Cisco Voice over IP

 

Port Numbers for QoS

 

All Cisco VoIP products utilize the same UDP port range (16384-32767) for RTP.

 

Note: The RTP protocol uses RTCP (Real Time Control Protocol) to control delivery of RTP packets. While RTP ports use even numbers, RTCP ports use odd numbers in the range of 16384-32767.

 

o        H.323/H.225 = TCP 1720

o        H.323/H.245 = TCP 11xxx (Standard Connect)

o        H.323/H.245 = TCP 1720 (Fast Connect)

o        H.323/H.225 RAS = TCP 1719

o        Skinny = TCP 2000-2002 (CM Encore)

o        ICCP = TCP 8001-8002 (CM Encore)

o        MGCP = UDP 2427, TCP 2428 (CM Encore)

o        SIP= UDP 5060, TCP 5060 (configurable)

 

class-map match-all voice-signaling
  match access-group 103
class-map match-all voice-traffic
  match access-group 102

 

access-list 102 permit udp any any range 16384 32767
access-list 103 permit tcp any eq 1720 any
access-list 103 permit tcp any any eq 1720

 

policy-map voice-policy
  class voice-signaling
   bandwidth 8
  class voice-traffic
    priority 48
  class class-default
   fair-queue

 

 

 

 

class-map match-all RTP

match ip precedence 5

 

policy-map OutboundPolicy

class RTP

set CoS 5

class class-default

set CoS 0

 

 

 

 

access-list 105 permit ip any any dscp ef
access-list 105 permit udp any any  range 16384 32767
access-list 105 permit ip any any precedence critical
!

class-map match-all voice

match access-group 105

 

policy-map VOIP
  class voice
    priority 48
  class class-default 
   fair-queue

 

 

Some particulary VoIP packet characteristics that have to be considered are:

 
  • VoIP travels on RTP protocol over UDP.
  • VoIP packets are very small. Payload is 20 to 150 bytes with a RTP/UDP/IP header of 40 bytes (IP=20 bytes; UDP=12 bytes; RTP=8 bytes). Then due to the high relation between header size and payload size the transmission of VoIP packets is not an efficient process.
  • Being VoIP a protocol to service a playback application (voice playback) its maximum end to end delay should be less that 150-200ms; 150ms is better. This, to guarantee the good quality of the sound to be transmitted.

 

RTPC - Realtime Transport Protocol Header Compression: RTP is a protocol used for carrying multimedia application traffic, including audio and video, over an IP network. RTP packets have a 40-byte header and typically a 20 to 150 payload. RTP protocol travels over UDP. Given the size of the IP/UDP/RTP header combination, it is inefficient to transmit those small payloads using an uncompressed header. RTPC is a technology that helps RTP run more efficiently, especially over lower-speed links, by compressing the RTP/UDP/IP header from 40 bytes to 2 to 5 bytes. This is especially beneficial for smaller packets (such as IP voice traffic) on slower links, where RTP header compression can reduce overhead and transmission delay significantly.

 

 

 

DSCP

Precedence

Purpose

0

0

Best effort

8

1

Class 1

16

2

Class 2

24

3

Class 3

32

4

Class 4

40

5

Express forwarding

48

6

Control

56

7

Control

 

 

Assured forwarding service

RFC2697 defined a "assured forwarding" service. This codified the idea of "bronze", "silver" and "gold" levels of network service above the Best Effort service.

DSCP

Service

0

Best effort

8

Class 1

10

Class 1, gold (AF11)

12

Class 1, silver (AF12)

14

Class 1, bronze (AF13)

16

Class 2

18

Class 2, gold (AF21)

20

Class 2, silver (AF22)

22

Class 2, bronze (AF23)

24

Class 3

26

Class 3, gold (AF31)

27

Class 3, silver (AF32)

30

Class 3, bronze (AF33)

32

Class 4

34

Class 4, gold (AF41)

36

Class 4, silver (AF42)

38

Class 4, bronze (AF43)

40

Express forwarding

48

Control

56

Control

Of course, the IETF couldn't use "loaded" terms like Gold, Silver and Bronze to describe networks services. Apparently numbers are less loaded, so the the RFC has descriptions like "AF21" rather than "Class 2, Gold".

