Request for Comments: 6872
Category: Standards Track
ISSN: 2070-1721
Bell Laboratories, Alcatel-Lucent
E. Burger, Ed.
Georgetown University
T. Anjali
Illinois Institute of Technology
H. Abdelnur
O. Festor
INRIA
February 2013
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The Common Log Format (CLF) for the Session Initiation Protocol (SIP):
Framework and Information Model
Abstract
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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
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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.
Copyright Notice
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Copyright © 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
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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
1. Introduction
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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
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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
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
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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
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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.
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
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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
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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.
5.1. SIP CLF and Call Detail Records
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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
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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:
- 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.
- The packet capture from these tools is not easily searchable by simple command-line tools for text processing.
- 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).
- 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.
- Using such tools and related packet capture libraries may imposes a dependency on a third-party library.
5.3. SIP CLF and Syslog
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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 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
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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
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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.-
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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. -
7. Challenges in Establishing a SIP CLF
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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.
8. Information Model
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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
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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). 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.
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
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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
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
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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.
+-------------------+--------------------+-------------------+ | 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
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Illustrative examples of SIP CLF follow.
9.1. UAC Registration
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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-1After 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
9.2. Direct Call between Alice and Bob
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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 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 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
9.3. Single Downstream Branch Call
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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: -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 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 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 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
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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 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
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The SIP CLF log appears from the vantage point of P2. The fields logged are shown below; the line numbers refer to Figure 2.
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: - 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 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 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 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 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 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 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-trThe 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.
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
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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:
- An attacker may gain access to view the log file, or may surreptitiously make a copy of the log file for later viewing.
- 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.
- 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 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, 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.
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
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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
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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
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[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.
13.2. Informative References
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[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.
Authors' Addresses
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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
USAEMail: 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
FranceEMail:
humbol@gmail.com
Olivier Festor
INRIA
INRIA - Nancy Grant Est
Campus Scientifique
54506, Vandoeuvre-les-Nancy Cedex
FranceEMail: Olivier.Festor@loria.fr