Network Working Group                                       R. Carlson
Request for Comments: 2583                                         ANL
Category: Informational                                     L. Winkler
                                                                   ANL
                                                              May 1999


            Guidelines for Next Hop Client (NHC) Developers

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Abstract

   This document provides guidelines for developers of the Next Hop
   Resolution Protocol Clients (NHC).  It assumes that the clients are
   directly connected to an ATM based NBMA network.  The same principles
   will apply to clients connected to other types of NBMA networks.  The
   intent is to define the interaction between the NHC code and the
   TCP/IP protocol stack of the local host operating system.  The NHC is
   capable of sending NHRP requests to a Next Hop Resolution Protocol
   Server (NHS) to resolve both inter and intra LIS addresses.  The NHS
   reply may be positive (ACK) indicating a short-cut path is available
   or negative (NAK) indicating that a shortcut is not available and the
   routed path must be used.  The NHC must cache (maintain state) for
   both the ACK and NAK replies in order to use the correct shortcut or
   routed path.  The NAK reply must be cached to avoid making repeated
   requests to the NHS when the routed path is being used.

2. Overview

   In the Classical IP over ATM model [1], an ATM attached host
   communicates with an ATMARP server to resolve IP to ATM address
   semantics.  This model supports the concept of a Logical IP Subnet
   (LIS) with intra LIS communications using direct PVCs/SVCs and inter
   LIS communications using IP routers to forward packets.  This model
   easily maps to the conventional LAN model of subnets and routers.
   The Next Hop Resolution Protocol (NHRP) [2] defines how the LIS model
   can be modified to allow direct ATM SVCs (shortcut paths) for inter
   LIS traffic.  With NHRP, nodes directly attached to an ATM network
   can bypass the IP routers and establish a direct switched virtual

   circuit to improve performance when needed.

   The NHS code replaces the ATMARP code in the ATMARP server.  Each NHS
   serves a set of destination client hosts and cooperates with other
   NHSs to resolve NHRP next hop requests within their own logical ATM
   network. The NHC to NHS and NHS to NHS protocol interactions are
   described in [2].  Other documents in the NHRP series define the
   general applicability [3] and the transition from ATMARP servers to
   NHSs [4].

   The NHC code replaces the ATMARP code in the local workstations.
   This code will take the destination IP address and map it into the
   ATM End Station Address (AESA) for both intra and inter LIS
   destinations.  The returned AESA will be stored in a local cache
   table.  In addition to storing the positive replies, the NHC will
   need to store the negative replies to avoid making repeated NHS calls
   when using the routed path.

   This document describes a base line method for caching the returned
   information.  Other methods may be used as long as the same
   functionality is provided.

3. IP Processing

   In the Classical IP LIS model [1] the TCP/IP protocol stack treats
   the ATM network as a simple data link layer protocol.  When an
   application sends data using the Classical IP protocol, IP performs a
   routing table lookup to determine if the destination is reachable via
   a local interface or whether an intermediate router is the next hop
   to the IP destination.

   If the destination is found to be local (e.g. in the same LIS as the
   source) the packet will be passed to the local ATM interface with the
   next hop IP address set to the destination nodes IP address.  At this
   point the ATMARP table will be searched to determine the ATM Address
   of the destination node.  If no ATMARP table entry is found an ATMARP
   request will be sent to the ATMARP server.  This server can reply
   with a positive (ACK) or negative (NAK) answer depending on the
   current information it has in its cache.  If an ACK is received the
   host's local ATMARP table is filled in appropriately and the source
   is now able to send IP datagrams to the destination.  If a NAK is
   returned, the calling application is notified of this error condition
   (e.g., ICMP destination unreachable).

   If the destination is found to be remote (e.g., in a different LIS
   from the source) the IP address of the next hop router is extracted
   from the IP routing table and the ATM Address of this router is
   looked up in the ATMARP table.  Since the router is in the same LIS

   as the source node, the ATMARP procedure described above will find
   the correct ATM Address or the packet will be marked as undeliverable
   and the user application will be notified of the error.

   The ATMARP service functions exactly as the existing ARP service
   provided on Ethernet broadcast networks.  Since the ARP service will
   only try and resolve addresses for nodes that are in a single IP
   subnet, the ARP table only needs to keep positive answers.  No state
   information is retained about failed mappings.

4. NHC Processing

   In this section we briefly describe what is required in order for a
   host to take advantage of shortcuts through the ATM network.  On the
   host, a NHC process initiates various NHRP requests in order to
   obtain access to the NHRP service. Within the ATM subnetwork, the
   ATMARP server is replaced with a NHS.  As defined in [4] the NHS is
   required to respond to both ATMARP and NHRP Resolution requests.  In
   the nodes wishing to take advantage of shortcut paths across the ATM
   subnetwork, the ATMARP client code must be replaced with NHC code.
   This allows the source node to ask for the ATM AESA of both local and
   remote nodes.  Finally the source node must be modified to know when
   it should ask for the ATM AESA of a remote node and when the local
   LIS router should be used.  These modifications are described in the
   remainder of this document.

