Internet Engineering Task Force P. Montessoro
Internet-Draft R. Bernardini
Expires: December 24, 2012 UniUD
June 22, 2012
REBOOK: a Network Resource Booking Algorithm
draft-montessoro-rebook-00
Abstract
This document describes REBOOK, a resource reservation algorithm for
deterministic and fast dynamic resource allocation and release.
Based on a stateful approach, it handles faults and network errors,
and recovers from route changes and unexpected flows shutdown. The
distributed scheme used to store flows information have guaranteed
constant complexity.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. REBOOK Description . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Message transport and format . . . . . . . . . . . . . . . 6
2.2. Node behaviour . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1. Start-up . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.2. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . 8
2.2.3. Data transmission . . . . . . . . . . . . . . . . . . 9
2.2.4. Reservation update . . . . . . . . . . . . . . . . . . 9
2.2.5. Path change . . . . . . . . . . . . . . . . . . . . . 9
2.2.6. Shutdown . . . . . . . . . . . . . . . . . . . . . . . 10
2.3. Commands . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4. Router behaviour . . . . . . . . . . . . . . . . . . . . . 11
2.4.1. On reception of a RESV request . . . . . . . . . . . . 11
2.4.2. On reception of a data packet with the reference
field . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4.3. On reception of a KPALV request . . . . . . . . . . . 12
2.4.4. On reception of a UP_RESV request . . . . . . . . . . 13
2.4.5. On reception of a RL_RESV request . . . . . . . . . . 13
2.4.6. Checking packet validity . . . . . . . . . . . . . . . 13
3. Using the IP Router Alert field . . . . . . . . . . . . . . . 14
4. Session Timeout . . . . . . . . . . . . . . . . . . . . . . . 14
5. What is a resource? . . . . . . . . . . . . . . . . . . . . . 15
6. Security consideration . . . . . . . . . . . . . . . . . . . . 15
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. References . . . . . . . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . . 16
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1. Introduction
A large number of applications prefer UDP as a transport protocol.
Among others, teleconferencing, streaming applications, real-time
communications, and networked multiplayer games; more generally,
applications whose information value and perceived quality strictly
relies on network delay, and also multi-cast services. Dynamics of
network load can impact negatively on multimedia applications;
however, UDP doesn't offer any native congestion control mechanism
and therefore it cannot guarantee Quality of Service (QoS). In
addition, congestion is often caused by these applications' high
bandwidth requirements.
On the other side, TCP provides a congestion control mechanism based
on message loss only, without any explicit knowledge about the
traffic load in intermediate network nodes along the communication
path. Therefore, both UDP and TCP sources need to be enhanced with a
congestion control algorithm not only aimed at reducing loss rate and
improving bandwidth utilization, but also at improving fairness
towards competing connections while satisfying related QoS
requirements. UDP flows should become TCP-friendly whereas UDP and
TCP flows should both become aware of network load in order to adjust
their transmission rate before losing packets.
In the last years, several methods have been introduced for QoS
control mechanisms, mainly based on adapting the sender's
transmission rate in accordance with the network congestion state
(see Sect. 2), but typically with such approaches no QoS guarantees
can be made effectively. The proposed algorithm, that we call
REBOOK, allows a control protocol to prevent congestion by reserving
enough resources for supporting a dynamically changing QoS level in
advance, therefore it would be the most straightforward approach to
manage the problem.
It must be outlined that REBOOK is not dependent on TCP/IP protocols,
as it can be used in any other packet switching network. Obviously,
in the following the Internet and the TCP/IP protocol suite will be
used as reference environment for its description.
REBOOK requires a hosting protocol to carry the algorithm's messages.
They can be handled by a dedicated signalling protocol, like RSVP
[RFC2205] or a new ad hoc protocol, or it can be embedded in a data
transport protocol, like TCP using the options field.
REBOOK is designed to handle virtually every transport-layer
connection or application-layer flows, not necessarily in real-time
or QoS only. In principle, except for very short flows the network
may benefit from the preventive declaration (and the consequent
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resource booking) of the amount of traffic that each connection is
going to generate: if the application knows the amount of granted
resources in advance, it can modulate the data transmission rate to
prevent packet loss.
