Adspecs are a part of path events. Adspecs carry information about the RSVP support that is possible in the network for a specified flow.

An adspec is created by the RSVP daemon when a sender calls rapi_sender(). The path message traverses the network to the receiver, and accumulates changes to the adspec when passing through each RSVP enabled network node. The receiver callback function is called with a path event. The path event has the original sender tspec, and the adspec that indicates the support that is possible through the network for that tspec. The receiver then calls rapi_reserve() to make a reservation based upon the information in the tspec and adspec. Then the reservation message traverses the network to the sender. The QoS reservation is set up in each RSVP node until the reservation message reaches the sender. The sender callback function is called with a reserve event. The sender then may adjust packet size or other factors in order to use the reservation effectively, based upon the reservation information.

When the application callback function is passed a non-NULL pointer for the adspec, then the pointer points to an IS_adspec_t data type:

    typedef struct {
            Gen_adspec_t    Gen_Adspec;
            rbool_t         CL_present;
            CL_adspec_t     CL_Adspec;
            rbool_t         Guar_present;
            Guar_adspec_t   Guar_Adspec;
    } IS_adspec_t;

The IS_adspec_t data type has several fields. The Gen_Adspec field is always present. If CL_present is true, then CL_Adspec contains a CL_adspec_t data type, which is a composite of the controlled flow support for the path. If Guar_present is true, then Guar_Adspec contains a Guar_adspec_t data type, which is a composite of the guaranteed flow support for the path.

The Gen_Adspec_t data type is shown below.

    typedef struct {
            rbool_t    servBreak;    /* break in service */
            u_int16_t  ISHops;       /* num Int-serv aware hops */
            float32_t  pathBW;       /* min path band width (bytes/sec) */
            u_int32_t  pathLatency;  /* min path latency (microseconds) */
            u_int16_t  composedMTU;  /* composed path MTU */
    } Gen_adspec_t;

If servBreak is true, then there is at least one hop on the path that does not have QoS support and QoS must be considered unreliable. ISHops specifies the number of hops that do have QoS support of some sort (controlled load or guaranteed). There are estimates for minimum path bandwidth and minimum delay through the path. The composedMTU is the maximum transmission unit that can be sent on the path without fragmentation of the packet. The composedMTU includes the IP packet header and whatever protocol headers (UDP, TCP, and so on) will be in the packet. Note that a particular type of service might have a different value for the composedMTU, due to implementation differences for various types of QoS.

The CL_adspec_t data type provides information specifically for controlled load flows on the path. Note that it is possible for there to be a break in service for controlled load even if QoS is supported at every node. Some nodes may only implement QoS for guaranteed flows.

    typedef struct {
            rbool_t    servBreak;    /* break in service */
            u_int16_t  ISHops;       /* num Int-serv aware hops */
            float32_t  pathBW;       /* min path band width (bytes/sec) */
            u_int32_t  pathLatency;  /* min path latency (microseconds) */
            u_int16_t  composedMTU;  /* composed path MTU */
    } CL_adspec_t;

The fields have the same significance as for Gen_adspec_t but refer only to path hops for which controlled load is supported.

The Guar_adspec_t data type provides information specifically for guaranteed flows on the path. Note that it is possible for there to be a break in service for guaranteed flows even if QoS is supported at every node. Some nodes may only implement controlled load QoS.

    typedef struct {
            rbool_t    servBreak;    /* break in service */
            u_int16_t  ISHops;       /* num Int-serv aware hops */
            float32_t  pathBW;       /* min path band width (bytes/sec) */
            u_int32_t  pathLatency;  /* min path latency (microseconds) */
            u_int16_t  composedMTU;  /* composed path MTU (bytes) */
            u_int32_t  Ctot;         /* total rate dep. err (bytes) */
            u_int32_t  Dtot;         /* total rate indep. err (microseconds) */
            u_int32_t  Csum;         /* reshaped rate dep. err (bytes) */
            u_int32_t  Dsum;         /* reshaped rate indep. err (microseconds) */
    } Guar_adspec_t;

The first five fields have the same significance as for Gen_adspec_t but refer only to path hops for which guaranteed flow is supported. The remaining fields are unique to guaranteed flows.