Internet Engineering Task Force Integrated Services WG
INTERNET-DRAFT S. Jackowski
draft-ietf-issll-isslow-svcmap-04.txt Deterministic Networks
D. Putzolu
Intel Architecture Labs
Nov 1998
Expires: 5/99
Network Element Service Specification for Low Speed 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
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet- Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
This draft is a product of the Integrated Services Working Group of
the Internet Engineering Task Force. Comments are solicited and
should be addressed to the working group's mailing list at int-
serv@isi.edu and/or the author(s).
Abstract
This document defines the service mappings for controlled load and
guaranteed services over low-bitrate networks. These low-bitrate
networks typically include components such as analog phone lines, ISDN
connections and sub-T1 rate links. The document specifies the per-
network element packet handling behavior, parameters required, traffic
specification, policing requirements, and traffic ordering
relationships which are required to provide both Guaranteed and
Controlled Load service capabilities. It also includes evaluation
criteria for elements providing the service.
This document is a product of the IETF ISSL working group and is based
on [1] and [2] which describe modifications to the PPP protocol to
enable these services.
Jackowski/Putzolu Expires 5/99 [Page 1]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
Table of Contents
1. Introduction 3
2. End to end Behavior 4
3. Motivation 5
4. Network Element Data Handling 5
4.01 Rate and delay 6
4.02 Link Aggregation 6
4.1 Controlled Load Versus Guaranteed Service 7
4.2 Controlled Load and Guaranteed Service Data Handling 7
4.3 Controlled Load and Guaranteed Service Class Mapping 8
5. Invocation Information 11
6. Exported Information 11
7. Policing 12
8. Ordering and Merging 12
9. Guidelines for Implementors 12
9.1 Bit and Byte Stuffing Considerations 12
9.2 Compression Considerations 13
9.3 Admission Control 14
9.4 Fragment Scheduling Considerations 15
10. Evaluation Criteria 16
11. Security Considerations 17
12. References 17
13. Authors' Addresses 17
Appendix A - Additional Admission Control Issues for PPP Links 18
Jackowski/Putzolu Expires 5/99 [Page 2]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
1. Introduction
The ISSLOW subgroup of the ISSL working group has focused on defining
mechanisms to permit flow differentiation and QoS capabilities for mixed
traffic over low speed links. This has been accomplished through a
series of extensions to the PPP protocol which permit fragmentation
and/or suspension of large packets in favor of packets on flows which
require QoS. These protocol extensions are presented in [1] and [2].
This document describes the service mapping required to implement the
controlled load and guaranteed services over these PPP protocol
extensions. It is modeled on the Network Element Service Specification
Template described in [3]. It is assumed that the ISSLOW Network
Element is one portion of a PPP service available to the system.
2. End-to-end Behavior
Unlike many of the other specific link layers addressed in the ISSL
working group, ISSLOW operates only over low speed point to point links
or connections. Examples of these links include dial up lines, ISDN
channels, and leased lines. As such, 'end to end' simply means between
two points. In today's inter/intranet environment, 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 ISSLOW links are applied on
the link 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 tend to be the most 'bandwidth constrained' along the
path.
ISSLOW services are only provided if both endpoints on the link support
ISSLOW. This is determined during the PPP negotiation. Because of the
unique characteristics of a point to point link with both endpoints
supporting ISSLOW, traffic is automatically shaped. That is, incoming
traffic will be TSpec conformant, and except for some special
considerations for Guaranteed Service (below), 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. As
described in [5], Guaranteed Service should approximate the
functionality of a leased line. Since ISSLOW runs over point to point
links, when rate control and delay bounds are provided for individual
flows, the link inherently acts like a leased circuit.
Thus, even for Guaranteed Service, because this hop is the most
bandwidth constrained, and because the connection is dual simplex
(e.g., not a shared link for send & receive), all admission control
decisions can be made locally.
