< draft-ietf-rsvp-refresh-reduct-04.txt		draft-ietf-rsvp-refresh-reduct-05.txt >
	Network Working Group Lou Berger		A new Request for Comments is now available in online RFC libraries.
	Internet Draft LabN Consulting, LLC
	Expiration Date: October 2000
	Der-Hwa Gan
	Juniper Networks, Inc.
	George Swallow
	Cisco Systems, Inc.
	Ping Pan
	Bell Labs, Lucent
	Franco Tommasi
	Simone Molendini
	University of Lecce
	April 2000
	RSVP Refresh Overhead Reduction Extensions
	draft-ietf-rsvp-refresh-reduct-04.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
	documents of the Internet Engineering Task Force (IETF), its areas,
	and its working groups. Note that other groups may also distribute
	working documents as Internet-Drafts.
	Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any
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	To view the current status of any Internet-Draft, please check the
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	Abstract
	This document describes a number of mechanisms that can be used to
	reduce processing overhead 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. The same extensions also
	support reliable RSVP message delivery on a per hop basis. These
	extension present no backwards compatibility issues.
	Contents
	1 Introduction and Background ............................... 3
	1.1 Trigger and Refresh Messages .............................. 4
	2 Refresh-Reduction-Capable Bit ............................. 5
	3 RSVP Bundle Message ....................................... 6
	3.1 Bundle Header ............................................. 6
	3.2 Message Formats ........................................... 7
	3.3 Sending RSVP Bundle Messages .............................. 8
	3.4 Receiving RSVP Bundle Messages ............................ 9
	4 MESSAGE_ID Extension ...................................... 9
	4.1 Modification of Standard Message Formats .................. 10
	4.2 MESSAGE_ID Objects ....................................... 10
	4.3 MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects ................ 11
	4.4 Ack Message Format ........................................ 12
	4.5 MESSAGE_ID Object Usage ................................... 13
	4.6 MESSAGE_ID_ACK Object and MESSAGE_ID_NACK Object Usage .... 15
	4.7 Multicast Considerations .................................. 16
	4.7.1 Reference RSVP/Routing Interface .......................... 17
	4.8 Compatibility ............................................. 17
	5 Summary Refresh Extension ................................. 18
	5.1 MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects .......... 19
	5.2 Srefresh Message Format ................................... 25
	5.3 Srefresh Message Usage .................................... 26
	5.4 Srefresh NACK ............................................. 29
	5.5 Preserving RSVP Soft State ................................ 29
	5.6 Compatibility ............................................. 30
	6 Reference Exponential Back-Off Procedures ................. 31
	6.1 Outline of Operation ...................................... 31
	6.2 Time Parameters ........................................... 31
	6.3 Example Retransmission Algorithm .......................... 32
	7 Acknowledgments ........................................... 32
	8 Security Considerations ................................... 33
	9 References ................................................ 33
	10 Authors' Addresses ........................................ 34
	Changes from previous version:
	o All suggest values removed to avoid implementation confusion.
	IANA has not responded to repeated requests for assignment.
	o Some minor text cleanup as suggested on mailing list.
	1. Introduction and Background
	Standard RSVP [RFC2205] maintains state via the generation of RSVP
	refresh messages. Refresh messages are used to both synchronize
	state between RSVP neighbors and to recover from lost RSVP messages.
	The use of Refresh messages to cover many possible failures has
	resulted in a number of operational problems. One problem relates to
	scaling, another relates to the reliability and latency of RSVP
	Signaling.
	The scaling problems are linked to the resource requirements (in
	terms of processing and memory) of running RSVP. The resource
	requirements increase proportionally with the number of sessions.
	Each session requires the generation, transmission, reception and
	processing of RSVP Path and Resv messages per refresh period.
	Supporting a large number of sessions, and the corresponding volume
	of refresh messages, presents a scaling problem.
	The reliability and latency problem occurs when a non-refresh RSVP
	message is lost in transmission. Standard RSVP [RFC2205] recovers
	from a lost message via 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 delay incurred in the establishment or the 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].
	Increasing the value of R provides linear improvement on transmission
	overhead, but at the cost of increasing the time it takes to
	synchronize state.
	One way to address the reliability and latency of RSVP Signaling is
	to decrease the refresh period R. Decreasing the value of R
	increases the 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
	messages. 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
	unnecessary period of time. Note that even when the refresh period
	is adjusted, the "cleanup timer" must still expire since tear
	messages are not retransmitted.
	The extensions defined in this document address both the refresh
	volume and the reliability issues with mechanisms other than
	adjusting refresh rate. The extensions are collectively referred to
	as the "Refresh Overhead Reduction" or the "Refresh Reduction"
	extensions. A Bundle message is defined to reduce overall message
	handling load. A MESSAGE_ID object is defined to reduce refresh
	message 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 support reliable RSVP message
	delivery on a per hop basis. 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 and to adjust to changes in routing.
	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 alters a reservation path, or a modification to an
	existing RSVP session or reservation. 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
	transmitted message, and are sent over the same path. Only Path and
	Resv messages can be refresh messages. Refresh messages are
	identical to the corresponding previously transmitted message, with
	some possible exceptions. Specifically, the checksum field, the
	flags field and the INTEGRITY object may differ in refresh messages.
