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Internet Draft Mick Seaman
Expires November 1997 3Com
draft-ietf-issll-802-01.txt Andrew Smith
Extreme Networks
Eric Crawley
Gigapacket Networks
June 1997
Integrated Services over IEEE 802.1D/802.1p Networks
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
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draft" or "work in progress."
Please check the I-D abstract listing contained in each Internet
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Internet Draft.
Abstract
This document describes the support of IETF Integrated Services over
LANs built from IEEE 802 network segments which may be interconnected by
draft standard IEEE P802.1p switches.
It describes the practical capabilities and limitations of this
technology for supporting Controlled Load [8] and Guaranteed Service [9]
using the inherent capabilities of the relevant 802 technologies [5],[6]
etc. and the proposed 802.1p queuing features in switches. IEEE P802.1p
[2] is a superset of the existing IEEE 802.1D bridging specification.
This document provides a functional model for the layer 3 to layer 2 and
user-to-network dialogue which supports admission control and defines
requirements for interoperability between switches. The special case of
such networks where the sender and receiver are located on the same
segment is also discussed.
This scheme expands on the ISSLL over 802 LANs framework described in
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[7]. It makes reference to an admission control signaling protocol
developed by the ISSLL WG which is known as the "Subnet Bandwidth
Manager". This is an extension to the IETF's RSVP protocol [4] and is
described in a separate document [10].
1. Introduction
The IEEE 802.1 Interworking Task Group is currently enhancing the basic
MAC Service provided in Bridged Local Area Networks (aka "switched
LANs"). As a supplement to the original IEEE MAC Bridges standard [1],
the update P802.1p [2] proposes differential traffic class queuing and
access to media on the basis of a "user_priority" signaled in frames.
In this document we
* review the meaning and use of user_priority in LANs and the frame
forwarding capabilities of a standard LAN switch.
* examine alternatives for identifying layer 2 traffic flows for
admission control.
* review the options available for policing traffic flows.
* derive requirements for consistent traffic class handling in a network
of switches and use these requirements to discuss queue handling
alternatives for 802.1p and the way in which these meet administrative
and interoperability goals.
* consider the benefits and limitations of this switched-based approach,
contrasting it with full router based RSVP implementation in terms of
complexity, utilisation of transmission resources and administrative
controls.
The model used is outlined in the "framework document" [7] which in
summary:
* partitions the admission control process into two separable
operations:
* an interaction between the user of the integrated service and the
local network elements ("provision of the service" in the terms of
802.1D) to confirm the availability of transmission resources for
traffic to be introduced.
* selection of an appropriate user_priority for that traffic on the
basis of the service and service parameters to be supported.
* distinguishes between the user to network interface above and the
mechanisms used by the switches ("support of the service"). These
include communication between the switches (network to network
signaling).
* describes a simple architecture for the provision and support of these
services, broken down into components with functional and interface
descriptions:
* a single "user" component: a layer-3 to layer-2 negotiation and
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translation component for both sending and receiving, with interfaces to
other components residing in the station.
* processes residing in a bridge/switch to handle admission control and
mapping requests, including proposals for actual traffic mappings to
user_priority values.
* identifies a need for a signaling protocol to carry admission control
requests between devices.
It will be noted that this document is written from the pragmatic
viewpoint that there will be a widely deployed network technology and we
are evaluating it for its ability to support some or all of the defined
IETF integrated services: this approach is intended to ensure
development of a system which can provide useful new capabilities in
existing (and soon to be deployed) network infrastructures.
2. Goals and Assumptions
It is assumed that typical subnetworks that are concerned about
quality-of-service will be"switch-rich": that is to say most
communication between end stations using integrated services support
will pass through at least one switch. The mechanisms and protocols
described will be trivially extensible to communicating systems on the
same shared media, but it is important not to allow problem
generalisation to complicate the practical application that we target:
the access characteristics of Ethernet and Token-Ring LANs are forcing a
trend to switch-rich topologies along with MAC enhancements to ensure
access predictability on half-duplex switch to switch links.
Note that we illustrate most examples in this document using RSVP as an
"upper-layer" QoS signaling protocol but there are actually no real
dependencies on this protocol: RSVP could be replaced by some other
dynamic protocol or else the requests could be made by network
management or other policy entities. In any event, no extra
modifications to the RSVP protocol are assumed.
There may be a heterogeneous mixture of switches with different
capabilities, all compliant with IEEE 802.1p, but implementing queuing
and forwarding mechanisms in a range from simple 2-queue per port,
strict priority, up to more complex multi-queue (maybe even one per-
flow) WFQ or other algorithms.
The problem is broken down into smaller independent pieces: this may
lead to sub-optimal usage of the network resources but we contend that
such benefits are often equivalent to very small improvements in network
efficiency in a LAN environment. Therefore, it is a goal that the
switches in the network operate using a much simpler set of information
than the RSVP engine in a router. In particular, it is assumed that such
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switches do not need to implement per-flow queuing and policing
(although they might do so).
It is a fundamental assumption of the int-serv model that flows are
isolated from each other throughout their transit across a network.
Intermediate queueing nodes are expected to police the traffic to ensure
that it conforms to the pre-agreed traffic flow specification. In the
architecture proposed here for mapping to layer-2, we diverge from that
assumption in the interests of simplicity: the policing function is
assumed to be implemented in the transmit schedulers of the layer-3
devices (end stations, routers). In the LAN environments envisioned, it
is reasonable to assume that end stations are "trusted" to adhere to
their agreed contracts at the inputs to the network and that we can
afford to over-allocate resources at admission -control time to
compensate for the inevitable extra jitter/bunching introduced by the
switched network itself.
These divergences have some implications on the receiver heterogeneity
that can be supported and the statistical multiplexing gains that might
have been exploited, especially for Controlled Load flows.
3. User Priority and Frame Forwarding in IEEE 802 Networks
3.1 General IEEE 802 Service Model
User_priority is a value associated with the transmission and reception
of all frames in the IEEE 802 service model: it is supplied by the
sender which is using the MAC service. It is provided along with the
data to a receiver using the MAC service. It may or may not be actually
carried over the network: Token- Ring/802.5 carries this value (encoded
in its FC octet), basic Ethernet/802.3 does not. 802.1p defines a way to
carry this value over the network in a consistent way on Ethernet, Token
Ring, FDDI or other MAC-layer media using an extended frame format. The
usage of user_priority is summarised below but is more fully described
in section 2.5 of 802.1D [1] and 802.1p [2] "Support of the Internal
Layer Service by Specific MAC Procedures" and readers are referred to
these documents for further information.
If the "user_priority" is carried explicitly in packets, its utility is
as a simple label in the data stream enabling packets in different
classes to be discriminated easily by downstream nodes without their
having to parse the packet in more detail.
