Network Working Group Bruce Davie
Internet Draft Cisco Systems, Inc.
Expiration Date: December 1999
Steve Casner
Cisco Systems, Inc.
Carol Iturralde
Cisco Systems, Inc.
David Oran
Cisco Systems, Inc.
John Wroclawski
MIT LCS
June 1999
Integrated Services in the Presence of Compressible Flows
draft-ietf-intserv-compress-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
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An Integrated Services router performs admission control and resource
allocation based on the information contained in a TSpec (among other
things). As currently defined, TSpecs convey information about the
data rate (using a token bucket) and range of packet sizes of the
flow in question. However, the TSpec may not be an accurate
representation of the resources needed to support the reservation if
the router is able to compress the data at the link level. This
draft describes an extension to the TSpec which enables a sender of
potentially compressible data to provide hints to int-serv routers
about the compressibility they may obtain. Routers which support
appropriate compression take advantage of the hint in their admission
control decisions and resource allocation procedures; other routers
ignore the hint. An initial application of this approach is to
notify routers performing RTP header compression that they may
allocate fewer resources to RTP flows.
Contents
1 Introduction ........................................... 2
2 Addition of a Hint to the Sender TSpec ................. 3
3 Admission Control and Resource Allocation .............. 5
4 Object Format .......................................... 8
4.1 Hint Numbering ......................................... 10
5 Backward Compatibility ................................. 10
6 Security Considerations ................................ 11
7 IANA Considerations .................................... 11
8 Acknowledgments ........................................ 12
9 References ............................................. 12
10 Authors' Addresses ..................................... 12
1. Introduction
In an Integrated Services network, RSVP [RFC 2205] may be used as a
signalling protocol by which end nodes and network elements exchange
information about resource requirements, resource availability, and
the establishment and removal of resource reservations. The
Integrated Services architecture currently defines two services,
Controlled-Load [RFC 2211] and Guaranteed [RFC 2212]. When
establishing a reservation using either service, RSVP requires a
variety of information to be provided by the sender(s) and
receiver(s) for a particular reservation which is used for the
purposes of admission control and allocation of resources to the
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reservation. Some of this information is provided by the receiver in
a FLOWSPEC object; some is provided by the sender in a SENDER_TSPEC
object [RFC 2210].
A situation that is not handled well by the current specs arises when
a router that is making an admission control decision is able to
perform some sort of compression on the flow for which a reservation
is requested. For example, suppose a router is able to perform
IP/UDP/RTP header compression on one of its interfaces [RFC 2508].
The bandwidth needed to accommodate a compressible flow on that
interface would be less than the amount contained in the
SENDER_TSPEC. Thus the router might erroneously reject a reservation
that could in fact have been accommodated. At the same time, the
sender is not at liberty to reduce its TSpec to account for the
compression of the data, since it does not know if the routers along
the path are in fact able to perform compression. Furthermore, it is
probable that only a subset of the routers on the path (e.g., those
connected to low-speed serial links) will perform compression.
This draft proposes that the sender provide a hint to network
elements regarding the compressibility of the data stream that it
will generate. Network elements may use this hint as an additional
piece of data when making admission control and resource allocation
decisions.
This draft is restricted to the case where compression is performed
only on a link-by-link basis, as with header compression. Other cases
(e.g., transcoding, audio silence detection) which would affect the
bandwidth consumed at all downstream nodes are for further study. In
these latter cases, it would be necessary to modify a sender TSpec as
it is passed through a compressing node. In the proposal presented
here, the sender TSpec that appears on the wire is never modified,
just as specified in [RFC 2210].
2. Addition of a Hint to the Sender TSpec
The appropriate place for a `compressibility hint' is the Sender
TSpec. The reasons for this choice are:
- The sender is the party who knows best what the data will look
like.
- Unlike the Adspec, the Sender TSpec is not modified in transit
- From the perspective of RSVP, the Sender TSpec is a set of
opaque parameters that are passed to `traffic control' (admission
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control and resource allocation); the compressibility hint is just
such a parameter.