	2. Refresh-Reduction-Capable Bit
	To indicate support for the refresh overhead reduction extensions, 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: Refresh (overhead) reduction capable
	When set, indicates that this node is willing and capable of
	receiving all the messages and objects described in this
	document. This includes the Bundle message described in
	Section 3, the MESSAGE_ID objects and Ack messages described
	in Section 4, and the MESSAGE_ID LIST objects and Srefresh
	message described in Section 5. This bit is meaningful only
	between RSVP neighbors.
	Nodes supporting the refresh overhead reduction extensions must also
	take care to recognize when a next hop stops sending RSVP messages
	with the Refresh-Reduction-Capable bit set. To cover this case,
	nodes supporting the refresh overhead reduction extensions MUST
	examine the flags field of each received RSVP message. If the flag
	changes from indicating support to indicating non-support then,
	unless configured otherwise, Srefresh messages (described in Section
	5) MUST NOT be used for subsequent state refreshes to that neighbor
	and Bundle messages (Section 3) MUST NOT be sent to that neighbor.
	Note, a node that supports reliable RSVP message delivery (Section 4)
	but not Bundle and Srefresh messages, will not set the Refresh-
	Reduction-Capable bit.
	3. 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 aggregate multiple RSVP messages within a single
	PDU. The term "bundling" is used to avoid confusion with RSVP
	reservation aggregation. The following subsections define the
	formats of the bundle header and the rules for including standard
	RSVP messages as part of the message.
	3.1. 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
	follows:
	Vers: 4 bits
	Protocol version number. This is version 1.
	Flags: 4 bits
	0x01: Refresh (overhead) reduction capable
	See Section 2.
	0x02-0x08: Reserved
	Msg type: 8 bits
	TBA = Bundle (Value not yet assigned by IANA)
	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
	purpose of computing the checksum. An all-zero value means
	that no checksum was transmitted. Because individual sub-
	messages may carry their own checksum as well as the INTEGRITY
	object for authentication, this field MAY be set to zero. Note
	that when the checksum is not computed, the header of the
	bundle message will not be covered by any checksum. If the
	checksum is computed, individual sub-messages MAY set their own
	checksum 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 Send_TTL with
	the IP TTL in a received message.
	RSVP length: 16 bits
	The total length of this RSVP Bundle message in bytes,
	including the bundle header and the sub-messages that follow.
	3.2. 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.
	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 | TBA (IANA) | RSVP checksum |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Send_TTL | (Reserved) | RSVP length |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	// First sub-message //
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	// More sub-messages.. //
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	3.3. Sending RSVP Bundle Messages
	Support for RSVP Bundle messages is optional. While message bundling
	helps in scaling RSVP, by reducing processing overhead and bandwidth
	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.
	RSVP Bundle messages can only be sent to RSVP neighbors that support
	bundling. Methods for discovering such information include: (1)
	manual configuration and (2) observing the Refresh-Reduction-Capable
	bit (see Section 2) in the received RSVP messages. RSVP Bundle
	messages MUST NOT be used if the RSVP neighbor does not support RSVP
	Bundle messages.
	RSVP Bundle messages are sent hop by hop between RSVP-capable nodes
	as "raw" IP datagrams with protocol number 46. The IP source address
	is an address local to the system that originated the Bundle message.
	The IP destination address is the RSVP neighbor for which the sub-
	messages are intended.
	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, which
	is approximately 64K bytes. If it exceeds the MTU, the datagram is
	fragmented by IP and reassembled at the recipient node.
	Implementations may choose to limit each RSVP Bundle message to the
	MTU size of the outgoing link, e.g. 1500 bytes. Implementations
	SHOULD also limit the amount of time that a message is delayed in
	order to be bundled. Different limits may be used for trigger and
	standard refresh messages. Trigger messages SHOULD be delayed a
	minimal amount of time. Refresh messages may be delayed up to their
	refresh interval. Note that messages related to the same Resv or
	Path state should not be delayed at different intervals in order to
	preserve ordering.
	If the RSVP neighbor is not known or changes in next hops cannot be
	identified via routing, Bundle messages MUST NOT be used. Note that
	when the routing next hop is not RSVP capable it will typically not
	be possible to identify changes in next hop.
	Any message that will be handled by the RSVP neighbor indicated in a
	Bundle Message's destination address may be included in the same
	message. This includes all RSVP messages that would be sent out a
	point-to-point link. It includes any message, such as a Resv,
	addressed to the same destination address. It also includes Path and
	PathTear messages when the next hop is known to be the destination
	and changes in next hops can be detected. Path and PathTear messages
	for multicast sessions MUST NOT be sent in Bundle messages when the
	outgoing link is not a point-to-point link or when the next hop does
	not support the refresh overhead reduction extensions.
	3.4. Receiving RSVP Bundle Messages
	If the local system does not recognize or does not wish to accept a
	Bundle message, the received messages shall be discarded without
	further analysis.
	The receiver next compares the Send_TTL with which a Bundle message
	is sent to the IP 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.
	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. With the exception of using the Send_TTL from the
	header of the Bundle message, each sub-message is processed as if it
	was received individually.
	4. MESSAGE_ID Extension
	Three new objects are defined as part of the MESSAGE_ID extension.
