Network Working Group Richard Price, Siemens/Roke Manor
INTERNET-DRAFT Robert Hancock, Siemens/Roke Manor
Expires: May 2002 Stephen McCann, Siemens/Roke Manor
Mark A West, Siemens/Roke Manor
Abigail Surtees, Siemens/Roke Manor
Paul Ollis, Siemens/Roke Manor
Qian Zhang, Microsoft Research Asia
Hongbin Liao, Microsoft Research Asia
Wenwu Zhu, Microsoft Research Asia
Ya-Qin Zhang, Microsoft Research Asia
21 November, 2001
TCP/IP Compression for ROHC
<draft-ietf-rohc-tcp-epic-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC-2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document is a submission to the IETF ROHC WG. Comments should be
directed to the mailing list of ROHC, rohc@cdt.luth.se.
Abstract
This draft describes a ROHC profile for the robust compression of
TCP/IP.
The RObust Header Compression [ROHC] scheme is designed to compress
packet headers over error prone channels. It is built around an
extensible core framework that can be tailored to compress new
protocol stacks by adding additional ROHC profiles.
The new profile for TCP/IP compression is provided by the Efficient
Protocol Independent Compression (EPIC-LITE) scheme.
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Table of contents
Status of this Memo................................................1
Abstract...........................................................1
1. Introduction...................................................2
3. ROHC Profile for compression of TCP/IP.........................3
4. The concept and framework of TAROC-C...........................5
4.1. TCP congestion window tracking.............................7
4.2. Compressor/decompressor state machine with TAROC-C........11
4.3. Compressor logic in TAROC-C...............................12
4.4. Decompressor logic in TAROC-C.............................14
4.5. Modes of operation........................................15
4.6. Implementation issues.....................................17
4.7. Performance of TAROC-C....................................17
5. Security considerations.......................................18
6. Acknowledgements..............................................18
7. References....................................................18
Appendix A. Packet types provided by ROHC framework..............21
A.1. CO packet................................................21
A.2. IR-DYN packet............................................22
A.3. IR packet................................................22
1. Introduction
This document describes a method for compressing TCP/IP headers
within the [ROHC] framework.
The new profile for TCP/IP compression is provided by the Efficient
Protocol Independent Compression (EPIC) scheme. EPIC takes as its
input a BNF description of the protocol stack to be compressed, and
derives a set of packet formats that can be used to quickly and
efficiently compress and decompress headers.
A TCP-Aware RObust Header Compression Control scheme, TAROC-C, is
also introduced in this draft. The key point of TAROC-C is the TCP
congestion window tracking mechanism, which can be used to improve
the efficiency of the window-based encoding and the performance of
the overall header compression scheme without sacrificing the
robustness. With the dynamic congestion window tracking, our scheme
can achieve good performance even when the feedback channel is not
available.
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2. Terminology
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].
3. ROHC Profile for compression of TCP/IP
This chapter describes a simple ROHC profile for the compression of
TCP/IP.
Note that the current TCP/IP profile is designed specifically to test
implementations of [EPIC]. The profile is not designed to compress
TCP/IP with a high level of efficiency.
The profile supports all TCP options (it does not compress the
options, but instead passes them through transparently as part of the
payload).
The profile for TCP/IP compression is given below:
profile_identifier 0xFFFF
max_formats 200
max_sets 1
bit_alignment 8
npatterns 224
CO_packet TCP-IP
; The profile identifier is a placeholder.
; The IR-DYN_packet and IR_packet parameters are not specified. This
; means that the IR-DYN and IR packets are generated using the same
; encoding method "TCP/IP" as for the CO packets.
