RFC 6320
Protocol for Access Node Control Mechanism in Broadband Networks
3. Access Node Control Protocol -- General Aspects
This section specifies aspects of the Access Node Control Protocol (ANCP) that are generally applicable.3.1. Protocol Version
ANCP messages contain an 8-bit protocol version field. For the protocol version specified in this document, the value of that field MUST be set to 50.3.2. ANCP Transport
This document specifies the use of TCP / IPsec+IKEv2 / IP for transport of ANCP messages. For further discussion of the use of IPsec and IKEv2, see Section 11. The present section deals with the TCP aspects. Other specifications may introduce additional transports in the future. In the case of ATM access, a separate permanent virtual circuit (PVC) that is a control channel and is capable of transporting IP MAY be configured between the NAS and the AN for ANCP messages. In the case of an Ethernet access/aggregation network, a typical practice is to send the Access Node Control Protocol messages over a dedicated Ethernet virtual LAN (VLAN) using a separate VLAN identifier (VLAN ID). When transported over TCP, ANCP messages MUST use an encapsulation consisting of a 4-byte header field prepended to the ANCP message as shown in Figure 3.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier (0x880C) | Length | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ ANCP Message ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Encapsulation of ANCP Messages over TCP/IP The fields of the encapsulating header are as follows: Identifier (16 bits): This identifies a GSMP or ANCP message. It MUST be set to 0x880C. Length (16 bits): Total length of the ANCP message in bytes, not including the 4-byte encapsulating header. The Access Node MUST initiate the TCP session to the NAS, using destination port 6068. This is necessary to avoid static address provisioning on the NAS for all the ANs that are being served by the NAS. It is easier to configure a given AN with the single IP address of the NAS that serves the AN. The NAS MUST listen on port 6068 for incoming connections from the Access Nodes. In the event of an ANCP transport protocol failure, all pending ANCP messages destined to the disconnected recipient SHOULD be discarded until the transport connection is re-established.3.3. Encoding of Text Fields
In ANCP, all text fields use UTF-8 encoding [RFC3629]. Note that US- ASCII characters have the same representation when coded as UTF-8 as they do when coded according to [US_ASCII]. When extracting text fields from a message, the ANCP agent MUST NOT assume that the fields are zero-terminated.
3.4. Treatment of Reserved and Unused Fields
ANCP messages contain a number of fields that are unused or reserved. Some fields are always unused (typically because they were inherited from GSMPv3 but are not useful in the ANCP context). Others are reserved in the current specification, but are provided for flexibility in future extensions to ANCP. Both reserved and unused fields MUST be set to zeroes by the sender and MUST be ignored by the receiver. Unused bits in a flag field are shown in figures as 'x'. The above requirement (sender set to zero, receiver ignore) applies to such unused bits.3.5. The ANCP Adjacency Protocol
ANCP uses the adjacency protocol to synchronize the NAS and Access Nodes and maintain the ANCP session. After the TCP connection is established, adjacency protocol messages MUST be exchanged as specified in this section. ANCP messages other than adjacency protocol messages MUST NOT be sent until the adjacency protocol has achieved synchronization.3.5.1. ANCP Adjacency Message Format
The ANCP adjacency message format is shown in Figure 4 below.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version | Message Type | Timer |M| Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sender Name | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Receiver Name | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sender Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Receiver Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PType |P Flag | Sender Instance | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Partition ID | Receiver Instance | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | # of Caps | Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Capability Fields ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: ANCP Adjacency Message Format The fields of the ANCP adjacency message are as follows: Version (8 bits): ANCP version, which is subject to negotiation. This is the key parameter by means of which ANCP messages can be distinguished from GSMP messages received over the same port. Message Type (8 bits): Always has value 10 (adjacency protocol). Timer (8 bits): The Timer field is used to negotiate the timer value used in the adjacency protocol with the peer. The timer specifies the nominal time between periodic adjacency protocol messages. It is a constant for the duration of an ANCP session. The Timer field is specified in units of 100 ms, with a default value of 250 (i.e., 25 seconds). M flag (1 bit): Used in the SYN message to prevent the NAS from synchronizing with another NAS and the AN from synchronizing with another AN. In the SYN message, it is always set to 1 by the NAS and to 0 by the AN. In other adjacency message types, it is always set to 0 by the sender and ignored by the receiver.
Code (7 bits): The adjacency protocol message type. It MUST have
one of the following values:
Code = 1: SYN;
Code = 2: SYNACK;
Code = 3: ACK;
Code = 4: RSTACK.
Sender Name (48 bits): For the SYN, SYNACK, and ACK messages, is the
identifier of the entity sending the message. The Sender Name is
a 48-bit quantity that is unique within the operational context of
the device. A 48-bit IEEE 802 Media Access Control (MAC) address,
if available, may be used for the Sender Name. If the Ethernet
encapsulation is used, the Sender Name MUST be the Source Address
from the MAC header. For the RSTACK message, the Sender Name
field is set to the value of the Receiver Name field from the
incoming message that caused the RSTACK message to be generated.
