Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Luigi Rizzo]

On Wed, Sep 29, 2021 at 1:17 AM Dave Taht <dave.taht@gmail.com> wrote:
>
> In today's rpm meeting I didn't quite manage to make a complicated
> point. This long-ago proposal
> of matt mathis's has often intrigued (inspired? frightened?) me:
>
> https://datatracker.ietf.org/doc/html/draft-mathis-iccrg-relentless-tcp-00
>
> where he proposed that a tcp variant have no response at all to loss
> or markings, merely
> replacing lost segments as they are requested, continually ramping up
> until the network
> basically explodes.


For a similar purpose, I use the following patch that ignores holes,
thus mostly defeating congestion control.

The nice thing is that you can just count sent and received bytes
in the application to estimate losses

commit 9c429c9644f2fd22d5fe2b6f2d4df6fb2a8962b2
Author: Luigi Rizzo <lrizzo@google.com>
Date:   Fri Oct 1 09:10:44 2021 -0700

    test: module parameter to ignore holes in TCP

    Sometimes, for testing, it is useful to let the TCP receiver ignore
    drops (and defeat congestion control) and accept all packets as if they
    were in sequence. This will show whether a connection is sender or
    receiver throttled.

    This patch implements the above with 3 /sys/module/tcp_input/parameters :
    - lossy_local_port, lossy_remote_port
        if non zero, indicate that sockets matching one of these ports
        will be set to ignore drops. (This socker flag could be set with a
        setsockopt(), but would require changes in the caller);
    - drop_freq
        if non zero, one every drop_freq packets will be artificially
        dropped on the receive side.

    Example
      echo 2345 > /sys/module/tcp_input/parameters/lossy_local_port
      echo 10 > /sys/module/tcp_input/parameters/drop_freq # drop one in 10

      ifconfig lo mtu 600
      nc -l 2345 > /tmp/a &
      MSG="this is a test this is a test this is a test count is"
      (for ((i=0;i<10000;i++)); do echo "${MSG} ${i}; $i"; done) | nc
127.0.0.1 2345

    The output file will have missing lines


diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 48d8a363319e..3bb8888a56af 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -225,7 +225,8 @@ struct tcp_sock {
        u8      compressed_ack;
        u8      dup_ack_counter:2,
                tlp_retrans:1,  /* TLP is a retransmission */
-               unused:5;
+               ignore_holes:1, /* ignore holes on rx. test only */
+               unused:4;
        u32     chrono_start;   /* Start time in jiffies of a TCP chrono */
        u32     chrono_stat[3]; /* Time in jiffies for chrono_stat stats */
        u8      chrono_type:2,  /* current chronograph type */
@@ -250,6 +251,7 @@ struct tcp_sock {
        u32     tlp_high_seq;   /* snd_nxt at the time of TLP */

        u32     tcp_tx_delay;   /* delay (in usec) added to TX packets */
+       u32     tcp_test_drops; /* artificial packet drops. test only */
        u64     tcp_wstamp_ns;  /* departure time for next sent data packet */
        u64     tcp_clock_cache; /* cache last tcp_clock_ns() (see
tcp_mstamp_refresh()) */

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 414c179c28e0..fd770a176a68 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2344,6 +2344,9 @@ static int tcp_recvmsg_locked(struct sock *sk,
struct msghdr *msg, size_t len,
                last = skb_peek_tail(&sk->sk_receive_queue);
                skb_queue_walk(&sk->sk_receive_queue, skb) {
                        last = skb;
+                       /* XXX if we allow holes, update copied_seq */
+                       if (tp->ignore_holes && before(*seq,
TCP_SKB_CB(skb)->seq))
+                               *seq = TCP_SKB_CB(skb)->seq;
                        /* Now that we have two receive queues this
                         * shouldn't happen.
                         */
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 246ab7b5e857..a5161ce78171 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -81,6 +81,14 @@
 #include <net/busy_poll.h>
 #include <net/mptcp.h>

+#include <linux/module.h>
+static int drop_freq; /* drop one pkt ever this many */
+module_param(drop_freq, int, 0644);
+static int lossy_local_port = 2345; /* drop pkts, ignore holes on this port */
+module_param(lossy_local_port, int, 0644);
+static int lossy_remote_port = 0; /* drop pkts, ignore holes on this port */
+module_param(lossy_remote_port, int, 0644);
+
 int sysctl_tcp_max_orphans __read_mostly = NR_FILE;

