Internet Draft
Network Working Group Lou Berger
Internet Draft LabN Consulting, LLC
Expiration Date: March 2000
Der-Hwa Gan
Juniper Networks, Inc.
George Swallow
Cisco Systems, Inc.
Ping Pan
Bell Labs, Lucent
September 1999
RSVP Refresh Reduction Extensions
draft-ietf-rsvp-refresh-reduct-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
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Abstract
This document describes a number of mechanisms that reduce the
refresh overhead of RSVP. The extensions can be used to reduce
processing requirements of refresh messages, eliminate the state
synchronization latency incurred when an RSVP message is lost and,
when desired, refreshing state without the transmission of whole
refresh messages. These extension present no backwards compatibility
issues.
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Contents
1 Introduction and Background ............................... 3
1.1 Trigger and Refresh Messages .............................. 4
2 RSVP Bundle Message ....................................... 4
2.1 Bundle Header ............................................. 5
2.2 Message Formats ........................................... 6
2.3 Sending RSVP Bundle Messages .............................. 6
2.4 Receiving RSVP Bundle Messages ............................ 7
2.5 Forwarding RSVP Bundle Messages ........................... 8
2.6 Bundle-Capable Bit ........................................ 8
3 MESSAGE_ID Extension ...................................... 9
3.1 MESSAGE_ID and MESSAGE_ID ACK Objects ..................... 10
3.2 Ack Message Format ........................................ 11
3.3 MESSAGE_ID Object Usage ................................... 12
3.4 MESSAGE_ID ACK Object Usage ............................... 14
3.5 Multicast Considerations .................................. 14
3.5.1 Reference RSVP/Routing Interface .......................... 16
3.6 Compatibility ............................................. 16
4 Summary Refresh Extension ................................. 17
4.1 MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects .......... 18
4.2 Srefresh Message Format ................................... 22
4.3 Srefresh Message Usage .................................... 23
4.4 Srefresh NACK ............................................. 25
4.5 Compatibility ............................................. 26
5 Reference Exponential Back-Off Procedures ................. 26
5.1 Outline of Operation ...................................... 26
5.2 Time Parameters ........................................... 27
5.3 Example Retransmission Algorithm .......................... 28
6 Acknowledgments ........................................... 29
7 Security Considerations ................................... 29
8 References ................................................ 29
9 Authors' Addresses ........................................ 30
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1. Introduction and Background
The resource requirements (in terms of CPU processing and memory) for
running RSVP on a router increases proportionally with the number of
sessions. Supporting a large number of sessions can present scaling
problems.
This document describes mechanisms to help alleviate one set of scal-
ing issues. RSVP Path and Resv messages must be periodically
refreshed to maintain state. The approach described effectively
reduces the volume of messages which must be periodically sent and
received, as well as the resources required to process refresh mes-
sages.
The described mechanisms also address issues of latency and reliabil-
ity of RSVP Signaling. The latency and reliability problem occurs
when a non-refresh RSVP message is lost in transmission. Standard
RSVP [RFC2205] maintains state via the generation of RSVP refresh
messages. In the face of transmission loss of RSVP messages, the
end-to-end latency of RSVP signaling is tied to the refresh interval
of the node(s) experiencing the loss. When end-to-end signaling is
limited by the refresh interval, the establishment or change of a
reservation may be beyond the range of what is acceptable for some
applications.
One way to address the refresh volume problem is to increase the
refresh period, "R" as defined in Section 3.7 of [RFC2205]. Increas-
ing the value of R provides linear improvement on transmission over-
head, but at the cost of increasing the time it takes to synchronize
state.
One way to address the latency and reliability of RSVP Signaling is
to decrease the refresh period R. Decreasing the value of R provides
increased probability that state will be installed in the face of
message loss, but at the cost of increasing refresh message rate and
associated processing requirements.
An additional issue is the time to deallocate resources after a tear
message is lost. RSVP does not retransmit ResvTear or PathTear mes-
sages. If the sole tear message transmitted is lost, then resources
will only be deallocated once the "cleanup timer" interval has
passed. This may result in resources being allocated for an unneces-
sary period of time. Note that adjusting the refresh period has no
impact on this issues since tear messages are not retransmitted.
The extensions defined in this document address both the refresh vol-
ume and the reliability issues with mechanisms other than adjusting
refresh rate. A Bundle message is defined to reduce overall message
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handling load. A MESSAGE_ID object is defined to reduce refresh mes-
sage processing by allowing the receiver to more readily identify an
unchanged message. A MESSAGE_ACK object is defined which can be used
to detect message loss and, when used in combination with the MES-
SAGE_ID object, can be used to suppress refresh messages altogether.
A summary refresh message is defined to enable refreshing state
without the transmission of whole refresh messages, while maintaining
RSVP's ability to indicate when state is lost or when next hops
change.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.1. Trigger and Refresh Messages
This document categorizes RSVP messages into two types: trigger and
refresh messages. Trigger messages are those RSVP messages that
advertise state or any other information not previously transmitted.
Trigger messages include messages advertising new state, a route
change that altered the reservation paths, or a reservation modifica-
tion by a downstream router. Trigger messages also include those
messages that include changes in non-RSVP processed objects, such as
changes in the Policy or ADSPEC objects.
Refresh messages represent previously advertised state and contain
exactly the same objects and same information as a previously trans-
mitted message. Only Path and Resv messages can be refresh messages.
Refresh messages are typically bit for bit identical to the corre-
sponding previously transmitted message, with the exception of the
the INTEGRITY object and the flags in the MESSAGE_ID object. These
flags and the INTEGRITY object are allowed to differ in refresh mes-
sages.
2. RSVP Bundle Message
An RSVP Bundle message consists of a bundle header followed by a body
consisting of a variable number of standard RSVP messages. A Bundle
message is used to aggregated multiple RSVP messages within a single
PDU. The term "bundling" is used to avoid confusion with RSVP reser-
vation aggregation. The following subsections define the formats of
the bundle header and the rules for including standard RSVP messages
as part of the message.
