Network Working Group R. Stewart
Internet-Draft P. Lei
Expires: December 9, 2004 Cisco Systems, Inc.
M. Tuexen
Univ. of Applied Sciences Muenster
June 10, 2004
Stream Control Transmission Protocol (SCTP) Packet Drop Reporting
draft-stewart-sctp-pktdrprep-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes a new chunk type for SCTP. This new chunk
type can be used by both endhosts and routers to report the loss of
SCTP datagrams due to errors in transmission or other drops not due
to congestion.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architectural Considerations . . . . . . . . . . . . . . . . . 3
4. New Chunk Types . . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Packet Drop Chunk (PKTDROP) . . . . . . . . . . . . . . . 5
5. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1 Sender of the packet drop . . . . . . . . . . . . . . . . 7
5.1.1 Middle box . . . . . . . . . . . . . . . . . . . . . . 7
5.1.2 End host . . . . . . . . . . . . . . . . . . . . . . . 7
5.2 Receiver side . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Recommended Variables . . . . . . . . . . . . . . . . . . . . 11
8. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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1. Introduction
The modern Internet has a wide variety of link types. A vast
majority of these link type present a very low bit error rate. In
recent years, however, a large number of higher bit error links are
becoming more wide spread for example satellite, 802.11, and 3G
cellular to name just a few. Often times one of the segments in the
path will realize that it is going to drop a packet due to bit
errors. When a drop does occur due to an error other than
congestion, the drop will be mistakenly interpreted as congestion in
the network.
This "mis-interpretation" of feed back may cause an SCTP sender to
drastically under utilize a link. Depending on how severe the error
rate, the sender may stay in a continual state of congestion
collapse, thus effecting performance in a very negative way over the
entire life of the association.
This draft proposes a new SCTP chunk type that can be used by a
sender to discover dropped packets in such a case. This chunk may
also be used by an SCTP receiver to report cases of window overrun or
received data that may have had bit errors.
2. 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].
3. Architectural Considerations
The Packet Drop Reports (PKTDROP) can be generated by an SCTP
endpoint or a middle box.
The SCTP endpoint can inform its peer that it has received an SCTP
packet, but the CRC32C was wrong. The peer can retransmit this
packet and does not need to adopt the window for congestion control
because this packet-loss is not related to congestion. It is also
possible for the endpoint to make clear that the receiver window was
overrun.
There are two scenarios where a middle box may send Packet Drop
Reports.
For the first scenario consider a middle box in the path between the
communicating SCTP endpoints (see Figure 1), which communicates with
a middle box peer. Please note that the middle box peer can be can
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be located at the same physical device that also runs the SCTP stack
or running on separate boxes providing a tunneling service. This
crucial point here is, that there is some protocol running between
the middle box and the middle box peer.
+------------+ +-----------------+
+-------| middle box +-----+ middle box peer +--------+
| +------------+ +-----------------+ |
| |
+----+-----+ +-----+----+
| SCTP | | SCTP |
| endpoint | | endpoint |
+----------+ +----------+
Figure 1
If they run a protocol below SCTP which provides an acknowledgment
service in a way that the sending middle box knows that a packet was
not received by the middle box peer and the packet was not dropped
due to congestion, then the sending middle box can also send a Packet
Drop Report back to the sending SCTP endpoint. It can also indicate
the current status of the send queue and the bandwidth limit between
the middle boxes if applicable.
In the other scenario there is only one middle box involved, which
means that there is no middle box specific communication, as shown in
Figure 2. In this case the middle box may want to send Packet Drop
Reports to report to the SCTP sender the number of queued data and a
possible bandwidth limitation between the middle box and the SCTP
receiver.
+------------+
+--------------------+ middle box +--------------------+
| +------------+ |
| |
+----+-----+ +-----+----+
| SCTP | | SCTP |
| endpoint | | endpoint |
+----------+ +----------+
Figure 2
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4. New Chunk Types
This section defines the new chunk type that will be used to report
dropped packets not due to congestion in the network. Figure 3
illustrates the new chunk types.
