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Private ATM addresses are conceptually based on hierarchical addressing domains. The address format is twenty bytes long and consists of two parts: the initial domain part (IDP) and the domain-specific part (DSP). (See Figure 5.9.) The IDP specifies an administration authority which has the responsibility for allocating and assigning values for the DSP. It is subdivided into the authority and format identifier (AFI) and the initial domain identifier (IDI). AFI specifies the format of the IDI, and the abstract syntax of the DSP field. The length of the AFI field is 1 byte. The IDI specifies the network addressing domain, from which the DSPs are allocated and the network addressing authority responsible for allocating values of the DSP from that domain. The following three IDIs have been defined by the ATM Forum:
a. DCC (data country code): This field specifies the country in which the address is registered. The country codes are specified in ISO 3166. These addresses are administered by the ISOs national member body in each country. The length of this field is two bytes, and the digits of the data country code are encoded using BCD.
b. ICD (international code designator): The ICD field identifies an authority which administers a coding scheme. This authority is responsible for the allocation of identifiers within this coding scheme to organizations. The registration authority for the international code designator is maintained by the British Standards Institute. The length of the field is two bytes and the digits of the international code designator are encoded using BCD.
c. E.164 addresses.
The DSP field consists of the high-order DSP (HO-DSP) field, the end system identifier (ESI) field and the selector (SEL) field. The coding for the HO-DSP field is specified by the authority or the coding scheme identified by the IDP. The authority determines how identifiers will be assigned and interpreted within that domain. The authority can create further subdomains. That is, it can define a number of subfields of the HO-DSP and use them to identify a lower authority which in turn defines the balance of HO-DSP. The content of these subfields describe a hierarchy of addressing authorities and convey a topological structure.
The end system identifier (ESI) is used to identify an end device. This identifier must be unique for a particular value of the IDP and HO-DSP fields. The end system identifier can also be globally unique by populating it with a 48-bit IEEE MAC address. Finally, the selector (SEL) field has only local significance to the end device; it is not used in routing. It is used to distinguish different destinations reachable at the end device.
Of interest is how IP addresses can be mapped to the NSAP structure. Figure 5.10 shows a mapping which can eliminate the use of ATMARP.
To show how the NSAP ATM addresses can be used, we refer to the ATM addressing scheme for the private ATM network, North Carolina Advanced Network (NCANet). NCANet is a production network that is used for research and education purposes. The ICD format was selected (see Figure 5.9). The US GOSIP coded HO-DSP field was used, which consists of: a 1-byte domain format identifier (DFI) field; a 3-byte administrative authority (AA) field; a 2-byte reserved field; a 2-byte routing domain number (RDN) field; and a 2-byte AREA field. The fields were populated as follows (in hexadecimal):
AFI = 47: Indicates that an ICD ATM format is used.
ICD = 0005: Indicates that a GOSIP (NIST) coded HO-DSP field is used.
DFI = 80: Indicates that the next 3 bytes represent the administrative authority; in this case, it indicates the Micro Electronics Center of North Carolina (MCNC), which is responsible for handling the regional traffic.
AA = FFEA00: Assigned to MCNC by GOSIP (NIST).
Reserved field = 0000.
RDN = xxxx: To be assigned by MCNC. For instance, North Carolina State University is part of NCANet, and it has been assigned the RND value of 0101.
AREA = yyyy: To be assigned by the RDN owner. For instance, a group of ATM addresses at North Carolina State University have been assigned the AREA value of 1114.
As a result, all ATM addresses of ATM end devices and ATM switches in NCANet have the following NSAP prefix (in hexadecimal):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
47 00 90 0 SEL
AFI ICD Unused IPv4 address
Figure 5.10 The NSAP ATM format for IP addresses.
SIGNALING IN ATM NETWORKS
The following address (in hexadecimal) is an example of the complete ATM address of an ATM switch in the ATM Lab of North Carolina State University:
The first thirteen bytes provide the prefix, equal to: 47.0005.80.FFEA00.0000.0101.1114. The next field is the end system identifier (ESI); it is populated with the value of 400000000223, which is the IEEE MAC address of the switch. The final field is the selector (SEL), which is populated with the value 00.