13.3.1 Last-modified Dates .............................82
     13.3.2 Entity Tag Cache Validators .....................82
     13.3.3 Weak and Strong Validators ......................82
     13.3.4 Rules for When to Use Entity Tags and Last-
     modified Dates..........................................85
     13.3.5 Non-validating Conditionals .....................86
    13.4 Response Cachability ...............................86
    13.5 Constructing Responses From Caches .................87
     13.5.1 End-to-end and Hop-by-hop Headers ...............88
     13.5.2 Non-modifiable Headers ..........................88
     13.5.3 Combining Headers ...............................89
     13.5.4 Combining Byte Ranges ...........................90
    13.6 Caching Negotiated Responses .......................90
    13.7 Shared and Non-Shared Caches .......................91
    13.8 Errors or Incomplete Response Cache Behavior .......91
    13.9 Side Effects of GET and HEAD .......................92
    13.10 Invalidation After Updates or Deletions ...........92
    13.11 Write-Through Mandatory ...........................93
    13.12 Cache Replacement .................................93
    13.13 History Lists .....................................93
   14 Header Field Definitions...............................94
    14.1 Accept .............................................95




 
RFC 2068                        HTTP/1.1                    January 1997


    14.2 Accept-Charset .....................................97
    14.3 Accept-Encoding ....................................97
    14.4 Accept-Language ....................................98
    14.5 Accept-Ranges ......................................99
    14.6 Age ................................................99
    14.7 Allow .............................................100
    14.8 Authorization .....................................100
    14.9 Cache-Control .....................................101
     14.9.1 What is Cachable ...............................103
     14.9.2 What May be Stored by Caches ...................103
     14.9.3 Modifications of the Basic Expiration Mechanism 104
     14.9.4 Cache Revalidation and Reload Controls .........105
     14.9.5 No-Transform Directive .........................107
     14.9.6 Cache Control Extensions .......................108
    14.10 Connection .......................................109
    14.11 Content-Base .....................................109
    14.12 Content-Encoding .................................110
    14.13 Content-Language .................................110
    14.14 Content-Length ...................................111
    14.15 Content-Location .................................112
    14.16 Content-MD5 ......................................113
    14.17 Content-Range ....................................114
    14.18 Content-Type .....................................116
    14.19 Date .............................................116
    14.20 ETag .............................................117
    14.21 Expires ..........................................117
    14.22 From .............................................118
    14.23 Host .............................................119
    14.24 If-Modified-Since ................................119
    14.25 If-Match .........................................121
    14.26 If-None-Match ....................................122
    14.27 If-Range .........................................123
    14.28 If-Unmodified-Since ..............................124
    14.29 Last-Modified ....................................124
    14.30 Location .........................................125
    14.31 Max-Forwards .....................................125
    14.32 Pragma ...........................................126
    14.33 Proxy-Authenticate ...............................127
    14.34 Proxy-Authorization ..............................127
    14.35 Public ...........................................127
    14.36 Range ............................................128
     14.36.1 Byte Ranges ...................................128
     14.36.2 Range Retrieval Requests ......................130
    14.37 Referer ..........................................131
    14.38 Retry-After ......................................131
    14.39 Server ...........................................132
    14.40 Transfer-Encoding ................................132
    14.41 Upgrade ..........................................132




 
RFC 2068                        HTTP/1.1                    January 1997


    14.42 User-Agent .......................................134
    14.43 Vary .............................................134
    14.44 Via ..............................................135
    14.45 Warning ..........................................137
    14.46 WWW-Authenticate .................................139
   15 Security Considerations...............................139
    15.1 Authentication of Clients .........................139
    15.2 Offering a Choice of Authentication Schemes .......140
    15.3 Abuse of Server Log Information ...................141
    15.4 Transfer of Sensitive Information .................141
    15.5 Attacks Based On File and Path Names ..............142
    15.6 Personal Information ..............................143
    15.7 Privacy Issues Connected to Accept Headers ........143
    15.8 DNS Spoofing ......................................144