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Mobile Services (ST 2010)Chapter 4: Mobile Internet

Axel Küpper

Service-centric NetworkingDeutsche Telekom Laboratories, TU Berlin

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Mobile ServicesSummer Term 2010

4 Mobile Internet4.1 Problem Statement

4.2 Mobile IP

4.3 Network Layer Support in GPRS/UMTS

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4.1 Problem StatementInternet Protocol - Overview

Internet Protocol

Primary protocol of the network layer used for transmission in networks that employ packet-switched data communication

Packets can be sent without establishing a connection before (unlike circuit-switched communication)

Packets can be lost

Packets may arrive at the receiver in another order than initially sent

Each data packet contains a header that (besides other things) fixes the IP address of the sender (source) and the address of the receiver (destination)

Packet passes different routers along the way from the source to the destination

Router receives a packet from a source or a previous router, gets the destination address from the IP header, and forwards the packet to the next router

3

Version IHL Type of Service Total Length

Identification Flags Fragment Offset

Time to Live Protocol (IP) Header Checksum

Source Address

Destination Address

Options and Padding (optional)

0 4 8 12 16 20 24 28 31

TCP/UDP/… payload

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4.1 Problem StatementInternet Protocol - Addressing

Internet is an interconnection of several networks

IP addresses refer to the network a host is connected to (network prefix) and to the host within that network (host number)

Two-Level Classful Hierarchy

Initial addressing scheme for fixing the length of network prefixes and number of hosts within a network

Class A

8-bit network prefix for 126 networks

16,777,214 hosts per network

Class B

16-bit prefix for 16,384 networks

65,534 hosts per network

Class C

24-bit prefix for 2,097,152 networks

254 hosts per network

Three-Level Subnet Hierarchy

Division of Class A, B, and C networks into smaller subnetworks that have a common, designated IP addressing routing index

Subnet mask fixes the length of the prefix (sum of length of network prefix and subnet number)

Breaks a network into smaller realms that may use existing network address space more efficiently

4

Network prefix Host number

Network prefix Subnet number Host number

8, 16, or 24 bits forclass A, B, or C

network prefixes

24, 16, or 8 bits forclass A, B, or C host number

Two-Level Classful Hierarchy

Three-Level Subnet Hierarchy

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4.1 Problem StatementRouting Example (I)

Problem: IP addresses contain routing information (network/subnet ID fixed by the subnet mask)

What happens if Host C moves from the 128.8 network to the 128.9.1 subnet?

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Host ADestination Subnet mask Route to128.8.1.2 255.255.255.255 SELF128.8.0.0 255.255.0.0 LAN 0128.9.00 255.255.0.0 128.8.1.1

RouterDestination Subnet mask Route to128.8.1.1 255.255.255.255 SELF128.9.1.1 255.255.255.255 SELF128.9.2.1 255.255.255.255 SELF128.8.0.0 255.255.0.0 LAN 0128.9.1.0 255.255.255.0 LAN 1128.9.2.0 255.255.255.0 LAN 2

Host BDestination Subnet mask Route to128.9.1.2 255.255.255.255 SELF128.9.1.0 255.255.255.0 LAN 0DEFAULT 128.9.1.1

Host CDestination Subnet mask Route to128.9.2.2 255.255.255.255 SELF128.9.2.0 255.255.255.0 LAN 0DEFAULT 128.9.2.1128.8 network

128.9.1 subnet 128.9.2 subnet

Host A

Host B Host CRouter

LAN 0

LAN 1 LAN 2

LAN 0

LAN 0LAN 0

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4.1 Problem StatementRouting Example (II)

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Host ADestination Subnet mask Route to128.8.1.2 255.255.255.255 SELF128.8.0.0 255.255.0.0 LAN 0128.9.00 255.255.0.0 128.8.1.1

RouterDestination Subnet mask Route to128.8.1.1 255.255.255.255 SELF128.9.1.1 255.255.255.255 SELF128.9.2.1 255.255.255.255 SELF128.8.0.0 255.255.0.0 LAN 0128.9.1.0 255.255.255.0 LAN 1128.9.2.0 255.255.255.0 LAN 2

Host BDestination Subnet mask Route to128.9.1.2 255.255.255.255 SELF128.9.1.0 255.255.255.0 LAN 0DEFAULT 128.9.1.1

Host CDestination Subnet mask Route to128.9.1.3 255.255.255.255 SELF128.9.1.0 255.255.255.0 LAN 0DEFAULT 128.9.1.1

128.8 network

128.9.1 subnet 128.9.2 subnet

Host A

Host B

Host C

Router

LAN 0

LAN 1 LAN 2

LAN 0

LAN 0

LAN 0

Because an IP address is not only a reference to a particular host, but also addresses the network and subnet the host is connected to, a host gets a new IP address when getting connected to another network

Original Internet Protocol does not support mobility!

