Transcript DS,ESS, Subnet and VLAN

July 2005
doc.: IEEE 802.11-05/0522r0
DS, ESS, Subnet and VLAN
Date: 2005-05-25
Authors:
Name
Company
Mike Moreton
STMicroelectronics
Address
Phone
email
[email protected]
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Submission
Slide 1
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
Abstract
This presentation describes the DS, ESS, Subnet and
VLAN concepts, and their inter-relation in the 802.11
environment.
Submission
Slide 2
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
Definitions of ESS from 802.11maD1.0
• 3.45 extended service set (ESS): A set of one or more
interconnected basic service sets (BSSs) and integrated
local area networks (LANs) that appears as a single BSS to
the logical link control (LLC) layer at any station
associated with one of those BSSs.
– “Appears as a single BSS” implies reachability – any STA can
send data to any other STA anywhere in the ESS using a layer 2
address.
– LLC is end-to-end (ignoring the translations when using Ethernet)
• An ESS is equivalent to a physical wired LAN. All
STAs are reachable from all other STAs.
Submission
Slide 3
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
Definition of DS from 802.11maD1.0
• 3.33 distribution system (DS): A system used to
interconnect a set of basic service sets (BSSs) and
integrated local area networks (LANs) to create an
extended service set (ESS).
– A building block of the ESS
– Connects BSSes together
– Could be a LAN, but can be anything that can forward a layer 2
frame to the correct destination AP, e.g. some proprietary
encapsulation and routing protocol within IP.
• Whatever the technology used by the DS, the service provided by the
DS is always in terms of layer 2 addresses.
Submission
Slide 4
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
Definition of subnet
• Can’t find an official definition!
• Originally a sub-division of an IP network (identified
by an IP network number), but now used
interchangeably as a term for all or part of an IP
network
– Note: The Internet is technically not an IP network – it’s what
connects the IP networks together
• Possible definition: “A subnet is part or all of an IP
Network such that all hosts may be reached directly
from all other hosts without transiting an IP router.”
• Note that an IP Router is also a host, so any router
must be reachable from all other hosts in the subnet.
Submission
Slide 5
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
VLAN
• A VLAN (virtual LAN) is defined as a level above the
physical LAN.
• Used where you want to limit connectivity
– Not all hosts on the physical LAN can talk to all other hosts on the
same physical LAN
– Sometimes the VLANs can be set-up to limit access to a particular
physical location
• For example, only the ethernet ports in the Accounts Department are
connected to the Accounts VLAN.
Submission
Slide 6
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
VLANs and 802.11
• 802.11 is a wireless technology, so limiting VLAN
connectivity on the basis of physical location makes
little sense.
• 802.11 uses 802.1X based authentication to prove
entitlement to use the network, not physical location.
• The most sensible configuration for VLANs in an
802.11 environment is for all VLANs to be accessible
via all APs
– RADIUS authentication returns the VLAN to which the STA
should be connected by the AP
• Quite acceptable for “unusual” configurations to
require extra manual configuration.
Submission
Slide 7
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
VLANs Conclusion
• In an 802.11 environment, every VLAN is accessible
from every point of the physical LAN
– Unless the user goes in for extensive manual configuration
• When considering reachability in an 802.11
environment can ignore VLAN
– It’s only the ESS that matters.
• An AP that is not configured to allow access to all
VLANs should not advertise that it is a member of the
ESS
Submission
Slide 8
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
ARID
• The aim of ARID (Access Router Identifier) is to determine
whether a candidate AP (for roaming to) can provide access to the
STA’s current router
– Potentially requires all routers connected to the subnet to be identified
– With multiple subnets running on the same LAN, the problem gets even
worse.
• If the candidate AP is part of the same ESS, then reachability is
guaranteed.
– The current router is a host on the ESS, so access is guaranteed through all
APs (except in perverse VLAN configurations)
• So no need for ARID, if you know the identity of the ESS is the
same as your current AP.
Submission
Slide 9
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
Why DS Identifier?
• From the previous discussion, what is needed is the
identity of the ESS
– But the TGu requirement uses “DS Identity”. Why?
• “ESSID” appears in the 1999 standard, but appears to
be a misprint. (Removed in 802.11maD1.0)
• To avoid confusion “DS Identity” was used in TGu
– But maybe caused more confusion than it avoided.
Submission
Slide 10
Mike Moreton, STMicroelectronics
July 2005
doc.: IEEE 802.11-05/0522r0
Is ESS Identifier in scope?
• TGr’s scope is intra-ESS handover. Isn’t an ESS
identifier for inter-ESS handover, and hence out of
scope?
– No. Having an ESS identifier increases the speed of intra-ESS
handover, and makes no difference to the speed of inter-ESS
handovers. It is hence very much in scope for TGr.
– TGr seem more open to this than in the past.
• Is it in scope for TGu, if TGr don’t do it?
– It’s difficult to see how any sort of ESS Identifier or ARID can be
in scope for TGu. It’s clearly a method for speeding up intra-BSS
handover, and if TGr decide not to do it, we should respect their
decision.
Submission
Slide 11
Mike Moreton, STMicroelectronics