rfc2225.txt

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5.1 Background

   In the LIS scenario, each separate administrative entity configures
   its hosts and routers within a LIS.  Each LIS operates and
   communicates independently of other LISs on the same ATM network.

   In the classical model, hosts communicate directly via ATM to other
   hosts within the same LIS using the ATMARP service as the mechanism
   for resolving target IP addresses to target ATM endpoint addresses.
   The ATMARP service has LIS scope only and serves all hosts in the
   LIS.  Communication to hosts located outside of the local LIS is
   provided via an IP router.  This router is an ATM endpoint attached
   to the ATM network that is configured as a member of one or more
   LISs.  This configuration MAY result in a number of disjoint LISs
   operating over the same ATM network.  Using this model hosts of
   differing IP subnets MUST communicate via an intermediate IP router
   even though it may be possible to open a direct VC between the two IP
   members over the ATM network.

   By default, the ATMARP service and the classical LIS routing model
   MUST be available to any IP member client in the LIS.







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RFC 2225                  IP and ARP over ATM                 April 1998


5.2 LIS Configuration Requirements

   The requirements for IP members (hosts, routers) operating in an ATM
   LIS configuration are:

   o   All members of the LIS have the same IP network/subnet number and
       address mask [8].

   o   All members within a LIS are directly connected to the ATM
       network.

   o   All members of a LIS MUST have a mechanism for resolving IP
       addresses to ATM addresses via ATMARP (based on [3]) and vice
       versa via InATMARP (based on [12]) when using SVCs.  Refer to
       Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.

   o   All members of a LIS MUST have a mechanism for resolving VCs to
       IP addresses via InATMARP (based on [12]) when using PVCs.  Refer
       to Section 8 "LIS ADDRESS RESOLUTION SERVICES" in this memo.

   o   All members within a LIS MUST be able to communicate via ATM with
       all other members in the same LIS; i.e., the Virtual Connection
       topology underlying the intercommunication among the members is
       fully meshed.

   The following list identifies the set of ATM specific parameters that
   MUST be implemented in each IP station connected to the ATM network:

   o   ATM Hardware Address (atm$ha).  The ATM address of the individual
       IP station.

   o   ATMARP Request Address list (atm$arp-req-list): atm$arp-req-list
       is a list containing one or more ATM addresses of individual
       ATMARP servers located within the LIS.  In an SVC environment,
       ATMARP servers are used to resolve target IP addresses to target
       ATM address via an ATMARP request and reply protocol.  ATMARP
       servers MUST have authoritative responsibility for resolving
       ATMARP requests of all IP members using SVCs located within the
       LIS.

   A LIS MUST have a single ATMARP service entry configured and
   available to all members of the LIS who use SVCs.

   In the case where there is only a single ATMARP server within the
   LIS, then all ATMARP clients MUST be configured identically to have
   only one non-null entry in atm$arp-req-list configured with the same
   address of the single ATMARP service.




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RFC 2225                  IP and ARP over ATM                 April 1998


   If the IP member is operating with PVCs only, then atm$arp-req-list
   MUST be configured with all null entries and the client MUST not make
   queries to either address resolution service.

   Within the restrictions mentioned above and in Section 8, local
   administration MUST decide which server address(es) are appropriate
   for atm$arp-req-list.

   By default, atm$arp-req-list MUST be configured using the MIB [18].

   Manual configuration of the addresses and address lists presented in
   this section is implementation dependent and beyond the scope of this
   document; i.e., this memo does not require any specific configuration
   method.  This memo does require that these addresses MUST be
   configured completely on the client, as appropriate for the LIS,
   prior to use by any service or operation detailed in this memo.

