Internet Draft                                    Paul Ferguson
March 12, 1998                              cisco Systems, Inc.
Expires in six months

                    Simple Differential Services:
     IP TOS and Precedence, Delay Indication, and Drop Preference

Status of this Memo

    This document is an Internet Draft.  Internet Drafts are working
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    Recent opinions and sentiments expressed in the Internet
    Service Provider (ISP) community, as well as the Internet
    community at-large, have voiced concern over the applicability
    and scalability of RSVP and the Integrated Service model in
    the global Internet infrastructure. Convincing arguments have
    been made for a differential services model which offers packet
    delivery services better than traditional best effort, especially
    in the face of congestion, yet not as resource intensive as RSVP.
    As a result, the Differentiated Service (diffserv) working group
    in the IETF has been examining methods to provide simpler, less
    resource intensive methods of offering differentiated services.
    This draft provides a practical method to use bit values expressed
    in the IP Type or Service (TOS) and IP precedence subfields of the
    TOS byte in the IP packet header for delay indication and packet drop
    preference, respectively.

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Table of Contents

    1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
    2.  Background . . . . . . . . . . . . . . . . . . . . . . . . 3
    3.  Architectural Significance . . . . . . . . . . . . . . . . 4
        3.1 Delay Indication . . . . . . . . . . . . . . . . . . . 5
        3.2 Queuing Packets with Delay Indication  . . . . . . . . 5
        3.3 Drop Preference  . . . . . . . . . . . . . . . . . . . 7
        3.4 Preferential Drop Mechanism at Intermediate Nodes  . . 7
        3.5 Passive and Active admission . . . . . . . . . . . . . 8
    4.  Summary. . . . . . . . . . . . . . . . . . . . . . . . . . 8
    5.  Security Considerations. . . . . . . . . . . . . . . . . . 9
    6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 9
    7.  References . . . . . . . . . . . . . . . . . . . . . . . . 9
    8.  Author's Address . . . . . . . . . . . . . . . . . . . . . 10

1. Introduction

   Differentiated Quality of Service (QoS) has become a watchword
   in the Internet community, at the expense of differentiated
   definitions of the concept. While many had hoped that RSVP would
   prove to be the mechanism to provide this functionality, a large
   contingent of Internet Service Providers (ISP's) have expressed
   concern over the amount of computational and memory overhead, as
   well as other system resources which might be consumed by managing
   thousands, or perhaps hundreds of thousands, of flows.

   As a result, the Differentiated Services (diffserv) working group
   has been examining several methods to define less resource intensive
   mechanisms to provide differentiated services -- somewhere between
   the traditional best effort model and the guaranteed [1] and
   controlled-load [2] service models defined in the Integrated
   Services architecture [3].

   This draft focuses on two fundamental issues -- a simplistic
   method to signify a "delay indicator" in packets, as well as
   a "drop preference" to indicate the relative priority of a
   particular packet as it is transited through the network.

2. Background

   One of the first questions that becomes immediately apparent in this
   scheme is, "What are we trying to accomplish?" As mentioned previously,
   there is a need for a QoS traffic scheme which provides reasonable
   levels of differentiation, but yet which does not impose the path and
   resource reservation state maintenance required by RSVP [4].

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   The first order of business is to examine methods which provide a
   simpler, less resource intensive method of differentiating traffic,
   but at the same time requires no fundamental change in deployed
   protocols, implementation, and only a modification in the interpreted
   semantics of bit values in the IP TOS and precedence fields, when
   used in conjunction with a differentiated services implementation.