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4.1 Problem StatementSolutions for Mobility Support in the Internet

Problem: TCP connection cannot survive IP address change

TCP connections are identified by the tuple (source address, source port, destination address, destination port)

TCP connection cannot survive any address change

Solution: inform all communication partners of the mobile node about the new address

Drawback: requires modification of existing, proven protocol software

Problem: Mobile hosts acting as servers are not reachable after an address change

Solution #1: simply assign a new, topologically correct address after the movement

Drawback: nobody knows about this new address – it is almost impossible to find a host on the Internet that has just changed its address

Solution #2: Use of Dynamic DNS (DynDNS) for updating the mapping between a logical domain name and an IP address

Drawback: because of caching methods, DynDNS is not able to cope with frequent updates

Solution #3: create dedicated routes for a mobile node

Drawback: routing tables are based on network prefixes – storing entries of millions of mobile nodes would not scale

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4.2 Mobile IPComponents and Addresses (I)

Mobile Node (MN) Mobile device that moves to other

networks and for which mobility support is to be provided

Keeps its IP address and can continuously communicate with any other system in the Internet as long as link-layer connectivity is given

Correspondent Node (CN) Communication partner of the MN

May invoke services offered by the MN

Home Network Subnet the MN belongs to

Responsible for assigning an IP address to the MN

No mobility support required inside the home network

Foreign Network Current subnet the MN visits and which is

not the home network

Internet

CN

Router

HomeNetwork Router HA

ForeignNetwork

MN

Router FA

CoA

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4.2 Mobile IPComponents and Addresses (II)

Foreign Agent

Implemented in the router of the foreign network

Acts a tunnel endpoint that decapsulates packets and forwards them to the MN

Care-of Address (CoA)

IP address associated with the MN in the foreign network

IP packets sent to the MN are delivered to the CoA, not directly to the MN‘s IP address

CoA marks the tunnel endpoint

Foreign Agent CoA

CoA is an IP address referring to the FA

FA is tunnel endpoint and forwards packets to the MN

Internet

CN

Router

HomeNetwork Router HA

ForeignNetwork

MN

Router FA

CoA

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g4.2 Mobile IP

Basic Procedure

CN → HA

CN transmits an IP datagram destined to the MN, with MN’s home address in the IP header

IP datagram is routed to the home network of the MN

HA → FA

At the home network, IP datagram is intercepted by the HA

HA encapsulates the datagram inside a new IP datagram with the MN’s CoA in its header

Encapsulated IP datagram is retransmitted and routed to the FA

FA → MN

FA decapsulates the original IP datagram and delivers it to the MN across the foreign network

MN → CN

When the MN sends a reply to CN, the associated datagram travels directly across the Internet to the CN 10

Internet

CN

Router

HomeNetwork Router HA

ForeignNetwork

MN

Router FA

CoA

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g4.2 Mobile IP

Alternative: “FA Inside”

Initial version of Mobile IP worked only with a FA in the foreign network

Alternative proposal: MN may act as its own FA

Tunnel is established between HA and MN, and MN is addressed by a Co-located CoA

Advantage: no enhancements need to made in the foreign network

Disadvantage: scarcity of IP addresses (if MN received a permanent address)

Co-located CoA

CoA is assigned to the MN, and hence is topologically correct

May be acquired using DHCP

May be assigned as a long-term address for use when the MN visits the foreign network

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Internet

CN

Router

HomeNetwork Router HA

ForeignNetwork

MN

Router

Co-located CoA

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g4.2 Mobile IP

How to Detect the Entering of a New Network? (I)

Agent Advertisement

HAs and FAs periodically broadcast their presence using an Agent Advertisement Message (extension of ICMP messages)

