MAG Schema Statistics

The MAG schema provides operational statistics that can be used for monitoring and troubleshooting the following products: HSGW, S-GW

This schema provides the following types of statistics:
  • Counter: A counter records incremental data cumulatively and rolls over when the counter limit is reached. All counter statistics are cumulative and reset only by one of the following methods: roll-over when limit is reached, after a system restart, or after a clear command is performed. The limit depends upon the data type.
  • Gauge: A gauge statistic indicates a single value; a snapshot representation of a single point in time within a defined time frame. The gauge changes to a new value with each snapshot though a value may repeat from one period to the next. The limit depends upon the data type.
  • Information: This type of statistic provides information, often intended to differentiate sets of statistics; for example, a VPN name or IP address. The type of information provided depends upon the data type.
The data type defines the format of the data for the value provided by the statistic. The following data types are used in statistics for this schema:
  • Int32/Int64: An integer, either 32-bit or 64-bit: For statistics with the Int32 data type, the roll-over to zero limit is 4,294,967,295. For statistics with the Int64 data type, the roll-over to zero limit is 18,446,744,073,709,551,615.
  • Float: A numeric value that can be represented fractionally; for example, 1.345.
  • String: A series of ASCII alphanumeric characters in a single grouping, usually pre-configured.

IMPORTANT:

Unless otherwise indicated, all statistics are counters and all statistics are standards-based.

Key Variables: Every schema has some variables which are typically referred to as 'key variables'. These key variables provide index markers to identify which object the statistics apply to. For example, in the card schema, the card number (variable %card%) uniquely identifies a card. For an HA service, the keys would be "%vpnname%" plus "%servname%", as the combination uniquely identifies an HA service. So, in a given measurement interval, one row of statistics will be generated per unique key. The schema keys are identified in the Description section of the table.


Table 1. Bulk Statistic Variables in the MAG Service Schema
Variables Description Data Type

vpnname

Description: The name of the context configured on the system that is currently facilitating the MAG service.

This is a key variable.

String

vpnid

Description: The identification number of the context configured on the system that is currently facilitating the MAG service. This is an internal reference number.

This is a key variable.

Int32

servname

Description: Displays the name of the MAG service for which the statistics are displayed.

This is a key variable.

String

servid

Description: The identification number of the service configured on the system that is currently facilitating the MAG service. This is an internal reference number.

This is a key variable.

Int32

sess-cur

Description: The total number of sessions currently established on this system.

Triggers:
  1. Incremented when new PDN is setup
  2. Decremented when PDN is torn down

Availability: per MAG service

Type: Gauge

Int32

bindupd

Description: The total number of all binding updates sent by this system.

Triggers: Incremented when PBU is sent

Availability: per MAG service

Int32

bindupd-init

Description: The total number of initial request transmit binding updates sent by this system.

Triggers: Incremented when initial PBU is sent

Availability: per MAG service

Int32

bindupd-initretrans

Description: The total number of initial request retransmit binding updates sent by this system.

Triggers: Incremented when initial PBU is retransmitted

Availability: per MAG service

Int32

bindupd-renew

Description: The total number of renew request transmit binding updates sent by this system.

Triggers: Incremented when renew PBU is sent

Availability: per MAG service

Int32

bindupd-renewretrans

Description: The total number of renew request retransmit binding updates sent by this system.

Triggers: Incremented when renew PBU is retransmitted

Availability: per MAG service

Int32

bindupd-dereg

Description: The total number of deregistration request transmit binding updates sent by this system.

Triggers: Incremented when dereg PBU is sent

Availability: per MAG service

Int32

bindupd-deregretrans

Description: The total number of deregistration request retransmit binding updates sent by this system.

Triggers: Incremented when dereg PBU is retransmitted

Availability: per MAG service

Int32

bindack

Description: The total number of all binding acknowledgements received by this system.

Triggers: Incremented when PBA is received

Availability: per MAG service

Int32

bindack-error

Description: The total number of all binding acknowledgements, with errors, received by this system.

