





Internet Engineering Task Force (IETF)                   V. Gurbani, Ed.
Request for Comments: 6872             Bell Laboratories, Alcatel-Lucent
Category: Standards Track                                 E. Burger, Ed.
ISSN: 2070-1721                                    Georgetown University
                                                               T. Anjali
                                        Illinois Institute of Technology
                                                             H. Abdelnur
                                                               O. Festor
                                                                   INRIA
                                                           February 2013


 The Common Log Format (CLF) for the Session Initiation Protocol (SIP):
                    Framework and Information Model

Abstract

   Well-known web servers such as Apache and web proxies like Squid
   support event logging using a common log format.  The logs produced
   using these de facto standard formats are invaluable to system
   administrators for troubleshooting a server and tool writers to craft
   tools that mine the log files and produce reports and trends.
   Furthermore, these log files can also be used to train anomaly
   detection systems and feed events into a security event management
   system.  The Session Initiation Protocol (SIP) does not have a common
   log format, and, as a result, each server supports a distinct log
   format that makes it unnecessarily complex to produce tools to do
   trend analysis and security detection.  This document describes a
   framework, including requirements and analysis of existing
   approaches, and specifies an information model for development of a
   SIP common log file format that can be used uniformly by user agents,
   proxies, registrars, and redirect servers as well as back-to-back
   user agents.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6872.




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Copyright Notice

   Copyright (c) 2013 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 ....................................................3
   2. Terminology .....................................................4
   3. Problem Statement ...............................................4
   4. What SIP CLF Is and What It Is Not ..............................5
   5. Alternative Approaches to SIP CLF ...............................5
      5.1. SIP CLF and Call Detail Records ............................6
      5.2. SIP CLF and Packet Capture Tools ...........................6
      5.3. SIP CLF and Syslog .........................................7
      5.4. SIP CLF and IPFIX ..........................................8
   6. Motivation and Use Cases ........................................8
   7. Challenges in Establishing a SIP CLF ...........................10
   8. Information Model ..............................................11
      8.1. SIP CLF Mandatory Fields ..................................11
      8.2. Mandatory Fields and SIP Entities .........................13
   9. Examples .......................................................14
      9.1. UAC Registration ..........................................15
      9.2. Direct Call between Alice and Bob .........................17
      9.3. Single Downstream Branch Call .............................20
      9.4. Forked Call ...............................................25
   10. Security Considerations .......................................35
   11. Operational Guidance ..........................................37
   12. Acknowledgments ...............................................37
   13. References ....................................................37
      13.1. Normative References .....................................37
      13.2. Informative References ...................................38









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1.  Introduction

   Servers executing on Internet hosts produce log records as part of
   their normal operations.  Some log records are, in essence, a summary
   of an application-layer protocol data unit (PDU) that captures, in
   precise terms, an event that was processed by the server.  These log
   records serve many purposes including analysis and troubleshooting.

   Well-known web servers such as Apache and web proxies like Squid
   support event logging using a Common Log Format (CLF), the common
   structure for logging requests and responses serviced by the web
   server.  It can be argued that a good part of the success of Apache
   has been its CLF because it allowed third parties to produce tools
   that analyzed the data and generated traffic reports and trends.  The
   Apache CLF has been so successful that not only did it become the de
   facto standard in producing logging data for web servers but also
   many commercial web servers can be configured to produce logs in this
   format.  An example of the Apache CLF is depicted next:

             %h      %l     %u       %t   \"%r\"   %s    %b
        remotehost rfc931 authuser [date] request status bytes

   remotehost:  Remote hostname (or IP number if DNS hostname is not
                available or if DNSLookup is Off.

   rfc931:      The remote logname of the user.

   authuser:    The username by which the user has authenticated
                himself.

   [date]:      Date and time of the request.

   request:     The request line exactly as it came from the client.

   status:      The HTTP status code returned to the client.

   bytes:       The content-length of the document transferred.

   The inspiration for the SIP CLF is the Apache CLF.  However, the
   state machinery for an HTTP transaction is much simpler than that of
   the SIP transaction (as evidenced in Section 7).  The SIP CLF needs
   to do considerably more.

   This document outlines the problem statement that argues for a SIP
   CLF.  In addition, it provides an information model pertaining to the
   minimum set of SIP headers and fields that must be logged.  This
   document does not prescribe a specific representation format for the




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   SIP CLF record and, instead, allows other documents to define a
   representation format.  [RFC6873] is an example of a representation
   format that provides a UTF-8-based logging scheme.

2.  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 RFC 2119 [RFC2119].

   RFC 3261 [RFC3261] defines additional terms used in this document
   that are specific to the SIP domain such as "proxy"; "registrar";
   "redirect server"; "user agent server" or "UAS"; "user agent client"
   or "UAC"; "back-to-back user agent" or "B2BUA"; "dialog";
   "transaction"; "server transaction".

   This document uses the term "SIP server" that is defined to include
   the following SIP entities: user agent server, registrar, redirect
   server, a SIP proxy in the role of user agent server, and a B2BUA in
   the role of a user agent server.

3.  Problem Statement

   The Session Initiation Protocol (SIP) [RFC3261] is an Internet
   multimedia session signaling protocol.  A typical deployment of SIP
   in an enterprise will consist of SIP entities from multiple vendors.
   Each SIP entity produces logs using a proprietary format.  The result
   of multiplicity of the log file formats is the inability of the
   support staff to easily trace a call from one entity to another or
   even to craft common tools that will perform trend analysis,
   debugging and troubleshooting problems uniformly across the SIP
   entities from multiple vendors.

