3. INTERNET LAYER PROTOCOLS
3.1 INTRODUCTION
The Robustness Principle: "Be liberal in what you accept, and conservative in what you send" is particularly important in the Internet layer, where one misbehaving host can deny Internet service to many other hosts.
The protocol standards used in the Internet layer are:
o RFC-791 [IP:1] defines the IP protocol and gives an
introduction to the architecture of the Internet.
o RFC-792 [IP:2] defines ICMP, which provides routing,
diagnostic and error functionality for IP. Although ICMP
messages are encapsulated within IP datagrams, ICMP
processing is considered to be (and is typically implemented
as) part of the IP layer. See Section 3.2.2.
o RFC-950 [IP:3] defines the mandatory subnet extension to the
addressing architecture.
o RFC-1112 [IP:4] defines the Internet Group Management
Protocol IGMP, as part of a recommended extension to hosts
and to the host-gateway interface to support Internet-wide
multicasting at the IP level. See Section 3.2.3.
The target of an IP multicast may be an arbitrary group of
Internet hosts. IP multicasting is designed as a natural
extension of the link-layer multicasting facilities of some
networks, and it provides a standard means for local access
to such link-layer multicasting facilities.
Other important references are listed in Section 5 of this document.
The Internet layer of host software MUST implement both IP and ICMP. See Section 3.3.7 for the requirements on support of IGMP.
The host IP layer has two basic functions: (1) choose the "next hop" gateway or host for outgoing IP datagrams and (2) reassemble incoming IP datagrams. The IP layer may also (3) implement intentional fragmentation of outgoing datagrams. Finally, the IP layer must (4) provide diagnostic and error functionality. We expect that IP layer functions may increase somewhat in the future, as further Internet control and management facilities are developed.
For normal datagrams, the processing is straightforward. For incoming datagrams, the IP layer:
(1) verifies that the datagram is correctly formatted;
(2) verifies that it is destined to the local host;
(3) processes options;
(4) reassembles the datagram if necessary; and
(5) passes the encapsulated message to the appropriate transport-layer protocol module.
For outgoing datagrams, the IP layer:
(1) sets any fields not set by the transport layer;
(2) selects the correct first hop on the connected network (a process called "routing");
(3) fragments the datagram if necessary and if intentional fragmentation is implemented (see Section 3.3.3); and
(4) passes the packet(s) to the appropriate link-layer driver.
A host is said to be multihomed if it has multiple IP addresses. Multihoming introduces considerable confusion and complexity into the protocol suite, and it is an area in which the Internet architecture falls seriously short of solving all problems. There are two distinct problem areas in multihoming:
(1) Local multihoming -- the host itself is multihomed; or
(2) Remote multihoming -- the local host needs to communicate with a remote multihomed host.
At present, remote multihoming MUST be handled at the application layer, as discussed in the companion RFC [INTRO:1]. A host MAY support local multihoming, which is discussed in this document, and in particular in Section 3.3.4.
Any host that forwards datagrams generated by another host is acting as a gateway and MUST also meet the specifications laid out in the gateway requirements RFC [INTRO:2]. An Internet host that includes embedded gateway code MUST have a configuration switch to disable the gateway function, and this switch MUST default to the
non-gateway mode. In this mode, a datagram arriving through one interface will not be forwarded to another host or gateway (unless it is source-routed), regardless of whether the host is single- homed or multihomed. The host software MUST NOT automatically move into gateway mode if the host has more than one interface, as the operator of the machine may neither want to provide that service nor be competent to do so.
In the following, the action specified in certain cases is to "silently discard" a received datagram. This means that the datagram will be discarded without further processing and that the host will not send any ICMP error message (see Section 3.2.2) as a result. However, for diagnosis of problems a host SHOULD provide the capability of logging the error (see Section 1.2.3), including the contents of the silently-discarded datagram, and SHOULD record the event in a statistics counter.
DISCUSSION: Silent discard of erroneous datagrams is generally intended to prevent "broadcast storms".
3.2 PROTOCOL WALK-THROUGH
3.2.1 Internet Protocol -- IP
3.2.1.1 Version Number: RFC-791 Section 3.1
A datagram whose version number is not 4 **MUST** be silently
discarded.
3.2.1.2 Checksum: RFC-791 Section 3.1
A host **MUST** verify the IP header checksum on every received
datagram and silently discard every datagram that has a bad
checksum.
3.2.1.3 Addressing: RFC-791 Section 3.2
There are now five classes of IP addresses: Class A through
Class E. Class D addresses are used for IP multicasting
[IP:4], while Class E addresses are reserved for
experimental use.
A multicast (Class D) address is a 28-bit logical address
that stands for a group of hosts, and may be either
permanent or transient. Permanent multicast addresses are
allocated by the Internet Assigned Number Authority
[INTRO:6], while transient addresses may be allocated
dynamically to transient groups. Group membership is
determined dynamically using IGMP [IP:4].
We now summarize the important special cases for Class A, B,
and C IP addresses, using the following notation for an IP
address:
`{ <Network-number>, <Host-number> }`
or
`{ <Network-number>, <Subnet-number>, <Host-number> }`
and the notation "-1" for a field that contains all 1 bits.
This notation is not intended to imply that the 1-bits in an
address mask need be contiguous.
(a) `{ 0, 0 }`
This host on this network. **MUST** NOT be sent, except as
a source address as part of an initialization procedure
by which the host learns its own IP address.
See also Section 3.3.6 for a non-standard use of `{0,0}`.
(b) `{ 0, <Host-number> }`
Specified host on this network. It **MUST** NOT be sent,
except as a source address as part of an initialization
procedure by which the host learns its full IP address.
(c) `{ -1, -1 }`
Limited broadcast. It **MUST** NOT be used as a source
address.
A datagram with this destination address will be
received by every host on the connected physical
network but will not be forwarded outside that network.
(d) `{ <Network-number>, -1 }`
Directed broadcast to the specified network. It **MUST**
NOT be used as a source address.
(e) `{ <Network-number>, <Subnet-number>, -1 }`
Directed broadcast to the specified subnet. It **MUST**
NOT be used as a source address.
(f) `{ <Network-number>, -1, -1 }`
Directed broadcast to all subnets of the specified
subnetted network. It **MUST** NOT be used as a source
address.
(g) `{ 127, <any> }`
Internal host loopback address. Addresses of this form
**MUST** NOT appear outside a host.
The `<Network-number>` is administratively assigned so that
its value will be unique in the entire world.
IP addresses are not permitted to have the value 0 or -1 for
any of the `<Host-number>`, `<Network-number>`, or `<Subnet-number>` fields (except in the special cases listed above).
This implies that each of these fields will be at least two
bits long.
For further discussion of broadcast addresses, see Section
3.3.6.
A host **MUST** support the subnet extensions to IP [IP:3]. As
a result, there will be an address mask of the form:
`{-1, -1, 0}` associated with each of the host's local IP
addresses; see Sections 3.2.2.9 and 3.3.1.1.
When a host sends any datagram, the IP source address **MUST**
be one of its own IP addresses (but not a broadcast or
multicast address).
A host **MUST** silently discard an incoming datagram that is
not destined for the host. An incoming datagram is destined
for the host if the datagram's destination address field is:
(1) (one of) the host's IP address(es); or
(2) an IP broadcast address valid for the connected
network; or
(3) the address for a multicast group of which the host is
a member on the incoming physical interface.
For most purposes, a datagram addressed to a broadcast or
multicast destination is processed as if it had been
addressed to one of the host's IP addresses; we use the term
"specific-destination address" for the equivalent local IP
address of the host. The specific-destination address is
defined to be the destination address in the IP header
unless the header contains a broadcast or multicast address,
in which case the specific-destination is an IP address
assigned to the physical interface on which the datagram
arrived.
A host **MUST** silently discard an incoming datagram containing
an IP source address that is invalid by the rules of this
section. This validation could be done in either the IP
layer or by each protocol in the transport layer.
DISCUSSION:
A mis-addressed datagram might be caused by a link-
layer broadcast of a unicast datagram or by a gateway
or host that is confused or mis-configured.
An architectural goal for Internet hosts was to allow
IP addresses to be featureless 32-bit numbers, avoiding
algorithms that required a knowledge of the IP address
format. Otherwise, any future change in the format or
interpretation of IP addresses will require host
software changes. However, validation of broadcast and
multicast addresses violates this goal; a few other
violations are described elsewhere in this document.
Implementers should be aware that applications
depending upon the all-subnets directed broadcast
address (f) may be unusable on some networks. All-
subnets broadcast is not widely implemented in vendor
gateways at present, and even when it is implemented, a
particular network administration may disable it in the
gateway configuration.
3.2.1.4 Fragmentation and Reassembly: RFC-791 Section 3.2
The Internet model requires that every host support
reassembly. See Sections 3.3.2 and 3.3.3 for the
requirements on fragmentation and reassembly.
3.2.1.5 Identification: RFC-791 Section 3.2
When sending an identical copy of an earlier datagram, a
host **MAY** optionally retain the same Identification field in
the copy.
