The Common Log Format (CLF) for the Session Initiation Protocol (SIP)
DOCNAME

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 trouble-shooting 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 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. We propose a common log file format for SIP servers that can be used uniformly by proxies, registrars, redirect servers as well as back-to-back user agents.

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/.

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This Internet-Draft will expire on December 10, 2010.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1.  Terminology
2.  Introduction
3.  Problem statement
4.  What SIP CLF is and what it is not
5.  Alternative approaches to SIP CLF
    5.1.  SIP CLF and CDRs
    5.2.  SIP CLF and Wireshark packet capture
6.  Motivation and use cases
7.  Challenges in establishing a SIP CLF
8.  Data model
    8.1.  SIP CLF data model elements for an UAC
    8.2.  SIP CLF data model elements for an UAS
    8.3.  SIP CLF data model elements for a proxy
9.  Examples
    9.1.  UAC registering with a proxy
    9.2.  Direct call between Alice and Bob
    9.3.  Single downstream branch call
    9.4.  Forked call
10.  Security Considerations
11.  Operational guidance
12.  IANA Considerations
13.  Acknowledgments
14.  References
    14.1.  Normative References
    14.2.  Informative References
§  Authors' Addresses

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 RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

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

2.  Introduction

Servers executing on Internet hosts produce log records as part of their normal operations. A log record is, 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 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 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 a HTTP transaction is much simpler than that of the SIP transaction (as evidenced in Section 7 (Challenges in establishing a SIP CLF)). The SIP CLF needs to do considerably more.

3.  Problem statement

The Session Initiation Protocol (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261](SIP) is an Internet multimedia session signaling protocol that is increasingly used for other services besides session establishment. A typical deployment of SIP in an enterprise will consist of SIP entities from multiple vendors. Currently, if these entities are capable of producing a log file of the transactions being handled by them, the log files are produced in 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 of multiple vendors.

SIP does not currently have a CLF 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. For example, a SIP administrator should be able to issue a concise command to discover relationships between transactions or to search a certain dialog or transaction.

Note: The exact form of the "concise command" is left unspecified until the working group agrees to one or more formats for encoding the fields.

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

The SIP CLF is amenable to easy parsing and lends itself well to creating other innovative tools.

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.

5.  Alternative approaches to SIP CLF

It is perhaps tempting to consider other approaches --- which though not standardized, are in wide enough use in networks today --- to determine whether or not a SIP CLF would benefit a SIP network consisting of multi-vendor products. The two existing approaches that approximate what SIP CLF does are Call Detail Records (CDRs) and Wireshark packet sniffing.

5.1.  SIP CLF and CDRs

CDRs are used in operator networks widely and with the adoption of SIP, standardization bodies such as 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.

One, 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 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.

Two, 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 light- weight 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 VoIP networks. Universities, research laboratories, and small-to-medium size companies are deploying SIP-based VoIP solutions on networks owned and managed by them. Much 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 Wireshark packet capture

Wireshark is a popular raw packet capture tool. It contains filters that can understand SIP at the protocol level and break down a captured message into its individual header components. While Wireshark is appropriate to capture and view discrete SIP messages, it does not suffice to serve in the same capacity as SIP CLF for two reasons.

First, while the Wireshark format saves bulk of the information needed to create transaction and dialog state, the Wireshark format is a binary format that does not lend itself very well to being manipulated by text-based tools. Second and more importantly, if the SIP messages are exchanged over a TLS-oriented transport, Wireshark will be unable to decrypt them and render them as individual SIP headers.

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 CLF 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 CLF 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.

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 12:00 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] (Rieck, K., Wahl, S., Laskov, P., Domschitz, P., and K-R. Muller, “A Self-learning System for Detection of Anomalous SIP Messages,” 2008.). 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 trouble shooting 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 (Security Considerations)) 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.

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, and 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 pend at a proxy as it forks branches downstream or at a user agent server while it alerts the user. RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [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 (UAS), it is beneficial to derive a CLF format that supports back-to-back user agent (B2BUA) behavior, which may vary considerably depending on the specific nature of the B2BUA.

8.  Data model

The following SIP CLF fields are defined as 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.

Source:port:

The DNS name or IP address of the upstream client, including the port number. The port number must be separated from the DNS name or IP address by a single ':'.

Destination:port:

The DNS name or IP address of the downstream server, including the port number. The port number must be separated from the DNS name or IP address by a single ':'.