Receiver Name (48 bits) For the SYN, SYNACK, and ACK messages, is
the name of the entity that the sender of the message believes is
at the far end of the link. If the sender of the message does not
know the name of the entity at the far end of the link, this field
SHOULD be set to zero. For the RSTACK message, the Receiver Name
field is set to the value of the Sender Name field from the
incoming message that caused the RSTACK message to be generated.
Sender Port (32 bits): For the SYN, SYNACK, and ACK messages, is the
local port number of the link across which the message is being
sent. For the RSTACK message, the Sender Port field is set to the
value of the Receiver Port field from the incoming message that
caused the RSTACK message to be generated.
Receiver Port (32 bits): For the SYN, SYNACK, and ACK messages, is
what the sender believes is the local port number for the link,
allocated by the entity at the far end of the link. If the sender
of the message does not know the port number at the far end of the
link, this field SHOULD be set to zero. For the RSTACK message,
the Receiver Port field is set to the value of the Sender Port
field from the incoming message that caused the RSTACK message to
be generated.
PType (4 bits): PType is used to specify if partitions are used and
how the Partition ID is negotiated.
Type of partition being requested:
0 - no partition;
1 - fixed partition request;
2 - fixed partition assigned.
P Flag (4 bits): Used to indicate the type of partition request.
1 - new adjacency;
2 - recovered adjacency.
In case of a conflict between the peers' views of the value of the
P Flag, the lower value is used.
Sender Instance (24 bits): For the SYN, SYNACK, and ACK messages, is
the sender's instance number for the link to the peer. It is used
to detect when the link comes back up after going down or when the
identity of the entity at the other end of the link changes. The
instance number is a 24-bit number that is guaranteed to be unique
within the recent past and to change when the link or node comes
back up after going down. Zero is not a valid instance number.
For the RSTACK message, the Sender Instance field is set to the
value of the Receiver Instance field from the incoming message
that caused the RSTACK message to be generated.
Partition ID (8 bits): Field used to associate the message with a
specific partition of the AN. The value of this field is
negotiated during the adjacency procedure. The AN makes the final
decision, but will consider a request from the NAS. If the AN
does not support partitions, the value of this field MUST be 0.
Otherwise, it MUST be non-zero.
Receiver Instance (24 bits): For the SYN, SYNACK, and ACK messages,
is what the sender believes is the current instance number for the
link, allocated by the entity at the far end of the link. If the
sender of the message does not know the current instance number at
the far end of the link, this field SHOULD be set to zero. For
the RSTACK message, the Receiver Instance field is set to the
value of the Sender Instance field from the incoming message that
caused the RSTACK message to be generated.
Reserved (8 bits): Reserved for use by a future version of this
specification.
# of Caps (8 bits): Indicates the number of Capability fields that
follow.
Total Length (16 bits): Indicates the total number of bytes occupied
by the Capability fields that follow.
Capability Fields: Each Capability field indicates one ANCP
capability supported by the sender of the adjacency message.
Negotiation of a common set of capabilities to be supported within
the ANCP session is described below. The detailed format of a
Capability field is shown in Figure 5 and described below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Capability Type | Capability Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
~ Capability Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Capability Field
The sub-fields of this structure are as follows:
Capability Type (16 bits): Indicates the specific capability
supported. An IANA registry exists for values of this sub-field.
The values specified by this document are listed below.
Capability Length (16 bits): The number of bytes of data contained
in the Capability Data sub-field, excluding padding. If the
definition of a particular capability includes no capability data,
the value of the Capability Length sub-field is zero.
Capability Data (as indicated by Capability Length): Contains data
associated with the capability as specified for that capability.
If the definition of a particular capability includes no
capability data, the Capability Data sub-field is absent (has zero
length). Otherwise, the Capability Data sub-field MUST be padded
with zeroes as required to terminate on a 4-byte word boundary.
The possibility of specifying capability data provides the
flexibility to advertise more than the mere presence or absence of
a capability if needed.
The following capabilities are defined for ANCP as applied to DSL
access:
o Capability Type: DSL Topology Discovery = 0x01
Access technology: DSL
Length (in bytes): 0
Capability Data: NULL
For the detailed protocol specification of this capability, see
Section 6.
o Capability Type: DSL Line Configuration = 0x02
Access technology: DSL
Length (in bytes): 0
Capability Data: NULL
For the detailed protocol specification of this capability, see
Section 7.
o Capability Type: DSL Remote Line Connectivity Testing = 0x04
Access technology: DSL
Length (in bytes): 0
Capability Data: NULL
For the detailed protocol specification of this capability, see
Section 8.
