Mobile Communication

IT2402 MOBILE COMMUNICATION
UNIT – IV
Dr.A.Kathirvel, Professor and Head, Dept of IT
Anand Institute of Higher Technology, Chennai

Unit - IV
MOBILE NETWORK AND TRANSPORT LAYERS
Mobile IP – Dynamic Host Configuration Protocol-
Mobile Ad Hoc Routing Protocols– Multicast routing-
TCP over Wireless Networks – Indirect TCP –
Snooping TCP – Mobile TCP – Fast Retransmit / Fast
Recovery – Transmission/Timeout Freezing-Selective
Retransmission – Transaction Oriented TCP- TCP over
2.5 / 3G wireless Networks

Why Mobile IP?
 What do cellular networks and wireless LANs provide?
Wireless connectivity
Mobility at the data link layer
 What is Dynamic Host Configuration Protocol (DHCP)?
It provides local IP addresses for mobile hosts
Is not secure
Does not maintain network connectivity when moving around
 What they do not provide:
Transparent connectivity at the network layer
Mobility with local access
 The difference between mobility and nomadicity!

What is Mobile IP?
Mobile IP provides network layer mobility
Provides seamless roaming
‘‘Extends’’ the home network over the entire
Internet

IP Overview (1/3)
 IP Addressing :
Dotted Decimal Notation: 32 bits (4x8) used to represent
IPv4 addresses - 192.19.241.18
Network Prefix and Host Portions: p - prefix, h - host, p + h
= 32. If p = 24 then h = 32 - 24 = 8. Using above address the
network prefix will be 192.19.241 and host will be 18. For
those of you familiar with subnet masks, “p” represents
the number of 1’s in the subnet mask. If p = 24, subnet
mask is 255.255.255.0, if p = 26, subnet mask is
255.255.255.192.

IP Overview (2/3)
 IP Routing:
Network prefix is used for routing. Routing tables are used to look
up next hop and the interface on the router that is to be used.
In the routing tables we use the following notation: target/prefix
length, e.g., 192.19.241.0/24, or 192.19.241.192/26.
If two subnet masks/prefixes fit the address, the one with the
largest prefix is chosen for routing. E.g., a router with the
following 3 entries in its table: 7.7.7.99/32 (p=32 host specific)
and 7.7.7.0/24 (0<p<32 network prefix) and 0.0.0.0/0 (p=0
default) will use entry 2 for an IP packet with destination 7.7.7.1
and entry 3 for destination 192.33.14.12.

IP Overview (3/3)
 Domain Name System (DNS): used to translate a host name to an IP
address. A host sends a query to a server to obtain the IP address of
a destination of which it only has the host name.
 Link Layer Addresses - Address Resolution Protocol (ARP):
Once a host has the IP address of a destination it then needs to
finds its layer 2 address or the layer 2 address of the next hop on
the path. A broadcast message is sent and the targeted host
responds with its layer 2 address.
A proxy ARP is a response by a node for another node that cannot
respond at the time the request is made (e.g. the node is a
mobiel node and not on its host net at the time, its home agent
will respond in its stead).
A gratuitous ARP, is a reply to no ARP request, used by a node
that just joins the network and wants to make its address known.
Can be used by a mobile node upon its return to its home net.

Motivation for Mobile IP
 IP Routing
 based on IP destination address, network prefix (e.g. 129.13.42)
determines physical subnet
 change of physical subnet implies change of IP address to have a
topologically correct address (standard IP) or needs special entries in the
routing tables
 Specific routes to end-systems?
 requires changing all routing table entries to forward packets to the right
destination
 does not scale with the number of mobile hosts and frequent changes in
the location, security problems
 Changing the IP-address?
 adjust the host IP address depending on the current location
 almost impossible to find a mobile system, DNS updates take long time
 TCP connections break, security problems

What Mobile IP does:
 Mobile IP solves the following problems:
if a node moves without changing its IP address it will be unable to
receive its packets,
if a node changes its IP address it will have to terminate and restart
its ongoing connections everytime it moves to a new network area
(new network prefix).
 Mobile IP is a routing protocol with a very specific purpose.
 Mobile IP is a network layer solution to node mobility in the Internet.
 Mobile IP is not a complete solution to mobility, changes to the
transport protocols need to be made for a better solution (i.e., the
transport layers are unaware of the mobile node’s point of attachment
and it might be useful if, e.g., TCP knew that a wireless link was being
used!).

Requirements to Mobile IP
 Transparency
mobile end-systems keep their IP address
continuation of communication after interruption of link possible
point of connection to the fixed network can be changed
 Compatibility
support of the same layer 2 protocols as IP
no changes to current end-systems and routers required
mobile end-systems can communicate with fixed systems
 Security
authentication of all registration messages
 Efficiency and scalability
only little additional messages to the mobile system required
(connection typically via a low bandwidth radio link)
world-wide support of a large number of mobile systems in the
whole Internet

Mobile IP Terminology
 Mobile Node (MN)
 system (node) that can change the point of connection to the network without
changing its IP address
 Home Agent (HA)
 system in the home network of the MN, typically a router
 registers the location of the MN, tunnels IP datagrams to the COA
 Foreign Agent (FA)
 system in the current foreign network of the MN, typically a router
 forwards the tunneled datagrams to the MN, typically also the default router
for the MN
 Care-of Address (COA)
 address of the current tunnel end-point for the MN (at FA or MN)
 actual location of the MN from an IP point of view
 can be chosen, e.g., via DHCP
 Correspondent Node (CN)
 communication partner

Mobile IP Operation: Summary
Consists of 3 steps:
Agent discovery,
Registration, and
Routing/Tunneling

Operation Summary (1/3)
 Agent Advertisement/Discovery: consists of broadcast
messages used by mobiles to detect that they have moved
and are required to register with a new FA.
FAs send agent advertisements
MNs can solicit for agents if they have not heard an agent
advertisement in awhile or use some other mechanism to
obtain a COA or temp. IP address (e.g. DHCP).
MNs know they are home when they recognize their HA.