	The objects are the MESSAGE_ID object, the MESSAGE_ID_ACK object, and
	the MESSAGE_ID_NACK objects. The first two objects are used to
	support acknowledgments and reliable RSVP message delivery. The last
	object is used to support the summary refresh extension described in
	Section 5. The MESSAGE_ID object can also be used to simply provide
	a shorthand indication of when the message carrying the object is a
	refresh message. Such information can be used by the receiving node
	to reduce refresh processing requirements.
	Message identification and acknowledgment is done on a per hop basis.
	All types of MESSAGE_ID objects contain a message identifier. The
	identifier MUST be unique on a per object generator's IP address
	basis. No more than one MESSAGE_ID object may be included in an RSVP
	message. Each message containing a MESSAGE_ID object may be
	acknowledged via a MESSAGE_ID_ACK object, when so indicated.
	MESSAGE_ID_ACK and MESSAGE_ID_NACK objects may be sent piggy-backed
	in unrelated RSVP messages or in RSVP Ack messages. RSVP messages
	carrying any of the three object types may be included in a bundle
	message. When included, each object is treated as if it were
	contained in a standard, non-bundled, RSVP message.
	4.1. Modification of Standard Message Formats
	The MESSAGE_ID, MESSAGE_ID_ACK and MESSAGE_ID_NACK objects may be
	included in the standard RSVP messages, as defined in [RFC2205].
	When included, one or more MESSAGE_ID_ACK or MESSAGE_ID_NACK objects
	MUST immediately follow the INTEGRITY object. When no INTEGRITY
	object is present, the MESSAGE_ID_ACK or MESSAGE_ID_NACK objects MUST
	immediately follow the message or sub-message header. Only one
	MESSAGE_ID object MAY be included in a message or sub-message and it
	MUST follow any present MESSAGE_ID_ACK or MESSAGE_ID_NACK objects.
	When no MESSAGE_ID_ACK or MESSAGE_ID_NACK objects are present, the
	MESSAGE_ID object MUST immediately follow the INTEGRITY object. When
	no INTEGRITY object is present, the MESSAGE_ID object MUST
	immediately follow the message or sub-message header.
	The ordering of the ACK objects for all standard RSVP messages is:
	<Common Header> [ <INTEGRITY> ]
	[ [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>] ... ]
	[ <MESSAGE_ID> ]
	4.2. MESSAGE_ID Objects
	MESSAGE_ID Class = TBA (Value not yet assigned by IANA of form
	0bbbbbbb)
	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_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Flags: 8 bits
	0x01 = ACK_Desired flag
	Indicates that the sender requests the receiver to send an
	acknowledgment for the message.
	Epoch: 24 bits
	A value that indicates when the Message_Identifier sequence has
	reset. SHOULD be randomly generated each time a node reboots
	or the RSVP agent is restarted. The value SHOULD NOT be the
	same as was used when the node was last operational. This
	value MUST NOT be changed during normal operation.
	Message_Identifier: 32 bits
	When combined with the message generator's IP address, the
	Message_Identifier field uniquely identifies a message. The
	values placed in this field change incrementally and only
	decrease when the Epoch changes or when the value wraps.
	4.3. MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects
	MESSAGE_ID_ACK Class = TBA (Value not yet assigned by IANA of form
	0bbbbbbb)
	MESSAGE_ID_ACK object
	Class = MESSAGE_ID_ACK 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_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	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 copied from the message being acknowledged.
	Message_Identifier: 32 bits
	The Message_Identifier field copied from the message being
	acknowledged.
	MESSAGE_ID_NACK object
	Class = MESSAGE_ID_ACK Class, C_Type = 2
	Definition is the same as the MESSAGE_ID_ACK object.
	4.4. Ack Message Format
	Ack messages carry one or more MESSAGE_ID_ACK or MESSAGE_ID_NACK
	objects. They MUST NOT contain any MESSAGE_ID objects. Ack messages
	are sent between neighboring 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 messages with RSVP_HOP
	objects, such as Path and Resv messages, the address is found in the
	RSVP_HOP object. For other messages, such as ResvConf, the
	associated IP address is the source address in the IP header. The IP
	source address is an address of the node that sends the Ack message.
	The Ack message format is as follows:
	<ACK Message> ::= <Common Header> [ <INTEGRITY> ]
	<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>
	[ [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>] ... ]
	For Ack messages, the Msg Type field of the Common Header MUST be set
	to TBA (Value is to be assigned by IANA).
	Section 4.6 provides guidance on when an Ack message should be used
	and when MESSAGE_ID objects should be sent piggy-backed in other RSVP
	messages.
	4.5. MESSAGE_ID Object Usage
	The MESSAGE_ID object may be included in any RSVP message other than
	the Ack and Bundle messages. The MESSAGE_ID object is always
	generated and processed over a single hop between RSVP neighbors.
	The IP address of the object generator, i.e., the node that creates
	the object, is represented in a per RSVP message type specific
	fashion. For messages with RSVP_HOP objects, such as Path and Resv
	messages, the generator's IP address is found in the RSVP_HOP object.
	For other messages, such as ResvConf message, the generator's IP
	address is the source address in the IP header. Note that MESSAGE_ID
	objects can only be used in a Bundle sub-messages, but not in a
	Bundle message. As is always the case with the Bundle message, each
	sub-message is processed as if it was received individually. 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
	Message_Identifier field. 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_Identifier 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
	represents. The combination of Message_Identifier and Epoch can also
	be used to detect out of order messages. When a node is sending a
	refresh message with a MESSAGE_ID object, it SHOULD use the same
	Message_Identifier 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_Identifier value MUST have a value that
	is greater than any other value previously used with the same Epoch
	field value. A value is considered to have been used when it has
	been sent in any message using the associated IP address with the
	same Epoch field value.
