Page 226 - Asterisk™: The Future of Telephony

TCP’s high processing overhead, state management, and acknowledgment of arrival
work well for transmitting large amounts of data, but they simply aren’t efficient
enough for real-time media communications.

User Datagram Protocol
Unlike TCP, the User Datagram Protocol (UDP) does not offer any sort of delivery
guarantee. Packets are placed on the wire as quickly as possible and released into the
world to find their way to their final destinations, with no word back as to whether
they got there or not. Since UDP itself does not offer any kind of guarantee that the
†
data will arrive, it achieves its efficiency by spending very little effort on what it is
transporting.

TCP is a more “socially responsible” protocol because the bandwidth
is more evenly distributed to clients connecting to a server. As the per-
centage of UDP traffic increases, it is possible that a network could
become overwhelmed.

Stream Control Transmission Protocol
Approved by the IETF as a proposed standard in RFC 2960, SCTP is a relatively new
transport protocol. From the ground up, it was designed to address the shortcomings
of both TCP and UDP, especially as related to the types of services that used to be
delivered over circuit-switched telephony networks.
Some of the goals of SCTP were:
• Better congestion-avoidance techniques (specifically, avoiding Denial of Service
attacks)
• Strict sequencing of data delivery
• Lower latency for improved real-time transmissions
By overcoming the major shortcomings of TCP and UDP, the SCTP developers hoped
to create a robust protocol for the transmission of SS7 and other types of PSTN signaling
over an IP-based network.

Differentiated Service

Differentiated service, or DiffServ, is not so much a QoS mechanism as a method by
which traffic can be flagged and given specific treatment. Obviously, DiffServ can help
to provide QoS by allowing certain types of packets to take precedence over others.

† Keep in mind that the upper-layer protocols or applications can implement their own packet-
acknowledgment systems.

198 | Chapter 8: Protocols for VoIP