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Network Working Group                                       R. CarlsonRequest for Comments: 2583                                         ANLCategory: Informational                                     L. Winkler                                                                   ANL                                                              May 1999Guidelines for Next Hop Client (NHC) DevelopersStatus 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 virtualCarlson & Winkler            Informational                      [Page 1]

RFC 2583             Guidelines for NHC Developers              May 1999   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 LISCarlson & Winkler            Informational                      [Page 2]

RFC 2583             Guidelines for NHC Developers              May 1999   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.Carlson & Winkler            Informational                      [Page 3]

RFC 2583             Guidelines for NHC Developers              May 1999   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.Carlson & Winkler            Informational                      [Page 4]

RFC 2583             Guidelines for NHC Developers              May 1999   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.Carlson & Winkler            Informational                      [Page 5]

RFC 2583             Guidelines for NHC Developers              May 19995.2 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 allowsCarlson & Winkler            Informational                      [Page 6]

RFC 2583             Guidelines for NHC Developers              May 1999   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, increasingCarlson & Winkler            Informational                      [Page 7]

RFC 2583             Guidelines for NHC Developers              May 1999          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.gov9. References:   [1] Laubach, M. and J. Halpern, "Classical IP and ARP over ATM",RFC2225, 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.Carlson & Winkler            Informational                      [Page 8]

RFC 2583             Guidelines for NHC Developers              May 199910.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by   the Internet Society.Carlson & Winkler            Informational                      [Page 9]