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ChapterFive.md

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Chapter Five UDP and its Application

The Transport Layer

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UDP VS TCP

UDP TCP
Datagram oriented Stream oriented
Unreliable, connectionless Reliable, connection-oriented
Simple Complex
Unicast and multicast Unicast only
Commonly used for network control signaling services - Network management (SNMP), routing(RIP), naming (DNS), etc. Currently used by most Internet applications - Web (HTTP), email (SMTP), file transfer (FTP), terminal (telnet), etc.

UDP - User Datagram Protocol

  • UDP supports unreliable transmissions of datagrams
  • UDP merely extends the host-to-to-host delivery service of IP datagram to an application-to-application service
  • The only thing that UDP adds is multiplexing and demultiplexing

UDP Format

See UDP Format

Port Numbers

  • UDP (and TCP) use port numbers to identify applications

  • A globally unique flow of host application can be identified by a 5-tuple <Src. IP, Dst IP, Src. Port, Dst. Port, Protocol No.>

  • There are 65,535 UDP ports available per host

    • Dynamic/private , used by clients, randomly picked, >49,151 (per IANA)
    • Registered, used by ordinary user processes, 1024 – 49,151
    • Well-known, used by servers, fixed, 1~1023

  • Ephemeral Port: short-lived port used as transport protocol port automatically allocated from a predefined range by the TCP/IP stack software:

    • The IANA suggests 49,152 to 65,535 as "dynamic and/or private ports.“ •The BSD uses ports 1,024 through 4,999 as ephemeral ports

    • Many Linux kernels use 32,768 to 61,000 specified in /proc/sys/net/ipv4/ip_local_port_range

    • MS Windows OS’ through Server 2003 use the range 1,025 to 5,000 as ephemeral ports; use the IANA range since Windows Vista and Server 2008

    • FreeBSD uses the IANA port range since release 4.6.

About Port 0

  • Port 0 is a reserved port by IANA

  • Many OS’ allows a source port of 0 from a high layer application for connecting to a remote host. The OS automatically reassigns an ephemeral port when encapsulating the application data to a transport segment

  • No traffic should flow over Internet using port 0 although different OS' have different ways of handling traffic using port 0

  • The external behavior of handling port 0 in different OS's can be “fingerprinted” with a set of tests to send TCP or UDP packet from source port 0 to different destination ports

  • It is highly recommend that one should block any traffic using this port at your firewall so no program should be listening on port 0 and no program should connect from port 0

Maximum Transmission Unit (MTU)

  • The frame size limit of the data link protocol specifies a limit on the size of the IP datagram that can be encapsulated by the protocol.

  • This limit is called Maximum Transmission Unit (MTU)

  • MTUs various for different data link protocols:

    Ethernet: 1500

    802.2/802.3: 1492

    802.5: 4464

    FDDI: 4352

    ATM AAL5: 9180

    PPP: 296 (low delay)

  • What if the size of an IP datagram exceeds the MTU?

    IP datagram is fragmented into smaller units.

  • What if the route contains networks with different MTUs?

    The smallest MTU of any data link is used as the Path MTU.

IP Fragmentation

  • Host A sends a large IP datagram to Host B.
  • How does the intermediate router handle this?
    • IP router splits the datagram into several fragments.
    • Fragmentation requires that the data portion of every fragment except the last be a multiple of 8-bytes.

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Where is Fragmentation done?

  • Fragmentation can be done at the sender and at intermediate routers.
  • The same datagram can be fragmented several times.
  • Reassembly of original datagram is only done at destination hosts.

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What’s involved in Fragmentation?

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  • Identification is the same in all fragments.
  • Flags field contains
    • a reserved bit, must be zero
    • a Don’t Fragment (DF) bit that can be set, and
    • a More Fragments (MF) bit.
  • Fragment Offset contains the offset (in 8-byte units) of current fragment in the original datagram.
  • Total Length is changed to be the size of the fragment.

Fragmentation through Multiple Links

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  • The ID field stays the same for all fragments of a datagram sent by a sender to allow for reassemble

  • The fragment offset is relative to the datagram sent by the sender.

  • Two fragments created on X.25 link (offsets 0, 69)

    • 576 – 20 (IP header) = 556; 552 divides by 8 as 69.
    • First fragment: Offset 0, bytes 1552; second fragment: Offset 69, bytes 5531000

  • Each fragment is fragmented further on the PPP link

    • ID stays the same on all fragments
    • Fragment offset on the second set of fragments is relative to the original (0, 34, 68, 69, 103) > 296-20=276; 272/8 = 34

If the Don’t Fragment flag is set...

  • If fragmentation is needed, and the Don’t Fragment flag is set, The router drops the datagram and sends an ICMP unreachable error message to the source.

  • This can be used in Path MTU Discovery to find the smallest MTU along a path.

  • The format of an ICMP unreachable error message(See icmp_unreachable_error_message:

    icmp_unreachable_message

Path MTU Discovery

  • A host sends a set of IP datagrams with various lengths and the “don’t fragment” bit set

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