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Versions: (draft-zhou-behave-syslog-nat-logging) 00 01 02 03 04 05 06

Behave Working Group                                             Z. Chen
Internet-Draft                                             China Telecom
Intended status: Standards Track                                 C. Zhou
Expires: July 29, 2014                                           T. Tsou
                                                          T. Taylor, Ed.
                                                     Huawei Technologies
                                                        January 25, 2014


                     Syslog Format for NAT Logging
                draft-ietf-behave-syslog-nat-logging-06

Abstract

   NAT devices are required to log events like creation and deletion of
   translations and information about the resources the NAT is managing.
   The logs are required to identify an attacker or a host that was used
   to launch malicious attacks, and for various other purposes of
   accounting and management.  Since there is no standard way of logging
   this information, different NAT devices behave differently.  The lack
   of a consistent way makes it difficult to write the collector
   applications that would receive this data and process it to present
   useful information.

   This document describes the information that is required to be logged
   by the NAT devices.  It goes on to standardize formats for reporting
   these events and parameters using SYSLOG (RFC 5424).  A companion
   document specifies formats for reporting the same events and
   parameters using IPFIX (RFC 7011).  Applicability statements are
   provided in this document and its companion to guide operators and
   implementors in their choice of which technology to use for logging.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 29, 2014.



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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Deployment Considerations . . . . . . . . . . . . . . . . . .   6
     2.1.  Static and Dynamic NATs . . . . . . . . . . . . . . . . .   6
     2.2.  Realms and Address Pools  . . . . . . . . . . . . . . . .   7
       2.2.1.  Address Pools . . . . . . . . . . . . . . . . . . . .   7
     2.3.  NAT Logging Requirements For Different Transition Methods   8
     2.4.  Subscriber Identification . . . . . . . . . . . . . . . .   9
     2.5.  The Port Control Protocol (PCP) . . . . . . . . . . . . .  10
     2.6.  Logging At the Customer Edge  . . . . . . . . . . . . . .  10
   3.  NAT-Related Events and Parameters . . . . . . . . . . . . . .  10
     3.1.  Events Relating To Allocation Of Resources To Hosts . . .  10
       3.1.1.  NAT Address Mapping Creation and Deletion . . . . . .  11
       3.1.2.  NAT Address and Port Mapping Creation and Deletion  .  12
       3.1.3.  NAT Session Creation and Deletion . . . . . . . . . .  14
         3.1.3.1.  Destination Logging . . . . . . . . . . . . . . .  17
       3.1.4.  Port Range Allocation and Deallocation  . . . . . . .  17
     3.2.  Threshold Events  . . . . . . . . . . . . . . . . . . . .  19
       3.2.1.  Address Pool High- and Low-Water-Mark Threshold
               Events  . . . . . . . . . . . . . . . . . . . . . . .  19
       3.2.2.  Global Address Mapping High-Water-Mark Threshold
               Event . . . . . . . . . . . . . . . . . . . . . . . .  20
       3.2.3.  Global Address and Port Mapping High-Water-Mark
               Threshold Event . . . . . . . . . . . . . . . . . . .  21
       3.2.4.  Subscriber-Specific Address and Port Mapping
               Threshold Event . . . . . . . . . . . . . . . . . . .  22
     3.3.  Limit-Related Events  . . . . . . . . . . . . . . . . . .  22
       3.3.1.  Global Address Mapping Limit Exceeded . . . . . . . .  22
       3.3.2.  Global Address and Port Mapping Limit Exceeded  . . .  23
       3.3.3.  Global Limit On Number of Active Hosts Exceeded . . .  24
       3.3.4.  Subscriber-Specific Limit On Number of Address and



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               Port Mappings Exceeded  . . . . . . . . . . . . . . .  25
       3.3.5.  Global Limit On Number Of Fragments Pending
               Reassembly Exceeded . . . . . . . . . . . . . . . . .  26
   4.  SYSLOG Applicability  . . . . . . . . . . . . . . . . . . . .  27
   5.  SYSLOG Record Format For NAT Logging  . . . . . . . . . . . .  27
     5.1.  SYSLOG HEADER Fields  . . . . . . . . . . . . . . . . . .  28
     5.2.  Parameter Encodings . . . . . . . . . . . . . . . . . . .  29
       5.2.1.  General Encoding Rules  . . . . . . . . . . . . . . .  32
       5.2.2.  Special Cases . . . . . . . . . . . . . . . . . . . .  32
       5.2.3.  Relationship To Objects In the NAT MIB  . . . . . . .  33
     5.3.  Encoding Of Complete Log Report For Each Event Type . . .  35
       5.3.1.  Encoding of Events Relating To Allocation Of
               Resources To Hosts  . . . . . . . . . . . . . . . . .  35
         5.3.1.1.  NAT Address Mapping Creation and Deletion . . . .  36
         5.3.1.2.  NAT Address and Port Mapping Creation and
                   Deletion  . . . . . . . . . . . . . . . . . . . .  37
         5.3.1.3.  NAT Session Creation and Deletion . . . . . . . .  39
         5.3.1.4.  Port Range Allocation and Deallocation  . . . . .  41
       5.3.2.  Encoding of Threshold Events  . . . . . . . . . . . .  43
         5.3.2.1.  NAT Address Pool High- and Low-Water-Mark
                   Threshold Events  . . . . . . . . . . . . . . . .  43
         5.3.2.2.  Global Address Mapping High-Water-Mark
                   Threshold Exceeded  . . . . . . . . . . . . . . .  44
         5.3.2.3.  Global Address and Port Mapping High-Water-Mark
                   Threshold Event . . . . . . . . . . . . . . . . .  45
         5.3.2.4.  Subscriber-Specific Address and Port Mapping
                   High-Water-Mark Threshold Event . . . . . . . . .  45
       5.3.3.  Encoding of Limit Events  . . . . . . . . . . . . . .  46
         5.3.3.1.  Global Address Mapping Limit Exceeded . . . . . .  46
         5.3.3.2.  Global Address and Port Mapping Limit Exceeded  .  47
         5.3.3.3.  Global Limit On Number of Active Hosts Exceeded .  48
         5.3.3.4.  Subscriber-Specific Limit On Number of Address
                   and Port Mappings Exceeded  . . . . . . . . . . .  49
         5.3.3.5.  Pending Fragment Limit Exceeded . . . . . . . . .  50
   6.  Management Considerations . . . . . . . . . . . . . . . . . .  51
     6.1.  General Requirements For Control Of Logging . . . . . . .  51
       6.1.1.  Configuration of PRI Value  . . . . . . . . . . . . .  51
       6.1.2.  Ability For Each Collector To Detect Lost Event
               Reports . . . . . . . . . . . . . . . . . . . . . . .  52
       6.1.3.  Ability To Rate Limit Or Disable Event Reports  . . .  52
     6.2.  Setting Limits and Thresholds . . . . . . . . . . . . . .  53
     6.3.  Other Management Requirements . . . . . . . . . . . . . .  54
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  55
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  55
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  58
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  58
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  60
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  61



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1.  Introduction

   This document deals with logging of NAT activity in two categories:
   NAT translations and NAT resource usage.

   Operators already need to record the addresses assigned to
   subscribers at any point in time, for operational and regulatory
   reasons.  When operators introduce NAT devices that support address
   sharing (e.g., Carrier Grade NATs (CGNs)) into their network,
   additional information has to be logged.  This document and
   [I-D.behave-ipfix-nat-logging] are provided in order to standardize
   the events and parameters to be recorded, using SYSLOG [RFC5424] and
   IPFIX [RFC7011] respectively.  The same content is proposed to be
   logged by both documents.

   In addition to records of subscriber activity, some operators use
   logs to indicate when utilization of critical resources is
   approaching or has reached limits set by the operator or
   implementation.  This document and the IPFIX document therefore
   provide logs in two categories: thresholds exceeded and limits
   exceeded.  Operators have the alternative to receive the threshold
   limits as SNMP notifications (see the NAT MIB [I-D.behave-NAT-MIB]).

   Detailed logging requirements will vary depending on the context in
   which they are used.  For example, different methods for transition
   from IPv4 to IPv6 require different events and different parameters
   to be logged.  Section 2 covers this topic.

   Section 3 provides a detailed description of the events that need
   logging and the parameters that may be required in the logs.
   Section 3.1 describes events related to subscriber activity,
   Section 3.2 covers threshold events, and Section 3.3 covers events
   where hard limits have been reached.

   The use of SYSLOG [RFC5424] has advantages and disadvantages compared
   with the use of IPFIX [RFC7011].  Section 4 provides a statement of
   applicability for the SYSLOG approach.

   Section 5 specifies SYSLOG record formats for logging of the events
   and parameters described in Section 3.  Section 5.1 describes the
   SYSLOG header format for each report, Section 5.2 lists and describes
   the encoding of parameters that can appear in the logs, and
   Section 5.3 specifies the encoding of the body of each event report.
   The definitions provide the flexibility to vary actual log contents
   based on the requirements of the particular deployment.






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1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [RFC2119].

   This document makes frequent reference to the NAT MIB.  That
   reference is to the document [I-D.behave-NAT-MIB].

   This document makes frequent reference to NAT behaviours defined in
   [RFC4784].  In particular it refers to

   o  the recommended pooling behaviour "pooled" and its contrary
      pooling behaviour "arbitrary"; and

   o  the recommended mapping behaviour "endpoint-independent" and its
      contrary mapping behaviour "endpoint-dependent".

   This document uses the term "address mapping" to denote an
   association between an internal IP address and an IP address in a
   selected external realm.  See Section 2.2 for a further discussion of
   this process.

      The natMapIntAddrTable in the NAT MIB provides details on all
      currently active address mappings.  Note that this table is
      applicable only when NAT pooling behaviour is "paired".

   This document uses the [RFC4787] term "address and port mapping" to
   denote a three-tuple association between an internal IP address and
   port and an IP address and port in a selected external realm, or
   between an internal <IP address, ICMP identifier> pair and an <IP
   address, ICMP identifier> pair in the selected realm.  For
   implementations which maintain a Binding Information Base (BIB) (as
   described in Section 2 of [RFC6146], for example), the content of a
   BIB entry is an address and port mapping.

      The natMappingTable in the NAT MIB provides details on all
      currently active address and port mappings.

   This document uses the term "session" as it is defined in [RFC2663],
   Section 2.3.  From the point of view of this document, session
   creation involves the combination of a source address and port
   mapping with a mapping between internal and external destination
   address and port to create a full five-tuple mapping.

   Except where a clear distinction is necessary, this document uses the
   abbreviation "NAT" to encompass both Network Address Translation (NAT



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   in the strict sense) and Network Address and Port Translation (NAPT).
   The event report descriptions provided in this document apply to
   NAPT, and can be simplified for pure NAT operation.

   To match the terminology used by the NAT MIB, this document uses the
   term "subscriber" to denote any device being served by the NAT,
   whether individual host or customer edge router.  That is, despite
   the carrier-oriented terminology, the intended scope of applicability
   of this document is both to NATs in the carrier network and managed
   NATs in the customer network.

   Finally, with two exceptions, when the terms "source" or
   "destination" are used below, they denote the source and destination
   of packets that are flowing from the internal to the external realm,
   regardless of the direction of session establishment or the direction
   of flow of an individual packet.  The exceptions relate to the global
   address and port mapping limit event and the pending fragment limit
   event, when the actual source and destination addresses in the header
   of the packet that hit the limit are reported.

2.  Deployment Considerations

2.1.  Static and Dynamic NATs

   A NAT controls a set of resources in the form of one or more pools of
   external addresses.  If the NAT also does port translation (i.e., it
   is a NAPT), it also controls the sets of UDP and TCP port numbers and
   ICMP identifiers associated with each external address.

   Logging requirements for a NAT depend heavily on its resource
   allocation strategy.  NATs can be classed as static or dynamic
   depending on whether the resources provided to individual users are
   pre-configured or allocated in real time as the NAT recognizes new
   flows.

   Static assignments can be logged at configuration time by the NAT or
   by network infrastructure.  The logging volume associated with static
   assignments will be relatively low, of the order of the volume of
   user logons.

   Dynamic assignments typically require both more detail in the logs
   and a higher volume of logs in total.  A traditional Network Address
   Port Translator (NAPT) as described in [RFC3022] and following the
   recommendations of [RFC4787] and [RFC5382] will generate a new
   address and port mapping each time it encounters a new internal
   <address, port> combination.





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   For statistical reasons, static assignments support lower address
   sharing ratios than fully dynamic assignments as exemplified by the
   traditional NAPT.  The sharing ratio can be increased while
   restraining log volumes by assigning ports to users in multi-port
   increments as required rather than assigning just one port at a time.
   A subscriber may start with no initial allocation, or may start with
   an initial permanent allocation to which temporary increments are
   added when the initial set is all being used.  See [RFC6264] and
   [I-D.tsou-behave-natx4-log-reduction] for details.  If this strategy
   is followed, logging will be required only when an increment is
   allocated or reclaimed rather than every time an internal <address,
   port> combination is mapped to an external <address, port>.

2.2.  Realms and Address Pools

   A realm identifies an IP numbering space.  A NAT session always maps
   between an internal and an external realm.  In simple NAT
   configurations, it may be possible to identify a default internal
   realm and/or a default external realm for all sessions.  In more
   complex NAT configurations a given realm may be an internal realm for
   some sessions and an external realm for others.  Realms without
   subscriber sites are always external.

   Address pools are associated with specific realms in their external
   role.

   It is necessary to define multiple realms when the NAT supports
   overlapping IP numbering spaces.  In such a case, the NAT must
   determine the source realm and subscriber using additional
   information associated with the incoming packet.  See further
   discussion in Section 2.4.