Triggers: Incremented when PBA received has some error

Availability: per MAG service

Int32

bindack-accept

Description: The total number of all binding acknowledgements received, and accepted by this system or the specified service.

Triggers: Incremented when PBA has Accept status code

Availability: per MAG service

Int32

bindack-denied

Description: The total number of all binding acknowledgements received, but denied by this system or the specified service.

Triggers: Incremented when PBA has deny status code

Availability: per MAG service

Int32

bindack-init

Description: The total number of all binding acknowledgements - initial reply received by this system.

Triggers: Incremented when initial PBA is received

Availability: per MAG service

Int32

bindack-renew

Description: The total number of all binding acknowledgements - renew reply received by this system.

Triggers: Incremented when renew PBA is received

Availability: per MAG service

Int32

bindack-dereg

Description: The total number of all binding acknowledgements - deregistration reply received by this system.

Triggers: Incremented when dereg PBA is received

Availability: per MAG service

Int32

deniedlma-noresource

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to insufficient resources.

Triggers: Incremented when PBA received with code Insufficient Resources

Availability: per MAG service

Int32

deniedlma-mismatchid

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to mismatched IDs.

Triggers: Incremented when PBA received with code Mismatched ID

Availability: per MAG service

Int32

deniedlma-mnauthfail

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to mobile node authorization failures.

Triggers: Incremented when PBA received with code MN-Auth failure

Availability: per MAG service

Int32

deniedlma-adminproh

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to admin prohibited conditions.

Triggers: Incremented when PBA received with code Admin Prohibited

Availability: per MAG service

Int32

deniedlma-msgidrqd

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to missing message IDs.

Triggers: Incremented when PBA received with code Mesg ID Required

Availability: per MAG service

Int32

deniedlma-dadfailed

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to DAD failures.

Triggers: Incremented when PBA received with code DAD failed

Availability: per MAG service

Int32

deniedlma-homesubnet

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to incorrect home subnet.

Triggers: Incremented when PBA received with code Not Home Subnet

Availability: per MAG service

Int32

deniedlma-seqoow

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to sequence out of window conditions.

Triggers: Incremented when PBA received with code Sequence out of window

Availability: per MAG service

Int32

deniedlma-typchgdis

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to registration type change disallowed.

Triggers: Incremented when PBA received with code Reg Type change disallowed

Availability: per MAG service

Int32

deniedlma-unspec

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to unspecified reasons.

Triggers: Incremented when PBA received with code Unspecified reason

Availability: per MAG service

Int32

deniedlma-servauthfailed

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to failed service authorizations.

Triggers: Incremented when PBA received with code Service Authorisation failed

Availability: per MAG service

Int32

deniedlma-proxyreg

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to proxy registration not being enabled.

Triggers: Incremented when PBA received with code Proxy Reg not enabled

Availability: per MAG service

Int32

deniedlma-timestamp

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to timestamp mismatches.

Triggers: Incremented when PBA received with code Time mismatch

Availability: per MAG service

Int32

deniedlma-timestamplower

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to lower than expected timestamps.

Triggers: Incremented when PBA received with code Timestamp lower than expected

Availability: per MAG service

Int32

deniedlma-mismnid

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to missing mobile node ID options.

Triggers: Incremented when PBA received with code Missing MN-ID option

Availability: per MAG service

Int32

deniedlma-mishnp

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to missing HNP options.

Triggers: Incremented when PBA received with code Missing HNP option

Availability: per MAG service

Int32

deniedlma-misaccesstech

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to missing access technology options.

Triggers: Incremented when PBA received with code MissingAccess Tech option

Availability: per MAG service

Int32

deniedlma-mishandoffind

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to missing handoff indication options.

Triggers: Incremented when PBA received with code Missing Handoff indicator option

Availability: per MAG service

Int32

deniedlma-notauthhnp

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to not being authorized for HNP.

Triggers: Incremented when PBA received with code Not Authorized for HNP

Availability: per MAG service

Int32

deniedlma-notlmamobile

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due incorrect LMA for mobility.

