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Internet Engineering Steering Group int-area Ping This document describes a network diagnostic tool called PROBE. PROBE is similar to PING in that it can be used to query the status of a probed interface, but it differs from PING in that it does not require bidirectional connectivity between the probing and probed interfaces. Instead, PROBE requires bidirectional connectivity between the probing interface and a proxy interface. The proxy interface can reside on the same node as the probed interface, or it can reside on a node to which the probed interface is directly connected. This document updates RFC 4884 and obsoletes RFC 8335. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Internet Area Area mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Network operators use PING to test bidirectional connectivity between two interfaces. For the purposes of this document, these interfaces are called the probing and probed interfaces. PING sends an ICMP Echo Request message from the probing interface to the probed interface. The probing interface resides on a probing node while the probed interface resides on a probed node. If the probed interface receives the ICMP Echo Request message, it returns an ICMP Echo Reply. When the probing interface receives the ICMP Echo Reply, it has verified bidirectional connectivity between the probing and probed interfaces. Specifically, it has verified that:

• The probing node can reach the probed interface.
• The probed interface is active.
• The probed node can reach the probing interface.
• The probing interface is active.

This document describes a network diagnostic tool called PROBE. PROBE is similar to PING in that it can be used to query the status of a probed interface, but it differs from PING in that it does not require bidirectional connectivity between the probing and probed interfaces. Instead, PROBE requires bidirectional connectivity between the probing interface and a proxy interface. The proxy interface can reside on the same node as the probed interface, or it can reside on a node to which the probed interface is directly connected. of this document describes scenarios in which this characteristic is useful. Like PING, PROBE executes on a probing node. It sends an ICMP Extended Echo Request message from a local interface, called the probing interface, to a proxy interface. The proxy interface resides on a proxy node. The ICMP Extended Echo Request contains an ICMP Extension Structure and the ICMP Extension Structure contains an Interface Identification Object. The Interface Identification Object identifies the probed interface. The probed interface can reside on or directly connect to the proxy node. When the proxy interface receives the ICMP Extended Echo Request, the proxy node executes access control procedures. If access is granted, the proxy node determines the status of the probed interface and returns an ICMP Extended Echo Reply message. The ICMP Extended Echo Reply indicates the status of the probed interface. If the probed interface resides on the proxy node, PROBE determines the status of the probed interface as it would determine its oper-status. If oper-status is equal to 'up' (1), PROBE reports that the probed interface is active. Otherwise, PROBE reports that the probed interface is inactive. If the probed interface resides on a node that is directly connected to the proxy node, and the probed interface appears in the IPv4 Address Resolution Protocol (ARP) table or IPv6 Neighbor Cache, PROBE reports interface reachability. Otherwise, PROBE reports that the table entry does not exist.

Terminology This document uses the following terms:

• Probing interface: The interface that sends the ICMP Extended Echo Request.
• Probing node: The node upon which the probing interface resides.
• Proxy interface: The interface to which the ICMP Extended Echo Request message is sent.
• Proxy node: The node upon which the proxy interface resides.
• Probed interface: The interface whose status is being queried.
• Probed node: The node upon which the probed interface resides. If the proxy interface and the probed interface reside upon the same node, the proxy node is also the probed node. Otherwise, the proxy node is directly connected to the probed node.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

A note on this document's use of ICMP Extensions This document defines a unique use of ICMP Extensions : while normally, ICMP Extensions are defined to start at a given point and continue to the end of the packet, if the extension object is an Interface Identification Object as defined in this memo, then the extension structure (including the checksum) consists only of that single ICMP Extension Object. This is done to maintain compatibility with the initial set of implementations of RFC8335, which behave this way. New uses of ICMP Extensions, and in fact uses of Extended Echo using some object other than the Interface Identification Object, SHOULD NOT behave this way. Uses other than defined in this memo SHOULD treat the ICMP Extension Structure as extending to the end of the packet as defines.

ICMP Extended Echo Request The ICMP Extended Echo Request message is defined for both ICMPv4 and ICMPv6. Like any ICMP message, the ICMP Extended Echo Request message is encapsulated in an IP header. The ICMPv4 version of the Extended Echo Request message is encapsulated in an IPv4 header, while the ICMPv6 version is encapsulated in an IPv6 header. depicts the ICMP Extended Echo Request message.