 Registration: used by a MN to inform the FA that it is visiting.
The new care of address of the MN is sent to the HA.
Registration expires, duration is negotiated during
registration
Mobile must re-register before it expires
All registrations are authenticated
The MN sends a regristration request in to the FA which
passes it along to the home agent. The HA responds to the
FA which then informs the MN that all is in order and
registration is complete.

 Routing/Encapsulation/Tunneling: consists of the delivery of
the packets to the mobile node at its current care of address.
Sender does not need to know that the destination is a
MN.
HA intercepts all packets for the MN and passes them
along to MN using a tunnel.
MN communicates directly with the CN.
Referred to as Triangle Routing

Example network
mobile end-system
Internet
router
router
router
end-system
FA
HA
MN
home network
foreign
network
(physical home network
for the MN)
(current physical network
for the MN)
CN

Data transfer to the mobile system
Internet
sender
FA
HA
MN
home network
foreign
network
receiver
1
2
3
1. Sender sends to the IP address of MN,
HA intercepts packet (proxy ARP)
2. HA tunnels packet to COA, here FA,
by encapsulation
3. FA forwards the packet
to the MN
CN

Data transfer from the mobile system
Internet
receiver
FA
HA
MN
home network
foreign
network
sender
1
1. Sender sends to the IP address
of the receiver as usual,
FA works as default router
CN

Overview
CN
router
HA
router
FA
Internet
router
1.
2.
3.
home
network
MN
foreign
network
4.
CN
router
HA
router
FA
Internet
router
home
network
MN
foreign
network
COA

Network integration
 Agent Advertisement Discovery
 HA and FA periodically send advertisement messages into their physical
subnets
 MN listens to these messages and detects, if it is in the home or a foreign
network (standard case for home network)
 MN reads a COA from the FA advertisement messages
 Registration (always limited lifetime!)
 MN signals COA to the HA via the FA, HA acknowledges via FA to MN
 these actions have to be secured by authentication
 Routing/Encapsulation/Tunneling
 HA advertises the IP address of the MN (as for fixed systems), i.e. standard
routing information
 packets to the MN are sent to the HA,
 independent of changes in COA/FA

Agent advertisement
preference level 1
router address 1
#addresses
type
addr. size lifetime
checksum
COA 1
COA 2
type sequence numberlength
0 7 8 15 16 312423
code
preference level 2
router address 2
. . .
registration lifetime
. . .
R B H F M G V reserved

Registration
t
MN HA
t
MN FA HA

Mobile IP registration request
home agent
home address
type lifetime
0 7 8 15 16 312423
rsv
identification
COA
extensions . . .
S B DMGV

Processing Registration Messages (1/3)
 A MN, depending on which registration scenario it is in, will figure what addresses to
use in the various fields of the Registration request message.
 Link layer addresses are tricky:
 A MN may not use ARP if it is using a FA COA. It needs to use the address of
the FA as the destination address.
If it is using a collocated COA, then it uses ARP to locate the default router
using its COA as source. Note that if the ‘R’ bit is set is uses the FA address as
the destination address.
For de-registration is uses ARP to locate the HA link address and it uses its
own home address for the ARP message.
 For network layer addresses (i.e., IP addresses):
It uses the FA address as destination address when using the FA COA and its
own home address as the source address.
If using a collocated COA it uses its COA as source address and the HA address
as destination address. Note that if the ‘R’ bit is set then is must use the same
addresses as for the FA COA scenario.
For de-registration it uses its own home address as source and the HA address
as destination.

 For the FA:
 A FA may refuse a Registration request for a number of reasons:
lifetime too long, authentication failed, requested tunneling not
supported, cannot handle another MN (current load too high).
 If an FA does not refuse the request it relays it to the HA. Relaying is
different from forwarding as the FA is required to process the packet
and create new headers.
 Some important fields of the request message are recorded for use
later on: MN link layer address, MN IP address, UDP source port, HA IP
address, identification number and requested lifetime.
 Regarding a Registration reply message, the FA can refuse it and send a
decline to the MN is it finds the reply from the HA to be invalid.
Otherwise it updates its list of visiting MNs and begins acting on behalf
of the MN.

 For a HA
The HA will determine, as the FA did, whether it will accept
the request. If it does not it returns a code in the reply
message indicating the cause of the failed request.
If the request is accepted, the reply is sent back by
reversing all the IP addresses and UDP port numbers.
The HA updates the binding table corresponding to that
MN dependent upon the nature of the request.

