RFC 8156
DHCPv6 Failover Protocol
Pages: 96
Proposed Standard
Proposed Standard
Part 3 of 5 – Pages 40 to 65
6. Connection Management
Communication between failover partners takes place over a long-lived TCP connection. This connection is always initiated by the primary server, and if the long-lived connection is lost it is the responsibility of the primary server to attempt to reconnect to the secondary server. The detailed process used by the primary server when initiating a connection and by the secondary server when responding to a connection attempt as documented in Section 6.1 is followed each time a connection is established, regardless of any previous connection between the failover partners.6.1. Creating Connections
Every primary server implementing the failover protocol MUST periodically attempt to create a TCP connection to the dhcp-failover port (647) of all of its configured partners, where the period is implementation dependent and SHOULD be configurable. In the event that a connection has been rejected by a CONNECTREPLY message with an OPTION_STATUS_CODE option contained in it or a DISCONNECT message, a server SHOULD reduce the frequency with which it attempts to connect to that server, but it MUST continue to attempt to connect periodically. Every secondary server implementing the failover protocol MUST listen for TCP connection attempts on the dhcp-failover port (647) from a primary server. After a primary server successfully establishes a TCP connection to a secondary server, it MUST continue the connection process as described in Section 8.2 of [RFC7653]. In the language of that section, the primary failover server operates as the "requestor" and the secondary failover server operates as the "DHCP server". The message that is sent over the newly established connection is a CONNECT message, instead of an ACTIVELEASEQUERY message. When a secondary server receives a connection attempt, the only information that the secondary server has is the IP address of the partner initiating a connection. If it has any relationships with the connecting server for which it is a secondary server, it should
operate as described in Section 9.1 of [RFC7653], with the exception that instead of waiting for an Active Leasequery message it will wait for a CONNECT message. Once it has received the CONNECT message, it will use the information in that message to determine which relationship this connection is to service. If it has no secondary relationships with the connecting server, it MUST drop the connection. To summarize -- a primary server MUST use a connection that it has initiated in order to send a CONNECT message. Every server that is a secondary server in a relationship MUST listen for CONNECT messages from the primary server. When the CONNECT and CONNECTREPLY exchange successfully produces a working failover connection, the next message sent over a new connection is a STATE message. See Section 6.3. Upon the receipt of the STATE message, the receiver can consider communications "OK".6.1.1. Sending a CONNECT Message
The CONNECT message is sent with information about the failover configuration on the primary server. The message MUST contain at least the following information in the options area: o OPTION_F_PROTOCOL_VERSION containing the protocol version that the primary server will use when sending failover messages. o OPTION_F_MCLT containing the configured MCLT. o OPTION_F_KEEPALIVE_TIME containing the number of seconds (an interval) within which the server must receive a message from its partner, or it will assume that communications from the partner are not "OK". o OPTION_F_MAX_UNACKED_BNDUPD containing the maximum number of BNDUPD messages that this server is prepared to accept over the failover connection without causing the connection to block. This implements application-level flow control over the connection, so that a flood of BNDUPD messages does not cause the connection to block and thereby prevent other messages from being transmitted over the connection and received by the failover partner.
o OPTION_F_RELATIONSHIP_NAME containing the name of the failover
relationship to which this connection applies. If there is no
OPTION_F_RELATIONSHIP_NAME in the CONNECT message, it indicates
that there is only a single relationship between this pair of
primary and secondary servers.
o OPTION_F_CONNECT_FLAGS containing information about certain
attributes of the connecting servers.
6.1.2. Receiving a CONNECT Message
A server receiving a CONNECT message must process the information in
the message and decide whether or not to accept the connection. The
processing is performed as follows:
o sent-time - The secondary server checks the sent-time to see if it
is within 5 seconds of its current time. See Section 7.1. If it
is not, return ExcessiveTimeSkew in the OPTION_STATUS_CODE to
reject the CONNECT message.
o OPTION_F_PROTOCOL_VERSION - The secondary server decides if the
protocol version of the primary server is supported by the
secondary server. If it is not, return NotSupported in the
OPTION_STATUS_CODE to reject the CONNECT message.
o OPTION_F_MCLT - Use this MCLT supplied by the primary server.
Remember this MCLT, and use it until a different MCLT is supplied
by some subsequent CONNECT message.
o OPTION_F_KEEPALIVE_TIME - Remember the keepalive-time as the
FO_KEEPALIVE_TIME (Section 6.5) when implementing the
Unreachability Detection algorithm described in Section 6.6.
o OPTION_F_MAX_UNACKED_BNDUPD - Ensure that the maximum amount of
unacked BNDUPD messages queued to the primary server never exceeds
the value in the OPTION_F_MAX_UNACKED_BNDUPD option.
o OPTION_F_CONNECT_FLAGS - Ensure that the secondary server can
process information from the primary server as specified in the
flags. For example, if the secondary server cannot process prefix
delegation with variable-sized prefixes delegated from the same
delegable prefix and the primary server says that it can, the
secondary should reject the connection.