2.2.1.  Address Pools

   An address pool is a mechanism for configuring the set of addresses
   to which a given internal address can be mapped in a given realm.
   The pool may be used simply to ration the available addresses within
   that realm, or may be selected for other reasons such as to add
   additional semantics (e.g., type of service required) to the external
   address within the target realm.  Clearly a given internal address
   may be mapped into more than one address pool at a given time.

   The model of an address pool assumed in this document and in the NAT
   MIB is that the pool offers a fixed range of port/ICMP identifier
   values, the same over all addresses within the pool.  How these are
   allocated to individual mappings depends on the pooling behaviour.
   With a pooling behaviour of "arbitrary", the NAT can select any
   address in the pool with a free port value for the required protocol



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   and map the internal address to it.  With the recommended pooling
   behaviour of "paired", the NAT restricts itself to finding a free
   port at the address to which the internal address is already mapped,
   if there is one.

   From this description, one can see that ports are a limited resource,
   subject to exhaustion at the pool level and, with "paired" behaviour,
   at the level of the individual address.  Log events are defined in
   Section 3.2.1 that allow monitoring of port utilization at the pool
   level.  Section 6.2 discusses how the thresholds for triggering these
   events should be varied depending on pooling behaviour.

2.3.  NAT Logging Requirements For Different Transition Methods

   A number of transition technologies have been or are being developed
   to aid in the transition from IPv4 to IPv6. 6rd [RFC5969] and DS-Lite
   [RFC6333] are at the deployment stage.  Several 'stateless'
   technologies: Public IPv4 over IPv6 [RFC7040], MAP-E
   [I-D.softwire-map], and Lightweight 4over6 [I-D.softwire-lw4over6]
   have seen experimental deployment and are in the process of being
   standardized at the time of writing of this document.

   Of the technologies just listed, 6rd and Public IPv4 over IPv6 do not
   involve NATs and hence need not be considered further.  The other
   techniques involve NAT at the customer edge, at the border router, or
   both, and hence are in scope.

   A DS-Lite Address Family Transition Router (AFTR) includes a large-
   scale session-stateful NAT44 processing potentially millions of
   sessions per second.  The special character of AFTR operation over
   that of a traditional NAT44 is that the source IPv4 addresses of the
   internal hosts will not be unique.  As a consequence, identification
   of the realm and subscriber from which the packet was sent needs to
   include an additional identifier associated with the subscriber host.
   For basic DS-Lite, this will be the IPv6 address used to encapsulate
   the packets outgoing from the host.  See Section 6.6 of [RFC6333].
   For gateway-initiated DS-Lite [RFC6674], two identifiers are needed:
   an identifier of the softwire from the gateway to the NAT, and an
   identifier associated with the incoming tunnel to the gateway.

   The DS-Lite customer edge equipment (the 'B4') may also perform NAT44
   functions, similar to the functions performed by traditional NAT44
   devices.

   As a NAT44, the DS-Lite AFTR may be fully dynamic, or may allocate
   ports in increments as described in the previous section.





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   Lightweight 4over6 [I-D.softwire-lw4over6] and MAP-E
   [I-D.softwire-map] both require NAT44 operation at the customer edge.
   In both cases the resource allocation strategy is static.  Thus any
   logging of resource allocation for these two transition techniques
   can be done by the network at configuration time.

2.4.  Subscriber Identification

   The ability to identify the particular subscriber involved in an
   event is required for the events defined in Section 3.1, and
   desirable for technician follow-up for those defined in Section 3.2.4
   and Section 3.3.

   As mentioned above, in some NAT configurations the source address is
   insufficient to identify an individual subscriber because of
   overlapping address space, and additional information is required.
   For example, if the NAT supports DS-Lite [RFC 6333], the source
   address of incoming packets from DS-Lite subscribers will always be
   in the range 192.0.0/29.  The additional information required in this
   case is the IPv6 address of the encapsulating header.

   The natSubscribersTable in the NAT MIB contains the additional
   information needed, if any, to identify each subscriber.  Thus it is
   sufficient to include the index to this table in the event report to
   provide the needed identification.  However, this implies that for
   full interpretation of the event report, the configuration
   information stored in the natSubscribersTable must be stored (along
   with AAA information relating the additional identifiers to the
   subscriber profiles, which must be stored in any event).  To relieve
   the operator of the need to store the configuration data (given that
   the logs may be needed months or years after they were recorded), the
   reports specified in Section 3.1 include the additional identifying
   information that is found in the natSubscribersTable.

   This document standardizes the presentation of the following possible
   additional classifying information within NAT-related log reports:

   o  interface index [RFC2863];

   o  VLAN index [RFC4363];

   o  VPN identifier [RFC4265];

   o  DS-Lite encapsulating IPv6 address [RFC6333].

   Which of these is actually used in a given NAT depends on
   implementation and deployment.




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      Gateway-Initiated DS-Lite [RFC6674] identifiers could also be
      specified, but it seems premature to do so because it is not clear
      which of the variety of possibilities presented in Section 6 and
      Appendix A.2 of [RFC6674] are actually being deployed.

2.5.  The Port Control Protocol (PCP)

   The Port Control Protocol (PCP) [RFC6887] and its port set extension
   [I-D.pcp-port-set] can be viewed as a way to provision ports by other
   means.  However, PCP can be invoked on a per-flow basis, so the
   volume of logs generated by a PCP server can be closer to the volume
   associated with a fully dynamic NAT.  The volume really depends on
   how PCP is being used in a specific network.

2.6.  Logging At the Customer Edge

   Logging at the customer edge (or at the ISP edge for NATs protecting
   the ISP's internal networks) may be done by the customer for purposes
   of internal management, or by the ISP for its own administrative and
   regulatory purposes.  Given the likelihood of a high internal
   community of interest, it is possible but unlikely that a NAT at the
   edge of a large enterprise network processes a number of new packet
   flows per second which is comparable to the volume handled by a
   carrier grade NAT.  Most customer edge NATs will handle a much
   smaller volume of flows.

3.  NAT-Related Events and Parameters

   The events which follow were initially gleaned, in the words of the
   authors of [I-D.behave-ipfix-nat-logging], from [RFC4787] and
   [RFC5382].  Some details were subsequently informed by the discussion
   in Section 2 and by provisions within the NAT MIB.  Section 4 of
   [RFC6888] also provides a brief statement of logging requirements for
   carrier grade NATs.

   In SYSLOG, the timestamp and the event type will appear in the log
   header rather than as an explicit part of the structured data portion
   of the log.  Hence they are omitted from the parameter tabulations
   that follow.

   Parameters marked CONDITIONAL are REQUIRED under some circumstances
   but not others.  Details are provided for each event.

3.1.  Events Relating To Allocation Of Resources To Hosts

   Setting up a NAT session proceeds in a series of logical steps:
   creation of an address mapping, creation of an address and port
   mapping, and finally, creation of the session.



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   The reports corresponding to these three steps are defined in
   Section 3.1.1, Section 3.1.2, and Section 3.1.3 respectively.  Which
   of these reports is enabled depends on the NAT implementation and
   operator preferences, subject to the considerations of the next
   paragraph.

   If the NAT implements the recommended pooling behaviour of "paired",
   address mapping creation is an event distinct in general from the
   creation of a subsequent address and port mapping based on that
   address mapping.  However, if the pooling behaviour is "arbitrary"
   [RFC4787], the two events occur simultaneously and there is no point
   in reporting both.  Similarly, if the NAT implements the recommended
   mapping behaviour of "endpoint-independent mapping", the two events
   of address and port mapping creation and session creation based on
   that mapping are distinct and may meaningfully be reported
   separately.  However, if the mapping behaviour is "endpoint-
   dependent", the two events occur simultaneously and it is only
   meaningful to report session creation.

   The fourth report type in this section describes the bulk allocation
   of ports to an address mapping, which the NAT may implement if the
   pooling behaviour is "paired" [RFC4787].  It, along with the other
   reports, is needed to provide complete accountability for resources
   allocated to the subscriber.

3.1.1.  NAT Address Mapping Creation and Deletion

   Two specific events are provided:

   o  NAT address mapping creation;

   o  NAT address mapping deletion.

   Implementations MUST NOT report these events unless pooling behaviour
   is "paired".

   Address mapping is discussed in detail in Section 2.2.

   One address mapping creation event is associated with potentially
   many succeeding address and port mapping creation events, as
   individual port values are mapped for specific protocols.  Similarly,
   an address mapping deletion event may be associated with potentially
   many address and port mapping deletion events, which may have
   preceded it over a period of time or may occur at the same time as a
   result of the address unbinding.

   The address mapping events take the following specific parameters:




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   o  NAT instance identifier (CONDITIONAL);

   o  Source subscriber index (MANDATORY);

   o  Additional source subscriber classifier value as recognized at the
      ingress to the internal realm (CONDITIONAL);

   o  Internal realm (CONDITIONAL);

   o  Internal address type (MANDATORY);

   o  Internal source IP address (MANDATORY);

   o  External realm (CONDITIONAL);

   o  External address type (MANDATORY);

   o  External source IP address (MANDATORY);

   o  Trigger for address mapping creation or deletion (OPTIONAL):

      *  outgoing packet;

      *  administrative action (e.g., via the Port Control Protocol
         [RFC6887]); or

      *  autonomous action of the NAT.

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   o  Additional source subscriber classifier value REQUIRED if the
      internal source IP address is not enough to identify the
      subscriber unambiguously, else MUST NOT appear.

   o  Internal or external realm REQUIRED if not the default internal or
      external realm, respectively, else MAY appear.

3.1.2.  NAT Address and Port Mapping Creation and Deletion

   The address and port mapping creation or deletion event reports the
   addition or deletion of an address and port mapping as defined in
   Section 1.1.  If the implementation maintains a Binding Information
   Base (BIB), this is equivalent to the creation or deletion of a BIB
   entry.  Implementations MUST support the generation of the address
   and port mapping creation/deletion event reports if they implement



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   the recommended mapping behaviour "endpoint-independent".  They MAY
   support reporting of these events in the contrary case.

   The address and port mapping creation/deletion event report provides
   the same information as the session creation/deletion event, except
   for the destination-related fields and (in general) timestamp values
   in the latter.  With "endpoint-independent" mapping behaviour, one
   address and port mapping creation event is associated with
   potentially many succeeding session creation events.  Similarly, an
   address and port mapping deletion event will be associated with
   potentially many session deletion events, which may have preceded it
   over a period of time or may occur at the same time as a result of
   the address and port mapping deletion.

   Operators should disable the reporting of address and port mapping
   creation and deletion events when destination logging is enabled,
   because of the redundancy between the address and port mapping and
   session event reports.  However, if destination logging is disabled
   and the NAT uses the recommended "endpoint-independent" mapping
   behaviour, it is the session events that are redundant and should be
   disabled.

   The following specific events are defined:

   o  NAT address and port mapping creation

   o  NAT address and port mapping deletion

   These take the same parameters for all types of NAT.  The internal
   realm, subscriber-identifying information, internal source IP
   address, external realm, and external source IP address capture the
   underlying address mapping.  The port values and protocol are unique
   to the address and port mapping.

   The parameters for the address and port mapping creation/deletion
   event are:

   o  NAT instance identifier (CONDITIONAL);

   o  Source subscriber index (MANDATORY);

   o  Additional source subscriber classifier value as recognized at the
      ingress to the internal realm (CONDITIONAL);

   o  Internal realm (CONDITIONAL);

   o  Internal address type (MANDATORY);




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   o  Internal source IP address (MANDATORY);

   o  Internal source port or ICMP identifier (MANDATORY);

   o  External realm (CONDITIONAL);

   o  External address type (MANDATORY);

   o  External source IP address (MANDATORY);

   o  External source port or ICMP identifier (MANDATORY);

   o  Protocol identifier (MANDATORY);

   o  Trigger for address and port mapping creation or deletion
      (OPTIONAL):

      *  outgoing packet received;

      *  incoming packet received;

      *  administrative action (e.g., via the Port Control Protocol
         [RFC6887]); or

      *  deletion of the underlying address mapping (applicable only if
         pooling behaviour is "paired" [RFC4787]).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   o  Additional source subscriber classifier value REQUIRED if the
      internal source IP address is not enough to identify the
      subscriber unambiguously, else MUST NOT appear.

   o  Internal or external realm REQUIRED if not the default internal or
      external realm, respectively, else MAY appear.

3.1.3.  NAT Session Creation and Deletion

   A NAT session creation or deletion event is logged when a address and
   port mapping is further bound to or unbound from a specific
   destination address and port in the external realm.  One to many
   sessions can be based on the same address and port mapping.






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   Implementations MUST provide a means for the operator to specify
   whether destination information is to be included in the reports of
   these events (see discussion below).

   The following specific events are defined:

   o  NAT session creation

   o  NAT session deletion

   These take the same parameters for all types of NAT.  Parameters
   "internal realm" through "protocol identifier" capture the underlying
   address and port mapping.  Subsequent parameters capture the
   destination address and destination subscriber identity (if
   applicable).

   The parameters for the session creation/deletion event are:

   o  NAT instance identifier (CONDITIONAL);

   o  Internal source subscriber index, equal to the natSubscriberIndex
      value in the natSubscribersTable in the NAT MIB (MANDATORY);

   o  Additional internal subscriber classifier value (CONDITIONAL);

   o  Internal realm (CONDITIONAL);

   o  Internal address type (MANDATORY);

   o  Internal source IP address (MANDATORY);

   o  Internal source port or ICMP identifier (MANDATORY);

   o  External realm (CONDITIONAL);

   o  External address type (MANDATORY);

   o  External source IP address (MANDATORY);

   o  External source port or ICMP identifier (MANDATORY);

   o  Protocol identifier (MANDATORY);

   o  Internal destination IP address (CONDITIONAL);

   o  Internal destination port or ICMP identifier (CONDITIONAL);

   o  Destination subscriber index (CONDITIONAL);



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   o  Additional destination subscriber classifier value as recognized
      at the ingress to the external realm (CONDITIONAL);

   o  External destination IP address (CONDITIONAL);

   o  External destination port or ICMP identifier (CONDITIONAL);

   o  Trigger for session creation or deletion (OPTIONAL):

      *  outgoing packet received;

      *  incoming packet received;

      *  administrative action (e.g., via the Port Control Protocol
         [RFC6887]); or

      *  deletion of the underlying address and port mapping (applicable
         only if the NAT mapping behaviour is "endpoint-independent".