DISCUSSION:
Some Internet protocol experts have maintained that
when a host sends an identical copy of an earlier
datagram, the new copy should contain the same
Identification value as the original. There are two
suggested advantages: (1) if the datagrams are
fragmented and some of the fragments are lost, the
receiver may be able to reconstruct a complete datagram
from fragments of the original and the copies; (2) a
congested gateway might use the IP Identification field
(and Fragment Offset) to discard duplicate datagrams
from the queue.
However, the observed patterns of datagram loss in the
Internet do not favor the probability of retransmitted
fragments filling reassembly gaps, while other
mechanisms (e.g., TCP repacketizing upon
retransmission) tend to prevent retransmission of an
identical datagram [IP:9]. Therefore, we believe that
retransmitting the same Identification field is not
useful. Also, a connectionless transport protocol like
UDP would require the cooperation of the application
programs to retain the same Identification value in
identical datagrams.
3.2.1.6 Type-of-Service: RFC-791 Section 3.2
The "Type-of-Service" byte in the IP header is divided into
two sections: the Precedence field (high-order 3 bits), and
a field that is customarily called "Type-of-Service" or
"TOS" (low-order 5 bits). In this document, all references
to "TOS" or the "TOS field" refer to the low-order 5 bits
only.
The Precedence field is intended for Department of Defense
applications of the Internet protocols. The use of non-zero
values in this field is outside the scope of this document
and the IP standard specification. Vendors should consult
the Defense Communication Agency (DCA) for guidance on the
IP Precedence field and its implications for other protocol
layers. However, vendors should note that the use of
precedence will most likely require that its value be passed
between protocol layers in just the same way as the TOS
field is passed.
The IP layer **MUST** provide a means for the transport layer to
set the TOS field of every datagram that is sent; the
default is all zero bits. The IP layer **SHOULD** pass received
TOS values up to the transport layer.
The particular link-layer mappings of TOS contained in RFC-
795 **SHOULD** NOT be implemented.
DISCUSSION:
While the TOS field has been little used in the past,
it is expected to play an increasing role in the near
future. The TOS field is expected to be used to
control two aspects of gateway operations: routing and
queueing algorithms. See Section 2 of [INTRO:1] for
the requirements on application programs to specify TOS
values.
The TOS field may also be mapped into link-layer
service selectors. This has been applied to provide
effective sharing of serial lines by different classes
of TCP traffic, for example. However, the mappings
suggested in RFC-795 for networks that were included in
the Internet as of 1981 are now obsolete.
3.2.1.7 Time-to-Live: RFC-791 Section 3.2
A host **MUST** NOT send a datagram with a Time-to-Live (TTL)
value of zero.
A host **MUST** NOT discard a datagram just because it was
received with TTL less than 2.
The IP layer **MUST** provide a means for the transport layer to
set the TTL field of every datagram that is sent. When a
fixed TTL value is used, it **MUST** be configurable. The
current suggested value will be published in the "Assigned
Numbers" RFC.
DISCUSSION:
The TTL field has two functions: limit the lifetime of
TCP segments (see RFC-793 [TCP:1], p. 28), and
terminate Internet routing loops. Although TTL is a
time in seconds, it also has some attributes of a hop-
count, since each gateway is required to reduce the TTL
field by at least one.
The intent is that TTL expiration will cause a datagram
to be discarded by a gateway but not by the destination
host; however, hosts that act as gateways by forwarding
datagrams must follow the gateway rules for TTL.
A higher-layer protocol may want to set the TTL in
order to implement an "expanding scope" search for some
Internet resource. This is used by some diagnostic
tools, and is expected to be useful for locating the
"nearest" server of a given class using IP
multicasting, for example. A particular transport
protocol may also want to specify its own TTL bound on
maximum datagram lifetime.
A fixed value must be at least big enough for the
Internet "diameter," i.e., the longest possible path.
A reasonable value is about twice the diameter, to
allow for continued Internet growth.
3.2.1.8 Options: RFC-791 Section 3.2
There **MUST** be a means for the transport layer to specify IP
options to be included in transmitted IP datagrams (see
Section 3.4).
All IP options (except NOP or END-OF-LIST) received in
datagrams **MUST** be passed to the transport layer (or to ICMP
processing when the datagram is an ICMP message). The IP
and transport layer **MUST** each interpret those IP options
that they understand and silently ignore the others.
Later sections of this document discuss specific IP option
support required by each of ICMP, TCP, and UDP.
DISCUSSION:
Passing all received IP options to the transport layer
is a deliberate "violation of strict layering" that is
designed to ease the introduction of new transport-
relevant IP options in the future. Each layer must
pick out any options that are relevant to its own
processing and ignore the rest. For this purpose,
every IP option except NOP and END-OF-LIST will include
a specification of its own length.
This document does not define the order in which a
receiver must process multiple options in the same IP
header. Hosts sending multiple options must be aware
that this introduces an ambiguity in the meaning of
certain options when combined with a source-route
option.
IMPLEMENTATION:
The IP layer must not crash as the result of an option
length that is outside the possible range. For
example, erroneous option lengths have been observed to
put some IP implementations into infinite loops.
Here are the requirements for specific IP options:
(a) Security Option
Some environments require the Security option in every
datagram; such a requirement is outside the scope of
this document and the IP standard specification. Note,
however, that the security options described in RFC-791
and RFC-1038 are obsolete. For DoD applications,
vendors should consult [IP:8] for guidance.
(b) Stream Identifier Option
This option is obsolete; it **SHOULD** NOT be sent, and it
**MUST** be silently ignored if received.
(c) Source Route Options
A host **MUST** support originating a source route and **MUST**
be able to act as the final destination of a source
route.
If host receives a datagram containing a completed
source route (i.e., the pointer points beyond the last
field), the datagram has reached its final destination;
the option as received (the recorded route) **MUST** be
passed up to the transport layer (or to ICMP message
processing). This recorded route will be reversed and
used to form a return source route for reply datagrams
(see discussion of IP Options in Section 4). When a
return source route is built, it **MUST** be correctly
formed even if the recorded route included the source
host (see case (B) in the discussion below).
An IP header containing more than one Source Route
option **MUST** NOT be sent; the effect on routing of
multiple Source Route options is implementation-
specific.
Section 3.3.5 presents the rules for a host acting as
an intermediate hop in a source route, i.e., forwarding
a source-routed datagram.
DISCUSSION:
If a source-routed datagram is fragmented, each
fragment will contain a copy of the source route.
Since the processing of IP options (including a
source route) must precede reassembly, the
original datagram will not be reassembled until
the final destination is reached.
Suppose a source routed datagram is to be routed
from host S to host D via gateways G1, G2, ... Gn.
There was an ambiguity in the specification over
whether the source route option in a datagram sent
out by S should be (A) or (B):
(A): `{>>G2, G3, ... Gn, D}` <--- CORRECT
(B): `{S, >>G2, G3, ... Gn, D}` <---- WRONG
(where >> represents the pointer). If (A) is
sent, the datagram received at D will contain the
option: `{G1, G2, ... Gn >>}`, with S and D as the
IP source and destination addresses. If (B) were
sent, the datagram received at D would again
contain S and D as the same IP source and
destination addresses, but the option would be:
`{S, G1, ...Gn >>}`; i.e., the originating host
would be the first hop in the route.
(d) Record Route Option
Implementation of originating and processing the Record
Route option is **OPTIONAL**.
(e) Timestamp Option
Implementation of originating and processing the
Timestamp option is **OPTIONAL**. If it is implemented,
the following rules apply:
o The originating host **MUST** record a timestamp in a
Timestamp option whose Internet address fields are
not pre-specified or whose first pre-specified
address is the host's interface address.
o The destination host **MUST** (if possible) add the
current timestamp to a Timestamp option before
passing the option to the transport layer or to
ICMP for processing.
o A timestamp value **MUST** follow the rules given in
Section 3.2.2.8 for the ICMP Timestamp message.
3.2.2 Internet Control Message Protocol -- ICMP
ICMP messages are grouped into two classes.
-
ICMP error messages:
Destination Unreachable (see Section 3.2.2.1)
Redirect (see Section 3.2.2.2)
Source Quench (see Section 3.2.2.3)
Time Exceeded (see Section 3.2.2.4)
Parameter Problem (see Section 3.2.2.5) -
ICMP query messages:
Echo (see Section 3.2.2.6)
Information (see Section 3.2.2.7)
Timestamp (see Section 3.2.2.8)
Address Mask (see Section 3.2.2.9)
If an ICMP message of unknown type is received, it MUST be silently discarded.
Every ICMP error message includes the Internet header and at least the first 8 data octets of the datagram that triggered the error; more than 8 octets MAY be sent; this header and data MUST be unchanged from the received datagram.
In those cases where the Internet layer is required to pass an ICMP error message to the transport layer, the IP protocol number MUST be extracted from the original header and used to select the appropriate transport protocol entity to handle the error.
An ICMP error message SHOULD be sent with normal (i.e., zero) TOS bits.
