aboutsummaryrefslogblamecommitdiffstats
path: root/vendor/sabre/dav/docs/draft-ietf-httpbis-p6-cache-11.txt
blob: 35b8756153633f080e61899969d91e8fc8e2cedb (plain) (tree)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352















































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































                                                                        
HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2616 (if approved)                                  J. Gettys
Intended status: Standards Track                          Alcatel-Lucent
Expires: February 5, 2011                                       J. Mogul
                                                                      HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                      M. Nottingham, Ed.

                                                         J. Reschke, Ed.
                                                              greenbytes
                                                          August 4, 2010


                       HTTP/1.1, part 6: Caching
                     draft-ietf-httpbis-p6-cache-11

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems.  This document is Part 6 of the seven-part specification
   that defines the protocol referred to as "HTTP/1.1" and, taken
   together, obsoletes RFC 2616.  Part 6 defines requirements on HTTP
   caches and the associated header fields that control cache behavior
   or indicate cacheable response messages.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (ietf-http-wg@w3.org).  The current issues list is
   at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
   documents (including fancy diffs) can be found at
   <http://tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix C.12.

Status of This Memo




Fielding, et al.        Expires February 5, 2011                [Page 1]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 5, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6



Fielding, et al.        Expires February 5, 2011                [Page 2]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


     1.4.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
       1.4.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7
       1.4.2.  ABNF Rules defined in other Parts of the
               Specification  . . . . . . . . . . . . . . . . . . . .  7
   2.  Cache Operation  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  7
       2.1.1.  Storing Partial and Incomplete Responses . . . . . . .  8
     2.2.  Constructing Responses from Caches . . . . . . . . . . . .  9
     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 10
       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 11
       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 12
       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 13
     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 14
     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 14
     2.6.  Shared Caching of Authenticated Responses  . . . . . . . . 15
     2.7.  Caching Negotiated Responses . . . . . . . . . . . . . . . 16
     2.8.  Combining Responses  . . . . . . . . . . . . . . . . . . . 16
   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 17
     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 18
       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 18
       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 20
       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 22
     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 24
     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 28
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
     5.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 28
     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 29
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 30
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 31
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 31
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 31
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 33
     C.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 33
     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33
     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 34
     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35



Fielding, et al.        Expires February 5, 2011                [Page 3]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35
     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 36
     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 36
     C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 37
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37














































Fielding, et al.        Expires February 5, 2011                [Page 4]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


1.  Introduction

   HTTP is typically used for distributed information systems, where
   performance can be improved by the use of response caches.  This
   document defines aspects of HTTP/1.1 related to caching and reusing
   response messages.

1.1.  Purpose

   An HTTP cache is a local store of response messages and the subsystem
   that controls its message storage, retrieval, and deletion.  A cache
   stores cacheable responses in order to reduce the response time and
   network bandwidth consumption on future, equivalent requests.  Any
   client or server MAY employ a cache, though a cache cannot be used by
   a server that is acting as a tunnel.

   Caching would be useless if it did not significantly improve
   performance.  The goal of caching in HTTP/1.1 is to reuse a prior
   response message to satisfy a current request.  In some cases, a
   stored response can be reused without the need for a network request,
   reducing latency and network round-trips; a "freshness" mechanism is
   used for this purpose (see Section 2.3).  Even when a new request is
   required, it is often possible to reuse all or parts of the payload
   of a prior response to satisfy the request, thereby reducing network
   bandwidth usage; a "validation" mechanism is used for this purpose
   (see Section 2.4).

1.2.  Terminology

   This specification uses a number of terms to refer to the roles
   played by participants in, and objects of, HTTP caching.

   cacheable

      A response is cacheable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests.
      Even when a response is cacheable, there might be additional
      constraints on whether a cache can use the cached copy to satisfy
      a particular request.

   explicit expiration time

      The time at which the origin server intends that a representation
      no longer be returned by a cache without further validation.







Fielding, et al.        Expires February 5, 2011                [Page 5]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   heuristic expiration time

      An expiration time assigned by a cache when no explicit expiration
      time is available.

   age

      The age of a response is the time since it was sent by, or
      successfully validated with, the origin server.

   first-hand

      A response is first-hand if the freshness model is not in use;
      i.e., its age is 0.

   freshness lifetime

      The length of time between the generation of a response and its
      expiration time.

   fresh

      A response is fresh if its age has not yet exceeded its freshness
      lifetime.

   stale

      A response is stale if its age has passed its freshness lifetime
      (either explicit or heuristic).

   validator

      A protocol element (e.g., an entity-tag or a Last-Modified time)
      that is used to find out whether a stored response has an
      equivalent copy of a representation.

   shared cache

      A cache that is accessible to more than one user.  A non-shared
      cache is dedicated to a single user.

1.3.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   An implementation is not compliant if it fails to satisfy one or more



Fielding, et al.        Expires February 5, 2011                [Page 6]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   of the "MUST" or "REQUIRED" level requirements for the protocols it
   implements.  An implementation that satisfies all the "MUST" or
   "REQUIRED" level and all the "SHOULD" level requirements for its
   protocols is said to be "unconditionally compliant"; one that
   satisfies all the "MUST" level requirements but not all the "SHOULD"
   level requirements for its protocols is said to be "conditionally
   compliant".

1.4.  Syntax Notation

   This specification uses the ABNF syntax defined in Section 1.2 of
   [Part1] (which extends the syntax defined in [RFC5234] with a list
   rule).  Appendix B shows the collected ABNF, with the list rule
   expanded.

   The following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
   sequence of data), SP (space), VCHAR (any visible USASCII character),
   and WSP (whitespace).

1.4.1.  Core Rules

   The core rules below are defined in Section 1.2.2 of [Part1]:

     quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
     token         = <token, defined in [Part1], Section 1.2.2>
     OWS           = <OWS, defined in [Part1], Section 1.2.2>

1.4.2.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:

     field-name    = <field-name, defined in [Part1], Section 3.2>
     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
     port          = <port, defined in [Part1], Section 2.6>
     pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
     uri-host      = <uri-host, defined in [Part1], Section 2.6>

2.  Cache Operation

2.1.  Response Cacheability

   A cache MUST NOT store a response to any request, unless:

   o  The request method is understood by the cache and defined as being
      cacheable, and



Fielding, et al.        Expires February 5, 2011                [Page 7]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   o  the response status code is understood by the cache, and

   o  the "no-store" cache directive (see Section 3.2) does not appear
      in request or response headers, and

   o  the "private" cache response directive (see Section 3.2.2 does not
      appear in the response, if the cache is shared, and

   o  the "Authorization" header (see Section 3.1 of [Part7]) does not
      appear in the request, if the cache is shared, unless the response
      explicitly allows it (see Section 2.6), and

   o  the response either:

      *  contains an Expires header (see Section 3.3), or

      *  contains a max-age response cache directive (see
         Section 3.2.2), or

      *  contains a s-maxage response cache directive and the cache is
         shared, or

      *  contains a Cache Control Extension (see Section 3.2.3) that
         allows it to be cached, or

      *  has a status code that can be served with heuristic freshness
         (see Section 2.3.1.1).

