UDP, TCP/IP, and IP Multicast

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UDP, TCP/IP, and IP Multicast

• COM S 414
• Sunny Gleason, Vivek Uppal
• Tuesday, October 23rd, 2001

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In This Lecture

• We will build on understanding of IP (Internet
  Protocol)
• UDP User Datagram Protocol
• Unreliable, packet-based protocol
• TCP Transmission Control Protocol
• Reliable, connection-oriented, stream-based
  protocol
• IP Multicast (if time allows)
• Facilities for delivering datagrams to multiple
  recipients
• We wont discuss ICMP (Internet Control Message
  Protocol), but you can look it up if you want

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Where To Find More Info

• For More Practical Information
• Network Programming in Java
• The Java Custom Networking Trailhttp//java.sun.c
  om/docs/books/tutorial/networking/sockets/http//
  java.sun.com/docs/books/tutorial/networking/datagr
  ams/
• Network Programming in C
• Books by W. Richard Stevens HIGHLY recommended!
• TCP/IP Illustrated Series
• UNIX Network Programming, Vol. 1
• Kernel Source Real Protocol Stacks
• Linux TCP/IP Stack
• http//www.kernel.org/pub/linux/kernel/v2.4/
• OpenBSD TCP/IP Stack
• ftp//ftp.openbsd.org/pub/OpenBSD/src/sys/netinet/

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Where to Find More Info

• Papers, Lecture Notes and RFCs
• TCP Congestion Control
• Van Jacobson, Congestion Avoidance and Control,
  1988
• Internet RFC Series http//www.rfc-editor.org/
• CS514 - Fall 2000 Lecture Notes
• Birman, Kenneth. Building Secure and Reliable
  Network Applications. 1995.

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First, some definitions

• Keep the OSI Layers in mind!
• Address
• An identifier, following an addressing
  convention, which allows a machine to be uniquely
  identified
• MAC Address, or Hardware Address
• Numeric address used by Ethernet (data-link
  layer)
• Might look like 00022D0868F8
• IP Address
• Numeric address used by IP (network layer)
• Might look like 128.84.133.221

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First, some definitions

• Packet, or Datagram
• self-contained unit of information
• consists of a header and body
• Packet Header
• For now, realize that it includes source address,
  destination address
• With layered model, nesting of headers

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First, some definitions

• Local Area Network (LAN)
• Group of machines sharing a common communications
  medium (such as Ethernet)
• High data rates, private wires, shorter
  distances
• Wide Area Network (WAN)
• spans a greater geographic area, may depend on
  publicly available network structures(telephone
  system, leased lines, satellites)

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First, some definitions

• Router
• Machine that moves packets from one network to a
  network that is closer to the destination
• (Based on a routing table, which may change)
• Bridge
• A machine that indiscriminantly replicates
  packets between two LANs
• typically not as smart but faster than a router
• Gateway
• A machine that routes packets from the LAN to the
  WAN (What is a Firewall?)

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First, some definitions

• Port
• In UDP and TCP, a number which the kernel uses to
  deliver datagrams to the appropriate application
• For instance HTTP is port 80, SMTP is port 25,
  Telnet is port 23, DNS is port 53, FTP is port 21
• In this model, receivers agree to wait for
  datagrams on a specified port
• Socket address, port

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The Internet

• A network based on the Internet Protocol (IP)

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The Internet

• Routes IP Datagrams from point A to point B
  unreliably

B 128.84.154.132
A 171.64.14.203
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Unreliably?

• What good is that?
• Packet loss rate is extremely low (ltlt 1)
• Packets usually dropped by overloaded routers (as
  well see later)
• This is good enough for us to build the User
  Datagram Protocol (UDP)

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UDP

• For applications where IP guarantees of
  reliability are good-enough
• Streaming multimedia, stock quotes
• Extends IP packet with source port, destination
  port
• In addition, provides fragmentation (and checksum)

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Fragmentation in UDP

• Very simple splits large UDP datagram into
  multiple IP datagrams, each with a sequence
  number
• Marks fragmented bit in the UDP header
• If one fragment is lost, the whole UDP packet is
  discarded
• UDP datagrams are discarded if checksum fails

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The UDP API

• No-frills! Basically, you
• Create a socket address, port
• Send data to a remote socket
• Receive data on a given socket
• No guarantees about reliability, or even the
  ordering in which datagrams are received
• How can we get around this?

