high volume of expensive leased lines. Overprovisioning might
therefore be prohibitively expensive.
- The radio base stations are spread over a wide geographical
area and are in general situated a large distance from the
backbone.
3) Backbone of the wireless network: the requirements imposed by this
network are similar to the requirements imposed by other types of
backbone networks.
Due to these very different characteristics and requirements, often
contradictory, different QoS signaling solutions might be needed in
each of the three network parts.
8.5. Session Mobility
In this scenario, a session is moved from one end-system to another.
Ongoing sessions are kept and QoS parameters need to be adapted,
since it is very likely that the new device provides different
capabilities. Note that it is open which entity initiates the move,
which implies that the NSIS Initiator might be triggered by different
entities.
User mobility (i.e., a user changing the device and therefore moving
the sessions to the new device) is considered to be a special case
within the session mobility scenario.
Note that this scenario is different from terminal mobility. The
terminal (end-system) has not moved to a different access point.
Both terminals are still connected to an IP network at their original
points.
The issues include:
1) Keeping the QoS guarantees negotiated implies that the end-
point(s) of communication are changed without changing the s.
2) The trigger of the session move might be the user or any other
party involved in the session.
8.6. QoS Reservation/Negotiation from Access to Core Network
The scenario includes the signaling between access networks and core
networks in order to setup and change reservations together with
potential negotiation.
The issues to be solved in this scenario are different from previous
ones.
1) The entity of reservation is most likely an aggregate.
2) The time scales of states might be different (long living states
of aggregates, less often re-negotiation).
3) The specification of the traffic (amount of traffic), a particular
QoS is guaranteed for, needs to be changed. E.g., in case
additional flows are added to the aggregate, the traffic
specification of the flow needs to be added if it is not already
included in the aggregates specification.
4) The flow specification is more complex including network addresses
and sets of different address for the source as well as for the
destination of the flow.
8.7. QoS Reservation/Negotiation Over Administrative Boundaries
Signaling between two or more core networks to provide QoS is handled
in this scenario. This might also include access to core signaling
over administrative boundaries. Compared to the previous one it adds
the case, where the two networks are not in the same administrative
domain. Basically, it is the inter-domain/inter-provider signaling
which is handled in here.
The domain boundary is the critical issue to be resolved. Which of
various flavors of issues a QoS signaling protocol has to be
concerned with.
1) Competing administrations: Normally, only basic information should
be exchanged, if the signaling is between competing
administrations. Specifically information about core network
internals (e.g., topology, technology, etc.) should not be
exchanged. Some information exchange about the "access points" of
the core networks (which is topology information as well) may be
required, to be exchanged, because it is needed for proper
signaling.
2) Additionally, as in scenario 4, signaling most likely is based on
aggregates, with all the issues raise there.
3) Authorization: It is critical that the NSIS Initiator is
authorized to perform a QoS path setup.
4) Accountability: It is important to notice that signaling might be
used as an entity to charge money for, therefore the
interoperation with accounting needs to be available.
8.8. QoS Signaling Between PSTN Gateways and Backbone Routers
A PSTN gateway (i.e., host) requires information from the network
regarding its ability to transport voice traffic across the network.
The voice quality will suffer due to packet loss, latency and jitter.
Signaling is used to identify and admit a flow for which these
impairments are minimized. In addition, the disposition of the
signaling request is used to allow the PSTN GW to make a call routing
decision before the call is actually accepted and delivered to the
final destination.
PSTN gateways may handle thousands of calls simultaneously and there
may be hundreds of PSTN gateways in a single provider network. These
numbers are likely to increase as the size of the network increases.
The point being that scalability is a major issue.
There are several ways that a PSTN gateway can acquire assurances
that a network can carry its traffic across the network. These
include:
1. Over-provisioning a high availability network.
2. Handling admission control through some policy server that has a
global view of the network and its resources.
3. Per PSTN GW pair admission control.
4. Per call admission control (where a call is defined as the 5-tuple
used to carry a single RTP flow).
Item 1 requires no signaling at all and is therefore outside the
scope of this working group.