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2.1. Bundle Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Flags | Msg type | RSVP checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send_TTL | (Reserved) | RSVP length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the bundle header is identical to the format of the
RSVP common header [RFC2205]. The fields in the header are as fol-
lows:
Vers: 4 bits
Protocol version number. This is version 1.
Flags: 4 bits
0x01: Bundle capable
If set, indicates to RSVP neighbors that this node is willing
and capable of receiving bundle messages. This bit is mean-
ingful only between adjacent RSVP neighbors.
0x02-0x08: Reserved
Msg type: 8 bits
12 = Bundle
RSVP checksum: 16 bits
The one's complement of the one's complement sum of the entire
message, with the checksum field replaced by zero for the pur-
pose of computing the checksum. An all-zero value means that
no checksum was transmitted. Because individual sub-messages
carry their own checksum as well as the INTEGRITY object for
authentication, this field MAY be set to zero.
Send_TTL: 8 bits
The IP TTL value with which the message was sent. This is used
by RSVP to detect a non-RSVP hop by comparing the IP TTL that a
Bundle message sent to the TTL in the received message.
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RSVP length: 16 bits
The total length of this RSVP Bundle message in bytes, includ-
ing the bundle header and the sub-messages that follow.
2.2. Message Formats
An RSVP Bundle message must contain at least one sub-message. A sub-
message MAY be any message type except for another Bundle message.
Current valid sub-messages are RSVP Path, PathTear, PathErr, Resv,
ResvTear, ResvErr, ResvConf or Ack
Empty RSVP Bundle messages SHOULD NOT be sent. A Bundle message MUST
NOT include another RSVP Bundle message as a sub-message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Flags | 12 | RSVP checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send_TTL | (Reserved) | RSVP length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// First sub-message //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// More sub-messages.. //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3. Sending RSVP Bundle Messages
RSVP Bundle messages are sent hop by hop between RSVP-capable neigh-
bors as "raw" IP datagrams with protocol number 46. Raw IP datagrams
are also intended to be used between an end system and the first/last
hop router, although it is also possible to encapsulate RSVP messages
as UDP datagrams for end-system communication that cannot perform raw
network I/O.
RSVP Bundle messages MUST not be used if the next hop RSVP neighbor
does not support RSVP Bundle messages. Methods for discovering such
information include: (1) manual configuration and (2) observing the
Bundle-capable bit (see the description that follows) in the received
RSVP messages. If the next hop RSVP neighbor is not known or changes
in next hops cannot be identified via routing, Bundle messages MUST
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NOT be sent. Note that when the routing next hop is not RSVP capable
it will typically not be possible to identify changes in next hop.
Support for RSVP Bundle messages is optional. While message bundling
helps in scaling RSVP, and in reducing processing overhead and band-
width consumption, a node is not required to transmit every standard
RSVP message in a Bundle message. A node MUST always be ready to
receive standard RSVP messages.
The IP source address is local to the system that originated the Bun-
dle message. The IP destination address is the next hop node for
which the sub-messages are intended. These addresses need not be
identical to those used if the sub-messages were sent as standard
RSVP messages.
For example, the IP source address of Path and PathTear messages is
the address of the sender it describes, while the IP destination
address is the DestAddress for the session. These end-to-end
addresses are overridden by hop-by-hop addresses while encapsulated
in a Bundle message. These addresses can easily be restored from the
SENDER_TEMPLATE and SESSION objects within Path and PathTear mes-
sages. For Path and PathTear messages, the next hop node can be
identified either via a received ACK or from a received corresponding
Resv message. Path and PathTear messages for multicast sessions MUST
NOT be sent in Bundle messages except when the outgoing link is a
point-to-point link and it is known that the next hop is RSVP capa-
ble.
RSVP Bundle messages SHOULD NOT be sent with the Router Alert IP
option in their IP headers. This is because Bundle messages are
addressed directly to RSVP neighbors.
Each RSVP Bundle message MUST occupy exactly one IP datagram. If it
exceeds the MTU, the datagram is fragmented by IP and reassembled at
the recipient node. A single RSVP Bundle message MUST NOT exceed the
maximum IP datagram size, which is approximately 64K bytes.
2.4. Receiving RSVP Bundle Messages
If the local system does not recognize or does not wish to accept an
Bundle message, the received messages shall be discarded without fur-
ther analysis.
The receiver next compares the IP TTL with which a Bundle message is
sent to the TTL with which it is received. If a non-RSVP hop is
detected, the number of non-RSVP hops is recorded. It is used later
in processing of sub-messages.
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Next, the receiver verifies the version number and checksum of the
RSVP Bundle message and discards the message if any mismatch is
found.
The receiver then starts decapsulating individual sub-messages. Each
sub-message has its own complete message length and authentication
information. Each sub-message is processed as if it was received
individually.
2.5. Forwarding RSVP Bundle Messages
When an RSVP router receives a Bundle messages which is not addressed
to one of it's IP addresses, it SHALL forward the message. Non-RSVP
routers will treat RSVP Bundle messages as any other IP datagram.
2.6. Bundle-Capable Bit
To support message bundling, an additional capability bit is added to
the common RSVP header, which is defined in [RFC2205].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Flags | Msg Type | RSVP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send_TTL | (Reserved) | RSVP Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 4 bits
0x01: Bundle capable
If set, indicates to RSVP neighbors that this node is willing
and capable of receiving Bundle messages. This bit is mean-
ingful only between adjacent RSVP neighbors.
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3. MESSAGE_ID Extension
Two new objects are defined as part of the MESSAGE_ID extension. The
two object types are the MESSAGE_ID object and the MESSAGE_ID ACK
object. The objects are used to support acknowledgments. When
refreshes are normally generated, the MESSAGE_ID object can also be
used to simply provide a shorthand indication of when a message rep-
resents new state. Such information can be used on the receiving
node to reduce refresh processing requirements.