Apart from making the job of desktop or wiring-closet switches easier,
an explicit field means they do not have to change hardware or software
as the rules for classifying packets evolve (e.g. based on new protocols
or new policies). More sophisticated layer-3 switches, perhaps deployed
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towards the core of a network, can provide added value here by
performing the classification more accurately and, hence, utilising
network resources more efficiently or providing better protection of
flows from one another: this appears to be a good economic choice since
there are likely to be very many more desktop/wiring closet switches in
a network than switches requiring layer-3 functionality.
The IEEE 802 specifications make no assumptions about how user_priority
is to be used by end stations or by the network. In particular it can
only be considered a "priority" in a loose sense: although the current
802.1p draft defines static priority queuing as the default mode of
operation of switches that implement multiple queues (user_priority is
defined as a 3-bit quantity so strict priority queueing would give value
7 = high priority, 0 = low priority). The general switch algorithm is as
follows: packets are placed onto a particular queue based on the
received user_priority (from the packet if a 802.1p header or 802.5
network was used, invented according to some local policy if not). The
selection of queue is based on a mapping from user_priority
[0,1,2,3,4,5,6 or 7] onto the number of available queues.Note that
switches may implement any number of queues from 1 upwards and it may
not be visible externally, except through any advertised switch
parameters and the its admission control behaviour, which user_priority
values get mapped to the same vs. Different queues internally.Other
algorithms that a switch might implement might include e.g. weighted
fair queueuing, round robin.
In particular, IEEE makes no recommendations about how a sender should
select the value for user_priority: one of the main purposes of this
current document is to propose such usage rules and how to communicate
the semantics of the values between switches, end- stations and routers.
For the remainder of this document we use the term "traffic class" when
discussing the treatment of packets with one of the user_priority
values.
3.2 Ethernet/802.3
There is no explicit traffic class or user_priority field carried in
Ethernet packets. This means that user_priority must be regenerated at a
downstream receiver or switch according to some defaults or by parsing
further into higher-layer protocol fields in the packet. Alternatively,
the IEEE 802.1Q encapsulation [11] may be used which provides an
explicit traffic class field on top of an basic MAC format.
For the different IP packet encapsulations used over Ethernet/802.3, it
will be necessary to adjust any admission- control calculations
according to the framing and to the padding requirements:
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Encapsulation Framing Overhead IP MTU
bytes/pkt bytes
IP EtherType (ip_len<=46 bytes) 64-ip_len 1500
(1500>=ip_len>=46 bytes) 18 1500
IP EtherType over 802.1p/Q (ip_len<=42) 64-ip_len 1500*
(1500>=ip_len>=42 bytes) 22 1500*
IP EtherType over LLC/SNAP (ip_len<=40) 64-ip_len 1492
(1500>=ip_len>=40 bytes) 24 1492
* note that the draft IEEE 802.1Q specification exceeds the IEEE 802.3
maximum packet length values by 4 bytes.
3.3 Token-Ring/802.5
The token ring standard [6] provides a priority mechanism that can be
used to control both the queuing of packets for transmission and the
access of packets to the shared media. The priority mechanisms are
implemented using bits within the Access Control (AC) and the Frame
Control (FC) fields of a LLC frame. The first three bits of the AC
field, the Token Priority bits, together with the last three bits of the
AC field, the Reservation bits, regulate which stations get access to
the ring. The last three bits of the FC field of an LLC frame, the User
Priority bits, are obtained from the higher layer in the user_priority
parameter when it requests transmission of a packet. This parameter also
establishes the Access Priority used by the MAC. The user_priority value
is conveyed end-to-end by the User Priority bits in the FC field and is
typically preserved through Token-Ring bridges of all types. In all
cases, 0 is the lowest priority.
Token-Ring also uses a concept of Reserved Priority: this relates to the
value of priority which a station uses to reserve the token for the next
transmission on the ring. When a free token is circulating, only a
station having an Access Priority greater than or equal to the Reserved
Priority in the token will be allowed to seize the token for
transmission. Readers are referred to [14] for further discussion of
this topic.
A token ring station is theoretically capable of separately queuing each
of the eight levels of requested user priority and then transmitting
frames in order of priority. A station sets Reservation bits according
to the user priority of frames that are queued for transmission in the
highest priority queue. This allows the access mechanism to ensure that
the frame with the highest priority throughout the entire ring will be
transmitted before any lower priority frame. Annex I to the IEEE 802.5
token ring standard recommends that stations send/relay frames as
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follows:
Application user_priority
non-time-critical data 0
- 1
- 2
- 3
LAN management 4
time-sensitive data 5
real-time-critical data 6
MAC frames 7
To reduce frame jitter associated with high-priority traffic, the annex
also recommends that only one frame be transmitted per token and that
the maximum information field size be 4399 octets whenever delay-
sensitive traffic is traversing the ring. Most existing implementations
of token ring bridges forward all LLC frames with a default access
priority of 4. Annex I recommends that bridges forward LLC frames that
have a user priorities greater that 4 with a reservation equal to the
user priority (although the draft IEEE P802.1p [2] permits network
management override this behaviour). The capabilities provided by token
ring's user and reservation priorities and by IEEE 802.1p can provide
effective support for Integrated Services flows that request QoS using
RSVP. These mechanisms can provide, with few or no additions to the
token ring architecture, bandwidth guarantees with the network flow
control necessary to support such guarantees.
For the different IP packet encapsulations used over Token Ring/802.5,
it will be necessary to adjust any admission-control calculations
according to the framing requirements:
Encapsulation Framing Overhead IP MTU
bytes/pkt bytes
IP EtherType over 802.1p/Q 29 4370*
IP EtherType over LLC/SNAP 25 4370*
*the suggested MTU from RFC 1042 [13] is 4464 bytes but there are issues
related to discovering what the maximum supported MTU between any two
points both within and between Token Ring subnets. We recommend here an
MTU consistent with the 802.5 Annex I recommendation.
4. Integrated services through layer-2 switches
4.1 Summary of switch characteristics
For the sake of illustration, we divide layer-2 bridges/switches into
several categories, based on the level of sophistication of their QoS
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and software protocol capabilities: these categories are not intended to
represent all possible implementation choices but, instead, to aid
discussion of what QoS capabilities can be expected from a network made
of these devices.
Class I - 802.1p priority queueuing between traffic classes.
- No multicast heterogeneity.
- 802.1p GARP/GMRP pruning of individual multicast addresses.
Class II As (I) plus:
- can map received user_priority on a per-input-port basis to
some internal set of canonical values.
- can map internal canonical values onto transmitted
user_priority on a per-output-port basis giving some limited form of
multicast heterogeneity.
- maybe implements IGMP snooping for pruning.
Class III As (II) plus:
- per-flow classification
- maybe per-flow policing and/or reshaping
- WFQ or other transmit scheduling (probably not per-flow) 4.2
Queueing
Connectionless packet-based networks in general, and LAN-switched
networks in particular, work today because of scaling choices in network
provisioning. Consciously or (more usually) unconsciously, enough excess
bandwidth and buffering is provisioned in the network to absorb the
traffic sourced by higher-layer protocols or cause their transmission
windows to run out, on a statistical basis, so that the network is only
overloaded for a short duration and the average expected loading is less
than 60% (usually much less).
