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rfc:rfc1349

Network Working Group P. Almquist Request for Comments: 1349 Consultant Updates: RFCs 1248, 1247, 1195, July 1992

       1123, 1122, 1060, 791
           Type of Service in the Internet Protocol Suite

Status of This Memo

 This document specifies an IAB standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "IAB
 Official Protocol Standards" for the standardization state and status
 of this protocol.  Distribution of this memo is unlimited.

Summary

 This memo changes and clarifies some aspects of the semantics of the
 Type of Service octet in the Internet Protocol (IP) header.  The
 handling of IP Type of Service by both hosts and routers is specified
 in some detail.
 This memo defines a new TOS value for requesting that the network
 minimize the monetary cost of transmitting a datagram.  A number of
 additional new TOS values are reserved for future experimentation and
 standardization.  The ability to request that transmission be
 optimized along multiple axes (previously accomplished by setting
 multiple TOS bits simultaneously) is removed.  Thus, for example, a
 single datagram can no longer request that the network simultaneously
 minimize delay and maximize throughput.
 In addition, there is a minor conflict between the Host Requirements
 (RFC-1122 and RFC-1123) and a number of other standards concerning
 the sizes of the fields in the Type of Service octet.  This memo
 resolves that conflict.

Table of Contents

 1.  Introduction ...............................................    3
 2.  Goals and Philosophy .......................................    3
 3.  Specification of the Type of Service Octet .................    4
 4.  Specification of the TOS Field .............................    5

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 5.  Use of the TOS Field in the Internet Protocols .............    6
    5.1  Internet Control Message Protocol (ICMP) ...............    6
    5.2  Transport Protocols ....................................    7
    5.3  Application Protocols ..................................    7
 6.  ICMP and the TOS Facility ..................................    8
    6.1  Destination Unreachable ................................    8
    6.2  Redirect ...............................................    9
 7.  Use of the TOS Field in Routing ............................    9
    7.1  Host Routing ...........................................   10
    7.2  Forwarding .............................................   12
 8.  Other consequences of TOS ..................................   13
 APPENDIX A.  Updates to Other Specifications ...................   14
    A.1  RFC-792 (ICMP) .........................................   14
    A.2  RFC-1060 (Assigned Numbers) ............................   14
    A.3  RFC-1122 and RFC-1123 (Host Requirements) ..............   16
    A.4  RFC-1195 (Integrated IS-IS) ............................   16
    A.5  RFC-1247 (OSPF) and RFC-1248 (OSPF MIB) ................   17
 APPENDIX B.  Rationale .........................................   18
    B.1  The Minimize Monetary Cost TOS Value ...................   18
    B.2  The Specification of the TOS Field .....................   19
    B.3  The Choice of Weak TOS Routing .........................   21
    B.4  The Retention of Longest Match Routing .................   22
    B.5  The Use of Destination Unreachable .....................   23
 APPENDIX C.  Limitations of the TOS Mechanism ..................   24
    C.1  Inherent Limitations ...................................   24
    C.2  Limitations of this Specification ......................   25
 References .....................................................   27
 Acknowledgements ...............................................   28
 Security Considerations ........................................   28
 Author's Address ...............................................   28

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1. Introduction

 Paths through the Internet vary widely in the quality of service they
 provide.  Some paths are more reliable than others.  Some impose high
 call setup or per-packet charges, while others do not do usage-based
 charging.  Throughput and delay also vary widely.  Often there are
 tradeoffs: the path that provides the highest throughput may well not
 be the one that provides the lowest delay or the lowest monetary
 cost.  Therefore, the "optimal" path for a packet to follow through
 the Internet may depend on the needs of the application and its user.
 Because the Internet itself has no direct knowledge of how to
 optimize the path for a particular application or user, the IP
 protocol [11] provides a (rather limited) facility for upper layer
 protocols to convey hints to the Internet Layer about how the
 tradeoffs should be made for the particular packet.  This facility is
 the "Type of Service" facility, abbreviated as the "TOS facility" in
 this memo.
 Although the TOS facility has been a part of the IP specification
 since the beginning, it has been little used in the past.  However,
 the Internet host specification [1,2] now mandates that hosts use the
 TOS facility.  Additionally, routing protocols (including OSPF [10]
 and Integrated IS-IS [7]) have been developed which can compute
 routes separately for each type of service.  These new routing
 protocols make it practical for routers to consider the requested
 type of service when making routing decisions.
 This specification defines in detail how hosts and routers use the
 TOS facility.  Section 2 introduces the primary considerations that
 motivated the design choices in this specification.  Sections 3 and 4
 describe the Type of Service octet in the IP header and the values
 which the TOS field of that octet may contain.  Section 5 describes
 how a host (or router) chooses appropriate values to insert into the
 TOS fields of the IP datagrams it originates.  Sections 6 and 7
 describe the ICMP Destination Unreachable and Redirect messages and
 how TOS affects path choice by both hosts and routers.  Section 8
 describes some additional ways in which TOS may optionally affect
 packet processing.  Appendix A describes how this specification
 updates a number of existing specifications.  Appendices B and C
 expand on the discussion in Section 2.

2. Goals and Philosophy

 The fundamental rule that guided this specification is that a host
 should never be penalized for using the TOS facility.  If a host
 makes appropriate use of the TOS facility, its network service should
 be at least as good as (and hopefully better than) it would have been

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 if the host had not used the facility.  This goal was considered
 particularly important because it is unlikely that any specification
 which did not meet this goal, no matter how good it might be in other
 respects, would ever become widely deployed and used.  A particular
 consequence of this goal is that if a network cannot provide the TOS
 requested in a packet, the network does not discard the packet but
 instead delivers it the same way it would have been delivered had
 none of the TOS bits been set.
 Even though the TOS facility has not been widely used in the past, it
 is a goal of this memo to be as compatible as possible with existing
 practice.  Primarily this means that existing host implementations
 should not interact badly with hosts and routers which implement the
 specifications of this memo, since TOS support is almost non-existent
 in routers which predate this specification.  However, this memo does
 attempt to be compatible with the treatment of IP TOS in OSPF and
 Integrated IS-IS.
 Because the Internet community does not have much experience with
 TOS, it is important that this specification allow easy definition
 and deployment of new and experimental types of service.  This goal
 has had a significant impact on this specification.  In particular,
 it led to the decision to fix permanently the size of the TOS field
 and to the decision that hosts and routers should be able to handle a
 new type of service correctly without having to understand its
 semantics.
 Appendix B of this memo provides a more detailed explanation of the
 rationale behind particular aspects of this specification.

