Network Working Group Lou Berger Internet Draft LabN Consulting, LLC Expiration Date: April 2000 Der-Hwa Gan Juniper Networks, Inc. George Swallow Cisco Systems, Inc. Ping Pan Bell Labs, Lucent October 1999 RSVP Refresh Reduction Extensions draft-ietf-rsvp-refresh-reduct-01.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 time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. The same extensions also support reliable RSVP message delivery. These extension present no backwards compatibility issues. Berger, et al. [Page 1] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 Contents 1 Introduction and Background ............................... 3 1.1 Trigger and Refresh Messages .............................. 4 2 RSVP Bundle Message ....................................... 5 2.1 Bundle Header ............................................. 5 2.2 Message Formats ........................................... 6 2.3 Sending RSVP Bundle Messages .............................. 7 2.4 Receiving RSVP Bundle Messages ............................ 8 2.5 Forwarding RSVP Bundle Messages ........................... 8 2.6 Bundle-Capable Bit ........................................ 8 3 MESSAGE_ID Extension ...................................... 9 3.1 MESSAGE_ID Objects ....................................... 10 3.2 MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects ................ 11 3.3 Ack Message Format ........................................ 12 3.4 MESSAGE_ID Object Usage ................................... 12 3.5 MESSAGE_ID_ACK Object and MESSAGE_ID_NACK Object Usage .... 14 3.6 Multicast Considerations .................................. 15 3.6.1 Reference RSVP/Routing Interface .......................... 15 3.7 Compatibility ............................................. 16 4 Summary Refresh Extension ................................. 16 4.1 MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects .......... 18 4.2 Srefresh Message Format ................................... 21 4.3 Srefresh Message Usage .................................... 22 4.4 Srefresh NACK ............................................. 25 4.5 Compatibility ............................................. 25 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 Berger, et al. [Page 2] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 Changes from previous version o Integrated feedback from working group chairs. o No substantive changes were made to how extensions operate. o Renamed Message_ID field to Message_Identifier. o Broke MESSAGE_ID class into MESSAGE_ID, MESSAGE_ID_ACK and MESSAGE_ID_LIST classes. o Removed some text left over from previous versions and removed reference to UDP encapsulation. o Many other editorial and clarification related changes. 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 scaling 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 messages. The described mechanisms also address issues of latency and reliability 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]. 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 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. Berger, et al. [Page 3] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 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 volume and the reliability issues with mechanisms other than adjusting refresh rate. 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. 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 modification 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 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 the exception of the INTEGRITY object, the flags in the MESSAGE_ID object and the RSVP Checksum. The checksum, the flags and the INTEGRITY object are allowed to differ in refresh messages. Berger, et al. [Page 4] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 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. 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 follows: 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 meaningful only between adjacent RSVP neighbors. 0x02-0x08: Reserved Msg type: 8 bits 12 = Bundle Berger, et al. [Page 5] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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. 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 IP TTL that a Bundle message sent to the TTL in the 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. 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. 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.. // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Berger, et al. [Page 6] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 2.3. 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 MAY be sent to RSVP neighbors that support the message. 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. RSVP Bundle messages MUST not be used if the RSVP neighbor does not support RSVP Bundle messages. If the RSVP neighbor is not known or changes in next hops cannot be identified via routing, Bundle messages MUST 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. 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. 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. Implementations may choose to limit each RSVP Bundle message to the MTU size of the outgoing link, e.g. 1500 bytes. 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 know 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 is not Bundle capable. Berger, et al. [Page 7] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 further 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. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Berger, et al. [Page 8] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 Flags: 4 bits 0x01: Bundle capable When set, indicates to that this node is willing and capable of receiving Bundle messages. This bit is meaningful only between RSVP neighbors. 3. 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 4. The MESSAGE_ID object can also be used to simply provide a shorthand indication of when a message represents 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 and MESSAGE_ID_NACK objects may be sent piggy-backed in unrelated RSVP messages or in RSVP Ack messages. All three object types may be included in a bundle sub-message. When included, the object is treated as if it were contained in a standard, non-bundled, RSVP message. When present, 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. Berger, et al. [Page 9] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 The ordering of the ACK objects are: [ ] [ | ... ] [ ] 3.1. MESSAGE_ID 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_Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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 4.5 for description of handling by receiver. 0x40 = 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. 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. This Berger, et al. [Page 10] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 field is ordered and only decreases in value when the Epoch changes or the value wraps. 3.2. MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects MESSAGE_ID_ACK Class = 167 (Value to be assigned by IANA of form 10bbbbbb) 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 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 4.5 for description of handling by receiver. 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 same as MESSAGE_ID_ACK object. Berger, et al. [Page 11] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 3.3. 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 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 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 and Bundle messages, 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: ::= [ ] | [ ... ] [ ... ] For Ack messages, the Msg Type field of the Common Header MUST be set to 13 (This is a suggested value, the permanent value is to be assigned by IANA). 3.4. 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 processed hop-by-hop. 