4. The NSIS Transport Layer Protocol
This section describes the overall functionality required from the
NTLP. It mentions possible protocol components within the NTLP layer
and the different possible addressing modes that can be utilized, as
well as the assumed transport and state management functionality.
The interfaces between NTLP and the layers above and below it are
identified, with a description of the identity elements that appear
on these interfaces.
This discussion is not intended to design the NTLP or even to
enumerate design options, although some are included as examples.
The goal is to provide a general discussion of required functionality
and to highlight some of the issues associated with this.
4.1. Internal Protocol Components
The NTLP includes all functionality below the signaling application
layer and above the IP layer. The functionality that is required
within the NTLP is outlined in Section 3.2.4, with some more details
in Sections 3.2.5 and 4.3.
Some NTLP functionality could be provided via components operating as
sublayers within the NTLP design. For example, if specific transport
capabilities are required (such as congestion avoidance,
retransmission, and security), then existing protocols (such as
TCP+TLS or DCCP+IPsec) could be incorporated into the NTLP. This
possibility is not required or excluded by this framework.
If peer-peer addressing (Section 4.2) is used for some messages, then
active next-peer discovery functionality will be required within the
NTLP to support the explicit addressing of these messages. This
could use message exchanges for dynamic peer discovery as a sublayer
within the NTLP; there could also be an interface to external
mechanisms to carry out this function.
^ +------------------+ +-------------------------+
| | | | NSIS Specific Functions |
| | | | .............|
NSIS | | Monolithic | |+----------+. Peer .|
Transport | | Protocol | || Existing |. Discovery .|
Layer | | | || Protocol |. Aspects .|
| | | |+----------+.............|
V +------------------+ +-------------------------+
Figure 6: Options for NTLP Structure
There are two ways to address a signaling message being transmitted
between NTLP peers:
o peer-peer, where the message is addressed to a neighboring NSIS
entity that is known to be closer to the destination NE.
o end-to-end, where the message is addressed to the flow destination
directly and intercepted by an intervening NE.
With peer-peer addressing, an NE will determine the address of the
next NE based on the payload of the message (and potentially on the
previous NE). This requires that the address of the destination NE
be derivable from the information present in the payload, either by
using some local routing table or through participation in active
peer discovery message exchanges. Peer-peer addressing inherently
supports tunneling of messages between NEs, and is equally applicable
to the path-coupled and path-decoupled cases.
In the case of end-to-end addressing, the message is addressed to the
data flow receiver, and (some of) the NEs along the data path
intercept the messages. The routing of the messages should follow
exactly the same path as the associated data flow (but see
Section 5.1.1 on this point). Note that securing messages sent this
way raises some interesting security issues (these are discussed in
). In addition, it is a matter of the protocol design what should
be used as the source address of the message (the flow source or
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
4.3. Classical Transport Functions
The NSIS signaling protocols are responsible for transporting
(signaling) data around the network; in general, this requires
functionality such as congestion management, reliability, and so on.
This section discusses how much of this functionality should be
provided within the NTLP. It appears that this doesn't affect the
basic way in which the NSLP/NTLP layers relate to each other (e.g.,
in terms of the semantics of the inter-layer interaction); it is much
more a question of the overall performance/complexity tradeoff
implied by placing certain functions within each layer.
Note that, per the discussion at the end of Section 3.2.3, there may
be cases where intermediate nodes wish to modify messages in transit
even though they do not perform full signaling application
processing. In this case, not all the following functionality would
be invoked at every intermediate node.
The following functionality is assumed to lie within the NTLP:
1. Bundling together of small messages (comparable to ) can be
provided locally by the NTLP as an option, if desired; it doesn't
affect the operation of the network elsewhere. The NTLP should
always support unbundling, to avoid the cost of negotiating the
feature as an option. (The related function of refresh
summarization -- where objects in a refresh message are replaced
with a reference to a previous message identifier -- is left to
NSLPs, which can then do this in a way tuned to the state
management requirements of the signaling application. Additional
transparent compression functionality could be added to the NTLP
design later as a local option.) Note that end-to-end addressed
messages for different flows cannot be bundled safely unless the
next node on the outgoing interface is known to be NSIS-aware.
2. When needed, message fragmentation should be provided by the
NTLP. The use of IP fragmentation for large messages may lead to
reduced reliability and may be incompatible with some addressing
schemes. Therefore, this functionality should be provided within
the NTLP as a service for NSLPs that generate large messages.
How the NTLP determines and accommodates Maximum Transmission
Unit (MTU) constraints is left as a matter of protocol design.
To avoid imposing the cost of reassembly on intermediate nodes,
the fragmentation scheme used should allow for the independent
forwarding of individual fragments towards a node hosting an
3. There can be significant benefits for signaling applications if
state-changing messages are delivered reliably (as introduced in
 for RSVP; see also the more general analysis of ). This
does not change any assumption about the use of soft-state by
NSLPs to manage signaling application state, and it leaves the
responsibility for detecting and recovering from application
layer error conditions in the NSLP. However, it means that such
functionality does not need to be tuned to handle fast recovery
from message loss due to congestion or corruption in the lower
layers, and it also means that the NTLP can prevent the
amplification of message loss rates caused by fragmentation.
