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Updated by: 8335 PROPOSED STANDARD
Errata Exist
Network Working Group R. Bonica
Request for Comments: 4884 Juniper Networks
Updates: 792, 4443 D. Gan
Category: Standards Track Consultant
D. Tappan
Consultant
C. Pignataro
Cisco Systems, Inc.
April 2007
Extended ICMP to Support Multi-Part Messages
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.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
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.
Bonica, et al. Standards Track [Page 1]
RFC 4884 Multi-Part ICMP Messages April 2007
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.
Table of Contents
1. Introduction ....................................................3
2. Conventions Used in This Document ...............................4
3. Summary of Changes to ICMP ......................................4
4. ICMP Extensibility ..............................................4
4.1. ICMPv4 Destination Unreachable .............................7
4.2. ICMPv4 Time Exceeded .......................................8
4.3. ICMPv4 Parameter Problem ...................................8
4.4. ICMPv6 Destination Unreachable .............................9
4.5. ICMPv6 Time Exceeded .......................................9
4.6. ICMP Messages That Can Be Extended ........................10
5. Backwards Compatibility ........................................10
5.1. Classic Application Receives ICMP Message with
Extensions ................................................12
5.2. Non-Compliant Application Receives ICMP Message
with No Extensions ........................................12
5.3. Non-Compliant Application Receives ICMP Message
with Compliant Extensions .................................13
5.4. Compliant Application Receives ICMP Message with
No Extensions .............................................14
5.5. Compliant Application Receives ICMP Message with
Non-Compliant Extensions ..................................14
6. Interaction with Network Address Translation ...................14
7. The ICMP Extension Structure ...................................15
8. ICMP Extension Objects .........................................16
9. Security Considerations ........................................16
10. IANA Considerations ...........................................17
11. Acknowledgments ...............................................17
12. References ....................................................17
12.1. Normative References .....................................17
12.2. Informative References ...................................17
Bonica, et al. Standards Track [Page 2]
RFC 4884 Multi-Part ICMP Messages April 2007
1. Introduction
This document redefines selected ICMPv4 [RFC0792] and ICMPv6
[RFC4443] messages to include an extension structure and a length
attribute. The extension structure supports multi-part ICMP
operation. Protocol designers can make an ICMP message carry
additional information by encoding that information in the extension
structure.
This document also addresses a fundamental problem in ICMP
extensibility. All of the ICMP messages addressed by this memo
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.
Application software infers the length of the "original datagram"
field from the total length of the ICMP message. If an extension
structure were appended to the message without adding a length
attribute for the "original datagram" field, the message would become
unparsable. Specifically, application software would not be able to
determine where the "original datagram" field ends and where the
extension structure begins. Therefore, this document proposes a
length attribute as well as an extension structure that is appended
to the ICMP message.
The current memo also addresses backwards compatibility with existing
ICMP implementations that either do not implement the extensions
defined herein or implement them without adding the required length
attributes. In particular, this document addresses backwards
compatibility with certain, widely deployed, MPLS-aware ICMPv4
implementations that send the extensions defined herein without
adding the required length attribute.
The current memo does not define any ICMP extension objects. It
defines only the extension header and a common header that all
extension objects share. [UNNUMBERED], [ROUTING-INST], and
[MPLS-ICMP] provide sample applications of the ICMP Extension Object.
The above mentioned memos share a common characteristic. They all
append information to the ICMP Time Expired message for consumption
by TRACEROUTE. In this case, as in many others, appending
information to the existing ICMP Time Expired Message is preferable
to defining a new message and emitting two messages whenever a packet
is dropped due to TTL expiration.
Bonica, et al. Standards Track [Page 3]
RFC 4884 Multi-Part ICMP Messages April 2007
2. Conventions Used in This Document
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 [RFC2119].
3. Summary of Changes to ICMP
The following is a summary of changes to ICMP that are introduced by
this memo:
An ICMP Extension Structure MAY be appended to ICMPv4 Destination
Unreachable, Time Exceeded, and Parameter Problem messages.
An ICMP Extension Structure MAY be appended to ICMPv6 Destination
Unreachable, and Time Exceeded messages.