; The encoding methods used by the TCP/IP profile are given below:
TCP-IP = IPv4-header
TCP-header
msn
msn = C(MSN-LSB(4,-1,90%)) | C(MSN-LSB(7,-1,9%)) |
MSN-IRREGULAR(16,1%)
IPv4-header = version
header_len
tos
ecn
length
ip-id
rf_flag
df_flag
mf_flag
offset
ttl
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protocol
ip_chksum
src_address
dst_address
version = STATIC-KNOWN(4,4)
header_len = STATIC-KNOWN(4,5)
tos = C(STATIC(99%)) | IRREGULAR(6,1%)
ecn = IRREGULAR(2,100%)
length = IRREGULAR(16)
ip-id = C(LSB(4,-1,90%)) | C(LSB(6,-1,8%)) |
C(LSB(8,-1,1%)) | IRREGULAR(16,1%)
rf_flag = VALUE(1,0,100%)
df_flag = IRREGULAR(1,100%)
mf_flag = VALUE(1,0,99%) | VALUE(1,1,1%)
offset = C(STATIC(99%)) | IRREGULAR(13,1%)
ttl = C(STATIC(99%)) | IRREGULAR(8,1%)
protocol = STATIC-KNOWN(8,6)
ip_chksum = IRREGULAR(16,100%)
src_address = STATIC-UNKNOWN(32)
dst_address = STATIC-UNKNOWN(32)
TCP-header = source_port
dest_port
seqno
ackno
data_offset
flags
window
tcp_chksum
urg_ptr
source_port = STATIC-UNKNOWN(16)
dest_port = STATIC-UNKNOWN(16)
seqno = C(LSB(8,63,80%)) | C(LSB(14,127,10%)) |
C(LSB(20,1023,5%)) | IRREGULAR(32,5%)
ackno = C(LSB(8,-1,80%)) | C(LSB(14,-1,10%)) |
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C(LSB(20,-1,5%)) | IRREGULAR(32,5%)
data_offset = IRREGULAR(4,100%)
window = C(STATIC(80%)) | C(LSB(12,63,10%)) |
IRREGULAR(16,10%)
tcp_chksum = IRREGULAR(16,100%)
urg_ptr = C(STATIC(99%)) | IRREGULAR(16,1%)
flags = reserved
cwr
ece
urg
ack
psh
rst
syn
fin
reserved = C(STATIC(90%)) | IRREGULAR(4,10%)
cwr = VALUE(1,0,80%) | VALUE(1,1,20%)
ece = VALUE(1,0,80%) | VALUE(1,1,20%)
urg = VALUE(1,0,99%) | VALUE(1,1,1%)
ack = VALUE(1,1,99%) | VALUE(1,0,1%)
psh = IRREGULAR(1,100%)
rst = VALUE(1,0,99%) | VALUE(1,1,1%)
syn = VALUE(1,0,99%) | VALUE(1,1,1%)
fin = VALUE(1,0,95%) | VALUE(1,1,5%)
4. The concept and framework of TAROC-C
This section first describes the concept of the TCP-aware robust
header compression control (TAROC-C) mechanism and then discusses how
this concept leads to a better performance when used over unreliable
links.
To design suitable mechanisms for efficient compression of all TCP/IP
header fields, it would be important to analyze their change patterns
first. It is known that the change patterns of several TCP fields
(for example, Sequence Number, Acknowledgement Number, Window, etc.)
are completely different from the ones of RTP, which had already
discussed in detail in [ROHC], and are very hard to predict. Thus, it
is hard to encode these fields with k-LSB both efficiently and
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robustly. On the other hand, Window-based LSB encoding [ROHC], which
does not assume the linear changing pattern of the target header
fields, is more suitable to encode those TCP fields both efficiently
and robustly.
The main idea of TAROC-C, the control mechanism of TAROC, is the
combination of the Window-based LSB encoding (W-LSB encoding) and
dynamically TCP congestion window tracking. In W-LSB encoding, a
sliding window (VSW), which equals to value r mentioned in Section
6.4, is maintained on the compressor side. The compressor gets
inconsistent with the decompressor only when the reference value on
the decompressor side is out of this VSW. By keeping the sliding
window large enough, the compressor rarely gets out of
synchronization with the decompressor.
However, the larger the sliding window is, the less the header
compression gains. To shrink the window size, the compressor needs
some form of feedback to get sufficient confidence that a certain
value will not be used as a reference by the decompressor. Then the
window can be advanced by removing that value and all other values
older than it. Obviously, when a feedback channel is available,
confidence can be achieved by proactive feedback in the form of ACKs
from the decompressor. A feedback channel, however, is unavailable or
expensive in some environments. In this Internet draft, a mechanism
based on dynamically tracking TCP congestion window is proposed to
explore such feedbacks from the nature feedback-loop of TCP protocol
itself.
Since TCP is a window-based protocol, a new segment cannot be
transmitted without getting the acknowledgment of segment in the
previous window. Upon receiving the new segment, the compressor can
get enough confidence that the decompressor has received the segment
in the previous window and then shrink the sliding window by removing
all the values older than that segment.
As originally outlined in [CONG1] and specified in [CONG2], TCP is
incorporated with four congestion control algorithms: slow-start,
congestion-avoidance, fast retransmit, and fast recovery. The
effective window of TCP is mainly controlled by the congestion window
and may change during the entire connection life. TAROC-C designs a
mechanism to track the dynamics of TCP congestion window, and control
the sliding window of W-LSB encoding by the estimated congestion
window. By combining the W-LSB encoding and TCP congestion window
tracking, TAROC can achieve better performance over high bit-error-
rate links.
Note that in one-way TCP traffic, only the information about sequence
number or acknowledgment number is available for tracking TCP
congestion window. TAROC-C does not require that all one-way TCP
traffics must cross the same compressor. The detail will be described
in the following sections.
The TAROC scheme achieves its compression gain by establishing state
information at both ends of the link, i.e., at the compressor and at
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the decompressor. Header compression with TAROC can be characterized
as an interaction between two state machines, one compressor machine
and one decompressor machine, each instantiated once per context.
In the rest of this session, the TCP congestion window tracking
algorithm, the state machines in the TCP/IP header compression
framework, and the logics of the compressor/decompressor, are
described in detail.
4.1. TCP congestion window tracking
4.1.1. General principle of congestion window tracking
The general principle of congestion window tracking is as follows.
