RFC 5945
Resource Reservation Protocol (RSVP) Proxy Approaches
Pages: 50
Informational
Informational
Part 2 of 3 – Pages 9 to 34
4. RSVP Proxy Approaches
This section discusses fundamental RSVP proxy approaches.4.1. Path-Triggered Receiver Proxy
In this approach, it is assumed that the sender is RSVP-capable and takes full care of the synchronization between application requirements and RSVP reservations. With this approach, the RSVP Receiver Proxy uses the RSVP Path messages generated by the sender as the cue for establishing the RSVP reservation on behalf of the receiver. The RSVP Receiver Proxy is effectively acting as a slave making reservations (on behalf of the receiver) under the sender's control. This changes somewhat the usual RSVP reservation model where reservations are normally controlled by receivers. Such a change greatly facilitates operations in the scenario of interest here, which is where the receiver is not RSVP-capable. Indeed, it allows the RSVP Receiver Proxy to remain application-unaware by taking advantage of the application awareness and RSVP awareness of the sender. With the Path-Triggered RSVP Receiver Proxy approach, the RSVP router may be configured to use receipt of a regular RSVP Path message as the trigger for RSVP Receiver Proxy behavior. On receipt of the RSVP Path message, the RSVP Receiver Proxy: 1. establishes the RSVP Path state as per regular RSVP processing. 2. identifies the downstream interface towards the receiver. 3. sinks the Path message. 4. behaves as if a Resv message (whose details are discussed below) was received on the downstream interface. This includes performing admission control on the downstream interface, establishing a Resv state (in case of successful admission
control), and forwarding the Resv message upstream, sending
periodic refreshes of the Resv message and tearing down the
reservation if the Path state is torn down.
In order to build the Resv message, the RSVP Receiver Proxy can take
into account information received in the Path message. For example,
the RSVP Receiver Proxy may compose a FLOWSPEC object for the Resv
message that mirrors the SENDER_TSPEC object in the received Path
message (as an RSVP-capable receiver would typically do).
Operation of the Path-Triggered Receiver Proxy in the case of a
successful reservation is illustrated in Figure 3.
|****| *** *** |**********| |----|
| S |---------*r*----------*r*---------| RSVP |----------| R |
|****| *** *** | Receiver | |----|
| Proxy |
|**********|
---Path---> ----Path----> ---Path---->
<--Resv---> <---Resv----- <--Resv----
==================RSVP===============>
**********************************************************>
|****| RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|****| |----| *** router
***> media flow
==> segment of flow path protected by RSVP reservation
Figure 3: Path-Triggered RSVP Receiver Proxy
In case the reservation establishment is rejected (for example,
because of an admission control failure on a regular RSVP router on
the path between the RSVP-capable sender and the RSVP Receiver
Proxy), a ResvErr message will be generated as per conventional RSVP
operations and will travel downstream towards the RSVP Receiver
Proxy. While this ensures that the RSVP Receiver Proxy is aware of
the reservation failure, conventional RSVP procedures do not cater to
the notification of the sender of the reservation failure. Operation
of the Path-Triggered RSVP Receiver Proxy in the case of an admission
control failure is illustrated in Figure 4.
|****| *** *** |**********| |----| | S |---------*r*----------*r*---------| RSVP |----------| R | |****| *** *** | Receiver | |----| | Proxy | |**********| ---Path---> ----Path----> ---Path----> <---Resv----- <--Resv------ ---ResvErr---> --ResvErr---> ===================RSVP===============> **********************************************************> |****| RSVP-capable |----| Non-RSVP-capable *** | S | Sender | R | Receiver *r* regular RSVP |****| |----| *** router ***> media flow ==> segment of flow path protected by RSVP reservation Figure 4: Path-Triggered RSVP Receiver Proxy with Failure Since, as explained above, in this scenario involving the RSVP Receiver Proxy, synchronization between an application and an RSVP reservation is generally performed by the sender, notifying the sender of reservation failure is needed. [RFC5946] specifies RSVP extensions allowing such sender notification in the case of reservation failure in the presence of a Path-Triggered RSVP Receiver Proxy.4.1.1. Mechanisms for Maximizing the Reservation Span
The presence in the flow path of a Path-Triggered RSVP Receiver Proxy (for a given flow) that strictly behaves as described previously would cause the Path message to be terminated and a Resv message to be generated towards the sender. When the receiver is indeed not RSVP-capable and there is no other RSVP Receiver Proxy downstream on the flow path, this achieves the best achievable result of establishing an RSVP reservation as far downstream as the RSVP Receiver Proxy.
