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Network Working Group D. Wessels Request for Comments: 2187 K. Claffy Category: Informational National Laboratory for Applied

                                                  Network Research/UCSD
                                                         September 1997
      Application of Internet Cache Protocol (ICP), version 2

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.


 This document describes the application of ICPv2 (Internet Cache
 Protocol version 2, RFC2186) to Web caching.  ICPv2 is a lightweight
 message format used for communication among Web caches.  Several
 independent caching implementations now use ICP[3,5], making it
 important to codify the existing practical uses of ICP for those
 trying to implement, deploy, and extend its use.
 ICP queries and replies refer to the existence of URLs (or objects)
 in neighbor caches.  Caches exchange ICP messages and use the
 gathered information to select the most appropriate location from
 which to retrieve an object.  A companion document (RFC2186)
 describes the format and syntax of the protocol itself.  In this
 document we focus on issues of ICP deployment, efficiency, security,
 and interaction with other aspects of Web traffic behavior.

Table of Contents

 1.   Introduction.................................................  2
 2.   Web Cache Hierarchies........................................  3
 3.   What is the Added Value of ICP?..............................  5
 4.   Example Configuration of ICP Hierarchy.......................  5
   4.1. Configuring the `' cache.................  6
   4.2. Configuring the `' cache......................  6
 5.   Applying the Protocol........................................  7
   5.1. Sending ICP Queries........................................  8
   5.2. Receiving ICP Queries and Sending Replies.................. 10
   5.3. Receiving ICP Replies...................................... 11
   5.4. ICP Options................................................ 13
 6.   Firewalls.................................................... 14
 7.   Multicast.................................................... 14
 8.   Lessons Learned.............................................. 16
   8.1. Differences Between ICP and HTTP........................... 16

Wessels & Claffy Informational [Page 1] RFC 2187 ICP September 1997

   8.2. Parents, Siblings, Hits and Misses......................... 16
   8.3. Different Roles of ICP..................................... 17
   8.4. Protocol Design Flaws of ICPv2............................. 17
 9.   Security Considerations...................................... 18
   9.1. Inserting Bogus ICP Queries................................ 19
   9.2. Inserting Bogus ICP Replies................................ 19
   9.3. Eavesdropping.............................................. 20
   9.4. Blocking ICP Messages...................................... 20
   9.5. Delaying ICP Messages...................................... 20
   9.6. Denial of Service.......................................... 20
   9.7. Altering ICP Fields........................................ 21
   9.8. Summary.................................................... 22
 10.  References................................................... 23
 11.  Acknowledgments.............................................. 24
 12.  Authors' Addresses........................................... 24

1. Introduction

 ICP is a lightweight message format used for communicating among Web
 caches.  ICP is used to exchange hints about the existence of URLs in
 neighbor caches.  Caches exchange ICP queries and replies to gather
 information for use in selecting the most appropriate location from
 which to retrieve an object.
 This document describes the implementation of ICP in software.  For a
 description of the protocol and message format, please refer to the
 companion document (RFC2186).  We avoid making judgments about
 whether or how ICP should be used in particular Web caching
 configurations.  ICP may be a "net win" in some situations, and a
 "net loss" in others.  We recognize that certain practices described
 in this document are suboptimal. Some of these exist for historical
 reasons.  Some aspects have been improved in later versions.  Since
 this document only serves to describe current practices, we focus on
 documenting rather than evaluating.  However, we do address known
 security problems and other shortcomings.
 The remainder of this document is written as follows.  We first
 describe Web cache hierarchies, explain motivation for using ICP, and
 demonstrate how to configure its use in cache hierarchies.  We then
 provide a step-by-step description of an ICP query-response
 transaction.  We then discuss ICP interaction with firewalls, and
 briefly touch on multicasting ICP.  We end with lessons with have
 learned during the protocol development and deployement thus far, and
 the canonical security considerations.
 ICP was initially developed by Peter Danzig, et. al.  at the
 University of Southern California as a central part of hierarchical
 caching in the Harvest research project[3].

Wessels & Claffy Informational [Page 2] RFC 2187 ICP September 1997

2. Web Cache Hierarchies

 A single Web cache will reduce the amount of traffic generated by the
 clients behind it.  Similarly, a group of Web caches can benefit by
 sharing another cache in much the same way.  Researchers on the
 Harvest project envisioned that it would be important to connect Web
 caches hierarchically.  In a cache hierarchy (or mesh) one cache
 establishes peering relationships with its neighbor caches.  There
 are two types of relationship: parent and sibling.  A parent cache is
 essentially one level up in a cache hierarchy.  A sibling cache is on
 the same level.  The terms "neighbor" and "peer" are used to refer to
 either parents or siblings which are a single "cache-hop" away.
 Figure 1 shows a simple hierarchy configuration.
 But what does it mean to be "on the same level" or "one level up?"
 The general flow of document requests is up the hierarchy.  When a
 cache does not hold a requested object, it may ask via ICP whether
 any of its neighbor caches has the object.  If any of the neighbors
 does have the requested object (i.e., a "neighbor hit"), then the
 cache will request it from them.  If none of the neighbors has the
 object (a "neighbor miss"), then the cache must forward the request
 either to a parent, or directly to the origin server.  The essential
 difference between a parent and sibling is that a "neighbor hit" may
 be fetched from either one, but a "neighbor miss" may NOT be fetched
 from a sibling.  In other words, in a sibling relationship, a cache
 can only ask to retrieve objects that the sibling already has cached,
 whereas the same cache can ask a parent to retrieve any object
 regardless of whether or not it is cached.  A parent cache's role is

