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RFC 1122


Requirements for Internet Hosts - Communication Layers

Part 2 of 5, p. 21 to 47
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      All Internet systems, both hosts and gateways, have the same
      requirements for link layer protocols.  These requirements are
      given in Chapter 3 of "Requirements for Internet Gateways"
      [INTRO:2], augmented with the material in this section.




      2.3.1  Trailer Protocol Negotiation

         The trailer protocol [LINK:1] for link-layer encapsulation MAY
         be used, but only when it has been verified that both systems
         (host or gateway) involved in the link-layer communication
         implement trailers.  If the system does not dynamically
         negotiate use of the trailer protocol on a per-destination
         basis, the default configuration MUST disable the protocol.

              The trailer protocol is a link-layer encapsulation
              technique that rearranges the data contents of packets
              sent on the physical network.  In some cases, trailers
              improve the throughput of higher layer protocols by
              reducing the amount of data copying within the operating
              system.  Higher layer protocols are unaware of trailer
              use, but both the sending and receiving host MUST
              understand the protocol if it is used.

              Improper use of trailers can result in very confusing
              symptoms.  Only packets with specific size attributes are
              encapsulated using trailers, and typically only a small
              fraction of the packets being exchanged have these
              attributes.  Thus, if a system using trailers exchanges
              packets with a system that does not, some packets
              disappear into a black hole while others are delivered

              On an Ethernet, packets encapsulated with trailers use a
              distinct Ethernet type [LINK:1], and trailer negotiation
              is performed at the time that ARP is used to discover the
              link-layer address of a destination system.

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              Specifically, the ARP exchange is completed in the usual
              manner using the normal IP protocol type, but a host that
              wants to speak trailers will send an additional "trailer
              ARP reply" packet, i.e., an ARP reply that specifies the
              trailer encapsulation protocol type but otherwise has the
              format of a normal ARP reply.  If a host configured to use
              trailers receives a trailer ARP reply message from a
              remote machine, it can add that machine to the list of
              machines that understand trailers, e.g., by marking the
              corresponding entry in the ARP cache.

              Hosts wishing to receive trailer encapsulations send
              trailer ARP replies whenever they complete exchanges of
              normal ARP messages for IP.  Thus, a host that received an
              ARP request for its IP protocol address would send a
              trailer ARP reply in addition to the normal IP ARP reply;
              a host that sent the IP ARP request would send a trailer
              ARP reply when it received the corresponding IP ARP reply.
              In this way, either the requesting or responding host in
              an IP ARP exchange may request that it receive trailer

              This scheme, using extra trailer ARP reply packets rather
              than sending an ARP request for the trailer protocol type,
              was designed to avoid a continuous exchange of ARP packets
              with a misbehaving host that, contrary to any
              specification or common sense, responded to an ARP reply
              for trailers with another ARP reply for IP.  This problem
              is avoided by sending a trailer ARP reply in response to
              an IP ARP reply only when the IP ARP reply answers an
              outstanding request; this is true when the hardware
              address for the host is still unknown when the IP ARP
              reply is received.  A trailer ARP reply may always be sent
              along with an IP ARP reply responding to an IP ARP

      2.3.2  Address Resolution Protocol -- ARP
  ARP Cache Validation

            An implementation of the Address Resolution Protocol (ARP)
            [LINK:2] MUST provide a mechanism to flush out-of-date cache
            entries.  If this mechanism involves a timeout, it SHOULD be
            possible to configure the timeout value.

            A mechanism to prevent ARP flooding (repeatedly sending an
            ARP Request for the same IP address, at a high rate) MUST be
            included.  The recommended maximum rate is 1 per second per

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                 The ARP specification [LINK:2] suggests but does not
                 require a timeout mechanism to invalidate cache entries
                 when hosts change their Ethernet addresses.  The
                 prevalence of proxy ARP (see Section 2.4 of [INTRO:2])
                 has significantly increased the likelihood that cache
                 entries in hosts will become invalid, and therefore
                 some ARP-cache invalidation mechanism is now required
                 for hosts.  Even in the absence of proxy ARP, a long-
                 period cache timeout is useful in order to
                 automatically correct any bad ARP data that might have
                 been cached.

                 Four mechanisms have been used, sometimes in
                 combination, to flush out-of-date cache entries.

                 (1)  Timeout -- Periodically time out cache entries,
                      even if they are in use.  Note that this timeout
                      should be restarted when the cache entry is
                      "refreshed" (by observing the source fields,
                      regardless of target address, of an ARP broadcast
                      from the system in question).  For proxy ARP
                      situations, the timeout needs to be on the order
                      of a minute.