However, if the eventual receiver was in fact RSVP-capable, it would be prevented from participating in RSVP signaling, since it does not receive any Path message. As a result, the RSVP reservation would only span a subset of the path it could actually span. A similar sub-optimality would exist with multiple Receiver Proxies in the path of the flow: the first Receiver Proxy may prevent the Path message from reaching the second one and therefore prevent the reservation from extending down to the second Receiver Proxy. It is desirable that, in the presence of Path-Triggered RSVP Receiver Proxies and of a mix of RSVP-capable and non-RSVP-capable receivers, the RSVP reservation spans as much of the flow path as possible. This can be achieved dynamically (avoiding tedious specific configuration), using the mechanisms described in Sections 4.1.1.1 and 4.1.1.2.4.1.1.1. Dynamic Discovery of Downstream RSVP Functionality
When generating a proxy Resv message upstream, a Receiver Proxy may be configured to perform dynamic discovery of downstream RSVP functionality. To that end, when generating the proxy Resv message upstream, the Receiver Proxy forwards the Path message downstream instead of terminating it. This allows an RSVP-capable receiver (or a downstream Receiver Proxy) to respond to the Path with an upstream Resv message. On receipt of a Resv message, the Receiver Proxy internally converts its state from a proxied reservation to a regular midpoint RSVP behavior. From then on, everything proceeds as if the RSVP router had behaved as a regular RSVP router at reservation establishment (as opposed to having behaved as an RSVP Receiver Proxy for that flow). The RSVP Receiver Proxy behavior for dynamic discovery of downstream RSVP functionality is illustrated in Figure 5 and is also discussed in Section 4.1 of [RFC5946].
|****| *** |**********| |----| | S |---------*r*---------| RSVP |---| R1 | |****| *** | Receiver | |----| | Proxy | | | | | |****| | |------------| R2 | |**********| |****| ---Path---> --Path---> (R1) (R1) \-------Path--> / (R1) <--Resv--- <---Resv--- ================RSVP===> **************************************> ---Path---> --Path---> (R2) (R2) \-------------Path----> / (R2) <--Resv--- <---Resv--- <----Resv--- ================RSVP===========================> ***********************************************> |****| RSVP-capable |----| non-RSVP-capable |****| RSVP-capable | S | Sender | R | Receiver | R | Receiver |****| |----| |****| *** *r* regular RSVP *** router (R1) = Path message contains a Session object whose destination is R1 ***> media flow ==> segment of flow path protected by RSVP reservation Figure 5: Dynamic Discovery of Downstream RSVP Functionality This dynamic discovery mechanism has the benefit that new (or upgraded) RSVP endpoints will automatically and seamlessly be able to take advantage of end-to-end reservations, without impacting the
ability of a Receiver Proxy to proxy RSVP for other, non-RSVP-capable
endpoints. This mechanism also achieves the goal of automatically
discovering the longest possible RSVP-supporting segment in a network
with multiple Receiver Proxies along the path. This mechanism
dynamically adjusts to any topology and routing change. Also, this
mechanism dynamically handles the situation in which a receiver was
RSVP-capable and for some reason (e.g., software downgrade) no longer
is. Finally, this approach requires no new RSVP protocol extensions
and no configuration changes to the Receiver Proxy as new RSVP-
capable endpoints come and go.
The only identified drawbacks to this approach are:
o If admission control fails on the segment between the Receiver
Proxy and the RSVP-capable receiver, the receiver will get a
ResvErr and can take application-level signaling steps to
terminate the call. However, the Receiver Proxy has already sent
a Resv upstream for this flow, so the sender will see a "false"
reservation that is not truly end-to-end. The actual admission
control status will resolve itself in a short while, but the
sender will need to roll back any permanent action (such as
billing) that may have been taken on receipt of the phantom Resv.
Note that if the second receiver is also a Receiver Proxy that is
not participating in application signaling, it will convert the
received ResvErr into a PathErr that will be received by the
sender.
o If there is no RSVP-capable receiver (or other Receiver Proxy)
downstream of the Receiver Proxy, then the Path messages sent by
the Receiver Proxy every RSVP refresh interval (e.g., 30 seconds
by default) will never be responded to. However, these messages
consume a small amount of bandwidth, and in addition would install
some RSVP state on RSVP-capable midpoint nodes downstream of the
first Receiver Proxy. This is seen as a very minor sub-
optimality. We also observe that such resources would be consumed
anyways if the receiver was RSVP-capable. Still, if deemed
necessary, to mitigate this, the Receiver Proxy can tear down any
unanswered downstream Path state and stop sending Path messages
for the flow (or only send them at much lower frequency) as
further discussed in [RFC5946].