   In this context, a cache has "understood" a request method or a
   response status code if it recognises it and implements any cache-
   specific behaviour.  In particular, 206 Partial Content responses
   cannot be cached by an implementation that does not handle partial
   content (see Section 2.1.1).

   Note that in normal operation, most caches will not store a response
   that has neither a cache validator nor an explicit expiration time,
   as such responses are not usually useful to store.  However, caches
   are not prohibited from storing such responses.

2.1.1.  Storing Partial and Incomplete Responses

   A cache that receives an incomplete response (for example, with fewer
   bytes of data than specified in a Content-Length header) can store
   the response, but MUST treat it as a partial response [Part5].
   Partial responses can be combined as described in Section 4 of
   [Part5]; the result might be a full response or might still be
   partial.  A cache MUST NOT return a partial response to a client
   without explicitly marking it as such using the 206 (Partial Content)



Fielding, et al.        Expires February 5, 2011                [Page 8]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   status code.

   A cache that does not support the Range and Content-Range headers
   MUST NOT store incomplete or partial responses.

2.2.  Constructing Responses from Caches

   For a presented request, a cache MUST NOT return a stored response,
   unless:

   o  The presented effective request URI (Section 4.3 of [Part1]) and
      that of the stored response match, and

   o  the request method associated with the stored response allows it
      to be used for the presented request, and

   o  selecting request-headers nominated by the stored response (if
      any) match those presented (see Section 2.7), and

   o  the presented request and stored response are free from directives
      that would prevent its use (see Section 3.2 and Section 3.4), and

   o  the stored response is either:

      *  fresh (see Section 2.3), or

      *  allowed to be served stale (see Section 2.3.3), or

      *  successfully validated (see Section 2.4).

   When a stored response is used to satisfy a request without
   validation, caches MUST include a single Age header field
   (Section 3.1) in the response with a value equal to the stored
   response's current_age; see Section 2.3.2.

   Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
   be written through the cache to the origin server; i.e., a cache must
   not reply to such a request before having forwarded the request and
   having received a corresponding response.

   Also, note that unsafe requests might invalidate already stored
   responses; see Section 2.5.

   Caches MUST use the most recent response (as determined by the Date
   header) when more than one suitable response is stored.  They can
   also forward a request with "Cache-Control: max-age=0" or "Cache-
   Control: no-cache" to disambiguate which response to use.




Fielding, et al.        Expires February 5, 2011                [Page 9]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


2.3.  Freshness Model

   When a response is "fresh" in the cache, it can be used to satisfy
   subsequent requests without contacting the origin server, thereby
   improving efficiency.

   The primary mechanism for determining freshness is for an origin
   server to provide an explicit expiration time in the future, using
   either the Expires header (Section 3.3) or the max-age response cache
   directive (Section 3.2.2).  Generally, origin servers will assign
   future explicit expiration times to responses in the belief that the
   representation is not likely to change in a semantically significant
   way before the expiration time is reached.

   If an origin server wishes to force a cache to validate every
   request, it can assign an explicit expiration time in the past to
   indicate that the response is already stale.  Compliant caches will
   validate the cached response before reusing it for subsequent
   requests.

   Since origin servers do not always provide explicit expiration times,
   HTTP caches MAY assign heuristic expiration times when explicit times
   are not specified, employing algorithms that use other header values
   (such as the Last-Modified time) to estimate a plausible expiration
   time.  The HTTP/1.1 specification does not provide specific
   algorithms, but does impose worst-case constraints on their results.

   The calculation to determine if a response is fresh is:

      response_is_fresh = (freshness_lifetime > current_age)

   The freshness_lifetime is defined in Section 2.3.1; the current_age
   is defined in Section 2.3.2.

   Additionally, clients might need to influence freshness calculation.
   They can do this using several request cache directives, with the
   effect of either increasing or loosening constraints on freshness.
   See Section 3.2.1.

   [[ISSUE-no-req-for-directives: there are not requirements directly
   applying to cache-request-directives and freshness.]]

   Note that freshness applies only to cache operation; it cannot be
   used to force a user agent to refresh its display or reload a
   resource.  See Section 4 for an explanation of the difference between
   caches and history mechanisms.





Fielding, et al.        Expires February 5, 2011               [Page 10]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


2.3.1.  Calculating Freshness Lifetime

   A cache can calculate the freshness lifetime (denoted as
   freshness_lifetime) of a response by using the first match of:

   o  If the cache is shared and the s-maxage response cache directive
      (Section 3.2.2) is present, use its value, or

   o  If the max-age response cache directive (Section 3.2.2) is
      present, use its value, or

   o  If the Expires response header (Section 3.3) is present, use its
      value minus the value of the Date response header, or

   o  Otherwise, no explicit expiration time is present in the response.
      A heuristic freshness lifetime might be applicable; see
      Section 2.3.1.1.

   Note that this calculation is not vulnerable to clock skew, since all
   of the information comes from the origin server.

2.3.1.1.  Calculating Heuristic Freshness

   If no explicit expiration time is present in a stored response that
   has a status code whose definition allows heuristic freshness to be
   used (including the following in Section 8 of [Part2]: 200, 203, 206,
   300, 301 and 410), a heuristic expiration time MAY be calculated.
   Heuristics MUST NOT be used for response status codes that do not
   explicitly allow it.

   When a heuristic is used to calculate freshness lifetime, the cache
   SHOULD attach a Warning header with a 113 warn-code to the response
   if its current_age is more than 24 hours and such a warning is not
   already present.

   Also, if the response has a Last-Modified header (Section 6.6 of
   [Part4]), the heuristic expiration value SHOULD be no more than some
   fraction of the interval since that time.  A typical setting of this
   fraction might be 10%.

      Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
      not calculate heuristic freshness for URLs with query components
      (i.e., those containing '?').  In practice, this has not been
      widely implemented.  Therefore, servers are encouraged to send
      explicit directives (e.g., Cache-Control: no-cache) if they wish
      to preclude caching.





