[PATCH 2/3] net: bootp: BOOTP/DHCPv4 retransmission improvements

[Lothar Waßmann]

Hi,

On Mon, 25 Sep 2023 13:29:34 -0700 seanedmond at linux.microsoft.com wrote:
> From: Sean Edmond <seanedmond at microsoft.com>
> 
> This patch introduces 3 improvements to align with RFC 951:
> - retransmission backoff interval maximum is configurable
> - initial retranmission backoff interval is configurable
> - transaction ID is kept the same for each BOOTP/DHCPv4 request
> 
> In applications where thousands of nodes are serviced by a single DHCP
> server, maximizing the retransmission backoff interval at 2 seconds (the
> current u-boot default) exerts high pressure on the DHCP server and
> network layer.
> 
> RFC 951 “7.2. Client Retransmission Strategy” states that the
> retransmission backoff interval should maximize at 60 seconds.  This
> patch allows the interval to be configurable using the environment
> variable "bootpretransmitperiodmax"
> 
> The initial retranmission backoff period defaults to 250ms, which is
> also too small for these scenarios with many clients.  This patch makes
> the initial retransmission interval to be configurable using the
> environment variable "bootpretransmitperiodinit".
> 
> Also, on a retransmission it is not expected for the transaction ID to
> change (only the 'secs' field should be updated). Let's save the
> transaction ID and use the same transaction ID for each BOOTP/DHCPv4
> exchange.
> 
> Signed-off-by: Sean Edmond <seanedmond at microsoft.com>
> ---
>  net/bootp.c | 63 +++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 47 insertions(+), 16 deletions(-)
> 
> diff --git a/net/bootp.c b/net/bootp.c
> index 013d54c7ed..7248536cc4 100644
> --- a/net/bootp.c
> +++ b/net/bootp.c
> @@ -42,6 +42,17 @@
>   */
>  #define TIMEOUT_MS	((3 + (CONFIG_NET_RETRY_COUNT * 5)) * 1000)
>  
> +/*
> + * According to rfc951 : 7.2. Client Retransmission Strategy
> + * "After the 'average' backoff reaches about 60 seconds, it should be
> + * increased no further, but still randomized."
> + *
> + * U-Boot has saturated this backoff at 2 seconds for a long time.
> + * To modify, set the environment variable "bootpretransmitperiodmax"
> + */
> +#define RETRANSMIT_PERIOD_MAX_MS	2000
> +#define RETRANSMIT_PERIOD_INIT_MS	250
> +
>  #define PORT_BOOTPS	67		/* BOOTP server UDP port */
>  #define PORT_BOOTPC	68		/* BOOTP client UDP port */
>  
> @@ -56,6 +67,7 @@
>  u32		bootp_ids[CFG_BOOTP_ID_CACHE_SIZE];
>  unsigned int	bootp_num_ids;
>  int		bootp_try;
> +u32		bootp_id;
>  ulong		bootp_start;
>  ulong		bootp_timeout;
>  char net_nis_domain[32] = {0,}; /* Our NIS domain */
> @@ -63,6 +75,7 @@ char net_hostname[32] = {0,}; /* Our hostname */
>  char net_root_path[CONFIG_BOOTP_MAX_ROOT_PATH_LEN] = {0,}; /* Our bootpath */
>  
>  static ulong time_taken_max;
> +static u32   retransmit_period_max_ms;
>  
>  #if defined(CONFIG_CMD_DHCP)
>  static dhcp_state_t dhcp_state = INIT;
> @@ -417,8 +430,8 @@ static void bootp_timeout_handler(void)
>  		}
>  	} else {
>  		bootp_timeout *= 2;
> -		if (bootp_timeout > 2000)
> -			bootp_timeout = 2000;
> +		if (bootp_timeout > retransmit_period_max_ms)
> +			bootp_timeout = retransmit_period_max_ms;
>  		net_set_timeout_handler(bootp_timeout, bootp_timeout_handler);
>  		bootp_request();
>  	}
> @@ -714,10 +727,18 @@ static int bootp_extended(u8 *e)
>  
>  void bootp_reset(void)
>  {
> +	char *ep;  /* Environment pointer */
> +
>  	bootp_num_ids = 0;
>  	bootp_try = 0;
>  	bootp_start = get_timer(0);
> -	bootp_timeout = 250;
> +
> +	ep = env_get("bootpretransmitperiodinit");
> +	if (ep)
> +		bootp_timeout = dectoul(ep, NULL);
> +	else
> +		bootp_timeout = RETRANSMIT_PERIOD_INIT_MS;
> +
bootp_timeout = env_get_ulong("bootpretransmitperiodinit", 0, RETRANSMIT_PERIOD_INIT_MS);
does the same...

>  }
>  
>  void bootp_request(void)
> @@ -729,7 +750,6 @@ void bootp_request(void)
>  #ifdef CONFIG_BOOTP_RANDOM_DELAY
>  	ulong rand_ms;
>  #endif
> -	u32 bootp_id;
>  	struct in_addr zero_ip;
>  	struct in_addr bcast_ip;
>  	char *ep;  /* Environment pointer */
> @@ -745,6 +765,12 @@ void bootp_request(void)
>  	else
>  		time_taken_max = TIMEOUT_MS;
>  
> +	ep = env_get("bootpretransmitperiodmax");
> +	if (ep)
> +		retransmit_period_max_ms = dectoul(ep, NULL);
> +	else
> +		retransmit_period_max_ms = RETRANSMIT_PERIOD_MAX_MS;
> +
see above


Lothar Waßmann