MNs listen to the broadcast …

… to detect whether they entered a new subnet

… to detect whether the new subnet is their home or foreign network

… to receive special features of the new subnet

… to receive a CoA if they entered a new foreign network

Agent Solicitations

Agent Advertisements are only broadcast periodically

MN may explicitly solicit for an Agent Advertisement Message if it needs it immediately

Agent Advertisement Message

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Type Code Checksum

#Addresses Addr. Size Lifetime

Router address 1

Preference level 1

0 4 8 12 16 20 24 28 31

Router address 2

Preference level 2

Type=16 Lentgh Sequence number

Registration lifetime RBHFMGrT reserved

CoA 1

CoA 2St

and

ard

ICM

P m

essa

geEx

ten

sio

ns

for

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g4.2 Mobile IP

How to Detect the Entering of a New Network? (II)

MN may move from one subnet to another without the IP layer being aware of it

Agent discovery process enables the MN to detect such a move

Use of lifetime field

MN receiving an Agent Advertisement Message from an agent records the lifetime field as a timer

If the timer expires before the MN has received the next message, it assumes that it has lost contact to that agent

If, in the meantime, it has received an AA message from another agent (and that advertisement has not yet expired), the MN registers with this new agent

If it has not received an AA message, MN uses Agent Solicitation

Use of network prefix

MN checks whether any newly received AA is on the same network as the MN’s current CoA

If not, MN assumes that it has moved and may register with the agent whose advertisement it has just received.

13

Router FA Router FA

Agent Advertisement

Agent Advertisement

Agent Advertisement

Agent Advertisement

Agent Advertisement

Agent Solicitation

Agent Advertisement

MN

AA

Bro

adca

stA

ASo

licit

atio

n

Old New

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g4.2 Mobile IP

Registration with a Foreign Network

Once an MN has detected that it entered a (new) foreign network, it has to register with the FA and alert the HA.

MN sends a registration request to the FA

FA relays request to the HA

HA accepts or denies request and sends a registration reply to the FA

FA forwards reply to the MN

If the MN uses a co-located CoA, then it registers directly with its HA

14

RegistrationRequest

RegistrationRequest

RegistrationReplyRegistration

Reply

Router HA

ForeignNetwork

MNRouter FA

Registration via the FA:

RegistrationRequest

RegistrationReply

Router HA

HomeNetwork

MN

Registration directly with the HA:

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4.2 Mobile IPTunneling and Encapsulation (I)

Tunnel

Establishes a virtual pipe for data packets between a tunnel entry and a tunnel endpoint

Packets entering a tunnel are forwarded inside the tunnel and leave the tunnel unchanged

Tunneling, i.e., sending a packet through a tunnel is achieved by using encapsulation

Whole tunnel is considered as single hop from the packet’s point of view

Tunneling allows the MN to behave as if it were attached directly to the home network

Encapsulation/Decapsulation

Encapsulation: mechanism of taking a packet consisting of packet header and data and putting it into the data part of the new packet

Decapsulation: taking a packet out of the data part of another packet

Encapsulation/decapsulation are operations typically performed when a packet is transferred from a higher protocol layer to a lower one (or from a lower layer to a higher layer respectively)

Mobile IP uses encapsulation/decapsulation within the same layer

Original IP header

Original data

New dataNew IP header

Outer header

Inner header

Original dataOR

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4.2 Mobile IPTunneling and Encapsulation (II)

IP-in-IP Encapsulation

Simple encapsulation of one IP packet into another one

Mandatory of Mobile IP

Drawback: several redundant fields

Minimal Encapsulation

Removes redundant fields

Source address in the inner header is omitted if original sender is identical to the HA

Version IHL Type of Service Total Length

Identification Flags Fragment Offset

Time to Live IP-in-IP Header Checksum

Source Address (HA Address)

Destination Address (Care-of Address)

0 4 8 12 16 20 24 28 31

TCP/UDP/… payload

Version IHL Type of Service Total Length

Identification Flags Fragment Offset

Time to Live IPv4 Header Checksum

Source Address (CN Address)

Destination Address (Home address of MN)

0 4 8 12 16 20 24 28 31

Version IHL Type of Service Total Length

Identification Flags Fragment Offset

Time to Live Min. Encaps. Header Checksum

Source Address (HA Address)

