8. BGP Finite State Machine (FSM)
- BGP Finite State Machine (FSM)
The data structures and FSM described in this document are conceptual and do not have to be implemented precisely as described here, as long as the implementations support the described functionality and they exhibit the same externally visible behavior.
This section specifies the BGP operation in terms of a Finite State Machine (FSM). The section falls into two parts:
1) Description of Events for the State machine (Section 8.1)
2) Description of the FSM (Section 8.2)
Session attributes required (mandatory) for each connection are:
1) State
2) ConnectRetryCounter
3) ConnectRetryTimer
4) ConnectRetryTime
5) HoldTimer
6) HoldTime
7) KeepaliveTimer
8) KeepaliveTime
The state session attribute indicates the current state of the BGP FSM. The ConnectRetryCounter indicates the number of times a BGP peer has tried to establish a peer session.
The mandatory attributes related to timers are described in Section 10. Each timer has a "timer" and a "time" (the initial value).
The optional Session attributes are listed below. These optional attributes may be supported, either per connection or per local system:
1) AcceptConnectionsUnconfiguredPeers
2) AllowAutomaticStart
3) AllowAutomaticStop
4) CollisionDetectEstablishedState
5) DampPeerOscillations
6) DelayOpen
7) DelayOpenTime
8) DelayOpenTimer
9) IdleHoldTime
10) IdleHoldTimer
11) PassiveTcpEstablishment
12) SendNOTIFICATIONwithoutOPEN
13) TrackTcpState
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The optional session attributes support different features of the BGP functionality that have implications for the BGP FSM state transitions. Two groups of the attributes which relate to timers are:
group 1: DelayOpen, DelayOpenTime, DelayOpenTimer
group 2: DampPeerOscillations, IdleHoldTime, IdleHoldTimer
The first parameter (DelayOpen, DampPeerOscillations) is an optional attribute that indicates that the Timer function is active. The "Time" value specifies the initial value for the "Timer" (DelayOpenTime, IdleHoldTime). The "Timer" specifies the actual timer.
Please refer to Section 8.1.1 for an explanation of the interaction between these optional attributes and the events signaled to the state machine. Section 8.2.1.3 also provides a short overview of the different types of optional attributes (flags or timers).
8.1. Events for the BGP FSM
8.1.1. Optional Events Linked to Optional Session Attributes
The Inputs to the BGP FSM are events. Events can either be mandatory or optional. Some optional events are linked to optional session attributes. Optional session attributes enable several groups of FSM functionality.
The linkage between FSM functionality, events, and the optional session attributes are described below.
Group 1: Automatic Administrative Events (Start/Stop)
Optional Session Attributes: AllowAutomaticStart,
AllowAutomaticStop,
DampPeerOscillations,
IdleHoldTime, IdleHoldTimer
Option 1: AllowAutomaticStart
Description: A BGP peer connection can be started and stopped
by administrative control. This administrative
control can either be manual, based on operator
intervention, or under the control of logic that
is specific to a BGP implementation. The term
"automatic" refers to a start being issued to the
BGP peer connection FSM when such logic determines
that the BGP peer connection should be restarted.
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The AllowAutomaticStart attribute specifies that
this BGP connection supports automatic starting of
the BGP connection.
If the BGP implementation supports
AllowAutomaticStart, the peer may be repeatedly
restarted. Three other options control the rate
at which the automatic restart occurs:
DampPeerOscillations, IdleHoldTime, and the
IdleHoldTimer.
The DampPeerOscillations option specifies that the
implementation engages additional logic to damp
the oscillations of BGP peers in the face of
sequences of automatic start and automatic stop.
IdleHoldTime specifies the length of time the BGP
peer is held in the Idle state prior to allowing
the next automatic restart. The IdleHoldTimer is
the timer that holds the peer in Idle state.
An example of DampPeerOscillations logic is an
increase of the IdleHoldTime value if a BGP peer
oscillates connectivity (connected/disconnected)
repeatedly within a time period. To engage this
logic, a peer could connect and disconnect 10
times within 5 minutes. The IdleHoldTime value
would be reset from 0 to 120 seconds.
Values: TRUE or FALSE
Option 2: AllowAutomaticStop
Description: This BGP peer session optional attribute indicates
that the BGP connection allows "automatic"
stopping of the BGP connection. An "automatic"
stop is defined as a stop under the control of
implementation-specific logic. The
implementation-specific logic is outside the scope
of this specification.
Values: TRUE or FALSE
Option 3: DampPeerOscillations
Description: The DampPeerOscillations optional session
attribute indicates that the BGP connection is
using logic that damps BGP peer oscillations in
the Idle State.
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Value: TRUE or FALSE
Option 4: IdleHoldTime
Description: The IdleHoldTime is the value that is set in the
IdleHoldTimer.
Values: Time in seconds
Option 5: IdleHoldTimer
Description: The IdleHoldTimer aids in controlling BGP peer
oscillation. The IdleHoldTimer is used to keep
the BGP peer in Idle for a particular duration.
The IdleHoldTimer_Expires event is described in
Section 8.1.3.
