Internet Draft Robert Hancock
Eleanor Hepworth
Andrew McDonald
Siemens/Roke Manor Research
Document: draft-hancock-nsis-sender-
receiver-00.txt
Expires: April 2003 October 2002
Sender and Receiver Orientation Issues in NSIS
draft-hancock-nsis-sender-receiver-00.txt
Status of this Memo
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all provisions of Section 10 of RFC2026 [1].
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Abstract
The NSIS working group is considering protocols for signaling for
resources for a traffic flow along its path in the network. The
requirements for such signaling are being developed in [2] and a
framework in [3].
It is clear from existing work that there are many interrelated
issues with NSIS signaling, concerning the respective roles of the
two ends of the communication path. These issues include route
finding, authorisation, state management requirements, localization
of negotiation, and so on. The wide variety of problems involved
hinders progress in deciding what approach NSIS should adopt. This
Internet Draft attempts to provide a summary of these issues and
suggests a way of structuring further analysis. It is not expected
that this document should have a long term existence.
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Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [4].
Table of Contents
1. Introduction, Terminology, and Scope...........................2
1.1 Data and Signaling Flows ...................................2
1.2 Status of Existing Protocols ...............................4
1.3 Protocol Layering Assumptions ..............................4
2. Constraints on Sender/Receiver Orientation.....................5
2.1 Signaling Message Routing ..................................5
2.2 User Application Triggering ................................5
2.3 Renegotiation ..............................................6
2.4 'Service' Authorization ....................................6
2.5 Localized Signaling Support ................................8
2.6 Protocol - Protocol Interactions ...........................9
2.7 Multicast Support ..........................................9
2.8 Something Unpleasant about NAT .............................9
2.9 Summary ...................................................10
3. Possible Approaches...........................................10
3.1 Fix on One Paradigm .......................................10
3.2 Allow Both Paradigms ......................................11
3.3 Choose Separately for Each Protocol Component .............11
3.4 Implications of a Layered Choice ..........................12
4. Additional Considerations.....................................12
4.1 Bidirectional Reservations ................................12
4.2 Path-Decoupled Signaling ..................................13
5. Conclusions...................................................14
Acknowledgments..................................................15
Author's Addresses...............................................15
Full Copyright Statement.........................................15
1. Introduction, Terminology, and Scope
Unless otherwise stated, this document follows the terminology given
in the current NSIS framework [3].
1.1 Data and Signaling Flows
For the bulk of this document, we are concerned with path-coupled
signaling for a single unidirectional flow, as shown in Figure 1
(additional considerations are given in section 4). The node that is
sending the user data packets is called the 'sender' and the node
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sinking them the 'receiver'; these packets pass through one or more
routers.
+--------+ +-+ +-+ +-+ +--------+
| Sender |------->|R|------->|R|------->|R|------->|Receiver|
+--------+ +-+ +-+ +-+ +--------+
----------> = Flow of user data packets
Figure 1: Sender and Receiver
In the case of path-coupled NSIS signaling, there are signaling nodes
(NSIS entities) along the data path. The NSIS initiator (NI)
notionally controls the signaling (e.g. at application request),
whereas the NSIS responder (NR) terminates the signaling at the far
end; there may be one of more NSIS forwarders (NF) between the two.
The NI and NR do not have to be colocated with sender and receiver
(e.g. they could be at first/last hop access routers); nor do they
have to be the same 'way round' as the sender and receiver. This
leads to two different cases for analysis. Figure 2 shows the 'sender
initiated' case, and Figure 3 shows the 'receiver initiated' case.
+--------+ +--+ +--+ +--+ +--------+
| Sender |------->|NI|------>|NF|------>|NR|------>|Receiver|
+--------+ +--+ +--+ +--+ +--------+
========> ========>
========> = Flow of NSIS 'control'
Figure 2: Sender Initiation
+--------+ +--+ +--+ +--+ +--------+
| Sender |------->|NR|------>|NF|------>|NI|------>|Receiver|
+--------+ +--+ +--+ +--+ +--------+
<======== <========
========> = Flow of NSIS 'control'
Figure 3: Receiver Initiation
One of the basic open issues in NSIS is whether one or both of these
models should be supported, and in either case, what is the real
difference in functionality between the NI and NR; to put it another
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way, how 'directional' is the relationship between NSIS entities
(which up to now has not really been defined).
It is the purpose of this document to gather together some of the
information about this subject and propose a way forward.
1.2 Status of Existing Protocols
The principle existing path-coupled signaling protocol is RSVP [5].
RSVP is commonly described as 'receiver initiated', although there
are some subtleties in this categorization.
