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Network Working Group Lou Berger
Internet Draft LabN Consulting
Expiration Date: December 1999
Der-Hwa Gan
Juniper Networks, Inc.
George Swallow
Cisco Systems, Inc.
Ping Pan
Bell Labs, Lucent
June 1999
RSVP Refresh Reduction Extensions
draft-berger-rsvp-refresh-reduct-03.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
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Abstract
This document describes a number of mechanisms that reduce the
refresh overhead of RSVP. The extensions can be used to reduce
processing requirements of refresh messages, eliminate the state
synchronization latency incurred when an RSVP message is lost and,
when desired, suppress the generation of refresh messages. An
extension to support detection of when an RSVP neighbor resets its
state is also defined. These extension present no backwards
compatibility issues.
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Contents
1 Introduction and Background ............................... 4
1.1 Trigger and Refresh Messages .............................. 5
2 RSVP Bundle Message ....................................... 5
2.1 Bundle Header ............................................. 6
2.2 Message Formats ........................................... 7
2.3 Sending RSVP Bundle Messages .............................. 7
2.4 Receiving RSVP Bundle Messages ............................ 8
2.5 Forwarding RSVP Bundle Messages ........................... 9
2.6 Bundle-Capable Bit ........................................ 9
3 MESSAGE_ID Extension ...................................... 10
3.1 MESSAGE_ID Object ......................................... 11
3.2 Ack Message Format ........................................ 13
3.3 MESSAGE_ID Object Usage ................................... 13
3.4 MESSAGE_ID ACK Object Usage ............................... 16
3.5 Multicast Considerations .................................. 16
3.5.1 Reference RSVP/Routing Interface .......................... 18
3.6 Compatibility ............................................. 18
4 Summary Refresh Extension ................................. 19
4.1 Srefresh Message Format ................................... 20
4.2 Srefresh Message Usage .................................... 21
4.3 Srefresh NACK ............................................. 22
4.4 Compatibility ............................................. 23
5 Hello Extension ........................................... 23
5.1 Hello Message Format ...................................... 24
5.2 HELLO Object .............................................. 25
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5.3 Hello Message Usage ....................................... 26
5.4 Multi-Link Considerations ................................. 27
5.5 Compatibility ............................................. 27
6 Reference Exponential Back-Off Procedures ................. 28
6.1 Outline of Operation ...................................... 28
6.2 Time Parameters ........................................... 28
6.3 Example Retransmission Algorithm .......................... 29
7 Acknowledgments ........................................... 30
8 Security Considerations ................................... 30
9 References ................................................ 31
10 Authors' Addresses ........................................ 32
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1. Introduction and Background
The resource requirements (in terms of CPU processing and memory) for
running RSVP on a router increases proportionally with the number of
sessions. Supporting a large number of sessions can present scaling
problems.
This document describes mechanisms to help alleviate one set of scal-
ing issues. RSVP Path and Resv messages must be periodically
refreshed to maintain state. The approach described effectively
reduces the volume of messages which must be periodically sent and
received, as well as the resources required to process refresh mes-
sages.
The described mechanisms also address issues of latency and reliabil-
ity of RSVP Signaling. The latency and reliability problem occurs
when a non-refresh RSVP message is lost in transmission. Standard
RSVP [RFC2205] maintains state via the generation of RSVP refresh
messages. In the face of transmission loss of RSVP messages, the
end-to-end latency of RSVP signaling is tied to the refresh interval
of the node(s) experiencing the loss. When end-to-end signaling is
limited by the refresh interval, the establishment or change of a
reservation may be beyond the range of what is acceptable for some
applications.
One way to address the refresh volume problem is to increase the
refresh period, "R" as defined in section 3.7 of [RFC2205]. Increas-
ing the value of R provides linear improvement on transmission over-
head, but at the cost of increasing the time it takes to synchronize
state.
One way to address the latency and reliability of RSVP Signaling is
to decrease the refresh period R. Decreasing the value of R provides
increased probability that state will be installed in the face of
message loss, but at the cost of increasing refresh message rate and
associated processing requirements.
An additional issue is the time to deallocate resources after a tear
message is lost. RSVP does not retransmit ResvTear or PathTear mes-
sages. If the sole tear message transmitted is lost, then resources
will only be deallocated once the "cleanup timer" interval has
passed. This may result in resources being allocated for an unneces-
sary period of time. Note that adjusting the refresh period has no
impact on this issues since tear messages are not retransmitted.
The extensions defined in this document address both the refresh vol-
ume and the reliability issues with mechanisms other than adjusting
refresh rate. A Bundle message is defined to reduce overall message
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handling load. A Message_ID object is defined to reduce refresh mes-
sage processing by allowing the receiver to more readily identify an
unchanged message. A Message_ACK object is defined which can be used
to detect message loss and, when used in combination with the Mes-
sage_ID object, can be used to suppress refresh messages altogether.
A summary refresh message is defined to enable refreshing state
without the transmission of whole refresh messages, while maintaining
RSVP's ability to indicate when state is lost or when next hops
change. Finally, a hello protocol is defined to allow detection of
the loss of a neighbor node or a reset of it's RSVP state informa-
tion.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.1. Trigger and Refresh Messages
This document categorizes RSVP messages into two types: trigger and
refresh messages. Trigger messages are those RSVP messages that
advertise state or any other information not previously transmitted.
Trigger messages include messages advertising new state, a route
change that altered the reservation paths, or a reservation modifica-
tion by a downstream router. Trigger messages also include those
messages that include changes in non-RSVP processed objects, such as
changes in the Policy or ADSPEC objects.
Refresh messages represent previously advertised state and contain
exactly the same objects and same information as a previously trans-
mitted message. Only Path and Resv messages can be refresh messages.
Refresh messages are typically bit for bit identical to the corre-
sponding previously transmitted message, with the exception of the
flags in the MESSAGE_ID object. These flags allowed to differ in
refresh messages.
