ICP and the Squid Web Cache

Duanc Wessels

k Claffy

August 13, 1997

• 元智大學系統實驗室 宮春富 2000/01/26

Outline

⊙ Introduction

⊙ Internet Cache Protocol

⊙ Implementation of ICP in Squid

⊙ ICP Delays

What is Caching

⊙ Caching has proven a useful technique for reducing end user

experienced latency on the Web.

⊙ Caching is effective because many Web Documents are

requested much more than once.

⊙ Cache is the intermediate storage of copies of popular Web

documents close to the end users.

HTTP

⊙ An HTTP request is comprised of three major parts: a request

method, a URL, and a set of request headers.

⊙ An HTTP reply consists of a numeric result code, a set of

reply headers, and an optional reply body.

⊙ GET -> download; POST -> upload.

⊙Max-age directive: age refers to the elapsed time since the

origin server provide the data.

Cache Hierarchical

⊙ A set of child cache share a common parent cache.

⊙ A simple hierarchy is not appropriate to all situations.

⊙ The ICP is to provide a quick and efficient method of

intercache communication, offers a mechanism for

establishing complex cache hierarchies.

PARENT

CHILD CHILDCHILD

ICP Message Format

⊙ A cache will query its peers by sending each one an

ICP_QUERY message.

⊙ The peer will reply with either an ICP_HIT or ICP_MISS.

⊙ Other codes: ICP_DENIED 、 ICP_HIT_OBJ.

OPCODE VERSION PACKET LENGTHREQUEST NUMBER

OPTIONSPADDING

SENDER HOST ADDRESS

0 31

ICP Transport

⊙ ICP could use TCP or UDP as the underlying delivery

protocol.

⊙ A UDP is simpler to implement because each cache needs to

maintain only a single UDP socket.

⊙ A ICP is intended as unreliable protocol and TCP would

actually be detrimental.

⊙ One advantage: a cache can quickly parse and interpret an ICP

message.

⊙ Two disadvantages: ICP doesn’t match HTTP; ICP increase

the request latency by at least the network round-trip time to a

neighbor cache.

ICP vs. HTTP

ICP Query Algorithm

⊙ Squid supports the ability to restrict the range of ICP_QUERY

messages it will send to different peers.

⊙ The cache_host_domain option lets one specify which

domains to query for a given peer.

⊙ Another Squid configuration parameter, hierarchy_stoplist,

allows one to exclude certain requests from the ICP query

algorithm.

⊙ Extract and parses the URL. (ICP_INVALID)

⊙ Check local access controls. (ICP_DENIED)

⊙ Lookup the given URL. (ICP_MISS)

⊙ If object is small enough, return an ICP_HIT_OBJ message.

⊙ Otherwise, return an ICP_HIT message.

Processing an ICP query

⊙ Squid collects replies until it receives an ICP_HIT or until all

ICP_MISS replies arrive.

⊙When receiving an ICP_HIT, Squid begins retrieving the

object from that peer.

⊙ If ICP_HIT_OBJ reply is first arrive, Squid just takes the

object data from the ICP message payload.

⊙ If no hit reply, then Squid retrieves the object from the parent.

Collect ICP replies

⊙ One of the peers becoming unreachable would significantly

increase the chances of suffering the two-second timeout.

⊙We designate a peer as dead when it fails to reply to 20

consecutive ICP queries.

⊙We still send the ICP_QUERY messages to dead peers, we

just don’t expect to receive replies from them.

Detecting Unreachable Peers

⊙ The Squid will return ICP_MISS_NOFETCH instead of

ICP_MISS message.

⊙ This feature allows this parent to continue serving hits, but

take itself out of the peer selection process for misses.

More Network Failure

INTERNETPARENT

ROUTER

CHILD CHILD

A

B

⊙ One problem is that it makes the UDP packet quite a bit larger.

⊙ Another problem is they require more time to generate.

⊙ The payload must actually consist of the URL followed by the

object data.

ICP_HIT_OBJ

⊙We don’t claim these measurements prove that hierarchical

caching with ICP gives improved performance.

⊙We suspect it depends on the regional and/or local network

situation.

⊙We used a special program to alternate between sending

ICMP echo request and ICP_QUERY messages.

ICP Delay