Routing/Tunneling (1/5)
 Routing a packet to a MN involves the following:
A router on the home link, possibly the HA, advertises
reachability to the network prefix of the MN’s home
address.
All packets are therefore routed to the MN’s home link.
A HA intercepts the packets for the MN and tunnels a copy
to each COA in the binding table.
At the foreign link either the MN extracts the packet
(collocated COA) or the FA extracts the packet and
forwards it to the MN.

 A HA can use one of two methods to intercept a MN’s packets:
The HA is a router with multiple network interfaces. In that
case it advertises reachability to the MN’s home network
prefix.
The HA is not a router with multiple interfaces. It must use
ARP to receive the MN’s packets. It either responds to ARP
requests on behalf of the MN (proxy ARP) or uses
gratuitous ARPs to inform the home network that it is
receiving the MN’s IP packets. This is to update any ARP
caches that hosts and other devices might have.

 How to ‘fool’ the routing table into handling tunneled packets at
the HA?
A virtual interface is used to do the encapsulation.
A packet destined for the MN is handled by the routing
routine as all received IP packets are.
The routing table has a host specific entry for the MN. This
host specific entry is used to route the packet to a virtual
interface that basically consists of a process that does
encapsulation.
Once encapsulation has been performed the packet is sent to
be processed by the routing routine again. This time the
destination address is the COA and it is routed normally.

 How to ‘fool’ the routing table into handling tunneled packets at the
FA?
The same procedure is used as above.
A packet coming in with a COA that is one of the FA addresses’ is
handled by the routing routine.
A host specific address (its own address) in the routing table
points to the higher layers and the packet is passed on to a virtual
interface.
The virtual interface consists of a process that decapsulates the
packet and re-routes it to the routing routine.
The routing routine routes the packet normally based upon a host
specific entry that is the MN’s home address (for which it has the
link layer address!).

• How does a MN route its packets?
– It needs to find a router to send all its packets to.
– It can select a router in one of a number of ways dependent upon
whether it has a FA COA or a collocated COA.
– Having a FA COA does not imply that the MN needs to use it as its
default router for sending packets. It can use any router that sends
advertisements or that is advertised in the Agent Advertisement
message.
– If the MN is using a collocated COA it needs to listen for router
advertisements or is it hears none, use DHCP to find the default
router.
– Determining the link layer address is another issue. Collocated COA
MNs can use ARP. FA COA must note the link layer address when
they receive router advertisements or agent advertisements.

Encapsulation Process
original IP header original data
new datanew IP header
outer header inner header original data

Types of Encapsulation
 Three types of encapsulation protocols are specified for Mobile IP:
IP-in-IP encapsulation: required to be supported. Full IP header
added to the original IP packet. The new header contains HA
address as source and Care of Address as destination.
Minimal encapsulation: optional. Requires less overhead but
requires changes to the original header. Destination address is
changed to Care of Address and Source IP address is maintained
as is.
Generic Routing Encapsulation (GRE): optional. Allows packets
of a different protocol suite to be encapsulated by another
protocol suite.
 Type of tunneling/encapsulation supported is indicated in
registration.

IP in IP Encapsulation
IP in IP encapsulation (mandatory in RFC 2003)
tunnel between HA and COA
Care-of address COA
IP address of HA
TTL
IP identification
IP-in-IP IP checksum
flags fragment offset
lengthTOSver. IHL
IP address of MN
IP address of CN
TTL
IP identification
lay. 4 prot. IP checksum
lengthTOSver. IHL
TCP/UDP/ ... payload

Minimum Encapsulation
 Minimal encapsulation (optional)
avoids repetition of identical fields
e.g. TTL, IHL, version, TOS
only applicable for unfragmented packets, no space left for
fragment identification
care-of address COA
IP address of HA
TTL
IP identification
min. encap. IP checksum
lengthTOSver. IHL
IP address of MN
original sender IP address (if S=1)
Slay. 4 protoc. IP checksum
reserved

Generic Routing Encapsulation
original
header
original data
new datanew header
outer header
GRE
header
original data
original
header
Care-of address COA
IP address of HA
TTL
IP identification
GRE IP checksum
lengthTOSver. IHL
IP address of MN
IP address of CN
TTL
IP identification
lay. 4 prot. IP checksum
lengthTOSver. IHL
routing (optional)
sequence number (optional)
key (optional)
offset (optional)checksum (optional)
protocolrec. rsv. ver.CRK S s

Routing techniques
 Triangle Routing: tunneling in its simplest form has all packets go to
home network (HA) and then sent to MN via a tunnel.
This involves two IP routes that need to be set-up, one original and
the second the tunnel route.
Causes unnecessary network overhead and adds to the latency.
 Route optimization: allows the correspondent node to learn the current
location of the MN and tunnel its own packets directly. Problems arise
with
mobility: correspondent node has to update/maintain its cache.
authentication: HA has to communicate with the correspondent
node to do authentication, i.e., security association is with HA not
with MN.