An alternative to putting this information in the TSpec would be to
use an additional object in the RSVP PATH message. While this could
be made to work for RSVP, it does not address the issue of how to get
the same information to an intserv router when mechanisms other than
RSVP are used to reserve resources. It would also imply a change to
RSVP message processing just for the purposes of getting more
information to entities that are logically not part of RSVP
(admission control and resource allocation). The inclusion of the
information in the TSpec seems preferable and more consistent with
the Integrated Services architecture.
The contents of the hint are likely to vary depending on the exact
scenario. The hint needs to tell the routers that receive it:
- the type of compression that is possible on this flow (e.g.
IP/UDP/RTP);
- enough information to enable a router to determine the likely
compression ratio that may be achieved.
In a simple case such as IP/UDP/RTP header compression, it may be
sufficient to tell the routers nothing more than the fact that
IP/UDP/RTP data is being sent. Knowing this fact, the maximum packet
size of the flow (from the TSpec), and the local conditions at the
router, may be sufficient to allow the router to determine the
reduction in bandwidth that compression will allow. In other cases,
it may be helpful or necessary for the sender to include additional
quantitative information to assist in the calculation of the
compression ratio. To handle these cases, additional parameters
containing various amounts of information may be added to the sender
TSpec. Details of the encoding of these parameters, following the
approach originally described in [RFC 2210] are described below.
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3. Admission Control and Resource Allocation
Integrated Services routers make admission control and resource
allocation decisions based on, among other things, information in the
sender TSpec. If a router receives a sender TSpec which contains a
compressibility hint, it may use the hint to calculate a `compressed
TSpec' which can be used as input to the admission control and
resource allocation processes in place of the TSpec provided by the
sender. To make this concrete, consider the following simple example.
A router receives a reservation request for controlled load service
where:
- The Sender TSpec and Receiver TSpec contain identical token
bucket parameters;
- The rate parameter in the token bucket (r) is 48 kbps;
- The token bucket depth (b) is 120 bytes;
- The maximum packet size (M) in the TSpecs is 120 bytes;
- The minimum policed unit (m) is 64 bytes;
- The Sender TSpec contains a compressibility hint indicating that
the data is IP/UDP/RTP;
- The compressibility hint includes a compression factor of 0.7,
meaning that IP/UDP/RTP header compression will cause a reduction
in bandwidth consumed at the link level by a factor of 0.7 (the
result of compressing 40 bytes of IP/UDP/RTP header to 4 bytes on
a 120 byte packet)
- The interface on which the reservation is to be installed is
able to perform IP/UDP/RTP header compression.
The router may thus conclude that it can scale down the token bucket
parameters r and b by a factor of 0.7, i.e., to 33.6 kbps and 84
bytes respectively. M may be scaled down by the same factor (to 84
bytes), but a different calculation should be used for m. If the
sender actually sends a packet of size m, its header may be
compressed from 40 bytes to 4, thus reducing the packet to 28 bytes;
this value should be used for m.
Note that if the source always sends packets of the same size and
IP/UDP/RTP always works perfectly, the compression factor is not
strictly needed. The router can independently determine that it can
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compress the 40 bytes of IP/UDP/RTP header to 4 bytes (with high
probability). To determine the worst-case (smallest) gain provided by
compression, it can assume that the sender always sends maximum sized
packets at 48 kbps, i.e. a 120 byte packet every 20 milliseconds. The
router can conclude that these packets would be compressed to 84
bytes, yielding a token bucket rate of 33.6 kbps and a token bucket
depth of 84 bytes as before. If the sender is willing to allow an
independent calculation of compression gain by the router, the
explicit compression factor may be omitted from the TSpec. Details of
the TSpec encoding are provided below.
