There have been discussions on whether `s' is fixed in CCID 2/3 or whether,
in spite of RFC 4341/2, `s' is variable. To resolve the discussion into
material which is practically implementable and yet does not defy existing
standards-track documents, a simple algorithmic strategy is suggested below.
An aside to the discussion was the PACKET_SIZE socket option, but in actual
fact this option is of no use other than causing unnecessary confusion: for
CCID 2 it is entirely irrelevant, and for CCID 3 (and its variations) it is
redundant and not useful due to argumentation below.
Strategy
----------
1/ Remove the PACKET_SIZE socket options as they don't help with the problem;
I have therefore updated Ian's patch to be used standalone [attached].
2/ In the initialisation code, set ccid3hctx_s = ccid3hcrx_s = 0 instead of the
TFRC_STD_PACKET_SIZE (which would give wrong estimates)
3/ Update ccid3hc{rx,tx}_s using the `payload len' value taken each time from
* dccp_sendmsg (after the check against dccp_mss_cache)
* dccp_v{4,6}_rcv (only on Data/DataAck packets)
using a moving average with a large q = 8/10 ... 9/10:
s <-- q * s + (1-q) * len
4/ Don't use MTU minus header length for `s' (justified below).
Thus, if the application is well-behaved and sends only `fixed' size packets, step
(3) reduces to a no-op. Otherwise, it slowly converges to a long-term value. The
exact/optimal weight of q can be found through experimentation, but 9/10 seems
conservative enough. The algorithm above is in accordance with recent modifications
suggested in [HFPW06]. It could be extended to guard against excessive changes in
packet length for CCID 2 as suggested in [RFC 4341, sec. 5.3].
R A T I O N A L E
The problem in assuming that `s' may vary and in allowing it to be set to
some other (but fixed) value, such as the path MTU minus header/option
lengths, lies in required changes to the loss rate estimation algorithm.
References which explicitly warn against this are given below; in both
[Wid00, p. 21] and [FHP+00, 3.1.2] it is pointed out that this part has taken
much discussion and testing; for good reasons, since any changes endanger
both efficiency and fairness wrt competing TCP flows.
Theorems and numerical examples that attest that inaccuracies lead to either
non-TCP-friendly or suboptimal application behaviour can be found in [RR99];
trends to be confirmed later by a much more comprehensive analysis in [VLB05].
These findings were validated and confirmed by Widmer et al in [WBLB04]. This
article, as well as the earlier technical report [Vas00] warn against using the MTU
as `fixed' packet size parameter of the throughput equation, in such scenarios where
the application is allowed to send variable-sized packets. To solve the problem of a
non-`fixed' s, Widmer et al introduce a number of changes to the loss estimation
algorithm in [WBLB04].
Similar ideas and a confirmation that the loss interval estimation needs updating when `s' may
vary can be found in [FK06]; where a constant of s=1460 is plugged into the throughput equation.
Consequently adjustments to estimating the loss event rate do follow; and an upper bound on the
sending rate is additionally imposed to support using non-`fixed' s.
In summary, using variable packet sizes is not well understood and even less well specified. There are
several publications which explicitly warn against clamping `s' to the path MTU [Vas00, WBLB04,VLB05,
RR99] and thereby allowing applications to be liberal with (the length of) what they send.
References
--------------
[RR99] Ramesh, Sridhar and Injong Rhee. Issues in TCP Model-Based Flow
Control. Technical report, TR-99-15, NCSU, North Carolina State
University, Raleigh, 1999.
[VLB05] Vojnovic, Milan and Jean-Yves Le Boudec. On the long-run behavior
of equation-based rate control. IEEE/ACM Transactions on
Networking (TON), 13(3):568--581, 6/2005.
[WBLB04] Widmer, Jörg, Catherine Boutremans and Jean-Yves Le Boudec.
End-to-End Congestion Control for TCP-Friendly Flows with
Variable Packet Size. ACM SIGCOMM Computer Communication Review,
34(2):137--151, 4/2004.
[Vas00] Vasallo, Pedro Reviriego. Variable Packet Size Equation Based
Congestion Control. Technical Report, tr-00-008, ICSI, 4/2000.
[FK06] Floyd, Sally and Eddie Kohler. TCP Friendly Rate Control (TFRC):
the Small-Packet (SP) Variant. draft-ietf-dccp-tfrc-voip-05.txt,
1/3/2006.
