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RFC 4080 - Next Steps in Signaling (NSIS): Framework

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Network Working Group                                         R. Hancock
Request for Comments: 4080                                   Siemens/RMR
Category: Informational                                   G. Karagiannis
                                           University of Twente/Ericsson
                                                             J. Loughney
                                                        S. Van den Bosch
                                                               June 2005

               Next Steps in Signaling (NSIS): Framework

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 (2005).


   The Next Steps in Signaling (NSIS) working group is considering
   protocols for signaling information about a data flow along its path
   in the network.  The NSIS suite of protocols is envisioned to support
   various signaling applications that need to install and/or manipulate
   such state in the network.  Based on existing work on signaling
   requirements, this document proposes an architectural framework for
   these signaling protocols.

   This document provides a model for the network entities that take
   part in such signaling, and for the relationship between signaling
   and the rest of network operation.  We decompose the overall
   signaling protocol suite into a generic (lower) layer, with separate
   upper layers for each specific signaling application.

Table of Contents

   1. Introduction ....................................................3
      1.1. Definition of the Signaling Problem ........................3
      1.2. Scope and Structure of the NSIS Framework ..................3
   2. Terminology .....................................................4
   3. Overview of Signaling Scenarios and Protocol Structure ..........6
      3.1. Fundamental Signaling Concepts .............................6
           3.1.1. Simple Network and Signaling Topology ...............6

           3.1.2. Path-Coupled and Path-Decoupled Signaling ...........7
           3.1.3. Signaling to Hosts, Networks, and Proxies ...........8
           3.1.4. Signaling Messages and Network Control State .......10
           3.1.5. Data Flows and Sessions ............................10
      3.2. Layer Model for the Protocol Suite ........................11
           3.2.1. Layer Model Overview ...............................11
           3.2.2. Layer Split Concept ................................12
           3.2.3. Bypassing Intermediate Nodes .......................13
           3.2.4. Core NSIS Transport Layer Functionality ............15
           3.2.5. State Management Functionality .....................16
           3.2.6. Path-Decoupled Operation ...........................17
      3.3. Signaling Application Properties ..........................18
           3.3.1. Sender/Receiver Orientation ........................18
           3.3.2. Uni- and Bi-Directional Operation ..................19
           3.3.3. Heterogeneous Operation ............................19
           3.3.4. Aggregation ........................................20
           3.3.5. Peer-Peer and End-End Relationships ................21
           3.3.6. Acknowledgements and Notifications .................21
           3.3.7. Security and Other AAA Issues ......................22
   4. The NSIS Transport Layer Protocol ..............................23
      4.1. Internal Protocol Components ..............................23
      4.2. Addressing ................................................24
      4.3. Classical Transport Functions .............................24
      4.4. Lower Layer Interfaces ....................................26
      4.5. Upper Layer Services ......................................27
      4.6. Identity Elements .........................................28
           4.6.1. Flow Identification ................................28
           4.6.2. Session Identification .............................28
           4.6.3. Signaling Application Identification ...............29
      4.7. Security Properties .......................................30
   5. Interactions with Other Protocols ..............................30
      5.1. IP Routing Interactions ...................................30
           5.1.1. Load Sharing and Policy-Based Forwarding ...........31
           5.1.2. Route Changes ......................................31
      5.2. Mobility and Multihoming Interactions .....................33
      5.3. Interactions with NATs ....................................36
      5.4. Interactions with IP Tunneling ............................36
   6. Signaling Applications .........................................37
      6.1. Signaling for Quality of Service ..........................37
           6.1.1. Protocol Message Semantics .........................38
           6.1.2. State Management ...................................39
           6.1.3. Route Changes and QoS Reservations .................39
           6.1.4. Resource Management Interactions ...................41
      6.2. Other Signaling Applications ..............................42
   7. Security Considerations ........................................42
   8. References .....................................................43
      8.1. Normative References ......................................43
      8.2. Informative References ....................................44

1.  Introduction

1.1.  Definition of the Signaling Problem

   The Next Steps in Signaling (NSIS) working group is considering
   protocols for signaling information about a data flow along its path
   in the network.

   It is assumed that the path taken by the data flow is already
   determined by network configuration and routing protocols,
   independently of the signaling itself; that is, signaling to set up
   the routes themselves is not considered.  Instead, the signaling
   simply interacts with nodes along the data flow path.  Additional
   simplifications are that the actual signaling messages pass directly
   through these nodes themselves (i.e., the 'path-coupled' case; see
   Section 3.1.2) and that only unicast data flows are considered.

   The signaling problem in this sense is very similar to that addressed
   by RSVP.  However, there are two generalizations.  First, the
   intention is that components of the NSIS protocol suite will be
   usable in different parts of the Internet, for different needs,
   without requiring a complete end-to-end deployment (in particular,
   the signaling protocol messages may not need to run all the way
   between the data flow endpoints).

   Second, the signaling is intended for more purposes than just QoS
   (resource reservation).  The basic mechanism to achieve this
   flexibility is to divide the signaling protocol stack into two
   layers: a generic (lower) layer, and an upper layer specific to each
   signaling application.  The scope of NSIS work is to define both the
   generic protocol and, initially, upper layers suitable for QoS
   signaling (similar to the corresponding functionality in RSVP) and
   middlebox signaling.  Further applications may be considered later.

1.2.  Scope and Structure of the NSIS Framework

   The underlying requirements for signaling in the context of NSIS are
   defined in [1] and a separate security threats document [2]; other
   related requirements can be found in [3] and [4] for QoS/Mobility and
   middlebox communication, respectively.  This framework does not
   replace or update these requirements.  Discussions about lessons to
   be learned from existing signaling and resource management protocols
   are contained in separate analysis documents [5], [6].

   The role of this framework is to explain how NSIS signaling should
   work within the broader networking context, and to describe the
   overall structure of the protocol suite itself.  Therefore, it

   discusses important protocol considerations such as routing,
   mobility, security, and interactions with network 'resource'
   management (in the broadest sense).

   The basic context for NSIS protocols is given in Section 3.
   Section 3.1 describes the fundamental elements of NSIS protocol
   operation in comparison to RSVP [7]; in particular, Section 3.1.3
   describes more general signaling scenarios, and Section 3.1.4 defines
   a broader class of signaling applications for which the NSIS
   protocols should be useful.  The two-layer protocol architecture that
   supports this generality is described in Section 3.2, and Section 3.3
   gives examples of the ways in which particular signaling application
   properties can be accommodated within signaling layer protocol

   The overall functionality required from the lower (generic) protocol
   layer is described in Section 4.  This is not intended to define the
   detailed design of the protocol or even design options, although some
   are described as examples.  It describes the interfaces between this
   lower-layer protocol and the IP layer (below) and signaling
   application protocols (above), including the identifier elements that
   appear on these interfaces (Section 4.6).  Following this, Section 5
   describes how signaling applications that use the NSIS protocols can
   interact sensibly with network layer operations; specifically,
   routing (and re-routing), IP mobility, and network address
   translation (NAT).

   Section 6 describes particular signaling applications.  The example
   of signaling for QoS (comparable to core RSVP QoS signaling
   functionality) is given in detail in Section 6.1, which describes
   both the signaling application specific protocol and example modes of
   interaction with network resource management and other deployment
   aspects.  However, note that these examples are included only as
   background and for explanation; we do not intend to define an
   over-arching architecture for carrying out resource management in the
   Internet.  Further possible signaling applications are outlined in
   Section 6.2.

2.  Terminology

   Classifier: an entity that selects packets based on their contents
      according to defined rules.

   [Data] flow: a stream of packets from sender to receiver that is a
      distinguishable subset of a packet stream.  Each flow is
      distinguished by some flow identifier (see Section 4.6.1).

   Edge node: an (NSIS-capable) node on the boundary of some
      administrative domain.

   Interior nodes: the set of (NSIS-capable) nodes that form an
      administrative domain, excluding the edge nodes.

   NSIS Entity (NE): the function within a node that implements an NSIS
      protocol.  In the case of path-coupled signaling, the NE will
      always be on the data path.

   NSIS Signaling Layer Protocol (NSLP): generic term for an NSIS
      protocol component that supports a specific signaling application.
      See also Section 3.2.1.

   NSIS Transport Layer Protocol (NTLP): placeholder name for the NSIS
      protocol component that will support lower-layer (signaling
      application-independent) functions.  See also Section 3.2.1.

   Path-coupled signaling: a mode of signaling in which the signaling
      messages follow a path that is tied to the data messages.

   Path-decoupled signaling: signaling for state manipulation related to
      data flows, but only loosely coupled to the data path; e.g., at
      the AS level.

   Peer discovery: the act of locating and/or selecting which NSIS peer
      to carry out signaling exchanges with for a specific data flow.

   Peer relationship: signaling relationship between two adjacent NSIS
      entities (i.e., NEs with no other NEs between them).

   Receiver: the node in the network that is receiving the data packets
      in a flow.

   Sender: the node in the network that is sending the data packets in a

   Session: application layer flow of information for which some network
      control state information is to be manipulated or monitored (see
      Section 3.1.5).

   Signaling application: the purpose of the NSIS signaling.  A
      signaling application could be QoS management, firewall control,
      and so on.  Totally distinct from any specific user application.

3.  Overview of Signaling Scenarios and Protocol Structure

3.1.  Fundamental Signaling Concepts

3.1.1.  Simple Network and Signaling Topology

   The NSIS suite of protocols is envisioned to support various
   signaling applications that need to install and/or manipulate state
   in the network.  This state is related to a data flow and is
   installed and maintained on the NSIS Entities (NEs) along the data
   flow path through the network; not every node has to contain an NE.
   The basic protocol concepts do not depend on the signaling
   application, but the details of operation and the information carried
   do.  This section discusses the basic entities involved with
   signaling as well as interfaces between them.

   Two NSIS entities that communicate directly are said to be in a 'peer
   relationship'.  This concept might loosely be described as an 'NSIS
   hop'; however, there is no implication that it corresponds to a
   single IP hop.  Either or both NEs might store some state information
   about the other, but there is no assumption that they necessarily
   establish a long-term signaling connection between themselves.

   It is common to consider a network as composed of various domains
   (e.g., for administrative or routing purposes), and the operation of
   signaling protocols may be influenced by these domain boundaries.
   However, it seems there is no reason to expect that an 'NSIS domain'
   should exactly overlap with an IP domain (AS, area), but it is likely
   that its boundaries would consist of boundaries (segments) of one or
   several IP domains.

   Figure 1 shows a diagram of nearly the simplest possible signaling
   configuration.  A single data flow is running from an application in
   the sender to the receiver via routers R1, R2, and R3.  Each host and
   two of the routers contain NEs that exchange signaling messages --
   possibly in both directions -- about the flow.  This scenario is
   essentially the same as that considered by RSVP for QoS signaling;
   the main difference is that here we make no assumptions about the
   particular sequence of signaling messages that will be invoked.