With the advent of time-critical traffic such overprovisioning has
become far less easy to achieve. Time critical frames may find
themselves queued for annoyingly long periods of time behind temporary
bursts of file transfer traffic, particularly at network bottleneck
points, e.g. at the 100 Mb/s to 10 Mb/s transition that might occur
between the riser to the wiring closet and the final link to the user
from a desktop switch. In this case, however, if it is known (guaranteed
by application design, merely expected on the basis of statistics, or
just that this is all that the network guarantees to support) that the
time critical traffic is a small fraction of the total bandwidth, it
suffices to give it strict priority over the "normal" traffic. The worst
case delay experienced by the time critical traffic is roughly the
maximum transmission time of a maximum length non-time-critical frame -
less than a millisecond for 10 Mb/s Ethernet, and well below an end to
end budget based on human perception times.
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When more than one "priority" service is to be offered by a network
element e.g. it supports Controlled-Load as well as Guaranteed Service,
the queuing discipline becomes more complex. In order to provide the
required isolation between the service classes, it will probably be
necessary to queue them separately. There is then an issue of how to
service the queues - a combination of admission control and maybe
weighted fair queuing may be required in such cases. As with the service
specifications themselves, it is not the place for this document to
specify queuing algorithms, merely to observe that the external
behaviour meet the services' requirements.
4.3 Multicast Heterogeneity
IEEE 802.1D and 802.1p specify a basic model for multicast whereby a
switch performs multicast routing decisions based on the destination
address: this would produce a list of output ports to which the packet
should be forwarded. In its default mode, such a switch would use any
user_priority value in received packets to enqueue the packets at each
output port. All of the classes of switch identified above can support
this operation.
At layer-3, the int-serv model allows heterogeneous multicast flows
where different branches of a tree can have different types of
reservations for a given multicast destination, or even supports the
notion that some trees will have some branches with reserved flows and
some using best effort (default) service.
If a switch is selecting per-port output queues based only on the
incoming user_priority, as described by 802.1p, it must treat all
branches of all multicast sessions within that user_priority class with
the same queuing mechanism: no heterogeneity is then possible.I If a
switch were to implement a separate user_priority mapping at each output
port, as described under "Class II switch" above, then some limited form
of receiver heterogeneity can be supported e.g. forwarding of traffic as
user_priority 4 on one branch where receivers have performed admission
control reservations and as user_priority 0 on one where they have not.
We assume that per-user_priority queuing without taking account of input
or output ports is the minimum standard functionality for systems in a
LAN environment (Class I switch, as defined above). More functional
layer-2 switches or even layer-3 switches (a.k.a. routers) can be used
if even more flexible forms of heterogeneity are considered necessary:
their behaviour is well standardised.
4.4 Override of incoming user_priority
In some cases, a network administrator may not trust the user_priority
values contained in packets from a source and may which to map these
into some more suitable set of values. Alternatively, due perhaps to
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equipment limitations or transition periods, values may need to be
mapped to/from different regions of a network.
Some switches may implement such a function on input that maps received
user_priority into some internal set of values (this table is known in
802.1p as the "user_priority regeneration table"). These values can then
be mapped using the output table described above onto outgoing
user_priority values: these same mappings must also be used when
applying admission control to requests that use the user_priority values
(see e.g. [10]). More sophisticated approaches may also be envisioned
where a device polices traffic flows and adjusts their onward
user_priority based on their conformance to the admitted traffic flow
specifications.
4.5 Remapping of non-conformant aggregated flows
One other topic under discussion in the int-serv context is how to
handle the traffic for data flows from sources that are exceeding their
currently agreed traffic contract with the network. An approach that
shows much promise is to treat such traffic with "somewhat less than
best effort" service in order to protect traffic that is normally given
"best effort" service from having to back off (such traffic is often
"adaptive" using TCP or other congestion control algorithms and it would
be unfair to penalise it due to badly behaved traffic from reserved
flows which are usually set up by non-adaptive applications).
A solution here might be to assign normal best effort traffic to one
user_priority and to label excess non-conformant traffic as a "lower"
user_priority. This topic is further discussed below.
5. Selecting traffic classes
One fundamental question is "who gets to decide what the classes mean
and who gets access to them?" One approach would be for the meanings of
the classes to be "well-known": we would then need to standardise a set
of classes e.g. 1 = best effort, 2 = controlled- load, 3 = guaranteed
(loose delay bound, high bandwidth), 4 = guaranteed (slightly tighter
delay) etc. The values to encode in such a table in end stations, in
isolation from the network to which they are connected, is
problematical: one approach could be to define one user_priority value
per int-serv service and leave it at that (reserving the rest of the
combinations for future traffic classes - there are sure to be plenty!).
We propose here a more flexible mapping: clients ask "the network" which
user_priority traffic class to use for a given traffic flow, as
categorised by its flow-spec and layer-2 endpoints. The network provides
a value back to the requester which is appropriate to the current
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network topology, load conditions, other admitted flows etc. The task of
configuring switches with this mapping (e.g. through network management,
a switch-switch protocol or via some network-wide QoS-mapping directory
service) is an order of magnitude less complex than performing the same
function in end stations. Also, when new services (or other network
reconfigurations) are added to such a network, the network elements will
typically be the ones to be upgraded with new queuing algorithms etc.
and can be provided with new mappings at this time.
Given the need for a new session or "flow" requiring some QoS support, a
client then needs answers to the following questions:
1. which traffic class do I add this flow to?
The client needs to know how to label the packets of the flow as it
places them into the network.
2. who do I ask/tell?
The proposed model is that a client ask "the network" which
user_priority traffic class to use for a given traffic flow. This has
several benefits as compared to a model which allows clients to select a
class for themselves.
3. how do I ask/tell them?
A request/response protocol is needed between client and network: in
fact, the request can be piggy-backed onto an admission control request
and the response can be piggy-backed onto an admission control
acknowledgment: this "one pass" assignment has the benefit of completing
the admission control in a timely way and reducing the exposure to
changing conditions which could occur if clients cached the knowledge
for extensive periods.
The network (i.e. the first network element encountered downstream from
the client) must then answer the following questions:
1. which traffic class do I add this flow to?
This is a packing problem, difficult to solve in general, but many
simplifying assumptions can be made: presumably some simple form of
allocation can be done without a more complex scheme able to dynamically
shift flows around between classes.
2. which traffic class has worst-case parameters which meet the needs of
this flow?
This might be an ordering/comparison problem: which of two service
classes is "better" than another? Again, we can make this tractable by
observing that all of the current int-serv classes can be ranked (best
effort <= Controlled Load <= Guaranteed Service) in a simple manner. If
any classes are implemented in the future that cannot be simply ranked
then the issue can be finessed by either a priori knowledge about what
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classes are supported or by configuration.
and return the chosen user_priority value to the client.
