One-way Delay Measurement Based on
Reference DelayChina MobileBeijing100053Chinaliyangzn@chinamobile.comChina MobileBeijing100053Chinasuntao@chinamobile.comChina MobileBeijing100053Chinayanghongwei@chinamobile.comChina MobileBeijing100053Chinachendanyang@chinamobile.comHuawei156 Beiqing Rd., Haidian DistrictBeijingChinawangyali11@huawei.com
Transport
Network Working Groupreference delay;delay measurementThe end-to-end network one-way delay is an important performance
metric in the 5G network. For realizing the accurate one-way delay
measurement, existing methods requires the end-to-end deployment of
accurate clock synchronization mechanism, such as PTP or GPS, which
results in relatively high deployment cost. Another method can derive
the one-way delay from the round-trip delay. In this case, since the
delay of the downlink and uplink of the 5G network may be asymmetric,
the measurement accuracy is relatively low. Hence, this document
introduces a method to measure the end-to-end network one-way delay
based on a reference delay guaranteed by deterministic networking
without clock synchronization.With the gradual promotion of new-generation network technologies
(such as 5G networks) and their application in various industries, SLA
guarantees for network quality become more and more important. For
example, different 5G services have different requirements for network
performance indicators such as delay, jitter, packet loss, and
bandwidth. Among them, the 5G network delay is defined as end-to-end
one-way delay of the network. Real-time and accurate measurement of the
end-to-end one-way delay is very important for the SLA guarantee of
network services, and has become an urgent and important
requirement.As shown in figure 1, 5G network HD video surveillance service is a
common scenario having requirement of end-to-end one-way delay
measurement. In this case, one end of the network is a high-definition
surveillance camera in the wireless access side, and the other end of
the network is a video server. The end-to-end one-way delay from the
surveillance camera to the video server is the sum of T1, T2, T3 and T4,
which is composed of delay in wireless access network, optical
transmission network, 5G core network, and IP data network.The existing one-way delay measurement solutions are divided into two
types. One type of mechanism to calculate one-way delay is based on the
measurement of round-trip delay. However, for example, because upstream
traffic and downstream traffic do not share the same path in 5G network,
the accuracy of the end-to-end one-way delay calculated from the
round-trip delay is low. Another type of mechanism is in-band OAM with
accurate network time synchronization mechanism , such as NTP or PTP.The one-way delay measurement solution based on precise network time
synchronization requires the deployment of an end-to-end time
synchronization mechanism. The current time synchronization accuracy
based on the NTP protocol can only reach millisecond level, which cannot
fully meet the measurement accuracy requirements. The time
synchronization accuracy based on the GPS module or the PTP protocol can
meet the requirements. However, because many data centers are actually
located underground or in rooms without GPS signals, so GPS clock
information cannot be continuously obtained for time synchronization.
For time synchronization solutions based on the PTP protocol, each
device in the wireless access network, 5G transport network, and 5G core
network must support the PTP protocol, which is unrealistic at the
moment. So the one-way delay measurement solution based on precise
end-to-end time synchronization is expensive and difficult to be
deployed.This document introduces a one-way delay measurement mechanism for
Deterministic Networking (DetNet) . The one-way
delay measurement is based on a stable one-way delay of a reference
DetNet packet, named as reference delay, which is known in advance and
has extremely low jitter. We can use the reference delay provided by the
reference DetNet packet to derive the one-way delay of other common
service packets.NTP Network Time ProtocolPTP Precision Time ProtocolSLA Service Level AgreementThe key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 when, and only
when, they appear in all capitals, as shown here.The end-to-end one-way delay of a reference packet with a stable
delay in the network can be used as a reference delay, denoted as Dref,
which is known in advance and has extremely low jitter. This section
will describe in detail the end-to-end one-way delay measurement method
based on reference delay of the reference packet. Assume that the
end-to-end one-way delay from the sender to the receiver is measured, as
shown in figure 2. The intermediate network devices other than the
sender and receiver are hidden in the figure.The measurement steps are shown in figure 3, which describe the
measurement steps at the sender side and receiver side respectively.