       Sender                                               Receiver
   +-----------+      +----+      +----+      +----+      +-----------+
   |Application|----->| R1 |----->| R2 |----->| R3 |----->|Application|
   |   +--+    |      |+--+|      |+--+|      +----+      |   +--+    |
   |   |NE|====|======||NE||======||NE||==================|===|NE|    |
   |   +--+    |      |+--+|      |+--+|                  |   +--+    |
   +-----------+      +----+      +----+                  +-----------+

      |NE| = NSIS      ==== = Signaling    ---> = Data flow messages
      +--+   Entity           Messages            (unidirectional)

                 Figure 1: Simple Signaling and Data Flows

3.1.2.  Path-Coupled and Path-Decoupled Signaling

   We can consider two basic paradigms for resource reservation
   signaling, which we refer to as "path-coupled" and "path-decoupled".

   In the path-coupled case, signaling messages are routed only through
   NEs that are on the data path.  They do not have to reach all the
   nodes on the data path.  (For example, there could be intermediate
   signaling-unaware nodes, or the presence of proxies such as those
   shown in Figure 2 could prevent the signaling from reaching the path
   end points.)  Between adjacent NEs, the route taken by signaling and
   data might diverge.  The path-coupled case can be supported by
   various addressing styles, with messages either explicitly addressed
   to the neighbor on-path NE, or addressed identically to the data
   packets, but also with the router alert option (see [8] and [9]), and
   intercepted.  These cases are considered in Section 4.2.  In the
   second case, some network configurations may split the signaling and
   data paths (see Section 5.1.1); this is considered an error case for
   path-coupled signaling.

   In the path-decoupled case, signaling messages are routed to nodes
   (NEs) that are not assumed to be on the data path, but that are
   (presumably) aware of it.  Signaling messages will always be directly
   addressed to the neighbor NE, and the signaling endpoints may have no
   relation at all with the ultimate data sender or receiver.  The
   implications of path-decoupled operation for the NSIS protocols are
   considered briefly in Section 3.2.6; however, the initial goal of
   NSIS and this framework is to concentrate mainly on the path-coupled

3.1.3.  Signaling to Hosts, Networks, and Proxies

   There are different possible triggers for the signaling protocols.
   Among them are user applications (that are using NSIS signaling
   services), other signaling applications, network management actions,
   some network events, and so on.  The variety of possible triggers
   requires that the signaling can be initiated and terminated in the
   different parts of the network: hosts, domain boundary nodes (edge
   nodes), or interior domain nodes.

   The NSIS protocol suite extends the RSVP model to consider this wider
   variety of possible signaling exchanges.  As well as the basic
   end-to-end model already described, examples such as end-to-edge and
   edge-to-edge can be considered.  The edge-to-edge case might involve
   the edge nodes communicating directly, as well as via the interior

   Although the end-to-edge (host-to-network) scenario requires only
   intra-domain signaling, the other cases might need inter-domain NSIS
   signaling as well if the signaling endpoints (hosts or network edges)
   are connected to different domains.  Depending on the trust relation
   between concatenated NSIS domains, the edge-to-edge scenario might
   cover a single domain or multiple concatenated NSIS domains.  The
   latter case assumes the existence of trust relations between domains.

   In some cases, it is desired to be able to initiate and/or terminate
   NSIS signaling not from the end host that sends/receives the data
   flow, but from some other entities in the network that can be called
   signaling proxies.  There could be various reasons for this:
   signaling on behalf of the end hosts that are not NSIS-aware,
   consolidation of the customer accounting (authentication,
   authorization) in respect to consumed application and transport
   resources, security considerations, limitation of the physical
   connection between host and network, and so on.  This configuration
   can be considered a kind of "proxy on the data path"; see Figure 2.

                 Proxy1                        Proxy2
   +------+      +----+    +----+    +----+    +----+      +--------+
   |Sender|-...->|Appl|--->| R  |--->| R  |--->|Appl|-...->|Receiver|
   |      |      |+--+|    |+--+|    |+--+|    |+--+|      |        |
   +------+      ||NE||====||NE||====||NE||====||NE||      +--------+
                 |+--+|    |+--+|    |+--+|    |+--+|
                 +----+    +----+    +----+    +----+

      |NE| = NSIS      ==== = Signaling    ---> = Data flow messages
      +--+   Entity           Messages            (unidirectional)

      Appl = signaling application

                      Figure 2: "On path" NSIS proxy

   This configuration presents two specific challenges for the

   o  A proxy that terminates signaling on behalf of the NSIS-unaware
      host (or part of the network) should be able to determine that it
      is the last NSIS-aware node along the path.

   o  Where a proxy initiates NSIS signaling on behalf of the NSIS-
      unaware host, interworking with some other "local" technology
      might be required (for example, to provide QoS reservation from
      proxy to the end host in the case of a QoS signaling application).

   +------+      +----+      +----+      +----+      +--------+
   |Sender|----->| PA |----->| R2 |----->| R3 |----->|Receiver|
   |      |      |+--+|      |+--+|      +----+      |  +--+  |
   +------+      ||NE||======||NE||==================|==|NE|  |
                 |+--+|      |+--+|                  |  +--+  |
                 +-..-+      +----+                  +--------+

            Appl = signaling         PA = Proxy for signaling
                   application            application

                      Figure 3: "Off path" NSIS proxy

   Another possible configuration, shown in Figure 3, is where an NE can
   send and receive signaling information to a remote processor.  The
   NSIS protocols may or may not be suitable for this remote
   interaction, but in any case it is not currently part of the NSIS
   problem.  This configuration is supported by considering the NE a
   proxy at the signaling application level.  This is a natural
   implementation approach for some policy control and centralized
   control architectures; see also Section 6.1.4.

3.1.4.  Signaling Messages and Network Control State

   The distinguishing features of the signaling supported by the NSIS
   protocols are that it is related to specific flows (rather than to
   network operation in general), and that it involves nodes in the
   network (rather than running transparently between the end hosts).

   Therefore, each signaling application (upper-layer) protocol must
   carry per-flow information for the aspects of network-internal
   operation that are of interest to that signaling application.  An
   example for the case of an RSVP-like QoS signaling application would
   be state data representing resource reservations.  However, more
   generally, the per-flow information might be related to some other
   control function in routers and middleboxes along the path.  Indeed,
   the signaling might simply be used to gather per-flow information,
   without modifying network operation at all.

   We call this information 'network control state' generically.
   Signaling messages may install, modify, refresh, or simply read this
   state from network elements for particular data flows.  Usually a
   network element will also manage this information at the per-flow
   level, although coarser-grained ('per-class') state management is
   also possible.

3.1.5.  Data Flows and Sessions

   Formally, a data flow is a (unidirectional) sequence of packets
   between the same endpoints that all follow a unique path through the
   network (determined by IP routing and other network configuration).
   A flow is defined by a packet classifier (in the simplest cases, just
   the destination address and topological origin are needed).  In
   general we assume that when discussing only the data flow path, we
   only need to consider 'simple' fixed classifiers (e.g., IPv4 5-tuple
   or equivalent).

   A session is an application layer concept for an exchange of packets
   between two endpoints, for which some network state is to be
   allocated or monitored.  In simple cases, a session may map to a
   specific flow; however, signaling applications are allowed to create

   more flexible flow:session relationships.  (Note that this concept of
   'session' is different from that of RSVP, which defines a session as
   a flow with a specific destination address and transport protocol.
   The NSIS usage is closer to the session concepts of higher-layer

   The simplest service provided by NSIS signaling protocols is the
   management of network control state at the level of a specific flow,
   as described in the previous subsection.  In particular, it should be
   possible to monitor routing updates as they change the path taken by
   a flow and, for example, update network state appropriately.  This is
   no different from the case for RSVP (local path repair).  Where there
   is a 1:1 flow:session relationship, this is all that is required.

   However, for some more complex scenarios (especially mobility and
   multihoming related ones; see [1] and the mobility discussion of
   [5]), it is desirable to update the flow:session mapping during the
   session lifetime.  For example, a new flow can be added, and the old
   one deleted (and maybe in that order, for a 'make-before-break'
   handover), effectively transferring the network control state between
   data flows to keep it associated with the same session.  Such updates
   are best managed by the end systems (generally, systems that
   understand the flow:session mapping and are aware of the packet
   classifier change).  To enable this, it must be possible to relate
   signaling messages to sessions as well as to data flows.  A session
   identifier (Section 4.6.2) is one component of the solution.

3.2.  Layer Model for the Protocol Suite

3.2.1.  Layer Model Overview

   In order to achieve a modular solution for the NSIS requirements, the
   NSIS protocol suite will be structured in two layers:

   o  a 'signaling transport' layer, responsible for moving signaling
      messages around, which should be independent of any particular
      signaling application; and

   o  a 'signaling application' layer, which contains functionality such
      as message formats and sequences, specific to a particular
      signaling application.

   For the purpose of this document, we use the term 'NSIS Transport
   Layer Protocol' (NTLP) to refer to the component that will be used in
   the transport layer.  We also use the term 'NSIS Signaling Layer
   Protocol' (NSLP) to refer generically to any protocol within the
   signaling application layer; in the end, there will be several NSLPs,
   largely independent of each other.  These relationships are

   illustrated in Figure 4.  Note that the NTLP may or may not have an
   interesting internal structure (e.g., including existing transport
   protocols), but that is not relevant at this level of description.

                 ^                     +-----------------+
                 |                     | NSIS Signaling  |
                 |                     | Layer Protocol  |
         NSIS    |    +----------------| for middleboxes |
       Signaling |    | NSIS Signaling |        +-----------------+
         Layer   |    | Layer Protocol +--------| NSIS Signaling  |
                 |    |     for QoS     |       | Layer Protocol  |
                 |    +-----------------+       |    for ...      |
                 V                              +-----------------+
         NSIS    ^         +--------------------------------+
       Transport |         | NSIS Transport Layer Protocol  |
         Layer   V         +--------------------------------+
                           .      IP and lower layers       .
                           .                                .

                    Figure 4: NSIS Protocol Components

   Note that not every generic function has to be located in the NTLP.
   Another option would be to have re-usable components within the
   signaling application layer.  Functionality within the NTLP should be
   restricted to what interacts strongly with other transport and
   lower-layer operations.

3.2.2.  Layer Split Concept

   This section describes the basic concepts underlying the
   functionality of the NTLP.  First, we make a working assumption that
   the protocol mechanisms of the NTLP operate only between adjacent NEs
   (informally, the NTLP is a 'hop-by-hop' protocol), whereas any
   larger-scope issues (including e2e aspects) are left to the upper

   The way in which the NTLP works can be described as follows: When a
   signaling message is ready to be sent from one NE, it is given to the
   NTLP along with information about what flow it is for; it is then up
   to the NTLP to get it to the next NE along the path (upstream or
   downstream), where it is received and the responsibility of the NTLP
   ends.  Note that there is no assumption here about how the messages
   are actually addressed (this is a protocol design issue, and the

   options are outlined in Section 4.2).  The key point is that the NTLP
   for a given NE does not use any knowledge about addresses,
   capabilities, or status of any NEs other than its direct peers.