Reliable delivery functionality is invoked by the NSLP on a
message-by-message basis and is always optional to use.
4. The NTLP should not allow signaling messages to cause congestion
in the network (i.e., at the IP layer). Congestion could be
caused by retransmission of lost signaling packets or by upper
layer actions (e.g., a flood of signaling updates to recover from
a route change). In some cases, it may be possible to engineer
the network to ensure that signaling cannot overload it; in
others, the NTLP would have to detect congestion and to adapt the
rate at which it allows signaling messages to be transmitted.
Principles of congestion control in Internet protocols are given
in . The NTLP may or may not be able to detect overload in
the control plane itself (e.g., an NSLP-aware node several
NTLP-hops away that cannot keep up with the incoming message
rate) and indicate this as a flow-control condition to local
signaling applications. However, for both the congestion and
overload cases, it is up to the signaling applications themselves
to adapt their behavior accordingly.
4.4. Lower Layer Interfaces
The NTLP interacts with 'lower layers' of the protocol stack for the
purposes of sending and receiving signaling messages. This framework
places the lower boundary of the NTLP at the IP layer. The interface
to the lower layer is therefore very simple:
o The NTLP sends raw IP packets
o The NTLP receives raw IP packets. In the case of peer-peer
addressing, they have been addressed directly to it. In the case
of end-to-end addressing, this will be achieved by intercepting
packets that have been marked in some special way (by special
protocol number or by some option interpreted within the IP layer,
such as the router alert option).
o The NTLP receives indications from the IP layer (including local
forwarding tables and routing protocol state) that provide some
information about route changes and similar events (see
For correct message routing, the NTLP needs to have some information
about link and IP layer configuration of the local networking stack.
In general, it needs to know how to select the outgoing interface for
a signaling message and where this must match the interface that will
be used by the corresponding flow. This might be as simple as just
allowing the IP layer to handle the message using its own routing
table. There is no intention to do something different from IP
routing (for end-to-end addressed messages); however, some hosts
allow applications to bypass routing for their data flows, and the
NTLP processing must account for this. Further network layer
information would be needed to handle scoped addresses (if such
things ever exist).
Configuration of lower-layer operation to handle flows in particular
ways is handled by the signaling application.
4.5. Upper Layer Services
The NTLP offers transport-layer services to higher-layer signaling
applications for two purposes: sending and receiving signaling
messages, and exchanging control and feedback information.
For sending and receiving messages, two basic control primitives are
o Send Message, to allow the signaling application to pass data to
the NTLP for transport.
o Receive Message, to allow the NTLP to pass received data to the
The NTLP and signaling application may also want to exchange other
control information, such as the following:
o Signaling application registration/de-registration, so that
particular signaling application instances can register their
presence with the transport layer. This may also require some
identifier to be agreed upon between the NTLP and signaling
application to support the exchange of further control information
and to allow the de-multiplexing of incoming data.
o NTLP configuration, allowing signaling applications to indicate
what optional NTLP features they want to use, and to configure
NTLP operation, such as controlling what transport layer state
should be maintained.
o Error messages, to allow the NTLP to indicate error conditions to
the signaling application, and vice versa.
o Feedback information, such as route change indications so that the
signaling application can decide what action to take.
4.6. Identity Elements
4.6.1. Flow Identification
The flow identification is a method of identifying a flow in a unique
way. All packets associated with the same flow will be identified by
the same flow identifier. The key aspect of the flow identifier is
to provide enough information such that the signaling flow receives
the same treatment along the data path as the actual data itself;
i.e., consistent behavior is applied to the signaling and data flows
by a NAT or policy-based forwarding engine.
Information that could be used in flow identification may include:
o source IP address;
o destination IP address;
o protocol identifier and higher layer (port) addressing;
o flow label (typical for IPv6);
o SPI field for IPsec encapsulated data; and
o DSCP/TOS field.
It is assumed that at most limited wildcarding on these identifiers
We assume here that the flow identification is not hidden within the
NSLP, but is explicitly part of the NTLP. The justification for this
is that being able to do NSIS processing, even at a node which was
unaware of the specific signaling application (see Section 3.2.3)
might be valuable. An example scenario would be messages passing
through an addressing boundary where the flow identification had to
4.6.2. Session Identification
There are circumstances in which being able to refer to signaling
application state independently of the underlying flow is important.
For example, if the address of one of the flow endpoints changes due
to a mobility event, it is desirable to be able to change the flow
identifier without having to install a completely new reservation.
The session identifier provides a method to correlate the signaling
about the different flows with the same network control state.
The session identifier is essentially a signaling application
concept, since it is only used in non-trivial state management
actions that are application specific. However, we assume here that
it should be visible within the NTLP. This enables it to be used to
control NTLP behavior; for example, by controlling how the transport
layer should forward packets belonging to this session (as opposed to
this signaling application). In addition, the session identifier can
be used by the NTLP to demultiplex received signaling messages
between multiple instances of the same signaling application, if such
an operational scenario is supported (see Section 4.6.3 for more
information on signaling application identification).
