INTERNET-DRAFT HAK GOO LEE Expires: March 2003 PYUNG SOO KIM YOUNG KEUN KIM CHU KYO SHIN SAMSUNG ELECTRONICS Optimal Detecting Increases in PMTU Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [RFC2026]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document presents a new method for the detection of increases in PMTU using the newly defined Hop-by-Hop option header. To detect increases in a path's PMTU, a node does not increase its assumed PMTU unconditionally without considering network status, but measures its real PMTU, and then replaces the previous PMTU with new one. To measure node's real PMTU, the node sends the IP packet with the newly defined Hop-by-Hop option header to the destination node right before a timer expires. This can eliminate the chance of occurrence of packets being discarded and Packet Too Big messages being generated. Table of Contents Abstract ............................................................1 1 Introduction.......................................................2 2 Proposed Method : Overview.........................................2 2.1 Measurement DIP Option Format................................4 2.2 Response DIP Option Format...................................4 3 Node Requirements for Proposed Method..............................5 3.1. Source Node and Destination Node............................5 3.2. Nodes on Routing Path.......................................5 4 Operation Procedure................................................6 5 References ........................................................6 6 Authors' Addresses ................................................6 LEE et al. Expires March 2003 [Page 1] Internet Draft Optimal Detecting Increases in PMTU Oct 2002 1. Introduction Due to changes in the routing topology, the Path MTU (PMTU) of a path may change over time. Therefore, in Path MTU Discovery [RFC1981], to detect increases in a path's PMTU, a node periodically increases its assumed PMTU although this is done infrequently. However, in this method, the assumed PMTU is increased by the node's link MTU unconditionally. This may result in packets being discarded and Packet Too Big messages being generated. Therefore, this document presents a new method for the detection of increases in PMTU using the newly defined Hop-by-Hop Option header. In this new method, to detect increases in a path's PMTU, a node does not increase its assumed PMTU unconditionally without considering network status, but measures its real PMTU, and then replaces the previous PMTU with new one. To measure node's real PMTU, the node sends the IP packet with the newly defined Hop-by-Hop option header to the destination node right before a timer expires. In this document, this timer will be called the detection timer. The recommended setting for this detection timer is also 10 minutes as [RFC1981]. Note that the newly obtained PMTU is always greater than or equal to the previous one. After then, the new PMTU is used to send packets. This can eliminate the chance of occurrence of packets being discarded and Packet Too Big messages being generated, which might occur when the assumed PMTU is increased unconditionally by the detection timer in PMTU Discovery [RFC1981]. The proposed method in this document can provide the best-conditioned network environment because the optimized PMTU is obtained for changes in the routing topology. 2. Proposed Method : Overview This section gives a brief overview of the new method for the detection of increases in PMTU. A router on routing path examines only IP header, Hop-by-Hop option header and routing header in extension header. Other extension headers and data are examined only by a source node and destination node. In the new method, Hop-by-Hop option header is used to measure increases in a path's PMTU The Hop-by-Hop option header and option is defined as below. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Hdr Ext Len | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | . . . Options . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1 Hop-by-Hop Option Header LEE et al. Expires March 2003 [Page 2] Internet Draft Optimal Detecting Increases in PMTU Oct 2002 Currently, Hop-by-Hop option headers are Pad1, PadN, Jumbo Payload, and Router Alert options. This document defines a new option. It is called 'Detection of Increasing PMTU option (DIP option)'. Then, the IP packet including Hop-by-Hop option header with DIP option has the following format. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| Traffic Class | Flow Label | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload Length | Next Header | Hop Limint | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Source Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Destination Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | 0 | Option Type | Opt Data Len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Data(PMTU) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Dummy Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | .... +-+ Option Type 8-bit identifier of the type of option. Opt Data Len 8-bit unsigned integer. Length of the Option Data field of this option, in octets. Option Data(PMTU) Measured PMTU value with 4 bytes Figure 2 IP Packet with DIP option The Option Type identifiers are internally encoded such that their highest-order two bits specify the action that must be taken when the router does not recognize the Option Type. The third highest order bit of the Option Type specifies whether or not the Option Data (PMTU) of that option can change en-route to the packet's final destination. The other five bits are defined arbitrarily. The DIP option has two Option Types. The one is Measurement DIP Option and the other is Response DIP Option, which will be shown in following subsections. If any of the routers en-route cannot recognize the Option Types, it discards the packet silently. LEE et al. Expires March 2003 [Page 3] Internet Draft Optimal Detecting Increases in PMTU Oct 2002 2.1 Measurement DIP Option Format The measurement DIP option format is defined to measure real PMTU as shown in Figure 2. The source node sends the IP packet with this measurement DIP option format to the destination node. