Network Working Group J. Manner
Request for Comments: 4094 X. Fu
Category: Informational May 2005

Analysis of Existing Quality-of-Service Signaling Protocols

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

This document reviews some of the existing Quality of Service (QoS)
signaling protocols for an IP network. The goal here is to learn
from them and to avoid common misconceptions. Further, we need to
avoid mistakes during the design and implementation of any new
protocol in this area.

Table of Contents

1. IntrodUCtion ....................................................3
2. RSVP and RSVP Extensions ........................................4
2.1. Basic Design ...............................................4
2.1.1. Signaling Model .....................................4
2.1.2. Soft State ..........................................5
2.1.3. Two-Pass Signaling Message Exchanges ................5
2.1.4. Receiver-Based Resource Reservation .................5
2.1.5. Separation of QoS Signaling from Routing ............5
2.2. RSVP Extensions ............................................6
2.2.1. Simple Tunneling ....................................6
2.2.2. IPsec Interface .....................................6
2.2.3. Policy Interface ....................................6
2.2.4. Refresh Reduction ...................................7
2.2.5. RSVP over RSVP ......................................8
2.2.6. IEEE 802-Style LAN Interface ........................8
2.2.7. ATM Interface .......................................9
2.2.8. DiffServ Interface ..................................9
2.2.9. Null Service Type ...................................9
2.2.10. MPLS Traffic Engineering ..........................10
2.2.11. GMPLS and RSVP-TE .................................11
2.2.12. GMPLS Operation at UNI and E-NNI Reference
Points ............................................12
2.2.13. MPLS and GMPLS Future Extensions ..................12
2.2.14. ITU-T H.323 Interface .............................13
2.2.15. 3GPP Interface ....................................13
2.3. Extensions for New Deployment Scenarios ...................14
2.4. Conclusion ................................................15
3. RSVP Transport Mechanism Issues ................................16
3.1. Messaging Reliability .....................................16
3.2. Message Packing ...........................................17
3.3. MTU Problem ...............................................17
3.4. RSVP-TE vs. Signaling Protocol for RT Applications ........18

3.5. What Would Be a Better Alternative? .......................18
4. RSVP Protocol Performance Issues ...............................19
4.1. Processing Overhead .......................................19
4.2. Bandwidth Consumption .....................................20
5. RSVP Security and Mobility .....................................21
5.1. Security ..................................................21
5.2. Mobility Support ..........................................22
6. Other QoS Signaling Proposals ..................................23
6.1. Tenet and ST-II ...........................................23
6.2. YESSIR ....................................................24
6.2.1. Reservation Functionality ..........................24
6.2.2. Conclusion .........................................25
6.3. Boomerang .................................................25
6.3.1. Reservation Functionality ..........................25
6.3.2. Conclusions ........................................26
6.4. INSIGNIA ..................................................26
7. Inter-Domain Signaling .........................................27
7.1. BGRP ......................................................27
7.2. SICAP .....................................................27
7.3. DARIS .....................................................28
8. Security Considerations ........................................30
9. Summary ........................................................30
10. Contributors ..................................................31
11. Acknowledgements ..............................................31
12. Appendix A: Comparison of RSVP to the NSIS Requirements .......32
13. Normative References ..........................................38
14. Informative References ........................................38

1. Introduction

This document reviews some of the existing QoS signaling protocols
for an IP network. The goal here is to learn from them and to avoid
common misconceptions. Further, we need to avoid mistakes during the
design and implementation of any new protocol in this area.

There have been a number of historic attempts to deliver QoS or
generic signaling to the Internet. In the early years, it was
believed that multicast would be popular for the majority of
communications; thus, both RSVP and earlier ST-II were designed in a
way that is multicast-oriented.

ST-II was developed as a reservation protocol for point-to-multipoint
communication. However, since it is sender-initiated, it does not
scale with the number of receivers in a multicast group. Its
processing is fairly complex. Since every sender needs to set up its
own reservation, the total amount of reservation states is large.
RSVP was then designed to provide support for multipoint-to-
multipoint reservation setup in a more efficient way. However, its
complexity, scalability, and ability to meet new requirements have

been criticized.

YESSIR (YEt another Sender Session Internet Reservations) [PS98] and
Boomerang [FNM+99] are examples of protocols designed after RSVP.
Both were meant to be simpler than RSVP. YESSIR is an extension to
RTCP, whereas Boomerang is used with ICMP.

Previously, a lot of work has been targeted at creating a new
signaling protocol for resource control. Istvan Cselenyi suggested
having a QoSSIG BOF in IETF47, for identifying problems in QoS
signaling, but failed to get enough support [URL1]. Some people
argued, "in many ways, RSVP improved upon ST-2, and it did start out
simpler, but it resulted in a design with complexity and
scalability", while others thought that "new knowledge and
requirements" made RSVP insufficient. Some concluded that there is
no simpler way to handle the same problem than RSVP.

Michael Welzl organized a special session "ABR to the Internet" in
SCI 2001, and gathered some inputs for requesting an "ABR to the
Internet" BOF in IETF#51, which was intended to introduce eXPlicit
rate-feedback-related mechanisms for the Internet (e2e, edge2edge).
This failed because of "missing community interest".

OPENSIG [URL2] has been involved in the Internet signaling for years.
Ping Pan initiated a SIGLITE [URL3] BOF mailing list to investigate
lightweight Internet signaling. Finally, NSIS BOF was successful,
and the NSIS WG was formed.

The most mature and original protocols are presented in their own
sections, and other QoS signaling protocols are presented in later
subsections. The presented protocols are chosen based on relevance
to the work within NSIS. The aim is not to review every existing
protocol.

2. RSVP and RSVP Extensions

RSVP (the Resource Reservation Protocol) [ZDSZ93] [RFC2205] [BEBH96]
has evolved from ST-II to provide end-to-end QoS signaling services
for application data streams. Hosts use RSVP to request a specific
quality of service (QoS) from the network for particular application
flows. Routers use RSVP to deliver QoS requests to all routers along
the data path. RSVP also can maintain and refresh states for a
requested QoS application flow.

By original design, RSVP fits well into the framework of the
Integrated Services (IntServ) [RFC2210] [BEBH96] with certain
modularity and scalability.

RSVP carries QoS signaling messages through the network, visiting
each node along the data path. To make a resource reservation at a
node, the RSVP module communicates with two local decision modules,
admission control and policy control. Admission control determines
whether the node has sufficient available resources to supply the
requested QoS. Policy control provides authorization for the QoS
request. If either check fails, the RSVP module returns an error

notification to the application process that originated the request.
If both checks succeed, the RSVP module sets parameters in a packet
classifier and packet scheduler to oBTain the desired QoS.