4.1.1.2. Selective Receiver Proxy and Sender Control of Receiver Proxy
An RSVP Receiver Proxy can be selective about the sessions that it
terminates, based on local policy decision. For example, an edge
router functioning as a Receiver Proxy may behave as a proxy only for
Path messages that are actually going to exit the domain in question,
and not for Path messages that are transiting through it but stay
within the domain. As another example, the Receiver Proxy may be configurable to only proxy for flows addressed to a given destination address or destination address ranges (for which end devices are known to not be RSVP-capable). The decision to proxy a Resv for a Path may also be based on information signaled from the sender in the Path message. For example, the sender may identify the type of application or flow in the Application Identity policy element ([RFC2872]) in the Path, and the Receiver Proxy may be configured to proxy for only certain types of flows. Or, if the sender knows (for example, through application signaling) that the receiver is RSVP-capable, the sender can include an indication in a policy element to any Receiver Proxy that it ought not to terminate the Path (or conversely, if the receiver is known not to support RSVP, the sender could include an indication to Receiver Proxies that they ought to generate a proxy Resv message). The Receiver Proxy Control policy element specified in Section 4.2 of [RFC5946] can be used for that purpose.4.2. Path-Triggered Sender Proxy for Reverse Direction
In this approach, it is assumed that one endpoint is RSVP-capable and takes full care of the synchronization between application requirements and RSVP reservations. This endpoint is the sender for one flow direction (which we refer to as the "forward" direction) and is the receiver for the flow in the opposite direction (which we refer to as the "reverse" direction). With the Path-Triggered Sender Proxy for Reverse Direction approach, the RSVP proxy uses the RSVP signaling generated by the receiver (for the reverse direction) as the cue for initiating RSVP signaling for the reservation in the reverse direction. More precisely, the RSVP proxy can take the creation (or maintenance or teardown) of a Path state by the receiver as the cue to create (or maintain or tear down, respectively) a Path state towards the receiver. Thus, the RSVP proxy is effectively acting as a Sender Proxy for the reverse direction under the control of the receiver (for the reverse direction). Note that this assumes a degree of symmetry, for example, in terms of bandwidth for the two directions of the flow (as is currently typical for IP telephony). The signaling flow for the Path-Triggered Sender Proxy for Reverse Direction is illustrated in Figure 6. Path messages generated by the receiver need to transit via the RSVP Sender Proxy that is on the path from the sender to the receiver. In some topologies, this will always be the case: for example, where the sender is on a stub network hanging off the RSVP Sender Proxy or
where there is no asymmetric routing (such that if an RSVP Sender Proxy is on the path from receiver to sender, then it is also on the path from sender to receiver). In some topologies (such as those involving asymmetric routing), this may not always happen naturally. Measures to ensure this does happen in these topologies are outside the scope of this document. |****| *** *** |**********| |----| | R |---------*r*----------*r*---------| RSVP |----------| S | |****| *** *** | Sender | |----| | Proxy | |**********| ---Path---> ----Path----> ---Path----> <--Path---> <---Path----- <--Path---- ---Resv---> ----Resv----> ---Resv----> <================RSVP================== <********************************************************** |****| RSVP-capable |----| Non-RSVP-capable *** | R | Receiver for | S | Sender for *r* regular RSVP |****| reverse direction |----| reverse direction *** router ***> media flow ==> segment of flow path protected by RSVP reservation in reverse direction Figure 6: Path-Triggered Sender Proxy for Reverse Direction Of course, the RSVP proxy may simultaneously (and typically will) also act as the Path-Triggered Receiver Proxy for the forward direction, as defined in Section 4.1. Such an approach is most useful in situations involving RSVP reservations in both directions for symmetric flows. This is illustrated in Figure 7.
|****| *** *** |----------| |----| |S/R |---------*r*----------*r*---------| RSVP |----------|S/R | |****| *** *** | Receiver | |----| | & Sender | | Proxy | |----------| ---Path---> ----Path----> ---Path----> <--Resv---> <---Resv----- <--Resv---- <--Path---> <---Path----- <--Path---- ---Resv---> ----Resv----> ---Resv----> ================RSVP==================> <================RSVP================== **********************************************************> <********************************************************** |****| RSVP-capable |----| Non-RSVP-capable *** |S/R | Sender and |S/R | Sender and *r* regular RSVP |****| Receiver |----| Receiver *** router ***> media flow ==> segment of flow path protected by RSVP reservation in forward and in reverse direction Figure 7: Path Triggered Receiver and Sender Proxy With the Path-Triggered Sender Proxy for Reverse Direction approach, the RSVP router may be configurable to use receipt of a regular RSVP Path message as the trigger for Sender Proxy for Reverse Direction behavior. On receipt of the RSVP Path message for the forward direction, the RSVP Sender Receiver Proxy: 1. sinks the Path message. 2. behaves as if a Path message for the reverse direction (whose details are discussed below) had been received by the Sender Proxy. This includes establishing the corresponding Path state, forwarding the Path message downstream, sending periodic
refreshes of the Path message, and tearing down the Path in the
reverse direction when the Path state in the forward direction is
torn down.
In order to build the Path message for the reverse direction, the
RSVP Sender Proxy can take into account information in the received
Path message for the forward direction. For example, the RSVP Sender
Proxy may mirror the SENDER_TSPEC object in the received Path
message.
We observe that this approach does not require any extensions to the
existing RSVP protocol.
In the case where reservations are required in both directions (as
shown in Figure 7), the RSVP-capable device simply needs to behave as
a regular RSVP sender and RSVP receiver. It need not be aware that
an RSVP proxy happens to be used, and the Path message it sent for
the forward reservation also acts as the trigger for establishment of
the reverse reservation. However, in the case where a reservation is
only required in the reverse direction (as shown in Figure 6), the
RSVP-capable device has to generate Path messages in order to trigger
the reverse-direction reservation even if no reservation is required
in the forward direction. Although this is not in violation of
[RFC2205], it may not be the default behavior of an RSVP-capable
device and therefore may need a behavioral change specifically to
facilitate operation of the Path-Triggered Sender Proxy for Reverse
Direction.
