Originally, HTTP was designed as a stateless protocol. Each request is treated as an independent transaction that is unrelated to any previous request. In order to enable interactions with multiple requests, session cookies were introduced. A session cookie is a piece of data that is sent with each request, enabling the server to relate the request to an HTTP session.
Session cookies are a workaround for introducing state into the otherwise stateless HTTP protocol. On a protocol level, HTTP/2 could make session cookies obsolete, because it supports long-running connections. However, as we have seen in the Hello, World! example, the HTTP session state does not use the underlying HTTP/2 connection state. Both states are maintained independent of each other.
The reason is that HTTP sessions based on session cookies are so widely used in current HTTP applications. From the HTTP/2 FAQ:
If HTTP/2 had introduced a new HTTP session state mechanism, it would have meant that the new protocol was incompatible with the existing Web.
HTTP/2 still uses session cookies. This post shows how to implement them.
This post will
- show how headers are exchanged with HTTP/2.
- show how HTTP/2’s Header Compression (HPACK) maintains a connection state which is independent of the HTTP session state.
- provide the lessons learned when implementing cookie headers in the h2c command line tool.
HTTP/2 Header Frames
In HTTP/1, each request and each response is a single message. In HTTP/2, each request and each response is represented by a stream which is composed of multiple frames. The most important frame types are the HEADERS type, carrying the information that used to be in HTTP/1 message headers, and the DATA type, carrying the equivalent of HTTP/1 message bodies. Each frame has a stream ID to indicate which request or with response it belongs to.
The following examples from the HTTP/2 RFC shows HTTP/1.1 requests and responses, with illustrations of equivalent HTTP/2 requests and responses.
HEADERS frames have a size limit. If the content becomes too big, it is split into an initial HEADERS frame followed by one or more CONTINUATION frames.
HTTP/2 Connection State
HTTP/1 headers are repetitive and verbose, causing unnecessary network traffic. For example, the Chrome browser typically includes a dozen headers in each request. Among other things, each request sent by the Chrome browser contains the same 133 Bytes User Agent header.
HTTP/2 addresses this by using HTTP Header Compression (HPACK):
- Headers are represented in compressed binary format, not ASCII text.
- Once a server or a client receives a new header, it remembers the header in a dynamic table. Instead of including the header again in the next request, the request may include an index to the existing entry in the dynamic table.
The dynamic table maintained by the server and client can be seen as a connection state, which is altered by updating existing headers or inserting new headers.
Using the Header Compression described above, cookies need to be sent only once to put them into the dynamic table, and subsequent requests include a reference to the entry in the dynamic table.
In order to try this, I extended the h2c command line client from the Hello, World!
example with an option
-i to view the headers received, and with a
command to set headers. The headers are referenced in the
dynamic table with each subsequent request, unless
h2c unset is called.
The Hello, World! example can now be run as follows:
It took some attempts to get the implementation of the
h2c set command right:
First Attempt: HEADERS Frame with Single Header
The first attempt to implementing the
h2c set command was to send out a HEADERS frame
with the header provided in the command line, so that the server inserts this header
into its dynamic table.
However, it turned out that HEADER frames with only one header are malformed according to the HTTP/2 spec:
All HTTP/2 requests MUST include exactly one valid value for the :method :schema and :path unless it is a CONNECT request. An HTTP request that omits mandatory pseudo-header fields is malformed.
Second Attempt: Continuation
The second attempt was to split the headers into a HEADERS frame which is sent out with
h2c set command, and a CONTINUATION frames which is sent with the next
and provides the missing :method and other mandatory fields.
This works well with the current Wildfly 9 Beta, but the HTTP/2 specification indicates that it actually shouldn’t work:
All pseudo-header fields MUST appear in the header block before regular header fields. Any request or response that contains a pseudo-header field that appears in a header block after a regular header field MUST be treated as malformed
The composed header includes the cookie before the :method, which is illegal.
As a side note, some code comments in Wildfly’s Undertow source code indicate that developers don’t feel very confident about CONTINUATION frames:
Third Attempt: Caching the Header Client Side
The trivial, but most robust implementation is not to send anything with the
h2c set command, but to remember the headers on the client side and include
them with the next request. This works, but it seems a bit boring, because
it does not implement any fancy way of setting the header independently of the
This post showed how session cookies are implemented in HTTP/2 in a way that is compatible with existing Web applications. In order to remain compatible with existing web application semantics, HTTP/2 did not change the way HTTP session are maintained.
However, for some server-to-server APIs it might not be a requirement to be compatible with existing HTTP session semantics. For these APIs, it might be more natural to use HTTP/2 connections for implementing sessions, and not to revert to HTTP/1 compatible session cookies. Future posts will investigate how this can be achieved.