2.1. Basic Design

The design of RSVP distinguished itself by a number of fundamental
ways; particularly, soft state management, two-pass signaling message
exchanges, receiver-based resource reservation, and separation of QoS
signaling from routing.

2.1.1. Signaling Model

The RSVP signaling model is based on a special handling of multicast.
The sender of a multicast flow advertises the traffic characteristics
periodically to the receivers via "Path" messages. Upon receipt of
an advertisement, a receiver may generate a "Resv" message to reserve
resources along the flow path from the sender. Receiver reservations
may be heterogeneous. To accommodate the multipoint-to-multipoint
multicast applications, RSVP was designed to support a vector of
reservation attributes called the "style". A style describes whether
all senders of a multicast group share a single reservation and which
receiver is applied. The "Scope" object additionally provides the
explicit list of senders.

2.1.2. Soft State

Because the number of receivers in a multicast flow is likely to
change, and the flow of delivery paths might change during the life
of an application flow, RSVP takes a soft-state approach in its
design, creating and removing the protocol states (Path and Resv
states) in routers and hosts incrementally over time. RSVP sends
periodic refresh messages (Path and Resv) to maintain its states and
to recover from occasional lost messages. In the absence of refresh
messages, the RSVP states automatically time out and are deleted.
States may also be deleted explicitly by PathTear, PathErr with
Path_State_Removed flag, or ResvTear Message.

2.1.3. Two-Pass Signaling Message Exchanges

The receiver in an application flow is responsible for requesting the
desired QoS from the sender. To do this, the receiver issues an RSVP
QoS request on behalf of the local application. The request
propagates to all routers in reverse direction of the data paths
toward the sender. In this process, RSVP requests might be merged,
resulting in a protocol that scales well when there are a large
number of receivers.

2.1.4. Receiver-Based Resource Reservation

Receiver-initiation is critical for RSVP to set up multicast sessions
with a large number of heterogeneous receivers. A receiver initiates
a reservation request at a leaf of the multicast distribution tree,
traveling toward the sender. Whenever a reservation is found to
already exist in a node in the distribution tree, the new request
will be merged with the existing reservation. This could result in

fewer signaling operations for the RSVP nodes in the multicast tree
close to the sender but could introduce a restriction to receiver-
initiation.

2.1.5. Separation of QoS Signaling from Routing

RSVP messages follow normal IP routing. RSVP is not a routing
protocol, but rather is designed to operate with current and future
unicast and multicast routing protocols. The routing protocols are
responsible for choosing the routes to use to forward packets, and
RSVP consults local routing tables to obtain routes. RSVP is
responsible only for reservation setup along a data path.

A number of messages and objects have been defined for the protocol.
A detailed description is given in [RFC2205].

2.2. RSVP Extensions

RSVP [RFC2205] was originally designed to support real-time
applications over the Internet. Over the past several years, the
demand for multicast-capable real-time teleconferencing, which many
people had envisioned to be one of the key Internet applications that
could benefit from network-wide deployment of RSVP, has never
materialized. Instead, RSVP-TE [RFC3209], a RSVP extension for
traffic engineering, has been widely deployed by a large number of
network providers to support MPLS applications.

There are a large number of protocol extensions based on RSVP. Some
provide additional features, such as security and scalability, to the
original protocol. Some introduce additional interfaces to other
services, such as DiffServ. And some simply define new applications,
such as MPLS and GMPLS, that are completely irrelevant from
protocol"s original intent.

In this section, we list only IETF-based RFCs and a limited set of
other organizations" specifications. Informational RFCs (e.g.,
RFC2998 [RFC2998]) and work-in-progress I-Ds (e.g., proxy) are not
covered here.

2.2.1. Simple Tunneling

[RFC2746] describes an IP tunneling enhancement mechanism that allows
RSVP to make reservations across all IP-in-IP tunnels, basically by
recursively applying RSVP over the tunnel portion of the path.

2.2.2. IPsec Interface

RSVP can support IPsec on a per-address, per-protocol basis instead
of on a per flow basis. [RFC2207] extends RSVP by using the IPsec
Security Parameter Index (SPI) in place of the UDP/TCP-like ports.
This introduces a new FILTER_SPEC object, which contains the IPsec
SPI, and a new SESSION object.

2.2.3. Policy Interface

[RFC2750] specifies the format of POLICY_DATA objects and RSVP"s
handling of policy events. It introduces objects that are
interpreted only by policy-aware nodes (PEPs) that interact with
policy decision points (PDPs). Nodes that are unable to interpret
the POLICY_DATA objects are called policy-ignorant nodes (PINs). The
content of the POLICY_DATA object itself is protected only between
PEPs and therefore provides end-to-middle or middle-to-middle

security.

[RFC2749] specifies the usage of COPS policy services in RSVP
environments. [RFC3181] specifies a preemption priority policy
element (PREEMPTION_PRI) for use by RSVP POLICY_DATA Object.
[RFC3520] describes how authorization provided by a separate protocol
(such as SIP) can be reused with the help of an authorization token
within RSVP. The token might therefore contain either the authorized
information itself (e.g., QoS parameters) or a reference to those
values. The token might be unprotected (which is strongly
discouraged) or protected based on symmetric or asymmetric
cryptography. Moreover, the document describes how to provide the
host with encoded session authorization information as a POLICY_DATA
object.

2.2.4. Refresh Reduction

[RFC2961] describes mechanisms to reduce processing overhead
requirements of refresh messages, eliminate the state synchronization
latency incurred when an RSVP message is lost, and refresh state
without the transmission of whole refresh messages. It defines the
following objects: MESSAGE_ID, MESSAGE_ID_ACK, MESSAGE_ID_NACK,
MESSAGE_ID LIST, MESSAGE_ID SRC_LIST, and MESSAGE_ID MCAST_LIST
objects. Three new RSVP message types are defined:

1) Bundle messages consist of a bundle header followed by a body
consisting one or more standard RSVP messages. Bundle messages
help in scaling RSVP to reduce processing overhead and bandwidth
consumption.

2) ACK messages carry one or more MESSAGE_ID_ACK or MESSAGE_ID_NACK
objects. ACK messages are sent between neighboring RSVP nodes to
detect message loss and to support reliable RSVP message delivery
on a per-hop basis.

3) Srefresh messages carry one or more MESSAGE_ID LIST, MESSAGE_ID
SRC_LIST, and MESSAGE_ID MCAST_LIST objects. They correspond to
Path and Resv messages that establish the states. Srefresh
messages are used to refresh RSVP states without transmitting
standard Path or Resv messages.