A CONNECT message SHOULD always be followed by a CONNECTREPLY
message, to either (1) accept the connection or (2) reject the
connection by including an OPTION_STATUS_CODE option with a
status-code indicating the reason for the rejection. If accepting
the connection attempt, then send a CONNECTREPLY message with the
following information:
o OPTION_F_PROTOCOL_VERSION containing the protocol version being
used by the secondary server when sending failover messages.
o OPTION_F_MCLT containing the MCLT currently in use on the
secondary server. This MUST equal the MCLT that was in the
OPTION_F_MCLT option in the CONNECT message.
o OPTION_F_KEEPALIVE_TIME containing the number of seconds (an
interval) within which the server must receive a message from its
partner, or it will assume that communications from the partner
are not "OK".
o OPTION_F_MAX_UNACKED_BNDUPD containing the maximum number of
BNDUPD messages that this server is prepared to accept over the
failover connection without causing the connection to block. This
implements application-level flow control over the connection, so
that a flood of BNDUPD messages does not cause the connection to
block and thereby prevent other messages from being transmitted
over the connection and received by the failover partner.
o OPTION_F_CONNECT_FLAGS containing information describing the
attributes of the secondary server that the primary needs to
know about.
After sending a CONNECTREPLY message to accept the primary server's
CONNECT message, the secondary server MUST send a STATE message (see
Section 6.3).
6.1.3. Receiving a CONNECTREPLY Message
A server receiving a CONNECTREPLY message must process the
information in the message and decide whether or not to continue to
employ the connection. The processing is performed as follows:
o OPTION_F_PROTOCOL_VERSION - The primary server decides if the
protocol version in use by the secondary server is supported by
the primary server. If it is not, send a DISCONNECT message and
drop the connection. If it is supported, continue processing. It
is possible that the primary and secondary servers will each be
sending different versions of the protocol to the other server.
The extent to which this is supported will be defined partly by
as-yet-unknown differences in the protocols that the versions
represent and partly by the capabilities of the two
implementations involved in the failover relationship.
o OPTION_F_MCLT - Compare the MCLT received with the configured
MCLT. If they are different, send a DISCONNECT message and drop
the connection.
o OPTION_F_KEEPALIVE_TIME - Remember the keepalive-time as the
FO_KEEPALIVE_TIME (Section 6.5) when implementing the
Unreachability Detection algorithm described in Section 6.6.
o OPTION_F_MAX_UNACKED_BNDUPD - Ensure that the maximum amount of
unacked BNDUPD messages queued to the secondary server never
exceeds the value in the OPTION_F_MAX_UNACKED_BNDUPD option.
o OPTION_F_CONNECT_FLAGS - Ensure that the primary server can
process information from the secondary server as specified in the
flags. For example, if the primary server cannot process prefix
delegation with variable-sized prefixes delegated from the same
delegable prefix and the secondary server says that it can, the
primary should drop the connection.
After receiving a CONNECTREPLY message that accepted the primary
server's CONNECT message, the primary server MUST send a STATE
message (see Section 6.3).
6.2. Endpoint Identification
A failover endpoint is always associated with a set of DHCP prefixes
that are configured on the DHCP server where the endpoint appears. A
DHCP prefix MUST NOT be associated with more than one failover
endpoint.
The failover protocol SHOULD be configured with one failover
relationship between each pair of failover servers. In this case,
there is one failover endpoint for that relationship on each failover
partner. This failover relationship MUST have a unique name.
Any failover endpoint can take actions and hold unique states.
This document frequently describes the behavior of the failover
protocol in terms of primary and secondary servers, not primary and
secondary failover endpoints. However, it is important to remember
that every "server" described in this document is in reality a
failover endpoint that resides in a particular process and that
several failover endpoints may reside in the same server process.
It is not the case that there is a unique failover endpoint for each prefix that participates in a failover relationship. On one server, there is (typically) one failover endpoint per partner, regardless of how many prefixes are managed by that combination of partner and role. On a particular server, any given prefix that participates in failover will be associated with exactly one failover endpoint. When a connection is received from the partner, the unique failover endpoint to which the message is directed is determined solely by the IPv6 address of the partner, the relationship name, and the role of the receiving server.6.3. Sending a STATE Message
A server MUST send a STATE message to its failover partner whenever the state of the failover endpoint changes. Sending the occasional duplicate STATE message will not cause any problems; note, however, that not updating the failover partner with information about a failover endpoint state change can, in many cases, cause the entire failover protocol to be inoperative. The STATE message is sent with information about the endpoint state of the failover relationship. The STATE message MUST contain at least the following information in the options area: o OPTION_F_SERVER_STATE containing the state of this failover endpoint. o OPTION_F_SERVER_FLAGS containing the flag values associated with this failover endpoint. o OPTION_F_START_TIME_OF_STATE containing the time when this became the state of this failover endpoint. o OPTION_F_PARTNER_DOWN_TIME containing the time that this failover endpoint went into PARTNER-DOWN state if this server is in PARTNER-DOWN state. If this server isn't in PARTNER-DOWN state, do not include this option. The server sending a STATE message SHOULD ensure that this information is written to stable storage prior to enqueuing it to its failover partner.
6.4. Receiving a STATE Message
A server receiving a STATE message must process the information in the message and decide how to react to the information. The processing is performed as follows: o OPTION_F_SERVER_STATE - If this represents a change in state for the failover partner, react according to the instructions in Section 8.1. If the state is not PARTNER-DOWN, clear any memory of the partner-down-time. o OPTION_F_SERVER_FLAGS - Remember these flags in an appropriate data area so they can be referenced later. o OPTION_F_START_TIME_OF_STATE - Remember this information in an appropriate data area so it can be referenced later. o OPTION_F_PARTNER_DOWN_TIME - If the value of the OPTION_F_SERVER_STATE is PARTNER-DOWN, remember this information in an appropriate data area so it can be referenced later. A server receiving a STATE message SHOULD ensure that this information is written to stable storage.6.5. Connection Maintenance Parameters
The following parameters and timers are used to ensure the integrity of the connections between two failover servers. Parameter Default Description --------------------------------------------------------------------- FO_KEEPALIVE_TIMER timer counts down to time connection assumed dead due to lack of messages FO_KEEPALIVE_TIME 60 maximum time server will consider connection still up with no messages FO_CONTACT_PER_KEEPALIVE_TIME 4 number of CONTACT messages to send during partner's FO_KEEPALIVE_TIME period FO_SEND_TIMER timer counts down to time to send next CONTACT message
FO_SEND_TIME 15 maximum time to wait between
sending CONTACT messages
if no other traffic.
