1 UDP—User Datagram Protocol An unreliable, connectionless transport layer protocol UDP format....

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UDP—User Datagram Protocol• An unreliable, connectionless transport layer protocol• UDP format. See picture • Two additional functions beyond IP:

– Demultiplexing: deliver to different upper layer entities such as DNS, RTP, SNMP based on the destination port # in the header. i.e., UDP can support multiple applications in the same end systems.

– (Optionally) check the integrity of entire UDP. (recall IP only checks the integrity of IP header.)

• If source does not want to compute checksum, fill checksum with all 0s.

• If compute checksum and the checksum happens to be 0s, then fill all 1s.

• UDP checksum computation is similar to IP checksum, with two more:– Add extra 0s to entire datagram if not multiple of 16 bits.

– Add pseudoheader to the beginning of datagram. UDP pseudoheader

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Source Port Destination Port

UDP Length UDP Checksum

Data

0 16 31

Figure 8.16

UDP datagram

Back to UDP—User Datagram Protocol

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0 0 0 0 0 0 0 0 Protocol = 17 UDP Length

Source IP Address

Destination IP Address

0 8 16 31

Figure 8.17

UDP pseudoheader

1.Pseudoheader is to ensure that the datagram has indeed reached the correct destination host and port.2. The padding of 0s and pseudoheader is only for the computation of checksum and not be transmitted.

Back to UDP—User Datagram Protocol

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TCP—transmission control protocol• TCP functionality

– Provides connection-oriented, reliable, in-sequence, byte-stream service

– Provides a logical full-duplex (two way) connection

– Provides flow-control by advertised window.

– Provides congestion control by congestion window.

– Support multiple applications in the same end systems.

• TCP establishes connection by setting up variables that are used in two peer TCP entities. Most important variables are initial sequence numbers.

• TCP uses Selective Repeat ARQ.

• TCP terminates each direction of connection independently, allowing data to continue flowing in one direction after closing the other direction.

• TCP does not keep messages boundaries and treats data as byte stream. e.g, when source sends out two chunks of data with length 400 and 600 bytes, the receiver may receive data in chunks of 300, 400, and 300 bytes, or 100 and 900 bytes.

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TCP operations1. TCP delivers byte stream.See picture2. TCP deals with old packets from old connections by several

methods. See picture3. TCP uses sliding-window to implement reliable transfer of

byte stream. See picture4. TCP uses advertised window for flow control.5. Adaptive timer:

1. tout = tRTT+4dRTT ,

2. tRTT(new) = tRTT(old) +(1-)n , dRTT(new)=dRTT(old) + (1-)(n-tRTT)

3. Where n is the time from transmitting a segment until receiving its ACK. , are in 0 to 1 with being 7/8 and being ¼ typically. tRTT is mean round-trip-time, dRTT is average of deviation.

6. TCP uses congestion window for congestion control. See picture

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byte stream

Send buffer

segments

Receive buffer

byte stream

Application Application

ACKs

Transmitter Receiver

Figure 8.18

TCP byte stream

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Host A Host B

SYN, Seq_no = n

SYN, Seq_no = n, ACK, Ack_no = n+1

Seq_no = n+1, ACK, Ack_no = n+1Delayed segment withSeq_no = n+2will be accepted

Figure 8.23Back to TCP operations

Question: How does TCP prevent old packets of old connections?

An old segment could not be distinguished from current ones

–Using long (32 bit) sequence number

–Random initial sequence number

-- set a timer at the end of a connection to clear all lost packets from this connection. As a result, that an old packet from an old connection conflicts with packets in current connection is very low!!

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Octetstransmitted

and ACKed

Rlast highest-numbered octet not yet read by the applicationRnext next expected octetRnew highest numbered octet receivedcorrectlyRlast+WR-1 highest-numbered octet that can be accommodated in receive buffer

Transmitter Receiver

Receive Window

Slast

Slast+WS-1

...

Send Window

Srecent

Rnext

... ...

Slast+WA-1

RlastRlast+WR+1

Slast oldest unacknowledged octetSrecent highest-numbered transmitted octetSlast+WA-1 highest-numbered octet that can be transmittedSlast+WS-1 highest-numbered octet that can be accepted from the application

Rnew

Figure 8.19Back to TCP operations

TCP uses Selective-Repeat ARQ

Note: 1. Rnew highest bytes received correctly, which are out-of sequence bytes.