Message identification and acknowledgment is done on a hop-by-hop
basis. Acknowledgment is handled independent of SESSION or message
type. Both types of MESSAGE_ID objects contain a message identifier.
The identifier MUST be unique on a per source IP address basis across
messages sent by an RSVP node and received by a particular node. No
more than one MESSAGE_ID object may be included in an RSVP message.
Each message containing an MESSAGE_ID object may be acknowledged via
a MESSAGE_ID ACK object. MESSAGE_ID ACK objects may be sent piggy-
backed in unrelated RSVP messages or in RSVP Ack messages.
Either type of MESSAGE_ID object may be included in a bundle sub-mes-
sage. When included, the object is treated as if it were contained
in a standard, unbundled, RSVP message. Only one MESSAGE_ID object
MAY be included in a (sub)message and it MUST follow any present MES-
SAGE_ID ACK objects. When no MESSAGE_ID ACK objects are present, the
MESSAGE_ID object MUST immediately follow the INTEGRITY object. When
no INTEGRITY object is present, the MESSAGE_ID object MUST immedi-
ately follow the (sub)message header.
When present, one or more MESSAGE_ID ACK objects MUST immediately
follow the INTEGRITY object. When no INTEGRITY object is present,
the MESSAGE_ID ACK objects MUST immediately follow the the (sub)mes-
sage header. An MESSAGE_ID ACK object may only be included in a mes-
sage when the message's IP destination address matches the unicast
address of the node that generated the message(s) being acknowledged.
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3.1. MESSAGE_ID and MESSAGE_ID ACK Objects
MESSAGE_ID Class = 166 (Value to be assigned by IANA of form
10bbbbbb)
MESSAGE_ID object
Class = MESSAGE_ID Class, C_Type = 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 8 bits
0x80 = Summary_Capable flag
Indicates that the sender supports the summary refresh
extension. This flag MUST be set if the node supports the
summary refresh extension. See Section 4.4 for description
of handling by receiver.
0x40 = ACK_Desired flag
Indicates that the sender is willing to accept a message
acknowledgment. Acknowledgments MUST be silently ignored
when they are sent in response to messages whose
ACK_Desired flag is not set.
Epoch: 24 bits
A value that indicates when the Message_ID sequence has reset.
SHOULD be randomly generated each time a node reboots. This
value MUST NOT be changed during normal operation.
Message_ID: 32 bits
When combined with the message generator's IP address, the Mes-
sage_ID field uniquely identifies a message. This field is
ordered and only decreases in value when the Epoch changes or
the value wraps.
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MESSAGE_ID ACK object
Class = MESSAGE_ID Class, C_Type = 2
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 8 bits
0x80 = Summary_Capable flag
Indicates that the sender supports the summary refresh
extension. This flag MUST be set if the node supports the
summary refresh extension. See Section 4.4 for description
of handling by receiver.
0x40 = Refresh_NACK flag
Indicates that no state was found corresponding to the
indicated message identifier. This flag SHALL ONLY be set
when the matching Epoch and Message_ID field values were
received in a Summary Refresh message, and MUST NOT be set
in response to a MESSAGE_ID object received in any other
message. See Section 4 for details.
Epoch: 24 bits
The Epoch field copied from the message being acknowledged.
Message_ID: 32 bits
The Message_ID field copied from the message being acknowl-
edged.
3.2. Ack Message Format
Ack messages carry one or more MESSAGE_ID ACK objects. They MUST NOT
contain any MESSAGE_ID objects. Ack messages are sent hop-by-hop
between RSVP nodes. The IP destination address of an Ack message is
the unicast address of the node that generated the message(s) being
acknowledged. For Path, PathTear, Resv, and RervErr messages this is
taken from the RSVP_HOP Object. For PathErr and ResvErr messages
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this is taken from the message's source address. The IP source
address is an address of the node that sends the Ack message.
The Ack message format is as follows:
::= [ ]
[ ... ]
For Ack messages, the Msg Type field of the Common Header MUST be set
to 13 (Value to be assigned by IANA).
3.3. MESSAGE_ID Object Usage
The MESSAGE_ID object may be included in any RSVP message other than
the Ack message. The MESSAGE_ID object is always generated and pro-
cessed hop-by-hop. The IP address of the object generator is repre-
sented in a per RSVP message type specific fashion. For Path and
PathTear messages the generator's IP address is contained in the
RSVP_HOP. For Resv, ResvTear, PathErr, ResvErr, ResvConf and Bundle
messages the generator's IP address is the source address in the IP
header. Note that MESSAGE_ID objects can be used in both a Bundle
message and its sub-messages. As is always the case with the Bundle
message, each sub-message is processed as if it was received individ-
ually. This includes processing of MESSAGE_ID objects.
The Epoch field contains a generator selected value. The value is
used to indicate when the sender resets the values used in the Mes-
sage_ID field. This information is used by the receiver to detect
out of order messages. On startup, a node SHOULD randomly select a
value to be used in the Epoch field. The node SHOULD ensure that the
selected value is not the same as was used when the node was last
operational. The value MUST NOT be changed unless the node or the
RSVP agent is restarted.
The Message_ID field contains a generator selected value. This
value, when combined with the generator's IP address, identifies a
particular RSVP message and the specific state information it repre-
sents. When a node is sending a refresh message with a MESSAGE_ID
object, it SHOULD use the same Message_ID value that was used in the
RSVP message that first advertised the state being refreshed. When a
node is sending a trigger message, the Message_ID value MUST have a
value that is greater than any other previously used value. A value
is considered to have been used when it has been sent in any message
using the associated IP address. Note that this 32-bit value MAY
wrap.