Optimization of packet forwarding
Change of FA
packets on-the-fly during the change can be lost
new FA informs old FA to avoid packet loss, old FA
now forwards remaining packets to new FA
this information also enables the old FA to release
resources for the MN

Change of foreign agent
CN HA FAold FAnew MN
t
request
update
ACK
data data
MN changes
location
registration
update
ACK
data
data data
warning
update
ACK
data
data
registration

Problems with Triangle Routing
 Triangle routing has the MN correspond directly with the CN
using its home address as the SA
Firewalls at the foreign network may not allow that
Multicasting: if a MN is to participate in a multicast group, it
needs to use a reverse tunnel to maintain its association with
the home network.
TTL: a MN might have a TTL that is suitable for communication
when it is in its HM. This TTL may not be sufficient when
moving around (longer routes possibly). When using a reverse
tunnel, it only counts as a single hop. A MN does not want to
change the TTL everytime it moves.
 Solution: reverse tunneling

Reverse tunneling (RFC 2344)
Internet
receiver
FA
HA
MN
home network
foreign
network
sender
3
2
1
1. MN sends to FA
2. FA tunnels packets to HA
by encapsulation
3. HA forwards the packet to the
receiver (standard case)
CN

Mobile IP with reverse tunneling
 Routers accept often only “topologically correct“ addresses
(firewall!)
a packet from the MN encapsulated by the FA is now
topologically correct
 Multicast and TTL problems solved
 Reverse tunneling does not solve
all problems with firewalls, the reverse tunnel can be abused to
circumvent security mechanisms (tunnel hijacking)
optimization of data paths, i.e. packets will be forwarded
through the tunnel via the HA to a sender (longer routes)
 The new standard is backwards compatible
the extensions can be implemented easily

Mobile IP and IPv6
 Mobile IP was developed for IPv4, but IPv6 simplifies the protocols
security is integrated and not an add-on, authentication of
registration is included
COA can be assigned via auto-configuration (DHCPv6 is one
candidate), every node has address auto configuration
no need for a separate FA, all routers perform router advertisement
which can be used instead of the special agent advertisement
MN can signal a sender directly the COA, sending via HA not needed
in this case (automatic path optimization)
soft hand-over, i.e. without packet loss, between two subnets is
supported
MN sends the new COA to its old router
the old router encapsulates all incoming packets for the MN and
forwards them to the new COA
authentication is always granted

Problems with Mobile IP
 Security
 authentication with FA problematic, for the FA typically belongs to
another organization
 no protocol for key management and key distribution has been
standardized in the Internet
 patent and export restrictions
 Firewalls
 typically mobile IP cannot be used together with firewalls, special set-
ups are needed (such as reverse tunneling)
 QoS
 many new reservations in case of RSVP
 tunneling makes it hard to give a flow of packets a special treatment
needed for the QoS
 Security, firewalls, QoS etc. are topics of current research and discussions!

Security in Mobile IP
 Security requirements (Security Architecture for the Internet Protocol, RFC 1825)
 Integrity
any changes to data between sender and receiver can be detected by the
receiver
 Authentication
sender address is really the address of the sender and all data received is
really data sent by this sender
 Confidentiality
only sender and receiver can read the data
 Non-Repudiation
sender cannot deny sending of data
 Traffic Analysis
creation of traffic and user profiles should not be possible
 Replay Protection
receivers can detect replay of messages

not encrypted encrypted
IP security architecture (1/2)
 Two or more partners have to negotiate security mechanisms to setup a
security association
 typically, all partners choose the same parameters and mechanisms
 Two headers have been defined for securing IP packets:
 Authentication-Header
guarantees integrity and authenticity of IP packets
if asymmetric encryption schemes are used, non-repudiation can
also be guaranteed
 Encapsulation Security Payload
protects confidentiality between communication partners
Authentification-HeaderIP-Header UDP/TCP-Paketauthentication headerIP header UDP/TCP data
ESP headerIP header encrypted data

 Mobile Security Association for registrations
 parameters for the mobile host (MH), home agent (HA), and foreign
agent (FA)
 Extensions of the IP security architecture
 extended authentication of registration
 prevention of replays of registrations
time stamps: 32 bit time stamps + 32 bit random number
responses: 32 bit random number (MH) + 32 bit random number (HA)
registration reply
registration request
registration request
IP security architecture (2/2)
MH FA HA
registration reply
MH-HA authentication
MH-FA authentication FA-HA authentication

Key distribution
 Home agent distributes session keys
 foreign agent has a security association with the home agent
 mobile host registers a new binding at the home agent
 home agent answers with a new session key for foreign agent
and mobile node
FA MH
HA
response:
EHA-FA {session key}
EHA-MH {session key}

DHCP: Dynamic Host Configuration
Protocol
 Application
 simplification of installation and maintenance of networked computers
 supplies systems with all necessary information, such as IP address, DNS server
address, domain name, subnet mask, default router etc.
 enables automatic integration of systems into an Intranet or the Internet, can be
used to acquire a COA for Mobile IP
 Client/Server-Model
 the client sends via a MAC broadcast a request to the DHCP server (might be via
a DHCP relay)
client relay
clientserver
DHCPDISCOVER
DHCPDISCOVER

DHCP - protocol mechanisms
server
(not selected)
client server
(selected)initialization
collection of replies
selection of configuration
initialization completed
release
confirmation of
configuration
delete context
determine the
configuration
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
(reject)
DHCPACK
DHCPRELEASE
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
(options)
determine the
configuration

DHCP characteristics
 Server
several servers can be configured for DHCP, coordination not
yet standardized (i.e., manual configuration)
 Renewal of configurations
IP addresses have to be requested periodically, simplified
protocol
 Options
available for routers, subnet mask, NTP (network time
protocol) timeserver, SLP (service location protocol)
directory, DNS (domain name system)
 Big security problems!
no authentication of DHCP information specified