2.2.5. RSVP over RSVP

[RFC3175] allows installation of one or more aggregated reservations
in an aggregation region; thus, the number of individual RSVP
sessions can be reduced. The protocol type is swapped from RSVP to
RSVP-E2E-IGNORE in E2E (standard) Path, PathTear, and ResvConf
messages when they enter the aggregation region, and is swapped back
when they leave. In addition to a new PathErr code
(NEW_AGGREGATE_NEEDED), three new objects are introduced:

1) SESSION object, which contains two values: the IP Address of the
aggregate session destination, and the Differentiated Services
Code Point (DSCP) that it will use on the E2E data the reservation
contains.

2) SENDER_TEMPLATE object, which identifies the aggregating router
for the aggregate reservation.

3) FILTER_SPEC object, which identifies the aggregating router for
the aggregate reservation, and is syntactically identical to the

SENDER_TEMPLATE object.

From the perspective of RSVP signaling and the handling of data
packets in the aggregation region, these cases are equivalent to that
of aggregating E2E RSVP reservations. The only difference is that
E2E RSVP signaling does not take place and cannot therefore be used
as a trigger, so some additional knowledge is required for setting up
the aggregate reservation.

2.2.6. IEEE 802-Style LAN Interface

[RFC2814] introduces an RSVP LAN_NHOP address object that keeps track
of the next L3 hop as the PATH message traverses an L2 domain between
two L3 entities (RSVP PHOP and NHOP nodes). Both layer-2 and layer-3
addresses are included in the LAN_NHOP; the RSVP_HOP_L2 object is
used to include the Layer-2 address (L2ADDR) of the previous hop,
complementing the L3 address information included in the RSVP_HOP
object (RSVP_HOP_L3 address).

To provide sufficient information for debugging or resource
management, RSVP diagnostic messages (DREQ and DREP) are defined in
[RFC2745] to collect and report RSVP state information along the path
from a receiver to a specific sender.

2.2.7. ATM Interface

[RFC2379] and [RFC2380] define RSVP over ATM implementation
guidelines and requirements to interwork with the ATM (Forum) UNI
3.x/4.0. [RFC2380] states that the RSVP (control) messages and RSVP
associated data packets must not be sent on the same virtual circuits
(VCs), and that an explicit release of RSVP associated QoS VCs must
be performed once the VC for forwarding RSVP control messages
terminates. Although a separate control VC is also possible for
forwarding RSVP control messages, [RFC2379] recommends creating a
best-effort short-cut first (if one does not exist), which can allow
setting up RSVP-triggered VCs to use the best-effort end-point. (A
short-cut is a point-to-point VC where the two end-points are located
in different IP subnets.) For data flows, the subnet senders must
establish all QoS VCs, and the RSVP-enabled subnet receiver must be
able to accept incoming QoS VCs. RSVP must request that the
configurable inactivity timers of VCs be set to "infinite". If it is
too complex to do this at the VC receiver, RSVP over ATM
implementations are required not to use an inactivity timer to clear
any received connections. For dynamic QoS, the replacement of VC
should be done gracefully.

2.2.8. DiffServ Interface

RFC2996 [RFC2996] introduces a DCLASS Object to carry Differentiated
Services Code Points (DSCPs) in RSVP message objects. If the network
element determines that the RSVP request is admissible to the
DiffServ network, one or more DSCPs corresponding to the behavior
aggregate are determined, and will be carried by the DCLASS Object
added to the RESV message upstream toward the RSVP sender.

2.2.9. Null Service Type

For some applications, service parameters are specified by the

network, not by the application; e.g., enterprise resource planning
(ERP) applications. The Null Service [RFC2997] allows applications
to identify themselves to network QoS policy agents using RSVP
signaling, but does not require them to specify resource
requirements. QoS policy agents in the network respond by applying
QoS policies appropriate for the application (as determined by the
network administrator). The RSVP sender offers the new service type,
"Null Service Type", in the ADSPEC that is included with the PATH
message. A new TSPEC corresponding to the new service type is added
to the SENDER_TSPEC. In addition, the RSVP sender will typically
include with the PATH message policy objects identifying the user,
application and sub-flow, which will be used for network nodes to
manage the correspondent traffic flow.

2.2.10. MPLS Traffic Engineering

RSVP-TE [RFC3209] specifies the core extensions to RSVP for
establishing constraint-based explicitly routed LSPs in MPLS networks
using RSVP as a signaling protocol. RSVP-TE is intended for use by
label switching routers (as well as hosts) to establish and maintain
LSP-tunnels and to reserve network resources for such LSP-tunnels.

RFC3209 defines a new Hello message (for rapid node failure
detection).

RFC3209 also defines new C-Types (LSP_TUNNEL_IPv4 and
LSP_TUNNEL_IPv6) for the SESSION, SENDER_TEMPLATE, and FILTER_SPEC
objects. Here, a session is the association of LSPs that support the
LSP-tunnel. The traffic on an LSP can be classified as the set of
packets that are assigned the same MPLS label value at the
originating node of an LSP-tunnel.

The following 5 new objects are also defined:

1) EXPLICIT_ROUTE object (ERO), which is incorporated into RSVP Path
messages, encapsulating a concatenation of hops that constitutes
the explicitly routed path. Using this object, the paths taken by
label-switched RSVP-MPLS flows can be pre-determined independently
of conventional IP routing.

2) LABEL_REQUEST object. To establish an LSP tunnel, the sender can
create a Path message with a LABEL_REQUEST object. A node that
sends a LABEL_REQUEST object MUST be ready to accept and correctly
process a LABEL object in the corresponding Resv messages.

3) LABEL object. Each node that receives a Resv message containing a
LABEL object uses that label for outgoing traffic associated with
this LSP tunnel.

4) SESSION_ATTRIBUTE object, which can be added to Path messages to
aid in session identification and diagnostics. Additional control
information, such as setup and holding priorities, resource
affinities, and local-protection, are also included in this
object.

5) RECORD_ROUTE object (RRO). The RECORD_ROUTE object may appear in
both Path and Resv messages. It is used to collect detailed path
information and is useful for loop detection and for diagnostics.

Section 5 of [RFC3270] further specifies the extensions to RSVP to

establish LSPs supporting DiffServ in MPLS networks, introducing a
new DIFFSERV Object (applicable in the Path messages), and using
pre-configured or signaled "EXP<-->PHB mapping" (e.g., [RFC3270]).