From:

The From URI, including the tag. Whilst one may question the value of the From URI in light of RFC4744 (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.) [RFC4474], the From URI, nonetheless, imparts some information. For one, the From tag is important and, in the case of a REGISTER request, the From URI can provide information on whether this was a third-party registration or a first-party one.

To:

The To URI, including tag.

Callid:

The Call-ID.

CSeq:

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 data model elements provide this correlation.

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 (Examples) 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 (Examples) for usage.)

Finally, the SIP CLF should be extensible such that future SIP methods, headers and bodies can be represented as well. Besides the mandatory fields listed above, all other SIP headers will appear as an ordered pairs of header field names and values.

This data model applies to all SIP entities --- a UAC, UAS, Proxy, a B2BUA, registrar and redirect server. Note that a B2BUA is a degenerate case of a proxy and as such the SIP CLF field layout format prescribed for a proxy is equally applicable to the B2BUA. Similarly, registrars and redirect servers are a degenerate case of a UAS, and as such the SIP CLF field layout prescribed for a UAS is equally applicable to registrars and redirect servers.

The following sections specify the individual SIP CLF data model elements that form a log record for specific instance of a SIP entity. We limit our specification to using the minimum data model elements. 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.

8.1.  SIP CLF data model elements for an UAC

When an UAC generates a request, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq R-URI Destination-IP:port Client-Txn
       To From Call-ID

Similarly, when an UAC receives a response, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq Source-IP:port Status Client-Txn To

8.2.  SIP CLF data model elements for an UAS

When an UAS receives a request, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq R-URI Source-IP:port Server-Txn To From
       Call-ID

Similarly, when an UAS generates a response, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq Destination-IP:port Status Server-Txn

8.3.  SIP CLF data model elements for a proxy

When the UAS half of a SIP proxy receives a request, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq R-URI Source:port Server-Txn To From
       Call-ID

Similarly, when a UAS half of a SIP proxy generates a response, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq Destination:port Status Server-Txn Client-Txn
       To

The Client-Txn may be empty (or null) since a downstream branch may not have been created when the response log record is generated. Imagine a proxy receiving an INVITE request and generating a "100 Trying" response. At the time the provisional response is generated, the proxy may not have progressed the INVITE transaction to the point of creating a client transaction or a downstream destination. Thus, it is acceptable for these fields to be empty (or null.)

When an UAC-half of a SIP proxy generates a request, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq  R-URI Destination:port Server-Txn
       Client-Txn To From Call-ID

Similarly, when an UAC-half receives a response, the following data model elements --- in the order specified below --- are used to create a SIP CLF record that is subsequently logged:

       Timestamp CSeq Source:port Status Server-Txn Client-Txn To

9.  Examples

In the examples below, we use the horizontal dash ("-") to denote empty (or null) elements. Similarly, the CSeq header field is represented by Method-Number (e.g., INVITE-32). It is important to note that the syntax for the examples in this section is for illustration purposes only, and is not a specific representation of a logging format. It is expected that one or more documents will outline specific formats for logging.

The examples use only the mandatory data elements defined in Section 8 (Data model). Extension elements are not considered.

There are five principals in the examples below. They are 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.

Principal to IP address asignment

Illustrative examples of SIP CLF follow. These examples use the <allOneLine> tag defined in [RFC4475] (Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J., and H. Schulzrinne, “Session Initiation Protocol (SIP) Torture Test Messages,” May 2006.) to logically denote a single line.

9.1.  UAC registering with a proxy

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. The mandatory data model elements correspond to those listed in Section 8.1 (SIP CLF data model elements for an UAC).

     <allOneLine>
     1275930743.699 REGISTER-1 sip:example.com 198.51.100.10:5060
     ty7u7 sip:example.com sip:alice@example.com;tag=76yhh
     f81-d4-f6@example.com
     </allOneLine>

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. The mandatory data elements correspond to those listed in Section 8.1 (SIP CLF data model elements for an UAC).

     <allOneLine>
     1275930744.100 REGISTER-1 198.51.100.10:5060 200 ty7u7
     sip:example.com;tag=reg-98j
     <allOneLine>

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 Alice's UAC point of view. 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).

