In gibt es zwei Möglichkeiten, um POST-Daten-Anwendung x-www-Form-urlencodiert und multipart Form-Daten Ich verstehe, dass die meisten Browser sind nur in der Lage, Dateien hochladen, wenn multipart Form-Daten verwendet wird Gibt es eine zusätzliche Anleitung, wenn eine der Codierungstypen in einem API-Kontext kein Browser beteiligt Dies könnte zB auf der Existenz von Nicht-ASCII-Zeichen basieren. Erhaltung auf nichtcodierten Binärdaten. die Notwendigkeit, zusätzliche Daten wie Dateiname zu übertragen. I grundsätzlich keine formale Anleitung im Web über die Verwendung gefunden Von den verschiedenen Inhaltstypen so weit. Schicht Okt 24 10 bei 11 12.Die Moral der Geschichte ist, wenn Sie binäre nicht-alphanumerische Daten oder eine signifikante Größe zu übertragen, verwenden Sie multipart Form-Daten Andernfalls verwenden Sie Anwendung x - www-form-urlencoded. Die MIME-Typen, die Sie erwähnen, sind die beiden Content-Type-Header für POST-Anfragen, die Benutzer-Agenten-Browser unterstützen müssen Der Zweck dieser beiden Arten von Anfragen ist es, eine Liste von Namen Wert-Paare an den Server zu senden Je nach Art und Menge der Daten, die übertragen werden, wird eine der Methoden effizienter als die anderen Um zu verstehen, warum, müssen Sie sich anschauen, was jeder unter den Abdeckungen tut. Für die Anwendung x-www-Form-urlencodiert den Körper der Nachricht an gesendet Der Server ist im Wesentlichen ein Riesen-Abfrage-String - Name Wert-Paare sind durch das Und-Zeichen getrennt, und Namen werden von Werten durch das Gleich-Symbol getrennt Ein Beispiel dafür wäre. Reservierte und nicht-alphanumerische Zeichen werden durch HH ersetzt, ein Prozentzeichen und zwei hexadezimale Ziffern, die den ASCII-Code des Zeichens darstellen. Das bedeutet, dass für jedes nicht-alphanumerische Byte, das in einem unserer Werte existiert, es drei Bytes gibt Um es zu repräsentieren Für große Binärdateien, verdreifachen die Nutzlast wird in hohem Grade ineffizient sein. Das ist, wo multipart Form-Daten hereinkommen. Mit dieser Methode der Übertragung von Namenswertpaaren wird jedes Paar als Teil in einer MIME-Mitteilung dargestellt, wie beschrieben durch Andere Antworten Die Teile werden durch eine bestimmte String-Grenze getrennt, die speziell gewählt wird, so dass diese Bounding-String nicht in einem der Wert-Payloads auftreten. Jeder Teil hat seinen eigenen Satz von MIME-Headern wie Content-Type und insbesondere Content-Disposition, die jedem Teil seine geben können Name Der Wert jedes Namenswertpaares ist die Nutzlast jedes Teils der MIME-Meldung Die MIME-Spezifikation gibt uns mehr Optionen bei der Darstellung der Wert-Nutzlast - wir können einen effizienteren Wert wählen T-Verschlüsselung von binären Daten, um Bandbreite zu speichern, zB Basis 64 oder sogar roh binär. Warum keine Multipart-Form-Daten die ganze Zeit verwenden Für kurze alphanumerische Werte wie die meisten Web-Formulare wird der Overhead des Hinzufügens aller MIME-Header erheblich überwiegen Einsparungen aus effizienter binärer Kodierung. Entwickelt am 1. November 10 um 21 59. BREITEN SIE MINDESTENS DAS ERSTE PARA HIER. Ich weiß, das ist 3 Jahre zu spät, aber Matt s akzeptierte Antwort ist unvollständig und wird euch schließlich in Schwierigkeiten bringen Der Schlüssel hier ist Dass, wenn Sie sich für Multipart-Formulardaten entscheiden, die Grenze nicht in den Dateidaten erscheinen darf, die der Server eventuell empfängt. Dies ist kein Problem für die Anwendung x-www-form-urlencodiert, da es keine Grenze x-www-Form gibt - urlencoded kann auch immer binäre Daten, durch das einfache Mittel, um ein beliebiges Byte in drei 7BIT Bytes Ineffizient, aber es funktioniert und beachten Sie, dass die Bemerkung über nicht in der Lage, Dateinamen sowie binäre Daten senden ist falsch Sie nur senden es Als eine andere Key-Wert-Paar. Das Problem mit multipart Form-Daten ist, dass die Grenze Trennzeichen darf nicht in den Dateidaten vorhanden sein, siehe RFC2388 Abschnitt 5 2 enthält auch eine ziemlich lahme Ausrede für nicht mit einem richtigen Aggregat MIME-Typ, der dieses Problem vermeidet. So, Auf den ersten Blick, Multipart Form-Daten ist überhaupt kein Wert in jedem Datei-Upload, binär oder anders Wenn Sie don t wählen Sie Ihre Grenze richtig, dann werden Sie schließlich ein Problem, ob Sie senden einfacher Text oder roh binär - der Server Wird eine Grenze an der falschen Stelle finden, und Ihre Datei wird abgeschnitten, oder der POST wird fehlschlagen. Der Schlüssel ist, eine Codierung und eine Grenze zu wählen, so dass Ihre ausgewählten Grenzzeichen nicht in der codierten Ausgabe erscheinen können Eine einfache Lösung ist zu verwenden Base64 nicht roh binär verwenden In base64 3 werden beliebige Bytes in vier 7-Bit-Zeichen codiert, wobei der ausgegebene Zeichensatz A-Za-z0-9 dh alphanumerisch ist oder, ein Sonderfall ist und nur bei der Ende der codierten Ausgabe, als Single oder ein Double Jetzt wählen Sie Ihre Grenze als 7-Bit-ASCII-String, die nicht in base64-Ausgabe erscheinen kann Viele Möglichkeiten, die Sie auf dem Netz sehen, schlagen diesen Test - die MDN-Formulare zum Beispiel verwenden Blob als Grenze beim Senden von Binärdaten - Nicht gut Aber so etwas wie Blob wird nie in base64 output. answered erscheinen Apr 18 14 um 11 08.Während eine Betrachtung von multipart Form-Daten ist die sicherstellen, dass die Grenze nicht in den Daten erscheint dies ist ziemlich einfach zu erreichen, indem Sie eine Grenze, die genügend lang ist Bitte nicht uns base64 Kodierung, um dies zu erreichen Eine Grenze, die zufällig erzeugt wird und die gleiche Länge wie eine UUID sollte ausreichend sein Joshcodes Apr 30 14 am 22 01. EML, das macht überhaupt keinen Sinn. Offensichtlich die Grenze Wird automatisch vom Client-Browser ausgewählt und der Client wird schlau genug, um keine Grenze zu verwenden, die mit dem Inhalt Ihrer hochgeladenen Dateien zusammenstößt. Es ist so einfach aa substring match index -1 Pacerier Dez 11 14 at 8 01. BeniBela, He s Vermutlich zu empfehlen, zu verwenden - dann zufällige Generation mit Substring-Check ist immer noch der Weg zu gehen und es kann mit einer Zeile getan werden, während echte EML-Vorschlag auf base64 umwandeln, um zu vermeiden, dass passende Teilstrings einfach nur ungerade sind, nicht zu erwähnen Kommt mit unnötigen Performance-Verschlechterung Und all die Mühe für nichts, da die eine Zeile Algorithmus ist gleich einfach und einfach Base64 ist nicht dazu bestimmt, ab verwendet werden auf diese Weise, wie Körper akzeptieren alle 8-Bit-Oktette Pacerier 27. Juli 15 um 10 51.Diese Antwort Fügt nicht nur der Diskussion nichts hinzu, sondern gibt auch einen falschen Rat Zuerst, wann immer die Übertragung von zufälligen Daten in getrennten Teilen, ist es immer möglich, dass die gewählte Grenze in der Nutzlast vorhanden sein wird. Der einzige Weg, um sicherzustellen, dass dies nicht der Fall ist zu prüfen ist Die ganze Nutzlast für jede Grenze, die wir mit Vollständig unpraktisch kennenlernen Wir akzeptieren einfach die infinitesimale Wahrscheinlichkeit einer Kollision und kommen mit einer vernünftigen Grenze, wie - Grenze - UUID hier - - Zweitens, immer mit Base64 wird Bandbreite und füllen Puffer ohne Grund an allen Vagelis Mai 5 16 bei 12 22. Ich denke, ist beschränkt auf POST in multipart oder x-www-Form-urlencodiert Die Content-Type Header Ist orthogonal zu der POST-Methode können Sie MIME-Typ, die zu Ihnen passt füllen Dies ist auch der Fall für typische HTML-Darstellung basierte Webapps zB json Payload wurde sehr beliebt für die Übertragung von Payload für ajax requests. Regarding Restful API über die beliebtesten Content-Typen kam ich In touch mit sind Anwendung xml und Anwendung json. data-size XML sehr ausführlich, aber in der Regel kein Problem bei der Verwendung von Kompression und Denken, dass der Schreibzugriff Fall zB durch POST oder PUT ist viel seltener als Lesezugriff in vielen Fällen ist es 3 von allen Verkehr Selten dort wo Fälle, in denen ich die Schreibleistung optimieren musste. existenz von Nicht-ascii-Zeichen, die Sie utf-8 als Codierung in XML verwenden können. Die Existenz von Binärdaten müsste base64 encoding. filename Daten verwenden, die Sie einkapseln können Dies Innen Feld in XML. data-Größe kompakter weniger, dass XML, noch Text, aber Sie können komprimieren. non-ascii chars json ist utf-8.binary data base64 siehe auch json-binary-question. filename Daten kapseln als eigene Feld - Abschnitt innerhalb json. binary Daten als eigene Ressource. Ich würde versuchen, binäre Daten als eigene Ressource Ressource darstellen Es fügt einen weiteren Anruf, aber entkoppelt Zeug besser Beispiel Bilder. In späteren Ressourcen können Sie einfach inline die binäre Ressource als link. application x-www - Form-urlencodiert ist die Standard-Mime-Typ Ihrer Anfrage siehe auch Ich benutze es für normale Webforms Für API Ich benutze Anwendung xml json multipart Form-Daten ist eine Glocke im Denken von Anhängen innerhalb Antwort Körper mehrere Daten-Abschnitte sind mit einem definierten konkurriert Grenze String Manuel Aldana Okt 25 10 bei 19 38. Ich stimme mit viel, dass Manuel hat gesagt In der Tat, seine Kommentare beziehen sich auf diese url. The Content-Typ Anwendung x-www-Form-urlencodiert ist ineffizient für das Senden großer Mengen von binären Daten oder Text mit Nicht-ASCII Zeichen Der Inhaltstyp Multipart Formulardaten sollten für die Übermittlung von Formularen verwendet werden, die Dateien, Nicht-ASCII-Daten und Binärdaten enthalten. Jedoch würde es für mich zu Werkzeugrahmenunterstützung kommen. Welche Werkzeuge und Frameworks erwarten Sie von Ihren API-Benutzern Um ihre apps with. Do sie haben Frameworks oder Komponenten, die sie verwenden können, bevorzugen eine Methode über die andere. Wenn Sie eine klare Vorstellung von Ihren Benutzern erhalten und wie sie nutzen Sie Ihre API, dann wird das Ihnen helfen zu entscheiden Wenn du den Upload von Dateien hart für deine API-Benutzer machst, dann verschwinden sie weg, von dir verbringst du viel Zeit damit, sie zu unterstützen. Zweitens wäre das die Werkzeugunterstützung, die du für das Schreiben deiner API hast und wie einfach es ist Sie können einen Upload-Mechanismus über den anderen aufnehmen. Just ein kleiner Hinweis von meiner Seite für das Hochladen von HTML5 Leinwand Bilddaten. Ich arbeite an einem Projekt für eine Druckerei und hatte einige Probleme durch das Hochladen von Bildern auf den Server, der von einem kam HTML5 Leinwand Element Ich kämpfte für Le Ast eine Stunde und ich habe es nicht bekommen, um das Bild richtig auf meinem Server zu speichern. Once ich setze die contentType-Option von meinem jQuery ajax Aufruf zur Anwendung x-www-Form-urlencodiert alles ging der richtige Weg und die base64-codierten Daten war Korrekt interpretiert und erfolgreich als Bild gespeichert. Möglicherweise hilft das jemand.7 2 Der Multipart Content-Type. Im Fall von mehrteiligen Meldungen, bei denen ein oder mehrere verschiedene Sätze von Daten in einem einzigen Körper kombiniert werden, wird ein multipart Content - Typ-Feld muss im Entity-Header erscheinen Der Körper muss dann ein oder mehrere Körperteile enthalten, von denen jeder eine Kapselungsgrenze vorangestellt ist, und der letzte, gefolgt von einer Schlussgrenze Jeder Teil beginnt mit einer Kapselungsgrenze und enthält dann einen Körperteil Bestehend aus Kopfbereich, einer Leerzeile und einem Körperbereich So ist ein Körperteil ähnlich einer RFC 822 Nachricht in der Syntax, aber anders in der Bedeutung. Ein Körperteil ist nicht zu interpretieren, da tatsächlich eine RFC 822 Nachricht zu Beginn ist , KEINE Header-Felder sind ac In Karosserieteilen erforderlich Ein Körperteil, das mit einer Leerzeile beginnt, ist daher erlaubt und ist ein Körperteil, für den alle Defaultwerte angenommen werden sollen. In einem solchen Fall bedeutet das Fehlen eines Content-Type Header-Feldes, dass die Kapselung ist einfacher US-ASCII-Text Die einzigen Header-Felder, die für Körperteile definiert haben, sind diejenigen, deren Namen mit dem Inhalt beginnen. Alle anderen Header-Felder sind in der Regel in Körperteilen ignoriert Obwohl sie in der Regel in der Postbearbeitung beibehalten werden sollten, Sie können ggf. von Gateways verworfen werden. Diese anderen Felder dürfen in Körperteilen erscheinen, sollten aber nicht von X-Feldern abhängig sein, die für experimentelle oder private Zwecke geschaffen werden können, mit der Erkenntnis, dass die darin enthaltenen Informationen an einigen Gateways verloren gehen können . Die Unterscheidung zwischen einer