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 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 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. 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. Berger, et al. [Page 12] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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. 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 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. 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 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 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 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 identify 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 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. Berger, et al. [Page 13] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 processes 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. 3.5. 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 received MESSAGE_ID_NACK objects is described in further detail in Section 4.4. 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. Berger, et al. [Page 14] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 3.6. 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 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 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 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 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 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. 3.6.1. 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 Berger, et al. [Page 15] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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. 3.7. 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 messages per standard RSVP. 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. 4. 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 Berger, et al. [Page 16] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 may only 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. 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. Berger, et al. [Page 17] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 4.1. MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects MESSAGE_ID LIST object MESSAGE_ID_ACK Class = TBD (Value to be 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. A variable number of Message_Identifiers may be included. Berger, et al. [Page 18] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. A variable number of 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. Berger, et al. [Page 19] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 Source_IP_Address: 32 bits The IP address corresponding to the sender of the Path state being refreshed. MESSAGE_ID MCAST_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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Multicast_ | | Message_Identifier_ | | Tuple | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : | // : // | : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Multicast_ | | Message_Identifier_ | | Tuple | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Where a Multicast_Message_Identifier_Tuple consists of: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message_Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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 trigger message that advertised the state being refreshed. Berger, et al. [Page 20] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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. A variable number of 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. 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 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 destination, 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 Berger, et al. [Page 21] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 source address associate 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 and Bundle messages, 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 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 aggregate 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: ::= [ ] | ::= | [ ] ::= [ ... ] For Srefresh messages, the Msg Type field of the Common Header MUST be set to 15 (This is a suggested value, the permanent value is to be assigned by IANA). 4.3. 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 SHOULD be suppressed. A state's refresh interval is not affected by the use of Srefresh message 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. Berger, et al. [Page 22] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 4.2, may be included in the same Srefresh message. Identifying Resv state that can refreshed using the same Srefresh message is fairly straightforward. Identifying which Path state may be included is a little more complex. Independent of the state being refreshed, 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. 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. When using this method, the destination address in the IP header of the Srefresh message is always the next hop's address. 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 Berger, et al. [Page 23] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 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 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 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 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 Berger, et al. [Page 24] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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. 4.4. Srefresh NACK Srefresh NACKs are used to indicated 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 3.5. 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. 4.5. Compatibility Nodes supporting the summary refresh extension advertise their support 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 Berger, et al. [Page 25] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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. 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 Summary_Capable flag received in a MESSAGE_ID or MESSAGE_ID_ACK object. Summary_Capable flags received in MESSAGE_ID_NACK objects SHOULD be ignored. 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 The following is one possible 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 ACK_Desired flag set. Upon reception, a receiving node acknowledges the arrival of the message by sending back an message acknowledgment (that is, a corresponding MESSAGE_ID_ACK object.) When the sending node receives this message acknowledgment for a Path or Resv message, it will automatically 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. Until the message acknowledgment is received, the sending node will retransmit the message. The interval between retransmissions is governed 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 specification even when the receiving node does not support message acknowledgment. Berger, et al. [Page 26] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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. 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). Berger, et al. [Page 27] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 retransmission 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 MESSAGE_ID_ACK object, it will change the retransmission interval Rk to 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. Berger, et al. [Page 28] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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 Fred Baker, Bob Braden, Roch Guerin, David Mankins, Henning Schulzrinne, Andreas Terzis and Masanobu Yuhara for specific feedback 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, November 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. Berger, et al. [Page 29] Internet Draft draft-ietf-rsvp-refresh-reduct-01.txt October 1999 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]