draft-ietf-mpls-ldp-state-03.txt   draft-ietf-mpls-ldp-state-04.txt 
MPLS Working Group Christophe Boscher A new Request for Comments is now available in online RFC libraries.
Internet Draft Pierrick Cheval
Expiration Date: July 2000 Alcatel
Liwen Wu
Cisco
Eric Gray
Lucent
January 2000
LDP State Machine
draft-ietf-mpls-ldp-state-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.
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Distribution of this memo is unlimited. RFC 3215
Copyright Notice Title: LDP State Machine
Author(s): C. Boscher, P. Cheval, L. Wu, E. Gray
Status: Informational
Date: January 2002
Mailbox: christophe.boscher@alcatel.fr,
pierrick.cheval@space.alcatel.fr, liwwu@cisco.com,
eric.gray@sandburst.com
Pages: 78
Characters: 117278
Updates/Obsoletes/SeeAlso: NONE
Copyright (C) The Internet Society (1998). All Rights Reserved. I-D Tag: draft-ietf-mpls-ldp-state-04.txt
1. Abstract URL: ftp://ftp.rfc-editor.org/in-notes/rfc3215.txt
In the current LDP specification [4], there is no state machine This document provides state machine tables for ATM (Asynchronous
specified for processing LDP messages. We think that defining a Transfer Mode) switch LSRs. In the current LDP specification, there
common state machine is very important for interoperability between is no state machine specified for processing LDP messages. We think
different LDP and CR-LDP implementations. that defining a common state machine is very important for
interoperability between different LDP and CR-LDP implementations.
This document provides state machine tables for ATM switch LSRs. We We begin in section 1 by defining a list of terminologies. Then in
begin in section 2 by defining a list of terminologies. Then in section 2, we propose two sets of state machine tables for ATM switch
section 3, we propose two sets of state machine tables for ATM switch LSRs that use downstream-on-demand mode, one method can be used for
LSRs which use downstream-on-demand mode, one method can be used for
non-vc merge capable ATM LSRs, while the other one can be used for non-vc merge capable ATM LSRs, while the other one can be used for
the vc-merge capable ATM LSRs. In section 4, we provides a state the vc-merge capable ATM LSRs. In section 3, we provides a state
machine for downstream unsolicited mode ATM LSRs. machine for downstream unsolicited mode ATM LSRs.
We focus on the LDP state machines and the associated control blocks We focus on the LDP state machines and the associated control blocks
used for establishing and maintaining LSPs. We do not describe state used for establishing and maintaining LSPs. We do not describe state
machines for the "LDP controller" which is in charge of LDP session machines for the "LDP controller" that is in charge of LDP session
initialization, address mapping messages management, routing initialization, address mapping messages management, routing
interface, etc. which is defined in the LDP specification [4]. interface, etc. that is defined in the LDP specification.
Even though the state machines in this document are specific for Even though the state machines in this document are specific for
ATM-LSR, they can be easily adapted for other types of LSRs. ATM-LSR, they can be easily adapted for other types of LSRs.
2. Terminology This document is a product of the Multiprotocol Label Switching
Working Group of the IETF.
- LDP-REQUEST: LDP Label Request message
- LDP-MAPPING: LDP Label Mapping message
- LDP-WITHDRAW: LDP Label Withdraw message
- LDP-RELEASE: LDP Label Release message
- LDP-ABORT: LDP Abort message used to abort a LSP setup.
- LDP-NAK: LDP Notification message used to reject an LDP message.
3. State Machine for Downstream-on-Demand Mode
In this draft, we provide two sets of state machines: one for the ATM
LSR which does not have VC-merge capability, and the other for the
ATM LSR which does have VC-merge capability.
State machine descriptions are given in terms of control blocks,
states, events, response actions and state transitions. Control
blocks contain the information that is required to support handling
of events. A control block may also contain any additional
information that is required either of any specific implementation or
in support of other required functions. In every case, additional
information required to support the procedures defined in the LDP
specification [4] or management objects defined in the LDP MIB [3]
would be stored in a specific LDP implementation - either as part of
the control block structure or in some other way.
The state machines cover both independent LSP control and ordered LSP
control.
Loop detection and loop prevention messages will be processed as
specified in [4]. The impact of loop detection and loop prevention
messages on state transitions is left for further study.
3.0 An LSR's Behavior in the Case of a Next Hop Change
When there is a topology change and an LSR detects a new better next
hop for an LSP, it may behave in 2 different ways:
1) It tries to do a "local repair". This means that it extends the
LSP through the new next hop, releases the old path from this LSR
forward and then splices into this newly extended LSP.
2) If the LSP is created with the "pinned" option (CR-LDP[5]), the
LSR ignores the new next hop change, and the LSP stays unchanged. The
LSR may decide to send an LDP-MAPPING containing attributes for this
New Next Hop (NH) that have changed.
3.1. ATM Switch LSR with No VC-merge Capability
In an MPLS domain where some ATM LSRs do not have VC-merge
capability, downstream-on-demand mode is required for these ATM LSRs
[1]. Also, "conservative label retention mode" is required in this
case [1].
For each LSP, there are 2 kinds of state machines involved:
1) the LSP Control Block and its state machine which can be used to
handle normal LSP setup. It is created when the LSR receives a new
LDP Request and it is deleted when the LSP of this request is torn
down.
2) the Next Hop Trigger Control Block and its state machine which is
used to handle switching over to a better LSP through a different
next hop. It is created when the LSR decides to switch over to a
better next hop and it is deleted when the LSR finishes switching
over to the better next hop. This state machine uses a timer (and
has corresponding states) to ensure that switch over occurs in a
timely fashion after a routing transient has had time to stabilize.
3.1.1 LSP Control Block
For each LSP, an LSP Control Block is defined which may contain the
following information:
- Upstream Label Request ID (assigned by the upstream LSR), which
is the 'Message Id' in the Label Request Message received from the
upstream LSR.
- Downstream Label Request ID (assigned by this LSR itself), which
is 'Message Id' in the Label Request Message sent to the downstream
LSR.
- Upstream LDP Identifier
- Downstream LDP Identifier
- State
- FEC
- Upstream Label (assigned by this LSR)
- Downstream Label (assigned by the downstream LSR)
- Trigger Control Block Pointer, (Only used at the ingress LSR of a
LSP) that points to the control block that triggers setting up this
LSP or tearing down this LSP.
- Next Hop Control Block Pointer, which points to the control block
which is used for switching over to a better LSP.
The following index combinations can be used to locate a unique LSP
Control Block:
- Downstream Label and Downstream LDP Identifier, or
- Upstream Label and Upstream LDP Identifier, or
- Downstream Label Request ID and Downstream LDP Identifier
- Upstream Label Request ID and Upstream LDP Identifier
Here is the relationship between different control blocks, the detail
definition of Next Hop Trigger Control Block is described in section
3.1.6.
For example, an LSP which transit through (LSR-A, LSR-B, LSR-C, LSR-
D):
LSR-A ----> LSR-B ---> LSR-C ---> LSR-D
The control blocks in LSR-A are:
+-----------------------+
| Trigger Control Block |
| (e.g, by config) |
+-----------------------+
^
|(Trigger Control block pointer)
|
|
+-----------------------+
| LSP Control Block |
+-----------------------+
When LSR-B detects a better next hop to LSR-D through LSR-E, and it
decides to switch over to it, so control blocks in LSR-B are:
+-----------------------+
| LSP Control Block |
| (original LSP) |
+-----------------------+
(LSP ^ |
Control | | (Next Hop Trigger Control Block Pointer)
Block | |
Pointer) | v
+--------------------------------+
| Next Hop Trigger Control Block |
+--------------------------------+
^ |
(Trigger | | (New Next Hop LSP
Control | | Control Block Pointer)
Block | |
Pointer)| |
| v
+------------------------+
| LSP Control Block |
| (for LSP: LSR-B, LSR-E,|
| LSR-D) |
+------------------------+
3.1.2 States
This section describes the various states that are used in the state
machine for the ATM non VC-merge LSR.
-- IDLE
This is the initial LSP state, when the LSP Control Block is created.
-- RESPONSE_AWAITED
This state means that the LSR has received and processed an LDP-
REQUEST from an upstream LSR, or it has received an internal set up
request. It has sent a new LDP-REQUEST towards a downstream LSR. The
LSR is waiting for the LDP-MAPPING from the downstream LSR.
-- ESTABLISHED
This state means that the LSR has received the LDP-MAPPING from the
downstream LSR and the LSP is up and operational.
-- RELEASE_AWAITED
This state means that the LSR has sent a LDP-WITHDRAW upstream and is
waiting for the LDP-RELEASE before freeing up the label resource.
3.1.3 Events
-- LDP Request
The LSR receives an LDP-REQUEST from an upstream LSR.
-- LDP Mapping
The LSR receives an LDP-MAPPING from a downstream LSR.
-- LDP Release
The LSR receives an LDP-RELEASE from an upstream LSR.
-- LDP Withdraw
The LSR receives an LDP-WITHDRAW from a downstream LSR.
-- LDP Upstream Abort
The LSR receives an LDP-ABORT from an upstream LSR.
-- LDP Downstream NAK The LSR receives an LDP-NAK (notification) from
an downstream LSR.
-- Upstream Lost
The LSR loses its LDP session with an upstream LDP peer.
-- Downstream Lost
The LSR loses its LDP session with a downstream LDP peer.