3. Specification of the Type of Service Octet

 The TOS facility is one of the features of the Type of Service octet
 in the IP datagram header.  The Type of Service octet consists of
 three fields:
              0     1     2     3     4     5     6     7
           +-----+-----+-----+-----+-----+-----+-----+-----+
           |                 |                       |     |
           |   PRECEDENCE    |          TOS          | MBZ |
           |                 |                       |     |
           +-----+-----+-----+-----+-----+-----+-----+-----+
 The first field, labeled "PRECEDENCE" above, is intended to denote
 the importance or priority of the datagram.  This field is not
 discussed in detail in this memo.
 The second field, labeled "TOS" above, denotes how the network should

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 make tradeoffs between throughput, delay, reliability, and cost.  The
 TOS field is the primary topic of this memo.
 The last field, labeled "MBZ" (for "must be zero") above, is
 currently unused.  The originator of a datagram sets this field to
 zero (unless participating in an Internet protocol experiment which
 makes use of that bit).  Routers and recipients of datagrams ignore
 the value of this field.  This field is copied on fragmentation.
 In the past there has been some confusion about the size of the TOS
 field.  RFC-791 defined it as a three bit field, including bits 3-5
 in the figure above.  It included bit 6 in the MBZ field.  RFC-1122
 added bits 6 and 7 to the TOS field, eliminating the MBZ field.  This
 memo redefines the TOS field to be the four bits shown in the figure
 above.  The reasons for choosing to make the TOS field four bits wide
 can be found in Appendix B.2.

4. Specification of the TOS Field

 As was stated just above, this memo redefines the TOS field as a four
 bit field.  Also contrary to RFC-791, this memo defines the TOS field
 as a single enumerated value rather than as a set of bits (where each
 bit has its own meaning).  This memo defines the semantics of the
 following TOS field values (expressed as binary numbers):
                  1000   --   minimize delay
                  0100   --   maximize throughput
                  0010   --   maximize reliability
                  0001   --   minimize monetary cost
                  0000   --   normal service
 The values used in the TOS field are referred to in this memo as "TOS
 values", and the value of the TOS field of an IP packet is referred
 to in this memo as the "requested TOS".  The TOS field value 0000 is
 referred to in this memo as the "default TOS."
 Because this specification redefines TOS values to be integers rather
 than sets of bits, computing the logical OR of two TOS values is no
 longer meaningful.  For example, it would be a serious error for a
 router to choose a low delay path for a packet whose requested TOS
 was 1110 simply because the router noted that the former "delay bit"
 was set.
 Although the semantics of values other than the five listed above are
 not defined by this memo, they are perfectly legal TOS values, and
 hosts and routers must not preclude their use in any way.  As will
 become clear after reading the remainder of this memo, only the
 default TOS is in any way special.  A host or router need not (and

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 except as described in Section 8 should not) make any distinction
 between TOS values whose semantics are defined by this memo and those
 that are not.
 It is important to note the use of the words "minimize" and
 "maximize" in the definitions of values for the TOS field.  For
 example, setting the TOS field to 1000 (minimize delay) does not
 guarantee that the path taken by the datagram will have a delay that
 the user considers "low".  The network will attempt to choose the
 lowest delay path available, based on its (often imperfect)
 information about path delay.  The network will not discard the
 datagram simply because it believes that the delay of the available
 paths is "too high" (actually, the network manager can override this
 behavior through creative use of routing metrics, but this is
 strongly discouraged: setting the TOS field is intended to give
 better service when it is available, rather than to deny service when
 it is not).

5. Use of the TOS Field in the Internet Protocols

 For the TOS facility to be useful, the TOS fields in IP packets must
 be filled in with reasonable values.  This section discusses how
 protocols above IP choose appropriate values.
 5.1  Internet Control Message Protocol (ICMP)
    ICMP [8,9,12] defines a number of messages for performing error
    reporting and diagnostic functions for the Internet Layer.  This
    section describes how a host or router chooses appropriate TOS
    values for ICMP messages it originates.  The TOS facility also
    affects the origination and processing of ICMP Redirects and ICMP
    Destination Unreachables, but that is the topic of Section 6.
    For purposes of this discussion, it is useful to divide ICMP
    messages into three classes:
     o   ICMP error messages include ICMP message types 3 (Destination
         Unreachable), 4 (Source Quench), 5 (Redirect), 11 (Time
         Exceeded), and 12 (Parameter Problem).
     o   ICMP request messages include ICMP message types 8 (Echo), 10
         (Router Solicitation), 13 (Timestamp), 15 (Information
         Request -- now obsolete), and 17 (Address Mask Request).
     o   ICMP reply messages include ICMP message types 0 (Echo
         Reply), 9 (Router Advertisement), 14 (Timestamp Reply), 16
         (Information Reply -- also obsolete), and 18 (Address Mask
         Reply).

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    An ICMP error message is always sent with the default TOS (0000).
    An ICMP request message may be sent with any value in the TOS
    field.  A mechanism to allow the user to specify the TOS value to
    be used would be a useful feature in many applications that
    generate ICMP request messages.
    An ICMP reply message is sent with the same value in the TOS field
    as was used in the corresponding ICMP request message.
 5.2  Transport Protocols
    When sending a datagram, a transport protocol uses the TOS
    requested by the application.  There is no requirement that both
    ends of a transport connection use the same TOS.  For example, the
    sending side of a bulk data transfer application should request
    that throughput be maximized, whereas the receiving side might
    request that delay be minimized (assuming that it is primarily
    sending small acknowledgement packets).  It may be useful for a
    transport protocol to provide applications with a mechanism for
    learning the value of the TOS field that accompanied the most
    recently received data.
    It is quite permissible to switch to a different TOS in the middle
    of a connection if the nature of the traffic being generated
    changes.  An example of this would be SMTP, which spends part of
    its time doing bulk data transfer and part of its time exchanging
    short command messages and responses.
    TCP [13] should use the same TOS for datagrams containing only TCP
    control information as it does for datagrams which contain user
    data.  Although it might seem intuitively correct to always
    request that the network minimize delay for segments containing
    acknowledgements but no data, doing so could corrupt TCP's round
    trip time estimates.
 5.3  Application Protocols
    Applications are responsible for choosing appropriate TOS values
    for any traffic they originate.  The Assigned Numbers document
    [15] lists the TOS values to be used by a number of common network
    applications.  For other applications, it is the responsibility of
    the application's designer or programmer to make a suitable
    choice, based on the nature of the traffic to be originated by the
    application.
    It is essential for many sorts of network diagnostic applications,
    and desirable for other applications, that the user of the