The above mentioned messages include an "original datagram" field,
and the message formats are updated to specify a length attribute
for the "original datagram" field.
When the ICMP Extension Structure is appended to an ICMP message
and that ICMP message contains an "original datagram" field, the
"original datagram" field MUST contain at least 128 octets.
When the ICMP Extension Structure is appended to an ICMPv4 message
and that ICMPv4 message contains an "original datagram" field, the
"original datagram" field MUST be zero padded to the nearest
32-bit boundary.
When the ICMP Extension Structure is appended to an ICMPv6 message
and that ICMPv6 message contains an "original datagram" field, the
"original datagram" field MUST be zero padded to the nearest
64-bit boundary.
ICMP messages defined in the future SHOULD indicate whether or not
they support the extension mechanism defined in this
specification. It is recommended that all new messages support
extensions.
4. ICMP Extensibility
RFC 792 defines the following ICMPv4 message types:
- Destination Unreachable
- Time Exceeded
Bonica, et al. Standards Track [Page 4]
RFC 4884 Multi-Part ICMP Messages April 2007
- Parameter Problem
- Source Quench
- Redirect
- Echo Request/Reply
- Timestamp/Timestamp Reply
- Information Request/Information Reply
[RFC1191] reserves bits for the "Next-Hop MTU" field in the
Destination Unreachable message.
RFC 4443 defines the following ICMPv6 message types:
- Destination Unreachable
- Packet Too Big
- Time Exceeded
- Parameter Problem
- Echo Request/Reply
Many ICMP messages are extensible as currently defined. Protocol
designers can extend ICMP messages by simply appending fields or data
structures to them.
However, the following ICMP messages are not extensible as currently
defined:
- ICMPv4 Destination Unreachable (type = 3)
- ICMPv4 Time Exceeded (type = 11)
- ICMPv4 Parameter Problem (type = 12)
- ICMPv6 Destination Unreachable (type = 1)
- ICMPv6 Packet Too Big (type = 2)
- ICMPv6 Time Exceeded (type = 3)
- ICMPv6 Parameter Problem (type = 4)
Bonica, et al. Standards Track [Page 5]
RFC 4884 Multi-Part ICMP Messages April 2007
These messages contain an "original datagram" field which represents
the leading octets of the datagram to which the ICMP message is a
response. RFC 792 defines the "original datagram" field for ICMPv4
messages. In RFC 792, the "original datagram" field includes the IP
header plus the next eight octets of the original datagram.
[RFC1812] extends the "original datagram" field to contain as many
octets as possible without causing the ICMP message to exceed the
minimum IPv4 reassembly buffer size (i.e., 576 octets). RFC 4443
defines the "original datagram" field for ICMPv6 messages. In RFC
4443, the "original datagram" field always contained as many octets
as possible without causing the ICMP message to exceed the minimum
IPv6 MTU (i.e., 1280 octets).
Unfortunately, the "original datagram" field lacks a length
attribute. Application software infers the length of this field from
the total length of the ICMP message. If an extension structure were
appended to the message without adding a length attribute for the
"original datagram" field, the message would become unparsable.
Specifically, application software would not be able to determine
where the "original datagram" field ends and where the extension
structure begins.
In order to solve this problem, this memo introduces an 8-bit length
attribute to the following ICMPv4 messages.
- Destination Unreachable (type = 3)
- Time Exceeded (type = 11)
- Parameter Problem (type = 12)
It also introduces an 8-bit length attribute to the following ICMPv6
messages.
- Destination Unreachable (type = 1)
- Time Exceeded (type = 3)
The length attribute MUST be specified when the ICMP Extension
Structure is appended to the above mentioned ICMP messages.
The length attribute represents the length of the "original datagram"
field. Space for the length attribute is claimed from reserved
octets, whose value was previously required to be zero.
For ICMPv4 messages, the length attribute represents 32-bit words.
When the length attribute is specified, the "original datagram" field
MUST be zero padded to the nearest 32-bit boundary. Because the
Bonica, et al. Standards Track [Page 6]
RFC 4884 Multi-Part ICMP Messages April 2007
sixth octet of each of the impacted ICMPv4 messages was reserved for
future use, this octet was selected as the location of the length
attribute in ICMPv4.