2. RSVP Bundle Message
An RSVP Bundle message consists of a bundle header followed by a body
consisting of a variable number of standard RSVP messages. A Bundle
message is used to aggregated multiple RSVP messages within a single
PDU. The term "bundling" is used to avoid confusion with RSVP reser-
vation aggregation. The following subsections define the formats of
the bundle header and the rules for including standard RSVP messages
as part of the message.
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2.1. Bundle Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Flags | Msg type | RSVP checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send_TTL | (Reserved) | RSVP length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the bundle header is identical to the format of the
RSVP common header [RFC2205]. The fields in the header are as fol-
lows:
Vers: 4 bits
Protocol version number. This is version 1.
Flags: 4 bits
0x01: Bundle capable
If set, indicates to RSVP neighbors that this node is willing
and capable of receiving bundle messages. This bit is mean-
ingful only between adjacent RSVP neighbors.
0x02-0x08: Reserved
Msg type: 8 bits
12 = Bundle
RSVP checksum: 16 bits
The one's complement of the one's complement sum of the entire
message, with the checksum field replaced by zero for the pur-
pose of computing the checksum. An all-zero value means that
no checksum was transmitted. Because individual sub-messages
carry their own checksum as well as the INTEGRITY object for
authentication, this field MAY be set to zero.
Send_TTL: 8 bits
The IP TTL value with which the message was sent. This is used
by RSVP to detect a non-RSVP hop by comparing the IP TTL that a
Bundle message sent to the TTL in the received message.
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RSVP length: 16 bits
The total length of this RSVP Bundle message in bytes, includ-
ing the bundle header and the sub-messages that follow.
2.2. Message Formats
An RSVP Bundle message must contain at least one sub-message. A sub-
message MAY be any message type except for another Bundle message.
Current valid sub-messages are RSVP Path, PathTear, PathErr, Resv,
ResvTear, ResvErr, ResvConf, Ack or Hello messages.
Empty RSVP Bundle messages SHOULD NOT be sent. A Bundle message MUST
NOT include another RSVP Bundle message as a sub-message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Flags | 12 | RSVP checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send_TTL | (Reserved) | RSVP length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// First sub-message //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// More sub-messages.. //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.3. Sending RSVP Bundle Messages
RSVP Bundle messages are sent hop by hop between RSVP-capable neigh-
bors as "raw" IP datagrams with protocol number 46. Raw IP datagrams
are also intended to be used between an end system and the first/last
hop router, although it is also possible to encapsulate RSVP messages
as UDP datagrams for end-system communication that cannot perform raw
network I/O.
RSVP Bundle messages MUST not be used if the next hop RSVP neighbor
does not support RSVP Bundle messages. Methods for discovering such
information include: (1) manual configuration and (2) observing the
Bundle-capable bit (see the description that follows) in the received
RSVP messages. If the next hop RSVP neighbor is not known or changes
in next hops cannot be identified via routing, Bundle messages MUST
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NOT be sent. Note that when the routing next hop is not RSVP capable
it will typically not be possible to identify changes in next hop.
Support for RSVP Bundle messages is optional. While message bundling
helps in scaling RSVP, and in reducing processing overhead and band-
width consumption, a node is not required to transmit every standard
RSVP message in a Bundle message. A node MUST always be ready to
receive standard RSVP messages.
The IP source address is local to the system that originated the Bun-
dle message. The IP destination address is the next hop node for
which the sub-messages are intended. These addresses need not be
identical to those used if the sub-messages were sent as standard
RSVP messages.
For example, the IP source address of Path and PathTear messages is
the address of the sender it describes, while the IP destination
address is the DestAddress for the session. These end-to-end
addresses are overridden by hop-by-hop addresses while encapsulated
in a Bundle message. These addresses can easily be restored from the
SENDER_TEMPLATE and SESSION objects within Path and PathTear mes-
sages. For Path and PathTear messages, the next hop node can be
identified either via a received ACK or from a received corresponding
Resv message. Path and PathTear messages for multicast sessions MUST
NOT be sent in Bundle messages except when the outgoing interface is
a point-to-point interface and it is known that the next hop is RSVP
capable.
RSVP Bundle messages SHOULD NOT be sent with the Router Alert IP
option in their IP headers. This is because Bundle messages are
addressed directly to RSVP neighbors.
Each RSVP Bundle message MUST occupy exactly one IP datagram. If it
exceeds the MTU, the datagram is fragmented by IP and reassembled at
the recipient node. A single RSVP Bundle message MUST NOT exceed the
maximum IP datagram size, which is approximately 64K bytes.
2.4. Receiving RSVP Bundle Messages
If the local system does not recognize or does not wish to accept an
Bundle message, the received messages shall be discarded without fur-
ther analysis.
The receiver next compares the IP TTL with which a Bundle message is
sent to the TTL with which it is received. If a non-RSVP hop is
detected, the number of non-RSVP hops is recorded. It is used later
in processing of sub-messages.
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Next, the receiver verifies the version number and checksum of the
RSVP Bundle message and discards the message if any mismatch is
found.
The receiver then starts decapsulating individual sub-messages. Each
sub-message has its own complete message length and authentication
information. Each sub-message is processed per standard RSVP.
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.
When individual sub-messages are being forwarded, they MAY be encap-
sulated in another Bundle message before sending to the next hop
neighbor. The Send_TTL field in the sub-messages should be decre-
mented properly before transmission.
2.6. Bundle-Capable Bit
To support message bundling, an additional capability bit is added to
the common RSVP header, which is defined in [RFC2205].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Flags | Msg Type | RSVP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send_TTL | (Reserved) | RSVP Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 4 bits
0x01: Bundle capable
If set, indicates to RSVP neighbors that this node is willing
and capable of receiving Bundle messages. This bit is mean-
ingful only between adjacent RSVP neighbors.
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3. MESSAGE_ID Extension
Two new objects are defined as part of the MESSAGE_ID extension. The
two object types are the MESSAGE_ID object and the MESSAGE_ID ACK
object. The objects are used to support acknowledgments and, when
used in conjunction with the Hello Extension described in Section 5,
to indicate when refresh messages are not needed after an acknowledg-
ment. When refreshes are normally generated, 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 objects may be sent piggy-
backed in unrelated RSVP messages or in RSVP Ack messages.