-- Internal SetUp
For some reason, e.g. a configuration request of a traffic
engineering tunnel, or recognizing a new FEC could trigger an
Internal SetUp event to set up a new LSP from this node.
-- Internal Destroy
The LSR send an Internal Destroy event to tear down an LSP.
-- Internal Cross-Connect
The LSR send an Internal Cross-Connect to splice two LSPs into one
LSP. This happens when a LSR switchs over to a better next hop
-- Internal New NH
The LSR decides to switch over the better next hop.
3.1.4 State Transitions
The following diagram describes briefly the state transitions.
+-------------------+
| |<-------------------+
+-------->| IDLE | |
| | |----------+ |
| +-------------------+ | |
|(LDP Release) | | |
|(LDP Upstream |(LDP Request 1) | | (LDP Release)
| Abort |(Internal SetUp) | | (Upstream Lost)
|(Internal Destroy) | | |
|(Upstream Lost) v | |
| +-------------------+ | |
+---------| | | |
| RESPONSE_AWAITED | | |
+---------| | | |
| +-------------------+ | |
| | | |
|(Downstream Lost) |(LDP Mapping) | |
|(LDP Downstream | | |
| NAK) | +---------------+ |
| | | (LDP Request 2) |
| | | |
| v v |
| +-------------------+ (LDP Withdraw 1) |
| | | (Internal Destroy) |
| | ESTABLISHED |------------>-------+
| | | |
| +-------------------+ |
| | |
| | |
| |(LDP Withdraw 2) | (LDP Upstream
| |(Downstream Lost) | Abort)
| | |
| v |
| +-------------------+ |
| | | |
+-------->| RELEASE_AWAITED |------------>-------+
| |
+-------------------+
3.1.5 State Machine
3.1.5.1 State -- "IDLE"
State: IDLE
Event: LDP Request
New State: Depends upon the action routine.
Actions:
If this LSR is the LSP Egress or Proxy Egress [2]
Then:
Choose an upstream label, connect this upstream label to the local
IP forwarding module, allocate the resources, send the LDP-MAPPING
upstream with the upstream label, and go to the new state
`ESTABLISHED'.
else
Obtain a next hop (or interface) with the FEC specified in the
LDP-REQUEST, propagate the LDP-REQUEST, with newly assigned Message
ID by this LSR, towards the obtained next hop, and go to the new
state `RESPONSE_AWAITED'.
If the LSR uses the independent control mode [2], choose an
upstream label, connect this upstream label to the local IP
forwarding module, go to the ESTABLISHED state and send an LDP-
MAPPING upstream with the upstream label.
If unable to process the request for any reason, issue an LDP-NAK to
the sender with the appropriate error code, go to IDLE and delete the
LSP Control Block.
State: IDLE
Event: LDP Mapping
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Release
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Withdraw
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Upstream Abort
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Downstream NAK
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Upstream Lost
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Downstream Lost
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Internal SetUp
New State: RESPONSE_AWAITED
Actions:
Set up the Trigger Control Block pointer,
Obtain a next hop (or interface) with the FEC specified in the
Internal SetUp message, send a LDP-REQUEST towards the obtained next
hop, and go to the new state `RESPONSE_AWAITED'.
State: IDLE
Event: Internal Destroy
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal Cross-Connect
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal New NH
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
3.1.5.2 State -- "RESPONSE_AWAITED"
State: RESPONSE_AWAITED
Event: LDP Request
New State: RESPONSE_AWAITED
Actions:
Ignore the event. It is an internal implementation error. A non VC
merge ATM LSR must create a new LSP control block for a new LDP
request.
State: RESPONSE_AWAITED
Event: LDP Mapping
New State: ESTABLISHED
Actions:
1) If the LSP is triggered by the local router (Trigger Control Block
Pointer is not zero), send event `Internal LSP UP' to the Trigger
control block.
2) Else If the LSR uses the ordered control mode, choose an upstream
label.
3) Connect the upstream label to the downstream label. Allocate the
resources. Propagate the LDP-MAPPING upstream with the upstream
label.
If unable to process the message, disconnect the upstream label from
the downstream label, free the upstream label, release the resources,
send an LDP-RELEASE downstream and an LDP-NAK upstream with status
(No Label Resources [4]), go to IDLE and delete the LSP Control
Block.
State: RESPONSE_AWAITED
Event: LDP Release
New State: IDLE
Actions:
If the LSR uses the independent control mode, free the upstream
label.
Send an LDP-ABORT downstream, go to IDLE and delete the LSP Control
Block.
Note: This should only occur if the LSR uses the independent control
mode. In the ordered control mode, no upstream label mapping will
have been sent corresponding to this LSP while waiting for a label
mapping from downstream.
State: RESPONSE_AWAITED
Event: LDP Withdraw
New State: RESPONSE_AWAITED
Actions:
Ignore the event. It's a protocol error from the downstream LSR.
State: RESPONSE_AWAITED
Event: LDP Upstream Abort
New State: IDLE
Actions:
If the LSR uses the independent control mode, free the upstream
label.
Send an LDP-ABORT downstream.
Delete the LSP Control Block.
State: RESPONSE_AWAITED
Event: LDP Downstream NAK
New State: Depends on the action routine.
Actions:
1. If the LSP is triggered by the local router (Trigger Control Block
Pointer is not zero), send event `Internal LSP DOWN' to the Trigger
control block, go to IDLE and delete the LSP Control Block.
2. Else If the LSR uses the independent control mode, send an LDP-
WITHDRAW upstream and go to state `RELEASE_AWAITED'.
If the LSR uses the ordered control mode, send an LDP-NAK upstream,
go to IDLE and delete the LSP Control Block.
State: RESPONSE_AWAITED
Event: Upstream Lost
New State: IDLE
Actions:
If the LSR uses the independent control mode, free the upstream
label.
Send an LDP-ABORT downstream, go to IDLE and delete the LSP Control
Block.
State: RESPONSE_AWAITED
Event: Downstream Lost
New State: Depends on the action routine.
Actions:
1. If the LSP is triggered by the local router (Trigger Control Block
Pointer is not zero), send event `Internal LSP DOWN' to the trigger
control block, go to IDLE and delete the LSP Control Block.
2. Else, If the LSR uses the independent control mode, free the
upstream label and send an LDP-WITHDRAW upstream and go to state
`RELEASE_AWAITED'.
If the LSR uses the ordered control mode, send an LDP-NAK upstream
(with a status `No Route' [4]), go to IDLE and delete the LSP Control
Block.
State: RESPONSE_AWAITED
Event: Internal SetUp
New State: RESPONSE_AWAITED
Actions:
Ignore, it is an internal implementation error.
State: RESPONSE_AWAITED
Event: Internal Destroy
New State: IDLE
Actions:
Send an LDP-ABORT downstream, go to IDLE and delete the LSP Control
Block.
State: RESPONSE_AWAITED
Event: Internal Cross-Connect
New State: RESPONSE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
State: RESPONSE_AWAITED
Event: Internal New NH
New State: RESPONSE_AWAITED
Actions:
Send LDP-ABORT to the old downstream, and send LDP-REQUEST to the new
next hop.
3.1.5.3 State --"ESTABLISHED"
State: ESTABLISHED
Event: LDP Request
New State: ESTABLISHED
Actions:
Ignore the event. It's an internal implementation error. For non
VC-merge ATM LSR, a new LSP control block is always created for each
LDP request.
State: ESTABLISHED
Event: LDP Mapping
New State: ESTABLISHED
Actions:
Process the LDP-MAPPING, which may contain the new attributes of the
label mapping and then propagate the LDP-MAPPING upstream.
State: ESTABLISHED
Event: LDP Release
New State: IDLE
Actions:
Disconnect the upstream label from the downstream label.
Free the upstream label.
Free the resources.
Send event `Internal Destroy' to the Next Hop Trigger Control Block
if it was in the middle of switching over to the better next hop.
Propagate the LDP-RELEASE downstream if the LSR is not the egress for
the LSP, go to IDLE and delete the LSP Control Block.
State: ESTABLISHED
Event: LDP Withdraw
New State: Depends on the action routine.
Actions:
1) Free the resources and send LDP-RELEASE downstream.
2) If it is independent control mode, set the state to `IDLE', create
a internal LDP Request with the information in the LSP Control Block,
and pass event `LDP Request' to its own state machine.
3) Else for the ordered control mode
3.1) If the LSP is triggered to be setup by itself (e.g it is the
ingress LSR of this LSP), send event `Internal LSP Down' to the
trigger control block, go to IDLE and delete the LSP Control Block.
3.2) Else, if it is triggered by the incoming LDP Request,
Disconnect the upstream label from the downstream label. Propagate
the LDP-WITHDRAW upstream and go to state `RELEASE_AWAITED'.
3.3) If the LSP is in the middle of switching over to a better LSP,
send event `Internal Destroy' to the state machine of its New Next
Hop LSP Control Block, go to IDLE and delete the LSP Control Block.
State: ESTABLISHED
Event: LDP Upstream Abort
New State: ESTABLISHED
Actions:
Ignore the event.
Note: This scenario can occur if the upstream LSR sends a LDP-ABORT
at about the same time as the local LSR sends a LDP-MAPPING. In this
situation, it should be up to exactly one of the two LSRs as to
whether or not the label that was sent remains valid. The LDP
specification [4] procedures leave the choice to the upstream LSR
which must send an LDP-RELEASE if it will not use the label provided.