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    application be able to override the TOS value(s) which the
    application would otherwise choose.
    The Assigned Numbers document is revised and reissued
    periodically.  Until RFC-1060, the edition current as this is
    being written, has been superceded, readers should consult
    Appendix A.2 of this memo.

6. ICMP and the TOS Facility

 Routers communicate routing information to hosts using the ICMP
 protocol [12].  This section describes how support for the TOS
 facility affects the origination and interpretation of ICMP Redirect
 messages and certain types of ICMP Destination Unreachable messages.
 This memo does not define any new extensions to the ICMP protocol.
 6.1  Destination Unreachable
    The ICMP Destination Unreachable message contains a code which
    describes the reason that the destination is unreachable.  There
    are four codes [1,12] which are particularly relevant to the topic
    of this memo:
       0 -- network unreachable
       1 -- host unreachable
      11 -- network unreachable for type of service
      12 -- host unreachable for type of service
    A router generates a code 11 or code 12 Destination Unreachable
    when an unreachable destination (network or host) would have been
    reachable had a different TOS value been specified.  A router
    generates a code 0 or code 1 Destination Unreachable in other
    cases.
    A host receiving a Destination Unreachable message containing any
    of these codes should recognize that it may result from a routing
    transient.  The host should therefore interpret the message as
    only a hint, not proof, that the specified destination is
    unreachable.
    The use of codes 11 and 12 may seem contrary to the statement in
    Section 2 that packets should not be discarded simply because the
    requested TOS cannot be provided.  The rationale for having these
    codes and the limited cases in which they are expected to be used
    are described in Appendix B.5.

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 6.2  Redirect
    The ICMP Redirect message also includes a code, which specifies
    the class of datagrams to which the Redirect applies.  There are
    currently four codes defined:
       0 -- redirect datagrams for the network
       1 -- redirect datagrams for the host
       2 -- redirect datagrams for the type of service and network
       3 -- redirect datagrams for the type of service and host
    A router generates a code 3 Redirect when the Redirect applies
    only to IP packets which request a particular TOS value.  A router
    generates a code 1 Redirect instead when the the optimal next hop
    on the path to the destination would be the same for any TOS
    value.  In order to minimize the potential for host confusion,
    routers should refrain from using codes 0 and 2 in Redirects
    [3,6].
    Although the current Internet Host specification [1] only requires
    hosts to correctly handle code 0 and code 1 Redirects, a host
    should also correctly handle code 2 and code 3 Redirects, as
    described in Section 7.1 of this memo.  If a host does not, it is
    better for the host to treat code 2 as equivalent to code 0 and
    code 3 as equivalent to code 1 than for the host to simply ignore
    code 2 and code 3 Redirects.

7. Use of the TOS Field in Routing

 Both hosts and routers should consider the value of the TOS field of
 a datagram when choosing an appropriate path to get the datagram to
 its destination.  The mechanisms for doing so are discussed in this
 section.
 Whether a packet's TOS value actually affects the path it takes
 inside of a particular routing domain is a choice made by the routing
 domain's network manager.  In many routing domains the paths are
 sufficiently homogeneous in nature that there is no reason for
 routers to choose different paths based up the TOS field in a
 datagram.  Inside such a routing domain, the network manager may
 choose to limit the size of the routing database and of routing
 protocol updates by only defining routes for the default (0000) TOS.
 Neither hosts nor routers should need to have any explicit knowledge
 of whether TOS affects routing in the local routing domain.

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 7.1  Host Routing
    When a host (which is not also a router) wishes to send an IP
    packet to a destination on another network or subnet, it needs to
    choose an appropriate router to send the packet to.  According to
    the IP Architecture, it does so by maintaining a route cache and a
    list of default routers.  Each entry in the route cache lists a
    destination (IP address) and the appropriate router to use to
    reach that destination.  The host learns the information stored in
    its route cache through the ICMP Redirect mechanism.  The host
    learns the list of default routers either from static
    configuration information or by using the ICMP Router Discovery
    mechanism [8].  When the host wishes to send an IP packet, it
    searches its route cache for a route matching the destination
    address in the packet.  If one is found it is used; if not, the
    packet is sent to one of the default routers.  All of this is
    described in greater detail in section 3.3.1 of RFC-1122 [1].
    Adding support for the TOS facility changes the host routing
    procedure only slightly.  In the following, it is assumed that (in
    accordance with the current Internet Host specification [1]) the
    host treats code 0 (redirect datagrams for the network) Redirects
    as if they were code 1 (redirect datagrams for the host)
    Redirects.  Similarly, it is assumed that the host treats code 2
    (redirect datagrams for the network and type of service) Redirects
    as if they were code 3 (redirect datagrams for the host and type
    of service) Redirects.  Readers considering violating these
    assumptions should be aware that long and careful consideration of
    the way in which Redirects are treated is necessary to avoid
    situations where every packet sent to some destination provokes a
    Redirect.  Because these assumptions match the recommendations of
    Internet Host specification, that careful consideration is beyond
    the scope of this memo.
    As was described in Section 6.2, some ICMP Redirects apply only to
    IP packets which request a particular TOS.  Thus, a host (at least
    conceptually) needs to store two types of entries in its route
    cache:
     type 1: { destination, TOS, router }
     type 2: { destination, *, router }
    where type 1 entries result from the receipt of code 3 (or code 1)
    Redirects and type 2 entries result from the receipt of code 2 (or
    code 0) Redirects.

