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Keywords: IP, subnet
Network Working Group Y. Rekhter
Request for Comments: 1937 Cisco Systems
Category: Informational D. Kandlur
T.J. Watson Research Center, IBM Corp.
May 1996
"Local/Remote" Forwarding Decision in Switched Data Link Subnetworks
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
The IP architecture assumes that each Data Link subnetwork is labeled
with a single IP subnet number. A pair of hosts with the same subnet
number communicate directly (with no routers); a pair of hosts with
different subnet numbers always communicate through one or more
routers. As indicated in RFC1620, these assumptions may be too
restrictive for large data networks, and specifically for networks
based on switched virtual circuit (SVC) based technologies (e.g. ATM,
Frame Relay, X.25), as these assumptions impose constraints on
communication among hosts and routers through a network. The
restrictions may preclude full utilization of the capabilities
provided by the underlying SVC-based Data Link subnetwork. This
document describes extensions to the IP architecture that relaxes
these constraints, thus enabling the full utilization of the services
provided by SVC-based Data Link subnetworks.
1. Background
The following briefly recaptures the concept of the IP Subnet. The
topology is assumed to be composed of hosts and routers
interconnected via links (Data Link subnetworks). An IP address of a
host with an interface attached to a particular link is a tuple
<prefix length, address prefix, host number>, where host number is
unique within the subnet address prefix. When a host needs to send
an IP packet to a destination, the host needs to determine whether
the destination address identifies an interface that is connected to
one of the links the host is attached to, or not. This referred to
as the "local/remote" decision. The outcome of the "local/remote"
decision is based on (a) the destination address, and (b) the address
and the prefix length associated with the the local interfaces. If
the outcome is "local", then the host resolves the IP address to a
Link Layer address (e.g. by using ARP), and then sends the packet
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directly to that destination (using the Link layer services). If the
outcome is "remote", then the host uses one of its first-hop routers
(thus relying on the services provided by IP routing).
To summarize, two of the important attributes of the IP subnet model
are:
hosts with a common subnet address prefix are assumed to be
attached to a common link (subnetwork), and thus communicate with
each other directly, without any routers - "local";
hosts with different subnet address prefixes are assumed to be
attached to different links (subnetworks), and thus communicate
with each other only through routers - "remote".
A typical example of applying the IP subnet architecture to an SVC-
based Data Link subnetwork is "Classical IP and ARP over ATM"
(RFC1577). RFC1577 provides support for ATM deployment that follows
the traditional IP subnet model and introduces the notion of a
Logical IP Subnetwork (LIS). The consequence of this model is that a
host is required to setup an ATM SVC to any host within its LIS; for
destinations outside its LIS the host must forward packets through a
router. It is important to stress that this "local/remote" decision
is based solely on the information carried by the destination address
and the address and prefix lengths associated with the local
interfaces.
2. Motivations
The diversity of TCP/IP applications results in a wide range of
traffic characteristics. Some applications last for a very short
time and generate only a small number of packets between a pair of
communicating hosts (e.g. ping, DNS). Other applications have a short
lifetime, but generate a relatively large volume of packets (e.g.
FTP). There are also applications that have a relatively long
lifetime, but generate relatively few packets (e.g. Telnet).
Finally, we anticipate the emergence of applications that have a
relatively long lifetime and generate a large volume of packets (e.g.
video-conferencing).
SVC-based Data Link subnetworks offer certain unique capabilities
that are not present in other (non-SVC) subnetworks (e.g. Ethernet,
Token Ring). The ability to dynamically establish and tear-down SVCs
between communicating entities attached to an SVC-based Data Link
subnetwork enables the dynamic dedication and redistribution of
certain communication resources (e.g. bandwidth) among the entities.
This dedication and redistribution of resources could be accomplished
by relying solely on the mechanism(s) provided by the Data Link
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layer.
The unique capabilities provided by SVC-based Data Link subnetworks
do not come "for free". The mechanisms that provide dedication and
redistribution of resources have certain overhead (e.g. the time
needed to establish an SVC, resources associated with maintaining a
state for an SVC). There may also be a monetary cost associated with
establishing and maintaining an SVC. Therefore, it is very important
to be cognizant of such an overhead and to carefully balance the
benefits provided by the mechanisms against the overhead introduced
by such mechanisms.