ICMP Extended Echo Request Message

IP Header fields:

• Source Address: The Source Address identifies the probing interface. It MUST be a valid IPv4 or IPv6 unicast address.
• Destination Address: The Destination Address identifies the proxy interface. It MUST be a unicast address.

ICMP fields:

• Type: Extended Echo Request. The value for ICMPv4 is 42. The value for ICMPv6 is 160.
• Code: MUST be set to 0 and MUST be ignored upon receipt.
• Checksum: For ICMPv4, see RFC 792. For ICMPv6, see RFC 4443.
• Identifier: An Identifier to aid in matching Extended Echo Replies to Extended Echo Requests. May be 0.
• Sequence Number: A Sequence Number to aid in matching Extended Echo Replies to Extended Echo Requests. May be 0.
• Reserved: This field MUST be set to 0 and ignored upon receipt.
• L (local): The L-bit is set if the probed interface resides on the proxy node. The L-bit is clear if the probed interface is directly connected to the proxy node.
• ICMP Extension Structure: The ICMP Extension Structure contains an Interface Identification Object that identifies the probed interface. The checksum in the ICMP Extension structure covers the Interface Identification Object but not any (optional) data that follows.

Section 7 of defines the ICMP Extension Structure. As per RFC 4884, the Extension Structure contains exactly one Extension Header followed by one or more objects. When applied to the ICMP Extended Echo Request message, the Extension Object(s) define the operation to perform. In the PROBE application, the ICMP Extension Structure MUST contain exactly one instance of the Interface Identification Object, and the ICMP Extension Structure does not cover the rest of the packet; it ends at the end of the single Interface Identification Object, and what follows is simply optional data. The behavior when it contains a different Extension Object is not defined by this memo but may be defined in the future. If the L-bit is set, the Interface Identification Object can identify the probed interface by name, index, or address. If the L-bit is clear, the Interface Identification Object MUST identify the probed interface by address. If the Interface Identification Object identifies the probed interface by address, that address can be a member of any address family. For example, an ICMPv4 Extended Echo Request message can carry an Interface Identification Object that identifies the probed interface by IPv4, IPv6, or IEEE 802 address. Likewise, an ICMPv6 Extended Echo Request message can carry an Interface Identification Object that identifies the probed interface by IPv4, IPv6, or IEEE 802 address. The Interface Identification Object MAY be followed by an optional data section, which is not interpreted but is simply present to be copied to the ICMP Extended Echo Reply.

Interface Identification Object The Interface Identification Object identifies the probed interface by name, index, or address. Like any other ICMP Extension Object, it contains an Object Header and Object Payload. The Object Header contains the following fields:

• Class-Num: Interface Identification Object. The value is 3.
• C-Type: Values are (1) Identifies Interface by Name, (2) Identifies Interface by Index, and (3) Identifies Interface by Address.
• Length: Length of the object, measured in octets, including the Object Header and Object Payload.

If the Interface Identification Object identifies the probed interface by name, the Object Payload MUST be the interface name as defined in . If the Object Payload would not otherwise terminate on a 32-bit boundary, it MUST be padded with ASCII NUL characters, adjusting the Length accordingly. If the Interface Identification Object identifies the probed interface by index, the length is equal to 8 and the payload contains the if-index . If the Interface Identification Object identifies the probed interface by address, the payload is as depicted in .

Interface Identification Object - C-Type 3 Payload

Payload fields are defined as follows:

• Address Family Identifier (AFI): This 16-bit field identifies the type of address represented by the Address field. All values found in the IANA registry of Address Family Numbers (available from ) are valid in this field.
• Address Length: Number of significant bytes contained by the Address field. (The Address field contains significant bytes and padding bytes.)
• Reserved: This field MUST be set to 0 and ignored upon receipt.
• Address: This variable-length field represents an address associated with the probed interface. If the address field would not otherwise terminate on a 32-bit boundary, it MUST be padded with zeroes.