2. Advertised window WA: Srecent – Slast WA =WR – ( Rnew – Rlast)

… … …

Advertised window

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Congestionwindow

10

5

15

20

0

Round-trip times

Slowstart

Congestionavoidance

Congestion occurs

Threshold

Figure 7.63

Back to TCP operations

Dynamics of TCP congestion window

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TCP protocol• TCP segment See Segment format

– TCP pseudoheader. See pseudoheader

• TCP connection establishment. See establishment– Client-server application See socket

• TCP Data transfer– Sliding window with window sliding on byte basis– Flow control and piggybacking See flow control

• TCP connection termination– After receiving ACK for previous data, but no more data

to send, the TCP will terminate the connection in its direction by issuing an FIN segment. Graceful termination

• TCP state transition diagram

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Source Port Destination Port

Sequence Number

Acknowledgement Number

Checksum Urgent Pointer

Options Padding

0 4 10 16 24 31

URG

ACK

PSH

RST

SYN

FIN

HeaderLength Reserved (Advertised) Window Size

Data

Figure 8.20

Back to TCP protocol TCP segment format

1.SYN: request to set a connection. 2. RST: tell the receiver to abort the connection.3. FIN: tell receiver this is the final segment, no more data, i.e, close the connection in this direction.4. ACK: tell the receiver (or sender) that the value is the field of acknowledgment number is valid.5. PSH: tell the receiving TCP entity to pass the data to the application immediately.6. URG: tell the receiver that the Urgent Pointer is valid.

Urgent Pointer: this pointer added to the sequence number points to the last byte of the “Urgent Data”, (the data that needs immediately delivery).

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0 0 0 0 0 0 0 0 Protocol = 6 TCP Segment Length

Source IP Address

Destination IP Address

0 8 16 31

Figure 8.21

Back to TCP protocol

TCP pseudoheader

The padding of 0s and pseudoheader is only used in computationof checksum but not be transmitted, as in UDP checksum.

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Host A Host B

SYN, Seq_no = x

SYN, Seq_no = y, ACK, Ack_no = x+1

Seq_no = x+1, ACK, Ack_no = y+1

Figure 8.22


Three-way handshake to set up connection

1. Random initial SN2. Initial SNs in two directions are different3. Initial SNs for two connections are different.4. It should be clear here that

what setting up connection means: both A and B know that they will exchange data, and go into ready state to send and receive data. Most important is that

they agree upon the initial SNs.

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Host A (Client) Host B (Server)

SYN, Seq_no = x

SYN, Seq_no = y, ACK, Ack_no = x+1

Seq_no = x+1, ACK, Ack_no = y+1

socketbindlistenaccept (blocks)

socketconnect (blocks)

connect returns

accept returnsread (blocks)

writeread (blocks)

read returns

writeread (blocks)

read returns

request message

reply message

Figure 8.24


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Host A Host B

Seq_no = 2000, Ack_no = 1, Win = 1024, Data = 2000-3023




t1

t2

t3

t4

Seq_no = 1, Ack_no = 2000, Win = 2048, No Data t0

Figure 8.25Back to TCP protocol

TCP window flow control

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FIN, seq = 5086

ACK = 5087

Data (150 bytes), seq. = 303, ACK = 5087

FIN, seq. =453, ACK = 5087

ACK = 454

Host A Host B

ACK = 453

Figure 8.27

Back to TCP protocol TCP graceful termination

Question: is terminationeasier than establishment?Or to say, is it possiblethat a connection is closed when both of two parties confirm with each other?

No, Saying goodbyeis hard to do.Famous blue-red armies problem.

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CLOSED

LISTEN

SYN_RCVD

ESTABLISHED

CLOSING

TIME_WAIT

SYN_SENT

FIN_WAIT_1

CLOSE_WAIT

LAST_ACK

FIN_WAIT_2

active open,create TCB

send SYN

passive open,create TCB

send SYN

receive SYN,

send SYN, ACK

receive

RST

receiveACK receive SYN, ACK,send ACKapplic.

close,sendFIN

applic. clo

se,

send FIN

receive FIN,send ACK

receive FINsend ACK

receive FIN, ACK

send ACKreceive

ACK

receive FINsend ACK

receiveACK

applic. closesend FIN

receiveACK

applic. closeor timeout,delete TCB

2MSL timeoutdelete TCB

receive SYN,send ACK

applic.close

Figure 8.28

Thick lines: normal client statesDashed lines: normal server states


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Sequence number wraparound and timestamps

• Original TCP specification for MSL (Maximum Segment Lifetime) is 2 minutes.