From the point of view of the act of resource reservation, RSVP is
clearly receiver initiated, in that the receiver is responsible for
generating the RESV message which actually defines the QoS that the
receiver wants for incoming traffic. This RESV message can also be
accompanied with security-related policy information to support the
request (see [6]). The primary motivation behind adopting receiver
initiation for resource reservation appears to have been multicast
support, as described in [7].
On the other hand, key elements of RSVP operation (RESV routing and
route change detection) depend on the PATH message which is generated
by the sender, and can be seen as triggering the RESV message (at
least the first one). This sender-generated message also contains
QoS-related information (and can even contain policy elements).
We can therefore see RSVP as containing several functions, some
sender oriented and some receiver oriented. It might be that this
distinction should be carried over into a successor protocol.
1.3 Protocol Layering Assumptions
The working assumption in the NSIS group is the signaling protocol
should be 'layered' in two parts (see section 4 of [3] for more
details), and this is consistent with several protocol proposals,
such as CSTP/ALSP [8] and others too provocative to mention here.
In this document, we refer to these layers as follows:
*) The 'NSIS Base Protocol' (NBP), handling message routing aspects
specific to path-coupled signaling; it may include transport-layer-
like functionality (reliability, congestion control and so on) or be
layered on an existing transport protocol.
*) 'A Signaling Application Protocol' (ASAP), a 'placeholder' for
one of many possible protocols which handle particular signaling
applications (QoS, middlebox control, and so on).
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2. Constraints on Sender/Receiver Orientation
Depending on the particular NSIS function (or specific signaling
application function) under consideration, it may be much easier to
implement it in a sender or receiver 'oriented' way. This section
summarizes these various constraints or influences.
2.1 Signaling Message Routing
Regardless of the particular signaling application in question, path-
coupled signaling requires the capability of message routing along
the path from sender to receiver. It appears that there are only two
methods for the signaling protocol to acquire awareness of the route:
*) Using a PATH mechanism similar to RSVP.
*) Using local topology information (e.g. from a routing protocol,
or local configuration).
Signaling message routing, which is a function of the NBP layer,
should therefore be sender oriented, possibly with the ability to use
additional information sources if available.
A related question is whether signaling messages need to be routed
with or against the data flow (or both). (So far as we can tell, the
NBP layer only sends and receives messages over a single NSIS hop, so
the question only applies to the ASAP layer. It applies both to
'real' signaling application messages and probably also to
application-specific error notifications.) If messages need to be
routed against the data flow, this has implications for the need to
store reverse-path message routing state at intermediate nodes.
The conclusion therefore seems to be that the NBP layer should be
able to operate in a sender-oriented mode, but what state it needs to
store depends on ASAP layer requirements.
2.2 User Application Triggering
Ultimately, the NSIS signaling is supporting the requirements of some
user application (e.g. a VoIP or other media capability). It is
likely that sometimes, only one 'party' will have a clear view on
what to request, e.g. what is the appropriate QoS, or even what are
the flow identification characteristics (port numbers or flow labels
may be allocated only at the sender).
Even if both ends know, still one end probably knows first and
communicates the information via upper layer exchanges; therefore,
fixing sender or receiver orientation for NSIS signaling may impose
additional roundtrip delays compared to an 'optimised' solution.
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The constraints here are probably both
*) Signaling application specific, and
*) User application specific.
2.3 Renegotiation
There has been some discussion (requirement 5.6.3 of [2] and section
3.3.2 of [3]) of the need for flexibility in which entities can
renegotiate aspects of a reservation - for example, whether the
sender or receiver should be able to do this, or the initiator or
responder, or whether it should be possible from within the network.
This is probably a question which depends on the ASAP layer. If
additional flexibility has to be supported for renegotiation compared
to initial reservation setup, then this will be an additional source
of complexity. Note that some of the motivation for this flexibility
is (presumably) to allow localized renegotiation, which is also
discussed in section 2.5.
2.4 'Service' Authorization
When any 'resource' is being requested from the network, in some
cases the use of this resource must be authorised (or somehow
verified to be compatible with a network's internal policy
requirements).
It is a hard question to work out how authorisation approaches might
impact on the sender/receiver orientation aspects. For example, it is
possible that current inter-provider peering agreements would favour
a 'sender-initiated' authorisation approach, since typically the
traffic originator 'pays' for traffic. On the other hand, in mobile
environments, the mobile user may be prepared to authorise a resource
request for both directions; a firewall application may only accept
resource requests from one side.