Item 2 is really a better informed version of 1, but it is also
outside the scope of this working group as it relies on a particular
telephony signaling protocol rather than a packet admission control
protocol.
Item 3 is initially attractive, as it appears to have reasonable
scaling properties, however, its scaling properties only are
effective in cases where there are relatively few PSTN GWs. In the
more general case where a PSTN GW reduces to a single IP phone
sitting behind some access network, the opportunities for aggregation
are reduced and the problem reduces to item 4.
Item 4 is the most general case. However, it has the most difficult
scaling problems. The objective here is to place the requirements on
Item 4 such that a scalable per-flow admission control protocol or
protocol suite may be developed.
The case where per-flow signaling extends to individual IP end-points
allows the inclusion of IP phones on cable, DSL, wireless or other
access networks in this scenario.
Call Scenario
A PSTN GW signals end-to-end for some 5-tuple defined flow a
bandwidth and QoS requirement. Note that the 5-tuple might include
masking/wildcarding. The access network admits this flow according
to its local policy and the specific details of the access
technology.
At the edge router (i.e., border node), the flow is admitted, again
with an optional authentication process, possibly involving an
external policy server. Note that the relationship between the PSTN
GW and the policy server and the routers and the policy server is
outside the scope of NSIS. The edge router then admits the flow into
the core of the network, possibly using some aggregation technique.
At the interior nodes, the NSIS host-to-host signaling should either
be ignored or invisible as the Edge router performed the admission
control decision to some aggregate.
At the inter-provider router (i.e., border node), again the NSIS
host-to-host signaling should either be ignored or invisible, as the
Edge router has performed an admission control decision about an
aggregate across a carrier network.
8.9. PSTN Trunking Gateway
One of the use cases for the NSIS signaling protocol is the scenario
of interconnecting PSTN gateways with an IP network that supports
QoS.
Four different scenarios are considered here.
1. In-band QoS signaling is used. In this case the Media Gateway
(MG) will be acting as the NSIS Initiator and the Edge Router (ER)
will be the NSIS Forwarder. Hence, the ER should do admission
control (into pre-provisioned traffic trunks) for the individual
traffic flows. This scenario is not further considered here.
2. Out-of-band signaling in a single domain, the NSIS forwarder is
integrated in the Media Gateway Controller (MGC). In this case no
NSIS protocol is required.
3. Out-of-band signaling in a single domain, the NSIS forwarder is a
separate box. In this case NSIS signaling is used between the MGC
and the NSIS Forwarder.
4. Out-of-band signaling between multiple domains, the NSIS Forwarder
(which may be integrated in the MGC) triggers the NSIS Forwarder
of the next domain.
When the out-of-band QoS signaling is used the Media Gateway
Controller (MGC) will be acting as the NSIS Initiator.
In the second scenario the voice provider manages a set of traffic
trunks that are leased from a network provider. The MGC does the
admission control in this case. Since the NSIS Forwarder acts both
as a NSIS Initiator and a NSIS Forwarder, no NSIS signaling is
required. This scenario is shown in Figure 3.
+-------------+ ISUP/SIGTRAN +-----+ +-----+
| SS7 network |---------------------| MGC |--------------| SS7 |
+-------------+ +-------+-----+---------+ +-----+
: / : \
: / : \
: / +--------:----------+ \
: MEGACO / / : \ \
: / / +-----+ \ \
: / / | NMS | \ \
: / | +-----+ | \
: : | | :
+--------------+ +----+ | bandwidth pipe (SLS) | +----+
| PSTN network |--| MG |--|ER|======================|ER|-| MG |--
+--------------+ +----+ \ / +----+
\ QoS network /
+-------------------+
Figure 3: PSTN trunking gateway scenario
In the third scenario, the voice provider does not lease traffic
trunks in the network. Another entity may lease traffic trunks and
may use a NSIS Forwarder to do per-flow admission control. In this
case the NSIS signaling is used between the MGC and the NSIS
Forwarder, which is a separate box here. Hence, the MGC acts only as
a NSIS Initiator. This scenario is depicted in Figure 4.