In addition to the adjacency messages whose format is shown in
Figure 6, ANCP adjacency procedures use the Adjacency Update message
(Figure 6) to inform other NASs controlling the same AN partition
when a particular NAS joins or loses an adjacency with that
partition.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version | Message Type | Result| Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Partition ID | Transaction Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |I| SubMessage Number | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: The Adjacency Update Message The Adjacency Update message is identical to the general ANCP message header described in Section 3.6, but the field settings are in part specific to the Adjacency Update message. The fields in this message are as follows: Version (8 bits): The ANCP version negotiated and running in this adjacency. Message Type (8 bits): Always 85. Result (4 bits): Set to Ignore (0). Code (12 bits): Set to the total number of adjacencies currently established on this partition, from the point of view of the AN. Partition ID (8 bits): The partition identifier of the partition for which this notification is being sent. Transaction Identifier (24 bits): MUST be set to 0. I (1 bit), SubMessage number (15 bits): Set as described in Section 3.6.1.7. Length (16 bits): Set as described in Section 3.6.1.8.3.5.2. ANCP Adjacency Procedures
3.5.2.1. Overview
The ANCP adjacency protocol operates symmetrically between the NAS and the AN. In the absence of errors or race conditions, each peer sends a SYN message, receives a SYNACK message in acknowledgement, and completes the establishment of the adjacency by sending an ACK message. Through this exchange, each peer learns the values of the Name, Port, and Instance parameters identifying the other peer, and
the two peers negotiate the values of the Version, Timer, P Flag, and Partition ID parameters and the set of capabilities that the adjacency will support. Once the adjacency has been established, its liveness is periodically tested. The peers engage in an ACK message exchange at a frequency determined by the negotiated value of the Timer field. If an inconsistency, loss of contact, or protocol violation is detected, the detecting peer can force a restart of the synchronization process by sending an RSTACK message to the other end. Once an adjacency has been established, if more than one NAS has established an adjacency to the same partition, then the AN sends an Adjacency Update message to each such NAS to let it know how many established adjacencies the partition currently supports. Similarly, if an adjacency is lost, the AN sends an Adjacency Update message to each of the remaining adjacent NASs to let them know about the change in status.3.5.2.2. Adjacency Protocol State Machine
The adjacency protocol is described by the following rules and state tables. It begins with the sending of a SYN by each end as soon as the transport connection has been established. If at any point the operations A, B, C, or "Verify Adjacent State" defined below detect a mismatch, a log SHOULD be generated, identifying the fields concerned and the expected and received values for each. The rules and state tables use the following operations: o The "Record Adjacency State" operation is defined in Section 3.5.2.3.2. o The "Verify Adjacency State" operation consists of verifying that the contents of the incoming SYNACK message match the adjacency state values previously recorded. o The procedure "Reset the link" is defined as: 1. Generate a new instance number for the link. 2. Delete the peer verifier (set to zero the values of Sender Instance, Sender Port, and Sender Name previously stored by the "Record Adjacency State" operation). 3. Send a SYN message (Section 3.5.2.3.1).
4. Enter the SYNSENT state.
o The state tables use the following Boolean terms and operators.
A. The Sender Instance in the incoming message matches the value
stored from a previous message by the "Record Adjacency State"
operation.
B. The Sender Instance, Sender Port, Sender Name, and Partition
ID fields in the incoming message match the values stored from
a previous message by the "Record Adjacency State" operation.
C. The Receiver Instance, Receiver Port, Receiver Name, and
Partition ID fields in the incoming message match the values
of the Sender Instance, Sender Port, Sender Name, and
Partition ID currently sent in outgoing SYN, SYNACK, and ACK
messages, except that the NAS always accepts the Partition ID
value presented to it in a SYN or SYNACK message.
"&&" Represents the logical AND operation.
"||" Represents the logical OR operation.
"!" Represents the logical negation (NOT) operation.
o A timer is required for the periodic generation of SYN, SYNACK,
and ACK messages. The value of the timer is negotiated in the
Timer field. The period of the timer is unspecified, but a value
of 25 seconds is suggested. Note that since ANCP uses a reliable
transport protocol, the timer is unlikely to expire in any state
other than ESTAB.
There are two independent events: the timer expires, and a packet
arrives. The processing rules for these events are:
Timer Expires: Reset Timer
If state = SYNSENT Send SYN
If state = SYNRCVD Send SYNACK
If state = ESTAB Send ACK
Packet Arrives:
If incoming message is an RSTACK:
If (A && C && !SYNSENT) Reset the link
Else discard the message.
If incoming message is a SYN, SYNACK, or ACK:
Response defined by the following state tables.
If incoming message is any other ANCP message and state !=
ESTAB:
Discard incoming message.
If state = SYNSENT Send SYN (Note 1)
If state = SYNRCVD Send SYNACK (Note 1)
Note 1: No more than two SYN or SYNACK messages should be sent
within any time period of length defined by the timer.
o State synchronization across a link is considered to be achieved
when the protocol reaches the ESTAB state. All ANCP messages,
other than adjacency protocol messages, that are received before
synchronization is achieved will be discarded.