4.3. Inspection-Triggered Proxy
In this approach, it is assumed that the RSVP proxy is on the data
path of "packets of interest", that it can inspect such packets on
the fly as they transit through it, and that it can infer information
from these packets of interest to determine what RSVP reservations
need to be established, as well as when and with what characteristics
(possibly also using some configured information).
One example of "packets of interest" could be application-level
signaling. An RSVP proxy capable of inspecting SIP signaling for a
multimedia session or RTSP signaling for video streaming can obtain
from such signaling information about when a multimedia session is up
or when a video is going to be streamed. It can also identify the
addresses and ports of senders and receivers and can determine the
bandwidth of the corresponding flows. It can also determine when the
reservation is no longer needed and tear it down. Thus, such an RSVP
proxy can determine all necessary information to synchronize RSVP
reservations to application requirements. This is illustrated in
Figure 8.
|-------------| | Application | | Signaling | | Entity | |-------------| / \ / \ / \ </////////////////////// \\\\\\\\\\\\\\\\\\\\\\\\> |----| |********| *** |********| |----| | S |--------| RSVP |------*r*--------| RSVP |----------| R | |----| | Proxy | *** | Proxy | |----| |********| |********| =======RSVP=======> ********************************************************> |----| Non-RSVP-capable |----| Non-RSVP-capable *** | S | Sender | R | Receiver *r* regular RSVP |----| |----| *** router </\> application-level signaling ***> media flow ==> segment of flow path protected by RSVP reservation Figure 8: Inspection-Triggered RSVP Proxy Another example of "packets of interest" could be transport control messages (e.g., the Real-time Transport Control Protocol (RTCP) [RFC3550]) traveling alongside the application flow itself (i.e., media packets). An RSVP proxy capable of detecting the transit of packets from a particular flow can attempt to establish a reservation corresponding to that flow. Characteristics of the reservation may be derived by various methods such as from configuration, flow measurement, or a combination of those. However, these methods usually come with their respective operational drawbacks: configuration involves an operational cost and may hinder introduction of new applications, and measurement is reactive so that accurate reservation may lag actual traffic. In the case of reservation failure, the Inspection-Triggered RSVP Proxy does not have a direct mechanism for notifying the application (since it is not participating itself actively in application
signaling) so that the application is not in a position to take appropriate action (for example, terminate the corresponding session). To mitigate this problem, the Inspection-Triggered RSVP Proxy may differently mark the Differentiated Services codepoint (DSCP) ([RFC2474]) of flows for which an RSVP reservation has been successfully proxied from the flows for which a reservation is not in place. In some situations, the Inspection-Triggered Proxy might be able to modify the "packets of interest" (e.g., application signaling messages) to convey some hint to applications that the corresponding flows cannot be guaranteed by RSVP reservations. With the Inspection-Triggered Proxy approach, the RSVP proxy is effectively required to attempt to build application awareness by traffic inspection and then is somewhat limited in the actions it can take in case of reservation failure. Depending on the "packets of interest" used by the RSVP proxy to trigger the reservation, there is a risk that the RSVP proxy will end up establishing a reservation for a media flow that actually never starts. However, this can be mitigated by the timing out and tearing down of an unnecessary reservation by the RSVP proxy when no corresponding media flow is observed. This flow observation and timeout approach can also be used to tear down reservations that were rightfully established for a flow but are no longer needed because the flow stopped. The Inspection-Triggered approach is also subject to the general limitations associated with data inspection. This includes being impeded by encryption or tunneling, or being dependent on some topology constraints such as relying on the fact that both the packets of interest and the corresponding flow packets always transit through the same RSVP proxy. Nonetheless, this may be a useful approach in specific environments. Note also that this approach does not require any change to the RSVP protocol. With the Inspection-Triggered RSVP Proxy approach, the RSVP router may be configurable to use and interpret some specific packets of interest as the trigger for RSVP Receiver Proxy behavior. When operating off signaling traffic, the Inspection-Triggered RSVP Proxy may be able to detect from the signaling that the endpoint is capable of establishing an RSVP reservation (e.g., in the case of SIP, via the inspection of the [RFC3312]/[RFC4032] precondition), in which case it would not behave as a proxy for that endpoint. Also, the Inspection-Triggered RSVP Proxy may inspect RSVP signaling, and if it sees RSVP signaling for the flow of interest, it can disable its Sender Proxy behavior for that flow (or that sender). Optionally, through RSVP signaling inspection, the Sender Proxy might
also gradually "learn" (possibly with some timeout) which sender is RSVP-capable and which is not. These mechanisms can facilitate gradual and dynamic migration from the proxy model towards the end- to-end RSVP model as more and more endpoints become RSVP-capable.4.4. STUN-Triggered Proxy
In this approach, the RSVP proxy takes advantage of the application awareness provided by the Session Traversal Utilities for NAT (STUN) ([RFC5389]) signaling to synchronize RSVP reservations with application requirements. The STUN signaling is sent from endpoint to endpoint. This is illustrated in Figure 9. In this approach, a STUN message triggers the RSVP proxy. |----| |********| *** |********| |----| | S |--------| RSVP |------*r*--------| RSVP |----------| R | |----| | Proxy | *** | Proxy | |----| |********| |********| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^> =======RSVP=======> ********************************************************> |----| Non-RSVP-capable |----| Non-RSVP-capable *** | S | Sender | R | Receiver *r* regular RSVP |----| |----| *** router ^^^> STUN message flow (over same UDP ports as media flow) ==> segment of flow path protected by RSVP reservation ***> RTP media flow Figure 9: STUN-Triggered Proxy For unicast flows, [RFC5245] is a widely adopted approach for Network Address Translator (NAT) traversal. For our purposes of triggering RSVP proxy behavior, we rely on the Interactive Connectivity Establishment (ICE) protocol's connectivity check, which is based on the exchange of STUN Binding Request messages between hosts to verify connectivity (see Section 2.2 of [RFC5245]). The STUN message could also include (yet to be specified) STUN attributes to indicate information such as the bandwidth and application requesting the flow, which would allow the RSVP proxy agent to create an