   Without adding a new signaling protocol (which has, in and of itself,
   been a topic of much dissension), the most logical place to consider for
   placing indicators for differentiation is in the IP packet header. If we
   examine the IP header, we find the 8 bit TOS field could indeed be used
   to convey differential service information [Figure 1] [5].  This is due
   to the fact that this byte has historically not been widely used in the

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |Version|  IHL  |Type of Service|          Total Length         |
   |         Identification        |Flags|      Fragment Offset    |
   |  Time to Live |    Protocol   |         Header Checksum       |
   |                       Source Address                          |
   |                    Destination Address                        |
   |                    Options                    |    Padding    |

                    Example IPv4 Datagram Header [5]

                               [Figure 1]

                0     1     2     3     4     5     6     7
             |                 |                       |     |
             |   PRECEDENCE    |          TOS          | MBZ |
             |                 |                       |     |

                         Type of Service Byte [6]

                               [Figure 2]

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   As it exists today, RFC791 [5] suggests using the following
   semantics of bit values which might be contained in the
   precedence field, as depicted in [Figure 2]:

          111 - Network Control
          110 - Internetwork Control
          101 - CRITIC/ECP
          100 - Flash Override
          011 - Flash
          010 - Immediate
          001 - Priority
          000 - Routine

   Additionally, RFC1349 [6] describes the semantics of the bit values
   which might be contained in the 4 bit TOS field, also depicted in
   [Figure 2]:

          1000   --   minimize delay
          0100   --   maximize throughput
          0010   --   maximize reliability
          0001   --   minimize monetary cost
          0000   --   normal service

   This proposal suggests modifying the semantics of the values
   of both of these fields when used in a differential services
   implementation. It should be noted that as both of these bit value
   semantics are currently defined in RFC791 and RFC1349, neither is
   (arguably) widely used for traffic differentiation in practice

3. Architectural Significance

   The objective in this approach is to provide a simplistic method
   in which to indicate the intrinsic value and relative priority of
   each packet presented to the network for transmission. Consider
   the following topology:

     h2---->+-->r--->R---->R--->R-----> [towards destination]

   Consider h1 and h2 as the "hosts" which desire some sort of
   "better-then-best-effort" service -- h1 may send traffic
   which is somewhat delay sensitive (for example, real-time
   loss-tolerant video), and h2 may send traffic which is not
   delay sensitive (for example, batch ftp traffic), but is
   considered to be of higher relative priority than traditional
   best effort traffic (e.g. e-mail).

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   In this particular example, packets originating from h1 may have
   a higher delay sensitivity indication (as well as a lower drop
   preference indication) than other traffic in the network, while
   traffic originating from h2 may only indicate a lower drop

   Each of the tools mentioned in this draft have very specific
   architectural significance, or in other words, the effectiveness
   each tool is dependent on where & how it is used at various
   locations in the network topology.

3.1 Delay Indication

   It should be again be mentioned that this approach does not provide
   a priori knowledge of end-to-end network delay or maximal queuing
   delay, as done with the Integrated Services guaranteed service
   model. What it does attempt to provide, however, is a mechanism
   to identify packets which belong to traffic flows for which predictable
   queuing behavior is desired.

   The following semantics (Delay Indication) is proposed for differential
   services implementations, effectively modifying existing RFC1349 TOS
   subfield semantics:

   Bit            Delay
   Value          Indication

   11__           High delay sensitivity
   10__           Medium delay sensitivity
   01__           Low delay sensitivity
   00__           No delay sensitivity

   The low-order two bits (indicated by underscores in the chart above)
   are not used in this scheme in the interest of coding efficiency and
   should be values of zero (0). It is believed that three levels of
   delay indication (high, medium, low delay sensitivity), in addition
   to "no delay sensitivity," is sufficient for preferential non-FIFO
   queuing and preferential transmission servicing schemes. In any
   event, the reserved low-order two bits may be used in the future
   for expansion of this scheme should it be determined that more
   levels of delay indication is required.

3.2 Queuing Packets with Delay Indication

   There are perhaps several methods which could be used to
   map packets with the delay indication (expressed in Section
   3.1) to specific queues as a mechanism to provide predicable
   transmission behavior.  This relies on the queuing discipline
   used, as well as the queue servicing and transmission algorithm.

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   The most interesting, and perhaps most effective, method of doing
   this is to map packets to different CBQ (Class Based Queuing) [7]

   It should be noted that to provide effective differentiation for
   packets with non-zero (e.g. high, medium, low delay sensitivity)
   delay indication, a consistent CBQ model should be deployed on each
   router in the end-to-end traffic path.

