Header Structure of RTP

The following figure shows the RTP header structure -

• version (V): 2 bits
  This field identifies the version of RTP. The version is 2 upto RFC 1889.
• padding (P): 1 bit
  If the padding bit is set, the packet contains one or more additional padding octets at the end which are not part of the payload. The last octet of the padding contains a count of how many padding octets should be ignored. Padding may be needed by some encryption algorithms with fixed block sizes or for carrying several RTP packets in a lower-layer protocol data unit.
• extension (X): 1 bit
  If the extension bit is set, the fixed header is followed by exactly one header extension.
• CSRC count (CC): 4 bits
  The CSRC count contains the number of CSRC identifiers that follow the fixed header.
• marker (M): 1 bit
  Marker bit is used by specific applications to serve a purpose of its own. We will discuss this in more detail when we study Application Level Framing.
• payload type (PT): 7 bits
  This field identifies the format (e.g. encoding) of the RTP payload and determines its interpretation by the application. This field is not intended for multiplexing separate media.
• sequence number: 16 bits
  The sequence number increments by one for each RTP data packet sent, and may be used by the receiver to detect packet loss and to restore packet sequence. The initial value of the sequence number is random (unpredictable).
• timestamp: 32 bits
  The timestamp reflects the sampling instant of the first octet in the RTP data packet. The sampling instant must be derived from a clock that increments monotonically and linearly in time to allow synchronization and jitter calculations.
• SSRC: 32 bits
  The SSRC field identifies the synchronization source. This identifier is chosen randomly, with the intent that no two synchronization sources within the same RTP session will have the same SSRC identifier.
• CSRC list: 0 to 15 items, 32 bits each
  The CSRC list identifies the contributing sources for the payload contained in this packet. The number of identifiers is given by the CC field. If there are more than 15 contributing sources, only 15 may be identified. CSRC identifiers are inserted by mixers, using the SSRC identifiers of contributing sources.

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NTP on Cisco 3550

We had someone email us asking for help with configuring a 3550 as an NTP server. The short answer is it can’t be done. The long answer from Cisco’s site is as follows:

Configuring NTP

The Catalyst 2950, 2955, 3550 and 3560 switches do not have a hardware-supported clock, and they cannot function as an NTP master clock to which peers synchronize themselves when an external NTP source is not available. These switches also have no hardware support for a calendar. As a result, the ntp update-calendar and the ntp master global configuration commands are not available.

(Cisco 1100 APs also do not have a hardware-supported clock - will check on other APs and update)

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Cisco Announces Definitive Agreement to Acquire PostPath

PostPath’s Email and Calendaring Software to Enhance Cisco’s WebEx Collaboration Platform

SAN JOSE, Calif. - August 27, 2008 - Building upon its commitment to provide a comprehensive collaboration portfolio, Cisco today announced its intent to acquire privately held PostPath, Inc., a provider of innovative email and calendaring software. Based in Mountain View, Calif. with additional development operations in Sofia, Bulgaria, PostPath will enhance the existing email and calendaring capabilities of Cisco’s WebEx Connect collaboration platform.

In today’s fast-paced business environment, effective, adaptive collaboration is critical to creating and sustaining a competitive advantage. With PostPath’s software, Cisco will extend the e-mail and calendar functionality of its flexible software-as-a-service (SaaS)-based collaborative platform that includes instant messaging, voice, video, data, document management and Web 2.0 applications. This combination will enable customers to use collaboration to accelerate business processes, within and between businesses.

In today’s fast-paced business environment, effective, adaptive collaboration is critical to creating and sustaining a competitive advantage. With PostPath’s software, Cisco will extend the e-mail and calendar functionality of its flexible software-as-a-service (SaaS)-based collaborative platform that includes instant messaging, voice, video, data, document management and Web 2.0 applications. This combination will enable customers to use collaboration to accelerate business processes, within and between businesses.

“The acquisition of PostPath complements our strategy to develop an integrated collaboration platform designed for how we work today and into the future, providing real productivity gains and a more satisfying user experience”, said Doug Dennerline, Cisco senior vice president, Collaboration Software Group. “Our ‘cloud-based’ delivery model offers our customers rapid deployment and compelling economics.”

PostPath offers a Linux-based e-mail, calendaring and collaboration solution. It is interoperable with many other e-mail solutions and provides a browser-independent AJAX Web client. In addition, PostPath’s software is compatible with a number of mobile clients.

PostPath’s software is highly secure and scalable, and it incorporates innovative Web 2.0 architectures to meet the requirements of large enterprises and small businesses alike to provide Cisco customers with a next-generation user experience.

The PostPath acquisition exemplifies Cisco’s “build, buy, and partner” innovation strategy to move quickly into new markets and capture key market transitions. In addition to internal software innovations, Cisco actively employs investments in, and acquisitions of, other companies to support its software strategy; recent purchases include industry leaders WebEx, IronPort and Securent.

Under the terms of the agreement, Cisco will pay approximately $215 million in exchange for all shares of PostPath. The transaction will be accounted for in accordance with generally accepted accounting principles. The acquisition is subject to various standard closing conditions and is expected to be complete in Cisco’s first quarter of fiscal year 2009. Upon completion of the acquisition, PostPath employees will become part of the Cisco Collaboration Software Group (CSG). CSG is part of the recently established Software Group, consisting of Cisco’s major software businesses; including the IOS network operating system, network and service management, Unified Communications solutions, policy management, and SaaS offerings.

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