Note that the client may be either an end station, router or a first
switch which may be acting as a proxy for a client which does not
participate in these protocols for whatever reason. Note also that a
device e.g. a server or router, may choose to implement both the
"client" as well as the "network" portion of this model so that it can
select its own user_priority values: such an implementation would,
however, be discouraged unless the device really does have a close tie-
in with the network topology and resource allocation policies but would
work in some cases where there is known over- provisioning of resources.
6. Flow Identification
Several previous proposals for int-serv over lower-layers have treated
switches very much as a special case of routers: in particular, that
switches along the data path will make packet handling decisions based
on the RSVP flow and filter specifications and use them to classify the
corresponding data packets. However, filtering to the per-flow level
becomes cost-prohibitive with increasing switch speed: devices with such
filtering capabilities are unlikely to have a very different
implementation cost to IP routers, in which case we must question
whether a specification oriented toward switched networks is of any
benefit at all.
This document proposes that "aggregated flow" identification based on
user_priority be the minimum required of switches.
7. Reserving Network Resources - Admission Control
So far we have not discussed admission control. In fact, without
admission control it is possible to scratchbuild a LAN network of some
size capable of supporting real-time services, providing that the
traffic fits within certain scaling constraints (relative link speeds,
numbers of ports etc. - see below). This is not surprising since it is
possible to run a fair approximation to real time services on small LANs
today with no admission control or help from encoded priority bits.
Imagine a campus network providing dedicated 10 Mbps connections to each
user. Each floor of each building supports up to 96 users, organized
into groups of 24, with each group being supported by a 100 Mbps
downlink to a basement switch which concentrates 5 floors (20 x 100
Mbps) and a data center (4 x 100 Mbps) to a 1 Gbps link to an 8 Gbps
central campus switch, which in turn hooks 6 buildings together (with 2
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x 1 Gbps full duplex links to support a corporate server farm). Such a
network could support 1.5 Mb/s of voice/video from every user to any
other user or (for half the population) the server farm, provided the
video ran high priority: this gives 3000 users, all with desktop video
conferencing running along with file transfer/email etc. In such a
network RSVP's role would be limited to ensuring resource availability
at the communicating end stations and for connection to the wide area.
In such a network, a discussion as to the best service policy to apply
to high and low priority queues may prove academic: while it is true
that "normal" traffic may be delayed by bunches of high priority frames,
queuing theory tells us that the average queue occupancy in the high
priority queue at any switch port will be somewhat less than 1 (with
real user behaviour, i.e. not all watching video conferences all the
time) it should be far less. A cheaper alternative to buying equipment
with a fancy queue service policy may be to buy equipment with more
bandwidth to lower the average link utilisation by a few per cent.
In practice a number of objections can be made to such a simple
solution. There may be long established expensive equipment in the
network which does not provide all the bandwidth required. There will be
considerable concern over who is allowed to say what traffic is high
priority. There may be a wish to give some form of "prioritised" service
to crucial business applications, above that given to experimental
video-conferencing. The task that faces us is to provide a degree of
control without making that control so elaborate to implement that the
control-oriented solution is not simply rejected in favor of providing
yet more bandwidth, at a lower cost.
The proposed admission control mechanism requires a query-response
interaction with the network returning a "YES/NO" answer and, if
successful, a user_priority value with which to tag the data frames of
this flow.
The relevant int-serv specifications describe the parameters which need
to be considered when making an admission control decision at each node
in the network path between sender and receiver. We discuss how to
calculate these parameters for different network technologies below but
we do not specify admission control algorithms or mechanisms as to how
to progress the admission control process across the network. One such
mechanism is described as SBM in [10].
Where there are multiple mechanisms in use for allocating resources e.g.
some combination of SBM and network management, it will be necessary to
ensure that network resources are partitioned amongst the different
mechanisms in some way: this could be by configuration or maybe by
having the mechanisms allocate from a common resource pool within any
device.
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8. Mapping of integrated services to layer-2 in layer-3 devices
8.1 Layer-3 client
We assume the same client model as int-serv and RSVP where we use the
term "client" to mean the entity handling QoS in the layer-3 device at
each end of a layer-2 hop (e.g. end-station, router). The sending client
itself is responsible for local admission control and scheduling packets
onto its link in accordance with the service agreed. Just as in the
int-serv model, this involves per-flow schedulers (a.k.a. shapers) in
every such data source.
The client is running an RSVP process which presents a session
establishment interface to applications, signals RSVP over the network,
programs a scheduler and classifier in the driver and interfaces to a
policy control module. In particular, RSVP also interfaces to a local
admission control module: it is this entity that we focus on here.
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The following diagram is taken from the RSVP specification [4]:
_____________________________
| _______ |
| | | _______ |
| |Appli- | | | | RSVP
| | cation| | RSVP <-------------------->
| | <--> | |
| | | |process| _____ |
| |_._____| | -->Polcy||
| | |__.__._| |Cntrl||
| |data | | |_____||
|===|===========|==|==========|
| | --------| | _____ |
| | | | ---->Admis||
| _V__V_ ___V____ |Cntrl||
| | | | | |_____||
| |Class-| | Packet | |
| | ifier|==>Schedulr|====================>
| |______| |________| | data
| |
|_____________________________|
Figure 1 - RSVP in Sending Hosts
Note that we illustrate examples in this document using RSVP as the
"upper-layer" signaling protocol but there are no actual dependencies on
this protocol: RSVP could be replaced by some other dynamic protocol or
else the requests could be made by network management or other policy
entities.
8.2 Requests to layer-2
The local admission control entity within a client is responsible for
mapping these layer-3 requests into layer-2 language.
The upper-layer entity requests from ISSLL:
"May I reserve for traffic with <traffic characteristic> with
<performance requirements> from <here> to <there> and how
should I label it?"
where
<traffic characteristic> = Flow Spec, Tspec, Rspec (e.g.
bandwidth, burstiness, MTU etc.)
<performance requirements> = latency, jitter bounds etc.
<here> = IP address(es)
<there> = IP address(es) - may be multicast
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8.3 Sender
The ISSLL functionality in the sender is illustrated below and may be
summarised as:
* maps the endpoints of the conversation to layer-2 addresses in the
LAN, so it can figure out what traffic is really going where (probably
makes reference to the ARP protocol cache for unicast or an algorithmic
mapping for multicast destinations).
* applies local admission control on outgoing link and driver
* formats a SBM request to the network with the mapped addresses and
filter/flow specs
* receives response from the network and reports the YES/NO admission
control answer back to the upper layer entity, along with any negotiated
modifications to the session parameters.