appropriately sized reservation for each flow. Including such new
STUN attributes in the ICE connectivity check messages would
facilitate operation of the RSVP proxy. To ensure RSVP reservations
are only established when needed, the RSVP proxy needs to
distinguish, among all the STUN messages, the ones that reflect (with
high likelihood) an actual upcoming media flow. This can be achieved
by identifying the STUN messages associated with an ICE connectivity
check. In turn, this can be achieved through (some combination of)
the following checks:
o if, as discussed above, new STUN attributes (e.g., conveying the
flow bandwidth) are indeed defined in the future in view of
facilitating STUN-Triggered reservations, then the presence of
these attributes would reveal that the STUN message is part of an
ICE connectivity check.
o the presence of the PRIORITY, USE-CANDIDATE, ICE-CONTROLLED, or
ICE-CONTROLLING attributes reveals that the STUN message is part
of an ICE connectivity check.
o the RSVP proxy may wait for a STUN message containing the USE-
CANDIDATE attribute indicating the selected ICE "path" to trigger
reservation only for the selected "path". This allows the RSVP
proxy to only trigger a reservation for the "path" actually
selected and therefore for the media flow that will actually be
established (for example, when ICE is being used for IPv4/v6 path
selection).
o the RSVP proxy configuration could contain some information
facilitating determination of when to perform RSVP proxy
reservation and when not to. For example, the RSVP proxy
configuration could contain the IP addresses of the STUN servers
such that STUN messages to/from those addresses are known to not
be part of an ICE connectivity check. As another example, the
RSVP proxy configuration could contain information identifying the
set of Differentiated Services codepoint (DSCP) values that the
media flows requiring reservation use, so that STUN messages not
using one of these DSCP values are known to not be part of an ICE
connectivity check.
Despite these checks, there is always a potential risk that the RSVP
proxy will end up establishing a reservation for a media flow that
actually never starts. However, this is limited to situations in
which the end-systems are interested enough in establishing
connectivity for a flow but never transmit. Also, this can be
mitigated by timing out and tear down of an unnecessary reservation
by the RSVP proxy when no corresponding media flow is observed.
The RSVP proxy agent can inform endpoints of an RSVP reservation failure implicitly by dropping the ICE connectivity check message or explicitly by sending ICMP messages back to the endpoint. This allows reasonably effective synchronization between RSVP reservations handled by the RSVP proxies and the application running on non-RSVP- capable endpoints. It also has the benefits of operating through NATs. For multicast flows (or certain kinds of unicast flows that don't or can't use ICE), a STUN Indication message [RFC5389] could be used to carry the (yet to be defined) STUN attributes mentioned earlier to indicate the flow bandwidth, thereby providing a benefit similar to the ICE connectivity check. STUN Indication messages are not acknowledged by the receiver and have the same scalability as the underlying multicast flow. The corresponding extensions to ICE and STUN for such a STUN- Triggered RSVP Proxy approach are beyond the scope of this document. They may be defined in the future in a separate document. As the STUN-Triggered RSVP Proxy approach uses STUN in a way (i.e., to trigger reservations) that is beyond its initial intended purpose, the potential security implications need to be considered by the operator. ICE connectivity checks are not always used for all flows. When the STUN-Triggered RSVP Proxy approach is used, it can establish RSVP reservations for flows for which ICE connectivity is performed. However, the STUN-Triggered RSVP Proxy will not establish a reservation for flows for which an ICE connectivity check is not performed. Those flows either will not benefit from an RSVP reservation or can benefit from an RSVP reservation established through other means (end-to-end RSVP, other forms of RSVP proxy). The STUN-Triggered approach relies on interception and inspection of STUN messages. Thus, this approach may be impeded by encryption or tunneling.4.5. Application_Entity-Controlled Proxy
In this approach, it is assumed that an entity involved in the application-level signaling controls an RSVP proxy that is located in the data path of the application flows (i.e., "on-path"). With this approach, the RSVP proxy does not itself attempt to determine the application reservation requirements. Instead, the RSVP proxy is instructed by the entity participating in application-level signaling to establish, maintain, and tear down reservations as needed by the application flows. In other words, with this approach, the solution for synchronizing RSVP signaling with application-level requirements
is to rely on an application-level signaling entity that controls an RSVP proxy function that sits in the flow data path. This approach allows control of an RSVP Sender Proxy, an RSVP Receiver Proxy, or both. Operation of the Application_Entity-Controlled Proxy is illustrated in Figure 10. |---------| |---------| /////////| App |////\\\\| App |\\\\\\\\ / | Entity | | Entity | \ / |---------| |---------| \ / // \\ \ / // \\ \ / // \\ \ / // \\ \ / // \\ \ |----| |********| *** |*********| |----| | S |----------| |------*r*-------| |---------| R | |----| | RSVP | *** | RSVP | |----| | Sender | | Receiver| | Proxy | | Proxy | |********| |*********| =======RSVP=======> ********************************************************> |----| Non-RSVP-capable |----| Non-RSVP-capable *** | S | Sender | R | Receiver *r* regular RSVP |----| |----| *** router ***> media flow ==> segment of flow path protected by RSVP reservation /\ Application signaling (e.g., SIP) // RSVP proxy control interface Figure 10: Application_Entity-Controlled Proxy As an example, the Application_Entity-Controlled Proxy may be used in the context of SIP servers ([RFC3261]) or Session Border Controllers (SBCs) (see [RFC5853] for a description of SBCs) to establish RSVP reservations for multimedia sessions. In that case, the application entity may be the signaling component of the SBC.
