SSAP?

[ashraf_can2]

Hi there! I hope someone will be kind enough to help me with the following question:

If DSAP (Destination Service Access Protocol) from the Ethernet header identifies the type of layer 3 protocol that follows, then what is SSAP for?

Thank you in advance for your help.

Best wishes,

Ashraf

[lgijssel · Accepted solution]

The DSAP field contains a Destination Service Access Point. A DSAP is a unique user-level address that identifies the higher-level protocol used on the destination machine.

The DSAP address is one byte in length. The least significant bit in the DSAP identifies whether an individual or a group of individuals should receive the packet. The remaining seven bits, or the most significant bits of the DSAP, are the address.

The SSAP field contains a Source Service Access Point. An SSAP is a unique user-level address that identifies the higher-level protocol used on the source machine. The SSAP and the DSAP must be the same in order for two nodes to communicate.

http://docs.hp.com/en/98194-90053/ch02s04.html

regards,

Leo

[ashraf_can2]

Thank you very much for your help!

Kind regards,

Ashraf

[helmutkraus]

Hi Ashraf,

as I understand LLC, SAPs are "Service Access Points".

First of all, SAPs do not necessarily denominate a layer 3 protocol, but also (and rather often, look at IEEE802.1d) other L2 services.

The concept of D-SAP and S-SAP simply follows the idea that not only the destination (DSAP) protocol might be of any interest, but the originating protocol also (SSAP).

But of course, usually both source and destination protocol are of the same type.

Example:

One way of transporting IP over LLC is to use the IEEE defined LLC SAP value for IP: 0x96

Because mostly the IP source comunicates with an IP destination instance, both SSAP and DSAP hold the value 0x96.