Chunk Type Chunk Name
--------------------------------------------------------------
0x81 Packet Drop Chunk (PKTDROP)
Figure 3
It should be noted that the PKTDROP Chunk format requires the
receiver to ignore the chunk if it is not understood. This is
accomplished as described in RFC2960 [3] section 3.2. by the use of
the upper bit of the chunk type.
4.1 Packet Drop Chunk (PKTDROP)
This chunk is used to communicate to the remote endpoint the
purposeful dropping of a packet which is NOT due to congestion or the
current state of a network bottleneck.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x81 | Flags=TBM | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Bandwidth or Maximum Rwnd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of data on queue |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Truncated Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dropped SCTP Packet |
\ (No IP header Included - optional) /
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Type : 8 bits - This value MUST be set to 0x81 for all packet
drop chunks.
Flags : 8 bits - The lower 3 bits of this field are used to
identify various properties about the packet report:
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+--Bit--+-Set Value-+----------------Meaning---------
| T | 00000100 | This bit informs the receiver
| | | the packet was truncated to
| | | fit. If this bit is set then the
| | | truncated length holds the
| | | original packet length (from the
| | | IP header).
+-------+-----------+-------------------------------
| B | 00000010 | This bit informs the receiver
| | | that a BAD CRC32C was detected
| | | by an SCTP endpoint.
+-------+-----------+-------------------------------
| M | 00000001 | This bit informs the receiver
| | | that the source of the packet is
| | | a middle box, not the endhost.
| | | This also tells the receiver to
| | | look for the Peers Verification tag
| | | in the packet. This is equivalent
| | | to the T bit in an ABORT or
| | | SHUTDOWN COMPLETE packet.
+-------+-----------+-------------------------------
Chunk Length : 16 bits unsigned int - This value holds the
length of the chunk including the chunk header.
Link Bandwidth or Maximum Rwnd : 32 bits unsigned int - If the M
bit is set to '1', this value holds the bandwidth capacity in bytes
per second of the link the middle box is connected to aka the
bottleneck bandwidth being sent towards. If the M bit is set to '0'
then this value holds the maximum allowable Rwnd of the peer. This
value is normally the same value as that found in the INIT or
INIT-ACK's a_rwnd field.
Size of data on queue : 32 bits unsigned int - This value
represents the current number of bytes of data onqueue towards the
link or reader. In the case of a middle box (M bit set to '1'), this
will inform the receiver how much data is currently in queue towards
the bottleneck, if the link layer is reliable (e.g. a Reliable Link
Protocol) this number will also include any inflight data over the
link. In the case of an endhost (M bit set to '0') this will tell
the receiver how much data is still un-read or held for reassembly by
the remote SCTP endpoint.
Truncated Length : 16 bits unsigned int - This value is set to
the original size of the SCTP packet that was dropped. The size does
NOT include the IP header on any other IP option field (i.e. it is
the size of the SCTP payload within the IP packet). This value is
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only valid if the T bit is set to '1'.
Reserved : 16 bits unsigned int - This value SHOULD be set to '0' by
the sender and MUST be ignored by the receiver.
Data Field: variable - This field is variable and usually holds the
packet that was dropped or a portion of it if the T bit is set. In
some instances a middle box may send a packet drop report without
this data. In such a case, it is reporting to the SCTP sender the
current bandwidth and NOT reporting a dropped packet.
5. Procedures
5.1 Sender of the packet drop
5.1.1 Middle box
Periodically a middle box may realize that it cannot transmit a chunk
due to errors in transmission. In such a case the middle box SHOULD
compose a packet drop chunk to send back to the sender of the dropped
SCTP packet. The middle box MUST set the M bit to one and copy into
the SCTP common header the verification tag found in the packet to be
dropped. The IP addresses and SCTP ports MUST be swapped so that the
receiver of the packet drop will identify the packet drop report with
the correct SCTP association.