   The protocol processing described in [2] states a source may query a
   NHS for the ATM AESA of a destination node.  However as is pointed
   out in [5], to achieve shortcut paths through the ATM network, it is
   not enough to simply replace the ATMARP client code with the NHC
   code.  This is because the source host will never ask the NHS for the
   ATM AESA of a node in a remote LIS.  When the source consults the IP
   routing table, it performs the local/remote test, before the NHC code
   is processed.  As a result, the IP address of the next hop router
   will be used by the NHC instead of the IP address of the remote
   (inter LIS) host.  The NHC code must ignore the result of the IP
   routing table lookup and perform its own local/remote test.

   The NHC must perform the following functions:

   1.     Test to see if the destination node is `local' to this LIS.
          If so use the existing ATMARP rules described in [1].
   2.     If not; send an NHRP message to the local NHS and attempt to
          setup a `shortcut' path.  If successful; save the IP to ATM
          AESA mapping in the local NHC cache.
   3.     If not successful; use the routed path and save this state in
          the NHC cache so future requests don't test for a shortcut
          again.

   4.     Allow user application to override system default operation
          and explicitly request a shortcut or routed path for a flow.

   It is required that this routed path state will be maintained in the
   same manner as the existing ATMARP service.  That is a timer will be
   used to expire old information and some administrative function
   exists to manually delete data if needed.

5. Need for State

   It is obvious that the IP to ATM AESA mappings should be maintained
   in a local cache to improve network performance.  This soft state is
   maintained in today's ARP and ATMARP systems using timers to purge
   old or unused data.  The NHC will maintain both inter and intra LIS
   IP to ATM Address mappings in the same manner.  It may be less
   obvious that an NHC will also need to maintain this same soft state
   for inter LIS mappings using the routed path.  If this state is not
   maintained, the source node will send requests to the NHS asking if a
   shortcut path can be setup every time a packet is sent over the
   routed path.

   Some of the features of this state are:

   1.     Cache lookups must be fast as they are done on every packet.
   2.     The cache lookup must be on the destination IP address instead
          of the next-hop router IP address.
   3.     Both ACK and NAK data should be cached for the length of the
          holding time parameter in the NHRP response.

   Since state must be maintained, the questions of where to maintain
   it, how to manually managed it, and how to selectively override it
   need to be addressed.  No matter where this state information is
   kept, a method for manually examining and changing this state
   information must be provided.  This is essential to insure that the
   network is operating properly.

   There are several possible locations for storing this state
   information, they are:

   1.     Store state in the `ARP' table.  This is the traditional
          location for this IP to ATM address mappings.  This table must
          be extended to handle the caching of negative (routed path)
          information. This solution provides a system wide service that
          may be used by the NHC.
   2.     Store state in the IP routing table.  This is the traditional
          location for the local/remote state information.

   3.     Store state in an ATM MIB structure.  This is the traditional
          location for storing ATM VCC data.  It also provides a system
          wide service that is geared toward ATM services.  This avoids
          munging the `ARP' table to hold negative data.
   4.     Store state in the TCP Process Control Block.  This allows a
          per process tailoring of shortcut or routed path information.
          This works well for TCP connections, but not UDP style
          services.
   5.     Store state in the socket structure.  This also allows per
          process tailoring of the state information.
   6.     Store state in a newly defined table.

   The NHC should also support both local (per-process) and global
   (per-system) state.  This would allow a system wide default while
   allowing a specific application to tailor the operation for a
   specific task.  For example assume a site runs both a DNS server and
   FTP server on a single host.  Inter LIS communications to the DNS
   server should take the routed path to avoid setup overhead.  While an
   FTP session would benefit from the shortcut path to improve
   performance.  Supporting both operations from a single client will
   require both a global state (e.g. use shortcut for FTP) and a local
   state (e.g. use routed path for DNS).

5.1 Using TCP

   TCP is a connection orientated protocol that provides per-process
   state information using a TCP Protocol Control Block (PCB).  This PCB
   can be used to save the shortcut/routed path state information. Using
   a quad-state flag that shows the USE_SHORT_CUT, TRY_SHORT_CUT,
   USE_ROUTED_PATH, or TRY_ROUTED_PATH states would allow each process
   to use the service it chooses.  The advantage of this approach is
   that it allows per flow control over the use of the shortcut or
   routed path.  The disadvantage is that this PCB is only created for
   TCP connections.  UDP connections will only use the system default
   action.

   A second option is to store this information in the socket PCB and
   use the socket function (setsockopt) to save this information.  This
   option will allow both TCP and UDP applications to set a per flow
   action to override the system default operation.  To enable this
   option, the IP kernel code will need to be modified to allow this
   quad-state flag to be set.  In addition this flag will need to be
   checked when each packet is sent to determine the if the shortcut or
   routed path is being used.