State: ESTABLISHED
Event: LDP Downstream NAK
New State: ESTABLISHED
Actions:
Ignore the event. It is a protocol error from the downstream LSR.
The downstream LSR should always LSP-WITHDRAW to tear down the LSP
when the LSP is established.
State: ESTABLISHED
Event: Upstream Lost
New State: IDLE
Actions:
Disconnect the upstream label from the downstream label.
Free the upstream label.
Send event `Internal Destroy' to the Next Hop Trigger Control Block
if it was in the middle of switching over to the better next hop.
Free the resources.
Propagate an LDP-RELEASE downstream, go to IDLE and delete the LSP
Control Block.
State: ESTABLISHED
Event: Downstream Lost
New State: Depends on the action routine.
Actions:
1) If the LSP is triggered by the local router (Trigger Control Block
Pointer is not zero), send event `Internal LSP NAK' to the Trigger
control block, go to IDLE and delete the LSP Control Block.
2) Else, disconnect the upstream label from the downstream label.
Propagate an LDP-WITHDRAW upstream and go to `RELEASE_AWAITED' state.
3) Send event `Internal Destroy' to the Next Hop Trigger Control
Block if it was in the middle of switching over to the better next
hop.
State: ESTABLISHED
Event: Internal Setup
New State: ESTABLISHED
Actions:
Ignore, it is an internal implementation error.
State: ESTABLISHED
Event: Internal Destroy
New State: IDLE
Actions:
Disconnect the upstream label from the downstream label if it is not
the ingress of the LSP.
Free the resources.
Send an LDP-RELEASE downstream, go to IDLE and delete the LSP Control
Block.
State: ESTABLISHED
Event: Internal Cross-Connect
New State: ESTABLISHED
Actions:
Connect the upstream label to the downstream label
May need to send a new LDP-MAPPING upstream with the attributes from
the new next hop.
Reset Trigger Control Block Pointer to zero.
State: ESTABLISHED
Event: Internal New NH
New State: ESTABLISHED
Actions:
1) If the LSR was in the middle of switching over to a better next
hop (Next Hop Trigger Control Block Pointer is not zero), it send
`Internal New NH' to that control block.
2) Else, create a new Next Hop Trigger Control Block, set Next Hop
Trigger Control Block pointer which points this control block. And
then pass `Internal New NH' to this control block.
3.1.5.4 State --"RELEASE_AWAITED"
State: RELEASE_AWAITED
Event: LDP Request
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
State: RELEASE_AWAITED
Event: LDP Mapping
New State: RELEASE_AWAITED
Actions:
It is a protocol error from the downstream LDP peer, but anyway send
a LDP-RELEASE downstream.
State: RELEASE_AWAITED
Event: LDP Release
New State: IDLE
Actions:
1) Free the upstream label
2) Delete the control block.
State: RELEASE_AWAITED
Event: LDP Withdraw
New State: RELEASE_AWAITED
Actions:
It is a protocol error from the downstream LDP peer, but send a LDP-
RELEASE anyway.
State: RELEASE_AWAITED
Event: LDP Upstream Abort
New State: IDLE
Actions:
1) Free the upstream label
2) Delete the control block.
State: RELEASE_AWAITED
Event: LDP Downstream NAK
New State: RELEASE_AWAITED
Actions:
Ignore the event. Continue waiting for the LDP-RELEASE from upstream.
State: RELEASE_AWAITED
Event: Upstream Lost
New State: IDLE
Actions:
1) Free the upstream label
2) Delete the control block.
State: RELEASE_AWAITED
Event: Downstream Lost
New State: RELEASE_AWAITED
Actions:
Ignore the event. Continue waiting for the LDP-RELEASE from upstream.
State: RELEASE_AWAITED
Event: Internal SetUp
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
State: RELEASE_AWAITED
Event: Internal Destroy
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
State: RELEASE_AWAITED
Event: Internal Cross-Connect
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
3.1.6 Handling the Next Hop Change
When an LSR detects a better next hop, it may decides to establish a
new LSP through this next hop. For example, an LSR is configured as
"local repair", or the LSR is configured as "global repair" and it is
the ingress end of a LSP. It can then create a Next Hop Trigger
Control Block and use the state machine of Next Hop Trigger Control
Block to establish a new LSP through the better next hop.
3.1.6.1 Next Hop Trigger Control Block
-- State
-- LSP Control Block Pointer, which points to the original LSP
control block.
-- New Next Hop LSP Control Block Pointer, which points to the LSP
Control Block that is setting up an LSP through the new next hop.
3.1.6.2 States
-- IDLE
This is the initial LSP state, when the Trigger_Control_Block is
created.
-- NEW_NH_RETRY
This is the state where an LSR waits for a retry timer to expire and
then tries to establish an LSP through a new next hop.
-- NEW_NH_RESPONSE_AWAITED
This is the state where an LSR is in the middle of establishing a new
LSP through a new next hop. It has triggered a LSP control block to
send an LDP-REQUEST towards the new next hop and is waiting for the
LDP-MAPPING.
3.1.6.3 Events
-- Internal New NH
The LSR detects there is a new next hop for a FEC.
-- Internal Retry Timeout
The LSP retry timer expires.
-- Internal LSP UP
The LSP to the new Next Hop is UP
-- Internal LSP NAK
The LSP through the new next hop could not get set up
-- Internal Destroy
This event is triggered when the LSR lost the LDP session with its
upstream neighbor.
3.1.6.4 State Transition for next hop change
+---------------------+
| |
| IDLE |<------------+
| | |
+---------------------+ |
| |
| |
| (Internal New NH) |
| |
v |
+---------------------+ |
| | |
| NEW_NH_RETRY |----------->-+
| | (Internal |
+---------------------+ Destroy) |
| |
| |
| (Internal retry timeout) |
| |
v |
+---------------------+ |
| | (Internal |
| NEW_NH_RESPONSE | Destroy) |
| _AWAITED |----------->-+
| | |
+---------------------+ |
| |
| (Internal LSP UP) |
| (Internal LSP NAK) |
+------------------------>-+
3.01.3 State Machine
3.01.3.1 State -- "IDLE"
State: IDLE
Event: Internal New NH
New State: NEW_NH_RETRY
Actions:
Start the LSP retry timer and go to the `NEW_NH_RETRY' state.
State: IDLE
Event: Internal retry timeout
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal LSP UP
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal LSP NAK
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal destroy
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
3.01.3.2 State -- "NEW_NH_RETRY"
State: NEW_NH_RETRY
Event: Internal New NH
New State: NEW_NH_RETRY
Actions:
Restart the LSP retry timer.
State: NEW_NH_RETRY
Event: Internal retry timeout
New State: Depends on action routine.
Actions:
If the new next hop is the same one as the old next hop, go to IDLE
and delete the control block.
Otherwise, create an LSP control block which will try to establish a
new LSP through the new next hop, send event `Internal Setup' to its
state machine and go to NEW_NH_RESPONSE_AWAITED.
State: NEW_NH_RETRY
Event: Internal LSP UP
New State: NEW_NH_RETRY
Actions:
Ignore. It is an internal implementation error.
State: NEW_NH_RETRY
Event: Internal LSP NAK
New State: NEW_NH_RETRY
Actions:
Ignore. It is an internal implementation error.
State: NEW_NH_RETRY
Event: Internal destroy
New State: IDLE
Actions:
Stop the timer, go to IDLE and delete the control block.
3.01.3.3 State -- "NEW_NH_RESPONSE_AWAITED"
State: NEW_NH_RESPONSE_AWAITED
Event: Internal New NH
New State: NEW_NH_RETRY
Actions:
Restart the LSP retry timer, send `Internal destroy' to the control
block of the LSP for the new next hop and go to the `NEW_NH_RETRY'
state.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal retry timeout
New State: NEW_NH_RESPONSE_AWAITED
Actions:
Ignore. It is an internal implementation error.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal LSP UP
New State: IDLE
Actions:
Send event `Internal cross-connect' event to the LSP control block of
the new next hop.
Send event `Internal destroy' event to the original LSP control
block.
Then go to IDLE and delete the control block.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal LSP NAK
New State: IDLE
Actions:
Delete the control block.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal destroy
New State: IDLE
Actions:
Send event `Internal destroy' the control block for the new LSP
through the new next hop.
3.1.7 LDP Related Message Handling
- If an LSR receives an LDP-REQUEST from an upstream LSR:
a) If this is a duplicate request, discard the message. A duplicate
request means that there is a LSP Control Block which has same FEC,
same Upstream Label Request ID and same Upstream LDP Session
Identifier.
b) Otherwise, create a new LSP Control Block, store the relevant
information from the message into the control block, then pass the
event `LDP Request' to its state machine.
- If an LSR receives an LDP-MAPPING from a downstream LSR:
a) Extract the 'Label Request Message ID' field and from the LDP-
MAPPING.
b) Find an LSP Control Block which has the same Downstream Label
Request ID and the same Downstream LDP Session Identifier.
c) If an LSP Control Block is found, pass the event `LDP Mapping' to
its state machine.
d) If there is no matching LSP Control Block found, then try to find
an LSP Control Block which has the same Downstream Label and the same
Downstream LDP Session Identifier.
e) If an LSP Control Block is found, pass the event `LDP Mapping' to
its state machine.
f) Otherwise, ignore the LDP-MAPPING and send a LDP-RELEASE
downstream.
