Network Working Group A. Morton Request for Comments: 5618 AT&T Labs Updates: 5357 K. Hedayat Category: Standards Track EXFO August 2009
Mixed Security Mode for the Two-Way Active Measurement Protocol (TWAMP)
This memo describes a simple extension to TWAMP (the Two-Way Active Measurement Protocol). The extension adds the option to use different security modes in the TWAMP-Control and TWAMP-Test protocols simultaneously. The memo also describes a new IANA registry for additional features, called the TWAMP Modes registry.
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
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The Two-Way Active Measurement Protocol (TWAMP) [RFC5357] is an extension of the One-Way Active Measurement Protocol (OWAMP) [RFC4656]. The TWAMP specification gathered wide review as it approached completion, and the by-products were several recommendations for new features in TWAMP. There is a growing number of TWAMP implementations at present, and widespread usage is expected. There are even devices that are designed to test implementations for protocol compliance.
This memo describes a simple extension for TWAMP: the option to use different security modes in the TWAMP-Control and TWAMP-Test protocols (mixed security mode). It also describes a new IANA registry for additional features, called the TWAMP Modes registry.
When the Server and Control-Client have agreed to use the mixed security mode during control connection setup, then the Control- Client, the Server, the Session-Sender, and the Session-Reflector MUST all conform to the requirements of this mode as described in Sections 3, 4, and 5.
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].
The purpose of this memo is to describe and specify an extension for TWAMP [RFC5357], and to request the establishment of a registry for future TWAMP extensions.
The scope of the memo is limited to specifications of the following:
o Extension of the modes of operation through assignment of one new value in the Modes field (see Section 3.1 of [RFC4656]), while retaining backward compatibility with TWAMP [RFC5357] implementations. This value adds the OPTIONAL ability to use different security modes in the TWAMP-Control and TWAMP-Test protocols. The motivation for this extension is to permit the low-packet-rate TWAMP-Control protocol to utilize a stronger mode of integrity protection than that used in the TWAMP-Test protocol.
The TWAMP-Control protocol is a derivative of the OWAMP-Control protocol, and coordinates a two-way measurement capability. All TWAMP-Control messages are similar in format and follow similar guidelines to those defined in Section 3 of [RFC4656], with the exceptions described in TWAMP [RFC5357] and in the following sections.
All OWAMP-Control messages apply to TWAMP-Control, except for the Fetch-Session command.
TWAMP-Control connection establishment follows the same procedure defined in Section 3.1 of [RFC4656]. This extended mode assigns one new bit position (and value) to allow the Test protocol security mode to operate in Unauthenticated mode, while the Control protocol operates in Encrypted mode. With this extension, the complete set of TWAMP Mode values are as follows:
8 Unauth. TEST protocol, new bit position (3) Encrypted CONTROL
In the original OWAMP and TWAMP Modes field, setting bit position 0, 1, or 2 indicated the security mode of the Control protocol, and the Test protocol inherited the same mode (see Section 4 of [RFC4656]).
In this extension to TWAMP, when the Control-Client sets Modes Field bit position 3, it SHALL discontinue the inheritance of the security mode in the Test protocol, and each protocol's mode SHALL be as specified below. When the desired TWAMP-Test protocol mode is identical to the Control Session mode, the corresponding Modes Field bit (position 0, 1, or 2) SHALL be set by the Control-Client. The table below gives the various combinations of integrity protection that are permissible in TWAMP (with this extension). The TWAMP- Control and TWAMP-Test protocols SHALL use the mode in each column corresponding to the bit position set in the Modes Field.
Note that the TWAMP-Control protocol security measures are identical in the Authenticated and Encrypted Modes. Therefore, only one new bit position (3) is needed to convey the single mixed security mode.
The value of the Modes Field sent by the Server in the Server- Greeting message is the bit-wise OR of the modes (bit positions) that it is willing to support during this session. Thus, the last four
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bits of the 32-bit Modes Field are used. When no other features are activated, the first 28 bits MUST be zero. A client conforming to this extension of [RFC5357] MAY ignore the values in the first 28 bits of the Modes Field, or it MAY support other features that are communicated in these bit positions.
Other ways in which TWAMP extends OWAMP are described in [RFC5357].
The TWAMP-Test protocol is similar to the OWAMP-Test protocol [RFC4656] with the exception that the Session-Reflector transmits test packets to the Session-Sender in response to each test packet it receives. TWAMP [RFC5357] defines two different test packet formats: one for packets transmitted by the Session-Sender and one for packets transmitted by the Session-Reflector. As with the OWAMP-Test protocol, there are three security modes that also determine the test packet format: unauthenticated, authenticated, and encrypted. This TWAMP extension makes it possible to use TWAMP-Test Unauthenticated mode regardless of the mode used in the TWAMP-Control protocol.
When the Server has identified the ability to support the mixed security mode, the Control-Client has selected the mixed security mode in its Set-Up-Response, and the Server has responded with a zero Accept field in the Server-Start message, these extensions are REQUIRED.
The extended mixed mode of operation permits stronger security/ integrity protection on the TWAMP-Control protocol while simultaneously emphasizing accuracy or efficiency on the TWAMP-Test protocol, thus making it possible to increase overall security when compared to the previous options (when resource constraints would have forced less security for TWAMP-Control and conditions are such that use of unauthenticated TWAMP-Test is not a significant concern).
The security considerations that apply to any active measurement of live networks are relevant here as well. See [RFC4656] and [RFC5357].
This memo adds one security mode bit position/value beyond those in the OWAMP-Control specification [RFC4656], and describes behavior when the new mode is used. According to this document, IANA created a registry for the TWAMP Modes field. This field is a recognized extension mechanism for TWAMP.
IANA created a TWAMP Modes registry. TWAMP Modes are specified in TWAMP Server Greeting messages and Set-up Response messages consistent with Section 3.1 of [RFC4656] and Section 3.1 of [RFC5357], and extended by this memo. Modes are currently indicated by setting single bits in the 32-bit Modes Field. However, more complex encoding may be used in the future. Thus, this registry can contain a total of 2^32 possible assignments.
Because the TWAMP Modes registry can contain a maximum of 2^32 values, and because TWAMP is an IETF protocol, this registry must be updated only by "IETF Review" as specified in [RFC5226] (an RFC documenting registry use that is approved by the IESG). For the
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TWAMP Modes registry, we expect that new features will be assigned using monotonically increasing single bit positions and in the range [0-31], unless there is a good reason to do otherwise (more complex encoding than single bit positions may be used in the future, to access the 2^32 value space).