INTERNET-DRAFT Steve Jackowski/Deterministic Networks
Expires: August 1999 David Putzolu/Intel Architecture Labs
February 3, 1999
Network Element Service Specification for Low Speed Networks
draft-ietf-issl-isslow-svcmap-05.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. Internet-Drafts are working
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Abstract
A set of companion documents describe an architecture for providing
integrated services over low-bitrate links, such as modem lines,
ISDN B-channels, and sub-T1 links [1, 2, 3, 4]. The main components
of the architecture are: a set of real-time encapsulation formats for
asynchronous and synchronous low-bitrate links, a header compression
architecture optimized for real-time flows, elements of negotiation
protocols used between routers (or between hosts and routers), and
announcement protocols used by applications to allow this negotiation
to take place.
This document defines the service mappings for controlled load [5] and
guaranteed [6] services for use with the real-time encapsulation part
of the architecture. The approach takes the form of a set of guidelines
and considerations for implementing these services, along with
evaluation criteria for elements providing these services.
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Table of Contents
1. Introduction 3
1.1 Specification Language 3
2. End to End Behavior 3
3. Motivation 4
4. Network Element Data Handling Requirements 5
4.1 Rate and Delay 5
4.2 Link Aggregation 6
4.3 Controlled Load Versus Guaranteed Service 6
4.4 Controlled Load and Guaranteed Service Data Handling 7
4.5 Controlled Load and Guaranteed Service Class Mapping 7
5. Invocation Information 10
6. Exported Information 10
7. Ordering and Merging 10
8. Guidelines for Implementors 10
8.1 PPP Bit and Byte Stuffing Effects on Admission Control 10
8.2 Compression Considerations 11
8.3 Admission Control 11
8.4 Fragment Scheduling Considerations 13
9. Evaluation Criteria 14
10. Security Considerations 14
11. References 15
12. Authors' Addresses 16
Acknowledgements 16
Appendix A. Admission Control Considerations for POTS Modems 16
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1. Introduction
In addition to the ``best-effort'' services the Internet is well-known
for, other types of services (``integrated services'') are being
developed and deployed in the Internet. These services support special
handling of traffic based on bandwidth, latency, and other requirements
that cannot usually be met using ``best-effort'' service.
This document defines how to map integrated services ``controlled
load'' [5] and ``guaranteed'' [6] services on to low-
bandwidth links. The architecture and mechanisms used to implement
these services on such links are defined in a set of companion
documents. The mechanisms defined in these documents include both
compression of flows (for bandwidth savings) [4] and a set of
extensions to the PPP protocol which permit fragmentation [2]
or suspension [3] of large packets in favor of packets from flows
with more stringent service requirements.
1.1. Specification Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
2. End-to-end Behavior
Unlike other link layers the links referred to in this document operate
only over low speed point to point links or connections. Examples
of the kinds of links addressed here include dial-up lines, ISDN
channels, and leased lines. In this context, 'end to end' simply means
between two points. In typical inter/intranet environments, this will
include:
- host to directly connected host.
- host to/from network access device (router or switch).
- Edge device (subnet router or switch) to/from router or switch.
- In rare circumstances, the link may run from backbone router to
backbone router.
Thus, the endpoints are two network elements as described above. The
Controlled Load and Guaranteed services for the links addressed here
are applied between these elements and often represent the first or last
wide area hop in a true end to end service. It is important to note
that these links can be the most ``bandwidth constrained'' along the
path.
The services utilized in mapping integrated services to these links
are only provided if both endpoints on the link support the architecture
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and mechanisms referenced above. Support for these mechanisms is
determined during the PPP negotiation. Because of the unique
characteristics of a point to point link with both endpoints
supporting these mechanisms, traffic is automatically shaped.