An ICMP error message MUST NOT be sent as the result of receiving:
-
an ICMP error message, or
-
a datagram destined to an IP broadcast or IP multicast address, or
-
a datagram sent as a link-layer broadcast, or
-
a non-initial fragment, or
-
a datagram whose source address does not define a single host -- e.g., a zero address, a loopback address, a broadcast address, a multicast address, or a Class E address.
NOTE: THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY REQUIREMENT ELSEWHERE IN THIS DOCUMENT FOR SENDING ICMP ERROR MESSAGES.
DISCUSSION: These rules will prevent the "broadcast storms" that have resulted from hosts returning ICMP error messages in response to broadcast datagrams. For example, a broadcast UDP segment to a non-existent port could trigger a flood of ICMP Destination Unreachable datagrams from all machines that do not have a client for that destination port. On a large Ethernet, the resulting collisions can render the network useless for a second or more.
Every datagram that is broadcast on the connected network
should have a valid IP broadcast address as its IP
destination (see Section 3.3.6). However, some hosts
violate this rule. To be certain to detect broadcast
datagrams, therefore, hosts are required to check for a
link-layer broadcast as well as an IP-layer broadcast
address.
IMPLEMENTATION: This requires that the link layer inform the IP layer when a link-layer broadcast datagram has been received; see Section 2.4.
3.2.2.1 Destination Unreachable: RFC-792
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).
3.2.2.2 Redirect: RFC-792
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).
3.2.2.3 Source Quench: RFC-792
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.
3.2.2.4 Time Exceeded: RFC-792
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.
3.2.2.5 Parameter Problem: RFC-792
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.
3.2.2.6 Echo Request/Reply: RFC-792
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.
3.2.2.7 Information Request/Reply: RFC-792
A host **SHOULD** NOT implement these messages.
DISCUSSION:
The Information Request/Reply pair was intended to
support self-configuring systems such as diskless
workstations, to allow them to discover their IP
network numbers at boot time. However, the RARP and
BOOTP protocols provide better mechanisms for a host to
discover its own IP address.
3.2.2.8 Timestamp and Timestamp Reply: RFC-792
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.
3.2.2.9 Address Mask Request/Reply: RFC-950
A host **MUST** support the first, and **MAY** implement all three,
of the following methods for determining the address mask(s)
corresponding to its IP address(es):
(1) static configuration information;
(2) obtaining the address mask(s) dynamically as a side-
effect of the system initialization process (see
[INTRO:1]); and
(3) sending ICMP Address Mask Request(s) and receiving ICMP
Address Mask Reply(s).
The choice of method to be used in a particular host **MUST** be
configurable.
When method (3), the use of Address Mask messages, is
enabled, then:
(a) When it initializes, the host **MUST** broadcast an Address
Mask Request message on the connected network
corresponding to the IP address. It **MUST** retransmit
this message a small number of times if it does not
receive an immediate Address Mask Reply.
(b) Until it has received an Address Mask Reply, the host
**SHOULD** assume a mask appropriate for the address class
of the IP address, i.e., assume that the connected
network is not subnetted.
(c) The first Address Mask Reply message received **MUST** be
used to set the address mask corresponding to the
particular local IP address. This is true even if the
first Address Mask Reply message is "unsolicited", in
which case it will have been broadcast and may arrive
after the host has ceased to retransmit Address Mask
Requests. Once the mask has been set by an Address
Mask Reply, later Address Mask Reply messages **MUST** be
(silently) ignored.
Conversely, if Address Mask messages are disabled, then no
ICMP Address Mask Requests will be sent, and any ICMP
Address Mask Replies received for that local IP address **MUST**
be (silently) ignored.
A host **SHOULD** make some reasonableness check on any address
mask it installs; see IMPLEMENTATION section below.
A system **MUST** NOT send an Address Mask Reply unless it is an
authoritative agent for address masks. An authoritative
agent may be a host or a gateway, but it **MUST** be explicitly
configured as a address mask agent. Receiving an address
mask via an Address Mask Reply does not give the receiver
authority and **MUST** NOT be used as the basis for issuing
Address Mask Replies.
With a statically configured address mask, there **SHOULD** be
an additional configuration flag that determines whether the
host is to act as an authoritative agent for this mask,
i.e., whether it will answer Address Mask Request messages
using this mask.
If it is configured as an agent, the host **MUST** broadcast an
Address Mask Reply for the mask on the appropriate interface
when it initializes.
See "System Initialization" in [INTRO:1] for more
information about the use of Address Mask Request/Reply
messages.
DISCUSSION
Hosts that casually send Address Mask Replies with
invalid address masks have often been a serious
nuisance. To prevent this, Address Mask Replies ought
to be sent only by authoritative agents that have been
selected by explicit administrative action.
When an authoritative agent receives an Address Mask
Request message, it will send a unicast Address Mask
Reply to the source IP address. If the network part of
this address is zero (see (a) and (b) in 3.2.1.3), the
Reply will be broadcast.
Getting no reply to its Address Mask Request messages,
a host will assume there is no agent and use an
unsubnetted mask, but the agent may be only temporarily
unreachable. An agent will broadcast an unsolicited
Address Mask Reply whenever it initializes, in order to
update the masks of all hosts that have initialized in
the meantime.
IMPLEMENTATION:
The following reasonableness check on an address mask
is suggested: the mask is not all 1 bits, and it is
either zero or else the 8 highest-order bits are on.
3.2.3 Internet Group Management Protocol IGMP
IGMP [IP:4] is a protocol used between hosts and gateways on a single network to establish hosts' membership in particular multicast groups. The gateways use this information, in conjunction with a multicast routing protocol, to support IP multicasting across the Internet.
At this time, implementation of IGMP is OPTIONAL; see Section
3.3.7 for more information. Without IGMP, a host can still
participate in multicasting local to its connected networks.
3.3 SPECIFIC ISSUES
3.3.1 Routing Outbound Datagrams
The IP layer chooses the correct next hop for each datagram it sends. If the destination is on a connected network, the datagram is sent directly to the destination host; otherwise, it has to be routed to a gateway on a connected network.
3.3.1.1 Local/Remote Decision
To decide if the destination is on a connected network, the
following algorithm **MUST** be used [see IP:3]:
(a) The address mask (particular to a local IP address for
a multihomed host) is a 32-bit mask that selects the
network number and subnet number fields of the
corresponding IP address.
(b) If the IP destination address bits extracted by the
address mask match the IP source address bits extracted
by the same mask, then the destination is on the
corresponding connected network, and the datagram is to
be transmitted directly to the destination host.
(c) If not, then the destination is accessible only through
a gateway. Selection of a gateway is described below
(3.3.1.2).
A special-case destination address is handled as follows:
* For a limited broadcast or a multicast address, simply
pass the datagram to the link layer for the appropriate
interface.
* For a (network or subnet) directed broadcast, the
datagram can use the standard routing algorithms.
The host IP layer **MUST** operate correctly in a minimal
network environment, and in particular, when there are no
gateways. For example, if the IP layer of a host insists on
finding at least one gateway to initialize, the host will be
unable to operate on a single isolated broadcast net.
3.3.1.2 Gateway Selection
To efficiently route a series of datagrams to the same
destination, the source host **MUST** keep a "route cache" of
mappings to next-hop gateways. A host uses the following
basic algorithm on this cache to route a datagram; this
algorithm is designed to put the primary routing burden on
the gateways [IP:11].
(a) If the route cache contains no information for a
particular destination, the host chooses a "default"
gateway and sends the datagram to it. It also builds a
corresponding Route Cache entry.
(b) If that gateway is not the best next hop to the
destination, the gateway will forward the datagram to
the best next-hop gateway and return an ICMP Redirect
message to the source host.
(c) When it receives a Redirect, the host updates the
next-hop gateway in the appropriate route cache entry,
so later datagrams to the same destination will go
directly to the best gateway.
Since the subnet mask appropriate to the destination address
is generally not known, a Network Redirect message **SHOULD** be
treated identically to a Host Redirect message; i.e., the
cache entry for the destination host (only) would be updated
(or created, if an entry for that host did not exist) for
the new gateway.
DISCUSSION:
This recommendation is to protect against gateways that
erroneously send Network Redirects for a subnetted
network, in violation of the gateway requirements
[INTRO:2].
When there is no route cache entry for the destination host
address (and the destination is not on the connected
network), the IP layer **MUST** pick a gateway from its list of
"default" gateways. The IP layer **MUST** support multiple
default gateways.
As an extra feature, a host IP layer **MAY** implement a table
of "static routes". Each such static route **MAY** include a
flag specifying whether it may be overridden by ICMP
Redirects.
DISCUSSION:
A host generally needs to know at least one default
gateway to get started. This information can be
obtained from a configuration file or else from the
host startup sequence, e.g., the BOOTP protocol (see
[INTRO:1]).
It has been suggested that a host can augment its list
of default gateways by recording any new gateways it
learns about. For example, it can record every gateway
to which it is ever redirected. Such a feature, while
possibly useful in some circumstances, may cause
problems in other cases (e.g., gateways are not all
equal), and it is not recommended.