Fielding, et al.        Expires February 5, 2011               [Page 11]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


2.3.2.  Calculating Age

   HTTP/1.1 uses the Age response-header to convey the estimated age of
   the response message when obtained from a cache.  The Age field value
   is the cache's estimate of the amount of time since the response was
   generated or validated by the origin server.  In essence, the Age
   value is the sum of the time that the response has been resident in
   each of the caches along the path from the origin server, plus the
   amount of time it has been in transit along network paths.

   The following data is used for the age calculation:

   age_value

      The term "age_value" denotes the value of the Age header
      (Section 3.1), in a form appropriate for arithmetic operation; or
      0, if not available.

   date_value

      HTTP/1.1 requires origin servers to send a Date header, if
      possible, with every response, giving the time at which the
      response was generated.  The term "date_value" denotes the value
      of the Date header, in a form appropriate for arithmetic
      operations.  See Section 9.3 of [Part1] for the definition of the
      Date header, and for requirements regarding responses without a
      Date response header.

   now

      The term "now" means "the current value of the clock at the host
      performing the calculation".  Hosts that use HTTP, but especially
      hosts running origin servers and caches, SHOULD use NTP
      ([RFC1305]) or some similar protocol to synchronize their clocks
      to a globally accurate time standard.

   request_time

      The current value of the clock at the host at the time the request
      resulting in the stored response was made.

   response_time

      The current value of the clock at the host at the time the
      response was received.

   A response's age can be calculated in two entirely independent ways:




Fielding, et al.        Expires February 5, 2011               [Page 12]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   1.  the "apparent_age": response_time minus date_value, if the local
       clock is reasonably well synchronized to the origin server's
       clock.  If the result is negative, the result is replaced by
       zero.

   2.  the "corrected_age_value", if all of the caches along the
       response path implement HTTP/1.1; note this value MUST be
       interpreted relative to the time the request was initiated, not
       the time that the response was received.


     apparent_age = max(0, response_time - date_value);

     response_delay = response_time - request_time;
     corrected_age_value = age_value + response_delay;

   These are combined as

     corrected_initial_age = max(apparent_age, corrected_age_value);

   The current_age of a stored response can then be calculated by adding
   the amount of time (in seconds) since the stored response was last
   validated by the origin server to the corrected_initial_age.

     resident_time = now - response_time;
     current_age = corrected_initial_age + resident_time;

2.3.3.  Serving Stale Responses

   A "stale" response is one that either has explicit expiry information
   or is allowed to have heuristic expiry calculated, but is not fresh
   according to the calculations in Section 2.3.

   Caches MUST NOT return a stale response if it is prohibited by an
   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
   cache directive, a "must-revalidate" cache-response-directive, or an
   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
   see Section 3.2.2).

   Caches SHOULD NOT return stale responses unless they are disconnected
   (i.e., it cannot contact the origin server or otherwise find a
   forward path) or otherwise explicitly allowed (e.g., the max-stale
   request directive; see Section 3.2.1).

   Stale responses SHOULD have a Warning header with the 110 warn-code
   (see Section 3.6).  Likewise, the 112 warn-code SHOULD be sent on
   stale responses if the cache is disconnected.




Fielding, et al.        Expires February 5, 2011               [Page 13]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   If a cache receives a first-hand response (either an entire response,
   or a 304 (Not Modified) response) that it would normally forward to
   the requesting client, and the received response is no longer fresh,
   the cache SHOULD forward it to the requesting client without adding a
   new Warning (but without removing any existing Warning headers).  A
   cache SHOULD NOT attempt to validate a response simply because that
   response became stale in transit.

2.4.  Validation Model

   When a cache has one or more stored responses for a requested URI,
   but cannot serve any of them (e.g., because they are not fresh, or
   one cannot be selected; see Section 2.7), it can use the conditional
   request mechanism [Part4] in the forwarded request to give the origin
   server an opportunity to both select a valid stored response to be
   used, and to update it.  This process is known as "validating" or
   "revalidating" the stored response.

   When sending such a conditional request, the cache SHOULD add an If-
   Modified-Since header whose value is that of the Last-Modified header
   from the selected (see Section 2.7) stored response, if available.

   Additionally, the cache SHOULD add an If-None-Match header whose
   value is that of the ETag header(s) from all responses stored for the
   requested URI, if present.  However, if any of the stored responses
   contains only partial content, its entity-tag SHOULD NOT be included
   in the If-None-Match header field unless the request is for a range
   that would be fully satisfied by that stored response.

   A 304 (Not Modified) response status code indicates that the stored
   response can be updated and reused; see Section 2.8.

   A full response (i.e., one with a response body) indicates that none
   of the stored responses nominated in the conditional request is
   suitable.  Instead, the full response SHOULD be used to satisfy the
   request and MAY replace the stored response.

   If a cache receives a 5xx response while attempting to validate a
   response, it MAY either forward this response to the requesting
   client, or act as if the server failed to respond.  In the latter
   case, it MAY return a previously stored response (see Section 2.3.3).

2.5.  Request Methods that Invalidate

   Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
   for changing state on the origin server, intervening caches can use
   them to keep their contents up-to-date.




Fielding, et al.        Expires February 5, 2011               [Page 14]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   The following HTTP methods MUST cause a cache to invalidate the
   effective Request URI (Section 4.3 of [Part1]) as well as the URI(s)
   in the Location and Content-Location headers (if present):

   o  PUT

   o  DELETE

   o  POST

   An invalidation based on a URI from a Location or Content-Location
   header MUST NOT be performed if the host part of that URI differs
   from the host part in the effective request URI (Section 4.3 of
   [Part1]).  This helps prevent denial of service attacks.

   A cache that passes through requests for methods it does not
   understand SHOULD invalidate the effective request URI (Section 4.3
   of [Part1]).

   Here, "invalidate" means that the cache will either remove all stored
   responses related to the effective request URI, or will mark these as
   "invalid" and in need of a mandatory validation before they can be
   returned in response to a subsequent request.

   Note that this does not guarantee that all appropriate responses are
   invalidated.  For example, the request that caused the change at the
   origin server might not have gone through the cache where a response
   is stored.

2.6.  Shared Caching of Authenticated Responses

   Shared caches MUST NOT use a cached response to a request with an
   Authorization header (Section 3.1 of [Part7]) to satisfy any
   subsequent request unless a cache directive that allows such
   responses to be stored is present in the response.

   In this specification, the following Cache-Control response
   directives (Section 3.2.2) have such an effect: must-revalidate,
   public, s-maxage.

   Note that cached responses that contain the "must-revalidate" and/or
   "s-maxage" response directives are not allowed to be served stale
   (Section 2.3.3) by shared caches.  In particular, a response with
   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
   satisfy a subsequent request without revalidating it on the origin
   server.