* stores any resulting user_priority to be associated with this session
in a "802 header" lookup table for use when sending any future data
packets.
from IP from RSVP
____|____________|____________
| | | |
| __V____ ___V___ |
| | | | | |
| | Addr |<->| | | SBM signaling
| |mapping| | SBM |<------------------------>
| |_______| |Client | |
| ___|___ | | |
| | |<->| | |
| | 802 | |_______| |
| | header| / | | |
| |_______| / | | |
| | / | | _____ |
| | +-----/ | +->|Local| |
| __V_V_ _____V__ |Admis| |
| | | | | |Cntrl| |
| |Class-| | Packet | |_____| |
| | ifier|==>Schedulr|======================>
| |______| |________| | data
|______________________________|
Figure 2 - ISSLL in End-station Sender
ISSLL manageable objects in the sender:
802 header table
Local admission control resource status
L2 additions to classifier/scheduler int-serv tables
8.4 Receiver
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The ISSLL functionality in the receiver is a good deal simpler. It is
summarised below and is illustrated by the following picture:
* handles any received SBM protocol indications.
* applies local admission control to see if a request can be supported
with appropriate local receive resources.
* passes indications up to RSVP if OK.
* accepts confirmations from RSVP and relays them back via SBM signaling
towards the requester.
* may program a receive classifier and scheduler, if any is used, to
identify traffic classes of received packets and accord them appropriate
treatment e.g. reserve some buffers for particular traffic classes.
* programs receiver to strip any 802 header information from received
packets.
to RSVP to IP
^ ^
____|____________|___________
| | | |
| __|____ | |
| | | | |
SBM signaling | | SBM | ___|___ |
<-----------------> |Client | | Strip | |
| |_______| |802 hdr| |
| | \ |_______| |
| __v___ \ ^ |
| | Local |\ | |
| | Admis | \ | |
| | Cntrl | \ | |
| |_______| \ | |
| ______ v___|____ |
| |Class-| | Packet | |
===================>| ifier|==>|Scheduler| |
data | |______| |_________| |
|_____________________________|
Figure 3 - ISSLL in End-station Receiver
9. Layer-2 Switch Functions
9.1 Switch Model
In this model of layer-2 switch behaviour, we define the following
entities within the switch:
* Local admission control - one of these on each port accounts for the
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available bandwidth on the link attached to that port. For half-duplex
links, this involves taking account of the resources allocated to both
transmit and receive flows. For full-duplex, the input port accountant's
task is trivial.
* Input SBM module: one instance on each port, performs the "network"
side of the signaling protocol for peering with clients or other
switches. Also holds knowledge of the mappings of int-serv classes to
user_priority.
* SBM propagation - relays requests that have passed admission control
at the input port to the relevant output ports' SBM modules. This will
require access to the switch's forwarding table (layer-2 "routing table"
- cf. RSVP model) and port spanning-tree states.
* Output SBM module - forwards requests to the next layer-2 or -3
network hop.
* Classifier, Queueing and Scheduler - these functions are basically as
described by the Forwarding Process of IEEE 802.1p (see section 3.7 of
[2]). The Classifier module identifies the relevant QoS information from
incoming packets and uses this, together with the normal bridge
forwarding database, to decide to which output queue of which output
port to enqueue the packet. In Class I switches, this information is the
"regenerated user_priority" parameter which has already been decoded by
the receiving MAC service and potentially re-mapped by the 802.1p
forwarding process (see description in section 3.7.3 of [2]). This does
not preclude more sophisticated classification rules which may be
applied in more complex Class III switches e.g. matching on individual
int-serv flows.
The Queueing and Scheduler module holds the output queues for ports and
provides the algorithm for servicing the queues for transmission onto
the output link in order to provide the promised int-serv service.
Switches will implement one or more output queues per port and all will
implement at least a basic strict priority dequeueing algorithm as their
default, in accordance with 802.1p.
* Ingress traffic class mapper and policing - as described in 802.1p
section 3.7. This optional module may check on whether the data within
traffic classes are conforming to the patterns currently agreed:
switches may police this and discard or re-map packets. The default
behaviour is to pass things through unchanged.
* Egress traffic class mapper - as described in 802.1p section 3.7. This
optional module may apply re-mapping of traffic classes e.g. on a per-
output port basis. The default behaviour is to pass things through
unchanged.
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These are shown by the following diagram which is a superset of the IEEE
802.1D/802.1p bridge model:
_______________________________
| _____ ______ ______ |
SBM signaling | | | | | | | | SBM signaling
<------------------>| IN |<->| SBM |<->| OUT |<---------------->
| | SBM | | prop.| | SBM | |
| |_____| |______| |______| |
| / | ^ / | |
______________| / | | | | |_____________
| \ / __V__ | | __V__ / |
| \ ____/ |Local| | | |Local| / |
| \ / |Admis| | | |Admis| / |
| \/ |Cntrl| | | |Cntrl| / |
| _____V \ |_____| | | |_____| / _____ |
| |traff | \ ___|__ V_______ / |egrss| |
| |class | \ |Filter| |Queue & | / |traff| |
| |map & |=====|==========>|Data- |=| Packet |=|===>|class| |
| |police| | | base| |Schedule| | |map | |
| |______| | |______| |________| | |_____| |
|____^_________|_______________________________|______|______|
data in | |data out
========+ +========>
Figure 4 - ISSLL in Switches
9.2 Admission Control
On reception of an admission control request, a switch performs the
following actions:
* ingress SBM module translates any received user_priority or else
selects a layer-2 traffic class which appears compatible with the
request and whose use does not violate any administrative policies in
force. In effect, it matches up the requested service with those
available in each of the user_priority classes and chooses the "best"
one. It ensures that, if this reservation is successful, the selected
value is passed back to the client.
* ingress SBM observes the current state of allocation of resources on
the input port/link and then determines whether the new resource
allocation from the mapped traffic class would be excessive. The request
is passed to the reservation propagator if accepted so far.
* reservation propagator relays the request to the bandwidth accountants
on each of the switch's outbound links to which this reservation would
apply (implied interface to routing/forwarding database).
* egress bandwidth accountant observes the current state of allocation
of queueing resources on its outbound port and bandwidth on the link
itself and determines whether the new allocation would be excessive.
Note that this is only the local decision of this switch hop: each
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further layer-2 hop through the network gets a chance to veto the
request as it passes along.
* the request, if accepted by this switch, is then passed on down the
line on each output link selected. Any user_priority described in the
forwarded request must be translated according to any egress mapping
table.
* if accepted, the switch must notify the client of the user_priority to
use for packets belonging to this flow. Note that this is a
"provisional YES" - we assume an optimistic approach here: later
switches can still say "NO" later.
* if this switch wishes to reject the request, it can do so by notifying
the original client (by means of its layer-2 address).
10. Mappings from intserv service models to IEEE 802
It is assumed that admission control will be applied when deciding
whether or not to admit a new flow through a given network element and
that a device sending onto a link will be proxying the parameters and
admission control decisions on behalf of that link: this process will
require the device to be able to determine (by estimation, measurement
or calculation) several parameters. It is assumed that details of the
potential flow are provided to the device by some means (e.g. a
signaling protocol, network management). The service definition
specifications themselves provide some implementation guidance as to how
to calculate some of these quantities.
The accuracy of calculation of these parameters may not be very
critical: indeed it is an assumption of this model's being used with
relatively simple Class I switches that they merely provide values to
describe the device and admit flows conservatively.