A static route is typically a particular preset mapping
from destination host or network into a particular
next-hop gateway; it might also depend on the Type-of-
Service (see next section). Static routes would be set
up by system administrators to override the normal
automatic routing mechanism, to handle exceptional
situations. However, any static routing information is
a potential source of failure as configurations change
or equipment fails.
3.3.1.3 Route Cache
Each route cache entry needs to include the following
fields:
(1) Local IP address (for a multihomed host)
(2) Destination IP address
(3) Type(s)-of-Service
(4) Next-hop gateway IP address
Field (2) **MAY** be the full IP address of the destination
host, or only the destination network number. Field (3),
the TOS, **SHOULD** be included.
See Section 3.3.4.2 for a discussion of the implications of
multihoming for the lookup procedure in this cache.
DISCUSSION:
Including the Type-of-Service field in the route cache
and considering it in the host route algorithm will
provide the necessary mechanism for the future when
Type-of-Service routing is commonly used in the
Internet. See Section 3.2.1.6.
Each route cache entry defines the endpoints of an
Internet path. Although the connecting path may change
dynamically in an arbitrary way, the transmission
characteristics of the path tend to remain
approximately constant over a time period longer than a
single typical host-host transport connection.
Therefore, a route cache entry is a natural place to
cache data on the properties of the path. Examples of
such properties might be the maximum unfragmented
datagram size (see Section 3.3.3), or the average
round-trip delay measured by a transport protocol.
This data will generally be both gathered and used by a
higher layer protocol, e.g., by TCP, or by an
application using UDP. Experiments are currently in
progress on caching path properties in this manner.
There is no consensus on whether the route cache should
be keyed on destination host addresses alone, or allow
both host and network addresses. Those who favor the
use of only host addresses argue that:
(1) As required in Section 3.3.1.2, Redirect messages
will generally result in entries keyed on
destination host addresses; the simplest and most
general scheme would be to use host addresses
always.
(2) The IP layer may not always know the address mask
for a network address in a complex subnetted
environment.
(3) The use of only host addresses allows the
destination address to be used as a pure 32-bit
number, which may allow the Internet architecture
to be more easily extended in the future without
any change to the hosts.
The opposing view is that allowing a mixture of
destination hosts and networks in the route cache:
(1) Saves memory space.
(2) Leads to a simpler data structure, easily
combining the cache with the tables of default and
static routes (see below).
(3) Provides a more useful place to cache path
properties, as discussed earlier.
IMPLEMENTATION:
The cache needs to be large enough to include entries
for the maximum number of destination hosts that may be
in use at one time.
A route cache entry may also include control
information used to choose an entry for replacement.
This might take the form of a "recently used" bit, a
use count, or a last-used timestamp, for example. It
is recommended that it include the time of last
modification of the entry, for diagnostic purposes.
An implementation may wish to reduce the overhead of
scanning the route cache for every datagram to be
transmitted. This may be accomplished with a hash
table to speed the lookup, or by giving a connection-
oriented transport protocol a "hint" or temporary
handle on the appropriate cache entry, to be passed to
the IP layer with each subsequent datagram.
Although we have described the route cache, the lists
of default gateways, and a table of static routes as
conceptually distinct, in practice they may be combined
into a single "routing table" data structure.
3.3.1.4 Dead Gateway Detection
The IP layer **MUST** be able to detect the failure of a "next-
hop" gateway that is listed in its route cache and to choose
an alternate gateway (see Section 3.3.1.5).
Dead gateway detection is covered in some detail in RFC-816
[IP:11]. Experience to date has not produced a complete
algorithm which is totally satisfactory, though it has
identified several forbidden paths and promising techniques.
* A particular gateway **SHOULD** NOT be used indefinitely in
the absence of positive indications that it is
functioning.
* Active probes such as "pinging" (i.e., using an ICMP
Echo Request/Reply exchange) are expensive and scale
poorly. In particular, hosts **MUST** NOT actively check
the status of a first-hop gateway by simply pinging the
gateway continuously.
* Even when it is the only effective way to verify a
gateway's status, pinging **MUST** be used only when
traffic is being sent to the gateway and when there is
no other positive indication to suggest that the
gateway is functioning.
* To avoid pinging, the layers above and/or below the
Internet layer **SHOULD** be able to give "advice" on the
status of route cache entries when either positive
(gateway OK) or negative (gateway dead) information is
available.
DISCUSSION:
If an implementation does not include an adequate
mechanism for detecting a dead gateway and re-routing,
a gateway failure may cause datagrams to apparently
vanish into a "black hole". This failure can be
extremely confusing for users and difficult for network
personnel to debug.
The dead-gateway detection mechanism must not cause
unacceptable load on the host, on connected networks,
or on first-hop gateway(s). The exact constraints on
the timeliness of dead gateway detection and on
acceptable load may vary somewhat depending on the
nature of the host's mission, but a host generally
needs to detect a failed first-hop gateway quickly
enough that transport-layer connections will not break
before an alternate gateway can be selected.
Passing advice from other layers of the protocol stack
complicates the interfaces between the layers, but it
is the preferred approach to dead gateway detection.
Advice can come from almost any part of the IP/TCP
architecture, but it is expected to come primarily from
the transport and link layers. Here are some possible
sources for gateway advice:
o TCP or any connection-oriented transport protocol
should be able to give negative advice, e.g.,
triggered by excessive retransmissions.
o TCP may give positive advice when (new) data is
acknowledged. Even though the route may be
asymmetric, an ACK for new data proves that the
acknowleged data must have been transmitted
successfully.
o An ICMP Redirect message from a particular gateway
should be used as positive advice about that
gateway.
o Link-layer information that reliably detects and
reports host failures (e.g., ARPANET Destination
Dead messages) should be used as negative advice.
o Failure to ARP or to re-validate ARP mappings may
be used as negative advice for the corresponding
IP address.
o Packets arriving from a particular link-layer
address are evidence that the system at this
address is alive. However, turning this
information into advice about gateways requires
mapping the link-layer address into an IP address,
and then checking that IP address against the
gateways pointed to by the route cache. This is
probably prohibitively inefficient.
Note that positive advice that is given for every
datagram received may cause unacceptable overhead in
the implementation.
While advice might be passed using required arguments
in all interfaces to the IP layer, some transport and
application layer protocols cannot deduce the correct
advice. These interfaces must therefore allow a
neutral value for advice, since either always-positive
or always-negative advice leads to incorrect behavior.
There is another technique for dead gateway detection
that has been commonly used but is not recommended.
This technique depends upon the host passively
receiving ("wiretapping") the Interior Gateway Protocol
(IGP) datagrams that the gateways are broadcasting to
each other. This approach has the drawback that a host
needs to recognize all the interior gateway protocols
that gateways may use (see [INTRO:2]). In addition, it
only works on a broadcast network.
At present, pinging (i.e., using ICMP Echo messages) is
the mechanism for gateway probing when absolutely
required. A successful ping guarantees that the
addressed interface and its associated machine are up,
but it does not guarantee that the machine is a gateway
as opposed to a host. The normal inference is that if
a Redirect or other evidence indicates that a machine
was a gateway, successful pings will indicate that the
machine is still up and hence still a gateway.
However, since a host silently discards packets that a
gateway would forward or redirect, this assumption
could sometimes fail. To avoid this problem, a new
ICMP message under development will ask "are you a
gateway?"
IMPLEMENTATION:
The following specific algorithm has been suggested:
o Associate a "reroute timer" with each gateway
pointed to by the route cache. Initialize the
timer to a value Tr, which must be small enough to
allow detection of a dead gateway before transport
connections time out.
o Positive advice would reset the reroute timer to
Tr. Negative advice would reduce or zero the
reroute timer.
o Whenever the IP layer used a particular gateway to
route a datagram, it would check the corresponding
reroute timer. If the timer had expired (reached
zero), the IP layer would send a ping to the
gateway, followed immediately by the datagram.
o The ping (ICMP Echo) would be sent again if
necessary, up to N times. If no ping reply was
received in N tries, the gateway would be assumed
to have failed, and a new first-hop gateway would
be chosen for all cache entries pointing to the
failed gateway.
Note that the size of Tr is inversely related to the
amount of advice available. Tr should be large enough
to insure that:
* Any pinging will be at a low level (e.g., <10%) of
all packets sent to a gateway from the host, AND
* pinging is infrequent (e.g., every 3 minutes)
Since the recommended algorithm is concerned with the
gateways pointed to by route cache entries, rather than
the cache entries themselves, a two level data
structure (perhaps coordinated with ARP or similar
caches) may be desirable for implementing a route
cache.
3.3.1.5 New Gateway Selection
If the failed gateway is not the current default, the IP
layer can immediately switch to a default gateway. If it is
the current default that failed, the IP layer **MUST** select a
different default gateway (assuming more than one default is
known) for the failed route and for establishing new routes.