+-------------+ ISUP/SIGTRAN +-----+ +-----+
| SS7 network |---------------------| MGC |--------------| SS7 |
+-------------+ +-------+-----+---------+ +-----+
: / : \
: / +-----+ \
: / | NF | \
: / +-----+ \
: / : \
: / +--------:----------+ \
: MEGACO : / : \ :
: : / +-----+ \ :
: : / | NMS | \ :
: : | +-----+ | :
: : | | :
+--------------+ +----+ | bandwidth pipe (SLS) | +----+
| PSTN network |--| MG |--|ER|======================|ER|-| MG |--
+--------------+ +----+ \ / +----+
\ QoS network /
+-------------------+
Figure 4: PSTN trunking gateway scenario
In the fourth scenario multiple transport domains are involved. In
the originating network either the MGC may have an overview on the
resources of the overlay network or a separate NSIS Forwarder will
have the overview. Hence, depending on this either the MGC or the
NSIS Forwarder of the originating domain will contact the NSIS
Forwarder of the next domain. The MGC always acts as a NSIS
Initiator and may also be acting as a NSIS Forwarder in the first
domain.
8.10. An Application Requests End-to-End QoS Path from the Network
This is actually the conceptually simplest case. A multimedia
application requests a guaranteed service from an IP network. We
assume here that the application is somehow able to specify the
network service. The characteristics here are that many hosts might
do it, but that the requested service is low capacity (bounded by the
access line). Note that there is an issue of scaling in the number
of applications requesting this service in the core of the network.
8.11. QOS for Virtual Private Networks
In a Virtual Private Network (VPN), a variety of tunnels might be
used between its edges. These tunnels could be for example, IPSec,
GRE, and IP-IP. One of the most significant issues in VPNs is
related to how a flow is identified and what quality a flow gets. A
flow identification might consist among others of the transport
protocol port numbers. In an IP-Sec tunnel this will be problematic
since the transport protocol information is encrypted.
There are two types of L3 VPNs, distinguished by where the endpoints
of the tunnels exist. The endpoints of the tunnels may either be on
the customer (CPE) or the provider equipment or provider edge (PE).
Virtual Private networks are also likely to request bandwidth or
other type of service in addition to the premium services the PSTN GW
are likely to use.
8.11.1. Tunnel End Points at the Customer Premises
When the endpoints are the CPE, the CPE may want to signal across the
public IP network for a particular amount of bandwidth and QoS for
the tunnel aggregate. Such signaling may be useful when a customer
wants to vary their network cost with demand, rather than paying a
flat rate. Such signaling exists between the two CPE routers.
Intermediate access and edge routers perform the same exact call
admission control, authentication and aggregation functions performed
by the corresponding routers in the PSTN GW scenario with the
exception that the endpoints are the CPE tunnel endpoints rather than
PSTN GWs and the 5-tuple used to describe the RTP flow is replaced
with the corresponding flow spec to uniquely identify the tunnels.
Tunnels may be of any variety (e.g., IP-Sec, GRE, IP-IP).
In such a scenario, NSIS would actually allow partly for customer
managed VPNs, which means a customer can setup VPNs by subsequent
NSIS signaling to various end-point. Plus the tunnel end-points are
not necessarily bound to an application. The customer administrator
might be the one triggering NSIS signaling.
8.11.2. Tunnel End Points at the Provider Premises
In the case were the tunnel end-points exist on the provider edge,
requests for bandwidth may be signaled either per flow, where a flow
is defined from a customers address space, or between customer sites.
In the case of per flow signaling, the PE router must map the
bandwidth request to the tunnel carrying traffic to the destination
specified in the flow spec. Such a tunnel is a member of an
aggregate to which the flow must be admitted. In this case, the
operation of admission control is very similar to the case of the
PSTN GW with the additional level of indirection imposed by the VPN
tunnel. Therefore, authentication, accounting and policing may be
required on the PE router.
In the case of per site signaling, a site would need to be
identified. This may be accomplished by specifying the network
serviced at that site through an IP prefix. In this case, the
admission control function is performed on the aggregate to the PE
router connected to the site in question.
9. References
9.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[RSVP] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S. and S.
