As a result of me wanting to brush up on my QoS skills, I'm jumping ahead to Chapter 12 in the book this week. And so begins "Classification and Marking," a saga in two parts, where our hero sets out to study the fields that can be marked, the mechanics of MQC (modular QoS CLI), and Cisco's tools such as Class-Based Marking.
First, though, let me briefly introduce some ideas about how queuing tool concepts will be working together. Classification is for marking packets so you can determine which queue they'll go into. This feature doesn't affect QoS per se by dealing with latency, or jitter, or bandwidth allocation, but it is the foundation for the other tools. Other Cisco tools that will affect QoS characteristics include the drop policy (affects loss), scheduling between queues (affects every QoS characteristic), and maximum queue length (affects loss and delay).
QoS Related Packet Fields
The IP header, LAN trunking headers, ATM cell header, and Frame Relay header all have at least one field that can be marked for QoS purposes. The book spends most of its time on IP Precedence and DSCP. This is because IP packet headers are retained during routing; layer 2 headers are removed when routing is done.
RFC 791 - defines the IP packet header, including a one byte field called Type of Service (ToS)- see section 3.1 in the RFC. ToS was further subdivided and the 3 higher order bits are known as the IP Precedence field. As a historical side note, most of the precedence names were taken straight from military message coding.
Differentiated Services - when DiffServ came along, it redesignated the ToS field and it became known as the DiffServ field. IPP was replaced by a 6 bit field called the DSCP or Differentiated Services Code Point. RFC 3168 defined the lower two bits of the DiffServ field as being used for QoS Explicit Congestion Notification (ECN),
Per-Hop Behaviors (PHBs) - RFC 2475 defines these as "differential treatment an individual packet receives, as implemented by queue service disciplines and/or queue management disciplines." The RFC explicitly states it is defining the behaviors and not the implementation mechanics, since the latter rapidly evolve with technology. Class Selector (CS) per-hop behaviors are used for backwards compatibility with IP precedence marking.
RFC 2598 - defines Expedited Forwarding class (DSCP decimal value 46). EF packets get queuing priority but should also be policed to prevent them from choking out other traffic. It also helps if you've properly estimated your bandwidth usage by EF traffic, but that is another story.
RFC 2597 - defines the Assured Forwarding PHB, with 4 classes inside it for queuing, and 3 levels of drop probability inside each queue. Thus, AF has 12 DSCP values it may set. The names of the values are formatted as AFxy where x is one of the 4 queues and y is one of the 3 drop probability settings. Higher x values correspond to better priority queuing treatment. Ex: AF13 is the lowest setting combination and AF41 is the highest.
Ethernet LAN Class of Service
Ethernet supports a 3 bit QoS field in 802.1Q or ISL trunking headers. 802.1Q defines the 3 most significant bits of the Tag Control field as the QoS field; it calls these bits the user priority bits. In the ISL header, the 3 least significant bits of the 1 byte User field are defined as QoS bits. They are known as the Class of Service (CoS0 field. Most people call Layer 2 QoS settings "CoS" bits, regardless of the actual trunking method in use.
Marking WAN Traffic
Both Frame Relay and ATM have a single bit used for QoS, but these are intended to identify only the drop probability of a cell or frame. Frame Relay calls this Discard Eligibility (DE) and ATM calls this bit Cell Loss Priority (CLP).
If you are using Multiprotocol Label Switching (MPLS) then you can use the MPLS Experimental (EXP) field, 3 bits intended for QoS marking. It is common to see DSCP or IP Precedence markings remapped into the EXP field at the boundary of an MPLS network.
Where Classification and Marking is done
Non-IP header fields will only exist in certain parts of the network, so you can end up doing classification and marking at multiple places. The rules on where to mark traffic are as follows:
Field Location Length
IP Precedence IP Header 3 bits
IP DSCP IP Header 6 bits
DS Field IP Header 1 byte
ToS Byte IP Header 1 byte
CoS ISL & 802.1Q Header 3 bits
Discard Eligible (DE) Frame header 1 bit
Cell Loss Priority ATM cell header 1 bit
MPLS Experimental MPLS Header 3 bits
First, though, let me briefly introduce some ideas about how queuing tool concepts will be working together. Classification is for marking packets so you can determine which queue they'll go into. This feature doesn't affect QoS per se by dealing with latency, or jitter, or bandwidth allocation, but it is the foundation for the other tools. Other Cisco tools that will affect QoS characteristics include the drop policy (affects loss), scheduling between queues (affects every QoS characteristic), and maximum queue length (affects loss and delay).