DISCUSSION:
When a gateway does fail, the other gateways on the
connected network will learn of the failure through
some inter-gateway routing protocol. However, this
will not happen instantaneously, since gateway routing
protocols typically have a settling time of 30-60
seconds. If the host switches to an alternative
gateway before the gateways have agreed on the failure,
the new target gateway will probably forward the
datagram to the failed gateway and send a Redirect back
to the host pointing to the failed gateway (!). The
result is likely to be a rapid oscillation in the
contents of the host's route cache during the gateway
settling period. It has been proposed that the dead-
gateway logic should include some hysteresis mechanism
to prevent such oscillations. However, experience has
not shown any harm from such oscillations, since
service cannot be restored to the host until the
gateways' routing information does settle down.
IMPLEMENTATION:
One implementation technique for choosing a new default
gateway is to simply round-robin among the default
gateways in the host's list. Another is to rank the
gateways in priority order, and when the current
default gateway is not the highest priority one, to
"ping" the higher-priority gateways slowly to detect
when they return to service. This pinging can be at a
very low rate, e.g., 0.005 per second.
3.3.1.6 Initialization
The following information **MUST** be configurable:
(1) IP address(es).
(2) Address mask(s).
(3) A list of default gateways, with a preference level.
A manual method of entering this configuration data **MUST** be
provided. In addition, a variety of methods can be used to
determine this information dynamically; see the section on
"Host Initialization" in [INTRO:1].
DISCUSSION:
Some host implementations use "wiretapping" of gateway
protocols on a broadcast network to learn what gateways
exist. A standard method for default gateway discovery
is under development.
3.3.2 Reassembly
The IP layer MUST implement reassembly of IP datagrams.
We designate the largest datagram size that can be reassembled by EMTU_R ("Effective MTU to receive"); this is sometimes called the "reassembly buffer size". EMTU_R MUST be greater than or equal to 576, SHOULD be either configurable or indefinite, and SHOULD be greater than or equal to the MTU of the connected network(s).
DISCUSSION: A fixed EMTU_R limit should not be built into the code because some application layer protocols require EMTU_R values larger than 576.
IMPLEMENTATION: An implementation may use a contiguous reassembly buffer for each datagram, or it may use a more complex data structure that places no definite limit on the reassembled datagram size; in the latter case, EMTU_R is said to be
"indefinite".
Logically, reassembly is performed by simply copying each
fragment into the packet buffer at the proper offset.
Note that fragments may overlap if successive
retransmissions use different packetizing but the same
reassembly Id.
The tricky part of reassembly is the bookkeeping to
determine when all bytes of the datagram have been
reassembled. We recommend Clark's algorithm [IP:10] that
requires no additional data space for the bookkeeping.
However, note that, contrary to [IP:10], the first
fragment header needs to be saved for inclusion in a
possible ICMP Time Exceeded (Reassembly Timeout) message.
There MUST be a mechanism by which the transport layer can learn MMS_R, the maximum message size that can be received and reassembled in an IP datagram (see GET_MAXSIZES calls in Section 3.4). If EMTU_R is not indefinite, then the value of MMS_R is given by:
MMS_R = EMTU_R - 20
since 20 is the minimum size of an IP header.
There MUST be a reassembly timeout. The reassembly timeout value SHOULD be a fixed value, not set from the remaining TTL. It is recommended that the value lie between 60 seconds and 120 seconds. If this timeout expires, the partially-reassembled datagram MUST be discarded and an ICMP Time Exceeded message sent to the source host (if fragment zero has been received).
DISCUSSION: The IP specification says that the reassembly timeout should be the remaining TTL from the IP header, but this does not work well because gateways generally treat TTL as a simple hop count rather than an elapsed time. If the reassembly timeout is too small, datagrams will be discarded unnecessarily, and communication may fail. The timeout needs to be at least as large as the typical maximum delay across the Internet. A realistic minimum reassembly timeout would be 60 seconds.
It has been suggested that a cache might be kept of
round-trip times measured by transport protocols for
various destinations, and that these values might be used
to dynamically determine a reasonable reassembly timeout
value. Further investigation of this approach is
required.
If the reassembly timeout is set too high, buffer
resources in the receiving host will be tied up too long,
and the MSL (Maximum Segment Lifetime) [TCP:1] will be
larger than necessary. The MSL controls the maximum rate
at which fragmented datagrams can be sent using distinct
values of the 16-bit Ident field; a larger MSL lowers the
maximum rate. The TCP specification [TCP:1] arbitrarily
assumes a value of 2 minutes for MSL. This sets an upper
limit on a reasonable reassembly timeout value.
3.3.3 Fragmentation
Optionally, the IP layer MAY implement a mechanism to fragment outgoing datagrams intentionally.
We designate by EMTU_S ("Effective MTU for sending") the maximum IP datagram size that may be sent, for a particular combination of IP source and destination addresses and perhaps TOS.
A host MUST implement a mechanism to allow the transport layer
to learn MMS_S, the maximum transport-layer message size that
may be sent for a given {source, destination, TOS} triplet (see
GET_MAXSIZES call in Section 3.4). If no local fragmentation
is performed, the value of MMS_S will be:
MMS_S = EMTU_S - `<IP header size>`
and EMTU_S must be less than or equal to the MTU of the network
interface corresponding to the source address of the datagram.
Note that <IP header size> in this equation will be 20, unless
the IP reserves space to insert IP options for its own purposes
in addition to any options inserted by the transport layer.
A host that does not implement local fragmentation MUST ensure that the transport layer (for TCP) or the application layer (for UDP) obtains MMS_S from the IP layer and does not send a datagram exceeding MMS_S in size.
It is generally desirable to avoid local fragmentation and to
choose EMTU_S low enough to avoid fragmentation in any gateway
along the path. In the absence of actual knowledge of the
minimum MTU along the path, the IP layer SHOULD use
EMTU_S <= 576 whenever the destination address is not on a
connected network, and otherwise use the connected network's
MTU.
The MTU of each physical interface MUST be configurable.
A host IP layer implementation MAY have a configuration flag "All-Subnets-MTU", indicating that the MTU of the connected network is to be used for destinations on different subnets within the same network, but not for other networks. Thus, this flag causes the network class mask, rather than the subnet address mask, to be used to choose an EMTU_S. For a multihomed host, an "All-Subnets-MTU" flag is needed for each network interface.
DISCUSSION: Picking the correct datagram size to use when sending data is a complex topic [IP:9].
(a) In general, no host is required to accept an IP
datagram larger than 576 bytes (including header and
data), so a host must not send a larger datagram
without explicit knowledge or prior arrangement with
the destination host. Thus, MMS_S is only an upper
bound on the datagram size that a transport protocol
may send; even when MMS_S exceeds 556, the transport
layer must limit its messages to 556 bytes in the
absence of other knowledge about the destination
host.
(b) Some transport protocols (e.g., TCP) provide a way to
explicitly inform the sender about the largest
datagram the other end can receive and reassemble
[IP:7]. There is no corresponding mechanism in the
IP layer.
A transport protocol that assumes an EMTU_R larger
than 576 (see Section 3.3.2), can send a datagram of
this larger size to another host that implements the
same protocol.
(c) Hosts should ideally limit their EMTU_S for a given
destination to the minimum MTU of all the networks
along the path, to avoid any fragmentation. IP
fragmentation, while formally correct, can create a
serious transport protocol performance problem,
because loss of a single fragment means all the
fragments in the segment must be retransmitted
[IP:9].
Since nearly all networks in the Internet currently
support an MTU of 576 or greater, we strongly recommend
the use of 576 for datagrams sent to non-local networks.
It has been suggested that a host could determine the MTU
over a given path by sending a zero-offset datagram
fragment and waiting for the receiver to time out the
reassembly (which cannot complete!) and return an ICMP
Time Exceeded message. This message would include the
largest remaining fragment header in its body. More
direct mechanisms are being experimented with, but have
not yet been adopted (see e.g., RFC-1063).
3.3.4 Local Multihoming
3.3.4.1 Introduction
A multihomed host has multiple IP addresses, which we may
think of as "logical interfaces". These logical interfaces
may be associated with one or more physical interfaces, and
these physical interfaces may be connected to the same or
different networks.
Here are some important cases of multihoming:
(a) Multiple Logical Networks
The Internet architects envisioned that each physical
network would have a single unique IP network (or
subnet) number. However, LAN administrators have
sometimes found it useful to violate this assumption,
operating a LAN with multiple logical networks per
physical connected network.
If a host connected to such a physical network is
configured to handle traffic for each of N different
logical networks, then the host will have N logical
interfaces. These could share a single physical
interface, or might use N physical interfaces to the
same network.
(b) Multiple Logical Hosts
When a host has multiple IP addresses that all have the
same `<Network-number>` part (and the same `<Subnet-number>` part, if any), the logical interfaces are known
as "logical hosts". These logical interfaces might
share a single physical interface or might use separate
physical interfaces to the same physical network.
(c) Simple Multihoming
In this case, each logical interface is mapped into a
separate physical interface and each physical interface
is connected to a different physical network. The term
"multihoming" was originally applied only to this case,
but it is now applied more generally.
A host with embedded gateway functionality will
typically fall into the simple multihoming case. Note,
however, that a host may be simply multihomed without
containing an embedded gateway, i.e., without
forwarding datagrams from one connected network to
another.