RSVP-TE provides a way to indicate an unnumbered link in its Explicit
Route and Record Route Objects through [RFC3477]. This specifies the
following extensions:

- An Unnumbered Interface ID Subobject, which is a subobject of the
Explicit Route Object (ERO) used to specify unnumbered links.

- An LSP_TUNNEL_INTERFACE_ID Object, to allow the adjacent LSR to
form or use an identifier for an unnumbered Forwarding Adjacency.

- A new subobject of the Record Route Object, used to record that the
LSP path traversed an unnumbered link.

2.2.11. GMPLS and RSVP-TE

GMPLS RSVP-TE [RFC3473] is an extension of RSVP-TE. It enables the
provisioning of data-paths within networks supporting a variety of
switching types including packet and cell switching networks, layer
two networks, TDM networks, and photonic networks.

It defines the new Notify message (for general event notification),
which may contain notifications being sent, with respect to each
listed LSP, both upstream and downstream. Notify messages can be
used for expedited notification of failures and other events to nodes
responsible for restoring failed LSPs. A Notify message is sent
without the router alert option.

A number of new RSVP-TE (sub)objects are defined in GMPLS RSVP-TE for
general uses of MPLS:

- a Generalized Label Request Object;

- a Generalized Label Object;

- a Suggested Label Object;

- a Label Set Object (to restrict label choice);

- an Upstream_Label object (to support bidirectional LSPs);

- a Label ERO subobject;

- IF_ID RSVP_HOP objects (IPv4 & IPv6; to identify interfaces in
out-of-band signaling or in bundled links);

- IF_ID ERROR_SPEC objects (IPv4 & IPv6; to identify interfaces in
out-of-band signaling or in bundled links);

- an Acceptable Label Set object (to support negotiation of label
values in particular for bidirectional LSPs)

- a Notify Request object (may be inserted in a Path or Resv message
to indicate where a notification of LSP failure is to be sent)

- a Restart_Cap Object (used on Hello messages to identify recovery
capabilities)

- an Admin Status Object (to notify each node along the path of the
status of the LSP, and to control that state).

2.2.12. GMPLS Operation at UNI and E-NNI Reference Points

The ITU-T defines network reference points that separate
administrative or operational parts of the network. The reference
points are designated as:

- User to Network Interfaces (UNIs) if they lie between the user or
user network and the core network, or

- External Network to Network Interfaces (E-NNIs) if they lie between

peer networks, network domains, or subnetworks.

GMPLS is applicable to the UNI and E-NNI without further
modification, and no new messages, objects, or C-Types are required.
See [OVERLAY].

2.2.13. MPLS and GMPLS Future Extensions

At the time of writing, MPLS and GMPLS are being extended by the MPLS
and CCAMP Working Groups to support additional sophisticated
functions. This will inevitably lead to the introduction of new
C-Types for existing objects, and to the requirement for new objects
(CNums). It is possible that new messages will also be introduced.

Some of the key features and functions being introduced include the
following:

- Protection and restoration. Features will be developed to provide
- end-to-end protection
- segment protection
- various protection schemes (1+1, 1:1, 1:n)
- support of extra traffic on backup LSPs
- Diverse path establishment for protection and load sharing.
- Establishment of point-to-multipoint paths.
- Inter-area and inter-AS path establishment with
- explicit path control
- bandwidth reservation
- path diversity
- Support for the requirements of Automatic Switched Optical Network
(ASON) signaling as defined by the ITU-T, including call and
connection separation.
- Crankback during LSP setup.

2.2.14. ITU-T H.323 Interface

ITU-T H.323 ([H.323]) recommends the IntServ resource reservation
procedure using RSVP. The information as to whether an endpoint
supports RSVP should be conveyed during the H.245 [H.245] capability
exchange phase, by setting appropriate qOSMode fields. If both
endpoints are RSVP-capable, when opening an H.245 logical channel, a
receiver port ID should be conveyed to the sender by the
openLogicalChannelAck message. Only after that can a "Path - Resv -
ResvConf" process take place. The timer of waiting for ResvConf
message will be set by the endpoint. If this timer expires or RSVP
reservation fails at any point during an H.323 call, the action is up
to the vendor. Once a ResvConf message is sent or received, the
endpoints should stop reservation timers and resume with the H.323
call procedures. Only explicit release of reservations are supported
in [H.323]. Before sending a closeLogicalChannel message for a
stream, a sender should send a PathTear message if an RSVP session
has been previous created for that stream. After receiving a
closeLogicalChannel, a receiver should send a ResvTear similarly.
Only the FF style is supported, even for point-to-multipoint calls.

2.2.15. 3GPP Interface

Third Generation Partnership Project (3GPP) TS 23.207
([3GPP-TS23207]) specifies the QoS signaling procedure with policy
control within the Universal Mobile Telecommunications System (UMTS)
end-to-end QoS architecture. When using RSVP, the signaling source
and/or destination are the User Equipments (UEs, devices that allow

users Access to network services) that locate in the Mobile
Originating (MO) side and the Mobile Terminating (MT) side. An RSVP
signaling process can either trigger or be triggered by the (COPS)
PDP Context establishment/modification process. The operation of
refreshing states is not specified in [3GPP-TS23207]. If a
bidirectional reservation is needed, the RSVP signaling exchange must
be performed twice between the end-points. The authorization token
and flow identifier(s) in a policy data object should be included in
the RSVP messages sent by the UE, if the token is available in the
UE. When both RSVP and Service-based Local Policy are used, the
Gateway GPRS Support Node (GGSN, the access point of the network)
should use the policy information to decide whether to accept and
forward Path/Resv messages.

2.3. Extensions for New Deployment Scenarios

As a well-acknowledged protocol in the Internet, RSVP is expected
more and more to provide a more generic service for various signaling
applications. However, RSVP messages were designed in a way to
support end-to-end QoS signaling optimally. To meet the increasing
demand that a signaling protocol also operate in host-to-edge and
edge-to-edge ways, and that it serve for some other signaling
purposes in addition to end-to-end QoS signaling, RSVP needs to be
made more flexible and applicable for more generic signaling.

RSVP proxies [BEGD02] extend RSVP by originating or receiving the
RSVP message on behalf of the end node(s), so that applications may
still benefit from reservations that are not truly end-to-end.
However, there are certainly scenarios where an application would
want to explicitly convey its non-QoS purposed (as well as QoS) data
from a host into the network, or from an ingress node to an egress
node of an administrative domain. It must do so without burdening
the network with excess messaging overhead. Typical examples are an
end host desiring a firewall service from its provider"s network and
MPLS label setup within an MPLS domain.