                 (2)  Unicast Poll -- Actively poll the remote host by
                      periodically sending a point-to-point ARP Request
                      to it, and delete the entry if no ARP Reply is
                      received from N successive polls.  Again, the
                      timeout should be on the order of a minute, and
                      typically N is 2.

                 (3)  Link-Layer Advice -- If the link-layer driver
                      detects a delivery problem, flush the
                      corresponding ARP cache entry.

                 (4)  Higher-layer Advice -- Provide a call from the
                      Internet layer to the link layer to indicate a
                      delivery problem.  The effect of this call would
                      be to invalidate the corresponding cache entry.
                      This call would be analogous to the
                      "ADVISE_DELIVPROB()" call from the transport layer
                      to the Internet layer (see Section 3.4), and in
                      fact the ADVISE_DELIVPROB routine might in turn
                      call the link-layer advice routine to invalidate

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                      the ARP cache entry.

                 Approaches (1) and (2) involve ARP cache timeouts on
                 the order of a minute or less.  In the absence of proxy
                 ARP, a timeout this short could create noticeable
                 overhead traffic on a very large Ethernet.  Therefore,
                 it may be necessary to configure a host to lengthen the
                 ARP cache timeout.
  ARP Packet Queue

            The link layer SHOULD save (rather than discard) at least
            one (the latest) packet of each set of packets destined to
            the same unresolved IP address, and transmit the saved
            packet when the address has been resolved.

                 Failure to follow this recommendation causes the first
                 packet of every exchange to be lost.  Although higher-
                 layer protocols can generally cope with packet loss by
                 retransmission, packet loss does impact performance.
                 For example, loss of a TCP open request causes the
                 initial round-trip time estimate to be inflated.  UDP-
                 based applications such as the Domain Name System are
                 more seriously affected.

      2.3.3  Ethernet and IEEE 802 Encapsulation

         The IP encapsulation for Ethernets is described in RFC-894
         [LINK:3], while RFC-1042 [LINK:4] describes the IP
         encapsulation for IEEE 802 networks.  RFC-1042 elaborates and
         replaces the discussion in Section 3.4 of [INTRO:2].

         Every Internet host connected to a 10Mbps Ethernet cable:

         o    MUST be able to send and receive packets using RFC-894

         o    SHOULD be able to receive RFC-1042 packets, intermixed
              with RFC-894 packets; and

         o    MAY be able to send packets using RFC-1042 encapsulation.

         An Internet host that implements sending both the RFC-894 and
         the RFC-1042 encapsulations MUST provide a configuration switch
         to select which is sent, and this switch MUST default to RFC-

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         Note that the standard IP encapsulation in RFC-1042 does not
         use the protocol id value (K1=6) that IEEE reserved for IP;
         instead, it uses a value (K1=170) that implies an extension
         (the "SNAP") which can be used to hold the Ether-Type field.
         An Internet system MUST NOT send 802 packets using K1=6.

         Address translation from Internet addresses to link-layer
         addresses on Ethernet and IEEE 802 networks MUST be managed by
         the Address Resolution Protocol (ARP).

         The MTU for an Ethernet is 1500 and for 802.3 is 1492.

              The IEEE 802.3 specification provides for operation over a
              10Mbps Ethernet cable, in which case Ethernet and IEEE
              802.3 frames can be physically intermixed.  A receiver can
              distinguish Ethernet and 802.3 frames by the value of the
              802.3 Length field; this two-octet field coincides in the
              header with the Ether-Type field of an Ethernet frame.  In
              particular, the 802.3 Length field must be less than or
              equal to 1500, while all valid Ether-Type values are
              greater than 1500.

              Another compatibility problem arises with link-layer
              broadcasts.  A broadcast sent with one framing will not be
              seen by hosts that can receive only the other framing.

              The provisions of this section were designed to provide
              direct interoperation between 894-capable and 1042-capable
              systems on the same cable, to the maximum extent possible.
              It is intended to support the present situation where
              894-only systems predominate, while providing an easy
              transition to a possible future in which 1042-capable
              systems become common.

              Note that 894-only systems cannot interoperate directly
              with 1042-only systems.  If the two system types are set
              up as two different logical networks on the same cable,
              they can communicate only through an IP gateway.
              Furthermore, it is not useful or even possible for a
              dual-format host to discover automatically which format to
              send, because of the problem of link-layer broadcasts.


      The packet receive interface between the IP layer and the link
      layer MUST include a flag to indicate whether the incoming packet
      was addressed to a link-layer broadcast address.