Jamin, "Resource Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
10. Authors' Addresses
Marcus Brunner (Editor)
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
D-69115 Heidelberg
Germany
EMail: brunner@netlab.nec.de
Robert Hancock
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
EMail: robert.hancock@roke.co.uk
Eleanor Hepworth
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
EMail: eleanor.hepworth@roke.co.uk
Cornelia Kappler
Siemens AG
Berlin 13623
Germany
EMail: cornelia.kappler@siemens.com
Hannes Tschofenig
Siemens AG
Otto-Hahn-Ring 6
81739 Munchen
Germany
EMail: Hannes.Tschofenig@mchp.siemens.de
11. Full Copyright Statement
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ISO Plain Language
On this page
PROBLEMSignalling protocols set up and maintain state in the network, for quality of service, for label distribution and for talking to middleboxes. The ones built for one network, or one technology, do not carry across administrative and technology boundaries.
MECHANISMThe memo starts from eleven scenarios, defines a small model of who takes part in a signalling exchange, and then lists requirements in ten groups: architecture, flows, messaging, control information, performance, flexibility, security, mobility, interworking and operation.
WHAT THE MEMO DELIVERSThe requirements are deliberately independent of any one application, so the same signalling protocol can serve resource reservation, MPLS label distribution and middlebox control. The memo states no protocol of its own and lists no problems with existing ones.
§1What This Memo Is For
This document is the product of the Next Steps in Signaling Working Group. It defines requirements for signalling across different network environments, and it does not list any problems of existing signalling protocols such as RSVP.
To derive requirements for signalling it is necessary first to have an idea of the scope within which they apply, so the memo lists use cases and scenarios where an NSIS protocol could be applied. The scenarios are used to help derive the requirements and to test the requirements against use cases.
The requirements listed are independent of any application, although resource reservation and quality of service related issues are used as examples within the text. Quality of service is not the only field where signalling is used in the Internet: signalling might also be used as a communication protocol to set up and maintain state in middleboxes. The document does not cover requirements in relation to some networking areas, in particular interaction with host and site multihoming, and leaves these for future analysis.
This is a shopping list, not a design. The authors take real situations where signalling would be useful, work out what they have in common, and write down what any protocol has to do to serve them all.
§2The Pieces of the Model
The memo lists the terms it uses most often. They cannot be made precise without a more complete architectural model and they are not meant to prescribe any solution.
An NSIS Entity is the function within a node that implements an NSIS protocol; in the case of path-coupled signalling it will always be on the data path. An NSIS Initiator is the entity that starts signalling to set up or manipulate network state. An NSIS Forwarder is the entity between an initiator and a responder, which may interact with local state management functions in the network and also propagates signalling further through the network. An NSIS Responder is the entity that terminates signalling and can optionally interact with applications as well.
A flow is a traffic stream, a sequence of IP packets between two end systems, for which a specific packet level treatment is provided; it can be unicast, uni- or bi-directional, or multicast, and for multicast a flow can diverge as it propagates toward the receiver. The data path is the route across the networks taken by a flow or aggregate, and the signalling path is the route taken by a signalling flow or aggregate.
Path-coupled signalling is a mode where the signalling messages follow a path tied to the data packets, so they are routed only through nodes that are in the data path. Path-decoupled signalling has independent data and signalling paths: signalling messages are routed to nodes that are not assumed to be on the data path but which are aware of it, and are always directly addressed to the neighbouring entity, so the initiator and responder may have no relation at all with the ultimate data sender or receiver.
The scope of NSIS is the interaction between the initiator, the forwarders and the responder, including a protocol to carry the information and the syntax and semantics of what is exchanged. The initiator might be in the end system or elsewhere in the network, and its distinguishing feature is that it acts on triggers coming directly or indirectly from the higher layers in the end systems; it needs to map the services they request and provides feedback information which might be used by transport layer algorithms or adaptive applications. The forwarder does not interact with higher layers, but with the initiator, the responder and possibly other forwarders. The responder might be in an end system or in other equipment, and its distinguishing feature is that it responds to requests at the end of a signalling path. The signalling path traverses an underlying network covering one or more IP hops, which might use locally different technology, so in the quality of service example a forwarder maps service specific information to technology related QoS parameters and receives indications of success or failure. The network can be seen at the level of domains and subdomains rather than individual routers, since domains are assumed to be administrative entities, so security requirements might apply differently between domains and within one.