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The ACK_Desired flag is set when the MESSAGE_ID object generator is
capable of accepting MESSAGE_ID ACK objects. Such information can be
used to ensure reliable delivery of error and confirm messages and to
support fast refreshes in the face of network loss. Nodes setting
the ACK_Desired flag SHOULD retransmit unacknowledged messages at a
more rapid interval than the standard refresh period until the mes-
sage is acknowledged or until a "rapid" retry limit is reached.
Rapid retransmission rate SHOULD be based on well known exponential
back-off procedures. See Section 5 for details on one exponential
back-off retransmission approach. Note that nodes setting the
ACK_Desired flag for unicast sessions, do not need to track the iden-
tify of the next hop since all that is expected is an ACK, not an ACK
from a specific next hop. Issues relate to multicast sessions are
covered in a later section. The ACK_Desired flag will typically be
set only in trigger messages. The ACK_Desired flag MAY be set in
refresh messages.
Nodes processing incoming MESSAGE_ID objects SHOULD check to see if a
newly received message is out of order and can be ignored. Out of
order messages can be identified by examining the values in the Epoch
and Message_ID fields. If the Epoch value differs from the value
previously received from the message sender, the receiver MUST fully
processes the message. If the Epoch values match and the Message_ID
value is greater than the largest value previously received from the
sender, the receiver MUST fully processes the message. If the value
is less than the largest value previously received from the sender,
then the receiver SHOULD check the value previously received for the
state associated with the message. This check should be performed
for the currently defined messages: Path, Resv, PathTear, ResvTear,
PathErr and ResvErr. If no local state information can be associated
with the message, the receiver MUST fully processes the message. If
local state can be associated with the message and the received Mes-
sage_ID value is less than the most recently received value associ-
ated with the state, the message SHOULD be ignored, i.e., silently
dropped.
Nodes receiving messages containing MESSAGE_ID objects SHOULD use the
information in the objects to aid in determining if a message repre-
sents new state or a state refresh. Note that state is only
refreshed in Path and Resv messages. If the received Epoch values
differs from the value previously received from the message sender,
the message is a trigger message and the receiver MUST fully pro-
cesses the message. If a Path or Resv message contains the same Mes-
sage_ID value that was used in the most recently received message for
the same session and, for Path messages, SENDER_TEMPLATE then the
receiver SHOULD treat the message as a state refresh. If the Mes-
sage_ID value is grater than the most recently received value, the
receiver MUST fully processes the message. If the Message_ID value
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is less than the most recently received value, the receiver SHOULD
ignore the message.
Nodes receiving a non-out of order message containing a MESSAGE_ID
object with the ACK_Desired flag set, SHOULD respond with a MES-
SAGE_ID ACK object. Note that MESSAGE_ID objects received in mes-
sages containing errors, i.e., are not syntactically valid, MUST NOT
be acknowledged. PathErr and ResvErr messages SHOULD be treated as
implicit acknowledgments.
3.4. MESSAGE_ID ACK Object Usage
The MESSAGE_ID ACK object is used to acknowledge receipt of messages
containing MESSAGE_ID objects that were sent with the ACK_Desired
flag set. The Epoch and Message_ID fields of a MESSAGE_ID ACK object
MUST have the same value as was received. A MESSAGE_ID ACK object
MUST NOT be generated in response to a received MESSAGE_ID object
when the ACK_Desired flag is not set, except as noted in Section 4.3.
A MESSAGE_ID ACK object MAY be sent in any RSVP message that has an
IP destination address matching the generator of the associated MES-
SAGE_ID object. The MESSAGE_ID ACK object will not typically be
included in the non hop-by-hop Path, PathTear and ResvConf messages.
When no appropriate message is available, one or more MESSAGE_ID ACK
objects SHOULD be sent in an Ack message. Implementations SHOULD
include MESSAGE_ID ACK objects in standard RSVP messages when possi-
ble.
MESSAGE_ID ACK objects received with the Refresh_NACK flag set MUST
process the object as described in Section 4.3. Upon receiving a
MESSAGE_ID ACK object with the Refresh_NACK flag not set, a node
SHOULD stop retransmitting the message at the "rapid" retry rate.
3.5. Multicast Considerations
Path and PathTear messages may be sent to IP multicast destination
addresses. When the destination is a multicast address, it is possi-
ble that a single message containing a single MESSAGE_ID object will
be received by multiple RSVP next hops. When the ACK_Desired flag is
set in this case, acknowledgment processing is more complex. There
are a number of issues to be addressed including ACK implosion, num-
ber acknowledgments to be expected and handling of new receivers.
ACK implosion occurs when each receiver responds to the MESSAGE_ID
object at approximately the same time. This can lead to a poten-
tially large number of MESSAGE_ID ACK objects being simultaneously
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delivered to the message generator. To address this case, the
receiver MUST wait a random interval prior to acknowledging a MES-
SAGE_ID object received in a message destined to a multicast address.
The random interval SHOULD be between zero (0) and a configured maxi-
mum time. The configured maximum SHOULD be set in proportion to the
refresh and "rapid" retransmission interval, i.e, such that the maxi-
mum back-off time does not result in retransmission.
A more fundamental issue is the number of acknowledgments that the
upstream node, i.e., the message generator, should expect. The num-
ber of acknowledgments that should be expected is the same as the
number of RSVP next hops. In the router-to-router case, the number
of next hops can usually be obtained from routing. When hosts are
either the upstream node or the next hops, the number of next hops
will typically not be readily available. Another case where the num-
ber of RSVP next hops will typically not be known is when there are
non-RSVP routers between the message generator and the RSVP next
hops.
When the number of next hops is not known, the message generator
SHOULD only expect a single response. The result of this behavior
will be special retransmission handling until the message is deliv-
ered to at least one next hop, then followed by standard RSVP
refreshes. Refresh messages will synchronize state with any next
hops that don't receive the original message.