   Furthermore, the log file must be easily accessible by command-line
   tools for simple text processing.  This allows ad hoc queries against
   the elements in the log file to retrieve a log record.  Furthermore,
   the log file must be in a format that allows for rapid searches of a
   particular log record (or records).  Because of the large number of
   records expected in the log file, the records must be in a format
   that allows for rapid scanning and ease of skipping records that do
   not match a search criterion.  Finally, the generation of the log
   file must not impose undue burden on the SIP implementation in the
   form of additional libraries that may not be uniformly available on
   different platforms and operating environments where a SIP entity
   generating a log file record may be found.






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   SIP does not currently have a common log format, and this document
   serves to provide the rationale to establish a SIP CLF and identifies
   the required minimal information that must appear in any SIP CLF
   record.

4.  What SIP CLF Is and What It Is Not

   The SIP CLF is a standardized manner of producing a log file.  This
   format can be used by SIP clients, SIP servers, proxies, and B2BUAs.
   The SIP CLF is simply an easily digestible log of currently occurring
   events and past transactions.  It contains enough information to
   allow humans and automata to derive relationships between discrete
   transactions handled at a SIP entity or to search for a certain
   dialog or a related set of transactions.

   The SIP CLF is amenable to quick parsing (i.e., well-delimited
   fields), and it is platform and operating system neutral.

   Due to the structure imposed by delimited fields, the SIP CLF is
   amenable to easy parsing and lends itself well to creating other
   innovative tools such as logfile parsers and trend analytic engines.

   The SIP CLF is not a billing tool.  It is not expected that
   enterprises will bill customers based on SIP CLF.  The SIP CLF
   records events at the signaling layer only and does not attempt to
   correlate the veracity of these events with the media layer.  Thus,
   it cannot be used to trigger customer billing.

   The SIP CLF is not a quality of service (QoS) measurement tool.  If
   QoS is defined as measuring the mean opinion score (MOS) of the
   received media, then SIP CLF does not aid in this task since it does
   not summarize events at the media layer.

   Finally, the SIP CLF is not a tool for supporting lawful intercept.

5.  Alternative Approaches to SIP CLF

   The sipclf working group discussed four alternative approaches to
   determine whether they fill the requirements of what is desired of a
   SIP CLF outlined in Section 3.  We conclude that while every scheme
   discussed below comes with its advantages, its disadvantages may
   preclude it from being used as a SIP CLF.  However, we stress that
   the information model contained in this document can be used to
   develop alternative representation formats when desired.  Currently,
   [RFC6873] is an example of a representation format that provides a
   UTF-8-based logging scheme that meets all the requirements of Section
   3.




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5.1.  SIP CLF and Call Detail Records

   Call Detail Records (CDRs) are used in operator networks widely and
   with the adoption of SIP, standardization bodies such as the Third
   Generation Partnership Project (3GPP) have subsequently defined SIP-
   related CDRs as well.  Today, CDRs are used to implement the
   functionality approximated by SIP CLF; however, there are important
   differences.

   First, SIP CLF operates natively at the transaction layer and
   maintains enough information in the information elements being logged
   that dialog-related data can be subsequently derived from the
   transaction logs.  Thus, esoteric SIP fields and parameters like the
   To header (including tags), the From header (including tags), the
   Command Sequence (CSeq) number, etc., are logged in SIP CLF.  By
   contrast, a CDR is used mostly for charging and thus saves
   information to facilitate that very aspect.  A CDR will most
   certainly log the public user identification of a party requesting a
   service (which may not correspond to the From header) and the public
   user identification of the party called party (which may not
   correspond to the To header).  Furthermore, the sequence numbers
   maintained by the CDR may not correspond to the SIP CSeq header.
   Thus, it will be hard to piece together the state of a dialog through
   a sequence of CDR records.

   Second, a CDR record will, in all probability, be generated at a SIP
   entity performing some form of proxy-like functionality of a B2BUA
   providing some service.  By contrast, SIP CLF is lightweight enough
   that it can be generated by a canonical SIP user agent server and
   user agent client as well, including those that execute on resource
   constrained devices (mobile phones).

   Finally, SIP is also being deployed outside of operator-managed Voice
   over IP (VoIP) networks.  Universities, research laboratories, and
   small-to medium-sized companies are deploying SIP-based VoIP
   solutions on networks owned and managed by them.  Many of the latter
   constituencies will not have an interest in generating CDRs, but they
   will like to have a concise representation of the messages being
   handled by the SIP entities in a common format.

5.2.  SIP CLF and Packet Capture Tools

   Wireshark and tcpdump are popular raw packet capture tools.
   Wireshark even contains filters that can understand SIP at the
   protocol level and break down a captured message into its individual
   header components.  While packet capture tools are appropriate to
   capture and view discrete SIP messages, they do not suffice to serve
   in the same capacity as SIP CLF for the following reasons:



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   o  Using packet capturing tools will not eliminate the need for
      agreeing to a common set of fields to represent a SIP CLF record.
      This common understanding is important for interoperability to
      allow one implementation to read a log file written by a different
      implementation.

   o  The packet capture from these tools is not easily searchable by
      simple command-line tools for text processing.

   o  Using packet capture tools requires that the underlying libraries
      related to packet capture be available for all platforms on which
      a SIP server or a SIP client can execute.  Given the different
      platforms on which a SIP client or server runs --- mobile, fixed
      host, tablet, etc. --- this may become an inhibiting factor when
      compared to the SIP client or server producing a SIP CLF record
      natively (the SIP client or server has already parsed the SIP
      message for operation on it; therefore, it seems reasonable to
      have it write the parsed tokens out to persistent store in an
      agreed upon format).

   o  If SIP messages are exchanged over a secure transport (TLS)
      packet, capture tools will be unable to decrypt them and render
      them as individual SIP headers.

   o  Using such tools and related packet capture libraries may imposes
      a dependency on a third-party library.