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 1 1|0 0 1 1 1|0 0 0 0 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | P M T U | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 01 - Discard the packet 1 - Option Data may change en-route 00111 - Option Type Option Type - 103 Option Length - 4 PMTU - 4 Octets Figure 3 Measurement DIP Option Format 2.2 Response DIP Option Format The response DIP option format is defined to response measured real PMTU as shown in Figure 3. The destination node sends the IP packet with this response DIP option format to the source node. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 1 0|0 0 1 1 1|0 0 0 0 0 1 0 0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | P M T U | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 01 - Discard the packet 0 - Option Data does not change en-route 00111 - Option Type Option Type - 71 Option Length - 4 PMTU - 4 Octets Figure 4 Measurement DIP Option Format As described on Path MTU Discovery [RFC 1981], a source node sends IP packet with Measurement DIP Option to a destination node before the detection timer expires so that it wants to increase PMTU value. Default MTU value is link MTU of the source node. While the packet is passing through nodes en-route, the following action occurs. - Compare PMTU in Measurement DIP Option and Link MTU of next hop. - Store the smaller value between them on MTU field of the packet. LEE et al. Expires March 2003 [Page 4] Internet Draft Optimal Detecting Increases in PMTU Oct 2002 By doing the same action on routers en-route, PMTU field of the IP packet with Measurement DIP Option is updated to have minimum PMTU. This is the optimal PMTU value. The destination node that receives the IP packet returns back this value to the source node using Response DIP Option. The source node that receives the optimal PMTU value determines the transmission length of a packet. This method can reduce the number of ICMP-Packet Too Big Error message, otherwise happens more frequently. This also reduce waste of network resource 3. Node Requirements for Proposed Method 3.1. Source Node and Destination Node A source node does not change the PMTU value for some amount of time after it comes to know PMTU based on PMTU Discovery [RFC1981]. When the source node wants to increase PMTU by the detection timer, it sends the IP packet with Measurement DIP Option to a destination. The PMTU field of Measurement DIP Option of the packet contains the value of link MTU of the source node and the data field contains dummy data. If the packet is discarded by a router that does not understand the packet format on routing, no response returns to the source node. For this reason, the source node uses a timer while transmitting the packet. This timer will be called the response timer. If there is no response until the response timer expires, the source node increase the PMTU as described in Path MTU Discovery [RFC1981]. Whenever a destination node receives the IP packet with Measurement DIP Option, it must response the IP packet with Response DIP Option immediately. By the Option Type, this packet cannot be modified by routers on routing path. 3.2. Nodes on Routing Path Nodes on a routing path are routers. When the IP packet with Measurement DIP Option arrives on these routers, they compare the PMTU in Measurement DIP Option and link MTU of the next hop. They select a smaller value between them and forward the packet with a PMTU value. After doing the same procedure, the final destination node comes to know minimum PMTU. If the IP packet with Response DIP Option arrives on theses routers, they forward the packet without any modification. If routers can not recognize two DIP Options, it discard the packet silently without any error message such as ICMP-Parameter Problem since the first two bits of the DIP Option is 01. LEE et al. Expires March 2003 [Page 5] Internet Draft Optimal Detecting Increases in PMTU Oct 2002 4. Operation Procedure When the source node wants to increase PMTU by the detection timer, it sends the IP packet with Measurement DIP Option to a destination. In this case, the Option Type in Measurement DIP Option is 103. While the packet is en-route, intermediate routers compare the PMTU in Measurement DIP Option and link MTU of the next hop. They select a smaller value between them, store the value to PMTU field of Measurement DIP Option and forward the packet with the updated PMTU. PMTU of the packet arrived at the destination node becomes minimum PMTU. The destination node sends the IP packet Response DIP Option to inform the source node. In this case, the Option Type in Measurement DIP Option is 71. After then, the source node sends packets using new PMTU. If there are routers that this feature is not implemented during transitional period, that is, if routers implements only previous PMTU Discovery [RFC 1981] on routing path, the DIP Options defined newly in this document is unknown parameter. Thus, in this case the router can silently discard the packet without ICMP-Parameter problem by the highest-order two bits, 01, in Option Type field. Additionally, the source node operates the response timer when transmitting the packet with DIP option. If the source node does not receive the IP packet with Response DIP Option until the response timer expires, the source node increase PMTU as described in Path MTU Discovery [RFC1981]. 5 References [RFC1981] Path MTU Discovery for IP version 6 (RFC 1981) 6 Authors' Addresses HAK GOO LEE Digital Media R&D Center, SAMSUNG ELECTRONICS Tel : +82 31 200 9309 Fax : +82 31 200 3147 E-mail : solited@samsung.co.kr PYUNG SOO KIM Digital Media R&D Center, SAMSUNG ELECTRONICS Tel : +82 31 200 4635 Fax : +82 31 200 3147 E-mail : kimps@samsung.com YOUNG KEUN KIM Digital Media R&D Center, SAMSUNG ELECTRONICS Tel : +82 31 200 3772 Fax : +82 31 200 3147 E-mail : ykksam@samsung.com CHU KYO SHIN Digital Media R&D Center, SAMSUNG ELECTRONICS Tel : +82 31 200 8318 Fax : +82 31 200 3147 E-mail : kyoshin@samsung.com LEE et al. Expires March 2003 [Page 6]