Another issue is handling new receivers. It is possible that after
sending a Path message and handling of expected number of acknowledg-
ments that a new receiver joins the group. In this case a new Path
message must be sent to the new receiver. When normal refresh pro-
cessing is occurring, there is no issue. When normal refresh pro-
cessing is suppressed, a Path message must still be generated. In
the router-to-router case, the identification of new next hops can
usually be obtained from routing. When hosts are either the upstream
node or the next hops, the identification of new next hops will typi-
cally not be possible. Another case where the identification of new
RSVP next hops will typically not be possible is when there are non-
RSVP routers between the message generator and the RSVP next hops.
When identification of new next hops is not possible, the message
generator SHOULD only expect a single response. The result of this
behavior will be special retransmission handling until the message is
delivered to at least one next hop, then followed by standard RSVP
refreshes. Refresh messages will synchronize state with any next
hops that don't receive the original message either due to loss or
not yet being a group member.
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3.5.1. Reference RSVP/Routing Interface
When using the MESSAGE_ID extension with multicast sessions it is
preferable for RSVP to obtain the number of next hops from routing
and to be notified when that number changes. The interface between
routing and RSVP is purely an implementation issue. Since RSVP
[RFC2205] describes a reference routing interface, we present a ver-
sion of the RSVP/routing interface updated to provide number of next
hop information. See [RFC2205] for previously defined parameters and
function description.
o Route Query
Mcast_Route_Query( [ SrcAddress, ] DestAddress,
Notify_flag )
-> [ IncInterface, ] OutInterface_list,
NHops_list
o Route Change Notification
Mcast_Route_Change( ) -> [ SrcAddress, ] DestAddress,
[ IncInterface, ] OutInterface_list,
NHops_list
NHops_list provides the number of multicast group members
reachable via each OutInterface_list entry.
3.6. Compatibility
There are no backward compatibility issues raised by the MESSAGE_ID
Class. The MESSAGE_ID Class has an assigned value whose form is
10bbbbbb. Per RSVP [RFC2205], classes with values of this form must
be ignored and not forwarded by nodes not supporting the class. When
the receiver of a MESSAGE_ID object does not support the class, the
object will be silently ignored. The generator of the MESSAGE_ID
object will not see any acknowledgments and therefore refresh mes-
sages per standard RSVP. Lastly, since the MESSAGE_ID ACK object can
only be issued in response to the MESSAGE_ID object, there are no
possible issues with this object or Ack messages.
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4. Summary Refresh Extension
The Summary Refresh extension enables the refreshing of RSVP state
without the transmission of standard Path or Resv messages. The ben-
efits of the described extension are that it reduces the amount of
information that must be transmitted and processed in order to main-
tain RSVP state synchronization. Importantly, the described exten-
sion preserves RSVP's ability to handle non-RSVP next hops and to
adjust to changes in routing. This extension cannot be used with
Path or Resv messages that contain any change from previously trans-
mitted messages, i.e, are not refresh messages.
The summary refresh extension uses the previously defined MESSAGE_ID
object class, the ACK message, and a new Srefresh message. The new
message carries a list of Message_ID fields corresponding to the Path
and Resv states that are to be refreshed. The Message_ID fields are
carried in one of three Srefresh related MESSAGE_ID class objects.
The three Srefresh related MESSAGE_ID class objects are the MES-
SAGE_ID LIST object, the MESSAGE_ID SRC_LIST object, and the MES-
SAGE_ID MCAST_LIST object. The MESSAGE_ID LIST object is made up of
a list of Message_ID fields that were originally advertised in MES-
SAGE_ID objects that shared a common Epoch value. The MESSAGE_ID
SRC_LIST object is used for multicast sessions over multi-access net-
works and adds the sender's IP address. The MESSAGE_ID MCAST_LIST
object is used for multicast sessions over point-to-point links and
adds the sender's IP address and the Session IP address.
An RSVP node receiving an Srefresh message, matches each received
listed Message_ID field with installed Path or Resv state. All
matching state is updated as if a normal RSVP refresh message has
been received. If matching state cannot be found, then the Srefresh
message sender is notified via a refresh NACK.
A refresh NACK is indicated by setting the Refresh_NACK flag in the
MESSAGE_ID ACK object. The rules for sending a MESSAGE_ID ACK object
with the Refresh_NACK flag set are the same as was described in the
previous section. This includes sending MESSAGE_ID ACK object both
piggy-backed in unrelated RSVP messages or in RSVP ACK messages.
Nodes supporting the described extension can advertise their support
and detect if an RSVP neighbor also supports the extension. This is
accomplished via a flag in the MESSAGE_ID and MESSAGE_ID ACK objects.
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4.1. MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects
The MESSAGE_ID LIST, MESSAGE_ID SRC_LIST and MESSAGE_ID MCAST_LIST
objects are part of the MESSAGE_ID class. See Section 3.1 for class
value.
MESSAGE_ID LIST object
Class = MESSAGE_ID Class, C_Type = 3
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : |
// : //
| : |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 8 bits
No flags are currently defined. This field MUST be zero on
transmission and ignored on receipt.
Epoch: 24 bits
The Epoch field from the MESSAGE_ID object corresponding to the
state being refreshed.
Message_ID: 32 bits
The Message_ID field from the MESSAGE_ID object corresponding
to the state being refreshed. A variable number of Message_IDs
may be included. The number may range from one up to as many
as can be carried without the Srefresh Message exceeding the
MTU.
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MESSAGE_ID SRC_LIST object
Class = MESSAGE_ID Class, C_Type = 4
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source_ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID_Tuple |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : |
// : //
| : |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source_ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID_Tuple |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where a Source_Message_ID_Tuple consists of:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source_IP_Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 8 bits
No flags are currently defined. This field MUST be zero on
transmission and ignored on receipt.
Epoch: 24 bits
The Epoch field from the MESSAGE_ID object corresponding to the
state being refreshed.