Created from partner's
FO_KEEPALIVE_TIME divided by
FO_CONTACT_PER_KEEPALIVE_TIME
6.6. Unreachability Detection
Each partner MUST maintain an FO_SEND_TIMER for each failover
connection. The FO_SEND_TIMER for a particular connection is reset
to FO_SEND_TIME every time any message is transmitted on that
connection, and the timer counts down once per second. If the timer
reaches zero, a CONTACT message is transmitted on that connection and
the timer for that connection is reset to FO_SEND_TIME. The CONTACT
message may be transmitted at any time. An implementation MAY use
additional mechanisms to detect partner unreachability.
The FO_SEND_TIME is initialized from the configured FO_KEEPALIVE_TIME
divided by FO_CONTACT_PER_KEEPALIVE_TIME. When a CONNECT or
CONNECTREPLY message is received on a connection, the received
OPTION_F_KEEPALIVE_TIME option is checked, and the value in that
option is used to calculate the FO_SEND_TIME for that connection by
dividing the value received by the configured
FO_CONTACT_PER_KEEPALIVE_TIME.
Each partner MUST maintain an FO_KEEPALIVE_TIMER for each failover
connection. This timer is initialized to FO_KEEPALIVE_TIME and
counts down once per second. It is reset to FO_KEEPALIVE_TIME
whenever a message is received on that connection. If it ever
reaches zero, that connection is considered dead. In addition, the
FO_KEEPALIVE_TIME for that connection MUST be sent to the failover
partner on every CONNECT or CONNECTREPLY message in the
OPTION_F_KEEPALIVE_TIME option.
7. Binding Updates and Acks
7.1. Time Skew
Partners exchange information about known lease states. To reliably
compare a known lease state with an update received from a partner,
servers must be able to reliably compare the times stored in the
known lease state with the times received in the update. The
failover protocol adopts the simple approach of requiring that the
failover partners use some mechanism to synchronize the clocks on the
two servers to within an accuracy of roughly 5 seconds.
A mechanism to measure and track relative time differences between servers is necessary to ensure this synchronization. To do so, each message contains the time of the transmission in the sent-time field of the message (see Section 5.2). The transmitting server MUST set this as close to the actual transmission as possible. The receiving partner MUST store its own timestamp of reception as close to the actual reception as possible. The received timestamp information is then compared with the local timestamp.7.2. Information Model
In most DHCP servers, a lease on an IPv6 address or a prefix can take on several different binding-status values, sometimes also called "lease states". While no two DHCP server implementations will have exactly the same possible binding-status values, [RFC3315] enforces some commonality among the general semantics of the binding-status values used by various DHCP server implementations. In order to transmit binding database updates between one server and another using the failover protocol, some common binding-status values must be defined. It is not expected that these values correspond to any actual implementation of DHCPv6 in a DHCP server, but rather that the binding-status values defined in this document should be convertible back and forth between those defined below and those in use by many DHCP server implementations. The lease binding-status values defined for the failover protocol are listed below. Unless otherwise noted below, there MAY be client information associated with each of these binding-status values. ACTIVE - The lease is assigned to a client. Client identification data MUST appear. EXPIRED - This value indicates that a client's binding on a given lease has expired. When the partner acks the BNDUPD message of an expired lease, the server sets its internal state to PENDING-FREE. Client identification SHOULD appear. RELEASED - This value indicates that a client sent a RELEASE message. When the partner acks the BNDUPD message of a released lease, the server sets its internal state to PENDING-FREE. Client identification SHOULD appear.
PENDING-FREE - Once a lease is expired or released, its state becomes
PENDING-FREE. Depending on which algorithm was used to allocate a
given lease, PENDING-FREE may mean either FREE or FREE-BACKUP.
Implementations do not have to implement this PENDING-FREE state
but may choose to switch to the destination state directly. For
clarity of representation, this transitional PENDING-FREE state is
treated as a separate state.
FREE - This value is used when a DHCP server needs to communicate
that a lease is unused by any client, but it was not just
released, expired, or reset by a network administrator. When the
partner acks the BNDUPD message of a FREE lease, the server marks
the lease as available for assignment by the primary server. Note
that on a secondary server running in PARTNER-DOWN state, after
waiting the MCLT, the lease MAY be allocated to a client by the
secondary server. Client identification MAY appear and indicates,
as a hint, the last client to have used this lease.
FREE-BACKUP - This value indicates that this lease can be allocated
by the secondary server to a client at any time. Note that on the
primary server running in PARTNER-DOWN state, after waiting the
MCLT, the lease MAY be allocated to a client by the primary server
if the proportional algorithm was used. Client identification MAY
appear and indicates, as a hint, the last client to have used this
lease.