The actual values of the DSCP for the Assured Forwarding service make sense once you realise that they leave the last bit of the DSCP as 0 to allow for possible future developments with the Assured Forwarding service.

Expedited forwarding service

RFC2598 defined a "expedited forwarding" service. This service is designed to allow ISPs to offer a service with attributes similar to a "leased line". This service offers the ultimate in low loss, low latency and low jitter by ensuring that there is always sufficent room in output queues for the contracted expedited forwarding traffic.

The Expedited Forwarding service has a DSCP of 46.

DSCP

Service

0

Best effort

8

Class 1

10

Class 1, gold (AF11)

12

Class 1, silver (AF12)

14

Class 1, bronze (AF13)

16

Class 2

18

Class 2, gold (AF21)

20

Class 2, silver (AF22)

22

Class 2, bronze (AF23)

24

Class 3

26

Class 3, gold (AF31)

27

Class 3, silver (AF32)

30

Class 3, bronze (AF33)

32

Class 4

34

Class 4, gold (AF41)

36

Class 4, silver (AF42)

38

Class 4, bronze (AF43)

40

Express forwarding

46

Expedited forwarding (EF)

48

Control

56

Control

 

 

RFC791 gives Precedence the values in the table below

Precedence

Purpose

0

Routine

1

Priority

2

Immediate

3

Flash

4

Flash Override

5

CRITIC/ECP

6

Internetwork Control

7

Network Control

 

 

This table compares QoS values for IP precedence, Differentiated Services Code Point (DSCP) and Multiprotocol Label Switching (MPLS) Experimental (EXP) values along with standard values used in Cisco IOS® Software for configuration. For an introduction to this topic, refer to QoS for VoIP.

Per Hop Behavior (PHB)

DSCP value

DSCP bits

IP precedence and Class of Service (COS) value (xyz000)

Comment:

MPLS EXP bits

Default

0

000000

0=routine
  • Best effort

 

 

1

000001

 

 

 

 

2

000010

 

 

 

 

3

000011

 

 

 

 

4

000100

 

 

 

 

5

000101

 

 

 

 

6

000110

 

 

 

 

7

000111

 

 

 

CS1

8

001000

1=priority

 

 

 

9

001001

 

 

 

AF11

10

001010

 

  • Assured forwarding class 1 (001)
  • Drop priority 010 (1)
  • Dropped last in AF1 class

 

 

11

001011

 

 

 

AF12

12

001100

 

  • Assured forwarding class 1 (001)
  • Drop priority 100 (2)
  • Dropped 2nd in AF1 class

 

 

13

001101

 

 

 

AF13

14

001110

 

  • Assured forwarding class 1 (001)
  • Drop priority 110 (3)
  • Dropped first in AF1 class

 

 

15

001111

 

 

 

CS2

16

010000

2=immediate

 

1

 

17

010001

 

 

 

AF21

18

010010

 

  • Assured forwarding class 2 (010)
  • Drop priority 010 (1)
  • Dropped last in AF2 class

 

 

19

010011

 

 

 

AF22

20

010100

 

  • Assured forwarding class 2 (010)
  • Drop priority 100 (2)
  • Dropped 2nd in AF2 class

 

 

21

010101

 

 

 

AF23

22

010110

 

  • Assured forwarding class 2 (010)
  • Drop priority 110 (3)
  • Dropped first in AF2 class

 

 

23

010111

 

 

 

CS3

24

011000

3=Flash

 

2

 

25

011001

 

 

 

AF31

26

011010

 

  • Assured forwarding class 3 (011)
  • Drop priority 1 (010)
  • Dropped last in AF3 class

 

 

27

011011

 

 

 

AF32

28

011100

 

  • Assured forwarding class 3 (011)
  • Drop priority 2 (100)
  • Dropped 2nd in AF3 class

 

 

29

011101

 

 

 

AF33

30

011110

 

  • Assured forwarding class 3 (011)
  • Drop priority 3 (110)
  • Dropped first in AF3 class