5.3.  SIP CLF and Syslog

   The syslog protocol [RFC5424] conveys event notification messages
   from an originator to a collector.  While the syslog protocol
   provides a packet format and transport mechanism, it does not
   describe any storage format for syslog messages.  Pragmatically,
   while the syslog protocol itself does not describe a storage format,
   the collector will write the arriving messages into a disk file.  A
   new problem arises due to the general nature of syslog: the disk file
   will contain log messages from many originators, not just SIP
   entities.  This imposes an additional burden of discarding all
   extraneous records when analyzing the disk file for SIP CLF records
   of interest.  SIP CLF records are best stored in a log file that is
   easily searchable by command-line tools.

   Other drawbacks of using syslog include the unavailability of the
   collector under certain scenarios (a mobile SIP phone may be unable
   to find a collector to which it should send the messages), and the
   need to have syslog-specific libraries available for each platform on
   which the SIP server or the SIP client can execute.  Finally, because
   of the frequency and size of SIP log messages, it is not desirable to



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   send every SIP CLF log message to the collector.  Instead, a
   judicious use of syslog could be that only certain events -- those
   that are pertinent from a network situational awareness perspective,
   or those that include a periodic statistical summary of the messages
   processed -- are sent to the collector.

5.4.  SIP CLF and IPFIX

   The IP Flow Information Export (IPFIX) protocol [RFC5101] allows
   network administrators to aggregate IP packets characterized by some
   commonality (similar packet header fields, one or more
   characteristics of the packet itself) into a flow that can be
   subsequently collected and sent to other elements for analysis and
   monitoring.  However, IPFIX is not a logging format and does not
   produce a log file that can be examined by ad hoc text processing
   tools.

6.  Motivation and Use Cases

   As SIP becomes pervasive in multiple business domains and ubiquitous
   in academic and research environments, it is beneficial to establish
   a CLF for the following reasons:

   Common reference for interpreting events:  In a laboratory
      environment or an enterprise service offering, there will
      typically be SIP entities from multiple vendors participating in
      routing requests.  Absent a common log format, each entity will
      produce output records in a native format, making it hard to
      establish commonality for tools that operate on the log file.

   Writing common tools:  A common log format allows independent tool
      providers to craft tools and applications that interpret the CLF
      data to produce insightful trend analysis and detailed traffic
      reports.  The format should be such that it retains the ability to
      be read by humans and processed using traditional Unix text
      processing tools.

   Session correlation across diverse processing elements:  In
      operational SIP networks, a request will typically be processed by
      more than one SIP server.  A SIP CLF will allow the network
      operator to trace the progression of the request (or a set of
      requests) as they traverse through the different servers to
      establish a concise diagnostic trail of a SIP session.

            Note that tracing the request through a set of servers is
            considerably less challenging if all the servers belong to
            the same administrative domain.




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   Message correlation across transactions:  A SIP CLF can enable a
      quick lookup of all messages that comprise a transaction (e.g.,
      "Find all messages corresponding to server transaction X,
      including all forked branches.").

   Message correlation across dialogs:  A SIP CLF can correlate
      transactions that comprise a dialog (e.g., "Find all messages for
      dialog created by Call-ID C, From tag F and To tag T.").

   Trend analysis:  A SIP CLF allows an administrator to collect data
      and spot patterns or trends in the information (e.g., "What is the
      domain where the most sessions are routed to between 9:00 AM and
      1:00 PM?").

   Train anomaly detection systems:  A SIP CLF will allow for the
      training of anomaly detection systems that once trained can
      monitor the CLF file to trigger an alarm on the subsequent
      deviations from accepted patterns in the data set.  Currently,
      anomaly detection systems monitor the network and parse raw
      packets that comprise a SIP message -- a process that is
      unsuitable for anomaly detection systems [rieck2008].  With all
      the necessary event data at their disposal, network operations
      managers and information technology operation managers are in a
      much better position to correlate, aggregate, and prioritize log
      data to maintain situational awareness.

   Testing:  A SIP CLF allows for automatic testing of SIP equipment by
      writing tools that can parse a SIP CLF file to ensure behavior of
      a device under test.

   Troubleshooting:  A SIP CLF can enable cursory troubleshooting of a
      SIP entity (e.g., "How long did it take to generate a final
      response for the INVITE associated with Call-ID X?").

   Offline analysis:  A SIP CLF allows for offline analysis of the data
      gathered.  Once a SIP CLF file has been generated, it can be
      transported (subject to the security considerations in Section 10)
      to a host with appropriate computing resources to perform
      subsequent analysis.

   Real-time monitoring:  A SIP CLF allows administrators to visually
      notice the events occurring at a SIP entity in real-time providing
      accurate situational awareness.








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7.  Challenges in Establishing a SIP CLF

   Establishing a CLF for SIP is a challenging task.  The behavior of a
   SIP entity is more complex when compared to the equivalent HTTP
   entity.

   Base protocol services such as parallel or serial forking elicit
   multiple final responses.  Ensuing delays between sending a request
   and receiving a final response all add complexity when considering
   what fields should comprise a CLF and in what manner.  Furthermore,
   unlike HTTP, SIP groups multiple discrete transactions into a dialog,
   and these transactions may arrive at a varying inter-arrival rate at
   a proxy.  For example, the BYE transaction usually arrives much after
   the corresponding INVITE transaction was received, serviced, and
   expunged from the transaction list.  Nonetheless, it is advantageous
   to relate these transactions such that automata or a human monitoring
   the log file can construct a set consisting of related transactions.