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    When a host wants to send a packet, it first searches the route
    cache for a type 1 entry whose destination matches the destination
    address of the packet and whose TOS matches the requested TOS in
    the packet.  If it doesn't find one, the host searches its route
    cache again, this time looking for a type 2 entry whose
    destination matches the destination address of the packet.  If
    either of these searches finds a matching entry, the packet is
    sent to the router listed in the matching entry.  Otherwise, the
    packet is sent to one of the routers on the list of default
    routers.
    When a host creates (or updates) a type 2 entry, it must flush
    from its route cache any type 1 entries which have the same
    destination.  This is necessary for correctness, since the type 1
    entry may be obsolete but would continue to be used if it weren't
    flushed because type 1 entries are always preferred over type 2
    entries.
    However, the converse is not true: when a host creates a type 1
    entry, it should not flush a type 2 entry that has the same
    destination.  In this case, the type 1 entry will properly
    override the type 2 entry for packets whose destination address
    and requested TOS match the type 1 entry.  Because the type 2
    entry may well specify the correct router for some TOS values
    other than the one specified in the type 1 entry, saving the type
    2 entry will likely cut down on the number of Redirects which the
    host would otherwise receive.  This savings can potentially be
    substantial if one of the Redirects which was avoided would have
    created a new type 2 entry (thereby causing the new type 1 entry
    to be flushed).  That can happen, for example, if only some of the
    routers on the local net are part of a routing domain that
    computes separate routes for each TOS.
    As an alternative, a host may treat all Redirects as if they were
    code 3 (redirect datagrams for hosts and type of service)
    Redirects.  This alternative allows the host to have only type 1
    route cache entries, thereby simplifying route lookup and
    eliminating the need for the rules in the previous two paragraphs.
    The disadvantage of this approach is that it increases the size of
    the route cache and the amount of Redirect traffic if the host
    sends packets with a variety of requested TOS's to a destination
    for which the host should use the same router regardless of the
    requested TOS.  There is not yet sufficient experience with the
    TOS facility to know whether that disadvantage would be serious
    enough in practice to outweigh the simplicity of this approach.
    Despite RFC-1122, some hosts acquire their routing information by
    "wiretapping" a routing protocol instead of by using the

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    mechanisms described above.  Such hosts will need to follow the
    procedures described in Section 7.2 (except of course that hosts
    will not send ICMP Destination Unreachables or ICMP Redirects).
 7.2  Forwarding
    A router in the Internet should be able to consider the value of
    the TOS field when choosing an appropriate path over which to
    forward an IP packet.  How a router does this is a part of the
    more general issue of how a router picks appropriate paths.  This
    larger issue can be extremely complex [4], and is beyond the scope
    of this memo.  This discussion should therefore be considered only
    an overview.  Implementors should consult the Router Requirements
    specification [3] and the the specifications of the routing
    protocols they implement for details.
    A router associates a TOS value with each route in its forwarding
    table.  The value can be any of the possible values of the TOS
    field in an IP datagram (including those values whose semantics
    are yet to be defined).  Any routes learned using routing
    protocols which support TOS are assigned appropriate TOS value by
    those protocols.  Routes learned using other routing protocols are
    always assigned the default TOS value (0000).  Static routes have
    their TOS values assigned by the network manager.
    When a router wants to forward a packet, it first looks up the
    destination address in its forwarding table.  This yields a set of
    candidate routes.  The set may be empty (if the destination is
    unreachable), or it may contain one or more routes to the
    destination.  If the set is not empty, the TOS values of the
    routes in the set are examined.  If the set contains a route whose
    TOS exactly matches the TOS field of the packet being forwarded
    then that route is chosen.  If not but the set contains a route
    with the default TOS then that route is chosen.
    If no route is found, or if the the chosen route has an infinite
    metric, the destination is considered to be unreachable.  The
    packet is discarded and an ICMP Destination Unreachable is
    returned to the source.  Normally, the Unreachable uses code 0
    (Network unreachable) or 1 (Host unreachable).  If, however, a
    route to the destination exists which has a different TOS value
    and a non-infinite metric then code 11 (Network unreachable for
    type of service) or code 12 (Host unreachable for type of service)
    must be used instead.

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8. Other consequences of TOS

 The TOS field in a datagram primarily affects the path chosen through
 the network, but an implementor may choose to have TOS also affect
 other aspects of how the datagram is handled.  For example, a host or
 router might choose to give preferential queuing on network output
 queues to datagrams which have requested that delay be minimized.
 Similarly, a router forced by overload to discard packets might
 attempt to avoid discarding packets that have requested that
 reliability be maximized.  At least one paper [14] has explored these
 ideas in some detail, but little is known about how well such special
 handling would work in practice.
 Additionally, some Link Layer protocols have their own quality of
 service mechanisms.  When a router or host transmits an IP packet, it
 might request from the Link Layer a quality of service as close as
 possible to the one requested in the TOS field in the IP header.
 Long ago an attempt (RFC-795) was made to codify how this might be
 done, but that document describes Link Layer protocols which have
 since become obsolete and no more recent document on the subject has
 been written.

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APPENDIX A. Updates to Other Specifications

 While this memo is primarily an update to the IP protocol
 specification [11], it also peripherally affects a number of other
 specifications.  This appendix describes those peripheral effects.
 This information is included in an appendix rather than in the main
 body of the document because most if not all of these other
 specifications will be updated in the future.  As that happens, the
 information included in this appendix will become obsolete.
 A.1  RFC-792 (ICMP)
    RFC-792 [12] defines a set of codes indicating reasons why a
    destination is unreachable.  This memo describes the use of two
    additional codes:
      11 -- network unreachable for type of service
      12 -- host unreachable for type of service
    These codes were defined in RFC-1122 [1] but were not included in
    RFC-792.
 A.2  RFC-1060 (Assigned Numbers)
    RFC-1060 [15] describes the old interpretation of the TOS field
    (as three independent bits, with no way to specify that monetary
    cost should be minimized).  Although it is likely obvious how the
    values in RFC-1060 ought to be interpreted in light of this memo,
    the information from that RFC is reproduced here.  The only actual
    changes are for ICMP (to conform to Section 5.1 of this memo) and
    NNTP:
  1. —- Type-of-Service Value —–
       Protocol           TOS Value
       TELNET (1)         1000                 (minimize delay)
       FTP
         Control          1000                 (minimize delay)
         Data (2)         0100                 (maximize throughput)
       TFTP               1000                 (minimize delay)
       SMTP (3)
         Command phase    1000                 (minimize delay)
         DATA phase       0100                 (maximize throughput)