5.3 LIS Router Additional Configuration

   It is RECOMMENDED that routers providing LIS functionality over the
   ATM network also support the ability to interconnect multiple LISs.
   Routers that wish to provide interconnection of differing LISs MUST
   be able to support multiple sets of these parameters (one set for
   each connected LIS) and be able to associate each set of parameters
   to a specific IP network/ subnet number.  In addition, it is
   RECOMMENDED that a router be able to provide this multiple LIS
   support with a single physical ATM interface that may have one or
   more individual ATM endpoint addresses.   NOTE: this does not
   necessarily mean different End System Identifiers (ESIs) when NSAPAs
   are used.  The last octet of an NSAPA is the NSAPA Selector (SEL)
   field which can be used to differentiate up to 256 different LISs for
   the same ESI.  (Refer to Section 5.1.3.1, "Private Networks" in [9].)

6.  IP PACKET FORMAT

   Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation as
   described in [2].  LLC/SNAP encapsulation is the default packet
   format for IP datagrams.

   This memo recognizes that other encapsulation methods may be used
   however, in the absence of other knowledge or agreement, LLC/SNAP
   encapsulation is the default.

   This memo recognizes that end-to-end signaling within ATM may allow
   negotiation of encapsulation method on a per-VC basis.






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RFC 2225                  IP and ARP over ATM                 April 1998


7.  DEFAULT VALUE FOR IP MTU OVER ATM AAL5

   Protocols in wide use throughout the Internet, such as the Network
   File System (NFS), currently use large frame sizes (e.g., 8 KB).
   Empirical evidence with various applications over the Transmission
   Control Protocol (TCP) indicates that larger Maximum Transmission
   Unit (MTU) sizes for the Internet Protocol (IP) tend to give better
   performance.  Fragmentation of IP datagrams is known to be highly
   undesirable [16].  It is desirable to reduce fragmentation in the
   network and thereby enhance performance by having the IP Maximum
   Transmission Unit (MTU) for AAL5 be reasonably large.  NFS defaults
   to an 8192 byte frame size.  Allowing for RPC/XDR, UDP, IP, and LLC
   headers, NFS would prefer a default MTU of at least 8300 octets.
   Routers can sometimes perform better with larger packet sizes because
   most of the performance costs in routers relate to "packets handled"
   rather than "bytes transferred".  So, there are a number of good
   reasons to have a reasonably large default MTU value for IP over ATM
   AAL5.

   RFC 1209 specifies the IP MTU over SMDS to be 9180 octets, which is
   larger than 8300 octets but still in the same range [1].  There is no
   good reason for the default MTU of IP over ATM AAL5 to be different
   from IP over SMDS, given that they will be the same magnitude.
   Having the two be the same size will be helpful in interoperability
   and will also help reduce incidence of IP fragmentation.

   Therefore, the default IP MTU for use with ATM AAL5 shall be 9180
   octets.  All implementations compliant and conformant with this
   specification shall support at least the default IP MTU value for use
   over ATM AAL5.

7.1  Permanent Virtual Circuits

   Implementations which only support Permanent Virtual Circuits (PVCs)
   will (by definition) not implement any ATM signalling protocol.  Such
   implementations shall use the default IP MTU value of 9180 octets
   unless both parties have agreed in advance to use some other IP MTU
   value via some mechanism not specified here.

7.2  Switched Virtual Circuits

   Implementations that support Switched Virtual Circuits (SVCs) MUST
   attempt to negotiate the AAL CPCS-SDU size using the ATM signalling
   protocol.  The industry standard ATM signalling protocol uses two
   different parts of the Information Element named "AAL Parameters" to
   exchange information on the MTU over the ATM circuit being setup [9].
   The Forward Maximum CPCS-SDU Size field contains the value over the
   path from the calling party to the called party.  The Backwards



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RFC 2225                  IP and ARP over ATM                 April 1998


   Maximum CPCS-SDU Size Identifier field contains the value over the
   path from the called party to the calling party.  The ATM Forum
   specifies the valid values of this identifier as 1 to 65535
   inclusive.  Note that the ATM Forum's User-to-Network-Interface (UNI)
   signalling permits the MTU in one direction to be different from the
   MTU in the opposite direction, so the Forward Maximum CPCS-SDU Size
   Identifier might have a different value from the Backwards Maximum
   CPCS-SDU Size Identifier on the same connection.