 #define FLAG_DATA              0x01 /* Incoming frame contained data.
         */
@@ -5806,6 +5814,16 @@ void tcp_rcv_established(struct sock *sk,
struct sk_buff *skb)

        tp->rx_opt.saw_tstamp = 0;

+       if (tp->ignore_holes) {
+               const u32 seq = TCP_SKB_CB(skb)->seq;
+               if (drop_freq && tp->tcp_test_drops++ >= drop_freq) {
+                       tp->tcp_test_drops = 0;
+                       goto discard;   /* artificial drop */
+               }
+               if (after(seq, tp->rcv_nxt)) /* Pretend this is in order */
+                       tcp_rcv_nxt_update(tp, seq);
+       }
+
        /*      pred_flags is 0xS?10 << 16 + snd_wnd
         *      if header_prediction is to be made
         *      'S' will always be tp->tcp_header_len >> 2
@@ -5986,12 +6004,25 @@ void tcp_init_transfer(struct sock *sk, int
bpf_op, struct sk_buff *skb)
        tcp_init_buffer_space(sk);
 }

+/* Testing. Conditionally set tp->ignore_holes. Should be a setsockopt */
+static void tcp_update_ignore_holes(struct sock *sk)
+{
+       if (READ_ONCE(lossy_local_port) == ntohs(inet_sk(sk)->inet_sport) ||
+           READ_ONCE(lossy_remote_port) == ntohs(inet_sk(sk)->inet_dport)) {
+               pr_info("XXX ignore holes for ports local %d remote %d\n",
+                       ntohs(inet_sk(sk)->inet_sport),
+                       ntohs(inet_sk(sk)->inet_dport));
+               tcp_sk(sk)->ignore_holes = 1;
+       }
+}
+
 void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
 {
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);

        tcp_set_state(sk, TCP_ESTABLISHED);
+       tcp_update_ignore_holes(sk);
        icsk->icsk_ack.lrcvtime = tcp_jiffies32;

        if (skb) {
@@ -6476,6 +6507,7 @@ int tcp_rcv_state_process(struct sock *sk,
struct sk_buff *skb)
                }
                smp_mb();
                tcp_set_state(sk, TCP_ESTABLISHED);
+               tcp_update_ignore_holes(sk);
                sk->sk_state_change(sk);

                /* Note, that this wakeup is only for marginal crossed SYN case.

Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Bob McMahon]

[-- Attachment #1.1: Type: text/plain, Size: 8007 bytes --]

hmm, this looks interesting to a test & measurement guy. Can it be done
with a setsockopt? I might want to add this as an iperf2 option,
particularly if it's broadly available,

Thanks,
Bob

On Fri, Oct 1, 2021 at 9:22 AM Luigi Rizzo <rizzo@iet.unipi.it> wrote:

> On Wed, Sep 29, 2021 at 1:17 AM Dave Taht <dave.taht@gmail.com> wrote:
> >
> > In today's rpm meeting I didn't quite manage to make a complicated
> > point. This long-ago proposal
> > of matt mathis's has often intrigued (inspired? frightened?) me:
> >
> >
> https://datatracker.ietf.org/doc/html/draft-mathis-iccrg-relentless-tcp-00
> >
> > where he proposed that a tcp variant have no response at all to loss
> > or markings, merely
> > replacing lost segments as they are requested, continually ramping up
> > until the network
> > basically explodes.
>
>
> For a similar purpose, I use the following patch that ignores holes,
> thus mostly defeating congestion control.
>
> The nice thing is that you can just count sent and received bytes
> in the application to estimate losses
>
> commit 9c429c9644f2fd22d5fe2b6f2d4df6fb2a8962b2
> Author: Luigi Rizzo <lrizzo@google.com>
> Date:   Fri Oct 1 09:10:44 2021 -0700
>
>     test: module parameter to ignore holes in TCP
>
>     Sometimes, for testing, it is useful to let the TCP receiver ignore
>     drops (and defeat congestion control) and accept all packets as if they
>     were in sequence. This will show whether a connection is sender or
>     receiver throttled.
>
>     This patch implements the above with 3
> /sys/module/tcp_input/parameters :
>     - lossy_local_port, lossy_remote_port
>         if non zero, indicate that sockets matching one of these ports
>         will be set to ignore drops. (This socker flag could be set with a
>         setsockopt(), but would require changes in the caller);
>     - drop_freq
>         if non zero, one every drop_freq packets will be artificially
>         dropped on the receive side.
>
>     Example
>       echo 2345 > /sys/module/tcp_input/parameters/lossy_local_port
>       echo 10 > /sys/module/tcp_input/parameters/drop_freq # drop one in 10
>
>       ifconfig lo mtu 600
>       nc -l 2345 > /tmp/a &
>       MSG="this is a test this is a test this is a test count is"
>       (for ((i=0;i<10000;i++)); do echo "${MSG} ${i}; $i"; done) | nc
> 127.0.0.1 2345
>
>     The output file will have missing lines
>
>
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 48d8a363319e..3bb8888a56af 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -225,7 +225,8 @@ struct tcp_sock {
>         u8      compressed_ack;
>         u8      dup_ack_counter:2,
>                 tlp_retrans:1,  /* TLP is a retransmission */
> -               unused:5;
> +               ignore_holes:1, /* ignore holes on rx. test only */
> +               unused:4;
>         u32     chrono_start;   /* Start time in jiffies of a TCP chrono */
>         u32     chrono_stat[3]; /* Time in jiffies for chrono_stat stats */
>         u8      chrono_type:2,  /* current chronograph type */
> @@ -250,6 +251,7 @@ struct tcp_sock {
>         u32     tlp_high_seq;   /* snd_nxt at the time of TLP */
>
>         u32     tcp_tx_delay;   /* delay (in usec) added to TX packets */
> +       u32     tcp_test_drops; /* artificial packet drops. test only */
>         u64     tcp_wstamp_ns;  /* departure time for next sent data
> packet */
>         u64     tcp_clock_cache; /* cache last tcp_clock_ns() (see
> tcp_mstamp_refresh()) */
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 414c179c28e0..fd770a176a68 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2344,6 +2344,9 @@ static int tcp_recvmsg_locked(struct sock *sk,
> struct msghdr *msg, size_t len,
>                 last = skb_peek_tail(&sk->sk_receive_queue);
>                 skb_queue_walk(&sk->sk_receive_queue, skb) {
>                         last = skb;
> +                       /* XXX if we allow holes, update copied_seq */
> +                       if (tp->ignore_holes && before(*seq,
> TCP_SKB_CB(skb)->seq))
> +                               *seq = TCP_SKB_CB(skb)->seq;
>                         /* Now that we have two receive queues this
>                          * shouldn't happen.
>                          */
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 246ab7b5e857..a5161ce78171 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -81,6 +81,14 @@
>  #include <net/busy_poll.h>
>  #include <net/mptcp.h>
>
> +#include <linux/module.h>
> +static int drop_freq; /* drop one pkt ever this many */
> +module_param(drop_freq, int, 0644);
> +static int lossy_local_port = 2345; /* drop pkts, ignore holes on this
> port */
> +module_param(lossy_local_port, int, 0644);
> +static int lossy_remote_port = 0; /* drop pkts, ignore holes on this port
> */
> +module_param(lossy_remote_port, int, 0644);
> +
>  int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
>
>  #define FLAG_DATA              0x01 /* Incoming frame contained data.
>          */
> @@ -5806,6 +5814,16 @@ void tcp_rcv_established(struct sock *sk,
> struct sk_buff *skb)
>
>         tp->rx_opt.saw_tstamp = 0;
>
> +       if (tp->ignore_holes) {
> +               const u32 seq = TCP_SKB_CB(skb)->seq;
> +               if (drop_freq && tp->tcp_test_drops++ >= drop_freq) {
> +                       tp->tcp_test_drops = 0;
> +                       goto discard;   /* artificial drop */
> +               }
> +               if (after(seq, tp->rcv_nxt)) /* Pretend this is in order */
> +                       tcp_rcv_nxt_update(tp, seq);
> +       }
> +
>         /*      pred_flags is 0xS?10 << 16 + snd_wnd
>          *      if header_prediction is to be made
>          *      'S' will always be tp->tcp_header_len >> 2
> @@ -5986,12 +6004,25 @@ void tcp_init_transfer(struct sock *sk, int
> bpf_op, struct sk_buff *skb)
>         tcp_init_buffer_space(sk);
>  }
>
> +/* Testing. Conditionally set tp->ignore_holes. Should be a setsockopt */
> +static void tcp_update_ignore_holes(struct sock *sk)
> +{
> +       if (READ_ONCE(lossy_local_port) == ntohs(inet_sk(sk)->inet_sport)
> ||
> +           READ_ONCE(lossy_remote_port) ==
> ntohs(inet_sk(sk)->inet_dport)) {
> +               pr_info("XXX ignore holes for ports local %d remote %d\n",
> +                       ntohs(inet_sk(sk)->inet_sport),
> +                       ntohs(inet_sk(sk)->inet_dport));
> +               tcp_sk(sk)->ignore_holes = 1;
> +       }
> +}
> +
>  void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
>  {
>         struct tcp_sock *tp = tcp_sk(sk);
>         struct inet_connection_sock *icsk = inet_csk(sk);
>
>         tcp_set_state(sk, TCP_ESTABLISHED);
> +       tcp_update_ignore_holes(sk);
>         icsk->icsk_ack.lrcvtime = tcp_jiffies32;
>
>         if (skb) {
> @@ -6476,6 +6507,7 @@ int tcp_rcv_state_process(struct sock *sk,
> struct sk_buff *skb)
>                 }
>                 smp_mb();
>                 tcp_set_state(sk, TCP_ESTABLISHED);
> +               tcp_update_ignore_holes(sk);
>                 sk->sk_state_change(sk);
>
>                 /* Note, that this wakeup is only for marginal crossed SYN
> case.
>