This case presents the most difficult routing problems.
The choice of interface (i.e., the choice of first-hop
network) may significantly affect performance or even
reachability of remote parts of the Internet.
Finally, we note another possibility that is NOT
multihoming: one logical interface may be bound to multiple
physical interfaces, in order to increase the reliability or
throughput between directly connected machines by providing
alternative physical paths between them. For instance, two
systems might be connected by multiple point-to-point links.
We call this "link-layer multiplexing". With link-layer
multiplexing, the protocols above the link layer are unaware
that multiple physical interfaces are present; the link-
layer device driver is responsible for multiplexing and
routing packets across the physical interfaces.
In the Internet protocol architecture, a transport protocol
instance ("entity") has no address of its own, but instead
uses a single Internet Protocol (IP) address. This has
implications for the IP, transport, and application layers,
and for the interfaces between them. In particular, the
application software may have to be aware of the multiple IP
addresses of a multihomed host; in other cases, the choice
can be made within the network software.
3.3.4.2 Multihoming Requirements
The following general rules apply to the selection of an IP
source address for sending a datagram from a multihomed
host.
(1) If the datagram is sent in response to a received
datagram, the source address for the response **SHOULD** be
the specific-destination address of the request. See
Sections 4.1.3.5 and 4.2.3.7 and the "General Issues"
section of [INTRO:1] for more specific requirements on
higher layers.
Otherwise, a source address must be selected.
(2) An application **MUST** be able to explicitly specify the
source address for initiating a connection or a
request.
(3) In the absence of such a specification, the networking
software **MUST** choose a source address. Rules for this
choice are described below.
There are two key requirement issues related to multihoming:
(A) A host **MAY** silently discard an incoming datagram whose
destination address does not correspond to the physical
interface through which it is received.
(B) A host **MAY** restrict itself to sending (non-source-
routed) IP datagrams only through the physical
interface that corresponds to the IP source address of
the datagrams.
DISCUSSION:
Internet host implementors have used two different
conceptual models for multihoming, briefly summarized
in the following discussion. This document takes no
stand on which model is preferred; each seems to have a
place. This ambivalence is reflected in the issues (A)
and (B) being optional.
o Strong ES Model
The Strong ES (End System, i.e., host) model
emphasizes the host/gateway (ES/IS) distinction,
and would therefore substitute **MUST** for **MAY** in
issues (A) and (B) above. It tends to model a
multihomed host as a set of logical hosts within
the same physical host.
With respect to (A), proponents of the Strong ES
model note that automatic Internet routing
mechanisms could not route a datagram to a
physical interface that did not correspond to the
destination address.
Under the Strong ES model, the route computation
for an outgoing datagram is the mapping:
route(src IP addr, dest IP addr, TOS)
-> gateway
Here the source address is included as a parameter
in order to select a gateway that is directly
reachable on the corresponding physical interface.
Note that this model logically requires that in
general there be at least one default gateway, and
preferably multiple defaults, for each IP source
address.
o Weak ES Model
This view de-emphasizes the ES/IS distinction, and
would therefore substitute **MUST** NOT for **MAY** in
issues (A) and (B). This model may be the more
natural one for hosts that wiretap gateway routing
protocols, and is necessary for hosts that have
embedded gateway functionality.
The Weak ES Model may cause the Redirect mechanism
to fail. If a datagram is sent out a physical
interface that does not correspond to the
destination address, the first-hop gateway will
not realize when it needs to send a Redirect. On
the other hand, if the host has embedded gateway
functionality, then it has routing information
without listening to Redirects.
In the Weak ES model, the route computation for an
outgoing datagram is the mapping:
route(dest IP addr, TOS) -> gateway, interface
3.3.4.3 Choosing a Source Address
DISCUSSION:
When it sends an initial connection request (e.g., a
TCP "SYN" segment) or a datagram service request (e.g.,
a UDP-based query), the transport layer on a multihomed
host needs to know which source address to use. If the
application does not specify it, the transport layer
must ask the IP layer to perform the conceptual
mapping:
GET_SRCADDR(remote IP addr, TOS)
-> local IP address
Here TOS is the Type-of-Service value (see Section
3.2.1.6), and the result is the desired source address.
The following rules are suggested for implementing this
mapping:
(a) If the remote Internet address lies on one of the
(sub-) nets to which the host is directly
connected, a corresponding source address may be
chosen, unless the corresponding interface is
known to be down.
(b) The route cache may be consulted, to see if there
is an active route to the specified destination
network through any network interface; if so, a
local IP address corresponding to that interface
may be chosen.
(c) The table of static routes, if any (see Section
3.3.1.2) may be similarly consulted.
(d) The default gateways may be consulted. If these
gateways are assigned to different interfaces, the
interface corresponding to the gateway with the
highest preference may be chosen.
In the future, there may be a defined way for a
multihomed host to ask the gateways on all connected
networks for advice about the best network to use for a
given destination.
IMPLEMENTATION:
It will be noted that this process is essentially the
same as datagram routing (see Section 3.3.1), and
therefore hosts may be able to combine the
implementation of the two functions.
3.3.5 Source Route Forwarding
Subject to restrictions given below, a host MAY be able to act as an intermediate hop in a source route, forwarding a source- routed datagram to the next specified hop.
However, in performing this gateway-like function, the host MUST obey all the relevant rules for a gateway forwarding source-routed datagrams [INTRO:2]. This includes the following specific provisions, which override the corresponding host provisions given earlier in this document:
(A) TTL (ref. Section 3.2.1.7)
The TTL field **MUST** be decremented and the datagram perhaps
discarded as specified for a gateway in [INTRO:2].
(B) ICMP Destination Unreachable (ref. Section 3.2.2.1)
A host **MUST** be able to generate Destination Unreachable
messages with the following codes:
4 (Fragmentation Required but DF Set) when a source-
routed datagram cannot be fragmented to fit into the
target network;
5 (Source Route Failed) when a source-routed datagram
cannot be forwarded, e.g., because of a routing
problem or because the next hop of a strict source
route is not on a connected network.
(C) IP Source Address (ref. Section 3.2.1.3)
A source-routed datagram being forwarded **MAY** (and normally
will) have a source address that is not one of the IP
addresses of the forwarding host.
(D) Record Route Option (ref. Section 3.2.1.8d)
A host that is forwarding a source-routed datagram
containing a Record Route option **MUST** update that option,
if it has room.
(E) Timestamp Option (ref. Section 3.2.1.8e)
A host that is forwarding a source-routed datagram
containing a Timestamp Option **MUST** add the current
timestamp to that option, according to the rules for this
option.
To define the rules restricting host forwarding of source- routed datagrams, we use the term "local source-routing" if the next hop will be through the same physical interface through which the datagram arrived; otherwise, it is "non-local source-routing".
o A host is permitted to perform local source-routing
without restriction.
o A host that supports non-local source-routing **MUST** have a
configurable switch to disable forwarding, and this switch
**MUST** default to disabled.
o The host **MUST** satisfy all gateway requirements for
configurable policy filters [INTRO:2] restricting non-
local forwarding.
If a host receives a datagram with an incomplete source route but does not forward it for some reason, the host SHOULD return an ICMP Destination Unreachable (code 5, Source Route Failed) message, unless the datagram was itself an ICMP error message.
3.3.6 Broadcasts
Section 3.2.1.3 defined the four standard IP broadcast address forms:
Limited Broadcast: `{-1, -1}`
Directed Broadcast: `{<Network-number>,-1}`
Subnet Directed Broadcast:
`{<Network-number>,<Subnet-number>,-1}`
All-Subnets Directed Broadcast: `{<Network-number>,-1,-1}`
A host MUST recognize any of these forms in the destination address of an incoming datagram.
There is a class of hosts* that use non-standard broadcast address forms, substituting 0 for -1. All hosts SHOULD
*4.2BSD Unix and its derivatives, but not 4.3BSD.
recognize and accept any of these non-standard broadcast addresses as the destination address of an incoming datagram. A host MAY optionally have a configuration option to choose the 0 or the -1 form of broadcast address, for each physical interface, but this option SHOULD default to the standard (-1) form.
When a host sends a datagram to a link-layer broadcast address, the IP destination address MUST be a legal IP broadcast or IP multicast address.
A host SHOULD silently discard a datagram that is received via a link-layer broadcast (see Section 2.4) but does not specify an IP multicast or broadcast destination address.
Hosts SHOULD use the Limited Broadcast address to broadcast to a connected network.
DISCUSSION: Using the Limited Broadcast address instead of a Directed Broadcast address may improve system robustness. Problems are often caused by machines that do not understand the plethora of broadcast addresses (see Section 3.2.1.3), or that may have different ideas about which broadcast addresses are in use. The prime example of the latter is machines that do not understand subnetting but are attached to a subnetted net. Sending a Subnet Broadcast for the connected network will confuse those machines, which will see it as a message to some other host.
There has been discussion on whether a datagram addressed
to the Limited Broadcast address ought to be sent from all
the interfaces of a multihomed host. This specification
takes no stand on the issue.