3.5.2.2.1. State Tables
State: SYNSENT
+===================================================================+
| Condition | Action | New State |
+=================+=====================================+===========+
| SYNACK && C | Update Peer Verifier; Send ACK | ESTAB |
+-----------------+-------------------------------------+-----------+
| SYNACK && !C | Send RSTACK | SYNSENT |
+-----------------+-------------------------------------+-----------+
| SYN | Update Peer Verifier; Send SYNACK | SYNRCVD |
+-----------------+-------------------------------------+-----------+
| ACK | Send RSTACK | SYNSENT |
+===================================================================+
State: SYNRCVD
+===================================================================+
| Condition | Action | New State |
+=================+=====================================+===========+
| SYNACK && C | Verify Adjacency State; Send ACK | ESTAB |
+-----------------+-------------------------------------+-----------+
| SYNACK && !C | Send RSTACK | SYNRCVD |
+-----------------+-------------------------------------+-----------+
| SYN | Record Adjacency State; Send SYNACK | SYNRCVD |
+-----------------+-------------------------------------+-----------+
| ACK && B && C | Send ACK | ESTAB |
+-----------------+-------------------------------------+-----------+
| ACK && !(B && C)| Send RSTACK | SYNRCVD |
+===================================================================+
State: ESTAB
+===================================================================+
| Condition | Action | New State |
+=================+=====================================+===========+
| SYN || SYNACK | Send ACK (Note 2) | ESTAB |
+-----------------+-------------------------------------+-----------+
| ACK && B && C | Send ACK (Note 3) | ESTAB |
+-----------------+-------------------------------------+-----------+
| ACK && !(B && C)| Send RSTACK | ESTAB |
+===================================================================+
Note 2: No more than two ACKs should be sent within any time period
of length defined by the timer. Thus, one ACK MUST be sent every
time the timer expires. In addition, one further ACK may be sent
between timer expirations if the incoming message is a SYN or SYNACK.
This additional ACK allows the adjacency protocol to reach
synchronization more quickly.
Note 3: No more than one ACK should be sent within any time period of
length defined by the timer.
3.5.2.3. The Adjacency Protocol SYN Message
3.5.2.3.1. Action by the Sender
The SYN message is sent in accordance with the state tables just
described. The sender sets the individual fields as follows:
Version: SHOULD be set to the highest version of ANCP that the
sender supports.
Message Type: MUST be set to 10.
Timer: SHOULD be set to the value configured in the AN or NAS
sending the message.
M Flag: MUST be set to 1 by the NAS, and 0 by the AN.
Code: MUST be set to 1 (SYN).
Sender Name: Set as described in Section 3.5.1.
Receiver Name: SHOULD be set to 0.
Sender Port: Set as described in Section 3.5.1.
Receiver Port: SHOULD be set to 0.
PType: Set according to the following rules:
Settings by the AN:
0 - the AN does not support partitions;
2 - the value of Partition ID contained in this message is
assigned to the current partition.
Settings by the NAS:
0 - the NAS leaves the decision on partitioning to the AN
(RECOMMENDED setting);
1 - the NAS requests that the AN use the value of Partition
ID contained in this message for the current partition. The
NAS MAY use this setting even if it has already received a
SYN message from the AN, provided that the AN has indicated
support for partitions. The NAS MUST be prepared to use
whatever value it receives in a subsequent SYN or SYNACK
message, even if this differs from the requested value.
P Flag: Set to the mode of adjacency setup (new adjacency vs.
recovered adjacency) requested by the sender. Warning: setting P
Flag=1 runs the risk of state mismatch because ANCP does not
provide the means for the NAS to audit the current state of the
AN.
Sender Instance: Set as described in Section 3.5.1. Partition ID: MUST be set to 0 if PType=0; otherwise, set to the assigned or requested partition identifier value. Receiver Instance: SHOULD be set to 0. # of Caps: MUST be set to the number of Capability fields that follow. Total Length: MUST be set to the total number of bytes in the Capability fields that follow. Capability Fields: One Capability field MUST be present for each ANCP capability for which the sender wishes to advertise support.3.5.2.3.2. Action by the Receiver
Upon receiving a validly formed SYN message, the receiver first checks the value of the Version field. If this value is not within the range of ANCP versions that the receiver supports, the message MUST be silently ignored. Similarly, the message is silently ignored if the M flag is 0 and the receiver is an AN or if the M flag is 1 and the receiver is a NAS. If these checks are passed and the receiver is in ESTAB state, it returns an ACK (as indicated by the ESTAB state table in Section 3.5.2.2.1). The contents of the ACK MUST reflect the adjacency state as previously recorded by the receiver. Otherwise, the receiver MUST perform the "Record Adjacency State" operation by recording the following fields: Version: The supported Version value received in the SYN message. This value MUST be used for all subsequent ANCP messages sent during the life of the adjacency. Timer: The larger of the Timer value received in the SYN message and the value with which the receiver is configured. Sender Name: The value of the Sender Name field in the SYN message just received. Receiver Name: The value used by the receiver in the Sender Name field of SYN, SYNACK, and ACK messages it sends in this adjacency.