Destination Address (Care-of Address)

TCP/UDP/… payload

S Header Checksum

Destination Address (Home Address of MN)

Source Address (CN Address, is not present if S=0)

IPv4 reserved

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4.2 Mobile IPHA Realizations

Home Agent (HA)

Tunnel for packets toward the MN starts at the HA

HA maintains a location registry, which stores the CoA of all MNs that have their origins in the home network

HA Realization #1

HA is implemented on a router that is responsible for the home network

Good solution as all packets have to pass the router anyway

HA Realization #2

HA could be implemented on an arbitrary node in the subnet

Disadvantage: double crossing of the router by all packets if the MN is in a foreign network

HA Realization #3

No home network, but only a virtual home network represented by a router and all MNs are always connected to foreign networks

HA is again implemented on the router

Internet

CN

Router

HomeNetwork Router HA

ForeignNetwork

MN

Router FA

CoA

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g4.2 Mobile IP

Optimization (I)

Problem: Triangular Routing

With basic Mobile IP, all packets to the MN have to go through the HA

May cause unnecessary overhead for the network between CN and HA, but also between HA and CoA, depending on the current location of the MN

Figure: although the communicating computers might only be a few meters away, the packets have to travel around the world

18

Router FA

CN MN

Router HA

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g4.2 Mobile IP

Optimization (II)

Idea: Optimize routing by informing the CN of the current location of the MN

Binding cache: part of the local routing table of the CN

Introduction of four additional messages

Binding request

Any CN that wants to know the current location of an MN sends a binding request to the HA

If HA is allowed to disclose the MN’s location, it returns a binding update

Binding update

Used by the HA to inform the CN about the current location of the MN

Contains the MN’s fixed address and CoA

Binding acknowledgement

CN returns an acknowledgement after receiving an update

Binding warning

If an FA decapsulates a packet for an MN, but is not the MN’s current FA, it sends a warning to the CN

Warning contains the MN’s fixed home address

After receiving a warning, the CN knows that the MN has probably moved, and initiates a binding request 19

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g4.2 Mobile IP

Optimization (III)

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MN changeslocation

Data

Router FA

New

CN

DataDataUpdate

ACK

DataData

RegistrationUpdate

ACK

DataData

DataWarningRequest

Update

ACK

DataData

Router HA Router FA

Old

MN

1

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4

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g4.3 Network Layer Support in GPRS/UMTS

GPRS/UMTS Basic Procedures

Before data can be transmitted between the UE and an external host, some preparations are necessary

UE must be introduced to the network

Appropriate SGSN must be found (according to the user’s location)

Features and capabilities of the data service must be negotiated

An GGSN must be selected and a setup between UE and GGSN has to be established

Path for routing data packets has to prepared for tunneling

Three procedures

GPRS Attach

Activation of PDP Context

Data Transfer

GERAN Packet-switched domain

Internet

UE

BTS GGSNBSC SGSN

GPRS Attach

Connection Setup (Activation of a PDP Context)

Transmission of the User Data

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g4.3 Network Layer Support in GPRS/UMTS

PDP Context

PDP Context

PDP: Packet Data Protocol

Describes characteristics of the session

Contains routing information for packet transfer between a UE and a GGSN to have access to an external packet-switching network

Stored in the mobile station, the SGSN, and the GGSN

Once a mobile station has an active PDP context, it is visible for the external network and can send and receive packets

Each UE may be assigned several PDP contexts for …

… getting access to different networks

… different services

… different charging methods

Access Point Name (APN)

Logical name for the desired data network

Refers to the GGSN enabling access to that network

PDP type Type of the packet network used (e.g.,

IPv4 or IPv6)

PDP address Address assigned to the mobile station

(e.g. an IP address) Static address: permanent IP address of

UE Dynamic address: dynamically assigned,

usually for the duration of a session

QoS class Four QoS parameters: service

precedence, reliability, delay, and throughput

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g4.3 Network Layer Support in GPRS/UMTS

GPRS/UMTS Packet Addresses

Packet Data Protocol (PDP) Address

Address of a UE in the format of the used PDP (e.g., IP address)

Static

UE permanently owns a PDP address assigned by the operator of the user’s home GSM network

