[j-nsp] BGP tcp-mss

[david.roy at orange-ftgroup.com]

Hi all,

Do you why, when I set the tcp-mss to 4096 between to BGP neighbors, this one is well exchange between the TCP Connection Establishment but when I check the mss via the command "show system connection extensive" I see that the mss used is 2048 (the half).

R1 config - lo0 172.20.223.6 :

  neighbor 172.20.223.5 {
        tcp-mss 4096;
    }
R2 config - lo0 172.20.223.5

  neighbor 172.20.223.6 {
        tcp-mss 4096;
    }

One Link between R1 and R2 with MTU set to 4484.

TCPDUMP output on R2 - we see that 4096 is well exchanged :

15:49:29.681732 Out IP 172.20.223.5.55025 > 172.20.223.6.179: S 2338565447:2338565447(0) win 16384 <mss 4096,nop,wscale 0,nop,nop,timestamp 2417320394 0,sackOK,eol>
15:49:29.682439  In IP 172.20.223.6.179 > 172.20.223.5.55025: S 1139135010:1139135010(0) ack 2338565448 win 16384 <mss 4096,nop,wscale 0,nop,nop,timestamp 455578404 2417320394,sackOK,eol>
15:49:29.682455 Out IP 172.20.223.5.55025 > 172.20.223.6.179: . ack 1 win 16384 <nop,nop,timestamp 2417320394 455578404>


On R2 : show system connection extensive | find 172.20.223.6

tcp4       0      0  172.20.223.5.55025                            172.20.223.6.179                              ESTABLISHED
   sndsbcc:          0 sndsbmbcnt:          0  sndsbmbmax:     131072
sndsblowat:       2048 sndsbhiwat:      16384
   rcvsbcc:          0 rcvsbmbcnt:          0  rcvsbmbmax:     131072
rcvsblowat:          1 rcvsbhiwat:      16384
   proc id:       1131  proc name:        rpd
       iss: 2338565447      sndup: 2338567245
    snduna: 2338567245     sndnxt: 2338567245      sndwnd:      16384
    sndmax: 2338567245    sndcwnd:       4096 sndssthresh: 1073725440
       irs: 1139135010      rcvup: 1139136242
    rcvnxt: 1139136261     rcvadv: 1139152645      rcvwnd:      16384
       rtt:          0       srtt:       2230        rttv:        574
    rxtcur:       1200   rxtshift:          0       rtseq: 2338567226
    rttmin:       1000  mss:       2048


N.B: Release 9.3

Thank you
Regards,
David


David Roy
Orange France - RBCI IP Technical Assistance Center
Tel.   +33(0)299876472
Mob. +33(0)685522213
Email. david.roy at orange-ftgroup.com<mailto:david.roy at orange-ftgroup.com>



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