Fielding, et al.        Expires February 5, 2011               [Page 15]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


2.7.  Caching Negotiated Responses

   When a cache receives a request that can be satisfied by a stored
   response that has a Vary header field (Section 3.5), it MUST NOT use
   that response unless all of the selecting request-headers nominated
   by the Vary header match in both the original request (i.e., that
   associated with the stored response), and the presented request.

   The selecting request-headers from two requests are defined to match
   if and only if those in the first request can be transformed to those
   in the second request by applying any of the following:

   o  adding or removing whitespace, where allowed in the header's
      syntax

   o  combining multiple message-header fields with the same field name
      (see Section 3.2 of [Part1])

   o  normalizing both header values in a way that is known to have
      identical semantics, according to the header's specification
      (e.g., re-ordering field values when order is not significant;
      case-normalization, where values are defined to be case-
      insensitive)

   If (after any normalization that might take place) a header field is
   absent from a request, it can only match another request if it is
   also absent there.

   A Vary header field-value of "*" always fails to match, and
   subsequent requests to that resource can only be properly interpreted
   by the origin server.

   The stored response with matching selecting request-headers is known
   as the selected response.

   If no selected response is available, the cache MAY forward the
   presented request to the origin server in a conditional request; see
   Section 2.4.

2.8.  Combining Responses

   When a cache receives a 304 (Not Modified) response or a 206 (Partial
   Content) response (in this section, the "new" response"), it needs to
   created an updated response by combining the stored response with the
   new one, so that the updated response can be used to satisfy the
   request, and potentially update the cached response.

   If the new response contains an ETag, it identifies the stored



Fielding, et al.        Expires February 5, 2011               [Page 16]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   response to use.  [[TODO-mention-CL: might need language about
   Content-Location here]][[TODO-select-for-combine: Shouldn't this be
   the selected response?]]

   If the new response's status code is 206 (partial content), both the
   stored and new responses MUST have validators, and those validators
   MUST match using the strong comparison function (see Section 4 of
   [Part4]).  Otherwise, the responses MUST NOT be combined.

   The stored response headers are used as those of the updated
   response, except that

   o  any stored Warning headers with warn-code 1xx (see Section 3.6)
      MUST be deleted.

   o  any stored Warning headers with warn-code 2xx MUST be retained.

   o  any other headers provided in the new response MUST replace all
      instances of the corresponding headers from the stored response.

   The updated response headers MUST be used to replace those of the
   stored response in cache (unless the stored response is removed from
   cache).  In the case of a 206 response, the combined representation
   MAY be stored.

3.  Header Field Definitions

   This section defines the syntax and semantics of HTTP/1.1 header
   fields related to caching.

3.1.  Age

   The "Age" response-header field conveys the sender's estimate of the
   amount of time since the response was generated or successfully
   validated at the origin server.  Age values are calculated as
   specified in Section 2.3.2.

     Age   = "Age" ":" OWS Age-v
     Age-v = delta-seconds

   Age field-values are non-negative integers, representing time in
   seconds.

     delta-seconds  = 1*DIGIT

   If a cache receives a value larger than the largest positive integer
   it can represent, or if any of its age calculations overflows, it
   MUST transmit an Age header with a field-value of 2147483648 (2^31).



Fielding, et al.        Expires February 5, 2011               [Page 17]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   Caches SHOULD use an arithmetic type of at least 31 bits of range.

   The presence of an Age header field in a response implies that a
   response is not first-hand.  However, the converse is not true, since
   HTTP/1.0 caches might not implement the Age header field.

3.2.  Cache-Control

   The "Cache-Control" general-header field is used to specify
   directives for caches along the request/response chain.  Such cache
   directives are unidirectional in that the presence of a directive in
   a request does not imply that the same directive is to be given in
   the response.

   HTTP/1.1 caches MUST obey the requirements of the Cache-Control
   directives defined in this section.  See Section 3.2.3 for
   information about how Cache-Control directives defined elsewhere are
   handled.

      Note: HTTP/1.0 caches might not implement Cache-Control and might
      only implement Pragma: no-cache (see Section 3.4).

   Cache directives MUST be passed through by a proxy or gateway
   application, regardless of their significance to that application,
   since the directives might be applicable to all recipients along the
   request/response chain.  It is not possible to target a directive to
   a specific cache.

     Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
     Cache-Control-v = 1#cache-directive

     cache-directive = cache-request-directive
        / cache-response-directive

     cache-extension = token [ "=" ( token / quoted-string ) ]

3.2.1.  Request Cache-Control Directives

     cache-request-directive =
          "no-cache"
        / "no-store"
        / "max-age" "=" delta-seconds
        / "max-stale" [ "=" delta-seconds ]
        / "min-fresh" "=" delta-seconds
        / "no-transform"
        / "only-if-cached"
        / cache-extension




Fielding, et al.        Expires February 5, 2011               [Page 18]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   no-cache

      The no-cache request directive indicates that a stored response
      MUST NOT be used to satisfy the request without successful
      validation on the origin server.

   no-store

      The no-store request directive indicates that a cache MUST NOT
      store any part of either this request or any response to it.  This
      directive applies to both non-shared and shared caches.  "MUST NOT
      store" in this context means that the cache MUST NOT intentionally
      store the information in non-volatile storage, and MUST make a
      best-effort attempt to remove the information from volatile
      storage as promptly as possible after forwarding it.

      This directive is NOT a reliable or sufficient mechanism for
      ensuring privacy.  In particular, malicious or compromised caches
      might not recognize or obey this directive, and communications
      networks might be vulnerable to eavesdropping.

   max-age

      The max-age request directive indicates that the client is willing
      to accept a response whose age is no greater than the specified
      time in seconds.  Unless the max-stale request directive is also
      present, the client is not willing to accept a stale response.

   max-stale

      The max-stale request directive indicates that the client is
      willing to accept a response that has exceeded its expiration
      time.  If max-stale is assigned a value, then the client is
      willing to accept a response that has exceeded its expiration time
      by no more than the specified number of seconds.  If no value is
      assigned to max-stale, then the client is willing to accept a
      stale response of any age.

   min-fresh

      The min-fresh request directive indicates that the client is
      willing to accept a response whose freshness lifetime is no less
      than its current age plus the specified time in seconds.  That is,
      the client wants a response that will still be fresh for at least
      the specified number of seconds.