ABANDONED - This value indicates that a lease is considered unusable
by the DHCP system. The primary reason for entering such a state
is the reception of a DECLINE message for the lease. Client
identification MAY appear.
RESET - This value indicates that this lease was made available by an
operator command. This is a distinct state so that the reason
that the lease became FREE can be determined. Client
identification MAY appear.
Which binding-status values are associated with a timeout is
implementation dependent. Some binding-status values, such as
ACTIVE, will have a timeout value in all implementations, while
others, such as ABANDONED, will have a timeout value in some
implementations and not in others. In some implementations, a
binding-status value may be associated with a timeout in some
circumstances and not in others. The receipt of a BNDUPD message
with a particular binding-status value and an
OPTION_F_STATE_EXPIRATION_TIME indicates that this particular
binding-status value is associated with a timeout.
The lease state machine is presented in Figure 2. Most states are stationary, i.e., the lease stays in a given state until an external event triggers transition to another state. The only transitive state is PENDING-FREE. Once it is reached, the state machine immediately transitions to either FREE or FREE-BACKUP state. +---------+ /------------->| ACTIVE |<--------------\ | +---------+ | | | | | | | /--(8)--/ (3) \--(9)-\ | | | | | | | V V V | | +-------+ +--------+ +---------+ | | |EXPIRED| |RELEASED| |ABANDONED| | | +-------+ +--------+ +---------+ | | | | | | | | | (10) | | | | V | | | | +---------+ | | | | | RESET | | | | | +---------+ | | | | | | | \--(4)--\ (4) /--(4)--/ | | | | | | (1) V V V (2) | /---------\ | | | PENDING-| | | | FREE | | | \---------/ | | | | | | /-(5)--/ \-(6)-\ | | | | | | V V | | +-------+ +-----------+ | \----| FREE |<--(7)-->|FREE-BACKUP|-----/ +-------+ +-----------+ PENDING-FREE transition Figure 2: Lease State Machine
Transitions between states will result from the following events:
(1) The primary server allocates a lease.
(2) The secondary server allocates a lease.
(3) The client sends RELEASE, and the lease is released.
(4) The partner acknowledges the state change. This transition MAY
also occur if the server is in PARTNER-DOWN state and the MCLT
has passed since the entry into RELEASED, EXPIRED, or RESET
states.
(5) The lease belongs to a pool that is governed by proportional
allocation, or independent allocation is used and this lease
belongs to the primary server's pool.
(6) The lease belongs to a pool that is governed by independent
allocation, and the lease belongs to the secondary server.
(7) A pool rebalance event occurs (POOLREQ/POOLRESP messages are
exchanged). Delegable prefixes belonging to the primary server
can be assigned to the secondary server's pool (transition from
FREE to FREE-BACKUP) or vice versa.
(8) The lease has expired.
(9) A DECLINE message is received, or a lease is deemed unusable
for other reasons.
(10) An administrative action is taken to restore an abandoned lease
to a usable state. This transition MAY occur due to
implementation-specific handling of an ABANDONED lease. One
possible example of this is a Neighbor Discovery or ICMPv6 Echo
check to see if the address is still in use.
The lease that is no longer in use (due to expiration or release) becomes PENDING-FREE. Depending on what allocation algorithm is used, the lease that is no longer in use returns to the primary pool (FREE) or the secondary pool (FREE-BACKUP). The conditions for specific transitions are depicted in Figure 3. +----------------+---------+-----------+ | \ Lease owner| | | | \----------\ | Primary | Secondary | |Algorithm \ | | | +----------------+---------+-----------+ | Proportional | FREE |FREE-BACKUP| | Independent | FREE | FREE | +----------------+---------+-----------+ Figure 3: PENDING-FREE State Transitions7.3. Times Required for Exchanging Binding Updates
Each server must keep track of the following specific times beyond those required by the base DHCP specification [RFC3315]. expiration-time The greatest lifetime that this server has ever acked to its failover partner in a BNDREPLY message. acked-partner-lifetime The greatest lifetime that the failover partner has ever acked to this server in a BNDREPLY message. partner-lifetime The time value that will be sent (or that has been sent) to the partner to indicate the time after which the partner can consider the lease expired. When a BNDUPD message is received, this value can be updated from the received OPTION_F_EXPIRATION_TIME. client-last-transaction-time The time when this server most recently interacted with the client associated with this lease. partner-raw-clt-time The time when the partner most recently interacted with the client associated with this lease. This time remains exactly as it was received by this server and MUST NOT be adjusted in any way.
start-time-of-state
The time when the binding-status of this lease was changed to its
current value.
state-expiration-time
The time when the current state of this lease will expire.
7.4. Sending Binding Updates
Every BNDUPD message contains information about either (1) a single
client binding in an OPTION_CLIENT_DATA option that includes IAADDR
or IAPREFIX options associated with that client or (2) a single
prefix lease in an OPTION_IAPREFIX option for prefixes that are
currently not associated with any clients.