Ad hoc networks
 Standard Mobile IP needs an infrastructure
 Home Agent/Foreign Agent in the fixed network
 DNS, routing etc. are not designed for mobility
 Sometimes there is no infrastructure!
 remote areas, ad-hoc meetings, disaster areas
 cost can also be an argument against an infrastructure!
 Main topic: routing
 no default router available
 every node should be able to forward
A B C

Routing examples for an ad hoc
network
N1
N4
N2
N5
N3
N1
N4
N2
N5
N3
good link
weak link
time = t1 time = t2

Traditional routing algorithms
 Distance Vector
 periodic exchange of messages with all physical neighbors that contain
information about who can be reached at what distance
 selection of the shortest path if several paths available
 Link State
 periodic notification of all routers about the current state of all physical
links
 router get a complete picture of the network
 Example
 ARPA packet radio network (1973), DV-Routing
every 7.5s exchange of routing tables including link quality
updating of tables also by reception of packets
routing problems solved with limited flooding

Problems of traditional routing algorithms
 Dynamics of the topology
frequent changes of connections, connection quality, participants
 Limited performance of mobile systems
periodic updates of routing tables need energy without
contributing to the transmission of user data, sleep modes
difficult to realize
limited bandwidth of the system is reduced even more due to the
exchange of routing information
links can be asymmetric, i.e., they can have a direction dependent
transmission quality
 Problem
protocols have been designed for fixed networks with infrequent
changes and typically assume symmetric links

DSDV (Destination Sequenced
Distance Vector)
 Expansion of distance vector routing
 Sequence numbers for all routing updates
assures in-order execution of all updates
avoids loops and inconsistencies
 Decrease of update frequency
store time between first and best announcement of a path
inhibit update if it seems to be unstable (based on the
stored time values)

Dynamic source routing I
 Split routing into discovering a path and maintaining a path
 Discover a path
only if a path for sending packets to a certain destination is
needed and no path is currently available
 Maintaining a path
only while the path is in use one has to make sure that it
can be used continuously
 No periodic updates needed!

Dynamic source routing II
 Path discovery
 broadcast a packet with destination address and unique ID
 if a station receives a broadcast packet
if the station is the receiver (i.e., has the correct destination address)
then return the packet to the sender (path was collected in the packet)
if the packet has already been received earlier (identified via ID) then
discard the packet
otherwise, append own address and broadcast packet
 sender receives packet with the current path (address list)
 Optimizations
 limit broadcasting if maximum diameter of the network is known
 caching of address lists (i.e. paths) with help of passing packets
stations can use the cached information for path discovery (own paths
or paths for other hosts)

Dynamic Source Routing III
 Maintaining paths
after sending a packet
wait for a layer 2 acknowledgement (if applicable)
listen into the medium to detect if other stations
forward the packet (if possible)
request an explicit acknowledgement
if a station encounters problems it can inform the sender
of a packet or look-up a new path locally

Clustering of ad-hoc networks
Internet
super cluster
cluster

Interference-based routing
 Routing based on assumptions about
interference between signals
S1
N5
N3
N4
N1
N2
R1
R2N6
N8
S2
N9
N7
neighbors
(i.e. within radio range)

Examples for interference based
routing
 Least Interference Routing (LIR)
calculate the cost of a path based on the number of stations
that can receive a transmission
 Max-Min Residual Capacity Routing (MMRCR)
calculate the cost of a path based on a probability function
of successful transmissions and interference
 Least Resistance Routing (LRR)
calculate the cost of a path based on interference, jamming
and other transmissions
 LIR is very simple to implement, only information from direct
neighbors is necessary

Multicast routing
 Unicast: single source sends to a single destination
 Multicast: hosts are part of a multicast group
packet sent by any member of a group are received by all
 Useful for
multiparty videoconference
distance learning
resource location

Multicast group
 Associates a set of senders and receivers with each other
but independent of them
created either when a sender starts sending from a group
or a receiver expresses interest in receiving
even if no one else is there!
 Sender does not need to know receivers’ identities
rendezvous point

Addressing
 Multicast group in the Internet has its own Class D address
looks like a host address, but isn’t
 Senders send to the address
 Receivers anywhere in the world request packets from that
address
 “Magic” is in associating the two: dynamic directory service
 Four problems
which groups are currently active
how to express interest in joining a group
discovering the set of receivers in a group
delivering data to members of a group

Expanding ring search
 A way to use multicast groups for resource discovery
 Routers decrement TTL when forwarding
 Sender sets TTL and multicasts
reaches all receivers <= TTL hops away
 Discovers local resources first
 Since heavily loaded servers can keep quiet, automatically
distributes load

Multicast flavors
 Unicast: point to point
 Multicast:
point to multipoint
multipoint to multipoint
 Can simulate point to multipoint by a set of point to point
unicasts
 Can simulate multipoint to multipoint by a set of point to
multipoint multicasts
 The difference is efficiency

Example
 Suppose A wants to talk to B, G, H, I, B to A, G, H, I
 With unicast, 4 messages sent from each source
links AC, BC carry a packet in triplicate
 With point to multipoint multicast, 1 message sent from each
source
but requires establishment of two separate multicast
groups
 With multipoint to multipoint multicast, 1 message sent from
each source,
single multicast group

Shortest path tree
 Ideally, want to send exactly one multicast packet per link
forms a multicast tree rooted at sender
 Optimal multicast tree provides shortest path from sender to
every receiver
shortest-path tree rooted at sender

Issues in wide-area multicast
 Difficult because
sources may join and leave dynamically
need to dynamically update shortest-path tree
leaves of tree are often members of broadcast LAN
would like to exploit LAN broadcast capability
would like a receiver to join or leave without explicitly
notifying sender
otherwise it will not scale

Multicast in a broadcast LAN
 Wide area multicast can exploit a LAN’s broadcast capability
 E.g. Ethernet will multicast all packets with multicast bit set on
destination address
 Two problems:
what multicast MAC address corresponds to a given Class
D IP address?
does the LAN have contain any members for a given group
(why do we need to know this?)

