Apache Module mod_http2

H2Direct Directive

This directive toggles the usage of the HTTP/2 Direct Mode. This should be used inside a <VirtualHost> section to enable direct HTTP/2 communication for that virtual host.

Direct communication means that if the first bytes received by the server on a connection match the HTTP/2 preamble, the HTTP/2 protocol is switched to immediately without further negotiation. This mode is defined in RFC 7540 for the cleartext (h2c) case. Its use on TLS connections not mandated by the standard.

When a server/vhost does not have h2 or h2c enabled via <Protocols>, the connection is never inspected for a HTTP/2 preamble. H2Direct does not matter then. This is important for connections that use protocols where an initial read might hang indefinitely, such as NNTP.

For clients that have out-of-band knowledge about a server supporting h2c, direct HTTP/2 saves the client from having to perform an HTTP/1.1 upgrade, resulting in better performance and avoiding the Upgrade restrictions on request bodies.

This makes direct h2c attractive for server to server communication as well, when the connection can be trusted or is secured by other means.

H2Direct on

H2MaxSessionStreams Directive

This directive sets the maximum number of active streams per HTTP/2 session (e.g. connection) that the server allows. A stream is active if it is not idle or closed according to RFC 7540.

H2MaxSessionStreams 20

H2MaxWorkerIdleSeconds Directive

This directive sets the maximum number of seconds a h2 worker may idle until it shuts itself down. This only happens while the number of h2 workers exceeds H2MinWorkers.

H2MaxWorkerIdleSeconds 20

H2MaxWorkers Directive

This directive sets the maximum number of worker threads to spawn per child process for HTTP/2 processing. If this directive is not used, mod_http2 will chose a value suitable for the mpm module loaded.

H2MaxWorkers 20

H2MinWorkers Directive

This directive sets the minimum number of worker threads to spawn per child process for HTTP/2 processing. If this directive is not used, mod_http2 will chose a value suitable for the mpm module loaded.

H2MinWorkers 10

H2ModernTLSOnly Directive

This directive toggles the security checks on HTTP/2 connections in TLS mode (https:). This can be used server wide or for specific <VirtualHost>s.

The security checks require that the TSL protocol is at least TLSv1.2 and that none of the ciphers listed in RFC 7540, Appendix A is used. These checks will be extended once new security requirements come into place.

The name stems from the Security/Server Side TLS definitions at mozilla where "modern compatibility" is defined. Mozilla Firefox and other browsers require modern compatibility for HTTP/2 connections. As everything in OpSec, this is a moving target and can be expected to evolve in the future.

One purpose of having these checks in mod_http2 is to enforce this security level for all connections, not only those from browsers. The other purpose is to prevent the negotiation of HTTP/2 as a protocol should the requirements not be met.

Ultimately, the security of the TLS connection is determined by the server configuration directives for mod_ssl.

H2ModernTLSOnly off

H2Push Directive

This directive toggles the usage of the HTTP/2 server push protocol feature. This should be used inside a <VirtualHost> section to enable direct HTTP/2 communication for that virtual host.

The HTTP/2 protocol allows the server to push other resources to a client when it asked for a particular one. This is helpful if those resources are connected in some way and the client can be expected to ask for it anyway. The pushing then saves the time it takes the client to ask for the resources itself. On the other hand, pushing resources the client never needs or already has is a waste of bandwidth.

Server pushes are detected by inspecting the Link headers of responses (see https://tools.ietf.org/html/rfc5988 for the specification). When a link thus specified has the rel=preload attribute, it is treated as a resource to be pushed.

Link headers in responses are either set by the application or can be configured via mod_headers as:

<Location /index.html>
    Header add Link "</css/site.css>;rel=preload"
    Header add Link "</images/logo.jpg>;rel=preload"
</Location>

As the example shows, there can be several link headers added to a response, resulting in several pushes being triggered. There are no checks in the module to avoid pushing the same resource twice or more to one client. Use with care.

HTTP/2 server pushes are enabled by default. This directive allows it to be switch off on all resources of this server/virtual host.

H2Push off

Last but not least, pushes happen only when the client signals its willingness to accept those. Most browsers do, some, like Safari 9, do not. Also, pushes also only happen for resources from the same authority as the original response is for.

H2PushPriority Directive

This directive defines the priority handling of pushed responses based on the content-type of the response. This is usually defined per server config, but may also appear in a virtual host.

HTTP/2 server pushes are always related to a client request. Each such request/response pairs, or streams have a dependency and a weight, together defining the priority of a stream.

When a stream depends on another, say X depends on Y, then Y gets all bandwidth before X gets any. Note that this does not men that Y will block X. If Y has no data to send, all bandwidth allocated to Y can be used by X.

When a stream has more than one dependant, say X1 and X2 both depend on Y, the weight determines the bandwidth allocation. If X1 and X2 have the same weight, they both get half of the available bandwdith. If the weight of X1 is twice as large as that for X2, X1 gets twice the bandwidth of X2.

Ultimately, every stream depends on the root stream which gets all the bandwidht available, but never sends anything. So all its bandwidth is distributed by weight among its children. Which either have data to send or distribute the bandwidth to their own children. And so on. If none of the children have data to send, that bandwidth get distributed somewhere else according to the same rules.

The purpose of this priority system is to always make use of available bandwidth while allowing precedence and weight to be given to specific streams. Since, normally, all streams are initiated by the client, it is also the one that sets these priorities.

Only when such a stream results in a PUSH, gets the server to decide what the initial priority of such a pushed stream is. In the examples below, X is the client stream. It depends on Y and the server decides to PUSH streams P1 and P2 onto X.