   The NTLP in the receiving NE either forwards the message directly or,
   if there is an appropriate signaling application locally, passes it
   upwards for further processing; the signaling application can then
   generate another message to be sent via the NTLP.  In this way,
   larger-scope (including end-to-end) message delivery is achieved.

   This definition relates to NTLP operation.  It does not restrict the
   ability of an NSLP to send messages by other means.  For example, an
   NE in the middle or end of the signaling path could send a message
   directly to the other end as a notification or acknowledgement of
   some signaling application event.  However, the issues in sending
   such messages (endpoint discovery, security, NAT traversal, and so
   on) are so different from the direct peer-peer case that there is no
   benefit in extending the NTLP to include such non-local
   functionality.  Instead, an NSLP that requires such messages and
   wants to avoid traversing the path of NEs should use some other
   existing transport protocol.  For example, UDP or DCCP would be a
   good match for many of the scenarios that have been proposed.
   Acknowledgements and notifications of this type are considered
   further in Section 3.3.6.

   One motivation for restricting the NTLP to peer-relationship scope is
   that if there are any options or variants in design approach -- or,
   worse, in basic functionality -- it is easier to manage the resulting
   complexity if it only impacts direct peers rather than potentially
   the whole Internet.

3.2.3.  Bypassing Intermediate Nodes

   Because the NSIS problem includes multiple signaling applications, it
   is very likely that a particular NSLP will only be implemented on a
   subset of the NSIS-aware nodes on a path, as shown in Figure 5.  In
   addition, a node inside an aggregation region will still wish to
   ignore signaling messages that are per-flow, even if they are for a
   signaling application that the node is generally able to process.

               +------+    +------+    +------+    +------+
               |  NE  |    |  NE  |    |  NE  |    |  NE  |
               |+----+|    |      |    |+----+|    |+----+|
               ||NSLP||    |      |    ||NSLP||    ||NSLP||
               || 1  ||    |      |    || 2  ||    || 1  ||
               |+----+|    |      |    |+----+|    |+----+|
               |  ||  |    |      |    |      |    |  ||  |
               |+----+|    |+----+|    |+----+|    |+----+|
               |+----+|    |+----+|    |+----+|    |+----+|
               +------+    +------+    +------+    +------+

               Figure 5: Signaling with Heterogeneous NSLPs

   Where signaling messages traverse such NSIS-aware intermediate nodes,
   it is desirable to process them at the lowest level possible (in
   particular, on the fastest path).  In order to offer a non-trivial
   message transfer service (in terms of security, reliability and so
   on) to the peer NSLP nodes, it is important that the NTLP at
   intermediate nodes is as transparent as possible; that is, it carries
   out minimal processing.  In addition, if intermediate nodes have to
   do slow-path processing of all NSIS messages, this eliminates many of
   the scaling benefits of aggregation, unless tunneling is used.

   Considering first the case of messages sent with the router alert
   option, there are two complementary methods to achieve this bypassing
   of intermediate NEs:

   o  At the IP layer, a set of protocol numbers or a range of values in
      the router alert option can be used.  In this way, messages can be
      marked with an implied granularity, and routers can choose to
      apply further slow-path processing only to configured subsets of
      messages.  This is the method used in [10] to distinguish per-flow
      and per-aggregate signaling.

   o  The NTLP could process the message but determine that there was no
      local signaling application it was relevant to.  At this stage,
      the message can be returned unchanged to the IP layer for normal
      forwarding; the intermediate NE has effectively chosen to be
      transparent to the message in question.

   In both cases, the existence of the intermediate NE is totally hidden
   from the NSLP nodes.  If later stages of the signaling use directly
   addressed messages (e.g., for reverse routing), they will not involve
   the intermediate NE at all, except perhaps as a normal router.

   There may be cases where the intermediate NE would like to do some
   restricted protocol processing, such as the following:

   o  Translating addresses in message payloads (compare Section 4.6.1);
      note that this would have to be done to messages passing in both
      directions through a node.

   o  Updating signaling application payloads with local status
      information (e.g., path property measurement inside a domain).

   If this can be done without fully terminating the NSIS protocols, it
   would allow a more lightweight implementation of the intermediate NE,
   and a more direct 'end-to-end' NTLP association between the peer
   NSLPs where the signaling application is fully processed.  On the
   other hand, this is only possible with a limited class of possible
   NTLP designs, and makes it harder for the NTLP to offer a security
   service (since messages have to be partially protected).  The
   feasibility of this approach will be evaluated during the NTLP

3.2.4.  Core NSIS Transport Layer Functionality

   This section describes the basic functionality to be supported by the
   NTLP.  Note that the overall signaling solution will always be the
   result of joint operation of both the NTLP and the signaling layer
   protocols (NSLPs); for example, we can always assume that an NSLP is
   operating above the NTLP and taking care of end-to-end issues (e.g.,
   recovery of messages after restarts).

   Therefore, NTLP functionality is essentially just efficient upstream
   and downstream peer-peer message delivery, in a wide variety of
   network scenarios.  Message delivery includes the act of locating
   and/or selecting which NTLP peer to carry out signaling exchanges
   with for a specific data flow.  This discovery might be an active
   process (using specific signaling packets) or a passive process (a
   side effect of using a particular addressing mode).  In addition, it
   appears that the NTLP can sensibly carry out many of the functions
   that enable signaling messages to pass through middleboxes, since
   this is closely related to the problem of routing the signaling
   messages in the first place.  Further details about NTLP
   functionality are contained in Sections 3.2.5 and 4.3.

3.2.5.  State Management Functionality

   Internet signaling requires the existence and management of state
   within the network for several reasons.  This section describes how
   state management functionality is split across the NSIS layers.
   (Note that how the NTLP internal state is managed is a matter for its
   design and indeed implementation.)

   1.  Conceptually, the NTLP provides a uniform message delivery
       service.  It is unaware of the difference in state semantics
       between different types of signaling application messages (e.g.,
       whether a message changes, just refreshes signaling application
       state, or even has nothing to with signaling application state at

   2.  An NTLP instance processes and, if necessary, forwards all
       signaling application messages "immediately".  (It might offer
       different service classes, but these would be distinguished by,
       for example, reliability or priority, not by state aspects.)
       This means that the NTLP does not know explicit timer or message
       sequence information for the signaling application; and that
       signaling application messages pass immediately through an
       NSLP-unaware node.  (Their timing cannot be jittered there, nor
       can messages be stored up to be re-sent on a new path in case of
       a later re-routing event.)

   3.  Within any node, it is an implementation decision whether to
       generate/jitter/filter refreshes separately within each signaling
       application that needs this functionality, or to integrate it
       with the NTLP implementation as a generic "soft-state management
       toolbox".  The choice doesn't affect the NTLP specification at
       all.  Implementations might piggyback NTLP soft-state refresh
       information (if the NTLP works this way) on signaling application
       messages, or they might even combine soft-state management
       between layers.  The state machines of the NTLP and NSLPs remain
       logically independent, but an implementation is free to allow
       them to interact to reduce the load on the network to the same
       level that would be achieved by a monolithic model.

   4.  It may be helpful for signaling applications to receive
       state-management related 'triggers' from the NTLP indicating that
       a peer has failed or become available ("down/up notifications").
       These triggers would be about adjacent NTLP peers, rather than
       signaling application peers.  We can consider this another case
       of route change detection/notification (which the NTLP is also
       allowed to do anyway).  However, apart from generating such

       triggers, the NTLP takes no action itself on such events, other
       than to ensure that subsequent signaling messages are routed

   5.  The existence of these triggers doesn't replace NSLP refreshes as
       the mechanism for maintaining liveness at the signaling
       application level.  In this sense, up/down notifications are
       advisories that allow faster reaction to events in the network,
       but that shouldn't be built into NSLP semantics.  (This is
       essentially the same distinction, with the same rationale, that
       SNMP makes between notifications and normal message exchanges.)

3.2.6.  Path-Decoupled Operation

   Path-decoupled signaling is defined as signaling for state
   installation along the data path, without the restriction of passing
   only through nodes that are located on the data path.  Signaling
   messages can be routed to nodes that are off the data path, but that
   are (presumably) aware of it.  This allows a looser coupling between
   signaling and data plane nodes (e.g., at the autonomous system
   level).  Although support for path-decoupled operation is not one of
   the initial goals of the NSIS work, this section is included for
   completeness and to capture some initial considerations for future

   The main advantages of path-decoupled signaling are ease of
   deployment and support of additional functionality.  The ease of
   deployment comes from a restriction of the number of impacted nodes
   in case of deployment and/or upgrade of an NSLP.  Path-decoupled
   signaling would allow, for instance, deploying a solution without
   upgrading any of the routers in the data plane.  Additional
   functionality that can be supported includes the use of off-path
   proxies to support authorization or accounting architectures.

   There are potentially significant differences in the way that the two
   signaling paradigms should be analyzed.  Using a single centralized
   off-path NE may increase the requirements in terms of message
   handling; on the other hand, path-decoupled signaling is equally
   applicable to distributed off-path entities.  Failure recovery
   scenarios need to be analyzed differently because fate-sharing
   between data and control planes can no longer be assumed.
   Furthermore, the interpretation of sender/receiver orientation
   becomes less natural.  With the local operation of the NTLP, the
   impact of path-decoupled signaling on the routing of signaling
   messages is presumably restricted to the problem of peer
   determination.  The assumption that the off-path NSIS nodes are
   loosely tied to the data path suggests, however, that peer
   determination can still be based on L3 routing information.  This

   means that a path-decoupled signaling solution could be implemented
   using a lower-layer protocol presenting the same service interface to
   NSLPs as the path-coupled NTLP.  A new message transport protocol
   (possibly derived from the path-coupled NTLP) would be needed, but
   NSLP specifications and the inter-layer interaction would be
   unchanged from the path-coupled case.

3.3.  Signaling Application Properties

   It is clear that many signaling applications will require specific
   protocol behavior in their NSLP.  This section outlines some of the
   options for NSLP behavior; further work on selecting from these
   options would depend on detailed analysis of the signaling
   application in question.