Sender Port: The value of the Sender Port field in the SYN message
just received.
Receiver Port: The value used by the receiver in the Sender Port
field of SYN, SYNACK, and ACK messages it sends in this adjacency.
Sender Instance: The value of the Sender Instance field in the SYN
message just received.
P Flag: The lesser of the value determined by local policy and the
value received in the SYN message. That is, preference is given
to "0 - New adjacency" if there is a conflict.
Partition ID: If the SYN receiver is the AN, this is set to 0 if the
AN does not support partitions or to the non-zero value of the
partition identifier it chooses to assign otherwise. If the SYN
receiver is the NAS, this is set to the value of the Partition ID
field copied from the SYN.
Receiver Instance: The value used by the receiver in the Sender
Instance field of SYN, SYNACK, and ACK messages it sends in this
adjacency.
Capabilities: The set of ANCP capabilities that were offered in the
SYN and are supported by the receiver.
3.5.2.4. The Adjacency Protocol SYNACK Message
3.5.2.4.1. Action by the Sender
The SYNACK is sent in response to a successfully received SYN
message, as indicated by the state tables. The Version, Timer, P
Flag, and Partition ID fields MUST be populated with the values
recorded as part of adjacency state. The # of Caps, Total Length,
and Capability fields MUST also be populated in accordance with the
Capabilities recorded as part of adjacency state. The remaining
fields of the SYNACK message MUST be populated as follows:
Message Type: MUST be 10.
M flag: MUST be set to 0.
Code: MUST be 2 (SYNACK).
PType: MUST be 0 if the Partition ID value is 0 or 2 if the
Partition ID value is non-zero.
Sender Name: MUST be set to the Receiver Name value recorded as part
of adjacency state.
Receiver Name: MUST be set to the Sender Name value recorded as part
of adjacency state.
Sender Port: MUST be set to the Receiver Port value recorded as part
of adjacency state.
Receiver Port: MUST be set to the Sender Port value recorded as part
of adjacency state.
Sender Instance: MUST be set to the Receiver Instance value recorded
as part of adjacency state.
Receiver Instance: MUST be set to the Sender Instance value recorded
as part of adjacency state.
If the set of capabilities recorded in the adjacency state is empty,
then after sending the SYNACK the sender MUST raise an alarm to
management, halt the adjacency procedure, and tear down the TCP
session if it is not being used by another adjacency. The sender MAY
also terminate the IPsec security association if no other adjacency
is using it.
3.5.2.4.2. Action by the Receiver
As indicated by the state tables, the receiver of a SYNACK first
checks that the Receiver Name, Receiver Port, and Receiver Instance
values match the Sender Name, Sender Port, and Sender Instance values
it sent in SYN message that is being acknowledged. The AN also
checks that the PType and Partition ID match. If any of these checks
fail, the receiver sends an RSTACK as described in Section 3.5.2.6.1.
The receiver next checks whether the set of capabilities provided in
the SYNACK is empty. If so, the receiver MUST raise an alarm to
management and halt the adjacency procedure.
Assuming that the SYNACK passes these checks, two cases arise. The
first possibility is that the receiver has already recorded adjacency
state. This will occur if the SYNACK is received while the receiver
is in SYNRCVD state. In this case, the Version, Timer, Sender Name,
Sender Port, Sender Instance, P Flag, and capability-related fields
in the SYNACK MUST match those recorded as part of adjacency state.
If a mismatch is detected, the receiver sends an RSTACK. This is the
"Verify Adjacency State" procedure shown in the SYNRCVD state table.
If, on the other hand, the SYNACK is received while the receiver is in SYNSENT state, the receiver MUST record session state as described in Section 3.5.2.3.2. In either case, if the receiver is the NAS, it MUST accept the Partition ID value provided in the SYNACK, updating its recorded adjacency state if necessary.3.5.2.5. The Adjacency Protocol ACK Message
3.5.2.5.1. Actions by the Sender
As indicated by the state tables, the ACK message is sent in a number of different circumstances. The main-line usages are as a response to SYNACK, leading directly to the ESTAB state, and as a periodic test of liveness once the ESTAB state has been reached. The sender MUST populate the ACK from recorded adjacency state, exactly as described in Section 3.5.2.4.1. The only difference is that Code MUST be set to 3 (ACK).3.5.2.5.2. Actions by the Receiver
The required actions by the receiver are specified by the state tables. In addition to the checks B and C, the receiver SHOULD verify that the remaining contents of the ACK match the recorded adjacency state at the receiver. If that check fails, the receiver MUST send an RSTACK as described in Section 3.5.2.6.1. Once the adjacency has been established, either peer can initiate the ACK exchange that tests for liveness. To meet the restrictions on ACK frequency laid down in the notes to the state tables, it is desirable that only one such exchange occur during any one interval. Hence, if a peer receives an ACK when in ESTAB state, it MUST reply to that ACK as directed by the state tables, but SHOULD NOT initiate another ACK exchange in the same interval. To meet this objective, the receiver MUST reset its timer when it receives an ACK while in ESTAB state. It is, of course, possible that two exchanges happen because of race conditions.
