UIT - Secteur de la normalisation des télécommunications
ITU - Telecommunication Standardization Sector
UIT - Sector de Normalización de las Telecomunicaciones

Study Period 1997-2000

  16	  

Geneva, 14-25 September 1998
	  E
Question(s):	13/16

SOURCE*:	
	　　　Canon Inc.
             		    Matsushita Electric Industrial Co., Ltd.
                         Mitsubishi Electric Corp.
                         NEC Corporation
	           NTT Corporation
　                     Oki Electric Industry Co., Ltd.
                         Ricoh Co., Ltd.
　                     Toshiba Corporation

TITLE:	PROPOSAL OF TCP SUPPORT ON H.323 ANNEX D
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1.ABSTRACT
	We discussed the effect of facsimile communication in both case of TCP and UDP. As a result,  it was clarified UDP does not guarantee the reliability of facsimile communication when the packet losses occur.
	This contribution proposes that TCP should be mandatory as COM16-60-E describes, and the description of condition for selecting TCP or UDP should be added to the H.323 Annex D.
2.	DISCUSSION
	COM16-60-E H.323 Annex D describes TCP is mandatory and UDP is option support for real-time facsimile procedures. While, T.38 Annex B describes both TCP and UDP are mandatory.
	However two implementation terminals are required for interoperability, so it desirable to specify one of which methods to take for mandatory.
	
2.1 Comparison of TCP and UDP
	The differences between TCP and UDP in capability are shown in Table.1.
(1)	Packet Losses
	When using UDP,  the network with packet losses need the transport mechanism for recovery of packet losses. Therefore, T.38 is provided with UDPTL(UDP Transport Layer). As the message of UDPTL has redundancy, the packet losses can be recovered if they do not occur continuously. 
	But, in the network with more than 3 hops of routing, the packet losses occur continuously at the  maximum of fifty five(55) percent in total. Because the losses are caused by congestion of traffic sent from back-bone to local line.(shown in FIGURE1) UDP may disconnect the facsimile connections on the network without QoS, so retransmission by ECM is needed. However, most of facsimile terminals of home usage are not implemented with the ECM. For this reason, UDP does not guarantee the reliability of facsimile communication. 
	On the other hand, TCP can retransmit the lost packets through transport procedures. But TCP may  disconnect a facsimile connection after a certain T.30 timer-over as the slide windows mechanism delays detecting the packet losses and the retransmission packets increase. However, this problem can be solved by preparing the  buffer absorbing the jitter of facsimile communication, and spoofing facsimile terminal in facsimile Gateways and terminals. The facsimile communications can allows larger jitter than voice can because of its bigger transmission delay tolerance. In fact, we had good results of TCP implementation enough to guarantee T.30 timing.     

	

	
	Condition：Load Time = 1 minutes, Packet Size = 256 bytes, Load Traffic = 80kbit/s, 96kbit/s 
	                  FAX:UDP, Packet Size = 256 bytes, Traffic = 9600 bit/s
Load						
Traffic	1 packet loss	2 
packet losses	3
 packet losses	4 
packet losses	More 5
packet losses	
80kbit/s	78%	13%	7%	1%	1%	
96kbit/s	45%	30%	13%	9%	3%	
FIGURE1 Rate of continuous packet losses to total packet losses on UDP

(2)Bandwidth
	The benefit of UDP is a small delay time of transmission, but the bandwidth increases than TCP for the redundancy of message. If using UDP, the more large bandwidth is needed in the network.  
	
	(3)Round Trip Time
	The round trip time of TCP is longer than UDP, because of  waiting ACK signal. Since the critical timing on the T.30 is 5 seconds, so the allowable round trip time of TCP including a waiting ACK is 2.5seconds, and the delay time should be 1.25 seconds.
	In contrast, UDP is allowed 2.5 seconds for the delay time, because UDP does not wait a ACK signal. In case of TCP requiring the shake hand procedures, the ACK signal is not needed in TCP. Because the response packets can include ACK signal in TCP by appending Positive Acknowledgment. Therefore, under the most critical T.30 condition, the limit of delay time is same between TCP and UDP.   
	
Table.1 Comparison of TCP and UDP 
Item	TCP	UDP with UDPTL	
Packet Losses	Retransmissions by TCP 	Redundancy Message
or Retransmissions by ECM	
Bandwidth	facsimile capacity
+TCP header overhead	facsimile capacity x n
+UDP header overhead
(n=more than 2)	
Round Trip Time	Less than 1.25s
(Wait ACK Packets)
But, Less than 2.5s
(Positive Acknowledgment)	Less than 2.5s	

2.2 Network Usage Condition by TCP and UDP
	For the reason(discussion of 2.1), it is beneficial H.323 Annex D describe the condition to select TCP or UDP for an environment. 
(1)	TCP : It is effective when the bandwidth for facsimile communication is small and the possibility of packet losses is large. For example, the Internet of today, and the Intranet with small bandwidth. 
(2)	UDP：It is effective when the bandwidth for facsimile communication is large and the possibility of packet losses is small. For example, the Internet of further, and the Intranet with large bandwidth. 
Table.2 Network Usage Condition by TCP and UDP
Environmental Condition		            Selection	
Capacity or 
Tolerance of Bandwidth 	Small	TCP	
for Facsimile	Large	UDP with UDPTL	
Packet Losses	Larger*1	TCP	
	Small*2	UDP with UDPTL	
	*1: Continuous packet losses can be recovered by retransmission of TCP transport protocol.
	*2: Continuous packet losses can be recovered by retransmission algorithm of an end-terminal. 
2.3 Transport Protocol Negotiation
As mentioned 2.2,  the best selection of  TCP or UDP should be decided based on the condition of network. When interworking between H.323 Annex D implementation  and T.38 implementation, the selection of each protocols can be negotiated by the capability procedures.     

3.CONCLUSION
	As the SG8 June meeting, T.38 Annex B was decided that TCP and UDP were mandatory. But the discussion of facsimile transmission in SG8 has not presented enough data that shows UDP is mandatory. This contribution discusses this points in detail and shows UDP does not guarantee the reliability of facsimile communication, specifically the usage of UDP is limited to the network with small packet losses. For this reason, it is undesirable that UDP is mandatory.
	While, as mentioned 2.1, TCP is the better selection under the condition in packet losses and bandwidth.
	
	PROPOSAL:
(1) 	We propose that TCP should be mandatory and UDP option in H.323 Annex D as COM16-60-E describes.
(2) 	We propose that the description(as mentioned Table2) of condition for selecting TCP or UDP should be added to the H.323 Annex D section4.2 “Opening the Facsimile channels”. 

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