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   o  Additional source subscriber classifier value REQUIRED if the
      internal source IP address is not enough to identify the
      subscriber unambiguously, else MUST NOT appear.

   o  Internal or external realm REQUIRED if not the default internal or
      external realm, respectively, else MAY appear.

   o  Internal destination address and port REQUIRED if destination
      logging is enabled and these need to be remapped to external
      destination address and port.  Otherwise, if destination logging
      is disabled, they MUST NOT appear, and if destination logging is
      enabled, they SHOULD NOT appear because of redundancy.

   o  External destination subscriber index REQUIRED if destination
      logging is enabled and the destination is a subscriber served by
      the NAT, else MUST NOT appear.

   o  Additional external subscriber classifier value REQUIRED if
      destination logging is enabled and the destination is a subscriber
      served by the NAT and the external destination address is not
      enough to identify the external destination subscriber
      unambiguously, else MUST NOT appear.





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   o  External destination address and port REQUIRED if destination
      logging is enabled, else MUST NOT appear.

3.1.3.1.  Destination Logging

   The logging of destination address and port is undesirable, for
   several reasons.  [RFC6888] recommends against destination logging
   because of the privacy issues it creates.  From an operator's point
   of view, destination logging is costly not just because of the volume
   of logs it will generate, but because the NAT now has to carry
   additional session state so that it only needs to log once per
   session between two transport end points rather than logging every
   packet.  Finally, [RFC4787], etc.  recommend the use of endpoint-
   independent mapping to maximize the ability of applications to
   operate through the NAT.  In that case, most of the contents of the
   session creation event report will be repeated for one destination
   after another.

   One possibility is that the implementation provides the operator with
   the ability to log destinations only for particular subscribers or
   particular mapped addresses on a special study basis.  This facility
   could be used for trouble-shooting or malicious activity tracing in
   particular cases as required.  If such a capability is provided, the
   implementation MUST report destination information for sessions
   matching the specified criteria, but MUST NOT report these events for
   other sessions.

3.1.4.  Port Range Allocation and Deallocation

   This event is recorded at a hybrid NAT whenever the set of ports
   allocated to a given address mapping changes.  It is assumed that
   when ports are allocated in bulk, the same values are allocated for
   all protocols.

   The following specific events are defined:

   o  Port range allocation;

   o  Port range deallocation.

   The parameters for these events are:

   o  NAT instance identifier (CONDITIONAL);

   o  Source subscriber index (MANDATORY);

   o  Additional source subscriber classifier value as recognized at the
      ingress to the internal realm (CONDITIONAL);



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   o  Internal realm (CONDITIONAL);

   o  Internal address type (MANDATORY);

   o  Internal source IP address (MANDATORY);

   o  External realm (CONDITIONAL);

   o  External address type (MANDATORY);

   o  External source IP address (MANDATORY);

   o  Lowest port number of the range being allocated or deallocated
      (MANDATORY).

   o  Highest port number of the range being allocated or deallocated
      (MANDATORY).

   o  Trigger for port range allocation or deallocation (OPTIONAL):

      *  outgoing packet received;

      *  incoming packet received;

      *  administrative action (e.g., via the Port Control Protocol
         [RFC6887]); or

      *  autonomous action of the NAT.

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   o  Additional source subscriber classifier value REQUIRED if the
      internal source IP address is not enough to identify the
      subscriber unambiguously, else MUST NOT appear.

   o  Internal or external realm REQUIRED if not the default internal or
      external realm, respectively, else MAY appear.

   It will be necessary to use multiple event reports to report more
   complex allocations or deallocations.








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3.2.  Threshold Events

   The events of this section are based on thresholds set by the
   operator within the NAT MIB.  Cross-references to the associated MIB
   objects are provided for each event.  With the exception of the
   address pool low-water-mark event, the threshold events provide early
   warning of potential dropped packets due to resource exhaustion or
   administrator-imposed limits.

3.2.1.  Address Pool High- and Low-Water-Mark Threshold Events

   Two specific events provide reports on address pool utilization:

   o  High-water-mark threshold reached or exceeded;

   o  Low-water-mark threshold reached or under-shot.

   Depending on deployment the operator has the alternative of using the
   SNMP notifications natNotifPoolWater-MarkHigh and natNotifPoolWater-
   MarkLow defined in the NAT MIB rather than logging these events.

   Address pools are discussed in Section 2.2.1.  The
   natPoolTable object in the NAT MIB provides access to parameters
   describing the utilization level of address-port combinations within
   a given pool.  Since a new mapping cannot be allocated unless a
   mappable address and a free port on that address are available, it is
   important to know when the available set of address-port combinations
   within a given pool is nearing exhaustion.  Hence the
   natPoolTable contains a high-water-mark threshold settable by the
   operator.  An address pool high-water-mark event report is generated
   when a new mapping into the pool is requested and aggregate address-
   port utilization is equal to or greater the threshold.

   Similarly it can be of interest to know when a pool is under-
   utilized.  Hence the natPoolTable also provides a low-water-mark
   threshold.  An address pool low-water-mark event report is generated
   when aggregate address-port utilization is equal to or less than the
   low-water-mark threshold.

   Section 6.2 discusses factors affecting the choice of the threshold
   values.

   The high-water-mark threshold event provides a warning that the
   address-port combinations offered by the pool are nearing exhaustion.
   Upon exhaustion, subscribers may be unable to establish new
   connections because no address has enough free port values left to be
   allocated to an address mapping ("address exhaustion").  This applies
   to the case of "paired" pooling behaviour, where typically an address



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   will not be allocated unless it has a sufficient number of free
   ports.  Alternatively, new connections cannot be established simply
   because no address in the pool has a free port number for the
   required protocol ("port exhaustion").

   Packets triggering failed attempts to establish new connections due
   to address exhaustion are included in the following NAT MIB dropped
   packet counters:

   o  globally, natResourceErrors in the natCounters table;

   o  per protocol, natProtocolResourceErrors in natProtocolTable;

   o  per subscriber, natSubscriberResourceErrors in
      natSubscribersTable.

   Packets triggering failed attempts to establish new connections due
   to port exhaustion are counted in the following NAT MIB dropped
   packet counters:

   o  globally, natOutOfPortErrors in the natCounters table;

   o  per protocol, natProtocolOutOfPortErrors in natProtocolTable;

   o  per subscriber, natSubscriberOutOfPortErrors in
      natSubscribersTable.

   An address pool threshold event report contains the following
   specific parameters:

   o  NAT instance identifier (CONDITIONAL);

   o  Pool identifier (MANDATORY);

   o  The threshold value set by the administrator (MANDATORY).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

3.2.2.  Global Address Mapping High-Water-Mark Threshold Event

   One specific event allows monitoring of the total number of mappings
   between internal and external addresses:

   o  Address mapping high-water-mark threshold exceeded.




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   Implementations MUST NOT generate this event report unless the
   pooling behaviour is "paired".  Depending on deployment, operators
   can choose instead to use the SNMP notification natNotifAddrMappings
   defined in the NAT MIB.

   The NAT MIB displays cumulative counts of address mappings created
   and removed in the natCounters table.  When the difference between
   these two counters is greater than the threshold
   natAddrMapNotifyThreshold provided in the natLimits table the global
   address binding high-water-mark threshold event is reported.

   The specific parameters provided by this event report are:

   o  NAT instance identifier (CONDITIONAL);

   o  Current number of active address mappings (MANDATORY).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

3.2.3.  Global Address and Port Mapping High-Water-Mark Threshold Event

   One specific event allows monitoring of the total number of active
   address and port mappings.  Where the NAT implements a BIB, this is
   equivalent to the total number of BIB entries.

   o  address and port mapping high-water-mark threshold exceeded.

   Depending on deployment, operators can choose instead to use the SNMP
   notification natNotifMappings defined in the NAT MIB.

   The NAT MIB displays cumulative counts of address and port mappings
   created and removed in the natCounters table.  When the difference
   between these two counters is greater than the threshold
   natMappingsNotifyThreshold provided in the natLimits table the global
   mapping high-water-mark threshold event is reported.

   The specific parameters provided by this event report are:

   o  NAT instance identifier (CONDITIONAL);

   o  Current number of active address and port mappings (MANDATORY).

   Conditions:





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   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

3.2.4.  Subscriber-Specific Address and Port Mapping Threshold Event

   An event is provided to allow monitoring of the total number of
   active mappings per subscriber:

   o  Subscriber-specific mapping high-water-mark threshold exceeded.

   Depending on deployment, operators can choose instead to use the SNMP
   notification natNotifSubscriberMappings defined in the NAT MIB.

   The NAT MIB displays cumulative counts of address and port mappings
   created and removed per subscriber in the natSubscribersTable.  When
   the difference between these two counters is greater than the
   threshold natSubscriberMapNotifyThresh provided in that table the
   subscriber address and port mapping high-water-mark threshold event
   is reported.

   The specific parameters provided by this event report are:

   o  NAT instance identifier (CONDITIONAL);

   o  Index of the affected subscriber (MANDATORY).

   o  Current number of active mappings for this subscriber (MANDATORY).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

3.3.  Limit-Related Events

   The events of this section are generated when hard limits set by the
   operator are exceeded.  The consequence for service will be dropped
   packets.  As with the threshold events, the description of each
   report includes cross-references to the associated MIB objects.

3.3.1.  Global Address Mapping Limit Exceeded

   The global address mapping limit exceeded event is reported when a
   new address mapping is requested but the total number of address
   mappings would exceed an administrative limit if it were added.  The
   limit is given by object natLimitAddressMappings in the natLimits
   table of the NAT MIB.  MIB counters giving number of packets dropped
   due to resource limitations including this one are:



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   o  globally, natResourceErrors in the natCounters table;

   o  per protocol, natProtocolResourceErrors in natProtocolTable;

   o  per subscriber, natSubscriberResourceErrors in
      natSubscribersTable.

   Implementations MUST NOT generate this event report unless the
   pooling behaviour is "paired".  Depending on deployment, operators
   can choose instead to use the SNMP notification natNotifAddrMappings
   defined in the NAT MIB.

   The parameters for this event are:

   o  NAT instance identifier (CONDITIONAL);

   o  Index of the affected subscriber (MANDATORY).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   The subscriber index is provided to allow the operator to correlate
   the event with any subscriber complaints or possible abuse.

3.3.2.  Global Address and Port Mapping Limit Exceeded

   The global address and port mapping limit exceeded event is reported
   when a new address and port mapping is requested but the total number
   of address and port mappings would exceed an administrative limit if
   it were added.  The limit is given by object natLimitMappings in the
   natLimits table of the NAT MIB.  MIB counters giving number of
   packets dropped due to resource limitations including this one are:

   o  globally, natResourceErrors in the natCounters table;

   o  per protocol, natProtocolResourceErrors in natProtocolTable;

   o  per subscriber, natSubscriberResourceErrors in
      natSubscribersTable.

   The parameters for this event are:

   o  NAT instance identifier (CONDITIONAL);

   o  Index of the internal subscriber (CONDITIONAL);




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   o  Index of the external subscriber (CONDITIONAL);

   o  Source realm of the triggering packet (MANDATORY);

   o  Incoming packet header IP address type (CONDITIONAL);

   o  Incoming packet source IP address (CONDITIONAL).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   o  The index of the internal subscriber is REQUIRED if the mapping
      was triggered by a packet outgoing from the internal to the
      external realm, else MUST NOT appear.

   o  The index of the external subscriber is REQUIRED if the mapping
      was triggered by a packet incoming from a subscriber served by the
      NAT and located in the external realm (i.e., using an address
      mapping created previously by the internal subscriber), else MUST
      NOT appear.

   o  The address type and source IP address from the initiating packet
      are REQUIRED if the mapping was triggered by a packet incoming
      from a purely external realm (i.e., using an address mapping
      created previously by the internal subscriber), else MAY appear.

   The subscriber index or packet source address is provided to allow
   the operator to correlate the event with any subscriber complaints or
   possible abuse.

3.3.3.  Global Limit On Number of Active Hosts Exceeded

   The global limit on number of active hosts exceeded event is reported
   when an address mapping is requested (at least at the logical level)
   for a host with no previous active mappings, but the total number of
   active hosts would exceed an administrative limit if it were added.
   The limit is given by object natLimitSubscribers in the natLimits
   table of the NAT MIB.  MIB counters giving number of packets dropped
   due to resource limitations including this one are:

   o  globally, natResourceErrors in the natCounters table;

   o  per protocol, natProtocolResourceErrors in natProtocolTable;

   o  per subscriber, natSubscriberResourceErrors in
      natSubscribersTable.



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   The parameters for this event are:

   o  NAT instance identifier (CONDITIONAL);

   o  Index of the affected subscriber (MANDATORY).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   The subscriber index is provided to allow the operator to correlate
   the event with any subscriber complaints.