3.3.1.  Sender/Receiver Orientation

   In some signaling applications, a node at one end of the data flow
   takes responsibility for requesting special treatment (such as a
   resource reservation) from the network.  Which end may depend on the
   signaling application, or on characteristics of the network

   In a sender-initiated approach, the sender of the data flow requests
   and maintains the treatment for that flow.  In a receiver-initiated
   approach, the receiver of the data flow requests and maintains the
   treatment for that flow.  The NTLP itself has no freedom in this
   area: Next NTLP peers have to be discovered in the sender-to-receiver
   direction, but after that the default assumption is that signaling is
   possible both upstream and downstream (unless a signaling application
   specifically indicates that this is not required).  This implies that
   backward routing state must be maintained by the NTLP or that
   backward routing information must be available in the signaling

   The sender- and receiver-initiated approaches have several
   differences in their operational characteristics.  The main ones are
   as follows:

   o  In a receiver-initiated approach, the signaling messages traveling
      from the receiver to the sender must be backward routed such that
      they follow exactly the same path as was followed by the signaling
      messages belonging to the same flow traveling from the sender to
      the receiver.  In a sender-initiated approach, provided that
      acknowledgements and notifications can be delivered securely to
      the sending node, backward routing is not necessary, and nodes do
      not have to maintain backward routing state.

   o  In a sender-initiated approach, a mobile node can initiate a
      reservation for its outgoing flows as soon as it has moved to
      another roaming subnetwork.  In a receiver-initiated approach, a
      mobile node has to inform the receiver about its handover, thus
      allowing the receiver to initiate a reservation for these flows.
      For incoming flows, the reverse argument applies.

   o  In general, setup and modification will be fastest if the node
      responsible for authorizing these actions can initiate them
      directly within the NSLP.  A mismatch between authorizing and
      initiating NEs will cause additional message exchanges, either in
      the NSLP or in a protocol executed prior to NSIS invocation.
      Depending on how the authorization for a particular signaling
      application is done, this may favor either sender- or receiver-
      initiated signaling.  Note that this may complicate modification
      of network control state for existing flows.

3.3.2.  Uni- and Bi-Directional Operation

   For some signaling applications and scenarios, signaling may only be
   considered for a unidirectional data flow.  However, in other cases,
   there may be interesting relationships in the signaling between the
   two flows of a bi-directional session; an example is QoS for a voice
   call.  Note that the path in the two directions may differ due to
   asymmetric routing.  In the basic case, bi-directional signaling can
   simply use a separate instance of the same signaling mechanism in
   each direction.

   In constrained topologies where parts of the route are symmetric, it
   may be possible to use a more unified approach to bi-directional
   signaling; e.g., carrying the two signaling directions in common
   messages.  This optimization might be used for example to make mobile
   QoS signaling more efficient.

   In either case, the correlation of the signaling for the two flow
   directions is carried out in the NSLP.  The NTLP would simply be
   enabled to bundle the messages together.

3.3.3.  Heterogeneous Operation

   It is likely that the appropriate way to describe the state for which
   NSIS is signaling will vary from one part of the network to another
   (depending on the signaling application).  For example, in the QoS
   case, resource descriptions that are valid for inter-domain links
   will probably be different from those useful for intra-domain
   operation (and the latter will differ from one domain to another).

   One way to address this issue is to consider the state description
   used within the NSLP as carried in globally-understood objects and
   locally-understood objects.  The local objects are only applicable
   for intra-domain signaling, while the global objects are mainly used
   in inter-domain signaling.  Note that the local objects are still
   part of the protocol but are inserted, used, and removed by one
   single domain.

   The purpose of this division is to provide additional flexibility in
   defining the objects carried by the NSLP such that only the objects
   applicable in a particular setting are used.  One approach for
   reflecting the distinction is that local objects could be put into
   separate local messages that are initiated and terminated within one
   single domain; an alternative is that they could be "stacked" within
   the NSLP messages that are used anyway for inter-domain signaling.

3.3.4.  Aggregation

   It is a well-known problem that per-flow signaling in large-scale
   networks presents scaling challenges because of the large number of
   flows that may traverse individual nodes.

   The possibilities for aggregation at the level of the NTLP are quite
   limited; the primary scaling approach for path-coupled signaling is
   for a signaling application to group flows together and to perform
   signaling for the aggregate, rather than for the flows individually.
   The aggregate may be created in a number of ways; for example, the
   individual flows may be sent down a tunnel, or given a common
   Differentiated Services Code Point (DSCP) marking.  The aggregation
   and de-aggregation points perform per flow signaling, but nodes
   within the aggregation region should only be forced to process
   signaling messages for the aggregate.  This depends on the ability of
   the interior nodes to ignore the per-flow signaling as discussed in
   Section 3.2.3.

   Individual NSLPs will need to specify what aggregation means in their
   context, and how it should be performed.  For example, in the QoS
   context it is possible to add together the resources specified in a
   number of separate reservations.  In the case of other applications,
   such as signaling to NATs and firewalls, the feasibility (and even
   the meaning) of aggregation is less clear.

3.3.5.  Peer-Peer and End-End Relationships

   The assumption in this framework is that the NTLP will operate
   'locally'; that is, just over the scope of a single peer
   relationship.  End-to-end operation is built up by concatenating
   these relationships.  Non-local operation (if any) will take place in

   The peering relations may also have an impact on the required amount
   of state at each NSIS entity.  When direct interaction with remote
   peers is not allowed, it may be required to keep track of the path
   that a message has followed through the network.  This could be
   achieved by keeping per-flow state at the NSIS entities, as is done
   in RSVP.  Another approach would be to maintain a record route object
   in the messages; this object would be carried within the NSIS
   protocols, rather than depend on the route-recording functionality
   provided by the IP layer.

3.3.6.  Acknowledgements and Notifications

   We are assuming that the NTLP provides a simple message transfer
   service, and that any acknowledgements or notifications it generates
   are handled purely internally (and apply within the scope of a single
   NTLP peer relationship).

   However, we expect that some signaling applications will require
   acknowledgements regarding the failure/success of state installation
   along the data path, and this will be an NSLP function.

   Acknowledgements can be sent along the sequence of NTLP peer
   relationships towards the signaling initiator, which relieves the
   requirements on the security associations that need to be maintained
   by NEs and that can allow NAT traversal in both directions.  (If this
   direction is towards the sender, it implies maintaining reverse
   routing state in the NTLP.)  In certain circumstances (e.g., trusted
   domains), an optimization could be to send acknowledgements directly
   to the signaling initiator outside the NTLP (see Section 3.2.2),
   although any such approach would have to take into account the
   necessity of handling denial of service attacks launched from outside
   the network.

   The semantics of the acknowledgement messages are of particular
   importance.  An NE sending a message could assume responsibility for
   the entire downstream chain of NEs, indicating (for instance) the
   availability of reserved resources for the entire downstream path.
   Alternatively, the message could have a more local meaning,
   indicating (for instance) that a certain failure or degradation
   occurred at a particular point in the network.

   Notifications differ from acknowledgements because they are not
   (necessarily) generated in response to other signaling messages.
   This means that it may not be obvious how to determine where the
   notification should be sent.  Other than that, the same
   considerations apply as for acknowledgements.  One useful distinction
   to make would be to differentiate between notifications that trigger
   a signaling action and others that don't.  The security requirements
   for the latter are less stringent, which means they could be sent
   directly to the NE they are destined for (provided that this NE can
   be determined).

3.3.7.  Security and Other AAA Issues

   In some cases, it will be possible to achieve the necessary level of
   signaling security by using basic 'channel security' mechanisms [11]
   at the level of the NTLP, and the possibilities are described in
   Section 4.7.  In other cases, signaling applications may have
   specific security requirements, in which case they are free to invoke
   their own authentication and key exchange mechanisms and to apply
   'object security' to specific fields within the NSLP messages.

   In addition to authentication, the authorization (to manipulate
   network control state) has to be considered as functionality above
   the NTLP level, since it will be entirely application specific.
   Indeed, authorization decisions may be handed off to a third party in
   the protocol (e.g., for QoS, the resource management function as
   described in Section 6.1.4).  Many different authorization models are
   possible, and the variations impact:

   o  what message flows take place -- for example, whether
      authorization information is carried along with a control state
      modification request or is sent in the reverse direction in
      response to it;

   o  what administrative relationships are required -- for example,
      whether authorization takes place only between peer signaling
      applications, or over longer distances.

   Because the NTLP operates only between adjacent peers and places no
   constraints on the direction or order in which signaling applications
   can send messages, these authorization aspects are left open to be
   defined by each NSLP.  Further background discussion of this issue is
   contained in [12].

4.  The NSIS Transport Layer Protocol

   This section describes the overall functionality required from the
   NTLP.  It mentions possible protocol components within the NTLP layer
   and the different possible addressing modes that can be utilized, as
   well as the assumed transport and state management functionality.
   The interfaces between NTLP and the layers above and below it are
   identified, with a description of the identity elements that appear
   on these interfaces.

   This discussion is not intended to design the NTLP or even to
   enumerate design options, although some are included as examples.
   The goal is to provide a general discussion of required functionality
   and to highlight some of the issues associated with this.

4.1.  Internal Protocol Components

   The NTLP includes all functionality below the signaling application
   layer and above the IP layer.  The functionality that is required
   within the NTLP is outlined in Section 3.2.4, with some more details
   in Sections 3.2.5 and 4.3.

   Some NTLP functionality could be provided via components operating as
   sublayers within the NTLP design.  For example, if specific transport
   capabilities are required (such as congestion avoidance,
   retransmission, and security), then existing protocols (such as
   TCP+TLS or DCCP+IPsec) could be incorporated into the NTLP.  This
   possibility is not required or excluded by this framework.

   If peer-peer addressing (Section 4.2) is used for some messages, then
   active next-peer discovery functionality will be required within the
   NTLP to support the explicit addressing of these messages.  This
   could use message exchanges for dynamic peer discovery as a sublayer
   within the NTLP; there could also be an interface to external
   mechanisms to carry out this function.

                ====================      ===========================
             ^  +------------------+      +-------------------------+
             |  |                  |      | NSIS Specific Functions |
             |  |                  |      |            .............|
      NSIS   |  |    Monolithic    |      |+----------+.   Peer    .|
   Transport |  |     Protocol     |      || Existing |. Discovery .|
     Layer   |  |                  |      || Protocol |.  Aspects  .|
             |  |                  |      |+----------+.............|
             V  +------------------+      +-------------------------+
                ====================      ===========================

                   Figure 6: Options for NTLP Structure

4.2.  Addressing

   There are two ways to address a signaling message being transmitted
   between NTLP peers:

   o  peer-peer, where the message is addressed to a neighboring NSIS
      entity that is known to be closer to the destination NE.

   o  end-to-end, where the message is addressed to the flow destination
      directly and intercepted by an intervening NE.

   With peer-peer addressing, an NE will determine the address of the
   next NE based on the payload of the message (and potentially on the
   previous NE).  This requires that the address of the destination NE
   be derivable from the information present in the payload, either by
   using some local routing table or through participation in active
   peer discovery message exchanges.  Peer-peer addressing inherently
   supports tunneling of messages between NEs, and is equally applicable
   to the path-coupled and path-decoupled cases.