Wessels & Claffy Informational [Page 3] RFC 2187 ICP September 1997

   T H E   I N T E R N E T
     |          ||
     |          ||
     |          ||
     |          ||
     |      +----------------------+
     |      |                      |
     |      |        PARENT        |
     |      |        CACHE         |
     |      |                      |
     |      +----------------------+
     |          ||
   DIRECT       ||
     |          ||
     |         HITS
     |         AND
     |        MISSES
     |       RESOLVED
     |          ||
     |          ||
     |          ||
     V          \/
 +------------------+                    +------------------+
 |                  |                    |                  |
 |      LOCAL       |/--------HITS-------|     SIBLING      |
 |      CACHE       |\------RESOLVED-----|      CACHE       |
 |                  |                    |                  |
 +------------------+                    +------------------+
    |  |  |  |  |
    |  |  |  |  |
    |  |  |  |  |
    V  V  V  V  V
 FIGURE 1: A Simple Web cache hierarchy.  The local cache can retrieve
 hits from sibling caches, hits and misses from parent caches, and
 some requests directly from origin servers.
 to provide "transit" for the request if necessary, and accordingly
 parent caches are ideally located within or on the way to a transit
 Internet service provider (ISP).
 Squid and Harvest allow for complex hierarchical configurations.  For
 example, one could specify that a given neighbor be used for only a
 certain class of requests, such as URLs from a specific DNS domain.

Wessels & Claffy Informational [Page 4] RFC 2187 ICP September 1997

 Additionally, it is possible to treat a neighbor as a sibling for
 some requests and as a parent for others.
 The cache hierarchy model described here includes a number of
 features to prevent top-level caches from becoming choke points.  One
 is the ability to restrict parents as just described previously (by
 domains).  Another optimization is that the cache only forwards
 cachable requests to its neighbors.  A large class of Web requests
 are inherently uncachable, including: requests requiring certain
 types of authentication, session-encrypted data, highly personalized
 responses, and certain types of database queries.  Lower level caches
 should handle these requests directly rather than burdening parent

3. What is the Added Value of ICP?

 Although it is possible to maintain cache hierarchies without using
 ICP, the lack of ICP or something similar prohibits the existence of
 sibling meta-communicative relationships, i.e., mechanisms to query
 nearby caches about a given document.
 One concern over the use of ICP is the additional delay that an ICP
 query/reply exchange contributes to an HTTP transaction.  However, if
 the ICP query can locate the object in a nearby neighbor cache, then
 the ICP delay may be more than offset by the faster delivery of the
 data from the neighbor.  In order to minimize ICP delays, the caches
 (as well as the protocol itself) are designed to return ICP requests
 quickly.  Indeed, the application does minimal processing of the ICP
 request, most ICP-related delay is due to transmission on the
 ICP also serves to provide an indication of neighbor reachability.
 If ICP replies from a neighbor fail to arrive, then either the
 network path is congested (or down), or the cache application is not
 running on the ICP-queried neighbor machine.  In either case, the
 cache should not use this neighbor at this time.  Additionally,
 because an idle cache can turn around the replies faster than a busy
 one, all other things being equal, ICP provides some form of load

4. Example Configuration of ICP Hierarchy

 Configuring caches within a hierarchy requires establishing peering
 relationships, which currently involves manual configuration at both
 peering endpoints.  One cache must indicate that the other is a
 parent or sibling.  The other cache will most likely have to add the
 first cache to its access control lists.

Wessels & Claffy Informational [Page 5] RFC 2187 ICP September 1997

 Below we show some sample configuration lines for a hypothetical
 situation.  We have two caches, one operated by an ISP, and another
 operated by a customer.  First we describe how the customer would
 configure his cache to peer with the ISP.  Second, we describe how
 the ISP would allow the customer access to its cache.

4.1. Configuring the `' cache

 In Squid, to configure parents and siblings in a hierarchy, a
 `cache_host' directive is entered into the configuration file.  The
 format is:
     cache_host hostname type http-port icp-port [options]
 Where type is either `parent', `sibling', or `multicast'.  For our
 example, it would be:
     cache_host parent 8080 3130
 This configuration will cause the customer cache to resolve most
 cache misses through the parent (`cgi-bin' and non-GET requests would
 be resolved directly).  Utilizing the parent may be undesirable for
 certain servers, such as servers also in the domain.  To
 always handle such local domains directly, the customer would add
 this to his configuration file:
 It may also be the case that the customer wants to use the ISP cache
 only for a specific subset of DNS domains.  The need to limit
 requests this way is actually more common for higher levels of cache
 hierarchies, but it is illustrated here nonetheless.  To limit the
 ISP cache to a subset of DNS domains, the customer would use:
     cache_host_domain com net org
 Then, any requests which are NOT in the .com, .net, or .org domains
 would be handled directly.