   If the calling party wishes to use the default MTU it shall still
   include the "AAL Parameters" information element with the default
   values for the Maximum CPCS-SDU Size as part of the SETUP message of
   the ATM signalling protocol [9].  If the calling party desires to use
   a different value than the default, it shall include the "AAL
   Parameters" information element with the desired value for the
   Maximum CPCS-SDU Size as part of the SETUP message of the ATM
   Signalling Protocol.  The called party will respond using the same
   information elements and identifiers in its CONNECT message response
   [9].

   If the called party receives a SETUP message containing the "Maximum
   CPCS-SDU Size" in the AAL Parameters information element, it shall
   handle the Forward and Backward Maximum CPCS-SDU Size Identifier as
   follows:

   a)  If it is able to accept the ATM MTU values proposed by the SETUP
       message, it shall include an AAL Parameters information element
       in its response.  The Forward and Backwards Maximum CPCS-SDU Size
       fields shall be present and their values shall be equal to the
       corresponding values in the SETUP message.

   b)  If it wishes a smaller ATM MTU size than that proposed, then it
       shall set the values of the Maximum CPCS-SDU Size in the AAL
       Parameters information elements equal to the desired value in the
       CONNECT message responding to the original SETUP message.

   c)  If the calling endpoint receives a CONNECT message that does not
       contain the AAL Parameters Information Element, but the
       corresponding SETUP message did contain the AAL Parameters
       Information element (including the forward and backward CPCS-SDU
       Size fields), it shall clear the call with cause "AAL Parameters
       cannot be supported".

   d)  If either endpoint receives a STATUS message with cause
       "Information Element Non-existent or Not Implemented" or cause
       "Access Information Discarded", and with a diagnostic field





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RFC 2225                  IP and ARP over ATM                 April 1998


       indicating the AAL Parameters Information Element identifier, it
       shall clear the call with cause "AAL Parameters cannot be
       supported."

   e)  If either endpoint receives CPCS-SDUs in excess of the negotiated
       MTU size, it may use IP fragmentation or may clear the call with
       cause "AAL Parameters cannot be supported".  In this case, an
       error has occurred either due to a fault in an end system or in
       the ATM network.  The error should be noted by ATM network
       management for human examination and intervention.

   If the called endpoint incorrectly includes the Forward and Backward
   Maximum CPCS-SDU Size fields in the CONNECT messages (e.g., because
   the original SETUP message did not include these fields) or it sets
   these fields to an invalid value, then the calling party shall clear
   the call with cause "Invalid Information Element Contents".

7.3  Path MTU Discovery Required

   The Path MTU Discovery mechanism is Internet Standard RFC 1191 [17]
   and is an important mechanism for reducing IP fragmentation in the
   Internet.  This mechanism is particularly important because new
   subnet ATM uses a default MTU sizes significantly different from
   older subnet technologies such as Ethernet and FDDI.

   In order to ensure good performance throughout the Internet and also
   to permit IP to take full advantage of the potentially larger IP
   datagram sizes supported by ATM, all router implementations that
   comply or conform with this specification must also implement the IP
   Path MTU Discovery mechanism as defined in RFC 1191 and clarified by
   RFC 1435 [14].  Host implementations should implement the IP Path MTU
   Discovery mechanism as defined in RFC 1191.

8.  LIS ADDRESS RESOLUTION SERVICES

8.1 ATM-based ARP and InARP Equivalent Services

   Address resolution within an ATM LIS SHALL make use of the ATM
   Address Resolution Protocol (ATMARP) (based on [3]) and the Inverse
   ATM Address Resolution Protocol (InATMARP) (based on [12]) and as
   defined in this memo.  ATMARP is the same protocol as the ARP
   protocol presented in [3] with extensions needed to support address
   resolution in a unicast server ATM environment.  InATMARP is the same
   protocol as the original InARP protocol presented in [12] but applied
   to ATM networks.  All IP stations MUST support these protocols as
   updated and extended in this memo.  Use of these protocols differs
   depending on whether PVCs or SVCs are used.




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