This RSVP proxy approach does not require any extension to the RSVP protocol. However, it relies on an RSVP proxy control interface allowing control of the RSVP proxy by an application signaling entity. This RSVP proxy control interface is beyond the scope of this document. Candidate protocols for realizing such an interface include the IETF Network Configuration (NETCONF) Protocol ([RFC4741], [RFC5277]), the Web Services protocol ([W3C]), the QoS Policy Information Model (QPIM) ([RFC3644]), and Diameter ([RFC3588]). This interface can rely on soft states or hard states. Clearly, when hard states are used, those need to be converted appropriately by the RSVP proxy entities into the corresponding RSVP soft states. As an example, [RFC5866] is intended to allow control of RSVP proxy via Diameter. In general, the application entity is not expected to maintain awareness of which RSVP Receiver Proxy is on the path to which destination. However, in the particular cases where it does so reliably, we observe that the application entity could control the RSVP Sender Proxy and Receiver Proxy so that aggregate RSVP reservations are used between those, instead of one reservation per flow. For example, these aggregate reservations could be of the RSVP-AGGREGATE type, as specified in [RFC3175], or of the GENERIC- AGGREGATE type, as specified in [RFC4860]. Such aggregate reservations could be used so that a single reservation can be used for multiple (possibly all) application flows transiting via the same RSVP Sender Proxy and the same RSVP Receiver Proxy. For situations in which only the RSVP Sender Proxy has to be controlled by this interface, the interface may be realized through the simple use of RSVP itself, over a Generic Routing Encapsulation (GRE) tunnel from the application entity to the RSVP Sender Proxy. This particular case is further discussed in Section 4.5.1. Another particular case of interest is where the application signaling entity resides on the same device as the RSVP proxy. In that case, this interface may be trivially realized as an internal API. An example environment based on this particular case is illustrated in Section 4.5.2. The application entity controlling the RSVP proxy (e.g., a SIP Call Agent) would often be aware of a number of endpoint capabilities, and it has to be aware of which endpoint can be best "served" by which RSVP proxy anyways. So it is reasonable to assume that such an application is aware of whether a given endpoint is RSVP-capable or not. The application may also consider the QoS preconditions and QoS mechanisms signaled by an endpoint as per [RFC3312]/[RFC4032] and [RFC5432]. The information about endpoint RSVP capability can then be used by the application to decide whether to trigger proxy behavior or not for a given endpoint. This can facilitate gradual
and dynamic migration from the proxy model towards the end-to-end RSVP model as more and more endpoints become RSVP-capable. In some environments, the application entities (e.g., SIP back-to- back user agents) that need to control the RSVP proxies would already be deployed independently of the use, or not, of the Application_Entity-Controlled Proxy approach. In this case, the activation of the RSVP proxy approach should not introduce significant disruption in the application signaling path. In some environments, additional application entities may need to be deployed to control the RSVP proxies. In this case, the network operator needs to consider the associated risks of disruption to the application signaling path.4.5.1. Application_Entity-Controlled Sender Proxy Using "RSVP over GRE"
This approach is simply a particular case of the more general Application_Entity-Controlled Proxy, but where only RSVP Sender Proxies need to be controlled by the application, and where RSVP is effectively used as the control protocol between the application- signaling entity and the RSVP Sender Proxy. In this approach, the RSVP messages (e.g., RSVP Path message) are effectively generated by the application entity and logically "tunneled" to the RSVP Sender Proxy via GRE tunneling. This is to ensure that the RSVP messages follow the exact same path as the flow they protect (as required by RSVP operations) on the segment of the end-to-end path that is to be subject to RSVP reservations. Figure 11 illustrates such an environment.
