netdev - Re: [PATCH 3/5] tcp/dccp: Consolidate common code for RFC 3390 conversion

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Date:	Mon, 30 Aug 2010 15:59:34 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Gerrit Renker <gerrit@....abdn.ac.uk>
cc:	David Miller <davem@...emloft.net>, dccp@...r.kernel.org,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 3/5] tcp/dccp: Consolidate common code for RFC 3390
 conversion

On Mon, 30 Aug 2010, Gerrit Renker wrote:

> This patch consolidates initial-window code common to TCP and CCID-2:
>  * TCP uses RFC 3390 in a packet-oriented manner (tcp_input.c) and
>  * CCID-2 uses RFC 3390 in packet-oriented manner (RFC 4341).
> 
> Signed-off-by: Gerrit Renker <gerrit@....abdn.ac.uk>
> ---
>  include/net/tcp.h      |   15 +++++++++++++++
>  net/dccp/ccids/ccid2.c |    8 ++------
>  net/ipv4/tcp_input.c   |   17 ++---------------
>  3 files changed, 19 insertions(+), 21 deletions(-)
> 
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -789,6 +789,21 @@ static inline __u32 tcp_current_ssthresh(const struct sock *sk)
>  /* Use define here intentionally to get WARN_ON location shown at the caller */
>  #define tcp_verify_left_out(tp)	WARN_ON(tcp_left_out(tp) > tp->packets_out)
>  
> +/*
> + * Convert RFC 3390 larger initial window into an equivalent number of packets.
> + *
> + * John Heffner states:
> + *
> + *	The RFC specifies a window of no more than 4380 bytes
> + *	unless 2*MSS > 4380.  Reading the pseudocode in the RFC
> + *	is a bit misleading because they use a clamp at 4380 bytes
> + *	rather than a multiplier in the relevant range.
> + */
> +static inline u32 rfc3390_bytes_to_packets(const u32 smss)
> +{
> +	return smss <= 1095 ? 4 : (smss > 1460 ? 2 : 3);
> +}
> +
>  extern void tcp_enter_cwr(struct sock *sk, const int set_ssthresh);
>  extern __u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst);
>  
> --- a/net/dccp/ccids/ccid2.c
> +++ b/net/dccp/ccids/ccid2.c
> @@ -641,12 +641,8 @@ static int ccid2_hc_tx_init(struct ccid *ccid, struct sock *sk)
>  	/* RFC 4341, 5: initialise ssthresh to arbitrarily high (max) value */
>  	hc->tx_ssthresh = ~0U;
>  
> -	/*
> -	 * RFC 4341, 5: "The cwnd parameter is initialized to at most four
> -	 * packets for new connections, following the rules from [RFC3390]".
> -	 * We need to convert the bytes of RFC3390 into the packets of RFC 4341.
> -	 */
> -	hc->tx_cwnd = clamp(4380U / dp->dccps_mss_cache, 2U, 4U);
> +	/* Use larger initial windows (RFC 4341, section 5). */
> +	hc->tx_cwnd = rfc3390_bytes_to_packets(dp->dccps_mss_cache);
>  
>  	/* Make sure that Ack Ratio is enabled and within bounds. */
>  	max_ratio = DIV_ROUND_UP(hc->tx_cwnd, 2);
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -805,25 +805,12 @@ void tcp_update_metrics(struct sock *sk)
>  	}
>  }
>  
> -/* Numbers are taken from RFC3390.
> - *
> - * John Heffner states:
> - *
> - *	The RFC specifies a window of no more than 4380 bytes
> - *	unless 2*MSS > 4380.  Reading the pseudocode in the RFC
> - *	is a bit misleading because they use a clamp at 4380 bytes
> - *	rather than use a multiplier in the relevant range.
> - */
>  __u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst)
>  {
>  	__u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);
>  
> -	if (!cwnd) {
> -		if (tp->mss_cache > 1460)
> -			cwnd = 2;
> -		else
> -			cwnd = (tp->mss_cache > 1095) ? 3 : 4;
> -	}
> +	if (!cwnd)
> +		cwnd = rfc3390_bytes_to_packets(tp->mss_cache);
>  	return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
>  }

What is spelled out in tcp_select_initial_window might also need to follow 
the same logic?

-- 
 i.
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