10.1 General characterisation parameters
There are some general parameters that a device will need to use and/or
supply for all service types:
- Ingress link
- Egress links and their MTUs, framing overheads and minimum packet
sizes (see media-specific information presented above).
- available path bandwidth: updated hop-by-hop by any device along the
path of the flow.
- minimum latency
10.2 Parameters to implement Guaranteed Service
A network element must be able to determine the following parameters:
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- Constant delay bound through this device (in addition to any value
provided by "minimum latency" above) and up to the receiver at the next
network element for the packets of this flow if it were to be admitted:
this would include any access latency bound to the outgoing link as well
as propagation delay across that link.
- Rate-proportional delay bound through this device and up to the
receiver at the next network element for the packets of this flow if it
were to be admitted.
- Receive resources that would need to be associated with this flow
(e.g. buffering, bandwidth) if it were to be admitted and not suffer
packet loss if it kept within its supplied Tspec/Rspec.
- Transmit resources that would need to be associated with this flow
(e.g. buffering, bandwidth, constant- and rate-proportional delay
bounds) if it were to be admitted.
10.3 Parameters to implement Controlled Load
A network element must be able to determine the following parameters
which can be extracted from [8]:
- Receive resources that would need to be associated with this flow
(e.g. buffering) if it were to be admitted.
- Transmit resources that would need to be associated with this flow
(e.g. buffering) if it were to be admitted.
10.4 Parameters to implement Best Effort
For a network element to implement best effort service there are no
explicit parameters that need to be characterised.
10.5 Mapping to IEEE 802 user_priority
There are many options available for mapping aggregations of flows
described by int-serv service models (Best Effort, Controlled Load, and
Guaranteed are the services considered here) onto user_priority classes.
There currently exists very little practical experience with particular
mappings to help make a determination as to the "best" mapping. In that
spirit, the following options are presented in order to stimulate
experimentation in this area. Note, this does not dictate what
mechanisms/algorithms a network element (e.g. an Ethernet switch) needs
to perform to implement these mappings: this is an implementation choice
and does not matter so long as the requirements for the particular
service model are met. Having said that, we do explore below the ability
of a switch implementing strict priority queueing to support some or all
of the service types under discussion: this is worthwhile because this
is likely to be the most widely deployed dequeueing algorithm in simple
switches as it is the default specified in 802.1p.
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In order to reduce the administrative problems , such a mapping table is
held by *switches* (and routers if desired) but generally not by end-
station hosts and is a read-write table. The values proposed below are
defaults and can be overridden by management control so long as all
switches agree to some extent (the required level of agreement requires
further analysis).
It is possible that some form of network-wide lookup service could be
implemented that serviced requests from clients e.g. traffic_class =
getQoSbyName("H.323 video") and notified switches of what sorts of
traffic categories they were likely to encounter and how to allocate
those requests into traffic classes: such mechanisms are for further
study.
Proposal: A Simple Scheme
user_priority Service
0 "less than" Best Effort
1 Best Effort
2 reserved
3 reserved
4 Controlled Load
5 Guaranteed Service, 100ms bound
6 Guaranteed Service, 10ms bound
7 reserved
In this proposal, all traffic that uses the controlled load service is
mapped to a single 802.1p user_priority whilst that for guaranteed
service is placed into one of two user_priority classes with different
delay bounds. Unreserved best effort traffic is mapped to another.
The use of classes 4, 5 and 6 for Controlled Load and Guaranteed Service
is somewhat arbitrary as long as they are increasing. Any two classes
greater than Best Effort can be used as long as GS is "greater" than CL
although those proposed here have the advantage that, for transit
through 802.1p switches with only two-level strict priority queuing,
they both get "high priority" treatment (the current 802.1p default
split is 0-3 and 4-7 for a device with 2 queues). The choice of delay
bound is also arbitrary but potentially very significant: this can lead
to a much more efficient allocation of resources as well as greater
(though still not very good) isolation between flows.
The "less than best effort" class might be useful for devices that wish
to tag packets that are exceeding a committed network capacity and can
be optionally discarded by a downstream device. Note, this is not
*required* by any current int-serv models but is under study.
The advantage to this approach is that it puts some real delay bounds on
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the Guaranteed Service without adding any additional complexity to the
other services. It still ignores the amount of *bandwidth* available
for each class. This should behave reasonably well as long as all
traffic for CL and GS flows does not exceed any resource capacities in
the device. Some isolation between very delay-critical GS and less
critical GS flows is provided but there is still an overall assumption
that flows will in general be well- behaved. In addition, this mapping
still leaves room for future service models.
Expanding the number of classes for CL service is not as appealing since
there is no need to map to a particular delay bound. There may be cases
where an administrator might map CL onto more classes for particular
bandwidths or policy levels. It may also be desirable to further
subdivide CL traffic in cases where the itis frequently non-conformant
for certain applications.
11. Network Topology Scenarios
11.1 Switched networks using priority scheduling algorithms
In general, the int-serv standards work has tried to avoid any
specification of scheduling algorithms, instead relying on implementers
to deduce appropriate algorithms from the service definitions and on
users to apply measurable benchmarks to check for conformance. However,
since one standards' body has chosen to specify a single default
scheduling algorithm for switches [2], it seems appropriate to examine
to some degree, how well this "implementation" might actually support
some or all of the int-serv services.
If the mappings of Proposal A above are applied in a switch implementing
strict priority queueing between the 8 traffic classes (7 = highest)
then the result will be that all Guaranteed Service packets will be
transmitted in preference to any other service. Controlled Load packets
will be transmitted next, with everything else waiting until both of
these queues are empty. If the admission control algorithms in use on
the switch ensure that the sum of the "promised" bandwidth of all of the
GS and CL sessions are never allowed to exceed the available link
bandwidth then things are looking good.
11.2 Full-duplex switched networks
We have up to now ignored the MAC access protocol. On a full-duplex
switched LAN (of either Ethernet or Token-Ring types - the MAC algorithm
is, by definition, unimportant) this can be factored in to the
characterisation parameters advertised by the device since the access
latency is well controlled (jitter = one largest packet time). Some
example characteristics (approximate):
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Type Speed Max Pkt Max Access
Length Latency
Ethernet 10Mbps 1.2ms 1.2ms
100Mbps 120us 120us
1Gbps 12us 12us
Token-Ring 4Mbps 9ms 9ms
16Mbps 9ms 9ms
FDDI 100Mbps 360us 8.4ms
These delays should be also be considered in the context of speed- of-
light delays of e.g. ~400ns for typical 100m UTP links and ~7us for
typical 2km multimode fibre links.
Therefore we see Full-Duplex switched network topologies as offering
good QoS capabilities for both Controlled Load and Guaranteed Service.
11.3 Shared-media Ethernet networks
We have not mentioned the difficulty of dealing with allocation on a
single shared CSMA/CD segment: as soon as any CSMA/CD algorithm is
introduced then the ability to provide any form of Guaranteed Service is
seriously compromised in the absence of any tight coupling between the
multiple senders on the link. There are a number of reasons for not
offering a better solution for this issue.