All information about a particular client binding MUST be contained
in a single OPTION_CLIENT_DATA option (see Section 4.1.2.2 of
[RFC5007]). The OPTION_CLIENT_DATA option contains at least the data
shown below in its client-options section:
o OPTION_CLIENTID containing the DUID of the client most recently
associated with this lease MUST appear.
o OPTION_LQ_BASE_TIME containing the absolute time that the
information was placed in this OPTION_CLIENT_DATA option (see
Section 6.3.1 of [RFC7653]) MUST appear.
o OPTION_VSS (see Section 3.4 of [RFC6607]). This option MUST NOT
appear if the information in this OPTION_CLIENT_DATA option is
associated with the global, default VPN. This option MUST appear
if the information in this OPTION_CLIENT_DATA option is associated
with a VPN other than the global, default VPN. Support of
[RFC6607] is not required, and if it is not supported, then an
OPTION_VSS MUST NOT appear. If [RFC6607] is supported, then an
OPTION_VSS MUST appear if and only if a VPN other than the global,
default VPN is used.
o OPTION_F_RECONFIGURE_DATA containing the time and reconfigure key,
if any.
o OPTION_LQ_RELAY_DATA containing information described in
Section 4.1.2.4 of [RFC5007], if any exists.
o OPTION_IA_NA or OPTION_IA_TA for an IPv6 address, or OPTION_IA_PD
for an IPv6 prefix. More than one of either of these options MAY
appear if more than one of them are associated with this client.
At least one of an OPTION_IA_NA, OPTION_IA_TA, or OPTION_IA_PD
must appear.
* IAID - Identity Association used by the client, while obtaining
a given lease. Note that (1) one client may use many IAIDs
simultaneously and (2) IAIDs for OPTION_IA_NA, OPTION_IA_TA,
and OPTION_IA_PD are orthogonal number spaces.
* T1 time sent to client.
* T2 time sent to client.
* Inside of the IA_NA-options, IA_TA-options, or IA_PD-options
sections:
+ OPTION_IAADDR for an IPv6 address or an OPTION_IAPREFIX for
an IPv6 prefix MUST appear.
- IPv6 address or IPv6 prefix (with length).
- Preferred lifetime sent to client.
- Valid lifetime sent to client.
- Inside of the IAaddr-options or IAprefix-options:
o OPTION_F_BINDING_STATUS containing the binding-status
MUST appear.
o OPTION_F_START_TIME_OF_STATE containing the
start-time-of-state MUST appear.
o OPTION_F_STATE_EXPIRATION_TIME (absolute) containing
the state-expiration-time*.
o OPTION_CLT_TIME (relative) containing the
client-last-transaction-time. See [RFC5007] for a
description of this option.
o OPTION_F_PARTNER_LIFETIME (absolute) containing the
partner-lifetime*.
o OPTION_F_PARTNER_RAW_CLT_TIME (absolute) containing
the partner-raw-clt-time.
o OPTION_F_EXPIRATION_TIME (absolute) containing the
expiration-time*.
o OPTION_CLIENT_FQDN containing the FQDN information
associated with this lease and client, if any.
Information about a prefix lease is contained in a single
OPTION_IAPREFIX option. Only a single OPTION_IAPREFIX option may
appear in a BNDUPD message outside of an OPTION_CLIENT_DATA option.
In detail:
o OPTION_IAPREFIX for a prefix lease.
* IPv6 prefix (with length).
* Inside of the IAprefix-options section:
+ OPTION_VSS (see Section 3.4 of [RFC6607]). This option
MUST NOT appear if the information in this OPTION_IAPREFIX
option is associated with the global, default VPN. This
option MUST appear if the information in this
OPTION_IAPREFIX option is associated with a VPN other than
the global, default VPN. Support of [RFC6607] is not
required, and if it is not supported, then an OPTION_VSS
MUST NOT appear. If [RFC6607] is supported, then an
OPTION_VSS MUST appear if and only if a VPN other than the
global, default VPN is used.
+ OPTION_LQ_BASE_TIME containing the absolute time that this
information was placed in this OPTIONS_IAPREFIX option (see
Section 6.3.1 of [RFC7653]) MUST appear.
+ OPTION_F_BINDING_STATUS containing the binding-status MUST
appear.
+ OPTION_F_START_TIME_OF_STATE containing the
start-time-of-state MUST appear.
+ OPTION_F_STATE_EXPIRATION_TIME (absolute) containing the
state-expiration-time*.
+ OPTION_F_PARTNER_LIFETIME (absolute) containing the
partner-lifetime*.
+ OPTION_F_EXPIRATION_TIME (absolute) containing the
expiration-time*.
Items marked with a single asterisk (*) MUST appear only if the value in the OPTION_F_BINDING_STATUS is associated with a timeout; otherwise, it MUST NOT appear. See Section 7.2 for details. The OPTION_CLT_TIME MUST, if it appears, be the time that the server last interacted with the DHCP client. It MUST NOT be, for instance, the time that the lease expired if there has been no interaction with the DHCP client in question. A server SHOULD be prepared to clean up DNS information once the lease expires or is released. See Section 9 for a detailed discussion about DNS update. Another reason the partner may be interested in keeping additional data is to enable better support for Leasequery [RFC5007], Bulk Leasequery [RFC5460], or Active Leasequery [RFC7653], some of which feature queries based on Relay-ID, link address, or Remote-ID.7.5. Receiving Binding Updates
7.5.1. Monitoring Time Skew
The sent-time from the failover message is compared with the current time of the receiving server as recorded when it received the message. The difference is noted, and if it is greater than 5 seconds the receiving server SHOULD drop the connection. A message SHOULD be logged to signal the reason for the connection being dropped. Any time can be before, after, or essentially the same as another time. Any time that ends up being +/- 5 seconds of another time SHOULD be considered to be representing the same time when performing a comparison between two times.7.5.2. Acknowledging Reception
Upon acceptance of a binding update, the server MUST notify its partner that it has processed the binding update (and updated its lease state database if necessary) by sending a BNDREPLY message. A server MUST NOT send the BNDREPLY message before its binding database is updated.
