File: vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Resources/config/ssi.xml

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="ssi" class="Symfony\Component\HttpKernel\HttpCache\Ssi" />

        <service id="ssi_listener" class="Symfony\Component\HttpKernel\EventListener\SurrogateListener">
            <tag name="kernel.event_subscriber" />
            <argument type="service" id="ssi" on-invalid="ignore" />
        </service>
    </services>
</container>