udp by SanjuDudeja


									           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
   – (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   1
                        UDP datagram

     0                       16                       31

          Source Port              Destination Port

          UDP Length               UDP Checksum


Back to UDP—User Datagram Protocol
                                                           Figure 8.16
Back to UDP—User Datagram Protocol

                            UDP pseudoheader

      0              8                   16                                31

                                 Source IP Address

                               Destination IP Address

          00000000       Protocol = 17                  UDP Length

      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.
                                                                     Figure 8.17
     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.

                         TCP operations
1. TCP delivers byte stream.See picture
2. TCP deals with old packets from old connections by
   several methods. See picture
3. TCP uses sliding-window to implement reliable transfer of
   byte stream. See picture
4. 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
                      TCP byte stream

                Application               Application

              byte stream                      byte stream


Transmitter                                                 Receiver
                Send buffer                Receive buffer


                                                                Figure 8.18
An old segment could not be distinguished from current ones
                       Host A                                                     Host B

                                        Delayed segment with
                                        Seq_no = n+2
                                        will be accepted

    Question: How does TCP prevent old packets of old connections?
             –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!!
Back to TCP operations                                                                           Figure 8.23
                  TCP uses Selective-Repeat ARQ
                      Transmitter                                 Receiver

                                                                Receive Window
                  Send Window
                                      Slast+WS-         Rlast                 Rlast+WR+1

                  ...        ...      1
                                       ...                      …      …       …
     Octets                                                         Rnext   Rnew
  transmitted Slast     Srecent    Slast+WA-1
  and ACKed
                                       Advertised window
                                                     Rlast highest-numbered octet not yet read
    Slast oldest unacknowledged octet                by the application
    Srecent highest-numbered transmitted octet       Rnext next expected octet
    Slast+WA-1 highest-numbered octet that           Rnew highest numbered octet received
    can be transmitted                               correctly
    Slast+WS-1 highest-numbered octet that           Rlast+WR-1 highest-numbered octet that
    can be accepted from the application             can be accommodated in receive buffer
   Note: 1. Rnew highest bytes received correctly, which are out-of sequence bytes.
         2. Advertised window WA: Srecent – Slast  WA =WR – ( Rnew – Rlast)
Back to TCP operations                                                             Figure 8.19
        Dynamics of TCP congestion window

             Congestion          Congestion occurs
       20    avoidance




                                  Round-trip times

Back to TCP operations                                      9
                                                     Figure 7.63
                    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
 Back to TCP protocol                         TCP segment format
          0            4              10              16                    24                      31

                            Source Port                            Destination Port

                                                 Sequence Number

                                             Acknowledgement Number

              Header                       U A P R S F
              Length       Reserved        R C S S Y I        (Advertised) Window Size
                                           GKH T N N
                           Checksum                                   Urgent Pointer

                                           Options                                    Padding


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).
                                                                                                Figure 8.20
Back to TCP protocol

                    TCP pseudoheader

         0              8                     16                                      31

                                    Source IP Address

                                  Destination IP Address

             00000000       Protocol = 6                   TCP Segment Length

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

                                                                                Figure 8.21
Back to TCP protocol
                                   Host A                Host B

1. Random initial SN
2. Initial SNs in two
   directions are different
3. Initial SNs for two
   connections are different.
4. It should be clear here that
   what setting up connection
      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.
             Three-way handshake to set up connection
                                                        Figure 8.22
Back to TCP protocol
           Host A (Client)   Host B (Server)
       socket                         listen
connect (blocks)                      accept (blocks)

connect returns

    read (blocks)                     accept returns
                                      read (blocks)

                                       read returns

                                      read (blocks)
     read returns
                                               Figure 8.24
                       TCP window flow control
             Host A                              Host B






Back to TCP protocol                                       Figure 8.25
Back to TCP protocol
                                   TCP graceful termination

                              Host A                 Host B
Question: is termination
easier than establishment?
Or to say, is it possible
that a connection is closed
when both of two parties
confirm with each other?

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

                                                              Figure 8.27
  Thick lines: normal client states
  Dashed lines: normal server states      CLOSED
                                passive open,    applic.
                                  create TCB     close

                                                                           applic. close
                                                                           or timeout,
                SYN_RCVD               receive SYN,        SYN_SENT        delete TCB
                                       send ACK

              close,                    ESTABLISHED

                FIN_WAIT_1                      CLOSING


                                          TIME_WAIT         2MSL timeout
                                                            delete TCB                     17
Back to TCP protocol                                                                 Figure 8.28
   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, (2328)/(1.544  106) = 6 hours
   – T-3 line, (2328)/(45  106) = 12 minutes
   – OC-48 line, (2328)/(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, 232232=264 is big enough right now.
             Internet routing protocols
• Autonomous system (AS)
   – A set of routers or networks technically administrated by a single
   – 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

 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

     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

                        RIP message format
    0               8                   16            31
        Command           Version              Zero
          Address Family Identifier            Zero
                                  IP Address