- If an LSR receives an LDP-RELEASE from an upstream LSR:
a) Find an LSP Control Block which has the same Upstream Label and
the same Upstream LDP Session Identifier.
b) If an LSP Control Block is found, pass the event `LDP Release' to
its state machine.
c) Otherwise, ignore the message.
- If an LSR receives an LDP-WITHDRAW from a downstream LSR:
a) Find an LSP Control Block which has the same Downstream Label and
the same Downstream LDP Session Identifier.
b) If an LSP Control Block is found, pass the event `LDP Withdraw' to
its state machine.
c) Otherwise, ignore the LDP-WITHDRAW and send a LDP-RELEASE
downstream.
- If an upstream LDP peer is lost:
a) Find all the LSP Control Blocks whose upstream LDP peer is that
LSR.
b) Then pass the event `Upstream Lost' to their state machines.
- If a downstream LDP peer is lost:
a) Find all the LSP Control Blocks whose downstream LDP peer is that
LSR.
b) Then pass the event `Downstream Lost' to their state machines.
- If the LSR detects a new next hop for an FEC:
For each LSP which needs "local repair", or it needs "global repair"
and it is the ingress of the LSP, pass event "Internal New NH" to its
state machine.
- If an LSR receives an LDP-Abort from an upstream LSR:
a) Extract the LDP Request ID value from the LDP-Abort message.
b) Find an LSP Control Block which has the same Upstream Label
Request ID and the same Upstream LDP Session Identifier.
c) If an LSP Control Block is found, pass the event `LDP Upstream
Abort' to its state machine.
d) Otherwise, ignore the message.
- If the LSR receives an LDP-NAK from a downstream LSR:
a) Extract the LDP Request ID value from the LDP-NAK.
b) Find an LSP Control Block which has the same Downstream Label
Request ID and the same Downstream LDP Session Identifier.
c) If an LSP Control Block is found, pass the event `LDP Downstream
NAK' to its state machine.
d) Otherwise, ignore the message.
3.2. ATM Switch LSR with VC-merge
3.2.1 VC-merge
A VC-merge capable LSR can map multiple incoming labels (VPI/VCI)
into one outgoing label. It is possible that this LSR can only merge
a limited number of incoming labels into a single outgoing label. As
described in [2], suppose, for example, that due to some hardware
limitation a node is capable of merging four incoming labels into a
single outgoing label. Suppose however, that this particular node has
six incoming labels arriving at it for a particular FEC. In this
case, this node may merge these into two outgoing labels.
When an upstream LSR has a limited merging capability, it is
difficult for a downstream LSR to know how many labels should be
assigned to each FEC. In this case, downstream-on-demand is
recommended.
3.2.2 Control Block
There are 3 types of control blocks involved: Upstream LSP Control
Block, Downstream LSP Control Block, and Next Hop Trigger Control
Block.
There is one Upstream LSP Control Block for each LDP-REQUEST
received.
There is one Downstream LSP Control Block for each unique LDP-REQUEST
sent to a downstream LSR. There can be multiple Downstream LSP
Control Blocks per FEC in an LSR. This can be the result of an
upstream LSR asking for a label for an FEC. This LSR must assign a
unique upstream label and it can not merge this upstream label into
an existing downstream label for this FEC.
There is one Next Hop Trigger Control Block for each FEC for which a
better next hop has been detected and the LSR has decided to switch
to this better next hop. It could be the result of "local repair" or
"global repair" if the LSR is the ingress LSR of the LSP.
A Downstream LSP Control Block contains a list of pointers to
Upstream LSP Control Blocks or the Next Hop Trigger Control Block.
This means that this LSR has decided to map the multiple labels
listed in the Upstream LSP Control Blocks and the Next Hop Trigger
Control Block into a single label listed in the Downstream LSP
Control Block.
An Upstream LSP Control Block may contain the following information:
- Upstream LDP Session Identifier
- State
- Upstream Label (assigned by this LSR)
- Downstream LSP Control Block pointer
- Upstream LDP Request ID (assigned by the upstream LSR in
downstream-on-demand mode)
- Next_Hop_Trigger_Block pointer
Upstream Label and Upstream LDP Session Identifier can be used to
locate a unique Upstream LSP Control Block.
If an LSR is using downstream-on-demand mode, it can use the Upstream
LDP Request ID and the Upstream LDP Session Identifier to locate a
unique Upstream LSP Control Block.
An Next_Hop_Trigger LSP Control Block may contain the following
information:
- Upstream LSP Control Block pointer, which points to the one
which is needed to switch over to the better next hop
- State
- Downstream LSP Control Block pointer
A Downstream LSP Control Block may contain the following information:
- FEC
- State
- Downstream LDP Session Identifier
- list of pointers to the Upstream LSP Control Blocks or the
Trigger_Control_Blocks which are merged at this LSR for this
FEC
- Downstream Label (assigned by the downstream LSR)
- Downstream Label Request ID (assigned by the LSR itself if
it is using downstream-on-demand mode)
Downstream Label, Downstream LDP Session Identifier can be used to
locate a unique Downstream LSP Control Block.
If an LSR is using downstream-on-demand mode, it can also use the
Downstream Label Request ID and the Downstream LDP Session Identifier
to locate a unique Downstream LSP Control Block.
The following diagram details the relationship between these 2 types
of control blocks:
For example, the LSR has decided to merge 3 LDP-REQUESTs of a FEC
from upstream LSR1, LSR2, LSR3 into one LDP-REQUEST and sent it to a
downstream LSR4.
+---------------------+
| |
| Upstream_LSP_Control|
| _Block for Upstream|------+
| LSR1 | |
+---------------------+ |
|
+---------------------+ |
| | |
| Upstream_LSP_Control| | (merged into)
| _Block for Upstream |------+
| LSR2 | |
+---------------------+ | +------------------------------+
| | |
+---------------------+ +--->| Downstream LSP Control Block |
| Next_Hop_Trigger_ | | | for Downstream LSR4 |
| LSP Control Block |------+ | |
| | +------------------------------+
+---------------------+
3.2.3 State Machines for Downstream-on-demand Mode
The following sections describe the state machines used in
downstream-on-demand mode.
3.2.3.1 State of the Upstream LSP Control Block's State Machine
for Downstream-on-demand Mode
-- IDLE
This is the initial LSP state.
-- RESPONSE_AWAITED
This state means that the LSR has received and processed an LDP-
REQUEST from an upstream LSR, and has sent a new LDP-REQUEST towards
a downstream LSR. The LSR is waiting for the LDP-MAPPING from the
downstream LSR.
-- ESTABLISHED
This state means that the LSR has received the LDP-MAPPING from the
downstream LSR and the LSP is up and operational.
-- RELEASE_AWAITED
This state means that the LSR has sent a LDP-WITHDRAW upstream and is
waiting for the LDP-RELEASE before freeing up the label resource.
3.2.3.2 Events of the Upstream LSP Control Block's State Machine
for Downstream-on-demand Mode
-- LDP Request
The LSR receives an LDP-REQUEST from an upstream LSR.
-- Internal Downstream Mapping
This event is sent by one Downstream LSP Control Block's state
machine. This Downstream LSP Control Block is the merged Downstream
LSP Control Block of this Upstream LSP Control Block. The event is
the result of receiving an LDP-MAPPING by the Downstream LSP Control
Block's state machine.
-- LDP Release
The LSR receives an LDP-RELEASE from an upstream LSR.
-- Internal Downstream Withdraw
This event is sent by one Downstream LSP Control Block's state
machine. This Downstream LSP Control Block is the merged Downstream
LSP Control Block of this Upstream LSP Control Block. The event is
the result of receiving an LDP-WITHDRAW by the Downstream LSP Control
Block's state machine.
-- LDP Upstream Abort
The LSR receives an LDP-ABORT from an upstream LSR.
-- Internal Downstream NAK
This event is sent by one Downstream LSP Control Block's state
machine. This Downstream LSP Control Block is the merged Downstream
LSP Control Block of this Upstream LSP Control Block. The event is
the result of receiving an LDP-NAK by the Downstream LSP Control
Block's state machine, or it detects an error.
-- Upstream Lost
The LSR loses the LDP session with its upstream LDP peer.
-- Internal New NH
The LSR detects there is better next hop and decides to establish the
lsp through this better next hop
-- Internal Re-Cross-Connect
This event is used to trigger splicing into a different downstream
LSP. This can happens when it is switched over to a better LSP
through the new next hop.
3.2.3.3 State Transitions of the Upstream LSP Control Block's State
Machine for Downstream-on-demand Mode
+-------------------+
| |
+-------->| IDLE |<-------------------+
| | | |
| +-------------------+ |
|(LDP Abort) | |
|(Internal |(LDP Request) |
| Downstream NAK) | |
|(Upstream Lost) | (Upstream Lost) |
| v (LDP Release) |
| +-------------------+ |
| | | |
+---------| RESPONSE_AWAITED | |
| | |
+-------------------+ |
| |
|(Internal Downstream |
| mapping) |
| |
v |
+-------------------+ |
| | |
| ESTABLISHED |------->------------+
| | |
+-------------------+ |
| |
| |
|(Internal Downstream Withdraw) |
|(Internal Downstream NAK) |
v |
+-------------------+ (LDP Upstream |
| | Abort) |
|RELEASE_AWAITED |------->------------+
| |
+-------------------+
3.2.3.4 Upstream LSP Control Block's State Machine
for Downstream-on-demand Mode
3.2.3.4.1 State -- "IDLE"
State: IDLE
Event: LDP Request
New State: Depends upon the action routine.
