Internet DRAFT - draft-gg-udt
draft-gg-udt
Internet Engineering Task Force Yunhong Gu
Internet Draft Robert L. Grossman
Intended status: Informational University of Illinois at Chicago
Expires: April 15, 2008 October 2007
UDT: UDP-based Data Transfer Protocol
draft-gg-udt-02.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document proposes UDT, or UDP based Data Transfer protocol, as
an alternative data transfer protocol for the situation when TCP does
not work well. One of the most common cases, and also the original
motivation of UDT, is to overcome TCP's inefficiency in high
bandwidth-delay product (BDP) networks. Another important target use
scenario is to allow networking researchers, students, and
application developers to easily implement and deploy new data
transfer algorithms and protocols.
UDT is completely based on UDP. UDT is connection oriented, unicast,
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and duplex. It supports both reliable data streaming and partial
reliable messaging. The congestion control module is an open
framework that can be used to implement and/or deploy different
control algorithms. UDT also have a native/default control algorithm
based on AIMD rate control.
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Table of Contents
1. Introduction...................................................4
2. Packet Structures..............................................5
3. UDP Multiplexer................................................7
4. Timers.........................................................8
5. Connection Setup and shutdown..................................9
5.1 Client/Server Connection Setup.............................9
5.2 Rendezvous Connection Setup...............................10
5.3 Shutdown..................................................11
6. Data Sending and Receiving....................................11
6.1 The Sender's Algorithm....................................11
6.2 The Receiver's Algorithm..................................12
6.3 Flow Control..............................................15
6.4 Loss Information Compression Scheme.......................15
7. Configurable Congestion Control (CCC).........................15
7.1 CCC Interface.............................................15
7.2 UDT's Native Control Algorithm............................16
Security Considerations..........................................17
IANA Considerations..............................................18
Normative References.............................................18
Informative References...........................................18
Author's Addresses...............................................19
Full Copyright Statement.........................................20
Intellectual Property............................................20
Acknowledgment...................................................20
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1. Introduction
The Transmission Control Protocol (TCP) [RFC2581] has been very
successful and greatly contributes to the popularity of today's
Internet. Today TCP still contributes the majority of the traffic on
the Internet.
However, TCP is not perfect and it is not designed for every specific
applications. In the last several years, with the rapid advance of
optical networks and rich Internet applications, TCP has been found
inefficient as the network bandwidth delay product (BDP) increases.
Its AIMD (additive increase multiplicative decrease) algorithm
reduces the TCP congestion window drastically but fails to recover it
to the available bandwidth quickly. Theoretical flow level analysis
has shown that TCP becomes more vulnerable to packet loss as the BDP
increases higher [LM97].
To overcome the TCP's inefficiency problem over high speed wide area
networks is the original motivation of UDT. Although there are new
TCP variants deployed today (for example, BiC TCP [XHR04] on Linux
and Compound TCP [TS06] on Windows), certain problems still exist.
For example, none of the new TCP variants address RTT unfairness, the
situation that connections with shorter RTT consume more bandwidth.
Moreover, as the Internet continues to evolve, new challenges and
requirements to the transport protocol will always emerge.
Researchers need a platform to rapidly develop and test new
algorithms and protocols. Network researchers and students can use
UDT to easily implement their ideas on transport protocols, in
particular congestion control algorithms, and conduct experiments
over real networks.
Finally, there are other situations when UDT can be found more
helpful than TCP. For example, UDP-based protocol is usually easier
for punching NAT firewall. For another example, TCP's congestion
control and reliability control is not desirable in certain
applications of VOIP, wireless communication, etc. Application
developers can modify UDT to suit their requirement.
Due to all those reasons and motivations described above, we believe
that it is necessary to design a well defined and developed UDP-based
data transfer protocol.
As its name suggest, UDT is built solely on the top of UDP [RFC768].
Both data and control packets are transferred using UDP. UDT is
connection-oriented in order to easily maintain congestion control,
reliability, and security. It is a unicast protocol while multicast
is not considered here. Finally, data can be transferred over UDT in
duplex.
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UDT supports both reliable data streaming and partial reliable
messaging. The data streaming semantics is similar to that of TCP,
while the messaging semantics is similar to a subset of SCTP
[RFC2960].