One of the key issues for using SVC-based Data Link subnetworks in
the TCP/IP environment is the issue of switched virtual circuit (SVC)
management. This includes SVC establishment and tear-down, class of
service specification, and SVC sharing. At one end of the spectrum
one could require SVC establishment between communicating entities
(on a common Data Link subnetwork) for any application. At the other
end of the spectrum, one could require communicating entities to
always go through a router, regardless of the application. Given the
diversity of TCP/IP applications, either extreme is likely to yield a
suboptimal solution with respect to the ability to efficiently
exploit capabilities provided by the underlying Data Link layer.
The traditional IP subnet model is too restrictive for flexible and
adaptive use of SVC-based Data Link subnetworks - the use of a
subnetwork is driven by information completely unrelated to the
characteristics of individual applications. To illustrate the
problem consider "Classical IP and ARP over ATM" (RFC1577). RFC1577
provides support for ATM deployment that follows the traditional IP
subnet model, and introduces the notion of a Logical IP Subnetwork
(LIS). The consequence of this model is that a host is required to
setup an SVC to any host within its LIS, and it must forward packets
to destinations outside its LIS through a router. This
"local/remote" forwarding decision, and consequently the SVC
management, is based solely on the information carried in the source
and destination addresses and the subnet mask associated with the
source address and has no relation to the nature of the applications
that generated these packets.
3. QoS/Traffic Driven "Local/Remote" Decision
Consider a host attached to an SVC-based Data Link subnetwork, and
assume that the "local/remote" decision the host could make is not
constrained by the IP subnet model. When such a host needs to send a
packet to a destination, the host might consider any of the following
options:
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Use a best-effort SVC to the first hop router.
Use an SVC to the first hop router dedicated to a particular type
of service (ie: predictive real time).
Use a dedicated SVC to the first hop router.
Use a best-effort SVC to a router closer to the destination than
the first hop router.
Use an SVC to a router closer to the destination than the first
hop router dedicated to a particular type of service.
Use a dedicated SVC to a router closer to the destination than the
first hop router.
Use a best-effort SVC directly to the destination (if the
destination is on the same Data Link subnetwork as the host).
Use an SVC directly to the destination dedicated to a particular
type of service (if the destination is on the same Data Link
subnetwork as the host).
Use a dedicated SVC directly to the destination (if the
destination is on the same Data Link subnetwork as the host).
In the above we observe that the forwarding decision at the host is
more flexible than the "local/remote" decision of the IP subnet
model. We also observe that the host's forwarding decision may take
into account QoS and/or traffic requirements of the applications
and/or cost factors associated with establishing and maintaining a
VC, and thus improve the overall SVC management. Therefore, removing
constraints imposed by the IP subnet model is an important step
towards better SVC management.
3.1 Extending the scope of possible "local" outcomes
A source may have an SVC (either dedicated or shared) to a
destination if both the source and the destination are on a common
Data Link subnetwork. The ability to create and use the SVC (either
dedicated or shared) is completely decoupled from the source and
destination IP addresses, but is instead coupled to the QoS and/or
traffic characteristics of the application. In other words, the
ability to establish a direct VC (either dedicated or shared) between
a pair of hosts on a common Data Link subnetwork has nothing to do
with the IP addresses of the hosts. In contrast with the IP subnet
model (or the LIS mode), the "local" outcome becomes divorced from
the addressing information.
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3.2 Allowing the "remote" outcome where applicable
A source may go through one or more routers to reach a destination if
either (a) the destination is not on the same Data Link subnetwork as
the source, or (b) the destination is on the same Data Link
subnetwork as the source, but the QoS and/or traffic requirements of
the application on the source do not justify a direct (either
dedicated or shared) VC.
When the destination is not on the same Data Link subnetwork as the
source, the source may select between either (a) using its first-hop
(default) router, or (b) establishing a "shortcut" to a router closer
to the destination than the first-hop router. The source should be
able to select between these two choices irrespective of the source
and destination IP addresses.
When the destination is on the same Data Link subnetwork as the
source, but the QoS and/or traffic requirements do not justify a
direct VC, the source should be able to go through a router
irrespective of the source and destination IP addresses.
In contrast with the IP subnet model (or the LIS model) the "remote"
outcome, and its particular option (first-hop router versus router
closer to the destination than the first-hop router), becomes
decoupled from the addressing information.