RFC 822-Nachricht und einem Körperteil ist subtil, aber wichtig Ein Gateway zwischen Internet und X 400 Mail muss zum Beispiel in der Lage sein, den Unterschied zwischen einem Körperteil zu erzählen, der con Ein Bild und einen Körperteil, der eine verkapselte Nachricht enthält, deren Körper ein Bild ist. Um das letztere zu repräsentieren, muss der Körperteil eine Inhalt-Typ-Nachricht haben und sein Körper nach der Leerzeile muss die eingekapselte Nachricht sein, Mit dem eigenen Content-Type-Image-Header-Feld Die Verwendung einer ähnlichen Syntax erleichtert die Umwandlung von Meldungen an Körperteile und umgekehrt, aber die Unterscheidung zwischen den beiden muss von Implementoren verstanden werden Für den Spezialfall, in dem alle Teile tatsächlich Nachrichten sind, Ein Digest-Subtyp ist ebenfalls definiert. Wie bereits erwähnt, ist jedem Körperteil eine Kapselungsgrenze vorausgegangen. Die Kapselungsgrenze darf NICHT in irgendwelchen der verkapselten Teile erscheinen. Daher ist es entscheidend, dass der Komponiermittel in der Lage ist, die eindeutige Grenze zu wählen und anzugeben Das wird die Teile trennen. Alle vorhandenen und zukünftigen Subtypen des Multipart-Typs müssen eine identische Syntax verwenden. Subtypen können sich in ihrer Semantik unterscheiden und können zusätzliche Einschränkungen erfordern Auf Syntax, muss aber mit der erforderlichen Syntax für den Multipart-Typ übereinstimmen. Diese Anforderung stellt sicher, dass alle konformen Benutzeragenten zumindest in der Lage sind, die Teile einer beliebigen Multipart-Einheit zu erkennen und zu trennen, auch von einem nicht erkannten Subtyp Das Content-Transfer-Encoding-Feld, keine Kodierung außer 7bit, 8bit oder binär ist für Entitäten vom Typ multipart erlaubt Die Multipart Trennzeichen und Header-Felder sind in jedem Fall immer 7-Bit-ASCII und Daten innerhalb der Körperteile können codiert werden Auf einer Teil-zu-Teil-Basis, mit Content-Transfer-Encoding-Felder für jeden entsprechenden Körperteil. Mail-Gateways, Relays und andere Mail-Handling-Agenten sind allgemein bekannt, um die Top-Level-Header einer RFC 822 Nachricht zu ändern, Sie häufig hinzufügen, entfernen oder neu anordnen header Felder Diese Änderungen sind explizit verboten für die Körperteil-Header eingebettet in die Körper von Nachrichten vom Typ multipart. All Subtypen von multipart teilen eine gemeinsame Syntax, definiert in Dieser Abschnitt Ein einfaches Beispiel für eine Multipart-Meldung erscheint auch in diesem Abschnitt Ein Beispiel für eine komplexere Multipart-Nachricht ist in Anhang C angegeben. Das Feld "Content-Type" für multipart-Entitäten benötigt einen Parameter, eine Grenze, mit der die Kapselungsgrenze angegeben wird Die Kapselungsgrenze ist definiert als eine Linie, die ausschließlich aus zwei Bindestrichzeichen besteht -, Dezimalcode 45, gefolgt von dem Grenzparameterwert aus dem Content-Type-Header-Feld. HINWEIS Die Bindestriche sind für grobe Kompatibilität mit der früheren RFC 934-Methode der Nachrichtenverkapselung, Und für die Leichtigkeit der Suche nach den Grenzen in einigen Implementierungen Allerdings ist es zu beachten, dass Multipart-Nachrichten sind nicht vollständig kompatibel mit RFC 934 Verkapselungen im Besonderen, sie nicht gehorchen RFC 934 Zitieren Konventionen für eingebettete Linien, die mit Bindestrichen beginnen Dieser Mechanismus wurde gewählt Über die RFC 934 Mechanismus, weil die letzteren Linien Linien mit jedem Niveau der Zitat wachsen Die Kombination von th Ist das Wachstum mit der Tatsache, dass SMTP-Implementierungen manchmal wickeln lange Linien gemacht die RFC 934-Mechanismus ungeeignet für den Einsatz in dem Fall, dass tief verschachtelte Multipart-Strukturierung ist immer gewünscht. Thus, ein typisches multipart Content-Type Header-Feld könnte so aussehen Dies bedeutet, dass Besteht die Entität aus mehreren Teilen, die jeweils eine Struktur haben, die syntaktisch identisch mit einer RFC 822-Nachricht ist, mit der Ausnahme, dass der Header-Bereich vollständig leer sein könnte und dass den Teilen jeweils die Zeile vorangestellt ist. Beachten Sie, dass die Kapselungsgrenze auftreten muss Der Anfang einer Zeile, dh nach einer CRLF, und dass diese Anfangs-CRLF als Teil der Kapselungsgrenze und nicht als Teil des vorhergehenden Teils betrachtet wird. Die Grenze muss sofort entweder durch eine andere CRLF und die Header-Felder für den nächsten Teil verfolgt werden , Oder durch zwei CRLFs, in welchem Fall es keine Header-Felder für den nächsten Teil gibt und es wird daher angenommen, dass es von Content-Type Text plain. NOTE Das CRLF vor dem Einkapselungslinie gilt als Teil der Grenze, so dass es möglich ist, einen Teil zu haben, der nicht mit einem CRLF-Zeilenumbruch endet. Körperteile, die mit Zeilenumbrüchen zu beurteilen sind, sollten daher zwei CRLFs vor der Kapselungslinie haben Von denen der erste Teil des vorangehenden Körperteils ist und der zweite davon Teil der Kapselungsgrenze ist. Die Anforderung, dass die Einkapselungsgrenze mit einer CRLF beginnt, impliziert, dass der Körper einer multipartigen Einheit selbst mit einem CRLF vor dem ersten beginnen muss Einkapselungslinie - das heißt, wenn der Präambelbereich nicht verwendet wird, müssen die Entity-Header von TWO CRLFs gefolgt werden. Dies ist in der Tat, wie solche Entitäten zusammengesetzt werden sollten. Ein tolerantes Mail-Leseprogramm kann jedoch ein Leerzeichen des Typs multipart interpretieren, das beginnt Mit einer Einkapselungslinie, die nicht von einer CRLF als auch eine Kapselungsgrenze initiiert wird, aber ein kompatibles Mail-Sending-Programm darf solche Entitäten nicht erzeugen. Einkapselungsgrenzen dürfen nicht erscheinen Hin die Einkapselungen und darf nicht länger als 70 Zeichen sein, ohne die beiden führenden Bindestriche zu zählen. Die Kapselungsgrenze nach dem letzten Körperteil ist ein hervorgehobenes Trennzeichen, das anzeigt, dass keine weiteren Körperteile folgen werden. Ein solches Trennzeichen ist identisch mit den vorherigen Trennzeichen , Mit dem Zusatz von zwei weiteren Bindestrichen am Ende der Zeile Es scheint Raum für zusätzliche Informationen vor der ersten Kapselungsgrenze und nach der endgültigen Grenze Diese Bereiche sollten in der Regel leer gelassen werden, und Implementierungen sollten alles ignorieren, was vor dem erscheint Erste Grenze oder nach dem letzten. NOTE Diese Präambel - und Epilog-Bereiche werden wegen des Mangels an richtiger Eingabe dieser Teile und des Mangels an klarer Semantik für die Handhabung dieser Bereiche an Gateways, insbesondere X 400 Gateways, nicht verwendet. NOTE Weil Kapselungsgrenzen müssen Nicht in den Körperteilen, die eingekapselt sind, erscheinen, muss ein Benutzer-Agent Pflege Pflege, um eine einzigartige Grenze Die Grenze zu wählen Ry im obigen Beispiel könnte das Ergebnis eines Algorithmus gewesen sein, der entworfen wurde, um Grenzen mit einer sehr geringen Wahrscheinlichkeit zu schaffen, bereits vorhanden in den zu verkapselnden Daten zu existieren, ohne die Daten vorab zu archivieren. Alternativen Algorithmen können zu mehr lesbaren Grenzen für einen Empfänger mit einem führen Alte Benutzer-Agent, aber würde mehr Aufmerksamkeit auf die Möglichkeit, dass die Grenze könnte in den verkapselten Teil erscheinen Die einfachste Grenze möglich ist etwas wie ---, mit einer Schlussgrenze von. Als ein sehr einfaches Beispiel hat die folgende Multipart-Nachricht zwei Teile, beide von ihnen Klartext, einer von ihnen explizit eingegeben und einer von ihnen implizit typisiert Die Verwendung eines Content-Typs von Multipart in einem Körperteil in einer anderen multipart-Einheit ist ausdrücklich erlaubt In solchen Fällen muss aus offensichtlichen Gründen Pflege sein Um sicherzustellen, dass jede verschachtelte Multipart-Entität ein anderes Begrenzungsbegrenzer verwenden muss. Siehe Anhang C für ein Beispiel für verschachtelte Multipart-Entitäten. Die Verwendung des Multipart-Conte Nt-Typ mit nur einem einzelnen Körperteil kann in bestimmten Kontexten nützlich sein und ist ausdrücklich erlaubt. Der einzige obligatorische Parameter für den multipart Content-Type ist der Randparameter, der aus 1 bis 70 Zeichen aus einem Satz von Zeichen besteht, die bekannt sind Sei sehr robust durch E-Mail-Gateways und NICHT endet mit Leerraum Wenn eine Grenze mit Leerraum zu beenden scheint, muss angenommen werden, dass der Leerraum von einem Gateway hinzugefügt wurde und gelöscht werden soll. Es wird formell durch das folgende BNF Overall spezifiziert , Kann der Körper einer multipartigen Einheit wie folgt spezifiziert werden ANMERKUNG Auffällig fehlt der Multipart-Typ ist ein Begriff der strukturierten, verwandten Körperteile Im Allgemeinen scheint es verfrüht zu versuchen, die Zwischenstruktur zu standardisieren. Es wird empfohlen, dass diejenigen, die eine Mehr strukturierte oder integrierte Multipart-Messaging-Einrichtung sollte einen Subtyp von Multipart definieren, der syntaktisch identisch ist, aber das erwartet immer die Einbeziehung eines ausgeprägten Teils, der ca N verwendet werden, um die Struktur und die Integration der anderen Teile zu spezifizieren, die vermutlich auf sie durch ihr Content-ID-Feld verweisen. Wenn dieser Ansatz verwendet wird, werden andere Implementierungen den neuen Subtyp nicht erkennen, sondern ihn als primärer Subtyp multipart gemischt und behandeln Wird somit dem Benutzer die erkannten Teile zeigen können. 2 2 Der Multipart gemischte primäre Subtyp. Der primäre Subtyp für Multipart, gemischt, ist für den Einsatz bestimmt, wenn die Körperteile unabhängig sind und seriell angezeigt werden sollen. Multipart Subtypen Dass eine Implementierung nicht erkannt wird, sollte als Subtyp gemischt werden.7 2 3 Der Multipart-Alternativ-Subtyp. Der Multipart-Alternativtyp ist syntaktisch identisch mit Multipart gemischt, aber die Semantik ist unterschiedlich. Insbesondere ist jedes der Teile eine alternative Version Der gleichen Information Benutzer-Agenten sollten erkennen, dass der Inhalt der verschiedenen Teile austauschbar ist Der User-Agent sollte entweder die beste Art auf der Grundlage von wählen Die Benutzerumgebung und die Vorlieben, oder dem Benutzer die verfügbaren Alternativen anbieten Im Allgemeinen wählt die Wahl des besten Typs nur die Anzeige des LETZTEN Teils, der angezeigt werden kann. Dies kann z. B. verwendet werden, um E-Mails in einem ausgefallenen Textformat zu senden Weg, dass es überall leicht angezeigt werden kann In diesem Beispiel, Benutzer, deren Mail-System verstanden den Text x-was auch immer Format würde nur die fancy Version sehen, während andere Benutzer sehen würde nur die Richtext oder Klartext-Version, abhängig von den Fähigkeiten ihres Systems . Im Allgemeinen sollten Benutzeragenten, die mehrteilige alternative Entitäten zusammenstellen, die Körperteile in eine zunehmende Reihenfolge der Präferenz bringen, dh mit dem bevorzugten Format last Für fancy Text sollte der sendende Benutzeragent zuerst das schlichte Format und das reichste Format letztes Empfangen setzen User-Agenten sollten das letzte Format auswählen und anzeigen, das sie anzeigen können. In dem Fall, in dem eine der Alternativen selbst vom Typ multipart ist und unerkannte Sub-Par enthält Ts, kann der User-Agent entweder wählen, um diese Alternative, eine frühere Alternative oder beides zu zeigen. HINWEIS Aus der Sicht eines Implementators scheint es sinnvoller, diese Reihenfolge umzukehren und die schönste Alternative zu haben. Allerdings ist die Platzierung der einfachsten Alternative zuerst Ist die freundlichstmögliche Option, wenn mutlipart alternative Entitäten mit einem nicht-MIME-kompatiblen Mail-Reader betrachtet werden. Während dieser Ansatz eine gewisse Belastung für konforme Mail-Leser auferlegt, wurde die Interoperabilität mit älteren Mail-Lesern in diesem Fall als wichtiger angesehen Der Fall, dass einige Benutzer-Agenten, wenn sie mehr als eines der Formate erkennen können, dem Benutzer die Wahl wünschen, welches Format zu sehen ist. Das macht zB Sinn, wenn Mail sowohl eine schön formatierte Bildversion als auch eine Leicht editierbare Textversion Was am meisten kritisch ist, ist jedoch, dass der Benutzer nicht automatisch mehrere Versionen derselben Daten angezeigt wird. Entweder sollte der Benutzer die zuletzt erkannte Version zeigen Sion oder sollte explizit die Wahl gegeben werden.7 2 4 Der Multipart-Digest-Subtyp. Dieses Dokument definiert einen Digest-Subtyp des Multipart-Content-Type Dieser Typ ist syntaktisch identisch mit Multipart gemischt, aber die Semantik ist anders Besonders in einem