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           Although the IP layer does not generally know link layer
           addresses (since every different network medium typically has
           a different address format), the broadcast address on a
           broadcast-capable medium is an important special case.  See
           Section 3.2.2, especially the DISCUSSION concerning broadcast

      The packet send interface between the IP and link layers MUST
      include the 5-bit TOS field (see Section

      The link layer MUST NOT report a Destination Unreachable error to
      IP solely because there is no ARP cache entry for a destination.


                                                  |       | | | |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
                                                  |       | | | | | |
Trailer encapsulation                             |2.3.1  | | |x| | |
Send Trailers by default without negotiation      |2.3.1  | | | | |x|
ARP                                               |2.3.2  | | | | | |
  Flush out-of-date ARP cache entries             ||x| | | | |
  Prevent ARP floods                              ||x| | | | |
  Cache timeout configurable                      || |x| | | |
  Save at least one (latest) unresolved pkt       || |x| | | |
Ethernet and IEEE 802 Encapsulation               |2.3.3  | | | | | |
  Host able to:                                   |2.3.3  | | | | | |
    Send & receive RFC-894 encapsulation          |2.3.3  |x| | | | |
    Receive RFC-1042 encapsulation                |2.3.3  | |x| | | |
    Send RFC-1042 encapsulation                   |2.3.3  | | |x| | |
      Then config. sw. to select, RFC-894 dflt    |2.3.3  |x| | | | |
  Send K1=6 encapsulation                         |2.3.3  | | | | |x|
  Use ARP on Ethernet and IEEE 802 nets           |2.3.3  |x| | | | |
Link layer report b'casts to IP layer             |2.4    |x| | | | |
IP layer pass TOS to link layer                   |2.4    |x| | | | |
No ARP cache entry treated as Dest. Unreach.      |2.4    | | | | |x|

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      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

      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

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      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

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      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.

           Silent discard of erroneous datagrams is generally intended
           to prevent "broadcast storms".


      3.2.1 Internet Protocol -- IP
  Version Number: RFC-791 Section 3.1

            A datagram whose version number is not 4 MUST be silently
  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
  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

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            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

                { <Network-number>, <Host-number> }

                { <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

                 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.

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            (f)  { <Network-number>, -1, -1 }

                 Directed broadcast to all subnets of the specified
                 subnetted network.  It MUST NOT be used as a source

            (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

            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 and

            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

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            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

            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.

                 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.
  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.
  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.

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                 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.
  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

            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

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            TOS values up to the transport layer.

            The particular link-layer mappings of TOS contained in RFC-
            795 SHOULD NOT be implemented.

                 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

                 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.
  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.

                 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.

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                 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.
  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.

                 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

                 The IP layer must not crash as the result of an option

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                 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

                 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-

                 Section 3.3.5 presents the rules for a host acting as
                 an intermediate hop in a source route, i.e., forwarding

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                 a source-routed datagram.

                      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.

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                 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 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
               Redirect                  (see Section
               Source Quench             (see Section
               Time Exceeded             (see Section
               Parameter Problem         (see Section

              ICMP query messages:

                Echo                     (see Section
                Information              (see Section
                Timestamp                (see Section
                Address Mask             (see Section

         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

         An ICMP error message SHOULD be sent with normal (i.e., zero)
         TOS bits.

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         An ICMP error message MUST NOT be sent as the result of

         *    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


              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

              This requires that the link layer inform the IP layer when
              a link-layer broadcast datagram has been received; see
              Section 2.4.
  Destination Unreachable: RFC-792

            The following additional codes are hereby defined:

                    6 = destination network unknown

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                    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

            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

            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
  ,, 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).
  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

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            accept both Host and Network Redirects and to process them
            as described in Section 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).
  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.

                 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
  Time Exceeded: RFC-792

            An incoming Time Exceeded message MUST be passed to the
            transport layer.

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                 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.
  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.

                 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
              Code 1 = required option is missing.

                 This variant is currently in use in the military
                 community for a missing security option.
  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.

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                 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
   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.
  Information Request/Reply: RFC-792

            A host SHOULD NOT implement these messages.

                 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.
  Timestamp and Timestamp Reply: RFC-792

            A host MAY implement Timestamp and Timestamp Reply.  If they
            are implemented, the following rules MUST be followed.

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            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.

Top      Up      ToC       Page 45   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

            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

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            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

                 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, 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.

                 The following reasonableness check on an address mask
                 is suggested: the mask is not all 1 bits, and it is

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                 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.

(page 47 continued on part 3)

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