Triggers: Incremented when PBA received with code Not LMA for Mobile

Availability: per MAG service

Int32

deniedlma-notauthproxyreg

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to not being authorized for proxy registration.

Triggers: Incremented when PBA received with code Not Authorized for Proxy Reg

Availability: per MAG service

Int32

deniedlma-bceprefix

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to BCE prefix mismatches.

Triggers: Incremented when PBA received with code BCE prefix set do not Match

Availability: per MAG service

Int32

deniedlma-grekey

Description: The total number of binding updates sent by this system or the specified service but denied by the LMA due to GRE key option required.

Triggers: Incremented when PBA received with code GRE key option required

Availability: per MAG service

Int32

bindack-errormishnp

Description: The total number of binding acknowledgements with missing HNP errors received by this system or the specified service.

Triggers: Incremented when PBA received that is discarded due to missing HNP option

Availability: per MAG service

Int32

bindack-errornai

Description: The total number of binding acknowledgements with missing NAI errors received by this system or the specified service.

Triggers: Incremented when PBA received that is discarded due to missing MN-ID option

Availability: per MAG service

Int32

bindack-errorhomeaddconf

Description: The total number of binding acknowledgements with home address conflict errors received by this system or the specified service.

Triggers: Incremented when PBA received that is discarded due to Home Address Conflict

Availability: per MAG service

Int32

bindack-errormatchreq

Description: The total number of binding acknowledgements with matching requests not found errors received by this system or the specified service.

Triggers: Incremented when PBA received that is discarded due to Matching Request not found

Availability: per MAG service

Int32

bindack-errorbadlyformed

Description: The total number of binding acknowledgements with badly formed message errors received by this system or the specified service.

Triggers: Incremented when PBA received that is discarded due to decode failure

Availability: per MAG service

Int32

bindack-errorchecksum

Description: The total number of binding acknowledgements with checksum errors received by this system or the specified service.

Triggers: Incremented when PBA received that is discarded due to Checksum error

Availability: per MAG service

Int32

bindack-errorsessnotfound

Description: The total number of binding acknowledgements with session not found errors received by this system or the specified service.

Triggers: Incremented when PBA received that is discarded due to session not found

Availability: per MAG service

Int32

bindrev-sent

Description: The total number of binding revocations sent by this system or the specified service.

Triggers: Incremented when BRI is sent by MAG service

Availability: per MAG service

Int32

bindrev-retriessent

Description: The total number of binding revocation retries sent by this system or the specified service.

Triggers: Incremented when BRI is retransmitted by MAG service

Availability: per MAG service

Int32

bindrev-ackrcvd

Description: The total number of binding revocation acknowledgements received by this system or the specified service.

Triggers: Incremented when BRA is received by MAG service

Availability: per MAG service

Int32

bindrev-notacked

Description: The total number of binding revocations sent, but not acknowledged, by this system or the specified service.

Triggers: Incremented when session is deleted after max BRI retires without receiving ACK

Availability: per MAG service

Int32

bindrev-rcvd

Description: The total number of binding revocations received by this system or the specified service.

Triggers: Incremented when BRI is received by MAG service

Availability: per MAG service

Int32

bindrev-acksent

Description: The total number of binding revocation acknowledgements sent by this system or the specified service.

Triggers: Incremented when BRA is sent by MAG service

Availability: per MAG service

Int32

rcvdbindrevtrig-reserved

Description: The total number of binding revocation trigger reasons received - reserved.

Triggers: Incremented when BRI is received with trigger reason - Reserved

Availability: per MAG service

Int32

rcvdbindrevtrig-unspecified

Description: The total number of binding revocation trigger reasons received - unspecified.

Triggers: Incremented when BRI is received with trigger reason - Unspecified

Availability: per MAG service

Int32

rcvdbindrevtrig-admin

Description: The total number of binding revocation trigger reasons received - administrative reason.

Triggers: Incremented when BRI is received with trigger reason - Administrative reason

Availability: per MAG service

Int32

rcvdbindrevtrig-maghoffsameatt

Description: The total number of binding revocation trigger reasons received - inter-MAG handoff-same ATT.

