Network Working Group Hong-ke. Zhang
Internet Draft Bo. Wang
Dong.Yang
Fei.Song
Huan.Yan
Beijing Jiaotong University
Expires: March 2009 September 16, 2008
Wireless Multi-path Transmission Using SCTP
draft-zhang-wcmt-sctp-00.txt
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Abstract
In this draft, WCMT-SCTP(WCMT:Wireless Concurrent Multi-path
Transfer)is proposed to allow a host to improve the transmission
throughput in wireless multi-hop networks using simultaneous multi-
path transmission of Stream Control Transmission Protocol. Relevant
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issues of WCMT-SCTP included are: (1) data transmission mode and path
selection, (2) path management and, (3) multi-path handover mechanism.
Transmission modes of WCMT-SCTP, and the path handover mechanism, are
designed to enhance the transmission throughput and solve the receive
buffer blocking problem of different wireless link status. Since the
transmission paths used for WCMT-SCTP may differ when an WCMT-SCTP
host moves, the multi-path handover problem is defined, and a
handover mechanism is proposed.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119
.
Table of Contents
1. Introduction................................................3
1.1.Wireless mobile sctp..................................3
1.2. SCTP multi-path transmission.........................3
1.3. Wireless mobile Multi-path transmission using SCTP....3
2. Head-of-line blocking and receive buffer blocking: Problem
Description....................................................4
2.1. Example Description..................................4
2.2. Discussion..........................................5
3. Transmission mechanism and path selection of WCMT-SCTP.....5
3.1.Transmission mechanism of WCMT-SCTP...................5
3.2.The retransmission mechanism of WCMT-SCTP protocol.....6
3.2.1 Distinguish different types of packet loss........6
3.2.2 Retransimission Mechanism of WCMT-SCTP...........7
3.3.Path selection........................................8
3.3.1 The trigger of path selection....................8
3.3.2 Path selection mechanism.........................8
4. Modifications to SCTP.....................................9
4.1.Chunks format modifications...........................9
4.2.Congestion control mechanism changed in WCMT-SCTP.....10
4.3.Path handover notification changed in WCMT-SCTP.......11
4.4.Other modifications have to be taken in WCMT-SCTP.....12
5. Further considerations...................................12
6. Security considerations..................................12
7. IANA considerations......................................12
8. References..............................................12
Intellectual Property Statement................................14
Disclaimer of Validity........................................14
Copyright Statement...........................................14
Acknowledgment................................................15
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1. Introduction
1.1.Wireless mobile sctp
Stream Control Transmission Protocol (SCTP)[1]is a transport layer
protocol that supports multihoming in nature. The SCTP ADDIP
Extention[2] enables an SCTP endpoint to dynamically add a new IP
address and delete an unnecessary IP address, and change the primary
IP address used for the association without terminating the
correspongding association. A new SCTP chunk type that the SCTP
Address Reconfiguration introduces is the SCTP ASCONF(Address
Configuration Change)chunk type. The SCTP ASCONF chunk is used to
notify the corresponding event to the remote endpoint when one of the
events such as ADD (to add a new IP address), DELETE (to delete
anunnecessary IP address), or CHANGE (to change the primary path)
occurs in an ongoing association. With the functions provided by SCTP
ADDIP Extension, SCTP becomes more powerful when it is adopted in
wireless mobile networks. The changed sctp protocol which is enabled
the address reconfiguration is called mobile sctp(msctp). Some
researches of mSCTP, which is based on the SCTP ADDIP Extension, are
also proposed to enable the mobility support of SCTP [3][4].
1.2. SCTP multi-path transmission
Currently, some researchers are proposed to enable SCTP to transmit
data through multiple paths to improve transmission throughput, e.g.,
IPCC-SCTP, LS-SCTP, Westwood SCTP, cmpSCTP, and mmpSCTP.
1.3. Wireless mobile Multi-path transmission using SCTP
The main scope of this draft is to investigate relevant issues of
multi-path transmission using SCTP in wireless multi-hop mobile
networks, and try to propose a protocol extension of SCTP, namely,
WCMT-SCTP. In this draft, a concurrent multi-path transmission
mechanism is proposed to improve the throughput under different
wireless network status. Furthermore, a possible path selection
policy that is used to select the paths used by WCMT-SCTP is also
proposed in this draft.
The scope of this draft also includes the path handover managements
of WCMT-SCTP. i.e., the substitute paths selection of WCMT-SCTP.
