GPRS Tunnelling Protocol: Wikis

Advertisements
  

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

From Wikipedia, the free encyclopedia

The Internet Protocol Suite
Application Layer
BGP · DHCP · DNS · FTP · GTP · HTTP · IMAP · IRC · Megaco · MGCP · NNTP · NTP · POP · RIP · RPC · RTP · RTSP · SDP · SIP · SMTP · SNMP · SOAP · SSH · Telnet · TLS/SSL · XMPP · (more)
Transport Layer
TCP · UDP · DCCP · SCTP · RSVP · ECN · (more)
Internet Layer
IP (IPv4, IPv6) · ICMP · ICMPv6 · IGMP · IPsec · (more)
Link Layer
ARP/InARP · NDP · OSPF · Tunnels (L2TP) · PPP · Media Access Control (Ethernet, DSL, ISDN, FDDI) · (more)

GPRS Tunnelling Protocol (GTP) is a group of IP-based communications protocols used to carry General Packet Radio Service (GPRS) within GSM and UMTS networks.

GTP can be decomposed into separate protocols, GTP-C, GTP-U and GTP'. GTP-C is used within the GPRS core network for signaling between Gateway GPRS Support Nodes (GGSN) and Serving GPRS Support Nodes (SGSN). This allows the SGSN to activate a session on a user's behalf (PDP context activation), to deactivate the same session, to adjust quality of service parameters, or to update a session for a subscriber who has just arrived from another SGSN.

GTP-U is used for carrying user data within the GPRS Core Network and between the Radio Access Network and the core network. The user data transported can be packets in any of IPv4, IPv6, or PPP formats.

GTP' (GTP prime) uses the same message structure as GTP-C and GTP-U, but has an independent function. It can be used for carrying charging data from the Charging Data Function (CDF) of the GSM or UMTS network to the Charging Gateway Function (CGF). In most cases, this should mean from many individual network elements such as the GGSNs to a centralized computer that delivers the charging data more conveniently to the network operator's billing center.

Different GTP variants are implemented by RNCs, SGSNs, GGSNs and CGFs within 3GPP networks. GPRS mobile stations (MSs) are connected to a SGSN without being aware of GTP.

GTP can be used with UDP or TCP. GTP version one is used only on UDP.

Contents

General features

All variants of GTP have certain features in common. The structure of the messages is the same, with a GTP header following the UDP/TCP header.

Advertisements

Header

GTPv1 headers contain the following fields:[1]

+ Bit 0-2 3 4 5 6 7 8-15 16-23 24-31
0 Version Protocol type Reserved Extension Header Flag Sequence Number Flag N-PDU Number Flag Message Type Total length
32 TEID
64 Sequence number N-PDU number Next extension header type
Version 
It is a 3-bit field. For GTPv1, this has a value of 1.
Protocol Type (PT) 
a 1-bit value that differentiates GTP (value 1) from GTP' (value 0).
Reserved
a 1-bit reserved field (must be 0).
Extension header flag(E)
a 1-bit value that states whether there is an extension header optional field.
Sequence number flag(S)
a 1-bit value that states whether there is a Sequence Number optional field.
N-PDU number flag(PN)
a 1-bit value that states whether there is a N-PDU number optional field.
Message Type
an 8-bit field that indicates the type of GTP message.
Length
a 16-bit field that indicates the length of the payload in bytes (rest of the packet following the mandatory 8-byte GTP header). Includes the optional fields.
Tunnel endpoint identifier (TEID)
A 32-bit field used to multiplex different connections in the same GTP tunnel.
Sequence number
an (optional) 16-bit field. This field exists if any of the E, S, or PN bits are on. The field must be interpreted only if the S bit is on.
N-PDU number
an (optional) 8-bit field. This field exists if any of the E, S, or PN bits are on. The field must be interpreted only if the PN bit is on.
Next extension header type
an (optional) 8-bit field. This field exists if any of the E, S, or PN bits are on. The field must be interpreted only if the E bit is on.

Next Extension Headers are as follows:

+ Bits 1-7 8-23 24-31
0 Total length Contents
... ...
... Contents Next extension header
Length
an 8-bit field. This field states the length of this extension header, including the length, the contents, and the next extension header field, in 4-octet units. The length must be a multiple of 4.
Contents
extension header contents.
Next extension header
an 8-bit field. It states the type of the next extension, or 0 if no next extension exists. This permits chaining several next extension headers.

Connectivity mechanisms

Apart from the common message structure, there is also a common mechanism for verifying connectivity from one GSN to another GSN. This uses two messages.

  • echo request
  • echo response

As often as every 60 seconds, a GSN can send an echo request to every other GSN with which it has an active connection. If the other end does not respond it can be treated as down and active connections to it deleted.

Apart from the two messages previously mentioned, there are no other messages common across all GTP variants[2] meaning that, for the most part, they effectively form three completely separate protocols.

GTP-C - GTP control

The GTP-C protocol is the control section of the GTP standard. When a subscriber requests a PDP context, the SGSN will send a create PDP context request GTP-C message to the GGSN giving details of the subscriber's request. The GGSN will then respond with a create PDP context response GTP-C message which will either give details of the PDP context actually activated or will indicate a failure and give a reason for that failure. This is a UDP message on port 2123.

