One document matched: draft-iyengar-sctp-cacc-01.txt
Differences from draft-iyengar-sctp-cacc-00.txt
Internet Engineering Task Force J. R. Iyengar,
Category: Internet Draft P. D. Amer
Expires: In six months University of Delaware
R. Stewart
Cisco Systems
I. Arias-Rodriguez
Nokia
June 30, 2002
Preventing SCTP Congestion Window Overgrowth During Changeover
draft-iyengar-sctp-cacc-01.txt
Status of this Memo
This document is an internet-draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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Abstract
SCTP [RFC2960] supports IP multihoming at the transport layer. SCTP
allows an association to span multiple local and peer IP addresses,
and allows the application to dynamically change the primary
destination during an active association. We present a problem in the
current SCTP specification that results in unnecessary retransmissions
and "TCP-unfriendly" growth of the sender's congestion window during
certain changeover conditions. We present the problem and propose an
algorithm called the Split Fast Retransmit Changeover Aware Congestion
Control algorithm (SFR-CACC) as a solution. We recommend the addition
of SFR-CACC to the SCTP specification [RFC2960].
Table of Contents
1 Introduction ................................................ 2
2 Congestion Window Overgrowth: Problem Description ........... 2
3 A Solution to the Problem: The SFR-CACC Algorithm ........... 4
4 Conclusion .................................................. 7
5 Security Considerations ..................................... 7
6 Acknowledgments ............................................. 7
7 Authors' Addresses .......................................... 7
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8 References ..................................................
1 Introduction
In an SCTP [RFC2960] association, the sender transmits data to its
peer's primary destination address. SCTP provides for
application-initiated changeovers so that the sending application can
move the outgoing traffic to another path by changing the sender's
primary destination address. We uncovered a problem in the
current SCTP specification that results in unnecessary retransmissions
and "TCP-unfriendly" growth of the sender's congestion window under
certain changeover conditions. We present the problem and propose an
algorithm called the Split Fast Retransmit Changeover Aware Congestion
Control (SFR-CACC) algorithm as a solution. We recommend the addition
of the SFR-CACC algorithm to the SCTP specification [RFC2960].
1.1 Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,SHOULD
NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear in
this document, are to be interpreted as described in [RFC2119].
2 Congestion Window Overgrowth: Problem Description
We present a specific example which illustrates the congestion window
overgrowth problem.
2.1 Example Description:
Consider the architecture shown below:
______ _________ ______
| | / \ | |
| |A1 <============== Path 1 ============> B1| |
| |<------------->| |<------------>| |
| Host | | Network | | Host |
| A | | | | B |
| |<------------->| |<------------>| |
| |A2 <============== Path 2 ============> B2| |
| | \_________/ | |
------ ------
Fig 1: Example Architecture
SCTP endpoints A and B have an association between them. Both
endpoints are multihomed, A with network interfaces A1 and A2, and B
with interfaces B1 and B2. More precisely, A1, A2, B1 and B2 are IP
addresses associated with link layer interfaces. Here we assume only
one address per interface, so address and interface are used
interchangeably.
All four addresses are bound to the SCTP association. For one of
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several possible reasons (e.g., path diversity, policy based routing,
load balancing), we assume in this example that the data traffic from
A to B1 is routed through A1, and from A to B2 is routed through A2.
Let C1 be the cwnd at A for destination B1, and C2 be the cwnd at A
for destination B2. C1 and C2 are denoted in terms of MTUs, not
bytes.
Consider the following sequence of events:
1) The sender (host A) initially sends data to the receiver (host B)
using primary destination address B1. This setting causes packets
to leave through A1. Assume these packets leave the
transport/network layers, and get buffered at A's link layer A1,
whereupon they get transmitted according to the channel's
availability. We refer to these TSNs (that is, packets) the first
group of TSNs.
