Network Working Group R. Clark
Request for Comments: 1683 M. Ammar
Category: Informational K. Calvert
Georgia Institute of Technology
August 1994
Multiprotocol Interoperability In IPng
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This document was submitted to the IETF IPng area in response to RFC
1550. Publication of this document does not imply acceptance by the
IPng area of any ideas expressed within. Comments should be
submitted to the big-internet@munnari.oz.au mailing list.
1. Executive Summary
The two most commonly cited issues motivating the introduction of
IPng are address depletion and routing table growth in IPv4. Further
motivation is the fact that the Internet is witnessing an increasing
diversity in the protocols and services found in the network. When
evaluating alternatives for IPng, we should consider how well each
alternative addresses the problems arising from this diversity. In
this document, we identify several features that affect a protocol's
ability to operate in a multiprotocol environment and propose the
incorporation of these features into IPng.
Our thesis, succinctly stated, is: The next generation Internet
Protocol should have features that support its use with a variety of
protocol architectures.
2. Introduction
The Internet is not a single protocol network [4]. While TCP/IP
remains the primary protocol suite, other protocols (e.g., IPX,
AppleTalk, OSI) exist either natively or encapsulated as data within
IP. As new protocols continue to be developed, we are likely to find
that a significant portion of the traffic in future networks is not
from single-protocol communications. It is important to recognize
that multiprotocol networking is not just a transition issue. For
instance, we will continue to see tunneling used to carry IPX traffic
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over the Internet between two Novell networks. Furthermore, the
introduction of IPng is not going to result in a near term
elimination of IPv4. Even when IPng becomes the primary protocol
used in the Internet, there will still be IPv4 systems in use. We
should consider such multiprotocol uses of the network as we design
future protocols that can efficiently handle mixed protocol traffic.
We have identified several issues related to the way in which
protocols operate in a multiprotocol environment. Many of these
issues have traditionally been deemed "less important" by protocol
designers since their goal was to optimize for the case where all
systems supported the same protocol. With the increasing diversity
of network protocols, this approach is no longer practical. By
addressing the issues outlined in this paper, we can simplify the
introduction of IPng to the Internet and reduce the risk for network
managers faced with the prospect of supporting a new protocol. This
will result in a faster, wider acceptance of IPng and increased
interoperability between Internet hosts. In addition, by designing
IPng to address these issues, we will make the introduction of future
protocols (IPng2) even easier.
The outline for this document is as follows. In Section 3 we
motivate the issues of multiprotocol networking with a discussion of
an example system. In Section 4 we describe three main techniques
for dealing with multiple protocols. This is followed in Section 5
by a description of the various protocol features that are important
for implementing these three techniques. We conclude in Section 6
with a summary of the issues raised.
3. Multiprotocol Systems
Consider the multiprotocol architecture depicted in Figure 1. A
system supporting this architecture provides a generic file-transfer
service using either the Internet or OSI protocol stacks. The
generic service presents the user with a consistent interface,
regardless of the actual protocols used. The user can transfer files
between this host and hosts supporting either of the single protocol
stacks presented in Figures 2a and 2b. To carry out this file
transfer, the user is not required to decide which protocols to use
or to adjust between different application interfaces.
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+-----------------------------------+
| File Transfer Service |
+-----------+-----------------------+
| | FTAM |
| +-----------------------+
| FTP | ISO 8823 |
| +-----------------------+
| | ISO 8327 |
| +-----------+-----------+
| |TP0/RFC1006| TP4 |
+-----------+-----------+ |
| TCP | |
+-----------+-----------+-----------+
| IP | CLNP |
+-----------+-----------------------+
Figure 1: Multiprotocol architecture providing file-transfer service
+-----------+ +-----------+ +-----------+ +-----------+
| FTP | | FTAM | | FTAM | | FTP |
+-----------+ +-----------+ +-----------+ +-----------+
| TCP | | ISO 8823 | | ISO 8823 | | TCP |
+-----------+ +-----------+ +-----------+ +-----------+
| IP | | ISO 8327 | | ISO 8327 | | CLNP |
+-----------+ +-----------+ +-----------+ +-----------+
| TP4 | |TP0/RFC1006|
+-----------+ +-----------+
| CLNP | | TCP |
+-----------+ +-----------+
| IP |
+-----------+
a) TCP/IP b) OSI c) RFC 1006 d) TUBA
Figure 2: Protocol stacks providing file-transfer service.
Figure 2c depicts a mixed stack architecture that provides the upper
layer OSI services using the Internet protocols. This is an example
of a "transition architecture" for providing OSI applications without
requiring a full OSI implementation. Figure 2d depicts a mixed stack
architecture that provides the upper layer Internet applications
using the OSI network protocol. In addition to communicating with
the two previous simple protocol stacks, the multiprotocol system of
Figure 1 includes all the protocols necessary to communicate with
these two new, mixed protocol stacks.