Dynamic

UE is assigned a new PDP address whenever it attaches to the network

Dynamic Home-PLMN Address: Dynamic address assigned by the user’s home PLMN

Dynamic Visited-PLMN Address: dynamic address assigned by the operator of the visited PLMN

GGSN is responsible for the allocation and deactivation of PDP addresses

Packet TMSI (P-TMSI)

Assigned during an GPRS attach procedure and after a location update

Used to page the UE when packets have to be delivered

Mapping between PDP address and P-TMSI by the SGSN makes the transmission of packets between GGSN and UE possible

Routing Area Identifier (RAI)

In order to optimize location management, GSM location areas are subdivided into several routing areas

RAI is transmitted from the MS to the network instead of the LAI

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g4.3 Network Layer Support in GPRS/UMTS

Quality of Service

Quality of Service Classes

Required to support diverse applications (ranging from real-time video conferencing to E-Mail)

Enable operators to offer different billing options

Service precedence Priority of a service in relation to other

services (high, normal, low)

Reliability Reliability of a service with regard to

probability of packet loss, packet duplication, wrong sequencing, packet corruption

Delay Maximum values for mean and 95% delay

with regard to end-to-end delay between UEs or between a UE and the serving GGSN

Throughput Peak bit rate and mean bit rate

Probability

Lost packet Duplicated packetOut of sequ.

packetCorrupted

packet Mean delay 95% delay

10-9

10-4

10-2

10-9

10-5

10-5

10-9

10-6

10-5

10-9

10-9

10-2

< 0.5

< 5

< 50

Best effort

< 1.5

< 25

< 250

Best effort

Delay class

1

2

3

4

Rel. class

1

2

3

Seconds

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g4.3 Network Layer Support in GPRS/UMTS

Multiple PDP Contexts

Multiple PDP contexts

UE with different PDP contexts

Operators may use different PDP contexts with different APNs for charging different services in the Internet differently, for example

Mobile Web

Email

Tethering

UE

BTS BSC SGSN

internet.carrier.de

wap.carrier.de

ims.carrier.de

mms.carrier.de

GGSNWAPGW

Internet

GGSN

GGSN

IMS

MMS Server

GGSN

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g4.3 Network Layer Support in GPRS/UMTS

PDP Context Example and Configuration

26

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Configuration of PDP contextCurrent PDP context(s)

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g4.3 Network Layer Support in GPRS/UMTS

GPRS Attach (I)

GPRS Attach is executed when subscriber switches on his device or he explicitly activates GPRS while in GSM mode

Result of GPRS Attach: current SGSN knows that the UE has activated GPRS

If UE was already registered with another SGSN, the HLR is notified about the new SGSN

HLR provides data for assembling a PDP context

Several PDP contexts can be set up for a UE such that the UE can get connected to several IP networks with different QoS

Alternative: different contexts for accessing the same network with different services and different QoS

HLRNew SGSNOld

SGSN

AttachRequest(P-TMSI/IMSI, old RAI, attach type, old P-TMSI signature,…)

IdentificationRequest(old RAI, attach type, old P-TMSI signature,…)IdentificationResponse(Cause, IMSI, authentication triplet,…)

AuthenticationGPRSUpdateLocation(SGSN number, SGSN address, IMSI,…)

CancelLocation(IMSI, cancellation type,…)

CancelLocationAck(IMSI,…)

InsertSubscriberData(IMSI, GPRS subscription data,…)

InsertSubscriberDataAck(IMSI, SGSN area restricted,…)

GPRSUpdateLocationAck(…)

AttachAccept(P-TMSI, P-TMSI signature,…)AttachComplete(…)

UE

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g4.3 Network Layer Support in GPRS/UMTS

GPRS Attach (II)

28

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g4.3 Network Layer Support in GPRS/UMTS

Activation of PDP Context

Ra-diusDNS DHCP

IP PDU arrives

SendRoutingInfoForGPRS(IMSI,…)SendRoutingInfoForGPRSAck(IMSI, SGSN address,…)

PDUNotificationRequest(PDP address,…)

PDUNotificationResponse(cause,…)

RequestPDPContextActivation(PDP address, APN,…)

ActivatePDPContextRequest(requested NSAPI, requested QoS, …)

