VoIP QoS - breaking voice and flapping interface ?!

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Jul 25th, 2007
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Hello everyone,


To practice QoS I build a trivial topology (attached image) to simulate a link bottleneck:

- A WAN connection 1.544 mbps between two LAN, both r1 and r2 are performing static NAT for the internal PC1 ,r1(inside local=172.16.1.1, inside global =10.10.10.11), r2(inside local=10.10.10.11, inside global=192.168.42.1), so PC1 appears to both PC2 and IPPBX as 192.168.42.1.


To simulate concurrent traffic in the network, PC1 is performing the following activities:

- Calling PC2 through IP PBX.

- read streaming audio from the media server rtsp traffic.

- http browsing.

- ftp downloading.

As you guess the goal is to guarantee an acceptable voice quality call between PC1 and PC2, so I applied QoS policy on router r2 as follow:


!!!!!!!!!

class-map match-all http-set

match protocol http

class-map match-all rtspplayer-set

match protocol rtspplayer

class-map match-all sip-voice-set

match protocol rtp

class-map match-all ftp-set

match protocol ftp

class-map match-all sip-voice-policy

match ip dscp ef

class-map match-all http-policy

match ip dscp af11

class-map match-all rtspplayer-policy

match ip dscp default

class-map match-all ftp-policy

match ip dscp af21

!

policy-map mypolicy

class sip-voice-policy

priority 500

class ftp-policy

bandwidth 150

random-detect dscp-based

class http-policy

bandwidth 100

random-detect dscp-based

class rtspplayer-policy

bandwidth 50

queue-limit 50

!

policy-map myset

class sip-voice-set

set ip dscp ef

class ftp-set

set ip dscp af21

class http-set

set ip dscp af11

class rtspplayer-set

set ip dscp default

!

interface Ethernet0/0

service-policy input myset

!

interface Serial0/0

service-policy output mypolicy

!!!!!!!!!


The result is a breaking voice quality (more comprehensible than without any QoS) and r2 e0/0 interface is continuously oscillating between up and down.


Even though I assigned a strict priority queue (LLQ) to the voice traffic (rtp) and allocated a bandwidth of 500kbps (theorically 2 calls need just 168kbps with overhead), the sniffer at PC1 is showing a variable jitter between (0-14ms) and a packet loss up to 43.74%, average audio throughput of 33.28kbps and an average packet delay of 13.72ms.

Any idea about this issue?

Thank you inadvance.

Abdel



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dgahm Wed, 07/25/2007 - 10:25
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You need to fix the flapping interface before anything else. Bad cable maybe?


A show policy-map interface s0/0 will tell you if the service policies are functioning correctly.


Please rate helpful posts.


Dave

abdel_n Thu, 07/26/2007 - 02:01
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Think you for you response,


The interface was not flapping before applying service policy to interfaces and it stops flapping after (no service-policy).


Here is the output of (sh policy-map interface s0/0) and voice traffic is being dropped, about 43.74% as mentioned by the sniffer before. I intentionally assigned more bandwidth than needed and with LLQ there should not be any drops.

I guess I?am missing something!

----


r2#sh policy-map int s0/0

Serial0/0


Service-policy output: mypolicy


Class-map: sip-voice-policy (match-all)

29727 packets, 6064308 bytes

5 minute offered rate 76000 bps, drop rate 0 bps

Match: ip dscp ef

Queueing

Strict Priority

Output Queue: Conversation 264

Bandwidth 500 (kbps) Burst 12500 (Bytes)

(pkts matched/bytes matched) 29689/6056556

(total drops/bytes drops) 1746/356184


Class-map: ftp-policy (match-all)

0 packets, 0 bytes

5 minute offered rate 0 bps, drop rate 0 bps

Match: ip dscp af21

Queueing

Output Queue: Conversation 265

Bandwidth 150 (kbps)

(pkts matched/bytes matched) 0/0

(depth/total drops/no-buffer drops) 0/0/0

exponential weight: 9

mean queue depth: 0


dscp Transmitted Random drop Tail drop Minimum Maximum Mark

pkts/bytes pkts/bytes pkts/bytes thresh thresh prob

af11 0/0 0/0 0/0 32 40 1/10



Class-map: http-policy (match-all)

614 packets, 439620 bytes

5 minute offered rate 0 bps, drop rate 0 bps

Match: ip dscp af11

Queueing

Output Queue: Conversation 266

Bandwidth 100 (kbps)

(pkts matched/bytes matched) 602/432122

(depth/total drops/no-buffer drops) 0/0/0

exponential weight: 9

mean queue depth: 0


dscp Transmitted Random drop Tail drop Minimum Maximum Mark

pkts/bytes pkts/bytes pkts/bytes thresh thresh prob

af11 614/439620 0/0 0/0 32 40 1/10


Class-map: rtspplayer-policy (match-all)

5810 packets, 6675919 bytes

5 minute offered rate 0 bps, drop rate 0 bps

Match: ip dscp default

Queueing

Output Queue: Conversation 267

Bandwidth 50 (kbps) Max Threshold 50 (packets)

(pkts matched/bytes matched) 5541/6449694

(depth/total drops/no-buffer drops) 5/5/2


Class-map: class-default (match-any)

395 packets, 73997 bytes

5 minute offered rate 0 bps, drop rate 0 bps

Match: any


dgahm Thu, 07/26/2007 - 14:52
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Interesting, never heard of that happening. What does a show policy-map interface e0/0 look like? I assume it the service policy on e0/0 that causes the flapping.


What version code? I just saw a notice about serious QOS faults in 12.4(11)T.

Hieu Cao Mon, 07/30/2007 - 12:58
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From your QoS config, you're assigning a total of 800 Kbps of bw - Do you have this command "max-reserved bandwidth 100" under your serial interface?


There are alot of drops for your sip-voice-policy, thus causing VoIP quality issue.


Bandwidth 500 (kbps) Burst 12500 (Bytes)

(pkts matched/bytes matched) 29689/6056556

(total drops/bytes drops) 1746/356184


Also, take a look at your switch port where the PCs are connected...any dropped packets there? and try removing "service-policy input myset" from your E0/0, then made the test calls again.


HTH,

hieu

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