CSCI 5273 Computer Networks Stevens, Chapter 11

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CSCI 5273Computer NetworksStevens, Chapter 11
12UDP with supplemental informationon the
POSIX interfaceBroadcast Multicast

• Dirk GrunwaldAssoc. ProfessorDept. of Computer
  ScienceUniversity of Colorado, Boulder

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Overview

• Basic UDP protocol
• IP Fragmentation reassembly
• Path MTU discovery
• ICMP Source Quench
• POSIX interface pragmatics

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UDP Protocol
IP Datagram
UDP Datagram
IPHeader
UDPHeader
UDPData
20 bytes
8 bytes
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UDP Header
16-bit Destination Port
16-bit Source Port
16-bit UDP Checksum (opt)
16-bit UDP Length
Data (if any)
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UDP Header
IPHeader
UDPHeader
UDPData
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UDP Checksum
IPHeader
UDPHeader
UDPData
IPPesudo-Header
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IP Pseudo-Header
32-bit Source IP address
32-bit Destination IP address
MBZ
Protocol
16-bit UDP Length
16-bit Destination Port
16-bit Source Port
16-bit UDP Checksum (opt)
16-bit UDP Length
Data (if any)
Possible odd byte
PAD
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UDP Checksum

• Checksum calculated like IP checksum, but use
  pseudo-IP header to insure packet arrived at
  proper host
• If transmitted checksum field is zero, it means
  sender didnt compute the checksum.
• If the computed checksum would be zero, its
  represented as 65535
• No packets with checksum errors are not reported

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IP Fragmentation

• When a router transits a packet that is too large
  for the MTU of the outgoing link, the packet is
  fragmented
• Fragmented packets are not reassembled until they
  reach their final destination
• Fragments may also be fragmented
• Fragments are identified using the datagram
  sequence
• Typically, if any fragment is lost, a router will
  discard all fragments. Routers usually only
  discover fragment loss if they drop the fragment
  themselves.
• The endpoint assumes fragments are lost after
  30-60 seconds

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Packets vs. Datagrams

• An IP datagram is the unit of end-to-end
  transmission at the IP layer (before
  fragmentation after reassembly)
• A packet is the unit of data passed between the
  IP layer and the link layer.
• A packet can be a complete IP datagram or a
  fragment

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IP Fragmentation
Payload
IP Header
Payload
IP Header
Payload
IP Header
More Fragementsis Set
Payload
IP Header
More Fragementsis NOT Set
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IP Fragmentation - Identifying Information
Payload
IP Header
Payload
IP Header
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IP Fragmentation
Payload
IP Header
Payload
IP Header
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IP Fragmentation Of Non-Final Fragments
Payload
IP Header
Payload
IP Header
Payload
IP Header
More Fragementsis Set
IP Header
Payload
Payload
IP Header
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IP Fragmentation Of Final Fragment
Payload
IP Header
Payload
IP Header
Payload
IP Header
Payload
IP Header
More Fragementsis Set
IP Header
More Fragementsis NOT Set
IP Header
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Dont Fragment

• Hosts must be able to receive packets of 576
  bytes, which means a 512-byte datagram wont be
  fragmented
• One of the IPv4 header flags specifies that this
  packet should be fragmented

16-bit Packet Identification
Fragment Offset
Reserved
Dont Fragment
MoreFragments
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ICMP Unreachable Error

• Attempting to fragment a fragment with dont
  fragment set generates an ICMP error packet
• ICMP type destination unreachable (type 3)
• code fragmentation required but dont fragment
  set (code 4)

Type (3)
Code (4)
Checksum
MTU of next network hop
MBZ
IP Header (including options)and first 8 bytes
of original IP datagram data
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MTU Discovery UsingDont Fragment Packets
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ICMP Source Quench

• If a router / host discards datagrams due to
  buffer overflows, it may send a ICMP source
  quench message
• I tried for 15 minutes to generate this on a slow
  host was unable to do so
• More likely to occur when e.g., routing to a
  dialup, but even that failed.
• Can be used by a protocol to slow down
  transmission rate (e.g., TCP)