3.3.7 IP Multicasting
A host SHOULD support local IP multicasting on all connected networks for which a mapping from Class D IP addresses to link-layer addresses has been specified (see below). Support for local IP multicasting includes sending multicast datagrams, joining multicast groups and receiving multicast datagrams, and leaving multicast groups. This implies support for all of [IP:4] except the IGMP protocol itself, which is OPTIONAL.
DISCUSSION: IGMP provides gateways that are capable of multicast routing with the information required to support IP multicasting across multiple networks. At this time, multicast-routing gateways are in the experimental stage and are not widely available. For hosts that are not connected to networks with multicast-routing gateways or that do not need to receive multicast datagrams originating on other networks, IGMP serves no purpose and is therefore optional for now. However, the rest of [IP:4] is currently recommended for the purpose of providing IP-layer access to local network multicast addressing, as a preferable alternative to local broadcast addressing. It is expected that IGMP will become recommended at some future date, when multicast-routing gateways have become more widely available.
If IGMP is not implemented, a host SHOULD still join the "all- hosts" group (224.0.0.1) when the IP layer is initialized and remain a member for as long as the IP layer is active.
DISCUSSION: Joining the "all-hosts" group will support strictly local uses of multicasting, e.g., a gateway discovery protocol, even if IGMP is not implemented.
The mapping of IP Class D addresses to local addresses is currently specified for the following types of networks:
o Ethernet/IEEE 802.3, as defined in [IP:4].
o Any network that supports broadcast but not multicast,
addressing: all IP Class D addresses map to the local
broadcast address.
o Any type of point-to-point link (e.g., SLIP or HDLC
links): no mapping required. All IP multicast datagrams
are sent as-is, inside the local framing.
Mappings for other types of networks will be specified in the future.
A host SHOULD provide a way for higher-layer protocols or applications to determine which of the host's connected network(s) support IP multicast addressing.
3.3.8 Error Reporting
Wherever practical, hosts MUST return ICMP error datagrams on detection of an error, except in those cases where returning an ICMP error message is specifically prohibited.
DISCUSSION: A common phenomenon in datagram networks is the "black hole disease": datagrams are sent out, but nothing comes back. Without any error datagrams, it is difficult for the user to figure out what the problem is.
3.4 INTERNET/TRANSPORT LAYER INTERFACE
The interface between the IP layer and the transport layer MUST provide full access to all the mechanisms of the IP layer, including options, Type-of-Service, and Time-to-Live. The transport layer MUST either have mechanisms to set these interface parameters, or provide a path to pass them through from an application, or both.
DISCUSSION: Applications are urged to make use of these mechanisms where applicable, even when the mechanisms are not currently effective in the Internet (e.g., TOS). This will allow these mechanisms to be immediately useful when they do become effective, without a large amount of retrofitting of host software.
We now describe a conceptual interface between the transport layer and the IP layer, as a set of procedure calls. This is an extension of the information in Section 3.3 of RFC-791 [IP:1].
-
Send Datagram
SEND(src, dst, prot, TOS, TTL, BufPTR, len, Id, DF, opt
=> result )where the parameters are defined in RFC-791. Passing an Id parameter is optional; see Section 3.2.1.5.
-
Receive Datagram
RECV(BufPTR, prot
=> result, src, dst, SpecDest, TOS, len, opt)All the parameters are defined in RFC-791, except for:
SpecDest = specific-destination address of datagram
(defined in Section 3.2.1.3)The result parameter dst contains the datagram's destination address. Since this may be a broadcast or multicast address, the SpecDest parameter (not shown in RFC-791) MUST be passed. The parameter opt contains all the IP options received in the datagram; these MUST also be passed to the transport layer.
-
Select Source Address
GET_SRCADDR(remote, TOS) -> local
remote = remote IP address
TOS = Type-of-Service
local = local IP addressSee Section 3.3.4.3.
-
Find Maximum Datagram Sizes
GET_MAXSIZES(local, remote, TOS) -> MMS_R, MMS_S
MMS_R = maximum receive transport-message size.
MMS_S = maximum send transport-message size.
(local, remote, TOS defined above)See Sections 3.3.2 and 3.3.3.
-
Advice on Delivery Success
ADVISE_DELIVPROB(sense, local, remote, TOS)Here the parameter sense is a 1-bit flag indicating whether positive or negative advice is being given; see the discussion in Section 3.3.1.4. The other parameters were defined earlier.
-
Send ICMP Message
SEND_ICMP(src, dst, TOS, TTL, BufPTR, len, Id, DF, opt)
-> result
(Parameters defined in RFC-791).Passing an Id parameter is optional; see Section 3.2.1.5. The transport layer MUST be able to send certain ICMP messages: Port Unreachable or any of the query-type messages. This function could be considered to be a special case of the SEND() call, of course; we describe it separately for clarity.
-
Receive ICMP Message
RECV_ICMP(BufPTR ) -> result, src, dst, len, opt
(Parameters defined in RFC-791).The IP layer MUST pass certain ICMP messages up to the appropriate transport-layer routine. This function could be considered to be a special case of the RECV() call, of course; we describe it separately for clarity.
For an ICMP error message, the data that is passed up MUST include the original Internet header plus all the octets of the original message that are included in the ICMP message. This data will be used by the transport layer to locate the connection state information, if any.
In particular, the following ICMP messages are to be passed up:
o Destination Unreachable
o Source Quench
o Echo Reply (to ICMP user interface, unless the Echo
Request originated in the IP layer)
o Timestamp Reply (to ICMP user interface)
o Time Exceeded
DISCUSSION: In the future, there may be additions to this interface to pass path data (see Section 3.3.1.3) between the IP and transport layers.
3.5 INTERNET LAYER REQUIREMENTS SUMMARY
| | | | |S| |
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| |M|O| |D|T|n
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| |T|D|Y|O|O|t
| FEATURE | SECTION | T | T | e |
|---|
| | | | | | |
Implement IP and ICMP |3.1 |x| | | | | Handle remote multihoming in application layer |3.1 |x| | | | | Support local multihoming |3.1 | | |x| | | Meet gateway specs if forward datagrams |3.1 |x| | | | | Configuration switch for embedded gateway |3.1 |x| | | | |1 Config switch default to non-gateway |3.1 |x| | | | |1 Auto-config based on number of interfaces |3.1 | | | | |x|1 Able to log discarded datagrams |3.1 | |x| | | | Record in counter |3.1 | |x| | | | | | | | | | | Silently discard Version != 4 |3.2.1.1 |x| | | | | Verify IP checksum, silently discard bad dgram |3.2.1.2 |x| | | | | Addressing: | | | | | | | Subnet addressing (RFC-950) |3.2.1.3 |x| | | | | Src address must be host's own IP address |3.2.1.3 |x| | | | | Silently discard datagram with bad dest addr |3.2.1.3 |x| | | | | Silently discard datagram with bad src addr |3.2.1.3 |x| | | | | Support reassembly |3.2.1.4 |x| | | | | Retain same Id field in identical datagram |3.2.1.5 | | |x| | | | | | | | | | TOS: | | | | | | | Allow transport layer to set TOS |3.2.1.6 |x| | | | | Pass received TOS up to transport layer |3.2.1.6 | |x| | | | Use RFC-795 link-layer mappings for TOS |3.2.1.6 | | | |x| | TTL: | | | | | | | Send packet with TTL of 0 |3.2.1.7 | | | | |x| Discard received packets with TTL < 2 |3.2.1.7 | | | | |x| Allow transport layer to set TTL |3.2.1.7 |x| | | | | Fixed TTL is configurable |3.2.1.7 |x| | | | | | | | | | | | IP Options: | | | | | | | Allow transport layer to send IP options |3.2.1.8 |x| | | | | Pass all IP options rcvd to higher layer |3.2.1.8 |x| | | | |