ICMP Extended Echo Reply The ICMP Extended Echo Reply message is defined for both ICMPv4 and ICMPv6. Like any ICMP message, the ICMP Extended Echo Reply message is encapsulated in an IP header. The ICMPv4 version of the Extended Echo Reply message is encapsulated in an IPv4 header, while the ICMPv6 version is encapsulated in an IPv6 header. depicts the ICMP Extended Echo Reply message.

ICMP Extended Echo Reply Message

IP Header fields:

• Source Address: Copied from the Destination Address field of the invoking Extended Echo Request message.
• Destination Address: Copied from the Source Address field of the invoking Extended Echo Request message.

ICMP fields:

• Type: Extended Echo Reply. The value for ICMPv4 is 43. The value for ICMPv6 is 161.
• Code: Values are (0) No Error, (1) Malformed Query, (2) No Such Interface, (3) No Such Table Entry, and (4) Multiple Interfaces Satisfy Query.
• Checksum: For ICMPv4, see RFC 792. For ICMPv6, see RFC 4443.
• Identifier: Copied from the Identifier field of the invoking Extended Echo Request packet.
• Sequence Number: Copied from the Sequence Number field of the invoking Extended Echo Request packet.
• State: If Code is not equal to 0, this field MUST be set to 0 and ignored upon receipt. Likewise, if the probed interface resides upon the proxy node, this field MUST be set to 0 and ignored upon receipt. Otherwise, this field reflects the state of the ARP table or Neighbor Cache entry associated with the probed interface. Values are (0) Reserved, (1) Incomplete, (2) Reachable, (3) Stale, (4) Delay, (5) Probe, and (6) Failed.
• Res: This field MUST be set to 0 and ignored upon receipt.
• A (Active): The A-bit is set if the Code is equal to 0, the probed interface resides on the proxy node, and the probed interface is active. Otherwise, the A-bit is clear.
• 4 (IPv4): The 4-bit is set if the A-bit is also set and IPv4 is running on the probed interface. Otherwise, the 4-bit is clear.
• 6 (IPv6): The 6-bit is set if the A-bit is also set and IPv6 is running on the probed interface. Otherwise, the 6-bit is clear.

ICMP Message Processing When a node receives an ICMP Extended Echo Request message and any of the following conditions apply, the node MUST silently discard the incoming message:

• The node does not recognize ICMP Extended Echo Request messages.
• The node has not explicitly enabled ICMP Extended Echo functionality.
• The incoming ICMP Extend Echo Request carries a Source Address that is not explicitly authorized for the L-bit setting of the incoming ICMP Extended Echo Request.
• The incoming ICMP Extend Echo Request carries a Source Address that is not explicitly authorized for the incoming ICMP Extended Echo Request type (i.e., by name, by if-index, or by address).
• The Source Address of the incoming message is not a unicast address.
• The Destination Address of the incoming message is a multicast address.

Otherwise, when a node receives an ICMPv4 Extended Echo Request, it MUST format an ICMP Extended Echo Reply as follows:

• Don't Fragment (DF) flag is 1
• More Fragments flag is 0
• Fragment Offset is 0
• TTL is 255
• Protocol is ICMP

When a node receives an ICMPv6 Extended Echo Request, it MUST format an ICMPv6 Extended Echo Reply as follows:

• Hop Limit is 255
• Next Header is ICMPv6

In either case, the responding node MUST do the following:

• Copy the Source Address from the Extended Echo Request message to the Destination Address of the Extended Echo Reply.
• Copy the Destination Address from the Extended Echo Request message to the Source Address of the Extended Echo Reply.
• Set the DiffServ codepoint to CS0.
• Set the ICMP Type to Extended Echo Reply.
• Copy the Identifier from the Extended Echo Request message to the Extended Echo Reply.
• Copy the Sequence Number from the Extended Echo Request message to the Extended Echo Reply.
• Set the Code field as described in .
• Set the State field to 0.
• Clear the A-bit, the 4-bit, and the 6-bit.
• If (1) the Code Field is equal to (0) No Error, (2) the L-bit is set, and (3) the probed interface is active, set the A-bit. Also, set the 4-bit and the 6-bit as appropriate.
• If the Code field is equal to (0) No Error and the L-bit is clear, then set the State field to reflect the state of the ARP table or Neighbor Cache entry that represents the probed interface.
• Copy the ICMP Extension Structure, ICMP Extension Object, and Data (if any) from the Extended Echo Request message.
• Set the Checksum appropriately.
• Forward the ICMP Extended Echo Reply to its destination.