1. Command: 1: request other routers to send routing information
                2: a response containing its routing information
2. Version: 1 or 2
3. 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.                         Figure 8.32
                  Internet multicast
• A packet is to be sent to multiple hosts with the same multicast address
• Class D multicast addresses: e.g.,
   – all systems on a LAN
   – all routers on a LAN
   – all OSPF routers on a LAN
   – 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

          G1                                                                G1
                  3                                                   7 2
          2                                                       4
              2 4                                 2                   3
                  1                       1
                                      5       5                              2
                                                      3                              G1
                      2                           4                       1 8
      S       1 1 3                                                              4   G1
               5 4
              1 2
                                      3                       6 3
              3                                                   4         G2

• Source S sends packets to multicast group G1
            Multicast Routing
• Multicast routing useful when a source wants to
  transmit its packets to several destinations
• 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
  Reverse-Path Broadcasting (RPB)
• Fact: Set of shortest paths to the source node S forms a tree that spans the
   – 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
   – If paths asymmetric, need to use link state info to compute shortest paths
      from S
    Example: Shortest Paths from S
            G1                                                               G1
                     3                                               7 2
            2                                                    4
                2 4                          2                       3
                1                        5                                   2
                                 5               3                               3   G1
                        2                    4
        S       1                                                                4   G1
                     1 3
                    5 4
                            2 4
                1 2
                                3                        6 3
                3                                            4             G2

• Spanning tree of shortest paths to node S and parent
  ports are shown in blue                            27
       Example: S sends a packet
             G1                                                               G1
                       3                                               7 2
              2                                                    4
                  2 4                          2                       3
                  1                        5                                   2
                                   5               3                               3   G1
                          2                    4
         S        1                                                                4   G1
                       1 3
                      5 4
                              2 4
                  1 2
                                  3                        6 3
                  3                                            4             G2

• S sends a packet to node 1
• Node 1 forwards to all ports, except parent port
     Example: Hop 1 nodes broadcast
              G1                                                              G1      
                       3                                               7 2
              2                                                    4
                  2 4                          2                       3
                  1                        5
                                   5               3
                                                                                   3       G1   
                          2                    4
          S            1 3                                                         4       G1   
                      5 4
                              2 4
                  1 2
                                  3                        6 3
                  3                                            4             G2

• Nodes 2, 3, 4, and 5 broadcast, except on parent ports
• All nodes, not only G1, receive packets
        Example: Broadcast continues
               G1                                                               G1
                        3                                               7 2
               2                                                    4
                   2 4                          2                       3
                   1                        5                                   2
                                    5               3                               3   G1
                           2                    4
          S        1                                                                4   G1
                        1 3
                       5 4
                               2 4
                   1 2
                                   3                        6 3
                   3                                            4             G2
• Truncated RPB (TRPB): Leaf routers do not broadcast if
  none of its attached hosts belong to packet’s multicast group
 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
   – 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                   31
              Multicast programming
• 2.1 Multicast addresses.
• 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.

              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/o
   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                                          34
             Other IPv6 Features
• Flexible support for options: more efficient and
  flexible options encoded in optional extension
• 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 35
  processing time in a router
                   IPv6 Header Format
     0         4                   12         16                  24                 31
         Version   Traffic Class                                  Flow Label
                     Payload Length                     Next Header            Hop Limit

                                         Source Address

                                        Destination Address

• 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
                     IPv6 Header Format
       0         4                   12         16                  24                 31
           Version   Traffic Class                                  Flow Label
                       Payload Length                     Next Header            Hop Limit

                                           Source Address

                                          Destination Address

• 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
                   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:                                             38
       • ::FFFF:

Address Types based on Prefixes
  Binary 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
  0000 011        Unassigned                           0.78
  0000 1          Unassigned                           3.12
  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 0          Unassigned                           3.12
  1111 10         Unassigned                           1.56
  1111 110        Unassigned                           0.78
  1111 1110 0     Unassigned                           0.2
  1111 1110 10    Link local use addresses             0.098
  1111 1110 11    Site local use addresses             0.098
                Special Purpose Addresses
       n bits          m bits          o bits           p bits         (125-m-n-o-p) bits

010 Registry ID      Provider ID    Subscriber ID       Subnet ID         Interface ID

  •   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

   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
  – ::
• IP-mapped addresses: 80 0’s + 16 1’s +
  – Denote IPv4 hosts & routers that do not support
    IPv6                                         42
   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
  – Encapsulate user packet insider IPv4 packet
  – Tunnel endpoint at source host, intermediate
    router, or destination host
  – Tunneling can be recursive                       43
      Migration from IPv4 to IPv6
                Tunnel head-end                     Tunnel tail-end        Destination


                      IPv6 header
       IPv6 network                                IPv4 header        IPv6 network
                                    IPv4 network

      Source                                                               Destination


       IPv6 network                                                   IPv6 network
    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.
                   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.

Deliver packets to mobile host through home agent and foreign agent


     Home                                       Foreign
     network                                    agent                Mobile




                                                                    Figure 8.29

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