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icmp-metarfc.txt
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INTERNET CONTROL MESSAGE PROTOCOL
Introduction
The Internet Protocol (IP) [1] is used for host-to-host datagram
service in a system of interconnected networks called the
Catenet [2]. The network connecting devices are called Gateways.
These gateways communicate between themselves for control purposes
via a Gateway to Gateway Protocol (GGP) [3,4]. Occasionally a
gateway or destination host will communicate with a source host, for
example, to report an error in datagram processing. For such
purposes this protocol, the Internet Control Message Protocol (ICMP),
is used. ICMP, uses the basic support of IP as if it were a higher
level protocol, however, ICMP is actually an integral part of IP, and
must be implemented by every IP module.
ICMP messages are sent in several situations: for example, when a
datagram cannot reach its destination, when the gateway does not have
the buffering capacity to forward a datagram, and when the gateway
can direct the host to send traffic on a shorter route.
The Internet Protocol is not designed to be absolutely reliable. The
purpose of these control messages is to provide feedback about
problems in the communication environment, not to make IP reliable.
There are still no guarantees that a datagram will be delivered or a
control message will be returned. Some datagrams may still be
undelivered without any report of their loss. The higher level
protocols that use IP must implement their own reliability procedures
if reliable communication is required.
The ICMP messages typically report errors in the processing of
datagrams. To avoid the infinite regress of messages about messages
etc., no ICMP messages are sent about ICMP messages. Also ICMP
messages are only sent about errors in handling fragment zero of
fragmented datagrams. (Fragment zero has the fragment offeset equal
zero).
Message Formats
ICMP messages are sent using the basic IP header. The first octet of
the data portion of the datagram is a ICMP type field; the value of
this field determines the format of the remaining data. Any field
labeled "unused" is reserved for later extensions and must be zero
when sent, but receivers should not use these fields (except to
include them in the checksum). Unless otherwise noted under the
individual format descriptions, the values of the internet header
fields are as follows:
Version
4
IHL
Internet header length in 32-bit words.
Type of Service
0
Total Length
Length of internet header and data in octets.
Identification, Flags, Fragment Offset
Used in fragmentation, see [1].
Time to Live
Time to live in seconds; as this field is decremented at each
machine in which the datagram is processed, the value in this
field should be at least as great as the number of gateways which
this datagram will traverse.
Protocol
ICMP = 1
Header Checksum
The 16 bit one's complement of the one's complement sum of all 16
bit words in the header. For computing the checksum, the checksum
field should be zero. This checksum may be replaced in the
future.
Source Address
The address of the gateway or host that composes the ICMP message.
Unless otherwise noted, this can be any of a gateway's addresses.
Destination Address
The address of the gateway or host to which the message should be
sent.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Destination Address
The source network and address from the original datagram's data.
ICMP Fields:
Type
3
Code
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable;
3 = port unreachable;
4 = fragmentation needed and DF set;
5 = source route failed.
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP Type.
For computing the checksum , the checksum field should be zero.
This checksum may be replaced in the future.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
If, according to the information in the gateway's routing tables,
the network specified in the internet destination field of a
datagram is unreachable, e.g., the distance to the network is
infinity, the gateway may send a destination unreachable message
to the internet source host of the datagram. In addition, in some
networks, the gateway may be able to determine if the internet
destination host is unreachable. Gateways in these networks may
send destination unreachable messages to the source host when the
destination host is unreachable.
If, in the destination host, the IP module cannot deliver the
datagram because the indicated protocol module or process port is
not active, the destination host may send a destination
unreachable message to the source host.
Another case is when a datagram must be fragmented to be forwarded
by a gateway yet the Don't Fragment flag is on. In this case the
gateway must discard the datagram and may return a destination
unreachable message.
Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and
3 may be received from a host.
Requirements for Internet Hosts
The following additional codes are hereby defined:
6 = destination network unknown.
7 = destination host unknown.
8 = source host isolated.
9 = communication with destination network
administratively prohibited.