Values: Time in seconds
Group 2: Unconfigured Peers
Optional Session Attributes: AcceptConnectionsUnconfiguredPeers
Option 1: AcceptConnectionsUnconfiguredPeers
Description: The BGP FSM optionally allows the acceptance of
BGP peer connections from neighbors that are not
pre-configured. The
"AcceptConnectionsUnconfiguredPeers" optional
session attribute allows the FSM to support the
state transitions that allow the implementation to
accept or reject these unconfigured peers.
The AcceptConnectionsUnconfiguredPeers has
security implications. Please refer to the BGP
Vulnerabilities document [RFC4272] for details.
Value: True or False
Group 3: TCP processing
Optional Session Attributes: PassiveTcpEstablishment,
TrackTcpState
Option 1: PassiveTcpEstablishment
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Description: This option indicates that the BGP FSM will
passively wait for the remote BGP peer to
establish the BGP TCP connection.
value: TRUE or FALSE
Option 2: TrackTcpState
Description: The BGP FSM normally tracks the end result of a
TCP connection attempt rather than individual TCP
messages. Optionally, the BGP FSM can support
additional interaction with the TCP connection
negotiation. The interaction with the TCP events
may increase the amount of logging the BGP peer
connection requires and the number of BGP FSM
changes.
Value: TRUE or FALSE
Group 4: BGP Message Processing
Optional Session Attributes: DelayOpen, DelayOpenTime,
DelayOpenTimer,
SendNOTIFICATIONwithoutOPEN,
CollisionDetectEstablishedState
Option 1: DelayOpen
Description: The DelayOpen optional session attribute allows
implementations to be configured to delay sending
an OPEN message for a specific time period
(DelayOpenTime). The delay allows the remote BGP
Peer time to send the first OPEN message.
Value: TRUE or FALSE
Option 2: DelayOpenTime
Description: The DelayOpenTime is the initial value set in the
DelayOpenTimer.
Value: Time in seconds
Option 3: DelayOpenTimer
Description: The DelayOpenTimer optional session attribute is
used to delay the sending of an OPEN message on a
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connection. The DelayOpenTimer_Expires event
(Event 12) is described in Section 8.1.3.
Value: Time in seconds
Option 4: SendNOTIFICATIONwithoutOPEN
Description: The SendNOTIFICATIONwithoutOPEN allows a peer to
send a NOTIFICATION without first sending an OPEN
message. Without this optional session attribute,
the BGP connection assumes that an OPEN message
must be sent by a peer prior to the peer sending a
NOTIFICATION message.
Value: True or False
Option 5: CollisionDetectEstablishedState
Description: Normally, a Detect Collision (see Section 6.8)
will be ignored in the Established state. This
optional session attribute indicates that this BGP
connection processes collisions in the Established
state.
Value: True or False
Note: The optional session attributes clarify the BGP FSM
description for existing features of BGP implementations.
The optional session attributes may be pre-defined for an
implementation and not readable via management interfaces
for existing correct implementations. As newer BGP MIBs
(version 2 and beyond) are supported, these fields will be
accessible via a management interface.
8.1.2. Administrative Events
An administrative event is an event in which the operator interface and BGP Policy engine signal the BGP-finite state machine to start or stop the BGP state machine. The basic start and stop indications are augmented by optional connection attributes that signal a certain type of start or stop mechanism to the BGP FSM. An example of this combination is Event 5, AutomaticStart_with_PassiveTcpEstablishment. With this event, the BGP implementation signals to the BGP FSM that the implementation is using an Automatic Start with the option to use a Passive TCP Establishment. The Passive TCP establishment signals that this BGP FSM will wait for the remote side to start the TCP establishment.
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Note that only Event 1 (ManualStart) and Event 2 (ManualStop) are mandatory administrative events. All other administrative events are optional (Events 3-8). Each event below has a name, definition, status (mandatory or optional), and the optional session attributes that SHOULD be set at each stage. When generating Event 1 through Event 8 for the BGP FSM, the conditions specified in the "Optional Attribute Status" section are verified. If any of these conditions are not satisfied, then the local system should log an FSM error.
The settings of optional session attributes may be implicit in some implementations, and therefore may not be set explicitly by an external operator action. Section 8.2.1.5 describes these implicit settings of the optional session attributes. The administrative states described below may also be implicit in some implementations and not directly configurable by an external operator.
Event 1: ManualStart
Definition: Local system administrator manually starts the peer
connection.
Status: Mandatory
Optional
Attribute
Status: The PassiveTcpEstablishment attribute SHOULD be set
to FALSE.
Event 2: ManualStop
Definition: Local system administrator manually stops the peer
connection.
Status: Mandatory
Optional
Attribute
Status: No interaction with any optional attributes.
Event 3: AutomaticStart
Definition: Local system automatically starts the BGP
connection.
Status: Optional, depending on local system
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Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE if this event occurs.
2) If the PassiveTcpEstablishment optional session
attribute is supported, it SHOULD be set to
FALSE.
3) If the DampPeerOscillations is supported, it
SHOULD be set to FALSE when this event occurs.