Fielding, et al.        Expires February 5, 2011               [Page 19]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   no-transform

      The no-transform request directive indicates that an intermediate
      cache or proxy MUST NOT change the Content-Encoding, Content-Range
      or Content-Type request headers, nor the request representation.

   only-if-cached

      The only-if-cached request directive indicates that the client
      only wishes to return a stored response.  If it receives this
      directive, a cache SHOULD either respond using a stored response
      that is consistent with the other constraints of the request, or
      respond with a 504 (Gateway Timeout) status code.  If a group of
      caches is being operated as a unified system with good internal
      connectivity, such a request MAY be forwarded within that group of
      caches.

3.2.2.  Response Cache-Control Directives

     cache-response-directive =
          "public"
        / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
        / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
        / "no-store"
        / "no-transform"
        / "must-revalidate"
        / "proxy-revalidate"
        / "max-age" "=" delta-seconds
        / "s-maxage" "=" delta-seconds
        / cache-extension

   public

      The public response directive indicates that the response MAY be
      cached, even if it would normally be non-cacheable or cacheable
      only within a non-shared cache.  (See also Authorization, Section
      3.1 of [Part7], for additional details.)

   private

      The private response directive indicates that the response message
      is intended for a single user and MUST NOT be stored by a shared
      cache.  A private (non-shared) cache MAY store the response.

      If the private response directive specifies one or more field-
      names, this requirement is limited to the field-values associated
      with the listed response headers.  That is, the specified field-
      names(s) MUST NOT be stored by a shared cache, whereas the



Fielding, et al.        Expires February 5, 2011               [Page 20]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


      remainder of the response message MAY be.

      Note: This usage of the word private only controls where the
      response can be stored; it cannot ensure the privacy of the
      message content.  Also, private response directives with field-
      names are often handled by implementations as if an unqualified
      private directive was received; i.e., the special handling for the
      qualified form is not widely implemented.

   no-cache

      The no-cache response directive indicates that the response MUST
      NOT be used to satisfy a subsequent request without successful
      validation on the origin server.  This allows an origin server to
      prevent a cache from using it to satisfy a request without
      contacting it, even by caches that have been configured to return
      stale responses.

      If the no-cache response directive specifies one or more field-
      names, this requirement is limited to the field-values associated
      with the listed response headers.  That is, the specified field-
      name(s) MUST NOT be sent in the response to a subsequent request
      without successful validation on the origin server.  This allows
      an origin server to prevent the re-use of certain header fields in
      a response, while still allowing caching of the rest of the
      response.

      Note: Most HTTP/1.0 caches will not recognize or obey this
      directive.  Also, no-cache response directives with field-names
      are often handled by implementations as if an unqualified no-cache
      directive was received; i.e., the special handling for the
      qualified form is not widely implemented.

   no-store

      The no-store response directive indicates that a cache MUST NOT
      store any part of either the immediate request or response.  This
      directive applies to both non-shared and shared caches.  "MUST NOT
      store" in this context means that the cache MUST NOT intentionally
      store the information in non-volatile storage, and MUST make a
      best-effort attempt to remove the information from volatile
      storage as promptly as possible after forwarding it.

      This directive is NOT a reliable or sufficient mechanism for
      ensuring privacy.  In particular, malicious or compromised caches
      might not recognize or obey this directive, and communications
      networks might be vulnerable to eavesdropping.




Fielding, et al.        Expires February 5, 2011               [Page 21]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   must-revalidate

      The must-revalidate response directive indicates that once it has
      become stale, the response MUST NOT be used to satisfy subsequent
      requests without successful validation on the origin server.

      The must-revalidate directive is necessary to support reliable
      operation for certain protocol features.  In all circumstances an
      HTTP/1.1 cache MUST obey the must-revalidate directive; in
      particular, if the cache cannot reach the origin server for any
      reason, it MUST generate a 504 (Gateway Timeout) response.

      Servers SHOULD send the must-revalidate directive if and only if
      failure to validate a request on the representation could result
      in incorrect operation, such as a silently unexecuted financial
      transaction.

   proxy-revalidate

      The proxy-revalidate response directive has the same meaning as
      the must-revalidate response directive, except that it does not
      apply to non-shared caches.

   max-age

      The max-age response directive indicates that response is to be
      considered stale after its age is greater than the specified
      number of seconds.

   s-maxage

      The s-maxage response directive indicates that, in shared caches,
      the maximum age specified by this directive overrides the maximum
      age specified by either the max-age directive or the Expires
      header.  The s-maxage directive also implies the semantics of the
      proxy-revalidate response directive.

   no-transform

      The no-transform response directive indicates that an intermediate
      cache or proxy MUST NOT change the Content-Encoding, Content-Range
      or Content-Type response headers, nor the response representation.

3.2.3.  Cache Control Extensions

   The Cache-Control header field can be extended through the use of one
   or more cache-extension tokens, each with an optional value.
   Informational extensions (those that do not require a change in cache



Fielding, et al.        Expires February 5, 2011               [Page 22]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   behavior) can be added without changing the semantics of other
   directives.  Behavioral extensions are designed to work by acting as
   modifiers to the existing base of cache directives.  Both the new
   directive and the standard directive are supplied, such that
   applications that do not understand the new directive will default to
   the behavior specified by the standard directive, and those that
   understand the new directive will recognize it as modifying the
   requirements associated with the standard directive.  In this way,
   extensions to the cache-control directives can be made without
   requiring changes to the base protocol.

   This extension mechanism depends on an HTTP cache obeying all of the
   cache-control directives defined for its native HTTP-version, obeying
   certain extensions, and ignoring all directives that it does not
   understand.

   For example, consider a hypothetical new response directive called
   "community" that acts as a modifier to the private directive.  We
   define this new directive to mean that, in addition to any non-shared
   cache, any cache that is shared only by members of the community
   named within its value may cache the response.  An origin server
   wishing to allow the UCI community to use an otherwise private
   response in their shared cache(s) could do so by including

     Cache-Control: private, community="UCI"

   A cache seeing this header field will act correctly even if the cache
   does not understand the community cache-extension, since it will also
   see and understand the private directive and thus default to the safe
   behavior.

   Unrecognized cache directives MUST be ignored; it is assumed that any
   cache directive likely to be unrecognized by an HTTP/1.1 cache will
   be combined with standard directives (or the response's default
   cacheability) such that the cache behavior will remain minimally
   correct even if the cache does not understand the extension(s).

   The HTTP Cache Directive Registry defines the name space for the
   cache directives.

   Registrations MUST include the following fields:

   o  Cache Directive Name

   o  Pointer to specification text

   Values to be added to this name space are subject to IETF review
   ([RFC5226], Section 4.1).



Fielding, et al.        Expires February 5, 2011               [Page 23]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   The registry itself is maintained at
   <http://www.iana.org/assignments/http-cache-directives>.

3.3.  Expires

   The "Expires" header field gives the date/time after which the
   response is considered stale.  See Section 2.3 for further discussion
   of the freshness model.