3.3.4.  Subscriber-Specific Limit On Number of Address and Port Mappings
        Exceeded

   The subscriber-specific limit on number of address and port mappings
   exceeded event is reported when a new mapping is requested, but the
   total number of active mappings for that subscriber would exceed an
   administrative limit if it were added.  The limit is given by object
   natSubscriberLimitMappings in natSubscribersTable in the NAT MIB.
   MIB counters giving number of packets dropped due to resource
   limitations including this one are:

   o  globally, natResourceErrors in the natCounters table;

   o  per protocol, natProtocolResourceErrors in natProtocolTable;

   o  per subscriber, natSubscriberResourceErrors in
      natSubscribersTable.

   The parameters for this event are:

   o  NAT instance identifier (CONDITIONAL);

   o  Index of the affected subscriber (MANDATORY).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   The subscriber index is provided to allow the operator to take
   administrative action or to correlate the event with any subscriber
   complaints or possible abuse.





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3.3.5.  Global Limit On Number Of Fragments Pending Reassembly Exceeded

   The global limit on number of fragments pending reassembly exceeded
   event is reported when a new fragment is received and the number of
   fragments currently awaiting reassembly is already equal to an
   administrative limit.  That limit is given by the natLimitFragments
   object in the natLimits table.  This event MUST NOT be reported
   unless the NAT supports the "receive fragments out of order" behavior
   [RFC4787].  MIB counters giving number of packets dropped due to
   resource limitations including this one are:

   o  globally, natResourceErrors in the natCounters table;

   o  per protocol, natProtocolResourceErrors in natProtocolTable;

   o  per subscriber, natSubscriberResourceErrors in
      natSubscribersTable.

   The parameters for this event provide the contents of the IP header
   of the received fragment that triggered it.  If the source of the
   packet is a subscriber served by the NAT and the subscriber index can
   be determined, it MUST also be included.

   o  NAT instance identifier (CONDITIONAL);

   o  Source realm of the packet (MANDATORY);

   o  Packet header IP address type (MANDATORY);

   o  Packet source IP address (MANDATORY);

   o  Packet destination IP address (MANDATORY);

   o  Source subscriber index (CONDITIONAL).

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   o  Source subscriber index REQUIRED if the source of the packet is a
      subscriber served by the NAT and can be determined, else MUST NOT
      appear.








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4.  SYSLOG Applicability

   The primary advantage of SYSLOG is the human readability and
   searchability of its contents.  In addition, it has built-in priority
   and other header fields that allow for separate routing of reports
   requiring management action.  Finally, it has a well-developed
   underpinning of transport and security protocol infrastructure.

   SYSLOG presents two obstacles to scalability: the fact that the
   records will typically be larger than records based on a binary
   protocol such as IPFIX, and, depending on the architectural context,
   the reduced performance of a router that is forced to do text
   manipulation in the data plane.  One has to conclude that for larger
   message volumes, IPFIX should be preferred as the reporting medium on
   the NAT itself.  It is possible that SYSLOG could be used as a back-
   end format on an off-board device processing IPFIX records in real
   time, but this would give a limited boost to scalability.  One
   concern expressed in list discussion is that when the SYSLOG
   formatting process gets overloaded records will be lost.

   As a result, the key question is what the practical cutoff point is
   for the expected volume of SYSLOG records, on-board or off-board the
   NAT.  This obviously depends on the computing power of the formatting
   platform, and also on the record lengths being generated.

   Information has been provided to the BEHAVE list at the time of
   writing to the effect that one production application is generating
   an average of 150,000 call detail records per second, varying in
   length from 500 to 1500 bytes.  Capacities several times this level
   have been reported involving shorter records, but this particular
   application has chosen to limit the average in order to handle peaks.

   As illustrated by the example in Section 5.3.1.3, if destination
   logging is enabled, typical record sizes for session event logs are
   in the order of 300 bytes, so throughput capacity should be higher
   than in the call detail case for the same amount of computing power.
   However, note that bursts of session deletion events may occur as a
   result of deletion of the underlying mapping or address mapping.

   In private communication, a discussant has noted a practical limit of
   a few hundred thousand SYSLOG records per second on a router.

5.  SYSLOG Record Format For NAT Logging

   This section describes the SYSLOG record format for NAT logging in
   terms of the field names used in [RFC5424] and specified in Section 6
   of that document.  In particular, this section specifies values for
   the APP-NAME and MSGID fields in the record header, the SD-ID



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   identifying the STRUCTURED-DATA section, and the PARAM-NAMEs and
   PARAM-VALUE types for the individual possible parameters within that
   section.  The specification is in three parts, covering the header,
   encoding of the individual parameters, and encoding of the complete
   log record for each event type.

5.1.  SYSLOG HEADER Fields

   Within the HEADER portion of the SYSLOG record, the priority (PRI)
   level is subject to local policy, but a Severity value of 6
   (Informational) is suggested for the events relating to creation and
   deletion of sessions, mappings, address mappings, and port
   allocation, combined with a suitable Facility value in the range
   16-23 (local use) to ensure routing to a secure collector.  The
   Facility value(s) for the threshold and limit events will presumably
   be chosen to route them to maintenance for immediate action and/or to
   provisioning for less urgent consideration.  The suggested value of
   Severity by event type is shown in Table 1, but in practice has a
   clear dependency on the context within which the NAT is operating.

   The TIMESTAMP field SHOULD be expressed with sufficient precision to
   distinguish non-simultaneous event occurrences, subject to the
   accuracy of the local clock.  This specification does not assume the
   ability to correlate the events reported by the subject device with
   events recorded by other devices, although that may be required for
   other reasons.  Hence from the point of view of this specification
   only relative rather than absolute accuracy is of interest.

   The HOSTNAME header field MUST identify the NAT device.  The value of
   the HOSTNAME field is subject to the preferences given in
   Section 6.2.4 of [RFC5424].

   The values of the APP-NAME and MSGID fields in the record header
   determine the semantics of the record.  To simplify log collection
   procedures, the APP-NAME value "NAT" MUST be used for the event
   reports specified in Section 5.3.1, the APP-NAME value "NATTHR" MUST
   be used for the event types defined in Section 5.3.2, and the APP-
   NAME value "NATLIM" MUST be used for the event types defined in
   Section 5.3.3.

   The MSGID values indicate the individual events.  They are listed in
   Table 1 for each of the events defined in Section 3.  The table also
   shows the SD-ID value used to label the event-specific STRUCTURED-
   DATA element.







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   +-------------------------+----------+---------+----------+---------+
   | Event                   | APP-NAME | MSGID   | Severity | SD-ID   |
   +-------------------------+----------+---------+----------+---------+
   | NAT address mapping     | NAT      | AMADD   | 6 info   | namap   |
   | creation                |          |         |          |         |
   | NAT address mapping     | NAT      | AMDEL   | 6 info   | namap   |
   | deletion                |          |         |          |         |
   | NAT address and port    | NAT      | APMADD  | 6 info   | napmap  |
   | mapping creation        |          |         |          |         |
   | NAT address and port    | NAT      | APMDEL  | 6 info   | napmap  |
   | mapping deletion        |          |         |          |         |
   | NAT session creation    | NAT      | SADD    | 6 info   | nsess   |
   | NAT session deletion    | NAT      | SDEL    | 6 info   | nsess   |
   | Port range allocation   | NAT      | PTADD   | 6 info   | nprng   |
   | Port range deallocation | NAT      | PTDEL   | 6 info   | nprng   |
   |                         |          |         |          |         |
   | Address pool high       | NATTHR   | POOLHT  | 4        | npool   |
   | threshold               |          |         | warning  |         |
   | Address pool low        | NATTHR   | POOLLT  | 6 info   | npool   |
   | threshold               |          |         |          |         |
   | Global address mapping  | NATTHR   | GAMHT   | 4        | ngamht  |
   | high threshold          |          |         | warning  |         |
   | Global address and port | NATTHR   | GAPMHT  | 4        | ngapmht |
   | mapping high threshold  |          |         | warning  |         |
   | Subscriber-specific     | NATTHR   | SAPMHT  | 5 notice | nsapmht |
   | mapping high threshold  |          |         |          |         |
   |                         |          |         |          |         |
   | Global address mapping  | NATLIM   | GAMLIM  | 3 error  | ngaml   |
   | limit                   |          |         |          |         |
   | Global address and port | NATLIM   | GAPMLIM | 3 error  | ngapml  |
   | mapping limit           |          |         |          |         |
   | Global active           | NATLIM   | GSLIM   | 3 error  | ngsl    |
   | subscriber limit        |          |         |          |         |
   | Subscriber-specific     | NATLIM   | SAPMLIM | 5 notice | nsapml  |
   | address and port        |          |         |          |         |
   | mapping limit           |          |         |          |         |
   | Pending fragment limit  | NATLIM   | FRAG    | 4        | nfpkt   |
   |                         |          |         | warning  |         |
   +-------------------------+----------+---------+----------+---------+

   Table 1: Recommended MSGID Encodings and Default Severity Values for
                      the Events Defined In Section 3

5.2.  Parameter Encodings

   This section describes how to encode the individual parameters that
   can appear in NAT-related logs.  The parameters are taken from the
   event descriptions in Section 3.  The PARAM-NAMEs, brief



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   descriptions, and encoding are listed in Table 2, with reference to
   the general and special case encoding rules which follow.

   +------------+-----------------------------------------+------------+
   | PARAM-NAME | Description                             | Encoding   |
   +------------+-----------------------------------------+------------+
   |            | Miscellaneous                           |            |
   |            |                                         |            |
   | NATINST    | NAT instance identifier                 | Text       |
   | TRIG       | Trigger for event                       | Special    |
   |            |                                         | case       |
   |            |                                         |            |
   |            | Subscriber-identifying information      |            |
   |            |                                         |            |
   | SSUBIX     | Source subscriber index                 | 32-bit     |
   |            |                                         | field      |
   | SIFIX      | Source subscriber ingress interface     | Special    |
   |            | index list                              | case       |
   | SVLAN      | Source subscriber ingress VLAN index    | 32-bit     |
   |            |                                         | field      |
   | SVPN       | Source subscriber ingress VPN Id        | Special    |
   |            |                                         | case       |
   | SV6ENC     | Source subscriber ingress RFC6333       | IPv6       |
   |            | encapsulating IPv6 address              | address    |
   | DSUBIX     | Destination subscriber index            | 32-bit     |
   |            |                                         | field      |
   | DIFIX      | Destination subscriber ingress          | Special    |
   |            | interface index list                    | case       |
   | DVLAN      | Destination subscriber ingress VLAN     | 32-bit     |
   |            | index                                   | field      |
   | DVPN       | Destination subscriber ingress VPN Id   | Special    |
   |            |                                         | case       |
   | DV6ENC     | Destination subscriber ingress RFC6333  | IPv6       |
   |            | encapsulating IPv6 address              | address    |
   |            |                                         |            |
   |            | Internal packet description             |            |
   |            |                                         |            |
   | IRLM       | Internal realm                          | Text       |
   | IATYP      | Internal IP address type                | "IPv4" or  |
   |            |                                         | "IPv6"     |
   | ISADDR     | Internal source IP address value        | IPv4 or    |
   |            |                                         | IPv6       |
   |            |                                         | address    |
   | ISPORT     | Internal source port or ICMP identifier | 16-bit     |
   |            | value                                   | field      |
   | IDADDR     | Internal destination IP address value   | IPv4 or    |
   |            |                                         | IPv6       |
   |            |                                         | address    |



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   | IDPORT     | Internal destination port or ICMP       | 16-bit     |
   |            | identifier value                        | field      |
   | PROTO      | Protocol identifier (from the IANA      | 8-bit      |
   |            | Assigned Internet Protocol Numbers      | field      |
   |            | registry)                               |            |
   |            |                                         |            |
   |            | External (mapped) packet description    |            |
   |            |                                         |            |
   | XRLM       | External realm                          | Text       |
   | XATYP      | External IP address type                | "IPv4" or  |
   |            |                                         | "IPv6"     |
   | XSADDR     | External source IP address value        | IPv4 or    |
   |            |                                         | IPv6       |
   |            |                                         | address    |
   | XSPORT     | External source port or ICMP identifier | 16-bit     |
   |            | value                                   | field      |
   | XDADDR     | External destination IP address value   | IPv4 or    |
   |            |                                         | IPv6       |
   |            |                                         | address    |
   | XDPORT     | External destination port or ICMP       | 16-bit     |
   |            | identifier value                        | field      |
   |            |                                         |            |
   |            | Port range description                  |            |
   |            |                                         |            |
   | PORTMN     | Port range lowest value                 | 16-bit     |
   |            |                                         | field      |
   | PORTMX     | Port range highest value                | 16-bit     |
   |            |                                         | field      |
   |            |                                         |            |
   |            | Values related to thresholds            |            |
   |            |                                         |            |
   | POOLID     | Address pool identifier                 | 32-bit     |
   |            |                                         | field      |
   | POOLHW     | Address pool high water mark threshold  | Unsigned   |
   |            |                                         | decimal    |
   | POOLID     | Address pool low water mark threshold   | Unsigned   |
   |            |                                         | decimal    |
   | GAMCNT     | Current global number of address        | Unsigned   |
   |            | mappings                                | decimal    |
   | GAPMCNT    | Current global number of address and    | Unsigned   |
   |            | port mappings                           | decimal    |
   | SAPMCNT    | Current subscriber-specific number of   | Unsigned   |
   |            | address and port mappings               | decimal    |
   |            |                                         |            |
   |            | Specific incoming packet description    |            |
   |            |                                         |            |
   | PSRLM      | Packet source realm                     | Text       |
   | PATYP      | Packet IP address type                  | "IPv4" or  |



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   |            |                                         | "IPv6"     |
   | PSADDR     | Packet source IP address                | IPv4 or    |
   |            |                                         | IPv6       |
   |            |                                         | address    |
   | PDADDR     | Packet destination IP address           | IPv4 or    |
   |            |                                         | IPv6       |
   |            |                                         | address    |
   +------------+-----------------------------------------+------------+

            Table 2: Parameters Used In NAT-Related Log Reports

5.2.1.  General Encoding Rules

   All fields MUST be encoded as 7-bit US ASCII [US-ASCII].