     <allOneLine>
     1275930743.699 INVITE-32 sip:bob@bob1.example.net
     203.0.113.1:5060 c-1-xt6 sip:bob@example.net
     sip:alice@example.com;tag=76yhh f82-d4-f7@example.com
     </allOneLine>

     <allOneLine>
     1275930745.002 INVITE-32 203.0.113.1:5060 180 c-1-xt6
     sip:bob@example.net;tag=b-in6-iu
     <allOneLine>

     <allOneLine>
     1275930746.100 INVITE-32 203.0.113.1:5060 200 c-1-xt6
     sip:bob@example.net;tag=b-in6-iu
     <allOneLine>

     <allOneLine>
     1275930746.120 ACK-32 sip:bob@bob1.example.net
     203.0.113.1:5060 c-1-xt6 sip:bob@example.net;tag=b-in6-iu
     sip:alice@example.com;tag=76yhh f82-d4-f7@example.com
     <allOneLine>

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 correspond to the viewpoint of P1. The log records are presented one per logical line and the line numbers refer to Figure 1 (Simple proxy-aided call flow)

     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

     <allOneLine>
1    1275930743.699 INVITE-43 sip:bob@example.net
     198.51.100.1:5060 s-1-tr sip:bob@example.net
     sip:alice@example.com;tag=al-1 tr-87h@example.com
     </allOneLine>

     <allOneLine>
2    1275930744.001 INVITE-43 198.51.100.1:5060 100 s-1-tr -
     sip:bob@example.net
     </allOneLine>

     <allOneLine>
3    1275930744.998 INVITE-43 sip:bob@bob1.example.net
     203.0.113.1:5060 s-1-tr c-1-tr sip:bob@example.net
     sip:alice@example.com;tag=a1-1 tr-87h@example.com
     </allOneLine>

     <allOneLine>
4    1275930745.200 INVITE-43 203.0.113.1:5060 100 s-1-tr c-1-tr
     sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
5    1275930745.800 INVITE-43 203.0.113.1:5060 180 s-1-tr c-1-tr
     sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
6    1275930746.009 INVITE-43 198.51.100.1:5060 180 s-1-tr c-1-tr
     sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
7    1275930747.120 INVITE-43 203.0.113.1:5060 200 s-1-tr c-1-tr
     sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
8    1275930747.300 INVITE-43 198.51.100.1:5060 200 s-1-tr c-1-tr
     sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
9    1275930748.201 ACK-43 sip:bob@bob1.example.net
     198.51.100.1:5060 s-1-tr sip:bob@example.net;tag=b1-1
     sip:alice@example.com;tag=al-1 tr-87h@example.com
     </allOneLine>

     <allOneLine>
10   1275930749.100 ACK-43 sip:bob@bob1.example.net
     203.0.113.1:5060 s-1-tr c-1-tr sip:bob@example.net;tag=b1-1
     sip:alice@example.com;tag=al-1 tr-87h@example.com
     </allOneLine>

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

Figure 2 (Forked call flow) depicts the call flow. The SIP CLF log records correspond to the viewpoint of P2. The log records are presented one per logical line and the line numbers refer to Figure 2 (Forked 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

     <allOneLine>
1    1275930743.699 INVITE-43 sip:bob@example.net
     198.51.100.1:5060 s-1-tr sip:bob@example.net
     sip:alice@example.com;tag=al-1 tr-87h@example.com
     </allOneLine>

     <allOneLine>
2    1275930744.001 INVITE-43 198.51.100.1:5060 100 s-1-tr -
     sip:bob@example.net
     </allOneLine>

     <allOneLine>
3    1275930744.998 INVITE-43 sip:bob@bob1.example.net
     203.0.113.1:5060 s-1-tr c-1-tr sip:bob@example.net
     sip:alice@example.com;tag=a1-1 tr-87h@example.com
     </allOneLine>

     <allOneLine>
4    1275930745.500 INVITE-43 sip:bob@bob2.example.net
     [2001:db8::9]:5060 s-1-tr c-2-tr sip:bob@example.net
     sip:alice@example.com;tag=a1-1 tr-87h@example.com
     </allOneLine>

     <allOneLine>
5    1275930745.800 INVITE-43 203.0.113.1:5060 100 s-1-tr
     c-1-tr sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
6    1275930746.100 INVITE-43 [2001:db8::9]:5060 100 s-1-tr
     c-2-tr sip:bob@example.net;tag=b1-2
     </allOneLine>

     <allOneLine>
7    1275930746.700 INVITE-43 [2001:db8::9]:5060 180 s-1-tr
     c-2-tr sip:bob@example.net;tag=b1-2
     </allOneLine>