   In the case of end-to-end addressing, the message is addressed to the
   data flow receiver, and (some of) the NEs along the data path
   intercept the messages.  The routing of the messages should follow
   exactly the same path as the associated data flow (but see
   Section 5.1.1 on this point).  Note that securing messages sent this
   way raises some interesting security issues (these are discussed in
   [2]).  In addition, it is a matter of the protocol design what should
   be used as the source address of the message (the flow source or
   signaling source).

   It is not possible at this stage to mandate one addressing mode or
   the other.  Indeed, each is necessary for some aspects of NTLP
   operation: In particular, initial discovery of the next downstream
   peer will usually require end-to-end addressing, whereas reverse
   routing will always require peer-peer addressing.  For other message
   types, the choice is a matter of protocol design.  The mode used is
   not visible to the NSLP, and the information needed in each case is
   available from the flow identifier (Section 4.6.1) or locally stored
   NTLP state.

4.3.  Classical Transport Functions

   The NSIS signaling protocols are responsible for transporting
   (signaling) data around the network; in general, this requires
   functionality such as congestion management, reliability, and so on.
   This section discusses how much of this functionality should be
   provided within the NTLP.  It appears that this doesn't affect the
   basic way in which the NSLP/NTLP layers relate to each other (e.g.,

   in terms of the semantics of the inter-layer interaction); it is much
   more a question of the overall performance/complexity tradeoff
   implied by placing certain functions within each layer.

   Note that, per the discussion at the end of Section 3.2.3, there may
   be cases where intermediate nodes wish to modify messages in transit
   even though they do not perform full signaling application
   processing.  In this case, not all the following functionality would
   be invoked at every intermediate node.

   The following functionality is assumed to lie within the NTLP:

   1.  Bundling together of small messages (comparable to [13]) can be
       provided locally by the NTLP as an option, if desired; it doesn't
       affect the operation of the network elsewhere.  The NTLP should
       always support unbundling, to avoid the cost of negotiating the
       feature as an option.  (The related function of refresh
       summarization -- where objects in a refresh message are replaced
       with a reference to a previous message identifier -- is left to
       NSLPs, which can then do this in a way tuned to the state
       management requirements of the signaling application.  Additional
       transparent compression functionality could be added to the NTLP
       design later as a local option.)  Note that end-to-end addressed
       messages for different flows cannot be bundled safely unless the
       next node on the outgoing interface is known to be NSIS-aware.

   2.  When needed, message fragmentation should be provided by the
       NTLP.  The use of IP fragmentation for large messages may lead to
       reduced reliability and may be incompatible with some addressing
       schemes.  Therefore, this functionality should be provided within
       the NTLP as a service for NSLPs that generate large messages.
       How the NTLP determines and accommodates Maximum Transmission
       Unit (MTU) constraints is left as a matter of protocol design.
       To avoid imposing the cost of reassembly on intermediate nodes,
       the fragmentation scheme used should allow for the independent
       forwarding of individual fragments towards a node hosting an
       interested NSLP.

   3.  There can be significant benefits for signaling applications if
       state-changing messages are delivered reliably (as introduced in
       [13] for RSVP; see also the more general analysis of [14]).  This
       does not change any assumption about the use of soft-state by
       NSLPs to manage signaling application state, and it leaves the
       responsibility for detecting and recovering from application
       layer error conditions in the NSLP.  However, it means that such
       functionality does not need to be tuned to handle fast recovery
       from message loss due to congestion or corruption in the lower
       layers, and it also means that the NTLP can prevent the

       amplification of message loss rates caused by fragmentation.
       Reliable delivery functionality is invoked by the NSLP on a
       message-by-message basis and is always optional to use.

   4.  The NTLP should not allow signaling messages to cause congestion
       in the network (i.e., at the IP layer).  Congestion could be
       caused by retransmission of lost signaling packets or by upper
       layer actions (e.g., a flood of signaling updates to recover from
       a route change).  In some cases, it may be possible to engineer
       the network to ensure that signaling cannot overload it; in
       others, the NTLP would have to detect congestion and to adapt the
       rate at which it allows signaling messages to be transmitted.
       Principles of congestion control in Internet protocols are given
       in [15].  The NTLP may or may not be able to detect overload in
       the control plane itself (e.g., an NSLP-aware node several
       NTLP-hops away that cannot keep up with the incoming message
       rate) and indicate this as a flow-control condition to local
       signaling applications.  However, for both the congestion and
       overload cases, it is up to the signaling applications themselves
       to adapt their behavior accordingly.

4.4.  Lower Layer Interfaces

   The NTLP interacts with 'lower layers' of the protocol stack for the
   purposes of sending and receiving signaling messages.  This framework
   places the lower boundary of the NTLP at the IP layer.  The interface
   to the lower layer is therefore very simple:

   o  The NTLP sends raw IP packets

   o  The NTLP receives raw IP packets.  In the case of peer-peer
      addressing, they have been addressed directly to it.  In the case
      of end-to-end addressing, this will be achieved by intercepting
      packets that have been marked in some special way (by special
      protocol number or by some option interpreted within the IP layer,
      such as the router alert option).

   o  The NTLP receives indications from the IP layer (including local
      forwarding tables and routing protocol state) that provide some
      information about route changes and similar events (see
      Section 5.1).

   For correct message routing, the NTLP needs to have some information
   about link and IP layer configuration of the local networking stack.
   In general, it needs to know how to select the outgoing interface for
   a signaling message and where this must match the interface that will
   be used by the corresponding flow.  This might be as simple as just
   allowing the IP layer to handle the message using its own routing

   table.  There is no intention to do something different from IP
   routing (for end-to-end addressed messages); however, some hosts
   allow applications to bypass routing for their data flows, and the
   NTLP processing must account for this.  Further network layer
   information would be needed to handle scoped addresses (if such
   things ever exist).

   Configuration of lower-layer operation to handle flows in particular
   ways is handled by the signaling application.

4.5.  Upper Layer Services

   The NTLP offers transport-layer services to higher-layer signaling
   applications for two purposes: sending and receiving signaling
   messages, and exchanging control and feedback information.

   For sending and receiving messages, two basic control primitives are

   o  Send Message, to allow the signaling application to pass data to
      the NTLP for transport.

   o  Receive Message, to allow the NTLP to pass received data to the
      signaling application.

   The NTLP and signaling application may also want to exchange other
   control information, such as the following:

   o  Signaling application registration/de-registration, so that
      particular signaling application instances can register their
      presence with the transport layer.  This may also require some
      identifier to be agreed upon between the NTLP and signaling
      application to support the exchange of further control information
      and to allow the de-multiplexing of incoming data.

   o  NTLP configuration, allowing signaling applications to indicate
      what optional NTLP features they want to use, and to configure
      NTLP operation, such as controlling what transport layer state
      should be maintained.

   o  Error messages, to allow the NTLP to indicate error conditions to
      the signaling application, and vice versa.

   o  Feedback information, such as route change indications so that the
      signaling application can decide what action to take.

4.6.  Identity Elements

4.6.1.  Flow Identification

   The flow identification is a method of identifying a flow in a unique
   way.  All packets associated with the same flow will be identified by
   the same flow identifier.  The key aspect of the flow identifier is
   to provide enough information such that the signaling flow receives
   the same treatment along the data path as the actual data itself;
   i.e., consistent behavior is applied to the signaling and data flows
   by a NAT or policy-based forwarding engine.

   Information that could be used in flow identification may include:

   o  source IP address;

   o  destination IP address;

   o  protocol identifier and higher layer (port) addressing;

   o  flow label (typical for IPv6);

   o  SPI field for IPsec encapsulated data; and

   o  DSCP/TOS field.

   It is assumed that at most limited wildcarding on these identifiers
   is needed.

   We assume here that the flow identification is not hidden within the
   NSLP, but is explicitly part of the NTLP.  The justification for this
   is that being able to do NSIS processing, even at a node which was
   unaware of the specific signaling application (see Section 3.2.3)
   might be valuable.  An example scenario would be messages passing
   through an addressing boundary where the flow identification had to
   be re-written.

4.6.2.  Session Identification

   There are circumstances in which being able to refer to signaling
   application state independently of the underlying flow is important.
   For example, if the address of one of the flow endpoints changes due
   to a mobility event, it is desirable to be able to change the flow
   identifier without having to install a completely new reservation.
   The session identifier provides a method to correlate the signaling
   about the different flows with the same network control state.

   The session identifier is essentially a signaling application
   concept, since it is only used in non-trivial state management
   actions that are application specific.  However, we assume here that
   it should be visible within the NTLP.  This enables it to be used to
   control NTLP behavior; for example, by controlling how the transport
   layer should forward packets belonging to this session (as opposed to
   this signaling application).  In addition, the session identifier can
   be used by the NTLP to demultiplex received signaling messages
   between multiple instances of the same signaling application, if such
   an operational scenario is supported (see Section 4.6.3 for more
   information on signaling application identification).

   To be useful for mobility support, the session identifier should be
   globally unique, and it should not be modified end-to-end.  It is
   well known that it is practically impossible to generate identifiers
   in a way that guarantees this property; however, using a large random
   number makes it highly likely.  In any case, the NTLP ascribes no
   valuable semantics to the identifier (such as 'session ownership');
   this problem is left to the signaling application, which may be able
   to secure it to be used for this purpose.

4.6.3.  Signaling Application Identification

   Because the NTLP can be used to support several NSLP types, there is
   a need to identify which type a particular signaling message exchange
   is being used for.  This is to support:

   o  processing of incoming messages -- the NTLP should be able to
      demultiplex these towards the appropriate signaling applications;

   o  processing of general messages at an NSIS-aware intermediate node
      -- if the node does not handle the specific signaling application,
      it should be able to make a forwarding decision without having to
      parse upper-layer information.

   No position is taken on the form of the signaling application
   identifier, or even the structure of the signaling application
   'space': free-standing applications, potentially overlapping groups
   of capabilities, etc.  These details should not influence the rest of
   the NTLP design.

4.7.  Security Properties

   It is assumed that the only security service required within the NTLP
   is channel security.  Channel security requires a security
   association to be established between the signaling endpoints, which
   is carried out via some authentication and key management exchange.
   This functionality could be provided by reusing a standard protocol.

   In order to protect a particular signaling exchange, the NSIS entity
   needs to select the security association that it has in place with
   the next NSIS entity that will be receiving the signaling message.
   The ease of doing this depends on the addressing model in use by the
   NTLP (see Section 4.2).

   Channel security can provide many different types of protection to
   signaling exchanges, including integrity and replay protection and
   encryption.  It is not clear which of these is required at the NTLP
   layer, although most channel security mechanisms support them all.
   It is also not clear how tightly an NSLP can 'bind' to the channel
   security service provided by the NTLP.