Either type of MESSAGE_ID object may be included in a bundle sub-mes-
sage. When included, the object is treated as if it were contained
in a standard, unbundled, RSVP message. Only one MESSAGE_ID object
MAY be included in a (sub)message and it MUST follow any present MES-
SAGE_ID ACK objects. When no MESSAGE_ID ACK objects are present, the
MESSAGE_ID object MUST immediately follow the INTEGRITY object. When
no INTEGRITY object is present, the MESSAGE_ID object MUST immedi-
ately follow the (sub)message header.
When present, one or more MESSAGE_ID ACK objects MUST immediately
follow the INTEGRITY object. When no INTEGRITY object is present,
the MESSAGE_ID ACK objects MUST immediately follow the the (sub)mes-
sage header. An MESSAGE_ID ACK object may only be included in a mes-
sage when the message's IP destination address matches the unicast
address of the node that generated the message(s) being acknowledged.
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3.1. MESSAGE_ID Object
MESSAGE_ID Class = 166 (Value to be assigned by IANA of form
10bbbbbb)
MESSAGE_ID object
Class = MESSAGE_ID Class, C_Type = 1
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 8 bits
0x80 = Summary_Capable flag
Indicates that the sender supports the summary refresh
extension. This flag MUST be set if the node supports the
summary refresh extension. See Section 4.4 for description
of handling by receiver.
0x40 = ACK_Desired flag
Indicates that the sender is willing to accept a message
acknowledgment. Acknowledgments MUST be silently ignored
when they are sent in response to messages whose
ACK_Desired flag is not set. This flag MUST be set when
the Last_Refresh flag is set.
0x20 = Last_Refresh flag
Used in Resv and Path refresh messages to indicate that the
sender will not be sending further refreshes. When set,
the ACK_Desired flag MUST also be set. This flag MUST NOT
be set when the HELLO messages are not being exchanged with
the neighboring RSVP node.
Epoch: 24 bits
A value that indicates when the Message_ID sequence has reset.
SHOULD be randomly generated each time a node reboots. This value
MUST NOT be changed during normal operation.
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Message_ID: 32 bits
When combined with the message generator's IP address, the Message_ID
field uniquely identifies a message. This field is ordered and only
decreases in value when the Epoch changes or the value wraps.
MESSAGE_ID ACK object
Class = MESSAGE_ID Class, C_Type = 2
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags: 8 bits
0x80 = Summary_Capable flag
Indicates that the sender supports the summary refresh
extension. This flag MUST be set if the node supports the
summary refresh extension. See Section 4.4 for description
of handling by receiver.
0x40 = Refresh_NACK flag
Indicates that no state was found corresponding to the
indicated message identifier. This flag SHALL ONLY be set
when the matching Epoch and Message_ID field values were
received in a Summary Refresh message, and MUST NOT be set
in response to a MESSAGE_ID object received in any other
message. See Section 4 for details.
Epoch: 24 bits
The Epoch field copied from the message being acknowledged.
Message_ID: 32 bits
The Message_ID field copied from the message being acknowl-
edged.
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3.2. Ack Message Format
Ack messages carry one or more MESSAGE_ID ACK objects. They MUST NOT
contain any MESSAGE_ID objects. Ack messages are sent hop-by-hop
between RSVP nodes. The IP destination address of an Ack message is
the unicast address of the node that generated the message(s) being
acknowledged. For Path, PathTear, Resv, and RervErr messages this is
taken from the RSVP_HOP Object. For PathErr and ResvErr messages
this is taken from the message's source address. The IP source
address is an address of the node that sends the Ack message.
The Ack message format is as follows:
<ACK Message> ::= <Common Header> [ <INTEGRITY> ]
<MESSAGE_ID ACK>
[ <MESSAGE_ID ACK> ... ]
For Ack messages, the Msg Type field of the Common Header MUST be set
to 13 (Value to be assigned by IANA).
3.3. MESSAGE_ID Object Usage
The MESSAGE_ID object may be included in any RSVP message other than
the Ack message. The MESSAGE_ID object is always generated and pro-
cessed hop-by-hop. The IP address of the object generator is repre-
sented in a per RSVP message type specific fashion. For Path and
PathTear messages the generator's IP address is contained in the
RSVP_HOP. For Resv, ResvTear, PathErr, ResvErr, ResvConf and Bundle
messages the generator's IP address is the source address in the IP
header.
The Epoch field contains a generator selected value. The value is
used to indicate when the sender resets the values used in the Mes-
sage_ID field. This information is used by the receiver to detect
out of order messages. On startup, a node SHOULD randomly select a
value to be used in the Epoch field. The node SHOULD ensure that the
selected value is not the same as was used when the node was last
operational. The value MUST NOT be changed unless the node or the
RSVP agent is restarted.
The Message_ID field contains a generator selected value. This
value, when combined with the generator's IP address, identifies a
particular RSVP message and the specific state information it repre-
sents. When a node is sending a refresh message with a MESSAGE_ID
object, it SHOULD use the same Message_ID value that was used in the
RSVP message that first advertised the state being refreshed. When a
node is sending a trigger message, the Message_ID value MUST have a
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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 is
capable of accepting MESSAGE_ID ACK objects. Such information can be
used to ensure reliable delivery of error and confirm messages and to
support fast refreshes in the face of network loss. Nodes setting
the ACK_Desired flag SHOULD retransmit unacknowledged messages at a
more rapid interval than the standard refresh period until the mes-
sage is acknowledged or until a "rapid" retry limit is reached.
Rapid retransmission rate SHOULD be based on well known exponential
back-off procedures. See Section 6 for details on one exponential
back-off retransmission approach. Note that nodes setting the
ACK_Desired flag for unicast sessions, do not need to track the iden-
tify of the next hop since all that is expected is an ACK, not an ACK
from a specific next hop. Issues relate to multicast sessions are
covered in a later section. The ACK_Desired flag will typically be
set only in trigger messages. The ACK_Desired flag MAY be set in
refresh messages.