To be useful for mobility support, the session identifier should be
globally unique, and it should not be modified end-to-end. It is
well known that it is practically impossible to generate identifiers
in a way that guarantees this property; however, using a large random
number makes it highly likely. In any case, the NTLP ascribes no
valuable semantics to the identifier (such as 'session ownership');
this problem is left to the signaling application, which may be able
to secure it to be used for this purpose.
4.6.3. Signaling Application Identification
Because the NTLP can be used to support several NSLP types, there is
a need to identify which type a particular signaling message exchange
is being used for. This is to support:
o processing of incoming messages -- the NTLP should be able to
demultiplex these towards the appropriate signaling applications;
o processing of general messages at an NSIS-aware intermediate node
-- if the node does not handle the specific signaling application,
it should be able to make a forwarding decision without having to
parse upper-layer information.
No position is taken on the form of the signaling application
identifier, or even the structure of the signaling application
'space': free-standing applications, potentially overlapping groups
of capabilities, etc. These details should not influence the rest of
the NTLP design.
4.7. Security Properties
It is assumed that the only security service required within the NTLP
is channel security. Channel security requires a security
association to be established between the signaling endpoints, which
is carried out via some authentication and key management exchange.
This functionality could be provided by reusing a standard protocol.
In order to protect a particular signaling exchange, the NSIS entity
needs to select the security association that it has in place with
the next NSIS entity that will be receiving the signaling message.
The ease of doing this depends on the addressing model in use by the
NTLP (see Section 4.2).
Channel security can provide many different types of protection to
signaling exchanges, including integrity and replay protection and
encryption. It is not clear which of these is required at the NTLP
layer, although most channel security mechanisms support them all.
It is also not clear how tightly an NSLP can 'bind' to the channel
security service provided by the NTLP.
Channel security can also be applied to the signaling messages with
differing granularity; i.e., all or parts of the signaling message
may be protected. For example, if the flow is traversing a NAT, only
the parts of the message that do not need to be processed by the NAT
should be protected. (Alternatively, if the NAT takes part in NTLP
security procedures, it only needs to be given access to the message
fields containing addresses, often just the flow id.) Which parts of
the NTLP messages need protecting is an open question, as is what
type of protection should be applied to each.
5. Interactions with Other Protocols
5.1. IP Routing Interactions
The NTLP is responsible for determining the next node to be visited
by the signaling protocol. For path-coupled signaling, this next
node should be one that will be visited by the data flow. In
practice, this peer discovery will be approximate, as any node could
use any feature of the peer discovery packet to route it differently
from the corresponding data flow packets. Divergence between the
data and signaling paths can occur due to load sharing or load
balancing (Section 5.1.1). An example specific to the case of QoS is
given in Section 6.1.1. Route changes cause a temporary divergence
between the data path and the path on which signaling state has been
installed. The occurrence, detection, and impact of route changes is
described in Section 5.1.2. A description of this issue in the
context of QoS is given in Section 6.1.2.
5.1.1. Load Sharing and Policy-Based Forwarding
Load sharing or load balancing is a network optimization technique
that exploits the existence of multiple paths to the same destination
in order to obtain benefits in terms of protection, resource
efficiency, or network stability. It has been proposed for a number
of routing protocols, such as OSPF  and others. In general, load
sharing means that packet forwarding will take into account header
fields in addition to the destination address; a general discussion
of such techniques and the problems they cause is provided in .
The significance of load sharing in the context of NSIS is that
routing of signaling messages using end-to-end addressing does not
guarantee that these messages will follow the data path. Policy-
based forwarding for data packets -- where the outgoing link is
selected based on policy information about fields additional to the
packet destination address -- has the same impact. Signaling and
data packets may diverge because of both of these techniques.
If signaling packets are given source and destination addresses
identical to data packets, signaling and data may still diverge
because of layer-4 load balancing (based on protocol or port). Such
techniques would also cause ICMP errors to be misdirected to the
source of the data because of source address spoofing. If signaling
packets are made identical in the complete 5-tuple, divergence may
still occur because of the presence of router alert options. The
same ICMP misdirection applies, and it becomes difficult for the end
systems to distinguish between data and signaling packets. Finally,
QoS routing techniques may base the routing decision on any field in
the packet header (e.g., DSCP).
5.1.2. Route Changes
In a connectionless network, each packet is independently routed
based on its header information. Whenever a better route towards the
destination becomes available, this route is installed in the
forwarding table and will be used for all subsequent (data and
signaling) packets. This can cause a divergence between the path
along which state has been installed and the path along which
forwarding will actually take place. The problem of route changes is
reduced if route pinning is performed. Route pinning refers to the
independence of the path taken by certain data packets from
reachability changes caused by routing updates from an Interior
Gateway Protocol (OSPF, IS-IS) or an Exterior Gateway Protocol (BGP).
Nothing about NSIS signaling prevents route pinning from being used
as a network engineering technique, provided that it is done in a way
that preserves the common routing of signaling and data. However,
even if route pinning is used, it cannot be depended on to prevent
all route changes (for example, in the case of link failures).
