Dynamic Packet Transport

Last updated March 26, 2019

Dynamic Packet Transport (DPT) is a Cisco transport protocol designed for use in optical fiber ring networks. In overview, it is quite similar to POS and DTM. It was one of the major influences on the Resilient Packet Ring/802.17 standard.

An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to transmit light between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer excessively. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers.

A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring. Data travels from node to node, with each node along the way handling every packet.

Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol (PPP) over SDH or SONET, which are both standard protocols for communicating digital information using lasers or light emitting diodes (LEDs) over optical fibre at high line rates. POS is defined by RFC 2615 as PPP over SONET/SDH. PPP is the Point to Point Protocol that was designed as a standard method of communicating over point-to-point links. Since SONET/SDH uses point-to-point circuits, PPP is well suited for use over these links. Scrambling is performed during insertion of the PPP packets into the SONET/SDH frame to solve various security attacks including denial-of-service attacks and the imitation of SONET/SDH alarms. This modification was justified as cost-effective because the scrambling algorithm was already used by the standard used to transport ATM cells over SONET/SDH. However, scrambling can optionally be disabled to allow a node to be compatible with another node that uses the now obsoleted RFC 1619 version of Packet over SONET/SDH which lacks the scrambler.

Protocol design

DPT is implemented as two counter-rotating rings. This means the network is composed of two completely separate rings of fiber that are both able to transmit data concurrently. This design provides for redundancy in case of a fiber cut or link failure, and increased throughput in common situations. DPT as opposed to POS or normal SONET/SDH is able to use both rings at the same time whereas POS only uses one ring under normal circumstances but switches to the second upon failure of the first. Cisco claims that DPT can run with double the bit-rate of POS due to this characteristic. DPT is not a PPP whereas POS is, this means that traffic between two nodes of a DPT ring does not affect intermediate nodes. With the introduction of DPT came the introduction of another Cisco developed MAC layer protocol, Spatial Reuse Protocol or SRP. The use of SRP in conjunction with DPT makes it possible for DPT to communicate with the physical layer.

In computer networking, Point-to-Point Protocol (PPP) is a data link layer communications protocol used to establish a direct connection between two nodes. It connects two routers directly without any host or any other networking device in between. It can provide connection authentication, transmission encryption, and compression.

Spatial Reuse Protocol is a networking protocol developed by Cisco. It is a MAC-layer protocol for ring-based packet internetworking that is commonly used in optical fiber ring networks. Ideas from the protocol are reflected in parts of the IEEE 802.17 Resilient Packet Ring (RPR) standard.

Types of data in DPT networks

As with most other lower layer protocols, there are methods for communicating not only application data between the nodes of a DPT network. It is necessary for the nodes to be able to communicate control data between each other in case of a fiber cut or link failure so the nodes can forward traffic on the appropriate interfaces and maintain network connectivity. Both control packets, and data packets are transmitted on both rings in order to maintain connectivity and full bandwidth utilization in normal situations; but once a failure occurs, the control data will notify the applicable routers of the failure and all the routers will switch to using only their active interfaces for data and control packets.

DPT packet structure

The structure of a DPT Packet is quite similar to that of Ethernet. It contains a source and destination MAC address (both 48-bits long), a protocol type identifier (used for identifying the upper layer protocol contained in the payload), and a frame check sequence used to validate the data.

Ethernet is a family of computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1983 as IEEE 802.3, and has since retained a good deal of backward compatibility and been refined to support higher bit rates and longer link distances. Over time, Ethernet has largely replaced competing wired LAN technologies such as Token Ring, FDDI and ARCNET.

A frame check sequence (FCS) refers to an error-detecting code added to a frame in a communications protocol. Frames are used to send payload data from a source to a destination.

DPT topologies

Both DPT and SRP are independent of their physical layers. This means that the DPT protocol can operate above several physical mediums such as SONET/SDH, Gigabit Ethernet, and others. As aforementioned, DPT is composed of two rings for fault tolerance and increased throughput. The method for switching between these two rings in the event of a failure is called Intelligent Protection Switching, or IPS. This ensures that a fiber cut or link failure (layer 1 error) will be rectified and IP traffic will be resumed within 50 ms. DPT also contains a "plug and play" feature which dynamically fetches the MAC addresses of neighboring devices which provides for very simple configuration with little to no setup prior to functional data transfer.

In computer networking, Gigabit Ethernet is the various technologies for transmitting Ethernet frames at a rate of a gigabit per second, as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet in wired local networks, as a result of being considerably faster. The cables and equipment are very similar to previous standards and have been very common and economical since 2010.

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