This document defines UDT's protocol specification. The detailed
description and performance analysis can be found in [GG07], and a
fully functional reference implementation can be found at [UDT].
2. Packet Structures
UDT has two kinds of packets: the data packets and the control
packets. They are distinguished by the 1st bit (flag bit) of the
packet header.
The data packet header structure is as following.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Packet Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|FF |O| Message Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time Stamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Socket ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The data packet header starts with 0. Packet sequence number uses the
following 31 bits after the flag bit. UDT uses packet based
sequencing, i.e., the sequence number is increased by 1 for each sent
data packet in the order of packet sending. Sequence number is
wrapped after it is increased to the maximum number (2^31 - 1).
The next 32-bit field in the header is for the messaging. The first
two bits "FF" flags the position of the packet is a message. "10" is
the first packet, "01" is the last one, "11" is the only packet, and
"00" is any packets in the middle. The third bit "O" means if the
message should be delivered in order (1) or not (0). A message to be
delivered in order requires that all previous messages must be either
delivered or dropped. The rest 29 bits is the message number, similar
to packet sequence number (but independent). A UDT message may
contain multiple UDT packets.
Following are the 32-bit time stamp when the packet is sent and the
destination socket ID. The time stamp is a relative value starting
from the time when the connection is set up. The time stamp
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information is not required by UDT or its native control algorithm.
It is included only in case that a user defined control algorithm may
require the information (See Section 6).
The Destination ID is used for UDP multiplexer. Multiple UDT socket
can be bound on the same UDP port and this UDT ID is used to
differentiate the UDT connections.
If the flag bit of a UDT packet is 1, then it is a control packet and
parsed according to the following structure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| type | Reserved | ACK Seq. No. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Message Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time Stamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Socket ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Control Information Field ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There are 8 types of control packets in UDT and the type information
is put in bit field 1 - 4 of the header. The contents of the
following fields depend on the packet type. The first 128 bits must
exist in the packet header, whereas there may be an empty control
information field, depending on the packet type.
Particularly, UDT uses sub-sequencing for ACK packet. Each ACK packet
is assigned a unique increasing 16-bit sequence number, which is
independent of the data packet sequence number. The ACK sequence
number uses bits 16 - 31 in the control packet header. The ACK
sequence number ranges from 0 to (2^16 - 1). Bits 16 - 31 are
undefined in other control packets.
TYPE 0000: Protocol Connection Handshake
Control Info:
1) 32 bits: UDT version
2) 32 bits: Socket Type (STREAM or DGRAM)
3) 32 bits: initial sequence number
4) 32 bits: packet size
5) 32 bits: maximum flow window size
6) 32 bits: connection type (regular or rendezvous)
7) 32 bits: socket ID
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TYPE 0001: Keep-alive
Control Info: None
TYPE 0010: Acknowledgement (ACK)
bits 16-31: ACK sequence number
Control Info:
1) 32 bits: The packet sequence number to which all the
previous packets have been received (excluding)
2) 32 bits: RTT (in microseconds)
3) 32 bits: RTT variance
4) 32 bits: Available buffer size (in bytes)
5) 32 bits: Packets receiving rate (in number of packets
per second)
6) 32 bits: Estimated link capacity (in number of packets
per second)
TYPE 0011: Negative Acknowledgement (NAK)
Control Info:
1) 32 bits integer array of compressed loss information
(see section 3.9).
TYPE 0100: Unused
TYPE 0101: Shutdown
Control Info: None
TYPE 0110: Acknowledgement of Acknowledgement (ACK2)
bits 16-31: ACK sequence number
Control Info: None
TYPE 0111: Message Drop Request:
Bits 35-64: Message number
Control Info:
1) 32 bits: First sequence number in the message
2) 32 bits: Last sequence number in the message
TYPE 1111: Explained by bits 4 - 15, reserved for user defined
Control Packet
Finally, Time Stamp and Destination Socket ID also exist in the
control packets.
3. UDP Multiplexer
A UDP multiplexer is used to handle concurrent UDT connections
sharing the same UDP port. The multiplexer dispatch incoming UDT
packets to the corresponding UDT sockets according to the destination
socket ID in the packet header.
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One multiplexer is used for all UDT connections bound to the same UDP
port. That is, UDT sockets on different UDP port will be handled by
different multiplexer.