	The ACK_Desired flag is set when the MESSAGE_ID object generator
	wants a MESSAGE_ID_ACK object sent in response to the message. Such
	information can be used to ensure reliable delivery of RSVP messages
	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 message 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 6 for an example of an exponential back-off retransmission
	approach. Suggested default values are an initial retransmission
	timeout of 500ms, a power of 2 exponential back-off and a default
	retry limit of 3. The ACK_Desired flag will typically be set only in
	trigger messages. The ACK_Desired flag MAY be set in refresh
	messages. Issues relate to multicast sessions are covered in a later
	section.
	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 SHOULD be ignored, i.e., silently dropped. Out of
	order messages can be identified by examining the values in the Epoch
	and Message_Identifier fields. To determine ordering, the received
	Epoch value must match the value previously received from the message
	sender. If the values differ then the receiver MUST NOT treat the
	message as out of order. When the Epoch values match and the
	Message_Identifier 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 any message that installs or changes
	state. (Includes at least: Path, Resv, PathTear, ResvTear, PathErr
	and ResvErr.) If no local state information can be associated with
	the message, the receiver MUST NOT treat the message as out of order.
	If local state can be associated with the message and the received
	Message_Identifier value is less than the most recently received
	value associated with the state, the message SHOULD be treated as
	being out of order.
	Note that the 32-bit Message_Identifier value MAY wrap. To cover the
	wrap case, the following expression may be used to test if a newly
	received Message_Identifier value is less than a previously received
	value:
	if ((int) old_id - (int) new_id > 0) {
	new value is less than old value;
	}
	MESSAGE_ID objects of messages that are not out of order SHOULD be
	used to aid in determining if the message represents 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 process the message. If a Path
	or Resv message contains the same Message_Identifier 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 Message_Identifier value is
	greater than the most recently received value, the receiver MUST
	fully processes the message. When fully processing a Path or Resv
	message, the receiver MUST store the received Message_Identifier
	value as part of the local Path or Resv state for future reference.
	Nodes receiving a non-out of order message containing a MESSAGE_ID
	object with the ACK_Desired flag set, SHOULD respond with a
	MESSAGE_ID_ACK object. Note that MESSAGE_ID objects received in
	messages containing errors, i.e., are not syntactically valid, MUST
	NOT be acknowledged. PathErr and ResvErr messages SHOULD be treated
	as implicit acknowledgments.
	4.6. MESSAGE_ID_ACK Object and MESSAGE_ID_NACK 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. 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.
	The MESSAGE_ID_NACK object is used as part of the summary refresh
	extension. The generation and processing of MESSAGE_ID_NACK objects
	is described in further detail in Section 5.4.
	MESSAGE_ID_ACK and MESSAGE_ID_NACK objects MAY be sent in any RSVP
	message that has an IP destination address matching the generator of
	the associated MESSAGE_ID object. This means that the objects 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 objects SHOULD be sent in an Ack message. Implementations
	SHOULD include MESSAGE_ID_ACK and MESSAGE_ID_NACK objects in standard
	RSVP messages when possible.
	Implementations SHOULD limit the amount of time that an object is
	delayed in order to be piggy-backed or sent in an Ack message.
	Different limits may be used for MESSAGE_ID_ACK and MESSAGE_ID_NACK
	objects. MESSAGE_ID_ACK objects are used to detect link transmission
	losses. If an ACK object is delayed too long, the corresponding
	message will be retransmitted. To avoid such retransmission, ACK
	objects SHOULD be delayed a minimal amount of time. A delay time
	equal to the link transit time MAY be used. MESSAGE_ID_NACK objects
	may be delayed an independent and longer time, although additional
	delay increases the amount of time a desired reservation is not
	installed.
	4.7. Multicast Considerations
	Path and PathTear messages may be sent to IP multicast destination
	addresses. When the destination is a multicast address, it is
	possible 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,
	number of 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
	potentially large number of MESSAGE_ID_ACK objects being
	simultaneously delivered to the message generator. To address this
	case, the receiver MUST wait a random interval prior to acknowledging
	a MESSAGE_ID object received in a message destined to a multicast
	address. The random interval SHOULD be between zero (0) and a
	configured maximum time. The configured maximum SHOULD be set in
	proportion to the refresh and "rapid" retransmission interval, i.e,
	such that the maximum time before sending an acknowledgment does not
	result in retransmission. It should be noted that ACK implosion is
	being addressed by spreading acknowledgments out in time, not by ACK
	suppression.
	A more fundamental issue is the number of acknowledgments that the
	upstream node, i.e., the message generator, should expect. The
	number 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 often 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
	number 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
	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.
	4.7.1. 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, a version of the
	RSVP/routing interface updated to provide number of next hop
	information is presented. 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.
