Transcript 휴대인터넷 무선환경에서의 효과적인 Broadcast/Multicast 적용방안

TCP/IP
Illustrated Volume I
Internet Protocol
2005. 3. 16
백일우
Basic Features


Unreliable, connectionless Delivery
Best Effort Service


No Guarantee that IP datagram successfully get to its Dest.
When Something wrong, IP runs A simple error Handling Algorithm
 ICMP : Congestion, Redirect, Packet Arrived or Not
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
Each Datagram is Handled Independently, so Delivered out of order
Fragmentation/Reassembly For supporting network Interface which has
variable frame sizes
Network Type
IP MTU (BYTE)
Ethernet(Ethernet II)
1500
Ethernet(802.3)
1492
Token Ring(802.5)
4440 – 17940(Token Ring의 Holding Time에 따라 다름)
FDD I
1600
Frame Relay
4352
Minimun MTU
576
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IP Header
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Version
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Only V4 and V6 are used
Other is not used
3
IP Header
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Header Length
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Range is 4bits, so it can present values 0~15
Specifies the length of the IP packet header in 32 bit words
Header Length = Real Value Times(*) 4byte(32bits)
 If value is 5, Real Length is 5*4 = 20 (bytes)

Header Length valid Range
 5 ~ 15
 Minimun 20 ~ Maximum 60 (5*4 ~ 15*4)

It is only can be presented by a multiple of 4
 So, If Can not be presented by 4-multiple, Must add ‘Padding’ to be
type of 4-multiple
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IP Header
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Type of Service (TOS)
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Specifies the parameters for the type of service requested
The parameters may be utilized by networks to define the handling
of the datagram during transport
Precedence
 Basically Setted by 000, the others is not allowed
 Delay
 0 : Normal Delay, 1: Low Delay
 1 for sound, movie and Logon request
 Throughput
 0 : normal Throughput, 1: High throughput
 If 1, a Router select the way which has the best BW
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IP Header

Precedence
Precedence Value
Precedence
000
Routine
001
Priority
010
Immediate
011
Flash
100
Flash Override
101
CRITIC/ECP
110
Internetwork Control
111
Network Control
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IP Header
 Reliability
 0 : normal Reliability, 1: High Reliability
 As so many traffics are driven to some router, the router decide which
packet is less significant with Reliability Field, and Drop it.
 Cost
 0 : normal Cost, 1 : Low Cost
 Decide which way is more reasonable, checking this field.
 Reserved
 Last field is always set by 0 (MBZ : Must Be Zero), So routers ignore this
field.
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Total Length

Contains the length of the datagram (16 bits) – MAX : 65535 bytes
 IP header + IP Payload
 MAX(65535 bytes) possible, but most link-Layer will fragment this
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IP Header

Identification (16bits)
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
Used to identify the fragments of one datagram from those of
another.
Flags
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R, Reserved. 1 bit. Should be cleared to 0.
DF, Don’t Fragment, 1bit.
 Controls the fragmentation of the datagram
 0 : Fragment if necessary, 1 : Nope! Fragment

MF, More fragments. 1 bit.
 Indicates if the datagram contains additional fragments
 0 : This is the last Fragment, 1: More Fragments follows after this

Fragment Offset (13 bits)

Used to direct the reassembly of a fragmented datagram
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IP Header – Fragmentation
More Fragments follow
<Identification & Fragmentation>
Identification
123
0000 001
123
1000 001
123
Last Fragment
2,300 bytes
2000 000
MTU = 2,300
<Fragment Offet 역할>
MTU = 1,000
ROUTER
A
B
123
C
D
IP Header
IP Header
PayLoad
IP Header
A까지
PayLoad
B까지
IP Header
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PayLoad
IP Header
C까지
PayLoad
D까지
IP Header
H7
H1
H2
R3
H8
H3
Network 4
(Point-to-Point)
Network 2(Ethernet)
R1
R2
H4
1.
Ethernet MTU : 1500
2.
FDDII MTU : 4500
3.
Suppose PtoP MTU is
532(included Header)
4.
H1 datagram : 1420
Network 3(FDDI)
H5
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IP Header