Actions:
If this LSR is the LSP Egress or Proxy Egress [2],
Then:
choose an upstream label, allocate the resources, connect this
upstream label to the local IP forwarding module, send an LDP-
MAPPING upstream with the upstream label and go to the state
`ESTABLISHED'.
else
Obtain a next hop (or interface). Find a Downstream LSP Control
Block which has the same FEC and the same next hop and also is able
to merge more input labels. If not found, create a new Downstream
LSP Control Block with the state `IDLE'.
If the state of the Downstream LSP Control Block is `ESTABLISHED',
choose an upstream label, connect the upstream label with the
downstream label and send an LDP-MAPPING upstream with the upstream
label, and go to the state `ESTABLISHED'.
If the state of Downstream LSP Control Block is not `ESTABLISHED',
set the state of the Upstream LSP Control Block to
`RESPONSE_AWAITED'. If the LSR use the independent control mode
[2], choose an upstream label, and send an LDP-MAPPING upstream.
Pass the event `Internal AddUpstream' to the Downstream LSP Control
Block's state machine.
If unable to process the request for any reason, issue an LDP-NAK to
the sender with the appropriate error code, go to IDLE and delete the
control block.
State: IDLE
Event: Internal Downstream Mapping
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Release
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Internal Downstream Withdraw
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Upstream Abort
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Internal Downstream NAK
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Upstream Lost
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Internal Re-Cross-Connect
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Internal New NH
New State: IDLE
Actions:
Ignore the event. It is an internal implementation error.
3.2.3.4.2 State -- "RESPONSE_AWAITED"
State: RESPONSE_AWAITED
Event: LDP Request
New State: RESPONSE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
State: RESPONSE_AWAITED
Event: Internal Downstream Mapping
New State: Depends on the action routine.
Actions:
If the LSR uses the ordered control mode, assign an upstream label,
connect the upstream label to the downstream label and allocate the
resources, send an LDP-MAPPING upstream with the upstream label and
go to `ESTABLISHED'.
If unable to process the message for any reason, issue an LDP-NAK
upstream and an LDP-RELEASE downstream, go to IDLE and delete the
control block.
State: RESPONSE_AWAITED
Event: LDP Release
New State: RESPONSE_AWAITED
Actions
Ignore the event. It is a protocol error from the upstream peer.
State: RESPONSE_AWAITED
Event: Internal Downstream Withdraw
New State: RESPONSE_AWAITED
Actions
Ignore the event. It is an internal implementation error.
State: RESPONSE_AWAITED
Event: LDP Upstream Abort
New State: IDLE
Actions
If the LSR uses the independent control mode, free the upstream label
and the resources.
Send the event `Internal DeleteUpstream' to its Downstream LSP
Control Block's state machine.
Delete the control block.
State: RESPONSE_AWAITED
Event: Internal Downstream NAK
New State: IDLE
Actions:
If the LSR uses the independent control mode, free the upstream label
and the resources. Then, send an LDP-WITHDRAW upstream.
If the LSR uses the ordered control mode, propagate the LDP-NAK
upstream.
Delete the control block.
State: RESPONSE_AWAITED
Event: Upstream Lost
New State: IDLE
Actions
If the LSR uses the independent control mode, free the upstream label
and the resources.
Send the event `Internal DeleteUpstream' to its Downstream LSP
Control Block's state machine.
Delete the control block.
State: RESPONSE_AWAITED
Event: Internal Re-Cross-Connect
New State: RESPONSE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
State: RESPONSE_AWAITED
Event: Internal New NH
New State: depends on the actions
Actions:
Send event `Internal DeleteUpstream' to its old downstream control
block.
Find a Downstream LSP Control Block which has the same FEC and the
same next hop and also is able to merge more input labels. If not
found, create a new Downstream LSP Control Block with the state
`IDLE'.
If the state of the Downstream LSP Control Block is `ESTABLISHED',
choose an upstream label, connect the upstream label with the
downstream label and send an LDP-MAPPING upstream with the upstream
label, and go to the state `ESTABLISHED'.
If the state of Downstream LSP Control Block is not `ESTABLISHED',
set the state of the Upstream LSP Control Block to
`RESPONSE_AWAITED'.
Pass the event `Internal AddUpstream' to the new Downstream LSP
Control Block's state machine.
3.2.3.4.3 State -- "ESTABLISHED"
State: ESTABLISHED
Event: LDP Request
New State: ESTABLISHED
Actions
Ignore the event. It is an internal implementation error.
State: ESTABLISHED
Event: Internal Downstream Mapping
New State: ESTABLISHED
Actions
Process the new attributes of the mapping and then propagate the
LDP-MAPPING upstream.
State: ESTABLISHED
Event: LDP Release
New State: IDLE
Actions
Disconnect the upstream label from the downstream label, free the
upstream label and resources.
Send the event `Internal DeleteUpstream' to its Downstream LSP
Control Block's state machine.
Send the event `Internal Destroy' to the Next_Hop_Trigger_Block's
state machine if the LSR was in the middle of switching over to the
better next hop.
Delete the control block.
State: ESTABLISHED
Event: Internal Downstream Withdraw
New State: Depends on the action routine.
Actions
If it uses independent mode, set its state to `IDLE' and create a
internal `LDP Request' and send to its own state machine.
Else
Disconnect the upstream label from the downstream label.
Propagate the LDP-WITHDRAW upstream and go to state
`RELEASE_AWAITED'.
Send the event `Internal Destroy' to the Next_Hop_Trigger_Block's
state machine if the LSR was in the middle of switching over to the
better next hop.
State: ESTABLISHED
Event: LDP Upstream Abort
New State: ESTABLISHED
Actions
Ignore the event.
Note: This scenario can occur if the upstream LSR sends a LDP-ABORT
at about the same time as the local LSR sends a LDP-MAPPING. In this
situation, it should be up to exactly one of the two LSRs as to
whether or not the label that was sent remains valid. The LDP
specification [4] procedures leave the choice to the upstream LSR
which must send an LDP-RELEASE if it will not use the label provided.
State: ESTABLISHED
Event: Internal Downstream NAK
New State: Depends on the action routine.
Actions:
If it uses independent mode, set its state to `IDLE' and create a
internal `LDP Request' and send to its own state machine.
Else
Disconnect the upstream label from the downstream label
Send an LDP-WITHDRAW upstream and go to state `RELEASE_AWAITED'.
Send the event `Internal Destroy' to the Next_Hop_Trigger_Block's
state machine if the LSR was in the middle of switching over to the
better next hop.
State: ESTABLISHED
Event: Upstream Lost
New State: IDLE
Actions:
Disconnect the upstream label from the downstream label, free the
upstream label and the resources.
Send the event `Internal DeleteUpstream' to its Downstream LSP
Control Block's state machine.
Send the event `Internal Destroy' to the Next_Hop_Trigger_Block's
state machine if the LSR was in the middle of switching over to the
better next hop.
Delete the control block.
State: ESTABLISH
Event: Internal Re-Cross-Connect
New State: ESTABLISH
Actions:
Reconnect the upstream label to the new downstream label.
Send the event `Internal DeleteUpstream' to its old Downstream LSP
Control Block's state machine.
State: ESTABLISH
Event: Internal New NH
New State: ESTABLISH
Actions:
Create a new Next_Hop_Trigger_Control_Block and pass event `Internal
New NH' to its state machine.
3.2.3.4.4 State -- "RELEASE_AWAITED"
State: RELEASE_AWAITED
Event: LDP Request
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is a protocol error from the upstream LSR.
State: RELEASE_AWAITED
Event: Internal Downstream Mapping
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
State: RELEASE_AWAITED
Event: LDP Release
New State: IDLE
Actions:
Free the upstream label resource and delete the control block.
State: RELEASE_AWAITED
Event: Internal Downstream Withdraw
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is a protocol error from the downstream LSR.
State: RELEASE_AWAITED
Event: LDP Upstream Abort
New State: IDLE
Actions:
Free the upstream label resource and delete the control block.
State: RELEASE_AWAITED
Event: Internal Downstream NAK
New State: RELEASE_AWAITED
Actions:
Ignore the event. And continue waiting for the LDP-RELEASE.
State: RELEASE_AWAITED
Event: Upstream Lost
New State: IDLE
Actions:
Free the upstream label resource and delete the control block.
State: RELEASE_AWAITED
Event: Internal New NH
New State: RELEASE_AWAITED
Actions:
Ignore the event. And continue waiting for the LDP-RELEASE.
State: RELEASE_AWAITED
Event: Internal Re-Cross-Connect
New State: RELEASE_AWAITED
Actions:
Ignore the event. It is an internal implementation error.
3.2.3.5 State of the Downstream LSP Control Block's State Machine
for Downstream-on-demand Mode
-- IDLE
This is the initial LSP state.
-- RESPONSE_AWAITED
This state means that the LSR has received an LDP-REQUEST from an
upstream LSR, has processed the LDP-REQUEST, and has sent a new LDP-
REQUEST towards a downstream LSR. The LSR is waiting for the LDP-
MAPPING from the downstream LSR.
-- ESTABLISHED
This state means that the LSR has received the LDP-MAPPING from the
downstream LSR and the LSP is up and operational.