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2. Head-of-line blocking and receive buffer blocking: Problem
Description
2.1. Example Description
We present a specific example which illustrates the head-of-line
blocking and receive buffer blocking problem in the concurrent multi-
path transfer.
Consider the architecture shown below:
------- _________ _____
| | / \ | |
| |A1 <============== Path 1 ============> B1| |
| |<------------->| |<------------>| |
| Host | | Wireless | | Host |
| A | | Network | | B |
| | | | | |
| |<------------->| |<------------>| |
| |A2 <============== Path 2 ============> B2| |
| | \_________/ | |
------ ------
Figure 1: Example Architecure
SCTP endpoints A and B have an association between them. Both
endpoints are multihomed, A posses network interfaces A1 and A2,and B
posses interfaces B1 and B2. More precisely, A1, A2, B1 and B2 are
associated with link layer interfaces. A1 and B1, A2 and B2 are bound
to one SCTP association. in this example, We assume that the data
traffic are transmitted through these four interfaces simultaneously.
Thus it forms two paths: path1 is the data traffic from A to B1 which
is routed through A1, and path2 is the data traffic from A to B2
which is routed through A2. Both path1 and path2 have a certain
packet loss rate. Each host has one transport receive buffer.
Now, consider the following sequence of events:
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1)
The sender (host A) initially sends data to the receiver (host B)
using both destination address B1 and B2.
2)
Because there is only one receive buffer, if there is a path
failure or packet error occurred on the path1, the receiver has to
wait for retransmissions to come through, and is unable to deliver
subsequent TSNs to the application (some of which were sent over
path1). These subsequent TSNs (time sequence number) are held in
the transport layer receive buffer and the peer-rwnd(receive
window) is reduced sharply, so the path2 gets into congestion.
Thus path2 causes blocking of the receive buffer, preventing data
from being sent on either path and reduce over all throughput.
3)
If there are multiple data streams exist in one SCTP association,
consecutive TSN number may be assigned to different streams. For
example, stream1 is assigned TSN number 1, 3, 5, 7, 9, and stream2
is assigned TSN number 2, 4, 6, 8, 10. If receiver receives data
packets with TSN number 1, 2, 3, 4, 5, 7, 9, 10, it can deliver
the data of the stream 1 to application absolutely, because data
stream is SCTP protocol is completely logic independently. But in
real transfer process in traditional transport protocol, the
receiver has to wait data packet with TSN number 6 and 8. This
instance causes the head-of-line blocking problem.
2.2. Discussion
The receive buffer blocking and head-of-line blocking problems are
existed in all traditional CMT transfer. The essential of this
problem is that all paths using a shared buffer and global variable
TSN is used to calculate the receive order of each path.
3.
Transmission mechanism and path selection of WCMT-SCTP
3.1.Transmission mechanism of WCMT-SCTP
WCMT-SCTP protocol has the following characters:
1) WCMT-SCTP has multi send buffers and multi receive buffers.
2) WCMT-SCTP has a per-path congestion control mechanism. The
original SCTP only allows for CWND adjustments when the association
cumulative TSN Ack point is updated by an incoming SACK. This is
correct when at most one path is used at any given time and
consecutive packets arrive in order at the receiver, but in the CMT
situation, the assumption of ordered reception does not hold anymore,
and SACKs, may acknowledge one or more chunks that were received in
order on different paths, which do not update the association
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cumulative TSN Ack point. This problem can be easily solved by
introducing a per-path Ack point and updating it after processing a
meaningful SACK.
3) In order to solve the receive buffer blocking and head-of-line
blocking problems in the CMT situation, it proposed to bind one of
multiple data streams with one of the multi-path. The sender can
transfer multiple data streams in the same time, but if there is no
path failure packet loss is reported, the certain data stream can
only be transferred on the certain path.
4) In WCMT-SCTP, the protocol uses stream identifier (SID) and path
sequence number (PSN) instead of TSN as the adjustment of CWND.
Because the data stream is bind with the path, so using these two
parameters can uniquely identify per-path CWND.
5) The retransmission mechanism of WCMT-SCTP protocol is different as
used in original SCTP protocol. It depends on types of packet loss.
3.2.The retransmission mechanism of WCMT-SCTP protocol
3.2.1 Distinguish different types of packet loss
In wireless network, packet losses can be categorized as three major
types.