To generalize the above discussion, assume that the Sender TSpec
consists of values (r, b, p, M, m), that the explicit compression
factor provided by the sender is f, and that the number of bytes
saved by compression is N, independent of packet size. The parameters
in the compressed TSpec would be:
r' = r * f
b' = b * f
p' = p
M' = M-N
m' = m-N
The calculations for M' and m' hold only in the case where the
compression algorithm reduces packets by a certain number of bytes
independent of content or length of the packet, as is true for header
compression. Other compression algorithms may not have this property.
In determining the value of N, the router may need to make worst case
assumptions about the number of bytes that may be removed by
compression, which depends on such factors as the presence of UDP
checksums and the linearity of RTP timestamps.
All these adjusted values are used in the compressed TSpec. The
router's admission control and resource allocation algorithms should
behave as if the sender TSpec contained those values. [RFC 2205]
provides a set of rules by which sender and receiver TSpecs are
combined to calculate a single `effective' TSpec that is passed to
admission control. When a reservation covering multiple senders is to
be installed, it is necessary to reduce each sender TSpec by its
appropriate compression factor. The set of sender TSpecs that apply
to a single reservation on an interface are added together to form
the effective sender TSpec, which is passed to traffic control. The
effective receiver TSpec need not be modified; traffic control takes
the greatest lower bound of these two TSpecs when making its
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admission control and resource allocation decisions.
The handling of the receiver RSpec depends on whether controlled load
or guaranteed service is used. In the case of controlled load, no
additional processing of RSpec is needed. However, a guaranteed
service RSpec contains a rate term R which does need to be adjusted
downwards to account for compression. To determine how R should be
adjusted, we note that the receiver has chose R to meet a certain
delay goal, and that the terms in the delay equation that depend on R
are b/R and C/R (when the peak rate is large). The burstsize b in
this case is the sum of the burstsizes of all the senders for this
reservation, and each of these numbers has been scaled down by the
appropriate compression factor. Thus, R should be scaled down by the
ratio
f_avg = (b1*f1 + b2*f2 + ... + bn*fn)/(b1 + b2 + ... bn)
where bk is the burstsize of sender k and fk is the corresponding
compression factor for this sender. Note that this is equivalent to
multiplying R by f in the case where all senders use the same
compression factor f.
To prevent an increase in delay caused by the C/R term when the
reduced value of R is used for the reservation, it is necessary for
this hop to `inflate' its value of C by dividing it by f_avg. This
will cause the contribution to delay made by this hop's C term to be
what the receiver would expect when it chooses its value of R.
There are certain risks in adjusting the resource requirements
downwards for the purposes of admission control and resource
allocation. Most compression algorithms are not completely
deterministic, and thus there is a risk that a flow will turn out to
be less compressible than had been assumed by admission control. This
risk is reduced by the use of the explicit compression factor
provided by the sender, and may be minimized if the router makes
worst case assumptions about the amount of compression that may be
achieved. This is somewhat analogous to the tradeoff between making
worst case assumptions when performing admission control or making
more optimistic assumptions, as in the case of measurement-based
admission control. If a flow turns out to be less compressible that
had been assumed when performing admission control, any extra traffic
will need to be policed according to normal intserv rules. For
example, if the router assumed that the 48 kbps stream above could be
compressed to 33.6 kbps and it was ultimately possible to compress it
to 35 kbps, the extra 1.4 kbps would be treated as excess. The exact
treatment of such excess is service dependent.
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A similar scenario may arise if a sender claims that data for a
certain session is compressible when in fact it is not, or overstates
the extent of its compressibility. This might cause the flow to be
erroneously admitted, and would cause insufficient resources to be
allocated to it. To prevent such behavior from adversely affecting
other reserved flows, any flow that sends a compressibility hint
should be policed (in any router that has made use of the hint for
its admission control) on the assumption that it is indeed
compressible, i.e. using the compressed TSpec. That is, if the flow
is found to be less compressible than advertised, the extra traffic
that must be forwarded by the router above the compressed TSpec will
be policed according to intserv rules appropriate for the service.