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  1. —- Type-of-Service Value —–
       Protocol           TOS Value
       Domain Name Service
         UDP Query        1000                 (minimize delay)
         TCP Query        0000
         Zone Transfer    0100                 (maximize throughput)
       NNTP               0001                 (minimize monetary cost)
       ICMP
         Errors           0000
         Requests         0000 (4)
         Responses        <same as request> (4)
       Any IGP            0010                 (maximize reliability)
       EGP                0000
       SNMP               0010                 (maximize reliability)
       BOOTP              0000
       Notes:
        (1) Includes all interactive user protocols (e.g., rlogin).
        (2) Includes all bulk data transfer protocols (e.g., rcp).
        (3) If the implementation does not support changing the TOS
            during the lifetime of the connection, then the
            recommended TOS on opening the connection is the default
            TOS (0000).
        (4) Although ICMP request messages are normally sent with the
            default TOS, there are sometimes good reasons why they
            would be sent with some other TOS value.  An ICMP response
            always uses the same TOS value as was used in the
            corresponding ICMP request message.  See Section 5.1 of
            this memo.
       An application may (at the request of the user) substitute 0001
       (minimize monetary cost) for any of the above values.
       This appendix is expected to be obsoleted by the next revision
       of the Assigned Numbers document.

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 A.3  RFC-1122 and RFC-1123 (Host Requirements)
    The use of the TOS field by hosts is described in detail in
    RFC-1122 [1] and RFC-1123 [2].  The information provided there is
    still correct, except that:
     (1) The TOS field is four bits wide rather than five bits wide.
         The requirements that refer to the TOS field should refer
         only to the four bits that make up the TOS field.
     (2) An application may set bit 6 of the TOS octet to a non-zero
         value (but still must not set bit 7 to a non-zero value).
    These details will presumably be corrected in the next revision of
    the Host Requirements specification, at which time this appendix
    can be considered obsolete.
 A.4  RFC-1195 (Integrated IS-IS)
    Integrated IS-IS (sometimes known as Dual IS-IS) has multiple
    metrics for each route.  Which of the metrics is used to route a
    particular IP packet is determined by the TOS field in the packet.
    This is described in detail in section 3.5 of RFC-1195 [7].
    The mapping from the value of the TOS field to an appropriate
    Integrated IS-IS metric is described by a table in that section.
    Although the specification in this memo is intended to be
    substantially compatible with Integrated IS-IS, the extension of
    the TOS field to four bits and the addition of a TOS value
    requesting "minimize monetary cost" require minor modifications to
    that table, as shown here:
       The IP TOS octet is mapped onto the four available metrics as
       follows:
       Bits 0-2 (Precedence): (unchanged from RFC-1195)
       Bits 3-6 (TOS):
          0000    (all normal)               Use default metric
          1000    (minimize delay)           Use delay metric
          0100    (maximize throughput)      Use default metric
          0010    (maximize reliability)     Use reliability metric
          0001    (minimize monetary cost)   Use cost metric
          other                              Use default metric
       Bit 7 (MBZ): This bit is ignored by Integrated IS-IS.

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    It is expected that the next revision of the Integrated IS-IS
    specification will include this corrected table, at which time
    this appendix can be considered obsolete.
 A.5  RFC-1247 (OSPF) and RFC-1248 (OSPF MIB)
    Although the specification in this memo is intended to be
    substantially compatible with OSPF, the extension of the TOS field
    to four bits requires minor modifications to the section that
    describes the encoding of TOS values in Link State Advertisements,
    described in section 12.3 of RFC-1247 [10].  The encoding is
    summarized in Table 17 of that memo; what follows is an updated
    version of table 17.  The numbers in the first column are decimal
    integers, and the numbers in the second column are binary TOS
    values:
              OSPF encoding   TOS
              _____________________________________________
              0               0000   normal service
              2               0001   minimize monetary cost
              4               0010   maximize reliability
              6               0011
              8               0100   maximize throughput
              10              0101
              12              0110
              14              0111
              16              1000   minimize delay
              18              1001
              20              1010
              22              1011
              24              1100
              26              1101
              28              1110
              30              1111
    The OSPF MIB, described in RFC-1248 [5], is entirely consistent
    with this memo except for the textual comment which describes the
    mapping of the old TOS flag bits into TOSType values.  TOSType
    values use the same encoding of TOS values as OSPF's Link State
    Advertisements do, so the above table also describes the mapping
    between TOSType values (the first column) and TOS field values
    (the second column).
    If RFC-1247 and RFC-1248 are revised in the future, it is expected
    that this information will be incorporated into the revised
    versions.  At that time, this appendix may be considered obsolete.

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APPENDIX B. Rationale

 The main body of this memo has described the details of how TOS
 facility works.  This appendix is for those who wonder why it works
 that way.
 Much of what is in this document can be explained by the simple fact
 that the goal of this document is to provide a clear and complete
 specification of the existing TOS facility rather than to design from
 scratch a new quality of service mechanism for IP.  While this memo
 does amend the facility in some small and carefully considered ways
 discussed below, the desirability of compatibility with existing
 specifications and uses of the TOS facility [1,2,7,10,11] was never
 in doubt.  This goal of backwards compatibility determined the broad
 outlines and many of the details of this specification.
 Much of the rest of this specification was determined by two
 additional goals, which were described more fully in Section 2.  The
 first was that hosts should never be penalized for using the TOS
 facility, since that would likely ensure that it would never be
 widely deployed.  The second was that the specification should make
 it easy, or at least possible, to define and deploy new types of
 service in the future.
 The three goals above did not eliminate all need for engineering
 choices, however, and in a few cases the goals proved to be in
 conflict with each other.  The remainder of this appendix discusses
 the rationale behind some of these engineering choices.
 B.1  The Minimize Monetary Cost TOS Value
    Because the Internet is becoming increasingly commercialized, a
    number of participants in the IETF's Router Requirements Working
    Group felt it would be important to have a TOS value which would
    allow a user to declare that monetary cost was more important than
    other qualities of the service.
    There was considerable debate over what exactly this value should
    mean.  Some felt, for example, that the TOS value should mean
    "must not cost money".  This was rejected for several reasons.
    Because it would request a particular level of service (cost = 0)
    rather than merely requesting that some service attribute be
    minimized or maximized, it would not only philosophically at odds
    with the other TOS values but would require special code in both
    hosts and routers.  Also, it would not be helpful to users who
    want their packets to travel via the least-cost path but can
    accept some level of cost when necessary.  Finally, since whether
    any particular routing domain considers the TOS field when routing