3.5.2.6. The Adjacency Protocol RSTACK Message
3.5.2.6.1. Action by the Sender
The RSTACK is sent in response to various error conditions as indicated by the state tables. In general, it leads to a restart of adjacency negotiations (although this takes a few steps when the original sender of the RSTACK is in ESTAB state). As indicated in Section 3.5.1, the Sender Name, Port, and Instance fields in the RSTACK MUST be copied from the Receiver, Name, Port, and Instance fields in the message that caused the RSTACK to be sent. Similarly, the Receiver identifier fields in the RSTACK MUST be copied from the corresponding Sender identifier fields in the message that triggered the RSTACK. If the sender has recorded adjacency state, the Version, Timer, PType, P Flag, Partition ID, and capability-related fields SHOULD be set based on the recorded adjacency state. Otherwise, they SHOULD be the same as the sender would send in a SYN message. The Message Type MUST be 10, the M flag MUST be 0, and Code MUST be 4 (RSTACK).3.5.2.6.2. Action by the Receiver
The receiver of an RSTACK MAY attempt to diagnose the problem that caused the RSTACK to be generated by comparing its own adjacency state with the contents of the RSTACK. However, the primary purpose of the RSTACK is to trigger action as prescribed by Section 3.5.2.2.3.5.2.7. Loss of Synchronization
Loss of synchronization MAY be declared if after synchronization is achieved: o no valid ANCP messages are received in any period of time in excess of three times the value of the Timer field negotiated in the adjacency protocol messages, or o a mismatch in adjacency state is detected. In either case, the peer detecting the condition MUST send an RSTACK to the other peer, as directed in Section 3.5.2.6.1, in order to initiate resynchronization. While re-establishing synchronization with a controller, a switch SHOULD maintain its connection state, deferring the decision about resetting the state until after synchronization is re-established.
Once synchronization is re-established, the decision about resetting the connection state SHOULD be made based on the negotiated value of the P Flag.3.6. ANCP General Message Formats
This section describes the general format of ANCP messages other than the adjacency messages. See Figure 7. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version | Message Type | Result| Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Partition ID | Transaction Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |I| SubMessage Number | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Message Payload ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: ANCP General Message Format3.6.1. The ANCP Message Header
A complete explanation of the ANCP general message header fields follows.3.6.1.1. Version Field (8 bits)
This field carries the version of ANCP that was agreed upon for the session during adjacency negotiation.3.6.1.2. Message Type Field (8 bits)
This field indicates the ANCP message type. Message type values are registered in an IANA registry.3.6.1.3. Result Field (4 bits)
In request messages, the Result field indicates the circumstances under which a response is required. ANCP specifies what Result value each request message type should have. In responses, the Result field indicates either Success (0x3) or Failure (0x4), as the case may be.
Ignore: Res = 0x0 - Treat this field as a "no operation" and follow
the response procedures specified for the received message type.
Nack: Res = 0x1 - Result value indicating that a response is
expected to the request only in cases of failure caused during the
processing of the message contents or of the contained
directive(s).
AckAll: Res = 0x2 - Result value indicating that a response to the
message is requested in all cases.
Success: Res = 0x3 - Result value indicating that this is a response
and that the request was executed successfully. The Result Code
field for a successful result is typically 0, but it MAY take on
other values as specified for particular message types.
Failure: Res = 0x4 - Result value indicating that this is a response
and that the request was not executed successfully. The receiver
of the response SHOULD take further action as indicated by the
Result Code value and any diagnostic data contained in a Status-
Info TLV included in the response.
3.6.1.4. Result Code Field (12 bits)
This field gives further information concerning the result in a
response message. It is mostly used to pass an error code in a
failure response, but it can also be used to give further information
in a success response message or an event message. In a request
message, the Result Code field is not used and MUST be set to 0x0 (No
result).
A number of Result Code values are specified below. Specification of
additional Result Code values in extensions or updates to this
document MUST include the following information:
o Result Code value;
o One-line description;
o Where condition detected (control application or ANCP agent);
o Further description (if any);
o Required additional information in the response message;
o Target (control application or ANCP agent at the peer that sent
the original request);
o Action RECOMMENDED for the receiving ANCP agent.