The Last_Refresh flag may be set in Path and Resv messages when the
MESSAGE_ID object generator is exchanging Hello messages, described
in Section 5, with the next hop RSVP node. When a refresh message
with the Last_Refresh flag set is generated, normal refresh genera-
tion MUST continue until the message containing the Last_Refresh flag
is acknowledged. Although, messages removing state advertised in
such messages MUST be retransmit until acknowledged or a maximum
retry limit is reached in order to cover certain packet loss condi-
tions. Messages removing state include PathTear and ResvTear.
When sending MESSAGE_ID objects with the Last_Refresh flag set, spe-
cial care must be taken to properly advertise state. Specifically,
refresh processing MUST continue per standard RSVP processing until
after a acknowledgment is received. Suppression of refresh process-
ing MAY ONLY occur after an acknowledgment is received for a MES-
SAGE_ID object with the Last_Refresh flag set. Note that the
Last_Refresh flag MAY ONLY be set when the RSVP next hop is exchang-
ing Hello messages with the message generator.
When a Path message for a new session arrives, the RSVP next hop may
not always be known. When the RSVP next hop is not known, the
Last_Refresh flag MUST NOT be set. Once the next hop of a unicast
session is identified, only then may the Last_Refresh flag be set.
(Issues relate to multicast sessions are covered in a later section.)
There are several ways to identify the RSVP next hop of a new unicast
session. Some are more conservative than other, e.g., waiting for a
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Resv message versus checking if the other end of a PPP link supports
Hello messages. Since there are no interoperability issues, the spe-
cific mechanism used to identify the RSVP next hop of a new session
is a specific implementation choice. In most implementations, it is
expected that an advertiser of Path state will do standard refresh
processing until either an ACK is received for a Path message adver-
tising a new session, or a corresponding Resv message is received.
Nodes processing incoming MESSAGE_ID objects SHOULD check to see if a
newly received message is out of order and can be ignored. Out of
order messages can be identified by examining the values in the Epoch
and Message_ID fields. If the Epoch value differs from the value
previously received from the message sender, the receiver MUST fully
processes the message. If the Epoch values match and the Message_ID
value is greater than the largest value previously received from the
sender, the receiver MUST fully processes the message. If the value
is less than the largest value previously received from the sender,
then the receiver SHOULD check the value previously received for the
state associated with the message. This check should be performed
for the currently defined messages: Path, Resv, PathTear, ResvTear,
PathErr and ResvErr. If no local state information can be associated
with the message, the receiver MUST fully processes the message. If
local state can be associated with the message and the received Mes-
sage_ID value is less than the most recently received value associ-
ated with the state, the message SHOULD be ignored, i.e., silently
dropped.
Nodes receiving messages containing MESSAGE_ID objects SHOULD use the
information in the objects to aid in determining if a message repre-
sents new state or a state refresh. Note that state is only
refreshed in Path and Resv messages. If the received Epoch values
differs from the value previously received from the message sender,
the message is a trigger message and the receiver MUST fully pro-
cesses the message. If a Path or Resv message contains the same Mes-
sage_ID value that was used in the most recently received message for
the same session and, for Path messages, SENDER_TEMPLATE then the
receiver SHOULD treat the message as a state refresh. If the Mes-
sage_ID value is grater than the most recently received value, the
receiver MUST fully processes the message. If the Message_ID value
is less than the most recently received value, the receiver SHOULD
ignore the message.
Nodes receiving a non-out of order message containing a MESSAGE_ID
object with the ACK_Desired flag set, SHOULD respond with a MES-
SAGE_ID ACK object. If a node supports the Hello extension it MUST
also check the Last_Refresh flag of received Resv and Path messages.
If the flag is set, the receiver MUST NOT timeout state associated
with associated message. The receiver MUST also be prepared to
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properly process refresh messages. Messages containing a MESSAGE_ID
ACK object with the Last_Refresh flag set MUST NOT be acknowledged
when either the receiving node doesn't support the Hello extension or
Hello messages aren't being exchanged with the message generator.
3.4. MESSAGE_ID ACK Object Usage
The MESSAGE_ID ACK object is used to acknowledge receipt of messages
containing MESSAGE_ID objects that were sent with the ACK_Desired
flag set. The Epoch and Message_ID fields of a MESSAGE_ID ACK object
MUST have the same value as was received. A MESSAGE_ID ACK object
MUST NOT be generated in response to a received MESSAGE_ID object
when the ACK_Desired flag is not set, except as noted in Section 4.3.
A MESSAGE_ID ACK object MAY be sent in any RSVP message that has an
IP destination address matching the generator of the associated MES-
SAGE_ID object. The MESSAGE_ID ACK object will not typically be
included in the non hop-by-hop Path, PathTear and ResvConf messages.
When no appropriate message is available, one or more MESSAGE_ID ACK
objects SHOULD be sent in an Ack message. Implementations SHOULD
include MESSAGE_ID ACK objects in standard RSVP messages when possi-
ble.
MESSAGE_ID ACK objects received with the Refresh_NACK flag set MUST
process the object as described in Section 4.3. Upon receiving a
MESSAGE_ID ACK object with the Refresh_NACK flag not set, a node
SHOULD stop retransmitting the message at the "rapid" retry rate. If
the received object also has the Last_Refresh flag set, normal
refresh generation SHOULD be suppressed for the associated state. As
previously mentioned, special care must be taken to properly adver-
tise state when sending MESSAGE_ID objects with the Last_Refresh flag
set, see section 3.3.
3.5. Multicast Considerations
Path and PathTear messages may be sent to IP multicast destination
addresses. When the destination is a multicast address, it is possi-
ble that a single message containing a single MESSAGE_ID object will
be received by multiple RSVP next hops. When the ACK_Desired flag is
set in this case, acknowledgment processing is more complex. There
are a number of issues to be addressed including ACK implosion, num-
ber acknowledgments to be expected and handling of new receivers.