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    is a choice made by the network manager, a user requiring a free
    path might not get one if the packet has to pass through a routing
    domain that does not consider TOS in its routing decisions.
    Some proposed a slight variant: a TOS value which would mean "I am
    willing to pay money to have this packet delivered".  This
    proposal suffers most of the same shortcomings as the previous one
    and turns out to have an additional interesting quirk: because of
    the algorithms specified in Section 7.2, any packet which used
    this TOS value would prefer links that cost money over equally
    good free links.  Thus, such a TOS value would almost be
    equivalent to a "maximize monetary cost" value!
    It seems likely that in the future users may need some mechanism
    to express the maximum amount they are willing to pay to have a
    packet delivered.  However, an IP option would be a more
    appropriate mechanism, since there are precedents for having IP
    options that all routers are required to honor, and an IP option
    could include parameters such as the maximum amount the user was
    willing to pay.  Thus, the TOS value defined in this memo merely
    requests that the network "minimize monetary cost".
 B.2  The Specification of the TOS Field
    There were four goals that guided the decision to have a four bit
    TOS field and the specification of that field's values:
     (1) To define a new type of service requesting that the network
         "minimize monetary cost"
     (2) To remain as compatible as possible with existing
         specifications and uses of the TOS facility
     (3) To allow for the definition and deployment of new types of
         service in the future
     (4) To permanently fix the size of the TOS field
    The last goal may seem surprising, but turns out to be necessary
    for routing to work correctly when new types of service are
    deployed.  If routers have different ideas about the size of the
    TOS field they make inconsistent decisions that may lead to
    routing loops.
    At first glance goals (3) and (4) seem to be pretty much mutually
    exclusive.  The IP header currently has only three unused bits, so
    at most three new type of service bits could be defined without
    resorting to the impractical step of changing the IP header

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    format.  Since one of them would need to be allocated to meet goal
    (1), at most two bits could be reserved for new or experimental
    types of service.  Not only is it questionable whether two would
    be enough, but it is improbable that the IETF and IAB would allow
    all of the currently unused bits to be permanently reserved for
    types of service which might or might or might not ever be
    defined.
    However, some (if not most of) the possible combinations of the
    individual bits would not be useful.  Clearly, setting all of the
    bits would be equivalent to setting none of the bits, since
    setting all of the bits would indicate that none of the types of
    optimization was any more important than any of the others.
    Although one could perhaps assign reasonable semantics to most
    pairs of bits, it is unclear that the range of network service
    provided by various paths could usefully be subdivided in so fine
    a manner.  If some of these non-useful combinations of bits could
    be assigned to new types of service then it would be possible to
    meet goal (3) and goal (4) without having to use up all of the
    remaining reserved bits in the IP header.  The obvious way to do
    that was to change the interpretation of TOS values so that they
    were integers rather than independently settable bits.
    The integers were chosen to be compatible with the bit definitions
    found in RFC-791.  Thus, for example, setting the TOS field to
    1000 (minimize delay) sets bit 3 of the Type of Service octet; bit
    3 is defined as the Low Delay bit in RFC-791.  This memo only
    defines values which correspond to setting a single one of the
    RFC-791 bits, since setting multiple TOS bits does not seem to be
    a common practice.  According to [15], none of the common TCP/IP
    applications currently set multiple TOS bits.  However, TOS values
    corresponding to particular combinations of the RFC-791 bits could
    be defined if and when they are determined to be useful.
    The new TOS value for "minimize monetary cost" needed to be one
    which would not be too terribly misconstrued by preexisting
    implementations.  This seemed to imply that the value should be
    one which left all of the RFC-791 bits clear.  That would require
    expanding the TOS field, but would allow old implementations to
    treat packets which request minimization of monetary cost (TOS
    0001) as if they had requested the default TOS.  This is not a
    perfect solution since (as described above) changing the size of
    the TOS field could cause routing loops if some routers were to
    route based on a three bit TOS field and others were to route
    based on a four bit TOS field.  Fortunately, this should not be
    much of a problem in practice because routers which route based on
    a three bit TOS field are very rare as this is being written and
    will only become more so once this specification is published.

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    Because of those considerations, and also in order to allow a
    reasonable number of TOS values for future definition, it seemed
    desirable to expand the TOS field.  That left the question of how
    much to expand it.  Expanding it to five bits would allow
    considerable future expansion (27 new TOS values) and would be
    consistent with Host Requirements, but would reduce to one the
    number of reserved bits in the IP header.  Expanding the TOS field
    to four bits would restrict future expansion to more modest levels
    (11 new TOS values), but would leave an additional IP header bit
    free.  The IETF's Router Requirements Working Group concluded that
    a four bits wide TOS field allow enough values for future use and
    that consistency with Host Requirements was inadequate
    justification for unnecessarily increasing the size of the TOS
    field.
 B.3  The Choice of Weak TOS Routing
    "Ruminations on the Next Hop" [4] describes three alternative ways
    of routing based on the TOS field.  Briefly, they are:
     (1) Strong TOS --
         a route may be used only if its TOS exactly matches the TOS
         in the datagram being routed.  If there is no route with the
         requested TOS, the packet is discarded.
     (2) Weak TOS --
         like Strong TOS, except that a route with the default TOS
         (0000) is used if there is no route that has the requested
         TOS.  If there is no route with either the requested TOS or
         the default TOS, the packet is discarded.
     (3) Very Weak TOS --
         like Weak TOS, except that a route with the numerically
         smallest TOS is used if there is no route that has either the
         requested TOS or the default TOS.
    This specification has adopted Weak TOS.
    Strong TOS was quickly rejected.  Because it requires that each
    router a packet traverses have a route with the requested TOS,
    packets which requested non-zero TOS values would have (at least
    until the TOS facility becomes widely used) a high probability of
    being discarded as undeliverable.  This violates the principle
    (described in Section 2) that hosts should not be penalized for
    choosing non-zero TOS values.
    The choice between Weak TOS and Very Weak TOS was not as
    straightforward.  Weak TOS was chosen because it is slightly

