Reliable Server Pooling Policies

The protocols defined in , and 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 adaptive and non-adaptive. The selection of the pool element is performed by two different entities, the ENRP server and the pool user. Some of the consequences for policies which are not stateless are described in and . Therefore this document describes not only packet formats but also gives a detailed description of the procedures to be followed at the ENRP servers and the pool users to implement each server policy.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

The term load is a value specifying how much a pool element's resources are currently utilized. 0x00000000 states, that the pool element is not utilized (0%), 0xffffffff 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.

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 higher weight of denotes that the pool element is capable of providing better service than a pool element having a lower weight. 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.

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.

The ENRP 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 handle resolution request the ENRP 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.

A pool user SHOULD use the list of pool elements returned by the ENRP 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.

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 = 0x8 | Length = 0x8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x00000001 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

The ENRP server SHOULD follow the same rules as for RR but initialize and modify the circular list differently. The ENRP 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.

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

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 = 0x8 | Length = 0xc | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x00000002 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Weight (32 bits, unsigned integer): Weight constant for the WRR process.

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

The ENRP 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.

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 ENRP server.

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 = 0x8 | Length = 0x8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x00000003 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

The ENRP 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.

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 ENRP server.

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 = 0x8 | Length = 0xc | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x00000004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Weight (32 bits, unsigned integer): Weight constant for the WRAND process.

The Priority (PRIO) policy can be used to select always a pool element with the highest priority.

The ENRP server MUST select the pool elements with the highest priorities. They MUST be reported in decreasing order. If multiple pool elements have the same priority, they may be listed in any order.

The pool user MUST select the active pool element with the highest priority.

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 = 0x8 | Length = 0xc | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x00000005 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Priority | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Priority (32 bits, unsigned integer): Larger numbers mean higher priorities.

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

The ENRP 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 among these elements. The returned list of pool elements MUST be sorted ascending by load value.

The pool user should try to use the pool elements returned from the list in the order returned by the ENRP server. A subsequent call for handle 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.

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 = 0x8 | Length = 0xc | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x40000001 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Load | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Load (32 bits, unsigned integer): Current load of the pool element.

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.

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 1. 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 * load degradation value.

See LU policy.

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 = 0x8 | Length = 0x10 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x40000002 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Load | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Load Degradation | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Load (32 bits, unsigned integer): Current load of the pool element. Load Degradation (32 bits, unsigned integer): Load Degradation constant of the pool element.

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).

The ENRP 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 among these elements. The returned list of pool elements MUST be sorted ascending by the sum of load and degradation value.

The pool user should try to use the pool elements returned from the list in the order returned by the ENRP server. A subsequent call for handle 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.

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 = 0x8 | Length = 0x10 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x40000003 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Load | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Load Degradation | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Load (32 bits, unsigned integer): Current load of the pool element. Load Degradation (32 bits, unsigned integer): Load Degradation constant of the pool element.

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 A, utilized with load_A, to be selected is (0xFFFFFFFF - load_A) / (sum(0xFFFFFFFF-load_x)), i.e. this PE's unload part related to the whole pool unload rate.

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

See WRAND policy.

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 = 0x8 | Length = 0xc | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Policy Type = 0x40000004 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Load | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Load (32 bits, unsigned integer): Current load of the pool element.

The security threats regarding RSerPool have been analyzed in RSerPool threats. The server policy descriptions in this document do not add any other threats.

[NOTE to RFC-Editor: "RFCXXXX" is to be replaced by the RFC number you assign this document. ] This document (RFCXXX) is the reference for all registrations described in this section. All registrations need to be listed on an RSerPool specific page.

RSerPool Policy Types which are 4 byte values have to be maintained by IANA. The format of the policy type value is defined as follows:

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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |X|A| Policy Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X: If set to 1, the policy is user-defined and not standardized. All standards policies reserved by the IETF use X=0. A: If set to 1, the policy is adaptive. Otherwise, it is non-adaptive. Policy Number: The actual number of the policy. Nine initial Policy Types should be assigned and maintained in a new table "RSerPool Policy Types":

Value Policy Reference ----- --------- --------- 0x00000000 (reserved, invalid value) RFCXXXX 0x00000001 Round Robin RFCXXXX 0x00000002 Weighted Round Robin RFCXXXX 0x00000003 Random RFCXXXX 0x00000004 Weighted Random RFCXXXX 0x00000005 Priority RFCXXXX 0x00000006 (reserved by IETF) RFCXXXX ... 0x3fffffff (reserved by IETF) RFCXXXX 0x40000000 (reserved, invalid value) RFCXXXX 0x40000001 Least Used RFCXXXX 0x40000002 Least Used with Degradation RFCXXXX 0x40000003 Priority Least Used RFCXXXX 0x40000004 Randomized Least Used RFCXXXX 0x40000005 (reserved by IETF) RFCXXXX ... 0x7fffffff (reserved by IETF) RFCXXXX 0x80000000 (private use, non-standard policy) RFCXXXX ... 0xffffffff (private use, non-standard policy) RFCXXXX For registering at IANA an RSerPool Policy Type in this table a request has to be made to assign such a number. This number must be unique and use the appropiate upper bits. The "Specification Required" policy of MUST be applied. The Policy Type space from 0x80000000 to 0xffffffff is designated for private use.

The reference implementation of RSerPool and the policies described in this document is available at and described in .