   Complete IPv6 addresses MUST be presented according to the rules
   specified in Sections 4 and 5 of [RFC5952], without a succeeding
   prefix length.  The Section 5 rules MUST NOT be applied unless the
   address can be distinguished as having an IPv4 address embedded in
   the lower 32 bits solely from the IPv6 prefix portion (e.g., based on
   well-known prefix, flag), without external information.  In such
   cases, the IPv6 prefix portion MUST be presented according to the
   Section 4 rules.  Stand-alone IPv6 prefixes (i.e., outside of special
   addresses) MUST be presented according to the Section 4 rules, with
   the slash character (/) appended, followed by a decimal value with
   leading zeroes suppressed, giving the prefix length (0 to 127) in
   bits.

   Similarly, complete IPv4 addresses MUST be presented in dotted
   decimal format, with no succeeding prefix length.  IPv4 prefixes MUST
   be presented as if they were full addresses, with the slash character
   (/) appended, followed by a decimal value with leading zeroes
   suppressed, giving the prefix length (0 to 31) in bits.

   N-bit fields and unsigned decimals are both presented as unsigned
   decimal integers with no leading zeroes.

5.2.2.  Special Cases

   Three special cases are identified in Table 2: encoding of the
   interface index list (PARAM-NAMEs SIFIX and DIFIX), encoding of the
   VPN identifier (PARAM-NAMEs SVPN and DVPN), and encoding of the
   trigger for resource allocation events (PARAM-NAME TRIG).

   The interface index list is presented as a series of individual
   interface indexes separated by commas, e.g., SIFIX="5,15".  Each
   individual interface index is presented as a 32-bit field (i.e., as
   an unsigned decimal integer with no leading zeroes).



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   The VPN Identifier is standardized in [RFC2685], and consists of a
   three octet VPN Authority (Organizationally Unique Identifier, OUI)
   followed by a four octet VPN index identifying the VPN according to
   OUI.  For SYSLOG, the OUI portion is presented as a string of six
   hexadecimal digits in lower case.  The VPN index is presented as a
   32-bit field.  A colon (:) is used to separate the OUI from the
   succeeding index value.  The OUI and separator MAY be omitted.  If
   so, the applicable OUI is the default value for the NAT instance.

   The trigger is an enumeration of text values which were not spelled
   out in the table itself for lack of space.  The possible values for
   TRIG are:

   "OPKT":  outgoing packet received at NAT.

   "IPKT":  incoming packet received at NAT.

   "ADMIN":  administrative action.

   "APMDEL":  deletion of the underlying address and port mapping.

   "AMDEL":  deletion of the underlying address mapping.

   "AUTO":  autonomous action of the NAT.

   The values applicable for any specific event are a subset of this
   list and are spelled out for each event in Section 5.3.

5.2.3.  Relationship To Objects In the NAT MIB

   Table 3 lists the parameters in the same order as Table 2 and relates
   each parameter to its corresponding object in the NAT MIB.

   +------------------------+------------------------------------------+
   | PARAM-NAME             | Related MIB Object(s)                    |
   +------------------------+------------------------------------------+
   | Miscellaneous          |                                          |
   |                        |                                          |
   | NATINST                | natInstanceAlias in natInstanceTable     |
   | TRIG                   | None                                     |
   |                        |                                          |
   | Subscriber-identifying |                                          |
   | information            |                                          |
   |                        |                                          |
   | SSUBIX                 | natSubscriberIndex in                    |
   |                        | natSubscribersTable                      |
   | SIFIX                  | natSubsInterfaceIndex in                 |
   |                        | natSubsInterfaceIdentifierTable          |



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   | SVLAN                  | natSubscriberVlanIdentifier in           |
   |                        | natSubscribersTable                      |
   | SVPN                   | natSubscriberVpnIdentifier in            |
   |                        | natSubscribersTable                      |
   | SV6ENC                 | natSubscriberIPEncapsIdType and          |
   |                        | natSubscriberIPEncapsIdAddr in           |
   |                        | natSubscribersTable                      |
   | DSUBIX                 | natSubscriberIndex in                    |
   |                        | natSubscribersTable                      |
   | DIFIX                  | natSubsInterfaceIndex in                 |
   |                        | natSubsInterfaceIdentifierTable          |
   | DVLAN                  | natSubscriberVlanIdentifier in           |
   |                        | natSubscribersTable                      |
   | DVPN                   | natSubscriberVpnIdentifier in            |
   |                        | natSubscribersTable                      |
   | DV6ENC                 | natSubscriberIPEncapsIdType and          |
   |                        | natSubscriberIPEncapsIdAddr in           |
   |                        | natSubscribersTable                      |
   |                        |                                          |
   | Internal packet        |                                          |
   | description            |                                          |
   |                        |                                          |
   | IRLM                   | natSubscriberRealm in                    |
   |                        | natSubscribersTable                      |
   | IATYP                  | natMapIntAddrIntType in                  |
   |                        | natMapIntAddrTable or                    |
   |                        | natMappingIntAddressType in              |
   |                        | natMappingTable                          |
   | ISADDR                 | natMapIntAddrInt in natMapIntAddrTable   |
   |                        | or natMappingIntAddress in               |
   |                        | natMappingTable                          |
   | ISPORT                 | natMappingIntPort in natMappingTable     |
   | IDADDR                 | None                                     |
   | IDPORT                 | None                                     |
   | PROTO                  | natMappingProto in natMappingTable       |
   |                        |                                          |
   | External (mapped)      |                                          |
   | packet description     |                                          |
   |                        |                                          |
   | XRLM                   | natPoolRealm in natPoolTable             |
   | XATYP                  | natMapIntAddrExtType in                  |
   |                        | natMapIntAddrTable or                    |
   |                        | natMappingExtAddressType in              |
   |                        | natMappingTable                          |
   | XSADDR                 | natMapIntAddrExt in natMapIntAddrTable   |
   |                        | or natMappingExtAddress in               |
   |                        | natMappingTable                          |
   | XSPORT                 | natMappingExtPort in natMappingTable     |



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   | XDADDR                 | None                                     |
   | XDPORT                 | None                                     |
   |                        |                                          |
   | Port range description |                                          |
   |                        |                                          |
   | PORTMN                 | None                                     |
   | PORTMX                 | None                                     |
   |                        |                                          |
   | Values related to      |                                          |
   | thresholds             |                                          |
   |                        |                                          |
   | POOLID                 | natPoolIndex in natPoolTable             |
   | POOLHW                 | natPoolWatermarkHigh in natPoolTable     |
   | POOLLW                 | natPoolWatermarkLow in natPoolTable      |
   | GAMCNT                 | natAddressMappingCreations -             |
   |                        | natAddressMappingRemovals in             |
   |                        | natCountersTable                         |
   | GAPMCNT                | natMappingCreations - natMappingRemovals |
   |                        | in natCountersTable                      |
   | SAPMCNT                | natSubscriberMappingCreations -          |
   |                        | natSubscriberMappingRemovals in          |
   |                        | natSubscribersTable                      |
   |                        |                                          |
   | Specific incoming      |                                          |
   | packet description     |                                          |
   |                        |                                          |
   | PSRLM                  | natSubscriberRealm in                    |
   |                        | natSubscribersTable in the case of a     |
   |                        | packet originated by an identifiable     |
   |                        | subscriber                               |
   | PATYP                  | None                                     |
   | PSADDR                 | None                                     |
   | PDADDR                 | None                                     |
   +------------------------+------------------------------------------+

       Table 3: Relationship of Parameters To Objects In the NAT MIB

5.3.  Encoding Of Complete Log Report For Each Event Type

   This section describes the complete NAT-related contents of the logs
   used to report the events listed in Table 1.

5.3.1.  Encoding of Events Relating To Allocation Of Resources To Hosts

   As indicated in Section 5.1, the event reports specified in this
   section MUST have APP-NAME="NAT" in the message header.





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5.3.1.1.  NAT Address Mapping Creation and Deletion

   As shown in Table 1:

   o  NAT address mapping creation event is indicated by MSGID set to
      "AMADD";

   o  NAT address mapping deletion event is indicated by MSGID set to
      "AMDEL".

   For both events, the associated SD-ELEMENT is tagged by SD-ID
   "namap".  The contents of the namap SD-ELEMENT are shown in Table 4.
   The requirements for these contents are derived from the description
   in Section 3.1.1.

   +------------------------------------+----------------+-------------+
   | Description                        | PARAM-NAME     | Requirement |
   +------------------------------------+----------------+-------------+
   | NAT instance identifier            | NATINST        | CONDITIONAL |
   | Source subscriber index            | SSUBIX         | MANDATORY   |
   | Additional source subscriber       | One of SIFIX,  | CONDITIONAL |
   | classifier value as recognized at  | SVLAN, SVPN,   |             |
   | the ingress to the internal realm  | or SV6ENC      |             |
   | Internal realm                     | IRLM           | CONDITIONAL |
   | Internal address type              | IATYP          | MANDATORY   |
   | Internal source IP address         | ISADDR         | MANDATORY   |
   | External realm                     | XRLM           | CONDITIONAL |
   | External address type              | XATYP          | MANDATORY   |
   | External source IP address         | XSADDR         | MANDATORY   |
   | Trigger for address mapping        | TRIG           | OPTIONAL    |
   | creation or deletion               |                |             |
   +------------------------------------+----------------+-------------+

    Table 4: Contents Of the SD-ELEMENT Section For Logging the Address
                   Mapping Creation and Deletion Events

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   o  One of SIFIX, SVLAN, SVPN, or SV6ENC REQUIRED if the internal
      source IP address is not enough to identify the subscriber
      unambiguously, else MUST NOT appear.

   o  IRLM or XRLM REQUIRED if not the default internal or external
      realm, respectively, else MAY appear.




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   For the AMADD event type (MSGID), TRIG can take on the values "OPKT"
   or "ADMIN".  For the AMDEL event type, TRIG can take on the values
   "ADMIN" or "AUTO".

   Example: DS-Lite AFTR.  One NAT instance.  Multiple internal IPv4
   realms containing the subscribers, divided by higher-level IPv6
   prefix (details unnecessary).  One default global IPv4 external
   realm.  Intra-subscriber sessions use mappings into this realm.

   Subscriber A in realm Internal05 sends an outgoing packet, causing
   the creation of an address mapping from the DS-Lite well-known
   address 192.0.0.2 to the global IPv4 address 198.51.100.127.
   Subscriber A's encapsulating IPv6 tunnel address is
   2001:db8:a5e6:39b0:bd6a:35ad:1d33:6df6.

   The event report for the address mapping creation is as follows (line
   folded into several for presentation):

      <142>1 2013-05-07T22:14:15.03487Z record.example.net NAT 5063
      AMADD [namap SSUBIX="489321"
      SV6ENC="2001:db8:a5e6:3900:bd6a:35ad:1d33:6df6" IRLM="Internal05"
      IATYP="IPv4" ISADDR="192.0.0.2" XATYP="IPv4"
      XSADDR="198.51.100.127"
      TRIG="OPKT"]

   Character count is about 240.

5.3.1.2.  NAT Address and Port Mapping Creation and Deletion

   As shown in Table 1:

   o  NAT address and port mapping creation event is indicated by MSGID
      set to "APMADD";

   o  NAT mapping deletion event is indicated by MSGID set to "APMDEL".

   For both events, the associated SD-ELEMENT is tagged by SD-ID
   "napmap".  The contents of the nmap SD-ELEMENT are shown in Table 5.
   The requirements for these contents are derived from the description
   in Section 3.1.2.











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   +------------------------------------+----------------+-------------+
   | PARAM-NAME                         | Description    | Requirement |
   +------------------------------------+----------------+-------------+
   | NAT instance identifier            | NATINST        | CONDITIONAL |
   | Source subscriber index            | SSUBIX         | MANDATORY   |
   | Additional source subscriber       | One of SIFIX,  | CONDITIONAL |
   | classifier value as recognized at  | SVLAN, SVPN,   |             |
   | the ingress to the internal realm  | or SV6ENC      |             |
   | Internal realm                     | IRLM           | CONDITIONAL |
   | Internal address type              | IATYP          | MANDATORY   |
   | Internal source IP address         | ISADDR         | MANDATORY   |
   | Internal source port or ICMP       | ISPORT         | MANDATORY   |
   | identifier                         |                |             |
   | External realm                     | XRLM           | CONDITIONAL |
   | External address type              | XATYP          | MANDATORY   |
   | External source IP address         | XSADDR         | MANDATORY   |
   | External source port or ICMP       | XSPORT         | MANDATORY   |
   | identifier                         |                |             |
   | Protocol identifier                | PROTO          | MANDATORY   |
   | Trigger for address and port       | TRIG           | OPTIONAL    |
   | mapping creation or deletion       |                |             |
   +------------------------------------+----------------+-------------+

    Table 5: Contents Of the SD-ELEMENT Section For Logging the mapping
                       Creation and Deletion Events

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   o  One of SIFIX, SVLAN, SVPN, or SV6ENC REQUIRED if the internal
      source IP address is not enough to identify the subscriber
      unambiguously, else MUST NOT appear.

   o  IRLM or XRLM REQUIRED if not the default internal or external
      realm, respectively, else MAY appear.

   For the APMADD event type (MSGID), TRIG can take on the values
   "OPKT", IPKT", or "ADMIN".

      Note: it is not clear how the internal source port is selected if
      an address and port mapping is triggered by an incoming TCP
      packet.  The NAT could select one based on its knowledge of
      subscriber port usage, but this knowledge may be incomplete.  Some
      type of negotiation may be necessary, or else TCP address and port
      mappings can only be triggered by outbound packets as in the
      example below.