• How long will it take to wrap around 32 bit sequence number when 232=4,294,967,296 bytes have been sent (maximum window size=231)– T-1 line, (2328)/(1.544 106) = 6 hours– T-3 line, (2328)/(45 106) = 12 minutes– OC-48 line, (2328)/(2.4 109) = 14 seconds !!!

• When sequence number wrap around, the wraparounded sequence number will confuse with previous sequence number.

• Solution: optional timestamp field (32 bits) in TCP header, thus, 232232=264 is big enough right now.

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Internet routing protocols• Autonomous system (AS)

– A set of routers or networks technically administrated by a single organization.

– No restriction that an AS must run a single routing protocol– Only requirement is that from outside, an AS presents a consistent picture of

which ASs are reachable through it.

• Three types of ASs:– Stub AS: has only a single connection to outside.– Multihomed AS: has multiple connections to outside, but refuses to carry out

transit traffic– Transit AS: multiple connections to outside and carry transit traffic.

• ASs need to be assigned globally unique AS number (ASN)

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Classification of Internet routing protocols

• IGP (Interior Gateway Protocol):– For routers to communicate within an AS and relies on

IP address to construct paths.– Provides a map of a county dealing with how to reach

each building.– RIP (Routing Information Protocol): distance vector– OSPF (Open Shortest Path First): link state

• EGP (Exterior Gateway Protocol): – For routers to communicate among different ASs and

relies on AS numbers to construct AS paths. – Provides a map of a country, connecting each county.– BGP (Border Gateway Protocol): (distance) path vector

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RIP—Routing Information Protocol• Distance vector• On top of UDP with port #520• Metric is number of hops

– Maximum number of hops is 15, 16 stands for infinity– Using split-horizon with poisoned reverse.– May speed up convergence by triggered updates.

• Routers exchange distance vector every 30 seconds– If a router does not receive distance vector from its

neighbor X within 180 seconds, the link to X is considered broken and the router sets the cost to X is 16 (infinity).

• RIP-2 contains more information: subnet mask, next hop, routing domain, authentication, CIDR

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Command Version Zero

Address Family Identifier Zero

IP Address

Zero

Zero

Metric

0 8 16 31

. . .

Figure 8.32

RIP message format

1. Command: 1: request other routers to send routing information 2: a response containing its routing information

2. Version: 1 or 23. Up to 25 routing information message 3.1 Family identifier: only 2 for IP address 3.2 IP address: can be a host address or a network address 3.3 Metric: 1—15. 16 indicates infinity Problems of RIP: not scalable, slow convergence, counting-to-infinity, therefore replaced By OSPF in 1979.

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Internet multicast• A packet is to be sent to multiple hosts with the same multicast address• Class D multicast addresses: e.g.,

– 224.0.0.1 all systems on a LAN– 224.0.0.2 all routers on a LAN– 224.0.0.5 all OSPF routers on a LAN– 224.0.0.6 all designated OSPF routers on a LAN

• It is not efficient to implement multicast by unicast, i.e., the source sends a separate copy for every destination.

• Reverse-path broadcasting / multicasting, each packet is transmitted once per link

• IGMP (Internet Group Management Protocol): allow a user to join a multicast group and let routers collect multicast group membership information.

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Multicasting

S

G1 G1

G1

G1

G3

1

2

3

4

5

6

7

81

2

3

45

1

23

4 1 2

34

5

1 2

3

1

23

1

2

1 2

34

1

2 3

4

G2

G3

3

4

• Source S sends packets to multicast group G1

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Multicast Routing• Multicast routing useful when a source wants to

transmit its packets to several destinations simultaneously

• Relying on unicast routing by transmitting each copy of packet separately works, but can be very inefficient if number of destinations is large

• Typical applications is multi-party conferencing over the Internet

• Example: Multicast Backbone (MBONE) uses reverse path multicasting

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Reverse-Path Broadcasting (RPB)

• Fact: Set of shortest paths to the source node S forms a tree that spans the network– Approach: Follow paths in reverse direction

• Assume each router knows current shortest path to S– Upon receipt of a multicast packet, router records the packet’s source

address and the port it arrives on– If shortest path to source is through same port (“parent port”), router

forwards the packet to all other ports– Else, drops the packet

• Loops are suppressed; each packet forwarded by a router exactly once• Implicitly assume shortest path to source S is same as shortest path from

source– If paths asymmetric, need to use link state info to compute shortest paths

from S

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Example: Shortest Paths from S