|-------------| ////////////| Application |\\\\\\\\\ / | Entity | \ / |-------------| \ / /=/ \ / /=/ \ / /=/ \ / /=/ \ / /=/ \ / /=/ \ / /=/ \ / /=/ \ |----| |********| *** |****| | S |-----------| RSVP |-----------*r*-----------------| R | |----| | Sender | *** |****| | Proxy | |********| =========RSVP====================> *****************************************************> |----| non-RSVP-capable |----| RSVP-capable *** | S | Sender | R | Receiver *r* regular RSVP |----| |----| *** router ***> media flow ==> segment of flow path protected by RSVP reservation /\ Application-level signaling /=/ GRE-tunneled RSVP (Path messages) Figure 11: Application_Entity-Controlled Sender Proxy via "RSVP over GRE" With the Application_Entity-Controlled Sender Proxy using "RSVP Over GRE", the application entity: o generates a Path message on behalf of the sender, corresponding to the reservation needed by the application, and maintains the corresponding Path state. The Path message built by the application entity is exactly the same as would be built by the actual sender (if it was RSVP-capable), with one single exception, which is that the application entity puts its own IP address as the RSVP previous hop. In particular, it is recommended that the
source address of the Path message built by the application entity
be set to the IP address of the sender (not of the application
entity). This helps ensure that, in the presence of non-RSVP
routers and of load-balancing in the network where the load-
balancing algorithm takes into account the source IP address, the
Path message generated by the application entity follows the exact
same path as the actual stream sourced by the sender.
o encapsulates the Path message into a GRE tunnel whose destination
address is the RSVP Sender Proxy, i.e., an RSVP router sitting on
the data path for the flow (and upstream of the segment that
requires QoS guarantees via RSVP reservation).
o processes the corresponding received RSVP messages (including Resv
messages) as per regular RSVP.
o synchronizes the RSVP reservation state with application-level
requirements and signaling.
Note that since the application entity encodes its own IP address as
the previous RSVP hop inside the [RFC2205] RSVP_HOP object of the
Path message, the RSVP router terminating the GRE tunnel naturally
addresses all the RSVP messages traveling upstream hop-by-hop (such
as Resv messages) to the application entity (without having to
encapsulate those in a reverse-direction GRE tunnel towards the
application entity).
4.5.2. Application_Entity-Controlled Proxy via Co-Location
This approach is simply a particular case of the more general
Application_Entity-Controlled Proxy, but where the application entity
is co-located with the RSVP proxy. As an example, Session Border
Controllers (SBCs) with on-board SIP agents could implement RSVP
proxy functions and make use of such an approach to achieve session
admission control over the SBC-to-SBC segment using RSVP signaling.
Figure 12 illustrates operations of the Application_Entity-Controlled
RSVP Proxy via co-location.
|---------| |---------| ////////| App |////////\\\\\\\| App |\\\\\\\\\ / | Entity | | Entity | \ / | | | | \ |----| |*********| *** |*********| |----| | S |--------| RSVP |------*r*------| RSVP |---------| R | |----| | Sender | *** | Receiver| |----| | Proxy | | Proxy | |*********| |*********| =======RSVP======> *******************************************************> |----| Non-RSVP-capable |----| Non-RSVP-capable *** | S | Sender | R | Receiver *r* regular RSVP |----| |----| *** router ***> media flow ==> segment of flow path protected by RSVP reservation /\ Application-level signaling Figure 12: Application_Entity-Controlled Proxy via Co-Location This RSVP proxy approach does not require any protocol extensions. We also observe that when multiple sessions are to be established on paths sharing the same RSVP Sender Proxy and the same RSVP Receiver Proxy, the RSVP proxies have the option to establish aggregate RSVP reservations (as defined in ([RFC3175] or [RFC4860]) for a group of sessions, instead of establishing one RSVP reservation per session.4.6. Policy_Server-Controlled Proxy
In this approach, it is assumed that a policy server, which is located in the control plane of the network, controls an RSVP proxy that is located in the data path of the application flows (i.e., "on- path"). In turn, the policy server is triggered by an entity involved in the application-level signaling. With this approach, the RSVP proxy does not itself attempt to determine the application reservation requirements, but instead is instructed by the policy server to establish, maintain, and tear down reservations as needed by the application flows. Moreover, the entity participating in application-level signaling does not attempt to understand the specific reservation mechanism (i.e., RSVP) or the topology of the network layer, but instead it simply asks the policy server to
perform (or tear down) a reservation. In other words, with this approach, the solution for synchronizing RSVP signaling with application-level requirements is to rely on an application-level entity that controls a policy server that, in turn, controls an RSVP proxy function that sits in the flow data path. This approach allows control of an RSVP Sender Proxy, an RSVP Receiver Proxy, or both. Operation of the Policy_Server-Controlled proxy is illustrated in Figure 13.