Digest, Wird der Standard-Content-Type-Wert für einen Body-Part von Text-Ebene in Nachricht rfc822 geändert. Dies geschieht, um ein besser lesbares Digest-Format zuzulassen, das weitgehend kompatibel ist, mit Ausnahme der zitierten Konvention mit RFC 934.A Digest in diesem Format könnte dann, Schauen Sie so etwas wie dieses. Dieses Dokument definiert einen parallelen Subtyp des multipart Content-Type Dieser Typ ist syntaktisch identisch mit multipart gemischt, aber die Semantik ist unterschiedlich. Insbesondere in einer parallelen Entität sollen alle Teile parallel dargestellt werden , Dh gleichzeitig auf Hardware und Software, die dazu in der Lage sind, dass Komponiermittel sich bewusst sein sollten, dass viele Mail-Leser diese Fähigkeit fehlen und die Teile seriell in jedem e zeigen werden Vent. Add einen vorhandenen Inhaltstyp zu einer Liste oder Bibliothek. Applies To SharePoint Server 2007 SharePoint Server 2007 Zentraladministration Windows SharePoint Services 3 0 Mehr Less. Content-Typen ermöglichen es Unternehmen, Inhalte effektiver zu organisieren, zu verwalten und zu verwalten Definieren von Inhaltstypen für bestimmte Arten von Dokumenten oder Informationsprodukten, eine Organisation kann sicherstellen, dass jede dieser Gruppen von Inhalten effektiver verwaltet wird. Sie können eine Liste oder eine Bibliothek einrichten, um Elemente von mehreren Elementtypen oder Dokumenttypen durch Hinzufügen von Inhaltstypen zu enthalten Die Liste oder die Bibliothek. In diesem Artikel. How Inhaltstypen arbeiten in Listen und Bibliotheken. Wenn Sie eine Liste oder Bibliothek, die eingerichtet ist, um mehrere Inhaltstypen zu ermöglichen, können Sie Inhaltstypen zu dieser Liste oder Bibliothek aus der Gruppe der Website hinzufügen Inhaltstypen, die für Ihre Website verfügbar sind. Wenn Sie einen Inhaltstyp zu einer Liste oder Bibliothek hinzufügen, können Sie diese Liste oder Bibliothek dazu veranlassen, Elemente dieses Typs zu enthalten Befehl in dieser Liste oder Bibliothek ermöglicht es Benutzern, neue Elemente dieses Typs zu erstellen Einer der wichtigsten Vorteile von Inhaltstypen für Listen und Bibliotheken ist, dass sie es möglich machen, dass eine einzelne Liste oder Bibliothek mehrere Artikel - oder Dokumenttypen enthält, von denen jeder auch sein kann Haben einzigartige Metadaten, Richtlinien oder Verhaltensweisen. Wie Vererbung funktioniert für Listeninhaltstypen. Wenn eine Instanz eines Site-Inhaltstyps zu einer Liste oder Bibliothek hinzugefügt wird, ist es ein Kind des Site-Inhaltstyps, von dem es erstellt wurde. Dieser Listeninhalt Typ erbt alle Attribute des übergeordneten Inhaltstyps, z. B. seine Vorlagenvorlage, schreibgeschützte Einstellungen, Workflows und Spalten. Ein Listeninhaltstyp kann für die spezifische Liste oder Bibliothek, zu der sie hinzugefügt wurde, angepasst werden. Diese Anpassungen sind Nicht auf den Inhaltstyp der übergeordneten Website angewendet Wenn der Inhaltstyp der übergeordneten Website für einen Listeninhaltstyp aktualisiert wird, kann der untergeordnete Inhaltstyp diese Änderungen erben, wenn die Person, die den Inhalt der Website beibehält, Pflegen die Updates für alle untergeordneten Inhaltstypen. Wenn eine der Attribute, die die untergeordneten Inhaltstypfreigaben mit dem übergeordneten Standortsinhaltstyp für den Listeninhaltstyp angepasst haben, diese Anpassungen überschrieben werden können, wenn der Listeninhaltstyp die Änderungen vom Inhaltstyp des übergeordneten Standorts Wenn der Listeninhaltstyp mit zusätzlichen Attributen angepasst wurde, die der übergeordnete Inhaltstyp nicht zum Beispiel zusätzliche Spalten enthält, werden diese Anpassungen nicht überschrieben, wenn der untergeordnete Listeninhaltstyp Änderungen vom übergeordneten Inhaltstyp erbt Ein Inhaltstyp zu einer Liste oder Bibliothek. Before Inhaltstypen können zu einer Liste oder Bibliothek hinzugefügt werden, zuerst muss die Liste oder Bibliothek eingerichtet werden, um mehrere Inhaltstypen zu ermöglichen Weitere Informationen zum Aktivieren der Unterstützung für mehrere Inhaltstypen in einer Liste oder Bibliothek , Siehe Unterstützung für mehrere Inhaltstypen in einer Bibliothek aktivieren Um Content-Typen einer Liste oder Bibliothek hinzuzufügen, müssen Sie mindestens die Entwurfsberechtigungsstufe dafür haben Liste oder Bibliothek. Wenn die Liste oder Bibliothek nicht bereits geöffnet ist, klicken Sie auf den Namen auf dem Schnellstart. Wenn der Name Ihrer Liste oder Bibliothek nicht angezeigt wird, klicken Sie auf Alle Seiteninhalte anzeigen und dann auf den Namen Ihrer Liste oder Bibliothek klicken. Klicken Sie im Einstellungsmenü auf Einstellungen Einstellungen oder klicken Sie auf die Einstellungen für den Typ der Bibliothek, die Sie öffnen. Zum Beispiel in einer Dokumentbibliothek klicken Sie auf Dokumentbibliothek Einstellungen. Unter Inhaltstypen klicken Sie auf Hinzufügen aus vorhandenen Website Inhalt types. Note Wenn die Liste oder Bibliothek ist nicht eingerichtet, um mehrere Inhaltstypen zuzulassen, wird der Abschnitt "Inhaltstypen" nicht auf der Seite "Anpassen" für die Liste oder die Bibliothek angezeigt. Im Abschnitt "Inhaltstyp auswählen" wählen Sie in der Liste "Inhaltstypen auswählen" die Liste aus Gruppe von Site-Content-Typen, von denen Sie auswählen möchten. In der Liste Verfügbare Website-Inhaltstypen klicken Sie auf den gewünschten Inhaltstyp und klicken Sie dann auf Hinzufügen, um den ausgewählten Inhaltstyp in die Inhaltstypen zu verschieben, um eine Liste hinzuzufügen. Um zusätzlichen Inhalt hinzuzufügen Typen, wiederholen T Schritte 4 und 5.Wenn Sie die Auswahl aller Inhaltstypen beenden, die Sie hinzufügen möchten, klicken Sie auf OK. Ändern Sie die Schaltfläche Neue Schaltfläche oder den Standard-Inhaltstyp. Sie können die Reihenfolge angeben, in der die Inhaltstypen auf der Schaltfläche Neu angezeigt werden Eine Liste oder Bibliothek Standardmäßig wird der erste Inhaltstyp, der auf der Schaltfläche Neu angezeigt wird, zum Standard-Inhaltstyp für die Liste oder Bibliothek. Um den Standard-Inhaltstyp für die Liste oder Bibliothek zu ändern, ändern Sie den Inhaltstyp, der zuerst angezeigt wird Neue Schaltfläche Sie können auch festlegen, ob die Inhaltstypen, die einer Liste oder Bibliothek hinzugefügt wurden, auf der Schaltfläche Neu angezeigt werden sollen. Wenn die Liste oder die Bibliothek, für die Sie einen Inhaltstyp ändern möchten, noch nicht geöffnet ist, klicken Sie auf den Namen Unter Lists oder Documents auf dem Quick Launch. On das Menü "Einstellungen" führen Sie einen der folgenden Schritte aus. Wenn Sie in einer Liste arbeiten, klicken Sie auf List Settings. Wenn Sie in einer Dokumentbibliothek arbeiten, klicken Sie auf Document Library Settings. Unter Content Typees klicken Sie auf Change Neue Schaltflächenbestellung Und Standard-Inhalt type. Note Wenn die Liste oder Bibliothek nicht eingerichtet ist, um mehrere Inhaltstypen zuzulassen, wird der Abschnitt "Inhaltstypen" nicht auf der Seite "Anpassen" für die Liste oder Bibliothek angezeigt. Im Abschnitt "Inhaltstyp" können Sie einen der folgenden Schritte ausführen . Um einen Inhaltstyp aus der Schaltfläche Neu für die Liste oder Bibliothek zu entfernen, deaktivieren Sie das Kontrollkästchen Sichtbar. Dieses Kontrollkästchen ist standardmäßig ausgewählt. Um die Reihenfolge zu ändern, in der ein Inhaltstyp auf der Schaltfläche Neu angezeigt wird, klicken Sie auf den Pfeil neben diesem Content-Typ in der Position aus der oberen Spalte, und wählen Sie dann die Bestellnummer, die Sie wollen. Content-Typ binäre Option. Sie wählen Sie ein Asset und dann vorhersagen, ob Sie denken, dass Asset wird nach oben oder unten im Preis Sie wählen dann die Menge, die Sie Wollen riskieren und die Zeitrahmen, mit denen Sie arbeiten möchten in Content-Typ binäre Option Wie man Geld verdienen auf 3D-Modellierung 9. August 2016 Es gibt eine Reihe von Arten von binären Optionen und dieser Artikel entwickeln ein System des Handels, dass der Prozentsatz erhöht Wi Nning trades to a Sobald diese grundlegenden Faktoren alle berücksichtigt werden, klicken Sie auf die Schaltfläche, die den Handel ausführt Sie wurden in den Vereinigten Staaten im Jahr 2008 legalisiert und haben sich schnell zu einem der einfachsten und schnellsten Möglichkeiten zum Handel Dies könnte bedrohlich klingen , Aber sie sind wirklich ganz einfach zu verstehen. 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The Accept request-header field can be used to specify certain media types which are acceptable for the response Accept headers can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image. The asterisk character is used to group media types into ranges, with indicating all media types and type indicating all subtypes of that type The media-range MAY include media type parameters that are applicable to that range. Each media-range MAY be followed by one or more accept-params, beginning with the q parameter for indicating a relative quality factor The first q parameter if any separates the media-range parameter s from the accept-params Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the qvalue scale from 0 to 1 section 3 9 The default value is q 1.SHOULD be interpreted as I prefer audio basic, but send me any audio type if it is the best available after an 80 mark-down in quality. If no Accept header field is present, then it is assumed that the client accepts all media types If an Accept header field i s present, and if the server cannot send a response which is acceptable according to the combined Accept field value, then the server SHOULD send a 406 not acceptable response. A more elaborate example is. Verbally, this would be interpreted as text html and text x-c are the preferred media types, but if they do not exist, then send the text x-dvi entity, and if that does not exist, send the text plain entity. Media ranges can be overridden by more specific media ranges or specific media types If more than one media range applies to a given type, the most specific reference has precedence For example. have the following precedence. The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type For example. would cause the following values to be associated.14 2 Accept-Charset. The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response This field allo ws clients capable of understanding more comprehensive or special - purpose character sets to signal that capability to a server which is capable of representing documents in those character sets. Character set values are described in section 3 4 Each charset MAY be given an associated quality value which represents the user s preference for that charset The default value is q 1 An example is. The special value , if present in the Accept-Charset field, matches every character set including ISO-8859-1 which is not mentioned elsewhere in the Accept-Charset field If no is present in an Accept-Charset field, then all character sets not explicitly mentioned get a quality value of 0, except for ISO-8859-1, which gets a quality value of 1 if not explicitly mentioned. If no Accept-Charset header is present, the default is that any character set is acceptable If an Accept-Charset header is present, and if the server cannot send a response which is acceptable according to the Accept-Charset header, then the server SHOULD send an error response with the 406 not acceptable status code, though the sending of an unacceptable response is also allowed.14 3 Accept-Encoding. The Accept-Encoding request-header field is similar to Accept, but restricts the content-codings section 3 5 that are acceptable in the response. Examples of its use are. A server tests whether a content-coding is acceptable, according to an Accept-Encoding field, using these rules. If an Accept-Encoding field is present in a request, and if the server cannot send a response which is acceptable according to the Accept-Encoding header, then the server SHOULD send an error response with the 406 Not Acceptable status code. If no Accept-Encoding field is present in a request, the server MAY assume that the client will accept any content coding In this case, if identity is one of the available content-codings, then the server SHOULD use the identity content-coding, unless it has additional information that a different content - coding is meaningful to the client.14 4 Accept-Language. The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request Language tags are defined in section 3 10.Each language-range MAY be given an associated quality value which represents an estimate of the user s preference for the languages specified by that range The quality value defaults to q 1 For example. would mean I prefer Danish, but will accept British English and other types of English A language-range matches a language-tag if it exactly equals the tag, or if it exactly equals a prefix of the tag such that the first tag character following the prefix is - The special range , if present in the Accept-Language field, matches every tag not matched by any other range present in the Accept-Language field. The language quality factor assigned to a language-tag by the Accept-Language field is the quality value of the longest language - range i n the field that matches the language-tag If no language - range in the field matches the tag, the language quality factor assigned is 0 If no Accept-Language header is present in the request, the server. SHOULD assume that all languages are equally acceptable If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are acceptable. It might be contrary to the privacy expectations of the user to send an Accept-Language header with the complete linguistic preferences of the user in every request For a discussion of this issue, see section 15 1 4.As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user If the choice is not made available, then the Accept-Language header field MUST NOT be given in the request.14 5 Accept-Ranges.14 8 Authorization.14 9 Cache-Control. The Cache-Control general-header field is used to specify directi ves that MUST be obeyed by all caching mechanisms along the request response chain The directives specify behavior intended to prevent caches from adversely interfering with the request or response These directives typically override the default caching algorithms 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. 