The default priority rule is:

H2PushPriority * After 16

which reads as 'Send a pushed stream of any content-type depending on the client stream with weight 16'. And so P1 and P2 will be send after X and, as they have equal weight, share bandwidth equally among themselves.

H2PushPriority text/css Interleaved 256

which reads as 'Send any CSS resource on the same dependency and weight as the client stream'. If P1 has content-type 'text/css', it will depend on Y (as does X) and its effective weight will be calculated as P1ew = Xw * (P1w / 256). With P1w being 256, this will make the effective weight the same as the weight of X. If both X and P1 have data to send, bandwidth will be allocated to both equally.

With Pw specified as 512, a pushed, interleaved stream would get double the weight of X. With 128 only half as much. Note that effective weights are always capped at 256.

H2PushPriority application/json Before

This says that any pushed stream of content type 'application/json' should be send out before X. This makes P1 dependent on Y and X dependent on P1. So, X will be stalled as long as P1 has data to send. The effective weight is inherited from the client stream. Specifying a weight is not allowed.

Be aware that the effect of priority specifications is limited by the available server resources. If a server does not have workers available for pushed streams, the data for the stream may only ever arrive when other streams have been finished.

Last, but not least, there are some specifics of the syntax to be used in this directive:

1. '*' is the only special content-type that matches all others. 'image/*' will not work.
2. The default dependency is 'After'.
3. There are also default weights: for 'After' it is 16, 'interleaved' is 256.

H2PushPriority application/json 32         # an After rule
H2PushPriority image/jpeg before           # weight inherited
H2PushPriority text/css   interleaved      # weight 256 default

H2SerializeHeaders Directive

This directive toggles if HTTP/2 requests shall be serialized in HTTP/1.1 format for processing by httpd core or if received binary data shall be passed into the request_recs directly.

Serialization will lower performance, but gives more backward compatibility in case custom filters/hooks need it.

H2SerializeHeaders on

H2SessionExtraFiles Directive

This directive sets maximum number of extra file handles a HTTP/2 session is allowed to use. A file handle is counted as extra when it is transferred from a h2 worker thread to the main HTTP/2 connection handling. This commonly happens when serving static files.

Depending on the processing model configured on the server, the number of connections times number of active streams may exceed the number of file handles for the process. On the other hand, converting every file into memory bytes early results in too many buffer writes. This option helps to mitigate that.

The number of file handles used by a server process is then in the order of:

(h2_connections * extra_files) + (h2_max_worker)

H2SessionExtraFiles 10

H2StreamMaxMemSize Directive

This directive sets the maximum number of outgoing data bytes buffered in memory for an active streams. This memory is not allocated per stream as such. Allocations are counted against this limit when they are about to be done. Stream processing freezes when the limit has been reached and will only continue when buffered data has been sent out to the client.

H2StreamMaxMemSize 128000

H2TLSCoolDownSecs Directive

This directive sets the number of seconds of idle time on a TLS connection before the TLS write size falls back to small (~1300 bytes) length. This can be used server wide or for specific <VirtualHost>s.

See <H2TLSWarmUpSize> for a description of TLS warmup. H2TLSCoolDownSecs reflects the fact that connections may deteriorate over time (and TCP flow adjusts) for idle connections as well. It is beneficial to overall performance to fall back to the pre-warmup phase after a number of seconds that no data has been sent.

In deployments where connections can be considered reliable, this timer can be disabled by setting it to 0.

The following example sets the seconds to zero, effectively disabling any cool down. Warmed up TLS connections stay on maximum record size.

H2TLSCoolDownSecs 0

H2TLSWarmUpSize Directive

This directive sets the number of bytes to be sent in small TLS records (~1300 bytes) until doing maximum sized writes (16k) on https: HTTP/2 connections. This can be used server wide or for specific <VirtualHost>s.

Measurements by google performance labs show that best performance on TLS connections is reached, if initial record sizes stay below the MTU level, to allow a complete record to fit into an IP packet.

While TCP adjust its flow-control and window sizes, longer TLS records can get stuck in queues or get lost and need retransmission. This is of course true for all packets. TLS however needs the whole record in order to decrypt it. Any missing bytes at the end will stall usage of the received ones.

After a sufficient number of bytes have been send successfully, the TCP state of the connection is stable and maximum TLS record sizes (16 KB) can be used for optimal performance.

In deployments where servers are reached locally or over reliable connections only, the value might be decreased with 0 disabling any warmup phase altogether.

The following example sets the size to zero, effectively disabling any warmup phase.

H2TLSWarmUpSize 0

H2Upgrade Directive

This directive toggles the usage of the HTTP/1.1 Upgrade method for switching to HTTP/2. This should be used inside a <VirtualHost> section to enable Upgrades to HTTP/2 for that virtual host.

This method of switching protocols is defined in HTTP/1.1 and uses the "Upgrade" header (thus the name) to announce willingness to use another protocol. This may happen on any request of a HTTP/1.1 connection.

This method of protocol switching is enabled by default on cleartext (potential h2c) connections and disabled on TLS (potential h2), as mandated by RFC 7540.

Please be aware that Upgrades are only accepted for requests that carry no body. POSTs and PUTs with content will never trigger an upgrade to HTTP/2. See <H2Direct> for an alternative to Upgrade.

This mode only has an effect when h2 or h2c is enabled via the <Protocols>.

H2Upgrade on

H2WindowSize Directive

This directive sets the size of the window that is used for flow control from client to server and limits the amount of data the server has to buffer. The client will stop sending on a stream once the limit has been reached until the server announces more available space (as it has processed some of the data).

This limit affects only request bodies, not its meta data such as headers. Also, it has no effect on response bodies as the window size for those are managed by the clients.

H2WindowSize 128000