10 = communication with destination host
administratively prohibited.
11 = network unreachable for type of service.
12 = host unreachable for type of service.
A host SHOULD generate Destination Unreachable messages with
code:
2 (Protocol Unreachable), when the designated transport
protocol is not supported; or
3 (Port Unreachable), when the designated transport
protocol (e.g., UDP) is unable to demultiplex the
datagram but has no protocol mechanism to inform the
sender.
A Destination Unreachable message that is received MUST be
reported to the transport layer. The transport layer SHOULD
use the information appropriately; for example, see Sections
4.1.3.3, 4.2.3.9, and 4.2.4 below. A transport protocol
that has its own mechanism for notifying the sender that a
port is unreachable (e.g., TCP, which sends RST segments)
MUST nevertheless accept an ICMP Port Unreachable for the
same purpose.
A Destination Unreachable message that is received with code
0 (Net), 1 (Host), or 5 (Bad Source Route) may result from a
routing transient and MUST therefore be interpreted as only
a hint, not proof, that the specified destination is
unreachable [IP:11]. For example, it MUST NOT be used as
proof of a dead gateway (see Section 3.3.1).
Requirements for IP Version 4 Routers
If a router cannot forward a packet because it has no routes at all
(including no default route) to the destination specified in the
packet, then the router MUST generate a Destination Unreachable, Code
0 (Network Unreachable) ICMP message. If the router does have routes
to the destination network specified in the packet but the TOS
specified for the routes is neither the default TOS (0000) nor the
TOS of the packet that the router is attempting to route, then the
router MUST generate a Destination Unreachable, Code 11 (Network
Unreachable for TOS) ICMP message.
If a packet is to be forwarded to a host on a network that is
directly connected to the router (i.e., the router is the last-hop
router) and the router has ascertained that there is no path to the
destination host then the router MUST generate a Destination
Unreachable, Code 1 (Host Unreachable) ICMP message. If a packet is
to be forwarded to a host that is on a network that is directly
connected to the router and the router cannot forward the packet
because no route to the destination has a TOS that is either equal to
the TOS requested in the packet or is the default TOS (0000) then the
router MUST generate a Destination Unreachable, Code 12 (Host
Unreachable for TOS) ICMP message.
DISCUSSION
The intent is that a router generates the "generic" host/network
unreachable if it has no path at all (including default routes) to
the destination. If the router has one or more paths to the
destination, but none of those paths have an acceptable TOS, then
the router generates the "unreachable for TOS" message.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Destination Address
The source network and address from the original datagram's data.
ICMP Fields:
Type
11
Code
0 = time to live exceeded in transit;
1 = fragment reassembly time exceeded.
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP Type.
For computing the checksum , the checksum field should be zero.
This checksum may be replaced in the future.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
If the gateway processing a datagram finds the time to live field
is zero it must discard the datagram. The gateway may also notify
the source host via the time exceeded message.
If a host reassembling a fragmented datagram cannot complete the
reassembly due to missing fragments within its time limit it
discards the datagram, and it may send a time exceeded message.
If fragment zero is not available then no time exceeded need be
sent at all.
Code 0 may be received from a gateway. Code 1 may be received
from a host.
Requirements for Internet Hosts
An incoming Time Exceeded message MUST be passed to the
transport layer.
DISCUSSION:
A gateway will send a Time Exceeded Code 0 (In Transit)
message when it discards a datagram due to an expired
TTL field. This indicates either a gateway routing
loop or too small an initial TTL value.
A host may receive a Time Exceeded Code 1 (Reassembly
Timeout) message from a destination host that has timed
out and discarded an incomplete datagram; see Section
3.3.2 below. In the future, receipt of this message
might be part of some "MTU discovery" procedure, to
discover the maximum datagram size that can be sent on
the path without fragmentation.
Requirements for IP Version 4 Routers
When a router is forwarding a packet and the TTL field of the packet
is reduced to 0, the requirements of section [5.2.3.8] apply.
