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.

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.

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.

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.

(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.

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 address

See 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| | | | | | |H| |F | | | | |O|M|o | | |S| |U|U|o | | |H| |L|S|t | |M|O| |D|T|n | |U|U|M| | |o | |S|L|A|N|N|t | |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.

3.1 INTRODUCTION​

3.2 PROTOCOL WALK-THROUGH​

3.2.1 Internet Protocol -- IP​

3.2.2 Internet Control Message Protocol -- ICMP​

3.3.2 below. In the future, receipt of this message​

3.2.3 Internet Group Management Protocol IGMP​

3.3.7 for more information. Without IGMP, a host can still​

3.3 SPECIFIC ISSUES​

3.3.1 Routing Outbound Datagrams​

3.3.2 Reassembly​

3.3.3 Fragmentation​

3.3.4 Local Multihoming​

3.3.5 Source Route Forwarding​

3.3.6 Broadcasts​

3.3.7 IP Multicasting​

3.3.8 Error Reporting​

3.4 INTERNET/TRANSPORT LAYER INTERFACE​

3.5 INTERNET LAYER REQUIREMENTS SUMMARY​