4.2. Configuring the `' cache

 To configure the query-receiving side of the cache peer
 relationship one uses access lists, similar to those used in routing
 peers.  The access lists support a large degree of customization in
 the peering relationship.  If there are no access lines present, the
 cache allows the request by default.

Wessels & Claffy Informational [Page 6] RFC 2187 ICP September 1997

 Note that the cache need not explicitly specify the
 customer cache as a peer, nor is the type of relationship encoded
 within the ICP query itself.  The access control entries regulate the
 relationships between this cache and its neighbors.  For our example,
 the ISP would use:
     acl src Customer
     http_access allow Customer
     icp_access  allow Customer
 This defines an access control entry named `Customer' which specifies
 a source IP address of the customer cache machine.  The customer
 cache would then be allowed to make any request to both the HTTP and
 ICP ports (including cache misses).  This configuration implies that
 the ISP cache is a parent of the customer.
 If the ISP wanted to enforce a sibling relationship, it would need to
 deny access to cache misses.  This would be done as follows:
     miss_access deny Customer
 Of course the ISP should also communicate this to the customer, so
 that the customer will change his configuration from parent to
 sibling.  Otherwise, if the customer requests an object not in the
 ISP cache, an error message is generated.

5. Applying the Protocol

 The following sections describe the ICP implementation in the
 Harvest[3] (research version) and Squid Web cache[5] packages.  In
 terms of version numbers, this means version 1.4pl2 for Harvest and
 version 1.1.10 for Squid.
 The basic sequence of events in an ICP transaction is as follows:
 1.   Local cache receives an HTTP[1] request from a cache client.
 2.   The local cache sends ICP queries (section 5.1).
 3.   The peer cache(s) receive the queries and send ICP replies
      (section 5.2).
 4.   The local cache receives the ICP replies and decides where to
      forward the request (section 5.3).

Wessels & Claffy Informational [Page 7] RFC 2187 ICP September 1997

5.1. Sending ICP Queries

5.1.1. Determine whether to use ICP at all

 Not every HTTP request requires an ICP query to be sent.  Obviously,
 cache hits will not need ICP because the request is satisfied
 immediately.  For origin servers very close to the cache, we do not
 want to use any neighbor caches.  In Squid and Harvest, the
 administrator specifies what constitutes a `local' server with the
 `local_domain' and `local_ip' configuration options.  The cache
 always contacts a local server directly, never querying a peer cache.
 There are other classes of requests that the cache (or the
 administrator) may prefer to forward directly to the origin server.
 In Squid and Harvest, one such class includes all non-GET request
 methods.  A Squid cache can also be configured to not use peers for
 URLs matching the `hierarchy_stoplist'.
 In order for an HTTP request to yield an ICP transaction, it must:
 o    not be a cache hit
 o    not be to a local server
 o    be a GET request, and
 o    not match the `hierarchy_stoplist' configuration.
 We call this a "hierarchical" request.  A "non-hierarchical" request
 is one that doesn't generate any ICP traffic.  To avoid processing
 requests that are likely to lower cache efficiency, one can configure
 the cache to not consult the hierarchy for URLs that contain certain
 strings (e.g. `cgi_bin').

5.1.2. Determine which peers to query

 By default, a cache sends an ICP_OP_QUERY message to each peer,
 unless any one of the following are true:
 o    Restrictions prevent querying a peer for this request, based on
      the configuration directive `cache_host_domain', which specifies
      a set of DNS domains (from the URLs) for which the peer should
      or should not be queried.  In Squid, a more flexible directive
      ('cache_host_acl') supports restrictions on other parts of the
      request (method, port number, source, etc.).

Wessels & Claffy Informational [Page 8] RFC 2187 ICP September 1997

 o    The peer is a sibling, and the HTTP request includes a "Pragma:
      no-cache" header.  This is because the sibling would be asked to
      transit the request, which is not allowed.
 o    The peer is configured to never be sent ICP queries (i.e. with
      the `no-query' option).
 If the determination yields only one queryable ICP peer, and the
 Squid configuration directive `single_parent_bypass' is set, then one
 can bypass waiting for the single ICP response and just send the HTTP
 request directly to the peer cache.
 The Squid configuration option `source_ping' configures a Squid cache
 to send a ping to the original source simultaneous with its ICP
 queries, in case the origin is closer than any of the caches.

5.1.3. Calculate the expected number of ICP replies

 Harvest and Squid want to maximize the chance to get a HIT reply from
 one of the peers.  Therefore, the cache waits for all ICP replies to
 be received.  Normally, we expect to receive an ICP reply for each
 query sent, except:
 o    When the peer is believed to be down.  If the peer is down Squid
      and Harvest continue to send it ICP queries, but do not expect
      the peer to reply.  When an ICP reply is again received from the
      peer, its status will be changed to up.
      The determination of up/down status has varied a little bit as
      the Harvest and Squid software evolved.  Both Harvest and Squid
      mark a peer down when it fails to reply to 20 consecutive ICP
      queries.  Squid also marks a peer down when a TCP connection
      fails, and up again when a diagnostic TCP connection succeeds.
 o    When sending to a multicast address.  In this case we'll
      probably expect to receive more than one reply, and have no way
      to definitively determine how many to expect.  We discuss
      multicast issues in section 7 below.

