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Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

This document describes server pool policies for Reliable Server Pooling including considerations for implementing them at name servers and pool users.

Table of Contents

1.  Introduction
2.  Terminology and Definitions
    2.1  Load
    2.2  Weight
3.  Static Policies
    3.1  Round Robin Policy
        3.1.1  Description
        3.1.2  Name Server Considerations
        3.1.3  Pool User Considerations
        3.1.4  Pool Member Selection Policy Parameter
    3.2  Weighted Round Robin Policy
        3.2.1  Description
        3.2.2  Name Server Considerations
        3.2.3  Pool User Considerations
        3.2.4  Pool Member Selection Policy Parameter
    3.3  Random Policy
        3.3.1  Description
        3.3.2  Name Server Considerations
        3.3.3  Pool User Considerations
        3.3.4  Pool Member Selection Policy Parameter
    3.4  Weighted Random Policy
        3.4.1  Description
        3.4.2  Name Server Considerations
        3.4.3  Pool User Considerations
        3.4.4  Pool Member Selection Policy Parameter
4.  Dynamic Policies
    4.1  Least Used Policy
        4.1.1  Description
        4.1.2  Name Server Considerations
        4.1.3  Pool User Considerations
        4.1.4  Pool Member Selection Policy Parameter
    4.2  Least Used with Degradation Policy
        4.2.1  Description
        4.2.2  Name Server Considerations
        4.2.3  Pool User Considerations
        4.2.4  Pool Member Selection Policy Parameter
    4.3  Priority Least Used Policy
        4.3.1  Description
        4.3.2  Name Server Considerations
        4.3.3  Pool User Considerations
        4.3.4  Pool Member Selection Policy Parameter
    4.4  Randomized Least Used Policy
        4.4.1  Description
        4.4.2  Name Server Considerations
        4.4.3  Pool User Considerations
        4.4.4  Pool Member Selection Policy Parameter
5.  Security Considerations
6.  IANA Considerations
§.  Normative References
§.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1. Introduction

The protocols defined in ENRPXie, Q., Stewart, R. and M. Stillman, Enpoint Name Resolution Protocol (ENRP), October 2003.[5], ASAPStewart, R., Xie, Q., Stillman, M. and M. Tuexen, Aggregate Server Access Protocol (ASAP), October 2003.[4] and ParametersStewart, R., Xie, Q. and M. Tuexen, Aggregate Server Access Protocol (ASAP) and Endpoint Name Resolution (ENRP) Parameters, October 2003.[3] support a variety of server policies. Some of the policies use dynamic load information of the pool elements and others do not. Therefore, we classify them as dynamic and static. The selection of the pool user is performed by two different entities. Some of the consequences for policies which are not stateless are described in PerformanceDreibholz, T., Rathgeb, E. and M. Tuexen, Load Distribution Performance of the Reliable Server Pooling Framework, .[7].

Therefore this document describes not only packet formats but also gives a detailed description of the procedures to be followed at the name servers and the pool users to implement each server policy.

2. Terminology and Definitions

2.1 Load

The term load is a value specifying how much a pool element's resources are currently utilized. 0x000000 states, that the pool element is not utilized (0%), 0xffffff states that it is fully utilized (100%). Defining what utilization means is application-dependent and out of the scope of RSerPool. However, it is required that all pool elements of the same pool using load information have the same definition of load.

For example, load may define the current amount of users out of a maximum on a FTP server, the CPU usage of a database server or the memory utilization of a compute service.

2.2 Weight

Weight defines a pool element's service capacity relatively to other pool elements of the same pool. Theoretically, there is no upper limit for weight values (although limited by datatype size). Defining what value weights compare is application-dependent and out of the scope of RSerPool. However, it is required that all pool elements of the same pool using weight information have the same definition of weight.

A weight of 0 denotes that the pool element is not capable of providing any service, a weight of 2*n denotes that the pool element is capable of providing a two times better service than a pool element having weight n.

