As per Relevance of the word subnetwork, we have this rfc below:
Network Working Group Y.
Request for Comments: 1937 Cisco
Category: Informational D.
T.J. Watson Research Center, IBM Corp
May 1996
"Local/Remote" Forwarding Decision in Switched Data Link
Status of this
This memo provides information for the Internet community. This
does not specify an Internet standard of any kind. Distribution
this memo is unlimited
The IP architecture assumes that each Data Link subnetwork is
with a single IP subnet number. A pair of hosts with the same
number communicate directly (with no routers); a pair of hosts
different subnet numbers always communicate through one or
routers. As indicated in RFC1620, these assumptions may be
restrictive for large data networks, and specifically for
based on switched virtual circuit (SVC) based technologies (e.g. ATM
Frame Relay, X.25), as these assumptions impose constraints
communication among hosts and routers through a network.
restrictions may preclude full utilization of the
provided by the underlying SVC-based Data Link subnetwork.
document describes extensions to the IP architecture that
these constraints, thus enabling the full utilization of the
provided by SVC-based Data Link subnetworks
1.
The following briefly recaptures the concept of the IP Subnet.
topology is assumed to be composed of hosts and
interconnected via links (Data Link subnetworks). An IP address of
host with an interface attached to a particular link is a
, where host number
unique within the subnet address prefix. When a host needs to
an IP packet to a destination, the host needs to determine
the destination address identifies an interface that is connected
one of the links the host is attached to, or not. This referred
as the "local/remote" decision. The outcome of the "local/remote
decision is based on (a) the destination address, and (b) the
and the prefix length associated with the the local interfaces.
the outcome is "local", then the host resolves the IP address to
Link Layer address (e.g. by using ARP), and then sends the
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directly to that destination (using the Link layer services). If
outcome is "remote", then the host uses one of its first-hop
(thus relying on the services provided by IP routing).
To summarize, two of the important attributes of the IP subnet
are
hosts with a common subnet address prefix are assumed to
attached to a common link (subnetwork), and thus communicate
each other directly, without any routers - "local";
hosts with different subnet address prefixes are assumed to
attached to different links (subnetworks), and thus
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
the traditional IP subnet model and introduces the notion of
Logical IP Subnetwork (LIS). The consequence of this model is that
host is required to setup an ATM SVC to any host within its LIS;
destinations outside its LIS the host must forward packets through
router. It is important to stress that this "local/remote"
is based solely on the information carried by the destination
and the address and prefix lengths associated with the
interfaces
2.
The diversity of TCP/IP applications results in a wide range
traffic characteristics. Some applications last for a very
time and generate only a small number of packets between a pair
communicating hosts (e.g. ping, DNS). Other applications have a
lifetime, but generate a relatively large volume of packets (e.g
FTP). There are also applications that have a relatively
lifetime, but generate relatively few packets (e.g. Telnet).
Finally, we anticipate the emergence of applications that have
relatively long lifetime and generate a large volume of packets (e.g
video-conferencing).
SVC-based Data Link subnetworks offer certain unique
that are not present in other (non-SVC) subnetworks (e.g. Ethernet
Token Ring). The ability to dynamically establish and tear-down
between communicating entities attached to an SVC-based Data
subnetwork enables the dynamic dedication and redistribution
certain communication resources (e.g. bandwidth) among the entities
This dedication and redistribution of resources could be
by relying solely on the mechanism(s) provided by the Data
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layer
The unique capabilities provided by SVC-based Data Link
do not come "for free". The mechanisms that provide dedication
redistribution of resources have certain overhead (e.g. the
needed to establish an SVC, resources associated with maintaining
state for an SVC). There may also be a monetary cost associated
establishing and maintaining an SVC. Therefore, it is very
to be cognizant of such an overhead and to carefully balance
benefits provided by the mechanisms against the overhead
by such mechanisms
One of the key issues for using SVC-based Data Link subnetworks
the TCP/IP environment is the issue of switched virtual circuit (SVC
management. This includes SVC establishment and tear-down, class
service specification, and SVC sharing. At one end of the
one could require SVC establishment between communicating
(on a common Data Link subnetwork) for any application. At the
end of the spectrum, one could require communicating entities
always go through a router, regardless of the application. Given
diversity of TCP/IP applications, either extreme is likely to yield
suboptimal solution with respect to the ability to
exploit capabilities provided by the underlying Data Link layer
The traditional IP subnet model is too restrictive for flexible
adaptive use of SVC-based Data Link subnetworks - the use of
subnetwork is driven by information completely unrelated to
characteristics of individual applications. To illustrate
problem consider "Classical IP and ARP over ATM" (RFC1577). RFC1577
provides support for ATM deployment that follows the traditional
subnet model, and introduces the notion of a Logical IP
(LIS). The consequence of this model is that a host is required
setup an SVC to any host within its LIS, and it must forward
to destinations outside its LIS through a router.
"local/remote" forwarding decision, and consequently the
management, is based solely on the information carried in the
and destination addresses and the subnet mask associated with
source address and has no relation to the nature of the
that generated these packets
3. QoS/Traffic Driven "Local/Remote"
Consider a host attached to an SVC-based Data Link subnetwork,
assume that the "local/remote" decision the host could make is
constrained by the IP subnet model. When such a host needs to send
packet to a destination, the host might consider any of the
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
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
the first hop router
Use an SVC to a router closer to the destination than the
hop router dedicated to a particular type of service
Use a dedicated SVC to a router closer to the destination than
first hop router
Use a best-effort SVC directly to the destination (if
destination is on the same Data Link subnetwork as the host).
Use an SVC directly to the destination dedicated to a
type of service (if the destination is on the same Data
subnetwork as the host).