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    simpler to implement and because it is consistent with the OSPF
    and Integrated IS-IS specifications.  In addition, many dislike
    Very Weak TOS because its algorithm for choosing a route when none
    of the available routes have either the requested or the default
    TOS cannot be justified by intuition (there is no reason to
    believe that having a numerically smaller TOS makes a route
    better).  Since a router would need to understand the semantics of
    all of the TOS values to make a more intelligent choice, there
    seems to be no reasonable way to fix this particular deficiency of
    Very Weak TOS.
    In practice it is expected that the choice between Weak TOS and
    Very Weak TOS will make little practical difference, since (except
    where the network manager has intentionally set things up
    otherwise) there will be a route with the default TOS to any
    destination for which there is a route with any other TOS.
 B.4  The Retention of Longest Match Routing
    An interesting issue is how early in the route choice process TOS
    should be considered.  There seem to be two obvious possibilities:
     (1) Find the set of routes that best match the destination
         address of the packet.  From among those, choose the route
         which best matches the requested TOS.
     (2) Find the set of routes that best match the requested TOS.
         From among those, choose the route which best matches the
         destination address of the packet.
    The two approaches are believed to support an identical set of
    routing policies.  Which of the two allows the simpler
    configuration and minimizes the amount of routing information that
    needs to be passed around seems to depend on the topology, though
    some believe that the second option has a slight edge in this
    regard.
    Under the first option, if the network manager neglects some
    pieces of the configuration the likely consequence is that some
    packets which would benefit from TOS-specific routes will be
    routed as if they had requested the default TOS.  Under the second
    option, however, a network manager can easily (accidently)
    configure things in such a way that packets which request a
    certain TOS and should be delivered locally will instead follow a
    default route for that TOS and be dumped into the Internet.  Thus,
    the first option would seem to have a slight edge with regard to
    robustness in the face of errors by the network manager.

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    It has been also been suggested that the first option provides the
    additional benefit of allowing loop-free routing in routing
    domains which contain both routers that consider TOS in their
    routing decisions and routers that do not.  Whether that is true
    in all cases is unknown.  It is certainly the case, however, that
    under the second option it would not work to mix routers that
    consider TOS and routers which do not in the same routing domain.
    All in all, there were no truly compelling arguments for choosing
    one way or the other, but it was nontheless necessary to make a
    choice: if different routers were to make the choice differently,
    chaos (in the form of routing loops) would result.  The mechanisms
    specified in this memo reflect the first option because that will
    probably be more intuitive to most network managers.  Internet
    routing has traditionally chosen the route which best matches the
    destination address, with other mechanisms serving merely as tie-
    breakers.  The first option is consistent with that tradition.
 B.5  The Use of Destination Unreachable
    Perhaps the most contentious and least defensible part of this
    specification is that a packet can be discarded because the
    destination is considered to be unreachable even though a packet
    to the same destination but requesting a different TOS would have
    been deliverable.  This would seem to fall perilously close to
    violating the principle that hosts should never be penalized for
    requesting non-default TOS values in packets they originate.
    This can happen in only three, somewhat unusual, cases:
     (1) There is a route to the packet's destination which has the
         TOS value requested in the packet, but the route has an
         infinite metric.
     (2) The only routes to the packet's destination have TOS values
         other than the one requested in the packet.  One of them has
         the default TOS, but it has an infinite metric.
     (3) The only routes to the packet's destination have TOS values
         other than the one requested in the packet.  None of them
         have the default TOS.
    It is commonly accepted that a router which has a default route
    should nonetheless discard a packet if the router has a more
    specific route to the destination in its forwarding table but that
    route has an infinite metric.  The first two cases seem to be
    analogous to that rule.

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    In addition, it is worth noting that, except perhaps during brief
    transients resulting from topology changes, routes with infinite
    metrics occur only as the result of deliberate action (or serious
    error) on the part of the network manager.  Thus, packets are
    unlikely to be discarded unless the network manager has taken
    deliberate action to cause them to be.  Some people believe that
    this is an important feature of the specification, allowing the
    network to (for example) keep packets which have requested that
    cost be minimized off of a link that is so expensive that the
    network manager feels confident that the users would want their
    packets to be dropped.  Others (including the author of this memo)
    believe that this "feature" will prove not to be useful, and that
    other mechanisms may be required for access controls on links, but
    couldn't justify changing this specification in the ways necessary
    to eliminate the "feature".
    Case (3) above is more problematic.  It could have been avoided by
    using Very Weak TOS, but that idea was rejected for the reasons
    discussed in Appendix B.3.  Some suggested that case (3) could be
    fixed by relaxing longest match routing (described in Appendix
    B.4), but that idea was rejected because it would add complexity
    to routers without necessarily making their routing choices
    particularly more intuitive.  It is also worth noting that this is
    another case that a network manager has to try rather hard to
    create: since OSPF and Integrated IS-IS both enforce the
    constraint that there must be a route with the default TOS to any
    destination for which there is a route with a non-zero TOS, a
    network manager would have to await the development of a new
    routing protocol or create the problem with static routes.  The
    eventual conclusion was that any fix to case (3) was worse than
    the problem.