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UDP Pragmatics (review from code)

• UDP port and TCP ports are separate name spaces
• UDP port 80 doesnt mean the same thing as TCP
  port 80
• UDP ports are unique to a specific interface
• port 80 on loopback is not the same as port 80 on
  eth0
• Most POSIX/UNIX systems let you specify
  wildcards
• IPADDR_ANY is a special address (0.0.0.0) that is
  a wild card interface address

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Using netstat to see ports
current-45 netstat -n -a Active Internet
connections (including servers) Proto Recv-Q
Send-Q Local Address Foreign Address
State tcp 0 0 128.138.202.9222
128.138.241.121813 ESTABLISHED tcp
0 0 0.0.0.06000 0.0.0.0
LISTEN tcp 0 0 0.0.0.022
0.0.0.0 LISTEN tcp
0 0 0.0.0.01024 0.0.0.0
LISTEN tcp 0 0 0.0.0.0758
0.0.0.0 LISTEN tcp
0 0 0.0.0.025 0.0.0.0
LISTEN tcp 0 0 0.0.0.0113
0.0.0.0 LISTEN tcp
0 0 0.0.0.079 0.0.0.0
LISTEN tcp 0 0 0.0.0.0512
0.0.0.0 LISTEN tcp
0 0 0.0.0.0513 0.0.0.0
LISTEN tcp 0 0 0.0.0.0514
0.0.0.0 LISTEN tcp
0 0 0.0.0.023 0.0.0.0
LISTEN tcp 0 0 0.0.0.021
0.0.0.0 LISTEN tcp
0 0 0.0.0.037 0.0.0.0
LISTEN tcp 0 0 0.0.0.013
0.0.0.0 LISTEN tcp
0 0 0.0.0.0111 0.0.0.0
LISTEN udp 0 0 0.0.0.08000
0.0.0.0 udp 0 0
0.0.0.0768 0.0.0.0 udp 0
0 0.0.0.0770 0.0.0.0 udp
0 0 0.0.0.0177 0.0.0.0
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Using netstat to see interfaces
current-45 netstat -n -a . udp 0
0 128.138.202.928000 0.0.0.0 udp 0
0 127.0.0.18000 0.0.0.0 udp
0 0 0.0.0.0769 0.0.0.0 udp
0 0 0.0.0.0768 0.0.0.0
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System Calls Used

• Socket
• Create an endpoint on the local system
• Bind
• Specify the local interface and port for the
  endpoint
• Connection
• Specify the remote interface and port for the
  endpoint
• setsockopt / getsockopt
• Modify various default properties

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Bound Connected Sockets

• Until bind is called, a socket is not bound
• Cant receive messages (havent specified port)
• When you send using an unbound socket, its
  bound to an ephemeral port
• Until connect is called, a socket is not
  connected
• Sending messages on an unconnected socket
  requires that you specify the destination address
  each time.
• If you do call connect, you can only receive
  messages on the connected socket from that the
  specified remote endpoint

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POSIX socket interface

• send
• Send a message on a connected socket
• sendto
• Send a datagram to a specified IP address. The
  socket can be unconnected.
• recv
• Receive a datagram from a bound socket
• recvfrom
• Receive a datagram and record the source IP
  address
• recvmsg
• Essentially like recvfrom, but arguments packed
  in a struct

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One Last POSIX call - select

• Select lets you wait on multiple file descriptors
  to become available, or for a timeout to occur
• include ltsys/time.hgt
• int select(
• int nfds,
• fd_set readfds,
• fd_set writefds,
• fd_set exceptfds,
• struct timeval timeout)
• Youll need this for next homework!

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Common UDP Server Pattern
socket
recvfrom
setsockopt
sendto
bind
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Common UDP Server Pattern
socket
sendto
setsockopt
recvfrom