Three roles: whoever asks for the service, whoever passes the request along, and whoever answers at the far end. The request normally travels along the same path as the data, but the model also allows a separate signalling path.
NSIS InitiatorNSIS ForwarderNSIS Responder
- REQUESTThe initiator asks for a service, mapping it from the higher layers.
- REQUESTForwarders pass the request on and map it to local parameters.
- RESPONSEThe responder answers at the end of the signalling path.
- RESPONSEThe answer comes back to the initiator, which reports it upwards.
§4What Is Assumed and What Is Excluded
The memo states its assumptions. Signalling could run end-to-end, end-to-edge, edge-to-edge or network-to-network between providers, depending on what point in the network acts as the initiator and how far the signalling propagates. In general the memo expects forwarders to become more dense towards the edges of the network, but this is not a requirement: an over-provisioned domain might contain no forwarders at all and be signalling transparent, while at the other extreme forwarders might be placed at every router, in which case QoS provisioning can be carried out locally without further signalling.
Pure end-to-end signalling that is not interpreted anywhere in the network is not considered, since that is a higher layer issue for protocols such as SIP. Where signalling covers several domains, the memo does not exclude different signalling protocols being used in each domain; it only places requirements on the universality of the control information being transported, with the goal of allowing protocols matched to the characteristics of the portion of the network being traversed. The memo also assumes the service definitions an initiator can ask for are known in advance of the signalling protocol running, that there are means for discovering NSIS entities so that signalling peers are known, and that NSIS assumes layer 3 routing, with the determination of the next data node not done by NSIS.
The exclusions are just as long. Development of specific mechanisms and algorithms for application and transport layer adaptation, and the protocols that would support it, are not considered. Specific mechanisms such as APIs for interaction between transport and applications and the network layer are not considered, except to clarify the requirements on negotiation capabilities and information semantics. Specific mechanisms and protocols for provisioning or other network control functions within a domain are not considered, the goal being to reuse existing functions and protocols unchanged, although NSIS itself can be used for signalling within a domain. Interaction with resource management or other internal state management capabilities is not considered. Security implications related to multicasting are outside the scope. Service definitions, and in particular QoS services and classes, are out of scope: only the base signalling protocol for transporting the service information is addressed. Handoff decision and trigger sources are out of scope too: an NSIS protocol is not used to trigger handoffs in mobile IP nor to decide whether to hand off, and the underlying assumption is that the route comes first, defining the path, and the signalling comes after it, following the path.
The memo is careful about what it is not deciding. It does not design a protocol, does not define what a quality of service is, and does not say how a router should hand a phone over to a new base station.
§5The Requirement Groups
Two general and potentially contradictory goals run through the requirements: the solution should be applicable in a very wide range of scenarios, and at the same time be lightweight in implementation complexity and resource consumption in the entities that implement it. One approach is that the solution could deal with certain requirements through modular components or capabilities which are optional to implement or use in individual nodes.
Architecture and design goals: NSIS should provide a mechanism to check whether state is available without setting it up, for example checking resource availability without performing a reservation, which is required in some scenarios such as the mobile terminal case. It must be designed modularly, so that fewer features give lighter implementations, and mutually exclusive solutions are supported, such as working over any kind of network, from narrowband to broadband and from error-prone to reliable. It must decouple the protocol from the information it carries, so the two can develop independently and the control information can be carried within other protocols, including application layer ones, though the information carried still needs to be standardised or interoperability is difficult. It must be independent of the paradigm and mechanism of network control, so that the same signalling can be used with various QoS technologies. It should be able to carry opaque objects interpreted only by some nodes.
Signalling flows: the protocol must work in scenarios such as host-to-network-to-host, edge-to-edge, user-to-network and network-to-network. It must support path-coupled signalling and may support path-decoupled signalling. It should allow the internal structure of a domain to be hidden from end nodes and other networks, for business reasons among others, and it should be possible for signalling for some flows to traverse path segments transparently, without interpretation at the forwarders inside them, as with a subdomain in a core network that only interprets signalling for aggregates established at the domain edge.