After filling out the IP and SCTP headers, the sender MUST copy in
all or part of the SCTP packet being dropped not including the IP and
SCTP header (i.e. starting at the first SCTP chunk). The sender of
the packet drop report MUST assure that the packet fits into a single
MTU, truncating the packet and setting the T bit if necessary. If
the middle box truncates the packet to fit in a single MTU, the
middle box MUST copy the original length of the SCTP packet into the
Truncated length field.
The middle box sending the drop packet report SHOULD also total up
the data that is inflight (towards the destination, of the dropped
packet) and the data that is inqueue awaiting transmission, placing
this size in 'size of data on queue' field. The sender SHOULD also
place the link bandwidth, in bytes per second, in the 'bandwidth'
field. The receiving SCTP endpoint should use this information to
adjust its congestion control parameters.
5.1.2 End host
An SCTP endhost MAY want to send a packet drop for one of two
reasons, the SCTP sender has overrun the local receivers rwnd or the
inbound packet failed its CRC-32 check.
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If the SCTP endhost detects a bad CRC-32 it will still use the SCTP
common header to attempt to locate the association. If a valid
association is found and the verification tag are correct, chances
are good that the common header was not damaged and thus the found
TCB can be used to generate a drop report with the rest of the SCTP
packet.
In either case the receiver that is sending the drop report MUST copy
the packet, with possible truncation as described above. The sender
of the drop report MUST set the M bit to 0 and place the verification
tag of the peer in the outbound packet. The sender of the drop
report should also place the maximum rwnd value in the 'Maximum Rwnd'
field, and should place the number of bytes unread in the 'data on
queue' field. Note that the unread byte count MUST include data in
any local buffer not yet read by the user, data pending reassembly
and data awaiting stream re-ordering.
5.2 Receiver side
When receiving a Packet Drop report the SCTP endpoint will want to
examine the drop report and based on the information possibly
retransmit lost information to the peer. The receiver SHOULD verify
that the sender actually had a packet by comparing some of the data
that was dropped to the data that was sent. This is done to assure
the sender that a malicious receiver is not attempting to induce a
retransmission of a congestion related dropped packet. The following
list illustrates the handling procedure by chunk type for dropped
packets.
DATA - For a data chunk drop, the receiver SHOULD locate the
identified DATA chunk and mark it for retransmission. The DATA
chunk should be treated just as if it had been marked for fast
retransmit with the exception that no adjustment should be made to
the value of cwnd (providing that the receiver can validate a
portion of the packet as being what was sent).
SACK - For a lost SACK chunk, a receiver MAY wish to send out a
new SACK illustrating the current receiver conditions.
INIT - For a INIT chunk, the receiver SHOULD resend the INIT
restarting its local T-1 timer.
HEARTBEAT REQUEST - For a heartbeat request, the receiver SHOULD
resend a heartbeat to the source address of the packet.
SHUTDOWN - For a shutdown request, the SCTP receiver SHOULD resend
the shutdown request.
SHUTDOWN ACKNOWLEDGEMENT - For a shutdown acknowledgement the
receiver SHOULD resend the SHUTDOWN-ACK.
COOKIE ECHO - For a Cookie Echo the receiver SHOULD retransmit the
lost COOKIE ECHO, restarting any cookie timer.
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COOKIE ACKNOWLEDGMENT - For a lost cookie-ack a receiver should
retransmit a cookie-ack to the peer.
ASCONF - For a lost ASCONF, the receiver SHOULD retransmit the
ASCONF restarting any timer associated with the ASCONF.
FORWARD TSN - For a lost forward TSN the endpoint SHOULD resend a
new forward TSN reporting the current value that the TSN should be
advanced to. Note this may not be the same information as that
contained in the dropped chunk.
Packet Drop - It is possible that a packet drop report sent by
this endpoint, may be lost. In such a case the receiver MAY wish
to keep a list of recent drop reports and resend this list to the
peer.