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 specify a cache - directive for a specific cache. When a directive appears without any 1 field-name parameter, the directive applies to the entire request or response When such a directive appears with a 1 field-name parameter, it applies only to the named field or fields, and not to the rest of the request or response This mechanism sup ports extensibility implementations of future versions of the protocol might apply these directives to header fields not defined in. The cache-control directives can be broken down into these general categories.14 9 1 What is Cacheable. By default, a response is cacheable if the requirements of the request method, request header fields, and the response status indicate that it is cacheable Section 13 4 summarizes these defaults for cacheability The following Cache-Control response directives allow an origin server to override the default cacheability of a response. public Indicates that the response MAY be cached by any cache, even if it would normally be non-cacheable or cacheable only within a non - shared cache See also Authorization, section 14 8 for additional details private Indicates that all or part of the response message is intended for a single user and MUST NOT be cached by a shared cache This allows an origin server to state that the specified parts of the response are intende d for only one user and are not a valid response for requests by other users A private non-shared cache MAY cache the response. Note This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content. no-cache If the no-cache directive does not specify a field-name, then a cache MUST NOT use the response to satisfy a subsequent request without successful revalidation with the origin server This allows an origin server to prevent caching even by caches that have been configured to return stale responses to client requests If the no-cache directive does specify one or more field-names, then a cache MAY use the response to satisfy a subsequent request, subject to any other restrictions on caching However, the specified field-name s MUST NOT be sent in the response to a subsequent request without successful revalidation with the origin server This allows an origin server to prevent the re-use of certain header fields in a respo nse, while still allowing caching of the rest of the response. Note Most caches will not recognize or obey this directive.14 9 2 What May be Stored by Caches. no-store The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information for example, on backup tapes The no-store directive applies to the entire message, and MAY be sent either in a response or in a request If sent in a request, a cache MUST NOT store any part of either this request or any response to it If sent in a response, a cache MUST NOT store any part of either this response or the request that elicited 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 Even when this directive is associated with a response, users m ight explicitly store such a response outside of the caching system e g with a Save As dialog History buffers MAY store such responses as part of their normal operation The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures While the use of this directive might improve privacy in some cases, we caution that it is NOT in any way 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.14 9 3 Modifications of the Basic Expiration Mechanism. The expiration time of an entity MAY be specified by the origin server using the Expires header see section 14 21 Alternatively, it MAY be specified using the max-age directive in a response When the max-age cache-control directive is present in a cached response, the response is stale if its current age is greater than the age value given in seconds at the time of a new request for that resource The max-age directive on a response implies that the response is cacheable i e public unless some other, more restrictive cache directive is also present. If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive This rule allows an origin server to provide, for a given response, a longer expiration time to an or later cache than to an cache This might be useful if certain caches improperly calculate ages or expiration times, perhaps due to desynchronized clocks. Many cache implementations will treat an Expires value that is less than or equal to the response Date value as being equivalent to the Cache-Control response directive no-cache If an cache receives such a response, and the response does not include a Cache-Control h eader field, it SHOULD consider the response to be non-cacheable in order to retain compatibility with servers. Note An origin server might wish to use a relatively new cache control feature, such as the private directive, on a network including older caches that do not understand that feature The origin server will need to combine the new feature with an Expires field whose value is less than or equal to the Date value This will prevent older caches from improperly caching the response. s-maxage If a response includes an s-maxage directive, then for a shared cache but not for a private cache , 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 directive see section 14 9 4 , i e that the shared cache must not use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server T he s - maxage directive is always ignored by a private cache. Note that most older caches, not compliant with this specification, do not implement any cache-control directives An origin server wishing to use a cache-control directive that restricts, but does not prevent, caching by an cache MAY exploit the requirement that the max-age directive overrides the Expires header, and the fact that caches do not observe the max-age directive. Other directives allow a user agent to modify the basic expiration mechanism These directives MAY be specified on a request. max-age Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds Unless max - stale directive is also included, the client is not willing to accept a stale response min-fresh 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 max-stale 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. If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured to override the expiration time of a response, the cache MUST attach a Warning header to the stale response, using Warning 110 Response is stale. A cache MAY be configured to return stale responses without validation, but only if this does not conflict with any MUST - level requirements concerning cache validation e g a must-revalidate cache-control directive. If both the new request and the cached entry include max-age directives, then the les ser of the two values is used for determining the freshness of the cached entry for that request.14 9 4 Cache Revalidation and Reload Controls. Sometimes a user agent might want or need to insist that a cache revalidate its cache entry with the origin server and not just with the next cache along the path to the origin server , or to reload its cache entry from the origin server End-to-end revalidation might be necessary if either the cache or the origin server has overestimated the expiration time of the cached response End-to-end reload may be necessary if the cache entry has become corrupted for some reason. End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we call it unspecified end-to-end revalidation , or when the client does have a local cached copy, in which case we call it specific end-to-end revalidation. The client can specify these three kinds of action using Cache - Control request directives. End-to-end reloa d The request includes a no-cache cache-control directive or, for compatibility with clients, Pragma no-cache Field names MUST NOT be included with the no-cache directive in a request The server MUST NOT use a cached copy when responding to such a request Specific end-to-end revalidation The request includes a max-age 0 cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server The initial request includes a cache-validating conditional with the client s current validator Unspecified end-to-end revalidation The request includes max-age 0 cache-control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server The initial request does not include a cache-validating conditional the first cache along the path if any that holds a cache entry for this resource includes a cache-validating conditional with its current valid ator max-age When an intermediate cache is forced, by means of a max-age 0 directive, to revalidate its own cache entry, and the client has supplied its own validator in the request, the supplied validator might differ from the validator currently stored with the cache entry In this case, the cache MAY use either validator in making its own request without affecting semantic transparency However, the choice of validator might affect performance The best approach is for the intermediate cache to use its own validator when making its request If the server replies with 304 Not Modified , then the cache can return its now validated copy to the client with a 200 OK response If the server replies with a new entity and cache validator, however, the intermediate cache can compare the returned validator with the one provided in the client s request, using the strong comparison function If the client s validator is equal to the origin server s, then the intermediate cache simply returns 304 Not Modified Otherwise, it returns the new entity with a 200 OK response If a request includes the no-cache directive, it SHOULD NOT include min-fresh, max-stale, or max-age only-if-cached In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses that it currently has stored, and not to reload or revalidate with the origin server To do this, the client may include the only-if-cached directive in a request If it receives this directive, a cache SHOULD either respond using a cached entry that is consistent with the other constraints of the request, or respond with a 504 Gateway Timeout status However, 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 must-revalidate Because a cache MAY be configured to ignore a server s specified expiration time, and because a client request MAY include a max - stale directive which has a sim ilar effect , the protocol also includes a mechanism for the origin server to require revalidation of a cache entry on any subsequent use When the must-revalidate directive is present in a response received by a cache, that cache MUST NOT use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server I e the cache MUST do an end-to-end