[PATCH v2 2/3] net: bootp: BOOTP/DHCPv4 retransmission improvements
Sean Edmond
seanedmond at linux.microsoft.com
Sat Nov 4 02:04:55 CET 2023
On 2023-10-23 11:06 p.m., Heinrich Schuchardt wrote:
> On 10/24/23 02:21, 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
>
> %s/maximize at/be limited to/
>
>> patch allows the interval to be configurable using the environment
>> variable "bootpretransmitperiodmax"
>
> If there is an RFC defining the value, why do we need an environment
> variable?
>
>>
>> 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 | 56 ++++++++++++++++++++++++++++++++++++++---------------
>> 1 file changed, 40 insertions(+), 16 deletions(-)
>>
>> diff --git a/net/bootp.c b/net/bootp.c
>> index 6800290963..bab17a9ceb 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
>
> This does not match RFC 951. Please, why shouldn't we use the standard
> value by default?
>
>> +#define RETRANSMIT_PERIOD_INIT_MS 250
>
> This constant should be described too.
>
>> +
>> #ifndef CFG_DHCP_MIN_EXT_LEN /* minimal length of extension
>> list */
>> #define CFG_DHCP_MIN_EXT_LEN 64
>> #endif
>> @@ -53,6 +64,7 @@
>> 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 */
>> @@ -60,6 +72,7 @@ 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;
>> @@ -414,8 +427,8 @@ 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;
>
> RFC 951 requires that the backoff time is randomized.
>
I added randomization in v3. Note, in that update I haven't put any
guards around the use of rand()/srand() (as is the case in net_rand.h).
Perhaps it's safe to assume it will always be present for inclusion of net?
> Best regards
>
> Heinrich
>
>> net_set_timeout_handler(bootp_timeout, bootp_timeout_handler);
>> bootp_request();
>> }
>> @@ -711,10 +724,14 @@ 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;
>> +
>> + bootp_timeout = env_get_ulong("bootpretransmitperiodinit", 10,
>> RETRANSMIT_PERIOD_INIT_MS);
>> +
>> }
>>
>> void bootp_request(void)
>> @@ -726,7 +743,6 @@ 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 */
>> @@ -742,6 +758,9 @@ void bootp_request(void)
>> else
>> time_taken_max = TIMEOUT_MS;
>>
>> + retransmit_period_max_ms =
>> env_get_ulong("bootpretransmitperiodmax", 10,
>> + RETRANSMIT_PERIOD_MAX_MS);
>> +
>> #ifdef CONFIG_BOOTP_RANDOM_DELAY /* Random BOOTP delay */
>> if (bootp_try == 0)
>> srand_mac();
>> @@ -801,18 +820,23 @@ void bootp_request(void)
>> extlen = bootp_extended((u8 *)bp->bp_vend);
>> #endif
>>
>> - /*
>> - * Bootp ID is the lower 4 bytes of our ethernet address
>> - * plus the current time in ms.
>> - */
>> - bootp_id = ((u32)net_ethaddr[2] << 24)
>> - | ((u32)net_ethaddr[3] << 16)
>> - | ((u32)net_ethaddr[4] << 8)
>> - | (u32)net_ethaddr[5];
>> - bootp_id += get_timer(0);
>> - bootp_id = htonl(bootp_id);
>> - bootp_add_id(bootp_id);
>> - net_copy_u32(&bp->bp_id, &bootp_id);
>> + /* Only generate a new transaction ID for each new BOOTP request */
>> + if (bootp_try == 1) {
>> + /*
>> + * Bootp ID is the lower 4 bytes of our ethernet address
>> + * plus the current time in ms.
>> + */
>> + bootp_id = ((u32)net_ethaddr[2] << 24)
>> + | ((u32)net_ethaddr[3] << 16)
>> + | ((u32)net_ethaddr[4] << 8)
>> + | (u32)net_ethaddr[5];
>> + bootp_id += get_timer(0);
>> + bootp_id = htonl(bootp_id);
>> + bootp_add_id(bootp_id);
>> + net_copy_u32(&bp->bp_id, &bootp_id);
>> + } else {
>> + net_copy_u32(&bp->bp_id, &bootp_id);
>> + }
>>
>> /*
>> * Calculate proper packet lengths taking into account the
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