Class D to MAC translation
 Multiple Class D addresses map to the same MAC address
 Well-known translation algorithm => no need for a translation
table
01 00 5E
23 bits copied from IP address
IEEE 802 MAC Address
Class D IP
address
Ignore
d
‘1110’ = Class D
indication
Multicast bit Reserved
bit

Internet Group Management Protocol
 Detects if a LAN has any members for a particular group
If no members, then we can prune the shortest path tree for
that group by telling parent
 Router periodically broadcasts a query message
 Hosts reply with the list of groups they are interested in
 To suppress traffic
reply after random timeout
broadcast reply
if someone else has expressed interest in a group, drop out
 To receive multicast packets:
translate from class D to MAC and configure adapter

Wide area multicast
 Assume
each endpoint is a router
a router can use IGMP to discover all the members in its
LAN that want to subscribe to each multicast group
 Goal
distribute packets coming from any sender directed to a
given group to all routers on the path to a group member

Simplest solution
 Flood packets from a source to entire network
 If a router has not seen a packet before, forward it to all
interfaces except the incoming one
 Pros
simple
always works!
 Cons
routers receive duplicate packets
detecting that a packet is a duplicate requires storage,
which can be expensive for long multicast sessions

A clever solution
 Reverse path forwarding
 Rule
forward packet from S to all interfaces if and only if packet
arrives on the interface that corresponds to the shortest
path to S
no need to remember past packets
C need not forward packet received from D

Cleverer
 Don’t send a packet downstream if you are not on the
shortest path from the downstream router to the source
 C need not forward packet from A to E
 Potential confusion if downstream router has a choice of
shortest paths to source (see figure on previous slide)

Pruning
 RPF does not completely eliminate unnecessary transmissions
 B and C get packets even though they do not need it
 Pruning => router tells parent in tree to stop forwarding
 Can be associated either with a multicast group or with a
source and group
trades selectivity for router memory

Rejoining
 What if host on C’s LAN wants to receive messages from A
after a previous prune by C?
IGMP lets C know of host’s interest
C can send a join(group, A) message to B, which
propagates it to A
or, periodically flood a message; C refrains from pruning

A problem
 Reverse path forwarding requires a router to know shortest
path to a source
known from routing table
 Doesn’t work if some routers do not support multicast
virtual links between multicast-capable routers
shortest path to A from E is not C, but F
 Two problems
 how to build virtual links
 how to construct routing table for a network with virtual
links

Tunnels
 Why do we need them?
 Consider packet sent from A to F via multicast-incapable D
 If packet’s destination is Class D, D drops it
 If destination is F’s address, F doesn’t know multicast address!
 So, put packet destination as F, but carry multicast address
internally
 Encapsulate IP in IP => set protocol type to IP-in-IP

Multicast routing protocol
 Interface on “shortest path” to source depends on whether
path is real or virtual
 Shortest path from E to A is not through C, but F
so packets from F will be flooded, but not from C
 Need to discover shortest paths only taking multicast-capable
routers into account
DVMRP

DVMRP
 Distance-vector Multicast routing protocol
 Very similar to RIP
distance vector
hop count metric
 Used in conjunction with
flood-and-prune (to determine memberships)
prunes store per-source and per-group information
reverse-path forwarding (to decide where to forward a packet)
explicit join messages to reduce join latency (but no source
info, so still need flooding)

MOSPF
 Multicast extension to OSPF
 Routers flood group membership information with LSPs
 Each router independently computes shortest-path tree that
only includes multicast-capable routers
no need to flood and prune
 Complex
interactions with external and summary records
need storage per group per link
need to compute shortest path tree per source and group

Core-based trees
 Problems with DVMRP-oriented approach
need to periodically flood and prune to determine group
members
need to source per-source and per-group prune records at
each router
 Key idea with core-based tree
coordinate multicast with a core router
host sends a join request to core router
routers along path mark incoming interface for forwarding

Example
 Pros
routers not part of a group are not involved in pruning
explicit join/leave makes membership changes faster
router needs to store only one record per group
 Cons
all multicast traffic traverses core, which is a bottleneck
traffic travels on non-optimal paths

Protocol independent multicast (PIM)
 Tries to bring together best aspects of CBT and DVMRP
 Choose different strategies depending on whether multicast
tree is dense or sparse
flood and prune good for dense groups
only need a few prunes
CBT needs explicit join per source/group
CBT good for sparse groups
 Dense mode PIM == DVMRP
 Sparse mode PIM is similar to CBT
but receivers can switch from CBT to a shortest-path tree

PIM (contd.)
 In CBT, E must send to core
 In PIM, B discovers shorter path to E (by looking at unicast
routing table)
sends join message directly to E
sends prune message towards core
 Core no longer bottleneck
 Survives failure of core

More on core
 Renamed a rendezvous point
because it no longer carries all the traffic like a CBT core
 Rendezvous points periodically send “I am alive” messages
downstream
 Leaf routers set timer on receipt
 If timer goes off, send a join request to alternative rendezvous
point
 Problems
how to decide whether to use dense or sparse mode?
how to determine “best” rendezvous point?

