Type of service

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The service type field in the IPv4 header has various destinations over the years, and has been defined in different ways by five RFCs. The modern redefinition of the ToS field is the area of ​​the Six-bit Differentiated Code Point (DSCP) and the two-bit Explicit Congestion Notification (ECN) field. Although the Differential Service is somewhat backwards compatible with ToS, ECN does not.

The ToS field can specify datagram priorities and request routes for low, high, or highly reliable delay services.

Based on these ToS values, a packet will be placed in a prioritized exit queue, or take the route with the appropriate latency, throughput, or reliability.

In practice, ToS field has never seen extensive use outside the US Department of Defense network. However, many experimental, research, and deployment work focuses on how to utilize these eight bits, resulting in the current definition of DS fields.

• Interpretation history for IP Precedence, TOS & amp; DSCP

1) The Service field type in the IP header was originally defined in RFC 791. This definition is largely derived from AS DoD Specification JANAP-128, which defines the message priority.

2) It defines a mechanism for assigning priority to each IP packet, as well as mechanisms for requesting special treatment such as high throughput, high reliability or low latency etc...

3) In practice, only the IP Precedence section in the field has ever been used outside the US DoD network. Some US DoD networks do use delay bits for route selection between oceanic cable lines and Satellite Communications (SATCOM) lines when both lines exist.

4) In the simplest, the higher the IP Precedence field value, the higher the IP packet priority.

5) In the update version: RFC 1349 Bit Monetary Cost was introduced (this bit was previously marked "Requested for Future Use").

6) In the update version: RFC 2474 definition of the whole field is changed.

7) This is now called "DS" (Differentiated Services Services) and the top 6 bits contain a value called "DSCP" (Differential Service Code Code).

8) Since RFC 3168, the remaining two bits (two least significant bits) are used for Explicit Congestion Notification

Video Type of service

Implementation

Before termination, the Service Type field is set as follows.

Presence is a 3-bit field that treats high-priority packages more important than other packages. If a router gets stuck and needs to discard some packets, it will discard packets that have the lowest priority first. Although the precedence field is part of version 4, it was never used.

The last bit of the Type of Service (bit 7) is defined as "Must Be Zero". Since the last three bits pass through many definitions prior to RFC 2474 (see below), the documentation and implementation may be confusing and contradictory.

DSCP and ECN

RFC 2474 (released in December 1998) reserves the first six bits of DS (or IPv4 ToS) fields for the DiffServ Code Points (DSCP), and RFC 3168 reserves the last two bits for Explicit Congestion Notification.

• DSCP & lt; = & gt; IP Priority Conversion Table

• DSCP Name Description

CS: Class Selector (RFC 2474)

AFxy: Secured Forwarding (x = class, y = precedence statement) (RFC 2597)

EF: Expedited Forwarding (RFC 3246)

• "DSCP & lt; = & gt; TOS & lt; = & gt; IP Presence" Value Conversion table

DSCP (Differentiated Service Code Code), TOS (Service type)

Examples based on the table above:

Note: In the table shown above, TOS is considered in Decimal Format, in many TOS routers expressed in hex format. In this case the conversion to Decimal is required.

IPP = 1 in decimal, or 001 in bits. That gives 001 00000 for the entire ToS field, or ToS = 32. DSCP field is: 001000 00, or DSCP = 8 (the last 2 bits are not used related to priority, so we start counting on the third bit).

1) Traditionally, the first three IP bits (- & gt; RFC 791) bits should be used in TOS Application Routing (RFC 1583- & gt; OSPF, IS-IS) but no applications are used widely support it.

2) The TOS field is then redefined as the Differentiated Services Code Point (DSCP- & gt; RFC 2474) consisting of the first 6 bits and 2 bits used for the TCP mechanism called Explicit Congestion Notification (ECN) defined in the RFC 3168.

3) TOS defines "Service type" and represents the 2nd byte in the IP datagram

4) Concept summary for 3 QoS values: IP Precedence, ToS, DSCP

• IP Precedence - 0 ~ 7 (Traditional and simple way of QoS) - & gt; ToS: 0 ~ 224 (too complicated) - & gt; DSCP: 0 ~ 56 (Value optimized)
• IP Precedence, TOS (RFC 791, RFC 1583) - & gt; DSCP (RFC 2474) - & gt; ECN (RFC 3168)

Maps Type of service

See also

• Service class
• Service quality
• Explicit Congestion Notice

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References

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Further reading

• John Evans, Clarence Filsfils (2007). Deploy IP and MPLS QoS for Multiservice Networks: Theory and Practice . Morgan Kaufmann. ISBN: 978-0123705495.

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External links

• Linux Advanced Routing & amp; Control traffic. How to set byte bys through IPChains
• Simple Traffic Queue according to ToS values ​​

Source of the article : Wikipedia