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digest-url= url

This option is specific to Cache Digests. See Section 10.7.

allow-miss

This option instructs Squid to omit the Cache-Control: only-if-cached directive for requests sent to a sibling. You should use this only if the neighbor has enabled the icp_hit_stale directive and isn't using a miss_access list.

max-conn= n

This option places a limit on the number of simultaneous connections that Squid can open to the neighbor. When this limit is reached, Squid excludes the neighbor from its selection algorithm.

htcp

This option designates the neighbor as an HTCP server. In other words, Squid sends HTCP queries, instead of ICP, to the neighbor. Note that Squid doesn't accept ICP and HTCP queries on the same port. When you add this option, don't forget to change the icp-port value as well. See Section 10.8.1. HTCP support requires the —enable-htcp option when running ./configure.

carp-load-factor= f

This option makes the neighbor, which must be a parent, a member of a CARP array. The sum of all f values, for all parents, must equal 1. I cover CARP in Section 10.9. CARP support requires the —enable-carp option when running ./configure.

Neighbor State

Squid keeps a variety of statistics and state information about each of its neighbors. One of the most important is whether Squid thinks the neighbor is alive (up) or dead (down). The neighbor's alive/dead state affects many aspects Squid's selection procedures. The algorithm for determining the alive/dead state is a little bit complicated, so I'll go through it here. If you want to follow along in the source code, look at the neighborUp( ) function.

Squid uses both TCP (HTTP) and UDP (ICP/HTCP) communication to determine the state. The TCP state defaults to alive, but changes to dead if 10 consecutive TCP connections fail. When this happens, Squid initiates probe connections, no more than once every connect_timeout time period (the global directive, not the cache_peer option). The state remains dead until one of the probe connections succeeds.

If the no-query option isn't set (meaning Squid is sending ICP/HTCP queries to the neighbor), the UDP layer communication also factors into the alive/dead algorithm. The UDP state defaults to alive, but changes to dead if Squid doesn't get any ICP/HTCP replies for a certain amount of time—the value of the dead_peer_timeout directive.

Squid also marks a neighbor dead if its hostname doesn't resolve to any IP addresses. When Squid determines a neighbor is dead, it writes an entry in cache.log. Here's an example:

2003/09/29 01:13:46| Detected DEAD Sibling: bo2.us.ircache.net/3128/3130

When communication with the neighbor is reestablished, Squid logs a message like this:

2003/09/29 01:13:49| Detected REVIVED Sibling: bo2.us.ircache.net/3128/3130

A neighbor's state affects neighbor-selection algorithms in the following ways:

Squid doesn't expect to receive ICP/HTCP replies from dead neighbors. Squid sends ICP queries to dead neighbors no more than once each dead_peer_timeout interval. See Appendix A.

A dead parent is excluded from the following algorithms: Cache Digests, round-robin parent, first up parent, default parent, and closest parent.

CARP is special: any failed TCP connections (not the 10 required to become dead) excludes the parent from the CARP algorithm.

There is no way to force Squid to send HTTP requests to a dead neighbor. If all neighbors are dead, Squid will try connecting to the origin server. If you don't allow Squid to talk to the origin server (with never_direct, for example), Squid returns a cannot forward error message:

This request could not be forwarded to the origin server or to any

parent caches. The most likely cause for this error is that:

* The cache administrator does not allow this cache to make

direct connections to origin servers, and

* All configured parent caches are currently unreachable.

Altering the Relationship

The neighbor_type_domain