3.2.3.6 Events of the Downstream LSP Control Block's State Machine
for Downstream-on-demand Mode
-- Internal AddUpstream
This event is sent by an Upstream LSP Control Block's state machine
when it is created.
-- Internal DeleteUpstream
This event is sent by an Upstream LSP Control Block's state machine
when it is deleted.
-- LDP Mapping
The LSR receives an LDP-MAPPING from a downstream LSR.
-- LDP Withdraw
The LSR receives an LDP-WITHDRAW from a downstream LSR.
-- LDP Downstream NAK
The LSR receives an LDP-NAK from a downstream LSR.
-- Downstream Lost
The LSR loses the LDP session with its downstream LSR.
3.2.3.7 State Transitions of the Downstream LSP Control Block's
State Machine for Downstream-on-demand mode
+-------------------+
| |
| IDLE |<--------------+
| | |(last Internal
+-------------------+ | DeleteUpstream)
| |(LDP Withdraw)
|(1st Internal AddUpstream)|
| |(LDP Downstream
v | NAK)
+-------------------+ |(Downstream
| | | Lost)
| RESPONSE_AWAITED |---------->----+
| | |
+-------------------+ |
| |
|(LDP Mapping) |
| |
v |
+-------------------+ |
| | |
| ESTABLISHED |-------->------+
| |
+-------------------+
3.2.3.8 Downstream LSP Control Block's State Machine for
Downstream-on-demand Mode.
3.2.3.8.1 State -- "IDLE"
State: IDLE
Event: Internal AddUpstream
New State: RESPONSE_AWAITED
Actions
Initialize the list of pointers in the Upstream LSP Control Block to
contain the newly added upstream pointer.
Send a new LDP-REQUEST downstream and go to the state
`RESPONSE_AWAITED'.
State: IDLE
Event: Internal DeleteUpstream
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Mapping
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Withdraw
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: LDP Downstream NAK
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Downstream Lost
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
3.2.3.8.2 State -- "RESPONSE_AWAITED"
State: RESPONSE_AWAITED
Event: Internal AddUpstream
New State: RESPONSE_AWAITED
Actions
Add the pointer to new Upstream LSP Control Block to the Upstream LSP
Control Blocks pointer list.
State: RESPONSE_AWAITED
Event: Internal DeleteUpstream
New State: Depend on the action routine
Actions
Delete the Upstream LSP Control Block pointer from the Upstream LSP
Control Block pointers list.
If the list becomes empty, release the resources, send an LDP-Abort
downstream, go to IDLE and then delete the control block.
State: RESPONSE_AWAITED
Event: LDP Mapping
New State: ESTABLISHED
Actions
For each Upstream LSP Control Block in the Upstream LSP Control Block
pointers list, pass the event `Internal Downstream Mapping' to its
state machine.
State: RESPONSE_AWAITED
Event: LDP Withdraw
New State: RESPONSE_AWAITED
Actions
It is a protocol error from the downstream LDP peer; send a LDP-
RELEASE downstream
State: RESPONSE_AWAITED
Event: LDP Downstream NAK
New State: IDLE
Actions
For each Upstream LSP Control Block in the Upstream LSP Control Block
pointers list, pass the event `Internal Downstream NAK' to its state
machine.
Release the resources, and delete the control block.
State: RESPONSE_AWAITED
Event: Downstream Lost
New State: IDLE
Actions
For each Upstream LSP Control Block in the Upstream LSP Control Block
pointers list, pass the event `Internal Downstream NAK' to its state
machine.
Release the resources, and delete the control block.
3.2.3.8.3 State -- "ESTABLISHED"
State: ESTABLISHED
Event: Internal AddUpstream
New State: ESTABLISHED
Actions
Add the pointer to new Upstream LSP Control Block to the Upstream LSP
Control Block pointers list.
State: ESTABLISHED
Event: Internal DeleteUpstream
New State: Depends on the action routine.
Actions
Delete the pointer of Upstream LSP Control Block from its Upstream
LSP Control Block pointers list.
If the list becomes empty, release the resources, send an LDP-RELEASE
downstream, go to IDLE and then delete the control block.
Otherwise, remain in the ESTABLISHED state.
State: ESTABLISHED
Event: LDP Mapping
New State: ESTABLISHED
Actions
For each Upstream LSP Control Block in the Upstream LSP Control Block
pointers list, pass the event `Internal Downstream mapping' to its
state machine.
State: ESTABLISHED
Event: LDP Withdraw
New State: IDLE
Actions
For each Upstream LSP Control Block in the Upstream LSP Control Block
pointers list, pass the event `Internal Downstream withdraw' to its
state machine.
Release the resources, and delete the control block and send LDP-
RELEASE downstream.
State: ESTABLISHED
Event: LDP Downstream NAK
New State: ESTABLISHED
Actions
It is a protocol error from the downstream LDP peer.
3.2.3.9 State of the Next_Hop_Trigger_Control_Block's State Machine
for Downstream-on-demand Mode
-- IDLE
This is the initial LSP state.
-- NEW_NH_RETRY
This is the state where an LSR waits for a retry timer to expire and
then tries to establish an LSP through a new next hop.
-- NEW_NH_RESPONSE_AWAITED
This state means that the LSR has sent a new LDP-REQUEST towards a
downstream LSR. The LSR is waiting for the LDP-MAPPING from the
downstream LSR.
3.2.3.10 Events of the Next_Hop_Trigger_Control_Block's State Machine
for Downstream-on-demand Mode
-- Internal New NH
Trigger to setup an LSP through a better next hop.
-- Internal Downstream Mapping
This event is sent by one Downstream LSP Control Block's state
machine. This Downstream LSP Control Block is the merged Downstream
LSP Control Block of this Upstream LSP Control Block. The event is
the result of receiving an LDP-MAPPING by the Downstream LSP Control
Block's state machine.
-- Internal Downstream NAK
This event is sent by one Downstream LSP Control Block's state
machine. This Downstream LSP Control Block is the merged Downstream
LSP Control Block of this Upstream LSP Control Block. The event is
the result of receiving an LDP-NAK by the Downstream LSP Control
Block's state machine, or it detects an error.
-- Internal Destroy This event is used to stop the procedure of
switching over to the better next hop.
3.2.3.11 State Transitions of the Next_Hop_Trigger_Control_Block's State
Machine for Downstream-on-demand Mode
+---------------------+
| |
| IDLE |<------------+
| | |
+---------------------+ |
| |
| |
| (Internal New NH) |
| |
v |
+---------------------+ |
| | |
| NEW_NH_RETRY |----------->-+
| | (Internal |
+---------------------+ Destroy) |
| |
| |
| (Internal retry timeout) |
| |
v |
+---------------------+ |
| | (Internal |
| NEW_NH_RESPONSE | Destroy) |
| _AWAITED |----------->-+
| | |
+---------------------+ |
| |
| (Internal Downstream |
| Mapping |
| (Internal Downstream |
| NAK) |
+------------------------>-+
3.2.3.12 State Machine
3.2.3.12.1 State -- "IDLE"
State: IDLE
Event: Internal New NH
New State: NEW_NH_RETRY
Actions:
Start the LSP retry timer and go to the `NEW_NH_RETRY' state.
State: IDLE
Event: Internal retry timeout
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal Downstream Mapping
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal Downstream NAK
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
State: IDLE
Event: Internal destroy
New State: IDLE
Actions:
Ignore. It is an internal implementation error.
3.2.3.12.2 State -- "NEW_NH_RETRY"
State: NEW_NH_RETRY
Event: Internal New NH
New State: NEW_NH_RETRY
Actions:
Restart the LSP retry timer.
State: NEW_NH_RETRY
Event: Internal retry timeout
New State: Depends on the action routine.
Actions:
If the new next hop is the same one as the old next hop, go to IDLE
and delete the control block.
Otherwise, go to NEW_NH_RESPONSE_AWAITED, find an downstream LSP
control block which go through the same next hop for the same FEC, if
there is no one, create one, and pass `Internal AddUpstream' event to
its state machine.
State: NEW_NH_RETRY
Event: Internal Downstream Mapping
New State: NEW_NH_RETRY
Actions:
Ignore. It is an internal implementation error.
State: NEW_NH_RETRY
Event: Internal Downstream NAK
New State: NEW_NH_RETRY
Actions:
Ignore. It is an internal implementation error.
State: NEW_NH_RETRY
Event: Internal destroy
New State: IDLE
Actions:
Stop the timer and delete the control block.
3.2.3.12.3 State -- "NEW_NH_RESPONSE_AWAITED"
State: NEW_NH_RESPONSE_AWAITED
Event: Internal New NH
New State: NEW_NH_RETRY
Actions:
Restart the LSP retry timer and send event `Internal destroy' to the
control block of the LSP for the new next hop.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal retry timeout
New State: NEW_NH_RESPONSE_AWAITED
Actions:
Ignore. It is an internal implementation error.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal Downstream Mapping
New State: IDLE
Actions:
Send event `Internal Re-cross-connect' event to the upstream LSP
control block of the new next hop.
Send event `DeleteUpstream' event to the downstream LSP control block
of the the new next hop, since the upstream has spliced into the new
next hop.
Delete the control block.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal Downstream NAK
New State: IDLE
Actions:
Delete the control block.