That is, incoming traffic will be TSpec conformant and the
admission control function can make decisions based on local state:
it does not need to coordinate with the network element on the other
end of the link. Furthermore, since the context that these services
is implemented in is a point to point link, when rate control and
delay bounds are provided for individual flows, the link inherently
acts like a leased circuit (that is, it is not shared with any other
sources of traffic). The non-shared nature of these links, along with
the fact that point-to-point links are typically dual simplex
(i.e., the send and receive channels are separate) is what allows all
admission control decisions to be made locally.
3. Motivation
Many applications are now beginning to make more and more use of such
services. As this use grows, the likelihood of a low bandwidth link of
some sort being present in the end to end connection grows. In order to
meet the service requirements of these applications, it will be necessary
to provide these enhanced services on the low bandwidth links in the
end to end connection. The presence of integrated services on low
bandwidth links is critical to providing a good end to end service
because it is precisely on these links that traffic is most likely to
experience undesirable effects on latency, jitter, and bandwidth.
The negative effects on service can stem both from traditional causes
of poor service as well as low bandwidth link specific causes. The
traditional cause of poor service is multiple packet queuing effects,
and is alleviated by intelligent ordering of outgoing packets on
links supporting enhanced services. Low bandwidth links introduce an
additional cause for latency - the time for a MTU-sized packet to
transit such a link is often large enough to be a significant
component in the end to end latency and jitter requirements of
flows requiring enhanced services. As such, mechanisms for allowing
packets to be suspended or fragmented so as to allow transmission of
packets with more stringent service requirements are necessary,
along with guidelines for mapping the enhanced services defined in
integrated services onto these mechanisms. It is the particular
considerations specific to low bandwidth links in providing a good
end to end service that this document focuses on.
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4. Network Element Data Handling Requirements
The Network Service element may be implemented in hardware or
the available bandwidth by minimizing header overhead associated with
software. As described in [2] and [3], for systems which can perform
bit-oriented transmission control, the suspend/resume approach optimizes
MLPPP fragmentation. For systems which provide frame-oriented
transmission control, the fragmentation approach can be implemented with
no hardware changes. Choice of suspend/resume versus fragmentation
should be made based on the hardware's capability to handle the new HDLC
framing described in [2] and the system overhead associated with byte by
byte scanning (required by suspend/resume).
To provide controlled load or guaranteed service with the suspend/resume
approach, when a packet for an admitted flow (QoS packet) arrives
during transmission of a best effort packet and continued
transmission of the best effort packet would violate delay constraints
of the QoS service flows, the best effort packet is preempted, the QoS
packet/fragments are added to the transmission, and the best effort
packet transmission is then resumed: usually all in one transmission.
The receiving station separates the best effort packet from the embedded
QoS packet's fragments. It is also conceivable that one QoS flow's packet
might suspend another flow's packet if the delivery deadline of the new
packet is earlier than the current packet.
For systems which use fragmentation where suspend/resume is not
possible, any packets longer than the maximum tolerable delay for packets
from enhanced service flows are fragmented prior to transmission so that
a short packet for another flow can be interleaved between fragments of
a larger packet and still meet the transmission deadline for the
flow requiring enhanced services.
Note that the fragmentation discussed in this document refers to
multilink PPP (MLPPP) fragmentation and associated MCMLPPP modifications
as described in [2], not IP or other layer 3 fragmentation. MLPPP
fragmentation is local to the PPP link, and does not affect end-to-end
(IP) MTU.
4.1. Rate and Delay
One assumption made about the nature of point to point links is that
rate, transmission time and delay are fixed and consistent. The rate
of the link is assumed to be determined at connection time, and the
devices on the link (adapters, modems, DSU/CSUs, etc) traditionally
exhibit fixed delay characteristics. Unfortunately these assumptions
do not always hold true. Certain link types (e.g., POTS modems) can
vary both in terms of data rates and in terms of delay characteristics.
Implementations of these services on such links need to adjust their
admission control policies to reflect these characteristics. Refer
to Appendix A for more considerations on specific link characteristics.
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4.2. Link Aggregation
Although certain link types, like ISDN, permit dynamic allocation of
bandwidth across multiple links, it is assumed that the Admission
Control service will consider the impact of multiple physical links over
the point to point logical connection.