|---------| /////////////| App |\\\\\\\\\\\\\\ / | Entity | \ / |---------| \ / I \ / I \ / |----------| \ / | Policy | \ / | Server | \ / |----------| \ / // \\ \ / // \\ \ / // \\ \ |----| |********| *** |*********| |----| | S |-----------| |------*r*-----| |----------| R | |----| | RSVP | *** | RSVP | |----| | Sender | | Receiver| | Proxy | | Proxy | |********| |*********| =====RSVP========> **********************************************************> |----| Non-RSVP-capable |----| Non-RSVP-capable *** | S | Sender | R | Receiver *r* regular RSVP |----| |----| *** router ***> media flow ==> segment of flow path protected by RSVP reservation /\ Application signaling (e.g., SIP) // RSVP proxy control interface I Interface between application entity and policy server Figure 13: Policy_Server-Controlled Proxy This RSVP proxy approach does not require any extension to the RSVP protocol. However, as with the Application_Entity-Controlled Proxy approach presented in Figure 10, this approach relies on an RSVP proxy control interface allowing control of the RSVP proxy (by the policy server in this case). This RSVP proxy control interface is beyond the scope of this document. Considerations about candidate protocols for realizing such an interface can be found in
Section 4.5. Again, for situations in which only the RSVP Sender Proxy has to be controlled by this interface, the interface may be realized through the simple use of RSVP itself, over a GRE tunnel from the policy server to the RSVP Sender Proxy. This is similar to what is presented in Section 4.5.1, except that the "RSVP over GRE" interface is used in this case by the policy server (instead of the application entity). The interface between the application entity and the policy server is beyond the scope of this document.4.7. RSVP-Signaling-Triggered Proxy
An RSVP proxy can also be triggered and controlled through extended RSVP signaling from the remote end that is RSVP-capable (and supports these RSVP extensions for proxy control). For example, an RSVP- capable sender could send a new or extended RSVP message explicitly requesting an RSVP proxy on the path towards the receiver to behave as an RSVP Receiver Proxy and also to trigger a reverse-direction reservation, thus also behaving as an RSVP Sender Proxy. The new or extended RSVP message sent by the sender could also include attributes (e.g., bandwidth) for the reservations to be signaled by the RSVP proxy. The challenges in these explicit signaling schemes include the following: o How can the nodes determine when a reservation request ought to be proxied and when it should not, and accordingly invoke appropriate signaling procedures? o How does the node sending the messages explicitly triggering the proxy know where the proxy is located, e.g., determine an IP address of the proxy that should reply to the signaling? o How is all the information needed by a Sender Proxy to generate a Path message actually communicated to the proxy? An example of such a mechanism is presented in [QOS-MOBILE]. This scheme is primarily targeted to local access network reservations whereby an end host can request resource reservations for both incoming and outgoing flows only over the access network. This may be useful in environments where the access network is typically the bottleneck while the core is comparatively over-provisioned, as may be the case with a number of radio access technologies. In this proposal, messages targeted to the proxy are flagged with one bit in all RSVP messages. Similarly, all RSVP messages sent back by the proxy are also flagged. The use of such a flag allows
differentiating between proxied and end-to-end reservations. For triggering an RSVP Receiver Proxy, the sender of the data sends a Path message that is marked with the mentioned flag. The Receiver Proxy is located on the signaling and data path, eventually gets the Path message, and replies back with a Resv message. A node triggers an RSVP Sender Proxy with a newly defined Path_Request message, which instructs the proxy to send Path messages towards the triggering node. The node then replies back with a Resv. More details can be found in [QOS-MOBILE]. Such an RSVP-Signaling-Triggered Proxy approach would require RSVP signaling extensions (that are outside the scope of this document). However, it could provide more flexibility in the control of the proxy behavior (e.g., control of reverse reservation parameters) than would the Path-Triggered approaches defined in Section 4.1 and Section 4.2. Through potential corresponding protocol extensions, an RSVP- Signaling-Triggered Proxy approach could facilitate operation (e.g., reduce or avoid the need for associated configuration) in hybrid environments involving both reservations established end-to-end and reservations established via RSVP proxies. For example, [QOS-MOBILE] proposed a mechanism allowing an end-system to control whether a reservation can be handled by an RSVP proxy on the path, or is to be established end-to-end.4.8. Reachability Considerations
There may be situations in which the RSVP Receiver Proxy is reachable by the sender, while the receiver itself is not. In such situations, it is possible that the RSVP Receiver Proxy is not always aware that the receiver is unreachable, and consequently may accept to establish an RSVP reservation on behalf of that receiver. This would result in unnecessary reservation establishment and unnecessary network resource consumption. This is not considered a significant practical concern for a number of reasons. First, in many cases, if the receiver is not reachable from the sender, it will not be reachable for application signaling either, and so application-level session establishment will not be possible in the first place. Secondly, where the receiver is unreachable from the sender but is reachable for application-level signaling (say, because session establishment is performed through an off-path SIP agent that uses a different logical topology to communicate with the receiver), then the sender may detect that the receiver is unreachable before attempting reservation establishment. This may be achieved through mechanisms such as ICE's connectivity check ([RFC5245]). Finally, even if the sender does not detect that
the receiver is unreachable before triggering the RSVP reservation establishment, it is very likely that the application will quickly realize this lack of connectivity (e.g., the human accepting the phone call on the receiver side will not hear the human's voice on the sender side) and therefore tear down the session (e.g., hang up the phone), which in turn will trigger RSVP reservation release. Nonetheless, it is recommended that network administrators consider the above in light of their particular environment when deploying RSVP proxies. The mirror considerations apply for situations involving an RSVP Sender Proxy and where the sender cannot reach the destination while the RSVP Sender Proxy can.
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