5.1.4. Install timeout event

 Because ICP uses UDP as underlying transport, ICP queries and replies
 may sometimes be dropped by the network.  The cache installs a
 timeout event in case not all of the expected replies arrive.  By
 default Squid and Harvest use a two-second timeout.  If object
 retrieval has not commenced when the timeout occurs, a source is
 selected as described in section 5.3.9 below.

Wessels & Claffy Informational [Page 9] RFC 2187 ICP September 1997

5.2. Receiving ICP Queries and Sending Replies

 When an ICP_OP_QUERY message is received, the cache examines it and
 decides which reply message is to be sent.  It will send one of the
 following reply opcodes, tested for use in the order listed:

5.2.1. ICP_OP_ERR

 The URL is extracted from the payload and parsed.  If parsing fails,
 an ICP_OP_ERR message is returned.


 The access controls are checked.  If the peer is not allowed to make
 this request, ICP_OP_DENIED is returned.  Squid counts the number of
 ICP_OP_DENIED messages sent to each peer.  If more than 95% of more
 than 100 replies have been denied, then no reply is sent at all.
 This prevents misconfigured caches from endlessly sending unnecessary
 ICP messages back and forth.

5.2.3. ICP_OP_HIT

 If the cache reaches this point without already matching one of the
 previous  opcodes, it means the request is allowed and we must
 determine if it will be HIT or MISS, so we check if the URL exists in
 the local cache.  If so, and if the cached entry is fresh for at
 least the next 30 seconds, we can return an ICP_OP_HIT message.  The
 stale/fresh determination uses the local refresh (or TTL) rules.
 Note that a race condition exists for ICP_OP_HIT replies to sibling
 peers.  The ICP_OP_HIT means that a subsequent HTTP request for the
 named URL would result in a cache hit.  We assume that the HTTP
 request will come very quickly after the ICP_OP_HIT.  However, there
 is a slight chance that the object might be purged from this cache
 before the HTTP request is received.  If this happens, and the
 replying peer has applied Squid's `miss_access' configuration then
 the user will receive a very confusing access denied message. ICP_OP_HIT_OBJ

 Before returning the ICP_OP_HIT message, we see if we can send an
 ICP_OP_HIT_OBJ message instead.  We can use ICP_OP_HIT_OBJ if:
 o    The ICP_OP_QUERY message had the ICP_FLAG_HIT_OBJ flag set.

Wessels & Claffy Informational [Page 10] RFC 2187 ICP September 1997

 o    The entire object (plus URL) will fit in an ICP message.  The
      maximum ICP message size is 16 Kbytes, but an application may
      choose to set a smaller maximum value for ICP_OP_HIT_OBJ
 Normally ICP replies are sent immediately after the query is
 received, but the ICP_OP_HIT_OBJ message cannot be sent until the
 object data is available to copy into the reply message.  For Squid
 and Harvest this means the object must be "swapped in" from disk if
 it is not already in memory.  Therefore, on average, an
 ICP_OP_HIT_OBJ reply will have higher latency than ICP_OP_HIT.


 At this point we have a cache miss.  ICP has two types of miss
 replies.  If the cache does not want the peer to request the object
 from it, it sends an ICP_OP_MISS_NOFETCH message.

5.2.5. ICP_OP_MISS

 Finally, an ICP_OP_MISS reply is returned as the default.  If the
 replying cache is a parent of the querying cache, the ICP_OP_MISS
 indicates an invitation to fetch the URL through the replying cache.

5.3. Receiving ICP Replies

 Some ICP replies will be ignored; specifically, when any of the
 following are true:
 o    The reply message originated from an unknown peer.
 o    The object named by the URL does not exist.
 o    The object is already being fetched.


 If more than 95% of more than 100 replies from a peer cache have been
 ICP_OP_DENIED, then such a high denial rate most likely indicates a
 configuration error, either locally or at the peer.  For this reason,
 no further queries will be sent to the peer for the duration of the
 cache process.

5.3.2. ICP_OP_HIT

 Object retrieval commences immediately from the replying peer.

Wessels & Claffy Informational [Page 11] RFC 2187 ICP September 1997


 The object data is extracted from the ICP message and the retrieval
 is complete.  If there is some problem with the ICP_OP_HIT_OBJ
 message (e.g. missing data) the reply will be treated like a standard


 Object retrieval commences immediately from the origin server because
 the ICP_OP_SECHO reply arrived prior to any ICP_OP_HIT's.  If an
 ICP_OP_HIT had arrived prior, this ICP_OP_SECHO reply would be
 ignored because the retrieval has already started.


 An ICP_OP_DECHO reply is handled like an ICP_OP_MISS.  Non-ICP peers
 must always be configured as parents; a non-ICP sibling makes no
 sense.  One serious problem with the ICP_OP_DECHO feature is that
 since it bounces messages off the peer's UDP echo port, it does not
 indicate that the peer cache is actually running -- only that network
 connectivity exists between the pair.