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   For the APMDEL event type, TRIG can take on the values "ADMIN",
   "AMDEL", or "AUTO".

   Example: The triggering outgoing packet in the previous case was a
   TCP packet with internal source port 49178.  As well as triggering
   the creation of an address mapping, the packet triggers the creation
   of an address and port mapping between that port and an external
   source port 6803.  The corresponding mapping creation report would
   look like this:

      <142>1 2013-05-07T22:14:15.03487Z record.example.net NAT 5063
      APMADD [napmap SSUBIX="489321"
      SV6ENC="2001:db8:a5e6:3900:bd6a:35ad:1d33:6df6" IRLM="Internal05"
      IATYP="IPv4" ISADDR="192.0.0.2" ISPORT="49178"
      XATYP="IPv4" XSADDR="198.51.100.127" XSPORT=6803"
      PROTO="6" TRIG="OPKT"]

   Character count is about 280.

5.3.1.3.  NAT Session Creation and Deletion

   As shown in Table 1:

   o  NAT session creation event is indicated by MSGID set to "SADD";

   o  NAT session deletion event is indicated by MSGID set to "SDEL".

   For both events, the associated SD-ELEMENT is tagged by SD-ID
   "nsess".  The contents of the nsess SD-ELEMENT are shown in Table 6.
   The requirements for these contents are derived from the description
   in Section 3.1.3.




















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   +-------------------------------------+---------------+-------------+
   | PARAM-NAME                          | Description   | Requirement |
   +-------------------------------------+---------------+-------------+
   | NAT instance identifier             | NATINST       | CONDITIONAL |
   | Source subscriber index             | SSUBIX        | MANDATORY   |
   | Additional source subscriber        | One of SIFIX, | CONDITIONAL |
   | classifier value as recognized at   | SVLAN, SVPN,  |             |
   | the ingress to the internal realm   | or SV6ENC     |             |
   | Internal realm                      | IRLM          | CONDITIONAL |
   | Internal address type               | IATYP         | MANDATORY   |
   | Internal source IP address          | ISADDR        | MANDATORY   |
   | Internal source port or ICMP        | ISPORT        | MANDATORY   |
   | identifier                          |               |             |
   | External realm                      | XRLM          | CONDITIONAL |
   | External address type               | XATYP         | MANDATORY   |
   | External source IP address          | XSADDR        | MANDATORY   |
   | External source port or ICMP        | XSPORT        | MANDATORY   |
   | identifier                          |               |             |
   | Protocol identifier                 | PROTO         | MANDATORY   |
   | Internal destination IP address     | IDADDR        | CONDITIONAL |
   | Internal destination port or ICMP   | IDPORT        | CONDITIONAL |
   | identifier                          |               |             |
   | Destination subscriber index        | DSUBIX        | CONDITIONAL |
   | Additional destination subscriber   | One of DIFIX, | CONDITIONAL |
   | classifier value as recognized at   | DVLAN, DVPN,  |             |
   | the ingress to the external realm   | or DV6ENC     |             |
   | External destination IP address     | XDADDR        | CONDITIONAL |
   | External destination port or ICMP   | XDPORT        | CONDITIONAL |
   | identifier                          |               |             |
   | Trigger for session creation or     | TRIG          | OPTIONAL    |
   | deletion                            |               |             |
   +-------------------------------------+---------------+-------------+

    Table 6: Contents Of the SD-ELEMENT Section For Logging the Session
                       Creation and Deletion Events

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   o  One of SIFIX, SVLAN, SVPN, or SV6ENC REQUIRED if the internal
      source IP address is not enough to identify the subscriber
      unambiguously, else MUST NOT appear.

   o  IRLM or XRLM REQUIRED if not the default internal or external
      realm, respectively, else MAY appear.




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   o  IDADDR and IDPORT REQUIRED if destination logging is enabled and
      these need to be remapped to external destination address and
      port.  Otherwise, if destination logging is disabled, they MUST
      NOT appear, and if destination logging is enabled, they SHOULD NOT
      appear because of redundancy.

   o  DSUBIX REQUIRED if destination logging is enabled and the
      destination is a subscriber served by the NAT, else MUST NOT
      appear.

   o  One of DIFIX, DVLAN, DVPN, or DV6ENC REQUIRED if destination
      logging is enabled and the destination is a subscriber served by
      the NAT and the external destination address is not enough to
      identify the external destination subscriber unambiguously, else
      MUST NOT appear.

   o  XDADDR and XDPORT REQUIRED if destination logging is enabled, else
      MUST NOT appear.

   For the SADD event type (MSGID), TRIG can take on the values "OPKT",
   IPKT", or "ADMIN".  For the SDEL event type, TRIG can take on the
   values "ADMIN", "MDEL", or "AUTO".

   Example: destination logging is enabled.  The outgoing packet that
   triggered the address and port mapping in the previous section was
   sent to 192.0.2.57 port 80.  The session creation event report
   appears as follows:

      <142>1 2013-05-07T22:14:15.03487Z record.example.net NAT 5063
      SESSADD [nsess SSUBIX="489321"
      SV6ENC="2001:db8:a5e6:3900:bd6a:35ad:1d33:6df6" IRLM="Internal05"
      IATYP="IPv4" ISADDR="192.0.0.2" ISPORT="49178"
      XATYP="IPv4" XSADDR="198.51.100.127" XSPORT=6803"
      PROTO="6" XDADDR="192.0.2.57" XDPORT="80" TRIG="OPKT"]

   Character count is about 310.

5.3.1.4.  Port Range Allocation and Deallocation

   As shown in Table 1:

   o  Port range allocation event is indicated by MSGID set to "PTADD";

   o  Port range deallocation event is indicated by MSGID set to
      "PTDEL".

   For both events, the associated SD-ELEMENT is tagged by SD-ID
   "nprng".  The contents of the npset SD-ELEMENT are shown in Table 7.



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   The requirements for these contents are derived from the description
   in Section 3.1.4.

   +------------------------------------+----------------+-------------+
   | PARAM-NAME                         | Description    | Requirement |
   +------------------------------------+----------------+-------------+
   | NAT instance identifier            | NATINST        | CONDITIONAL |
   | Source subscriber index            | SSUBIX         | MANDATORY   |
   | Additional source subscriber       | One of SIFIX,  | CONDITIONAL |
   | classifier value as recognized at  | SVLAN, SVPN,   |             |
   | the ingress to the internal realm  | or SV6ENC      |             |
   | Internal realm                     | IRLM           | CONDITIONAL |
   | Internal address type              | IATYP          | MANDATORY   |
   | Internal source IP address         | ISADDR         | MANDATORY   |
   | External realm                     | XRLM           | CONDITIONAL |
   | External address type              | XATYP          | MANDATORY   |
   | External source IP address         | XSADDR         | MANDATORY   |
   | Port range lowest value            | PORTMN         | MANDATORY   |
   | Port range highest value           | PORTMX         | MANDATORY   |
   | Trigger for port range allocation  | TRIG           | OPTIONAL    |
   | or deallocation                    |                |             |
   +------------------------------------+----------------+-------------+

   Table 7: Contents Of the SD-ELEMENT Section For Logging the Port Set
                    Allocation and Deallocation Events

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   o  One of SIFIX, SVLAN, SVPN, or SV6ENC REQUIRED if the internal
      source IP address is not enough to identify the subscriber
      unambiguously, else MUST NOT appear.

   o  IRLM or XRLM REQUIRED if not the default internal or external
      realm, respectively, else MAY appear.

   For the PTADD event type (MSGID), TRIG can take on the values "OPKT",
   "IPKT", "ADMIN", or "AUTO".  For the PTDEL event type, TRIG can take
   on the values "ADMIN" or "AUTO".

   Consider an example where the range 1024-1535 is allocated to the
   address mapping on which the example in Section 5.3.1.1 is based.
   The corresponding port range allocation report would look like this:

      <142>1 2013-05-07T22:14:15.03487Z record.example.net NAT 5063
      PTADD [nprng SSUBIX="489321"



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      SV6ENC="2001:db8:a5e6:3900:bd6a:35ad:1d33:6df6" IRLM="Internal05"
      IATYP="IPv4" ISADDR="192.0.0.2" XATYP="IPv4"
      XSADDR="198.51.100.127"
      PORTMN="1024" PORTMX="1535" TRIG="OPKT"]

   Character count is about 270.

5.3.2.  Encoding of Threshold Events

   As indicated in Section 5.1, the event reports specified in this
   section MUST have APP-NAME="NATTHR" in the SYSLOG message header.

5.3.2.1.  NAT Address Pool High- and Low-Water-Mark Threshold Events

   As shown in Table 1:

   o  NAT address pool high-water-mark threshold event is indicated by
      MSGID set to "POOLHT";

   o  NAT address pool low-water-mark threshold event is indicated by
      MSGID set to "POOLLT".

   For both events, the associated SD-ELEMENT is tagged by SD-ID
   "npool".  The contents of the npool SD-ELEMENT are shown in Table 8.
   The requirements for these contents are derived from the description
   in Section 3.2.1.

   +------------------------------+----------------------+-------------+
   | PARAM-NAME                   | Description          | Requirement |
   +------------------------------+----------------------+-------------+
   | NAT instance identifier      | NATINST              | CONDITIONAL |
   | Pool identifier              | POOLID               | MANDATORY   |
   | The threshold value set by   | POOLHW or POOLLW as  | CONDITIONAL |
   | the administrator            | applicable           |             |
   +------------------------------+----------------------+-------------+

    Table 8: Contents Of the SD-ELEMENT Section For Logging the Address
              Pool High- and Low-Water-Mark Threshold Events

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   o  POOLHW REQUIRED for high-water-mark event, else MUST NOT appear.

   o  POOLLW REQUIRED for low-water-mark event, else MUST NOT appear.




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   Example, assuming a high-water-mark threshold of 80% aggregate
   address-port utilization::

      <132>1 2013-08-15T09:15:16.08716Z record.example.net NATTHR 5025
      POOLHT [npool POOLID="13" POOLHW="80"]

   Character count is about 105.

5.3.2.2.  Global Address Mapping High-Water-Mark Threshold Exceeded

   As shown in Table 1:

   o  Global address mapping high-water-mark threshold event is
      indicated by MSGID set to "GAMHT"; and

   o  the associated SD-ELEMENT is tagged by SD-ID "ngamht".

   The contents of the ngamht SD-ELEMENT are shown in Table 9.  The
   requirements for these contents are derived from the description in
   Section 3.2.2.

   +---------------------------------------+-------------+-------------+
   | PARAM-NAME                            | Description | Requirement |
   +---------------------------------------+-------------+-------------+
   | NAT instance identifier               | NATINST     | CONDITIONAL |
   | Current number of active address      | GAMCNT      | MANDATORY   |
   | mappings                              |             |             |
   +---------------------------------------+-------------+-------------+

    Table 9: Contents Of the SD-ELEMENT Section For Logging the Global
                Address Map High-Water-Mark Threshold Event

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   Example, assuming a threshold was set to 690000, already exceeded.
   As a result, prior events of this type were detected and logged,
   unless they were suppressed by the sort of controls discussed in
   Section 6.

      <132>1 2013-08-15T09:15:16.08716Z record.example.net NATTHR 5025
      GAMHT [ngamht GAMCNT="690015"]

   Character count is about 95.





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5.3.2.3.  Global Address and Port Mapping High-Water-Mark Threshold
          Event

   As shown in Table 1:

   o  Global address and port mapping high-water-mark threshold event is
      indicated by MSGID set to "GAPMHT"; and

   o  the associated SD-ELEMENT is tagged by SD-ID "ngapmht".

   The contents of the ngmht SD-ELEMENT are shown in Table 10.  The
   requirements for these contents are derived from the description in
   Section 3.2.3.

   +---------------------------------------+-------------+-------------+
   | PARAM-NAME                            | Description | Requirement |
   +---------------------------------------+-------------+-------------+
   | NAT instance identifier               | NATINST     | CONDITIONAL |
   | Current global number of address and  | GAPMCNT     | MANDATORY   |
   | port mappings                         |             |             |
   +---------------------------------------+-------------+-------------+

    Table 10: Contents Of the SD-ELEMENT Section For Logging the Global
         Address and Port Mapping High-Water-Mark Threshold Event

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   Example: suppose the threshold was set to 2000000, so it has already
   been exceeded.  As in the previous section, prior events of this type
   were detected and logged, unless they were suppressed by the sort of
   controls discussed in Section 6.

      <132>1 2013-08-15T09:15:16.08716Z record.example.net NATTHR 5025
      GAPMHT [ngapmht GAPMCNT="2000023"]

   Character count is about 100.

5.3.2.4.  Subscriber-Specific Address and Port Mapping High-Water-Mark
          Threshold Event

   As shown in Table 1:

   o  Subscriber-specific address and port mapping high-water-mark
      threshold event is indicated by MSGID set to "SAPMHT"; and




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   o  the associated SD-ELEMENT is tagged by SD-ID "nsapmht".

   The contents of the nsapmht SD-ELEMENT are shown in Table 11.  The
   requirements for these contents are derived from the description in
   Section 3.2.4.

   +---------------------------------------+-------------+-------------+
   | PARAM-NAME                            | Description | Requirement |
   +---------------------------------------+-------------+-------------+
   | NAT instance identifier               | NATINST     | CONDITIONAL |
   | Index of the affected subscriber      | SSUBIX      | MANDATORY   |
   | Current number of address and port    | SAPMCNT     | MANDATORY   |
   | mappings for this subscriber          |             |             |
   +---------------------------------------+-------------+-------------+

       Table 11: Contents Of the SD-ELEMENT Section For Logging the
       Subscriber-Specific Address and Port Mapping High-Water-Mark
                              Threshold Event

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   Example: suppose the threshold was set to 1500 and the number of
   mappings for this subscriber has been increasing.  Then this is the
   first threshold-exceeded event detected of what could possibly be a
   series of such events until subscriber consumption of outgoing ports
   drops below threshold again.