For ICMPv6 messages, the length attribute represents 64-bit words.
When the length attribute is specified, the "original datagram" field
MUST be zero padded to the nearest 64-bit boundary. Because the
fifth octet of each of the impacted ICMPv6 messages was reserved for
future use, this octet was selected as the location of the length
attribute in ICMPv6.
In order to achieve backwards compatibility, when the ICMP Extension
Structure is appended to an ICMP message and that ICMP message
contains an "original datagram" field, the "original datagram" field
MUST contain at least 128 octets. If the original datagram did not
contain 128 octets, the "original datagram" field MUST be zero padded
to 128 octets. (See Section 5.1 for rationale.)
The following sub-sections depict length attribute as it has been
introduced to selected ICMP messages.
4.1. ICMPv4 Destination Unreachable
Figure 1 depicts the ICMPv4 Destination Unreachable Message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused | Length | Next-Hop MTU* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + leading octets of original datagram |
| |
| // |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ICMPv4 Destination Unreachable
The syntax and semantics of all fields are unchanged from RFC 792.
However, a length attribute is added to the second word. The length
attribute represents length of the padded "original datagram" field,
measured in 32-bit words.
* The Next-Hop MTU field is not required in all cases. It is
depicted only to demonstrate that those bits are not available for
assignment in this memo.
Bonica, et al. Standards Track [Page 7]
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4.2. ICMPv4 Time Exceeded
Figure 2 depicts the ICMPv4 Time Exceeded Message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused | Length | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + leading octets of original datagram |
| |
| // |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ICMPv4 Time Exceeded
The syntax and semantics of all fields are unchanged from RFC 792,
except for a length attribute which is added to the second word. The
length attribute represents length of the padded "original datagram"
field, measured in 32-bit words.
4.3. ICMPv4 Parameter Problem
Figure 3 depicts the ICMPv4 Parameter Problem Message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer | Length | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + leading octets of original datagram |
| |
| // |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ICMPv4 Parameter Problem
The syntax and semantics of all fields are unchanged from RFC 792,
except for a length attribute which is added to the second word. The
length attribute represents length of the padded "original datagram"
field, measured in 32-bit words.
Bonica, et al. Standards Track [Page 8]
RFC 4884 Multi-Part ICMP Messages April 2007
4.4. ICMPv6 Destination Unreachable
Figure 4 depicts the ICMPv6 Destination Unreachable Message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as possible without the ICMPv6 packet +
| exceeding the minimum IPv6 MTU [RFC4443] |
Figure 4: ICMPv6 Destination Unreachable
The syntax and semantics of all fields are unchanged from RFC 4443.
However, a length attribute is added to the second word. The length
attribute represents length of the padded "original datagram" field,
measured in 64-bit words.
4.5. ICMPv6 Time Exceeded
Figure 5 depicts the ICMPv6 Time Exceeded Message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
+ as possible without the ICMPv6 packet +
| exceeding the minimum IPv6 MTU [RFC4443] |
Figure 5: ICMPv6 Time Exceeded
The syntax and semantics of all fields are unchanged from RFC 4443,
except for a length attribute which is added to the second word. The
length attribute represents length of the padded "original datagram"
field, measured in 64-bit words.
Bonica, et al. Standards Track [Page 9]
RFC 4884 Multi-Part ICMP Messages April 2007
4.6. ICMP Messages That Can Be Extended
The ICMP Extension Structure MAY be appended to messages of the
following types:
- ICMPv4 Destination Unreachable
- ICMPv4 Time Exceeded
- ICMPv4 Parameter Problem
- ICMPv6 Destination Unreachable
- ICMPv6 Time Exceeded
The ICMP Extension Structure MUST NOT be appended to any of the other
ICMP messages mentioned in Section 4. Extensions were not defined
for the ICMPv6 "Packet Too Big" and "Parameter Problem" messages
because these messages lack space for a length attribute.