	4.8. Compatibility
	All nodes sending messages with the Refresh-Reduction-Capable bit set
	will support the MESSAGE_ID Extension. There are no backward
	compatibility issues raised by the MESSAGE_ID Class with nodes that
	do not set the Refresh-Reduction-Capable bit. The MESSAGE_ID Class
	has an assigned value whose form is 0bbbbbbb. Per RSVP [RFC2205],
	classes with values of this form must be rejected with an "Unknown
	Object Class" error by nodes not supporting the class. When the
	receiver of a MESSAGE_ID object does not support the class, a
	corresponding error message will be generated. The generator of the
	MESSAGE_ID object will see the error and then MUST re-send the
	original message without the MESSAGE_ID object. In this case, the
	message generator MAY still choose to retransmit messages at the
	"rapid" retransmission interval. Lastly, since the MESSAGE_ID_ACK
	class can only be issued in response to the MESSAGE_ID object, there
	are no possible issues with this class or Ack messages. A node MAY
	support the MESSAGE_ID Extension without supporting the other refresh
	overhead reduction extensions.
	5. Summary Refresh Extension
	The summary refresh extension enables the refreshing of RSVP state
	without the transmission of standard Path or Resv messages. The
	benefits of the described extension are that it reduces the amount of
	information that must be transmitted and processed in order to
	maintain RSVP state synchronization. Importantly, the described
	extension 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
	transmitted messages, i.e, are trigger messages.
	The summary refresh extension builds on the previously defined
	MESSAGE_ID extension. Only state that was previously advertised in
	Path and Resv messages containing MESSAGE_ID objects can be refreshed
	via the summary refresh extension.
	The summary refresh extension uses the objects and the ACK message
	previously defined as part of the MESSAGE_ID extension, and a new
	Srefresh message. The new message carries a list of
	Message_Identifier fields corresponding to the Path and Resv trigger
	messages that established the state. The Message_Identifier fields
	are carried in one of three Srefresh related objects. The three
	objects are the MESSAGE_ID LIST object, the MESSAGE_ID SRC_LIST
	object, and the MESSAGE_ID MCAST_LIST object.
	The MESSAGE_ID LIST object is used to refresh all Resv state, and
	Path state of unicast sessions. It is made up of a list of
	Message_Identifier fields that were originally advertised in
	MESSAGE_ID objects. The other two objects are used to refresh Path
	state of multicast sessions. A node receiving a summary refresh for
	multicast path state will at times need source and group information.
	These two objects provide this information. The objects differ in
	the information they contain and how they are sent. Both carry
	Message_Identifier fields and corresponding source IP addresses. The
	MESSAGE_ID SRC_LIST is sent in messages addressed to the session's
	multicast IP address. The MESSAGE_ID MCAST_LIST object adds the
	group address and is sent in messages addressed to the RSVP next hop.
	The MESSAGE_ID MCAST_LIST is normally used on point-to-point links.
	An RSVP node receiving an Srefresh message, matches each listed
	Message_Identifier 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 sent via the MESSAGE_ID_NACK object. As described
	in the previous section, the rules for sending a MESSAGE_ID_NACK
	object are the same as for sending a MESSAGE_ID_ACK object. This
	includes sending MESSAGE_ID_NACK object both piggy-backed in
	unrelated RSVP messages or in RSVP ACK messages.
	5.1. MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects
	MESSAGE_ID LIST object
	MESSAGE_ID_LIST Class = TBA (Value not yet assigned by IANA of form
	0bbbbbbb)
	Class = MESSAGE_ID_LIST 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_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| : |
	// : //
	| : |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	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
	trigger message that advertised the state being refreshed.
	Message_Identifier: 32 bits
	The Message_Identifier field from the MESSAGE_ID object
	corresponding to the trigger message that advertised the state
	being refreshed. One or more Message_Identifiers may be
	included.
	IPv4/MESSAGE_ID SRC_LIST object
	Class = MESSAGE_ID_LIST 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 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Source_ |
	| Message_Identifier_Tuple |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| : |
	// : //
	| : |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Source_ |
	| Message_Identifier_Tuple |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Where a Source_Message_Identifier_Tuple consists of:
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Source_IP_Address |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	IPv6/MESSAGE_ID SRC_LIST object
	Class = MESSAGE_ID_LIST 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 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| IPv6_Source_ |
	| Message_Identifier_Tuple |
	| |
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| : |
	// : //
	| : |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| IPv6_Source_ |
	| Message_Identifier_Tuple |
	| |
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Where a IPv6 Source_Message_Identifier_Tuple consists of:
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| IPv6 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
	trigger message that advertised the state being refreshed.
	Message_Identifier: 32 bits
	The Message_Identifier field from the MESSAGE_ID object
	corresponding to the trigger message that advertised the Path
	state being refreshed. One or more Message_Identifiers may be
	included. Each Message_Identifier MUST be followed by the
	source IP address corresponding to the sender described in the
	Path state being refreshed.
	Source_IP_Address: 32 bits
	The IP address corresponding to the sender of the Path state
	being refreshed.
	IPv4/MESSAGE_ID MCAST_LIST object
	Class = MESSAGE_ID_LIST 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 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Multicast_ |
	| Message_Identifier_ |
	| Tuple |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| : |
	// : //
	| : |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Multicast_ |
	| Message_Identifier_ |
	| Tuple |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Where a Multicast_Message_Identifier_Tuple consists of:
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Source_IP_Address |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Destination_IP_Address |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	IPv6/MESSAGE_ID MCAST_LIST object
	Class = MESSAGE_ID_LIST 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 |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| |
	| |
	| IPv6 Multicast_ |
	| Message_Identifier_ |
	| Tuple |
	| |
	| |
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| : |
	// : //
	| : |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| |
	| |
	| IPv6 Multicast_ |
	| Message_Identifier_ |
	| Tuple |
	| |
	| |
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Where a IPv6 Multicast_Message_Identifier_Tuple consists of:
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Message_Identifier |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| IPv6 Source_IP_Address |
	| |
	| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |
	| IPv6 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
	trigger message that advertised the state being refreshed.