7.5.3. Processing Binding Updates
When a BNDUPD message is received, it MUST contain either a single OPTION_CLIENT_DATA option or a single OPTION_IAPREFIX option. When analyzing a BNDUPD message from a partner server, if there is insufficient information in the BNDUPD message to process it, then it is rejected with an OPTION_STATUS_CODE of "MissingBindingInformation". The server receiving a BNDUPD message from its partner must evaluate the received information in each OPTION_CLIENT_DATA or IAPREFIX option to see if it is consistent with the server's already-known state and, if it is not, decide to accept or reject the information. Section 7.5.4 provides details regarding how the server makes this determination. A server receiving a BNDUPD message MUST respond to the sender of that message with a BNDREPLY message that contains the same transaction-id as the BNDUPD message. This BNDREPLY message MUST contain either a single OPTION_CLIENT_DATA option or a single OPTION_IAPREFIX option, corresponding to whatever was received in the BNDUPD message. An OPTION_CLIENT_DATA option or an OPTION_IAPREFIX option in the BNDREPLY message that is accepted SHOULD NOT contain an OPTION_STATUS_CODE unless a status message needs to be sent to the failover partner, in which case it SHOULD include an OPTION_STATUS_CODE option with a status-code indicating success and whatever message is needed. To indicate rejection of the information in an OPTION_CLIENT_DATA option or an OPTION_IAPREFIX option, an OPTION_STATUS_CODE SHOULD be included with a status-code indicating an error in the OPTION_CLIENT_DATA option or OPTION_IAPREFIX option in the BNDREPLY message.7.5.4. Accept or Reject?
The first task in processing the information in an OPTION_CLIENT_DATA option or OPTION_IAPREFIX option is to extract the client information (if any) and lease information out of the option and to access the address lease or prefix lease information in the server's binding database. If an OPTION_VSS option is specified in the OPTION_CLIENT_DATA option or OPTION_IAPREFIX option and the VPN specified in the OPTION_VSS option does not appear in the configuration of the receiving server,
then reject the entire OPTION_CLIENT_DATA option or OPTION_IAPREFIX option by including an OPTION_STATUS_CODE option with a status-code of "ConfigurationConflict". If the lease specified in the OPTION_CLIENT_DATA option or OPTION_IAPREFIX option is not a lease associated with the failover endpoint that received the OPTION_CLIENT_DATA option, then reject it by including an OPTION_STATUS_CODE option with a status-code of "ConfigurationConflict". In general, acceptance or rejection is based on the comparison of two different time values -- one from the OPTION_CLIENT_DATA option or OPTION_IAPREFIX option in the BNDUPD message, and one from the receiving server's binding database associated with the address or prefix lease found in the BNDUPD message. The time for the BNDUPD message where the OPTION_F_BINDING_STATUS is ACTIVE, EXPIRED, or RELEASED is the OPTION_CLT_TIME if one appears, or the OPTION_F_START_TIME_OF_STATE if one does not. For other binding-status values, the time for the BNDUPD message is the later of (1) the OPTION_CLT_TIME if one appears or (2) the OPTION_F_START_TIME_OF_STATE. The time for the lease in the server's binding database is the client-last-transaction-time if one appears, or the start-time-of-state if one does not. The basic approach is to compare these times, and if the one from the BNDUPD message is clearly later, then accept the information in the OPTION_CLIENT_DATA option or OPTION_IAPREFIX option. If the one from the server's binding database is clearly later, then reject the information in the BNDUPD message. The challenge comes when they are essentially the same (i.e., +/- 5 seconds). In this case, they are considered identical, despite the minor differences. Figure 4 shows a table containing the rules for dealing with all of these situations. binding-status in received OPTION_CLIENT_DATA or OPTION_IAPREFIX binding-status in receiving server's FREE RESET lease state DB ACTIVE EXPIRED RELEASED FREE-BACKUP ABANDONED --------------------------------------------------------------------- ACTIVE accept(3) time(1) accept time(1) accept EXPIRED accept accept accept accept accept RELEASED accept accept accept accept accept FREE/FREE-BACKUP accept accept accept accept accept RESET time(2) accept accept accept accept ABANDONED accept accept accept accept accept Figure 4: Conflict Resolution
accept: If the time value in the OPTION_CLIENT_DATA option or
OPTION_IAPREFIX option is later than the time value in the
server's binding database, accept it, else reject it.
time(1): If the current time is later than the receiving server's
state-expiration-time, accept it, else reject it.
time(2): If the OPTION_CLT_TIME value (if it appears) in the
OPTION_CLIENT_DATA is later than the start-time-of-state in the
receiving server's binding, accept it, else reject it.
accept,time(1),time(2): If rejecting, use a status-code of
"OutdatedBindingInformation".
accept(3): If the clients in an OPTION_CLIENT_DATA option and in a
receiving server's binding differ, then if time(2) or the
receiving server is a secondary accept it, else reject it with a
status-code of "AddressInUse". If the clients match, accept the
update.
The lease update may be accepted or rejected. If a lease is rejected
with "OutdatedBindingInformation", then the flag in the lease that
indicates that the partner should be updated with the information in
this lease SHOULD be set; otherwise, it SHOULD NOT be changed. If
this flag was previously not set, then an update MAY be transmitted
immediately to the partner (though the BNDREPLY to this BNDUPD
message SHOULD be sent first). If this flag was previously set, an
update SHOULD NOT be transmitted immediately to the partner. In this
case, an update will be sent during the next periodic scan, but not
immediately, thus preventing a possible update storm should the
servers be unable to agree. Ultimately, the server with the most
recent binding information should have its update accepted by its
partner.
