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Versions: 00 02 03 rfc2583                                              
INTERNET-DRAFT                                         R. Carlson
                                                       L. Winkler
                                                       April, 1999

           Guidelines for Next Hop Client (NHC) Developers

1. Status of this memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups.  Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
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time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".

To view the entire list of current Internet-Drafts, please check the
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ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

2. 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.

3. 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

  Carlson, Winkler                                        [Page 1]

  Internet Draft                                       April, 1999

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.

4. 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

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.

  Carlson, Winkler                                        [Page 2]

  Internet Draft                                       April, 1999

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.

5. 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 test to see if the destination node is able to accept
          a `shortcut' path.  If so save the IP to ATM AESA mapping in
          the local NHC cache.
     3.   If not; 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.

6. Need for State

  Carlson, Winkler                                        [Page 3]

  Internet Draft                                       April, 1999

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

  Carlson, Winkler                                        [Page 4]

  Internet Draft                                       April, 1999

(e.g. use shortcut for FTP) and a local state (e.g. use routed path
for DNS).

6.1 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.

6.2 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.

6.3 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.

7. 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.

  Carlson, Winkler                                        [Page 5]

  Internet Draft                                       April, 1999

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)

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:
     1.   USE_SYSTEM_DEFAULT.  Use the shortcut or routed path based
          on the system configuration information for this
          application.  (This is the default behavior.)
     2.   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.
     3.   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
     4.   USE_ROUTED_PATH.  Use the routed path regardless of whether
          a shortcut exists or not.
     5.   TRY_ROUTED_PATH.  If a shortcut doesn't exist, don't try and
          create it, use the routed path instead.

8. Security

Some of the security issues are addressed in the other NHRP documents.
Some specific questions for the NHC are:
     *    How do we verify the returned ATM Address?  Do we need to?
          [If it is bogus then things don't work right?   Is this the
          same as spoofing?  Can I spoof both the ATM Address and IP
          address of another host?]

  Carlson, Winkler                                        [Page 6]

  Internet Draft                                       April, 1999

     *    Is there a denial of service attack possible?
     *    Are there any problems with having the state stored on a per
          process bases?
     *    Are there any problems with having the user request/set a
          specific path?

9. Authors Address

Richard Carlson
Argonne National Laboratory

Linda Winkler
Argonne National Laboratory

10. References:

[1] "Classical IP and ARP over ATM", RFC-2225, M. Laubach J. Halpern,
April 1998.
[2] "NBMA Next Hop Resolution Protocol (NHRP)", RFC-2332, J. Luciani
D. Katz D. Piscitello B. Cole N. Doraswamy, April 1998.
[3] "NHRP Protocol Applicability Statement", RFC-2333, D. Cansever,
April 1998.
[4] "Classical IP to NHRP Transition", RFC-2336, J. Luciani, July
[5] "Local/Remote Forwarding Decision in Switched Data link
Subnetworks", RFC-1937, Y. Rekhter & D. Kandlur, May 1996.

  Carlson, Winkler                                        [Page 7]