5.3.6. ICP_OP_MISS

 If the peer is a sibling, the ICP_OP_MISS reply is ignored.
 Otherwise, the peer may be "remembered" for future use in case no HIT
 replies are received later (section 5.3.9).
 Harvest and Squid remember the first parent to return an ICP_OP_MISS
 message.  With Squid, the parents may be weighted so that the "first
 parent to miss" may not actually be the first reply received.  We
 call this the FIRST_PARENT_MISS.  Remember that sibling misses are
 entirely ignored, we only care about misses from parents.  The parent
 miss RTT's can be weighted because sometimes the closest parent is
 not the one people want to use.
 Also, recent versions of Squid may remember the parent with the
 lowest RTT to the origin server, using the ICP_FLAG_SRC_RTT option.
 We call this the CLOSEST_PARENT_MISS.


 This reply is essentially ignored.  A cache must not forward a
 request to a peer that returns ICP_OP_MISS_NOFETCH.

Wessels & Claffy Informational [Page 12] RFC 2187 ICP September 1997

5.3.8. ICP_OP_ERR

 Silently ignored.

5.3.9. When all peers MISS.

 For ICP_OP_HIT and ICP_OP_SECHO the request is forwarded immediately.
 For ICP_OP_HIT_OBJ there is no need to forward the request.  For all
 other reply opcodes, we wait until the expected number of replies
 have been received.  When we have all of the expected replies, or
 when the query timeout occurs, it is time to forward the request.
 Since MISS replies were received from all peers, we must either
 select a parent cache or the origin server.
 o    If the peers are using the ICP_FLAG_SRC_RTT feature, we forward
      the request to the peer with the lowest RTT to the origin
      server.  If the local cache is also measuring RTT's to origin
      servers, and is closer than any of the parents, the request is
      forwarded directly to the origin server.
 o    If there is a FIRST_PARENT_MISS parent available, the request
      will be forwarded there.
 o    If the ICP query/reply exchange did not produce any appropriate
      parents, the request will be sent directly to the origin server
      (unless firewall restrictions prevent it).

5.4. ICP Options

 The following options were added to Squid to support some new
 features while maintaining backward compatibility with the Harvest


 This flag is off by default and will be set in an ICP_OP_QUERY
 message only if these three criteria are met:
 o    It is enabled in the cache configuration file with `udp_hit_obj
 o    The peer must be using ICP version 2.
 o    The HTTP request must not include the "Pragma: no-cache" header.

Wessels & Claffy Informational [Page 13] RFC 2187 ICP September 1997


 This flag is off by default and will be set in an ICP_OP_QUERY
 message only if these two criteria are met:
 o    It is enabled in the cache configuration file with `query_icmp
 o    The peer must be using ICP version 2.

6. Firewalls

 Operating a Web cache behind a firewall or in a private network poses
 some interesting problems.  The hard part is figuring out whether the
 cache is able to connect to the origin server.  Harvest and Squid
 provide an `inside_firewall' configuration directive to list DNS
 domains on the near side of a firewall.  Everything else is assumed
 to be on the far side of a firewall.  Squid also has a `firewall_ip'
 directive so that inside hosts can be specified by IP addresses as
 In a simple configuration, a Squid cache behind a firewall will have
 only one parent cache (which is on the firewall itself).  In this
 case, Squid must use that parent for all servers beyond the firewall,
 so there is no need to utilize ICP.
 In a more complex configuration, there may be a number of peer caches
 also behind the firewall.  Here, ICP may be used to check for cache
 hits in the peers.  Occasionally, when ICP is being used, there may
 not be any replies received.  If the cache were not behind a
 firewall, the request would be forwarded directly to the origin
 server.  But in this situation, the cache must pick a parent cache,
 either randomly or due to configuration information.  For example,
 Squid allows a parent cache to be designated as a default choice when
 no others are available.

7. Multicast

 For efficient distribution, a cache may deliver ICP queries to a
 multicast address, and neighbor caches may join the multicast group
 to receive such queries.
 Current practice is that caches send ICP replies only to unicast
 addresses, for several reasons:
 o    Multicasting ICP replies would not reduce the number of packets