The multiplexer maintains two queues. The sending queue includes the
sockets with at least one packet scheduled for sending. The UDT
sockets in the sending queue are ordered by the next packet sending
time. A high performance timer is maintained by the sending queue and
when it is time for the first socket in the queue to send its packet,
the packet will be sent and the socket will be removed. If there is
new packet to be sent, the socket will be re-inserted to the queue.
The receiving queue reads incoming packets and dispatches them to the
corresponding sockets. If the destination ID is 0, the packet will be
sent to the listening socket (if there is any), or to a socket that
is in rendezvous connection phase. (See Section 5.)
Similar to the sending queue, the receiving queue also maintains a
list of sockets waiting for incoming packets. The receiving queue
scan the list to check if any timer expires for each socket every SYN
(SYN = 0.01 second, defined in Section 4). The list is ordered by the
checking time, which means if for one socket that was last checked
within an SYN time, there is no need to check the rest on the queue.
Note that the UDP multiplexer is only used so that multiple UDT
sockets can be bound to the same UDP port. It is not a must in order
to support a UDT data transfer session.
4. Timers
UDT uses four timers to trigger different periodical events. Each
event has its own period and they are all independent. They use the
system time as origins and should process wrapping if the system time
wraps.
For a certain periodical event E in UDT, suppose the time variable is
ET and its period is p. If E is set or reset at system time t0 (ET =
t0), then at any time t1, (t1 - ET >= p) is the condition to check if
E should be triggered.
The four timers are ACK, NAK, EXP and SND. SND is used in the sender
only for rate-based packet sending (see Section 6.1), whereas the
other three are used in the receiver only.
ACK is used to trigger an acknowledgement (ACK). Its period is set by
the congestion control module. However, UDT will send an ACK no
longer than every 0.01 second, even though the congestion control
does not need timer-based ACK. Here, 0.01 second is defined as the
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SYN time, or synchronization time, and it affects many of the other
timers used in UDT.
NAK is used to trigger a negative acknowledgement (NAK). Its period
is dynamically updated to 4 * RTT_+ RTTVar + SYN, where RTTVar is the
variance of RTT samples.
EXP is used to trigger data packets retransmission and maintain
connection status. Its period is dynamically updated to 4 * RTT +
RTTVar + SYN.
The recommended granularity of their periods is microseconds. The
system time is queried after each time bounded UDP receiving (there
will be additional necessary data processing time if a UDP packet is
received) to check if any of the ACK, NAK, or EXP event should be
triggered. The timeout value of UDP receiving should be at least SYN.
In the rest of this document, a name of a time variable will be used
to represent the associated event, the variable itself, or the value
of its period, depending on the context. For example, ACK can mean
either the ACK event or the value of ACK period.
5. Connection Setup and shutdown
UDT supports two different connection setup methods, the traditional
client/server mode and the rendezvous mode. In the latter mode, both
UDT sockets connect to each other at (approximately) the same time.
The UDT client (in rendezvous mode, both peer are clients) sends a
handshake request (type 0 control packet) to the server or the peer
side. The handshake packet has the following information:
1) UDT version: this value is for compatibility purpose. The
current version is 4.
2) Socket Type: STREAM (0) or DGRAM (1).
3) Initial Sequence Number: It is the initial data packet sequence
number that the UDT entity that sends this handshake will use to
send out data packets. This should be a random value.
4) Packet Size: the maximum size of a data packet (including all
headers). This is usually the value of MTU.
5) Maximum Flow Window Size: The maximum flow window size. This
value may not be necessary; however, it is needed in the current
reference implementation.
6) Connection Type. This information is used to differential the
connection setup modes and request/response.
7) Socket ID. The client UDT socket ID.
5.1 Client/Server Connection Setup
One UDT entity starts first as the server (listener). The server
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accepts and processes incoming connection request, and creates new
UDT socket for each new connection.
A client that wants to connect to the server will send a handshake
packet first. The client should keep on sending the handshake packet
every constant interval (the implementation should decide this
interval according to the balance between response time and system
overhead) until it receives a response handshake from the server or a
timeout timer expires.