Triggers: Incremented when BRI is received with trigger reason - Inter MAG Handoff - same Access Type

Availability: per MAG service

Int32

rcvdbindrevtrig-maghoff-unknown

Description: The total number of binding revocation trigger reasons received - inter-MAG - unknown handoff.

Triggers: Incremented when BRI is received with trigger reason - Inter MAG - unknown handoff

Availability: per MAG service

Int32

rcvdbindrevtrig-maghoff-diffatt

Description: The total number of binding revocation trigger reasons received - inter-MAG handoff-diff ATT.

Triggers: Incremented when BRI is received with trigger reason - Inter MAG Handoff - diff Access type

Availability: per MAG service

Int32

rcvdbindrevtrig-perpeer

Description: The total number of binding revocation trigger reasons received - per-peer policy.

Triggers: Incremented when BRI is received with trigger reason - Per peer policy

Availability: per MAG service

Int32

rcvdbindrevtrig-nodelocal

Description: The total number of binding revocation trigger reasons received - revoking node local policy.

Triggers: Incremented when BRI is received with trigger reason - Node local policy

Availability: per MAG service

Int32

rcvdbindrevtrig-userinitsess

Description: The total number of binding revocation trigger reasons received - user initiated session term.

Triggers: Incremented when BRI is received with trigger reason - User Initiated Sess Termination

Availability: per MAG service

Int32

rcvdbindrevtrig-accessnwsess

Description: The total number of binding revocation trigger reasons received - access network session term.

Triggers: Incremented when BRI is received with trigger reason - Access network initiated Term

Availability: per MAG service

Int32

rcvdbindrevtrig-ipv4hoabind

Description: The total number of binding revocation trigger reasons received - IPv4 HoA binding only.

Triggers: Incremented when BRI is received with trigger reason - IPv4 HoA binding only

Availability: per MAG service

Int32

rcvdbindrevtrig-syncbce

Description: The total number of binding revocation trigger reasons received - out-of sync BCE state.

Triggers: Incremented when BRI is received with trigger reason - BCE Out of Sync

Availability: per MAG service

Int32

rcvdbindrevtrig-unknown

Description: The total number of binding revocation trigger reasons received - unknown.

Triggers: Incremented when BRI is received with trigger reason - Unknown

Availability: per MAG service

Int32

sentrevack-success

Description: The total number of revocation ACK status sent - success.

Triggers: Incremented when BRA is sent with status Success

Availability: per MAG service

Int32

sentrevack-partialsuccess

Description: The total number of revocation ACK status sent - partial-success.

Triggers: Incremented when BRA is sent with status Partial Success

Availability: per MAG service

Int32

sentrevack-nobinding

Description: The total number of revocation ACK status sent - binding-does-not-exist.

Triggers: Incremented when BRA is sent with status Binding Does Not exist

Availability: per MAG service

Int32

sentrevack-noipv4hoabind

Description: The total number of revocation ACK status sent - no IPv4-HoA-bind.

Triggers: Incremented when BRA is sent with status No IPv4 HoA Binding

Availability: per MAG service

Int32

sentrevack-revocnotauth

Description: The total number of revocation ACK status sent - global-revoc-not-authorized.

Triggers: Incremented when BRA is sent with status Global Revoc Not Authorized

Availability: per MAG service

Int32

sentrevack-bindingnotidentified

Description: The total number of revocation ACK status sent - cannot-identify-binding.

Triggers: Incremented when BRA is sent with status Cannot Identify binding

Availability: per MAG service

Int32

sentrevack-revocfailmnattch

Description: The total number of revocation ACK status sent - revoc-failed-MN-attached.

Triggers: Incremented when BRA is sent with status Revoc Failed MN Attached

Availability: per MAG service

Int32

sentrevack-unknown

Description: The total number of revocation ACK status sent - unknown.

Triggers: Incremented when BRA is sent with status unknown

Availability: per MAG service

Int32

bindrev-discardtotal

Description: The total number of binding revocation acknowledgements received and discarded by this system or the specified service.