   Simple Delay Indication to CBQ mapping example:

      delay      +-----------------------------+
    sensitivity  |     CBQ queue depth         |
                 |                             |
                 |                             |
                 |    +-------------------+    |
      0100-+     | +->|                   |-+  |
           |     | |  +-------------------+ |  |   towards
           |     | |                        |  |   destinations
           +-------+  +----------------+    +-------->
      1000----------->|                |---------------->
           +-------+  +----------------+     +------->
           |     | |                         | |
           |     | |  +--------+             | |
      1100-+     | +->|        |-------------+ |
                 |    +--------+               |
                 |                             |


   packets with       CBQ queue
   delay              depths proportional
   indication         to relative transmission

   The CBQ queue servicing algorithm may be implementation specific,
   but it is important to note that the desired functionality is for
   packets with a "higher" delay sensitivity indication to be
   transmitted prior to those with "lower" delay sensitivity indication.

   It is also important to note that this is simply an example of
   how the delay indication bits could be used, not a mandatory
   implementation. We believe it is prudent to allow implementors
   to determine how these mechanics will be deployed -- it is arguably
   only important at this point to agree on the bit value semantics.

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3.3 Drop Preference

   There is at least one known implementation of preferential
   packet drop which uses the values expressed in the IP precedence
   subfield of the TOS byte, as described in [5], and this draft
   effectively modifies the semantics of the existing IP precedence
   values defined below [5].  Instead, a suggested interpretation
   of these values is provided below which modifies the existing
   semantics when used in conjunction with a differential services

    Bit      RFC791               Relative
   Value    Semantics          Drop Preference

    111 - Network Control         Lowest
    110 - Internetwork Control       .
    101 - CRITIC/ECP                 .
    100 - Flash Override             .
    011 - Flash                      .
    010 - Immediate                  .
    001 - Priority                   .
    000 - Routine                 Highest

   The values which fall between 110 (Internetwork Control)
   and 001 (Priority) can be used to represent any arbitrary
   incremental drop preference between "lowest" and "highest"
   drop preference values, 111 and 000 respectively. This is
   could be considered a matter of local policy.

3.4 Preferential Drop Mechanism at Intermediate Nodes

   It is important to note that if congestion is not experienced
   at an intermediate transit node, then no action (e.g. packet
   discard) is taken -- packets are forwarded in the order in which
   they are received, unless the packet contains a non-zero delay

   The concept of preferential drop, or packet discard, only becomes
   an issue should congestion be imminent. The most effective method
   of mitigating congestion is to use Random Early Detection (RED)
   [8], such that congestion and global synchronization [9] (and
   ultimately congestion collapse) is avoided to the maximum extent

   However, basic RED functionality can be considered to be
   somewhat "fair," in that packets are pseudo-randomly selected
   for discard as the queue depth(s) reaches an arbitrary,
   administratively-defined threshold. What is needed is a more

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   "unfair," or preferential, method of selecting packets for
   discard when the queue depth(s) exceed the threshold, thus providing
   a prejudicial mechanism for providing differentiated services.

   A modified RED behavior is discussed in [10], whereas the relative
   priority of packets (as discussed in Section 3.2) is considered in
   the packet discard decision process.

   This "unfair," or "enhanced," RED should be implemented on each node
   (R) in the traffic transit path.

   Again, it is also important to note that this is simply an example
   of how the drop preference bits could be used, not a mandatory
   implementation. It is prudent to allow implementors to determine
   how these mechanics will be deployed -- again, it is only important
   to agree on the bit value semantics.

3.5 Passive and Active admission

   There are two possible scenarios for how packets are marked as
   they enter the network. The hosts (for example h1 and h2) could
   set the TOS subfield values (delay indication) and IP precedence
   subfield values (drop preference) in their IP packets as they see
   fit, and the first-hop ingress router (r) could simply accept traffic
   "as is" without examining or policing it, and simply forward it on
   it's way unimpeded. This is passive admission.