Handling route changes requires the presence of three processes in
the signaling protocol:
1. route change detection
2. installation of state on the new path
3. removal of state on the old path
Many route change detection methods can be used, some needing
explicit protocol support, and some of which are implementation-
internal. They differ in their speed of reaction and in the types of
change they can detect. In rough order of increasing applicability,
they can be summarized as follows:
1. monitoring changes in local forwarding table state
2. monitoring topology changes in a link-state routing protocol
3. inference from changes in data packet TTL
4. inference from loss of packet stream in a flow-aware router
5. inference from changes in signaling packet TTL
6. changed route of an end-to-end addressed signaling packet
7. changed route of a specific end-to-end addressed probe packet
These methods can be categorized as being based on network monitoring
(methods 1-2), on data packet monitoring (methods 3-4) and on
monitoring signaling protocol messages (methods 5-7); method 6 is the
baseline method of RSVP. The network monitoring methods can only
detect local changes; in particular, method 1 can only detect an
event that changes the immediate next downstream hop, and method 2
can only detect changes within the scope of the link-state protocol.
Methods 5-7, which are contingent on monitoring signaling messages,
become less effective as soft-state refresh rates are reduced.
When a route change has been detected, it is important that state is
installed as quickly as possible along the new path. It is not
guaranteed that the new path will be able to provide the same
characteristics that were available on the old path. To avoid
duplicate state installation or, worse, rejection of the signaling
message because of previously installed state, it is important to be
able to recognize the new signaling message as belonging to an
existing session. In this respect, we distinguish between route
changes with associated change of the flow identification (e.g., in
case of a mobility event when the IP source might change) and route
changes without change of the flow identification (e.g., in case of a
link failure along the path). The former case requires an identifier
independent from the flow identification; i.e., the session
identifier (Section 4.6.2). Mobility issues are discussed in more
detail in Section 5.2.
When state has been installed along the new path, the existing state
on the old path needs to be removed. With the soft-state principle,
this will happen automatically because of the lack of refresh
messages. Depending on the refresh timer, however, it may be
required to tear down this state much faster (e.g., because it is
tied to an accounting record). In that case, the teardown message
needs to be able to distinguish between the new path and the old
In some environments, it is desirable to provide connectivity and
per-flow or per-class state management with high-availability
characteristics; i.e., with rapid transparent recovery, even in the
presence of route changes. This may require interactions with
protocols that are used to manage the routing in this case, such as
Virtual Router Redundancy Protocol (VRRP) .
Our basic assumption about such interactions is that the NTLP would
be responsible for detecting the route change and ensuring that
signaling messages were re-routed consistently (in the same way as
the data traffic). However, further state re-synchronization
(including failover between 'main' and 'standby' nodes in the high
availability case) would be the responsibility of the signaling
application and its NSLP, and would possibly be triggered by the
5.2. Mobility and Multihoming Interactions
The issues associated with mobility and multihoming are a
generalization of the basic route change case of the previous
section. As well as the fact that packets for a given session are no
longer traveling over a single topological path, the following extra
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
4. Conversely, the re-routing may be rapid and routine (unlike
network-internal route changes), increasing the importance of
rapid state release on old paths.
The interactions between mobility and signaling have been extensively
analyzed in recent years, primarily in the context of RSVP and Mobile
IP interaction (e.g., the mobility discussion of ), but also in
that of other types of network (e.g., ). A general review of the
fundamental interactions is given in , which provides further
details on many of the subjects considered in this section.
We assume that the signaling will refer to 'outer' IP headers when
defining the flows it is controlling. There are two main reasons for
this. The first is that the data plane will usually be unable to
work in terms of anything else when implementing per-flow treatment
(e.g., we cannot expect that a router will analyze inner headers to
decide how to schedule packets). The second reason is that we are
implicitly relying on the security provided by the network
infrastructure to ensure that the correct packets are given the
special treatment being signaled for, and this is built on the
relationship between packet source and destination addresses and
network topology. (This is essentially the same approach that is
used as the basis of route optimization security in Mobile IPv6
.) The consequence of this assumption is that we see the packet
streams to (or from) different addresses as different flows. Where a
flow is carried inside a tunnel, it is seen as a different flow
again. The encapsulation issues (point (2) above) are therefore to
be handled the same way as other tunneling cases (Section 5.4).
Therefore, the most critical aspect is that multiple flows are being
used, and the signaling for them needs to be correlated. This is the
intended role of the session identifier (see Section 4.6.2, which
also describes some of the security requirements for such an
identifier). Although the session identifier is visible at the NTLP,
the signaling application is responsible for performing the
correlation (and for doing so securely). The NTLP responsibility is
limited to delivering the signaling messages for each flow between
the correct signaling application peers. The locations at which the
correlation takes place are the end system and the signaling-
application-aware node in the network where the flows meet. (This
node is generally referred to as the "crossover router"; it can be
anywhere in the network.)