The compressor imitates the congestion control behavior of TCP upon
receiving each segment, in the meantime, estimates the congestion
window (cwnd) and the slow start threshold (ssthresh). Besides the
requirement of accuracy, there are also some other requirements for
the congestion window tracking algorithms:
- Simplex link. The tracking algorithm SHOULD always only take
Sequence Number or Acknowledgment Number of a one-way TCP
traffic into consideration. It SHOULD NOT use Sequence Number
and Acknowledgment Number of that traffic simultaneously.
- Misordering resilience. The tracking algorithm SHOULD work
well while receiving misordered segments.
- Multiple-links. The tracking algorithm SHOULD work well when
not all the one-way TCP traffics are crossing the same link.
- Slightly overestimation. If the tracking algorithm cannot
guarantee the accuracy of the estimated cwnd and ssthresh, it is
RECOMMANDED that it produces a slightly overestimated one.
The following sections will describe two congestion window tracking
algorithms, which use Sequence Number and Acknowledgment Number of a
one-way TCP traffic, respectively.
4.1.2. Congestion window tracking based on Sequence Number
This algorithm (Algorithm SEQ) contains 3 states: SLOW-START,
CONGESTION-AVOIDANCE, and FAST-RECOVERY, which are equivalent to the
states in TCP congestion control algorithms. It maintains 2 variables:
cwnd and ssthresh.
+-------------+
| |
---------------->| CONGESTION- |
| | AVOIDANCE |
| ----| |<---
+------------+ | +-------------+ |
| | | |
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| SLOW-START | | |
| | | +-------------+ |
+------------+ | | | |
| |-->| FAST- |----
| | RECOVERY |
---------------->| |
+-------------+
Initially, this algorithm starts in state SLOW-START with ssthresh
set to ISSTHRESH and cwnd set to IW.
Upon receiving a segment, if it is the first segment, which is not
necessary to be the SYN segment, the algorithm sets the current
maximum Sequence Number (CMAXSN) and the current minimum Sequence
Number (CMINSN) to this segment's sequence number; otherwise, the
algorithm takes a procedure according to the current state.
- SLOW-START
* If the new Sequence Number (NSN) is larger than CMAXSN,
increase cwnd by the distance between NSN and CMAXSN, and
update CMAXSN and CMINSN based on the following rules:
CMAXSN = NSN
if (CMAXSN - CMINSN) > cwnd)
CMINSN = cwnd - CMAXSN;
If the cwnd is larger than ssthresh, the algorithm transits to
CONGESTION-AVOIDANCE state;
* If the distance between NSN and CMAXSN is less than or equal
to 3*MSS, ignore it;
* If the distance is larger than 3*MSS, halve the cwnd and set
ssthresh to MAX(cwnd, 2*MSS). After that, the algorithm
transits into FAST-RECOVERY state.
- CONGESTION-AVOIDANCE
* If NSN is larger than CMAXSN, increase cwnd by ((NSN-
CMAXSN)*MSS)/cwnd and then update CMAXSN and CMINSN based on
the following rules:
CMAXSN = NSN
if (CMAXSN - CMINSN) > cwnd)
CMINSN = cwnd - CMAXSN;
* If the distance between NSN and CMAXSN is less than or equal
to 3*MSS, ignore it;
* If the distance is larger than 3*MSS, halve the cwnd and set
ssthresh to MAX(cwnd, 2*MSS). After that, the algorithm
transits into FAST-RECOVERY state.
- FAST-RECOVERY
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* If NSN is larger than or equal to CMAXSN + cwnd, the algorithm
transits into CONGESTION-AVOIDANCE state;
* Otherwise, ignore it.
In this algorithm, MSS is denoted as the estimated maximum segment
size. The implementation can use the MTU of the link as an
approximation of this value. ISSHRESH and IW are the initial values
of ssthresh and cwnd, respectively. ISSTHRESH MAY be arbitrarily high.
IW SHOULD be set to 4*MSS.
4.1.3. Congestion window tracking based on Acknowledgment Number
This algorithm (Algorithm ACK) maintains 3 states: SLOW-START,
CONGESTION-AVOIDANCE and FAST-RECOVERY, which are equivalent to the
states in TCP congestion control algorithms. It also maintains 2
variables: cwnd and ssthresh.
+-------------+
| |
---------------->| CONGESTION- |
| | AVOIDANCE |
| ----| |<---
+------------+ | +-------------+ |
| | | |
| SLOW-START | | |
| | | +-------------+ |
+------------+ | | | |
| |-->| FAST- |----
| | RECOVERY |
---------------->| |
+-------------+
Initially, this algorithm starts in state SLOW-START with ssthresh
set to ISSTHRESH and cwnd set to IW.
Upon receiving a segment, if it is the first segment, which is not
necessary to be the SYN segment, the algorithm sets the current
maximum Acknowledgment Number (CMAXACK) to this segment's
acknowledgment number; otherwise, the algorithm takes a procedure
according to the current state.