   The presence of an Expires field does not imply that the original
   resource will change or cease to exist at, before, or after that
   time.

   The field-value is an absolute date and time as defined by HTTP-date
   in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format.

     Expires   = "Expires" ":" OWS Expires-v
     Expires-v = HTTP-date

   For example

     Expires: Thu, 01 Dec 1994 16:00:00 GMT

      Note: If a response includes a Cache-Control field with the max-
      age directive (see Section 3.2.2), that directive overrides the
      Expires field.  Likewise, the s-maxage directive overrides Expires
      in shared caches.

   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
   the future.

   HTTP/1.1 clients and caches MUST treat other invalid date formats,
   especially including the value "0", as in the past (i.e., "already
   expired").

3.4.  Pragma

   The "Pragma" general-header field is used to include implementation-
   specific directives that might apply to any recipient along the
   request/response chain.  All pragma directives specify optional
   behavior from the viewpoint of the protocol; however, some systems
   MAY require that behavior be consistent with the directives.

     Pragma            = "Pragma" ":" OWS Pragma-v
     Pragma-v          = 1#pragma-directive
     pragma-directive  = "no-cache" / extension-pragma
     extension-pragma  = token [ "=" ( token / quoted-string ) ]




Fielding, et al.        Expires February 5, 2011               [Page 24]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   When the no-cache directive is present in a request message, an
   application SHOULD forward the request toward the origin server even
   if it has a cached copy of what is being requested.  This pragma
   directive has the same semantics as the no-cache response directive
   (see Section 3.2.2) and is defined here for backward compatibility
   with HTTP/1.0.  Clients SHOULD include both header fields when a no-
   cache request is sent to a server not known to be HTTP/1.1 compliant.
   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
   sent "Cache-Control: no-cache".

      Note: Because the meaning of "Pragma: no-cache" as a response-
      header field is not actually specified, it does not provide a
      reliable replacement for "Cache-Control: no-cache" in a response.

   This mechanism is deprecated; no new Pragma directives will be
   defined in HTTP.

3.5.  Vary

   The "Vary" response-header field conveys the set of request-header
   fields that were used to select the representation.

   Caches use this information, in part, to determine whether a stored
   response can be used to satisfy a given request; see Section 2.7.
   determines, while the response is fresh, whether a cache is permitted
   to use the response to reply to a subsequent request without
   validation; see Section 2.7.

   In uncacheable or stale responses, the Vary field value advises the
   user agent about the criteria that were used to select the
   representation.

     Vary   = "Vary" ":" OWS Vary-v
     Vary-v = "*" / 1#field-name

   The set of header fields named by the Vary field value is known as
   the selecting request-headers.

   Servers SHOULD include a Vary header field with any cacheable
   response that is subject to server-driven negotiation.  Doing so
   allows a cache to properly interpret future requests on that resource
   and informs the user agent about the presence of negotiation on that
   resource.  A server MAY include a Vary header field with a non-
   cacheable response that is subject to server-driven negotiation,
   since this might provide the user agent with useful information about
   the dimensions over which the response varies at the time of the
   response.




Fielding, et al.        Expires February 5, 2011               [Page 25]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   A Vary field value of "*" signals that unspecified parameters not
   limited to the request-headers (e.g., the network address of the
   client), play a role in the selection of the response representation;
   therefore, a cache cannot determine whether this response is
   appropriate.  The "*" value MUST NOT be generated by a proxy server.

   The field-names given are not limited to the set of standard request-
   header fields defined by this specification.  Field names are case-
   insensitive.

3.6.  Warning

   The "Warning" general-header field is used to carry additional
   information about the status or transformation of a message that
   might not be reflected in the message.  This information is typically
   used to warn about possible incorrectness introduced by caching
   operations or transformations applied to the payload of the message.

   Warnings can be used for other purposes, both cache-related and
   otherwise.  The use of a warning, rather than an error status code,
   distinguishes these responses from true failures.

   Warning headers can in general be applied to any message, however
   some warn-codes are specific to caches and can only be applied to
   response messages.

     Warning    = "Warning" ":" OWS Warning-v
     Warning-v  = 1#warning-value

     warning-value = warn-code SP warn-agent SP warn-text
                                           [SP warn-date]

     warn-code  = 3DIGIT
     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
                     ; the name or pseudonym of the server adding
                     ; the Warning header, for use in debugging
     warn-text  = quoted-string
     warn-date  = DQUOTE HTTP-date DQUOTE

   Multiple warnings can be attached to a response (either by the origin
   server or by a cache), including multiple warnings with the same code
   number, only differing in warn-text.

   When this occurs, the user agent SHOULD inform the user of as many of
   them as possible, in the order that they appear in the response.

   Systems that generate multiple Warning headers SHOULD order them with
   this user agent behavior in mind.  New Warning headers SHOULD be



Fielding, et al.        Expires February 5, 2011               [Page 26]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   added after any existing Warning headers.

   Warnings are assigned three digit warn-codes.  The first digit
   indicates whether the Warning is required to be deleted from a stored
   response after validation:

   o  1xx Warnings describe the freshness or validation status of the
      response, and so MUST be deleted by caches after validation.  They
      can only be generated by a cache when validating a cached entry,
      and MUST NOT be generated in any other situation.

   o  2xx Warnings describe some aspect of the representation that is
      not rectified by a validation (for example, a lossy compression of
      the representation) and MUST NOT be deleted by caches after
      validation, unless a full response is returned, in which case they
      MUST be.

   If an implementation sends a message with one or more Warning headers
   to a receiver whose version is HTTP/1.0 or lower, then the sender
   MUST include in each warning-value a warn-date that matches the Date
   header in the message.

   If an implementation receives a message with a warning-value that
   includes a warn-date, and that warn-date is different from the Date
   value in the response, then that warning-value MUST be deleted from
   the message before storing, forwarding, or using it. (preventing the
   consequences of naive caching of Warning header fields.)  If all of
   the warning-values are deleted for this reason, the Warning header
   MUST be deleted as well.

   The following warn-codes are defined by this specification, each with
   a recommended warn-text in English, and a description of its meaning.

   110 Response is stale

      SHOULD be included whenever the returned response is stale.

   111 Revalidation failed

      SHOULD be included if a cache returns a stale response because an
      attempt to validate the response failed, due to an inability to
      reach the server.

   112 Disconnected operation

      SHOULD be included if the cache is intentionally disconnected from
      the rest of the network for a period of time.




Fielding, et al.        Expires February 5, 2011               [Page 27]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   113 Heuristic expiration

      SHOULD be included if the cache heuristically chose a freshness
      lifetime greater than 24 hours and the response's age is greater
      than 24 hours.