Wessels & Claffy Informational [Page 14] RFC 2187 ICP September 1997

 o    It prevents other group members from receiving unexpected
 o    The reply should follow unicast routing paths to indicate
      (unicast) connectivity between the receiver and the sender since
      the subsequent HTTP request will be unicast routed.
 Trust is an important aspect of inter-cache relationships.  A Web
 cache should not automatically trust any cache which replies to a
 multicast ICP query.  Caches should ignore ICP messages from
 addresses not specifically configured as neighbors.  Otherwise, one
 could easily pollute a cache mesh by running an illegitimate cache
 and having it join a group, return ICP_OP_HIT for all requests, and
 then deliver bogus content.
 When sending to multicast groups, cache administrators must be
 careful to use the minimum multicast TTL required to reach all group
 members.  Joining a multicast group requires no special privileges
 and there is no way to prevent anyone from joining "your" group.  Two
 groups of caches utilizing the same multicast address could overlap,
 which would cause a cache to receive ICP replies from unknown
 neighbors.  The unknown neighbors would not be used to retrieve the
 object data, but the cache would constantly receive ICP replies that
 it must always ignore.
 To prevent an overlapping cache mesh, caches should thus limit the
 scope of their ICP queries with appropriate TTLs; an application such
 as mtrace[6] can determine appropriate multicast TTLs.
 As mentioned in section 5.1.3, we need to estimate the number of
 expected replies for an ICP_OP_QUERY message.  For unicast we expect
 one reply for each query if the peer is up.  However, for multicast
 we generally expect more than one reply, but have no way of knowing
 exactly how many replies to expect.  Squid regularly (every 15
 minutes) sends out test ICP_OP_QUERY messages to only the multicast
 group peers.  As with a real ICP query, a timeout event is installed
 and the replies are counted until the timeout occurs.  We have found
 that the received count varies considerably.  Therefore, the number
 of replies to expect is calculated as a moving average, rounded down
 to the nearest integer.

Wessels & Claffy Informational [Page 15] RFC 2187 ICP September 1997

8. Lessons Learned

8.1. Differences Between ICP and HTTP

 ICP is notably different from HTTP.  HTTP supports a rich and
 sophisticated set of features.  In contrast, ICP was designed to be
 simple, small, and efficient.  HTTP request and reply headers consist
 of lines of ASCII text delimited by a CRLF pair, whereas ICP uses a
 fixed size header and represents numbers in binary.  The only thing
 ICP and HTTP have in common is the URL.
 Note that the ICP message does not even include the HTTP request
 method.  The original implementation assumed that only GET requests
 would be cachable and there would be no need to locate non-GET
 requests in neighbor caches.  Thus, the current version of ICP does
 not accommodate non-GET requests, although the next version of this
 protocol will likely include a field for the request method.
 HTTP defines features that are important for caching but not
 expressible with the current ICP protocol.  Among these are Pragma:
 no-cache, If-Modified-Since, and all of the Cache-Control features of
 HTTP/1.1.  An ICP_OP_HIT_OBJ message may deliver an object which may
 not obey all of the request header constraints.  These differences
 between ICP and HTTP are the reason we discourage the use of the
 ICP_OP_HIT_OBJ feature.

8.2. Parents, Siblings, Hits and Misses

 Note that the ICP message does not have a field to indicate the
 intent of the querying cache.  That is, nowhere in the ICP request or
 reply does it say that the two caches have a sibling or parent
 relationship.  A sibling cache can only respond with HIT or MISS, not
 "you can retrieve this from me" or "you can not retrieve this from
 me."  The querying cache must apply the HIT or MISS reply to its
 local configuration to prevent it from resolving misses through a
 sibling cache.  This constraint is awkward, because this aspect of
 the relationship can be configured only in the cache originating the
 requests, and indirectly via the access controls configured in the
 queried cache as described earlier in section 4.2.

Wessels & Claffy Informational [Page 16] RFC 2187 ICP September 1997

8.3. Different Roles of ICP

 There are two different understandings of what exactly the role of
 ICP is in a cache mesh.  One understanding is that ICP's role is only
 object location, specifically, to provide hints about whether or not
 a named object exists in a neighbor cache.  An implied assumption is
 that cache hits are highly desirable, and ICP is used to maximize the
 chance of getting them.  If an ICP message is lost due to congestion,
 then nothing significant is lost; the request will be satisfied
 ICP is increasingly being tasked to fill a more complex role:
 conveying cache usage policy.  For example, many organizations (e.g.
 universities) will install a Web cache on the border of their
 network.  Such organizations may be happy to establish sibling
 relationships with other, nearby caches, subject to the following
 o    Any of the organization's customers or users may request any
      object (cached or not).
 o    Anyone may request an object already in the cache.
 o    Anyone may request any object from the organization's servers
      behind the cache.
 o    All other requests are denied; specifically, the organization
      will not provide transit for requests in which neither the
      client nor the server falls within its domain.
 To successfully convey policy the ICP exchange must very accurately
 predict the result (hit, miss) of a subsequent HTTP request.  The
 result may often depend on other request fields, such as Cache-
 Control.  So it's not possible for ICP to accurately predict the
 result without more, or perhaps all, of the HTTP request.

8.4. Protocol Design Flaws of ICPv2

 We recognize certain flaws with the original design of ICP, and make
 note of them so that future versions can avoid the same mistakes.
 o    The NULL-terminated URL in the payload field requires stepping
      through the message an octet at a time to find some of the
      fields (i.e. the beginning of object data in an ICP_OP_HIT_OBJ

Wessels & Claffy Informational [Page 17] RFC 2187 ICP September 1997

 o    Two fields (Sender Host Address and Requester Host Address) are
      IPv4 specific.  However, neither of these fields are used in
      practice; they are normally zero-filled.  If IP addresses have a
      role in the ICP message, there needs to be an address family
      descriptor for each address, and clients need to be able to say
      whether they want to hear IPv6 responses or not.
 o    Options are limited to 32 option flags and 32 bits of option
      data.  This should be more like TCP, with an option descriptor
      followed by option data.
 o    Although currently used as the cache key, the URL string no
      longer serves this role adequately.  Some HTTP responses now
      vary according to the requestor's User-Agent and other headers.
      A cache key must incorporate all non-transport headers present
      in the client's request.  All non-hop-by-hop request headers
      should be sent in an ICP query.
 o    ICPv2 uses different opcode values for queries and responses.
      ICP should use the same opcode for both sides of a two-sided
      transaction, with a "query/response" indicator telling which
      side is which.
 o    ICPv2 does not include any authentication fields.

