Last modified 17 months ago Last modified on 09/15/15 23:54:14

Support for QoS (TOS/DSCP, WMM, 802.1p)

Table of Contents

  1. Introduction on QoS
    1. Layer 2: IEEE 802.1p for Ethernet
    2. Layer 2: WMM
    3. Layer 3: DSCP
    4. Layer 3: Other
  2. Availability
    1. Summary
    2. Linux
    3. MacOS X
    4. Windows and Windows Mobile
    5. Symbian S60 3rd Ed
  3. Objective
  4. Design
    1. Portable High Level API
    2. Fine Grained Control API
  5. Limitations
  6. Using QoS in PJSIP Applications
    1. PJSUA-LIB
    2. Low Level Transports
  7. References

This article describes the QoS support in PJSIP and how to use it.

Introduction on QoS

QoS settings are available for both Layer 2 and Layer 3 of TCP/IP protocols:

Layer 2: IEEE 802.1p for Ethernet

IEEE 802.1p tagging will mark frames sent by a host for prioritized delivery using a 3-bit Priority field in the virtual local area network (VLAN) header of the Ethernet frame. The VLAN header is placed inside the Ethernet header, between the Source Address field and either the Length field (for an IEEE 802.3 frame) or the EtherType field (for an Ethernet II frame).

Layer 2: WMM

At the Network Interface layer for IEEE 802.11 wireless, the Wi-Fi Alliance certification for Wi-Fi Multimedia (WMM) defines four access categories for prioritizing network traffic. These access categories are (in order of highest to lowest priority) voice, video, best-effort, and background. Host support for WMM prioritization requires that both wireless network adapters and their drivers support WMM. Wireless access points (APs) must have WMM enabled.

Layer 3: DSCP

At the Internet layer, you can use Differentiated Services/Diffserv and set the value of the Differentiated Services Code Point (DSCP) in the IP header. As defined in RFC 2472, the DSCP value is the high-order 6 bits of the IP version 4 (IPv4) TOS field and the IP version 6 (IPv6) Traffic Class field.

Layer 3: Other

Other mechanisms exist (such as RSVP, IntServ) but this will not be implemented.



The following table summarizes the availability/accessability of various QoS settings on platforms that PJSIP supports. "XXX is supported" row shows whether the OS is able to set that QoS setting. Whether that setting can be controlled programmatically depends on "XXX is user settable" row. For example, on Windows Mobile 6 (WM6), both DSCP and WMM priority can be changed by the OS, but these settings are applied based on IP_DSCP_TRAFFIC_TYPE and user (i.e. PJLIB) cannot directly change the DSCP and WMM prio settings.

Win2k/older XP, Vista, WM2003, WM5 WM6 Symbian S60 Linux MacOS X iPhone Android
High level API Yes No Yes Yes Yes Yes Yes Yes1)
API backend qos_bsd.c2) qos_dummy.c qos_wm.c qos_symbian.cpp qos_bsd.c qos_bsd.c qos_bsd.c qos_bsd.c
DSCP is supported Yes No Yes Yes Yes Yes Yes Yes
DSCP is user settable Yes No No Yes Yes Yes Yes Yes
WMM prio is supported No No Yes Yes No No No No
WMM prio is user settable No No No No No No No No
SO_PRIORITY is supported No No No No Yes Yes Yes Yes
SO_PRIORITY is settable No No No No Yes Yes Yes Yes3)


1) Via PJSUA2 API TransportConfig, fields qosType and qosParams.
2) On win32, sock_qos_dummy.c is used by default. Set PJ_QOS_IMPLEMENTATION to PJ_QOS_BSD to enable the use of sock_qos_bsd.c.
3) In our test, setting SO_PRIORITY showed no error, but wireshark traffic revealed that it's not set.


DSCP is available via IP TOS option.

Ethernet 802.1p tagging is done by setting setsockopt(SO_PRIORITY) option of the socket, then with the set_egress_map option of the vconfig utility to convert this to set vlan-qos field of the packet.

WMM is not known to be available.


DSCP is available via IP TOS option.

Windows and Windows Mobile

(It's a mess!)

DSCP is settable with setsockopt() on Windows 2000 or older, but Windows would silently ignore this call on WinXP or later, unless administrator modifies the registry. On Windows 2000, Windows XP, and Windows Server 2003, GQoS (Generic QoS) API is the standard API, but this API may not be supported in the future. On Vista and Windows 7, the is a new QoS2 API, also known as Quality Windows Audio-Video Experience (qWAVE).

IEEE 802.1p tagging is available via Traffic Control (TC) API, available on Windows XP SP2, but this needs administrator access. For Vista and later, it's in qWAVE.

WMM is available for mobile platforms on Windows Mobile 6 platform and Windows Embedded CE 6, via setsockopt(IP_DSCP_TRAFFIC_TYPE). qWAVE supports this as well.