Event 4: ManualStart_with_PassiveTcpEstablishment
Definition: Local system administrator manually starts the peer
connection, but has PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment optional
attribute indicates that the peer will listen prior
to establishing the connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The PassiveTcpEstablishment attribute SHOULD be
set to TRUE if this event occurs.
2) The DampPeerOscillations attribute SHOULD be set
to FALSE when this event occurs.
Event 5: AutomaticStart_with_PassiveTcpEstablishment
Definition: Local system automatically starts the BGP
connection with the PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment optional
attribute indicates that the peer will listen prior
to establishing a connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE.
2) The PassiveTcpEstablishment attribute SHOULD be
set to TRUE.
3) If the DampPeerOscillations attribute is
supported, the DampPeerOscillations SHOULD be
set to FALSE.
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Event 6: AutomaticStart_with_DampPeerOscillations
Definition: Local system automatically starts the BGP peer
connection with peer oscillation damping enabled.
The exact method of damping persistent peer
oscillations is determined by the implementation
and is outside the scope of this document.
Status: Optional, depending on local system.
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE.
2) The DampPeerOscillations attribute SHOULD be set
to TRUE.
3) The PassiveTcpEstablishment attribute SHOULD be
set to FALSE.
Event 7: AutomaticStart_with_DampPeerOscillations_and_
PassiveTcpEstablishment
Definition: Local system automatically starts the BGP peer
connection with peer oscillation damping enabled
and PassiveTcpEstablishment enabled. The exact
method of damping persistent peer oscillations is
determined by the implementation and is outside the
scope of this document.
Status: Optional, depending on local system
Optional
Attributes
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE.
2) The DampPeerOscillations attribute SHOULD be set
to TRUE.
3) The PassiveTcpEstablishment attribute SHOULD be
set to TRUE.
Event 8: AutomaticStop
Definition: Local system automatically stops the BGP
connection.
An example of an automatic stop event is exceeding
the number of prefixes for a given peer and the
local system automatically disconnecting the peer.
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Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStop attribute SHOULD be TRUE.
8.1.3. Timer Events
Event 9: ConnectRetryTimer_Expires
Definition: An event generated when the ConnectRetryTimer
expires.
Status: Mandatory
Event 10: HoldTimer_Expires
Definition: An event generated when the HoldTimer expires.
Status: Mandatory
Event 11: KeepaliveTimer_Expires
Definition: An event generated when the KeepaliveTimer expires.
Status: Mandatory
Event 12: DelayOpenTimer_Expires
Definition: An event generated when the DelayOpenTimer expires.
Status: Optional
Optional
Attribute
Status: If this event occurs,
1) DelayOpen attribute SHOULD be set to TRUE,
2) DelayOpenTime attribute SHOULD be supported,
3) DelayOpenTimer SHOULD be supported.
Event 13: IdleHoldTimer_Expires
Definition: An event generated when the IdleHoldTimer expires,
indicating that the BGP connection has completed
waiting for the back-off period to prevent BGP peer
oscillation.
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The IdleHoldTimer is only used when the persistent
peer oscillation damping function is enabled by
setting the DampPeerOscillations optional attribute
to TRUE.
Implementations not implementing the persistent
peer oscillation damping function may not have the
IdleHoldTimer.
Status: Optional
Optional
Attribute
Status: If this event occurs:
1) DampPeerOscillations attribute SHOULD be set to
TRUE.
2) IdleHoldTimer SHOULD have just expired.
8.1.4. TCP Connection-Based Events
Event 14: TcpConnection_Valid
Definition: Event indicating the local system reception of a
TCP connection request with a valid source IP
address, TCP port, destination IP address, and TCP
Port. The definition of invalid source and invalid
destination IP address is determined by the
implementation.
BGP's destination port SHOULD be port 179, as
defined by IANA.
TCP connection request is denoted by the local
system receiving a TCP SYN.
Status: Optional
Optional
Attribute
Status: 1) The TrackTcpState attribute SHOULD be set to
TRUE if this event occurs.
Event 15: Tcp_CR_Invalid
Definition: Event indicating the local system reception of a
TCP connection request with either an invalid
source address or port number, or an invalid
destination address or port number.
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BGP destination port number SHOULD be 179, as
defined by IANA.
A TCP connection request occurs when the local
system receives a TCP SYN.
Status: Optional
Optional
Attribute
Status: 1) The TrackTcpState attribute should be set to
TRUE if this event occurs.
Event 16: Tcp_CR_Acked
Definition: Event indicating the local system's request to
establish a TCP connection to the remote peer.
The local system's TCP connection sent a TCP SYN,
received a TCP SYN/ACK message, and sent a TCP ACK.
Status: Mandatory
Event 17: TcpConnectionConfirmed
Definition: Event indicating that the local system has received
a confirmation that the TCP connection has been
established by the remote site.
The remote peer's TCP engine sent a TCP SYN. The
local peer's TCP engine sent a SYN, ACK message and
now has received a final ACK.
Status: Mandatory
Event 18: TcpConnectionFails
Definition: Event indicating that the local system has received
a TCP connection failure notice.
The remote BGP peer's TCP machine could have sent a
FIN. The local peer would respond with a FIN-ACK.
