Network Working Group
   Internet Draft                                   R. Hancock (editor)
                                            Siemens/Roke Manor Research
                                                            I. Freytsis
                                                      Cetacean Networks
                                                         G. Karagiannis
                                          University of Twente/Ericsson
                                                            J. Loughney
                                                       S. Van den Bosch
   Document: draft-ietf-nsis-fw-05.txt
   Expires: April 2004                                     October 2003

                    Next Steps in Signaling: Framework

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   The list of Internet-Draft Shadow Directories can be accessed at


   The Next Steps in Signaling working group is considering protocols
   for signaling information about a data flow along its path in the
   network. Based on existing work on signaling requirements, this
   document proposes an architectural framework for such signaling

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

Hancock et al.           Expires - April 2004                 [Page 1]

                  Next Steps in Signaling: Framework      October 2003

   layers for each specific signaling application. An initial proposal
   for the split between these layers is given, describing the overall
   functionality of the lower layer, and discussing the ways that upper
   layer behavior can be adapted to specific signaling application

   This framework also considers the general interactions between
   signaling and other network layer functions, specifically routing,
   mobility, and address translators. The different events that impact
   signaling operation are described, along with how their handling
   should be divided between the generic and application-specific
   layers. Finally, an example signaling application (for Quality of
   Service) is described in more detail.

Table of Contents

   1 Introduction ...................................................3
     1.1   Definition of the Signaling Problem ......................3
     1.2   Scope and Structure of the NSIS Framework ................4
   2 Terminology ....................................................5
   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 NTLP Functionality ...............................15
     3.2.5   State Management Functionality ........................15
     3.2.6   Path De-Coupled Operation .............................16
     3.3   Signaling Application Properties ........................17
     3.3.1   Sender/Receiver Orientation ...........................17
     3.3.2   Uni- and Bi-Directional Operation .....................18
     3.3.3   Heterogeneous Operation ...............................19
     3.3.4   Aggregation ...........................................19
     3.3.5   Peer-Peer and End-End Relationships ...................20
     3.3.6   Acknowledgements and Notifications ....................20
     3.3.7   Security and Other AAA Issues .........................21
   4 The NSIS Transport Layer Protocol .............................21
     4.1   Internal Protocol Components ............................22
     4.2   Addressing ..............................................22
     4.3   Classical Transport Functions ...........................23
     4.4   Lower Layer Interfaces ..................................25
     4.5   Upper Layer Services ....................................25

Hancock et al.           Expires - April 2004                 [Page 2]

                  Next Steps in Signaling: Framework      October 2003

     4.6   Identity Elements .......................................26
     4.6.1   Flow Identification ...................................26
     4.6.2   Session Identification ................................27
     4.6.3   Signaling Application Identification ..................27
     4.7   Security Properties .....................................28
   5 Interactions with Other Protocols .............................28
     5.1   IP Routing Interactions .................................28
     5.1.1   Load Sharing and Policy-Based Forwarding ..............29
     5.1.2   Route Changes .........................................29
     5.2   Mobility and Multihoming Interactions ...................31
     5.3   Interactions with NATs ..................................33
     5.4   Interactions with IP Tunneling ..........................34
   6 Signaling Applications ........................................35
     6.1   Signaling for Quality of Service ........................35
     6.1.1   Protocol Message Semantics ............................36
     6.1.2   State Management ......................................36
     6.1.3   Route Changes and QoS Reservations ....................37
     6.1.4   Resource Management Interactions ......................38
     6.2   Other Signaling Applications ............................39
   7 Security Considerations .......................................40
   Normative References.............................................41
   Informative References...........................................41
   Authors' Addresses...............................................44
   Intellectual Property Considerations.............................44
   Full Copyright Statement.........................................45

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,
   independent 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. Firstly, the
   intention is that components of the NSIS protocol suite will be

Hancock et al.           Expires - April 2004                 [Page 3]

                  Next Steps in Signaling: Framework      October 2003

   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).

   Secondly, 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]. 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 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 behavior.

   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

Hancock et al.           Expires - April 2004                 [Page 4]

                  Next Steps in Signaling: Framework      October 2003

   interact sensibly with network layer operations, specifically routing
   (and re-routing), IP mobility, and network address translation.

   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; it is not intended 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 which selects packets based on their contents
   according to defined rules.

   [Data] flow - a stream of packets from sender to receiver which 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 which form an
   administrative domain, excluding the edge nodes.

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

   NSIS 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 where 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.