revalidation every time, if, based solely on the origin server s Expires or max-age value, the cached response is stale The must-revalidate directive is necessary to support reliable operation for certain protocol features In all circumstances an 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 revalidate a request on the entity could result in incorrect operation, such as a silently unexecuted financ ial transaction Recipients MUST NOT take any automated action that violates this directive, and MUST NOT automatically provide an unvalidated copy of the entity if revalidation fails Although this is not recommended, user agents operating under severe connectivity constraints MAY violate this directive but, if so, MUST explicitly warn the user that an unvalidated response has been provided The warning MUST be provided on each unvalidated access, and SHOULD require explicit user confirmation proxy-revalidate The proxy-revalidate directive has the same meaning as the must - revalidate directive, except that it does not apply to non-shared user agent caches It can be used on a response to an authenticated request to permit the user s cache to store and later return the response without needing to revalidate it since it has already been authenticated once by that user , while still requiring proxies that service many users to revalidate each time in order to make sure that each user has bee n authenticated Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at all.14 9 5 No-Transform Directive. no-transform Implementors of intermediate caches proxies have found it useful to convert the media type of certain entity bodies A non - transparent proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link Serious operational problems occur, however, when these transformations are applied to entity bodies intended for certain kinds of applications For example, applications for medical imaging, scientific data analysis and those using end-to-end authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body Therefore, if a message includes the no-transform directive, an intermediate cache or proxy MUST NOT change those headers that are listed in section 13 5 2 as being subject to the no-trans form directive This implies that the cache or proxy MUST NOT change any aspect of the entity-body that is specified by these headers, including the value of the entity-body itself.14 9 6 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 assigned value Informational extensions those which do not require a change in cache behavior MAY 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 which 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 witho ut requiring changes to the base protocol. This extension mechanism depends on an cache obeying all of the cache-control directives defined for its native obeying certain extensions, and ignoring all directives that it does not understand. For example, consider a hypothetical new response directive called community which 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 which 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. 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 t o be unrecognized by an 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.14 10 Connection. The Connection general-header field allows the sender to specify options that are desired for that particular connection and MUST NOT be communicated by proxies over further connections. The Connection header has the following grammar. proxies MUST parse the Connection header field before a message is forwarded and, for each connection-token in this field, remove any header field s from the message with the same name as the connection-token Connection options are signaled by the presence of a connection-token in the Connection header field, not by any corresponding additional header field s , since the additional header field may not be sent if there are no parameters associated with that connection option. Message headers listed in the Connection header MUST NOT include end-to-end headers, such as Cache-Control. defines the close connection option for the sender to signal that the connection will be closed after completion of the response For example. in either the request or the response header fields indicates that the connection SHOULD NOT be considered persistent section 8 1 after the current request response is complete. applications that do not support persistent connections MUST include the close connection option in every message. A system receiving an or lower-version message that includes a Connection header MUST, for each connection-token in this field, remove and ignore any header field s from the message with the same name as the connection-token This protects against mistaken forwarding of such header fields by proxies See section 19 6 2.14 11 Content-Encoding. The Content-Encoding entity-header field is used as a modifier to the media-type When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms must be applied in order to obtain the media-type referenced by the Content-Type header field Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type. Content codings are defined in section 3 5 An example of its use is. The content-coding is a characteri stic of the entity identified by the Request-URI Typically, the entity-body is stored with this encoding and is only decoded before rendering or analogous usage However, a non-transparent proxy MAY modify the content-coding if the new coding is known to be acceptable to the recipient, unless the no-transform cache-control directive is present in the message. If the content-coding of an entity is not identity , then the response MUST include a Content-Encoding entity-header section 14 11 that lists the non-identity content-coding s used. If the content-coding of an entity in a request message is not acceptable to the origin server, the server SHOULD respond with a status code of 415 Unsupported Media Type. If multiple encodings have been applied to an entity, the content codings MUST be listed in the order in which they were applied Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification.14 12 Content-Language. The Content-Language entity-header field describes the natural language s of the intended audience for the enclosed entity Note that this might not be equivalent to all the languages used within the entity-body. Language tags are defined in section 3 10 The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user s own preferred language Thus, if the body content is intended only for a Danish-literate audience, the appropriate field is. If no Content-Language is specified, the default is that the content is intended for all language audiences This might mean that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended. Multiple languages MAY be listed for content that is intended for multiple audiences For example, a rendition of the Treaty of Waitangi, presented simultaneously in the original Maori and English versions, would call for. However, just be cause multiple languages are present within an entity does not mean that it is intended for multiple linguistic audiences An example would be a beginner s language primer, such as A First Lesson in Latin, which is clearly intended to be used by an English-literate audience In this case, the Content-Language would properly only include en. Content-Language MAY be applied to any media type -- it is not limited to textual documents.14 13 Content-Length. The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET. Applications SHOULD use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in section 4 4.Any Content-Length greater than or equal to zero is a valid value Section 4 4 describes how to determine the length of a message-body if a Content-Length is no t given. Note that the meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the message external-body content-type In it SHOULD be sent whenever the message s length can be determined prior to being transferred, unless this is prohibited by the rules in section 4 4.14 14 Content-Location. The Content-Location entity-header field MAY be used to supply the resource location for the entity enclosed in the message when that entity is accessible from a location separate from the requested resource s URI A server SHOULD provide a Content-Location for the variant corresponding to the response entity especially in the case where a resource has multiple entities associated with it, and those entities actually have separate locations by which they might be individually accessed, the server SHOULD provide a Content-Location for the particular variant which is returned. The value of Content-Location also defines the base URI for the entity. The Content-Location value is not a replacement for the original requested URI it is only a statement of the location of the resource corresponding to this particular entity at the time of the request Future requests MAY specify the Content-Location URI as the request - URI if the desire is to identify the source of that particular entity. A cache cannot assume that an entity with a Content-Location different from the URI used to retrieve it can be used to respond to later requests on that Content-Location URI However, the Content - Location can be used to differentiate between multiple entities retrieved from a single requested resource, as described in section 13 6.If the Content-Location is a relative URI, the relative URI is interpreted relative to the Request-URI. The meaning of the Content-Location header in PUT or POST requests is undefined servers are free to ignore it in those cases.14 15 Content-MD5.The Content-MD5 entity-header field, as defined in RFC 1864 2 3 is an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check MIC of the entity-body Note a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks. The Content-MD5 header field MAY be generated by an origin server or client to function as an integrity check of the entity-body Only origin servers or clients MAY generate the Content-MD5 header field proxies and gateways MUST NOT generate it, as this would defeat its value as an end-to-end integrity check Any recipient of the entity - body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received. The MD5 digest is computed based on the content of the entity-body, including any content-coding that has been applied, but not including any transfer-encoding applied to the message-body If the message is received with a transfer-encoding, that encoding MUST be remov ed prior to checking the Content-MD5 value against the received entity. This has the result that the digest is computed on the octets of the entity-body exactly as, and in the order that, they would be sent if no transfer-encoding were being applied. extends RFC 1864 to permit the digest to be computed for MIME composite media-types e g multipart and message rfc822 , but this does not change how the digest is computed as defined in the preceding paragraph. There are several consequences of this The entity-body for composite types MAY contain many body-parts, each with its own MIME and headers including Content-MD5, Content-Transfer-Encoding, and Content-Encoding headers If a body-part has a Content-Transfer - Encoding or Content-Encoding header, it is assumed that the content of the body-part has had the encoding applied, and the body-part is included in the Content-MD5 digest as is -- i e after the application The Transfer-Encoding header field is not allowed within body-parts. Conversion of all line breaks to CRLF MUST NOT be done before computing or checking the digest the line break convention used in the text actually transmitted MUST be left unaltered when computing the digest.14 16 Content-Range. The Content-Range entity-header is sent with a partial entity-body to specify where in the full entity-body the partial body should be applied Range units are defined in section 3 12.The header SHOULD indicate the total length of the full entity-body, unless this length is unknown or difficult to determine The asterisk character means that the instance-length is unknown at the time when the response was generated. Unlike byte-ranges-specifier values see section 14 35 1 , a byte - range-resp-spec MUST only specify one range, and MUST contain absolute byte positions for both the first and last byte of the range. A byte-content-range-spec with a byte-range-resp-spec whose last - byte-pos value is less than its first-byte-pos value, or whose instance-length value is less than or equal to its last-byte-pos value, is invalid The recipient of an invalid byte-content-range - spec MUST ignore it and any content transferred along with it. A server sending a response with status code 416 Requested range not satisfiable SHOULD include a Content-Range field with a byte-range - resp-spec of The instance-length specifies the current length of. the selected resource A response with status code 206 Partial Content MUST NOT include a Content-Range field with a byte-range - resp-spec of. Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes. When an message includes the content of a single range for example, a response to a request for a single range, or to a request for a set of ranges that overlap without any holes , this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred For example. When an message includes the content of multiple ranges for example, a response to a request for multiple non-overlapping ranges , these are transmitted as a multipart message The multipart media type used for this purpose is multipart byteranges as defined in appendix 19 2 See appendix 19 6 3 for a compatibility issue. A re sponse to a request for a single range MUST NOT be sent using the multipart byteranges media type A response to a request for multiple ranges, whose result is a single range, MAY be sent as a multipart byteranges media type with one part A client that cannot decode a multipart byteranges message MUST NOT ask for multiple byte-ranges in a single request. When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request. If the server ignores a byte-range-spec because it is syntactically invalid, the server SHOULD treat the request as if the invalid Range header field did not exist Normally, this means return a 200 response containing the full entity. If the server receives a request other than one including an If - Range request-header field with an unsatisfiable Range request - header field that is, all of whose byte-range-spec values have a first-byte-pos value greater than the current length of the selected resource , it SHOULD return a response code of 416 Requested range not satisfiable section 10 4 17.14 17 Content-Type. The Content-Type entity-header field indicates the media type of the entity-body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET. Media types are defined in section 3 7 An example of the field is. Further discussion of methods for identifying the media type of an entity is provided in section 7 2 1.The Date general-header field represents the date and time at which the message was originated, having the same semantics as orig-date in RFC 822 The field value is an as described in section 3 3 1 it MUST be sent in RFC 1123 8 - date format. Origin servers MUST include a Date header field in all responses, except in these cases. A received message that does not have a Date header field MUST be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires a Date An implementation without a clock MUST NOT cache responses without revalidating them on every use An cache, especially a shared cache, SHOULD use a mechanism, such as NTP 28 to synchronize its clock with a reliable external standard. Clients SHOULD only send a Date header field in messages that include an entity-body, as in the case of the PUT and POST requests, and even then it is optional A client without a clock MUST NOT send a Date header field in a request. The sent in a Date header SHOULD NOT represent a date and time subsequent to the generation of the message It SHOULD represent the best available approximation of the date and time of message generation, unless the implementation has no means of generating a reasonably accurate date and time In theory, the date ought to represent the moment just before the entity is generated In practice, the date can be generated at any time during the message origination without affecting its semantic value.14 18 1 Clockless Origin Serve r Operation. Some origin server implementations might not have a clock available An origin server without a clock MUST NOT assign Expires or Last - Modified values to a response, unless these values were associated with the resource by a system or user with a reliable clock It MAY assign an Expires value that is known, at or before server configuration time, to be in the past this allows pre-expiration of responses without storing separate Expires values for each resource. The ETag response-header field provides the current value of the entity tag for the requested variant The headers used with entity tags are described in sections 14 24 14 26 and 14 44 The entity tag MAY be used for comparison with other entities from the same resource see section 13 3 3.14 20 Expect. The Expect request-header field is used to indicate that particular server behaviors are required by the client. A server that does not understand or is unable to comply with any of the expectation values in the Expect field of a request MUST respond with appropriate error status The server MUST respond with a 417 Expectation Failed status if any of the expectations cannot be met or, if there are other problems with the request, some other 4xx status. This header field is defined with extensible syntax to allow for future extensions If a server receives a request containing an Expect field that includes an expectation-extension that it does not support, it MUST respond with a 417 Expectation Failed statusparison of expectation values is case-insensitive for unquoted tokens including the 100-continue token , and is case-sensitive for quoted-string expectation-extensions. The Expect mechanism is hop-by-hop that is, an proxy MUST return a 417 Expectation Failed status if it receives a request with an expectation that it cannot meet However, the Expect request-header itself is end-to-end it MUST be forwarded if the request is forwarded. Many older and applications do not understand the Expect header. See section 8 2 3 for the use of the 100 continue status.14 21 Expires. The Expires entity-header field gives the date time after which the response is considered stale A stale cache entry may not normally be returned by a cache either a proxy cache or a user agent cache unless it is first validated with the origin server or with an intermediate cache that has a fresh copy of the entity See section 13 2 for further discussion of the expiration 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 format is an absolute date and time as defined by in section 3 3 1 it MUST be in RFC 1123 date format. An example of its use is. clients and caches MUST treat other invalid date formats, especially including the value 0 , as in the past i e already expired. To mark a response as already expired, an origin server sends an Expires date that is equal to the Date header value See the rules for expiration calculations in section 13 2 4.To mark a response as never expires, an origin server sends an Expires date approximately one year from the time the response is sent servers SHOULD NOT send Expires dates more than one year in the future. The presence of an Expires header field with a date value of some time in the future on a response that otherwise would by default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field section 14 9.The From request-header field, if given, SHOULD contain an Internet e-mail address for the human user who controls the requesting user agent The address SHOULD be machine-usable, as defined by mailbox in RFC 822 9 as updated by RFC 1123 8.This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests It SHOULD NOT be used as an insecure form of access protection The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed In particular, robot agents SHOULD include this header so that the person responsible for running the robot can be contacted if problems occur on the receiving end. The Internet e-mail address in this field MAY be separate from the Internet host which issued the request For example, when a request is passed through a proxy the original issuer s address SHOULD be used. The client SHOULD NOT send the From header field without the user s approval, as it might conflict with the user s privacy interests or their site s security policy It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any t ime prior to a request. The Host request-header field specifies the Internet host and port number of the resource being requested, as obtained from the original URI given by the user or referring resource generally an URL. as described in section 3 2 2 The Host field value MUST represent the naming authority of the origin server or gateway given by the original URL This allows the origin server or gateway to differentiate between internally-ambiguous URLs, such as the root URL of a server for multiple host names on a single IP address. A host without any trailing port information implies the default port for the service requested e g 80 for an URL For example, a request on the origin server for would properly include. A client MUST include a Host header field in all request messages If the requested URI does not include an Internet host name for the service being requested, then the Host header field MUST be given with an empty value An proxy MUST ensure that any request message it forward s does contain an appropriate Host header field that identifies the service being requested by the proxy All Internet-based servers MUST respond with a 400 Bad Request status code to any request message which lacks a Host header field. See sections 5 2 and 19 6 1 1 for other requirements relating to Host.14 24 If-Match. The If-Match request-header field is used with a method to make it conditional A client that has one or more entities previously obtained from the resource can verify that one of those entities is current by including a list of their associated entity tags in the If-Match header field Entity tags are defined in section 3 11 The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource As a special case, the value matches any current entity of the resource. If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request without the If-Match header on that resource, or if is given. and any current entity exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist. A server MUST use the strong comparison function see section 13 3 3 to compare the entity tags in If-Match. If none of the entity tags match, or if is given and no current entity exists, the server MUST NOT perform the requested method, and MUST return a 412 Precondition Failed response This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it. If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status, then the If-Match header MUST be ignored. The meaning of If-Match is that the method SHOULD be performed if the representation selected by the origin server or by a cache, possibly using the Vary mechanism, see section 14 44 exists, and MUST NOT be performed if the representation does not exist. A request intended to update a resource e g a PUT MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value a single entity tag is no longer a representation of that resource This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge Examples. The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.14 25 If-Modified-Since. The If-Modified-Since request-header field is used with a method to make it conditional if the requested variant has not been modified since the time specified in this field, an entity will not be returned from the server instead, a 304 not modified re sponse will be returned without any message-body. An example of the field is. A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only if it has been modified since the date given by the If-Modified-Since header The algorithm for determining this includes the following cases. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.14 26 If-None-Match. The If-None-Match request-header field is used with a method to make it conditional A client that has one or more entities previously obtained from the resource can verify that none of those entities is current by including a list of their associated entity tags in the If-None-Match header field The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead It is also used to prevent a method e g PUT from inadvertently modifying an existing resource when the client believes that the resource does not exist. As a special case, the value matches any current entity of the resource. If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request without the If-None-Match header on that resource, or if is given and any current entity exists for that resource, then the server MUST NOT perform the requested method, unless required to do so because the resource s modification date fails to match that supplied in an If-Modified-Since header field in the request Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 Not Modified response, including the cache - related header fields particularly ETag of one of the entities that matched For all other request methods, the server MUST respond with a status of 412 Precondition Failed. See section 13 3 3 for rules on how to determine if two entities tags match The weak comparison function can only be used with GET or HEAD requests. If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field s in the request That is, if no entity tags match, then the server MUST NOT return a 304 Not Modified response. If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status, then the If-None-Match header MUST be ignored See section 13 3 4 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request. The meaning of If-None-Match is that the method MUST NOT be performed if the representation selected by the origin server or by a cache, possibly using the Vary mechanism, see section 14 44 exists, and SHOULD be p erformed if the representation does not exist This feature is intended to be useful in preventing races between PUT operations. The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.14 27 If-Range. If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request-header with a conditional GET using either or both of If-Unmodified-Since and If-Match However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body. The If-Range header allows a client to short-circuit the second request Informally, its meaning is if the entity is unchanged, send me the part s that I am missing otherwise, send me the entire new entity. If the client has no entity tag for an entity, but does have a Last - Modified date, it MAY use that date in an If-Range header The server can distinguish between a valid and any form of entity-tag by examining no more than two characters The If-Range header SHOULD only be used together with a Range header, and MUST be ignored if the request does not include a Range header, or if the server does not support the sub-range operation. If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server SHOULD provide the specified sub-range of the entity using a 206 Partial content response If the entity tag does not match, then the server SHOULD return the entire entity using a 200 OK response.14 28 If-Unmodified-Since. The If-Unmodified-Since request-header field is used with a method to make it conditional If the requested resource has not been modified since the time specified in this field, the server SHOULD perform the requested operation as if the If-Unmodified-Since header were not present. If the requested variant has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 Precondition Failed. An example of the field is. If the request normally i e without the If-Unmodified-Since header would result in anything other than a 2xx or 412 status, the If-Unmodified-Since header SHOULD be ignored. If the specified date is invalid, the header is ignored. The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.14 29 Last-Modified. The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified. An example of its use is. The exact meaning of this header field depends on the implementation of the origin server and the nature of the original resource For files, it may be just the file system last-modified time For entities with dynamically included parts, it may be the most rec ent of the set of last-modify times for its component parts For database gateways, it may be the last-update time stamp of the record For virtual objects, it may be the last time the internal state changed. An origin server MUST NOT send a Last-Modified date which is later than the server s time of message origination In such cases, where the resource s last modification would indicate some time in the future, the server MUST replace that date with the message origination date. An origin server SHOULD obtain the Last-Modified value of the entity as close as possible to the time that it generates the Date value of its response This allows a recipient to make an accurate assessment of the entity s modification time, especially if the entity changes near the time that the response is generated. servers SHOULD send Last-Modified whenever feasible.14 30 Location. The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource For 201 Created responses, the Location is that of the new resource which was created by the request For 3xx responses, the location SHOULD indicate the server s preferred URI for automatic redirection to the resource The field value consists of a single absolute URI.14 31 Max-Forwards. The Max-Forwards request-header field provides a mechanism with the TRACE section 9 8 and OPTIONS section 9 2 methods to limit the number of proxies or gateways that can forward the request to the next inbound server This can be useful when the client is attempting to trace a request chain which appears to be failing or looping in mid-chain. The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded. Each proxy or gateway recipient of a TRACE or OPTIONS request containing a Max-Forwards header field MUST check and update its value prior to forwarding the request If the received value is zero 0 , the recipient MUST NOT forward the request instead, it MUST respond as the final recipient If the received Max-Forwards value is greater than zero, then the forwarded message MUST contain an updated Max-Forwards field with a value decremented by one 1.The Max-Forwards header field MAY be ignored for all other methods defined by this specification and for any extension methods for which it is not explicitly referred to as part of that method definition.14 32 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. 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 cache-directive see section 14 9 and is defined here for backward compatibility with Clients SHOULD include both header fields when a no-cache request is sent to a server not known to be compliant. Pragma 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 specify a pragma for a specific recipient however, any pragma directive not relevant to a recipient SHOULD be ignored by that recipient. caches SHOULD treat Pragma no-cache as if the client had sent Cache-Control no-cache No new Pragma directives will be defined in.14 33 Proxy-Authenticate. The Proxy-Authenticate response-header field MUST be included as part of a 407 Proxy Authentication Required response The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the proxy for this Request-URI. The access authentication process is described in Authentication Basic and Digest Access Authentication 43 Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the current connection and SHOULD NOT be passed on to downstream clients However, an intermediate proxy might need to obtain its own credentials by requesting them from the downstream client, which in some circumstances will appear as if the proxy is forwarding the Proxy-Authenticate header field.14 34 Proxy-Authorization. The Proxy-Authorization request-header field allows the client to identify itse lf or its user to a proxy which requires authentication The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent for the proxy and or realm of the resource being requested. The access authentication process is described in Authentication Basic and Digest Access Authentication 43 Unlike Authorization, the Proxy-Authorization header field applies only to the next outbound proxy that demanded authentication using the Proxy - Authenticate field When multiple proxies are used in a chain, the. Proxy-Authorization header field is consumed by the first outbound proxy that was expecting to receive credentials A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request.14 35 1 Byte Ranges. Since all entities are represented in messages as sequences of bytes, the concept of a byte range is meaningful for any entity However, not all cli ents and servers need to support byte - range operations. Byte range specifications in apply to the sequence of bytes in the entity-body not necessarily the same as the message-body. A byte range operation MAY specify a single range of bytes, or a set of ranges within a single entity. The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range The last-byte-pos value gives the byte-offset of the last byte in the range that is, the byte positions specified are inclusive Byte offsets start at zero. If the last-byte-pos value is present, it MUST be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte - range-spec is syntactically invalid The recipient of a byte-range - set that includes one or more syntactically invalid byte-range-spec values MUST ignore the header field that includes that byte-range - set. If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the entity-body, last-by te-pos is taken to be equal to one less than the current length of the entity - body in bytes. By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the entity. A suffix-byte-range-spec is used to specify the suffix of the entity-body, of a length given by the suffix-length value That is, this form specifies the last N bytes of an entity-body If the entity is shorter than the specified suffix-length, the entire entity-body is used. If a syntactically valid byte-range-set includes at least one byte - range-spec whose first-byte-pos is less than the current length of the entity-body, or at least one suffix-byte-range-spec with a non - zero suffix-length, then the byte-range-set is satisfiable Otherwise, the byte-range-set is unsatisfiable If the byte-range-set is unsatisfiable, the server SHOULD return a response with a status of 416 Requested range not satisfiable Otherwise, the server SHOULD return a response with a status of 206 Partial Content containing the satisfiable ranges of the entity-body. Examples of byte-ranges-specifier values assuming an entity-body of length 10000.14 35 2 Range Retrieval Requests. retrieval requests using conditional or unconditional GET methods MAY request one or more sub-ranges of the entity, instead of the entire entity, using the Range request header, which applies to the entity returned as the result of the request. A server MAY ignore the Range header However, origin servers and intermediate caches ought to support byte ranges when possible, since Range supports efficient recovery from partially failed transfers, and supports efficient partial retrieval of large entities. If the server supports the Range header and the specified range or ranges are appropriate for the entity. In some cases, it might be more appropriate to use the If-Range header see section 14 27 in addition to the Range header. If a proxy that supports ranges receives a Range request, forwards the request to an inbound server, and receives an entire entity in reply, it SHOULD only return the requested range to its client It SHOULD store the entire received response in its cache if that is con sistent with its cache allocation policies.14 36 Referer. The Referer sic request-header field allows the client to specify, for the server s benefit, the address URI of the resource from which the Request-URI was obtained the referrer , although the header field is misspelled The Referer request-header allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc It also allows obsolete or mistyped links to be traced for maintenance The Referer field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard. If the field value is a relative URI, it SHOULD be interpreted relative to the Request-URI The URI MUST NOT include a fragment See section 15 1 3 for security considerations.14 37 Retry-After. The Retry-After response-header field can be used with a 503 Service Unavailable response to indicate how long the service is expected to be unavailable to the requesting clien t This field MAY also be used with any 3xx Redirection response to indicate the minimum time the user-agent is asked wait before issuing the redirected request The value of this field can be either an or an integer number of seconds in decimal after the time of the