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It is likely that many future network systems will be configured to
support multiple protocols including IPng. As the IPng protocol is
deployed, it is unreasonable to expect that users will be willing to
give up any aspect of their current connectivity for the promise of a
better future. In reality, most IPng installations will be made "in
addition to" the current protocols. The resulting systems will
resemble Figure 1 in that they will be able to communicate with
systems supporting several different protocols.
Unfortunately, in most current examples, the architecture of Figure 1
is implemented as independent protocol stacks. This means that even
though both TCP and CLNP exist on the system, there is no way to use
TCP and CLNP in the same communication. The problem with current
implementations of architectures like Figure 1 is that they are
designed as co-existence architectures and are not integrated
interoperability systems. We believe future systems should include
mechanisms to overcome this traditional limitation. By integrating
the components of multiple protocol stacks in a systematic way, we
can interoperate with hosts supporting any of the individual stacks
as well as those supporting various combinations of the stacks.
In order to effectively use multiple protocols, a system must
identify which of the available protocols to use for a given
communication task. We call this the Protocol Determination [2]
task. In performing this task, a system determines the combination
of protocols necessary to provide the needed service. For achieving
interoperability, protocols are selected from the intersection of
those supported on the systems that must communicate.
4. Multiprotocol Techniques
In this section we identify three main techniques to dealing with
multiprotocol networks that are in use today and will continue to be
used in the Internet. The first two techniques, tunneling and
conversion, are categorized as intermediate-system techniques in that
they are designed to achieve multiprotocol support without changing
the end-systems. The third technique explicitly calls for the
support of multiple protocols in end-systems. By describing these
techniques here, we can motivate the need for the specific protocol
features described in Section 5.
4.1 Encapsulation/Tunneling
Encapsulation or tunneling is commonly used when two networks that
support a common protocol must be connected using a third
intermediate network running a different protocol. Protocol packets
from the two end networks are carried as data within the protocol of
the intermediate network. This technique is only appropriate when
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both end-systems support the same protocol stack. It does not
provide interoperability between these end systems and systems that
only support the protocol stack in the intermediate network. Some
examples of this technique are: a mechanism for providing the OSI
transport services on top of the Internet protocols [13],
encapsulating IEEE 802.2 frames in IPX network packets [5], tunneling
IPX [10] and AppleTalk traffic over the Internet backbone. We expect
IPng to be used for tunneling other network protocols over IPng and
to be encapsulated.
4.2 Translation/Conversion
Despite their known limitations [8], translation or conversion
gateways are another technique for handling multiple protocols [11,
12]. These gateways perform direct conversion of network traffic
from one protocol to another. The most common examples of conversion
gateways are the many electronic mail gateways now in use in the
Internet. In certain cases it may also be feasible to perform
conversion of lower layer protocols such as the network layer. This
technique has been suggested as part of the transition plan for some
of the current IPng proposals [3, 15].
4.3 Multiprotocol End-Systems
We expect that IPng will be introduced as an additional protocol in
many network systems. This means that IPng should be able to coexist
with other protocols on both end- and intermediate-systems.
Specifically, IPng should be designed to support the Protocol
Determination task described in Section 3.
One technique that we consider for solving the Protocol Determination
problem is to employ a directory service in distributing system
protocol configuration information. We have developed and
implemented mechanism for using the Internet Domain Name System (DNS)
[6, 7] to distribute this protocol information [2]. Using this
mechanism, a multiprotocol host can determine the protocol
configuration of a desired host when it retrieves the network address
for that host. Then the multiprotocol host can match the
configuration of the desired host to its own configuration and
determine which protocols should be used to carry out the requested
communication service.
Another alternative to determining protocol information about another
host is Protocol Discovery. Using this approach, a host determines
which protocols to use by trial-and-error with the protocols
currently available. The initiating host monitors successive
attempts to communicate and uses the information gained from that
monitoring to build a knowledge base of the possible protocols of the
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remote system.
This knowledge is used to determine whether or not a communication
link can be established and if it can, which protocol should be used.
An important aspect of the Protocol Discovery approach is that it
requires an error and control feedback system similar to ICMP [9],
but with additional functionality (See Section 5).
5. Protocol Features
In this section we identify features that affect a protocol's ability
to support the multiprotocol techniques described in the previous
section. These features indicate specific areas that should be
considered when comparing proposed protocols. We present two
different types of protocol features: those that should be included
as part of the IPng protocol standard, and those that should be
considered as part of the implementation and deployment requirements
for IPng.
5.1 Protocol Standard Features
o Addressing
A significant problem in dealing with multiprotocol networks is
that most of the popular network protocols use different
addressing mechanisms. The problem is not just with different
lengths but also with different semantics (e.g., hierarchical vs.
flat addresses). In order to accommodate these multiple formats,
IPng should have the flexibility to incorporate many address
formats within its addressing mechanism.