State: NEW_NH_RESPONSE_AWAITED
Event: Internal destroy
New State: IDLE
Actions:
Send event `Internal DeleteUpstream' the control block for the new
LSP through the new next hop.
3.2.4 LDP Related Message Processing
- If an LSR receives an LDP-REQUEST:
a) If this is a duplicate request, discard the message. A duplicate
request means that there is a LSP Control Block which has same FEC,
same Upstream Label Request ID and same Upstream LDP Session
Identifier.
b) Otherwise, create a new Upstream LSP Control Block. Then pass the
event `LDP Request' to this Upstream LSP Control Block's state
machine.
- If an LSR receives an LDP-MAPPING:
Locate a Downstream LSP Control Block which has the same FEC, the
same Downstream LDP Session Identifier and the same Downstream Label.
If a Downstream LSP Control Block is found, pass the event `LDP
Mapping' to its state table. This could mean that the attributes of
label binding have changed.
Otherwise, use the Downstream LDP request ID (the 'Label Request
Message ID' field in the LDP-MAPPING) and Downstream LDP Session
Identifier to locate the Downstream LSP Control Block and pass the
event `LDP Mapping' to its state machine. If no Downstream LSP
Control Block is found, ignore the message.
- If an LSR receives an LDP-RELEASE:
Locate an Upstream LSP Control Block which has the same FEC, the same
Upstream Label, the same Upstream LDP Session Identifier. If no
Upstream LSP Control Block is found, ignore the message. If an
Upstream LSP Control Block is found, send the event `LDP Release' to
its state machine.
- If an LSR receives an LDP-WITHDRAW:
Find a Downstream LSP Control Block which has the same FEC, the same
Downstream LDP Session Identifier and the same Downstream Label.
Pass the event `LDP Withdraw' to its state machines.
- If an Upstream LDP peer is lost:
Pass the event `Upstream Lost' to the state machines of all the
Upstream LSP Control Blocks whose upstream LDP peer is that LSR.
- If a Downstream LDP peer is lost:
Pass the event `Downstream Lost' to the state machines of all the
Downstream LSP Control Blocks whose the downstream LDP peer is that
LSR.
- If a next hop of an FEC is changed:
For all the Upstream LSP Control Blocks which are infected by this
change, pass the event `Internal New NH' to their state machines.
- If an LSR receives an LDP-ABORT from an upstream LSR:
Use the Upstream LDP Request ID and Upstream LDP Session Identifier
to locate the Upstream LSP Control Block and pass the event `LDP
Abort' to its state machine.
- If an LSR receives an LDP-NAK from a downstream LSR:
Use the Downstream LDP Request ID and Downstream Session Identifier
to locate a Downstream_LSP_control_block and pass the event `LDP
Downstream NAK' to its state machine.
4. State Machine for Downstream Unsolicited
The following sections describe the state machines for the ATM-LSR
which uses downstream unsolicited mode.
While both independent LSP control and ordered LSP control modes are
possible, only the ordered mode is taken into account, because the
independent LSP control mode uses the liberal label retention mode
and so is considered burning too many ATM resources.
In downstream unsolicited mode, multiple path is not supported in
this version and will be For Further Study (FFS). We suspect with
multiple next hops and Downstream mode, it is easy to get into a loop
condition.
4.0 Control Block
There are 2 types of control blocks involved: Upstream LSP Control
Block, Downstream LSP Control Block.
There is a list of Upstream LSP Control Blocks for each FEC in the
routing table, with each one corresponding to a LDP peer. A Upstream
LSP Control Block is created for each FEC when there is a label ready
to be distributed to that upstream. It is deleted when the FEC is
deleted from the FEC table, or the LDP peer disappears, or the
downstream label is withdrawed.
There is one Downstream LSP Control Blocks for each FEC in the
routing table. It is created when the FEC is inserted into the
forwarding table and deleted when the FEC is removed from the
forwarding table.
An Upstream LSP Control Block may contain the following information:
- Upstream LDP Session Identifier
- State
- Upstream Label (assigned by this LSR)
- FEC
Upstream Label and Upstream LDP Session Identifier, or FEC and
Upstream LDP Session Identifier can be used to locate a unique
Upstream LSP Control Block.
A Downstream LSP Control Block may contain the following information:
- FEC
- State
- Downstream LDP Session Identifier
- Downstream Label (assigned by the downstream LSR)
- Downstream Label Request ID (assigned by the LSR itself)
Downstream Label and Downstream LDP Session Identifier, or FEC and
Downstream LDP Session Identifier can be used to locate a unique
Downstream LSP Control Block.
4.1 States of the Upstream LSP Control Block's State Machine
for Downstream Mode
-- IDLE
This is the initial LSP state.
-- ESTABLISHED
This state means that the LSR has received the LDP-MAPPING from the
downstream LSR and the LSP is up and operational.
-- RELEASE_AWAITED
This state means that the LSR is waiting for the LDP-RELEASE in
respond to the LDP-WITHDRAW sent by this LSR.
-- RESOURCES_AWAITED
This state means that the LSR is waiting for the label resources.
4.2 Events of the Upstream LSP Control Block's State Machine
for Downstream Mode
-- Internal Downstream Mapping
This event is sent by one Downstream LSP Control Block's state
machine. The event is the result of receiving an LDP-MAPPING by the
Downstream LSP Control Block's state machine. Or when the LDP peer is
discovered and there is a downstream Label available for this FEC.
-- LDP Release
The LSR receives an LDP-RELEASE from an upstream LSR.
-- Internal Withdraw
This event is sent by Downstream LSP Control Block's state machine.
The event is the result of receiving an LDP-WITHDRAW by the
Downstream LSP Control Block's state machine.
-- Resource Available
This event means the local resource (such as label) becomes
available.
-- Delete FEC
This event means that either the FEC is removed from the forwarding
table.
-- Upstream Lost
This event means that the upstream LDP peer is lost.
4.3 State Transitions of Upstream LSP Control Block's State
Machine for Downstream Mode
|
|(created when
|a label is to be distributed
| to the LDP peer)
v
+-------------------+
| |
| IDLE |<--------------+
| | |
+-------------------+ |
| |(LDP Release)
| |
| |
| |
|(Internal Downstream |
+-------------------| Mapping) |
| | |
|(no label resource)v |
| +-------------------+ |
| | | |
| +-----| ESTABLISHED |---------------+
| | | | ^
| | +-------------------+ |
| |(delete FEC) ^ |
| |(Internal |(Resource Available) | (LDP Release)
| | Withdraw) | | (Internal
| | | | Downstream
| | | | Withdraw)
| | +-------------------+ |
+--------->| | |
| |RESOURCES_AWAITED |---------------+
| | | |
| +-------------------+ |
| |
| (Internal Downstream Withdraw) |(LDP Release)
| +-------------------+ |
| | | |
+---->| RELEASE_AWAITED |---------------+
| |
+-------------------+
4.4 Upstream LSP Control Block's State Machine for
Downstream Mode
4.4.1 : State -- "IDLE"
State: IDLE
Event: Internal Downstream mapping
New State: Depends on the action routine.
Actions
Choose an upstream label, connect the upstream label with the
downstream label, propagate the LDP-MAPPING upstream and go to state
`ESTABLISHED'
If there is no resource for the upstream label, go to state
`RESOURCE_AWAITED'.
State: IDLE
Event: LDP Release
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Internal Downstream Withdraw
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Resource Available
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Delete FEC
New State: IDLE
Actions
Delete the control block.
State: IDLE
Event: Upstream Lost
New State: IDLE
Actions
Delete the control block.
4.4.2 : State -- "ESTABLISHED"
State: ESTABLISHED
Event: Internal Downstream Mapping
New State: ESTABLISHED
Actions
Process the new attributes of the new mapping message.
Propagate the LDP-MAPPING upstream.
State: ESTABLISHED
Event: LDP Release
New State: IDLE
Actions
Disconnect upstream label from downstream label.
Release the upstream label resource
Delete the control block.
State: ESTABLISHED
Event: Internal Downstream Withdraw
New State: RELEASE_AWAITED
Actions
Disconnect upstream label from downstream label.
Propagate the LDP-WITHDRAW upstream.
State: ESTABLISHED
Event: Resource Available
New State: ESTABLISHED
Actions
Ignore the event. It is an internal implementation error.
State: ESTABLISHED
Event: Delete FEC
New State: RELEASE_AWAITED
Actions
Send a LDP-WITHDRAW upstream.
State: ESTABLISHED
Event: Upstream Lost
New State: IDLE
Actions
Release the upstream label and delete the control block.
4.4.2 : State -- "RELEASE_AWAITED"
State: RELEASE_AWAITED
Event: Internal Downstream Mapping
New State: RELEASE_AWAITED
Actions
Ignore the message.
State: RELEASE_AWAITED
Event: LDP Release
New State: IDLE
Actions
Release the upstream label and delete the control block.
State: RELEASE_AWAITED
Event: Internal Downstream Withdraw
New State: RELEASE_AWAITED
Actions
Ignore the event.
State: RELEASE_AWAITED
Event: Resource Available
New State: RELEASE_AWAITED
Actions
Ignore the event. It is an internal implementation error.
State: RELEASE_AWAITED
Event: Delete FEC
New State: RELEASE_AWAITED
Actions
Do nothing.
State: RELEASE_AWAITED
Event: Upstream Lost
New State: IDLE
Actions
Release the upstream label and delete the control block.