Hancock et al.           Expires - April 2004                 [Page 5]

                  Next Steps in Signaling: Framework      October 2003

   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 which is receiving the data
   packets in a flow.

   Sender - the node in the network which is sending the data packets in
   a flow.

   Session - application layer flow of information for which some
   network control state information is to be manipulated or monitored
   (see section 4.6.2).

   Signaling application - the purpose of the NSIS signaling: a service
   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

Hancock et al.           Expires - April 2004                 [Page 6]

                  Next Steps in Signaling: Framework      October 2003

   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 which 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 we make no assumptions here 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
   nodes (NEs) that are in the data path. They do not have to reach all
   the nodes on the data path (for example, there could be proxies
   distinct from the sender and receiver as described in section 3.1.3,
   or intermediate signaling-unaware nodes); and 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) which are not assumed to be on the data path, but which are

Hancock et al.           Expires - April 2004                 [Page 7]

                  Next Steps in Signaling: Framework      October 2003

   (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.
   Amongst 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 nodes.

   While 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 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 the 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 as a kind of "proxy on the data path", see Figure 2.

Hancock et al.           Expires - April 2004                 [Page 8]

                  Next Steps in Signaling: Framework      October 2003

                 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 2 specific challenges for the signaling:
   *) A proxy that terminates signaling on behalf of the NSIS-unaware
   host (or part of the network) should be able to make determination
   that it is a last NSIS aware node along the path.
   *) 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 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 as a
   proxy at the signaling application level. This is a natural

Hancock et al.           Expires - April 2004                 [Page 9]

                  Next Steps in Signaling: Framework      October 2003

   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 interesting 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 generically 'network control state'.
   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 which 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 a (unidirectional) flow
   of information between two endpoints, for which some network state is
   to be allocated or monitored. (Note that this use of the term
   'session' is not identical to the usage in RSVP. It is closer to the
   session concept of, for example, the Session Initiation Protocol.)

   The simplest service provided by NSIS signaling protocols is
   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

Hancock et al.           Expires - April 2004                [Page 10]

                  Next Steps in Signaling: Framework      October 2003

   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 which
   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 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 2 layers:
    *) a 'signaling transport' layer, responsible for moving signaling
   messages around, which should be independent of any particular
   signaling application; and
    *) 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.

Hancock et al.           Expires - April 2004                [Page 11]

                  Next Steps in Signaling: Framework      October 2003

                 ^                     +-----------------+
                 |                     | 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 that which 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. Firstly, 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 layers.

   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 (up- or down-
   stream), 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

Hancock et al.           Expires - April 2004                [Page 12]

                  Next Steps in Signaling: Framework      October 2003

   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 of or acknowledgement for
   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 which 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 only 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 which are per-flow, even if they are for a
   signaling application which the node is able to process in general.

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

               Figure 5: Signaling with Heterogeneous NSLPs

Hancock et al.           Expires - April 2004                [Page 13]

                  Next Steps in Signaling: Framework      October 2003

   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 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:

   *) At the IP layer, a set of protocol numbers can be used, or a range
   of values in the router alert option. 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.

   *) 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 involved
   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, for example:
   *) Translating addresses in message payloads (compare section 4.6.1);
   note this would have to be done to messages passing both directions
   through a node.
   *) 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,
   this 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 design.

Hancock et al.           Expires - April 2004                [Page 14]

                  Next Steps in Signaling: Framework      October 2003

3.2.4  Core NTLP 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 NSLP and NTLP operation; 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 of
   enabling 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 message (e.g. whether a
   message changes or just refreshes signaling application state, or
   even has nothing to with signaling application state at all).

   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 e.g. by reliability or
   priority, not 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 paths in case of
   a later re-routing event).

   3. Within any node, it is an implementation decision whether to
   generate/jitter/filter refreshes either separately within each
   signaling application that needs this functionality, or to integrate

Hancock et al.           Expires - April 2004                [Page 15]

                  Next Steps in Signaling: Framework      October 2003

   it with the NTLP implementation as a generic "soft-state management
   toolbox"; the choice doesn't affect the NTLP specification at all.
   Implementations might piggy-back NTLP soft-state refresh information
   (if the NTLP works this way) on signaling application messages, or
   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 as would be achieved by a monolithic

   4. It may be helpful for signaling applications to receive state-
   management related 'triggers' from the NTLP, 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 as 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 correctly 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 which
   allow faster reaction to events in the network, but shouldn't be
   built into NSLP semantics. (This is essentially the same distinction
   - with the same rationale - as SNMP makes between traps and normal
   message exchanges.)