7.5.5. Accepting Updates
When the information in an OPTION_CLIENT_DATA option or
OPTION_IAPREFIX option has been accepted, some of that information is
stored in the receiving server's binding database, and a
corresponding OPTION_CLIENT_DATA option or OPTION_IAPREFIX option is
entered into a BNDREPLY message. The information to enter into the
OPTION_CLIENT_DATA option or OPTION_IAPREFIX option in the BNDREPLY
message is described in Section 7.6.
The information contained in an accepted OPTION_CLIENT_DATA option is
stored in the receiving server's binding database as follows:
1. The OPTION_CLIENTID is used to find the client.
2. The other data contained in the top level of the
OPTION_CLIENT_DATA option is stored with the client as
appropriate.
3. For each of the OPTION_IA_NA, OPTION_IA_TA, or OPTION_IA_PD
options in the OPTION_CLIENT_DATA option and for each of the
OPTION_IAADDR or OPTION_IAPREFIX options in the IA_* options:
a. OPTION_F_BINDING_STATUS is stored as the binding-status.
b. OPTION_F_PARTNER_LIFETIME is stored in the expiration-time.
c. OPTION_F_STATE_EXPIRATION_TIME is stored in the
state-expiration-time.
d. OPTION_CLT_TIME [RFC5007] is stored in the
partner-raw-clt-time.
e. OPTION_F_PARTNER_RAW_CLT_TIME replaces the
client-last-transaction-time if it is later than the current
client-last-transaction-time.
f. OPTION_F_EXPIRATION_TIME replaces the partner-lifetime if it
is later than the current partner-lifetime.
The information contained in an accepted single OPTION_IAPREFIX
option that is not contained in an OPTION_CLIENT_DATA option is
stored in the receiving server's binding database as follows:
1. The IPv6 prefix is used to find the prefix.
2. Inside of the IAprefix-options section:
a. OPTION_F_BINDING_STATUS is stored as the binding-status.
b. OPTION_F_PARTNER_LIFETIME (if any) is stored in the
expiration-time.
c. OPTION_F_STATE_EXPIRATION_TIME (if any) is stored in the
state-expiration-time.
d. OPTION_F_EXPIRATION_TIME (if any) replaces the
partner-lifetime if it is later than the current
partner-lifetime.
7.6. Sending Binding Replies
A server MUST respond to every BNDUPD message with a BNDREPLY
message. The BNDREPLY message MUST contain an OPTION_CLIENT_DATA
option if the BNDUPD message contained an OPTION_CLIENT_DATA option,
or it MUST contain an OPTION_IAPREFIX option if the BNDUPD message
contained an OPTION_IAPREFIX option. The BNDREPLY message MUST have
the same transaction-id as the BNDUPD message to which it is a
response.
Acceptance or rejection of all of or a particular part of the BNDUPD
message is signaled with an OPTION_STATUS_CODE option. An
OPTION_STATUS_CODE option containing a status-code representing an
error is significant, while an OPTION_STATUS_CODE option whose
status-code contains success is considered informational but does not
affect the processing of the BNDREPLY message when it is received by
the server that sent the BNDUPD message.
Rejection of all of or part of the information in a BNDUPD message is
signaled in a BNDREPLY message by using the OPTION_STATUS_CODE
message with an error in the status-code field. This rejection can
take place at either of two levels -- the top level of the option
hierarchy or the bottom level of the option hierarchy:
1. Entire BNDUPD: The OPTION_STATUS_CODE containing an error is
present in the outermost option of the BNDREPLY message -- either
the single OPTION_CLIENT_DATA option or the single
OPTION_IAPREFIX option. An example of this sort of error might
be that an OPTION_VSS option was present and specified a VPN that
might not exist in the receiving server.
2. Single address or prefix: The OPTION_STATUS_CODE containing an
error is present in a single IAADDR or IAPREFIX option that is
itself contained in an OPTION_IA_NA, OPTION_IA_TA, or
OPTION_IA_PD option. An example of this sort of error might be
that a particular IPv6 address was specified in an IAADDR option
that doesn't appear in the receiving server's configuration.
Rejection occurring at either of these levels indicates rejection of
all of the information contained in the option (including any other
options contained in that option) where the OPTION_STATUS_CODE option
containing an error appears. The converse is not true -- an
OPTION_STATUS_CODE option containing success does not signify that
all of the contained information has been accepted.
If the BNDREPLY message contains an OPTION_CLIENT_DATA option, then
the OPTION_CLIENT_DATA option MUST contain at least the data shown
below in its client-options section:
o OPTION_CLIENTID containing the DUID of the client most recently
associated with this IPv6 address.
o OPTION_VSS from the BNDUPD message, if any.
o OPTION_IA_NA or OPTION_IA_TA for an IPv6 address or OPTION_IA_PD
for an IPv6 prefix. More than one of either of these options MAY
appear if there are more than one of them associated with this
client.
* Inside of the IA_NA-options, IA_TA-options, or IA_PD-options
sections:
+ OPTION_IAADDR for an IPv6 address or an OPTION_IAPREFIX for
an IPv6 prefix.
- IPv6 address or IPv6 prefix (with length).
