RFC2583 - Guidelines for Next Hop Client (NHC) Developers

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　　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)", RFC2332, April 1998.

[3] Cansever, D., "NHRP Protocol Applicability Statement", RFC2333,
April 1998.

[4] Luciani, J., "Classical IP to NHRP Transition", RFC2336, July
1998.

[5] Rekhter, Y. and D. Kandlur, "Local/Remote Forwarding Decision in
Switched Data link Subnetworks", RFC1937, May 1996.

10. Full Copyright Statement

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

This document and translations of it may be copied and furnished to
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Acknowledgement

Funding for the RFCEditor function is currently provided by
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