Another possibility is that the local peer
indicated a timeout in the TCP connection and
downed the connection.
Status: Mandatory
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8.1.5. BGP Message-Based Events
Event 19: BGPOpen
Definition: An event is generated when a valid OPEN message has
been received.
Status: Mandatory
Optional
Attribute
Status: 1) The DelayOpen optional attribute SHOULD be set
to FALSE.
2) The DelayOpenTimer SHOULD not be running.
Event 20: BGPOpen with DelayOpenTimer running
Definition: An event is generated when a valid OPEN message has
been received for a peer that has a successfully
established transport connection and is currently
delaying the sending of a BGP open message.
Status: Optional
Optional
Attribute
Status: 1) The DelayOpen attribute SHOULD be set to TRUE.
2) The DelayOpenTimer SHOULD be running.
Event 21: BGPHeaderErr
Definition: An event is generated when a received BGP message
header is not valid.
Status: Mandatory
Event 22: BGPOpenMsgErr
Definition: An event is generated when an OPEN message has been
received with errors.
Status: Mandatory
Event 23: OpenCollisionDump
Definition: An event generated administratively when a
connection collision has been detected while
processing an incoming OPEN message and this
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connection has been selected to be disconnected.
See Section 6.8 for more information on collision
detection.
Event 23 is an administrative action generated by
implementation logic that determines whether this
connection needs to be dropped per the rules in
Section 6.8. This event may occur if the FSM is
implemented as two linked state machines.
Status: Optional
Optional
Attribute
Status: If the state machine is to process this event in
the Established state,
1) CollisionDetectEstablishedState optional
attribute SHOULD be set to TRUE.
Please note: The OpenCollisionDump event can occur
in Idle, Connect, Active, OpenSent, and OpenConfirm
without any optional attributes being set.
Event 24: NotifMsgVerErr
Definition: An event is generated when a NOTIFICATION message
with "version error" is received.
Status: Mandatory
Event 25: NotifMsg
Definition: An event is generated when a NOTIFICATION message
is received and the error code is anything but
"version error".
Status: Mandatory
Event 26: KeepAliveMsg
Definition: An event is generated when a KEEPALIVE message is
received.
Status: Mandatory
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Event 27: UpdateMsg
Definition: An event is generated when a valid UPDATE message
is received.
Status: Mandatory
Event 28: UpdateMsgErr
Definition: An event is generated when an invalid UPDATE
message is received.
Status: Mandatory
8.2. Description of FSM
8.2.1. FSM Definition
BGP MUST maintain a separate FSM for each configured peer. Each BGP peer paired in a potential connection will attempt to connect to the other, unless configured to remain in the idle state, or configured to remain passive. For the purpose of this discussion, the active or connecting side of the TCP connection (the side of a TCP connection sending the first TCP SYN packet) is called outgoing. The passive or listening side (the sender of the first SYN/ACK) is called an incoming connection. (See Section 8.2.1.1 for information on the terms active and passive used below.)
A BGP implementation MUST connect to and listen on TCP port 179 for incoming connections in addition to trying to connect to peers. For each incoming connection, a state machine MUST be instantiated. There exists a period in which the identity of the peer on the other end of an incoming connection is known, but the BGP identifier is not known. During this time, both an incoming and outgoing connection may exist for the same configured peering. This is referred to as a connection collision (see Section 6.8).
A BGP implementation will have, at most, one FSM for each configured peering, plus one FSM for each incoming TCP connection for which the peer has not yet been identified. Each FSM corresponds to exactly one TCP connection.
There may be more than one connection between a pair of peers if the connections are configured to use a different pair of IP addresses. This is referred to as multiple "configured peerings" to the same peer.
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8.2.1.1. Terms "active" and "passive"
The terms active and passive have been in the Internet operator's vocabulary for almost a decade and have proven useful. The words active and passive have slightly different meanings when applied to a TCP connection or a peer. There is only one active side and one passive side to any one TCP connection, per the definition above and the state machine below. When a BGP speaker is configured as active, it may end up on either the active or passive side of the connection that eventually gets established. Once the TCP connection is completed, it doesn't matter which end was active and which was passive. The only difference is in which side of the TCP connection has port number 179.
8.2.1.2. FSM and Collision Detection
There is one FSM per BGP connection. When the connection collision occurs prior to determining what peer a connection is associated with, there may be two connections for one peer. After the connection collision is resolved (see Section 6.8), the FSM for the connection that is closed SHOULD be disposed.
8.2.1.3. FSM and Optional Session Attributes
Optional Session Attributes specify either attributes that act as flags (TRUE or FALSE) or optional timers. For optional attributes that act as flags, if the optional session attribute can be set to TRUE on the system, the corresponding BGP FSM actions must be supported. For example, if the following options can be set in a BGP implementation: AutoStart and PassiveTcpEstablishment, then Events 3, 4 and 5 must be supported. If an Optional Session attribute cannot be set to TRUE, the events supporting that set of options do not have to be supported.
Each of the optional timers (DelayOpenTimer and IdleHoldTimer) has a group of attributes that are:
- flag indicating support,
- Time set in Timer
- Timer.