After queuing for retransmission any lost chunks, the sender MUST
also examine the bandwidth and queue fields taking into consideration
the source. If the M bit is set to '0' then the source of the drop
report was the SCTP peer. In such a case the receiver MUST
immediately adjust its peer rwnd by taking the value in the 'Maximum
Rwnd' field, subtracting the value of the 'data on queue' field and
any data in-flight.
If the sender is a middle box, M bit set to '1', the receiver MAY
adjust the cwnd for the source address of the drop packet by applying
the following algorithm if the current RTT of the link is larger than
the variable RTO.Large. A receiver of a packet drop report MUST NOT
adjust its cwnd if the RTT is or has ever been measured to be less
than or equal to RTO.Large.
Establish the True RTT using the values normally
used in calculating the RTO, set this value in
milliseconds into the variable 'rtt'.
rtt = (lastsa >> 2) + lastsv) >> 1;
Validate that an adjustment can be made.
if ((pd.chunk_flags & M_BIT) != M_BIT)
return /* make no adjustment */
if ( rtt < RTO.Large)
return /* make no adjustment */
Set 'bottle_bw' to the value found in the Link Bandwidth
field.
bottle_bw = ntohl(pd.bottle_bw);
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Set 'on_queue' to the value found in the size on data queue
field.
on_queue = ntohl(pd.current_onq);
Adjust the on_queue for any in-flight data that may yet
not have arrived at the bottle neck.
if(on_queue < flight_size) {
on_queue = flight_size;
}
Calculate the bandwidth available by multiplying the bottle_bw
variable times the rtt and dividing the result by a thousand.
Call this value 'bw_avail'.
bw_avail = (bottle_bw*rtt)/1000;
If more is 'on_queue' than the current value of 'bw_avail' a
negative congestion window adjustment is needed.
if (on_queue > bw_avail) {
Clear the partial bytes acked field.
partial_bytes_acked = 0;
Subtract the bw_avail from the current on_queue call this
value the 'decrease'.
decrease = on_queue - bw_avail;
Subtact the value in 'decrease' from the current congestion
window.
cwnd -= decrease;
If the current flight size is larger than this new congestion
window, set the congestion window to the current flight size.
if (flight_size > cwnd) {
cwnd = flight_size;
}
If the current congestion window is smaller than a
single MTU set the current congestion window to 1 MTU.
if (cwnd <= mtu) {
cwnd = mtu;
}
Set ssthresh to the current congestion window minus 1 byte.
ssthresh = cwnd - 1;
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} else {
Otherwise an increase is needed. Calculate the increase value
'incr' by taking the minimum of one fourth the bw_avail minus
the size on queue OR the MTU size times max burst (whichever
is smaller).
incr = min(((bw_avail - on_queue) >> 2),
((int)asoc.max_burst * (int)mtu));
Add this value to the current congestion window
cwnd += incr;
After making an increase to the congestion window verify that
the value of cwnd is smaller than or equal to the bw_avail if
not, set the cwnd to the value of bw_avail.
if (cwnd > bw_avail) {
cwnd = bw_avail;
}
}
6. Security Considerations
TBD
7. Recommended Variables
The following are the recommended values for variables defined within
this document:
RTO.Large - 500 Milliseconds.
8 Normative References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
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"Stream Control Transmission Protocol", RFC 2960, October 2000.
Authors' Addresses
Randall R. Stewart
Cisco Systems, Inc.
8735 West Higgins Road
Suite 300
Chicago, IL 60631
USA
Phone:
EMail: rrs@cisco.com
Peter Lei
Cisco Systems, Inc.
8735 West Higgins Road
Suite 300
Chicago, IL 60631
USA
Phone:
EMail: peterlei@cisco.com
Michael Tuexen
Univ. of Applied Sciences Muenster
Stegerwaldstr. 39
48565 Steinfurt
Germany
EMail: tuexen@fh-muenster.de
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