When the router is reassembling a packet that is destined for the
router, it is acting as an Internet host. [INTRO:2]'s reassembly
requirements therefore apply.
When the router receives (i.e., is destined for the router) a Time
Exceeded message, it MUST comply with [INTRO:2].
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 | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Destination Address
The source network and address from the original datagram's data.
ICMP Fields:
Type
12
Code
0 = pointer indicates the error.
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP Type.
For computing the checksum , the checksum field should be zero.
This checksum may be replaced in the future.
Pointer
If code = 0, identifies the octet where an error was detected.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
If the gateway or host processing a datagram finds a problem with
the header parameters such that it cannot complete processing the
datagram it must discard the datagram. One potential source of
such a problem is with incorrect arguments in an option. The
gateway or host may also notify the source host via the parameter
problem message. This message is only sent if the error caused
the datagram to be discarded.
The pointer identifies the octet of the original datagram's header
where the error was detected (it may be in the middle of an
option). For example, 1 indicates something is wrong with the
Type of Service, and (if there are options present) 20 indicates
something is wrong with the type code of the first option.
Code 0 may be received from a gateway or a host.
Requirements for Internet Hosts
A host SHOULD generate Parameter Problem messages. An
incoming Parameter Problem message MUST be passed to the
transport layer, and it MAY be reported to the user.
DISCUSSION:
The ICMP Parameter Problem message is sent to the
source host for any problem not specifically covered by
another ICMP message. Receipt of a Parameter Problem
message generally indicates some local or remote
implementation error.
A new variant on the Parameter Problem message is hereby
defined:
Code 1 = required option is missing.
DISCUSSION:
This variant is currently in use in the military
community for a missing security option.
Requirements for IP Version 4 Routers
A router MUST generate a Parameter Problem message for any error not
specifically covered by another ICMP message. The IP header field or
IP option including the byte indicated by the pointer field MUST be
included unchanged in the IP header returned with this ICMP message.
Section [4.3.2] defines an exception to this requirement.
A new variant of the Parameter Problem message was defined in
[INTRO:2]:
Code 1 = required option is missing.
DISCUSSION
This variant is currently in use in the military community for a
missing security option.
Source Quench 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Destination Address
The source network and address of the original datagram's data.
ICMP Fields:
Type
4
Code
0
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP Type.
For computing the checksum , the checksum field should be zero.
This checksum may be replaced in the future.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
A gateway may discard internet datagrams if it does not have the
buffer space needed to queue the datagrams for output to the next
network on the route to the destination network. If a gateway
discards a datagram, it may send a source quench message to the
internet source host of the datagram. A destination host may also
send a source quench message if datagrams arrive too fast to be
processed. The source quench message is a request to the host to
cut back the rate at which it is sending traffic to the internet
destination. The gateway may send a source quench message for
every message that it discards. On receipt of a source quench
message, the source host should cut back the rate at which it is
sending traffic to the specified destination until it no longer
receives source quench messages from the gateway. The source host
can then gradually increase the rate at which it sends traffic to
the destination until it again receives source quench messages.
The gateway or host may send the source quench message when it
approaches its capacity limit rather than waiting until the
capacity is exceeded. This means that the data datagram which
triggered the source quench message may be delivered.
Code 0 may be received from a gateway or a host.
Requirements for Internet Hosts
A host MAY send a Source Quench message if it is
approaching, or has reached, the point at which it is forced
to discard incoming datagrams due to a shortage of
reassembly buffers or other resources. See Section 2.2.3 of
[INTRO:2] for suggestions on when to send Source Quench.
If a Source Quench message is received, the IP layer MUST
report it to the transport layer (or ICMP processing). In
general, the transport or application layer SHOULD implement
a mechanism to respond to Source Quench for any protocol
that can send a sequence of datagrams to the same
destination and which can reasonably be expected to maintain
enough state information to make this feasible. See Section
4 for the handling of Source Quench by TCP and UDP.