The two optional timers show this format:
DelayOpenTimer: DelayOpen, DelayOpenTime, DelayOpenTimer
IdleHoldTimer: DampPeerOscillations, IdleHoldTime,
IdleHoldTimer
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If the flag indicating support for an optional timer (DelayOpen or DampPeerOscillations) cannot be set to TRUE, the timers and events supporting that option do not have to be supported.
8.2.1.4. FSM Event Numbers
The Event numbers (1-28) utilized in this state machine description aid in specifying the behavior of the BGP state machine. Implementations MAY use these numbers to provide network management information. The exact form of an FSM or the FSM events are specific to each implementation.
8.2.1.5. FSM Actions that are Implementation Dependent
At certain points, the BGP FSM specifies that BGP initialization will occur or that BGP resources will be deleted. The initialization of the BGP FSM and the associated resources depend on the policy portion of the BGP implementation. The details of these actions are outside the scope of the FSM document.
8.2.2. Finite State Machine
Idle state:
Initially, the BGP peer FSM is in the Idle state. Hereafter, the
BGP peer FSM will be shortened to BGP FSM.
In this state, BGP FSM refuses all incoming BGP connections for
this peer. No resources are allocated to the peer. In response
to a ManualStart event (Event 1) or an AutomaticStart event (Event
3), the local system:
- initializes all BGP resources for the peer connection,
- sets ConnectRetryCounter to zero,
- starts the ConnectRetryTimer with the initial value,
- initiates a TCP connection to the other BGP peer,
- listens for a connection that may be initiated by the remote
BGP peer, and
- changes its state to Connect.
The ManualStop event (Event 2) and AutomaticStop (Event 8) event
are ignored in the Idle state.
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In response to a ManualStart_with_PassiveTcpEstablishment event
(Event 4) or AutomaticStart_with_PassiveTcpEstablishment event
(Event 5), the local system:
- initializes all BGP resources,
- sets the ConnectRetryCounter to zero,
- starts the ConnectRetryTimer with the initial value,
- listens for a connection that may be initiated by the remote
peer, and
- changes its state to Active.
The exact value of the ConnectRetryTimer is a local matter, but it
SHOULD be sufficiently large to allow TCP initialization.
If the DampPeerOscillations attribute is set to TRUE, the
following three additional events may occur within the Idle state:
- AutomaticStart_with_DampPeerOscillations (Event 6),
- AutomaticStart_with_DampPeerOscillations_and_
PassiveTcpEstablishment (Event 7),
- IdleHoldTimer_Expires (Event 13).
Upon receiving these 3 events, the local system will use these
events to prevent peer oscillations. The method of preventing
persistent peer oscillation is outside the scope of this document.
Any other event (Events 9-12, 15-28) received in the Idle state
does not cause change in the state of the local system.
Connect State:
In this state, BGP FSM is waiting for the TCP connection to be
completed.
The start events (Events 1, 3-7) are ignored in the Connect state.
In response to a ManualStop event (Event 2), the local system:
- drops the TCP connection,
- releases all BGP resources,
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- sets ConnectRetryCounter to zero,
- stops the ConnectRetryTimer and sets ConnectRetryTimer to
zero, and
- changes its state to Idle.
In response to the ConnectRetryTimer_Expires event (Event 9), the
local system:
- drops the TCP connection,
- restarts the ConnectRetryTimer,
- stops the DelayOpenTimer and resets the timer to zero,
- initiates a TCP connection to the other BGP peer,
- continues to listen for a connection that may be initiated by
the remote BGP peer, and
- stays in the Connect state.
If the DelayOpenTimer_Expires event (Event 12) occurs in the
Connect state, the local system:
- sends an OPEN message to its peer,
- sets the HoldTimer to a large value, and
- changes its state to OpenSent.
If the BGP FSM receives a TcpConnection_Valid event (Event 14),
the TCP connection is processed, and the connection remains in the
Connect state.
If the BGP FSM receives a Tcp_CR_Invalid event (Event 15), the
local system rejects the TCP connection, and the connection
remains in the Connect state.
If the TCP connection succeeds (Event 16 or Event 17), the local
system checks the DelayOpen attribute prior to processing. If the
DelayOpen attribute is set to TRUE, the local system:
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- sets the DelayOpenTimer to the initial value, and
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- stays in the Connect state.
If the DelayOpen attribute is set to FALSE, the local system:
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- completes BGP initialization
- sends an OPEN message to its peer,
- sets the HoldTimer to a large value, and
- changes its state to OpenSent.
A HoldTimer value of 4 minutes is suggested.
If the TCP connection fails (Event 18), the local system checks
the DelayOpenTimer. If the DelayOpenTimer is running, the local
system:
- restarts the ConnectRetryTimer with the initial value,
- stops the DelayOpenTimer and resets its value to zero,
- continues to listen for a connection that may be initiated by
the remote BGP peer, and
- changes its state to Active.
If the DelayOpenTimer is not running, the local system:
- stops the ConnectRetryTimer to zero,
- drops the TCP connection,
- releases all BGP resources, and
- changes its state to Idle.