RSVP requires support from network routers and user space
applications. Domains not supporting RSVP are traversed
transparently. Unfortunately, like other IP options, RSVP messages
implemented by way of IP alert option may be dropped by some routers
[FJ02]. Although applications need to be built with RSVP libraries,
one article presents a mechanism that would allow any host to benefit
from RSVP mechanisms without applications" awareness [MHS02].

A somewhat similar deployment benefit can be gained from the
Localized RSVP (LRSVP) [JR03] [MSK+04]. The documents present the
concept of local RSVP-based reservation that alone can be used to
trigger reservation within an access network. In those cases, an
end-host may request QoS from its own access network without the

cooperation of a correspondent node outside the access network. This
would be especially helpful when the correspondent node is unaware of
RSVP. A proxy node responds to the messages sent by the end host and
enables both upstream and downstream reservations. Furthermore, the
scheme allows for faster reservation repairs following a handover by
triggering the proxy to assist in an RSVP local repair.

Still, in end-hosts that are low in processing power and
functionality, having an RSVP daemon run and take care of the
signaling may introduce unnecessary overhead. One article [Kars01]
proposes to create a remote API so that the daemon would in fact run
on the end-host"s default router and the end-host application would
send its requests to that daemon.

Another potential problem lies in the limited size of signaled data
due to the limitation of message size. An RSVP message must fit
entirely into a single non-fragmented IP datagram. Bundle messages
[RFC2961] can aggregate multiple RSVP messages within a single PDU,
but they still only occupy one IP datagram (i.e., approximately 64K).
If it exceeds the MTU, the datagram is fragmented by IP and
reassembled at the recipient node.

2.4. Conclusion

A good signaling protocol should be transparent to the applications.
RSVP has proven to be a very well designed protocol. However, it has
a number of fundamental protocol design issues that require more
careful re-evaluation.

The design of RSVP was originally targeted at multicast applications.
The result has been that the message processing within nodes is
somewhat heavy, mainly due to flow merging. Still, merging rules
should not appear in the specification as they are QoS-specific.

RSVP has a comprehensive set of filtering styles, including
Wildcard-Filter (WF), Fixed-Filter (FF), and Shared-Explicit (SE),
and is not tied to certain QoS objects. (RSVP is not tied to IntServ
Guaranteed Service/Controlled Load (GS/CL) specifications.) Objects
were designed to be modular, but Xspecs (TSPEC, etc.) are more or
less QoS-specific and should be more generalized; there is no clear
layering/separation between the signaled data and signaling protocol.

RSVP uses a soft state mechanism to maintain states and allows each
node to define its own refresh timer. The protocol is also
independent of underlying routing protocols. Still, in mobile
networks the movement of the mobile nodes may not properly trigger a
reservation refresh for the new path, and therefore a mobile node may
be left without a reservation up to the length of the refresh timer.

Furthermore, RSVP does not work properly with changing end-point
identifiers; that is, if one of the IP addresses of a mobile node
changes, the filters may not be able to identify the flow that had a
reservation.

From the security point of view, RSVP does provide the basic building

blocks for deploying the protocol in various environments to protect
its messages from forgery and modification. Hop-by-hop protection is
provided. However, the current RSVP security mechanism does not
provide non-repudiation and protection against message deletion; the
two-way peer authentication and key management procedures are still
missing.

Finally, since the publication of the RSVP standard, tens of
extensions have emerged that allow for much wider deployment than
RSVP was originally designed for -- for instance, the Subnet
Bandwidth Manager, the NULL service type, aggregation, operation over
tunneling, and MPLS, as well as diagnostic messages.

Domains not supporting RSVP are traversed transparently by default.
Unfortunately, like other IP options, RSVP messages implemented by
way of IP alert option may be dropped by some routers. Also, the
maximal size of RSVP message is limited.

The transport mechanisms, performance, security, and mobility issues
are detailed in the following sections.

3. RSVP Transport Mechanism Issues

3.1. Messaging Reliability

RSVP messages are defined as a new IP protocol (that is, a new ptype
in the IP header). RSVP Path messages must be delivered end-to-end.
For the transit routers to intercept the Path messages, a new IP
Router Alert option [RFC2113] was introduced. This design is simple
to implement and efficient to run. As shown from the experiments in
[PS00], with minor kernel changes IP option processing introduces
very little overhead on a Free BSD box.

However, RSVP does not have a good message delivery mechanism. If a
message is lost on the wire, the next re-transmit cycle by the
network would be one soft-state refresh interval later. By default,
a soft-state refresh interval is 30 seconds.

To overcome this problem, [PS97] introduced a staged refresh timer
mechanism, which has been defined as a RSVP extension in [RFC2961].
The staged refresh timer mechanism retransmits RSVP messages until
the receiving node acknowledges. It can address the reliability
problem in RSVP.

However, during the mechanism"s implementation, a lot of effort had
to be spent on per-session timer maintenance, message retransmission
(e.g., avoid message bursts), and message sequencing. In addition,
we have to make an effort to try to separate the transport functions
from protocol processing. For example, if a protocol extension
requires a natural RSVP session time-out (such as RSVP-TE one-to-one
fast-reroute [FAST-REROUTE]), we have to turn off the staged refresh
timers.

3.2. Message Packing

According to RSVP [RFC2205], each RSVP message can only contain
information for one session. In a network that has a reasonably
large number of RSVP sessions, this constraint imposes a heavy
processing burden on the routers. Many router OSes are based on
UNIX. [PS00] showed that the UNIX socket I/O processing is not very

sensitive to packet size. In fact, processing small packets takes
almost as much CPU overhead as processing large ones. However,
processing too many individual messages can easily cause congestion
at socket I/O interfaces.

To overcome this problem, RFC2961 introduced the message bundling
mechanism. The bundling mechanism packs multiple RSVP messages
between two adjacent nodes into a single packet. In one deployed
router platform, the bundling mechanism has improved the number of
RSVP sessions that a router can handle from 2,000 to over 7,000.

3.3. MTU Problem

RSVP does not support message fragmentation and reassembly at
protocol level. If the size of a RSVP message is larger than the
link MTU, the message will be fragmented. The routers simply cannot
detect and process RSVP message fragments.

There is no solution for the MTU problem. Fortunately, at places
where RSVP-TE has been used, either the amount of per-session RSVP
data is never too large, or the link MTU is adjustable; PPP and Frame
Relay can always increase or decrease the MTU sizes. For example, on
some routers, a Frame Relay interface can support a link MTU size up
to 9600 bytes. Currently, the RSVP MTU problem is not a realistic
concern in MPLS networks.