3.2.6  Path De-Coupled 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 off the data path, but which are
   (presumably) aware of it. This allows a looser coupling between
   signaling and data plane nodes, e.g. at the autonomous system level.

   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. It 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

Hancock et al.           Expires - April 2004                [Page 16]

                  Next Steps in Signaling: Framework      October 2003

   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 plane can no longer be assumed. Furthermore,
   the interpretation of sender/receiver orientation becomes less
   natural. With the local operation of 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 characteristics of the network deployment.

   A sender-initiated approach is when 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 time the default assumption is
   that signaling is possible both upstream and downstream (unless
   possibly a signaling application specifically indicates 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 message.

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

Hancock et al.           Expires - April 2004                [Page 17]

                  Next Steps in Signaling: Framework      October 2003

   *) 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 acknowledgements
   and notifications can be securely delivered to the sending node,
   backward routing is not necessary, and nodes do not have to maintain
   backward routing state.
   *) 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.
   *) 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 modifications of network control state for
   existing flows.
   *) Where the signaling is looking for the last (nearest to receiver)
   NE on the data path, receiver oriented signaling is most efficient;
   sender orientation would be possible, but take more messages.
   *) In either case, the initiator can generally be informed faster
   about reservation failures than the remote end.

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 between the signaling for 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.

Hancock et al.           Expires - April 2004                [Page 18]

                  Next Steps in Signaling: Framework      October 2003

   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 NSIS is
   signaling for will vary from one part of the network to another
   (depending on 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 present 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 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 DSCP
   marking. The aggregation and deaggregation points perform per flow
   signaling, but nodes within the aggregation region should only be
   forced to process signaling messages for the aggregate, somehow
   ignoring the per-flow signaling (see 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

Hancock et al.           Expires - April 2004                [Page 19]

                  Next Steps in Signaling: Framework      October 2003

   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 NSLPs.

   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 can be achieved
   by keeping per-flow state at the NSIS entities or by maintaining a
   record route object in the messages.

3.3.6  Acknowledgements and Notifications

   We are assuming that the NTLP provides a simple message transfer
   service, and 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 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 can be to send acknowledgements directly to
   the signaling initiator outside the NTLP (see section 3.2.2).

   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

Hancock et al.           Expires - April 2004                [Page 20]

                  Next Steps in Signaling: Framework      October 2003

   means that it may not be obvious 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 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 apply 'object
   security' to specific fields within the NSLP messages.

   In addition to authentication, the authorisation (to manipulate
   network control state) has to be considered as functionality above
   the NTLP level, since it will be entirely application specific.
   Indeed, authorisation 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 authorisation models are
   possible, and the variations impact:
   *) what message flows take place - for example, whether authorisation
   information is carried along with a control state modification
   request, or is sent in the reverse direction in response to it;
   *) what administrative relationships are required - for example,
   whether authorisation 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 authorisation 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.

Hancock et al.           Expires - April 2004                [Page 21]

                  Next Steps in Signaling: Framework      October 2003

   It is not the intention of this discussion 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, security and so on, 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:
    *) peer-peer, where the message is addressed to a neighboring NSIS
   entity that is known to be closer to the destination NE.
    *) end-to-end, where the message is addressed to the flow
   destination directly, and intercepted by an intervening NE.

Hancock et al.           Expires - April 2004                [Page 22]

                  Next Steps in Signaling: Framework      October 2003

   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 the address of the destination NE to 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]).

   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, following 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 of 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

Hancock et al.           Expires - April 2004                [Page 23]

                  Next Steps in Signaling: Framework      October 2003

      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. Message fragmentation should be provided by the NTLP when needed.
      For NSLPs that generate large messages, it will be necessary to
      fragment them efficiently within the network, where the use of IP
      fragmentation may lead to reduced reliability and be incompatible
      with some addressing schemes. 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 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 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 other
      cases, the NTLP would have to detect congestion and 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 which 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.

Hancock et al.           Expires - April 2004                [Page 24]

                  Next Steps in Signaling: Framework      October 2003

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:
   *) The NTLP sends raw IP packets
   *) 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).
   *) The NTLP receives indications from the IP layer regarding 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, 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 handled scoped addresses (if such
   things ever will 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
   *) Send Message, to allow the signaling application to pass data to
   the NTLP for transport.
   *) 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:

Hancock et al.           Expires - April 2004                [Page 25]

                  Next Steps in Signaling: Framework      October 2003

   *) 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 between the NTLP and signaling application to
   allow support the exchange of further control information and to
   allow the de-multiplexing of incoming data.
   *) 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
   *) Error messages, to allow the NTLP to indicate error conditions to
   the signaling application and vice versa.
   *) 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 that 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:
   *) source IP address;
   *) destination IP address;
   *) protocol identifier and higher layer (port) addressing;
   *) flow label (typical for IPv6);
   *) SPI field for IPsec encapsulated data;
   *) DSCP/TOS field
   It is assumed that wildcarding on these identifiers is not needed
   (further analysis may be required).