   ACK requests in SIP need careful consideration as well.  In SIP, an
   ACK is a special method that is associated with an INVITE only.  It
   does not require a response; furthermore, if it is acknowledging a
   non-2xx response, then the ACK is considered part of the original
   INVITE transaction.  If it is acknowledging a 2xx-class response,
   then the ACK is a separate transaction consisting of a request only
   (i.e., there is not a response for an ACK request).  CANCEL is
   another method that is tied to an INVITE transaction, but unlike ACK,
   the CANCEL request elicits a final response.

   While most requests elicit a response immediately, the INVITE request
   in SIP can remain in a pending state at a proxy as it forks branches
   downstream or at a user agent server while it alerts the user.
   [RFC3261] instructs the server transaction to send a 1xx-class
   provisional response if a final response is delayed for more than 200
   ms.  A SIP CLF log file needs to include such provisional responses
   because they help train automata associated with anomaly detection
   systems and provide some positive feedback for a human observer
   monitoring the log file.

   Finally, beyond supporting native SIP actors such as proxies,
   registrars, redirect servers, and user agent servers (UASs), it is
   beneficial to derive a common log format that supports B2BUA
   behavior, which may vary considerably depending on the specific
   nature of the B2BUA.








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8.  Information Model

   This document defines the mandatory fields that MUST occur in a SIP
   CLF record.  The maximum size (in number of bytes) for a SIP CLF
   field is 4096 bytes.  This limit is the same regardless of whether
   the SIP CLF field is a meta-field (see "Timestamp" and
   "Directionality" defined below) or a normal SIP header.  If the body
   of the SIP message is to be logged, it MUST conform to this limit as
   well.

   SIP bodies may contain characters that do not form a valid UTF-8
   sequence.  As such, the logging of bodies requires understanding
   trade-offs with respect to a specific logging format to determine if
   the body can be logged as is or some encoding will be required.  The
   specific syntax and semantics used to log SIP bodies MUST be defined
   by the specific representation format document used to generate the
   SIP CLF record.

   The information model supports extensibility by providing the
   capability to log "optional fields".  Optional fields are those SIP
   header fields (or field components) that are not mandatory (see
   Section 8.1 for the mandatory field list).  Optional fields may
   contain SIP headers or other elements present in a SIP message (for
   example, the Reason-Phrase element from the Status-Line production
   rule in RFC 3261 [RFC3261]).  Optional fields may also contain
   additional information that a particular vendor desires to log.  The
   specific syntax and semantics to be accorded to optional fields MUST
   be defined by the specific representation format used to generate the
   SIP CLF record.

8.1.  SIP CLF Mandatory Fields

   The following SIP CLF fields are defined as the minimal information
   that MUST appear in any SIP CLF record:

   Timestamp:  Date and time of the request or response represented as
      the number of seconds and milliseconds since the Unix epoch.

   Message type:  An indicator of whether the SIP message is a request
      or a response.  The allowable values for this field are 'R' (for
      Request) and 'r' (for response).

   Directionality:  An indicator of whether the SIP message is received
      by the SIP entity or sent by the SIP entity.  The allowable values
      for this field are 's' (for message sent) and 'r' (for message
      received).





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   Transport:  The transport over which a SIP message is sent or
      received.  The allowable values for the transport are governed by
      the "transport" production rule in Section 25.1 of RFC 3261
      [RFC3261].

   Source-address:  The IPv4 or IPv6 address of the sender of the SIP
      message.

   Source-port:  The source port number of the sender of the SIP
      message.

   Destination-address:  The IPv4 or IPv6 address of the recipient of
      the SIP message.

   Destination-port:  The port number of the recipient of the SIP
      message.

   From:  The From URI.  For the sake of brevity, URI parameters should
      not be logged.

   From tag:  The tag parameter of the From header.

   To:  The To URI.  For the sake of brevity, URI parameters should not
      be logged.

   To tag:  The tag parameter of the To header.  Note that the tag
      parameter will be absent in the initial request that forms a
      dialog.

   Callid:  The Call-ID.

   CSeq-Method:  The method from the CSeq header.

   CSeq-Number:  The number from the CSeq header.

   R-URI:  The Request-URI, including any URI parameters.

   Status:  The SIP response status code.

   SIP proxies may fork, creating several client transactions that
   correlate to a single server transaction.  Responses arriving on
   these client transactions or new requests (CANCEL, ACK) sent on the
   client transaction need log file entries that correlate with a server
   transaction.  Similarly, a B2BUA may create one or more client
   transactions in response to an incoming request.  These transactions
   will require correlation as well.  The last two information model
   elements provide this correlation.




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   Server-Txn:  Server transaction identification code - the transaction
      identifier associated with the server transaction.
      Implementations can reuse the server transaction identifier (the
      topmost branch-id of the incoming request, with or without the
      magic cookie), or they could generate a unique identification
      string for a server transaction (this identifier needs to be
      locally unique to the server only).  This identifier is used to
      correlate ACKs and CANCELs to an INVITE transaction; it is also
      used to aid in forking as explained later in this section.  (See
      Section 9 for usage.)

   Client-Txn:  Client transaction identification code - this field is
      used to associate client transactions with a server transaction
      for forking proxies or B2BUAs.  Upon forking, implementations can
      reuse the value they inserted into the topmost Via header's branch
      parameter, or they can generate a unique identification string for
      the client transaction.  (See Section 9 for usage.)

   This information model applies to all SIP entities --- a UAC, UAS,
   proxy, B2BUA, registrar, and redirect server.  The SIP CLF fields
   prescribed for a proxy are equally applicable to the B2BUA.
   Similarly, the SIP CLF fields prescribed for a UAS are equally
   applicable to registrars and redirect servers.