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Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Luigi Rizzo]

On Fri, Oct 1, 2021 at 6:33 PM Bob McMahon <bob.mcmahon@broadcom.com> wrote:
>
> hmm, this looks interesting to a test & measurement guy. Can it be done with a setsockopt? I might want to add this as an iperf2 option, particularly if it's broadly available,


I would be happy to submit it as one or two upstream patches --
perhaps one to implement
the basic "ignore_holes" + setsockopt(), and another mechanism (if
there isn't one
already) to override defaults sockopts on certain sockets.

I do think we need more readily available testing tool,

cheers
luigi

Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Matt Mathis]

[-- Attachment #1: Type: text/plain, Size: 1803 bytes --]

Please don't send this upstream.   It would makesTCP into the
evil transport from hell.    Modern loss recovery is robust enough to run
hardcoded cwnd=<blat> and persistent losses, Please don't make this too
easy for people who are intent on getting their "fair" share of the network
before the greedy people.

Dave overlooked an important detail in relentless TCP: It reduced the
window by exactly the losses, such that the presented load was exactly the
quantity of data successfully delivered on the previous RTT.   I have
forgotten the details of the increase function, but it was something Reno
like but only on loss less RTTs.



If you want to adapt TCP
Thanks,
--MM--
The best way to predict the future is to create it.  - Alan Kay

We must not tolerate intolerance;
       however our response must be carefully measured:
            too strong would be hypocritical and risks spiraling out of
control;
            too weak risks being mistaken for tacit approval.


On Fri, Oct 1, 2021 at 11:02 AM Luigi Rizzo via Rpm <
rpm@lists.bufferbloat.net> wrote:

> On Fri, Oct 1, 2021 at 6:33 PM Bob McMahon <bob.mcmahon@broadcom.com>
> wrote:
> >
> > hmm, this looks interesting to a test & measurement guy. Can it be done
> with a setsockopt? I might want to add this as an iperf2 option,
> particularly if it's broadly available,
>
>
> I would be happy to submit it as one or two upstream patches --
> perhaps one to implement
> the basic "ignore_holes" + setsockopt(), and another mechanism (if
> there isn't one
> already) to override defaults sockopts on certain sockets.
>
> I do think we need more readily available testing tool,
>
> cheers
> luigi
> _______________________________________________
> Rpm mailing list
> Rpm@lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/rpm
>

[-- Attachment #2: Type: text/html, Size: 2756 bytes --]

Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Dave Taht]

On Mon, Oct 4, 2021 at 3:45 PM Matt Mathis via Rpm
<rpm@lists.bufferbloat.net> wrote:
>
> Please don't send this upstream.   It would makesTCP into the evil transport from hell.

You mean it isn't already? :)

There are a lot of test tools in the world (like pktgen) that are
designed to do evil things to the network,
why not evilTCP(tm)? Too many tools synthesize evil one way traffic
and people draw incorrect conclusions
from those....

>  Modern loss recovery is robust enough to run hardcoded cwnd=<blat> and persistent losses, Please don't make this too easy for people who are intent on getting their "fair" share of the network before the greedy people.
>
> Dave overlooked an important detail in relentless TCP:

It had been a while, and it was intended to be a complex point. But
thx for the correction.