   199 Miscellaneous warning

      The warning text can include arbitrary information to be presented
      to a human user, or logged.  A system receiving this warning MUST
      NOT take any automated action, besides presenting the warning to
      the user.

   214 Transformation applied

      MUST be added by an intermediate proxy if it applies any
      transformation to the representation, such as changing the
      content-coding, media-type, or modifying the representation data,
      unless this Warning code already appears in the response.

   299 Miscellaneous persistent warning

      The warning text can include arbitrary information to be presented
      to a human user, or logged.  A system receiving this warning MUST
      NOT take any automated action.

4.  History Lists

   User agents often have history mechanisms, such as "Back" buttons and
   history lists, that can be used to redisplay a representation
   retrieved earlier in a session.

   The freshness model (Section 2.3) does not necessarily apply to
   history mechanisms.  I.e., a history mechanism can display a previous
   representation even if it has expired.

   This does not prohibit the history mechanism from telling the user
   that a view might be stale, or from honoring cache directives (e.g.,
   Cache-Control: no-store).

5.  IANA Considerations

5.1.  Cache Directive Registry

   The registration procedure for HTTP Cache Directives is defined by
   Section 3.2.3 of this document.

   The HTTP Cache Directive Registry shall be created at



Fielding, et al.        Expires February 5, 2011               [Page 28]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   <http://www.iana.org/assignments/http-cache-directives> and be
   populated with the registrations below:

   +------------------------+------------------------------+
   | Cache Directive        | Reference                    |
   +------------------------+------------------------------+
   | max-age                | Section 3.2.1, Section 3.2.2 |
   | max-stale              | Section 3.2.1                |
   | min-fresh              | Section 3.2.1                |
   | must-revalidate        | Section 3.2.2                |
   | no-cache               | Section 3.2.1, Section 3.2.2 |
   | no-store               | Section 3.2.1, Section 3.2.2 |
   | no-transform           | Section 3.2.1, Section 3.2.2 |
   | only-if-cached         | Section 3.2.1                |
   | private                | Section 3.2.2                |
   | proxy-revalidate       | Section 3.2.2                |
   | public                 | Section 3.2.2                |
   | s-maxage               | Section 3.2.2                |
   | stale-if-error         | [RFC5861], Section 4         |
   | stale-while-revalidate | [RFC5861], Section 3         |
   +------------------------+------------------------------+

5.2.  Header Field Registration

   The Message Header Field Registry located at <http://www.iana.org/
   assignments/message-headers/message-header-index.html> shall be
   updated with the permanent registrations below (see [RFC3864]):

   +-------------------+----------+----------+-------------+
   | Header Field Name | Protocol | Status   | Reference   |
   +-------------------+----------+----------+-------------+
   | Age               | http     | standard | Section 3.1 |
   | Cache-Control     | http     | standard | Section 3.2 |
   | Expires           | http     | standard | Section 3.3 |
   | Pragma            | http     | standard | Section 3.4 |
   | Vary              | http     | standard | Section 3.5 |
   | Warning           | http     | standard | Section 3.6 |
   +-------------------+----------+----------+-------------+

   The change controller is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

6.  Security Considerations

   Caches expose additional potential vulnerabilities, since the
   contents of the cache represent an attractive target for malicious
   exploitation.  Because cache contents persist after an HTTP request
   is complete, an attack on the cache can reveal information long after



Fielding, et al.        Expires February 5, 2011               [Page 29]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   a user believes that the information has been removed from the
   network.  Therefore, cache contents need to be protected as sensitive
   information.

7.  Acknowledgments

   Much of the content and presentation of the caching design is due to
   suggestions and comments from individuals including: Shel Kaphan,
   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.

8.  References

8.1.  Normative References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
              and Message Parsing", draft-ietf-httpbis-p1-messaging-11
              (work in progress), August 2010.

   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
              Semantics", draft-ietf-httpbis-p2-semantics-11 (work in
              progress), August 2010.

   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
              Requests", draft-ietf-httpbis-p4-conditional-11 (work in
              progress), August 2010.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-11 (work
              in progress), August 2010.

   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
              draft-ietf-httpbis-p7-auth-11 (work in progress),
              August 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax



Fielding, et al.        Expires February 5, 2011               [Page 30]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


              Specifications: ABNF", STD 68, RFC 5234, January 2008.

8.2.  Informative References

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, April 2010.

Appendix A.  Changes from RFC 2616

   Make the specified age calculation algorithm less conservative.
   (Section 2.3.2)

   Remove requirement to consider Content-Location in successful
   responses in order to determine the appropriate response to use.
   (Section 2.4)

   Clarify denial of service attack avoidance requirement.
   (Section 2.5)

   Do not mention RFC 2047 encoding and multiple languages in Warning
   headers anymore, as these aspects never were implemented.
   (Section 3.6)

Appendix B.  Collected ABNF

   Age = "Age:" OWS Age-v
   Age-v = delta-seconds

   Cache-Control = "Cache-Control:" OWS Cache-Control-v
   Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
    cache-directive ] )

   Expires = "Expires:" OWS Expires-v



Fielding, et al.        Expires February 5, 2011               [Page 31]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   Expires-v = HTTP-date

   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>

   OWS = <OWS, defined in [Part1], Section 1.2.2>

   Pragma = "Pragma:" OWS Pragma-v
   Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
    pragma-directive ] )

   Vary = "Vary:" OWS Vary-v
   Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
    ] ) )

   Warning = "Warning:" OWS Warning-v
   Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
    ] )

   cache-directive = cache-request-directive / cache-response-directive
   cache-extension = token [ "=" ( token / quoted-string ) ]
   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
    cache-extension
   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
    ) / ( "s-maxage=" delta-seconds ) / cache-extension

   delta-seconds = 1*DIGIT

   extension-pragma = token [ "=" ( token / quoted-string ) ]

   field-name = <field-name, defined in [Part1], Section 3.2>

   port = <port, defined in [Part1], Section 2.6>
   pragma-directive = "no-cache" / extension-pragma
   pseudonym = <pseudonym, defined in [Part1], Section 9.9>

   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>

   token = <token, defined in [Part1], Section 1.2.2>

   uri-host = <uri-host, defined in [Part1], Section 2.6>

   warn-agent = ( uri-host [ ":" port ] ) / pseudonym



Fielding, et al.        Expires February 5, 2011               [Page 32]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   warn-code = 3DIGIT
   warn-date = DQUOTE HTTP-date DQUOTE
   warn-text = quoted-string
   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
    ]

   ABNF diagnostics:

   ; Age defined but not used
   ; Cache-Control defined but not used
   ; Expires defined but not used
   ; Pragma defined but not used
   ; Vary defined but not used
   ; Warning defined but not used

Appendix C.  Change Log (to be removed by RFC Editor before publication)

C.1.  Since RFC2616

   Extracted relevant partitions from [RFC2616].