- SLOW-START
* If the new Acknowledgment Number (NEWACK) is larger than
CMAXACK, increase cwnd by the distance between NEWACK and
CMAXACK, set duplicate ack counter (NDUPACKS) to 0, and update
CMAXACK accordingly; If the cwnd is larger than ssthresh, the
algorithm transits to CONGESTION-AVOIDANCE state;
* If NEWACK is equal to CMAXACK, increase the NDUPACKS by 1. If
NDUPACKS is greater than 3, halve the cwnd and set ssthresh to
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MAX(cwnd, 2*MSS). Consequently, the algorithm transits into
FAST-RECOVERY state;
* Otherwise, set NDUPACKS to 0.
- CONGESTION-AVOIDANCE
* If NEWACK is larger than CMAXACK, increase cwnd by ((NEWACK-
CMAXACK)*MSS)/cwnd, set NDUPACKS to 0 and update CMAXACK
accordingly;
* If NEWACK is equal to CMAXACK, increase NDUPACKS by 1. If
NDUPACKS is greater than 3, halve the cwnd and set ssthresh to
MAX(cwnd, 2*MSS). After that, the algorithm transits into
FAST-RECOVERY state;
* Otherwise, set NDUPACKS to 0.
- FAST-RECOVERY
* If NEWACK is larger than CMAXACK, set NDUPACKS to 0.
Consequently, the algorithm transits into CONGESTION-AVOID
state;
* Otherwise, ignore it.
In this algorithm, MSS is denoted as the estimated maximum segment
size. The implementation can use the MTU of the link as an
approximation of this value. ISSHRESH and IW are the initial values
of ssthresh and cwnd, respectively. ISSTHRESH MAY be arbitrarily high.
IW SHOULD be set to 4*MSS.
4.1.4. Further discussion on congestion window tracking
In some cases, it is inevitable for the tracking algorithms to
overestimate the TCP congestion window. Also, it SHOULD be avoided
that the estimated congestion window gets significantly smaller that
the actual one. For all of these cases, TAROC simply applies two
boundaries on the estimated congestion window size. One of the two
boundaries is the MIN boundary, which is the minimum congestion
window size and whose value is determined according to the [INITWIN];
the other boundary is the MAX boundary, which is the maximum
congestion window size. There are two possible approaches to setting
this MAX boundary. One is to select a commonly used maximum TCP
socket buffer size. The other one is to use the simple equation
W=sqrt(8/3*l), where W is the maximum window size and l is the
typical packet loss rate.
If ECN mechanism is deployed, according to [RFC-2481] and [ECN], the
TCP sender will set the CWR (Congestion Window Reduced) flag in the
TCP header of the first new data packet sent after the window
reduction, and the TCP receiver will reset the ECN-Echo flag back to
0 after receiving a packet with CWR flag set. Thus, the CWR flag and
the ECN-Echo flag's transition from 1 to 0 can be used as another
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indication of congestion combined with other mechanisms mentioned in
the tracking algorithm.
4.2. Compressor/decompressor state machine with TAROC-C
4.2.1. Compressor states
There are three compressor states in TAROC: Initialization and
Refresh (IR) state, First Order (FO), and Second Order (SO) states.
The compressor starts in the lowest compression state (IR) and
transits gradually to the higher compression state. The compressor
will always operate in the highest possible compression state, under
the constraint that the compressor is sufficiently confident that the
decompressor has the information necessary to decompress a header,
which is compressed according to the state.
+----------+
| |
+----------+ | FO State | +----------+
| | <--------> | | <--------> | |
| IR State | +----------+ | SO State |
| | <----------------------------------> | |
+----------+ +----------+
4.2.1.1. Initialization and Refresh (IR) state
The purpose of IR state is to initialize or refresh the static parts
of the context at the decompressor. In this state, the compressor
sends full header periodically with an exponentially increasing
period, which is so-called compression slow-start [RFC-2507]. The
compressor leaves the IR state only when it is confident that the
decompressor has correctly received the static information.
To compress short-lived TCP transfers more efficiently, the
compressor should speed up the initial process. The compressor enters
the IR state when it receives the packet with SYN bit set and sends
IR packet. When it receives the first data packet of the transfer, it
should transit to FO state because that means the decompressor has
received the packet with SYN bit set and established the context
successfully at its side. Using this mechanism can significantly
reduce the number of context initiation headers.
4.2.1.2. First Order (FO) State
The purpose of FO state is to efficiently transmit the difference
between the two consecutive packets in the TCP stream. When operating
in this state, the compressor and the decompressor should have the
same context. Only compressed packet is transmitted from the
compressor to the decompressor in this state. The compressor transits
back to IR state only when it finds that the context of decompressor
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may be inconsistent, or there are remarkable changes in the TCP/IP
header.
4.2.1.3. Second Order (SO) State
The purpose of SO state is to efficiently transmit the fixed-payload
data. The compressor enters this state when it is sufficiently
confident that the decompressor has got the constant payload size of
the data transferring.
The compressor leaves this state and transits to the FO state when
the current payload size no longer conforms to the constant payload.
The compressor transits back to IR state only when it finds that the
context of decompressor may be inconsistent, or there are remarkable
changes in the TCP/IP header.
4.2.2. Decompressor states
The decompressor starts in its lowest compression state, "No Context"
and gradually transits to higher state, "Full Context". The
decompressor state machine normally never leaves the "Full Context"
state once it has entered this state.