Therefore, the service authorisation constraints on sender/receiver
orientation are both
*) Signaling application dependent, and
*) Network policy dependent (although it may be the case that for
any given signaling application, there is a single 'natural'
authorisation direction). Indeed, even for a single path, the network
policy may change at provider boundaries.
One reason why sender/receiver authorisation has an impact on
signaling flows is the state management aspects while a request is
being authorised end to end. For example, Figure 4 shows a 'initiator
authorised' signaling flow: messages flowing in the direction
NI-->NF-->NR can carry their own authorisation data (they could even
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carry it idempotently/statelessly), which could allow very simple
authorisation processing at intermediate nodes.
+--+ |
|NI| |
+--+ 1: Resource request (with |
\ authorisation data) for |
\ first segment of data path |
\ |
_| |
+---+ 2: Authorisation verified by NF1 | T
|NF1| and request admitted; resource | I
+---+ request propagated to next segment | M
\ | E
\ 3: Resource request for |
\ second segment of data path |
_| |
+---+ |
|NF2| V
+---+ V
. V
. V
Figure 4: Message Flow for Initiator Authorisation
However, the 'responder authorised' situation is more complex, since
the actual authorisation data has to come from the remote end of the
signaling exchange, and intermediate nodes may have to retain state
waiting for this to arrive, as shown in Figure 5.
The conclusion from this part of the discussion is that:
*) Either the initiator or the responder might be responsible for
authorisation aspects (depending on the discussion above), but
*) If the responder is responsible, the NBP will have to handle
messages in both directions, and intermediate nodes will have to
handle more local state storage.
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+--+
|NI| |
+--+ |
\ 1: Resource request for |
\ first segment of path |
\ |
_| |
+--+ 2: Resource request |
{|NF| propagated |
{+--+ to next segment |
{ \ |
{ \ 3: Resource request for | T
During steps { \ second segment of path | I
2 to 5: { _| | M
NF awaiting { +--+ | E
authorisation { |NR| 4: NR generates |
information { +--+ authorisation info |
from NR { / |
{ / 5: Authorisation |
{ / information from NR |
{ |_ for second segment |
{+--+ V
{|NF| V
+--+ V
. V
.
Figure 5: Message Flows for Responder Authorisation
2.5 Localized Signaling Support
Technical approaches for localization of signaling have already been
discussed in the context of RSVP, for example in [9] and [10]. There
are several reasons why it may be desirable to localize the scope of
some aspect of the signaling, such as:
*) Only one endpoint may be generally NSIS aware (e.g. because the
other endpoint has no motivation to implement it, or because it is a
legacy device).
*) Only one endpoint may be aware of the specific ASAP which is
relevant.
*) One endpoint may be mobile and wish to manage aspects of its
reservations locally to improve handover performance.
Regardless of the motivation, the end result is that in some
scenarios, an endpoint will probably wish to carry out both sender
and receiver oriented signaling over some local region of the
network, i.e. for incoming and outgoing packets for a bi-directional
session. Ideally this would be done both:
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*) for the NBP layer (although we have said in 2.1 that this is
hard), and
*) for the ASAP layer.
In practice, the mechanism for localizing signaling will be some kind
of proxy, and the difficulty in the NBP layer is precisely the
difficulty in locating the proxy using purely local signaling. Given
the proxy location, however, the ASAP layer signaling between it and
the end point then suffers from all the same constraints related to
sender/receiver orientation as in the end to end case.
2.6 Protocol - Protocol Interactions
As well as operating locally (in isolation), NSIS signaling will have
to interact with other protocols, such as RSVP in other parts of the
network. Also, several NSIS deployment scenarios consider NSIS
interacting with itself in a 'layered' style, or end-to-end NSIS
using edge-to-edge signaling for intradomain provisioning (see for
examples sections 3.2 and 7 of [3]).
In these circumstances, NSIS is at least partly at the mercy of these
other protocols or other instances of itself, to be initiated and to
respond in a compatible way at the protocol interworking boundary. In
particular, to interwork with RSVP, NSIS signaling may have to be
able to operate in compatible way (e.g. receiver oriented for
reservation).
2.7 Multicast Support
Multicast support is the primary justification for the receiver
orientation of the reservation signaling in RSVP. The reason is that
this naturally allows for progressive state merging from large
numbers of receivers back towards the senders, thereby allowing
better scalability. For the most general multicast case, this
conclusion seems unchallenged (although restricted multicast
scenarios, such SSM [11] or multicast with homogeneous receivers,
other options may be possible).
Multicast support is not an initial requirement for NSIS protocol
work. However, in the future, it might be desirable to extend parts
of NSIS to support multicast signaling applications, in which case
particular sorts of receiver orientation should not be permanently
excluded.
