rfc1122.txt

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   2.1  INTRODUCTION

      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.

   2.2  PROTOCOL WALK-THROUGH

      None.

   2.3  SPECIFIC ISSUES

      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.

         DISCUSSION:
              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
              successfully.

         IMPLEMENTATION:
              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
              encapsulations.

              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
              request.

      2.3.2  Address Resolution Protocol -- ARP

         2.3.2.1  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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            destination.

            DISCUSSION:
                 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.

            IMPLEMENTATION:
                 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.

         2.3.2.2  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.

            DISCUSSION:
                 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
              encapsulation;

         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-
         894.



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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.

         DISCUSSION:
              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.

   2.4  LINK/INTERNET LAYER INTERFACE

      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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