ACK implosion occurs when each receiver responds to the MESSAGE_ID
object at approximately the same time. This can lead to a poten-
tially large number of MESSAGE_ID ACK objects being simultaneously
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delivered to the message generator. To address this case, the
receiver MUST wait a random interval prior to acknowledging a MES-
SAGE_ID object received in a message destined to a multicast address.
The random interval SHOULD be between zero (0) and a configured maxi-
mum time. The configured maximum SHOULD be set in proportion to the
refresh and "rapid" retransmission interval, i.e, such that the maxi-
mum back-off time does not result in retransmission.
A more fundamental issue is the number of acknowledgments that the
upstream node, i.e., the message generator, should expect. The num-
ber of acknowledgments that should be expected is the same as the
number of RSVP next hops. In the router-to-router case, the number
of next hops can usually be obtained from routing. When hosts are
either the upstream node or the next hops, the number of next hops
will typically not be readily available. Another case where the num-
ber of RSVP next hops will typically not be known is when there are
non-RSVP routers between the message generator and the RSVP next
hops.
When the number of next hops is not known, the message generator
SHOULD only expect a single response and MUST NOT set the
Last_Refresh flag in MESSAGE_ID objects. The result of this behavior
will be special retransmission handling until the message is deliv-
ered to at least one next hop, then followed by standard RSVP
refreshes. Standard refresh messages will synchronize state with any
next hops that don't receive the original message.
Another issue is handling new receivers. It is possible that after
sending a Path message and handling of expected number of acknowledg-
ments that a new receiver joins the group. In this case a new Path
message must be sent to the new receiver. When normal refresh pro-
cessing is occurring, there is no issue. When normal refresh pro-
cessing is suppressed, a Path message must still be generated. In
the router-to-router case, the identification of new next hops can
usually be obtained from routing. When hosts are either the upstream
node or the next hops, the identification of new next hops will typi-
cally not be possible. Another case where the identification of new
RSVP next hops will typically not be possible is when there are non-
RSVP routers between the message generator and the RSVP next hops.
When identification of new next hops is not possible, the message
generator SHOULD only expect a single response and MUST NOT set the
Last_Refresh flag in MESSAGE_ID objects. The result of this behavior
will be special retransmission handling until the message is deliv-
ered to at least one next hop, then followed by standard RSVP
refreshes. Standard refresh messages will synchronize state with any
next hops that don't receive the original message either due to loss
or not yet being a group member.
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There is one additional minor issue with multiple next hops. The
issue is handling a combination of standard-refresh and non-refresh
next hops, i.e., Hello messages are being exchanged with some neigh-
boring nodes but not with others. When this case occurs, refreshes
MUST be generated per standard RSVP and the Last_Refresh flag MUST
NOT be set.
3.5.1. Reference RSVP/Routing Interface
When using the MESSAGE_ID extension with multicast sessions it is
preferable for RSVP to obtain the number of next hops from routing
and to be notified when that number changes. The interface between
routing and RSVP is purely an implementation issue. Since RSVP
[RFC2205] describes a reference routing interface, we present a ver-
sion of the RSVP/routing interface updated to provide number of next
hop information. See [RFC2205] for previously defined parameters and
function description.
o Route Query
Mcast_Route_Query( [ SrcAddress, ] DestAddress,
Notify_flag )
-> [ IncInterface, ] OutInterface_list,
NHops_list
o Route Change Notification
Mcast_Route_Change( ) -> [ SrcAddress, ] DestAddress,
[ IncInterface, ] OutInterface_list,
NHops_list
NHops_list provides the number of multicast group members
reachable via each OutInterface_list entry.
3.6. Compatibility
There are no backward compatibility issues raised by the MESSAGE_ID
Class. The MESSAGE_ID Class has an assigned value whose form is
10bbbbbb. Per RSVP [RFC2205], classes with values of this form must
be ignored and not forwarded by nodes not supporting the class. When
the receiver of a MESSAGE_ID object does not support the class, the
object will be silently ignored. The generator of the MESSAGE_ID
object will not see any acknowledgments and therefore refresh mes-
sages per standard RSVP. Lastly, since the MESSAGE_ID ACK object can
only be issued in response to the MESSAGE_ID object, there are no
possible issues with this object or Ack messages.
Implementations supporting Path and Resv state refresh suppression
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via the MESSAGE_ID object's Last_Refresh flag MUST also support the
Hello extension. Such implementations SHOULD initiate Hello process-
ing and MUST be able to respond to Hello 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 ben-
efits of the described extension are that it reduces the amount of
information that must be transmitted and processed in order to main-
tain RSVP state synchronization. Importantly, the described exten-
sion preserves RSVP's ability to handle non-RSVP next hops and to
adjust to changes in routing. This extension cannot be used with
Path or Resv messages that contain any change from previously trans-
mitted messages, i.e, are not refresh messages.
The summary refresh extension uses the previously defined MESSAGE_ID
object class, the ACK message, and a new Srefresh message. The new
message carries a list of MESSAGE_ID objects corresponding to the
Path and Resv states that are to be refreshed. An RSVP node receiv-
ing an Srefresh message, matches each received MESSAGE_ID object with
installed Path or Resv state. All matching state is updated as if a
normal RSVP refresh message is received. If matching state cannot be
found, then the Srefresh message sender is notified via a refresh
NACK.
Since Srefresh messages can carry multiple MESSAGE_ID objects, Sre-
fresh messages are not expected to be sent in an RSVP aggregate mes-
sages. The flags field of MESSAGE_ID objects carried in Srefresh
messages may be set.
A refresh NACK is indicated by setting the Refresh_NACK flag in the
MESSAGE_ID ACK object. The rules for sending a MESSAGE_ID ACK object
with the Refresh_NACK flag set are the same as was described in the
previous section. This includes sending MESSAGE_ID ACK object both
piggy-backed in unrelated RSVP messages or in RSVP ACK messages.
Nodes supporting the described extension can advertise their support
and detect if an RSVP neighbor also supports the extension. This is
accomplished via flag in the MESSAGE_ID class objects.