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Message_ID: 32 bits
The Message_ID field from the MESSAGE_ID object corresponding
to the Path state being refreshed. A variable number of Mes-
sage_IDs may be included. The number may range from one up to
as many as can be carried without the Srefresh Message exceed-
ing the MTU. Each Message_ID MUST be followed by the source IP
address corresponding to the sender of the Path state being
refreshed.
Source_IP_Address: 32 bits
The source IP address corresponding to the sender of the Path
state being refreshed.
MESSAGE_ID MCAST_LIST object
Class = MESSAGE_ID Class, C_Type = 5
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast_ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID_ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tuple |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : |
// : //
| : |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast_ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID_ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tuple |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Where a Multicast_Message_ID_Tuple consists of:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source_IP_Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination_IP_Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 8 bits
No flags are currently defined. This field MUST be zero on
transmission and ignored on receipt.
Epoch: 24 bits
The Epoch field from the MESSAGE_ID object corresponding to the
state being refreshed.
Message_ID: 32 bits
The Message_ID field from the MESSAGE_ID object corresponding
to the Path state being refreshed. A variable number of Mes-
sage_IDs may be included. The number may range from one up to
as many as can be carried without the Srefresh Message exceed-
ing the MTU. Each Message_ID MUST be followed by the source IP
address corresponding to the sender of the Path state being
refreshed, and the destination IP address of the session.
Source_IP_Address: 32 bits
The source IP address corresponding to the sender of the Path
state being refreshed.
Destination_IP_Address: 32 bits
The destination IP address corresponding to the session of the
Path state being refreshed.
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4.2. Srefresh Message Format
Srefresh messages carry one or more MESSAGE_ID LIST, MESSAGE_ID
SRC_LIST, and MESSAGE_ID MCAST_LIST objects. MESSAGE_ID LIST and
MESSAGE_ID MCAST_LIST objects MAY be carried in the same Srefresh
message. MESSAGE_ID SRC_LIST can not be combined in Srefresh mes-
sages with the other objects. A single Srefresh message MAY refresh
both Path or Resv state.
Srefresh messages carrying Message_ID fields corresponding to Path
state SHOULD be sent with a destination IP address equal to the
address carried in the corresponding SESSION objects. The destina-
tion IP address MAY be set to the RSVP next hop when the next hop is
known to be RSVP capable and either (a) the session is unicast or (b)
the outgoing interface is a point-to-point link. Srefresh messages
carrying Message_ID fields corresponding to Resv state MUST be sent
with an destination IP address set to the Resv state's previous hop.
Srefresh messages sent to a multicast destination, MUST contain MES-
SAGE_ID SRC_LIST objects and MUST NOT include any MESSAGE_ID LIST or
MESSAGE_ID MCAST_LIST objects. Srefresh messages sent to the RSVP
next hop MAY contain either or both MESSAGE_ID LIST and MESSAGE_ID
MCAST_LIST objects, but MUST NOT include any MESSAGE_ID SRC_LIST
objects.
The source IP address of an Srefresh message is an address of the
node that generates the message. The source IP address MUST match
the addressed associate with the MESSAGE_ID objects when they were
included in a standard RSVP message. As previously mentioned, the
address associate with a MESSAGE_ID object is represented in a per
RSVP message type specific fashion. For Path and PathTear messages
the associated IP address is contained in the RSVP_HOP. For Resv,
ResvTear, PathErr, ResvErr, ResvConf and Bundle messages the associ-
ated IP address is the source address in the IP header.
Srefresh messages that are sent destined to a session's destination
IP address MUST be sent the Router Alert IP option in their IP head-
ers. Srefresh messages addressed directly to RSVP neighbors SHOULD
NOT be sent with the Router Alert IP option in their IP headers.
Each Srefresh message MUST occupy exactly one IP datagram. If it
exceeds the MTU, the datagram is fragmented by IP and reassembled at
the recipient node. A single RSVP Srefresh message MUST NOT exceed
the maximum IP datagram size, which is approximately 64K bytes. Sre-
fresh messages MAY be sent within an RSVP aggregate messages.
Although this is not expected since Srefresh messages can carry a
list of Message_ID fields within a single object.
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The Srefresh message format is as follows:
::= [ ]
|
::= [ ... ]
::= [ ... ]
::=
[ ... ]
For Srefresh messages, the Msg Type field of the Common Header MUST
be set to 14 (Value to be assigned by IANA).
4.3. Srefresh Message Usage
An Srefresh message MAY be generated to refresh Resv or Path state.
If an Srefresh message is used to refresh some particular state, then
the generation of a standard refresh message SHOULD be suppressed. A
state's refresh interval is not affected by the use of Srefresh mes-
sage based refreshes. An Srefresh message MUST NOT be used in place
of a trigger Path or Resv message, i.e., one that would advertise a
state change.
When generating an Srefresh message, a node SHOULD refresh as much
Path and Resv state as is possible by including the information from
as many MESSAGE_ID objects in the same Srefresh message. Only the
information from MESSAGE_ID objects that meet the previously
described source and destination IP address restrictions may be
included in the same Srefresh message. Identifying Resv state that
can refreshed using the same Srefresh message is fairly straightfor-
ward. Identifying which Path state may be included is a little more
complex.
Independent of the state being refreshed, only state that was previ-
ously advertised in Path and Resv messages containing MESSAGE_ID
objects can be refreshed via an Srefresh message. Srefresh message
based refreshes must preserve the state synchronization properties of
Path or Resv message based refreshes. Specifically, the use of Sre-
fresh messages MUST NOT result in state being timed-out at the RSVP
next hop. The period at which state is refreshed when using Srefresh
messages MAY be shorter than the period that would be used when using
Path or Resv message based refreshes, but it MUST NOT be longer. The
particular approach used to trigger Srefresh message based refreshes
is implementation specific. Some possibilities are triggering
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Srefresh message generation based on each state's refresh period or,
on a per interface basis, periodically generating Srefresh messages
to refresh all state that has not been refreshed within the state's
refresh interval. Other approaches are also possible.