For the sender side, a reference packet is sent. In the first step,
the sender gets ready to send a reference packet; in the second step,
the sender marks an egress timestamp Ts1 for the reference packet; in
the third step, the sender encapsulates the egress timestamp of the
reference packet in the measurement header of the reference packet; in
the fourth step, the sender sends the reference packet. For the target
packet, the sender side procedures are the same,we omit it for
simplicity. The sending time of the target packet is according to the
traffic model of real applications. On the other hand, the sender can
send the reference packet according to a fixed frequency or adjust the
sending frequency according to the link usage rate, so that the target
packet can always find a nearby reference packet to make sure that the
sending time interval between the reference packet and the target
packet is small.For the reference packet, the processing steps at the receiver are
shown in figure 3. In the first step, the reference packet arrives at
the receiver, and the receiver receives the reference packet; in the
second step, the receiver timestamps the reference packet at the
entrance, which is denoted as Tr1; in the third step, the receiver
decapsulates the measurement header of the reference packet to obtain
the sender side timestamp Ts1; in the fourth step, the receiver
records the timestamp information of Ts1 and Tr1; in the fifth step,
the receiver uses the source/destination pair obtained by
decapsulation in the third step as the search key, queries the
reference delay table and records the reference delay search result,
denoted as Dref.For the target packet, the processing steps at the receiver are
also shown in figure 3. In the first step, the target packet arrives
at the receiver, and the receiver receives the target packet; in the
second step, the receiver timestamps the target packet at the
entrance, which is denoted as Tr2; in the third step, the receiver
decapsulates the measurement header of the target packet to obtain the
sender side timestamp Ts2; in the fourth step, the receiver records
the timestamp information of Ts2 and Tr2; in the fifth step, the
receiver calculates the target one-way delay, which we want to
measure, according to the recorded timestamp information Ts1, Ts2,
Tr1, Tr2 and reference delay information Dref. The target one-way
delay of the target packet is recorded as Dtarget.Now we describe the fifth step of the receiver procedures for the
target packet in figure 3, that is, calculating the one-way delay
Dtarget of the target packet based on the recorded timestamp
information Ts1, Ts2, Tr1, Tr2 and the reference delay information
Dref. The calculation method is the core of this solution. For the
reference packet, leveraging the receiver timestamp minus the sender
timestamp, we can get Equation 1.Equation 1: Tr1 - Ts1 = Dref + Offset1where Offset1 is the time offset between the sender and the
receiver when the reference packet transmission occurs. Similarly, for
the target packet, we can get Equation 2 using the same method.Equation 2: Tr2 - Ts2 = Dtarget + Offset2where Offset2 is the time offset between the sender and the
receiver when the target packet transmission occurs. Assuming that the
sending time interval between the reference packet and the target
packet is very small, we can get that Offset1 = Offset2. By (Equation
2 - Equation 1), we can get Equation 3.Equation 3: Dtarget = (Tr2 + Ts1) - (Tr1 + Ts2) + DrefSo the one-way delay of the target packet can be calculated by
Equation 3.The sender encapsulates the timestamp information and
sender-receiver pair information in the measurement header of the sent
packet, as shown in figure 4. The position of measurement header is in
the option field of the TCP protocol header. The delay measurement
option format is defined in figure 5. The Length value is 8 octets,
which is in accordance with TCP option. The sender ID is one octet,
and the receiver ID is also one octet. The sender side timestamp is 4
octets, which can store accurate timestamp information.The end-to-end one-way delay includes three parts, namely the
transmission delay, the internal processing delay of the network
devices, and the internal queueing delay of the network devices. Among
them, fixed parts of the delay include transmission delay and internal
processing delay. The transmission delay is related to transmission
distance and transmission media. For example, in optical fiber, it is
about 5ns per meter. With transmission path and media determined, it is
basically a fixed value. The internal processing delay of a network
device includes processing delay of the device's internal pipeline or
processor and serial-to-parallel conversion delay of the interface,
which is related to in/out port rate of the device, message length and
forwarding behavior. The magnitude of the internal processing delay is
at microsecond level, and it is basically a fixed value related to the
chip design specifications of a particular network device. Variable part
of the delay is the internal queueing delay. The queueing delay of the
device internal buffer is related to the queue depth, queue scheduling
algorithm, message priority and message length. For each device along
the end-to-end path, the queueing delay can reach microsecond or even
millisecond level, depending on values of the above parameters and
network congestion state.With the continuous development of networking technologies and
application requirements, a series of new network technologies have
emerged which can guarantee bounded end-to-end delay and ultra small
jitter. For example, deterministic network, by
leveraging novel scheduling algorithms and packet priority settings, can
stabilize queuing delay of network device on the end-to-end path. As a
result, the end-to-end one-way delay is extremely low and bounded. So
packets transmitted by a deterministic network with delay guarantee can
be used as reference packets, and their end-to-end one-way delay can be
used as reference delays. The acquisition method of reference delay is
not limited to the above method based on deterministic network
technology.TBD.This document requests IANA to assign a Kind Number in TCP Option to
indicate TCP Delay Measurement option.IEEE Standard for a Precision Clock Synchronization Protocol
for Networked Measurement and Control SystemsIEEE