ICP and the Squid Web Cache

Duane Wessels and K. Claffy

산업공학과 조희권

Contents Introduction Related Work Internet Cache Protocol (ICP) Implementation of ICP in Squid ICP Delays Experiences and Issue with ICP Recent Work Conclusions & Critique

Introduction (1/2) Since WWW rose to popularity, much effort has

focused on reducing latency experienced by users.

Caching has proven a useful technique for reducing end user experienced latency in the Web.

Introduction (2/2) HTTP and Caching

HTTP has several basic features relevant to Web caching: conditional GET, If-modified-Since header.

Hierarchical Caching and ICP A group of Web caching can benefit from sharing

another cache. Fig.1: Simple cache hierarchy ICP provides a quick and efficient method of inter-

cache communication.

Related Work The most difficult part of any caching scheme is maintain

ing cache consistency. Harvest Research Software

ICP was originally developed as a component of the Harvest cache project.

Squid New features have been added to its ICP implementation for a r

esearch. Netcache

The commercial Harvest cache

Internet Cache Protocol (1/2) Message Format

Internet Cache Protocol (2/2) Transport

ICP could use either TCP or UDP, however it currently uses only UDP.

ICP vs. HTTP ICP is simple

Positive: quickly parsing and interpreting Negative: does not match all HTTP query and

increases the request latency

Implementation of ICP in Squid (1/3)

ICP Query Algorithm Squid sends an ICP_query message to each peer, unless:

Cache_host domain rules prevent use of the peer for the given URL

A TCP connection to the peer has failed within the last minute The peer has been configured locally with the no-query option

Processing an ICP query Extract and parses the URL Check local access controls Lookup the given URL

ICP_MISS, ICP_HIT, ICP_HIT_OBJ

Implementation of ICP in Squid (2/3)

Collecting ICP replies Squid collects replies until it receives an ICP_HIT, or

until all ICP_MISS replies arrive with default timeout (2seconds).

Lowest RTT first Detecting Unreachable Peers

Designate a peer as dead when it fails to reply to 20 consecutive ICP queries.

Implementation of ICP in Squid (3/3)

More Network Failures

Experiences and Issues with ICP

Siblings vs. Parents Freshness Parameters Hit of Miss? Security ICP Scales Poorly URLs

Recent Work Probing the Network

Squid supports domain restrictions The domain names are unrelated to network

topology Then, Squid acquires topology data with ICMP

Multicast To solve scaling and configuration

Conclusions ICP provides hints about the location of

Web servers There is a linear relationship between

cache load and average ICP delays Is it all worth it? For the foreseeable future, there is not

another viable alternative.

Critiques Strengths

ICP process is well described. Issue is also described clearly.

Weaknesses The advantage of hierarchy cache is not

represented well.