A specific example might be for IPng to have the ability to
include an IPv4 or IPX address as a subfield of the IPng address.
This would reduce the complexity of performing address conversion
by limiting the number of external mechanisms (e.g., lookup
tables) needed to convert an address. This reduction in
complexity would facilitate both tunneling and conversion. It
would also simplify the task of using IPng with legacy
applications which rely on a particular address format.
o Header Option Handling
In any widely used protocol, it is advantageous to define option
mechanisms for including header information that is not required
in all packets or is not yet defined. This is especially true in
multiprotocol networks where there is wide variation in the
requirements of protocol users. IPng should provide efficient,
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flexible support for future header options. This will better
accommodate the different user needs and will facilitate
conversion between IPng and other protocols with different
standard features.
As part of the support for protocol options, IPng should include a
mechanism for specifying how a system should handle unsupported
options. If a network system adds an option header, it should be
able to specify whether another system that does not support the
option should drop the packet, drop the packet and return an
error, forward it as is, or forward it without the option header.
The ability to request the "forward as is" option is important
when conversion is used. When two protocols have different
features, a converter may introduce an option header that is not
understood by an intermediate node but may be required for
interpretation of the packet at the ultimate destination. On the
other hand, consider the case where a source is using IPng with a
critical option like encryption. In this situation the user would
not want a conversion to be performed where the option was not
understood by the converter. The "drop the packet" or "drop and
return error" options would likely be used in this scenario.
o Multiplexing
The future Internet protocol should support the ability to
distinguish between multiple users of the network. This includes
the ability to handle traditional "transport layer" protocols like
TCP and UDP, as well as other payload types such as encapsulated
AppleTalk packets or future real-time protocols. This kind of
protocol multiplexing can be supported with an explicit header
field as in IPv4 or by reserving part of the address format as is
done with OSI NSEL's.
In a multiprotocol network there will likely be a large number of
different protocols running atop IPng. It should not be necessary
to use a transport layer protocol for the sole purpose of
providing multiplexing for the various network users. The cost of
this additional multiplexing is prohibitive for future high-speed
networks [14]. In order to avoid the need for an additional level
of multiplexing, the IPng should either use a payload selector
larger than the 8-bits used in IPv4 or provide an option for
including additional payload type information within the header.
o Status/Control Feedback
With multiple protocols, the correct transmission of a packet
might include encapsulation in another protocol and/or multiple
conversions to different protocols before the packet finally
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reaches its destination. This means that there are many different
places the transmission can fail and determining what went wrong
will be a challenge.
In order to handle this situation, a critical protocol feature in
multiprotocol networks is a powerful error reporting mechanism.
In addition to reporting traditional network level errors, such as
those reported by ICMP [9], the IPng error mechanism should
include feedback on tunneling and conversion failures. Also,
since it is impossible to know exactly which part of a packet is
an encapsulated header, it is important that the feedback
mechanism include as much of the failed packet as possible in the
returned error message.
In addition to providing new types of feedback, this mechanism
should support variable resolution such that a transmitting system
can request limited feedback or complete information about the
communication process. This level of control would greatly
facilitate the Protocol Discovery process described in Section
4.3. For example, a multiprotocol system could request maximal
feedback when it sends packets to a destination it has not
communicated with for some time. After the first few packets to
this "new" destination, the system would revert back to limited
feedback, freeing up the resources used by the network feedback
mechanisms.
Finally, it is important that the information provided by the
feedback mechanism be available outside the IPng implementation.
In multiprotocol networks it is often the case that the solution
to a communication problem requires an adjustment in one of the
protocols outside the network layer. In order for this to happen,
the other protocols must be able to access and interpret these
feedback messages.
o MTU Discovery or Fragmentation
A form of multiprotocol support that has long been a part of
networking is the use of diverse data link and physical layers.
One aspect of this support that affects the network layer is the
different Maximum Transmission Units (MTU) used by various media
formats. For efficiency, many protocols will attempt to avoid
fragmentation at intermediate nodes by using the largest packet
size possible, without exceeding the minimum MTU along the route.
To achieve this, a network protocol performs MTU discovery to find
the smallest MTU on a path.
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The choice of mechanism for dealing with differing MTUs is also
important when doing conversion or tunneling with multiple
protocols. When tunneling is performed by an intermediate node,
the resulting packets may be too large to meet the MTU
requirements. Similarly, if conversion at an intermediate node
results in a larger protocol header, the new packets may also be
too large. In both cases, it may be desirable to have the source
host reduce the transmission size used in order to prevent the
need for additional fragmentation. This information could be sent
to the source host as part of the previously described feedback
mechanism or as an additional MTU discovery message.