Triggers: Incremented when BRI is discarded

Availability: per MAG service

Int32

bindrev-discardsessnotfound

Description: The total number of binding revocation acknowledgements received and discarded, due to a session not found condition, by this system or the specified service.

Triggers: Incremented when BRI is discarded due to session not found

Availability: per MAG service

Int32

bindrev-discardbadreq

Description: The total number of binding revocation acknowledgements received and discarded, due to a badly formed request condition, by this system or the specified service.

Triggers: Incremented when BRI is discarded due to malformed request

Availability: per MAG service

Int32

bindrev-discarddecode

Description: The total number of binding revocation acknowledgements received and discarded, due to a decode error condition, by this system or the specified service.

Triggers: Incremented when BRI is discarded due to decode failure

Availability: per MAG service

Int32

bindrev-discardchecksum

Description: The total number of binding revocation acknowledgements received and discarded, due to a checksum error condition, by this system or the specified service.

Triggers: Incremented when BRI is discarded due to checksum error

Availability: per MAG service

Int32

bindrev-discardmsgtype

Description: The total number of binding revocation acknowledgements received and discarded, due to a invalid memory type condition, by this system or the specified service.

Triggers: Incremented when BRI is discarded due to invalid message type

Availability: per MAG service

Int32

bindrev-discardnomemory

Description: The total number of binding revocation acknowledgements received and discarded, due to insufficient memory, by this system or the specified service.

Triggers: Incremented when BRI is discarded due to memory alloc failure

Availability: per MAG service

Int32

rxpackets

Description: The total number of packets received by this system.

Triggers: Incremented when tunneled data packet is received

Availability: per MAG service

Int32

rxpackets-6in6

Description: The total number of IPv6-in-IPv6 tunnel packets received by this system.

Triggers: Incremented when tunneled IPv6 in IPv6 encapsulated data packet is received

Availability: per MAG service

Int32

rxpackets-4in6

Description: The total number of IPv4-in-IPv6 tunnel packets received by this system or the specified service.

Triggers: Incremented when tunneled IPv4 in IPv6 encapsulated data packet is received

Availability: per MAG service

Int32

rxpackets-ipv6greipv4

Description: The total number of IPv4-in-IPv6 GRE tunnel packets received by this system or the specified service.

Triggers: Incremented when tunneled IPv4 in IPv6 GRE encapsulated data packet is received

Availability: per MAG service

Int32

rxpackets-ipv6greipv6

Description: The total number of IPv6-in-IPv6 GRE tunnel packets received by this system or the specified service.

Triggers: Incremented when tunneled IPv6 in IPv6 GREncapsulated data packet is received

Availability: per MAG service

Int32

rxoctets

Description: The total number of octets received by this system.

Triggers: Incremented with number of bytes received when tunneled data packet is received

Availability: per MAG service

Int32

rxoctets-6in6

Description: The total number of IPv6-in-IPv6 tunnel octets received by this system.

Triggers: Incremented with number of IPv6 in IPv6 bytes received when tunneled data packet is received

Availability: per MAG service

Int32

rxoctets-4in6

Description: The total number of IPv4-in-IPv6 tunnel octets received by this system or the specified service.

Triggers: Incremented with number of IPv4 in IPv6 bytes received when tunneled data packet is received

Availability: per MAG service

Int32

rxoctets-ipv6greipv4

Description: The total number of IPv4-in-IPv6 GRE tunnel octets received by this system or the specified service.

Triggers: Incremented with number of IPv4 in IPv6 GRE bytes received when tunneled data packet is received

Availability: per MAG service

Int32

rxoctets-ipv6greipv6

Description: The total number of IPv6-in-IPv6 GRE tunnel octets received by this system or the specified service.

Triggers: Incremented with number of IPv6 in IPv6 GRE bytes received when tunneled data packet is received

Availability: per MAG service

Int32

dataerror

Description: The total number of data errors received by this system.

Triggers: Incremented when error is encountered on processing data packet received from the tunnel

Availability: per MAG service

Int32

dataerror-proto

Description: The total number of protocol type data errors received by this system.