2.8 Something Unpleasant about NAT
The existence of NATs poses some special problems for signaling
protocols, since they change the header information in packets
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downstream from the sender in a way which may not be predictable
before the data flow along the path is actually active (e.g. if
dynamic address sharing is taking place).
The consequence of this is that, even if we would naturally imagine a
certain signaling operation being controlled from the receiver, this
may not be possible because the receiver does not know how to refer
to the flow in the first place. Therefore, the signaling has to at
least involve the sender as well, probably in cooperation with the
receiver (and NAT) as well.
2.9 Summary
The overall conclusion of this section is that there are all sorts of
reasons why:
*) Sender orientiation may be required for some functions or in some
scenarios;
*) Receiver orientation may be required for other functions or other
scenarios;
*) Sender and receiver orientiation have different costs and
complexities (e.g. in state management or latency) associated with
them.
The choice between sender and receiver orientation therefore appears
as a classic rock and hard place dilemma, especially given the
natural desire to build a solution that is not overwhelmed by
complexity or option negotiation.
3. Possible Approaches
This section presents three possible approaches to resolving this
conundrum.
3.1 Fix on One Paradigm
Initially, the most attractive possibility would be to fix on a
single paradigm and impose it throughout the NSIS work.
However, it seems impossible to imagine that a single paradigm will
support all the requirements and scenarios under discussion; even the
baseline RSVP approach, summarized in 1.2, covers only some of the
possibilities, and in some scenarios simpler sender-only solutions
are possible. A wider set of options might also make incremental
deployment (which could be a critical issue) more achievable.
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3.2 Allow Both Paradigms
The opposite approach is to allow everything - all aspects of NSIS -
to be both sender and receiver oriented. The basic danger here is of
overwhelming the NSIS protocols with excessive complexity, since they
may well have to operate differently depending on which direction
they are working in. It would also make it more difficult to
implement a minimal subset of NSIS for particularly constrained
environments.
Even if the NSIS protocols could be specified and implemented, the
variety of options would pose some operational problems. It might be
that both sender and receiver would attempt to initiate the signaling
protocol and cause a protocol collision (or indeed that neither of
them would). The necessary remedy for this would be to introduce yet
another component of the NSIS protocol, to negotiate which end should
take the initiative.
3.3 Choose Separately for Each Protocol Component
A third way is to select between sender and receiver orientation
independently for each component; provided the inter-component
interactions can be controlled, this should then allow better fitting
of protocol behavior to the constraints identified above.
Specifically, we could imagine the following:
The NBP layer would be (universally) sender oriented, the same way as
the RSVP PATH message (possibly also allowing for other peer
discovery mechanisms and proxy usage).
The ASAP layer would be either sender or receiver oriented, depending
on the signaling application in question. There might even be
different variants for different deployment scenarios (e.g. a sender-
oriented intra-domain QoS signaling application, which worked with a
receiver-oriented inter-domain counterpart at domain boundaries).
The operation of the NBP and ASAP layers would be interdependent to
some extent. The dependencies would include:
*) A receiver-oriented ASAP would suffer from (at least) a single
end-to-end delay, waiting for the NBP layer to complete establishing
the signaling path. However, this delay is probably an unavoidable
consequence of whatever constraints meant the ASAP was receiver-
oriented in the first place.
*) The NBP might unnecessarily store reverse-path state for a purely
sender-oriented ASAP (in other words, one which required no receiver-
to-sender messages). This could be fine tuned by allowing the ASAP to
invoke the NBP in a mode which didn't store such state.
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3.4 Implications of a Layered Choice
Splitting the responsibility in this way and leaving the selection to
the ASAP layer represents quite a significant shift in thinking
compared to current protocols. There are therefore some dangers.
The first danger is of excessive flexibility. On the other hand, the
flexibility is a consequence of the NSIS requirements and
constraints. This approach does allow simpler solutions in particular
environments (e.g. for specific ASAP layers).
The split decision probably has implications for the way state is
managed between the layers, especially where different layers are in
different protocol states in the interior of the network. This
clearly needs further analysis.
If the choice between sender and receiver initiation is really a
matter for the ASAP layer, the implication is that the messages
visible in the NBP should be somewhat neutral in content. The
existing NSIS framework (section 4.3.2 of [3]) may be too specific in
this regard. Also, the basic NI/NF/NR concepts may have to be split
depending on the NBP/ASAP layer.
4. Additional Considerations
The adoption of a split approach for sender/receiver orientation
could have some implications for other aspects of NSIS-related work
beyond the basic unicast path-coupled case. These are summarized
here.