DISCUSSION:
A Source Quench may be generated by the target host or
by some gateway in the path of a datagram. The host
receiving a Source Quench should throttle itself back
for a period of time, then gradually increase the
transmission rate again. The mechanism to respond to
Source Quench may be in the transport layer (for
connection-oriented protocols like TCP) or in the
application layer (for protocols that are built on top
of UDP).
A mechanism has been proposed [IP:14] to make the IP
layer respond directly to Source Quench by controlling
the rate at which datagrams are sent, however, this
proposal is currently experimental and not currently
recommended.
Requirements for IP Version 4 Routers
A router SHOULD NOT originate ICMP Source Quench messages. As
specified in Section [4.3.2], a router that does originate Source
Quench messages MUST be able to limit the rate at which they are
generated.
DISCUSSION
Research seems to suggest that Source Quench consumes network
bandwidth but is an ineffective (and unfair) antidote to
congestion. See, for example, [INTERNET:9] and [INTERNET:10].
Section [5.3.6] discusses the current thinking on how routers
ought to deal with overload and network congestion.
A router MAY ignore any ICMP Source Quench messages it receives.
DISCUSSION
A router itself may receive a Source Quench as the result of
originating a packet sent to another router or host. Such
datagrams might be, e.g., an EGP update sent to another router, or
a telnet stream sent to a host. A mechanism has been proposed
([INTERNET:11], [INTERNET:12]) to make the IP layer respond
directly to Source Quench by controlling the rate at which packets
are sent, however, this proposal is currently experimental and not
currently recommended.
Redirect 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gateway Internet Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Destination Address
The source network and address of the original datagram's data.
ICMP Fields:
Type
5
Code
0 = Redirect datagrams for the Network.
1 = Redirect datagrams for the Host.
2 = Redirect datagrams for the Type of Service and Network.
3 = Redirect datagrams for the Type of Service and Host.
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP Type.
For computing the checksum , the checksum field should be zero.
This checksum may be replaced in the future.
Gateway Internet Address
Address of the gateway to which traffic for the network specified
in the internet destination network field of the original
datagram's data should be sent.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
The gateway sends a redirect message to a host in the following
situation. A gateway, G1, receives an internet datagram from a
host on a network to which the gateway is attached. The gateway,
G1, checks its routing table and obtains the address of the next
gateway, G2, on the route to the datagram's internet destination
network, X. If G2 and the host identified by the internet source
address of the datagram are on the same network, a redirect
message is sent to the host. The redirect message advises the
host to send its traffic for network X directly to gateway G2 as
this is a shorter path to the destination. The gateway forwards
the original datagram's data to its internet destination.
For datagrams with the IP source route options and the gateway
address in the destination address field, a redirect message is
not sent even if there is a better route to the ultimate
destination than the next address in the source route.
Codes 0, 1, 2, and 3 may be received from a gateway.
Requirements for Internet Hosts
A host SHOULD NOT send an ICMP Redirect message; Redirects
are to be sent only by gateways.
A host receiving a Redirect message MUST update its routing
information accordingly. Every host MUST be prepared to
accept both Host and Network Redirects and to process them
as described in Section 3.3.1.2 below.
A Redirect message SHOULD be silently discarded if the new
gateway address it specifies is not on the same connected
(sub-) net through which the Redirect arrived [INTRO:2,
Appendix A], or if the source of the Redirect is not the
current first-hop gateway for the specified destination (see
Section 3.3.1).
Requirements for IP Version 4 Routers
The ICMP Redirect message is generated to inform a local host that it
should use a different next hop router for certain traffic.
Contrary to [INTRO:2], a router MAY ignore ICMP Redirects when
choosing a path for a packet originated by the router if the router
is running a routing protocol or if forwarding is enabled on the
router and on the interface over which the packet is being sent.
Echo or Echo Reply 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-
IP Fields:
Addresses
The address of the source in an echo message will be the
destination of the echo reply message. To form an echo reply
message, the source and destination addresses are simply reversed,
the type code changed to 0, and the checksum recomputed.
IP Fields:
Type
8 for echo message;
0 for echo reply message.
