Timestamps for Network Measurements

Original draft: 29 June 2004
Revised: 26 August 2004

1. Introduction

Timestamps are an integral part of network measurements. In order to exchange these measurements, we need a framework for describing the timestamps associated with the measurements. Different types of measurement contexts will have different requirements and capabilities in this regard, so we attempt to produce a framework for describing
timestamps and not just a single "correct" format. XML and XML Schema will be used where wire formats are discussed.

2. Notational Conventions

2.1. XML Namespaces

To avoid conflicting with or inventing a GGF namespace convention in this document, we simply use the namespace prefix nm: for all XML elements associated with this specification. Whatever namespace nm: resolves to, this is the target namespace for all schema fragments. Similarly, the prefix xsd: will be used for the W3C XML Schema namespace http://www.w3.org/2001/XMLSchema.

2.2. XML Element Names

Element names are all in "TitleCase".

3. Base timestamp format

The "base" timestamp is nothing more than a container in the NM-WG
namespace. The XML Schema declaration for this is simply:

  <xsd:complexType name="nm:Timestamp"/>

4. Preferred timestamp format

In absence of other considerations, the preferred timestamp format is the Network Time Protocol (NTP) [NTP] format, a 64-bit number where the first 32 bits are the seconds and the last 32 bits are the fractional seconds. To this we add two sub-fields, resolution and accuracy. Resolution is a 64-bit number in the same format as the timestamp, indicating the smallest time that can occur between two successive timestamp values. Accuracy is a place-holder so the semantics are not defined. Both resolution and accuracy are optional.

This results in the following XML Schema:


  <xsd:element name="NTPTimestamp">
    <xsd:complexType>
      <xsd:complexContent>
        <xsd:extension base="nm:Timestamp">
          <xsd:sequence>
            <xsd:element name="Time" type="nm:Hex64"/>
            <xsd:element name="Resolution" 
             type="nm:Hex64" minOccurs="0"/>
            <xsd:element name="Accuracy" minOccurs="0"/>
          </xsd:sequence>
        </xsd:extension>
      </xsd:complexContent>
    </xsd:complexType>
  </xsd:element>
  <xsd:simpleType name="Hex64">
    <xsd:restriction base="xsd:hexBinary">
      <xsd:length value="8" fixed="true"/>
    </xsd:restriction>
  </xsd:simpleType>

5. Other timestamp formats

 5.1. ISO8601 Timestamp

The preferred human-understandable timestamp format is a particular variation of ISO8601 [ISO8601], "YYYY-MM-DDThh:mm:ss.<frac>Z". The <frac> represents fractional seconds. For example, "2004-06-29T17:11:13.163853Z". This is one legal format for the XML Schema 'dateTime' type, so for simplicity's sake the ISO 8601 timestamp schema uses the XML Schema type directly (empty restriction).

      <xsd:element name="ISOTimestamp">
      <xsd:complexType>
        <xsd:complexContent>
          <xsd:extension base="nm:Timestamp">
            <xsd:sequence>
              <xsd:element name="Time">
                <xsd:simpleType>
                  <xsd:restriction base='xsd:dateTime'/>
                </xsd:simpleType>
              </xsd:element>
            </xsd:sequence>
          </xsd:extension>
        </xsd:complexContent>
      </xsd:complexType>
    </xsd:element>


    5.2. Single-source timestamp

 The above timestamps are mostly intended to represent a globally synchronized clock. To represent a single-source clock, the preferred timestamp format is instead a long integer number of nanoseconds and a source identifier. The format of the source identifier is not stated, but implementations should recognize IPv4 and IPv6 addresses.

      <xsd:element name="SingleSourceTimestamp">
      <xsd:complexType>
        <xsd:complexContent>
          <xsd:extension base="nm:Timestamp">
            <xsd:sequence>
            <xsd:element name="Time" type="xsd:unsignedLong"/>
            <xsd:element name="Source" type="xsd:string" minOccurs="0"/>
          </xsd:sequence>
          </xsd:extension>
        </xsd:complexContent>
      </xsd:complexType>
    </xsd:element>


6. Full XML Schema

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:nm="http://www.ggf.org/nmwg"
targetNamespace="http://www.ggf.org/nmwg">
  <xsd:complexType name="Timestamp"/>
    <xsd:element name="NTPTimestamp">
      <xsd:complexType>
        <xsd:complexContent>
          <xsd:extension base="nm:Timestamp">
            <xsd:sequence>
              <xsd:element name="Time" type="nm:Hex64"/>
              <xsd:element name="Resolution"
                  type="nm:Hex64" minOccurs="0"/>
              <xsd:element name="Accuracy" minOccurs="0"/>
            </xsd:sequence>
          </xsd:extension>
        </xsd:complexContent>
      </xsd:complexType>
    </xsd:element>
  <xsd:simpleType name="Hex64">
    <xsd:restriction base="xsd:hexBinary">
      <xsd:length value="8" fixed="true"/>
    </xsd:restriction>
  </xsd:simpleType>
    <xsd:element name="ISOTimestamp">
      <xsd:complexType>
        <xsd:complexContent>
          <xsd:extension base="nm:Timestamp">
            <xsd:sequence>
              <xsd:element name="Time">
                <xsd:simpleType>
                  <xsd:restriction base='xsd:dateTime'/>
                </xsd:simpleType>
              </xsd:element>
            </xsd:sequence>
          </xsd:extension>
        </xsd:complexContent>
      </xsd:complexType>
    </xsd:element>
    <xsd:element name="SingleSourceTimestamp">
      <xsd:complexType>
        <xsd:complexContent>
          <xsd:extension base="nm:Timestamp">
            <xsd:sequence>
            <xsd:element name="Time" type="xsd:unsignedLong"/>
            <xsd:element name="Source" type="xsd:string" minOccurs="0"/>
          </xsd:sequence>
          </xsd:extension>
        </xsd:complexContent>
      </xsd:complexType>
    </xsd:element>
</xsd:schema>


7. Examples

7.1. NTP timestamp

   <?xml version='1.0'?>
   <nm:NTPTimestamp xmlns:nm="http://www.ggf.org/nmwg>
         <Time>DEADBEEFDEADBEEF</Time>
   </nm:NTPTimestamp>

7.2. ISO 8601 timestamp

   <?xml version='1.0'?>
   <nm:ISOTimestamp xmlns:nm="http://www.ggf.org/nmwg">
         <Time>2002-12-30T14:43:21.100012Z</Time>
   </nm:ISOTimestamp>

7.3 Single-source timestamp

   <?xml version='1.0'?>
   <nm:SingleSourceTimestamp xmlns:nm="http://www.ggf.org/nmwg">
         <Time>12345678987654321</Time>
	   <Source>192.168.1.1</Source>	
   </nm:SingleSourceTimestamp>

8. References

[ISO8601] http://www.iso.org/
[NTP]  http://www.ntp.org/

9. Authors

Dan Gunter, dkgunter@lbl.gov