INTERNET-DRAFT                                      Vinod Valloppillil
<draft-vinod-carp-v1-03.txt>                     Microsoft Corporation
                                                         Keith W. Ross
                                            University of Pennsylvania
                                                           26 Feb 1998
                                                   Expires August 1998

                Cache Array Routing Protocol v1.0

Status of this Memo

 This document is an Internet-Draft.  Internet-Drafts are working
 documents of the Internet Engineering Task Force (IETF), its areas,
 and its working groups.  Note that other groups may also distribute
 working documents as Internet-Drafts.

 Internet-Drafts are draft documents valid for a maximum of six months
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 material or to cite them other than as ``work in progress.''

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  This draft documents the Cache Array Routing Protocol (CARP) v1.0
  for dividing URL-space among an array of loosely coupled proxy

  An HTTP client agent (either a proxy server or a client browser)
  which implements CARP v1.0 can allocate and intelligently route
  requests for the correct URLs to any member of the Proxy
  Array.  Due to the resulting sorting of requests through these
  proxies, duplication of cache contents is eliminated and global
  cache hit rates are improved.

Valloppillil                                           [Page 1]

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Table of Contents

  1.  Overview........................................ 2
  2.  Proxy Array Membership Table.................... 3
      2.1  Global Information......................... 3
      2.2  Member Information......................... 4
  3.  Routing Function................................ 5
      3.1  Hash Function.............................. 5
      3.2  Hash Combination........................... 6
      3.3  Load Factor................................ 7
      3.4  Route Selection............................ 7
      3.5  Member Failure Routing..................... 7
  4.  Client-Side Implementation...................... 7
  5.  Versioning...................................... 7
  6.  Security Considerations......................... 8
  7.  Open Issues..................................... 8
  8.  Acknowledgements................................ 8
  9.  References...................................... 8
  10. Author's Information............................ 9

Valloppillil                                           [Page 2]

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1. Overview

  The Cache Array Routing Protocol describes a distributed caching
  protocol based on

        1) a known membership list of loosely coupled proxies and
        2) a hash function for dividing URL space among those proxies

  The Proxy Array Membership Table is defined as a plain ASCII text
  file retrieved from an Array Configuration URL.  This document does
  NOT describe how this table is constructed, merely the format of
  the fields used by agents implementing.

  The hash function plus routing algorithm defined in this document
  take member proxies described in the Proxy Array Membership Table
  and make an on-the-fly determination as to which Proxy Array member
  should be the proper receptacle for a cached version of a resource
  keyed by URL.

  Downstream agents may then access the cached resource by forwarding
  the proxied HTTP request [5] for that resource to the appropriate
  member of the Proxy Array.

Valloppillil                                           [Page 3]

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2. Proxy Array Membership Table

  The Proxy Array Membership Table is a plain-text ASCII file which
  can be published from a URL.

  The format of the table is:

  Proxy Array Information/<Version number>
  ArrayEnabled:  <0 | 1>
  ConfigID:  <opaque string>
  ArrayName:  <opaque string>
  ListTTL:  <minutes until next check>

  <name> <IP addr> <listening port> <table URL> <agent str>
  <statetime> <status UP | DOWN> <load factor> <cache size>

2.1 Global Information

  These are fields that describe the array itself and are not specific
  to any one member of an array

  Global information is terminated in the Proxy Array Membership Table
  by a CR/LF/CR/LF.

2.1.1 Version number

  The version number for implementations of this specification is

2.1.2 ArrayEnabled

  This field allows proxies to advertise their implementation of CARP
  v1 even if they are not members of a Proxy Array.

2.1.3 ConfigID

  ConfigID is an opaque number no larger than 32bits similar to an
  ETag in HTTP 1.1.  It is used to track the current state of an
  Array table and may be used to match multiple yet independently
  published copies of the Proxy Array Membership Table.

2.1.4 ArrayName

  ArrayName is an opaque string which is used to provide a convenient
  administrative name for a given array.