However, when and if RSVP is used for end-user applications, for
which network security is an essential and critical concern, it is
possible that the size of RSVP messages can be larger than the link
MTU. Note that end-users will most likely have to deal with a small
1500-byte Ethernet MTU.

3.4. RSVP-TE vs. Signaling Protocol for RT Applications

RSVP-TE works in an environment that is different from what the
original RSVP has been designed for: in MPLS networks, the RSVP
sessions that are used to support Label-Switched Paths (LSPs) do not
change frequently.

In fact, the network operators typically set up the MPLS LSPs so that
they cannot switch too quickly. For example, the operators often
regulate the Constraint-based Shortest Path First (CSPF) computation
interval to prevent or delay a large volume of user traffic from
shifting from one session to another during LSP path optimization.
(CSPF is a routing algorithm that operates from the network edge to
compute the "most" optimal routes for the LSPs.) As a result, RSVP-
TE does not have to handle a large amount of "triggered" (new or
modified) messages. Most of the messages are refresh messages,
which can be handled by the mechanisms introduced in RFC2961. In
particular, in the Summary Refresh extension [RFC2961], each RSVP
refresh message can be represented as a 4-byte ID. The routers can
simply exchange the IDs to refresh RSVP sessions. With the full
implementation of RFC2961, MPLS routers do not have any RSVP scaling
issue. On one deployed router platform, it can support over 50,000
RSVP sessions in a stable backbone network.

On the other hand, in many of the new applications for which a

signaling protocol is required, the user session duration can be
relatively short. The dynamics of adding/dropping user sessions
could introduce a large number of "triggered" messages in the
network. This can clearly introduce a substantial amount of
processing overhead to the routers. This is one area where a new
signaling protocol may be needed to reduce the processing complexity
in the resource reservation process.

3.5. What Would Be a Better Alternative?

A good signaling protocol should be transparent to the applications.
On the other hand, the design of a signaling protocol must take the
intended and potential applications into consideration.

With the addition of RFC2961, RSVP-TE is sufficient to support its
intended application, MPLS, within the backbone. There is no
significant transport-layer problem that needs to be solved.

In the last several years, a number of new applications have emerged
that are proposed to need IP signaling, beyond the traditional ones
associated with quality of service and resource allocation. On-path
firewall control/NAT traversal (synergistic with the midcom design of
[RFC3303]) is one of these. There are far-out applications such as
depositing active network code in network devices. Next-generation
signaling protocols dealing with novel applications, with network
security requirements, and with the MTU problems described above,
will prevent the re-use of the existing RSVP transport mechanism.

If a new transport protocol is needed, the protocol must be able to
handle the following:

- reliable messaging;

- message packing;

- the MTU problem;

- small triggered message volume.

4. RSVP Protocol Performance Issues

4.1. Processing Overhead

By "processing overhead" we mean the amount of processing required to
handle messages belonging to a reservation session. This is the
processing required in addition to the processing needed for routing
an (ordinary) IP packet. The processing overhead of RSVP originates
from two major issues:

1) Complexity of the protocol elements. First, RSVP itself is per-
flow based; thus the number of states is proportional to RSVP
session number. Path and Resv states have to be maintained in
each RSVP router for each session (and Path state also has to
record the reverse route for the correspondent Resv message).
Second, being receiver-initiated, RSVP optimizes various merging
operations for multicast reservations while the Resv message is
processed. To handle multicast, other mechanisms such as
reservation styles, scope object, and blockade state, are also
required to be presented in the basic protocol. This not only
adds sources of failures and errors, but also complicates the
state machine [Fu02]. Third, the same RSVP signaling messages are
used not only for maintaining the state, but also for dealing with

recovery of signaling message loss and discovery of route change.
Thus, although protocol elements that represent the actual data
(e.g., QoS parameters) specification are separated from signaling
elements, the processing overhead needed for all RSVP messages is
not marginal. Finally, the possible variations of the order and
existence of objects increases the complexity of message parsing
and internal message and state representation.

2) Implementation-specific Overhead. There are two ways to send and
receive RSVP messages: either as "raw" IP datagrams with protocol
number 46, or as encapsulated UDP datagrams, which increase the
efficiency of RSVP processing. Typical RSVP implementations are
user-space daemons interacting with the kernel; thus, state
management, message sending, and reception would affect the
efficiency of the protocol processing. For example, in the recent
version of the implementation described in [KSS01], the relative
execution costs for the message sending/reception system calls
"sendto", "select", and "recvmsg" were 14-16%, 6-7%, 9-10%,
individually, of the total execution cost. [KSS01] also found
that state (memory) management can use up to 17-18% of the total
execution cost, but it is possible to decrease that cost to 6-7%,
if appropriate action is taken to replace the standard memory
management with dedicated memory management for state information.
RSVP/routing, RSVP/policy control, and RSVP/traffic control
interfaces can also pose different overhead depending on
implementation. For example, the RSVP/routing overhead has been
measured to be approximately 11-12% of the total execution cost
[KSS01].

4.2. Bandwidth Consumption

By "bandwidth consumption" we mean the amount of bandwidth used
during the lifetime of a session: to set up a reservation session, to
keep the session alive, and finally to close it.

RSVP messages are sent either to trigger a new reservation or to
refresh an existing reservation. In standard RSVP, Path/Resv
messages are used for triggering and refreshing/recovering
reservations, identically, which results in an increased size of
refresh messages. The hop-by-hop refreshment may reduce the
bandwidth consumption for RSVP, but could result in more sources of
error/failure events. [RFC2961] presents a way to bundle standard
RSVP messages and reduces the refreshment redundancy by Srefresh
message.

Thus, the following formula represents the bandwidth consumption in
bytes for an RSVP session lasting n seconds:

F(n) = (bP + bR) + ((n/Ri) * (bP + bR)) + bPt

bP: IP payload size of Path message
bR: IP payload size of Resv message
bPt: IP payload size of Path Tear message
Ri: refresh interval

For example, for a simple Controlled Load reservation without
security and identification requirements (where bP is 172 bytes, bR
is 92, bPt is 44 bytes, and Ri is 30 seconds), the bandwidth

consumption would be as follows:

F(n) = (172 + 92) + ((n/30) * (172 + 92)) + 44

= 308 + (264n/30) bytes

5. RSVP Security and Mobility

5.1. Security

To allow a process on a system to securely identify the owner and the
application of the communicating process (e.g., user id) and to
convey this information in RSVP messages (PATH or RESV) in a secure
manner, [RFC3182] specifies the encoding of identities as RSVP
POLICY_DATA Object. However, to provide ironclad security
protection, cryptographic authentication combined with authorization
has to be provided. Such a functionality is typically offered by
authentication and key exchange protocols. Solely including a user
identifier is insufficient.