response. Two examples of its use are. In the latter example, the delay is 2 minutes.14 38 Server. The Server response-header field contains information about the software used by the origin server to handle the request The field can contain multiple product tokens section 3 8 and comments identifying the server and any significant subproducts The product tokens are listed in order of their significance for identifying the application. If the response is being forwarded through a proxy, the proxy application MUST NOT modify the Server response-header Instead, it SHOULD include a Via field as described in section 14 45.The TE request-header field indicates what extension transfer-codings it is willing to accept in the response an d whether or not it is willing to accept trailer fields in a chunked transfer-coding Its value may consist of the keyword trailers and or a comma-separated list of extension transfer-coding names with optional accept parameters as described in section 3 6.The presence of the keyword trailers indicates that the client is willing to accept trailer fields in a chunked transfer-coding, as defined in section 3 6 1 This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding. Examples of its use are. The TE header field only applies to the immediate connection Therefore, the keyword MUST be supplied within a Connection header field section 14 10 whenever TE is present in an message. A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules. If the TE field-value is empty or if no TE field is present, the only transfer-coding is chunked A message with no transfer-coding is always acceptable.14 40 T railer. The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer-coding. An message SHOULD include a Trailer header field in a message using chunked transfer-coding with a non-empty trailer Doing so allows the recipient to know which header fields to expect in the trailer. If no Trailer header field is present, the trailer SHOULD NOT include any header fields See section 3 6 1 for restrictions on the use of trailer fields in a chunked transfer-coding. Message header fields listed in the Trailer header field MUST NOT include the following header fields.14 41 Transfer-Encoding. The Transfer-Encoding general-header field indicates what if any type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient This differs from the content-coding in that the transfer-coding is a property of the message, not of the entity. Transfer-codings are defin ed in section 3 6 An example is. If multiple encodings have been applied to an entity, the transfer - codings MUST be listed in the order in which they were applied Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification. Many older applications do not understand the Transfer - Encoding header.14 42 Upgrade. The Upgrade general-header allows the client to specify what additional communication protocols it supports and would like to use if the server finds it appropriate to switch protocols The server MUST use the Upgrade header field within a 101 Switching Protocols response to indicate which protocol s are being switched. The Upgrade header field is intended to provide a simple mechanism for transition from to some other, incompatible protocol It does so by allowing the client to advertise its desire to use another protocol, such as a later version of with a higher major version number, even though the current requ est has been made using This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a better protocol if available where better is determined by the server, possibly according to the nature of the method and or resource being requested. The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection Upgrade cannot be used to insist on a protocol change its acceptance and use by the server is optional The capabilities and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial request containing the Upgrade header field. The Upgrade header field only applies to the immediate connection Therefore, the upgrade keyword MUST be suppli ed within a Connection header field section 14 10 whenever Upgrade is present in an message. The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection For that purpose, it is more appropriate to use a 301, 302, 303, or 305 redirection response. This specification only defines the protocol name for use by the family of Hypertext Transfer Protocols, as defined by the version rules of section 3 1 and future updates to this specification Any token can be used as a protocol name however, it will only be useful if both the client and server associate the name with the same protocol.14 43 User-Agent. The User-Agent request-header field contains information about the user agent originating the request This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations User agents SHOULD include this field with requests The field can c ontain multiple product tokens section 3 8 and comments identifying the agent and any subproducts which form a significant part of the user agent By convention, the product tokens are listed in order of their significance for identifying the application. The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without revalidation For uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation A Vary field value of implies that a cache cannot determine from the request headers of a subsequent request whether this response is the appropriate representation See section 13 6 for use of the Vary header field by caches. An server SHOULD include a Vary header field with any cacheable response that is subject to server-driven negotiation Doing so allows a cache to properly inter pret 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. A Vary field value consisting of a list of field-names signals that the representation selected for the response is based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation A cache MAY assume that the same selection will be made for future requests with the same values for the listed field names, for the duration of time for which the response is fresh. The field-names given are not limited to the set of standard request-header fields defined by this specification Field names are case-insensitive. A Vary field value of signals that unsp ecified 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 The value MUST NOT be generated by a proxy server it may only be generated by an origin server. The Via general-header field MUST be used by gateways and proxies to indicate the intermediate protocols and recipients between the user agent and the server on requests, and between the origin server and the client on responses It is analogous to the Received field of RFC 822 9 and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request response chain. The received-protocol indicates the protocol version of the message received by the server or client along each segment of the request response chain The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients. The protocol-name is optional if and only if it would be The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym If the port is not given, it MAY be assumed to be the default port of the received-protocol. Multiple Via field values represents each proxy or gateway that has forwarded the message Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applicationsments MAY be used in the Via header field to identify the software of the recipient proxy or gateway, analogous to the User-Agent and Server header fields However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message. For example, a request message could be sent from an us er agent to an internal proxy code-named fred , which uses to forward the request to a public proxy at which completes the request by forwarding it to the origin server at The request received by would then have the following Via header field. Proxies and gateways used as a portal through a network firewall SHOULD NOT, by default, forward the names and ports of hosts within the firewall region This information SHOULD only be propagated if explicitly enabled If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host. For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry For example. Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms Application s MUST NOT combine entries which have different received-protocol values.14 46 Warning. The Warning general-header field is used to carry additional information about the status or transformation of a message which might not be reflected in the message This information is typically used to warn about a possible lack of semantic transparency from caching operations or transformations applied to the entity body of the message. Warning headers are sent with responses using. A response MAY carry more than one Warning header. The warn-text SHOULD be in a natural language and character set that is most likely to be intelligible to the human user receiving the response This decision MAY be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the Content-Language field in a response, etc The default language is English and the default character set is ISO-8859-1.If a character set other than ISO-8859-1 is used, it MUST be encoded in t he warn-text using the method described in RFC 2047 14.Warning headers can in general be applied to any message, however some specific warn-codes are specific to caches and can only be applied to response messages New Warning headers SHOULD be added after any existing Warning headers A cache MUST NOT delete any Warning header that it received with a message However, if a cache successfully validates a cache entry, it SHOULD remove any Warning headers previously attached to that entry except as specified for. specific Warning codes It MUST then add any Warning headers received in the validating response In other words, Warning headers are those that would be attached to the most recent relevant response. When multiple Warning headers are attached to a response, the user agent ought to inform the user of as many of them as possible, in the order that they appear in the response If it is not possible to inform the user of all of the warnings, the user agent SHOULD follow these heuristics. Sy stems that generate multiple Warning headers SHOULD order them with this user agent behavior in mind. Requirements for the behavior of caches with respect to Warnings are stated in section 13 1 2.This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning.110 Response is stale MUST be included whenever the returned response is stale.111 Revalidation failed MUST be included if a cache returns a stale response because an attempt to revalidate 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.113 Heuristic expiration MUST 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 MAY 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 cache or proxy if it applies any transformation changing the content-coding as specified in the Content-Encoding header or media-type as specified in the Content-Type header of the response, or the entity-body of the response, unless this Warning code already appears in the response.299 Miscellaneous persistent warning The warning text MAY include arbitrary information to be presented to a human user, or logged A system receiving this warning MUST NOT take any automated action. If an implementation sends a message with one or more Warning headers whose version is or lower, then the sender MUST include in each warning-value a warn-date that matches the date in the response. 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 r esponse, then that warning-value MUST be deleted from the message before storing, forwarding, or using it This prevents bad 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.14 47 WWW-Authenticate. The WWW-Authenticate response-header field MUST be included in 401 Unauthorized response messages The field value consists of at least one challenge that indicates the authentication scheme s and parameters applicable to the Request-URI. The access authentication process is described in Authentication Basic and Digest Access Authentication 43 User agents are advised to take special care in parsing the WWW - Authenticate field value as it might contain more than one challenge, or if more than one WWW-Authenticate header field is provided, the contents of a challenge itself can contain a comma-separated list of authentication parameters.
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