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Adding Reliability to UDP

• Timeouts Acknowledgements
• Receiver sends acks of received datagrams
• If sender does not receive ack within a certain
  time, retransmit the packet
• Sequence Numbers
• Sender marks datagrams with sequence numbers
• Receiver uses sequence numbers to restore order
  to the datagrams, and ignore duplicates
• What if we have 100 or more concurrent
  applications? Is this efficient?

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TCP

• A TCP connection is defined by
• src_addr, src_port, dst_addr, dst_port
• Note symmetry at both ends of connection
• Thus, sender is a receiver and vice-versa
• The goal a reliable, stream-based,
  connection-oriented protocol
• Reliable data gets through or connection
  breaks
• Stream-based imagine reading a file in-order
• Connection-oriented point-to-point
• How is it all done?

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Vivek Presents

• The inner workings of the TCP protocol
• Any questions before we move on?

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TCP

• TCP Stream Protocol
• 3-way Handshake
• Closing a connection
• Acknowledgments
• Sliding Window
• Flow Control
• RED

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TCP -- Stream Protocol

• Connection oriented
• like a telephone connection
• Needs set up before the transfer starts.
• Reliable, point to point communication.
• In order delivery
• No loss or duplication.
• Flow Control and error correction
• Duplex connections

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3 Way Hand Shake

TCP is connection Oriented
Connection initiated by a 3 - way handshake
Takes 3 packets Protection against
duplicate Syn Packets
A
B
Syn
Syn, Ack Of Syn
Ack Of Syn
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Basic 3 Way Handshake

• TCP A
  TCP B
• SEQ ACK CTL

1. CLOSED
LISTEN
2. SYN-SENT ? lt100gt
ltSYNgt ? SYN-RECV
3. ESTABLISH ? lt300gt lt101gt ltSYN,ACKgt ?
SYN-RECV
4. ESTABLISH ? lt101gt lt301gt ltACKgt
? ESTABLISH
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Duplicate Recovery

• TCP A
  TCP B
• SEQ ACK CTL

1. CLOSED LISTEN
2. SYN-SENT ? lt100gt
ltSYNgt ...
3. (duplicate) ... lt90gt
ltSYNgt ? SYN-RECV
4. ? lt300gt lt91gt
ltSYN,ACKgt ? (duplicate)
5. ? lt91gt
ltRSTgt ? LISTEN
6. ... ? lt100gt
ltSYNgt ? SYN-RECV
7. SYN-SENT ? lt400gt lt101gt ltSYN,ACKgt ?
SYN-RECV
8. ESTABLISH ? lt101gt lt401gt ltACKgt
? ESTABLISH
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3 Way Handshake

• It ensures that both sides are ready to
  transmit data, and that both ends know that the
  other end is ready before transmission actually
  starts.
• It allows both sides to pick the initial
  sequence number to use.

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Closing a Connection
Send a Fin packet before tearing the
connection
A
B
Both processes must send Fin packets
separately for closing the connection in that
direction
Fin, Ack
Ack of Fin
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Closing a Connection

• TCP A
  TCP B
• SEQ ACK CTL

1. ESTABLISHED
ESTABLISHED
2. (Close) FIN ? lt100gt
lt300gt ltFIN,ACKgt ?
3. FIN lt300gt lt101gt ltACKgt
? CLOSE-WAIT
4. (Close) lt300gt
lt101gt ltFIN,ACKgt ? LAST-ACK
5. ? lt101gt lt301gt ltACKgt
? CLOSED
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Acknowledgements

• Receiver acks only the last in-order packet
  received
• Send nacks for out-of-order packets
• Sender resends the first unacknowledged packet
• timeout typically set to 1.5 round trip times

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Sliding Window
The sender window has k segments (buffers)
Initially Empty
Initially Empty
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Sliding Window
Send message mi
mi
mi
Empty
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Sliding Window
mi mi1 mik
ack
mi mi1
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Sliding Window
mi2 mi3 mik1
mi mi1 Have been acked
ack
mi2 mi3
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TCP Congestion Control

• Dynamically adjust window size
• Sender should not swamp the receiver both sides
  advertise maximum window size
• Linear increase -- When packets are getting
  through, increment the window size by 1.
• When a packet is dropped, halve the window size,
  and double the retransmission timeouts --
  exponential backoff.
• Also called TCP fairness/friendliness