   The next section specifies the individual SIP CLF information model
   elements that form a log record for specific instances of a SIP
   entity.  It is understood that a SIP CLF record is extensible using
   extension mechanisms appropriate to the specific representation used
   to generate the SIP CLF record.  This document, however, does not
   prescribe a specific representation format, and it limits the
   discussion to the mandatory data elements described above.

8.2.  Mandatory Fields and SIP Entities

   Each SIP CLF record must contain all the mandatory information model
   elements outlined in Section 8.1.  This document does not specify a
   representation of a logging format; it is expected that other
   documents will do so.

   An element may not always have an appropriate value to provide for
   one of these fields, for example, the R-URI field is not applicable
   when logging a response, the Status field is not applicable when
   logging a request, the To tag is not known when a request is first
   sent out, etc.  As all the mandatory fields are required to appear in
   the SIP CLF record, the representation document MUST define how to
   indicate a field that is not applicable in the context that the SIP





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   CLF record was generated.  Similarly, to handle parsing errors in a
   field, the representation document MUST define a means to indicate
   that a field cannot be parsed.

   The Client-Txn field is always applicable to a UAC.  The Server-Txn
   field does not apply to a UAC unless the element is also acting as a
   UAS, and the message associated to this log record corresponds to a
   message handled by that UAS.  For instance, a proxy forwarding a
   request will populate both the Client-Txn and Server-Txn fields in
   the record corresponding to the forwarded request.

   The Server-Txn field is always applicable to a UAS.  The Client-Txn
   field does not apply to a UAS unless the element is also acting as a
   UAC, and the message associated to this log record corresponds to a
   message handled by that UAC.  For instance, a proxy forwarding a
   response will populate both the Server-Txn and Client-Txn fields in
   the record corresponding to the forwarded response.  However, a proxy
   would only populate the Client-Txn field when creating a log record
   corresponding to a request.

9.  Examples

   The examples use only the mandatory data elements defined in Section
   8.1.  Extension elements are not considered and neither are SIP
   bodies.  When a given mandatory field is not applicable to a SIP
   entity, we use the horizontal dash ("-") to represent it.

   There are five principals in the examples below.  They are the
   following: Alice, the initiator of requests.  Alice's user agent uses
   IPv4 address 198.51.100.1, port 5060.  P1 is a proxy that Alice's
   request traverse on their way to Bob, the recipient of the requests.
   P1 also acts as a registrar to Alice.  P1 uses an IPv4 address of
   198.51.100.10, port 5060.  Bob has two instances of his user agent
   running on different hosts.  The first instance uses an IPv4 address
   of 203.0.113.1, port 5060 and the second instance uses an IPv6
   address of 2001:db8::9, port 5060.  P2 is a proxy responsible for
   Bob's domain.  Table 1 summarizes these addresses.














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        +-------------------+--------------------+-------------------+
        | Principal         | IP:port            | Host/Domain name  |
        +-------------------+--------------------+-------------------+
        | Alice             | 198.51.100.1:5060  | alice.example.com |
        | P1                | 198.51.100.10:5060 | p1.example.com    |
        | P2                | 203.0.113.200:5060 | p2.example.net    |
        | Bob UA instance 1 | 203.0.113.1:5060   | bob1.example.net  |
        | Bob UA instance 2 | [2001:db8::9]:5060 | bob2.example.net  |
        +-------------------+--------------------+-------------------+

                    Principal to IP Address Assignment

                                  Table 1

   Illustrative examples of SIP CLF follow.

9.1.  UAC Registration

   Alice sends a registration registrar P1 and receives a 2xx-class
   response.  The register requests causes Alice's UAC to produce a log
   record shown below.

        Timestamp: 1275930743.699
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 1
        CSeq-Method: REGISTER
        R-URI: sip:example.com
        Destination-address: 198.51.100.10
        Destination-port: 5060
        Source-address: 198.51.100.1
        Source-port: 5060
        To: sip:example.com
        To tag: -
        From: sip:alice@example.com
        From tag: 76yhh
        Call-ID: f81-d4-f6@example.com
        Status: -
        Server-Txn: -
        Client-Txn: c-tr-1










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   After some time, Alice's UAC will receive a response from the
   registrar.  The response causes Alice's agent to produce a log record
   shown below.

        Timestamp: 1275930744.100
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 1
        CSeq-Method: REGISTER
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 198.51.100.10
        Source-port: 5060
        To: sip:example.com
        To tag: reg-1-xtr
        From: sip:alice@example.com
        From tag: 76yhh
        Call-ID: f81-d4-f6@example.com
        Status: 100
        Server-Txn: -
        Client-Txn: c-tr-1




























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9.2.  Direct Call between Alice and Bob

   In this example, Alice sends a session initiation request directly to
   Bob's agent (instance 1).  Bob's agent accepts the session
   invitation.  We first present the SIP CLF logging from the vantage
   point of Alice's UAC.  In line 1, Alice's user agent sends out the
   INVITE.  Shortly, it receives a "180 Ringing" (line 2), followed by a
   "200 OK" response (line 3).  Upon the receipt of the 2xx-class
   response, Alice's user agent sends out an ACK request (line 4).