The server, when receiving a handshake packet, compares the packet
size and maximum window size with its own values and set its own
values as the smaller ones. The result values are also sent back to
the client by a response handshake packet, together with the server's
version and initial sequence number. The server is ready for
sending/receiving data right after this step is finished. However, it
must send back response packet as long as it receives any further
handshakes from the same client.
The client can start sending/receiving data once it gets a response
handshake packet from the server. Further response handshake
messages, if received any, should be omitted.
The connection type from the client should be set to 1 and the
response from the server should be set to -1.
The client should also check if the response is from the server that
the original request was sent to.
5.2 Rendezvous Connection Setup
In this mode, both clients send a connect request to each other at
the same time. The initial connection type is set to 0. Once a peer
receives a connection request, it sends back a response. If the
connection type is 0, then the response sends back -1; if the
connection type is -1, then the response sends back -2; No response
will be sent for -2 request.
The rendezvous peer does the same check on the handshake messages
(version, packet size, window size, etc.) as described in Section
5.1. In addition, the peer only process the connection request from
the address it has sent a connection request to. Finally, rendezvous
connection will be rejected by a regular UDT server (listener).
A peer initializes the connection when it receives -1 response.
The rendezvous connection setup is useful when both peers are behind
firewall.
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5.3 Shutdown
If one of the connected UDT entities is being closed, it will send a
shutdown message to the peer side. The peer side, after received this
message, will also be closed. This shutdown message, delivered using
UDP, is only sent once and not guaranteed to be received. If the
message is not received, the peer side will be closed after 16
continuous EXP timeout (see section 3.5). However, the total timeout
value should be between 3 seconds and 30 seconds.
6. Data Sending and Receiving
Each UDT entity has two logical parts: the sender and the receiver.
The sender sends (and retransmits) application data according to flow
control and congestion control. The receiver receives both data
packets and control packets, and sends out control packets according
to the received packets and timers as well.
The receiver is responsible for triggering and processing all control
events, including congestion control and reliability control, and
their related mechanisms.
Note that when a UDT multiplexer is implemented, the sender and the
receiver will be called from the multiplexer. Otherwise, each UDT
entity maintains the running of its own sender and receiver.
UDT always tries to pack application data into fixed size packets
(the maximum packet size negotiated during connection setup), unless
there is not enough data to be sent.
We explained the rationale of some of the UDT data sending/receiving
schemes in [GHG04b].
6.1 The Sender's Algorithm
Data Structures and Variables:
1) Sender's Loss List: The sender's loss list is used to store the
sequence numbers of the lost packets fed back by the receiver
through NAK packets. The numbers are stored in increasing order.
Data Sending Algorithm:
1) If the sender's loss list is not empty, retransmit the first
packet in the list and remove it from the list. Go to 5).
2) In messaging mode, if the packets has been the loss list for a
time more than the application specified TTL, send a message drop
request and remove all related packets from the loss list. Go to
1).
3) Wait until there is application data to be sent.
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4)
a. If the number of unacknowledged packets exceeds the
flow/congestion window size, wait until an ACK comes. Go to
1).
b. Pack a new data packet and send it out.
5) If the sequence number of the current packet is 16n, where n is an
integer, go to 2).
6) Wait (SND - t) time, where SND is the inter-packet interval
updated by congestion control and t is the total time used by step
1 to step 5. Go to 1).
6.2 The Receiver's Algorithm
Data Structures and Variables:
1) Receiver's Loss List: It is a list of tuples whose values include:
the sequence numbers of detected lost data packets, the latest
feedback time of each tuple, and a parameter k that is the number
of times each one has been fed back in NAK. Values are stored in
the increasing order of packet sequence numbers.
2) ACK History Window: A circular array of each sent ACK and the time
it is sent out. The most recent value will overwrite the oldest
one if no more free space in the array.
3) PKT History Window: A circular array that records the arrival time
of each data packet.
4) Packet Pair Window: A circular array that records the time
interval between each probing packet pair.
5) LRSN: A variable to record the largest received data packet
sequence number. LRSN is initialized to the initial sequence
number minus 1.
6) ExpCount: A variable to record number of continuous EXP time-out
events.
Data Receiving Algorithm:
1) Query the system time to check if ACK, NAK, or EXP timer has
expired. If there is any, process the event (as described below
in this section) and reset the associated time variables. For
ACK, also check the ACK packet interval.