4.1 Bidirectional Reservations
NSIS work (especially requirements work) has discussed the case of
'bidirectional' reservations, in other words, signaling for both
directions of a point-to-point data flow. The baseline approach for
this feature (see section 3.2.7 of [3]) is to simply combine a pair
of unidirectional reservations, which is then covered by the previous
discussion.
However, a 'true' bi-directional reservation (integrating the
signaling for each direction) would also be interesting in some
applications. Topologically, this would only be possible over a path
segment that was symmetrically routed.
Following the split layer approach of section 3.3, it seems that
asking for bi-directional protocol within the NBP layer is not
meaningful, since in general, even if the route is symmetric, NBP
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layer procedures have to operate asymmetrically while finding this
out. However, it could be possible for the NBP layer to detect this
symmetry (i.e. correlate the routes for incoming and outgoing flows)
and provide this as an enhanced service interface to the ASAP layer.
Whether the ASAP layer can or must use this capability to set up a
bi-directional reservation using that interface is probably very much
dependent on the signaling application and possibly scenario in
question. It seems likely that the logical behavior (to do with state
management, message sequences and so on) is the same as just a sender
and receiver initiated reservation; however, the sending and
reception of the messages in pairs might enable more efficient local
processing.
4.2 Path-Decoupled Signaling
Although NSIS does not currently have path-decoupled signaling in its
scope, it is worth pointing out here some issues that may be special
related to sender/receiver aspects in the path-decoupled case.
The main issue with path-decoupled signaling is that once the
signaling endpoints are not on the data path, it is no longer an
unambiguous topological decision to categorize one of them as being
related to the sender and the other to the receiver (see Figure 6).
+--------+ +-+
| Sender |------->|R|
+--------+ +-+\
\
\
+--+ \ +--+
|NI|============>|NR|
+--+ \ +--+
\
\
\+-+ +--------+
|R|------->|Receiver|
+-+ +--------+
----------> = Flow of user data packets
========> = Flow of NSIS 'control'
Figure 6: Path-Decoupled Signaling Topology
If there is to be an attempt to re-use path-coupled NSIS signaling in
this type of environment, and the signaling depends significantly on
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sender and receiver orientation, it will be necessary to work out how
to match these concepts in the path-decoupled case.
One approach to this would be to place the responsibility for 'path-
orientation' in the NBP layer or its equivalent (which has to be
modified anyway for the path-decoupled case to support off-path
nodes). This layer will also have to have some more explicit
(application layer?) interaction with the data sender and receiver,
just to trigger the signaling process in the first place. However,
once this is done, the ASAP layer (at least in terms of message
exchanges) might operate in almost exactly the same way as in the
path-coupled case.
5. Conclusions
This document has no conclusions. However, it proposes a method for
reasoning (possibly constructively) about the sender/receiver
orientation possibilities. Implications for the requirements and
framework, and consequences for path-decoupled signaling, have also
been identified.
References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Brunner, M., "Requirements for QoS Signaling Protocols", draft-
ietf-nsis-req-04.txt (work in progress), August 2002
3 Freytsis, I., R. E. Hancock, G. Karagiannis, J. Loughney, S. van
den Bosch, "Next Steps in Signaling: Framework", draft-ietf-nsis-
fw-00.txt (work in progress), October 2002
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
5 Braden, R. et al., "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", RFC 2205, September 1997
6 Herzog, S., "RSVP Extensions for Policy Control", RFC 2750,
January 2000
7 Braden, R. et al., "Integrated Services in the Internet
Architecture: an Overview", RFC 1633, June 1994
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NSIS: Sender/Receiver Issues October 2002
8 Braden, R., "A Two-Level Architecture for Internet Signaling",
draft-braden-2level-signal-arch-00.txt (work in progress),
November 2001 (expired)
9 Gai, S. et al., "RSVP Proxy", draft-ietf-rsvp-proxy-03.txt (work
in progress), March 2002
10 Manner, J., et al., "Localized RSVP", draft-manner-lrsvp-00.txt
(work in progress), May 2002
11 Bhattacharyya, S. et al., "An Overview of Source-Specific
Multicast (SSM)", draft-ietf-ssm-overview-03.txt (work in
progress), March 2002
Acknowledgments
The authors would like to thank all their colleagues and fellow
participants in the NSIS working group for exposing the complexities
and subtleties in this subject area.
Author's Addresses
{Robert Hancock, Eleanor Hepworth, Andrew McDonald}
Roke Manor Research
Old Salisbury Lane
Romsey
Hampshire
SO51 0ZN
United Kingdom
email: {robert.hancock|eleanor.hepworth|andrew.mcdonald}@roke.co.uk
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