Although much work has been done in the past on finding the crossover
router directly from information held in particular mobility
signaling protocols, the initial focus of NSIS work should be a
solution that is not tightly bound to any single mobility approach.
In other words, it should be possible to determine the crossover
router based on NSIS signaling. (This doesn't rule out the
possibility that some implementations may be able to do this
discovery faster; e.g., by being tightly integrated with local
mobility management protocols. This is directly comparable to
spotting route changes in fixed networks by being routing aware.)
Note that the crossover router discovery may involve end-to-end
signaling exchanges (especially for flows towards the mobile or
multihomed node), which raises a latency concern. On the other hand,
end-to-end signaling will have been necessary in any case, at the
application level not only to communicate changed addresses, but also
to update packet classifiers along the path. It is a matter for
further analysis to decide how these exchanges could be combined or
carried out in parallel.
On the shared part of the path, signaling is needed at least to
update the packet classifiers to include the new flow, although if
correlation with the existing flow is possible it should be possible
to bypass any policy or admission control processing. State
installation on the new path (and possibly release on the old one)
are also required. Which entity (one of the end hosts or the
crossover router) controls all these procedures depends on which
entities are authorized to carry out network state manipulations, so
this is therefore a matter of signaling application and NSLP design.
The approach may depend on the sender/receiver orientation of the
original signaling (see Section 3.3.1). In addition, in the mobility
case, the old path may no longer be directly accessible to the mobile
node; inter-access-router communication may be required to release
state in these circumstances.
The frequency of handovers in some network types makes fast handover
support protocols desirable, for selecting the optimal access router
for handover (for example, ), and for transferring state
information to avoid having to regenerate it in the new access router
after handover (for example, ). Both of these procedures could
have strong interactions with signaling protocols. The access router
selection might depend on the network control state that could be
supported on the path through the new access router. Transfer of
signaling application state or NTLP/NSLP protocol state may be a
candidate for context transfer.
5.3. Interactions with NATs
Because at least some messages will almost inevitably contain
addresses and possibly higher-layer information as payload, we must
consider the interaction with address translation devices (NATs).
These considerations apply both to 'traditional' NATs of various
types (as defined in ) as well as some IPv4/v6 transition
mechanisms, such as Stateless IP/ICMP Translation (SIIT) .
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  or similar
techniques to detect and recover from the presence of the NAT. Even
then, NSIS protocols would have to use a well-known encapsulation
(TCP/UDP/ICMP) to avoid being dropped by more cautious low-end NAT
A simple 'NSIS-aware' NAT would require flow identification
information to be in the clear and not to be integrity protected. An
alternative conceptual approach is to consider the NAT functionality
part of message processing itself, in which case the translating node
can take part natively in any NSIS protocol security mechanisms.
Depending on NSIS protocol layering, it would be possible for this
processing to be done in an NSIS entity that was otherwise ignorant
of any particular signaling applications. This is the motivation for
including basic flow identification information in the NTLP
Note that all of this discussion is independent of the use of a
specific NSLP for general control of NATs (and firewalls). That case
is considered in Section 6.2.
5.4. Interactions with IP Tunneling
Tunneling is used in the Internet for a number of reasons, such as
flow aggregation, IPv4/6 transition mechanisms, mobile IP, virtual
private networking, and so on. An NSIS solution must continue to
work in the presence of these techniques. The presence of the tunnel
should not cause problems for end-to-end signaling, and it should
also be possible to use NSIS signaling to control the treatment of
the packets carrying the tunneled data.
It is assumed that the NSIS approach will be similar to that of ,
where the signaling for the end-to-end data flow is tunneled along
with that data flow and is invisible to nodes along the path of the
tunnel (other than the endpoints). This provides backwards
compatibility with networks where the tunnel endpoints do not support
the NSIS protocols. We assume that NEs will not unwrap tunnel
encapsulations to find and process tunneled signaling messages.
To signal for the packets carrying the tunneled data, the tunnel is
considered a new data flow in its own right, and NSIS signaling is
applied to it recursively. This requires signaling support in at
least one tunnel endpoint. In some cases (where the signaling
initiator is at the opposite end of the data flow from the tunnel
initiator; i.e., in the case of receiver initiated signaling), the
ability to provide a binding between the original flow identification
and that for the tunneled flow is needed. It is left open here
whether this should be an NTLP or an NSLP function.
6. Signaling Applications
This section gives an overview of NSLPs for particular signaling
applications. The assumption is that the NSLP uses the generic
functionality of the NTLP given earlier; this section describes
specific aspects of NSLP operation. It includes simple examples that
are intended to clarify how NSLPs fit into the framework. It does
not replace or even form part of the formal NSLP protocol
specifications; in particular, initial designs are being developed
for NSLPs for resource reservation  and middlebox communication
6.1. Signaling for Quality of Service
In the case of signaling for QoS, all the basic NSIS concepts of
Section 3.1 apply. In addition, there is an assumed directionality
of the signaling process, in that one end of the signaling flow takes
responsibility for actually requesting the resource. This leads to
the following definitions:
o QoS NSIS Initiator (QNI): the signaling entity that makes the
resource request, usually as a result of user application request.
o QoS NSIS Responder (QNR): the signaling entity that acts as the
endpoint for the signaling and that can optionally interact with
applications as well.
o QoS NSIS Forwarder (QNF): a signaling entity between a QNI and QNR
that propagates NSIS signaling further through the network.