        Timestamp: 1275930743.699
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 32
        CSeq-Method: INVITE
        R-URI: sip:bob@bob1.example.net
        Destination-address: 203.0.113.1
        Destination-port: 5060
        Source-address: 198.51.100.1
        Source-port: 5060
        To: sip:bob@bob1.example.net
        To tag: -
        From: sip:alice@example.com
        From tag: 76yhh
        Call-ID: f82-d4-f7@example.com
        Status: -
        Server-Txn: -
        Client-Txn: c-1-xt6






















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        Timestamp: 1275930745.002
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 32
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b-in6-iu
        From: sip:alice@example.com
        From tag: 76yhh
        Call-ID: f82-d4-f7@example.com
        Status: 180
        Server-Txn: -
        Client-Txn: c-1-xt6

        Timestamp: 1275930746.100
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 32
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b-in6-iu
        From: sip:alice@example.com
        From tag: 76yhh
        Call-ID: f82-d4-f7@example.com
        Status: 200
        Server-Txn: -
        Client-Txn: c-1-xt6












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        Timestamp: 1275930746.120
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 32
        CSeq-Method: ACK
        R-URI: sip:bob@bob1.example.net
        Destination-address: 203.0.113.1
        Destination-port: 5060
        Source-address: 198.51.100.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b-in6-iu
        From: sip:alice@example.com
        From tag: 76yhh
        Call-ID: f82-d4-f7@example.com
        Status: -
        Server-Txn: -
        Client-Txn: c-1-xt6
































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9.3.  Single Downstream Branch Call

   In this example, Alice sends a session invitation request to Bob
   through proxy P1, which inserts a Record-Route header causing
   subsequent requests between Alice and Bob to traverse the proxy.  The
   SIP CLF log records appears from the vantage point of P1.  The line
   numbers below refer to Figure 1.

        Alice             P1             Bob
         +---INV--------->|               |  Line 1
         |                |               |
         |<---------100---+               |  Line 2
         |                |               |
         |                +---INV-------->|  Line 3
         |                |               |
         |                |<--------100---+  Line 4
         |                |               |
         |                |<--------180---+  Line 5
         |                |               |
         |<---------180---+               |  Line 6
         |                |               |
         |                |<--------200---+  Line 7
         |                |               |
         |<---------200---+               |  Line 8
         |                |               |
         +---ACK--------->|               |  Line 9
         |                |               |
         |                |---ACK-------->|  Line 10

                  Figure 1: Simple Proxy-Aided Call Flow





















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   1    Timestamp: 1275930743.699
        Message Type: R
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: sip:bob@example.net
        Destination-address: 198.51.100.10
        Destination-port: 5060
        Source-address: 198.51.100.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: -
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: -
        Server-Txn: s-x-tr
        Client-Txn: -

   Note that, at this point, P1 has created a server transaction
   identification code and populated the SIP CLF field Server-Txn with
   it.  P1 has not yet created a client transaction identification code;
   thus, Client-Txn contains a "-".

   2    Timestamp: 1275930744.001
        Message Type: r
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 198.51.100.10
        Source-port: 5060
        To: sip:bob@example.net
        To tag: -
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: 100
        Server-Txn: s-x-tr
        Client-Txn: -







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   In line 3 below, P1 has created a client transaction identification
   code for the downstream branch and populated the SIP CLF field
   Client-Txn.

   3    Timestamp: 1275930744.998
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: sip:bob@bob1.example.net
        Destination-address: 203.0.113.1
        Destination-port: 5060
        Source-address: 198.51.100.10
        Source-port: 5060
        To: sip:bob@example.net
        To tag: -
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: -
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr

   4    Timestamp: 1275930745.200
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.10
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: 100
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr








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   5    Timestamp: 1275930745.800
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.10
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: 180
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr

   6    Timestamp: 1275930746.009
        Message Type: r
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 198.51.100.10
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: 180
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr












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   7    Timestamp: 1275930747.120
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.10
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: 200
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr

   8    Timestamp: 1275930747.300
        Message Type: r
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 198.51.100.10
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: 200
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr












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   9    Timestamp: 1275930749.100
        Message Type: R
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: ACK
        R-URI: sip:bob@example.net
        Destination-address: 198.51.100.10
        Destination-port: 5060
        Source-address: 198.51.100.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: -
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr

   10   Timestamp: 1275930749.100
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: ACK
        R-URI: sip:bob@bob1.example.net
        Destination-address: 203.0.113.1
        Destination-port: 5060
        Source-address: 198.51.100.10
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: al-1
        Call-ID: tr-87h@example.com
        Status: -
        Server-Txn: s-x-tr
        Client-Txn: c-x-tr

9.4.  Forked Call

   In this example, Alice sends a session invitation to Bob's proxy, P2.
   P2 forks the session invitation request to two registered endpoints
   corresponding to Bob's address-of-record.  Both endpoints respond
   with provisional responses.  Shortly thereafter, one of Bob's user
   agent instances accepts the call, causing P2 to send a CANCEL request
   to the second user agent.  P2 does not Record-Route; therefore, the



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   subsequent ACK request from Alice to Bob's user agent does not
   traverse through P2 (and is not shown below).

   Figure 2 depicts the call flow.
                           Bob            Bob
        Alice      P2   (Instance 1) (Instance 2)
         +---INV--->|          |         |  Line 1
         |          |          |         |
         |<---100---+          |         |  Line 2
         |          |          |         |
         |          +---INV--->|         |  Line 3
         |          |          |         |
         |          +---INV----+-------->|  Line 4
         |          |          |         |
         |          |<---100---+         |  Line 5
         |          |          |         |
         |          |<---------+---100---+  Line 6
         |          |          |         |
         |          |<---180---+---------+  Line 7
         |          |          |         |
         |<---180---+          |         |  Line 8
         |          |          |         |
         |          |<---180---+         |  Line 9
         |          |          |         |
         |<---180---+          |         |  Line 10
         |          |          |         |
         |          |<---200---+         |  Line 11
         |          |          |         |
         |<---200---+          |         |  Line 12
         |          |          |         |
         |          +---CANCEL-+-------->|  Line 13
         |          |          |         |
         |          |<---------+---487---+  Line 14
         |          |          |         |
         |          +---ACK----+-------->|  Line 15
         |          |          |         |
         |          |<---------+---200---+  Line 16

                        Figure 2: Forked Call Flow

   The SIP CLF log appears from the vantage point of P2.  The fields
   logged are shown below; the line numbers refer to Figure 2.