+--------------+ +--------------+
| No Context | <---> | Full Context |
+--------------+ +--------------+
4.3. Compressor logic in TAROC-C
In TAROC-C, the compressor will start in the IR state and perform
different logics in different states. The following sub-sections will
describe the logic for each compressor sate in detail.
4.3.1. IR state
The operations of compressor in IR state can be summarized as follows:
a) Upon receiving a packet, the compressor sends IR or IR-DYN packet
on the following conditions: 1) if it is the turn to send full
header packet according to compression slow-start, i.e. after
sending F_PERIOD compressed packets; 2) if the packet to be sent
is a retransmission of the packet in VSW and it was sent as IR or
IR-DYN packet previously. Otherwise, the compressor compresses
the packet using W-LSB encoding. If the compressor enters the IR
state for the first time or the static part of the TCP flow has
changed, it will send IR packet. Otherwise, it will send IR-DYN
packet because the decompressor has known the static part.
b) The packet is added into VSW as a potential reference after it
has been sent out. The compressor then invokes the Algorithm SEQ
and Algorithm ACK to track the congestion windows of the two one-
way traffics with different directions in a TCP connection.
Suppose that the estimated congestion windows are cwnd_seq and
cwnd_ack, while the estimated slow start thresholds are
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ssthresh_seq and ssthresh_ack, respectively. Let W(cwnd_seq,
ssthresh_seq, cwnd_ack, ssthresh_ack) = K*MAX(MAX(cwnd_seq,
2*ssthresh_seq), MAX(cwnd_ack, 2*ssthresh_ack)). If the size of
VSW is larger than W(cwnd_seq, ssthresh_seq, cwnd_ack,
ssthresh_ack), the VSW can be shrunk. K is an implementation
parameter that will be further discussed in Section 5.6.
c) After sending F_PERIOD compressed packets, F_PERIOD SHOULD be
doubled. If it gets larger than W(cwnd_seq, ssthresh_seq,
cwnd_ack, ssthresh_ack), the compressor transits to FO or SO
state. If the compressor finds that the payload size of
consecutive packets is a constant value and one of such packets
is removed from the VSW, which means the decompressor has known
the exact value of the constant size, it may transit to SO state.
Otherwise it will transit to the FO state.
4.3.2. FO state
The operations of the compressor in the FO state can be summarized as
follows:
a) Upon receiving a packet, if it falls behind the VSW, i.e. it is
older than all the packets in VSW; the compressor transits to IR
state. Otherwise, the compressor compresses it using W-LSB encoding
and sends it.
b) The packet is added into VSW as a potential reference after it has
been sent out. The compressor then invokes the Algorithm SEQ and
Algorithm ACK to track the congestion windows of the two one-way
traffics with different directions in a TCP connection. Suppose
that the estimated congestion windows are cwnd_seq and cwnd_ack,
while the estimated slow start thresholds are ssthresh_seq and
ssthresh_ack, respectively. Let W(cwnd_seq, ssthresh_seq, cwnd_ack,
ssthresh_ack) = K*MAX(MAX(cwnd_seq, 2*ssthresh_seq), MAX(cwnd_ack,
2*ssthresh_ack)). If the size of VSW is larger than W(cwnd_seq,
ssthresh_seq, cwnd_ack, ssthresh_ack), the VSW can be shrunk. K is
also an implementation parameter, which can be set to the same
value as in the IR state.
c) If the VSW contains only one packet, which means there is a long
jump in the packet sequence number or acknowledge number, the
compressor will transit to the IR state and re-initialize the
algorithm for tracking TCP congestion window. Here, a segment
causes a long jump when the distance between its sequence number
(or acknowledgment number) and CMAXSN (or CMAXACK) is larger than
the estimated congestion window size, i.e.,
|sequence number (acknowledgement number) û CMAXSN (CMAXACK)| >
estimated congestion window size.
d) If the compressor finds that the payload size of consecutive
packets is a constant value and one of such packets has been
removed from the VSW, which means the decompressor has known the
exact value of the constant size, it may transit to the SO state.
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e) If the static context of transfers changed, the compressor will
transit to the IR state and re-initialize the algorithms for
tracking TCP congestion window.
4.3.3. SO state
The operations of the compressor in the SO state can be summarized as
follows:
a) Upon receiving a packet, if it falls behind the VSW, i.e. it is
older than all the packets in VSW; the compressor transits to IR
state. Otherwise, the compressor compresses it using fixed-payload
encoding and sends it.
b) The packet is added into VSW as a potential reference after it has
been sent out. The compressor then invokes the Algorithm SEQ and
Algorithm ACK to track the congestion windows of the two one-way
traffics with different directions in a TCP connection. Suppose
that the estimated congestion windows are cwnd_seq and cwnd_ack,
while the estimated slow start thresholds are ssthresh_seq and
ssthresh_ack, respectively. Let W(cwnd_seq, ssthresh_seq, cwnd_ack,
ssthresh_ack) = K*MAX(MAX(cwnd_seq, 2*ssthresh_seq), MAX(cwnd_ack,
2*ssthresh_ack)). If the size of VSW is larger than W(cwnd_seq,
ssthresh_seq, cwnd_ack, ssthresh_ack), the VSW can be shrunk. K is
an implementation parameter, which can be set to the same value as
in the IR state.
c) If the VSW contains only one packet, which means there is a long
jump in the packet sequence number or acknowledge number, the
compressor will transit to the IR state and re-initialize the
algorithms for tracking TCP congestion window.
d) If the payload size of the packets in VSW doesn't keep constant,
the compressor transits to the FO state.
e) If the static context of transfers changed, the compressor will
transit to the IR state and re-initialize the algorithms for
tracking TCP congestion window.