91
Transport Layer
E.g. HTTP (used by web services)
typically uses TCP
 Reliable transport between client
and server required
TCP
 Steam oriented, not transaction
oriented
 Network friendly: time-out
 congestion
 slow down transmission
Well known – TCP guesses quite often
wrong in wireless and mobile networks
 Packet loss due to transmission
errors
 Packet loss due to change of
network
Result
 Severe performance degradation
Client Server
Connection
setup
Data
transmission
Connection
release
TCP SYN
TCP SYN/ACK
TCP ACK
HTTP request
HTTP response
GPRS: 500ms!
>15 s
no data

92
Motivation I
 Transport protocols typically designed for
 Fixed end-systems
 Fixed, wired networks
 Research activities
 Performance
 Congestion control
 Efficient retransmissions
 TCP congestion control
 packet loss in fixed networks typically due to (temporary) overload
situations
 router have to discard packets as soon as the buffers are full
 TCP recognizes congestion only indirect via missing
acknowledgements, retransmissions unwise, they would only
contribute to the congestion and make it even worse
 slow-start algorithm as reaction

93
Motivation II
 TCP slow-start algorithm
 sender calculates a congestion window for a receiver
 start with a congestion window size equal to one segment
 exponential increase of the congestion window up to the congestion
threshold, then linear increase
 missing acknowledgement causes the reduction of the congestion
threshold to one half of the current congestion window
 congestion window starts again with one segment
 TCP fast retransmit/fast recovery
 TCP sends an acknowledgement only after receiving a packet
 if a sender receives several acknowledgements for the same packet, this
is due to a gap in received packets at the receiver
 however, the receiver got all packets up to the gap and is actually
receiving packets
 therefore, packet loss is not due to congestion, continue with current
congestion window (do not use slow-start)

94
Influences of mobility on TCP-mechanisms
 TCP assumes congestion if packets are dropped
typically wrong in wireless networks, here we often have packet
loss due to transmission errors
furthermore, mobility itself can cause packet loss, if e.g. a mobile
node roams from one access point (e.g. foreign agent in Mobile
IP) to another while there are still packets in transit to the wrong
access point and forwarding is not possible
 The performance of an unchanged TCP degrades severely
however, TCP cannot be changed fundamentally due to the large
base of installation in the fixed network, TCP for mobility has to
remain compatible
the basic TCP mechanisms keep the whole Internet together

95
Early approach: Indirect TCP I
 Indirect TCP or I-TCP segments the connection
 no changes to the TCP protocol for hosts connected to the wired Internet, millions of
computers use (variants of) this protocol
 optimized TCP protocol for mobile hosts
 splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no
real end-to-end connection any longer
 hosts in the fixed part of the net do not notice the characteristics of the wireless part
mobile host
access point
(foreign agent) „wired“ Internet
„wireless“ TCP standard TCP

96
I-TCP socket and state migration
mobile host
access point2
Internet
access point1
socket migration
and state transfer

97
Indirect TCP II
 Advantages
 no changes in the fixed network necessary, no changes for the hosts
(TCP protocol) necessary, all current optimizations to TCP still work
 transmission errors on the wireless link do not propagate into the
fixed network
 simple to control, mobile TCP is used only for one hop between, e.g.,
a foreign agent and mobile host
 therefore, a very fast retransmission of packets is possible, the short
delay on the mobile hop is known
 Disadvantages
 loss of end-to-end semantics, an acknowledgement to a sender does
now not any longer mean that a receiver really got a packet, foreign
agents might crash
 higher latency possible due to buffering of data within the foreign
agent and forwarding to a new foreign agent

98
Early approach: Snooping TCP I
 Transparent extension of TCP within the foreign agent
 buffering of packets sent to the mobile host
 lost packets on the wireless link (both directions!) will be retransmitted
immediately by the mobile host or foreign agent, respectively (so called
“local” retransmission)
 the foreign agent therefore “snoops” the packet flow and recognizes
acknowledgements in both directions, it also filters ACKs
 changes of TCP only within the foreign agent
„wired“ Internet
buffering of data
end-to-end TCP connection
local retransmission correspondent
hostforeign
agent
mobile
host
snooping of ACKs

99
Snooping TCP II
 Data transfer to the mobile host
 FA buffers data until it receives ACK of the MH, FA detects packet loss via
duplicated ACKs or time-out
 fast retransmission possible, transparent for the fixed network
 Data transfer from the mobile host
 FA detects packet loss on the wireless link via sequence numbers, FA answers
directly with a NACK to the MH
 MH can now retransmit data with only a very short delay
 Integration of the MAC layer
 MAC layer often has similar mechanisms to those of TCP
 thus, the MAC layer can already detect duplicated packets due to
retransmissions and discard them
 Problems
 snooping TCP does not isolate the wireless link as good as I-TCP
 snooping might be useless depending on encryption schemes