   Channel security can also be applied to the signaling messages with
   differing granularity; i.e., all or parts of the signaling message
   may be protected.  For example, if the flow is traversing a NAT, only
   the parts of the message that do not need to be processed by the NAT
   should be protected.  (Alternatively, if the NAT takes part in NTLP
   security procedures, it only needs to be given access to the message
   fields containing addresses, often just the flow id.)  Which parts of
   the NTLP messages need protecting is an open question, as is what
   type of protection should be applied to each.

5.  Interactions with Other Protocols

5.1.  IP Routing Interactions

   The NTLP is responsible for determining the next node to be visited
   by the signaling protocol.  For path-coupled signaling, this next
   node should be one that will be visited by the data flow.  In
   practice, this peer discovery will be approximate, as any node could
   use any feature of the peer discovery packet to route it differently
   from the corresponding data flow packets.  Divergence between the
   data and signaling paths can occur due to load sharing or load
   balancing (Section 5.1.1).  An example specific to the case of QoS is
   given in Section 6.1.1.  Route changes cause a temporary divergence
   between the data path and the path on which signaling state has been
   installed.  The occurrence, detection, and impact of route changes is
   described in Section 5.1.2.  A description of this issue in the
   context of QoS is given in Section 6.1.2.

5.1.1.  Load Sharing and Policy-Based Forwarding

   Load sharing or load balancing is a network optimization technique
   that exploits the existence of multiple paths to the same destination
   in order to obtain benefits in terms of protection, resource
   efficiency, or network stability.  It has been proposed for a number
   of routing protocols, such as OSPF [16] and others.  In general, load
   sharing means that packet forwarding will take into account header
   fields in addition to the destination address; a general discussion
   of such techniques and the problems they cause is provided in [17].

   The significance of load sharing in the context of NSIS is that
   routing of signaling messages using end-to-end addressing does not
   guarantee that these messages will follow the data path.  Policy-
   based forwarding for data packets -- where the outgoing link is
   selected based on policy information about fields additional to the
   packet destination address -- has the same impact.  Signaling and
   data packets may diverge because of both of these techniques.

   If signaling packets are given source and destination addresses
   identical to data packets, signaling and data may still diverge
   because of layer-4 load balancing (based on protocol or port).  Such
   techniques would also cause ICMP errors to be misdirected to the
   source of the data because of source address spoofing.  If signaling
   packets are made identical in the complete 5-tuple, divergence may
   still occur because of the presence of router alert options.  The
   same ICMP misdirection applies, and it becomes difficult for the end
   systems to distinguish between data and signaling packets.  Finally,
   QoS routing techniques may base the routing decision on any field in
   the packet header (e.g., DSCP).

5.1.2.  Route Changes

   In a connectionless network, each packet is independently routed
   based on its header information.  Whenever a better route towards the
   destination becomes available, this route is installed in the
   forwarding table and will be used for all subsequent (data and
   signaling) packets.  This can cause a divergence between the path
   along which state has been installed and the path along which
   forwarding will actually take place.  The problem of route changes is
   reduced if route pinning is performed.  Route pinning refers to the
   independence of the path taken by certain data packets from
   reachability changes caused by routing updates from an Interior
   Gateway Protocol (OSPF, IS-IS) or an Exterior Gateway Protocol (BGP).
   Nothing about NSIS signaling prevents route pinning from being used
   as a network engineering technique, provided that it is done in a way

   that preserves the common routing of signaling and data.  However,
   even if route pinning is used, it cannot be depended on to prevent
   all route changes (for example, in the case of link failures).

   Handling route changes requires the presence of three processes in
   the signaling protocol:

   1.  route change detection

   2.  installation of state on the new path

   3.  removal of state on the old path

   Many route change detection methods can be used, some needing
   explicit protocol support, and some of which are implementation-
   internal.  They differ in their speed of reaction and in the types of
   change they can detect.  In rough order of increasing applicability,
   they can be summarized as follows:

   1.  monitoring changes in local forwarding table state

   2.  monitoring topology changes in a link-state routing protocol

   3.  inference from changes in data packet TTL

   4.  inference from loss of packet stream in a flow-aware router

   5.  inference from changes in signaling packet TTL

   6.  changed route of an end-to-end addressed signaling packet

   7.  changed route of a specific end-to-end addressed probe packet

   These methods can be categorized as being based on network monitoring
   (methods 1-2), on data packet monitoring (methods 3-4) and on
   monitoring signaling protocol messages (methods 5-7); method 6 is the
   baseline method of RSVP.  The network monitoring methods can only
   detect local changes; in particular, method 1 can only detect an
   event that changes the immediate next downstream hop, and method 2
   can only detect changes within the scope of the link-state protocol.
   Methods 5-7, which are contingent on monitoring signaling messages,
   become less effective as soft-state refresh rates are reduced.

   When a route change has been detected, it is important that state is
   installed as quickly as possible along the new path.  It is not
   guaranteed that the new path will be able to provide the same
   characteristics that were available on the old path.  To avoid
   duplicate state installation or, worse, rejection of the signaling

   message because of previously installed state, it is important to be
   able to recognize the new signaling message as belonging to an
   existing session.  In this respect, we distinguish between route
   changes with associated change of the flow identification (e.g., in
   case of a mobility event when the IP source might change) and route
   changes without change of the flow identification (e.g., in case of a
   link failure along the path).  The former case requires an identifier
   independent from the flow identification; i.e., the session
   identifier (Section 4.6.2).  Mobility issues are discussed in more
   detail in Section 5.2.

   When state has been installed along the new path, the existing state
   on the old path needs to be removed.  With the soft-state principle,
   this will happen automatically because of the lack of refresh
   messages.  Depending on the refresh timer, however, it may be
   required to tear down this state much faster (e.g., because it is
   tied to an accounting record).  In that case, the teardown message
   needs to be able to distinguish between the new path and the old

   In some environments, it is desirable to provide connectivity and
   per-flow or per-class state management with high-availability
   characteristics; i.e., with rapid transparent recovery, even in the
   presence of route changes.  This may require interactions with
   protocols that are used to manage the routing in this case, such as
   Virtual Router Redundancy Protocol (VRRP) [18].

   Our basic assumption about such interactions is that the NTLP would
   be responsible for detecting the route change and ensuring that
   signaling messages were re-routed consistently (in the same way as
   the data traffic).  However, further state re-synchronization
   (including failover between 'main' and 'standby' nodes in the high
   availability case) would be the responsibility of the signaling
   application and its NSLP, and would possibly be triggered by the

5.2.  Mobility and Multihoming Interactions

   The issues associated with mobility and multihoming are a
   generalization of the basic route change case of the previous
   section.  As well as the fact that packets for a given session are no
   longer traveling over a single topological path, the following extra
   considerations arise:

   1.  The use of IP-layer mobility and multihoming means that more than
       one IP source or destination address will be associated with a
       single session.  The same applies if application-layer solutions
       (e.g., SIP-based approaches) are used.

   2.  Mobile IP and associated protocols use some special
       encapsulations for some segments of the data path.

   3.  The double route may persist for some time in the network (e.g.,
       in the case of a 'make-before-break' handover being done by a
       multihomed host).

   4.  Conversely, the re-routing may be rapid and routine (unlike
       network-internal route changes), increasing the importance of
       rapid state release on old paths.

   The interactions between mobility and signaling have been extensively
   analyzed in recent years, primarily in the context of RSVP and Mobile
   IP interaction (e.g., the mobility discussion of [5]), but also in
   that of other types of network (e.g., [19]).  A general review of the
   fundamental interactions is given in [20], which provides further
   details on many of the subjects considered in this section.

   We assume that the signaling will refer to 'outer' IP headers when
   defining the flows it is controlling.  There are two main reasons for
   this.  The first is that the data plane will usually be unable to
   work in terms of anything else when implementing per-flow treatment
   (e.g., we cannot expect that a router will analyze inner headers to
   decide how to schedule packets).  The second reason is that we are
   implicitly relying on the security provided by the network
   infrastructure to ensure that the correct packets are given the
   special treatment being signaled for, and this is built on the
   relationship between packet source and destination addresses and
   network topology.  (This is essentially the same approach that is
   used as the basis of route optimization security in Mobile IPv6
   [21].)  The consequence of this assumption is that we see the packet
   streams to (or from) different addresses as different flows.  Where a
   flow is carried inside a tunnel, it is seen as a different flow
   again.  The encapsulation issues (point (2) above) are therefore to
   be handled the same way as other tunneling cases (Section 5.4).

   Therefore, the most critical aspect is that multiple flows are being
   used, and the signaling for them needs to be correlated.  This is the
   intended role of the session identifier (see Section 4.6.2, which
   also describes some of the security requirements for such an
   identifier).  Although the session identifier is visible at the NTLP,
   the signaling application is responsible for performing the
   correlation (and for doing so securely).  The NTLP responsibility is
   limited to delivering the signaling messages for each flow between
   the correct signaling application peers.  The locations at which the
   correlation takes place are the end system and the signaling-

   application-aware node in the network where the flows meet.  (This
   node is generally referred to as the "crossover router"; it can be
   anywhere in the network.)

   Although much work has been done in the past on finding the crossover
   router directly from information held in particular mobility
   signaling protocols, the initial focus of NSIS work should be a
   solution that is not tightly bound to any single mobility approach.
   In other words, it should be possible to determine the crossover
   router based on NSIS signaling.  (This doesn't rule out the
   possibility that some implementations may be able to do this
   discovery faster; e.g., by being tightly integrated with local
   mobility management protocols.  This is directly comparable to
   spotting route changes in fixed networks by being routing aware.)

   Note that the crossover router discovery may involve end-to-end
   signaling exchanges (especially for flows towards the mobile or
   multihomed node), which raises a latency concern.  On the other hand,
   end-to-end signaling will have been necessary in any case, at the
   application level not only to communicate changed addresses, but also
   to update packet classifiers along the path.  It is a matter for
   further analysis to decide how these exchanges could be combined or
   carried out in parallel.

   On the shared part of the path, signaling is needed at least to
   update the packet classifiers to include the new flow, although if
   correlation with the existing flow is possible it should be possible
   to bypass any policy or admission control processing.  State
   installation on the new path (and possibly release on the old one)
   are also required.  Which entity (one of the end hosts or the
   crossover router) controls all these procedures depends on which
   entities are authorized to carry out network state manipulations, so
   this is therefore a matter of signaling application and NSLP design.
   The approach may depend on the sender/receiver orientation of the
   original signaling (see Section 3.3.1).  In addition, in the mobility
   case, the old path may no longer be directly accessible to the mobile
   node; inter-access-router communication may be required to release
   state in these circumstances.

   The frequency of handovers in some network types makes fast handover
   support protocols desirable, for selecting the optimal access router
   for handover (for example, [22]), and for transferring state
   information to avoid having to regenerate it in the new access router
   after handover (for example, [23]).  Both of these procedures could
   have strong interactions with signaling protocols.  The access router
   selection might depend on the network control state that could be

   supported on the path through the new access router.  Transfer of
   signaling application state or NTLP/NSLP protocol state may be a
   candidate for context transfer.