Messaging: explicit erasure of state must be possible when state is no longer needed, and state must be released automatically after a failure, to prevent stale state in the network. Notifications should be possible upstream, and both establishment and refusal to set up state must be notified. Local information exchange must be allowed.
Control information: it should be possible to say which parameters may be changed, so that an end-to-end request is not gradually mutated as it passes through nodes. Local scope elements should be added and removed as a message enters and leaves a domain. State must be addressed independently of flow identification, because flows resulting from a handoff may have changed end points but still have the same service requirement. Modification of already established state should be seamless, and grouping the signalling for several micro-flows into one message may be provided, for the optimisation of setup delay, though there must not be any transactional semantic associated with the grouping.
Performance: NSIS must be scalable in the number of messages received by a signalling partner, in the number of hand-offs in mobile environments, in the number of interactions for setting up state, in the amount of state per entity in the core, and in CPU usage on end terminals and intermediate nodes. It must work in Internet scale deployments, and its performance must degrade gracefully rather than catastrophically under overload. It should allow low latency in setup, and must allow low bandwidth consumption, since small sets of scenarios call for it, mainly those where wireless links are involved. It should allow the load on devices to be constrained, through message aggregation, header compression, minimising functionality or ignoring signalling in core nodes.
Flexibility: flow aggregation must be allowed, including the capability to select and change the level of aggregation. Placement of the initiator and responder must be flexible, and network initiated signalling and termination must be allowed in scenarios such as PSTN gateways, some VPNs and mobility. Either the initiator or the responder should be able to initiate a change of state, and the protocol should support network initiated state change, used when the network can no longer guarantee resources and wants to downgrade a reservation. Both unidirectional and bidirectional state setup should be possible, with the goal of bidirectional setup being an optimisation in terms of setup delay.
Mobility: after a handover the states may need to be completely or partially re-established, requested by the mobile node itself or triggered by the access point it is attached to, and the re-establishment must be as quick as possible so that the mobile node does not experience service interruption or degradation. It should be localised and not require end-to-end signalling.
Interworking: the protocol must interwork with IP tunnelling, particularly IPsec tunnels, which mainly affects the identification of flows because parts of the information commonly used for flow identification may not be accessible due to encryption. It must not be constrained to IPv4 or IPv6, and must be independent of the charging model. It should provide hooks for AAA protocols so that usage records can be collected, and should work with seamless handoff protocols such as context transfer and candidate access router discovery. It must work with traditional layer 3 routing, in particular when routes change, so that state on the old route is released and state on the new route is established.
Operational: the architecture should allow the operator to assign signalling messages a certain transport quality, which gives a way to trade off signalling latency against the impact of the messages on devices, and this means the protocol messages should be detectable where their priority is assigned and procedures must be defined for a request that stays unanswered. Any unit participating in signalling must not cause further damage to other systems when it has to go out of service, and entities should be able to detect a malfunctioning peer and notify the initiator or another entity, or handle the problem themselves by switching to a backup entity, in which case state must be synchronised between the primary and the backup.
The requirements fall into ten groups. The short version: the protocol must be modular and cheap, must let the network hide its own shape, must clean up after itself when things fail, must be fast to set up and slow to flood, and must keep working when a phone moves to a different base station.