Code
0
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP Type.
For computing the checksum , the checksum field should be zero.
If the total length is odd, the received data is padded with one
octet of zeros for computing the checksum. This checksum may be
replaced in the future.
Identifier
If code = 0, an identifier to aid in matching echos and replies,
may be zero.
Sequence Number
If code = 0, a sequence number to aid in matching echos and
replies, may be zero.
Description
The data received in the echo message must be returned in the echo
reply message.
The identifier and sequence number may be used by the echo sender
to aid in matching the replies with the echo requests. For
example, the identifier might be used like a port in TCP or UDP to
identify a session, and the sequence number might be incremented
on each echo request sent. The echoer returns these same values
in the echo reply.
Code 0 may be received from a gateway or a host.
Requirements for Internet Hosts
Every host MUST implement an ICMP Echo server function that
receives Echo Requests and sends corresponding Echo Replies.
A host SHOULD also implement an application-layer interface
for sending an Echo Request and receiving an Echo Reply, for
diagnostic purposes.
An ICMP Echo Request destined to an IP broadcast or IP
multicast address MAY be silently discarded.
DISCUSSION:
This neutral provision results from a passionate debate
between those who feel that ICMP Echo to a broadcast
address provides a valuable diagnostic capability and
those who feel that misuse of this feature can too
easily create packet storms.
The IP source address in an ICMP Echo Reply MUST be the same
as the specific-destination address (defined in Section
3.2.1.3) of the corresponding ICMP Echo Request message.
Data received in an ICMP Echo Request MUST be entirely
included in the resulting Echo Reply. However, if sending
the Echo Reply requires intentional fragmentation that is
not implemented, the datagram MUST be truncated to maximum
transmission size (see Section 3.3.3) and sent.
Echo Reply messages MUST be passed to the ICMP user
interface, unless the corresponding Echo Request originated
in the IP layer.
If a Record Route and/or Time Stamp option is received in an
ICMP Echo Request, this option (these options) SHOULD be
updated to include the current host and included in the IP
header of the Echo Reply message, without "truncation".
Thus, the recorded route will be for the entire round trip.
If a Source Route option is received in an ICMP Echo
Request, the return route MUST be reversed and used as a
Source Route option for the Echo Reply message.
Requirements for IP Version 4 Routers
A router MUST implement an ICMP Echo server function that receives
Echo Requests sent to the router, and sends corresponding Echo
Replies. A router MUST be prepared to receive, reassemble and echo
an ICMP Echo Request datagram at least as the maximum of 576 and the
MTUs of all the connected networks.
The Echo server function MAY choose not to respond to ICMP echo
requests addressed to IP broadcast or IP multicast addresses.
A router SHOULD have a configuration option that, if enabled, causes
the router to silently ignore all ICMP echo requests; if provided,
this option MUST default to allowing responses.
DISCUSSION
The neutral provision about responding to broadcast and multicast
Echo Requests derives from [INTRO:2]'s "Echo Request/Reply"
section.
As stated in Section [10.3.3], a router MUST also implement a
user/application-layer interface for sending an Echo Request and
receiving an Echo Reply, for diagnostic purposes. All ICMP Echo
Reply messages MUST be passed to this interface.
The IP source address in an ICMP Echo Reply MUST be the same as the
specific-destination address of the corresponding ICMP Echo Request
message.
Data received in an ICMP Echo Request MUST be entirely included in
the resulting Echo Reply.
If a Record Route and/or Timestamp option is received in an ICMP Echo
Request, this option (these options) SHOULD be updated to include the
current router and included in the IP header of the Echo Reply
message, without truncation. Thus, the recorded route will be for
the entire round trip.
If a Source Route option is received in an ICMP Echo Request, the
return route MUST be reversed and used as a Source Route option for
the Echo Reply message, unless the router is aware of policy that
would prevent the delivery of the message.
Timestamp or Timestamp Reply 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originate Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transmit Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
Addresses
The address of the source in a timestamp message will be the
destination of the timestamp reply message. To form a timestamp
reply message, the source and destination addresses are simply
reversed, the type code changed to 14, and the checksum
recomputed.
