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Network Working Group                                         R. Stewart
Internet-Draft                                            Adara Networks
Intended status: Standards Track                                  P. Lei
Expires: July 19, 2014                               Cisco Systems, Inc.
                                                               M. Tuexen
                                      Univ. of Applied Sciences Muenster
                                                        January 15, 2014

   Stream Control Transmission Protocol (SCTP) Packet Drop Reporting


   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 19, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Architectural Considerations  . . . . . . . . . . . . . . . .   3
   4.  New Chunk Types . . . . . . . . . . . . . . . . . . . . . . .   4
     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  . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Receiver side . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Recommended Variables . . . . . . . . . . . . . . . . . . . .  12
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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 by any transport protocol.

   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

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   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

   they appear in this document, are to be interpreted as described in
   RFC2119 [RFC2119].

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

   There are two scenarios where a middle box may send Packet Drop

   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
   be located at the same physical device that also runs the SCTP stack
   or running on separate boxes providing a tunneling service.  The
   crucial point here is, that there is some protocol running between
   the middle box and the middle box peer.

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                   +------------+     +-----------------+
           +-------| 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

           +--------------------+ middle box +--------------------+
           |                    +------------+                    |
           |                                                      |
      +----+-----+                                          +-----+----+
      |   SCTP   |                                          |   SCTP   |
      | endpoint |                                          | endpoint |
      +----------+                                          +----------+

                                 Figure 2

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.

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        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 [RFC2960] 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=CTBM  |      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---------
      |  C    | 00001000  | This bit transforms the link
      |       |           | bandwidth and queue size fields
      |       |           | from a byte count to a packet
      |       |           | count. This will be set only
      |       |           | by middle boxes that cannot
      |       |           | determine byte counts.
      |  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

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   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 or any other IP option field (i.e. it is the
   size of the SCTP payload within the IP packet).  This value is 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

   A packet drop chunk MUST NOT be send in response to a packet
   containing an ABORT chunk or a packet drop chunk.

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 SCTP sender of the
   dropped 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

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   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-32c check.

   If the SCTP endhost detects a bad CRC-32c 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

   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

   1.   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).

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   2.   SACK - For a lost SACK chunk, a receiver MAY wish to send out a
        new SACK illustrating the current receiver conditions.

   3.   INIT - For a INIT chunk, the receiver SHOULD resend the INIT
        restarting its local T-1 timer.

   4.   HEARTBEAT REQUEST - For a heartbeat request, the receiver SHOULD
        resend a heartbeat to the source address of the packet.

   5.   SHUTDOWN - For a shutdown request, the SCTP receiver SHOULD
        resend the shutdown request.

   6.   SHUTDOWN ACKNOWLEDGEMENT - For a shutdown acknowledgement the
        receiver SHOULD resend the SHUTDOWN-ACK.

   7.   COOKIE ECHO - For a Cookie Echo the receiver SHOULD retransmit
        the lost COOKIE ECHO, restarting any cookie timer.

   8.   COOKIE ACKNOWLEDGMENT - For a lost cookie-ack a receiver should
        retransmit a cookie-ack to the peer.

   9.   ASCONF - For a lost ASCONF, the receiver SHOULD retransmit the
        ASCONF restarting any timer associated with the ASCONF.

   10.  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.

   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'.

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   rtt = (lastsa >> 2) + lastsv) >> 1;

   Validate that an adjustment can be made.

   if ((pd.chunk_flags AND M_BIT) != M_BIT)

   if ( rtt < RTO.Large)

   Set 'bottle_bw' to the value found in the Link Bandwidth

   bottle_bw = ntohl(pd.bottle_bw);

   Set 'on_queue' to the value found in the size on data queue

   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'.

       overrun = on_queue - bw_avail;

       Undo any congestion adjustment if a SACK has been processed.

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       if (seen_a_sack_this_packet)
            cwnd = prev_cwnd

       Calculate the portion of the onqueue data that is
       caused by this endpoints in-flight data.

       seg_inflight = flight_size / mtu
       seg_onqueue = on_queue / mtu
       my_portion = (overrun * seg_inflight)/seg_onqueue;

       If we have already adjusted the cwnd, indicated by the
       fact that the cwnd is larger than the flight size, we
       adjust our portion down by a smaller amount i.e the
       amount we have already adjusted it previously.

       if( cwnd > flight_size )
            adjust =  cwnd - flight_size;
        if( adjust > my_portion)
            my_portion = 0;
                    my_portion -= adjust

       Adjust the cwnd downward by our calculated amount.

       cwnd -= my_portion

       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;

   } 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

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       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


7.  Recommended Variables

   The following are the recommended values for variables defined within
   this document:

   RTO.Large - 500 Milliseconds.

8.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

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Authors' Addresses

   Randall R. Stewart
   Adara Networks
   2150 First Street
   San Jose, CA  29036

   Email: randall@lakerest.net

   Peter Lei
   Cisco Systems, Inc.
   8735 West Higgins Road
   Suite 300
   Chicago, IL  60631

   Email: peterlei@cisco.com

   Michael Tuexen
   Univ. of Applied Sciences Muenster
   Stegerwaldstr. 39
   48565 Steinfurt

   Email: tuexen@fh-muenster.de

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