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   1    Timestamp: 1275930743.699
        Message Type: R
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: sip:bob@example.net
        Destination-address: 203.0.113.200
        Destination-port: 5060
        Source-address: 198.51.100.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: -
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: -
        Server-Txn: s-1-tr
        Client-Txn: -

   2    Timestamp: 1275930744.001
        Message Type: r
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: -
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 100
        Server-Txn: s-1-tr
        Client-Txn: -












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   3    Timestamp: 1275930744.998
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: sip:bob@bob1.example.net
        Destination-address: 203.0.113.1
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: -
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: -
        Server-Txn: s-1-tr
        Client-Txn: c-1-tr

   4    Timestamp: 1275930745.500
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: sip:bob@bob2.example.net
        Destination-address: [2001:db8::9]
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: -
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: -
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr












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   5    Timestamp: 1275930745.800
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 203.0.113.200
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1=-1
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com 100
        Status: 100
        Server-Txn: s-1-tr
        Client-Txn: c-1-tr

   6    Timestamp: 1275930746.100
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 203.0.113.200
        Destination-port: udp
        Source-address: [2001:db8::9]
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b2-2
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 100
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr












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   7    Timestamp: 1275930746.700
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 203.0.113.200
        Destination-port: udp
        Source-address: [2001:db8::9]
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b2-2
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 180
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr

   8    Timestamp: 1275930746.990
        Message Type: r
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b2-2
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 180
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr












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   9    Timestamp: 1275930747.100
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 203.0.113.200
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com 100
        Status: 180
        Server-Txn: s-1-tr
        Client-Txn: c-1-tr

   10   Timestamp: 1275930747.300
        Message Type: r
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 180
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr












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   11   Timestamp: 1275930747.800
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 203.0.113.200
        Destination-port: 5060
        Source-address: 203.0.113.1
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com 100
        Status: 200
        Server-Txn: s-1-tr
        Client-Txn: c-1-tr

   12   Timestamp: 1275930748.000
        Message Type: r
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 198.51.100.1
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b1-1
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 200
        Server-Txn: s-1-tr
        Client-Txn: c-1-tr












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   13   Timestamp: 1275930748.201
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: CANCEL
        R-URI: sip:bob@bob2.example.net
        Destination-address: [2001:db8::9]
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b2-2
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: -
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr

   14   Timestamp: 1275930748.300
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: INVITE
        R-URI: -
        Destination-address: 203.0.113.200
        Destination-port: udp
        Source-address: [2001:db8::9]
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b2-2
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 487
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr












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   15   Timestamp: 1275930748.355
        Message Type: R
        Directionality: s
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: ACK
        R-URI: sip:bob@bob2.example.net
        Destination-address: [2001:db8::9]
        Destination-port: 5060
        Source-address: 203.0.113.200
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b2-2
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: -
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr

   16   Timestamp: 1275930748.698
        Message Type: r
        Directionality: r
        Transport: udp
        CSeq-Number: 43
        CSeq-Method: CANCEL
        R-URI: -
        Destination-address: 203.0.113.200
        Destination-port: udp
        Source-address: [2001:db8::9]
        Source-port: 5060
        To: sip:bob@example.net
        To tag: b2-2
        From: sip:alice@example.com
        From tag: a1-1
        Call-ID: tr-88h@example.com
        Status: 200
        Server-Txn: s-1-tr
        Client-Txn: c-2-tr

   The above SIP CLF log makes it easy to search for a specific
   transaction or a state of the session.  Searching for the string
   "c-1-tr" on the log records will readily yield the information that
   an INVITE was sent to sip:bob@bob1.example.com, it elicited a 100
   followed by a 180 and then a 200.  Because the ACK request in this
   case would be exchanged end-to-end, this element does not see (and
   therefore will not log) the ACK.




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   Searching for "c-2-tr" yields a more complex scenario of sending an
   INVITE to sip:bob@bob2.example.net, receiving 100 and 180.  However,
   the log makes it apparent that the request to
   sip:bob@bob2.example.net was subsequently CANCEL'ed before a final
   response was generated, and that the pending INVITE returned a 487.
   The ACK to the final non-2xx response and a 200 to the CANCEL request
   complete the exchange on that branch.

10.  Security Considerations

   A log file by its nature reveals both the state of the entity
   producing it and the nature of the information being logged.  To the
   extent that this state should not be publicly accessible and that the
   information is to be considered private, appropriate file and
   directory permissions attached to the log file SHOULD be used.  It is
   outside the scope of this document to specify how to protect the log
   file while it is stored on disk; however, certain precautions can be
   taken.  Operators SHOULD consider using common administrative
   features such as disk encryption and securing log files [schneier-1].
   Operators SHOULD also consider hardening the machine on which the log
   file is stored by restricting physical access to the host as well as
   restricting access to the file itself.  Depending on the specific
   operating system and environment, the file and directory permissions
   SHOULD be set to be most restrictive such that the file is not
   publicly readable and writable and the directory where the file is
   stored is not publicly accessible.

   The following threats may be considered for the log file while it is
   stored:

   o  An attacker may gain access to view the log file, or may
      surreptitiously make a copy of the log file for later viewing.

   o  An attacker who is unable to eavesdrop on real-time SIP traffic on
      the network, but, nonetheless, can access the log file, is able to
      easily mount replay attack or other attacks that result from
      channel eavesdropping.  Encrypting SIP traffic does not help here
      because the SIP entity generating the log file would have
      decrypted the message for processing and subsequent logging.

   o  An attacker may delete parts of --- or indeed, the whole --- file.

