Charles Perkins Nokia Alessio Casati Lucent Technologies Generic Tunneling Protocol (GTP) draft-casati-gtp-00.txt ABSTRACT Some discussion has been lately going on on the standardization of GRE within the IETF. GRE has an Impact on standard track protocols such as Mobile IP. A Generic Tunneling Protocol (GTP) sharing the same format and byte order of the 3GPP GTP protocol ( 3G TS 29.060) benefits from the commonality of the encapsulation format used in IP mobility support protocols in different families of 3G standards. GTP HEADER 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ver|*|*|*|S|R| Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Tunnel Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number | Resv/Exp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The GTP header is a variable length header. It is used to encapulate packets over any network layer or transport layer protocol (e.g. it can be transported over UDP/IP, in which case it SHALL use port number TBD [Note:-likely to be the 3386 port, currenlty allocated to GTP defined by 3GPP]). Two flags signal the presence of optional fields: the R flag and the S flag. The R flag is used to signal the need to interpret the Reserved/Experimental field. The S flag is used to signal the presence of the GTP Sequence Number field. If any of these flags are set, the length of the header is 12 octets and the fields corresponding to the flags that are set SHALL be evaluated by the receiver. The bits marked with '*' are currently unused and reserved for experimental use and they MUST not be interpreted. - The version field value is the version of the GTP protocol. This document specifies GTP version 1. - The type field defines possible different uses of the GTP. It shall be set to 0XFF when user traffic is transported. - The lenght Field defines the length of the GTP encapsulated packet (that is the length of the user datagram encasulated using the GTP protocol). - The Tunnel Identifier field is used as a multiplexing field. - The Sequence number field defines the value of the sequence number of the GTP encapsulated data. This field is present and SHALL be interpreted when the S Flag is set to 1. - The Resv/Exp field is present and SHALL be interpreted when the R flag is set to 1. Author's Addresses: Alessio Casati Lucent Technologies Sigma Building Windmill Hill Business Park Wiltshire, SN5 6P United Kingdom email: acasati@lucent.com phone: +44 179388 3861 Charles E. Perkins Communications Systems Lab Nokia Research Center 313 Fairchild Drive Mountain View, California 94043 USA Phone: +1-650 625-2986 EMail: charliep@iprg.nokia.com Fax: +1 650 625-2502