Security Functions

DNSQuery(APN,…)DNSResponse(GGSN IP address,…)

CreatePDPContextRequest(APN,PDPaddress,negotiatedQoS…)

RadiusAuthenticationRequest

DHCP address request

RadiusAuthenticationResponse

DHCP address response

CreatePDPContextResponse(Cause,PDPaddress,negotiatedQoS…)ActivatePDPContextAccept(SAPI, negotiatedQoS, …)N

etw

ork

-in

itia

ted

PD

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text

Act

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UE-

init

. PD

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on

text

Act

ivat

ion

HLRGGSNSGSNUE

Not used if IPaddressesare static

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g4.3 Network Layer Support in GPRS/UMTS

Data Transfer

After activation of the PDP context, the logical path between UE and external network is defined and can be used for the exchange of user data

GPRS/UMTS applies same principles for mobility support during the data transfer as Mobile IP: tunneling/encapsulation

Gateway Tunneling Protocol (GTP) between GGSN (Home Agent) and SGSN (Foreign Agent)

GPRS: tunnel between GGSN and SGSN

UMTS: tunnel between GGSN and RNC

LTE: tunnel between gateway and eNB

Internet

IP sourceIP destination

Header User dataHeader Payload

SGSN AddressTunnel Identifier

IP PacketGTP Packet

GGSNSGSN

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g4.3 Network Layer Support in GPRS/UMTS

GPRS/UMTS Roaming Example

UEAccess networkOperator A

BTS BSC

GERANUE

Access networks

BTSBSC

GGSN

SGSN

Host

Internet

BG

GGSN

SGSN

BG

Inter-operatorBackboneNetwork

1

24

3

Mobile-originated traffic

Mobile-terminated traffic (home network)

User roams to foreign network

Mobile-terminated traffic (foreign network)

1

2

3

4

Access networkOperator B

Core network(Intra-operator

backbone)Operator A

Core network(Intra-operator backbone)Operator B

Local AreaNetwork

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g4.3 Network Layer Support in GPRS/UMTS

GPRS versus UMTS versus LTE

32

GPRS (2.5G)

Internet

IMS

BTSUE

GTP

LTE (4G)

InterneteNBUE GTP

UMTS (3G)

Internet

IMS

Node BUEGTPGTP

Base Transceiver StationBase Station ControlerGeneral Packet Radio ServicesGateway Tunneling ProtocolGateway GPRS Support Node

IP Multimedia SubsystemRadio Network ControlerServing GPRS Support NodeSystem Architecture Evolution Gateway

GGSNSGSN

GGSNSGSN

BSC

RNC

SAEGW

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g4.3 Network Layer Support in GPRS/UMTS

Protocol Layer in GPRS and UMTS

33

Layer 2

Layer 1

UDP/TCP

IP

IP

Layer 1

Layer 2

Layer 1

IP

Layer 1

Layer 2

UDP/TCP

Layer 2

IP

UDP/TCP

MAC

Layer 1

RLC

PDCP

Application

Layer 1

IP

Layer 1

Layer 2

UDP/TCP

MAC

RLC

PDCP

UTRAN

PDP context IPIP

GTP-UGTP-UGTP-UGTP-U

GGSNSGSNRNC

Layer 2

Layer 1

UDP/TCP

IP

IP

Layer 1

Layer 2

Layer 1

IP

Layer 1

Layer 2

UDP/TCP

Layer 2

BSSGP

LLC

MAC

Layer 1

RLC

SNDCP

Application

Layer 1

BSSGP

Layer 1

Layer 2MAC

RLC

GERAN

PDP context IPIP

GTP-UGTP-USNDCP

GGSNSGSNBSC

LLC

Gateway Tunneling Protocol – User PlaneGSM/EDGE Radio Access NetworkLogical Link ControlMedium Access Control

Packet Data Convergence ProtocolRadio Link ControlRadio Network ControlerServing GPRS Support Node

Subnetwork Dependent Convergence ProtocolTransmission Control ProtocolUMTS Terrestrial Radio Access NetworkUser Datagram Protocol

Base Station ControlerBase Station System GPRS ProtocolInternet ProtocolGateway GPRS Support Node

GPRS Transmission Plane (User Plane)

UMTS Transmission Plane (User Plane)