The MPLS-Multi-Protocol Label Switching Label is appended to the IP-Internet Protocol version 4 packet.  The Label consists of the Label 20 bits, EXPerimental 3-bits, Stack 1-bit and TTL-Time-To-Live 8-bits.  In a PWE3-PPVNP-Provider Provisioned Virtual Private Network, there may be more than label.  To begin with, IP-Internet Protocol packets may have a number of labels or “tags” attached to them.  MPLS-Multi-Protocol Label Switching is just one type of label.  Here are some terms associated with labeling: Push – add a label, Swap – replace the label and Pop – remove the label.  Click on the image for the animation.

Here is the IPv4 packet with the MPLS-Multi-Protocol Label Switching label.  This tutorial shows how DSCP-Differentiated Services Code Point with MPLS-Multi-Protocol Label Switching EXP-EXPerimental bits can work together or independently to provide QoS-Quality of Service.

DiffServ- Differentiated Services

- Uses first six bits of IPv4 TOS-Type of Service of IPv4 or IPv6 header to convey DSCP-Differentiated Services Code Point

- Last two bits not CU-Currently Unused saved for experimental use

- DSCP selects PHB-Per Hop Behavior

- RFC2597 defines 12 DSCPs with four AF-Assured Forwarding classes – AF1(1-3), AF2(1-3), AF3(1-3) and AF4(1-3).

- Each AF is one instance of forwarding behavior or BA-Behavior Aggregate with some assigned level of queuing resources and three drop precedences

- BA-Behavior Aggregate is the core value which determines the DSCP

NOTE: DSCP REPLACES the TOS bits with its own but is backward compatible with the 3 Precendence bits so that non-DSCP routers will not conflict with DSCP bits (called mapping).

Here’s where DSCP gets complicated.  For VoIP priority routing, EF-Express Forwarding is used on Cisco routers.  According to Cisco, “EF-Express forwarding is a low-loss, low-latency, low-jitter, assured-bandwidth, end-to-end service.”  The DSCP value for EF is 46 (0×101110).  Often values are indicated in hexadecimal such as 0x3F.  Hexadecimal uses base16 format with a 0x prefix followed by eight bits.  HSD-High Speed Data is the default with no guaranteed priority.  The DSCP value for Default is 0 (0×000000).  AF-Assured Forwarding is based on four levels of priority or “drop precedence” (the higher the drop precedence the packets to be are discarded first).  Drop precedence is usually high-medium-low).  This means there are 12 levels of AF from DSCP 10 to 38.  DSCP values map to Ethernet CoS values such as DSCP 0-9 is COS 0 and DSCP 53-63 is COS 7. 

DSCP priority routing is part of the WRR-Weighted Round Robin and other queuing techniques (next), bandwidth and other factors.  Please note the DSCP varies between manufacturers, so check with specific vendors for DSCP values.  See WRR, Deficit Round Robin, Queuing and other terms in TECHtionary for more details.

WRR-Weighted Round Robin assigns weights (queuing priority values such as 1, 2, 3) to each queue connection (bandwidth) not necessarily the protocol.  The data traffic in each queue is processed (serviced) in proportion (relation) to its weight in a round-robin fashion such as 1-2-3, 3-2-1, etc.).   WRR is optimal when there are small packet sizes and connections are long such as voice telephone calls.

Uniform mode – replace DSCP-Differentiated Services Code Point with MPLS-Multi-Protocol Label Switching EXP-EXPerimental bits.  This may require some mapping if DSCP uses more than the three Precedence bits.  The key point is that MPLS Traffic Engineering or TE and MPLS-DiffServ (using DSCP codes) can operate simultaneously and independently.  Both can work together or separately to provide QoS-Quality of Service.

Pipe and Short-Pipe assume that the SP-Service Provider and the Customer want different QoS-Quality of Service policies.  That is, the Service Provider transfers the DSCP-Differentiated Services Code Point without changing the values transparently.  SEE THE ANIMATION for details – click on either image.

Marker is the DSCP-Differentiated Services Code Point – traffic priority

Meter is the traffic control – speed limit

Shaping is the traffic cop – stop unless you have a priority DSCP

Droppers – like it sounds – if no priority then drop-discard-delete IP packet

All classifier, meter, marker dropper, queue and scheduler functions are called TCB – traffic conditioning blocks for TCB – taking care of business.

In summary, DSCP uses existing IPv4 and IPv6 packets whereas MPLS-Multi-Protocol Label Switching ADDs

a label or tag to the IPpacket.