Each of these entities will interact with a resource management
function (RMF) that actually allocates network resources (router
buffers, interface bandwidth, and so on).
Note that there is no constraint on which end of the signaling flow
should take the QNI role: With respect to the data flow direction, it
could be at the sending or receiving end.
6.1.1. Protocol Message Semantics
The QoS NSLP will include a set of messages to carry out resource
reservations along the signaling path. A possible set of message
semantics for the QoS NSLP is shown below. Note that the 'direction'
column in the table below only indicates the 'orientation' of the
message. Messages can be originated and absorbed at QNF nodes as
well as the QNI or QNR; an example might be QNFs at the edge of a
domain exchanging messages to set up resources for a flow across a
it. Note that it is left open if the responder can release or modify
a reservation, during or after setup. This seems mainly a matter of
assumptions about authorization, and the possibilities might depend
on resource type specifics.
The table also explicitly includes a refresh operation. This does
nothing to a reservation except extend its lifetime, and it is one
possible state management mechanism (see next section).
| Operation | Direction | Operation |
| Request | I-->R | Create a new reservation for a flow |
| | | |
| Modify | I-->R | Modify an existing reservation |
| | (&R-->I?) | |
| | | |
| Release | I-->R | Delete (tear down) an existing |
| | (&R-->I?) | reservation |
| | | |
| Accept/ | R-->I | Confirm (possibly modified?) or reject a |
| Reject | | reservation request |
| | | |
| Notify | I-->R & | Report an event detected within the |
| | R-->I | network |
| | | |
| Refresh | I-->R | State management (see Section 6.1.2) |
6.1.2. State Management
The primary purpose of NSIS is to manage state information along the
path taken by a data flow. The issues regarding state management
within the NTLP (state related to message transport) are described in
Section 4. The QoS NSLP will typically have to handle additional
state related to the desired resource reservation to be made.
There two critical issues to be considered in building a robust NSLP
to handle this problem:
o The protocol must be scalable. It should allow minimization of
the resource reservation state-storage demands that it implies for
intermediate nodes; in particular, storage of state per 'micro'
flow is likely to be impossible except at the very edge of the
network. A QoS signaling application might require per-flow or
lower granularity state; examples of each for the case of QoS
would be IntServ  or RMD  (per 'class' state),
o The protocol must be robust against failure and other conditions
that imply that the stored resource reservation state has to be
moved or removed.
For resource reservations, soft-state management is typically used as
a general robustness mechanism. According to the discussion of
Section 3.2.5, the soft-state protocol mechanisms are built into the
NSLP for the specific signaling application that needs them; the NTLP
sees this simply as a sequence of (presumably identical) messages.
6.1.3. Route Changes and QoS Reservations
In this section, we will explore the expected interaction between
resource signaling and routing updates (the precise source of routing
updates does not matter). The normal operation of the NSIS protocol
will lead to the situation depicted in Figure 7, where the reserved
resources match the data path.
reserved +-----+ reserved +-----+
=========>| QNF |===========>| QNF |
Figure 7: Normal NSIS Protocol Operation
A route change can occur while such a reservation is in place. The
route change will be installed immediately, and any data will be
forwarded on the new path. This situation is depicted Figure 8.
Resource reservation on the new path will only be started once the
next control message is routed along the new path. This means that
there is a certain time interval during which resources are not
reserved on (part of) the data path, and certain delay or
drop-sensitive applications will require that this time interval be
minimized. Several techniques to achieve this could be considered.
As an example, RSVP  has the concept of local repair, whereby the
router may be triggered by a route change. In that case, the RSVP
node can start sending PATH messages directly after the route has
been changed. Note that this option may not be available if no
per-flow state is kept in the QNF. Another approach would be to
pre-install backup state, and it would be the responsibility of the
QoS-NSLP to do this. However, mechanisms for identifying backup
paths and routing the necessary signaling messages along them are not
currently considered in the NSIS requirements and framework.
reserved +-----+ reserved +-----+
=========>| QNF |===========>| QNF |
\ || +-----+
| ===========>| QNF |
Figure 8: Route Change
The new path might not be able to provide the same guarantees that
were available on the old path. Therefore, it might be desirable for
the QNF to wait until resources have been reserved on the new path
before allowing the route change to be installed (unless, of course,
the old path no longer exists). However, delaying the route change
installation while waiting for reservation setup needs careful
analysis of the interaction with the routing protocol being used, in
order to avoid routing loops.
Another example related to route changes is denoted as severe
congestion and is explained in . This solution adapts to a route
change when a route change creates congestion on the new routed path.