	Message_Identifier: 32 bits
	The Message_Identifier field from the MESSAGE_ID object
	corresponding to the trigger message that advertised the Path
	state being refreshed. One or more Message_Identifiers may be
	included. Each Message_Identifier 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 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.
	5.2. 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
	messages with the other objects. A single Srefresh message MAY
	refresh both Path and Resv state.
	Srefresh messages carrying Message_Identifier fields corresponding to
	Path state are normally sent with a destination IP address equal to
	the address carried in the corresponding SESSION objects. The
	destination 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_Identifier fields corresponding to Resv
	state MUST be sent with a destination IP address set to the Resv
	state's previous hop.
	Srefresh messages sent to a multicast session's destination IP
	address, MUST contain MESSAGE_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 address associate with the MESSAGE_ID objects when they were
	included in a standard RSVP message. As previously mentioned, the
	source address associated with a MESSAGE_ID object is represented in
	a per RSVP message type specific fashion. For messages with RSVP_HOP
	objects, such as Path and Resv messages, the address is found in the
	RSVP_HOP object. For other messages, such as ResvConf message, the
	associated IP address is the source address in the IP header.
	Srefresh messages that are addressed to a session's destination IP
	address MUST be sent with the Router Alert IP option in their IP
	headers. 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. Srefresh messages MAY be sent within an RSVP
	Bundle messages. Although this is not expected since Srefresh
	messages can carry a list of Message_Identifier fields within a
	single object. Implementations may choose to limit each Srefresh
	message to the MTU size of the outgoing link, e.g. 1500 bytes.
	The Srefresh message format is:
	<Srefresh Message> ::= <Common Header> [ <INTEGRITY> ]
	[ [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>] ... ]
	[ <MESSAGE_ID> ]
	<srefresh list> | <source srefresh list>
	<srefresh list> ::= <MESSAGE_ID LIST> | <MESSAGE_ID MCAST_LIST>
	[ <srefresh list> ]
	<source srefresh list> ::= <MESSAGE_ID SRC_LIST>
	[ <source srefresh list> ]
	For Srefresh messages, the Msg Type field of the Common Header MUST
	be set to TBA (Value not yet assigned by IANA).
	5.3. Srefresh Message Usage
	An Srefresh message may be generated to refresh Resv and Path state.
	If an Srefresh message is used to refresh some particular state, then
	the generation of a standard refresh message for that particular
	state SHOULD be suppressed. A state's refresh interval is not
	affected by the use of Srefresh message based refreshes.
	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 source and
	destination IP address restrictions, as described in Sections 5.2,
	may be included in the same Srefresh message. Identifying Resv state
	that can be refreshed using the same Srefresh message is fairly
	straightforward. Identifying which Path state may be included is a
	little more complex.
	Only state that was previously 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 Srefresh 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 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. A
	default Srefresh message generation interval of 30 seconds is
	suggested for nodes that do not dynamically calculate a generation
	interval.
	When generating an Srefresh message, there are two methods for
	identifying 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.
	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.
	The use of this method MAY be administratively configured. When
	using this method, the destination address in the IP header of the
	Srefresh message is usually the next hop's address. When the use of
	this method is administratively configured, the destination address
	should be the well known group address 224.0.0.14. When the outgoing
	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
	Srefresh 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
	message MESSAGE_ID information matching each identified state's
	previously used object. For all Resv state and for Path state of
	unicast sessions, the information is added to the message in a
	MESSAGE_ID LIST object that has a matching Epoch value. (Note only
	one Epoch value will be in use during normal operation.) 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 destination address of the Srefresh message is the RSVP next
	hop and the outgoing interface is a point-to-point link. In this
	case the information is added to the message in a 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 a MESSAGE_ID
	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
	transmits the message out the proper interface.
	Upon receiving an Srefresh message, the node MUST attempt to identify
	matching installed Path or Resv state. Matching is done based on the
	source address in the IP header of the Srefresh message, the object
	type and each Message_Identifier field. If matching state can be
	found, then the receiving node MUST update the matching state
	information as if a standard refresh message had been received. If
	matching state cannot be identified, then an Srefresh NACK MUST be
	generated corresponding to the unmatched Message_Identifier field.
	Message_Identifier fields received in MESSAGE_ID LIST objects may
	correspond to any Resv state or to Path state of unicast sessions.
	Message_Identifier fields received in MESSAGE_ID SRC_LIST or
	MCAST_LIST objects correspond to Path state of multicast sessions.
	An additional check must be performed to determine if a NACK should
	be generated for unmatched Message_Identifier fields associated with
	Path state of multicast sessions, i.e. fields that were carried in
	MESSAGE_ID SRC_LIST or MCAST_LIST objects. The receiving node must
	check to see if the node would forward data packets originated from
	the source corresponding to the unmatched field. This check,
	commonly known as an RPF check, is performed based on the source and
	group information carried in the MESSAGE_ID SRC_LIST and MCAST_LIST
	objects. In both objects the IP address of the source is listed
	immediately after the corresponding Message_Identifier field. The
	group address is listed immediately after the source IP address in
	MESSAGE_ID MCAST_LIST objects. The group address is the message's
	destination IP address when MESSAGE_ID SRC_LIST objects are used.