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

Hancock et al.           Expires - April 2004                [Page 26]

                  Next Steps in Signaling: Framework      October 2003

4.6.2  Session Identification

   There are circumstances where it is important to be able to refer to
   signaling application state independently of the underlying flow.
   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 which 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 use for this purpose.

4.6.3  Signaling Application Identification

   Since 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:
   *) processing of incoming messages - the NTLP should be able to
   demultiplex these towards the appropriate signaling applications;
   *) 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
   NTLP design.

Hancock et al.           Expires - April 2004                [Page 27]

                  Next Steps in Signaling: Framework      October 2003

4.7 Security Properties

   It is assumed that the only security service required within 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.

   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). It is an open
   question as to which parts of the NTLP messages need protecting, and
   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 data and
   signaling path 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.

Hancock et al.           Expires - April 2004                [Page 28]

                  Next Steps in Signaling: Framework      October 2003

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. The significance of load sharing in
   the context of NSIS is that, if the load sharing mechanism in use
   will forward packets on any basis other than the destination address,
   routing of signaling messages using end-to-end addressing does not
   guarantee that the 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.

   Load balancing techniques have been proposed for a number of routing
   protocols, such as OSPF equal cost paths [16] and others. Typically,
   based on the load reported from a particular path, load balancing
   determines which fraction of the data to direct to that path.
   Incoming packets are subject to a (source, destination address) hash
   computation, and effective load sharing is accomplished by means of
   adjusting the hash thresholds.

   If signaling packets are given source and destination addresses
   identical to data packets, signaling and data packets may still
   diverge because of layer 4 load-balancing (based on protocol or port
   number). Such techniques would also cause ICMP errors to be
   misdirected to the source of the data because of the source address
   spoofing. If signaling packets are made identical in the complete
   five-tuple, divergence may still occur because of the presence of
   router alert options. In this case, the same ICMP misdirection
   applies. Additionally, 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, ...).

   Many load-balancing implementations use the first n bytes of the
   packet header (including SA, DA and protocol) in the hash function.
   In this case, the general considerations above regarding SA/DA or
   five-tuple based forwarding continue to apply.

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

Hancock et al.           Expires - April 2004                [Page 29]

                  Next Steps in Signaling: Framework      October 2003

   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 being used as a
   network engineering technique, provided it is done in a way which
   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 the types of
   change they can detect. In rough order of increasing applicability,
   they can be summarized as:
   a) monitoring changes in local interface state
   b) monitoring topology changes in a link-state routing protocol
   c) inference from changes in data packet TTL
   d) inference from loss of packet stream in a flow-aware router
   e) inference from changes in signaling packet TTL
   f) changed route of an end-to-end addressed signaling packet
   g) changed route of a specific end-to-end addressed probe packet

   These methods can be categorized as being based on network monitoring
   (method a-b), based on data packet monitoring (method c-d) and based
   on monitoring signaling protocol messages (method e-g); method f is
   the baseline method of RSVP. Methods 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. In order to be
   able 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

Hancock et al.           Expires - April 2004                [Page 30]

                  Next Steps in Signaling: Framework      October 2003

   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 path.

   In some environments, it is desired 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 need interactions with protocols
   which are used to manage the routing in this case, such as VRRP [17].

   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 appropriately along with data
   traffic; but that the 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, possibly triggered by the NTLP.

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
   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.

Hancock et al.           Expires - April 2004                [Page 31]

                  Next Steps in Signaling: Framework      October 2003

   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 the
   context of other types of network (e.g. [18]); a general review of
   the fundamental interactions is given in [19], which provides further
   details on many of the subjects considered in this section.

   We are assuming that the signaling will refer to 'outer' IP headers
   when defining the flows it is controlling. There 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 a router will analyse 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 [20]). The consequence of this
   assumption is that we see the packet streams to (or from) different
   addresses as different flows, and where a flow is carried inside a
   tunnel this 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).