2) Assume as the first group of TSNs is being transmitted through A1,
that the sender's application changes the primary destination to
B2, thereby causing any new data from the sender to be sent to
B2. In the example, we assume C2 = 2 at the moment of changeover
and new TSNs (second group of TSNs) are now transmitted to the new
primary, B2. This new primary destination causes new TSNs to
leave the sender through A2. Concurrently, the packets buffered
earlier at A1 are still being transmitted. Previous packets sent
through A1, and the packets sent through A2, can arrive at the
receiver B in an interleaved fashion on interfaces B1 and B2,
respectively. This reordering is introduced as a result of
changeover.
3) The receiver starts reporting gaps as soon as it notices
reordering. If the receiver communicates four missing reports to
the sender before all original transmissions of the first group
have been acked, the sender will start retransmitting the unacked
TSNs on path 2.
4) The SACKs for the original transmission of the first group of TSNs
reach A on A1. Since the sender cannot distinguish between SACKs
generated by transmissions from SACKs generated by
retransmissions, the SACKs now received by A on A1 end up acking
the retransmissions of the first group of TSNs, incorrectly
crediting C2 instead of C1. This behaviour whereby SACKs for
original transmissions incorrectly ack retransmissions continues
until all original transmissions of the first group are
retransmitted to B2. Thus, the SACKs from the original
transmissions cause C2 to grow (possibly drastically) from wrong
interpretation of the feedback.
2.2 Discussion
Our preliminary investigation shows that the problem occurs for a
range of {end-to-end delay, end-to-end available bandwidth, MTU}
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settings. [SCTP_IYENGAR_2002a, SCTP_IYENGAR_2002b] give a more
detailed description and analysis of the problem. From the general
model developed in [SCTP_IYENGAR_2002b], we have found that whenever
a changeover is made to a higher quality path (i.e., lower end-to-end
delay, higher end-to-end available bandwidth path), there is a
likelihood of TCP-unfriendly cwnd growth and unnecessary
retransmissions. We also note that the bigger the quality improvement
that the new path provides, the larger the TCP-unfriendly growth and
number of false retransmissions will be.
The congestion window overgrowth (i.e., TCP-unfriendly congestion
window growth) problem exists even if buffering of the first group
occurs not at the sender's link layer, but in a router along the path
(in the example architecture, path 1). In essence, the transport
layers at the endpoints can be thought of as the sending and
receiving entities, and the buffering could potentially be
distributed anywhere along the end-to-end path.
3 Solution to the Problem: The SFR-CACC Algorithm
The problem of TCP-unfriendly cwnd growth occurs due to incorrect fast
retransmissions. These incorrect retransmissions occur because the
congestion control algorithm at the sender is unaware of the
occurrence of a changeover, and is hence unable to identify reordering
introduced due to changeover. In [SCTP_IYENGAR_2002b], we propose the
Changeover Aware Congestion Control algorithms (CACC) - the
Conservative CACC algorithm (C-CACC), and the Split Fast Retransmit
CACC algorithm (SFR-CACC), which curb the TCP-unfriendly cwnd growth
by avoiding these unnecessary fast retransmissions. Of the three
algorithms, C-CACC has the disadvantage that in the face of loss, a
lot of TSNs could potentially have to wait for an RTO when they could
have been fast retransmitted. SFR-CACC alleviates this disadvantage.
The key idea in SFR-CACC is to maintain state at the sender on a
per-destination basis when a changeover happens. On the receipt of a
SACK, the sender uses this state to selectively increase the missing
report count for TSNs in the retransmission list. In SFR-CACC, we
further make the following observation: the reordering observed during
changeover happens because TSNs which are supposed to reach the
receiver in-sequence end up reaching the receiver in concurrent
groups, in-sequence within each group. With this observation, we
reason that the Fast Retransmit algorithm can be applied independently
within each group. That is, on the receipt of a SACK, if we can
estimate the TSN(s) that causes this SACK to be sent from the
receiver, we can use the SACK to increment missing report counts
within the causative TSN(s)'s group. Our estimate is conservative, if
a SACK could have been caused by TSNs in multiple groups, this SACK
will be used to increment missing report counts only for TSNs sent to
the current primary destination, if any. In the case where multiple
changeovers cycle back to a destination while the CHANGEOVER_ACTIVE is
still set, CYCLING_CHANGEOVER is set to indicate a double switch to
the destination. The CYCLING_CHANGEOVER flag is used to mark TSNs in
only the latest group sent to the current primary destination, thus
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preventing incorrect marking of TSNs in any other changeover
range. SFR-CACC also enables Fast Retransmit for TSNs which could have
timed out on some destination, but were retransmitted on the current
primary destination after the latest changeover to the current primary
destination.