5.3.  Interactions with NATs

   Because at least some messages will almost inevitably contain
   addresses and possibly higher-layer information as payload, we must
   consider the interaction with address translation devices (NATs).
   These considerations apply both to 'traditional' NATs of various
   types (as defined in [24]) as well as some IPv4/v6 transition
   mechanisms, such as Stateless IP/ICMP Translation (SIIT) [25].

   In the simplest case of an NSIS-unaware NAT in the path, payloads
   will be uncorrected, and signaling will refer to the flow
   incorrectly.  Applications could attempt to use STUN [26] or similar
   techniques to detect and recover from the presence of the NAT.  Even
   then, NSIS protocols would have to use a well-known encapsulation
   (TCP/UDP/ICMP) to avoid being dropped by more cautious low-end NAT

   A simple 'NSIS-aware' NAT would require flow identification
   information to be in the clear and not to be integrity protected.  An
   alternative conceptual approach is to consider the NAT functionality
   part of message processing itself, in which case the translating node
   can take part natively in any NSIS protocol security mechanisms.
   Depending on NSIS protocol layering, it would be possible for this
   processing to be done in an NSIS entity that was otherwise ignorant
   of any particular signaling applications.  This is the motivation for
   including basic flow identification information in the NTLP
   (Section 4.6.1).

   Note that all of this discussion is independent of the use of a
   specific NSLP for general control of NATs (and firewalls).  That case
   is considered in Section 6.2.

5.4.  Interactions with IP Tunneling

   Tunneling is used in the Internet for a number of reasons, such as
   flow aggregation, IPv4/6 transition mechanisms, mobile IP, virtual
   private networking, and so on.  An NSIS solution must continue to
   work in the presence of these techniques.  The presence of the tunnel
   should not cause problems for end-to-end signaling, and it should
   also be possible to use NSIS signaling to control the treatment of
   the packets carrying the tunneled data.

   It is assumed that the NSIS approach will be similar to that of [27],
   where the signaling for the end-to-end data flow is tunneled along
   with that data flow and is invisible to nodes along the path of the
   tunnel (other than the endpoints).  This provides backwards
   compatibility with networks where the tunnel endpoints do not support
   the NSIS protocols.  We assume that NEs will not unwrap tunnel
   encapsulations to find and process tunneled signaling messages.

   To signal for the packets carrying the tunneled data, the tunnel is
   considered a new data flow in its own right, and NSIS signaling is
   applied to it recursively.  This requires signaling support in at
   least one tunnel endpoint.  In some cases (where the signaling
   initiator is at the opposite end of the data flow from the tunnel
   initiator; i.e., in the case of receiver initiated signaling), the
   ability to provide a binding between the original flow identification
   and that for the tunneled flow is needed.  It is left open here
   whether this should be an NTLP or an NSLP function.

6.  Signaling Applications

   This section gives an overview of NSLPs for particular signaling
   applications.  The assumption is that the NSLP uses the generic
   functionality of the NTLP given earlier; this section describes
   specific aspects of NSLP operation.  It includes simple examples that
   are intended to clarify how NSLPs fit into the framework.  It does
   not replace or even form part of the formal NSLP protocol
   specifications; in particular, initial designs are being developed
   for NSLPs for resource reservation [28] and middlebox communication

6.1.  Signaling for Quality of Service

   In the case of signaling for QoS, all the basic NSIS concepts of
   Section 3.1 apply.  In addition, there is an assumed directionality
   of the signaling process, in that one end of the signaling flow takes
   responsibility for actually requesting the resource.  This leads to
   the following definitions:

   o  QoS NSIS Initiator (QNI): the signaling entity that makes the
      resource request, usually as a result of user application request.

   o  QoS NSIS Responder (QNR): the signaling entity that acts as the
      endpoint for the signaling and that can optionally interact with
      applications as well.

   o  QoS NSIS Forwarder (QNF): a signaling entity between a QNI and QNR
      that propagates NSIS signaling further through the network.

   Each of these entities will interact with a resource management
   function (RMF) that actually allocates network resources (router
   buffers, interface bandwidth, and so on).

   Note that there is no constraint on which end of the signaling flow
   should take the QNI role: With respect to the data flow direction, it
   could be at the sending or receiving end.

6.1.1.  Protocol Message Semantics

   The QoS NSLP will include a set of messages to carry out resource
   reservations along the signaling path.  A possible set of message
   semantics for the QoS NSLP is shown below.  Note that the 'direction'
   column in the table below only indicates the 'orientation' of the
   message.  Messages can be originated and absorbed at QNF nodes as
   well as the QNI or QNR; an example might be QNFs at the edge of a
   domain exchanging messages to set up resources for a flow across a
   it.  Note that it is left open if the responder can release or modify
   a reservation, during or after setup.  This seems mainly a matter of
   assumptions about authorization, and the possibilities might depend
   on resource type specifics.

   The table also explicitly includes a refresh operation.  This does
   nothing to a reservation except extend its lifetime, and it is one
   possible state management mechanism (see next section).

   | Operation | Direction |                 Operation                 |
   |  Request  |   I-->R   |    Create a new reservation for a flow    |
   |           |           |                                           |
   |   Modify  |   I-->R   |       Modify an existing reservation      |
   |           | (&R-->I?) |                                           |
   |           |           |                                           |
   |  Release  |   I-->R   |       Delete (tear down) an existing      |
   |           | (&R-->I?) |                reservation                |
   |           |           |                                           |
   |  Accept/  |   R-->I   |  Confirm (possibly modified?) or reject a |
   |   Reject  |           |            reservation request            |
   |           |           |                                           |
   |   Notify  |  I-->R &  |    Report an event detected within the    |
   |           |   R-->I   |                  network                  |
   |           |           |                                           |
   |  Refresh  |   I-->R   |    State management (see Section 6.1.2)   |

6.1.2.  State Management

   The primary purpose of NSIS is to manage state information along the
   path taken by a data flow.  The issues regarding state management
   within the NTLP (state related to message transport) are described in
   Section 4.  The QoS NSLP will typically have to handle additional
   state related to the desired resource reservation to be made.

   There two critical issues to be considered in building a robust NSLP
   to handle this problem:

   o  The protocol must be scalable.  It should allow minimization of
      the resource reservation state-storage demands that it implies for
      intermediate nodes; in particular, storage of state per 'micro'
      flow is likely to be impossible except at the very edge of the
      network.  A QoS signaling application might require per-flow or
      lower granularity state; examples of each for the case of QoS
      would be IntServ [30] or RMD [31] (per 'class' state),

   o  The protocol must be robust against failure and other conditions
      that imply that the stored resource reservation state has to be
      moved or removed.

   For resource reservations, soft-state management is typically used as
   a general robustness mechanism.  According to the discussion of
   Section 3.2.5, the soft-state protocol mechanisms are built into the
   NSLP for the specific signaling application that needs them; the NTLP
   sees this simply as a sequence of (presumably identical) messages.

6.1.3.  Route Changes and QoS Reservations

   In this section, we will explore the expected interaction between
   resource signaling and routing updates (the precise source of routing
   updates does not matter).  The normal operation of the NSIS protocol
   will lead to the situation depicted in Figure 7, where the reserved
   resources match the data path.

                   reserved +-----+  reserved  +-----+
                  =========>| QNF |===========>| QNF |
                            +-----+            +-----+
                                 data path

                 Figure 7: Normal NSIS Protocol Operation

   A route change can occur while such a reservation is in place.  The
   route change will be installed immediately, and any data will be
   forwarded on the new path.  This situation is depicted Figure 8.

   Resource reservation on the new path will only be started once the
   next control message is routed along the new path.  This means that
   there is a certain time interval during which resources are not
   reserved on (part of) the data path, and certain delay or
   drop-sensitive applications will require that this time interval be
   minimized.  Several techniques to achieve this could be considered.
   As an example, RSVP [7] has the concept of local repair, whereby the
   router may be triggered by a route change.  In that case, the RSVP
   node can start sending PATH messages directly after the route has
   been changed.  Note that this option may not be available if no
   per-flow state is kept in the QNF.  Another approach would be to
   pre-install backup state, and it would be the responsibility of the
   QoS-NSLP to do this.  However, mechanisms for identifying backup
   paths and routing the necessary signaling messages along them are not
   currently considered in the NSIS requirements and framework.

                              Route update
                       reserved +-----+  reserved  +-----+
                      =========>| QNF |===========>| QNF |
                                +-----+            +-----+
                       --------   ||
                               \  ||           +-----+
                                |  ===========>| QNF |
                                |              +-----+
                                  data path

                          Figure 8: Route Change

   The new path might not be able to provide the same guarantees that
   were available on the old path.  Therefore, it might be desirable for
   the QNF to wait until resources have been reserved on the new path
   before allowing the route change to be installed (unless, of course,
   the old path no longer exists).  However, delaying the route change
   installation while waiting for reservation setup needs careful
   analysis of the interaction with the routing protocol being used, in
   order to avoid routing loops.

   Another example related to route changes is denoted as severe
   congestion and is explained in [31].  This solution adapts to a route
   change when a route change creates congestion on the new routed path.

6.1.4.  Resource Management Interactions

   The QoS NSLP itself is not involved in any specific resource
   allocation or management techniques.  The definition of an NSLP for
   resource reservation with Quality of Service, however, implies the
   notion of admission control.  For a QoS NSLP, the measure of
   signaling success will be the ability to reserve resources from the
   total resource pool that is provisioned in the network.  We define
   the function responsible for allocating this resource pool as the
   Resource Management Function (RMF).  The RMF is responsible for all
   resource provisioning, monitoring, and assurance functions in the

   A QoS NSLP will rely on the RMF to do resource management and to
   provide inputs for admission control.  In this model, the RMF acts as
   a server towards client NSLP(s).  Note, however, that the RMF may in
   turn use another NSLP instance to do the actual resource provisioning
   in the network.  In this case, the RMF acts as the initiator (client)
   of an NSLP.

   This essentially corresponds to a multi-level signaling paradigm,
   with an 'upper' level handling internetworking QoS signaling
   (possibly running end-to-end), and a 'lower' level handling the more
   specialized intra-domain QoS signaling (running between just the
   edges of the network).  (See [10], [32], and [33] for a discussion of
   similar architectures.)  Given that NSIS signaling is already
   supposed to be able to support multiple instances of NSLPs for a
   given flow and limited scope (e.g., edge-to-edge) operation, it is
   not currently clear that supporting the multi-level model leads to
   any new protocol requirements for the QoS NSLP.