1400 bytes split into pieces of 512 bytes
Start of header
Iden = A
1
Offset = 0
Rest of Header
512 data bytes
Start of header
Iden = A
0
Start of header
Offset = 0
Iden = A
1
Offset = 512
Rest of Header
Rest of Header
1400 data bytes
512 data bytes
> 1400 = 512 + 512 + 376
Start of header
Iden = A
0
Offset = 1024
Rest of Header
376 data bytes
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IP Header
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TTL (Time To live) – 8bits
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
A timer field used to track the lifetime of the datagram. When the
TTL field is decremented down to zero, the datagram is discarded
Protocol – 8bits

This field specifies the next encapsulated protocol
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IP Routing - Intro
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Forwarding
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Routing
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That is, Send the packet which is sent from outside to reasonable
path, checking the routing table
That is, Configuring the routing table to make the router send the
packet to the reasonable path
Routing Table Contains..,

Destination IP addr
 This can be either a complete host addr, or a network addr, as
specified by the flag field


IP addr of a next-hop router
Flags
 One specifies whether Dest IP addr is Addr of Network or Host
 Another says whether the next-hop router field is really a next-hop
router or a directly connected interface.
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IP Routing - Intro
Minimal Encapsulation
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IP Routing - Intro

Key Points from previous examples
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All hosts and routers in this example used a default route. Indeed,
most host and some routers can use a default route for everything
other than destinations on Local networks
The Destination IP addr in the datagram never changes. All the
routing decision are based on this destination address
In previous Example, Both Ethernets encapsulated header containing
the next-hop’s ethernet address, but the SLIP link did NOT
The Ethernet addresses are normally obtained using ARP
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Subnetting & Subnet Mask
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Purpose
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For efficient IP address usage, and Reducing the size of routing
tables
Subnet
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One network Number of IPs can be assigned in many physical
Networks
Conditions
 Seen from outside as if It were single Network
 That is, Router can select one path for them
 Campus can be good example
 All nodes on each subnet must be configured with same subnet mask
to share Just one network number

Subnet Mask
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IP organization Could be more specific
 Network and host to Network, subnet, and host
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Subnetting & Subnet Mask
Network Number
Host Number
Class B
Subnet Mask : 255.255.255.128
111111111111111111111
Subnet Number : 128.96.34.0
128.96.34.15
00000000
Subnet mask(255.255.255.0)
Network Number
128.96.34.1
H1
Subnet ID Host ID
Subnetted Address
R1
128.96.34.130
Subnet Mask : 255.255.255.128
Subnet Number : 128.96.34. 128
1.
H1 -> H2로 보내고자 한다면,
2.
And 연산을 한다
1.
128.96.34.139
H3
R2
H2
3.
H1의 서브넷과 일치 않함!!
1.
128.96.33.1
128.96.33.14
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결과 : 128.96.34.128
H1과 H2는 다른
서브넷상에 있다는 것을
알수 있음
Subnetting & Subnet Mask
<Subnetted Forwarding Table>
Subnet Number

Subnet Mask
Next Hop
128.96.34.0
255.255.255.128
Interface 0
128.96.34.128
255.255.255.128
Interface 1
128.96.33.0
255.255.255.0
R2
H1 -> H2
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R1 : 128.96.34.139 AND 255.255.255.128
 Result : 128.96.34.128

Compare 128.96.34.0 with 128.96.34.128 -> X
 Compare 128.96.34.0 with next entry

Through this works, R1 forward to H2 using interface 1
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Question

패킷 단편화후 그 단편화된 Datagram들이 꼭 순서에 맞게
도착하지 않음 .
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Host에서는 맨 마지막 조각이 오고 나서 재조립을 시작하는가.
Flag M = 0 이라는 마지막 조각이 먼저 오게 되면, 언제까지
기다려야 되나.
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