Note that because of the load balancing effect of Multilink PPP (MLPPP),
two 64 Kbps links should exhibit the delay and transmission
characteristics of a single 128 Kbps link. However, MLPPP
implementations may approach load balancing and fragmentation
differently. The mechanism used should be taken into consideration when
implementing the scheduler (especially token bucket) for packets,
fragments, and suspend/resume on top of existing MLPPP services to
ensure that adequate rate and delay characteristics are maintained.
4.3. Controlled Load versus Guaranteed Service
With most link layers, Guaranteed Service (GS) requires more tightly
controlled service by the Network Element, and in most cases, acceptance
of a Guaranteed Service request results in over-provisioning of link
level resources. Controlled Load (CL) Service is usually less
constrained and permits more flexibility in scheduling of packets for
the link. However, due to the characteristics of some types of
point to point links (e.g., low bandwidth links such as ISDN, POTS,
etc.), providing Controlled Load service may actually be equally or
more difficult than Guaranteed Service for certain kinds of traffic.
Controlled Load requires that the delay associated with packet
transmission be 'closely approximating unloaded best effort service.'
Because the links being discussed are not shared (i.e., point-to-point)
links, unloaded best effort service means that best effort packets
will incur no more than burst packet delay: M/r where M is the maximum
packet size and r is the transmission rate. Thus, the maximum permitted
delay for a Controlled Load packet (CLmDELAY) is bounded by M/r + P/r
where P is the size of the outgoing packet.
In the case where the transmission rate is relatively high, the
difference in queuing delay between a packet being sent on a
totally unloaded link & a lightly loaded link is relatively small.
Consider a scenario wherein a 10MBit/s link is being used to carry
CL traffic with a burst size of 30 bytes. If the link is totally
unloaded, the time to transmit a burst will be 0.024ms. If BE traffic
is also being carried on the link, perhaps with a packet size of
1000 bytes, and a CL packet is queued after a BE packet, the time
before it is transmitted grows to 0.824ms. While this is a 3400%
increase, it actually only changes the overall delay for the QoS
packet by less than a millisecond - a relatively small amount in the
end-to-end latency expected for a large fraction of CL flows. In
contrast to this, consider the same traffic being carried over a
relatively slow link with a 56Kbit/s data rate. The burst transmit
time for a CL packet on an unloaded link would be approximately 4ms.
If a single BE packet was queued before a CL packet, the transmit time
for the CL packet would grow to 147ms - an increase in transmission
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time of over 140ms. This change is quite significant in the timescales
used for many CL reservations.
Given the special considerations associated with scheduling CL flows on
low bandwidth links, "standard" assumptions and implementations of
controlled load service may not result in expected performance.
Implementors must careful review all assumptions and parameters in
order to ensure correct functioning.
4.4. Controlled Load and Guaranteed Service Data Handling
Upon arrival of a QoS flow's packet, the Network Element determines
if the packet is conformant. If it is not, Policing is applied
(see Policing). Conformant means:
1) The flow does not exceed the associated TSpec peak rate (RSpec rate
for Guaranteed Service: rT+b with T=time period).
2) The packet does not cause a token bucket overflow.
If the packet is conformant, it is compressed as required, fragmented
(if necessary), and scheduled. If there is no conflicting best effort
traffic, the packet is queued along with the rest of conformant QoS
traffic and scheduled with respect to any other enhanced services flows
such that its transmission deadline is met.
For the suspend/resume implementation to achieve controlled load, any
packets being transmitted whose transmission would violate
the CLmDELAY are suspended. Otherwise, the QoS packet/fragments are
scheduled ahead of any queued best effort traffic.
For CL Fragmentation implementations, the packet/fragment is scheduled
ahead of any best effort packets. Note that all best effort packets
must be divided into fragments less than or equal to the smallest MRU
(or associated fragment size) of all the QoS flows. This incurs at
most one fragment delay for the QoS traffic: closely equivalent to
unloaded best effort service.