Jackowski/Putzolu Expires 5/99 [Page 3]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt May, 1997
3. Motivation
Previous sections described the motivation for the ISSLOW capabilities.
Dial up users are now treating their relatively low-bitrate connections
as they would a higher speed connection. They are mixing multiple flows
of data and expect performance similar to what they see in a LAN
environment. However, it is deployment of realtime applications which
is the primary motivation for hosts to implement Integrated Services.
Realtime applications typically have tight delay constraints to achieve
consistent performance, requiring small packets. When these are
mixed with flows consisting of large packets (e.g. HTTP, FTP), delay
variance increases and absolute delay suffers as these small packets
wait in the queue behind even a single large packet being transmitted on
the link. Because of the jitter tolerance and adaptive nature of many
modern realtime applications, just providing a controlled load
service would satisfy most of the needs of this class of applications.
Another consideration in handling of packets over low speed links occurs
when looking at the end-to-end issues. The low speed link is
usually just one hop on a longer path between endpoints. As such, it is
usually the limiting factor in performance. While this needs to be
considered in the host to router configuration, it becomes more critical
between edge devices and backbone routers where there is a multiuser
subnet as source and destination for traffic and a low-bitrate link to
the router. To ensure some performance bounds end-to-end, guaranteed
service should be considered over these links even if it cannot be
offered end to end in the network.
4. Network Element Data Handling Requirements
The ISSLOW Network Service element may be implemented in hardware or
software. As described in [1] and [2], for systems which can perform
bit-oriented transmission control, the suspend/resume approach optimizes
the available bandwidth by minimizing header overhead associated with
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 [1] 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 IntServ 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 fragments. It is also conceivable that one IntServe Flow's packet
might suspend another flow's packet if the delivery deadline of the new
packet is earlier than the current packet.
Jackowski/Putzolu Expires 5/99 [Page 4]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
For systems which use fragmentation, since suspend/resume is not
possible, all packets longer than the maximum tolerable delay for an
IntServ packet are fragmented prior to transmission so that a short
packet for another flow can be interleaved between fragments of a large
packet and still meet the transmission deadline for the IntServ flow.
Note that the fragmentation discussed in this document refers to
multilink PPP (MLPPP) fragmentation and associated MCMLPPP modifications
as described in [1], not IP or other layer 3 fragmentation. MLPPP
fragmentation is local to the PPP link, and does not affect end-to-end
(IP) MTU.
4.01 Rate and Delay
ISSLOW assumes that the nature of point to point links is such that
rate, transmission time and delay are fixed and consistent. The rate of
the link is determined at connection time, and the devices on the link
(adapters, modems, DSU/CSUs, etc) exhibit fixed delay characteristics.
Unfortunately this is not always true.
POTS modems can have varying rates, but the rate for a particular
POTS modem connection tends to converge over time to a particular
value as the modems adjust to line conditions. Implementations
may need to adjust their admission control policies to reflect
this convergence. Refer to Appendix A for more considerations
on link characteristics and associated delay.
4.02 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.
4.1 Controlled Load versus Guaranteed Service
With most link layers, Guaranteed Service 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.
Jackowski/Putzolu Expires 5/99 [Page 5]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
Controlled Load requires that the delay associated with packet
transmission be 'closely equivalent to unloaded best effort service.'
Because ISSLOW operates over 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 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.
4.2 Controlled Load and Guaranteed Service Data Handling
Upon arrival of a QoS flow's packet, the ISSLOW 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 IntServ 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.
Jackowski/Putzolu Expires 5/99 [Page 6]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
4.3 Controlled Load and Guaranteed Service Class Mappings
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 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 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 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
Jackowski/Putzolu Expires 5/99 [Page 7]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
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)
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 and non-conformant traffic be logically separated
from conformant 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 and
non-conformant 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/non-conformant traffic. Whether BE and non-
conformant 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.