If an OPEN message is received while the DelayOpenTimer is running
(Event 20), the local system:
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- completes the BGP initialization,
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- stops and clears the DelayOpenTimer (sets the value to zero),
- sends an OPEN message,
- sends a KEEPALIVE message,
- if the HoldTimer initial value is non-zero,
- starts the KeepaliveTimer with the initial value and
- resets the HoldTimer to the negotiated value,
else, if the HoldTimer initial value is zero,
- resets the KeepaliveTimer and
- resets the HoldTimer value to zero,
- and changes its state to OpenConfirm.
If the value of the autonomous system field is the same as the
local Autonomous System number, set the connection status to an
internal connection; otherwise it will be "external".
If BGP message header checking (Event 21) or OPEN message checking
detects an error (Event 22) (see Section 6.2), the local system:
- (optionally) If the SendNOTIFICATIONwithoutOPEN attribute is
set to TRUE, then the local system first sends a NOTIFICATION
message with the appropriate error code, and then
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a NOTIFICATION message is received with a version error (Event
24), the local system checks the DelayOpenTimer. If the
DelayOpenTimer is running, the local system:
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- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- stops and resets the DelayOpenTimer (sets to zero),
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
If the DelayOpenTimer is not running, the local system:
- stops the ConnectRetryTimer and sets the ConnectRetryTimer to
zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- performs peer oscillation damping if the DampPeerOscillations
attribute is set to True, and
- changes its state to Idle.
In response to any other events (Events 8, 10-11, 13, 19, 23,
25-28), the local system:
- if the ConnectRetryTimer is running, stops and resets the
ConnectRetryTimer (sets to zero),
- if the DelayOpenTimer is running, stops and resets the
DelayOpenTimer (sets to zero),
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- performs peer oscillation damping if the DampPeerOscillations
attribute is set to True, and
- changes its state to Idle.
RFC 4271 BGP-4 January 2006
Active State:
In this state, BGP FSM is trying to acquire a peer by listening
for, and accepting, a TCP connection.
The start events (Events 1, 3-7) are ignored in the Active state.
In response to a ManualStop event (Event 2), the local system:
- If the DelayOpenTimer is running and the
SendNOTIFICATIONwithoutOPEN session attribute is set, the
local system sends a NOTIFICATION with a Cease,
- releases all BGP resources including stopping the
DelayOpenTimer
- drops the TCP connection,
- sets ConnectRetryCounter to zero,
- stops the ConnectRetryTimer and sets the ConnectRetryTimer to
zero, and
- changes its state to Idle.
In response to a ConnectRetryTimer_Expires event (Event 9), the
local system:
- restarts the ConnectRetryTimer (with initial value),
- initiates a TCP connection to the other BGP peer,
- continues to listen for a TCP connection that may be initiated
by a remote BGP peer, and
- changes its state to Connect.
If the local system receives a DelayOpenTimer_Expires event (Event
12), the local system:
- sets the ConnectRetryTimer to zero,
- stops and clears the DelayOpenTimer (set to zero),
- completes the BGP initialization,
- sends the OPEN message to its remote peer,
RFC 4271 BGP-4 January 2006
- sets its hold timer to a large value, and
- changes its state to OpenSent.
A HoldTimer value of 4 minutes is also suggested for this state
transition.
If the local system receives a TcpConnection_Valid event (Event
14), the local system processes the TCP connection flags and stays
in the Active state.
If the local system receives a Tcp_CR_Invalid event (Event 15),
the local system rejects the TCP connection and stays in the
Active State.
In response to the success of a TCP connection (Event 16 or Event
17), the local system checks the DelayOpen optional attribute
prior to processing.
If the DelayOpen attribute is set to TRUE, the local system:
- stops the ConnectRetryTimer and sets the ConnectRetryTimer
to zero,
- sets the DelayOpenTimer to the initial value
(DelayOpenTime), and
- stays in the Active state.
If the DelayOpen attribute is set to FALSE, the local system:
- sets the ConnectRetryTimer to zero,
- completes the BGP initialization,
- sends the OPEN message to its peer,
- sets its HoldTimer to a large value, and
- changes its state to OpenSent.
A HoldTimer value of 4 minutes is suggested as a "large value" for
the HoldTimer.
If the local system receives a TcpConnectionFails event (Event
18), the local system:
- restarts the ConnectRetryTimer (with the initial value),
RFC 4271 BGP-4 January 2006
- stops and clears the DelayOpenTimer (sets the value to zero),
- releases all BGP resource,
- increments the ConnectRetryCounter by 1,
- optionally performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If an OPEN message is received and the DelayOpenTimer is running
(Event 20), the local system:
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- stops and clears the DelayOpenTimer (sets to zero),
- completes the BGP initialization,
- sends an OPEN message,
- sends a KEEPALIVE message,
- if the HoldTimer value is non-zero,
- starts the KeepaliveTimer to initial value,
- resets the HoldTimer to the negotiated value,
else if the HoldTimer is zero
- resets the KeepaliveTimer (set to zero),
- resets the HoldTimer to zero, and
- changes its state to OpenConfirm.
If the value of the autonomous system field is the same as the
local Autonomous System number, set the connection status to an
internal connection; otherwise it will be external.