To provide hop-by-hop integrity and authentication of RSVP messages,
an RSVP message may contain an INTEGRITY object ([RFC2747]) using a
keyed message digest. Since intermediate routers need to modify and
process the content of the signaling message, a hop-by-hop security
architecture based on a chain-of-trust is used. However, with the
different usage of RSVP as described throughout this document and
with new requirements, a re-evaluation of the original assumptions
might be necessary.

RFC2747 provides protection against forgery and message modification.
However, this does not provide non-repudiation or protect against
message deletion. In the current RSVP security scheme, the two-way
peer authentication and key management procedures are still missing.

The security issues have been well analyzed in [Tsch03].

5.2. Mobility Support

Two issues raise concern when a mobile node (MN) uses RSVP: the flow
identifier and reservation refresh. When an MN changes locations, it
may need to change one of its assigned IP addresses. An MN may have
an IP address by which it is reachable by nodes outside the access
network, and an IP address used to support local mobility management.
Depending on the mobility management mechanism, a handover may force
a change in any of these addresses. As a consequence, the filters
associated with a reservation may not identify the flow anymore, and
the resource reservation is ineffective until a refresh with a new
set of filters is initialized.

The second issue relates to following the movement of a mobile node.
RFC2205 defines that Path messages can perform a local repair of
reservation paths. When the route between the communicating end
hosts changes, a Path message will set the state of the reservation
on the new route, and a subsequent Resv message will make the
resource reservation. Therefore, by sending a Resv message a host
cannot alone update the reservation, and thus it cannot perform a
local repair before a Path message has passed. Also, in order to
provide fast adaptation to routing changes without the overhead of
short refresh periods, the local routing protocol module can notify

the RSVP process of route changes for particular destinations. The
RSVP process should use this information to trigger a quick refresh
of state for these destinations, using the new route (Section 3.6,
[RFC2205]). However, not all local mobility protocols affect routing
directly in routers (not even Mobile IP), and thus mobility may not
be noticed at RSVP routers. Therefore, it may take a relatively long
time before a reservation is refreshed following a handover.

There have been several designs for extensions to RSVP to allow for
more seamless mobility. One solution is presented in [MSK+04], in
which one section discusses the coupling of RSVP and the mobility
management mechanisms and proposes small extensions to RSVP to handle
the handover event better, among other things. The extension allows
the mobile host to request a Path for the downstream reservation when
a handover has happened.

Another example is Mobile RSVP (MRSVP) [TBA01], which is an extension
to standard RSVP. It is based on advance reservations, where
neighboring access points keep resources reserved for mobile nodes
moving to their coverage area. When a mobile node requests
resources, the neighboring access points are checked, too, and a
passive reservation is done around the mobile nodes" current
location.

The problem with the various "advance reservation" schemes is that
they require topological information of the access network and,
usually, advance knowledge of the handover event. Furthermore, the
way the resources reserved in advance are used in the neighboring
service areas is an open issue. A good overview of these different
schemes can be found in [MA01].

The interactions of RSVP and Mobile IP have been well documented in
[Thom02].

6. Other QoS Signaling Proposals

6.1. Tenet and ST-II

Tenet and ST-II are two original QoS signaling protocols for the
Internet.

In the original Tenet architecture [BFM+96], the receiver sends a
reservation request toward the source. Each network node along the
way makes the reservation. Once the request arrives at the source,
the source sends another Relax message back toward to the receiver,
and has the option to modify the previous reservation at each node.

ST-II [RFC1819] basically works in the following way: a sender
originates a Connect message to a set of receivers. Each
intermediate node determines the next hop subnets, and makes
reservations on the links going to these next hops. Upon receiving a
Connect indication, a receiver must send back either an Accept or a
Refuse message to the sender. In the case of an Accept, the receiver
may further reduce the resource request by updating the returned flow
specifications.

ST-II consists of two protocols: ST for the data transport and the
Stream Control Message Protocol (SCMP) for all control functions.
ST is simple and contains only a single PDU format, which is designed

for fast and efficient data forwarding in order to achieve low
communication delays. SCMP packets are transferred within ST
packets.

ST-II has no built-in soft states; thus, it requires that the network
be responsible for correctness. It is sender-initiated, and the
overhead for ST-II to handle group membership dynamics is higher than
that for RSVP [MESZ94]. ST-II does not provide security, but
[RFC1819] describes some objects related to charging.

6.2. YESSIR

YESSIR (YEt another Sender Session Internet Reservations) [PS98] is a
resource reservation protocol that seeks to simplify the process of
establishing reserved flows while preserving many unique features
introduced in RSVP. Simplicity is measured in terms of control
message processing, data packet processing, and user-level
flexibility. Features such as robustness, advertising network
service availability, and resource sharing among multiple senders are
also supported in the proposal.

The proposed mechanism generates reservation requests by senders to
reduce the processing overhead. It is built as an extension to the
Real-Time Transport Control Protocol (RTCP), taking advantage of
Real-Time Protocol (RTP). YESSIR also introduces a concept called
partial reservation, in which, for certain types of applications, the
reservation requests can be passed to the next hop, even though there
are not enough resources on a local node. The local node can rely on
optimized retries to complete the reservations.

6.2.1. Reservation Functionality

YESSIR [PS98] was designed for one-way, sender-initiated end-to-end
resource reservation. It also uses soft state to maintain states.
It supports resource query (similar to RSVP diagnosis message),
advertising (similar to RSVP ADSPEC), shared reservation, partial
reservations, and flow merging.

To support multicast, YESSIR simplifies the reservation styles to
individual and shared reservation styles. Individual reservations
are made separately for each sender, whereas shared reservations
allocate resources that can be used by all senders in an RTP session.
Although RSVP supports shared reservation (SE and WF styles) from the
receiver"s direction, YESSIR handles the shared reservation style
from the sender"s direction; thus, new receivers can re-use the
existing reservation of the previous sender.

It has been shown that the YESSIR one-pass reservation model has
better performance and lower processing cost than a regular two-way
signaling protocol, such as RSVP [PS98]. The bandwidth consumption
of YESSIR is somewhat lower than that of, for example, RSVP, because
it does not require additional IP and transport headers. Bandwidth
consumption is limited to the extension header size.

YESSIR does not have any particular support for mobility, and the
security of YESSIR relies on RTP/RTCP security measures.