• Spanning tree of shortest paths to node S and parent ports are shown in blue

S

G1 G1

G1

G1

G3

1

2

3

4

5

6

7

81

2

3

45

1

23

4 1 2

34

5

1 2

3

1

23

1

2

1 2

34

1

2 3

4

G2

G3

3

4

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Example: S sends a packet

• S sends a packet to node 1• Node 1 forwards to all ports, except parent port

S

G1 G1

G1

G1

G3

1

2

3

4

5

6

7

81

2

3

45

1

23

4 1 2

34

5

1 2

3

1

23

1

2

1 2

34

1

2 3

4

G2

G3

3

4

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Example: Hop 1 nodes broadcast

• Nodes 2, 3, 4, and 5 broadcast, except on parent ports• All nodes, not only G1, receive packets

S

G1 G1

G1

G1

G3

1

2

3

4

5

6

7

81

2

3

45

1

23

4 1 2

34

5

1 2

3

1

23

1

2

1 2

34

1

2 3

4

G2

G3

3

4

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Example: Broadcast continues

• Truncated RPB (TRPB): Leaf routers do not broadcast if none of its attached hosts belong to packet’s multicast group

S

G1 G1

G1

G1

G3

1

2

3

4

5

6

7

81

2

3

45

1

23

4 1 2

34

5

1 2

3

1

23

1

2

1 2

34

1

2 3

4

G2

G3

3

4

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Internet Group Management Protocol (IGMP)

• Internet Group Management Protocol: – Host can join a multicast group by sending an IGMP message to

its router• Each multicast router periodically sends an IGMP query

message to check whether there are hosts belonging to multicast groups– Hosts respond with list of multicast groups they belong to– Hosts randomize response time; cancel response if other hosts

reply with same membership• Routers determine which multicast groups are associated

with a certain port • Routers only forward packets on ports that have hosts

belonging to the multicast group

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Multicast programming• 2.1 Multicast addresses.

– 224.0.0.0---247.255.255.255 • 2.2 Levels of conformance.

– 0: no, 1: sending, 2: receiving• 2.3 Sending Multicast Datagrams.

– Open UDP socket, and send to multicast address – TTL

• 0 Restricted to the same host. • 1 Restricted to the same subnet. • <32 Restricted to the same site, organization or department. • <64 Restricted to the same region. • <128 Restricted to the same continent. • <255 Unrestricted in scope. Global.

• 2.4 Receiving Multicast Datagrams. – Joining multicast group– Drop multicast group

• Mapping of IP Multicast Addresses to Ethernet/FDDI addresses.

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Multicast functions

• int getsockopt(int s, int level, int optname, void* optval, int* optlen);

• int setsockopt(int s, int level, int optname, const void* optval, int optlen);

• setsockopt() getsockopt() • IP_MULTICAST_LOOP yes yes • IP_MULTICAST_TTL yes yes • IP_MULTICAST_IF yes yes • IP_ADD_MEMBERSHIP yes no • IP_DROP_MEMBERSHIP yes no

• http://www.ibiblio.org/pub/Linux/docs/HOWTO/other-formats/html_single/Multicast-HOWTO.html#ss2.1

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IPv6 (IPng): IPv4 is very successful but the victim of its own success.

• Longer address field:– 128 bits can support up to 3.4 x 1038 hosts

• Simplified header format: – Simpler format to speed up processing of each header– All fields are of fixed size– IPv4 vs IPv6 fields:

• Same: Version• Dropped: Header length, ID/flags/frag offset, header checksum• Replaced:

– Datagram length by Payload length– Protocol type by Next header– TTL by Hop limit– TOS by traffic class

• New: Flow label

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Other IPv6 Features• Flexible support for options: more efficient and

flexible options encoded in optional extension headers• Flow label capability: “flow label” to identify a

packet flow that requires a certain QoS• Security: built-in authentication and confidentiality• Large packets: supports payloads that are longer than

64 K bytes, called jumbo payloads.• Fragmentation at source only: source should check

the minimum MTU along the path• No checksum field: removed to reduce packet

processing time in a router

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IPv6 Header Format

• Version field same size, same location• Traffic class to support differentiated services• Flow: sequence of packets from particular source to particular

destination for which source requires special handling

Version Traffic Class Flow Label

Payload Length Next Header Hop Limit

Source Address

Destination Address

0 4 12 16 24 31

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IPv6 Header Format

• Payload length: length of data excluding header, up to 65535 B• Next header: type of extension header that follows basic header• Hop limit: # hops packet can travel before being dropped by a router