4.4. Decompressor logic in TAROC-C
The logic of the decompressor is simpler compared to the compressor.
4.4.1. No Context State
The decompressor starts in this state. Upon receiving an IR or IR-DYN
packet, the decompressor should verify the correctness of its header
by TCP checksum. If the verification succeeds, the decompressor will
update the context and use this packet as the reference packet. After
that, the decompressor will pass it to the system's network layer and
transit to Full Context State. Conformed to ROHC framework [ROHC],
only IR or IR-DYN packets may be decompressed in No Context state.
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4.4.2. Full Context State
The operations of decompressor in Full Context state can be
summarized as follows:
a) Upon receiving an IR or IR-DYN packet, the decompressor should
verify the correctness of its header by TCP checksum. If the
verification succeeds, the decompressor will update the context and
use this packet as the reference packet. Consequently, the
decompressor will convert the packet into the original packet and
pass it to the network layer of the system.
b) Upon receiving the other type of packet, the decompressor will
decompress it. After that, the decompressor MUST verify the
correctness of the decompressed packet by the TCP checksum. If the
verification succeeds, the decompressor passes it to the system's
network layer. Then the decompressor will use it as the reference
value if this packet is not older than the current reference packet.
c) If consequent N packets fail to be decompressed, the decompressor
should transit downwards to No Context State. N is an implementation
parameter that will be further discussed in Section 8.6.
4.5. Modes of operation
There are three modes in ROHC framework, called Unidirectional, Bi-
directional Optimistic, and Bi-directional Reliable mode,
respectively. The mode transitions are conformed to ROHC framework.
However, the operations of each mode are different.
4.5.1. Unidirectional mode -- U-mode
When in U-mode, packets are sent in one direction only: from
compressor to decompressor. Therefore, feedbacks from decompressor to
the compressor are unavailable under this mode.
In the U-mode, the compressor and decompressor logic is the same as
the discussion in section 8.3 and 8.4.
4.5.2. Bi-directional Optimistic mode -- O-mode
When in O-mode, a feedback channel is used to send error recovery
requests and (optionally) acknowledgments of significant context
updates from the decompressor to the compressor. In this mode, the
VSW will be shrunk more efficiently.
4.5.2.1. Compressor states and logic (O-mode)
Following rules should be combined with the action defined in section
8.3.
In the IR state, the compressor can transit to the FO or SO state
once it receives a valid ACK(O) for an IR packet sent (an ACK(O) can
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INTERNET-DRAFT TCP/IP Compression for ROHC 21 November, 2001
only be valid if it refers to a packet sent earlier). If the packet
referred by the feedback is in the VSW, the compressor will remove
the packets older than the referred packet from the VSW window.
Because ACK(O) means that the packet referred by ACK(O) has been the
reference of the decompressor, the compressor doesn't need to keep
older packets.
If the compressor is in the FO or SO state, it will remove the
packets older than the referred packet from the VSW window.
Upon receiving an NACK(O), the compressor transits back to IR state.
4.5.2.2. Decompressor states and logic (O-mode)
The decompression states and the state transition logic are the same
as in the Unidirectional case (see section 8.5.1.). What differs is
the feedback logic.
Below, rules are defined stating which feedback to use when.
When an IR packet passes the verification, send an ACK(O). When an
IR-DYN packet or other packet is correctly decompressed, optionally
send an ACK(O). When any packet fails the verification, send an
NACK(O).
4.5.3. Bi-directional Reliable mode -- R-mode
The R-mode are a more intensive usage of the feedback channel and a
stricter logic at both the compressor and the decompressor that
prevents loss of context synchronization between the compressor and
decompressor except for very high residual bit error rates. Feedback
is sent to acknowledge all context updates. In this mode, the VSW
will be shrunk with the highest efficiency.
4.5.3.1. Compressor states and logic (R-mode)
Following rules should be reparation to the action defined in section
8.3.
In IR state, the compressor should transit to the FO or SO state only
when it receives a valid ACK(R) for an IR or IR-DYN packet sent (an
ACK(R) can only be valid if it refers to an packet sent earlier). If
the packet referred by the feedback is in the VSW, the compressor
will remove the packets older than the referred packet from the VSW
window. Because ACK(R) means that the packet referred by ACK(R) has
been the reference of the decompressor; the compressor doesn't need
to keep older packets.
If the compressor is in the FO or SO state, when it receives a valid
ACK(R), it will remove the packets older than the referred packet
from the VSW window. In this mode, the compressor need not use window
tracking, because feedback can shrink VSW efficiently and robustly.