6.2.2. Conclusion

YESSIR requires support in applications since it is an integral part

of RTCP. Similarly, it requires network routers to inspect RTCP
packets to identify reservation requests and refreshes. Routers
unaware of YESSIR forward the RTCP packets transparently.

6.3. Boomerang

Boomerang [FNM+99] is a another resource reservation protocol for IP
networks. The protocol has only one message type and a single
signaling loop for reservation setup and teardown, and it has no
requirements on the far end node. Instead, it concentrates the
intelligence in the Initiating Node (IN).

In addition, the Boomerang protocol allows for sender- or receiver-
oriented reservations and resource query. Flows are identified with
the common 5-tuple, and the QoS can be specified by various means;
e.g., service class and bit rate. In the initial implementation,
Boomerang messages are transported in ICMP ECHO/REPLY messages.

6.3.1. Reservation Functionality

Boomerang can only be used for unicast sessions; no support for
multicast exists. The requested QoS can be specified with various
methods, and both ends of a communication session can make a
reservation for their transmitted flow.

The authors of Boomerang show in [FNS02] that the processing of the
protocol is considerably lower than that of the ISI RSVP daemon
implementation. However, this is mainly due to the limited
functionality provided by the protocol compared to that provided by
RSVP.

Boomerang messages are quite short and consume a relatively low
amount of link bandwidth. This is due to the limited functionality
of the protocol; for example, no security-specific information or
policy-based interaction is provided. Being sender oriented, the
bandwidth consumption mostly affects the downstream direction, from
the sender to the receiver.

As Boomerang is sender oriented, there is no need to store backward
information. This reduces the signaling required. The rest of the
issues that were identified with RSVP apply with Boomerang. No
security mechanism is specified for Boomerang.

The Boomerang protocol has deployment issues similar to those of any
host-network-host protocol. It requires an implementation at both
communicating nodes and in routers. Boomerang-unaware routers should
be able to forward Boomerang messages transparently. Still,
firewalls often drop ICMP packets, making the protocol useless.

6.3.2. Conclusions

Boomerang seems to be a very lightweight protocol and efficient in
its own scenarios. However, the apparent low processing overhead and
bandwidth consumption results from the limited functionality. No
support for multicast or any security features are present, which
allows for a different functionality than RSVP, which the authors
like to compare Boomerang to.

6.4. INSIGNIA

INSIGNIA [LGZC00] is proposed as a very simple signaling mechanism
for supporting QoS in mobile ad-hoc networks. It avoids the need for

separate signaling by carrying the QoS signaling data along with the
normal data in IP packets using IP packet header options. This
approach, known as "in-band signaling", is proposed as more suitable
in the rapidly changing environment of mobile networks since the
signaled QoS information is not tied to a particular path. It also
allows the flows to be rapidly established and, thus, is suitable for
short-lived and dynamic flows.

INSIGNIA aims to minimize signaling by reducing the number of
parameters that are provided to the network. It assumes that real-
time flows may tolerate some loss, but are very delay sensitive so
that the only QoS information needed is the required minimum and
maximum bandwidth.

The INSIGNIA protocol operates at the network layer and assumes that
link status sensing and access schemes are provided by lower-layer
entities. The usefulness of the scheme depends on the MAC layers,
but this is undefined, so INSIGNIA can run over any MAC layer. The
protocol requires that each router maintains per-flow state.

The INSIGNIA system implicitly supports mobility. A near-minimal
amount of information is exchanged with the network. To achieve
this, INSIGNIA makes many assumptions about the nature of traffic
that a source will send. This may also simplify admission control
and buffer allocation. The system basically assumes that "real-time"
will be defined as a maximum delay, and the user can simply request
real-time service for a particular quantity of traffic.

After handover, data that was transmitted to the old base station can
be forwarded to the new base station, so no data loss should occur.
However, there is no way to differentiate between re-routed and new
traffic, so priority cannot be given to handover traffic, for
example.

INSIGNIA, however, (completely) lacks a security framework and does
not investigate how to secure signaled QoS data in an ad-hoc network,
where relatively weak trust or even no trust exists between the
participating nodes. Therefore, authorization and charging
especially might be a challenge. The security protection of in-band
signaling is costly since the data delivery itself experiences
increased latency if security processing is done hop-by-hop. Because
the QoS signaling information is encoded into the flow label and
end-to-end addressing is used, it is very difficult to provide
security other than IPsec in tunnel mode.

7. Inter-Domain Signaling

This section gives a short overview of protocols designed for inter-
domain signaling.

7.1. BGRP

Border Gateway Reservation Protocol (BGRP) [BGRP] is a signaling
protocol for inter-domain aggregated resource reservation for unicast
traffic. BGRP builds a sink tree for each of the stub domains. Each
sink tree aggregates bandwidth reservations from all data sources in
the network. BGRP maintains these aggregated reservations using soft

state and relies on Differentiated Services for data forwarding.

In terms of message processing load, BGRP scales state storage and
bandwidth. Because backbone routers only maintain the sink tree
information, the total number of reservations at each router scales
linearly with the number of Internet domains.

7.2. SICAP

SICAP (Shared-segment Inter-domain Control Aggregation protocol)
[SGV03] is an inter-domain signaling solution that performs shared-
segment aggregation [SGV02] on the Autonomous System (AS) level in
order to reduce state required at Boundary Routers (BRs). SICAP
performs aggregation based on path segments that different
reservations share. Thus, reservations may be merged into aggregates
that do not necessarily extend all the way to the reservation"s
destination. The motivation for creating "shorter" aggregates is
that, on one hand, their ability to accommodate future requests more
easily, and, on the other hand, the minimization of aggregates
created and consequently, the reduction of state required to manage
established reservations. However, in contrast to the sink-tree
approach (used by BGRP [BGRP]), the shared-segment approach
introduces intermediate de-aggregation locations. These are ASes
where aggregates may experience "re-aggregation". At these
locations, routers that perform aggregation (AS egress routers) have
to keep track of the mapping between reservations and aggregates.
One possible way to do this is to keep each reservation identifier
and the corresponding resources stored at each aggregator. However,
this solution incurs a high state penalty. SICAP avoids this state
penalty by keeping track of the mapping between aggregates and
reservations at the level of destination domains rather than
explicitly map individual reservations to aggregates. In other
Words, SICAP maintains, per aggregate, a list of the destination
prefixes advertised by the destination AS an aggregate provides
access to.

Pan et al. show that BGRP scales well in terms of control state,
message processing, and bandwidth efficiency, when compared