5. Backwards Compatibility
ICMP messages can be categorized as follows:
- Messages that do not include any ICMP extensions
- Messages that include non-compliant ICMP extensions
- Messages that includes compliant ICMP extensions
Any ICMP implementation can send a message that does not include
extensions. ICMP implementations produced prior to 1999 are not
known to send ICMP extensions.
Some ICMP implementations, produced between 1999 and the time of this
publication, may send a non-compliant version of ICMP extensions
described in this memo. Specifically, these implementations may
append the ICMP Extension Structure to the Time Exceeded and
Destination Unreachable messages. When they do this, they send
exactly 128 octets representing the original datagram, zero padding
if required. They also calculate checksums as described in this
document. However, they do not specify a length attribute to be
associated with the "original datagram" field.
It is assumed that ICMP implementations produced in the future will
send ICMP extensions that are compliant with this specification.
Bonica, et al. Standards Track [Page 10]
RFC 4884 Multi-Part ICMP Messages April 2007
Likewise, applications that consume ICMP messages can be categorized
as follows:
- Classic applications
- Non-compliant applications
- Compliant applications
Classic applications do not parse extensions defined in this memo.
They are insensitive to the length attribute that is associated with
the "original datagram" field.
Non-compliant implementations parse the extensions defined in this
memo, but only in conjunction with the Time Expired and Destination
Unreachable messages. They require the "original datagram" field to
contain exactly 128 octets and are insensitive to the length
attribute that is associated with the "original datagram" field.
Non-compliant applications were produced between 1999 and the time of
publication of this memo.
Compliant applications comply fully with the specifications of this
document.
In order to demonstrate backwards compatibility, Table 1 describes
how members of each application category would parse each category of
ICMP message.
+----------------+----------------+----------------+----------------+
| | No Extensions | Non-compliant | Compliant |
| | | Extensions | Extensions |
+----------------+----------------+----------------+----------------+
| Classic | - | Section 5.1 | Section 5.1 |
| Application | | | |
| | | | |
| Non-compliant | Section 5.2 | - | Section 5.3 |
| Application | | | |
| | | | |
| Compliant | Section 5.4 | Section 5.5 | - |
| Application | | | |
+----------------+----------------+----------------+----------------+
Table 1
In the table above, cells that contain a dash represent the nominal
case and require no explanation. In the following sections, we
assume that the ICMP message type is "Time Exceeded".
Bonica, et al. Standards Track [Page 11]
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5.1. Classic Application Receives ICMP Message with Extensions
When a classic application receives an ICMP message that includes
extensions, it will incorrectly interpret those extensions as being
part of the "original datagram" field. Fortunately, the extensions
are guaranteed to begin at least 128 octets beyond the beginning of
the "original datagram" field. So, only those ICMP applications that
process the 129th octet of the "original datagram" field will be
adversely effected. To date, only two applications falling into this
category have been identified, and the degree to which they are
effected is minimal.
Some TCP stacks, when they receive an ICMP message, verify the
checksum in the original datagram field [ATTACKS]. If the checksum
is incorrect, the TCP stack discards the ICMP message for security
reasons. If the trailing octets of the original datagram field are
overwritten by ICMP extensions, the TCP stack will discard an ICMP
message that it would not otherwise have discarded. The impact of
this issue is considered to be minimal because many ICMP messages are
discarded for other reasons (e.g., ICMP filtering, network
congestion, checksum was incorrect because original datagram field
was truncated.)
Another theoretically possible, but highly improbably scenario occurs
when ICMP extensions overwrite the portion of the original datagram
field that represents the TCP header, causing the TCP stack to
operate upon the wrong TCP connection. This scenario is highly
unlikely because it occurs only when the TCP header appears at or
beyond the 128th octet of the original datagram field and then only
when the extensions approximate a valid TCP header.
5.2. Non-Compliant Application Receives ICMP Message with No Extensions
When a non-compliant ICMPv4 application receives a message that
contains no extensions, the application examines the total length of
the ICMPv4 message. If the total ICMPv4 message length is less than
the length of its IP header plus 144 octets, the application
correctly determines that the message does not contain any
extensions.