2.1.5 ListTTL

  ListTTL is the number of seconds for which an HTTP client entity
  should consider the current table image valid.  After ListTTL
  has expired, that client should retrieve a new copy of
  the Proxy Array Membership Table.

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2.2 Member Information

  The following fields are published per member in an array and are
  separated by single spaces.  The end of an array member's record is
  terminated by a CR/LF.

2.2.1 Name

  The name of the proxy server.  Typically this is the fully qualified
  DNS name.  Downstream HTTP agents should use resolution of this name
  to determine how to connect to this proxy.

2.2.2 IP Addr

  The IP address that other proxy servers within this array should use
  to connect to this proxy server.  This is necessary for proxy
  servers which may be hosted on multi-hommed servers where requests
  are only accepted by one of the interfaces.

  If this field is not published in the table, name resolution may be
  used to find a proxy IP address

2.2.3 Listening Port

  The TCP port number this proxy is expecting requests on.

2.2.4 Table URL

  A URL which may be maintained by this proxy server on which a copy
  of the array membership table can be found.

  This entry does not have to be unique per proxy server.  In such
  cases, the URL should be identical to the URL from which the
  downstream client requested this table.

2.2.5 Agent String

  An opaque string identifying the vendor / version of the
  proxy Server in the Array Membership Table.

Valloppillil                                           [Page 5]

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2.2.7 Statetime

  How long a Proxy Server has been in its current state and has been
  a member of this table.  This is useful for dynamic generation of
  the Array Membership Table where the host generating the table has
  knowledge of the proxy's operational status.

  This field is expressed in seconds and is an unsighed 32 bit value.

2.2.8 Status

  Status provides a simple text string indicating whether a member
  proxy is currently able to handle requests (UP) or refused a
  connection when last contacted (DOWN).

2.2.9 Load Factor

  Load Factor is a relative amount of the total load for an array that
  should be handled by any given member of the array.

  Load Factor is specified as an integer and relative weight is
  computed against other integer values in the table.

2.2.10 Cache Size

  Cache size is an informational field that indicates the size of the
  cache held by a particular member of an array.

  Cache size is specified in Megabytes (MB) and represents the
  maximum potential size of a disk cache for this server.

3. Routing Function

  Once an agent has a Proxy Array Membership Table.  It uses a
  mathematical hash function to determine which of the members of
  the array should be the receptacle of a particular URL request.

  This routing function involves constructing n "scores" using a hash
  of the request URL plus a hash of each of the k proxies in the Proxy
  Array Membership Table.

  Both the URL and the proxy names are hashed in order to minimize the
  disruption of target routes if a member of the target array can't
  be contacted.

  Hashes of the URL and proxy name are constructed using the algorithm
  described in 3.1 and combined using the algorithm described in 3.2.

Valloppillil                                           [Page 6]

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3.1. Hash Function

  The hash function outputs a 32 bit unsigned integers based on a
  zero-terminated ASCII input string.  The machine name and domain
  names of the URL, the protocol, and the machine names of each member
  proxy should be evaluated in lower case since that portion of the
  URL is case insensitive.

  Because irreversibility and strong cryptographic features are
  unnecessary for this application, a very simple and fast hash
  function based on the bitwise left rotate operator is used.

  For (each char in URL):
        URL_Hash +=  _rotl(URL_Hash, 19) + char ;

  Member proxy hashes are computed in a similar manner:

  For (each char in MemberProxyName):
        MemberProxy_Hash +=  _rotl(MemberProxy_Hash, 19) + char ;

  Becaues member names are often similar to each other, their hash
  values are further spread across hash space via the following
  additional operations:

  MemberProxy_Hash += MemberProxy_Hash * 0x62531965 ;
  MemberProxy_Hash = _rotl (MemberProxy_Hash, 21) ;

3.2. Hash Combination

  Hashes are combined by first exclusive or-ing (XOR) the URL hash by
  the machine name and then multiplying by a constant and performing
  a bitwise rotation.