QoS Related Packet Fields
The IP header, LAN trunking headers, ATM cell header, and Frame Relay header all have at least one field that can be marked for QoS purposes. The book spends most of its time on IP Precedence and DSCP. This is because IP packet headers are retained during routing; layer 2 headers are removed when routing is done.
RFC 791 - defines the IP packet header, including a one byte field called Type of Service (ToS)- see section 3.1 in the RFC. ToS was further subdivided and the 3 higher order bits are known as the IP Precedence field. As a historical side note, most of the precedence names were taken straight from military message coding.
Differentiated Services - when DiffServ came along, it redesignated the ToS field and it became known as the DiffServ field. IPP was replaced by a 6 bit field called the DSCP or Differentiated Services Code Point. RFC 3168 defined the lower two bits of the DiffServ field as being used for QoS Explicit Congestion Notification (ECN),
Per-Hop Behaviors (PHBs) - RFC 2475 defines these as "differential treatment an individual packet receives, as implemented by queue service disciplines and/or queue management disciplines." The RFC explicitly states it is defining the behaviors and not the implementation mechanics, since the latter rapidly evolve with technology. Class Selector (CS) per-hop behaviors are used for backwards compatibility with IP precedence marking.
RFC 2598 - defines Expedited Forwarding class (DSCP decimal value 46). EF packets get queuing priority but should also be policed to prevent them from choking out other traffic. It also helps if you've properly estimated your bandwidth usage by EF traffic, but that is another story.
RFC 2597 - defines the Assured Forwarding PHB, with 4 classes inside it for queuing, and 3 levels of drop probability inside each queue. Thus, AF has 12 DSCP values it may set. The names of the values are formatted as AFxy where x is one of the 4 queues and y is one of the 3 drop probability settings. Higher x values correspond to better priority queuing treatment. Ex: AF13 is the lowest setting combination and AF41 is the highest.
Ethernet LAN Class of Service
Ethernet supports a 3 bit QoS field in 802.1Q or ISL trunking headers. 802.1Q defines the 3 most significant bits of the Tag Control field as the QoS field; it calls these bits the user priority bits. In the ISL header, the 3 least significant bits of the 1 byte User field are defined as QoS bits. They are known as the Class of Service (CoS0 field. Most people call Layer 2 QoS settings "CoS" bits, regardless of the actual trunking method in use.
Marking WAN Traffic
Both Frame Relay and ATM have a single bit used for QoS, but these are intended to identify only the drop probability of a cell or frame. Frame Relay calls this Discard Eligibility (DE) and ATM calls this bit Cell Loss Priority (CLP).
If you are using Multiprotocol Label Switching (MPLS) then you can use the MPLS Experimental (EXP) field, 3 bits intended for QoS marking. It is common to see DSCP or IP Precedence markings remapped into the EXP field at the boundary of an MPLS network.
Where Classification and Marking is done
Non-IP header fields will only exist in certain parts of the network, so you can end up doing classification and marking at multiple places. The rules on where to mark traffic are as follows:
- Classification of traffic: on ingress only (provided the interface supports the particular header field, i.e. EXP, CoS, etc)
- Marking: on egress only, again provided the header is supported
Field Location Length
IP Precedence IP Header 3 bits
IP DSCP IP Header 6 bits
DS Field IP Header 1 byte
ToS Byte IP Header 1 byte
CoS ISL & 802.1Q Header 3 bits
Discard Eligible (DE) Frame header 1 bit
Cell Loss Priority ATM cell header 1 bit
MPLS Experimental MPLS Header 3 bits
Being a king he had authority Download Videos Tips to ask questions. Conn requested her, consequently, all of the questions that he might believe of, for it's not daily that a girl drives from the ocean.
ReplyDelete