Firstly, we do not believe this is a truly solvable problem: it would
seem to require a new MAC protocol. Those who are interested in solving
this problem per se should probably be following the BLAM developments
in 802.3 but we would be suspicious of the interoperability
characteristics of a series of new software MACs running above the
traditional 802.3 MAC.
Secondly, we are not convinced that it is really an interesting problem.
While not everyone in the world is buying desktop switches today and
there will be end stations living on repeated segments for some time to
come, the number of switches is going up and the number of stations on
repeated segments is going down. This trend is proceeding to the point
that we may be happy with a solution which assumes that any network
conversation requiring resource reservations will take place through at
least one switch (be it layer-2 or layer-3). Put another way, the
easiest QoS upgrade to a layer-2 network is to install segment
switching: only when has been done is it worthwhile to investigate more
complex solutions involving admission control.
Thirdly, in the core of the network (as opposed to at the edges), there
does not seem to be enough economic benefit for repeated segment
solutions as opposed to switched solutions. While repeated solutions
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*may* be 50% cheaper, their cost impact on the entire network is
amortised across all of the edge ports. There may be special
circumstances in the future (e.g. Gigabit buffered repeaters) but these
have differing characteristics to existing CSMA/CD repeaters anyway.
Type Speed Max Pkt Max Access
Length Latency
Ethernet 10Mbps 1.2ms unbounded
100Mbps 120us unbounded
1Gbps 12us unbounded
11.4 Half-duplex switched Ethernet networks
Many of the same arguments for sub-optimal support of Guaranteed Service
apply to half-duplex switched Ethernet as to shared media: in essence,
this topology is a medium that *is* shared between at least two senders
contending for each packet transmission opportunity. Unless these are
tightly coupled and cooperative then there is always the chance that the
junk traffic of one will interfere with the other's important traffic.
Such coupling would seem to need some form of modifications to the MAC
protocol (see above).
Notwithstanding this, these topologies do seem to offer the chance to
provide Controlled Load service: with the knowledge that there are only
a small limited number (e.g. two) of potential senders that are both
using prioritisation for their CL traffic (with admission control for
those CL flows based on the knowledge of the number of potential
senders) over best effort, the media access characteristics, whilst not
deterministic in the true mathematical sense, are somewhat predictable.
This is probably a close enough approximation to CL to be useful.
Type Speed Max Pkt Max Access
Length Latency
Ethernet 10Mbps 1.2ms unbounded
100Mbps 120us unbounded
1Gbps 12us unbounded
11.5 Half-duplex and shared Token Ring networks
In a shared Token Ring network, the network access time for high
priority traffic at any station is bounded and is given by (N+1)*THTmax,
where N is the number of stations sending high priority traffic and
THTmax is the maximum token holding time [14]. This assumes that network
adapters have priority queues so that reservation of the token is done
for traffic with the highest priority currently queued in the adapter.
It is easy to see that access times can be improved by reducing N or
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THTmax. The recommended default for THTmax is 10 ms [6]. N is an
integer from 2 to 256 for a shared ring and 2 for a switched half duplex
topology. A similar analysis applies for FDDI. Using default values
gives:
Type Speed Max Pkt Max Access
Length Latency
Token-Ring 4/16Mbps shared 9ms 2570ms
4/16Mbps switched 9ms 30ms
FDDI 100Mbps 360us 8ms
Given that access time is bounded, it is possible to provide an upper
bound for end-to-end delays as required by Guaranteed Service assuming
that traffic of this class uses the highest priority allowable for user
traffic. The actual number of stations that send traffic mapped into
the same traffic class as GS may vary over time but, from an admission
control standpoint, this value is needed a priori. The admission
control entity must therefore use a fixed value for N, which may be the
total number of stations on the ring or some lower value if it is
desired to keep the offered delay guarantees smaller. If the value of N
used is lower than the total number of stations on the ring, admission
control must ensure that the number of stations sending high priority
traffic never exceeds this number. This approach allows admission
control to estimate worst case access delays assuming that all of the N
stations are sending high priority data even though, in most cases, this
will mean that delays are significantly overestimated.
Assuming that Controlled Load flows use a traffic class lower than that
used by GS, no upper-bound on access latency can be provided for CL
flows. However, CL flows will receive better service than best effort
flows.
Note that, on many existing shared token rings, bridges will transmit
frames using an Access Priority (see section 3.3) value 4 irrespective
of the user_priority carried in the frame control field of the frame.
Therefore, existing bridges would need to be reconfigured or modified
before the above access time bounds can actually be used.
12. Signaling protocol
The mechanisms described in this document make use of a signaling
protocol for devices to communicate their admission control requests
across the network: the service definitions to be provided by such a
protocol are described below. The candidate IETF protocol for this
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purpose is called "Subnet Bandwidth Manager" and is described in [10].
In all these cases, appropriate delete/cleanup mechanisms will also have
to be provided for when sessions are torn down. All interactions are
assumed to provide read as well as write capabilities.
12.1 Client service definitions
The following interfaces are identified from Figures 2 and 3:
SBM <-> Address mapping
This is a simple lookup function which may cause ARP protocol
interactions, may be just a lookup of an existing ARP cache entry or may
be an algorithmic mapping. The layer-2 addresses are needed by SBM for
inclusion in its signaling messages to/from switches which avoids the
switches having to perform the mapping and, hence, have knowledge of
layer-3 information for the complete subnet:
l2_addr = map_address( ip_addr )
SBM <-> Session/802 header
This is for notifying the transmit path of how to associate
user_priority values with the traffic of each outgoing session: the
transmit path will provide the user_priority value when it requests a
MAC-layer transmit operation for each packet (user_priority is one of
the parameters defined by the IEEE 802 service model):
bind_802_header( sessionid, user_priority )
SBM <-> Classifier/Scheduler
This is for notifying transmit classifier/scheduler of additional
layer-2 information associated with scheduling the transmission of a
session's packets (may be unused in some cases):
bind_l2sessioninfo( sessionid, l2_header, traffic_class )
SBM <-> Local Admission Control
For applying local admission control for a session e.g. is there enough
transmit bandwidth still uncommitted for this potential new session? Are
there sufficient receive buffers? This should commit the necessary
resources if OK: it will be necessary to release these resources if a
later stage of the session setup process fails.
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status = admit_l2txsession( Tspec, flowspec )
status = admit_l2rxsession( Rspec, flowspec )
SBM <-> RSVP - this is outlined above in section 8.2 and fully described
in [10].
12.2 Switch service definitions
The following interfaces are identified from Figure 4:
SBM <-> Classifier
This is for notifying receive classifier of how to match up incoming
layer-2 information with the associated traffic class: it may in some
cases consist of a set of read-only default mappings:
bind_l2classifierinfo( l2_header, traffic_class )
SBM <-> Queue and Packet Scheduler
This is for notifying transmit scheduler of additional layer-2
information associated with a given traffic class (it may be unused in
some cases):
bind_l2schedulerinfo( l2_header, traffic_class )
SBM <-> Local Admission Control
As for host above.