The 144-octet sum is derived from 8 octets for the first two words of
the ICMPv4 Time Exceeded message, 128 octets for the "original
datagram" field, 4 octets for the ICMP Extension Header, and 4 octets
for a single ICMP Object header. All of these octets would be
required if extensions were present.
Bonica, et al. Standards Track [Page 12]
RFC 4884 Multi-Part ICMP Messages April 2007
If the ICMPv4 payload contains 144 octets or more, the application
must examine the 137th octet to determine whether it represents a
valid ICMPv4 Extension Header. In order to represent a valid
Extension Header, it must contain a valid version number and
checksum. If it does not contain a valid version number and
checksum, the application correctly determines that the message does
not contain any extensions.
Non-compliant applications assume that the ICMPv4 Extension Structure
begins on the 137th octet of the Time Exceeded message, after a
128-octet field representing the padded "original datagram" message.
It is possible that a non-compliant application will parse an ICMPv4
message incorrectly under the following conditions:
- the message does not contain extensions
- the original datagram field contains 144 octets or more
- selected octets of the original datagram field represent the
correct values for an extension header version number and
checksum
Although this is possible, it is very unlikely.
A similar analysis can be performed for ICMPv6. However, the numeric
constants would change as appropriate.
5.3. Non-Compliant Application Receives ICMP Message with Compliant
Extensions
When a non-compliant application receives a message that contains
compliant ICMP extensions, it will parse those extensions correctly
only if the "original datagram" field contains exactly 128 octets.
This is because non-compliant applications are insensitive to the
length attribute that is associated with the "original datagram"
field. (They assume its value to be 128.)
Provided that the entire ICMP message does not exceed the minimum
reassembly buffer size (576 octets for ICMPv4 or 1280 octets for
ICMPv6), there is no upper limit upon the length of the "original
datagram" field. However, each implementation will decide how many
octets to include. Those wishing to be backward compatible with non-
compliant TRACEROUTE implementations will include exactly 128 octets.
Those not requiring compatibility with non-compliant TRACEROUTE
applications may include more octets.
Bonica, et al. Standards Track [Page 13]
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5.4. Compliant Application Receives ICMP Message with No Extensions
When a compliant application receives an ICMP message, it examines
the length attribute that is associated with the "original datagram"
field. If the length attribute is zero, the compliant application
MUST determine that the message contains no extensions.
5.5. Compliant Application Receives ICMP Message with Non-Compliant
Extensions
When a compliant application receives an ICMP message, it examines
the length attribute that is associated with the "original datagram"
field. If the length attribute is zero, the compliant application
MUST determine that the message contains no extensions. In this
case, that determination is technically correct, but not backwards
compatible with the non-compliant implementation that originated the
ICMP message.
So, to ease transition yet encourage compliant implementation,
compliant TRACEROUTE implementations MUST include a non-default
operation mode to also interpret non-compliant responses.
Specifically, when a TRACEROUTE application operating in non-
compliant mode receives a sufficiently long ICMP message that does
not specify a length attribute, it will parse for a valid extension
header at a fixed location, assuming a 128-octet "original datagram"
field. If the application detects a valid version and checksum, it
will treat the octets that follow as an extension structure.
6. Interaction with Network Address Translation
The ICMP extensions defined in this memo do not interfere with
Network Address Translation. [RFC3022] permits traditional NAT
devices to modify selected fields within ICMP messages. These fields
include the "original datagram" field mentioned above. However, if a
NAT device modifies the "original datagram" field, it should modify
only the leading octets of that field, which represent the outermost
IP header. Because the outermost IP header is guaranteed to be
contained by the first 128 octets of the "original datagram" field,
ICMP extensions and NAT will not interfere with one another.
It is conceivable that a NAT implementation might overstep the
restrictions of RFC 3022 and overwrite the length attribute specified
by this memo. If a NAT implementation were to overwrite the length
attribute with zeros, the resulting packet will be indistinguishable
from a packet that was generated by a non-compliant ICMP
implementation. See Section 5.5 for packet details and a discussion
of backwards compatibility.