>It reduced the window by exactly the losses, such that the presented load was exactly the quantity of data successfully delivered on the previous RTT.   I have forgotten the details of the increase function, but it was something Reno like but only on loss less RTTs.

What I was looking for was a test tool that mimicked enough of tcp's
coupled ack behavior to give more predictable results, when
looking for expected behavior from an AQM (or policer). What I had
been using before was a udp flood, but that was dicy (and very
misleading) on non-duplex wireless.

That, and a small nuclear reactor so I'd never have to put more diesel
in my (still broken) boat.

>
>
> If you want to adapt TCP
> Thanks,
> --MM--
> The best way to predict the future is to create it.  - Alan Kay
>
> We must not tolerate intolerance;
>        however our response must be carefully measured:
>             too strong would be hypocritical and risks spiraling out of control;
>             too weak risks being mistaken for tacit approval.
>
>
> On Fri, Oct 1, 2021 at 11:02 AM Luigi Rizzo via Rpm <rpm@lists.bufferbloat.net> wrote:
>>
>> On Fri, Oct 1, 2021 at 6:33 PM Bob McMahon <bob.mcmahon@broadcom.com> wrote:
>> >
>> > hmm, this looks interesting to a test & measurement guy. Can it be done with a setsockopt? I might want to add this as an iperf2 option, particularly if it's broadly available,
>>
>>
>> I would be happy to submit it as one or two upstream patches --
>> perhaps one to implement
>> the basic "ignore_holes" + setsockopt(), and another mechanism (if
>> there isn't one
>> already) to override defaults sockopts on certain sockets.
>>
>> I do think we need more readily available testing tool,
>>
>> cheers
>> luigi
>> _______________________________________________
>> Rpm mailing list
>> Rpm@lists.bufferbloat.net
>> https://lists.bufferbloat.net/listinfo/rpm
>
> _______________________________________________
> Rpm mailing list
> Rpm@lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/rpm



-- 
Fixing Starlink's Latencies: https://www.youtube.com/watch?v=c9gLo6Xrwgw

Dave Täht CEO, TekLibre, LLC

Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Matt Mathis]

[-- Attachment #1: Type: text/plain, Size: 3773 bytes --]

Spewing crap is easy, but useless except to annoy people.   EvilTCP would
actually reliably deliver data and could do extremely useful work, at the
expense of other users.

Thanks,
--MM--
The best way to predict the future is to create it.  - Alan Kay

We must not tolerate intolerance;
       however our response must be carefully measured:
            too strong would be hypocritical and risks spiraling out of
control;
            too weak risks being mistaken for tacit approval.


On Mon, Oct 4, 2021 at 4:16 PM Dave Taht <dave.taht@gmail.com> wrote:

> On Mon, Oct 4, 2021 at 3:45 PM Matt Mathis via Rpm
> <rpm@lists.bufferbloat.net> wrote:
> >
> > Please don't send this upstream.   It would makesTCP into the evil
> transport from hell.
>
> You mean it isn't already? :)
>
> There are a lot of test tools in the world (like pktgen) that are
> designed to do evil things to the network,
> why not evilTCP(tm)? Too many tools synthesize evil one way traffic
> and people draw incorrect conclusions
> from those....
>
> >  Modern loss recovery is robust enough to run hardcoded cwnd=<blat> and
> persistent losses, Please don't make this too easy for people who are
> intent on getting their "fair" share of the network before the greedy
> people.
> >
> > Dave overlooked an important detail in relentless TCP:
>
> It had been a while, and it was intended to be a complex point. But
> thx for the correction.
>
> >It reduced the window by exactly the losses, such that the presented load
> was exactly the quantity of data successfully delivered on the previous
> RTT.   I have forgotten the details of the increase function, but it was
> something Reno like but only on loss less RTTs.
>
> What I was looking for was a test tool that mimicked enough of tcp's
> coupled ack behavior to give more predictable results, when
> looking for expected behavior from an AQM (or policer). What I had
> been using before was a udp flood, but that was dicy (and very
> misleading) on non-duplex wireless.
>
> That, and a small nuclear reactor so I'd never have to put more diesel
> in my (still broken) boat.
>
> >
> >
> > If you want to adapt TCP
> > Thanks,
> > --MM--
> > The best way to predict the future is to create it.  - Alan Kay
> >
> > We must not tolerate intolerance;
> >        however our response must be carefully measured:
> >             too strong would be hypocritical and risks spiraling out of
> control;
> >             too weak risks being mistaken for tacit approval.
> >
> >
> > On Fri, Oct 1, 2021 at 11:02 AM Luigi Rizzo via Rpm <
> rpm@lists.bufferbloat.net> wrote:
> >>
> >> On Fri, Oct 1, 2021 at 6:33 PM Bob McMahon <bob.mcmahon@broadcom.com>
> wrote:
> >> >
> >> > hmm, this looks interesting to a test & measurement guy. Can it be
> done with a setsockopt? I might want to add this as an iperf2 option,
> particularly if it's broadly available,
> >>
> >>
> >> I would be happy to submit it as one or two upstream patches --
> >> perhaps one to implement
> >> the basic "ignore_holes" + setsockopt(), and another mechanism (if
> >> there isn't one
> >> already) to override defaults sockopts on certain sockets.
> >>
> >> I do think we need more readily available testing tool,
> >>
> >> cheers
> >> luigi
> >> _______________________________________________
> >> Rpm mailing list
> >> Rpm@lists.bufferbloat.net
> >> https://lists.bufferbloat.net/listinfo/rpm
> >
> > _______________________________________________
> > Rpm mailing list
> > Rpm@lists.bufferbloat.net
> > https://lists.bufferbloat.net/listinfo/rpm
>
>
>
> --
> Fixing Starlink's Latencies: https://www.youtube.com/watch?v=c9gLo6Xrwgw
>
> Dave Täht CEO, TekLibre, LLC
>

