Internet Protocol Authentication Header

Prerequisite: Internet Protocol version 6 (IPv6) Header
IP Authentication Header is used to provide connection-less integrity and data origin authentication. There are two main advantages that Authentication Header provides,

  • Message Integrity –
    It means, message is not modified while coming from source.
  • Source Authentication –
    It means, source is exactly source from whom we were expecting data.

When packet is sent from source A to Destination B, it consists of data that we need to send and header which consist of information regarding packet. Authentication Header verifies origin of data and also payload to confirm if there has been modification done in between, during transmission between source and destination.

However, in transit, values of some IP header fields might change (like- Hop count, options, extension headers). So, values of such fields cannot be protected from Authentication header. Authentication header cannot protect every field of IP header. It provides protection to fields which are essential to be protected.

Authentication Header :
The question may arise, that how IP header will know that adjacent Extension header is Authentication Header. Well, there is protocol field in IP Header which tells type of header that is present in packet. So, protocol field in IP Header should have value of “51” in order to detect Authentication Header.



  1. Next Header –
    Next Header is 8-bit field that identifies type of header present after Authentication Header. In case of TCP, UDP or destination header or some other extension header it will store correspondence IP protocol number . Like, number 4 in this field will indicate IPv4, number 41 will indicate IPv6 and number 6 will indicate TCP.


  2. Payload Length –
    Payload length is length of Authentication header and here we use scaling factor of 4. Whatever be size of header, divide it by 4 and then subtract by 2. We are subtracting by 2 because we’re not counting first 8 bytes of Authentication header, which is first two row of picture given above. It means we are not including Next Header, Payload length, Reserved and Security Parameter index in calculating payload length. Like, say if payload length is given to be X. Then (X+2)*4 will be original Authentication header length.


  3. Reserved –
    This is 16-bit field which is set to “zero” by sender as this field is reserved for future use.


  4. Security Parameter Index (SPI) –
    It is arbitrary 32-bit field. It is very important field which identifies all packets which belongs to present connection. If we’re sending data from Source A to Destination B. Both A and B will already know algorithm and key they are going to use. So for Authentication, hashing function and key will be required which only source and destination will know about. Secret key between A and B is exchanged by method of Diffie Hellman algorithm. So Hashing algorithm and secret key for Security parameter index of connection will be fixed. Before data transfer starts security association needs to be established.

    In Security Association, both parties needs to communicate prior to data exchange. Security association tells what is security parameter index, hashing algorithm and secret key that are being used.


  5. Sequence Number –
    This unsigned 32-bit field contains counter value that increases by one for each packet sent. Every packet will need sequence number. It will start from 0 and will go till 232 – 1 and there will be no wrap around. Say, if all sequence numbers are over and none of it is left but we cannot wrap around as it is not allowed. So, we will end connection and re-establish connection again to resume transfer of remaining data from sequence number 0. Basically sequence numbers are used to stop replay attack.

    In Replay attack, if same message is sent twice or more, receiver won’t be able to know if both messages are sent from a single source or not. Say, I am requesting 100$ from receiver and Intruder in between asked for another 100$. Receiver won’t be able to know that there is intruder in between.


  6. Authentication Data (Integrity Check Value) –
    Authentication data is variable length field that contains Integrity Check Value (ICV) for packet. Using hashing algorithm and secret key, sender will create message digest which will be sent to receiver. Receiver on other hand will use same hashing algorithm and secret key. If both message digest matches then receiver will accept data. Otherwise, receiver will discard it by saying that message has been modified in between. So basically, authentication data is used to verify integrity of transmission. Also length of Authentication data depends upon hashing algorithm you choose.

Conclusion :
How Authentication Header can be useful ?

  • Message Integrity also known as Connection-less Integrity
  • Source Authentication
  • Replay attack protection

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