3.1 Variables Introduced
In SFR-CACC, four per-destination variables are introduced:
1) CHANGEOVER_ACTIVE - a flag which indicates the occurrence of
a changeover.
2) CYCLING_CHANGEOVER - a flag which indicates whether the
change of primary is the first switch to this destination address
during an active switch. We refer to the time during which the
CHANGEOVER_ACTIVE is 1 as an active switch. This flag is
used to determine primary switches cycling through destination
address space.
3) next_tsn_at_change - an unsigned integer, which stores the next
TSN to be used by the sender, at the moment of changeover.
4) cacc_saw_newack - a temporary flag, which is used during the
processing of a SACK to estimate the causative TSN(s)'s group.
3.2 The SFR-CACC Algorithm
Upon the receipt of a request to change the primary destination
address, on the data structure for the new primary destination,
the sender MUST do the following:
1) If CHANGEOVER_ACTIVE is set, then there was a switch to this
destination address earlier. The sender MUST set
CYCLING_CHANGEOVER to indicate that this switch is a double
switch to the same destination address.
2) The sender MUST set CHANGEOVER_ACTIVE to indicate that a
changeover has occurred.
3) The sender MUST store the next TSN to be sent in
next_tsn_at_change.
On receipt of a SACK the sender SHOULD execute the following statements:
1) If the cumulative ack in the SACK passes next_tsn_at_change on the
current primary, the CHANGEOVER_ACTIVE flag SHOULD be
cleared. The CYCLING_CHANGEOVER flag SHOULD also be cleared
for all destinations.
2) If the SACK contains gap acks and the flag CHANGEOVER_ACTIVE
is set the receiver of the SACK MUST take the following actions:
A) Initialize the cacc_saw_newack to 0 for all destination
addresses.
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B) For each TSN t being acked that has not been acked in any
SACK so far, set cacc_saw_newack to 1 for the destination that
the TSN was sent to.
3) If the missing report count for TSN t is to be incremented
according to [RFC2960] and [SCTP_STEWART_2002], and
CHANGEOVER_ACTIVE is set, then the sender MUST further
execute steps 3.1 and 3.2 to determine if the missing report count
for TSN t SHOULD NOT be incremented.
3.1) If CYCLING_CHANGEOVER is 0, the sender SHOULD execute
steps C, D, F.
C) Let count_of_newacks be the number of destinations for
which cacc_saw_newack is set.
D) If count_of_newacks is greater than or equal to 2, and t
was not sent to the current primary then the sender MUST
NOT increment missing report count for t.
F) If count_of_newacks is less than 2, let d be the
destination to which t was sent. If cacc_saw_newack is 0
for destination d, then the sender MUST NOT increment
missing report count for t.
3.2) Else if CYCLING_CHANGEOVER is 1, and t is less than
next_tsn_at_change of the current primary, then the sender
MUST NOT increment missing report count for t.
3.3) If 3.1 and 3.2 do not dictate that the missing report count
for t should not be incremented, then the sender SHOULD
increment missing report count for t (according to [RFC2960]
and [SCTP_STEWART_2002]).