Upon receiving an NACK(O), the compressor transits back to IR state.
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4.5.3.2. Decompressor states and logic (R-mode)
Below, rules are defined stating which feedback to use when.
@When a packet is correctly decompressed and updates the context,
send an ACK(R).
@When any packet fails the verification, send a NACK(R).
The frequency of updating context will be discussed in section 8.6.
4.6. Implementation issues
4.6.1. Determine the value K
As mentioned above, the VSW SHOULD be shrunk when its size gets
larger than K*MAX(MAX(cwnd_seq, 2*ssthresh_seq), MAX(cwnd_ack,
2*ssthresh_ack)). Since the Fast Recovery algorithm was introduced in
TCP, several TCP variants had been proposed, which are different only
in the behaviors of Fast Recovery. Some of them need several RTTs to
be recovered from multiple losses in a window. Ideally, they may send
L*W/2 packets in this stage, where L is the number of lost packets
and W is the size of the congestion window where error occurs. Some
recent work [TCPREQ] on improving TCP performance allows to transmit
packets even when receiving duplicate acknowledgments. Due to the
above concerns, it'd better keep K large enough so as to prevent
shrinking VSW without enough confidence that corresponding packets
had been successfully received.
Considering the bandwidth-limited environments and the limited
receiver buffer, a practical range of K is around 1~2. From the
simulation results, K=1 is good enough for most cases.
4.6.2. Determine the value N
We should distinguish out of synchronization from the packet errors
cause by the link. So considering the error condition of the link, N
should be higher than the packet burst error length, a practical
range of N is around 8~10.
4.6.3. Determine the frequency of updating context
The choice of the frequency of updating context, ACK(R), is a balance
between the efficiency and robustness, i.e. sending ACK(R) more
frequently improves the compression robustness but adds more system
overhead, and the vice versa. From a practical view, the ACK(R)
SHOULD be sent for every 4~8 successfully decompressed packets.
4.7. Performance of TAROC-C
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The Simulations results (see Appendix B in [TAROC-3]) demonstrate the
effectiveness of control mechanism TAROC-C and corresponding header
compression scheme.
5. Security considerations
EPIC-LITE generates compressed header formats for direct use in ROHC
profiles. Consequently the security considerations for EPIC-LITE
match those of [ROHC].
6. Acknowledgements
Header compression schemes from [ROHC] have been important sources of
ideas and knowledge. Basic Huffman encoding [HUFF] was enhanced for
the specific tasks of robust, efficient header compression.
Thanks to
Carsten Bormann (cabo@tzi.org)
Christian Schmidt (christian.schmidt@icn.siemens.de)
Max Riegel (maximilian.riegel@icn.siemens.de)
for valuable input and review.
7. References
[ROHC] "RObust Header Compression (ROHC)", Carsten Bormann et
al, RFC3095, Internet Engineering Task Force, July 2001
[EPIC] "Framework for EPIC-LITE", Richard Price et al,
<draft-ietf-rohc-epic-lite-00.txt>, Internet
Engineering Task Force, October 23, 2001
[HUFF] "The Data Compression Book", Mark Nelson and Jean-Loup
Gailly, M&T Books, 1995
[RFC-1144] "Compressing TCP/IP Headers for Low-Speed Serial
Links", V. Jacobson, Internet Engineering Task Force,
February 1990
[RFC-1951] "DEFLATE Compressed Data Format Specification version
1.3", P. Deutsch, Internet Engineering Task Force, May
1996
[RFC-2026] "The Internet Standards Process - Revision 3", Scott
Bradner, Internet Engineering Task Force, October 1996
[RFC-2119] "Key words for use in RFCs to Indicate Requirement
Levels", Scott Bradner, Internet Engineering Task
Force, March 1997
Price et al. [PAGE 18]
INTERNET-DRAFT TCP/IP Compression for ROHC 21 November, 2001
[RFC-2507] M. Degermark, B. Nordgren, and S. Pink, "IP Header
Compression", Internet Engineering Task Force, February
1999
[CONG1] "Congestion avoidance and control", V. Jacobson, In ACM
SIGCOMM '88, 1988
[CONG2] "TCP Congestion Control", M. Allman, V. Paxson, and W.
R. Stevens, RFC 2581, April 1999
[RFC-2481] "A Proposal to add Explicit Congestion Notification
(ECN) to IP", K. Ramakrishnan, S. Floyd, Internet
Engineering Task Force, January 1999
[ECN] "The Addition of Explicit Congestion Notification (ECN)
to IP", K. K. RamaKrishnan, Sally Floyd, D. Black,
Internet Draft (work in progress), June, 2001. <draft-
ietf-tsvwg-ecn-04.txt>
[TCPREQ] "Requirements for ROHC IP/TCP header compression", L-E.