If BGP message header checking (Event 21) or OPEN message checking
detects an error (Event 22) (see Section 6.2), the local system:
RFC 4271 BGP-4 January 2006
- (optionally) sends a NOTIFICATION message with the appropriate
error code if the SendNOTIFICATIONwithoutOPEN attribute is set
to TRUE,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a NOTIFICATION message is received with a version error (Event
24), the local system checks the DelayOpenTimer. If the
DelayOpenTimer is running, the local system:
- stops the ConnectRetryTimer (if running) and sets the
ConnectRetryTimer to zero,
- stops and resets the DelayOpenTimer (sets to zero),
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
If the DelayOpenTimer is not running, the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
RFC 4271 BGP-4 January 2006
In response to any other event (Events 8, 10-11, 13, 19, 23,
25-28), the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by one,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
OpenSent:
In this state, BGP FSM waits for an OPEN message from its peer.
The start events (Events 1, 3-7) are ignored in the OpenSent
state.
If a ManualStop event (Event 2) is issued in the OpenSent state,
the local system:
- sends the NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- sets the ConnectRetryCounter to zero, and
- changes its state to Idle.
If an AutomaticStop event (Event 8) is issued in the OpenSent
state, the local system:
- sends the NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- releases all the BGP resources,
- drops the TCP connection,
RFC 4271 BGP-4 January 2006
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the HoldTimer_Expires (Event 10), the local system:
- sends a NOTIFICATION message with the error code Hold Timer
Expired,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a TcpConnection_Valid (Event 14), Tcp_CR_Acked (Event 16), or a
TcpConnectionConfirmed event (Event 17) is received, a second TCP
connection may be in progress. This second TCP connection is
tracked per Connection Collision processing (Section 6.8) until an
OPEN message is received.
A TCP Connection Request for an Invalid port (Tcp_CR_Invalid
(Event 15)) is ignored.
If a TcpConnectionFails event (Event 18) is received, the local
system:
- closes the BGP connection,
- restarts the ConnectRetryTimer,
- continues to listen for a connection that may be initiated by
the remote BGP peer, and
- changes its state to Active.
RFC 4271 BGP-4 January 2006
When an OPEN message is received, all fields are checked for
correctness. If there are no errors in the OPEN message (Event
19), the local system:
- resets the DelayOpenTimer to zero,
- sets the BGP ConnectRetryTimer to zero,
- sends a KEEPALIVE message, and
- sets a KeepaliveTimer (via the text below)
- sets the HoldTimer according to the negotiated value (see
Section 4.2),
- changes its state to OpenConfirm.
If the negotiated hold time value is zero, then the HoldTimer and
KeepaliveTimer are not started. If the value of the Autonomous
System field is the same as the local Autonomous System number,
then the connection is an "internal" connection; otherwise, it is
an "external" connection. (This will impact UPDATE processing as
described below.)
If the BGP message header checking (Event 21) or OPEN message
checking detects an error (Event 22)(see Section 6.2), the local
system:
- sends a NOTIFICATION message with the appropriate error code,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is TRUE, and
- changes its state to Idle.
Collision detection mechanisms (Section 6.8) need to be applied
when a valid BGP OPEN message is received (Event 19 or Event 20).
Please refer to Section 6.8 for the details of the comparison. A
RFC 4271 BGP-4 January 2006
CollisionDetectDump event occurs when the BGP implementation
determines, by means outside the scope of this document, that a
connection collision has occurred.
If a connection in the OpenSent state is determined to be the
connection that must be closed, an OpenCollisionDump (Event 23) is
signaled to the state machine. If such an event is received in
the OpenSent state, the local system:
- sends a NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If a NOTIFICATION message is received with a version error (Event
24), the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
In response to any other event (Events 9, 11-13, 20, 25-28), the
local system:
- sends the NOTIFICATION with the Error Code Finite State
Machine Error,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
RFC 4271 BGP-4 January 2006
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
OpenConfirm State:
In this state, BGP waits for a KEEPALIVE or NOTIFICATION message.
Any start event (Events 1, 3-7) is ignored in the OpenConfirm
state.
In response to a ManualStop event (Event 2) initiated by the
operator, the local system:
- sends the NOTIFICATION message with a Cease,
- releases all BGP resources,
- drops the TCP connection,
- sets the ConnectRetryCounter to zero,
- sets the ConnectRetryTimer to zero, and
- changes its state to Idle.
In response to the AutomaticStop event initiated by the system
(Event 8), the local system:
- sends the NOTIFICATION message with a Cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the HoldTimer_Expires event (Event 10) occurs before a
KEEPALIVE message is received, the local system:
RFC 4271 BGP-4 January 2006
- sends the NOTIFICATION message with the Error Code Hold Timer
Expired,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the local system receives a KeepaliveTimer_Expires event (Event
11), the local system:
- sends a KEEPALIVE message,
- restarts the KeepaliveTimer, and
- remains in the OpenConfirmed state.
In the event of a TcpConnection_Valid event (Event 14), or the
success of a TCP connection (Event 16 or Event 17) while in
OpenConfirm, the local system needs to track the second
connection.