| Group | What it asks for |
|---|---|
| Architecture and design goals | Modular design, protocol separated from the information carried, independent of how the network is controlled |
| Signalling flows | Works wherever the initiator, forwarders and responder sit, path-coupled and optionally path-decoupled |
| Messaging | State can be erased explicitly and is released automatically after a failure |
| Control information | Mutability of parameters, local domain information, state addressed apart from the flow |
| Performance | Scalable, low latency where needed, low bandwidth on wireless links, graceful under overload |
| Flexibility | Flow aggregation, flexible placement, one or both directions, network initiated changes |
| Security THIS RFC | Authentication, authorisation, integrity, replay protection, hop-by-hop security, denial of service |
| Mobility | Service re-established quickly and locally after a handover |
| Interworking | IP tunnelling, either IP version, charging models, AAA hooks, handoff protocols, ordinary routing |
| Operational | Transport quality for signalling messages, graceful failure, detection of a faulty peer |
§5.7Security Requirements in Detail
The NSIS protocol must provide means for security, but it must be allowed that nodes implementing it do not have to use those means. A signalling protocol must make provision for enabling entities to be authenticated against each other using strong mechanisms, where strong means that weak plain-text password mechanisms must not be used. It must provide means to authorise state setup requests, which demands a hook to interact with a policy entity so that an authenticated entity can be associated with authorisation data and the request verified; authorisation prevents state setup by unauthorised entities, setups that violate policies, and theft of service, and it also limits denial of service attacks caused by unrestricted state setups.
It must provide means to protect message payloads against modification, to prevent an adversary from modifying parts of a signalling message and from mounting denial of service or theft of service attacks against network elements taking part in the exchange. It must provide means to detect old, already transmitted, signalling messages, so that they cannot be replayed, and the solution must cover synchronisation problems in the case of a restart or crash of a participating element.
Channel security between signalling entities must be implemented. It is a well known concept in quality of service and other signalling protocols to have intermediate nodes actively participate in the protocol and modify the messages as required by processing rules. This requirement does not exclude end-to-end or network-to-network security: end-to-end security between the initiator and the responder may be used to protect non-mutable data fields, and network-to-network security refers to protection over various hops but not end to end.
Identity confidentiality should be supported, to enable privacy and avoid profiling of entities by an adversary eavesdropping the signalling traffic, and the identity used in authentication may be hidden to a limited extent from the network the initiator is attached to, though it must provide enough information for the nodes in the access network to collect accounting data. Network topology hiding may be supported, though it might conflict with a diagnostic capability.
Prevention of denial of service attacks should be provided, and to do it effectively the security and protocol mechanisms must have low computational complexity to verify a state setup request before authenticating the requester, and should not require large resource consumption on the entities before a successful authentication is done. Confidentiality of signalling messages in a hop-by-hop manner should be provided, since an adversary who can listen to signalling channels learns which data channels are worth listening to; most messages must be protected hop by hop because the entities taking part in the protocol have to be able to read and eventually modify them. Finally, where existing states have to be modified, a session identifier is needed to identify the established state uniquely, and the protocol must provide security protection to stop adversaries modifying state.
Signalling messages travel through routers that must read them, so they are protected hop by hop rather than end to end. The rules also insist that checking a request is cheap, so that a flood of signalling cannot be used to overwhelm the network.
- AuthenticationEntities authenticate each other with strong mechanisms, never plain-text passwords
- AuthorisationA policy hook decides who may set up state, which also limits denial of service
- IntegrityMessage payloads are protected against modification
- Replay protectionOld messages are detected, including after a restart or crash
- Hop-by-hop securityEach leg is protected, since intermediate nodes must read and change the message
- Denial of serviceVerifying a request before authenticating it must be cheap
- ConfidentialitySignalling content is hidden hop by hop
- Ownership of stateA session identifier names the established state, and changes to it are protected
§8The Scenarios Behind the Requirements
The appendix holds the use cases the requirements were derived from and tested against. They cover terminal mobility, wireless networks, an example scenario for 3G wireless networks, the wired part of a wireless network, session mobility, and quality of service reservation and negotiation from an access network into a core network and over administrative boundaries.
There are also scenarios for quality of service signalling between PSTN gateways and backbone routers, for a PSTN trunking gateway, for an application that asks the network for an end-to-end quality of service path, and for quality of service in virtual private networks, with the tunnel endpoints at the customer premises and at the provider premises treated separately.
The scenarios cover a phone moving between base stations, wireless and 3G networks, sessions that follow a user, reservations crossing into a core network and across organisational boundaries, telephone gateways, and quality of service inside VPNs. They are the cases the requirements were checked against.
§7References
[RSVP]RFC 2205Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1, Functional Specification", RFC 2205, September 1997.September 1997[RFC3234]RFC 3234Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, February 2002.February 2002[KEYWORDS]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997