5.2 Using UDP

   UDP is a connectionless orientated protocol that doesn't provide any
   support for state information.  It relies on the application to
   provide the necessary state information.  In this case where should
   the state be stored?  The user application could store this itself
   and pass this down to the kernel in some manner.  Another option is
   to store this information in an ATM MIB structure.  A third option is
   to allow a socket option (setsockopt) that the user application can
   set to override the default behavior.

5.3 Using ICMP

   In keeping with the tradition of using ICMP echo packets for Internet
   management functions (e.g. ping, traceroute) then it will be
   necessary to allow these applications to run over the shortcut and
   routed paths.  The user will need to be able to specify which path to
   use and a default action needs to be defined too.

6. Conclusions

   NHRP provides new services and functionality for IP nodes using ATM
   networks.  To use these services the client must store state
   information that describes whether a destination node is reachable
   via a shortcut or a routed path.

   The state information should be stored on a global per-application
   basis with per-process override functionality.  This allows short
   lived functions (e.g. DNS requests) and long lived requests (e.g. ftp
   sessions) to use different paths.  Storing state only based on the
   destination address means that all processes must use the same path
   and this creates unreasonable demands on the network.  To accomplish
   this the /etc/services file should be modified to carry a new flag to
   indicate the per-application default (shortcut vs. routed path)
   behavior.

   This state information is required to avoid having the client make a
   call to the NHS for every packet it sends along the routed path.  It
   is recommended that the IP routing table be modified to support a new
   flag.  This flag will indicate whether the NHS returned an ACK or NAK
   to the NHRP request.

   In addition, application programmers and system administrators
   require the ability to explicitly request a specific service (e.g.
   use the routed path or shortcut path).  This includes the ability to
   verify network operation by specifying how ICMP echo requests (e.g.
   ping, traceroute) are handled.  The NHC must support the manual
   setting of this state information.  A new socket option that allows

   the user to specify the operation needs to be supported.

   To support this capability a new socket option will be created to
   allow the user application to control the operation of a particular
   connection (flow).  This option will allow the user to specify that a
   connection use one of the following:

   *      USE_SYSTEM_DEFAULT.  Use the shortcut or routed path based on
          the system configuration information for this application.
          (This is the default behavior.)
   *      USE_SHORT_CUT.  If a shortcut path exists, then use it to
          deliver the data.  If it doesn't exist, then try and create
          it.  If the shortcut cannot be created, fail the connection
          and notify the user.
   *      TRY_SHORT_CUT.  If a shortcut path exists, then use it to
          deliver the data.  If it doesn't exist, then try and create
          it.  If the shortcut cannot be created, try using the routed
          path.
   *      USE_ROUTED_PATH.  Use the routed path regardless of whether a
          shortcut exists or not.
   *      TRY_ROUTED_PATH.  If a shortcut doesn't exist, don't try and
          create it, use the routed path instead.

7. Security

   The security issues for NHRP are addressed in other NHRP documents
   [2,3].  Some specific security issues for the NHC developer are
   discussed below.

   *      Address spoofing at the IP or ATM layer may allow an attacker
          to hi-jack an IP connection or service. This threat may be
          reduced by limiting the scope of the ATM routing domain.  In
          this way only trusted IP hosts will be able to reach and use
          the services of the NHS.
   *      Denial of service attacks may be launched at both the IP and
          ATM layers of the NHS.  At the ATM layer, the attacker may
          repeatedly generate signaling messages that consuming system
          resources thus preventing NHCs from using the NHS services.
          At the IP layer, the attacker may register false IP to ATM
          mappings thus preventing a NHC from registering the correct IP
          to ATM mapping.
   *      When a NHC creates or accepts a short-cut path it bypasses the
          site border router.  Therefore, any security features in the
          border router are also bypassed.  This threat may be reduced
          by limiting the scope of the ATM routing domain, increasing

          security features in the NHC host, allowing the NHS to
          evaluate security features when short-cut paths are requested
          or a compination of all of these methods.

8. Authors' Addresses

   Richard Carlson
   Argonne National Laboratory

   EMail: RACarlson@anl.gov


   Linda Winkler
   Argonne National Laboratory

   EMail: lwinkler@anl.gov

9. References:

   [1] Laubach, M. and J. Halpern, "Classical IP and ARP over ATM", RFC
       2225, April 1998.

   [2] Luciani, J., Katz, D., Piscitello, D., Cole B. and N. Doraswamy,
       "NBMA Next Hop Resolution Protocol (NHRP)", RFC 2332, April 1998.

   [3] Cansever, D., "NHRP Protocol Applicability Statement", RFC 2333,
       April 1998.

   [4] Luciani, J., "Classical IP to NHRP Transition", RFC 2336, July
       1998.

   [5] Rekhter, Y. and D. Kandlur, "Local/Remote Forwarding Decision in
       Switched Data link Subnetworks", RFC 1937, May 1996.

10.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

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