ICMP Fields:
Type
13 for timestamp message;
14 for timestamp reply message.
Code
0
Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the ICMP message starting with the ICMP Type.
For computing the checksum , the checksum field should be zero.
This checksum may be replaced in the future.
Identifier
If code = 0, an identifier to aid in matching timestamp and
replies, may be zero.
Sequence Number
If code = 0, a sequence number to aid in matching timestamp and
replies, may be zero.
Description
The data received (a timestamp) in the message is returned in the
reply together with an additional timestamp. The timestamp is 32
bits of milliseconds since midnight UT. One use of these
timestamps is described by Mills [5].
The Originate Timestamp is the time the sender last touched the
message before sending it, the Receive Timestamp is the time the
echoer first touched it on receipt, and the Transmit Timestamp is
the time the echoer last touched the message on sending it.
If the time is not available in miliseconds or cannot be provided
with respect to midnight UT then any time can be inserted in a
timestamp provided the high order bit of the timestamp is also set
to indicate this non-standard value.
The identifier and sequence number may be used by the echo sender
to aid in matching the replies with the requests. For example,
the identifier might be used like a port in TCP or UDP to identify
a session, and the sequence number might be incremented on each
request sent. The destination returns these same values in the
reply.
Code 0 may be received from a gateway or a host.
Requirements for Internet Hosts
A host MAY implement Timestamp and Timestamp Reply. If they
are implemented, the following rules MUST be followed.
o The ICMP Timestamp server function returns a Timestamp
Reply to every Timestamp message that is received. If
this function is implemented, it SHOULD be designed for
minimum variability in delay (e.g., implemented in the
kernel to avoid delay in scheduling a user process).
The following cases for Timestamp are to be handled
according to the corresponding rules for ICMP Echo:
o An ICMP Timestamp Request message to an IP broadcast or
IP multicast address MAY be silently discarded.
o The IP source address in an ICMP Timestamp Reply MUST
be the same as the specific-destination address of the
corresponding Timestamp Request message.
o If a Source-route option is received in an ICMP Echo
Request, the return route MUST be reversed and used as
a Source Route option for the Timestamp Reply message.
o If a Record Route and/or Timestamp option is received
in a Timestamp Request, this (these) option(s) SHOULD
be updated to include the current host and included in
the IP header of the Timestamp Reply message.
o Incoming Timestamp Reply messages MUST be passed up to
the ICMP user interface.
The preferred form for a timestamp value (the "standard
value") is in units of milliseconds since midnight Universal
Time. However, it may be difficult to provide this value
with millisecond resolution. For example, many systems use
clocks that update only at line frequency, 50 or 60 times
per second. Therefore, some latitude is allowed in a
"standard value":
(a) A "standard value" MUST be updated at least 15 times
per second (i.e., at most the six low-order bits of the
value may be undefined).
(b) The accuracy of a "standard value" MUST approximate
that of operator-set CPU clocks, i.e., correct within a
few minutes.
Requirements for IP Version 4 Routers
A router MAY implement Timestamp and Timestamp Reply. If they are
implemented then:
o The ICMP Timestamp server function MUST return a Timestamp Reply to
every Timestamp message that is received. It SHOULD be designed
for minimum variability in delay.
o An ICMP Timestamp Request message to an IP broadcast or IP
multicast address MAY be silently discarded.
o The IP source address in an ICMP Timestamp Reply MUST be the same
as the specific-destination address of the corresponding Timestamp
Request message.
o If a Source Route option is received in an ICMP Timestamp Request,
the return route MUST be reversed and used as a Source Route
option for the Timestamp Reply message, unless the router is aware
of policy that would prevent the delivery of the message.
o If a Record Route and/or Timestamp option is received in a
Timestamp Request, this (these) option(s) SHOULD be updated to
include the current router and included in the IP header of the
Timestamp Reply message.
o If the router provides an application-layer interface for sending