[HFPW06] draft-floyd-rfc3448bis-00.txt
[Wid00] Widmer, Jörg. Equation-Based Congestion Control. Diploma Thesis,
Department of Mathematics and Computer Science, University of
Mannheim, Germany, 2/2000.
[FHP+00] Floyd, Sally, Mark Handley, Jitendra Padhye and Jörg Widmer.
Equation-Based Congestion Control for Unicast Applications. ACM
SIGCOMM Computer Communication Review, 30(4):43--56, 10/2000.
diff --git a/include/linux/dccp.h b/include/linux/dccp.h
index d6f4ec4..628035f 100644
--- a/include/linux/dccp.h
+++ b/include/linux/dccp.h
@@ -196,7 +196,6 @@ struct dccp_so_feat {
};
/* DCCP socket options */
-#define DCCP_SOCKOPT_PACKET_SIZE 1
#define DCCP_SOCKOPT_SERVICE 2
#define DCCP_SOCKOPT_CHANGE_L 3
#define DCCP_SOCKOPT_CHANGE_R 4
@@ -464,7 +463,6 @@ struct dccp_sock {
struct dccp_service_list *dccps_service_list;
struct timeval dccps_timestamp_time;
__u32 dccps_timestamp_echo;
- __u32 dccps_packet_size;
__u16 dccps_l_ack_ratio;
__u16 dccps_r_ack_ratio;
unsigned long dccps_ndp_count;
diff --git a/net/dccp/ccids/ccid3.c b/net/dccp/ccids/ccid3.c
index cec23ad..aa8f19e 100644
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -652,11 +652,7 @@ static int ccid3_hc_tx_init(struct ccid
struct dccp_sock *dp = dccp_sk(sk);
struct ccid3_hc_tx_sock *hctx = ccid_priv(ccid);
- if (dp->dccps_packet_size >= TFRC_MIN_PACKET_SIZE &&
- dp->dccps_packet_size <= TFRC_MAX_PACKET_SIZE)
- hctx->ccid3hctx_s = dp->dccps_packet_size;
- else
- hctx->ccid3hctx_s = TFRC_STD_PACKET_SIZE;
+ hctx->ccid3hctx_s = TFRC_STD_PACKET_SIZE;
/* Set transmission rate to 1 packet per second */
hctx->ccid3hctx_x = hctx->ccid3hctx_s;
@@ -1125,11 +1121,7 @@ static int ccid3_hc_rx_init(struct ccid
ccid3_pr_debug("%s, sk=%p\n", dccp_role(sk), sk);
- if (dp->dccps_packet_size >= TFRC_MIN_PACKET_SIZE &&
- dp->dccps_packet_size <= TFRC_MAX_PACKET_SIZE)
- hcrx->ccid3hcrx_s = dp->dccps_packet_size;
- else
- hcrx->ccid3hcrx_s = TFRC_STD_PACKET_SIZE;
+ hcrx->ccid3hcrx_s = TFRC_STD_PACKET_SIZE;
hcrx->ccid3hcrx_state = TFRC_RSTATE_NO_DATA;
INIT_LIST_HEAD(&hcrx->ccid3hcrx_hist);
diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index d3e6e81..69ba5c3 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -451,8 +451,7 @@ out_free_val:
static int do_dccp_setsockopt(struct sock *sk, int level, int optname,
char __user *optval, int optlen)
{
- struct dccp_sock *dp;
- int err;
+ int err = 0;
int val;
if (optlen < sizeof(int))
@@ -465,14 +464,8 @@ static int do_dccp_setsockopt(struct soc
return dccp_setsockopt_service(sk, val, optval, optlen);
lock_sock(sk);
- dp = dccp_sk(sk);
- err = 0;
switch (optname) {
- case DCCP_SOCKOPT_PACKET_SIZE:
- dp->dccps_packet_size = val;
- break;
-
case DCCP_SOCKOPT_CHANGE_L:
if (optlen != sizeof(struct dccp_so_feat))
err = -EINVAL;
@@ -568,10 +561,6 @@ static int do_dccp_getsockopt(struct soc
dp = dccp_sk(sk);
switch (optname) {
- case DCCP_SOCKOPT_PACKET_SIZE:
- val = dp->dccps_packet_size;
- len = sizeof(dp->dccps_packet_size);
- break;
case DCCP_SOCKOPT_SERVICE:
return dccp_getsockopt_service(sk, len,
(__be32 __user *)optval, optlen);