- Inside of the IAaddr-options or IAprefix-options:
o OPTION_STATUS_CODE containing an error code, or
containing a success code if a message is required.
An OPTION_STATUS_CODE option SHOULD NOT appear with a
success code unless a message associated with the
success code needs to be included. The lack of an
OPTION_STATUS_CODE option is an indication of success.
o OPTION_F_BINDING_STATUS containing the binding-status
received in the BNDUPD message.
o OPTION_F_STATE_EXPIRATION_TIME (absolute) containing
the state-expiration-time received in the BNDUPD
message.
o OPTION_F_PARTNER_LIFETIME_SENT (absolute) containing a
duplicate of the OPTION_F_PARTNER_LIFETIME received in
the BNDUPD message.
If the BNDREPLY message contains a single OPTION_IAPREFIX option not
contained in an OPTION_CLIENT_DATA option, then the OPTION_IAPREFIX
option MUST contain at least the data shown below:
o IPv6 prefix (with length).
o IAprefix-options:
* OPTION_VSS from the BNDUPD message, if any.
* OPTION_STATUS_CODE containing an error code, or containing a
success code if a message is required. If the information in
the corresponding OPTION_IAPREFIX in the BNDUPD message was
accepted and no status message was required (which is the usual
case), no OPTION_STATUS_CODE option appears.
* OPTION_F_BINDING_STATUS containing the binding-status received
in the BNDREPLY message.
* OPTION_F_STATE_EXPIRATION_TIME (absolute) containing the
state-expiration-time received in the BNDREPLY message.
* OPTION_F_PARTNER_LIFETIME_SENT (absolute) containing a
duplicate of the OPTION_F_PARTNER_LIFETIME received in the
BNDREPLY message.
7.7. Receiving Binding Acks
When a BNDREPLY message is received, the overall OPTION_CLIENT_DATA
option or the overall OPTION_IAPREFIX option may contain an
OPTION_STATUS_CODE containing an error that represents a rejection of
the entire BNDUPD message. An enclosed OPTION_IA_NA, OPTION_IA_TA,
or OPTION_IA_PD option may also contain an OPTION_STATUS_CODE
containing an error that indicates that everything in the containing
option has been rejected. Alternatively, an individual IAADDR or
IAPREFIX option may contain an OPTION_STATUS_CODE option containing
an error that indicates that the IAADDR or IAPREFIX option has been
rejected. An OPTION_STATUS_CODE containing a success code has no
bearing on the acceptance status of the BNDREPLY message at any
level.
Receipt of a rejection (or a part of a BNDREPLY message that has been
rejected) requires no processing, other than remembering that it has
been encountered.
The information contained in the BNDREPLY message in an
OPTION_CLIENT_DATA that represents an acceptance is stored with the
appropriate client and lease, as follows:
1. The OPTION_CLIENTID is used to find the client.
2. For each of the OPTION_IA_NA, OPTION_IA_TA, or OPTION_IA_PD
options in the OPTION_CLIENT_DATA option and for each of the
OPTION_IAADDR or OPTION_IAPREFIX options they contain:
a. OPTION_F_PARTNER_LIFETIME_SENT is stored in the
acked-partner-lifetime.
b. The partner-lifetime is set to 0 to indicate that no more
information needs to be sent to the partner.
Alternatively, the BNDREPLY message may contain a single
OPTION_IAPREFIX option not contained in an OPTION_CLIENT_DATA option,
representing information concerning a single prefix lease. If the
IAprefix-options section of the OPTION_IAPREFIX option contains an
OPTION_STATUS_CODE representing an error, then it is considered a
rejection of the corresponding BNDUPD message. If the
OPTION_IAPREFIX option does not contain an OPTION_STATUS_CODE option
or if the OPTION_STATUS_CODE option contains a success status, then
the three items in the following list are stored in the lease state
database, in the section associated with the prefix lease represented
by the OPTION_IAPREFIX option.
1. OPTION_F_BINDING_STATUS containing the binding-status received in
the BNDREPLY message.
2. OPTION_F_STATE_EXPIRATION_TIME (absolute) containing the
state-expiration-time received in the BNDREPLY message.
3. OPTION_F_PARTNER_LIFETIME_SENT (absolute) containing a duplicate
of the OPTION_F_PARTNER_LIFETIME received in the BNDREPLY
message.
7.8. BNDUPD/BNDREPLY Data Flow
Figure 5 shows the relationship of the times described in Section 7.3 to the options used to transmit them. It also relates the times on one failover partner to the other failover partner. ----------------------- BNDUPD --------------------------------- Source on OPTION_F in Storage on Sending Server -> BNDUPD message -> Receiving Server [always update] partner-lifetime PARTNER_LIFETIME expiration-time client-last-transaction-time CLT_TIME partner-raw-clt-time start-time-of-state START_TIME_OF_STATE start-time-of-state state-expiration-time STATE_EXPIRATION_TIME state-expiration-time [update only if received > current] expiration-time EXPIRATION_TIME partner-lifetime partner-raw-clt-time PARTNER_RAW_CLT_TIME client-last-transaction-time ----------------------- BNDREPLY ------------------------------- Storage on OPTION_F in Storage on Receiving Server <- BNDUPD message <- Sending Server [always update] acked-partner-lifetime PARTNER_LIFETIME_SENT duplicate of received PARTNER_LIFETIME (nothing to update) STATE_EXPIRATION_TIME state-expiration-time ---------------------------------------------------------------- Figure 5: BNDUPD and BNDREPLY Time Handling
(next page on part 4)