      <133>1 2013-08-15T09:15:16.08853Z record.example.net NATTHR 5025
      SAPMHT [nsapmht SSUBIX="489321" SAPMCNT="1501"]

   Character count is about 115.

5.3.3.  Encoding of Limit Events

   As indicated in Section 5.1, the event reports specified in this
   section MUST have APP-NAME="NATLIM" in the SYSLOG message header.

5.3.3.1.  Global Address Mapping Limit Exceeded

   As shown in Table 1:

   o  Global address mapping limit exceeded event is indicated by MSGID
      set to "GAMLIM"; and

   o  the associated SD-ELEMENT is tagged by SD-ID "ngaml".



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   The contents of the ngaml SD-ELEMENT are shown in Table 12.  The
   requirements for these contents are derived from the description in
   Section 3.3.1.

     +----------------------------------+-------------+-------------+
     | PARAM-NAME                       | Description | Requirement |
     +----------------------------------+-------------+-------------+
     | NAT instance identifier          | NATINST     | CONDITIONAL |
     | Index of the affected subscriber | SSUBIX      | MANDATORY   |
     +----------------------------------+-------------+-------------+

    Table 12: Contents Of the SD-ELEMENT Section For Logging the Global
                     Address Map Limit Exceeded Event

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   Example:

      <131>1 2013-08-15T09:15:16.08716Z record.example.net NATLIM 5025
      GAMLIM [ngaml NATINST="VRF-Cust-X" SSUBIX="278067"]

   Character count is about 115.

5.3.3.2.  Global Address and Port Mapping Limit Exceeded

   As shown in Table 1:

   o  Global adress and port mapping limit exceeded event is indicated
      by MSGID set to "GAPMLIM"; and

   o  the associated SD-ELEMENT is tagged by SD-ID "ngapml".

   The contents of the ngapml SD-ELEMENT are shown in Table 13.  The
   requirements for these contents are derived from the description in
   Section 3.3.2.













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   +---------------------------------------+-------------+-------------+
   | PARAM-NAME                            | Description | Requirement |
   +---------------------------------------+-------------+-------------+
   | NAT instance identifier               | NATINST     | CONDITIONAL |
   | Index of the internal subscriber      | SSUBIX      | CONDITIONAL |
   | Index of the external subscriber      | DSUBIX      | CONDITIONAL |
   | Source realm of the triggering packet | PSRLM       | MANDATORY   |
   | Incoming packet header IP address     | PATYP       | CONDITIONAL |
   | type                                  |             |             |
   | Incoming packet source IP address     | PSADDR      | CONDITIONAL |
   +---------------------------------------+-------------+-------------+

    Table 13: Contents Of the SD-ELEMENT Section For Logging the Global
               Address and Port Mapping Limit Exceeded Event

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   o  SSUBIX REQUIRED if the mapping was triggered by a packet outgoing
      from the internal to the external realm, else MUST NOT appear.

   o  DSUBIX is REQUIRED if the mapping was triggered by a packet
      incoming from a subscriber served by the NAT and located in the
      external realm (i.e., using an address mapping created previously
      by the internal subscriber), else MUST NOT appear.

   o  PATYP and PSADDR from the initiating packet are REQUIRED if the
      mapping was triggered by a packet incoming from a purely external
      realm (i.e., using an address mapping created previously by the
      internal subscriber), else MAY appear.

   Example: limit event triggered by a packet coming from 192.0.2.57 in
   realm "externv4".

      <131>1 2013-08-15T09:15:16.08716Z record.example.net NATLIM 5025
      GAPMLIM [ngapml NATINST="VRF-Cust-X" PSRLM="externv4"
      PATYP="IPv4" PSADDR="192.0.2.57"]

   Character count is about 150.

5.3.3.3.  Global Limit On Number of Active Hosts Exceeded

   As shown in Table 1:

   o  Global active hosts limit exceeded event is indicated by MSGID set
      to "GSLIM"; and



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   o  the associated SD-ELEMENT is tagged by SD-ID "ngsl".

   The contents of the ngsl SD-ELEMENT are shown in Table 14.  The
   requirements for these contents are derived from the description in
   Section 3.3.3.

     +----------------------------------+-------------+-------------+
     | PARAM-NAME                       | Description | Requirement |
     +----------------------------------+-------------+-------------+
     | NAT instance identifier          | NATINST     | CONDITIONAL |
     | Index of the affected subscriber | SSUBIX      | MANDATORY   |
     +----------------------------------+-------------+-------------+

    Table 14: Contents Of the SD-ELEMENT Section For Logging the Global
                     Active Host Limit Exceeded Event

   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   An example would look exactly like that in Section 5.3.3.1 with the
   substitution of GSLIM for GAMLIM and ngsl for ngaml.

5.3.3.4.  Subscriber-Specific Limit On Number of Address and Port
          Mappings Exceeded

   As shown in Table 1:

   o  Subscriber-specific mapping limit exceeded event is indicated by
      MSGID set to "SMLIM"; and

   o  the associated SD-ELEMENT is tagged by SD-ID "nsml".

   The contents of the nsml SD-ELEMENT are shown in Table 15.  The
   requirements for these contents are derived from the description in
   Section 3.3.4.

     +----------------------------------+-------------+-------------+
     | PARAM-NAME                       | Description | Requirement |
     +----------------------------------+-------------+-------------+
     | NAT instance identifier          | NATINST     | CONDITIONAL |
     | Index of the affected subscriber | SSUBIX      | MANDATORY   |
     +----------------------------------+-------------+-------------+

       Table 15: Contents Of the SD-ELEMENT Section For Logging the
             Subscriber-Specific Mapping Limit Exceeded Event




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   Conditions:

   o  NATINST REQUIRED if device supports more than one instance, else
      MAY appear.

   An example would look exactly like that in Section 5.3.3.1 with the
   substitution of SAPMLIM for GAMLIM and nsapml for ngaml.

5.3.3.5.  Pending Fragment Limit Exceeded

   As shown in Table 1:

   o  Pending fragment limit exceeded event is indicated by MSGID set to
      "FRAG"; and

   o  the associated SD-ELEMENT is tagged by SD-ID "nfpkt".

   The contents of the nfpkt SD-ELEMENT are shown in Table 16.  The
   requirements for these contents are derived from the description in
   Section 3.3.5.

       +-------------------------------+-------------+-------------+
       | PARAM-NAME                    | Description | Requirement |
       +-------------------------------+-------------+-------------+
       | NAT instance identifier       | NATINST     | CONDITIONAL |
       | Source realm of the packet    | PSRLM       | MANDATORY   |
       | Packet header IP address type | PATYP       | MANDATORY   |
       | Packet source IP address      | PSADDR      | MANDATORY   |
       | Packet destination IP address | PDADDR      | MANDATORY   |
       | Source subscriber index       | SSUBIX      | CONDITIONAL |
       +-------------------------------+-------------+-------------+

   Table 16: Contents Of the SD-ELEMENT Section For Logging the Pending
                       Fragment Limit Exceeded Event

   Conditions:

   o  NAT instance identifier REQUIRED if device supports more than one
      instance, else MAY appear.

   o  Source subscriber index REQUIRED if the source of the packet is a
      subscriber served by the NAT and can be determined, else MUST NOT
      appear.

   Example: assuming the packet passing the limit came from an internal
   host and was dropped as a result of the limit.

      <132>1 2013-08-15T09:15:16.08Z record.example.net NATLIM 5025



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      FRAG [nfpkt PSRLM="DsLite-089" PATYP="IPv4" PSADDR="192.0.0.2"
      PDADDR="203.0.113.26" SSUBIX="32791"]

   Character count is about 160.

6.  Management Considerations

   This section considers requirements for management of the log system
   to support logging of the events described above.  It first covers
   requirements applicable to log management in general.  Any additional
   standardization required to fulfil these requirements is out of scope
   of the present document.  Subsequent sub-sections discuss management
   issues related to specific event report types.  The identifiers PRI,
   APP-NAME, and MSGID used below refer to fields in the SYSLOG header
   [RFC5424]

6.1.  General Requirements For Control Of Logging

   This document assumes that any implementation provides the following
   capabilities, discussed in more detail below:

   o  ability to configure the PRI value of each event report type at
      the granularity of (APP-NAME, MSGID) combination;

   o  ability at each collector to determine that event reports that it
      should have received have been lost.  The required granularity is
      at least at the level of PRI and may be finer for some event
      types.

   o  ability to configure criteria to automatically suppress the
      generation of event reports while the criteria are met, at the
      granularity of (APP-NAME, MSGID) combination.

6.1.1.  Configuration of PRI Value

   The PRI value is composed of two numbers, the Facility value and the
   Severity.  It may be used at the origin for selecting logs to streams
   being dispatched to different collectors, and in applications beyond
   the collectors to prioritize display of logs to operators.  The event
   reports in this document have been structured such that the Severity
   level varies between event types as represented by (APP-NAME, MSGID)
   combination.  As an extreme example, the address pool high-water-mark
   threshold event (APP-NAME="NATTHR", MSGID="POOLHT") is obviously more
   urgent than the low-water-mark threshold event (APP-NAME="NATTHR",
   MSGID="POOLLT").

   To some extent, this document tries to simplify message routing by
   making a general distinction between event types recording the



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   allocation of resources to hosts (with APP-NAME="NAT") and events of
   interest to operations and maintenance (with APP-NAME="NATTHR" and
   APP-NAME="NATLIM").  The need to provide different Severity levels
   for different event types remains.

6.1.2.  Ability For Each Collector To Detect Lost Event Reports

   Operators have a need to know when a given collector has not received
   all of the event reports it should have.  It probably does not matter
   if less-important events are tracked at the granularity of event type
   (APP-NAME, MSGID combination), by APP-NAME, or just by PRI value.

   The event types defined in this document relating to allocation of
   resources to hosts are a special case.  Regulatory requirements or
   the possibility that such reports might be introduced into court in
   cases such as abuse impose a requirement that the record of
   allocations to a particular host be complete.  This requirement is
   important enough to be stated in the Security Considerations section
   (Section 7), where the implementation of signed SYSLOG messages
   [RFC5848], which also provides message sequencing, is mandated as
   part of this specification.

   In deploying [RFC5848], the operator needs to decide the level of
   granularity of tracking, whether it should be over the whole set of
   reports covered by APP-NAME="NAT" or at a finer level.  This
   judgement has to be tempered by local circumstances.  One point to
   note is that since both creations/allocations and deletions/
   deallocations are recorded, a certain amount of redundancy is
   available in the reports being generated.  However, without both the
   creation and deletion timestamps, there is no definitive evidence of
   the specific period of time during which the resources concerned were
   allocated to a specific host.

6.1.3.  Ability To Rate Limit Or Disable Event Reports

   The event report types specified with APP-NAME="NATTHR" and APP-
   NAME="NATLIM" all relate to thresholds or limits.  By their nature,
   events of this sort will come in bursts.  The threshold or limit will
   be hit, the resource concerned will remain busy for a period, then
   pressure on the resource will ease.  Depending on the resource,
   possibly hundreds of instances of the event concerned will be
   detected during a single busy period.

   Where repeated events involve the same resource, it makes little
   sense to report all of them, since the NAT MIB counters provide the
   necessary information more succinctly.  On the other hand, it can be
   useful to know that the fragmentation limit, for instance, is being
   hit by successive packets from the same source address.



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   As a result of these considerations, this document requires that
   implementations MUST provide means to configure limits on the rate at
   which event reports of a given type (APP-NAME, MSGID combination) are
   generated.  It is RECOMMENDED that it be possible to specify two
   values per (APP-NAME, MSGID) combination:

   o  minimum time between initial instances of a given event report
      type;

   o  maximum number of instances of the event report to generate per
      busy period.

   Regardless of the detailed method the implementation provides for
   specifying the rate limiting of individual event report types, all
   implementations MUST allow the operator to indicate through
   configuration that a given event report type is to be completely
   disabled.  This is particularly required to disable logging of either
   session or mapping creations and deletions when not required (see
   discussion in Section 3.1.2).  It is also required when the operator
   prefers to receive threshold event notifications via SNMP rather than
   SYSLOG.

   The ability to rate limit or disable event reports MUST NOT interfere
   with the requirement to detect lost messages.  This has implications
   for any sequence numbering used for that purpose.  It is RECOMMENDED
   in any event that the implementation provide MIB counters of numbers
   of messages not generated due to rate limiting by event type
   supported.  If this is done, counters for disabled event report types
   SHOULD NOT be incremented, since that could require keeping
   unnecessary additional state.

6.2.  Setting Limits and Thresholds

   The "NATTHR" and "NATLIM" events specified in this document depend on
   the thresholds and limits configured in the NAT MIB
   [I-D.behave-NAT-MIB].  The limits have to do with policy in some
   cases (e.g., most especially the subscriber-specific limits), but
   generally depend on the implementation and the device in which it is
   deployed.

   The purpose of high-water-mark thresholds is, of course, to give
   sufficient advance warning that utilization of a particular resource
   is approaching its limit, so that appropriate provisioning or
   reconfiguration action can be undertaken to preserve target service
   levels on the NAT device.  Thus the following general principles
   apply:





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   o  A high-water-mark threshold should be derived as a percentage of
      the relevant limit.

   o  The more quickly that utilization of a given resource can build
      up, the lower the threshold must be to provide an adequate
      response time.

   o  Some limits are more important than others in terms of their
      effect on overall service levels provided by the NAT device.  To
      focus attention on the more important limits, their corresponding
      thresholds should be set lower than those for less-important
      limits, all other things being equal.