In addition to any suggested action in the text that follows, a count
of the number of times a given non-zero Result Code value was
received SHOULD be provided for management. Where an action includes
the re-sending of a request, a given request SHOULD NOT be re-sent
more than once.
This document specifies the following Result Code values.
Result Code value: 0x2
* One-line description: Invalid request message
* Where condition detected: ANCP agent
* Further description: The request was a properly formed message
that violates the protocol through its timing or direction of
transmission. The most likely reason for this outcome in the
field will be a race condition.
* Required additional information in the response message: None,
if the response message is of the same type as the request. As
specified in Section 4.2, if the response message is a Generic
Response message.
* Target: ANCP agent at the peer that sent the original request
* Action RECOMMENDED for the receiving ANCP agent: The original
request MAY be re-sent once only after a short delay. Inform
the control application with appropriate identification of the
failed transaction if the second attempt fails or no second
attempt is made.
Result Code value: 0x6
* One-line description: One or more of the specified ports are
down
* Where condition detected: Control application
* Further description (if any): This Result Code value indicates
a state mismatch between the NAS and AN control applications,
possibly due to a race condition.
* Required additional information in the response message: If the
request identified multiple access lines or the response is a
Generic Response message, then the response MUST contain a
Status-Info TLV encapsulating TLV(s) containing the line
identifier(s) of the access lines that are not operational.
* Target: Control application at the peer that sent the original
request
* Action RECOMMENDED for the receiving ANCP agent: Indicate the
error and forward the line identifier(s) to the control
application.
Result Code value: 0x13
* One-line description: Out of resources
* Where condition detected: ANCP protocol layer or control
application
* Further description (e.g., memory exhausted): This Result Code
value MUST be reported only by the AN, and indicates a
condition that is probably unrelated to specific access lines
(although it may be related to the specific request).
* Required additional information in the response message: None,
if the response message is of the same type as the request. As
specified in Section 4.2, if the response message is a Generic
Response message.
* Target: ANCP agent at the peer that sent the original request
* Action RECOMMENDED for the receiving ANCP agent: If the NAS
receives this Result Code value from multiple requests for the
same AN in a short interval, it SHOULD reduce the rate at which
it sends requests in proportion to the rate at which requests
are failing with Result Code = 19. It MAY retry individual
requests. If only a specific request is failing with Result
Code = 19, the ANCP agent in the NAS MAY request the control
application to decompose the request into simpler components if
this is possible.
Result Code value: 0x51
* One-line description: Request message type not implemented
* Where condition detected: ANCP agent
* Further description: This could indicate a mismatch in protocol
version or capability state. It is also possible that support
of a specific message is optional within some ANCP capability.
* Required additional information in the response message: None,
if the response message is of the same type as the request. As
specified in Section 4.2, if the response message is a Generic
Response message.
* Target: ANCP agent at the peer that sent the original request
* Action RECOMMENDED for the receiving ANCP agent: If the
receiver of this Result Code value expects that support of the
message type concerned is mandatory according to the
capabilities negotiated for the session, it MAY re-send the
message in case the message was corrupted in transit the first
time. If that fails, and use of the message type cannot be
avoided, the ANCP agent MAY reset the adjacency by sending an
RSTACK adjacency message as described in Section 3.5.2.6.1,
where Sender and Receiver Name, Port, and Instance are taken
from recorded adjacency state. If a reset does not eliminate
the problem, the receiving ANCP agent SHOULD raise an alarm to
management and then cease to operate.
Result Code value: 0x53
* One-line description: Malformed message
* Where condition detected: ANCP agent
* Further description: This could be the result of corruption in
transit, or an error in implementation at one end or the other.
* Required additional information in the response message: None,
if the response message is of the same type as the request. As
specified in Section 4.2, if the response message is a Generic
Response message.
* Target: ANCP agent at the peer that sent the original request
* Action RECOMMENDED for the receiving ANCP agent: The request
SHOULD be re-sent once to eliminate the possibility of in-
transit corruption.
Result Code value: 0x54
* One-line description: Mandatory TLV missing
* Where condition detected: ANCP agent
* Further description: None
* Required additional information in the response message: The
response message MUST contain a Status-Info message that
encapsulates an instance of each missing mandatory TLV, where
the length is set to zero and the value field is empty (i.e.,
only the 4-byte TLV header is present).
* Target: ANCP agent at the peer that sent the original request
* Action RECOMMENDED for the receiving ANCP agent: Re-send the
message with the missing TLV(s), if possible. Otherwise,
report the error to the control application with an indication
of the missing information required to construct the missing
TLV(s).
Result Code value: 0x55
* One-line description: Invalid TLV contents
* Where condition detected: ANCP agent
* Further description: The contents of one or more TLVs in the
request do not match the specifications provided for the those
TLVs.