REBOOK implementation will bring to a sender-oriented protocol,
unlike RSVP which is receiver oriented. The node initiating a
connection is responsible for resource reservation request, based on
the amount and type of data to be transmitted, application
constraints and, possibly, SLA (Service Level Agreement) parameters.
While the connection is active the amount of granted resources may be
reduced to allow the activation of new flows in an almost-congested
network or may be increased if switching nodes become less loaded.
These events are acknowledged by the sender that will consequently
adapt its transmission rate. RSVP is receiver-oriented mainly
because it is designed to support single-cast and multi-cast flows as
well.
REBOOK does not rely on any special network feature: it works even if
only part of the network is REBOOK-aware as the resource reservation
is effective even if part of a flow traverses unaware routers. There
are no special requirements to routing, which can be asymmetrical
(transmitting and receiving flows can follow different paths),
except, obviously, its stability: in normal conditions data packets
and control messages must follow the same route for the duration of
the connection. Anyhow, if unexpected events occur, REBOOK
identifies route changes and recovers from errors and faults in the
network.
REBOOK does not rely on special hardware in routers either. Its
status storage scheme allows direct access to table entries without
any hardware look-up feature, simply using conventional memory
architecture. This makes its implementation faster and cheaper than
today's typical solutions provided by hardware hashing. Finally,
REBOOK does not require any improvement in the switching fabric. No
additional memory for queues and buffers nor different packet
handling. On the contrary, the router architecture will drive the
resource granting phase, depending on available resources left after
previous reservations.
It is worth observing that REBOOK is not another virtual circuit
technique; it is an add-on to conventional routers to improve
performance and features. For example, it does not require any
interaction with the routing protocols; it can work along routes
traversing several Autonomous Systems and routers using different
routing protocols; it does not require to be supported by all the
routers along the path nor for the supporting routers to be
contiguous; it does need neither hierarchy nor partitioning. In
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respect of existing virtual circuit techniques, the novelty consists
in making each router autonomous in handling its REBOOK data
structures and the effects that routing dynamics have on them,
without any routing-aware signalling protocol. All these features
overcome most of the difficulties that limit a wide deployment of
MPLS or ATM, for example. Moreover, in case of routing instability
or faults, REBOOK can fail in enhancing router performance, but
packets are still forwarded in a best-effort, lookup-driven mode.
REBOOK is scalable since its computational cost is constant,
regardless of the number of active flows, yielding a solution to per-
flow resource reservation and packet handling, without the need of
aggregations. Moreover, it is intrinsically secure since each router
uses only the information generated in a previous phase by itself,
allowing any form of fast and secure symmetrical cryptography without
the need of a key exchange.
REBOOK is based on a distributed linked data structure in the sense
that it makes use of pointers to memory locations or indexes to table
entries containing information stored in the routers that are very
likely to be accessed in the future. Unlike conventional linked data
structures, pointers are not stored within the router they address,
but in the end nodes, in the packets, and in adjacent routers. When
a packet is received by a router, the pointer to the table entry to
be used for its processing is found in the packet itself and look-up
procedures are avoided. To keep the pointers consistent in a dynamic
environment, where route changes may send packets to routers
different than the ones the pointers refer to, a specific integrity
check is adopted. It makes REBOOK completely autonomous in
identifying faults, errors and route changes. The overhead
introduced by this integrity check is a critical issue. Thanks to
careful design of the distributed linked data structure, experimental
data demonstrate that this overhead is overcome by the speedup
provided by the look-up-free access to table entries.
REBOOK is a soft state algorithm. Reservations no longer refreshed
by keep-alive messages (due to errors, route changes or end node
faults) are removed by a low priority table cleanup task active in
the routers. This is the only part of the algorithm whose cost is
linear in the number of active flows, instead of constant. However,
the experimental results in [rebook] report 100 ms CPU time to handle
a 10,000,000 flows RR table, and in current implementation this
procedure is activated every 15 s. Thanks to this procedure and to
the consistency check, REBOOK messages are not required to be
acknowledged.