   The most critical aspect is therefore the fact that multiple flows
   are being used, and the signaling for them needs to be correlated
   together. 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, it is the signaling application which is responsible for
   performing the correlation (and 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 to have
   a solution which 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

Hancock et al.           Expires - April 2004                [Page 32]

                  Next Steps in Signaling: Framework      October 2003

   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, both at
   the application level (to communicate changed addresses) and 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 authorised 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 encourages the
   consideration of fast handover support protocols, for selection of
   the optimal access router to hand over to (for example, [21]), and
   transfer of state information to avoid having to regenerate it in the
   new access router after handover (for example, [22]). Both these
   procedures could have strong interactions with signaling protocols,
   the former because a selection criterion might be what network
   control state could be supported on the path through the new access
   router, the latter because 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 [23]) as well as some IPv4/v6 transition
   mechanisms such as SIIT [24].

Hancock et al.           Expires - April 2004                [Page 33]

                  Next Steps in Signaling: Framework      October 2003

   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 [25] 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 devices.

   A simple 'NSIS-aware' NAT would require flow identification
   information to be in the clear and not integrity protected. An
   alternative conceptual approach is to consider the NAT functionality
   being 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 which 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). This 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, i.e. 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 [26],
   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 as a new data flow in its own right, and NSIS signaling is
   applied recursively to it. 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), there
   needs to be the ability to provide a binding between the original

Hancock et al.           Expires - April 2004                [Page 34]

                  Next Steps in Signaling: Framework      October 2003

   flow identification and that for the tunneled flow. 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 is intended to clarify by
   simple examples 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 [27] and middlebox communication [28].

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:
   *) QoS NSIS Initiator (QNI): the signaling entity which makes the
   resource request, usually as a result of user application request.
   *) QoS NSIS Responder (QNR): the signaling entity that acts as the
   endpoint for the signaling and can optionally interact with
   applications as well.
   *) QoS NSIS Forwarder (QNF): the signaling entity between a QNI and
   QNR which propagates NSIS signaling further through the network.
   Each of these entities will interact with a resource management
   function (RMF) which 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.

   Note the continued assumption is that the NTLP works with standard
   (i.e. best-effort) layer 3 routing. There are, however, several
   proposals for the introduction of QoS awareness in routing protocols.
   All of these essentially lead to the existence of multiple paths
   (with different QoS) towards the same destination. As such, they also
   contain an inherent risk for a divergence between control plane and
   data plane, similar to the load sharing case. Clearly, any QoS NSLP
   needs to be able to handle this type of routing, although, provided
   the NTLP continues to deliver signaling messages correctly, the
   impact on the QoS NSLP protocol design itself should be limited.

Hancock et al.           Expires - April 2004                [Page 35]

                  Next Steps in Signaling: Framework      October 2003

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 is one
   possible state management mechanism (see next section).

    | Operation |Direction|                 Semantics                |
    |  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           |
    |           |(&R-->I?)|           existing 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 prime 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:

Hancock et al.           Expires - April 2004                [Page 36]

                  Next Steps in Signaling: Framework      October 2003

   *) 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 [29] or
   RMD [30] (per 'class' state) respectively.
   *) The protocol must be robust against failure and other conditions,
   which 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. To minimize this time interval
   several techniques could be considered. As an example, RSVP [7] has
   the concept of local repair, where 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 NF.

Hancock et al.           Expires - April 2004                [Page 37]

                  Next Steps in Signaling: Framework      October 2003

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

                          Figure 8: Route Change

   It is not guaranteed that the new path will be able to provide the
   same guarantees that were available on the old path. Therefore, in a
   more desirable scenario, the QNF should wait until resources have
   been reserved on the new path before installing the route change
   (unless of course the old path no longer exists). The route change
   procedure then consists of the following steps:
   1. QNF receives a route announcement,
   2. Refresh messages are forwarded along the current path,
   3. A copy of the refresh message is re-marked as a request and sent
   along the new path that was announced,
   4. When the QNF has been acknowledged about the reservations on the
   new path, the route will be installed and data will flow along it.

   Another example related to route changes is denoted as severe
   congestion and is explained in [30]. This solution adapts to a route
   change, when a route change creates congestion on the new routed

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

Hancock et al.           Expires - April 2004                [Page 38]

                  Next Steps in Signaling: Framework      October 2003

   a server towards client NSLP(s). It is noted, 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 intradomain QoS signaling, running between just the edges
   of the network (see [10], [31], and [32] 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) NF-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 which
   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.