Version Traffic Class Flow Label

Payload Length Next Header Hop Limit

Source Address

Destination Address

0 4 12 16 24 31

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IPv6 Addressing• Address Categories

– Unicast: single network interface– Multicast: group of network interfaces, typically at different

locations. Packet sent to all.– Anycast: group of network interfaces. Packet sent to only one

interface in group, e.g. nearest.• Hexadecimal notation

– Groups of 16 bits represented by 4 hex digits– Separated by colons

• 4BF5:AA12:0216:FEBC:BA5F:039A:BE9A:2176

– Shortened forms:• 4BF5:0000:0000:0000:BA5F:039A:000A:2176 • To 4BF5:0:0:0:BA5F:39A:A:2176• To 4BF5::BA5F:39A:A:2176

– Mixed notation:• ::FFFF:128.155.12.198

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Example

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Address Types based on PrefixesBinary prefix Types Percentage of address space

0000 0000 Reserved 0.39

0000 0001 Unassigned 0.39

0000 001 ISO network addresses 0.78

0000 010 IPX network addresses 0.78



0001 Unassigned 6.25

001 Unassigned 12.5

010 Provider-based unicast addresses 12.5

011 Unassigned 12.5

100 Geographic-based unicast addresses 12.5

101 Unassigned 12.5

110 Unassigned 12.5

1110 Unassigned 6.25




1111 1110 0 Unassigned 0.2

1111 1110 10 Link local use addresses 0.098

1111 1110 11 Site local use addresses 0.098

1111 1111 Multicast addresses 0.39

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Special Purpose Addresses

• Provider-based Addresses: 010 prefix

– Assigned by providers to their customers

– Hierarchical structure promotes aggregation

• Registry ID: ARIN, RIPE, APNIC

• ISP

• Subscriber ID: subnet ID & interface ID

• Local Addresses: do not connect to global Internet

– Link-local: for single link

– Site-local: for single site

– Designed to facilitate transition to connection to Internet

010 Registry ID Provider ID Subscriber ID Subnet ID Interface ID

n bits m bits o bits p bits (125-m-n-o-p) bits

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Special Purpose Addresses• Unspecified Address: 0::0

– Used by source station to learn own address

• Loopback Address: ::1• IPv4-compatible addresses: 96 0’s + IPv4

– For tunneling by IPv6 routers connected to IPv4 networks

– ::135.150.10.247

• IP-mapped addresses: 80 0’s + 16 1’s + IPv4– Denote IPv4 hosts & routers that do not support IPv6

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Migration from IPv4 to IPv6

• Gradual transition from IPv4 to IPv6• Dual IP stacks: routers run IPv4 & IPv6

– Type field used to direct packet to IP version

• IPv6 islands can tunnel across IPv4 networks– Encapsulate user packet insider IPv4 packet– Tunnel endpoint at source host, intermediate

router, or destination host– Tunneling can be recursive

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Migration from IPv4 to IPv6

Source Destination

IPv6 network IPv6 network

Link

(b)

Source Destination

IPv6 networkIPv4 network

IPv6 network

Tunnel

Tunnel head-end Tunnel tail-end

IPv6 headerIPv4 header

(a)

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DHCP (Dynamic Host Configuration Protocol)

• A host broadcasts a DHCP discovery message in its physical network for an IP address.

• Server(s) reply with DHCP offer message• The host selects one IP address and broadcasts a

DHCP request message including the IP address• The selected server allocates the IP address and

sends back a DHCP ACK message with a lease time T, two thresholds T1 (=0.5T), T2(=0.875T)– when T1 expires, the host asks the server for extension.– If T2 expire, the host broadcasts DHCP request to any

server on the network– If T expires, the host relinquishes the IP address and

reapply from scratch.

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Mobile IP

• Mobile host, home agent, foreign agent• If mobile host is currently at the same network

with HA (home agent), the packet to the mobile host will be broadcast to it.

• If mobile host moves to another network, the mobile host will register itself with FA (foreign

agent) and gets a new care-of IP address. Then packet is sent to HA, which will forward to the FA and FA continues to forward to destination.

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Home agent

Foreign agent

Home network

Foreign network

Internet

Correspondent host

Mobile host

1

2

3

Figure 8.29

Deliver packets to mobile host through home agent and foreign agent

1 UDP—User Datagram Protocol An unreliable, connectionless transport layer protocol UDP format....

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Transcript of 1 UDP—User Datagram Protocol An unreliable, connectionless transport layer protocol UDP format....