   Public access to the SIP log file creates more of a privacy leak when
   compared to an adversary eavesdropping cleartext SIP traffic on the
   network.  If all SIP traffic on a network segment is encrypted, then
   as noted above, special attention must be directed to the file and
   directory permissions associated with the log file to preserve




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   privacy such that only a privileged user can access the contents of
   the log file.

   Transporting SIP CLF files across the network pose special challenges
   as well.  The following threats may be considered for transferring
   log files or while transferring individual log records:

   o  An attacker may view the records;

   o  An attacker may modify the records in transit or insert previously
      captured records into the stream;

   o  An attacker may remove records in transit, or may stage a man-in-
      the-middle attack to deliver a partially or entirely falsified log
      file.

   It is also outside the scope of this document to specify protection
   methods for log files or log records that are being transferred
   between hosts; however, certain precautions can be taken.  Operators
   SHOULD require mutual authentication, channel confidentiality, and
   channel integrity while transferring the log file.  The use of a
   secure shell transport layer protocol [RFC4253] or TLS [RFC5246]
   accomplishes this.

   Even with such care, sensitive information can be leaked during or
   after the transfer.  SIP CLF fields like IP addresses and URIs
   contain potentially sensitive information.  Before transferring the
   log file across domains, operators SHOULD ensure that any fields that
   contain sensitive information are appropriately anonymized or
   obfuscated.  A specification for a format that describes which fields
   are obfuscated and with what characteristics (e.g., what correlations
   still work) is needed to allow interoperable but privacy-friendly
   exchange of SIP CLF between administrative domains.  Such a
   specification is not attempted here, but is for further study.

   The SIP CLF represents the minimum fields that lend themselves to
   trend analysis and serve as information that may be deemed useful.
   Other formats can be defined that include more headers (and the body)
   from Section 8.1.  However, where to draw a judicial line regarding
   the inclusion of non-mandatory headers can be challenging.  Clearly,
   the more information a SIP entity logs, the longer time the logging
   process will take, the more disk space the log entry will consume,
   and the more potentially sensitive information could be breached.
   Therefore, adequate trade-offs should be taken in account when
   logging more fields than the ones recommended in Section 8.1.






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   Implementers need to pay particular attention to buffer handling when
   reading or writing log files.  SIP CLF entries can be unbounded in
   length.  It would be reasonable for a full dump of a SIP message to
   be thousands of octets long.  This is of particular importance to CLF
   log parsers, as a SIP CLF log writers may add one or more extension
   fields to the message to be logged.

11.  Operational Guidance

   SIP CLF log files will take up a substantial amount of disk space
   depending on traffic volume at a processing entity and the amount of
   information being logged.  As such, any organization using SIP CLF
   should establish operational procedures for file rollovers and
   periodic retrieval of logs before rollover as appropriate to the
   needs of the organization.

   Listing such operational guidelines in this document is out of scope
   for this work.

12.  Acknowledgments

   Members of the sipping, dispatch, ipfix, and syslog working groups
   provided invaluable input to the formulation of the document.  These
   include Benoit Claise, Spencer Dawkins, John Elwell, David
   Harrington, Christer Holmberg, Hadriel Kaplan, Atsushi Kobayashi,
   Jiri Kuthan, Scott Lawrence, Chris Lonvick, Peter Musgrave, Simon
   Perreault, Adam Roach, Dan Romascanu, Robert Sparks, Brian Trammell,
   Dale Worley, Theo Zourzouvillys, and others that we have undoubtedly,
   but inadvertently, missed.

   Rainer Gerhards, David Harrington, Cullen Jennings, and Gonzalo
   Salgueiro helped tremendously in discussions related to arriving at
   the beginnings of an information model.

13.  References

13.1.  Normative References

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

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.






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13.2.  Informative References

   [RFC4253]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, January 2006.

   [RFC5101]  Claise, B., "Specification of the IP Flow Information
              Export (IPFIX) Protocol for the Exchange of IP Traffic
              Flow Information", RFC 5101, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

   [RFC6873]  Salgueiro, G., Gurbani, V., and A. B. Roach, "Format for
              the Session Initiation Protocol (SIP) Common Log Format
              (CLF)", RFC 6873, February 2013.

   [rieck2008]
              Rieck, K., Wahl, S., Laskov, P., Domschitz, P., and K-R.
              Muller, "A Self-learning System for Detection of Anomalous
              SIP Messages", Principles, Systems and Applications of IP
              Telecommunications Services and Security for Next
              Generation Networks (IPTComm), LNCS 5310, pp. 90-106,
              2008.

   [schneier-1]
              Schneier, B. and J. Kelsey, "Secure audit logs to support
              computer forensics", ACM Transactions on Information and
              System Security (TISSEC), 2(2), pp. 159,176, May 1999.





















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Authors' Addresses

   Vijay K. Gurbani (editor)
   Bell Laboratories, Alcatel-Lucent
   1960 Lucent Lane
   Naperville, IL  60566
   USA

   EMail: vkg@bell-labs.com


   Eric W. Burger (editor)
   Georgetown University
   USA

   EMail: eburger@standardstrack.com
   URI:   http://www.standardstrack.com


   Tricha Anjali
   Illinois Institute of Technology
   316 Siegel Hall
   Chicago, IL  60616
   USA

   EMail: tricha@ece.iit.edu


   Humberto Abdelnur
   INRIA
   INRIA - Nancy Grant Est
   Campus Scientifique
   54506, Vandoeuvre-les-Nancy Cedex
   France

   EMail: humbol@gmail.com


   Olivier Festor
   INRIA
   INRIA - Nancy Grant Est
   Campus Scientifique
   54506, Vandoeuvre-les-Nancy Cedex
   France

   EMail: Olivier.Festor@loria.fr





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