For Guaranteed Service for both fragmentation and suspend/resume, the
scheduler determines if continued transmission of the best effort packet
being transmitted would cause delay greater than the acceptable delay.
If so, the best effort packet is preempted or, in the case of
fragmentation, the QoS packet is scheduled ahead of the rest of the
best effort packets' fragments.
4.5. Controlled Load and Guaranteed Service Class Mapping
Supporting integrated services over PPP links which implement MCML or
RTF can be accomplished in several ways. Guidelines for mapping these
services to PPP links, and specifically, to the classes provided by the
suspend/resume and fragmentation mechanisms mentioned above, are
presented below. Note that these guidelines assume that some sort
of signaling is used to indicate desired quality of service to both
the sender and receiver of a flow over a PPP link. These guidelines
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also assume that it is unlikely that a series of PPP links be
connected to each other. It is noted that even if a series of PPP
links were to be connected together, it is likely that each link
would have different characteristics, and further, that frames would
have to be reassembled at the terminus of each link for error
correction purposes, requiring that class assignment be performed on
each hop of the link, rather than just forwarding frames with
identical segmentation or fragmentation. These assumptions remove any
requirement on the service-mapping implementation that quality of
service information be implicit in the class selection applied to
particular flows, allowing the sender of an integrated services flow on
a PPP link complete freedom in assigning classes to flows (or to
packets within flows).
One important observation that must be made is that the classes that
MCML and RTF provide can be viewed purely as PPP-specific
segmentation/fragmentation mechanisms. That is, while the class number
must remain constant on an intra-packet basis, it may vary on an inter-
packet basis for all flows transiting a PPP link. Actual assignment of
particular flows to fixed classes is unnecessary, as the class numbers
are not required to have any meaning other than in the context of
identifying the membership of fragments/segments as part of a single
packet. This consideration is very important, in that it offers
implementers with a large degree of flexibility in providing integrated
services over PPP links. This observation implies that the queuing
discipline used to differentiate different flows does not have any ties
to the class numbers used. This point is sufficiently important that an
example is provided below.
Consider a PPP link using the MCML short sequence number fragment
format (that is, four classes are provided). Assume that in addition
to carrying best effort traffic, this link is carrying four guaranteed
service flows, A, B, C, D, and E. Further assume that the link capacity
is 100kbit/s and the latency is 100ms. Finally, assume the BE traffic
is sufficient to keep the pipe full at all times and that GS flows A-E
are each 10kbit/s and all have delay bounds of 145ms.
Time(ms) Action
0 BE traffic is queued up
0 2kbit fragment from 10kbit packet of BE traffic sent, cls 0 (...)
8 2kbit fragment from BE sent, cls 0 (10kbit BE packet done)
9 8kbit packet from flow A arrives
10 2kbit fragment from A sent, cls 1 (8kbit flow A packet start)
11 8kbit packet from flow B arrives
12 2kbit fragment from B sent, cls 2 (8kbit flow B packet start)
13 8kbit packets from flows C, D, and E arrive
14 2kbit fragment from C sent, cls 3 (8kbit flow C packet start)
16 2kbit fragment from D sent, cls 0 (8kbit flow D packet start)
18 2kbit fragment from A sent, cls 1
20 2kbit fragment from B sent, cls 2
22 2kbit fragment from A sent, cls 1
24 2kbit fragment from A sent, cls 1 (8kbit flow A packet done)
26 2kbit fragment from E sent, cls 1 (8kbit flow E packet start)
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27 8kbit packet from flow A arrives
28 2kbit fragment from B sent, cls 2
30 2kbit fragment from C sent, cls 3
32 2kbit fragment from E sent, cls 1
34 2kbit fragment from B sent, cls 2 (8kbit flow B packet done)
36 2kbit fragment from E sent, cls 1
38 2kbit fragment flow A sent, cls 2 (8kbit flow A packet start)
(etc.)