   In practice, thresholds will require tuning to fit the particular
   characteristics of the NAT device and its users.

   The setting of the high-water-mark-thresholds for address pools
   (Section 3.2.1) poses additional challenges.  The problem is that the
   bottleneck for port availability will generally be a single protocol,
   which may vary from one time to another.  However, the threshold is
   based on overall port utilization.  If port usage is such that one
   protocol generally predominates, the required threshold value has to
   be lower than if usage is more balanced between protocols.  Clearly
   the appropriate threshold value depends on the characteristics of the
   traffic handled by the particular address pool concerned.

   Pooling behaviour adds another factor for consideration.  With a
   pooling behaviour of "arbitrary" [RFC4787], port utilization for the
   bottleneck protocol can be quite high before service levels offered
   by the pool are in danger.  On the other hand, with a pooling
   behaviour of "paired", possible utilization levels will be much lower
   because typically a number of port values will be reserved to each
   address mapping and only some of those will be in use on the average.
   The difference between "arbitrary" and "paired" utilization for a
   given level of service may be quite dramatic.

6.3.  Other Management Requirements

   The identification of internal realms is contingent on the the
   existence and applicability of default internal and external realms.
   If the implementation is capable of supporting more than one internal
   or external realm, it MUST provide the means for the operator to
   specify which realm is the default internal and/or external realm, as
   the case may be.







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7.  Security Considerations

   When logs are being recorded for regulatory reasons or as potential
   evidence in abuse cases, preservation of their integrity and
   authentication of their origin is essential.  To achieve this result,
   signed SYSLOG messages [RFC5848] MUST be implemented as part of this
   specification.  It is RECOMMENDED that the operator deploy [RFC5848]
   where local requirements on integrity and authentication of origin
   are stringent.  In conjunction with [RFC5848] and as recommended in
   Section 3 of that document, TLS transport as specified in [RFC5425]
   SHOULD be used between the origin and the collector(s) and MUST be
   implemented.  Section 5.2.1 of [RFC5848] specifies the minimum
   support for Key Blob Type that must be provided by implementations of
   that specification.

   Access to the logs defined in Section 3.1 and Section 5.3.1 while the
   reported assignments are in force could improve an attacker's chance
   of hijacking a session through port-guessing.  Even after an
   assignment has expired, the information in the logs SHOULD be treated
   as confidential, since, if revealed, it could help an attacker trace
   sessions back to a particular user or user location.  It is therefore
   RECOMMENDED that these logs be transported securely, using [RFC5425],
   for example, even if [RFC5848] is not deployed, that they be stored
   securely at the collector, and that access to them at the collector
   and in applications be tightly controlled.

   The logs defined in Section 3.2 and Section 3.3 are less sensitive in
   general, but since many of them contain the subscriber identifier,
   they could be used to get some sense of subscriber activity.  The
   fragmentation limit event provides actual packet header contents.
   Operators SHOULD at the least deploy secure transport to ensure that
   this information is not misused.

8.  IANA Considerations

   This document requests IANA to make the following assignments to the
   SYSLOG Structured Data ID Values registry.  RFCxxxx refers to the
   present document when approved.

   Some PARAM-NAMES appear under more than one SD-ID in Table 17.
   Formally, a parameter used with more than one event is registered as
   multiple separate parameters, one for each event report in which it
   is used.  However, there is no reason to change either the PARAM-NAME
   or the encoding of the PARAM-VALUE between different instances of the
   same parameter if the parameters have the same meaning in both event
   reports.





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   While a number of parameters are marked CONDITIONAL in the body of
   this document, the SYSLOG registry provides only for MANDATORY and
   OPTIONAL parameters.  All CONDITIONAL parameters have been placed in
   the OPTIONAL category in Table 17.

   +----------------+--------------------+-----------------+-----------+
   | Structured     | Structured Data    | Required or     | Reference |
   | Data ID        | Parameter          | Optional        |           |
   +----------------+--------------------+-----------------+-----------+
   | namap          |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   |                | SIFIX              | OPTIONAL        | RFCxxxx   |
   |                | SVLAN              | OPTIONAL        | RFCxxxx   |
   |                | SVPN               | OPTIONAL        | RFCxxxx   |
   |                | SV6ENC             | OPTIONAL        | RFCxxxx   |
   |                | IRLM               | OPTIONAL        | RFCxxxx   |
   |                | IATYP              | MANDATORY       | RFCxxxx   |
   |                | ISADDR             | MANDATORY       | RFCxxxx   |
   |                | XRLM               | OPTIONAL        | RFCxxxx   |
   |                | XATYP              | MANDATORY       | RFCxxxx   |
   |                | XSADDR             | MANDATORY       | RFCxxxx   |
   |                | TRIG               | OPTIONAL        | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | napmap         |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   |                | SIFIX              | OPTIONAL        | RFCxxxx   |
   |                | SVLAN              | OPTIONAL        | RFCxxxx   |
   |                | SVPN               | OPTIONAL        | RFCxxxx   |
   |                | SV6ENC             | OPTIONAL        | RFCxxxx   |
   |                | IRLM               | OPTIONAL        | RFCxxxx   |
   |                | IATYP              | MANDATORY       | RFCxxxx   |
   |                | ISADDR             | MANDATORY       | RFCxxxx   |
   |                | ISPORT             | MANDATORY       | RFCxxxx   |
   |                | XRLM               | OPTIONAL        | RFCxxxx   |
   |                | XATYP              | MANDATORY       | RFCxxxx   |
   |                | XSADDR             | MANDATORY       | RFCxxxx   |
   |                | XSPORT             | MANDATORY       | RFCxxxx   |
   |                | PROTO              | MANDATORY       | RFCxxxx   |
   |                | TRIG               | OPTIONAL        | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | nsess          |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   |                | SIFIX              | OPTIONAL        | RFCxxxx   |
   |                | SVLAN              | OPTIONAL        | RFCxxxx   |
   |                | SVPN               | OPTIONAL        | RFCxxxx   |



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   |                | SV6ENC             | OPTIONAL        | RFCxxxx   |
   |                | IRLM               | OPTIONAL        | RFCxxxx   |
   |                | IATYP              | MANDATORY       | RFCxxxx   |
   |                | ISADDR             | MANDATORY       | RFCxxxx   |
   |                | ISPORT             | MANDATORY       | RFCxxxx   |
   |                | XRLM               | OPTIONAL        | RFCxxxx   |
   |                | XATYP              | MANDATORY       | RFCxxxx   |
   |                | XSADDR             | MANDATORY       | RFCxxxx   |
   |                | XSPORT             | MANDATORY       | RFCxxxx   |
   |                | PROTO              | MANDATORY       | RFCxxxx   |
   |                | IDADDR             | OPTIONAL        | RFCxxxx   |
   |                | IDPORT             | OPTIONAL        | RFCxxxx   |
   |                | DSUBIX             | OPTIONAL        | RFCxxxx   |
   |                | DIFIX              | OPTIONAL        | RFCxxxx   |
   |                | DVLAN              | OPTIONAL        | RFCxxxx   |
   |                | DVPN               | OPTIONAL        | RFCxxxx   |
   |                | DV6ENC             | OPTIONAL        | RFCxxxx   |
   |                | XDADDR             | OPTIONAL        | RFCxxxx   |
   |                | XDPORT             | OPTIONAL        | RFCxxxx   |
   |                | TRIG               | OPTIONAL        | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | nprng          |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   |                | SIFIX              | OPTIONAL        | RFCxxxx   |
   |                | SVLAN              | OPTIONAL        | RFCxxxx   |
   |                | SVPN               | OPTIONAL        | RFCxxxx   |
   |                | SV6ENC             | OPTIONAL        | RFCxxxx   |
   |                | IRLM               | OPTIONAL        | RFCxxxx   |
   |                | IATYP              | MANDATORY       | RFCxxxx   |
   |                | ISADDR             | MANDATORY       | RFCxxxx   |
   |                | XRLM               | OPTIONAL        | RFCxxxx   |
   |                | XATYP              | MANDATORY       | RFCxxxx   |
   |                | XSADDR             | MANDATORY       | RFCxxxx   |
   |                | PORTMN             | MANDATORY       | RFCxxxx   |
   |                | PORTMX             | MANDATORY       | RFCxxxx   |
   |                | TRIG               | OPTIONAL        | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | npool          |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | POOLID             | MANDATORY       | RFCxxxx   |
   |                | POOLLT             | OPTIONAL        | RFCxxxx   |
   |                | POOLHT             | OPTIONAL        | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | ngamht         |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | GAMCNT             | MANDATORY       | RFCxxxx   |
   | ----           | ----               | ----            | ----      |



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   | ngapmht        |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | GAPMCNT            | MANDATORY       | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | nsapmht        |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   |                | SAPMCNT            | MANDATORY       | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | ngaml          |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | ngapml         |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | OPTIONAL        | RFCxxxx   |
   |                | DSUBIX             | OPTIONAL        | RFCxxxx   |
   |                | PSRLM              | MANDATORY       | RFCxxxx   |
   |                | PATYP              | OPTIONAL        | RFCxxxx   |
   |                | PSADDR             | OPTIONAL        | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | ngsl           |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | nsapml         |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | SSUBIX             | MANDATORY       | RFCxxxx   |
   | ----           | ----               | ----            | ----      |
   | nfpkt          |                    | OPTIONAL        | RFCxxxx   |
   |                | NATINST            | OPTIONAL        | RFCxxxx   |
   |                | PSRLM              | MANDATORY       | RFCxxxx   |
   |                | PATYP              | MANDATORY       | RFCxxxx   |
   |                | PSADDR             | MANDATORY       | RFCxxxx   |
   |                | PDADDR             | MANDATORY       | RFCxxxx   |
   |                | SSUBIX             | OPTIONAL        | RFCxxxx   |
   +----------------+--------------------+-----------------+-----------+

            Table 17: NAT-Related STRUCTURED-DATA Registrations

9.  References

9.1.  Normative References

   [I-D.behave-NAT-MIB]
              Perreault, S., Tsou, T., and S. Sivakumar, "Additional
              Managed Objects for Network Address Translators (NAT)
              (Work in progress)", September 2013.



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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations", RFC
              2663, August 1999.

   [RFC2685]  Fox, B. and B. Gleeson, "Virtual Private Networks
              Identifier", RFC 2685, September 1999.

   [RFC2863]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group
              MIB", RFC 2863, June 2000.

   [RFC4265]  Schliesser, B. and T. Nadeau, "Definition of Textual
              Conventions for Virtual Private Network (VPN) Management",
              RFC 4265, November 2005.

   [RFC4363]  Levi, D. and D. Harrington, "Definitions of Managed
              Objects for Bridges with Traffic Classes, Multicast
              Filtering, and Virtual LAN Extensions", RFC 4363, January
              2006.

   [RFC4784]  Carroll, C. and F. Quick, "Verizon Wireless Dynamic Mobile
              IP Key Update for cdma2000(R) Networks", RFC 4784, June
              2007.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

   [RFC5425]  Miao, F., Ma, Y., and J. Salowey, "Transport Layer
              Security (TLS) Transport Mapping for Syslog", RFC 5425,
              March 2009.

   [RFC5848]  Kelsey, J., Callas, J., and A. Clemm, "Signed Syslog
              Messages", RFC 5848, May 2010.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.



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   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

   [US-ASCII]
              American National Standards Institute, , "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

9.2.  Informative References

   [I-D.behave-ipfix-nat-logging]
              Sivakumar, S. and R. Penno, "IPFIX Information Elements
              for logging NAT Events (Work in progress)", August 2013.

   [I-D.pcp-port-set]
              Sun, Q., Boucadair, M., Sivakumar, S., Zhou, C., Tsou, T.,
              and S. Perreault, "Port Control Protocol (PCP) Extension
              for Port Set Allocation (Work in progress)", July 2013.

   [I-D.softwire-lw4over6]
              Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the DS-Lite
              Architecture (Work in progress)", July 2013.

   [I-D.softwire-map]
              Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, "Mapping of Address and Port
              with Encapsulation (MAP) (Work in progress)", August 2013.

   [I-D.tsou-behave-natx4-log-reduction]
              Tsou, T., Li, W., and T. Taylor, "Port Management To
              Reduce Logging In Large-Scale NATs (Work in progress)",
              July 2013.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001.

   [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
              Private Network (VPN) Terminology", RFC 4026, March 2005.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.






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   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification", RFC
              5969, August 2010.

   [RFC6264]  Jiang, S., Guo, D., and B. Carpenter, "An Incremental
              Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264,
              June 2011.

   [RFC6674]  Brockners, F., Gundavelli, S., Speicher, S., and D. Ward,
              "Gateway-Initiated Dual-Stack Lite Deployment", RFC 6674,
              July 2012.

   [RFC6887]  Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
              Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
              2013.

   [RFC6888]  Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common Requirements for Carrier-Grade NATs
              (CGNs)", BCP 127, RFC 6888, April 2013.

   [RFC7011]  Claise, B., Trammell, B., and P. Aitken, "Specification of
              the IP Flow Information Export (IPFIX) Protocol for the
              Exchange of Flow Information", STD 77, RFC 7011, September
              2013.

   [RFC7040]  Cui, Y., Wu, J., Wu, P., Vautrin, O., and Y. Lee, "Public
              IPv4-over-IPv6 Access Network", RFC 7040, November 2013.

Authors' Addresses

   Zhonghua Chen
   China Telecom
   P.R. China

   Email: 18918588897@189.cn


   Cathy Zhou
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: cathy.zhou@huawei.com







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   Tina Tsou
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: tina.tsou.zouting@huawei.com


   T. Taylor (editor)
   Huawei Technologies
   Ottawa
   Canada

   Email: tom.taylor.stds@gmail.com




































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