CCIE RS – Written – Network Principles – Explain UDP Operations

Explain UDP Operations

User Datagram Protocol

  • Traffic is connectionless
    • No handshake or reliability
    • No guarantee of message delivery
  • Smaller than TCP traffic
  • Ideal for Voice and Video
    • Cannot wait on delayed packets
    • Lack of retransmission delays
  • Transaction oriented
  • Simple – used by DHCP and TFTP
  • Stateless – Suitable for large number of clients
  • Unidirectional communication
  • UDP header – 4 fields, each 2 bytes (16 bits)
    • Source port number – ID’s senders port
      • Assumed to be the port to reply on
    • Destination port number – ID’s receivers port and is required
    • Length – Length in bytes of the UDP header and data
      • Min is 8 bytes
    • Checksum – May be used for error-checking of header and data
      • Optional in IPv4
      • Mandatory in IPv6


TCP Starvation / UDP Dominance

Occurs when TCP and UDP streams are assigned to the same QOS class

  • UDP has no flow control to cause it to back off when congestion occurs
  • TCP ends up backing off allowing more bandwidth for UDP to the point where UDP completely takes over

Avoid by assigning TCP and UDP to different classes and separate in the best possible way


End to end delay

Jitter is a variance of latency

  • Jitter resolved through buffering

RTP/RTCP Concepts

RTP – Real-time Transport Protocol

  • Delivers voice and video over IP networks
  • Runs over UDP
  • Used in conjunction with RCTP
  • Often used with signaling protocols (SIP, Session Initiation Protocol)
  • Header
    • Min size of 12 bytes
      • Version (2 bits) – Version of protocol
      • P (Padding, 1 bit)
      • X (Extension) 1 bit
      • CC (CSRC count) 4 bits
      • M (Marker) 1 bit
      • PT (Payload Type) 7 bits – Format of payload for application
      • Sequence Number (16 bits) – Used by receiver to detect packet loss. RTP does not specify an action on packet loss
      • Timestamp (32 bit)
      • SSRC 32 bits – Synchronization source
      • CSRC – 32bits each
      • Header Extension (optional)
  • RFC 3550

From CCNA Collab Notes

  • Operates at transport level of OSI model
    • UDP based traffic, does not require acknowledgement
  • UDP provides port numbers and header checksum
  • RTP adds timestamps and sequence numbers to header information
  • Random port – 16,384 <> 32,767
    • Always even number
  • Devices setup point to point RTP stream, one in each direction

RTCP – RTP Control Protocol

  • Monitor transmission stats and QOS
  • Aids in synchronization of multiple streams
  • Stats include
    • Transmitted octect and packet counts
    • Packet loss, delay variation and RTD
  • Application can use this information to take an action
    • Such as – Choose different codec

From CCNA Collab Notes

  • Reports statistics between 2 devices in the call
    • Packet count
    • Packet delay
    • Packet loss
    • Jitter (delay variations)
  • UDP based traffic
    • Random port – 16,384 <> 32,767
      • Always odd number
  • Separate session from RTP
  • Devices send RTCP packet once every 5 seconds

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