Layer 3 QoS: IP ToS, Precedence, CoS, IntServ, and DiffServ Codepoints

QoS definitions vary by service and approach chosen. For data communication networks, typical QoS characteristics and metrics include bandwidth, delay (latency), delay variation (jitter), and reliability, as follows:

Bandwidth? Peak data rate (PDR), sustained data rate (SDR), minimum data rate (MDR).
Delay/latency? End-to-end or round-trip delay, delay variation (jitter), node-processing delay.
Reliability? Availability (as percent of uptime), mean time between failures/mean time to repair (MTBF/MTTR), errors, and packet loss.

The IP header contains a Type of Service (ToS) field (see Example 13-12). Applications can set the three precedence bits of this ToS field at the network interface card (NIC) level according to their requirements.

Example 13-12. IPv4 Header with ToS Field


    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   | Version |  IHL  | Type of Service |          Total Length     |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |         Identification         | Flags |      Fragment Offset |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |  Time to Live |    Protocol   |         Header Checksum       |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                       Source Address                          |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                    Destination Address                        |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   |                    Options                     |    Padding   |

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In the context of IP QoS considerations, a 3-bit field in the ToS byte of the IP header is referred to as precedence (see Example 13-13). Using IP precedence, a network administrator can assign values from 0 (the default) to 7 to classify and prioritize types of traffic.

Example 13-13. ToS and Precedence


       0     1     2     3     4     5     6     7

      +-----+-----+-----+-----+-----+-----+-----+-----+

      |                 |     |           |     |     |

      |    PRECEDENCE   | STRM|RELIABILITY| S/R |SPEED|

      |                 |     |           |     |     |

      +-----+-----+-----+-----+-----+-----+-----+-----+

Many applications and routers support IP precedence. The ToS and differentiated services (DiffServ) approach directly tag the traffic itself, which therefore contains in-band QoS markings. An out-band approach is the Resource Reservation Protocol (RSVP). An integrated services (IntServ) approach provides end-to-end QoS in IP networks and relies on per-flow state information and integration with RSVP as a signaling protocol at every involved hop. (IntServ is considered to have some weaknesses.)

DiffServ takes a simpler approach with less signaling overhead and no QoS-aware intermediate network nodes for the entire path. Packets are classified and marked to receive a particular per-hop forwarding behavior on nodes along their path (RFC 2475). The DiffServ (DS) field is supposed to succeed the IPv4 ToS field in the IPv4 header, which is deprecated and in IPv6 context "rejuvenated" as the traffic-class octet (see Example 13-14).

NOTE

For DiffServ internals, see RFC 2474, "Definition of Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers."

Example 13-14. DiffServ Codepoints


The DS field structure is presented below (RFC 2474):



        0   1   2   3   4   5   6   7

      +---+---+---+---+---+---+---+---+

      |         DSCP          |  CU   |

      +---+---+---+---+---+---+---+---+



        DSCP: Differentiated services codepoint

        CU:   Currently unused (reserved)

Note that when you are dealing with DiffServ, two expressions are used frequently: PHB (per-hop behavior) and DSCP (DiffServ codepoint). In current architectures, IP precedence values are mapped into DSCPs.