9. Security Considerations

 Security is an issue with ICP over UDP because of its connectionless
 nature.  Below we consider various vulnerabilities and methods of
 attack, and their implications.
 Our first line of defense is to check the source IP address of the
 ICP message, e.g. as given by recvfrom(2).  ICP query messages should
 be processed if the access control rules allow the querying address
 access to the cache.  However, ICP reply messages must only be
 accepted from known neighbors; a cache must ignore replies from
 unknown addresses.
 Because we trust the validity of an address in an IP packet, ICP is
 susceptible to IP address spoofing.  In this document we address some
 consequences of IP address spoofing.  Normally, spoofed addresses can
 only be detected by routers, not by hosts.  However, the IP
 Authentication Header[7,8] can be used underneath ICP to provide
 cryptographic authentication of the entire IP packet containing the
 ICP protocol, thus eliminating the risk of IP address spoofing.

Wessels & Claffy Informational [Page 18] RFC 2187 ICP September 1997

9.1. Inserting Bogus ICP Queries

 Processing an ICP_OP_QUERY message has no known security
 implications, so long as the requesting address is granted access to
 the cache.

9.2. Inserting Bogus ICP Replies

 Here we are concerned with a third party generating ICP reply
 messages which are returned to the querying cache before the real
 reply arrives, or before any replies arrive.  The third party may
 insert bogus ICP replies which appear to come from legitimate
 neighbors.  There are three vulnerabilities:
 o    Preventing a certain neighbor from being used
      If a third-party could send an ICP_OP_MISS_NOFETCH reply back
      before the real reply arrived, the (falsified) neighbor would
      not be used.
      A third-party could blast a cache with ICP_OP_DENIED messages
      until the threshold described in section 5.3.1 is reached,
      thereby causing the neighbor relationship to be temporarily
 o    Forcing a certain neighbor to be used
      If a third-party could send an ICP_OP_HIT reply back before the
      real reply arrived, the (falsified) neighbor would be used.
      This may violate the terms of a sibling relationship; ICP_OP_HIT
      replies mean a subsequent HTTP request will also be a hit.
      Similarly, if bogus ICP_OP_SECHO messages can be generated, the
      cache would retrieve requests directly from the origin server.

o Cache poisoning

      The ICP_OP_HIT_OBJ message is especially sensitive to security
      issues since it contains actual object data.  In combination
      with IP address spoofing, this option opens up the likely
      possibility of having the cache polluted with invalid objects.

Wessels & Claffy Informational [Page 19] RFC 2187 ICP September 1997

9.3. Eavesdropping

 Multicasting ICP queries provides a very simple method for others to
 "snoop" on ICP messages.  If enabling multicast, cache administrators
 should configure the application to use the minimum required
 multicast TTL, using a tool such as mtrace[6].  Note that the IP
 Encapsulating Security Payload [7,9] mechanism can be used to provide
 protection against eavesdropping of ICP messages.
 Eavesdropping on ICP traffic can provide third parties with a list of
 URLs being browsed by cache users.  Because the Requestor Host
 Address is zero-filled by Squid and Harvest, the URLs cannot be
 mapped back to individual host systems.
 By default, Squid and Harvest do not send ICP messages for URLs
 containing `cgi-bin' or `?'.  These URLs sometimes contain sensitive
 information as argument parameters.  Cache administrators need to be
 aware that altering the configuration to make ICP queries for such
 URLs may expose sensitive information to outsiders, especially when
 multicast is used.

9.4. Blocking ICP Messages

 Intentionally blocked (or discarded) ICP queries or replies will
 appear to reflect link failure or congestion, and will prevent the
 use of a neighbor as well as lead to timeouts (see section 5.1.4).
 If all messages are blocked, the cache will assume the neighbor is
 down and remove it from the selection algorithm.  However, if, for
 example, every other query is blocked, the neighbor will remain
 "alive," but every other request will suffer the ICP timeout.

9.5. Delaying ICP Messages

 The neighbor selection algorithm normally waits for all ICP MISS
 replies to arrive.  Delaying queries or replies, so that they arrive
 later than they normally would, will cause additional delay for the
 subsequent HTTP request.  Of course, if messages are delayed so that
 they arrive after the timeout, the behavior is the same as "blocking"

9.6. Denial of Service

 A denial-of-service attack, where the ICP port is flooded with a
 continuous stream of bogus messages has three vulnerabilities:
 o    The application may log every bogus ICP message and eventually
      fill up a disk partition.