3.3 Discussion
The SFR-CACC algorithm maintains state information during a
changeover, and uses this information to avoid incorrect fast
retransmissions. Consequently, this algorithm prevents the
TCP-unfriendly cwnd growth. This algorithm has the added advantage
that no extra bits are added to any packets, and thus the load on the
wire and the network is not increased. SFR-CACC is also capable of
handling multiple changeovers. One disadvantage of SFR-CACC is that
there is added complexity at the sender to maintain and use the added
state variables. Some of the TSNs on the old primary may also not be
eligible for Fast Retransmit. To quantify the number of TSNs which
will be ineligible for Fast Retransmit in the face of loss, let us
assume that only one changeover is performed, and that SACKs are not
lost. Under these assumptions, potentially only the last four packets
sent to the old primary destination will be forced to be retransmitted
with an RTO instead of a Fast Retransmit. In other words, under the
stated assumptions, if a TSN that is lost has at least four packets
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successfully transmitted after it to the same destination, then the
TSN will be retransmitted via Fast Retransmit.
4 Conclusion
The general consensus at the IETF has been to dissuade the usage of
SCTP's multihoming feature for simultaneous data transfer to the
multiple destination addresses, largely due to insufficient research
in the area. Though there is some amount of simultaneous data transfer
in the described scenario, this phenomenon is an effect of changing
the primary destination; not necessarily a result of an application
intending to simultaneously transfer data over the multiple paths.
Among other reasons, this changeover could be initiated by an
application searching for a better path to the peer host for a long
session, or attempting to perform a smoother failover.
We recommend the addition of SFR-CACC to SCTP [RFC2960] to alleviate
the problem of TCP-unfriendly cwnd growth and unnecessary fast
retransmissions during a changeover. We have implemented the SFR-CACC
algorithm in the NetBSD/FreeBSD release for the KAME stack
[SCTP_WEB_KAME, SCTP_WEB_SCTPHOME]. The implementation uses three
additional flags and one TSN marker per-destination, as described in
section 3.2. Approximately twenty lines of C code were needed to
facilitate SFR-CACC, most of which will be executed only when a
changeover is performed in an association.
5 Security Considerations
This document discusses a congestion control issue during changeover
in SCTP. This does not raise any new security issues with SCTP.
6 Acknowledgments
The authors would like to thank Vern Paxson, Mark Allman, Phillip
Conrad, Armando Caro and Jerry Heinz for providing comments and input.
7 Authors' Addresses
Janardhan R. Iyengar
Department of Computer & Information Sciences
University of Delaware
103 Smith Hall
Newark, DE 19716, USA
email: iyengar@cis.udel.edu
Paul D. Amer
Department of Computer & Information Sciences
University of Delaware
103 Smith Hall
Newark, DE 19716, USA
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email: amer@cis.udel.edu
Randall R. Stewart
24 Burning Bush Trail
Crystal Lake, IL 60012, USA
email: rrs@cisco.com
Ivan Arias-Rodriguez
Nokia Research Center
PO Box 407
FIN-00045 Nokia Group
Finland
email: ivan.arias-rodriguez@nokia.com
8 References
[RFC2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer,
T. Taylor, I. Rytina, M. Kalla, L. Zhang, V. Paxson. "Stream Control
Transmission Protocol". Proposed Standard (RFC2960), IETF, October
2000.
[SCTP_STEWART_2002] Randall Stewart, Lyndon Ong, Ivan Arias-Rodriguez,
Kacheong Poon, Armando L. Caro, Jr. "SCTP Implementor's Guide".
Internet Draft: draft-ietf-tsvwg-sctpimpguide-05.txt, IETF, May
2002. (work in progress)
[SCTP_IYENGAR_2002a] J. R. Iyengar, Armando L. Caro Jr., Paul D. Amer,
Gerard J. Heinz, Randall Stewart. "SCTP Congestion Window Overgrowth
During Changeover". Proc. SCI 2002, July 2002, Orlando. (to appear)
[SCTP_IYENGAR_2002b] J. R. Iyengar, Armando L. Caro Jr., Paul D. Amer,
Gerard J. Heinz, Randall Stewart. "Preventing SCTP Congestion Window
Overgrowth During Changeover". Technical Report XX-XX, Department of
Computer and Information Sciences, University of Delaware.
[SCTP_WEB_KAME] Webpage of the KAME Project, http://www.kame.org
[SCTP_WEB_SCTPHOME] The SCTP Homepage, http://www.sctp.org
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