IP layer silently ignore unknown options |3.2.1.8 |x| | | | | Security option |3.2.1.8a| | |x| | | Send Stream Identifier option |3.2.1.8b| | | |x| | Silently ignore Stream Identifer option |3.2.1.8b|x| | | | | Record Route option |3.2.1.8d| | |x| | | Timestamp option |3.2.1.8e| | |x| | | Source Route Option: | | | | | | | Originate & terminate Source Route options |3.2.1.8c|x| | | | | Datagram with completed SR passed up to TL |3.2.1.8c|x| | | | | Build correct (non-redundant) return route |3.2.1.8c|x| | | | | Send multiple SR options in one header |3.2.1.8c| | | | |x| | | | | | | | ICMP: | | | | | | | Silently discard ICMP msg with unknown type |3.2.2 |x| | | | | Include more than 8 octets of orig datagram |3.2.2 | | |x| | | Included octets same as received |3.2.2 |x| | | | | Demux ICMP Error to transport protocol |3.2.2 |x| | | | | Send ICMP error message with TOS=0 |3.2.2 | |x| | | | Send ICMP error message for: | | | | | | |
- ICMP error msg |3.2.2 | | | | |x|
- IP b'cast or IP m'cast |3.2.2 | | | | |x|
- Link-layer b'cast |3.2.2 | | | | |x|
- Non-initial fragment |3.2.2 | | | | |x|
- Datagram with non-unique src address |3.2.2 | | | | |x| Return ICMP error msgs (when not prohibited) |3.3.8 |x| | | | | | | | | | | | Dest Unreachable: | | | | | | | Generate Dest Unreachable (code 2/3) |3.2.2.1 | |x| | | | Pass ICMP Dest Unreachable to higher layer |3.2.2.1 |x| | | | | Higher layer act on Dest Unreach |3.2.2.1 | |x| | | | Interpret Dest Unreach as only hint |3.2.2.1 |x| | | | | Redirect: | | | | | | | Host send Redirect |3.2.2.2 | | | |x| | Update route cache when recv Redirect |3.2.2.2 |x| | | | | Handle both Host and Net Redirects |3.2.2.2 |x| | | | | Discard illegal Redirect |3.2.2.2 | |x| | | | Source Quench: | | | | | | | Send Source Quench if buffering exceeded |3.2.2.3 | | |x| | | Pass Source Quench to higher layer |3.2.2.3 |x| | | | | Higher layer act on Source Quench |3.2.2.3 | |x| | | | Time Exceeded: pass to higher layer |3.2.2.4 |x| | | | | Parameter Problem: | | | | | | | Send Parameter Problem messages |3.2.2.5 | |x| | | | Pass Parameter Problem to higher layer |3.2.2.5 |x| | | | | Report Parameter Problem to user |3.2.2.5 | | |x| | | | | | | | | | ICMP Echo Request or Reply: | | | | | | | Echo server and Echo client |3.2.2.6 |x| | | | |
Echo client |3.2.2.6 | |x| | | |
Discard Echo Request to broadcast address |3.2.2.6 | | |x| | |
Discard Echo Request to multicast address |3.2.2.6 | | |x| | |
Use specific-dest addr as Echo Reply src |3.2.2.6 |x| | | | |
Send same data in Echo Reply |3.2.2.6 |x| | | | |
Pass Echo Reply to higher layer |3.2.2.6 |x| | | | |
Reflect Record Route, Time Stamp options |3.2.2.6 | |x| | | |
Reverse and reflect Source Route option |3.2.2.6 |x| | | | |
| | | | | | |
ICMP Information Request or Reply: |3.2.2.7 | | | |x| | ICMP Timestamp and Timestamp Reply: |3.2.2.8 | | |x| | | Minimize delay variability |3.2.2.8 | |x| | | |1 Silently discard b'cast Timestamp |3.2.2.8 | | |x| | |1 Silently discard m'cast Timestamp |3.2.2.8 | | |x| | |1 Use specific-dest addr as TS Reply src |3.2.2.8 |x| | | | |1 Reflect Record Route, Time Stamp options |3.2.2.6 | |x| | | |1 Reverse and reflect Source Route option |3.2.2.8 |x| | | | |1 Pass Timestamp Reply to higher layer |3.2.2.8 |x| | | | |1 Obey rules for "standard value" |3.2.2.8 |x| | | | |1 | | | | | | | ICMP Address Mask Request and Reply: | | | | | | | Addr Mask source configurable |3.2.2.9 |x| | | | | Support static configuration of addr mask |3.2.2.9 |x| | | | | Get addr mask dynamically during booting |3.2.2.9 | | |x| | | Get addr via ICMP Addr Mask Request/Reply |3.2.2.9 | | |x| | | Retransmit Addr Mask Req if no Reply |3.2.2.9 |x| | | | |3 Assume default mask if no Reply |3.2.2.9 | |x| | | |3 Update address mask from first Reply only |3.2.2.9 |x| | | | |3 Reasonableness check on Addr Mask |3.2.2.9 | |x| | | | Send unauthorized Addr Mask Reply msgs |3.2.2.9 | | | | |x| Explicitly configured to be agent |3.2.2.9 |x| | | | | Static config=> Addr-Mask-Authoritative flag |3.2.2.9 | |x| | | | Broadcast Addr Mask Reply when init. |3.2.2.9 |x| | | | |3 | | | | | | | ROUTING OUTBOUND DATAGRAMS: | | | | | | | Use address mask in local/remote decision |3.3.1.1 |x| | | | | Operate with no gateways on conn network |3.3.1.1 |x| | | | | Maintain "route cache" of next-hop gateways |3.3.1.2 |x| | | | | Treat Host and Net Redirect the same |3.3.1.2 | |x| | | | If no cache entry, use default gateway |3.3.1.2 |x| | | | | Support multiple default gateways |3.3.1.2 |x| | | | | Provide table of static routes |3.3.1.2 | | |x| | | Flag: route overridable by Redirects |3.3.1.2 | | |x| | | Key route cache on host, not net address |3.3.1.3 | | |x| | | Include TOS in route cache |3.3.1.3 | |x| | | | | | | | | | | Able to detect failure of next-hop gateway |3.3.1.4 |x| | | | | Assume route is good forever |3.3.1.4 | | | |x| |
Ping gateways continuously |3.3.1.4 | | | | |x| Ping only when traffic being sent |3.3.1.4 |x| | | | | Ping only when no positive indication |3.3.1.4 |x| | | | | Higher and lower layers give advice |3.3.1.4 | |x| | | | Switch from failed default g'way to another |3.3.1.5 |x| | | | | Manual method of entering config info |3.3.1.6 |x| | | | | | | | | | | | REASSEMBLY and FRAGMENTATION: | | | | | | | Able to reassemble incoming datagrams |3.3.2 |x| | | | | At least 576 byte datagrams |3.3.2 |x| | | | | EMTU_R configurable or indefinite |3.3.2 | |x| | | | Transport layer able to learn MMS_R |3.3.2 |x| | | | | Send ICMP Time Exceeded on reassembly timeout |3.3.2 |x| | | | | Fixed reassembly timeout value |3.3.2 | |x| | | | | | | | | | | Pass MMS_S to higher layers |3.3.3 |x| | | | | Local fragmentation of outgoing packets |3.3.3 | | |x| | | Else don't send bigger than MMS_S |3.3.3 |x| | | | | Send max 576 to off-net destination |3.3.3 | |x| | | | All-Subnets-MTU configuration flag |3.3.3 | | |x| | | | | | | | | | MULTIHOMING: | | | | | | | Reply with same addr as spec-dest addr |3.3.4.2 | |x| | | | Allow application to choose local IP addr |3.3.4.2 |x| | | | | Silently discard d'gram in "wrong" interface |3.3.4.2 | | |x| | | Only send d'gram through "right" interface |3.3.4.2 | | |x| | |4 | | | | | | | SOURCE-ROUTE FORWARDING: | | | | | | | Forward datagram with Source Route option |3.3.5 | | |x| | |1 Obey corresponding gateway rules |3.3.5 |x| | | | |1 Update TTL by gateway rules |3.3.5 |x| | | | |1 Able to generate ICMP err code 4, 5 |3.3.5 |x| | | | |1 IP src addr not local host |3.3.5 | | |x| | |1 Update Timestamp, Record Route options |3.3.5 |x| | | | |1 Configurable switch for non-local SRing |3.3.5 |x| | | | |1 Defaults to OFF |3.3.5 |x| | | | |1 Satisfy gwy access rules for non-local SRing |3.3.5 |x| | | | |1 If not forward, send Dest Unreach (cd 5) |3.3.5 | |x| | | |2 | | | | | | | BROADCAST: | | | | | | | Broadcast addr as IP source addr |3.2.1.3 | | | | |x| Receive 0 or -1 broadcast formats OK |3.3.6 | |x| | | | Config'ble option to send 0 or -1 b'cast |3.3.6 | | |x| | | Default to -1 broadcast |3.3.6 | |x| | | | Recognize all broadcast address formats |3.3.6 |x| | | | | Use IP b'cast/m'cast addr in link-layer b'cast |3.3.6 |x| | | | | Silently discard link-layer-only b'cast dg's |3.3.6 | |x| | | | Use Limited Broadcast addr for connected net |3.3.6 | |x| | | |
| | | | | | |
MULTICAST: | | | | | | | Support local IP multicasting (RFC-1112) |3.3.7 | |x| | | | Support IGMP (RFC-1112) |3.3.7 | | |x| | | Join all-hosts group at startup |3.3.7 | |x| | | | Higher layers learn i'face m'cast capability |3.3.7 | |x| | | | | | | | | | | INTERFACE: | | | | | | | Allow transport layer to use all IP mechanisms |3.4 |x| | | | | Pass interface ident up to transport layer |3.4 |x| | | | | Pass all IP options up to transport layer |3.4 |x| | | | | Transport layer can send certain ICMP messages |3.4 |x| | | | | Pass spec'd ICMP messages up to transp. layer |3.4 |x| | | | | Include IP hdr+8 octets or more from orig. |3.4 |x| | | | | Able to leap tall buildings at a single bound |3.5 | |x| | | |
Footnotes:
(1) Only if feature is implemented.
(2) This requirement is overruled if datagram is an ICMP error message.
(3) Only if feature is implemented and is configured "on".
(4) Unless has embedded gateway functionality or is source routed.