   The RMF may or may not be co-located with a QNF (note that
   co-location with a QNI/QNR can be handled logically as a combination
   between QNF and QNI/QNR).  To cater for both cases, we define a
   (possibly logical) QNF-RMF interface.  Over this interface,
   information may be provided from the RMF about monitoring, resource
   availability, topology, and configuration.  In the other direction,
   the interface may be used to trigger requests for resource
   provisioning.  One way to formalize the interface between the QNF and
   the RMF is via a Service Level Agreement (SLA).  The SLA may be
   static or it may be dynamically updated by means of a negotiation
   protocol.  Such a protocol is outside the scope of NSIS.

   There is no assumed restriction on the placement of the RMF.  It may
   be a centralized RMF per domain, several off-path distributed RMFs,
   or an on-path RMF per router.  The advantages and disadvantages of
   both approaches are well-known.  Centralization typically allows
   decisions to be taken using more global information, with more

   efficient resource utilization as a result.  It also facilitates
   deployment or upgrade of policies.  Distribution allows local
   decision processes and rapid response to data path changes.

6.2.  Other Signaling Applications

   As well as the use for 'traditional' QoS signaling, it should be
   possible to develop NSLPs for other signaling applications that
   operate on different types of network control state.  One specific
   case is setting up flow-related state in middleboxes (firewalls,
   NATs, and so on).  Requirements for such communication are given in
   [4].  Other examples include network monitoring and testing, and
   tunnel endpoint discovery.

7.  Security Considerations

   This document describes a framework for signaling protocols that
   assumes a two-layer decomposition, with a common lower layer (NTLP)
   supporting a family of signaling-application-specific upper-layer
   protocols (NSLPs).  The overall security considerations for the
   signaling therefore depend on the joint security properties assumed
   or demanded for each layer.

   Security for the NTLP is discussed in Section 4.7.  We have assumed
   that, apart from being resistant to denial of service attacks against
   itself, the main role of the NTLP will be to provide message
   protection over the scope of a single peer relationship, between
   adjacent signaling application entities.  (See Section 3.2.3 for a
   discussion of the case where these entities are separated by more
   than one NTLP hop.)  These functions can ideally be provided by an
   existing channel security mechanism, preferably using an external key
   management mechanism based on mutual authentication.  Examples of
   possible mechanisms are TLS, IPsec and SSH.  However, there are
   interactions between the actual choice of security protocol and the
   rest of the NTLP design.  Primarily, most existing channel security
   mechanisms require explicit identification of the peers involved at
   the network and/or transport level.  This conflicts with those
   aspects of path-coupled signaling operation (e.g., discovery) where
   this information is not even implicitly available because peer
   identities are unknown; the impact of this 'next-hop problem' on RSVP
   design is discussed in the security properties document [6] and also
   influences many parts of the threat analysis [2].  Therefore, this
   framework does not mandate the use of any specific channel security
   protocol; instead, this has to be integrated with the design of the
   NTLP as a whole.

   Security for the NSLPs is entirely dependent on signaling application
   requirements.  In some cases, no additional protection may be
   required compared to what is provided by the NTLP.  In other cases,
   more sophisticated object-level protection and the use of public-
   key-based solutions may be required.  In addition, the NSLP needs to
   consider the authorization requirements of the signaling application.
   Authorization is a complex topic, for which a very brief overview is
   provided in Section 3.3.7.

   Another factor is that NTLP security mechanisms operate only locally,
   whereas NSLP mechanisms may also need to operate over larger regions
   (not just between adjacent peers), especially for authorization
   aspects.  This complicates the analysis of basing signaling
   application security on NTLP protection.

   An additional concern for signaling applications is the session
   identifier security issue (Sections 4.6.2 and 5.2).  The purpose of
   this identifier is to decouple session identification (as a handle
   for network control state) from session "location" (i.e., the data
   flow endpoints).  The identifier/locator distinction has been
   extensively discussed in the user plane for end-to-end data flows,
   and is known to lead to non-trivial security issues in binding the
   two together again.  Our problem is the analogue in the control
   plane, and is at least similarly complex, because of the need to
   involve nodes in the interior of the network as well.

   Further work on this and other security design will depend on a
   refinement of the NSIS threats work begun in [2].

8.  References

8.1.  Normative References

   [1]   Brunner, M., "Requirements for Signaling Protocols", RFC 3726,
         April 2004.

   [2]   Tschofenig, H. and D. Kroeselberg, "Security Threats for Next
         Steps in Signaling (NSIS)", RFC 4081, June 2005.

   [3]   Chaskar, H., "Requirements of a Quality of Service (QoS)
         Solution for Mobile IP", RFC 3583, September 2003.

   [4]   Swale, R., Mart, P., Sijben, P., Brim, S., and M. Shore,
         "Middlebox Communications (midcom) Protocol Requirements",
         RFC 3304, August 2002.

8.2.  Informative References

   [5]   Manner, J. and X. Fu, "Analysis of Existing Quality of Service
         Signaling Protocols", Work in Progress, December 2004.

   [6]   Tschofenig, H., "RSVP Security Properties", Work in Progress,
         February 2005.

   [7]   Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
         "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
         Specification", RFC 2205, September 1997.

   [8]   Katz, D., "IP Router Alert Option", RFC 2113, February 1997.

   [9]   Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
         RFC 2711, October 1999.

   [10]  Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
         "Aggregation of RSVP for IPv4 and IPv6 Reservations", RFC 3175,
         September 2001.

   [11]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
         Security Considerations", BCP 72, RFC 3552, July 2003.

   [12]  Tschofenig, H., "NSIS Authentication, Authorization and
         Accounting Issues", Work in Progress, March 2003.

   [13]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F., and S.
         Molendini, "RSVP Refresh Overhead Reduction Extensions",
         RFC 2961, April 2001.

   [14]  Ji, P., Ge, Z., Kurose, J., and D. Townsley, "A Comparison of
         Hard-State and Soft-State Signaling Protocols", Computer
         Communication Review, Volume 33, Number 4, October 2003.

   [15]  Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914,
         September 2000.

   [16]  Apostolopoulos, G., Kamat, S., Williams, D., Guerin, R., Orda,
         A., and T. Przygienda, "QoS Routing Mechanisms and OSPF
         Extensions", RFC 2676, August 1999.

   [17]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
         Multicast Next-Hop Selection", RFC 2991, November 2000.

   [18]  Hinden, R., "Virtual Router Redundancy Protocol (VRRP)", RFC
         3768, April 2004.

   [19]  Heijenk, G., Karagiannis, G., Rexhepi, V., and L. Westberg,
         "DiffServ Resource Management in IP-based Radio Access
         Networks", Proceedings of 4th International Symposium on
         Wireless Personal Multimedia Communications WPMC'01, September
         9 - 12 2001.

   [20]  Manner, J., Lopez, A., Mihailovic, A., Velayos, H., Hepworth,
         E., and Y. Khouaja, "Evaluation of Mobility and QoS
         Interaction", Computer Networks Volume 38, Issue 2, p. 137-163,
         5 February 2002.

   [21]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
         IPv6", RFC 3775, June 2004.

   [22]  Liebsch, M., Ed., Singh, A., Ed., Chaskar, H., Funato, D., and
         E. Shim, "Candidate Access Router Discovery (CARD)", Work in
         Progress, May 2005.

   [23]  Kempf, J., "Problem Description: Reasons For Performing Context
         Transfers Between Nodes in an IP Access Network", RFC 3374,
         September 2002.

   [24]  Srisuresh, P. and M. Holdrege, "IP Network Address Translator
         (NAT) Terminology and Considerations", RFC 2663, August 1999.

   [25]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)",
         RFC 2765, February 2000.

   [26]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
         - Simple Traversal of User Datagram Protocol (UDP) Through
         Network Address Translators (NATs)", RFC 3489, March 2003.

   [27]  Terzis, A., Krawczyk, J., Wroclawski, J., and L. Zhang, "RSVP
         Operation Over IP Tunnels", RFC 2746, January 2000.

   [28]  Bosch, S., Karagiannis, G., and A. McDonald, "NSLP for
         Quality-of-Service signaling", Work in Progress, February 2005.

   [29]  Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer Protocol
         (NSLP)", Work in Progress, February 2005.

   [30]  Braden, R., Clark, D., and S. Shenker, "Integrated Services in
         the Internet Architecture: an Overview", RFC 1633, June 1994.

   [31]  Westberg, L., Csaszar, A., Karagiannis, G., Marquetant, A.,
         Partain, D., Pop, O., Rexhepi, V., Szabo, R., and A. Takacs,
         "Resource Management in Diffserv (RMD): A Functionality and
         Performance Behavior Overview", Seventh International Workshop
         on Protocols for High-Speed networks PfHSN 2002, 22 - 24
         April 2002.

   [32]  Ferrari, D., Banerjea, A., and H. Zhang, "Network Support for
         Multimedia - A Discussion of the Tenet Approach",
         Berkeley TR-92-072, November 1992.

   [33]  Nichols, K., Jacobson, V., and L. Zhang, "A Two-bit
         Differentiated Services Architecture for the Internet",
         RFC 2638, July 1999.

Appendix A.  Contributors

   Several parts of the introductory sections of this document (in
   particular, in Sections 3.1 and 3.3) are based on contributions from
   Ilya Freytsis, then of Cetacean Networks, Inc.

   Bob Braden originally proposed "A Two-Level Architecture for Internet
   Signalling" as an Internet-Draft in November 2001.  This document
   served as an important starting point for the framework discussed
   herein, and the authors owe a debt of gratitude to Bob for this

Appendix B.  Acknowledgements

   The authors would like to thank Bob Braden, Maarten Buchli, Eleanor
   Hepworth, Andrew McDonald, Melinda Shore, and Hannes Tschofenig for
   significant contributions in particular areas of this document.  In
   addition, the authors would like to acknowledge Cedric Aoun, Attila
   Bader, Anders Bergsten, Roland Bless, Marcus Brunner, Louise Burness,
   Xiaoming Fu, Ruediger Geib, Danny Goderis, Kim Hui, Cornelia Kappler,
   Sung Hycuk Lee, Thanh Tra Luu, Mac McTiffin, Paulo Mendes, Hans De
   Neve, Ping Pan, David Partain, Vlora Rexhepi, Henning Schulzrinne,
   Tom Taylor, Michael Thomas, Daniel Warren, Michael Welzl, Lars
   Westberg, and Lixia Zhang for insights and inputs during this and
   previous framework activities.  Dave Oran, Michael Richardson, and
   Alex Zinin provided valuable comments during the final review stages.

Authors' Addresses

   Robert Hancock
   Siemens/Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN

   EMail: robert.hancock@roke.co.uk

   Georgios Karagiannis
   University of Twente
   P.O. BOX 217
   7500 AE Enschede
   The Netherlands

   EMail: g.karagiannis@ewi.utwente.nl

   John Loughney
   Nokia Research Center
   11-13 Itamerenkatu
   Helsinki  00180

   EMail: john.loughney@nokia.com

   Sven Van den Bosch
   Francis Wellesplein 1
   B-2018 Antwerpen

   EMail: sven.van_den_bosch@alcatel.be

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