Symbian S60 3rd Ed

Both DSCP and WMM is supported via RSocket::SetOpt() with will set both Layer 2 and Layer 3 QoS settings accordingly.


The objective of this ticket is to add new API to PJLIB socket API to enable manipulation of the QoS parameters above in a uniform and portable manner.


Based on the above, the following API is proposed.

Declare the following "standard" traffic types.

typedef enum pj_qos_type
} pj_qos_type;

The traffic classes above will determine how the Layer 2 and 3 QoS settings will be used. The standard mapping between the classes above to the corresponding Layer 2 and 3 settings are as follows:

PJLIB Traffic Type IP DSCP WMM 802.1p
BEST_EFFORT 0x00 BE (Bulk Effort) 0
BACKGROUND 0x08 BK (Bulk) 2
VIDEO 0x28 VI (Video) 5
VOICE 0x30 VO (Voice) 6
CONTROL 0x38 VO (Voice) 7

There are two sets of API provided to manipulate the QoS parameters.

Portable High Level API

The first set of API is:

// Set QoS parameters
PJ_DECL(pj_status_t) pj_sock_set_qos_type(pj_sock_t sock,
                                          pj_qos_type val);

// Get QoS parameters
PJ_DECL(pj_status_t) pj_sock_get_qos_type(pj_sock_t sock,
                                          pj_qos_type *p_val);

The API will set the traffic type according to the DSCP class, for both Layer 2 and Layer 3 QoS settings, where it's available. If any of the layer QoS setting is not settable, the API will silently ignore it. If both layers are not setable, the API will return error.

The API above is the recommended use of QoS, since it is the most portable across all platforms.

Fine Grained Control API

The second set of API is intended for application that wants to fine tune the QoS parameters.

The Layer 2 and 3 QoS parameters are stored in pj_qos_params structure:

typedef enum pj_qos_flag
   PJ_QOS_PARAM_HAS_802_1_P = 2,
} pj_qos_flag;

typedef enum pj_qos_wmm_prio
} pj_qos_wmm_prio;

typedef struct pj_qos_params
   pj_uint8_t      flags;    // Determines which values to 
                             // set, bitmask of pj_qos_flag
   pj_uint8_t      dscp_val; // DSCP value to set
   pj_uint8_t      so_prio;  // SO_PRIORITY value
   pj_qos_wmm_prio wmm_prio; // WMM priority value
} pj_qos_params;

The second set of API with more fine-grained control over the parameters are:

// Retrieve QoS params for the specified traffic type
PJ_DECL(pj_status_t) pj_qos_get_params(pj_qos_type type, 
                                       pj_qos_params *p);

// Set QoS parameters to the socket
PJ_DECL(pj_status_t) pj_sock_set_qos_params(pj_sock_t sock,
                                            const pj_qos_params *p);

// Get QoS parameters from the socket
PJ_DECL(pj_status_t) pj_sock_get_qos_params(pj_sock_t sock,
                                            pj_qos_params *p);


The pj_sock_set/get_qos_params() APIs are not portable, and it's probably only going to be implemented on Linux. Application should always try to use pj_sock_set_qos_type() instead.


Win32 currently is not be implemented.

Using QoS in PJSIP Applications


On PJSUA-LIB, QoS parameters have been added to pjsua_transport_config. Please see pjsua_transport_config reference for more info.


To set QoS of RTP/RTCP traffic to Voice type (this will activate the appropriate DSCP, WMM, and SO_PRIORITY settings, if the OS supports it):

 // Media transport setting is configurable on per account basis
 pjsua_acc_config acc_cfg;

 // Set account settings
 // Set media transport settings (listening start port etc) according to app settings
 // Set media transport traffic type to Voice
 acc_cfg.rtp_cfg.qos_type = PJ_QOS_TYPE_VOICE;

 // Create account with this config
 pjsua_acc_add(&acc_cfg, ...);

To tag SIP transport traffic with a specific DSCP value (in this case, DSCP CS3 or value 24). Note that not all platforms allow this, see the table above:

 pjsua_transport_config sip_tcfg;

 // Set listening port etc according to app settings
 // Set QoS to DSCP CS3 (DSCP value 24)
 sip_tcfg.qos_params.flags = PJ_QOS_PARAM_HAS_DSCP;
 sip_tcfg.qos_params.dscp_val = 24;

 // Create SIP transport with this config
 pjsua_transport_create(..., &sip_tcfg, ...);

Low Level Transports

If you are using the low level transports (such as SIP transports, media transports, or STUN/TURN/ICE transports) directly instead of from PJSUA-LIB, the QoS settings are available in one of its creation parameters. Hint: they are normally named as qos_type and qos_params.


  1. QoS Support in Windows - good intro for QoS on Windows and in general
  2. WMM (Wi-Fi Multimedia) (Windows Mobile 6)
  3. VoIP developer guidelines for S60
  4. WiFi QoS Support in Windows Vista: WMM part 2