IP option 20, Router Alert

Description Glossary RFCs Publications Obsolete RFCs


Protocol suite: TCP/IP.
Protocol type:Connectionless network layer protocol.
Base protocol: IP, Internet Protocol.
Option length:4 bytes.
Host implementation:
Router implementation:
Links: IANA: IP option numbers.

The IP Router Alert option can be used to notify transit routers to more closely examine the contents of an IP packet. This is useful for, but not limited to, new protocols that are addressed to a destination but require relatively complex processing in routers along the path.

MAC header IP header IP option 20

IP Option 20:

0001020304050607 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Type Length Router Alert Value

Type. 8 bits. Set to 20.

C Class Option

C, Copy flag. 1 bit. Set to 1.
This option is to be copied into all fragments.

Class. 2 bits. Cleared to 0.
The option is a control option.

Option. 5 bits. Set to 20.
The IP option number.

Length. 8 bits. Set to 4.
Total length of the option in bytes.

Router Alert Value. 16 bits.
Specifies what action the router should take.

0 Router shall examine packet.



[RFC 2113] IP Router Alert Option.


Obsolete RFCs:

Description Glossary RFCs Publications Obsolete RFCs