Summarizing, the main features of REBOOK are
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Soft state: Reservations not refreshed by keep-alive messages are
automatically removed.
Scalability: The computational cost for routing and connection
handling is constant with the number of active flows. The only
cost that grows linearly with the number of active flows is the
removal of expired session, but experimental data shows that this
is negligible.
Consistency checks: Routers can easily check (in constant time) if
the received packet is actually a valid REBOOK-related packet.
This makes it possible for routers to identify faults, errors and
route changes.
Safety: Each router uses only the information generated in a
previous phase by itself, this makes it possible to use efficient
cryptographic protection techniques.
2. REBOOK Description
In a REBOOK setup there is a _source node_ that wants to reserve
resources to send data to a _target node_ with a guaranteed quality
of service. Before beginning the streaming, the source node reserves
the required resources by sending REBOOK requests to the target node.
The intermediate REBOOK-capable routers will analyze the REBOOK
requests and assign resources to the newly opened connection. This
section describes the details of the interaction between the source
node, the target node and the intermediate routers.
2.1. Message transport and format
It is worth emphasizing that REBOOK is not a protocol, but an
algorithm for resource reservations that can be "embedded" in many
different protocol. Because of this, we do not specify format for
REBOOK requests or how they are transmitted over the network, leaving
the choice of those details to the application employing REBOOK.
This is similar to what it is done in other protocols like TFRC (TCP-
Friendly Rate Control [RFC5348]) that requests that the source node
will send some data (e.g., estimated RTT, timestamps, ...) to the
receiver and that the receiver will send back some feedback data
(e.g., loss probability), but it leaves open how those data are
embedded in the protocol employing TFRC.
About the way REBOOK requests are transported, we can observe that
there are two possible approaches
o Via a specific protocol, "parallel" to the one used to stream the
data.
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o Piggy-backed to the data stream.
Both solutions are reasonable and the choice will be a matter of
convenience of the specific application.
2.2. Node behaviour
In this section we describe an overview of the behavior of the
different REBOOK nodes (source, target and routers).
2.2.1. Start-up
o Suppose that a source node needs to stream data to the target
using a guaranteed QoS. The node decides to reserve the required
resources using REBOOK. Toward such a goal, the node sends a
REBOOK RESV request (see Section 2.3) to the target node.
o The packet with the RESV request travels over the network, passing
through several routers. If a router is REBOOK-capable, it
reserves the requested resources (if available) and update its
internal tables by adding an entry for the new stream(see
Section 2.4 for details). If not enough resources are available,
the router can decide to reserve less than the required resource
amount and write this information in the request.
o In order to be able to recover later the information about the
registered stream, the router appends to the RESV packet a
_reference_ to the new entry.
Although the "reference" written by the router can be thought
as the memory address of the entry associated with the stream,
it is worth emphasizing that a reference is used only by the
router that wrote it, so that, to the other nodes the reference
appears as an opaque string of bits. This implies that the
reference is not constrained to be an actual address and that
the router can give it any meaning without compromising
interoperability. For example, the reference could be an
_index_ to the table, possibly extended with some CRC and
encrypted to check for packet validity and improve security
(see Section 2.4.6).
o The RESV request arrives to the target. The RESV packet now
contains the amount of resources reserved along the path and the
list of table references added by the intermediate REBOOK-capable.
The target sends back to the source the received packet with a
RESV_ACK request (see Section 2.3). Now the source knows the
amount of reserved resources and the reference list.
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2.2.2. Keep-Alive
Periodically, starting right after receiving the RESV_ACK, the source
sends keep-alive KPALV requests to the target node. The KPALV
request carries the following data
o The amount of reserved resources
o The list of table references inserted by the routers
When a router receives a KPALV message, first it checks if it is a
valid KPALV request relative to an already opened stream. If the
check is positive
1. It updates the amount of reserved resources.
In order to understand why this is necessary, consider the
following case: source S wants to send data to target T;
suppose that the canonical required bandwidth is 3 Mbit/s, but
that transmission can still be done with good enough quality
as soon as the bandwidth is at least 512 kbit/s (for example,
the source could encode the data at a lower quality). Suppose
also that between S and T there are two routers A and B, the
latter with only 1 Mbit/s available. When A processes the
RESV request, it has enough resources and decide to reserve 3
Mbit/s to S; however, since B has not enough bandwidth it
reserves only 1 Mbit/s to S. Note that A is wasting 2 Mbit/s,
since S will never be able to use more than 1 Mbit/s, but A
reserved 3 Mbit/s to S. However, since the KPALV packet sent
by S will contain the amount of reserved resources, A can see
that only 1 Mbit/s will be used by S and will update
accordingly its tables.