GTP-U - GTP user data tunneling

GTP-U is, in effect a relatively simple IP based tunneling protocol which permits many tunnels between each set of end points. When used in the UMTS, each subscriber will have one or more tunnel, one for each PDP context they have active plus, possibly separate tunnels for specific connections with different quality of service requirements.

The separate tunnels are identified by a (Tunnel Endpoint Identifier) in the GTP-U messages, which should be a dynamically allocated random number. If this random number is of cryptographic quality, then it will provide a measure of security against certain attacks. Even so, the requirement of the 3GPP standard is that all GTP traffic, including user data should be sent within secure private networks, not directly connected to the Internet. This happens on UDP port 2152.

GTP' - charging transfer

The GTP' protocol is used to transfer charging data to the Charging Gateway Function. GTP' uses TCP/UDP port 3386.

Within the GPRS core network

GTP is the primary protocol used in the GPRS core network. It is the protocol which allows end users of a GSM or UMTS network to move from place to place whilst continuing to connect to the Internet as if from one location at the GGSN. It does this by carrying the subscriber's data from the subscriber's current SGSN to the GGSN which is handling the subscriber's session. Three forms of GTP are used by the GPRS core network.

  • GTP-U for transfer of user data in separated tunnels for each PDP context
  • GTP-C for control reasons including:
    • setup and deletion of PDP contexts
    • verification of GSN reachability
    • updates; e.g., as subscribers move from one SGSN to another.
  • GTP' for transfer of charging data from GSNs to the charging function.

GGSNs and SGSNs (collectively known as GSNs) listen for GTP-C messages on UDP port 2123 and for GTP-U messages on port 2152. This communication happens within a single network or may, in the case of international roaming, happen internationally, probably across a GPRS roaming exchange (GRX).

The Charging Gateway Function (CGF) listens to GTP' messages sent from the GSNs on TCP/UDP port 3386. The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred. However, this communication which occurs within one network is less standardized and may, depending on the vendor and configuration options, use proprietary encoding or even an entirely proprietary system.

Use on the IuPS interface

GTP-U is used on the IuPS between the GPRS core network and the RAN, however the GTP-C protocol is not used. In this case, RANAP is used as a control protocol and establishes GTP-U tunnels between the SGSN and the radio network controller (RNC).

Protocol stack

 ????
IP (user)
GTP
UDP
IP
Layer 2 (e.g., WAN or Ethernet)

GTP-U protocol stack

GTP can be used with UDP or TCP. GTP version one is used only on UDP.

As of 2004 there are two versions defined, version 0 and version 1. Version 0 and version 1 differ considerably in structure. In version 0, the signalling protocol (the protocol which sets up the tunnels by activating the PDP context) is combined with the tunneling protocol on one port. Version 1 is actually effectively two protocols, one for control (called GTP-C) and one for user data tunneling (called GTP-U).

GTP-U is also used to transport user data from the RNC to the SGSN in UMTS networks. However, in this case signalling is done using RANAP instead of GTP-C.

Historical GTP versions

The original version of GTP (version 0) had considerable differences from the current version (version 1):

  • the tunnel identification was non-random;
  • options were provided for transporting X.25;
  • the fixed port number 3386 was used for all functions (not just charging as in GTPv1);
  • TCP was allowed as a transport option instead of UDP, but support for this was optional;
  • subscription-related fields such as quality of service were more limited.

The non-random TEID in version 0 represented a security problem if an attacker had access to any roaming partner's network, or could find some other way to remotely send packets to the GPRS backbone. Version 0 is going out of use and being replaced by version 1 in almost all networks. Even so, the standard for the newer version states that the older version must be supported by the GSN. Fortunately, however the use of different port numbers allows easy blocking of version 0 through simple IP access lists.

GTP standardization

GTP was originally standardized within ETSI (GSM standard 09.60). With the creation of the UMTS standards this was moved over to the 3GPP which, as of 2005 maintains it as 3GPP standard 29.060. GTP' uses the same message format, but its special uses are covered in standard 32.295 along with the standardized formats for the charging data it transfers.

Later versions of TS 29.060 deprecate GTPv1/v0 interworking such that there is no fallback in the event that the GSN does not support the higher version.

GTPv2 (for evolved packet services) went into draft in early 2008 and was released in December of that year. GTPv2 offers fallback to GTPv1 via the earlier "Version Not Supported" mechanism but explicitly offers no support for fallback to GTPv0.

See also

Notes

  1. ^ 3GPP TS 29.060 section 6
  2. ^ 3GPP TS 29.060 section 7

References

  • GSM standard 09.60, ETSI, 1996-98, this standard covers the original version 0 of GTP.
  • 3GPP TS 29.060 V6.9.0 (2005-06), 3rd Generation Partnership Project, 650 Route des Lucioles - Sophia Antipolis, Valbonne - FRANCE, 2005-06. This is the primary standard defining all of the GTP variants for GTP version 1.
  • 3GPP TS 32.295 V6.1.0 (2005-06), 3rd Generation Partnership Project, 650 Route des Lucioles - Sophia Antipolis, Valbonne - FRANCE, 2005-06. This standard covers using GTP for charging.
  • 3GPP TS 29.274 V8.1.0 (2009-03), 3rd Generation Partnership Project, 650 Route des Lucioles - Sophia Antipolis, Valbonne - FRANCE, 2009-03. GTPv2 for evolved GPRS.

External links


Advertisements






Got something to say? Make a comment.
Your name
Your email address
Message