For example, weight may define a compute service's computation capacity. That is, a pool element of weight 100 will complete a work package in half of the time compared to a pool element of weight 50.

3. Static Policies

3.1 Round Robin Policy

3.1.1 Description

The Round Robin (RR) policy is a very simple and efficient policy which requires state. This policy is denoted as the default policy and MUST be supported by all RSerPool components.

3.1.2 Name Server Considerations

The name server SHOULD hold the pool elements of each server pool in a circular list and SHOULD store a pointer to one of the elements, called the head. On reception of a name resolution request the name server SHOULD return the pool elements from the circular list starting with head. Then head SHOULD be advanced by one element.

Using this algorithm it is made sure that not all lists presented to the pool users start with the same element.

3.1.3 Pool User Considerations

A pool user SHOULD use the list of pool elements returned by the name server in a round robin fashion, starting with the first. If all elements of the list have been used it should start from the beginning again until the information is out of date.

3.1.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x6           |      Length = 0x8             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x1   |                (reserved)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Reserved: 24 bits, SHOULD be set to 0.

3.2 Weighted Round Robin Policy

3.2.1 Description

The Weighted Round Robin (WRR) policy is a generalization of the RR policy. If all weights are 1 then WRR is just RR.

3.2.2 Name Server Considerations

The name server SHOULD follow the same rules as for RR but initialize and modify the circular list differently. The name server puts each pool element possibly multiple times into the list such that:

• The ratio of the number of occurrences of a pool element to the list length is the same as the ratio of the weight of that pool element to the sum of weights.
• Each pool element is inserted as distributed as possible in the circular list.

3.2.3 Pool User Considerations

The pool user SHOULD follow the same rules as for RR.

3.2.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x6           |      Length = 0x8             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x2   |                 Weight                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Weight: Weight constant for the WRR process.

3.3 Random Policy

3.3.1 Description

The Random (RAND) policy is a very simple stateless policy.

3.3.2 Name Server Considerations

The name server selects at most the requested number of pool elements from the list of pool elements. Each element MUST NOT be reported more than once to the pool user.

3.3.3 Pool User Considerations

Each time the pool user must select one pool element it does this by randomly selecting one element from the list of pool elements received from the name server.

3.3.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x6           |      Length = 0x8             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x3   |                (reserved)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Reserved: 24 bits, SHOULD be set to 0.

3.4 Weighted Random Policy

3.4.1 Description

The Weighted Random (WRAND) policy is a generalization of the RAND policy, adding a weight for each pool element entry. RAND is equal to WRAND having all weights set to 1.

3.4.2 Name Server Considerations

The name server SHOULD select at most the requested number of pool elements randomly from the list of pool elements. Each element MUST NOT be reported more than once to the pool user. The probability of selecting a pool element should be the ratio of the weight of that pool element to the sum of weights.

3.4.3 Pool User Considerations

Each time the pool user must select one pool element it does this by randomly selecting one element from the list of pool elements received from the name server.

3.4.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x6           |      Length = 0x8             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x4   |                 Weight                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Weight: Weight constant for the WRAND process.

4. Dynamic Policies

4.1 Least Used Policy

4.1.1 Description

The Least Used (LU) policy uses load information provided by the pool elements to select the lowest-loaded pool elements within the pool.

4.1.2 Name Server Considerations

The name server SHOULD select at most the requested number of pool elements. Their load values SHOULD be the lowest possible ones within the pool. Each element MUST NOT be reported more than once to the pool user. If there is a choice of equal-loaded pool elements, round robin selection SHOULD be made between these elements. The returned list of pool elements MUST be sorted ascending by load value.

4.1.3 Pool User Considerations

The pool user should try to use the pool elements returned from the list in the order returned by the name server. A subsequent call for name resolution may result in the same list. Thereofore, it is RECOMMENDED for a pool user to request multiple entries in order to have a sufficient amount of feasible backup entries available.