If a TCP connection is attempted with an invalid port (Event 15),
the local system will ignore the second connection attempt.
If the local system receives a TcpConnectionFails event (Event 18)
from the underlying TCP or a NOTIFICATION message (Event 25), the
local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
RFC 4271 BGP-4 January 2006
- changes its state to Idle.
If the local system receives a NOTIFICATION message with a version
error (NotifMsgVerErr (Event 24)), the local system:
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection, and
- changes its state to Idle.
If the local system receives a valid OPEN message (BGPOpen (Event
19)), the collision detect function is processed per Section 6.8.
If this connection is to be dropped due to connection collision,
the local system:
- sends a NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection (send TCP FIN),
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking (BGPHeaderErr
(Event 21)) or OPEN message checking detects an error (see Section
6.2) (BGPOpenMsgErr (Event 22)), the local system:
- sends a NOTIFICATION message with the appropriate error code,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
RFC 4271 BGP-4 January 2006
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If, during the processing of another OPEN message, the BGP
implementation determines, by a means outside the scope of this
document, that a connection collision has occurred and this
connection is to be closed, the local system will issue an
OpenCollisionDump event (Event 23). When the local system
receives an OpenCollisionDump event (Event 23), the local system:
- sends a NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the local system receives a KEEPALIVE message (KeepAliveMsg
(Event 26)), the local system:
- restarts the HoldTimer and
- changes its state to Established.
In response to any other event (Events 9, 12-13, 20, 27-28), the
local system:
- sends a NOTIFICATION with a code of Finite State Machine
Error,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
RFC 4271 BGP-4 January 2006
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
Established State:
In the Established state, the BGP FSM can exchange UPDATE,
NOTIFICATION, and KEEPALIVE messages with its peer.
Any Start event (Events 1, 3-7) is ignored in the Established
state.
In response to a ManualStop event (initiated by an operator)
(Event 2), the local system:
- sends the NOTIFICATION message with a Cease,
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases BGP resources,
- drops the TCP connection,
- sets the ConnectRetryCounter to zero, and
- changes its state to Idle.
In response to an AutomaticStop event (Event 8), the local system:
- sends a NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero
- deletes all routes associated with this connection,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
RFC 4271 BGP-4 January 2006
One reason for an AutomaticStop event is: A BGP receives an UPDATE
messages with a number of prefixes for a given peer such that the
total prefixes received exceeds the maximum number of prefixes
configured. The local system automatically disconnects the peer.
If the HoldTimer_Expires event occurs (Event 10), the local
system:
- sends a NOTIFICATION message with the Error Code Hold Timer
Expired,
- sets the ConnectRetryTimer to zero,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
If the KeepaliveTimer_Expires event occurs (Event 11), the local
system:
- sends a KEEPALIVE message, and
- restarts its KeepaliveTimer, unless the negotiated HoldTime
value is zero.
Each time the local system sends a KEEPALIVE or UPDATE message, it
restarts its KeepaliveTimer, unless the negotiated HoldTime value
is zero.
A TcpConnection_Valid (Event 14), received for a valid port, will
cause the second connection to be tracked.
An invalid TCP connection (Tcp_CR_Invalid event (Event 15)) will
be ignored.
In response to an indication that the TCP connection is
successfully established (Event 16 or Event 17), the second
connection SHALL be tracked until it sends an OPEN message.
RFC 4271 BGP-4 January 2006
If a valid OPEN message (BGPOpen (Event 19)) is received, and if
the CollisionDetectEstablishedState optional attribute is TRUE,
the OPEN message will be checked to see if it collides (Section
6.8) with any other connection. If the BGP implementation
determines that this connection needs to be terminated, it will
process an OpenCollisionDump event (Event 23). If this connection
needs to be terminated, the local system:
- sends a NOTIFICATION with a Cease,
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations is set to TRUE, and
- changes its state to Idle.
If the local system receives a NOTIFICATION message (Event 24 or
Event 25) or a TcpConnectionFails (Event 18) from the underlying
TCP, the local system:
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases all the BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- changes its state to Idle.
RFC 4271 BGP-4 January 2006
If the local system receives a KEEPALIVE message (Event 26), the
local system:
- restarts its HoldTimer, if the negotiated HoldTime value is
non-zero, and
- remains in the Established state.
If the local system receives an UPDATE message (Event 27), the
local system:
- processes the message,
- restarts its HoldTimer, if the negotiated HoldTime value is
non-zero, and
- remains in the Established state.
If the local system receives an UPDATE message, and the UPDATE
message error handling procedure (see Section 6.3) detects an
error (Event 28), the local system:
- sends a NOTIFICATION message with an Update error,
- sets the ConnectRetryTimer to zero,
- deletes all routes associated with this connection,
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.
In response to any other event (Events 9, 12-13, 20-22), the local
system:
- sends a NOTIFICATION message with the Error Code Finite State
Machine Error,
- deletes all routes associated with this connection,
- sets the ConnectRetryTimer to zero,
RFC 4271 BGP-4 January 2006
- releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCounter by 1,
- (optionally) performs peer oscillation damping if the
DampPeerOscillations attribute is set to TRUE, and
- changes its state to Idle.