	The receiving node only generates an Srefresh NACK when the node
	would forward packets to the identified group from the listed sender.
	If the node would forward multicast data packets from a listed sender
	and there is a corresponding unmatched Message_Identifier field, then
	an appropriate Srefresh NACK MUST be generated. If the node would
	not forward packets to the identified group from a listed sender, a
	corresponding unmatched Message_Identifier field is silently ignored.
	5.4. Srefresh NACK
	Srefresh NACKs are used to indicate that a received
	Message_Identifier field carried in MESSAGE_ID LIST, SRC_LIST, or
	MCAST_LIST object does not match any installed state. This may occur
	for a number of reasons including, for example, a route change. An
	Srefresh NACK is encoded in a MESSAGE_ID_NACK object. When
	generating an Srefresh NACK, the epoch and Message_Identifier fields
	of the MESSAGE_ID_NACK object MUST have the same value as was
	received. MESSAGE_ID_NACK objects are transmitted as described in
	Section 4.6.
	Received MESSAGE_ID_NACK objects indicate that the object generator
	does not have any installed state matching the object. Upon
	receiving a MESSAGE_ID_NACK object, the receiver performs an
	installed Path or Resv state lookup based on the Epoch and
	Message_Identifier 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 identify any
	installed state, then no action is required.
	5.5. Preserving RSVP Soft State
	As discussed in [RFC2205], RSVP uses soft state to address a large
	class of potential errors. RSVP does this by periodically sending a
	full representation of installed state in Resv and Path messages.
	Srefresh messages are used in place of the periodic sending of
	standard Path and Resv refresh messages. While this provides scaling
	benefits and protects against common network events such as packet
	loss or routing change, it does not provide exactly the same error
	recovery properties. An example error that could potentially be
	recovered from via standard messages but not with Srefresh messages
	is internal corruption of state. This section recommends two methods
	that can be used to better preserve RSVP's soft state error recovery
	mechanism. Both mechanisms are supported using existing protocol
	messages.
	The first mechanism uses a checksum or other algorithm to detect a
	previously unnoticed change in internal state. This mechanism does
	not protect against internal state corruption. It just covers the
	case where a trigger message should have been sent, but was not.
	When sending a Path or Resv trigger message, a node should run a
	checksum or other algorithm, such as [MD5], over the internal state
	and store the result. The choice of algorithm is an administrative
	decision. Periodically the node should rerun the algorithm and
	compare the new result with the stored result. If the values differ,
	then a corresponding standard Path or Resv refresh message should be
	sent and the new value should be stored. The recomputation period
	should be set based on the computation resources of the node and the
	reliability requirements of the network.
	The second mechanism is simply to periodically send standard Path and
	Resv refresh messages. Since this mechanism uses standard refresh
	messages, it can recover from the same set of errors as standard
	RSVP. When using this mechanism, the period that standard refresh
	messages are sent must be longer than the interval that Srefresh
	messages are generated in order to gain the benefits of using the
	summary refresh extension. When a standard refresh message is sent,
	a corresponding summary refresh SHOULD NOT be sent during the same
	refresh period. When a node supports the periodic generation of
	standard refresh messages while Srefreshes are being used, the
	frequency of generation of standard refresh messages relative to the
	generation of summary refreshes SHOULD be configurable by the network
	administrator.
	5.6. Compatibility
	Nodes supporting the summary refresh extension advertise their
	support via the Refresh-Reduction-Capable bit in the RSVP message
	header. This enables nodes supporting the extension 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
	messages in all cases when all RSVP nodes in a network are known to
	support the summary refresh extension. This is useful since when
	operating in this mode, the extension properly adjusts to the case of
	non-RSVP next hops and changes in routing.
	Per section 2, nodes supporting the summary refresh extension must
	also take care to recognize when a next hop stops sending RSVP
	messages with the Refresh-Reduction-Capable bit set.
	6. Reference Exponential Back-Off Procedures
	This section is based on [Pan] and provides an example of how to
	implement exponential back-off for retransmission of messages
	awaiting acknowledgment, see Section 4.5. Implementations MAY choose
	to use the described procedures.
	6.1. Outline of Operation
	The following is one possible mechanism for exponential back-off
	retransmission of an unacknowledged RSVP message: When sending such a
	message, a node inserts a MESSAGE_ID object with the ACK_Desired flag
	set. The sending node will retransmit the message until a message
	acknowledgment is received or the message has been transmitted a
	maximum number of times. Upon reception, a receiving node
	acknowledges the arrival of the message by sending back a message
	acknowledgment (that is, a corresponding MESSAGE_ID_ACK object.)
	When the sending node receives the acknowledgment retransmission of
	the message is stopped. The interval between retransmissions is
	governed by a rapid retransmission timer. The rapid retransmission
	timer starts at a small interval and increases exponentially until it
	reaches a threshold.
	6.2. 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.
	Rapid retry limit Rl:
	Rl is the maximum number of times a message will be transmitted
	without being acknowledged.
