This was not an easy chapter – even the author(s) said take a break at least four time during the chapter. Even so close to passing the Cisco QOS exam this was still tough going and took me at a week of reading and re-reading. I know i will be reading this section again at some point once the book is done.
A few terms to know –
assured forwarding (AF)
CoS packet forwarding class that provides a group of values you can define and includes four subclasses, AF1, AF2, AF3, and AF4, each with three drop probabilities, low, medium, and high.
behavior aggregate (BA) classifier
Feature that can be used to determine the forwarding treatment for each packet. The behavior aggregate classifier maps a code point to a forwarding class and loss priority. The loss priority is used later in the work flow to select one of the two drop profiles used by random early detection (RED).
best effort (BE)
CoS packet forwarding class that provides no service profile. For the BE forwarding class, loss priority is typically not carried in a code point, and random early detection (RED) drop profiles are more aggressive.
class of service (CoS)
Method of classifying traffic on a packet-by-packet basis, using information in the type-of-service (ToS) byte to assign traffic flows to different service levels.
Differentiated Services (DiffServ)
Services based on RFC 2474, Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers. The DiffServ method of CoS uses the type-of-service (ToS) byte to identify different packet flows on a packet-by-packet basis. DiffServ adds a Class Selector code point (CSCP) and a DiffServ code point (DSCP).
DiffServ code point (DSCP) values
Values for a 6-bit field defined in IP packet headers that can be used to enforce class-of-service (CoS) distinctions.
Drop probabilities for different levels of buffer fullness that are used by random early detection (RED) to determine when to drop packets from a given J-series or SRX-series device scheduling queue.
expedited forwarding (EF)
CoS packet forwarding class that provides end-to-end service with low loss, low latency, low jitter, and assured bandwidth.
multifield (MF) classifier
Firewall filter that scans through a variety of packet fields to determine the forwarding class and loss priority for a packet and polices traffic to a specific bandwidth and burst size. Typically, a classifier performs matching operations on the selected fields against a configured value.
network control (NC)
CoS packet forwarding class that is typically high priority because it supports protocol control.
Packet loss priority bit. Used to identify packets that have experienced congestion or are from a transmission that exceeded a service provider’s customer service license agreement. A J-series or SRX-series device can use the PLP bit as part of a congestion control strategy. The bit can be configured on an interface or in a filter.
Feature that limits the amount of traffic passing into or out of an interface. It is an essential component of firewall filters that is designed to thwart denial-of-service (DoS) attacks. A policer applies rate limits on bandwidth and burst size for traffic on a particular J-series device interface.
Applying rate and burst size limits to traffic on an interface.
random early detection (RED)
Gradual drop profile for a given class, used for congestion avoidance. RED attempts to anticipate congestion and reacts by dropping a small percentage of packets from the tail of the queue to prevent congestion.
Guide that the device follows when applying services. A rule consists of a match direction and one or more terms.