2. It stores in the table entry associated with the stream the
reference written by the next router. See Section 2.4 for
details about how this value is used.
KPALV messages are expected to be received at regular intervals. If
KPALV packets are not received for a time exceeding a threshold
(possibly depending on the path RTT), the router will suppose the
session expired and will release the reserved resources.
Finally, observe that the target usually ignores the KPALV packets,
unless there is a change in the reservation. See Section 2.2.4 for
details.
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2.2.3. Data transmission
In the simplest implementation of REBOOK, no special handling is
required for data packets. The routers will access to the routing
tables in the usual way.
However, in order to speed-up the access to the table, avoiding the
look-up procedure, the source can include in every transmitted data
packet the reference value inserted by the first router. (See
Section 2.4 for details about how this value is used.) Since this
value must be used by the routers, the router must know (i) that the
packet belongs to a REBOOK stream and (ii) where the reference is
stored. A possible solution, employing the IP Router Alert field
[RFC2113] is described in Section 3.
2.2.4. Reservation update
In some cases it could be necessary to update the resource reserved.
For example, in a video streaming application the user could decide
to receive a better quality (but more expensive) version of the
video, so the source needs to reserve more resources. Another case
where it is necessary to update the reservation is when some routers
in the path are not REBOOK capable, so that their service is of best-
effort type. If congestion at the non-REBOOK router happens, the
source detects it by using known congestion control procedures
(e.g.,TFRC [RFC5348]) and it can decide to update the reserved
bandwidth. Finally, a third case is when a router receives a request
that cannot satisfy and it can decide to reduce the amount of
resources assigned to another node in order to satisfy the new
request.
In all those cases, the reservation can be changed by using a KPALV
request with the new reservation. If the source wants to reduce the
allocated resources, it just generates the request with the new
allocation value; if an intermediate router wants to reduce the
reserved bandwidth, it will wait for the first KPALV packet to arrive
and it will update the reservation value in it.
When the target receives a KPALV packet with different resource
values, it sends to the source a PRL_ACK (see Section 2.3) with the
new values.
Finally, if the source desires to increase the reserved resources, it
can use the request UP_RESV (see Section 2.3).
2.2.5. Path change
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2.2.6. Shutdown
When the transmission is concluded, the source can release the
reserved resources by sending a RL_RESV request. This will cause the
intermediate routers to release the allocated resources.
2.3. Commands
RESV Sent at the beginning of a session. When it leaves the source
it contains the amount of required resources (R_req) and the
minimum admissible amount of resources (R_min) and the current
amount of reserved resources (R_curr), initialized to R_req. The
request is modified by intermediate routers as follows
* If the router cannot allocate R_curr resources, it writes in
R_curr the reserved amount of resources.
* The router creates a new entry in its tables for the new stream
and append to the RESV request a "reference" to the new entry.
RESV_ACK Sent by the target to the source when the target receives
the RESV requests. Its payload stores the value of R_curr in the
received RESV request and the list of router references.
KPALV Keep-alive request sent by the source to the target. They
carry the values of R_min, R_req and R_curr, together with the
router references. They are used, beside for keeping the
connection "hot" to that the routers release the resources, for
updating the resource reservation. They are ignored by the target
node, unless the resource reservation has been changed.
PRL_ACK Sent by the target to the source when a KPALV with changed
reservation is received. Its payload stores the value of R_curr
in the received RESV request and the list of router references.
UP_RESV Sent by the source to increase the amount of reserved
resources. It is handled like a RESV request, with the difference
that the stream entry in the router tables must by already
present.