C.2.  Since draft-ietf-httpbis-p6-cache-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
      (<http://purl.org/NET/http-errata#trailer-hop>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
      after Update or Delete"
      (<http://purl.org/NET/http-errata#invalidupd>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
      Informative references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
      typo"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
      header text"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
      references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
      Reference"





Fielding, et al.        Expires February 5, 2011               [Page 33]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
      to-date references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
      13.2.2"

   Other changes:

   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)

C.3.  Since draft-ietf-httpbis-p6-cache-01

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
      used"

   Other changes:

   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)

   o  Add explicit references to BNF syntax and rules imported from
      other parts of the specification.

C.4.  Since draft-ietf-httpbis-p6-cache-02

   Ongoing work on IANA Message Header Registration
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):

   o  Reference RFC 3984, and update header registrations for headers
      defined in this document.

C.5.  Since draft-ietf-httpbis-p6-cache-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
      classification"

C.6.  Since draft-ietf-httpbis-p6-cache-04

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Use "/" instead of "|" for alternatives.




Fielding, et al.        Expires February 5, 2011               [Page 34]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      value format definitions.

C.7.  Since draft-ietf-httpbis-p6-cache-05

   This is a total rewrite of this part of the specification.

   Affected issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
      1xx Warn-Codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
      13.5.1 and 13.5.2"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role of
      Warning and Semantic Transparency in Caching"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
      Caching"

   In addition: Final work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Add appendix containing collected and expanded ABNF, reorganize
      ABNF introduction.

C.8.  Since draft-ietf-httpbis-p6-cache-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
      numeric protocol elements"

   Affected issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: Vary and non-
      existant headers

C.9.  Since draft-ietf-httpbis-p6-cache-07

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
      1xx Warn-Codes"



Fielding, et al.        Expires February 5, 2011               [Page 35]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
      Location on 304 responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
      no-cache CC directives with headers"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
      warn-text"

C.10.  Since draft-ietf-httpbis-p6-cache-08

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
      negotiated responses from cache: header-specific canonicalization"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
      directives on history lists"

   Affected issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
      and caching

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
      13.5.1 and 13.5.2"

C.11.  Since draft-ietf-httpbis-p6-cache-09

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/29>: "Age
      calculation"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/168>: "Clarify
      differences between / requirements for request and response CC
      directives"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/174>: "Caching
      authenticated responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/208>: "IANA registry
      for cache-control directives"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
      caching of URLs with query components"



Fielding, et al.        Expires February 5, 2011               [Page 36]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
      requested resource's URI"

C.12.  Since draft-ietf-httpbis-p6-cache-10

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
      entity / representation / variant terminology"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
      removing the 'changes from 2068' sections"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
      heuristic caching for new status codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
      heuristic caching for new status codes"

   o  Clean up TODOs and prose in "Combining Responses."

Index

   A
      age  6
      Age header  17

   C
      cache  5
      Cache Directives
         max-age  19, 22
         max-stale  19
         min-fresh  19
         must-revalidate  22
         no-cache  19, 21
         no-store  19, 21
         no-transform  20, 22
         only-if-cached  20
         private  20
         proxy-revalidate  22
         public  20
         s-maxage  22
      Cache-Control header  18
      cacheable  5

   E



Fielding, et al.        Expires February 5, 2011               [Page 37]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


      Expires header  24
      explicit expiration time  5

   F
      first-hand  6
      fresh  6
      freshness lifetime  6

   G
      Grammar
         Age  17
         Age-v  17
         Cache-Control  18
         Cache-Control-v  18
         cache-extension  18
         cache-request-directive  18
         cache-response-directive  20
         delta-seconds  17
         Expires  24
         Expires-v  24
         extension-pragma  24
         Pragma  24
         pragma-directive  24
         Pragma-v  24
         Vary  25
         Vary-v  25
         warn-agent  26
         warn-code  26
         warn-date  26
         warn-text  26
         Warning  26
         Warning-v  26
         warning-value  26

   H
      Headers
         Age  17
         Cache-Control  18
         Expires  24
         Pragma  24
         Vary  25
         Warning  26
      heuristic expiration time  5

   M
      max-age
         Cache Directive  19, 22
      max-stale



Fielding, et al.        Expires February 5, 2011               [Page 38]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


         Cache Directive  19
      min-fresh
         Cache Directive  19
      must-revalidate
         Cache Directive  22

   N
      no-cache
         Cache Directive  19, 21
      no-store
         Cache Directive  19, 21
      no-transform
         Cache Directive  20, 22

   O
      only-if-cached
         Cache Directive  20

   P
      Pragma header  24
      private
         Cache Directive  20
      proxy-revalidate
         Cache Directive  22
      public
         Cache Directive  20

   S
      s-maxage
         Cache Directive  22
      stale  6

   V
      validator  6
      Vary header  25

   W
      Warning header  26













Fielding, et al.        Expires February 5, 2011               [Page 39]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


Authors' Addresses

   Roy T. Fielding (editor)
   Day Software
   23 Corporate Plaza DR, Suite 280
   Newport Beach, CA  92660
   USA

   Phone: +1-949-706-5300
   Fax:   +1-949-706-5305
   EMail: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   Jim Gettys
   Alcatel-Lucent Bell Labs
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   EMail: jg@freedesktop.org
   URI:   http://gettys.wordpress.com/


   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304
   USA

   EMail: JeffMogul@acm.org


   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052
   USA

   EMail: henrikn@microsoft.com










Fielding, et al.        Expires February 5, 2011               [Page 40]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   Larry Masinter
   Adobe Systems, Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: LMM@acm.org
   URI:   http://larry.masinter.net/


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052

   EMail: paulle@microsoft.com


   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139
   USA

   EMail: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/


   Yves Lafon (editor)
   World Wide Web Consortium
   W3C / ERCIM
   2004, rte des Lucioles
   Sophia-Antipolis, AM  06902
   France

   EMail: ylafon@w3.org
   URI:   http://www.raubacapeu.net/people/yves/


   Mark Nottingham (editor)

   EMail: mnot@mnot.net
   URI:   http://www.mnot.net/






Fielding, et al.        Expires February 5, 2011               [Page 41]

Internet-Draft              HTTP/1.1, Part 6                 August 2010


   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/









































Fielding, et al.        Expires February 5, 2011               [Page 42]