  All final and intermediate values are 32 bit unsigned integers.

  Combined_Hash = (URL_hash ^ MemberProxy_Hash) ;
  Combined_Hash += Combined_Hash * 0x62531965 ;
  Combined_Hash = _rotl(Combined_Hash, 21) ;

Valloppillil                                           [Page 7]

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3.3. Load Factor

  Support for array members with differing HTTP processing & caching
  capacity is achieved by multiplying each of the combined hash values
  by a Load Factor Multiplier.

  The Load Factor Multiplier for an individual member is calculated by
  taking each member's relative Load Factor and applying the
  following formula:

  The Load Factor Multiplier must be calculated from the smallest
  P_k to the largest P_k.  The sum of all P_k's must be 1.

  For each proxy server 1,...,K, the Load Factor Multiplier, X_k, is
  calculated iteratively as follows:

  All X_n values are 32 bit floating point numbers.

  X_1 = pow ((K*p_1), (1/K))

  X_k = ([K-k+1] * [P_k - P_{k-1}])/(X_1 * X_2 * ... * X_{k-1})
  X_k += pow ((X_{k-1}, {K-k+1})
  X_k = pow (X_k, {1/(K-k+1)})


  X_k = Load Factor Multiplier for proxy k
  K = number of proxies in an array
  P_k = relative percent of the load that proxy k should handle

  This is then combined with the previously computed hashes as

  Resultant_value = Combined_Hash * X_k

3.4. Route Selection

  The "score" for a particular combination of URL plus proxy is its
  resultant value. Once the agent determines the scores of the
  K proxies, it routes the URL query to the proxy with the highest

3.5. Member Failure Routing

  If a proxy can not contact the designated member of a proxy array
  in order to forward an HTTP request, that proxy should route
  the request to the second highest scoring proxy in the target array.

4. Client-side implementation

  CARP can be implemented on client-side HTTP browsers via the
  use of the Proxy AutoConfig file described in [1] and [2].

5. Versioning

  If a downstream proxy receives an Array Membership Table with a
  greater version # than that proxy is able to parse, it should
  fall back to simple proxy request routing to any administrator
  defined upstream proxy server.

Valloppillil                                           [Page 8]

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6. Security Considerations

  This draft does not discuss relevant security considerations.

7. Open Issues

8. Acknowledgements

  The author would like to thank Brian Smith, Kip Compton, and
  Kerry Schwartz for their assistance in preparing this document.

  Most of the architecture & design of CARP stem from work conducted
  by Brian Smith at Microsoft Corp.

9. References

  [1] Luotonen, Ari., "Navigator Proxy Auto-Config File Format",
 Netscape Corporation,
 relnotes/demo/proxy-live.html, March 1996.

  [3] Wessels, Duane., "Internet Cache Protocol Version 2", http://ds., March 21,

  [4] Sharp Corporation., "Super Proxy Script",, August 9, 1996.

  [5] Fielding, R., et. al, "Hypertext Transfer Protocol -- HTTP/1.1",
 RFC 2068, UC Irvine, January 1997.

  [6] Valloppillil & Cohen, "Hierarchical HTTP Routing Protocol",,
 April 21, 1997.

  [7] Thaler, David & Ravishankar, Chinya.  "Using Name-Based
 Mappings to Increase Hit Rates,"  ACM/IEEE Transactions on Networking.
 to appear.

Valloppillil                                           [Page 9]

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10.  Author Information

    Vinod Valloppillil
    Microsoft Corporation
    One Microsoft Way
    Redmond, WA 98052

    Phone:  1.206.703.3460
    Email:  VinodV@Microsoft.Com

    Keith W. Ross
    University of Pennsylvania
    Department of Systems Engineering
    Philadelphia, PA  19104

    Phone:  1.215.898.6069
    Email:  Ross@UPenn.Edu

Expires August 1998