Code Field Processing The Code field MUST be set to (1) Malformed Query if any of the following conditions apply:

• The ICMP Extended Echo Request does not include an ICMP Extension Structure.
• The ICMP Extension Structure does not include exactly one Interface Identification Object.
• The ICMP Extension Structure checksum is 0 or incorrect.
• The L-bit is clear and the Interface Identification Object identifies the probed interface by name or if-index.
• The query is otherwise malformed.

The Code field MUST be set to (2) No Such Interface if the L-bit is set and the ICMP Extension Structure does not identify an interface that resides on the proxy node. The Code field MUST be set to (3) No Such Table Entry if the L-bit is clear and the address found in the Interface Identification Object does not appear in the IPv4 Address Resolution Protocol (ARP) table or the IPv6 Neighbor Cache. The Code field MUST be set to (4) Multiple Interfaces Satisfy Query if any of the following conditions apply:

• The L-bit is set and the ICMP Extension Structure identifies more than one interface that resides in the proxy node.
• The L-bit is clear and the address found in the Interface Identification Object maps to multiple IPv4 ARP or IPv6 Neighbor Cache entries.

Otherwise, the Code field MUST be set to (0) No Error.

Use Cases In the scenarios listed below, network operators can use PROBE to determine the status of a probed interface but cannot use PING for the same purpose. In all scenarios, assume bidirectional connectivity between the probing and proxy interfaces. However, bidirectional connectivity between the probing and probed interfaces is lacking.

• The probed interface is unnumbered.
• The probing and probed interfaces are not directly connected to one another. The probed interface has an IPv6 link-local address but does not have a more globally scoped address.
• The probing interface runs IPv4 only while the probed interface runs IPv6 only.
• The probing interface runs IPv6 only while the probed interface runs IPv4 only.
• For lack of a route, the probing node cannot reach the probed interface.

Updates to RFC 4884 Section 4.6 of provides a list of extensible ICMP messages (i.e., messages that can carry the ICMP Extension Structure). This document adds the ICMP Extended Echo Request message and the ICMP Extended Echo Reply message to that list.

Change History

Changes from RFC 8335 This document updates to clarify the handling of extra data beyond the ICMP Extension Structure, that data is echoed in the response packet, and checksum handling in the ICMP Extension Structure. Specifically,

• Updated to reflect the presence of the ICMP Extension Object and additional data.
• Updated to mention the ICMP Extension Structure checksum, and extra verbosity about how the Extension Structure does not cover the rest of the packet.
• Updated to reflect the presence of the ICMP Extension Structure and additional data.
• Added a step in about copying data from the request to the response.
• Added a step in about validating the ICMP Extension Structure checksum.
• Added section to suggest human-readable display of PROBE responses
• Clarified in that the length of an ifName Object is adjusted when padding is added.

Changes from draft-fenner-intarea-probe-clarification-00

• Changed "NULL" to "NUL" when referring to the ASCII control character, per RFC20.
• Consistently refer to interface name and index using their yang names, not SNMP names.
• Added [] around the Data following the ICMP Extension Structure in and to indicate that it is optional.

Changes from draft-fenner-intarea-probe-clarification-01

• Updated the section on ICMP Extension header format to say that different ICMP Extension Option headers may be present, and if they are, the mechanism is not as specified in this memo.

Changes from draft-fenner-intarea-probe-clarification-02

• Made a stronger statement about not copying this behavior in
• Renamed to rfc8335bis and made WG document

Changes from draft-int-intarea-rfc8335bis-00

• Changed "For the operations in this memo" to "In the PROBE application" to better align with draft-ietf-6man-icmpv6-reflection

IANA Considerations IANA is requested to update the references for the below actions from to refer to this document. IANA has performed the following actions:

• Added the following to the "ICMP Type Numbers" registry: Added the following to the "Type 42 - Extended Echo Request" subregistry:
• Added the following to the "ICMPv6 'type' Numbers" registry: Added the following to the "Type 160 - Extended Echo Request" subregistry:
• Added the following to the "ICMP Type Numbers" registry: Added the following to the "Type 43 - Extended Echo Reply" subregistry:
• Added the following to the "ICMPv6 'type' Numbers" registry: Added the following to the "Type 161 - Extended Echo Reply" subregistry:
• Added the following to the "ICMP Extension Object Classes and Class Sub-types" registry: Added the following C-types to the "Sub-types - Class 3 - Interface Identification Object" subregistry: C-Type values are assigned on a First Come First Serve (FCFS) basis with a range of 0-255.

