RFC2583 - Guidelines for Next Hop Client (NHC) Developers

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.

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

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 RFCEditor function is currently provided by

the Internet Society.