RL_RESV Sent by the source at the end of the session. The routers
will release the allocated resources and will remove the stream
entry from their tables.
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2.4. Router behaviour
In this section we describe the expected behavior of a REBOOK-capable
router, with reference to a specific implementation. It is clear
that the implementation details are mostly a private matter of the
router and that they can be changed as long as the router behavior
"seen" by the rest of the network does not change.
In the reference implementation chosen here the router uses two
tables
o The usual _routing table_ mapping destinations to router ports
o A _stream table_ addressed via the reference that the router
writes in the RESV packet. Each entry of the table contains
information relative to the specific stream and it is expected to
contain at least
* The current values of R_min, R_req and R_curr
* The reference of the next router
* The session expiration time T_exp. If no KPALV packets are
received before T_exp, the session is considered expired and
the allocated resources released
* A reference to the routing table pointing to the entry
associated with the target address.
2.4.1. On reception of a RESV request
When the router receives a RESV request
o It creates a new entry in the stream table for the new stream.
Let REF be the reference to the new entry. It look-ups the
routing information required for the stream and set the routing
table reference in the stream table entry accordingly.
o It checks the amount of resources that it can allocate to the
stream and update the stream table accordingly. If the allocated
resources are less the R_curr value in the RESV request, update
the RESV value to the actual amount of allocated resources.
o It appends to the RESV request the value of REF and forward the
updated request to the next hop.
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2.4.2. On reception of a data packet with the reference field
As explained in Section 2.2.3, the source can optionally include in
the data packet the reference belonging to the first router. In this
case, when the router receives a packet that is marked as a REBOOK
packet
o It checks for the packet validity. This is done both for security
and for detecting events like route changes and faults. The check
can be done in constant time by checking the REBOOK reference in
the packet, see Section 2.4.6 for details. If the check is
positive, continue, otherwise the REBOOK reference is invalidated
and the packet is handled as a normal one.
o By using the router reference field embedded in the packet, it
accesses the entry in the stream table.
o It copies the router reference stored in the table in the
corresponding data packet field. Note that in this way the next
REBOOK-capable router will find in the RRef field the reference
value that it wrote in the RESV packet during the setup phase.
o It finds the corresponding entry in the routing table and forward
the packet toward the right output port.
o Finally, note that the access to the table at constant cost allows
to implement new features, e.g., accounting and security controls.
Note that the operations above require a constant time, independent
on the number of entries in the stream table.
2.4.3. On reception of a KPALV request
When the router receives a KPALV
o It checks for the packet validity. This is done both for security
and for detecting events like route changes and faults. The check
can be done in constant time by checking the REBOOK reference in
the packet, see Section 2.4.6 for details. If the check is
positive, continue, otherwise discard the packet (GIUSTO?)
o It finds the corresponding entries in the stream table.
o It updates the expiring time T_exp of the session
o If the value of R_curr in the KPALV request is lower than the
corresponding value stored in the table, it updates the table (and
release the corresponding resources too)
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o If the router lowered the resources assigned to the stream because
of some resource re-assignation, it update the R_curr value in the
KPALV request
o The (possibly updated) KPALV request is forwarded to the next hop
2.4.4. On reception of a UP_RESV request
The behavior is similar to the case of reception of a RESV request,
with the difference that the stream must already have an entry in the
stream table
2.4.5. On reception of a RL_RESV request
When the router receives a RL_RESV
o It checks its validity (see Section 2.4.6). If the check is
positive, continue, otherwise discard the packet (GIUSTO?)
o It deletes the entry associated to the stream from the stream
table and releases the associated resources.
2.4.6. Checking packet validity
For safety reasons, a router must check if the received REBOOK packet
is a valid one. The check can be done by exploiting some
redundancies in the REBOOK structure
o When a router receives for the first time a RESV request, it opens
a new entry in the stream table and initialize it with the data
stored in the request, including the FlowID, that is, the (source
address, target address) pair, where each address is, of course, a
(host address, port) pair. When a data packet or a REBOOK request
is received, the router can check that the FlowID of the received
request matches the FlowID stored in the stream table.
o If the router use only B <32 bits of the 32 bits of the reference,
it can use the remaining 32-B bits for the validity check. A
possible approach is to fill the unused 32-B bits with parity bits
and encrypting the result. The check for reference validity is
immediate and it is very difficult to forge a valid reference.