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Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Matt Mathis]

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Thanks,
--MM--
Evil is defined by mortals who think they know "The Truth" and use force to
apply it to others.
-------------------------------------------
Matt Mathis  (Email is best)
Home & mobile: 412-654-7529 please leave a message if you must call.



On Fri, Oct 1, 2021 at 11:02 AM Luigi Rizzo via Rpm <
rpm@lists.bufferbloat.net> wrote:

> On Fri, Oct 1, 2021 at 6:33 PM Bob McMahon <bob.mcmahon@broadcom.com>
> wrote:
> >
> > hmm, this looks interesting to a test & measurement guy. Can it be done
> with a setsockopt? I might want to add this as an iperf2 option,
> particularly if it's broadly available,
>
>
> I would be happy to submit it as one or two upstream patches --
> perhaps one to implement
> the basic "ignore_holes" + setsockopt(), and another mechanism (if
> there isn't one
> already) to override defaults sockopts on certain sockets.
>
> I do think we need more readily available testing tool,
>
> cheers
> luigi
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Re: [Rpm] [Bloat] Relentless congestion control for testing purposes

[Jonathan Morton]

> On 29 Sep, 2021, at 2:17 am, Dave Taht <dave.taht@gmail.com> wrote:
> 
> In today's rpm meeting I didn't quite manage to make a complicated point. This long-ago proposal of matt mathis's has often intrigued (inspired? frightened?) me:
> 
> https://datatracker.ietf.org/doc/html/draft-mathis-iccrg-relentless-tcp-00
> 
> where he proposed that a tcp variant have no response at all to loss or markings, merely replacing lost segments as they are requested, continually ramping up until the network basically explodes.

I think "no response at all" is overstating it.  Right in the abstract, it is described as removing the lost segments from the cwnd; ie. only acked segments result in new segments being transmitted (modulo the 2-segment minimum).  In this sense, Relentless TCP is an AIAD algorithm much like DCTCP, to be classified distinctly from Reno (AIMD) and Scalable TCP (MIMD).

   Relentless congestion control is a simple modification that can be
   applied to almost any AIMD style congestion control: instead of
   applying a multiplicative reduction to cwnd after a loss, cwnd is
   reduced by the number of lost segments.  It can be modeled as a
   strict implementation of van Jacobson's Packet Conservation
   Principle.  During recovery, new segments are injected into the
   network in exact accordance with the segments that are reported to
   have been delivered to the receiver by the returning ACKs.

Obviously, an AIAD congestion control would not coexist nicely with AIMD based traffic.  We know this directly from experience with DCTCP.  It cannot therefore be recommended for general use on the Internet.  This is acknowledged extensively in Mathis' draft.

> In the context of *testing* bidirectional network behaviors in particular, seeing tcp tested more than unicast udp has been, in more labs, has long been on my mind.

Yes, as a tool specifically for testing with, and distributed with copious warnings against attempting to use it more generally, this might be interesting.

 - Jonathan Morton