Bonica, et al. Standards Track [Page 14]
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7. The ICMP Extension Structure
This memo proposes an optional ICMP Extension Structure that can be
appended to the ICMP messages referenced in Section 4.6 of this
document.
The Extension Structure contains exactly one Extension Header
followed by one or more objects. Having received an ICMP message
with extensions, application software MAY process selected objects
while ignoring others. The presence of an unrecognized object does
not imply that an ICMP message is malformed.
As stated above, the total length of the ICMP message, including
extensions, MUST NOT exceed the minimum reassembly buffer size.
Figure 6 depicts the ICMP Extension Header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| (Reserved) | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: ICMP Extension Header
The fields of the ICMP Extension Header are as follows:
Version: 4 bits
ICMP extension version number. This is version 2.
Reserved: 12 bits
Must be set to 0.
Checksum: 16 bits
The one's complement of the one's complement sum of the data
structure, with the checksum field replaced by zero for the
purpose of computing the checksum. An all-zero value means that
no checksum was transmitted. See Section 5.2 for a description of
how this field is used.
Bonica, et al. Standards Track [Page 15]
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8. ICMP Extension Objects
Each extension object contains one or more 32-bit words, representing
an object header and payload. All object headers share a common
format. Figure 7 depicts the object header and payload.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Class-Num | C-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| // (Object payload) // |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Object Header and Payload
An object header has the following fields:
Length: 16 bits
Length of the object, measured in octets, including the object
header and object payload.
Class-Num: 8 bits
Identifies object class.
C-Type: 8 bits
Identifies object sub-type.
9. Security Considerations
Upon receipt of an ICMP message, application software must check it
for syntactic correctness. The extension checksum must be verified.
Improperly specified length attributes and other syntax problems may
result in buffer overruns.
This memo does not define the conditions under which a router sends
an ICMP message. Therefore, it does not expose routers to any new
denial-of-service attacks. Routers may need to limit the rate at
which ICMP messages are sent.
Bonica, et al. Standards Track [Page 16]
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10. IANA Considerations
The ICMP Extension Object header contains two 8-bit fields: The
Class-Num identifies the object class, and the C-Type identifies the
class sub-type. Sub-type values are defined relative to a specific
object class value, and are defined per class.
IANA has established a registry of ICMP extension objects classes and
class sub-types. There are no values assigned within this document
to maintain. Object classes 0xF7 - 0xFF are reserved for private
use. Object class values are assignable on a first-come-first-serve
basis. The policy for assigning sub-type values should be defined in
the document defining new class values.
11. Acknowledgments
Thanks to Pekka Nikander, Mark Doll, Fernando Gont, Joe Touch,
Christian Voiqt, and Sharon Chrisholm for their comments regarding
this document.
12. References
12.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD
5, RFC 792, September 1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC
1191, November 1990.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
March 2006.
12.2. Informative References
[UNNUMBERED] Atlas, A., Bonica, R., Rivers, JR., Shen, N., and E.
Chen, "ICMP Extensions for Unnumbered Interfaces",
Work in Progress, March 2007.
Bonica, et al. Standards Track [Page 17]
RFC 4884 Multi-Part ICMP Messages April 2007
[MPLS-ICMP] Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
"ICMP Extensions for MultiProtocol Label Switching",
Work in Progress, January 2007.
[ATTACKS] Gont, F., "ICMP attacks against TCP", Work in
Progress, October 2006.
[ROUTING-INST] Shen, N. and E. Chen, "ICMP Extensions for Routing
Instances", Work in Progress, November 2006.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
Authors' Addresses
Ronald P. Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
US
EMail: rbonica@juniper.net
Der-Hwa Gan
Consultant
EMail: derhwagan@yahoo.com
Daniel C. Tappan
Consultant
EMail: Dan.Tappan@gmail.com
Carlos Pignataro
Cisco Systems, Inc.
7025 Kit Creek Road
Research Triangle Park, NC 27709
US
EMail: cpignata@cisco.com
Bonica, et al. Standards Track [Page 18]
RFC 4884 Multi-Part ICMP Messages April 2007
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Bonica, et al. Standards Track [Page 19]
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