All codes mentioned above are assigned on an FCFS basis with a range of 0-255.

Security Considerations The following are legitimate uses of PROBE:

• to determine the operational status of an interface.
• to determine which protocols (e.g., IPv4 or IPv6) are active on an interface.

However, malicious parties can use PROBE to obtain additional information. For example, a malicious party can use PROBE to discover interface names. Having discovered an interface name, the malicious party may be able to infer additional information. Additional information may include:

• interface bandwidth
• the type of device that supports the interface (e.g., vendor identity)
• the operating system version that the above-mentioned device executes

Understanding this risk, network operators establish policies that restrict access to ICMP Extended Echo functionality. In order to enforce these policies, nodes that support ICMP Extended Echo functionality MUST support the following configuration options:

• Enable/disable ICMP Extended Echo functionality. By default, ICMP Extend Echo functionality is disabled.
• Define enabled L-bit settings. By default, the option to set the L-bit is enabled and the option to clear the L-bit is disabled.
• Define enabled query types (i.e., by name, by index, or by address); by default, all query types are disabled.
• For each enabled query type, define the prefixes from which ICMP Extended Echo Request messages are permitted.
• For each interface, determine whether ICMP Echo Request messages are accepted.

When a node receives an ICMP Extended Echo Request message that it is not configured to support, it MUST silently discard the message. See for details. PROBE must not leak information about one Virtual Private Network (VPN) into another. Therefore, when a node receives an ICMP Extended Echo Request and the proxy interface is in a different VPN than the probed interface, the node MUST return an ICMP Extended Echo Reply with error code equal to (2) No Such Interface. In order to protect local resources, implementations SHOULD rate-limit incoming ICMP Extended Echo Request messages.

References Normative References This document redefines selected ICMP messages to support multi-part operation. A multi-part ICMP message carries all of the information that ICMP messages carried previously, as well as additional information that applications may require. Multi-part messages are supported by an ICMP extension structure. The extension structure is situated at the end of the ICMP message. It includes an extension header followed by one or more extension objects. Each extension object contains an object header and object payload. All object headers share a common format. This document further redefines the above mentioned ICMP messages by specifying a length attribute. All of the currently defined ICMP messages to which an extension structure can be appended include an "original datagram" field. The "original datagram" field contains the initial octets of the datagram that elicited the ICMP error message. Although the original datagram field is of variable length, the ICMP message does not include a field that specifies its length. Therefore, in order to facilitate message parsing, this document allocates eight previously reserved bits to reflect the length of the "original datagram" field. The proposed modifications change the requirements for ICMP compliance. The impact of these changes on compliant implementations is discussed, and new requirements for future implementations are presented. This memo updates RFC 792 and RFC 4443. [STANDARDS-TRACK] The purpose of this RFC is to present a method of Converting Protocol Addresses (e.g., IP addresses) to Local Network Addresses (e.g., Ethernet addresses). This is an issue of general concern in the ARPA Internet Community at this time. The method proposed here is presented for your consideration and comment. This is not the specification of an Internet Standard. This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK] This document defines a YANG data model for the management of network interfaces. It is expected that interface-type-specific data models augment the generic interfaces data model defined in this document. The data model includes definitions for configuration and system state (status information and counters for the collection of statistics). The YANG data model in this document conforms to the Network Management Datastore Architecture (NMDA) defined in RFC 8342. This document obsoletes RFC 7223. This document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol). ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6). [STANDARDS-TRACK] This document describes a network diagnostic tool called PROBE. PROBE is similar to PING in that it can be used to query the status of a probed interface, but it differs from PING in that it does not require bidirectional connectivity between the probing and probed interfaces. Instead, PROBE requires bidirectional connectivity between the probing interface and a proxy interface. The proxy interface can reside on the same node as the probed interface, or it can reside on a node to which the probed interface is directly connected. This document updates RFC 4884. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This memo is an introductory guide to many of the most commonly- available TCP/IP and Internet tools and utilities. It also describes discussion lists accessible from the Internet, ways to obtain Internet and TCP/IP documents, and some resources that help users weave their way through the Internet. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. This document describes service classes configured with Diffserv and recommends how they can be used and how to construct them using Differentiated Services Code Points (DSCPs), traffic conditioners, Per-Hop Behaviors (PHBs), and Active Queue Management (AQM) mechanisms. There is no intrinsic requirement that particular DSCPs, traffic conditioners, PHBs, and AQM be used for a certain service class, but as a policy and for interoperability it is useful to apply them consistently. This memo provides information for the Internet community. IANA