This example shows several things. First, multiple flows may share the
same class, particularly in the case where there are more flows than
classes. More importantly, there is no reason that a particular flow
must be assigned to a fixed class - the only requirement is that a each
packet, when fragmented, must have the same class value assigned to all
fragments.
One suggestion to implementers of integrated services on MCML and RTF
links is that all BE traffic may be logically separated from QoS traffic,
and mapped to a fragmentable (MCML classes 0-3 in short sequence number
fragment format, 0-15 in long sequence number fragment format) or
suspendable (RTF classes 0-6) class. Since BE traffic will in most
implementations not be scheduled for transmission except when a link
is empty (that is, no CL or GS traffic is ready for transmission), it
is possible to recommend use of class number 0 for BE traffic.
Treatment of non-conformant QoS traffic is a policy and implementation
issue. It is recommended that policing of flows containing non-
conformant traffic always be done at the level of granularity of
individual packets rather than at a finer grained level. In
particular, network elements scheduling flows for transmission
that drop non-conformant traffic should drop entire packets rather
than dropping individual fragments of packets belonging to non-
conformant traffic. For those network elements whose implementation
allows forwarding of non-conformant traffic when link bandwidth is
available rather than dropping the traffic, the implementation should
fragment packets of such traffic to the smallest MTU of all
admitted CL flows so as to ensure that CLmDELAY targets are met.
Whether BE and traffic are treated differently in regards to transmission
(e.g., BE is given priority access over non-conformant traffic to the
link) or whether within each type of traffic special treatment is
afforded to individual flows (e.g., WFQ, RED, etc.) is implementation
dependent.
In the case where fewer reservations are expected than the total number
of classes negotiated for a PPP link, it is possible to assign
individual flows to fixed class numbers. This assignment is useful in
the case where the protocol identifier associated with one or more
flows is known at LCP negotiation time and the bandwidth of the
connection is relatively small. If these conditions hold true, then for
those flows that are known, a specific class can optionally be assigned
to them and the prefix elision PPP option can be used for those classes
to achieve a small bandwidth savings.
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5. Invocation Information
To invoke Controlled Load and Guaranteed Services, both traffic
characteristics and the flow itself must be identified to the Network
Element. Several methods can be employed to identify the flows. For
RSVP, classification can be used to identify the flows. For non-RSVP
implementations, other mechanisms have been described such as the
FlowID field in IP Version 6 and the TOS field in IP version 4.
If the Network Service Element is running on a system that doesn't
support application or proxy use of the TOS or FlowID fields, then
classification must be applied and:
As described in [4], Controlled Load Service is invoked by specifying
the flow's traffic characteristics through a TSpec (see [5]).
As described in [5], Guaranteed Service is invoked by specifying the
flow's TSpec and a requested reservation via an RSpec (see [6]).
6. Exported Information
For Controlled Load Service, there is no requirement to export
information.
For Guaranteed service, both C and D terms for delay computations must
be made available for export through the Adspec or other means. See
Sections 9.1 (Admission Control) for guidelines on computing the C and D
terms.
See [4] and [5] for additional information on Exported Information.
7. Ordering and Merging
Refer to [4] and [5] for TSpec and RSpec ordering and merging
guidelines.
8. Guidelines for Implementors
8.1. PPP Bit and Byte Stuffing Effects on Admission Control
An important consideration in performing admission control for PPP
links is reductions in effective link rate due to bit stuffing. Typical
bit stuffing algorithms can result in as much as 20% additional
overhead. Thus, admission control implementations for guaranteed
service over links where bit stuffing is used should take the RSpec
rate of all flows and multiply by 1.2 in determining whether a new flow
can be admitted or not. Admission control implementations for
controlled load reservations may use a similar algorithm using the
TSpec peak rate or may attempt to measure the actual degree of
expansion occurring on a link due to bit stuffing. This
characterization can then be used to adjust the calculated remaining
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link capacity. Such measurements must be used cautiously, in that the
degree of bit stuffing that occurs may vary significantly, both in an
inter- and intra-flow fashion.