Note that RSVP does not generally require flows to be policed at
every hop. To quote [RFC 2205]
Some QoS services may require traffic policing at some or all of
(1) the edge of the network, (2) a merging point for data from
multiple senders, and/or (3) a branch point where traffic flow
from upstream may be greater than the downstream reservation being
requested. RSVP knows where such points occur and must so
indicate to the traffic control mechanism.
For the purposes of policing, a router which makes use of the
compressibility hint in a sender TSpec should behave as if it is at
the edge of the network, because it is in a position to receive
traffic from a sender that, while it passed through policing at the
real network edge, may still need to be policed if the amount of data
sent exceeds the amount described by the compressed TSpec.
4. Object Format
The compressibility hint may be included in the sender TSpec using
the encoding rules of Appendix A in [RFC 2210]. The complete sender
TSpec is as follows:
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31 24 23 16 15 8 7 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 | 0 (a) | reserved | 10 (b) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2 | 1 (c) |0| reserved | 9 (d) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3 | 127 (e) | 0 (f) | 5 (g) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4 | Token Bucket Rate [r] (32-bit IEEE floating point number) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5 | Token Bucket Size [b] (32-bit IEEE floating point number) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6 | Peak Data Rate [p] (32-bit IEEE floating point number) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7 | Minimum Policed Unit [m] (32-bit integer) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 | Maximum Packet Size [M] (32-bit integer) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9 | 126 (h) | 0 (i) | 2 (j) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10 | Hint (assigned number) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
11 | Compression factor [f] (32-bit IEEE floating point number) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(a) - Message format version number (0)
(b) - Overall length (10 words not including header)
(c) - Service header, service number 1 (default/global information)
(d) - Length of service 1 data, 9 words not including header
(e) - Parameter ID, parameter 127 (Token_Bucket_TSpec)
(f) - Parameter 127 flags (none set)
(g) - Parameter 127 length, 5 words not including header
(h) - Parameter ID, parameter 126 (Compression_Hint)
(i) - Parameter 126 flags (none set)
(j) - Parameter 126 length, 2 words not including header
The difference between this TSpec and the one described in [RFC 2210]
is that the overall length contained in the first word is increased
by 3, as is the length of the `service 1 data', and the original
TSpec parameters are followed by a new parameter, the compressibility
hint. This parameter contains the standard parameter header, and an
assigned number indicating the type of compression that is possible
on this data. Different values of the hint would imply different
compression algorithms may be applied to the data. Details of the
numbering scheme for hints appear below.
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Following the hint value is the compression factor f, expressed as a
32 bit IEEE floating point number. The valid range for this factor is
(0,1]. A sender that does not know what value to use here or wishes
to leave the compression factor calculation to the routers'
discretion may use the reserved value 0 to indicate this fact.
In some cases, additional quantitative information about the traffic
may be required to enable a router to determine the amount of
compression possible. In this case, a different encoding of the
parameter would be required.
Note that the Compression_Hint is, like the Token_Bucket_Tspec, not
specific to a single service, and thus has a parameter value less
than 128. It is also included as part of the default/global
information (service number 1).
4.1. Hint Numbering
We propose that the hints be represented by a 32 bit field, with the
high order 16 bits being the IP-compression-protocol number as
defined in [RFC 1332] and [RFC 2509]. The low order 16 bits are a
sub-option for the cases where the IP-compression-protocol number
alone is not sufficient for int-serv purposes. The following hint
values are required at the time of writing:
- hint = 0x002d0000: IP/TCP data that may be compressed according
to [RFC 1144]
- hint = 0x00610000: IP data that may be compressed according to
[RFC 2507]
- hint = 0x00610100: IP/UDP/RTP data that may be compressed
according to [RFC 2508]
5. Backward Compatibility
It is desirable that an intserv router which receives this new TSpec
format and does not understand the compressibility hint should
silently ignore the hint rather than rejecting the entire TSpec (or
the message containing it) as malformed. While [RFC 2210] clearly
specifies the format of TSpecs in a way that they can be parsed even
when they contain unknown parameters, it does not specify what action
should be taken when unknown objects are received. Thus it is quite
possible that some RSVP implementations will discard PATH messages
containing a TSpec with the compressibility hint. In such a case, the
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router should send a PathErr message to the sending host. The message
should indicate a malformed TSpec (Error code 21, Sub-code 04). The
host may conclude that the hint caused the problem and send a new
PATH without the hint.