The PROBE Application The PROBE application accepts input parameters, sets a counter, and enters a loop to be exited when the counter is equal to 0. On each iteration of the loop, PROBE emits an ICMP Extended Echo Request, decrements the counter, sets a timer, and waits. The ICMP Extended Echo Request includes an Identifier and a Sequence Number. If an ICMP Extended Echo Reply carrying the same Identifier and Sequence Number arrives, PROBE relays information returned by that message to its user. However, on each iteration of the loop, PROBE waits for the timer to expire regardless of whether an Extended Echo Reply message arrives. PROBE accepts the following parameters:

• Count
• Wait
• Probing Interface Address
• Hop Count
• Proxy Interface Address
• Local
• Probed Interface Identifier

Count is a positive integer whose default value is 3. Count determines the number of times that PROBE iterates through the above-mentioned loop. Wait is a positive integer whose minimum and default values are 1. Wait determines the duration of the above-mentioned timer, measured in seconds. Probing Interface Address specifies the Source Address of the ICMP Extended Echo Request. The Probing Interface Address MUST be a unicast address and MUST identify an interface that resides on the probing node. The Proxy Interface Address identifies the interface to which the ICMP Extended Echo Request message is sent. It must be an IPv4 or IPv6 unicast address. If it is an IPv4 address, PROBE emits an ICMPv4 message. If it is an IPv6 address, PROBE emits an ICMPv6 message. Local is a boolean value. It is TRUE if the proxy and probed interfaces both reside on the same node. Otherwise, it is FALSE. The Probed Interface Identifier identifies the probed interface. It is one of the following:

• an interface name;
• an address from any address family (e.g., IPv4, IPv6, IEEE 802, 48-bit MAC, or 64-bit MAC); or
• an if-index.

If the Probed Interface Identifier is an address, it does not need to be of the same address family as the proxy interface address. For example, PROBE accepts an IPv4 Proxy Interface Address and an IPv6 Probed Interface Identifier.

Information Display For the PING application, the primary available piece of information is the fact that we received an ICMP Echo Reply. Therefore, the appropriate information to display is all of the available information about the received reply, e.g., size, ttl, etc. However, with PROBE, the primary piece of information is the reported status of the probed interface: the code, status, A, 4, and 6 fields. It's appropriate to convert the combination of the returned values into a "human-readable" response. For example, an application may perform these steps:

• If the code field is non-zero, print the code value as described in .
• If the code field is zero, then if the L field sent is zero, print the state value as described in .
• Otherwise, the L field sent is 1; print the state represented by the A, 4, and 6 bits. Sample textual translations for these bits are shown in .

Sample translations for bit settings

A	4	6	Text
0	0	0	Interface inactive
1	0	0	Interface active, with no ipv4 or ipv6 running
1	0	1	Interface active, with ipv6 running
1	1	0	Interface active, with ipv4 running
1	1	1	Interface active, with ipv4 and ipv6 running

Acknowledgments Thanks to Sowmini Varadhan, Jeff Haas, Carlos Pignataro, Jonathan Looney, Dave Thaler, Mikio Hara, Joel Halpern, Yaron Sheffer, Stefan Winter, Jean-Michel Combes, Amanda Barber, and Joe Touch for their thoughtful review of this document.