When generating an Srefresh message, there are two methods for iden-
tifying which Path state may be refreshed in a specific message. In
both cases, the previously mentioned refresh interval and source IP
address restrictions must be followed. The primary method is to
include only those sessions that share the same destination IP
address in the same Srefresh message. When using this method, the
destination address of each session MUST be the same as the destina-
tion address in the IP header of the Srefresh message.
The secondary method for identifying which Path state may be
refreshed within a single Srefresh message is an optimization. This
method MAY be used when the next hop is known to support RSVP and
when either (a) the session is unicast or (b) the outgoing interface
is a point-to-point link. This method MUST NOT be used when the next
hop is not known to support RSVP or when the outgoing interface is to
a multi-access network and the session is to a multicast address.
When using this method, the destination address in the IP header of
the Srefresh message is always the next hop's address. When the out-
going interface is a point-to-point link, all Path state associated
with sessions advertised out the interface SHOULD be included in the
same Srefresh message. When the outgoing interface is not a point-
to-point link, all unicast session Path state SHOULD be included in
the same Srefresh message.
Identifying which Resv state may be refreshed within a single Sre-
fresh message is based simply on the source and destination IP
addresses. Any state that was previously advertised in Resv messages
with the same IP addresses as an Srefresh message MAY be included.
After identifying the Path and Resv state that can be included in a
particular Srefresh message, the message generator adds to the mes-
sage MESSAGE_ID information matching each identified state's previ-
ously used object. For all Resv state and for Path state of unicast
sessions, the information is added to the message in an MESSAGE_ID
LIST object that has a matching Epoch value. If no matching object
exists, then a new MESSAGE_ID LIST object is created. Path state of
multicast sessions may be added to the same message when the destina-
tion address of the Srefresh message is the RSVP next hop. In this
case the information is added to the message in an MESSAGE_ID
MCAST_LIST object that has a matching Epoch value. If no matching
object exists, then a new MESSAGE_ID MCAST_LIST object is created.
When the destination address of the message is a multicast address,
then identified information is added to the message in an MESSAGE_ID
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SRC_LIST object that has a matching Epoch value. If no matching
object exists, then a new MESSAGE_ID SRC_LIST object is created.
Once the Srefresh message is composed, the message generator trans-
mits the message out the proper interface.
Upon receiving an Srefresh message, a receiver MUST attempt to iden-
tify matching Path or Resv state. Matching is done based on the
source address in the IP header of the Srefresh message and each MES-
SAGE_ID class object contained in the message. For each received
Message_ID field, the receiver performs an installed state lookup
based on the received value and the received object type. If match-
ing state can be found, then the receiver MUST update the matching
state information as if a standard refresh had been received. If the
receiver cannot identify any installed state matching a Message_ID
field associated with Resv state or Path state of unicast session,
i.e. carried in a MESSAGE_ID LIST Object, then an Srefresh NACK MUST
be generated corresponding to each unmatched Message_ID field in a
MESSAGE_ID LIST object. For unmatched Message_ID fields associated
with Path state of multicast sessions, an RPF check must be performed
in order to determine if a NACK should be generated. The RPF check
is performed based on the source and group information carried in the
MESSAGE_ID SRC_LIST and MCAST_LIST objects. The receiver only gener-
ates an Srefresh NACK when the receiver would forward packets to the
identified group from the listed sender. If the receiver would for-
ward multicast data packets from a listed sender and there is a cor-
responding unmatched Message_ID field, then an appropriate Srefresh
NACK MUST be generated. If the receiver is not forwarding packets to
the identified group from a listed sender, a corresponding unmatched
Message_ID field is silently ignored.
4.4. Srefresh NACK
Srefresh NACKs are used to indicated that a received MESSAGE_ID LIST,
SRC_LIST, or MCAST_LIST object does not match any installed state.
An Srefresh NACK is encoded in a MESSAGE_ID ACK object with the
Refresh_NACK flag set. When generating an Srefresh NACK, The epoch
and Message_ID fields of a MESSAGE_ID ACK object MUST have the same
value as was received. Objects with the Refresh_NACK flag set are
transmitted as previously described, see Section 3.4.
MESSAGE_ID ACK objects received with the Refresh_NACK flag set indi-
cate that the object generator does not have any installed state
matching the object. Upon receiving a MESSAGE_ID ACK object with the
Refresh_NACK flag set, the receiver performs an installed Path or
Resv state lookup based on the values contained in the object. If
matching state is found, then the receiver MUST transmit the matching
state via a standard Path or Resv message. If the receiver cannot
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identify any installed state, then no action is required.
4.5. Compatibility
Nodes supporting the Summary Refresh extension advertise their sup-
port via the Summary_Capable flag in all MESSAGE_ID and MESSAGE_ID
ACK objects transmitted by the node. Support is also implied when a
node transmits an Srefresh Message. This enables supporting nodes to
detect each other. When it is not known if a next hop supports the
extension, standard Path and Resv message based refreshes MUST be
used. Note that when the routing next hop does not support RSVP, it
will not always be possible to detect if the RSVP next hop supports
the Summary Refresh extension. Therefore, when the routing next hop
is not RSVP capable the Srefresh message based refresh SHOULD NOT be
used. A node MAY be administratively configured to use Srefresh mes-
sages in all cases when all RSVP nodes in a network are known to sup-
port the Summary Refresh extension.
Nodes supporting the Summary Refresh extension must also take care to
recognize when a next hop stops sending MESSAGE_ID and MESSAGE_ID ACK
objects with the Summary_Capable flag set. To cover this case, nodes
supporting the Summary Refresh extension MUST examine each received
Summary_Capable flag. If the flag changes from indicating support to
indicating non-support then Srefresh messages MUST NOT be used for
subsequent state refreshes to that neighbor.