Triggers: Incremented when packet received with invalid protocol type

Availability: per MAG service

Int32

dataerror-invpktlen

Description: The total number of invalid packet length data errors received by this system.

Triggers: Incremented when packet received with invalid length

Availability: per MAG service

Int32

dataerror-nosess

Description: The total number of no session found data errors received by this system.

Triggers: Incremented when packet received with session not found

Availability: per MAG service

Int32

txpackets

Description: The total number of packets sent by this system.

Triggers: Incremented when tunneled data packet is sent

Availability: per MAG service

Int32

txpackets-6in6

Description: The total number of IPv6-in-IPv6 tunnel packets sent by this system.

Triggers: Incremented when tunneled IPv6 in IPv6 encapsulated data packet is sent

Availability: per MAG service

Int32

txpackets-4in6

Description: The total number of IPv4-in-IPv6 tunnel packets sent by this system or the specified service.

Triggers: Incremented when tunneled IPv4 in IPv6 encapsulated data packet is sent

Availability: per MAG service

Int32

txpackets-ipv6greipv4

Description: The total number of IPv4-in-IPv6 GRE tunnel packets sent by this system or the specified service.

Triggers: Incremented when tunneled IPv4 in IPv6 GRE encapsulated data packet is sent

Availability: per MAG service

Int32

txpackets-ipv6greipv6

Description: The total number of IPv6-in-IPv6 GRE tunnel packets sent by this system or the specified service.

Triggers: Incremented when tunneled IPv6 in IPv6 GREncapsulated data packet is sent

Availability: per MAG service

Int32

txoctets

Description: The total number of octets sent by this system.

Triggers: Incremented with number of bytes sent when tunneled data packet is sent

Availability: per MAG service

Int32

txoctets-6in6

Description: The total number of IPv6-in-IPv6 tunnel octets sent by this system.

Triggers: Incremented with number of IPv6 in IPv6 bytes sent when tunneled data packet is sent

Availability: per MAG service

Int32

txoctets-4in6

Description: The total number of IPv4-in-IPv6 tunnel octets sent by this system or the specified service.

Triggers: Incremented with number of IPv4 in IPv6 bytes sent when tunneled data packet is sent

Availability: per MAG service

Int32

txoctets-ipv6greipv4

Description: The total number of IPv4-in-IPv6 GRE tunnel octets sent by this system or the specified service.

Triggers: Incremented with number of IPv4 in IPv6 GRE bytes sent when tunneled data packet is sent

Availability: per MAG service

Int32

txoctets-ipv6greipv6

Description: The total number of IPv6-in-IPv6 GRE tunnel octets sent by this system or the specified service.

Triggers: Incremented with number of IPv6 in IPv6 GRE bytes sent when tunneled data packet is sent

Availability: per MAG service

Int32

disc

Description: The total number of disconnects initiated by this system.

Triggers: Incremented when MAG session disconnects

Availability: per MAG service

Int32

disclifetime

Description: The total number of disconnects due to lifetime expiry initiated by this system.

Triggers: Incremented when MAG session is disconnected due to lifetime expiry

Availability: per MAG service

Int32

discaccessinit

Description: The total number of disconnects due to de-registrations initiated by this system or the specified service.

Triggers: Incremented when MAG session is disconnected due to access initiated term

Availability: per MAG service

Int32

discadmin

Description: The total number of disconnects due to admin drops initiated by this system.

Triggers: Incremented when MAG session disconnects due to Admin reasons

Availability: per MAG service

Int32

discother

Description: The total number of disconnects due to “other reasons” initiated by this system.

Triggers: Incremented when MAG session is disconnected due to misc reasons

Availability: per MAG service

Int32

disclmarevoc

Description: The total number of disconnects due to LMA revocations received by this system or the specified service.

Triggers: Incremented when MAG session disconnects due to revocation from LMA

Availability: per MAG service

Int32

IMPORTANT:

For information on statistics that are common to all schema see the Statistics and Counters Overview chapter.