APPENDIX C. Limitations of the TOS Mechanism

 It is important to note that the TOS facility has some limitations.
 Some are consequences of engineering choices made in this
 specification.  Others, referred to as "inherent limitations" below,
 could probably not have been avoided without either replacing the TOS
 facility defined in RFC-791 or accepting that things wouldn't work
 right until all routers in the Internet supported the TOS facility.
 C.1  Inherent Limitations
    The most important of the inherent limitations is that the TOS
    facility is strictly an advisory mechanism.  It is not an
    appropriate mechanism for requesting service guarantees.  There
    are two reasons why this is so:

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RFC 1349 Type of Service July 1992

     (1) Not all networks will consider the value of the TOS field
         when deciding how to handle and route packets.  Partly this
         is a transition issue: there will be a (probably lengthy)
         period when some networks will use equipment that predates
         this specification.  Even long term, however, many networks
         will not be able to provide better service by considering the
         value of the TOS field.  For example, the best path through a
         network composed of a homogeneous collection of
         interconnected LANs is probably the same for any possible TOS
         value.  Inside such a network, it would make little sense to
         require routers and routing protocols to do the extra work
         needed to consider the value of the TOS field when forwarding
         packets.
     (2) The TOS mechanism is not powerful enough to allow an
         application to quantify the level of service it desires.  For
         example, an application may use the TOS field to request that
         the network choose a path which maximizes throughput, but
         cannot use that mechanism to say that it needs or wants a
         particular number of kilobytes or megabytes per second.
         Because the network cannot know what the application
         requires, it would be inappropriate for the network to decide
         to discard a packet which requested maximal throughput
         because no "high throughput" path was available.
    The inability to provide resource guarantees is a serious drawback
    for certain kinds of network applications.  For example, a system
    using packetized voice simply creates network congestion when the
    available bandwidth is inadequate to deliver intelligible speech.
    Likewise, the network oughtn't even bother to deliver a voice
    packet that has suffered more delay in the network than the
    application can tolerate.  Unfortunately, resource guarantees are
    problematic in connectionless networks.  Internet researchers are
    actively studying this problem, and are optimistic that they will
    be able to invent ways in which the Internet Architecture can
    evolve to support resource guarantees while preserving the
    advantages of connectionless networking.
 C.2  Limitations of this Specification
    There are a couple of additional limitations of the TOS facility
    which are not inherent limitations but instead are consequences of
    engineering choices made in this specification:
     (1) Routing is not really optimal for some TOS values.  This is
         because optimal routing for those TOS values would require
         that routing protocols be cognizant of the semantics of the
         TOS values and use special algorithms to compute routes for

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RFC 1349 Type of Service July 1992

         them.  For example, routing protocols traditionally compute
         the metric for a path by summing the costs of the individual
         links that make up the path.  However, to maximize
         reliability, a routing protocol would instead have to compute
         a metric which was the product of the probabilities of
         successful delivery over each of the individual links in the
         path.  While this limitation is in some sense a limitation of
         current routing protocols rather than of this specification,
         this specification contributes to the problem by specifying
         that there are a number of legal TOS values that have no
         currently defined semantics.
     (2) This specification assumes that network managers will do "the
         right thing".  If a routing domain uses TOS, the network
         manager must configure the routers in such a way that a
         reasonable path is chosen for each TOS.  While this ought not
         to be terribly difficult, a network manager could accidently
         or intentionally violate our rule that using the TOS facility
         should provide service at least as good as not using it.

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RFC 1349 Type of Service July 1992

References

[1]   Internet Engineering Task Force (R. Braden, Editor),
      "Requirements for Internet Hosts -- Communication Layers", RFC
      1122, USC/Information Sciences Institute, October 1989.
[2]   Internet Engineering Task Force (R. Braden, Editor),
      "Requirements for Internet Hosts -- Application and Support",
      RFC 1123, USC/Information Sciences Institute, October 1989.
[3]   Almquist, P., "Requirements for IP Routers", Work in progress.
[4]   Almquist, P., "Ruminations on the Next Hop", Work in progress.
[5]   Baker, F. and R. Coltun, "OSPF Version 2 Management Information
      Base", RFC 1248, ACC, Computer Science Center, August 1991.
[6]   Braden, R. and J. Postel, "Requirements for Internet Gateways",
      RFC 1009, USC/Information Sciences Institute, June 1987.
[7]   Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
      Environments", RFC 1195, Digital Equipment Corporation, December
      1990.
[8]   Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox
      PARC, September 1991.
[9]   Mogul, J. and J. Postel, "Internet Standard Subnetting
      Procedure", RFC 950, USC/Information Sciences Institute, August
      1985.

[10] Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc., July 1991.

[11] Postel, J., "Internet Protocol", RFC 791, DARPA, September 1981.

[12] Postel, J., "Internet Control Message Protocol", RFC 792, DARPA,

      September 1981.

[13] Postel, J., "Transmission Control Protocol", RFC 793, DARPA,

      September 1981.

[14] Prue, W. and J. Postel, "A Queuing Algorithm to Provide Type-

      of-Service for IP Links", RFC 1046, USC/Information Sciences
      Institute, February 1988.

[15] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1060,

      USC/Information Sciences Institute, March 1990.

Almquist [Page 27]

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Acknowledgements

 Some of the ideas presented in this memo are based on discussions
 held by the IETF's Router Requirements Working Group.  Much of the
 specification of the treatment of Type of Service by hosts is merely
 a restatement of the ideas of the IETF's former Host Requirements
 Working Group, as captured in RFC-1122 and RFC-1123.  The author is
 indebted to John Moy and Ross Callon for their assistance and
 cooperation in achieving consistency among the OSPF specification,
 the Integrated IS-IS specification, and this memo.
 This memo has been substantially improved as the result of thoughtful
 comments from a number of reviewers, including Dave Borman, Bob
 Braden, Ross Callon, Vint Cerf, Noel Chiappa, Deborah Estrin, Phill
 Gross, Bob Hinden, Steve Huston, Jon Postel, Greg Vaudreuil, John
 Wobus, and the Router Requirements Working Group.
 The initial work on this memo was done while its author was an
 employee of BARRNet.  Their support is gratefully acknowledged.

Security Considerations

 This memo does not explicitly discuss security issues.  The author
 does not believe that the specifications in this memo either weaken
 or enhance the security of the IP Protocol or of the other protocols
 mentioned herein.

Author's Address

 Philip Almquist
 214 Cole Street, Suite 2
 San Francisco, CA 94117-1916
 Phone: 415-752-2427
 Email: almquist@Jessica.Stanford.EDU

Almquist [Page 28]

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