Byte stuffing is also used on many PPP links, most frequently on POTS
modems when using the v.42 protocol. Byte stuffing poses a difficult
problem to admission control, particularly in the case of guaranteed
service, due to its highly variable nature. In the worse case, byte
stuffing can result in a doubling of frame sizes. As a consequence, a
strict implementation of admission control for guaranteed load on byte
stuffed PPP links should double the RSpec of link traffic in making
flow admission decisions. As with bit stuffing, implementations of
controlled load service admission control algorithms for links with
byte stuffing may attempt to determine average packet expansion via
observation or may use the theoretical worst case values.
8.2. Compression Considerations
The architecture for providing integrated services over low bandwidth
links uses several PPP options to negotiate link configuration as
described in [4, 8]. When deciding whether to admit a flow, Admission
Control must compute the impact of the following on MTU size, rate,
and fragment size:
Header compression: Van Jacobson or Casner-Jacobson.
Prefix Elision.
CCP.
CRTP.
Fragment header option used.
Fragmentation versus suspend/resume approach.
If any of the compression options are implemented for the connection,
the actual transmission rate, and thus the bandwidth required of the
link, will be reduced by the compression method(s) used.
Prefix elision can take advantage of mapping flows to MLPPP classes
to elide prefixes which cannot be compressed at higher layers. By
establishing agreement across the link, the sender may elide a prefix for
a certain class of traffic and upon receiving packets in that class, the
receiver can restore the prefix.
Both compression gain and elision gain must be included as described in
the admission control section below.
8.3. Admission Control
Admission Control must decide whether to admit a flow based on rate and
delay. Assume the following:
LinkRate is the rate of the link.
MTU is the maximum transmission unit from a protocol.
MRU is the maximum receive unit for a particular link.
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CMTU is the maximum size of the MTU after compression is applied.
eMTU is the maximum effective size of the MTU after fragmentation.
FRAG is the fragment size including MLPPP header/trailers.
Header is the size of the header/trailers/framing for MLPPP/Fragments.
pHeader is the additional header/framing overhead associated with
suspend/resume. This should include FSE and worst case stuffing
overhead.
pDelay is the delay associated with suspend/resume packets.
b is the bucket depth in bytes
R is the requested Rate.
D is the fixed overhead delay for the link (Modem, DSU, etc).
C is the delay associated with transmission and fragmentation.
eRate is the effective rate after compression and fragmentation.
The D term may be configured by an administrative tool once the network
is installed; it may be computed using the Adspec or other real-time
measurement means; or it may be available from hardware during link
setup and/or PPP negotiation. Refer to Appendix A for more
considerations on PPP link characteristics and delays.
Admission Control must compute CMTU, eMTU, and eRate for Controlled Load
Service, and it must compute CMTU, eMTU, eRate, and C for Guaranteed
Service:
To determine whether the requested rate is available, Admission Control
must compute the effective rate of the request (eRate) - worst case - as
follows:
#_of_Fragments = (CMTU + FRAG)/(FRAG-Header)
eMTU = (#_of_Fragments) * FRAG
eRate = eMTU/CMTU * R
Admission Control should compare the eRate of the request against the
remaining bandwidth available to determine if the requested rate can be
delivered.
For Controlled Load Service, a flow can be admitted as long as there is
sufficient bandwidth available (after the above computation) to meet the
rate requirement, and if there is sufficient buffer space (sum of the
token bucket sizes does not exceed the buffer capacity). While some
statistical multiplexing could be done in computing admissibility, the
nature of the low-bitrate links could make this approach risky as any
delay incurred to address a temporary overcommitment could be difficult
to amortize.
Guaranteed Service requires delay computations. These computation are
based on the standard formula for delay:
Delay = b/R + C/R + D
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Note that for suspend/resume, an additional term is required:
pDelay = b/R + C/R + D + pHeader/R.