     <allOneLine>
8    1275930746.990 INVITE-43 198.51.100.1:5060 180 s-1-tr
     c-2-tr sip:bob@example.net;tag=b1-2
     <allOneLine>

     <allOneLine>
9    1275930747.100 INVITE-43 203.0.113.1:5060 180 s-1-tr
     c-1-tr sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
10   1275930747.300 INVITE-43 198.51.100.1:5060 180 s-1-tr
     c-1-tr sip:bob@example.net;tag=b1-1
     </allOneLine>

     <allOneLine>
11   1275930747.800 INVITE-43 203.0.113.1:5060 200 s-1-tr
     c-1-tr sip:bob@example.net;tag=b1-1
     </allOneLine>

    <allOneLine>
12  1275930748.000 INVITE-43 198.51.100.1:5060 200 s-1-tr
    c-1-tr sip:bob@example.net;tag=b1-1
    </allOneLine>

    <allOneLine>
13  1275930748.201 CANCEL-43 sip:bob@bob2.example.net
    [2001:db8::9]:5060 s-1-tr c-2-tr sip:bob@example.net
    sip:alice@example.com;tag=a1-1 tr-87h@example.com
    </allOneLine>

    <allOneLine>
14  1275930748.991 INVITE-43 [2001:db8::9]:5060 487 s-1-tr c-2-tr
    sip:bob@example.net;tag=b1-2
    </allOneLine>

    <allOneLine>
15  1275930749.455 ACK-43 sip:bob@bob2.example.net [2001:db8::9]:5060
    s-1-tr c-2-tr sip:bob@example.net;tag=b1-2
    sip:alice@example.com;tag=a1-1 tr-87h@example.com
    </allOneLine>

    <allOneLine>
16  1275930750.001 CANCEL-43 [2001:db8::9]:5060 200 s-1-tr c-2-tr
    sip:bob@example.net;tag=b1-2
    </allOneLine>

The above SIP CLF log makes it easy to search for specific transactions or a state of the session. On a Linux/Unix system, a command of "grep c-1-tr" on the above log 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. The absence of the ACK request signifies that the ACK was exchanged end-to-end.

A command of "grep 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. The following threats may be considered for the log file while it is stored:

• An attacker may gain access to view the log file, or may surreptitiously make a copy of the log file for later vieweing;
• An attacker may mount a replay attack by modifying existing records in the log file or inserting new records;
• An attacker may delete parts of --- or indeed, the whole --- file.

It is outside the scope of this document to specify how to protect the log file while it is stored on disk. However, operators may consider using common administrative features such as disk encryption and securing log files [schneier‑1] (Schneier, B. and J. Kelsey, “Secure audit logs to support computer forensics,” May 1999.). Operators may also consider hardening the machine on which the log files are stored by restricting physical access to the host as well as restricting access to the files themselves.

In the worst case, public access to the SIP log file provides the same information that an adversary can gain using network sniffing tools (assuming that the SIP traffic is in clear text.) 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 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:

• An attacker may view the records;
• An attacker may modify the records in transit or insert previously captured records into the stream;
• 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, operators may consider using common security protocols described in [RFC3552] (Rescorla, E. and B. Korver, “Guidelines for Writing RFC Text on Security Considerations,” July 2003.) to transfer log files or individual records. Alternatively, the log file may be transferred through bulk methods that also guarantees integrity, or at least detects and alerts to modification attempts.

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 (Data model). 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 tradeoffs should be taken in account when logging more fields than the ones recommended in Section 8 (Data model).

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 substantive amount of disk space depending on traffic volume at a processing entity and the amount of information being logged. As such, any enterprise using SIP CLF should establish operational procedures for file rollovers as appropriate to the needs of the organization.

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

NOTE: Preliminary volume analysis was presented to the working group mailing list during the Anaheim IETF (please see http://www.ietf.org/mail-archive/web/sip-clf/current/msg00123.html for the analysis.) An open question is whether the working group thinks that this analysis should be put in this document.

12.  IANA Considerations

This document does not require any considerations from IANA.

13.  Acknowledgments

Members of the sipping, dispatch, ipfix and syslog working groups provided invaluable input to the formulation of the draft. These include Benoit Claise, Spencer Dawkins, John Elwell, David Harrington, Christer Holmberg, Hadriel Kaplan, Atsushi Kobayashi, Jiri Kuthan, Scott Lawrence, Chris Lonvick, 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 a data model.

14.  References

14.1. Normative References

14.2. Informative References

Authors' Addresses