SBM <-> Traffic Class Map and Police
Optional configuration of any layer-2 policing function and/or
user_priority remapping that might be implemented on input to a switch:
bind_l2classmapping( in_user_priority, remap_user_priority )
bind_l2policing( l2_header, traffic_characteristics )
SBM <-> Filtering Database
SBM propagation rules need access to the layer-2 forwarding database to
determine where to forward SBM messages (analogous to RSRR interface in
L3 RSVP):
output_portlist = lookup_l2dest( l2_addr )
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13. Compatibility and Interoperability with existing equipment
Layer-2-only "standard" 802.1p switches will have to work together with
routers and layer-3 switches. Wide deployment of such 802.1p switches is
envisaged, in a number of roles in the network. "Desktop switches" will
provide dedicated 10/100 Mbps links to end stations at costs
comparable/compatible with NICs/adapter cards. Very high speed core
switches may act as central campus switching points for layer 3 devices.
Real network deployments provide a wide range of examples today. The
question is "what functionality beyond that of the basic 802.1D bridge
should such 802.1p switches provide?". In the abstract the answer is
"whatever they can do to broaden the applicability of the switching
solution while still being economically distinct from the layer 3
switches in their cost of acquisition, speed/bandwidth, cost of
ownership and administration". Broadening the applicability means both
addressing the needs of new traffic types and building larger switched
networks (or making larger portions of existing networks switched). Thus
one could imagine a network in which every device (along a network path)
was layer-3 capable/intrusive into the full data stream; or one in which
only the edge devices were pure layer-2; or one in which every alternate
device lacked layer-3 functionality; or most do - excluding some key
control points such as router firewalls, for example. Whatever the mix,
the solution has to interoperate with these layer-3 QoS-aware devices.
Of course, where int-serv flows pass through equipment which is ignorant
of priority queuing and which places all packets through the same
queuing/overload-dropping path, it is obvious that some of the
characteristics of the flow get more difficult to support. Suitable
courses of action in the cases where sufficient bandwidth or buffering
is not available are of the form:
(a) buy more (and bigger) routers
(b) buy more capable switches
(c) rearrange the network topology: 802.1Q VLANs [11] may help here.
(d) buy more bandwidth
It would also be possible to pass more information between switches
about the capabilities of their neighbours and to route around non-
QoS-capable switches: such methods are for further study.
14. Justification
An obvious comment is that this is all too complex, it's what RSVP is
doing already, why do we think we can do better by reinventing the
solution to this problem at layer-2?
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The key is that we do not have to tackle the full problem space of RSVP:
there are a number of simple scenarios that cover a considerable
proportion of the real situations that occur: all we have to do here is
cover 99% of the territory at significantly lower cost and leave the
other applications to full RSVP running in strategically positioned
high-function switches or routers. This will allow a significant
reduction in overall network cost (equipment and ownership). This
approach does mean that we have to discuss real life situations instead
of abstract topologies that "could happen".
Sometimes, for example, simple bandwidth configuration in a few switches
e.g. to avoid overloading particular trunk links, can be used to
overcome bottlenecks due to the network topology: if there are issues
with overloading end station "last hops", RSVP in the end stations would
exert the correct controls simply by examining local resources without
much tie-in to the layer-2 topology. In this case there has been no need
to resort to any form of complex topology computation and much
complexity has been avoided.
In the more general case, there remains work to be done. This will need
to be done against the background constraint that the changing of queue
service policies and the addition of extra functionality to support new
service disciplines will proceed at the rate of hardware product
development cycles and advance implementations of new algorithms may be
pursued reluctantly or without the necessary 20/20 foresight.
However, compared to the alternative of no traffic classes at all, there
is substantial benefit in even the simplest of approaches (e.g. 2-4
queues with straight priority), so there is significant reward for doing
something: wide acceptance of that "something" probably means that even
the simplest queue service disciplines will be provided for.
15. References
[1] ISO/IEC 10038, ANSI/IEEE Std 802.1D-1993 "MAC Bridges"
[2] "Supplement to MAC Bridges: Traffic Class Expediting and
Dynamic Multicast Filtering", May 1997, IEEE P802.1p/D6
[3] "Integrated Services in the Internet Architecture: an Overview"
RFC1633, June 1994
[4] "Resource Reservation Protocol (RSVP) - Version 1 Functional
Specification", Internet Draft, June 1997
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<draft-ietf-rsvp-spec-16.[ps,txt]>
[5] "Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) Access Method and Physical Layer Specifications"
ANSI/IEEE Std 802.3-1985.
[6] "Token-Ring Access Method and Physical Layer Specifications"
ANSI/IEEE Std 802.5-1995
[7] "A Framework for Providing Integrated Services Over Shared and
Switched LAN Technologies", Internet Draft, May 1997
<draft-ietf-issll-is802-framework-02>
[8] "Specification of the Controlled-Load Network Element Service",
Internet Draft, May 1997,
<draft-ietf-intserv-ctrl-load-svc-05.txt>
[9] "Specification of Guaranteed Quality of Service",
Internet Draft, February 1997,
<draft-ietf-intserv-guaranteed-svc-07.txt>
[10] "SBM (Subnet Bandwidth Manager): A Proposal for Admission
Control over Ethernet", Internet Draft, June 1997
<draft-yavatkar-sbm-ethernet-04>
[11] "Draft Standard for Virtual Bridged Local Area Networks",
May 1997, IEEE P802.1Q/D6
[12] "General Characterization Parameters for Integrated
Service Network Elements", Internet Draft, November 1996
<draft-ietf-intserv-charac-02.txt>
[13] "A Standard for the Transmission of IP Datagrams over IEEE
802 Networks", RFC 1042, February 1988
[14] "The Use of Priorities on Token-Ring Networks for Multimedia
Traffic", C. Bisdikian, B. V. Patel, F. Schaffa and M.
Willebeek-LeMair, IEEE Network, Nov/Dec 1995.
16. Security Considerations
There are no known security issues over and above those inherent in the
Integrated Services architecture and the network technologies referenced
by this document.
17. Acknowledgments
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This document draws heavily on the work of the ISSLL WG of the IETF and
the IEEE P802.1 Interworking Task Group. In particular, it includes
previous work on Token-Ring by Anoop Ghanwani, Wayne Pace and Vijay
Srinivasan.
18. Authors' addresses
Mick Seaman
3Com Corp.
5400 Bayfront Plaza
Santa Clara CA 95052-8145
USA
+1 (408) 764 5000
mick_seaman@3com.com
Andrew Smith
Extreme Networks
10460 Bandley Drive
Cupertino CA 95014
USA
+1 (408) 863 2821
andrew@extremenetworks.com
Eric Crawley
Gigapacket Networks
25 Porter Rd.
Littleton MA 01460
USA
+1 (508) 486 0665
esc@gigapacket.com
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