Note that since the reference value is an opaque 32-bit string for
the other nodes, this type of checks can be implemented privately
by the router without problems of interoperability.
o Another low-cost validity check supposes that the free entries in
the stream table are organized in a linked list. In this case the
router initializes the empty stream table by joining its entries
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in random order, making unpredictable the order the entries will
be used by the router. Moreover, in order to avoid a fast reuse
of the same entry, released entries can be inserted at the end of
the free list, while allocated entries can be extracted from the
head.
In order to check the validity of the REBOOK packet, the router
can verify that the referenced entry is really used.
3. Using the IP Router Alert field
A possible solution to embed REBOOK information inside data packet is
to exploit the IP Router Alert field. This will require the
allocation of a suitable code-point in the IANA maintained registry.
A REBOOK value in the IP Router Alert field will signal to the router
that the 32-bit reference value has been stored between the next
level protocol header and the data payload.
4. Session Timeout
As already explained, because of the soft state nature of REBOOK,
sessions that are not refreshed by keep-alive messages are considered
expired and their resources released. An important issue is the
choice of the timeout interval: if it is too short, the source will
send many keep-alive packets, increasing the overhead; if it is too
long, the resources associated with a crashed session (i.e., a
session where the source crashed) can remain unavailable for a long
time (until the session expires), reducing the efficiency of the
router. Note that it is important that the source knows the session
timeout, in order to choose the keep-alive frequency.
Some possible solutions about the choice of the session timeout are
the following
Fixed The timeout value could be fixed by the protocol. This is
maybe the simplest solution and it has the advantage of a low
overhead, but maybe is too rigid.
RTT based The timeout could be fixed to a multiple of the round trip
time (RTT) between the source and the target. The source would
send keep-alive packet with a frequency that depends on the RTT
and the expiration time should be long enough to keep in account
the probability that a keep-alive packet gets lost. This overhead
of this solution is low. It is important that the difference
between the RTT estimated by the router and the RTT estimated by
the source is small.
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Timeout as a resource With this approach the timeout is considered a
resource that can be negotiated by using the REBOOK mechanism (see
Section 5). Therefore, the source will send its timeout request
in the RESV packet and the routers will change the timeout value
in the RESV request according to their needs. Moreover, timeout
can be changed dynamically during the session by the usual REBOOK
procedure. The drawback of this approach is that it introduces
overhead in the RESV and KPALV packets. Note that with this
approach, the session timeout is the smallest among the timeouts
chosen by the routers.
5. What is a resource?
In this document we talked generically about "resources" to be
reserved, without describing in details what a resource is.
Typically one can expect bandwidth to be a negotiable resource, but
other types of resources are possible. For example, the source could
want to improve the latency by assigning to the stream a higher
priority, so that the router will store the packets in a high-
priority queue. Another example of possible resource is the session
timeout (see Section 4). Depending on the application using REBOOK,
it could prove useful to keep the set of resources extensible, maybe
by using a Type-Length-Value format, so that routers that do not
recognize some resources can skip them.
6. Security consideration
To be written
7. IANA considerations
To be written
8. References
8.1. References
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
February 1997.
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, September 2008.
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8.2. Informative References
[rebook] Montessoro, P. and D. Daniele, "REBOOK: A Deterministic,
Robust and Scalable Resource Booking Algorithm", J Netw
Syst Manage DOI 10.1007/s10922-010-9167-8, may 2010.
Authors' Addresses
Pierluca Montessoro
University of Udine
Via delle Scienze 208
Udine 33100
Italy
Phone: +39-0432-55-8286
EMail: montessoro@uniud.it
Riccardo Bernardini
University of Udine
Via delle Scienze 208
Udine 33100
Italy
Phone: +39-0432-55-8271
EMail: riccardo.bernardini@uniud.it
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