Hancock et al.           Expires - April 2004                [Page 39]

                  Next Steps in Signaling: Framework      October 2003

7 Security Considerations

   This document describes a framework for signaling protocols which
   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 the 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 most likely be provided by some kind of channel
   security mechanism using an external key management mechanism based
   on mutual authentication. In addition, the NTLP should be resilient
   against denial of service attacks on the protocol itself.

   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 authorisation requirements of the signaling application.
   Authorisation 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 authorisation
   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].

Hancock et al.           Expires - April 2004                [Page 40]

                  Next Steps in Signaling: Framework      October 2003

Normative References

   1  Brunner, M., "Requirements for Signaling Protocols", draft-ietf-
      nsis-req-09.txt (work in progress), August 2003

   2  Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
      draft-ietf-nsis-threats-02.txt (work in progress), June 2003

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

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

Informative References

   5  Manner, J., Fu, X. and P. Pan, "Analysis of Existing Quality of
      Service Signaling Protocols", draft-ietf-nsis-signalling-analysis-
      02.txt (work in progress), June 2003

   6  Tschofenig, H., "RSVP Security Properties", draft-ietf-nsis-rsvp-
      sec-properties-02.txt (work in progress), June 2003

   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", RFC2113, 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., Buechli, M., Van den Bosch, S. and H. Schulzrinne,
      "NSIS Authentication, Authorization and Accounting Issues", draft-
      tschofenig-nsis-aaa-issues-01.txt (work in progress), March 2003

Hancock et al.           Expires - April 2004                [Page 41]

                  Next Steps in Signaling: Framework      October 2003

   13 Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F. and S.
      Molendini, "RSVP Refresh Overhead Reduction Extensions", RFC2961,
      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., Williams, D., Kamat, S., Guerin, R., Orda, A.
      and T. Przygienda, "QoS Routing Mechanisms and OSPF Extensions",
      RFC 2676, August 1999

   17 Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel, D., Hunt,
      P., Higginson, P., Shand, M. and A. Lindem, "Virtual Router
      Redundancy Protocol", RFC2338, April 1998

   18 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

   19 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, 5 February 2002, pp 137-163

   20 Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in IPv6",
      draft-ietf-mobileip-ipv6-24.txt (work in progress), June 2003

   21 Trossen, D., Krishnamurthi, G., Chaskar, H. and J. Kempf, "Issues
      in candidate access router discovery for seamless IP-level
      handoffs", draft-ietf-seamoby-cardiscovery-issues-04.txt (work in
      progress), October 2002

   22 Kempf, J., "Problem Description: Reasons For Performing Context
      Transfers Between Nodes in an IP Access Network", RFC3374,
      September 2002

   23 Srisuresh, P. and M. Holdrege, "IP Network Address Translator
      (NAT) Terminology and Considerations", RFC2663, August 1999

   24 Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)",
      RFC2765, February 2000

Hancock et al.           Expires - April 2004                [Page 42]

                  Next Steps in Signaling: Framework      October 2003

   25 Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
      Simple Traversal of User Datagram Protocol (UDP) Through Network
      Address Translators (NATs)", RFC3489, March 2003

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

   27 Van den Bosch, S., Karagiannis, G. and A. McDonald, "NSLP for
      Quality-of-                         -
                 Service Signaling", draft ietf-nsis-qos-nslp-00.txt
      (work in progress), September 2003

   28 Stiemerling, M., Tschofenig, H., Martin, M. and C. Aoun, "A
      NAT/Firewall NSIS Signaling Layer Protocol (NSLP)", draft-ietf-
      nsis-nslp-natfw-00 (work in progress), October 2003

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

   30 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

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

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


   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 draft. 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, 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.

Hancock et al.           Expires - April 2004                [Page 43]

                  Next Steps in Signaling: Framework      October 2003

Authors' Addresses

   Robert Hancock (editor)
   Roke Manor Research
   Old Salisbury Lane
   SO51 0ZN
   United Kingdom

   Ilya Freytsis
   Cetacean Networks Inc.
   100 Arboretum Drive
   Portsmouth, NH 03801

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

   John Loughney
   Nokia Research Center
   11-13 Italahdenkatu
   00180 Helsinki

   Sven Van den Bosch
   Francis Wellesplein 1
   B-2018 Antwerpen

Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and

Hancock et al.           Expires - April 2004                [Page 44]

                  Next Steps in Signaling: Framework      October 2003

   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementers or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved. This
   document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns. This
   document and the information contained herein is provided on an "AS

Hancock et al.           Expires - April 2004                [Page 45]