5. Reference Exponential Back-Off Procedures
This section is based on [Pan] and provides an example of how to
implement exponential back-off. Implementations MAY choose to use
the described procedures.
5.1. Outline of Operation
We propose the following feedback mechanism for exponential back-off
retransmission of an RSVP message: When sending such a message, a
node inserts a MESSAGE_ID object with the the ACK_Desired flag set.
Upon reception, a receiving node acknowledges the arrival of the mes-
sage by sending back an message acknowledgment (that is, a corre-
sponding MESSAGE_ID ACK object.) When the sending node receives this
message acknowledgment for a Path or Resv message, it will automati-
cally scale back the retransmission rate for these messages for the
flow. If the trigger message was for a different message type, no
other further action is required.
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Until the message acknowledgment is received, the sending node will
retransmit the message. The interval between retransmissions is gov-
erned by a rapid retransmission timer. The rapid retransmission
timer starts at a small interval which increases exponentially until
it reaches a threshold. From that point on, the sending node will
use a fixed timer to refresh Path and Resv messages and stop re-
transmitting other messages. This mechanism is designed so that the
message load is only slightly larger than in the current specifica-
tion even when the receiving node does not support message acknowl-
edgment.
5.2. Time Parameters
The described procedures make use of the following time parameters.
All parameters are per interface.
Rapid retransmission interval Rf:
Rf is the initial retransmission interval for unacknowledged
messages. After sending the message for the first time, the
sending node will schedule a retransmission after Rf seconds.
The value of Rf could be as small as the round trip time (RTT)
between a sending and a receiving node, if known. Unless a node
knows that all receiving nodes support echo-replies, a slightly
larger configurable value is suggested.
Slow refresh interval Rs:
The sending node retransmits Path and Resv messages with this
interval after it has determined that the receiving node will
generate MESSAGE_ID ACK objects. To reduce the number of
unnecessary retransmissions in a stable network, Rs can be set
to a large value. The value of Rs should be configurable per
each egress interface.
Fixed retransmission interval R:
A node retransmits the trigger message with the interval Rf*(1 +
Delta)**i until the retransmission interval reaches the fixed
retransmission interval R or a message acknowledgment has been
received. If no acknowledgment has been received, the node
continues to retransmit Resv and Path messages every R seconds.
By default R should be the same value as the retransmission
interval in the current RSVP specification.
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Increment value Delta:
Delta governs the speed with which the sender increases the
retransmission interval. The ratio of two successive
retransmission intervals is (1 + Delta).
5.3. Example Retransmission Algorithm
After a sending node transmits a message containing a MESSAGE_ID
object with the ACK_Desired flag set, it should immediately schedule
a retransmission after Rf seconds. If a corresponding MESSAGE_ID ACK
object is received earlier than Rf seconds, then retransmission
SHOULD be canceled. Otherwise, it will retransmit the message after
(1 + Delta)*Rf seconds. The staged retransmission will continue
until either an appropriate MESSAGE_ID ACK object is received, or the
retransmission interval has been increased to R. Once the retrans-
mission interval has been increased to R, Path and Resv messages will
be refreshed within the interval R. Other messages will not be
retransmitted.
The implementation of exponential back-off retransmission is simple.
A sending node can use the following algorithm after transmitting a
message containing a MESSAGE_ID object with the ACK_Desired flag set:
On initial transmission initialize: Rk = Rf
if (Rk < R) {
retransmit the message;
wake up after Rk seconds;
Rk = Rk * (1 + Delta);
exit;
}
else { /* no reply from receivers for too long: */
Rk = R;
if (message is a Path or Resv Message) {
send out a refresh message;
wake up after Rk seconds;
exit;
}
else {
clean up any associated state and resources;
exit;
}
}
Asynchronously, when a sending node receives a corresponding MES-
SAGE_ID ACK object, it will change the retransmission interval Rk to
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Rs and, for non Path or Resv messages, clean up any associated state
and resources. Note that the transmitting node does not advertise
the use of the described exponential back-off procedures via the
TIME_VALUE object.
6. Acknowledgments
This document represents ideas and comments from the MPLS-TE design
team and participants in the RSVP Working Group's interim meeting.
Thanks to Yoram Bernet, Fred Baker, Roch Guerin, Henning Schulzrinne,
Andreas Terzis, David Mankins and Masanobu Yuhara for specific feed-
back on the document.
Portions of this work are based on work done by Masanobu Yuhara and
Mayumi Tomikawa [Yuhara].
7. Security Considerations
No new security issues are raised in this document. See [RFC2205]
for a general discussion on RSVP security issues.
8. References
[Pan] Pan, P., Schulzrinne, H., "Staged Refresh Timers for RSVP,"
Global Internet'97, Phoenix, AZ, Nov. 1997.
http://www.ctr.columbia.edu/~pan/papers/timergi.ps
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119.
[RFC2205] Braden, R. Ed. et al, "Resource ReserVation Protocol
-- Version 1 Functional Specification", RFC 2205,
September 1997.
[Yuhara] Yuhara, M., Tomikawa, M. "RSVP Extensions for ID-based
Refreshes," Internet Draft,
draft-yuhara-rsvp-refresh-00.txt, April 1999.
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9. Authors' Addresses
Lou Berger
LabN Consulting, LLC
Voice: +1 301 468 9228
Email: lberger@labn.net
Der-Hwa Gan
Juniper Networks, Inc.
385 Ravendale Drive
Mountain View, CA 94043
Voice: +1 650 526 8074
Email: dhg@juniper.net
George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
Voice: +1 978 244 8143
Email: swallow@cisco.com
Ping Pan
Bell Labs, Lucent
101 Crawfords Corner Road, Room 4C-508
Holmdel, NJ 07733
USA
Phone: +1 732 332 6744
Email: pingpan@dnrc.bell-labs.com
Berger, et al. [Page 30]