Congestion loss: It occurs in the transmit nodes in wireless networks
because of bandwidth and buffer limitations that result in buffer
over-flows. In wireless network, it could not be the main reason of
packet loss.
Error Loss: It occurs at random over a wireless channel due to
transmission errors, caused mainly by impairments such as noise,
interference and multi-path fading.
Path failure loss: It occurs in the situation that the original path
is inexistence and packets are re-routed to the new path. Consecutive
packets may be lost due to futile transmissions over old path. Path
failure loss is also one of the main sources of packet losses in
wireless communication.
The difference between path failure loss and congestion loss as well
as error loss is that path failure loss is usually detected by a time
out, while error loss and congestion loss are detected by a sender
receiving duplicate SACKs.
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3.2.2 Retransimission Mechanism of WCMT-SCTP
When the packet loss is detected by receiving duplicate SACKs, a
WCMT-SCTP sender uses a failure detection threshold called DMR (data
max retransmission). When a sender experiences more than DMR
consecutive retransmission of the same loss packets and no one is
succeed, it will try to reach other active destination, the original
destination is marked as failed. Only heartbeats are sent to the
failed destination. The original destination returns to the active
state when the sender receives a heartbeat ack from the destination
again. The default DMR value of WCMT-SCTP is 3. The finite state
machine is shown below:
-----------------------
| Path is "Active" |<-----
| Send Data | |
----------------------- |
| |
| |
\|/ |
---------------------- No |
| Adjust Received the |------>|
| same Duplicate-SACK? | |
---------------------- |
| |
| Yes |
\|/ |
---------------------- No |
| Adjust Consecutive |------>
| Retransmit Time> DMR? |
----------------------
|
| Yes
\|/
------------------------
/ Path is "Failure" \
| Change to a New Path |
\ /
------------------------
When the path is failure, the packet loss is detected by timeout. In
this situation, if twice consecutive timeout is happened, the
original destination is marked as failed immediately, and the sender
will try to reach a new active destination. The status of failed
destination turns into active is as same as that of error loss
process. The finite state machine is shown below:
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-----------------------
| Path is "Active" |<-----
| Send Data | |
----------------------- |
| |
| |
\|/ |
---------------------- No |
| Adjust Consecutive |------
| Timeout Time> 2? |
----------------------
|
| Yes
\|/
------------------------
/ Path is "Failure" \
| Change to a New Path |
\ /
------------------------
3.3.Path selection
3.3.1 The trigger of path selection
When the total number of available transmission paths is more than the
number of paths that needs to be used for WCMT-SCTP, i.e., the path
number is larger than the number of data streams. a path selection
mechanism is needed to select which paths should be used. The path
selection mechanism can be triggered in the following conditions:
1. Association initiation: the initial setup of an association in a
multihomed host to start the data transmission.
2. New path added: a new path is discovered as available and added
into the association.
3. Path inactive: A path that is used by WCMT-SCTP is determined as
"inactive" by the association.
3.3.2 Path selection mechanism
In condition "Association Initiation", when the multihomed host is
going to setup an association, the multihomed host connects to its
peer endpoint using one of its available addresses. Then the
multihomed host exchanges the information of all available IP
addresses with its peer endpoint in initialization. When the
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initialization is complete, WCMT-SCTP triggers the path selection
mechanism to select the paths which has the minimum RTT time to use
for the following transmission. If the number of data streams is
smaller than or equal to the path number, one path will bind with one
data streams. If the number of data streams is less than the path
number, some of the largest bandwidth paths will bind with more than
one data streams.
Condition "New path added" occurs when the multihomed host added a
new IP and found a new path between source and receiver. The path
selection mechanism can be triggered to determine whether the newly
added transmission path is better than the one that is currently used.
Condition "Path inactive" occurs when one of the transmit paths
repeatlly fails and determined as "inactive" by the association. Thus,
WCMT-SCTP will trigger the path selection mechanism to select another
transmission path to replace the failed path.
4.
Modifications to SCTP
On the basis of the aforementioned issues and design, some
modifications must be made to SCTP to solve the problems and support
the new features of WCMT-SCTP.
4.1.Chunks format modifications
A new parameter path sequence number (PSN) is used to substitute
transmit sequence number (TSN). PSN represents sequence of SCTP data
chunk transmitted through a path, it has similar function as TSN in
standard SCTP.