4.4.2 : State -- "RESOURCE_AWAITED"
State: RESOURCE_AWAITED
Event: Internal Downstream Mapping
New State: RESOURCE_AWAITED
Actions
Ignore the message.
State: RESOURCE_AWAITED
Event: LDP Release
New State: RESOURCE_AWAITED
Actions
Ignore the message. It is an internal implementation error.
State: RESOURCE_AWAITED
Event: Internal Downstream Withdraw
New State: IDLE
Actions
Delete the control block.
State: RESOURCE_AWAITED
Event: Resource Available
New State: ESTABLISHED
Actions
Allocate an upstream label, connect the upstream label with the
downstream label, and send LDP-MAPPING upstream.
State: RESOURCE_AWAITED
Event: Delete FEC
New State: IDLE
Actions
Delete the control block.
State: RESOURCE_AWAITED
Event: Upstream Lost
New State: IDLE
Actions
Delete the control block.
4.5 State of the Downstream LSP Control Block's State Machine
for Downstream Mode
-- IDLE
This is the initial LSP state.
-- ESTABLISHED
This state means that the LSR has received the LDP-MAPPING from the
downstream LSR.
3.2.4.6 Events of the Downstream LSP Control Block's State Machine
for Downstream Mode
-- LDP Mapping
The LSR receives an LDP-MAPPING from a downstream LSR.
-- LDP Withdraw
The LSR receives an LDP-WITHDRAW from a downstream LSR.
-- Delete FEC
The FEC is deleted from the forwarding table.
-- Next Hop Change
The next hop for this FEC is change to different LSR.
-- Downstream Lost
The downstream peer is gone.
4.7 State Transitions of Downstream LSP Control Block's State
Machine for Downstream Mode
|
|(FEC is being added into the forwarding table)
v
+-------------------+
| |
| IDLE |<--------------+
| | |
+-------------------+ |
| |
| |(LDP Withdraw)
| |(Internal New NH)
| |(Downstream Lost)
| (LDP Mapping) |
| |
v |
+-------------------+ |
| | |
| ESTABLISHED |---------------+
| |
+-------------------+
|
|(FEC is deleted from the forwarding table)
v
4.8 Downstream LSP Control Block's State Machine
for Downstream Mode
4.8.1 : State -- "IDLE"
State: IDLE
Event: LDP mapping
New State: ESTABLISHED
Actions
For all the LDP peers except the downstream LSR which assigned the
label, create an Upstream LSP Control Block, and pass the event
`Internal Downstream Mapping' to each of the Upstream LSP Control
Block's state machines.
State: IDLE
Event: LDP withdraw
New State: IDLE
Actions
Ignore the event. It is an internal implementation error.
State: IDLE
Event: Delete FEC
New State: IDLE
Actions
Delete the control block.
State: IDLE
Event: Next Hop Change
New State: IDLE
Actions
Ignore the event.
State: IDLE
Event: Downstream Lost
New State: IDLE
Actions
Ignore the event.
4.8.1 : State -- "ESTABLISHED"
State: ESTABLISHED
Event: LDP mapping
New State: ESTABLISHED
Actions
For each Upstream_LSP_control_block of this FEC, pass event `Internal
downstream mapping' to its state machine.
State: ESTABLISHED
Event: LDP withdraw
New State: IDLE
Actions
For each Upstream_LSP_control_block for this FEC, pass event
`Internal downstream Withdraw' to its state machine.
Send a LDP Withdraw downstream.
State: ESTABLISHED
Event: Delete FEC
New State: IDLE
Actions
Send LDP-RELEASE downstream and delete the control block.
State: ESTABLISHED
Event: Next Hop Change
New State: IDLE
Actions
For each Upstream_LSP_control_block for this FEC, pass event
`Internal downstream Withdraw' to its state machine.
Send LDP-REQUEST to the new next hop.
State: ESTABLISHED
Event: Downstream Lost
New State: IDLE
Actions
Send LDP-WITHDRAW to all Upstream_Control_Block's state machine of
this FEC.
4.5 LDP Related Message Processing for downstream mode.
- If an LSR receives an LDP-REQUEST:
If there is a next hop for this FEC and there is a
Downstream_Control_Block for this FEC whose state is `ESTABLISHED',
create a new Upstream_Control_Block and pass `internal Mapping' event
to its state machine.
- If an LSR receives an LDP-MAPPING:
Locate a Downstream LSP Control Block which has the same FEC, the
same Downstream LDP Session Identifier and the same Downstream Label.
If a Downstream LSP Control Block is found, pass the event `LDP
Mapping' to its state table. This could mean that the attributes of
label binding have changed.
Otherwise, if there is no matching Downstream LSP Control Block
found, find a Downstream LSP Control Block of this FEC and its next
hop is the this downstream peer, pass the event `LDP Mapping' to its
state machine.
- If an LSR receives an LDP-RELEASE:
Locate an Upstream LSP Control Block which has the same FEC, the same
Upstream Label, the same Upstream LDP Session Identifier. If no
Upstream LSP Control Block is found, ignore the message. If an
Upstream LSP Control Block is found, send the event `LDP Release' to
its state machine.
- If an LSR receives an LDP-WITHDRAW:
Find a Downstream LSP Control Block which has the same FEC, the same
Downstream LDP Session Identifier and the same Downstream Label.
Pass the event `LDP Withdraw' to its state machines.
- If an Upstream LDP peer is lost:
Pass the event `Upstream Lost' to the state machines of all the
Upstream LSP Control Blocks whose upstream LDP peer is that LSR.
- If a Downstream LDP peer is lost:
Pass the event `Label Withdraw' to the state machines of all the
Downstream LSP Control Blocks whose the downstream LDP peer is that
LSR.
- If a next hop of an FEC is changed:
Find all the Downstream LSP Control Blocks which has the same FEC and
the same next hop and pass the event `Next Hop Change' to their state
machine
- If there is a FEC being added to the forwarding table
Create a new Downstream LSP Control Block with state `IDLE'
- If the FEC is deleted from the forwarding table
Send the `Delete FEC' event to the its control block.
- If an LSR receives an LDP-NAK from an upstream LSR:
Ignore the message. An LDP-NAK should never appear in the
downstream-mode LSR
- If an LSR receives an LDP-NAK from a downstream LSR:
Ignore the message. It is a protocol error from the downstream LSR.
5. Security Considerations
This document is provided as an informational extension of the LDP
specification [4]. State machines presented here are intended to
clarify procedures defined in the LDP specification, but do not
supplant or override definitions and procedures provided there.
Implementations of a state machine may be vulnerable to spurious
events generated by an external source. In this document, events fall
in two categories: internal events and external events caused by
receipt of an LDP message.
LDP messages may be protected using mechanisms described in the LDP
specification. See "Security Considerations" in the LDP specification
[4].
Security considerations relating to generation of spurious internal
events are not addressed in this document.
6. Acknowledgements
The authors would like to acknowledge the helpful comments and
suggestions of the following people: Bob Thomas, Myunghee Son and
Adrian Farrel.
7. Authors' Address
Christophe Boscher
Alcatel
Le Mail
44700 Orvault
France
Phone: (33) 251781828
Email: christophe.boscher@alcatel.fr
Pierrick Cheval
Alcatel
5 rue Noel-Pons
92734 Nanterre Cedex
France
Phone: (33) 146524027
Email: pierrick.cheval@alcatel.fr
Eric Gray
Lucent Technologies, Inc.
P.O. Box 710
Durham, NH 03824
Phone: (603) 659-3386
Email: ewgray@lucent.com
Liwen Wu
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
U.S.A
Phone: 978-244-3087
Email:liwwu@cisco.com
8. References This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
1."MPLS Using LDP and ATM Switching", Bruce Davie, Jeremy Lawrence, This announcement is sent to the IETF list and the RFC-DIST list.
Keith McCloghrie, Yakov Rekhter, Eric Rosen, George Swallow, Paul Requests to be added to or deleted from the IETF distribution list
Doolan, work in progress, Internet Draft, <draft-ietf-mpls-atm-02.txt> should be sent to IETF-REQUEST@IETF.ORG. Requests to be
added to or deleted from the RFC-DIST distribution list should
be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
2."Multiprotocol Label Switching Architecture", Eric C Rosen, Arun Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
Viswanathan, Ross Callon, work in progress, Internet Draft, <draft- an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
ietf-mpls-arch-06.txt> help: ways_to_get_rfcs. For example:
3."Definition of Managed Objects for the Multiprotocol Label Switching, To: rfc-info@RFC-EDITOR.ORG
Label Distribution Protocol (LDP)", Joan Cucchiara, Hans Sjostrand, Subject: getting rfcs
James V. Luciani, work in progress, Internet Draft, <draft-ietf-mpls-
ldp-mib-03.txt>
4. "LDP Specification", Loa Andersson, Paul Doolan, Nancy Feldman, help: ways_to_get_rfcs
Andre Fredette, Bob Thomas, work in progress, Internet Draft, <draft-
ietf-mpls-ldp-06.txt>
5. "Constraint-Based LSP Set up Using LDP", Bilel Jamoussi, et al., Requests for special distribution should be addressed to either the
work in progress, Internet Draft, <draft-ietf-mpls-cr-ldp-03.txt> author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
specifically noted otherwise on the RFC itself, all RFCs are for
unlimited distribution.echo
Submissions for Requests for Comments should be sent to
RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
Authors, for further information.
 End of changes. 

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