Application of Internet Cache Protocol (ICP), version 2

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.

Abstract

   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 `proxy.customer.org' cache.................  6
     4.2. Configuring the `cache.isp.com' 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
     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].

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
     T H E   I N T E R N E T
   ===========================
       |          ||
       |          ||
       |          ||
       |          ||
       |      +----------------------+
       |      |                      |
       |      |        PARENT        |
       |      |        CACHE         |
       |      |                      |
       |      +----------------------+
       |          ||
     DIRECT       ||
   RETRIEVALS     ||
       |          ||
       |         HITS
       |         AND
       |        MISSES
       |       RESOLVED
       |          ||
       |          ||
       |          ||
       V          \/
   +------------------+                    +------------------+
   |                  |                    |                  |
   |      LOCAL       |/--------HITS-------|     SIBLING      |
   |      CACHE       |\------RESOLVED-----|      CACHE       |
   |                  |                    |                  |
   +------------------+                    +------------------+
      |  |  |  |  |
      |  |  |  |  |
      |  |  |  |  |
      V  V  V  V  V
   ===================
      CACHE CLIENTS

   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.

   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
   caches.

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
   network.

   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
   balancing.

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.

   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 `proxy.customer.org' 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 cache.isp.com 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 customer.org domain.  To
   always handle such local domains directly, the customer would add
   this to his configuration file:

       local_domain customer.org

   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 cache.isp.com 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.isp.com' 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.

   Note that the cache.isp.com 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  proxy.customer.org
       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).

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.).

   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.

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.

5.2.2.  ICP_OP_DENIED

   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.

5.2.3.1.  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.

   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
        replies.

   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.

5.2.4.  ICP_OP_MISS_NOFETCH

   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.

5.3.1.  ICP_OP_DENIED

   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.

5.3.3.  ICP_OP_HIT_OBJ

   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
   ICP_OP_HIT.

5.3.4.  ICP_OP_SECHO

   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.

5.3.5.  ICP_OP_DECHO

   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.

5.3.7.  ICP_OP_MISS_NOFETCH

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

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
   implementation.

5.4.1.  ICP_FLAG_HIT_OBJ

   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
        on'.

   o    The peer must be using ICP version 2.

   o    The HTTP request must not include the "Pragma: no-cache" header.

5.4.2.  ICP_FLAG_SRC_RTT

   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
        on'.

   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
   well.

   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
        sent.

   o    It prevents other group members from receiving unexpected
        replies.

   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.

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.

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
   regardless.

   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
   terms:

   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
        message).

   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.

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
        terminated.

   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.

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"
   above.

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.

   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.

   Opcode

      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.

   Version

      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.

      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.

   Options

      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.

   URL

      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:

        -    If the cache is behind a firewall and cannot directly
             connect to the origin server.

        -    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,
   http://harvest.transarc.com/.

   [4] Wessels D., Claffy K., "ICP and the Squid Web Cache", National
   Laboratory for Applied Network Research,
   http://www.nlanr.net/~wessels/Papers/icp-squid.ps.gz.

   [5] Wessels D., "The Squid Internet Object Cache", National
   Laboratory for Applied Network Research,
   http://squid.nlanr.net/Squid/

   [6] mtrace, Xerox PARC, ftp://ftp.parc.xerox.com/pub/net-
   research/ipmulti/.

   [7] Atkinson, R., "Security Architecture for the Internet Protocol",
   RFC 1825, NRL, August 1995.

   [8] Atkinson, R., "IP Authentication Header", RFC 1826, NRL, August
   1995.

   [9] Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC
   1827, NRL, August 1995.

11.  Acknowledgments

   The authors wish to thank Paul A Vixie <paul@vix.com> for providing
   excellent feedback on this document, Martin Hamilton
   <martin@mrrl.lut.ac.uk> for pushing the development of multicast ICP,
   Eric Rescorla <ekr@terisa.com> and Randall Atkinson <rja@home.net>
   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

   EMail: wessels@nlanr.net


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

   EMail: kc@nlanr.net