4.1.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x6           |      Length = 0x8             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x5   |                 Load                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Load: Current load of the pool element.

4.2 Least Used with Degradation Policy

4.2.1 Description

The Least Used with Degradation (LUD) policy extends the LU policy by a load degradation value describing the pool element's load increment when a new service association is accepted.

4.2.2 Name Server Considerations

For every pool element entry, a degradation counter MUST be stored. When a pool element entry is added or updated by registration or reregistration, this counter MUST be set to 0. When an entry is selected for being returned to a pool user, the internal degradation counter MUST be incremented by the entry's load degradation constant. The selection of pool element entries is handled like for LU, except that the selected pool element entries SHOULD have the lowest possible sum of load value + degradation counter.

4.2.3 Pool User Considerations

See LU policy.

4.2.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x6           |      Length = 0xc             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x6   |                  Load                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+
   |  (reserved)   |              Load Degradation                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Load: Current load of the pool element.
• Reserved: 8 bits, SHOULD be set to 0.
• Load Degradation: Load Degradation constant of the pool element.

4.3 Priority Least Used Policy

4.3.1 Description

The Priority Least Used (PLU) policy uses load information provided by the pool elements to select the lowest-loaded pool elements within the pool under the assumption that a new application request is accepted by the pool elements. Therefore, the pool elements also have to specify load degradation information.

Example: Pool elements A and B are loaded by 50%, but the load of A will increase due to a new application request only by 10% while B will be fully loaded. PLU allows to specify this load degradation in the policy information, the selection is made on the lowest sum of load and degradation value. That is, A will be selected (50+10=60) instead of B (50+50=100).

4.3.2 Name Server Considerations

The name server SHOULD select at most the requested number of pool elements. Their sums of load + degradation SHOULD be the lowest possible ones within the pool. Each element MUST NOT be reported more than once to the pool user. If there is a choice of equal-valued pool element entries, round robin SHOULD be made between these elements. The returned list of pool elements MUST be sorted ascending by the sum of load and degradation value.

4.3.3 Pool User Considerations

The pool user should try to use the pool elements returned from the list in the order returned by the name server. A subsequent call for name resolution may result in the same list. Therefore, it is RECOMMENDED for a pool user to request multiple entries in order to have a sufficient amount of feasible backup entries available.

4.3.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x6           |      Length = 0xc             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x7   |                 Load                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+
   |  (reserved)   |              Load Degradation                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Load: Current load of the pool element.
• Load Degradation: Load Degradation constant of the pool element.

4.4 Randomized Least Used Policy

4.4.1 Description

The Randomized Least Used (RLU) policy combines LU and WRAND. That is, the pool element entries are selected randomly; the probability for a pool element entry to be selected is the ratio of 100%-load to the sum of all pool elements' load values.

4.4.2 Name Server Considerations

The name server SHOULD behave like WRAND, having every PE's weight set to (0xffffff - Load value provided by the pool element).

4.4.3 Pool User Considerations

See WRAND policy.

4.4.4 Pool Member Selection Policy Parameter

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Param Type = 0x7           |      Length = 0x8             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Policy=0x9   |                 Load                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+

• Load: Current load of the pool element.

5. Security Considerations

The security threats regarding RSerPool have been analyzed in RSerPool threatsStillman, M., Threats Introduced by Rserpool and Requirements for Security in response to Threats, October 2003.[6]. The server policy descriptions in this document do not add any other threats.

6. IANA Considerations

IANA keeps a list of Policy Types which are 1 byte values. The Policy values used in this document are:

  Value     Policy
  -----     ---------
  0x00      (reserved by IETF)
  0x01      Round Robin
  0x02      Weighted Round Robin
  0x03      Random
  0x04      Weighted Random
  0x05      Least Used
  0x06      Least Used with Degradation
  0x07      Priority Least Used
  0x09      Randomized Least Used
  others    (reserved by IETF)

7. References

7.1 Normative References

7.2 Informative References

Authors' Addresses

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