Wessels & Claffy Informational [Page 20] RFC 2187 ICP September 1997

 o    The socket receive queue may fill up, causing legitimate
      messages to be dropped.
 o    The host may waste some CPU cycles receiving the bogus messages.

9.7. Altering ICP Fields

 Here we assume a third party is able to change one or more of the ICP
 reply message fields.
    Changing the opcode field is much like inserting bogus messages
    described above.  Changing a hit to a miss would prevent the peer
    from being used.  Changing a miss to a hit would force the peer to
    be used.
    Altering the ICP version field may have unpredictable consequences
    if the new version number is recognized and supported.  The
    receiving application should ignore messages with invalid version
    numbers.  At the time of this writing, both version numbers 2 and
    3 are in use.  These two versions use some fields (e.g. Options)
    in a slightly different manner.
 Message Length
    An incorrect message length should be detected by the receiving
    application as an invalid ICP message.
 Request Number
    The request number is often used as a part of the cache key.
    Harvest does not use the request number.  Squid uses the request
    number in conjunction with the URL to create a cache key.
    Altering the request number will cause a lookup of the cache key
    to fail.  This is similar to blocking the ICP reply altogether.

Wessels & Claffy Informational [Page 21] RFC 2187 ICP September 1997

    There is no requirement that a cache use both the URL and the
    request number to locate HTTP requests with outstanding ICP
    queries (however both Squid and Harvest do).  The request number
    must always be the same in the query and the reply.  However, if
    the querying cache uses only the request number to locate pending
    requests, there is some possibility that a replying cache might
    increment the request number in the reply to give the false
    impression that the two caches are closer than they really are.
    In other words, assuming that there are a few ICP requests "in
    flight" at any given time, incrementing the reply request number
    trick the querying cache into seeing a smaller round-trip time
    than really exists.
    There is little risk in having the Options bitfields altered.  Any
    option bit must only be set in a reply if it was also set in a
    query.  Changing a bit from clear to set is detectable by the
    querying cache, and such a message must be ignored.  Changing a
    bit from set to clear is allowed and has no negative side effects.
 Option Data
    ICP_FLAG_SRC_RTT is the only option which uses the Option Data
    field.  Altering the RTT values returned here can affect the
    neighbor selection algorithm, either forcing or preventing the use
    of a neighbor.
    The URL and Request Number are used to generate the cache key.
    Altering the URL will cause a lookup of the cache key to fail, and
    the ICP reply to be entirely ignored.  This is similar to blocking
    the ICP reply altogether.

9.8. Summary

 o    ICP_OP_HIT_OBJ is particularly vulnerable to security problems
      because it includes object data.  For this, and other reasons,
      its use is discouraged.
 o    Falsifying, altering, inserting, or blocking ICP messages can
      cause an HTTP request to fail only in two situations:
  1. If the cache is behind a firewall and cannot directly

connect to the origin server.

Wessels & Claffy Informational [Page 22] RFC 2187 ICP September 1997

  1. If a false ICP_OP_HIT reply causes the HTTP request to be

forwarded to a sibling, where the request is a cache miss

           and the sibling refuses to continue forwarding the request
           on behalf of the originating cache.
 o    Falsifying, altering, inserting, or blocking ICP messages can
      easily cause HTTP requests to be forwarded (or not forwarded) to
      certain neighbors.  If the neighbor cache has also been
      compromised, then it could serve bogus content and pollute a
      cache hierarchy.
 o    Blocking or delaying ICP messages can cause HTTP request to be
      further delayed, but still satisfied.

10. References

 [1] Fielding, R., et. al, "Hypertext Transfer Protocol -- HTTP/1.1",
 RFC 2068, UC Irvine, January 1997.
 [2] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
 Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota,
 December 1994.
 [3] Bowman M., Danzig P., Hardy D., Manber U., Schwartz M., and
 Wessels D., "The Harvest Information Discovery and Access System",
 Internet Research Task Force - Resource Discovery,
 [4] Wessels D., Claffy K., "ICP and the Squid Web Cache", National
 Laboratory for Applied Network Research,
 [5] Wessels D., "The Squid Internet Object Cache", National
 Laboratory for Applied Network Research,
 [6] mtrace, Xerox PARC,
 [7] Atkinson, R., "Security Architecture for the Internet Protocol",
 RFC 1825, NRL, August 1995.
 [8] Atkinson, R., "IP Authentication Header", RFC 1826, NRL, August
 [9] Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC
 1827, NRL, August 1995.

Wessels & Claffy Informational [Page 23] RFC 2187 ICP September 1997

11. Acknowledgments

 The authors wish to thank Paul A Vixie <> for providing
 excellent feedback on this document, Martin Hamilton
 <> for pushing the development of multicast ICP,
 Eric Rescorla <> and Randall Atkinson <>
 for assisting with security issues, and especially Allyn Romanow for
 keeping us on the right track.

12. Authors' Addresses

 Duane Wessels
 National Laboratory for Applied Network Research
 10100 Hopkins Drive
 La Jolla, CA 92093
 K. Claffy
 National Laboratory for Applied Network Research
 10100 Hopkins Drive
 La Jolla, CA 92093

Wessels & Claffy Informational [Page 24]

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