Jonsson, Internet Draft (work in progress), June 20,
2001
[INITWIN] "Increasing TCP's Initial Window", M. Allman, S. Floyd,
and C. Partridge, Internet Draft (work in progress),
May 2001. <draft-ietf-tsvwg-initwin-00.txt>
[TAROC-4] H. Liao, Q. Zhang, W. Zhu, and Y.-Q. Zhang, ôTCP-Aware
RObust Header Compression (TAROC)ö, Internet Draft
(work in progress), Nov. 2001. <draft-ietf-rohc-taroc-
04.txt>
8. Authors' addresses
Richard Price Tel: +44 1794 833681
Email: richard.price@roke.co.uk
Robert Hancock Tel: +44 1794 833601
Email: robert.hancock@roke.co.uk
Stephen McCann Tel: +44 1794 833341
Email: stephen.mccann@roke.co.uk
Mark A West Tel: +44 1794 833311
Email: mark.a.west@roke.co.uk
Abigail Surtees Tel: +44 1794 833131
Email: abigail.surtees@roke.co.uk
Paul Ollis Tel: +44 1794 833168
Email: paul.ollis@roke.co.uk
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
Price et al. [PAGE 19]
INTERNET-DRAFT TCP/IP Compression for ROHC 21 November, 2001
United Kingdom
Qian Zhang Tel: +86 10 62617711-3135
Email: qianz@microsoft.com
HongBin Liao Tel: +86 10 62617711-3156
Email: i-hbliao@microsoft.com
Wenwu Zhu Tel: +86 10 62617711-5405
Email: wwzhu@microsoft.com
Ya-Qin Zhang Tel: +86 10 62617711
Email: yzhang@microsoft.com
Microsoft Research Asia
Beijing Sigma Center
No.49, Zhichun Road, Haidian District
Beijing 100080, P.R.C.
Price et al. [PAGE 20]
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Appendix A. Packet types provided by ROHC framework
In addition to the standard CO (compressed) packets, the [ROHC]
framework contains two special packet types designed to help
synchronize the context at the compressor and decompressor. An IR
(Initialization and Refresh) packet associates a context with a
certain ROHC profile, and transmits the value of all fields including
those which remain constant throughout the lifetime of the context.
An IR-DYN (Dynamic Initialization and Refresh) packet associates a
context with a ROHC profile, and additionally transmits the value of
any fields except those that remain constant for the lifetime of the
context. An IR-DYN packet cannot be used to completely initialize a
new context, but it is usually smaller than a full IR packet.
[ROHC] also defines a general compressed packet that can be used to
encapsulate CO, IR and IR-DYN packets. The general packet format
includes a CID (Context Identifier) to indicate the context to which
the compressed packet belongs. It also includes a packet type
indicator to specify whether the packet is a feedback, initialization
or general compressed packet, whether it is segmented, and whether it
contains padding.
The following packet type indicators are reserved in the overall
[ROHC] framework:
1110: Padding or Add-CID octet
11110: Feedback
11111000: IR-DYN packet
1111110: IR packet
1111111: Segment
Any packet types not indicated by the bit pattern 111XXXXX can be
used by individual [ROHC] profiles such as the TCP/IP profile.
A.1. CO packet
The compressed (CO) packet type is the basic compressed packet
offered by EPIC-LITE. CO packets can be used to transmit data between
the compressor and decompressor with a high level of efficiency, and
can cope with most irregularities in the packet stream.
The location of an EPIC-LITE CO packet within the general ROHC packet
is shown below:
0 7
--- --- --- --- --- --- --- ---
| Add-CID octet | if for CID 1-15 and small CIDs
+---+--- --- --- ---+--- --- ---+
| EPIC-LITE CO packet | 1 octet
+---+--- ---+---+---+--- --- ---+
| |
/ 0, 1, or 2 octets of CID / 1 or 2 octets if large CIDs
| |
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+---+---+---+---+---+---+---+---+
/ EPIC-LITE CO packet / variable
+---+---+---+---+---+---+---+---+
Figure 1 : Format of CO packet generated by EPIC-LITE
Note that CO packets are decompressed relative to the context stored
at the decompressor. If the compressor has not yet initialized this
context, or suspects that it has become invalidated, then a CO packet
cannot be sent.
A.2. IR-DYN packet
The structure of the IR-DYN packet used by EPIC-LITE is shown below:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for CID 1-15 and small CIDs
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 | 0 | IR-DYN type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ EPIC-LITE IR-DYN packet / variable length
| |
+---+---+---+---+---+---+---+---+
Figure 2 : Format of IR-DYN packet generated by EPIC-LITE
The Profile field associates the context with a certain profile. It
transmits the 8 least significant bits of the EPIC-LITE
profile_identifier parameter described in Section 7.1. Furthermore,
the polynomial used to calculate the CRC is defined in Section 6.12.
A.3. IR packet
The structure of the IR packet used by EPIC-LITE is shown below:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for CID 1-15 and small CIDs
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 | D | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
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+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ EPIC-LITE IR packet / variable length
| |
+---+---+---+---+---+---+---+---+
Figure 3 : Format of IR packet generated by EPIC-LITE
Note that the D bit is currently always set to 1 (as specified in
[ROHC]), since the IR packet generated by EPIC-LITE always compresses
every field in the header. A version of the IR packet that only
compresses static fields may be introduced in future.
Price et al. [PAGE 23]