This term exists because of the additional overhead associated with the
suspend/resume headers created when suspending a packet. In the worse
case, every transmission of a QoS packet could require suspension of a
best effort packet and thus incur the overhead. In most networks, this
term will be nominal at most. However, on some low-bitrate links, the
overhead may be worth computing.
Since MLPPP includes fragmentation, the C term is not fixed and must be
represented by the worse case fragmentation as computed in the effective
MTU size:
C = eMTU.
Note that because the links under consideration are point to point,
Guaranteed Service can be offered over a link without any
negotiated agreement from the next hop. However, if these services
are used in conjunction with RSVP, the C and D values above should be
used in the Adspec.
8.4. Fragment Scheduling Considerations
As described in Section 4, large packets should be fragmented to a size
sufficiently small to allow higher priority flows to get a hold of the
line quickly enough to not violate their reservation constraints. As
such, the upper bound for fragment sizes should be no larger than the
smallest MTU of all QoS flows. While a very small fragment size
is desirable from the point of view of meeting QoS deadlines, the
overhead associated with highly granular fragmentation makes it
necessary to strike a balance between these considerations. While this
document will not specify a particular scheduling algorithm, the
following example should help illustrate the issue:
Assume we have three different priority flows, A, B, and C.
Packets from flow C take 100ms, flow B takes 30ms, and flow A takes 30ms
to transmit. B's required maximum latency is 70ms, while A's is 50ms.
The above scenario results in flows B and C needing to be segmented
into 20ms long fragments - that way a lower priority frame will hold
the link at most 20ms before A gets to the link, taking another
30ms to transit, totaling 50ms - all well and good. B has a
problem, however - in the scenario where a fragment from C is just
starting to transmit the link when packets from A and B arrive (call
this time 0). The fragment from C will transmit until time 20ms.
After that, the A packet will transmit - finishing by time 50ms,
just in time. At this point, the fragment from B starts to
transmit - taking 30ms more, finishing by time 80ms (thus violating
its reservation).
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The important point above the scenario is not that it is possible
to overcommit a link, but that a link can be underutilized
by using too large a fragment size - in the above case, a 10ms
fragment size would have allowed both A and B to honor their
reservations, a 20ms size does not.
9. Evaluation Criteria
For Controlled Load Service, the network element must ensure that
the service requested via the TSpec is delivered to the requesting QoS
flow such that the PPP link appears to be a lightly loaded link.
As a baseline, it is suggested that performance measurements on
throughput, delay, and packet error measurements be performed on an
unloaded link with just the QoS flow using various packet sizes. The
baseline should measure performance for both conformant and non-
conformant traffic when for overloading the link with a single flow.
Once these measurements are complete, measurements of the
Controlled Load Service should be performed as follows:
1) Request QoS flows in the presence of best effort traffic and ensure
that the QoS flows' performance approximate the unloaded baseline
measurements.
2) Request QoS flows whose aggregate throughput would exceed the link
capacity. Admission Control should deny these service requests or admit
them as best effort only.
3) Generate traffic on a QoS flow which exceeds its TSpec commitment.
Ensure recovery of the flow once the traffic becomes conformant.
For Guaranteed Service:
1) Ensure that Admission Control will deny service requests or convert
them to best effort when link capacity or delay bounds would be
exceeded.
2) On a best-efforts loaded link, ensure that the number of lost packets
does not exceed those established in the baseline measurements.
3) On a best-efforts loaded link, ensure that delay and rate commitments
can be met for QoS flows.
4) With multiple QoS flows, ensure that an admission of additional QoS
flows does not cause a violation in rate, error rate, or delay
constraints of any QoS flow.
10. Security Considerations
General security considerations for MLPPP and PPP links are
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addressed in RFC 1990 and RFC 1661, respectively. Security
considerations relevant to RSVP, used as the signaling protocol
for integrated services, are discussed in RFC 2209.