The data chunk format of WCMT-SCTP is defined as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+
| Type=0x00 |Reserved| Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+
| Path Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+
| Stream Identifier | Stream Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+
| Variable Length User Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+
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The SACK chunk format of WCMT-SCTP is defined as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=3 | Flags=0 | Length=variable |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cumulative PSN ACK |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time Stamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID #1 | rwnd 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID #N | rwnd N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num of Fragments=M | Num of Dup=k |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap ACK #1 start | Gap ACK #1 end |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gap ACK #M start | Gap ACK #M end |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate PSN #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Duplicate PSN #K |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The time stamp is used to determine the order of the SACKs, the
receiver of the old SACK does not update its view of the rwnd of the
peer.
4.2.Congestion control mechanism changed in WCMT-SCTP
PSN is used in WCMT-SCTP for congestion control. The sender keeps a
mapping relationship table between stream identifiers (SID) and the
corresponding paths. So the protocol can use PSN and SID instead of TSN
to achieve the congestion control mechanism on each path.
The CWND updating mechanism and gap report process of WCMT-SCTP is as
follows :
Sender Side Behavior:
(1)CWND updating on SACKs
On receipt of a SACK
for each destination di
for each PSN being acked by the SACK and yet not been acked do
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if PSN.dst=di then
di.num_acked_PSN_old=di.num_acked_PSN
increment di.num_acked_PSN by1;
if(SIDi_PSNn_outstanding=TRUE) then
Decrement di.num_out_SIDi by 1;
end
end
end
increment di.CWND by min(di.num_acked_PSN-di.num_acked_PSN_old,PMTU);
end
(2)GAP Report Process
On receipt of a SACK with GAP reports:
for each SIDi_PSNm that are reported as missing do
if(SIDi_PSNn_outstanding=TRUE&&di_num_out_SIDi!=0) then
increment the missing report count for SIDi_SSNm;
else
do not increment the missing report count for SIDi_PSNm;
end
end
4.3.Path handover notification changed in WCMT-SCTP
The SCTP ADDIP Extention allows an SCTP host to dynamically add,
delete, and change the primary address of ongoing association without
interrupting it. But the ADDIP Extention function is only used for
the situation of one primary path used. In multiple primary paths
situation,new chunk type to announce which primary path has to be
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replaced.(this need a new chunk type, we can discuss the format of
the new chunk)
4.4.Other modifications have to be taken in WCMT-SCTP
In order to avoid spurious retransmission, SACKs must be sent to the
new destination address after the path handover occurs at the sender.
The CWND of the sender should not grow when receiving the
acknowledgments of the PSNs that are transmitted through the old
path after the path handover occurs at the sender.
5.
Further considerations
For the practical implementation of WCMT-SCTP, path selection should
be performed for each type of the wireless access technology. But
the WCMT-SCTP can not know which access technology the host use, so
the probability solution is using cross layer design. For example,
in the process of association established, the initial transport
packet can take some information of routing layer or PHY/MAC layer.
Thus the path selection mechanism can be successful performed.
6.
Security considerations
Security considerations are not discussed in this memo.
7.
IANA Considerations
This document has no actions for IANA.
8.
References
[1]R. Stewart, "Stream Control Transmission Protocol," RFC 4960, IETF,
September 2007.
[2]R. Stewart, Q. Xie et. al., "Stream Control Transmission
Protocol(SCTP)Dynamic Address Reconfiguration, " RFC 5061, IETF,
September 2007.
[3]S. J. Koh, et al., "Architecture of Mobile SCTP for IP Mobility
Support," draft-sjkoh-sctp-mobility-02.txt, June 2003
[4]M. Riegel, M. Tuexen, "Mobile SCTP," draft-riegel-tuexen-mobile-sctp-
06.txt, October 2004.
Authors' Addresses:
Hong-ke Zhang
Next generation Internet research center
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School of electronic and information engineering
Beijing Jiaotong University
Beijing, China
Email:hkzhang@bjtu.edu.cn
Bo Wang
Next generation Internet research center
School of electronic and information engineering
Beijing Jiaotong University
Beijing, China
Email:wangbolulu@163.com
Dong Yang
Next generation Internet research center
School of electronic and information engineering
Beijing Jiaotong University
Beijing, China
Email:youngmanyd@163.com
Fei Song
Next generation Internet research center
School of electronic and information engineering
Beijing Jiaotong University
Beijing, China
Email:song2000fei@gmail.com
Huan Yan
Next generation Internet research center
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School of electronic and information engineering
Beijing Jiaotong University
Beijing, China
Email:fanfan_821@163.com
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