Shulou(Shulou.com)06/01 Report--

Look at the location of ToS from the beginning of IP:

ToS is the type of service, and this makes sense only if the network device is able to support (recognize the ToS field in the IP header) to recognize the ToS field. Otherwise, it's all empty talk.

Let's start with the meaning of specific fields:

Tos field length is 8bit

Pre-3bit field:

As a preferred sub-field, it is now obsolete. The default value of this field is 000. according to the result of wireshark packet capture, it indicates:

In the 8bit field of the ToS field in most IP headers, the default value for the first three subfields is 000, which is populated to 000 by default. However, in some specific scenarios, such as the exchange of OSPF-related data between network devices in the OSPF protocol, this field is useful:

4bit field:

These four fields are combined to represent the service category corresponding to the Datagram, which is different in this application layer. The types of services mentioned here refer to:

1000-minimize delay # minimum delay

0100-maximize throughput # maximum Throughput

0010-maximize reliability # highest reliability

0001-minimize monetary cost # minimum fee

0000-normal service # General Services

The ToS field in the header of IP can only indicate one service category, that is, there can be at most one bit field in this 4bit field that is 1.

Take a look at the corresponding values of this 4bit field for different applications:

The translation is:

Minimum delay, corresponding to delay-sensitive applications, such as telnet and human login.

The maximum throughput corresponds to the applications with high throughput requirements, such as FTP file applications, which have higher requirements for file transfer throughput.

Applications with the highest reliability and high requirements for network transmission reliability, such as applications using SNMP, routing protocols and so on.

Minimum cost, such as NNTP user network news and so on.

1bit field:

The end of this 1bit, which is not used, must be forced to 0

Summary:

In this field, for a normal IP message, the value of the first 3bit is 000. this priority subfield is not needed.

The 4bit field represents the class of service, and only one bit field can be 1 in the 4bit field

In the 1bit field, this field is not used but must be forced to 0.

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