Transcript IP Datagrams - University of Maine System

IP Datagrams
Lecture 12
November 8, 2000
Frames v. Datagrams
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Both provide a clear and concise delineation
around the data.
Both have a header (and maybe a footer).
Frames encompass much smaller amounts of
data. RS-232 framing is 5-8 data bits.
Datagrams are variable length, anywhere from
a single octet to 64K octets payload.
Frames v. Datagrams
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Frames are media-dependent. Ethernet
framing cannot be used on a FDDI network.
Datagrams are media-independent.
Connection v. Connectionless
Service
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The two can be compared much like P-T-P
connections and shared medium connections.
Connection service essentially opens a
channel between the two machines and
communicates over the channel.
Connectionless service is where data is
forwarded from one node to the next. When
data reaches the destination, an ACK is sent.
Connection v. Connectionless
Service (cont.)
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Both models exist with TCP/IP!
Connection service = TCP
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Guaranteed delivery
Full duplex communication
Point-to-point communication
Connectionless service = UDP
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Unreliable
Non-sequential delivery
Connection v. Connectionless
Service (cont.)
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IP’s default delivery method is UDP (User
Datagram Protocol).
Connection service delivery was an
afterthought.
Universal, Virtual Packets
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The goal of IP datagrams is to be entirely
media-independent.
This independence leads to a universal packet
scheme that all IP devices recognize.
The protocol software handles the creation and
interpretation of the internet packets. Virtual
packets.
IP Datagram
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IP datagram = IP packet
Payload (data) is not a
fixed size
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One octet to 64K octets
Header
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Source IP address
Destination IP address
Payload size
CRC
And some other stuff…
Forwarding a Datagram
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Because datagrams are a connectionless
communication, they are forwarded from node
to node.
At each step, the router (node) inspects the
destination address of the datagram and
forwards it to the appropriate interface.
Simple Datagram Forwarding
Datagram Forwarding with a
Routing Table
Network Address
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From our subnetting discussion, we’ve already
seen how the network address can be
determined from the IP address and the
netmask.
192.4.10.3 & 255.255.255.0 == 192.4.10.0
With the network address, the router can
determine the correct next hop.
Best-Effort Delivery
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Although IP makes the best-effort of datagram
delivery, it does not guarantee proper handling
of:
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Datagram duplication
Delayed or out-of-order delivery
Corruption of data
Datagram loss
Other protocol layers are responsible for error
handling.
IP Datagram Header
IP Datagram Header (cont.)
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Version (4 bits)
Header Length (4 bits) – length of the header in 32 bit
words. 5 minimum.
Service Type (8 bits) – Indication of the quality of
service desired.
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Bits 0-2: Precedence
Bit 3: 0 = Normal Delay, 1 = Low Delay
Bit 4: 0 = Normal Throughput, 1 = High Throughput
Bit 5: 0 = Normal Reliability, 1 = High Reliability
Bits 6-7: Reserved for future use
IP Datagram Header (cont.)
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Precedence (from Service Type)
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111: Network control
110: Internetwork control
101: CRITIC/ECP
100: Flash Override
011: Flash
010: Immediate
001: Priority
000: Routine
IP Datagram Header(cont.)
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Total Length (16 bits): Total length of the datagram,
measured in octets, including header and data.
Identification (16 bits): A value assigned to aid in
assembly of fragments.
Flags (3 bits): Various Control Flags.
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Bit 0: Reserved. Must be 0.
Bit 1: (DF) 0 = May Fragment, 1 = Don’t Fragment
Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments
Time to Live (8 bits): Maximum time the datagram is
allowed to exist in the system. Each router that handles
the datagram decrements the TTL by 1.
IP Datagram Header (cont.)
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Fragment Offset (13 bits): Designates where in the
datagram a fragment belongs. First fragment is 0.
Protocol (8 bits): Indicates which Transport Layer
protocol the datagram is passed to.
Header Checksum (16 bits): Checksum that covers the
header only
Source address (32 bits)
Destination address (32 bits)
IP Options (variable): usually don’t appear in
datagrams, but must be implemented in IP stacks.
IP Transmission
Datagram Transmission
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A datagram is an untransmissible unit in itself.
In order for a datagram to be sent across a
network, it must be encapsulated into a frame.
Keep in mind, the framing method is mediadependent!
As a datagram traverses the Internet, it
remains intact, but the framing around the
datagram changes at every node!
IP Encapsulation
Internet Transmission
Transmission (cont.)
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Remember that as a packet traverses a
network, the payload remains the same (the
datagram), but the framing is created and
destroyed at each hop.
MTU & Datagram Size
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MTU – Maximum Transmissible Unit
Consider the following example:
MTU & Fragmentation
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H1 sending data to H2.
H1’s network allows for 1500 byte transmissions, but
H2’s network is only 1000 bytes.
Assuming one 1500 byte transmission, the router will
take the 1500 byte datagram, and fragment it into two
smaller datagrams, both beneath the 1000 byte MTU
restriction in H2’s network.
The fragmentation flags will be set in the datagram
header.
MTU & Fragmentation (cont.)
Fragment Reassembly
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The process of combining all of the datagram
fragments is called reassembly.
IP specifies that reassembly is to occur at the
destination address!
Advantages:
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Intermediate router requires less processing
Routers can send the fragments on to the source,
regardless of the path! Routing changes would not
disturb the transmission.
Fragment Reassembly (cont.)
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In the below example, R2 does not combine
fragments, so the datagrams sent onto Net 3
are 1000 bytes, not 1500 bytes!
Fragment Reassembly (cont.)
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If one fragment is lost in the transmission, the receiver
cannot combine the fragments into the datagram!
The receiver will hold the fragments for a period of
time, after which all fragments are discarded.
IP is “all or nothing”. If all fragments aren’t received, the
transmission is useless.
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TCP v. UDP
Fragmenting a Fragment
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Can it be done? You bet.
IP designed such that each time a transmission
is fragmented, a new IP header is applied to
the subsequent fragments.