2) Start time bounded UDP receiving. If no packet arrives, go to 1).
1) Reset the ExpCount to 1. If there is no unacknowledged data
packet, or if this is an ACK or NAK control packet, reset the EXP
timer.
3) Check the flag bit of the packet header. If it is a control
packet, process it according to its type and go to 1).
4) If the sequence number of the current data packet is 16n + 1,
where n is an integer, record the time interval between this
packet and the last data packet in the Packet Pair Window.
5) Record the packet arrival time in PKT History Window.
6)
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a. If the sequence number of the current data packet is greater
than LRSN + 1, put all the sequence numbers between (but
excluding) these two values into the receiver's loss list and
send them to the sender in an NAK packet.
b. If the sequence number is less than LRSN, remove it from the
receiver's loss list.
7) Update LRSN. Go to 1).
ACK Event Processing:
1) Find the sequence number prior to which all the packets have been
received by the receiver (ACK number) according to the following
rule: if the receiver's loss list is empty, the ACK number is LRSN
+ 1; otherwise it is the smallest sequence number in the
receiver's loss list.
2) If (a) the ACK number equals to the largest ACK number ever
acknowledged by ACK2, or (b) it is equal to the ACK number in the
last ACK and the time interval between this two ACK packets is
less than 2 RTTs, stop (do not send this ACK).
3) Assign this ACK a unique increasing ACK sequence number. Pack the
ACK packet with RTT, RTT Variance, and flow window size (available
receiver buffer size). If this ACK is not triggered by ACK timers,
send out a Light ACK and stop.
4) Calculate the packet arrival speed according to the following
algorithm:
Calculate the median value of the last 16 packet arrival
intervals (AI) using the values stored in PKT History Window.
In these 16 values, remove those either greater than AI*8 or
less than AI/8. If more than 8 values are left, calculate the
average of the left values AI', and the packet arrival speed is
1/AI' (number of packets per second). Otherwise, return 0.
5) Calculate the estimated link capacity according to the following
algorithm:
Calculate the median value of the last 16 packet pair
intervals (PI) using the values in Packet Pair Window, and the
link capacity is 1/PI (number of packets per second).
6) Pack the packet arrival speed and estimated link capacity into the
ACK packet and send it out.
7) Record the ACK sequence number, ACK number and the departure time
of this ACK in the ACK History Window.
NAK Event Processing:
Search the receiver's loss list, find out all those sequence numbers
whose last feedback time is k*RTT before, where k is initialized as 2
and increased by 1 each time the number is fed back. Compress
(according to section 6.4) and send these numbers back to the sender
in an NAK packet.
EXP Event Processing:
1) Put all the unacknowledged packets into the sender's loss list.
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2) If (ExpCount > 16) and at least 3 seconds has elapsed since last
time ExpCount is reset to 1, or, 3 minutes has elapsed, close the
UDT connection and exit.
3) If the sender's loss list is empty, send a keep-alive packet to
the peer side.
4) Increase ExpCount by 1.
On ACK packet received:
1) Update the largest acknowledged sequence number.
2) Send back an ACK2 with the same ACK sequence number in this ACK.
3) Update RTT and RTTVar.
4) Update both ACK and NAK period to 4 * RTT + RTTVar + SYN.
5) Update flow window size.
6) If this is a Light ACK, stop.
7) Update packet arrival rate: A = (A * 7 + a) / 8, where a is the
value carried in the ACK.
8) Update estimated link capacity: B = (B * 7 + b) / 8, where b is
the value carried in the ACK.
9) Update sender's buffer (by releasing the buffer that has been
acknowledged).
10)Update sender's loss list (by removing all those that has been
acknowledged).
On NAK packet received:
1) Add all sequence numbers carried in the NAK into the sender's loss
list.
2) Update the SND period by rate control (see section 3.6).
3) Reset the EXP time variable.
On ACK2 packet received:
1) Locate the related ACK in the ACK History Window according to the
ACK sequence number in this ACK2.
2) Update the largest ACK number ever been acknowledged.
3) Calculate new rtt according to the ACK2 arrival time and the ACK
departure time, and update the RTT value as: RTT = (RTT * 7 +
rtt) / 8.