A specific security consideration relevant to providing quality
of service over PPP links appears when relying on either observed
or theoretical average packet expansion during admission control
due to bit- or byte-stuffing. Implementations based on these
packet-expansion values contain a potential vulnerability to
denial of service attacks. An adversary could intentionally send
traffic that will result in worst case bit- or byte stuffing
packet expansion. This in turn could result in quality of service
guarantees not being met for other flows due to overly permissive
admission control. This potential denial of service attack
argues strongly for using a worst case expansion factor in
admission control calculations, even for controlled load service.
Beyond the considerations documented above, this document introduces
no new security issues on top of those discussed in the companion
ISSLL documents [5] and [10]. Any use of these service mappings
assumes that all requests for service are authenticated
appropriately.
11. References
[1] C. Bormann, ``Providing integrated services over low-bitrate
links'', Work in Progress (draft-ietf-issll-isslow-04.txt),
August 1998.
[2] C. Bormann, ``The Multi-Class Extension to Multi-Link PPP'',
Work in Progress (draft-ietf-issl-isslow-mcml-04.txt),
August 1998.
[3] C. Bormann, ``PPP in a real-time oriented HDLC-like framing'',
Work in Progress (draft-ietf-issl-isslow-rtf-03.txt),
August 1998.
[4] S. Casner, V. Jacobson, ``Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links'', Work in Progress (draft-ietf-avt-
crtp-05.txt), July 1998.
[5] J. Wroclawski, ``Specification of the Controlled-Load Network
Element Service'', RFC 2211, September 1997.
[6] C. Partridge, R. Guerin, ``Specification of Guaranteed Quality
of Service'', RFC 2212, September 1997.
[7] S. Shenker, J. Wroclawski, ``General Characterization Parameters
for Integrated Service Network Elements'', RFC 2215,
September 1997.
[8] V. Jacobson, "TCP/IP Compression for Low-Speed Serial Links,"
RFC 1144.
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12. Authors' Addresses:
Steve Jackowski
Deterministic Networks, Inc.
245M Mt Hermon Rd, #140
Scotts Valley, CA 95060
stevej@DeterministicNetworks.com
(408)813-6294
David Putzolu
Intel Architecture Labs (IAL)
JF3-206-H10
2111 NE 25th Avenue
Hillsboro, OR 97124-5961
David.Putzolu@intel.com
(503) 264-4510
Acknowledgements
This document draws heavily on the work of the ISSLL WG of the IETF.
Appendix A. Admission Control Considerations for POTS Modems
The protocols used in current implementations of POTS modems can
exhibit significant changes in link rate and delay over the duration of
a connection. Admission control and link scheduling algorithms used
with these devices must be prepared to compensate for this variability
in order to provide a robust implementation of integrated services.
Link rate on POTS modems is typically reported at connection time. This
value may change over the duration of the connection. The v.34
protocol, used in most POTS modems, is adaptive to link conditions, and
is able to recalibrate transmission rate multiple times over the
duration of a connection. Typically this will result in a small (~10%)
increase in transmission rate over the initial connection within the
first minute of a call. It is important to note, however, that other
results are possible as well, including decreases in available
bandwidth. Admission control algorithms must take such changes into
consideration as they occur, and implementations must be able to
gracefully handle the pathological case where link rate actually drops
below the currently reserved capacity of a link.
Delay experienced by traffic over POTS modems can vary significantly
over time. Unlike link rate, the delay often does not converge to a
stable value. The v.42 protocol is used in most POTS modems to provide
link-layer reliability. This reliability, which is implemented via
retransmission, can cause frames to experience significant delays.
Retransmissions also implicitly steal link bandwidth from other
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traffic. These delays and reductions in link bandwidth make it
extremely difficult to honor a guaranteed service reservation. On a
link that is actually lightly or moderately loaded, a controlled load
service can to some extent accept such events as part of the behavior
of a lightly loaded link. Unfortunately, as actual link utilization
increases, v.42 retransmissions have the potential of stealing larger
and larger fractions of available link bandwidth; making even
controlled load service difficult to offer at high link utilization
when retransmissions occur.
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