	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).
	6.3. 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
	rapid retry limit, Rl, has been reached.
	A sending node can use the following algorithm when transmitting a
	message containing a MESSAGE_ID object with the ACK_Desired flag set:
	Prior to initial transmission initialize: Rk = Rf and Rn = 0
	while (Rn++ < Rl) {
	transmit the message;
	wake up after Rk seconds;
	Rk = Rk * (1 + Delta);
	}
	/* acknowledged or no reply from receiver for too long: */
	do any needed clean up;
	exit;
	Asynchronously, when a sending node receives a corresponding
	MESSAGE_ID_ACK object, it will change the retry count, Rn, to Rl.
	Note that the transmitting node does not advertise the use of the
	described exponential back-off procedures via the TIME_VALUE object.
	7. 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 Bob Braden, Lixia Zhang, Fred Baker, Adrian Farrel, Roch
	Guerin, Kireeti Kompella, David Mankins, Henning Schulzrinne, Andreas
	Terzis, Lan Wang and Masanobu Yuhara for specific feedback on the
	various versions of the document.
	Portions of this work are based on work done by Masanobu Yuhara and
	Mayumi Tomikawa [Yuhara].
	8. Security Considerations
	No new security issues are raised in this document. See [RFC2205]
	for a general discussion on RSVP security issues.
	9. References		RFC 2961
	[Pan] Pan, P., Schulzrinne, H., "Staged Refresh Timers for RSVP,"		Title: RSVP Refresh Overhead Reduction Extensions
	Global Internet'97, Phoenix, AZ, November 1997.		Author(s): L. Berger, D. Gan, G. Swallow, P. Pan, F. Tommasi,
	http://www.ctr.columbia.edu/~pan/papers/timergi.ps		S. Molendini
			Status: Standards Track
			Date: April 2001
			Mailbox: lberger@labn.net, dhg@juniper.net,
			swallow@cisco.com, pingpan@juniper.net,
			franco.tommasi@unile.it, molendini@ultra5.unile.it
			Pages: 34
			Characters: 81675
			Updates/Obsoletes/SeeAlso: None
	[MD5] Rivest, R., "The MD5 Message-Digest Algorithm," RFC 1321,		I-D Tag: draft-ietf-rsvp-refresh-reduct-05.txt
	April 1992.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		URL: ftp://ftp.rfc-editor.org/in-notes/rfc2961.txt
	Requirement Levels," RFC 2119.
	[RFC2205] Braden, R. Ed. et al, "Resource ReserVation Protocol		This document describes a number of mechanisms that can be used to
	-- Version 1 Functional Specification", RFC 2205,		reduce processing overhead requirements of refresh messages, eliminate
	September 1997.		the state synchronization latency incurred when an RSVP (Resource
			ReserVation Protocol) message is lost and, when desired, refreshing
			state without the transmission of whole refresh messages. The same
			extensions also support reliable RSVP message delivery on a per hop
			basis. These extension present no backwards compatibility issues.
	[Yuhara] Yuhara, M., Tomikawa, M. "RSVP Extensions for ID-based		This document is a prodcut of the Resource Reservation Setup Protocol
	Refreshes," Internet Draft,		Working Group of the IETF.
	draft-yuhara-rsvp-refresh-00.txt, April 1999.
	10. Authors' Addresses		This is now a Proposed Standard Protocol.
	Lou Berger		This document specifies an Internet standards track protocol for
	LabN Consulting, LLC		the Internet community, and requests discussion and suggestions
	Voice: +1 301 468 9228		for improvements. Please refer to the current edition of the
	Email: lberger@labn.net		"Internet Official Protocol Standards" (STD 1) for the
			standardization state and status of this protocol. Distribution
			of this memo is unlimited.
	Der-Hwa Gan		This announcement is sent to the IETF list and the RFC-DIST list.
	Juniper Networks, Inc.		Requests to be added to or deleted from the IETF distribution list
	385 Ravendale Drive		should be sent to IETF-REQUEST@IETF.ORG. Requests to be
	Mountain View, CA 94043		added to or deleted from the RFC-DIST distribution list should
	Voice: +1 650 526 8074		be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
	Email: dhg@juniper.net
	George Swallow		Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
	Cisco Systems, Inc.		an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
	250 Apollo Drive		help: ways_to_get_rfcs. For example:
	Chelmsford, MA 01824
	Voice: +1 978 244 8143
	Email: swallow@cisco.com
	Ping Pan		To: rfc-info@RFC-EDITOR.ORG
	Bell Labs, Lucent		Subject: getting rfcs
	101 Crawfords Corner Road, Room 4C-508
	Holmdel, NJ 07733
	Phone: +1 732 332 6744
	Email: pingpan@dnrc.bell-labs.com
	Franco Tommasi		help: ways_to_get_rfcs
	University of Lecce, Fac. Ingegneria
	Via Monteroni 73100 Lecce, ITALY
	Email: tommasi@ilenic.unile.it
	Simone Molendini		Requests for special distribution should be addressed to either the
	University of Lecce, Fac. Ingegneria		author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
	Via Monteroni 73100 Lecce, ITALY		specifically noted otherwise on the RFC itself, all RFCs are for
	Email: molendini@ultra5.unile.it		unlimited distribution.echo
			Submissions for Requests for Comments should be sent to
			RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
			Authors, for further information.
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