4) Update RTTVar by: RTTVar = (RTTVar * 3 + abs(RTT - rtt)) / 4.
5) Update both ACK and NAK period to 4 * RTT + RTTVar + SYN.
On message drop request received:
1) Tag all packets belong to the message in the receiver buffer so
that they will not be read.
2) Remove all corresponding packets in the receiver's loss list.
On Keep-alive packet received:
Do nothing.
On Handshake/Shutdown packet received:
See Section 5.
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6.3 Flow Control
The flow control window size is 16 initially.
On ACK packet received:
The flow window size is updated to the receiver's available buffer
size.
6.4 Loss Information Compression Scheme
The loss information carried in an NAK packet is an array of 32-bit
integers. If an integer in the array is a normal sequence number (1st
bit is 0), it means that the packet with this sequence number is
lost; if the 1st bit is 1, it means all the packets starting from
(including) this number to (including) the next number in the array
(whose 1st bit must be 0) are lost.
For example, the following information carried in an NAK:
0x00000002, 0x80000006, 0x0000000B, 0x0000000E
means packets with sequence number 2, 6, 7, 8, 9, 10, 11, and 14 are
lost.
7. Configurable Congestion Control (CCC)
The congestion control in UDT is an open framework so that user-
defined control algorithm can be easily implemented and switched.
Particularly, the native control algorithm is also implemented by
this framework.
The user-defined algorithm may redefine several control routines to
read and adjust several UDT parameters. The routines will be called
when certain event occurs. For example, when an ACK is received, the
control algorithm may increase the congestion window size.
7.1 CCC Interface
UDT allow users to access two congestion control parameters: the
congestion window size and the inter-packet sending interval. Users
may adjust these two parameters to realize window-based control,
rate-based control, or a hybrid approach.
In addition, the following parameters should also be exposed.
1) RTT
2) Maximum Segment/Packet Size
3) Estimated Bandwidth
4) The maximum packet sequence number that has been sent so far
5) Packet arriving rate at the receiver side
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A UDT implementation may expose additional parameters as well. This
information can be used in user-defined congestion control algorithms
to adjust the packet sending rate.
The following control events can be redefined via CCC (e.g., by a
callback function).
1) init: when the UDT socket is connected.
2) close: when the UDT socket is closed.
3) onACK: when ACK is received.
4) onLOSS: when NACK is received.
5) onTimeout: when timeout occurs.
6) onPktSent: when a data packet is sent.
7) onPktRecv: when a data packet is received.
Users can also adjust the following parameters in the user-defined
control algorithms.
1) ACK interval: An ACK may be sent every fixed number of packets.
User may define this interval. If this value is -1, then it means
no ACK will be sent based on packet interval.
2) ACK Timer: An ACK will also be sent every fixed time interval.
This is mandatory in UDT. The maximum and default ACK time
interval is SYN.
3) RTO: UDT uses 4 * RTT + RTTVar to compute RTO. Users may redefine
this.
Detailed description and discussion of UDT/CCC can be found in
[GG05].
7.2 UDT's Native Control Algorithm
UDT has a native and default control algorithm, which will be used if
no user-defined algorithm is implemented and configured. The native
UDT algorithm should be implemented using CCC.
UDT's native algorithm is a hybrid congestion control algorithm,
hence it adjusts both the congestion window size and the inter-packet
interval. The native algorithm uses timer-based ACK and the ACK
interval is SYN.
The initial congestion window size is 16 packets and the initial
inter-packet interval is 0. The algorithm start with Slow Start phase
until the first ACK or NAK arrives.
On ACK packet received:
1) If it is in slow start phase, set the congestion window size to
the product of packet arrival rate and (RTT + SYN). Slow Start
ends. Stop.
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UDT October 2007
2) Set the congestion window size (CWND) to: CWND = A + 16.
3) The number of sent packets to be increased in the next SYN period
(inc) is calculated as:
if (B <= C)
inc = 1/PS;
else
inc = max(10^(ceil(log10((B-C)*PS*8))) * Beta/PS, 1/PS);
where B is the estimated link capacity and C is the current
sending speed. All are counted as packets per second. PS is the
fixed size of UDT packet counted in bytes. Beta is a constant
value of 0.0000015.
