[c-nsp] Fwd: IPV6 Path MTU DIscovery Test
Oliver Boehmer (oboehmer)
oboehmer at cisco.com
Wed Aug 1 11:03:59 EDT 2012
well, you don't have to use TCP, but it's the most convinient as you have the client prepared to react to the packet-too-big ICMP/ICMPv6 messages from intermediate nodes, and see the effect it has on the MSS. You can't test this just by sending icmp packets from the CLI?!
And to your question: yes, it is the MSS reported, and the below output suggests that you have PMTUD enabled on the routers and you're using a min MTU of 1500 along the path, otherwise the nodes would have used MSS=532 or something low like this..
oli
From: Xu Hu [mailto:jstuxuhu0816 at gmail.com]
Sent: 01 August 2012 15:54
To: Oliver Boehmer (oboehmer)
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Fwd: IPV6 Path MTU DIscovery Test
Must use the TCP packets? I want to just use the ping packet to test the effect of PMTUD. Is any simple way to test this feature?
Another thing, if use the iBGP to test, i find one show ip bgp neighbor command as below, the MSS you mentioned is the "Datagrams (max data segment is 1460 bytes):" ?
Thanks for your help.
Hu Xu
Router# show ip bgp neighbors 172.16.232.178
BGP neighbor is 172.16.232.178, remote AS 10, external link
Index 1, Offset 0, Mask 0x2
Inbound soft reconfiguration allowed
BGP version 4, remote router ID 172.16.232.178
BGP state = Established, table version = 27, up for 00:06:12
Last read 00:00:12, hold time is 180, keepalive interval is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 19 messages, 0 notifications, 0 in queue
Sent 17 messages, 0 notifications, 0 in queue
Inbound path policy configured
Route map for incoming advertisements is testing
Connections established 2; dropped 1
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 172.16.232.181, Local port: 11002
Foreign host: 172.16.232.178, Foreign port: 179
Enqueued packets for retransmit: 0, input: 0, saved: 0
Event Timers (current time is 0x530C294):
Timer Starts Wakeups Next
Retrans 12 0 0x0
TimeWait 0 0 0x0
AckHold 12 10 0x0
SendWnd 0 0 0x0
KeepAlive 0 0 0x0
GiveUp 0 0 0x0
PmtuAger 0 0 0x0
iss: 133981889 snduna: 133982166 sndnxt: 133982166 sndwnd: 16108
irs: 3317025518 rcvnxt: 3317025810 rcvwnd: 16093 delrcvwnd: 291
SRTT: 441 ms, RTTO: 2784 ms, RTV: 951 ms, KRTT: 0 ms
minRTT: 0 ms, maxRTT: 300 ms, ACK hold: 300 ms
Flags: higher precedence, nagle
Datagrams (max data segment is 1460 bytes):
Rcvd: 15 (out of order: 0), with data: 12, total data bytes: 291
Sent: 23 (retransmit: 0), with data: 11, total data bytes: 276
2012/8/1 Oliver Boehmer (oboehmer) <oboehmer at cisco.com<mailto:oboehmer at cisco.com>>
Well, you need to create TCP connections to see the effect of PMTUD.. you could build an iBGP session across the test network, and play around with the minimum link MTU along the path. The MSS (shown in show bgp neighbor ..) should adapt as you change it..
oli
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net<mailto:cisco-nsp-bounces at puck.nether.net> [mailto:cisco-nsp-<mailto:cisco-nsp->
> bounces at puck.nether.net<mailto:bounces at puck.nether.net>] On Behalf Of Xu Hu
> Sent: 01 August 2012 02:33
> To: cisco-nsp at puck.nether.net<mailto:cisco-nsp at puck.nether.net>
> Subject: [c-nsp] Fwd: IPV6 Path MTU DIscovery Test
>
> Anybody have any good idea?
>
> Thanks advance for any inputs,
> Xu Hu
>
> Begin forwarded message:
>
> > From: TJ <trejrco at gmail.com<mailto:trejrco at gmail.com>>
> > Date: 1 August, 2012 5:23:46 GMT+08:00
> > To: Xu Hu <jstuxuhu0816 at gmail.com<mailto:jstuxuhu0816 at gmail.com>>
> > Subject: Re: [c-nsp] IPV6 Path MTU DIscovery Test
> > Reply-To: trejrco at gmail.com<mailto:trejrco at gmail.com>
> >
> > In order to test it, take one of the links along the path and change the
> (IPv6) MTU to something smaller - say 1300B. Then send large packets, with
> WireShark running. And no, you cannot disable PMTUD in IPv6.
> >
> > HTH!
> > /TJ
> >
> >
> > On Tue, Jul 31, 2012 at 6:43 AM, Xu Hu <jstuxuhu0816 at gmail.com<mailto:jstuxuhu0816 at gmail.com>> wrote:
> > Hi Experts,
> >
> > I have some queries about the PMTU for IPv6.
> > 1. From my understanding, path MTU discovery in IPv6 allows a host to
> > dynamically discover and adjust to differences in the MTU size of every
> > link along a given data path, So the fragmentation is handled by the
> > source, if i want to test this feature in the IPv6 network, anyone have
> any
> > good ideas?
> > For example, as below topology, if i want to test the Path MTU discovery,
> > how to test, i can ping from CE-1 to CE--2 with a specific IPv6 packet
> > size.
> > CE-1------PE-1------------PE-2--------CE-2
> >
> > 2. If i need to configure the Path MTU discovery, just one command
> > mentioned in Cisco.com, which is "ipv6 flowset" under configuration mode.
> > So if i want to disable that in test scenario, just using the "no ipv6
> > flowset" is okay? Cisco don't mentioned that.
> > By default, it is enable for ip routers or not?
> >
> > Thanks for any inputs.
> >
> > Best Regards,
> > Hu Xu
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