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There are many differences between the behaviour of a T.38 terminal and a T.38 gateway. However, some functions are common to both types of T.38 entity, particular in the IP network interface.
The T.38 specification defines an ASN.1 schema for the messages which pass between T.38 entities. These messages are called IFP (Internet Fascimile Protocol) messages. Their format is independent of the transport used to carry them. However, there are currently two slightly different versions of the ASN.1 schema. This is due to a typo in the original version of the T.38 specification. The protocol negotiation which occurs just before T.38 communication resolves which version will be used. Although the typo was corrected several years ago, it is still very more common to find implementations which only support the original buggy version.