3.3 Sufficient conditions for direct connectivity
The ability of a host to establish an SVC to a peer on a common
switched Data Link subnetwork is predicated on its knowledge of the
Link Layer address of the peer or an intermediate point closer to the
destination. This document assumes the existence of mechanism(s)
that can provide the host with this information. Some of the possible
alternatives are NHRP, ARP, or static configuration; other
alternatives are not precluded. The ability to acquire the Link
Layer address of the peer should not be viewed as an indication that
the host and the peer can establish an SVC - the two may be on
different Data Link subnetworks, or may be on a common Data Link
subnetwork that is partitioned.
3.4 Some of the implications
Since the "local/remote" decision would depend on factors other than
the addresses of the source and the destination, a pair of hosts may
simultaneously be using two different means to reach each other,
forwarding traffic for applications with different QoS/and or traffic
characteristics differently.
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3.5 Address assignment
It is expected that if the total number of hosts and routers on a
common SVC-based Data Link subnetwork is sufficiently large, then the
hosts and routers could be partitioned into groups, called Local
Addressing Groups (LAGs). Each LAG would have hosts and routers. The
routers within a LAG would act as the first-hop routers for the hosts
in the LAG. If the total number of hosts and routers is not large,
then all these hosts and routers could form a single LAG. Criteria
for determining LAG sizes are outside the scope of this document.
To provide scalable routing each LAG should be given an IP address
prefix, and elements within the LAG should be assigned addresses out
of this prefix. The routers in a LAG would then advertise (via
appropriate routing protocols) routes to the prefix associated with
the LAG. These routes would be advertised as "directly reachable"
(with metric 0). Thus, routers within a LAG would act as the last-hop
routers for the hosts within the LAG.
4. Conclusions
Different approaches to SVC-based Data Link subnetworks used by
TCP/IP yield substantially different results with respect to the
ability of TCP/IP applications to efficiently exploit the
functionality provided by such subnetworks. For example, in the case
of ATM both LAN Emulation [LANE] and "classical" IP over ATM
[RFC1577] localize host changes below the IP layer, and therefore may
be good first steps in the ATM deployment. However, these approaches
alone are likely to be inadequate for the full utilization of ATM.
It appears that any model that does not allow SVC management based on
QoS and/or traffic requirements will preempt the full use of SVC-
based Data Link subnetworks. Enabling more direct connectivity for
applications that could benefit from the functionality provided by
SVC-based Data Link subnetworks, while relying on strict hop by hop
paths for other applications, could facilitate exploration of the
capabilities provided by these subnetworks.
While this document does not define any specific coupling between
various QoS, traffic characteristics and other parameters, and SVC
management, it is important to stress that efforts towards
standardization of various QoS, traffic characteristics, and other
parameters than an application could use (through an appropriate API)
to influence SVC management are essential for flexible and adaptive
use of SVC-based Data Link subnetworks.
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The proposed model utilizes the SVC-based infrastructure for the
applications that could benefit from the capabilities supported
within such an infrastructure, and takes advantage of a router-based
overlay for all other applications. As such it provides a balanced
mix of router-based and switch-based infrastructures, where the
balance could be determined by the applications requirements.
5. Security Considerations
Security issues are not discussed in this memo.
6. Acknowledgements
The authors would like to thank Joel Halpern (NewBridge), Allison
Mankin (ISI), Tony Li (cisco Systems), Andrew Smith (BayNetworks),
and Curtis Villamizar (ANS) for their review and comments.
References
[LANE] "LAN Emulation over ATM specification - version 1", ATM Forum,
Feb.95.
[Postel 81] Postel, J., Sunshine, C., Cohen, D., "The ARPA Internet
Protocol", Computer Networks, 5, pp. 261-271, 1983.
[RFC792] Postel, J., "Internet Control Message Protocol- DARPA
Internet Program Protocol Specification", STD 5, RFC 792, ISI,
September 1981.
[RFC1122] Braden, R., Editor, "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, USC/ISI, October 1989.
[RFC1577] Laubach, M., "Classical IP and ARP over ATM", January 1994.
[RFC1620] Braden, R., Postel, J., Rekhter, Y., "Internet Architecture
Extensions for Shared Media", May 1994.
[RFC1755] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E.,
Malis, A., "ATM Signalling Support for IP over ATM", January 1995.
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14. Authors' Addresses
Yakov Rekhter
Cisco Systems
170 West Tasman Drive,
San Jose, CA 95134-1706
Phone: (914) 528-0090
EMail: yakov@cisco.com
Dilip Kandlur
T.J. Watson Research Center IBM Corporation
P.O. Box 704
Yorktown Heights, NY 10598
Phone: (914) 784-7722
EMail: kandlur@watson.ibm.com
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