Jackowski/Putzolu Expires 5/99 [Page 8]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
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, filtering can be used to identify the flows. For non-RSVP
implementations, mechanisms such as the FlowID field in IP Version 6, or
the TOS field in IP version 4 can be used. As described above, the
Network Element can then use the specified values to apply the
appropriate QoS and to map the flows to a particular class.
A number of major router and switch vendors currently support use of
the TOS bits to signal priority and class of service. As a result,
applications and host proxies have been developed to enable users and
network managers to apply policies requesting differential services via
TOS. Use of these bits is the easiest way to identify flows without
the overhead of filtering.
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
filtering 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. Policing
Policing is applied to non-conformant QoS traffic and to best effort
traffic whose transmission would violate the Controlled Load or
Guaranteed Service constraints. This document does not designate a
specific packet scheduling implementation. Ideally, best effort traffic
can be serviced through separate queues and a weighted queuing mechanism,
or when a conflict with QoS traffic arises, best-efforts traffic can
simply be discarded.
Both Controlled Load and Guaranteed Services permit scheduling of non-
conformant traffic as well as the option to discard non-conformant
packets. See [4] and [5] for additional information on policing options
for Controlled Load and Guaranteed Services.
Jackowski/Putzolu Expires 5/99 [Page 9]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
8. Ordering and Merging
Refer to [4] and [5] for TSpec and RSpec ordering and merging
guidelines.
9. Guidelines for Implementors
9.1. PPP Bit Stuffing 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
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.
9.2 Compression Considerations
The ISSLOW specification supports several PPP options. 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-Jacobsen.
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.
Jackowski/Putzolu Expires 5/99 [Page 9]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
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.
9.3 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.
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 realtime
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.
Jackowski/Putzolu Expires 5/99 [Page 10]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
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
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 ISSLOW runs on point to point links, 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.
9.4 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 efficient link utilization, 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:
Jackowski/Putzolu Expires 5/99 [Page 11]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
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).
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.
10. Evaluation Criteria
For Controlled Load Service, the ISSLOW 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.
Jackowski/Putzolu Expires 5/99 [Page 12]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
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.
11. Security Considerations
General security considerations for PPP links are
addressed elsewhere. 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 permission 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.
12. References
[1] C. Bormann "The Multi-Class Extension to Multilink PPP"
Internet Draft, July 1997, <draft-ietf-issl-isslow-mcml-04.txt>
[2] C. Bormann "PPP in a real-time oriented HDLC-like framing"
Internet Draft, July 1997, <draft-ietf-issl-isslow-rtf-04.txt>
[3] rfc2216 -- Network Element Service Specification Template. S.
Shenker, J. Wroclawski. September 1997
[4] rfc2211 -- Specification of the Controlled-Load Network
Element Service. J. Wroclawski. September 1997
[5] rfc2212 -- Specification of Guaranteed Quality of Service. S.
Shenker, C. Partridge, R. Guerin. September 1997
[6] rfc2215 -- General Characterization Parameters for Integrated
Service Network Elements. S. Shenker, J. Wroclawski. September 1997.
Jackowski/Putzolu Expires 5/99 [Page 13]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
13 Author's Address:
Steve Jackowski
Deterministic Networks, Inc.
245M Mt Hermon Rd, #140
Scotts Valley, CA 95060
stevej@DeterministicNetworks.com
(408)813-6294
David M. Putzolu
Intel Architecture Labs (IAL)
JF3-206-H10
2111 NE 25th Avenue
Hillsboro, OR 97124-5961
David.Putzolu@intel.com
(503) 264-4510
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
traffic. These delays and reductions in link bandwidth make it
extremely difficult to honor a guaranteed service reservation. On a
Jackowski/Putzolu Expires 5/99 [Page 14]
INTERNET-DRAFT draft-ietf-issl-isslow-svcmap-04.txt Nov 1998
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.
Jackowski/Putzolu Expires 5/99 [Page 15]