6.1.4. Resource Management Interactions
The QoS NSLP itself is not involved in any specific resource
allocation or management techniques. The definition of an NSLP for
resource reservation with Quality of Service, however, implies the
notion of admission control. For a QoS NSLP, the measure of
signaling success will be the ability to reserve resources from the
total resource pool that is provisioned in the network. We define
the function responsible for allocating this resource pool as the
Resource Management Function (RMF). The RMF is responsible for all
resource provisioning, monitoring, and assurance functions in the
A QoS NSLP will rely on the RMF to do resource management and to
provide inputs for admission control. In this model, the RMF acts as
a server towards client NSLP(s). Note, however, that the RMF may in
turn use another NSLP instance to do the actual resource provisioning
in the network. In this case, the RMF acts as the initiator (client)
of an NSLP.
This essentially corresponds to a multi-level signaling paradigm,
with an 'upper' level handling internetworking QoS signaling
(possibly running end-to-end), and a 'lower' level handling the more
specialized intra-domain QoS signaling (running between just the
edges of the network). (See , , and  for a discussion of
similar architectures.) Given that NSIS signaling is already
supposed to be able to support multiple instances of NSLPs for a
given flow and limited scope (e.g., edge-to-edge) operation, it is
not currently clear that supporting the multi-level model leads to
any new protocol requirements for the QoS NSLP.
The RMF may or may not be co-located with a QNF (note that
co-location with a QNI/QNR can be handled logically as a combination
between QNF and QNI/QNR). To cater for both cases, we define a
(possibly logical) QNF-RMF interface. Over this interface,
information may be provided from the RMF about monitoring, resource
availability, topology, and configuration. In the other direction,
the interface may be used to trigger requests for resource
provisioning. One way to formalize the interface between the QNF and
the RMF is via a Service Level Agreement (SLA). The SLA may be
static or it may be dynamically updated by means of a negotiation
protocol. Such a protocol is outside the scope of NSIS.
There is no assumed restriction on the placement of the RMF. It may
be a centralized RMF per domain, several off-path distributed RMFs,
or an on-path RMF per router. The advantages and disadvantages of
both approaches are well-known. Centralization typically allows
decisions to be taken using more global information, with more
efficient resource utilization as a result. It also facilitates
deployment or upgrade of policies. Distribution allows local
decision processes and rapid response to data path changes.
6.2. Other Signaling Applications
As well as the use for 'traditional' QoS signaling, it should be
possible to develop NSLPs for other signaling applications that
operate on different types of network control state. One specific
case is setting up flow-related state in middleboxes (firewalls,
NATs, and so on). Requirements for such communication are given in
. Other examples include network monitoring and testing, and
tunnel endpoint discovery.
7. Security Considerations
This document describes a framework for signaling protocols that
assumes a two-layer decomposition, with a common lower layer (NTLP)
supporting a family of signaling-application-specific upper-layer
protocols (NSLPs). The overall security considerations for the
signaling therefore depend on the joint security properties assumed
or demanded for each layer.
Security for the NTLP is discussed in Section 4.7. We have assumed
that, apart from being resistant to denial of service attacks against
itself, the main role of the NTLP will be to provide message
protection over the scope of a single peer relationship, between
adjacent signaling application entities. (See Section 3.2.3 for a
discussion of the case where these entities are separated by more
than one NTLP hop.) These functions can ideally be provided by an
existing channel security mechanism, preferably using an external key
management mechanism based on mutual authentication. Examples of
possible mechanisms are TLS, IPsec and SSH. However, there are
interactions between the actual choice of security protocol and the
rest of the NTLP design. Primarily, most existing channel security
mechanisms require explicit identification of the peers involved at
the network and/or transport level. This conflicts with those
aspects of path-coupled signaling operation (e.g., discovery) where
this information is not even implicitly available because peer
identities are unknown; the impact of this 'next-hop problem' on RSVP
design is discussed in the security properties document  and also
influences many parts of the threat analysis . Therefore, this
framework does not mandate the use of any specific channel security
protocol; instead, this has to be integrated with the design of the
NTLP as a whole.
Security for the NSLPs is entirely dependent on signaling application
requirements. In some cases, no additional protection may be
required compared to what is provided by the NTLP. In other cases,
more sophisticated object-level protection and the use of public-
key-based solutions may be required. In addition, the NSLP needs to
consider the authorization requirements of the signaling application.
Authorization is a complex topic, for which a very brief overview is
provided in Section 3.3.7.
Another factor is that NTLP security mechanisms operate only locally,
whereas NSLP mechanisms may also need to operate over larger regions
(not just between adjacent peers), especially for authorization
aspects. This complicates the analysis of basing signaling
application security on NTLP protection.
An additional concern for signaling applications is the session
identifier security issue (Sections 4.6.2 and 5.2). The purpose of
this identifier is to decouple session identification (as a handle
for network control state) from session "location" (i.e., the data
flow endpoints). The identifier/locator distinction has been
extensively discussed in the user plane for end-to-end data flows,
and is known to lead to non-trivial security issues in binding the
two together again. Our problem is the analogue in the control
plane, and is at least similarly complex, because of the need to
involve nodes in the interior of the network as well.
Further work on this and other security design will depend on a
refinement of the NSIS threats work begun in .