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4.1. Srefresh Message Format
Srefresh messages carry one or more MESSAGE_ID objects. A single
Srefresh message MAY refresh both Path or Resv state. Srefresh mes-
sages carrying MESSAGE_ID objects corresponding to Path state SHOULD
be 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 capa-
ble and the session is unicast or the outgoing interface is a point-
to-point interface. Srefresh messages carrying MESSAGE_ID objects
corresponding to Resv state MUST be sent with an destination IP
address set to the Resv state's previous hop.
The source IP address of an Srefresh message is an address of the
node that generates the message. The source IP address MUST match
the addressed associate with the MESSAGE_ID objects when they were
included in a standard RSVP message. As previously mentioned, the
address associate with a MESSAGE_ID object is represented in a per
RSVP message type specific fashion. For Path and PathTear messages
the associated IP address is contained in the RSVP_HOP. For Resv,
ResvTear, PathErr, ResvErr, ResvConf and Bundle messages the associ-
ated IP address is the source address in the IP header.
Srefresh messages that are sent destined to a session's destination
IP address MUST be sent the Router Alert IP option in their IP head-
ers. Srefresh messages addressed directly to RSVP neighbors SHOULD
NOT be sent with the Router Alert IP option in their IP headers.
Each Srefresh message MUST occupy exactly one IP datagram. If it
exceeds the MTU, the datagram is fragmented by IP and reassembled at
the recipient node. A single RSVP Srefresh message MUST NOT exceed
the maximum IP datagram size, which is approximately 64K bytes.
The Srefresh message format is as follows:
<Srefresh Message> ::= <Common Header> [ <INTEGRITY> ]
<MESSAGE_ID>
[ <MESSAGE_ID> ... ]
For Srefresh messages, the Msg Type field of the Common Header MUST
be set to 14 (Value to be assigned by IANA).
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4.2. Srefresh Message Usage
An Srefresh message MAY be generated to refresh Resv or Path state.
If an Srefresh message is used to refresh some particular state, then
the generation of a standard refresh message SHOULD be suppressed. A
state's refresh interval is not affected by the use of Srefresh mes-
sage based refreshes. An Srefresh message MUST NOT be used in place
of a trigger Path or Resv message, i.e., one that would advertise a
state change.
When generating an Srefresh message, a node SHOULD refresh as much
Path and Resv state as is possible by including as many MESSAGE_ID
objects in the same Srefresh message. Only MESSAGE_ID objects that
meet the previously described source and destination IP address
restrictions may be included in the same Srefresh message. Identify-
ing 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 previ-
ously advertised in Path and Resv messages containing MESSAGE_ID
objects can be refreshed via an Srefresh message. Srefresh message
based refreshes must also have 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 iden-
tifying which Path state may be refreshed in a specific message. In
both cases, the previously mentioned refresh interval and source IP
address restrictions must be followed. The primary method is to
include only those sessions that share the same destination IP
address in the same Srefresh message. When using this method, the
destination address of each session MUST be the same as the destina-
tion address in the IP header of the Srefresh message.
The secondary method for identifying which Path state may be
refreshed within a single Srefresh message is an optimization. This
method MAY be used when the next hop is known to support RSVP and
either the session is unicast or the outgoing interface is a point-
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to-point interface. 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 interface, all Path state advertised
out the interface SHOULD be included in the same Srefresh message.
When the outgoing interface is not a point-to-point interface, all
unicast session Path state SHOULD be included in the same Srefresh
message.
Identifying which Resv state may be refreshed within a single Sre-
fresh message is based simply on the source and destination IP
addresses. Any state that was previously advertised in Resv messages
with the same IP addresses as an Srefresh message MAY be included.
After identifying the Path and Resv state that can be included in a
particular Srefresh message, the message generator adds to the mes-
sage a MESSAGE_ID object matching each identified state's previously
used object. Once the Srefresh message is composed, the message gen-
erator transmits the message out the proper interface.
Upon receiving an Srefresh message, a receiver MUST attempt to iden-
tify matching Path or Resv state. Matching is done based on the
source address in the IP header of the Srefresh message and each MES-
SAGE_ID object contained in the message. For each received MES-
SAGE_ID object, the receiver performs an installed state lookup based
on the values contained in the object. If matching state can be
found, then the receiver MUST update the matching state information
as if a standard refresh had been received. The receiver MUST also
process the flags contained in the MESSAGE_ID object per Sections 3
and 4.4. If the receiver cannot identify any matching installed
state, then a Srefresh NACK MUST be generated corresponding to the
unmatched MESSAGE_ID object.
4.3. Srefresh NACK
Srefresh NACKs are used to indicated that a received MESSAGE_ID
object does not match any installed state. An Srefresh NACK is
encoded in a MESSAGE_ID ACK object with the Refresh_NACK flag set.
When generating an Srefresh NACK, The epoch and Message_ID fields of
a MESSAGE_ID ACK object MUST have the same value as was received.
Objects with the Refresh_NACK flag set are transmitted as previously
described, see Section 3.4.
MESSAGE_ID ACK objects received with the Refresh_NACK flag set indi-
cate that the object generator does not have any installed state
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matching the object. Upon receiving a MESSAGE_ID ACK object with the
Refresh_NACK flag set, the receiver performs an installed Path or
Resv state lookup based on the values contained in the object. If
matching state is found, then the receiver MUST transmit the matching
state via a standard Path or Resv message. If the receiver cannot
identify any installed state, then no action is required.
4.4. Compatibility
Nodes supporting the Summary Refresh extension advertise their sup-
port via the Summary_Capable flag in all MESSAGE_ID call objects
transmitted by the node. This enables supporting nodes to detect
each other. When it is not known if a next hop supports the exten-
sion, standard Path and Resv message based refreshes MUST be used.