For the purposes of this draft, it would be preferable if unknown
TSpec parameters could be silently ignored. In the case where a
parameter is silently ignored, the node should behave as if that
parameter were not present, but leave the unknown parameter intact in
the object that it forwards. We propose that this should be the
default for unknown parameters of the type described in [RFC 2210].
It is possible that some future modifications to [RFC 2210] will
require unknown parameter types to cause an error response. This
situation is analogous to RSVP's handling of unknown objects, which
allows for three different response to an unknown object, based on
the highest two bits of the Class-Num. One way to handle this would
be to divide the parameter space further than already done in [RFC
2216]. For example, parameter numbers of the form x1xxxxxx could be
silently ignored if unrecognized, while parameter numbers of the form
x0xxxxxx could cause an error response if unrecognized. (The meaning
of the highest order bit is already fixed by [RFC 2216].) A third
possibility exists, which is to remove the unrecognized parameter
before forwarding, but this does not seem to be useful.
6. Security Considerations
The extensions defined in this document pose essentially the same
security risks as those of [RFC 2210]. The risk that a sender will
falsely declare his data to be compressible is equivalent to the
sender providing an insufficiently large TSpec and is dealt with in
the same way.
7. IANA Considerations
This draft relies on IANA-assigned numbers for the compression scheme
hint. Where possible the existing numbering scheme for compression
algorithm identification in PPP has been used, but it may in the
future be necessary for IANA to assign hint numbers purely for the
purposes of int-serv.
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8. Acknowledgments
Carsten Borman and Mike DiBiasio provided much helpful feedback on
this draft.
9. References
[RFC1144] Jacobson, V. Compressing TCP/IP Headers for Low-Speed
Serial Links, RFC 1144, February 1990.
[RFC 1332] McGregor, G. The PPP Internet Protocol Control Protocol
(IPCP). RFC 1332, May 1992.
[RFC 2205] Braden, R. et al. Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification, RFC 2205, Sep. 1997.
[RFC 2210] Wroclawski, J. The Use of RSVP with IETF Integrated
Services, RFC 2210, Sep. 1997.
[RFC 2211] Wroclawski, J. Specification of the Controlled-Load
Network Element Service, RFC 2211, Sep. 1997.
[RFC 2212] Shenker, S., Partridge, C., and Guerin, R. Specification
of Guaranteed Quality of Service, RFC 2212, Sep. 1997.
[RFC 2216] Shenker, S., and Wroclawski, J. Network Element Service
Specification Template, RFC 2216, Sep. 1997.
[RFC 2507] Degermark, M., Nordgren, B. and S. Pink. Header
Compression for IP, RFC 2507, February 1999.
[RFC 2508] Casner, S. and V. Jacobson. Compressing IP/UDP/RTP Headers
for Low-Speed Serial Links, RFC 2508, February 1999.
[RFC 2509] Engan, M., Casner, S., and Bormann, C. IP Header
Compression over PPP, RFC 2509, February 1999.
10. Authors' Addresses
Bruce Davie
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: bsd@cisco.com
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Stephen L. Casner
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: casner@cisco.com
Carol Iturralde
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: cei@cisco.com
Dave Oran
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: oran@cisco.com
John Wroclawski
MIT Laboratory for Computer Science
545 Technology Sq.
Cambridge, MA 02139
E-mail: jtw@lcs.mit.edu
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