分类： 系统运维
2007-09-26 00:11:45
2.6. Path MTU
Standard RSVP [1] and Int-Serv [11] provide the RSVP sender with the
minimum MTU available between the sender and the receiver. This path
MTU identification capability is also provided for LSPs established
via RSVP.
//标准RSVP和Int-Serv能够发现一个发送者和接收者都认可的最小的MTU给RSVP
//sender。该MTU发现功能同样可以提供给RSVP创建的LSP。
Path MTU information is carried, depending on which is present, in
the Integrated Services or Null Service objects. When using
Integrated Services objects, path MTU is provided based on the
procedures defined in [11]. Path MTU identification when using Null
Service objects is defined in [16].
//MTU信息的的携带，取决于它是Int-Serv还是Null Service。当是Int-Serv
//对象，MTU通过[11]中定义的流程发现。当是Null Service对象，MTU通过
//[16]中定义的流程发现。
Awduche, et al. Standards Track [Page 13]
RFC 3209 Extensions to RSVP for LSP Tunnels December 2001
With standard RSVP, the path MTU information is used by the sender to
check which IP packets exceed the path MTU. For packets that exceed
the path MTU, the sender either fragments the packets or, when the IP
datagram has the "Don't Fragment" bit set, issues an ICMP destination
unreachable message. This path MTU related handling is also required
for LSPs established via RSVP.
//标准RSVP中，首节点使用MTU信息判断哪些IP包超过了路径MTU。对于超过的，
//如果"Fragment"标志位被设置，将发送一个目的不可达的ICMP消息，否则，
//IP包被分片。一场处理流程同样适用于RSVP创建的LSP。
The following algorithm applies to all unlabeled IP datagrams and to
any labeled packets which the node knows to be IP datagrams, to which
labels need to be added before forwarding. For labeled packets the
bottom of stack is found, the IP header examined.
//以下算法适用于所有未打标签的IP包，和所有打了标签但节点分辨出是IP包，
//且在转发前需要添加标签。******。
Using the terminology defined in [5], an LSR MUST execute the
following algorithm:
//用[5]中定义的术语，LSR必须执行以下算法。
1. Let N be the number of bytes in the label stack (i.e, 4 times the
number of label stack entries) including labels to be added by
this node.
//设N为包含本节点添加的标签的标签栈的字节数。
2. Let M be the smaller of the "Maximum Initially Labeled IP Datagram
Size" or of (Path MTU - N).
//设M为"出始标签IP包大小"的最小值或路径MTU-N(即实际MTU-标签栈字的
//大小)。
When the size of an IPv4 datagram (without labels) exceeds the value
of M,
//当IPv4包大小(除去标签)超过M，
If the DF bit is not set in the IPv4 header, then
//如果IPv4头中分片标志未设置，
(a) the datagram MUST be broken into fragments, each of whose
size is no greater than M, and
//(a)数据包必须分割成片段，每个片段不能大于M，然后
(b) each fragment MUST be labeled and then forwarded.
//(b)每个片段必须打标签再转发。
If the DF bit is set in the IPv4 header, then
//如果IPv4头中分片标志设置，
(a) the datagram MUST NOT be forwarded
//(a)不能转发该数据包。
(b) Create an ICMP Destination Unreachable Message:
//(b)创建一个ICMP目的不可达消息：
i. set its Code field [12] to "Fragmentation Required and
DF Set",
//i.设置编码字段[12]为"需要分片且DF位已设置"
ii. set its Next-Hop MTU field [13] to M
//设置ICMP的下一跳MTU字段为M。
(c) If possible, transmit the ICMP Destination Unreachable
Message to the source of the of the discarded datagram.
//(c)如果可能，关于该丢弃的数据包，发送该目的不可达ICMP消息到
//首节点。
When the size of an IPv6 datagram (without labels) exceeds the
value of M,
//当IPv6的数据包(除标签)的大小超过M。
Awduche, et al. Standards Track [Page 14]
RFC 3209 Extensions to RSVP for LSP Tunnels December 2001
(a) the datagram MUST NOT be forwarded
//(a)不能转发该数据包。
(b) Create an ICMP Packet too Big Message with the Next-Hop
link MTU field [14] set to M
//创建一个包超大ICMP消息，且下一跳MTU字段为M。
(c) If possible, transmit the ICMP Packet too Big Message to
the source of the of the discarded datagram.
//(c)如果可能，关于该丢弃的数据包，发送该包超大ICMP消息到
//首节点。