8.1. Normative References
 Brunner, M., "Requirements for Signaling Protocols", RFC 3726,
 Tschofenig, H. and D. Kroeselberg, "Security Threats for Next
Steps in Signaling (NSIS)", RFC 4081, June 2005.
 Chaskar, H., "Requirements of a Quality of Service (QoS)
Solution for Mobile IP", RFC 3583, September 2003.
 Swale, R., Mart, P., Sijben, P., Brim, S., and M. Shore,
"Middlebox Communications (midcom) Protocol Requirements",
RFC 3304, August 2002.
8.2. Informative References
 Manner, J. and X. Fu, "Analysis of Existing Quality of Service
Signaling Protocols", Work in Progress, December 2004.
 Tschofenig, H., "RSVP Security Properties", Work in Progress,
 Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
 Katz, D., "IP Router Alert Option", RFC 2113, February 1997.
 Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
RFC 2711, October 1999.
 Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
"Aggregation of RSVP for IPv4 and IPv6 Reservations", RFC 3175,
 Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
Security Considerations", BCP 72, RFC 3552, July 2003.
 Tschofenig, H., "NSIS Authentication, Authorization and
Accounting Issues", Work in Progress, March 2003.
 Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F., and S.
Molendini, "RSVP Refresh Overhead Reduction Extensions",
RFC 2961, April 2001.
 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.
 Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914,
 Apostolopoulos, G., Kamat, S., Williams, D., Guerin, R., Orda,
A., and T. Przygienda, "QoS Routing Mechanisms and OSPF
Extensions", RFC 2676, August 1999.
 Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
Multicast Next-Hop Selection", RFC 2991, November 2000.
 Hinden, R., "Virtual Router Redundancy Protocol (VRRP)", RFC
3768, April 2004.
 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.
 Manner, J., Lopez, A., Mihailovic, A., Velayos, H., Hepworth,
E., and Y. Khouaja, "Evaluation of Mobility and QoS
Interaction", Computer Networks Volume 38, Issue 2, p. 137-163,
5 February 2002.
 Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
 Liebsch, M., Ed., Singh, A., Ed., Chaskar, H., Funato, D., and
E. Shim, "Candidate Access Router Discovery (CARD)", Work in
Progress, May 2005.
 Kempf, J., "Problem Description: Reasons For Performing Context
Transfers Between Nodes in an IP Access Network", RFC 3374,
 Srisuresh, P. and M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
 Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)",
RFC 2765, February 2000.
 Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
- Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
 Terzis, A., Krawczyk, J., Wroclawski, J., and L. Zhang, "RSVP
Operation Over IP Tunnels", RFC 2746, January 2000.
 Bosch, S., Karagiannis, G., and A. McDonald, "NSLP for
Quality-of-Service signaling", Work in Progress, February 2005.
 Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer Protocol
(NSLP)", Work in Progress, February 2005.
 Braden, R., Clark, D., and S. Shenker, "Integrated Services in
the Internet Architecture: an Overview", RFC 1633, June 1994.
 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
 Ferrari, D., Banerjea, A., and H. Zhang, "Network Support for
Multimedia - A Discussion of the Tenet Approach",
Berkeley TR-92-072, November 1992.
 Nichols, K., Jacobson, V., and L. Zhang, "A Two-bit
Differentiated Services Architecture for the Internet",
RFC 2638, July 1999.
Appendix A. Contributors
Several parts of the introductory sections of this document (in
particular, in Sections 3.1 and 3.3) are based on contributions from
Ilya Freytsis, then of Cetacean Networks, Inc.
Bob Braden originally proposed "A Two-Level Architecture for Internet
Signalling" as an Internet-Draft in November 2001. This document
served as an important starting point for the framework discussed
herein, and the authors owe a debt of gratitude to Bob for this
Appendix B. Acknowledgements
The authors would like to thank Bob Braden, Maarten Buchli, Eleanor
Hepworth, Andrew McDonald, Melinda Shore, and Hannes Tschofenig for
significant contributions in particular areas of this document. In
addition, the authors would like to acknowledge Cedric Aoun, Attila
Bader, Anders Bergsten, Roland Bless, Marcus Brunner, Louise Burness,
Xiaoming Fu, Ruediger Geib, Danny Goderis, Kim Hui, Cornelia Kappler,
Sung Hycuk Lee, Thanh Tra Luu, Mac McTiffin, Paulo Mendes, Hans De
Neve, Ping Pan, David Partain, Vlora Rexhepi, Henning Schulzrinne,
Tom Taylor, Michael Thomas, Daniel Warren, Michael Welzl, Lars
Westberg, and Lixia Zhang for insights and inputs during this and
previous framework activities. Dave Oran, Michael Richardson, and
Alex Zinin provided valuable comments during the final review stages.
Siemens/Roke Manor Research
Old Salisbury Lane
Romsey, Hampshire SO51 0ZN
University of Twente
P.O. BOX 217
7500 AE Enschede
Nokia Research Center
Sven Van den Bosch
Francis Wellesplein 1
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights 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; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat 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 on-line IPR repository at
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
Funding for the RFC Editor function is currently provided by the