Note that when the routing next hop does not support RSVP, it will
not always be possible to detect if the RSVP next hop supports the
Summary Refresh extension. Therefore, when the routing next hop is
not RSVP capable the Srefresh message based refresh SHOULD NOT be
used. A node MAY be administratively configured to use Srefresh mes-
sages in all cases when all RSVP nodes in a network are known to sup-
port the Summary Refresh extension.
Nodes supporting the Summary Refresh extension must also take care to
recognize when a next hop stops sending MESSAGE_ID objects with the
Summary_Capable flag set. To cover this case, nodes supporting the
Summary Refresh extension MUST examine each received Summary_Capable
flag. If the flag changes from indicating support to indicating non-
support then Srefresh messages MUST NOT be used for subsequent state
refreshes to that neighbor.
5. Hello Extension
The RSVP Hello extension enables RSVP nodes to detect a loss of a
neighboring node's state information. In standard RSVP, such detec-
tion occurs as a consequence of RSVP's soft state model. When
refresh message generation is suppressed via the previously discussed
Last_Refresh flag processing, the Hello extension is needed to
address this failure case. The Hello extensions is not intended to
provide a link failure detection mechanism, particularly in the case
of multiple parallel unnumbered links.
The Hello extension is specifically designed so that one side can use
the mechanism while the other side does not. Neighbor RSVP state
tracking may be initiated at any time. This includes when neighbors
first learn about each other, or just when neighbors are sharing Resv
or Path state. As previously stated, all implementations supporting
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state refresh suppression are required to also support the Hello
Extension.
The Hello extension is composed of a Hello message, a HELLO REQUEST
object and a HELLO ACK object. Hello processing between two neigh-
bors supports independent selection of, typically configured, failure
detection intervals. Each neighbor can autonomously issue HELLO
REQUEST objects. Each request is answered by an acknowledgment.
Hello Messages also contain enough information so that one neighbor
can suppress issuing hello requests and still perform neighbor fail-
ure detection. A Hello message may be included as a sub-message
within a bundle message.
Neighbor state tracking is accomplished by collecting and storing a
neighbor's state "instance" value. If a change in value is seen or
if the neighbor is not properly reporting the locally advertised
value, then the neighbor is presumed to have reset it's RSVP state.
When communication is lost with a neighbor, then the instance value
advertised to that neighbor is also changed. The HELLO objects pro-
vide a mechanism for polling for and providing an RSVP state instance
value. A poll request also includes the sender's instance value.
This allows the receiver of a poll to optionally treat the poll as an
implicit poll response. This optional handling is an optimization
that can reduce the total number of polls and responses processed by
a pair of neighbors. In all cases, when both sides support the opti-
mization the result will be only one set of polls and responses per
failure detection interval. Depending on selected intervals, the
same benefit can occur even when only one neighbor supports the opti-
mization.
5.1. Hello Message Format
Hello Messages are always sent between two RSVP neighbors. The IP
source address is the IP address of the sending node. The IP desti-
nation address is the IP address of the neighbor node.
HELLO messages SHOULD be exchanged between immediate RSVP neighbors.
When HELLO messages are being the exchanged between immediate neigh-
bors, the IP TTL field of all outgoing HELLO messages SHOULD be set
to 1.
The Hello message format is as follows:
<Hello Message> ::= <Common Header> [ <INTEGRITY> ]
<HELLO>
For Hello messages, the Msg Type field of the Common Header MUST be
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set to 14 (Value to be assigned by IANA).
5.2. HELLO Object
HELLO Class = 22 (Value to be assigned by IANA of form 0bbbbbbb)
HELLO REQUEST object
Class = HELLO 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src_Instance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dst_Instance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
HELLO ACK object
Class = HELLO 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Src_Instance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dst_Instance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Src_Instance: 32 bits
a 32 bit value that represents the sender's RSVP agent's state.
The advertiser maintains a per neighbor representation/value.
This value MUST change when the agent is reset, when the node
reboots, or when communication is lost to the neighboring node
and otherwise remains the same. This field MUST NOT be set to
zero (0).
Dst_Instance: 32 bits
The most recently received Src_Instance value received from the
neighbor. This field MUST be be set to zero (0) when no value
has ever been seen from the neighbor.
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5.3. Hello Message Usage
A Hello message containing a HELLO REQUEST object MUST be generated
for each neighbor who's state is being tracked. When generating a
message containing a HELLO REQUEST object, the sender fills in the
Src_Instance field with a value representing it's per neighbor RSVP
agent state. This value MUST NOT change while the agent is maintain-
ing any RSVP state with the corresponding neighbor. The sender also
fills in the Dst_Instance field with the Src_Instance value most
recently received from the neighbor. If no value has ever been
received from the neighbor, a value of zero (0) is used. The genera-
tion of a message SHOULD be skipped when a HELLO REQUEST object was
received from the destination node within the failure detection
interval.
On receipt of a message containing a HELLO REQUEST object, the
receiver MUST generate a Hello message containing a HELLO ACK object.
The receiver SHOULD also verify that the neighbor has not reset.
This is done by comparing the sender's Src_Instance field value with
the previously received value. If the value differs, than the neigh-
bor has reset and all state associated with the neighbor MUST be
"expired" and cleaned up per standard RSVP processing. Additionally,
all Path state advertised to the neighbor MUST be refreshed. The
receiver SHOULD also verify that the neighbor is reflecting back the
receiver's Instance value. This is done by comparing the received
Dst_Instance field with the Src_Instance field value most recently
transmitted to that neighbor. If the neighbor continues to advertise
a wrong non-zero value after a configured number of intervals, then a
node MUST treat the neighbor as if communication has been lost. In
this case all state associated with the neighbor MUST be "expired"
and cleaned up per standard RSVP processing. Additionally, the
Src_Instance value advertised in the HELLO ACK object MUST be be dif-
ferent from the previously advertised value. This new value MUST
continue to be advertised to the corresponding neighbor until a reset
or reboot occurs, or until another communication failure is detected.