   The alternative approach is for the first-hop ingress router (r)
   to actively profile, monitor, and police traffic which is presented
   to it for transit. The method of performing this active admission
   control can be implementation-specific, but there is at least one
   method that we understand, and which works well. This is the use of
   a token-bucket implemented on (r), or perhaps multiple token-buckets,
   which uses thresholds to mark packets based upon their compliance
   or non-compliance (or both) to a bit rate threshold. A similar
   method for marking packets in this fashion is discussed in [11].
   The ingress router is also an ideal point in the topology to mark
   packet indicators insofar as host address or application (TCP or
   UDP port).

4. Summary

   The mechanisms discussed in this proposal are simple and effective
   methods to provide reasonably predictable service differentiation
   for traffic presented to the network. It should be noted, however,
   that the effectiveness of the deployment of these tools relies
   somewhat on the engineering of the network architecture, in that if
   the network is drastically under-provisioned and severely congested,
   these tools become noticeably less effective. This is the intrinsic
   difference between service quality and quality of service (QoS), in

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   that if the network is poorly provisioned or excessively unstable
   (poor service quality), the effectiveness of any tool that attempts
   to provide service differentiation (QoS) is effectively diminished.

   Having said that, however, traffic entering the network will indeed
   be provided preferential queuing and transmission if the delay
   indication bits are set accordingly, and traffic will be discarded
   at intermediate nodes, based on congestion (queue occupancy) in
   accordance with the drop preference indicated in the packets

   These use of delay indication and drop preference are not
   necessarily reliant upon one another -- they can be used
   independently or in conjunction with one another. However, use
   of the delay indication bits requires the use of preferential
   queuing and transmission at each router in the transit path,
   and likewise, use of the drop preference bits requires that a
   preferential drop mechanism, similar to the ones discussed above,
   be used on each intermediate node in the transit path, in order
   for these mechanisms to be effective.

5. Security Considerations

   Inherently, it may be necessary to provide some sort of policy
   mechanism at the network ingress node to ensure that only
   "authorized" entities or applications can obtain preferential
   use of network resources. This is a local policy matter, and
   the implementation of such policy mechanisms are not discussed
   in this memo.

6. Acknowledgments

   Special thanks to Juha Heinanen (Telia) for his assistance
   in reviewing the *-00.txt revision memo.

7. References

   [1] RFC2212, "Specification of Guaranteed Quality of Service,"
    S. Shenker, C. Partridge, R. Guerin, September 1997.

   [2] RFC2211, "Specification of the Controlled-Load Network Element
    Service," J. Wroclawski, September 1997.

   [3] RFC1633, "Integrated Services in the Internet Architecture: An
    Overview," R. Braden, D. Clark, S. Shenker, June 1994.

   [4] RFC2205, "Resource ReSerVation Protocol (RSVP) Version 1
    Functional Specification," R. Braden, L. Zhang, S. Berson,
    S. Herzog, S. Jamin, September 1997.

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    SPECIFICATION," September 1981.

   [6] RFC1349, "Type of Service in the Internet Protocol Suite,"
    P. Almquist. July 1992.

   [7] "Link-sharing and Resource Management Models for Packet
    Networks," Floyd, S., and Jacobson, V., IEEE/ACM Transactions
    on Networking, Vol. 3 No. 4, pp. 365-386, August 1995.

   [8] "Random Early Detection Gateways for Congestion Avoidance,"
    S. Floyd, V. Jacobson, IEEE/ACM Transactions on Networking,
    v.1, n.4, August 1993.

   [9] "Oscillating Behavior of Network Traffic: A Case Study
    Simulation," L. Zhang, D. Clark, Internetwork: Research and
    Experience, Volume 1, Number 2, John Wiley & Sons, 1990, pages

   [10] "Understanding TCP Dynamics in an Integrated Services
    Internet," W. Feng, D. Kandlur, D. Saha, K. Shin, NOSSDAV
    '97, May 1997.

   [11] "Adaptive Packet Marking for Providing Differentiated
    Services in the Internet," W. Feng, D. Kandlur, D. Saha,
    K. Shin,  U. Michigan CSE-TR-347-97, October 1997.

8. Author's address

   Paul Ferguson
   cisco Systems, Inc.
   400 Herndon Parkway
   Herndon, VA  USA 20170

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