4) The SND period is updated as:
SND = (SND * SYN) / (SND * inc + SYN).
These four parameters are used in rate decrease, and their initial
values are in the parentheses: AvgNAKNum (1), NAKCount (1),
DecCount(1), LastDecSeq (initial sequence number - 1).
We define a congestion epoch as the period between two NAKs in which
the first biggest lost packet sequence number is greater than the
LastDecSeq, which is the biggest sequence number when last time the
packet sending rate is decreased.
AvgNAKNum is the average number of NAKs in a congestion epoch.
NAKCount is the current number of NAKs in the current epoch.
On NAK packet received:
1) If it is in slow start phase, set inter-packet interval to
1/recvrate. Slow start ends. Stop.
2) If this NAK starts a new congestion epoch, increase inter-packet
interval (snd) to snd = snd * 1.125; Update AvgNAKNum, reset
NAKCount to 1, and compute DecRandom to a random (average
distribution) number between 1 and AvgNAKNum. Update LastDecSeq.
Stop.
3) If DecCount <= 5, and NAKCount == DecCount * DecRandom:
a. Update SND period: SND = SND * 1.125;
b. Increase DecCount by 1;
c. Record the current largest sent sequence number (LastDecSeq).
The native UDT control algorithm is designed for bulk data transfer
over high BDP networks. [GHG04a]
Security Considerations
UDT does not have its specific security mechanism, whereas it depends
on the application to provide authentication and lower layer to
provide security mechanisms, if necessary. However, UDT
implementations should check each arrived packets are from the
expected source, since UDP is connectionless.
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UDT October 2007
IANA Considerations
This document has no actions for IANA.
Normative References
[RFC768] J. Postel, RFC 768: User Datagram Protocol, Aug. 1980.
Informative References
[GG07] Yunhong Gu and Robert L. Grossman, UDT: UDP-based Data
Transfer for High-Speed Wide Area Networks, Computer Networks
(Elsevier). Volume 51, Issue 7. May 2007.
[GG05] Yunhong Gu and Robert L. Grossman, Supporting Configurable
Congestion Control in Data Transport Services, SC 2005, Nov 12 -
18, Seattle, WA, USA.
[GHG04b] Yunhong Gu, Xinwei Hong, and Robert L. Grossman, Experiences
in Design and Implementation of a High Performance Transport
Protocol, SC 2004, Nov 6 - 12, Pittsburgh, PA, USA.
[GHG04a] Yunhong Gu, Xinwei Hong, and Robert L. Grossman, An Analysis
of AIMD Algorithms with Decreasing Increases, First Workshop on
Networks for Grid Applications (Gridnets 2004), Oct. 29, San Jose,
CA, USA.
[LM97] T. V. Lakshman and U. Madhow, The Performance of TCP/IP for
Networks with High Bandwidth-Delay Products and Random Loss,
IEEE/ACM Trans. on Networking, vol. 5 no 3, July 1997, pp. 336-
350.
[RFC2581] Allman, M., Paxson, V. and W. Stevens, TCP Congestion
Control, RFC 2581, April 1999.
[RFC2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Chwarzbauer,
T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson. Stream
Control Transmission Protocol. RFC 2960, IETF, October 2000.
[TS06] K. Tan, Jingmin Song, Qian Zhang, Murari Sridharan, A Compound
TCP Approach for High-speed and Long Distance Networks, in IEEE
Infocom, April 2006, Barcelona, Spain.
[UDT] UDT: UDP-based Data Transfer, URL http://udt.sf.net.
[XHR04] Lisong Xu, Khaled Harfoush, and Injong Rhee, Binary Increase
Congestion Control for Fast Long-Distance Networks, INFOCOM 2004.
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UDT October 2007
Author's Addresses
Yunhong Gu
National Center for Data Mining
University of Illinois at Chicago
713 SEO, M/C 249, 851 S Morgan St
Chicago, IL 60607, USA
Phone: +1 (312) 413-9576
Email: yunhong@lac.uic.edu
Robert Grossman
National Center for Data Mining
University of Illinois at Chicago
727 SEO, M/C 249, 851 S Morgan St
Chicago, IL 60607, USA
Phone: +1 (312) 413-2176
Email: grossman@uic.edu
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UDT October 2007
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