5.2 Implementation/Deployment Features
o Switching
We define switching in a protocol as the capability to
simultaneously use more than one different underlying protocol
[1]. In network layer protocols, this implies using different
datalink layers. For example, it may be necessary to select
between the 802.3 LLC and traditional Ethernet interfaces when
connecting a host to an "ethernet" network. Additionally, in some
systems IPng will not be used directly over a datalink layer but
will be encapsulated within another network protocol before being
transmitted. It is important that IPng be designed to support
different underlying datalink services and that it provide
mechanisms allowing IPng users to specify which of the available
services should be used.
o Directory Service Requirements
While not specifically a part of the IPng protocol, it is clear
that the future Internet will include a directory service for
obtaining address information for IPng. In light of this, there
are some features of the directory service that should be
considered vis-a-vis their support for multiple protocols.
First, the directory service should be able to distribute address
formats for several different protocol families, not just IPng and
IPv4. This is necessary for the use of tunneling, conversion, and
the support of multiprotocol systems. Second, the directory
service should include support for distributing protocol
configuration information in addition to addressing information
for the network hosts. This feature will support the protocol
determination task to be carried out by multiprotocol systems [2].
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6. Conclusion
Future networks will incorporate multiple protocols to meet diverse
user requirements. Because of this, we are likely to find that a
significant portion of the traffic in the Internet will not be from
single-protocol communications (e.g., TCPng/IPng). This will not
just be true of near term, transitional networks but will remain as a
reality for most of the Internet. As we pursue the selection of
IPng, we should consider the special needs of multiprotocol networks.
In particular, IPng should include mechanisms to handle mixed
protocol traffic that includes tunneling, conversion, and
multiprotocol end-systems.
7. Acknowledgments
The authors would like to acknowledge the support for this work by a
grant from the National Science Foundation (NCR-9305115) and the
TRANSOPEN project of the Army Research Lab (formerly AIRMICS) under
contract number DAKF11-91-D-0004.
8. References
[1] Clark, R., Ammar, M., and K. Calvert, "Multi-protocol
architectures as a paradigm for achieving inter-operability", In
Proceedings of IEEE INFOCOM, April 1993.
[2] Clark, R., Calvert, K. and M. Ammar, "On the use of directory
services to support multiprotocol interoperability", To appear in
proceedings of IEEE INFOCOM, 1994. Technical Report GIT-CC-93/56,
College of Computing, Georgia Institute of Technology, ATLANTA,
GA 30332-0280, August 1993.
[3] Gilligan, R., Nordmark, E., and B. Hinden, "IPAE: the SIPP
Interoperability and Transition Mechanism, Work in Progress,
November 1993.
[4] Leiner, B., and Y. Rekhter, "The Multiprotocol Internet", RFC
1560, USRA, IBM, December 1993.
[5] McLaughlin, L., "Standard for the Transmission of 802.2 Packets
over IPX Networks", RFC 1132, The Wollongong Group, November
1989.
[6] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
13, RFC 1034, USC/Information Sciences Institute, November 1987.
Clark, Ammar & Calvert [Page 10]
RFC 1683 Multiprotocol Interoperability In IPng August 1994
[7] Mockapetris, P., "Domain Names - Implementation and
Specification. STD 13, RFC 1035, USC/Information Sciences
Institute, November 1987.
[8] Padlipsky, M., Gateways, Architectures, and Heffalumps", RFC 875,
MITRE, September 1982.
[9] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
USC/Information Sciences Institute, September 1981.
[10] Provan, D., "Tunneling IPX Traffic Through IP Networks", RFC
1234, Novell, Inc., June 1991.
[11] Rose, M., "The Open Book", Prentice-Hall, Englewood Cliffs, New
Jersey, 1990.
[12] Rose, M., "The ISO Development Environment User's Manual -
Version 7.0.", Performance Systems International, July 1991.
[13] Rose, M., and D. Cass, "ISO Transport Services on top of the
TCP", STD 35, RFC 1006, Northrop Research and Technology Center,
May 1987.
[14] Tennenhouse, D., "Layered multiplexing considered harmful", In
IFIP Workshop on Protocols for High-Speed Networks. Elsevier, May
1989.
[15] Ullmann, R., "CATNIP: Common architecture technology for next-
generation internet protocol", Work in Progress, October 1993.
9. Security Considerations
Security issues are not discussed in this memo.
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10. Authors' Addresses
Russell J. Clark
College of Computing Georgia Institute of Technology
Atlanta, GA 30332-0280
EMail: rjc@cc.gatech.edu
Mostafa H. Ammar
College of Computing Georgia Institute of Technology
Atlanta, GA 30332-0280
EMail: ammar@cc.gatech.edu
Kenneth L. Calvert
College of Computing Georgia Institute of Technology
Atlanta, GA 30332-0280
EMail: calvert@cc.gatech.edu
Clark, Ammar & Calvert [Page 12]