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TCP Slow start

• Might take some time to get to the maximum
  possible window size
• Optimization
• Exponential increase to start with.
• Then follow the linear increase exponential back
  off when the first packet is lost

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RED

• Random Early Detection
• Idea is very simple
• Router senses that load is increasing
• It simply notices that it has less available
  memory for buffering
• This is because packets are entering faster than
  they can be forwarded

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RED

• Picks a packet at random and discards it
• Even though perhaps it could be forwarded
• Receiver detects the loss and sends a NACK
• The network isnt completely overloaded yet so
  the NACK gets through
• Sender chokes back

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Sunny Presents

• IP Multicast
• Any questions before we move on?
• Note Slides were stolen from CS514 FA2000 Web
  site

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Unicast to multiple hosts
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Multicast to multiple hosts
to group
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Why do multicast?

• Send to a group, not to individual hosts
• Reduces overhead in sender
• Reduces bandwidth consumption in network
• Reduces latency seen by receivers (all receive
  at the same time, in theory)

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Logical addressing

• Multicast groups handled by network
• Senders, receivers do not need to know each
  others identities
• Group persists as long as it has at least one
  member
• a rendezvous mechanism

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Applications

• Teleconferencing
• Distance learning
• Multimedia streaming
• Directory service lookup
• ...

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Multicasting for resource location

• Expanding-ring search
• We want to find an instance of a resource
  (database, etc) which is close by
• Use multicast with IP time-to-live (TTL) values

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Time-to-live and hop counts

• TTL is a counter in the packet header
• Decrement at each hop through a router
• When TTL reaches zero, the packet is dropped
• special values for global and regional TTL
  (use with care!)

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Expanding-ring search
Find me a database, TTL1
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Expanding-ring search
Find me a database, TTL2
Im a database, what can I do for you?
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Multicast addresses

• Class D IP addresses for group
• 224.0.0.0 to 239.255.255.255
• Treated like any other IP address can send from
  it or listen to it
• In practice, use UDP as well (more on this later)

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Multicast at the LAN level

• Ethernet is a broadcast medium all network cards
  see all packets
• Register the multicast address in the network
  card
• only pass matching packets to OS
• all other packets are ignored

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Multicast beyond the LAN

• We would like to multicast between hosts on
  different LANs
• LANs are joined together directly by bridges
• or can be connected through the Internet by a
  sequence of routers
• need an inter-LAN (WAN) protocol
• (in fact, this is rarely enabled!)

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A naive approach

• We want to send multicasts everywhere where there
  are group members
• use flooding to send multicast between routers
• when we get to a LAN, use regular (Ethernet)
  multicast

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Multicast by flooding
non-member
group member
router
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Multicast by flooding
non-member
group member
router
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Why simple flooding doesnt work
non-member
group member
router
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Why simple flooding doesnt work
wasted!
non-member
group member
router
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Multicast flooding

• Not a scalable mechanism
• every LAN sees every multicast
• every WAN router sees every multicast wastes
  bandwidth, CPU
• Requires a two-part solution
• determining LAN group members
• omitting WAN routers from multicast

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Multicast trees

• Shortest-path tree to all multicast members,
  rooted at sender
• But must be computed independently by each router
• And must be dynamically adjusted for joins and
  leaves

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A multicast tree
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A multicast tree
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IGMP

• Internet Group Management Protocol (Deering and
  Cheriton)
• Developed from work in V distributed operating
  system
• introduced notion of process groups (Cheriton and
  Zwaenepol)
• groups for services, e.g. name resolution, remote
  paging

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IGMP

• Detects if a multicast group has any members
  within a LAN
• Query and report messages
• router sends query of group membership
  periodically
• hosts report groups theyre in

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IGMP
Internet
Who is a member?
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IGMP
Internet
I am
I am
I am
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IGMP
Internet
I am
I am
I am
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Avoiding overloading

• Report packets may overload router
• upon getting a query, each group member sets a
  timer
• if it sees a report for its group before the
  timer expires, it suppresses its report
• otherwise reports on expiration

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THE END!

• Any questions?
• Slides will be put up on the web
• If interested, check out the sources for more
  information