100
Early approach: Mobile TCP
 Special handling of lengthy and/or frequent disconnections
 M-TCP splits as I-TCP does
 unmodified TCP fixed network to supervisory host (SH)
 optimized TCP SH to MH
 Supervisory host
 no caching, no retransmission
 monitors all packets, if disconnection detected
set sender window size to 0
sender automatically goes into persistent mode
 old or new SH reopen the window
 Advantages: maintains semantics, supports disconnection, no buffer
forwarding
 Disadvantages: loss on wireless link propagated into fixed network and
adapted TCP on wireless link

101
Fast retransmit/fast recovery
 Change of foreign agent often results in packet loss
 TCP reacts with slow-start although there is no congestion
 Forced fast retransmit
 as soon as the mobile host has registered with a new foreign agent, the
MH sends duplicated acknowledgements on purpose
 this forces the fast retransmit mode at the communication partners
 additionally, the TCP on the MH is forced to continue sending with the
actual window size and not to go into slow-start after registration
 Advantage
 simple changes result in significant higher performance
 Disadvantage
 further mix of IP and TCP, no transparent approach

102
Transmission/time-out freezing
 Mobile hosts can be disconnected for a longer time
 no packet exchange possible, e.g., in a tunnel, disconnection due to
overloaded cells or mux. with higher priority traffic
 TCP disconnects after time-out completely
 TCP freezing
 MAC layer is often able to detect interruption in advance
 MAC can inform TCP layer of upcoming loss of connection
 TCP stops sending, but does now not assume a congested link
 MAC layer signals again if reconnected
 Advantage
 scheme is independent of data
 Disadvantage
 TCP on mobile host has to be changed, mechanism depends on MAC layer

103
Selective retransmission
 TCP acknowledgements are often cumulative
 ACK n acknowledges correct and in-sequence receipt of packets up to n
 if single packets are missing quite often a whole packet sequence
beginning at the gap has to be retransmitted (go-back-n), thus wasting
bandwidth
 Selective retransmission as one solution
 RFC2018 allows for acknowledgements of single packets, not only
acknowledgements of in-sequence packet streams without gaps
 sender can now retransmit only the missing packets
 Advantage : much higher efficiency
 Disadvantage: more complex software in a receiver, more buffer needed at
the receiver

104
Transaction oriented TCP
 TCP phases
connection setup, data transmission, connection release
using 3-way-handshake needs 3 packets for setup and release,
respectively
thus, even short messages need a minimum of 7 packets!
 Transaction oriented TCP
RFC1644, T-TCP, describes a TCP version to avoid this overhead
connection setup, data transfer and connection release can be
combined
thus, only 2 or 3 packets are needed
 Advantage: efficiency
 Disadvantage: requires changed TCP and mobility not longer
transparent

105
Comparison of different approaches for a “mobile” TCP
Approach Mechanism Advantages Disadvantages
Indirect TCP splits TCP connection
into two connections
isolation of wireless
link, simple
loss of TCP semantics,
higher latency at
handover
Snooping TCP “snoops” data and
acknowledgements, local
retransmission
transparent for end-to-
end connection, MAC
integration possible
problematic with
encryption, bad isolation
of wireless link
M-TCP splits TCP connection,
chokes sender via
window size
Maintains end-to-end
semantics, handles
long term and frequent
disconnections
Bad isolation of wireless
link, processing
overhead due to
bandwidth management
Fast retransmit/
fast recovery
avoids slow-start after
roaming
simple and efficient mixed layers, not
transparent
Transmission/
time-out freezing
freezes TCP state at
disconnect, resumes
after reconnection
independent of content
or encryption, works for
longer interrupts
changes in TCP
required, MAC
dependant
Selective
retransmission
retransmit only lost data very efficient slightly more complex
receiver software, more
buffer needed
Transaction
oriented TCP
combine connection
setup/release and data
transmission
Efficient for certain
applications
changes in TCP
required, not transparent

106
TCP Improvements I
 Initial research work
 Indirect TCP, Snoop TCP, M-TCP, T/TCP, SACK, Transmission/time-out freezing,
 TCP over 2.5/3G wireless networks
 Fine tuning today’s TCP
 Learn to live with
Data rates: 64 kbit/s up, 115-384 kbit/s down; asymmetry: 3-6, but also up
to 1000 (broadcast systems), periodic allocation/release of channels
High latency, high jitter, packet loss
 Suggestions
Large (initial) sending windows, large maximum transfer unit, selective
acknowledgement, explicit congestion notification, time stamp, no header
compression
 Already in use
i-mode running over FOMA
WAP 2.0 (“TCP with wireless profile”)
pRTT
MSS
BW
*
*93.0

• max. TCP BandWidth
• Max. Segment Size
• Round Trip Time
• loss probability

107
TCP Improvements II
 Performance enhancing proxies (PEP, RFC 3135)
 Transport layer
Local retransmissions and acknowledgements
 Additionally on the application layer
Content filtering, compression, picture downscaling
E.g., Internet/WAP gateways
Web service gateways?
 Big problem: breaks end-to-end semantics
Disables use of IP security
Choose between PEP and security!
 More open issues
 RFC 3150 (slow links)
Recommends header compression, no timestamp
 RFC 3155 (links with errors)
States that explicit congestion notification cannot be used
 In contrast to 2.5G/3G recommendations!
Mobile system
PEP
Comm. partner
wireless
Internet

Mobile Communication

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