* Required additional information in the response message: The
response MUST contain a Status-Info TLV encapsulating the
erroneous TLVs copied from the original request.
* Target: ANCP agent at the peer that sent the original request
* Action RECOMMENDED for the receiving ANCP agent: Correct the
error and re-send the request, if possible. Otherwise, report
the error to the control application with an indication of the
erroneous information associated with the invalid TLV(s).
Result Code value: 0x500
* One-line description: One or more of the specified ports do not
exist
* Where condition detected: Control application
* Further description (if any): This may indicate a configuration
mismatch between the AN and the NAS or Authentication,
Authorization, and Accounting (AAA).
* Required additional information in the response message: If the
request identified multiple access lines or the response is a
Generic Response message, then the response MUST contain a
Status-Info TLV encapsulating TLV(s) containing the rejected
line identifier(s).
* Target: Control application at the peer that sent the original
request
* Action RECOMMENDED for the receiving ANCP agent: Indicate the
error and forward the line identifiers to the control
application.
3.6.1.5. Partition ID (8 bits)
The Partition ID field MUST contain the value that was negotiated for
Partition ID during the adjacency procedure as described above.
3.6.1.6. Transaction ID (24 bits)
The Transaction ID is set by the sender of a request message to
associate a response message with the original request message.
Unless otherwise specified for a given message type, the Transaction
ID in request messages MUST be set to a value in the range
(1, 2^24 - 1). When used in this manner, the Transaction ID
sequencing MUST be maintained independently for each message type
within each ANCP adjacency. Furthermore, it SHOULD be incremented by
1 for each new message of the given type, cycling back to 1 after
running the full range. For event messages, the Transaction ID
SHOULD be set to zero.
Unless otherwise specified, the default behavior for all ANCP
responses is that the value of the Transaction ID MUST be copied from
the corresponding request message.
3.6.1.7. I Flag and SubMessage Number (1 + 15 bits)
In GSMPv3, these provide a mechanism for message fragmentation.
Because ANCP uses TCP transport, this mechanism is unnecessary. An
ANCP agent MUST set the I Flag and subMessage Number fields to 1 to
signify "no fragmentation".
3.6.1.8. Length (16 bits)
This field MUST be set to the length of the ANCP message in bytes, including its header fields and message body but excluding the 4-byte encapsulating header defined in Section 3.2.3.6.2. The ANCP Message Body
The detailed contents of the message payload portion of a given ANCP message can vary with the capability in the context of which it is being used. However, the general format consists of zero or more fixed fields, followed by a variable amount of data in the form of Type-Length-Value (TLV) data structures. The general format of a TLV is shown in Figure 8: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type (IANA registered) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Value ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: General TLV Format The fields of a TLV are defined as follows: Type (16 bits): The TLV Type is an unsigned value identifying the TLV type and nature of its contents. An IANA registry has been established for ANCP TLV Type codes. Length (16 bits): The number of bytes of data in the Value field of the TLV, excluding any padding required to bring this TLV to a 4-byte word boundary (see "Value" below). If a TLV contains other TLVs, any padding in the contained TLVs MUST be included in the value of Length. Depending on the specification of the TLV, the value of Length can be zero, a constant for all instances of the TLV, or a varying quantity. Value (variable): The actual data carried by the TLV, if any. The Value field in each TLV MUST be padded with zeroes as required to align with a 4-byte word boundary. The Value field of a TLV MAY include fixed fields and/or other TLVs.
Unless otherwise specified, TLVs MAY be added to a message in any order. If the recipient of a message does not understand a particular TLV, it MUST silently ignore it. A number of TLVs are specified in the remainder of this document.3.7. General Principles for the Design of ANCP Messages
ANCP allows for two messaging constructs to support request/response interaction: a. The same message type is used for both the request message and the response message. The Result and Result Code field settings are used to differentiate between request and response messages. b. The request and response messages use two different message types. The first approach is illustrated by the protocol specifications in Section 8.4, the second by specifications in Section 6.4. The purpose of this section is to provide more details about the second approach in order to allow the use of this messaging construct for the development of additional ANCP extensions. As Section 3.6 indicated, all ANCP messages other than adjacency messages share a common header format. When the response message type is different from that of the request, the specification of the request message will typically indicate that the Result field is set to Ignore (0x0) and provide procedures indicating explicitly when the receiver should generate a response and what message type it should use. The Transaction ID field is used to distinguish between multiple request messages of the same type and to associate a response message to a request. Specifications of ANCP messages for applications not requiring response correlation SHOULD indicate that the Transaction ID MUST be set to zero in requests. Applications that require response correlation SHOULD refer to the Transaction ID behavior described in Section 3.6.1. The specification for a response message SHOULD indicate in all cases that the value of the Transaction Identifier MUST be set to that of the corresponding request message. This allows the requester to establish whether or not correlation is needed (by setting a non-zero or zero value for the Transaction ID).
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