Use a dedicated SVC directly to the destination (if
destination is on the same Data Link subnetwork as the host).
In the above we observe that the forwarding decision at the host
more flexible than the "local/remote" decision of the IP
model. We also observe that the host's forwarding decision may
into account QoS and/or traffic requirements of the
and/or cost factors associated with establishing and maintaining
VC, and thus improve the overall SVC management. Therefore,
constraints imposed by the IP subnet model is an important
towards better SVC management
3.1 Extending the scope of possible "local"
A source may have an SVC (either dedicated or shared) to
destination if both the source and the destination are on a
Data Link subnetwork. The ability to create and use the SVC (
dedicated or shared) is completely decoupled from the source
destination IP addresses, but is instead coupled to the QoS and/
traffic characteristics of the application. In other words,
ability to establish a direct VC (either dedicated or shared)
a pair of hosts on a common Data Link subnetwork has nothing to
with the IP addresses of the hosts. In contrast with the IP
model (or the LIS mode), the "local" outcome becomes divorced
the addressing information
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3.2 Allowing the "remote" outcome where
A source may go through one or more routers to reach a destination
either (a) the destination is not on the same Data Link subnetwork
the source, or (b) the destination is on the same Data
subnetwork as the source, but the QoS and/or traffic requirements
the application on the source do not justify a direct (
dedicated or shared) VC
When the destination is not on the same Data Link subnetwork as
source, the source may select between either (a) using its first-
(default) router, or (b) establishing a "shortcut" to a router
to the destination than the first-hop router. The source should
able to select between these two choices irrespective of the
and destination IP addresses
When the destination is on the same Data Link subnetwork as
source, but the QoS and/or traffic requirements do not justify
direct VC, the source should be able to go through a
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
closer to the destination than the first-hop router),
decoupled from the addressing information
3.3 Sufficient conditions for direct
The ability of a host to establish an SVC to a peer on a
switched Data Link subnetwork is predicated on its knowledge of
Link Layer address of the peer or an intermediate point closer to
destination. This document assumes the existence of mechanism(s
that can provide the host with this information. Some of the
alternatives are NHRP, ARP, or static configuration;
alternatives are not precluded. The ability to acquire the
Layer address of the peer should not be viewed as an indication
the host and the peer can establish an SVC - the two may be
different Data Link subnetworks, or may be on a common Data
subnetwork that is partitioned
3.4 Some of the
Since the "local/remote" decision would depend on factors other
the addresses of the source and the destination, a pair of hosts
simultaneously be using two different means to reach each other
forwarding traffic for applications with different QoS/and or
characteristics differently
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3.5 Address
It is expected that if the total number of hosts and routers on
common SVC-based Data Link subnetwork is sufficiently large, then
hosts and routers could be partitioned into groups, called
Addressing Groups (LAGs). Each LAG would have hosts and routers.
routers within a LAG would act as the first-hop routers for the
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.
for determining LAG sizes are outside the scope of this document
To provide scalable routing each LAG should be given an IP
prefix, and elements within the LAG should be assigned addresses
of this prefix. The routers in a LAG would then advertise (
appropriate routing protocols) routes to the prefix associated
the LAG. These routes would be advertised as "directly reachable
(with metric 0). Thus, routers within a LAG would act as the last-
routers for the hosts within the LAG
4.
Different approaches to SVC-based Data Link subnetworks used
TCP/IP yield substantially different results with respect to
ability of TCP/IP applications to efficiently exploit
functionality provided by such subnetworks. For example, in the
of ATM both LAN Emulation [LANE] and "classical" IP over
[RFC1577] localize host changes below the IP layer, and therefore
be good first steps in the ATM deployment. However, these
alone are likely to be inadequate for the full utilization of ATM
It appears that any model that does not allow SVC management based
QoS and/or traffic requirements will preempt the full use of SVC
based Data Link subnetworks. Enabling more direct connectivity
applications that could benefit from the functionality provided
SVC-based Data Link subnetworks, while relying on strict hop by
paths for other applications, could facilitate exploration of
capabilities provided by these subnetworks
While this document does not define any specific coupling
various QoS, traffic characteristics and other parameters, and
management, it is important to stress that efforts
standardization of various QoS, traffic characteristics, and
parameters than an application could use (through an appropriate API
to influence SVC management are essential for flexible and
use of SVC-based Data Link subnetworks
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The proposed model utilizes the SVC-based infrastructure for
applications that could benefit from the capabilities
within such an infrastructure, and takes advantage of a router-
overlay for all other applications. As such it provides a
mix of router-based and switch-based infrastructures, where
balance could be determined by the applications requirements
5. Security
Security issues are not discussed in this memo
6.
The authors would like to thank Joel Halpern (NewBridge),
Mankin (ISI), Tony Li (cisco Systems), Andrew Smith (BayNetworks),
and Curtis Villamizar (ANS) for their review and comments
[LANE] "LAN Emulation over ATM specification - version 1", ATM Forum
Feb.95.
[Postel 81] Postel, J., Sunshine, C., Cohen, D., "The ARPA
Protocol", Computer Networks, 5, pp. 261-271, 1983.
[RFC792] Postel, J., "Internet Control Message Protocol-
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
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'
Yakov
Cisco
170 West Tasman Drive
San Jose, CA 95134-1706
Phone: (914) 528-0090
EMail: yakov@cisco.
Dilip
T.J. Watson Research Center IBM
P.O. Box 704
Yorktown Heights, NY 10598
Phone: (914) 784-7722
EMail: kandlur@watson.ibm.
Rekhter & Kandlur Informational [Page 8]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
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