On receipt of a message containing a HELLO ACK object, the receiver
MUST verify that the neighbor has not reset. This is done by compar-
ing the sender's Src_Instance field value with the previously
received value. If the value differs, than the neighbor has reset
and all state associated with the neighbor MUST be "expired" and
cleaned up per standard RSVP processing. Additionally, all Path
state advertised to the neighbor MUST be refreshed. The receiver
MUST also verify that the neighbor is reflecting back the receiver's
Instance value. If the neighbor advertises a wrong value in the
Dst_Instance field, then a node MUST treat the neighbor as if commu-
nication has been lost. In this case all state associated with the
neighbor MUST be "expired" and cleaned up per standard RSVP
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processing.
If no Instance values are received, via either REQUEST or ACK
objects, from a neighbor within a configured number of failure detec-
tion intervals, then a node MUST presume that it cannot communicate
with the neighbor. When this occurs, all state associated with the
neighbor MUST be "expired" and cleaned up per standard RSVP process-
ing, and all Path state advertised to the neighbor MUST be refreshed.
If any state is removed or needs to be refreshed as a result of this
case, then a new HELLO message MUST be immediately issued with a
Src_Instance value different then the one advertised in the previous
HELLO message. This new value MUST continue to be advertised to the
corresponding neighbor until a reset or reboot occurs, or until
another communication failure is detected.
5.4. Multi-Link Considerations
As previously noted, the Hello extension is targeted at detecting
node failures not per link failures. When there is only one link
between neighboring nodes or when all links between a pair of nodes
fail, the distinction between node and link failures is not really
meaningful and handling of such failures has already been covered.
When there are multiple links shared between neighbors, there are
special considerations. When the links between neighbors are num-
bered, then Hellos MUST be run on each link and the previously
described mechanisms apply.
When the links are unnumbered, link failure detection MUST be pro-
vided by some means other than Hellos. Each node SHOULD use a single
Hello exchange with the neighbor. When a node removes state due to a
link failure, the node MUST advertise the removal of the state, via
appropriate Tear messages, over a non-failed link. The case where
all links have failed, is the same as the no received value case men-
tioned in the previous section.
5.5. Compatibility
The Hello extension is fully backwards compatible. The Hello class
is assigned a class value of the form 0bbbbbbb. Depending on the
implementation, implementations that don't support the extension will
either silently discard Hello messages or will respond with an
"Unknown Object Class" error. In either case the sender will fail to
see an acknowledgment for the issued Hello. When a Hello sender does
not receive an acknowledgment, it MUST NOT send MESSAGE_ID objects
with the Last_Refresh flag set. This restriction will preclude
neighbors from getting out of RSVP state synchronization.
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Implementations supporting the Hello extension MUST also support the
MESSAGE_ID extension and refresh suppression.
6. 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.
6.1. Outline of Operation
We propose the following feedback mechanism for exponential back-off
retransmission of an RSVP message: When sending such a message, a
node inserts a MESSAGE_ID object with the the ACK_Desired flag set.
Upon reception, a receiving node acknowledges the arrival of the mes-
sage by sending back an message acknowledgment (that is, a corre-
sponding MESSAGE_ID ACK object.) When the sending node receives this
message acknowledgment for a Path or Resv message, it will automati-
cally scale back the retransmission rate for these messages for the
flow. If the trigger message was for a different message type, no
other further action is required.
Until the message acknowledgment is received, the sending node will
retransmit the message. The interval between retransmissions is gov-
erned by a rapid retransmission timer. The rapid retransmission
timer starts at a small interval which increases exponentially until
it reaches a threshold. From that point on, the sending node will
use a fixed timer to refresh Path and Resv messages and stop re-
transmitting other messages. This mechanism is designed so that the
message load is only slightly larger than in the current specifica-
tion even when the receiving node does not support message acknowl-
edgment.
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. Unless a node
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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).
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
retransmission interval has been increased to R. Once the retrans-
mission interval has been increased to R, Path and Resv messages will
be refreshed within the interval R. Other messages will not be
retransmitted.
The implementation of exponential back-off retransmission is simple.
A sending node can use the following algorithm after transmitting a
message containing a MESSAGE_ID object with the ACK_Desired flag set:
Berger, et al. [Page 29]
Internet Draft draft-berger-rsvp-refresh-reduct-03.txt June 1999
if (Rk < R) {
Rk = Rk * (1 + Delta);
retransmit the message;
wake up after Rk seconds;
exit;
}
else { /* no reply from receivers for too long: */
Rk = R;
if (message is a Path or Resv Message) {
send out a refresh message;
wake up after Rk seconds;
exit;
}
else {
clean up any associated state and resources;
exit;
}
}
Asynchronously, when a sending node receives a corresponding MES-
SAGE_ID ACK object, it will change the retransmission interval Rk to
Rs and, for non Path or Resv messages, clean up any associated state
and resources.
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 Yoram Bernet, Fred Baker, Roch Guerin, Henning Schulzrinne,
Andreas Terzis, David Mankins and Masanobu Yuhara for specific feed-
back on the document.
Portions of this work are based on work done by Masanobu Yuhara and
Mayumi Tomikawa [Yuhara].
8. Security Considerations
No new security issues are raised in this document. See [RFC2205]
for a general discussion on RSVP security issues.
Berger, et al. [Page 30]
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9. References
[Awduche] Awduche, D. et al. Requirements for Traffic Engineering
over MPLS, Internet Draft,
draft-awduche-mpls-traffic-eng-00.txt, April 1998.
[Pan] Pan, P., Schulzrinne, H., "Staged Refresh Timers for RSVP,"
Global Internet'97, Phoenix, AZ, Nov. 1997.
http://www.ctr.columbia.edu/~pan/papers/timergi.ps
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119.
[RFC2205] Braden, R. Ed. et al, "Resource ReserVation Protocol
-- Version 1 Functional Specification", RFC 2205,
September 1997.
[Yuhara] Yuhara, M., Tomikawa, M. "RSVP Extensions for ID-based
Refreshes," Internet Draft,
draft-yuhara-rsvp-refresh-00.txt, April 1999.
Berger, et al. [Page 31]
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10. Authors' Addresses
Lou Berger
LabN Consulting
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 32]
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